Genesis of Precambrian iron and manganese deposits: Proceedings of the Kiev Symposium, 20-25 August 1970

Genesis of Precambrian iron and manganese deposits

Genèse des formations précambriennes de fer et de manganèse

Proceedings of the Kiev Symposium,

Actes du colloque de Kiev,

20-25 August 1970

20-25 août 1970

i\

'

Résumés e n français

Unesco

Paris 1973

Earth sciences

Sciences de la terre

9

In this series / Dans cette collection: 1. The seismicity of the earth,1953-1965, by J. P.Rothé / La séismicité du globe, 1953-1965,par J. Rothé. 2. Gondwana stratigraphy.IUGS Symposium,Buenos Aires, 1-15 October 1967 / La estratigrafía del Gondwana. Coloquio de la UICG,Buenos Aires, 1-15 de octubre de 1967. 3. Mineral map o$ Africa. Explanatory note / Carte minérale de l’Afrique.Notice explicative. 1/10O00 000. 4. Carte tectonique internationalede l’Afrique.Notice explicative / International tectonic map of Africa. Explanatory note. 1/5O00 000. 5. Notes on geomagneticobservatoryand survey practice,

by K. A. Wienert. 5. Méthodes d‘observationet de prospectiongéomagnétiques, par K. A. Wienert. 6. Tectoniquede l’Afrique/ Tectonics of Africa. 7. Geology of saline deposits.Proceedingsof the Hanover Symposium,15-21 May 1968 / Géologie des dépôts salins. Actes du Colloque de Hanovre, 15-21 mai 1968. 8. The surveillance and prediction of volcanic activity. A review of methods and techniques. 9. Genesis of Precambrianiron and manganese deposits. Proceedingsof the Kiev Symposium,20-25 August 1970 I Genèse des formations précambriennes de fer et de manganèse.Actes du Colloque de Kiev,20-25 août 1970. 10. Carte géologique internationale de l’Europe et des régions riveraines de la Méditerranée. Notice explicative / Internationalgeological map of Europe and the Mediterranean region. Explanatory note. 1/5O00 O00

11. 11. 12. 13.

(Édition multilingue: français, anglais, allemand, espagnol,italien, russe/ Multilingual edition: French, English,German,Spanish,Italian,Russian). Geological map of Asia and the Far East. 1/5O00 000. Explanatory note. Second edition. Carte géologique de l’Asie et de l’Extrême-Orient. 1/5O00 000.Notice explicative. Deuxième édition. Geothermal energy. Review of research. Carte tectonique de l’Europeet des régions avoisinantes. 1/2500 000.Notice explicative/Tectonic map of Europe and adjacent areas. 112 500 000.Explanatory note. (A paraître/To be published.)

Published by the United Nations Educational,Scientific and Cultural Organization, 7 Place de Fontenoy, 75700 Paris Printed by Presses Universitaires de France,Vendôme Publié par l’organisationdes Nations Unies pour l’éducation,la science et la culture, 7,place de Fontenoy,75700 Paris Imprimerie des Presses Universitaires de France,Vendôme ISBN 92-3-001107-X (Paper/ Broché) ISBN 92-3-001108-8 (Cloth/Relié) L.C.NO 73-79858

0 Unesco 1973 Printed in France

The designations employed and the presentation of the material in this publication do not imply the expression of any opinion whatsoever on the part of the Unesco Secretariat concerning the legal status of any country or territory, or of its authorities,or concerning the delimitations of the frontiers of any country or territory.

Les désignations employées et la présentation adoptée ici ne sauraient être interprétées comme exprimant une prise de position du Secrétariat de l’Unesco sur le statutjuridique ou le régime d’un pays ou d‘un territoire quelconque,non plus que sur le tracé de ses frontières.

Foreword

Avant-propos

The Precambrian is of very special significancein the evols ution of the Earth’s crust.It representsalmo st seven-eighth ofthegeologicalhistory of our planet. During this period of time, lasting approximately 4,000million years, the basement of continentalland masses and the deposits of iron and manganese ore were formed.These latter are of world-wide significanceboth in quantity and extent. They form part of the natural resources of the geographical environment and their study is important both for developed and developing countries. The study of Precambrian rocks and ore deposits includes various theoretical and practical aspects-economic, mineralogical, geochemical, tectonic. The research methodology applied to the Precambrian is very specific and fundamentally different from that used for other geological eras. Straightforward time-stratigraphicalmethods are not applicable here because of the lack of palaeontological criteria, destroyed by metamorphism.Successive granitizations form a complex which is very often difficult to bring into conventional order. In an attempt to throw some light on these complex geological phenomena, Unesco, in collaboration with the International Association of Geochemistry and Cosmochemistry of the InternationalUnion of Geological Sciences and the Academy of Sciences of the Ukrainian S.S.R., organized a symposium on the geology and genesis of Precambrian iron-manganeseformations and ore deposits.At the invitation of the Academy of Sciences,the meeting was held in Kiev from 20 to 25 August 1970.Some sixty specialists coming from twelve countries met at the Main Conference Hall of the Academy of Sciences of the Ukrainian S.S.R. and presented papers at this meeting. The participants were welcomed by R . V. Babiychuk, Minister of Culture of the Ukrainian S.S.R. and Chairman of the Ukrainian National Commission for Unesco,Opening addresses were also given by Academician N.P. Semenenko, Chairman of the symposium, and D r K. Lange of the Natural Resources Research Division of Unesco. In order to provide a systematic consideration of the problems, the programme was divided into four sections:

L’ère précambrienne a une importance toute particulière dans l’évolution de l’écorce terrestre. Elle couvre les sept huitièmes de l’histoire géologique de notre planète. Pendant cette période, qui a duré approximativement quatre milliards d’années,se sont formés le socle des masses continentales et les gisements de fer et de manganèse. Ces gisements précambriens présentent un intérêt mondial à la fois sur le plan de la quantité et sur celui de l’étendue.Ils font partie des ressources naturelles du milieu géographique et leur étude est utile tant aux pays développés qu’à ceux qui sont en voie de développement. L’étude des roches et gisements de minerais précambriens revêt divers aspects théoriques et pratiques :économiques, minéralogiques, géochimiques, tectoniques. Les méthodes de recherches appliquées, lorsqu’il s’agit du Précambrien, sont très spécifiques et diffèrent fondamentalement de celles qui sont utilisées pour d‘autres ères géologiques. Les méthodes stratigraphiquesde datation ne sont pas applicables ici,faute de critères d‘ordre paléontologique, dont l’absence est due au métamorphisme. Les granitisations successives ont donné naissance à un ensemble complexe qu’il est très souvent difficile de classer dans l’ordre conventionnel. Afin d‘essayer d‘éclairer quelque peu ces phénomènes géologiques complexes, l’Unesco, en collaboration avec l’Association internationale de géochimie et de cosmochimie de l’Union internationale des sciences géologiques et l’Académie des sciences de la République socialiste soviétiqued’Ukraine,a organisé un colloque sur la géologie et la genèse des formations précambriennes de fer et de manganèse. Sur l’invitation de l’Académie des sciences, la réunion s’est tenue à Kiev du 20 au 25 août 1970. U n e soixantaine de spécialistes venus de douze pays se sont réunis dans la grande salle des conférences de l’Académie des sciencesde la Républiquesocialistesoviétique d‘Ukraine et ont présenté des communications. Les participants ont été accueillis par M. R . V. Babiychuk, ministre de la culture de la RSS d‘Ukraine et par le président de la Commission nationale ukrainienne pour l’Unesco; des discours d‘ouverture ont été prononcés par M . N. Semenenko,

I. Genesis and types of iron-silicate and ferruginous cherty formations,their position in geosynclinal sedimentary or volcanic sequences and the relation between these and analogousmanganese-bearingformations. II. Absolute age dating of iron-silicate and ferruginous formations and their position in the Precambrian stratigraphic sequence. Analogous formations from the Phanerozoic. III. Differing degrees of metamorphism,the mineral facies and the petrographic nomenclature of ferruginous rocks such as ferruginous quartzites, taconites, jaspilites, itabirites. IV. Genesis of high-gradesecondary iron and manganese ores from iron-silicate and ferruginous formations and ores, metasomatic processes and processes of oxidation in them. An exhibition of Precambrian/manganese rocks was arranged: consisting of samples from the U.S.S.R., in particular from the Ukrainian S.S.R.,as well as samples brought by foreign participants. Immediately following the meeting, from 25 to 30 August, a field trip was organized to the well-known Krivoyrog deposits. The Ukraine occupies a leading position in industrial mining and exploration of Precambrian iron formations, and this visit enabled participants in the symposium to make comparisons and correlations with rocks of Precambrian iron formations from elsewhere. The symposium was the first international gathering toprovide an opportunity for a wide exchange of results obtained through studies of rather intricate problems concerning the nature and specific features of the unique ironbearing metamorphic Precambrian strata of the Earth. As a result of a broad discussion of the presented papers, it was recommended that further studies be made on basic regularities of occurrence, distribution and genesis of Precambrian iron-manganeseore formations, with special attention to modern geological,mineralogical,geochemical,and geophysicalmethodsand researchtechniques. The symposium also drew attention to the need for the determination and detailed investigation of ironmanganese deposits, including studies of interrelations between chert-iron-manganesedeposits, including studies of interrelations between chert-iron-manganeseand volcanogenic formations. It was considered that one of the first tasks to be undertaken should be the systematizationand classification of the rocks of chert-iron-manganeseformations,the correlationofnomenclatureoftheserocksindifferentcountries, the elaboration of a unified system of nomenclature for iron rocks in different regions of the world, and the study of analogues of these rocks in conditions of different degrees of metamorphism. A second important task was also recommended: intensification of investigations on the problem of formation of secondary ores, study of characteristic features of these ores in zones of oxidation and supergene alterations along with formation of iron-richores related to hypogene processes.

académicien, président du colloque,et par M.K.Lange, de l'Unesco (Division des recherches relatives aux ressources naturelles). Afin d'assurer l'examen systématique des questions, le programme a été divisé en quatre sections. I. La genèse et les types de formation de silicate de fer et de chert ferrugineux,leur position dans les séquences sédimentaires ou volcaniques géosynclinales et les relations entre ces dernières et les formations manganésifères analogues. Il. La datation absolue des formations de fer et de silicate de fer et leur position dans la série stratigraphique précambrienne. Les formations analogues phanérozoïques. III. Différents degrés de métamorphisme,faciès des minéraux et nomenclaturepétrographique des roches ferrugineuses telles que les quartzites, taconites,jaspilites et itabirites ferrugineux. IV. La genèse des minerais de fer et de manganèse secondaires à haute teneur,à partir des formations de minerais de fer et de silicate de fer, les processus métasomatiques et les processus d'oxydation qui s'y rattachent. Une exposition de roches manganésées précambriennes a été présentée. Elle était composée d'échantillons provenant de l'URSS,en particulier de la RSS d'Ukraine, ainsi que d'échantillons apportés par des participants étrangers. Immédiatement après le colloque une visite,qui a duré du 25 au 30 août, a été organisée aux célèbres gisements de Krivoyrog. L'Ukraine occupe une place prépondérante dans l'exploitation minière industrielle et l'exploitation des formations de fer précambriennes et cette visite a permis aux participants de faire des comparaisons et d'établir des corrélations entre les roches des formations ferrugineuses précambriennes. Ce colloque a été la première rencontre internationale qui ait permis un large échange de résultats d'études consacrées à des questions relativement complexes concernant la nature et les caractéristiques des remarquables couches précambriennes métamorphiques ferrugineusesde l'écorce terrestre. A la suite d'une ample discussion des communications présentées, les participants ont estimé qu'il y avait lieu de procéder à d'autres études sur les constantes fondamentales de la présence, de la répartition et de la genèse des formations précambriennes des minerais de fer et de manganèse, en se préoccupant particulièrement des méthodes et des techniques modernes de recherche géologique, minéralogique, géochimique et géophysique. Ils ont en outre souligné la nécessité de délimiter et d'étudier en détail les gisements de fer et de manganèse en recherchant notamment les relations entre les formations de chert-fer-manganèseet les formations volcanogéniques. L'une des premières tâches devrait être, a-t-onestimé, de systématiser et de classer les roches des formations de chert-fer-manganèse,d'établir la concordance des nomenclatures de ces roches en vigueur dans différents pays, d'élaborer une nomenclature unifiée des roches ferrugi-

It was agreed that publication of the Proceedings of the symposium would be a valuable contribution to geological and geochemical sciences,and while the Academy of Sciences of the Ukrainian S.S.R.has undertaken to provide such a publicationin the Russian language,Unesco was asked to ensure publication in English [with summaries in French). The undoubted success of this symposium was assured on the one hand by the preparatory work undertaken by the InternationalAssociation of Geochemistry and Cosmochemistry, and in particular its President, Professor E.Ingerson,and on the other hand by the excellent organization of the meeting in Kiev by the Academy of Sciences of the Ukrainian S.S.R. Special thanks are due to the Chairman of the Organizing Committee, Professor N.P.Semenenko. The papers presented at the symposium are reproduced in this ninth volume of the Earth Sciences series. The selection of material, the points of view, and the opinions presented are those of the authors and are not necessarily endorsed by Unesco.

neuses des différentes régions du monde et d'étudier les roches analogues à différents degrés de métamorphisme. U n e autre tâche importante a également été recommandée :l'intensification des recherches sur la formation des minerais secondaires,l'étude des traits caractéristiques de ces minerais dans les zones d'oxydation et d'altération supergene ainsi que l'étude de la formation des minerais riches en fer liée aux processus internes. Les participants ayant estimé que la publication des Actes du colloque constitueraitune aide précieuse pour les sciences géologiques et géochimiques, l'Académie des sciences de la RSS d'Ukraine s'est chargée d'assurer cette publication en langue russe,et l'Unesco a été chargée d'en assurer la publication en anglais (avec résumés en français). Le succes incontestablede ce colloque est attribuable, d'une part, au travail préparatoire accompli par 1'Association internationalede géochimie et de cosinochimie,et en particulier par son président,le professeur E.Ingerson, et, d'autre part, à la façon remarquable dont l'Académie des sciences de la RSS d'Ukraine a organisé la réunion à Kiev. L e président du comité d'organisation, le professeur N. P. Semenenko, doit être tout particulièrement remercié. Le présent ouvrage, qui fait partie de la collection ( ( Sciences de la terre », reproduit les communications présentées au colloque. Les opinions qui y sont exprimées n'engagent évidemment que leurs auteurs.

Contents

Table des matières

Genesis and types of iron-silicate and ferruginous cherty formations,their position in geosynclinal sedimentary or volcanic sequences and the relation between these and analogous manganese bearing formations / Les types de formations de silicate de fer et de chert ferrugineux; leur genèse, leur position dans les séquences sédimentaires ou volcaniques géosynclinales et les relations entre ces dernières et les formations manganésifères analogues

The depositional environment of principal types of Precambrianiron-formations Milieux dans lesquels se sont déposés les principaux types deformationsprécambriennes defer [Résumé] G.A . Gross Archaean volcanogenic iron-formationof the Canadian shield Laformation defer volcanogéniquearchéenne du bouclier canadien[Résumé] A . M . Goodwin The facial nature of the Krivoyrog iron-formation Lesfaciès desformationsferrugineuses du Krivoyrog [Résumé] A,I. Tugarinov,I.A . Bergman and L.K.Gavrilova Jacobsitesfrom the Urandi manganese district,Bahia (Brazil) Jacobsites du district de manganèse d’Urandi,Bahia (Brésil) [Résumé] E.Ribeiro Filho Time-distribution and type-distributionof Precambrian iron-formationsin Australia Répartition de l’âgeet du type desformationsprécambriennesdefer en Austr.alie[Résumé] A . F.Trendall The origins of the jaspilitic iron ores of Australia Les origines des minerais de fer jaspilitique d’Australie[Résumé] R. T. Brandt Occurrence and origin of the iron ores of India Manifestationset origine des minerais de fer de l’Inde [Résumé] M . S. Krishnan Precambrian iron ores of sedimentary origin in Sweden Minerais defer précambriens 21 caractèressédimentaires,en Suède [Résumé] R . Frietsch The ferruginous-siliceousformations of the eastern part of the Baltic shield Lesformations defer siliceux dam lapartie orientale du bouclier baltique [Résumé] V. M.Chernov Precambrian ferruginous-siliceousformations associated with the Kursk Magnetic Anomaly Lesformations defer siliceux du Précambrien dans la région de l’anomaliemagnétique de Koursk [Résumé] N.A . Plaksenko,I. K . Koval and I. N.Shchogolev Structural-tectonicenvironments of iron-oreprocess in the Baltic shield Precambrian Environnement tectoniqueet structuraldesprocessus deformation de minerai defer dans le Précambrien du bouclier baltique [Résumé] P. M . Goryainov

15 20 23 33

35 39

41 47 49 55 59 66 69 75 77 82 85 86

89 94 95 98

Geology of the Precambrian cherty-ironformations of the Belgorod iron-oreregion Géologie desformationsprécambriennes defer siliceux dans le gisement de Belgorod [Résumé] Yu. S. Zaitsev Iron-formationand associated manganese in Brazil Formation de fer et de manganèse en association,au Brésil [Résumé] J. Van N.Dorr II The Precambrian iron and manganese deposits of the Anti-Atlas Gisements de minerai de jer et de mangatièse dans le Précambrien de l’Anti-Atlas[Résumé] G.Choubert and A.Faure-Muret Tectonic control of sedimentation and trace-element distribution in iron ores of central Minas Gerais (Brazil) Le contrôle tectonique de la sédimentation et la répartition des éléments-tracesclans les minerais de fer de la partie centrale de l’&tut de Minas Gerais,au Brésil [Résumé] A. L. M.Barbosa and J. H.Grossi Sad

1 o1 1Ó3 105 112

115 123 125

131

Absolute age dating of iron-silicate and ferruginous formations and their position in the Precambrian stratigraphic sequence. Analogous formations from the Phanerozoic / L a datation absolue des formations de fer et de silicate de fer et leur position dans la série stratigraphique précambrienne. Les formations phanérozoïques analogues

The iron-chertformationsof the Ukrainian shield Géologie et genèse desformations de fer siliceux du bouclier cristallin d’Ukraine [Résumé] N.I?. Semenenko Occurrences of manganese in the Guianas (South America) and their relation with fundamental structures Les indicesde manganèse dans les Guyanes (Amériquedu Sud) et leurs relations avec les structuresfondamentales [Résumé] B. Choubert Precambrian ferruginous-siliceousformations of Kazakhstan Les formations de fer siliceux dans le Précambrien du Kazakhstan [Résumé] I. P. Novokhatsky Geology and genesis of the Devonian banded iron-formationin Altai, western Siberia and eastern Kazakhstan Géologie et genèse de laformation dévonienne defer rubaiié dans I’Altai,la Sibérie occidentale et le Kazakhstan oriental Désumé] A . S. Kalugin Genesis of high-grade iron ores of Krivoyrog type Genèse des minerais de fer à haute teneur de Krivoyrog [Résumé] Y.N.Belevtsev Effusive iron-silicaformations and iron deposits of the Maly Khingan Les formations de fer siliceux eflusif et les gisements de fer du Maly Khingan [Résumé] E.V. Egorov and M.W.Timofeieva Effusive jasper iron-formationand iron ores of the Uda area L a formation du minerai de fer à jaspe effusifet les minerais defer de la région d’Ouda[Résumé] E.L. Shkolnik

135 141 143 150 153 156

159 164 167 177

181 184 187 189

Differing degrees of metamorphism, the mineral facies and the petrographic nomenclature of ferruginous rocks such as ferruginous quartzites, taconites, jaspilites, itabirites / Différents degrés de métamorphisme, faciès des minéraux et nomenclature pétrographique des roches ferrugineuses telles que quartzites ferrugineuses, taconites, jaspilites et itabirites

Mesabi, Gunflint and Cuyuna Ranges, Minnesota (United States of America)

193

Les chaînes de Mesabi, Gunflintet Cuyuna dans le Minnesota, aux ÉLats-Unisd’Amérique[Résumé] G.B. Morey

206

Physico-chemicalconditions of the metamorphism of cherty-ironrocks Les conditionsphysico-chimiquesdu métamorphisme desformations de fer siliceux [Résumé] Y.P.Melnik and R . I. Siroshtan The Serra do Navio manganese deposit (Brazil) Le gisement de manganèse de Serra do Navio, au Brésil [Résumé] W.Scarpelli

209 215 217 227

Genetic studies on the Precambrian manganese formations of India with particular reference to the effects of metamorphism 229 Étude génétique des formations de manganèse précambrien en Inde avec références particulières aux efsets du 239 métamorphisme [Résumé] S. Roy Precambrian ferruginous formations of the Aldan shield 243 Formationsferrifères du Précambrien inférieur du bouclier d’Aldan[Résumé] 246 I. D.Vorona,V. M . Kravchenko,V. A. Pervago and I. M . Frumkin O n the issue of genesis and metamorphism of ferromanganese formations in Kazakhstan 249 Formation et métamorphisme des roches ferrugineuses de diverses époques dans les provinces du Kuzakhstan [Résumé] 253 V. M . Shtsherbak,A . S. Kryukov and Z.T. Tilepov Genesis of high-grade secondary iron and manganese ores from iron-silicate and ferruginous formations and ores, metasomatic processes and processes of oxidation in them / Genèse des minerais de fer et de manganèse secondaires à haute teneur, à partir des formations de minerais de fer et de silicate de fer; processus métasomatiques et processus d’oxydation qui s’y rattachent

Iron-formationsof the Hamersley Group of Western Australia: type examples of varved Precambrian evaporites Formations de fer du groipe de Hamersley, en Australie occidentale :exemples typiquesd’évaporitesprécambriennes en varve [Résumé] A.F. Trendall Geology and iron ore deposits of Serra dos Carajás,Pará (Brazil) Géologie et dépôts de minerai de fer de la Serra dos Carajás,Pará,Brésil [Résumé] G . E.Tolbert,J. W.Tremaine,G.C.Melcher and C. B. Gomes Enrichment of banded iron ore, Kedia d’Idjil (Mauritania) Enrichissement des minerais zonés de fer de la Kedia d’ldjilen Mauritanie [Résumé] F.G.Percival Iron ores of the Hamersley Iron Province,Western Australia Les minerais de fer d’Hamersley,en Australie occidentale [Résumé] W.N.MacLeod Significance of carbon isotope variations in carbonates from the Biwabik Iron Formation, Minnesota Significationdes variationsdesproportions des isotopesdu carbone dans les carbonatesdes gisementsdefer de Biwabik,dans le Minnesota [Résumé] E.C. Perry Jr and F.C.Tan Genesis and supergene evolution of the Precambrian sedimentary manganese deposit at Moanda (Gabon) Genèse et évolution supergène du gisement sédimentaireprécambrien de manganèse de Moanda, au Gabon [Résumé] F. Weber The Belinga iron ore deposit (Gabon) Les minerais de fer de Bélinga,au Gabon [Résumé] S. J. Sims Itabiriteiron ores of the Liberia and Guyana shields Les minerais de fer d’itabirite du Libéria et du bouclier guyanais [Résumé] H. Gruss Structuralcontrol of the localization of rich iron ores of Krivoyrog Déterminationstructurale de la localisation des minerais de fer à haute teneur de Krivoyrog [Résumé] G.V. Tokhtuev Iron deposits of Michigan (United States of America) Gisements de fer du Michigan,aux États-Unisd’Amérique[Résumé] J. E.Gair

257 268 271 279 281 288 291 297 299 304 307 320 323 332 335 357

361 364 365 374

Problems of nomenclature for banded ferruginous-chertysedimentary rocks and their metamorphic equivalents

377

List of participants/Liste des participants

381

Genesis and types of iron-sihcateand ferruginous cherty formations,their position in geosynclinal sedimentary os volcanic sequences and the relation between these and analogous manganese-bearing formations

Les types de formations de silicate de fer et de chert ferrugineux; leur genèse, position dans les séquences sédimentaires ou volcaniques géosynclinales et les relations entre ces dernières et les formations manganésifères analogues

The depositional environment of principal types of Precambrian iron-formations G.A. Gross Geological Survey of Canada, Ottawa 4,Ontario (Canada)

Iron-formationscomposed of thinly bedded chert and iron minerals which contain at least 15 per cent iron are probably the most abundant chemically precipitated sedimentary rocks known. They occur in a wide variety of geological environmentsand because of the diversity in chemicalproperties of their elemental constituents are highly sensitive indicators of the depositional environments in which they formed.M u c h of the geologicalliteratureon these rockshas been based on separate iron ranges or formations and interpretations from these specific studies have been applied to the whole group of cherty ferruginous sediments. Interpretations and extrapolations are frequently made without distinguishing adequately the diversity in depositional, tectonic, chronological and host rock environments iii which the many different lithological varieties of these chemical sediments occur. The purpose of this paper is to distinguish differences between some of the principal geological environments where siliceous iron sediments occur and to recognize variations in the physical and chemical characteristics of banded cherty iron-formationsas found in these different environments.Itis necessaryto distinguish and define the various types of depositionalenvironmentsof these rocks before concepts and hypotheses pertaining to their origin and genesis can be satisfactorily evaluated and the geological significance of iron-formations fully appreciated. Of the broad group of iron-richsediments, only the banded cherty iron-formation sediments are considered in this paper. The oolitic chamosite-siderite-goethite clay-rich rocks commonly referred to as ironstones are recognized as a distinctly separate type of iron sediment,They formed in different environments than the cherty iron sediments and probably have a different origin and source of iron. The separategroup ofcherty ironsedimentswhich are associated with a wide variety of sedimentary and volcanic rocks indicate pronounced diversity in conditions in their sedimentary environments. The cherty iron-formations are chemically precipitated sediments and the many different sedimentary facies demonstrate the changes in physical and chemical environment during their deposition.The distinct

variationsingeologicalenvironment and physicaland chemical characteristics of the cherty iron-formations are such that it cannot be assumed that all of these rocks havesimilar sources of iron and silica and similar genetic affinities. It is highly probable that there are other fundamental factors affecting the origin of these sediments which have still not been recognized. Because there are relatively few examples of cherty iron-formationin rocks of Mesozoic age or younger and apparently no modern examples exist where banded cherty iron sediments are forming today,w e have no complete contemporary model or guide to the geological parameters affecting the origin of these special sediments. For these reasons investigations of the depositional environment of cherty iron sediments have to be comprehensive both in scope and in definition of sedimentary featuresand environmentif the mode of origin of these rocks is to be understood. Detailed comparisons of iron ranges throughout the world may provide a composite picture of the complex factors and conditions which contribute to the deposition of iron-formations. It has proved highly instructive to classify or group the cherty iron-formations according to general features and characteristics of their depositional environments and the kinds ofsedimentaryrocks associatedwith them.In Canada the name ‘Algomatype’ has been used in recent years to designate cherty iron-formationsand their equivalentfacies variants that are intimately associated with volcanic rocks and greywacke type sediments in eugeosynclinalbelts. The iron-formations associated with quartzites,dolomites and black slates in continental-shelfenvironments are classified as ‘LakeSuperior type’.This broad classification may not be entirely satisfactory for all occurrences of cherty ironformation,but it servesto distinguish the two main environments in which cherty iron sediments most frequently occur. The Lake Superiortype ofiron-formationformsprominent iron ranges of middle to late Precambrianage in nearly all of the shield areas of the world. Most of the geological literature on cherty iron-formationsis based on this type of iron sediment and it is the host rock, or protore, for

Unesco, 1973. Genesis of Precambrian iron and niunganese deposits. Proc. Kiev Syrnp., 1970. (Earth sciences, 9.)

15

G.A.Gross

the largest and best known iron ore deposits in cherty ironformations. Lake Superior type iron-formations are characteristically thin-banded cherty rocks with iron-rich layers representing various sedimentary facies. Oxide facies are composed of magnetite,hematite or mixtures of these oxide minerals which were deposited mainly as primary iron oxides. Silicate minerals in the silicate facies commonly range from greenalite and minnesotaite to stilpnomelane, cummingtonites and grunerite to hypersthene depending on their rank of metamorphism. Carbonate facies are representedpredominantly by siderite associated with magnetite or iron silicates but ankerite and ferrous dolomites are prevalent where carbonate is associated with hematite-rich facies.Sulphide facies of this type of iron-formationusually consist of fine-grainedcarbon-rich mudstones with interlayered chert or siliceous shale. Characteristic features of the various facies of this type of iron-formationhave been described by Gross (1965), James (1954)and others. Granules and oolites composed of both chert and iron minerals are typicaltexturalfeatures of these sediments and they are practically free of clastic material except in the transitional border zones or in distinct well-defined members within the formation. The alternate or rhythmic banding of iron-rich and iron-poor cherty layers, which normally range in thickness from a few millimetres to 1 metre,is a prominent feature.Individuallayers may pinch and swell to give a wavy-bandedmember or the uniformity of the layering may be disrupted by nodular or stubby lenses of chert and jasper, by rare occurrences of crossbedding, or by cherty forms resembling in shape and structure ‘Collenia’or ‘Crystozoan’growths in limestones formed by algal colonies.Tension,syneresisand desiccation cracks are present in some chert granules and nodules,and styolites are common.Textures and sedimentaryfeatures of this type of formation are remarkably alike in detail wherever examined, although certain sedimentary features are more prominent in some formations than in others. The close associations of this type of formation with quartzite and black carbonaceous shale, and commonly also with conglomerate dolomite, massive chert, chert breccia,and argillite,are recognized throughout the world. Volcanic rocks,either tuffs or flows,are not always directly associated with Superior type iron-formation,but they are nearly always present in some part of the stratigraphic succession.The sequencedolomite,quartzite,red and black ferruginous shale,iron-formation,black shale and argillite, in order from bottom to top,is so common on all continents that some investigatorshave been led to believe in the past that it is invariable. However, stratigraphic studies have shown that, although there is a persistent association of these sedimentary rocks, the successionmay differ in local areas; it does so for example in the Labrador geosyncline. Quartzite and red to black shale lie below the ironformation and black carbonaceous shale above it, but the presence of other sedimentary rocks and their position in the stratigraphic succession may vary from place to place, even in a single range or sedimentary belt. 16

Continuous stratigraphic members of Superior type iron-formation commonly extend for hundreds of miles along the margins of ancient continental platforms or geosynclinalbasins.The formationsmay vary in thickness from a few tens ofmetres to severalhundred metres and occasionally up to 1,000metres, but their persistence is truly remarkable. The rock successions in which the ironformations occur usually lie unconformably above highly metamorphosed gneisses,granites or amphibolites,and the iron-formations are, as a rule, in the lower part of the succession. In some places they are separated from the basement rocks by only a few metres of quartzite,grit and shale or,as in certain parts of the Gunflint Range, they lie directly on the basement rocks. However, in most areas they occur at least some hundreds of metres above the basement rocks. The Lake Superior type iron-formationsare present in late Precambrian rocks in nearly all parts of the world and possibly in some early Palaeozoic rocks (O’Rourke, 1961). They apparently formed in fairly shallow water on continental shelves or along the margins of continental shelves and miogeosynclinal basins, and consist of sediments derived from the adjacent land mass and also some material from the volcanic belts within the basin. It is still considered uncertain as to whether the iron and silica in this type of iron-formationwere derived from the eroding of a land mass or a volcanic source. This type of siliceous formation is the protore or host rock for the rich hematite-goethite orebodies of the Lake Superior region in the United States, Quebec-Labradorin Canada,north-westernAustralia, Orissa and Bihar states in India, Krivoyrog and Kursk areas in the U.S.S.R.,in Brazil and for many other major iron deposits in the world. Algoma type iron-formations are present in nearly all of the early Precambrianbelts of volcanic and sedimentary rocks in the Canadian shield, in parts of the Australian shield and in belts of similar rock of Palaeozoic and Mesozoic age in many other regions. This type of ironformation is characteristically thin-banded or laminated with interlayered bands of ferruginous grey or jasper chert and hematite and magnetite. Massive siderite and carbonate beds, iron-silicatemineral facies and iron-sulphidemineral facies are frequently associated in the formationbut are less abundant than the oxide facies.In the Michipicoten area of Ontario, massive siderite and pyrite-pyrrhotitebeds form part of the formation. Single iron-formationmembers of this type range from more than a hundred metres to less than 1 metre in thickness and rarely extend more than a few kilometres along strike.A number of these lenticular beds may be linked together or distributed en échelon throughout a belt of volcanic and sedimentary rocks. The Algoma type iron-formationsare intimately associated with various volcanic rocks including pillowed andesites, tuffs, pyroclastic rocks, or rhyolitic íìows and with greywacke,greygreen slate, or black carbonaceous slate. Tuff and finegrained clastic beds or ferruginouscherts are interbedded in the iron-formationand detailed stratigraphicsuccessions

The depositionalenvironment of principal types of Precambrian iron-formations

show heterogeneous lithological assemblages. These ironformations have streaked lamination or layering,and oolitic or granular textures are apparently absent, except in rare casesin post-Precambrianrocks.The associated rocks indicate a eugeosynclinal environment for their deposition and a closerelationshipin time and spaceto volcanic activity. The direct association of Algoma type iron-formations with centres of volcanism or volcanic activity is recognized in a number of volcanic belts in the Canadian shield. Rhyolitic and daciticvolcanicrocks are usually thickest and most abundant in the succession of volcanic sedimentary rocks in and around the ancient volcanic centres.In general the Algoma iron-formationsoverlap the bulk of the acidic volcanic material and are in turn covered by andesitic volcanic rocks and associated greywacke type of sediments. Sulphideand carbonatefacies of iron-formationoccur at or near the centres of volcanism and the oxide facies are usually distributed farther away,even where they are almost entirely enclosed by clastic sediments. Carbonate and silicate facies occur near the centres of volcanism, but a general zonalrelationship from sulphide through carbonate to oxide facies of Algoma type iron-formationis commonly found. This direct relationship between the type of ironformation facies and the various kinds and distribution of volcanicrocks leaves little doubt about the geneticrelationship of these cherty iron sediments and volcanic processes. Thin beds of graphitic schist or black carbon-rich mudstones are commonly associated with Algoma type iron-formationand occur mainly in parts of the succession where volcanic rocks are more abundant than the greywacke sediments. Much of the fine clastic material in the black mudstone may be derived from tuff and volcanic ash and collected in depressions in the depositional basin. Usually they contain pyrite and pyrrhotite and parts of them have appreciable amounts of lead, zinc and copper. Black mudstones of this type are closely associated with stratiform base metal sulphide deposits and are one of the common host rocks in which the thin-banded and layered sulphide beds occur. The black mudstones may be a facies of the Algoma type iron-formationand occur in the same bed or member as oxide and carbonate facies.They also occur as separate beds or horizons which are closely associated with thicker beds composed of other facies of ironformation. Algoma type iron-formations are widely distributed in the volcanic-sedimentary belts in the older parts of the Canadian shield and some of the better known examples of this type of iron-formationoccur in the Michipicoten District, near Kirkland Lake, Moose Mountain Mine, Timagami Lake, the Kapico Iron Range,north of Nakina, at Red Lake, Bruce Lake and Lake St Joseph in Ontario. Examples of Ordovician age occur near Bathurst in northern N e w Brunswick and northern Newfoundland and some of Mesozoic age on Vancouver Island. Iron-manganese formations of Algoma type are of particular interest but are relatively rare compared with the frequency of occurrence of the iron-rich beds. Ironmanganese formationswere deposited under much the same

conditions and in a similar geologicalenvironment to those for typical Algoma iron-formation.The manganese content may range from nearly pure cherty manganese sediment to cherty sediments with a low manganese to iron ratio. Examples of this type of cherty sediment are found in the Karazdhal range in the U.S.S.R. and near Woodstock in N e w Brunswick, Canada. These appear to be formed by volcanic exhalativeprocessesand are classified with Algoma type iron-formation. Nearly all of the cherty iron-formationscan be classified satisfactorily in these two principal environmental types. Many of the iron-formations and their associated rocks are highly metamorphosed and their sedimentary environments can only be interpreted from the relict sedimentary features that are still recognizable. Many other iron-formationsare not known in detail and their immediate geological setting or depositional environment has not been studied or reported. A n interesting iron-formationin an unusual geological setting extends along the Yukon and Mackenzie District border in north-western Canada. The Snake River ironformation forms a succession of jasper and blue hematite beds more than 150 metres thick which occur near the base of the Rapitan formation;a crudely stratified,poorly sorted conglomerate at least 1,500 metres thick. The Rapitan conglomerate lies between two angular unconformíties. It overlies a thick succession of dolomite, shale, gypsum and shale, shaly carbonate,limestone, and quartzite which may be Lower Cambrian in age but is believed to be Precambrian. The Rapitan conglomerate is overlain by dark shale and silty dolomite of late Cambrian age. The exact age of the Rapitan formation and the enclosed ironformation is still not known. The Rapitan formation as a whole is composed of conglomeratic siltstone and shale, siltstone and silty shale with about 10 per cent of its volume made up of rounded to subangular coarse fragments mostly in the 1-5 centimetres size range with isolated boulders up to 5 metres in dimension. The coarse fragments consist of carbonate, basic igneous rocks, sandstone,quartzite and shale in decreasing order of abundance. M u c h of the conglomeratic siltstone in the lower part of the formation associated with the iron-formationis highly ferruginous and dark red to maroon in colour. Parts of the Rapitan formation some distance from the thicker iron-formationcontain a high proportion of coarse fragmental volcanic rocks and considerable tuffaceous material. The iron-formationhas an average iron content of 46per cent and is composed ofinterlayered bright red jasper and fine-grained deep blue hematite beds which range in thickness from thin laminae to several centimetres. The jasper and hematite layers are mostly well-segregated,but some hematite beds have conspicuousroundnodules ofred, grey or buff chert 0.5-1 centimetre in size which may make up 20 per cent of the hematite layer. Granular or oolitic textured beds were not found in the iron-formation.Other common siliceous layers and beds are deep red to maroon in colour and are made up of very fine-grainedclastic m u d

17

G.A.Gross

in a highly siliceous matrix. There are numerous thin lenticular beds of coarser clastic material distributed throughout the iron-formation. Some of the fine-grained silty material is composed of tuff fragments and coarser fragments are similar in composition to the coarse fragments in the main conglomerate.T w o thin but continuous silty sandstone beds, one near the base and one near the top of the iron-formationsequence,have been used as horizon markers for correlation of detailed stratigraphy. Thin laminae and beds of ankeritic and dolomitic carbonate are interlayered with the chert and hematite in some parts of the iron sequence. The iron-formationappears fresh and there is little evidence of metamorphism. Primary sedimentary and diagenetic features are well preserved and much can be determined about the sedimentary environment and nature of these beds. Differential compaction features, slump and glide structures, intraformational breccias composed of cherty iron-formationfragments,scour and €ill structures,tension and syneresis cracks are all conspicuous throughout the iron-formation.Many of the coarsefragmentalbeds appear to have been m u d flows which spread over beds of partly consolidatediron-formationcausing distortionand disturbance of the underlying bedding in the iron-formation.The iron-formationoverlying the m u d flow is straight,undisturbed,horizontally bedded jasper and hematite. In some places mud flows were observed which had scoured and cut channels in the soft iron-formation5 metres deep,and tens of metres wide. Large blocks of iron-formationare suspended in the m u d flow and iron-formation fragments in the flow are most abundant near the walls of the channel. The suggestion has been made that some of the large isolated boulders found in the iron-formation,which caused warping and depression of the underlying chert beds,were rafted by ice and dropped in the soft semi-consolidated cherty iron-formation.Most of these lie along thin seams of conglomerate and tuffaceous material and the writer believes that this material is the product of explosive volcanism which took place during the deposition of the ironformation. N o volcanic vents or diatremes have been identified, but the occurrence of tuffaceous layers and volcanic materials in the conglomerate and iron-formation are evidence of volcanic activity during the deposition of these rocks. The thick lenticular iron-formationdescribed here is exposed over a width of 10 miles (16kilometres)and extends laterally for more than 30 miles (48 kilometres). It thins towards the east and west, is terminated at the uiiconformity surface to the north and its extent to the south, where it dips under youngerstrata,has not been determined. The total dimensions of this iron-formation,either for the thicker lenticular zone or for its completelateral extent,are not known. Thinner beds of lithologically similar ironformation,which may be a continuation of this same stratigraphic zone, have been observed in isolated occurrences for more than 200 miles (320kilometres) to the north-west and also for some considerable distance to the south-east. 18

The Rapitan formation represents a rapid filling of a deep basin depression with poorly sorted and stratified silty and conglomeratic material. Chemical precipitation of the iron and silica of the cherty iron-formationhas taken place at the same time as the inpouring of the silty conglomerate and the two types of sedimentation, clastic and chemical, have been superimposed on one another. The ironformationisfreshand relatively unmetamorphosed.Primary sedimentary features indicate that alternate chemical precipitation of silica-and hematite-richlayers was interrupted by the influx of m u d flows and conglomerate which spread over the partly consolidated chert and hematite,in places scouring channels in the soft iron-formation.The conglomerateand iron-formationarebelievedto have been deposited in a broad depression or basin on the ocean floor, and slumping and flow of unconsolidated rocks from adjacent fault scarps or basin shelves may have been triggered by movement along bordering faults or by explosive volcanic activity. Some of the fine-grainedclastic beds impregnated with hematite appear to be tuff or volcanic ash that settled in soft hematite ooze. The hematite and silica are believed to have been transported in solution by hot fumarolic waters and precipitated when these solutionswere discharged on the sea floor along fault zones (Gross, 1965). The Snake River iron-formation may be the product ofexhalative-sedimentaryprocessesand thereforehave a very closegenetic aíñnity to the main g o u p of iron-formations classified as Algoma type. The origin of the Snake River iron beds may be closely analogous to the siliceous iron, manganese and base-metal deposits at present being precipitated in the deeps of the Red Sea (James, 1969). The Snake River iron-formationrepresents a voluminous influx of chemicalIy precipitated iron and silica into a basin that was being rapidly filled by conglomerate and coarsesilt.There is no apparent genetic relationshipbetween the source and manner of derivation of the two types of sediment. In the case of the Algoma type iron-formations, the chemically precipitated iron and chert beds deposited contemporaneously with a great variety of volcanic and sedimentary rock and the specific genetic relationship between the chemical sediment and the various kinds of clastic and volcanic material is subject to conjecture and interpretation.Important empirical relationships of different facies of iron-formationwith certain phases of volcanic activity or kinds of volcanic rock and sediments, and the zonal distribution of different iron-formation facies and exhalative deposits around volcanic centres, leave little doubt that deposition of iron-formationand the volcanic rocks are both expressions of a common igneous-volcanic phenomenon. In the case ofthe Lake Superior type ofiron-formation, very thick successions of chemically precipitated silica and iron sediment have been deposited in sequences of normal and common types of continentalshelf sediment.In many of these areas there was contemporaneousvolcanic activity and deposition taking place along the outer edge of the shelf or basin. A possible common source for the iron and silica

The depositional environment of principal types of Precambrian iron-formations

in the iron-formation aiid the quartzites, dolomites and argillaceous sediments has been proposed by postulating deep chemicalweathering of a land mass and specialerosion and sedimentation conditions to account for the whole assemblage of sedimentary rocks. Geological models based entirely on these concepts of erosion, transportation and deposition of the iron and silica have not provided a satisfactory explanation for the origin of this type of ironformation.The problem of the sourceand origin of the iron and silica has not been solved conclusively by appealing to exhalative-sedimentaryprocesses related to volcanic activity in the adjacent volcanic belts. The writer believes, however, that the source of iron and silica most probably lies in the volcanic belt rather than in an eroded land mass. This opinion is based more on comparison of common features and aspects in the environments of Algoma and Lake Superiorformationsand analogies which may be made between the two types. It is expected that continuing detailed study of the depositional environments of both Algoma and Lake Superior types of iron-formation will provide examples of iron ranges depositedunder conditions intermediate between the volcanic eugeosyncline environment of the Algoma type and the stable continental shelf environment of the Lake Superior type. If this proves to be the case, then the two prominent types of environment now recognized can be considered as two depositional models or sedimentary expressions with the iron and silica derived or supplied from a common kind of source and by a common phenomenon. Recognition of the two principal types of cherty ironformation and characteristicfeatures of their depositional environment is an important step towards determining the critical or essential geological processes and features that are involved or related to the origin and development of these chemical sediments. Only some of these processes or features are mentioned here in a qualitative way and it is not possible in this short paper to elaborate on their significance or implication with regard to the source of iron and silica and the origin of cherty iron-formations. There are also many important economic implications related to these typical environments which are being considered in mineral exploration. Distinctive characteristics of the different kinds of iron ore derived from the principal types of iron-formationhave been recognized and described in the literature on iron-oredeposits and will not be elaborated here. Recognition of the characteristic features of the types of iron-formation plays an important part in the evaluation of newly discovered or developed iron-oredeposits.Probably one of the most significant factorsrelating to the type of iron-formationis recognition of the kind of manganese, base-metal or gold deposits that may be associated with it. Important stratiform base-metal sulphide deposits in the same geological environment as Algoma type ironformation are recognized as faciesvariants ofsulphidefacies or iron-formationand, like the iron-formation,are considered to be exhalative-sedimentary volcanic deposits. There is little doubt about the genetic relationship of these

stratiform base-metalsulphide deposits with sulphide,carboiiate and oxide facies of iron-formation,and recognition of this fact has fostered new and highly rewardingconcepts in mineral exploration in the Canadian shield.The empirical association of gold deposits and Algoma type ironformation has been recognizedfor many years.In the past, some have explained this relationship on a structuralbasis, believing that the brittle cherty iron-formationswere a favourablehost rock for quartzvein development.Evidence is n o w being accumulated to show that the carbonate and some of the sulphide facies of Algoma type iron-formation are source beds for gold and probably silver which were later concentrated in veins and stockworks associated with the iron-formation. It is noted that the composition and physical characteristics of some of the stratiform base-metal sulphide deposits associated with Algoma type iron-formationare very similarand directly comparablewith the contemporary layered siliceous sulphide sediment being deposited in the deeps of the Red Sea. It is highly probable that deposits in the Algoma type iron-formationand Red Sea environments are both products of deep-seatedmagmatic processes centred along major faults or tectonic features in the crust, Fumarolic activities and circulation of water caused by near-surface thermal gradients have probably given rise to the solution and transport of large quantities of silica and metallic ions in both cases.In the Algoma type environment there has been a prominent deposition of volcanic rock contemporaneous with the discharge of these metal bearing solutions and deposition of their salts,while in the Red Sea solutions are being discharged from the deepseated fault systems without active volcanism. Referring briefly to the global distribution of cherty iron-formations,it is noted that many of the major Precambrian iron-formationsof the world lie close to or parallel to the borders of the continentalmasses. This is the case for iron-formationnear the west coast of the African continent and those in South America along its east coast, and for the distribution of iron-formations in India and Australia. These iron-formations are Precambrian sediments in ancient shield terrain which may have been closely related to, or even parts of, the sanie depositional basins and tectonic belts prior to continental drifting and segmentation of the principal Precambrian land masses. The type of cherty iron-formation,its associated rocks and depositionalenvironment for each of these iron belts, need to be defined and compared in detail to determine whether the iron ranges now on the borders of different continents may at one time have formed parts of the same depositional basins and tectonic belts. This comparison of the type and environmentof iron-formationbelts is of course dependent on better determination of the age of sedimentation of the iron beds and much inore detailed information on the chronological sequence of events in each of the ironformation ranges. The writer believes that many of the Precambrian iron ranges and their depositional basins may be closely related to, ifnot parts of,the same sedimentarysequences of rocks 19

G.A Gross

which were separated during the segmentation and drifting of the continents. The iron-formations may be closely related to major deep-seated fault and tectonic systems of global dimensions which existed in the Precambrian land mass and have not been recognized because of continental drift.The separation of large volumes of iron and silica and their transportation by fumarolic water or by circulation of water currents caused by thermal gradients along these tectonic zones may be related to deep-seated igneous aiid volcanic processes.Ifthis is the case,we can then appreciate some fundamentalreasons and basic causes for finding this large group of cherty sediments in such a diversity of depo-

sitional environments,The fundamental reasons for finding voluminous sequences of silica, iron and other metallic elements on continental shelves, in volcanic-sedimentary rock assemblages in eugeosynclines, or in thick sequences of conglomerate,as in the case of the Snake River ironformations, will not be found by exclusive studies of the sedimentation in typical iron-formationenvironments. These answers will most likely be found through study of major tectonic features and the associated deep-seated igneous processes which may have had a common genetic relationshipto all of these distinctive sedimentary environments of iron-formation.

Résumé Milieux dans lesquels se sont déposés les principaux types de formations précambriennes de fer (G. A.Gross)

Les formations de fer, veinées de silex, qui sont réparties très largement dans toutes les régions du bouclier précambrien se rencontrent dans deux types principaux de milieux ; d'où le nom qui leur a été donné en Amérique du Nord : ( (Algoma ) )et ( (Lac Supérieur ) ) . L e type ((Algoma 1) est étroitement lié à la fois par sa genèse et par sa localisation aux roches volcaniques. O n pense qu'il a été produit par des processus d'exhalation volcanique dans un milieu eugéosynclinal.I1 consiste dans une grande variété de faciès sédimentaires qui vont de l'oxyde de fer siliceux aux faciès des carbonates,silicates et sulfures. Ce type est largement distribué dans les roches volcaniques archéennes dans tout l'ensemble du bouclier canadien. L e type ((Lac Supérieur ))s'est déposé sur la plateforme protérozoïque et dans les environs du plateau continental.Il est associé avec la quartzite,la dolomite et l'argile schisteuse noire, et avec du tuf en moindre quantité et d'autres roches volcaniques. C e type de formation de fer siliceux atteint des épaisseurs de plusieurs centaines de

pieds et est distribué de façon continue sur des centaines et m ê m e des milliers de kilomètres près de la ligne de côte des anciens continents. U n exemple remarquable de ce type de formation de fer est la région du lac Supérieur et le géosynclinal du Labrador dans le bouclier canadien. U n e image précise de la position relative des zones couvertes par le bouclier précambrien avant la dérive des continents est nécessaire pour l'étudedes milieux sédimentaires où s'est forméle fer siliceux.Les zones où l'on trouve le fer dans l'hémisphèrenord à l'intérieur des masses continentales actuelles ont pu être préservées dans un milieu phanérozoïque tectonique relativement stable. Les régions où se rencontre le fer près des bordures des masses continentales actuelles dans les régions équatoriales et dans l'hémisphère sud semblent avoir été fragmentées à la suite de la dérive des continents. Des comparaisons entre des milieux où se sont formés les dépôts des formations de fer devraient permettre de reconstruire les principales zones de dépôts de fer, les vastes plateaux continentaux ainsi que les environs des bassins où les deux types se sont déposés avant la dérive des continents.

Bibliography/ Bibliographie CANADA. GEOLOGICAL SURVEY OF CANADA. 1963. Geology of northern Yukon territory andnorthwestern district of Mackenzie. Ottawa, Geological Survey of Canada.(Map 10-1963 .) GOODWIN, A. M . 1962. Structure, stratigraphy and origin of

iron formations,Michipicoten area,Algoma district,Ontario, Canada. Bull. Geol. Soc. Amer., vol. 73, p. 561-86. GROSS, G. A.1965.Iron-formation,Snake River area, Yukon and Northwest territories; Report of activities; Field, 1964. Geol. Surv. Pap. Can., 65-1, p. 143. . 1965-68. Geology of iron deposits i?z Canada. Ottawa,

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Geological Survey of Canada. (Econoinic Geology Report no. 22.) Vol.I: General geology and evaluation of iron deposits (1965);Vol. II: Iron deposits, Appalachian and Grenville

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regions (1967); Vol. III: Iron ranges of the Labrador geosyncline (1968). JAMES, H.L. 1954.Sedimentary facies of iron-formation.Econ. Geol., no.49,p. 235. . 1966. Data of geochemistry,sixth edition, chapter W. Chemistry of the iron-richsedimentary rocks.Prof. Pap, US. Geol. Surv., 440-W. .1969. Comparison between Red Sea deposits and older ironstone and iron-formation; Hot brines and recent heavy metal deposits in the Red Sea. Edited by Egon. T. Degens and

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David A.Ross. New York, N.Y., Springer. O'ROURKE, J. E.1961,Paleozoic banded iron-formation.Econ. Ceol.,vol. 56, p. 331-61.

The depositional environment of principal types of Precambrian iron-formations

SAPOZHNIKOV, D. G. 1963. Karadzhal'skoe zhelezo-margantsevoe rnestorozhdenie [The Karadzhal iron-manganese deposit]. Transactions, Institute of the Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, no. 89,p. 12395. Moscow, U.S.S.R. Academy of Sciences. (In Russian.) (Unpublished translation by the Canada Department of the Secretary of State, Bureau for Translations).

ZELENOV, K.K.1958.O n the discharge of iron in solution into the Okhotsk Sea by thermal springs of the Ebeko volcano (Paramushir Island). C.R. Acad. sci. U.R.S.S.,vol. 120, p. 1089-92. (In Russian; English translation published by Consultants Bureau Inc.,1959,p. 497-500.) -. 1970. Survey of world iron ore resources. New York, N.Y., United Nations.(Sales no. E.69,II. C.4.)

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Archaean volcanogenic iron-formation of the Canadian shield A. M.Goodwin Department of Geology, University of Toronto,Canada

Introduction Iron-formationis widely distributed in Archaean (older than 2,500m.y.) rocks of the Canadian shield. Although individual iron-formations are comparatively small, their wide distributioncompensates in total quantity.Thus total estimated iron-ore reserves in Archaean iron-formation amount to 35,000million tons with an average content of 25-30 per cent Fe. This constitutes25 per cent of the total estimated iron-oreresource in Canada (Gross, 1968). This paper demonstrates the genetic relationship of Archaean iron-formationto volcanism by focusing on three increasing levels of relationships: (a)the Helen iron range

where the quantities of silica present in the iron-formation are equivalent to those leached from subjacent footwall volcanic rocks;(b) the Michipicoten area where basin analysis has revealed the genetic relationship of iron facies to basin bathymetry and volcanic centres; and (c) the Canadian shield with exclusive regional relationship ofArchaean ironformation to volcanic-richgreenstone belts.

Helen Iron Range The Michipicoten area, situated in southern Superior tectonic province (Fig. i), is underlain by Archaean

Michi picoten

FIG.1. Location of Michipicoten area and HeIen iron range. Unesco, 1973. Genesis of Precambrian iron and manganese deposits. Proc. Kiev Symp., 1970. (Earth sciences, 9.)

23

A,M.Goodwin

FIG.2. Geology of Helen iron range. supracrustalrocks of Michipicoten Group and by younger intrusions.In the centralpart,which includes the main iron range, the Helen iron-formation forms a part of a thick, varied volcanic succession.Situated at the top of thick felsic pyroclasticsand overlainby mafic flows,it occupies a unique stratigraphic position at an abrupt felsic-mafic volcanic interface (Fig. 2). Structurally,the Helen iron-formationand enclosing volcanic rocks have been overturned to the north about an east-trendingfold axis;the rocks dip steeply southward but face to the north.They plunge eastward at 30-45 degrees.

The main banded chert member is from 400to 1,000ft (120-300 m) thick. It is typically composed of alternating bands of white to grey chert and pale brown siliceous sider-

Stratigraphic tops

H E L E N IRON F O R M A T I O N

Thisiron-formationcomprises three distinctiveand mutually transitionalfacies which are in descending stratigraphicsuccession,banded chert,pyrite and sideritemembers (Fig. 3). In addition, thin discontinuous chert zones are present within and at the base of the siderite member. 24

-, i

FIG.3. Cross-sectionof Helen iron-formation.

<

-
Archaean volcanogenic iron-formationof the Canadian shield

ite.Individualbands range in thickness from barely perceptible lamina to 3 in (7.5cm) thick.The banding is generally irregular with much pinching, swelling and bifurcation of individual layers.Thin zones of sooty,black carbonaceous, sulphide-bearingchert up to 30 ft (9 m) thick and several hundred feet long are present. The banded chert member normally grades downwards to the pyrite, or sulphide,member, a lensy,discontinuous unit ranging in thickness from 50 ft (15 m) at the west end (1000 E in Fig. 2) to less than 10 ft (3 m) at the east end of the range (SO00 Ej. This member is composed of pyrite with some pyrrhotite, siderite, magnetite and chert. The pyrite normally has a well-developed granular texture, although veins and bands of dense, massive pyrite occur locally. The pyrite member grades down in turn to the siderite, or carbonate,member which formsa persistent tabular body ranging in thickness from less than 100ft (30 m)at the west end of the range to more than 350 ft (107 m) at the centre (3500 E-5000 E). It contains several hundred million tons of iron ore. The member is composed of fine-grainedsiderite with variable proportions of ankeriticcarbonate,calcite, quartz, sulphides, magnetite, chlorite and iron silicates. The siderite is light grey to brown to black in colour and is typically dense and structureless in appearance.Local fragments of volcanic tuff as well as some thin,persistent pyritiferous shaly units are present in the siderite member. A thin, discontinuous zone of banded chert normally 5-10 ft (1.5-3 m j thick but ranging to 35 ft (9.5m j thick commonly lies at the base of the siderite member. It conformably overlies the footwall volcanic rocks without discernible evidence of erosion or deformation of subjacent volcanic rocks. F O O T W A L L VOLCANIC COMPLEX

The footwall complex in the vicinity of the iron range is mainly composed of alternating rhyolite-dacitetuff, flow and breccia but also includes substantial masses of carbonatized diorite (metadiorite)and smallerquartz porphyry intrusions (Fig. 2). The footwall complex within 3,000 ft (914 m) stratigraphically of the iron-formation comprises (a> coarse grained rhyolitic pyroclastics,marking proximity to source vents to the west (1000 W to 3500 E); (b) finer grained dacitic tuffs and flows to the east (4500 E to 8000 E); these two principal volcanic facies which are transitionalthrough (c) a mixed volcanic zone (3500 E to 4500 E) have been traced down and plunge to the east at least 3,000ft (914m j below surface. The footwall volcanic complex is zoned in both lateral and vertical stratigraphic directions. Lateral stratigraphic zoning,described above,is a function of the original compositional zoning of the volcanic pile. Vertical stratigraphic zoning, on the other hand, is a function of progressive upward chemical alteration of footwall volcanic rocks. Alteration occurred while the volcanics were flat-lying and

during the general period of deposition of the Helen ironformation.The quantity of SiO,leached from the footwall volcanic rocks equals that present in the Helen iron-formation (Goodwin,1964). A zone of maximum chemical alteration of footwall volcanic rocks,on average 150 ft (46 ni) thick,immediately underlies the Helen iron-formation(Fig. 3). This zone is uniformly thick regardless of local wall-rock composition. Beneath this zone of maximum alteration, the underlying volcanic rocks have themselves been altered to stratigraphic depths of at least 3,000ft (914 m). The chemical nature of the wall-rockalteration is the same throughout varying only in degree. On this basis the footwall volcanic complex is divided in a vertical stratigraphicdirection into the ‘highly’ altered zone (approximately 150 ft (46 m) thick) and stratigraphically beneath this, the mildly altered-zone (several thousand feet thick). The chemistry of wall-rock alteration has been investigated by means of chemicalanalyses of fresh diamond drill core(Goodwin,1964).Fifty-fiveequally spaced intersections of footwall volcanics provided 1,375 samples. Each sample was analysed for SiO,,AlzOe, totalFe,Cao,M g O , MnO,S, Tio,and loss on ignition(CO,+ H,O). Inaddition,selected samples were analysed for Na,O, KzO, F e 0 and Fe,O,. These chemical data reveal that wall-rock alteration resulted in subtractionof SiO,and addition of the carbonate components-Feo, M g O , MnO and CO,.The Alzo,content of volcanic rocks in individual intersections has remained essentially constant despite the degree of chemical alteration (Goodwin, 1964). Typical oxide trends across footwall rhyolitic and dacitic assemblages respectively are shown in Figure 4. Initial average SiO,contentsin rhyolite of 72.3 per cent and in dacite of 59.8 per cent decrease stratigraphicallyupwards within 150 ft (46 m j of the ore zone (siderite member) to an average of 64.1 and 58.4 per cent SiO, respectively. Carbonate contents increase proportionately (Goodwin, 1964).

A systematic plot of SiO,:CO,+HzOcontents in the 55 intersectionsrepresented by the 1,375 wall-rock samples reveals that each weight per cent addition of CO,+ HzOin excess of 2 per cent has resulted in 2.8 weight per cent loss of SiO, in rhyolite and 2.4 weight per cent loss SiO, in dacite (unpublished data), The corresponding weight per cent carbonate addition is almost equal to the weightper cent SiO, loss. INTERPRETATION

Widespread chemical alteration of assorted volcanic rocks to stratigraphic depths of several thousand feet together with the presence of a uniformly thick,continuous,highly altered zone within assorted volcanics indicatesthat chemical alteration occurred as a function of depth in flat-lying volcanic rocks. The chemical data show that alteration was largely independent of local structuralfeatures such as faults and fractures as well as lithofacies boundaries. The 25

A. M.Goodwin

Rhyolitic

Volconics

Volconics

Docitic 1 [ U-3-49

70

50

'

1

\

50'

J

3 !

O

-

c

'5

I

--

._...

IO 51:

Feet

400

L io o> a

-

.d.. . . .m . .4 : . : . : . . .C.

5

O IO 5

Mgo

Co0 . MgO

o

300

200

I O0

O

Horizonto/ distance south of the ore contact

-

Feet

400

300

2O 0

Horizontal distance south

O

IO0

A

of the.ore contoct

FIG. 4. Oxide trends across footwall volcanics.

mechanism of alteration apparently involved upward migration of volcanic-derivedsolutionsby way of hot springs, fumaroles and similar migrational systems. This resulted in pervasive alteration of freshly deposited predominantly felsic pyroclastics. According to the theory, volcanic solutions and gases were released to the volcanic pile during this explosive volcanic phase. Together with availableground waters,they migrated upwards through freshly deposited felsic pyroclastics,replete with glassy,vesicular fragments,by way of available openings including pores, fragment boundaries and fractures as well as conventional passage ways such as conduits, pipes,fissures,necks and other orifices. During their upward migration, the primary,bicarbonate-sulphurcharged hydrous solutions promoted pervasive devitrification of volcanic glass and breakdown of primary rock silicates to secondary mineral assemblages together with release of surplus soluble SiO,. This material there26

upon joinedtheupward migrating iron bicarbonate-charged solutions. High confining pressures in the lower levels of the volcanic pile presumably restricted the degree of chemical alteration (mildly altered zone). Within 150 ft (46 m) of the top of the volcanic pile, however, the expanding internal pressures in the solutions gradually exceeded the confining pressuresexerted by the overlying rock and water columns. As a result chemical alteration in the upper 150 ft (46 m) attained a maximum ('highly' altered zone). The volcanic solutions,charged with primary,igneousderived iron bicarbonate and sulphidecomponentsand augmented with leached Sioz, thereupon entered the overlying aqueous environment whether sea or lake. Sequential precipitation of chemicalcomponentsresulted in accumulation of,in order,siderite,pyrite and banded chert members.The main environmental controls were probably, in order of effect: (a) release of pressure at the rock-water interface

Archaean volcanogenic iron-formationof the Canadian shield

with loss of CO,resulting in rapid, blanket deposition of massive siderite in proximity to centres of discharge and (b) gradual temperature decrease resulting in delayed precipitation of banded chert upon the main siderite mass. The intermediary position of the sulphide member may reflect similar pressure-temperaturecontrol in addition to the influence of selective biogenic activities (Table 1).

TABLE 1. Principalenvironmentalcontrols and chemicalproducts during precipitation of Helen iron-formation Subaqueous environment Surñcial (volcanic) environment

Near source

Increasing timedistance from source

As a result of structural deformation the original lithofacies pattern has been distorted to prevailing easterly trends.T o reconstructthe originalfacies pattern it has been necessary to unravel the superimposed structures and to consider the lithofacies within individualrock-stratigraphic units throughout the area.The net result reveals a predominant northerly trend of the various lithofacies within the main rock-stratigraphicunits. This northerly trend correspondsto a principal direction of basin configurationduring Michipicoten time. Accordingly an east-west stratigraphic section of the Michipicoten area which is oriented normal to this principal direction of basin configuration,transects the original lithofaciestrend. STRATIGRAPHY

~

Pressure Temperature PH Eh Salinity Organic activity Principalchemical products

High Higher Low Low

Nil Nil Sileceous bicarbonate and sulphide solutions; wallrock alteration

Lower HighIntermediate Intermediate Low Low High (?) Siderite Pyrite

Low Low Higher

Higher Higher High (?) Chert

In summary the main chemical components of the Helen iron-formation are considered to represent direct volcanic contributions.The characteristic threefold facies construction is attributed to variations in the aqueous environment during the period of precipitation and lithification.Careful evaluation of the chemical data has shown that the SiO,requirementsof the Helen iron-formationmay be reasonably met by chemicalleachingoffootwallvolcanics (Goodwin, 1964). Taken together with the intimate stratigraphic relations, this constitutes the main quantitative evidence for a direct volcanic origin.

Michipicoten basin The Michipicoten area,70 miles (112km) long by 30 miles (48km)broad,is underlainby mafic to felsicvolcanic rocks, clastic sediments and banded iron-formation,including the Helen formation discussed above, in addition to younger intrusions.Of present concern are the distribution,nature and relationship of the supracrustalrocks,particularly felsic pysoclastics, clastic sediments and iron-formation,to the originalbasin of deposition(Goodwinand Shklanka,1967). Michipicoten rocks have been complexly folded about east-trending and north-west-trendingáxes. This has resulted in complex, doubly plunging fold patterns in both longitudinal and cross-section(Fig. 5). In addition, numerous north-trendingfaults have disrupted the rocks.

Michipicoten stratigraphy exihibits volcanic and sedimentary facies of conventional ‘eugeosynclinal‘type. Lensoid rather than ‘layer-cake’ stratigraphicrelations predominate. Some units blanket the asea but most have limited distribution. The Michipicoten assemblage contains distinctive volcanic and sedimentary facies (Fig.6). The lowermost mafic volcanic member (1) is continuous across the area. It is overlain in the west by clastic sediments (3) which thin and become finer grained to the east, and by felsic volcanic masses (2)in the east which are,in turn, overlain by mafic volcanics (5, 6). Iron-formationis transitional from sedimentary association in the west to volcanic association in the east. Large masses of felsic volcanic rocks are present at Goudreau in the east, Magpie in the centre and W a w a in the south-centre.The felsicpiles contain a great assortment of andesite-dacite-rhyolite pyroclastics and flows.The more silicic types,especially rhyolite,are concentrated at the top of the piles. The coarser, more angular pyroclastic fragments as much as three ft (91 cm) in diameter are more common in thicker parts of the piles. Elsewhere tuff and tuff breccia predominate. Some andesite-dacitelava flows are present. Carbonatizationof the felsic pyroclastic rocks is a common feature. The felsicvolcanic masses are clearly products ofhighly explosive eruptions which rapidly produced thick,irregular, high-rising piles. The ‘coarse fragment-thickpile’ spatial relationship described above reflects proximity to eruptive sources probably central vents. IRON-FO R M A TIO N

Iron-formations are present in nearly all parts of the Michipicoten area. In central and eastern parts, they occur in volcanic rocks; enclosed, they typically occur at prominent felsic eruptive-maficeffusive contacts. In the western part of the area, laterally equivalent iron-formationsare enclosed in clastic sediments. Michipicoten iron-formations of volcanic association 27

A. M.Goodwin

I

I West

Idealized section of Michipicoten basin illustrating shore-to-depth relationship of iron facies.

Dore sediments: conglomerote;greywocke,shale.

I

S T R U C T U R A L SECTIONS

Felsic volcunics.

I/---I Geologhl boundary. [ T Fuult. = j Gradational ond approximoie boundory of iron facies.

FIG.5. Iron facies distribution in Michipicoten basin

in central and eastern parts of the area contain a threefold arrangement of, in descending order, banded chert, sulphide and carbonatemembers (Fig.7,Helen and Goudreau sections). The banded chert member is commonly 100-200ft (30-60m)thick; it reaches a maximum thickness of 1,000ft (304 m) at the Helen range in the central part but is thinner or absent to the east,The sulphidemember is commonly 10 to 30ft (3-9 m)thick but attains a maximum of 120ft (36m) at Goudreau in the eastern part of the area.The underlying carbonate member is a similarly lenticular and discontinuous unit composed mainly of iron-bearingcarbonateminerals with minor pyrite, magnetite, pyrrhotite and silicate minerals. The carbonate member is commonly less than 200 ft (60 m) thick; it reaches a maximum thickness of 350 ft (106 m) at the Helen iron range near Wawa. The member is predominantly sideritic in the central part of the area but becomes increasingly calcareous to the east. 28

Michipicoten iron-formations of sedimentary association in the western part of the area (Fig. 7,Kabenung section) comprise thick cherty units composed of thinly interbanded chert,siliceousmagnetite and jasper (hematitic chert). Individualzonesofiron-formationsmay reach thicknesses of 600 ft (180 m). This type of iron-formationis typically intercalated with greywacke and shale. Michipicoten iron facies are gradational across the area (Fig. 5). Oxide facies predominates in the west as at Kabenung Lake, carbonate facies in the centre, as at the Helen and Magpie ranges,and sulphidefaciesin the eastern part,as at Goudreau.The sinuous yet parallel distributions of the oxide-carbonateand carbonate-sulphidefaciesboundaries are shown in Figure 5. Examination of structural relations reveals that this sinuous pattern is due mainly to regional folding. Thus Michipicoten iron-formationsare arranged from

Archaean volcanogenic iron-formationof the Canadian shield

FIG.6. Reconstructed east-westsection of Michipicoten basin.

west to east in oxide, carbonate and sulphide facies,the three facies being broadly transitional one to the other across the area (Fig. 5). Considered relative to clastic sedimentary patterns, it is apparent that oxide iron facies is largely coincident with shallower water, conglomeratebearing clastic facies in the western part, the carbonate iron facies with deeper-water,conglomerate-freesediments in the central part, and the sulphide iron facies with still deeper-water, shaly sediments in the eastern part of the area.This relationship points strongly to common environmental controls during iron-silicaprecipitation. The principal factors were depth of water coupled with a transition from shallow water, and the
faciesupon finer grained felsic pyroclastic piles in the east. Michipicoten iron-formationsare viewed as'direct volcanicproducts because: (a) the major concentrations of iron (siderite and pyrite) overlie the thickest felsic volcanic piles thereby implying maximum iron accumulation in proximity to volcanic vents; (b) iron-formationwas deposited rapidly at the end of a mafic-to-felsicvolcanic cycle implying an end stage of igneousdifferentiation as a source of iron;(c) deposition ofiron-formationwas associatedwith pervasiveleaching of silica from freshly deposited felsic pyroclastics. In brief the relationship of Michipicoten iron-formationto volcanism is direct and impressive. Although the chemical components of Michipicoten iron-formationswere volcanically derived and thus associated with specific exhalative centres,yet the environmental parameters exerted by the existing basin configurationwere sufficient to have produced a general shelf-to-depthfacies pattern that conforms to the world-wide pattern of iron deposition as presented by James.Thus Michipicoten iron facies, both in vertical and lateral distribution, are attributed to depositional changes and transitions of environment within the Michipicoten basin. It is noteworthy that the predominantly cherty oxide facies iron-formationwhich is the most commonly recognized Archaean iron-formation is best developed in the sedimentary environment on the slopes of the basin some distance removed from volcanic exhalative sources. Thus within an Archaean basin, the distribution of cherty oxide facies iron-formationmay be used to locate the margins of the basin rather than specific volcanic sources (Goodwin and Ridler, 1970). 29

A. M.Goodwin

~

KABENUNG SECTION

HELEN SECTION

(Oxide Facies)

(Carbonate Facies)

GOUDREAU SECTION (Sulphide Facies)

Pyrite

Carbonate s-siderite Interbedded chert-magnetite

Banded chert

: 1 j'L'.,

c-limestone Rhyolite-dacite tuff, brôccia,flows

:IC. 7. Typical sections of oxide,carbonate and sulphide facies.

Canadian shield Archaean iron-formation is broadly distributed across the Canadian shield. Distribution patterns (Figs. S(a) and (b)) are based substantially on a compilation (Geol. Surv. Canada, Maps 13-1963 to 20-1963 inclusive and M a p 1187A) prepared by G.A . Gross in 1963, with some additions (Davidson,1969). Additional bands of Archaean iron-formationwill doubtless be revealed particularly in northern parts of the shield. Also certain ironbearing belts presently classified as Proterozoic in age may in futurebe reclassified as Archaean. However,considering that all parts ofthe Canadian shield willhave been mapped, at least on reconnaissance scale,by 1972,there is reason for confidence in present distribution patterns. 30

The term 'iron-formation'has been generally applied to layered rocks with 15per cent or more iron (Gross,1965). The most commonly recognized type is chert-magnetiteor oxide iron facies. Problems of classifying other types (including associated non-ferruginous chemical sediments) e.g. sulphide-bearing aiid calcareous facies, pose certain problems not yet satisfactorily resolved. For example, should the numerous thin, pyritiferous zones common in quartz-mica schist be classified as iron formation? What about massive sulphide ore deposits,e.g. Kidd Creek CuZn-Ag deposit, of presumed exhalative origin? A broader definition of the term iron-formationmay be required in future.In this event established distributionpatterns of Archaean iron-formationin the Canadian shield would require appropriaterevision.

84.

92.

100.

56.

76.

\

LEGEND

Other Precambrian rocks ; mainly grmitic Archaean Archaean Archaean Archáaan

iron-formation sedimentary rocks felsic volc~nicrocks mafic voicanic rocks

Boundary of shield

I

I

100.

9' 2

116.

100-

I

I

\

1

64'

92'

I

76'

76.

64.

bn'

Orher Precambrian rocks ; mainly grmitic Archaean iron-formation Archaean sedimentary rocks Archaean feisic volcanic rocks

.

Boundary of shield

-

Tcelonic boundary

o Scale rn Miles

106.

100.

92'

64'

(b)

FIG.8. Distribution of Archaean iron-formation in Canadian shield :(a) southern part ; (b) northern part.

76'

A. M.Goodwin

DISTRIBUTION

TECTONIC SETTING

The distribution of recognized iron-formationin Archaean rocks is shown in Figure 8(a) (southern shield) and 8(b) (northern shield), With few exceptions iron-formationis located either in volcanic-richbelts or in nearby sediments. Iron-formationwithin volcanic-richbelts is closely related to felsic volcanic centresrepresentedby felsicvolcanic rocks and cogenetic tuff and greywacke. The largest concentration of Archaean iron-formation is in Superior Province (Fig. 8(a)). At least thirty volcanicrich belts contain iron-formationmainly associated with thirty-fourestablished felsic concentrations each a volcanic centre. In Churchill Province iron-formationis present in six greenstone belts; the major iron-bearing belt is in the Rankin-Ennadai area west of Hudson Bay. In Slave Province a single,smalliron-bearingbelt has been recognized. Obviously the abundance of Archaean iron-formation is directly proportional to the number of volcanic-rich greenstone belts especially those containing felsic pyroclastics and related sediments.

The typical association of Archaean iron-formationis illustrated in a reconstructed section of the Archaean crust (Fig.9; Goodwin, 1968). According to this model the main lithofacies,arranged in lateralsuccessionoutwards from the more stable parts of the primitive crust, are: (a) sialic craton;(b) flyschoid facies;and (c) volcanogenic facies.The predominant orogenic characteristics of both the flyschoid and volcanogenic facies point to their accumulation in mobile belts corresponding to thin-crustal zones such as may be reasonably ascribed to the margins of,and between (i.e. intracratonic), expanding sialic cratons. The primitive sialic cratons of the Canadian shield are identified only on the basis of widespread clastic detritus of appropriate sialiccomposition.The exact nature and degree of stability of the presumed cratons are conjectural beyond their function as a source of sialic detritus. Flyschoid facies, which includes the common quartzfeldspar mica schist assemblage of the Canadian shield, represents orogenic detritus apparently derived by rapid weathering of sialic provenances and deposited in the manner of classical flysch. The facies is moderately thick (10,000-20,000 ft (3,048-6,096 m)) and tends to be lithologically uniform and stratigraphically continuous.

Flyschoid Facies

Craton

I

I

Volcanogenic Facies

I

Island Arc

Sedimentary.Basin

..

..

LEGEND

t

[1113Greywacke, argillite Granodiorite. ......... Dacite,rhyolite

Andesite

Basalt

&@ Gabbro, norite, etc. Voicaniciastics

a

FIG.9.Idealized crustal section showing Archaean lithofacies. 32

i

Approx. 100 Miles

iron-formation

Archaean volcanogenic iron-formationof the Canadian shield

The flyschoid facies is transitionalthrough a subfacies to the volcanogenic facies,the principal Archaean volcanicbearing facies.The volcanogenic facies,which is commonly called greenstone, is very thick (approximately 30,000ft (9,140 m)). It typically comprises basalt-andesite-rhyolite assemblages, assorted volcaiiiclastics, chemical sediments (iron-formation)and numerous intrusions. Evidence of tectonic instability during accumulation is widespread. Abundant pillow lavas indicate a prevailing subaqueous environment of accumulation. Iron-formationis common in the volcanogenic facies as well as iii transitional sub-faciesthe latter being dominantly volcaniclastic and located on the flanks of the volcanic piles. Locally some iron-formation is also present in marginal flyschoid zones adjoining volcanic assemblages.

ORIGIN

Weighted average compositions of Archaean volcanic rocks of the Canadian shield have been determined 011 the basis of detailed geochemical studies in four widely separated belts (Baragar and Goodwin, 1969). The results demonstrate that Archaean volcanic rocks have substantial CO, and H,O contents;these are listed in Table 2 with corresponding SiO, contents (Baragar and Goodwin, 1969). Thus theindicatedaveragevolatile content ofArchaean volcanic rocks is sufficient to have facilitated leaching of substantial quantities of SiO, from the volcanic rocks. Although the presence of CO,and H,O in the volcanic rocks does not constitute proof of pervasive leaching of SiO, at the time of deposition of iron-formation,yet the demon-

TABLE2 Weighted average percentage composition

CO,

HZ0

53.9

1.1

2.4

52.6

1.2

2.5

66.8

1.1

1.5

SiO,

Average Archaean volcanic belts Average Archaean mafic volcanic fraction Average Archaean felsic volcanic fraction

strated example of leaching of SO, from felsic volcanic rocks at the Helen iron range and the intimate shield-wide association of iron-formationwith volcanic rocks do support a silica-leachingmechanism for obtaining most if not all the large quantities of SiO, present in Archaean ironformation of the Canadian shield.

Acknowledgements I gratefully acknowledge the value of discussions with G.A.Gross on the distributionofArchaean iron-formation in the Canadian shield and with A. Davidson on recent investigations in the Rankiii-Ennadai belt of Northwest Territories. Some of the illustrationswere prepared by F.Jurgeneit and all were photographed for reproduction by P.B.O’Donovan, both of the Department of Geology, University of Toronto.

Résumé La formation de fer volcanogéniqsre archéenne du bouclier canadien (A. M.Goodwin) Des formations de fer rubanées datant de l’époque archéenne (c’est-à-direayant plus de 2,5milliards d’années) présentent des relations étroites de localisation et d’origine avec les roches volcaniques dans de nombreuses zones archéennes de ((greenstone >) du bouclier canadien. Dans la région de Michipicoten, dans la partie septentrionale de l’Ontario central,une formation de fer du type ((Algoma )) comprenant, de bas en haut, dans l’ordre stratigraphique, des faciès carbonate,sulfuré, et de silex est située à un contact stratigraphiqueremarquableentre les pyroclastitesfelsiques sous-jacenteset les coulées de lave mafique susjacentes. Des études détaillées de l’altération de la roche encaissante portant sur plus de 2 500 analyses complètes de roche couvrant une zone stratigraphiquede roches volcaniques sous-jacentes de plus de 5 kilomètres carrés et atteignant des profondeurs stratigraphiques de près d’un

kilomètre ont permis de définir la nature et le degré de son altération chimique,altération attribuée à des sources thermales contemporaines et à une activité fumérolique.Parmi les éléments qui composent la formation de fer on trouve : le fer,le manganèse et le gaz carbonique,qui se sont ajoutés aux roches volcaniques sous-jacentes et ont évidemment une originevolcanique plus profonde.Cependant SiO,a été lixivié des roches volcaniquesfelsiques immédiatement sousjacentes en quantités à peu près égales à celles présentes dans la formation de fer sus-jacente.Ainsi les relations de localisation et génétiques de cette formation de fer aux roches volcaniques et aux processus volcaniques sont à la fois directes et remarquables. Outre la construction du faciès vertical à l’intérieurde l’empilement des roches volcaniques mafiques et felsiques, les formations de fer de Michipicoten présentent des changements de faciès latéraux qui rappellent la construction du bassin original. Ainsi, les formations de fer où prédominent lesfacièsoxydés,carbonatéset sulfuréssont associées 33

A. M.Goodwin

avec des structures volcaniques sédimentaires de plus en plus profondes quand on progresse de l’ouest vers l’est à travers la région de Michipicoten. C e système de faciès latéraux rappelle le dépôt depuis le littoral jusqu’aux couches profondes de composants chimiques volcaniques, F e-M n -CO,-S-Sioz,sur des pentes inclinées vers l’est dans un bassin archéen primitif. Le milieu archéen a permis apparemment que se déposent des combinaisons d’oxydes (magnétite-hématite par exemple). Des relationsvolcaniques similaires se retrouvent dans plus de trente bandes de ((greenstone ))du bouclier canadien, où les formations de fer sont associées directement

avec des empilementsvolcaniques épais ou des assemblages clastiques voisins. Dans bien des régions du bouclier, les schémas de distribution :oxyde, carbonate, sulfures, sont des indicationsutiles sur la construction du bassin original. Bien qu’elles soient abondantessurtout dans larégion du lac Supérieur, les formations de fer archéennes sont assez communes dans les provinces de Slave et de Churchill pour mettre en évidence que les conditionsqui prévalaient dans la croûte ont favorisé le développement de la formation de fer volcanogénique dans toutes les parties de la croûte primitive précambrienne représentées dans le bouclier canadien.

Bibliography/ Bibliographie BARAGAR, W. R. A.; GOODWIN, A. M. 1969. Andesite and Archaean volcanisin in the Canadianshield.In: A.R.McBirney (ed.), Proceedings of the Andesite Conference. Bull. Ore. Dep. Geol.,no. 65.

DAVIDSON, A. 1969. Eskimo Point and Dawson Inlet mapareas. District of Keewatin (55E, north half, 55F, north half). Geol. Surv. Pap. Can. 70-1,Part A, p. 131-33. GOODWIN, A. M.1962. Structure,stratigraphy and origin of iron-formations,Michipicoten area,Algoma district,Ontario, Canada.Bull.geol. Soc. Amer., vol. 73,p. 561-86. . 1964. Geochemical studies at the Helen iron range.Econ. Geol., vol. 59, no. 4,p. 684-718.

__

34

--.1968.Preliminaryreconnaissanceof the Flin Flon volcanic belt,Manitobaandsaskatchewan.Geol.Surv.Pap. Can. 69-1. Part A,p. 165-68. ; RIDLER, R.H.1970. Abitibi orogenic belt. Geol. Surv. Pap. Can. 70-40, p. 1-24. ; SHKLANKA, R. 1967. Archaean volcano-tectonicbasins: form and pattern. Canad. J. Earth Sci., vol. 4, p. 777-95. GROSS, G.A. 1965.Geology of iron deposits in Canada. Geological Survey of Canada (Economic Geology Report no. 22),

__ __ -.

181 p. 1968. Detailed survey tabulates billions of tons of iron. North. Min., November 28,Annual Review Number,p. 51.

T he facial nature of the Krivoyrog . .. n

iron-Iorrnation

A. I. Tugarinov, I. A. Bergman

and

L. K.Gavrilova

Institute of Geochemistry and Analytical Chemistry, Academy of Sciences of the U.S.S.R., Moscow (Union of Soviet Socialist Republics)

The Krivoyrog iron-formationincludes a complex of metamorphosed sedimentary rocks of magnesium-ironand iron composition.Beginning with a talc horizon,it passes into a rhythmically constructed stratum of successively alternating shaleand ironhorizons.In the centralpart of the Krivoyrog Basin (Saksagan region), where the iron-formationis most intensively developed,up to seven rhythms are distinguishable. The thickness of the separate horizons varies over a wide range, averaging from tens to a few hundreds of metres. The abundance of rocks in the Krivoyrog iron-formation is shown in Table 1 according to data from a study in five sections (Skelevatski-Magnetitovy,Zelenovski district, Inguletz,Zholtaya Reka and Frunze mines).

The second peculiarity is that quartz-chert interlayers are integral elements of all rocks which compose shale and iron horizons. From the point of view of the general thickness of quartz-chert interlayers, three rock groups are distinguishable (Table 2).

TABLE 2 Rocks

Shales Iron cherts and jaspilites Ore-freeand low in ore cherts

Contents of quartz-chert interlayers (in volume %)

20-40 40-60 60-80

TABLE 1 Rocks

Iron cherts and jaspilites Magnesium-ironschists Ore-free and low in ore cherts Talc-containingrocks Carbonaceous shalesand cherts(carbonatealumosilicate) Alumosilicate shales and cherts Others

Abundance (ratio, %)

55 19 13 7 3 2.5 0.5

Structural peculiarities of the Krivoyrog iron-formation The first peculiarity of magnesium-ironand ironrocksis the rhvthmic character of their structure-the alternation of quartz-chert(quartzite) with interlayers which,as it will be shown below,form a successive series connected by mutual transitions: alumosilicate shales-magnesium-iron schistsiron cherts.

Between the rock groups shown in Table 2, all types of mutual transition are taking place. In other words, quartz-chertinterlayers alternate both with shale and ore interlayers in practically any quantitative ratios. Hence, to a certain degree quartz-chert interlayers are, with respect to shale-oreinterlayers,an independent componentformational peculiarity and not a peculiarity connected with a concrete composition of some rocks. The third peculiarityofthe Krivoyrog iron-formationis the dual nature of its structure.Within one formation there are depositions of two facial series, one at present representedby quartz-chertinterlayers and the second by slateore interlayers. Alumosilicate slates and iron cherts are two extreme subelements of the rhythm into which the depositions of the second series are differentiated.

Facial position of the iron-formation

~~i~~~~~~

N o data are available for a direct reconstruction of the region ofiron-sedimentaccumulationsand of their relationships to such types of sediments.At present,the alternation

Unesco, 1973. Genesis of Precambrian iron and nranganesr deposits. Proc. Kiev Symp., 1970. (Earth sciences, 9.)

35

A. I. Tugarinov,I. A.Bergman and L. K.Gavrilova

of rocks in the section,especially their composition and structure,are the sole sources of information. During the determination of the facial position of the Krivoyrog iron-formation,the following observations were taken into account:(a)the iron-formationreplaces in time the lower terrigenousformation,its thinnest fractions;(b) in the lower terrigenous formation only the ‘roots’of the ironformation can be found (cummingtonite shales,lrelicts of iron carbonates in phyllites and some other, less distinct, signs), on the other hand rudaceous sediments are lacking in the iron-formationitself;(c) the persistent character of the iron-formationand of the rocks composing it; (d) the high degree of differentiation of the material;(e) the distribution of alumina in the rocks,etc. All these features bear witness to a facial displacement of the iron-formationinto a region of pelagic conditionsrather than coastal ones with respect to pelites. In Palaeo-Cenozoicformations some types of carbonate deposits (pelitomorphic limestones) occupy a facial position of iron sediments.

Compositional peculiarities of the iron-formation From the point of view of mineral composition,the magnesium-iron shales-iron cherts (including jaspilites) are composed of one and the same suite of minerals, but in different quantitative ratios? magnesium iron carbonates, magnetite, cummingtonite(grunerite), chlorite,garnet, stilpnomelane, hematite, biotite. Within the Saksagan region a noticeable,and sometimes even a cardinal,role of the ore mineral in the composition of magnesium-iron and iron rocks is fulfilled by carbonates: siderite-sideroplesite(-pistomesite). Structuralinterrelationsdo not reflect the evolution of the mineral composition of rocks (Betekhtin, 1951), but to study them is an indispensableprecondition for singling out mineral parageneses of different ages and for the analysis of the paragenetic relationships of minerals. In iron rocks of the Krivoyrog iron-formation,beyond the oxidation zone, the fractured and ore zones, a quite definite sequence of structural replacement is revealed, shown by the series: siderite -+ garnet icummingtonite -+ magnetite> quartz +alkaline amphiboles -+hematite +chlorite istilpnomelane +biotite. A typical feature of structural interrelationsbetween minerals is their evolution in the process of regional metamorphism:in somecasespositional and geneticalconnexions are lost (siderite-magnetite, siderite-cummingtonite), in other cases reactional relationships arise which are of no genetic significance(garnet-cummingtonite,cummingtonitemagnetite). Contradictory views on the formation of magnetite, cummingtonite and other minerals are explained by the fact that investigators study rocks in which the structural interrelationshave ‘stopped’at differentmovements of their evolution. 36

Taking into account the character of structural interrelationships between minerals, we have singled out the following typical parageneses of the second temperature degree of regional metamorphism in magnesium-ironrocks -‘cummingtonite + magnetite’,in magnesium-ironrocks with pelitic matter-‘garnet + cummingtonite’,in rocks which are intermediate in composition-‘garnet + cummingtonite + magnetite’. Transition to the second degree of progressive metamorphism is associated with the appearance of rocks with different quantitative ratios of minerals of both degrees: ‘siderite + magnetite’, ‘siderite + cummingtonite’, ‘siderite + cummingtonite + magnetite’, etc. Rocks with higher temperature degrees of regional metamorphism have local spreading. Sideriteis the sole mineral which does not replace other iron minerals. In its turn it is replaced by cummingtonite, magnetite, chlorite,stilpnomelane and others. This is why strict proportionality in the content of siderite with each of the enumerated minerals has not been observed in bimineral rocks (e.g. siderite-magnetiticrocks); the inversely proportional dependence is frequently disturbed as a consequence of siderite displacement by quartz. The structuralinterrelationsbetween sideriteand magnetite, as they bear the most direct relation to the problem of the genesis of iron-formations,will now be dealt with in greater detail. Siderite and magnetite are the chief ore minerals of the Krivoyrog iron-formation outside the oxidation zone in areas with a low degree of progressive metamorphism (e.g. Frunze mine). In iron rocks where iron silicates and quartz either are absent or are contained in negligible amounts,siderite and magnetiteare positionally and geneticallyclosely connected. The following casesoccur:sideriteis solelyreplaced by magnetite; siderite is almost simultaneously replaced by quartz and magnetite (late quartz displaces sideritewhich has not been replaced by magnetite); siderite is solely replaced by quartz.With the increase of the quartzcontent a separation of magnetite from siderite occurs,chiefly at the expense of siderite, and the earlier existing genetic relationship between them is lost. Hematite3characterizes,in the paragenesis with chlorite and/or stilpnomelane,the regressive stage of regional metamorphism. The mineral-forming processes in rocks of the ironformation are concluded by biotite crystallization.It seems that its development was the consequence of the superimposed regional migmatization which, during the PostKrivoyrog time,covered the territory directly to the west of the Krivoyrog structural-facialzone. Let us first consider the chief regularities of minorelement distributionin iron ores in generalfrom the tectonic and the physico-chemicalaspects. 1. Cummingtonite is formed solely according to the reaction mentioned below. 2. Evaluated for shale ore interlayers. 3. Outside the zone of oxidation, the ore and fractured zones the hematite content amounts to 5 per cent, rarely more.

The facial nature of the Krivoyrog iron-formation

In the tectonic aspect,the regularitiesof minor-element distribution were studied by Strakhov (1947), according to w h o m ores formed in regions of just-completedfolding are distinguishable by the greatest concentration and the greatest diversity of minor elements. The least accumulations and the least diversity of minor elements are met in geosynclinalores.In this respect ores of platform regions occupy an intermediate position. Arkhangelsky and Kopchenova (1934) approached the interpretation of minor-element distribution from other standpoints.They have established that the chemical composition of iron ores depends on their formation conditions. Ores of an oxidizing medium contain substantially more admixtures (e.g. phosphorus, arsenic, vanadium, nickel, cobalt, chromium) than siderites and other ores of a reducing medium,i.e.the primary ore of Precambrianquartzites was siderite. When comparing both variants we can easily establish that the chief factor controlling the distribution of this group of elements is the physico-chemicalmedium of ore formationand theirtectonicpositionbecomeslesssignificant. Tables 3 and 4 show the distribution of typical minor elementsin the iron Krivoyrog cherts of the iron-formation.

For comparison,data on iron-formationsof similartectonic position and age, iron ores of oxygenous and oxygen-free media and clarkes of minor elements in sedimentary rocks are included. Analysis of the distribution of these elements permits the following conclusions to be drawn: In iron cherts and jaspilites of the Krivoyrog iron-formation (e.g. Kursk Magnetic Anomaly), elements which have a tendency to accumulate by sorption or by the formation of slightly soluble compounds of the type of arsenates or molybdates of ferric iron (vanadium,chromium, nickel,cobalt) are present in amounts smaller by as much as a half or even a whole order of magnitude than their clarkes. Iron cherts and jaspilites of different chemical composition and different degree of metamorphism do not differ in general in the character of distribution of this group of elements and their contents in samples from various sections are similar. In their content of minor elements iron cherts and jaspilites of the Krivoyrog (and Kursk Magnetic Anomaly) markedly differ from the marine iron ores of the oxygenous zone and, on the contrary,have much in c o m m o n with

TABLE 3. Distribution of sulphur, phosphorus and arsenic (X10-4 per cent) Region of deposit

Gershoig (ed.-in-chief

Krivoyrog

Rocks

Author

Magnetite cherts

S

770

As

P

410 no data-5 samples

Belevtsev, 1962) 50-1 sample (C

Martitic and specularite-martitic 170 no data-15 samples

jaspilites

Magnetitic,martitic and goethitehematite cherts and jaspilites K, Basic syncline Fomenko and Chernovsky Amphibole-magnetiticand carbonate(Belevtsev, ed.-in-chief, magnetitic cherts 1, 2 and 4 of iron1962) containing horizons 1,100 Sideritic rocks Fedorchenko (1965) 3,300 Krivoyrog Kursk Magnetic Anomaly, Illarionov (1965) Hematitic quartzites 310 Mikhailovskoye deposit ( ( 420 Specularite-magnetiticquartzites ( ( 610 Magnetitic quartzites Ore-freeand low ore quartzites 3,300 Kursk Magnetic Anomaly ( ( Cummiiigtonite-magnetiticquartzites 3,000 ( ( 1,480 Dolomite-magnetiticquartzites (< Hematite-magnetiticquartzites 670 (hematite :magnetite = 10 :90) Hematite-magnetititicquartzites 420 (hematite :magnetite = 40 : 60) Van Hise,Baley and Smith Carbonate cherts Lake Superior Negaunee ( (

(1 897) (< (< (<

Gunflint Ironwood Iron River

Ukrainian S.S.R.,Kerch basin France,LandresAmerrnont basin

Irving and Van Hise (1892)

(<

Huber (1959) James (1951) Vinogradov (1 962) Green (1953) Litvinenko (1964)

Clays and shales,average Limestones,average Oolitic ores

Coche et al. (1954,1955)

Minette ores

( ( ( (

70-13 samples 270 340 160

510 530 700 750 740 710 540

130 570 470 4,000 3,000 770 1,100 1,100 1,850 9,700 590 525

6.6 1,020

740 8,300

37

A.I. Tugarinov,I. A. Bergman and L. K. Gavrilova

TABLE 4. Distribution of the iron-groupelements,uranium and molybdenum ( X IO-* per cent Region of deposit

Tarkhanov

Ti

V

Cr

300

< 10

20

340

7

13

70

100

20

50 45

60 50

13

50

730 720

38 38

20 20

30

620 330

38 13

20 22

30

340

17

20

25 25 200

650 2,000

25 15 20

60

25 25 30

10

Clays and shales, average 4,500

670

130 1O0

95

20 3.2 2.0

Rocks

Author

30

According to Iron cherts and jaspilites authors Magnetite quartzites Plaksenko and Koval Specularite-magnetite (1967) quartzites Specularite quartzites Silicate-magnetitequartz(< ites

Gorlitski

M n

Ni

Co

U

Mo

Iron quartzites

(1969)

Novo-Yaltinskdeposit

,

Magnetite quartzites Specularite-magnetite quartzites Magnetite quartzites Specularite-magnetite quartzites Specularite-magnetite quartzites Specularite quartzites Iron quartzites

1O0 35

80

< 10 <10 1.10 1.0 5

6/

1.3

10

(1962)

zone Vinogradov Russian platform

(1962) Green (1953) Limestones,average Ronov 1956) Carbonate rocks

Baranov et

385

< 10

2

O? 7.5

-1

Carbonate rocks

O 1.3 4.3 2.1

al. (1956)

Gerasimova

Limestones

etal.(1969)

Volga-Ural,Emba regions and N.W. Caucasus Ukrainian S.S.R., Kerch basin

Katchenkov

Dolomites Siderites Carbonate rocks

5,400 200 1,300

80

-

10

30 470

3

32

8 20

1,200

9

24

16,000

590

6

1

-

2

(1959)

Litvinenko (i 964)and Kantor

11

Oolitic ores

(1937)

200

190

1. According to three out of five.

carbonate rocks of the Russian platforni,the Volga-Ural and E m b a regions and the North Caucasus. All these peculiarities and, in the first place, the lower contents than clarkes of minor elements which have a tendency to accumulate by sorption or by the formation of slightly soluble compounds with ferric iron, may be explained solely by the fact that iron was present in the primary sediment in a form in which neither sorption,nor formation of slightly soluble compounds, could manifest themselves.Such a form could only be the carbonate with all the ensuing consequences. A number of factors (such as the hardly probable 100 per cent reduction of iron oxides,the facial position of iron sedimentsbeing peculiar to chemogeniccarbonatesand not to iron oxides,the scale of the phenomenon,the absence 38

of typicalconcretionalforms,and composition peculiarities of magnesium-iron carbonates,including pistomesites unknown as diageneticformations) point to a chemogenicand not to a diagenetic nature of ferric carbonates and,hence, to the formation of iron cherts and jaspilites in the process of metamorphism of only chemogenic carbonates.

Conclusions The Krivoyrog iron-formationis a binary flyschoid formation which consists of two rhythm elements: quartz-chert and shale-oreinterlayers,the second rhythm element in its turn differentiating into two rhythm subelements-pelites and iron carbonates.The first corresponds to alumosilicate

The facial nature of the Krivoyrog iron-formation

shales, the second to iron cherts; magnesium-iron shales occupy an intermediate position corresponding to magnesium-ironmarls. The chemogenicnature of magnesium-ironcarbonates of the Krivoyrog iron-formation,which is in agreement with finds of magnesium-iron carbonates in pebbles of

conglomerates (Molass formation, according to Kalyaev (1965)), permits us to concludethatin theLower Proterozoic reducing conditions prevailed. The motive factor in the evolution of the iron-ore process in the history of the Earth was the progressive increase of oxygen content.

Résumé Les faciès des formations ferrugineuses du Krivoyrog

(A.I. Tugarinov, I. A. Bergman, L. K. Gavrilova) 1. Des recherches dans la zone de strates de Krivoyrog, axées sur la paragenèse minérale,mettent en évidence que le minerai de fer siliceux trouve son origine dans les roches carbonatées dont le dépôt a commencé par des carbonates de manganèse ferrugineux. Tous les minéraux qui sont venus par la suite ont été formés au cours de métamorphismes progressif et régressif. 2. L a comparaison des données recueillies sur les micro-éléments a montré que la région de Krivoyrog, dans son ensemble,a un contenu maximal de ces éléments dans

K,= K, et un contenu minimal accusé dans K,.L e minim u m est tout à fait comparable au contenu des mêmes éléments dans les faciès carbonatés à travers le monde. 3. A la lumière de ces observations,on peut suggérer une origine des formations de fer dans les carbonates primaires sédimentaires dont l’accumulation a eu lieu dans les conditions offertes par les mers ouvertes précambriennes en contraste avec les conditions côtières d’accumulation de K et le faciès K,de la plate-formecontinentale. 4.L a nature spécifiquement précambrienne des dépôts K, a été régénérée par l’accumulation de FeCO, (fer ferreux) avec une différenciationde certains micro-éléments typiques du Précambrien.

Bibliography/ Bibliographie ARKHANGELSKY, A. D.;KOPCHENOVA, E. V. 1934. O zavisimosti khimicheskogosostava osadochnykh zheleznykhrud ot usloviy ikh olrazovaniya.[Dependenceofthe chemicalcomposition of sedimentary iron ores on the conditions of their formation]. Bull. Mask. obshch. ispit. prirody, otd. geologii, no.2,p. 12. BETEKHTIN, A. G . 1951. Parageneticheskiye sootnosheniya i posledovatel’nost obrazovaniya mineralov [Paragenetic relations and sequence of mineral formation]. Rec. Russ. miner. soc., no. 2. COCHE, L.;DASTILLON, D.;DEUDON, M.;EMERY, P. 1954. Compléments à l’étude du bassin ferrifère de Lorraine. Le Bassin de Landres-Amermont. Paris, Centre de documentation sidérurgique. FEDORCHENKO, V. S. 1965. Mineralnyy sostav iskhodnykh porod i genezis kraslcovykh i krasko-maritovykh rud krivorozhskogo zhelezorudnogo basseina [Mineral composition of initial rocks andgenesis of krassyk and krassyk-martite ores of the Krivoyrog iron-ore basin]. Vol. 1-2 of dissertation,Krivoyrog. GLAGOLEV, A. A. 1966. Metamorfizm dokombriyskikh porod

KMA. [Metamorphism of Precambrian rocks of the Kursk Magnetic Anomal]. Moscow, Nauka. GREEN, J. 1953. Geochemical table of the elements for 1953. Bull. Geol. Soc. Amer., vol. 64, no.9. HUBER, N.K. 1959.Some aspects of the origin of the Ironwood iron-formation of Michigan and Wisconsin. Econ. Geol., vol. 54, p. 82-118. IRVING,R.D.; VANHISE, C.R. 1892.The Penokee iron-bearing seriesof MichiganandWisconsin. US.Geol.Surv.Monogr. 19.

JAMES, H. L. 1951. Iron-formation and associated rocks in the Iron River district, Michigan. Bull. Geol. Soc. Amer., vol. 62. KALYAEV, G . I. 1965. Tektonika dokembriya Ukrainskoy zhelezorudnoy provintsii [Tectonics of the Precambrian of the Ukrainian iron-ore province]. Kiev, Naukova Dumka. KANTOR, M . I. 1937. Geneziz kerchenskikh zheleznykh rud [Genesis of the Kerch iron ores]. Trudy konjerentsii PO genezisu rud zheleza, margantsa i aluminia.

PLAKSENKO, N.A. 1966. Glavneyshiye zakonomernosti zhelezorzidnogo osadkonakopleniya v dokembrii [Main regularities of iron-oresedimentation in the Precambrian]. Voronezhskogo University. PONOMAREV, M . S. 1964. K voprosu o sootnoshenii ‘zhelezorudnoy formacii’i fillitoarkozovoy tolshchi proterozoya Krivorozhskogo basseina. [On the correlation between iron-ore formationand phyllite-arkoseProterozoic series of Krivoyrog basin.]Soviet. Geol., MOSCOW, no. 7. STRAKHOV, N.M . 1947. Zhelezorudnye facii i ikh analogi v istorii Zemli. [Iron-ore facies and their analogues in the history of the earth]. Trudy inst. geol.nauk A N S S S R , no. 73. (Geological series,no. 22.) TARKHANOV, A. V. 1969. K voprosu o proiskhozhdenii zhelezistykhkvartsitov[Origin of ferruginousquartzites]. Probbmy obrazovaniya zhelezistykh porod dokembriya [Problems of formation of Precambrian ferruginousrocks]. Kiev,Naukova Dumka. VAN HISE, C. R.;BAYLEY, W . S. 1897. The Marquette ironbearing district of Michigan. US. Geol. Surv. Monogr. 28.

39

Jacobsites from the Urandi manganese district, Bahia (Brazil) E. Ribeiro Filho Instituto de Geociencias, Universidade de São Paulo, São Paulo (Brazil)

Introduction

Brazilian occurrences of jacobsite

The area where jacobsite associated with manganese minerals occurs is situated in the south-westpart of the state of Bahia, Brazil. It is found localized within the crystalline complex that forms part of the major geotectonic unit known as the Espinhaço Geosyncline. The deposits studied are restricted to the southern region of the belt of metamorphic rocks near Urandi (14'49' S, 42O38' W). The manganese deposits in this region of Brazil, as in most other occurrences, are associated with Precambrian rocks. These form a lower sequence of gneisses, granitic gneisses, schists and amphibolites overlain by an upper sequence of phyllites,green schists,metaconglomeratesand quartzites. The manganese deposits are always intercalated between the schists and phyllites. The age of the rocks in this region is between 463 and 791 m.y.,based on K/Ardeterminations. In the manganiferous district of Urandi, there are at least three different types of manganese ore deposits. The first type, which occurs in the Barreiro dos Campos mine, refers to a carbonate protore. The most common mineral of this protore, which could be structurally classified as mangano-dolomite,has a DTA curve very similar to that of kutnahorite. The two other types were both formed by regional metamorphism of syngenetic sediments with primary manganese oxides. The difference between these two types depends on the grade of metamorphism. In the Barnabé mine the manganese assemblage of minerals as well as the associated rocks shows evidence of high-grade metamorphism. In some deposits, such as Pedra Preta, where the associated rocks are green schists and sericitoschistsandjacobsiteand hausmanniteare absent in the ores, the grade of metamorphism is lower. A common feature of all manganese deposits of Urandi district is the occurrence of ore formed by weathering and supergene enrichment of protore. Thus, the paragenesis of the manganese ore minerals of Urandi district may be exemplified as shown in Table 1.

In Brazil,the first report of the discovery of jacobsite was from the Barnabé mine, which is situated near the town of LicÍnio de Almeida, Bahia (Ribeiro Filho, 1966). With the progress of studies on the manganiferous deposits of this region, it has been possible to identify several jacobsites, whose mineralogical, textural, chemical and etching test behaviour show some variations. However, the mode of association of jacobsite with other manganese minerals is not always identical. Jacobsite is always a mineral of metamorphic origin, formed in a stratified manganese deposit.It shows a granoblastic texture in polished section,with crystals of variable shape phose proportions range from 0.05to 3.0 m m . The most common diameters range between 0.2and 1.0m m . It is strongly magnetic and under reflected light,exhibits a rose-grey or olive-grey colour; colour is the most useful means of distinguishing jacobsite from magnetite. It is isotropic and inert with all reagents commonly used in etching tests. Nevertheless, some samples,including those found in the Urandi district,give a positive reaction with a 40 per cent aqueous solution of H F .Previously Roy (1959) observed this phenomenon in jacobsite from Andhra Pradesh. H e advanced the hypothesis that this reaction occurred only when jacobsite had a chemical composition approaching the transition to magnetite. However, jacobsites from the Urandi district, despite having a composition very close to magnetite according to Roy's data, give a negative reaction with HCl. Samples containing jacobsite, collected from various deposits have been examined in polished section by X-rays, and by chemical analysis. The results are given in Tables 2 and 3. At the Pedra Preta mine, magnetic ore is rare and is situated only at one oftheexploratorywork faces.Itcontains crystals up to 3.0 mm in diameter, which show the rosegrey colour in polished section and are isotropic. They give a positive reaction in the etching test using HF,but a negative reaction with all other commonly used reagents,

Unesco, 1973. Genesis of Precambrian iron and manganese deposits. Proc. Kiev Syinp., 1970. (Earth sciences, 9.)

41

E.Ribeiro Filho

TABLE1. Paragenesis of the manganese-oreminerals of Urandi district ~

Period of mineralization Minerals

Mine

Metasomatic ore

%dim.-Metam. ore

Barreiro dos Campos

Supergene ore

Mangano-dolomite Spessartite Rhodonite Quartz

Cryptomelane Hollandite Pyrolusite Todorokite Ramsdellite

7. -7-7-3-.

Pedra Preta

Alpha MnO, Mangano-magnetite Cryptomelane Pyrolusite

Barnabé

Bixbyite Jacobsite Hausmannite Manganite Alpha MnO, Cryptomelane Todorokite Pyrolusite

-7-7-.1

. .

. .

?-?

TABLE 2.X-raypowder data for jacobsite.Radiation FeKor,M n filter, Barnabé mine 1

2

4

3

i

i

hkl 111

220 311

222 400 422 333

440 531 620 533 622 444 551 642 553

800 660 555

d&)

d&)

4.85 2.96 2.53 2.42 2.09 1.71 1.61 1.48 1.41 1.32 1.27 1.26 1.21

4.84 2.96 2.52 2.41 2.10 1.71 1.61 1.48

4.88 2.98 2.55 2.43 2.10 1.72 1.62 1.49

-

1.42 1.33

1.12 1 .O9 1.O4 0.98 0.97 a,,=8.38

1.12

-

1.28 -

1.21

-

1 .o9 1 .O5

0.98 0.97 a, = 8.39

1.28 1.27 1.22 1.18 i .13 1.10 1 .O5

0.97 a,=8.43

1. Jacobsite from amphibolite. 2. Jacobsite with exsolved hematite. 3. Jacobsite crystals cut by quartz veins. 4. Granoblastic jacobsite crystals. 5. Granoblastic jacobsite crystals. 6. Granoblastic jacobsite crystals. 7. Granoblastic jacobsite crystals with exsolved hausmannite.

i

d(Å) 4.88 3.O0

2.55 2.43 2.11 1.72 1.62 1.49 1.42

-

1.28

-

1.22

5

6

-+

i

dí&

4.88 2.99 2.55 2.44 2.11 1.72 1 .63 1.49 1.43 1.33

1.29 1.27 1.22

-

1.18 1.13

1.10 1.O5 0.98 0.97 a,=8.46

1.10 1 .O5 0.99 0.97 a,=8.47

7

d(& 4.91

d&)

1/10

4.89

3 .O0

3.o0

2.56 2.45 2.12

2.56 2.45 2.12 1.73 1.63 1.50 1.43

40 40 100

1.73

1.63 1.50 1.43

-

1.29 1.27 1.22

-

1.13 1.10 1.O6 1.o0 0.98

a,=8.49

3

60 10 60 60 5 3

1.34

1.29 1.28 1.22 1.19 1.13 1.10 1 .O6 1 .o0

0.98 ao=8.49

20 5 10 3 ~~

5 40

20 10

40

Jacobsites from the Urandi manganese district,Bahia (Brazil)

TABLE 3. X-raypowder data of magnetite-hausmanniteseries.Radiation FeKcc,M n filter, Urandi district

i

111 220 311 222 400 422 333 440 531

620

4.83 2.95 2.51 2.40 2.O9 1.70 1.61 1.47

-

533

1.30 1.27

622 444

1.21

-

4.87 3 .O8 2.54 2.43 2.11 1.72 1.62 1.49 1.42 1.28 1.27 1.22

1.34 1.29 1.27 1.22 1.13 1.10 1 .O5 0.99 0.97

4.87 3 .O0 2.54 2.44 2.11 1.72 1.62 1.49 1.43 1.34 1.29 1.27 1.22 1.18 1.13 1.10 1 .O5 0.99 0.97

a, =8.46

a,,=8.46

-

-

551

-

642

-

553

1 .O9 1 .O4

-

1.12 1.10 1 .O5 0.99 0.97

a,,=8.37

a,=8.44

800 660 555

4.87 3 .O9 2.54 2.43 2.11 1.72 1.62 1.49

-

-

4.89 3 .O9 2.55 2.44 2.12 1.73 1.63 1.50 1.43

1.13 1.10 1 .O6 1 .o0 0.98

d(A) 4.94 3 .O0 2.55 2.45 2.12 1.73 1.63 1S O 1.43 1.34 1.29 1.28 1.22 1.18 1.13 1.10 1 .O6 1 .o0 0.98

a,=8.48

a,,=8.49

-

1.29 1.28 1.22

-

1/10

40 40 1O0 3

60 10 60 60 5 3

20 5 10 3 5

40 20 10 40

1. Mangano-magnetite from Pedra Preta manganese mine. 2. Jacobsite from Covão manganese deposit. 3. Vredenburgite from Feixe de Vara manganese deposit. 4. Vredenburgite from Piedade manganese deposit. 5. Jacobsite from P a u de Rego manganese deposit. 6. Vredenburgite from Piedade manganese deposit.

H,O,+H,SO, and concentrated H,SO,.W h e n the crystals are attacked by a 40per cent aqueous solutionof HF,they darken instantly showing clearly the exsolution texture of hematitein mangano-magnetite(widmanstetten texture). Thus the percentage of Mn,O, and the results of X-ray studies indicate that this mineral has to be at the beginning of the isomorphous series magnetite-hausmannite,as it has an excess of iron relative to manganese. At the Barnabé mine typicaljacobsites,jacobsites with exsolved hematite, and jacobsites with intergrown hausmannite occur. These have to be classified as vredenburgite. Jacobsite with intergrowth of hematite occurs in ore with a granobiastic texture having crystals of diameter between 0.05and 0.5mm.When the material is submitted to an etching test,the jacobsitehas a negative reaction with all common reagents and a positive reaction with a 40 per cent aqueous solution of HF. The HF accentuates the exsolution texture of the hematite in jacobsite (Fig. 1). Thus the results of X-ray analysis,as much as chemical analysis, and behaviour in the etching tests prove that we are dealing with jacobsite. The unit parameter, a,,= 8.397+ 0.003 A, shows that this mineral is almost the end member of the magnetite-hausmanniteseries.The chemical analysis, giving Mn,0,=17.36 molecular per cent,is also evidence that w e are dealing with jacobsite. In agreement with the phase diagram of Muan and

Somiya (1962), it is not possible to consider the exsolution mineral formed in an excess of manganese to be hausmannite; it may be bixbyite or hematite.Hematite is indicated by the colour of the exsolution lamellae in reflected light,as well as by the negativereaction to all reagentsused in the etching test. In our sample which has exsolution hematite,the percentage of Mn,O, rises to 44.70molecular per cent and the parameter ao= 8.498f 0.002A. There is a samplewith Mn,0,=34.64 molecular per cent, in which the exsolution hematite develops as rare and sparsely distributed grains. In other samples of magnetic ore from the Barnabéminetheproportion ofMn,O,is higher than54molecular perocent, and the unit-cell parameter is greater than 8.46 A , thus defining the mineral as 'vredenburgite (Table 4). Magnetic ores that containjacobsiteand vredenburgite also occur at other mines located close to theBarnabémine, such as Feixe de Vara, Covão, Piedade and Pau de Rego Mines. They are ores having a granoblastic texture with crystals of jacobsite or vredenburgite partially substituted by cryptomelane (Fig. 2), todorokite and pyrolusite. The minerals here defined as vredenburgite are differentiated on the basis of their chemical composition,because they do not contain an exsolution texture formed by lamellae of hausmannite in jacobsite. Crystals of vredenburgite are granular and generally homogeneous. W h e n 43

E. Ribeiro Filho

FIG.1. Hematite in jacobsite.

FIG.2. Cryptomelane veins cutting crystals of jacobsite. The cryptomelane was etched with SnCI, (x50).

44

Jacobcites from the Urandi manganese district,Bahia (Brazil)

FIG.3. Corroded crystal of jacobcite associated with M n O , minerals (x63).

FIG.4.‘Cranzon ore’ with jacobcite being encrusted by secondary MnO, minerals. 45

E.Ribeiro Filho

TABLE

4.Comparison of data on unit-cell diinensions of jacobsite and vredenburgite ~

ao&

References

Locality

8.44 8.505050.0005 8.452rL: 0.004

Sintético Weabonga,Australia Kiuragi mine,Japan

Mason (1943,1947) McAndrew (1952) Hirowatari and Myashisa

8.49 8.42-8.52 8.410-8.506 8.51+0.01 8.38-8.43 8.373 8.385k0.002 8.397+0.003 8.436-8.488 8.465-8.493

Jakobsberg,Sweden Tirodi and Kodur,India Andhra Pradesh,India Negev, Israel Buryat,U.S.S.R. Pedra Preta mine Amphibolite,Barnabé mine Barnabé mine Several deposits,Urandi Several deposits,Urandi

Ramdohr (1956) Mukherjee (1959) Roy (1959) Katz (1960) Rumyantsev (1965) Ribeiro Filho Ribeiro Filho Ribeiro Filho Ribeiro Filho Ribeiro Filho

(1955)

exsolution occurs,it shows as small grains of hausmaimite diffusely exsolved in jacobsite. The jacobsite crystals have been subject to a rate of cooling that was not sufficiently slow to permit the formation of exsolution lamellae. In some samples, the vredenburgite crystals appear to be corroded predominantly along preferential directions that correspond to the now totally altered exsolution lamellae (Figs. 3 and 4).

9'5

T

Conclusions Some conclusions may be inferred in the light of results obtained during the present investigation concerningjacobsite and the associated manganese minerals, The values of the unit parameter together with a comparison of the chemical analysis data of natural jacobsite agree with the graph given by Mason (1943) (Fig. 5), who studied the system Fe,O, -Mn,O,. H e concluded that the dimensions of the unit cell increased with increasing quantities of manganese. Hematite and hausmannite are exsolved in jacobsite based on the ratios of Mn,O, and Fe,O, found in the ores studied here. The ratios agree with the limits postulated from the phase diagrams of Mason (1943), Van Hook and Keith (1958), and Muan and Somiya (1962). The behaviour of jacobsite when submitted to the etching test shows that there may be a relationship between a positive reaction to HF and the manganese content, as already suggested by Roy (1959). On the other hand, the results with concentrated HC1 disagree, since Roy (1959) observed a positive reaction,whereas the jacobsite studied here gave a negative reaction. The variation in the unit-cell parameter of jacobsite based on the ratio Mn/Fe is more or less in agreement with the results obtained by Mason (1943). The discrepancies are probably caused by the fact that in natural jacobsite other chemicalelements may enter into the crystallinestructure replacing iron and/or manganese (Fig. 5). The mineralogicalassociationfound in the ore minerals 46

FIG.5. Variation of lattice dimensions in the system Fe,O,Mn,O, (o denotes findings of the presentinvestigation;rn signifies those of Mason (1943)). from the Urandi district is similar to that of other localities where metamorphic ores occur. An exception is a single locality where native copper, cuprite and malachite occur together with manganese minerals.

Jacobsites from the Urandi manganese district,Bahia (Brazil)

The observed textures associated with the identification of jacobsite gave rise to information useful in the interpretation of the paragenesis of the manganeseminerals studied. Where the magnetic mineral, jacobsite, occurs as a major component in lenticular ore bodies, it provides a sufficientlystrong anomaly to register during the use of the magnetometric prospecting method.

Acknowledgements The present study was made possible through the generous co-operationof Urandi S.A. Mining Company. Financial support for field and laboratoryresearch on the ore deposits came from F.A.P.E.S.P.(Fundação de Amparo à Pesquisa do Estado de São Paulo). All this assistance is gratefully acknowledged.

Résumé Jucobsites c h district de manganèse d’Urundi,Bahia, Brésil

(E. Ribeiro Filho) Le district de manganèse d‘Urandi est situé dans la partie sud-ouestde l’État de Bahia au Brésil. I1 est localisé à l’intérieur du complexe cristallin qui constitue une partie de l’ensemblegéotectonique importantconnu sous le n o m géosynclinal d‘ (( Espinhaço ». Les gisements étudiés sont limités à la partie méridionale de la ceinture de roches métamorphiques près d’Urandi. Les gisements de manganèse dans cette région du Brésil,comme dans beaucoup d’autres circonstances,sont associés à desrochesprécambriennes.Ces dernièresforment une séquence inférieure de gneiss, de gneiss granitiques,de schistes et d’amphibolites recouverte par une séquence de phyllites, de schistes verts, de métoconglomérats et de quartzites. Les dépôts de manganèse sont toujours intercalés entre les schistes et les phyllites. L’âge des roches précambriennes dans cette région se situe entre 463 et 791 millions d’années d‘après les délerminations potassium-argon. Dans le district manganifère d’Urandiil y a au moins trois types différents de gisements de minerai de manganèse d’après leur origine.Le premier type, qu’onrencontre dans la mine de Barreiro dos Campos, se rattache à un protore carbonaté.Le minerai le plus commun de ce protore, qui

pourrait être classé d‘après sa structure comme une mangano-dolomite,a une courbe de dosage à I’EDTAtrès semblable à celle d‘une kutnahorite. Les deux autres types ont été formés tous les deux par un métamorphisme régional de sédiments syngénétiques avec des oxydes primaires de manganèse.L a différenceentre ces deux types dépend du degré de métamorphisme. Dans la mine de Barnabé, les assemblages des minéraux manganiques aussi bien que les roches associées mettent en évidence un métamorphisme très avancé. Dans quelques gisements tels que ceux de Pedra Preta, où les roches associées sont des schistes verts et des schistes séricites et OU la jacobsite, I’hausmannite sont absents dans le minerai, le degré de métamorphisme est moins avancé. U n caractère commun de tous les gisements de manganèse du district d’Urandi est l’existence de minerais formés par l’altérationpar les agents atmosphériques et l’enrichissement supergène du protore. Les jacobsites provenant de gisements manganifères métamorphosés ont été identifiés par la diffraction des rayons X et l’examen de surfaces polies. Les pics de la jacobsite varient de 8,38 à 8,49 angströms. Les résultats obtenus montrent que les méthodes magnétométriques d’exploration peuvent être utilisées comme des outils précieux dans la région étudiée.

Bibliography/Bibliographie DEER, W. A.; HOWIE, M.A.;ZUSSMAN, M.A. 1962. Rock Forming Minerals, vol. 5, p. 77. New York, Wiley. 317 p. HIROWATARI, F.; MIASHISA, M.1955. Jacobsite from manganese deposit of Kiuragi mine. Min. Geol. (Soc.Min. Geol. Japan), vol. 5, no. 16, p. 95-107. KATZ, G.1960. Jacobsite from the Weger, Israel.Amer. Min., vol. 45, no. 6, p. 734-39. MCANDREW, J. 1952. The cell edge of jacobsite. Amer. Min., vol. 37,no. 5-6, p. 453-60. MASON, B. 1943. Mineralogical aspects of the system FeOFe,O,-MnO-Mn,O,. Geol. Fören. Stockh. Förh., vol. 65, UO.

2, p. 97-180.

-.

1947. Mineralogical Aspects of the system Fe,O,Mn,O,ZnMn,O,. Amer. Min., vol. 32,no. 7-8, p. 426-41. MONTORO, V. 1938. Miscibilita frai sesquiossidi di ferro e di manganese. Gazz. chim. ital., vol,68, p. 728-33. MUAN, A.;SOMIYA,S. 1962.The system iron oxide-manganese oxide in air. Amer. J. Sci., vol. 260, no. 3, p. 230-40. MUKHERJEE, B. 1959. An X-raystudy of manganese minerals. Miner. Mag., vol. 32, no. 247, p. 332-39. RAMDOHR, P. 1956. Die Manganerze. XX Int. geol. Congr., Mexico, vol. 1 (Symposium del manganeso), p. 19-73. RIBEIRO FILHO,E. 1966. Jacobsita de Licinio de Almeida, Bahia. Soc. Bras. Geol., vol. 15, no. 2, p. 43-8.

47

E.Ribeiro Filho

RIBEIRO FILHO, E.;ELLERT, N. 1969. Magnetometria relacionadaàs jazidasde manganês do sudoesteda Bahia.Mineraç. e Metall., vol. 49,no.289,p. 11-3. ROY,S. 1959. Variation in the etch behavior of jacobsite with different cell dimensions.Nature, vol. 183, p. 1256-7. -. 1965. Comparative study of the metamorphosed manganese protore of the world. Econ. Geol., vol. 60, no. 6, p. 1238-60.

RUMYANTESEV, G.S. 1965.Sostav i svoystva vnov’obnarugenih mineralov ryada magnetit-jacobsit v mestorogenii Magnetitovoye (Buryatskaya ASSR) [Composition and properties

of minerals of the magnetite-jacobsiteseries redetected in the Magnetitovoye deposit (Buryat A.S.S.R.)]. C.R.Acad. Sci. U.R.S.S., vol. 164,no. 5, p. 1143-6. STILLWELL,F.L.;EDWARDS, A. B. 1951. Jacobsite from the Tamworth district of N.S.W.Miner. Mag., vol. 29, no. 212, p. 538-41. VANHOOK, H.J.; KEITH, M . L. 1958.ThesystemFeso,.Amer. Min., vol.43, no. 1-2, p. 69-83. YUN,I. 1958. Experimental studies on magnetic and crystallographic characters of Fe-bearingmanganese oxides.Mem. Coll.Sci. Kyoto, series B,vol. 25, p. 125-37.

Discussion S. ROY.W h y have you distinguished between clMnO, and cryptomelane in the paragenesis table? I thought they were the same.

W . SCARPELLI.Could you give us an idea of the average M n and Fe content of the Urandi ores?

E.RIBEIRA FILHO. E.RIBEIRO FILHO. In some ores I know that both cryptomelane and hollandite occur together,but in some it was impossible to distinguish them. Because of this I preferred to give a general classification as crMnO,.

S.ROY.Does cryptomelanecoexist with jacobsite and hausmannite as a metamorphic mineral? It is rather unexpected -physico-chemically .

E.RIBEIRO FILHO. Cryptomelane,jacobsite and hausman\

nite coexist in the same ore. Probably the cryptomelane (aMnO,) is a secondary mineral which was formed later by weathering. In the slide of the polished section I showed some crystals of jacobsite which were cut by veins of cryptomelane.

S. Roy. ?ave you chemically analysed the jacobsite with a, = 8.46A which you call vredenbuyite? McAndrew (1952)described jacobsite with a, = 8.52A which has been chemically analysed and shown to belong to jacobsite field.

E.&BEIRO FILHO. Yes,I have the results of chemical analyses and I called it vredenburgite,based on the definition which was used by Mason.

48

%

Fe ..D

40-53

1-2.4

45-52

1 .O-57

0.25-4.60

39-45

10.5-16.5

0.40-13.50

Mn

SiO,

%

Ba0

%

P%

Pedra Preta mine

0.60-3.0

0.70-14.0

0.05-0.21

Barreiro dos Campos mine Barnabé mine

0.05-0.13

0.008-0.12

-

0.012-0.31

I. P.NOVOKHATSKY. What is your opinion on the presence of copper-rich minerals?

E.RIBEIRO FILHO. In one of the slides I tried to show the relationshipbetween copperminerals andquartzvein.Ithink that the native copper was formed later than the M n oxide. What is theroleofsilicaintheprotores? I.P.NOVOKHATSKY.

E.RIBEIRO FILHO. Most of the M n deposits of the Urandi area are very low in SiO,. Only in the Barreiro dos Campos mine was rhodonite found. There are no silicatic protores in the area.

Time-distribution and type-distribution of Precambrian iron-formations in Australia A. F. Trendall Geological Survey of Western Australia,Perth

Introduction Brief reviews of the Precambrian banded iron-formations of Western Australia, with particular reference to their associatedmineral deposits,havebeen givenby Miles (1953) and MacLeod (1965).N o complete account of the Precambrian iron-formations of the Australian continent exists, and this contribution is intended to fill this gap. Little new information is presented in this paper, which is compiled largely from published sources. The Australian Precambrian iron-formations are here classifiedinto six divisions.The criteria forthis classification are geographic separation,age,lithological type and stratigraphic geometry.With minor exceptions,discussed below, all these four criteria are equally applicable to all six ironformation divisions.After a summary of the six divisions the following characters of each are discussed sequentially: age and regional geological environment;lithology;stratigraphic geometry.

Australian political divisions The mainland continent of Australia forms a single political unit administered by a federal or commonwealth government. There are six mainland states, namely N e w South Wales, Northern Territory, Queensland,South Australia, Victoria, and Western Australia (Fig. 1). These,with the island State of Tasmania to the south,constitute the C o m monwealth of Australia. This explanation is given to avoid the confusion sometimes caused by the use of indefinitegeographic terms,such as ‘southernAustralia’,in parallel with precisely defined political terms, such as ‘SouthAustralia’.

The six categories of iron-formation The six divisions used in the following descriptive summaries are listed below and their regionallocationsmarked with equivalent numbers in Figure 1.

1. Iron-formationsof the Yilgarn Block and Pilbara Block of Western Australia. 2. Iron-formations of the Hamersley Group of Western Australia. 3. Iron-formations of the Cleve Metamorphics of South Australia,best known from the Middleback Range. 4. Hematite-rich sediments of the Yampi Sound area of Western Australia. 5. The Roper Bar and Constance Range iron-formations of the Northern Territory and Queensland. 6.The HolowilenaIron-Formationand Braemar Iron-Formation of South Australia. These six divisions exclude some minor occurrences which are strongly metamorphosed and of uncertain age and relationships.

Age and regional geological environment IRON-FORMATIONSO F T H E YILGARN B L O C K A N D PILBARA B L O C K

These two ancient stable blocks are the only extensive areas of rocks older than about 2,400 m.y. in the Australian continent. The northern Pilbara Block (Fig. i), with an area of about 47,000km2,is bounded by the Indian Ocean to the north and by the basal unconformity of the overlying Mount Bruce Supergroup (see below) to the south.Its eastern margin is poorly known. The much larger Yilgarn Block, to the south, has an area of about 603,000 k m z . It is bounded to the west by a major fault, the Darling Fault, which defined by its movement throughout Phanerozoic time the sharp eastern margin of the neighbouring Perth Basin. T o the north-west,as well as along the southern and south-easternmargins,the Yilgarn Block has boundaries against younger metamorphicrocks.The nature of these boundaries is not well known, neither is it clear whether the adjacent younger rocks are re-metamorphosed parts of the block or subsequently generated crust. The north-western boundary of the block is the

Unesco, 1973. Genesis of Precambrian iron und manganese deposits. Proc. Kiev Symp.,1970. (Earth sciences, 9.)

49

2

A. F.Trendall

50

Q

R

t n

n

<1

... :::: ... .......

BOU@ B

Time-distributionand type-distributionof Precambrian iron-formationsin Australia

basal unconformity of the overlying younger Precambrian Bangemall Group, while the eastern boundary is the basal unconformity of the Mesozoic rocks of the Officer Basin. Within both these blocks the geology is closely comparable with that of the Archaean 'nuclei'of other continents. Sinuous belts of tightly folded metasediments and metavolcanic rocks encircle extensive areas of granitic rocks, often with some foliation defining broad domes. At their margins the granites are usually more strongly foliated and, although on a regional scale they appear to be structurally basal to the adjacent sedimentary sequences,in detail their contacts are discordant.Close to the granites the metasediments have undergone thermal metamorphism, although, with the exception of the south-westernpart of the Yilgarn Block,the generalmetamorphic grade of the metasediments and metavolcanics is low. The stratified sedimentary and volcanic rocks, whose curvilinear outcrops are traditionally known as 'greenstone belts', form almost exactly a quarter of the total area of the two blocks. Over much of the eastern part of the Yilgarn Block the belts have a pronounced north-north-westerly trend, but elsewhere their direction is less clearly aligned. Banded iron-formations occur in the sedimentary successionsofmost of the belts,but details oflocalstratigraphy are only now being systematically recorded,so that the proportion of iron-formationpresent is not known, nor its status in the Stratigraphic sequence. However,in the Kurnalpi area (3Oa-3l0S;121" 30'-123"00' E) Williams (1969) has related iron-formationoccurrence to a quiet interval between successive depositional cycles, beginning with a thick basic and intermediatevolcanic (extrusive and intrusive) successionwith some intercalated sediment and ending with intermediate and acid lavas, breccias, agglomerates and tuffs,with interbedded greywacke,shale,siltstone and sandstone.Each depositional cycle has a likely total thickness of about 12,000 m . It is unlikely, in this area, that banded iron-formationmakes up as much as 1 per cent of the total volume of the associated depositional pile. Compston and Arriens (1968) have summarized the available geochronological data for the age of the depositional sequences, and thus of their included iron-formations,in the Pilbara and Yilgarn Blocks. Granite ages vary from about 3,000 m.y. for the Pilbara Block to a range of 2,900 to 2,600m.y. for the Yilgarn Block. Some acid volcanics in the Pilbara Block succession must be at least as old as 3,000m.y. and,although the Yilgarn Block evidence is unsatisfactory, some of the eastern volcanics are at least as old as 2,670&30 m.y. Thus, although all the banded iron-formationsof this division appear to be older than this latter figure,it is still uncertain what the total age range may be between various occurrences.

IRON-FORMATIONS OF T H E HAMERSLEY G R O U P

The Hamersley Group is one of three constituentgroups of the Mount Bruce Supergroup;conformably below it lies the Fortescue Group, and above it, with some local discon-

tinuity,the Wyloo Group. All three groups were laid down sequentially in an ovoid depositionalbasin (the Hamersley Basin) about 500 km long and 250 km wide, with a westnorth-westerlyelongation. The Hamersley Basin developed by the steady sinking of a presumed southern extension of what is now the Pilbara Block and may have originally extended over the whole of the presently exposed area of this. However, the present outcrop of the remaining part of the Mount Bruce Supergroup is bounded (Fig. 1) to the south and east by sediments of the unconformably overlying younger Precambrian Bangemall Group, to the north by its eroded outcrop termination exposing the Pilbara Block, and to the west by the overlyingMesozoic sedimentsofthePhanerozoicCarnarvon Basin. In the northern part of the Mount Bruce Supergroup outcrop both the Fortescue Group and Hamersley Group have a southerly dip of only a few degrees,and are unmetamorphosed.In the central outcrop area folding is open and dips are gentle,but along the southern edge there is strong folding of all three groups. The banded iron-formationsof the Hamersley Basin are virtually confined to the Hamersley Group. They form some 1,145 m,or over 40 per cent, of its total thickness of 2,500 m,in five main stratigraphic units which are set out in another paper in this volume.Even taking the most generous estimatefor the original totalvolume of the Mount Bruce Supergroup,the Hamersley Group iron-formations could not have constituted less than 7 per cent of the total material of the basin. The beginning of deposition in the Hamersley Basin was at about 2,200 to 2,250 may.,an age obtained by Compston and Arriens (1968) from Fortescue Group material.The Woongarra Volcanics,acid lavas of the Hamersley Group, gave an age of 2,000k 100 m.y., and Wyloo Group acid igneous rocks one of 2,020I165 m.y. IRON-FORMATIONS O F T H E CLEVE M E T A M O R P H I C S

The Gawler Block of South Australia is a massif of metamorphic rocks which formed the stable south-easternmargin of the later Precambrian Adelaide Geosyncline (Fig. l, see also Parkin, 1969). The western parts of the block consist largely of granite,gneiss and migmatite.In the east iron-formationsand other metasediments,includingquartzites,mica schists and amphibolites are present also. Miles (1955) placed the iron-formationsand most of the metasediments in the Middleback Group, and most of the graniticand gneissicrocksinan underlyingGneiss Complex. It now appears established (Parkin, 1969) ñrstly that granite and gneiss also occur stratigraphically above the Middleback Group, and secondly that the iron-formations of the Middleback Group occur at the base of more than 9,000m of metasediments to which, with the underlying gneisses, the name Cleve Metamorphics is now applied. The stratigraphicsection given by Parkin (1969) shows two main bands of iron-formation,the lower about 300 m and the upper about 150 m thick,separated by a thin layer of 51

A.F. Trendall

schist,Some 9,000 m above these,another iron-formation about 50 m thick occurs, so that iron-formation forms about 5 per cent of the sedimentary material above and including the Middleback Group. Parkin’s (1969) map shows the whole sequence tightly folded about axial planes dipping very steeply east,and striking roughly north-south, but swingingnorth-westerlyand south-westerlyin thenorthern and southern parts of their outcrop respectively. Whitten (1966)has correlatedthese Middleback Group ironformations with minor occurrences of iron-formationsin the northern part of the Gawler Block (including the Wilgena H i l l Jaspilite;see below), and in the pre-Adelaide Geosyncline metamorphic rocks on the other (eastern) side of the geosyncline. Gneissic granulites from the Gneiss Complex of the southern Gawler Block have an age of 1,780-t120 m.y., whereas granites farther north which transect the metasediments of the Cleve Metamorphics give ages of 1,550 1:70 and 1,590 30 m.y. (Compston and Arriens, 1968). Apparently the older (granulite) age sets a minimum depositionalage for the the Middleback Group iron-formations.

HEMATITE-RICH SEDIMENTS O F T H E YAMPI S O U N D AREA

The Kimberley Basin is a,relatively undisturbed sedimentary basin in the northernmost part of Western Australia, bounded on its south-westernand south-eastern sides by metamorphicbelts,which it unconformably overlies (Fig. 1; see also Gellatly, Derrick and Plumb, 1968). Three main sedimentary groups, successively the Speewah, Kimberley and Bastion Groups, were laid down in the basin, with a combined thickness of about 3,000-5,000 m . Sandstone is by far the most abundant constituent,with siltstone,conglomerate,volcanic rocks and carbonates also present. The uppermost unit of the Kimberley Group, the Pentecost Sandstone,is about 1,000m thick and has very local developments of hematitic conglomerate,sandstone, quartzite and schist (siltstone). In the Yampi Sound area (Reid, 1965) the hematite content reaches ore proportions, and there are also noteworthy enrichments at scattered localities along the eastern basin edge. At Yampi Sound sediments with over 60 per cent of hematite are interbedded cyclically with normal clastic sediments at a range of scales. The total thickness of iron-richsediment may be as much as 200 m,representing theoretically about 4-7 per cent of the total sedimentary thickness, but the lateral restriction means that much less than 1 per cent of the total basin volume is hematitic. Dating ofthe sediments and volcanics of the Kimberley Basin by Bofinger is summarizedby Compston and Arriens (1968). Deposition began at about 1,820m.y. and appears to have been relatively fast,since a high shale gave a date of 1,790+ 60 m.y. The hematite sediments thus have a depositional age of about 1,800m.y.

52

R O P E R B A R A N D C O N S T A N C E R A N G E IRONSTONES

The name ‘CarpentariaProvince’was applied by McDougall et al.(1965)to the area of depositionof a thick sequence of unmetamorphosed Precambrian sedimentary rocks laid down in three contiguous basins in the northern part of the Northern Territory and Queensland.The large central basin is called the McArthur Basin, its smaller south-eastern neighbour the South Nicholson Basin (Fig.l), and an overlapping northern part the Wessel Basin. In the McArthur Basin four sedimentary suites,with a combined thickness of about 12,000m,were deposited in successionthroughout the basin.The uppermost of these,the Roper Group,varies between 1,800and 4,500min thickness,and consistsmainly of siltstone and shale with prominent sandstone units. Near the top of the Roper Group there are thin beds of oolitic ironstone,known formally as the Sherwin Ironstone Member of the McMinn Formation,of the Maiwok Sub-Group (5A of Fig. 1). The outcrop of the member is shown by Randal (1963), Dum (1963u, 19636) and by Plumb and Paine (1964), who give a maximum thickness of about 20 m-less than 0.5 per cent of the total basin sedimentation thickness. In the South Nicholson Basin the Roper Group is believed by McDougall et al. (1965) to be represented by the South Nicholson Group, with a thickness of about 5,800 m; it consists mainly of siltstone and sandstone. Within the Mullera Formationofthis group (Carter,Brooks and Walker,1961)there are up to 10ferruginousbeds,each up to 12 m thick, with a total stratigraphic thickness of about 90 m (5B of Fig. 1). Carter and Öpik (1961)show the outcrop of this ironstone. Harms (1965) places it in a separate Train Range Ironstone Formation. McDougall et al.(1965)argue a minimum depositional age of 1,390420 m.y. for one formation of the Roper Group. The age of the granitic basament of the basin, at 1,800?50 m.y.,and intermediate ages upwards which are consistent with the stratigraphic succession, suggest that this age is approximately correct,and that a depositional age for these ironstones of 1,400 m.y.is likely.

H O L O W I L E N A A N D B R A E M A R IRON-FORMATIONS

The Adelaide Geosynclide (Fig. 1) is a great belt of relatively unaltered Precambrian sediments extending northwesterly across South Australia. A n excellent summary of these rocks has recently been given by Thomson (in Parkin, 1969). The varied sedimentary successionof the geosyncline attains a maximum thickness of about 25,000m,and has been divided into four major units :the CaUanna Beds,the Burra Group, the Umberatana Group and the Wilpena Group. The sediments are mainly lagoonal and shallow marine, and accumulated slowly on a gently sinking downwarp. The Umberatana Group, whose thickness varies from about 3,500 to 6,100 m,has glacial sediments in the upper and lower parts, separated by a non-glacialsequence.The

Time-distributionand type-distributionof Precambrian iron-formationsin Australia

lower glacial sediments-the Yudnamutana Sub-Groupconsist largely of an immense thickness (over 5,000 m) of massive or stratified tillite and related rocks,but contain, very locally,two unusual iron-formationunits. The Braemar Iron-Formation(Whitten, 1970) lies at the top of this glacial sequence,with a thickness of about 600 m.Most of this thickness consists of shale and tillite, but about one-fifthof the total thickness consistsof as many as 30 beds, up to about 12 m thick, of highly ferruginous sediment,with or without an admixture of dropped glacial erratics. The regional outcrop of the Braemar Iron-Formation is shown by Mirams (1962). The Holowilena Iron-Formation, or Ironstone, is described by Dalgarno and Johnson (1965) as a glacigene siltstoneclose to the base of the Yudnamutana Sub-Group. The occurrences lie about 100 km west of those of the Braemar Iron-Formation(Dalgarno and Johnson, 1966). N o information on thickness is published. The Umberatana Group has not been dated, but Compston and Arriens (1968) give an age range of 850600 m.y. for the underlying Burra Group, and by correlation with glacial rocks in the Kimberley area of Western Australia these iron-formationswould have a depositional age of about 750 m.y.

Lithology IRON-FORMATIONS O F T H E Y I L G A R N B L O C K A N D PILBARA B L O C K

In these rocks bands of red, white, brown, yellow, pale green-grey,or clear chert alternatewith dark iron-richbands consisting largely of iron oxide, usually with some silica. The chert bands consist of crystalline quartz of average grain diameter about 20-30 p, although in more strongly metamorphosed areas the quartz mosaic may have a grain diameter of over 1 m m . Most of the brown and yellow chert is due to weathering, but it is possible that both red and white chert persist in depth. The most common iron oxide is magnetite, especially in deep samples,and Miles (1941) has suggested that magnetite is the only primary oxide.The evidence seems inconclusive, however, and hematite and magnetite may both be primary. Rarely does hematite, other than martite,occur together with magnetite.Goethite is always secondary. In areas of low metamorphism these rocks consist almost entirely of two minerals, but silicate and carbonate appear in areas of higher regional metamorphism (Baxter, 1965). The individualbands vary from less than 1 mm in thickness up to about 15 m m . Although no systematicmeasurements exist, the median band thickness of both chert and iron-richmaterial is probably about 5 m m . In most rock there are, over any stratigraphic thickness involving about 20 or more bands,roughly equal thicknesses of each of the two band types. Since the usual Fe content is about 30 per cent this implies an average silica content in the iron-rich bands of about 15 per cent.

IRON-FORMATIONS O F T H E H A M E R S L E Y G R O U P

The lithology of these iron-formationshas been described in detail by Trendall and Blockley (1969). Like the older iron-formations of the Pilbara and Yilgarn Blocks they are typically banded, with silica-rich chert bands alternating with iron-richbands, called ‘chert-matrix’ by Trendall and Blockley (1969). Taking all five units (see above) as a group,their lithology differs from that of the older iron-formationin the following respects:(a) the chert bands tend to be thicker, with a median thicknessinthe one measured part of 7.9mm, and a proportion of much thicker bands (12per cent above 30mm);(b) most of the chert mesobandshave afine internal lamination(microbanding)definedby iron-bearingminerals, so that the cherts have a significant iron content; (c) carbonate minerals, including major ankerite, siderite and highly magnesian ‘siderites’,as well as minor dolomite and calcite, are important constituents of the iron-formation; (d) at many levels there is an internalcyclicityofchert types; (e) ‘primary’magnetite and hematite are present and commonly Co-exist;(f) sheet silicates (stilpnomelane and minnesotaite) and riebeckite are significant, but erratically distributed,constituents. Among the five main stratigraphic units of iron-formation in the Hamersley Group there are characteristicand consistent differences in thickness distribution of the banding,in the degree of development of cyclicity,in the amount and distribution of riebeckite,in the nature of the common sheet silicate,and in the distribution of interbedded shale. IRON-FORMATIONS OF T H E CLEVE M E T A M O R P H I C S

The iron-formationsof the Middleback Group have been described by Miles (1955) and Edwards (1955), who refer to them as banded hematite quartzites.The following summary by O w e n and Whitehead (1965) is worth direct quotation: ‘Typically these are layered rocks which at the surface are composed essentially of quartz and iron oxides with, in some localities,relicts of silicates. Silicification of silicateand carbonatemineralsis not uncommon.The bands range in width from microscopic to about 1 c m and the iron content of the rock as a whole is commonly between 24 and 34 per cent. ‘Whereencountered at depth by diamond drill holes, these rocks have a much more varied mineral composition. Magnetite is the predominant iron oxide, non-aluminous amphiboles such as grunerite, cummingtonite, actinolite and tremolite are common, and carbonate minerals (excluding siderite) occur in some bands, lenses and zones. Accessory minerals include minor but persistent apatite which tends to occur along some bands, and minor pyrite and pyrrhotite which have migrated and recrystallized.’ The metamorphism of these banded iron-formations make their lithological comparison with either of the two preceding categories difficult.Possibly the least altered representative is the Wilgena H i l l Jaspilite described by 53

A. F. Trendall

Whitten (1968). This is a finely laminated rock with 25 per cent hematite, 75 per cent quartz and a trace of magnetite. Whitten believes at least some of the hematite to be primary.

HEMATITE-RICH SEDIMENTS OF T H E Y A M P I S O U N D AREA

Canavan and Edwards (1938)have described the lithology and mineralogy of these rocks. The hematite occurs in quartzites, sandstones, conglomerates and shales, all of which are intergradationalvarieties of similar clastic sediments.The hematite sandstonesand quartzites are generally banded in hematite-richand quartz-richbands. This banding defines the stratification, and ripple-mark, crossbedding and small-scalegraded bedding are as well developed as in normal sandstones. Unlike any of the ironformations of the above three categories both quartz and hematite are inrounded and clearly detritalgrains,although both have locally recrystallized into an even mosaic and often hematite acts as an interstitial cement to the quartz grains. In the more pelitic rocks the hematite tends to take the textural position of the sericite in the interstratified ironpoor siltstones. Conglomerate occurs in which rounded quartz cobbles 5-15 c m in diameter lie in a matrix of hematite;more rarely the cobbles also consist of hematite. In summary these sediments are closely similar to normal clastic sediments,but have hematite locally occupying the textural position of either the clastic grains, or their matrix,or both.

ROPER B A R A N D C O N S T A N C E R A N G E IRONSTONES

The lithology of these oolitic ironstones has been summarized by Canavan (1965) and Harms (1965) respectively, largely from the work of Edwards (in both cases) and of Whitehead (in the second); Canavan and Harms provide references to this previous work. In both places recent weathering affects the rocks for up to 30 m below the surface, and the following descriptions apply only to unoxidized material. Of the Constance Range rocks Harms (1965) summarizes: <. . .the ironstonesconsists of oolites of ochreous or finely crystalline hematite,siderite and/or chamosite and silica grains set in a matrix of siderite, hematite, minor microcrystalline quartz, and carbon. The mineral referred to as chamosite has not been deñnitely identified; it resembles chamosite, greenalite, or glauconite, and may include all these minerals. The oolites vary from 0.2 mm to 3 mm in diameter and the successive shells may be composed of different iron minerals.In the groundmass,siderite frequently forms crystals up to several centimetres across, enclosing oolites and quartz grains in a Fontainebleau texture’. In the Roper Bar rocks (Canavan, 1965) ‘unoxidized 54

ore consists of quartz grains in a sideritematrix with oolites ofred ochreous hematite and occasionally of chamosite. . . , The bottom ironstone bed consists of closely packed oolites of red ochreous hematite up to 3 mm in diameter. Detrital quartz grains occur in the matrix and also form the nuclei of some oolites’. The average iron content in both areas is about 45 per cent.

H O L O W I L E N A A N D B R A E M A R IRON-FORMATIONS

Bucknell’s (1970) description of these rocks shows them to resemble siltstones in which magnetite euhedra about 2070 p in diameter emphasize, by systematic variations of concentration from about 3 to 80 per cent,the lamination of the rocks. Some of the magnetite is usually altered to martite, and this proportion varies from 3 to 100 per cent. Often, some fine-grained flaky hematite is present in addition to martite. Other minerals present to varying extents are chlorite, stilpnomelane, biotite, quartz, feldspar and carbonates.The magnetite euhedra are believed to represent a primary precipitate, and to have been penecontemporaneously reworked, and locally concentrated in ripple crests.

Stratigraphic geometry By ‘stratigraphicgeometry’here is meant the ratio of thickness to lateral extent at various scales. The heading is included because the recently demonstrated fine-scalestratigraphic continuity of the Hamersley Group iron-formations (Trendall and Blockley, 1969) has set a standard against which other iron-formationsmay usefully be compared.

IRON-FORMATIONS O F T H E Y I L G A R N B L O C K A N D PILBARA B L O C K

Many iron-formations(or jaspilites) of these blocks are tightly contorted,and reliable thickness estimates are rare. It seemsunlikely that any exceeds a true thickness of 65 m, and most are no thicker than about 10 m. The present state of mapping does not permit an accurate estimate of lateral extent. However, recently published aeromagnetic maps indicate strike lengths of up to 75 km. Within this distance it is uncertain whether the anomaly is due to one or several iron-formations,and no detailed intervalsubdivision of any of these iron-formationshas been carried out to allow investigation of small-scalecontinuity.

IRON-FORMATIONS O F THE H A M E R S L E Y G R O U P

The present area of the Hamersley Group is about 85,000km2,and within selected levels of the group the likelihood of outcrop-wide lateral continuity of laminae less

Time-distributionand type-distributionof Precambrian iron-formationsin Australia

than 1 mm thick, within thin chert bands of the iron-formation, has been demonstrated (Trendall and Blockley, 1969). The probable original continuity of such laminae over the full basinal area of about 100,000 km2is argued, and this degree of lateral correlation is believed to be possible at any selected level within the iron-formationsof the Hamersley Group;that is, over a total stratigraphic thickness of about 1,000m within the 2,500 m total thickness of the group.

R O P E R BAR A N D C O N S T A N C E R A N G E IRONSTONES

Each of these appears to have been deposited initially over a maximum area of about 10,000km2, althoughinformation is vague since there is a tendency to overlook areas of no potential economic interest.In both places the ferruginous beds are discontinuous laterally; no precise details are published. H O L O W I L E N A A N D B R A E M A R IRON-FORMATIONS

IRON-FORMATIONS OF THE CLEVE M E T A M O R P H I C S

Whitten (1966), in his correlation chart, has implied an initial area of deposition of these iron-formationsat least equal to that of the Hamersley Group iron-formations. However,the scattered nature of the occurrences and the lack of any detailed stratigraphy to support the correlation makes it necessary to reserve final judgement on this (not improbable) proposition. Certainly,the original extent of the Middleback Group iron-formationsis unlikely to have been less than the 10,000 km2 implied by Parkin (1969). Within the iron-formations no record is available of the lateral continuity of internal subdivisions.

HEMATITE-RICH SEDIMENTS O F T H E YAMPI S O U N D AREA

These are very local,and it seems unlikely that these sandstones are notably ferruginous over areas greater than a few tens of square kilometres.It is not known,within this order of area, how the small-scale stratigraphy varies laterally.

Thomson (in Parkin, 1969) has shown these formations with a depositional area of about 10,000 km2.For the Braemar Iron-FormationWhitten (1970) has emphasized 'the lenticular nature of the formations as a whole and of individualmembers of it'. At Razorback Ridge,where the formation is best developed,ironstone bands severalmetres thick pinch out laterally within a kilometre.

Summary and conclusions Among the Australian Precambrian sedimentary basins which contains iron-rich sediments the following conclusions seem valid: 1. Banded iron-formationin the generally accepted sense is apparently confined to sediments with a depositional age of 2,000 m.y. or older. 2. In such sediments banded iron-formation is quantitatively more significant at about 2,000 m.y., although it occursfrom the earliestrecord ofsedimentationonwards. 3. There is a progressive decrease in the abundance of all types of iron-rich sediment after about 2,000m.y. There is little consensus of opinion about the origin of any of these sediments,and this has not been dealt with in this factual summary.

Résumé Répartition de l'âge et du type des formations pi4xmbriennes de fer en Australie (A. F. Trendall)

Les formations précambriennes du fer du continent australien peuvent être classées en six groupes,qu'on peut définir aussi bien par leur localisation géographique que par le type lithologique ou la géométrie stratigraphique,ou encore l'âge.Ces groupes,dans l'ordre des âges décroissant, sont les suivants :(a)les formations de fer dans le massif de Yilgarn Block et dans celui de Pilbara, dans l'ouest de l'Australie :ces massifs sont situés le premier entre les méridiens 116" et 123" est et les parallèles 26" et 34" sud et le second entre les méridiens 118"et 121" est et les parallèles 20"et 22" sud ; (b) les formations de fer des monts Hamersley dans l'Australie occidentale, qui se situent dans un

bassin formé sur des roches précambriennes plus anciennes situé approximativement entre les méridiens de 116" et 122" est et les parallèles de 21" et 23" sud entre les deux massifs indiqués en (a) ; (c) les formations de fer des gisements métamorphiques de Cleve en Australie méridionale, dont leprincipal affleurementlinéaireest situé dans la chaîne de Middleback à la longitude de 137" est entre les latitudes de 33" et 34" sud ; (d) les sédiments riches en hématites de la région du Yampi Sound dans l'ouestde l'Australie situés approximativement à la longitudede 124"est et à la latitude de 16" sud ; (e) les formations de Roper Bar et de Constance Range du McArthur Basin, situées respectivement à environ 134" de longitude est et 15" de latitude sud et 138" de longitude est et 18" de latitude sud ; (f) les formations de fer d'Holowilena et de Braemar,dans le sud de l'Australie, 55

A. F.Trendall

qui étaient initialementdéposées sur une surface s'étendant approximativement entre les méridiens de 138" et 141" est et les parallèles de 31" et 33" sud. Ces six divisionsignorent quelques apparitions de moindre importance qui sont fortement métamorphosées et d'âge et de relation incertains. D u point de vue lithologique,(a), (b) et (c) sont toutes des formations zonées. Entre (a)et (b) les différences lithobgiques sont les suivantes :les carbonates de (a) sont rares, les bancs de silex sont relativementpeu épais,la périodicité n'est pas apparente,le quartz et la magnétite sont de loin les éléments dominants;les carbonatesde (b) sont un constituant important, les silex sont d'épaisseur variable, on observe une périodicité complexe,et thématite,stilpnomelane et riebeckite sont des constituants significatifs en plus du quartz,de la magnétite et des carbonates.Lesformations de fer zonées de (c) sont pour la plupart trop fortement métamorphosées pour présenter une similitude approchée avec.(a) ou (b), mais eíles semblent se rapprocher de (b). Celles de (d), (e) et (f) manquent toutes de zonalité ; (d) est un sédimentclastiqueriche en hématites,(e) est un sédiment

pisolitique d'hématite-sidéritechamositeet (f) est une roche d'association glaciaire dans laquelle magnétite à grain fin et hématite auraient pu,être précipitées simultanément. Quant à la géométriestratigraphique,le seulcontraste significatif est celui qui existe entre les formations relativement peu épaisses et peu étendues latéralement de (d) et la plus grande puissance et l'extension latérale de celles de (b) ;les formations de fer zonées de (c) sont en général trop énergiquement plissées pour permettre une comparaison stratigraphique,tandis que la stratigraphie des formations de fer non zonées n'est pas aussi significative.L'âge auquel se sont déposées les diverses formations de fer de (a)se situe entre 3 O00 et 2 700 millions d'années, tandis que celui de (b) est d'environ 2 O00 millions d'années. Les formations de (c) furent métamorphosées il y a environ 1 780 millions d'années et peuvent avoir le m ê m e âge que celles de (b). Les sédiments de Yampi (d) ont environ 1 810 millions d'années tandis que ceux de (e) ont probablement un peu plus de 1 500 millions d'années. Ceux de (f) remontentau voisinage de 750millions d'années.

Bibliography/ Bibliographie BAXTER, J. L.1965.Petrology of a banded iron-formation, Koolanooka Hills, Western Australia. Honours thesis,University of Western Australia, 130 p. (Unpublished.) BUCKNELL, M . J. 1970.Petrologicalreports.In:G .F.Whitten, 1970,p. 91-114 (this reference list). CANAVAN, F. 1965. Iron ore deposits of Roper Bar. In: J. McAndrew (ed.), Geology of Australian ore deposits, p. 212-15. Melbourne, Australasian Institute of Mining and Metallurgy. (8th Commonw. Min. Metall. Congr. Publ. no. 1.) CANAVAN, F.;EDWARDS, A. B.1938. The iron ores of Yampi Sound, Western Australia. Proc. Aust. Inst. Min. Engrs., no. 110, p. 59-101.

CARTER, E. K.;BROOKES, J. H.; WALKER, K.R. 1961. The Precambrian mineral belt of northwestern Queensland. Bull. Bur. Min. Resour. Aust., no. 51, 343 p. CARTER, E.K.; ÖPIK, A.A. 1961.L a w n Hill. 17 p. (Bureau of Mineral Resources 1 :250,000 explanatory notes series.) COMPSTON, W.; ARRIENS, P. A. 1968. The Precambrian geochronology of Australia. Canad. J. Earth Sci., vol. 5, p. 561-83.

DALGARNO, C. R.; JOHNSON, J. E.1965. The Holowilena ironstone, a Sturtian glacigene unit. Quart. Notes Geol. Surv. S.Aust., no. 13, p. 2-4. -. 1966. 1 :250,000 geological atlas series sheet H 54-13. Parachilna,South Australia Geological Survey. DUNN, P. R. 1963a. Hodgson Downs, N.T.16 p. (Bureau of Mineral Resources 1 :250,000 explanatory notes series.) -.19636. Urapunga,N.T.17 p. (Bureau of Mineral Resources 1 :250,000explanatory notes series.) EDWARDS, A. B. 1955. Banded hematite quartzites from the Middleback Range,South Australia.In: K.R. Miles, 1955, p. 206-10 (this reference list). GELLATLY, D . C.;DERRICK, A. M.; PLUMB, K.A. 1968. Proterozoic palaeocurrent directions in the Kimberley region, 56

northwestern Australia.Bureau of Mineral Resources Record no. 141, 10 p. (Unpublished.) HARMS, J. E. 1965. Iron ore deposits of Constance Range. In: J. McAndrew (ed.), Geology of Australian ore deposits,p. 26469. Melbourne,Australasian Institute of Mining and Metallurgy (8th Commonw. Min. Metall. Congr.Publ.no. 1.) MACLEOD, W . N. 1965. Banded iron-formations of Western Australia.In: J. McAndrew (ed.), Geology of Australian ore deposits, p. 113-17. Melbourne, Australasian Institute of Mining and Metallurgy (8th Commonw.Min. Metall.Congr. Publ. no. 1.) MCDOUGALL, I.;DUNN, P. R.; COMPSTON, W.; WEBB, A.W.; RICHARDS, J. R.;BOFINGER, W.M . 1965.Isotopic age determinations on Precambrianrocks of the Carpentarian Region, Northern Territory, Australia. J. Geol. Soc. Aust., vol. 12, no. 1,p. 67-90. MILES,K.R.1941. Magnetite-hematiterelations in the banded iron-formationsof Western Australia.Proc. Aust. Inst. Min. Engrs, no. 124, p. 193-201. . 1953. Banded iron-formations in Western Australia. In: A.B.Edwards(ed.), Geology ofAustralianore deposits, p. 11554. 1st ed.,Melbourne,AustralasianInstitute of Mining and Metallurgy (5th Emp. Min. Metall. Congr.). . 1955. The geology and iron ore resources of the Middleback Range area. Bull. S. Aust. Geol. Surv.,no. 33, 247 p. MIRAMS, R. C. 1962. The geology of the Manunda Military Sheet. Rep. Invest. Geol. Suvv. S. Aust., no. 19, 39 p. OWEN,H.B.;WHITEHEAD, Sylvia, 1965. Iron ore deposits of IronKnob and theMiddlehack Ranges.In:J. McAndrew (ed.), Geology of Australian ore deposits,p.301-11.Melbourne,AustralasianInstituteof Mining and Metallurgy (8th Coinmonw. Min. Metall. Congr. Publ. no. 1.) PARKIN, L.W .(ed.) 1969.Handbook of South Australiungeology. South Australia Geological Survey. 268 p. PLUMB, K.A.;PAINE, A.G.L.1964.Mount Young,N.T.,19 p.

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(Bureau of Mineral Resources 1 : 250,000 explanatory notes series.) RANDAL, M . A. 1963.Katherine,N . T.26 p. (Bureauof Mineral Resources 1 :250,000explanatory notes series.) REID,I. W.1965. Iron ore deposits of Yampi Sound. In: J. McAndrew (ed.), Geology ofAustralian ore deposits,p. 12631. Melbourne,Australasian Institute of Mining and Metallurgy (8th Commonw. Min. Metall. Congr. Publ. no. 1.) TRENDALL, A.F.;BLOCKLEY, J. G.1969.The iron-formationsof the Hamersley Group,Western Australia, with special reference to the associated crocidolites.Bull.W.Aust. Geol. Surv., no. 119, 353 p.

WHITTEN, G.F.1966.Suggested correlation of iron ore deposits within South Australia. Quart. Notes Geol. Surv. S. Aust., no. 18,p. 7-11. . 1968.The section of iron-formations,Tarcoola District. Quart. Notes Geol. Surv. S. Aust. no. 26, p. 4-7. -. 1970.The investigationand exploitation of the Razorback Ridge iron deposit.Rept. Invest. S.Aust. Geol. Surv.no. 33, 165 p. WILLIAMS, I. R.1969.Explanatory notes on the Kurnalpi 1/250,000 geological sheet, Western Australia. Rec.Geol. Surv.W.Aust.,

no. 197011.

Discussion R . P.PETROV.What terminology is used for iron-formation rocks by Australian geologists? What term is preferable: chert,jaspilite, itabirite, taconite,iron quartzite? A . F. TRENDALL. The following definitions are used by the Geological Survey of Western Australia: 1. Iron-formation is a chemical sedimentary rock with a high iron content;in addition to ‘bandediron-formation’ (defined below) it includes the high-iron shales, slates, carbonate rocks and mixed oolitic rocks frequently associated with Precambrian banded iron-formations,as well as Phanerozoic chamositic sediments. 2. Banded iron-jormation (often shortened to BIF)is a chemical sedimentary rock consisting in its least metamorphosed state of successive thin layers (mesobands) of fine-grainedquartz,iron oxides,carbonates and silicates ofvarious proportions. Some nondefining characteristics are:(a) there is usually a widerangebetween therelatively low iron content of quartz-rich(chert) mesobands and the high iron content of adjacent magnetite-rich or hematite-richmesobands; (b) the total iron content of samples taken across the banding over widths about 10 times the average mesoband thickness is often in the range of 15 to 35 per cent; (c) most banded iron-formation is Precambrian. 3. Jaspilite is a rock with alternating thin bands of jasper (red chert) and some other material,usually either black iron oxides or chert of another colour. With these definitions,most jaspilite is a special type of banded iron-formation,and banded iron-formationis itself

a special type of iron-€ormation.Outside the Geological Survey of Western Australia jaspilite is sometimes used in a broader sense,more or less synonymously with banded iron-formation. The Brazilian term itabirite and the North American term taconite are not used in Australia. Personally I would regard both as special types of banded iron-formation, although some massive granular taconite is only slightly banded. Banded iron-formation (including jaspilite) is known in India as banded hematite quartzite, but many English-speakinggeologists,including those in Australia, do not use this term because quartzite usually means a quartz-rich sandstone which has been either thoroughly cemented or metamorphosed. All persons working on iron-rich sediments should include exact definitions of their terms in their publications.

R.T.BRANDT. Is it justifiable to extend the term iron-formation to include iron-bearing sediments, such as the oolitic iron-stonesof Roper Bar and Constance Range? The latter sediments have quitedifferent characteristicsand compositions from the banded iron-chertsand jaspilites which are usually understood to be the essential rock types of banded iron-formation. A . F.TRENDALL. Yes. A general term is needed to cover all iron-richsedimentary rocks,and ‘iron-formation’ seems to be the best one available.I would be glad to consider any suggested alternative,but have not yet met one.

The origins of the jaspilitic iron ores of Australia R. T.Brandt Goldsworthy Mining Ltd, Port Hedland,Western Australia

Introduction Iron-oredeposits in Australia are of several different types and can be broadly classified,in order of economic importance,into four categories,as follows. 1. Deposits within or directly associated with Precambrian banded iron-formations,generally known in Australia as jaspilites.These deposits are here described as jaspilitic iron ores and are the subject of this paper. 2. Sedimentary iron deposits younger than banded ironformations and of different character. 3. Massive depositswith igneousaffiliations,usually known as magmatic-type deposits. 4. Superficialiron-richlaterites,conglomerates,etc. By far the most important are the ores of the first category, of which there are potential reserves amounting to thousands of millions of tons,mostly in Western Australia.The protores or source rocks of these deposits are banded ironformations or jaspilites of Archaean and Proterozoic age. The other types of deposit listed above are outside the scope of this paper and will not be discussed here.

Banded iron-formations The iron-formationsare conspicuously banded iron-silica rocks quite similar to the itabirites of Brazil, the taconites of North America and equivalentformations in other parts of the world. Silica is present as finely granular or cryptocrystalline quartz or chert.The iron is in the form of oxide, carbonate,silicateor occasionallysulphideand is invariably oxidized in the zone of weathering to hematite or limonite, giving rise to the characteristicred and white or brown and white striped rock known in Australia as jaspilite. The character of the surface rock depends on the composition of the originalrock from which it is derived.Some hematite jaspilites may be primary oxide facies iron-formations; others are oxidation products of magnetite-carbonate rocks into which they pass below the zone of weathering. Iron silicate rocks, on oxidation,yield limonite and

clay material and are commonly represented at the surface by banded limonitic cherts or shaly rocks. Pyritic iron-formations are characterized by gossanous outcrops. The typical protores of hematite ore deposits are hematite-rich jaspilites,usually with some magnetite more or less oxidized to martite. Drillholes have shown that magnetite is often the dominant iron mineral in the primary zone. The presence of siderite or iron silicates in the protore appears to be detrimental to the formation of hematite deposits, though much limonite is commonly associated with this type of iron-formation. Thejaspilitesof the Archaean and Proterozoic are similar mineralogically but differ greatly in extent, uniformity and geological associations.Those of the Archaean, in the Yilgarn and Pilbara Blocks, are closely associated with greenstone volcanics, which generally underlie them, and are relatively thin and non-persistentunits which finger out or changein facieslaterallyinto cherts,fineclasticsediments or tuffaceous rocks.They were evidently deposited in small and relatively short-livedbasins in extensive volcanic terrains. The grade of metamorphism is low to medium in general and high in the vicinity of granitic plutons. Iron-ore deposits occur where the jaspilites are unusually thick and particularly where they have been tectonicallythickened by intricate internal folding. The Proterozoic iron-formationsare individually far more extensive than those of the Archaean and constitute stratigraphic units of great persistence and uniformity in conformable sequences of sedimentary and volcanic rocks. Thin jaspilite marker beds in the Median Belt can be traced in outcrop for hundreds of miles with little change in composition or thickness,indicatingvery extensivedepositional basins and a uniformity of sedimentary environmentalconditionsover considerableareas.The grade ofmetamorphism in the Median Belt is uniformly low,but in the Middleback Ranges is much higher, the associated rocks being metamorphosed to schists,quartzites, amphibolites, etc. The association of the iron-formations with volcanic rocks is less evident in the Proterozoic than it is in the Archaean, but thin volcanic and tuffaceous horizons occur

Unesco, 1973. Genesis of Precambrian iron and manganese deposits. Proc. Kiev Symp., 1970. (Earth sciences, 9.)

59

R. T.Brandt

throughoutthe sedimentarysuccessionsand volcanic shards and pyroclastic fragments have been identified by L a Berge (1966)in the principal iron-formationof the Median Belt. In the present writer’s opinion the field evidence shows that contemporaneous volcanic activity occurred and was probably a necessary accompaniment to the deposition of the banded iron-formations. In the Middleback Ranges the most productive ironformation is underlain by dolomitic rocks, a fact which has also been notedin South Africa (Alberts and Ortlepp,1961). This is thought to be of possible significance as a pointer to the activity of magnesian solutions in the genesis of the ore deposits.The association with dolomite does not exist in the Archaean because of the absence of carbonate rocks in that era, but magnesian solutions may well have been active in association with the basic igneous rocks that are abundant in the Archaean.

Iron-ore deposits GENERAL

Iron-ore deposits formed in situ in jaspilites by assumed chemical weathering processes are often referred to as Lake Superior type deposits. This rather unsatisfactory term rests on the assumption,unjustified in the present author’s opinion,that all the major deposits of the North American Lake Superior provinceare ofthe sametypeand wereformed inthe sameway.Itis difficultto reconcilethe greatdifferences in character between, for example,the extensive sheet-like deposits of the Mesabi Range and the deep,discrete,dykecontrolled orebodies of the Gogebic Range, and to regard both as manifestations of the same genetic process seems illogical. Concentrationsofhematite or limoniteinjaspilitecould theoretically have been formed in a number of different ways which may be broadly summarizedas follows:(a) concentration during sedimentation or diagenesis (syngenetic concentration); (b) concentrationby Precambriansupergene weathering processes, shortly after or even during the period of sedimentation; (c) concentration by hypogene (metamorphicand/origneous) processes either before,during or after folding of the beds; (d) concentration by supergene processes after folding, uplift and exposure of the protore beds to erosion. One of the difficulties in the genetic interpretation of iron-oredeposits arises from the fact that they are generally well exposed and form high ground, any covering rocks which may once have existed having been removed by erosion. In the absence of any direct evidence of the depth at which a deposit was formed,it may be difficult to decide whether its superficialsituation and limited depth are original or are due to geological structure and the fortuitous level of current erosion.Cases in point occur in the Middleback Ranges,where many prominent hill-formingdeposits are shallow because they occupy the cores of deeply eroded synclines, the adjacent anticlinal portions having been 60

removed.In one such deposit a depth zonal arrangement, from siliceous through talcose to carbonate-richore, has been noted and roughly parallels the present land surface. This might be interpreted as evidence of a recent supergene origin,but a more likely explanation is that the zoning is a hypogene post-folding phenomenon and its parallelism with the existing topography is merely a reflection of the high resistance of the uppermost zone to erosion. The Thabazimbideposit in South Africa exhibits similarfeatures (Alberts and Ortlepp, 1961). Australian jaspilitic iron ores can be classified into three types,which are here given the following descriptive names: (a) crust type; (b) derived type; (c) lode type.The first two types,which account for by far the largest tonnages of ore,were unquestionably formed by post-folding supergene processes. The third type, which constitutes very important high-gradereserves and includes the Middleback Ranges deposits,is ofless-certainorigin.The present author advocates a hypogene metamorphic origin similar to that proposed by Dorr (1965) for the high grade hematites of Brazil,but syngeneticor pre-foldingsupergene origins cannot be excluded from consideration.

C R U S T TYPE DEPOSITS

Crust type deposits are essentially surficial crusts of hematite and limonite formed in situ on tilted or folded protore beds in the zone of weathering. They are very numerous and widespread in the Median Belt and the Pilbara Block, much less abundant in the Yilgarn Block and rare in the Gawler Block. They range from shallow limonitic cappings of a lateritic nature to relatively thick concentrations of hematite replacing jaspilites in areas of deep weathering. These deposits are similar in all essential respects to the supergene iron ores of Brazil described by Dorr (1964)and are undoubtedly attributable to the same genetic process, namely downward leaching of the protore beds by surface waters during climatic periods of high rainfall and high temperature. The process,as described by Dorr,involves two kinds of iron concentration,namely residual enrichment by preferentialremoval of silica in solution,and secondary cementation by solution of iron and its redeposition as limonite in the spaces vacated by the silica. Residual enrichment is the earlier and deeper process which, under favourable conditions,may reach depths approaching 1,000ft (304m), though the average is much less. It produces concentrations of porous and disaggregated hematite in which the hematite or martite laminae of the original jaspilite,after removal of the silica,have usually broken up and slumped together.Some secondary limonite is nearly always present as a cementing material or coating on the hematite fragments, but is subordinate in amount to the original hematite. Secondary cementationis a shallowerprocess by which the hematite fragments near the surface become hydrated and cemented together by secondary limonite, forming a

The origins of the aspilitic iron ores of Australia

hard surface limonitic capping of lateritic character known in Brazil as canga. The hard capping is nearly always underlain by a collapsed zone,characterizedby slump structures, cavebreccias and opencaverns due to leaching.Theprincipal concentrations of residual hematite are found below this zone. A hard canga capping, according to Dorr (1964), is essential for the preservation of the underlying residual hematite, which would otherwise, because of its largely disaggregated nature, be rapidly removed by mechanical erosion.Part of the iron which forms the capping is derived not from the rocks originally above, but from below, by precipitation from solutions drawn upwards by capillarity during dry periods of the year. By this means the capping renews itself continuously,as fast as its upper surface is eroded. The controlling factors in the formation of crust type depositsare climateand drainage,the chemicaland physical nature of the protore beds, and geological structure. The fact that Australian crust type deposits are virtually confined to the tropical northern region which, though semiarid at present,had a hot and humid climate in the past, indicates that without the necessary climatic conditions these deposits are not formed,even though suitable protore beds exist. The optimum physiographic conditions would be good sub-surface drainage, a low and steadily falling water-tableand a slow rate of mechanical erosion. The nature of the protore beds is also a powerful controlling factor.Themost favourableformationsfor hematite deposits are hematite jaspilites or those in which primary magnetite has been oxidized to martite. The highest-grade deposits consist of hematite residual from the original jaspilite, with a minimum of secondary limonite. There is evidence in some cases that secondaryironhas been precipitated not as limonitebut as massive,fine-grainedcolloform hematite, which cements and further enriches the deposit. Carbonate and silicate iron-formations do not normally contain hematite deposits but yield much limonite and commonly carry extensive canga cappings, underlain by thoroughlylimonitized zones which are often collapsed and cavernous due to leaching out of silica. D u e to their mode offormation,crust type deposits are generally shallow in relation to their horizontal dimensions and have very irregular basal profiles, due to differences in the depth of iron enrichment from bed to bed. Sedimentary structures are normally preserved in the ore on a large and a small scale.Compositionaldifferences in the protore beds are reflected by differences in the proportions of hematite, limonite,silica,alumina,etc.,in the ore.The laminationsof the jaspilite are preserved as a plate-likestructure,causing the hematite to split easily into thin slabs. The principal physical controls of ore emplacement are the bedded structure and state of fracturing of the host rocks.Iron enrichment is deepest and most thorough where the jaspilites are well-jointedor shattered in the vicinity of faults. Where the beds are unfractured and only mildly deformed there may be no enrichment at all. Areas of open folding with fairly steep dips and numerous cross fractures

appear to be the most favourable. The deepest hematite concentrations are often found in the cores of plunging synclinal structures,especially where these are floored by relatively impervious shaly limoniticstrata.A typical crosssectionof such a deposit is shown in Figure 1. The synclines evidently acted as conduits in which downward-moving silica-leachingsolutions became channelled.The same feature has been noted in Venezuela (Ruckmick, 1963). The grades of crust type hematite ores range up to 66-67 per cent Fe. Material below an arbitrary grade of 55 per cent Fe is classed as iron-enrichedjaspilite and not as iron ore. The degree of iron enrichment that can be achieved by silica leaching is limited by the presence of small amounts of combined water and other non-leachable impurities, which are enriched at the same time as the iron.Consequently the ore has a somewhathigher content of alumina,phosphorus and combined water than the parent jaspilite. The Median Belt of Western Australia has enormous reserves, estimated at about 8,000million tons, of crust type hematits of grade from 55-66 per cent iron,with a phosphorus content between 0.03 and 0.16 per cent. The exceptionally high grade deposits at Mount Tom Price and Mount Whaleback together contain nearly 1,000 million tons of ore of an average grade higher than 64 per cent Fe, including some which assays up to 69 per cent Fe and is very low in phosphorus.It appears possible that these very high assays,which are said to be above the limit attainable by hematites formed by supergene processes (Dorr, 1965), may be from lode type and not crust type ore. The Median Belt region is an elevated undulating plateau with deeply weathered convexly rounded hill profiles and flat-flooredvalleys formed by an old erosion surface which is under dissection by present drainages.The deposits are related to this old surface, known as the Hamersley surface,which is believed to have been perfected some time during the Tertiary and was then upwarped and vigorously incised by rejuvenated drainages. The beginnings of the iron-enrichmentprocess could thus date back at least as far as the Mesozoic and possibly even earlier,and it may even be proceeding at a reduced rate at the present day,though current erosion is chiefly engaged in dissecting and destroying the previously formed deposits. The crust type deposits of the Pilbara Block are of similar composition to those of the Median Belt but of somewhat lower average grade and very much smaller, probably because the thin, closely folded and metamorphosed jaspilites of the Archaean did not lend themselves to supergene iron enrichment to the extent that the thicker Proterozoic formations did.The total reserves are probably somewhere between 100 and 200 million tons in numerous small scattered deposits, many of which are too small to be economically workable. The Pilbara Archaean area is relatively flat and low lying and once carried a cover of Mesozoic sediments. Supergene iron enrichment probably dates from the removalof this cover in the Tertiary,though it could have commenced much earlier, in early or preMesozoic times. 61

R. T.Brandt

MAfNLY HEMA TI TE ENR/CtfMENT

MAfNL Y LfMONf TE ENRfCHMENT JASPfL i TE

E[ .. . ......

Scale of Feet (approx 1 O

I

500

1000

I

I

FIG.1. Section of typical crust type iron-oredeposit. DERIVED TYPE DEPOSITS

Derived type deposits are extensively developed in the Median Belt ofWestern Australiaand arenumerous,though very much smaller,in the Pilbara Block. The deposits are surficial accumulations of limonite derived from jaspilite but not formed in situ, the iron having migrated a short or a long distance from its source. In Brazil and Venezuela these deposits are classed as cangas(Dorr,1964;Ruckmick, 1963), no distinction being made between the in situ canga cappings on crust type deposits and the transported cangas which commonly surround them as apronson hill slopesand in valleys.Thereis infact no real distinction,as the one type passes gradationally into the other, but in Australia the enormous tonnages of transported cangas that exist independently ofcrusttype depositsmake it necessaryto consider them as a separate type of deposit,namely derived type. Derived type depositshave clearly originatedfrom both mechanical and chemical weathering of jaspilites,whereby detrital accumulations were formed and were subject to supergene leaching of silica,hydration of original hematite and other iron minerals and the deposition of much limonite cementing and replacing the original rock fragments. Their formationhas involved the downslope transportation of enormous amounts of iron as detritus, in solution, or both. The deposits range from limonite-cementedscrees on slopes,which are sometimes continuous with in situ canga 62

cappings on hilltops,to thick valley-flooraccumulations of massive and pisolitic limonite without detrital material. The latter variety is very extensively developed in the catchment area of the Robe river,which drains the northwestern part of the Median Belt. The pisolitic limonites form extensive mesa deposits 100 ft (30 m) or more in thickness,which are deeply dissected by the present rivers. The upper surfaces of the mesas have been shown to be continuous with the Tertiary Hamersley erosion surface on the hill-formingjaspilites to the south-east(Fig. 2), from which it can be inferred that the limonites were formed during the period of this extinct erosion cycle and are thus essentially contemporaneouswith the crust type deposits in the jaspilites. While there is no dispute as to the origin of the limonite,which was derived by erosion of large areas ofjaspilite within the drainage basin of the ancestral Robe river and its tributaries,some controversyexistsregardingthemanner in which the iron was transported and deposited. In the headwaterareasaremarginal canga apronsaround theflanks of jaspilite hills which consist largely of cemented jaspilite fragments.This variety passes downstream into mesaform pisolitic limonite, which occupies extensive valley floors eroded on softer rocks. Harms and Morgan (1964) regard the mesa deposits as bog iron ores formed by chemical precipitation of iron in large lakes or swamps. MacLeod (1966)interprets them as sheet-likeaccumulations of jaspi-

The origins of the jaspilitic iron ores of Australia

2 H A M E RSLEY SUR FA

I

CRUST

TYPE /RON OR€ DEPOSíT

2 IN srru GANGA 3 DETR/TAL CANGA 4 PrsoL r n c L rMoNrrE FIG.2. Illustrating relationship of derived type deposits to Hamersley erosion surface. litic detritus so thoroughly leached by iron-chargedwaters and replaced by limonite that their original detrital character is lost. Certain Lower Cretaceous conglomerates in the lower Robe river valley have been shown to be older than the mesaform limonites, which are therefore of late Mesozoic or Tertiary age.It seemsreasonableto regard them as essentially cogenetic with the crust type hematite deposits,both being products of the prolonged chemical and mechanical degradation process which culminated in the attainment of maturity of the Tertiary Hamersley erosion surface. The abundance of limonite suggests that the chief source of the iron may have been largeareasof carbonate or silicateironformation. Total reserves of mesaform pisolitic limonite in the Median Belt are estimated at about 6,000 million tons.The grade ranges from 40-60 per cent Fe, with considerable amounts in the range 52-58 per cent,with silica and alumina less than 10 per cent, combined water 10-12 per cent and phosphorusless than 0.05per cent.This materialconstitutes excellent beneficiating ore which can be upgraded to 6365 per cent by removal of water. Limonitic deposits, including canga cappings, invariably contain smallamounts ofhematite and maghemite, which is developed mostly at the surface as thin crusts on outcrops and loose fragments.Evidently hydrated iron oxides are unstable in the present semi-aridclimate and dehydrate slowly when exposed to the atmosphere.This possibly represents the beginnings of the processby which canga-type cappings of hard surface hematite were formed under conditions of extreme aridity, as in Mauritania (Gross and Strangway,1961; Baldwin and Gross, 1967).

LODE TYPE DEPOSITS

Compared with the huge crust and derived type deposits of the Median Belt, lode type deposits are small,the largest known being less than 100 million tons. They are of sporadic occurrence in the Archaean jaspilites of the Yilgarn

and PilbaraBlocks and in the South Australian Middleback Ranges,but have not been recognized in the Median Belt, though there seems to be no compelling reason why they should not occur there. The Western Australian Archaean deposits appear to be exact parallels of the Brazilian high grade hematites described by Dorr (1965).They are structurally controlled replacement bodies of massive hematite in jaspilite,which commonly have the form of narrow,steeply dipping lenses similar to certain hydrothermal metalliferous lodes. Some are of saddle-reeftype in the cores of folds.The lenses are generally concordant with the bedding of the jaspilite and have sharp contacts with it except at the extremities,where the hematite interfingers with jaspilite and passes gradationally into it. The metasomatic origin of the hematite is clearly shown by the preservation in it of the laminated structure of the jaspilite,as bands of slightly different texture or porosity in massive hematite. Some silica expelled during replacement segregatesin scattered blebs,pods and random veinlets of crystalline quartz,which are of common occurrence in the marginal transition zones of the hematite bodies. This type of siliceous hematite is entirely different from that formed by incompletesupergeneleachingin crust type deposits,the hematite being very pure and the quartz recrystallized and often coarse-grained. Lode type deposits have no discerniblegeneticrelationship to present or past land surfaces,their exposure at the surface being due to the fortuitouslevel of current erosion. Small lenses without surface outcrops have been encountered in drilling and it is possibly only a matter of time before large 'blind' orebodies with no surfaceexpression are discovered by underground or geophysical methods. The control of ore emplacement is both stratigraphic and structural.The typical host rocks are hematite or martitejaspiliteswith primary iron contentsup to 40per cent F e or more, especially the variety containing red hematitestainedjasper in place of chert.As in Brazil,the relationship of the deposits to geological structures is their most conspicuous feature. They are localized in zones of relatively intense deformation, where the host formation is 63

R. T.Brandt

r \

1000

O

1000

2000

E d JASPlL/JE CHERT

SECTION A - B

QUARTZ/JE

LODE TYPE ORE CRUST JYPE ORE mmmmn BASIC DYKE

3

No. OF OREBODY

FIG.3. Plan and section of Mount Goldsworthy orebodies.

tectonically thickened by internal crumpling,due to drag on fold limbs or against faults. The relationship to faulting is very Weil exemplified at Mount Goldsworthy in the Pilbara Block (Fig. 3). The geology of this area has been described in previous publications (Matheson et al., 1965;Brandt, 1964, 1966). The host rock is an unusually thick Archaean iron-formation with a steep northerly dip,which has been further thickened locally by crumplingagainst an obliqueverticaltranscurrent fault with a horizontal displacement of about 2miles (3 km). The beds on the south side of the fault are thrown into a series of drag folds plunging westwards at about 45",and are also crumpled in the opposite sense about vertical axes. Lode and crust type hematite deposits occur in three stratigraphicpositions marked by iron-richjaspilites within the formation.The lode type bodies are conformable lenses which are localized against the fault and plunge westwards at 45" in conformity with the drag folds. The largest lens, known as No. 1 orebody (Fig. 3), has an abrupt and steep westerly termination,apparently due to a complexZ-shaped vertical fold in the adjacent beds. This structure is also the locus of a narrow vertical dyke which may have played a significantpart in the ore localization.No,3 orebody occurs stratigraphically below No. 1, outcrops against the fault to the east of it,and its westward-plungingextension has been locatedundergroundbelow No. 1. Part of the lode type ore of No. 3 orebody is directly overlain by a body of younger crust type ore which follows the outcrop of the same jaspi-

64

lite protore bed. The structural controls are thus the two sets of crumplings associated with the fault, namely the westward-plunging drag folds and the vertical Z-shaped folds, which between them furnish a favourable structural environment for the deposition of lode type ore. Another less-thoroughly explored lode type deposit occurs at Shay Gap, 40 miles (64 km) east of Mount Goldsworthy.Here the orebody is a long narrow conformable lens in jaspilite dipping north-eastwardsat about 45" (Fig. 4). The ore is localized near the intersection of two

FIG.4.Plan of the Shay Gap lode orebody.

\ '

The origins of the jaspilitic iron ores of Australia

structures. One is a set of drag folds plunging eastwards, obliquely to the strike of the beds. The other is a possible fault or sharp flexure trending northwards, against which the thickest part of the orebody terminates abruptly. The same jaspilite formation extends uninterruptedly for many miles in both directions,but most of it is devoid of suitable structuresand therefore devoid of ore. In South Australia, the deposits of the Middleback Ranges occur in jaspilites which are folded about northsouth axes.The folds are themselves gently cross-foldedand interrupted by faults, dykes and other transgressive structures, so that the plunge of the main folds undulates, changing in direction and steepness from point to point. The terrain is deeply eroded,the main ranges being formed by the deepest parts of the structure,where fold synclines are intersected by plunge synclines. The hematite deposits are generally conformable with the jaspilite bedding and occur on the limbs and in the troughs of synclines. The apparent shallowness of some of the deposits and their occurrence in basin-shaped synclines once led to the idea that they were of supergene origin.On many other grounds, however,it seems reasonablycertain that they are lode type deposits deeply eroded, with the synclines preserved and the adjacent anticlinal portionsremoved. As in the Pilbara Block, the Middleback deposits are localized by particular structural features.Where the jaspilites are regularly folded and plunge uniformly there is generallyno ore.Depositswhich occupy fold-limbpositions have been shown to be associated with local drag folds on the limbs. The principal synclinal deposits occur in modified basin-likestructures where the fold plunge changes or where the folds are interrupted by cross-faults,dykes or sharp flexures. Some deposits are delimited by amphibolitized basic dykes (Fig. 5). Most of the dykes have been shown to be older than the ore, but a few are younger (Miles, 1954). An interesting feature of the deposit illustrated in Figure 5 is the occurrence locally of highly manganiferous ore where,it is believed,the hematite has replaced part of a bed of manganiferous dolomite (Miles, 1954). Lode type ores have grades from 69 per cent Fe downwards.In some deposits the bulk of the ore has a grade of 66 per cent Fe or higher,but the silica content of the thinner marginal portions brings the average grade lower.The only important impurity is silica,the alumina and phosphorus contents being significantly less than in crust type ores of similar grade.Limonite is very scarce in the ore and where it is present it can usually be attributed to hydration by subsequentsupergeneprocesses.The ores consist ofmassive hematite with a little martite or magnetite. The texture is usually granular,but quite often micaceous or schistoseand occasionally specular. Most varieties have an inherited banded structure,often more or less contorted and cut by veinlets of later coarse-grained hematite. Sometimes the banding imparts a fissility which causes the ore to split easily into thin slabs, a characteristic which is developed much more strongly in crust type ores. Most lode type hematite is hard,compact and resistant to weathering, but some ore of identical composition is

CROSS -SECTION EAST

WEST

+ -_-

+ + ++++'

LONGITUDINAL SECTION

-__----.--

SOUTH

NORTH

Scale of Feet

u

1000

O

1000

I

I

I

2000

I

JASPIL /TE

SCHIST

GNEISS COMPLEX AMPHIBOLITE (BASIC INTRUSIVES)

/RON ORE

-

FAULT

FIG.5. Section of the Iron Monarch orebody, Middleback Ranges (by courtesy of the Australasian Instituteof Mining and Metallurgy). soft, porous and weathers easily. The soft ores are sometimes hardened by weathering at the surface but more often are leached,limonitic and covered by thin canga cappings. This indicates that supergene solutions must have been instrumental in enlarging the pore spaces by dissolving out siliceous impurities and some hematite, but whether this represents the sole origin of the soft ores, or whether the porosity has a more deep-seated origin, is still uncertain. Lode type deposits are clearly of different origin from crust type deposits and much older.A Precambrian age for the Middleback deposits is indicated by the presence of water-wornhematite pebbles in nearby Cambrian conglomerates (Miles, 1954). Similar pebbles occur in Proterozoic conglomerates in the Pilbara Block. A syngenetic origin for the hematite, as advocated by Baldwin and Gross (1967) 65

R.T.Brandt

for the deep hematite ores of Mauritania,would seem to be invalidated by the obviously metasomatic character of the ore and its localization by tectonic structures. The same would apply to any hypothesis of supergene iron concentration before folding. The origin favoured by the present author,at least for the Western Australian lode type deposits,is that proposed by Dorr (1965) for the high-gradehematites of Brazil. This postulates that the deposits were formed at elevated temperatures and pressures by hypogene fluids,possibly activated by and partly derived from igneous bodies at depth. The preservation of folded structures in the ore indicates that the process took place mainly after folding of the beds. The hematite is chemically similar to that of crust type deposits,thoughwith a smallercontent ofcertainimpurities, and so in all probability was derived from the same source, namely the protore beds themselves. The deposits characteristically occur where the jaspilite protores have been thickened by folding,a process which must have involved the splitting apart of bedding planes and the creation of voids suitable for penetration by hypogene solutions. It is significant that deposits do not occur where the jaspilites are tectonically thinned or where they are tightly and plastically folded in a manner which would preclude the existence of open spaces between bedding surfaces. It could be assumed that the solutionstravelled mainly along open bedding planes in an up-dip or up-plungedirection,dissolving iron at depth and re-precipitatingit as hematite in place of silica at higher levels in zones of reduced pressure. The jaspilites below a deposit should therefore be impoverishedin iron and those above it should be enriched in silica. Field evidence in support of this hypothesis is very scanty and inconclusive,but very iron-poorjaspilites have been intersected by deep drillholes below hematite deposits in the Pilbara Block,and one diamond drill core showed jaspilite with the hematite bands leached out. H y drothermally leached and silicifiedjaspilites have been noted in the Middleback Ranges (Miles, 1954). Evidence of hydrothermal argilic wall rock alteration adjacent to hematite orebodies at Mount Goldsworthy has been mentioned previously (Brandt, 1966). The Middleback Ranges deposits have various features not possessed by those of the Western Australian Archaean and the writer is at a disadvantage in having no personal

knowledge of these deposits. The genetic hypothesis proposed by Miles (1954) is essentially in accord with that outlined above, but other investigators (Catley, 1963; Owen, 1964) have different ideas on which the writer does not feel qualified to comment. In conclusion the writer wishes to draw attention to the widespread association of deep hematite deposits with basic dykes, not only in Australia but elsewhere, such as the Gogebic Range in the Lake Superior province.It is felt that these dykes could have played a more substantial role than that of fortuitous structural barriers impounding supergene or hypogene solutions.Talc is a minor accessory constituent of many lode type hematites and is especially abundant in the Iron Duke area of the Middleback Ranges where zones of siliceous,talcose and carbonate-richhematite and magnetite, in descending succession, have been identified. The formation of talc implies the presence of magnesium in the solutions. Owen and Whitehead (1965) envisage magnesium bicarbonate solutions connected with the intrusion of dolerite dykes as the agents by which the iron, silica and carbonates were mobilized and selectively re-precipitated.The dykes, which commonly occur within or marginal to the deposits, could thus be channels by which iron-chargedmagnesian solutions arose and entered the beds. The common occurrence of dolomite in association with the deposits also suggests the activity of magnesian solutions.

Conclusion Australianjaspilitic iron ores are in this paper classified on a geneticbasis into three types,which have their equivalents under different names in other parts of the world. Though probably far from comprehensive,it is felt that this classification could, with suitable additions, eventually form a basis for one of world-wide application,but this will have to await further knowledge of the genesis of these deposits and the adoption of a world-wideterminology.

Acknowledgement The author wishes to thank the management of Goldsworthy Mining Ltd for permission to publish this paper.

Résumé Les origines des minerais de fer jaspilitique d'Australie (R. T. Brandt) Les plus importants gisements de minerai de fer en Australie,avec des réserves potentielles s'élevant à des milliards de tonnes,sont ceux qui sont directement associés aux formations zonées précambriennes de fer ou jaspilites et, de

66

ce fait, ont été groupés sous la désignation de minerais de fer jaspilitique. Les formations de fer zonées, généralement désignées en Australie sous le nom de jaspilites, sont des membres remarquablesdes séries archéenneset protérozoïques du sud et de l'ouest de l'Australie. Elles sont en tout point comparables aux formations de fer du m ê m e âge dans les autres

The origins of the jaspilitic iron ores of Australia

parties du monde.Elles comprennentles hématites-jaspilites zonéeset les quartz-magnétitesoxydés superficiellement,les roches ferreuses carbonatées et silicatées.Les minerais primitifs (protores) de la plupart des gisements d'hématite sont constituésd'hématites et dejaspilites contenantde la magnétite. Quelques gisements limonitiquessont associés au faciès du bicarbonate et du silicate de fer. Les minerais de fer peuvent être classés en trois catégories sous les descriptions suivantes :(a) type ((croûte )); (b) type ((dérivé )); (c) type ((filon D. Gisements du type ((cuolite D. On les connaît aussi sous le n o m de type supergène ou type ((Lac Supérieur n. Ce sont les plus abondants.Ce sont essentiellement des croûtes superficielles d'hématite secondaire et de limonite formées in situ par lixiviation de silices provenant des lits des minerais primitifs au cours des désagrégationset décompositions présentes et passées. Quoique fréquents, ces dépôts sont discontinus et de dimensions, forme et profondeur très variables, car les facteurs déterminants de leur formation sont le climat et le drainage, la structure géologique et la nature physico-chimique des lits des minerais primitifs. Gisements du type ((dérivé 1). On les connaît aussi sous le n o m de gisements de Canga. Ce sont des accumulations sur les versants et dans les vallées de limonite provenant de jaspilite par décomposition et désagrégation mécanique et chimique,dont le résultat est l'élimination de la silice, l'hydratation de l'hématite originaleet des autres minéraux ferreux et le transport le long des pentes de la plus grande partie du fer soit en solution,soit c o m m e détritus rocheux ou les deux. Les dépôts vont des talus de limonite agglomérée aux accumulations épaisses dans les fonds de vallée

de limonite pisolitique sans matériel détritique. L'assodation étroite entre les gisements type (< croûte ))et ceux du type ((dérivé 1) laisse à penser qu'ils ont pris naissance dans des régimes climatiques identiques ou similaires. Gisements du type ((filon». C e sont des corps de remplacement à structure contrôlée constitués d'hématite massive dans la jaspilite, généralement en accord avec la stratificationet n'ayant en apparence aucune relation avec les surfaces de désagrégation et de décomposition présentes ou passées. Certains dépôts se présentent comme des lentilles abruptes ressemblant à des filonsmétallifères,d'autres sont du type des gîtes en selle au cœur des plissements. Le minerai d'hématite est chimiquement analogue à celui des dépôts du type ((croûte ))à haute teneur,quoique d'une pureté légèrement plus grande, et, par suite, il provient probablement de la même origine, à savoir les minerais primitifs eux-mêmes,par quelque processus de reconcentration secondaire du fer. Il paraît vraisemblable d'en attribuer l'origine à la redistribution métamorphique du fer et de la silicependant le plissement,sans pour autant exclure la possibilité soit d'une origine syngénétique,soit de processus supergenes au cours des cycles de désagrégation et de décompositions précambriens. Tandis que les gisements du type ((filon >) sont distribués sporadiquementdans lesjaspilites du sud et de l'ouest de l'Australie, sans relation avec la géographie, ceux des types ((croûte ))ou K dérivé ))ne se trouvent guère que dans la région tropicale du nord, qui a eu un climat chaud et humide dans le passé géologique récent. L a dépendance de ces derniers dépôts à l'égard du climat se trouve ainsi clairement démontrée.

Bibliography/ Bibliographie ALBERTS, B. C.;ORTLEPP, J. A.L. 1961. Iron ore mining in South Africa. Proc. 7th Commoniv. Mirz. Metall. Congr., South Afiica. Institution of Mining and Metallurgy in South Africa. BALDWIN, A.B.; GROSS, W.H. 1967.Possible explanationsfor the localization ofresidualhematite ore on a Precambrianiron formation.Econ. Geol., vol. 62,no,1, p. 95. BRANDT, R.T. 1964.The iron ore deposits of the Mount Goldsworthy area,Port Hedland district,Western Australia. Proc. Aust. Inst. Min. Engrs.,no.211,p. 157. -. 1966. The genesis of the Mount Goldsworthy iron ore deposits of northwest Australia. Econ. Geol.,vol. 61,no. 6, p. 999.

CATLEY, D.E.1963.Some aspectsof the genesis ofthe Iron Duke iron orebody and associated rocks. Proc. Aust. Inst. Min. Engrs., no. 208, p. 81. DANIELS, J. L. 1966.The Proterozoic geology of the north-west division of Western Australia. Proc. Aiwt. Inst. Min.Engrs., no. 219,p. 17.

DORR, J. Van N.II 1964.Supergeneiron ores of Minas Gerais, Brazil. Econ. Geol.,vol. 59, no. 7,p. 1203. --. 1965. Nature and origin of the high grade hematite ores of Minas Gerais, Brazil. Econ. Geol., vol. 60, no. 1, p. 1.

GROSS,W.H.; STRANGWAY,D.W . 1961.Remanent magnetism and the origin of hard hematites in Precambrian banded iron formation.Econ. Geol.,vol. 56, no. 8, p. 1345. HARMS, J. E.;MORGAN, B.D.1964.Pisoliticlimonitedeposits in northwest Australia.Proc. Ausi. Inst. Min. Engrs.,no. 212, p. 91. LABERGE, G.L.1966.Altered pyroclasticrocksiniron-formation in the Hamersley Range, Western Australia. Econ. Geol., vol. 61, no. 1, p. 147. LIDDY,J. C. 1968. The jaspilite iron ores of Australia. Econ. Geol., vol. 63, no. 7, p. 815. MCANDREW, J. (ed.) 1965.Geology of Australasian ore deposits. Melbourne, Australasian Institute of Mining and Metallurgy (8th Commonw. Min. Metall. Congr. Publ. no. 1). MACLEOD, W . N.1966. The geology and iron deposits of the Hamersley Range area,Western Australia. Bull.geol. Surv. W.Aust., no. 117. MATHESON, R. S.; ANDREWS, P. B.;BRANDT,R. T.;LIDDICOAT, W. K. 1965. Iron ore deposits of the Port Hedland district.In: J. McAndrew (ed.), Geology of Australasian ore deposits,p. 132.Melbourne,Australasian Institute of Mining and Metallurgy (8th Commonw. Min. Metall. Congr. Publ. no. 1).

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R. T.Brandt

MILES, K. R. 1954. The geology and iron ore resources of the Middleback Range area. Bull. Geol. Surv. S. Aust., no. 33. OWEN, H.B. 1964.The geology of the Iron Monarch orebody. Proc. Aust. Inst. Min.Engrs, no. 209,p. 43.

OWEN, H.B.;WHITEHEAD, S. 1965. Iron ore deposits of Iron

Knob and the Middleback Ranges. In: J. McAndrew (ed.), Geology of Australusiun ore deposits, p. 301.Melbourne,Australasian Institute of Mining and Metallurgy (8th Commonw. Min. Metall. Congr. Publ. no. 1). RUCKMICK, J. C. 1963. The iron ores of Cerro Bolivar,Venezuela.Econ. Geol., vol. 58, no. 2, p. 218.

Discussion J. Van N.DORR II. In Brazil at least much confusion has been caused by superimposition of supergene process on hypogene lode deposits. Such superposition obscures contact relations and criteria within ore bodies. R. T. BRANDT.This phenomenon of imposition of supergene processes on the sites of previous hypogene action is very common in Australia too. Pure lode type hematite bodies are not usually much affected by supergene action, but the adjacent jaspilites are, and may be converted to crust type ore enveloping lode type ores in such a way that it is difficult to tell where one begins and the other ends. It seems quite feasible that the hydrothermal wall-rock alteration associated with lode type ore deposits renders the jaspilites more susceptible to supergene action at a later date. Consequently, the two types of ore often occur together in intimate association. G.A. GROSS. In the case of the Middleback iron deposits what criteria do you suggest which require that these deposits are of hypogene and not of supergene origin?They appear to be remarkably similar to the Shefferville ore in Labrador, Canada, which are undoubtedly of supergene origin.

R. T.BRANDT.Criteria which are strongly suggestive of a hypogene origin, although not conclusive, are the metamorphosed state of the iron ore and associated rocks, the metasomatic replacementcharacter of the ore and its localization by tectonic structures, and the level of erosion, which has exposed the deepest parts of the folded structures

68

and demonstrates that the ore bodies now exposed were once buried to great depth.It is difficult to visualize a supergene process giving rise to deposits of this kind,which have been buried, metamorphosed and then exposed by deep erosion.

R.P. PETROV.Is it possible to refer to the third type you have mentioned as a lode type? Was the type formed by the filling up of cavities, or by the replacement of the host jaspilites?

R.T.BRANDT.The lode types ores were definitely formed by metasomatic replacements of jaspilites. Mineralogical and textural evidence indicates that this replacement took place at elevated temperatures under hypogene conditions. J. E. GAIR. Can you comment on whether deposits related to deep dykes occur at intersections of dykes and iron-formation that open upward toward the surfaceor downward? Clarification of this relationship might favour a supergene origin if the ore is in upward-opening structural traps, or a hypogene origin if the ore is in downward-openingstructural traps.

R.T.BRANDT.The dykes I have observed have all been vertical, transverse to the strike of steeply dipping jaspilites, and hematite ore bodies terminate laterally against them.Ifthese dykes did in fact act as structuraltraps which impounded mineralizing solutions,they do not provide any clues as to whether the solutions travelled upwards, downwards or sideways.

Occurrence and origin of the iron ores of India M.S. Krishnan Hyderabad (India)

Introduction

The total thickness of the succession is of the order of 2,000-2,500m . The rocks have been isoclinally folded with

India's iron deposits fall into three major types: (a) Banded hematite-quartzitesor jaspilites of the Lake Superior type and banded magnetite quartz deposits which may be considered a variant of this type. This is the most extensively developed and,at present,the only exploited type. (b) Sedimentary beds of siderite intercalated with beds of shales. (c) Magmatic segregations,generally lensoid in shape,consisting of titaniferous (and vanadiferous or chromiferous) magnetite associated with intrusive masses of gabbro or olivene bearing rocks. Figure 1 shows the distribution of the important iron deposits in India.

Banded hematite ores These constitute the major deposits in India and are being actively exploited at several places, both for internal consumption and for export. They are derived from and are closely associated with banded hematite-quartzite(BHQ) also known as banded hematitejasperorjaspilite.Themajor deposits are those of Singhbhum district of Bihar and adjacent parts of Orissa, Drug, Raipur and Bastar districts of Madhya Pradesh, G o a territory and Mysore. The BHQ in Orissa forms part of the sedimentary succession of the Iron Ore Series described by Jones (1934), consisting of the following formations: upper shales with volcanics;BHQs;lower shales;pink and purple sandstones with some limestones; sandy and conglomerate beds followed by phyllitic shales and tuffs and basic lavas. The most important group of this type occurs in South Singhbhum,Bonai,Keonjhar and Mayurbhanj districts of Bihar and Orissa (Fig. 2), the last being separated from others by the large exposure of Singhbhum granite. The Iron Ore Series is intruded by Singhbhum granite,which occupies a large area and contains numerous inclusions of the country rocks which have been metamorphosed and partly assimilated.The Iron Ore Series which is intruded by the granite is believed to have an age exceeding 2,100 m.y.

the axes in an NNE.-SSW.direction with steepWNW.dip. The BHQ is exposed as ridges along the anticlinal axes, while the shales occupy the less elevated synclinal portions. The most prominent fold, which is also marked along its crest by massive hematite deposits, extends from Gua (22"13'N.,85'23'E.) to Chendongra (21"43'N., 85'06'E.) over a distance of some 56k m .T w o other parallel anticlines also expose numerous deposits. At the northern end,these turn sharply eastward and follow the southern margin of the Singhbhuin thrust zone, but only incomplete sections are seen in this part. The thickness of the BHQ is rather variable,being of the order of 1,000m in the Korhadi river section in Bonai district,but only about 350 m in the main range on the border of Keonjhar and Singhbhum districts. The B H Q s consist of thin, parallel alternating layers of hematite and jasper or chert. The individual layers vary in thickness from 1 to 20 mm or more, but the average in typical exposures is from about 3 to 5 m m . In addition to the major folds and fractures seen in the whole formation, there are small-scalestructures shown by individual layers over smalldistances.The layers often show intricatefolding, contortion and.faulting on a minute scale. These may be attributed to plastic deformation and readjustments during consolidation of the strata and also during the process of enrichment which involved solution and transport of material.Although the layers are generally uniform in thickness, occasional bulging and thinning may be seen. The ore layers consist mainly of hematite which is massive or lamellar.In some bands small octahedralcrystalsand grains of magnetite are found.Some of this ore is of the character of martite. The silica bands consist of very fine-grained cherty or chalcedonic material whose colour varies from white through lavender and light red to dark red to brown and black. The colour depends upon the amount and the density of distribution of the iron mineral in each band. The iron in the siliceous bands occurs in the form of thin flakes and dust of red hematite or tiny grains and needles of magnetite. Sometimes these may be distributed in lenticular

Unesco, 1973. Genesis of Precantbrian iron und manganese deposirs. Proc. Kiev Symp., 1970. (Earth sciences, 9.)

'69

M.S. Krishnan

36;

68"

72O I

76O I

80' I

84' I

8$

92O

9$

100"

I

3Z0

2E

24

20

I 6"

12"

9O

FIG.1. Map of India showing the more important iron ore deposits.Bihar and Orissa: 1. Singhbhum;2.Bonai &. Keonjhar; 3. Palamau.Bombuy: 4.Goa; 5.Ratnagiri. Central Provinces:

6.Chanda-Drug;7.Bastar. Hyderabad: 8 , Adilabad. Madras: 9. Salem; 10. Kurnool. Mysore: 11. Bababudan. Putialu: 12. Narnaul. Himachal: 13. Mandi.

or irregularclots.Inthe depositsat Noamundi,Jones (1934) has recorded the presence of occasional rhombic shapes in silica which he regards as pseudomorphs of silica after siderite. Some of these crystals were actually found to be colourless or light grey siderite. Such pseudomorphs are, however, not common in the oxide deposit. In a few deposits in Orissa iron carbonate is prominently developed in the banded iron-formation.Forinstance Acharya et al. (1968) have described the deposit of Kanill (21°45'N., 85'5'E.) in which the alternating dadhar H bands are from 1 to 2 mm thick,being composed of tiny euhedral crystals of colourless to pale grey siderite which are from 0.2to 0.35 mm across and show light brown colouring along their margin. Quantitative measurements under the microscope indicate that the three constituents of the different bands, namely siderite, chalcedony and hematite,are present in roughly equal amounts.Occasionally crystals of magnetite are found amidst the siderite or hematite bands. Similar observations have also been made in a few other deposits in Orissa.

Under the microscope the siliceous bands are seen to consistof very fine-grainedquartz showingunduloseextinction. The silica bands contain ñakes of red hematite and grains of dark magnetite and sometimes martite. When hematite ñakes are abundant the silica band assumes a red or brown colour.Occasionally small crystals of siderite are also present. Spencer and Percival (1952) have recorded the occurrence of micro-spheruliticstructure in the hematite bands, which they attribute to the shrinkage of original colloidal hydroxide during consolidation of the BHQ. They have also noted that the B H Q is free from clastic sediments, a fact confirmed by other observers. This characteristic indicates that the BHQ was deposited in quiet and fairly deep waters some distance from the shore. There is lateralvariation within the BHQ. It may pass into the solid thick band of hematite without any silica or into shaly-lookinghematite or into lenses of fine-grained dusty nearly blue-black crystalline hematite mixed with a certain amount of martite. Partially enriched masses of

70

Occurrence and origin of the iron ores of India

15'

I

-

22

oc

-42 FIG.2. Iron ore deposits of the Singhbhum-Keonjhar-Bonairegion,Bihar and Orissa.

BHQ may also be found amidst rich hematitic ore. The analyses given in Table 1 by Percival (1931) indicate such

B A S T A R DISTRICT (MADHYA PRADESH)

partial enrichment.

According to Crookshank (1938) the deposits of the Bailadila Range occur along two parallel ridges,separated by a valley. This area is located between 18"35' and 18'45'N. and roughly along 81"13'E. (Fig. 1). The two ridges are synclinal,while the valley is along an eroded anticline.The deposits occur in the BHQ and the immediately underlying ferruginous schists of the Bailadila Series which are of Precambrian age. The BHQ has a thickness of 400-500m.

TABLE 1. Analyses of hematite jasper (percentages)

Fe SiO, A1203

I

II

III

20.60 69.00 2.01

30.50 54.24 1.47

52.30 22.30 3.56

71

M.S. Krishnan

, I

.. .

i

FIG.3. Deposits of banded magnetite-quartzites of Salem and neighbouring areas, Madras. 72

"'I i

Occurrence and origin of the iron ores of India

There are fourteen large hematite deposits located on the two ridges which run practically N.-S. The ore at the surface is generally massive and compact,but in some cases a few feet at the top is composed of porous hematitelooking like laterite,but really of high grade. The high grade ore at the outcrop yielded on analysis 66-68 per cent Fe,0.060.12 per cent P and less than 0.05 per cent S. Similar deposits also occur at and near Rowghat along two ridges which run N.-S. on the east and west of Kolur (19"55'N., 81'8'E.; Fig. 1).

iron enrichment has been only partial. These ores contain only 40-45 per cent Fe and are therefore not worked at present.

Banded magnetite quartzites Deposits of these rocks are found mainly in southern India, particularly in southern Mysore, in the district of Salam and Tiruchirapalliof Madras and inthe Guntur and Nellore district of Andhra.

D R U G DISTRICT ( M A D H Y A PRADESH)

MADRAS

Hematite deposits derived from BHQ occur at four or five places in the Rajhara H i l l (2Oo34'N.,81"5'E.,Fig. 1) and its neighbourhood along a zig-zagridge several kilometres long. The average ore at and near the surface contains 65-69per cent Fe,0.5-2.0per cent SO,,0.10-0.20per cent M n , 0.05-0.07per cent P and 0.05per cent S. G O A A N D MYSORE

Similar ores associated with BHQ are found in the Dharwarian formations of the Dharwar district (now in Mysore State) and in the G o a territory (Fig. 1). In the Dharwar district the BHQ forms part of a succession consisting of chlorite and hornblende schists, phyllites, conglomerates and quartzites. The deposits in this area are small and unimportant. Dharwarian formations similar to those of the Dharwar district occur also along the border between Ratnagiri and Goa and at three or four places within the Goa territory (Bicholim, Sirigao, Kosti, etc.). In all these deposits the original BHQ has been converted into hematite. The deposits are, however, not as rich as those of Orissa and Madhya Pradesh,but contain 58-62 per cent Fe. Some of the material may be rather flaky and schistosehematite and lateritic in appearance. Several deposits of hematite associated with BHQ are known in central and northern Mysore, where they form part of the Dharwarian rocks. In some places the banded rocks also contain magnetite. In southern Mysore particularlythe BHQ is found to have been converted to magnetite quartzite because of the metamorphism to which it has been subjected. A synclinorium of Dharwar formations including BHQ phyllites and amphibolites occurs in the Sandur area of Bellary district. The structure is tightly folded along NNE.-SSE. axes,with a steep dip towards the ENE.T w o large groups of hematite deposits have been developed along two parallel ridges. The associated phyllites contain numerous secondary manganese ore deposits. A few depositsalso occur in the Chityal Hills (19"5'N., 78"45'E., Fig. 1) and their neighbourhood in Andhra Pradesh.This group contains both hematite and magnetite in varying proportions and the grade of ore is poor as the

Banded magnetite quartzites occur as part of the sedimentary successionwhich has suffered regionalmetamorphism. The various associated rock types are chlorite-and micaceous quartz-schists, quartzites, phyllites etc. There are generally some metamorphosed basic igneous rocks in the older part of the succession,these being now seen as amphibole schists or amphibolites with or without garnet. The rocks in this region are folded along NNE.S S W . direction and two or three parallel bands are found on the flanks of the folds. The structures are cut across by well-marked faults at the southern end along the Attur valley, where a few ore bodies occur in a sheared and disturbed condition. Several hillocks expose magnetitequartzites very prominently and in some cases they form perpendicular cliffs sometimes 150 m high,as in Godumalai 78"22'E.; about 16 m east of Salem town (11"38'N., Fig. 3). In the deposits of Perumamalai some 10 km east of Godumalai the ore bands are found to be sheared and disturbed.The individuallayers are up to 2 c m in thickness. The magnetite shows alteration to niartite along octahedral planes. According to Gokhale et al. (1961) the associated rocks show sedimentarycharacterssuch as current bedding. The hill called Kanjamalai (Fig. 3), 7 k m long and 4 km wide,is situated 10 kni W S W .of Salem town. It is a basinshaped structure in which all the exposures are concentric and show dips towards the centre.The rocks exposed in the hill are amphibolites(which are usually garnetiferous) at the base,overlainby magnetite-quartzites,sericite and chloriteschists, phyllites and talc-schists,amidst which are found two other bands of magnetite-quartzitesin the higher part of the succession.This structure is flanked on either side by the Peninsular Gneisses,whose age is probably around 2,500m.y.The succession in the Kanjamalai appears to be younger than the gneisses. The three parallel bands of magnetite-quartzitesin the hill have thicknesses of 30 m, 1 O " m and 10 m respectively. The magnetite-quartzite formation is conspicuously banded like the BHQ,but the individual bands are more irregularbecause of movements during metamorphism.The magnetite and quartz grains are medium to coarsely crystalline and the bands show appreciable variation in thickness, even within short distances.A few bands show the presence of grunerite, which has apparently been formed 73

M.S. Krichnan

by the reactionofthe magnetite with silica.Such occurrences are more common in southern Mysore and Bastar. The magnetite quartzite constitutes the ore which contains 3550 per cent Fe (38 per cent average), 41-56 per cent SiO,, 0.2-2.7per cent Al,O,,0.1-1.5per cent lime, 0.1-2.6per cent M g O , 0.017-0.193per cent P and negligible S. The rock can be easily crushed and the magnetite concentrated electromagnetically.The degree of fineness of crushing to free all the magnetite is variable, but grinding to minus sixty mesh size is adequate in most cases. The magnetite concentrate is rich in iron, the content ranging usually between 60 and 65 per cent.

MYSORE

A few deposits of magnetite quartzite occur, especially in southern Mysore, of which the more important are the Kudremukh deposits near the western coast and a few in Tumkur and Mysore. A N D H R A PRADESH

In the Nellore and Guntur districts of Andhra Pradesh there are several hillocks showing bands of magnetite quartzite. They are found between 15'15' and 15O48'N.; 79"27'and 8O003'E. along a narrow arcuate belt. They were originally described by Foote (1879) and form part of the Precambrian succession,consisting of quartzites and micaceousschistssurroundedby granitic gneisses,charnockites and amphibole schists.They occur in two groups of several exposures (a) the Gundlakamma group in the north and (b) the Ongole Group in the south near the town of Ongole (15"30'N., 80O3'E.). The southern group appears to be continuous,although covered by soil between the hillocks. It shows two or three bands of magnetite quartzite of which the middle or the upper one may be the thickest.The rocks form an anticlinorium with its axis trending NNE.-SSW. The folds also plunge to the NNE.and have a general steep easterly dip. The northern group of deposits turns sharply towards the west and apparently formspart of the plunging folds. Sastry (1967)has given a general descriptionof individual exposures and their possible structure. According to Sastry et al. (1968), who described the southern group,the fold axes trend N.-S.with cross folding movement along NNW.-SSE. axes. The final folding was imposed on the strata with NNE.-SSW. axes. The magnetite quartzites are interbeddedwith garnetiferous quartzites, hypersthene quartzites and ferruginous schists. The magnetite quartzite passes along the strike into the garnetiferousquartzite or pyroxene-bearingmagnetite rocks.The pyroxene is variablein compositionand may be ferro-hypersthene salite or jeffersonite. In the hypersthene-bearingrocks the magnetite is occasionally bordered by thin rims of garnet.Green spinelis also seen in a few thin sections. The mineral assemblage as well as the textural charac14

ters show that the rocks have been subjected to medium to high grade metamorphism resulting in the production of hypersthene and garnet. The average rock contains about 35 per cent Fe,the rest being mainly silica.It is possible to concentrate the magnetite by electromagneticmeans,bringing the iron content to about 60 per cent.

B A S T A R D I S T R I C T( M A D H Y A PRADESH)

The Bailadila series, associated with large hematite deposits, lie to the west of, and apparently superimposed upon, the Bengal series. These consist of amphibolite, quartzite,crystallinelimestone and a variety of schists. The youngest formations lying on top of the hills east of Bailadila are banded magnetite quartzites. According to Chatterjee (1968) the bands consist of alternating layers of quartz and magnetite. Cumingtonite and an amphibole, identified as ferro-hastingsite,occur amidst the magnetite. Grunerite is fairly common and it may often contain lamellae of martite. The minerals in the bands show marked parallelism to the banding. The amphiboles are elongated and are sometimes poikilitic with inclusionsof magnetite.The amphiboles have resulted from the interaction between magnetite and silica. Occasionally almandite and hastingsite are found, indicating that the original rock contained some alumina and calcareous constituents. Bands of magnetite quartzite are also found intruded by granite in thisarea.Riebeckite and aegirineare developed in the magnetite quartzite as a result of metasomatism during the intrusion of the granite. It is of interest to note that the magnetite-quartzitesdo not show any hypersthene in this region,leading to the inference that the temperature of metamorphism was not high enough for its formation.

Sedimentary siderite deposits Clay ironstone derived from sedimentary siderite beds occurs mainly in the Ranigunj coalfield of West Bengal and the Auranga coalfield of Bihar,some distance to the west. Early surveys showed that the siderite beds occurred in the stratigraphic unit named the 'ironstone shales', which form the middle part of the Lower Gondwana group. The ironstone shales, which are of Middle Permian age, lie above the Barakar series and below the Raniganj series, both of which are coal-bearing.The estimated thickness of the Ironstone shales was approximately 420-450 m. Data from outcrops and from a shaft 15 m deep, showed the ironstone(limoniteand goethite) to form thin layers5-25 c m thick,the proportion of the material in the formation being about 6 per cent. Later examination of this formation by Hughes (1874) and by Walker (1914) established that: the Ironstone shale formation has a total thickness in the Ranigunjcoalfield of 300 m and can be traced over a length of 53-55 km; the ironstone bands are intercalated with shales;the layers of ironstone do not persist for long dis-

Occurrence and origin of the iron ores of India

tances either along the strike or dip, but when one layer disappears another begins to appear slightly above or below. From numerous analyses of the material which was used as iron ore in blast furnaces at Kulti in the same coalfield the range and average composition is given in Table 2. TABLE 2. Analyses of ironstone,Ranigunj coalfield Constituent

Iron Manganese Silica Phosphorus Moisture

Range (per cent)

Average (per cent)

39 .OO-47.70 0.57- 3.62 16.00-21.81 0.23- 1.37 1.00- 5.10

45.20 1.85 18.O5 0.72 1.77

As these analyses are old they do not show the content of alumina,sulphur and other constituents.These oreswere replaced by hematite from the newly discovered banded ferruginous formations in Orissa from about 1914. At a depth of 15-20 m the limonitic ore changes into siderite which is generally granular in texture. This shows that the material as deposited was siderite and that the carbonatewas converted into hydroxideby meteoricwaters. The sideritebeds are generally fairly pure,but in some cases are mixed with a small amount of detrital clay and sand. The repeated occurrence oflayersof sideritein the Ironstone shale formation indicates that deposition was in shallow waters under reducing conditions,but subject to periodic inundations of clastic material. Similar beds of Ironstone shaleshavealsobeennoticedto occur intheAurangacoalfield in southernBihar, along the same tectonic trough in which the Ranigunj and a few other coalfields are now located. The total area covered by this formation in the Ranigunj coalñelds has been estimated as a little over 100 km2.

converted into steatitic material by post-magmaticchanges. The augite in the gabbro is usually uralitized. As the area is thickly forested the exposures are not good, but the distribution of magnetite debris gives an idea of the extent of the deposits. The magnetite ore bodies are composed of magnetite with subordinate hematite. Polished sections of material viewed under the reflecting microscope show the presence ofmuch magnetite enclosinglamellaeofilmenite,coulsonite (Fe-V-oxide), hematite, rutile and goethite and a little apatite. The occurrence of ilmenite along the octahedral planes in the magnetite suggests that the two minerals originally formed solid solutions and that the ilmenite was exsolved on cooling. Coulsonite occurs as minute grains or needles closely associated with the ilmenite. Sometimes magnetite and ilmenite show graphic intergrowths. Dunn and Dey (1937) postulated that part of the hematite may also have been originally present in solid solution in the magnetite. The magnetite ore contains both titanium and vanadium. The vanadium oxide content generally ranges between 0.6and 4.84 per cent.The titanium oxide content is much more, ranging from 10 to 25 per cent. Similar deposits are also present in the Simlipal Hills of the Mayurbhanj district,but they have not been investigated in detail.The deposits near the Singhbhum-Mayurbhanj border are believed to be large enough to yield a few million tons of magnetite ore. The close association of the magnetite with gabbroid and olivine rocks,and its titanium and vanadium content clearly indicate that it is a product of segregationfrom ultramaficmagmas. These ores are now receiving attention for the extraction of their vanadium content for the manufacture of ferro-vanadium.

MYSORE

Titaniferous magnetite Numerous large lensoid and vein-likebodies of titaniferous magnetite occur near the border of the Singhbhum and Mayurbhanj district of Bihar and Orissa. Though their occurrence has been known since 1908, they were described in some detail much later by Dunn (1937). Several occurrences are found between 22'16' and 22'29'N.; 86"15' and 86"20'E. They are found traversing gabbros and serpentinized olivine bearing rocks which have in some cases been

Several lensoid bodies of magnetite occur in or along the borders of ultramafic rocks such as gabbros, peridotites, saxonites,etc.,in parts of Mysore. They contain approximately 60 per cent Fe, 1 per cent SiO,, less than 2 per cent Alzo,and very low S and P.The titanium oxide content ranges up to a maximum of 12 per cent and there is always a small quantity (less than 3 per cent) of chromic oxide.A little vanadium may also be present. These ore bodies are small in dimension and may have only a limited industrial importance.

Résumé Manifestations et origine des minerais de fer de l'Inde

(M.S. Krishnan) On ne rencontre pratiquement de gisements de minerai de fer que dans la partie péninsulaire de l'Inde. Ils sont de

quatre types :(a) jaspe-hématiterubané ; (b) quartzite-magnétiterubanée ;c) magnétite titanifère ;d) carbonate de fer sédimentaire décomposé en limonite. Les trois premiers datent du Précambrien tandis que le dernier est de l'âge du Gondwana inférieur (Permien).

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M.S. Krishnan

Les strates de jaspe-hématiterubané ont été déposées dans la succession sédimentaire du système de Dharwar de Mysore, dans les séries de minerai de fer d'Orissa et dans les séries de Bailadila de Madhya Pradesh,c'est-à-dire à une époque comprise entre 2,3et 2,5 milliards d'années. Dans-tous les cas,la succession contient des laves basiques d'origine sous-marine en m ê m e temps que des grès, des argiles schisteuses et des roches ferrugineuses.Les straticules dans les roches ferrugineuses sont alternativement de l'hématiteet du jaspe,dont l'épaisseur varie de 3 à 5 mètres, mais avec certainesvariations. Ces strates et les minerais de fer qui en dérivent sont du type 'Lac Supérieur', avec toutefois cette exception que dans quelques cas les dépôts originaux contenaient de la sidérite au lieu d'hématite et que les silicates de fer sont virtuellement absents. Il semble raisonnable d'assigner une origine volcanique sous-marine à une partie du fer. La formationferrugineuse a été transforméeen gisements de riches minerais d'hématite compacte à la surface.Elle peut atteindre une profondeur dépassant 100mètres. Les lentilles de minerai brun foncé poussiéreux (poussière bleue) qu'on trouve à une certaine profondeur dans la plupart des gisements sont attribués à la cristallisation de l'hydroxydeferriqueoriginalen hématitefinement cristallisée pendant une période de lixiviation des dépôts par les eaux météoriques. C e minerai poussiéreux est cons-

titué presque uniquement d'hématite pure avec, dans certains cas, des traînées de kaolin blanc. Le jaspe-hématiterubané a été transformé en quartzitemagnétite rubanée dans quelques-unsdes dépôts de la partie méridionale de Mysore et dans les régions de Madras et d'Andhra où ils ont subi un métamorphisme modéré. Ce sont des couches à cristaux grossiers,mais en tout cas très semblables aux couches de jaspe-hématite. Dans le Singhbhum oriental (Bihar), dans la partie sud de Mysore,on rencontre des lentilles de magnétite titanifère associées avec des roches ultramafiques telles que des pyroxéniteset des gabbros.Ces lentillessont apparemment des ségrégations magmatiques. L a magnétite contient jusqu'à 15 % d'oxyde de titane ; elle contient aussi jusqu'à 2 % d'oxyde de vanadium dans le Singhbhum et une quantité équivalente d'oxyde de chrome dans le Mysore. Ces minerais n'ont pas encore été exploités. On trouve des minerais de sidérite sédimentaire au Bengale-Occidental,dans un horizon stratigraphique qui se situe entre les formations de Barakar et de Ranigunj avec présence de charbon. L'épaisseur de cette formation est de l'ordre de 300 mètres. Elle consiste en de nombreux rubans minces de minerais sidéritiquesentremêlés à l'argile schisteuse,la straticule de sidérite totalisant le dixième de l'épaisseur totale.

Bibliography/ Bibliographie ACHARYA, S.; AHMED, S. I. S.; SARANGI,K. 1968, Carbonate

__ . 1893. The iron ore resources and iron industries of the

facies of iron-formationin Kandadhar Hills, Orissa. Bull. Geocliem. Soc.India, vol. 3,no. 2. CHATTERJEE, A. 1968. Petromineralogy and stability relations of metamorphosed iron-formationsand amphibolites of an area east of Bailadila Range, Bastar district,M.P. Quart. J. Geol. Soc. India, vol. 40,p. 257-74. CROOKSHANK, H. 1938.The iron ores of Bailadila Range,Bastar State. Trans. Min. Geol. Inst. India, vol. 34,p. 255-82. ' DUNN, J. A. 1937. Mineral deposits of Eastern Singhbhum. M e m . Geol. Surv.India, vol. 61,no. 1, p. 214-23. DUNN, J. A.;DEY, A.K.1937.Vanadium bearing titaniferous iron ores of Singhbhum and Mayurbhanj. Trans. Min. Geol. Inst. Indfa, vol. 31,no. 3, p. 117-83. FOOTE, R. B. 1879. On the geological structure of the eastern coast from latitude 15" to Masulipatam. Mem. Geol. Surv. India, vol. 16,no. 1. GOKHALE, K.V. G. K.; BAGCHI,T.C. 1961.Preliminary investigation of banded iron ore formations of Perumalai Hills Salem district, Madras. Quart. J. Geol. Soc. India, vol. 33, no. 2,p. 49-53. HOLLAND, T. H. 1892.Preliminary report on the iron ores and iron industries of the Salem district.Rec. Geol. Surv. India, vol. 25, p. 136-59.

Southern districts of Madras Presidency.Imp. Jnst. Handbook of Commercial Products (Calcutta), no. 8,24 p.

76

HUGHES, T.W.H.1874. Notes on the raw inaterials on iron smelting in the Ranigunj coalfield. Rec. geol. Surv.India, vol. 7,p. 20-3, 122-4. JONES, H.C.1934.Iron ore deposits of Bihar and Orissa.Mem. geol. Surv. India, vol. 63, no. 2. KRISHNAN, M . S. 1954.Iron ores,iron and steel,Bull.geol. Surv. India, vol. 9. PERCIVAL, F.G. 1931.The iron-oresof Noamundi. Trans.Min. geol. Inst. India,vol. 26,p. 169-271. SASTRY,A. V. R.;VAIDWYANADHAN, R. 1968. Structure and petrography of the quartz-magnetiteand associated rocks of Vemparla area, Nellore district.J. geol. Soc. India. vol. 9, no. 1, p. 49-51. SASTRY,T.H.1967.Some structuralfeaturesof Ongole bandde magnetite quartzites. J. Indian Geosci. Ass., vol. 7,p. 67-84. SPENCER,E.;PERCIVAL, F. G.1952. The structure and origin of the banded hematite-jaspersof Singhbhum, India. Econ. Geol., vol. 41,no. 4,p. 365-83. WALKER, H.1914.Note on the Geological Survey of Raniganj coalfield. Trans. Min. Geol. Inst. India, vol. 7,p. 226-79.

Precambrian iron ores of sedimentary origin in Sweden R. Frietsch Geological Survey of Sweden,Stockholm

Introduction In Sweden Precambrianiron ores with sedimentaryfeatures are encountered in two separate geographical regions, namely central and northern Sweden. Only the ores of central Sweden have been mined and, due to their low content of phosphorus and sulphur, these were the main source of iron in Sweden until the end of the last century, when the basic steel-makingprocesses made it possible to utilize the apatite-richores of the country.

Non-apatitic iron ores of central Sweden The following presentation of the non-apatiticiron ores of central Sweden is mainly based on the papers of Geijer and Magnusson (Geijer and Magnusson 1944, 19520, 1952ó; Magnusson 1953,1960).These ores areknown ina very great number of deposits which occur in a broad, semi-circular zone west of Stockholm.The ores belong to a metamorphic volcanic-sedimentarycomplex of Svecofennian age, which in this part of Sweden is the oldest unit of the Precambrian. The complex begins with a sequence of acid volcanics with intercalations of limestone-dolomiteand clastic sediments. The iron ores lie in these rocks. This volcanic sequence is followed by an upper section mainly consisting of detrital sediments. The volcanics are divided into a lower part with an extremely sodic, quartz-keratophyriccomposition and an upper part ofpredominantly potassic,rhyolitic composition. The volcanics are called hälleflintas, leptites or leptitegneisses according to their grade of inetamorphism.Best preserved are the hüllefliirztas,which are only found in restricted areas, The supracrustalrocks were folded in connexioii with the intrusion of the oldest group of Svecofennian granites. These form concordant intrusions,consisting of a differentiated series the first members of which are gabbros and

diorites. The volcanic-sedimentarycomplex was metamorphosed,the hülleflintas being altered to leptites.During this epoch the rocks were subject to widespread metasomatic alterations.As the main element added through this action was magnesium, the process has been called ‘magnesiametasomatism’. The leptites have, by this process, been changed into rocks characterized by minerals rich in magnesium and ferrous iron (i.e. cordierite and anthophyllite). After a non-orogenicperiod marked by the intrusionof greenstone dykes there occurred a migmatization of the supracrustalrocks in connexion with the intrusion of the late Svecofennian granite group. The granites cross-cut the structures and are accompanied by large amounts of pegmatite. The age of the pegmatites,which represent the last phase of the Svecofennian orogeny,is about 1,800m.y. (Welin and Blomqvist, 1964). The non-apatiticiron ores of central Sweden all occur in the lower,volcanic section of the volcanic-sedimentary complex and are divided into two main groups, namely quartz-banded ores and skarn- and limestone ores. The ores of the latter group are further divided into a nonmanganiferous type and a manganiferous type, the limit put at 1 per cent Mn. This manganese limit has not only a chemical-technicalmeaning,it also divides the ores stratigraphically:the manganiferous type occurs in the potassic leptites and the non-manganiferous one is in most cases found in the sodic volcanics.

Q U A R T Z - B A N D E D ORES

These are characterized by a regular alternation of thin layers of iron oxide and quartz.The iron oxides are hematite or magnetite or both. Magnetite which has usually been formed from hematite indicates a higher degree of metamorphism.The content of iron varies between 30 and 50per cent;phosphorus (as apatite) is in most cases very low and manganese is usually lower than 0.1per cent. Besides the iron oxides and quartz,these ores contain

Unesco, 1973. Genesis of Precambrian iron and manganese deposits. Proc. Kiev Synip., 1970. (Earth sciences, 9.)

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small amounts of gangue forming layers or irregular aggregates.Where the ore is associated with carbonate rocks the later metamorphism has given rise to skarn silicates such as andradite, diopside and tremolite. Where leptite material has been present in the ore, epidote and hornblende may belong to the skarn association. According to Geijer and Magnusson (19526) the quartz-bandedores of central Sweden were originally deposited as chemicalsedimentsand the sourceof the iron and silica must be sought in volcanic emanations. The original iron mineral is believed to have been hematite or,in part, limonite, the well-preservedbanding and the purity of the bands excluding minerals like siderite or greenalite. The skarn silicatesmentioned above are formed through internal reactionsbetween iron,silicaand carbonatematerialand due to higher temperatures caused by the intrusion of the older granites.Inconnexionwith the magnesia-metasomaticalterations there has further been a rather intensive formation of skarn,which is richer in magnesium than that mentioned above.

S K A R N A N D LIMESTONE ORES P O O R IN M A N G A N E S E

These form a rather inhomogeneous group low both in phosphorus and sulphur. The ore mineral is magnetite which is more or less intimately associated with skarn minerals or carbonates.The distributionof magnetite and skarn minerals is usually very irregular, but in some deposits a fairly regular stratification of ore and skarn occurs. The skarn is of two different types,rich either in C a or in Mg. T o the Ca-rich skarn type belong andradite, diopsidehedenbergite and actinolite, which are always associated with limestones or dolomites. The Mg-rich skarn type is characterized by anthophyllite-gedrite,cummingtonite,talc,forsterite,humite minerals and serpentine.The Mg-rich skarns are younger than the Ca-richskarns being alteration products of the Ca-rich skarns formed in connexion with the magnesia-metasomatism.

LIMESTONE ORES

The limestone ores differ from the skarn ores by having little or no skarn. They consist of magnetite and limestone or dolomite. This type of ore shows a fairly distinct stratification and is thus considered to be of sedimentary origin.

MANGANIFEROUS SKARN ORES

These are mostly developed as limestones ores. They are low in phosphorus. The content of sulphur is in several depositshigher than 0.2per cent.The ore mineral is magnetite. The main skarn minerals are spessartite,dannemorite, knebelite or manganiferous fayalite.A pronounced strati78

ficationis a rather common feature,and these ores are also considered to have been originally sedimentary. The skarn is a product of internal reactions, as in the quartz-banded ores. The origin of the skarn iron ores poor in manganese has been the subject of much discussion. According to Geijer and Magnusson (1952a, 19526), the skarn ores might be either sedimentary deposits later affected by a regional metamorphism or true pyrometasomatic deposits. In the first case the skarn originated through internal reactions betweenthe present iron,silicaand carbonatematerial. In the second case the skarn is a result of the addition of iron, magnesia and silica by magnesia-metasomatism to pre-existingsedimentary iron ores or limestones-dolomites. For some of the deposits with a Mg-rich skarn there is strong evidence for an origin by replacement,as they contain E-or F-bearingminerals (ludwigite,fluoborite,humite) typical of contact metasomatic deposits. Regarding the origin of the skarn ores poor in manganese, Geijer and Magnusson (19526) stated that ‘mostof the economically important deposits belong to the originally sedimentary ones, but are more or less intensely “worked over” and rearrangedinconnexion with the intrusionofthefirst group of Archaean granites,possibly with some additions of iron’. Later Magnusson (1953,1960) modified this view and indicated that all skarn ores might have been sediments from the beginning. This opinion is mainly based on the fact that there are transitions between the quartz-banded ores and the skarn ores and between the skarn ores and the limestone ores. Geijer (1959), however,defended the existence of the ‘primary skarn’ores by stressing,among other things,the importance of the borate minerals.

Non-apatitic iron ores of northern Sweden The non-apatiticiron ores in northern Sweden occur in the Norrbottencounty,and most of them lie within a wide zone extending roughly E.-W.on both sides of Kiruna. Scattered occurrences are found to the south of this zone. The ores occur in supracrustalrocks of Precambrian age. On the regional m a p of Norrbotten (Qdman, 1957) the Precambrian was divided into an older, Svecofennian (Svionian) cycle and a younger, Karelian cycle.In the iron ore zone detailed mapping during the last decade (Offerberg, 1967; Padget, 1970) has required abandonning the subdivision into Svecofennian and Karelian cycles as outlined by Ödman. A m o n g the supracrustalsfour different groups can be discerned, but no major unconformity has been observed between them. A tentative stratigraphic scheme for the northern part of Norrbotten is given in Table 1. The oldest supracrustal rocks belong to a greenstone group which is mainly built up of greenstones and porphyrites. The greenstones are mainly spilitic, often pillowbearing effusives of basaltic composition.In the greenstones

Precambrian iron ores of sedimentary origin in Sweden

TABLE 1. Stratigraphic scheme for the northern part of

Norrbotten county Age (Million years)

Iron ores

Granite group (granites (‘Lina granite’) with pegmatite and aplite) Quartzitegroup (quartzitic sandstoneswith conglomer1,605-l,635 ate and phyllite) Apatite iron ore Porphyry group (acid and (Kiruna type) intermediatevolcanicswith intercalations of basic volcanics) 1,880 Granodioritegroup (granodioriteswith dioriteand gabbro) Schist-conglomerategroup (mica-schistswith intercalations of biotite-richquartzites, and conglomerates) Greenstone group (basic vol- Skarn iron ores canics with intercalations of and quartzdetrital and chemical sedibanded iron ments) ores 2,800 Granite north of Kiruna . 1,540

occur, mainly in the stratigraphically higher parts, intercalations oftuffs,tuffites,phyllites,graphite-bearingschists, limestones-dolomites,mark and cherts. The porphyrites, which are most widespread near and west of Kiruna,have a basaltic or andesitic composition.T o the north of Kiruna the greenstones are underlain by a granite which forms the basement of the volcanics.The granite has an age of about 2,800 m.y. (unpublished radiometric determination by Kouvo). The greenstone group is overlain by mica-schistsand conglomerates of moderate thickness. This schist-conglomerate group has a ratherrestricted extension,but is a distinct marker horizon.It is succeeded by a porphyry group which, in the iron ore-bearing zone, occurs mainly in the central and western parts. This group is built up of predominantly sodic volcanics of rhyolitic or keratophyric composition. Age determinations show that the volcanics at Kiruna and westwards have an age a little over 1,600m.y. (Welin, 1970).

The porphyry group is overlain by a quartzite group of restricted extent. The quartzites contain intercalations of conglomerate and, occasionally,phyllite. In the iron ore-bearing zone two groups of intrusives can be discerned besides the ‘basement’granite north of Kiruna. The older intrusive group, formerly called the Haparanda granite series by Ödman (1957), is a differentiated series with gabbro,diorite and granodiorite,of which the last-mentionedis the most wide-spread.The rocks of the granodioritegroup form concordantintrusionsin the greenstonegroup and the schist-conglomerategroup and intersect them.The geologicalrelationship to the porphyry group is not known. Age determinations made on gabbros and

granodioritesin the southern part of the Norrbotten county, show an age of 1,880m.y. (Welin,1970). This means that the granodioritegroup is older than the porphyry group and the quartzite group. The younger intrusivegroup which cuts all supracrustal rocks and the rocks of the granodiorite group, is built up of granites, Radiometric age determinations of the Lina granite,which is the most widespread one of the somewhat different granites that belong to this group,has given an age of 1,540 m.y. (Welin, 1970). T w o groups of non-apatiticiron ores can be discerned in Norrbotten,namely the quartz-bandediron ores and the skarn iron ores.

QUARTZ-BANDED ORES

These are quartzites in which magnetite and skarn minerals occur in a more or less banded fashion. The quartzites are locally rather high in iron,but the average grade is mostly low and seldom exceeds 20 per cent. The most common skarn minerals are cummingtonite-grünerite,clinoenstatitehypersthene,hornblendeand almandite.The magnetite and the skarn minerals are often accompanied by small amounts of pyrite and pyrrhotite.The content of phosphorus is less than 0.1 per cent. Manganese seems to be low in most deposits. The quartz-bandedores occur in connexion with detrital or chemical sediments in greenstones of the greenstone group,usually in the stratigraphicallyhigher part of it. Partly the ores are found very near the bottom of the schist-conglomerategroup.The ores,which form layer-like bodies concordant with the strike of the host rock,are often associated with limestones or dolomites or occur in the same stratigraphic position as these rocks, The quartz-banded iron ores of northern Sweden are considered to have been originally (chemical) sediments. Geijer’s (1925) observation that small, rounded aggregates pigmented with magnetite occur in the Käymäjärvi deposit is of genetic interest. These are most likely metamorphosed granules of greenalite or some other iron silicate.Otherwise there is no knowledge of the original iron mineral. These ores were formed by a deposition of iron and silica with varying amounts of limestone or dolomite in the final stage ofthe basic volcanism that gaverise to the greenstone group. Through later metamorphic processes the iron-bearingsediments recrystallized and the carbonatematerial reactedwith iron and silica, giving rise to the skarn minerals. Quartz-banded iron ores also occur outside the iron ore-bearingzone, in the southern part of the Norrbotten county,but differ somewhat from those described above. The ores are not associated with basic volcanics but with acid ones. The immediate wall rock may be made up of sediments,such as mica-schistsor quartzites, but the ores occur also directly in the volcanics. The most common skarn minerals are diopside, tremolite-actinolite,garnet, epidote and biotite. The content of manganese in some deposits is rather high, in cases rising to 7 per cent. This

79

R. Frietsch

element is concentrated in layers of skarn or carbonate in the ore.In some deposits hematite formed secondarilyfrom magnetite is found in subordinate amounts.

S K A R N ORES

These ora inNorrbotten have the same mode ofappearance as the main part of the quartz-bandedores in the county. They occur in the greenstone group, mostly in the stratigraphically higher part,where they form lens-shapedbodies concordant with the host rocks, which are either chemical or detrital sediments or the greenstones proper. In many cases the skarn ores are associated with limestonesor dolomites or lie in the same stratigraphic position as these. The ore mineral is magnetite or, exceptionally, also hematite. The amount of iron varies in most cases between 30 and 40 per cent. The ores usually contain small amounts of pyrite and pyrrhotite. The content of sulphur usually exceeds 1 per cent.The content of phosphorus,in the form of apatite,is in most cases less than 0.1 per cent, but in some deposits the content is higher and rises locally to 1 or 2 per cent. The content of manganese is usually less than 0.2per cent. The skarn minerals that accompany the ore in large amounts are evenly distributed in the ore or form independent masses or layers.A rather c o m m o n featureis a layering between magnetite and the skarn minerals. Sometimes a layering of calcite with magnetite and skarn minerals is observed. Among the skarn minerals tremolite-actinolite, diopside, phlogopite and serpentine dominate. The skarn iron ores of Norrbotten were considered by Geijer (1931) and Geijer &Magnusson (19526) as pyrometasomatic replacing limestones and dolomites.This view is supported by the fact that in the Junosuando deposit at Masugnsbyn fluorine occurs in the skarn,mainly bound to chondrodite. The iron-bearing solutions in this deposit should,in the opinion of Geijer, emanate from a pethite granite which forms the foot wall of the ore. However, a sedimentary origin for the skarn iron ores in northern Sweden has postulated most recently by the present author (Frietsch, 1966, 19676, 1970). The main reasons for a sedimentary mode offormation are that the skarn iron ores and the quartz-banded iron ores, of which the latter are undoubtedly of sedimentary origin, both occur in connexion with sediments in the greenstone group and almost in the same stratigraphic position. In some deposits the two types occur intermingled with each other and a clear division between them is not possible. The skarn-layeringand the more rare carbonatelayering in the skarn ores is probably a relict sedimentary texture. As regards the relationship between the older group o€ intrusives and the skarn ores, there now exist indications that the formation of the ore is older than this group. Thus in the small Juolovanjärvet deposit the ore, with a folded skarn-banding,is cut by a granodioritewhich most likely belongs to the older intrusive group (Frietsch, 19676).

80

There is also evidence that the perthite granite at Junosuandocannot have given rise to the ore in the deposit, as the granite cuts the ore and contains inclusions of the skarn. Welin and Blomqvist (1966) investigated the age of an uraninite-bearing sample of the skarn ore from this deposit. The uraninite is enclosed in chondrodite which follows the magnetite. An age of 1,775-1,845 m.y. was obtained. The age of the perthite granite on the other hand is 1,540 m.y. (Welin, 1970). Further it must be mentioned that the magnetite in the skarn iron ores and the quartz-bandediron ores has a similar trace element distribution (Frietsch, 1970). O f special interest is the relatively high content of magnesium in both types.In the skarn ores it can reach 5.1per cent (mean value 0.96 per cent) and in the quartz-banded ores 3.3 per cent (mean value 0.53 per cent). D u e to the above-mentioned facts, it is most proble that the skarn iron ores,as are the quartz-bandediron ores, in northern Sweden are original sediments in which iron, silica and carbonates have been deposited in varying proportions simultaneously with the sediments in which the ores occur. The deposition of this iron-bearingformation took place mainly in the final stage of the basic volcanism that gave rise to the greenstone group. In connexion with later regional metamorphic processes the original constituents of the ores have recrystallized and reacted with each other giving rise to the skarn minerals. At the same time the ores were in many cases mobilized and thus here the primary sedimentary features have been almost totally obliterated.

Comparisons between non-apatiticiron ores

in central and northern Sweden In summary the non-apatiticiron ores in centraland northern Sweden are most probably of sedimentary origin. Some fundamental differences in the composition of the ore and the gangue ofthe two regionsare here ascribed to differences in the depositional environment.W h e n compared, the following features are relevant. HOST R O C K S

In both regions the iron ores occur in volcanic rocks, in central Sweden in acid-intermediateones and in northern Sweden in basic ones. Quartz-banded ores in the southern part of the Norrbotten county also lie in acid volcanics.In both regions the ores are in many cases associated with limestones-dolomitesor in the same stratigraphic position as these. The age of the volcanicsin central Sweden is more than 1,800 m.y. and in northern Sweden between 1,880 and 2,800m.y.; a more precise dating is not possible at the moment.

Precambrian iron ores of sedimentary origin in Sweden

IRON MINERALS

In centralSweden the ironis believed to have been deposited in the ferric state (hematite or limonite) but altered to magnetite by later metamorphism. Hematite is often preserved in the quartz-banded ores, especially when such deposits have been subject to only relatively weak metamorphism. Otherwise the present ore mineral is magnetite. As pointed out by the present author (Frietsch,1967a) the alteration of hematite to magnetite, which is believed to have occurred in connexion with the foldingof the volcanics and the intrusion of the older Svecofennian granites, was coupled with reducing conditions.During this epoch there occurred the magnesia-metasomatismby which sulphides were formed also.Oxygen fugacitywas then low.The existence of reducing conditions is further supported by the fact that the metasomatism resulted in the formation of silicates rich in magnesium and ferrous iron, silicates with ferric iron not being formed. In the non-apatiticiron ores in northern Sweden there is no knowledge of the original form in which the iron was precipitated, except that in the quartz-banded ore at Käymäjärvi pseudomorphs occur after greenaliteor some other iron silicate. Magnetite is, except when hematite is formed secondarily after magnetite,the only iron oxide.That magnetite and not hematite is the present iron mineral is possibly due to the fact that the ores, especially the skarn ores, contain small amounts of iron sulphides. They are probably of syngenetic origin deposited together with the iron (Frietsch,1966). The presence of the sulphides means a reducing milieu and magnetite is, therefore, probably the primary iron mineral in many cases. Through later oxidation magnetite has been changed to hematite, but only in those deposits where sulphides are missing. GANGUE

The quartz-banded ores of central Sweden have a gangue that is composed of Ca-Mg-richor Ca-FeS+-richsilicates, while the gangue in the quartz-banded ores in northern Sweden is composed of FeZ+-richor Fe2+-Mg-richsilicates. The skarn that follows the quartz-banded ores in the southern part of the Norrbotten county is Ca-Mg-richor Ca-FeZ+-richand is thus rather similar, to the skarn following the quartz-banded ores and skarn ores in central Sweden. On the whole there seems to be a similarity both in composition and geologic milieu between the quartzbanded iron ores in the southern part of Norrbotten and in central Sweden. Also the gangue in the skarn iron ores in central Swedenand in northern Sweden differs in composition.The skarn silicates in central Sweden are in part rich in C a and in part rich in M g , the later ones being clearly later and formed through the magnesia-metasomatism.In northern Sweden the skarn silicates are Ca-Mg-richor Mg-rich.The Ca-Mg-richskarns are found in almost every ore deposit, the Mg-rich skarns being less common.

The internal relationship between the Ca-Mg-richsilicates (tremolite, diopside) and the Mg-rich silicates (serpentine,phlogopite) is imperfectlyknown. In some deposits there seems to be a tendency for the Ca-Mg-richsilicates to form independentmasses outsidethe ore and the Mg-rich silicates to be distributed in the ore itself. There are indications from some deposits that the Mg-rich silicates are later than the Ca-Mg-richsilicates,the order of formation being diopsidetremolite-serpentine.

S-P-MN-CONTENT

As previously pointed out, the non-apatitic iron ores in northern Sweden in most cases contain small amounts of iron-bearingsulphides,while the iron oresin centralSweden are more or less freefrom sulphides.The same is true of the phosphorus content. The relatively high content of phosphorus foundin some skarn ore depositsinnorthern Sweden is certainly a primary feature,as thereis no sign of secondary addition.One more important chemical difference between the non-apatiticiron ores in both regions,is that manganese-bearingores are relatively abundantin central Sweden, but are almost missing in northern Sweden. In the latter region the precipitation of iron was not accompanied by manganese. A G E OF METAMORPHISM

For the non-apatiticiron ores of central Sweden it seems obvious that the metamorphism with recrystallization and skarn formation occurred in connexion with the folding of the volcanic-sedimentarycomplex and the intrusion of the older group of the Svecofennian granites. Additional changes took place in connexion with late Svecofenniangranites. For the non-apatiticiron ores of northern Sweden it is less clear if the metamorphism affecting the ore was related to the older granodiorite group or the younger Lina granite. The present author previously considered the Lina granite as most important in this connexion, the granodiorite group having only a smaller effect (Frietsch,1966), but is now inclined to believe that the older intrusives had a significantinfluencetoo.The greater age of the skarn formation compared with the Lina granite is shown by the radiometric age determinationsfrom Masugnsbyn,the uraninite in the skarn having an age of 1,775-1,845 m.y. It is possible that the metamorphism in part is older than the granodiorite group. The only proof for this view is the earlier mentioned observation from the Juolovanjärvet deposit which shows that the ore and the skarn had recrystallized before the intrusion of the granodiorite.This does not, however, exclude the possibility that the recrystallization-skarn formation and the granodiorite intrusion belong to the same process, the time gap between these being relatively small.

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Résumé Minerais de fer précambriens ù caractères sédimentaires, en

Suède (R.Frietsch)

A part les minerais de fer oolithique de l'époque jurassique dans le sud de la Suède,les autres minerais de fer suédois qui présentent des caractères sédimentaires remontent au Précambrien. Ils sont confinés dans deux régions géographiques distinctes,l'une au centre de la Suède et l'autre au nord. Dans les deux cas, on trouve des minerais veinés de quartz avec faible teneur en phosphore. Dans la région centrale de la Suède, le minerai consiste en couches alternées de quartzet d'hématite, l'hématiteétant en généralplus ou moins remplacéepar de la magnétite.Dans les minerais du nord de la Suède, oii le minerai est la magnétite, avec de moindres proportions d'hématite, la structure rubanée est en quelque sorte moins prononcée. Les minerais veinés de quartz ont dû être à l'origine des sédiments chimiques, forméspar la m ê m e activité magmatique qui a engendré les appareils volcaniques où on les trouve. Plus tard, des processus métamorphiques en relation avec le plissement des

systèmes volcaniques et l'intrusion de granite ont produit la recristallisationet la réorganisationinterne des minerais, comme par exemple la formation de silicates de ((skarn 1) là où les carbonates étaient associés au fer et à la silice.Dans les deux régions on rencontre aussi du ((skarn 1) pauvre en phosphore et constitué de magnétite et de silicates de ( (skarn )I. Dans de nombreux gisements,on note une stratification plus ou moins prononcée de magnétite,de silicates et parfois de carbonates.L'origine de ces minerais a donné lieu à bien des discussions,car on y observe à la fois des caractères sédimentaires et pyrométasomatiques. On possède toutefois, maintenant, des indications montrant que presque tout le minerai ((skarn ))de la Suède centrale est sédimentaire,mais les caractères sédimentaires originaux ont été, la plupart du temps, oblitérés par des processus ultérieurs.Le minerai ((skarn 1) du nord de la Suèdesemble aussiêtre constituéde sédimentsmétamorphosés.Cette opinion est basée sur le fait,entre autres,que la magnétite dans les minerais veinés de quartz et les minerais ((skarn 1) ont une composition géochimique voisine.

Bibliography/Bibliographie FRIETSCH, R. 1966. Berggrund och malmer i Svappavaarafältet, norra Sverige [Geology and ores of the Svappavaara region, northern Sweden]. Sverig. geol. Unders. Afh., C 604. (In Swedish with English summary.) --. 1 9 6 7 ~ .The relationship between magnetite and hematite in ihe iron ores of the Kiruna type and some other iron ore types. Sverig. geol. Unders. Afh., C 625. . 19676. On the relative age of the skarn iron ores and the Haparanda granite series in the county of Norrbotten,northern Sweden.Geol.Fören. Stocklz. Förh.,no.89,p. 116-18. . 1970. Trace elements in magnetite and hematite mainly from northern Sweden.Sverig. geol. Unders. Afh., C 646. GEIJER, P. 1925. Eulysitic iron ores in northern Sweden.Sverig.

__

geol. Unders.Afh., C 324.

__ . 193 1. Berggrunden inom malmtrakten Kiruna-GällivarePajala [The geology within the ore region Kiuna-GällivarePajala]. Sverig. geol. Unders. Afk., C 366. (Tn Swedish with English summary.) . 1959. Några aspekter av skarnmalmsprobleineni Bergslagen [Some aspects regarding the problems of the skarn iron ores in the region of Bergslagen]. Geol. Faren. Stockh. Förh., no. 81, p. 514-34. (In Swedish with English sunmary.) GEIJER, P.;MAGNUSSON, N .H.1944.D e mellansvenskajärnmalmernas geologi [The geology of the iron ores of middle Sweden]. Sverig. geol. Unders. Afh., Ca35. . 1952a. Geological history of the iron ores of central Sweden.XVZIZ Ini.geol. Congr., Great Britain 1948,PartXIII,

__

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MAGNUSSON, N.H.1953.Mulmgeologi[oregeology],Stockholm, Jernkontoret. -. 1960. Iron and sulphide ores of central Sweden.Guide to excursionsnos.A 26 and C 21. XXZ Int. geol. Congr.,Norden, 1960. Geological Survey of Sweden. MAGNUSSON, N.H.; THORSLUND, P.;BROTZEN, F.;ASKLUND, B.; KULLING, O.,1960.Descriptionto accompany the map of the pre-quarternaryrocks of Sweden. Sverig. geol. Unders. Afh., Ba16.

ÖDMAN, O.H.1957. Beskrivning till berggrundskarta över urberget i Norrbottens län [Descriptionof the geological map of the primary rocks in Norrbotten county]. Sverig. geol. Unders. Afh., Ca 41. (In Swedish with English summary.) OFFERBERG, J. 1967. Beskrivning till berggrundskartbladen Kiruna NV,NO,SV,SO [Descriptionof the geological maps ofsectionsKiruna NW., NE.,SW.,SE.]. Sverig.geol. Unders. Afh.,Af 1-4 (In Swedish with English summary.) PADGET, P. 1970. Description of the geological maps Tärendö NW.,NE.,SW.,SE. Sverig. geol. Unders. Afh., Af 5-8. (In press.) WELIN, E. 1970. Den svekofenniska orogena zonen i norra Sverige.En preliminar diskussion[TheSvecofennianorogenian zone in northern Sweden. A preliminary discussion]. Geol. Füren. Stockh. Förh. (In press.) (In Swedish with English summary.) WELIN E.;BLOMQVIST, G.1964. Age measurements on radioactive minerals from Sweden. Geol. W r e n . Stoclch. Förh.,

p. 84-9.

--. 19526. The iron ores of Sweden. In: Symposium sur les gisements de fer du monde. XZX Congr. G o l . Znt., Alger, p. 477-99.

82

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no. 86, p. 33-50. 1966. Further age measurements on radioactive minerals from Sweden.Geol. Fören. Stoclch. Förh., no. 88, p. 3-18.

Precambrian iron ores of sedimentary origin in Sweden

Discussion R.T.BRANDT. What is the relationship,if any,between the sedimentary iron ores of northern Sweden and the massive magnetite deposits of Kiruna and Gällivare?

ted with the Lina granite. There are, however, indications that the formation of scapolitein some cases is much older.

V. M.KRAVCHENKO. W i l l you show on your regional map R.FRIETSCH. There is no relationship between them. The

the ‘Stabby’and ‘Maria’deposits?

sedimentary iron ores and the iron ores of the Kiruna type are quite different in mineralogy,host rock,geological appearance, etc.

R.FRIETSCH. I regret that I do not know of the existence of such deposits.Isuppose they are situatedin centralSweden.

A.S.KALUGIN. What is your opinion as to the origin of the

V. M.KRAVCHENKO. Have any definitely magmatic basic

Grengesberg deposit?

rocks been found in the stratigraphicsection of host rocks in these deposits?

R.FRIETSCH. The deposit belongs to theKiruna type of ore, and should thus be of magmatic origin,

V. M. CHERNOV. Has the Svecofennian basement been

R.FRIETSCH. Such rocks have been found among the basic lavas (greenstones) in the greenstone group. The gabbros belong in most cases to the older group of intrusives.

found?

R.FRIETSCH. No,thereis no knowledge of such a basement.

V. M.CHERNOV. What is the age of the scapolitizationand metasomatism?

R.FRIETSCH. The scapolitizationis most probably connec-

V. M.KRAVCHENKO. What is the absolute age of the orebearing rocks of the deposits cited? R. FRIETSCH. Unfortunately there are no radiometric age determinations. The rocks of the greenstone group seem to have an age between 1,880 m.y. (older intrusive group) and 2,800m.y. (granite basement).

83

The ferruginous-siliceousformations of the eastern part of the Baltic shield V. M.Chernov Karelian Branch of the Academy of Sciences of the U.S.S.R. Geology Institute

The Precambrian of the eastern part of the Baltic shield is divisible into four chronological units or structural stages differing in their tectonic structure,rock composition and peculiarities of magmatism and metamorphism. The lowest structural stage (the Early Proterozoic (Archaean) foundation) is composed of various gneisses,granitoids,amphibolites and migmatites, which have been subjected to intensive deformation and deep, often repeated, metamorphism and ultra-metamorphism. The second structural stage is characterized by the development of multiple folded, mottled Early Proterozoic primary sedimentary and sedimentary-volcanicserieswhich have undergone changes from greenschist facies to amphibolite facies of regional metamorphism. The third structural stage is characterized by the primary development of terrigenous, often rudaceous, deposits, accompanied by carbonate,shungite, pelite rocks and basic volcanites occurring mainly in interstructures of various types (through lines,etc.). The sediments of this structuralstageare distinguished by their low grade ofmetamorphism, are of Middle Proterozoic age and are interpreted by many investigatorsas formations of the orogenic and subplatform stages in the development of Karelides. The fourth structuralstage-a platform mantle,which is preserved over a small area-is composed of nonmetamorphosed and weakly dislocated terrigenous Jotnian sediments. The ferruginous-siliceousformations are confined exclusively to deposits of the second structural stage belonging chronologically to the Lower Proterozoic (2,6002,000 m.y.). Some deposits of hematite-martite ore are known also in the territory of Karelia in deposits of the third structural stage. The results of studies of geology, lithology and geochemical properties of the iron-ore series of the KolaKarelian region during the past fifteen to twenty years allow us to distinguish between the three genetic types of ferruginous-siliceousand ferriferous formations,differing in paragenetic rock associations, palaeotectonic and palaeofacial conditions of sedimentation as well as in

their stratigraphic position in the Precambrian sequence. The following types can be distinguished:leptite-porphyric series of ferruginous-siliceousformations; spilitediabasic ferruginous-siliceousformations; clastogene ferruginous formations. The leptite-porphyric series of ferrugirious-siliceousformations. These are widely distributed in the territory of

western Karelia in the deposits of the Himola series and are genetically connected with Lower Proterozoic intensivepersilicic volcanism. During recent years ferruginous-siliceous rocks, paragenetically connected with persilicic effusions, have been distinguished in the Kola peninsula in the formations of the Kola series (Olenyegora suite). The ferruginous-siliceousformations of this genetic type have been studied mostly in Karelia where, according to paragenetic rock associations,the schistose-leptiteand leptite-porphyricformations are prominent among them. The schistose-leptiteferruginous-siliceousformation comprises the lower part of the Himola series and, according to its volume, is in conformity with the first sedimentaryeruptive cycle of this series.It is composed of various primary sedimentary and eruptive rocks metamorphosed to different degrees among which appear conglomerates,gritstones,arkoses and various aluminiferous gneisses (garnetbiotite and staurolite). Less-commontypes which occur are tuff breccia, quartz-biotite tuffaceous shales, graphitic, muscovitic, sericitic, talcose and chloritic shales, amphibolitic paraschists and para-amphibolites. Ferruginoussiliceous rocks, mainly biotite, riebeckite and grunerite varieties, are located in the upper part of the formation, where they alternaterhythmically with the above-mentioned shales and gneisses. It is characteristic of the schistose-leptiteferruginoussiliceous formation that besides eruptive and ferruginoussiliceous rocks, in its constitution there is a widespread development of metamorphosed terrigenous deposits and a facial variation from section to section, caused by the pinching out of lithological units and lateral changes in rock type along the strike. The leptite-porphyricferruginous-siliceousformation

Unesco, 1973. Genesis of Precambrian iron and manganese deposits. Proc. Kiev Symp., 1970. (Earth sciences, 9.)

85

V. M.Chernov

conforms in volume to the deposits of the Himola series and in its lower part consists of persilicic volcanic rocks transformed into leptite gneisses, porphyroids, halleginta and various tufogenic crystalline schists and leptites. The widespread developmentof volcanites deposited in aqueous medium (rhythmic lamination in tuffaceous shales) shows that the development of this formation took place in underwater conditions and was accompanied by intensive volcanism.Along the strike of the western Karelian structuralfacial zone, the main features of the paragenesis of rocks of the leptite-porphyricferruginous-siliceousformation remain unaltered, due perhaps to monotypic palaeofacial conditions of sedimentation. Spilitic-diubusic ferriiginous-siliceous formations. These are widespread in the Kola peninsula (Kola and Tundra series) and in Karelia in the deposits of the Himola and Parandov series. In contradistinctionto the ferruginous-siliceousformationsof the leptite-porphyricseries,theferruginous-siliceousformationsn o w describedare paragenetically closely connected with volcanic series of basic composition. Amphiboliticshales,amphibolites and pyroxene-hornblende gneisses, formed as the result of metamorphic transformations,mainly of basic effusive rocks and their tuffs,are the main members of the rock associationsof these formations. Horizons of paracharnockites and aluminiferous gneisses are often met in the Kola peninsula. Ferruginoussiliceous rocks and graphitic shales enriched by sulphides are observed in the form of thin (0.5-15 m)beds and lenses among the rocks mentioned. A rhythmic structure,consisting of reiterationsin the sequenceof regularly built rock bands 50-200 m thick composed in the lower parts of metamorphosed volcanites of basic composition and in the upper parts of ferruginous-siliceousrocks and paraschists, is a characteristic lithological peculiarity of the ferruginoussiliceous formations of Karelia. Clustogene iron ores. The martite-hematitein thin deposits and partings is developed mainly in the southern and south-westernpart of Karelia (Prionezhye,Suojärvi,Tulomozero, Janisjärvij in the Jatulian deposits of the third structural stage. In contradistinction to volcanogenic ferruginous-siliceousformations these ores do not accumulate by volcanic-sedimentary processes. They associate only with terrigenous rocks. The correlation of sections and the paragenetic analysis of the iron-ore strata of Karelia and

the Kola peninsula allow us to distinguish two large epochs of iron accumulation connected with certain stages of the tectonic development of this territory. Duringthe geosynclinal period,pertaining to the Lower Proterozoic (2,600f 100-2,000m.y.), processes of volcanism proceeded in the Karelian and Kola-Norwegian geosyncline zone. Under the influence of these processes ferruginous-siliceousformations of sedimentary-volcanogenic origin were developed. Moreover,there is a certain dependence in the distributionof ferruginous-siliceousformations upon palaeotectonic conditions of development. Areas ofleptite-porphyricformationsare distinguished, accordingto geophysicalinvestigations,by great thicknesses of the earth‘s crust and the ‘graniticlayer’. Intensive persilicic volcanism,the thickness of the ‘granitelayer’and the earth’scrust,the wide distribution of terrigenous sediments, breaks in sedimentation and comparatively thin layers of sediments all confirm that a geoanticlinal régime of sedimentation took place during the development of ferruginous-siliceousformations of the leptite-porphyricseries. The spilitic-diabasic ferruginous-siliceousformations were developed in different tectonic conditions. Spatially they are associated with zones of intensive warping and abyssal fractures limiting the Lapland-White Sea and Murmansk blocks. These ‘greenrock‘ warps are cliaracterized by the thinness of the earth’s crust and ‘granite layer’.It is possible that such a differencein the structureof the earth’s crust in the eastern part of the Baltic shield determined the different types of initial Early Proterozoic volcanism and the difference in palaeotectonic conditions of the development of leptite-porphyricand spilite-diabasic ferruginous-siliceousformations. The second epoch of iron sedimentation is in connexion with the orogeny stage of tectonic development (Middle Proterozoic 2,000-1,750m.y.). During this period the Karelian geosyncline zone was transformed into a foldmountain country with accumulations characteristic of this period of orogenic and subplatform deposition of formations of Sariola and Jatulia (molasse; arenaceous, carbonate-terrigenous,shungitej. Ferruginous rocks, developed mainly by clastogenic processes, appeared due to the destruction and weathering of Lower Proterozoic volcanogenic thicknesses and ferruginous-siliceousformations.

Résumé Les formations de fer siliceux duns lu partie orientale du bouclier baltique (V. M.Chernov)

1. L’analyse des formations et la corrélation des formations géologiques des structures plissées protérozoïques des Karélides permettent de choisir dans la partie orientale du bouclier baltique deux époques significatives d’accumulation de fer correspondant à des stades 86

bien définis du développement tectonique de cette région. 2. L’époque la plus significative de l’accumulation du fer correspond à la période du Protérozoïque inférieur du développement géologico-tectonique 2 600-2O00 millions d’années) alors que le bouclier baltique était une vaste région géosynclinale. 3. Dépendant des conditions paléotectoniques de sédimentation, deux séries de formations de fer siliceux se sont

The ferruginous-siIiceousformations of the eastern part of the Baltic shield

formées pendant cette période. Elles sont liées génétiquement à des manifestations de volcanisme acide et basique. Les séries leptite-porphyriquesdes formations de fer siliceux, qui se sont développées dans la zone karélienne des karélides, se sont formées dans des conditions de régime ( (géoanticlinal 1) de sédimentation dans un vaste bloc intragéosynclinal composé de roches sialiques du début de l'archéen. 4.Les formations de fer siliceux des séries spilite-diabasiques montrent une tendance généralevers les régions de submersion intensive et de haute perméabilité de la croûte (failles profondes). Sur le bouclier baltique,elles sont confinées à la dépression de ((greenstone N. 5. La deuxième époque du Protérozoïque où l'on ob-

serve l'accumulation du fer sur une grande échelle est en relationavec un stadeorogéniquede développementde cette région (Protérozoïque moyen, 2 000-1 750 millions d'années). Pendant cette période, des formations de fer clastogène se sont constituéessurleterritoirede Karélie,résultant de la destruction des strates du géosynclinal du Protérozoïque inférieur. 6.Il n'y a aucune roche ferreuse au stade orogénique de développement dans la péninsule Kolsky. 7. Les informations données ci-dessus permettent de classer les formations de fer siliceux du Précambriensituées dans la partie orientale du bouclier baltique en fonction de la tectonique.

Discussion A. M.GOODWIN. What is the definition of persilicic volcanics? D o they belong to the calc-alkaline series?

V. M.CHERNOV. These are metamorphosed lavas and tuffs of dacite and keratophyre composition.Yes,they do.

87

Precambrian ferruginous-siliceousformations associated with the Kursk Magnetic Anomaly N.A. Plaksenko, I. K.Koval, I. N.Shchogolev

The area of the Kursk Magnetic Anomaly (KMA)is huge, about 450 km long and 130-150 k m wide (Fig. i). In this area there are two rather well-distinguishedlines of magnetic anomalies:northeast and south-west. Their position and configurationare due to steeply dipping beds of ferruginous quartzite in belts of metamorphic rocks. The structure of these beds is not simple. Besides the clearly distinguished,long magnetic anomalies there are many local short anomalies,which are situated both between the main lines and outside them or on their continuation.The crystalline basement of the KMA region is covered by sedimentary rocks of Palaeozoic,Mesozoic,and Cenozoic ages, the thickness of which is 30 m in the north-east and 300m in the south and south-west .The basementis cutby numerous boreholes and is exposed in iron ore pits. In the Precambrian rocks associated with the KMA there are two time-structuralunits: the lower (Archaean) includes different gneisses,migmatites, granites and other rocks of the Oboyanskaya series,and volcanic-sedimentary rocks, products of the metamorphism of spilitic-keratophyric rocks and quartz porphyries of the Mikhailovskaya series. The upper time-structuralunit consists of Proterozoic rocks which are separated from the Archaean rocks by a stratigraphic and structural disconformity. The Proterozoic rocks are separated into two series: Kursk (lower Proterozoic) and Oskol,which is called the Kurbakin series (lower and middle Proterozoic) in the north-west part of the area. The Oskol (Kurbakin) series overlying the Kursk series with disconformity is clastic ferruginous quartzite, detrital sediment rich in iron, sandstone-shale,shale-carbonaceous rock,and acid effusive rocks. W e presented samples of rocks of the Kurbakin series containing relicts of algae to the PollenologicalLaboratory of Voronezh State University; remnants of the simplest algae were found(Daminorites Eichw .,Oscillatorites Shcp., Leiomarginata Umnova, Rifenites Naum., Turuchanica Tim.,Brochosophosphaera facetus Schep.,Trachysophosphaera Naum., Leiosphaeridia tipa Volcova), establishing the middle Proterozoic age of the rocks. Ferruginous-siliceousrocks (ferruginous quartzites and

gneisses) recur several times in a cross-sectionof the Precambrian rocks of the K M A . Non-ferruginous rocks enclosing them are closely connected with them in origin and at the same time are separated from one another by disconformities. It is possible to determine four genetically independent formations of ferruginous-siliceousrocks of different ages,which successively alternated during the history of the region.They are: (a) ferruginous-siliceousgneiss (partings of magnetite gneiss in rocks of the Oboyan series); (b)ferruginous-siliceousmetabasite (thickpartingsofquartz, silicates, and magnetite in rocks of the Mikhailov series). (c) ferruginous-siliceousslate (thick mass of ferruginous quartziteofKursk series enclosed in schist); (d) ferruginoussiliceous clastic rock (clastic ferruginous quartzite and rich fragmental ore of Oskol-Kurbakin series).

Ferruginous-siliceous gneiss The rocks of this formation occur in the Kursk-Besedino region of the KMA. Ferruginous-siliceous rocks which are conformable in pyroxene-amphibole,garnet, and pyroxene-plagioclase gneisses and other rocks are characterized by quartz-magnetite-pyroxene(hypersthene) and garnet-magnetite-hypersthene composition.They are massive and usually vaguely banded or,rarely,distinctly banded. Interbeds and lenses of ferruginous-siliceousrock have thicknessesof from 0.8m to 35 m and are of small extent.

Ferruginous-siliceous metabasite At present this formation is distinguishedconditionally.Its ferruginous-siliceousrocks are amphibole-magnetite and chlorite-magnetitequartzites of subore grade,which occur as thin interbeds at the bottom of the upper effusive-sedimentary (keratophyre-shale)suite of the Mikhailov series and alternate with quartz-chlorite,albite-chlorite-biotite, and albite-chlorite-amphiboleslates.

Unesco, 1973. Genesis of Precambrian iron and manganese deposits. Proc. Kiev Symp.,1970. (Earth sciences, 9.)

89

N.A.Plaksenko,I. K.Koval and 1. N.Shchogolev

\

0 Q

Nizhnedevitsk

orshechnoe

Belgorod

q

FIG.1. Areal map of the Kursk Magnetic Anomaly. Magnetic anomalies:1, connected with ferruginousquartzites of the Kursk

series; 2, connected with ferruginous quartzites of supposed

Ferruginous-siliceous slate

wards, rocks underlying ferruginous quartzite give way to metamorphic sandstones,metagravelites,grusses,and conglomerates of Archaean age and also to a metamorphosed crust of weathering. Ferruginous-siliceousrocks are magnetite quartziteand micaceous hematite-magnetite quartzite with various proportions of magnetite and hematite (jaspilites). A zoning of authogenic minerals is typical. The dimensions of this formation are huge and its productivity (in iron) is the greatestin the region.Below,we give its principal structural characteristics.

This formation consists of ferruginous quartzite of the Kursk series,which is conformable in a sequence of metamorphosed terrigenous sandstone-shaledeposits. Ferruginousquartzites are directly underlain and overlapped by phyllitic, carbonaceous slate and crystalline quartz-sericite and garnet-biotite slate. This formation occurs on various rocks of the eroded Archaean basement with stratigraphic and structural unconformity. Dowii90

Obojan and Mikhailov series.

Precambrian ferruginous-siliceousformations associated with the Kursk Magnetic Anomaly

CLASSIFICATION O F FERRUGINOUSSILICEOUS R O C K S

Among the varied ferruginous-siliceousrocks that compose the iron ore bands of the formation it is possible to distinguish a limited number of types, the original sediments of which were formed under certain facies conditions and at definiteplaces in the facies profile (cross-section)of the formation.The main genetic types of ferruginous-siliceous rock of the formationare as follows:(a) low-gradeore (with magnetite) and non-ore-bearingsilicate-carbonatequartzite (non-ore silicate-carbonate-protoxidefacies); (b) silicate (cummingtonite)-magnetite quartzite with ferromagnesian carbonates (ore carbonate-silicate-ferruginousoxide-protoxide facies); (c) magnetite quartzite (ore-bearingmagnetite protoxide-oxidefacies); (d) micaceous hematite-magnetite quartzite; (e) magnetite-micaceous hematite quartzite; (f) micaceous hematite quartzite. Types (d) to (f) belong to the ore-bearing hematiteoxide facies. Each of the types may be characterized by a certain mineralogical composition, by geochemical peculiarities,textures and structures, and by a regular position in the cross-sectionof the formation. R E G U L A R ALTERNATION O F R O C K S IN T H E FACIES PROFILE O F T H E FORMATION

The composition of the iron ore suite is not identical in different parts of the K M A region. This is due to considerable variation in the thickness and to changesin the facies of ferruginousquartzite,and to varying numbers offerruginous interbeds and bands in the iron ore suite.However,if these genetic types of ferruginous-siliceousrock are arranged according to the order of their abundanceas they alternate with underlying and overlying rocks, ignoring interbedding which involves the adjacent types,w e obtain a generalized cross-sectionofthe ferruginoussilicateslateformation of the following kind (from top to bottom): (a) overlying schist-phylliticcarbonaceous slate-schist;(b) quartzite (non-oreor silicate-bearingand carbonaceouswith small amount ofore); (c) quartzite(silicate-bearingand magnetitic with ferromagnesian carbonates); (d) magnetite quartzite; (e) magnetite-micaceous hematite and micaceous hematite quartzites;(f) micaceous hematite-magnetite quartzite; (g) magnetite quartzite; (h) ore-bearing silicate-magnetite quartzite; (i) quartzite (non-ore-bearingor carbonaceous silicate-bearingwith small amount of ore). In places, the quartzite has interbeds and lenses of carbonaceousmagnetite ore and amphibole schist; (j) carbonaceous magnetite orewith sulphidesand pyrite-carbonaceousorewith magnetite. Sulphide-bearing ore occurs close to the contact with schist and in the schist itself. Carbonaceous magnetite ore occurs mostly at the contact with barren quartzite and penetrates into the quartzite; (k) phyllitic carbonaceous slate and schist, considerably pyritized in its upper part. In places,the slate and schist have interbedsof sulphideand carbonaceous magnetite ore; (1) arkosic metasandstone,

barren (non-ferruginous)quartzite containing blastopsammitic structure,metagraveliteand metaconglomerate. Carbonaceousmagnetite and pyritic carbonaceousores occur locally in the formation.Omitting such local occurrences of ore,w e have an ideal cross-sectionof the ferruginous-siliceous shale formation, which can be taken as a manifestation of one simple cycle of sedimentation characterized by similar conditions of sedimentation at the beginning and end of the cycle. The position of some facies in such a cross-section,from the shore seaward to depth, is as follows: Non-ore facies: blastopsammitic barren quartzite-metasandstone-carbonaceousbarren silicate-bearingquartzite or with a small amount of ore (silicate-carbonaceous facies). Ore-bearing facies: silicate-magnetite quartzite (carbonaceous-silicate-magnetitic-hematitic facies). The composition of ore minerals, carbonates and silicates regularly changes towards the deeper deposits,beginning with the silicate-carbonaceousfacies part ot the profile. An ideal faciesprofile of the formationformed under the condition of gradual lowering of the bottom from the shore toward the deepest part of the basin, as represented by the distribution of sedimentsaccording to their grain sizes;with increasing depth pelitic material loses its importance and ferruginous-siliceous colloids increase in importance. Organic matter also is disseminated in accordance with grain sizes of sediment;it is enormously concentrated in the pelagic zone of pelite deposition, but gradually decreases in the deeper water deposits. There was a decrease of E h near the contacts of shale and barren quartzite, and moderate hydrogen sulphide contamination, resultinginthe appearance in the shaleof a vague sulphide subfacies of the local protoxide facies (represented by carbonate-magnetiteand pyritic carbonaceous ores). Farther from the shore of the basin, with a decrease of organic matter and a decrease of activity of decayed organic matter, the process of reduction gradually gave way to reduction-oxidation processes, which in turn passed into oxidation processes, resulting in a gradual increase in the importance of iron oxide minerals from the shallowest to the deepest part of the ore-bearing portion of the faciesprofile.Accordingly,we can precisely outline an authigenic mineral zoning as a feature of the facies profile of the ferruginous-siliceousslate formation (Fig. 2). The sedimentation cycles represented in the cross-sectionof the formation demonstrate a regular alteration of conditions of sedimentation from less stable and more shallow in the eastern part of the KMA basin (northeastern belt) to more stable and deeper water in the western part (south western belt). This accounts for the increased importance of ferruginous-siliceousrocks of the oxide facies and the presence of the ferruginous quartzite series toward the west and south-west. Ferruginousquartziteshaving a thickness ofless than 200m and deposited under littoral conditions, mainly in the eastern wing of the north-easternbelt of the K M A , are 91

N.A. Plaksenko,I. K.Koval and I. N.Shchogolev

FIG.2. Authigenous-mineralogicalzoning of facies section of ferruginous-siliceous-slateformation. 1, low ore and barren quartzites;2,cummingtonite-magnetitequartzites;3, magnetite quartzites;4,hematite-magnetitequartzites;5,pyrite; 6,siderite-pistomesite;7, iron silicates/cummingtonite; 8, magnetite; 9,hematite.

an ore-bearingfacies and consist almost entirely of protoxide and oxide-protoxide minerals. The ferruginous quartzite is interbedded with a relatively large thickness of schist. Towards the south-westernbelt of the K M A , the proportion offerruginous quartzite to schist increases sharply and the proportion of oxide facies minerals increases in the ferruginous quartzite. Deposition of sediments of the south-westernbelt is believed to have taken place,in general,in deeper water than did sediments in the north-eastern belt. W e conclude that the iron-ore building process was sensitive,especially in the shallow water portion of ore facies,to the slightest alteration of conditions of sedimentation,which resulted in a direct relationshipbetweenthethickness ofdifferentferruginous quartzites and their genetic type of facies. A regularchangeof geochemical and other propertiesofthe ferruginous-siliceousrocks and their main rock-forming minerals takesplace in the faciesprofile of the formation. The essence of the regular geochemical changes is illustrated by Table 1.

TABLE 1.Systematicchangesin the geochemical and other properties offerruginous-siliceousrockson thefaciesprofile of theferruginoussiliceous slate formation of KMA T h e facies profile of the ferruginous-siliceousslate formation Facies with ore

Facies without ore

The main qualitative characteristics of the rocks and minerals Schists

Chemical composition (percentage) Fe general Fe solvent Fe silicate Fez03 Fe0 SiO,

Tio,

A1203

MnO Ca0 MgO P S C free

Ratiosofthemean contentsofsome SiO, : Fe solution components of the rocks Fe,O,: Fe0 Alzo,:SiO, Tio,: Alzo, M n : Fe solution P : Fe solution C a 0 : MgO

Ti : Y Sr : Ba G e : Fe 92

6.84 4.95 1.89 3.47 5.62 59.06 0.57

16.71 0.05 0.69 2.34 0.035 O -43 0.45 11.18 0.61 0.28 0.03

Silicate-

Silicate

carbonate; low-ore an barren quartzites

magnetite with carbonate quartzites

26.25 17.29 8.96 15.70 19.75 49.69 0.24 2.45 0.15 2.18

34.30 31.10 3.20 28.31 17.77 44.76 0.19 1.94 0.075 1.97 2.33 0.072 0.225 0.19

3.O8

0.075 0.43 0.22 2.68 0.79 0.05 0.09 0.0064 0.0042 0.70 6.45 3.6

0.0084 0.0071 0.29 10.9 1.6 O .o0006 0.00003

1.34 1.59 O.043 0.10 0.0021 0.0023 0.84 3.o0 1.o

0.000016

Hematitic Magnetite quartzites

34.59 33.08 1.51 33-21

14.66 41.83 0.21 0.92 0.07 1.90 1.96 0.072 0.119 0.12 1.18 2.26 0.022 0.23 0.0013 0.0021 0.96 2.40 1.30 0.000024

Specular Magnetite iron hematitespecular magnetite iron (hematite) quartzites quartzites

36.89 35.81 1.O8 39.09 11.81 40.39 0.15

0.68 0.045 1.74 1.86 0.064 0.053 0.074 1.O6 3.39 0.017 0.22 0.0011 0.0018 0.94 1.37 1.28 0.000012

39.61 38.67 0.94 48.09 7.62 39.24 0.08 0.41 0.035 1.25

-

0.048 0.030

-

0.94 6.31 0.01 0.19 0.0008 0.0010

-

1.33 1.20 0.000008

Precambrian ferruginous-siliceousformations associated with the Kursk Magnetic Anomaly

T h e facies profile of the ferruginous-siliceousslate formation Facies with ore

Facies without ore The main qualitative characteristics ot the rocks and minerals Schists

The mean contents of move widespread trace elements (percentage) In the rocks Mn Ti

0.11 0.0284 0.0044 0.0047 0.0058

0.067 0.0117 0.0038 0.0030 0.0021

0.0010

0.0010

0.040

0.006 0.006 0.0005 0.020 0.019 0.0040 0.0072 0.0029 0.0010

Mn Ti V

-

-

-

Ge

-

-

-

0.017 0.34 0.031 0.0657 0.034

0.049 0.047 0.023 0.013 0.0128

Cu

Ni Co

Sr Ba Ge

Mn Tn V Cu

Ni Ge

In hematite

In quartz

Silicate magnetite with carbonate quartzites

O.042 0.218 0.020 0.009 0.009 0.0025 0.08 0.05 0.0003 0.31 0.025

V

In magnetite

Silicatecarbonate; low-ore an barren quartzites

Ba Mn Ti CU Sr

Ba

0.011

0.00045 0.30 0.026 0.008 0.0062 0.10 0.082 0.008 0.041 0.00065 0.0012

-

-

0.012 0.038

0.011 0.019 0.0069

Magnetite Pyrite

Refractive index of carbonate

0.054 0.0075 0.0031

0.0034 0.0023 0.0008 0.0072 0.0053 0.0008

0.019 0.0056 0.0034 0.0053 0.0031 0.00087 0.019 0.007 0.0032 0.00074 0.019 0.0098 0.011 0.007 0.0078 0.0066

526.4 1261

Magnetite Pyrite

Thermo EDS(mv)

quartzites

21.12 52.07

Mean reflectivity for 441-688 n m Magnetite light waves Pyrite Microhardness (kg/mm2)

Hematitic

Magnetite

1.8381.855

Ferruginous-siliceous-clasticformation This formation is not widespread in the K M A region occurring only where beds of ferruginous quartzite in the ferruginous-siliceousformation are overlapped by other rocks with disconformity. The clasticferruginous-siliceousrocks ofthisformation are the products mainly of disintegration,rewashing, and

1.8271.873

1.6981.722

Specular Magnetite iron hematitespecular magnetite iron (hematite) quartzites quartzites

0.040 0.0037 0.0027 0.0032 0.0022

0.028 O .O024 0.0018 0.0025 0.0020

0.0064 0.0050 0.00045 0.013 0.0037 0.0031 0.0048 0.0029 0.00065 0.005 0.0063 0.0028 0.00053 0.011 0.0038

0.0040 0.0034 0.00032 0.07 0.0022 0.0021 0.0047 0.0026 0.00047 0.0028 0.0030 O .O023 0.0004 0.004 0.0026 0.005 0.0042

-

0.008

0.0043 0.0064 0.0057 20.18 49.42 531.9 1275

-

0.0050

0.0055 20.07 47.13

539.2 1278

0.400 0.788

0.410 O.800

0.410 0.830

1.6961.720

1.6851.723

1.6801.704

redeposition of ferruginous quartzite from the ferruginoussiliceous slate formation. In some places, they have pronounced typicalclastic texture,but elsewhere clastic texture is obscure. The differentiation of material into rhythmically alternating iron-rich layers and coarse, sandy, well-laminated ferruginouslayers and rudaceous texture,are typical of the formation. 93

N.A.Plaksenko,I. K. Koval and I. N.Shchogolev

Summary A comparison of the ferruginous-siliceousrocks in the terrigenous-sedimentaryand volcanic-sedimentarysequence enables us to establish considerable distinctions between them. The distinctions relate to the nature of the interlayering of ferruginous and volcanic rocks compared with sedimentary rocks; to the composition and genesis of the rocks that both immediately enclose and separate the ferruginous beds; to the amount of ferruginous material and the composition of the ferruginous zones; to the productivity of the ferruginous suites, i.e.to the amount of ferruginousrock relative to the total thickness of the suites; to the number and thickness of the separate ferruginous layers in suites;to the mineral composition of the ferruginous quartzites and the texture or internal structureof the ferruginous layers; and to the geochemical characteristics and other features. In general, the Precambrian ferruginous beds of vol-

canic associationdiffer chemically from those of terrigenous association in having more M n , M g , Cu, Ca, Co, S, Ba, P and Ni,and less Ge, V and Sr. The ratio of Ti to V generally exceeds 25 in the ferruginous-volcanic association and is less than 25 in the ferruginous-terrigenous association.The ratio of Sr to Ba is greater than 1 in the volcanic association and less than 1 in the non-volcanic association. Thus,it is quite possible to determine the formational association of the Precambrian ferruginous quartzites. These facts make groundless the attempts of some investigators to explain the origin of all ferruginous-siliceous rocks of Precambrianage in terms of the volcanic-sedimentary hypotheses. It is clear that the methods of sedimentary geology, lithofacies and stratigraphic analysis play a great role in the solution of the most complex problems of the origin of the metamorphosed Precambrian ferruginous-siliceous rocks.

Résumé Les formations de fer siliceux du Précambrien dans la région de l'anomalie magnétique de Koursk (N.A. Plaksenko, I. K.Koval et I. N.Shchogolev) On peut distinguer quatre types de formations de silex ferrugineux génétiquement indépendants dans une section des formations précambriennes de région de l'anomalie magnétique de Koursk. Elles se remplacent l'une l'autre dans le temps et apparaissent dans des complexes rocheux séparés l'un de l'autre par des ruptures stratigraphiques. Ces formations sont les suivantes (du bas vers le haut) : 1. Ferrugineux-siliceuxgneissique; 2. Ferrugineux-siliceux métabasique ; 3. Ferrugineux-siliceuxschisteux ; 4.Ferrugineux-siliceux-clastogène. Les deux premières appartiennent à l'Archéen. Elles ont été peu étudiées et leur interrelation n'est pas claire. Les formations de schistes siliceux-ferrugineux se rencontrent avec des interruptions dans la croûte métamorphosée des roches archéennes désagrégées par les facteurs météorologiques et qui remontent au Protérozoïque inférieur. C'est la formation la plus productive et par conséquent la mieux étudiée. L a formation clastogène-siliceuse-ferrugineuse s'est développéelocalement.D'après les microfossiles trouvés dans ces roches, elles appartiennent au Protérozoïque moyen. Tous les caractères géologiques des roches siliceuseset ferrugineuses et des roches encaissantes sont caractérisés

94

dails chaque formation par une individualité spécifique et un caractère qui reflète les conditions de leur formation. Pour le moment, la genèse de deux formations seulement peut être caractérisée avec quelque sûreté :celle des formations schisteuses-siliceuses-ferrugineuses et celle des formations clastogènes-siliceuses-ferrugineuses. Les roches siliceuses-ferruguieuses(silex ferrugineux) de la première ont une nature colloïde terrigène sédimentaire ; le fer et la silice, qui participèrent à leur formation, proviennent de la croûte de désagrégation des gneiss archéens,amphibolites et autres roches ferrugineuses.Cela est démontré par tout un ensemble de caractères géochimiques, lithologiques, pétrographiques, minéralogiques, géologiques et autres. Les roches siliceuses ferrugineusesde la formation clastogène se rencontrent sur la surface érodée par l'eau,de la formation schisteuse-ferrugineuse-siliceuse.Elles sont le produit de ((washout ))métamorphoséet de redéposition de quartzites ferrugineuses et des argiles schisteuses qui les composent. La comparaison de tous les caractères et propriétés dont on dispose pour la recherche sur les roches ferrugineuses des formations volcanogéniques et sédimentaires montre la possibilité d'établir des diagnostics dans lesquels on peut avoir confiance. Cette communication est surtout consacrée à l'examen de ces caractères et propriétés.

Structural-tectonic environments of ironore process in the Baltic shield Precambrian P. M.Goryainov Kola Branch of the Academy of Science of the U.S.S.R.

Within the Baltic crystalline shield,as well as in the other Precambrianregions,iron ores of the cherty-ironseries are broadly developed.Acknowledging in the most cases their supracrustalcharacter,the investigatorsof the Baltic shield Precambrian point out the potentially high correlative properties of cherty-ironrocks.Thus,each regional stratigraphical scheme of the Precambrian involves data concerning iron ore genesis. In this case, in the various schemes, the essential discrepancies arise in the evaluation of events in the lower Precambrian,including the determination of Archaean-Proterozoic boundary, i.e. the period to which the process of ore formationis commonly related. Kratz (1963), in his characteristics of the West Karelian synclinal sub-zone of karelides, points out that within its range the Archaean formations are broadly abundant. The Archaean formations are presented by gneissosegranites and gneissose-diorites,often migmatized. Supra-

FIG.1. Geological scheme of Kolmozero-Voroiiya river line (using data ofKharitonov,Garifullinand Maslennikov): I. (Granites): (a) oligoclase gneissose-granitesof the basement (in the north-east part granites (gneissose-granites)of Murmansk block adjoin this line); (b) young microclinic granites;2.Granitic conglomerates; 3. Metabasites: (a) hornblende amphibolites after diabases, porphyrites, mandelstones with lenses of volcanic

crustal rocks of the Lower Proterozoic attributed to the Gimolian series compose volcanogene and sedimentaryvolcanogene formations.According to Chernov (1964)the strata of Kostomuksh deposit comprise tuff breccia, ferruginous,quartzites,leptites,h¿iZZejfi?ztaafter acid lavas and tuffs, and also amphibolic shales and amphibolites.In the basement of the cross-section granitic conglomerates with plagioclase-amphiboliccement are detected. The rocks of iron ore strata of the Kostomuksh's deposit form the narrow synclinal zone enclosed between the block ledges of the Archaean foundation. On the Kola Peninsula the relationships between the rocks of cherty-ironformation and the basement complex are recorded in detail only in the region of KoImozeroVoronya (Fig. 1). Here,in the basement of the formation, the basal granite conglomeratesoccurring on the oligoclase gneissose granite are distinguishable. The rocks of the cherty-ironformationoccur between the blocks of gneissose

conglomerates; (b) meta- and ultrabasites of Olenii ridge; 4.Lenses of magnetitic quartzites;5. Quartz porphyry, quartz

albitophyre, apokeratophyres ('porphyroides'); 6. Polymictic conglomerates of Poros suite; 7. Sedimentary rocks of Poros suite (aluminiferous shales,flyschoid bed of quartzites,shales, conglomerates); 8.Faults.

Unesco, 1973. Genesis of Precumbrian iron und manganese deposiis. Proc. Kiev Sump.,1970. (Earth sciences, 9.)

95

P. M.Goryainov

granites in the form of a narrow linear zone spreading over 100km;from the north the oligoclasebiotite porphyroblastic gneisses (‘gneissose granites’) of the Murmansk block adjoin this zone. According to Maslennikov (1969) the complex of supracrustalrocks of the Kolmozero-Voronya zone consists of two suites of different ages. The lower Polmos suite, with the discontinuity and with basal conglomerates in the base, occurs on the foundation rocks.It is represented by hornblende-amphibolites with relicts of ophitic and porphyritic structures and of amygdaloid textures,In the lower part of amphiboliteseries some thin lenses of magnetite quartzites occur. In the upper part of the series occur the numerous lenticular bodies of acid volcanites-leptites;quartz-porphyry,quartz albitophyre, with perfectly preserved relict structures of effusive rocks. In the amphibolite series are found the lenses of ovoid amphibolites. It is supposed that some small concordant lenses of ‘ovoid’amphibolites, according to their petrological and geological features, are to be considered as metamorphosed volcanic conglomerates. The essentially plagioclasic isolations here are supposed to be the explosive fragments of hypabyssal rocks analogous to the overlying acid volcanites which are cemented by volcanic material. The overlying Poros suite, with the interruption and with polymictic conglomerates in the base, occurs both on the amphibolites and quartz-porphyryof the Polmos suite and foundation rocks. This section,surprisingly,resembles the Kiruna section taking into account that,firstly,Kiruna volcanites are the deeper magmatic differentiates and, secondly, apatite-magnetiticore manifestations of the Kiruna type are absent in the section of the KolmozeroVoronya zone.A comparison of the two sectionsis given in Table 1. The structural relationships between the rocks of cherty-ironformation and basement in the industrial Iman-

dra lake region on the Kola Peninsula seem to be the most complicated.The main structure in Imandra lake region is defined by an oval arrangement of narrow iron ore bands and corresponding magnetic anomalies. Ferrous quartzites form large lenses wedging out to the sides, stretching up to 4 km and having thicknesses to up 300 m. They are underlain by massive amphibolites with interlayers of biotite gneisses (often with sulphides and graphite), and overlain by aluminiferous gneisses, banded amphibole gneisses, and leptites. Despite the intense metamorphism for the majority of rock varieties,the relict features of their volcanic origin are determined.Amphibolites preserve the signs of effusive diabases, porphyrites, mandelstones; leptites are the metamorphosed acid and intermediate volcanites, quartz keratophyres, quartz porphyry, dacitic porphyrites. The rocks of cherty-ironformationin the Imandra lake region are contiguous to coarse-grained,often porphyroblastic gneissoid rocks occurring inside the ovals and also on their outer side. These gneissose rocks are mainly oligoclase-biotitic,with 10-40per cent quartz content, or hornblende (or pyroxene)-oligoclase-biotiticones.The presence of microcline is not necessary. The cherty-ironformationwithin the main structure of the Imandra lake region and underlying gneissose granites and migmatites were considered to be isoclinally folded. Only in the process of detailed study of samples along the transverse profile did it become evident that the sharp, distinct ‘intense’gneissosity of migniatized gneissose granites is not the only structural element. It appeared that,in the rocks between the bands specifyingthe gneissosity,short fragmentary parts appear where biotite is orientedobliquely to the superimposed secondary gneissosity.Orientation of this ‘hatching’is surprisingly stable.The identified submeridional extension of this weak ‘hatch’orientation does not coincide with the spatial orientation of the north-western

TABLE1 Kiruna

Basement

Busal level

Kolmozero-Voronya

‘Ancientgneisses’-oligoclase gneissose granite and gneisses.

Oligoclase gneissose granites.

Granite conglomeratesin gneissose and carbonace-

Granite conglomeratesin cement of biotite gneisses.

ous cement.

Discontinuity

Basic lavas,spilites,diabase porphyrites,metamandelstones,and amphibolites after them. Lenses of jaspilite,and tufogenerocks. Series of Kirunavaara volcanic conglomerateswith fragments of plutonic abyssal analogues of overlying keratophyres. Keratophyres and quartz keratophyres with apatite magnetitic ores,quartz porphyry.

Hornblende amphibolites-massive and bandedwith relicts of porphyritic,ophitic structures,and amygdaloidtextures,Lensesof magnetite quartzites. ‘Ovoid‘amphibolites-volcanic conglomerateswith acid volcanic fragments(?).

Upper Khauki complex-Vakko series-quartzites, polymictic conglomerates,flyschoid series of

Poros suite-polymictic conglomerates,aluminiferous shales,quartzites.

phyllites.

96

Lenses and bodies of quartz porphyry, and quartz albitophyres in amphibolites.

Structural-tectonicenvironments of iron-oreprocess in the Baltic shield Precambrian

secondary distinct gneissosity. Hence, the cherty-ironformation, with its deep and north-western extension of gneissosity and the complex of migmatized biotite gneisses enclosed inside it, refers to structurally autonomous forms of different ages.The contacts of these complexes form the thick zone of blastomylonites where alkalinemetasomatites and subalkaline granite rocks are developed. Thus, it may be admitted that accumulation of volcanogenic cherty-ironformation of the region has taken place under the conditions of differentiation of the crust into foundationblocks already outlined.Between the cracks occurred the eruption of volcanites succeeded by sedimentation. In the south Pechenga zone of the Kola Peninsula (Allarechensk district) some small bodies of magnetite quartzites occur in amphibolites, aluminous gneisses and other leucocratic supracrustal rocks (apparently leptites). Ferruginous quartzites are here characterized by stratigraphic and lateral zoning exactly the same as in the Imandra lake region and possess all the genetic features of this class of supracrustal rocks. The structure of this region is a combination of isometric or gently oval blocks (more often called ‘domes’) of Archaean granitoid rocks-oligoclase gneissose granites and gneissose granodiorites and,chiefly,linear metabasitic (amphibolitic) series with lenses of magnetitic and magnetite-cummingtonitequartzites or crystalloschists.

Conclusions Thus, within the Baltic shield to such a level as ‘ironore’, the Precambrian corresponds to not only qualitativelycomparable events,but also to a qualitatively similar state of the Earth’s crust when those events took place. N o matter whether ferro-siliceous formations form linear zones of complex structure specified by a combination of isometric foundation blocks and their circumfluent supracrustaliron ore rocks,the cherty-ironformationsalways occupy another and higher structuralposition than the rocks composingthe blocks of Archaean basement. These basement rocks are characterized by monotonous composition: oligoclase biotite gneissose granites, gneissose granodiorites, gneissose diorites, more rarely amphibole or hypersthene gneisses (Karelia and the Kola Peninsula); gneisses of foundation,gneissose granites and gneissose syenites in Finland and Sweden. The cherty-iron formation and the complex of basement rocks often have obvious features of structural autonomy.The rocks of the basement are commonly characterizedby the gentle submeridional orientation of structural elements, but younger cherty-ironformations are north-west trending with highdip elements.The rocks of the foundationmay carry traces of two orientations of plane elements: primary and superimposed. If tectonic movements acquire a sharply differentiated character the blocks of the basement lose their shape and elongation;one can see this phenomenon in the Imandra lake region. Here the structural features of the

basement rocks slightly differ from those of the cherty-iron formation. Eventually the blocks of the basement may be converted into narrow elongated bodies which, due to the coming diaphthoresis, cannot be differentiated from the rocks of cherty-iron formation contained between them. Such areas developed for example, to the east from the Monchya-Volchya tundras and in the central Kola area (Chudiavr Lake, Volslipachk, Semb Lake, etc.). They are accompanied by narrow zones, elongated according the strike of the rocks, in which the granulite associations of minerals are distinguished.The appearance of the narrow bands of the granulite facies of the rocks, as well as the different block pattern of the basement, are both, in our opinion, manifestations of the coherent sharply differentiated movements during the process of folding. According to the present stratigraphicalsubdivision of the Precambrian,the rocks of the basement and the chertyiron formationmust be referred to the Archaean and Lower Proterozoic respectively, as was accepted for Karelia by Kratz (1963). Subdivision of the Archaean into Upper, Lower, Catarchean is not geologically substantiated.The differentiation of the crust into isometric blocks must be considered as the distinctive feature of Archaean geology. The position of Lower Proterozoic mobile zones was strongly governed by the Archaean blocks’ pattern. Sufficient volcanism and sedimentation were confined to the interblock parts, grouping as narrow linear zones. The volcanic processes in these zones of the Baltic shield began everywhere with extrusions (frequently submarine) of basic lavas-spilites and diabases.Further evolution of volcanism and the iron-oreprocess connected with it were indicated by the degree of the magma’s differentiation and developed towards either very acid differentiates (e.g. lceratophyric and quartz-porphyriticseries of Kiruna with the rich apatite-magnetiteores) or normally acid and intermediate rocks (e.g. the series of quartz porphyries, albitophyres, dacitic porphyries (leptites) of the Imandra lake region and Karelia with iron quartzites). The volcanic process could, al last, end with the same basic volcanites which formed at the beginning of this process, but without the development of large bodies of the ferruginous quartzites (metabasite-amphibolitebeds of the Kola Peninsula). According to Peive (1956) the volcanites of the platforms are characterized by greater differentiationthan the volcanites of the orogenic belts. If this is so, the Lower Proterozoic volcaiiites and ores of the Kiruna must reflect the formation of platform conditions. Thus, the Lower Proterozoic mobile zones could differ according to the degree of ‘orogeny’,which was the main criterion for the formational variety of the iron-bearing process which one can see in the Lower Proterozoic of the Baltic shield. Inregarding the isometricblock structures as structures of different ages which reflect the primary differentiationof the basement,the formationofinterblockmobile zones and intensive deposition in those zones, we must take into account other concepts. A characteristic feature of Early Cambrian structures of all shields was noticed a long time ago;most of these structures are interpreted as domes. 97

P. M . Goryainov

Eskola (1948) considers that the combination of the infra- and supracrustal structuresis greatly attributable to the penetration of gneiss domes into sedimentary rocks. Kranck (1959) and Wegman (1930) explained these struc-

tures in terms of granitic diapirism. Kalyaev (1970),accenting the principles of granitic diapirism, supposes that sedimentary-volcanogenicrocks of the Ukrainian shield are folded by the increasing granitization.

Résumé Environnement tectonique et structural des processus de fosmation du minerai de fer duns le Précumbsien du bouclier baltique (P.M . Goryainov)

L a différenciation du socle en blocs isométriques a eu lieu avant l’accumulation des formations de silex ferrugineux volcanogène sédimentaire. Pendant le plissement du Protérozoïque inférieur,ces blocs ont pu perdre leur contour original et au cours du processus de granitisation contemporain les signes d’une indépendance structurale du bouclier et du complexe du minerai de fer sont devenus moins distincts. L a comparaison des complexes de minerai de fer de

Karelija,en Suède centraleet septentrionale,et de la péninsule de Kola a montré que, d’abord,elles occupent toutes la m ê m e position structurale pénétrant dans le deuxième stade structural et, ensuite, qu’on y trouve de fortes corrélations qui permettent d’identifier des événements géologiques qui pourraient sembler absolument incomparables. L a présence de silex ferrugineux,calcifère,d‘amphibolites et de gneiss alumineux dans la zone des roches supérieuresde la croûte dénote un âge K non archéen ) ) plusjeune de toutes ces roches. Ceci est une des indications qui permettent de différencier les complexes archéens et protérozoïques.

Bibliography/ Bibliographie CHERNOV, V. M.1964. Stratigraphy and sedimentaryconditions of volcanogene (leptitic) cherty-ironformations in Karelia. Moscow-Leningrad,Nauka. ESKOLA, P. E. 1948. The problem of mantled gneiss domes. Quart. J. geol. Soc., Lond., vol. 104, no. 4. __ . 1967. Precambrian of Finland. Docembrii Scandimvii [Precambrian of Scandinavia]. Moscow, MIR. GEIJER, P. 1931. Berggrunden inom malmtrakten KirunaGallivarePajala [Geology of the ore area Kiruna-GallivarePajaia]. Sverig. geol. Unders. Afh., series C,no. 366. KALYAEV, Y.I. 1970.The problem of connection of granitoid magmatism and folding of the basement.Geotektoniku,no.1. KRANCK, E.H.1959.On the folding movements in the zone of the basement.Geol. Rdsch., no. 46.

KRATZ, K.O.1963.Geologyofthe Kareliakarelides.Leningrad, Academy of Sciences of the U.S.S.R. MASLENNIKOV, V.A.1969.The most ancient Precambrian of the Kola Peninsula (geology and geologic time). Thesis.Moscow, Institute of Geochemistry and Organic Chemistry of the Academy of Sciences of the U.S.S.R. PEIVE,A. V. 1956. The connection between deposition, folding, magmatism and mineral deposits and tectonics. Glavneishie typy glubinnych razlomov [Main types of deep faults]. Bull.Acad. Sci. URSS,geology series,no. 3. WEGMAN, C.E.1930.Uber Diapirismus.C.R.Soc. geol. Finl., no. 92.

Discussion A. M. GOODWIN. In the Canadian shield the oldest rock assemblages of Archaean age contain great thicknesses of clastic sediments,mainly greywacke and shale.Would you care to comment?

Karelian assemblages ofthe lower parts of the Baltic shield, Strange as it may seem,some difficultiesin correlation seem to be due to age-determination figures.

V. M . CHERNOV. What is the stratigraphic relationship P. M. GORYAINOV. I a m familiar with some data on the Canadian shield (Lake Superior and Michipicoten area). I believe that the so-calledArchaean supercrystallinerocks, cherty iron rocks included, may be compared with the 98

between the Olenegorsk suite and the Kola series?

P.M.GORYAINOV. The gneissiciron ore assemblage (Olenegorsk suite) and the Kola series are not co-ordinateunits.

Structural-tectonicenvironments of iron-oreprocess in the Baltic shield Precambrian

The Kola series is the Archaean undissected basement.The rocks of the gneissic iron ore (Olenegorsk suite) assemblage are Lower Proterozoic.

W.M.CHERNOV. Are you certain about the basement of the iron ore beds of Kola Peninsula?

P. M. GORYAINOV. In general, yes. In some cases estabV. M. CHERNOV. Are there any basal conglomerates between the dome structures which you refer to the basement and iron ore beds? If so, what is the thickness of the conglomerate, its composition and that of the matrix?

lishing the basements presents a problem because the rocks have undergone crushing and metasomatism. It is then difficult to distinguish between the basement and the rocks of the second structural stage (the gneissic iron ore beds).

V. M.CHERNOV. What is the attitude of the geologists of P.M.GORYAINOV. The basal conglomerates occur between iron ore beds and the basement blocks (not domes) in the

the Northwestern Geological Survey to your conception?

Kolmozero-Voronya region (Kola Peninsula), in the Kostamuksh deposits (Karelia) and in Kiruna (Sweden). The conglomerate is composed of rounded granite fragments.

P. M. GORYAINOV. M y conception is relatively new and it has not yet been verified.Future investigationswill reveal how well-grounded it is.

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Geology of the Precambrian cherty-iron formations of the Belgorod iron-ore region Yu. S. Zaitsev Voronezh Geological Prospecting Expedition, Ministry of Geology of the Russian Soviet Federated Socialist Republic, Voronezh (U.S.S.R.>

The Kursk Magnetic Anomaly (KMA)territory is covered with two separatebands of gravimetric-magneticanomalies, the most intensive ones being produced by steeply dipping iron quartzites. The north-east band consists of the Stary Oskol and Novy Oskol iron-oreregions.The south-west band includes the anomalies of the Belgorod and IgovMikhailovsky regions (Fig. 1). The Belgorod iron-oreregion (BIR)is a linear northwest trending zone of crystalline rocks in the south-east area of the KMA (12,000 kin2). The width of the zone ranges from 80 to 100 km and it extends over 150 km. In the BIR the crystalline rocks lie at a depth of 350750 m and are covered with Mezozoic and Palaeozoic sedimentary rocks. The Precambrian basement dips gently (7 m per 1 km) south-west towards the Dnieper-Donetz Basin. The cross-sectionof the BIR includes two structural stages divided by the boundary surface with the absolute age of 2,600&100m.y. The lower stage consists of repeatedly dislocated sedimentary-volcanogenic rocks locally granitized and magnetized;the upper stage is mainly composed of sedimentary-volcanogenicrocks forming a distinct structural-facialzone of the Proterozoic geosyncline. Studies of the crystalline rock sections make it possible to subdivide the BIR iron-ore formation into three types using Plaksenko’sclassification(1966): volcanogenic chertyiron; slate cherty-iron;clastogenic cherty-iron.Each formation characterized a certain period in the evolution of the old basement thus defining the principal stratigraphic unit of the Precambrian. Volcanogenic cherty-iron type. Rocks of this type differ from others in their chemical composition.In spite of the restricted occurrence in these rocks of the cycle sequence from chlorite and biotite-chloriteslates to stilpnomelanemagnetite slates and quartzites, available data show that silica, ferric iron, manganese and calcium increase. Alumina, ferrous iron and magnesium appear to decrease. Thus, the cherty-iron rocks of the Oboyan-Mikhailovsky series of the BIR represent a cherty-ironformation analogous to the Gimo1 unit of the Baltic Shield as well as

to the ferruginous quartzites of the Konsko-Verkhovtsevo series of the Ukrainian Precambrian. Slate cherty-iron type.Rocks of this iron-oreformation reflect continental deposition prevailing during the early Proterozoic time of geosynclinal development. The formation is a unit of the Kursk metamorphic series.In monocline sections it is characterized by facial changes of ferruginous quartzites deposited under the conditions of shallow sea (slightly ferruginous coarse-bandedand silicate rocks) grading into deep-seafacies(finely banded magnetite and micaironquartzites). References on itscompositionandstructural elements are given in the reportsof Plaksenko,Chaikin,etc. Stratigraphic division of the ferruginous quartzites of the BIR is now available only in the Yakovlevo and Gostishchevo deposits,where Chaikin and Rusinovich counted up to seven horizons of quartzites. The odd-numbered quartzites correspond to martite (magnetite)-hydro-liematite rocks and ferriferous silicates;even-numbered ones to martite (magnetite) rocks with mica-iron facies. The data show that the quartzites grade into the underlying rocks. The slate cherty-ironformation of the BIR has evident facial changes in submeridional and north-east directions. The thickness of the mica-ironfacies decreases and locally one or more primary oxide facies are absent. Ferrous, ferrous carbonate and silicate (magnetite,silicate quartzites and ferruginous silicate slates) facies prevail. Facial changes of deep sea sediments to shallower ones extending eastward and north-eastwardshow that the nearshore line of the depositional basin of the BIR was somewhere to the east of the Prokhorovsko-Korochanskgravimetric and magnetic anomalies. In the same direction,the metamorphism of cherty-ironrocks tends to increase from metamorphic slate (phyllonite) to gneiss. Clastogenic cherty-iron type.The formation of this type is a part of the Oskol series and generally occurs everywhere. Cherty-ironrocks are metamorphosed re-deposited sediments of the slate cherty-ironformation and are composed of conglomerates,gritstones,debris of ferruginous quartzites,metasandstones and slates.

Unesco, 1973. Genesis of Precambrian iron and manganese deposits. Proc. Kiev Symp., 1970. (Earth sciences, 9.)

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Yu.S. Zaitsev

FIG.1. Generalized map of the Precambrian of the Voronezh crystalline shield. (Zaitsev and Bogdanov, 1969.) 1. Upper Proterozoic microline and plagioclase-microlinegranite;2. Rapakivi granite, syenite, granosyenite; 3. Subplatform gabbronorite, olivine gabbro-norite,olivine gabbro-dolerite,gabbrodolerite, gabbro-diabase;4. Middle Proterozoic late orogenic basite-hyperbacite:peridotite, pyroxenite, dunite, olivinite as well as their metamorphosed differentiates, gabbro, gabbronorite,norite, gabbro-diorite,olivine gabbro and their veined differentiates;5. Syntectonicplagioclase granitoids;6.Early and

synorogenic gabbro, gabbro-diorite,gabbro-amphibolite rare meta-ultrabasite;orthogneiss;7.LateSvekofeno-Karelianfolding (1,700-2,000m.y.); 8. Early and Late Svekofeno-Karelides (1,700-2,600my.); 9.Early Karelian (Kursk) folding;10.Belomoride analogue influenced by the Late Svekofeno-Karelian folding; 11. Presvekofeno-Kareliangranitoids; 12.Belomoride analogue influenced by the Early Karelian (Kursk) folding; 13. Presvekofeno-Karelianmedian massif; 14.Ancient cores of Presvekofeno-Karelianfolding;15. Deep faults; 16.Bounds of the Belgorod iron-oreregion of the KMA.

Along the strike these rocks grade into cherty-iron rocks locally called ‘conglomeratic’or ‘nodular’quartzites. Martite mica-ironquartzites average about 50 m in thickness, ‘nodular’ones being up to 30 m thick. In contrast to the quartzites of the Kursk series,the orebands ofcoarsemartite mica-ironquartzitesofthe Oskol series commonly have sand-like structure. The sandy appearance is due to oval and octahedral martite grains of about 1.5 mm in flaky bands of hematite. Semi-orebands are mainly composed of oval,lenticular and round jasper debris of brick-red and pink colour averaging 2-3 mm occasionally 8 mm. Flaky material of iron glance is commonly interstitial and does not form separate bands,as it may be seen in the quartzites of the slate cherty-ironformation. Occasionally jasper debris occupies 25-40 per cent of rock volume.The

rocks are composed of iron hydroxides evenly dispersed or in patches. ‘Conglomeratic’quartzites have lenticular echelon-likedebris of hornfels in silicate cherty-ironrocks. Conglomerates and gritstones grading into metasandstones,metasiltstones and slates are abundantin the crosssection of the clastogenic cherty-ironformation.The bands of conglomerates range from 5 to 30 m.W h e n products of the Kursk quartzites erosion,the conglomerates consist of quartzitedebris(mainly semi-orerocks) cementedby quartz, hydromica and ferruginous material. Psammitic and psephitic components usually contain jasper hornfels debris, granitoid and quartz pebbles. They are commonly of elongated, lens-flattened,round and irregular form.Typical magnetite (martite) quartzites of the slate cherty-ironformation as well as ore quartzites are absent in the debris material.

102

Geology of the Precambrian cherty-ironforniations of the Belgorod iron-oreregion

The conglomerates, gritstones and metasandstones grade into martite-mica-iron-sericiteslates and metasiltstones. The contact of intergrading rocks with the underlying psephitic ones is not sharp. Martite mica-iron sericite slates and metasiltstones range from 1 to 20 m in thickness, which is proportional to the thickness of conglomeratesand metasandstones. Thus, according to its geological and sedimentary features,the clastogene cherty-ironformation of the BIR corresponds to the carbonaceousterrigenous formation of the Upper Krivoyrog series of the Ukrainian crystalline shield. On the basis of the geological evidence, the BIR is a complex multistage unit, Comprising extensively dislocated and variously metamorphosed Precambrian rocks of the Belgorod synclinorium.The rocks of the basement include two structural stages. The contact between them is welldefined by folding, magmatic activity,regional metamorphism and old weathering. These stages characterize primary geosynclineand geosynclinecycles ofthe Precambrian. Stratigraphicdivision of the crystalline rocks involving lithologicaland stratigraphicmethods as well as the formation analyses must also be based on the main geological features of the iron-oreformations. These features make it possibleto develop reliably correlatedstratigraphicsections. The data obtained show that a great period of evolution corresponds to a certain geological formation with specific features of sedimentation and volcanic activity. The earliest stage-the Archaean-is characterized by a volcanogenic cherty-ironformation associatedwith volcanogenic spilite keratophyre rocks. The formation was deposited in a changing redox environment depending on the

volcanoes' position in the basin of sedimentation.Ferrous iron is more abundant near the shore line (carbonaceous and silicate forms). In the open sea environment ferric iron prevails. Ferrous iron material is much less abundant with decreasing content of alumina,magnesium and titanium. Absence of hematite in this formation within the BIR shows a relatively low oxidizing environmentand an abundance of volcanoes. In this respect our results coincide with those of N.Strakhov concerning iron reduction in cherty ores of exhalation type. Silicate cherty-ironrocks of the BIR may be referred to this type. In the Lower Proterozoic,when the developmentof the mobile zone began, the slate cherty-ironrocks were formed. They consist of the intergradingmetasandstones,slates and ferruginous quartzites. Environmental conditions of this formation are characterized by the changes of the ore and non-orefacies (from silicate and carbonateto magnetite and hematite) which reflect the increase of the depth from the shore line. The changes in the structure of the geosyncline occurred in the Lower and Middle Proterozoic time and are characterizedby the formation ofclastogenerocks involving the material of disintegration and redeposition of the metamorphosed rocks of the Kursk series. The subdivision of the BIR iron-oreformation into different genetic cherty-iron types corresponding to the stages of tectonic and magmatic activity probably reflects the general tendency of the iron ore formations in the early Proterozoic. This tendency may spread over the other regions of the Voronezh crystalline massif and other nearby regions in the East-Europeanplatform.

Résumé Géologie desformations précambriennes de fer. siliceux dans le gisement de Belgorod (Yu. S. Zaitsev)

1. La région du gisement de fer de Belgorod, située dans la structure précambrienne de l'anomalie magnétique de Koursk, est une formation complexe à plusieurs étages constituée par les structures cristallines fortement disloquées et différemment métamorphosées qui forment le synclinorium de Belgorod.Les roches basiques y forment deux étages structuraux,séparés l'un de l'autrepar un étage de plissement,par des manifestations d'activité magmatique, par une ancienne croûte de désagrégation.Ces étages représentent deux périodes de développement : la période progéosynclinale et la période géosynclinale inhérente. 2. La classification stratigraphique des roches cristal-

lines vise à clarifierles particularités et les principes de développement des formations de fer qui sont les plus typiques dans la section précambrienne. I1 se trouve qu'à grande échelle chaque étage correspond à un typebien défini deformation de minerai de fer présentant des particularités spécifiques des processus de sédimentation et d'activité volcanique séquentiels dans le temps. 3. Trois étages de formationde fer siliceux peuvent être distingués :(a) fer siliceux volcanogénique ; (b) fer siliceux schisteux ; (c) fer siliceux clastogène. 4.L a séparation des différents types génétiques de formations de fer de la région de Belgorod en complexes individuels superficiels de la base cristalline semble refléter les principes généraux de la formation du minerai de fer dans le Précambrien ancien.

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Iron-formation and associated manganese in Brazil’ J. Van N.Dorr II United States Geological Survey Washington,D.C.(United States)

Introduction This paper summarizes data on the major iron-formations2 in Brazil, the related manganiferous deposits, and some unrelated ones, and thus provides a frame of reference for the more detailed discussions of individual areas to be presented to this symposiumby m y colleaguesD r s Barbosa, Grossi, Scarpellí and Tolbert. Unfortunately,because we were separated by great distances, it was not possible to consult during the preparation of our individual papers. I trust that no discrepanciesgreater than customary between geologists will appear. At the request of D r Xngerson, I shall also discuss a manganese-iron deposit of probable Cambrian and Ordovician age because it seems quite similar to, although richer than, most Precambrian deposits aiid, being essentially unmetamorphosed,may throw some light on those older deposits.In these discussions,I shall try to approach the deposits from the point of view of their sedimentary environments rather than from their detailed mineralogy,economic potential,epigenetic alteration or metamorphic history, although of course these factors cannot be ignored. That the banded iron-formationsenclosed in Precambrian sedimentary and metamorphic rocks are sedimentary in origin seems so widely accepted today by geologists that thare is no need to labour the point. The acceptance of the concept of sedimentary facies in iron-formationdefined by James (1954)is also widely accepted, although epigenetic processes such as weathering,metasomatic and hydrothermal activity and metamorphism may obscure the original nature of these facies. I believe that everyone also accepts the evidence that the iron-formations were dominantly chemical sediments, although in some places contaminated by detrital debris. So far only two facies of iron-formationhave been found in Brazil,the oxide and the carbonate.Of these,the oxide is by far the more widespread both in time and space. It is of course quite possible that much more carbonatefacies iron-formationwill be found;the rock oxidizes at the surface to a weathering product almost indistinguishable

from that of oxide-faciesiron-formation. Only by explorationbelow the zone of oxidationcan this rock be definitely identified. Until the work of Gair (1962) and Matheson (1956), the presence of sideriticiron-formationin Brazil had not been established. It is not as widely recognized that the manganiferous sediments also were deposited in sedimentary facies similar to,and to some extent parallel to,the sedimentaryfacies of the iron-formations,although commonly they are not interbanded with chert. T o m y knowledge,the sulphide-facies has never been reported in sedimentary manganese deposits, although carbonate-faciesand oxide-facies are very common. Silicate-facies exists if one accepts the premise that braunite may be a primary sedimentary or diagenetic mineral. Other manganese silicate minerals are commonly regarded as hydrothermal,igneous or metamorphic minerals,although the line between hydrothermal and sedimentary deposits becomes quite vague in volcanogene sedimentary manganese deposits.Braunite,bementite,and neotocite may be primary sedimentary minerals in such cases.Fortunately,in Brazil w e do not have to consider such borderline deposits,for they have not yet been identified;here we have only carbonate-and oxide-facies manganiferous deposits. Precambrianrocks crop out in perhaps half the area of Brazil,or an area of about 4million km2. These shield rocks are found from the northernmost to southernmost extremities of the country and from the easternmost areas to those farthest west. Most of these Precambrian rocks are metasedimentary; the task of unravelling their relative and absolute ages is just beginning and it will be many years before all the complexities are resolved and accurate correlations made. Deep weathering in much of Brazil and extreme difficulty of travel in the forested areas have made detailed geologic work difficult and for this very reason 1. Publication authorized by the Director, United States Geological Survey. 2. Iron-formation was defined by James (1954) as a ‘chemical sediment, typically thin-bedded or laminated, containing 15 per cent or more iron of sedimentary origin, commonly but not necessarily containing layers of chert’.

Unesco, 1973. Genesis of Precumùriun iron u n d r?iungunese deposits. Proc. Kiev Symp., 1970. (Earth sciences, 9.)

1 OS

J. Van N.Dorr Il

Brazil is still a country of major undiscovered resources. Brazil's major known iron-formationdepositsare those in Minas Gerais in central Brazil (Fig. i), in Pará in the Amazon area and in Mato Grosso. Smaller deposits are known in Ceará,Bahia and Amapá. Important manganese deposits are found in Minas Gerais, Bahia, Goiás (not shown), Amapá, and Mato Grosso. The deposits in Mato Grosso are Cambrian and Ordovician in age,the others are Precambrian. There is no reason to suppose that all the iron or mangagnese deposits of significant size have been discovered,and care should be exercised in projecting patterns from the deposits now known.

Deposits in Minas Gerais The only carbonate-facies iron-formationyet known in Brazil is in the Nova Lima Group of the Rio das Velhas Series in Minas Gerais, a eugeosynclinal suite of sedimentary rocks not less than 5,000m thick dated as being older than 2,700 m.y. (Aldrich et al.,1964) by Rb-Sr analysis of muscovite formed in a contact aureole.The most complete description of these ferruginousrocks is that by Gair (1962). They are typical banded metachert-siderite containing varying quantities of magnetite. The lenses of this rock are relatively thin,ranging to perhaps 75 m but generally less, and ranging in length from a few tens of metres to perhaps 10 km or more. Commonly,they are only a few kilometres to a few hundred metres in strike length. The enclosing

O'

12'

24'

FIG.1. Map showingdistribution ofmajor iron (Fe)and manganese (Mn) deposits in Brazil and part of Bolivia.

106

Iron-formationand associated manganese in Brazil

rocks are now phyllite and schist; the original sediments were argillaceous sandstones, claystones, locally carbonaceous, tuffaceous clays and tuffs and probably some extrusive volcanic rocks. The original volcanic rocks are thought to have been mafic or intermediaterocks.The ironformation beds are interbedded with phyllite aiid may grade into ferruginous phyllite, carbonaceous phyllite or white quartzite (metachert) along strike. Methane has been detected in the same group of rocks in the nearby Morro Velho gold mine, probably derived from the organic material in the carbonaceous phyllites, which are widespread.The evidencefor a reducing environment during deposition of the carbonate-faciesiron-forma tion seems strong. The carbonate-faciesiron-formationin this region has not been altered to significant bodies of iron ore of usable grade either by metasomatism or by supergene enrichment, even though very largebodies ofhigh-gradeiron ore of both types of origin are found in younger oxide-faciesiron-formation nearby. This may be because carbonate-faciesironformation generally forms smalllenses,has most of its iron in the divalent form and is thus fugitive under weathering conditions compared to trivalent iron in the oxide-facies, or because the carbonate-faciesiron-formationis more plastic than the relatively brittle oxide-facies formation, thus reducing permeability, or a combination of factors. A discontinuous zone of manganese silicate-carbonate rock is found in a belt of rocks correlated with the Rio das Velhas Series; the belt stretches some 200 km north-east from São João del Rey. These metasedimentary manganiferous rocks are enclosed in graphitic phyllite, phyllite, schist and amphibolite. It is not yet known whether the amphibolite is metasedimentary or metavolcanic in origin. The metasediments are similar to the rocks that contain the carbonate-faciesiron-formation,but the zone of manganese silicate and carbonate cannot be confidently correlated stratigraphically with the zone containing the iron-formation. The manganese silicate-carbonateis believed to be the meíamorphic equivalentof original silty manganese carbonate beds deposited in a reducing environment, attested by the uniform but small content of free carbon in the ore (Dorr,Coelho and Horen, 1956) and in the wall rocks. Where the original manganese carbonate content was high and the sediment was relatively uncontaminated by detrital material, metamorphism did not form abundant silicate minerals;where there was a large admixture of silt and clay, spessartite,rhodonite,and many other silicates were formed at the expense of rhodochrosite.Weathering has produced large masses of manganese oxide from the manganese carbonate lenses,but the silicates did not yield significantquantities of oxide ore on weathering. Thus,these carbonate-faciesmanganese- and iron-formations were formed in a eugeosynclinalenvironment,iron and manganese were separated in space during sedimentation, and significant bodies of the two types of deposit were not laid down together,although both are widespread. There is no good evidence as to the source of the manganese and iron in the Rio das Velhas rocks.Although

-

volcanism was active in the generalregion,as attested by the tuffaceous sediments and probable extrusive rocks in the suite, it cannot be proved that these, or thermal waters emanating from volcanic sources, have any genetic connexion with the manganese or iron.The enormous thickness of Rio das Velhas clasticrocks is good evidence that a large land mass was being eroded during deposition of the sediments, and this might have furnished ample supplies of these elements to the basin of deposition. Spatially within a few hundred metres to a few tens of kilometresfrom these Rio das Velhas iron-and manganeseformations,but separated from them by vast reaches of time, are the manganese and iron deposits of the Minas Series.The Minas Series overlies the older rocks with profound angular and erosional unconformity; an orogenic event separates the two series. Much of the Minas Series was laid down in a miogeosynclinal or platform environment, and the unit is about 3,500m thick (Dorr, 1969). The age of the rocks is still uncertain;they were probably deposited between 2,200and 1,350 m.y. ago. The iron-formation of the Minas Series will be described in some detail by Professor Barbosa;I shall merely sketch in some of the more important points concerning the relation of manganese to this iron-formation,which crops out widely in the Quadrilátero Ferrífero, a large iron-rich area centring at about 20°15'S., 37O3O'W. The principal iron-formation,known as the Cauê Itabirite,is typically made up ofbanded quartzand hematite;the pre-metamorphicrock was chert and hematite, and magnetite was probably present locally in significant amounts. The formation was continuous for a minimum distance of 150 km in an east-west direction and 100 km in a north-south direction; the original thickness was probably about 250-300 m . Significant intercalated clastic sediments have not been found, although locally the rock contains some clay. Much more important from the view-point of sedimentary environment is the presence of dolomite interbedded with the iron-formation;in some cases it makes a threefold layering with the quartz and hematite, and in others it substitutes for the quartz bands. Where dolomite is abundant, magnetite is much more common; it is not certain whether this is a diagenetic or a metamorphic feature.Beds of dolomite approximately 1 m to 20m thick may also be intercalated in the iron-formation.Gradationally overlying the iron-formationis a thick formation largely composedofdolomitemarble,dolomiticphyllite,and minor dolomiticiron-formation.Iron carbonateminerals have not been found in the dolomitic iron-formationexcept in very minor quantity as epigenetic minerals,thus the rock cannot be considered carbonate-faciesiron-formationeven though it may contain much carbonate. The iron is predominantly in a trivalent state. Although the manganese content of most of the ironformation is very low, lenses of manganiferous rock enriched by supergeneprocessesinto usable ore deposits range in size to as much as 5 million tons and contain between 30 and 48 per cent Mn. Individual lenses are more than 1 km in strike length in very few cases;normally they are a few 107

J. Van N.Dorr II

hundred metres long, but a single stratigraphic zone containing manganiferous lenses may be more than 10 k m in length.The manganiferous lenses are usually less than 3 m thick. They are more common in the upper part of the formation, as are the dolomitic iron-formationsand dolomite lenses.The manganiferous lenses are in places,althoughnot invariably,closely associated with the presence of dolomite in the rock. In contrast to the manganiferous beds in the Rio das Velhas Series, these manganiferous sedimentary rocks are within the iron-formationitself. Unfortunately,because of the deep weathering and the fact that the enclosing rocks are too soft to mine to great depths without excessive timbering, exploration of the deposits lias not gone to a depth at which unaltered rock is found. Thus, the tenor, mineralogy and the general character of the originalrock from which these deposits formed by supergene concentration are unknown. Smaller deposits in the dolomite overlying the iron-formationmay give a clue as to the origin of these deposits; they are known to have been derived from manganoan dolomite containing from 5 to 40 clarkes of M n . It is quite probable that some of the deposits in the iron-formationwere also derived from manganoan dolomite,either interlayered in the iron-formation in thin bands,as in dolomitic itabirite,or as somewhat thicker beds. It is also probable that some manganese oxide was deposited synchronously with iron oxide in the ironformation without dolomite,and was concentrated during weathering. The p H during the deposition of the manganiferous iron-formationmust have fluctuated slightly on either side of 7.8,the limestone fence of Krumbein and Garrels (1952), as shown by the intermittent deposition of dolomite. The Eh may have been around O during depositionof the manganoan carbonate,as the oxidation potentialneeded to convert ferrousiron to ferric iron is much less than that required to oxidize divalent manganese to tri- or quadrivalent manganese, and the iron would oxidize first (Mason, 1949). The manganese would very possibly be deposited as manganoan dolomite or limestone.W h e n the Eh and the p H were higher,the manganese might well have been deposited with the iron in oxide form. Dolomite with included primary manganese oxide is not known in fresh rocks in the region. Although the rocks have been metamorphosed to the greenschist facies and higher, manganese silicates are rare in the Minas Series,having been found only in manganiferous phyllite, not in dolomite or iron-formation. Both the oxide-facies iron-formationand the manganiferous rocks ofthe Minas Serieshave been enriched to ore grade by supergene enrichment (Dorr, 1964). High-grade hematite deposits of great size and extreme purity have been formed by metasomatic enrichment during the last metamorphism that affected all the Precambrian rocks of the region (Dorr, 1965). The source of the manganese and iron in the Cauê Itabirite and the overlying dolomite cannot be proved. It is very probable that they were derived from the weathering of the Rio das Velhas Series,which the Minas Series transgresses; an ample source of both elements is present here 1 O8

and it is known that rocks of the Rio das Velhas were peneplained before Minas time. Volcanic rocks are rare in the lower and middle Minas Series;in the upper Minas Series they overlie hundreds of metres of nonvolcanic miogeosynclinal sediments that had been deposited on the dolomite overlying the Cauê Itabirite.

Deposits in Bahia In the Urandí district of southern Bahia, centred about 14'50' S., 42"40'W,, iron-formationand economic manganese deposits are also known. The area is remote and a detailed geologicmap of the district as a whole has yet to be completed. Exposures are very poor and weathering is intense. It is understood that the rocks have been metamorphosed to a somewhat higher grade than those in the Quadrilátero Ferrífero,although the argillaceous rocks are still classified as phyllite. Jacobsite is an ore mineral (Ribeiro, 1966) and the ore zones with jacobsite can be traced by magnetometer (Ribeiro and Ellert,1969).The enclosing rocks have been correlated with the Minas Series,although this correlationis not absolutely certain. In any case, the Urandí manganese deposits are lenticular, some are closely associated with iron-formation, and both the iron-formationand most ofthe originalmanganiferous sediments seem to have been oxide-facies.Recause some of the ore is extremely pulverulent,similar to some of the manganese ore derived from dolomite in the Quadrilátero Ferrífero, it is possible that some of the ore was derived by weathering of manganoan dolomite or manganiferous phyllite.N o volcanic rocks contemporary with the original rocks have been reported in the region. Farther north in Bahia,in the regions of Nazaré and Jacobina, manganese oxide deposits derived by supergene enrichment ofmanganiferousphyllite have produced a small tonnage of commercial ore, but these poorly exposed and superficiallyexplored deposits throw little light on the origin of the manganese,It seems probable that the manganese was an oxide sediment syngenetic with the siltstone or mudstone from which the manganiferous phyllite was formed. The sedimentary suite was probably miogeosynclinal or platform in depositional environment. The State of Bahia contains many lenses of iron-formation, some of large size, in the highly metamorphosed Precambrian metasedimentary rocks. None of these lenses have been thoroughly studied and the geologic environment is not clear. Metamorphism has transformed much of the iron into rather coarse-grainedmagnetite and, although it seems probable that the original facies was oxide, this cannot be confidently affirmed in the present state of our knowledge.

Deposits in Amapá The Serra do Navío is a major manganiferous ore-producing district in the Territory of Amapá, north of the

'

Iron-formationand associated manganese in Brazil

Amazon at about 0°59'N., 52"05'W. These deposits will be described for this symposium by Dr Scarpelli,who has studied them in detail in extensiveexcavations and in many thousands of metres of drill core.The area has few natural outcrops, has been deeply weathered, and is covered by dense rain forest. The structure is highly complex. The rocks of the Serra do Navío district belong to the Serra do Navío and the underlying Jornal Groups of the Amapá Series.The age of these dominantly metasedimentary rocks is unknown,but they are older than 1,800 m.y. (Almeida et al., 1968). They may well be contemporary with the Imataca Series,the rocks containing great oxidefacies iron-formations and unimportant manganese deposits in Venezuela. Also includedin theAmapá Series are metasedimentary rocks ofthe Santa Maria Group,containing importantironformation some 85 km from Serra do Navío. The relative stratigraphic position of the Serra do Navío and Jornal Groups and the Santa Maria Group is not certain; they could be contemporaneous, or the Santa Maria Group could be older than the others, as suggested by Nagell (1962). In the rain forest such matters are not easily clarified.The Santa Maria Group iron-formationis oxide-facies. The Jornal Group is largely amphibolite, considered by Nagell (1962) to be a metasediment, but by Scarpelli (1966) to be an ortho-amphibolite.Work is in progress to determine the origin of this rock, the most consistent in composition and the most widespread of the rocks of the Amapá Series. Scarpelli informs me that he now considers the rock a para-amphibolite(writtencommunication,1970). Overlying the Jornal Group is the Serra do Navío Group. Whether or not the contact is conformable is not clear. The Serra do Navío Group consists of dominant quartz-biotite-garnetschist. Scarpelli has subdivided this into three facies: quartzose, biotitic, and graphitic. The manganiferous rocks are in the graphitic facies and, where pure, consist of rhodochrosite marble with very minor rhodonite.Calcite marble lenses are also present.The graphitic facies may contain as much as 20 per cent graphite in the schist. Rhodochrosite was clearly the original manganesemineral in the rock; as in the Morro da Mina deposit in Minas Gerais and many similar deposits elsewhere in the world,it recrystallized during metamorphism,but new minerals were not iormed in the pure rhodochrosite lenses. Where the rhodochrosjte was mixed with clay and other detrital sediments,a suite ofmanganese silicateminerals formed which will be described by D r Scarpelli.Silicateminerals are abundant on the walls of the carbonate lenses. The rhodochrosite in the protore ranges from 2 to 99 per cent. Scarpelli has shown that tlie three facies of the Serra do Navío Group were deposited cyclically;as many as three cycles are present in some localities,each representing tens of metres of sediments. Nagell (1962) suggested that the original sediments were deposited in a euxinic environment, as indicated by the high carbon content of the enclosing rocks and also by the relatively high concentration of arsenic in the oxide ore derived from the protore. Scarpelli

further suggests,and I concur, that the sedimentswere deposited in an unstable shelf or lagoonal environment. In the general area of the Serra do Navío manganese deposits,patches of ferruginous laterite cover considerable areas of the high plateaux. I do not know whether these represent lenticular iron-formations in the bedrock or whether they are merely the surface concentration of hydrated iron over iron-richigneousor metasedimentaryrocks expected under these climatic and physiographic conditions, as are found in so many parts of West Africa.The essential point is that the original carbonate-faciesmanganese sediments are separated spatially, although not necessarily in time, from iron-formation,as was found to be the case in Minas Gerais.

Deposits in Pará In the State of Pará near 6"S.,51"20'W.,an extensivearea underlain by thick iron-formationwas recently found by D r Tolbert,who will describe the deposit. I had the privilege of visiting tlie region in 1968,but much more has been learned about it by drilling and surface geology since that time. The area is one of the most remote and difficult to traverse of any in the rain forest of Brazil and the rocks are deeply weathered,thus we may expect years to elapse before w e know most of the details of the geology. Judging from what I could observe, the iron-formationwas oxide-facies. According to Dr Tremaine (oral commuiiicatioii,1970), it is associatedwith quartzite and underlain by conglomerate. I saw 110 rocks that appeared to be volcanic, but outcrops are rare and scattered and the region vast.From this association the iron-formationseems to have been deposited in a platform environment.Manganese ispresent in the general region,but we do not yet know whether or not it is in the same stratigraphicunit as tlie iron-formation.W e may confidentlyexpect anotable incrementofknowledgeconcerning these matters in the future.

Deposits in Mato Grosso and adjacent Bolivia One ofthe largest and highest-gradeknown depositsofironformation and of unenriched sedimentary manganese oxide in the world is found in a geologically homogeneous area astride the boundary of Brazil and Bolivia near latitude 19"15'S. In Brazil this is in the state of Mato Grosso. Almost all the known manganese is on the Braziliaii side of the border; the iron-formationwith which it is ínterstratified is found in enormous quantity on both sides of the border.The best known area is in Morro do Urucum (Dorr, 1945). The economically interesting rocks are in the Band' Alta Formation of the Jacadigo Series. The age of these rocks is not certain, as diagnostic fossils have not been described.For many years the rocks were considered to be Silurian in age; some geologists,including Shatskiy (1954) 1 o9

J. Van N.Dorr II

and, for many years,myself, believed that a late Precambrian age was more probable. Recent regional work by Almeida (written communication,1969)led him to attribute a Cambrian and Ordovician age to the Jacadigo Series. I have heard at second hand that a United Nations geological team working in Bolivia found brachiopods in the ironformation,but have not seen this in print and cannotvouch for the accuracy of this statement. Conglomerates in the Jacadigo Series contain cobbles of a nearby granite, which has been radiometricallydated at a minimum age of 888 m.y., and of metamorphosed rocks of the Corumba Series, a metamorphism dated radiometrically as about 550 m.y. (Almeida and Hassui, unpublished data). For these reasons I concur with Almeida's assignment of these strata to the Cambrian and Ordovician. Unlike all the other ferruginousrocks described above, the Jacadigo Series is only gently and slightly folded and is unmetamorphosed, The degree of weathering and of supergene enrichment is very minor indeed; the rocks are somewhat leached at the surface, but mechanical erosion here dominates over chemical and fresh rock is close to,or at, the surface in most exposures. The iron-formationis very resistantto erosion and stands in high buttes and mesas bounded by steep slopes and nearly vertical cliffs. The Jacadigo Series consists of a basal formation some 350 m thick composed of clastic rocks,dominantly coarsegrained and dominantly arkosic,togetherwith local channel sandsand puddingstoneconglomeratesand someapparently lacustrine beds,the whole cemented by calcium carbonate. Crossbedding attests continentaland near-shoreconditions. Gradationally overlying this thick clastic formation is a formation about 100 m thick made up largely of jasper, massively bedded,a cliff-formingunit.Above thetransitional zone, the formation contains very little clastic material, although much of the jasper is colitic. This material is not banded and, although the iron content is perhaps 20 per cent,it could not be called a banded iron-formation. Above the jasper formation, with rather abrupt but completely conformable contact, lies the Band' Alta formation.It is not less than 350 m thick,composed of banded hematite-jasperrock containing lenses and beds of manganese oxide and of detrital rocks, some quite coarse. The iron-formationconsists of alternating bands of quite pure, very finely crystalline blue hematite and of red jasper. The hematite bands are generally 1 c m or less in thickness but reach 10 c m locally;the jasper bands are slightly thinner but may range to several tens of centimetres in thickness, though this is rare. Except where contaminated by detrital material both the hematite and jasper bands are essentially monomineralic. Beds of detrital material and of manganese oxide are intercalated within the banded iron-formation.The detrital material rangesfrom well-sortedmedium-grainedsandstone to poorly sorted conglomeratic rock with boulders as much as 30 c m and more in diameter. The boulders appear to be granitic; near the surface all are so altered by throughpassing waters that the identificationis not secure,for they now consist of iron-stainedclay and quartz. Much of the 110

coarsermaterialis quite angular.The medium-grainedsandstone is,in contrast,moderately rounded.The detrital material is in beds ranging from a few centimetres thick to zones 30 m thick in Morro do Urucum.None of the coarser detrital beds have great lateral extent. The manganese oxide (cryptomelane) beds range from 1 c m to more than 6 m thick.T w o main beds are known in Morro do Urucum,both in the lower part of the formation. The lower and most widespread bed averages almost 2 m in thickness,the upper bed perhaps 1 m.An unknown,but probably considerable,part of the manganese beds has been removed by erosion;the part of the main bed that remains is not less than 5 km2in extent. The upper bed in Morro do Urucum is about 3.3 km2in extent. Other manganese oxide beds are known in the region in this formation;none are apparently as thick or widespread as the main bed of Morro do Urucum. The manganese oxide beds are almost all intercalated between clasticbeds in the iron-formation.Commonly these clastic beds are only a few centimetresto tens of centimetres thick; the clastic beds below the manganese oxide beds are well-sortedmedium-grained sandstone. In many places the overlying beds are similar but locally the overlying clastic bed contains large boulders and cobbles in a poorly sorted medium- to fine-grainedsandy and clayey matrix; in such areasthe overlying clastic beds may be more than 1 m thick. Detrital grains occur widely scatteredin the manganese bed; they are rare in the iron-formation.Average analyses of the iron-formationand of the manganese oxide lenses in Morro do Urucum (Dorr, 1945), are given in Table 1. TABLE1 Manganiferous beds average (%)

Fe

11.1 (range8-16) 45.6(range39.4-50.7) Si02 1.25 '%,O, 1.74 MgO 0.13 Ca0 0.20 Kz0 3.52 Iron-silicaratio 8.8:1

Mn

Iron-manganese ratio 0.24 1

Banded hematite average (%)

56.9(range48.7-62.1) 0.08(range 0.005-0.60) 17.3 0.65 0.06 0.06 0.20

3.29:1 710: 1

A number of complete analyses of the manganese ores and the iron-formationmade by the United States Geological Survey laboratories are also quoted (Table 2)from Dorr (1945).

All the data so far given apply to Morro do Urucum. Investigations,almost all unpublished,of adjacent areas in Brazil and Bolivia have been carried on in recent years by many geologists and engineers and Ihave been able to learn of some of the results. Some of the information is particularly important in giving clues to the sedimentary environment in which the Jacadigo Series was deposited. About fifteen years ago the Bolivian Government spon-

J. Van N.Dorr II

200 million tons of manganese ore were present before erosion;I now consider this estimate,made before the discovery of the ore in Rabicho and Mutum, to be extremely conservative,perhaps by an order of magnitude. Although Shatskiy (1954)suggested that the iron and manganese might have been derived from the weathering of the older iron-formationsof the Precambrian shield, this hardly seems probable,as the nearest large known deposits are some,1,500to 1,700km from the Urucum deposits.The nearer ones, and possibly the others also, were probably covered by youngerrocks during deposition of the Jacadigo Series.Beneath the Jacadigo Series lies a complex of highly metamorphosed Precambrian crystalline rocks older than the granite mentioned above.It is conceivable that weathering of this complex supplied the iron and manganese.The scale of this,like most major iron-formations, is so vast that the problem of ultimate source is not easily solved.

Summary To summarize,major deposits of both oxide-an carbonatefacies iron-formation,and manganiferous sediments of Precambrian and early Palaeozoic age are known in Brazil. In several places, the oxide-faciesiron-formationis closely associated with manganiferous rocks, the latter being interbedded with the iron-formation.However.in no known case

is banded iron-formation closely associated with carbonate-faciesmanganese deposits,even though both iron-and manganese-formationmay occur in the same sedimentary unit. In all cases, carbonate-facies manganese-formation is closely associated with unusually carbonaceous sediments.That this condition is not peculiar to Brazil is shown by the presence of such ores in Africa; there oxide-facies iron and manganese deposits on a very large scale are interbedded in the Kuruman District of South Africa,whereas the carbonate-facies sedimentary manganese deposits of Ghana, the Ivory Coast, Upper Volta and, probably, the Congo are not associated closely with iron-formation.Graphitic or carbonaceous sediments are associated. India showsus that oxide-faciesmanganese-formationsmay occur without oxide-faciesiron-formation,and Gabon shows us that carbonate-faciesiron-formationmay be followed iii the sedimentary sequence by carbonate-facies manganese-formation, there also associated with carbonaceous rocks. All these deposits show that oxide-faciessediments are more likely to be found in a platform or miogeosynclinal environment,but the rule is not invariable;a number of carbonate-faciesmanganese-formationsare found in platform or estuarian environments. By the same token, a eugeosynclinalenvironmentseems to be the most favourable for carbonate-faciesmanganese-formationsand iron-formations too. I know of no major oxide-faciesmanganese-formation deposited in a clearly eugeosynclinal environment.

Résumé Formation de fer et de munganèss ell ussociution, air Brésil

(J. Van N.Dorr II) A u Brésil, des concentrations de fer et de manganèse sont connues des points extrêmes au nord et au sud du bouclier. Les minerais commerciaux, à l’exception des minerais de manganèse près de Corumba dans le Mato Grosso, ont été formés épigénétiquement à partir de formations de fer rubanées et de roches métasédimentaires riches en manganèse par de nombreux processus. Le degré de métamorphisme varie largement. Dans certaines régions, les sédiments mangaiiifères sont étroitement associés aux formations de fer. Dans d’autres,l’associationest équivoque. Dans d’autres, on ne connaît aucune association. L’âge des formations de fer rubanées et des roches sédimentaires manganésifères va du Cambro-Ordovicien dans le Mato Grosso à plus de 2,7 milliards d‘années dans l’État de Minas Gerais. Les principales formations de fer sont celles du Mato Grosso, celle de la partie centrale de Minas Gerais (entre 2200 et 1350 millions d‘années) et celle de Para,récemment découverte et non encore datée, quoique presque sûrement précambrienne,peut-êtrem ê m e du milieu du Précambrien. Les plus grands gisements de manganèse sont les couches non métamorphosées du Paléozoïque inférieur interstratifiées avec la formation de fer du Mato Grosso, mais sans doute de bien plus grandes quan112

tités de manganèse se sont déposées au moyen Précambrien et au début de cette période.La plus grande partie du manganèse précambrien est maintenant métamorphosée en minéraux silicatés réfractaires au processus de décomposition et inutilisable dans l’industrie. L’un des divers dépôts de manganèse supergénés d’Amapa a été formé par la décomposition de picrotéphroïte; c’est le seul dépôt commercial important que je connaisse qui soit formé en grande partie de silicates. Les sédiments originaux y dépassent l’âge de 1,8 milliard d’années. Toutes les formations de fer connues ont un faciès d‘oxyde ou de carbonate. L a forination de fer à iaciès carbonaté est une série engéosynclinale de plus de 2,7 milliards d’années dans la partie centrale de Minas Gerais. Elle est constituée de lentilles qui en général n’ontpas plus de quelques dizaines de mètres d’épaisseuret quelque 10kilomètres de long. O n trouve beaucoup de lentilles de ce genre dans une même zone,qu’elles caractérisent ainsi sur une vaste surface. La même série contient aussi, sur une zone de quelque 200 kilomètres de long,des minéraux silicatés de manganèse provenant de vases carbonées et de boues contenant du carbonate de manganèse. Localement, le carbonate de manganèse était assez épais et assez pur pour rester chimiquement inaltéré par le métamorphisme; le minerai commercial provient d‘une oxydation superficielle.La teneur constante en carbone libre est le signe d’un

Iron-formationand associated manganese in Brazil

TABLE 2. Complete analyses of manganese and iron ores1,Morro do Urucum, Brazil (from Dorr, 1945) ~

ho2

sairipie NO .I

MnO

SiOz 17eZO:J Alzo3

MgO

Ca0 Na20

K20 1120- I I p TioZ Pz05

V203

Nioz

COO

13iiO

Liz0 As203 Sb203 Cu0

l'ho

Sn02 C P ~ O : ~s

Total

~

0.05

4.80

0.19

Manganese ores - narrrpies fmn bod N ~i . 1.74 0.08 0.01 Nane ~ ~ " 0.31 e 0.05 ...............

.33

.1ï

4.07

1.11

2.11

.lu

.91

0.01

Nuno

.a

.zu

3.63

.44

2.33

.o5

.34

xone

.42

.$o

.i9

3.82

.4ï

2.18

.u

.....

.72

.....

'i~ne

.o4

.GG

.....

Nunc

.o2

NO"^

.35

mne

.33

.%

71.65

1.0.1

1.26 i5,gï

2.50

NOW

a

61.87

6.24

.40 14.24

1.73

0.07

10

1.04

0.22

72.45

2.58

.56 15.41

14 69.82 20 61.18

3.48

.u8 16.59

1.53

3.05

23.07

2.20

.23

.IB

.30

:j.uo

.4ï

2.51

.14 .21

ZG

67.19

3.55

.20

2.03

1.03

3.42

.I4

2.59

.1<J .IO

.21j

~9.90

.O.%

.53

3.21

.22

2.82

.I0

2.61)

1.77 16.46 .YO 18.08 1.71 18.13

2.16

28

2.51

.3G

.O7

.SO

2.57

.51

4.39

1.50

1.03

Z.IIR 0.09

0.02

0.63

3.43

.24

.40

1.40

.i5

.za

.50

4.48 2.59

.so

.I1 .i1 Nane

.50

2.03

.o5

.i7 .13 .OG

4.28

.19

1.58

.12 .O4

NoDe

NOnC

.46

3.47

.22

2.34

infi 66.30

5

Gs.10

,64

17.10

23

ï o . j ~ 3.31 05.43 4.64

1.31

19.71

33

72.10

1.77

1.33

17.43

105

69.90

2.80

.96

17.19

15

, 3 6 14.80

.<JZ

.Y? 1.90

1.94

'Pr. Xone .IO None Nono

Manganese ores 0.14 I 2.56 0.05 0.83 N O

None TI-.

.I4

,2a .o5

mne ~~~e

............... Y Y . ~

N~~~ 0.004 hbne .i4 .......... Xons None .s3 ................................... .i5 ........................................ .95 . . . . . . . . . . . . . . . .................... ,35 ,002 N~~~ N~~~ NOM NO"^ .......... .26 ,002 N ~ ~ N~~~ I ~ N~~~ N~~~ .......... .o7

0.001

99.91 100.4.1 100.03

99.83

100.43

- samples ïrarri bed Wo.2 ~ o n e 0.20

0.33

xonc

.io

.i6

None None

.16

None None

None

.30

~ C

0.01

.41

None

.28

.O8 None

.z4

0.001

~ r .

N~~~

xanc

xone

mne

rqone

0.003 N~~~~

T ~ . xeric

.......... 9 y . m .......... ss.ae

...............'xone nune .lis ............... xone None ...............io0.11 None ,002 None Nons None Tr. . None .......... 99.69 .a1

Iron ores

F~ 11

..... .....

F~ 25

.....

1 7 ~ IO

0.71

27.8s

70.74

.o9

9.02

119.87

.4a

.o1 11.40

87.60

.a6

0.36

.............................. 0.01 0.17 ......................... ..... ...........o.007 0.017 NO"^ 0.54 .......... 0.30 ..... NOIE .i9 .................... ,O.iU .036 .......... ................. 0.14 .......... Tr. Tr.

1. Complete analyses m a d e in the laboratories of the Geological Survey. 2. Location of samples shown on plates 5 and G of Dorr, 1945.

sored investigations in the part of the Jacadigo Series extending into that country in the Serrania de Mutum, and a diamond drill hole about 250 m deep was sunk in the ironformation.This is said to have revealed an average content of about 50 per cent Fe and 25 per cent SO,;unfortunately Ido not have data on the details ofthe stratigraphyrevealed. N o manganese was found. Several years later a team of German geologists working in the area found small lowgrade lenses of manganese oxide. On the Brazilian side of the border it has been reported that important lenses of manganese oxide have been found in the iron-formation.I know no details of the occurrence. Thirty years ago, I hastily visited the Mutum area and have the impression that the clastic content of the iron-formationis much less than in Morro do Urucum. In the Serra do Rabicho,about 15 km to the north-east of Morro do Urucum,Haralyi reports(oral communication, 1968) that manganese is present in the iron-formation in a detrital facies,forming the cement in the detrital rocks in some places.It is not knownhow extensive this lens may be; Haralyi states that the potassium and iron contents of the manganese are lower than that at Morro do Urucum. Thus,judging from the stratigraphicevidence in hand and subject to correction as more information becomes available from this remote part of South America,it would appear that the iron-formationand the included manganese oxide beds were chemical sediments laid down in an extensive basin filled,in the first instance,by continental clastic sediments and later by chemical sediments.The unusually high content of potassium (3.5 per cent K,O)in the manganese ore suggeststhat the waters of the basin were not ordinary sea-water,The essential absence of vanadium,nickel, arsenic,antimony,copper, lead,and tin and the relatively low content of barium and cobalt point in the same direction and also militate against any volcanic contribution.The

very high state of oxidation of the iron and manganese ores ( M n O J M n O = 23.7) certainly points to a strongly oxidizing environment. The evidence of the association of the manganese oxide beds with clastic beds in the iron-formation clearly indicates an abrupt change in the sedimentary environment that caused precipitation of manganese, for the iron-formationitselfcontainsless than 1 Clarke of Mn. The deposition ofjasper was then almost completely inhibited, for much of the SiO, in the manganese beds, only 1.25 per cent in all, is in detrital grains and secondary cryptocrystallinesilica.Whether the water in the basin from which the iron-formaiionwas deposited was fresh or salt is thus uncertain. I suspect that the basin was estuarian or lacustrine,and that an arid climatemay have concentrated the waters to a considerable extent and contributed to the lack of detrital material during most of the time the ironformation and manganese deposits were being deposited. Rapid and temporary changes in climate may have brought in sudden and local incursions of detrital material and changed the composition of the waters in such a manner that the manganese oxide was deposited. The very local distribution of the coarse clastic material and its angular nature suggest that turbidity currents may have been the agent of transportation. The intermixing of these coarse sediments with manganese oxides and the greater amount of detrital sediments to the north-east suggest that the shoreline may have been to the north-east and not far from the present margin of outcrop of the Jacadigo Series. The ultimate origin of the manganese and iron is purely speculative. The quantities involved are enormous. Alvorad0 (1970)estimated 40,000million tons of iron-formation in Bolivia and 10,000million tons in Brazil;this represents small erosional remnants of the original extent. I estimated (Dorr, 1945) that perhaps 500,000 million tons were originally present. I also estimated (Dorr, 1945) that perhaps 111

s9.aa 100. 60 100.29

Iron-formationand associated manganese in Brazil

milieu euxinique au cours de la formation du dépôt. Le faciès sédimentaire original ainsi que le milieu environnant au moment où s’estformé le dépôt de centaines de lentilles minces de formation ferreuse hautement métamorphosées et les roches de silicate de manganèse dans le gneiss et le schiste des régions du bouclier sont inconnus. Le graphite que l’on trouve dans certaines lentilles de silicate de manganèse suggère un faciès original carbonaté. A Amapa, un dépôt cyclique de carbonate de manganèse syngénétique presque pur, en couches de 30 mètres d‘épaisseur avec grès, argile schisteuse très carbonée et boue, indique un milieu environnant euxinique. L a plus grande partie du minerai d‘oxyde supergène provient de ce carbonate. Les principales formations de fer au Brésil sont des plates-formesà faciès d‘oxyde ou des dépôts miogéosynclinaux. Tous les gisements importants de minerai de fer ont été formés épigénétiquement à partir de telles roches. Les sédimentsgraduellementsous-jacentssont transgressifs.Les principales formations de fer à faciès d‘oxyde ont plus de 200mètres d’épaisseur.Elles sont continues sur des surfaces qui couvrent des centaines,voire des milliers, de kilomètres carrés ; leur extension originale a dû être beaucoup plus grande. Les sédiments détritiques qui y sont inclus ont un caractèrelocal et sont de peu d’importance.L a teneur varie de 30 % à 50 % de fer. L a plupart des formations de fer à faciès d’oxyde

contiennent moins d’un Clarke de manganèse, mais des lentilles ou des zones importantes atteignant 6 mètres d’épaisseur dans la formation de fer peuvent contenir une formation de fer riche en manganèse ou, dans le cas du Mato Grosso, peuvent contenir des couches d‘oxyde de manganèse syngénétique s’étendant sur plusieurs kilomètres carrés, de 1 à G mètres d’épaisseur et contenant jusqu’à 48 % de manganèse. L a formation de fer à faciès d‘oxyde manganésifère n’a jamais été reconnue dans un état avant désintégration,on présume que le manganèse apparaît sous forme d‘oxyde syngénétique en basse concentration. D e vastes gisements de manganèse ferrugineux ont été formés à partir de ces roches. Dans la formation de fer précambrienne,les lentillesmanganésifères tendent à se concentrer là où la formation de fer est en transitionvers une formation de dolomite superposée, illustrant l’action du p H sur le dépôt.La dolomitepeut contenirjusqu’à30 clarkes de manganèse sous forme de calcite ou dolomite manganique. Ainsi au Brésil, la formation de fer à faciès d’oxyde se trouve en grandes masses continues étroitement associées avec le manganèse, tandis que la formation de fer à faciès de carbonate se trouve en masses discontinuesminces séparées des dépôts de carbonate de manganèse dans la même séquence de roches. Les sédiments riches en carbonate de manganèse sont beaucoup plus étendus dans le temps et dansl’espaceque les formations de fer à faciès de carbonate.

Bibliography/Bibliographie ALDRICH, L.T.; HART, S. R.; TILTON, G. R.; DAVIS, G.L.; RAMA, S.N. I.;STEIGER, R.;RICHARDS, J. R.;GERKEN, J. S. 1964.IsotopeGeology.Annual Report Director,Dept.Terrestrial Magnetism: Carnegie Znst. Washington Yearbook 63,p. 33-340.

ALMEIDA, F.F.M.; MELCHER, G.C.;CORDANI, U . G.; KAWASHITA, K.; VANDOROS, P. 1968. Radiometric age determinations from northern Brazil. Bol. Soc. brus. Geol., vol. 17, no. 1, p. 3-14. ALVORADO, B.1970.Iron ore deposits of South America.Survey of world iron ore resources, p. 302-380, New York, N.Y., United Nations Department of Economics and Social Affairs, (United Nations Publications Sales No. E. 69. II. C. 4, ST/ECA/ll3 ,) DORR, J.Van N.II 1945.Manganese and iron deposits of Morro do Urucum, Mato Grosso, Brazil. Bull. US. geol. Surv., 946-A, p. 1-47. 1964. Supergene iron ores of Minas Gerais,Brazil.Econ. Geol., vol. 59, no. 7, p. 1203-40. -. 1965.Nature and origin of the high-gradehematiteores of Minas Gerais,Brazil. Econ. Geol., vol. 60, no. 1, p. 1-46. . 1969.Physiographic,stratigraphic,and structuraldevelop-

-.

ment of the QuadriláteroFerrífero,Minas Gerais,Brazil.Prof. Pap. US.Geol. Surv., 641-A, p. 1-110.

DORR, J. Van N.II;COELHO, I.S.;HOREN,A.1956.The manganese deposits of Minas Gerais,Brazil.XX Znt. geol. Congr., Mexico City,1956, Symposium sobre yacimientos de manganeso, vol. 3, p. 279-346.

GAIR, J. E. 1962. Geology and ore deposits of the Nova Lima and Rio Acima quadrangles,Minas Gerais,Brazil.Prof. Pup. U.S.Geol. Surv., 341-A, p. 1-67.

JAMES, H. L. 1954.Sedimentary facies of iron-formations.Econ. Geol., vol. 49, no. 3, p. 235-93. KRUMBEIN, W. C.;GARRELS, R. M.1952. Origin and classification of chemical sediments in terms of p H and oxidationreduction potentials. J. Geol., vol. 60, p. 1-33. MASON, B. 1949. Oxidation and reduction in geochemistry. J. Geol., vol. 57, p. 62-72. MATHESON, A. F. 1956. The St. John del Rey Mining Co., Limited, Minas Gerais,Brazil;history,geology,and mineral resources.Bull.Canad. Znst. Min., vol.49, no.525,p. 37-43. NAGELL, R.H.1962.Geology ofthe Serra do Navío manganese district,Brazil. Econ. Geol., vol. 57, no. 4, p. 481-98. RIBEIRO FILHO, E. 1966.Jacobsita de Licinio de Almeida,Bahia. Bol. Soc. bras. Geol., vol. 15, no. 2, p. 43-8. RIBEIRO FILHO, E.;ELLERT, N.1969.Magnetometria relacionada a jazidas de manganêc do sudoeste da Bahia. Mineraç. e Metall., vol. 49, no. 289, p. 11-13. SCARPELLI, W. 1966. Aspectos genéticos e metámorficos das rochas do distrito de Serra do Navío, Território Federal do Amapá, Brazil. Anais da VI Conferencia Geológica das Guianas. Avulso Dep. nac. Prod. min., Rio de J., Div. Geol.e Minerai,no. 41, p. 37-57. SHATSKIY, N.S. 1954. O n manganiferous formations and the metallogeny of manganese, Paper 1. Volcanogenic-sedimentary manganiferous formations.Int. Geol. Rev., vol. 6, no. 6, 1964,p. 1030-56.(Translated by V.P. Sokoloff from original article in Akad. Nuuk. SSSR, Zzvesstiya, Seriya Geologicheskayu, 1954,no. 4, p. 3-37.)

113

The Precambrian iron and manganese deposits of the Anti-Atlas G.Choubert and A. Faure-Muret Museum National d'Histoire Naturelle, Paris (France)

A short outline of the Precambrian of the Anti-Atlas The Precambrian of the Anti-Atlas is subdivided into six systemsof which the first five were terminated by important orogenies,resulting in folded and granitized belts, whereas the sixth forms the base of the Palaeozoic cover of earlier folded zones.During the first two systems the Precambrian basement became enlarged aiid resulted in: 1. The Kerdoiis system (Archaean) which granitized at 2,600 m.y. The Zagorides, trending in an E-W direction, are the orogenic belt which corresponds to this system. 2. The Senaga system (Pvecambrian I) is of early Precambrian age. The orogenic belt that this system constitutes are the Berberidesto which three-foldphases correspond and in which granitizationhas been dated at 1,940,1,850 and 1,750m.y. The strike varies around the southerly direction. These two belts cratoiiized the Anti-Atlas in such a way as to cause subsequent belts to mould themselves against the northern front of this craton. They are: 3. The El Gruara system (Precambrian I-II) which is characterized by important volcanic activity and by the emplacement of ultrabasites.The central Anti-Atlasides have been dated at about 1600 to 1680 m.y. 4. Tfie Limestone and Quartzife system (Precambrian II) which form most of the epicontinental cover of the three previous belts.They are neverthelessstrongly folded and locally granitized (1450 to 1500 m.y.). These may be referred to as the Western Anti-Atlasides. 5. The Eastern Anti-Atlasides (900to 1050 m.y.) constitute the last Precambrian orogenic belt. Like the other two this belt is folded in an E-W direction,moulds the front of the ancient craton and constitutes the Siroua-Sarhro systeni(Precambrian11-111) which begins with volcanites and tillites dated at 1250 and 1350 m.y. The granitization accompanying this belt is extremely important (Tifnout and Ouzellarh granites).

6. The last Precambrian system begins with the important ensemble of acid volcanites of the Ouarzazate series, which were covered by the Adoudounian dolomites.The sedimentary cycle of the Lower Adoudounian was followed by an important regressionseparating formations of the sedimentary cycle and embodying the Lower Adoudounian and the Lower Cambrian.The age of this ensemble ranges from 900 to 550 m.y. There are no important iron or manganese deposits in the first two systems; only some veinlets of oligoclase or veins of quartz with oligoclase and hematite have been located. Among the latter,the more important is the twin orebody of Bou Tazoult on the eastern side of the Archaean massif of Ifni (Fig. 1). Iron-oredeposits are found in the following two systems: (a) at the base of the Precambrian 1-11, and (b) in the schists of the Precambrian Il. The manganese deposits are connected with the acid volcanic series of the platform, partly at the base of the Precambrian 11-111 and partly in the Precambrian II (Ouarzazate series).

The iron ore deposits of the Precambrian 1-11 The extreme base of the Precambrian1-11 is locally underlined by a ferruginous formation, the oligoclase schists, belonging undoubtedly to the large family of itabirites (Choubert, 1963). This formation can be observed notably at the southern boundary of the Precambrian window (Boutonniere) at E l Graara (Central Anti-Atlas south of Ait Ahmane), where it constitutes the ferruginous band of Guelb el Hadid (Fig. 1). These oligoclase schists overlie unconformably the gneiss of Oued Assemiil, which is undoubtedly Precambrian I. It is therefore an ore deposit of the platform cover and consequently closely coiinected with the itabirites. Discovered in 1949 (Jouravsky, 1953) this formation can contain up to 66 per cent Feto,(4650 per cent Fe) and a siliceous residue of 24 per cent. In thin section this rock consists uniquely of well-assortedand generally intensely crenulated oligoclase grains and lamellae

Unesco, 1973. Genesis of Precanibriun iron und inringmese deposits. Proc. Kiev Symr,.,1970. (Earth sciences, 9.)

115

The Precambrian iron and manganese deposits of the Anti-Atlas

and of granoblastic quartz. In places the oligoclase schist It is therefore may pass laterally into biotite-mica-schists. relatively intensely metamorphosed (amphibolite facies). This has probably taken place at origin,and the formation may now be observed as a lenticular body which marks the contact of the Precambrian 1-11 with the Precambrian I. In some parts at Guelb el Hadid it can be followed over nearly 2 km;the maximum thickness is 50 m . Elsewhere it measures no more than a few decimetres or it may be discontinuous. The oligoclaseschists are actually known to occur over nearly 20 km,stretching from Ihrtem in the west along the northern flank of the Takroumt Massif and again to the east of Hassi Atlatat, where they emphasize the axes of anticlinal structures. They are deposits of sedimentary origin connected with the transgression of the Precambrian 1-11 on to the flattened Berberides,but it is not certain whether they are a marine formation due to that transgression or a continental formation deposited on the gneiss before the transgression-a type of lateritic crust. The ferruginous formation may be overlain by metamorphic limestones. However, in the presence of overthrusts, folding and crenulations in the oligoclase schists, it is often difficult to recognize the exact relationship between these two formations.At Guelb el Hadid some small lenticles of limestone measuring 10-20 c m can be observed, closely intermingled with the oligoclase schists and crenulated simultaneously with them. The presence of these limestones could indicate a marine origin.

Indications of the presence of iron in the Precambrian II It is also in the eastern part of the El Graara window (central Anti-Atlas), notably near El Bleida, where there are indications of iron in the schists of the Precambrian II. This system begins here with an important body of quartz diorite which has replaced the original stratigraphic unit for this system, i.e. the limestones of the base. The El Orf quartzites (1529 m) form the second stratigraphic unit; they are present in the shape of a great upright lens 4 k m long. The third unit consists of basic volcanites,preceded by some schists and followed by a thick complex of schist and flysch.Weak metamorphism resulted in the formation of greenschists.Ironis also present here as blackish, ferruginous schists with oligoclase. In thin section, they are rocks formed almost uniquely of oligoclase flakes, needles or rounded lamellae and of sericite, well aligned along the beds. There are also some quartz grains,biotite lamellae and chlorite present, as well as rocks formed of quartz and sericite with very fine bands and clouds of oligoclase.Theserocks are situated both in the schist band, which separates the quartzites from the volcanites,and in the schists which overlie the latter, for instance in the Gardens of Bleida. These ferruginous occurrences in the schists of the Precambrian II seem of little importance; neither the Fe content nor the tonnages have so far been

evaluated, and they are mentioned here merely as a point of interest. O n the other hand, the same supra-volcanite schists may possibly have Cu mineralization,and investigations are at present being made.

Deposits of manganese in the Precambrian 11-111 Only one deposit of M n dated Precambrian 11-111merits description. This is the stratiform deposit of Idikel (the village situated below the mine), or alternatively named Aferni (after the nearby mountain pass) (Fig. i), The deposit is situated to the east of Tafraoute (western AntiAtlas) on top of a cliff.It is interstratiñedwith an essentially conglomeratic series about 200 m thick which borders the Archaean massif of Kerdous to the east. This series is the lateral, eastern, reduced equivalent of the thick series of Anzi (1000m), whose schists and sandstonesconstitutethe country to the west of Tafraoute.The Anzi seriessurmounts an importantvolcanic complex,which is essentially rhyolitic and which,up to a short time ago,used to be confused with the massive rhyolitic series of the Precambrian III. Elsewhere a thin rhyolitic flow can be observed at the base of the detrital series of Idikel. The characteristics of the deposit,as studied and described by Bouladon and Jouravsky (1956), are as follows: 'Theconglomeraticseries of Idikelis of fluviatile origin and contains intercalations of micaceous pelites (or psammites) with floating mica,which originated in the Archaean schists and granites. One of these intercalations (2-3 m thick) comprises a sandstone zone and red pelites with volcanic constituents. It is accompanied by a red dolomite zone of lacustrine origin containing 4-12 per cent M n O . The manganese deposit is interstratified in this red zone and irregularly bedded,the main bed attaining a thickness of 1.5-2 m.Other non-exploitablebeds are situated sometimes above,sometimes below, the main bed. The outcrop can be followed over nearly 2 km and dips 20-25" to the east. The rich mineral belt is 800 m long and has been exploited over a width of 150 m . Beyond that the mineralized bed is replaced by dolomite poor in manganese.The marketable grade had 37-51 per cent M n with 1-13 per cent SO,,5-10 per cent Bao, 0.02-1.3 per cent Pb, etc. An average of 10,000-20,000 tons per yearswas mined. Exploitation commenced about 1951 and was discontinued about 1959. The total minerals extracted was 100,000tons.' The mineral consists of braunite and barium-bearing psilomelane with a little haussmannite and pyrolusite. Rhodonite and rhodochrosite appear locally, the latter close to the dolomites. Hematite, baryte, quartz and albite and different micas are also present.O n the other hand,chalcopyrite and other Cu minerals,baryte,oligoclase and quartz are found in the veinlets which cut the mineralized beds, as well as some acmite and spessartine garnet. The presence of these silicates, for instance rhodonite, indicates a local rise in temperature which was subsequent to the formation of the orebody. The Anzi series could perhaps be locally 117

G.Choubert and A. Faure-Muret

lightly metamorphosed,i .e.newly formed biotite is present in the rhyolites of Oued Oudrar not far from Anzi. Several hypotheses can be put forward regarding the origin of the Idikel deposit. 1. According to Bouladon and Jouravsky (1956) it is a hydrothermal syngenetic deposit. ‘The Idikel deposit was presumably formed during sedimentation which took place at the side of a closed and probably lacustrine basin and emanated from magmatic mineralizing solutions’.Elsewhere Jouravsky (1963) speaks of the Idikel deposit as an oxidation mineral in a superficial zone of carbonatized mineral, doubtlessly in relation with the manganiferous dolomite. 2. The red dolomites, apparently not accompanied by manganese deposits,frequently outcrop on the eastern boundary of the Kerdous Massif (here and there at Izerbi, the Agoujgal window, etc.). These lakes are undoubtedly later than the rhyolitic volcanic activity, which marks the beginning of the sedimentary cycle of the Precambrian II-III. The destruction of these volcanoes,which comprise the zones richest in manganese, and the concentration of manganiferous products in theselakes,could perhaps explain the origin of the Idikel deposit thus making it unnecessary to evoke a hydrothermal origin.It can therefore be assumed that a situation obtains here similar to the Cretaceous manganese deposits along the northern boundary of the Anti-Atlas. The deposit would thereforebe syngenetic sedimentary, perhaps enriched by an oxidation process. 3. Finally,accordingto a third hypothesis,it is an epigenetic hydrothermal deposit:the mineralized zone would have been formed by substitution in the manganiferous dolomite and the embanked pelites by hydrothermal activity. According to the isopach curves by Mixus (in Bouladon and Jouravsky,1956) the greater part of the mineralized zone is represented by a rectilinear band orientated N-S and measuring 50-100m.The thickestparts of the deposit (1.52 m) are aligned to this zone,whereas the manganiferous dolomites border it continuously,except where the deposit is intercepted by the cliff.This rectilinear shape of the mineral zone could explain the feeder of manganiferous solutions,which would have given place to a substitutiondeposit in the dolomite. Before passing on to the following section,the presence of a non-exploitedmanganese lode deposit should be mentioned in the Precambrian 11-111 in the Tidili area, which is situated at the border of the Haut Atlas with the AntiAtlas (Anfid village). This deposit is of interest because of the presence of rhodonite and spessartine which indicate a rise in temperature (250”) at the time this deposit was formed (Bouladon and Proust, 1959). Orientation of these lode deposits is ENE-WSW;one of them attains 1 m in thickness. Their paragenesis comprises,moreover,rhodonite (75 per cent Si0,Mn) and spesSartine (40.25 per cent MnO), psilomelane, polianite and some hydrated oxides of M n (wad). It may be concluded therefrom that the M i l deposits

in the Precambrian 11-111 formed at a higher temperature than those in the Precambrian III, to be discussed in the following section. In fact, the latter contain neither rhodonite nor spessartine.

Manganese deposits in the Upper Precambrian (Precambrian íII) W e owe our present knowledgeof the manganese depositsof the PrecambrianIII above all to Jouravsky.Their study was commended by Neltner (1934),andBondon and Frey (1937) provided us with descriptions and metallogenic details. More detailed studies were made during the 1950s by Jouravsky,accompanied by Bouladon and,later,by Pouit. These authors divide the M n deposits in the Precambrian III into two great groups: (a) the lode deposits, and (b) the stratiform deposits.The former are very numerous, above all in the region south of Ouarzazate (90 k m E-W and 60 km N-S); but they have also been located between the axial plane of Taliouine to the west and as far as east as Sarhro,a distance of about 250 km.Small stratiform lenticular deposits are often associated with lode deposits;but important stratiform deposits (Fig. 1) are not known except in the synclinalbasin ofTiouine (40k m west of Ouarzazate), which measures about 10 x 15 km The Precambrian III of the Ouarzazate region consists of a succession of slightly unconformable volcanic complexes.Those of the Lower PrecambrianIII are essentially andesitic,whereas those of the Middle PrecambrianIII are predominantly rhyolitic (ignimbrites,lavas, tuffs) but generally contain some andesites, latites, even trachytes. Finally, the Upper Precambrian III is characterized by its detrital continental sedimentary basins of the same periods as the lacustrinelimestones.Its rhyolites are generally alkaline, and the andesites are usually porphyritic. The majority of these lode deposits appear in the rhyolites of the Middle Precambrian III. The Tiouine basin, however,consists of conglomerates,sandstones and pelites of the Upper PrecambrianIII.A description of the characteristics of these two groups follows.

L O D E DEPOSITS O F M A N G A N E S E IN T H E P R E C A M B R I A N III

According to Jouravsky (1963)there are more than 200lode deposits of manganese inthe Ouarzazatearea,which extends 90 km E-W and 60 km N-S (Fig. 1,inset). Together they have provided nearly 400,000tons of mineral containingup to 50 per cent M n . Lately,their output has been considerably lower and most of these lode deposits are no longer worked, except by local workers. These lode deposits appear mostly in groups or ‘swarms’.Jouravsky claims that each swarm affects but a single unit or volcanic complex, a single flow or strata of

The Precambrian iron and manganese deposits of the Anti-Atlas

mannite,polianite,pyrolusite,psilomelane,todorokite,etc., accompanied by hematite, goethite, barium, quartz, dolomite, calcite,etc. However, the type associationis simpler: braunite-cryptomelane-hollanditeaccompanied by hematite, quartz and barite. Jouravsky has stated that these minerals crystallized together during the samemineralogicalprocesses,the braunite forming first. This mineral composition hardly changes with depth. B a 0 enrichment occurs near the surface. Jouravsky concluded therefrom that therewas no superficial oxidation posterior to mineralization;thus, according to him,mineralizationhere is hypogene.It must be added here that the manganese lode deposits developed mainly in the Middle PrecambrianIII,above all in the rhyolites,but also in the andesites, whereas they are rare in the Lower Precambrian III, which is characterized by development of andesites. Finally, M n lode deposits are practically nonexistent in the detrital formations of the Upper Precambrian III, though they are volcanic formations, as for example the Tiouine series. The same applies to the Adoudounian cover (equivalent of the Russian Wendían). It may be concluded that there is a close genetic liaison between the lode deposits and the enclosing flows.The metallogenic processes which led to the formation of lode deposits represent undoubtedly in some way the end of volcanic activity of each mineral complex. Jouravsky qualified these lode

rhyolite,ignimbrite or andesite (Fig. 2). The lode deposits continue only in exceptional cases from one volcanic unit to another and are bounded top and bottom within the same unit. The adjacent unit may also contain lode deposits,but these are not in any way connected with those in adjacent units. It can therefore be concluded that it is a succession of generations of lode deposits, each generation accompanying a different unit or volcanic complex. According to the same author, the lode deposits are never deeply situated.None has ever been exploited deeper down than 100 m.The most important ones extended to a depth of 9095 m (Bou Ouzgouar and Tizgui el Illane). Elsewhere exploitation depth did not exceed a dozen or so metres. In most cases, the lode deposit became rapidly wedged in depth in such a way that it became difficult to recognize or to follow the fracture,for instance,the deposit of Taourat which tailed out at -62m. M n is less frequently replaced by hematite (e.g. the underground workings at Charlot and at Tizgui el Illane). The length of the lode deposits may,however, be considerable, above all in the case of thin deposits poor in gangue mineral, i.e.Tindaf 800 m,Bou Ouzgouar 400m . The lode deposits, with dolomite as gangue mineral, may attain a thickness of 5 m,but are rarely longer than 50 m. The mineral composition is constant,the overall composition being braunite, cryptomelane, hollandite, hauss-

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El Borj Stratiform deposit

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El Borj

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+\.Iy ;+ + + + W-f+zt;,;:+2;y + + + t + + + +

+ + +t + + + + + + + + \ + I + - .

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+ + + ++

+ + + + + + + . . . . .

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FIG.2.Sectional diagrams of the thread deposits at El Borj (A)and Tizgui el Illane (B) (Central Anti-Atlas) (Taken from Jouravsky, 1963). 119

G.Choubert and A.Faure-Muret

deposits as epithermal and closely connected to volcanism.

It could also be concluded that the lode deposits were emplaced very near the topographic surface and evidently in a continental environment. Jouravsky states that saline precipitation of M n took place where the hydrothermal hypogene solutions encountered the superficialwaters rich in oxygen. In fact,the salts of trivalentand quadrivalentM n become unstablein diluted solutions and form colloidal Mn(OH),, which precipitates. Furthermore the addition of oxygen necessary for the precipitation of oxidized minerals (here 1.27per cent) would be clearly deficient.Later it will be seen that the percentage is 2.66 for the minerals of the stratiform deposit at Tiouine. The lode deposits would thereforehave formed under conditions of relative lack of oxygen.

STRATIFORM M A N G A N E S E DEPOSITS

IN T H E P R E C A M B R I A N III

As stated previously, stratiform deposits of M n of some importanceexist only in the sedimentary basin of the Upper Precambrian III at Tiouine-Oufrent. There are three such deposits (Fig. 1): Tiouine, Migoudene, and Oufrent, the latter being situated on the eastern side of the synclinal basin. Elsewhere,in harmony with the main swarms of lode deposits,stratiformlenses ofmanganese areknown to occur, but they are always of minor iniportance. In the area of

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Tiouine, there is a series of lode deposits, the most important being at Tindaf and Taourat.They are situated in the rhyolites and ignimbrites of the Middle Precambrian underlying the Tiouine Series (Fig. 3). This is a sedimentary series of continental origin,red and nearly 1000 m thick. It contains only rarely some isolated flows of trachyte or andesite. However,in other basins of the Upper Precambrian III,volcanism is both andesitic and rhyolitic and can assume great importance. At the western side of the basin, the Tiouine series begins with coarse conglomerates (tillites?), which dip 1525"to the east.Towardsthe centre ofthe basin the conglomerates become medium to fine,thereafter passing into red sandstones and pelites (Fig. 3). The latter have the pronounced red which characterizes the Tiouine basin and the sedimentaryrocks of the Upper PrecambrianIII generally. This successionis evidently not cyclic,but at all levels there are recurrences of coarse facies in finer facies. In particular, the zones of conglomerates are separated by layers or beds of pelites. O n the other hand,the diminution of the dimensions of the detrital constituents together with the lateral passing of the conglomeratesinto sandstones and even pelites can be observed in each of the beds going from the side of the basin towards its centre. Lacustrine limestones with flints containing Collenia are intercalated in the red pelites. These limestones,developed particularly between Oued Ihrir and Migoudene, are also known to occur on the eastern flank of the syncline in the region of Oufrent.

ESE

1400m

.1300m

.1100m

FIG. 3. Section of the stratiform deposit at Tiouine showing lateral transition from coarse facies to fine facies (Taken from Colson, in Bouladon and Jouravsky, 1955). 1. Pudding-stones 120

with very largeclasts (tillites?); 2.Normal pudding-stones;3.Fine pudding-stonesgoing into sandstones and pelites;4.Mineralized bunch being worked;5. Subjacent 'principal' ignimbrite.

The Precambrian iron and manganese deposits of the Anti-Atlas

The Tiouine series would appear to be an infilling of this basin by scatter deposits, the detrital material being carried by the floods of transitory inundationswhich were, however, of short duration. This picture is very different from that evoked by Bouladon and Jouravsky (1955),who envisaged a permanent lake into which various tributaries discharged material. In our opinion only the limestones mentioned above would be lacustrine,whereas the red beds would be comparable with the continental formations of the red sandstonetype which generally are assumed to have formed in a semi-arid climate. The tectonic basin of Tiouine is bounded to the west by important tectonic disturbances.Transverse or oblique faults mark its edge, and these have also been located in the orebody.

The manganese mineralizationis present as interstratified beds in the conglomerates;these beds are associated with the pelites and intercalatedwith them.At Tiouine there are more than twenty beds grouped in three layers:the basal beds;the exploitedstrata;the upper strata.Their description is part of the present report.However, here are some data by Bouladon and Jouravsky (1955) for each of these successions of strata (Fig. 4). The lowest beds start at the first pelitic levels some metres above the last ignimbrites.The first level may attain 50 c m and may extend more than 150 m from the border fault.The second level is situated 20 m higher and consists of three beds containing poor mineral. The exploited layers are situated 50 m higher up. They may consist of up to fifteen beds distributed over 20 m as is

B

/a

O

FIG. 4.Stratiform deposit at Tiouine:Section(A)and panoramic view (B)of the western edge of the basin. The pudding-stone series is in contact through faults with the ignimbrites (taken from Bouladon and Jouravsky, 1955). 1, 2. Mineralized levels

rh yoiitec

(pelites, fine pudding-stones); 3, 4.Pudding-stones,some with very large clasts (tillites?); 5. Complexes of ignimbrites,tuffs

and breccias.

121

G.Choubert and A.Faure-Muret

usual in the pelitic layers. Thickness is reduced to 4 m . Extension is 1 k m maximum over 300 m according to dip. Updip the beds wedge out rapidly. Downdip they become impoverished and are subdivided into a multitude of layers which disappear slowly into the pelites or sandstones. Certain mineral beds may be eroded by the overlying conglomerates u hich may contain pebbles of mineralized pelites. The upper sequenceoflayers is situated 30m aboveand occurs only in the northern part of the deposit,and consists merely of 4 thin,poor beds. The mineral consists of braunite and of cryptorrielane minerals of the hollandite-cryptomelane-coronaditeseries. Gangue minerals are not abundant and when present consist of quartz, fibrous silica and barytine. Within the mineral the same constituents are found as in the non-mineralized pelites, such as debris of glass,mica,quartz,etc.This could be an indication of the nature of the mineral formed by substitution in the pelites. The mineralization varies from one sequence of layers to another. Also the lower beds are richer in hollandite than the exploited beds and therefore have braunite and cryptomelane enrichment.On the other hand, the percentage of P b and of alkalis varies within the same beds from updip to downdip, the Pb being more abundant updip and the alkalis downdip. Barytine exists only in the lowest sequence. At Migoudene this mineral is more abundant and is accompanied by quartzand hematite.Locally thelattermay even form whole beds. The average percentages of marketable M n from Tiouine and its principal impuritiesare:42-48 per cent Mn; 6-21 per cent SiO,;2-9.5 per cent Bao;0.2-2.1per cent Pb. It is,therefore,poorer than the mineral from the lode deposits and by comparison a little richer in B a 0 and Pb. Its degree of oxidation is higher; 2.66 per cent against 1.27 per cent. The Tiouine deposit was commercially exploited between 1937 and 1962. During this period 50,000 tons of metallurgical rock mineral were mined. Bouladon and Jouravsky (1955, 1956) consider the Tiouine deposit to be hydrothermal and syngenetic; according to their hypothesis the mineralizing solutions poured into a hypothetical lake. In our opinion the mineral would be hydrothermal pene-epigenetic immediately after the deposition of each bed of pelite. A description of the metallogenic characteristics follows: Its paragenesis is notably the same as that of the lode deposits of the Ouarzazate region, the latter being richer in braunite and poorer in hollandite.They are therefore of common origin. The rich mineralized beds are situated near the border fault, which bounds the basin to the west. It has been stated earlier in this paper that enrichment occurs along the transverse faults. Near the same faults,the ignimbrites underlying the deposit show impregnation and substitution by M n . The manganiferous solutions ‘rose’,therefore,along,these fissures. The approximate contemporaneity of the Tiouine deposits 122

and their mineralization have already been shown by Neltner (1934), who noted in particular the break in certain mineral beds by overlying conglomerates as well as the presence of mineralized pebbles in these conglomerates.This has been regarded as proof for the syngenetic nature of the deposits. Furthermore,these conglomerates may themselves be impregnated by the manganese near the mineralized beds. Finally,the latter replacethe pelite zones or associatethemselves with them. The fact that they contain the same detrital constituentsas the pelites (vitreous debris,mica, quartz) mentioned earlier on, could show that they are substitution deposits and not deposits formed by precipitation of manganese at the bottom of the lake, as was stated by Bouladon and Jouravsky (1955,1956). The formation of the Tiouine stratiform deposits would, therefore, appear to be a complex process which consisted of interaction of three phenomena: infilling of the Tiouine basin by continental deposits; probably abrupt activity of faults during deposition; additions of hydrothermal solutions, doubtlessly related to the re-activation of the faults and, like the latter, taking place during several successive stages.In this way each part of the deposit would be affected by the faults, then impregnated by the solutions. The resulting conglomerate could have eroded the beds which would then have become mineralized, and thereafter the cycle could recommence. This mineralizing thermalism was undoubtedly coiinected with the end of volcanic activity and thus stratiform and lode deposits have a common origin; both would be hydrothermal epigenetic deposits. The morphology of the lode depositswould be attributableto the Middle and Lower PrecambrianIII,whereas the stratiform deposits are characteristic of the Upper Precambrian III. Bouladon and Jouravsky (1955, 1956) quote as comparable deposits those of the Thuringer Wald and the Harz (porphyries,breccia and tuffs of the Permian) and the Tertiary deposits in California and N e w Mexico. In Morocco, the manganese lode deposits may be observed in the volcanoes of the Mio-Pliocenein the Melilla area.

Metallogenic problems of the manganese deposits It thus follows that in the Anti-Atlasthe Precambrian III was an epoch of intense volcanic activity and also a particularly metallogenic one.These two phenomena, that of predominantly rhyolitic volcanism and of mineralizationby thermal inanganiferoussolutions,appear to be closely connected. Detailed studies by Jouravsky and Bouladon made it possible to construct a picture of the mode of arrival of these hot solutions,not only in the fissures which traverse the lava or ignimbrite flows, but also in the sedimentary series of continental origin. However, this reconstruction ofthe mechanism raises a certainnumber ofproblems which have so far not been solved.

The Precambrian iron and manganese deposits of the Anti-Atlas

First, why are the M n deposits concentrated above all in the Ouarzazate region over an area measuring 90 x 60 k m ? The volcanic outcrops of the Precambrian III extend farther than that to east and west; in fact, they characterizea belt of 250 km.However,moving away from the Ouarzazate region, one can note that the ore deposits become more and more rare and disappear altogether. Outside the Ouarzazate area only six manganiferous belts are known, i.e. two to the west (north of Taliouine) and four to the east (in the eastern Sarhro). The rhyolites themselves are very poor in M n O , and lateral segregationor leachingmust thereforebe discounted. Elsewhere Jouravsky and Bouladon have shown mineralization to be very superficial and to have taken place in the last flows. Neither can it be shown that certain volcanoes were more conducive to the formation of manganese deposits than others.The selectivefactor,ifit exists,of certain rhyolitic flows would be a very local factor,because in the 90 x 60 km mineralized zone south of Ouarzazate there are several groups of volcanoes with very different volcanic sequences which seem to have had no influence on the distribution of the ore deposits. Finally,lode deposits of manganese are also found in the andesites (Tachgagalt,E l Borj,etc.), and one deposit is even situated in the granites of the substratum of the Precambrian 11-111, i.e. the small orebody in the Tamassirt granite (Siroua track) close to the boundary with the PrecambrianIII. O n the other hand, of the various Upper PrecambrianIII sedimentary or volcanosedimentary basins, only that of Tiouine-Oufrenthas been mineralized. The conclusion may therefore be drawn that both lode and stratiform deposits are hydrothermal, but that the origin of the manganese and of the volcanic material is not

necessarily the same. It would appear,however, that volcanism was the factor which set in train the as yet unknown generating processes responsible for the formation of the manganese deposits. Second, the invariable selective paragenesis of the Precambrian II M n deposits must be emphasized. Chemically, oxides of manganese-always plumbiferous-predominate over all other constituents,i.e.hematite,barytine, quartz and sometimes dolomite (the latter is present in very small,but not negligible,quantities). Therefore, the metallogenic process provoked by the rhyolitic volcanism of the Precambrian111 evolved in a selective way in providing M n , Fe, Pb, Ba, Si and, eventually, also Ca, M g and some alkalis. In our present state of knowledge this phenomenon remains inexplicable,but the origin of this mineralization is, however, incontestably connected with rhyolitic volcanism,which is characteristic only of some areas to the exclusion of others and is distributed iii the former over a vast region of 80 x 50 km.This phenomenon occurs only during volcanic epochs and,like eruptions,seems to repeat itself. It must be noted that it is not peculiar to thePrecambrianIll but that it has also accompanied the rhyolitic volcanism of the Precambrian 11-111. Despite our knowledge regarding the process which causes the emplacement of the lode and stratiform deposits, the real problem has as yet not been resolved: W h y is this mineralization connected with volcanism, above all acid volcanism? Whence did this mineralizationcome? Are there deep metallogenic ‘reservoirs’connected to certain magmatic ‘reservoirs’ of the volcanoes? Or is this metallogenic process autogenic in certain more or less deep zones and under conditions known to be connected with volcanism?

Résumé Gisements de minerai de fer et de marzgarzèse dans le Précambrien de l’Anti-Atlas(G.Choubert et A,Faure-Muret)

Les gisements de minerai de fer et de manganèse précambriens se rencontrent dans le Précambrien moyen et supérieur de l’Anti-Atlas. Dans l’Anti-Atlas,qui est situé dans la partie méridionale du Maroc, on ne rencontre pas moins de six systèmes successifs de Précambrien;chacun d’eux est plissé,granitisé et se superpose au précédent dans la plus complète discordance. Les deux premiers systèmes,qui sont les plus anciens, à savoir le système de Kerdous, dont l’âge est de 2,6 milliards d‘années (archéen), et le système de Zenaga, dont l’âge est de 1950 à 1 850 millions d’années (Précambrien inférieur), ne contiennent aucun gisement de fer ou de manganèse. Le système suivant est celui d‘El Graara, dont l’âge

est compris entre I 650 et 1 600 millions d‘années. I1 présente à sa base une couche importante d’oligistoschistequi pourrait être comparé aux itabirites.Dans le quatrième système, on rencontre d‘autres oligistoschistes,moins importants et moins continuscaractérisés par des calcaires oncholites, des quartzites et des schistes-séricitesplissés datant de 1 500 à 1 450 millions d’années. Le manganèse caractérise les deux derniers systèmes : le système Anzi-Siroua-Sarhro plissé et granitisé d’âge compris entre 1 050 et 900 millions d’années; le système Ouarzazate-ouedAdoudou, qui appartient au Précambrien supérieur (900-575 millions d’années). On observe dans ces deux systèmes un développement important de volcaniques acides du début du cycle ((( volcanisme subséquent ))). Ils sont accompagnésde nombreux gisements de minerai de manganèse veinés ou stratifiés. Les derniers d’entre eux sont, pense-t-on,d’origine volcano-sédimentaire.

123

G.Choubert and A.Faure-Muret

Bibliography/ Bibliographie AGARD, J.; DESTOMBES, J.; VAN LECKWIJCK, W.1952. Fer, in Géologie des gîtes minéraux marocains.XIX Congr.géol. int. Alger 1952, Monogr. région.sir. Maroc na 1 et Notes Mém. Serv. géol. Maroc, Rabat,no. 87,p. 103-32. BLONDEL, F.;BONDON, J. 1936.Le manganèse dans la région du Siroua (Sud marocain). C.R.Acad. Sei. Paris, vol. 202, p. 958-9. BONDON, J.; FREY, R.1937. Sur la substitution de minéral de manganèse aux rhyolites cambriennes dans la région de Tiouine (Sud marocain). C.R.Soc. géol. Fr.,Paris,p. 201-3. BOULADON, J.; JOURAVSKY, G . 1952. Manganèse, in Géologie des gîtes minéraux marocains. XIX Congr. géol. int. Alger 1952, Monogr. région. sir. Maroc no I et Notes Mém. Serv. géol. Maroc, Rabat, no. 87, p. 45-80. . . 1955.Les gisements de manganèse de Tiouine (Sud marocain). Notes M é m . Serv. géol. Maroc, Rabat, no. 125, ~

180 p.

.

,.1956. Les gîtes de manganèse du Maroc (suivi d‘une description des gisements du Précambrien III). XX Congr.

géol. int. Mexico, 1956, Symposium sobre Yacimentos de Munganeso, vol. 2, p. 217-48. ; PROUST, F. 1959. Filon manganésifère à rhodonite et spessartite dans le Précambrien II du Haut-Atlas.Mines et

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Céol., Rabat, no. 8,p. 45-6. CHOUBERT, G.1952. Histoire géologique du domaine de l’AntiAtlas,in Géologie du Maroc.XIX Congr.géol. int. Alger 1952, Monogr. région. Sér. Maroc no 6,et Notes Mém. Serv. géol. Maroc, Rabat, no. 100, p. 75-194. . 1963. Histoiregéologique du Précambriende l’Anti-Atlas. T.I.Notes Mém. Serv. géol. Maroc, Rabat, no. 162, 352 p. DESPUJOLS, P. 1934.Aperçu sur la géologie et sur les gisements

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miniers de la zone française du protectorat marocain. La Science au Maroc, p. 102.Casablanca,Association Française Pour l’Avancement des Sciences (58th session). JOURAVSKY,G.1953. Sur la composition minéralogique et chimique des minerais de manganèse des gisements encaissés dans les formations volcaniques du Précambrien III (région d‘Ouarzazate). Notes Mém.Serv.giol.Maroc, Rabat,no.117, Notes T.7, p. 289-308. .1963.Filons de manganèsedans les f?rmations volcaniques du Précambrien JI1 de 1’Anti-Atlas-Etude métallogénique. Notes Mém. Serv. géol. Maroc, Rabat,no. 170,Notes T. 22, p. 81-92. ;DESTOMBES, J. 1961. Les différents types de minéralisations dans le domaine de 1’Anti-Atlas.Leur cadre géologique et les méthodes de leur prospection. Mines et Géol., Rabat, no. 13, p. 19-57. LIZAUR Y ROLDAN, J. DE. 1848. Nota sobre unos criaderos de manganeso en el valle del Rio de Oro (Melilla). Notas Inst. geol. España. Madrid,no. 18. NELTNER, L.1934.Le manganèsedanslespossessions françaises. Les resso~~rces minérales de la France d’outre-mer, vol. 2, Paris,Bureau Géol. et Min. Colon. POUIT, G.;JOURAVSKY, G.1960. Gisement de manganèse de Tizi n’Isdid (région de I’Ounein,Haut-Atlas). Mines et Géol., Rabat,no. 11, p. 21-9. .1962.Le gîte de manganèse d‘AitIsgelt(Anti-Atlas), __; un exemple de minéralisation pénécontemporaine de la série encaissante.Mines et Géol., Rabat, no. 17, p. 41-7. SITTER, L.U.DE; HUYSE, W.R.;LAGAAIJ, R.1951. Manganese ores in Eastern Morocco.Geol. en Mij,b., Amsterdam,no.2, p. 52-7.

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Discussion R.P.PETROV. Are the terms ‘itabirite’,‘taconite’and ‘jaspilite’ synonymous? W h y do African geologists prefer to use the term ‘itabirite’?

G.CHOUBERT. I think that geologists who are at work in Morocco, including Jouravsky, have never analysed the thallium and tungsten contents.I have never heard of such analyses either.

G.CHOUBERT. In France ‘itabirite’is used for ferruginous quartzite or ferruginous schist. French geologists working in Africa use this term with the same meaning.

S. ROY. Have you observed any consistent vertical zoning among manganese minerals from higher oxides to lower oxides and silicates along depth?

S. ROY.Did you determine the thallium and tungsten contents of the volcanogenic manganese ores? Hewett and Fleischer have shown that volcanogenic and hydrothermal deposits have much higher thallium and tungsten contents than those formed as nonvolcanogenic sediments.

G.CHOUBERT. As a rule there are no zones in the manganese veins of Anti-Atlas.Jouravsky stated that in veins the Ba0 content is sometimes higher at the surface than at a depth.

124

Tectonic control of sedimentation and trace-element distribution in iron ores of central Minas Gerais (Brazil) A. L. M.Barbosa and J. H.GrossiSad Universidade de Minas Gerais, Belo Horizonte (Brazil)

Introduction Metasedimentary oxide-faciesiron-formations are rather frequent in the Precambrian of the state of Minas Gerais, Brazil. The geology of Minas Gerais is not known in sufficient detail to elucidate the mutual relations of all these formations. The area where the most important iron-ore deposits are located,the Quadrilátero Ferrífero,has ironformations at several stratigraphic levels, but they are neither continuous nor reach high-grade values except at one definite level,namely the Itabira Group of the Minas Series.The conditions of sedimentation of the Minas Series are thus an important part of the history of these ores.The type of ore and the sedimentaryenvironmentemergingfrom the over-allanalysis of the rocks are quite similar to those in other Precambrian regions of the world, although the Minas Series stands out among the youngest of the Precambrian iron-bearingstrata,all radiochronology methods pointing to an age of about one billion years. The present investigationwas aimed at finding the patterns of distribution of trace-elementsin several types of iron ores.The Casa de Pedra mine, near Congonhas,being one of the most important and best explored through drilling in the Quadrilátero Ferrífero,was chosen as a starting point. This paper summarizes the results of determinations made for this mine. A generalsurvey of the geology is given here for the benefit of those who are unacquainted with the literature on Brazilian iron ores.

Sedimentation and tectonic control in the iron-bearing Minas Series An outstanding amount of iron is found in chemical and detrital deposits,not only in the Minas Series,but also in older and younger metasediments of the Quadrilátero Ferrífero. Most of the iron has been recycled and,as a whole, mechanical processes, and chemical sedimentary processes at times of marked instability led to the dispersion of iron, while chemical precipitationunder quiet conditions formed

the most important iron-formation. This formation, an integral part of the so-calledItabira Group of the Minas Series, shows continuity over many kilometres,and is the only stratigraphic unit that houses metasomatic lenses of pure hematite ore. It thus appears that continuity of the iron-orebeds played an importantrole in their later enrichment through metamorphic metasomatism. The sedimentarybasin was elongated,with a minimum extent of 500 km N-S to NE-SW and minimum breadth of 160 km in a W-E direction.These figures were reached through geometrical development of major folds and thrust faults.Throughout its depositional cycles this was a mobile zone,with a tectonic control that closely parallels the evolution of Phanerozoic geosyiiclines,and ended,like these,by being compressed into a folded belt. A strict analogy with typical geosynclines does not hold out. The main differences are: (a)the sedimentary sequences were remarkably thinner here than for any Caledonian, Hercynian or Alpine geosyncline; (b) they were metamorphosed everywhere, so their non-metamorphic equivalents are unknown, contrary to the rule in younger miogeosynclines; (cj the iron-formationis a unique lithology, not exactly duplicated in younger geosynclinal deposits; (dj all lithologies have a lateral persistence not matched in the Phanerozoic sediments, and no drastic facies changes have been recorded. The Minas Series has been divided into a lower,clastic group (Caraça Group), a middle, dominantly chemical group (Itabira group) and an upper, dominantly clastic group (Piracicaba group). The latter is a somewhat artificial grouping of five formations, which can be more naturally grouped as a 2-2-1sequence,as follows:CercadinhoFecho do Funil Subgroup; Taboões-Barreiro Subgroup; Sabará formation. It is generally agreed that the Minas Series,although defined on the basis of lithology,represents a well-defined interval of Precambrian time. The five main divisions adopted above, namely, the first two groups, and the two subgroups plus the isolated formation of the Piracicaba group, are also time-significant.

Unesco, 1973. Genesis of Precambrian iron and manganese deposits. Proc. Kiev Sump., 1970. (Earth sciences, 9.)

125

A. L.M.Barbosa and J. H.Grossi Sad

Each one of the five main divisions is composed of two main lithologies,which can generally be mapped separately and essentially follow one another in an invariablemanner, but with gradational and intertonguing contacts resulting from partially simultaneous deposition. The Minas Series was preceded by deformation,metamorphism and subaerial erosion of earlier formations. Transgression over the mobile zone came from the east and the emerging land to the west supplied most of the detritus, except for the Sabará formation.The transgressive sheet of sand,n o w quartzite of the Caraça group,was followed by clay deposited in quiet water. The Itabira group resulted from a general lack of detrital supply under continued quiet conditions,together with other factors that led to the most extraordinary deposition of banded iron-formation.W h e n these factors changed,deposition reverted to more normal dolomite. The Cercadinho-Fechodo Funil subgroup shows the return of clastic deposition,with well-balancedrates of basin sinking and uplift in the source of sediments to the west. These shallow water sediments are iron-bearingand show a flysch-typelayering. Some layers are iron-rich,detrital iron being derived from then exposed parts of the underlying iron-formation.The Taboões-Barreirorepresents a starvation interval,combining rapid sinking with scarce detrital supply. It is followed by the Sabará formation, with themost typicalfeaturesof flysch deposits.This depositional cycle was closed with the molasse-like Itacolomi Series,followed by deformation leading to structural patterns very similar to the most typical Alpine structures.

Deposition of the iron-bearing Itabira group With the complete peneplanation of the supply area, only the chemical decomposition products capable of being transported in solution or in a colloidal state could be carried to the sedimentation basin at the Itabira time. Great tectonic stability and uniform climate were the conditions predominant at that time.The sedimentation basin was not too deep through the Quadrilátero area,and oxidizing conditions prevailed in its well-aerated waters. Slight p H variations determined the deposition of the lithologies which distinguish the two group formations:the Cauê formation, whose typical rock is itabirite,and the Gandarela formation, whose characteristic lithology is dolomite. Typical itabirite is a striped rock,constituted by alternate layers of quartz and iron oxide, this last being generally specularite and subordinately martite or magnetite. W e suppose that this rock looked,before metamorphism,like hematitic jaspilite of the Morro do Urucum, in Mato Grosso, thus being a chemical precipitation product. The local presence of phyllosilicates and amphiboles may indicate clayey detrital contamination. The base of the Caus formation is concordant with the layering of the Batata1 phyllite, and the contact, which is sharp, is probably an isochronic surface. It is possible that the itabirite layering represents an annual sedimentation cycle. 126

The transportation method and the control of iron deposition are controversial.Admitting the supply by currents fed by meteoric waters which extract iron from the preexisting continental waters, w e should remember that, according to Brusilovsky,thermodynamiccalculations favour the transport of Fe(OH), in molecular solution. Some authors have already speculated on the possible relation of the ferriferoussedimentswith volcanic activities. This influence is likely when the referred sediments are found in the same stratigraphic levels which typically volcanic rocks occupy,such as the Leptitic Series of Sweden, the Soudan formationof Minnesota,or perhaps the itabirite of the Nova Lima group in Minas Gerais. In the case of the Cauê itabirites,however,this association does not exist and if the iron ore owes its existence to volcanism,it is due in an indirect manner to the volume of carbonic gas which Precambrian volcanoes spewed into the terrestrial atmosphere and,at the time when the biosphere was compounded by the most primitive sea organisms, it was still not extracted on a considerable scale. A larger concentration of CO,in the atmospherenecessarilymeans a largerquantity of this gas in the sea-waterand, therefore,a less than normal p H . Under these circumstances, calcium carbonates would be maintained in solutionand iron oxide could be deposited,if it existed in sufficient concentrations.However, the p H was not much below normal as the carbonic acid would havereactedwith iron to form siderite,which does not occur in the Itabira group. On the other hand, the association of itabirites with dolomites shows that p H frequently returned to its more normal values above 7.8.Some problems of the genesis of the itabirites are thus still controversial. The principal component of the Gandarela formation is a dolomitic marble to which dolomitic itabirites associate with phyllites and chlorite schists. The passage between the Cauê and Gandarela formations is transitional and is situated in an extremely variable position in relation to the top and base of the group.The sedimentation of the Itabira time is characterized as predominantly ferriferous at the beginning and predominantly dolomitic at the end, one of these lithologies,however,not excluding the other at any time. The two formations are in fact lithofacies,in a large part synchronic.As a whole,the Itabira Group may exceed the width of 1000 m.,but generally it does not extend beyond 200 m.

Metasomatic origin of the high-grade ore The Precambrian sedimentation in the Quadrilátero Ferrífero was followed by intense deformation and metamorphism.Inthewaning stageof metamorphism a redistribution of the iron took place, with the nearly pure iron oxide bodies as an end product.This process was a metasomatic replacement of quartz and/or dolomite by iron oxide at tectonically favourable sites. The arguments for a metasomatic origin of high-grade iron ore have been discussed at length by Dorr and Barbosa.

Tectonic control of sedimentation and trace-elementdistribution in iron ores of Central Minas Gerais (Brazil)

The general evidence described in their paper has been confirmed by our more recent investigations in the Casa de Pedra mine. Here the evidence,if anything, is still more pressing,for many isolated itabirite masses,with ill-defined boundaries and only a few decimetres long,are entirely surrounded by hematite ore. It is more diflicult to establish whether the rock that was replaced was a so-calledsiliceous itabirite (without carbonates), or a dolomitic itabirite. In the specific case of Casa de Pedra our observations seem to confirm Guild's contention that the original rock was dolomitic. The high-gradeiron ore body is in contact with dolomiticitabirite and ferruginousdolomite,both ofwhich show local iron enrichment,with structures very similar to those of the high-gradeore. On the conditions prevailing at the time metasomatism took place, we can add nothing to what has already been established by Dorr and Barbosa.

Geochemical data OXIDATION RATIO O F T H E IRON O R E S

The oxidation ratio of oxide-facies iron-formation is primarily a function of the ratio of hematite to magnetite, aIthough maghemite in varying stages of oxidation often takes the place of magnetite. In the iron-formationsof the Quadrilátero.Ferrífero, specularhematite is always the dominant ore mineral. Magnetite can be seen in the high-gradeores at many places, sometimes in large crystals,but it is known to be present in lower grade itabirite, even if it is generally not megascopically visible in this rock. On most of the outcrops the iron-formationis magnetic enough to disturb the compass, and investigationsmade in Itabiraby Barbour showimportant amounts of Fe0 both in low-and high-gradeores. Using Barbour's data, we find that the magnetite/ hematite ratio in hard,unenriched itabirite is about 20 per cent,giving an Fe,O,/FeO ratio of 15.8. This ratio is shown at a depth of nearly 200 m . At shallower depths there is a steady increase in the oxidation ratio. The high-grade ore is more thoroughly oxidized than the itabirite.In the study mentioned above,only 6 per cent of the hematite ore had oxidation ratios below 35,against 18 per cent of the itabirite samples. The Itabira ores seem to be derived from non-calcareoussediments.On the other hand,in the deposit of Casa de Pedra,which was the object of the present geochemical investigation,the high-gradeore seems to result from replacement Óf dolomitic itabirite,and is distinctly richer in magnetite.

eral deposits), are practically non-existent.In a preliminary investigation made by Herz and Dutra,a small number of samples taken from some of the lithological units of the region discussed had their trace-elements analysed; however,none of these units were geochemically characterized. During a recent revision and up-dating of the mineral reservesof one of the largest iron ore deposits of the Quadrilátero Ferrífero (Casa de Pedra,District of Congonhas), w e had the opportunity to collect about 550 samples of several types of iron ores and associated rocks,as well as soils and laterites derived from thein. These samples had some of their trace-elementsdetermined to establish,for the specific types of ore,distribution standards which would permit the establishment of comparisonsand correlations with similar ores in the rest of the Quadrilátero Ferrífero.It was also hoped that ii would be possible to infer tlie nature of the original rocks of iron ore and the behaviour of the suite of the trace-elements during regional metamorphism (and metasomatism) and weathering. It is important to remember that the averages calculated for the elements investigated are arithmetical (we hope lo broach the problem from the statistical point of view in the future) and the comparisons in this case are largely approximate.

SAMPLING A N D MATERIAL PREPARATION

The samples were taken exclusively from material originatingfrom drill holes (vertical drilling with diamond bits). Samples from the following units were considered: (1) compact hematite;(2)laminated hematite;(3) schistose hematite; (4) fine granular hematite; (5) average granular hematite; (6) coarse granular hematite; (7) powdered hematite; (8) massive rocks with chlorite and amphibole; (9)chlorite amphibole schists;(1 1) poor itabirites;(12)rich itabirites;(13) dolomites; (14)dolomitic itabirites;(15) decomposed products of dolomilic itabirites and dolomites (clayey-mangano-limonitic material); (I6)ferruginousaluminous laterites. In view of the largeuniformity of the sampledmaterial, starting from the drill cores,we prepared a small volume sample along one or more metres of column core. In the laboratory,each samplewas crushed and ground to 1 O mesh, quartered,reduced in the pestle to 60 mesh and,with successive quartering and reduction,the material was taken to 200 mesh; 5 g were taken for analysis.

ANALYTICAL M E T H O D INVESTIGATION O F T H E COMPOSITION O F THE IRON O R E TRACE-ELEMENTS A N D ASSOCIATED R O C K S

Geochemical investigations of metamorphic and metasomatic rocks of tlie QuadriláteroFerrífero,as well as some of theweathering products (which sometimesconstitutemin-

The spectrographical method was employed using a total burn. The standards were synthesized in high purity iron oxide matrix (manufactured by Johnson Mattey Co. and having twenty elements;Ni, Co,Cu, Sc,Zr,V,Pb,Cr,Ba, Sr,M o , Be, N b , La, W,Sn, B,Ge, Ti, Mn). T o have a reproducible burn, the samples and standards were diluted in a sodium-quartz-graphitecompound. 127

A. L. M.Barbosa and J. H.Grossi Sad

The samples and standards were weighed within the craters of the graphite electrodes and burned in the electric arc of the spectrograph until they were totally consumed. Spectrograph: Jarrel-Ash,3.4 m focal distance,Ebert assembly with 15.000 traces per inch net. Source: JarrelAsh, with a continuous current are of 250 volts and 16 amperes. Film:Kodak SA-1 (ultraviolet) 5nd Kodak 1-N(visible). Spectral covering:2500 to 5000 A.Microphotometer: Jarrel-Ashprojector-comparatorM2 1000. The spectral lines used were those recommended by Ahrens and Taylor in Spectrochemical Analysis. For each element a work curve was constructed from 1000p p m to the minimum sensibility limit. The approximate limits of detection (ppm) for each element are:Ni, 5; Co, 2;Cu, 1; Sc,5;Zr,5;V,10;Pb, 10; Cr,5;Ba, 1; Sr,5;Ti,100;M n , 100. The following elements were investigated and were not detected (limits of detection in brackets); Mo(5), Be(2), Nb(20), La(50), W(20), Sn(5), B(10) and Ge(l0). The spectrochemical analysis were performed at the Geología e Sondagens Ltda.laboratory by C.V.Dutra and Dayse A.O.Lima. ORIENTATION O F THE INVESTIGATION

In fifty samples taken to determine which elements should be analysed preferentially, w e found that, of the list of elements mentioned above, only V, Ni,Cu, Cr, Ba, Ti, M n and Zr were usually present in the investigated rocks and ores. In the iron ores (hematite and itabirite), the elements Ni,Cu, Cr, Ba, and V systematically occurred and were chosen. In the dolomites, schists, laterites, etc., Ni, Cu, Cr, V and Zr were selected. RESULTS

Hematites

In the groups of approximately fifty samples of each of the hematite types which had their averages calculated, the results were extremely concordant. The consequences of these unexpected results are considered in another part of this paper. The combined results for 327 samples are as follows: V, detected in 324 samples; range 12-360 ppm; average 44.5 ppm. Cu, detected in 311 samples;range 2-340 ppm; average 19 ppm. Ni,detected in 219 samples; range 6250 ppm; average 22 ppm. Cr,detected in 315 samples; range 8-300 ppm; average 35 ppm. Ba, detected in 54 samples;range 30-1600 ppm; average 91 ppm. Histograms of the results revealed that a large percentage of the results approached average content of vanadium, copper,nickel and chromium given above,while for barium they did not. Other elements were estimated in 28 samples chosen at random from the 327.The following results were obtained: 128

Co,detected in 7samples;range 6-32 ppm;average 18 ppm. Zr, detected in 11 samples; range 20-60 ppm; average 31 ppm. Ti,detected in 25 samples; range 100-800 ppm; average 657 ppm. M n , detected in all samples;range 3002300 p p m (3 samples with values over 5000 p p m were ignored); average 1180 ppm . Sc,P,Sr, M o , Be,Nb, La,W,Sn,B and G e were not detected. Itabirites

The average results obtained for rich and poor itabirites were concordant.The combined results for 89 samples are as follows: V, detected in 73 samples; range 6-210 ppm; average 35 ppm. Cu, detected in 71 samples; range 2150 ppm; average 22ppm.Ni,detected in 70samples;range 7-170 ppm; average 20.5 ppm. Cr, detected in 80 samples; range 8-160 ppm; average 28.5 ppm. Ba, detected in 62 samples; range 34-1000 ppm; average 179 ppm. Histograms showed that a low percentage of results approached the average. The following results were obtained from 8 samples (4of each type of itabirite): Co, detected in 2 samples; 19 and 220 ppm; average 69 ppm. The result does not look significant. Zr, detected in 3 samples; 8, 20 and 24 ppm; average 17.3 ppm. Ti,detected in 6 samples;range 100500 ppm; average 216.6 ppm. M n , detected in all samples; range 500-3500 p p m (1 sample with a value over 5000p p m was ignored); average 1785 ppm. Sc,P,Sr, M o , Be,Nb, La,W,Sn,B and G e were not detected. Dolomitic itabirites

The following values for dolomitic itabirites can hardly be considered representative;they are only presented as an illustration. Only 6 samples were analysed and in many cases the values were lower than the sensitivity limit.V,detected in all samples;range 26-58 ppm; average 41.6ppm. Cu, 2 samples with 6 and 8 ppm; average 7 ppm. Ni, 2 samples with 10 and 20 ppm; average 15 ppm. Cr, detected in 5 samples;range 10-110ppm; average 38.5p p m (if we do not consider the 110 ppm, the average is 21 ppm). Ba, 3 samples with 10,36 and 46 ppm; average 27.3 ppm. T w o of these samplesshowed T iand M n ,with averages of 150 and 1600 ppm, respectively. Ferruginous dolomites

Only 12 samples were analysed and in general the results were in agreement. In 8 samples C u was not detected and in 4 samples Ni was not detected. V,range 12-45 ppm; average 30 ppm. Cu,range 13-76 ppm; average 31.5 ppm. Ni,range 8-31 ppm; average 12.5 ppm. Cr,range 1344 ppm; average 24.5 ppm. Zr,range 26-72 ppm; average 37.5 ppm.

Tectonic control of sedimentationand trace-elementdistribution in iron ores of Central Minas Gerais (Brazil)

Metupelites (massive or schistose)

C O M P A R I S O N S W I T H SELECTED R O C K SEQUENCES

Phyllites, schists and homophanous rocks (mainly chlorite, talc and amphibole) gave similar results and were grouped together.A total of 57 samples showed Cu,Ni,Cr,V,and Zr,and 4 samples we analysed Co,Sc,Ba, Sr,Ti and M n . V, range 35-300 ppm; average 107 ppm. Cu, range 2340 ppm; average 69 ppm. Ni, range 12-240 ppm; average 67 ppm. Cr, range 13-560 ppm; average 106 ppm. Ba, range 128-240 ppm; average 196 ppm. Co, range 36108 ppm; average 58 ppm. Zr, range 38-640 ppm; average 236 ppm. Ti,range 660-22,000 ppm; average 18,850ppm. M n , range 1,100-4,500 ppm; average 2525 ppm. Sc,range 36-46 ppm; average 40 ppm. Sr, range 16-42 ppm; average 25.3 ppm.

Vanadium

Only 7 samples were analysed. The results were in good agreement. The averages were: V, 123 ppm. Cu,47 ppm. Ni,39 ppm. Cr,163 pprn. Zr, 278.5 ppm.

The average results for the hematites and dolomitic itabirites are comparable,as are those for itabirites and ferruginous dolomites. The results for the four units are lie between 30 and 44.5 ppm. In hematite ores (‘quartz-hematite ore’) of central Sweden, twelve samples showed average values of 49 ppm; based on results presented by Landergreen. Our results are also comparable with the 40 p p m of ultramaficrocks.The carbonate content is 20 ppm on average. The metapelites show results (107 ppm) comparable with those for stratified clayey rock (130 ppm). Decomposition products (residual material over iron ores and associated rocks) set the vanadium in such a manner that there is a larger incorporation of metal in geologicallymore evolved material,i .e.in mangano-limoniticmaterial the average value is 74 p p m and in the laterites 130 ppm. All thevanadium is contained inthecrystallinestructure of the hematite and magnetite. The constantlylower values for the itabirites than for the high-grade hematite result from the fact that the itabirites contain, besides the iron oxides,quartz with practically no traces of vanadium.Probably the concentration of vanadium in the iron ores was determined at the time of metasomaticmetamorphism.The residual enrichment of the itabirite by weathering is essentially a process of quartz leaching, which slightly affected the iron oxides.

Laterites

Copper

Results for 23 samples were quite similar. The averages were: V, 130 ppm. Cu, 62.5 ppm. Ni,34.5 ppm. Cr, 123 ppm. Zr, 335 ppm. The results described above are summarized in Table 1 together with the averagesfor selected groups ofrocks from the literature.

The results for the hematites and itabirites are almost identical and intermediate between the values of the dolomitic itabirites on the onehand and ferruginous dolomites on the other.W e do not have informationto compare theseresults with those for iron ores of other regions.The values for the hematites and itabirites are identical to the values for

Mangano-fevruginous material

Physically the mangano-ferruginous material is consistent, with a considerable clayey fraction.Samples were taken in places where it was certain that the materialhad originated from dolomiticrock decomposition,27,ferruginousor not, and with a variable content ofpelitic material.In 13 samples we did not analyse Ba,and in 16 samples,Zr.

Red und yellow clays

TABLE 1. Average results for trace-elementsin iron ores and associated rocks (pprn) 2

3

4

5

6

7

8

9

10

11

41.6 7.0 15.0 21 .o 27.3

30.0 31.5 12.5 24.5

123 .O 47.0 39.0 163 .O

130.0 62.5 34.5 123.O

40 20 2,000

200

20

1O0

4 20 11

-

-

-

101.5

278.5

335.0

-

-

107.0 69.0 67.O 106.0 196.0 58 .O 236.0 18,850.0 2,525.0 40.0 25.3

74.0 24.0 60.0 51 .O 181.0

657.0 1,180.0

35.0 22.0 20.5 28.5 179.0 69.O 17.3 216.6 1,785.0

1

V Cu Ni Cr Ba Co Zr

Ti Mn sc Sr

44.5 19.0 22.0 35.0 91.O 18.0 31 .O

-

-

150.0 1,600.0

-

-

-

37.5

-

-

-

-

-

-

2,000 1 200

30

-

1,500

-

5

-

3O0

10

160 200 300 45 1O0 9,000 2,000 24 440

10

OJ 19 400 1,100

1 610

12

130 57 95 1 O0

580* 20 200

4,500 850* 10

450

1, Hematites; 2, Itabirites; 3, Dolomitic itabirites;4,Ferruginous dolomites; 5, Metapelites;6,Mangano-ferruginous material; 7, Clays; 8,Laterites; 9, Ultramafic rocks (Vinogradov, Geokhimiya, 1962); 10, Mafic rocks (Vinogradov, Geokhimiya, 1962); 11, Carbonates (Turekian and Wedepohl, Geof. Soc. Ant. Bull.,Vol. 72, 1961); 12, Stratified clayey rocks (Vinogradov; values with asterisk, Turekian and Wedepoht).

129

A.L. M.Barbosa and J. H.Grossi Sad

ultramafic rocks (20 ppm). The carbonate content is 4ppm on average,Rankama and Sahama gave average values of 20 ppm in limestone and 12.5pprn in dolomites.The metapelites give average results of 69ppm,similarto the average (57 ppm) found for stratified clayey rock. The decomposition products directly connected to the ferruginous dolomites were not enriched during the weathering process with regard to copper;the results show leaching of the element and its fixationin the laterites and clays.

Nickel and cobalt In the hematites the results for these elements are 22 and 18 ppm respectively,with a Co/Ni ratio of 0.8.In twelve samples of quartz-hematiteore from central Sweden we calculated values of 30 and 20ppm for N i and Co respectively (according to data by Landergreen) with a ColNi ratio of 0.7.The itabirites show average contents of 20.5ppm of nickel and 69 p p m cobalt. (The result for cobalt was obtained from only two very discrepant values.) The average nickel content of the itabirites is comparable with that in the hematites and,when a larger number of cobalt determinations in the itabirites is available, w e can calculate the Co/Ni ratio more accurately. The nickel contents of the hematites and itabirites are greater than the values for the dolomitic itabirites and ferruginous dolomites. In general terms,the results for these rocks are comparable with the world average for limestone (20 PPm). In the metapelites the Co/Niratio is 0.8,while for the stratified clayey rock the ratio is 0.2. In the mangano-ferruginousmaterial, clays and laterites the nickel shows average values comparable with the original rocks. Chromium

This element is distributed in the hematites,itabirites,dolomitic itabirites and ferruginous dolomites in quantitiescomparable with values given by Landergreen for ‘quartz-hematite ores’ of central Sweden. The results are noticeably higher than the world average for chromium in limestone. Apparently discrete concentration of the element occurred in the hematites during metasomatism. The average grade of the metapelites is identicalto that indicated for stratified clayey rock. In the mangano-ferruginousmaterial the average chromium content is greater than in the original rocks,while the clays and laterites show a comparable grade, inferior to that normally reported for similar materials. Thus the weathering products have an average grade superior to that of the original rocks. Barium

The distribution of this element in the iron ores and associated rocks is irregular and the contents are not comparable with the average given for selected rock sequences.In view 130

of its low ionic potential and large size, the barium tends to set by absorption in the clays;this was not confirmed in our case. Nor there was any relationship between the Ba and M n . A more detailed investigation of the distribution of Ba in the iron ores may explain this. Zirconium

This element is maintained at a comparable level in the hematites, itabirites and ferruginous dolomites. The averages obtained are similar to the world averages for the ultramafic rocks and limestone. In the metapelites the distribution of Zr is comparable with the average world values for stratified clayish rocks. The levels of this element in the decomposed products agree with what w e would expect during weathering, i.e. precipitation (by absorption) occurs in the hydrolysed material. Titanium

A progressive enrichment in this element occurs, which it is difficult to explain. The values are not comparable with the world average for specific types of rock. In the case of metapelites,the average found is exceptionally high. Manganese

The results for manganese are comparable with those for the Swedish iron ores. Itabirites and dolomitic itabirites show identical grades in M n , while the hematite shows a markedly lower grade. The average obtained for itabirites and dolomitic itabirites is similar to the world average for ultramaficrocks,while the averageforhematite corresponds to that for limestone. The metapelites are notably Mn-enriched. Scandium and strontium

Values were obtained only for metapelites and they are not comparable with world averages for specific rock types.

Conclusions The preliminary study of the distribution of the traceelements in iron ores and associated rocks of one region of the Quadrilátero Ferrífero suggests: 1. The high-gradehematite bodies present a distributionof trace-elementsessentially comparable with poorer ferriferous rocks. As the investigation stopped at the time of the study of samples of total rock, it is not yet possible to draw conclusions on the migration of these elements in the course of the generating metamorphism metasomatic process of these masses of high-gradeiron minerals. 2. The same results are found with representative samples of materials which had suffered weathering alterations,

Tectonic control of sedimentation and trace-elementdistribution in iron ores of Central Minas Gerais (Brazil)

i.e. enriched itabirite and masses of high-gradefriable hematite. 3. The weathering products show average values comparable with each other and superior to those of the original formations;the results obtained were those expected. 4. In general, the average values for some elements of the hematite-itabirite-ferruginousdolomitic itabirite sequence are concordant with the world averages recog-

nizable for ultramafic rocks, while other elements show averages comparable with limestone rocks. Some elements have averages comparable with either ultramafic rocks or limestones. 5. The metapelites associated with the iron ores are really metamorphosed stratified clayey rocks which are,despite the total absence of quartz,sedimentspoor in combined silica.

Résumé L e contrôle tectonique de la sédimentation et la répartition des éléments-traces dans les minerais de fer de la partie centrale de l’État de Minas Gerais au Brésil (A.L. M.Barbosa

et J. H.Grossi Sad) A u cours du Précambrien, des sédiments riches en fer se sont déposésà diversesépoques dans l’État de Minas Gerais, au Brésil. Bien que des quantités extraordinaires de fer se trouvent dans des dépôts tant chimiques que détritiques, d‘une façon générale,les processus mécaniques et les processus sédimentaires chimiques dans des conditions calmes ont contribué à la création de la formation de fer la plus importante.Cetteformationest continue sur des kilomètres, et elle est la seule source de lentilles métasomatiques de minerai d’hématite pure,la continuitéjouant un rôle dans son enrichissement. Le bassin sédimentaire était allongé, son extension étant d’au moins 500 kilomètres entre les directions nord-sudet nord-est sud-ouestavec une largeur minimale de 160kilomètresdans la directionest-ouest.Tout au long des cycles de déposition,cette zone a été mobile, sous des actions tectoniques étroitement parallèles à l’évolution des géosynclinaux phanérozoïques. Les différences essentiellessontlessuivantes :(a) lesséquencessédimentaires étaient remarquablementplus minces ; (b) la métamorphose est générale;(c) la formation de fer n’a pas sa réplique dans les géosynclinaux phanérozoïques; (d) aucun changement extraordinaire dans le faciès n’a été observé. Cette communication traite spécialement des dépôts des séries de Minas qui contiennent la plus importante formation de fer. Cette série a été diviséeen plusieurs groupes : un groupe clastiqueinférieur (groupe de Caraça), un groupe moyen essentiellement chimique (groupe d‘Itabira) et un groupe supérieur,surtout clastique (groupe de Piracicaba). Le doyen des auteurs de cette communication considère comme arbitraire le dernier de ces trois groupes. I1 lui semble qu’il serait plus naturel de réunir le sous-groupeCercadinho-Fechodo Funil,le sous-groupeTaboões-Barreiroet

-

la formation de Sabará. II est généralement admis que la série de Minas représente une époque bien définie de la période précambrienne,Les cinq divisions principales indiquées ci-dessus ont aussi une signification chronologique. Chacune est composée de deux lithologies dominantes,décrites c o m m e des formations,avec un ordre chronologique quasi constant,mais avec des contacts graduels et interdigités qui résultent en partie de leur dépôt simultané. L a série de Minas a été précédée par la déformation, le métamorphisme et l’érosionsubaérienne des formations antérieures. La transgression de la zone mobile est venue de l’estet les terres émergées à l’ouest fournirentla plupart des détritus,sauf pour la formation de Sabará. L a couche de sable quartzite du groupe de Caraça,qui a transgressé, a été suivie par un dépôt d‘argile en eau calme. L e groupe d’Itabira provient d‘une déficience générale de l’apport de détritus pendant des périodes calmes prolongées, qui, avec d‘autres facteurs,a contribué à former un extraordinaire dépôt d‘une formation de fer rubanée. Quand ces facteurs ont changé,le dépôt est revenu normalementà la dolomite. On observe dans le sous-groupeCercadinho-Fechodo Funil le retour au dépôt clastique avec des périodes bien équilibrées de surhaussement et d‘abaissement du bassin à l’origine occidentale des sédiments. Ces sédiments en eau peu profonde contiennent du fer et présentent une stratification du genre du flysch.Quelques couches sont riches en fer, le fer détritique provenant des parties de la formation de fer sous-jacentequi étaient alors exposées. Le groupe TaboõesBarreiro traduit une période de disette au cours de laquelle un enfoncement rapide s’esttrouvé conjugué avec un faible apport de détritus. L a formation de Sabará lui fait suite et présente les caractères typiques des dépôts de flysch. C e cycle de dépôts s’estrefermé avec la série du genre molasse d’Itacolomi,suivie d’une déformation qui a donné des systèmes structuraux très voisins des structures alpines les plus typiques.

131

Absolute age dating of iron-silicate and ferruginous formations and their position in the Precambrian stratigraphic sequence. Analogous formations from the Phanerozoic

La datation absolue des formations de fer et de silicate de fer et leur position dans la série stratigraphique précambrienne. Les formations phanérozoïques analogues

The iron-chert formations of the Ukrainian shield N.P. Semenenko Institute of Geochemistry and Physics of Minerals Academy of Sciences of the Ukrainian S.S.R., Kiev (U.S.S.R.)

Iron-chertformations ofthe Ukrainian Shield are developed in a number of synclinorial zones, representing different structural stages of the earth’s Precambrian crust (Fig. 1). The oldest is the Konsko-Belozyorsky synclinorial zone, where rocks dated back to 3,500 m.y. have been found. Metamorphic masses composing the Konsko-Belozyorsky synclinorium are formations of the oldest defined structural stage of the earth’s crust-presumably the first Precambrianmegacycle. Granites of the age of 3,100-3,400 m.y. are synchronous with rocks of the first Precambrian megacycle. The Konsko-Belozyorskyfolded zone stabilized after the intrusion of the Mokromoskovsky granites-2,7002,800m.y. ago; emplacement of the granites resulted in reworking of older rocks in the western border of the Konsky syncline by processes of granitization and greisenization.The above folded zone forms a stable block of the ancient crust. The iron-chertformations remain at lower metamorphic grade (i.e. schist and chert stages) only in their marginal parts, in contact with granites, as they approach a gneissic stage of metamorphism. Iron-chertformations are present in two cycles-in the Lower Konsky and the Upper Konsky series (Figs 2and 3). In the Lower Konsky series-in the Yulyevsky zone of the Konsky syncline as well as in the western zone of the Belozyorsky syncline-iron-chert layers alternate and intercalatewith thin interlayers of green schist or, at gneissic stages of metamorphism, with amphibolite, forming a rhythmical stratification of chemogenic iron-chertdeposits and mixed tuff representingvolcanogenic products of basic composition (metabasites). The masses of iron-chert-metabasite formations,in which chemogenic iron-chertproducts are parageneticallyrelated to tuff-volcanogenicmetabasites, attain a thickness of 300-500 m and extend for 10-15 km. In the upper depositional cycle of iron-chertrocks of the Upper Konsky series iron-chertlayers and iron-alumosilicate schists are rhythmically interbanded. Amongst the schists,keratophyre layers are observed as well as acid tuffschists and tuff-sandstones. Iron-chert-schist-keratophyre formation constitutes an

Upper Konsky series, replacing an underlying tuff-sandy schist keratophyre suite. In the Belozyorsky syncline iron-chertlayers form a uniform iron-richzone 100-300 m thick and 30-90 km long. Here, a secondary metasomatic enrichment took place which has resulted in formation of high-gradehematite ore deposits with an iron content to 68 per cent. In the Konsky syncline the upper iron suite is repformation which resented by iron-chert-keratophyre-schist reaches 100-200m thick and 10-20 k m long. However,the iron content of the iron-chertdeposits ranges along strike from 35 per cent to 20 and to 10per cent within an interval of 1-3 k m . Another zone of iron-chertdeposits is present in the Bazavluksky synclinorium.It comprises a number of large brachy-synclines-the Verkhovtsevsky, Sursky and Chertomlyksko-Solenovsky,-and is composed of metamorphosed iron-chertformations. The oldest minerals found here are 2700-2800 m.y. in age. The metamorphic formationsare found to contain two main cycles of iron-chert deposits corresponding to the Lower and Upper Bazavluksky series (Fig. 4). In the lower series of the Verkhovsevsky syncline,ironchert deposits are present mainly in the middle suite.Ironchert layersalternatehere with tuff-schist-metabasite layers. Four iron zones are distinguished each with iron-rich, iron-chertlayers. Individual beds of iron-chert layers do not exceed 20-25 m thick; a number of layers approach 20-40 per cent. The iron-chert-metabasiteformation is 300-500 m thick; the formation is traced in the area of the Verkhovtsevsky syncline for a distance of 50 km. Iron-chert intercalations are also found in the upper metabasite suite of the Lower Bazavluksky series of the Verkhovtsevsky syncline.The iron-chertinterlayers do not exceed 10-20c m thick;they amount to not more than 10per cent of the suite. In the Sursky syncline iron-chert metabasite formation is also observed in the lower metabasite series. It ranges from phyllite-greenschistto gneiss amphibolite metamorphism, but reaches a gneiss-pyroxene stage in

Unesco, 1973. Genesis of Precambrian iron and manganese deposits. Proc. Kiev Symp., 1970. (Earth sciences, 9.)

135

N.P.Semenenko

-. 5

Formations:

A g e provinces:

Iron-chert-schist Iron-chert-keratophyreschist @@Iron-chert-metabasite

Konsko-Belozyorsky , m. 2,700-3,500 m.y.

Iron-chert-ultrabasite Undivided iron-chert Enclosing formations: Rhythmical schist-terrigene Keratophyre (leptite) Porphyrite

aMetabasite Ultrabasite

136

.

*I

Bazavluksky, 2,300-2,700 m.y. Orekhovo-Pavlogradsky , 2,000-2,300 m.y. Korsaksky

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Saksagansky metabasite Krivoyroghsko-Kremenchugsky, 1,700-2,000 m.y. Synclines:

1. Belozyorsky 2. Konsky 3. Chertomlyksky 4. Sursky 5. Verkhovtsevsky 6. Inguletsky 7. Saksagansky 8. Annovsky 9. Zheltorechensky 10. fravoberegliny 11. Kremenchugsky

FIG.1. M a p showing distribution of iron-chert formations of Ukrainian Shield.

The iron-chertformations of the Ukrainian Shield

FIG.2.Diagrammatic seismo-geologicalsection of Belozyorsky syncline. 1. Granites and migmatites;2. Ultrabasite formation. Upper iron-chert-schist-keratophyreseries. 3, 4 and 5. Ironchert-keratophyre-schistformation: (3. Schist-iron-chertzone with keratophyre interlayers;4. Jron-chert zone; 5. Schistiron-chert zone); 6. Schist-tuff-sand keratophyre formation.

Lower iron-chert-metabasiteseries: 7 ,Porphyrite-schistformation; 8. Iron-schist-metabasiteformation (iron-chert and green schist zone); 9. Metabasite formation (green schists and amphibolites); 10. Tectonic dislocations; 12. Refraction and reflection boundaries (seismic evidence); 12. Faults (seismic evidence).

-

400 O (O0LOO 300 m

FIG.3. Diagrammatic section of the Konsky syncline. 1. Pink granites and migmatites; 2. Grey granites and migmatites. Iron-chert-schist-keratophyreformation of the VeselyanskaKirpotinsky suite (KB,). 3. Schist zone;4. Iron-chertzone; 5. Keratophyre-schist formation of the Veselyansko-Kirpotinsky subsuite (KBB).Ultrabasiteformation of the Veselyansky 6. Actinolites and chlorite-actinolite schists; suite (a,).

-

II I -

Talc-magnetites;8. Metabasite formation of the upper green schist suite (KN,);9. Iron-chert-metabasiteformation of the Julyevsky suite (KN,): schist-spilitezone and iron-chertzone; 10. Metabasite formation of the lower Julyevsky suite (KN,); 11. Tectonic dislocations.Districts:I,Kirpotinsky;II,Medium; III, Julyevsky.

sw

NO

FIG. 4. Diagrammatic section of Verkhovtsevsky syncline. 1. Granites and migmatites. Upper Bazavluksky iron-chertkeratophyre-schistseries: 2. Iron-chert-schist-keratophyre formation; 3. Schist-keratophyreformation;4.Ultrabasite formation. Lower Bazavluksky iron-chert-metabasiteseries:5.Upper metabasiteformationwith iron-chertinterlayers;6,7and 8.Ironchert-metabasiteformation (6. Amphibolites of the third dividing zone; 7. Actinolites and talcs of the second and first dividing zone; 8. The first, second, third and fourth packet of iron-chertlayers intercalated with green schists and apospilites); 9.Lower metabasite formation. 137

N.P.Semenenko

the Domotkansky side of the Verkhovtsevsky syncline. The second cycle of iron-chertdeposits is found in the upper suite of the Upper Bazavluksky series; sand-tuffschist-keratophyredeposits of the middle suite are transitional upwards to iron-chertand tuff-schist-phyllite interlayers. The iron-chertdeposits are associated paragenetically with products of the dacite-rhyoliticvolcanism. The thickness of iron-chert layers reaches 50-140 m . They are represented by magnetite-siderite-chlorite-quartz cherts. Ironchert layers are traced along strike for a distance of 5 km; further their facies pinch out. The iron ratio also changes. The iron-chertformation attains a total thickness of 300500 m,iron-chertlayers constituting 30-50 per cent of the formation. In the Sursky brachy-synclinethe thickness of the ironchert-schist-keratophyre formation ranges from 300 to 500 m;On the northern side of the brachy-synclinethe formation extends for 30 k m . Individual packets of iron-chertlenses attain a thickness of 90-100m . Iron content, in most cases,however,is not high, averaging 10-12 per cent and,rarely,30 per cent. In the Chertomlyksko-Solyonovsky brachy-syncline iron-chert-schist-keratophyre formation (up to 400m thick) is found in the Chertomlyksky band. Iron-chertbeds are 100-150m thick and 6 km long. The average iron content is 30-35 per cent. The formation consists of siderite-magnetite-quartzcherts. The third zone of iron-chert distribution is present in the Orekhovo-Pavlogradskysynclinorium which extends for 100k m . The age of the oldest minerals is 2,300 m.y. To the east of this zone another smaller iron-chert zone-the zone of the Korsaksky synclinorium-is developed. Some older minerals present in the area of the village of Stulnevo yield an age of 2,800m.y. Further to the east, in the area of the shield adjacent to Azovsk Sea, small iron-chertdeposits occur in the Mangushsky synclinewhich is evidently synchronous with the Orekhovo-Pavlogradsky zone. Iron-chert deposits of the Orekhovo-Pavlogradsky syncline are represented by two formations: iron-chertmetabasite and iron-chert-schist-gneiss. Folded structures are divided here into separate synclinal bands extending for 2-5 k m . The rocks are metamorphosed to amphibolite gneiss and pyroxene gneiss; ironchertrocks consistofcoarse hornblende-magnetitequartzite and hypersthene-magnetite quartzite. Iron-chert lenses range in thickness from 10-20 to 100 m and extend for 2-5 km.The faciesare less consistentand are often replaced with, and end in, granite. The folded zones represent an inverted synclinorium in which the rocks have been intensely granitized. The metabasite Saksagansky series of the Kremenchugsky region, with its iron-chert interlayers, evidently corresponds to the same structural stage. The main structural zone of development of ironchert depositsis the Krivoyroghsko-Kremenchugskysynclinorium,traced for 200 km in the meridional direction.The 138

age of the formation is found to be 2,000 to 1,800m.y.; the latter age however, is debatable. Sedimentation of the iron-chert-schistzone began with eruption of ultrabasic products represented by a talcose horizon 200 km long. The origin of the ultrabasic rocks is evidently connected with a deep fault. Keratophyre interlayers occur sometimes in the upper part of the talcose horizon as well, for example, in the Inguletsky syncline along the Timosheva ravine. The rhythmic banding of iron-chertlayers and alumosilicate schists in the Krivoyroghsky series is up to 1,500m thick. Such a thickness is reached only in two troughs-the Saksagansky and Kremenchugsky,each of 50 km extent. Iron-chert-schistformation of the Krivoyroghsky series is unconformably overlain by the Inguletsky series, which is characterized by a change of sedimentation to a flyschoid type.They occur as compressed isoclinal,echelon-likefolds extending along strike for a distance of 10-50 km. The central structure of the Krivoyroghsky syncline (Fig. 5) passes into the Tarapakovsky anticline in the west; this is closed by the Inguletsky (Likhmanovsky) syncline, truncated from the west by a thrust extending all along the strike.

FIG.5. Section of Krivoyroghsky series. 1. Inguletsky series; 2. Iron-chert-schistformation of Krivoyroghsky series;3. Talc horizon; 4. Sand-schistlower suite of Krivoyroghsky series. I,Inguletsky syncline;II,Tarapakovsky syncline; III,,2, 3, Krivoyroghsky syncline (III,, Western Krivoyroghsky trough; III,, Sovetsky anticline; III,, Eastern Krivoyroghsky trough); IV,Saksagansky anticline;V,Saksagansky syncline. The eastern limb of the Krivoyroghsky syncline is represented by the Saksagansky band which, for its own part, is complicated by the isoclinal Saksagansky syncline and anticline. The curve of the Krivoyroghsky syncline borders the southern margin of the Saksagansky trough,the northern margin being rimmed by the Ternovsky flexure. Within the

The iron-chertformations of the Ukrainian Shield

limits of the Saksagansky trough some layers are encountered which consist of iron-chertunits interbanded with schists. The total maximum thickness of iron-chert strata is 500-800 m;it amounts to 50-75 per cent of the sequence. The average iron content in the iron-chert strata is 3040per cent.Within the limits of the ñrst and second Saksagansky subsuites,a secondary metasomatic enrichment of iron-chertstrata occurred and development of iron-richdeposits took place. Ferruginous strata at the surface are oxidized and martitized;they change into magnetite hornfelses at depth. The cross-sectionsof the iron-chert-schistsuite of the Saksagansky and other troughs are shown in Figure 6. To the south of the Saksagansky trough, in the area of the Inguletsky syncline,a sharp decrease in thickness of the iron-chert-schistformation is observed for a distance of 20 k m . The total thickness of the formation here is 300-400 m;in places it is only 50 m ‘thick.Iron-chertrocks constitute 50 per cent of the formation.The formation is represented by the ñrst,second,fourth and fifth strata each separated by a bench of schists. To the north of the Saksagansky trough along the strike of the Annovsly syncline,a sudden decrease in thickness is observed along with a gradual pinching out of ironchert facies. Further to the north in the parallel ‘Zheltorechensky’ trough, the thickness of iron-chertdeposits increases again to 700 m.Still further to the north in the Pravobereghny region a limited thickness (100-300 m) of iron-chert-schist formation is found over a distance of 40 km,with variable

number of iron-chert strata. A facies replacement occurs along the strike of the schist and iron-chert layers; these constitute 50-70 per cent of the sequences as shown at the corresponding cross-sections(Fig. 6). The Kremenchugsky syncline is developed on the right bank of the Dnieper in the northern part of the synclinorium.It is similar to the structure of the Krivoyroghsky syncline. In the Kremenchugsky trough iron-chert-schist formations once again approach thicknesses of 1,500m.A total thickness of all the iron-chert strata in the Galeschinsky syncline attains 500-700 m.The strata constitute 60-70 per cent of the sequence. The presence of tufogeneschistsin the sediments of the Kxemenchugsky trough is a prominent feature,as is also a number of clastogene intercalations within the formation. Moroever,a pyrite horizon is encounteredin the iron-chertschist formation;it extends for a distance of 20 km. The lenses of pyrite in the horizon attain a thickness of 90 m . Deposition of pyrite in the iron-chertformation of the Kremenchugsky region testifies to an alternation of oxidation-reduction regimes during sedimentation. Tufogene schist intercalations found in the Kremenchugsky trough suggest a genetic relationship between ironchert-schistsand volcanogene formations. To concludethe consideration of environments of ironchert formations it should be noted that they are also present in the Ananyevsky band of the magnetic anomaly in the western part of the Shield on the river Bug. The present considerations show that cycles of deposition of iron-chertformations occurred repeatedly in structural

m IO0

-

200

-

300 .

400 .

500 . 600 .

100 .

ao0 . 900 .

4000

-

ííoo . ízoo 4300.

(400. 1500.

i600

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FIG.6. Sections of iron-chert-schistformation along Krivoyroghsko-Kremenchugsky synclinorium. 1. Iron-chert layers; 2.Iron-pooriron-chert-schistlayers;3, Schist layers. Regions:

I, Inguletsky;II, Saksagansky; III, Annovsky; IV, Zheltorechensky; V, Pravobereghny; VI, Kremenchugsky.

139

N.P. Semenenko

stages of the Ukrainian Precambrian ranging in age from 3,500m.y. to 1,800-1,700m.y. Depositional environments and paragenetic associations of interbedded rocks have led to the conclusion that iron-chertformations are chemogenic deposits related genetically to submarine volcanic areas of geosynclines. Evidently their deposition may have been related to periods of interruptionin volcanic eruption when submarinehydrothermal solfataric and fumarolic activities,developing over great areas,provided a special environment for deposition of siliceous-ironsedimentsprecipitatiiig in a colloidalform. During the periods of interruption, which considerably exceeded in duration the periods of eruptionitself,the delivery of tuff-schistsdetritus did not supress chemogenic ironchert deposits;these were not smothered in the mass of tuffaceous material but were evidently deposited in a border or peripheral playground of volcanic activity where remote schist-iron-chertdeposits were formed. According to the above paragenetic associations of rocks in interbedded iron-chertformations as illustrated in different structural zones, the author distinguishes three types of iron-formations: (a) iron-chert-schist,(b) ironchert-schist-keratophyreand jaspilite-leptite,and (c) ironchert-metabasite and iron-chert-ultrabasite(Fig. 7). Iron-chert-schistformation is characterized by the highest degree of facies consistency.Continuous iron-chert zones extend for 10-20 k m , often with a constant iron content of 30-35 per cent and,rarely,to 45 per cent. Ironchert layers constitute 50-75 per cent of the sequence.Individual iron-chert layers with minor schist intercalations attain a thickness of 100-300m . Schist interlayersare composed of mixed pelitic terrigenous-tufogenicand chemogenic iron-silicatedeposits alternating with thin centimetric intercalations of chemogenic siliceous deposits. They are remote formations, deposited in conditions of prolonged and persistent hydrothermal activity of submarine volcanic areas with limited supply of ashy material and a wide distribution of chemogenic colloidal sediments. The formation of the Krivoyroghsko-Kremenchugsky syncline zone and of the upper suite of the Belozyorsky

syncline is referred to this type; the formation passes into iron-schist-keratophyreonly in the northern part of its strike. Iron-chert-schist-keratophyreformation is made up of paragenetic associations of tuff-keratophyre-schist and ironchert deposits formed in the area of submarine volcanism of dacite-rhyoliticlavas. In the area of gneissic stages of metamorphism, acid volcanogenic products have been transformed into leptites;that is why they have acquired the name jaspilite-leptiteformations. A lower order of facies consistency and a greater diversity in iron content are their characteristic features. The proportion of iron-chertdeposits in the sequences is not high; it amounts to 20-40 per cent. However, individual lenses of iron-chertdeposits are present up to 100150m thick and 5-7km long.Iron contentis 30-35 per cent. The upper suite of the Verkhne-Konsky series, the Teplovsky band of the Verkhne-Bazavluksky series (the Verkhovtsevsky syncline), etc., are also referred to ironchert-schist-keratophyreformations. The above formations are formed in the area of submarine volcanism of rhyolite-daciteandesitic composition. Deposition of iron-chert sediments proper is related to periods of extrusive dormancy and development of long periods of submarine hydrothermal activity. Areas of acid volcanism are characterized by more abundant subvolcanic activity in comparison with areas of forbasaltic lavas;that is why iron-chert-schist-keratophyre mations are more intensively enriched with iron-chert deposits. Here they are of local extent only, being restricted to 5-10 km from the immediate proximity of volcanic foci. Iron-chert-metabasiteformationsare deposited in areas of development of submarine volcanism of basic lavas. These formationsoccur in the lower metabasite series of the Konsky, Belozyorsky, Verkhovtsevsky and Sursky synclines. They are characterized by a small number of ironchert intercalations of relatively limited thickness and extent. However, they compose in some places thick masses up to 500 m thick,in which siliceous intercalationsamount to 20-30 per cent.

I O

IOKrn

100 200

300

4ao 500 600 700

am

900 i000

m

mI m2 i s x i 3 [vvv4mi5 FIG.7. Types of iron-chert formations. I, Iron-chert-schist formation with 50-90 per cent of iron-chertdeposits;II,Ironchert-schist-keratophyreformation with 30-50 per cent of ironchert deposits; III, Iron-chert-metabasiteformation with 140

10-30and 10 per cent of iron-chertdeposits.1. Iron-chertlayers; 2. Schist layers; 3. Keratophyre layers; 4. Metabasite layers; 5. Ultrabasite layers.

The iron-chertformations of the Ukrainian Shield

The absence of accumulations of iron-chert beds of great extent associated with basic volcanics may evidently be explained by a lesser intensity of hydrothermal volcanogenic processes in such areas compared with areas of rhyolite-dacitevolcanism. At the saine time, development of iron-chert intercalations makes it possible to evaluate a regularity of interruptionsin extrusive volcanic processes observed during the accumulation of rather thick masses. W e observe a high proportion of iron-chertzones in a mass of tuff-volcanogenematerials. Iron-chert-ultrabasiteformations have an insignificant distribution in the Kudashevsky area of the Konsky and Verkhovtsevsky synclines. The alternation of low grade iron-chertinterlayers with ultrabasite schist is a prominent feature. The formations testify to manifestations of tuffaceous ultrabasite volcanism with extrusive interruptions also accompanied by submarine hydrothermal activity. Pure iron-chert rocks consists only of Feo, Fe,O, and SiO,+ H,O + CO,.Iron ratio calculated by the formula (Fe0 + 2 Fe,O,) . 100 Alzo,+ M g O + C a 0 + (Fe0+ 2 Fe,O,) ’ derived from chemical analysis,amounts to 95-100.Thus, in chemogene deposits we have a complete differentiation of iron and silica from other rock-formingoxides. With an increase in number of schist interlayers the iron ratio decreases to 60-90 per cent at the expense of admixtures of Alzo,and partly of M g O ; in metabasite iron-chertformations C a 0 is added as well. According to degree of oxidation,iron-chertrocks are divided into oxide, protoxide-oxide,oxide-protoxide and protoxide, characterized by the following coefficients (O), obtained from a chemical analysis by the formula

Oxide rocks: O= 10-33 consist only of hematite and quartz; Protoxide-oxide:O= 1.5-10 contain magnetite, hematite, quartz; magnetite, quartz. Oxide-protoxide: O= 0.5-1.5 consist of magnetite, iron, silicate, quartz;magnetite,iron silicate, siderite,quartz. Protoxide: O= 0.05-0.5 consist of iron silicate quartz; siderite,quartz. The first two groups-oxide and protoxide-oxide-are iron-chertore rocks in which iron is fully represented by magnetite and hematite. The third group-oxide-protoxide-is represented by semi-oreiron-chertrocks in which only some iron is in the form of magnetite and the rest is fixed in silicates or carbonate. The fourth group-protoxide-are nonmetalliferous iron-silicate-siliceous rocks in which all the iron is present in silicates or siderite. By metamorphic stages A slatystageis characterizedby banded iron-chertjaspers. A stage of phyllite and cherts is characterized by ironquartz cherts or jaspilites (non-silicateiron-chert ores). Gneissic stages of metamorphism are characterized by coarse-crystallineiron quartzites. According to iron ratio, iron-chertrocks are divided into iron-rich,with an iron content higher than 30 per cent, intermediate with 20-30 per cent, and iron-poorwith less than 20per cent.In primary,unchanged iron-chertdeposits an iron contenthigher than 45 per cent has not been encountered. Increase of iron content to 60-70 per cent and formation of clusters of high-gradeore deposits are related to secondary metasomatic processes of enrichment. In the process of oxidation iron-chertrocks are martitized to a great depth, exceeding 1 k m .

Résumé Géologie et genèse desformations de fer siliceux du bouclier cristallin d’Ukraine (N. P. Semenenko)

Des formations de silex ferrugineux se rencontrent à différents niveaux structuraux du Précambrien du bouclier cristallin ukrainien ; elles remontent à 3 500-1 700 millions d’années. Les zones de synclinoriums où se sont développéesles formationsde silexferrugineuxsontlessuivantes:KonkskoBelozersky,Bazavluksky,Orekhovo-Pavlogradsky, Priazovsky oriental et occidental, Krivorozhsko-Kremenchugsky.

L a relation entre les formations de silex ferrugineux et les régions volcaniques géosynclinalesest établie. O n peut distinguer les types suivants de formation : schistes de fer siliceux ; schistes kératophyre de fer siliceux; ultrabasites de fer siliceux. Les dépôts chémogéniques de silex ferrugineux sont représentés par des formations de leptochlorite siliceuse, de sidérite siliceuse, de pyrite siliceuse,et de silice ferrugineuse selon le potentiel redox Eh. Les roches de silex ferrugineuxmétarnorphiséessont classées en groupes ferriques, ferriques-ferreuxet ferreux.

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N.P.Semenenko

Discussion R. FRIETSCH. A small remark. The Grangesberg deposit

N.P. SEMENENKO. The amount of germanium is 20-30 g

cannot be used as an example of iron-chert-schist-keratophyre formation. The ore is mainly a massive magnetitehematite lying in metamorphosed lavas of intermediate composition. T o the ore field belong siliceous hematite impregnations (Lomberg type), but they are probably hydrothermaldeposits following the main volcanism.

per ton.

N.P.SEMENENKO. Ifirst became familiar with the Grangesberg deposit with the help of the Swedish geologist Kautski. There, in subsurface, beds of iron banded quartzites are interbedded with leptites. Some samples of these rocks are in the exposition of the symposium.

A.F. TRENDALL. Are the ages quoted in the paper ages of deposition or of metamorphism?

N.P. SEMENENKO. The ages of the rocks refer to the time of their metamorphism.The ages of the oldest minerals are mentioned.

A. F. TRENDALL. What methods were used to obtain them?

N.P. SEMENENKO. The ages were determined by the K-Ar R. FRIETSCH. What is the content of hydrogen in the hematite?

method for chert hornblendes,and by the U-Th-Pbmethod for accessory minerals.

N.P. SEMENENKO. The amount of reducing agent in the

S. J. SIMS. Where the iron chert has been granitized,is the

iron quartzite is sometimes sufficient to reduce hematite to magnetite on heating in a soldered tube.

granitic rock iron-rich?

N.P. SEMENENKO. The granites replacing or migmatizing Z.T.TILEPOV. What is the distribution of the elementsPb, Zn, Mo, G e and M n along the radius from the centre of igneous rocks?

N.P. SEMENENKO.Under the depositional conditions of iron-chert formations,these elements are not usually deposited.

J. H . GROSSI Sm.H o w much germanium is found associated with magnetite?

142

iron quartzites are not iron-rich;the products of assimilation are usually removed. The process takes place under conditions of magmatic distillation.

G . A.GROSS. Are the thicknesses of iron-formationmentioned accumulative thicknessesof chert layers,or are these the thicknesses of iron-formationdeveloped by folding?

N.P. SEMENENKO. The thicknesses cited are normal stratigraphic thicknesses.

Occurrences of manganese in the Guianas (South America) and their relation with fundamental structures B. Choubert Directeur de recherchesau Centre National de la Recherche Scientifique (France)

Occurrences of manganese have long been known to exist in the Precambrian formations of the Guiana shield. In Guyana (formerly British Guiana), they were discovered at the end of last century,and Harrison refers to them as early as 1908,but it was only about 1950 that active prospecting began, apparently as a result of the discovery in 1945 of a deposit in the Serra do Navío in the Brazilian territory of Amapá. Prospecting then spread to British Guiana in 1952, to French Guiana in 1955 and to Surinam. The results were somewhat disappointing from the commercial point of view, but the description of the geological features is of considerable interest and the pages which follow give a summary of what is known.

Basic features of the geological structure of the Guiana shield The Guiana shield is separated from the rest of South America by the Orinoco and Amazon rivers. In the north it plunges beneath the Atlantic Ocean over a distance of about 1,500 k m . Politically this vast area comprises several territories: Venezuelan Guiana,Guyana, Surinam and French Guiana, plus the Brazilian territories of Amapá, Amazonas and Rio Branco. The geological exploration of this vast territory is still far from complete. To the south,the investigations rarely extend below the second parallel: while Amazonia and the upper Orinoco basin are scarcely known. The Guianasare a very ancientpart of the earth‘s crust, the evolution of which appears to have stopped about 1,700m.y. ago.Vast stretches of the shield consist of basically granitic terrains.The terrains of sedimentary and volcanic origin are metamorphosed to various degrees. These terrains have been subdivided into a number of series, the names of which vary from country to country. All of them are assignable to the different stages of the geosynclinal evolution, and we get, in ascending order, the following series.

Lustre schists. Yuruari series in Venezuela, Barama series in Guyana,Lower Paramaca series in French Guiana and Surinam,containing thick beds of argillaceous schists, quartz phyllites with quartzite lenses, usually black carbonate rock,horizons of manganese (gondites) and some ferruginous quartzites with iron ore concentrations.Chlorite schists,spilites,keratophyres [Guyana) and small massifs of intrusive rock ranging from pyroxenite through diorites and gabbros to granodiorites. Ophiolitic volcanism. Volcanic series with schists: E l Callao and Pastora series in Venezuela, ‘volcanicseries’in Guyana, Upper Paramaca series in French Guiana with intercalary lavas and sediments: basalts andesites,dacites, rhyolites, pillow lava (Venezuela), amygdaloidal lavas, products of submarine volcanism, pyroclastic deposits, jaspers shifting to quartzites,argillaceous schists. Flysch. Caballape series in Venezuela, Cuyuni series in Guyana,Bonidoro series(north facies)in French Guiana: detrital rocks,a series of very thin layersof variable composition: conglomerates,greywacke,argillaceous schists,etc. Paramolusses (or lower molasses). K n o w n as Haïmaraka inGuyana,Rosebel and Armina in Surinam,Bonidoro (southern facies) and Orapu in French Guiana. Thick series consisting of arkoses and conglomerates, of argillaceous schists lying in transgression over older rock formations. Schists ranging in colours from grey to violet or sometimes greenish with black intercalations of carbonaceous substances. At the base are polymictic and monomictic conglomerates,mainly becoming important to the east. According to certain writers,the combined thicknesses of these Precambriandeposits reach 14,000m in Venezuela and 8,000m in French Guiana. Post-paroxysmalmolasse and products of terminalbasic volcanism, exhibiting marked discordance, overlie all the folded and eroded series described above, to form the Roraïma series which has remained sub-horizontaland consists of coarse pebbles, conglomerateswith lava coulees and dolerite and gabbro massifs and veins. This molasse, which has undergone no granitization, is not found in French Guiana. It forms a butte-témoin in Surinam and

Unesco, 1973. Genesis of Precambrian iron and manganese deposits. Proc. Kiev Symp., 1970.(Earth sciences, 9.)

143

B. Choubert

covers vast areas in Guyana and Venezuelan Guiana. Apart from these thick geosynclinalterrains,still older rocks appear in places which representthe pre-geosynclinal platform with its cover of volcanic and sedimentary rock. This platform is made up of catametamorphic facies (Venezuela, Guyana and Surinam) or mesometamorphic facies (French Guiana), while the cover is of rhyodacites and pelitic and sandstone series,plus iron ore. The formations were granitized contemporaneously with the series in the geosynclinal domain, and consequently belong to the same chain.

Occurrences of manganese: their composition Manganese is closely associated with what may be roughly called the volcano-sedimentary rocks and occurs as lenticular intercalations in the topmost levels of the Barama series (Guyana) and Lower Paramaca (French Guiana and Surinam). The main occurrences known, with indications of the 'primary' manganiferous rocks,are shown in Table 1.

TABLE 1. Main occurrences of manganese GRANITIZATION

The main sanitization dates back to between 2,050 and 1,750m.y. and produced the Caribbean granites which consist, on average, of monzonite granite with microcline, plagioclase and biotite. These rocks have engendered a large quantity of pegmatites and vast fields of felspathized rocks. Most of the earlier rocks were rejuvenated by this mechanism which makes their absolute ages identical with those of the granites. The Guyana granites are of an earlier age which is difficult to determine in view of the generalrejuvenatiai.They can be dated after the formation ofthe volcano-sedimentary series and before the deposition of the paramolasses for which they supplied the material by erosion. They must be deemed the outcome of a remobilization and remodelling of the earlier granitoids which were formed between 2,700 and 2,500 m.y.,that is, during the pre-geosynclinalperiod. They have, nevertheless, retained fairly constant characteristics,which are reflected by their chemical composition. They are relatively poor in potassium, with acid plagioclase and biotite with or without hornblende as their principal constituents: the commonest types are alternatively graiiodiorites and akerite granites.

TECTONICS

For a proper understanding of the tectonics of the chain,it should be remembered that what is today visible on the surface represents the geometry of the deepest zones of this Archaean edifice. It can be conjectured that erosion has removed a thickness of 5-10 km of an undeducible structure of which only the roots are visible. The wide granitized cupolas and the synclinoria in which the parametamorphic rocks have survived represent the folding of the bottom strata among which the dislocations have brought blocks of the pre-geosynclinalbasement to the surface here and there, in a more or less remodelled and rejuvenated state.

'Primary' manganiferous Longitude/latitude rock

Amapá

Serra do Navio, on the Gondites,carbonates, Amapari Mn 55"05' O"55' French Guiana

ObservatoryMountains Manganiferous schists 51"37' 4"08' ( ( x 52"08' 4"34' K a w Mountains to 52"05' 4"31' Quartz-rich Upper Sinnamary gondites approx. 52"50' 4'30' Gondites 54"13' 4"43' Mt Richard Ampouman (Maroni 54"25' 4"38' River) 53'45' 3"32' Grand Inini Surinam

Goeje Mountains

Gondites

54"Ol' 3"3O'

to 5490' 3"2W Poeketi, on the Tapa( ( nahoni ( ( Apoema Maripa,PiquéHeuvel Gondites,braunite (traces) Afoebaka,Brokopondo Impregnations,small veins inthe schists Gondites Adarnpada

54"35' 490' 54030' 4"37' 54"53' 4"42' 55"OO' 5'00' 56"50 4"23'

Guyana

Saxacalli, on the EsseGondites quibo ( ( Kutuau (Cuyuni basin) Residual gondite Tasawinni blocks Residual gondite Pipiani blocks Mainly gondite,plus Matthews Ridge braunite Mainly gondite,plus Arakaka braunite

58"40' 6"35' 59"20' 6O.53' 59"35' 7"28' 59"43' 7"22' 60"lO' 7"29'

59"58' 7"35'

A s can be seen, most of these are associated with the gondites. These are rocks of rather unusual composition, of which the essential part is spessartite garnet: Mn,AI, (SiO&. Manganiferous rocks of this type are also found

I44

Occurrences of manganese in the Guianas (South America) and their relation with fundamental structures

in Brazil, at Minas Gerais, where they were described by Derby as early as the beginning of the century (1908)under the name of queluzites (Morro da Mina). In the opinion of Hussak (1906),the silicates have their origin in the carbonates of M n under the action of metamorphism, and these carbonates probably represent the protore. In 1909, Fermor, in his study on M n deposits in India,gave rocks of the same group the name of gondites and it is this name which has gained acceptance in the literature. Over and abovethe manganiferous garnet,the gondites may contain quartz,amphiboles (tremolite,actinolite,grunerite), biotite, sericite, carbonaceous matter (LIPto 2 per cent), plus carbonates of M n (dialogite, etc.). The proportions of the mineral elements vary very considerably. The most frequent paragenesis in the Guianas is garnet + quartz,with additionsof mica or amphibole.As the garnet content rises we get actual garnet rocks, formed by the action of general metamorphism,and therefore to be distinguished from the skarns which, though rich in garnet, are products of contact metamorphism. The manganiferous garnet is generally white or grey and shows a wide range of variations in composition. It is thought to represent a mixture of garnets whose individual composition varies very widely-spessarites, pyrope, almandine, grossuralite, andradite-with a little Tio,and carbonaceousmatter. Generally speaking these rocks have undergone a profound alteration,with the formation of black oxides of M n (pyrolusite,polyanite,wad, etc.) which render observation difficult. To permit a better grasp of the geological characteristics of these deposits, descriptions of three of them are given below. A M P O U M A N FALLS, M T R I C H A R D (FRENCH GU IA NA )

The geology of the manganese occurrences is fairly well known in this region, which is traversed by the Maroni. In places,the river is as much as 4k m wide,and is dispersed in a multitude of channels which girdle innumerable islands

-,

.MARONI R I V E R

N. m

200

and rocks. Observation in detail is possible here, whereas in other sectors a thick decomposition layer carried the dense equatorial forest,and masks the hard rock. The Lower Paramaca (Fig. 1) outcrops extensively with nearby E.-W.alignments, shifting progressively to NE.-SW.as one moves eastward. Overlying it, and in marked unconformity with it, is the Bonidoro series,which begins with a polymictic conglomerate above which comes the usual succession of coarse arkosic sandstones. Between Ampouman Falls to the south and Boëli-Mofou to the north-that is, over a length of 1-2.5 km-it includes chloritoid quartzites of variable 'sandiness' with lenses of carbonate-rich rocks,black carbonaceous schists and quartzites (dark and light stratification), with glances and gondites rich in fine-or coarse-grainedgarnets and showing an undulating Stratification.In addition to small elongated massifs or veins of gabbros, the series contains diorites and dolerites. Directions of dip are N.or NNW.(30" to 45"). There are two levels rich in lenses of carbonate rock, with surface breadths of 200-300 m and intercalated in quartzose schists. The southern strip is flanked by manganiferous levels each about 100 m thick (see section) at distances of about 600 ni, on either side. According to Brouwer (1960), the garnet rock lenses form segregationsin the quartzose schists where the garnet it less abundant and the chloritoids very common, plus biotite, calcite, epidote and amphibole. The lenses are of all sizes, the smallest being only some 10 m thick. According to Jaffé, the composition of the Maroni garnets is 57 per cent spessartite to 33 per cent almandine and 10 per cent pyrope. Averaged out,they contain 14per cent M n O and 38.93 per cent SiO,. The lenses of black carbonaceous rock have a maximum length of 1 k m and a maximum thickness of 25 m , and are sometimes of argillaceous sandstone,sometimesentirely quartzites,in all cases very fine-grained.They show only traces of MnO (0.01per cent), but may have as much as 90 per cent of SiO,, 9.5 per cent of M,O,and 5.6 per cent of Cao. Southeast of this manganiferous zone, ferruginous rocks are found intercalated in talcose schists.The iron-ore outcrops (85 per cent Fe,O,) are separatedfrom the gondite band by transgressions of the Bonidoro series which make

S

c---

Pe,te SargougFa1l.s

Bkli Mofou Falls .

i

!lI

3

4 6

5

Ampouman Falls Y 12 4

O

FIG.1. Profile along the Maroni River to Ampouinan.Bonidoro: 1. Coarse sandstone; 2. Conglomerate. Lower Paramaca:

3. Quartzite schists with chloritoid;4.Gondite levels;5.Level of calcium carbonate rock lenses; 6.Dioritic rock dykes.

145

B. Choubert

it impossible to see the relationship of this indicator with the manganiferous mineralization. This is a lenticular deposit of low tonnage in association with altered talcose schists (goethite and specularite,with a little quartz: SiOz = 6.5 per cent). The belt traversed by the Maroni between Ampouman Falls and Boëli-Moufou Falls, extends for about 5 km westwards into Surinam territory and about 20 km NE. into French Guiana, or 29 k m in all. The metamorphism is more marked to the east, where the Paramaca is in contact with a considerablemassif o€Caribbean granite (Massif de l'Espérance). The two zones of fine-grainedgondites are intercalated in amphibole rock and compact amphiboles, which have a north-westerly dip and form a fairly high ridge with a lateritic revetment highly manganiferous in places at the base. The texture is often pisolitic. Ferruginous quartzites have been found in the same region,but their stratigraphic position is not well known. Transformed lavas (green rock) exist on the Maroni near the Surinam bank (Ampouman Falls), as well as inland on the N W . edge of the band.

M A R I PA

( sU R I N A M )

This deposit, the characteristics of which are known through the survey work of Holtrop,is in the basin of Sarah Creek (aright-banktributary of the River Surinam). T w o major indicators are known in this region: Maripa Heuvel and Piqué Heuvel (=hill). Several types of exploratory operations have been effected: detailed surveys, boring, trenching, driving horizontal galleries. Only the essential is mentioned here.Further details can be found in the book by Holtrop (1962). The Paramaca series here consists mainly of quartzite schists with chlorite,biotite and sericite. In the schists are lenticular intercalations of gondites,carbonaceous schists, ferruginous quartzites, with lenses several hundred metres to 2 km in length and about 100m thick.The carbonaceous schists occur mainly at the base of the gondite horizons. Elsewhere, the gondites may give place to carbonaceous schists. All this is explicable on the assumption that, in anaerobic conditions and as a result of variations ofp H and Eh, F e and M n precipitate alternately,M n in the form of carbonates and Fe in the form of oxides. In addition,the appearances of M n and carbonaceous matter are definitely connected,whereas the ferruginous quartzites appear to be independent. Several parallel alignments directed 10"N.to 20" W. of gondite lenses are known in a zone 700-800 m wide and, rightly or wrongly, it has been held that this distribution came from the folding of a single maiiganiferous bed. Depthwise, a few trial borings give the stratigraphic succession of the mineralized zones and w e find that there are many of thin gondite seams, intercalated in a yellowish-brown decomposition clay. They run to about fifteen (LA-120)for a depth of about 41 m,their thickness varying between 10 c m and a little more than 1 m. 146

Sample analyses of the manganiferous rock from two borings and a well 21 ni deep show an average composition of:MnO, = 33.28 per cent,Fe20,= 5.74 per cent,A1,0, = 7.89per cent, and SiO,= 36.38 per cent. In some places, concentrations of garnet account for 80-90 per cent of the rock and contain an average of 27 per cent of MnO. Holtrop calculates that there should be a reserve of about 143,000 tons of metallic M n at Maripa. M A T T H E W S R I D G E , A R A K A K A (NW. G U Y A N A )

Here the manganiferous rock is intercalated in the thick Barama schistoseseries,equivalent to the Lower Paramaca of French Guiana (see also Holtrop (1962)). Argillaceous and quartzose schists with sericite occur, with horizons rich in carbonaceous matter and sterile lenticular quartzites, including a roughly 300 m horizon of gondites, ferruginous quartzites and beds of braunite with intercalations of metamorphed chert. Investigations have even pin-pointed two manganiferous zones and thin conglomerate bands with pebbles averaging2.5 c m in diameter. The manganiferous zone, which is 150-175 m thick, E.-W.at Matthews Ridge,gradually swings to a NE.-SW. orientation eastwards when, after crossing the Arakaka creek,it runs along the River Barima for a total distance of 22 km.Dips are northerly with 50" to 80" inclination. Borings show the stratigraphy in depth to be as follows. The ñrst SO m are divided into two parts: metadolerites topping a manganiferous horizon,which itselfincorporates afurtherintercalatedseam ofmetadolerites.Below,in order of descent over a total depth of 1.5 m,w e have 55 c m of brown and yellow clay, then alternating thin beds of gondites and brown clays, or fourteen manganiferous beds in all in a thickness of 95 cm. In the upper part, lying between two beds of dolerite, w e have, at the top, five intercalations of gondites alternating with white clay (to a total thickness of 25 cm), then a series of dark and light clays to a total thickness of 1 m; and once again,two 3-4 c m beds of gondites sandwichedin a white clay,after a seam of black clay and a thin horizon of quartz shingle. The levels of braunite have an irregular geographical distribution and thicknesses ranging from 1 to 3 cm. O n the other hand, lenses of over 1.5 m thickness are found farther west. The manganiferous band appears to be, in all, some 30 k m long. Exploitation of the Matthews Ridge deposit started in 1960, with extraction planned at about 1,200 tons daily. Reserves were estimated at some 13 million tons.

EFFECTS OF ALTERATION IN T H E S U P E R G E N E Z O N E

In the geological description we only discussed 'primary' M n minerals. Near the surface, the silicates, car-

Occurrences of manganese in the Guianas (South America) and their relation with fundamental structures

bonates,etc.,have been transformedinto secondary oxides such as pyrolusite, polianite, psilomelane, manganite, cryptomelane,lithiophorite,or wad.Trial borings provide evidence of impoverishment of the ores as depth increases. In the first 30 m tenor in M n drops from 40 per cent to 12 per cent,which demonstrates the relative poverty of the primary deposits. Enrichment occurs above the hydrostatic level,which is very high in the Guianas,as these countries belong to the humid sub-equatorialzone. This process of supergene alteration and accumulation is at all points comparable to that of laterization. Over the whole Guiana peneplain the lateritic coverings have been formed at different epochs. At the base of these, accumulations of manganese are to be found here and there (Kaw Mountains,Observatory Mountains, Mt Richard, etc.) in the form of blocks embedded in the clay, pisoliths, nodules, often cemented by the laterite. The metal content of these secondary ores is relatively high and may exceed 30 per cent (Mt Richard). Large blocks embedded inresidualclay-at Tasawinniand Pipiani in Guyana or at Serra do Navío in Amapá-contain up to 40 per cent and 50 per cent of metallic M n , with some percentages of Fe,O,,Alzo,and SiO,(sometimes > 10 per cent). Through a diversity of treatments (washings, etc.), satisfactory concentrations can be secured. According to Nagell and Seara (1961)the metallic M n tenor of the commercial ore exported from Serra do Navío in 1957 was 48.5 per cent. The remainder is made up of Fe,O,,A1,0, and Sioz, with some other elements: C u 0 (0.07per cent), Ba0 (0.12per cent), As,O, (0.25 per cent), P, etc. The problemspresented in the Guianas by gondite-type deposits are not of a qualitativebut of a quantitative order. G E N E R A L CHARACTERISTICS O F T H E G U I A N A DEPOSITS A N D O C C U R R E N C E S

The characteristicswe have been describing have points in common, and this is equally true as regards the circumstances of the deposits’formation. In every region the primary ore consists mainly either of rocks with a manganiferous garnet content broadly described as gondites, or manganese silicates such as the braunites (Matthews Ridge, Guyana) or carbonates (dialogite) as at Serra do Navío in Amapá and at Upata in Venezuela. At Serra do Navío it has been observed that here and there the dialogite gives place to garnetiferous rock still containing dialogite.It is thus apparent that the garnet and the calcic amphibole (tremolite) are genetically related to the manganese carbonates.It is legitimateto think that the primary ore everywhere consisted of carbonates and that the subsequenttransformations are ascribableto the general metamorphism, which affords some confirmation of the earlier observations made by Derby in Brazil in 1901. The deposits are aligned in zones several kilometres long-12 kin in the K a w Mountains,about 30 km in the

Ampouman-Mt Richard sector, 30 km also at Matthews Ridge and Arakaka-with a breadth of 50-175 m . Where the breadth is above 175 m,it is as a result of folding. Within these zones, the ores form lenticular beds of a thickness varying €rom a few millimetres to severalmetres. Their length-in the Serra do Navío,for example-is sometimes as much as several kilometres. The lenses o€ore alternatewith ferruginousclays which are the result of the decomposition of more or less metamorphic argillaceous schists. These rocks contain chlorite, sericite,and often too (Ampouman) biotite and chloritoid and are fairly rich in quartz. Associated with them are sediments,always the same ones: black carbonaceous rock (argillaceous or quartzose) in lenses, quartzite lenses, ferruginous or non-ferruginous (elsewhere silicifiedin cherts), all of them,from the spatial point of view, having the same discontinuous character as the ore. There is no doubt whatever that the amphibolites which occasionallyaccompany the depositsare in very many cases of sedimentary origin. At Ampouman transitional members have been observed in the series from carbonate rocks to amphibolites. The latter predominate at Mt Richard,where there are no longer any carbonates and where the quartzose and black schistose and more or less graphitic rocks always go with the manganiferous levels. Again, it is certain that the slightly metamorphic carbonated rocks have generally been replaced by silica. A proportion of very fine-grained quartzites and cherts or phtanites are products of this secondary silification,which is ascribable less to a metamorphic process than to the influence of a hot, damp climate. W e are dealing with deposits of sedimentary origin. Chemical precipitation alone seems capable of ensuring an alternation of beds whose regularity is revealed by the borings. The relatively low iron content of the manganiferous levels (about 1-2 per cent in fresh rock) contrasts sharply with the abundance of the metal in the shale intercalations(up to more than 10 per cent of Fe,O,) and iii the fresh metamorphic schists, which are often rich in chloritoid (H,Fe Al,SiO,, whose iron tenor js frequently as high as 28 per cent). There is, therefore,nothing surprising in finding ferruginousintercalations (hematite,limonite) in some deposits (Saxacalli,for example), and that in numerous instances beds of quartzites of sedimentary origin are found contiguouswith,or close to,the manganiferous beds. A s regards the volcanic and eruptiverocks of the ophiolitic suite which exist in ihe deposits or near the occurrences of M n , their role in the accumulation of the ore continues to be obscure. Following the view of certain authorities (Taliaferro and Hudson in California,Routhier and Arnould in N e w Caledonia,Zanone in Ivory Coast), it is generally accepted that the M n deposits are genetically related to ophiolites, with jaspers and tuffs. The investigations carried out in the Guianas have not yielded additional proofs for this postulate. The presence in the Guianas of ophiolitic intrusions in the Archaean chain of geosynclinalorigin requires no further proof,but no deposit 147

B. Choubert

has so far provided evidence of the mineralizing role of these rocks in this shield. In some of the deposits mentioned earlier there are small massifs of eruptive rock consisting of quartzite diorites or gabbros (Ampouman) or metadolerites (Matthews Ridge), but these rocks were emplaced in the sediments which had already been deposited and their influence appears to stop at a slight increase in the iron tenors in their immediate neighbourhood. Webber (1952)has asserted the existence of mineralized ‘tuffs’in the Barama sector (Pipiani, etc.) in Guyana,but this is disputed by Holtrop (1962). Other authorities see a connexion between M n and the amphibolites to which they ascribe an igneousorigin.However,the discovery in French Guiana of an incontrovertible connexion between amphibolites and carbonate rocks of sedimentary origin renders this attribution suspect in most instances. It is consequently difficult to contend that the ophiolitic intrusions known to us are the source of the manganese subsequently deposited in the form of chemical precipitates.M y own opinion is rather that the Mii comes from the ultra-basic rocks emplaced along the deep fractures which occurred in the pre-geosynclinalbasement and were followed by the formation of troughs. The decomposition and leaching of these rocks prior to sedimentation could have supplied the manganesethus accumulated to the sediments in formation. I have already published a description of the gilbertite serpentinechangingto talcoseschistswith magnetite crystals in the K a w Mountains in French Guiana. In the severely altered state,everything is transformed into clay and becomes unrecognizable. Above this come laterite carapaces containing concentrations of M n oxides at the base.These concentrations can reasonably be assumed to be due to the process of M n migration under the action of the climatic decomposition of the subjacent basic rock; w e thus get an indirect proof of the connexion between these rocks and the accumulations of manganese. Ultrabasicrocks are also known in the south of French Guiana and of Surinam (Goeje Mountains), while Holtrop (1962)reports the existence of pyroxenites giving place to talc-schistsin the immediate neighbourhood of the Piqué Heuvel gondite deposits,to the N E . of Maripa. Both seem to be earlier than the Lower Paramaca schists and, ajòuti&, than the predominately andesite ophiolitic intrusions of the Upper Paramaca. This assumptionwould explaintherectilinear or slightly curved nature of the manganiferous zones,the narrowness of the zones (less than 200 m), as well as their length, which is relatively considerable along the line of the chain and much less in the normal direction. Conversely, the frequentlyregularpattern of the deposits,with alternationof Fe-richand Mn-richbeds,with the simultaneousformation of carbonate and siliceousstrata,accords with thephysicochemical modifications during the process of deposition accordingto the theories of Pieruccini(1956)on the geochemistry of M n deposits,which fit in perfectly with m y observations. 148

The concentrations of high tenor iron ore found here and there (Kaw Mountains,Ampouman,Mt Richard) close to the nianganiferouszones do not seem to be directly traceable to the concentration mechanisms just described, although they share a common origin with the M n (ultrabasic rocks). As regards the genesis of the ribboned ferruginous quartzites known as itabirites, it can safely be said that nowhere in the Guianas are they to be found in conjunction with the manganese deposits of the type just described. They do,on the other hand,characterize the vestiges of the former semi-platformand their mode of formation appears to be different,

G E N E R A L DISTRIBUTION IN RELATION T O T H E DEEP TECTONIC S T R U C T U R E

The problem now is to determine the extent to which there is an effective relation between the manganiferous concentrations and the troughs of a geosynclinal system. The present study has shown the existence in various deposits either of carbonate rocks (Ampouman), of argillaceous successions with horizons of shingle (Matthews Ridge), of quartzite successions (Upper Sinnamary) or again of phyllite successions,with fairly considerable quantities of ferruginous quartzitesin contact with older serpentines(Kaw). These variations in the formation of deposits hardly agree with the notion of uniformity generally evoked when one talks of lustred schists. T o attempt to shed light on this question let us turn to the ‘trend‘map for potassium established on the basis of the analyses of granites (Fig. 2). It satisfactorily reflects (seismic reflection model) the deep structure of the Guiana geosynclinal zone, in particular the distribution of troughs and intermediate masses (Choubert and Vistelius, 1969).

The geologicalm a p does not allow an exact idea of this distributionto be secured,as the depth to which erosion has reached leaves no more to be seen than mainly granitic regions and sedimentary zones representing only the roots of the former mountain structures. The isolines plotted from the seismicreflectionmodel revealthe existence of two zones of maximum K20tenors, separated by a zone of minimum tenors, all with an alignment varying from WNW.-ESE. to NW.-SE., i.e.along the line of the former chain. The maxima correspond to regions where the predominant granites are the Caribbean, considerably richer in potassium than the older Guiana formations,which on the contrary coincide with the zones of minimum tenors. The former indicate the old troughs, the others the intermediate masses. On plotting the known deposits and occurrences on the map, we find that they are all in the zone of minimum tenors,inside the 3.2per cent K20isoline,in other words, along the median ridge (‘Zwischengebirge’) separating the troughs or along the edge of the semi-platform (Marudi, Serra do Navío). This distribution strikes m e as interesting,

Occurrences of manganese in the Guianas (SouthAmerica) and their relation with fundamental structures

+ O

6+

6)

POO

300 km

+

4+

o+

I O0

+

+

+

i

+

61

+

60

58

'56

FIG. 2.Situation map of the occurrences ofmanganese according to the potassium isolines of the granites. 1. Occurrences of manganese (onecircle), manganese deposits (two circles); 2.Out-

crops of Lower Paramaca-Barama; 3. Isolines of K,O,'trends'; 4.Percentages of K,O tenor;5.Political frontiers.

for it explainsthevariationsincompositionofthe geological environment,the zone of intermediate masses having been subjected to frequent movements arising from the orogenic evolution of the troughs-two N.-S.eu-mio bi-couplesaccompanied by major longitudinal and transverse dislocations still evidenced by the highly variable directions which I observed in the parametamorphic series. In a work now in press (Choubert,1969a),I have tried to establish the geologicalcorrelations between the Guianas and West Africa. This comparison brings out the common

features not only in the lithostratigraphy,but also in the tectonic structureand the mineralization of the African and South American continents which are today separated by the Atlantic. The gondite manganese deposits known in the Ivory Coast and Ghana,represent the eastern extension of those discovered in the Guianas, and although their stratigraphy is still unknown, the geological context and mineralizationare found to offer close analogies.The same deposition conditions apparently existed between 2,400and 1,700m.y. over the entire Guiana-Ivory Coast chain.

1 49

B. Choubert

Résumé Les indices de manganèse clans les Guyanes (Amérique du Sud) et leurs dutions avec les structures foiidamentales (E.Choubert) Dans les Guyanes,le Précambrien est subdivisé en un certain nombre de séries d’origine sédimentaire. L’analyse lithostratigraphiquepermet d’établirqu’il s’agit d‘une succession géosynclinale. Ces séries, plissées et métamorphisées, forment les racines d’une chaîne archéenne, profondément érodée,dirigée du nord-ouest- sud-estvers l’ouest nord-ouest est-sud-est. L e granite qui couvre de grandes surfacesappartientti deux types principaux,le plus ancien étant moins riche en K,O que l’autre.Les isolignes de K,O,construites en partant de l’analysechimique des granites,mettent en évidence deux zones de maximums sensiblementparallèles, séparées par une zone de minimums, qui peuvent être considérées c o m m e des dépressions géosynclinales de part et d’autre

-

d‘une masse médiane. Dans le sud s’amorcele bord de la semi-plate-€orme qui marque lalimitedeI’airegéosynclinale. La répartition de ces indices de manganèse dépend de ces structures fondamentales, qui sont presque toujours concentrées le long de la chaîne médiane ou le long de la bordure de la semi-plate-forme.D u point de vue stratigraphique, elles sont liées à la partie sédimentaire volcanique de la chaîne ancienne appelée,en Guyane française et au Surinam,les séries deParamaca.Génétiquement,elles ont certainement une origine sédimentaire. Parfois, elles sont apparentées aux ((gonditesD, parfois elles représentent des concentrations qui ont pu provenir de phillites contenant une certaine quantité de manganèse. En divers points, lemanganeseest accompagné de concentrationslenticulaires de fer.Les processus qui ont conduit à un enrichissement secondaireont pris une place proéminente danslaformation des divers dépôts,le plus important étant celui de la Serra do Navío à Amapá (Guyane brésilienne).

Bibliography/Bibliographie BRACEWELL, S. 1947. The geology and mineral resources of british Guiana. Bull. Imp. Inst., Loncl., vol. XLV, no. 1, p. 47-59.

BROUWER, G. C. 1960. Sur la géologie et la métallogénie du massif de l’Espérance.Unpublished technical memorandum, Cayenne. . 1964. Feuille de Paul Isnard avec notice explicative, au l/lGO 000.Curte géologique détaillée de la France, Dép. de Za Guyane. Paris, ImprimerieNationale. CHOUBERT, B. 1954. Sur ¡es roches éruptives basiques des montagnes de K a w et de Roura (Guyane française). C.R.Acad. Sci., Paris,,t. 239, p. 185-7. . 1965. Etat de nos connaissances sur la gkologie de la Guyane française. B d . Soc. géol., vol. VII, p. 129-35. , 1969~. Les Guyano-Eburnëidesde l’Amériquedu Sud et de l’Afrique occidentale (essai de comparaison géologique). (In press.) -. 19690. Le Précambrien des Guyanes, BRGM Memo. (In press.) CHOUBERT,B.;BROUWER,G.C.1960. Stratigraphiede la série de Paramaca en Guyane française. C.R. Acad. Sei.,Paris, t. 251, p. 109-11. CHOUBERT, B.;VISTELIUS, A. B. 1969.Essai d’interprétationde la structure profonde des Guyanes par des moyens mathématiques.4th Venezuelan Geological Congress,Caracas,Ministerio de Minas e Hydrocarburos. (In press.) --. 1970.D e la relation entre la composition des granites du bouclier guyanais et la position de ceux-cidans les structures tectoniques majeures. (In press.)

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150

DERBY, O.A.1908. On the original type of the manganese ore deposits of the Quelus District,Minas Gerais,Brazil. An. J. Sci.,no.175,p. 213-16. DORR, J. Van N.II;PARK, C.F.; PANA, E.G .de.1950.Deposito de manganeso do Distrito da Serra do Navio, Territorio Federal do Amapá. Bol. Dep. Prod. min., Rio de J., no. 85. EBERT, H.1961.Gonditesand charnockites as Guide Horizon in the Brazilian Shield. Minutes, 5th Inter-Guiana Geol. Conf., Georgetown, 1959.

FERMOR, L.L.1909.Themanganese deposits ofIndia.Bull.geol. Siirv. Zndia, vol. 37. HARRISON, J. B. 1908. Geology of the GoId Fields of British Guiana. London,Dulau. HOLTROP, J. F. 1962. D e mangaanafzettingen van het Guiana Schlid [Themanganese depositsof the Guiana Shield]. Leiden (thesis). HUSSAK, E. 1906. Ueber die manganerzlager Brasiliens. 2. prukt. Geol.,no. 14,p. 237-9. JAPFE,F.;BROUWER, G.C.1959.Sur la présence de gonditesen Guyane française. C.R. Acad. Sei.,Paris, t. 249, p. 148-9. NAGELL, R. H.;SEARA, A.C.1961.The geology and mining of the Serra do Navío, Manganese deposit Amapá, Brazil. Minutes, 5th Inter-Guiam Geol. Conf.,Georgetown, 1959.

PrERuccmr,R.1956.Analcheconsiderazionesulcomportamento geochimico e sul ciclo geochimico del manganese. XX Znt. geol.Congr.,Mexico. Symposiumsobreyacimientosdemanganeso,t. I. WEBBER, D.N.1952. Manganese Deposits in the North-West Dist. Brit. Giiiana Geol. Surv., no. 23.

Occurrences of manganese in the Guianas (South America) and their relation with fundamental structures

Discussion J. H. GROSSI SAD. The queluzite from Lafayette (exQueluz), according to Derby and other geologists, is an Mn-protore with carbonate. Garnet is not essential in the mineralogy of the rock. You have a typical analysis of queluzite on page 6 of the French original. B. CHOUBERT. Queluzites and gondites are sedimentary rocks which have no stable composition. Besides, these rocks were metamorphosedunder quite differentconditions: epi-meso and catazone. Transition of dialogite rocks to garnet rocks was observed in Serra do Navío, Amapá. In addition,in the hot and humid climate of Guiana,carbonates are replaced by silicon and are transformed into jasper and granular quartzites. It is, therefore,impossible to set up any subdivisions in gonditoid rocks.

S. ROY.The gondites describedin this paper do not exactly simulate those of India first described and named by Fermor. The so-calledgondites of the Guianas and Brazil were originally made up of M n C O , with associated silica, whereas the Indian gondites are always carbonate-free. Under these circumstances should w e accept the term gondite for both carbonate-richand carbonate-freemanganiferousrocks,or should we revive the term queluzite for the South American manganese silicate-carbonateprotore? B.CHOUBERT. I have already answered a similarquestionby Professor Grossi Sad. Besides,it should be mentioned that the paragenesis of most Guiana gondites is garnet-quartz -a little mica and carbonaceous substances. E. RIBEIRO FILHO. According to Odman’s paper the rhodonite of the Morro da Mina was formed by a metasomatic process. I would say that the same happened in Chandi District. B. CHOUBERT. I have never observedrhodonitein the Guiana manganese deposits.U p to now rhodonite is known only in Serra do Navío (Amapá). G.A. GROSS.Are the volcanic rocks and amphibolites associated with the stratiform manganese deposits rich in potassium or in soda? Has this aspect been investigated?

B. CHOUBERT. This aspect has not been specially investigated,but I have observed the mineralogicalcomposition of amphibolites in sections of quite ordinary hornblende, plagioclase and quartz in different proportions.

W.SCARPELLI. In Serra do Navío the manganese-bearing metasedimentshave a lateralextension of more than 10km, and the manganese zones appear in all this extent.Also, all evidence points to a shallow deposition of the manganese rocks. H o w does this fit in with your idea of the carbonate being deposited in deep troughs of geosyncline?

On our map of K,O trends, calculated from B. CHOUBERT. the data of the Guiana garnetoids by means of echo models, we see that the KzOisoline content shows the distribution of deep troughs of geosynclines,which are dissected by a median ridge. All known manganese deposits are concentrated in the area of the median ridge or at the margin of the platform, i.e. where sedimentary conditions are often altered by the fluctuation of this unstable zone depending on syncline evolution. N o manganese deposits have been observed in the syncline zones. J.VANN.DORR II. You attribute thefree carbon in manganese silicate rocks to liberation of carbon from carbonates during formation of manganese silicate, a metamorphic product. On the other hand, where rhodochrosite is quite pure and practically no manganese silicate is formed, as in part of Amapá and Ghana deposits,free carbon is abundant; up to 20 per cent in Amapá and 7 per cent in Ghana. D o you not believe that this free carbon could not be derivedinstead from organicdebris,producing thereducing environment that caused rhodochrositeinstead of oxide to be deposited? B. CHOUBERT. Itis difficultto answer this question,sincethe presence of organisms in old and 2,000m.y. sediments has not been proved. Besides,I think that the high temperature and pressure ofmetamorphosed changes should have turned carbon into a carbonaceous substance, which, however, never reaches the graphite stage.

151

Precambrian ferruginous-siliceousformations of Kazakhstan I. P. Novokhatsky Instituteof Geological Sciences, Academy of Sciences, Kazakh S.S.R., Aima-Ata(U.S.S.R.)

Iron-formationsare widespread in the Precambrian metamorphosed sedimentary rocks of Kazakhstan. They occur in two main regions: the Ulutau region on the western margin of the Kazakh folded area and the Betpakdala region to the west of Lake Balkhash. The oldest deposits are of the Archaean Bekturan series, which consists of amphibolite, mica schist,plagioclase gneisses and quartzite. It is up to 4,000 m thick. It consists of arenaceous to argillaceous rocks that have been altered by intense metamorphism, sodium metasomatism and granitization. A thick sequence (up to 11,000 m) of metamorphic rocks forming the eastern limb of the Ulutau anticlinorium and the Karsakpay synclinorium is Lower Proterozoic in age. There are two important series in this sequence: a lower (Aralbay series) characterized by wide-spread development of liparite-dacitic and andesitic volcanitic rocks; and an upper (Karsakpay series) volcanic green schist series. Iron-formationis developed mainly in the Karsakpay series composed of quartz-sericiteand chlorite-quartz-sericite schist,marble, quartzite and iron-formation,as well as of porphyritoid and green schist formed from basalt and tuff.In the Aralbay series quartzite is less abundant. In the upper layers of the Karsakpay series porphyritoid and conglomerate occurs sporadically. These series are characterized by an approximate rhythmicity, with porphyritoid and green schist predominating in the lower layers of a given suite, and phyllite and schistose rock with horizons of iron-formation predominating in the upper layers. The following suites are distinguishedin the Karsakpay series: the Burmashin (750 m thick), the Balbraun (800 m thick), the Shaghyrlin (1,500m thick) and the Biit (1,100m thick). Iron-formationis most common in the Balbraun suite,where the largest deposits of iron ores (the Balbraun and the Kereghetas) are located. These deposits make up the Karsakpay synclinorium where up to nine horizons of ferruginous quartzite are found. The Middle Proterozoic in southern Alatau is rep-

resented by sedimentary and acid volcanitic rock. T w o series are distinguished there: the Zhiydin (2,000m thick) and the Maytyubin (9,000m thick). The deposits of the Bozdak series, which belong to the Upper Proterozoic,consist of basic and acid extensiverock with sandstone, conglomerate and limestone. The series is up to 3,000 m thick. Most iron-formationdeposits of the Ulutau region lie in the Balbraun suite of the Karsakpay synclinorium.In all, about twenty deposits of ferruginous quartzite are known in the region. They stretch to about 200 k m in a narrow strip that extends in a nearly north-south direction. The southern continuation of the strata beneath unconsolidated deposits has been traced by geophysical observations,and its trend changesfrom NS to NW.In the north iron-formationis traced up to the bend of the Ishim river, where it also is overlapped by younger deposits. The Balbraun field, the largest field situated to the south of Karsakpay settlement, is briefly described here. Quartzites,which are sometimes conglomeratic,occur in the base of the ore-bearing strata with angular unconformity on older rock.The older rocks are altered to quartzsericite,quartz-sericite-chlorite, and graphite-sericiteschist, quartzite,and metamorphosed greenstone.Higher up in the section schist is gradually replaced by iron-formationinterbanded with quartz-sericiteschist. The strata are gathered into a number of isoclinalfolds,with a western dip of the layers prevailing.In it a number of synclines stand out with cores bearing ferruginous quartzites (Fig. 1). Iron-formationis part of the metamorphosed Lower Proterozoic rock that forms complex folds with a northsouth strike. Ore outcrops here are about 5 k m long and 3.5 to 4 k m wide. Seven ore strips, representing narrow synclinal structures,crop out on the surface. ïheir length along strike is up to 500 m,with an apparent thickness of 200 m or more.U p to 30sheet-likedepositshave been found in the field. Iron-formationand quartz-sericiteschist alterate throughout the 60-100 m thick ore horizon. The main ore mineral is hematite. It occurs in flakes, 0.01-0.1mm in size, forming aggregates with quartz; it

Unesco, 1973. Genesis of Precambrian iron and nianganese deposiis. Proc. Kiev Symp., 1970.(Earth sciences, 9.)

153

I. P.Novokhatsky

FIG.1. Schematic geological section of the Karskpay synclinorium (Uzbekov,1960): 1. Ferruginousquartzites with interbeds of quartz-sericiteschists; 2. Quartz-sericiteand quartz-chlorite schists with interbeds of tuffaceous schists and quartzites;

3. Green tufogene schists;4.Quartzites with interbeds of green and quartz-sericiteschists occurring with sharp unconformity; 5. Basic effusives and their tuffs.

also occurs as separate strips from fractions of 1 mm to several millimetres thick. Laminar hematite, in the shape of euhedral crystals that sometimes cut the thin flaky aggregates,is less common. Martite is often represented by porphyroblasts in finegrained hematite and quartz between 0.01-1.5 mm in size. Magnetite occurs in the ores in small quantities as grains up to 1-1.5 mm in size that are often concentrated along the lamination.Pyrite,which is rare,occurs as single grains. Sideriteis an occasional constituent of the ores. Quartz,the main noii-metallic mineral in the ores, constitutes up to about 40 per cent of the rock. Sericite, chlorite, calcite, apatite, epidote and gypsum also occur in the ore horizons. The iron content of the ores varies from 20 to 63 per cent,with an average of 34-44 per cent.The average abundances of other elements are: silica, 29-34 per cent; phosphorus,0.15 per cent;sulphur,0.22per cent.The following elements were determined by spectrographic analysis: Pb, 0.0007per cent, Zn, 0.015 per cent, Cu, 0.001 per cent, Ba, 0.003per cent,V,0.0015 per cent,Ni,0.0005per cent and Ge, 0.00017per cent. Kereghetas is the southern continuation of the Balbraun field.The areas have analogous geological structures, except that in Kereghetas greenstone is more abundantand quartzite is less abundant. The strike of rocks is northsouth;the dip is steeply westward. Structurally the deposit representsa synclinalfold complicated by finer folding.The anticlinal areas are made up of greenstone,the synclines of schist with iron-formation.Sixteen ore bodies occur in five ore strips within the sequence and are buried by up to 200-250 m . The ores of Kereghetas are mineralogically similar to the Balbraun ores, but they are leaner; the average iron content here is 37.2 per cent.

The other deposits of the region are smaller,and they have not yet been studied in detail. All of them, except the Koldybayshoko deposit, occur in the same regional structure.

154

Aschitasty The Aschitasty deposit,which is at the latitude of the town of Arkalyk,lies to the north of the previously described deposits within the same structure. It was discovered recently by geophysicalprospecting,and is now being explored. The Precambrian rocks here are covered by younger unconsolidated deposits 40-100 m thick.The deposit lies at the southern part of the Tasoba syncline composed of metamorphic rocks includingchlorite-sericiteschist,porphyroid, amphibolite, quartzite and iron-formation.The strata are intruded by granitic rock and gabbro. The iron-formation horizon has been traced by drilling over an area of 13 km; it is up to 60 m thick. In contrast to the other fields,the ores of the Aschitasty deposit have been metamorphosed by the intrusions. Contact metamorphism has resulted in recrystallizationand coarsening of grain size of quartz, the formation of magnetite instead of hematite,and the formationof amphibole, epidote, apatite and carbonates. In spite of this, the ores have retained their thin banding. The iron content of the ores varies from 20 to GO per cent, or from 38 to 44 per cent on average. The sulphur content is 0.01-0.11 per cent and phosphorus content is 0.4-1 per cent.

Precambrian ferruginous-siliceousformations of Kazakhstan

Betpakdala region

0.0007per cent;zinc,0.007per cent;copper,0.001per cent; barium,0.0015 per cent; germanium,0.0003 per cent.

Betpakdala,to the west of Lake Balkhash,is another region in Kazakhstan in which iron-formation occurs in the Precambrian sequence.The deposits are located in two strips of metamorphic rocks with a NW orientation.Older rocks, some of which are Archaean, are distinguished by the extentto which they have been metamorphosed;they consist of gneisses and crystalline schists. Overlying them are Proterozoic metamorphic rocks-quartz-mica and quartzactinoleschistcontaining marble layers,quartzite and ironformation. There are two important areas here: the northern region including the Zhuantyube field Gvardeiskoye,and the southern region,known as the Temir zone. In the northern region,ferruginousquartzite crops out over a distance of about 15 km.Phanerozoic rocks,including Cambrian,Ordovician,Devonian,Carboniferous,Cretaceous and Palaeogene, occur within this region. Metamorphic rocks of the Precambrian are represented by quartz-chloriteand quartz-chlorite-sericiteschist of green and grey colour, porphyritoid, white and yellowish grey massive quartzite and greyish-black iron-formation.The iron-formation crops out within a belt of metamorphic schist and porphyroid over an area about 10 k m long and up to 4 k m wide. The metamorphic rocks here form a synclineelongated in the NW direction and complicated by smaller folds.The rocks dip steeply from about 65 to 82". There are twentyone are bodies that crop out in zones 800-3,500 m long and 10-100m wide. They are from 30 to 100 m apart. A magnetic survey has shown the presence of ore bodies up to 6,000m long which do not crop out. Parallel arrangement of the ore bodies is due to their repetition by folding. The ores are thin-bandedferruginous quartzite.Banding is caused by the alternation of laminae of hematite with laminae of quartz enriched by hematite. The thickness of separate layers varies from 0.1to 2-3 m m . Hematite, the main ore mineral, comprises 80-90 per cent of the total amount of the ore minerals. Martite and magnetite are less common. The composition of the Zhuantyube ores is shown in Table 1. The followingvariations in the content of the main elements in the samples were observed: iron,23.18-59.24per cent, average, 44.43 per cent; silica, 14.65-66.35 per cent; phosphorus (P,O,), 0.02-0.45 per cent; sulphur, 0.030.28 per cent. The following trace elements were determined spectrographically:vanadium,O .O007per cent;nickel,

The Zhuantyube (Gvardeiskoye) deposit has not yet been completely prospected so that its total strike length is not known. Geophysical exploration has revealed ore bodies which do not crop out. The second important area,a southern strip of ferruginous quartzites of the Betpakdala region,is situated in the central part of the Chu uplift, 100 k m south of the area described above. It is known as the Temir ore zone. Here the iron-formationis also related to strata of metamorphic rocks built up of quartz-mica, quartz-epidote-actinolite schist alternating with porphyritoid, marble, and quartzite and iron-formation.The strata are gathered into complex folds of NW strike with steep NE pitches (70-88"). The metamorphic rocks are intruded by granite in the direction of the folding. Within the metamorphic rocks light-greynon-metallic quartzites occur;these contain lenticular bodies of hematite quartzite, sometimes in an echelon-like pattern. Their thickness varies from 5 to 50 m,while their length varies from 50 to 500 m.The total length of the ore outcrops in various areas is 5-7.5 km,and for the whole zone it is up to 40 km. The thin banding of hematite quartzites is caused by the alternation of laminae enriched by hematite and magnetite0.1-5mm thick,with layers ofnon-metallic quartzite. Martite,hematite and magnetite are the main ore minerals. Martite, which forms isometric grains 0.2-0.5 m m in size, is most abundant. The iron content of the iron-formationvaries considerably;from 10-15 to 32-36 per cent increasingregularlyfrom west to east. The sulphur content is 0.03-0.35per cent and phosphorus comprises 0.08-0.11 per cent. The geology of the Temir zone has not been studied in detail. It differs from other regions gf analogous Precambrian formations in the nature of its ore mineralization. Its economic importancehas not yet been dearly evaluated. The most interesting area, 'Magnitny', lies in the southeastern part of the zone marked by an intensive anomaly up to 3,500y.It is overlainby unconsolidated deposits.

The features of ferruginous-siliceous formations of the Precambrian in Kazakhstan The iron-formationof the Precambrianin Kazakhstan have much in common with analogous formations in other

TABLE 1. The composition of the Zhuantyube ores (%) Fe

total

SiO,

Tio2

A1,03

FenoI

Fe0

MnO

Ca0

MgO

K20

NazO

PzOj

SO,

H,O

(ignition loss) Total ~ _ _ _ _ _

56.45 49.75 59.24

16.45 27.45 14.65

N o data N o data N o data

0.85 n.f.

0.25

80.28 69.29 81.85

0.36 1.65 1.22

n.f. n.f. n.f.

n.f. n.f. n.f.

0.27 0.14 0.17

0.10 0.10 0.10

0.10 0.10 0.10

0.36 0.28 0.45

0.01

0.03

n.f.

0.03 0.01

0.14

0.57 0.62 0.54

99.38 99.66 99.48

155

I. P.Novokhatsky

regions of the world. They are related to a thick series of metamorphic rocks subjected to intensive NS and NW folding. The estimated absolute age of the formations is 2,600-1,900may. The ore components are mainly hematite with accessory magnetite. The amount of magnetite increases in contact zones where iron-formationhas been intruded by granite. Silicate-magnetite ores are practically absent in iron-formationsof Kazakhstan,and this distinguishesthem from analogous formations of other regions. The iron content in hematite quartzite of Kazakhstan s not high, being rarely more than 20-40 per cent.

The ferruginous quartzites are also characterized by a low content of manganese, aluminium, phosphorus, sulphur, calcium, magnesium and such admixtures as lead, zinc,copper,barium, germanium and others. Greenstonesformed from basic extrusives make up an important component of the Precambrian metamorphic rocks. They frequently compose the lower part of a suite, while in the upper part various schists containing ironformation predominate. Cases of interbedding of quartzite with extrusives and their tuffs are rather rare;this indicates a gap between the deposition of effusives and the main bulk of iron-formation.

Résumé Les formations de fey siliceux dans le Précambrien du

Kazakhstan (I. P. Novokhatsky)

Les formations de fer siliceux du Précambrien sont très abondantes dans le Kazakhstan. On les observe parmi les rochesmétamorphiques du Protérozoïque,qui formentdeux larges ceintures,celles de Karsakpay et de Betpakdala. L a ceinture de Karsakpay est située dans la bordure occidentale de la région plissée de Kazakh et s’étendle long du méridien sur 400 kilomètres depuis le mont Arkalik, au nord, jusqu’à la rivière Beleuta, au sud. A l’intérieur de cette ceinture,on connaît environ une vingtaine de dépôts de minerai de fer siliceux, dont les plus grands sont ceux de Balbraun et de Keregetas. Le minerai de fer siliceux est concentré dans une strate épaisse de minerais métamorphiques du Protérozoïque supérieur réunis dans des plis complexes qui forment un grand nombre de synclinoria et anticlinoria.La strate est constituée de porphyrites,de schistes verts (avec des veines de porphyrite et parfois de marbre), des quartzites, de minerai de fer siliceux,des schistes de chlorite (avec des veines de minerai de fer siliceux), de micro-quartzitesgraphitiques, de couches de porphyrite et de schistes d‘actinolite-chlorite et d‘actinolite-épidote-chlorite.Les plus abondants sont les fers siliceux de la série de Karsakpay qui constituent un grand nombre de synclinaux qui s’étendentsur des roches plus anciennes. L a formation consiste en quartzites blanches et brunes, en quartz séricites gris-verdâtre,en schistes d’actinolite-chloriteet d’épidote-chloritealternant avec du fer siliceux et des quartzites sans minerai. La puissance de cette formation est d’environ 850 mètres. ,

Dans le gisement de Balbraun, on trouve du minerai de fer siliceux sur une longueur de 5 kilomètres de long et 300mètres de large. On observe jusqu’à 30 bancs différents de minerai à l’intérieur de cette bande. L’horizon de minerai qui forme la partie supérieure de la strate est une série alternée de minerai de fer siliceux avec des schistes de séricite-quartzet des quartzites sans 156

minerai et plus rarement avec des minerais massifs.L a puissance est d‘environ 60 mètres. Le minerai de fer siliceux constitue de façon très distincte des formationszonées d‘hématite,magnétite, quartz, chlorite et séricite et plus rarement d‘apatite, albite et carbonates. Les réserves de minerai sont évaluées à 158 millions de tonnes. D’autres dépôts dans la ceinture de Karsakpay sont semblablesà ceux de Balbraun mais sont de dimensions plus modestes. Dans les années récentes dans le nord de la ceinture de Karsakpay,dans la région du mont d‘Arkalik on a découvert du minerai de fer siliceux par les méthodes géophysiques. Le dépôt est maintenant connu sous le nom de ( (dépôt Ashchitast ». L’horizon de minerai de fer siliceux est connu sur une distance de 13 kilomètres sous une couverture qui peut atteindre 100 mètres d‘épaisseur.L a strate qui contient le minerai consiste en schistes de séricitechlorite, porphyroïdes, minerai de fer siliceuxet silex sans minerai. L a strate est brisée par des intrusions de granite et de gabbro. L’horizon du minerai a environ 60 mètres d‘épaisseur. Dans les minerais à part l’hématite,on rencontre aussi de la magnétite. L a teneur en fer varie de 20 à 60 %, la moyenne étantde 38,4 %. Les minerais peuventêtre concentrés facilement.On estime les réserves de minerai à environ 500 millions de tonnes. L a ceinture de Betpakdala de minerai de fer siliceux est limitée à la strate de minerais métamorphiques de la montagne de Tchuysk. Les roches métamorphiques sont présumées datées de l’époque protérozoïque. Elles s’étendent dans une direction nord-ouest.Elles sont caractérisées par une tectonique complexe et sont divisées en deux districts par une bande de dépôt du Paléozoïque moyen :la région nord (avec le dépôt de Zhuantijube)et la région sud. Dans la région nord,les dépôts paléozoïques sont représentéspar des roches métamorphiques rassemblées dans des plis abrupts de direction nord-ouest.Dans le dépôt de Zhuan-

Precambrian ferruginous-siliceousformationsof Kazakhstan

tijube,on observe sur une distance d’environ 10 kilomètres un grand nombre d’ameurementsde quartzite; ils représentent des couches (qui s’enfoncentrapidement) de minerais de fer siliceux rassemblées en plis complexes. Les minerais se présentent en minces couches de quartzites composées de veines alternées d’hématite et de quartz-hématite. Une veine a de 0,l à 2-3 mm d‘épaisseur. L’hématite est le minerai principal, la magnétite et la martite sont moins abondantes (10 à 20 %). La région sud (zone de Temirsk) diffère de la région nord à la fois par le caractère et par la composition du minerai. Ici, sur une distance de 40 kilomètres,011 observe une strate de roches métamorphiques,qui s’étend dans une direction au nord-ouest,avec un plongement raide prédominant vers le nord-est.L a strate est coupéepar une grande

intrusion de granite. Elle est composée de quartzites, schistes, quartz-micacés, schistes porphyroïdes, amphibolites et marbre. Parmi les quartzites,on rencontre des lits ou des formations lenticulaires de minerais de fer siliceux. Dans une bande de 2 kilomètres de large, on trouve trois gisements de minerai de fer siliceux dont l’épaisseur varie de 30 à 50 mètres. Le contenu en fer des minerais de fer siliceux varie largement,depuis quelques centièmes jusqu’à 32 %.

En ce qui concerne la genèse des formations de silex ferrugineux dans le Kazakhstan,les opinions diffèrent :certains auteurs considèrent qu’elles sont des formations sédimentaires typiques, d‘autres au contraire inclinent à leur assigner un caractère sédimentaire volcanogénique.

Discussion J. H.GROSSI SAD. Is the iron quartzite (quartz-hematite ore) a chemical or a clastic rock?

I. P. NOVOKHATSKY. Magnetite is of later origin than hematite. Martite is a common mineral of the supergene zone.

I. P. NOVOKHATSKY. Quartz and hematite are mainly chemical sediments.In some deposits of Kazakhstan quartz has certain featuresof clastogeneorigin (Temirore zone).

J. H . GROSSI SAD. What causes the ‘bedded’aspect of the iron ores of Kazakhstan in your opinion?

I. P. NOVOKHATSKY. The bedding of the iron ores is the result of irregularsedimentationcaused by different factors.

J. H.GROSSI SAD. What are the minerals containing germanium? Are they the iron minerals (hematite,magnetite, etc.)?

S.ROY. At Kumdikul (central Kazakhstan) the iron ores are fairly rich in manganese (4-7 per cent and higher). In view of different E h and p H limitations for the formation of iron and manganese as oxides, how do you explain this association?

I. P. NOVOKHATSKY. There are different quantitative correlationsbetween iron and manganese,from pure iron ores to manganese ores.The Eh and p H had a great effect during their deposition.

and magnetite.

S. ROY. H o w do you explain the facial transition of iron and manganese ores in Karazhal (central Kazakhstan)? D o you attribute it to changes of Eh and p H during deposition as suggested by Krauskopf (1956-57)?

R.T. BRANDT. About what percentage of the Balbraun ferruginous quartzite has an iron content greater than 60 per cent?What are the characteristicsof thesehigh-gradezones?

I. P. NOVOKHATSKY. Ore depositional facies are different -from oxide facies to carbonate manganese. In this case I agree with Krauskopf.

I.P.NOVOKHATSKY. The percentage of ferruginous quartz-

S. ROY. What precisely is the grade of metamorphism of the Karazhal ores? M y experience with Indian ores shows that in oxide facies jacobsite and hausmannite appear in manganese ores in a fairly high grade of regional metamorphism.

I. P. NOVOKHATSKY. Germanium is common in hematite

ite containing more than 60per cent of iron is rather small. High-gradeores are bound in narrow synclines.

R. FRIETSCH. The Aschitasty ore contains 0.4-1.0 per cent P. What mineral is the phosphorus bound to?

I. P. NOVOKHATSKY. The grade of metamorphism is not I. P. NOVOKHATSKY. Phosphorus is bound to apatite. R. FRIETSCH.What is the relationship between magnetitemartite-hematite?

high,it is connected with folding and thermal activity that led to the formation of jacobsite, hausmannite and some manganese silicates (spessartite,rhodonite,friedelite,pyrosmalite and others).

157

Geology and genesis of the Devonian banded iron-formation in Altai, western Siberia and eastern Kazakhstan A. S. Kalugin Siberia Research Institute of Geology Ministry of Geology of the U S.S.R.

General geology Deposits of hematite, magnetite and hematite and magnetite ores occur in banded iron-formation in the folded, Devonian volcanogene-sedimentarystrata of Altai in western Siberia and eastern Kazakhstan.The study of these ores provides new data concerning the source, environment, mechanism of ore formation, diagenesis, epigenesis and metamorphism of ores of this type in general.D u e to their inaccessibility many iron-ore deposits of Altai are not mined; however, the reserves are reported to be 3 million tons. The ore-bearing district is associated with the Altai Hercynian folds and the depositional environment is considered to have been volcanic island arcs or the margins of a marine basin in the Lower or Middle Devonian. Mineralization took place in an area 600 km long and 150 km wide, but those iron deposits which have not been eroded are preserved in an area of not more than 20,000km2.The ores are found mainly in the marine tuffaceous, pelitealeurite-psammiticlithofacies,but disappearwhere therudaceous,carbonaceous and terrigene-carbonaceousmarine or terrestrial sediments appear. Outcrops of ore have been studied in detail in an area of approximately 50 km2. A geologic section through the least-metamorphosed hematite ores follows (Figs. 1 and 2).

porphyry and quartz keratophyre tuffs. The ignimbrites contain coarse, detrital grains or quartz, acid plagioclase, albite and K-spar.Biotite and tuffaceous rock fragments with an effusive habit also occur. Devitrified pumice fragments, apatite, zircon, amphibole, titanomagnetite, ilmenite and pyroxene are observed under the microscope. The vitroclastics and crystalline fragments are welded into a pseudofluidalmass with a micro-felsitic,more rarely microspherolitic,texture.Depending upon irregularitiesin deposition,as well as the extent of erosion, the thickness of ignimbrite and tuff varies from several tens to hundreds of metres. The detrital ferruginous minerals, titano-magnetite, ilmenite, biotite, pyroboles and glass groundmass are irregularly replaced on a large scale by hydromica, quartz, calcite,magnesium chlorite,leucoxene and anatase. Alteration of ignimbrite and tuff is compared with processes of postvolcanic, hydrothermal rock metamorphism in regions of recent, aerial, explosive volcanism, which is of the argillization type,and is accompanied by the subtraction of large amounts of iron,involving hydromicatization,local opalization and limonitization.

Iron-ore deposits BASAL STRATA O F C L A S T I C R O C K S

R O C K S U N D E R L Y I N G T H E ORE DEPOSITS

The basement is composed of folded quartz and quartzfeldspathicsandstones,aleurolites,and conglomerates containing Lower Silurian marine fauna. Above the basement lies the only amygdaloidal diabase flow in the region. At Vodopadnyi (Fig. 1) a bed of ancient talus breccia rests on Silurian rocks.Below this breccia the rocks are fractured and have a shell-likecleavage caused by ancient subaerial weathering. Above the talus breccia and diabase flow,lying on Silurian rocks, are vitrocrystaloclastic ignimbrites, quartz

In the south-eastern Altai deposits, the subaerial ignimbrites and tuffs are overlapped by psammitic, gravel-bearing, rarely aleurite-pelitic laminated rocks. These rocks consist mainly of poorly rounded quartz grains,acid plagioclases, potash feldspar, and titanomagnetite as well as zircon derived from underlying tuffs.Because these psamniitic rocks also contain pyroclastics in the form of pumice forks and fragments,they are called tuffites,tuffogritstone, tuffosandstone,etc. The basal section is characterized by inclined bedding, graded in places, with rare, wave-like cross-bedding. In the Srednekedrovsdy magnetite-hematite ores of

Unesco, 1973. Genesis of Precumbriun iron und ?nunganese deposits. Proc. Kiev Syinp., 1970. (Earth sciences, 9.)

159

A. S. Kalugin

central Altai the ore-bearing horizon is composed of conglomerates,coarse sandstone and sandstones which consist of fragments of quartzite, silicified tuffs and effusive rocks with hematite concentrated in the fine-grainedlayers. D E S C R I P T I O N OF THE O R E - B E A R I N G B E D S

The basal rocks change rapidly upward into horizontal, rhythmicallybanded hematiteores and finer-grainedclastics. The thickness of the ore beds in the south-eastern Altai deposits attains 65 m,as compared with the normal 10-15 m. Jn many places the ore strata are eroded and overlapped by sandstonesor gritstonec containingabundant fragmentsof ore laminae.Owing to the mountainous relief, the ore beds are exposed vertically for 800-1,000m and they extend laterally for 15 km.There are five to seven beds with iron contents of 30-35 per cent; in the finergrained facies the grade increases to 40-45 per cent. The ore beds are commonly separated by thicker, coarser-grained tuffite layers, some of which are characterized by rhythmic, unidirectional,cross-beddingas well as graded bedding in the upper part of the section.In the psammite-aleurite lithofacies are ores that have been plastically deformed, resulting from landslide deformation, as well as ores that demonstrate a break in continuity. In these fractured ores, breccias with broken fragments as well as detrital pieces of ore layers are found in the tuffite cement. Non-metallic laminae and beds in the ores consist mainly ofreworked volcanoclasticsderived from underlying tuffites and tuffs. The ore laminae consist mainly of thin layers of compact hematite. Under the microscope hematite has micro- to cryptolepidoblastictexture with relicts of pelitomorphic and spherical bodies which range from 0.01 to 0.001 mm in size. Rarely, ferruginous carbonates make up ore laminae and constitute the core of hematite concretions found in carbonate-claylithofacies. In some beds of laminated hematite ore, particularly those containing ore-fragment breccias, the ore has been replaced by compact, wavy, pseudolaminated hematite, hydromica and quartzite,the latter minerals having been formed by the decomposition of silicate clastic material in the ore. The recent formation of authigenic tourmaline, a result of the admixture of clayey matter and thin-bedded clastic material, is c o m m o n in the ore laminae. The chemical composition of hematite ores in southeastern Altai is characterized by a high content of alkalis, particularly potassium, followed by alumina and titania. The presence of the latter two compoundsresulted froin the reworking of tuffs, tuffites, pyroclastics of the same age and clayey matter. The least-metamorphosedhematite ores of Altai have rhythmic horizontal laminae with alternating laminae of tuffite and hematite.Theselaminae aregenerally 0.5-1 .Ocm, but in places are thicker. 160

In addition to dominant rhythmic laminae, the Altai ores with psammitic tuffite laminae contain a diversity of structures, including sun cracks, agitation ripples, gas bubble cavities,and halite crystal imprints,which indicate shallow water deposition, including littoral zones. Agitation ripples, sun cracks, sedimentary breccias with large and small ore fragments,washout or erosional trenches and both cross and wave-like lamination in the aleurite-psammite lithofacies of the ore indicate that the ore was deposited in an environment of multiple sediment deposition that involved turbulent and oscillatory movements of the water. A more detailed study shows that the predominant,rhythmic alternation of ore and tuffite laminae displays all the features of graded bedding. The rhythmic,graded structure of the layers,persistent or regularly changing thicknesses and the presence of diastems along the borders of the rhythmic layers suggests that the ores may be a peculiar type of ore flysch. This interpretation is strengthened by the occurrence of various hieroglyphs, traces of crawling and burrowing organisms, etc. The ore laminae surface, which is evidently consolidated to some extent,has agitationripples indicating that the washout (erosionchannel) is the result of oscillatory water movement. Pelitomorphic or microspherulitic structures and indications of syneresis,which are evidences of high absorptive capability,suggest that the ore material was in a colloidal state dispersed in water. The ore-bearingpsammite-aleuriticlithofaciesinsoutheastern Altai changes to a pelite-aleuritic facies several kilometres away at the Albesin deposit. The tuffite and hematite laminae coalesce in rhythmic layers. This occurs both in the shallow psammitealeurites facies and in the pelite-aleurite lithofacies. The intrastratal channels are often not recognized because they are mainly developed in loosely consolidated sediments. Our investigations show that washout channels in the ore sediments occurred during the formation of the rhythmic laminae and at the time of water fluctuation and turbulence.A singlerhythmiclayer,which consisted of clastics in the lower part and ore,in the form of mud, in the upper part was deposited. Therefore the rhythmic layers, many centimetres thick in places, did not come from a single source but are the result of a long period of sedimentation. The thickness of rhythmiclayers is in direct proportion to the amount of material supplied and to the length of time between periods of deep turbulence. Therefore, an increase in the thickness of the layers may result from an accelerated rate of supply of material or,at a constant rate of supply,less-frequentturbulence. It may be concluded that the formation of rhythmic, gradational laminae in the iron sediments of Altai was controlled by the hydrodynamics of sedimentation. It is evident that rhythmic layers of similar types will always be equal to the amount of material that has accumulated between periods of intensive turbulence, although turbulence is capable of penetrating depths much greater than the thickness of a rhythmic layer.

Geology and genesis of the Devonian banded iron-formationin Altai,western Siberia and eastern Kazakhstan

O

O

/:.. :.::I. 5

-

lo 11

A

FIG. 1. Geological map of the hematite at Vodopadiiyi in the south-easternAltai. 1. Talus deposits,moraines,snow and firn. 2-6.Sandstones,mottled aleurolitesand conglomerates,tuffites, algal limestoneinterbeds,breccias with hematite ore fragments; 7.Volcanomictic sandstones, gritstones and tuffites, in places with fragmentsofhematite ores,dolomitic limestoneswith corals, brachiopods and algae;8.Rhythmically laminated hematite ores

Y

12

A

13

a

14

B

with largefragmentsof aerialplant fossilremains;9.Ignimbrites, tuffs, sparse quartz keratophyre and porphyry flows, redcoloured aleurolites, tephrolites;10.Aleurolites, quartz sandstones, conglomerates with quartz pebbles; laminae interbeds with brachiopods,bryozoans, trilobites; 11. Fault; 12. Plant fossil remains; 13. Fauna fossil remains; 14. Fauna fossil remains. 161

A. S. Kalugin

L+

I

X

+

X

X

FIG.2. Schematic geological section of the hematite deposits in south-easternAltai with patterns of main structural types of ores and host rocks. 1. Mottled aleurolites, sandstones, conglom-

162

I

+

f

X

X

X

X

x

X

I

erates with fragments and pieces in the basal part of bed; 2. Siliceous effusive flows with spheroidal jointing and colloform hematite deposits in fractures; 3. Dolomitized limestones with

Geology and genesis of the Devonian banded iron-formationin Altai, western Siberia and eastern Kazakhstan

Rhythmic layered texturesin shallow facies of ore beds have resulted from the movement of sediments by waves caused by wind. Evidence of shallow water also tends to support this.It is also possible that the wind effect on the water was a decisive factor concerning the rhythmic laminae in the more abyssal facies. In sedimentary basins in volcanic areas,seismic movements disturbingthe bottom and the mass of water may cause turbulence.The following data point out the probability of seismic disturbances of the Altai ore sediments. Hematite ores occur in the pelite-aleurolitic lithofacies of the Albesin deposit; the individual laminae are closely folded and the axial planes are vertical,thus excluding a landslide effect. Plication is preserved here inside the diagenetic concretions while the same layers are horizontal.In some cases it is observed that material composed of aleuritic laminae penetrates upward through two or three ore laminae,which are broken into ribbons and folded.This deformation may be due to intensive disturbances. Finally,breccia occurs with ore debris in sand cement, indicative of instantaneous turbulence and rapid precipitation without lateral movement.

U P P E R - O R E DEPOSITS

As shown in Figure 1, overlying the ore strata are volcanomictosite rocks, tuffites with lenses of dolomitic limestone, corals,brachiopods and algae of the Middle Devonian, Eifelian stage (the epoch characterized by extensive advances of the sea in Siberia). Globular quartz keratophyres are seen in the same sections. Deposition of ore ceases with the beginning of these open-seaconditions.

corals,brachiopods;along strike the limestones grade into argillites with hematite and ferrous carbonateconcretions;4.Volcanomicticsandstones,aleurolites,gritstones,tuffites with fragments of laminatedhematite ore;5.Rhythmically laminated ores of the alenropelitic lithofacies,with submarine slumps,seismic fractures and crushed beds, local mud current deposits with aerial plant fossil remains and ripples; 6.Laminated hematite ore of aleuro-psammiticlithofacies,agitation ripples,sun cracks, gas blister cavities,rhythmic, sometimes cross and wave-like bedding, submarine slumps,breccias with ore fragments,plant fossil remains, possibly partly autochthonous; 7. Basal tuff sandstones,tuff gritstones, tuff aleurolites;diagenetic hematite concretions; 8. Quartz porphyry and keratophyre ignimbrites and tuffs, sometimes effusives; veins and lenses of colloform

Gritstones, sandstones and breccia, with abundant suspended detritus consisting of underlying hematite ore, are found in many places above the ore strata. In places,tuffites with intercalationsof algallimestones are superimposed on the molasse suite of green and red calciferous sandstones,aleurolites and conglomerates.

EPIGENESIS P H E N O M E N O N A N D O R E DEPOSITION

In the Altai area, €erruginous quartzites are extremely varied as to the type and intensity of metamorphism. In south-easternAltai, the chlorite-hydromicaceousfacies of abyssal epigenesis is observed, while in the central and western Altai zones of green slates, contact hornfels and granitization are observed. In the process of metamorphism,argillaceousmaterial and vitroclastic ash are altered to thin crystalls-and lithoclastics.Blastic quartz-carbonates,albite,sericiteand chlorite aggregrates develop during the first stage of metamorphism.With increasingmetamorphism light and dark mica, actinolite, epidote,sphene and other minerals are formed. Idioblasts of magnetite, which are associated with quartz and mica halos, are generated at the expense of hematite. During these stages of metamorphism,many of the original textures are obliterated, but the banding remains in the psephitic lithofacies with relicts of gradational bedding. During high-grade contact metamorphism and in zones of granitization,hematite is completely replaced by magnetite and relicts of sedimentary structures and textures disappear without obscuring the thick bedding and stratification of the ore bodies.

dense hematite (black) with jasperoids hydromicatization (dotted) zones, barite veins (slanting hachures); prismatic jointing ignimbrites and rheoignimbrites with bomb zones; 9. Redcoloured aleurolites and sandstones; 10. Tephrolites with siliceous hematite,large Liesegang rings and red-colouredaleurolites and sandstonesin the fractures;11. Quartz porphyry and keratophyreignimbriteand tuffs,with underlyingrock fragments at the base; 12. Amygdaloidal diabase; 13. Conglomerate or fanglomerate;14.Old talus rock breccia;15.Quartz sandstones and conglomerates, carbonate interbeds with trilobites and brachiopods.(Structural stretchesweremadefrom specimensand outcrops.Sketch sizes are enlarged compared with the scale of the geological section and reduced compared with their natural size 5-50 times.) 163

A.S.Kalugin

Résumé Géologie et genèse de la fosmution dévonienne du tes subatzé dans I’Altai‘,la Sibérie occidentale et le Kuzalc/lstun oriental

(A.S. Kalugin) Des dépôts de quartzites ferrugineuses se rencontrent dans les monts Altaï parmi des dépôts sédimentairesvolcanogéniques,qui datent de façon évidente du Dévonien inférieur et moyen. Les strates à minerai sont essentiellement composées, dans leur partie inférieure, d’ignimbrites,de tufs de porphyres à quartz et d’albitophyre,de tufs et de roches effusives de porphyres trachitiques, de porphyrites trachyandésitiques, de roches kératophyres et diabasiques, Des manifestations d‘une ancienne activité solfatarique et de fumerolles avec évacuation de €er jusqu’à la surface d‘alors se rencontrent fréquemment dans ces roches. Des tufites avec participation de roches carbonatéeset siliceusesargileuses,enrichiesçà et là de manganèse et de phosphore, prédominent dans la zone qui surmonte le minerai. Les types de plissement des strates à minerai changent depuis le pli ouvert jusqu’au pli linéaire, avec un accroissement

correspondant de la transformation régionale des roches et des minerais depuis un faciès d’épigénèseprofonde jusqu’au gneiss et aux schistes verb avec un développement de la région des intrusionsgranitoïdes,du métamorphisme de contact et de la granitisation. Les quartzites ferrugineuses des monts Altaï se rencontrent dans les sédiments marins tufogènes libres de carbonateset sont déposées dans un faciès qui va depuis les zones littorales jusqu’aux zones bathypélagiques. Elles contiennent des produits entraînés des formations de solfatares-fumerollesprovenant de tufs sous-jacentset de cendres volcaniques abondantes fréquemmentrelavées. L a stratificatioiirythmique des minerais est déterminée par une alternance de vases du minerai original et vol
Discussion N.A,PLAKSENKO. In your report you have emphasized the role of clastic material in ferruginous quartzites. D o you find any changes in the granulometric composition of the rocks in the facial profile, i.e. in the direction from the primary source of sedimentationtowards the deepest zones of sedimentation?

nation of ferruginousand siliceous layersin ores of the iron quartzite type cannot be explained by accumulation of colloids alone,because the previously formed textureis broken down when the slightest disturbance is applied to colloids.

A.F.TRENDALL. What was the original nature and origin of the gas in the bubbles?

A.S.KALUGIN. In the Altai region ores like iron quartzites pertaining to shallow facies with sun cracks, agitation ripples,etc.,contain abundant psammitic-siltmaterial. On passing to deep-seafacies,characterized by the absence of sun cracks ripples,cross-beddings,etc.,we find a predominance of pelitic material.

A. S. KALUGIN. I think that the gas bubbles in shallow facies are probably formed because of air sealed by the near-shorewaves in the sand. This phenomenon has been described for recent sedimentations.

G.A. GROSS. You mention reproduction of some sedi-

I.A . BERGMAN. Have you studied the distribution of accessory elements (P,As, V, M o , etc.) in the ores? What are

mentary features by laboratory experiments. Would you comment briefly on the technique of these experiments?Did you use silica gels?

the main features of their distribution: composition,concentration, etc.? H o w does metamorphism affect the concentration of these elements?

A.S.KALUGIN. Using a technique similarto that of Moore

A . S. KALUGIN. The distributions and concentrations of accessory elements in the Altai ores were studied by me and also by S. I. Zubova, A. G.Guzman, V. G.Pononiarev,E.G.Kassandrov,F.V.Sukhorukov andV.E.Popov. The hematite ores contain relatively high amounts of barium and boron. Lhe phosphorus and manganese contents do not exceed tenths and hundredths of 1 per cent.The titanium concentrations are due to clastogene material. Sometimes low concentrations of copper occur. Lead and zinc occur in concentrations of hundredths of 1 per cent;

and Maynard,we obtained colloidalsediments offerric and silicon hydrates in artificially prepared sea-water.First, ferric hydrate precipitated out of this colloidalmixture and we observed a two-layercherty ferruginoussediment.After settling,which lasted from several days to 1-2 weeks, the sediment was again subjected to agitation and settled in a water colum of about 25-30 cm. During the settling we observed sedimentation of undifferentiated cherty-ferruginous inaterial.Our experiments show that rhythmic alter1 U4

Geology and genesis of the Devonian banded iron-formationin Altai,western Siberia and eastern Kazakhstan

molybdenum in concentrationsof thousandthsof 1 per cent. Slightly metamorphosed ferruginous quartzites are lean in barium. The ores sometimes contain sulphides of nonferrous metals due to epigenetic mineralization.

agent in the rocks. You have shown that in your calculations.

Yu. P. MELNIK. What are the metamorphic changes of silica (opal, quartz)?

Yu.P. MELNIK. What is the percentage of free carbon in the hematite quartzites?

A. S. KALUGIN. Under metamorphism the fragments of

A.S.KALUGIN. Free carbon occurs in the banded hematite

quartz and probably opal, as well as primary auihigeiiic cherty sediments,are transformed into grained,frequently granoblastic rocks.

ores of Altai, but no data are as yet availableon its content.

Yu.P.MELNIK. What arethefactorsgoverning theformation of magnetite from hematite? Is it only the rise in temperature, or also the presence of a reducing agent in the rocks? A. S. KALUGIN. The formation of magnetite requires, besides the rise in temperature, the presence of a reducing

Yu.P.MELNIK. Have you met with primary iron silicates?

A. S. KALUGIN. Kascandrov’s studies of the iron ores of the Kholzunsk depositin Altai revealed spheroidsofferruginous chlorite which may be primary unmetamorphosed authigenic silicates.

165

Genesis of high-grade iron ores of Krivoyrog type Y.N.Belevtsev Institute of Geochemistry and Physics of Metals, Academy of Sciences of the Ukrainian S.S.R.(U.S.S.R.)

Precambrian deposits of high-gradeiron ores are confined to thin-bandediron-chertyrocks which bear different names on different continents: banded hematite quartzite, chert, jaspilite, itabirite, taconite, jasper-like beds, ferruginous quartzites,etc. Iron-richrocks consist of iron-oreminerals (magnetite, martite, hematite), quartz, carbonates, various silicates (chlorite,sericite,biotite,amphibole)and,to a lesser extent, feldspar,talc and muscovite. The rocks coinposed of the above minerals are classified into two groups: schist and iron chert,and jaspilite(itabirite,taconite,quartzites,etc.). T w o principal varieties of schist are distinguished in terms of their chemical and mineral composition: alumosilicateschists,consisting of sericite,quartz,muscovite,and biotite;and iron alumosilicateschist composed ofmagnetite, chlorite,biotite, cummingtonite and quartz. Banded iron-richcherts consist of two types of bands: metalliferous,made up magnetite, martite or hematite with a subordinate content of quartz, carbonate and iron silicates;and iron-free,composed of quartz and iron silicates, and carbonates. The thickness of the bands ranges from 0.1 to 10-15 m m . Iron-quartz,iron-quartz-silicate, ironcarbonate-quartz,or quartz-silicatecherts are distinguished according to the combinationof variously composed intercalations in the rock. Among ferriferous cherts,jaspilites are distinguished by their fine banding, formed by ore and quartz interbeds under almost absolute absence of other minerals (carbonates and silicates). Beds of iron cherts are afew metres to 250-260m thick, alternating with slaty beds form iron-oresuites. The thickness of iron-oresuites may range from 200-300 to 2,000m. Sometimes effusive rocks represented by amphibolite,talccarbonate, and talc-serpentine varieties are also found in the suites. The majority of authors advocate a sedimentary-metamorphic origin of iron ore formations. Different opinions as to the formation of the cited rocks result from a different understanding of the role of sedimentation and volcanism in the formation of the primary composition of

the suite and the effects of its subsequent metamorphism. The Krivoyrog iron-orebasin occupies the central part of the Ukrainian shield, stretching in the submeridional direction from the southern to northern margin of the shield along the boundary of Upper Archaean rocks in the east and Lower Proterozoic rocks abundant in the west. From the point of view of structuralgeology the Krivoyrog basin is a deep, relatively narrow synclinal zone which developed from the marginal trough of a Lower Proterozoic geosyncline. The rocks that make up basin are termed ‘theKrivorozhsky series’,which is subdivided into three suites:lower -arkosic sandstone,conglomerate,and phyllite; middleconsisting of seven iron-chertyand jaspilite beds alternating with various slaty beds; upper-quartzite, sandstone,and various shales. All the above rocks underwent regional dynamo-thermal metamorphism to the greenschist and amphibolite stage. The iron-ore deposits are generally located in jaspilite and iron cherts of the middle suite. They are represented by metalliferous beds,thick flexure deposits,ore lodes,and pockets. Ore deposits are confined to folded or fold-faulted structures,where they are concentrated in groups or patterns that form ore fields. All the mineral deposits of the basin, in their turn, form three ore fields or ore regionscharacterizedby common geologic-structuralconditions, mineral composition and genetic features of ores; the Southern, Saksagan (central), and Northern ore fields or ore areas. The Southern ore field is located in the southern part of the Krivoyrog basin and is characterized by a predominant development of beds and pods of specularite-magnetite and chlorite-magnetiteore deposits that are confined to the upper part of the middle iron ore suite or to the lower subsuite of the Krivoyrog upper suite. The Saksagan ore field is situated in the central part of the Krivoyrog basin and is characterized by an abundance of compact and porous martite and friable goethitehematite-martite and goethite-hematite ores, that form rather complexly shaped deposits with most frequent ore

Unesco, 1973. Genesis of Precumúriun iron and mangunese deposits. Proc. Kiev Symp., 1970. (Earth sciences, 9.)

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columns,stock-likeand sheet-likedeposits,morphologically connected with the thick articulate deposits (Fig. 1). The northern ore field occupies the northern area of the basin and is characterizedby amphibole-magnetiteand hematite-magnetitecompact ores confined to complexlyfolded aiid faulted block structures. In the world iron-oredeposits of the Precambrian,as well as in the Krivoyrog basin and Kursk Magnetic Anomaly (KMA), three types of high-grade ores are encountered. Compact magnetite or silicate-magnetiteores are located in areas of fold-faultedstructures within rocks,considerably altered by magnesian-ironand iron metasomatism. Compact magnetite-hematite,mastite-hematiteand porous martite ores forming ore columns and sheet-likedeposits are abundant in complexly-folded aseas of iron rocks. Magnetite ores replace martite porous and massive ores at shallow or greater depths within one and the same deposit. Friable or soft hydrated ores are represented by goethitehematite-martiteand goethite-hematitevarieties that are largely developed within the superficial crust of weathering or in narrow deep zones of oxidation. The second type of ore constitutes the bulk of all Precambrian iron-ore deposits encountered on all continents of our globe. On the other hand, the first and third type of ore are lesscommon aiid oflocalsignificance,althoughthere are individual deposits and areas where they are the main, if not the only, ore bodies present (Anshan,northern part of the Krivoyrog basin, etc.).

Magnetite and silicate-magnetite ores These form the northern and part of the southern ore fieldsoftheKrivoyrogbasin,where they occur inamphibolemagnetitechertand shaleofthemiddlesuiteoftheKrivoyrog series. The deposits are confined to areas of metasomatic rock transformation into a magnetite-amphibole variety. The metasomatic nature of the mineralization has also controlled the morphology of ore bodies, governed by two

The Dzerzhinsky mine

The Kirov mine

The Karl Liebknecht mine

factors: (a) degree of tectonic preparedness of rocks (fold or fault deformation) furnishing the ways for downward moving ore-formingsolutions;and (b) abundance of amphibole, magnetite-amphibole slate and jaspilite, which are lithologically favourable for rock metasomatism. The ore bodies occur conformably with the host metamorphic rocks. Four principal varieties of ores are distinguished in terms o€mineral composition: (1) amphibole-magnetiteand amphibole-hematite-magnetite;(2)quartz-amphibole-magnetite (occasionally with hematite); (3) aegirine-amphibolemagnetite; (4)carbonate-magnetite-hematite ores. All these ores are relatively equigranular with a grain-sizeno more than tenths of 1 m m . The ores are compact,with an average porosity of 3-4 per cent. Formation of metasomatic iron-oredeposits is associated in time and spacewith the concluding stages of folding, when the rocks were in the state of the greatest tectonic strain causing high permeability for pore solutions. The role of the ore-enclosingstructures was played by zones of interstratal leaf-by-leafgliding, cleavage, and zones of increasedjointing and crumpling, generally originating on the slopes of large folded structures. In consequence, the ores develop in conformity with the rock bedding, enter into the composition of finely-folded structures and exibit no sharp boundaries with the host rocks. The flow of pore solutions through the weakened zones resulted in the phenomena of dissolution and redeposition of the material, accompanied by accumulation of commercial abundance of iron in favourable structures. The main ore-formingprocess resulting iii the emplacement of iron ores is magnesian-iron metasomatism; subsequent stages of metasomatism-alkaline, magnesiancalcium-carbonate,and silicification-caused complication of ore mineral composition and deterioration of its quality due to a dissolution and removal of iron during the stages of metasomatism. Two stages are distinguished in the ore-formingmetasomatic process, namely, magnesian-iron metasomatism proper and iron (iron ore) metasomatism. The magnesian-ironmetasomatism proper is the most active process, involvingrocks of different lithologic-petro-

The Bolshevik mine

The October Revolution mine The Frunce mine

FIG.1. Longitudinalprojection of iron-oredeposits in horizons IV and V of the Saksagan syncline.(The lower portion is shown schematically.)

1 U8

The XX Congress of CPSU mine

The Red Guards mine

Genesis of high-gradeiron ores of Krivoyrog type

graphic composition. It was manifested through the formation of magnesium and iron-bearingamphibole: cummingtonite, grunerite, and hornblende, which replaced biotite, chlorite, and garnet in various iron cherts and shales, as well as quartz and some iron-ore minerals. As a result, amphibole magnetite rocks originated. With a complete replacement of ore-forming minerals by amphibole in shales, amphibolitic cummingtonite (monomineralic) and magnetite-cummingtoniteshales appear at the expense of iron-rich cherts. Maximum discharge of magnesium together with bivalent iron from the solutions is confined to tectonicallyweakened zones-periclinal closures offolds,flexures,structuralinversions,etc. The amphibole or magnetite-amphiboleshale and chert that are deposited in such areas have the shape of irregularly stretched bands. They are gradually replaced,both along and across strike, first by amphibole-bearingiron-richchert or shale of quartzbiotite composition,and later, by amphibole-freeprimary equivalents of the above rocks. Iron or iron-oremetasomatism immediately followed the magnesian-ironmetasomatism proper.In the first stage of amphibole formation,the lack of oxygen prevented the formation of ferric oxides,with the result that iron accumulated in solution. An increase in redox-potentialbrought about removal of excess iron froin solution,first as magnetite and finally as hematite. Since iron metasomatism manifested itself immediately after magnesian-iron metasomatism in areas of nearly monomineralic amphibole slate,the essential control of the same structural features,resulting from intense folding, complicated by minor forms of tectonic dislocation, is emphasized.

The deposition of magnetite and hematite of ore generation took place in conformity with the banding in cummingtoniteshale,inmagnetite-cummingtoniteand magnetite-cummingtonite-chert . There is almost no evidence of ore emplacement in amphibole-freeiron cherts. Developing from amphibole or quartz-amphibole bands, the newly formed magnetite and hematite replaced amphibole and quartz. The content of magnetite in ores may vary and, consequently,within the ore deposits,areas of magnetite, hematite-magnetiteand magnetite-hematite ores connected by mutual transitionsare distinguished.The ores are granular, with textures following those of the original rocks. The most widespread ore texture is a distinctly banded one,which is characterized by a regular alternation of magnetite and amphibole bands or magnetite and amphibole and hematite bands. Mineralization started with the deposition of magnetite.If,in iron chertand amphiboliticshale,the magnetite content (the product of general dynamo-thermalmetamorphism; first generation magnetite) is 20-40 per cent, this may be increased to 60-80 per cent as a result of iron metasomatism,due to neocrystallization(second generation magnetite). In ores with massive texture it may reach 9095 per cent. The lamellae of the newly formed amphibole usually coat the grain aggregates of the magnetite or form intergrowthstructures.Irregular and elongated grain shape, seldom with crystallographicconfiguration,O.1-0.5 mm in diameter,ischaracteristicofthesecondgenerationmagnetite. Next to the second generation magnetite, the second generation hematite crystallized. It also developed in a m phibole and quartz-amphibolebands, replacing amphibole and quartz (Fig. 2). The hematite bands in ore consist of

FIG. 2. Example of iron-oremetasomatism. Black, amphiboles;grey,magnetite;white, hematite. 169

Y.N.Belevtsev

aggregates of closely intergrownlamellar or elongate grains forming looped or lattice-likenetworks. The stage of magnesian-ironmetasomatism terminated by deposition of the second generation hematite. Thus,the ultimate products of this stage were amphibole-magnetite, quartz-amphibole-magnetiteand ampliibole-hematite-niagnetíte iron ores, with amphibole mainly represented by cummingtonite, to a lesser extent,by grünerite and,exceptionally,by hornblende. The general scheme of iron-oreformatioil is pictured as follows. The iron-rich cherty sediments underwent dynamothermal metamorphism which caused dehydration of rocks and formation of metamorphic solutions enriched by mobile components (Mg, Fe, Ca, CO,)due to decomposition of authigenous iron silicates. In the final period of folding, the metamorphic solutionsmigrated into tectonically weakened jointed permeable cavities,which were represented by vertical and steeply dipping structures responsible for the upward character of the solutions movement with a sub-

D lm

2

FIG.3. Distribution of iron-oredeposits:(a) in theArtyom mine, horizon 220 m;(b) in the XX Party Congress mine, horizon 270 m;(c) in the Lenin mine, horizon 267 m. 1. (KiC)Schist

170

sequent release of ore-forming components. The lack of oxygen in the first stage of the process promoted development of magnesian-ironamphiboleand,later,with a greater supply of oxygen, of successively magnetite and hematite. In this way the magnesian-iron (iron ore) metasomatism originated,resulting in the formation of amphibolic rocks and iron ore deposits in areas of complexly-foldeddeformations within the iron ore suite of the Krivoyrog series.

Martite and martite-hematite ores The above-mentioned ores are extensively distributed throughout the Krivoyrog basin. They form numerous deposits of the Saksagan ore field as groups of ore columns located on synclinal limbs,or constitute Aexure deposits. The deposits of martite ores occur in iron-richchert and jaspilite of horizons IV, V,and VI (Fig. 3) occupying the upper portion of the iron ore suite. The ore content,or abundance,of deposits in different

4 m

S

m 6

m 7

horizon IV;2.(Ka)Iron horizon IV; 3. (Kac) Schist horizon V; 4.(KS)Iron horizon V, 5. (Kgc) Schist horizon VI; 6. (Kg)Iron horizon VI;7.Iron ore lode.

Genesis of high-gradeiron ores of Krivoyrog type

horizons varies widely. The ore coefficient1 of all iron-rich rocks of the iron ore suite amounts to 0.04;however, for different horizons the coefficient varies from O to 0.8-0.9. The greatest number of deposits is located in the Vth iron horizon composed of jaspilite.The content of iron there is about 37-42 per cent.The total area of ore deposits within this horizon is more than 70 per cent of the total metalliferous area of the Saksagan region. The ore coefficient for iron horizon V ranges, for different mines, from 0.12to 0.78.The rate of mineralization of the horizon is discontinuous along the strike, thus a chain of ore beds is observed within deposits. However, no ore beds are contained in the jaspilite horizon between the deposits. The ore coefficient of the VIth iron horizon is 0.024. Ore beds in this horizon constitute 26.4 per cent of the total mineralization area. The rock mineralization in this horizon is also uneven. The intensity of mineralization grows higher in the same places as in horizon V. Ore accumulations in horizon VI also alternate with ore-free or poorly mineralized areas. Such a relationship is observed throughout the Saksagan area and seems to result from the same causes as in horizon V. Consequently, the rate of mineralization of iron-rich chert horizons is clearly discontinuous.Thus,in the southern part of the Saksagaii area,within the synclinal closure (the Dzerzhinsky mine), an intensive mineralization is observed not only in chertyiron rocks,but also in shale horizons, which resulted in the formation of a single thick flexure bed. Further northwards in the Artyom mine, two lengthwise chains of ore beds are found, confined to iron horizons V and VI and separated by a thick layer of ore-free chert. In all other mines chains of ore beds are also clearly defined and confined to the V,VI and partly the VI1 horizons of iron chert and jaspilite. Along the strike of the chains alternate bunching and thinning of deposits may be observed, affecting all or nearly all horizons simultaneously. This phenomenon has resulted in lateral mineralizationbelts,traced in two,three, and sometimes four adjacent iron horizons,with ore-free areas separating the belts. Along the entire Saksagan band the following six members of lateral mineralization belts, separated by large nonmineralization areas, are clearly distinguished in plan: the belt of the Dzerzhinsky and Artyom mines, the Karl Liebknecht mine, the Bolshevik, October Revolution,and Frunze mines,the XX Party Congress mine, the Krasnaya Gvardiya mine, and, finally, the Lenin mine. The areas separating the above-mentioned ore belts in some cases contain a number of ore lodes. However, the rate of mineralization within them is negligible. The lateral belts of ore deposits are confined to areas of disturbances in plane-paralleloccurrence of one or several chert and jaspilite beds in the form of gentle bends and flexures. The bends are in fact flat cross-folds up to 100 m wide, with a height up to 100 m . Between the mines, where such bends of rocks are not observable at depths of 1,500-2,000 ni reached by boreholes, no ore beds are found. Moreover,in the Dzerzhinsky and Artyom

mines,a dense network of boreholes and mining pits makes it possible to clearly delineate a thick flexure body, dipping northwards in perfect conformity with the Saksagan syncline bend. Consequently, two types of ore-bearing structures are distinguished:the bend of the Saksagan syncline and cross-foldson its limbs (Fig. 1). Thus,the ore deposits in the Saksagan area are closely connected spatially and morphologically with the cross-fold-jointeddeformations and are essentially not present in non-folded rocks. The main cause of the mineralizationprocess involving a bed section or severalbeds was the ore and silicamobility, which rose under the recrystallization of rock material, and caused the concentration of ore into permeable zones of folding,fine-jointingand residual porosity. The ore interbeds pass from rocks into ores without any significantchanges in thickness.Often a certainincrease in thickness of ore bands is observed when additionalthin intercalations of newly formed hematite appear, generally at the boundary between ore and semi-oreinterbeds.With an intense near-contact folding the ore interbeds become somewhat thinner and may even break, but the thickness of any ore interbed is the same as it is in the non-mineralized rocks. However, when one deals with ore-freeand semi-oreinterbeds,their thickness sharply decreases in the course of transformation of iron-rich chert into ore. The interbeds wedge out leaving just a thin streak composed of fine relict plates and laminae of hematite that had previously been dispersed in the semi-oreintercalation (Fig. 4). The decrease in the thickness and the wedging out of semi-ore and ore-freeintercalations is a general pheiiomenon characteristic of various types of ores, including the nonoxidized magnetite ores of the Frunze mine. The thinning out zone is not very wide; in the vast majority of the contacts, it does not exceed 10-20 cm. Often an ore-free layer thins out abruptly disappearing in a distance of 1-2 cm, particularly at sharp tectonic contacts. Cases of gradual, gentle decrease in thickness of the intercalations for several dozens of centimetres and even metres are much less common. Owing to the wedging out of ore-free and semi-ore intercalations, the alternation of iron-rich rocks in ores is always accompanied by a considerable decrease in volume,which may be evaluated by studies on samples or direcily in mining pits. A substantialnumber ofmeasurements (morethan 100) made it possible to establish that out of a layer of iron-rich rocks 100 c m thick,an ore layer 45-80 c m thick is formed. Consequently,100 cm3 of rocks yield 45-80 cm3 of ore,the degree of compression or contraction ranging from 20 to 55 per cent. In none of the cases was the original volume of the rocks preserved in the process of mineralization. In some samples the contraction reaches 50 per cent, but the resulting ores still have up to 30 per cent porosity. In othersamplesthe contractiondoes not exceed 20-25per cent, resultinginvery massive ores with a porosityof4-6 per cent. 1. T h e ore coefficient of rock is determined by the ratio of ore deposit area on a certain horizon, to the total area of rocks in which these ores are located.

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FIG. 4.Martite chert (1) and ore (2) contact.Dark colour,quartz intercalations;light colour,martite intercalations. The numerous measurements cited above suggest that the amount of compression does not depend on the value of present porosity, since both massive and friable porous ores are affected. The total decrease in thickness as compared with the original rocks amounts to 15-20 per cent; this decrease,however,is compensated for at the expense of ore-folding. In individual cases where a complete mineralization of the entire iron zone took place,a certain general thinning out of the zone results.The magnetite ores in the Saksagan area have been found at considerable depths as individual lodes and sections of limited dimensions. These are distributed immediately below the known martite ores or as separate thin beds in non-oxidized iron-richrocks. Such ores have been encountered in the Kirov mine at a depth of 326 m,in the Lenin mine at a depth of 527 m,and in the Frunze mine at a depth of 280 m . In most ore deposits of the Saksagan area the ores are fully oxidized and their magnetite content does not exceed 3-5 per cent. The oxidized martite ores are not homogeneous in their composition and physical properties,In numerous deposits compact massive martite ores are encountered, which pass into porous friable ores. In the 172

southern and central parts of the Saksagan area the massive martite ores are not very common,but in the northern past large deposits composed of homogeneous porous ores are quite sare. Usually a complex structure of ore bodies prevails,with massive ore distributedas inclusionsin porous ores or forming a kind of a border at the contact of the ore-bedwith non-mineralizediron-richcherts.The dimensions of the massive ore inclusions range from several centimetres to several metres. The contacts of massive and porous oses along the strike of the banding are usually rather gradual; sharp contacts are mainly observed along fractures.N o changes in thickness of a massive ore layer as it passes into friable ores occur (Fig. 5). The folding deformation in porous ores, manifested in the form of plication,is perfectly analogous to the deformation in massive ores so far as its character and extent are concerned. The complexly folded bends and fractures of the joints pass from porous friable ores into massive ones without any changes. Throughout the friable ores remnants of massive ores occur,and niassive ores not infrequently pass into friable ones and then again into massive ores within the same

Genesis of high-gradeiron ores of Krivoyrog type

FIG.5. Contact of jaspilite with massive and porous ore. deposit. The massive ores become friable without any perceptible decrease in thickness.While a distinct thinning is observed at thejaspilite/massiveores boundary, the transition from massive to friable ores is not accompanied by any change in thickness. Porous ores differ from massive ones only by the value of porosity, the greatest porosity being characteristic of martite ore interlayers that were of a quartz-martitecomposition in the correspondingmassive ores. The ore minerals of massive ores do not change at the transition into porous ores; the microstructure, size,configuration of grains and grain aggregates, as well as relative quantities of martite and laminar hematite,remain the same. The study of some deposits has shown that friable martite ores are extensively developed near the surface. They are gradually replaced at depth by massive martite and martite-magnetite ores, and at greater depth they are invariably replaced by primary magnetite ores. The magnetite ores have a texture,conditionsof occurrence,confinement to folded structures and mutual transitions within one and the same deposit exactly the same as those of massive martite ores. The quantitative and qualitativerelatioiiships between massive and porous ores, as well as between porous ores and iron-richrocks, have shown that it is impossible to get high-grademartite ores through only supergene leaching of quartz out of normal jaspilite.A great number of the determinations of volume, specific weight, porosity and composition of the jaspilite and the resulting highgrade ores suggest that the supergene mineralization in the

conditions of iron immobility should have been inevitably accompanied by a great decrease in the jaspilite volume (up to 50 per cent of the original volume), with the corresponding ‘subsidences’in the jaspilite series.Numerous measurements (1,200)in 150 sections,through the actual thickness of the jaspilite horizons, have shown that thickness of mineralized and ore-freejaspilite horizons do not differ significantly. Calculations have shown that the present porous martite ores were formed at the expense of primary massive ores containing 50-57 per cent iron, and could not have been formed directly from jaspilite containing 37-40 per cent iron. The emplacement of primary massive ores is associated with the process of dynamo-thermalmetamorphism of iron-rich cherty rocks accompanied by transport of both iron and silica. The resulting primary ores were of a magnetite or hematite-magnetite composition, had low porosity, and contained about 53 per cent iron. In the process of oxidation the magnetite became oxidized to martite, while the removal of silica caused formation of highly porous (up to 25-26 per cent) martite ores with an iron content of 55-70 per cent.

Friable or soft hydrated ores Within the Krivoyrog basin such ores are abundant in the Saksagan area, where they form either separate beds or compositionally complex ore deposits together with 173

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martite ores. In terms of mineral composition, goethitehematite-niartiteand goethite-hematite ores may be distinguished.The main ore minerals are goethite,fine-grained hematite, martite and, to a lesser extent, hydrogoethite. Beds of these ores occur within the zone of oxidation in silicate-ironcherts and iron-rich schist.The formation of hydrated ores is directly related to deep-seated zones of oxidation and the ancient crust of weathering. These ores were formed by disaggregation of iron silicates and carbonates, oxidation of iron, and formation of fine-grained hematite, goethite, and hydrogoethite, as well as by the magnetite to martite transformation. Mining to SOO m depth and exploration boreholes to depths of 1,500-2,400m enabled us to get an idea of the depth and character of the oxidation processes. The most conspicuous development of supergene processes is observed in the middle suite rocks-in iron-richschist and chert. The supergene alteration of the middle suite rocks manifested itself non-uniformly.In some areas it reached depths of only several tens of metres, while in others it reached more than 2,000-2,400 m . The surficial crust of weathering is widely developed and was located by numerous boreholes throughout the territory of the iron ore basin.The depth of the crust varies widely: in slate horizons from 15-25 to 80-100 m;in ironrich zones from 20-35 to 150-200 m. The vast body of evidence accumulated in recent years makes it possible to construct geologic maps and sections of deep-seated oxidation zones in the Saksagan band at depths of 100 and 500 m from the Precambrian surface. In ten most characteristic sections of the Saksagan band both the crust of weathering and deep-seated oxidation zones may be observed. At present the linear zones of oxidation are traced with the help of deep structural drilling to depths of 1,3001,400 and even 2,400 m . However, the lower boundary of the oxidized rocks for most horizons has not been established. The depth of the supergene alteration of rocks seems to exceed 2,000-2,500 m . Manifestations of deepseated oxidationwithin the Saksagan band of the Krivoyrog iron ore basin are confined to zones of cross-folding,the contact of the upper and middle suites and fault zones. Linear zones of oxidized rocks are most abundant in iron horizons of the synclinal structure in the area. The deep-seatedzones,including severalstratigraphichorizons, strike in the meridional direction, following the principal trend of the structures in the area; at greater depths they plunge in conformity with folded and joint structures. The width of the oxidation zones ranges from several tens to 500-600 ni; in the south of the Saksagan area, in the Dzerzhinsky and Kirov mines, it reaches 1,000to 1,200m . The length of individual deep-seated zones ranges from 24 km in the north to 10 kni in the south of the area. The zones are separated by areas of unaltered rocks 200 m wide in the central part of the area and 1,000to 1,100m wide in the north. Within the Saksagan area five deep-seated zones of highly oxidized rocks are distinguished in the following

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mines: the first zone, in the area of the Lenin mine; the second zone, in the Rosa Luxembourg mine area; the third one, the XX Party Congress mine; the fourth zone, the Komintern and Bolshevik mines; the fifth, the Karl Liebknecht, Dzerzhinsky and Illych mines. The factual data suggest that the effect of the supergene processes lias generally manifested itself in a radical alteration of hypogene magnetite and silicate-magnetite ores and in the formation of the goethite-hematiteores and ‘shelestukhas’ldue to decompositionof iron chert,jaspilile, and schist. Percolation of surface waters downwards through the beds in the most jointed zones reached greater depths than in the host rocks and naturally caused a greater alteration in the hypogene ores. As a result of the oxidation of the magnetite ores, massive martite ores originated. In the process of oxidation of the magnetite ores, their texture, form,and dimensionsofindividualminerals did not change. The only change in the mineral composition due to oxidation of the magnetite ores is inartitization of magnetite. During the next and the more vigorous stage of development of supergene processes, porous martite ores were formed by the removal of silica. Comparison of the chemical composition of massive and porous ores has shown that the porous ores resulted from the complete removal of quartz from compact martite ores; this removal of quartz was not compensated for by any introduced material. This brought about a change in the porosity from 4-5 per cent in massive ores to 2530 per cent in porous ores. In the process of formation of porous ores, the content of quartz decreased from 1526 per cent in massive ores to 0.5-8 per cent in porous ores where oxidation of magnetite was more vigorous (confirmed by a decrease in bivalent iron to 0.6-0.7 per cent). The removal of silica caused a considerable increase in the iron content in ores; massive ores contain 75-80 per cent Fe,O,, while porous ores contain up to 97-98 per cent. The silicate-magnetite ores in supergene conditions have undergone considerable changes,which were revealed as alterations of iron silicates and carbonates and the oxidation of magnetite. During this process a considerable removal of Ca, M g , AI and Si took place. Together with decomposition of silicates, carbonates, and the removal of individual elements during supergene alteration of silicate-magnetiteores,fine-grainedand dispersed hematite originated, which caused notable enrichment in iron of the goethite-hematite-martite ores. In the second case the processes of weathering resulted in the formation of a new supergene type of ore at the expense of disaggregation of jaspilite and schist (‘shelestukhas’). Consequently, the supergene processes played an essential role in the formation of porous martite and goethite-hematite-martite ores as products of alteration in the magnetite and silicate-

1. In the Krivoyrog basin ‘shelestulchas’are leach jaspilites, from which a considerable part of SiO? is taken out, and residual quartz is transformed into a mealy substance. According to their content of iron (4552 per cent), they are related to iron-rich ores.

Genesis of high-gradeiron ores of Krivoyrog type

magnetite ores,as well as in the formation of 'shelestukhas' and goethite-hematiteores. A study of the relationships existing between various types of ores suggested a general scheme of their emplacement, with magnetite ores as primary (original) ores and all martite and silicate-martiteores as products of their subsequent alteration. Magnetite, silicate-magnetite,compact and friable martite, and soft hydrated ores are intimately interrelated,each type representing a certain stage in the development of the inineralization process. All ore deposits,irrespective of their stratigraphic or structural position, mineral or chemical composition, are epigenetic with respect to iron-silicate and cherty-iron sediments of the Krivoyrog series. Thus, the ore deposits were formed essentially during the secondary enrichment of iron-richrocks. A n exception is furnished by interbeds and nests, non-considerablewith regard to their volume and relatively rich in ore,which are found among jaspilites of sedimentary-metamorphic origin. The following two main processes of ore formation may be distinguished within the Krivoyrogbasin:hypogene, which is connected with the process of magnetite and silicate-magnetiteore bodies;and supergene,embracing the formation of goethite-hematite,partially goethite-hematitemartite ores and 'shelestukhas', as well as transformation of massive magnetite ores into porous and friable ones. The subdivision of ores into two genetic types (hypogene and supergene) is quite arbitrary,since the formation of ores is a very complicated historical process consisting of a consecutive and multistage evolution-transformation of rocks and ores from the period of sedimentation up to present mineralization in the crust of weathering. A historical sequence is as follows:sedimentation,diagenesis, dynamo-thermalmetamorphism, supergene alteration (associated with the ancient crust of weathering and,finally, Pre-Tertiaryand present supergene alteration). In different deposits the above processes manifested themselves in a number of ways. The distinctive features of composition,texture and modes of occuri-enceof the Krivoyrog deposits and other similar world deposits of Brazil, Canada and Australia are so original that they cannot be classed with any one of the known genetic types of iron ores (magmatic,hydrothermal). Among the main features are these: mineral composition of the ores is similar to that of the host rocks; the ores contain the same chemical elements as their enclosing rocks; the ore deposits are distributed in folded-jointed structures; metasomatic processes of mineralization are predominant; no alteration is observed in the rocks adjacent to the ores; a strict combination in time and space of the mineralization process with folding;absence of zoning in the mineral paragenesis; absence of either spatial or time dependence of the deposits on intrusive rocks. The cited featuresof ores and ore deposits make it possible to treat them as metamorphogene,originating in the process of dynamo-thermal metamorphism of iron-rich cherty rocks,

It is common knowledge that the main agents of metamorphism and, consequently, of mineralization are hydrostaticand unilateral pressure,temperature,and chemical activity of water solutions (Turner,1962). These agents cause recrystallization of rocks, considerable removal of the material,and metamorphic differentiation accompanied by rearrangement and concentration of certain metal components. In order to determine the principal aspects of the metamorphogene process of iron ore development, the following items will be briefly discussed:the nature of oreforming solutions; causes and routes of the solutions; sources of iron and forms of its transport; causes of the material precipitation during mineralization. Dehydration of primary sediments takes place during metamorphism. A sedimentary rock that has undergone the stage of diagenesis,still contains a considerable percentage of water present in two forms: free water in pores and coatings on the rock fragments and combined water within the rock material proper. According to Strakhov, the content of moisture in sand is 20-25 per cent; in siltstone, 30-60 per cent; in pelite ooze, 60-80 per cent. Much water is usually present in chemical and colloid sediments, as well as in the hydrates of silica, ferric oxide, iron silicate, etc. The most widespread minerals in sediments contain the following percentages of water: nontronite, beidellite,and montmorillonite,13.6 per cent; kaolinite, 13.96 per cent; hybbenite, 34.65 per cent, vermiculite, 19.96 per cent. This water was discharged from the rocks during lower, middle, and upper stages of metamorphism. Water, other than combined, has been isolated at temperatures up to 100"C. Hydroxyl water (combined) is isolated at temperatures higher than 300"C; from kaolinite it is discharged at 400-525" C;from montmorillonite, at 500-800" C; from hydromica, at 300600"C;from diaspore, at 5.50"C; from brucite, at 400500° C; and from chlorites at different temperatures up to 600-800" C. Most of this water is used for the formation and supplementationof metamorphic hydrothermal solutions. In the metamorphic rocks of Krivoyrog the percentage of water is commonly 1-2 per cent and seldom reaches 34 per cent. Enormous water masses must have been liberated in the process of dehydration and recrystallizationof the rocks. A colossal amount of metamorphic rocks in the Krivoyrog during metamorphism must have supplied more water than could be contained in magmatic rocks of the same volume. If 30-35 per cent by volume of water was contained in the Krivoyrog sediments that had undergone diagenesis, it implies that every 100 m3 of metamorphic rock discharged about 50 msof water. The total volume of water released during metamorphism is roughly estimated to have been half of the present volume of all the Krivoyrog basin rocks. This water promoted dissolution and recrystallization of the rocks under metamorphism and transported mobile components into areas of tectonic injection due to the inflow of metamorphic solutions.Thus the sources of metamorphic solutions were 175

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supplied by sedimentary and volcanic rocks that were undergoing metamorphism. They were produced at all stages of progressive metamorphism through the release of free and combined water from various minerals. The locations of ore deposits are likely ways for niovement of the metamorphic solutions. In the conditions of the Krivoyrog basin, ore deposits are confined to foldedjointed structures, through which ore-bearing solutions percolated and where they left iron. Numerous observations in mines, exposures and quarries, as well as studies on polished samples and sections,point to a closeconnection between ore mineralization and fine-jointingand porosity, and to ail absence of ore emplacements in large fractures and faults. Joints,clearly developed in the Krivoyrog rocks and ores are always post-orewith no ore mineralization within. The deposition of ores is confined to areas of optimum porosity and finejointing that resulted from the formation of folded structures. Consequently, folded-jointedzones favoured movement of ore solutions. In the Saksagan area the ore solutions became most mobile in the zone of a complex folded-jointed core of the Saksagan syncline in places of shearing strain of rocks. From these main channels the solutions branched off through transverse folded-jointedzones produced by individual flexures or gentle bends. Under such conditions, the movement of metamorphic solutions is confined to folded zones, where the most vigorous movement of the material took place accompanied by lamination and later by fine-jointing.Such a favourable combination is readily matched by an iron-oresuite,characterized by an alternate sequence of iron-quartzcherts and fine-grainedshales.The solutions squeezed first out of argillo-siltylayers and then also from the shales. These, together with those released through the draining of the iron-richcherty rocks proper, could not move within the iron-rich chert zones shielded by water-tight slates. Since the waters within each ironrich chert zone were separated from other zones, they could not mingle,with the result that more or less isolated mineralization zones were produced.Thus the ore-forming solutions moved through folded-jointedzones in iron rocks from the places of tectonic compression to areas of folding where tectonic stresses were relieved and conditions of lower pressures set in. The amount of iron transported for the formation of high-grade iron ores does not seem too impressive. Ironrich chert and jaspilite that turned into ores had previously contained ail average of 35-37 per cent iron, while in the ores the figure became 55-60 per cent. Ore deposits occupy as much as 4 per cent of the area of all iron-richrocks in such a productive areas as the Saksagan region. This implies that the supply of iron for the formation of highgrade ores amounted to no more than 2 per cent. Based on the actual geologic conditions, there are no data that might account for the addition of iron into the Krivoyrog iron-ore suite from magmatic intrusions of from other outside sources. It is postulated that the source of iron for the mineralization was an iron ore suite. This is cor176

roborated both by field and laboratory evidence: (a) transport of the material during mineralizationtook place within the iron ore suite and even within individual suitable horizons; (b) intimate spatial mineralogical and structural interrelations of iron ores and iron-richchert and jaspilite. Insoluble iron compounds are produced at high valency and are represented by oxides and hydroxides. Transformation of bivalent into trivalent iron takes place under the effect of free oxygen or interaction between bivalent iron minerals or oxygen-containing solutions. Essential causes of ore deposition include tectono-physical conditions (pressure regime, temperatures, and gaseous components of the solutions). Genesis of high-gradeiron ores of Krivoyrog type is considered as a natural-historicalprocess of irondeposition, consisting of progressive primary sedimentation, metamorphism and supergene processes. The earliest process ofiron depositionimplies sediineiitation and diagenesis of the iron-rich cherty material, which laid down the foundation of all iron-richrocks and a certain part of iron ores. The source material for the iron ore suite was derived from crystalline rocks of the Precambrian:gneisses,metabasites,ultrabasites,migmatites and granites. Sedimentation took place under the conditionsofgeosynclinalregimeoftheKrivoyrog-Kremenchug sub-geosyncline.Sedimentationis characterizedby a single transgressive cycle covering the formation of effusive and coarse-grainedrocks of the lower suite, chemogenic products of the middle suite, and clastogene material of the upper suite. Diagenesis resulted in crystallization of sediments as well as in its lithification. This period is characterized by the formation of hydrous minerals, such as chalcedony, opal, hydromica as well as chlorite, siderite, hematite, and pyrite. The second stage of iron concentration in rocks, culminatingin the formation of high-gradesores,is related to dynamo-thermalmetamorphism that manifested itself through the formation of the Krivoyrog folded structure. Folding, plastic flow, and lamination of iron-chertysediments caused heating up and circulation of metamorphic solutions,which resulted in the transport of iron, silica, magnesium, sodium, calcium, and aluminium, as well as in the recrystallizationof rocks and the formation of various mineral associations. Such mineral associations as quartz-magnetite, quartz-magnetite-siderite,sericite-biotite-quartz, chlorite-biotite-siderite-quartz,chlorite-magnetite, cummingtonite-magnetite-quartz,aegirine-quartz, hematite-magnetite-quartz,and others are characteristicof rocks of the middle suite. During the mentioned period the iron-richcherty sediments were transformed into crystalline iron-rich chert and shale. In areas of development of folding (mainly cross-folding) and fine-jointing,where metamorphic solutions were in vigorous circulation, the transport of rock components,primarily iron and silica, resulted in the formation of high-grade magnetite and hematite-magnetite ores.The tectonic compression active in certain portions of iron complexes brought about instability of quartz, its

Genesis of high-gradeiron ores of Krivoyrog type

dissolution and removal from the compressed zones. Ultimately, as a result of a relative enrichment in iron,highgrade ore bodies were formed. This period includes oreforming magnesian-ironand iron metasomatism that seem to have developed by the end of the metamorphic transformationsofrocks,reflecting a genetically regressivestage. T w o genetic types of ores pertaining to the metamorphic cycle are distinguished metamorphic iron ores making up the middle suite deposits; and metamorphosed high-grade iron ores occurring at the base of the upper suite. Supergene alteration was the third process of ore formation and modification, and it was responsible for the formation of deep-seated zones of oxidation of iron-rich rocks and high-grade iron ores. Supergene processes brought about considerable transport of iron, silica, and other elements, which caused modification of compact magnetite ores into friable higher-grade and chemically

pure martite ores, formation of martite ores in jaspilite, goethite-hematite-martiteores in silicate-ironchert,goethite-hematiteores in iron-silicateshale,etc. The data from geological observations are corroborated by experimental results and theoretical studies conducted by the Ukrainian Academy of Sciences. Exploratory boreholes established that high-gradeiron ores of the Saksagan area extend as far down as 2,0002,400 m. Despite a certain decrease in thickness and worsening in quality of ores with depth due to a gradual dying out of supergene processes, the ore bodies at a depth of about 2,000m are of substantial thickness and are composed of high-grade ores. This serves as the basis for an optimistic evaluation of the mineralization extent down to an ultimate depth range of the iron-ore suite in the Saksagan syncline lower band. The geophysical data give this depth in the central part as 3.5 km;in the northern part, up to 5 k m .

Résumé Genèse des minemis de fer ii huute teneur de ICrivoyrog

(Y.N.Belevtsev) Les minerais de fer à haute teneur du type de Krivoyrog résultent d‘un processus chronologique naturel d’accumulation du fer, consistant en des processus successifs de sédimentation primaire, métamorphique et hypergène. La sédimentation et la diagenèse des sédiments de fer siliceux qui sont à la base de toutes les roches ferrugineuses et d’une certaine partie des minerais de fer à haute teneur sont considéréescomme les processus les plus anciens d‘accumulation du fer. Les roches cristallines du Précambrien fournirent les matériaux pour la série des minerais de fer : gneiss, métabasites, ultrabasites, migmatites et granites, L’accumulationdes sédiments s’est produite à la faveur du régime géosynclinal du subgéosynclinal Krivoyrog-Kremenchug. La sédimentation est caractérisée par un cycle unique, transgressif, recouvrant la série inférieure des roches effusives, les matériaux chemogéniques de la série moyenne et les roches plastogenes de la série supérieure.Le résultat de la diagenèse a été la formation minérale, le réarrangement et la recristallisation du matériel et sa consolidation,c’est-à-diresa lithification.L a formation de minéraux hydriques tels que la calcédoine,l’opale,l’hydromica aussi bien que la chlorite,la sidérite,l’hématiteet la pyrite est caractéristique de cette période. Le second stade de concentration de fer dans les roches, qui a atteint son plus haut point lors de la formation des minerais à haute teneur, est associé avec le métamorphismedynamico-thermique,résultat de la formation de la structure plissée de Krivoyrog. Les plissements, le flux plastique et la formation en bancs des sédiments des silex ferrugineux ont causé le réchauffement et la circulation de solutions métamor-

phiques, avec pour conséquence la migration du fer, de la silice, du magnésium, du sodium, du calcium et de l’aluminium,la recristallisation des roches et diverses parageneses minérales. Les roches de la série moyenne sont caractérisées par des parageneses telles que quartzmagnétite,quartz-magnétite-sidérite, séricite-biotite-quartz, chlorite-biotite-sidérite-quartz, chlorite-magnétite, cummingtonite-magnétite-quartz, aegirine-quartz, hématitemagnétite-quartz, etc. Durant cette période, les sédiments de silex ferrugineux ont été transformés en cornéennes ferrugineuses cristallines et en schistes.Dans les zones de plissement actif (essentiellement transverses) et de jointements fins, OU les solutions niétamorphiquesétaient en circulation intense,la migration des composants rocheux, surtout de fer et de Sioz,ont eu pour résultat la formation de minerais à haute teneur de magnétites et d’hématite-magnétite. L a compression tectonique qui s’est exercée sur certaines portions des masses ferrugineusesse traduit par l’instabilité du quartz, causant sa dissolution et son évacuation des zones de compression;plus tard,dans ces zones,des gisements de minerais à haute teneur se sont formés à la suite de la concentration du fer. On distingue deux types génétiques de minerai associés avec le cycle métamorphique : (a) les minerais de fer métamorphiques qu’on rencontre dans les dépôts de la série moyenne; (b) les minerais de fer à haute teneur métamorphosés qu’on rencontre dans la région la plus basse de la série supérieure. Le troisième processus de formation du fer et d’altération a consisté dans une hypergenèse qui est à l’origine des zones profondes d‘oxydation de roches ferrugineuseset de minerais de fer à haute teneur. L‘hypergenèsea causé des migrations très significatives 177

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de fer, de Siozet d‘autres éléments. L e résultat en a été la transformation de minerais de magnétite compacte en des minerais de martite difîus de plus haute teneur et chimiquement pure,La formation de minerais de martite dans les jaspilites, la formation de minerais de goethitehématite-martitedans les schistes ferrugineux silicates,etc. Tous ces processus d‘accumulation du fer dans les sédiments et dans les roches sont traités chronologiquement dans leur développement successif depuis la période de sédimentation jusqu’à l’état actuel. Les résultatsdes observationset des étudesgéologiques sont confirmés par des études expérimentales et théoriques de 1’Académiedes sciences d’Ukraine. Les puits d’exploration ont permis d’établir que les

minerais à haute teneur de la région de Saksagans’étendent à une profondeur de 2 O00 à 2 400 mètres avec une épaisseur un peu plus faible et une qualité moindre du minerai au fur et à mesure qu’on s’enfonce, en raison de l’atténuation graduelle des processus hypergènes. Les minerais à une profondeur d‘environ 2 O00 mètres sont suffisamment épais et d’une haute qualité. Cela pourrait fort bien servir de base pour une évaluation optimiste d‘une extension possible de la minéralisation jusqu’aux plus grandes profondeurs de la série des minerais de fer dans les régions plus profondes de la charnière inférieure de Saksagan. Les données de la géophysique indiquent que la couche a une extension en profondeur de 3 3 kilomètres dans la partie centrale et de 5 kilomètres dans la partie nord.

Bibliography/Bibliographie ANDREATTA, S. 1954. Stoffrnobilisierung und Stoffbewegungen bei der tectonischenMetamorphose.Mineral. Mon., no. 1-2. BELEVTSEV,Y.N. 1951. Tipy rudnykh polei Krivorozhskikh zhelezorudnykh mestorozhdenii i soobrazheniya o genezise zheleznykh rud [Types of ore fields of Krivoyrog iron-ore deposits and considerations about genesis of iron ores]. Bidl.Acad. Sci. U.R.S.S., Geology series,no. 2. --. 1953. Proiskhozhdeniye zheleznykh rud Saksaganskogo raiona Krivogo Roga [Origin of iron ores in Saksagan region of Krivoyrog]. Geol. zhurn. Alcad. Nauk. U.S.S.R., vol.XIII, no. 3. --. 1955. Geologicheskaya struktura i metallogeniya Krivorozhskogo zhelezorudnogo basseina [Geological structure and metallogeny of Krivoyrog iron ore basin]. Geologiya i genesis rud Krivorozhskogo zhelezorudnogo basseina. Moscow, Academy of Sciences of the U.S.S.R.(Trudy soveshchaniya.) ; DUBINKINA, R.P.1952.Plotnyemartito-gematitovyerudy iz Saksaganskogo raiona Krivogo Roga [Compact martitehematite ores from Saksagan region of Krivoyrog]. C.R. Acad. Sci. U.R.S.S., vol. 96,no.2. BETEHTIN, A. G.1954. O metamorfìzovannykh mestorozhdeniyakh margantsa [On the metamorphosed depositsof manganese]. C.R.Acad. Sci. U.R.S.S., vol,XLXI,no.1. DOBROHOTOV, M. M. 1954. K voprosu o genezise bogatykh zheleznykh rud krivorozhskogo tipa [On the problem of genesis of rich ores of Krivoyrog type]. Razvedlra i ohrana nedr., no. 1. DUNN, J. A. 1941. The origin of the banded hematite ores in India. Econ. Geol., vol. 36, no. 4. FAIF,U. C.;FERHOOGEN, J. 1962. Total thermodynamical considerations. Metarnorficheslciye realctsiyi i melamorficheskiefatsiyi. p. 42.Moscow,Editions of Foreign Literature. FEDORCHENKO,V. S,1955.K voprosu o genezise ‘kraskovykh’ rud Krivorozhskogo basseina [On the problem of genesis of ‘coloured’ores of Krivoyrog basin]. Mineral. sb. LGO, no. 9. GERSHOIG, Y.G.1951. O prirode rudnogo minerala tak nazyvaemykh ‘kraskovykh’rud Krivorozhya [On the nature of ore mineral of so called ‘coloured’ores of Krivoyrog]. Mineral sb. LGO,no. 5. (Annals of the Leningrad GeologicalSociety.) . 1955. Genezis rud Krivogo Roga [Genesis of ores of Krivoyrog]. Geologiya i genesis rird Krivorozhskogo basseinu.

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Moscow, Academy of Sciences of the U.S.S.R. (Trudy soveshchaniya.) . 1957. Protsessy obrazovaniya zhelezorudnoi formatsii i zalezhei bogatykh rud Krivorozhskogobasseina [Processesof the formation of iron ore structures and depositsof rich ores of Krivoyrog basin]. Bull. Acad. Sci. U.R.S.S., Geology series,no. 10. GINZBURG, I. I. 1955. O gipergennykh protsessakh v Krivorozhskom basseine [On the supergene processes in Krivoyrog basin]. Geologiya i genesis rud Krivorozliskogo zhelezorudnogo busseina. Moscow, Academy of Sciences of the U.S.S.R.(Trudy soveshchaniya.) GRECHISNIKOV, N.P. 1955. K voprosu o genezise zheleznykh rud Saksaganskogo raiona. [On the problem of genesis of iron ores of Saksagan region]. Geologiya i genesis rud Krivorozhskogo zhelezorudnogo basseina. Moscow, Academy of Sciences of the U.S.S.R.(Trudy soveshchaniya.) GRUNER, J.W.1926.Magnetite-martite-hematite. Econ. Geol., vol. 21. __ . 1930. Hydrothermal oxidation and leaching experiments: their bearing on the origin of Lake Superiorheniatite-limonite ores,ECOJZ. Geol.,vol. 25. GUILD, P. W. 1953. Iron deposits of the Congonhas district, Minas Gerais,Brazil,Econ. Geol.,vol.48,no. 8. HIETANEN, A. 1954. O n the geochemistry of metamorphism. J. Tenn. Acad. Sci., vol. 29, no. 4. JAMES,H.L. 1953. Origin of the soft iron ores of Michigan, discussion.Econ. Geol., vol. 48,no. 8. KANIBOLOTSKY, P.M . 1941.K voprosu o geneziserud Krivogo Roga [On the problem of genesis of ores in Krivoyrog]. Dnepropetrovsky gos. in-t,vol. XXVII,no. 2. , 1946. Petrogenez porod i rud Krivorozhskogo zhelezorudnogo basseina, Chernovtsy [Petrogenesis of rocks and ores of Krivoyrog iron ore basin. Chernovtsy]. Moscow, Academy of Sciences of the U.S.S.R. KING, B. C.1954. Metasomatism in petrogenesis.Sci.Progr., vol. 42,no. 167. KORZHINSKY, D . S. 1954.Problemy izucheniyaKrivogo Roga i Kurskoi magnitnoi anomalii[Problems of studing of Krivoyrog and Kursk magnetic anomaly]. Geol. zliurn. Alcacl. Naulc. U.S.S.R., vol. XIV,no. 4. . 1955. Svyaz’bogatykh rud Krivogo Roga s protsessami

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Genesis of high-gradeiron ores of Krivoyrog type

kory vyvetrivaniya [Connection of rich ores in Krivoyrog with the processes of weathering crust]. Zlielezistye kvartsity i bogatye zlleleznye rsrdy Kirrskoy rnagnitnoy anomalii. Moscow, Academy of Sciences of the U.S.S.R. KOTLYAR, V.N.1953.O genezise zheleznykhrud Krivogo Roga [On the genesis of iron ores of Krivoyrog]. Gorny zhurn., no. 12. MANN, V.J. 1953.The relationof oxidation to the origin of soft iron ores of Michigan.Ecori. Geol.,vol. 4. MARTINENKO, L. I. 1950. K voprosu ob obrazovanii bogatykh rud Krivogo Roga [On the problem of the formationrich ores in Krivoyrog]. Uclzen.zap. Chernovitskogo in-ta,vol.8, seriya geol. geogruph. nasrlc, vol. 2. . 1955. Rol’ gipergennukh protsessov v obrazovanii rud Saksaganskoi polosy Krivogo Roga [Role of supergene processesinthe formationofores in SaksaganzoneofKrivoyrog].

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Geologiya igenesisrud Krivorozhskogo zhelezorirdnogo basseina.

Moscow, Academy of Sciences of the U.S.S.R.(Trudy soveshchaniya.) POLOVINKINA, Y.I. 1956.K voprosu o proiskhozhdenii zheleznykh rud Krivogo Roga [Onthe problem of the origin of iron ores of Krivoyrog,] Inform. sb. VSEGEI,Leningrad, no, 3. RAMDOHR, P. 1953. Uber Metamorphose und Secundare Mobilisierung. Geol. Rdsch., no. 42. ROBERTS, H.M.; BARTLEY, H.W.1943.Hydrothermal replacement in deep seated iron ore deposits of the Lake Superior Region. Econ. Geol.,vol. 38. SAKAMOTO, T.1950.The origin of the pre-Cambrianbanded iron ores. Amer. J. Sci.,vol. 248. SEMENENKO, N. P.1955.Sostoyaniye i zadachiizucheniyageologicheskoi istorii, genezisa rud i porod, a takzhe struktury mestorozhdeniiKrivorozhskogobasseina [Conditionand tasks of studing geological history, genesis of ores and rocks and

also structures of deposits of Krivoyrog basin]. Geologiya i genesis rud Krivorozhskogo busseina. Moscow, Academy of Sciences of the U.S.S.R. (Trudy soveshchaniya.) STARITSKY, Y. G. 1954. Genezis rud Saksaganskogo raiona Krivorozhskogo basseina [Genesis of ore of Saksaganregion of Krivoyrog basin]. Geol. zhsrrn. Akad. Nuuk. U.S.S.R., no. 3. SVITALSKY, N,I.1924.ZhelezorudnyemestorozhdoniyaKrivogo Rogai genezis ego rud [Iron ore deposits of Krivoyrog and genesis of its ores]. Izv. Geollcoma, t. 43, vol. I. TANATAR, I.I.1916.Nekotoryesoobrazheniyao geneziseKrivorozhskikh zheleznykh rud i vklyuchayuschikh ikh zhelezistykh kvartsitov [Some considerationsabout genesis of Krivoyrog iron ores ferriferous quartzrocks including]. Uzhny inzhener, no. 7-8. . 1926. Noveishiye vzglyady na proiskhozhdeniye poloschatykh zhelezistykli kvartsitov v svyazi s voprosami proiskhozhdeniya etikh porod i rud v Krivorozhskom basseina [The latest views on the origin of banded ferriferous quartz rocks in the connectionwith the problem of the origin of these rocks and ores in Krivoyrog basin]. Inzlienerny rabotiiilc., no. 1. TOCHILIN, M.S. 1953. O genotioheskikhvzaimootnosheniyakh mezhdu bogatymi rudami K M A i Krovogo Roga [On the genetic relations between rich ores of KMA and Krivoyrog]. Miizeral. sb. LGO.,no, 7. TOKHTUEV, G.V. 1955. K voprosu o genezise zheleznykh rud Krivorozhskogo basseina [On the problem of genesis of iron ores in Krivoyrog]. Zheleznye rudy KMA,Moscow,Academy of Sciences of the U.S.S.R. TURNER, J. 1962.Teaching on the metamorphic facies. Metamorjiches-kive reaktsiyi i metamorfickeskiye fatsiyi, p. 11. Moscow,Editions of Foreign Literature.

Hiscussion R. T. BRANDT. Where supergene alteration is found to extend to depths of 2,500m , is this alteration the result of present-dayground-wateractivity,or is it related to ancient, Precambrianweathering cycles?

S. J. SIMS.Does goethite ever occur in the supergene ores derived from iron-chertsediments?

Y.N.BELEVTSEV. Goethite nearly always occurs in ores formed from iron silicates (chlorite, biotite, etc.).

Y. N. BELEVTSEV. Supergene alteration in deep-seated oxidation zones took place after all the tectonic processes

S. J. SIMS.Is there any relationship between the goethite

and is of Precambrian or,possibly, of Palaeozoic age.

content and the depth below the surface?

R.T.BRANDT. Is the aerial crust of weathering,which has been located by numerous boreholes throughout the iron ore basin, a recent weathering crust or a Precambrian one?

Y.N.BELEVTSEV. In oxidation zones the goethite-containing ores disappear at depths of 400-600m.

P. M.GORYAINOV. Are rich aluminosilicate ores ever encountered outside iron quartzites?If so,how far outside?

Y.N.BELEVTSEV.I think it is Precambrian. Y. N. BELEVTSEV. High-grade aluminosilicate ores are G.CHOUBERT. What are the specificweights of the compact and the porous ores?

Y.N.BELEVTSEV. The specific weight of the porous ore is about 4.0to 4.5;the specific weight of the compact ore is somewhat lower-about 3.7 to 4.0.

generally formed from schists. They are most commonly encounterednear their contacts with ferruginoushorizons. P. M. GORYAINOV. What is the occurrence of high-grade ores in the magnetite quartzites of Krivoyrog?

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Y.N.Belevtsev

Y.N.BELEVTSEV.Magnetite ores are widespread in the magnetite cherts of northern Krivoyrog. P.M.GORYAINOV. As high-gradeores are mostly abundant in cross-foldingzones (secondary folding), their formation must have taken place in the presence of rocks already metamorphosed (primary folding). Therefore, at the m o -

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ment of ore formation the rocks contained little water. D o you not find a contradiction in the mineralization process model that you suggest?

Y.N.BELEVTSEV. The transverse structures are contemporaneous with the longitudinal,and belong to the same stage of folding and metamorphism.

Effusive iron-silica formations and iron deposits of the Maly Khingan E.W.Egorov and M.W.Timofeieva Far East Geological Service, Ministry of Geology of the Russian Soviet Federated Socialist Republic Khabarovsk,R.S.F.s.R.

The iron-silica formations of the Maly Khingan stretch for about 150 k m ,from a zone 40 km north to 10-12km south of the river Bidjan, southward to the Amur river (Fig. 1). The sedimentary complex known as the Khingan series has a total thickness of about 7.5 km,and is composed of terrigenous-carbonate rocks, highly metamorphosed in the lower part of the complex and weakly in the upper part. Iron-silicaformations and iron ore of Lower Cambrian age are situated in the upper part of the series between dolomite of the Murandavskaya suite below and schist of the Londokovskaya suite above. Rocks of the Khingan series are in isoclinal folds with steep limbs, complicated by transverse and longitudinal faults which break the whole sequenceinto isolated blocks. Beds of iron ore are preserved in downthrown blocks and in the cores of big synclinal structures. Iron ore crops out in narrow isolated discontinuous belts that trend southerly, concordantwith the general strike of the rocks. The Maly Khingan ore-bearing sequence is divided into three parts: underore, ore-bearing,and overore. The ore-bearing part consists of ferruginous and ferruginousmanganiferous quartzites. The ferruginous quartzites are banded magnetite, magnetite-hematite,and hematite. The ferruginous-manganiferous quartzites contain assemblages of silica-carbonate,braunite-hausmanite, braunite-hematite, and other iron and manganese minerals. Rocks bordering the belts of iron ore consist of volcanic and sedimentary-volcanic schists formed by lowtemperature regional metamorphism (greenschist facies). The majority of these schists are developed from tuffs and tuffites of basic and intermediate composition and may contain chert-hydromica,chert-chlorite,carbonaceous carbonate, carbonate-mica,and so on. Less important parts of the wall rock consist of expanded breccia of carbonate and carbonate-chlorite(schistoseksenotuffsofthe Schalstein type). Rarely there are interlayers of sedimentary carbonate rocks in the volcanic and sedimentary-volcanicschists. In the north-west part of the Maly Khingan mining district there are concentrated deposits consisting essen-

tially of magnetite. Here, in a single belt of outcrops 40km long,divided only by Cretaceouseffusives,are three iron deposits (Kostenginskoie, Sutarskoie, Kimkanskoie) with total ore reserves of about 1,000million tons. Seventy per cent of these are of magnetite quartzite and 30 per cent are of magnetite-hematite quartzite.Industrialmanganese ores are practically absent in the Maly Khingan district. The Kostenginskoie deposit is distinguished from the other two in the district by relatively weak tectonic and metamorphic features. The ore field of this deposit is 14 km long and its width, after unfolding,is 3.5 k m . Iron deposits end by the splitting and pinching out of beds of ore, by a decrease in iron content,and by passing into green breccia containing rare interbeds of non-ferruginous quartzite. In the Kostenginskoiedeposit the iron-bearingquartzites are between two sequences of massive unsorted and non-beddedvolcanic breccia,which rapidly pass into finergrained sediments near the iron ore. Interlayers of coarse clastic rock occur at the base and top of the ore body, but are absent in the middle. The ore is distinguished by horizontal banding. Graded bedding is observed in interlayers of fine-grained clastic tuff. All this confirms that the iron ore zone was deposited during an interlude in volcanic activity, in calm water. The uniform bedding and composition of the upper part of the Khingan series attest to the protracted existence of a vast basin in which carbonate deposition prevailed. At the beginning of deposition of the ore-bearing suite, no terrigenous material was being carried into the basin and a thickness of 500-1,000m of pure chemical dolomite of the Murandavskaya suite had been deposited. Sudden volcanic activity retarded the normal process of deposition and produced volcanites of the ore-bearing suite during a short episode, after which deposition of carbonate began again in the basin. Material of the ore-bearingrocks was transported to the basin of deposition by hydrothermal solutions and volcanic exhalations.W e observe channel-waysof solutions

Unesco, 1973. Genesis of Precanrbriaii iron and manganese deposits. Proc. Kiev Symp., 1970. (Earth sciences, 9.)

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E.V. Egorov and M.W.Timofeieva

FIG. 1. Map of geologicalfeaturesofthe Maly Khingan.1. Sedimentary-metamorphic complex of Proterozoic age; 2. Terrigenous-metamorphicsequence of Lower Cambrian(?) age,containingeffusiveiron-silicaformations;3.VolcanicrocksofLower and Middle Cretaceousage;4.Undivided effusive rocks of Cre-

taceous age and friablesedimentsofPalaeogeneage;5.Granitoid rocks ofPalaeozoicage;6.Faults;7.Main iron and iron-manganese ore deposits: I. Kimkanskoie;II. Sutarskoie;111.Kostenginskoie; IV. Kaylanskoie; V. Bidjanskoie; VI. Yujnokhinganskaya.

and exhalations in the form of hematitic microquartzitic silicified zones in places in the underlying dolomite. After volcanism and ferruginous sedimentation, the periodic deposition of tuffaceous material produced nonore layers-breccia and schist-containing chlorite and disseminated magnetite. In rock units consistiiig substaniially of hematite quartzite,interlayers of green brecciated tuff or schist are

not uncommon. Here, oreless layers of grey or brownishgrey jasper-like siliceous schist formed at the expense of fine-grainedclastic tuff having a silica content of 80-87 per cent.Banding ofthe hematite ore is broad,with thicknesses of 10-20 cm.With a decrease in the number and thickness of such noli-ferruginous interbeds iii hematitic quartzite, the iron content rises from 30-35 per cent to 35-40 per cent. The average content of iron in the ore is 25-30 per

182

Effusive iron-silicaformationsand iron deposits ofthe Maly Khingan

cent. Towards the hanging wall of the ore zone, ore solutions were depleted in iron, but still brought in a significant amount of silica which continued to accumulate after the deposition of the greatest part of the iron, giving rise to the non-ferruginousquartzite. Silica also was added to tuffs, tuffites, or chemical carbonate sediments above the ore-bearingzone. Iron introduced by solution into reducing environments in the basin led to the formation of sulphides. Siderite,magnetite, and hematite are present together through the entire cross-sectionof the ore zone without any sign that one mineral formed at the expense of the other. In magnetite quartzite, magnetite forms large crystals of idiomorphic habit which, where abundant in a bed, unite in polyhedral aggregates.Hematite forms dusty grains mainly in non-orebeds and in the intersticesbetween magnetite grains. Siderite may fill the interstices between iron oxide minerals or may form granoblastic growths with other carbonate and quartz in non-ore interlayers. N o oolitic, coiloinorphic,or concretionary structures are visible.These particular features indicate the independent formzttion of the different iron minerals. Differences in ironmineral content are considered to reflect gradual changes in the sedimentation environment caused by the mixing of hydrothermal solutions and volcaiiic exhalations with seawater. In the early stage of iron deposition, the alkaline environment in the basin played a significant role in the precipitation of magnetite and siderite. Continuous influxes of acid solutions from hydrothermal solutions and volcanic exhalations changed the p H of the sea-waterand under the new conditions hematite became more stable. Later,with a weakening of hydrothermalactivity,the basin environment was dominantly alkaline,which again led to the deposition of magnetite. Manganese and iron accumulated at somewhat different times. The highest content of manganese, 6-8 per cent, is registered in beds underlying beds of iron ore. Manganese-bearingsediments may be both schist and breccia. Manganese-richbeds may be at different levels, immediately under the ore or at some distance, stratigraphically,from the ore.Manganese-richlayers are composed of one or several lens-shaped silica-carbonateinterlayers, 0.1-0.5m thick, which are macroscopically indistinguishable from the wall-rocks. In the iron ore and in the upper layer of magnetite quartzite, manganese diminishes to 12 per cent. Among the manganese minerals are small quantities of rhodochrosite and manganocalcite and, very rarely, pyrolusite and psilomelane.Evidently the variations in manganese minerals depend directly upon the availability of COz. It is clear that most of the manganese enters complicated carbonates as parts of isomorphous mixtures. The manganese content increases by the enrichment of beds in pyroclastic material,especially of fine size. So,within the massive breccia beneath the ore zone there are sporadic lens-shapedinterlayers,enriched in manganese to 1-2 per cent,where manganese is concentrated mainly in the finestsize fractions of pyroclastic material.

Thus,the abundance of the coarse pyroclastic material in the Kostenginskoie iron deposit indicates deposition of the iron near volcanoes. T o the north on the strike of the ore-bearing suite, volcanic rocks become subordinate in the stratigraphic section at the Sutarskoie deposit, and give way to schist at the Kimkanskoie deposit. Unfortunately,the change ofiron ore facies in moving away from volcanic centres is difficult to trace across the Sutarskoie and Kimkanskoie deposits, which are located in contact zones of granite intrusions. The ore of these deposits is intensely metamorphosed and the primary proportions of hematite and magnetite are changed, with an increase in magnetite taking place. The ‘Southband’ district begins 25 Icm south-west of the Kostenginskoie deposit. The ore-bearinglayers of the ‘Southband‘ extend 40 km and constitute the Yujnokhinganskaya group of deposits. In the west part of the ‘South band’, magnetite ore predominates. Carbonate breccia occurs widely in the wall-rocks and in ore bodies. A bed of silica-rhodochrosite (manganese) ore, 1-3 m thick, is present at the base of the iron ore zone,but hausmanite-brauniteore appears at the base of some iron-ore zones of great thickness, with gradual transitionto the protoxide carbonate facies. By and large, the western part of the Yujnokhinganskaya group of deposits is very similar to the Kostenginskoie deposit. The eastern part of the Yujnokhinganskaya group differs sharply from the western part both by the character of ore and the lithology of the wall-rocks.Here, the orebearing zone contains two clearly separated beds, one of iron-manganeseat the base of the zone and one of iron ore, higher in the zone. The maximum thickness of the iron-manganesebed is 5-8 m and the minimum thickness of the iron ore bed is 18-20 m . The iron-manganesebed consists of banded hausmanite-rhodochrosite,hausmanitebraunite and braunite-hematite with an iron content of 8.6-11.0per cent and a manganese content of 19.7-21 per cent. The iron-ore bed consists almost completely of hematite interbedded with cherty jaspilite and fine-grained clastic chloritized carbonate breccia. Wall-rocks are composed of diverse schists with an admixture of carbonaceous matter and give way to carbonaceous and calcareous dolomites. Lens-shapedinterlayers of fine-grained clastic breccia are noted only in the underore zone. Thus, from west to east in the Yujnokhinganskaya deposit a change takes place from coarse clastic littoral sediments to deeper-waterdeposits.In the same direction, protoxide facies (magnetite and rhodochrosite) change to oxide facies (hematite and hausmanite-braunite), inverse to the distribution of such facies in normal sedimentary deposits,but according to Strahov (1965), a peculiarity of volcanic-sedimentary deposits. Therefore, the regular increase in the thickness and grade of manganese ore in the east part of the Yujnokhinganskaya deposits is attributed to the greater mobility of manganese than iron at a distance from volcanic sources. Such regularities are also observed in the ‘Eastern ore-bearing belt’ of the north part of the Maly Khingan 183

E.V. Egorov and M.W.Timofeieva

region,This belt, within which are seven deposits of iron and iron-manganese ore, is located 30 k m east of the Kostengiiiskoie deposit and is 50 k m long. In the north part of the ‘Eastern belt’, ore bodies consist of magnetite quartzite in which are rare members composed of magnetite-hematitequartzite. The thickness of ore bodies is 5-25 m . From north-east to south-west along the belt, the thickness of the ore zone increases and hematite quartzite appears. Still farther to the south, at the base of the ore bed, appear lens-shapedinterlayers of siliceous schist of a peculiar lilac colour, in which the content of iron and manganese increases southward. In the southmost part of the ‘Easternbelt’, ore bodies of the Bidjanskoie deposit form persistent beds, 40-50 m thick, but manganese beds, on the contrary, are extremely irregular. In a distance of about 100-200 rn their thickness may change from 3 to 25 m and their manganese content diminishes upward from 27.2 to 12.0per cent at the roof of the manganese-richbed. The highest content of manga-

nese-28-31 per cent-is found in the underlying so-called ‘lilac’schist. The ‘lilac’schist contains fine-grainedsemitransparent granoblastic quartz with dusty ore matter which is described by Mahinin (1952) as a mixture of hematite and hydrohematite.W e believe that these rocks formed from volcanic ash that had been enriched in lightweightparticles of acid composition by aerial differentiation. The enrichment of the cinders, in manganese up to 31 per cent and in iron up to 40-50 per cent, supports the possibility of aerial transfer of significantamounts of ore matter. It should be noted that the wall-rocks,especially those beneath the ore zone, consist almost entirely of uniform carbonaceous sericite-chertshale with an important content of angular fragments of quartz (up to 35 per cent) in cherty cement.Apparently,these rocks are also of volcanic origin.Thus,the Bidjanskoie deposit may be referred to as a volcanic-cherty formation, presumably owing a significant amount of its matter to aerial transfer.

Résumé L e s formations de fer siliceux efliisif et les gisements de fer. chi Maly Khingan (E. V. Egorov et M.W.Timofeieva)

Les formations de silex ferrugineux du Maly Khingan s’étendent sur une surface d‘environ 3 O00 k m 2 ; une bande de leurs affleurementss’étenddans la direction du méridien sur une distance d’environ 150 kilomètres. Au sud et au sud-estdu fleuve Amour, les minerais de fer d‘Aii’shan’sky (Chine) et les silex ferrugineux de la région de Lesozavodsk (Primorye) sont considérés comme étant leur continuation. Les formations de fer du Maly Khingan sont nettement stratifiées et peuvent toujours être distinguées dans la série de couches à minerais entre deux bancs épais de roches de carbonate terrigène. L’âge des séries à minerais semble appartenir au Cambrien inférieur,bien que cela ne puisse être admis sans réservepar suite du manque de déterminationsfaunistiques. Associée à la strate encaissante dans la structure complexe de plissements de rupture de la région et dans les stratifications au cœur des plissements synclinaux et des blocs affaissés,la série à minerais se manifeste à la surface sous la forme de bandes séparées qui s’étendent dans une direction méridienne. L a section de cette suite envisagée dans son ensemble est caractérisée par une structure stable à trois horizons : horizon inférieur ((( subore D) :schistes, tuf-schistiqueset tufites ; horizon moyen (N ore 1) : quartzite ferrugineuse et quartzite manganèse-fer avec des bancs de roches effusives ; horizon supérieur (K supra-oreN):schistes, tufites schisteuses,et, dans la partie supérieure :calcaires. Les roches qui contiennent ces séries,à l’extérieur des zones soumises à l’influence de l’intrusion,sont caractérisées par un faible degré de métamorphisme jusqu’au 184

faciès des schistes verts. Elles sont représentées par des argiles schisteuses siliceuses-séritiques,calcaires et carbonifères formées de roches finement fractionnées volcanoterrigènes, volcano-sédimentaires et sédimentaires et des tufites schisteusesgrossièrement ou finement fractionnéeset des tufs à composition basique. Les formations ferrugineuses dans l’horizon du minerai sont composées d’un banc unique de roches typique de la région entière :les silex ferrugino-rubanésabondent, alternent et s’imbriquentavec des tufites fractionnéesirrégulièrement et des tufs de composition basique. La puissance de l’horizon atteint de 30 à 60 mètres et elle est marquée par des fracturesséparées,par des dislocations et plus rarement par des coincements dans de petites régions. Les silex ferrugineux sont représentéspar des variétés de magnétite clastique, magnétite-hématite et hématites jaspées généralement observées conjointement. Les minerais deviennent essentiellement magnétiques avec de l’amphibole, du grenat et des biotites, qui apparaissent dans leur composition. L‘accroissement de la teneur en manganèse (en moyenne de 1 à 3 %)et l‘accroissementde la minéralisation du manganèse lorsque l’on s’enfoncesont typiques de l’horizondu minerai tout entier jusqu’àl’apparition d’une strate de ferro-manganèse épaisse de 3 à 4 mètres, qui est composée de braunite, braunite-hausmanite,hématitebraunite,rodochrosite-hausmaniteet d‘autres minerais avec un contenu en manganèse allant jusqu’à 30 ”/o. De fortes concentrations en manganèse à la base de l’horizoncaractérisent les parties sud et est de cette région, où les formations de minerai de fer sont étendues. L’origine des formations de fer dans le Maly Khingan doit être considérée comme sédimentaire-volcanogénique

Effusive iron-silicaformations and iron deposits of the Maly Khingan

ce que confirment les faits suivants :interstratification de silex ferrugineux avec des roches volcanogéniques ; confinement exclusif de la minéralisation aux strates volcanogéniques-sédimentaires; absence de sources possibles de déplacements des substances du minerai ; consistance dans la composition des formations de minerais de fer sur de vastes régions au cours de leur développement. Les gisements de minerai de fer et de ferro-manganèse et les manifestations de minerai dans le Maly Khingan diffèrent en dimensions et en réserves. Les plus grandes se rencontrent dans la partie nord-ouest de la région : le

gisement de Kimkanskoye a des réserves d’environ200 millions de tonnes, celui de Kosten’ginskoye a des réserves atteignant500millions de tonnes ;les minerais sont pauvres, siliceux et faiblement phosphoreux.La teneur moyenne en fer est de 30 à 40 %. Le gisement de Yuzhno-Khinganskyest le plus grand gisement de ferro-manganèse.I1 est constitué de diverses sections séparées.Les réserves totales de minerai de manganèse de ce groupe atteignent 6,7millions de tonnes avec un contenu moyen de 20 à 21 %de manganèse et de 9 à 11 % de fer.

Bibliography/Bibliographie MAHININ, W.A. 1952. Geological features of iron-manganese ore deposits of the ‘Eastband’in the Maly Khingan (unpublished).

STRAHOV,N.M . 1965.Geochemistry ofthe sedimentary manganese ore process.

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Effusive jasper iron-formationand iron ores of the U d a area E.L. Shkolnik Far East Geological Service, Ministry of Geology of the Russian Soviet Federated Socialist Republic,Khabarovsk (US.S.R.)

Preliminary prospecting of the Uda iron-ore area in the Far East of the U.S.S.R. (Khabarovsk Territory) has shown that the iron-formationand the ore are unusual in several respects. The U d a area covers about 12,00014,000km2 and extends 500 km along the right bank of the U d a River towards the Shantarsk Islands (Fig. 1).

A Okhotsk sea

Iron ore farrn~tio"

I Iran are Manganese ore

A Phosphorite racks

FIG.1. Diagrammatic chart of area. Over a hundred deposits as well as zones of mineralization and many magnetic anomalies have been formed in the area. Potential reserves of ore here exceed tens of billions of tons. Because fossil remains are absent and exposures are poor, the geology of the area is not yet well-established; stratigraphic relations within the iron-formationand the structural features of the ore-bearingbeds are still obscure. Alternating iron ore beds and terrigenous rocks of the formation occur in roughly parallel bands, striking northeastward for a distance of 100 km. Some geologists place the volcanically derived silicarich iron bearing rocks partly in the Uligdan series of the

Lower Cambrian and partly in the Upper Precambrian. They also presume that the formation was repeatedly exposed by folding. Other scientists believe that the sequence consists of several suites of interbedded cherty rock of volcanic origin and terrigenous rock. Another interpretation is that the formation is partially or entirely of Upper (Middle) Palaeozoic age. Intensively dislocated rocks of Palaeozoic age dip at angles of 60-70" and even 90".Ore bodies dip at the same angles. The general north-eastwardtrend of the formation is sometimes crossed by intricate folds. Cross folds in the ores are less common than in jasper-rich horizons. The formation is characterized by two sets of faults,one with a north-eastern (longitudinal) trend, the other with a north-western(transverse) trend. Blocks of Upper Palaeozoic granite and granodiorite intrude the north-eastern portion of the iron-formation,causing localmetamorphism of the host rocks. Iron and manganese ore and associated phosphorite rocks are located only in the volcanically derived jasperrich part of the formation, which sometimes contains smallamounts of sandstone,shale,limestone and dolomite. There is no ore in the terrigenous rocks. The formation consists mainly of jasper and argillaceous jasper. Volcanic rocks are less common, but in some places they predominate,laterally grading into jasper and,rarely,into terrigenous rocks. The volcanic rocks are areally related to limestone and dolomite or their epigenetically altered differentiates (silicified rock). Cherty rock members sometimes form tabular or lens-shaped bodies from tens to several hundred metres thick. They commonly alternate with basic lava, pyroclastic rock, iron and manganese ore, carbonate and terrigenous rock. Most jasper is banded; massive homogeneous zones are rare. Banding is due to alternation of jasper and argillaceous jasper containing 92-98 and 80-90 per cent SiOs, respectively. Occasionally banding is caused by the alteration of coloured jaspers. Iron minerals and subordinate amounts of hydromica and clay produce distinctive colour

Unesco, 1973. Geizesi.~ of Prccunzbriun iron wid marzgunese deposifs.Proc. Kiev Synzp., 1970. (Earth sciences, 9.)

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patterns depending upon whether they are evenly distributed or make up parallel bands in the rock. Fossil remnants are not usually abundant. Locally chert which contains no iron minerals does contain abundant radiolaria and spicules of siliceous sponges. Argillaceous jasper is darker and more fissile than common jasper, and has no conchoidal fracture. Thin chips of argillaceous jasper are not transparent. Lightcoloured jasper contains negligible amounts of iron minerals;it consists of 97-98 per cent total silica and 80-90 per cent free silica. Subordinate amount of Alzo3(1-3 per cent) and a high free silica content are characteristic of light-colouredjasper. An alumina content of 3-4 per cent is found in rocks transitional from common jasper to cherty iron slates. The content of Fe203i-Fe0varies from 3 to 4 per cent. In specimens grading from jasper to iron ore this figure ranges from 10 to 20 per cent with Alzo,increasing to 6 per cent. It should be noted that the alumina content in ores and in cherty iron slate is nearly the same. MnO,, Cao, M g O , PzO,concentrations vary between tenths and hundredths of one per cent. Jasper beds commonly alternate with basic lavas and occasionally with pyroclastic rocks.Lava flows may be 1015 m thick and about 1-2 km wide.The calc-alkalinerocks of the flows consist of basalt and basalt porphyry. Rarely, spilite with an iron content of 10-20 per cent forms complicated packets 5-20 km long. Pyroclastic rocks are less common than lava flows. The most common are basaltic breccia with some lithocrystalloclastic agglomerate and psephitic tuff. In many of the ore-bearing rocks, ore horizons are from 10 to 100 km long. In some sections,not exceeding hundreds of metres, the ore bodies form a chain-likepattern. Sometimes the sequence consists of alternating host rocks and echelon-like ore bodies. The number of ore bodies and their thickness vary considerably along the strike. Tabular ore bodies are rarely lenticular and have a complex structure. They are 200-300 m thick and may extend over 6 km,but are commonly 1-2 km long. Zones consisting of almost pure iron ore range from some centimetres to 50-60 m in thickness. The thickness of the ore bodies is directly proportional to their length. Banded ores are predominant. Banding results from the alternation of layers of different composition and different component ratios. Bands and lenses are irregular in thickness,from microscopic dimensions to tens of centimetres. The average thickness ranges from several millimetres to 5 cm. Random variations from millimetres to tens of centimetresin the thickness of quartz-richand iron oxide-rich layers results in an irregular banding pattern. Massive ore texture is not common. In some specimens it occurs as a result of recrystallization of the ore. Brecciated ore structure is extremely rare; unmetamorphosed and slightly metamorphosed ore consists of fine and very finegrains.The size of hematite grains is about 8 microns. Magnetite consists mainly of grains of about 50 microns. Two ore types arepredominant:magnetite and hematite

188

with slight admixtures of magnetite. Other ore minerals present in the area are siderite,leptochlorite,magnetitegoethite-lepidocrocite.Hematite occurs almost everywhere exceptin zonesofcontactmetamorphism.Brick-redhematite with colloid-like grains is the main oxide mineral. Under hydrothermal metamorphism it is transformed into darkgrey hematite (specularite). The amount of specularite in specimensvaries greatly.Magnetite is commonly present in hematite. Its percentage differs in the various stratigraphic sections. There is no evidence of a gradational transition from hematite ore to magnetite ore by relative increase of magnetite. Sulphides (pyrite, chalcopyrite) in all types of ores rarely exceed 1 per cent. Magnetite commonly contains sulphide blebs. Siderite contains 4-5 per cent manganese. Chlorites (thuringite-chamosite), which are particularly abundant in hematite ores (10-15 per cent), occur as flaky aggregates. Fine-grained quartz is abundant only in relatively poor ores. Silica is abundant in the ores as a constituent of silicates. Red hematite is commonly not present in magnetite ores. Fine, more or less isometric, crystals of magnetite occur with tiny inclusions of sulphides and non-metallic components. During metamorphism and recrystallization magnetite crystalsbecome ideomorphicand tend to increase in size. The available data indicatethat sideriteores and those with subordinate amounts of siderite are lenticular or elongated and about 10 c m thick. They may be widely distributed all over the area. Some specimens consist of pink-greymanganous siderite globules as large as 2-3 m m . Manganous siderite is rare and consists exclusively of pelitic particles (oligonite). These ores commonly contain sulphide, magnetite and iron carbonate. Despite their varying mineral composition, the ore types possess certain common features. For example, the iron content in oxide and hydroxide ores is not high. They form two groups of ore with an iron content of 4046 per cent and 29-32 per cent. However, some specimens of ore contain from 50 to 60 per cent of iron. Carbonate ores sometimes occur with an iron content, which varies sympathetically with that of manganese, from 10-15 per cent to 25-35 per cent. All types of ores are characterized by a high manganese content ranging from 1-4 per cent to 10-15 per cent. Oxide and to some extent carbonate ores are rich in silica.The SiO,content is roughly inversely proportional to the iron content. The content of alumina, in silicates, is about 4-6 per cent. All ores are slightly carbonaceous. CaOfMgO is about 1-3 per cent. Both high and low iron groups are characterized by a high phosphorus content :from 0.6 to 1.2 per cent in the first group and from 0.25 to 0.3 per cent in the second. Thus the content of P is proportional to that of Fe.The ores are slightly enriched in Ni,V,Cr,Cu, Zn, Pb, As, Ge, Yb. The ore has a great tendency to localize in certain types of host rocks.Statisticalanalysis,based on 150 exploration openings,shows that 80 per cent of the ore occurs in cherty rocks,15 per cent in terrigenousrocks,and only

Effusivejasper iron-formationand iron ores of the Uda area

about 4 per cent in lava flows. Iron ores are unevenly distributed in the area; most of the large ore bodies are in the western part. In addition, the following regular features should be noted: (a) contact between ores and cherty rocks is generally gradational; (b) distribution of iron minerals in jasper;(c) jasper and ore are chemogenic sediments;(d) ore and carbonate rocks represents different facies. A survey of world occurrences shows that the ironformation and ores of the type described in this report are quite rare. Similar ore occurs in deposits of California,

New Zealand, Japan and in the Lower Palaeozoic of Kazakhstan. All deposits are small. The number of different ore types, their geologic associations and the great area covered by the iron-formation may be cited as evidence that the U d a iron ore area is rather unusual. The texture of ores in the area is quite different from that of Precambrian jaspilite. Only in the Taikan basin are ores of the U d a area to soine extent similar to the Precambrian ones. The genesis of iron ore formation in the area inay probably be explained by underwater fumarole activity.

Résumé La formation de minerai de feu à jaspe effusif’et les minerais de feu de la région d’Oucla (E. I. Shkolnik)

Au cours des dernières années,on a découvert une formation de minerai de fer unique et peu métamorphosée du type eugéosynclinal et dont l’âgese situe entre le Précambrien récent et le Cambrien. Cette formation,qui a fait l’objet d’une étude préliminaire,est située dans le nord de Khabarovsky Krai le long de la rive droite de la rivière Uda. La prédominance de roches siliceuses du type jaspe et d’effusions de séries de diabase spirite est très significative.Les roches pyroclastiques,terrigènes et carbonatées y jouent un rôle secondaire. Cette formation s’étend à l’intérieur des limites de la région nord-est de la ceinture plissée de Mongolo-Okhotskydans une bande de quelques centaines de kilomètres de long et de quelques dizaines de kilomètres de large. Les minerais de fer sont associés dans le gisement et probablement reliés paragénétiquement avec des minerais de manganèse et des phosphorites.D e plus, ils sont exclusivement accompagnés dejaspeset souvent ils se transforment en jaspe ; bien que, parfois, on les rencontre dans des effusionsbasiques et,très rarement,dans des grès aleurolites. Les minerais de fer se présentent soit sous forme de nappe,soit sous forme de lentilles,qui atteignent quelques kilomètres de long et dont l’épaisseurvarie de 50à 70mètres. Ils sont généralement rassemblés dans des bancs complexes qui s’étendent en des horizons en chaîne de quelques dizaines de kilomètres de long et de plusieurs centaines de mètres d’épaisseur.

Les minerais sont le plus souvent rubanés dans leur structure, mais ces structures résultent rarement de l’altération des couches de minerai et de jaspe. Elles sont plus fréquemment constituées par l’intercalation de minerais de richesse et de qualité différentes. Les structures sont surtout à grains fins ou à grains extrêmement fins. On rencontre ici des minerais de magnétite ou de magnétite-hématite peu métamorphosés, avec, fréquemment, de minces lits de sidérite et de magnétite-sidérite. L a magnétite a une structure grainée et subidiomorphe. L‘hématite est représentée par une variété rouge dispersée de forme laminaire, le carbonate de fer est de la sphérosidérite avec des proportions variables de manganèse. L a faible altération des minerais de magnétite et de magnétitesidérite, l’absence d’indices et de pseudomorphoses dans le remplacement de la magnétite par l’hématitedénotent le caractère sédimentaire de la magnétite. Les formes minérales des combinaisons de fer dans les minerais et les jaspes sont les mêmes. La teneur en fer des minerais est de 31-46 %.Ils sont en général manganeux,phosphoreux et fortement siliceux (15 à 30 % de silice). L a proportion d’alumine est relativement stable et compriseentre 4et 6 %. Les minerais sont corrélativement pauvres en substances basiques. L’analyse du matériel recueilli montre que les minerais peuvent être considérés comme des dépôts chemogéniques de champs de fumerolles subaquatiqueslimités à des zones de paléofractures qui sont restées stables dans le temps et qui sont situées dans des régions de dépression.

189

Differing degrees of metamorphism, the mineral facies and the petrograpbic nomenclature of ferruginous rocks such as ferruginous quartzites, taconites, jaspilites, itabirites

Différents degrés de métamorphisme, faciès des minéraux et nomenclature pétrographique des roches ferrugineuses telles que quartzites ferrugineuses, taconites, jaspilites et itabirites

Mesabi, Gunflint and Cuyuna ranges, Minnesota G.B. Morey Minnesota Geological Survey (United States of America)

Introduction Iron-formationsof Middle Precambrian age in Minnesota are the source of a large part of the iron ore of the world. The iron ores are of two types, iiatural ore and magnetitebearing taconite. The magnetite-bearing ores occur within the iron-formationsas specific magnetite-richlayers which can be mined and concentrated by magnetic methods after fine grinding. The natural ores include iron-richconcentrates that formed locally by oxidation and leaching of the iron-formations and which can be shipped directly, and material that can be shipped after crushing and sizing,or can be beneficated using screening and washing or gravity methods to yield a high-ironproduct. Three geographic areas or ranges containing Middle Precambrianiron-formationsoccur within or partly within the political boundaries o€ Minnesota. T w o ranges, the Mesabi and Cuyuna,have been major producers, and the Mesabi range remains as the largest source of iron ore in the United States.The third,the Gunflint range,has never been an ore producer,but its geology has been extensively studied because of good exposures,slight deformation,and minor alteration.As such, it is a prototype of the other more highly altered and deformed ranges in Minnesota.

sequently were eroded extensively prior to deposition of Upper Precambrianstrata.Rocks of the Upper Precambrian (the Keweenawan System) bridge the time span between the Penokean orogeny and the beginning of the Palaeozoic era. There is a gap of almost a billion (lo0) years in the geologic record of northern Minnesota extending from the Late Precambrian to the Late Cretaceous. It seems probable that the area was eroded to its present bedrock topography and the natural iron ores were formed during this interval.Isolated patches of mostly non-marineCretaceous strata locally overlie the iron-formationon the central and eastern Mesabi range, and marine Cretaceous strata are present as an extensive sheet south of the western Mesabi range and on the Cuyuna range (Sloan, 1964). Northern Minnesota was glaciated during Pleistocene time and glacial material ranging in thickness from less than 20 ft (6 m)to more than 300ft (92m)mantles much of the area. Consequently,the iron-formationscrop out only on the Gunflint range and on the eastern part of the Mesabi range. However, thousands of drill holes have precisely defined the distribution of iron-formationon the western Mesabi and Cuyuna ranges.

Stratigraphy GEOLOGIC SETTING

The Precambrian sequence in the Lake Superior region (Fig. 1) is divided into the Lower, Middle, and Upper Precambrian (Goldich et al., 1961) which have radiometric time ranges respectively of greater than 2,500 m.y., 2,500-1,700m.y.,and 1,700-600m.y.These systems were interruptedby two major orogenies;the Lower Precambrian includesall rocks that were deformed,metamorphosed,and intruded by granitic rocks during the Algoman and older orogenies. These rocks form the ‘basement’underlying strata of Middle Precambrian age.The Middle Precambrian rocks were deformed, metamorphosed, and intruded by igneous rocks during the Penokean orogeny and sub-

With the exception of an unnamed dolomite unit that underlies the iron-formationon part of the Cuyuna range, all the Middle Precambriansedimentaryrocks in Minnesota are assigned to the Animikie group. The stratigraphic position of the dolomite is uncertain, but it appears to be separated from underlying and overlying rocks by unconformities(Grout and Wolff, 1955). Possibly it is equivalent to other dolomitic formations that occupy a similar stratigraphic position in Wisconsin and Michigan (Fig. 1). The correlation of the Animikie group with other Middle Precambrian rocks in the Lake Superior region has long been a problem. The term ‘Animikie’was first used by Hunt (1873) for exposures around Thunder Bay

Unesco, 1973. Genesis of Precambrian iron arid manganese deposits. Proc. Kiev Synzp., 1970. (Earth sciences, 9.)

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G.B. Morey

EXPLANATION OF SYMBOLS USED O N M A P PHANEROZOIC COVER UNCONFORMITY

AL-

KEWEENAWAN ROCUS UHCONFORMITY 2 -

UNCONFORMITY ,LLud ( B L A C ~U O N F O R M A T I O N REPLACES SYMBOLIC DOTS WHEN AT OR NEAR UNCONFORMITYI LOWER PRECAMBRIAN ROCKS AFFECTED BY P E N O K E A H EVENT

LITHOLOGIC Volcanic Rocks Basalt

j-1 G~aenstonetuff o n d breccia

]-k

SYMBOLS Crystalline

Sedimentary Rocks

Rocks

E-/

1-1

Sandstone and quartzite

Gabbro and granite of post-Middle

L m

Kewecnawan a g e Conpiornerote, and (Irkm.

Granitic rocks of port-Anmilie aqa

Fq

Greenston., in port with presarved pillow strucbre~ Graywacle-shah

pj Amphibolite

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FIG.1. Geologic m a p of the western Lake Superior area (modified from Trendall, 1968) and a stratigraphic summary of Precambrian rocks in the region (after James, 1960).

194

j

Gneissic gronils af Iate prc-Animikie age

Mesabi,Gunflint and Cuyuna ranges,Minnesota (United States of America)

on Lake Superior. Irving (1 883) equated the ‘Animikie’ with Murray’s (1857)‘Huronian’and since that time many workers have accepted this correlation (Van Hise and Clements,1901;Van Hise and Leith,1911;Clements,1903). Because of lithologic and stratigraphicsimilarities,the Animikie Group in Minnesota can be correlated with part of the Marquette Range Supergroup in Michigan (Fig. 1). However Cannon and Gair (1970) have suggested that ..continued investigationshave failed to show unequivocal correlation between Middle Precambrian rocks of Michigan and the Huronian Supergroup of Ontario . . .’. Therefore the term ‘Huronian’,especially in a time-stratigraphic sense,is no longer used in Minnesota. The Animikie rocks in Minnesota are inferred to be younger than 2,000m.y. (Hanson and Malotra, 1971); their actual age is still uncertain. Various attempts to determine the actual age have resulted in values ranging from, 1,685 &24 m.y. (Faure and Kovach, 1969) to 1,900 &200 m.y. (Hurley et al.,1962). As yet this problem is still unresolved. I.

M I N E R A L O G Y A N D TEXTURES O F T H E IRON-FORMATIONS

The Middle Precambrian iron-formations are ferruginous chert? containing from 25 to 30 per cent iron. Winchell (18933 originally referred to the lower part of the Biwabik iron-formationon the Mesabi range as ‘taconyte’because of a supposed correlation of these rocks and Taconic rocks of the eastern United States.Later Spurr (1894) used this term ’. . . as a designation of the iron-bearing rock in general . . .’, and by the turn of the century,the spelling had become firmly established as ‘taconite’.Through common usage, the word taconite has since been extended to include all rocks of the Biwabik iron-formationexcept the oxidized ores (Gruner, 1946), and has been used as an informal rock name to describe the iron-rich rocks in other iron-formations of the Lake Superior region. Most of the iron-formations in Minnesota, where unaffected by younger metamorphic events, are mineralogically complexrocks that are intermediatebetween James’ (1954) carbonate and silicate facies of iron-formation.The chief minerals are quartz, magnetite, hematite, iron carbonates, and iron silicates (Gruner, 1946; White, 1954). Most of the quartz is microcrystalline,although a few detrital particles as large as 0.2 mm in diameter are present.The iron silicates (greenalite,minnesotaite,stilpnomelane) may occur singly, but more commonly in combination with each other as matted or radiating aggregates. Generally they are difficult to differentiate because of intimate intermixing and fine-grainsize and thus they are generally treated as a group. Magnetite forms tiny octahedra;commonly these may also occur in irregularly banded, regularly banded, laminated, patchy, or mottled concentrations.The carbonates form small to large rhombs or irregularly rounded grains. In general, taconite composed dominantly of chert

with iron silicates or magnetite is apt to have a coarsegrained or granular texture, whereas rock that is mostly carbonates and/or iron silicates is apt to have a finegrained or slaty texture.Thus, two fundamentally different kinds of iron-formationcan be distinguished (Wolff,1917; Gruner, 1946; White, 1954) :(a) cherty taconite,which is characteristically massive, quartz-rich,and has a granular texture, and (bj slaty taconite, which is generally fine grained, finely laminated, and composed mostly of iron silicates and carbonates.

DESCRIPTION O F INDIVIDUAL RANGES

Mesabi range

The name Mesabi range refers to the subcrop belt of the Biwabik iron-formation,most of which is buried beneath glacial deposits (Fig. 2). This subcrop belt, one-quarter to three miles (0.4-4.8 km) in width, extends for about 120 miles (192 km) in an east-northeast direction. The eastern end ofthe iron-formationis truncated by the Duluth Complex of Middle Keweenawan (Late Precambrian) age. The western end is covered by Cretaceous strata and by thick glacial deposits;the extent and trend of the formation west of Grand Rapids are not as well known as on the remainder of the range. The Biwabik iron-formation,ranging in thicknessfrom 100 to 750 ft (30-225 m), is underlain by quartzite and impure argillite of the Pokegama Formation and overlain by the Virginia formation, a thick succession of dark-grey argillite and intercalated greywacke. The base of the ironformation is well defined by an abrupt change from ironpoor quartzite to iron-bearingand granular rocks.Throughout much of the range, the top of the formation is the top of a limestone-bearingunit containing little iron,but some iron silicates,and a few interbeds of granular chert (Fig.2). The limestone-bearing unit pinches out in the western Mesabi and to the west, the top of the iron-formationis the top of a cherty siderite unit. The cherty siderite unit fingers-out to the west,and on the western-most Mesabi the top oí‘ the iron-formationis placed at the top of a graphitic argillite unit that is commonly an iron-bearing rock. On the western-most Mesabi, several iron-bearing members, one as much as 200 ft (60 m) thick, are interlayered with the argillites of the Virginia Formation. Thus, there is ample evidence of interfingering between the Biwabik iron-formation and the Virginia formation in this area. Because recognizable lithologic units consisting of various proportions of rock strata having ‘cherty’ or ‘slaty’ textures occur over long distances, WOW (1917) was able to sub-dividethe iron-formationinto four units. From bottom to top they are: Lower Cherty, Lower Slaty, Upper Cherty,and Upper Slaty. These units,which have been retained subsequently as members (Gruner, 1946; White, 1954), can be traced along most of the main 195

G.B. Morey

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196

Mesabi, Gunflint and Cuyuna ranges, Minnesota (United States of America)

range and are recognizable even in the highly metamorphosed rocks near the Duluth complex. On the westernmost Mesabi, however, the four-fold subdivision is not present. The Lower Slaty member, including an ash-fall unit called the Intermediate slate, pinches out near Grand Rapids;to the west,the Lower and Upper Cherty members are joined as a single member. O n the basis of texture and mineralogy, the lower part of the formation is referred to as a Cherty member and the upper part as a Slaty member. Both the thickness and the iron content of the cherty rocks diminish westward. At the far western end of the range these rocks are only 20 ft (6 m) thick and contain almost no iron-bearing minerals. However, as elsewhere on the range, the basal conglomerate still contains algal structures. O n the other hand, the slaty rocks consist of a chert and abundant siderite and are intimately associated with argillites that generally have a higher iron content than the overlying Virginia formation. Because of vertical and lateral lithologic changes within members, the subdivision of the iron-formation is at places arbitrary,but nevertheless useful from both a genetic and an economic standpoint.In general,lithic units included in the Slaty members contain 40 per cent or more slaty taconite (White, 1954), although locally within a given member the proportion of thin-beddedrocks may be less. The slaty strata characteristically contain sparse iron oxides, and the associated granular or cherty rock most commonly are silicate-rich. The cherty members on the other hand contain 10-30 per cent slaty material and are rich in magnetite, although they also contain abundant cherty or cherty-silicatetaconite. Gunflint range

The Gunflint range is more or less continuously exposed from west of Gunflint Lake on the international boundary to Thunder Bay 011 Lake Superior,a distance of approximately 110 miles (176 km) (Fig. 3). Isolated exposures on the north shore of Lake Superior indicate that these rocks once extended at least an additional 70 miles (112 km) to the east. Rocks exposed on the Gunflint range are the northeastern extension of those on the Mesabi range; the two ranges are separated over a distance of approximately 40miles (64km)by the Duluth Complex.As on the Mesabi range, three formations comprise the Animikie group. Locally the Gunflint iron-formation is underlain by a quartzite and overlain by the Rove formation,an interbedded argillite and greywacke sequence. The basal quartzite is thin and locally absent; where present,it is commonly included as a basal member in the iron-formation.The Gunflint iron-formationand the Rove formation have a gradational contact and,as on the Mesabi range,the top of a limestone-bearing member is considered to be the top of the iron-formation. Goodwin (1956) divided the iron-formation into six sedimentaryfacies,each ofwhich '...is an areally restricted unit with unique lithic characteristics . . .' (Fig. 3). The

boundaries of these members do not coincide with the boundaries of the older four-fold classification scheme used on the Mesabi range,but fortunately the two schemes can be correlated with only slight difficulty. The lower-mostfacies of the Lower Gunflint member consists of algal chert and lies on basement or on the conglomerate facies. It is overlain by tuffaceous shale (equivalent to the Intermediate slate) which in turn is succeeded by a thick granular taconite facies which grades north-eastwardinto banded chert and carbonate. This unit in turn grades northeastward into a granular taconite facies. The basal facies of the Upper Gunflint member is confined largely to the south-westernpart of the district, where it consists of algal chert. The algal chert is overlain by a second tuffaceous shale that forms the most persistent unit in the iron-formation,making it a marker horizon of time-stratigraphicsignificance. T o the south-west, the shale is overlain by a thick taconite unit which grades north-eastward into banded chert and carbonate. The Limestone member forms the top of the Gunflint ironformation,and is a thin but persistent unit separating ironand silica-bearingrocks from the overlyingRove formation. The lower and upper tuffaceousshale units are products of explosive volcanism, as are the several lava flows of basaltic composition that occur in the Gunflint. The flows and the tuffaceous rocks are considered indicative of the contemporaneity of volcanism and iron-formation deposition. Cuyuna sange

The Cuyuna range is near the geographic centre of Minnesota (Fig. 4) in an area that is generally flat and mantled by a thick layer of Pleistocene glacial drift. Natural exposures of iron-formationare lacking; consequently the geologic interpretation is based on artificial exposures in open-pit mines, on diamond drilling, and on various geophysical techniques. The relationship of the Middle Precambrian rocks to the older rocks of the area is not adequately known. In a simple interpretation, the Middle Precambrian rocks occupy a complex north-east-trendingsynclinorium,bounded by older rocks, except at the north-east end where the structurewidens.The Middle Precambrian strata probably wedge out against older rocks to the west and south, connect northward with the Mesabi range, and disappear eastward under a thick Cretaceous and Pleistocene cover. The Troinmald,the main iron-formationof the Cuyuna range, is a distinct stratigraphic marker unit throughout the north range. It is underlain by the Mahnomen forrnation, an intercalated sequence of light-coloured argillite or slate and quartz-richsiltstone that is at least 2,000 ft (600 m) thick, and is overlain by at least 2,000ft (600 m) of the Rabbit Lake formation,an argillite and slate unit that contains some carbonaceousand ferruginous material. The iron-formationranges in thickness from 45 ft (13 m) to more than 500 ft (150 m) and is divided into two mappable units; (a) an iron oxide-rich,thick-bedded facies, 197

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G.B. Morey

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Mesabi, Gunflint and Cuyuna ranges, Minnesota (United States of America)

199

G.B. Morey

and (b) an iron silicate and iron carbonate-rich, thinbedded facies. In about one-third of the north range, the thick-bedded facies overlies the thin-bedded facies and grades downward into it; elsewhere each facies comprises the entire iron-formation. Intermixed chert and iron oxides in relatively massive beds characterize the thick-beddedfacies.The cherty layers in this facies have a granular texture.Most granules and oolites are composed of various combinations of quartz, carbonates, fine-grainediron silicates, magnetite, and hematite. Locally abundant amounts of detrital quartz occur near the top of the thick-beddedfacies,especially in chert beds having an oolitic texture. The thin-bedded facies is characterized by individual bedding laminae that are less than half an inch (1.25 cm) thick;many laminae are less than one-eighthinch(0.3 cm) thick,and some laminae can be distinguished only with a microscope. The principal minerals in this facies are siderite, magnetite, stilpnomelane, minnesotaite, chlorite, and quartz. The silicates, carbonates, and magnetite comprise various proportions of the facies in all parts of the range and from bottom to top of the unit; thus there is no apparent mineralogic zoning. Argillaceous layers are intercalated with the thin-bedded facies at several places; some of this material may be reworked and altered ash falls,

I N T E R R A N G E CORRELATIONS

There is little doubt that the rocks on the Mesabi and Gunflint ranges are correlative, as there are broad similarities in general stratigraphy, texture, and mineralogy (Broderick, 1920). The correlation of rocks on the Mesabi and Cuyana ranges is less certain in that they appear to have little stratigraphic similarity. However, exploration in an area approximately midway between the Mesabi and Cuyuna ranges (the Emily district) has shown a sequence of rocks intermediate between that found in either range. Commonly the Emily district has been considered part of the Cuyuna range because of structural similarities (cf. Grout and Wolff,1955); however,it has a stratigraphic succession much like that found at the western-most end of the Mesabi range. The lower-mostformation contains quartzite like that of the Pokegama formation intercalated with light-colouredsericitic argillite similar to that in the Mahnomen formation. The iron-formationin the Emily district consists of a lower cherty member 255-300 ft (78.5-92 m) thick, a thin-bedded member 25-90 ft (7.527 m) thick and a second cherty member of unknown thickness. In detail, the Lower Cherty member resembles stratigraphically the Lower Cherty member on the Mesabi range. A thick section of carbonaceous argillite, lithologically similar to both the Rabbit Lake and Virginia formation, overlies the main iron-formation. Thus, in general, the three-fold subdivision of the Animikie group extends from the Mesabi range to the Cuyuna range. Although some of the necessary detail is still lacking at this time, there is no evidence refuting a correlation between

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the Mesabi and Cuyuna ranges and the iron-formation is projected along a sinuous path from the Mesabi range through the Emily district to the iron-formation of the Cuyuna range. The correlated segments of Middle Precambrian iron-formation thus extend from 70 miles (1 i2 km)east of Thunder Bay, Ontario to the central part of Minnesota, a distance of about 400 miles (640 km). D EP O S I T I O N A L HISTORY

The Middle Precambrian rocks in Minnesota were deposited in part of an elongate basin, the configuration of which was probably controlled by a pre-existinggrain in the older rocks (Van Hise and Leith, 1911). The western limit of the basin is unknown, but White (1954) suggested that the western shoreline was located somewhat to the west of the present Animikie exposures;the rocks extended to the east beyond Thunder Bay, Ontario (Goodwin, 1956) and to the south into Wisconsin and northern Michigan,where typical eugeosynclinal accumulations are n o w exposed (James,1958). In Animikie time a sea spread slowly from south to north across a broad, relatively flat plain. During the sea’s advance, shallow water clastic sediments were deposited (Fig. 5(a)). Relative subsidence was greater in the southern part o€ the basin, but sediment infilling was more or less able to keep pace with subsidence. Thus a thick wedge of fine-grainedclastic detritus,fringed by a thin strand-line deposit of sandstone and conglomerate, was deposited prior to deposition of the iron-formation.The changefrom clastic to chemical sedimentation was abrupt (Fig. 5(b)) and there is no evidence that this change is not timetransgressive;it is possible that the iron-formation-clastic rock contact represents an onlap of considerable importance. This contact also indicates that stable equilibrium conditions were achieved between the source area and the depositional basin, and that during iron-formationdeposition,little or no clastic detritus was supplied to the basin beyond that which is found near the inferred position of the strand-line. The various textural and compositionalaspects of the iron-formationresult from deposition under differing environmental conditions, and a close relationship exists between the inferred physical and chemical environment,the composition of the precipitate, and its textural character. LaBerge (1967) suggested that the non-granular or slaty taconites are similar in many respects to siltstones or argillites, and that many of the granules in the cherty taconite were derived from texturally similar fine-grainedmaterial. In cherty taconites, granules commonly occur in strata having graded bedding or cross-bedding,and mixtures of granules with chert and carbonate pebbles, fragments of algal structures,oolites, and detrital quartz indicate that the granules behaved as particulate detritus(Mengel,1965). Microscopic features indicate that many granules were reworked from previously deposited material; the cherty taconites are somewhat akin to clastic limestones. Thus,

Mesabi, Gunflint and Cuyuna ranges,Minnesota (United States of America)

Mesabi

Emily

Greenschist Facies

Cuyuna Amphibolite Facies

: x x x

x x x.x x

Iron -Formation,

.,Volcanic5

x x x x

(4 FIG.5. Schematic diagrams illustrating the evolution of the Penokean geosyncline and associated orogeny in east-central Minnesota,(a), early Middle Precambrian time; (d), late Middle

Precambriantime;and (e), present configuration(modified from Goldich et al., 1961).

during iron-formation deposition, granule-bearing sediments were deposited in a shallow-water,agitated environment, whereas slaty or thin-bedded sediments were deposited in deeper, less active water. Fluctuations in the relative position of the strand-line or alternate basin deepening and infilling would result in a vertical sequence containing intercalated beds of granular and slaty rock types. The iron-formationsalso reflect certain aspects of the chemical environment of deposition (James, 1954, 1966). In general,oxide-richfacies are to be expected in shallow wave- and current-sweptareas,whereas iron sulphides are

deposited under quiet, deep, and stagnant conditions;the other facies are deposited under intermediate conditions, and more or less overlap these end members. Thus, the Trommald iron-formation was deposited in two distinct but gradationalenvironments.Relatively deep and reducing water yielded a thin-bedded facies consisting of chert, carbonates,and silicates and as the basin filled,shallower, probably agitated,oxidizing water yielded a thick-bedded granular facies consisting of chert and iron oxides. White (1954) outlined a similar lithofacies pattern in the Biwabik iron-formationand suggested that the intercalated cherty (oxide-silicate) and slaty (silicate-carbonate) 201

G.B. Morey

facies resulted from deposition under transgressing and regressing conditions. Goodwin (1956) suggested that a similar facies pattern in the Gunflint iron-formationresulted from deposition at various water depths during periods of crustal instability, and that subsidence periodically modified the basin configuration and, in turn, the facies distribution. Extensive and continuous instability ultimately modified the equilibrium state between basin and source area, resulting in a great influx of shaly material far into the basin (Fig. 5(c)). Volcanism has been suggested as the cause of the abrupt change from chemical to clastic sedimentation that characterized the entire basin in Minnesota. Although the basin in Minnesota was situated near a volcanic region, it was somewhat distant from the centre of igneous activity inasmuch as the proportion of recognizable volcanic material present is relatively small. Kowever,there is ample evidence of extensive volcanic activity elsewherein the basin. James (1958) noted that approximately equivalent rocks in northern Michigan contain much volcanic material and that individual formations are lenticular,indicating deposition in a series of small and short-livedbasins iii a volcanically active and tectonically unstable area. Thus is seems reasonable to postulate that tectonic instability in the eugeosynclinal part of the basin was reflected in Minnesota by fluctuations in the relative position of the strand-lineand in systematic changes in the kinds of sediments that were deposited (Morey, 1969; Morey and Ojakangas, 1970). During later stages of deposition, the basin became unstable (Fig. 5(d)), and the near axial part was folded and intruded by igneous rocks (Goldich et al.,1961). This deformation, called the Penokean orogeny, marked the end of Middle Precambrian deposition in Minnesota.

STRUCTURE

ïhe present structuralconfiguration of MiddlePrecambrian rocks in Minnesota (Fig. 5) has resulted from the partial superposition of at least two tectonic events. The rocks first were folded and metamorphosed during the Penokean orogeny. As most of this deformation was restricted to the axial region of the Penokean geosyncline,the structural complexity increases in a south-westerly direction. The rocks of the Cuyuna range were markedly affected, the rocks of the Mesabi range were moderately affected, and the rocks of the Gunflint range were only slightly affected. Later, in Middle Keweenawan time, the structural configuration of these rocks was modified through the formation of the Lake Superior syncline, the axis of which approximates the present position of Lake Superior. As a consequence,the strata on the Mesabi and Gunflintranges, being on the north limb of this structure,now dip gently south. The Cuyuna range,however,was little affected by this younger structural event. The Gunflint range is a homocline that strikes east-north-east and dips 5-15"SE, except near Middle Keweenawan intrusive rocks, where dips may be as high

202

as 60".Gravity faults are widespread and dominate the structure. Most faults are steeply dipping and have vertical movements ofas much as about 300 ft (90m) (Tanton, 1931); however, most displacements are small, ranging from 20 to 100 ft (6-30 m).The majority of the faults have an easterly or east-north-easterlytrend, but several have northerly or north-westerly trends. The structure of the Mesabi range superficially resembles, but is more complex than, that of the Gunflint range. It is a gently dipping homocline that strikes eastnorth-eastand dips 5"-15"SE.This general trand,however, is interrupted by several prominent structural features that have caused noticeable bends in the strike of the Animikie rocks or have produced pronounced changes in the outcrop width of the iron-formation.Among the more prominent structuralfeaturesare (Fig.2): (a) the 'VirginiaHorn', a broad cross-fold composed of a gentle south-westerly plunging syncline and a parallel anticline; (b) the Siphon structure,which may be either a fault or a northwardtrending monocline; (c) the Eiwabik fault, a fault about N 30"W , dips steeply south,and has a vertical displacement of about 200 ft (60 m);(d) several northerly trending crossfaults east of the 'Virginia Horn'; and (e) the Sugar Lake anticline, a broad southward-plunging structure west of Grand Rapids. In addition to these rather conspicuous structures, other less apparent but nevertheless important structural features have been delineated only recently (Marsden et al., 1969). For example, recent work in the Nashwauk-Keewatinarea (Fig. 6) has indicated the presence of a more extensive structural pattern than was previously recognized.This structurally complex area consists of several cross-faultsthat, together with faults and monoclines in the intervening areas, fan out and dip toward each other, forming broad steps into a centrai graben. Other minor structural features also are common, and include: (a)small folds and monoclines that locally produce steep dips in the iron-formation;(b) faults that commonly strike N75"W or N 20"W,and have displacements of less than 50 ft (15 m); and (c) very prominent joints having N 10"E,N40"W,or east-west directions (Gruner, 1924, 1946). These minor structures were important factors in localizing the extent and distribution of the natural ore bodies. Not all the structures on the Mesabi range can be related to a particular tectonic event. Sims et al. (196%) have inferred that many faults that cut the iron-formation represent rejuvenated movements on older structures that developed initially in Early Precambrian time. Gruner (1946) and White (1954) concluded that both the present southward dip and the joint system are consistent with, but not necessarily related to, the development of the Lake Superior syncline.Many of the other structures have trends consistent with their having been developed during the Penokean orogeny. Thus, it may be concluded that the structure of the Mesabi range is complex and is related in time to both tectonic events that affected the area. In contrast to the Mesabi range, the structure of the

Mesabi, Gunflint and Cuyuna ranges,Minnesota (United States of America)

E X P L A NAT I ON

-y Fault

SCALE

FIG. 6. Generalized geologic map of the Nashwauk-Keewatin area, Mesabi range,Minnesota showing the spatial relationship

between various structural features and natural ore bodies (Owens et al., 1968,and modified from Marsden et al., 1969).

Cuyuna range is the result of a single deformationalevent. The bedrock pattern is one of a large conspicuous syncline and several relatively inconspicuous but major anticlines. The axial planes of almost all the folds strike north-east and dip steeply south-east; in most places,the south-east limbs of the synclines are overturned. Drag-folds of all sizes are abundant, and have a normal and systematic relationship to the principal folds. Cross-folds or undulations of fold axes, or reversals of plunge directions,only are exposed locally although they may be widespread. In general, the axial trace of the cross-folds is about N1020"E. Faults thathave been definitely recognized and mapped (Schmidt, 1963) are limited in number and size. Although they may be important within a particular mine, they are insignificant to the district as a whok. Joints are abundant in most of the iron-formation, and served as major loci for the development of some of the Cuyuna

natural ores. The dominant joints strike about N 35"W and are vertical; hence they are approximately perpendicular to the fold axes.

METAMORPHISM

The Middle Precambrian sedimentary rocks have been affected by at least two metamorphic events, which can be readily distinguished.The first, a dynamo-thermalevent, occurred over a wide area in late Middle Precambrian time,and is considered to be a consequence of the Penokean orogeny. The second, a thermal event related to the emplacement of gabbroic rocks in Late Precambrian time, is superposed on the older event within a narrow aureole adjacent to the gabbroic rocks. The Gunflint iron-formationis inferred to be the least metamorphosed of the three iroii-formations.Greenalite, 203

G.B. Morey

commonly believed to be a primary mineral, is the major silicate mineral; stilpnomelane occurs only where the ironformation has been metamorphosed by Keweenawan intrusions,and minnesotaite is rare. In contrast to the Gunflint iron-formation, minnesotaite and stilpnomelane,minerals considered by some to be indicative of the greenschist facies (James, 1955), are abundant constituents in parts of the Biwabik ironformation. White (1954) has shown that the distribution of these silicate minerals is stratigraphicallycontrolled on a dominantly regional scale and he concluded, therefore, that they were primary in origin. However, textural data (French, 1968) indicate that much of the stilpnomelane and minnesotaite is secondary. Most of the minnesotaite is restricted to cherty layers, where it appears to have replaced greenalite, according to the reaction; greenalite + quartz = minnesotaite + water, On the other hand, stilpnomelane is restricted to slaty beds which, on the average, contain a higher percentage of alumina than do the cherty beds (Gruner, 1946). Because many workers agree that stilpnomelane is one of the first minerals to form in an iron chlorite-rich sediment (Yoder, 1957) or in a sediment containing volcanically-derived material (LaBerge, 1966), stilpnomelane may have formed in beds originally containing this material. Thus,it can be postulated that selective transport concentrated clay-size material such as chamosite and/or altered volcanic detritus into particular stratigraphichorizons,and that the original bulk coinpositioninfluencedthe mineral phase that formed. This explanation can account for the apparent stratigraphic control of the silicates. Unfortunately there is no textural evidence from the Biwabik iron-formationthat can be used to indicate with certainty the physical conditions under which the mineral assemblagesformed.Perry(personalcommunication,1970), however, has demonstrated, using oxygen-isotoperatios in quartz-magnetitepairs, that when the magnetite equilibrated,temperatures on most of the main Mesabi range never exceeded around 125"-150° C. If these temperatures are indicative of the conditions under which the silicates formed, any distinction between diagenesis and low grade metamorphism is meaningless. On the Cuyuna range, evidence for a Penokean metamorphic event is much more definitive.Schmidt (1963)has shown that minnesotaite and stilpnomelane are abundant constituents of the Trommald iron-formation; the ironformation is intercalated with schistose rocks that contain chlorite and biotite. Moreover, grunerite, considered indicative of the garnet grade of regional metamorphism (James, 1955), is present in the iron-formation in the southeastern part of the north range where garnet occurs in intercalated schistose rocks. The close association of metamorphic minerals in the iron-formation and in the intercalated pelitic rocks led Schmidt (1963) to conclude that the Cuyuna range was metamorphosed during the Penokean orogeny (later dated by Peterman (1966) as having occurred 1,750 m.y. ago) and that the silicate minerals in the iron-formationare a product of that event. 204

Inasmuch as there is no stratigraphic control of the distribution of any of the silicates, these minerals must have formed via reactions involving primary iron-formation minerals such as quartz, carbonates,and greenalite. A second metamorphic event,related to emplacement of Keweenawan gabbroic rocks,is superposed on the broad mineralogic pattern described above. The effects of this event are restricted to a narrow aureole in the east Mesabi district of the Mesabi range and to that part of the Gunflint range exposed in Minnesota. French (1968) demonstrated that metamorphic affects in the Biwabik iron-formation decrease from east to west away from the contact and was able to define four metamorphic zones.Zone 1 is unaltered taconite characterized by being fine-grainedand composed of quartz,iron oxides,iron carbonates,and the iron silicates chamosite, greenalite, minnesotaite, and stilpiiomelane. Zone 2 is a transitional zone exhibiting no mineralogic changes, but having considerable secondary replacement of the original minerals by quartz and ankerite. Zone 3 comprises moderately metamorphosed taconite characterized by the development of grunerite and by the disappearance of layered silicates and carbonates (Griffin and Morey, 1969). Zone 4 is highly metamorphosed taconite characterized by increased hardness and grain size and by the presence of iron-bearingpyroxenes.A similar zonation occurs on the Gunflint range, and Morey et ul. (in preparation) have recognized three metamorphic zones that are roughly equivalent to French's zones 2, 3, and 4.The mineral assemblages near the Duluth Complex on the Gunflint range are similar to those from the easternmost Mesabi range (French's zone 4) described by Bonnichsen (1969). These studies have shown that the metamorphism was largely isochemical and characterized chiefly by progressive loss of H20and CO,.There is no indication in either area that large quantities of components were introduced into the iron-formationsfrom the Duluth Complex. The moderate grade of metamorphism,characterized by grunerite and cummingtonite,is approximately equivalent to the garnet grade of regional metamorphism; pyroxene-bearing rocks are indicative of the sillimanite grade of regionalmetamorphism (James,1955). Perry and Bonnichsen (1966), using oxygen isotope fractionation in quartz-magnetite pairs, estimated that the Biwabik ironformation attained a temperature of between 700" and 750" C near the Duluth Complex contact. French (1968), using experimental data, concluded that the moderately metamorphosed taconite attained temperatures of 300400" C 2-3 miles (3-4.5 km) from the Duluth Complex contact.

Origin and distribution of the ores MAGNETITE T A C O N I T E ORES

Magnetite occurs throughout the unoxidized Middle Precambrian iron-formationsin amounts ranging from very minor to abundant.It may occur as (a) disseminations of

Mesabi, Gunflint and Cuyuna ranges,Minnesota (United States of America)

individualoctahedra,(b) aggregatesofindividualoctahedra, or (c) layered clusters formed by interconnectingaggregates of grains. Very fine-grained magnetite, which occurs in both granules and matrix as disseminated and diffuse crystals 5 microns or less in size, is probably primary in origin. However, definite secondary magnetite euhedra, 0.05 to 0.1 mm in size, commonly replace earlier iron silicates in granules and also surround and vein fine-grained siderite in the thin slaty bands associated with cherty taconite (LaBerge, 1964; French, 1968). Replacement of granules by magnetite is most common at their margins, yielding an inner core of greenalite or minnesotaite surrounded by a rim of coarser magnetite that in most situations preserves the outline of the granule;more rarely an entire granule may be pseudomorphosed by magnetite. Thus, in many cases, there is little relationship between the primary sedimentary texture and the distribution of much of the magnetite. Thick layers from which magnetite can be mined and concentrated using modern technology are found only on the Mesabi range.The ore bodies commonly occur within the cherty members; the slaty units are either too thin or too lean to be mined. The ore bodies are tabular stratigraphic units that have arbitrary boundaries defined by vertical and lateral changes in magnetite content and grain size. The lower cherty ore zone, or the middle part of the member, is of the greatest extent and is fairly uniform in magnetite content.In contrast,the Upper Cherty member is much less uniform in iron content, and the magnetite has a very erratic distribution;it is a less suitable,although workable, ore horizon. The magnetite ores occur in two principal areas, the Main Mesabi district between Nashwauk and Mesaba, and the East Mesabi district east of Mesaba.The distinction of these two areas is more than geographicalinasmuch as the East Mesabi ores were modified by metamorphic changes associatedwith the contact aureole of the Duluth Complex, West of Nashwauk there is a profound lateral change from magnetite- to carbonate-bearingstrata;consequently, only relatively small‘islands’or tongues of unoxidized magnetitebearing strata still exist. Minable magnetite deposits in the East Mesabi district are confined to the Upper Cherty and lower part of the Upper Slaty members. Although the iron-formationhas been modified to a completely recrystallized, sandy textured,granular rock near the Duluth Complex,the recoverable magnetite-concentratefrom any particularhorizon is aboutthe same as thatfromitsnon-metamorphicequivalent, even though the magnetite is somewhat coarser-grained. This may be due in part to the extensive development of an intimate magnetite-silicatefabricinthemoremetamorphosed rocks,making grinding and liberation more difficult. N A T U R A L ORES

Natural ore bodies occur only on the Mesabi and Cuyuna ranges. They have been described by Leith (19031, Wolff

(1915), Gruner (1946), White (19541, Grout and Wolff (1955), and Schmidt (1963), and will be described only briefly here. The ore bodies occur in a wide variety of shapes and sizes within the Trommald and Biwabik ironformations, where there is a marked correlation between their location and structural featuresthat caused fracturing in the original taconite (Fig. 6). The deposits range in shape from fillings along narrow fissures,through channeltype deposits that occupy a system of fractures,to blankettype deposits where extensive areas of ore formed in favourable stratigraphic horizons. In some ore bodies, ore was developed essentially in the entire iron-formationfrom the foot-wallto the hanging-wall. There is little doubt that the natural ores are the products of secondary oxidation and leaching of original iron-formations. The original minerals were oxidized to ferric oxides, mainly hematite and goethite, while at the same time, calcium, magnesium, and much of the silica were removed by leaching.There have been extensive discussions as to the source and nature of the oxidizing and leaching solutions, but only two principal hypotheses pertaining to the origin of the natural ores have been proposed. In the first,the oxidation and leaching are postulated to have been accomplished during weathering by downward meteoric waters; in the second, by upward or hot hydrothermal solutions.Although much data pertaining to the ore bodies have been obtained,no field,experimental, or theoretical evidence can be considered as absolutely indicative of either mechanism.However,there is a general concensus today that most of the Mesabi natural ores probably were developed through normal weathering processes (cf., Marsden et aZ., 1969). All the ore bodies are related to an erosion surface which, in general, is the present bedrock surface. They underlie either glacial drift, or in some areas a veneer of Cretaceous strata. The ore extends to various depths,but most is concentrated fairly near the surface. Thus it is inferred that surface waters following permeable zones, such as faults,joints, or fractures, acting over a long period of time, could have produced the observed configuration,distribution,texture,and composition of the Mesabi natural ores. The origin of the Cuyuna ores is somewhat more complex.Schmidt (1963)believes that the ores were formed by two different processes. The first, probably hydrothermal, process is characterized by deep oxidation and the formation of ore in a rough spatial relation to fractured zones near the southeast edge of the north range where deformation was most intense. The resultant ore bodies are large,deep,tabular,and hematite-rich.A second process apparently took place at a later time as the result of ordinary weathering. An irregular blanket of goethitic ore formed on all exposed surfaces of the unoxidized iron-formation,alongjoint planes previously opened during the hydrothermal event, and at the peripheries of all the pre-existinghydrothermal ore bodies. Except where developed along the edges of hydrothermal ores bodies, no second-stageore occurs more than 100 ft (30 m>below the present bedrock surface; the ore distribution, although 205

G.B. Morey

modified by erosion, indicates that the bedrock surface nearly parallels the present surface and was, therefore, one of low relief. The actual time of ore iormation is not completely documented. Peterman (1966) showed that the Cuyuna rocks were metamorphosed about 1,750 m.y. ago and later affected by ‘. . .“hydrothermal” leaching . . .’about 1,460 m.y. ago, an interpretation consistent with the geologic evidence outlined by Schmidt (1963). Unfortunately, the time of second-stage ore formation cannot be as precisely established.If it is inferred that both the secondstage Cuyuna ores and the Mesabi ores developed approximately during the same time-interval,some limits can be

placed on that interval,The ore deposits must have been formed prior to Late Cretaceous time on a previously developed bedrock surface of low relief. Parham (1970) has shown that a thick regolith was developed in Mesozoic time prior to the early Late Cretaceous on a peneplain that extended from Manitoba,Canada to at least southern Minnesota. Previously Symons (1966) suggested, on the basis of palaeomagnetic data, ‘. . .that meteoric solutions weathered the primary Animikie iron-formationsduring the Mesozoic-Cenozoic to form. . . ore deposits.’ Thus, it appears likely that the weathered natural ores were the consequence of a prolonged period of weathering during late Mesozoic time.

Résumé Les chaines de Mesabi, Gunflint et Cuyuna dans le Minnesota, aux États-Unis d’Amérique(G. B. Morey)

Le Minnesota est l’un des plus grands producteurs de minerai de fer du monde. L a plus grande partie du minerai provient des formations de fer du Précambrienmoyen dans les chaînes de Mesabi, Cuyuna et Gunflint. L a chaîne de Mesabi est une bande étroite de formation de fer qui s’étend sur près de 200 kilomètresà travers la partie septentrionale du Minnesota. C‘est le plus grand producteur du monde avec une production de 2,7 milliards de tonnes brutes de minerai depuis 1892. Sur ce total, environ 809 millions de tonnes brutes (soit 30 %)ont été concentrées soit à partir de minerais naturels à faible teneur,soit à partir de taconite contenant de la magnétite. Durant les quinze dernières années, la production de minerai naturel a diminué et,en 1968, 59 %de la production totale de la chaîne était du concentré de taconite. L’extrémité ouest de la chaîne de Mesabi disparaît sous des strates du Crétacé et du Pléistocène; cependant, la formation de fer suit un tracé sinueux pour se rattacher aux formations de fer de la chaîne de Guyuna dans la partie centre-estdu Minnesota. Depuis sa découverte en 1904, la chaîne de Cuyuna a produit et expédié environ 103 millions de tonnes brutes.Dans les années récentes,la production de minerai naturel a diminué de 81 %, passant de 3,6millions de tonnes en 1955 à 698O00 tonnes en 1968. Cependant, à l’inverse de ce qui se passe dans la chaîne de Mesabi, aucun concentré de taconite n’est en production courante. L a partie est de la chaîne de Mesabi est tronquée par le complexe de Duluth du Précambrien supérieur mais un prolongement de la formation de fer émerge de nouveau du complexe de Duluth ?i environ 60 kilomètres au nord-est sur la chaîne de Gunflint dans le district de Thunder Bay dans l’Ontario et dans la partie adjacente du Minnesota. On ne trouve aucun minerai naturel sur la chaîne de Gunñint et l’on ne peut guère espérer trouver du minerai de taconite que dans la petite partie de la chaîne qui se trouve dans le Minnesota. 206

L a minéralogie des formations de fer inoxydées comprend du quartz (silex), de la magnétite, de la sidérite, de la stilpnomelane et de la minnesotaïte avec de moindres quantités d’hématite,de calcite,de dolomite,de chamosite, de greenalite et de chlorite. A l’intérieur d‘une zone de contact métamorphiqueautour du complexe de Duluth,la formation de fer contient du quartz, de la magnétite, des amphiboles, des pyroxènes, du grenat et de la fayalite.L a teneur moyenne de la formation de fer inoxydée est d’environ 29 % de fer, 46 % de silice et 0,9 % d‘alumine. Quant à leur structure,les chaînes de Mesabi et du Gunflint présentent un homocline peu accusé de direction est-nord-estet plongent de 5 à 15” vers le sud-est.Cette direction générale est modifiée par plusieurs plissements en travers dirigés vers le nord et par de nombreuses failles d‘orientation nord-est,est et nord-ouest.Par contre, la structure de la chaîne de Cuyuna est complexe.Les roches sont étroitement plissées dans une série de plis de direction générale nord-est,localement isoclinales et généralement renversées vers le nord-ouest; on trouve des plissements en travers dirigés vers le nord et quelques petites failles. Des structuresmoins importantes(petits plissements,failles et monoclines) ont joué un rôle important pour la locaíisation des gisements de minerai naturel sur les chaînes de Mesabi et de Cuyuna. Les concentrations de minerai naturel offrent une grande variété de formes et de dimensions.Les donnéesgéologiqueset chimiquesdontnous disposonsactuellementindiquent que les minerais naturels sont le résultat de solutions qui se sont déplacées le long des zones perméables OU l’oxydation et la lixiviation se sont produites. L a source et la nature de ces solutions sont inconnues.Les observations faites sur la chaîne du Cuyuna laissent peiiser qu’il y a eu deux périodes d’altération,une période ancienne hydrothermalesuivie beaucoup plus tard par une période de désagrégation et de désintégration. Les observations faites sur la chaîne de Mesabi corroborent la thèse que la lixiviation et l’oxydation se sont produitesà l’époquecénozoïque par l’action des eaux de surface et des procédés normaux

Mesabi, Gunflint and Cuyuna ranges,Minnesota (United States of America)

de désagrégation. Les gisements de minerai naturel sont généralement composésde quartz,de martite,d’hématite et degoethiteet,en quantitémoindre,demagnétite,d‘oxyde de manganèseet de kaolinite.Bien que la chimiedes gisements

de minerai naturel soit étroitement reliée à la composition des strates dont ils sont dérivés, la teneur moyenne en fer est d’environ 59 %; la quantité de silice varie entre 2 et 10 % et celle d’aluminiumvarie de moins de 1 à 6 %.

Bibliography/Bibliographie BONNICHSEN, B. 3 969. Metamorphic pyroxenes and amphiboles in theBiwabikIronFormation,Dunka River Area,Minnesota.

.

Spec. Pap. Miner. Soc. Amer..,no. 2, p. 217-39.

BRODERICK, T.M.1920.Economic geology and stratigraphy of the Gunñint iron district, Minnesota. Econ. Geol., vol. 15, p. 422-52. CANNON, W.F.;GAIR, J. E.1970. A revision of stratigraphic nomenclature for Middle Precambrian rocks in northern Michigan. BdI. geol. Soc. Amer., vol. 81, p. 2843-6. CLEMENTS, J. 1903.The Vermilion iron-bearingdistrict of Minnesota. Monogr. U.S.geol. Sirrv.,no. 45,463 p. FAURE, G.;KOVACH, J. 1969. The age of the Gunflint Iron Formation of the Aniniikie Series in Ontario,Canada. Bull. geol. Soc. Amer., vol. 80, p. 1725-36. FRENCH, B.M . 1968.Progressive contact metamorphism of the Biwabik Iron-formation,Mesabi range, Minnesota. Bull. Mim.geol. Surv.,no. 45, 103 p. GOLDICH, S. S.;NIER, A.O.;BAADXAARD, H.; HOFFMAN, J. H.; KRUEGER, H.W. 1961. The Precambrian geology and geochronologyofMinnesota.Bull. Minn.geol.Surv.,no.41,193p. GOODWIN, A. M.1956. Facies relations in the Gunflint Ironformation.Econ. Geol., vol. 51, p. 565-95. GRIFFIN, W.L.;MOREY, G. B.1969.The geology of the Isaac Lake Quadrangle,St Louis County, Minnesota.Spec. Publ. Minn. geol. Sirrv.,no. 8,57 p. GROUT, F.F.; WOLFF, J.F.SR.1955.The geology ofthe Cuyuna district, Minnesota. Bull. Minn. geol. Surv., no. 36, 144 p. GRUNER, J. W.1924.Contributionsto the geology of the Mesabi range, with special reference to the magnetites of the ironbearing formation west of Mesaba. Bull. Minn. geol. Surv., no. 19, 71 p. -.

1946. The mineralogy andgeology of the tacoriites and iron ores of the Mesabi range, Minnesota. St Paul, Minnesota,

Office of the Commissioner of the Range Resources and Rehabilitation, 127 p. HANSON, G.N.; MALHOTRA, R.1971.K-Arages of mafic dikes and evidence for low-graderegional metamorphism in northeastern Minnesota. BuII. geol. Soc. Ainer. (in press). HUNT, T.S. 1873.The geognosticalhistory of the metals. Trans. Amer. Inst. min. (metall.) Engrs., vol. 1, p. 331-95. HURLEY, P. M.; FAIRBAIRN, H.W.; PINSON, W.H.; HOWER, J. 1962.Unmetamorphosedminerals in the Gunflint Formation used to test the age oftheAnimikie.J.Geol.,vol.70,p.489-92. IRVING, R.D.1883.The copper-bearingrocks of Lake Superior. Monogr. US.geol. Surv., no. 5, 464 p. JAMES, H.L.1954.Sedimentary facies of the Lake Superiorironformations.Econ. Geol., vol.49, p. 235-93. . 1955.Zones ofregionalmetamorphism in the Precambrian of northern Michigan. Bull. geol. Soc. Amer.., vol. 66, p. 1435-88. . 1958.Stratigraphy of pre-Keweenawanrocks in parts of northern Michigan, Prof. Pap. U S . geol. Surv., 3144, p. 27-44.

__

__

.

1960. Problems of stratigraphy and correlation of Precambrian rocks with particular reference to the Lake Superior region. Amer. J. Sei., Bradley volume, vol. 258-A, p. 104-44. 1966. Data of geochemistry, 6th Edition. Chapter W. Chemistry of the iron-richsedimentaryrocks.Prof. Pap. U.S. geol. Surv., 440-W,61 p.

LABERGE,G.L. 1964. Development of magnetite in ironformations of the Lake Superiorregion.Econ. Geol.,vol. 59, p. 1313-42. . 1966. Altered pyroclastic rocks in iron-formationin the Hamersley Range, Western Australia. Econ. Geol., vol. 61, p. 147-61. .1967. Evidence on the physical environment of ironformation deposition (Abc.).13th Annual Meeting Institute on Lake Superior Geology M a y I-2,1967,East Lansing,Michigan, p. 25.

LEITH, C.K . 1903. The Mesabi iron-bearingdistrict of Minnesota. Monogr. US.geol. Sirrv.,no. 42, 316 p. MARSDEN, R. W.; EMANUELSON, J. W.; OWENS, J. S.; WALKER, N.E.;WERNER, R. F. 1969. The Mesabi Iron Range, Minnesota.Ore deposits of the United States,vol. 1, chap.25, p. 518-37. New York, American Institute of Mining (and Metallurgical) Engineers. MENGEL, J. T.JR.1965.Precambrian taconite iron-formations. A special type of sandstone, Geological Society of Americu Program for 1965 Annual Meetings, Nov. 4-6, 1965, Kansas City, Missouri,Abstracts,p. 106. MOREY, G.B. 1969. The geology of the Middle Precambrian Rove Formation in northeastern Minnesota. Spec. Publ. Mim.geol. Surv., no. 7, 62 p. -; OJAKANFAS, R.W.1970. Sedimentology of the Middle Precambrian Thomson Formation, east-central Minnesota. Rep. Invest. Minn. geol. Surv.,no. 13, 32 p. MURRAY, A. 1957. Report for the year 1856. p. 145-90. Geological Survey of Canada (Report of Progress for 1853-5455-56). OWENS, J. S.;TROST, L.C.; MATTSON, L.A. 1968.Application of geology at the Butler and National Taconite Operations on the Mesabi range. Society of Mining Engineers reprint Number 68-1-345, Minneapolis Meeting, Sept. 1968. PARHAM, W.E.1970. Clay mineralogy and geology of Minnesota’s kaolin clays. Spec. Publ. Minn. geol. Surv., no. 11 (in press). PERRY, E.C.JR.; BONNICHSEN, B. 1966.Quartz and magnetite: oxygen-18-oxygen-16 fractionation in metamorphosed Biwabik Iron-formation.Science, vol. 153, p. 528-9. PETERMAN, 2.E. 1966. Rb-Sr dating of Middle Precambrian metasedimentary rocks of Minnesota. Bull. geol. Soc. Amer., vol. 77, p. 1031-44. SCHMIDT,R. G.1963.Geology and ore deposits of the Cuyuna North range,Minnesota.Prof. Pap. U.S.geol. Surv.,no.407, 96 p.

207

G.B. Morey

SIMS, P.K.; MOREY, G.B.;OJAKANGAS, R.w.; GRIFFIN, w.L. 19680. Stratigraphic and structural framework of the Vermilion district and adjacent areas,northeastern Minnesota (Abs.). 14thAnnual Institute on Lake Superior Geology,M a y 6-7, 1968, Superior, Wisconsin, p. 19-20. , . > . __ . 1968b.Preliminary geologic map of the

Vermilion district and adjacent areas, northern Minnesota. Minri. geol. Surv.Misc.Mup M-5. SLOAN,E.R. 1964;The Cretaceous system in Minnesota.Rep. Invest. M i m . geol. Surv., no. 5, 64 p.

SPURR, J. E.1894,The iron-bearingrocks of the Mesabi range in Minnesota.Bull. M i m . geol. Surv., no. 10,268 p. SYMONS,D.T.A.1966.A paleomagnetic study of the Gunflint, Mesabi, and Cuyuna iron ranges in the Lake Superiorregion. Econ. Geol., vol. 61,p. 1336-61. SANTON, T.L.1931.Fort William and Port Arthur,and Thunder Cape map areas,Thunder Bay district,Ontario.Mem. geol. Siirv. Can., no. 167,222 p. TRENDALL, A. F. 1968. Three great basins of Precambrian

208

banded iron formation deposition.A systematic comparison, Bull.geol. Soc. Amer., vol. 79,p. 1527-44. VAN Hrse, C.R.;CLEMENTS, J. M.1901. The Vermilion ironbearing district. 21st Annu. Rep. US. geol. Sirrv.,part 3, p. 401-9. ;LEITH, C. K.1911,The geology of the Lake Superior region.Monogr. U.S.geol. Surv., no. 52, 641 p. WHITE, D.A.1954.The stratigraphyand structureof the Mesabi range,Minnesota. Bull.Minn. geol. Surv., no. 38,92 p. WINCHELL, H.V. 1893. The Mesabi Iron Range. Annu. Rep. Minn.geol. Surv., vol. 20,p. 111-80. WOLFF, J. F.1915.Ore bodies ofthe Mesabi range.Engng.Min. J. (Press), vol. 100,p. 89-94, 135-9, 178-85,219-24. . 1917.Recent geologic developments on the Mesabi range, Minnesota. Trans.Amer. Inst.min. (metall.) Engrs.,vol. 56, p. 142-69. YODER, H.S. 1957.Isograd problems in metamorphosed ironrich sediments.Annu. Rep.Geophys. Lab.,p. 232-7.Washington,Carnegie Institute,(Yearbook 56).

Physico-chemicalconditions of the metamorphism of cherty-ironrocks Y.P.Melnik and R. I. Siroshtan Institute of Geochemistry and Physics of Minerals, Academy of Sciences of the Ukrainian S.S.R.

Cherty-iron rocks make up a considerable part of any iron-formation.These rocks are characterized by a variety of mineral phases (from hematite-bearing nonsilicate jaspilites to silicate-bearingcherts and slates), and by a similarity in bulk chemical composition.For example,jaspilites and slates contain different mineral phases, but are very similar to each other in their silica and total iron content, whereas other rock-formingcomponents are in subordinate quantities and do not play any important role in mineral formation. Thus, the chemical composition of such rocks is characterized by the predominance of iron and silica over other components. This characteristic enables us to separate cherty-iron rocks into a separate iron-siliceous isochemicalgroup.This peculiarity of mineral formation in the system Fe,O,-Fe-SiO, has been described by Korzhinsky (1940) and Semenenko (1966). Korzhinsky maintains that, at the early stages of metamorphism, the formation of paragenetic hematite to magnetite was accompanied by the inert behaviour of oxygen.Increased metamorphic alteration is accompanied by a certain activity of oxygen, which results in the replacement of hematite by magnetite ; the former becomes unstable in high-temperaturemineral associations. Semenenko considers that the activity of iron depends on the presence of ferrous oxide,which reacts with SiO,to form silicates; ferric oxide enters into reactions with silica only when chemical potential of Na,O is high. When Fe0 and Fe,O, are both present in the sediment, the ore mineral magnetite forms first; the remaining F e 0 reacts with SiO, to form ferrous silicates. The equilibrium of the iron ore minerals (hematite, magnetite and siderite) in metamorphic rocks was investigated both on the basis of thermodynamic calculation (Hawley and Robinson, 1948; Holland, 1959; Kornilov, 1969; Melnik, 1964a,b, 1966a, 196901,and on the basis of experimental data (French and Rosenberg,1965;Melnik, 19666;Seguin, 1968; Shunzo, 1966). Peculiarities of mineral equilibrium involving the participation of fayalite also have been studied (Melnik and Jarotschuk, 1966). The data obtained helped the more complete understanding

of the role of other components (graphite) and mineral associations (hematite +magnetite, magnetite +fayalitej in elucidating the conditions of metamorphism in chertyiron rocks, and in the creation of the controlled oxidation-reductionsystem-via buffers-that fix the fugacity of oxygen-foz (Eugster, 1961; French, 1966; dames and Howland, 1955). The physico-chemicalinvestigations cited above were, to a certain extent,approximate and not exhaustive because of the absence of thermodynamic constants for a number of rock-forming minerals (amphiboles, micas, chlorites), inaccuracy in calculations of tabular constants (siderite, ferrosilite, fayalite, etc.j, considerable discrepancies between calculated and experimental data,and the absence of the data pertaining to the characteristics of fluid phases under high pressure.Apart from this,many thermodynamic calculations were treated only approximately,without due regard to the effect of pressure. This paper reports thermodynamicanalysis of mineral equilibria carried out using a new system of thermodynamic constants,which included such hydrosilicates as grunerite and minnesotaite. All calculations have been made in accordance with a very precise technique with due regard to the effect of pressure on solid phases (correction for AV> and for fluid phases (correction for the fugacity coefficient -y>. Below w e consider the metamorphic peculiarities of cherty-ironrocks of different lithological composition.

Metamorphism of silicate iron-formation It is believed that, after the processes of sedimentation and diagenesis, in rocks of this type the stable silicate containing ferrous iron is a mineral of the minnesotaite type (ferrous taicj,silica is in excess, siderite is absent, the presence of ferriferous oxide is possible,and the fluid phase in the intergranularspace consists dominantly of water.

Unesco, 1973. Genesis of Precanzbriari iron and manganese deposits. froc. Kiev Syrnp., 1970. (Earth sciences, 9.)

209

Y ,P.Melnik and R.I.Siroshtan

Under progressive metamorphism the low-temperature transformation of minnesotaite to grunerite takes place according to the dehydration reaction:

7 Fe,Si,O,,(OH), tt 3 Fe,Si,O,,(OH), + 4 SiO,-I- 4 H20 (i) The P-T curve for this reaction (Fig. l), at low and moderate pressures, lies in the interval of 250°-280" C and is characterized by a reverse slope. It is worth mentioning that the given position of the curve cannot be thought of as reliably fixed because thermodynamic constants of minnesotaite are based on scarce experimental data. The absence of minnesotaite may testify to the beginning of metamorphism under green schist facies conditions. Grunerite is stable from the beginning stages of regional metamorphism and is a typicalmineral in iron cherty rocks that have been metamorphosed under both green schist and amphibolite facies conditions. But at the top of the amphibolite facies at temperatures of 640°-690"C grunerite is decomposed (Fig. 1) according to the reaction:

2 Fe,Si,OzZ(OH),

+. 7 Fe,SiO, + 9 SO,+ 2 H,O (2) 1°C 800

700

into minerals (fayalite and quartz) stable under granulitic facies conditions of association. Ferrous pyroxene-ferrosilite-bearing assemblages are not stable at any temperatures whenever the pressure is below 15,000 bar, as indicated by the positive value of AZ, for the solid phase reaction: 2 FeSiO,+Fe,SiO, + SiO, (3) but where more than 10-15 per cent molecular magnesium is present, the direction of the reaction is reversed, A summary mineral equilibria diagram with Ig fol-T co-ordinatesat a pressure of 5,000 bar is given in Figure 2. O

-10

-2c

I

1

-30

I P foz -4c

-50

'

600

green schist

arnphiboliie

facies

facies

-60

IO00

T'K 500

400

granulite facies I100

1200

-

FIG.2. Metamorphism of silicate iron-formation (diagram Ig fon-T).Diagram for P,= P/= C (Pa,o, P,?, Po,>= 5 kbar. Isolines for lgf;I,/fH,o are shown as dotted lines. Fe, iron; Hem,hematite; Mgt, magnetite;Fay, fayalite;Cru, grunerite; Min, minnesotaite.

As can be seen,silicate equilibria with magnetite according 300

to reactions

6 Fe,Si,0Z2(OH),

+7 O,

200

3 Fe,SiO, 100

FIG.1. P-T curves ofmetamorphic reactions in iron cherty rocks with excess of silica.To the right of the curve the predominant fluid component is shown. C,graphite; Hem,hematite; Mgt, magnetite;Fay,fayalite;Cru,grunerite;Min,minnesotaite;Sid, siderite. 21 o

3c 14 Fe,O, + 48 SiO,+ 6 H,O + O,e 2 Fe,O, + 3 SiO,

(4) (5)

are buffered and control, with stable T and P, = the fugacity of oxygen. The facies boundaries also are shown on this diagram. W e consider as very important the confirmation by thermodynamic data of the instability of hematite with ferrous silicate under P-T conditions characteristic of any metamorphic facies. Because in essential fluids water can consist of decompositionproducts only (not counting neutral gases), we

Physico-chemicalconditions of the metamorphism of cherty-ironrocks

'

Total diagrams of equilibria

i, Gru \

t

-6

-4

-2

O

c2

+4

16

-6

-4

-2

O

1.2

14

16

-6

-4

-2

1.2

O

t4

t6

Detailed diagrams of mineral equilibria

t4

I

I

+2 -3 -2 -I green schist facies

o

tl

-3

-2

*I

o

+i

amphibolite facies

-3

-2

-I

o

tl

granulite facies

(ai

(cl

FIG.3, Metamorphism of silicate iron-formation (diagrams In (a), (b) and (c), T=6OO0K (327O C),800"K

(527"C), and 1,000"K (727"C), respectively. Fe, iron; Hem, hematite; Mgt, magnetite;Fay,fayalite;Cru, grunerite.

can also construct diagrams with PHz0-PH2-T co-ordinates. However, diagrams with lg -lg fH,-T co-ordinates and, especially their isothermal section, are more useful. Such sections are shownin Figure 3 for temperaturescorrespondingto the changes in metamorphicfacies conditions.l From diagrams (b) and (c) the composition of fluid and, in particular,the hydrogen content,which is in equilibrium with the mineral association in question, can easily be defined. Pure water is an oxidizer in relation to ferrous silicate. When a considerable quantity of water enters the rocks, this can lead to replacement (partial or complete) of barren minerals by magnetite according the reactions:

quite possible in hydrothermal activity-is required for the oxidation of 1 g grunerite into magnetite. But for the oxidation of magnetite into hematite via a similar process (a variant of hypogene martitization) an enormous quantity of water is needed and, as such,the phenomenon can be only of local importance.

lgfE20-lgfHz).

3 Fe,Si,O,,(OH),+ 4H,O

= 7Fe304+ 24 SiO,+ 7 Hz (6) 3 Fe,Si04+ 2 H,O

= 2 Fe304+ 3 SiOz+ 2 H,. (7) Thus,under the above-mentionedamphibolite facies conditions, approximately 100-150 g pure water-a quantity

Metamorphism of carbonate iron-formation The diagnostic features of carbonate iron-formation are the occurrence of siderite in paragenesis with quartz and the occurrence of Fe oxides,both magnetite and hematite; hydrosilicates with ferrous iron do not occur. Thenature of carbonate iron-formationmetamorphism depends, to a certain extent, on the presence of hematite 1. Isobar numbers correspond to P,=P~=~(FH,IJ +PE,+ Po?),kbar. Thin incline lines-isobars

Ig fo,.

211

Y.P.Melnik and R.I. Siroshtan

because, in such cases,under comparatively low temperatures (300"C)and PeO1= 2,000bar (Fig. i) the following reaction is possible: FeCO, +Fe,O, = Fe,O,+ CO,.

(8)

But the equilibrium assemblage siderite + heinatite+magnetite depends greatly on pressure in as much as the slope of the P-T curve defining a decarbonatization reaction is much steeper than that for dehydration reactions. It is believed that Auid consists dominantly of carbon dioxide, which is why the above-mentionedreaction does not define precisely the lower temperaturelimit of the green schist facies and why sideriteand hematite sometimesoccur with grunerite at temperatures up to 390"-420"C and Pcoz = 5,000-7,000bar. After disappearance of hematite at the completion of this reaction, pure FeCO, remains unchanged because, at temperatures lower than 400'-500" C,the formation of magnetite from carbonate requires the presence of oxidizers that must be derived from outside the system.In Figure 4, with Pco2 = 5,000bar, the phase limit of bivariante quilibrium of the siderite +magnetite assemblage corresponds to the temperature interval 38O0-5OO0 C. Above 400°-500" C dissociation of siderite is theoretically possible according to the reaction: .3 FeCO, = Fe,O, +2 CO2+ CO

(9) but the proportion CO:CO,=l :2,as required by the O

-10

-20

1

equation,is metastable because of the dissociationofcarbon monoxide to form graphite:

2CO=C+COz. (10) It has been shown by many investigators that graphite, whether newly formed or already present in the rock, is an oxygen buffer which can regulatefo, and fc0 in the carbonate ñuid. The line of graphite stability divides the diagram (Fig. 4) into two parts. Only the minerals whose fieldsof stability are crossed by this line-siderite, magnetite and fayalite-can be found in equilibrium with graphite. Mineral associations in the shaded area of Figure 4 cannot exist,as it is physically impossibleto createsuch low values of fog in carbonate rocks. Mineral assemblages found in an unshaded field are stable only in the absence of graphite. Analogous observations should be taken into consideration when studying the isothermic sections of diagrams with lg fco2-lg feo co-ordinates for the temperatures of various metamorphic facies (Fig. 5). Five petrological conclusions drawn from the analysis of Figures 4 and 5 are as follows. First,heinatitecannotexist in equilibrium with graphite at any temperature. Under green schist faciesconditions, hematite must react to form siderite: 2 Fe,O, + C 4-3 CO,+4FeCO,

(1 1)

or magnetite:

6 Fe,O,

+C+

4Fe,O, 4-CO,

(12) depending mainly onfeo,. Under amphiboliteand granulite facies conditions,reduction is only possible according to reaction (12). Second,siderite is a stable mineral up to temperatures of the beginning of amphibolite facies. Third,the equilibrium transformation of siderite into fayalite is thermodynamically impossible as the stability fields of these minerals are separated at any temperature by the magnetite field along the graphite join.Reaction

2 FeCO,+ SiO,= Fe2Si04+2 CO,

(13) is not an equilibrium reaction.Only the phase transformation of siderite into magnetite by reaction (9) is possible, reduction of magnetite to fayalite then follows. Fourth,at temperatureshigher than 500"-600"C under amphibolite facies conditions,the association of magnetite with graphite (Fig. 5(b) and (c)) becomes unstable as a result of the reaction:

-3c

kfo,

1-41

-51

2 Fe,O, + C + 3 SiO,+3 Fe,Si04 + CO,. c5 1

500

600

700

800

900

T"K

1000

II00

1200

-

FIG. 4. Metamorphism of carbonate iron-formation(diagram lgfo,-T). Diagram for Ps= Pf= C(Pco,, Pco,Po-= 5 kbar.In a dotted line isolines Ig fe0/fco~are shown.Area of metastability under the line of graphite is shaded.C,graphite;Fe,iron;Hem, hematite; Mgt, magnetite; Fay, fayalite;Sid, siderite.

212

(14)

Finally,under granulite facies conditions,graphiteis stable only with fayalite. By using the diagrams (Figs. 4 and 5), one can find equilibrium fluid compositions. Because increased temperature causes the graphite field to become smaller, the carbon monoxide content in any fluid in equilibrium with graphite must increase. Carbon dioxide, as well as water, can be an oxidizer for silicates containing ferrous iron, but in this case still

Physico-chemicalconditions of the metamorphism of cherty-ironrocks

greater quantities are required. This is why formation of magnetite in such a way can hardly be of ore-making importance.

could appear in the course of conjugated oxidation-reduction reactions involving the participation of COz, evolved in the disassociation of carbonates, for example reaction (9) or the reaction

Metamorphism of silicate-carbonate iron-formation

3 Fe,SiO,

$2 Fe,O,

-1- 3 SiO,+2 CO (1 5)

with further dissociation of CO as per reaction (10). For this reason it is necessary in the analysis of mineral equilibria to build sectional diagrams along the line of graphite stability (Fig. 5). Using fGo, and fHs0 as independent variables, such a section at a constanttemperaturewill represent the surface on which oxygen fugacityis controlled everywhere by the presence of graphite (Fig. 6), as per the reaction: c -1- O, CO,. (16)

A great number of cherty-ironrocks are not represented by purely silicate or carbonate types, but by combined silicate-carbonateones.Because the rock-formingminerals are ferrous silicates and siderite,the presence of a certain quantity of graphite, in the role of oxygen buffer, is required. Graphite could be formed by the metamorphism of organic carbon originally present in the sediment, or it

I

+2 CO,

I

I

t6

+5

+4 +3

+2

+I

-4

-2

O

+2

+4

-4

+6

green schist facies ia)

.

-2

+2

0

+4

-4

+6

-2

O

amphibolite facies

granulite facies

íb)

(C)

FIG.5. Metamorphism of carbonate iron-formation (diagrams Ig fco,-lg feo). Numbers of isobars correspond to Ps=P~ =C(Pco2, Pco, Po,),kbar.Area of metastability is shaded.In

t2

t4

+6

(a), (b) and (c), T=600° K (327O C), 800"K (527' C), and 1,000"K (727"C), respectively. C,graphite; Fe, iron; H e m , hematite;Mgt, magnetite; Fay, fayalite; Sid, siderite.

+? C6

Ig fco2

I

+3

+2 ti

o

-1

41

+2

+3

+4

+5

o

.I

+1

green schist facies

amphibolite facies

granulite facies

(a)

íb)

(C)

FIG. 6.Metamorphism of silicate-carbonateiron-formation(diagrams Ig feo,-lg fH,o in the plane of graphitestability). Isobar numbers correspond to P,=PI=C(PC,?, PH~o, PH?,PGO,PO,), kbar. Thin incline lines denote isobars lg fH2;thin dotted lines

c2

+3

+4

+5

denoteisobars lg foz;thin hachures denote isobars Ig fco. In (a), (b) and (c), T = 600" K (327"C),800"K (527"C),and 1,000"K (727" C), respectively. Mgt, magnetite; Fay, fayalite; Gru, grunerite;Sid,siderite. 213

Y.P. Melnik and R.I. Siroshtan

This enables us to include in the diagram isobars of oxygen fugacity and isobars of carbon monoxide fugacity as per reaction (10). If foz and ffis0are known, one can easily determine for every point in the diagram the fugacity of hydrogen, according to the reaction of water decomposition: 2 H2O 2 H2-1- 0 2 (17) and draw corresponding isobars. The resulting diagrams make it possible to carry out a detailed analysis of mineral equilibria and to determine at a given T and P the content of any of five volatile components of the fluid phase. Let us consider, first of all,the relation of siderite and grunerite, as determined by the reaction: 7 FeCO,+ 8 Si02+ H,O

= Fe,Si,O,,(OH),+7 COz (18) Siderite+ grunerite (-I quartz) inequilibriumoccupy a wide field under P-T conditions of low-temperaturemetamorphism, and this is the main mineral association in green schist facies rocks (Fig. 6(a)). However, under amphibolite facies conditions (Fig. 6(b)) this mineral association theoretically can remain only under a very high fluid pressure of more than 7,000-8,000bar. Within this stability field the formation of grunerite depends not on temperature but rather on the relation of CO,and HzOin fluid.Thus,the main cause for the development of grunerite in metamorphic rocks is the presence of a sufficient quantity of water,which provides a stablehigh value for PHtoand the relation Pzz0:Pco,with the expenditure of water according reaction (18). One can assume that the development of grunerite through metamorphism is connected with processes involving the dehydration of chlorites,hydromicasand other hydro-minerals,to form the slatebeds which are interlayeredwith iron cherty formation. The equilibrium siderite+grunerite+magnetite is monovariant, and at pressures of 4,000-8,000 bar corresponds to a temperature of 420°-530" C.At higher temperatures, the bivariant equilibrium grunerite+magnetite exists, at 550°-650° C,it is replaced by the equilibrium grunerite +fayalite and above 670"-700"C by fayaliteonly (Fig. 6(c)). The above data prove that,in fluids of complex composition, temperatures of equilibrium reactions involving the participation of only one volatile component drop markedly,and the appearance of fayalite becomes possible under amphibolite facies conditions.

Metamorphism of oxide iron-formation The rocks of this type of iron-formationare represented by sediments originally having iron hydroxides that were transformed during diagenesis into goethite and, possibly, into hydromagnetite or magnetite. The transformation .of 214

goethite into hematite takes place prior to metamorphism at temperaturesup to 120°-180"C.Themetamorphic transformation of hematite into magnetite according to the reaction 6 FezO,&4 Fe,O, + O2 (19)

is possible only in the presence of reducing agents (free carbon,gaseous CO or HJ.Where no reducing agents are present, the association of hematite + magnetite is quite stablein all facies of metamorphism,includingthe granulite facies. The equilibrium of hematite with other minerals has been considered in previous sections of this report.

Certain peculiarities of low-temperature metamorphism of iron cherty formation A great number of iron cherty rocks which have undergone metamorphism to the green schist facies are characterized by the close and frequent interlayering of various beds that have different compositions and modes of formation. In some places, within a distance of some centimetres, a bed of siderite-hematiteis replaced by one of gruneritemagnetite,and silicate-richbeds having graphite are interlayered with hematite-magnetite beds that contain no free carbon.A detailed investigation of layered rocks has provided enough evidence to suppose that specific equilibrium conditions occur in a limited volume (mosaic or local equilibrium); this equilibrium volume is characterized by a fluid phase of quite different composition. Probably diffusion, not only of the solid phases, but also of the fluid phase as well, was limited at low temperatures. These separate volumes of the metamorphosed rocks can be considered as closed systems, and iron cherty formation,taken as a whole, can be treated as a number of closed systems. Under amphibolite facies conditions, there is a tendency towards the equalization between beds of the fluid composition and thus towards a reduction in the diversity of mineral associations. The analysis of separate groups of rocks that belong to different metamorphic facies reveals the presence of a certain metamorphic zoning in the iron cherty formations of the Ukrainian Shield.

Metamorphism of iron cherty formations and ore deposition Banded iron cherty rocks make poor ores,their value being determined by their magnetite contents. Magnetite crystallization probably took place in early stages of metamorphism via the reduction of hematite by carbon according to reaction (12),or by the reaction of hematite and

Physico-chemicalconditions of the metamorphism of cherty-ironrocks

siderite according to reaction (8), and under metamorphic conditionsnear the amphibolite facies grade by the thermal dissociation of siderite according to reaction (9). Thus, metamorphism at low and moderate temperatures contributes greatly to an increase in ore quality. However, the introduction of water at these metamorphic stages has a negative effect, as it results in the formation of grunerite according to reaction (18) instead of magnetite.

Under amphibolite and granulite facies conditions, a surplus of graphite leads to the development of silicate -grunerite or fayalite-at the expense of magnetite, thus reducing the ore quality to a certain extent. However, introduction of a considrablequantity of pure water sometimes may lead to a reverse process; that is oxidation of silicatesto magnetite.

Résumé Les conditions physico-chimiques du métamorphisme des jòrmatioiis de jeu siliceux (Y.P. Melnik, R.I. Siroshtan) 1. Les roches de fer siliceux,qui ont une grande extension dans les limites du bouclier ukrainien, se rencontrent non seulement en strates qui contiennent de riches dépôts de minerai de fer,mais aussi en minerais à faible teneur,dont la concentration est facile,Les formations de fer siliceux sont des sédiments chimiques métamorphosés ; le fer et la silice dans ces roches proviennent de fumerolles hydrothermales actives au cours d'un volcanisme subaquatique dans des zones géosynclinales. 2. Les roches de fer siliceux sont de différents types pétrographiques.En raison des proportions différentes des minerais,des silicates et des carbonates,ces roches peuvent être classées en trois types : les roches de minerai, dans lesquelles le composant principal est la magnétite ou l'hématite ; des roches à faible teneur, dans lesquelles le quartz joue un rôle important avec d'autres minerais ; les roches sans minerai, dans lesquelles seuls les silicates et le quartz sont les minéraux constituantla roche. 3. Les principaux minéraux des roches de fer siliceux (qui consistent en Sioz, Feo,Fe,O,, H,O et CO,)sont le quartz, la grunerite, la fayalite,la magnétite, l'hématite et la sidérite.L a silice est un composant en excès toujours présent dans les associationsminérales sous formede quartz. Il en est de m ê m e de Fe,O,,qu'on rencontre dans les conditions de basse température et qui détermine l'apparition de l'hématite. L a présence de grunerite ou de sidérite dépend du rapport H,O/CO,. 4.Suivant les conditions dans lesquelles les roches de fer siliceux se sont métamorphosées, on peut distinguer trois faciès de métamorphisme : le schiste vert, l'amphibolite et la granulite.Les roches de fer siliceux des trois faciès se rencontrent dans la région structurale KrivoyrogKremenchug. Cela reflète l'irrégularité des conditions thermodynamiquesau cours du métamorphisme.Des différences dans la composition minérale des roches de fer siliceux sont le résultat de changements dans les conditions du métamorphisme tout le long du filon. 5. Les paramètres physicochimiques de la stabilité de i'hématite, de la magnétite, de la sidérite,de la grunerite, de la fayalite et du graphitedans les minerais de fer siliceux métamorphosés sont à la base d'un nouveau système de

constantes thermodynamiques des minéraux. Les diagrammes d'équilibre minéral ont été établis pour des condi- '. tions P-T de faciès de schiste vert, d'amphibolite et de granulite pour des compositions H,O/CO,d'un fluide (en coordonnées log foi et T ; log fco, et log fco ; log fHz0 et log fH2; log fco, et 1% fH@ etc.). 6. Les limites supérieuresde température des associationsminérales typiques sont calculées à partir des données thermodynamiques ; elles s'accordent avec les observations pétrographiques : Sidérite+hématite +magnétite 200-400"C ; Sidérite+ grunerite+magnétite + graphite 370-500" C ; Sidérite +magnétite +graphite 400-550"C ; Fayalite +magnétite +graphite 500-600" C ; Grunerite +fayalite + quartz 640-690" C. 7. O n trouve que l'hématite ne peut pas coexister en équilibre avec les silicates de Fe2+ (grunerite, fayalite) ou avec le graphite dans les roches de fer siliceux quels que soient P,T et la composition du fluide.L a transformation métamorphique de la sidérite (+quartz) en fayalite est peu probable, puisque dans les conditions P-T du faciès de schiste vert la sidérite doit se transformer en grunerite ou en magnétite, suivant la valeur du rapport CO,/H,O dans le fluide. L'association magnétite -1- quartz + graphite devient instable dans les conditions du faciès amphibolitique et se transforme en grunerite ou fayalite stable, suivant la valeur de Ces processus expliquent l'abaissement de la qualité du minerai du fait du métamorphisme, résultat des transitions des minerais de fer (magnétite, sidérite) en silicates. 8. L a comparaison des diagrammes et des données pétrologiques permet de montrer que l'équilibre a le caractère d'une mosaïque aux premiers stades du niétaniorphisme. Les roches situées dans des régions séparées diffèrent beaucoup, après métamorphisme, quant à la composition du hide (variétésfo,,fEZo, fC,> et quant à la teneur en composés volatils, essentiellementl'eau. 9. Si l'on suit la composition minérale des roches de fer siliceux tout le long du filon de Krivoyrog-Kremenchug, on observe un zonage dans le faciès métamorphique. La partie centrale de cette région consiste en roches de faciès granulitique qui,en direction du nord et du sud,remplace les faciès amphibolitique et de schiste vert. Vers le sud de Krivoyrog le stade du métamorphisme est encore plus 215

Y.P.Melnik and R. I.Siroshtan

élevé, avec des roches représentées par un faciès aniphibolitique. Les variations qu'on observe dans les types de rocheç de fer siliceux dénotent des variations de P-T au

cours du métamorphisme.L'âge du métamorphisme est de 1 900 à 2 100 millions d'années d'après des datations par la méthode K-Ar.

Bibliography/ Bibliographie EUGSTER, H.1961. Physico-chemicalproblems of rocks and ores formation. Vol. I, Moscow, Academy of Sciences of the U.S.S.R. (In Russian.) FRENCH, B. M.1966. Rev. Geophys.,vol. 4,no. 2, p. 223. FRENCH, B. M.; ROSENBERG, P. E. 1965. Science, vol. 147, no. 3663, p. 1283. HAWLEY,J.E.; ROBINSON, S.C.1948.Econ.Geol.,vol.43,p. 603. HOLLAND, H.G.1959. Econ. Geol., vol. 54, no. 2. JAMES, H.L.;HOWLAND, A.L. 1955. Bull.geol. Soc. Amer., vol. 66, no. 12, p. 1580. KORNILOV, N.A.1969. C.R.Acad. Sei. URSS,vol. 184,no. 4, p. 939. (In Russian.) KORZHINSKY, D. S. 1940. Factors of mineral equilibria and mineralogical facies of depth, Bull. Inst. Geol. Acad. Sei. USSR,vol. 12, no. 5. (In Russian.) MELNIK, Y.P. 1964a. Acad. Sci. USSR,Geol, Rudn. Mestorozdenij, no. 5, p. 3. (In Russian.) __ . 19646. Acad. Sci. UIcSSR, Geol. Zum., vol. 5, no. 5, p. 16. (In Ukrainian.)

216

-.

1966a.In:Problemstheory and experimentin oreformation, p. 58. Kiev, Naukova Dumka. (In Russian.)

19666. In: Research on nature and artificialformation of mineuals, p. 120. Moscow, Nauka. (In Russian.) -. 1969a. In: Problems of genesis of precambriun iron rocks, p. 259. Kiev, Naukova Dumka. (InRussian.) -. 1969b. Acad. Sei. UIcSSR,Geol. Zurn., vol. 29, no. 4, p. 13. (In Russian.) MELNIK, Y.P.;JAROTSCHLK, M.A. 1966. In: Problems theory and experiment in ore formation, p. 98. Kiev, Naukova Dumka. (InRussian.) , 1970.Acad. Sci. USSR,ZapiskiMiner. Ob.,vol. 99,no.1, p. 3. (In Russian.) SEGUIN,M.1968. Nat.canad.,vol. 95,no. 6,p. 1195,p. 1217. SEMENENKO,N.P. 1966. Metamorphism of Mobile Zones, Kiev, -.

Naukova Dumka. (In Russian.) SHUNZO, Y.1966. Econ. Geol.,vol. 61,no. 4,p. 768.

The Serra do Navio manganese deposit (Brazil)' W.Scarpelli Industria e Comercio de Minerios S.A.(Brazil)

Introduction

The Gneisses

The Serra do Navio manganese deposit is in the Federal Territory of Amapá,in northern Brazil (Fig.1). Production started in 1957 and up to the end of 1969,10million metric tons of washed ore had been produced, most exported to North America and Europe.The average grade of the commercial,beneficiated ore varies from 48 to 50 per cent of manganese. Beneficiation consists of crushing,washing and classification by size and density. The deposit is part of the PrecambrianGuyana Shield, at the left bank of the River Amazon. In Serra do Navio and vicinity this shield is composed essentially of gneisses, amphibolites, schists and quartzites (Table 1) plus pegmatites and quartz veins.

Gneisses are the most common rocks in the neighbourhood of Serra do Navio. The other metamorphic rocks seem to occur as inclusions in them. The predominant type of gneiss is leucocratic and composed essentially of quartz, microline and/or oligoclase, and biotite, occasionally with dark hornblende-richzones parallel to the foliation. In some places a gneiss with very high quartz content forms prominent ridges. This type of gneiss probably is the product of metamorphism of a silica-richrock, possibly quartzite.

TABLE1. Stratigraphiccolumn of the Serra do Navio district Series

Group

General description

Lithologic units

Serra do Navio

Metasediments

Jornal

Amphibolites

Amphibolites Schists Quartzites

Gneisses

Gneisses

Manganese protores Graphitic facies Biotitic facies Quartzose facies

Amapá

???

-

The oxide ore bodies are the product of secondary enrichment of protores which occur in the Serra do Navio group, outcrop in topographic ridges,and are mined from open pits.

The Jornal group The amphibolites of the Jornal group are second to the gneisses in areal extent. They are not uniform in texture or mineralogical composition, varying considerably even in short distances.The predominant mineral is green hornblende, followed by andesine-oligoclase and, in variable percentages, magnetite, titanite, diopside, tremolite, carbonate, and sulphides.Quartz occurs in small veins. Differences in adjacent bands of amphibolite are conspicuous. The bands vary in grain size (fine to medium or coarse-grained), in structure (well or poorly developed foliation), in texture (presence or absence of oriented minerals) and in mineralogical composition.It is very possible that the amphibolites are derived from a heterogeneous rock sequence.This possibility is reinforced by the occurrence within the amphibolitesof belts of quartzites,gneisses and biotite schists concordant with the foliation. These belts show that the rock column from which the amphibolite originated by metamorphism was not homogeneous. From these observationsthe origin of the amphibolites cannot be inferred.It is relatively certain that at least part of the sequence is of sedimentary origin,as testified by the 1. With permission of Industria e comercio de Minerios S.A.,ICOMI, Rio de Janeiro, Brazil.

Unesco, 1973. Genesis of Precambrian iron and mungunese deposits. Proc. Kiev Synzp., 1970. (Earth sciences, 9.)

217

W.Scarpelli

* Y)

SCALE

-

1:6.000.000

CONVENTIONS

0

city Village

__t_

FIG.1. Geographic location of the Serra do Navio manganese district.

218

Roilroa,d

The Serra do Navío manganese deposit (Brazil)

as determined from thin sections. It must be emphasized that themineralpercentagesof these rocks changemarkedly from layer to layer and place to place, thus the tabulated data are given only to illustrate the variety of observed minerals and mineralogicalcomposition.Quartz and biotite are the most common minerals, followed by graphite,muscovite, sillimanite, garnet (usually almandine), plagioclase (oligoclase, rarely andesine), andalusite, sulphides, and other less frequent minerals. The predominant sulphide is pyrite, followed by chalcopyrite and arsenopyrite. The manganese protores occur as lenses in the upper part of the graphitic facies.There are two types of protore. The thicker and richer in manganese is composed essentially of rhodochrosite, followed by manganese-bearing silicates such as spessartite,tephroite and rhodonite. The thinner lenses,poorer in manganese,are composed of spessartite, amphiboles,quartz, and graphite. In the quartzose facies there are layers very rich in calcium carbonate and silicates. They occur in two types. The thicker,which can be described as marbles, are composed essentially of calcite pIus some calcium silicates,The thinner are composed of coarse-graineddiopside,tremolite, calcite, and pyrrhotite and can be called a calc-schist. The bedding of the metasediments, which survived at least three metamorphic phases and is recognizable in the field,was preserved by the developmentof the metamorphic foliation parallel to it (with some exceptions) and by the great differences in composition of individual layers.

belts of quartzites and the belts of diopside-calcite-tremolite-titanite.O n the other hand, some intercalated bands of gneiss have remnants of a typical porphyritic texture, indicative of an igneous origin. N o field or microscopic evidence was found indicativeof the origin of the amphibolite layers. At one point in the River Amapari there is a good exposure of the contact zone between the gneisses and the amphibolites,N o gradationalchange was observed in either of these two rocks toward the contact,which is parallel to their foliation.At the actual contact zone the gneiss alternateswith the amphibolite in a seriesofcontinuousunfolded bands, each one about 0.3-1 .Om thick. It is possible that there is no great difference in age between the rocks from which the gneisses and the amphibolites originated,as both had the same metamorphic history.

The Serra do Navio group Above the amphibolites there is a sequence of metasediments composed of quartzites,schists and carbonate-rich layers (Fig. 2). These units alternate in a relatively cyclic pattern and are subdivided into three distinct facies, a quartzose, a biotitic,and a graphitic. The minerals which occur in these facies are almost the same, but occur in variable percentages. Table 2 shows the mineral compositioii of these facies

TABLE 2. Composition of the metasedimeiitary facies of the Serra do Navio groupl Biotitic facies (13 samples)

Quartzose facies (16 samples) Mineral Average

( %)

Quartz Biotite Graphite Muscovite Plagioclase Silimanite Andalusite Staurolite Garnet Cordierite Tremolite Diopside Titanite Carbonate Hornblende Tourmaline Sulphide

Maximum and minimum

( %)

Number of positive samples

Average

( %)

Maximum and minimum

Graphitic facies (14 samples)

( %)

Number of positive samples

Average

(%)

60-3

16

29

50-20

13

25

10

53-20 8-traces

13

14

8-0

340

4

25-0

13 10 5

26

3

12 3

35-0

12 12 9 5

34 4

8

33-0 9-0 25-0

8

2

5 1 15

25-0 16-0 1-0

10

17-0 traces-O 35-0

6 5 11

7 6 6 O

30 9

4

4 traces 5

O

-

8-0

O 8 2 6 2 3

20-0

12

6-0 15-0 5-0

10

10

50-0

I

10-0 6-0

1 1 3 3

2

Epidote

3

Chlorite

1

7-0

5 6

4 traces 8 2 O O O O O

25-0 18-0

1 1 1 1

3-0

-

3-0 5-0 5-0

2

O O O

O O 12 9 1 4

4 3

-0

O traces traces O 2 1 5 5

Maximum and minimum

(P4)

38-8 340 45-15 13-0

Number of

positive samples

14 13

14 8

25-0

4

5-0 20-0

8 8 10 10

10-0 10-0

-

O O O

1-0

1

1-0

-

1 O

6-0

10 10

2-0 30-0 45-0

4 5

1. Excluding the manganese protorcs and the quartz-free calc-siliceous layers of the quartzose facies.

219

FIG. 2.Geologicalmap ofthe Serra do Navio District.Contour lines at 50 metre interval.Contacts are inferredfrom the known data. At the centrethe ‘y’shaped metasediments of the Serra do

Navio group (SNV)overlying the Jornal group (J), and, at West,the gneisses (G). The dotted areas represent the outcrops and float of the manganese ore bodies.

The Serra do Navio manganese deposit (Brazil)

MANGANESE PROTORES

There are two types of manganese protore,one carbonatic and one siliceous,or garnetiferous (Table 3). The carbonatic protore has an average 31 per cent of manganese and is composed principally of rhodochrosite, followed by spessartite,occasionally with veins and bands of tephroite and rhodonite. As accessories there are sulphides (sphalerite, niccolite,gersdorfite), graphite and orthoclase.The texture is mosaic,sometimes disturbed by shearing.In the groundmass of rhodochrosite spessartite crystals grew to a maxim u m diameter of 0.5-1.0mm. TABLE 3. Chemical analysis of protore samples A

Mn Fe SiO2 A1203

CO2

C Ca0 MgO

NazO K2O

s

As P Ignition loss

B

C

D

E

F

G

3.6 35.7 27.7 4.8 3.7 2.8 32.6 15.3 49.7 12.7 1.9 3.6 8.9 27.5 1.2 19.3 8.6 3.4 8.2 7.9 4.3 0.6 3.5 0.8 1.7 3.1 1.7 0.3 0.4 0.2 t0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 1 .o n.a. 1 .o 0.6 0.3 0.01 0.1 0.2 <0.1 na. n.a. 0.1 0.2 0.1 n.a. n.a. 0.03 0.04 0.04 0.03 0.08

36.6 31.8 1.3 0.4 6.6 3.5 2.9 2.6 33 $4 32.4 na. na. 4.9 8.1 2.9 3.7 na. 0.3 na. 0.1

24.1 3.6 34.7

na. n.a. 10.2

33.7 3.7 17.1 2.4 26.3 9.3 0.7

25.5

26.7

19.7

9.O

A, Carbonatic protore (TG-59, 106 m); B, Carbonatic protore (C2); C, Transition between garnetiferous and carbonatic protores (T6-72, 121 m); D,Carbonatic protore (TG-72, 124 m); E, Carbonatic protore (TG-72, 125 m); F, Transition between garnetiferous and carbonatic protores (T6-72, 128 m); G, Garnetiferous protore (TG-72, 130 m); n.a.. Not available.

The siliceousor garnetiferous protore has 5-25 per cent of manganese and is formed essentially of manganesebearing garnets up to 3.0mm in diameter. Graphite is a minor component, occurring as inclusions in the garnets or in interstices between the garnets. Quartz appears in small veins or between the garnets. A very fine-grained amphibole,probably manganesiferous,locally replaces the garnets. The carbonatic protore, the more important of the two, occurs as lenses of variable extent and thickness. The largest known lens in the Serra do Navio district is almost 1 km long,with a thickness of 20-30 m , the thicker parts in fold axial zones. On average, the lenses of this type of protore have thicknesses of 10-20m , and lengths of 200-400 m.The garnetiferous protore appears as layers, only occasionally thicker than 2.0 ni, generally at the contact between the carbonatic protore and the enclosing schists.It also inarks the lateral continuity of the manganiferous horizons away from the carbonatic lenses. The carbonaticprotore is the product ofmetamorphism

of the original manganese-richsediment,which was a carbonate almost free of silica-bearingclastic fragments. O n the other hand, the garnetiferous protore represents either the metamorphism of an impure manganiferous sediment or the product of metamorphic reactions between the carbonatic protore and the adjoining sediments.

Q U A R T Z O S E EACIES

The quartzose faciesis a fine-grainedmetamorphosedchert, with intercalations of calc-silicate layers. Its foliation is parallel to its bedding planes, which are recognizable by variations in mineral composition between adjacent layers. It frequently has calc-silicate layers (not represented in Table 3) which are coarse-grainedand composed of variable amounts of calcite, diopside, pyrrhotite,grossularite, and tremolite,plus some titanite. Probably the original sediment was a mixture of chert, calcite and some clay,and during diagenesis the carbonatic material was segregated from the siliceous in the form of intercalated and discontinuous lensoid layers.Later metamorphic reactions between the carbonatic and the siliceous minerals resulted in the calc-silicates.

BIOTITIC FACIES

The biotitic facies is a biotite-schist.Its foliation is very well developed and commonly discordant with the bedding planes.It has a grain size coarser than the other two facies due to a more intense recrystallization,which also obscured the bedding planes. Sillimanite, andalusite, and a pink almandine occur as porphyroblasts of several millimetres in a mass of quartz and biotite. The biotitic facies is the product of metamorphism of a pelitic sediment.In the sedimentary column it grades both into the quartzose facies and the graphitic facies.

GRAPHITIC FACIES

Darker than the other two facies, the graphitic facies has its high concentration of graphite as the only real distinction froin the other two. Sometimes it resembles the quartzose facies, with its fine-grain size, high quartz content, and very well-preservedbedding, and sometimesit is more like the biotitic facies,with its coarser grain size,good foliation, poorly recognizable bedding planes, and presence of larger porphyroblasts of andalusite. The graphitic facies is possibly the metamorphic product of a clayey sedimentrich in carbonaceousmaterialwhich probably originatedfrom organic matter accumulated with the clays and preserved through diagenesis and metamorphism. The free carbon did not originate through decompositionofcarbonates,because carbonaticlayers almost free of graphite are frequent in the quartzose and gra-phitic facies,none of them revealing signals of decomposition of 221

W.Scarpelli

phitic facies (with 31 per cent of the analysed contacts), and followed by the biotitic facies (with 25 per cent), the quartzosefacies(with22per cent), the garnetiferousprotore (with 16 per cent), and the carbonatic protore (with 6 per cent). From the frequency of observed contacts of each lithologic unit the probability that each rock unit would occur in contact with each one of the others if they occurred in a random order,and also the actual distribution of the contacts was calcdated by dividing the actual data by the values calculated for a random distribution of the contacts, a correlation coefficient was obtained that indicates if each pair of the rock units occur in contact more or less frequently than expected in a random distribution.Correlation coefficientsgreater than 1.Oindicatethose contactspreferred by nature and those of values lower than 1.0show the contacts which were not preferred. The clear predominance of the gradational over the nongradational contacts suggests that the changes in rock lithology during sedimentation proceeded preferentially in a gradative way. The transition zone of the gradational contacts is normally only a couple of metres thick, but it can be expected that it was thicker during sedimentation. Although some ofthe gradationalcontactscan be attributed to metamorphic reactions,the metamorphic conditions did not strongly favour these reactions, as is testified by the

the carbonates into graphite. Furthermore, the more graphitic zones have no sign of carbonates, although they sometimes are close to lenses of carbonates.

STRATIGRAPHIC SEQUENCE

I

In Serra do Navio there are several repetitions of each metasedimentary facies in the stratigraphiccolumn.A statistical verification of the vertical distribution of the facies and their mutual relations revealed a clear transitionalpattern between the facies and a tendency for cyclic recurrence of that pattern. This analysis was made possible by the availability of fresh rock core samples collected by diamond drill holes throughout the district. In a preliminary verification of the preferential associations between the rock units, 421 contacts between the metasedimentary facies and the manganese protores were investigated.At each contact it was noted which rock unit occurred at each side of the contact and whether the contact was gradational or not. Although they were known, the thickness of the rock units and the local structure were disregarded. Table 4 presents the numerical results of that analysis. The most common rock unit encountered was the gra-

TABLE 4.Contacts between the metasedimentary rock units Probability of contacts by random distribution

(%)

R o c k unit Name

Frequency

(%)

All contacts (421)

Actual distribution of contacts

M c

M g

G

B

Q

M c

16 33 27 24 7 37 30 26 17 G 31 9 23 - 36 32 6 B 25 8 21 41 - 30 3 Q 22 8 19 40 33 4 Standard deviation of the correlation coefficients = 0.45 Mc

Mg

6 16

Correlation coefficient (actual/probability)

( %)

-

Gradational contacts (306) Mc 5 - 17 34 27 22 Mg 16 6 38 31 25 20 G 32 7 24 - 38 31 4 B 26 7 22 43 28 O Q 21 6 20 41 33 2 Standard deviation of the correlation coefficients = 1.13 Nongradational contacts (11 5) Mc 9 - 15 30 25 30 Mg 14 10 - 32 27 31 9 G 27 12 19 - 32 37 11 B 23 12 18 35 35 13 Q 27 12 19 37 32 7 Standard deviation of the correlation coefficients = 0.04

-

-

M g

G

B

Q

44

29

13 8 38

14 15

-

60

-

M c

M g

G

B

Q

-

2.7

O .9 1.6

0.5 0.3

0.6 0.6

1.1

0.8

-

1.5

1.3

-

o .2

1.1 0.8

3.8

46

-

25 46

11

34

51

-

65

25 69

10

-

-

O 4 40

7

49

-

20 49

3.3 0.6 0.0

31

61

-

33 31

33

34

20 41 39

-

37

33

-

31 5

-

36 2 6

14

16 11 21

39 39

19

0.6

0.7 i .8

1.5

-

0.5 0.3 0.6 1.7

1.5

-

0.0 0.1 1.1

0.3

0.1 0.3

1.1 0.8

1.8

-

-

0.9

1.1 1 .o

1.3

0.7

0.7

1.3

1.1

1.1 1.1

0.9 0.9 1.1 0.6

Note: Mc,Carbonatic protore; M g , Garnetiferous protore; G , Graphitic facies; B, Biotitic facies; Q , Quartzose facies.

222

-

2.4 0.6 0.4 0.5

-

-

-

0.8

-

0.6

1.1

-

1.1

1.1

1 .o

-

The Serra do Navio manganese deposit (Brazil)

relatively large number of nongradational contacts. Probably metamorphic reactions played a relevant role in the formation of garnetiferous protore in the flanks of carbonatic protore, but it seems that this was the only case whereitwas relativelyeffectivein originatingtransitioiizones. The correlation coefficients and their standard deviations show great differences in distribution between the gradational and the nongradational contacts,While for the gradational contacts the standard deviation is 1.13,for the nongradational contacts it is only 0.04,indicating a tendency for gradational contacts to follow preferred orders, while contacts of the nongradationaltype follow a random distribution more closely. The data OF the gradational contacts permit the constructionof a sequence of the preferred gradationalcontacts in the following order: carbonatic protore/garnetiferous

protore; garnetiferous protore/graphitic facies; graphitic facies/biotiticfacies; biotitic facies/quartzosefacies. This sequence contains all the positive correlation coefficients (higher than 1 .O), for the gradational contacts. It indicates the other most probable rock unit into which a given rock unit is most likely to change. The vertical disposition of the sequence of contacts was analysed in detail for 160 contacts from the C-2 ore deposit (Fig. 2). This deposit is specifically good for this test since it has a thick and gently folded column of metasediments (Fig. 3), with no possibility of interference of overturned contacts. The method of calculation was the same as already described except that the vertical disposition of each two rock units in contact was annotated.These results are given in Table 5.

TABLE 5. Vertical sequence of the rnetasedimentary rock units (160 contacts from the C-2 ore deposit) Top-to-bottom correlation coefficients Rock unit at bottom of contact

All contacts (160 contacts)

Gradational contacts (124 contacts)

Nongradational contacts (3U contacts)

FreName

quency

M c

M g

G

B

Q

-

2.2

2.4 0.4 0.8 0.4

-

0.7 1.1

1.9 0.3 0.6

0.6 0.1 0.2

1.9 0.2

0.8 1.1 1.4 0.5

M c

M g

G

B

Q

-

3.4

-

0.8 1.4

2.4 0.2 0.3

0.0 0.0 0.2

2.4 0.2

0.4 0.7 1.1 0.3

M c

M g

G

B

Q

-

0.0

1.3 1.1 3.6 0.0

-

0.5 0.0

0.7 0.5 1.1

-

2.0 0.6 0.3

0.0 0.0

1.5 2.6 2.1 1.5

2.7

PA) Mc Mg G B

Q

11 18 29 22 20

-

2.8

-

2.7 0.2 0.0 0.5

-

2.8

-

-

-

Note;M c , Carbonatic protore; M g , Garnetiferous protore; G , Graphitic facies; B , Biotitic facies; Q,Quartzose facies.

The data on the transitional contacts suggests the following sucession of rock units to be the most common, from top to bottom: graphitic facies (coeff. = 1.4)............................................................................ garnetiferous protore (coeff. = 3.4) carbonatic Drotore (coeff. = 2.7) garnetiferous protore (coeff. = 2.4) graphitic facies (coeff. = 2.4) biotitic facies (coeff. = 2.8) quartzose facies (coeff. = 1.1) ........................................................................... graphitic facies All the other thirteen possible contacts were less favoured than the listed seven. The truly dominant sequence is composed of only five contacts, starting with the garnetiferous protore at the top and ending with the quartzose facies at the bottom. The majority of the nongradational contacts have the quartzose facies at the top. This Facies shows a marked tendency to have nongradational contact at the bottom,

which does not occur at its upper contact. The quartzose facies seems to constitute the lower unit of the sequence of metasedimentary units that grade upwards, one into another. From these data a model of the ideal sedimentary sequence can be inferred.It starts at the bottom with the quartzose facies and grades upwards first into the biotitic faciesand then into the graphitic facies:in which the manganese protores occur. Actually this model is disturbed by occurrences of rock units not in that order,but these occurrences do not usually disturb the general pattern and are commonly represented by layers thinner than the usual. Although the manganese protores occur essentially in the graphiticfacies,occasionally one of them is associated with the other two facies. These occurrences are actually minor and represented by lenses of protore only some decimetres thick.

CYCLICITY O F T H E SEQUENCES

There are several sequences of metasediments in Serra do Navio, each one following roughly the described pattern. In the ideal sequence, the graphitic facies occur at 223

E O

O m

o O

:: O

The Serra do Navio manganese deposit (Brazil)

the top,the quartzose facies at the bottom and the biotitic facies is intermediate between the two. As was already pointed out,exceptions to this ideal sequence are not rare. The ideal sequence has more chance to occur where the contacts are gradational. The lower contact'ofeach sequence is considered to be at the bottom of the quartzose facies, because it is here that there is the greatest number of nongradationalcontacts and the greatest differences in the rocks of the top and the bottom. In Serra do Navio the maximum number of sequences drilled in a single section was three,and in that section the complete column of metasediment was not out. In the mine benches and road cuts the weathering of the rocks and the poverty of outcrops makes the recognition of each sequence very difficult. ORIGIN OF THE METASEDIMENTS

As mentioned previously, the quartzose facies is a metachert with intercalated lime-richlayers, the biotitic facies is a meta-pelite and the graphitic facies a nietasediment rich in carbonaceous matter. They occur preferentially in that order, from bottom to top, and form repeated sequences,Coarse- and medium-grained clastics are notably absent from these rocks, which were essentially clayey and chemical sediments.This €act may indicate that their source was mature. The present thickness of each metamorphosed sequence,from the quartzose to the graphitic facies,varies generally from 40 to 60m . Of the three facies,the quartzose is the thickest and has better lateral continuity and homogeneity.The other two faciesarerathervariable inthickness and not very homogeneous laterally. Themeta-chertwith limeis a chemicalmarine sediment, deposited at shallow water depth and moderate temperatures. The p H was above 7.5,a necessary condition for the deposition of calcite. Such an environment was oxidizing and relatively free of clastics. Clastics are represented by a few clayey beds,now metamorphosed to biotite-schists. The gradual transitionof the meta-chert into the metapelite indicates a change in the conditions which favoured the deposition of chert,an increasein the rate of deposition of clays, or both. As the meta-pelitebecame gradually richer in organic materials, it changed into what is now the graphitic facies. This sediment was deposited in a reducing and probably restricted environment, of low p H and negative Eh, conditions also favourable for the deposition of the lenses of rhodochrosite. A possible model for the deposition of these sediments in their rhythmic sequence will now be described and is illustrated in Figure 4. The sedimentation probably initiated (Fig. 4(a)) on a shallow marine platform which received small amounts of clastics from an aajoining land. Cherty and limy beds formed farther from the shore line where the deposition of

OXIDIZING P H A S E

ia1

TRANSITIONAL PHASE

íbl

.

\

REDUCING P H A S E

(Cl

(JI

FIG.4.Schematic model of the sedimentary phases of the Serra do Navio group,as inferred from their tendency to form rhythmic sequences. (a) On a flat platform chert and lime form seaward while clay is deposited close to the shore line.(b) The clayey beds eventually cover the chert beds. Water depth diminishes and clayey beds rich in carbonaceous beds start to appear. The areas of deposition start changing to a reducing environment. (c) Organic-rich clayey beds continue to form. The shore line migrates seaward,leaving behind a swampy area and lagoons. Manganese carbonate precipitates as lenses in the strongly reducing lagoons. (d) After subsidence a new cycle of sedimentation starts, with chert seaward and clay landward, covering the previous cycle. In the early stages of subsidence clayey beds covered the manganese-richlenses. clasticswas minimal. At the same time clays were deposited closer to the land. As the sedimentation proceeded, the water depth decreased and the zone of deposition of clay moved seaward. The deposition of layers of chert was interrupted and replaced by layers of clay (Fig. 4(b)). Eventually, as the pile of sediments increased and the water depth decreased,the environment became reducing, with acid pH, negative E h and diminished water aeration. Organisms deposited with the clay were not decomposed. With the continuous accumulationof material,the environment slowly changed and probably a large swamp was formed, and several lagoons. At this phase the sedimentation became essentially laguna B, and in the lagoons, where the addition of clastics was minimal or absent, the manganese carbonate was precipitated (Fig. 4(c). The manganese probably stayed in solution as a carbonate during the whole process, precipitating when the environment was reducing and favourable for its deposition. Although the important manganese protores occur at the upper part of the graphitic facies,the manganese was also deposited during all the phases of deposition of the graphitic facies,as is demonstrated by the presence of thin protore layers throughout the graphitic facies. The next step of the model is a subsidence of the pla.tform,renewing the original conditions of the cycle (Fig. 4(d)). A thin layer of graphitic facies covering the 225

W.Scarpelli

protore is the first indication of that recurrence, but the greatest evidence is the presence of the quartzose facies covering the graphitic facies in nongradational contacts. These two facies represent two strongly different environments of deposition and by themselves suggest such sudden change. Actually there are exceptions to the stratigraphiccycle as it is described in the model. In some sequences the biotitic facies is locally absent,in others it occurs also as thin lenses inside the other two facies (this may represent some sort of sudden clastic sedimentation). Occasionally the graphiticfaciesor the quartzose faciesarelocally absent. These exceptions do not invalidate the model, which was constructed to put together the known tendencies of the sedimentation processes. Of course, the model might be modified in the light of additional data.

M E T A M O R P H I C PHASES

The metasediments were metamorphosed three times. The older metamorphism was dynamic, of the amphibolite facies of metamorphism (according to the classification of Turner, 1968), accompanied by minor folding. It had a strong component of load pressure, which favoured the development of foliation parallel to the bedding planes. Quartz,biotite,and plagioclase are the most common minerals of this phase and are preserved as oriented inclusions in younger porphyroblasts. As inclusions,they occur with mosaic texture, have a very fine grain size, and do not present evidence of tectonic deformation. Next a thermal metamorphism occurred, of the hornblende-hornfels facies of Turner (1968), with the growth of porphyroblasts of andalusite and almandine in the three metasedimentary facies,of grossularite,tremolite and diopside in the calcic zones of quartzose facies,of spessartite, rhodonite and tefroite in the protores, and of cordierite iri the biotitic facies. Andalusite is more frequent and coarser in the graphitic facies, where it is very rich of inclusions of graphite. The youngest metamorphism was a dynamic nietamorphism,of the amphibolite facies,characterized by rock deformation, folding and mineral recrystallization. The textures ofthe quartz-richzonesbecame sutured and broken minerals are common. In general only the porphyroblasts and their inclusions did not recrystallize or become deformed. In the three facies sillimanitereplaced biotite and andalusite and was accompanied by muscovite. In the protores and the calcic zones amphiboles developed across older minerals. During this metamorphic phase a pneumatolitic phase took place,characterizedby the formation of pegmatites and local tourmalinization. Samples of the metasediments were analysed by the Laboratory of Geochronology of the University of São Paulo, Brazil. The K-Armethod gave an age of 1,7101,770m .y. It is assumed that this age represents the age of the Iast dynamic metamorphism and folding.

226

STRUCTURE

The overall structure of the district is poorly known due to the scarcity of outcrops and the presence of identical rocks in several levels of the stratigraphiccolumn,making lateral correlations difficult. Some informations is known from drilling and mine exposures. The folds vary from open (Fig. 3) to closed, plunging northwest and southeast at small to moderate angles.Their axial planes dip northeast.In some areas they turn into isoclinalfolds,with theparallellimbsdipping steeply northeast. The carbonatic protore has its maximum thickness in the nose of closed folds due to plastic recrystallization and concentration of rhodochrosite in the axial zones. The biotitic facies occasionally appears deformed,like an incompetent layer.The quartzosefacies was more resistant to deformation,forming concentric folds,which helped to avoid strong tectonic deformation of the column of metasediments.

SECONDARY ENRICHMENT

The predominant ore mineral is cryptomelane,followed by pyrolusite and occasionally other minor manganiferous minerals. In hand specimens one can distinguish between an amorphous mass (essentially cryptomelane) and crystalline zones (pyrolusite). These minerals replace the manganese carbonates and silicates of the protores down to the water table, leaving behind decomposed remnants of the manganese silicates in some cases. The ore is in a continuous process of secondary enrichment. Vugs are formed during the weathering and subsequentlyfilledby successivebands ofcryptomelaneand/ or pyrolusite, in mammillary textures,probably formed by deposition from colloidal solutions. The high-grade ore replace the carbonatic protore in situ (Fig. 3), with the original bedding still occasionally recognizable. The garnetiferous protore produces an ore of lower grade, due to both its original lower content in manganese and the formation during weathering of stable minerals rich in silica and iron. The manganese oxides generally occur in the enclosing schists and quartzites only as small, isolated, and uneconomic replacementpockets and veins,but when the structure is favourable for their accumulation, as in the axial zones of synclines, the manganese oxides replace the weathered schists intensively,forming schistose ores,which arecharacterizedby theabundanceofremnantsofthe schists. Float ore is common inthe hill sidesbelow the outcrops of manganese ore. It is composed of detrital ore fragments plus spheres of manganese oxides chemically precipitated around a nucleus of quartz or other minerals (granzon). The bulk of the gangue is composed of kaolinite and goethite, produced by the weathering of schistose rocks and manganese silicates. These minerals are followed by gibbsite, also a product of weathering,and residual quartz and graphite.

The Serra do Navio manganese deposit (Brazil)

Résumé Le gisement de manganèse c h Serra do Navio, an Brésil (W.Scarpelli) Les dépôts de minerai de Serra do Navio se rencontrent dans une séquence métasédimentairequi se trouve,au point de vue stratigraphique,au-dessus d’une ceinture d‘amphibolites. Ces roches ont été métamorphosées au moins trois fois,et le métamorphisme le plus récent remonte à 1,7milliard d’années. Les sédimentsen cours de transformation se succèdent et peuvent être classés en trois faciès : siliceux,à biotite et à graphite.L a distributionverticale de ces trois faciès révèle une tendance à une périodicité rythmée. Ils forment des séquences sédimentairessuccessives,chacune d‘ellespartant à la base du faciès siliceux pour se développer plus haut dans le faciès à biotite, et enfin dans le faciès à graphite. Des lentilles de rhodochlosite, qui ont eu leur origine dans le gisement de minerais, se rencontrent à la partie supérieure du faciès à graphite, du sommet duquel part la séquencesuivante,avec encorele faciès siliceux à labase.On observe au moins trois séquencesde ce genre dans la région. On supposeque les sédimentsoriginauxse sont déposés dans un milieu néritique peu profond en cours de subsidence. On constate en général qu’à la base de chaque cycle sédimentaire se trouvent du chert et de la chaux, tendant

graduellement, en s’élevant,vers une zone pélitique qui plus tard s’estenrichie en matière organique. Il semble que chaque cycle a commencé après une transgressiondu niveau marin, les sédiments pélitiques se déposant du côté de la terre, tandis que les sédiments chimiques riches en silice se déposaient du côté de la mer. Au fur et à mesure que l’empilementdes sédiments croissait et que la zone pélitique recouvrait la zone riche en silice, la profondeur de l’eau diminuait. C‘est alors que les sédiments pélitiques s’enrichirent de débris organiques, Lorsque la profondeur de l’eau a atteint son minimum, la circulation de l’eau a été contrariée et quelques lagons se sont formés, où se sont trouvées des conditions favorables au dépôt de manganèse sous forme de carbonate. On peut encore distinguer trois phases de métamorphisme dans les sédiments au cours de leur transformation: deux phases dynamiques et une phase thermale.L a conséquence des deux métamorphismes dynamiques est la présence de structures plissées superposées. Les gisements de minerais actuels ont été formés par l’altération atmosphérique et par l’enrichissement subséquent de lentilles de protore riche en rhodochrosite, qui ontété remplacéspar des cryptomélaneset des pyrolusites.Le protore inaltéré contient environ 31 %de manganèse,tandis que les gisements supergenes en contiennent environ 50 %.

Bibliography/ Bibliographie CRESSMAN, E.R. 1962. Non detrital siliceous sediments.Prof. Pap. US.Geol. Surv., 440 T. DORR, J. VANN., II; PARK, C.F.,JR.; DE PAFIA, G.1949. Manganese deposits of the Serra do Navio district,Territory of Amapá, Brazil. Bull. US. Geol. Surv., 964 A. ; SOARES COELHO, I.; HOREN, A. 1956. The manganese deposits of Minas Gerais, Brazil. XX I72t. Geol. Congr., Mexico, vol. III,p. 279-346. KRAUSKOPF, K. B. 1967. Introduction to geochemistry, New York, N.Y., McGraw-Hill. KRUMBEIN, W. C.;GARRELS, R. M . 1952. Origin and classification of chemical sediments in terms of p H and oxidationreduction potentials. J. Geol., vol. 60, p. 1-33. LEINZ,V. 1948. Estudo genético do minério de manganês da Serra do Navio, Territorio do Amapá. Vol. 20,no. 2,p. 21121.Rio de Janeiro,Academia brasileira de ciências. NAGELL, R.H.1962.Geology of the Serra do Navio manganese district,Brazil.Econ. Geol.,vol. 57,p. 481-98.

__

PARK,C. F.,JR. 1956. Manganese ore deposits of the Serra do Navio district,FederalTerritoryof Amapá,Brazil.XXInt. Geol. Congr.,Mexico, vol. III,p. 347-76. SCARPELLI,W. 1966. Aspectos genéticos e metamórficos das rochas do distrito de Serra do Navio. Avulso Dep. nac. Prod. min., Rio de J.,no. 41,p. 37-56. . 1968.Precambrian metamorphic rocks of Serra do Navio, Brazil, unpublished work, Stanford University. SUJKOWSKI,ZB.L. 1958. Diagenesis. Bull. Amer. Ass. Petrol. Geol., vol. 2, p. 2692-717. TURNER, F. J. 1968.Metamorphic petrology, mineralogical und field aspects. New York,N.Y., McGraw-Hill. VALARELLI, J. V. 1966. Contribuição á mineralogia do minério de manganês da Serra do Navio, Amapá, Avulso Dep. nue. Prod. min., Rio de J., no. 42,p. 83-98.

227

W.Scarpelli

Discussion I. P. NOVOKHATSKY. What are the manganese minerals in carbonate rocks?

any idea why picrotephroite formed at this particular deposit (Chumko)?

W . SCARPELLI. In fresh carbonatic protore the manganese mineral sare rhodochrosite, spessartite plus rhodonite and tephroite.In the oxide ore the main manganese minerals are cryptomelme and pyrolusite.

W. SCARPELLI. Tephroite, like rhodonite, was formed through chemical reactions between silica and rhodochrosite.The quantity of silica and rhodochrositeinvolved in the reactions dictated whether rhodonite or tephroite was formed. All the rocks of the district, except the protore, show small quartz veins. In the protore we do not get veins of quartz,but we get veins of tephroite and/or rhodonite in areas where the enclosing rocks have quartz veinlets,W e accept the possibility that silica entered the protore along fractures and reacted with the carbonate forming the silicates, under high temperature, during the thermal metamorphism. Tephroite and rhodonite occur mainly as large crystals.

What is the depthofthe oxidationzone? I.P.NOVOKHATSKY.

W.SCARPELLI. Usually the oxidation goes down to 70100 m in the hills where the ore bodies occur.

R. T. BRANDT.What do you consider was the reason for the deposition of large amounts of manganese in these sediments, and where did it come from originally? W . SCARPELLI. The reason for such a deposition was the conjunction of favourable conditions for the accumulation of manganese in solution and the existence of environments favourablefor the deposition of the manganese as carbonates with minimum admixture of clay. W e do not know where the manganese came from.

J. VAN N.DORR. Can you tell us anything about the pelletizing plant for manganese oxide pellets under construction?

W.SCARPELLI. The pellet plant is under construction,and we expect that it will start operating in the second semester

J. H. GROSSI SAD.The second metamorphism is called

of 1971,producing the first manganese ore pellets in the world.

thermal. Why? Did you find intrusives related with this metamorphic phase?

B. CHOUBERT. Can you explain the technique you use to

W . SCARPELLI. W e called that metamorphism a thermal one because temperature seems to be the major or the only factor which affected the rocks. Porphyroblasts grew in all the rock units described, and they grew without deformation, indicating that pressure did not affect the rock units during that metamorphic phase. W e did not find intrusives related with this metamorphic phase.

J. VAN N.DORR. Most of the protore is MnCO,, but in one deposit it is picrotephroite,I believe. D o you have

228

obtain the numbers to calculate the correlations between elements of various formations.

W.SCARPELLI. I compared the real data obtained through detailed examination of all the diamond drill cores available with a set of theoreticalnumbers calculated to express the probability of occurrences of contacts at random. The coefficients represent the ratio between the real data and the theoretical set for a random distribution of contacts. They show h o w nature preferred some contacts over others.

Genetic studies on the Precambrian manganese formations of India with particular reference to the effects of metamorphism S. Roy Department of Geological Sciences,

Jadavpur University (India)

Extensive deposits of manganese ore occur in the Precambrian shield of the Indian subcontinent in the states of Madhya Pradesh, Maharashtra, Orissa, Bihar, Andhra Pradesh,Gujarat,Mysore and G o a (Fig.1). Both syngen-

etic and supergene epigenetic deposits are present,of which the first type is by far the majority (Roy,1966). This paper will deal with the syngenetic manganese formations consisting of interbanded manganese ores and manganese silicate rocks that show evidences of regional (and locally thermal) metamorphism to different grades.

Distribution and geologic setting

I

SCALE I W

O

IO0 Z P O MILES

I

ODELHI

The syngeneticmanganese formations of India occur in the Sausar (Madhya Pradesh-Maharashtra), Aravalli (Rajasthan-Madhya Pradesh), Champaner (Gujarat), Gangpur (Orissa) and Khondalite (Andhra Pradesh) groups, all meta-sedimentary formations of Precambrian age. The geochronological relationships of the above groups are given in Table 1. M A N G A N E S E FORMATIONS O F T H E SAUSAR G R O U P

FIG.1. Map showing manganese ore deposits in India.Madhya Pradesh-Maharashtra ore belt; 2. Gangpur-Bamra deposits; 3. Panch Mahals Dt deposits;4.Jhabua Dt deposits, Madhya Pradesh;5. Srikakulam Dt deposits,Andhra Pradesh;6.BonaiKeonjhar deposits, Orissa; 7. Kalahandi-Koraput-Patnadeposits, Orissa; 8. Sandur-Bellary deposits,Mysore;9.Shimoga Dt deposits, Mysore; 10. North Kanara deposits, Mysore; 11. Banswara deposits, Rajasthan; 12.Goa deposits.

In India,syngenetic manganese formations are best developed in the Sausar Group that covers the districts of Nagpur and Bhandara (Maliarashtra)and Chhindwara and Balaghat (Madhya Pradesh). The manganese formations run NE.-SW., E.-W.and NW.-SE.as an arcuate belt, the obtusebulge facing south,for more than 130niiles (208 km) with an average width of 20 miles (32 km). The Sausar Group is represented by a miogeosynclinal orthoquartzitecarbonate sequence and the geological succession has been established,based on the study of severalworkers (Narayanaswanii et al., 1963). The different formations of the Sausar Group are represented by pelitic, psammopelitic, psammitic and calcareous rocks (Fig.2). Igneous rocks are almost absent in the Sausar sequence,except for rare occurrences of late and post-tectonic granite plutons in northern Nagpur,northern Bhaiidara and Balagliat districts. The manganese formations of the Sausar Group are entirely stratigraphically controlled. They are particularly associated with pelitic rocks in the bottom, middle and

Unesco, 1973. Genesis of Precambrian iron and nianganesc deposits. Proc. Kiev Sump.,1970. (Earth sciences, 9.)

229

S. Roy

TABLE 1 (after Sarkar,1968)l Madhya Pradesh and Maharashtra

Satpnua Cycle

Sausar Group 846-986 m y .

Asavalli Cycle

Aravalli Groupz 950-1,500m.y. Sed.,c. 2,000m y .

Eastern

Orissa

Gujarat

Andhra Pradesh

Gangpur Group 846-946 m.y. Sed.,1,700-2,000m.y. Champaner Group Khondalite Group Phase II, c. 1,600my. Phase I, c. 2,650 my.

Ghat Cycle

1. The ages indicate closing of events. Sedimentation(Sed.) ages are also given where determined.

2.The Aravalli Group mainly covers the state of Rajasthan and partly Madhya Pradesh. Syngeneticmanganese formationsin this group are only found in Madhya Pradesh.

upper parts of the Mansar formation. Besides the occurrences in the Mansar formation, manganese deposits are also enclosed in Lohangi (marble and calc-silicaterocks) and Tirodi biotite gneiss formations on a very minor scale (Fig. 2). The manganese formations are independent beds of oxidic manganese ores and manganese silicate rocks (gonCHHINDWARA OT. GOYIARI WbDHONbMAHbRKUND AllEA

N A G P V R DT. GuMGAON-

RAMDONGRI-

UOHBAON AREI

MINSIR-KbNORL

JUNAWAWI A R E A

B H A N D A R A DT. CHIIL4- SITA-

sho*G'-Do*cR' BUZURG A R E I

ditel) that are intimately interbanded among themselves and with the enclosingpelitic meta-sedimentsof the Mansar formation. Primary sedimentary lamination is discernible 1. Regionally metamorphosed manganese silicate rocks, essentially composed of spessartite and quartz,with or without other manganese silicates. First named gondite by Fermor (1909), later elaborated by R o y and Mitra (1964) and Roy and Purkait (1968).

B A L A G H A T DT.

LEGEND TIIOOI-SlTAPbTORE RAMIAMh-IPETIA AREA AREA

I8ARWELIJlKWA bREA

BICHUA A N D J U N N A N I FDRMATIOU.

CHORBAOLIF D R M A T I O ~ .

M A N S A R FORMATICM

@

G R E E N S C H I S T FACIES

@

~ u ~ F ~ -ALMAMOIME ~ : E

-ALMANDIME @ KYAUITE SUBFACIES.

@

SILLIMANITE-ALMANDINEM U S C O V I T E SUBFACIES.

L O H A H G I FCRMATIOM.

SITASAOHGI FORMATION. TIRODI BIOTITE GNEISS FORMATION. B E O S ABSENT.

9

M A N G A N E S E ORE.

FIG.2. Synoptic diagram showing stratigraphicpositions of the manganese ore horizons in the manganese ore belt of Madhya 230

Pradesh and Maharashtra. (Thickness and lateral extent not to scale.)

Genetic studies on the Precambrian manganese formations of India with particular reference to the effects of metamorphism

in the ores themselves. The orebodies, gondite, and the pelitic nieta-sediments show concordant relationships and are Co-folded in different scales. These manganese formations were originally laid down as sediments from a nonvolcanogenic source. The pelitic rock of Mansar formation show effects of regionalmetamorphism to different grades.The rocks in the Bharweli-Ukwaarea,Balaghat Dt,Madhya Pradesh,have been metamorphosed to the quartz-albite-epidote-almandine subfacies of greenschist facies.At the Dongri BuzurgKurmura area,Bhandara Dt,Maharashtra,the same grade of metamorphism has been attained by the rocks,increasing progressively eastward to staurolite-almandineand kyanitealmandine subfacies (almandine-amphibolitefacies) in the Chikla-Sitasaongiarea.The major part of the Sausar tract, including the areas around Tirodi-Sitapatore,Netra-Ramrama (Balaghat Dt),Mansar-Kandri,Gurngaon-Ramdongri (NagpurDt)and GowariWadhona,Sitapar-Kachidhana (Chhindwara Dt)have been metamorphosed to sillimanite-almandine-muscovitesubfacies (almandine-amphibolite facies). Since the pelitic rocks and the manganese formation are associated intimately in a syngeneticsequence and were later metamorphosed together, it is assumed that both of them have been subjected to the same range of pressuretemperature changes.

M A N G A N ESE FORMATION O F THE G A N G P U R GROUP

The manganese formation occurs as an important member in the meta-sedimentarynon-volcanogenicsequence of the Gangpur Group. Syngenetic manganese orebodies and gondite occur interbanded and Co-folded in pelitic rocks in Ghoriajor-Monomunda area, Sundargarh Dt,Orissa. The manganese formation occupies the core of the Gangpur anticlinorium (Krishnan,1937) and constitutes,along with the pelitic rock,the Ghoriajor formation at the base of the Gangpur Group. The pelitic rocks of the area have been metamorphosed to staurolite-almandinesubfacies.

M A N G A N E S E FORMATIONS OF T H E ARAVALLI A N D C H A M P A N E R G R O U P S

The Aravalli Group is well represented in the states of Rajasthan and part of Madhya Pradesh (Jhabua Dt)and its southward continuation in Gujarat has been designated as the Champaner Group. The Aravalli (and equivalent Champaner) Group is constituted of quartzite, limestone and calc-silicaterocks,and phyllites and mica schists with syngenetic beds of manganese oxide ores and manganese silicate rocks. The syngenetic manganese orebodies at Shivarajpur (Panch Mahals Dt,Gujarat) are interbanded and Co-folded with phyllite and quartzite in a non-volcanogenic sedimentary sequence that has been regionally metamorphosed to the quartz-albite-muscovite-chlorite subfacies. Manganese orebodies and gondite,interbanded

with quartzite and phyllite at Kajlidongri, Jhabua Dt, Madhya Pradesh,have been metamorphosed to the quartzalbite-epidote-biotite subfacies. A n isolated deposit of manganese oxide ore and manganese silicate rock (koduritel), interbandedand Co-foldedwith wollastonite-diopside hornfels, has been thermally metamorphosed to the pyroxene-hornfels facies by porphyritic biotite-granite at Jothvad, Panch Mahals Dt,Gujarat. M A N G A N E S E FORMATIONS O F T H E KHONDALITE G R O U P

The Khondalite Group meta-sediments cover the eastern Ghats region of south India and are made up of rocks metamorphosed to the granulite facies (garnet-sillimanitegraphite granulite, calc-granulite,garnetiferous quartzite, charnockite etc.). At Kodur-Garividi-Devadaand Garbham, Srikakulam Dt,Andhra Pradesh,syngeneticmanganese orebodies (and rarely manganese silicate rocks) occur interbandedand Co-foldedwith the meta-sedimentarym e m bers of the Khondalite Group. Igneous rocks, other than much later granite plutons and pegmatite, are absent in this area. In the Kodur group of mines, the manganese orebody, occurring as conformable beds, is enclosed in calc-granulite,whereas in the Garbham area, the orebody occurs interbanded with garnetiferous quartzite. In both the areas, garnet-sillimanite-graphitegranulite are present in close association,though never in direct contact with the orebodies.

Mineralogy and texture The mineralogy of the manganese oxide orebodies and the manganese silicate rocks of the metamorphosed manganese formations of India has been summarized by Roy (1966). The mineral assemblages representing different grades of metamorphism have been tabulated in Tables 2 and 3. In these tables,the mineral assemblages of metamorphosed manganese formationsfrom other countries are also listed from various sources (Dorr et al.,1956;Hewett et al.,1961; Horen, 1953; Huebner, 1967; Hutton, 1957; McAndrew, 1952; Mohr, 1964; Roper, 1956; Segnit, 1962; Servant, 1956;Watanabe,1959;Westerveld,1961;Woodland,1939). Besides the metamorphosed manganese formations listed in Tables 2 and 3, several other important deposits deserve particular mention. These could not be included in the tables due to uncertainties about their precise grade of metamorphism. The metamorphosed manganese formations of Ghana (spessartite-rhodonite-rhodochrosite (Service, 1943)); Madagascar (spessartite-rhodonite-tephroite-apatite-quartz (Boulanger, 1956)); southern Ural (spessartite-rhodonite-bustamite-piedmontite (Betekhtine,in 1. T h e n a m e kodurite was proposed by Fermor (1909) to designate thermally metamorphosed manganese silicate rocks of spessartiteandradite garnet, potash felspar and apatite.

23 1

S. Roy

TABLE 2.Sedimentary deposits of manganese metamorphosed to greenschist facies Mineralogy Area

Type of deposit

Silicate-carbonate Oxide

India Shivarajpur,Gujarat

Silicate

Kajlidongri,Madhya Pradesh

Quartz-albite-muscovite-Braunite chlorite subfacies Quartz-albite-epidote- Braunite,bixbyite, Spessartite, biotite subfacies hollandite, rhodonite,blanfordite, jacobsite juddite,winchite, manganophylite, alurgite,quartz

Bharweli-Ukwa, Madhya Pradesh

Quartz-albite-epidote- Braunite,bixbyite, almandine subfacies hollandite

-

Dongri BuzurgKurmura, Maharashtra

Quartz-albite-epidote- Braunite, almandine subfacies hollandite, jacobsite, manganite

Spessartite, rhodonite, quartz

United States of Anierica Big IndianDeposit, Quartz-albite-muscoviteSierra Nevada chlorite subfacies

-

-

-

Rhodochrosite,rhodonite, tephroite

Smith Prospect,Sierra Zeolite or lower part Nevada of greenschist facies

Hausmannite

-

Rhodochrosite,rhodonite, tephroite,spessartite yellow IAsilicate

CalaverousFormation, Greenschistfacies Sierra Nevada

-

Buckeye Deposit, California

Hausmannite, braunite

Brazil Merid Mine, Minas Gerais

Blueschist facies

Quartz-albite-epidote- Hausmannite almandinesubfacies

Rhodochrosite,rhodonite, tephroite,spessartite, neotocite,bementite

-

-

United Kingdom Merionethshire,Wales Lower part of greenschist facies Australia TamworthDistrict, N e w South Wales N e w Zealand Western Otago Africa Tiéré

232

Greenschist facies

Rhodochrosite,bementite

IA and i2Â silicate

Rhodochrosite,manganoan calcite, rhodonite,pyroxmangite, tephroite,spessartite, kupfferite,bementine,neotocite, graphite,quartz

Rhodochrosite,spessartite, quartz

Hausmannite, jacobsite

Mangandolomite,rhodonite, knebelite,tephroite,quartz

-

Rhodochrosite,rhodonite, spessartite

Quartz-albite-muscovitechlorite subfacies

-

Quartz-albite-muscovitechlorite subfacies

Spessartite,quartz

-

Genetic studies on the Precambrian manganese formationsof India with particular reference to the effects of metamorphism

TABLE 3. Sedimentary manganese deposits metamorphosedto almandine-amphiboliteand pyroxene-hornfelsfacies Mineralogy

Area

Type of deposit Oxide

Silicate

India Chikla-Sitasaongiarea, Staurolite-almandine Braunite,bixbyite, Maharashtra and kyanite-almandine hollandite,jacobsite, subfacies vredenburgite

Spessartite,rhodonite, tirodite,alurgite, manganophyllite, plagioclase,quartz Spessartite,rhodonite, Tirodi-Stapatorearea, Sillimanite-almandine- Braunite,bixbyite, muscovite subfacies Madhya Pradesh hollandite,hausmannite, blanfordite,brown manganiferous pyroxene, jacobsite,vredenburgite winchite,juddite, tirodite,alurgite, manganophyllite, . apatite,quartz Spessartite rhodonite, Netra-Ramramaarea, Sillimanite-almandine- Braunite,bixbyite, manganoan diopside,brown Madhya Pradesh muscovite subfacies hollandite,hausmannite, manganiferous pyroxene, jacobsite,vredenburgite blanfordite,winchite, tirodite,piedmontite, manganophyllite,apatite, plagioclase,quartz Spessartite,rhodonite, Sillimanite-almandine- Braunite,bixbyite, Gowari-Wadhona, manganoan diopside,brown Madhya Pradesh muscovite subfacies hollandite,jacobsite, hausmannite,vredenburgite manganiferous pyroxene, blanfordite,winchite, tirodite,juddite, piedmontite, manganophyllite,apatite, calcite,quartz Spessartite,rhodonite, Kodur-Garbhamarea, Granulite facies Braunite,hollandite, apatite,quartz Andhra Pradesh jacobsite,hausmannite, vredenburgite

Norway Mount Brandnuten Africa Otjosondu

Almandine-amphibolite Braunite,jacobsite, facies hausmannite

Spessartite,rhodonite, quartz

Spessartite,rhodonite, Almandine-amphibolite Braunite,bixbyite, facies diopside,acmite,quartz hollandite,jacobsite, hausmannite,vredenburgite

India Jothvad,Gujarat (thermallymetamorphosed deposit)

Pyroxene,hornblende hornfels facies

Braunite,bixbyite,hollan- Spessartite,andradite, dite,hausinannite rhodonite,blanfordite, brown manganiferous pyroxene,winchite,alurgite, manganophyllite,apatite, quartz

Japan Noda-Tamagawa, Iwateprefecture (thermally metamorphosed deposit)

Pyroxene/hornblende hornfels facies

Hausmannite,braunite, manganosite,pyrochroite

-

Silicate-carbonate

-

-

-

-

-

-

-

-

Rhodochrosite,rhodonite,pyroxmangite, tephroite,spessartite, galaxite,bementite

233

S. Roy

Zvéreff,1953)); southernKhingan(rhodochrosite-bustamiterhodonite-tephroite and braunite-hausmannik-hematitemagnetite (Chebotarev, 1960)), and the Postmasburg-Thabazimbi area (braunite-bixbyite-hausmannite-jacobsite-vredenburgile-chalcophanite(De Villiers,1956)) belong to this group. The metamorphosed inanganese orebodies of India characteristically exhibit banding both on a macro- and micro-scale.In the quartz-albile-epidote-biotite subfaciesat Kajlidongri, relict colloform texture and relict crustified

fracture-fillingveins are exhibited by braunite and bixbyite (Figs.3 and 4). Such texturesindicatethat colloform higher oxides (pyrolusite,cryptomelane,etc.), originally deposited in sediments, were reduced by rising temperature to form braunite and bixbyite,though the broad textures remained virtually undisturbed.On recrystallization,bixbyite usually shows idioblastic habit (Fig. 5) owing to its strong force of crystallization.Equant and strain-freerecrystallized grains of hollandite, of varying size,often constitute entire bands (Fig. 6). In almandine-amphibolitefacies, vredenburgite

FIG. 3. Relict colloform texturewith bixbyite (white) at the core and braunite (light grey) forming the rim.A relict crustified vein showingthesetwo minerals is also seen.Kajlidongri.( x 90.)

FIG.4.Same featureas in Figure 3, further enlarged and showing details. Braunite (grey) at the core is enclosed by bixbyite which is further rimmed by braunite. The subhedralto euhedral habit of bixbyite is characteristic. Kajlidongri. (x 450.)

FIG.5. Idioblastic crystals of bixbyite in quartz gangue. Hematite (white) is released when part of bixbyite converted to braunite (grey). Bharweli. (x SOO.)

FIG.6. Strain-free equant grains of hollandite formed by recrystallization during metamorphism Gowari Wadhona. (x 200.)

234

Genetic studies on the Precambrian manganese formations of India with particular reference to the effects of metamorphism

showing widmanstatten intergrowth (due to exsolution) of hausmannite in jacobsite (Fig. 8), exhibits xenoblastic to granoblastic texture with braunite (Fig. 7).Deformation texures are commonly exhibited by the metamorphosed orebodies. Braunite and hollandite are stretched and elongated and show preferred dimensional orientation parallel to the banding,imparting a schistosity to the ore (Fig. 9). Translation twinning is developed in hollandite (Fig. 10) and hollandite-bixbyite-brauniteores show evidences ofmicrofolding (Fig.l l). Micrographic intergrowth between rhodonite and jacobsite (Fig. 12), observed in gonditemay be interpreted as due to oxidationof FeSi0,rich rhodonite or pyroxmangite (see Table 4). Such

intergrowth has also been described from Dongri Buzurg, India,by Chaudhuri (1967).The transformation of bixbyite to braunite along crystallographicplanes is characteristicin all grades of metamorphism (Fig. 13), and very often hematite is released from bixbyite during this transformation. The resultant braunite shows slightly different optical characters (cf. Roy, 1966) and is possibly lower in silica content1 than that formed directly by the

FIG.7,Brauniteand vredenburgite(hausmannitelamellaeetched black) exhibiting granoblastic texture in recrystallized ores.East of Dongri Buzurg. (x 125.)

FIG.8. Hausmannite (lamellae) and jacobsite (host) intergrown to form vredenburgite. Hausmannite (grey) has been largely oxidized to cryptomelane (white). Netra. (x 600.)

FIG.9.Deformed braunite (dark grey) and hollandite (white, pitted) showing preferred dimensional orientation parallel to banding. Bharweli. (x 125).

FIG.10. Translation twins formed in hollandite due to deformation. Gowari Wadhona. Crossed nicols.(x 250.)

1. This variety m a y refer to the silica-poor braunite described by De Villiers anci Herbstein (1967). This braunite was separated from a sample containing only braunite and bixbyite and analysed. T h e Si03 content was determined as 3.89 per cent. T h e possibility of contamination of bixbyite in the sample is, however, not entirely ruled out.

235

S. Roy

FIG.11. Microfolded bands of braunite and hollandite (coarsegrained). Bharweli. (x 110.)

FIG.12. Micrographic intergrowth between jacobsite (white) and rhodonite (black). Tirodi. (x 300.)

FIG.13. Braunite (darker grey) formed as a conversion product of bixbyite along the crystallographicdirections.Black portions represent pits. Bharweli.(x 400.)

FIG.14.Hausmannite (dark grey) in the periphery and cleavage planes of hollandite (white). Subrounded grains of braunite (medium grey) are also present in hollandite.Gowari Wadhona. (%300.)

reaction7MnO,+3 Si0,=3 Mn203. MnSiO,+O, (Table4). Hausmannite sometimes envelops hollandite grains and also occupies the cleavage planes of the latter (Fig. 14). The gondite generally shows xenoblastic to granoblastic texture. Banding with oxide ore or within its own components is common. Textural relationships indicate that the formation of spessartite and rhodonite overlapped and a few other manganiferous pyroxenes and amphiboles such as brown manganiferous pyroxene (Mn. aegirineaugite), tirodite and wiiicliite formed by metamorphic process from the original bulk composition,Prolific development of manganiferous silicates other than spessartite and rhodonite in gondite is, however, always related to alkaliaiid Fe3+ metasomatismfrom laterintrusivepegmatites (Roy, 1966;Roy and Mitra, 1964;Roy andPurkait,1968). 236

Discussion The trends of transformation of different phases of manganese with rising temperature,as determined in the laboratory and observed in natural assemblages,are shown in Table 4.Higher oxides ofmanganese (E,ß,y,and 8-MnO,) when present exclusively,convert to E-Mn,O, (bixbyite) at temperatures between 500"-600" C,and further to hausmannite at 877"C in air (Faulring et ul., 1960;Hahn and Muan, 1960; Klingsberg and Roy, 1959; Okada, 1959~; Ukai et al., 1956). Okada (19596, 1960) has shown that todorokite [(Mn, Ca)Mn,O, .2Hz0] and birnessite[S-MnO, :(Na,Ca)Mn,O1,2.8H,O] which,incidentally,are the main constituents of recent deep-seamanganese nodules, transform to hausmannite at 600"-700°C.

Genetic studies on the Precambrianmanganese formations of India with particular reference to the effects of metamorphism

TABLE 4.Phase transformation of manganese minerals with rising temperature 1.

-1

h-MnOz(GRYPT0MELANE) íA) ß-MnO?(PYROLUSiTEi 6-MnO;(NSUTITE)

550°-60dC-

ô-MnOz(B1RNESSITE)

B I X B YITE- 8770-

HAUSMANNITE

(L-Mn;!O3 1

-31c-

-

900.c

MANGANOSITE (MnO)

-P'YROCHROITE (BI i MniOH)21

-HOLLANDITE-500'~(BaR806)

- 9 5 0 ~-BIXBY ~ IT€ -565%

-PYROLUSITE-375.C - MANGANITE (D) iMn O21

2.

ß-MnOz(PYR0LUSITE) 6 -MnO2 INCUTITE) 6 -MnOz(BIRNESSITE)

+

sioz

(A)7Mn02+5¡02~3Mn~O3~nSiO3tO2 (BRAUNITE)

P S I L O M E L A N E(c) I(Bo.H2012Mn5 0101

W M n OOH)

MANGANOSITE -RHODOCHROSITE (MnDl IMn CO3)

(E)

2MnO2+2S102=2MnS103tC2 (RHODONITE)

-

3.

BEMENTITE i(Mn.Mg,Fe16 Si4QOH)181

4

PYROXMANGITE

5

(C)3Mn2 O3 Mn C103t6S102= 7 M n SIO3 i312 Oz

(BRAUNITE) BRAUNITE -RHODONITE (3Mn2OaMnSiO3) (MnSiO3)

2[IMn,FelS10~3/2 Oz- MnFegO4+MnSiO3+ S I O Z (PYROXM A N G IT E (JACOBSITE) (RHODONITE)

\

d Mn02(CRYPTOMELANE)

ß-Mn02iPYROLUSITE) 6-MnOz(NSUTITE)

+

IRON OXIDE OR

BIXBYITE

7 8

(LU

v n Co3 (RHODOCHROSITE) + Mn CO3 (RHODOCHROSITE)

(A)

+ MnzS104 (TEPHROITE)

-

JACOBSITE

AND/OR

(Mn Fez 04)

4MnOzIBIRNESSITE) 6

(RHODONITE)

60% Fe2031 +

Ci O 2 =

Mn C1O3 (RHODONITE)

MnSiOJ (RHODONITE) Ciop

=

+

MnZCi04 (TEPHROITE)

= 2 M n Si03 (RHODONITE)

CO2 +

VREDENBURGITE íMnFe204 WITH E X C E S S Mn304 IN SOLID SOLUTION)

(8)7 M n C O 3 + S102=3Mnz03.MnSIO~ ~ C ~ 2 M n C O ~ + S i 0 z = M n p C i 0 ~ ~ 2 C O ~ (BRAUNITE) (T E PH ROIT E )

CO2

(8)6 M n 2 si04 + O2 (TEPHROITE)

=

6 M n '51.03 t 2 M n 3 0 4 (RHODONITE) HAUSMANNITE

9

Manganite (y-MnOOH)transforms to pyrolusite at 375" C which is again reduced to bixbyite at 565"C and further to hausmannite at 950"C (Table 4, lD,Das Gupta, 1965). Groutite (cr-MnOOH)transforms to pyrolusite at 300" C (Lima-de-Faria and Lopes-Vieira, 1964) and at

higher temperatures, the usual transformation of pyrolusite to bixbyite and hausmannite follows. Pyrochroite [Mn(OH),] transforms to manganosite (MnO) at 32" C (Table 4, 1B; Klingsberg and Roy, 1959) which easily oxidizes to hausmannite. Psilomelane [(Ba,H,O),Mn,O,,] transforms to hollandite at about 550"C (Fleischer, 1960; Fleischer and Richmond, 1943;Wadsley,1950) and hollandite transforms to hausmannite at 900"C (Table 4, lC, Fleischer, 1964). Braunite and rhodonite are formed by reaction of higher oxides ofmanganesewith silica in rising temperature depending on the oxygen fugacity (Table 4, 2A, and 2B; Huebner, 1967; Roy, 1968). The low temperature manganese silicate, bementite [(Mn,Mg,Fe),Si,(O,OH),,]converts to braunite first and further to rhodonite at higher temperatures (Ito, 1961). Higher oxides of manganese, admixed with iron,convert first to bixbyite with varying amountsof Fe,O,(according to temperature(Mason,1944)) and then to jacobsite and/or vredenburgite with rising temperature. In a metamorphosed oxidic orebody, braunite and bixbyite may form together or one in exclusion of the other,

depending upon bulk composition (availability of silica, iron, etc.), temperature and oxygen fugacity (Muan, 1959~).Braunite,once formed,remains stable with increasing temperature as the presence of silica in the structure has a strong stabilizing effect on Mn2+ (Muan, 1959~7, 19593). Huebner (1967), discussed the possibility of reaction between braunite and silica to give rhodonite (Table 4, 2C)and Pavlovitch (1931) showed that braunite (8.92per cent Sioz), on heating to 1,400"C for five hours,converted to tephroite and an eutectic intergrowth of tephroite and hausmannite.From the study of natural deposits in India (quartz-albite-muscovite-chlorite to sillimanite-almandinemuscovite subfacies) and elsewhere, however, no such evidence of transformation of braunite or its reaction with other phases has been obtained. Evidences of formation of braunite as a conversion product of bixbyite (Fig. 13) are, on the other hand, ubiquitous in Indian ore deposits. The formation of hausmannite in metamorphosed oxidic (carbonate-free)ores, is a function of high temperature and concomitantreduction.Its presence,in the absence of jacobsite, indicates a bulk composition low in iron (cf. Jothvad deposit,Table 3). In highly metamorphosed ores with iron-rich bulk composition, hausmannite is associated with jacobsite and/or vredenburgite. From a bulk composition rich in manganese carbonate, however, hausmannite may form either by dissociation of rhodochrosite at high temperature (Table 4, 1E;Smith Prospect, 237

S. Roy

Buckeye deposit,United States; Merid mine, Brazil; Tamworth District,Australia,etc.) or by controlled supergene oxidation of the latter (Bricker, 1965). The formation of jacobsite is dependent on the iron content in the original bulk composition,temperature and oxygen fugacity.It has been reported from low grade metamorphic deposits (cf. Kajlidongri,Table 2) and even from unmetamorphosed colloidal ores (Dongri Buzurg; Roy, 1959; Morocco; Vincienne, 1956; Saint Béat, Pyrénées; Perseil, 1966). The manganese content of these jacobsites is invariablylow.The mineral is also very widely distributed in the high temperature assemblage of almandine-amphibolite facies in India. The composition of jacobsite varies as a, function of temperature and,with rising temperature, more and more Mn,O, goes into solid solution in Fe,O, (Mason, 1943; Muan and Somiya, 1962; Van Hook and Keith, 1958). The micrographic intergrowth of jacobsite and rhodonite recorded by Chaudhuri (1967)from Dongri Buzurg and by the author from Tirodi (Fig. 12), formed by oxidation of FeSi0,-rich rhodonite or pyroxmangite.In carbonate deposits where rhodochrosite has a high content of FeCO, in solid solution,the carbonate stability field is reduced and Fe-Mnspinel or jacobsite may form as a decarbonation product in high temperature (Huebner, 1967; cf. Tamworth District,Australia). The presence of vredenburgite ensures a high temperature and testifies to an original bulk composition rich in manganese and iron and a sufficiently slow cooling for the exsolution intergrowth to form, postdating deformation. The stability field of vredenburgite has been studied by Mason(1943),VanHookandKeith(1958), andYun(1958). Rhodochrosite, under laboratory conditions in air, dissociatesto M n O + COzat 610-635" C and the resultant manganosite rapidly oxidizes to hausmannite at 670735" C (Cuthbert and Rowland, 1947; Kulp et al.,1949, 1950, 1951). When rhodochrosite is associated with silica, the reaction at high temperature produces rhodonite (Table 4,6A). If the rhodochrosite is present in larger proportion with respect to silica (Mn:Si ratio very high), tephroite may be produced (Table 4, 6C, 7). Rliodochrosite may also react with silica with rising temperature and high oxygenfugacity to produce braunite (Table 4,6B; Huebner, 1967). The manganese silicates formed at elevated temperatures, either by reaction between higher oxides of manganese and silica or between manganese carbonate and silica, may take part in furtherreactionswith a riseof temperature. Thus rhodonitemay be oxidized to give rise to hausmannite and silica (Table 4,9; Huebner, 1967) and tephroite may react with silica to produce rhodonite (Table 4,8A), or it may be oxidized to form rhodonite and hausmannite (Table 4, 8B). Studies of metamorphosed manganese formations of India and the data accumulated on other deposits suggest that thenatureofthebulk compositionof syngeneticmanganiferous sedimentslargely determines the latermetamorphic reactions and the products. The unmetamorphosed syngenetic oxide sediments contain manganese largely in the 238

tetravalent state and only a minor part in divalent or trivalent state(Hokkaido,Japan:Hariya,1961;Cuba:Hewett, 1966;Irnini Tasdremt,Morocco: Bouladon and Jouravsky, 1956;Timna Dome,Israel:Bentor,1956;Deep Sea nodules: Mero, 1965; Roy, 1969). The manganese carbonate sediments contain divalent manganese and CO,(Usinsk and Labinsk deposits,U.S.S.R.: Varentsov, 1964; Urukut deposit, Hungary: Nemeth and Grasselly, 1966, etc.) and sometimes the oxidic sediments transgress to carbonates (Nikopol, Bol'she Tokmaksk and Chiatura deposits, U.S.S.R.: Varentsov, 1964). The syngenetic sediments sometimescontainlow-temperaturemanganiferous silicates, represented by layer silicates containing mainly divalent manganese and these phases are generally hydrated. As shown earlier,the metamorphosedmanganese formations of India are characterized by oxidic ores sharply interbanded with manganese silicate rocks (gondite) in most places. The mineralogy of the manganese ores and gondite indicates that during metamorphism the individual bands act as separate entities, and the transformation of phases and reactions with rising temperature proceeded according to the physico-chemicalconditions restricted to individual bands. The total absence of rhodochrosite and tephroite is characteristic,indicating that the original sediments were devoid of manganese carbonates (Roy, 1966, 1968;Roy and Purkait,1968), and were,by and large,made up of higher oxides of manganese with variable admixtures of silica,alumina, iron, etc. With the onset of metamorphism, braunite formed as the only metamorphic mineral in quartz-albite-muscovite-chlorite subfacies (Shivarajpur) and its formation,by reaction of higher oxides and silica, in preference to rhodonite,was possibly facilitated by high oxygen fugacity. Rhodonite and spessartite,together with other manganese silicates, appeared in quartz-albite-epidote-biotitesubfaciesat Kajlidongri,and were restricted to the gondite bands, separated from braunite-rich oxidic ore bands. In the quartz-albite-epidote-almandine subfacies at Dongri Buzurg-Kurmuraarea they are similarly developed, whereas at the same grade of metamorphism, they are absent in the Bharweli-Ultwa deposits. Thus, the formation of rhodonite by the reaction of higher oxides and silica is not controlled entirely by temperature, and its appearance in gondite bands, in preference to braunite,is a function ofoxygenfugacity.Spessartite-richgarnetusually appears in low grade metamorphic rocks (quartz-albitemuscovite-chloritesubfacies: Chimer,1960;Hutton, 1957; Woodland, 1939). Fermor (1909) suggested its formation by reaction between kaolinite,silica and manganese oxide, and Huebner (1967) considered that the decomposition of manganiferous sheet silicates with rising temperature gives rise to the formation of spessartite. However, braunite forms earlier than spessartiteduring metamorphism (Huebner, 1967). Bixbyite appeared first in the quartz-albiteepidote-biotite subfacies (Kajlidongri) and the relict colloform texture shown by the mineral together with braunite strongly indicatestheir derivationfrom originally colloforin higher oxides by transformation with rising temperature. Hollandite, accompanying braunite and bixbyite at Kajli-

Genetic studies on the Precambrian manganese formations of India with particular reference to thr effects of metamorphism

dongri, apparently formed by transformation of bariumbearing psilomelane at higher temperature. Low-manganesejacobsite,approaching the magnetite compositionfield, appeared first at Kajlidongri and its manganese content gradually increased at higher grades of metamorphism. At Chikla-Sitasaongi (staurolite-almandine and kyanite-almandinesubfacies), Tirodi-Sitapatore,Netra-Ramrama, Gowari Wadhona (sillimanite-almandine-muscovite subfacies) and Kodur-Carbham (granulite facies), India, jacobsite (Mn,O,-rich), hausmannite and vredenburgite appeared and continued in stable assemblage with braunite, bixbyite and hollandite. The appearance and stability of these phases at higher temperature has been explained by Muan and Somiya (1962) and by the previous discussions on the mineralogenetic trend of pure and admixed (with iron) higher oxides of manganese with rising temperature, summarized in Table 4.The thermally metamorphosed (pyroxene-hornfels facies) deposits of Jothvad exhibit a braunite-bixbyite-hollandite-hausmanniteassemblageinthe ores, the absence of jacobsite and vredenburgite being explained by low content of iron in the sediments. The gondite contains spessartite and rhodonite in staurolitealmandine, kyanite-almandine and sillimanite-alniandinemuscovite subfacies, which shows that the phases are stable at higher temperature. Spessartite and rhodonite are often accompanied by other manganiferous silicates. Iron-richrhodonite os pyroxmangite,however,oxidized to give rise to iron-poor rhodonite and jacobsite in micrographic intergrowth (Fig. 12) at Tirodi in the sillimanitealmandine-muscovitesubfacies. Manganese silicate-carbonate protore, developed in many parts of theworld,containsprincipally rhodochrosite, rhodonite,spessartite,tephroite and quartz(Tables2and 3). The mineral assemblages indicate that in the original bulk composition, manganese was present as carbonate (rhodochrosite) with admixtures of silica, alumina, etc. Rhodonite and tephroite were formed by decarbonation of rhodochrosite and reaction with silica in rising temperature (Table 4, 6A, 6C, 7, SA, 8B), though no direct relationship of the formation of rhodonite and tephroite to increasing grade of metamorphism has been established.Tephroitehas been describedfrom the SmithProspect,United States (Huebiier,1967) which has been metamorphosed only lo zeolite or lower greenschist facies,while

the mineral is absent (in similar bulk composition) in the same metamorphic grade at Merionethshire,United Kingdom, and Western Otago,N e w Zealand.It is also characteristically absent in the carbonate-freemanganese silicate deposits including the rocks of sillimanite-almandine-muscovite subfacies of India. Thus, temperature and total pressure cannot explain all the phase assemblages and intensive parameters of @Oz and p.0,are most important (cf. Huebner,1967). Most of the silicate-carbonate deposits are devoid of manganese oxide phases (excepting the supergene oxidation products). In rare cases,hausmannite, hausmannite-braunite, liausmannite-jacobsite and hausmannite-braunite-manganosite-pyrochroite are developed (Tables 2 and 3), apparently formed by decarbonation of rhodochrosite and ferroan rhodochrosite and by the reaction of rhodochrosite and silica at high oxygen fugacity. Thus, the syngenetic manganese ore deposits of India were originally laid down as higher oxides and show, on metamorphism, a reaction sequence characterized by the progressivereduction of manganese. The syngenetic oxides contained only minor amounts of silica and iron in the orebands, and these, with rising temperature,reacted to form braunite-bixbyite-hollandite-jacobsite-hausmannitevredenburgite assemblages. The interbanded gondites were originally represented by manganese oxide, considerable amounts of silica,clay,etc.,in an admixed sediment.The oxidic ores and gondite bands acted as separate entities during metamorphism. The latter, by reaction at rising temperature, gave rise to spessartite-rhodonite-quartzassemblages, often containing manganiferous pyroxenes, amphiboles and micas.Metamorphism of theIndianmanganese formations took place at oxygen fugacities higher than those in the carbonatic rhodochrosite-rhodonitetephroite deposits (cf. Smith Prospect, Western Otago, Merid mine, etc.). Huebner (1967) pointed out the great disparity between the oxygen fugacity of certain manganese deposits compared to the foz prevailing during metamorphism of non-manganiferouscountry rocks. H e suggested that oxygen is not free to pass between the manganese deposit and the surrounding system and the country rock is not responsible for imposing a high fo, on the manganese deposits,rather the manganese minerals themselves buffer or internally define the high oxygen fugacity.

Résumé Étude génétique des furmatiuits de manganèse précambrien en Inde avec références particulières aux effets du métamorplzisme (S.Roy)

Les masses minéralisées d'oxyde syngénétique de manganèse de l'Inde, intimement imbriquées avec des roches silicatées de manganèse (groupées ensemble en formatioii de manganèse), se rencontrent en lits continus ordonnés

stratigraphiquement dans les mêmes plissements que les quartzites de schiste pblitique, les calco-silicates et les marbres du groupe précambrien de Sausas (province de Madhya-Pradeshet Maharashtra), du groupe Aravalli (région de Kajlidongri,Madhya-Pradesh), du groupe Champaner (Shivarapour,Gujarat), du groupe de Gangour (Sundargarh District, Orissa) et du groupe de Khondalite (district de Srikakulam, Andhra-Pradesh). Les formations 239

S. Roy

syngénétiquesde manganèse ont éré métamorphoséesrégionalement en schistes verts et en faciès d'almandine-amphibolite.L'intensité du métamorphisme subi par la formation de manganèse a été déterminée par le degré précis atteint par le méta-sédiment pélitique englobé. A Jothvad, dans l'État de Gujarat (groupe de Champaner) la formation de manganèse a subi un métamorphisme de contact dans les faciès hornblende-hornfelset pyroxène-hornfels. Il a été possible de suivre les changements minéralogiques et structuraux dans le minerai d'oxyde de manganèse aux différents degrés du métamorphisme régional ou de contact.La braunite est le premier minéral métamorphique qui apparaisse et cela est la seule phase stable dans les dépôts des sous-facièsquartz-albite-muscovite-chlorite. La braunite a un domaine de stabilité très étendu et continue jusqu'au sous-faciès sillimanite-almandine-muscovite.La bixbyite (avec une faible teneur de Fe,O,) apparaît d'abord dans le sous-facièsquartz-albite-épidote-biotite en association avec le sous-facièsépidote-almandine.L'hausmannite, la jacobsite et la vredenburgite (présentant un développement enchevêtré & deux phases) apparaissent dans le sousfaciès staurolite-almandinecontinuantle sous-facièssillimanite-almandine-muscovite,en association avec la braunite et la bixbyite (avec une haute concentration de Fe,O,). Les formations minérales d'oxyde de manganèse présentent des changements dans leur contexture en fonction du degré de métamorphisme. Les minerais syngénétiques non métamorphosés sont constitués d'oxydss supérieurs à grains fins et présentent parfois une structure rubanée et des marques évidentes de dépôt à Yair libre.La survivance de la contexture et la structure rubanée encroûtée sont mises en évidence par la présence de bixbyite et de braunite dans les gisements de minerai du sous-facièsquartz-albiteépidote-biotite.Apparemment, là où le métamorphisme est peu développé,les oxydes sédimentaires supérieurs ont été recristallisés pour former de la braunite et de la bixbyite bien que la contexture originalen'ait pas disparu.La structure rubanée sédimentaire originale est conservée dans le minerai.Des contexturesparticulièresdues à la déformation abondent en particulier dans les minerais métamorphosés à faible teneur. D e telles contextures se traduisent par un allongement et une orientation dimensionnelle préférentielle dela braunite et de la hollandite,par des déformations et des déplacements des macles de la hollandite, par des

micro-plissements des filons de minerai, etc. Dans les minerais hautement métamorphosés,la texture granoblastique à grains grossiers est caractéristique. On trouve dans les minerais hautement métamorphosés une texture d'exsolution mise en évidence par la jacobsite et l'hausmannite (vredenburgite). On a observé la transformation de la bixbyite en braunite suivant les plans cristallographiques à tous les stades du métamorphisme. Une formation interstitielle,filons ou couches minces d'un minerai dans l'autre, se rencontre à l'occasion et peut résulter des phénomènes de croissance et de tension superficielle entre des particules qui sont devenues simultanément plus grossières. Les roches de silicate de manganèse métamorphosées régionalement (gondite) sont caractérisées par l'association spessartite-rhodoiiite-quartz-apatite-tirodite-pyroxène manganifère brun (aerigine-augite)-braunite (et autres oxydes inférieurs). L a blanfordite, la winchite, la juddíte, etc., se sont développées au contact de la gondite et de la pegmatite.Les kodurites (roches silicatées de manganèse résultant d'un contact métamorphiquej diffèrent des gondites par la composition des grenats (spessartite-andratitej et par I'abondance des feldspaths potassiques. L a paragenèse des phases minérales dans les roches de silicate de manganèse, en relation avec le degré de métamorphisme,a été établie. L a tendance minéralogénétique qu'on observe aux différents degrés de métamorphisme régional dans les gisements de minerai oxydiques et dans les roches silicatées de manganèse a été rattachée aux résultats déduits de l'étude de l'équilibre des phases. Xi a été établi que les associations minérales des minerais d'oxyde de manganèse aux différents stades du mktamorphisme proviennent de la réduction progressive des oxydes plus riches à l'origine, oxydes qu'on rencontre dans les systèmes ferrugineuxanalogues et au cours du métamorphisme d'associations de minéraux comportant de la rhodochrosite [cf. Brésil). Les associations minérales dans les roches silicatées de manganèse métamorphoséesavec absence de tephroiteet de rhodochrosite, indiquent aussi que les carbonates étaient originairement absents dans la composition d'ensemble et que les oxydes élevés, silicates,silice, alumine, etc.,réagirent par réduction progressive pour donner naissance aux associations de spessartite-rhodonite-quartz-braunite avec plus ou moins de pyroxènes manganifères, d'amphiboles et de micas,

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Genetic studies on the Precambrianmanganese formationsof India with particularreferenceto the effects of metamorphism

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Discussion J. VANN.DORR. Are any of the many iron-formationsin India contemporary with the manganese deposits of the Nagpur region?

S. ROY. Except for the Khondalite group, the mkganese ore bodies in India are not associated with graphitic sediments.It may, thus, be assumed that the reduction of

S. ROY. Syngenetic manganese deposits of the Sausar,

sedimentary manganese oxide minerals during metamorphism was effected by rise in temperature only.

Aravalli and Khondalite groups are not associated with contemporary iron-formations.In the Gangpur Group the syngenetic manganese formation is overlain by iron ore formation. In Sandur, Mysore State, iron-formation is associated with manganese ore bodies, but it is not absolutely certain whether some of these manganese ore bodies are syngenetic,rathermany ofthem are distinctly epigenetic.

Y.P. MELNIK. D o the temperatures of the phase transformation of manganese minerals (Table 4 and text) correspond to 1 bar pressure or to high pressures under metamorphism? S.ROY. The temperature values cited in Table 4 are taken from results of laboratory phase-equilibrium studies carried out by different workers in air. So these absolute values will be different during metamorphism according to prevalent foz,feo,, etc.

Y.P. MELNIK. What agents caused the reduction of the manganeseminerals:natural reducing compounds in rocks -carbon, etc.-or the temperature rise only?

242

I. P. NOVOKHATSKY. What is the temperature range for formation of hausmannite-braunite and tephroite-spessartite?

S. ROY.The temperature of formation of hausmannite varies according to the nature of the original sediments, i.e. carbonate or oxide, as shown in Table 4.In oxidic sediments of India it is formed at fairly high temperatures in almandine-amphibolitefacies. Once formed, braunite continues to be stable in higher grades. Spessartite forms in low grade (at the beginning of greenschist facies), and continues to persist in higher grades in gondite.The temperature of formation of tephroite depends very much on foz and feo., as pointed out in the paper. I. P.NOVOKHATSKY. H o w do you estimate pressure values in the process of metamorphism?

S.ROY. The pressure-temperaturevalues of the ore deposits were determined by referring to the enclosing pelitic metamorphites which were metamorphosed to the same grade.

Precambrian ferruginous formations of the Aldan shield I. D.Vorona, V. M.Kravchenko, V. A. Pervago and I. M.Frurnkin Yakutia Geological Service (U.S.S.R.)

The Aldan shield is an outcropping crystalline basement ridge on the southeastern margin of the Siberian platform and is a unique area in which to study laws governing iron-ore accumulation during Precambrian time. Old ferruginous formations within the shield consist of three genetic rock units, i .e. metamorphosed sedimentary ferrosiliceous (jaspilitic), metamorphogeneticferromagnesian and sedimentary oolite-hematitic (Table 1). Both the first and the second units are interspersedwith the Precambrian sequence, but they attain great thickness and comprise deposits of commercial value within the only two series, i .e,Deoss-Leglierian (Middle Archaean) and Subganian (Upper Archaean). The third unit does not contain any commercial deposits and it is therefore not discussed below.

Description of ore-bearing formations The Precambrian within the Archaean shield (Fig. 1) consists of Archaean formations forming a crystalline basement as well as of Proterozoic formations building up the old mantle of the platform. The following are geotectonic features occurring within the Archaean formations (from older to younger): Yengrian, TimptonoJeltulian,Olecmian and Subganian.Physicalmeasurements of geologic time also proved the Archaean (from 2,500 to 4,500 m.y.) age of the formations (Geochronology,1968). Muscovite-pegmatites crosscutting the Subganian strata were dated at 2,500 m.y., while pyroxenes belonging to Yengrian metamorphic rocks were dated at 4,500 m.y. old. Proterozoic strata consist of Udocan, Maimacan and Engilian deposits of Lower, Middle and Upper Proterozoic age respectively. As Maimacan and Engilian strata are not ferruginous they are not discussed below.

Deoss-Leglierianmetamorphogenetic ferromagnesian formation bearing magnetic iron ore Magnetic iron-ore deposits of this formation are not known in Precambrian strata found in other regions of the U.S.S.R. Similar Archaean skarn magnetite deposits have been found in the Central Sweden (Gejer, 1939; Gejer and Magnusson, 1955). Practically all commercial deposits of the types to be described herein are found in the central part of the Ungra-Timptoniangraben-synclinorium (Fig.1). The iron-ore deposits are found on the flanks or occasionally in the core of secondary and younger synclines. They are associated with two producing horizons in the Deoss-Leglierian strata which are each 400-500 m thick and are interbedded with barren bands having thicknesses of 500-800 m (Pucharev, 1959). The producing horizons are interbedded with beds and lenses of dolomite marbles and calciphyres up to 200 m thick, diopside rocks, amphibolites and various gneisses (amphibole,pyroxene, biotite, garnet, graphite, etc.), as well as with magnesia skarn metasomatic replacements which contain magnetite occurrences. Bed-like or flattened-lensore bodies lie conformably in the country rocks. The ore bodies strike over 2704,200 m and are 11-140 m thick. Serpentinized forsterite and diopside-magnetite varieties are predominant (about 75 per cent); clinohumite and sahlite-scapolite-magnetite ores are less commun (about 20 per cent), whereas hypersthene-,hornblendeand salite-feldspar-magnetitesare rare. Sulphides, i.e. pyrrhotite and, more rarely, pyrite and chalcopyrite,are common in all of the rock varieties. The ores have massive banded-lenticular and banded structure. The ores have medium-grained structure. The mean iron content in rich varieties of ore amounts to 45-50per cent,in disseminated ores to 35 per cent, and that of sulphur amounts to 1.52.5 per cent.

Unesco, 1973. Genesis of Precambrian iron and manganese deposits. Proc. Kiev Symp., 1970. (Earth sciences, 9.)

243

I. D.Vorona,V. M.Kravchenko,V. A.fervago and I.M.Frumkin

TABLE 1. Precambrian ferruginous formations of the Aldan shield Geologic age

Absolute Rock complex age (m.y.) or series

Lower Proterozoic

2,0002,400

Lower over Proterozoic- 2,500 Upper

Udocan

-

-

Subrranian Palaeoaulacogenes' From greenschistose to amphibolitic

Geosyncline

Amphibolitic

Olecmian

Middle Archaean

2,750

Superposed From DeossLeglierian Grabenamphibolitic synclinorium to granulitic series (pretoaulacogene?)

Yengriaii

Iron content and size of deposits

Terrigenous: quartzose sandstones, aleurolites, argillites

Sedimentary colite-hematite; the Atugeian and the Hugdin deposits

Laminated colitehematite ores

Possible minor deposits

Gabbrodiorite Terripenousigneous; quartzites, aluminiferous schists biotite and amphibole gneisses; orthoamphibolites, metaporphyrites

Ferro-siliceous (jaspilitic); the Ymalic and the Neliukin deposits

Banded ferruginous quartzites (magnetite, grüneritemagnetite, biotitemagnetite, etc.)

Major deposits of poor but easily beneficiated ores

Ophiolite

Ferro-siliceous; the Olecmian occurrence

Lenticularbanded ferruginous quartzites (magnetite, amphibolemagnetite)

Minor occurrences of poor but easily beneficiated ores

Ferromagnesian (metasomatically altered); the Tayojiioye and Desovian deposits

Massive, lenticular banded, magnetite ores containing forsterite, diopside, scapolite, serpentine, phlogopite

Major

Nil

2,750 3,100

4,500 (3,8005,400?)

Minerai composition and texture type of the iron ores

Protoplatformtroughs

Upper Archaean

Lower

Type of ferruginous formation and deposit

Regional Conjugate metamorphism magmatic facies formation

Archaean

Archaean

Genetic type and composition of the orebearing formation

Geotectonic position of the ore-bearing strata

Protogeosyncline

Granulitic

Nil

Igneoussedimentary; biotite, amphibole gneisses, schists, amphibolites

Gabbrodiorite Igneoussedimentary; dolomitic marbles, calciphyres, basic crystalline schists, metadiabases; amphibole, pyroxene and other gneisses

Ophiolite

Igneouschemogenic; quartzites, gneisses of high alumina content and basic ortho-schists Sedimentary; dolomitic marbles; calciphyres, pyroxene and amphibole gneisses

deposits of rich ores

Ferro-siliceous; Lenticular- Iron ore banded the Yagindya, occurrences, ferruginous minor Holednikan quartzites deposits deposits of (amphibole- poor but magnetite, easily pyroxene beneficiated magnetite) ores Minor Ferromagnesian Massive, deposits (metasomatically mottled lenticular, altered); the of rich ores Emeljac deposit banded magnetite ores containing forsterite and hypersthene

1. Aulacogene: narrow sub-longitudinaltrench-like synclinorium structure.

Magnetite ore deposits associated with magnesium-ferruginous formation are explored on the surface and down to the depths of 400-1,000 m . The depositsvary from minor ones estimated at 25-45 million tons to major ones of 3001,500 million tons. The Tayoshchnoe, Pionerskoye and Desobskoyedeposits,are among the largest ones located in

244

Leglierskaya and Dios-Khatiminskayasynclines. The potential reserves of this type of deposit (Pervago, 1966) are estimated at about 3 billion tons, including 983.8 million tons of commercial reserves.

FIG.1. Geologic-tectonic scheme of the Charo-Aldan iron province. 1. Cenozoic sediments in superimposed troughs; 2.Mesozoic-MiddleProterozoicsedimentsofthe platform cover; 3. Proterozoic sediments of the Bajkal folded zone;4.Lower Proterozoic, the Udocan complex in platform depressions; S. Upper Archaean, the Subganian complex in palaeoaulacogenes; 6.Upper Archaean, the Stanovoj complex in the outer structural-facieszone;7.Upper Archaean,the Olecmian complex in structural-facieszones, (a) outer, (b) internal; 8. Middle Archaean,Deoss-Leglierianseriesina protoaulacogene;9.Middle

Archaean, the Timptono-Jeltuliancomplex in structural-facies zones,(a) outer,(b) internal;10.Lower Archaean,the Yengrian complex in structural-facieszones, (a) outer (the Yengrian series), (b) internal (the Kurultian series). Deposits of iron ores : 11. Magnesia-iron-formation, (a) large deposits,(b) minor deposits;12.Iron-chertformation, (a) potentially large deposits,(b) minor deposits;13.Sedimentary oolite-hematiteformation; 14.Deep faults-structural sutures; 15. Faults.

Metamorphic sedimentary ferro-siliceousformation in the Subganian series

The main minerals of the ferruginous quartzites are magnetite,quartz and amphiboles;plagioclase, garnet and biotite are less common. Clinopyroxene and fibrolite are rare.Zircon,orthite and titanite are accessories.Magnetite occursin practically monomineralbands and is interspersed, to some extent, in quartz grains as small inclusions.Magnetite lying in the subsurface turns to martite. Amphiboles are represented by two types, actinolite-ferroactinolite (40-59 per cent iron-bearing)and cunimingtonite-grunerite (48-74 per cent grunerite molecule). The two varieties occur both together and separately. Table 2 shows the chemical composition of main mineralogical types of ferruginous quartzites. These characteristicsclassify the ferruginous quartzites as poor but easily beneficiated ores. Rare occurrences of rich metamorphic magnetite ores which contain 64per cent iron are associated with ferruginous quartzites (col. 5, Table 2). Iron-ore deposits ascribed to the Subganian ferrosiliceous formation are poorly studied and almost unexplored. From the aeromagnetic surveys,potential iron ore

Deposits of poor iron ores (ferruginous quartzites) of the Subganian series are isolated from the deposits described above. They are concentrated on the western margin of the Aldan shield,in some narrow sub-longitudinaltrenchlike synclinorium structures. The ore-bearing Subganian complex consists of ferruginous quartzitebeds 10-40 m thick,in places 100-200m, at the top of the sequence. These are lying among biotite gneisses occasionally interbedded with pyroxene-amphibole schists,amphiboles and quartzites.Ferruginous quartzites are commonly associated with a certain division of the sequence;less commonly these are interbedded with barren rocks up to 200 m thick.Ferruginousquartzitesform simple and complex beds and extend for several kilometres, but usually do not exceed 10k m . These are clearly banded and, less commonly, plicated.

245

I. D.Vorona,V. M . Kravchenko,V. A.Pervago and I. M . Frumkin

TABLE 2. Chemical composition of the Subganian ferruginous quartzites (%) Compounds

Loss on ignition Total H20+

Fe total Fe soluble

1

2

3

47.96 44.62 43.28 0.22 0.2 0.27 2.O3 2.06 0.08 35.72 24.63 36.14 16.54 9.71 21.78 0.08 0.04 O .40 2.68 2.72 1.33 0.62 1.33 1 .o 0.1 0.13 0.05 0.13 traces 0.08 0.13 0.14 0.04 0.02 nil 1.84 2.01 100.04 99.67 100.38 nil 0.09 32.76 34.66 38.15 37.0 29.5 26.3

4

5

48.4 0.13 0.21 49.14 0.79 0.03 0.015 0.63 0.29

5.21 0.14 1.94 60.55 29.66 0.07 3.34 0.35

-

traces

0.29 0.103 0.045 0.005 0.82 0.85 100.508 99.8 0.13 40.62 64.14 34.6

-

1. Hornblende-grünerite-magnetiteferruginous quartzite (Charsk deposit); 2. Grünerite-magnetite ferruginous quartzite (Charsk deposit); 3. Magnetite ferruginous quartzite (Tarikhan deposit); 4. Martite ferruginous quartzite (Tarikhan deposit); 5. Rich quartz-hornblendemagnetite ore, thin-banded (Olecmian occurrence).

reserves within the Subganian strata (down to 500 m) amount to about 5-7 billion tons. Thus,the PrecambrianCharsk-Aldaniron ore province is found within the central and western parts of the Aldan shield. The potential iron ore reserves in the province amount to 8-10 billion tons. Commercial deposits are associated with two Archaean formations, a metamorphogenetic magnesio-ferruginousformation and a metamorphosed sedimentary ferro-siliceous formation. Areas of their distribution are separated, thus forming two iron ore subprovinces,namely Aldan and Charsk (Kravchenko and Vorona, 1968). The two iron-ore formations occur repeatedly in the Precambrian sequence. Each formation becomes commercial only at one stratigraphic time, i.e. Middle and Upper Archaean. Linear-oriented synclinorium structures of the aulacogene types developed on the Archaean protoplatforms played a decisive role in forming and distributing comniercial iron-oreformations. Though iron-ore formations are sometimes found in geosynclinal structures,these never acquire commercial value. A closepositionalconnectionoftheiron-oreformations with the Archaean pre-orogenic formations of the basic igneous rocks was revealed which allows their relationships to be traced when studying the problem of the primary concentration of iron in iron ores.

Résumé Formations feuvifères du Précambrien inférieur du bouclier d’dldan (I.D.Vorona,V.M.Kravchenko,V. A. Pervago,

I. M. Frumkin) Les formations ferrifères du bouclier d’Aldan se caractérisent par leur vaste étendue,par une grande diversité génétique et par l’énorme période (au moins deux milliards d’années)qui a présidé à leur constitution. Par rapport aux autres régions du monde, c’est sur le

bouclier d‘Aldan qu’apparaîtau mieux l’histoiregéologique précambrienne du globe, ce qui offre d‘excellentes conditions concrètespour l’étudedes lois qui ont régi le processus de formation des gisements de fer précambriens. Cette étude fournit des renseignements de base sur les particularités géologiques,génétiqueset minéralogiques des formations ferrifères et sur les degrés de concentration du minerai de fer dans le bouclier d‘Aldan.

Bibliography /Bibliographie FROLOVA, N. V. 1951. O b usloviyach osadkonakopleniya v arheyskoy ere [On the conditions of accumulation of deposits in the Archaean]. Tr. Irlciitskogo gas. univ.,vol. 5, ser. geol., vip. 2. Moscow, Gosgeolizdat. FRUMKIN, I. M . 1967.Structurno-litologichesckiymethod cartirovaniya dokembriyskich obrazovaniy i rezultaty ego primeneniya na Aldanskom schite [Structural-lithologicalmethod of mapping of Precambrian formations and the results of its utilization on the Aldan Shield]. Problemy izucheniya geologii dolcembriya, Moscow, Nauka. . 1970. Napravlennost geologicheskogo razvitiya zemnoy kory Aldanskogo schita v arheiskoe vremya [Trend of geo-

-_ 24U

logical development of the earth crust of the Aldan Shield in the Archaean time]. Tectonica Sibiri.,vol. III, Moscow, Nauka. GEJER, P. 1939. The paragenesis of ludwigite in Swedish iron ores.Geol. Fören. Stoclth. Förh.,vol. 416, p. 61. ;MAGNUSSON, N.H.1955.Zheleznierudy Swecii [Iron ores of Sweden]. Zhelezorudnie mestorozlzdeniya mira, vol. 1, IL. Geochronologia dolcembria Sibirdtoy platformy i ee sclai~tchatogo obramlenia [Geochronology of Precambrian of the Siberia platform and its plicative framing]. Leningrad,Nauka, 1968.

KITSUL, V. I.; LASEBNICK, K.A. 1966.Geologia i petrographia docembriysckich kristallitchesckichobrazovaniyrayona sliania

Precambrianferruginous formations of the Aldan shield

Aldana i Ungry (k probleme ‘Ungrinsckogoclina’) [Geology and petrography of Precambrian crystalline formationsof the region of the confluenceof Aldan and Ungra (on the problem of ‘UngraWedge’)]. Geologia i petrologia docembria Aldansckogo schita, Moscow, Nauka. KRAVCHENKO, B.M.;VORONA,I.D.1968.Tcharsko-Aldanskaya. Zhelezorudnayaprovincia [Tcharsk-Aldaniron ore province]. Materialy po geologii i poleznym iscopaemytn Yalrutsclcoy ASSR.Yak. knign. izd.,vip. 18. Yak.

LEYTES, A. M.; MURATOV, M.V.; PHEDOROVSKY, V. S. 1970. Paleoavlacogeny i ich mesto v razvitii drevnih platform [Palaeoaulacogenes and their place in the development of ancient platforms]. DAN SSSR,vol. 191,no. 6. MARACKUSCHEV, A. A. 1958. Petrologia Taezhnogo Zhelezorudnogo mestorozhdeniya v archeeAldansckogoschita [Petrology of Taezhniy iron ore depositin the Archaean of the Aldan Shield]. Tr.DVF AN SSSR, ser. geol., vol. 5, Magadan.

NUZNOV, S. V.;YARMOLUCK, V. A. 1968. Novie dannye po stratigraphii dokembria na primere Aldanskogo schita [New data on Precambrianstratigraphyon the exampleof the Aldan Shield]. Sov. geologia, no. 5. PERVAGO, V. A. 1966. Aldanskaya Zhelezorudnaya provincia [Aldan iron ore province]. Moscow, Nedra. PUCHAREV, A. I. 1959.O geologii i osobennostyah lockalizatsii orudeneniya Yuzno-Yakutsckich Zhelezorudnich mestorogdeniy [On the geology and particularities of the localization of ores in the South-Yakutianiron ore deposits]. Geologia rudnich mestorogdeniy, no. 1.

SCI-IADYNIN, L.I. 1958.O geneziseYuzno-YakutsckychZhelezorudnyh mestorozhdeniy [Onthe genesis of the South-Yakutian iron ore deposits]. Izv. AN SSSR,ser. geol.,no. 1. SERDYUTCHENKO, D. P. 1960. i dr., Zheleznie rudy Yuznoy Yakutii [Iron ores of South Yakutia]. Moscow, Academy of Sciences.

Discussion A. M.GOODWIN. What radiometric techniques were used

I.D.VORONA.Mainly the K-Armethod for amphibolites,

to determine the absolute ages of the Archaean rocks?

micas and pyroxenes;to a lesser extent,the U-Pbmethod.

247

O n the issue of genesis and metamorphism of ferromanganese formations in Kazakhstan V. M.Shtsherbak,A. S. Kryukov and Z.T.Tilepov The Institute of Geological Sciences of the Academy of Sciences of the Kazakh S.S.R.

In Kazakhstan the ferruginous and ferromanganeseformations are found in most of the geological provinces and epochs (Fig. 1). Recently,apart from the well-knownEarly Proterozoic ferruginous quartzites in Karsakpay Kara Tau and Famensk, and ferromanganesian and siliceous-carbonaceous formations in the Atasuy region, new ferruginous and ferromanganesian formations and ore deposits of different epochs have been distinguished in the Bet-PakDala steppe, Uspensk Synclinorium, Chingiz Anticlinorium,the Altai and in Turgai. In the Bet-Pak-Dalasteppe of Kazakhstan within the Chu anticlinorium the Precambrian formationsof volcanic ferruginous chert and jaspilite have been distinguished. These have subjected to various stages of metamorphism. The anticlinorium includes Proterozoic, Cambrian,Ordovician, Devonian, Carboniferous,Cretaceous and Palaeogene formations. Proterozoic deposits with ferruginous formationsare porphyroids,slate,quartziteand ferruginous quartzite,stretching north-west for 50 km in a belt 15 km wide. Cambrian deposits are composed of sandstone,metamorphic schists,limestone,jasper quartziteand porphyrites. The Ordovician is represented by arkose and micaceous sandstone and platy flints; the Devonian is expressed by effusions of acid composition, while Carboniferous deposits are mainly sandstone, limestone and conglomerates. There are basic and ultrabasic intrusions of Upper Ordovician or Cambrian age in the north-easternpart of the anticlinorium.In most of the region all these rocks are over-

lain by friable sediments of the Mesozoic and the Cenozoic. The volcanic ferruginous chert occurs in the Proterozoic deposits in the north-westernpart of the anticlinorium. It is associated with the recently discovered large ore deposit at Zhuantobe (Gvardeiskoe) (Fig. 2), where the section of the formation has been most thoroughly investigated. The bedrock of the studied part of the section is composed of grey and light grey porphyroids (onquartzporphyry) with intercalations of green quartz schists. It is overlain by 50 m of ferruginous quartzite with lenses of porphyroid and green and grey quartz-sericiteslates. The next layer is also composed of ferruginous quartzites,but here they are black and brown, 55 m thick,with lenses of porphyroids. The total thickness of the Proterozoic orebearing deposits is approximately 700-750 m . At Zhuantobe more than twenty lenticular ore bodies between 5-50 m thick and 50-3,500 m long have been discovered. The ores in the deposit are laminated ferruginous quartzites, with hematite laminae 0.1-3 mm thick intercalated with quartzite-hematite.Associated minerals are magnetite (10-20per cent) and martite.Hematite occurs as lathes (0.01x 0.05mm) and plates (0.003x 0.008mm), forming a thick net. Crystals of magnetite are dispersed in the hematite mass as porphyroblastic grains often with an octahedral shape and dimensions 0.01-0.5mm. Quartz forms isometric grains with dimensions 0.007-0.01m m . The chemical composition of the ferruginous quartzites is given in Table 1.

TABLE1. Chemical composition of ferruginous quartzites at Zhuantobe Sample

Fe

No.

(tota,)

SiOs

Alzo3

Fe,O,

Fe0

1. 2.

56.45 49.75 59.24 39.75 23.32 46.22 37.95

16.45 27.45 14.65 39.07 65.70 29.73 43.73

0.85

80.28 69.29 81.85 56.52 32.34 67.14 54.21

0.36 1.65 1.22 0.83 0.88 0.83 1.39

3.

4. 5. 6. 7.

-

0.25 1.63 0.95 0.97 1.47

MnO

0.02 0.04 0.08 0.04

MgO

0.27 0.14 0.17 0.04 0.07 0.04 0.11

Ca0

0.75 0.30 0.25 0.20

KzO

Na,O

P20,

SO,

0.10 0.10 0.10 0.20

0.10

0.36 0.28 0.45

0.01

0.10 0.10

0.20

0.10

0.10

0.10 0.20 0.20 0.20

Unesco, 1973. Genesis of Precambrian iron and »fangonesedeposits. Proc. Kiev Symp.,1970. (Earth sciences, 9.)

0.14 0.39 0.01 0.01 0.06

H,O- irznition

0.03 0.03 0.01

0.57 0.62 0.54

'Ota1

99.38 99.68 99.48 100.24 100.71 99.34 101.33

249

V. M.Shtsherbak,A.S. Kryukov and Z.T.Tilepov

[ Y J 4 FIG.1, Regions of development of ferruginous and ferruginoussiliceous formations in Kazakhstan. 1. ferruginous-siliceous formations of the Precambrian;2. ferruginous-siliceousforma-

tions of the early Palaeozoic;3.siliceous-carbonateferruginouscherty formations of the middle Palaeozoic(Fammenian); 4.the iron-oredeposits.

The ore-bearing rock mass of Zhuantobe is strongly folded with dips of 65-85' (Fig. 2). Crumpled layers often occur. The appearance of boudinage structuresin ore-freeand low-ferrousquartzites,with sectionsof0.5m , in ferruginous quartzites is evidence of intensive dynamic reworking of rock mass. Regional metamorphism is only of the greenschist facies,as is judged by intensive chloritization,sericitization,presence of stilpnomelane in ores, little recrystallization of fine-grained hematite and its transition to magnetite, as well as preservation of rnicrolamination of quartz-hematite ores. N o metamorphic differentiation has been observed. The ferruginous chert formation occurs in the Protero-

zoic deposits in the central and south-easternparts of the Chu aiiticlinorium. The formation includes the epidotebiotite-chlorite-quartzand amphibole slates, ferruginous quartzites, dolomitized limestone and marble. The Temir group of quartz-hematite-magnetiteores also belongs to it. This formation differs from the formation of the first type by the absence of volcanics and a minor iron content.Wide occurrence of quartz-chlorite-epidote,quartz-biotite-epidote and amphibole slates in these rocks,together with the transitionoffine-grainedquartz-hematiteores intomediumand coarse-grained quartz-hematite-magnetiteores places this formation in the amphibole facies of metamorphism. In the southern part of the anticlinoriuin,microcline and biotite gneiss, gneissose granite, quartz-andalusite

250

O n the issue of genesis and metamorphism offerromanganeseformationsin Kazakhstan

Section A

- Ei

FIG.2. Schematic geological map of Zhuantobe deposit. 1. friable depositions of the Mesozoic and the Cenozoic; 2,quartz-chloriteand quartz-sericiteschists;3.ore-freequartzites;

4.porphyroids;5. ferruginous quartzites;6.dislocationswith a break in continuity.

slates and low-ferrousmagnetite quartzites belonging to granulite facies are included among the Proterozoic formations. The low ferrous content in the rocks of the formation is evidently conditioned by the high metamorphous differentiation and probably by the remoteness from the volcanic centre. In the north-easternpart of the Bet-Pak-Dalasteppe, along the eastern part ofZhailmy synclinal limb for 50 km, the Ordovician deposits appear as quartz conglomerates, polymict, sandstones,aleurolites, chlorite sericitic schists, quartzites and porphyrite. In the eastern part they are interrupted by the large Upper Palaeozoic granitoid intrusions. The overlying part of the rock mass, datable and known as the Kosagaly formation,interbedsthe persistent 300 m thick horizon of quartzites and jaspers with strata and lenses of ferrous and ferrous-manganese ores. These form the Tuyak-Kosagalygroup of deposits. The ores are intercalated with quartzites,jaspers and amphibole-epidote-chloriteslates. In the south andesite and basalt porphyrites are also intercalated in the sequence. The ore-bearing rock is of rhythmic fine-grainedcomposition and includes hematite and magnetite.Near Kosagaly H i l l it contains up to 13 per cent manganese. The presence of volcanics and jaspers in the ore-bearingsiliceous horizon is evidence for the volcanic sedimentary origin of the formation. At the contact zonesoflatePalaeozoicgranitoid intrusives, the ore-bearingrock mass appears to be of hornfels

texture.Some distance from the intrusives,along the crush zones and zones of ore jointing,the enclosing rocks are also metasomatically altered.In addition to contact metamorphism, another ore recrystallizationtook place as the result of which jasper quartzites,volcanites and cherts have been locally changed to new metasomatites of epidoteactinolite-chlorite,sometimes with small amounts of pyroxene and garnet, as well as massive magnetite ores with relicts of jasper quartzites,heavily modified volcanics and thin laminated magnetite-hematite ores. Metasomatic processes contributed to the formation of magnetite deposits in beds fiveto ten times thicker than those of the primary sedimentary magnetite-hematite ores. Ferruginous-cherty-carbonate-volcanicformations of the Fammenian stage are widely spread in superimposed troughs within the limits of the Uspenk synclinorium and Chinghiz anticlinorium. By analogy with the Atasuy ferruginous-manganesedeposits,theseformationsareregarded as having volcanic sedimentary origin.In the same synclinal fold, apart from the Atasuy deposits, there are volcanic sedimentary iron-ore deposits with a high intensity of metamorphism, mainly that of contact and contact-metasomatic character, and with a considerable differentiation of ore. Some researchers consider the magnetite ores and associated skarns of the Kentyube deposit to be the result of the metasomatic reworking of volcanic sedimentary ferruginous rocks of the Fammenian stage. 251

V. M.Shtsherbak,A.S. Kryukov and 2.T.Tilepov

The iron-oredeposits at Tortkul and Kirghizia in the East Karagaly ore region also serve as examples of formations of intensive metamorphic differentiation following local metasomatic phenomena. In the formation of the Tortkul deposit (Fig. 3) the sandstone,aleurolite,aleuropelite,limestone,andesite porphyrite and tuffs of the Fammenian participate. In the

middle part of the rock mass jasper and carbonate-siliceous sinters with magnetite-hematitemineralization occur. The Fammenian formations are conformably overlain by conglomerates and arkose sandstone. All these deposits form the brachy-syncline with submeridional shearing induced by the intrusion of Upper Palaeozoic gabbro-dioritesand granodioritesin the north-

\

FIG.3. Geological structural map of the Tortkul deposit. 1. recent deposits;2.tufogene conglomerates,arkose sandstone, limestone; 3. tufite, cherty aleurolite, tufogene sandstone, limestone; 4. sandstone, aleurolite, aleuropelite, limestone; 252

5. porphyrite and tuffs; 6.diorite porphyrite;7.granodiorite; 8. diorite;9.gabbro-diorite;10.skarn; 11. magnetite-martite ore; 12.hematite-magnetiteore.

O n the issue of genesis and metamorphism of ferromanganeseformations in Kazakhstan

west. In the south-eastern part of the brachy-syncline,the carbonate-siliceoussinters and jaspers over a distance of 1.4 k m include beds and lenses of thin-bandedmagnetitehematite ores, 50-400 m long and 5-30 m thick (Tortkul region I, Fig. 3). The ores have a rhythmic lamination structure.Fine laminae of hematite and magnetite-hematite (0.5-3 .O mm) with carbonate-chertsaleurite alternate with finerlaminae of carbonate-siliceoussinters and aleurolites. In the richest ores of this type the average content of iron is 50percent,sulphur 1.2percentand phosphorus0.02percent. As can be judged from the morphology of the deposit, the composition and structure of ores as well as their association with jaspers and volcanites, the finely laminated formations are of volcanic sedimentary origin. In the northern and southern parts of the brachysyncline there are also stratiform ore deposits associated with volcanic carbonate-chert formations (ore regions Tortkul II and III,Fig. 3), but there are also ore bodies of different types such as intersecting veins, stockworks and others. Magnetite ores are mainly associated with pyroxene-garnetskarns and albitized rocks,which appeared as an aftermath of the metasomatic phenomena in the zone of contact with the Upper Palaeozoic intrusion. In the skarn-magnetitelaminated deposits relicts of

thin-banded hematite-magnetite ores often occur. Sometimes breccias are observed,with fragments of thin-banded hematite-magnetiteores and cementskarns or metasomatic magnetite. All the above proves that magnetite ores of Tortkul II and III regions, related to the horizon of the carbonateferruginouschert formations,are the regenerated analogues of the primary sedimentary (volcanic sedimentary) magnetite-hematite ores of the Tortkul I region. In the orebearing rock mass, near the contact of the intrusion,hornfels has been formed and recrystallizationof the ores has takenplace.Then the metasomaticmetamorphism produced albitization and skarning of the intrusions and the orebearing rocks, as well as further recrystallization and redeposition of ores. The above examples, together with data from other provinces and deposits,show that the formation ofvolcanicsedimentary,ferruginousand ferruginous-manganeserocks occurred in Kazakhstan during the Precambrian and early to middle Palaeozoic. The compact deposits of rich and easily dressed magnetite ores (Kosagaly, Tuyak, Tortkul, Kentyube deposits and others), were formed at the expense of poor sedimentary,mainly hematite, concentrations by metamorphic differentiation and local metasomatism.

Résumé Formation et métamorphisme des roches ferrugineuses de rlivesses époques dans les psovinces du Kazakhstan

(V. M. Shtsherbak, A . S. Kryukov, Z.T. Tilepov) L'anticlinal de Chu, la région de minerais de TuyakKosagaly et le synclinorium d'Uspensk permettent de mettre en lumière certaines particularités de la formation des roches ferrugineuses et des métamorphismes de différentes époques dans le Kazakhstan. 1. Dans l'anticlinal de Chu, les formations précambriennes de silex ferrugineux volcanique et de jaspilite ont distinctement subi divers stades de métamorphisme. L a formation du premier type se rencontre dans la partie nordouest de l'anticlinal. Elle est représentée par des porphyroïdes, des quartzites ferrugineux et des ardoises. A cette formation est rattaché le vaste gisement de minerai de magnétite-hématitede Zhuantobe.Ici les volcanites s'intercalent avec des quartzites et des minerais de fer qui mettent en évidence une origine sédimentaire volcanique pour ce dernier. Si l'on en juge par une chloritisatioiiassez intense, par la présence de minerais de stilpnomélane,par la recristallisation partielle (jusqu'à 20 %)d'hématite à grains fins et sa transition en magnétite, ainsi que par la préservation de la microlamination des minerais de quartz-hématite,le métamorphisme régional des roches ferrugineuses de cette section de l'anticlinorium atteint le faciès des schistesverts. La formation des silex ferrugineux domine dans les parties centrales et sud-est de l'anticlinorium. Les ardoises mi-

grantes, les quartzites ferrugineux et les marbres dolimitisés qui encaissent toutes les manifestations de minerai de Temir peuvent être considérés comme appartenant à des faciès d'amphibole en raison de la présence de quartzchlorite-épidote,de quartz-biotite-épidote, de biotite-chlorite-quartzet d'ardoises amphiboles et de minerais à grains moyens et ñns de quartz hématite-magnétite.Cette formation est différente de la formation du premier type par l'intensité du métamorphisme, l'absence de volcanites et sa moindre teneur en fer. Ces quartzites ferrugineux semblent avoir leur origine dans un volcanisme ancien. 2.Le groupe de gisements de quartz-hématite-magnétite de Tuyak-Kosagalya ses limites dans la partie centrale des séries de Kosagaly (O&, constituée de grès polimicte, de quartzites,de quartzites à jaspe,de silex et de volcanites modifiées. L a présence de volcanites et de jaspes dans les roches à minerai, conjointement avec des minerais de quartz-hématite-magnétiteà grains fins, est un autre argument en faveur de l'origine volcanique sédimentaire des dépôts de Tuyak-Kosagaly(formation de silex ferrugineux volcaniques). La majorité des roches à minerai est purement schisteuse. Au niveau des quartzites,les structures de boudinage se distinguent tandis qu'on trouve des grès, volcanites et silex qui ont donné naissance à un développement intense de chlorite, actinolite, épidote, séricite, calcite, etc. L a composition pélitique des silex et des grès indique une recristallisation d'ensemble.Dans les minerais de quartz-hématite-magnétite,on a mis en évidence une 253

V. M.Shtsherbak,A.S. Kryukov and 2.T.Tilepov

recristallisationfaible et une augmentationde la dimension des grains de magnétite d‘un ordre de grandeur. Tout cela indique un degré peu avancé de métamorphisme régional dans la masse rocheuse à minerai jusqu’aufaciès de schiste vert et peut-être au premier stade du faciès d‘amphibole. Dans ce processus, aucune différenciation importante des éléments qui constituent le minerai ne s’est produite. Aux zones de contact des intrusions granitoïdes du Paléozoïque récent,la roche à minerai présente une texture de hornfels tandis qu’à quelque distance des intrusions, le long des zones d’écrasement et de celles de jointement des minerais, la roche encaissante change aussi de façon métasomatique. Parallèlementavec le métamorphismede contact,une autre recristallisationdu minerai s’estproduite, et, comme résultat de phénomènes métasomatiques qui se sont produits localement aux endroits où se trouvaient des quartzites à jaspe,des volcaniteset des silex,de nouvelles métasomatites d’épidote-actinolite-chlorite sont apparues quelquefois avec de petites quantités de grenat, ainsi que des minerais de magnétite massifs avec des restes de quartzite à jaspe, des volcanites profondément modifiées et des minerais de magnétite-hématiteen fineslamelles. Le fer,entre-temps,s’est redéposé, provenant des parties basses des couches plongeantes. Les processus métasomatiques ont contribué à la formation de dépôts de magnétite d’épaisseur 5 à 10 fois supérieureà celle des lits des minerais de magnétite-hématite sédimentaires primaires. 3. Les formations ferrugineuses(silex, carbonate) volcaniques de l’étage fammenien sont largement répandues dans les dépressions superposées dans les limites du syncli-

254

norium d‘Uspensk et de l’anticlinoriumde Chinghiz. L‘origine sédimentairevolcanique de ces formationsa été démontrée par de nombreux chercheurs. Sauf pour des dépôts d’Atasuy de la dépression de Zhailmyn dont le métamorphisme des roches a à peine atteint le stade du schiste vert, on considère que ces structurescontiennent des formations de carbonate, du fer siliceux, sédimentaire et volcanique, avec un haut degré de métamorphisme,essentiellement de contact, avec une certaine différenciation métamorphique. Les minerais de magnétite et les skarns du dépôt de Kenytuybe qui les accompagnent résultent d‘une reprise métasomatique des roches ferrugineuses sédimentaires volcaniques de l’étage fammenien. Les gisements Tortkul et Kirghibia peuvent aussi servir d’exemples de formations après une différenciation métamorphique intensive faisant suite à des phénomènes métasomatiqueslocaux. Au détriment des dépôts d’hématite-magnétiteen couches minces, les minerais de magnétite apparaissent ici en association avec les skarns. 4.Les exemples mentionnés ci-dessus indiquent que dans les limites du Précambrien du Kazakhstan,le Paléozoïque ancien et moyen a été caractériséà certainesépoques par des formations de roches ferrugineuses sédimentaires surtout d‘origine volcanique. Ces dernières ont eu à subir différents stades de métamorphisme. L a différenciation métasomatique des matières qui ont constitué le minerai a été surtout intense au cours de phénomènes métasomatiques locaux d‘où ont résulté des dépôts compactsde minerais de magnétite ayant une valeur commerciale (dépôts de Kosagaly, Tuyak, Kenytuybe, Tortkul et autres).

Genesis of high-grade secondary iron and manganese ores from ironsilicate and ferruginous formations and ores, metasomatic processes and processes of oxidation in them

Genese des minerais de fer et de manganese secondaires à haute teneur,à partir des formations de minerais de fer et de silicate de fer; processus métasomatiques et processus d'oxydation qui s'y rattachent

Iron-formationsof the Hamersley Group of Western Australia: type examples of varved Precambrian evaporites A. F.Trendall Geological Survey of Western Australia,Perth

.

Introduction

Hamersley Group iron-formations

The iron-formationsof the Precambrian Hamersley Group of Western Australia crop out over an area of about 85,000 km2 bounded by latitudes 21" and 23" S. and longitudes 116" and 122"E.Comparatively little was known of them until 1961,when systematicmapping ofthe Hamersley Range area by the Geological Survey of Western Australia began, under the supervision of W.N.MacLeod, concurrently with intensive private company exploration for iron ore. The main earlier geological records are those of Maitland (1909),Miles (1942),and Talbot (1920), who described the rocks collected by Talbot and others. These authors referred to the iron-formationeither as part of the 'Nullagine Series', or as of 'Nullagine age'; the term 'Nullagine' has now been replaced by the stratigraphic nomenclature given in this paper. MacLeod et ul. (1963) reported the results of the systematic mapping, and MacLeod (1966) later provided a comprehensive account of the geology of the area, with special emphasis on the economic iron deposits. Between 1964 and 1967 the Geological Survey carried out a study of the crocidolite deposits associated with the Hamersley Group iron-formations,involving attention to the general depositional environment of the iron-formationsand their subsequent diagenetic and deformational history. Various aspects of the results of this study were reported concurrently by Ryan and Blockley (1965), Trendall (1965a, 19656, 1966u, 19666, 1966c, 1968, 19691, Blockley (1967, 1969) and Trendall and Blockley (1968); the complete resultswere finallycompiledhyTrendal1and Blockley (i 969). Trendall and Blockley (1969) suggested that the Hamersley Group iron-formations were laid down as seasonally varved evaporitic chemical precipitates in a barred basin with a warm desert climate. M y purposes here are to summarize the evidence and arguments for this suggestion in a single short paper, to draw comparison between the Hamersley Group iron-formations,other varved evaporites and other iron-formations,and to examine the geological consequences of these comparisons.

R E G I O N A L SETTING

The Hamersley Group is one of three constituent groups of the Mount Bruce Supergroup;conformably below it lies the Fortescue Group, and above it, with some local discontinuity,the Wyloo Group. The present outcrop area of each of these three groups appears in Figure 1. They were laid down sequentially in an ovoid depositional basin (the Hamersley Basin) about 500km long and 250 km wide, with a west-north-westerly elongation, which formed by the steady depression of an evenly eroded surface of Archaean granites,metasediments and metavolcanic rocks. These older rocks of the basin floor have now been reexposed over a wide area around the present main outcrop area of the Mount Bruce Supergroup, and also within it in local inliers in anticlinal cores (Fig. 1). The Fortescue Group has a maximum thickness of 4,350 m, and consists largely of basic lava, pyroclastic rocks, sandstone and shale. The Hamersley Group is about 2,500 m thick and is characterized by an abundance of iron-formation;details of its stratigraphy appear below. The Wyloo Group consists of mixed clastic sediments with thick local developments of dolomite and basalt; it reaches a thickness of 9,500 m.

STRATIGRAPHY

The lithostratigraphicsubdivisionsof theHamersleyGroup, which have been formally named in accordance with the Australian Code of StratigraphicNomenclature (Geological Society of Australia, 1964), are shown in Figure 2. As emphasized by Trendall and Blockley (1969), these units are named principally as a convenience for field mapping. In practice the formal selection and naming of formations will depend upon accidents of physiographic development, the judgement of the mapping geologist,and the purpose and scale of the mapping;in consequencethe named units

Unesco, 1973. Genesis of Precanibrian iron and manganese deposils. Proc. Kiev Syi?7p., 1970. (Earth sciences, 9.)

257

A.F.Trendall

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LOCALITY MAP

PHANEROZOIC ROCKS

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UNCONFORMITY ARCHAEAN ROCKS

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MAJOR FOLD AXES: -r -SYNCLINAL

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STRIKEAND 'DIPOF BEDDING

FIG.1. Map showing the outcrop areas of the FortescueGroup, Hamersley Group and Wyloo Group,and their regional geological setting. 258

Iron-formations of the Hamersley Group of Western Australia: type examples of varved Precambrian evaporites

B F L G E E D A IRON FORMATION

v v V

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516

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WOONGARRA VOLCANICS

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IRON FORMATION

FIG.2. Stratigraphic column of the Hamersley Group, and internal details of the Dales Gorge Member of the Brockman Iron Formation. 259

A.F.Trendall

may not reflect the real sedimentological significanceof the rocks to which they are applied. This situation has arisen in the Hamersley Group, and from the upper part of the Fortescue Group to the base of the Woongarra Volcanics the most significant largescale regularity in the sequence of major sediment types seems to be an alternation between shale, or shale and dolomite,and iron-formation,with or without subordinate shale. The rearrangement of the formal units given in Table 1 illustrates this alternation: TABLE 1. No.

8. Weeli Wolli Formation 7. Yandicoogina Shale Member of Brockman Iron Formation 6. Joffre Member of Brockman Iron Formation 5. Whaleback Shale Member of Brockman Iron Formation 4.Dales Gorge Member of Brockman Iron Formation + top of Mt McRae Shale 3. Wittenoom Dolomite + Mt Sylvia Formation + lower part of Mt McRae Shale 2. Marra Mamba iron Formation 1. Jeerinah Formation (shale) of Fortescne Group

Approximate thickness in metres

STRUCTURE

In the northern part of the Hamersley Range area the Fortescue Group rests directly on Archaean rocks and dips off them southwards at only a few degrees. The rocks are perfectly undisturbed. Along the Hamersley Range itself the overlying Hamersley Group also has low dips, normally less than lo",and the range is formed by a broad open synclinorium.Southwardsfrom the Hamersley Range the intensity of folding steadily increases. Thus, in the central part of the area, dips of 30-40" are common, and the amplitude and wave-length of folding are sufficiently great for the complete succession from Archaean to Wyloo Group to be re-exposed. In the southernmost part of the Hamersley Group outcrop, the folds are smaller and tighter,and locally overturned beds occur.

185l

60 315

60 185

185 150-300

METAMORPHISM

The Hamersley Group has nowhere undergone metamorphism. Trendall (1966~) argued from the evidence of Hoering that no part of the Brockman Iron Formation had reached a temperature above 160°C. Since then Grubb (1967) has successfully synthesized fibrous riebeckite,which had previously been regarded as a metamorphic mineral,at 35"C. Oxygen isotopeanalysis (Becker and Clayton, personal communication) also suggests that the Hamersley Group iron-formationshave never reached higher temperatures than those of a normal geothermal gradient.

1. Figure 2 shows c. 450 m. 2. Figure 2: 150 m.

SCALES O F B A N D I N G

Each of the numbered iron-formationunits in this sequence (2, 4,6,8) is lithologically distinct from the other three, as well as from the iron-formationof the Boolgeeda Iron Formation. These differences are expressed in the field by such secondary features as topographic response to erosion and colour of the weathered rock-face, which depend on subtle primary differences in composition, sequence and regularity of the mesobands (see 'Scales of Banding', below) and in minor but consistent differences in mineralogy.

AGE

Acid lavas of the Woongarra Volcanics have been dated by the Rb-Sr method to give an isochron of 2,100 m.y. (Leggo et al.,1965), later modified to 2,000m.y.(Compston and Arriens, 1968). An age of 2,200m.y. is given by the Fortescue Group, from unsatisfactory material, and a minimum age for the Wyloo Group of 1,700 m.y. is given from intrusive granite. N o chronostratigraphic allocation of the Hamersley Group is here suggested. 260

All the iron-formationsof the Hamersley Group, except the Boolgeeda Iron Formation, are banded. For various reasons most work in the Hamersley Range area has been carried out on the Dales Gorge Member of the Brockman Iron Formation, and the follawing description of the different scales of banding in that member serves as a basis for later comparison with the banding in other ironformations of the Hamersley Group. In the Dales Gorge Member there are three distinct scales of banding (Trendall,1965b): macrobanding,mesobanding and microbanding. Macrobanding (Fig. 2)is the name given to the major alternation between the two contrasted lithologies of the member-banded iron-formation, and mixed shale and chert-siderite iron-formation. Seventeen macrobands of banded iron-formation, numbered upwards from BIFO to BIF16 and ranging in thickness in the type section (Trendall and Blockley, 1968) from. 2.29 m (BIF8) to 15.05 m (BIFló), alternate with sixteen macrobands of the mixed lithology,numbered upwards from S1 to S16 and ranging in type-section thickness from 0.62 m (S2) to 5.47 m (S16), to give a total of thirty-three iiumbered macrobands,

Iron-formationsof the Hamersley Group of Western Australia:type examples of varved Precambrian evaporites

Mesobanding (Fig. 3) is the name applied to the banding of the scale which is usually referred to in general use of the term ‘banded’iron-formation.Within the BIF macrobands of the Dales Gorge Member, there is a conspicuously striped succession of internally consistent bands of different composition of average thickness 7.9mm (860 measurements), and a range of 1 mm (by definition) to 66 mm.The major mesoband types in the Dales Gorge Member are chert (about 56 per cent of total thickness), chert-matrix (about 21 per cent) and magnetite (about 13 per cent). The mesobands forming the remaining 10 per cent of the thickness of the member consist mainly of stilpnomelane,carbonates,riebeckite,and minor miscellaneous types. Chert-matrixis the name given to the finegrained, iron-rich,homogeneous, structureless or finely laminated material,composed mainly of a variable mixture of quartz, carbonates (ankerite or siderite), stilpnomelane, hematite and magnetite, with which the chert mesobands alternate. Most chert mesobands are internally microbanded. Microbanding (Fig. 3) is an alternation of regularly repetitive laminae of even thickness.The microbands are defined by a varying content of some iron mineral,most commonly by either hematite, ankerite,siderite or stilpnomelane, or by some combination of these. The usual thickness of

A FIG.3. Mesobands and microbands in drillcore of banded ironformation of the Dales Gorge Member. The darker mesobands are of chert-matrix or magnetite, and the paler mesobands are of chert, within which the fine microbands are clearly

microbands (the combined thickness of one iron-poor and one iron-richlamina) is within the range of0.2-2.0m m . They are clearly visible to the naked eye as a light and dark colour alternation in fresh samples of the iron-formation. Of the five main stratigraphic units of iron-formation already referred to (2,4,6, 8 and the Boolgeeda Iron Formation) only the Dales Gorge Member (4)has clear macrobanding, although both the Joffre Member (6) and the Marra M a m b a Iron Formation (2) have thin iiitercalated stilpnomelane-rich shales.The Boolgeeda Iron Formation is exceptional in having only a faint lamination in otherwise massive black iron-formation, while the Weeli Wolli Formation (8) differs from the others in the presence of great thicknesses of thinly microbanded ironformation with no mesoband development. The Joffre Member (6), the Dales Gorge Member (4),and the Marra M a m b a Iron Formation (2) all exhibit mesobanding and microbanding, but there are distinctive differences in the proportions of the mesoband types.

LATERAL STRATIGRAPHIC CONTINUITY

With the conspicuous exception of one small area in the south-west, and to a lesser extent in the extreme east,

B visible. Note that the left-handand right-handparts of B are photographs of the same stratigraphic level from two drillholes separated by a distance of kilometres.

261

A.F.Trendall

the constituent formations of the Hamersley Group persist throughout the outcrop area. Local erosion of the upper formations of the group in the northern and eastern parts of the outcrop means that the lateral continuity of the basal Marra M a m b a IronFormationis capableofmore convincing demonstration than that of the uppermost Boolgeeda Iron Formation,whose outcrop area is now relatively small. The most accurate information available on regional thickness variation of any stratigraphic unit is that for the Dales Gorge Member. With the one exception already mentioned, the thirty-threenumbered macrobands of this member can be identified and measured throughout its outcrop area, providing not only thickness data for the member but also internal confirmation that a true depositional thickness unaffected by penecontemporaneous erosion is obtained. Trendall and Blockley (1969), report an extreme range in the thirty-onesections measured from 87 to 186 m,with a rate of change over the outcrop area of about 0.5 per cent per k m .Although in detail the pattern is complex, there is a general thinning of the member away from the central part of the outcrop area,where the thickest sections were recorded. In the Wittenoom Gorge area (22"52'S.; 117"08'E.), excellent core is available from the Dales Gorge Member to study correlation between a series of drillholes for crocidolite (blue asbestos) exploration. Mesoband correlation was quickly established between the most widely spaced drillholes (about 10 km) of this group, and was later extended,using both further drillholes (about 80 km) and surface exposures,over most of the available outcrop area, in some of the lower macrobands of the member selected for detailed study (Fig. 3; see also Trendall and Blockley, 1969). Within some chert mesobands of the Dales Gorge Member selected for detailed study lateral microband correlation equal to that of the mesobands has also been established (Trendall and Blockley, 1968). A difficulty facing any microband correlation is the usual regularity of the microband sequence: some degree of irregularity is needed before correlation can be convincingly argued. The greatest lateral distance over which microband correlation has so far been achieved is 296 km,but since continuity has always been found with sufficiently detailed search, it is provisionally accepted that microbanding, like mesobanding, is continuous over the whole Hamersley Group outcrop, not only within the Dales Gorge Member, but equally in all other iron-formationunits of the Hamersley Group in which microbanding occurs. In summary, lateral stratigraphic continuity in the Hamersley Group is almost outcrop-wide,not only at the formation and member level, but also at the scale of macrobands,mesobands and microbands.

INTERPRETATION O F M I C R O B A N D I N G

Some chert mesobands contain several hundred microbands which are believed to be continuous over the outcrop area 262

of the Hamersky Group. The three main features of microbanding so displayed (even spacing, constant repetition, wide lateral extent) are the points which both demand explanation and provide the main arguments for origin. The first question to be answered is whether microbands are primary sedimentary features or secondary-possibly some form of Liesegang banding which developed during diagenesis. This latter possibility is difficult to disprove, but Trendall and Blockley (1969)have summarized the relevant arguments and,for the purposes of present discussion,their conclusion that microbands are probably primary is here accepted. If they are primary, then the second question arises, of how such thin layers,successively siliceous and ferruginous, could have been laid down successively throughout such a vast area. It is virtually impossible to imagine any form of mechanical transport of material that could have achieved this, and the only reasonable conclusion is that the material is chemically precipitated; this is, of course, entirely consistent with both the chemical composition and the evident absence of normal clastic textures. If the microbanded chert results from chemical precipitation, a third main question follows: what events controlled the continuous regularity of the microbanding? The alternate precipitation of thin and even laminae of such disparate composition in regular pulses suggests some alternation of the basin chemistry with a regularity only matched in the natural surface environment by the two astronomicalrhythms controlledrespectively by the earth's rotation and revolution-the day and the year. In deciding which of these is more likely to have controlled microbanding, two lines of argument may be used. The mean microband thickness in 300 chert mesobands of the Dales Gorge Member was 0,65 mm (Trendall and Blockley, 1969). If it is assumed that the number of days in the year during deposition was 365 then an annual accumulation of 237 mm of material is indicated. In fact, it is likely that the rotation rate at that time was much higher (Wells,1963), so that this figure is a minimum. Nevertheless, it is unacceptably fast as a likely sedimentation rate for any depositional basin, and in terms of water circulationwould raise problems of even distribution and supply of material. The second argument is a consideration whether,by modern comparison,the alternation of day and night represent changes of sufficient physical intensity to radically affect the depositional chemistry of a body of water at least 85,000 kmzin extent. It seems unlikely, and both arguments lead to the conclusion that the year is a more likely primary control for the chemical precipitation of microbands: they are essentially nonglacial seasonal varves. If microbands are varves, it may be calculated from chemicalanalysis of microbanded cherts that about 22.5m g of iron were precipitated annually per square centimetre of the basin area. The fact that such estimates are independent of the microband thickness of the analysed cherts is consistent with the interpretation here proposed,

Iron-formationsof the Hamersley Group of Western Australia: type examples of varved Precambrian evaporites

origin be assumed for it, or indeed for any chert-matrix?

INTERPRETATION O F MESOBANDING

If it is assumed that the chert-matrix at Z is compacted If the microbands within a microbanded chert mesoband form a regular record of the passing years from the lower to the upper surface,then what events do these surfaces themselvesrepresent,at the transitionsfrom and into chertmatrix or magnetite? This question is answered here by reference to a type of laterally discontinuous or podded chert, which occurs in most of the Hamersley Group ironformations. In the description of mesobanding above, it was stated that mesobands have, relative to their thickness, extreme lateral continuity.This is not always so. At some levels strings of flat chert lenticles (often connected when their full areal extent is observable) appear to represent mesobands with local small-scalediscontinuity. Such pods are as commonly microbanded internally as are normal chert mesobands,and the behaviour of microbands at the lateral termination of pods is illustrated diagrammatically in Figure 4.The microbands are not sharply truncated at the chert pod margins, but pass smoothly into the adjacent chert-matrix,where it is normally represented by a pervasive lamination lacking the distinctive twofold compositional disparity of the chert microbands.

----------------------

--- zz-

A

chert, the history to be read from a regularly interbanded , sequence of chert-matrix and chert mesobands becomes apparent. The preserved microbands of a typical laterally continuous chert mesoband are read as a sequence of recorded annual depositional events. The transition to chert-matrix at its upper surface is now interpreted as the start of a period when similarly microbanded sediments were laid down which differed in their response to compaction.This hypothesis,originally argued for the Dales Gorge Member by Trendall later found strong supporting evidence in a type of continuously microbanded iron-formationrestricted to the Weeli W i l l 0 Formation. The nature of the control of compaction in the parent material has not so far been determined. It is assumed, however, that some very minor chemical or physical property critically controlled reaction to the pressure of continuing sedimentation, and that this property was controlled by rather regular changes in the environment of the depositional basin, to give the regular alternation of chert and chert-matrix mesobands (Fig. 3).

CYCLICITY O F M E S O B A N D SEQUENCE

-

FIG.4.Diagram showing the structuralbehaviour of microbands at the lateral termination of a chert pod.

It is deduced from the relationships in Figure 4 that the thickness of chert-matrix, t2, at Y,represents the compacted remains of chert similar to that of thickness t, which remains uncompacted at X.A closely similar situation has been similarly interpreted by Bryant and Koch (1969). The ratio t, / tz in Dales Gorge Member chert mesobands is commonly close to 7 :1. It is found that the total amount of iron present in the lesser thickness of chert-matrixapproximates to that in the equivalent greater thickness of chert. Trendall and Blockley (1 969) have argued in some detail the case for regarding chert-matrix as compacted chert or, alternatively,that chert and chertmatrix are the less strongly and more strongly compacted products of some unknown common parent. The next step of the argument follows naturally. The chert-matrix at Y is petrographically identical with that immediately above and below, in which lamination is not demonstrably continuous with microbanding in laterally adjacent chert. W h y , therefore;should a different

In the Dales Gorge Member, the Joffre Member, and in the Weeli Wolli Formation, there is a small-scale(about 1020 cm) cyclicity of mesoband type. In general, groups of coarsely microbanded cherts alternate with groups of finely microbanded cherts, alternating with chert-matrix and magnetite in the normal way, but there are other characteristics of the cherts which help to differentiate the two cherts and to define the cycles. These are seen by Trendall and Blockley (1969) as the reflection of long-term cyclic environmental changes, and their expression in the Dales Gorge Member and Joffre Member have been called by them the Calamina cyclothem and K n o w cyclothem respectively.

The Hamersley Basin and its environment INTRODUCTION

If the foregoing arguments are accepted, the deposition of the Hamersley Group iron-formationsmust be envisaged as involving the annual deposition of a thin basin-wide skin of precipitate with a clock-likeregularity for up to a million virtually undisturbed years continuously. The impression is one of exceptional stability. With such a close relationship of precipitation to the year, it is difficult to escape the conclusion that the triggering mechanism of theprecipitation lies in direct seasonal effects on the basinal environment. The simplest such mechanism to envisage 263

A.F.Trendall

appears to be an annual increase of the silica and iron concentrations above their permissible solubilities by evaporation. Trendall and Blockley have built up a detailed reconstruction of the depositional basin of the iron-formations based on this central suggestion,and have shown that it is consistent with many other features of these rocks. Their arguments are not repeated in detail in this summary oftheir reconstruction,but the main types of evidence used are indicated for each aspect of the reconstruction.

INITIATION O F T H E BASIN

The name Hamersley Basin is applied to the basin in which the Fortescue, Hamersley and Wyloo groups were successively deposited. The initial event in the life of the basin was the accumulationofthe thick and largely volcanic Fortescue Group. The sedimentary rocks interstratified with the lavas of this group bear abundant evidence of shallow-waterdeposition,while the lavasthemselves locally have pillow structure.During this period the whole surface of the accumulating material was probably almost flat, with local shallow lakes. N e restricted centres of this vulcanicity are apparent, and it may be assumed that the abundant dolerite dykes that transect the presently exposed areas of Archaean rocks represent the fissures along which the basalt sheets were comparatively quietly extruded.

SIZE, SHAPE A N D S T R U C T U R E O F T H E BASIN

The basin, during deposition of the Hamersley Group, is believed to have been ovoid in shape,with a long axis of about 500 km trending west-north-west and a shorter axis of about 250 km,to give an area of about 100,000 k m 2 . It is believed to have been enclosed on all but the northwestern side,where there was at least a partial connexion with the open ocean. The main argument for the shape of the basin is the isopach pattern of the Dales Gorge Member, which appears to be roughly followed by most other stratigraphic units of the basin. The limits of size are provided by an outward extrapolation of the isopachs, and by the stratigraphic impression of proximity to a basin margin along the eastern edge of the outcrop. It appears that the basin formed a rough ovoid enclosing on, and never extending far outside, the present outcrop. Closure of the basin is consistent with the isopach pattern,and is required by the suggestion that precipitation was triggered by evaporative concentration. A north-western oceanic connexion is argued partly by a local thickening of the Dales Gorge Member at that extremity of outcrop and partly by doubt concerning the possibility of maintaining a sufficiently delicate balance between evaporation and water intake (from where?-see below) to maintain a stable water level in the basin without benefit of control by an oceanic connexion. 264

D E P T H , CIRCULATION A N D IRON C O N T E N T O F W A T E R

From the perfect preservation of annual microbanding in what was probably a delicate gelatinous precipitate, it is argued that there was negligible bottom current, and no disturbance of the bottom by waves or storms. By modern comparison a minimum depth of 50-200 m is indicated (Kuenen, 1950). The validity of such a direct comparison is open to much discussion,however. Nothing definite is known of the likely turbulence of the atmosphere at that time, for example, and the possible presence of an algal raft (see below) would affect the validity of the argument. A second argument for water depth runs as follows: There is negligible lateral stratigraphic or lithological variation throughout the present outcrop. Therefore all parts probably had a similar environment and were laid down in water of the same order of depth. But certain slump structures (omitted from descriptions above) indicate at least some bottom slope towards the centre of the basin, as would be expected if this were the area of most rapid depression. Even if this were as little as 1 :1,000there would be a depth differential of 100 m between centre and edges. Therefore, some figure above this is a minimum depth estimate for iron-formationdeposition in the Hamersley Basin. A depth estimate of about 200 m is accepted for the sake of continuing discussions. A concentration of 10-20 ppm of iron seems reasonable for the water of the basin; this is vitally dependent on the atmosphereof the time,a complex issue which is not discussed here in detail. With this concentration range, each square centimetre column of basin water would contain 200-400 m g of iron. The annual deposition of 22.5 g of this represents only 5-10 per cent of the total available iron. If circulation were confined to the upper 100 m of the basin, iron concentrations of less than 20 ppm in the incoming water would be adequate to replace the precipitated iron,and would require circulation speeds of less than 1 m/sec, which is consistent with the evident lack of disturbance of the precipitated microbands after deposition. CONDITIONS O N T H E S U R R O U N D I N G L A N D

During the long stable periods of Hamersley Group ironformation deposition virtually no clastic debris was transported into the basin. Two contrasted explanations are conventionally available to explain this; either there were no rivers because the surrounding area was desert,or the existing rivers were so sluggish as to be incapable of carrying a significant undissolved load. The first of these hypotheses is preferred for three main reasons.First,the second hypothesis,although widely suggested in palaeogeographic interpretation,is inconsistent with the fact that all existing rivers suflicientlylarge and

Iron-formationsof the Hamersley Group of Western Australia:type examples of varved Precambrian evaporites

mature to have low velocities have deltas built from transported debris. And secondly and thirdly the desert hypothesis seems to fit both with the high evaporation rate,independently suggested as a trigger for precipitation, and with the apparent regularity of annual climatic conditions suggested by the regularity of microbanding. The maintenance of a desert climate through a least a million years suggests that the surrounding topography lacked mountainous areas tending to stimulate at least occasional storms, flash floods and consequent clastic intercalations in the iron-formations.

PALAEOLATITUDE

No good palaeomagnetic estimate of latitude is yet available for the Hamersley Group. Work so far (Irving and Green, 1958) is, frankly, preliminary, and based on too few data. If the arguments for desert climate, intensive evaporation and marked seasonal variation (to define the microbands) are accepted then alow latitudeoffthe equator, perhaps at one of the tropics,is indicated.

covered structuredskeletal carbonaceousbodies from chert of the Dales Gorge Member. Trendall and Blockley (1969) later found the same bodies in Marra Mamba Iron Formation chert. Algal stromatolites occur in the Carawine Dolomite (correlative with the Wittenoom Dolmite,Fig.2) in the extremeeastern part of the outcrop,and stromatolites occur in both the Fortescue and Wyloo Groups. There is thus abundant indication of life in the basin, though the nature and extent of this is uncertain.Trendall and Blockley (1969) have suggested the possible presence during iron-formationdeposition of a floating algal raft. The hypothesis has a number of advantages,for example: (a) by acting as a quantitative biochemical buffer between some simpleclimaticfactor,possibly annualinsolation,and the basin chemistry an algal layer tolerant of the suggested range of iron concentrationsmay provide an equal annual precipitation of iron regardless of a relatively spasmodic supply; (b) the interposition of a biochemical process may overcome the difficulty of finding any climaticfactor which is likely by itself to affect the basinal chemistry sufficiently to provide the chemical contrast between the parts of the microbands;(c) an algal raft may be a contributory cause of the apparent absence of any influence by waves or weather on the light and incoherent precipitate.

VULCANICITY IN A N D A R O U N D T H E BASIN

It has already been noted that the Hamersley Basin started with intense volcanic activity, resulting in the present Fortescue Group. It can be seen also from Figure 2 that the Woongarra Volcanics, a stratigraphically concordant sequence of acid tuffs and lavas about 450 m thick,separate the Boolgeeda Iron Formation from the remaining ironformations of the Hamersley Group. It is evident from these facts alone that there is a close general association between volcanic activity and the development of the basin. There is further evidence of a closer association in time of volcanic activity and the iron-formations.Within the Mount McRae Shale, the Dales Gorge Member (in the 5 macrobands) and the Joffre Member, stilpnomelane bands 1-5 c m thick have clearly defined relicts of volcanic shards,and are ash-falltuffs (Trendalland Blockley, 1969). Their presence was first noted by LaBerge (1966). Throughout Brockman Iron Formation time, at least, there was periodic explosive vulcanicity in the general vicinity of the basin. The significanceof this association between Hamersley Group iron-formationsand vulcanicity is discussed further in the final section of this paper.

LIFE IN T H E BASIN

Although Edge11 (1964),regarded some microbanded chert pods as silicifiedalgal (stromatolitic)growths,this no longer seems likely. However, many of the Hamersley Group shales are highly carbonaceous, and LaBerge (1967) dis-

SINKING R A T E A N D E N D O F T H E BASIN

With a number of assumptions, discussed at length by Trendall and Blockley (1969),the average rate of sinking of the Hamersley Basin during Fortescue Group and Hamersley Group tirnewasbetween 16,000and8,00Oyears/m. At the end of Hamersley Group time the basin was full in the sense (Dallmus,1958) that the floor of the basin was planar, not geoidal. The Wyloo Group was subsequently laid down in a marginal trough which formed only along the southern edge of the basin.

Are Hamersley Group Iron Formations exceptional? Judged by published descriptions of Precambrian ironformations,those of the Hamersley Group appear to have some features which are uncommon; notable among these are the striking regularity, abundance, and strength of expression of microbanding in many chert mesobands (Fig. 3). Of the 20 per cent of chert mesobands in the Dales Gorge Member which are not clearly microhanded,many have a faint trace of microbanding,and it would be possible to select a sequence of chert mesobands in which the microbanding varied by insensible degrees from its normal development, as in Figure 3, to non-existence. It may be postulated,from this, that all chert mesobands in Hamersley Group iron-formationswere initially microbanded, but that in some it has been obliterated by 265

A. F.Trendall

diagenetic processes,If no chert mesobands in Hamersley Group iron-formations bore microbanding, there would certainly be much less reason to regard them as at all unusual or distinctive.It follows,too,that if microbanding proved to be similarly present in many other iron-formations, not only would their differences from those of the Hamersley Group again lessen,but there would arise at least the possibility that the genetic hypotheses argued here,and in prior publications,are rather widely valid. Inm y own observation,microbanding is not as uncommon in other iron-formations as may appear from the literature. Attention has already been drawn (Trendall, 1968) to the equivalence of Hainersley Group microbands and both the finest laminae of the Transvaal System of South Africa (Cullen,1963), and the 'second order laminae' in iron-formation of the Marquette Range noted by Tyler and Twenhofel (1952). In Figure 5 are illustrated further examples from iron-formationsof different ages in South Africa,North America, India and Australia.The frequency with which examples of lamination at least geometrically comparable with microbanding may be discovered in many iron-formationswhen sought suggests that,for thepurposes of further discussion,the implicationsmay be explored of a supposition that many iron-formationshave an evaporitic origin.

FIG.5. Examples of microbanding in chert mesobands of formations other than those of the Hamersley Group,from different continents and of various ages. The vertical bar with each letter represents 1 m m . A. Archaean iron-forniation,Western Australia (MtCrawford;28"35' S., 122"23'E.;G S W A No. 2/3598). 266

Other varved evaporities in the stratigraphic record The two best known occurrences of varved evaporites in basins whose palaeogeography can be reconstructed as confidently as that of the Hamersley Basin are in the Permian rocks of Germany and of Texas-New Mexico. Varved evaporites have also been reported from,at least, the Devonian of Canada,the Permian of England (Stewart, 1963a), the Miocene of Sicily and the Pleistocene of Israel (Bentor,1968), but none of these has so far been studied closely in relation to basin development, The varves of the German Zechstein basin have been extensively recorded by Richter-Bernburg,most recently in a summary which gives references to earlier work (RichterBernburg, 1963). In a particular anhydrite band, about 2 m thick, there are about 1,200varves 0.5-3 mm thick which are defined by thin bituminous layers inthe anhydrite. Mostly the varves are rather evenly spaced (RichterBernburg,1963), but distinctively spaced sequences permit lateral varve correlation over an east-west distance of 320 km,and of 350 km from north to south, to give a total area of about 100,000km2over which the varves can be correlated. A general account of the Permian basins of West Texas

B.Archaean iron-formation,Canada (Timagaini;about 43"N., 77"W.). C. Transvaal System,SouthAfrica(Derbi,about28"S., 23' E.). D. Archaean iron-formation, Jndia (Noamundi; about 22"N.,85"E.). E. Archaean iron-formation,Canada (Kaministikwa;about 48"N., 89"W.).

Iron-formationsof the Hamersley Group of Western Australia: type examples of varved Precambrian evaporites

and southern N e w Mexico has been given by Hills (1968), who showsthe depositionallimitsofthe Castileevaporitedeposits to be rathercrudely ovoid,with an area of 17,500km2. Within this area, Anderson and his associates (Anderson and Kirkland, 1966;Anderson, personal communication) have established that the millimetre-scalecalcite-anhydrite couplets in the Castile anhydrite,which were first described as varves by Udden (1924),may be correlated with confidence throughout the depositional area. The varves of both these basins thus have their thickness,regularity,and lateral extent all comparablewith the microbands of the Hamersley Basin. In all three respects these evaporitic varves are geometrically distinct from all other forms of sedimentary stratification. ,

TABLE 2. Total original metal of Hamersley Group sediments, and comparison with metal content of average Precambrian crust (all figures in units of 1013 tons). Hamersley Group sediments

1

Si

7.7

Al Fe

0.6

Mg

Ca Na K Total

9.4 1.1 1.1 o.1 0.5 20.5

Precambrian crust for Hamersley G r o u p iron

2

86.2 21.1

Excess of column 2 over column 1

3

78.5 20.5

9.4 2.8

-

6.6 6.2

5.5 6.1

8.1 140.4

7.6 119.9

1.7

Are iron-formations the Precambrian evaporites? Althoughit has been argued that the HamersleyGroup ironformations originated by the annual accumulation of an iron-rich precipitate whose deposition was triggered by evaporation from a partially enclosed basin, the ultimate origin of the precipitated iron has not so far been discussed. Trendall and Blocltley (1969) gave revised statistics for an argument concerning this which was first put forward by Trendall (19654. The argument was designed initially to negate the classical weathering hypothesis for the derivation of the iron in iron-formations, as proposed by Gruner (1922) and later supported by, for example, Sakamoto (1950), James (1951, 1954, 1966), Lepp and Goldich (1964) and Govett (1966). It is based on the problem of disposal of surplus material after chemical ‘processing’(weathering) of enough Precambriancrust of average chemical composition to supply all the iron in the sediments of the Hamersley Group. The relevant data are displayedin Table 2.In column 1 appear the totalweights ofmetals and silicon in the Hamersley Group sediments,estimated from the assumed area of the basin, and the known proportions, average chemical compositions, and densities of the sediment types. In column 2 are shown the total weights of these constituents in the weight of average Precambrian crust needed to supply the controlling 9.4x 1013 tons of iron in column 1. Column 3 shows the difference between columns 1 and 2. The figures show that a weathering hypothesis for the derivation of the iron createsmore problems than it solves. If column 2 is recalculated in terms of total rock volume then 1,000,000km3of crust would have been required to produce the 120,000k1n3of the Hamersley Group.Where did all the iron-freesurplus go? If the weathering hypothesis solves no problems, Trendall and Blockley (1969)argued that some alternative

derivation of the Hamersley Group iron must be sought, and the constant vulcanicity associated in particular with the iron-formationsand in general with the basin makes a volcanic source the most immediately attractive,the more so as the efflux of iron in association with vulcanicity is a demonstrable modern occurrence. However, there is another alternative if the similarity of iron-formationsand later evaporites is pursued further. If the classical barred basin hypothesis for saline evaporite formation is followed, it may be that the comparatively low absolute concentrations of iron in the Hamersley Group depositionalwater suggested in prior discussion could easily have been produced by gentle continuouscirculationof oceanwater across a barrier separating the evaporative concentrate from the less ferruginous external body. It is widely accepted as a statistical truth that banded iron-formations are, if not completely confined to, at least very much more abundant, in the Precambrian rocks (James, 1966 and further references therin) in spite of converse arguments (O’Rourke,1961). It is equally clear that saline evaporites of the type implied by normal usage of the term have a comparable restriction to the Phanerozoic (Kozary,Dunlap and Humphrey, 1968; Stewart,1963b). Could it be that these two, hitherto unrelated, observations could be linked to each other via the varved evaporite hypothesis for Precambrian iron-formationsoutlined in this paper, and that the transition from ferruginous to saline evaporites near the end of Precambriantimes marks an abruptly transitional period of oceanic composition of fundamental significance in the surface geochemical evolution of the earth?

Acknowledgements This paper is published with the permission of the Director, Geological Survey of Western Australia.

267

A.F.Trendall

Résumé Formations de feu du groupe de Hameusley, en Australie occidentale :exemples typiques d'évuporiíes précumbriennes en vurve (A.F.Trendall)

Le groupe précambrien de Hamersley affleure sur une surface de 85 O00 k m 2 environ dans la partie nord-ouest de l'Australie,entre 116 et 112"Eet 21" et 23"s.Avec le groupe de Fortescue au-dessous et le groupe de Wyloo au-dessus,il fait partie du supergroupe du mont Bruce succession volcanique-sédimentaire dont l'épaisseur locale maximale est d'environ 10O00 mètres et qui s'est déposée dans un bassin ovoïde [le bassin de Hamersley) d'environ 500 km de long et 250 k m de large,avec un prolongement dans la direction ouest-nord-ouest. Le bassin existait il y a 2 200 à 1 800 millions d'années. Le groupe de Hamersley a une épaisseur de quelque 2 500 mètres, dont 1 O00 mètres environ sont une formation de fer qui se présente en cinq unités stratigraphiquesprincipales séparées par des filons-couches de schiste, dolomite, lave, tuf et dolérite. Une de ces cinq unités a été étudiée en détail :le Dales Gorge Member (180 mètres d'épaisseui;) dans la formation de fer de Brockman. On distingue à son intérieur trois échelles de formation de bandes : a) à grande échelle, des macrobandes, définies par 16 alternances de fines couches de schiste avec des formationsde fer plus épaisses ; 6) à échelle moyenne, des mésobandes, définies par des couches alternées de 1 à 30 mm d'épaisseur de silex noir et d'une ((matrice de silex n riche en fer ;et c) à petite échelle, des microbandes, de 0,2 à 2 mm d'épaisseur, définies par les lignes d'un minéral contenant du fer, espacées régulièrement dans les mésobandesde silex noir.Dans l'expression formation de fer ((en bande D, le type de bande normalement considéréest celui des mésobandes.Les macrobandes, les mésobandes et les microbandes du Dales Gorge Member s'étendent généralement sur toute la surface d'affleurement. Nous soutenons,principalementpour des raisonsde texture,

-

que : CI) les microbandes constituent la seule structure primaire préservée actuellement;h) les microbandes reflètent des alternances saisonnières de précipités colloïdaux originels riches et pauvres en fer (ce ne sont donc pas des varvesnon glaciales) ;c) les mésobandes exemptes de microbandes de matrice de silex noir sont le résultat du compactage de précipités également en varve, si bien que les mésobandes sont une structure secondaire et diagénétique.Les conditions dans le bassin de Hamersley et aux alentours ont été exceptionnellementstables.L a profondeur probable de l'eau pendant le dépôt de la formation de fer était de 50 à 250 mètres ; il y avait une circulation réduite vers un océan ouvert au nord-ouest,avec des courants internes de vitesse inférieureà 1 m/s et une teneur en fer de 10à 20ppm. L a région environnante était plate, avec un climat désertique, un drainage négligeable, une évaporation annuelle de 3 mètres environ et de fortes variations saisonnières, impliquant une latitude quasi tropicale. Des phénomènes volcaniquesintermittentsétaient fréquentset ils ont apporté probablement une grande quantité de matières ; le bassin baissait à l'allure moyenne d'environ 1 mètre en 15 O00 ans. L'interprétation donnée s'applique spécifiquement aux formations de fer du groupe de Hamersley. La comparaison détaillée de diverses formationsde fer révèle des différences importantes, qui peuvent être attribuées à une variété de facteurs. L'environnement le plus semblable à celui qui est supposé pour le bassin de Hamersley, d'après les données géologiques d'autres régions, semble être celui des évaporites permiennes des bassins de Delaware et de Zechstein,et ce n'est pas par hasard que la géométrie stratigraphique de ces derniers est aussi très similaire ; cependant,la composition chimiquedu contenuest très différente, et la question reste posée de savoir si cette composition reflète quelque circonstance spéciale de l'environnement local de Hamersley ou résulte réellement et nécessairement de l'évolution géochimique de la terre.

Bibliography /Bibliographie ANDERSON, R. Y.;KIRKLAND, D.W. 1966. Intrabasin varve correlation.Bull.geoí. Soc. Amer., vol. 77,no. 3, p. 241-56. BENTOR, Y.K. 1968. Salt deposits of the Dead Sea region. Spec. Pap. geol. Soc. Amer., no. 88,p. 339-56. BLOCKLEY, J. G. 1967. The crocidolite deposits of Marra Mamba, West Pilbara Goldfield. W.Aust. geol. Surv.Annu. Rep. for 1966,p. 71-3. . 1969.The stratigraphy of the Mount T o m Price ore body and its implication in the genesis of iron ore, W.Aust. pol.

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COMPSTON, W.; ARRIENS, P.A. 1968. The Precambrian GeochronologyofAustralia.Canad.J.EarthSci.,vol.5, p.561-83. CULLEN, D.J. 1963.Tectonic implications of banded ironstone formations.J. sediment. Petrol., vol. 33, p. 387-92. DALLMUS, K.F. 1958. Mechanics of basin evolution and its relation to the habitat of oil in the basin. In: L.G.Weeks (ed)., Habitat of oil, p. 883-931.Tulsa Ann. Assoc.Petroleum Geologist. EDGELL, H.S. 1964. Precambrian fossils from the Hamersley Range, Western Australia, and their use in stratigraphic correlation.J. geol. Soc. Aust., vol. 11, p. 235-61. Geological Society of Australia, 1964. Australian code of stratigraphic nomenclature. J. geol. Soc. Aust., vol. 11, p. 165-71.

Iron-formationsof the Hamersley Group of Western Australia: type examples of varved Precambrian evaporites

GOVETT, G.J. S. 1966. Origin of Banded Iron Formations. Bull. geol. Soc. Amer., vol. 77,p. 1191-1212. GRUBB, P.L.C.1967.Asbestos.C.S.I.R.O. Division of Applied Mineralogy Annual Rep. 1966-67, p. 6.

GRUNER, J. W.1922. Organic matter and the origin of the Biwabik iron-bearingformation of the Mesabi Range. Econ. Geol., vol. 17, p. 407-60. HILLS, J. M.1968. Permian basin field area,West Texas and south-eastern New Mexico. Spec. Pap. geol. Soc. Amer., no. 88,p. 17-27. IRVING, E.;GREEN, R. 1958. Polar wandering relative to Australia. Geophys. J., vol. 1, p. 64. JAMES, H.L.1951. Iron-formationand associated rocks in the Iron River district,Michigan.Bull. geol. Soc. Amer., vol. 62, p. 251-66. _- . 1954.Sedimentary facies of iron-formation.Econ. Geol., vol. 49,p. 235-91. , 1966.Data of geochemistry,6th edition.Chapter W . Chemistry of the Iron-richSedimentary Rocks.Prof, Pap. U S . geol. Surv., 440-W,61 p. KOZARY, M . T.;DUNLAP, J. C.;HUMPHREY, W . E. 1968. Incidence of saline deposits in geologic time.Spec. Pap. geol. Soc. Amer., no. 88, p. 43-57. KUENEN, P. H.1950. Marine geology. New York, Wiley. LABERGE, G.L. 1966. Altered pyroclastic rocks in iron-formation in the Hamersley Range, Western Australia. Econ. Geol., vol. 61, p. 147-61. -. 1967.Microfossils and Precambrian iron-formations.Bull. geol. Soc. Amer., vol.78,p. 331-42. LEGGO, P. J.;COMPSTON, W.; TRENDALL, A.F. 1965. Radiometric ages of some Precambrian rocks from the Northcrest Division of Western Australia. J. geol. Soc. Arcst., vol. 12, pt. 1, p. 53-65. LEPP, H.; GOLDICH, S. S. 1964.Origin of the Precambrianironformations.Econ. Geol., vol. 59, p. 1025-60. MCLEOD, W . N . 1966. The geology and iron deposits of the Hamersley Range area, Western Australia. Bull. W.Aust. geol. Surv., no. 117,170 p. MCLEOD, W.N.;DE LA HUNTY, L.E.;JONES, W . R.;HALLIGAN,R. 1963. A preliminary report on the Hamersley Iron Province, North-West Division. W.Aust. geol. Snrv. Annu. Rep.for 1962, p. 44-54. MAITLAND, A. G.1909.Geologicalinvestigations in the country lying between 21'30' and 25"30'S latitude and 113"30' and 118"30' E longitude, embracing parts of the Gascoyne, Ashburton and West Pilbara Goldfields.Bull. W.Aust. geol. Surv.,no. 15. MILES,K. R. 1942. The blue asbestos bearing banded ironformation of the Hamersley Range, Western Australia. Bull. W.Aust. geol. Surv., no. 100,37 p.

__

O'ROURKE, J.E.1961.Palaeozoicbanded iron-formations.Econ. Geol., vol. 56,p. 331-61. RICHTER-BERNBURG, G. 1963. Solar cycle and other climatic periods in varvitic evaporites.p. 510-9. In: A.E.M . Nairn (ed.), Problems in palaeoclimatology, London, New-York, Sydney,IntersciencePublisher. RYAN, G.R.; BLOCKLEY, J. G . 1965. Progress report on the Hamersley blue asbestos survey. Rec. W .Aust. geol. Surv., no. 1965/32.(Unpublished open file report.) SAKAMOTO,T. 1950.The origin of the Precambrianbanded iron ores. Amer. J. Sci.,vol. 248,p. 449-74. STEWART, F. H. 1963a. The Permian Lower Evaporites of Fordon in Yorkshire.Proc. Yorlcs. geol. Soc., vol. 34,pt. 1, no. 1, p. 1-44. . 19636.Marineevaporites.Prof:Pup.U.S.geol. Surv.440-Y. TALBOT, H. W.B. 1920. The geology and mineral resources of the North-West, Central and Eastern Divisions.Bull. W. Aust. geol. Surv., no. 83. TRENDALL, A. F.1965~.Origin of Precambrianiron-formations (Discussion). Econ. Geol., vol. 60,p. 1065-70. -. 1965b.Progress report on the Brockman Iron Formation in the Wittenoom-Yampirearea. W.Aust. geol. Surv. annu. Rep.for 1964, p. 55-65. . 1966a. Second progress report on the Brockman Iron Formation in the Wittenoom-Yampirearea. Rec. W.Aust. geol. Surv., no. 1966/1.(Unpublished open file report.) -. 19666. Second progress report on the Brockman Iron Formation in the Wittenoom-Yampirearea. W.Aust. geol. Surv. annu. Rep.for 1965, p. 75-87. __ . 1966c.Altered pyroclastic rocks in iron-formationin the Hamersley Range, Western Australia (Discussion). Econ. Geol., vol. 61,p. 1451-8. -. 1968. Three Great Basins of Precambrian Banded Iron Deposition.Bull.geol. Soc. Amer., vol. 79,p. 1527-44. -.1969.The JoffreMember inthe gorgessouthofWittenoom. W.Aust. geol.Surv. annu. Rep.for 1968, p. 53-7. TRENDALL, A. F.;BLOCKLEY, J. G.1968. Stratigraphy of the Dales Gorge Member of the Brockmann Iron Formation, in the Precambrian Hamersley Group of Western Australia. W.Aust. geol. Surv. annu. Rep.for 1967, p. 48-53. . 1969.The Iron Formations of the Precambrian Hamersley Group,Western Australia.Bull. W.Aust. geol.Surv.,no.119, 350 p. TYLER, S. A.;TWENHOFEL, W.H.1952. Sedimentation and stratigraphy of the Hurnian of Upper Michigan. Amer. J. Sci., vol. 250, p. 1-27, 118-51. UDDEN, J. A. 1924. Laminated anhydrite in Texas.Bull. geol. Soc. Amer., vol. 35, p. 347-54. WELLS, J. W.1963.Coral growth and geochromometry.Nature, Lund., vol. 197,no. 4871,p. 948-50.

Discussion A. S. KALUGIN. There are indications by LaBerge of the presence in the iron quartzites of Australia of clastic m a terials in the form of volcanic ash. W i l l you comment on this point?

A. F. TRENDALL. LaBerge's descriptions are accurate, although in a publisheddiscussionofthis paper inEconomic

Geology I criticized both his stratigraphic vagueness and the conclusionswhich he drew from the presence ofvolcanic material.Volcanic ash,replaced by stilpnomelaneoccurs in at least the Marra M a m b a iron-formation and the Dales Gorge member and Joffre member of the Brockman Iron Formation. The presence of this volcanoclastic material falling into the basin does not affect the origin for the 269

A.F.Trendall

iron-formation suggested in the paper. The volcanoclastic material is restricted to distinct bands, from a few centimetres to a few metres thick,which consist almost entirely of stilpnomelane, and does not occur in the banded ironformation itself.

the Hamersley basin that you suggest preclude direct correlation between Hamersley and Transvaal?

A.F.TRENDALL. The age of the Hamersley basin is rather

northern part of the basin, which are described in Bulletin 119 of the Geological Survey of Western Australia, do not affect the correlation of mesobands.

accurately known at 2,000m.y.; the age of the Transvaal and Cape Province basins could be about the same, but the interbedded volcanics from which an exact age could be obtained have not yet been dated. W e have heard at this Symposium of the difficulty of dating the Brazilian itabirites,Although there are obvious similarities among all three basins,it is dangerous to suppose,before evidence is availablefrom all three,that they necessarily have exactly the same age. There may be no overlap in time in the deposition of the iron-formation in each. The answer to the supplementary question is ‘Yes’-direct correlation is impossible.

I. P. NOVOKHATSKY. Are there any other indications of

E.C.PERRY. Is it likely that differential weathering would

evaporites?

affect all rocks equally (crust of average chemical coinposition) or might basaltic rocks be selectively weathered?

S. ROY.In case of folding, because of the difference in consistency of chert, hematite or magnetite bands, there should be pinching and ñowage in different bands. Does it not affect the stratigraphiccorrelation of the mesobands?

A.F.TRENDALL. Only in the extreme south of the basin, where the folding is intense. Certain minor folds in the

A . F.TRENDALL. I a m not sure what other indications of evaporites are expected in the question. Apart from the complete absence of terrigenous clastic material, the main indications of similarity between evaporites in the usual sense and iron-formationof the Hamersley Group are the three features of microbands to which attention is drawn in the paper: their regular repetition,their small thickness, and their very large areal extent.

A . F.TRENDALL. Differentialweathering is of course possible,but it is so difficult to set up any reasonable model for the provision of sufficient basalt to be weathered. The iron certainly was not derived from the basalts of the underlying Fortescue Group, which have normal basaltic composition.

N.A.PLAKSENKO.Could you tell us h o w the mineralogical I.P.NOVOKHATSKY. What is the approximateconcentration of readily soluble salts in the water of the evaporite basins?

A.F.TRENDALL. The concentrationsof the readily soluble salts in the waters of the Hamersley basin are not known quantitatively,but the presence of diageneticriebeckiteand stilpnomelane in the iron-formationsleaves no doubt that sodium and potassium were both present. E. C. PERRY.Comparing microbanding mineralogy and age, what relation in space and time do you envisage between the Hamersley basin,the South African basin and perhaps the Brazilian basin? Does the geometric shape of

270

composition of the iron-formation changes vertically and horizontally? It would be interesting to know the variation of iron ore minerals. A. F.TRENDALL. It was mentioned in the paper that the five main stratigraphic units of iron-formation are each chemically, mineralogically and lithologically distinct. It is not possible in a brief answer to give the details of these vertical differences and I can only refer Prof. Plaksenko to Bulletin 119 of the Geological Survey of Western Australia for a full account. As far as horizontal variation is concerned there appears to be no significant variation throughouttheareaofthebasin;this factisemphasized,since it is contraryto published accountsof other iron-formations.

Geology and iron ore deposits of Serra dos Carajás,Pará,Brazil G.E.Tolbert,J. W.Tremaine, G,C. Melcher and C. B. Gomes Cia. Vale do Rio Doce,Div. de Desenvolvimento,Brazil

Introduction The Serra dos Carajás iron deposits were discovered in August 1967, by geologists of the Companhia Meridional de Mineração while undertaking a systematic exploration programme for economic mineral deposits (Tolbert et al., 1968). Supported by boats, small aircraft and helicopters, exploration began on the Xingú River and progressed eastwards to the Itacaiunas River,a tributary of the Tocantins River, from where the initial penetration to the Carajás ranges was made. Additional iron depositsof the sametype, but considerably smaller in size, were also discovered 175 km west of the Serra dos Carajás and 30 km north-east of the town of São Felix do Xingú (6"19'-6"30'S. and 51 "41'-52"00' W .). All of the deposits are now controlled by a new company formed by Companhia Meridional de Mineração and Companhia Vale do Rio Doce. The Carajás region is in a remote,unexplored part of the Amazon rain forest,between two major southern tributaries of the Amazon, the Xingú and the Tocantins rivers (5"54'-6"33'S. and 49"35'-50°34'W.; see Fig. 1). Serra dos Carajás,or the Carajás Range,does not constitute a single mountain range,but, as defined in this report,refers to the two principal iron-bearingranges, Serra Norte and Serra Sul, and the intervening area. The field work to date has consisted of reconnaissance geologic mapping of the iron deposits at a scale of 1 :5,000, airborne and ground magnetometer surveys and topographic surveys. Detailed exploration, including geologic mapping of the ore bodies, diamond drilling and the excavation of adits,has been confined to one area, called N-1, where the main camp and an air-strip are located.

In the state ofPará,the Itacaiunas-Carajásregion,between the Tocantins and Xingú rivers, consists of fairly flat lowlands, a few widely scattered ridges and hills, and iron-bearingplateaux. Except for the plateaux, the region is covered by dense, tropical rain forest.North and west of the iron deposits are several long, narrow, westwardtrending ridges and chains oflow,rounded hills.Incontrast, the Carajás ranges are more extensive and sinuous. These uplands are 600-700 m in elevation and have a relief of 200-300 m . Their uniform elevation suggests they are erosional remnants of a former widespread peneplain. Where the Carajás ranges are underlain by iron-formation, a hard hematite crust or canga capping has prevented the growth of tall rain forest resulting in a series of clearings or savannah areas covered by low brush which stand out in contrast to the surrounding densejungle.Some clearings have small lakes or swamps in low areas. The topography of these clearings,which is strongly influenced by the underlying rocks,is formed by undulating plateaux and hills, or monadnocks, which rise as high as 100 m above the general plateau surface. These savannah, like plateaux,which coincide with the ore deposits,range from 1 to 30 km in length and from 500 to 2,000 m in width. Nearly vertical scarps, 10-20 m high, and 30-40" talus slopes,are common along the borders where undercutting of the canga rim has caused progressive erosion of the plateaux. The climate is tropical and temperatures during the day are often high; however, the plateaux enjoy a far less humid and more healthy climate than the lowlands. At the project site the average annual rainfall for 1968-69 was 180 cm,the wet season extending from November to May.

Physiography

Regional geology

Three physiographic features are prominent in the Carajás region:(a) jungle-coveredlowlands;(b) long,nearly straight ridges or chains of hills; (c) discontinuous iron-bearing plateaux.

The area underlain by the Carajás iron-formations constitutes a small part of an extensivePrecambrian terrain which extends from the Tocantins to the Madeira River and is bordered on the north by sediments of the Amazon basin.

Unesco, 1913. Genesis of Precunibriun iron and manganese deposifs.Proc. Kiev Symp.,1970. (Earth sciences, 9.)

271

G.E.Tolbert,J. W.Tremaine,G.C.Melcher and C.B. Gomec

5'

1 O'

i0

O

50 100 150 200km

1 55'

FIG.1. Index map showing location of Serra dos Carajás.

Before 1950 very little was known about the geology of southern Pará, except for occasional observationsmade by a few river expeditions.Recently,aerial photographs have provided some geologic information enabling geologists, under the auspices of the Departamento Nacional da Produção Mineral, to establish a generalized sequence of rock types in the Araguaia region (Barbosa et al., 1966). This sequence,which extends as far north as the Itacaiunas River,is composed of five separate units: Undifferentiated Precambrian rocks,consisting of granite, migmatite and paragneiss. Precambrian metasediments (greenschist facies) correlated with the Araxá Series (described in south-centralBrazil) which include mica schist, quartzite and paragneiss. Slightly metamorphosed phyllite with intercalations of quartzite,itabirite,conglomerate, greywacke and limestone of the Tocantins Series. Unmetamorphosed,EopalaeozoicGorotireFormationconsisting of sandstone and conglomerate. Carboniferoiis sediments of the Piauí Formation, which include carbonate rocks, sandstone,chert and shale. Parada et al. (1966) mapped the Rio Naja area in the Rio

272

Fresco region and found a north-east-trendingsequence of quartzites and itabirites which they named the Tocandera formation.This formation may correspond,in part,to the Tocantins Series. Almaraz (1967)dated various rocks (granite,amphibolite, migmatite) in the Marabá-Itacaiunas area using the potassium-argon'methodand obtained an average age of approximately 2,000m y . Almeida and co-workers(1968) have included this area in the 'Tocantins-Tapajóscraton', the last metamorphism of which is also dated at about 2,000m.y. Amara1 (in preparation) has recently obtained additional age information on rocks collected during the current project. His results also indicate that the last principal metamorphic event occurred at about 2,000m.y., as confirmed by determinationson seven samples of gneiss, amphiboliteand muscovite schistfrom the ItacaiunasRiver. The age of one of the samples, an amphibolite from the western part of the Serra de Itapirapé,situated about 65 km north-west of area N-1,was determined to be 3,2805113 m.y. Interestingly,this is the oldest date yet determined for a Brazilian rock.

Geology and iron ore deposits of Serra dos Carajás,Fará (Brazil)

A considerable amount of geologic information has resulted from current field work,including the completion of surveys in previously inaccessibleareas made possible by the utilization of helicopters. Owing to the extensive size of the region,the ubiquitous forest and laterite cover,and the complete lack of population or supply facilities,it has been extremely difficult to establish a stratigraphicsequence or to correlatethe principal lithologic units mapped during the current exploration programme with those enumerated by Barbosa et al. (1966). The writers recognize that some of the lithologies found at Serra dos Carajás are similar,in part,to those describedin the Araguaia region;forexample, quartzite and itabirite of the Tocantins Series and mica schist of the Araxá Series (?) have counterparts in the Carajás rocks. Nevertheless,only after considerably more work has been done on the structure and stratigraphy in both regions will reliable correlations be possible. Several rock types distinguished by geologic mapping are described below and shown in Figure 2. GNEISS, GRANITE A N D AMPHIBOLITE

Much of the Itacaiunas-Carajás region is underlain by rocks which are dominantly gneiss with subordinate granite, amphibolite and granulite. Many of the rocks called gneiss in the field are found to be mylonitized granite when examined under the microscope. The varieties of gneiss include orthogneiss,injection gneiss and migmatites. In the area west of the Itacaiunas River, permatite dikes cut granite,gneiss and metasediments. Bands of amphibolite several metres thick are commonly associated with gneiss and appear to be derived from mafic rocks. Blastophitic texture is preserved, and uralitized pyroxene or relicts of pyroxene are altered to hornblende along border zones. Some varieties with garnet crystals aligned parallel to the lineation may be of sedimentary origin. Granulite,with typical granoblastictexture,crops out in a few places. It is composed of quartz, andesine-labradorite,augite, hypersthene, garnet and biotite.

of phyllite and schists are very rare. Those few phyllite samples collected are composed of quartz and sericite with subordinate chlorite,biotite,actinolite,epidote and opaque minerals. Muscovite schist is the dominant mica schist, but chlorite, actinolite, biotite and graphite schists are also found. Most of these rocks, which are fine-grainedwith lepidoblastictexture,contain epidote,garnet,albite,microcline and carbonate. Deposits of manganese oxides believed to be derived from the oxidation of quartz-spessartite schist are found in a few places along ridges north-east and north-west of Serra Norte.

-

I R O N F O R M A T I O N (ITABIRITE)

Owing to the fact that leaching and enrichment have been so thorough in Serra dos Carajás,fresh,unaltered itabirite is rarely encountered. Ferruginous quartzite and itabirite are found on ridges north of the iron deposits which suggests that iron-rich sedimentation was not confined exclusively to the Carajás ranges. Itabirite is composed of alternating laminae of quartz and iron oxides that range in thickness from 0.05 mm to 10m m . The main minerals in this rock are quartz (recrystallized chert), magnetite and hematite. Secondary minerals are goethite,martite, gibbsite and, rarely, sericite. Xenomorphic quartz, both granular and elongate,occurs in two grain sizes. In places, very fine-grained quartz (0.0050.01mm) shows a typical mortar texture and appears to have a cataclastic origin. A possible third generation of quartz is represented by tabular crystals,with grain sizes from 0.005 mm to 0.03mm,that grow perpendicular to crystal faces and ñll the interstices of opaque minerals. Hematite is nearly always peripheral to magnetite grains evidencing the growth of hematite at the expense of magnetite. Idiomorphicmagnetite crystals,which are equidimei-isionaland submillimetric,are aligned parallel to the banded structure but,as observed in a few specimens,they truncate this structure,suggestinga later stage of magnetite crystallization.

QUARTZITE, PHYLLITE A N D MICA SCHIST SANDSTONE A N D C O N G L O M E R A T E

Pure,granoblastic quartzite is the dominant type with subordinate ferruginous schistose varieties. Sericite, epidote, biotite, limonite and pyrite are common mineral assemblages in these rocks. The grain size ranges from 0.1 mm to 1.2 mm;cataclastic and mylonitic textures are common. Most of the quartzitic rocks are metamorphosed to low grades, although the presence of garnet, amphibole and pyroxene indicate higher grades in the area north of Serra Norte toward the Itacaiunas River. Argillaceousmetasediments and schists are interbedded with iron-formationand undoubtedly constitutea large part of the section. However, intense tropical weathering has converted these rocks to reddish laterite and fresh exposures

The area between the western part of Serra Norte and Serra Sul is underlain by friable sandstone,quartzitic sandstone, quartzite and conglomerate.These rocks are considered to be younger than the metasediments and may be equivalent to the Gorotire Formation.The sandstone is composed of fine-grained,xenomorphicquartz with subordinatesericite, biotite, chlorite, zircon and opaques. Some varieties are conglomeratic with fragments (0.4mm to 1.5 c m in diameter) composed of quartzite and banded itabirite. The matrix is fine-grained quartz, iron oxide, sericite and biotite. Angular pebbles found in some places suggest that these sediments were derived from a local source. 273

G.E.Tolbert,J. W.Tremaine,G.C.Melcher and C.B.Gomes

SCALE O 2 4 6 8 1Okm

.., , .....' .. ,:.

Gorotire f o r m a t i o n I?). quartzitic s a n d s t o n e , partly c o n g l o m e r a t i c

Quartzite. phyllite. a n d m i c a w i t h intercalated. b a n d s of iron formatinn.(if). c o m m o n l y c a p p e d b y tanga

LlthologiC c o n t a c t

i

Fa U I t ,d a s e d w h e r e in f e rred. questioned w h e r e d o u b t f u l . mostly photogeologic interpretation

__-----Structural lineament: p h o t o geological interpretation

-i-+

A X ~ So f anticline or syncline with plunge: photogeological

Gneiss a n d granite

interpretation

FIG.2. Generalized geologic map of Serra dos Carajás region.

M A F I C ROCKS

Severalvarieties ofmaficrocks,probably of more than one age, are found in the region. A petrographic study of mafics from the Itacaiunas area resulted in the identification of diabase, olivine diabase, metadiabase, basalt,

274

aiid andesite porphyry. Diabase dikes with ophitic texture are the most common aiid are comprised of dominant pyruxene aiid plagioclase (generally labradorite) with minor quartz, olivine and biotite. A n age determination on andesite porphyry from the Itacaiunas River gave 507&99 m.y., which corresponds to the age of otlier

Geology and iron ore deposits of Serra dos Carajás,Pará (Brazil)

rocks of basaltic composition in thc Amazon region (Almaraz, 1967). Mafic intrusions control,to some extent,the physical boundaries of ore bodies, as disclosed by adits and drill holes at N-1 which have intersected both concordant and discordant mafic dikes and sills or have terminated in maficrocks. The dikes and sills range from less than 1 m to several tens of metres in thickness. Aerial photographs reveal several north-east-striking dikes (presumably diabase), as much as 10 km long, that intrude the rocks between Serra Norte and Serra Sul.Whether these tabular intrusives correspond to the mafics encountered at depth is not known. Both appear to be basaltic in composition but they may be of different ages.

Structure The geologic map of the Araguaia Project (Barbosa et al., 1966) shows a belt of north-trendingPrecambrian rocks parallel to the Araguaia and Tocantins Rivers,This structure was formerly recognized by Kegel (1965)who named it the ‘Araguaia-Tocantins lineament’. About 150 km south of Marabá the strike of these rocks curves to the west and in the Carajás region they trend west to westnorth-west.This major change in strike, also noted by Kegel (1965), coincides approximately with the eastern end of Serra Sul where the iron-formationturns southward (Fig.2). This important tectonic feature may be related to the structural complexity of the Carajás rocks as well as responsible for the eastward diversion of the Araguaia and Tocantins Rivers east of Marabá (Fig. 1). Deformation of the rocks in areas north and west of Serra dos Carajás produced simple, open folds with gentle dips and horizontal fold axes striking west to westnorth-west.In contrast to this relatively mild type of deformation,the Carajás rocks are complexly folded with west-north-west fold axes that plunge both east and west. Tight,crenulated folds,isoclinal in places,with amplitudes varying from a fewcentimetresto severalmetres, are typical features of in situ hematite and itabirite. Plastic flowage has caused thickening and thinning of beds. In addition to folding,block faulting and tilting have been instrumental in segmenting Serra Norte and Serra Sul into the several isolated plateaux which characterize those ranges (Fig. 2). Aerial photographs show a prominent fracture system oriented north-east and north to north-west; faults with apparent displacements of as much as several hundred metres have been discerned. A major lineament, interpreted as a fault, strikes west-north-west across the area between the two iron ranges and is tangential to the easternmost area of Serra Sul (Fig. 2). The structure of Serra Sul is somewhat less complex than that of Serra Norte. The western part of Serra Sul is intersected, and in places displaced, by north-striking faults,but the itabirite beds are generally more continuous than those at Serra Norte. The central sector of Serra Sul appears to be offset to the north relative to the eastern

and western blocks (Fig. 2). The area between the western parts of Serras Norte and Sul, underlain by arenaceous sediments and metasediments, is intersected by a system of north-east- and north-west-trending fractures with a ‘herringbone’pattern.

Iron deposits ITABIRITE P R O T O R E

The Carajás iron-formationhas been described generally in a preceding section.Its chemical and physical properties closely approximate oxide-facies iron-formation found in other parts of the region as well as in the Quadrilátero Ferrífero in Minas Gerais (Dorr, 1964, 1965). Comparison of the size of quartz grains in itabiritefrom the two regions, for the few data available,suggests that the coarser grain sizes are similar,but quartz in the Carajás itabirite tends to be finer-grained.Other facies of iron-formationhave not been found except for one 1.4 m interval of banded carbonate rock, probably dolomite, intersected in a drill hole. The iron content of unenriched iron-formationin the few samples that have been analysed ranges from 17 to 41 per cent Fe (Table 1). The average of these few analyses is not considered to be representative.The original thickness of the iron-formationwas probably a few hundred metres, but in some areas complex folding and faulting have multiplied the apparent thickness to as much as 1 k m .

TABLE1. Analyses of itabirite samples, Serra dos Carajás

No.

N1 N1 N1 MA

T2 T2

T5 51

Wt percentage, dry basis

Distances from adit portal, inmeters

Fe0

FeZO:. Si03 A1,0, PcOa Fe

68 69 165 Surface sample

6.53 1.15 1.00 0.18

51.23 39.39 23.35 5.45

38.47 56.19 60.84 70.39

0.48 0.55 0.37 0.68

0.11 0.07 0.09 0.06

40.90 28.44 17.11 20.26

The uniform composition and texture as well as the consistency in thickness of laminae provide evidence that this rock was a shallow water sediment consisting of alternating layers of silica and hydrous iron oxides. Subsequent burial and diagenesis converted silica to chert and removed water from the iron minerals. Metamorphism recrystallized chert to quartz and the iron minerals to magnetite and subordinate hematite. Most of the hematite observed today resulted from the oxidation of magnetite under surface or near-surface conditions.

FRIABLE O R E BODIES

Detailed mapping of the ore deposits has been completed only in the northern range (Fig. 3). Furthermore,the determination of the dimensions of ore bodies by subsurface 275

G.E.Tolbert,J. W.Tremaine,G.C.Melcher and C.B. Gomes

LEGEND Contact High-grade hematite, in place Fault 45 _I

H e m a t i t e canga

Strike and diD of bedding

He'rnatite-goethite c a n g a

Vertical b e d d i n g

+

0 I

I

SCALE 2 3

@ ... ..

4

.. .. ... . ..

5km

FIG. 3. Geologic map of Serra Norte showing distribution of ore bodies. exploration is limited to area N-1,because no drilling or excavation of adits has yet been started on other deposits. Therefore,the 'ore bodies' as shown on Figure 3, except for N-1,are delineated by surface mapping only. The major ore deposits underlie hills that stand above the surface of the plateaux. Although flanks of the hills are covered with canga, the crests are generally formed by hard, in situ hematite. Principal ore bodies are composed of friable hematite and occupy an irregular zone between outcropping hard hematite, or the canga capping, and unleached itabirite below (Fig. 4). The thickness of the ore bodies depends mainly on the extent to which meteoric water has been able to penetrate and leach the iron-formations. Friable ore,as observed in adits, is dark reddishbrown or metallic grey. Preliminary grain size analyses of a few samples show that 24 per cent of the ore is above one-quarterinch and 76 per cent is below this size. Partial sampling of the ore bodies in area N-1,which may or may not be representativeof other untested deposits, indicates an average grade of more than 65 per cent Fe (Table 2). The iron content in the N-1 ore bodies seems to be fairly constant to the depths attained by current 276

drilling (Fig. 4).Drill hole NlD14 shows a decrease in Fe and corresponding increases in alumina and silica in the 20-40 m interval where the hole intersects a diabase dike (Fig. 5). Owing to zones of partially leached itabirite, similar variations in grade appear between 120-200 in. A lower iron content accompanied by an increase in alumina and silica mark the presence of a dike between 20-30 m in drill hole NlDlO. The following ore minerals were identified (confirmed by X-ray analyses): hematite, magnetite, goethite and martite. The texture of friable hematite is granular or platy with two common grain sizes,0.03mm and 0.2111111. Xenomorphic grains of hematite commonly have an equidimensional or elongate habit.Tabular crystals of hematite or specularite, growing perprndicular to other hematite grains or filling cavities, probably represent a later stage of crystallization. Specularite also occurs in indurated zones along the walls of dikes. Magnetite crystals are equidimensional,idiomorphic and vary in size from 0.02mm to 0.05 mm.Although magnetite is generally oxidized to hematite, the converse has not been observed. Along grain boundaries and part-

Geology and iron ore deposits of Serra dos Carajás,Pará (Brazil)

Friable hematite

I ta biri te

Mafic intrusive

Geologic contact, d a s h e d w h e r e inferred

~-

-------

FIG. 4.Cross section of part of area N-1,Serra Norte,showing relations between1ore bodies, itabirite,canga and mafic intrusives. ing planes magnetite is altered to martite. The ore is generally slightly magnetic and, in a few drill holes, short intervalsof 1-2m contain as much as 40per cent magnetite; however, the average magnetite content of the ore is estimated to be only a few per cent. Goethite, which is the most important secondary mineral, replaces hematite along grain boundaries and fractures.Accompanied by gibbsite, it frequently fills interstices and cavities. H A R D HEMATITE

Indurated hematite occurs on the crests of hills, where it commonly retains the laminated structure of the protore, and, in addition, forms lenses and tabular bodies within soft ore deposits. It is composed of metallic-greyhematite and specularite and generally has a tenor of 66 per cent Fe or higher (Table 2). The surficial type of hard hematite forms a crust 1020 m thick and is usually more hydrous and higher in phosphorus than the underlying ores. In places it has been partially disintegrated and cemented by hematite or goethite.

TABLE 2.Chemical analyses of selected ores from Serra dos Carajás Wt percentage, dry Sample1

Fe

A B C D E

P

SiO? Also3 M n

63.47 0.228 0.97 68.13 0.079 0.47 66.58 0.068 0.77 67.55 0.035 0.61 69.11 0.014 0.68 Fe+ + E 3.42 E (Spectrographic analysis) 0.001-0.01 Ca, Pb 0.0002-0.002 C u 0.0005-0.005M g

2.64 1.13 3.72 2.58 0.03

0.04 0.04 0.62 0.09 0.03

Losson ignition

Tio, 0.44 0.20

0.019 0.020

0.02

0.003

-

< 0.005

-

-

7.05 2.52 0.93 2.24 0.62

< 0.02Zn,W,Sb, N a < 0.01 Sr

< 0.006 N b < 0.003 Cr, S n < 0.001 V, Ba, Bi, Ni,Co, M o < 0.0005 A g

1. Sample A: Canga, from area N-4, Serra Norte. Sample B: Hard, in situ hematite ore from surface, N-1, Serra Norte. Sample C : Friable ore from adit N1 T1, area N-1,Serra Norte. Sample D:Friable ore from surface, western part of Serra Sul. Sample E Hematite fines from adit N1 T1,area N-1, Serra Norte.

277

G.E.Tolbert,J. W.Tremaine,G.C.Melcher and C.B. Gomes

Drill hole NfDf4

Drill hole NfDfO

FIG.5.Profiles of drill holes NlDlO and NlD14 in area N-1, Serra Norte,showing variations in iron, silica and aluminium with depth.

Lenses of hard ore below the surface range from a few centimetresto several metres in thickness and extend along strike from less than 1 m to tens of metres. Normally this materialhas a massive texture,however,it may also exhibit relict bedding; both concordant and discordant relationships are observed with the enclosing rock. Although few data exist, it is believed that most of the hard hematite, other than the surface material, is the product of metamorphism and redeposition of iron oxides; it does not appear to be a residual concentration resulting from the leaching o€ silica by meteoric water.An alternateinterpretation, unsubstantiated by this study but possibly valid in some of tlie occurrences,advocates the primary deposition of iron minerals,with little or no silica,as lenses in banded iron sediments. Subsequent metamorphism recrystallized the iron minerals to iron oxides and deformation may or may not have displaced the hard iron lenses to discordant positions with respect to the enclosing rocks.

CANGA

Covering the slopes of the hills and the flat portions of plateaux is a ferriferous capping, or canga, that ranges 278

from 1 to 20 m in thickness (Fig. 4). Canga consists of unoriented grains, pebbles or fragments, cemented by hydrated iron oxides and minor clay. Fragmentalmaterial is composed mostly of hematite, goethite, itabirite, some magnetite,pebbles and blocks of previously formed canga, and clay minerals,Rarely, other rocks, such as phyllite or schist,comprise the cemented material. Fragments may be of millimetric size or as large as several decimetres in diameter. Canga varies from a hematite-richvariety found on the flanks of hills to progressively more hydrated types (goethite) near the plateau borders. Analyses of a few grab samples of this material contain above 60 per cent Fe and average 0.20per cent phosphorus (Table 2). Surface water draining through fractures in the canga erodes the underlying soft ore, phyllite and schist,carving out large caverns especially under the rims of the plateaux. Canga is considered to have formed by the mechanical decomposition of bedrock and subsequentcementation of fragments by chemically precipitated mixtures ofhydrated iron oxides, fine-grained iron oxides and minor clay. Erosion and transportation of surface rubble has resulted in the formation of cangain areaswhich arenot necessarilyunderlain by itabirite or ore deposits.

Geology and iron ore deposits of Serra dos Carajás,Pará (Brazil)

Summary and origin of ore deposits At this early stage in the study of the Carajás iron deposits not enough data are available to recapitulate in detail the events related to their origin. Thus the following summary is based partly on observable facts in the Serra dos Carajás and partly on comparisons with similar deposits. The evolution of the iron deposits began in early or middle Precambrian with the rhythmic deposition of iron-rich chemical precipitates,intercalated with siliceous and argillaceous sediments, in a shallow basin probably adjacent to or associated with a stable cratonic region. The basin of deposition was approximately 300 km long in an east-west direction and 50-60 km wide. Whether or not volcanic rocks formed part of the above sequence is unknown. The only rocks in the region with a possible volcanic origin deposited at this time are amphibolites and their exact stratigraphicrelation to the iron-formations and metasediments is not clear. Subsequently the unconsolidated iron sediments were buried and compacted.In middle Precambrian the basin

Géologie et dépôts de minerai de fer de lu Serra dos Carajás, Pará,Brésil (G. E.Tolbert,J. W . Tremaine,G.C. Melcher et C. B. Gomes)

En 1967, de nouveaux gisements de minerai de fer importants ont été découverts dans la région de la Serra dos Carajás dans la partie sud de l’État de Pará au Brésil (6” de latitude S, 51” de longitude O). Les recherches effectuées à ce jour comprennentl’établissementd’une carte photogéologique régionale et d‘une carte topographique d’une partie de la surface à l’échelle du 1/5000, quelques forages, quelques tunnels dans certains dépôts et des études minéralogiques. Ce programme, auquel s’ajouteun levémagnétométriquede surface,se poursuit.Les gisements sont localisés dans une série de plateaux de direction générale nord-ouestcoiffés de canga qui apparemment sont les flancs d’un synclinal régional qui plonge rapidement vers le nord-ouest.Ces flancs sont distants de 30 km dans la région ouest de Carajás et ils convergent vers le nez du synclinal à 100 k m à l’est.U n réseau de failles et de fractures pointant vers le nord-est et le nord-ouest constitue le caractère structural dominant de la région. Les plateaux sont discontinus et leur surfacevarie de un à quelques dizaines de kilomètres carrés. Ils s’élèvent approximativement à 500 mètres au-dessusdes terres basses couvertes de forêts et s’apparentent à d‘autres chaînes de la région, ce qui fait penser qu’ils sont peut-être les vestiges d’une surface d’érosion étendue.

was affected by an orogeny accompanied by complex folding, faulting aiid metamorphism. Folding, and to a lesser extent, faulting, exerted an important influence on the later development of ore bodies by increasing the thickness of the protore. lion-rich sediments were metamorphosed to itabirite. Later faulting and fracturing increased the permeability of the iron-formations.The fact that Serra dos Carajás was the locus of intense deformation is significant because relatively undeformed ironïormations in outlying areas with similar relief did not form ore deposits. The physiographic evolution of this region began with the development of an extensive erosion surface above which a few monadnocks of resistant iron-formationprotruded.Epeirogenic movement was responsiblefor uplifting the peneplain approximately 700 m,where most of it was destroyed by erosion leaving knobs and ridges of resistant quartzite and itabirite.The protective canga capping prevented the erosion of soft ore,but was sufficientlypermeable for meteoric water to leach silica from underlying itabirite resulting in the residual enrichment of iron oxides and the ore bodies observed today.

L a croûte de canga, dont l’épaisseur varie de 1 à 30 mètres,est constituéede fragments d’hématite et d‘autres composants ferrugineux liés par des oxydes de fer hydratés. Le minerai primitif (protore) consiste en formations de fer métamorphosées (itabirite) de l’époque précambrienne qui ont été vigoureusement plissées. Le minerai est constitué de plaquettes d’hématite à grain fin, friables, avec en second lieu de la magnétite ; cependant, en quelques endroits, le minerai est dur et massif. Quelques analyses préliminaires indiquent que la teneur est équivalente à celle des autres dépôts d’hématite à haute teneur qu’on rencontre dans le monde entier. O n observe des intrusions de filons mafiques dans les formations de fer ; leur épaisseur varie de quelques centimètres à plusieurs dizaines de mètres. Parmi les autres types de roche qu’on trouve dans la région figurent la phyllite,le quartzite,le grès et des roches cristallines telles que le granite et le gneiss. Les datations de ces roches cristallines par la méthode du potassium-argon indique un âge d‘environ 2 milliards d‘années. La lixiviation supergène de la silice semble avoir été la cause de la formation de la plupart de ces gisements de fer. Parmi les autres facteurs qui ont contribué au développement des gisements figurent la préservation de la surface d‘érosion et de la croûte ferrugineuse,les plissements et failles complexes des formations de fer,ainsi que le climat tropical.

279

G.E.Tolbert,J. W.Tremaine,G.C.Melcher and C.B. Gomes

Bibliography/Bibliograghie AMARAL,G. 1969. Nota previa sôbre o reconhecimento geocronológico do Pre-cambriano da região amazônica. XXIII Congresso Brasileiro de Geologia,Bol. Especial, no,1, p. 81-2. AMARAL,G.(In preparation.) Reconhecimentogeocronológico do Pre-cambrianoda região amazônica. ALMARAZ,J. S. U. 1967. Determinações K-Arna região do curso médio do Tocantins. Bol. Soc. bras. Geol., vol. 16, no. 1, p. 121-6. ALMEIDA, F.F.M.de. 1967.Origem e evolução da plataforma brasileira.Bol. Div.Geol. min. Rio de J., no. 241,36 p. ALMEIDA,F. F. M.de; MELCHER, G.C.;CORDANI, U.G.; KAWASHITA, K.;VANDOROS, P. 1968. Radiometric age determinations from northern Brazil. Bol. Soc. byas. Geol., vol. 17,p. 3-14. BARBOSA, O.;ANDRADE RAMOS, J. R.; GOMES, F. A. de;

HELMBOLD, R. 1966. Geologia estratigrafia, estrutural e economica da área do projeto Araguaia.Monogr. Div.Geol. min.,Rio de J., no.XIX,94 p. DORR, J. Van N. II 1964.Supergeneiron ores of Minas Gerais, Brazil.Econ. Geol.,vol. 59,no. 7,p. 1203-40. -. 1965. Nature and origin of the high-gradehematite ores of Minas Gerais,Brazil. Econ. Geol.,vol. 60,no. 1, p. 1-64. KEGEL, W. 1965. Lineament-tektonikin Nordwest-Brasilien. Geol. Rdsch., vol. 54,no. 2,p. 1240-60. PARADA,J. M.;FORMAN, J. M. A.;FERREIRA, J. P. R.; LEAL, J. F. 1966. Pesquisas minerais no Estado do Pará. Bol. Div. Geol. min.,Rio de J.,no. 235,24 p. TOLBERT, G . E.;SANTOS,B. A. dos; ALMEIDA, E. B. de; RITTER, J. E. 1968. Recente descoberta de ocorrências de minério de ferro no Estado do Pará. Mineraç. e Metall., vol. 48,no. 288, p. 253-6.

Discussion M.V. MITKEYEV. In your collection you have a specimen of hard hematite. How do you account for the origin of this ore? G .E.TOLBERT. The specimen of hard hematite exhibited is not a typical or representative type of ore from the Serra

280

dos Carajás. Hard hematite is rather rare. Although w e have very little information to date, we tend to attribute the origin of the hard hematite lenses to metamorphic differentiation,thas is, an origin similar to that advocated for similar ores in Minas Gerais (Brazil) by Professor J. Van N. Dorr.

Enrichment of banded iron ore, Kedia d’Idji1,Mauritania F. G.Percival Sadlers End, Haslemere, Surrey (United Kingdom)

Introduction The chief purpose of this paper is to record certain evidence which has a bearing on the age of leaching and enrichment of the iron ores of the Kedia d‘Idji1,Mauritania.In order to assist in evaluating the evidence a brief description of the ores and rock types is necessary. The Kedia d’Idjil (Fig. 1) is an inselberg of Precambrian rocks with quartzites, schists and banded ironstones along its northern flanks and heights, running mainly east-west for some 24 km,and dipping steeply to

the south. T o the west it narrows and descends to plain level near the village and fort of the former Fort Gouraud, n o w called F’Derik. Eastwards the range widens to a maximum north-south development of about 10 km. It rises to elevations of 300-500 m above the Saharan plain, which here consists mainly of orthogneisses covered with ‘reg’-a loosely consolitated blanket of sand, gravel and boulders. South of the belt of regularly bedded rocks a so-called‘breccia’blankets the hill, as shown in Figure 1. Towards both extremities of the range the strike has a marked twist to a NW.-SE. direction, and at these

LEGEND

1-1

m l m v)I

Reg

22.45’

11115111P

ZOUERATE MIHE TOWNSHIP

B.H.Q. &

Canga

m l..

/3

Breccia

Bedded Breccia

Cambrian Sandstone

Schists

Conglomerate

Visible Ore

e-

r

KEDIA D‘IDJIL I

n

i

z

3

4

5

6

7

a

9

ip

KILOMETRES

FIG.1. Geological map of the Kedia d’Idjil,somewhat simplified. Unesco, 1973. Genesis of Precumbriun iron and manganese deposits. Proc. Kiev Symp.,1970.(Earth sciences, 9.)

281

F. G.Percival

distortionsthe largest enriched ore bodies are located. The banded ironstones are formed of largely recrystallized bands of heinatite and quartzite. The bands are of varying thickness, but normally do not exceed 5 m m . These rocks are similar to the itabirites of Brazil and this term will be used for convenience,though at the mine they are commonly called BHQ (banded hematite quartzites). A series of approximately N.-S.faults crosses the range, marked by deep canyons. These have not been shown on the simplified map (Fig. 1). Blondel (1952) gave a valuable short account of the Kedia, and Blanchot (1955) gave more details of the deposits, their covering of breccia, and their relationship to the sub-horizontalPalaeozoic sandstones to the east,H e also described a Conglomerate containing boulders of this breccia-boulders whose size frequently attained that of a camel’s head-exposed at the ravine of O u m el Hbel on the southern flank of the range (Fig. 1). Both the breccia and the conglomerate will be more fully discussed lierein. Lethbridge and Percival (1954)wrote a general description of the occurrences of iron ore,and of the prospecting work in hand up to 1953, and papers on special aspects of the deposits have been published by Huvelin (1963) and by Baldwin and Gross (1967). A description of the mining development of the ore bodies was given by Audibert et al. (1964). To the east of the Kedia lie the Cambrian sandstones, with some dolomite, and with a very gentle easterly dip. They develop a NE.-SW. trending escarpment, a feature that continues for some 300 km south-westerly to the oasis of Atar. These sandstone overlie the Precambrian rocks (both the bedded rocks and the breccia) at the eastern end of the Kedia, but only to a moderate height, well below the crest of the range. Blanchot considers that

probably the higher levels were never covered and the Kedia stood out as an island in the Cambrian sea. At O u m el Hbel these Palaeozoic sandstones pass into the conglomerate facies mentioned above, containing large boulders of breccia. Blanchot suggests that this is a local beach development, and describes the passage eastwards, between Oumel Hbel and Chig,as marked by recurrencesof sandstone1-1 O m inextentinterstratified intheconglomerate. The folding of the itabirite series of the Kedia was completed long before the Cambrian was laid down and there is no evidence on the hill to suggest that the itabirites along the crest were covered at any time by rocks later than the Cambrian other than localized surface canga and laterite. The tectonic conditions at F’Derik and Tazadit were evidently favourable for leaching throughout a period of Precambriantime,whose duration is at present unknown, and on to the present time.

The Breccia d’ldjil Various theories of origin have been suggested for this unusual type of rock. It forms a blanket stretching from the southern outcrop limites of the steeply dipping itabirites southwards for distances up to 7 km or more, to form ravine walls more than 100 m high along the southern boundary of the Kedia. Earlier observers who first met it exposed along its northern borders unhesitatingly called it a tectonic breccia. Those who first saw it in the southern ravines were equally certain it was a conglomerate,having rounded boulders of itabirite with a mainly siliceous cement. East of Rouessa the itabirite in situ is in places preserved in the process of crumpling up to form breccia, undoubtedly tectonic (Fig. 2) and if one assumes, as

FIG.2. Photograph of itabirite,east of Rouessa, crumpling up to form breccia (‘banded breccia’). 282

Enrichment of banded iron ore,Kedia d‘Idjil (Mauritania)

seems probable, that this movement was a part of the general orogeny of the Kedia, then the formation of the breccia was contemporaneouswith the uplift of the range. In the deep ravines of the south the ‘breccia’boulders are well-rounded and obviously water-worn,with occasional lenticles of sand. These cemented rolled boulders have been transported and cannot be matched against each other. One may conclude that the whole breccia/conglomerate was formed on a steeply sloping,foundering shoreline, with the boulders becoming rolled and worn in the off-shore depths. The ‘crumpled’breccia just mentioned has been mapped as an E.-W. zone of ‘bedded breccia’ between the unbrecciated itabirite (with localized enriched ore bodies) and the completely brecciated area. This transition zone has been included with the itabiritein Figure 1. At a number of places minor enrichments in hematite of the breccia and bedded breccia occur.

Types of enriched ore As previously stated,the banded ironstones are similar to the Brazilian itabirites.They have been locally enriched in iron by the leaching-out of the siliceous bands, with or without the introduction of secondary iron oxide. Where secondary iron oxide has completely taken the place of the leached silica, massive hard hematite ores of great purity have been formed, with an iron content of 67 per cent or more. The deposit of F’Derik,at the western end of the range, is mainly of this type. The primary bands are fine-grained and massive. The secondary bands are less compact, and in some cases specular, and their secondary nature is shown by local stringers from band to band, crossing the primary bands. In other deposits along the Kedia, where the leached-out silica bands have not been replaced by secondary iron oxide the residual hematite bands may still be held in place by local transverse developments, but generally they tend to collapse, forming biscuity fragments locally called ‘plaquetteores’.In some parts the siliceous bands originally contained dispersed tiny grains of iron oxide, and this insoluble hematite remains in some places after the leaching of the silica, as a fine-grainedpowdery ore. In the Rouessa deposits,located about the middle of the Kedia Range, both plaquette and powdery ores occur, and also a moderate amount of hard ore in which the silica bands have been replaced by secondary iron oxide (Fig. 3;Percival, 1967). The foregoing ore types are all formed directly in place of bedded itabirite, but comparatively small occurrences of ore, formed by the enrichment of the breccia in iron,are seen locally, e.g.at Azouazil, and on a small peak 500 m south of eastern Rouessa. They are also reported in the upper part of the O u m el Hbel valley. These occurrences are south of the alignment of the main high-iron enrichments of bedded itabirite. Their textures are those of the breccia, but the eiirichment in iron may be partly an impregnation rather than a replacement of the itabirite fragments; the secondary iron oxide fills up crev-

I

I

Imm

FIG. 3. Rouessa hard ore,borehole RBI, depth 16.4m;microphoto, incident light, showing bands of primary (massive) hematite and bands of secondary specularite,with a stringer crossing the massive bands.

ices and other cavities,and moulds itself round the fragments. Recognizable itabirite is largely doubly recrystallized, and has the appearance of having been under a distorting pressure that has caused a coarse recrystallization of the quartz,with a concomitant flow of the more mobile hematite. Fragments of enriched bedded hematite in these occurrences are rather rare,but such fragments do exist.

The Oum el Hbel conglomerate In the paper on Fort Gouraud by Lethbridge and Percival (1954) it was suggested that it was not unlikely that the bulk of the leaching of silica from the banded rocks took place in Precambrian times.This surmise was prompted by the fact that certain Precambrianconglomerateswith which the present author had been familiar in India contained 283

F.G.Percival

FIG.4. Oum el Hbel conglomerate, filling the Precambrian valley.

pebbles of enriched hematite ore of a type very like that of F’Derik.Consequently when Blanchot (1955) reported the existence of a Cambrian conglomerate,on the southeast flank of the Kedia, the writer arranged to visit this deposit to searchfor possibleboulders ofenriched hematite. The cement in the Cambrian conglomerate is mainly siliceous. The conglomerate in situ proved to be infilling a Precambrian valley and spreading over on to its flanks (Figs. 4 and 5). Boulders of hematite were so numerous in the lower levels of this valley as to suggest that there had been some kind of placer segregation by gravity. The first impression was that the problem of age of enrichment was solved, and the hematite boulders were taken to be of massive ore of the F’Derik type. Later examination of the specimens collected shows that this is far too simple a solution.The hematite boulders are well-rounded,and there is no doubt whatever that the enrichment was completed before incorporation in the O u m el Hbel conglomerate. These boulders are intensely hard, and more difficult to break than the massive hematite of F’Derik. Most of them have a lower density (3.9-4.4)than that of F’Derik hematite (5.0).The normal itabirite of the Kedia has a density of about 3.4.The density of the unenriched breccia is, of course, much the same as that of the itabirite. Microscopic examination reveals the presence of a good amount of silica in most of the hematite boulders,and the textures of nearly all of them examined to date are those of enriched brecchia,and not of normal leached and enriched itabirite. Certain exceptions will be described later. Small areas of enriched breccia may have been sources of the O u m el Hbel hematite boulders and,if so,the enrichment of the breccia was Precambrian.One may also concede that a small number of non-brecciated enriched hematite 284

FIG.5.Oum el Hbel conglomerate,close view.

Enrichment of banded iron ore,Kedia d’Idjil (Mauritania)

boulders and fragments from the Rouessa hard hematite outcrops further north could have travelled southwards down the valley,but the distance of the O u m el Hbel conglomerate from the outcrop of the belt of itabirite that lends itself readily to enrichment is at least 7.5k m .

Textures of the hematite boulders of the Oum el Hbel conglomerate Before discussing the make-up of the hematite boulders it should be emphasized that whether one has an almost pure hematite boulder or one that is only moderately enriched, each is an isolated boulder embedded in the siliceousmatrix of the conglomerate,and the majority of the neighbouring boulders are rounded lumps of breccia with no suggestion of hematite enrichment,i.e.there is no indication whatever of any enrichment in hematite after the incorporation of the boulders in the conglomerate.What is in questionis whether or not some occurrences of hematite within the enriched boulders are fragments of leached and enriched itabirite comparable with the enriched ores of F’Derik,Rouessa or Tazadit. In any event, accepting the Cambrian age of the conglomerate,the enrichment inhematitewas Precambrian. T o date the author has found only one conglomerate boulder completely made up of hard hematite, with only a very small content of silica. It has been sectioned in two planes at right angles to each other. Some indication of bedding or banding is seen, but it is not well marked, and here is no close resemblancebetween this hematite and the

I

FIG.6. O u m el Hbel conglomerate hematite boulder; inicrophoto, incident light, showing alternating bands of primary

enriched bedded ores. It is not brecciated. A few cavities within the hematite have allowed larger crystals of hematite to develop,and numeroustiny cavities occur,some ofwhich contain small amounts of recrystallizedquartz. These cavities show some alignment, but it is not so well marked as the banding of the bedded ores, yet it is possible that this specimen is a boulder of high-iron bedded hematite that has suffered some recrystallization. A second hematite boulder from the conglomerate contains two portions that seem to be derived from enriched bedded ore of the Rouessa type. One (Fig. 6) shows alternating bands OS dense and less-densehematite, but with a small amount of silica remaining as scattered quartz. The other (Fig. 7)shows hematite bands that are undoubtedly originalhematite bands from itabirite.They are still almost in their originalrelative positions,but the intervening silica bands have been leached away and little,if any, secondary iron has taken their place. As a result,we get a group of plaquettes. Thus, of a number of hematite boulders examined, only two have been Sound which contain texturesresembling the enriched ores of Rouessa. This may seem a meagre collection,but considering the distance of the O u m el Hbel conglomerate from the Rouessa-Tazadit ore bodies, it is surprising that any such textures at all have survived. Further search may produce more examples. Occasional doubts have been raised as to the exact age of the O u m el Hbel conglomerate. Blanchot (1955) does not claim that it is of basal Cambrian age,but that it is a local facies of the Cambrian sandstone, and the author sees no adequate reason to question this judgement.

J

Imrn

(massive) and secondary hematite, the latter less dense, with some residual silica (grey).

285

F. G.Percival

I mm

FIG.7.Oum el Hbel conglomerate hematite boulder; microphoto, incident light. Most of the silica has been leached from

this fragment, leaving primary hematite bands collapsed as ‘plaquettes’.(hematite =white, quartz =grey, cavities=black.)

Established conclusions regarding age of enrichment

coeval with the orogeny of the Kedia as a wholelong before Cambrian times. Thus the commencement of leaching and iron enrichmentof the itabiriteofthe Kedia (and locally of the breccia) was at some time between the age of breccia formation and the age of depositionof the Cambrian conglomerate of O u m el Hbel. 6. O n general grounds,as the uplift,folding and distortion of the itabirites of the Kedia were Precambrian,it is reasonable to expect that Precambrian surface waters would initiate the leaching of silica and the residual enrichment in iron, and tce evidence of the hematite boulders of the O u m el Hbel conglomerate tends to confirm this.

The chief conclusions may now be summarized as follows. 1. Although the hematite boulders of the conglomerate have not been analysed,it is quite obvious from their density and from the appearance of the sections and polished specimens examined that the hematite content is much higher than that of the local unaltered itabirites or of the local breccia. The breccia from which these boulders were derived had been enriched in iron, and this enrichment was markedly earlier than the age of the conglomerate,i.e. it was Precanibrian. 2. One of the boulders contains plaquette fragments that are evidence of leaching of silica from itabirite. Thus leaching of silica had certainly commenced in Precambrian times. 3. One boulder shows a fragment with coarser and finer hematite banding,but this is not completely convincing because some of the silica remains. 4. It would in any event be only rarely that enriched hematite pieces from Rouessa would be carried southwards over a distanceof 7.5km,but as only a small number of boulders have been examined further work may reveal more specimens of Rouessa hard ore type. 5. The apparent absence of fragments of hematite in the breccia in general certainly suggests that iron enrichment was not well developed when the breccia was formed,but the age of breccia formation may have been 286

Age of enrichment of deposits elsewhere Evidence of the age of leaching and enrichment of itabirite is in some cases inconclusive and in other cases the age may only be indicated within wide limits. Veriezrtela. Ruckmick (1963) published a study of the silica content of spring waters emerging along the lower flanks of the iron ore bodies of Cerro Bolivar, Venezuala, from which he deduced that ‘ifthe assumptionis made that present climatic conditions have prevailed in the past’ the rate of removal of SiO,‘suggestthat the Cerro Bolivar ores have been developing for approximately 24 million years, or since the Oligocene’. Ruckmick emphasized that his

Enrichment of banded iron ore,Kedia d’Idjil (Mauritania)

calculations ‘do not represent a precise dating method because of the assumptionsinvolved,and because of several factors,such as rates of physical and chemical erosion at the surface, which are difficult to assess’.In spite of this warning, it may be considered that there is proof that leaching and enrichment at Cerro Bolivar date back only to the Oligocene.One should not assume,however,that the whole of the silica content of the Cerro Bolivar waters has come from the siliceous bands of the itabirite. As to the assumption that present climatic conditions have prevailed in the past, such an assumption would certainly not be justified in the area of our present survey in Mauritania for example. This is n o w a desert region, but deposits of laterite and canga on the Kedia indicate a former humid monsoon type of climate, and the very large numbers of neolithic and earlier artefacts found in the dried-up kale bed of the Sebkha d’Erguya,20 ltm east of the Kedia, suggest that this humid climate prevailed within human times. South Africa anù South- West Africa. The age of enrichment of the ores of Postmasburg is given as late Precambrian by Boardnian (1952). Strauss (1952) is less certain about the ore of Thabazimbi,but considers it as possible that the ore may be Pre-Waterburg(late Precambrian) in age. India. In 1931 the author (Percival, 1931) published photographs of an Indian conglomerate composed of pebbles of enriched hematite-a conglomerate :hat Dunn (1940)took as the basal conglomerateof his (Precambrian) Kolhan series. Occurrences of this conglomerate,locally with enriched heinatite pebbles, are widespread in the Singhbhum-Orissairon ore field, as recorded by Percival and Spencer (1940), and there is no doubt that leaching of the banded hematite jasper, and enrichment in iron, had commenced in Precambrian times in this region. Locally the conglomerate forms a high-grade direct-shippingore. Dunn (1940)states that enrichment ‘tookplace at any stage in the geological history of this area when conditions were suitable’,and concludes that ‘wheneverand wherever these rocks were exposed at the surface,and subjected to circulating waters during their geological history, re-arrangement of Fe,O,was in progress’. Western Aiistualiu. Discussing the ores of the Hamersley Province of Western Australia,Campana (1966) states that ‘the high grade iron mineralization must have been favoured by the Tertiary climatic cycle’ and adds more emphatically that this Tertiary cycle ‘brought about the Hamersley iron field’,but MacLeod (1966), discussing the same area,writes that though this process has operated on a limited scale,and may indeed be proceeding at the present time, the restriction of iron-formation to such a limited

time poses severe difficulties. ‘It would have been impossible to produce ore bodies of such grade and dimensions within these residuals after maturity of the surface for the reasonthat therewouldnothavebeenenoughironavailable.’ MacLeod suggests as an alternative theory of origin that ‘the ores are of great antiquity and that their formation has been going on concurrently with the degradation of the land surface. . . .The ore bodies are,in effect,residues rich in iron,much of which has been derivedfrom overlying iron-formationn o w removed by erosion’. From this brief review of the published data on the ages of leaching and enrichment of itabirite to produce high-grade natural ores, it appears that further investigations are desirable to give greater precision than has yet been achieved.

Comment The results of the present investigation are rather disappointing, as they do not establish the existence of masses or lenses of bedded enriched hematite at the time of formation of the Cambrian conglomerate,although proof that such masses existed may yet be found. All that is firmly established is that the processes of leaching and enrichment had commenced prior to the deposition of the O u m el Hbel conglomerate,and w e may deduce that these processes have been continuously in operation since then, to form ore bodies that have a vertical depth below peak outcrop at Tazadit of more than 400 m,with diminishing thickness at this depth. At F’Derik,where the deposit appears to be entirely of the hard massive type,the ore has been proved by drilling to about 250 m below peak outcrop and,at this depth,the base of the enriched ore had not been reached. The iron for the secondary enrichment was presumably derived from the weathering of overlying itabirite, as suggested by MacLeod for the Hamersley area, and with such depths of enriched ore a considerable cover of itabirite has been removed. These deep ore bodies are compatible with a long period of leaching,but the rate of this leaching would vary with climatic variation,both directly through temperature and humidity, and indirectly through their effect on vegetation and resultant humic acids.

Acknow1edgerneii.t The author wishes to express his thanks to the S.A.des Mines de Fer de Mauritanie for their hospitality and assistance in his visits to the O u m el Hbel area, and for their consent to the publication of this material.

287

F. G.Percival

Résumé Enrichissement des minerais zoné,sde fer de la Kedia d’Ic$ìZ, en Mauritunie (F.G.Percival)

La Kedia d’Idjilest un inselberg de roches précambriennes qui s’élèveà quelque 500mètres au-dessusde la plaine saharienne. Les minerais de fer de la Kedia s’étendentle long d’une zone d‘enrichissement de l’hématitedansles itabirites, qui s’allonge dans une direction sensiblement est-ouest le long de la crête de la chaîne sur une distance d’environ 24 k m . Au sud de cette zone, la chaîne a une couverture de brèches formée de blocs fracturés (ou partiellement roulés) d‘itabirite.Des enrichissementsmineurs de labrèche, d’âge incertain, se rencontrent localement. Les itabirites ont des pendages raides vers le sud. A l’est, et recouvrant la bordure orientale de la Kedia, des grès cambriens ont un pendage très peu accentué vers l’est. A Oum el Hbel,sur la bordure sud-sud-estde la Kedia,on

rencontre un conglomérat formé esseniiellement de blocs roulés de la brèche, ce qu’on interprète comme un faciès local du Cambrien. C e conglomérat contient un nombre modéré de blocs d‘hématite enrichie,mais à ce jour l’examen de la plupart d‘entre eux a révélé qu’ils avaient la contexture de la brèche et non celle régulièrement veinée, lixiviée et enrichie des masses d’hématites de la zone principale du minerai. Cependant,il est évident que ces blocs ont été enrichis en fer avant d‘être incorporésdansleconglomératcambrien, et certains des fragments qu’ilcontient mettent en évidence une lixiviation de la silice,avec formation d’hématiterésiduelle, ce qui conduit à une date précambrienne au moins pour le commencement de ces processus. L’âge de la formation d‘hématites enrichies similaires qu’onrencontre ailleurs fait l’objetd‘une courte discussion.

Bibliography/Bibliographie AUDIBERT, J.; CARUEL, P.;CHOUBERSKY, A. 1964.Development of the Kedia d’Id$ orebodies (S.A.des mines de fer de Mauritanie) MIFERMA, Islamic Republic of Mauritania. Symposium on OpencastMining, Quarrying andAlluvial Mining.

London,Institutionof Mining and Metallurgy (Paper no.20), 34 p.

BALDWIN,A.B.;GROSS,W . H.1967. Possible explanations for the localization of residual hematite ore on a Precambrian iron-formation.Econ. Geol., vol. 62,p. 95-108. BLANCHOT, A.1955.Le Précambriende Mauritanie occidentale. Bulletin de la Direction fédérale des mines et de la géologie,

no. 17. Dakar. BLONDEL,F. 1952.Les gisements de fer de l’AfriqueOccidentale française. Symposium sur les gisements de fer du monde. XX Congr. géol. int., Alger, t. I,p. 7-9. BOARDMAN, L. G.1952. Short description of the Postmasburg iron ore deposits. Symposium sur les gisements de fer du Monde. XIX Congr. géol. int., Alger, t. 1, p. 252-6. CAMPANA, B. 1966.Stratigraphic-structural-palaeoclimaticcontrols of the newly discovered iron ore deposits of Western Australia.Mineralium Deposita, Berlin, vol. 1, no. 1,p. 53-9. D u m ,J. A. 1940.The stratigraphy of South Singhbhum.M e m . geol. Surv. India, LXIII,part 3, p. 303-69. GROSS, G.A.1968. Geology of iron deposits in Canada.VoI. III Iron Ranges of the Labrador GeosyncZine. Ottawa, Geological Survey of Canada.179 p. (EconomicGeology Report No.22.) GRUNER, J. W.1946.The minera ogy andgeology of the Taconites and iron ores of the Mesabi Range, Minnesota. p. 1-127.

St. Paul, Minn., Commissioner,Iron Range Resources and Rehabilitation. GUILD,P. W.1953. Iron Deposits of the Congonhas District, Minas Gerais,Brazil. Econ. Geol., vol. 48, p. 639-76.

288

-1957. Geology and mineral resources of the Congonhas ,

District,Minas Gerais, Brazil, Prof. Pap. U.S.geol. Suvv., no. 290, p. 1-90. HUVELIN, P. 1963. Précisions sur la genèse de la brèche d‘Idjil (Fort Gouraud,Mauritanie). Chronologie de la formation des minerais de fer et de la brèche. Bull. Soc. géol. Fu.,t. IV, no. 2, p. 322-8. LETHBRJDGE,’ R. F.;PERCNAL,F. G.1954. Iron deposits at Fort Gouraud,Mauritania,French West Africa. Trans. Instn. Min. Metall., Lond., vol. 63, p. 285-98. MACLEOD, W.N.1966.The geology and iron deposits of the Hamersley Range area,Western Australia, Bull. geol. Surv. W.Airst., no. 117,p. 1-170. PERCIVAL,F.G,1931.The iron-oresof Noamundi.Trans. Min. geol. Inst. India, vol. 26, p. 169-271. -. 1967.Possible explanationsfor the localization of residual hematite ore on a Precambrian iron-formation.Discussion. Econ. Geol., vol. 62, p. 739-42. PERCIVAL, F. G.; SPENCER,E. 1940. Conglomerates and lavas in the Singhbhum-Orissairon ore series. Trans. Min. geol. Inst. India, vol. 35, p. 343-63. ROYCE, S. 1948. Discussion on paper by Roberts,H.M.and Bertley,M . W.-Replacement hematite deposits,Steep Rock Lake, Ontario, Trans. Anier. Inst. min. (metall.) Engrs., vol. 178, Mining Geology, p. 387. RUCKMICK, J. C. 1963. The iron ores of Cerro Bolivar,Venezuela,Econ. Geol., vol. 58, p. 218-36. STRAUSS,C. A. 1952.The deposits mined by the South African Iron and Steel Industrial Corporation Limited. Symposium sur les gisementsde fer du monde,XIX Congr.géol.int.,Alger, t. 1, p. 241-52.

Enrichment of banded iron ore,Kedia d’Idjil (Mauritania)

Discussion R . P.PETROV.Are the terms ‘magnetite-hematite quartzite’ and ‘itabirite’synonymous?What is the difference between itabirite and taconite?

F.G.PERCIVAL. There is great need for agreed definitions of itabirite,jaspilite, taconite, and also canga. I shall be very glad if some agreed definitions can be an outcome of this symposium. S. J. SIMS.Is there any structural control of the localization of the high-gradehard hematite?

F.G.PERCIVAL.There is no separate structural control for the hard high-graciehematite. All three types-hard, plaquette and powdery-occur Tazadit.

together at Rouessa and at

S. J. SIMS. D o you consider specularite to be of supergene origin?

G.CHOUBERT. If I remember correctly, the sandstones around the southern part of Kedia d’Idjilare not Cambrian, but older, approximately Upper Precambrian. They are older than the stromatolite limestones of Atar-Hanck, which are about 700-900 m.y. old (by determination of M m e Bertrand and M m e Raber).

F. G.PERCIVAL.Yes, I wrote that there is some doubt as to the exact age,but this still leaves the date of leaching as Precambrian.

G.CHOUBERT. What is the relationship betwzen the Kedia d‘Idjil quartzitesand the ancient granites on the north? D o the granites break through the quartzites,or do the quartzites overlie the transgressive granites?

F. G.PERCIVAL. N o boreholes have been made to reveal the junction of the Kedia iron-formationand the granites and I have no certain information on this.

F.G.PERCIVAL.Yes, but the supergenewaters may locally have been heated by intrusive veins of dolerite that cut the ore bodies.

289

Iron ores of the Hamersley Iron Province, Western Australia W.N.MacLeod Carpentaria Exploration Company Pty Ltd,West Perth

Introduction Systematic exploration of the iron ore potential of the Hamersley Range area of Western Australia commenced early in 1961. The region had long been recognized as one in which iron deposits were likely to occur as banded ironformations of great thickness and lateral extent had been recorded during the early geologicalreconnaissance surveys of the region. Production and export of iron ore commenced in 1966 from the Mount T o m Price deposit,operated by Hamersley Iron Pty Ltd. This was followed by the initiation of production from the Mt Whaleback deposit in 1969 by the Mount N e w m a n Mining Co. The productive capacity of the operating mines is now in excess of 30 million tons per annum. Reserves of ore containing more than 50 per cent iron are estimated to amount to 18,000million tons of which over half is comprised of hematite-goethite ore containing between 58 per cent and 65 per cent iron.

Physical features The Hamersley IronProvince is situated in the North-West Division of Western Australia, about 1,000k m north of Perth.The region is mountainous and arid and was virtually uninhabited prior to the commencementofiron ore mining. The iron province is defiled by the extent of the Precambrian Hamersley Group of sedimentary and volcanic rocks. This stratigraphic unit was originally deposited in a discrete sedimentary basin at least 85,000 km2in area. The distinctive lithology of the group is matched by an equally distinctive topography.The resistant banded iron-formations,totalling over 1,000m thick and separated by softer dolomiteand shale beds,weather out to bold sinuousridges and plateaux rising up to 500 m above the intervening valleys. The majority of hill summits in the region are gently domed and concordant in level.These domes and plateaux are remnants of an older land surface, possibly of early

Tertiary age. Stream rejuvenation,with a new erosion cycle commenced in later Tertiary time and has continued to the present day. This appears to have been initiated by a major regional upward of the entire Hamersley block. Most of the present rivers flow in valleys with steep-walledgorges in the lower parts and gentle upper slopes upon which are preserved deposits of older detrital material. This ancient profile is of considreable economic significance. Many of the large hematite deposits are found in erosion residuals of this older surface at higher levels. In the valleys conglomeratic ores and pisolitic limonite deposits form part of the lower ancient profile and also remain as erosion residuals.

Stratigraphy and lithology The Hamersley Group of Lower Proterozoic sediments and lavas was deposited in an ovoid basin about 500 km long and 250 k m wide.The group forms part of a major stratigraphic unit referred to as the Mount Bruce Supergroup which is a conformable succession of sediments and lavas about 10,000m thick. Radiometric age dating indicates that the Mt Bruce Supergroup was deposited between 2,200 and 1,800m.y. ago. It is subdivided as follows. TOP Wyloo Group (3,100m) Hamersley Group (2,700m) Fortescue Group (4,200 m) Banded iron-formation, chert and dolomite are the principal constituents of the Hamersley Group. No coarse clastic sediments have been recorded from the succession. The chemical sediments have been intruded by thick dolerite sills and are interbedded with acid lavas in the upper part. Banded iron-formations constitute over one-third of the total thickness of the group and these are the host and source rocks of the iron ore deposits. The sedimentary units of the Hamersley Group are remarkably persistent in lithology and thickness throughout the entire iron province.

Unesco, 1973. Genesis of Precainbriun iron und ~nuizguiiesedeposits. Proc. Kiev Synzp., 1970.(Earth sciences, 9.)

291

W.N.MacLeod

Table 1 illustrates the stratigraphical subdivision of the group.

TABLE 1. Subdivision of the Hamersley Group

,

Lithology

Formation

Thickness (metres)

TOP

Boolgeeda Iron Formation

-

Woongarra Volcanics Weeli Wolli Formation

Brockman Iron Formation ,

Mt McRae Shale Mt Sylvia Formation Wittenoom Dolomite Marra Mamba Iron Formation

Iron-formation, ferruginousshale Rhyolite flows,tuffs, iron-formation Iron-formation,shale, dolerite sills Iron-formation,chert, shale,dolomite Shale,siltstone, dolomite,chert Iron-formation,shale, chert Dolomite,shale,chert Iron-formation,chert, shale

220 600 550

675 1O0

40 150 200

Base

The iron ore deposits are associated with, and derived from, the three principal iron-formations, the Marra Mamba,Brockman and Boolgeeda.Of these,the Brockman Iron Formation is by far the most important and is the host and source rock for all the largest known deposits in the province. The Brockman Iron Formation has been subdivided into four members as follows(Trendalland Blockley,1970). TOP Yandicoogina Shale Member (100m) Joffre Member (360 m) Whaleback Shale Member (30 m) Dales Gorge Member (180 m) It was early recognized during the initial exploration of the province that most of the major hematite deposits occurred within the Dales Gorge Member. Further work has shown that the Joffre Member can also be an important host for ore under optimum structural conditions.

Structure of the Hamersley Iron Province The Hamersley Group sediments occupy an intermediate position between the generally flat-lyingrocks of the Fortescue Group and the quite strongly folded Wyloo Group on the southern side. The northern half of the Hamersley Basin is virtually undisturbed with the rocks gently warped 292

into a broad synclinorium with average dips of less than 5". There are local zones within this broad structure with strong,flexuringand faulting,but such features die out over short distances. The central zone of the basin is much more strongly folded and the topographic forms faithfully reflect the major structural units. T w o superimposed fold trends can be recognized,lying almost at right angles,and the combination of these has produced a striking echelon pattern of domes and basins.The limbs of the major structures in this part of the province dip between 30" and GO",but complex secondaryand tertiary fold patterns have been superimposed on these major structures,particularly in fold axial zones and adjacent to major fault zones. It is these minor structures that provide the most important loci for iron ore deposits within the province. Practically all the major ore bodies occur within this zone of moderate to strong folding in or adjacent to synclinal troughs in the parent banded iron-formation.In the extreme southern and western parts of the province tectonic forces have been at their strongest, with the development of major faults and tectonic slides which disrupt the normal stratigraphy. Within the basin there is a complete range from virtually flat-lying,undisturbed rocks to those which have been overturned and isoclinally folded. Despite the variations in intensity of folding the grade of metamorphism remains very low,and these iron-formationsprovide one of the best examples in the world ofunmetamorphosedbanded iron-formation.

Iron ore deposits Four principal types of iron ore have been recognized in the HamersleyIron Province.In a sense these ore types are transitionaland reflect differing degrees of the protracted processes of enrichment of the parent banded iron-formation and the differing environmentsin which the enrichment occurred. The ore types are summarized as follows. Hard mussive hematite oye (blue ore). Iron content usually greater than 64 per cent rising as high as 69per cent. Phosphorus generally less than 0.05 per cent and silica and alumina less than 1 per cent. L o w content of combined water. Hematite in randomly oriented small bladed crystals is the dominant mineral with only a minor content of goethite. Bunded hematite-goethite oye. Laminatedtexture of the original iron-formationis well preserved with alternating bands ofvariablehematite-goethite ratio.Sometimesporous with interlaminate cavities. Iron content within the range of 58-64 per cent iron,phosphorusup to 0.15per cent,but averaging about 0.10 per cent. Silica and alumina up to 3 per cent and combined water content as high as G per cent. This is the commonest ore type in the province and occurs in intimate admixture with, and is transitionalto,the hard massive hematite. Coriglomevutìc ores. A cemented scree of fragments of massive hard hematite in a matrix of friable goethite and

Iron ores of the Hainersley Iron Province,Western Australia

limonite.Usually occurs in association with the types described above and is intermediate in composition. Pisolific limonite oye.Confined to older drainage channels as terraces and mesas. Complex mineralogy with hematite,maghemite,goethite and limonite. Contains 50-60 per cent iron with low phosphorus and high water content of 10-12 per cent.

Hematite and hematite-goethiteores G E N E R A L CHARACTERISTICS

The hematite and hematite-goethiteores occur within the banded iron-formationof the Boolgeeda, Brockman and Marra M a m b a Iron Formations and minor local enrichments have been recorded in the thinner iron-formationsof the Weeli Wolli Formation. U p to the present time, all deposits of economic significance occur in the Brockman Iron Forniation and particularly,although not exclusively, in the Dales Gorge Member of this formation. The three principal iron-formationsare generally similar in over-allmineralogical composition (Table 2), with an original iron content of about 30 per cent, with the iron mainly in the form of magnetite. Silica and carbonate minerals comprise most of the remainder of the rocks with accompanying minor amounts of iron-bearing phyllosilicates, notably stilpnomelane and amphiboles, including riebeckite and crocidolite. The hematite-goethite ore bodies have originated by processes of supergene enrichment of this primary ironformation material and, in the province as a whole, all stagesof thisenrichinentcan be observed.Probably all ironformation exposed at or near the present surface is altered and enriched to some degree. The essentially transitional character of the process is observable both from the examination of outcrops and from the chemical data provided by the analysis of drill core. It is postulated that ground and meteoric waters are the principal agents of enrichment of the parent iron-formationand that the principal processes involved are the selectiveremoval in solution of most of the silica and carbonate and the concurrent redistribution and recrystallization of iron oxides within the system. Hematite-goethite ore zones occur in practically all areas where the BrockmanIronFormationis exposed.These range in extent from a few hectares to several square kilo-

metres and the thickness of the ore from a thin surface crust of a few metres thick to deep trough-like occurrences between 100and 300m thick.

DISTRIBUTION A N D M O D E O F O C C U R R E N C E

The largest hematite ore bodies are found along the major central axis of the elliptical Hamersley Basin and to the south of this axis. This distribution is clearly related to the stronger folding and faulting of the iron-formationsin the southern half of the basin. In the northern half, that is in the main Hamersley Range Synclinorium,ore occurrences are restrictedin size and grade in unfolded areas,but appear in areas of localized flexuring along the flanks and troughs of synclines.In some areas there is an extensive development of platy hematite-goethite ore on remnants of the older surface even where the rocks are unfolded. Such ore zones may contain a very substantial tonnage of enriched material but the grade is variable and such deposits would present more mining difficulties than the deeper and more compact ore bodies now being worked. The principal areas of ore deposits are in the Mt Brockman and Mt Turner Synclines,the Weeli Wolli Anticline, the Parraburdoo Range and the Ophthalmia Range and its satellite hills. These areas account for over 80 per cent oî the immense reserves of hematite ore known in the province and will doubtlesscontinue as the main productive areas for many decades to come.

Genesis of the hematite ores The hematite-goethite ore bodies of the Hamersley Iron Provinceappear to have originated as a result of the enrichment of the parent banded iron-formationunder supergene conditions.This is common to all hematite deposits in the province. Such variations as do exist in grade, ore texture and size of the deposits can be attributed to variations in the degree to which these supergene processes have operated. There is no convincing evidence to suggestthat the ores represent primary depositionalconcentrations.A hypogene origin seems equally improbable as the iron-formations surrounding the deposits are unmetamorphosed and have not been affected by igneous intrusions to any degree. 'The

TABLE 2. Compositionalrange of iron ores from the Hamersley Iron Province(in percentages) Brockman Iron Formation

Platy leached ore

Compact banded ore

Massive blue ore

Conglomerate ore

Pisolitic limonite ore

Fe P SiO,

25-40 0.05 40-55 1.5-3.5

58-63 0.05-0.15 2-5 0.1-0.3 0.01-0.05

66-69 0.02-0.05 0.1-0.5 2-3 0.01-0.05

60-64 0.05-0.10 2-4 2-3 0.01-0.05

52-60 0.02-0.06

A120,

55-60 0.05-0.15 6-8 2-3 0.01-0.05

S

Less than 0.5

4-10 1-3

0.01-0.05

293

W.N.MacLeod

Hamersley Group rocks are not intruded by granite and although there are many doleritic sills and dykes in the sediments,these have limited contact metamorphic effects only and cannot be regarded as agents for the large scale metasomatic transformations involved in the formations of these immense iron ore bodies. Meteoric and ground waters, operating over a long period of time, are believed to be the principal agents for the transformation and enrichment of the parent iron-formation.These agents have operated most effectively under certain repetitive structural controls. From the textural and mineralogical characteristics of the ores it can be inferred that the following processeshave operated during enrichment. 1. Leaching of silica and carbonatesfrom the protore,leaving an enriched residual product containing between 50 per cent and 60 per cent iron. The original magnetite is oxidized to hematite and some is hydrated to produce goethite and other hydrous iron oxides. 2. Infilling of the cavities and planar voids in the leached ore by reprecipitatediron oxides,principally goethite,to produce a more compact ore type in which remnants of the original bedding are still clearly discernible. The compactore commonly containsbetween 60 per cent and 64 per cent iron;silica is lower than the leached cavernousore,but thereis littledifferenceinphosphoruscontent. 3. Recrystallization of the iron oxides and dehydration to produce a blue-grey massive hematite ore which is virtually structureless.The material approaches the theoretical composition of pure hematite (69.94 per cent). Silica and alumina together amount to less than 1 per cent and the content of combined water has dropped to less than 0.5 per cent. The massive blue ore appears to represent the culmination of the process of enrichment. It is less commonly seen close to the surface and seems to be most abundant in the deeper parts of well-definedsynclinal troughs, as at Mt Whaleback. The observed top of the ore is usually a remnant of the old surface and commonly the ore extends downwards from this surface to the base of the Brockman Iron Formation against the underlying Mt McRae Shale. This consistent relationshipbetween the ore bodies and the old surface led earlier workers in the province to the belief that the hematite deposits were developed after,or in a late stage of, the attainment of maturity of this surface (Campana et al.,1964; MacLeod, 1966). That this process has operated on a limited scale seems undeniable. Many of the shallow hematitecrusts on all the major iron-formations probably have resulted from late stage surface enrichment in relation to the old land surface and could be classed essentially as laterites.However,to restrict the main period of ore formation to a limited period during the maturity of the old surface poses some severe difficulties when applied to the formation of all major ore deposits in the province. During the earlier erosion cycle, dissection of the Hamersley Group sediments had proceeded far enough to leave many thin and isolated residuals of the Brockman 294

Iron Formation as erosion residuals in synclinalcores. It is clear that segmentation of the iron-formationsby erosion was well advanced prior to the attainment of maturity of the Hamersley surface and well before the initiation of the present erosion cycle. Many of these isolated residuals provide some of the largest hematite deposits known in the province and are the sites of an almost wholesale transformation of Brockman Iron Formation into hematite-goethiteore. It would have been impossible to produce ore bodies of such grade and dimensions within these residuals after maturity of the surface as there would not have been enough iron available. The erosion residuals would have become closed systems and the scale of iron enrichment and concentration that has occurred could be expected to leave profound and observablechangesinthe surroundingiron-formation.Such changes are not seen and,infact,most of the iron-formation existing beside the ore bodies in the residuals is itself enriched in iron to some degree. An alternative theory of origin, which accords better with the observed characteristics and distribution of the ore bodies, is based on the proposition that the ores are of great antiquity and that their formation has been going on concurrently with the degradation of the land surface, and particularly with the continuing erosion of the Brockman Iron Formation.In short,the present ore zones represent the end products of a long continued process of physical breakdown by erosion and chemicalconcentration of iron in the original iron-formation.The ore bodies are, in effect, residues rich in iron much of which has been derived from formerly overlying iron-formation,now removed by erosion. The progressive downward enrichment of the ironformation has been effected by the percolation of waters to the water table from the time when the iron-formation was ñrst exposed by removal of the overlying cover of younger rocks. Rainwaters falling on the exposed ironformation dissolve minute quantities of iron and silica, which are carried down to near the water table. At the water table the iron is oxidized and precipitated in the ferric state but silica remains in solution. The zone of seasonal fluctuation of the water table thus becomes secondarily enriched in iroii. In folded areas and where there are strong fault zones,the movements of the groundwaters are canalized in impounding structures such as synclinal troughs or zones of intense drag folding on the limbs of major folds.It is in such sections that the process of enrichment has culminated and these are the loci of the largest and highest grade ore bodies in the province. In Brazil Dorr (1964) observed that the maximum degree of supergene enrichment of itabirite was to a product containing 63 per cent iron. From the analysis of a large tonnage of ore he placed an upper limit of 65.5per cent iron for enrichment by purely supergene processes. H e concluded that hematite cannot have formed directly by supergene enrichment and that an appreciable amount of goethite must remain. These conclusions are applicable to many ore zones

Iron ores of the Hamersley Iron Province,Western Australia

in the Hamersley Iron Province,but in the two large ore bodies now being mined the process of enrichment has proceeded well beyond the upper limit set by Dorr. Both contain a substantial proportion of a massive very pure hematite closeto the theoreticalcompositionand containing minimal amounts of goethite,silica,alumina and combined water. The pure hematite ore is closely intermingled with, and often transitional into,lower grade hematite-goethite ores so that it is difficult to dissociate the pure ore from the same pervasive cycle of enrichment that has produced the hematite-goethite ore and, at an earlier stage, the partially enriched iron-formation. T o achieve such purity it seems necessary to invoke metasomatic replacement of pre-existing ore and direct crystallization of hematite. Microscopic examination provides evidence that this has actually occurred,as the purer massive ore is seen to consist of plates and needles of hematite to the virtual exclusion of the martite octahedra which are so abundant in the hematite-goethiteores. These plates and needles appear to be a late stage crystallization and it is perhaps of some signiñcancethat the pure massive ore mainly occurs at depth.Inthe case of the Mt Whaleback and Mt T o m Price deposits these depths are of the order of between 100 and 200 m and, of course, these depths may have been much greater at the time when the material was formed. The recrystallization of hematite may be a temperature effect dependent on depth. A n implication of the theory of genesis by progressive downward percolation of iron rich groundwaters and concurrent erosional reduction of thickness of the ironformation, is that there may be substantial deposits of hematite in synclinal troughs which have not yet been eroded sufficiently deeply to expose the full extent of the zone of basal enrichment. The occurrence of very high grade ore at depths of over 300 m in the Mt Whaleback deposit is probably not a unique occurrence, although in that particular case the folding is strong and very deep troughs exist in the parent iron-formation. Enrichment of the iron-formation is probably influenced by variations in permeability. Folded and faulted areas, in addition to providing definite structures for the concentrationand channellingofwater movement,probably offer increased freedom of water movement through a host of minute fractures in the minor folds and faults. Minor folding extends down to microscopic amplitudes and such violently crenulated zones are common sites of ore formation. There is a frequent occurrence of major ore bodies near strong faults, as at Mt Whaleback and Parraburdoo.

Pisolitic limonite ores DISTRIBUTION

The pisolitic limonite ore occurs as residual cappings of rnesaform hills and flat-toppedridges aligned along the

courses of fossil drainage channels. In some areas it occurs as broad valley-fill deposits at the confluence of internal drainage systems. This ore type is represented in practically every drainage system in the province which in the past has drained extensive areas of the Brockman and Marra M a m b a Iron Formations.There can be little question that these highly ferruginous sediments are the ultimate source of the iron in the secondary riverine and lacustrine accumulations of the pisolitic limonite.The maximum development of this ore type is on the western side of the iron province in the Robe River and Duck Creek drainage systems. The grade of the pisolitic limonite (Table 2) ranges within the limits of 40-60 per cent iron, with the bulk of the material falling within the range of 52-58 per cent. Drilling has shown that there are mineable sections within many of the deposits with an average grade of 55-58 per cent iron. This richer ore has a total silica and alumina content well below 10 per cent and a combined water content of about 10-12 per cent. Phosphorusand sulphur are normally less than 0.05per cent and titania less than 0.20per cent. The resources of this ore type in the Hamersley Iron Province are estimated to be at least 6,000 million tons. U p to the present it has not been utilized, although it has been established that the ore can be pelletized to a product containing between 63 per cent and 65 per cent iron. It constitutes an important reserve for the future, but at present suffers in competition with direct shipping higher grade lump hematitic ores from elsewhere in the province. TEXTURE A N D M I N E R A L O G Y

The iron oxides present in the pisolitic ore include amorphous isotropic limonite, goethite,hematite and maghemite. These minerals occur as components of the pisoliths and in the matrix between the pisoliths. The pisolitic texture of the ore is its most striking feature,both in hand specimens and under the microscope. The majority of the pisoliths range between 1 and 3 mm in diameter, exceptionally attaining a size of 5 mm, and ranging down to diameters of 0.1 mm or less. Many of the larger pisoliths are made up of aggregates of smaller pisoliths. The matrix between the pisoliths usually consist of colloform isotropic limonite which is yellow to brown and often soft and ochrous, or black lustrous goethite. In some zones the limonitic matrix is continuous and imparts to the ore a vitreous lustre and sub-conchoidal fracture; in other zones there is a high porosity due to discontinuity of the matrix and such material is less coherent. Cavities and pore spaces are often partially filled with opaline silica and travertine. The pisoliths themselves are greatly diversified in colour, texture and mineralogical composition. Many are soft and ochrous and can be scratched and disintegrated 295

W.N.MacLeod

with the fingernail whereas others are hard and metallic. Under the microscope most pisoliths are seen to be composite bodies with pronounced concentric layering which reflects successive stages of accretion of iron oxides. The majority of pisoliths are cored with hematite and around these cores there is a succession of thin, alternating shells of goethite, hematite and maghemite. Harms and Morgan (1964)state that the proportion of hematite in the pisoliths decreases with increasing distance from the source iron-formations,aiid in the Robe River deposits the overall hematite content ranges from nil to 20 per cent. Some zones of the ore contain a high content of fossil wood fragments,now completely replaced by hematite or limonite with good preservation of the original cellular structure. Clay beds and lenses are commonly interbedded with the ore and a common feature is the appearance of clayfilled joints and pipes. Many of the deposits have a pronounced vertical jointing which is attributed to shrinkage during diagenesis and these joints have permitted the ingress of fine clays from above. GENESIS O F T H E PISOLITIC LIMONITE ORES

It seems a reasonable assumption that the deposits have a common mode of origin and all were formed at or about the same time. The occurrence of identical deposits in association with Archaean iron-formationselsewhere in the region, which are of similar age and geomorphological relationship to those derived from the Proterozoic sediments suggests that conditions favouring this type of iron ore accumulation were widespread. From the consistent distribution pattern of these deposits in relation to long established drainage systems emanating from plateaux and ranges of iron-formation, there can be little doubt that these rocks comprise the ultimate source of the iron in these secondary riverine deposits. Theories differ, however, as to the mechanisms involved in the transport of the iron and of the processes which have effected the deposition of the limonite. These concepts can be summarized as follows. 1. Direct clzemical precipitation as bog iron ore. This theory suggests that the iron accumulations are the product of chemical precipitation of iron directly from iron-chargedsolutions which have moved into the drainage channels,Chemical conditions in the channels have been such as to favour rapid deposition of iron. 2. Replacement aiid desilicatiotz of iron-formation detritus in the river channels by iron charged solution. This theory favours the transport of some iron into solution into the drainage channels but suggests that the limonite formation has been effected within a matrix of ironrich detritus which itself makes the major contribution to the final volume of the deposit. 3. Clastic accumulation of ferruginous detritus derived by weathering oJ the enriched iron-jormution. This theory is a derivation of the previous one,but differs in its sugges296

tion that much of the iron enrichment and hydration of the iron oxides has occurred as a result of weathering of the iron-formations in situ. Erosion has stripped weathered profile of ferruginousmaterials and deposited them in low gradient stretches of the drainage channels in the similar fashion to the deposition of a placer deposit. All three theories are in part complementary,and all have some measure of credibility as an explanation for all or some of the features of the deposits. However, it is felt that no single process can account for all the features of the limonite deposits as observed in the province as a whole.In all likelihood,all three processes outlined above, and possibly others not yet visualized, have operated to different degrees in different parts of the region. It is felt that the most important clue as to the genesis of the pisolitic limonite is provided by the geomorphologicalrelationshipsofthe deposits.As discussed previously, the most outstanding geomorphological features of the iron province are the widespread remnants of an earlier mature landscape of gently domed hills separated by broad valleys in which there are thick accumulations of colluvium. Iron-formation fragments and chert are practically the sole constituents of these colluvial deposits. In many localities in the headwater sections of the drainages an intimate and apparently transitional relationship between the pisolitic limonite and iron-formationscree can be observed. Development of the limonite only seems to occur on a major scale where there has been a long persistent drainage system. The limonite is less abundant where the drainage has been meandering or diffuse. The intimate relationship between the iron-formation scree and the pisolitic limonite is observable in the terraced deposits of the older colluvium in the headwater sections of the rivers. The gently sloping terraces merge upwards with the gently domed hills of Brockman Iron Formation on the one hand, and on the other merge outwards with mesaform remnants of pisolitic limonite in the drainage channels. In the upper sections of the terraces,away from the river,there is an abundance of cemented iron-formation scree locally transitional into hematite conglomerate or canga ore. On passing towards the centre zone of the drainage channel,there is commonly a zone of complex interminglingof pisolitic and conglomeratematerials which are physically contiguous with, and directly gradational into,pisolitic limonite.This limonite is essentially the same material as that found well downstream in the mesa deposits beyond the hills. It can be postulated that the pisolitic limonite in the river channels, and the intermingled zones of pisolite and conglomerate in the montane areas,are both manifestations of the same protracted cycle of transformation of ironstone scree to pisolitic limonite that has proceeded concurrently with the evolution of the old land surface. As the landscape has been progressively degraded, the iron-formation detritus has been continually fed into persisting drainage systems. The movement of ground and surface waters towards and along these channels has

Iron ores of the Hainersley Iron Province,Western Australia

effected a progressive transformation and enrichment of the detritus resulting in an equilibrium end product of pisolitic limonite. The transformation of the detritus would involve the introduction of iron and the selective removal of the bulk of the silica, and is essentially the same chemical transformationas is envisaged for the formation of the hematite ore zones. The drainage channels can be regarded as fulfillingthe same canalizing roles as synclinal troughs in the formation of in situ hematite deposits. This concept implies a cogenesis of the hematite and pisolitic ores;both have originated by the enrichment of banded iron-formation and the agents of enrichment have been ground and surface waters operating over a long period of time within restricted spatial limits. The process is not regarded as being initiated or dominated by a particular set of climatic or geomorphic controls. Rather it is a long continued progressive cycle of erosion, enrichment and downstream movement of the products of enrichment. For this reason, and for the lack of stratigraphic evidence, no definite age can be offered for the pisolitic limonite deposits. The process probably commenced when there were sufficiently large areas of iron-formationexposed to erosional processes to provide a predominance of iron-richscree to the drainage channels and continued until the rejuvenation of drainage in the current erosion cycle. Most rivers since then have established new base levels leaving the pisolitic ores, for the most part, well above water table.

Conglomerate ores The conglomerate ores commonly occur in association with the large hematite-goethitedeposits as scree mantles on the lower slopes of the hills. Some conglomerates are simply cemented iron ore scree shed from the iron ore zones, whereas others appear to have a more complex origin and appear to be the product of enrichment of ironformation scree by essentially the same processes of leaching of silica and iron migration as has occurred in the in situ hematite deposits. In the upper terraces of many of the river valleys there is an almost complete transition from pisolitic limonite in the lower part of the valley to conglomerate ore consisting of angular fragments of almost pure hematite set in a pisolitic limonite matrix. Most of the conglomerate ores occur in this terraced fashion as erosion residuals on mature valley terraces. The conglomerate ores have been assessed in some areas and appear to be intermediate in composition between the hematite-goethite ores and the pisolitic limonite (Table 2). Where they occur in large quantity they are an attractive mining proposition as crushing costs are low and the material is upgraded several per cent on crushing with loss of limonite fines.

Résumé Les minerais de jeu d’Harneusley, en Acwtualie occidentale (W.N.MacLeod) La province de Hamersley,en Australie occidentale, contient des réserves estimées à 17 milliards de tonnes de minerai d’hématite-goethiteréparties entre quelques centaines de zones de minerai sur une surface qui couvre approximativement 85 O00 m z. L a capacité de production actuelle de la province est de l’ordre de 35 millions de tonnes par an et pourra dépasser 50 millions de tonnes vers 1972. Les minerais de fer sont rattachés à trois formations de fer,rubanées,épaisses et étendues,totalisant 1 O00 mètres en épaisseur à l’intérieur du groupe d’Hamersley du Protérozoïque inférieur. Parmi les gisements de minerais, celui qui est le plus important et dont la teneur est la plus élevée se rencontre dans la formation de fer de Brockman, d’une épaisseur d‘environ 650 mètres, et surtout dans la partie inférieure de cette formation appelée Dales Gorge Member et épaisse d’environ 180 mètres. On pense que les minerais ont eu leur origine dans l’enrichissement supergène de ces formations de fer sous l’influence des eaux souterraines et météoriques. Les for-

mations de fer sont fortement plissées localement,mais ne sont pas métamorphosées et n’ont pas été affectées par les intrusions ignées de façon perceptible. Les formationsde fer apparentées contiennent entre 30 et 40 % d‘oxyde de fer -surtout de la magnétite,le reste de la roche étant constitué par de la silice sous forme de silex - et des carbonates minéraux, en particulier de la dolomite ferreuse.L’enrichissement s’est produit à la suite de processus complexes mettant en jeu l’oxydation et l’hydratation des oxydes de fer, la lixiviation sélective et le déplacement de la silice et des carbonates et le déplacement et la redéposition d’oxyde de fer à des niveaux inférieurs dans le système. Les dépôts de minerai de fer sont considérés comme des résidus altérés chimiquement, en équilibre avec l’environnementprésent,qui représentent le produit final de la décomposition physique et chimique prolongée des couches de la formation de fer originale. On considère que l’enrichissements’est produit depuis que les formations de fer sont entrées pour la première fois dans le circuit des agents météoriques,et,bien qu’influencé par les changements de climat, le processus général n’a aucune relation avec aucun cycle climatique. Dans certains dépôts, il y a quelque évidence d’une 297

Iron ores of the Hamersley Iron Province,Western Australia

cristallisation ultérieure du matériel enrichi aboutissant à un produit ñnal d‘une composition proche de la composition théorique de l’hématitepure. Certains sont en faveur d‘une origine épigénétique pour ce type de minerai. Les minerais d’hématite-goethitese sont plus favorablement développés dans les régions de plissement modéré ou violent ou au voisinage des zones de grandes failles. Les fosses synclinales sont un moyen de contrôle de la structure. De plus, la province contient de grands dépôts d’un minerai secondaire limonitique et pisolitique qu’on trouve

essentiellement dans les anciens canaux de drainage. On n’est pas fixé sur l’origine de ce type de minerai, mais il y a quelque raison de penser qu’il a une double origine : dépôt direct des oxydes de fer en solution et enrichissement par les eaux souterrainesdes dépôts considérablesde détritus riches en fer accumulés dans l’ancien système hydrographique descendant des monts Hamersley. Des minerais de conglomérat formés par la recimentation des talus d’hématite sur les pentes en dessous des grands dépôts d’hématite constituent une réserve appréciable de minerai.

Bibliography/ Bibliographie CAMPANA, B. et al. 1964. Discovery of the Hamersley Iron deposits.Puoc. Aust. Inst. Min. Engrs., no. 210. DORR, J. VANN.1964. Supergene iron ores of Minas Gerais, Brazil.Econ. Geol., vol. 59, no. 7. HARMS. J. E.;MORGAN, B.D . 1964.Pisolitic limonite deposits in Northwest Australia. Proc. Aust. Inst. Min. Engvs., no. 212.

298

MACLEOD, W.N. 1966. The geology and iron deposits of the Hamersley Range area, Western Australia. Bull. W.Aust. geol.Surv.,no. 117.

TRENDALL, A. F.;BLOCKLEY, J. G. 1970. Iron-formationsof the Precambrian Hamersley Group of Western Australia with special reference to crocidolite. BuII. W.Aust. geol. Surv., no. 119.

Significance of carbon isotope variations in carbonates from the Biwabik Iron Formation, Minnesota E. C.Perry Jr and F. C.Tan Department of Geology,University of Minnesota

Introduction

Experimental pro cedures

Most carbonates in a suite of 3-1 x lo9 year-old rocks from southern Africa and the Canadian shield have SC13 values1 within J. 2 per mil of PDB,an isotopic standard described by Urey et cil. (1951) which has a value typical of Phanerozoic marine carbonates (Perry and Tan, 1970 and in preparation). Carbon isotopic determinations on Precambrian carbonates by Hoering (1967) also give a range of values that is normal for Phanerozoic marine sediments. Carbonates from iron-formation have E l 3 values lower than PDB by as much as 18 per mil. The anomaly was first reported by Becker and Clayton (1970), who found that SC13 of limestones and dolomites stratigraphically above and below the Dales Gorge Member of the 1.9 x lo9 year-old Brockman Iron Formation of Western Australia is similar to that of normal Phanerozoic carbonates, but that õC13 of iron carbonates within the ironformation is about 10-15 per mil lower. They concluded that : ‘(1) the carbon isotope ratio in the world oceans has been nearly constant for at least 2x lo9 years; (2) the banded iron-formation of West Australia was deposited in a basin distinct from but with some connection to the ocean; (3) there probably was organic activity involved in either the transport of iron to the basin or in the precipitation of iron and silica. However, the possibility of juvenile os recycled carbon from volcanic sources supplying the lighter carbon, rather than organic activity, cannot be ruled out.’ Evidence about the contribution of carbon from volcanic sources is difficult to obtain and interpret. One clue may come from carbonates from iron-formations within the 3 x lo9 year-oldOnverwacht and Fig Tree Formations of South Africa which also have anomalously low ôC13 values (Perry and Tan, 1970 and in preparation). Limestones and dolomites,sometimes associated with volcanics, and even carbonates from tuffaceous sediments in these formations have normal õC1” values,suggesting that there is no volcanic carbon contribution.

Composite samples and a few individual specimens from four continuous drill cores of Biwabik Iron Formationwere analysed in this study. Composite samples were chosen because partial analyses were available (Pfleider et ul., 1968) and because there is no a priori way of determining equilibration volume within the iron-formation. Carbon and oxygen isotopic data on carbonates were obtained from CO,liberated by 100 per cent phosphoric acid by the technique of McCrea (1950). The Biwabik Iron Formation contains coexisting ankerite and siderite (French,1968),both of which react slowly with phosphoric acid, and successive gas fractions from iron-formation samples contain increasing proportions of gas from the carbonate with the slowest reaction rate. Analyses of such successive CO,fractions from mixed CaCO,-CaMg(CO,), assemblages is discussed by Epstein, Degens and Graf (1964). Pending accurate determination of the carbonate phases involved and of the phosphoric acid-carbonate oxygen isotope fractionation factors for the appropriate carbonates, we have arbitrarily used the oxygen isotope fractionation factor for calcite., Oxygen from SiO, (chert) was liberated with BrF, and converted to CO, by the technique of Clayton and Mayeda (1963). ôû18for chert is reported with respect to Standard Mean Ocean Water (SMOW)(Craig, 1961). Isotopic analyses of CO,gas were performed with a mass spectrometer having a 15 c m radius analyser and equipped with double inlet and double collector systems (McKinney et al., 1950). i.

SC'^ =

(C1z/C’2)sample

1

-i x i 000.Similarly, -1 x 1000. The standard

(O*R/O*a)sample (0,8,0,0)standard

1

used for

carbonates is PDB. 2. Since this paper was prepared a phosphoric acid-siderite fractionation factor has been reported by Fritz et RI. (1910). Using this factor would decrease our values of carbonates by about 1.6 per cent, but would have no significant effect on the relative values or o n any of the conclusions presented here.

Unesco, 1973. Genesis of Precamhriun iron and manganese deposits. Proc. Kiev Symp., 1970. (Earth sciences, 9.)

299

E.C.Perry Jr and F.C.Tan

Discussion of data Samples for this study were taken at intervals throughout the stratigraphic section of the Biwabik Iron Formation and the data are given in Tables 1 and 2. Stratigraphic logs of percentage iron from magnetite (Pfleider et al., 1968), sol8and 6CI3are shown for 3 cores in Figure 1. It is apparent from this figure that isotope ratios show a % $AGNE,JiTE 40 I

35

s O,’& -26 -25 -24 -23 -22

15

FE

o

30

-Li -20

(per m;i)

-19 -18 -17 -16 -15 -14 -13 -12 -11

-10 -9

strong stratigraphic trend. W e shall explore whether this is a primary trend or the result of isotopic reactions between minerals and other components of the system during diagenesis and metamorphism. The iron carbonate-rich top of the Upper Slaty Member, the Intermediate Slate Member, and the uppermost part of the Lower Cherty Member of the Biwabik Iron Formation are iron-formations in the sense of containing about 20 per cent or more Fe in the form of silicates and carbonates, but they contain little or no magnetite. A plot of 8 0 1 8 versus 6C13(Fig. 2) shows that these units occupy a field different from that occupied by magnetite-bearingiron-formationunits. In particular,most samples of magnetite-free iron-formations have a 6C1s greater than -4 per mil,whereas almost all magnetitebearing iron-formationshave a less than -7 per mil. The Intermediate Slate contains as much as 3 per cent reduced carbon (Gruner, 1946). In sample EP 2-66 this reduced carbon has a XC13of-33.2 per mil, a value which agreeswellwith Hoering’s(1967)value of SC13 PDB = -30.5per mil for carbon from the Thomson Formation, stratigraphically equivalent to the overlyingVirginia Formation, and 6Cl8PDB = 30.3 per mil for carbon from the correlative Gunflint Iron Formation. It also agrees well with the value of -33.1 per cent reported by Smith et al. (1970) for the HF residue in chert from the Gunflint Iron Formation.As shown in Figure 2, this large quanO bearing horizons

, .I

-4 S o s,!,&

1 ,

‘ .-.

- - _ _ _ - ~ ’

per mil)

/ -

e---

LOWER SLATY

-8

-

/

o \ I I

/

42-

A

I

A

I

I

o

-1

\ \

Magnetite bearing horizons

0

1-

-

0 0

A

-20 -20

-4 6

-i 2

DZ D5 07 Y 1258 EPZ-66 -0

-4

SO,,‘& per mil FIG.1. Core log showing 6 0 1 8 and 8Cl3of carbonates and percentage magnetite iron for three continuous core samples of Biwabik Iron Formation. 300

FIG.2. versus of carbonates from Biwabik Iron Formation showing fields of magnetite-bearingand magnetitefree units.

Significanceof carbon isotope variations in carbonates from the Biwabik Iron Formation,Minnesota

TABLE 1. Isotopicand related data for core samplesof Biwabik Iron Formation SC13(PDB) per mil

SOIn (PDB) per mil

Percentage Fe2

D 2 1620-1624.5 D 2 1697-1705

- 1.08 - 7.75 - 6.19

-12.18

6.23 16.30

10.8

D 2 1882.4-1886.2 D 2 2010-2045.8

-17.01 -12.20 -10.16

-15.14 -14.19 -14.33

29.72 27.57

18.0 16.6

Upper Cherty

D 2 2085-2090

- 8.44 - 6.47

-13.19

37.87

33.8

Lower Cherty

-13.40

D 2 2170-2175

-13.82 -13.81

-15.73 -16.69

29.40

25.5

Lower Cherty

D 5 524.1-530

- 4.58 - 1.45

- 8.99 - 8.70

17.42

D5 692.5-700 D 5 755-760 D 5 829-835 D 5 965-970

-10.00

-16.16 -14.86 -12.18 -11.82 -12.53

25.60 29.05 26.05 13.98

-11.15 -10.61

-17.48 - 2.39 - 2.14

Sample number'

D 5 975-985

D5 1070-1075 D 7 1083

D 7 1094

-13.38

- 7.99 - 3.87 - 3.51 - 4.00 - 3.47

EP 2-66

Stratigraphic unit

O

Upper Slaty Upper Cherty

O

10.9 12.2 14.8

Lower Cherty

Upper Slaty

Upper Cherty Upper Cherty

O

Lower Slaty Lower Cherty

19.43

O

Lower Cherty

-12.86 -12.18

27.73 10.91$

7.9 03

Lower Cherty Lower Cherty

- 1.46 - 2.88

-12.13 -10.19

10.773

03

Lower Cherty

- 2.48

-10.02 -10.00

-10.39 -11.03 - 9.17 - 9.20

-13.37

27.1 24.5 6.13

Lower Cherty Lower Cherty Lower Cherty

- 8.56

-16.69 - 5.84

- 2.50

D 7 1175-1180 0 7 1200-1205 D 7 1337

-13.88 -14.28

Percentage magnetite Fe*

- 3.71

-11.98

-13.40 -14.94

32.31 30.59 27.3P

-15.21 20-30

O

Intermediate Slate

Reaction time

0-2 hours 0-2 hours 2 hours1 day 0-1 day O-2.hours 2 hours14 days 0-2 hours 2 hours3 days 0-1 hour 1 hour21 days 0-1 hour 1 hour7 days 0-3 days 0-7 days 0-4 days 0-1 hour 1 hour3 days 0-1 hour 1 hour3 days 0-7 days 0-2 hours 2 hours4 days 4-7 days 0-1 hour 1 hour10 days 10-17 days 0-1 day 0-4 days 0-2 hours 2 hours4 days 4-7 days 0-1 hour

1. Core 2 was taken near Biwabik, Minnesota (Sec. 22, T58N, R16W); core 5 was taken near Buhl, Minnesota (Sec. 36, TSSN,R20W);core 7 was taken near Keewatin, Minnesota (Sec. 36, T57N, R22W). 2. Taken from Píìeider et al. (1968). 3. Single hand specimen, but analysis is for five foot composite.

tity of isotopically light carbon has not produced a Significant SC13shift in the carbonate. The Intermediate Slate is composed dominantly of siderite and chamosite (French, 1968) and is considered to have a volcanic ash component (Morey, this volume, page 193). W e conclude from the essentially normal 8Cl3 values in this unit that it is unlikely that the S O 3 anomaly

in iron-formations is caused by a direct contribution of volcanic carbon to the sedimentary basin. In a number of samples in which we have analysed successive fractions of CO, gas, these fractions shift according to a variety of trends (Tables 1 and 2, points connected by lines in Fig. 2). Because these shifts show no consistent trend, we reject the trivial interpretation 301

E.C. Perry Jr and F. C.Tan

TABLE 2.Isotopic data and iron oxide percentage forcomposite samples of Biwabik Iron Formation (Hanna y 1258), Cooky,Minnesota Sample number

47-74 74-93 93-121 121-1 58 158-1 72

172-194 194-208 208-241

.

SCL3 (PDB) per mil

-11.95 -12.64 -12.78 -12.36 -12.43 -11.92 -12.82 -12.33 -13.15

SO1B (PDB) per mil

Percentage magnetite

-14.97

41.9 35.9 30.4 30.5

1.2 O 4.2 8.3

28.7 27.7 34.1 27.3

2.3 1.3

-12.40

-12.92 -14.21

-17.66 -13.25

-15.85 -14.01

-14.78

Percentage hematite

1.8

3.2

Stratigraphic

Reaction time

unit

Lower Cherty Lower Cherty Lower Cherty

Lower Cherty Lower Cherty Lower Cherty Lower Cherty Lower Cherty Lower Cherty

0-1 day

0-1 day 0-1 day 0-1 day

1-3 days 0-1 day 0-1 day 0-1 day 0-1 day

-

voir of organic carbon (less than 30 per mil) to the carbonate reservoir (initially near O per mil); similar shifts may transfer oxygen from hematite to carbonate (a shift of over 20 per mil). Although this model is tentative, it can be extended to explain some of the perplexing features of deposition and diagenesis of ironformation. W e postulate that much of the magnetite in iron-formation was precipitated as ferric-oxide-hydroxidesimultaneously with a variable amount of organic matter. During diagenesis and low grade metamorphism the following reactions (or similar ones) occurred:

that they are the result of surface exchanges with atmospheric CO,. Instead, they probably represent carbonate components with differing reaction rates and different isotopic compositions and fractionation factors. Since the shifts are small in short core specimens (D7 samples with single footage numbers, Table i), we attribute the larger shifts to sampling intervals that exceed the volume of local equilibration. Although the shifts are a nuisance in the interpretation of data, they suggest that diagenesis and metamorphism in the Biwabik Iron Formation involved reactions in a number of small subsystems with little interaction between them. All but one of the samples from core Y 1258 contain hematite and magnetite. W e note, without explanation, that in this buffered system SC13varies only from 12.0to 13.2 per mil, whereas Sol8 ranges from -12.4 to 17.7per mil.

A more complete set of possible reactions is given by

Interpretation

Yui (1966). After exchange according to reaction (2), CO, is assumed to have escaped from the immediate system; because of the sandwich-likenature of the iron-

-

-

At least three interpretations of the data of Figures 1 and 2 are possible: Both Fe for magnetite and carbon for carbonate are derivedfrom a volcanic source characterized by low SC13. W e regard this as unlikely because the SC13 anomaly is not evident in the Intermediate Slate,the only unit of the Biwabik Iron Formationin which there is direct evidence (shards) of a volcanic contribution. Iron is transported or precipitated by some organic process which produces carbonate with a low S O 3 value. This is the model of Becker and Clayton (1970); its validity rests on whether the isotopic patterns in Figure 1 are primary stratigraphic features or are the diagenetic and metamorphic result of primary differences. Normal E 1 3 values for carbonates from zones containing about 20 per cent iron in the form of iron silicates and carbonates,but no magnetite,indicate that the SC13 anomaly is most probably not related to transportation of iron. W e propose here a model in which oxidation-reduction reactions during diagenesis and metamorphism produce shifts of carbon from an abundant reduced reser302

6Fe,03 -t C(orgUnic) +4Fe30, + CO, C1202 + FeCl3O33 C 1 3 0 , +FeC.120, 2Fe30, + 6C0,f-r GFeCO,$- O,

(1) (2) (3)

formation, gaseous products may continue to react in other zones, but large local variations in isotope ratios discussed previously suggest that interactionbetween zones was limited. W e are not aware of CO,-FeCO, isotope exchange experiments or calculations but, by analogy with dolomite (O'Neiland Epstein, 1966), the CO, produced by reaction (1) could be out of equilibrium with FeCO, by about 40per mil for carbon.This is probably sufficientto produce the shifts of approximately 10 per mil that we observe in the carbonate values. A similar shift occurs in 6 0 1 8 , but this is complicated by possible exchange reactionsin the system.Figure 1 shows the correlation in core 5 of Sol8between carbonates and coexisting chert. The large spread in carbonate values compared to the relative constancy of coexisting cherts suggests that oxygen exchange reactions associated with oxidation and reduction may not be completely masked. Clayton (1970)reports 5 per mil shifts iii SOI8 for magnetite compared to 1 per mil maximum shifts for chert in the Dales Gorge Member of the Brockman Iron Formation.H e attributes this to late exchange and homogenization of the chert,but one might speculate that the large

Significanceof carbon isotope variations in carbonatesfrom the Biwabik Iron Formation,Minnesota

variations result from oxidation reduction reactions that do not affect the chert. Table 3 shows $OISfor selected chert samples from core 5. TABLE 3.60ls(relativeto S M O W ) of selected chert (SiO,) samples from core 5 Feet

524-530 755-760 829-835 1070-1075 11 15-1 124

ô015

(per mil)

18.29 k .50 20.02 I..o2

20.80 19.68 19.83

Implications Our model can readily be extended to explain some of the intriguing features of iron-formation deposition. In this limited discussion we make no effort to deal extensively with the geochemical problems associated with ironformations. A summary of the literature and an extensive bibliography are given by James (1966). Thermodynamics of diagenesis and metamorphism are discussed by Yui (1966). One question which has concerned many people (Gruner, 1922; Lepp and Goldich, 1964) is how the large quantities of iron found in Precambrian iron-formations was transported. Our model is compatible with a generally reducing atmosphere in which ferrous iron could be transported to the site of deposition.Here it could be oxidized in the photo-syntheticzone by primitive organisms using the Fe++-+Fe+++ reaction as an electron donor, as suggested by Cloud (1968). Ferric oxide-hydroxidemight then be precipitated and incorporated in the sediment together with dead organisms. The present day Fe,O,/ Fe,O,ratio in iron-formationswould then depend on the depositionalratio of Fe,O,/C. Less than i per cent would be sufficient for reaction (1) to go to completion in a rock containing 30 per cent magnetite and would produce CO, for exchange in a mole ratio of CO,(reduction)/FeCOy -~ .in 4a sedimentcontaining 10per cent FeCO,. Present concentration of reduced carbon in iron-formationsis more than adequate for this mechanism. This model is consistent with the facies concept of James (1954)since organic matter in shallow water deposits would be rapidly oxidized by water of high Eh,whereas in deeper water organisms and Fe,O, would be deposited together. The model also provides an explanation for the puzzling coexistence of Fe,O, and Fe,O, in many ironformations(Eugster,1957). This coexistence merely implies that reaction (1) stopped when the local system became depleted in carbon.

Iron-formation of the Transvaal System in South Africa and the .Brochan Iron Formation in Western Australia typically contain finelaminae that Trendall (1968) has suggested may represent annual accumulations. This is, of course, consistent with a mechanism for precipitation of iron oxides which depends on seasonal maxima in organic activity. Magnetite and iron carbonates often display complex interrelated textures which have led several authors to postulate diagenetic or metamorphic conversion of FeCO, to Fe,O, (LaBerge, 1964; French,1968). Reaction (1) is probably an early reaction that stabilizes siderite by increasing CO,pressure (reaction (3)). Higher temperature can decompose siderite to produce magnetite (Yui,1966). This complicated system,coupled with variability in the size of the equilibrating system, probably explains why we cannot demonstrate a direct relationship between 8C13and Fe,O,/ FeCO,.

Summary and conclusions Figures 1 and 2 provide good circumstantialevidence that a negative 6C1,shift in iron-formationcarbonates is related to oxidation-reductionreactionsinvolving magnetite which permit exchange between two carbon reservoirs differing by about 3 0 per mil in their carbon isotope composition. As an example of one possible reaction,w e have postulated a model in which primary hematite could be converted to magnetite by oxidation of organic carbon;and w e have suggestedsome implicationsofthis model. Four approaches may be useful in testing this model: (a) more detailed correlationssuch as those shown in Figure 1;(b) a tracer study of hydrogen-deuterium ratios in chamosite and other hydrous iron-formationsilicates; (c) a study of end member assemblages such as hematite-carbonate;and (d) a close correlation of textural relations to isotopic data. W e are currentlymaking a much more detailed study of carbon and oxygen isotope variations in carbonates and of oxygen isotope variations in silicates and oxides of the Biwabik IronFormation (test 1). Our model may not be unique,but w e emphasize that carbon isotopes provide an important tracer that can be used to help understand the petrology of iron-formations.

Acknowledgments This research was sponsored by the Minnesota Geological Survey and the National Science Foundation (GP 10855). W e wish to acknowledge the help of L. A. Mattson, J. S. Owens and R.D.Scamfer of the Hanna Mining Co. and R . Bleifuss, G . B. Morey and P. K. Sims of the University of Minnesota.

303

E.C.Perry Jr and F.C. Tan

Résumé Signifrcatiori des variations cles proportions cles isotopes clu carborze datis les carbonates des gisements dejev de Biwabik, duns le Minnesota (E.C.Perry Jr. et F. C.Tan)

Les rapports C13/C1dans 2 les carbonates des formations de fer du Précambrieninférieur et moyen sont plus faibles que ceux des autres carbonates précambriens et des carbonates marins phanérozoïques.Dans la formation de fer de Biwabik,dans le Minnesota,qui appartient au Précambrien moyen,la perte de C13varie de 7 à 8 o/oe par rapport à la calcite crétacéenormale (PDB).D e faibles rapports C13/C12 ne caractérisent que les zones qui contiennent de la magnétite et dans lesquelles est absente l'ardoise intermédiaire, matière sidéritique finement laminée, sans magnétite, qui contient certains fragments volcaniques. Cependant, dans cette matière l'abondant carbone réduit voit le CI3 diminué dans la proportion de 33 'Ioo. L'auteur propose une réaction diagénétique d'oxydation-réductionavec production de magnétite à partir d'hématite, qui permettrait un échange entre le carbone organique et le carbone des carbonates :

U n e conversion réversible de magnétite en carbonate est aussi possible : 2Fe,0,

+ 6CO,* 6FeC0, + O,

r31

C e modèle propose le transport de fer comme Fe++, l'oxydation par des organismes photo-synthétiques en hydroxyde-oxyde (comme cela a été proposé par Cloud en 1968), la précipitation de l'oxyde ferrique hydraté avec des organismes morts, et une réaction diagénétique entre l'oxyde ferrique et les organismes morts, sauf quand ces organismes sont oxydés dans des eaux peu profondes à Eh élevé. C e modèle peut expliquer les feuillets à caractère saisonnier, semblables à des varves, avec coexistence de magnétite-hématite et certaines structures de remplacement mettant en jeu de la magnétite et du carbonate. Cela s'accorde avec le transport de fer Fe+, et n'introduit que de minimes modifications au concept de faciès de James (1954).

Bibliography/Bibliographie BECKER, R. H.; CLAYTON, R.N. 1970.C13/C12 ratios in a Precambrian banded iron-formationand their implications,Abstract in Trans. Arner. geophys. Un., no. 51, p. 452. CLAYTON, R. N.1970. Oxygen isotopes in ancient sediments. Abstract in the 14th annual report on research, The Petroleum Research Fzind,AmericanChemicalSociety,Washington,D.C. -; MAYEDA, T.K.1963.The use of bromine pentafluoride in the extractionof oxygen from oxides and silicates for isotopic analysis. Geochim. et cosrnoch. Acta, vol. 27,p. 43-52. CLOUD, P.E.Jr.1968.Atmospheric and hydrosphericevolution on the primitive earth. Science, vol. 160,p. 729-36. CRAIG, H.1961.Standards for reporting concentrationsof Deuterium and oxygen-18 in natural waters. Science, vol. 133, p. 3467. EPSTEIN, S.; GRAF, D . L.; DEGENS, E. T.1964.Oxygen isotope studies on the origin of dolomites.In:H.CRAIG, S.L.MILLER, G.J. WASSERBURG (eds.), Cosmic and isotopic chemistry, p. 169-180. Amsterdam, North Holland. EUGSTER,H.1957.Reduction and oxidation in metamorphism. In: P. H.ABELSON (ed.), Researches in geochemistry. p. 397426,New York,Wiley. FRENCH, B. M . 1968.Progressivecontact metamorphism of the Biwabik Iron-formation,Mesabi Range, Minnesota. Bull. Minn. geol. Surv., vol. 45. FRITZ, P.;BINDA,P.L.;FOLINSBEE, F.E.;KROUSE, H.R.1970. Isotope composition of diagenetic siderites from Cretaceous sedimentsof Western Canada. J. sediment. Petrol. (In press.) GRUNER, J.W.1922.The origin of sedimentaryiron-formations: the Biwabik forinaiion of the Mesabi Range. Econ. Geol., vol.17,p. 407-60. 304

-. 1946. Mineralogy

and geology of the Mesabi Range,

St Paul,Minn., Minnesota Office of the Commissioner of the Iron Range Resources and Rehabilitation. HOERING, T.C.1967.The organic geochemistry ofPrecambrian rocks. In: P. H.ABELSON (ed.), Researches in geochemistry, vol. 2.New York,Wiley. JAMES,H.L.1954.Sedimentary facies of iron-formation.Econ. Geol., vol. 49,p. 235-93. . 1966. Chemistry of iron rich sedimentary rocks. Prof. Pap. US.geol. Surv., 440-W. LABERGE, G.1964.Developmentof Magnetiteiniron-formations, The Lake SuperiorRegion.Econ. Geol.,vol.59,p.1313-42. LEPP, H.; GOLDICH, S. S. 1964. Origin of Precambrian ironformations.Econ. Geol., vol. 59,p. 1025-60. MCCREA, J. M . 1950. On the isotopic chemistry of carbonates and a paleotemperaturescale.J. chem. Phys., vol. 18,p. 84957. MCKINNEY, C.R.;MCCREA, J. M.; EPSTEIN, S.;ALLEN, H.A.; UREY, H.C. 1950. Improvementsin mass spectrometersfor the measurement of small differences in isotope abundance ratios.Rev. sei. Instrum., vol. 21,p. 724-30. O'NEIL, J. R.;EPSTEIN, S. 1966. Oxygen isotope fractionation in the system dolomite-calcite-carbondioxide. Science, vol. 152,p. 198-201. PERRY, E.C.Jr,TAN, F. C. 1970.Carbon and oxygen isotope ratios in 3,000 m.y. old rocks of southern Africa. Abstract in Geological Society of America Annual Meeting Program, Milwaukee. PFLEIDER, E.P.;MOREY, G.B.;BLEIFUSS, R. L.1968.Mesabi Deep Drilling Project, Progress Report no. I, Minnesota

-

Significanceof carbon isotopevariations in carbonates from the Biwabik Iron Formation,Minnesota

Section, A I M E Forty-fist Annual Meeting, Minneapolis, University of Minnesota. SCHOPF,J. W.; KAPLAN, I. R. 1970.Extractable SMITH,J. W.; organic matter in Precambrian cherts. Geochn. et cosmoch. Acta, vol. 34, p. 659-75. TRENDALL, A. F. 1968. Three great basins of Precambrian banded iron-formationdeposition: A systematic comparison. Bull. geol. Soc. Amer., vol. 79, p. 1527-44.

UREY, H.C.; LOWENSTAM, H.A.;EPSTEIN, S.;MCKINNEY, C.R. 1951.Measurement of paleotemperaturesand temperaturesof the Upper Cretaceous of England, Denmark,and the southeastern United States.Bull.geol. Soc. Ameu.,vol. 62,p. 399416. Yur, S. 1966.Decomposition of siderite to magnetite at lower oxygen fugacities:A thermochemical interpretation and geological implications. Econ. Geol., vol. 61,p. 768-76.

305

Genesis and supergene evolution of the Precambrian sedimentary manganese deposit at Moanda (Gabon) F. Weber Laboratoire de Géologie et de Paléontologie, Université de Strasbourg,France

Introduction The manganese deposit at Moanda lies in a region occupied by the Precambrian series of the Francevillian; this series derives its name from Franceville, the prefecture of the Haut-Ogooué. The occurrence of manganese in the region of Franceville was first described by Barrat in 1895. Later Babet, Choubert, Bergé, Nicault and Briot reported manganese severaltimes.After these discoveries,systematicprospecting was commenced in 1951 by the Bureau Minier de la France d’outre-Mer(BUMIFOM) in collaborationwith the United States Steel Corporation.In September 1953, the Compagnie Minière de l’Ogooué (COMILOG)was founded using French and American capital to continue investigating the deposit and to commence its exploitation. A total of 96 million United States dollars were required to start exploitation in 1962,most of this being needed for transport facilities (Vigia, 1963). The mine is an open-cut, the ore, enriched in situ by scouring, is carried by cable and railway to the harbour at Pointe Noire. The mean annual production is about 1,600,000metric tons of washed highgrade ore (48-52 per cent Mil). At this rate the reserves should last for more than 150 years.

The geological environment The Francevillianis a Precambriannon-metamorphicseries lying unconformably on the metamorphic basement of the D u Chaillu and North Gabon massives (Fig. 1). Towards the east it disappears under the overlying Cretaceous and Tertiary beds of the Batéké Plateaux. According to radiometric dating, the Francevillian is 1,740&20 m.y. old (Bonhomme et ul., 1965; Vidal, 1968; Bonhomme and Weber,1969).The measurements have been made on three series of samples by different methods; pelites (Rb-Sr), interstratified cinerites (K-Ar)and intrusive syenites (RbSr). The results of these different methods are in good agreement.

The Francevillian sediments are distributed on both sides of a median northwest-southeast ridge penetrated by large inliers of basement which are the moles of Asséo, Amieni and Ondili (Fig.2).This ridgewas a submarinerise at the time of deposition, the thickness of the sediments being weak there and the seriesinvolvingstratigraphicgaps. This ridge separatestwo palaeogeographicdomains. In the north-east, i.e. in the ‘Okondja deep’, the series is very thick (more than 3,000 m) and folded in the southwest there is an epicontinentaldomain where the series is less thick (1,500 m) and has only weak undulations and faults (Fig. 3). This epicontinentaldomain includesseveral basins separated by sills; in one of these basins, the Franceville basin, lies the Moanda ore deposit. From recent studies of the Commissariat à l’finergie Atomique and the Bureau de Recherches Géologiques et Minières,a lithographic scale has been established (Donnot and Weber, 1969). Five formations have been described, which are found throughout the series,with some variation (Fig. 4). The basal sandstone formation (FA of Figs 3 and 4) is an arkosic gritty encroachment of fluvio-deltaiccharacter which spread all over the Francevillian area. The basal sandstone formation is very thick (3-400 m) on the border of the Du Chaillu Massif, vhere it contains thoriferous conglomerates;elsewhere this formation fills depressions in the basement.It is not very thick on the median ridge and is lackinginparts ofit.The faciesbecome thinner from southwest to northeast. The lower pelitic and volcano-sedimentaryformation (FBofFigs 3 and 4).In the Okondja deep,where subsidence was the strongest,more than 1,500 m of argillo-sandysediments were deposited.A basic submarine fissure volcanism occurred in the northern part of the deep. Hyaloclastites were mixed with the terrigenous sediments in which sills and lavaflows with spilitizedpillow lavas were intercalated. On the submarine rises, where terrigenous supply was nonexistent, silicification took place. In the epicontinental basins, confined conditions prevailed and carbonate sedimentation took place. In the Lastoursville basin massive

Unesco, 1973. Gemsis of Precambrian iron und manganese deposits. Proc. Kiev Symp., 1970. (Earth sciences,9.)

307

CAMEROON

-h ?+ A

t

h

Ri0 M U N ¡ b

EKÉ ATEAU

O

o

Mc

L E G E NO,

u

ûuoternary Tertiary Secondary

[IIIIIITIIDD

Upper P r e c a m b r i a n Middle P r e c a m b r i o n

[ml Lower

N-Noya system C-Western C o n g o s y s t e m

c

F- Froncevillian series S I-Intermediary system

B-Bamba

Precambrian

o

50

100,

160

a c c o r d i n g to t h e geological m a p of

system

aYJtI I

A.EF. ~/2000.000, by G . G É A A R D 1958

FIG.1. Geological draft of Gabon, location of the Francevillian series.

Genesis and supergene evolution of the Precambrian sedimentary manganese deposit at M o a n d a (Gabon)

r

1 Oe'*"

North

G a b o n Massif

+

+

+

+

+

+ N'GOUTOU

REGION

+

+

ABE1 L L E S REGION

i

t

+

-

+

+

+ DU

20 + I20

+ I

+

+

+

+

t

+

+

CHAILLU

+ 50 K m

+

+

i

+

+

+

t

+

t

+

MASSIF

+

+

PLATEAU

+

+

+

+

+

I

FIG.2.Schematic m a p of the principal domains of the Francevillian. 309

F. Weber

D U C H A I L L U M A S S I F -F R A N C E V I L L E B A S I N- M O L E OF ONDILI

-

- A K I E N IBASIN

Syncline Of

-

O K O N D J A BASIN

&nticline Syncline Anticline of O k o n d i a of O y a b i of A m b i n d a

YCYé

NE 1mm>OO.,.

om-

* ..t**++ I * *

1

FIG.3. Schematic section across the Francevillian from Moanda to Okondja.

-

No rt h we st ern reg ion

South region

I

I

I

LASTOURSVILLE B A S I N

I

North-eastern region OKONDJA BASIN

I

I

i F R A N C E V I L L E BASI!

i

!

Olcunga

I

dolomlte5

basic

tuffs

cherts

ocid

tufts

black shales

a peiites

coarse-grained

i L.dzOmdnl

i

Volcanic racks basement

FIG.4. Diagrammatic scheme showing the correlations and variations of facies between the different domains of Francevillian. The sections were based on the top of the jasper formation FC.

310

Genesis and supergene evolution of the Precambrian sedimentary manganese deposit at Moanda (Gabon)

dolomites were deposited, while in the Franceville basin the subsidence was more marked and an argillo-sandysedimentation in which dolomites began to deposit,gradually passed to an ampelitic sedimentation.The latter was suddenly stopped by a renewal of erosion leading to the deposition of a sandstone stratum.In this basin the following succession can be observed downwards: b 30-40 m pelites and ampelites (Djoumou River pelites). FB2 a 30-100 m isogranular quartzy sandstones (Poubara sandstones). c 50-150 m ampelites with dolomitic and manganiferous layers. b 20-100 m sandstone pelites alternating with dolomitic sandstones;near rJ% ) the bottom, intraformational (Bangombé breccias. pelites) a 10-20 m greenish lustrous pelites; at the bottom, a conglomerate with quartzitic boulders and pelitic fragments. The Jasper formation (FCof Figs 3 and 4).Through a thickness of about 40 m , chert layers alternate with ampelites and cineriteswhich are the first signs of a new volcanic phase which developed in the FD formation. The upper ampelitic and volcano-sedimentaryformation (FD of Figs 3 and 4) is a thick deposit of ampelites mixed with pyroclasts. An acid volcanism of ignimbritic type induced the deposition of wide-spreadlayers of vitroclastic tuffs. At that time, the Francevillian was one basin only, open towards the Okondja deep;the thickness of the FD formation,which is about 150 m in the region of Franceville, increases towards the north-east and reaches more than 1,000m in the region of Okondja. The upper sandstone formation (FE of Figs 3 and 4) includes alternate layers of pelites and micaceous greywackes. After their deposition the Francevillian formations were deformed with a mild foldingin the region of Okondja and undulations, flexures and faults due to differential movements of compartments of the basement in the other regions. A last, undated, volcanic phase caused the emplacement of dolerite dikes.

1

-T-.

exploitation is carried out. The schematic section of the mineralized formation comprises downwards (Fig. 6): 0.10-0.40 m . (5) The argillo-sandy humic horizon. This horizon is leached of manganese and contains some pisolites. 5-6 m.(4)The loosepisolitic layer.The pisolites,3 to 6 mm in diameter, are almost perfectly spherical and inserted in a yellow ochrous earth mainly composed of goethite, gibbsite and some kaolinite.They consistof a core,which is usually an ore fragment, around which concentric layers of gibbsite,goethite and,more rarely,lithiophorite alternate.This bed (15 per cent Mn) is not exploited. 0.5-1 m . (3) The transition horizon. This more or less cuirassed horizon contains fragments of mineralized plates, aggregates of pisolites cemented by concretionary cryptomelane,and big blocks of a coarse-grainedcavernous feldspathic sandstone. 3-9 m (average thickness:5 m). (2) The platy horizon. This horizon is the main part of the productive bed. Its chemical composition is given in Table 1, columns 1 and 2.Plates of ore, one or more centimetres thick, as well as massive fragments, are inserted in an ochrous matrix containing small fragments of ore around which small pisolites occasionally developed. The plates generally have a layered structure and occur in almost horizontal beds, but show in detail many undulations. In places sink-hole depressions are formed together with vertical plates and elements originating from upper horizons (fragments of the transition horizon and pisolites). Here and there in the platy horizon massive concretionary blocks are found and at the bottom special facies occur, named from their appearance ‘heavy layered ore’, TABLE1. Chemical compositionof manganese aiid iron ores (yo) 1

THE MANGANIFEROUS PLATEAUX O F THE FRANCEVILLE BASIN

The manganese deposits occur in the form of a superficial layer covering, at an altitude of 600 m,several plateaux (Fig. 5) the most important of which are the Okouma plateau (mineralized surface: 13 km2)and the Bangombé plateau (mineralized surface: 19 km2)where present-day

3

4

5 ~

SiO, Alzo, Ca0

2-3.5 6-7

MgO

7.0 3.2 0.10 0.10

Na,O K2O

Tio, P

Summary description of the manganese and iron deposits

2

Fe

Mn Loss on heating

0.10-0.13 3-4 50-52

0.41 0.17 4.4 44

10.4 3.2 0.35 0.28 o.1 0.35 0.10 0.7 35.2

23 6.3 8.6 4.3 0.13 1.3 0.17 0.14 2.5 15

35.8 0.2 2.0 1.8 0.06 0.07 0.04 0.42 31.2 0.1s

30.2

29.1

13.6

1. M e a n composition of the ‘marketable ore’ of the B a n g o m b é plateau during the first months of exploitation; 2. Composition of the ore straight from the mine in the pit P 39 on the Bangombé plateau. These values give a good idea of the mean composition of the ore in situ, in the zone of initial exploitation of the Bangombé plateau; 3. Analysis of a sample of layered ore with rhodochrosite collected on the Bangombé plateau at the bottom of the mineralized bed; 4. M e a n composition of manganiferous ampelites from the Bangombé borehole (10 analyses); 5. Average of two chemical analyses effected in the silicate facies with greenalite of the Okouma-Bafoula iron-formation.

311

F. Weber

’

*

\\A

Abouka

+

-

o

IO Km

P l a t e a u x with h i g h g r o d e . .oxidized ore

Z o n e in w h i c h m o n g a n i f e r o u s subsists under cap-rock

P l a t e a u x with l o w g r a d e oxidized a r e

Z o n e in w h i c h t h e m a n g a n i t e r o u s f o r m a t i o n has b e e n e r o d e d

0 D e e p borehole FIG.5. Schematic section across the mineralized horizon (afterBouladon et al., 1965). 312

formation

Genesis and supergene evolution of the Precambrian sedimentarymanganese deposit at Moanda (Gabon)

1

PLATY HORIZON

I \ COMPACT-.

LAYER

-like ore

\-

-:h shales

I O

1

4m

FIG. 6.Location of the mineralized plateaux and probable extension of the manganiferous formation in the Franceville basin.

‘polypary-like ore’ and black scoriaceous ore’. At dif-

ferent levels rather dislocated thin beds of sandstones with manganiferous cement and of ferruginousred shales are interbeddedin the plates.The main constituentsof the ore are amorphous manganese hydroxides in which can develop polianite,lithiophorite,nsutiteand cryptomelane and in the matrix iron and aluminium hydroxides. 0.20-0.50m.(1) The compact basal layer. Generally a thin band (2-5 cm) of pyrolusite (a pseudomorphism of manganite into polianite) lies on the substratum. On top is a massive layered ore composed mainly of amorphous hydroxides, manganite, groutite, lithiophorite and nsutite. Rhodochrosite appears, either as beautiful pink crystals lining geodes,or in a less spectacular form at the lowest part of the deposit as the principal constituent of a greyish shaly ore which is epigenized into manganite and pyrolusite.The analysisof this ore is given in Table 1, column 3. The substratum is composed of subhorizontal or slightly wavy ampelites with rare intercalations of fine-grained

sandstones and dolomites which belong to the upper third of the FBI formation.Diaclases are sometimes filled with pyrolusite or rhodochrosite but, except for these accidental concentrations,the content of manganese in the ampelites of the substratum seems to be very low; according to the few analyses which have been effected,from 0.2per cent to 0.7 per cent M n O . True cuirasses are observed in the lower zones of the plateau. The transition horizon is hardened by the development of a cement made of bluish concretionary cryptomelane. This hardening attains progressively the underlying mineralized layer,cementing small plates and pisolites,but it stops at the shales of the substratum.These cuirasseshave often been incised by the brooks draining the plateau. Thus ‘cliffs’of massive ore and enormous boulders have been formed,which are found on the slopes and which signal the presence of the ore deposits.These brooks remove manganese which is deposited today in the form of an often rather thick wad coating.

313

F.Weber

T H E CARBONATE MANGANIFEROUS FORMATION OF T H E DEEP BANGOMBE BOREHOLE

In the centre of the Bangombé plateau a small plating of Poubara sandstones (FB,,)has been protected from erosion in a downfault compartment (Fig. 7). A deep borehole carried out by the COMILOG passed through these sandstones,then the pelites of Bangombé (FB,)and reached the basal sandstones (FA). This borehole showed that in the upper third of FB, is a very thick but low-gradecarbonate manganiferous formation with around 13 per cent M n through 75 m . Figure 8 shows the changesin manganese content in the deep borehole of Bangombé. In the lower two-thirds of FB,, the fluctuations of the amount of M n O (which is always below 1 per cent) parallel those of C a 0 and M g O ; manganese is related to the dolomitic facies.At a depth of about 130 m in the borehole, the content of M n O suddenly increases and reaches values between 20 and 30 per cent, whereas the contents of M g O and C a 0 remain unchanged. The manganese content suddenly decreases near 55 m depth,25 m under the bottom of the FB,,sandstones.In the last 25 m ofthe FB,formation,the manganese contents are about the same as in the lower two thirds. Figure 9 shows inmore detail the changesin manganese content in the manganiferous formation, compared with the variations in iron and phosphorus content.The manganiferous formation is preceded and accompanied at its bottom by an increasein the phosphorus and iron contents. At a first approximation,the three elements achieve their maximum concentrations in the following order: phosphorus, iron, then manganese. The manganiferous formation is mainly composed of ampelites with a few intercalatedsandstonesand dolomites, the total thickness of which does not exceed 10-15 per cent of the formation.Dolomites are more frequent towards the bottom,sandstones towardsthe top.The ampelites are very rich in carbonates,which occur most frequently in the form of small radiate fibrous concretions scattered in the matrix

which is made opaque by organic matter and pyrite;detrital elements (quartz and degraded micas between 20 and 50 p) are rare.The clay minerals are illiteand chlorite,illite being largely predominant. Small aggregates and lenses of secondary silica (chalcedony) are occasionally observed. The mean chemical composition of the manganiferous ampelites is given in Table 1, column 4. Manganese is associated with calcium and magnesium incarbonates:amanganiferous dolomiteand a calcicrhodochrosite,the average formulae of which are approximately (Mgn.8,MnOn.2). Ca(CO3)z and (Mno.9,Cao.,) CO,. Iron occurs essentially in the form of pyrite. The average mineralogical composition is approximately the following: Quartz, 11 per cent; Illite (+ chlo lori te), 23 per cent; Carbonates, 56 per cent (MnCO,, 31 per cent; Cacoa, 16per cent;MgCO,, 9 per cent); Pyrite,4per cefit;Organic matter, 6 per cent. The manganese content of the dolomitic and sandstone layers is lower than that of the ampelites (3-4 per cent Mn). This explains the rapid variations in the M n content in the profile of Figure 9,and the slightly lower mean content of M n in the manganiferous formation compared with the ampelites (13 per cent instead of 15 per cent). THE IRON FORMATION OF O K O U M A - B A F O U L A

In the Okouma and Bafoula plateaux in the periphery of the zone of ore deposits, the manganese ore lies upon a banded iron-formationabout 10 m thick. The following three facies have been described from the base: sulphide facies,carbonate facies and silicate facies. The sulphide facies is characterizedby a high percentage ofpyrite within a microcrystalline quartz-chalcedonious matrix containing apatite, chlorite, degraded micas and organic matter. The carbonate facies contains alternating siliceous and carbonate beds. The siliceous beds are composed of microcrystalline quartz, the elements of which (5-20 IJ,) are iecounou ~ l v .

Leconi Rlv.

NWExplalted zone

I 3001 ...... I

1

I

I

I

borehole

I . . . ...

BFB D~ F ~Enrlched

0 20 ~ ......1 .. ~

SE

.I'

..........* I

oxldixed ore

Lor-gr'ade carbonritid ore

FIG.7.Section across the Bangombé plateau through the deep borehole and the exploited zone. 314

....

F/!

,..a ...a

.......

..e. a......

350m

Conglomerates.with rolled quartzy boulders

a

Polygenic brecclas

a B

Micaceous finegrained . sandstones Alternation of finegrained pelites or black shaies' .

Dolomite Manganiferous carbonated black shales

Ankeritic dolomite

1

P

Fe

Mn

-

=?

5

10

15

20

25

%

O

2

4

6

8 10

12%

O 0.1 0.2 0.3 0.40.5 0.6 %

O Sandstone Diagrams of

Gritty breccia

Dolomite

Black shale

Carbonated black shale

Fe and P must be compared with the peaked line of the Mn diagram

FIG.9.Contents of manganese, iron and phosphorus in the manganiferous formation of the Bangombé borehole.

moulded one against the other.Isolated crystals and small aggregates of siderite are scattered in the quartz matrix. A little pyrite is also observed. In the carbonate beds the crystals of siderite are closely packed and occupy almost ali the rock;quartz,generallyfiner-grainedthan in the siliceous beds, remains in the interstitial space.Pyrite is rarer and a discrete green clay mineral (chlorite or greenalite) appears. The silicate facies generally contains greenalite as the principal iron-bearingmineral. A much rarer facies with ferristilpnomelane occurs at the top of the formation. Siderite is always present,more or less abundant,and some beds contain a little pyrite. These facies present a regular rhythmic interbedding of 0.5-5 mm thick silicated beds of alternate laminated or spherolitic texture. Siliceous beds analogousto those ofthe carbonatefacies,but finer-grained, are irregularly distributed,In the silicatebeds with laminar texture the greenalite fibres are perpendicular to the shaly structure, which is marked by a discontinuous line of organic matter. In the silicate beds of spherolitic texture greenalite occurs in the form of sheaf-likestructures.These beds are enriched in siderite,pyrite and a phosphate belonging to the apatite group. Sometimes, at the top of the silicate beds of spherolitic texture,a phosphate bed appears which is composed oflargespherolitesofapatite(150-300 p) within a microcrystalline matrix of quartz,siderite,apatite and silicates. The silicate facies is characterized by the greatest content of iron;its chemical compositionis given in Table 1, column 5. Note the low contents of manganese, calcium, 31 6

aluminium and alkali metals and the high content of silica. This chemical composition is similar to that of the other Precambrian iron-formations.

The role of supergene weathering in the genesis of the Moanda ore deposit HYPOTHESES A N D DISCUSSION

In the first descriptionsofthe Moanda ore deposit published by Baud (1954,1956), the author regarded it as a ‘residual deposit’ originating from a process analogous to that of the formation of laterites and bauxites, the manganese having originated in the Franceviliian rocks where its content does not exceed the ‘clarke’.Varentsov (1964)and Thienhaus (1967) argued from this that sediments poor in manganese (less than 1 per cent) were the original rock of high-gradesupergene ore deposits. In contrast, Bouladon et al. (1965) considered the Moanda ore deposit as a sedimentary deposit subsequently enriched by lateritization. From a metallogenic study, Bouladon describeda ‘primaryore’ with preserved layered structure;the main constituents of this ore are amorphous hydroxides and, in the basal bed, manganite and rhodochrosite. From these constituents,cryptomelane,nsutite, lithiophorite and polianite could have developed during lateritic weathering.

Genesis and supergeneevolution of the Precambrian sedimentary manganese deposit at Moanda (Gabon)

The discovery in the Bangombé borehole, at the top of the FBIformation,of a manganiferous formation whose stratigraphic position is identical to that occurring in the Moanda ore deposit reopens the problem of the origin of the deposit. It is likely that the ‘primary’ore of Bouladon resulted from the transformation in situ of a primary carbonate ore, analogous to that which is locally protected by faults in the small caved-incompartments of the borehole. This transformation would be the result of a first phase of supergene weathering earlier than that which produced the cuirasses and pisolites. The principal arguments in favour of this hypothesis are the following. Residues of layered ore with rhodochrosite occur at the bottom of the ore deposit. The replacement of rhodochrosite by manganite, which is itself transformed into pyrolusite,has been observed on several occasions (Bouladon, 1963; Bouladon et al.,1965;Weber, 1969). Note, however,that the layered ore with rhodochrosite differs from the manganiferous ampelites in that it contains three times the amount of Mn,in the nature of the carbonates (rhodochrositewithout substitution of M n by C a or Mg) and in its structure. Recrystallization of secondary rhodochrosite partly hides the primary structure of the sediment. It should be considered as a transitional facies,hardly evolved, but already transformed by supergene factors,rather than as aii intact residue of the primary carbonated ore. The microscopic structure of the manganiferous ampelites is partially preserved in the parts of the oxidized ore which have been the least transformed during recent secondaryreworking.In fact,the amorphous hydroxides which are considered by Bouladon (1963) as the principal ‘primary’constituentsof the ore,sometimes show a spotty structure(‘fishspawn’) imitating that of the carbonates in the manganiferous ampelites. The presence of gibbsite and kaolinite shows that the ore bed must have undergone a strong lateritic weathering. Indeed these minerals are lacking in the unweathered sediments of the Francevillian.Therefore, it is not surprising that residues of carbonate ore are so scarce on the plateau and found only at the very bottom of the deposit. Such a strong weathering necessarily destroyed most of the carbonate in the mineralized formation. N o borehole has ever met an interstratified bed of oxidized manganesebelow the weatheringzonein the Francevillian formation. The ore deposits occur at the top of the plateaux apparently coinciding with an ancient peneplain (Chatelin,1964). Thus, this carbonate formation is evidence of a formation which was originally much more extensive. After outcropping it was subjected to weathering and gave rise to the ore deposit.

GEOCHEMICAL BALANCE O F T H E SUPERGENE T R A N S F O R M A T I O N S O F T H E O R E DEPOSIT

On the basis of the hypothesis that the parent rock of the ore had a composition very near that of the carbonated manganiferous ampelites of the Bangombé borehole, using the isovolumetric method (Millot and Bonifas, 1955), an approximate balance of the transformations undergone by the ore by weathering can be drawn up. In Table 2 the average composition of manganiferous ampelites per unit volume is comparedwith that of the ore in situ in the deposit of the Bangombé plateau. Since the sedimentary structure of the ore was preserved in most of the mineralized bed, probably the volume did not vary much during weathering. TABLE2. Isovolumetric balance of the supergene enrichment of the ore of the B a n g o m b é borehole

Ore of the Bangombé plateau, in situ

282 g

213 g

Manganiferous ampelites Weight of 100 c m s

Mn SiO,

42

94 15

MgO

65 (quartz 31) 18 7.0 0.39 12

Ca0

24

A1203

Fe

P

19 9.4 0.36

0.21 0.21

The weathering oí‘ the ore resulted in an important enrichment in manganese, the weight of which per unit volume more than doubled. On the other hand calcium, magnesium and a high percentage of the silica were removed. The contents of alumina, iron and phosphorus remained relativelyconstantwith a weak enrichment in iron. For the other elements the existing data are not complete enough to draw up an exact balance. Note,however, that the sulphur originally combined with iron as pyrite has been almost completely removed; in the ‘marketable ore’the amount of sulphur is generally below 0.05per cent. The same is observed for the carbon of organic matter and for carbonates. On the other hand potassium and barium fixed in cryptoinelanehave been only partially removed. The manganese which concentrated in the ore deposit probably originated in the eroded upper part of the carbonate manganiferous ampelitic formation. An approximate balance showsthat only 20 per cent of the manganese originally contained in the 75 m thick ampelites did concentrate at the bottom of the bed in a 5 m thick layer. A small part (5 per cent) remains in the superficial pisolitic horizon, but most of it (75 per cent) has been lost. Applying the same balance horizontally,the percentage of the original manganese recoverable today for exploitation is no longer 20 per cent but only 1-2 per cent, since the manganiferous formation has been completely eroded over more than 90per cent of its surface (Fig. 5). 317

F.Weber

A T T E M P T TO R E C O N S T R U C T T H E ENRICHMENT PROCESS IN THE ORE

Superficial waters rich in oxygen and CO, percolated through the manganiferous formation after erosion. In the top horizon of the weathering profile, oxidation of pyrite produced sulphuric acid which reacted with carbonates and gave rise to sulphates and CO,.In the deep horizons, the presence of sulphates favoured the action of sulphoreducing bacteria which developed in this sediment rich in organic matter, oxidizing it to CO2and H,S. Acid and very corrosive waters percolated through the formation, but their oxidizing character decreased rapidly because of the formation of H,S. Carbonates were attacked and manganese dissolved in a bivalent state in the form of ions and manganous complexes. While percolating,the waters were enriched in bicarbonates and the p H increased, lowering the solubility of manganese considerably. The increased p H favoured the oxidation of manganous ions and complexes, resulting in the precipitation of manganese hydroxides, but in a sufficiently reducing medium, manganese also reprecipitated as the carbonate. Thus,at the bottom of the weathering profile,a manganese accumulation horizon could form by epigenesis of complex carbonates to manganese hydroxides and/or rhodochrosite. Iron did not show any tendency to follow manganese in its migration. It occurred principally in the form of pyrite which was attacked only under the highly oxidizing surface conditions. In the pyritic zone the dissolution of iron in the bivalent state would have required a considerable decrease of pH, incompatible with the buffering capacity of the carbonates. The mineralogical forms of iron and manganese differ in the parent rock, and this explains why these two elements, despite their similar chemical properties, had different destinies during the first phase of weathering of the ore,and why only manganese migrated and concentrated in the lower horizons of the weathering profile.

T H E S E C O N D A R Y TRANSFORMATIONS O F T H E ORE DEPOSIT

After this first phase, the ore was subjected to other transformationswhich have been shownby Bouladon (1963) to be more directly related to lateritization; at this time the cuirasses and pisolites were formed and the bed of enriched ore was completely oxidized and dislocated. The cuirasses resulted from horizontal migrations of manganese towards the depressed zones of the plateau within the mineralized horizon which was being dislocated. The pisolitic overlap is probably the deeply transformed residue of the upper horizons,which have been leached of manganese and relatively enriched in iron and alumina. Pisolites formed by concretion of these elements around small fragments of ore; this is the beginning of iron and 318

alumina incrustation. Since the pisolitic horizon is very homogeneous,it is clear that,unlike manganese, iron and aluminium hardly migrated. It is likely that the presence of manganese hindered the migration of iron; colloidal solutions of manganese hydroxide (Mn(OH),) are weakly acid and flocculate iron hydroxide (Fe(OH),) which is weakly basic. The excess manganese can then migrate forming almost pure manganese cuirasses, iron having been fixed in sitic. Thus the formation of pisolites seems to be complementary to that of cuirasses. In the mineralized bed,a redistributionof the elements is observed. In the fragments of massive layered ore, stratified solution cavities appeared in which the removal of manganese oxides left a limonitic residue having the same nature as the ochrous sterile matrix of the ore. The composition of this matrix-gibbsite, goethite and traces of kaolinite-shows that it is the result of strong lateritic weathering. Iron and alumina were fixed again in small pisolites analogous to those found in the pisolitic overlap. Redistribution of manganese resulted in an enrichment of the small plates at the expense of the intercalated beds, which are more or less completely leached; the autocatalyticpower of MnO, explains this redistribution.Massive cuirassed boulders developed locally within the mineralized horizon.

EVOLUTION OF T H E M O R P H O L O G I C A L CONDITIONS D U R I N G T H E SUPERGENE TRANSFORMATIONS O F T H E ORE DEPOSIT

The phase of ore enrichment took place beneath the groundwater level. The base level,slightly lower than the bottom of the manganiferous formation,probably ensured sufficient drainage and a continuous circulation of water through the formation, but it was probably higher than it is today.The present-daymorphological disposition does not allow the presence of a permanent water table in the mineralized bed. Lateritization and encrustation occurred in the zone of water table fluctuation. Lowering of the base level exposed the enriched horizon to weathering. It was dislocated and locally invaded by cuirasses. Then the process of enrichment stopped, except in the basal bed, where a permanent water table remained in the decimetres overlying the impermeable shales of the substratum.Here the rhodochrosite which formed initially in a lower horizon was epigenized to manganite. Finally, after another lowering of the base level, the ‘cuirasses’themselves were notched by erosion and partially dislocated. The manganiferous plateaux of the Francevilleregion, like other plateaux in that region settled on manganesedevoid formations, give evidence of a peneplain which could be related to the ‘inner peneplain’ of Cameroon, as defined by Segalen (1907). This peneplain would correspond to a cycle of erosion which started at the end of the Cretaceous or at the beginning of the Eocene;it would

Genesis and supergene evolution of the Precambrian sedimentary manganese deposit at Moanda (Gabon)

have been notched by erosion from the beginning of the Miocene, after epirogenetic movements which affected the African continent during the Late Tertiary and the Quaternary. Supposing these correlations are exact, the major phases (enrichment and cuirassement) of the supergene evolution of the Moanda ores can thus be referred to the Eocene cycle of erosion and peneplanation.

Genesis of the primary carbonate ore deposit ORIGIN O F M A N G A N E S E

The existence of an important volcanic activity during the deposition of the Francevillian sediments and the chertmanganese association in some deposits suggest a volcanic origin;volcano-sedimentarymanganese deposits are comm o n all over the world (Chatsky, 1954; Routhier, 1963) and generally related to siliceous rocks. Despite their locationin the domain of the continental shelf,it seems that the Moanda manganese deposits must be related to the spilitic volcanism of Okondja rather than to an ignimbritic volcanism. The ignimbritic volcanism of the Francevillian is particularly manifest in the FD formation,after the deposition of manganese,whereas the spilitic volcanism of Okondja occurred at the same time as the deposition. The acid tuffs of the Francevillian have very low contents of manganese (always below 0.15 per cent Mn). In contrast, the basic lavas and spilites of Okondja and the associated hyaloclastiteshave high contentsofmanganese, reaching sometimes 0.8 per cent (average:0.35per cent Mn). The spilitic volcanism of Okondja helps to explain the supply of important quantities of manganese to the basin during the deposition of the FBIsediments. T H E M O D E O F INTRODUCTION O F M A N G A N E S E IN T H E SEDIMENTARY BASIN

As emphasized by Bernard (1968),the hypothesis of ‘transvaporization’ (Brousse, 1968) throws new light on the problem of mineralizations related to spilitic complexes. The rise of the magma through a great depth of unconsolidated sediments impregnated with salt brine resulted in the formation of large quaniities of hydrothermal fluid. These hydrothermal solutions, leaching the sediments of their most mobile elements, led to a considerable enrichment of the sea-waters in heavy metals which were redistributed according to the rules of sedimentarymechanisms. In the case of the volcano-sedimentaryformation of Okondja,the sedimentsin which the magma intruded were principally hyaloclastites originating from anterior eruptions. These hyaloclastites were composed of lava rich in

manganese and therefore the mechanism of transvaporization could have enriched the sedimentary basin in this element.

THE SEDIMENTARY MECHANISMS O F MANGANESE CONCENTRATION

Other elements were dissolved at the same time as manganese,especially iron,calcium,magnesium and silica,which are much more abundant than manganese in the lavas. The sedimentary mechanisms of deposition sorted these elements and concentrated manganese in certain parts of the sedimentary basin. W e shall consider here the mechanisms responsiblefor the enrichment of manganese vis-àvis iron and calcium, whose geochemical behaviour is normally rather similar to that of manganese in a sedimentary environment. The partition of iron and manganese during sedimentary and volcanosedimentaryprocesses has been studied by several authors (Marchandise, 1956; Krauskopf, 1956; Michard, 1969). Krauskopf (1956) showed that one could not expect an important enrichment in the solutions compared with the lava since iron and manganese are leached in similar proportions. The partition of iron and manganese occurs during deposition by an early precipitation of iron, most of the iron compounds, especially sulphides and oxides, being less soluble than those of manganese. The manganiferous formation of Bangombé is preceded by iron enrichment, but in the formation of Okouma-Bafoula it is laterally that the associated iron deposit must be found.The suppliyng waters of the manganiferous basin deposited a part of their iron content on its periphery before flowing into it, while the conditions of precipitation of the manganese carbonate had not been reached. Thus a ferriferous deposit, almost devoid of manganese,formed laterally to the manganiferous deposit, but before it. W h e n the manganese precipitated in its turn, the iron which had remained in solution also precipitated, but the solution had been previously impoverished in iron which appears only in small proportions in the manganiferous deposit (Mn/Fe = 6). The solubility product of manganese carbonate is higher than that of calcium carbonate,but Michard (1968, 1969) showed that ‘the direct precipitation of manganous carbonate is generally impossible in marine environments because of the high proportion of calcium; in reducing environments one observes a coprecipitationresulting in a rather weak enrichment’.Michard showed,however, that calcareous beds can be enriched in manganese by a mechanism in which diffusion phenomena intervene in sediments between an oxidized superficial zone and a reducing deep zone. In the formation of the Bangombé borehole,manganese was fixed in the form of a mixed precipitate with calcium and magnesium carbonates. However, the rate of enrichment in manganese is higher than in the models established by Michard with the used parameters (concentration of sea-water,sedimentation velocity, etc.). In the 319

F.Weber

Franceville basin the waters must have been rather stroiigly enriched in manganese by volcanic exhalation,and during the Precambrian the calcium concentration of the sea was probably lower than today. Moreover, the diagenetic enrichment must have been favoured by a very low sedimentation velocity,which agrees with the ampelitic nature of the sediment. THE PALAEOGEOGRAPHIC SCHEME

The deposition of manganese took place in a coastal basin barred by submarine rises at a rather great distance (100 km or so) from the source of exhalative supplies related to spilitic volcanism. The reducing character of the sedimentary environment allowed manganese to remain in solution and to migrate over great distances. The introduction of manganese into coastal basins and the distribution of the different deposits in these basins can be interpreted by a scheme analogous to that proposed by Brongersma-Sanders(1965)for the Kupferschieferwhere sedimentation occurred at the boundary of the euxinic and evaporitic domains. The scheme was inspired by the mode of circulation of currents in present-daybays and estuaries, especially in the Cariaco gulf (Venezuela). A superficial current runs toward the margin of the basin, whereas a deep current runs toward the centre where water rises to the surface.This model explains thoroughly the distribution of the chemical deposits in the Francevillian.The elements carried by the deep currents coming from the open sea were deposited in the order of their increasing solubility (Fig. 10): iron, silica and phosphorus were deposited first on the submarinerises barring the gulf inouth;in the centre of the gulf (Franceville basin) calcium and magnesium began to deposit, fixing dissolved manganese according C a ,‘Mg

to the mechanism proposed above;lastly,most of calcium and magnesium constituted the dolomitic deposits in the nner part of the gulf (basin of Lastoursville).

Conc1usion The concentration of manganese in the Moanda ore deposit was accomplished in three stages: the first was magmatic (the spilitic volcanism of Okondja supplied lavas with a manganese concentration higher than the ‘clarke’of the earth’scrust); the second was sedimentary (and diagenetic) (manganese originating from lavas was concentrated in sediments); the third was supergene (manganese of sediments was concentrated in the present-day ore deposits). Let us consider the ‘rate of concentration’ corresponding to these three stages, i.e. the ratio of the manganese content of one stage to the manganese content of the previous one, the starting point being the mean ‘clarke’ of the earth‘s crust. TABLE 3. ‘Clarke’

Mean

Magmatic stage

Sedimentary Supergene stage stage

(%I

0.1

0.3

15

45

Rate of concentration

-

3

50

3

M n content

The highest rate of concentration occurred during the sedimentary stage,but the other stages were still necessary in order to make a high-grade ore deposit out of what would otherwise only have been a geochemical anomaly.

Mn (Ca,Ms)

Si, Fe, P (Mn)

FIG.10.Rongersma-Sander’s(1965) scheme applied to the Francevillian.

Résumé Genèse et évolution siqwgène du gisement sédimentaire précambrien de manganèse de M o a n d u , au Gabon (F.Weber)

Le gisement de manganèse de Moanda, mis en exploitation en 1962 par la Compagnie minière de l’Ogooué (COMILOG),produit annuellementenviron 1 600 O00 tonnes de minerai à haute teneur.L a couche exploitée, d‘une puissance moyenne de 5 mètres, forme l’entablement du 320

plateau de Bangombé, où elle couvre une superficie de plus de 19 km2. D’autres plateaux minéralisés existent dans la région. Le minerai est formé pour l’essentiel de plaquettes d‘oxydes et d‘hydroxydes de manganèse dans une matrice argileuse d’hydroxydes de fer et d’alumine,avec un peu de kaolinite.U n recouvrementpisolitique stérile de 5 k 6 mètres d‘épaisseur surmonte la couche minéralisée.Le substratum

Genesisand supergeneevolution of the Precambrian sedimentary manganese deposit at Moanda (Gabon)

est constitué de schistes noirs stériles du Francevillien. L e Francevillien est une série sédimentaire précambrienne, non métamorphique, dont l’âge est de 1740 20 millions d‘années, selon les datations radiométriques. L a série stratigraphique,tronquée par l’érosion dans la zone minéralisée, est conservée dans un petit compartiment effondré, situé au centre du plateau de Bangonibé. Un sondage profond y a mis en évidence une formation manganésifère, à faible teneur (13 % Mn) niais très puissante (75 mètres). L a base de cette formation correspond stratigraphiquement à la base de la couche exploitée. Cette formation est constituée de schistes noirs carbonatés légèrement pyriteux, pauvres en éléments détritiques. L e manganèse est associé au calcium et au magnésium dans des carbonates complexes. L e minerai en plaquettes dérive probablement de l’évolutionsur place de ces schistes noirs carbonatés manganésifères,sous l’action des agents supergenes.L’enrichissementet l’oxydationdu minerai se sont produits lorsque la formation manganésifère originelle affleurait sur une ancienne surface,actuellement entaillée par l’érosion; de nombreux témoins de cette surface subsistent dans la région. Des bilans isovolumétriques montrent qu’il n’y a pas eu simplement oxydation du manganèse et lessivage des

cationssolubles(Caet Mg). Le minerai se serait formé dans un horizon profond du profil d’altération.La partie inférieure de la formation manganésifère a été enrichie par un apport en manganèse provenant du lessivage des horizons supérieurs. Les carbonates complexes de Ca, M g , et M n ont ainsi été épigénisés par des oxydes et des hydroxydes de manganèse. U n e étape intermédiaire comportant épigénie des carbonates complexes par de la rhodochrosite doit sans doute être envisagée,au moins dans les horizons les plus profonds.L e recouvrement pisolitique proviendrait des horizons supérieurspartiellementlessivésen manganèse. Ultérieurement,par suite d‘un abaissement du niveau de la nappe, le minerai a subi une altération latéritique intense qui l’a démantelé et qui est responsable de la formation de cuirassements latéraux. L’origine du gisement carbonate peut être mise en relation avec un volcanisme spilitique qui se manifeste à l’époque du dépôt dans une fosse située à 100 km au nord-estdu gisement. L e dépôt de manganèse s’esteffectué en bordure de cette fosse dans des bassins épicontinentaux isolés du large par des barrières de hauts-fonds.U n e formation ferrifère rubanée siliceuse à sidérose pyrite et greenalite se rencontre autour du dépôt de manganèse et lui est antérieure.

Bibliography/Bibliographie BAUD,L. 1954. Notice explicative de la feuille Franceville-Est, Carte géologique de reconnaissance au 11500 000. Brazzaville, Direction des mines et de la géologie de l’A.-E.F., 34 p., et Chron. min. colon., no.221, p. 260-61. . 1956.Les gisements et indices de manganèse de l’A.-E.F. XXeCongr. géol. int.,Mexico. Colloque sur les gisements de manganèse,vol. II,p. 21-30. BERNARD, A. 1968. Introduction pétrographique et métallogénique sur le cycle géosynclinal et la métallogenèsecratonique. Conférences et séminaires de recyclage-Métallogénie, 1, III, p. 1624, Nancy, 10-14 juin 1968 (inédit). BONHOMME, M.;WEBER, F. 1969.Compléments à la géochronologie du bassin de Francevilleet de son environnement.5O Colloque de géologie africaine, Clermont, 1969, à paraître dans Ann. Fac. Sci. Clermont, fasc. Géol. Miner. -- ; FAVRE-MERCURET, R. 1965. Age par la méthode rubidium-strontiumdes sédiments du bassin de Franceville (Républiquegabonaise). Bull. S.Cartegéol.Als. Lorr., no.18, fasc. 4, p. 243-52. BOULADON, J. 1963. Le gisement de manganèse de Moanda (Gabon). Étude de la zone de première exploitation.Rapport BRGM,no. 5313/MPMG(janvier 1963) (inédit). ; WEBER, F.;VEYSSET, C.;FAVRE-MERCURET, R.1965. Sur la situation géologique et le type métallogénique du gisement de manganèse de Moanda, près de Franceville (République gabonaise). BuII. S. Carte géol. Als. Lorr., vol. 18, fasc. 4,

__

__

p. 253-76.

BRONGERSMA-SANDERS, M. 1965.Metals of Kupferschiefer supplied by normal sea water, Geol. Rdsch., Ed. 55, p. 365-75. BROUSSE, R.1968.In:AUBOUIN, J.; BROUSSE, R.;LEHMANN, J. P. 1968.Précis de géologie, VOI. I, 711 p., Paris, Dunod.

CHATELIN, Y. 1964. Notes de pédologie gabonaise. Cah. ORSTOM,vol.II,fasc.4,p. 3-28. CHATSKY, N.S. 1954. Sur les formations manganésifères et la métallogénie du manganèse.I:Les formationsmanganésifères volcanogènes-sédimentaires.Bull. Acad. Sci. URSS (Moscou), Série géologie, no.4,p. 3-37, [Englishtranslationin Int. geol. Rev., vol. 6,no. 6, p. 1030-56 (1964).] DONNOT, M.; WEBER, F. 1969. Carte géologique de reconnaissance au 11500 000.Franceville-Ouest,avec notice explicative. Paris,B R G M . (Aparaître.)

-~.

19696. Carte géologique de reconnaissance au 11500 000. Franceville-Est, avec notice explicative. Paris,

B R G M . (A paraître.) KRAUSKOPF, K.1956.Separation of manganese from iron in the formation of manganese deposits in volcanic associations. XXcCongr. géol. int., Mexico, 1956, Colloque sur les gisements de manganèse, vol. I, p. 119-31. MARCHANDISE, H.1956.Contribution à l’étude des gisementsde manganèse sédimentaire.XXeCongr. géol. int., Mexico, 1956, Colloque sur les gisements de manganèse, vol. I,p. 107-18. MICHARD, A. 1968. Coprécipitation de l’ion manganeux avec le carbonate de calcium. C.R. Acad. Sei.,Paris, no. 267, p. 1685-8.

1969. Contribution à M u d e du comportement du manganèse dans la sédimentation chimique. Thèse Faculté des sciences de Paris,194 p. MILLOT, G.;BONIFAS, M. 1955. Transformations isovolumétriques dans les phénomènes de latéritisation et bauxitisation. Bull. Carte géol. Als. Lorr., vol. 8, p. 3-10. ROUTHIER, P. 1963. Les gisements niéfall$ères. Paris, Masson. -.

1282 p.

321

F.Weber

SEGALEN,P. 1967.Les sols et la géomorphologie du Cameroun. Cuh. ORSTOM, Série pédologie, no. 2, p. 137-87. THIENHAW, R. 1967.MontangeologischeProbleme lateritischer Mangaiierz-Lagersttäten.Mineralihm Deposita, vol. 2, no.4, p. 253-70. VARENTSOV, I. M . 1964. Sedimentary manganese ores. Amsterdam, Elsevier. 119 p. VIDAL,P. 1968.La méthode potassium-argondans la datation-.

322

des séries sédimentaires.Application aux sédiments du bassin de Franceville.Thèse 3" cycle,Faculté des sciences de Stras-

bourg. 55 p. VIGIER, R. 1963. L'exploitation de la mine de manganèse de Moanda (Gabon). Ann. Min., Paris, p. 529-48. WEBER, F.1969.Une série précambrienne du Gabon,le Francevillien;sédimentologiegéochimie,relations avec les gîtes minéraux associés.ThèseFaculté des sciencesde Strasbourg.367 p.

The Belinga iron ore deposit (Gabon) S. J. Sims Bethlehem Steel Corporation,Pennsylvania (United States)

Introduction The Belinga iron ore deposit is the largest deposit in the Mekambo district,a vast, isolated,and largely unexplored area in the north-eastern part of the Gabonese Republic in West Equatorial Africa (Fig. i). The Mekambo district includes at least five deposits: Boka-Boka, Batouala, Belinga, Minkebe and Kokomeguel (Fig. 1). All of the deposits are similar in type and origin of iron ore; all have been derived from Precambrian iron-formation.It is estimated that within the Mekambo district there could be 1,000 million tons of iron ore averaging about 64 per cent Fe. Further exploration could well enlarge this figure. It is clear that this is an important area of undeveloped iron ore and will surely gain importance as known world supplies of iron ore are steadily consumed. The occurrence of iron ore in the Mekambo district has been recognized for many years and is briefly mentioned in several early reports (Barrot, 1895; Launay, 1903; Periquet,1911;Choubert,1937;Chochine,1938;Rouquette, 1938;Chochine,1950;Devigneand Plegat,1954;Aubague, 1955, 1956). Because of the remoteness of the district, it is only relatively recently that exploration has taken place. In 1954 the French Direction des Mines mapped the BokaBoka deposit and collected samples (Devigne and Plegat, 1954). In 1955 the Bethlehem Steel Corporation, in conjunction with the French Bureau Minier de la France d'outre-Mer,undertook a reconnaissance examination of the Boka-Boka deposit, and subsequently formed the Syndicat de Mekambo in order to study this promising deposit as well as the nearby Batouala deposit. In 1958, as it became obvious that the Mekambo district had indeed a large potential, the Société des Mines de Fer de Mekambo (SOMIFER)was formed in order to explore the much larger Belinga deposit. Accordingly, this area was explored from 1958 through 1962, and this paper is based on the results of that exploration effort. Since 1962 very little work has been done on the area. The Belinga area is 65 km north-east of the town of Makokou and is accessible by boat on the Ivindo River

o

IO

20

30

40

,

50

I

km

FIG.1. Map ofnorth-easternGabon showing the Mekambo iron district.Iron ore deposits are hachured and the area of Figure 2 is outlined.

Unesco, 1973. Genesis of Precambrian iron and ìnaizg'onese deposits.Proc. Kiev Symp., 1970. (Earth sciences, 9.)

323

S. J. Sims

from Makokou or by air to a small landing strip near the deposit (Fig. i). The deposit is located in an area roughly 20 km north-south by 5 km east-west. T h e latitude and longitude of the centre of the deposit are 13'14'E and 1'6' N. The Belinga deposit is in the equatorial rainforest, where rainfall averages about 2.5m per year.Severalnorthsouth trending ranges are in the area and rise up to 550 m above a gently rolling plain which has an average elevation of 500 m above sea level. Maximum relief in the area is about 550 m, and ranges from a low elevation at the Ivindo River of 450 m to a high elevation of 1,000m at the highest peak in the area. Topography ranges from moderate to rugged, with many oversteepened slopes. Vegetation is dense and ubiquitous,and exposures of rock are limited to scattered outcrops along the crests of the ranges and in a few stream courses. The Belinga deposit consists of six explored and four unexplored ore bodies situated along the crests of the ranges, the distribution of which is shown in Figure 2.

Exploration of the ore bodies was by means of adits and by surface geological and topographic mapping. Chame1 samples were taken at 2 m intervals along the adit walls. A total of 8,027 m in fifty-nine adits was driven, proving at least 515 million metric tons of iron ore averaging 64.2 per cent Fe,2.2 per cent SiO,, 3.5 per cent Alzo,, 0.122per cent P,and 3.8 per cent ignition loss (in these rocks this can be considered as equivalent to H,O +).An additional 50 million metric tons of iron ore of similar analysis are estimated in the four unexplored ore bodies. Previous exploration at Boka-Bokaand Batouala yielded about 300 million metric tons of iron ore. The over-all probable tonnage for the Mekambo district is, therefore, 865 million metric tons. Within the Mekambo district additional but unknown tonnages of ore exist in the Minkebe and Kokomeguel deposits. The north-easternpart of Gabon is a vast plateau of Lower Precambrian basement rocks consisting mainly of quartz diorites with scattered areas of amphibolites and iron-formation (Hudeley and Belmonte, 1966). Almost nothing is known of the structure and stratigraphy in this region. Included within íhis basement complex is the iron-formation,aregionallymetamorphosed layered quartziron oxide rock. Known exposures in Gabon extend over an arc-shaped area from Boka-Boka on the south-east through Batouala and Belinga to Minkebe on the north and Kokomeguel on the north-east. It is presumed that this iron-formationis a single unit or series, but this has not yet been established. Almost everywhere the basement complex has been weathered to laterite and lateritic clay from which almost all ofthemain mineralcomponents,except alumina,iron oxide, and silica,have been leached.However,original texturesand structures are preserved in many places allowing tentative identification of the parent rock. The iron-formation is much more resistantto weathering and erosion and consequently forms distinct ridges throughout the region.

Rock types IRON-FORMATION

BELINGA

AREA

:=----;I R O N F O R M A T I O N CONTACT.

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I

2

3

4

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I

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ORE BODY, EXPLORED

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skm

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FIG. 2. Map of the Belinga area showing the explored and unexplored ore zones and the distribution of iron-formation. 324

The iron ores are derived from, and are gradational to, the iron-formation.An arbitrary limit of 60 per cent Fe is defined as the boundary between ore and iron-formation in this paper. The iron-formationat Belinga is a regionally metamorphosed layered rock unit consisting of thin alternating layers,lamellae,and lenses of quartz and iron oxide and also clay (mainly as kaolinite). Because of structural complicatioiis and lack of data, no thickness for the ironformation has been measured.It is estimated to be between 100 m and 200 m thick. In places the iron-formation is almost entirely clay and hematite. The presence of clay in the iron-formation at Belinga has led to a subdivision of the iron-formation into three rock types based on the relative amounts of quartz,iron oxide and clay. These are: ítabirite, argillaceous itabirite and hematitic phyllite. The

The Belinga iron ore deposit (Gabon)

term itabirite is used herein as defined by Dorr and Barbosa (1963)in Brazil. If the iron-formationcontains over 5 per cent clay, it is termed argillaceous itabirite, and if it con-

tains less than 5 per cent quartz, it is termed hematitic phyllite. Figure 3 is a triangular diagram of the three mineral components showing the compositional fields of each type of iron-formation.This is an empirical diagram and represents arbitrary limits based on field identification of the three types of iron-formationduring mapping of the explorationadits.The chemical analyses of channelsamples for each type of iron-formation as mapped in the adits were converted to percentages of mineral components, and the mineral components for each type of iron-formation were then plotted on a triangular diagram. The lines separating the three types of iron-formationin Figure 3 are straightened and separate the fields of maximum concentrationof each type of iron-formation.This diagram was made from 725 points. IRON

OXIDE

ARGILLACEOUS ITABI RITE

CLAY 8

QUARTZ

BAUXITE

FIG.3. Triangular diagram showing the compositional fields of types of iron-formationat Belinga. Based on 725 points. The relative amounts of each type of iron-formation based on the intercept-distancein the adits are: itabirite, 47.1 per cent; argillaceous itabirite, 22.7 per cent; hematitic phyllite, 30.2 per cent. N o stratigraphic relationships have been worked out among these three types of iron-formation. Itabirite. This rock type consists of interlayered quartz and iron oxide. In general the layers are discontinuous, range in thickness from about 0.05 mm to 10 mm,and consist mainly of either one or the other component. Quartz layers are composed of a granoblastic mosaic of grains which typically show undulatory extinction and locally have strain lamellae. Quartz grains range in diameter from 0.01 mm to 0.2 m m . Contacts between grains vary from sharp to indistinct where very fine-grained

impurities are concentrated between the grains. In some samples quartz grains are elongate within the layering and are oriented parallel to isoclinal fold hinges. Iron oxide layers are composed of grains of hematite (typically with relict traces of magnetite in the cores of the hematite grains) and in places limonite (as partial replacement of hematite). In many layers the grains of hematite appear to have grown together forming an anhedral tabular mass of hematite. Iron oxide layers are approximately the same thickness as quartz layers. Contacts between layers are relatively sharp. Itabirite ranges from very friable to hard and massive, depending on the degree of weathering. Weathering of itabirite causes a break-down in intergranular contacts between quartz grains due to leaching of silica. The average chemical analyses (wt. per cent) of 446 samples of itabirite at Belinga is: Fe, 46.9; Mag. Fe, 5.9;SO,,30.3;P,0.047;Alzo3, 1.2;loss on ignition,1.1. These analyses include both fresh and weathered itabirites. For comparison, the average analysis (wt. per cent) for fifty-four samples of fresh itabirite is: Fe, 38.8; Mag. Fe, 5.8; Sioz,42.7;P,0.035;Alzo,, 0.8;loss on ignition,0.7. In two separate adits, itabirite rich in a prismatic mineral altered to limonite was noted. The prismatic form of the mineral strongly suggests amphibole, but because of the high degree of alteration, no positive identification was made.Amphiboles were identified in itabirite at BokaBoka as hornblende and riebeckite (Mekambo Syndicat, 1959), and these may well be present at Belinga too. This is not a widespread type of iron-formationat Belinga. Argillaceous itabirite. This rock type is an itabirite with over 5 per cent clay mineral (kaolinite) interlayered with quartz and iron oxide.The clay occurs in distinctlayers of about the same size as quartz and iron oxide layers and is also intermixed with iron oxide forming a groundmass for hematite grains. All samples of argillaceous itabirite observed were weathered and very friable. The average chemical analysis for 214 samples of argillaceous itabirite is (in wt. per cent): Fe, 49.4; Mag. Fe, 6.5; Sioz, 21.9; P, 0.082;A1,0,, 4.1;loss on ignition,3.0. Hematitic plzyllite. This type of iron-formationis composed of clay and hematite with scattered layers of granular quartz. The rock is very fine-grained and thinly layered, with layers alternating between hematite-richand clay-rich, but on a scale such that megascopically the rock appears nearly homogenous. The average grain size of hematite is about 0.02mm.In some places distinct layers of white clay up to 10 mm thick are present. Quartz layers range up to 10mm thick and are unevenly distributed. Hematitic phyllite is friable in all observed occurrences and is typically reddish. The average analysis of 235 samples of hematitic phyllite (in wt. per cent) is: Fe, 52.4; Mag. Fe, 8.1; Sioz, 9.3;P,0.090; A1203, 9.2; loss on ignition, 6.0. It should be noted that in the hematitic phyllites,Alzo, is in excess of the SiO,necessary to form clay mineral. In this case it is assumed that the excess Alzo,occurs as bauxite. N o fresh samples of hematitic phyllite were noted at Belinza. 325

S. J. Sims

OTHER R O C K S

Other rock types present in the area are almost completely altered to clay mineral (kaolinite). Relict textures and structures suggest the following types are represented:phyllite, schist, cataclastic rock and intrusive igneous rock. These rock types are interlayered with the iron-formation, range in thickness from about 1 m to severaltens of metres, and are typically in sharp contact with iron-formation. Lineations and drag folds are present in these rocks as in the iron-formation,indicating they are concordant. Scattered lenses of quartz occur throughout these rocks but are not typical, suggesting the original rocks were mainly quartz-free.The main valleys of the Belinga area are probably underlain by these rocks which were less resistant to erosion. Quartz veins cut all the rock and ore types (with the exceptionof hard massive ore) and occur as irregularmasses and as true veins, mainly discordant,and frequently with associated coarse crystals of specularite. Quartz veins are always deformed and the quartz in the veins is splintery and friable.Some of the quartz masses have spots of white clay suggesting altered feldspars and the possible presence of pegmatites.

ORIGIN

Only a brief statement is given concerning the origin of the iron-formationat Belinga. The itabirite is believed to have formed from ferruginous cherts by recrystallizationduring regional metamorphism. The argillaceous components of argillaceous itabirite and hematitic phyllite represent clastic interruptions during the predominantly chemical sedimentation of ferruginous cherts when shaly material was deposited.

Structure T w o generationsof folding are evident in the Belinga rocks. The earlier folding was isoclinal and was formed in response to metamorphicdeformation and recrystallization. These folds have attenuated limbs, thickened,sharp crests (Fig. 4), and are present throughout the area.They range in size from microscopic to at least several metres across. The hinge lines of these folds define a lineation throughout the area which is illustrated in Figure 5, a stereogram of 165 measured lineations.This shows a maximum concentration of points plunging 70"S,50" E and a rotation of points about a horizontal axis trending about N 17"E.The horizontal rotation is caused by the second generation of folding. The second generation of folds has nearly horizontal axes and open and irregularly shaped crests with many open cavities parallel to the layering. These folds are characteristic of brittle folding, and in places have an almost 326

lrn

FIG.4.Isoclinally folded itabirite. Adit 121, 87 m, Mombo Range,looking parallel to fold axes. N

s FIG.5. Stereogram of lineations in the Belinga area. Equal area net, lower hemisphere, 165 points, contours at 3, 6,9,12, 15 and 18 per cent.

The Belinga iron ore deposit (Gabon)

the layering is at an angle to bedding as illustrated, for example, in a hematitic phyllite where a granular quartz bed is cut by layeringand elsewherewhere a contact between schist and soft platy ore is at an angle to layering.In these places layering is a foliation.Figure 8 is a stereogram of poles to layering for 1,496points in the Belinga area and shows that the layering (bedding) is concentrated at about a strike of N 15"E and a dip of 30" S-E. Figure 8 also illustrates the second generation of folding by a scattering of points rotated about a nearly horizontal axis trending N 15"E.Isoclinalfolding would not be illustrated because both limbs of isoclinalfolds have nearly the same attitude. Based mainly on the idea that small scale structures reflect large scale structures and that the attitude of layers in the Belinga area is relatively consistent, it is suggested that the rocks of the area are isoclinally folded on a large scale and are thereby repeatedly exposed throughout the area.Also, as a result of this folding,the iron-formationis locally thickened, thereby providing favourable zones for iron ore development. N o major fractures were encountered in the adits,and consequently no faults are shown on Figure 2, although it is highly possible many faults will be uncovered when mining begins. A stereogram of poles to shear fractures measured in the adits shows a bimodal concentration of vertical planes trending about N-S and N 20"E,or nearly parallel to the second generation fold axes,suggesting that the shears may also have formed.inresponse to collapse of the iron-formationand iron ores. In the adits intraformational breccias were observed in places mainly in argillaceousitabirite.These breccias are

chevron structure. Figure 6 is a stereogram of 203 fold axes showing a concentration of axes nearly horizontaland trending N 18"E.These folds occur exclusively in leached iron-formation and iron ores (with the exception of hard massive ore) and are attributed to collapse due to leaching of silica. Figure 7 illustrates this type of folding. All of the rocks at Belinga are layered to varying degrees. The layering is considered parallel to the original bedding in most places, modified by recrystallizatioii,but neverthelessreflectingbedding,In some exposures,however, N

s FIG. 6.Stereogram of secoiid generationfold axes in the Belinga area.Equal area net,lower hemisphere,203 points, contours at 3, 6,9 and 12 per cent.

lm

FIG.7.Second generation collapsefolds in high grade soft platy ore. Adit 116E, 50 rn, Bakota South Range. 327

S. J. Sims

N

The preceding types of fracturing may well have influenced the permeability of the iron-formationand consequently may have been a control for ore formation.

Iron ores O

,s FIG.8. Stereogram of poles to layering in the Belinga area. Equal area net, lower hemisphere,1,496points,contours at 2, 4,6, 8 and 10 per cent.

typically less than 1 m thick and are concordant with the layering,at least where observed in the adits. These zones represent places where metamorphic folding exceeded the ability of the rocks to accommodate plastically to the deformation. In some thin sections the crests of microfolds are fracturedalong axial planes forming an axialplane cleavage.

The iron ores of Belinga occur on and beneath the crests and upper flanks of the ranges in ten ore zones (Fig. 2). The ore grades down dip to iron-formation,the bottom contact ranging from less than 1 m to over 100 m below the surface. The bottom contact is irregular because it interfingers with iron-formation.The typical shape of the ore bodies is, therefore, crudely tabular with the length parallel to the range,the width perpendicular to the range, and the thickness perpendicular to the upper surface of the range.With the possible exception of the hard massive ore, exploration results show that almost all of the ore at Belinga occurs within 100 m of the surface. A n example of typical ore occurrence at Belinga is illustrated in Figure 9,which shows the surficial nature of the ore grading downwards to iron-formation. The iron ores at Belinga are classified according to grade,texture,and structure.High grade ore contains over 66 per cent Fe, intermediate grade ore ranges from 60 per cent to 66 per cent Fe,and low grade ore ranges from 45 per cent to 60 per cent Fe.Only intermediate and high

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CZZl GANGA ORE

E%ZiCLAY SOIL ! X Z lINDURATED, HYDRATED, PLATY U SOFT, PLATY

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HEMATITIC PHYLLITE O CLAY (ALTERE0 SCHIST) D INTRUStVE

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FIG.9. Structure section, Section 117,Bakota South Range, showing position of supergene ore and repetition of units by isoclinal folding. 328

200

The Belinga iron ore deposit (Gabon)

grade ore are economic, so that the low grade ore can be considered only as enriched iron-formation. Ore textures range from soft (friable) to indurated (due to secondary limonite)to hard (compact)and structuresrangefrom granular to platy to massive.A given ore is then referred to by grade, texture and structure as for example, intermediate grade indurated platy ore. The Belinga ores (both high and intermediate grades) fall mainly into the following classifications:hard massive, indurated platy, soft platy,and soft granular. Other types, such as indurated granular and hard granular,are present, but are. The distribution of the ore types (both high and intermediate grades) at Belinga in the several ore bodies is shown in Table 1.

TABLE 1. Volume per cent distribution Ore body

Bakota North Bakota South Bakouele Mombo Babiel North Babiel South Kombi Total,Belinga

Soft platy Soft granular

50.0

7.8

45.3 65.8 40.5 13.9 57.2 88.9 46.1

12.1 .11.9

6.1 2.5 9.5 8.5

9.4

Indurated

38.7 8.5 20.3 53.4 83.6 33.3

O 33.4

Hard massive

3.5

34.1 4.0 O O O 2.6 11.1

It is noteworthy that soft platy and indurated platy ores are most prevalent and that hard massive ore is mainly restricted to one ore body. Soft granular ore is widely distributed, but does not make up a large percentage of the total. Typically,indurated platy ore occurs at the surface and grades downward to soft platy and soft granular ores. However, there are places where soft platy ore is directly beneath a thin soil cover. Hard massive ore on Bakota South Range occurs at the surface and continues to depths of at least 100 m.Contacts between ore types are characteristically gradational over a distance of several metres and are not necessarily defined by layering. Canga, a separate and unimportant type of ore at Belinga, is a rock composed of detrital material derived from iron ore and iron-formationand cemented by limonite. Canga is not widespread,occurs at the surface,and is rarely more than 2 m thick. The occurrence of canga at Belinga is in sharp contrast to that in Brazil (Dorr, 1964)where it makes up a considerable percentage of the iron ore. This is probably due to differences in erosion rates between the two areas. The average analysis of canga at Belinga is 61.3 per cent Fe, 0.158 per cent P, 0.7 per cent SO,, 5.3 per cent Alzo, and 5.2 per cent loss on ignition. The iron ores are composed mainly of hematite, with varying amounts of limonite and minor amounts of quartz and clay. Magnetite occurs only as remnants in hematite grains and makes up less than 10per cent of the iron oxide. Limonite occurs as rims around hematite,as linings in cavi-

ties, and as a ground mass for hematite grains. Limonite may also replace individualhematite grains,generally along a given granular layer, forming indurated plates. Hematite occurs in both granular and specularitic forms, the latter being present mainly in the hard massive ore but also in vugs in hard plates in soft platy ore. Granular hematite ranges from 0.005 m m to 0.5 mm in diameter and averages about 0.1 m m . Specularite blades occur mainly as outgrowths from granular hematite and in places specularite forms concordant layers composed of intergrown blades. In many samples of hard massive ore and some samples of soft platy ore,finely crystalline specularite lines open cavities in the ore. Specularite crystals have grown at the expense of quartz and in ores of high specularite content (hard massive ore) replacement relations between specularite and quartz are noteworthy. Quartz is present in most of the ores in amounts less than 5 per cent,where it occurs mainly as loose grains in pores. Quartz grains tend to be concentrated along layers parallel to the layering in the ore.Clay minerals are present in some of the ores, both as a primary compound and as coatings on fractures as the result of infiltrations from the surface. The iron ores are layered to various degrees. In hard massive ore layers are not well defined,but on close inspection contrastinggrain size and layersof slightlymore porous material define layering and isoclinal folding the same as observed in itabirite. In the soft layering is more distinct and is defined by plates of harder hematite in granular hematite, and by layers of contrasting grain size and porosity.Small scale isoclinalfolds are,for the most part,not preserved,having been obliterated by the second generation folding. In the indurated platy ores, plates of hematite cemented and partly replaced by limonite define layering. The ores show a range of porosity from a low of 8 per cent for some samples of hard massive ore to 55 per cent for some samples of soft granular ore. The porosity is reflected by the average in-placedensity for each ore type as shown in Table 2.

TABLE 2. Ore type

Hard massive Indurated platy Soft platy Soft granular

Density, in-place,tons/m3

4.2 3.6 2.9 2.6

The followingtable (Table 3) summarizesthe chemistry of the various ore types as shown by averages of channel samples for each type. N o analyses are available specifically for high grade soft granular ore, although experience shows that this type of ore is chemically similar to high grade soft platy ore. It is noteworthy that the A1,0, and loss on ignition analyses are relatively high for all ore types,especially the intermediate grades.This reflects the presence of limonite and clay

329

S. J. Sims

TABLE 3.

Inteurnediate guade ore

Fe Mag. Fe P SiO,

Loss on ignition Number ofanalyses

63.6 0.3 0.128 1.3 4.1

2.6 (96)

63.4 5.1 0.165 1.3

3.2 4.0 (415)

63.9 62.2 7.9 9.2 0.101 0.072 2.2 6.4 2.3 1.4 3.2 2.4 (514) (144)

High guade ore

Fe 67.3 67.2 67.5 Mag. Fe 0.4 6.8 5.9 No P 0.082 0.116 0.078 analyses SiO, 0.7 0.5 1.3 available A1203 1.7 1.4 1.2 1.4 0.9 Loss on ignition 1.2 Number ofanalyses (105) (205) (399)

in the ore, but mainly it reflects the presence of surficial lateritic clay which has infiltrated by means of meteoric water. The phosphorus content is also relatively high for those ores with high Alzo,and loss on ignition and is believed to be due mainly to the association of phosphorus and limonite. Experience at Belinga has shown a close relationship between limonite content and high phosphorus analyses.It should be noted that the above analysesare from channel samples that were not washed because the in-place analyses were needed. Consequently, infiltrated surface material along joints in the ore accounts in part for the unusually high Al,O,contents of the ores as shown in the channel samples. Granulometric studies on bulk samples of indurated platy and soft platy ores were made. They show that, for indurated platy ore,the average size analysis is 36 per cent plus 3/8” and 12 per cent minus 100 mesh, and for soft platy ore the analysisis 28 per cent plus 3/8” and 14per cent minus 100 mesh. These tests also showed that, in general, there is little chemicalvariation between size fractions for a given sample,but that Alzo3 is slightly higher in the coarser fractions and Si02 slightly higher in the finer fractions. Phosphorus content is nearly equal for all size fractions.

Origin of the ores The hypothesis presented herein for the origin of the ores at Belinga involves two generations of concentration of iron oxide diflering widely in time of concentration,method of concentration and type of ore produced, but with one process superimposed on the other. The first generation of iron oxide enrichment formed hard massive ore by metasomatic replacement of quartz in iron-formation by specular hematite. It is believed this 330

replacement took place after, or near the end of, Precambrian metamorphic deformation, probably as a result of hydrothermal activity associated with emplacement of igneous rocks. The evidence that suggests this origin of hard massive ore is as follows: 1. Structural details, the same as observed in ironformation, are preserved in hard massive ore. This includes the small isoclinal folds and a consistent linear direction. 2. Layers of intergrown specularitewere noted only in hard massive ore. 3. Replacement relations between specularite and quartz were noted only in hard massive ore. 4.Hard massive ore is found in one ore body,is not widely distributed as are the other types,and is not obviously related to the present-day surface. 5. Lenses ofhard massive ore up to 1 m thick were observed in unleached itabirite showing discordant contacts. A lens of this type is clearly metasomatic because small isoclinal folds are preserved in it, and is not supergene because the itabirite is not highly leached. 6. Polished sections of hard massive ore show no evidence of hydration or replacement of hematite by limonite. Hard ores tend to show the least amount of relict magnetite in hematite, which indicates a higher degree of replacement of magnetite by hematite than in the other ore types. 7.The presence of quartz veins with coarse specularite throughout the area attests to a period of hydrothermal activity.Smallvugs of specularitein the ores are thought to represent lenses of quartz which were incompletely replaced and later leached out. It is thought that certain zones of the iron-formationwere more permeable to hydrothermal fluids, perhaps due to favourable structures such as fold crests fractured during late-stagemetamorphism. In these zones quartz was replaced and hard massive ore was formed, the degree of replacement determining high or intermediate grades. It is also thought that a source of the iron could have been the oxidation of magnetite to hematite, evident in the ironformation throughout the area. This oxidation,for equal volumes,yields a slight amount of excess iron which could easily account for the enrichment.It shouldbe clearly noted that the above suggestion is speculative and much more information is needed to coníìrm it. At a much later geological time, when the ironformation was exposed to surface weathering, silica, predominantly quartz, was leached by percolating meteoric waters above the water table leaving a porous hematite-rich rock. Associated with the leaching of quartz was the alteration of the other rocks to clay. At and near the surface, iron oxide was partly hydrated forming the indurated platy ores. Hydration is seemingly a near surface phenomenon and as such may be related to vegetation, as pointed out by Ruckmick (1963) at Cerro Bolivar in Venezuela. The type of ore formed depended on the degree of leaching,degree ofhydration and the nature ofthe hematiterich layers and the distributioii of the quartz and hematite

The Belinga iron ore deposit (Gabon)

in the original iron-formation.For instance, argillaceous itabirite would yield alumina-richore, itabirite with dense hematite layers would yield platy ore, itabirite with less well-definedhematite layers would yield granular ore,and itabirite with partial leaching would yield intermediate grade ore.The leaching of quartz is undoubtedly continuing under present day conditions. The following observations suggest that the iron ores (other than hard massive ore) formed by ground water leaching of quartz from the iron-formation: 1. Iron ore is restricted to near the surface. 2. Iron ore grades to iron-formationat depths generally less than 100 m. 3. Density and porosity measurements show that if pore spaces in soft platy and soft granular ores were refilled with quartz,the density would correspondto an itabirite. 4. Collapse structures in the ores indicate removal of a large volume of material. 5. In two adits a down-dipgradation from iron ore to ironformation can actually be observed. 6. Silica contentmeasurements in springs and streamswere measured by Park in 1958 and showed a range of from near zero in adits in the ore zone to 14p p m in springs at the base of the ranges. The Ivindo Rover measured 9 ppm. These measurements show that SiO, is soluble under present day conditions. Figure 10 shows analyses taken from a typical exploration adit. This shows a decrease in Fe and loss on ignition (IL) and increase of SiO, with depth, reflecting a decrease in leaching of SiO, and hydration as distance below the surface is increased. Densitymeasurementsinplaceweremadeon100samples each of the four main ore types. Using only the densities obtained for soft platy and soft granular ores, and assuming the pore spaces were once filled with quartz and that the iron oxide mineral is hematite, a density was calculatedfor the assumed unleached quartz-hematiterock. From a total of 114 samples, the following results were

-

-

obtained: range of porosities: 30-55 per cent; average porosity: 36 per cent; range of calculated densities:3.1-4.2; average calculated density: 3.7. From the density, a composition of F e and SiO, was calculated assuming only hematite and quartz. Using the above data, the range in Fe for the unleached parent itabirite would be 20-52 per cent and the average would be 40.5per cent Fe. It is interesting to note that the average F e percentage for mainly fresh itabirite is 38.8per cent Fe. Therefore,the calculated F e content of pre-leacheditabirite (40.6per cent Fe) compares well with the actual average of fresh itabirite (38.8 per cent Fe) and strongly suggests that leaching alone can account for the soft platy and granular ores. It should be noted that the density measurements in these ores represent maximum values because the ores have been collapsedin part due to leaching of silica.Consequently, the Fe content as calculated would also be a maximum. In two adits the down dip gradation from ore to ironformation could be observed.In one of these adits a sample was taken in vertically dipping layers at the top of the adit in platy ore aiid at the bottom in itabirite, a separation down the dip of about 2 m. The results are shown in Table 4. In the other adit no comparativeanalyses are available, TABLE 4.

Fe Mag. Fe

62.3 3.3

P SiO, A1,0, Loss on ignition

0.032 7.8 1.3 1.2

55.8

1.3 0.040 19.2

3.0 1.0

7 .

cio2 r70 7

r

Y . Fe

i

............................................ O

10

eo

30

40

............................. ............ .....si02 ... ... 50

HORIZONTAL DISTANCE

60

FROM

70

80

O 90

96

PORTAL

FIG.10. Graph showing chemical analyses of channel samplesv. horizontal distance in metres from the portal. Adit 124,Bakouele Rangs.

331

S. S. Sims

but a striking contrast from quartz-freeplaty ore grading down dip at 60" to quartz-bearing soft itabirite was noted and is illustrated in Figure 11. It has been suggested by Park (1959)that hard massive ore in French Equatorial Africa may have formed by supergene replacement of quartz by hematite.It is possible that locally,on a small scale, some hematite has formed at the surface,but little evidence of this was observed at Belinga. For reasons cited above, the large mass of hard massive ore at Bakota South is not considered supergene. It is apparent that,because iron ore is not everywhere formed over iron-formation,there are some controls to supergene enrichment. Grain size of parent iron-formation may have been a factor, but more likely structural deformation was more important. In zones of greater metamor-

phic deformation,the iron-formationwas probably more permeable to meteoric waters, as it would have been to hydrothermal fluids.Thus the association of hypogene and supergene ore may be more than coincidence. In summary,then,the Belinga iron deposit is believed to have formed as the result of supergene leaching of silica from an iron-formationthat had been structurally thickened by isoclinal folding and enriched locally by hydrothermal replacement of quartz by hematite. The fortuitous combination of metasomatic replacement and structural thickening of the iron-formationaf Belinga during Precambrian deformationprepared a favourable localefor surficialleaching when the area was exposed to prolonged weathering under the stable geologicenvironment ofthe centralAfrican Shield.

FIG.11. Soft platy ore grading down-dipto itabirite.Note appearance of white granular quartz layers in the centre of the picture. Adit 115W,66.5m,Bakota North Range.

Résumé Les minerais de feu de Bélinga, au Gabon (S. J. Sims)

En raison des conditions tropicales extrêmes et de la cou-

Le gisement de minerai de fer de Bélinga, dans la partie nord-estdu Gabon, est le plus important gisement du district de Mekambo qui est,pour la plus grande partie,encore inexploré. Il a été découvert en 1955 et contient plus de 550 millions de tonnes de minerai avec une teneur en fer de 64 %. I1 se répartit entre six massifs de minerai explorés et au moins quatre encore inexplorés le long de crêtes de direction générale nord-sud.I1 occupe une surface totale d'environ 5 km sur 20 km dans la forêt équatorialehumide.

verture forestière,la géologie de la région est peu connue, et celle de la région de Bélinga a été interprétée par extrapolation entre les sections accessibles et d'après les caractères topographiques. Les minerais de fer proviennent de la formation de fer de Bélinga,à des degrés différents, et cette formation peut être subdivisée en trois types de roches ou de faciès :itabirite argileuse et phyllite hématitique. La distinction entre chaque type est faite d'après la proportion relative des trois composants principaux :hématite, quartz et argile (kaoli-

332

The Belinga iron ore deposit (Gabon)

nite). L'itabirite se présente sous forme d'hématite discrètement rubanée et de lamelles de quartz.L'itabirite argileuse est une itabirite dans laquelle on reconnaît des lamelles d'argile et la phyllite hématitique est intercalée et mélangée à l'argile et à l'hématite. D'autres types de roches de la région de Bélinga sont ia phyllite, le schiste, les roches cataclastiques et les roches intrusives.Toutes sont décomposées à l'état d'argile. Des filons de quartz coupant çà et là la formation de fer sont caractérisés par la présence de spécularite. Toutes les roches datent du Précambrien. L a formation de fer de Bélinga a une direction générale nord-sud.Elle plonge rapidementsurtout vers l'est. L a formation de fer est plus résistante à la désagrégation et forme des chaînes de collines. A u point de vue structural, la région est interprétée comme une zone de plissements isoclinaux,ce qui a pour effet la répétition des formations de fer le long d'une série de crêtes parallèles,La déformation à l'intérieur de la formation de fer est marquée par une structure rubanée et l'on y observe des plis isoclinaux avec des flancs atténués et des crêtes serrées formant une structure linéaire qui plonge rapidement.E n superposition à ces plis métamorphiques plus anciens,on note des plis horizontaux de petites dimensions dans les formations altérées de fer et dans le minerai de fer.On en conclut que le plissement horizontal plus récent a eu pour cause l'effondrementdû à la lixiviation de la silice de la formation de fer. Les minerais de fer ont été classés en fonction de leur teneur, de leur texture et de leur structure. Les minerais à haute teneur contiennent plus de 66 %de fer,les minerais intermédiaires entre 60 et 66 % de fer et les minerais à faible teneur de 45 à 60 % de fer. On estime que seuls les minerais à haute teneur et à teneur intermédiaireont, pour le moment, une valeur économique. Les minerais à haute teneur se présentent sous différentes formes : compact dur,compact lamellaire,tendre,compact tendre.On passe des structures massives à des structures lamellaires puis granuleuses, et de textures tendres (friables) à des textures indurées puis dures. L'auteur présente les analyses chimiques des différents types de minerai. O n passe progressivement d'un type de minerai à l'autre.

A l'exception du minerai compact et dur à haute teneur, les minerais ont été formés par lixiviation de la silice de la formation de fer. A cette lixiviation s'est associée une addition d'oxyde de fer hydraté à certains types de minerai. Toutefois, on connaît de nombreux exemples de minerai tendre à haute teneur dans lesquels on ne relève aucune hydratation ou seulement une faible hydratation. Par endroits,la densité et la porosité ont été mesurées sur de nombreux prélèvements de minerai à haute teneur ou à teneur intermédiaire et allant de la structure lamellaire tendre au minerai granuleux. Ces mesures montrent qu'un minerai à haute teneur peut provenir d'une itabirite par simple lixiviation du quartz; l'hydratation est en effet secondaire et limitée essentiellement aux couches voisines de ia surface. L'article donne des exemples de passage du minerai à l'itabirite lixiviée lamellairetendre et à l'itabirite renouvelée. L'effondrement du plissement, évident dans les minerais et la formation de fer lixivié,est la preuve d'un déplacement d'une importante quantité de sílice. Le minerai dur compact à haute teneur est sans doute d'origine sinmétamorphique.II a été formé par le remplacement métasomatique hydrothermal du quartz par de l'hématite durant les derniers stades du métamorphisnie. Ce processus a eu,apparemment,à Bélinga,un développement limité car ce type de minerai ne compte que pour 10 % de la totalité du minerai de fer. O n a reconnu de l'hématite compacte dure dans l'itabirite sous la forme de lentilles discordantes dans lesquelles la survivance de certaines structures de l'itabirite a été conservée. O n pense que durant les derniers stades du métamorphisme, après le plissement,des intrusionsignées accompagnéesde courants hydrothermaux ont pénétré la formation de fer localement. Le quartz a été remplacé localement dans la formation de fer.Tout cela a probablement été favorisé par les structures existantes.L a combinaison fortuite du remplacementmétasomatique et de l'épaississement structurai de la formation de fer pendant la déformation précambrienne a favorisé la lixiviation superficielle lorsque la région a été exposée à une altération prolongée sous l'environnement tectoni quement stable du bouclier africain central.

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Bibliography/Bibliographie AUBAGUE, M.1955.Les gisements de fer de la région MakokouMekambo. BdI.Div.Min.Géol. A.E.F.,no. 7, p. 61-7. .1956.Les gisementsde fer de la région Makokou-Mekambo (Massif du Djaddie-Djouahet de l'lvindo). Bull.Div.Min.

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Géol. A.E.F.,no. 8, p. 45-52.

BARROT, M.1895.Sur la géologie du Congo français.Ann. Min., Paris,9' Série,t. VII, p. 379-510. CHOCHINE, N.1938. Notes sur trois gisements de fer dans la zone F.Brazzaville,Gouvernement Général de I'AEF,Service des Mines (Unpublished.)

__ . 1950. Notice

explicative sur la feuille Malcolcou-Est.

Brazzaville, Gouvernement Général de I'AEF.16 p.

CHOUBERT, B. 1937. Étude géologique des terrains anciens du Gabon. Thèse, Paris,Rev. Géogr. Phys.,210 p. DEVIGNE, J. P.,PLEGAT, R. 1954.L e gisement de fer de BokaBoka. Rap. Annu.Serv.Géol. A.E.F.1954, p. 71-4. DORR, J. VANN.II 1964.Supergene iron ores of Minas Gerais, Brazil. Econ. Geol., vol. 59, p. 1203-40. DORR, J. VAN N.; BARBOSA, A. L. M.1963.Geology and ore deposits of the Itabira district,Brazil. Pyof. Pap. U.S.geol. SWV.,341-C, 110 p.

HUDELEY, H.; BELMONTE, Y.1966. Carte géologique de la Réprrblique gabonaise. Paris, Bureau de recherches géologiques et minières.

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S. J. Sims

LAUNAY, L.de. 1903.Les Richesses minérules de l’Afrique.Paris, Béranger. MEKAMno SYNDICAT1959.Gabon,French Equatorial Africa,the Boka-Bolcairon deposits,March, 1959.Bethlehem Steel Corporation private report,68 p. PARK, C.F.Jr. 1959.Origin of hard hematite in itabirite.Econ. Geol., vol. 54,p. 573-87.

PERIQUES, L. 1911.Mission d’étirdes au Gabon: Chemin de fer du Nord et Mission Iiydrogruyhique, Paris. ROUQUETTE, G.1938.Étude des gisenients de fer de Boka-Bolca, Cocotiodie,Ivindo (Gabon), Brazzaville,Gouvernement Général de I’AEF,Service des Mines. 63 p. RUCKMICK, J. C. 1963,The iron ores of Cerro Bolivar,Venezuela. Econ. Geol., vol. 58, p. 218-36.

Discussion J. VAN N.DORR. Does the distribution of the supergene ore suggest that ore formation was related to particular elevations? In other words is ore formation related to a particular erosion surface or peneplain?

S. J. SIMS.Hypogene ore makes up about 10 per cent of the total ore. B. CnouBERT.Is there any relationshipbetween the Belinga and Boka-Boka deposits in the eastern part of the region?

S. J. SIMS.Yes, formation of the supergene ores appears to be related to an ancient erosion surface. The elevation of ore bodies in the Mekambo District is roughly equal.

G.A.GROSS. What is the proportion of supergene ores to hypogene ores?

334

S. 5. SIMS.Yes, I believe that Boka-Boka is related to Belinga, perhaps by the same iron-formation.However, the precise relationship has not yet been established.

Itabirite iron ores of the Liberia and Guyana shields H.Gruss Gewerkschaft Exploration, Dusseldorf, Federal Republic of Germany

In 1969 world production of iron ores reached approximately 670 million tons, of which nearly one-third-about 210 million tons-was shipped from the producing countries to the consumers.Thus,Liberia,Sierra Leone and Venezuela nowadays may be considered to be the most important oversea’s iron ore suppliers for the United States of America and European industries. All ores of these countries were mined in itabirite iron ore deposits. Similarities and analogous qualities of the ores, as well as a common geology which reaches back to the oldest Precambrian,justify a mutual study and description o€ itabirite iron-formations of both the continents. The first part of the study, therefore,is a summing up of the genesis of itabiritic iron ores and in the second part a short description is given of all mines producing at present. It is no easy task to compile a summary on the sedimentation of itabirite iron-formations,their ages, degree of metamorphosis, orogenic modification, weathering and the result and formation of high grade ores on both shields. According to the political splitting of the areas on both sides of the ocean, each country started its own geological research. Thus, during the past decades, a multitude of conceptions on the geological structure of the various countries has been set up, and though they are valid for the country coiicerned, they often lack relationships to the neighbouring countries. The sedimentary and metamorphic change of facies of the Precambrian rocks, as well as their varying definitions and nomenclature, make it even more difficult to make a comparison. In spite of this, the author collected all interesting details concerning itabirite iron-formations,and with this material he attempted to compile a kind of summarywhich, however, cannot be considered as complete or infallible. It only shows the present state of knowledge regarding itabirite iron-formationsin those areas. During a check-up of the details, it also became evident that the deposits’ geological investigations progressed at different speedsand so they are often incomplete. Thus,when drafting the summary, at first the well-known

areas were described and, based on these, the geology of the lesser-known areas was treated.

Stratigraphical situation and age of the Precambrian and its itabirites in the Liberia and Guyana shields The data acquired through radiometric dating methods place the sedimentation of itabirites between 2,500 and 3,000 1n.y.in the Liberian as well as in the Guyana shield. These data are mainly obtained from gneisses, which comprise the cores of both shields. This is the case with the so-calledKasila-schists of Sierra Leone (3,200m.y.), large parts of the granitic or granulitic basements of this country (2,700-3,600m.y.), as well as with the gneisses of the itabirite deposits Bong Range (2,910-3,280 m.y.), Mano River (2,660-3,350m.y.) and Nimba (2,500m.y.) in Liberia.W e also find similar ages in the Guyana shield, where the gneisses of the itabiritic Imataca-seriesare also dated from 2,700-2,900m.y. (Allage data are obtained by whole rock analysis based on the Rb/Sr method.) Overlying this crystalline underground,there are more or less metamorphic sediments and igneous rocks.Contrary to most other Precambrian shields,they are not,however, separated by an unconformity from the footwall gneiss, but grade into each other depending on their degree of metamorphism.Thus, at least in the itabirite provinces of both the shields, up to now no real basement has been found. The metasediments overlying the gneiss consist of quartzites, quartz mica schists, amphibolites and igneous rocks, interlain by ítabirites. Though in Africa as well as in South America they bear different names, for instance Kambui-series (Sierra Leone), Nimba-series (Liberia), Siniandou-series(Guinea), Imataca-series(Venezuela), they may generally be considered to have a similar age (Fig. i). As metamorphosis of these Precambrian sediments resulted in the formation of the basal gneisses-as ascertained in Liberia (Bong Range) or Venezuela (1mataca)-

Unesco, 1913. Genesis of Precarribrinii iron and iitnizgnizese deposits. Proc. Kiev Syi>ip.,1970.(Earth sciences, 9.)

335

H.Gruss

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LI BERIA SHIELD

orogenesis

sedimentat ion

GUYANA SHIELD

folding and metamorphism 700-800 m ,y. (gneisses,NW-SE striking Liberia i Sierra Leone )

._....., ...

Torkwaien (Ghana ) (Malasse-facies)

orogenesis

-

OrOgWleS¡S

-

Roraima -Farmation 1 675 m.y. I 14 o I o s se -Fac i es )

metatexis - anatexis 1800 -2000 m.y. gran ¡tes and gneisses

metatexis -anatexis 1 800 - 2 OCO m.y. "yo un ger gro ni t es "

folding and regional metamorphism 72.500 73.000m.y.

folding and regional metamorphism 2 700-2.900m.y. "o 1der granites "

-

-r C O ._ A-.

O

-

U

._

sedimentation ..........__

I

i ta bi ri tes Imataca - Series metosediments Barama -Mazaruni -system Nimba -series Dahomeyen

O ul

C O U

unconformity 2 basement 2

kasila -schists (3200m.y.) "basement"( 2700-3.600m.y,)

?

FIG.1. Precambrian of the Liberia and Guyana shields.

the measured age of all these gneisses ought to be the same for these different series of metasediments and their itabirites. Therefore, gneisses and metasediments are defined as Precambrian I. Already, in the early Precambrian, the sediments were intensively folded, accompanied by a more or less vigorous regional metamorphosis (Liberia-green schist facies to amphibolitefacies). For this reason the geothermic gradient of this metamorphism in the Bong Range itabirite deposit (amphibolite facies) reached 5.5 kb and a temperature of 570-630" C,corresponding to a modification in a depth of 20 km. Sporadically, this regional metamorphism was more intense and led, in both shields, to the formation of gneisses and granite intrusions (Sula Mountains, Sierra Leone, Iwokrama-graniteGuyana). The coastal areas of both the shields were especially affected, whereas the intensity of the regional metamorphosis seems to diminish towards the interior. Based on investigations in Liberia, Leo and White (1968) declared that the age of this orogenesis and metamorphosis is in the Precambrian I-more precisely, lying between 2,500and 3,000 m.y. This is in accordance with the age of intruded granites in Guyana and in the Iwokrama-series (2,595 m.y.). It seems that in both the shields the gradient of orogenesis tends from the present coast towards the interior. This fact is valid for the area of the Imataca-series in Venezuela (S-vergence) as well as for the Liberian deposits 336

(Bong Range, Bomi H i l l with N-vergence). In the pasts of the shields situated near the coast, the degree of metamorphism is higher and a flat folding seems to prevail, whereas isocline-typefolding is likely to be found in the inland areas. This orogenesis did not, however, cause a consolidation of the two shields,as in later periods of the Precambrian thick series of igneous rocks and sediments were deposed on them (Precambrian 11). Examples are the Pastora-series of Venezuela, with an age of 1,6001,800 m.y.,the so-calledBirrimien (which is supposed to form large parts of the Eastern Liberian shield near the Ivory Coast, Ghana and Upper Volta) the age of which is stated as 1,800-2,000m.y. (Machens, 1966). U p to now no itabirites have been found in these middle Precambrian series of the Liberian shield,whereas, inthe Guyana shield,thisage could be valid forthe itabirites and gondites of the Amapa area in Brazil and probably even for the itabirites of Southern Surinam. The next discernible period in both shields is a second orogenesis and metamorphosis. It affected the rock sequencesof PrecambrianIas well as those ofPrecambrian II. It caused extended granitizations and the formation of gneisses in the Ivory Coast and in Liberia. In the Ivory Coast the age of this metamorphism is stated as 1,8002,000 m.y., whereas in the Bong Range itabirite deposit in Liberia it is at least 1,600 m.y. Here, through detailed mapping, it could be proved that the younger gneiss

Itabiriteiron ores of the Liberia and Guyana shields

border cuts across the already existing fold system and risesin anticlinalregions,while sinkingin synclinalregions. Contrary to the first metamorphism, the gradient was 3.5 kb,corresponding to a depth of 13 km and a temperature of 640 to 680" C.This orogenesis and metamorphism in the Liberian shield have their counterpartin the Guyana shield within the so-calledyounger granites aged between 1,800 and 2,000m.y. Thus,in both shields,a state of rigidnesswas reached, as younger Precambrian sediments belong to the molasses facies which is practically unfolded, not metamorphic and lies unconformably flatly on the crystalline underground. In the Guyana shield area this is called the Roraimaformation aged at least 1,675 m.y., and in the Liberian shield it is named Tarkwaien and is found in Ghana and Upper Volta (Fig. 1).

Sedimentation and facies of itabirite iron-formations At the moment the only detailed mapping describing the sedimentary facies relationships of the itabirites with the country rock have been carried out in Liberia, i.e. Bong Range (Stobernack, 1968), Nimba (Berge, 1968) and Goe Range (Berge, 1965). Based on these mappings, the sedimentary sequence of the strata begins everywhere with quartzites, with a thickness of several hundred metres. These sometimes become coarse-grainedand conglomeratelike and show some characteristics of an itabiritic sedimentation (Bong Range, Fig. 2; Goe Range). Overlying these is generally a series of quartz-muscovite(i.e. quartzbiotite) schists which, for instance in Nimba, reach a thicknessof700 m . It is not certainwhether the intercalated amphibolites may be considered as igneous rocks. Similar

sediments also form the footwall of the itabirites in the Imataca-seriesof Venezuela. After the sedimentation of these rocks, in the area of both shields a deposition of itabirites took place. However,it is evident that this happened only sporadically and the thickness varies. Thus, from the Imataca-seriesof Venezuela, it is known that the thickness of the itabirites reaches only a few metres in places. It is only in larger deposits that the thickness increased to several tens or hundreds of metres and was often increased due to a later folding.The following sedimentary thicknesses of itabirite have been noted: Cerro Boliva, 200 m; Bong Range, 20-80 m;Nimba, 250-400 m . It is certainly not coincidence that the areas with a relatively thick itabirite sedimentation were later transformed into synclinoria. It is quite evident that the synclinoria are syn-sedimentary,representing former areas of subsidence and troughs, thus accumulating larger sedimentation masses than tectonically more stable areas in the neighbourhood.In general, the area of sedimentation in which the itabirites were deposited probably resembled an epicontinental shelf. It is interesting that in the Bong Range deposit (Fig. 2), for instance, the itabirites are laterally intercalated with coarse-grained quartzites and finally grade into them. At the same time there is a similar change of facies in the footwall of the itabirites where quartz-muscovite-schistgradually grades into amphibolebiotite-schist.The amphibole-biotite-schist,as well as the coarse-grained quartzites,represent a kind of a synclinefacies, while quartz-muscovite-schistand itabirites are a marginal shelf facies. Similar facies relations are-though on a much larger scale-also valid for the itabirites of the Minas-series in Brazil (Eichler, 1967; Pflug, 1967). In the area of the Guyana and the Liberian shields the itabirites generally correspond to the oxide facies of James (1954); itabirites of the carbonate or -sulphidefacies

EN F

W

hematite-magnelileilobirile pegmaloid

-5

5

bonded

___

a

coarse-groined quartzite

-

with itabirite indications

gneiss

au

c

---

E€

òg

--. \;.

x qgb.A

ELS rquorlz -bonded amphibole- biotite-schist

-.

. . . . . . . . ............. . . . . . . . . . . . . . . . . . . . . . . . . . . .

"",*$y*. %\.

-_

-.

-.

.

-.

. . . . .

quortz-muscwite-sChi*t

-

. . . . . . . . . . . . . . .

. . . . . . . . . . . . .

cmetadalerite

with ore indications iinely banded biotite -quartzites and finely bonded quartzites

FIG. 2.Bong Range: Stratigraphy and Facies(Stobernack,1968). 337

H.Gruss

arepractically unknown. However, the oxide facies may be of a varying mineralcomposition.In both shieldsmagnetitequartz- and magnetite-hematite-quartz-itabiritesprevail; with admixtures of iron silicate of varying proportions (greenalite, grünerite, cummingtonite). Phyllite-banded itabirites, however, seem to be a special facies, where quartz is represented by chlorite,sericite and amphiboles. The itabirite deposits of Tonkolili (Sierra Leone), M a n o River and Wologisi (Liberia) in the Liberian shield belong to this type. The different itabirite facies seem to be deposited according to a certain rhythmic pattern. For instance mining operations at the Cerro Bolivar deposit show very clearly that the silicate itabirites mostly appeared at the borders of the synclines (i.e. at the footwall of the itabirite sequence) whereas in the core of the synclines (i.e. in the higher parts of the itabirite sequence) silicate-freeitabirites prevail. The same observation could be made in the socalled Northern deposit of Bong Range (Liberia), a relatively flatly folded part of the deposit.There,the itabirites, which have a thickness of 73 m,are divided into three successive zones,in each of which the following change of facies gradually proceeds: Footwall

Hanging wall

low grade Fe +high grade Fe high grade Fe-silicate-+ Fe-silicate-free high grade magnetite + low grade magnetite low grade hematite -+ high grade hematite Consequently,the degree of oxidation gradually increases from the base towards the hanging wall, where either an interruption of the sedimentation took place after which the deposition started anew,or the sedimentation suddenly encountered changed conditions which mark the beginning of every cycle. Similar cycles axe known from itabirite series of other Precambrian shields, such as the Minas-series in Brazil (Eichler, 1968; Gruss, 1966) or from Canada (Goodwin, 1956), and they find their counterpart in the oolitic minetteores of Lothringen (Bubenicek, 1960). The stratigraphic hanging wall of the itabirites is again schists with varying degrees of metamorphism,as for instance,in the Bong Range and Nimba deposits. Because of the exposed position of the present itabirite outcrops, these strata have already been eroded, thus no exact data on their facies and thickness proportions can be obtained today. Nevertheless, being the youngest known rocks of the Precambrian I, they are of stratigraphical interest.

Formation of high grade ores through metamorphic differentiation The economic significance of the itabirites in both shields for the world's iron ore industry lies in the occurrence of large deposits of high grade ores. These, however, have not been formed by sedimentation, but originate from 338

epigenetically modified itabirite iron-formations. Principally, two types of high grade ore can be distinguished. The first was already of economical interest thirty years ago, and it was formed by metamorphic differentiation. As already explained, the sedimentary sequence of Precambrian I with its itabirites underwent two orogeneses and periods of metamorphism in its history. The metamorphisms were especially pronounced in the central part of the orogene which today forms the coastal areas of both shields.In these regionstheitabirites sometimes came into direct contact with risinggneissicfrontsand deep seated intrusions of basic plutons. In such cases a metamorphic conversion of the itabirites into high grade ores took place. This compositional change-as a result of increased pressure and temperature conditions-sometimes caused the mobilization and removal of silica,whilst the itabirite's content of magnetite-hematitewas residually enriched and, after a recrystallization,formed massive, high grade ore bodies. Typical examples are the deposits of Bomi H i l l (Liberia) and El Pao (Venezuela). i l l (Fig. 13) alternating sedimentation of In Bomi H itabirites and chlorite schists of approximately 450 m thickness form a flat,east-west striking and north-vergent syncline, which is lying directly on granite gneiss. Observation has shown,that the removal of silica and enrichment of ore started metasomatically at the contact with the granite and continued along the stratification. It was combined with an extensive alkali-metasomatismand the dissolved silica was partly precipitated in overlying schists, The result of this metamorphic differentiation of an itabirite sequence, whose original thickness was about 80-100 m, is a 30 m thick layer of compact magnetite ore, which shows the same structure as the former itabirite. The overlying itabiritelayers,which did not come into contact with the granite and were separated from the basement by about 20-50m chloriteschist,however,were not influvnced by the metamorphic differentiation. The same conditions can be found in the E l Pao deposit of Venezuela (Figs. 7 and 8). There, an itabirite formation with a thickness of about 100 m is folded by a system of EW.-NS.-strikingsynclines into an underground of gneiss and granite. This direct contact of granite and itabirite did not cause the metamorphic differentiation of the latter,which is due to a later intrusion of a gabbroid magma. W h e n rising, the magma nearly always followed the contacts of itabirites and granites and caused the same metamorphic differentiationas described for Bomi Hill.The geological m a p of El Pao (Fig. 7) clearly shows that the high grade ores are bound to the contacts of gabbro with itabirite and not to those of itabirite with granite. This is also demonstrated by the cross-sectionthrough the deposit (Fig. 8) showing a flat itabirite syncline,the core of which consists of gabbro. Here,the itabirites were not influenced at the footwall, only the superior part of the itabirite sequence next to the gabbro was transformed into high grade ore. So far no reliable particulars can be given as to the age of the metamorphic formation of high grade ores.

Itabirite iron ores of the Liberia and Guyana shields

Most likely,it was the second orogeny and metamorphism which is responsiblefor such metamorphic formation.Thus the gneiss formation of the Bong Range deposit which is next to Bomi Hill, can be estimated at about 1,600 m.y. Certain dolerites of the Guyana shield (Roraima Plateau) show about the same age, and it seems possible that there is a relation between them and deeper situated gabbro intrusions like that of El Pao.

Formation of supergene high grade ores More frequently another type of high grade iron ore is found, which for several decades has been generally known as supergene for,mation.However, the first qualitative and quantitative study of this ore was carried out at Cerro BolivarinVenezuela by Ruckmick (1963).Meanwhile, similar investigations were carried out in Minas Gerais (Eichler,1967), which in general verify the results obtained by Ruckmick. Based on these results, the formation of high grade ores from itabirites is due mainly to a removal of the silica by rain or subsoil water and a relative upgrading of iron and alumina as residual formation. In both shield areas tropical climatic conditions have been prevalent during the youngest periods of the geological history.Today,an annual rainfall of 3,000mm is measured in the coastal region of the Liberian shield,which gradually diminished farther inland. Similar conditions are also encountered in the Guyana shield where, for instance, 1,700 mm are measured for the Cerro Bolivar area. These rainfallsalso affect the outcrop of itabirite iron-formations. Here especially the silica is leached by way of hydrolysis because the rain-water, containing only small amounts of carbon dioxide,is able to dissolve a considerable quantity of silica. However, the portions of iron and alumina dissolved are relatively small (Table 1). Thus the solubility of silica depends mainly on the solvent properties of the available quartz surface i.e. grain size. Of further importance is the period of time in which the rock becomes affected by water. Eichler's (1968) research in Minas Gerais,Brazil, show that it takes twentyTABLE i. Quantities of SiO,, Fe and Al leached by rain-water Area

Cerro Bolivar (Ruckmick, 1963) Minas Gerais, Brazil (Eichler, 1967) Minas Gerais, Brazil (Eichler, 1967)

Grain size of the itabirite (mm)

mg/'

SiO?

mg/l

mg/l

Fe

AI

0.05-0.15

10-15

0.05-0.1

Unknown

0.05-0.1

6.20

0.34

1.95

0.5-1.5

1.60

0.14

0.84

(a) Rainfall and temperatures. Solubility of SiO? in ground water'

-a9..i max.OC

5O0 rnm

40 o

3OoC - 1Lpprn SI02

12 2ooc

10 IOOC

8

300

6

200

4

100

2

O

XII

VI

I

(b) Solubility of itabirite in pure rain-water

.

28.1X.1965 30.1X.1965 11.X.1965 12.X.1965 23.X.1965 1.111.1966

O 2 13 14 25 153

6.75 6.45 5.80 5.78 5.65 4.45

+ 260 260 310 415 442 400

trace

-

-

5.60 6.20

0.24 0.34

0.38 1.95

,

14.2 16.1 20.0 19.8 20.1 20.5

FIG.3. Solubility of silica in subsoil waters of Minas Gerais, Brazil (Eichler, 1968).

five days for the waters to dissolve a considerable quantity of Sioz, and it is not during the period of maximum rainfall that subsoil waters contain most of the dissolved silica (0.5-2.0mg/l), but towards the end of the dry period (8-10 mg/l) (Fig. 3). The result of this leaching of itabirites is a weathering profile with a typical zonal structure (Fig. 4 and Table 2). The description of the weathering profile shows that the composition of weathering residue of zones A and B is identicalwith the supergene high grade ores,and is mainly dependent on the composition of the primary rocks, i.e. on itabirite facies. The supergene high grade ores of both the Guyana and Liberian shields rarely occur in the coastal areas, but are mostly found about 200 km inland. This is especially true for Nimba and Simandou deposits in Liberia and Guinea,as well as for Cerro Bolivar and San Isidro deposits in Venezuela. Considering the dependence on the grain size of the itabirites of quartz leaching,this geographical distribution has been caused by the varying degrees of metamorphosis. High grade metamorphism resulted in coarse-grain sizes 339

H.Gruss

TABLE 2. Weathering profile Zone

On silicate-itabirites

O n oxide-itabirites

Crust of limonitic cemented tabular hematites,highly clayey, sometimes compact, about 58 per cent Fe Loose admixture of tabular hematite and limonite crusts,mainly clayey matrix,brown, 55 per cent Fe Clayey,brown detrital itabirite,40-50 per cent Fe Silicate-itabirite, unweathered,hard, containing mica and amphibole,magnetitic, 35 per cent Fe

Crust of limonitic-hydrohematitic-cemented tabular hematites, compact,but porous, 62-67 per cent Fe Loose admixture of tabular hematites,in powdery matrix brown, black,blue,63-69 per cent Fe Loose,quartz-richdetrital itabirite,40-50 per cent Fe Oxide-itabirite, unweathered hard, hematitic or magnetitic, 35-40 per cent Fe

and small grain surfaceper unit volume in coastal itabirites. Corresponding to this, quartz leaching was not intensive but extensive, i.e. deep, and consequently only little upgraded cappings of weathered itabirites were formed there. However,these may be enriched to high grade concentrates at comparatively low cost (Bong Range,Liberia;Marampa, Sierra Leone; Maria Luisa and Piacoa,Venezuela). In the itabirite deposits located further inland and characterized by lower metamorphism and small grain sizes, intensive leaching and upgrading prevail, producing direct-shipping high grade ores (Nimba, Liberia-Guinea;Simandou, Guinea;Cerro Bolivar and San Isidro,Venezuela). In spite of its proved efficiency, the solubility of the silica is small and, therefore, it is important to know in which period of time high grade ores were formed. Ruckmick (1963) states that the formation of the high grade ores at Cerro Bolivar began about 24 m.y. ago, in other words, it is younger than Oligocene,in any case not older than Cretaceous. This date has been verified by other geological investigations: the high grade ores of Cerro Bolivar as well

cementation ore

ific

FIG.4. Typical weathering profile of itabirite iron ores in tropical climates (Thienhaus,1963). 340

%

sw

Itabiriteiron ores of the Liberia and Guyana shields

NE Mt Piérre Richoud

Grands R o c h e r s

I600 m

1500m 1L00m

-

1300 m

I

FIG. 5.Blue high grade ores of Nimba,Guinea (Gaertner,1961).

as those of San Isidro are related to an old peneplain, relicts of which are still found on the ore mountains. This levelling corresponds to the old Gondwana Peneplain which, in the Cerro Bolivar area, still has an altitude of 700-750 m , but shows an incline of 1” towards the north and is covered with sediments of the Neocomian (lower Cretaceous) and younger sediments north of the Orinoco river. This leads to the assumption that during the lower Cretaceous the Gondwana-Peneplainwas lying horizontally near sea level and, therefore, the Neocomian sea could transgress over it. After this an elevation to the present level must have taken place, followed by erosion and intersection of the peneplain and lowering of the water table. Most probably the ore formation started at the same time and, according to Ruckmick, might be Oligocene. The accuracy of this dating might be verified by the studies of King (1957), according to w h o m the northeast Brazilian shield was elevated during the early Tertiary until the Miocene,causing an erosion cycle (Sulamaricano cycle) and according to Eichler (1967)-led to the formation of the supergene high grade ores in the ‘Iron Quadrangle’ of Minas Gerais,Brazil. After detailed calculations Eichler also came to the conclusion that the ore formation began during the Oligocene (26 n1.y. ago). Similar conditions can be expected in the Liberian shield, where in the Nimba Mountains of Liberia and Guinea supergene high grade ores are levelling with an

altitude of 1,100-1,300 m,and may be considered as a relict of the Gondwana Peneplain. In the more coastal itabirites of Putu, Liberia, the sanie ores can be found at an altitude of 700 m . Thus, in the Liberian shield too, an inclination of the Gondwana Peneplain towards the coast is indicated. However, this theory is complicated by another type of high grade ore which is similar in physical composition and grain size to parts B,C and D of the normalweathering profile (Fig. 4), but shows an Fe content which is generally 2 per cent higher than the one of zone B,with less alumina, and is marked by a high amount of secondary hematite. These ores are metallic blue and can easily be distinguished from the so-calledbrown and black supergene high grades ores. They rather resemble the so-called ‘hematite ores’ of the ‘Iron Quadrangle’ in Brazil whose formation is considered to be hypogene-metasomatic (Dorr, 1959; Eichler, 1968), or those of north-west Australia, representing pure weathering formations (MacLeod, 1966). This already shows the different opinions regarding the genesis of the blue high grade ores in general as well as those of Nimba and Simandou,Guinea. Gaertner (1961), after geomorphological studies on the position of high grade ore bodies in the northern Nimba Mountains and Simandou chain in Guinea, came to the conclusion that the blue high grade ores are bound to a plateau with an altitude of 1,600-1,650 m and are dislocated by younger faulting (Fig. 5). The binding of the blue high grade ores to a higher and older plateau than 341

H.Gruss

1 O00 k m

1 Precambrian shields

0Roraima -formation / Tarkwaien

E E l

striking of structures

I '0

Itabirites

A

high-grade ores of metamorphous differentiation high-grncle ores of alteration

FIG.6. Itabirite iron ores of the Liberia and Guyana shields (Gruss, 1966).

342

Itabirite iron ores of the Liberia and Guyana shields

the Gondwana Peneplain, strengthens the argument that the blue high grade ores were formed by weathering and belong to a Precretaceous cycle. The tectonic dislocation can also be seen as a proof for the greater age of the blue high grade ores. The question as to what caused the varying mineral content (recrystallization of hematite) and the low alumina content in the blue ores still remains. So far, at least in West Africa, no corresponding studies have been made. However, investigations by Eichler (1967) carried out in Brazil give some details regarding the alumina content in subsoil waters. H e states that by hydrolysis of itabirites not only silica can be dissolved,but iron and also considerable quantities of alumina. Thus, the blue ores of higher levelling might be considered as more mature,than the supergene high grade ores of the younger Gondwana Peneplain. This is also in accordance with the results of morphological studies. Regarding the recrystallization of hematite, MacLeod's (1966) investigations are interesting. In higher parts of the weathering section,the cementation of supergene high grade ores in north-west Australia is mainly limonitic,in lower parts, however, hematitic. The author considers the blue high grade ores of the Nimba Mountains and Simandou chain as weathering formation, and he is of the opinion that the recrystallization and cementation of the high grade ores with hematite does not necessarily prove hydrogenic-metasomaticprocedures. With regard to the brown and black weathered high grade ores, it should be examined whether the physico-chemical conditions in the roots of blue high grade ores,which may have a depth of several hundred metres, permit the recrystallization of secondary hematites.

Geological relations between the itabirites of the Guyana and Liberian shield By comparing interpretations of facts presented in the previous sections,it is clear that the geology of the ironformations of both shields is nearly identical. It would be interesting to pay special attention to these relations, enabling the corresponding inferences to be drawn. The itabirites (Fig. 6), as well as their associated formations,belong to Precambrian I. They were deposited in a shelf-likesea area 2,500-3,000m.y. ago,and towards the end of this period were affected for the first time by an orogenic folding and metamorphosis. The cores of this orogene can today be found in the coastal areas of both shields. The vergence of this folding was directed towards the present inland areas. The second metamorphic modification affected both shields about 1,800 m.y. ago. In both shields the results of these transformations are the mostly coarse-graineditabirite deposits near the coast and the fine-graineditabirites farther inland. At the same time the formation of itabirite high grade ores took place by metamorphic differentiation in the central parts of the

orogene. As the metamorphosis in the marginal parts was less effective, no high grade ores were formed there. The finer-graineditabirites of these areas were predestined for the formation of supergene high grade ores, which are bound to the Gondwana Peneplain or older levellings of Precretaceous time. Besides the synchronous geological events, a remarkable symmetric structure for both shields can be observed,as for instance in the vergence of folding, zones of the same metamorphic grade and the distribution of ,differentitabirite formations and their high grade ores. These facts and the argument that coastal as well as tectonic structures of both shields fit perfectly together, may, therefore, be considered as a proof that in Precretaceous times the Guyana and Liberian shields formed a single unit and that at least the itabiritic provinces of the shields belonged to the same sedimentation basin which later was developed as geosyncline.During two orogenics this geosyncline was folded into a mountain chain with a symmetric structure and a marked crest zone,along which the orogene was divided into the Liberian and Guyana shields when the Gondwana Continent disintegrated during lower Cretaceous period.

El Pao (Venezuela) The El Pao iron ore deposit (Figs.7 and 8)has been known since 1926, but it was not until 1950 that Iron Mines Company of Venezuela was able to start full mining operations. The average analysis of the reserves is almost the same as the analysis of the shipped ore: 62.6 per cent Fe; 12.5 per cent SO,;3.5-4 per cent Alzo,;0.06 per cent P; 3.66 per cent ignition loss. Contrary to the Cerro Bolivar and San Isidro deposits, El Pao contains high grade itabirite ores formed by metamorphic differentiation.They form two flat synclines,one striking N.80" E.and covering an area of 1,000X 500 m, and a maximum depth of 350 m . This east-west striking syncline is followed,towards the north, by another striking N.20" E. and extending over an area of 700 x 500 m . Both synclines are parts of two main folding-directions, forming sort of a lattice (Fig. 7). As shown by the geological map and section (Fig. 8), the metasomatic mineralization of the deposit is always bound to the contacts of itabirite and intrusive gabbro. Thus, the hard ore body reaches a thickness of 10-50 m , with underlying high grade metamorphic itabirites, while the hanging wallformsan intrusive,medium-grainedgabbro (norite). The latter ñlls the whole trough circumscribed by the hard ore body. The metasomatic high grade ores consist of hematite and magnetite in varying proportions with grain sizes up to 10 m m . The following analysis is typical: 67.5-71.0 per cent Fe, 0.1-0.7 per cent Sioz, 0.1-4.0 per cent Alzo3,0.01-0.1 per cent S, 0.01-0.03 per cent P. All rocks are marked on the outcrop by a deep alteration,especially the itabirites. D u e to high grade metamorphism and the coarse-grainsizes (1-5 mm), weathering was 343

H.Gruss

-1

=

gabbrolnorite lump ore

I-[

I

1 km

itabirites t

]

I

gneiss

FIG.7. Geological m a p of El fao, Venezuela (Iron Mines of Venezuela, S.A.).

344

Itabirite iron ores of the Liberia and Guyana shields

I

I South

---___

,pre-mining

c _ c - - - - - - _ _

/

surfoce

600m

North

er t

'

+ + +

+

.L

i

+

+

+

I

+

+

+ +

+ I

r

+ +

L

+

,

*

t

l

L

C

+

*

+

+

+

i c

+

I

+

L

* +

&

+

+

+

+

A

+

+

-

+ +

+

+

.

*

lumo ore itabirites.weathered gabbro a: unweathered b:weathered gneiss a: Unweathered

b;weathered

I

200m l

I

1200rn

FIG.8.El Pao,Venezuela: cross-sectionL (IronMines of Venezuela, S.A.). not intensive, but extensive,i.e. it had a deep reaching effect,Thus,there was no formation of 'genuine' supergene high grade ores,but only concentrations,which are typical for zone C of the alteration profile of itabirites,i.e. the formation of siliceous fine ores. For a cut-off grade of 56 per cent Fe these ores show the following analysis: 56-62 per cent Fe, 6-10 per cent Sioz,2-4 per cent Alzo3, 2-5 per cent ignition loss. O n the average 1.16 tons of overburden per ton of shipping ore are to be moved. During mining operations both types of ore are mined simultaneously, crushed and screened,thus producing a direct-shippingore as described at the beginning and showing thefollowinggrain sizes:more than 51 mm (20.33 per cent), 13-51 mm (26.26 per cent), less than 13 mm (53.41 per cent).

Cerro Bolivar (Venezuela) This deposit was discovered at the beginning of the forties. Since 1954 it has been exploited by the Orinoco Mining Company.At a cut-offgrade of 55 per cent Fe,the average analysis is: 63.84 per cent Fe, 1.86 per cent Sioz,1.44per cent Alzo3, 0.10per cent P,5.11 per cent ignition loss. These deposits represent the relicts of a synclinorium of itabirite-bearing metasediments, which reaches from Cerro Bolivar 80 km east to the Rio Caroni.The supergene high grade ores are bound to the old Gondwana Peneplain which cuts the island mountains with its itabirite outcrops at approximately 700 m above sea level. The Cerro Bolivar deposit has a strike length of 20 km with outcrops up to 750 m wide. In this area an itabirite formation with 200 m of sedimentary thickness is isoclinally folded. The special synclines staggered to the

riglit can reach a depth of 200-250 m , divided by steeply rising anticlines of footwall-schists (Figs. 9 and 10). The iron ores belonging to the brown and black type of weathered high grade ores, show the typical profile already described. The unweathered rock (zone D) consists of fine to coarse-banded(0.05-2.0 cmj itabirites with 39 per cent F e and 42 per cent SiO,on average,and grain sizes of between 0.05 and 0.15 m m . The main iron mineral, besides magnetite,is specularite.However,the majority of the itabirites also contain F e silicates as muscovite, sericite and, less frequently, amphiboles and pyroxenes. The freshrockis overlainby a zone(C)ofsoftitabirites, which often is no more than 10 m thick. Technically,two types are distinguishedsiliceousfine ores(50-62per centFe, 6-10 per cent Sioz, 1 per cent Alzo,and 3 per cent ignition loss); softitabirites(45-55 per cent Fe, approximately 30per cent Sioz,0.5 per centAlzo3 and 1.5 per cent ignition loss). However, with a maximum 100 m depth, zone B is much thicker, consisting of black and brown supergene high grade ores, the black ores resulting from mostly non-silicateitabirites,the brown ones from silicate-bearing itabirites.The following analyses are characteristic:brown fine ores (62-64 per cent Fe,0-6 per cent Sioz,1 per cent Alzo3and 3 per cent ignition loss); black fine ores (6668 per cent Fe, 0-6 per cent SO,,1 per cent Alzo3and 0-3 per cent ignition loss). Experience showsthatbrown fine ores mostly appear on the rimsof a syncline,while the black fine ores predominate in the centre. About two-thirdsof the reserves of zone B consist of brown ores, the rest of black ores. At Cerro Bolivar the surface ores ofzoneA are 10-30 m thick. Depending on intensity of weathering and composition of the primary rock, the hard and lumpy material 345

H.Gruss

D

=

laterite itabirites high-grade ores

FIG.9. Geological map of Cerro Bolivar,Venezuela (Orinoco Mining Company).

NW

SE

A

7fim

SOOm 500m -

-

EEBI crustal ores black fines brown fines

a itabirites

O laterite, FIG.10.Cerro Bolivar,Venezuela: cross-sectionA-B (Orinoco Mining Company).

200m

TABLE 3. Inch/mesh

1.050 0.742 0.525 0.371 3

6 10

20 35

65 100

200

346

mm

26.6 18.85 13.33

9.42 6.68 3.23 1.65 0.83 0.42 0.21 O.147 0.074

Crustal ore crushed -100 m m

6.72 12.53 23.59

FineOre brown

Fine ore black

30.43

3.43

43.47

6.96 14.92 26.97

-

36.05

-

44.47 54.39 63.31 71.25 79.99 89.70 92.94

48.80 64.58 80.93 89.33 91.57 93.10 93.90

100.00

3i 00.00

33.01

46.16 57.34 66.15 73.25 79.16 81.93 100.00

shows the following cheniical analysis: crustal ores (6269 per cent Fe,0.1-6per cent Sioz,0.1-1.5 per cent AI& and 0-5 per cent ignition loss). In order to guarantee a sufficientgrade control during mining operations, the four zones described above are subdivided into thirty-sevenore-typeswhich differ more or less with regard to hardness, mineralogical composition, colour and chemical analysis. Table 3 showsthe grain size distributionafter screening.

Itabirite iron ores of the Liberia and Guyana shields

San Isidro (Venezuela) This mine,which is situated only 15 km south of the Cerro Bolivar deposit, was discovered in 1948 and since then has belonged to the Venezuelan Ministry of Mines and Hydrocarbons (State Reservation). The deposits of supergenehigh grade ores are based on a synclinorium of itabirite-bearingmetasediments, whose steep specialfolds mostly strikeW S W .-ENE. and integrate with a north-south striking system.The individualdeposits cover a total area of 50 kmz,and the relation to the Gondwana Peneplain at 700 m above sea level is clearly evident. Again, analogous to Cerro Bolivar, the substance of the deposit consists of supergene high grade ore of the brown and black type. Thus, the fresh itabirite rock of zone D presents a hard, mostly fine-banded(millimetres), sometimes also unbanded hematite/magnetite-quartzite(H:M :Q = 38 :22 :39 weight per cent), which also contains some iron silicate. Grain sizes range between 0.03 and 0.2 111111. The unweathered itabiritecontainsapproximately 42 per cent Fe and 39 per cent SiOzon average. D u e to the fine-grain of the itabirite, the overlying zone C is rather thin (10-20 mm). The following types of iron ore can be distinguished: siliceous fine ores (58 per

cent Fe,4 per cent Sioz, 0.5per cent Alzo3 and 5.0 per cent ignition loss); soft itabirites (50 per cent Fe, 30 per cent Sioz0.3 , per cent Alzo,and 2.8 per cent ignition loss). Zone B of San Isidro is much better developed than that of Cerro Bolivar, and reaches a maximum depth of 240 m (Figs. 11 and 12). Here, too-depending on the content of F e silicate in the itabirites-black and brown fine ores can be distinguished,which, at a cut-offgrade of 58 per cent Fe, show the following average analyses: brown fine ores (62 per cent Fe; 2.8 per cent Sioz,0.5 per cent Alzo3 and 4.0per cent ignition loss); black fine ores (67 per cent Fe, 0.8 per cent SiO,,0.5 per cent Alzo,and 2.8 per cent ignition loss). Contrary to Cerro Bolivar,the black fine ores prevail at San Isidro in a ratio of black to brown ores of 2 :1. At San Isidro the limonite crustal ores of zone A generally have a thickness of 10 m,with individual roots reaching to a depth of 30 m . At a cut-offgrade of 58 per cent Fe they show the following analysis:crustal ores (62-67 per cent Fe, 0.6-1.3 per cent Sioz,0.5-1.3 per cent Alzo3, 2.5-4.3 per cent ignition loss). Based on the above analyses and the distribution of reserves the following average composition can be calculated for the main deposit of San Isidro: 58 per cent Fe

%

x

O

0.5

1.0 Km

%

FIG.11. Sketch map of iron ore deposit San Isidro,Venezuela (Ministerio de Minas e hydrocarbones de Venezuela). 347

H. Gruss

SE

NW P 3-5

500m

4-

_. I I I

crustal ores black fines brown fines itabirites laterite

200 m

FIG.12. San Isidro,Venezuela: cross-section27

cut-off(65.14per cent Fe, 1.23 per cent %Oe, 0.59 per cent Alzo3 and 3.O5per cent ignitionloss); 55 per cent F e cut-off (63.3 per cent Fe, 3.0 per cent SiO,, 0.6per cent Alzo3 and 3.2 per cent ignition loss); 0.03 per cent M n , 0.05per cent Tio,,0.03 per cent P,0.01per cent S. Grain size distribution can be expected to be as in Table 4. During mining operations 0.05 tons of overburden are to be moved for each ton of ore (20 :1). Present plans of the Ministry of Mines and Hydrocarbonsprovide for a large-scaledevelopmentofthe deposit

TABLE 4. Inch/mesh

1.050 0.742 0.525 0.371 3

6 10 20 35 65 1O0 200

348

mm

26.6 18.85 13.33 9.42 6.68 3.23 1.65 0.83 0.42 0.21 0.147 0.074

Total percentage

8.2

-

20.5

-

37.1 50.2 60.8 70.7 77.1 82.2 85.3 100.0

of San Isidro, so that beginning in 1972, 4.2 million tons per year will be mined;from 1973, 2.5 million tons per year of this tonnage are to be delivered as pellets.

Bomi H i l l (Liberia) The Bomi H i l liron ore deposit in Liberia has been known since the beginning of the thirties, when for the first time it was geologically investigated by a Dutch firm. After the second World War the Liberia Mining Company Ltd bought the mining concession for the deposit and starting mining in 1951. The direct-shipping ore has the following chemical composition:64.5 per cent Fe;4.5 per cent SiOs;1.5per cent Alzo3; 0.13 per cent P; 0.12 per cent S. Of these ores,53 per cent is lump ore (11-37 mm)and 47 per cent fines (minus 11 mm). In addition, the mine disposes of larger reserves of itabiritic low grade ores. If weathered and suitablefor grinding,they can be upgraded by dressing (Humphrey Spirals and magnetic separator) to sinterfeed concentrates. The concentrate has following analysis: 64.0 per cent Fe; 6.0 per cent SiO,;1.0per cent Alzo8; 0.04-0.05 per cent P; 0.08-0.12per cent S. The BomiH i l ldeposit(Fig.13) represents an east-weststriking syncline with a steeply dipping southern limb and a flatly dipping northern limb. The syncline extends over 500 x 1,000m and has a depth of 180 m . Its core consists of a series of itabirite-bearingmetasediments,the basement

Itabirite iron ores of the Liberia and Guyana shields

- s-

lgoo

FT

-N-

800

700

- -

600

..... _ _ .

soo LOO 300

200 100

FIG.13. Bomi Hill: North-south cross-sectionthrough central part of main deposit (Zigtema,1968). of which is bordered by younger granite-gneiss.Directly contacting the granite,there is an ore body averaging 40m thick, composed of coarse, magnetitic high grade ore formed by a metamorphic differentiation. The hanging wall is formed by about 40 m of schist, 60 m of itabiritic low grade ores and again up to 60 m of schist, which are all removed as overburden and get only partly dressed.Besides this main deposit,in the continuation of the strike there are several smaller deposits, the main reserves of which are

also nined today. In the main deposit ore and overburden are in the ratio of 1 :3.6.

Bon!? Range (Liberia) The Bong Range itabirite deposit (Figs. 14 and 15) was discoveredabout the end of the thirtiesand has been worked since 1965 under the management ofBon Mining Company.

O I

low rn

Upper Ouartz-Biolile -Schist Itobirite

Coarse-groined Ouorlzite

Banded Gneiss

Quartzbonded Amphibole -Schist

Lower Cuortz -eio:ite -Schist Ouorlz-Muscovite-Schist

Sillimanite-Schist Amphibole -8iotile -Schist -+-- Anticline

[SJTI

Granitoid

Gneiss

Gneiss Front

/ Syncline fine-banded Biotite -0umtzite fine-bonded Ouortzile

FIG.14.Bong Range: geological map (Stobernack, 1968). 349

H.Gruss

E SE m /1.1 PI

Bong Peak

LOO

400

200

200.

O

0

C

D N m/NN

/ /'

400 Northern Deposit

'.

Eastern Zoweoh I

LOO

200

200

O

O

____.

B

A NNW

m/NN

LOO

Western Zaweoh I

SSE m/NN -400

INorthern

- 200

O,

FIG.15. Bong Range: cross-sections (Stobernack, 1968). 350

Itabirite iron ores of the Liberia and Guyana shields

TABLE 5. Probable

Proved

(million tons)

Possible

1. Zaweah I

232

-

2. Zaweah II 3. BongPeak

-

60 128

-

4. Gomma 5. Northern deposit Bong Range

-

98

330

__

188

15

15

The Bong Range area comprises four individual deposits,with geological reserves calculated and estimated as in Table 5. A total of 275 million tons of the reserves proved for Zaweah I and the Northern deposit, are mineable, with 235 million tons still to be exploited on 1 January 1970. The four ore bodies (1-4above) form a single itabirite syncline striking east-west and extending over 13 km. This syncline is steeply folded into foot-wallschists and reaches an outcrop width of up to 300 m , mainly in the western part. In front of the western end, and towards the north, the so-calledNorthern deposit is situated. It forms only a rather flat syncline. This ore body shows that the sedimentary thickness of the itabirite formation is not more than 80 m,and that larger widths of outcrop are due to steeply isoclinal folding. The itabirites, which are mostly of the mesozonalmetamorphic type, have an average grain size of 0.1mm, and belong to the oxide facies. Width of banding is, in general,between 1 and 10 mm.The primary mineral stock is formed by magnetite, hematite and quartz, in addition to varying proportions of iron silicates, as e.g. biotite, cummingtoniteand grunerite.Due to their relative coarseness,the itabiriteshave undergone a deep weathering which caused an oxidation of the magnetite (martitization) and a looseningof the rock bond,which,however,did not result in the formation of high grade ores. Thus, only zones C and D of the Bong Range deposit represent the characteristic profile of weathering. From this and also from the operating point of view, the following types of ore can be distinguished,starting from the top of the profile: 1. Spiral ores: zone C,soft itabirite,weathered; 11.5 per cent of the proved reserves;42.6per cent Fe,7.1per cent magnetite,37.1 per cent Sioz, 0.5per cent Alzo3, 0.05per cent P,0.008 per cent S;80 per cent-0.25 m m . 2. Ttmsitionalores: zoneC,medium hard itabirite,slightly weathered;13.5 per cent of the proved reserves;40.6 per cent Fe, 12,O per cent magnetite, 40.0 per cent SiO,, 0.6 par cent Alzo,,0.03 per cent P, 0.01 per cent S; 90 per cent-0.1 m m . 3. Magfietic ores: zone D,hard itabirite, unweathered; 75 per cent of the proved reserves; 37.4 per cent Fe, 35.2 per cent magnetite,42.0per cent SiO,,0.4 per cent Alzo3,0.04 per cent P, 0.03 per cent S; 90 per cent -0.1 mm.

The crude ore is mined by modern open-pitmethods (overburden ratio is 1 ton: 0.5-1.0ton), crushed, ground to liberation size and upgraded by means of Humphreys spirals and magnetic separators to a high grade concentrate. For a weight recovery of 4244 per cent and an Fe recovery of 70-74per cent,the averageBong Range concentrate analysis is: 65.16 per cent Fe, 9.64 per cent Feo, 7.00 per cent SO,, 0.28 per cent Al,O,, 0.034 per cent P, 0.022 per cent S,0.05 per cent M n , 0.05 per cent Cao, 0.06 per cent M g O , 0.00per cent Cu,0.60 pur cent ignition loss and 4.76 per cent moisture.

Nimba (Liberia) The Nimba deposit (Figs. 16,17 and is), considered to be the largest iron ore mine in Africa at present, is managed by Lamco Joint Venture Operating Company. The high grade ores of the Nimba Mountains originate from itabirites of the oxide facies which, in general, are fine-banded(0.5-5nim) and fine-grained (grain size 0.030.1 mm). The sedimentary thickness of the itabirites ranges from 250 to 400 m,the width of outcrop being often increased by isoclinal folding. Ore minerals are almost exclusively magnetite and hematite, while iron silicates are negligible. The fine-grained itabirites have undergone a sometimes deep weathering during their geological history resulting in the formation of high grade ores. Thus, high grade ores of two weathering cycles can be distinguished, the ones bound to the Cretaceous Gondwana Peneplain (f1,300 m above sea level) and others originating from older,higher situated levellings (+1,600m above sea level). The high grade ores ofthe Gondwana Peneplain usually form flat caps,reaching a depth of 75-100 m and representing thecompletetypicalprofile of supergenehigh grade ores. The cementation ores of zone A are 2-5 m thick on average,but sometimes also maintain a depth of 15-20 m . The hard, porous, limonitic ore has the following composition:63.5 per cent Fe,0.8per cent SiO,,2.8 per cent Alzo3, 5.5 per cent ignition loss. The brown’ ores of zone B contain a high proportion of fines and belong to the type of the brown (fblack) fine ores; their thickness may reach as much as 100 m . The followingchemical composition is typical:65.5per cent Fe, 1.5 per cent Sioz, 0.8per cent Alzo,,4.0 per cent ignition loss. Here too, the brown varieties seem to originate from iron silicate-bearingitabirites, whilst the black fine ores stem from itabirites free of iron silicate. The foot wall of the brown fine ores is formed by soft itabirites of zone C and is rather thin. These siliceous fine ores have the following composition: 50-60 per cent Fe, 10-20 per cent Sioz, 1 per cent Al,O,, The fine-grained,hard itabirites of zone D give the following analysis:38 per cent Fe, 42per cent Sioz,0.5 per cent Alzo3, 1.5 per cent ignition loss. Compared with the brown high grade ores bound to the Gondwana Peneplain,the blue high grade ores of older 351

BLUE ORES BROWN ORES 1M T ALPHA PHYLLITE I:.'=.'[ NIMBA ITABIRITE 7 4 GBAHM RIDGE PHYLLITE D Z i SEKA VALLEY AMPHIBOLE SCHIST

HIGH -GRADE ORES NIMBA SERIES YEKEPA SERIES FIG.16. Nimba area/Liberia (Lanco J. V. Co.) (Berge, 1968).

352

Itabirite iron ores of the Liberia and Guyana shields

NW YEKEPA

NIMBA SERIES

!

SE

NW

I

I ;

3 -

YEKEPA SERIES

NIMBA SERIES

SE

-

I

/ -\ ‘\

I

/I

\

L’

! 1300 rn

l500m

PROFILE 5 S O U T H C E N T R A L N I M B A HILL PROFILE 8 N O R T H C E N T R A L G B A H M G U E S T HOUSE HILL

MT. A L P H A PHYLLITE N I M B A ITABIRITE G B A H M RIDGE PHYLLITE S E K A VALLEY A M P H I B O L E SCHIST. YEKEPA SERIES O

PROFILE 13 N O R T H G B A H M - N I M B A RIDGE

1000

I

2000

3000rn 1

FIG.17. Nimba area: geologic cross-sections (Berge, 1968).

-NW-

-CEBh.25

FIG.18. Nimba: section across main ore body (Thienhaus, 1963).

Bh

1..1..1

hard ore soft ore

I

/

/ 100 rn

itabiriie I__) schist

353

/-

,.--' g

Contact

, , ' ,

Probable

,

O I

_______ fault t -__ Anticline L-Syncline t

1 I

Km

' P ,

Strike and direction of dip of foliotion

, 4 ' Vertical foliotion .. :;.::.. .. .

j2::v,..i!

Limit of open pi1 workings

A.

Radiometric a g e locality

FIG.19. Geologic m a p of the Mano River Mine area, Grand Cape Mount County, Liberia (White and Baker, 1968).

Itabirite iron ores of the Liberia and Guyana shields

TABLE 6. 100 per cent crude ore

-85 mm 63.0 per cent Fe 6.17 per cent Siû, 1.03 per cent A&O, 0.057 per cent P

37 per cent washed lump

43 per cent fine ore

20 per cent slimes

f 5 m m

0.25-5 m m 66.9 per cent Fe 3.1 per cent SO, 0.73 per cent Alzo, 0.048 per cent P 1.G per cent ignition loss

-0.25m m

64.5 per cent Fe 4.0 per cent SiO, 0.92per cent Alzo, 0.07per cent P 2.1 per cent ignition loss

periods of weathering do not show the typicalprofile. Their areal extension is rather limited, but they go as deep as 600 m below surface. Even so,the blue ores of the Nimba Mountains represent only the deepest, non-eroded roots of larger ore bodies, which, for instance,on Guinean territory are bound to levellingsat an altitude of 1,600-1,650m above sea level. Although the entire weathering section is no longer preserved, the blue high grade ores may be considered as ores of zone B. The following chemical composition is characteristic:67.8 per cent Fe, 1.5 per cent SO,, 0.5 per cent Al,O,, 1.5 per cent ignition loss. The blue ores are mostly fine ores, but a secondary hematite mineralization,to which this type of ore owes its colour,sometimes resulted in a cementation (medium hard ores), thus lump ore production ofthe blue ores after mining and crushing amounts to approximately 10per cent.Therefore, the blue ores correspond practically to zone B of the itabirites alteration profile. For a weight recovery of 98 per cent,see Table 6.The slimes are enriched by flotation of the tailings to a concentrate, which is pelletized. The pellets give the following chemical analysis: 63.9 per cent Fe, 5.2 per cent SiO,, 1.99 per cent Alzo3,0.065 per cent P, 0.76 per cent Cao, 0.40per cent M g O .

lump ore 9.5-150 mm (56-59 per cent Fe, 3-4.5 per cent 0.05 per cent P and 8 per cent SO,, 6-7.5 per cent AlzoB, ignition loss); fine ore 0.3-9.5 mm (56-59 per cent Fe, 3.0-4.5per cent SiO,, 4.5-6.5per cent Alzo3,0.05-0.06per cent P and 7.0-9.0 per cent ignition loss). The Mano River iron ore deposit consists of a series of metamorphic schists, amphibolites and itabirites (Fig. 20). This series reaches a total thickness of more than 300 m and forms flat,NE.-SW.-strikingsynclines,which on their footwall are bordered by younger gneisses. The metasedinients are marked by an abrupt change of facies,thus a stratigraphical subdivision of this series cannot be set up. According to James (1954),the intercalated itabirite horizons nearly always belong to the silicate facies,integrating and alternatingwith schistshozirontally as well as vertically over very short distances (Fig. 20). Because of the intensive weathering, investigations up to the present rarely showed hard, unweathered itabirites

A Hill

I Hill

Mano River (Liberia) The Mano River deposit (Fig. 19) was discovered and geologicallyinvestigatedat the end of the fifties,and in 1961 it was opened up by the National Iron Ore Company Ltd. Deposit A is already exhausted, whereas deposits H,I, No. 4 and J are mined; the ore bodies E,V,No. 5 and 6, however,have not yet been opened up. The calculationof ore reserves,as well as mining operations, is based on a cut-off grade of 50 per cent Fe;in addition, so-calledlean ores containing 45-50 per cent Fe are eliminated, which are separately mined and stocked. Ore and overburden are in the ratio of 1 :0.4. The crude ore is mined by modern open-pitmethods, dressed by washing and at present gives the following analysis:50-55 per cent Fe, 2-5 per cent SiO,,4-7 per cent Alzo,,0.02-0.06 per cent P, 10-13 per cent ignition loss. During the dressing process, which includes crushing, washing and screening, the following qualities of directshippingores are obtained (weight recovery 70-75 per cent):

H Hill

5 Hill

O

1000 METRES

- '

EXPLANATION FOR MAP AND SECTIONS

Vertical exaggerotion

ZX

0 High -Grade Ores

EcI

u 1 tromaf ic

z intrusives

Iron formation.schist. and amphibolite

Ea

5 LT m

,z 4 w

U

!x

.a

355

I ] schists

600

LOO 2 O0

O O

100

200 m

FIG.21. Geological map of Mesaboin hill/Marampa,Sierra Leone (Sierra Leone Development Company Ltd). 356

~

Itabirite iron ores of the Liberia and Guyana shields

of zone D . There are mostly banded magnetite/Fe silicate rocks (amphibolites). The overlying zone C consists of a friable, limonitecoloured, clayey rock detritus with an Fe content of 4054 per cent,showing the specificationof the so-calledlean ores. The ores of zone B towards the hanging wall are also mined. They are marked by intense leaching of the silica and enriched alumina,which caused the clayey composition of the material. The outcrop ores of zone A consist of a 10 m thick limonitecrustwith high Al content and is yielding lump ore.

Marampa (Sierra Leone) Managed by the Sierra Leone Development Company Ltd, the Marampa mine has been in production since 1933 without interruption. At a cut-offgrade of 37 per cent Fe the crude ore shows the following average composition: 40.0 per cent Fe, 32.0 per cent Sioz,4,5 per cent Alzo3, 0.2 per cent M n. The crude ore is mined by open-pit operation (ore: overburden= 1 :0.85) and upgraded by means of H u m phrey spirals to a high grade concentrate,bringing about a weight recovery of 42 per cent at present, which it is intended to increase to 50 per cent by improved dressing operations. The concentrate shows the following analysis and grain composition: 64.1 per cent Fe, 6.37 per cent Sioz,0.84 per cent Alzo3, 0.23 per cent Mn,0.008 per cent P and 0.65 per cent ignition loss;0.32-3.1 mm = 19.5 per

cent,-0.32 mm = 20.0 per cent,-0.25 mm = 22.5 per cent,-0.18 mm = 18.5 per cent,-0.125mm = 14.5 per cent,-O.O9mm = 1.5percentand-0.075mm = 3.5per cent. In the area of the Marampa deposit (altitudeapproximately 250 m), we find a series of highly metamorphic hematite quartzites and hematitic mica schists (Marampa schists), whichwere formedby itabirites of the oxide and silicatefacies.Thestrataare divided into thefollowinghorizons, starting from the top: upper hematite-quartzites,approximately 100 m;upper quartz-mica-schists,approximately 40m;middle hematite-schists,approximately 75 m;middle quartz-mica-schists,approximately 60 m;lower hematiteschists,approximately 40m;and lower quartz-mica-schists, > 100 m . The metasediments form a flat,north-south-striking syncline extending to about 500x 500m,and with a rolling pitch (Fig. 21); this structure is based on granites and gneisses of the Kasila-series. W h e n unweathered (zone D)the hematite-quartzites are hard,distinctly slaty rocks,composed of quartz,hematite aiid little biotite. Because of its coarseness (liberation size approximately 0.5 mm), no high grade ores were formed near the surface,but only enriched, soft itabirites, which may be placed into zone C of the typical weathering section.Their Fe content averages 49 per cent. As there is no zone B (weathered high grade ores), the clayey-lateritic crustal ore of zoneA is directly placed on it with a thickness of 5-9 m and depending on the aluminium content it may show 50-65 per cent Fe. While in former years zones A and C were mined, the present reserves originate mostly from zone D.

Résumé Les minerais de fer d’itabiuite d~iLibésia et du bouclieu guyanais (H. Gruss)

Le Précambrien du Libéria et du bouclier guyanais contient des dépôts de minerai de fer d’itabirite qui, pendant les deux dernières décennies, sont devenus de plus en plus importantsparticulièrementpour les États-Uniset l’Europe occidentalecomme sources de matières premières, avec une productionet une exportation qui se sont élevées à 37,8millions de tonnes en 1968. Les similitudes de la structure des minerais de fer d‘itabirite des deux continents sont dues à leur histoire géologique commune,qui remonte au plus ancien Précambrienet qui a pris un cours analoguem ê m e après la séparation des deux continents la période mésozoïque. Les itabirites des deux boucliers représentent les plus jeunes éléments des strates géosynclinales des métasédiments et des vulcanites, dont le substratum est ou bien connu ou ne peut être identifié. Ces roches furent plissées par des mouvements orogéniques,il y a 2,5 ou 3 milliards d’annéeset ont subi des altérationsmétainorphiques régio-

nales. Une autre métamorphose s’est produite il y a 1,8 à 2 milliards d’années avec des intrusions de gneiss et des intrusionsacides ou alcalines.Le géosynclinal précambrien formé de cette façon a une structure symétrique avec UR noyau métamorphique mésozonal ou catazonal et des bordures métamorphosées épizonalement,la direction des plissements allant toujours du centre vers l’extérieur.En conséquence,les dépôts d’itabirite du centre sont caractérisés par un haut degré de métamorphose, par la seule présence de plis aplatis, un grain grossier et, en partie, une différenciation métamorphique du minerai à haute teneur (Bomi Hill,El Pao), tandisque les dépôts d’itabirite périphériquessont,en général,faiblementmétamorphiques, aux plis fortement redressés et à grains fins. Après le plissement et le surhaussement, l’orogénie précambrienne a été nivelée à l’état de pénéplaine. C o m m e résultat de la dislocation du continent du Gondwana pendant le Crétacé supérieur, le synclinal s’est fendu le long de sa crête plongeante nord-ouest/sud-estet s’est séparé pour former les boucliers actuels de Libéria et de Guyane. 357

H.Gruss

L'érosion profonde et la désagrégation qui ont commencé à se produire après cette séparation ont conduit, sur les deux continents, à la formation d'itabirite enrichie (Marampa, Bong Range) et des dépôts très importants de minerais désintégrés à haute teneur qui forment aujourd'hui la base des exportations de minerais de fer des pays concernés (Nimba, Mano, Cerro Bolivar, San Isidro). Les itabirites des boucliers du Libéria et de la Guyane présentent en général des épaisseurs sédimentaires qui n'excèdent pas quelques mètres ;cependant dans les bassins de sédimentation qui ont évolué plus tard en synclinoriums, les épaisseurs ont augmenté pour atteindre 100 et même 250 mètres. I1 a été démontré que parfois les itabirites se sont déposées sur le bord des bassins et latéralement se sont entremêlées avec des sédiments clastiquesà grains grossiers, A l'occasion,la sédimentation a eu lieu au cours de nombreux cycles,chacun commençant avec un faciès de silicate qui se transforme en un faciès de magnétite et se termine par un faciès d'oxyde d'hématite. Dans les zones catamétamorphiques,il s'est produit une différenciation métamorphique des itabirites du faciès d'oxyde accompagnée essentiellement de la formation de magnétite et de minerais grumeleux à haute teneur (67 %de fer), qui n'existent pas dans les parties mésozonales et épizonales du synclinal. Au contraire,les minerais à haute teneur désagrégés et décomposés se sont développés aux époques fossiles ou récentes dans un climat tropical humide, présentant une section verticale typique qui dépend de la structure de la roche originale (silicate ou faciès d'oxyde). Zone Faciès-silicate

Faciès-oxyde

A

Croûte de lamelles d'hématite cimentées de limonitehydro-hématite

Croûte de lamelies d'hématite cimentées de limonite,haute concentration d'argile,

partiellement compacte, 50 %Fe

B

C

D

Mélange meuble et argileux de lamelles d'hématite et de croûtes de limonite,brun, 55 %Fe Argileux, désintégration de l'itabirite,brun, 40-50 %Fe Itabirite silicatée, dure, contenant du mica et de l'amphibole,avec magnétite, 35 % F e

compacte,mais poreuse, 62-67 %Fe Mélange meuble de lamelles d'hématite martite,brun,noir et même bleu,63-69 %Fe Meuble, désintégration de l'itabirite en quartzite (haute teneur), 40-50 %Fe Itabirite oxydée dure, avec hématite et magnétite, 35-40 % Fe

Dans les itabirites catamorphiques, c'est-à-dire à wains grossiers,la désagrégation,en raisonde lasurfacelimitée du grain par unité de volume, est étendue et pénètre profondément ; il n'en résulte pas la formation de minerai à haute teneur, mais plutôt la formationd'itabirites molles qui peuvent aisément être concentrées (Maramba, Bong Range). D'autre part, dans la zone épizonale métamorphique, la désagrégation,en raisonde la finesse du grain,a été trèsprofonde,le résultatétant la formation des minerais désagrégés à haute teneur (Nimba,Mano,Cerro Bolivar,San Isidro). Ces dépôts se rencontrent en liaison avec la pénéplaine crétacée (Gondwana) et sur des plans d'érosion plus récents. Le minerai désagrégé à haute teneur des pénéplaines plus anciennes et situées à un niveau plus élevé, pénètre plus profondément (à plus de 500 mètres) que celle des plans d'érosion plus jeunes (de 50 à 200mètres). Tandis que les minerais à haute teneur de cette dernière sont de couleur brune et noire,les minerais à haute teneur des plus anciennes pénéplaines sont caractérisées par une couleur bleue et une pénétration zonale radiculaire, cimentée par l'hématite qui, de l'avis de l'auteur, est supergène.

Bibliography /Bibliographie BERGE,J. W.1965. Contributions to the petrology of the Goe Range Area, Grand Bassa Co., Liberia. Bull. geol. Institn. Univ.,Uppsulu, vol. XLIII, p. 1-24. __ . 1968. A proposed structural and stratigraphic interpretation of the Nimba-Gbahm Ridge area, Liberia. Bull, geol. Soc. Liberia, vol. III, p. 28-44. BEURLEN,K. 1970. Geologie von Brusilien. Berlin/Stuttgart, Bornstraeger. BUBENICEK,M.L.1960. Recherches sur la constitution et la répartition du minerai de fer dans 1'Aalénien de Lorraine. Thèse,Faculté des sciences,Université de Nancy. BURCHARD, E. F.1930.The Pao deposits of iron ore in the State of Bolivar,Venezuela. Tech. Pitbl. A m r . Inst. Min. Engrs., no. 295, Class I, Min. Geol., no. 28, p. 1-27. DAHLKAMP, F.J.; KIRCHNER, G.1967. Die Itabiritlagerstätten in Surinam.Erzmetall., vol. XX,p. 209-14. 358

D o m ,J. VANN.IIet al.,1959.Esboçogeológico do Quadrilátero ferrifero de Minas GeraislBrasil.Rio de Janeiro,Departamento nacional de producção minera. (Publicação especial no. 1.) 115 p.

EICHLER, J. 1967. Das physikalisch-chemische Milieu bei der Verwitterung von Itabiriten in Minas Gerais/Brasilien.Chernie der Erde, vol. XXVI, p. 119-32. _- .1968.Geologieund Entstchung der itabiritischenReicherze im Eisernen Vierook von Minas Gerais/Brasilien. Habilitation thesis,Faculty for Sciences.Clausthal, Technical University.

FERENCIO, A. J. 1969.Geology of the San Isidro Ore Deposit, Venezuela.Mineral Deposita (Berl.),vol. 4,p. 283-97. GAERTNER, H.R. v. 1961. Bericht über die Bereisung der Eisenerz-Lagerstättevon Guinea. Unpublished report of Bundesanstaltfir Bodenforschung,Hannover.

Itabirite iron ores of the Liberia and Guyana shields

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GOODWIN, A. M. 1956. Facies relations in the Gunflint Iron Formation.Econ. Geol., vol. 51,p. 565-95. GRUSS, H . 1966. Itabiritische Eisenerze in Venezuela. Stuhl u. Eisen, Düsseldorf, vol. 86,p. 1177-89. JAMES, H.L. 1954.Sedimentary Facies of IronFormation.Econ. Geol., vol. 49,p. 235-93. KING, L. C.1957.A Geomorfologiu do Brasil Oriental. Rio de Janeiro,Instituto brasileiro de geografia,Conselho Nacional de Geografia. p. 256. LEO,G.W.; WHITE, R. W.1968. Geologic reconnaissance in Western Liberia, Repnblic of Liberia. Bureau of Natural Resources and Surveys,Geological Survey (unpublished). MACHENS, E. 1966.Zur GeotektonischenEntwicklung von Westafrika. 2.dtsch. geol. Ges., vol. 116,p. 589-98. MACLEOD, W.N.1966. Iron ore deposits of the Hamersley Range area. Bull. W.Aust. geol. Suuv.,no. 117. MARMO, V. 1956. Banded Ironstone of Kangari Hills,Sierra Leone.Econ. Geol., vol. 51,p. 799-811. PFLUG,R. 1967. Physikalische Altersbestimmungen aus dem Brasilianischen Schild. Tectonophysics, vol. 5,p. 381-411.

RUCKMICK, J. C. 1963. The iron ores of Cerro Bolivar,Venezuela,Econ. Geol., vol. 58,p. 218-36. STAM,J. C. 1963. Geology,petrology and iron deposits of the Guiana Shield,Venezuela. Econ. Geol.,vol. 58,p. 70-83. STOBERNACK,J. 1968. Stratigraphie und Metamorphose des präkambrischen Grundgebirges der Bong Range in Liberia. Thesis, Faculty of Sciences,Clausthal,Technical University. THIENHAUS, R. 1963. Neue Eisen- und Manganerzvorkoimien in West- und Zentralafrika.Stahl u. Eisen, Düsseldorf, vol. 83, p. 1089-98. WHITE,R.W .; BAKER, M .W.1968.Geology of the Mano River Mine Area.Bull.geo1. Soc. Liberiu,vol.III,p. 57-63 plus46-7. ZIGTEMA, A.;MCCRARY, J. R. 1968. Bomi H i l l Ores and their benefication. Bull. Geol. Min. & Met. Soc., (Monrovia), vol. III,p. 16-27. ZULOAGA, G. 1933. The geology of the iron deposits of the Sierra de Imataca,Venezuela. Tech. Publ. Amer. Inst. Min. Engrs., no. 516. Class I,Min. Geol., no. 44,p. 1-36.

359

Structural control of the localization of rich iron ores of Krivoyrog G.V. Tokhtuev Institute of Geochemistry and Physics of Minerals, Academy of Sciences of the Ukrainian S.S.R.

The Krivoyrog basin, one of the largest centres of iron ore mining, has been intensely studied and prospected. The general structure of the basin consists of a group of conjugatedsecond order foldswhich form a synclinorium 7 km wide and 70 km long. These folds are found in the following succession(from east to west): Saksagan syncline and anticline,Main Krivoyrog synclineand anticline,Main Krivoyrog syncline,Taranakholihman anticline, and Lihm a n (Iiiguletz) syncline. Each of the above folds forms the structural foundation of a separate ore field, which differs from the others not only in its structure,but also in the type of rocks, degree of metamorphism, ore type, and in its degree of weathering.

Saksagan ore field This field (Fig. 1) is very important because of its scale of ore mineralization.The field is located within the limits of two conjugate second order folds: the Saksagan syncline and Saksagan anticline.Both of these folds are complicated by a longitudinal thrust fault and by a series of smaller faults.The larger ore bodies are localized in the Saksagan syncline (90 per cent of the ore deposit). The Saksagan anticline is not as important. The Saksagan ore field is characterized by low grade metamorphic rocks of the green schist facies, and by a high intensity of oxidation that extends as deep as 2.5 km. Structural control of mineralization is indicated for this ore field by the observation that the rich ores are concentrated in the trough of the Saksagan syncline and are associated with deformation of its east limb. Small deposits in the Saksagan anticline are controlled by longitudinal dislocations and loop-shapedfoldings of layers.

FIG.1. Distribution of Krivoyrog ore fields. 1. Illych mine; 2. Dzerzhinsky mine; 3. Kirov mine; 4.Karl Liebknecht mine; 5.Komintern mine;6.Frunze mine;7.XX Party Congress mine; 8. Rosa Luxembourg mine; 9.Lenin mine.

S T R U C T U R A L TYPES O F DEPOSITS IN T H E S A K S A G A N O R E FIELD

The Saksagan ore field consists of eight separate deposits (Fig. 2). Each deposit has a particular structure that

Unesco, 1973. Genesis of Precambrian iron and mangunese deposits. Proc. Kiev Symp., 1970. (Earth sciences, 9.)

361

G.V. Tokhtuev

Vertical longitudinal section Saksagan syncline

FIG. 2. Structural types

of Saksagan ore field deposits. 1. deposits associated with the Saksagan trough bend; 2. deposits related to knots of compression and shear deformations on the Saksagan syncline limbs, connected with flexure-typemineral-

ization at depth; 3. deposits confined to transverse zones of compression and development of fold-faultdeformations on the Saksagan syncline limbs.

controls ore mineralization. The Saksagan field deposits can be divided into three structural types: 1. Deposits associated with the trough of the Saksagan syncline. For example, a section of the Communav mine at the south end of the Dzerzhinsky mining district,where the trough ofthe Saksagan synclineinterceptsthe surface. 2. Deposits associated with compressional knots and crosscutting folded fractures joining at depth with ore mineralization of the trough type. They are illustrated in sections of the Gigant mine, the Dzerzhinsky Saksagan mine, the Kirov mine, and the Karl Liebknecht mine. Trough mineralization of the Saksagan syncline plunges north and forms ore at depths ranging from 300 to 2,000m . Steeply plunging ore chimneys, belonging to the cross-cuttingzones of pressure and deformation, extend from the top of the ore. The Komintern deposit belongs to this structure type. 3. Deposits associated with cross-cuttingcompressivezones and the development of folded-faulteddeformations on the limb of the Saksagan syncline. T o this type belong the deposits of the northern part,of the Saksagan ore field where the depth of the trough of the Saksagan syncline is 3-4 km. It is not yet known whether ore concentration in the trough extends farther to the north.The followingmines are encountered:the Frunze, Twentieth Congress of the CPSU,Rosa Luxembourg and V. I. Lenin mines. Cross-cutting zones of deformation on the east limb of the Saksagan syncline,which make up the basic structure of almost aíl deposits of the Saksagan ore field, are regularly distributed along the syncline strike, accompanied by small folds,breaks, depots, cleavages,boudinage,etc. In these zones, 1 or 2 km wide, numerous ore bodies are located associated with high order structures.The distance between the cross-cuttingzones ranges from 2 to 3 km. Compression of the ferruginous strata of the fifth and sixth ferruginous horizons characteristically caused a decrease in thickness in the limits of the cross-cuttingzones of shearing as compared to the thickness of the horizons in non-ore

locations between mines. Especially, considerable decrease in the thickness of the ferruginous horizons is observed in places of intense metallization where boudinage structures were formed. The process of formation of boudinage structures consisted of the removal of quartz from the compressive zone,because quartz became unstable as the result of high stress,and was easily dissolved and removed by metamorphic solutions. Ore minerals were not mobile under such conditions, and they accumulated in the interboudinepinches,leading to the formationof orebodies. The increase of quartz solubility under high pressure has been proved experimentally by Syromjatnikov. Thus,in the formation of cross-cuttingzones of shearing the development of boudinage along with the development of folding and fracturingplayed an important role. It should also be mentioned that cross-cutting zones of shearing of some ferruginous horizons resulted in the development of parallel beds. For example,in the Komintern mine, ore bodies are located in the ñrst, second,fifth and sixth ferruginoushorizons. In some mines parallel beds are found in the fifth and sixth horizons, and in the Rosa Luxembourg, and V. I. Lenin mines there are five to six or more parailel chains of beds in the fifth and sixth ferruginous horizons. The beds in various strata are located strictly along the cross-cuttingzone of shearing.

362

S T R U C T U R A L TYPES O F O R E BODIES IN T H E S A K S A G A N O R E FIELD

Ore bodies are typically controlled by various high order folds or fractures,either belonging to the complex of the structure of the deposit, or being independent forms with their own characteristicorientation.The structures controlling ore bodies are different; they can be folds and flexures of various types and orders,zones of microfolding,various tectonic cross-cutting and diagonal ruptures, zones of interboudine pinches, zones of intensive development of jointing and cleavage,zones of breccia, etc.

Structural control of the localization of rich iron ores of Krivoyrog

The following are the structuraltypes of ore bodies in the Saksagan ore field: 1. Ore bodies belonging to the trough of the Saksagan syncline. 2. Ore bodies belonging to arched-up high order folds. 3. Ore bodies controlledby cross-cuttingsmall open folds. T o this structural type belong both large and small ore bodies, where the relationship of ore mineralization and small folding is so close that the localization of the ore body is easily determined by the orientation of the folds. 4. Ore bodies controlled by the zone of isoclinalfolds not undergoing the process of mineralization.The deposits of this structural type are located very close to and parallel with the zone of isoclinal folds,and plunge in the direction and angle of the hinges of the folds, which do not undergo the process of mineralization. 5. Ore bodies belonging to small flexural folds along the strike. 6. Ore bodies belonging to interboudine pinches in zones of macroboudinage development. This structural type is widely distributed not only in the Krivoyrog basin, but also in the Kremenchug and Belozersk areas as well. Interboudine ore bodies are controlled by the orientation of interboudine pinches,which either correspond to the direction of pods of included rocks, or to a diagonal direction which plunges south or north. 7. Ore bodies in zones or cross-cutting faults of high orders. 8. Ore bodies belonging to zones of longitudinal thrust faults. 9.Ore bodies in zones of intraformational and interformationalbreccias.Evidently,these zones are formed as the result of the release of points of tectonic strain where the plastic limit of the rocks was exceeded. These zones undergo the process of mineralization and make up small ore bodies of irregular wasted form. 10. Ore bodies in zones of thickening of cross-cutting shear joints. On the flanks of the Saksagan syncline is a system of closely spaced cross-cuttingjoints. The spacing of joints ranges from 10 to 20 c m and up to 50 cm. But in some intervals of 50-100 m occur zones of very closely spaced joints where the distances between joint planes are not more than 1-2 c m with simultaneous intensive development of cleavage. Such places are occasionally mineralized, making up small ore bodies controlled by the direction of the joint zones. 11. Ore bodies in zones of intensive development of two cross-cuttingshearjoints.This structuraltype is characterized by ore mineralization associated with the intersection of joints which control the position of the ore body in space. The above typical structural types of ore bodies in the Saksagan ore field are the most important. However, there are other types, in which the relationship of ore mineralization to the structures is less evident.

Northern ore field The Northern ore field is situated along the continuation of the Saksagan ore field directly to its north. It begins with a large flexural bend of the east flank of the syncline, beyond which the extension of the structures changes from NNE.to N N W . The Northern ore field is also characterized by a higher degree of rock metamorphism than the Saksagan field, by the extensive development of the process of metasomatism, and by a magnetite-type of rich iron ore. Structural types of deposits and beds of the Northern ore field are the following: 1. Depositsand beds belonging to faultedsurfacestructures. 2. Deposits and beds associated with steeply-dippingsynclinal folds and flexures.

Central Krivoyrog ore field The Central Krivoyrog ore field joins on its west to the southern part of the Saksagan ore field. The major structure of this field is the Main Krivoyrog syncline which makes up the central part of Krivoyrog synclinorium. It is complicated by complex folding of higher orders. Ore bodies occur along the complexly folded contacts of the rocks of the middle ore suite and cover the upper schist suite of the Krivoyrog series. Characteristic features of the Central Krivoyrog ore field are complexly folded structures complicated by disturbances of various types and orders, a low degree of metamorphism (green schist facies) and the development of chlorite-magnetite and carbonate-magnetiteores which have been transformed in the zone of oxidation into martite ores up to a depth of 150200 m . Deposits of the Central Krivoyrog ore field are controlled by one type of complicated steep folded structureof the third,fourth and higher orders in the zone of the contact of the iron ore suite with the overlying layers of the upper schist suite.

Tarapako-Lihman ore field The Tarapako-Lihman ore field is situated to the west of the Central Krivoyrog ore field. It belongs to a large structure of the second order, the Tarapako-Lihman anticline, which forms the west flank of the Krivoyrog synclinorium.Ore zones are located on the contact between ferruginousbeds and the upper suite,developing mainly on the flanks of the fold. They are complicated by numerous fractures and, more rarely, are distributed up to the crest of the anticline. This field is characterized by a high degree of metamorphism (zone of garnet-cummingtoniteschists), by the development of magnetite-rich ores, by a shallow depth of the zone of oxidation (60-100 m), and by small thicknesses (3-10 m) of bedding-plane ore bodies. 363

G.V. Tokhtuev

Deposits of the Tarapako-Lihmanore field belong to the following structural types: 1. Deposits and ore bodies on the flank of the anticline, which have been complicated by post-ore cross-cutting and longitudinal faults. 2. Deposits and ore bodies associated with arched-up folds of the Tarapako-Lihman anticline and folds of higher orders.

Inguletz (Lihman) ore field The Inguletz (Lihman) ore field is located along the western border of the Krivoyrog basin. The structural base for this ore field is the Lihman syncline, conjugated in its northern part with the Tarapako-Lihman anticline. It extends 30 km south of the other submeridional structures of the Krivoyrog synclinorium. The western limb of the Lihman synclines is in the main part and

is cut off by the Western Thrust.In the southern extremity of the fold, a part of the western limb is preserved in the limits of the Inguletz mine. In ternis of the types of ores and their stratigraphic relationships, the Inguletz ore field is analogous to the Tarapako-Lihman and Central Krivoyrog ore fields, However, the southern extremity of the ore field (Inguletz mine) is characterized by a thick trough mineralization in the Lihman syncline and a highly developed zone of oxidation.According to these characteristics,it approaches the Saksagan type of trough mineralization. The ore controlling structures of the Inguletz ore field produce two structural types of ore deposits and beds. 1. Deposits and ore bodies in the trough of the syncline (Inguletz mine). 2. Deposits and ore bodies in zones of shearing on the eastern limb of the syncline (Pahmanovsky mine and small exhausted deposits north of the InguletzRiver).

Résumé Détermination structurale de la localisation des minerais de fer à haute teneur de Krivoyrog (G. V.Tokhtuev)

1. Les relations structurales qui conduisent à la localisation des minerais de fer de Krivoyrog sont déterminées par des études de structures de différents ordres : minéralisation, screening, minerais. 2. L a région de faciès structural de KrivorozhskyKremenchugsky a une structure qui est définie par un synclinorium composé de roches d’une formation de fer siliceux.Elle est compliquéepar une fracturelongitudinale et couvre une étendue de 400 à 500 km. Ici le contrôle structural a été utilisé pour planifier et exécuter un levé géophysique à petite échelle au sol et aéroporté. 3. L a région de minerai de fer de Krivorozhsky (bassin) fait partie de la zone de faciès structural de Krivorozhsky-Kremenchugsky. Sa structure consiste en un groupe de larges plis conjugués qui s’étend sur 70 km dans la direction du gisement. Les minerais de fer sont ici déplacés. Les éléments structuraux servent alors à la prospection et à l’étude des perspectives. 4.Les gisements de minerai de fer qui composent le bassin de Krivorozhsky sont définis par de larges plis séparés du troisième ordre compliqués par des dislocations longitudinaleset une rupture de continuité (synclinaux de Saksagansky,de Likhmanovsky,Krivorozhsky et anticlinal de Tarapaco-Likhmanovsky). L a longueur du bassin de minerai est déterminée par les dix premiers kilomètres. Les facteurs structuraux sont utilisés ici pour une exploration préliminaire. 5. Les dépôts de minerai dans chacun des bassins de

364

Krivoyrog sont liés à des structures complexes, flancs de raccordements de quatrième ordre, courbes de larges plis. En général,c’est aux nœuds des plis transversaux,flexures et fractures, structures de microboudinage, etc., que la minéralisation est liée.L a dimension des dépôts de minerai est mesurée par les premiers kilomètres. Le contrôle structural est utilisé comme base pour la prospection détaillée des différentes mines (dépôts). 6.Les gisements de minerai des différents dépôts sont liés à des structures à minerais des ordres les plus élevés (différents types de plissement, structure de boudinage, zones de jointement intensif et de clivage,zones de brèche et de cataclase, dislocations avec des ruptures de continuité, différents types de déplacements, etc.). L a relation structurale de localisation et de minéralisation apparaît ici d’une façon particulièrement claire et précise et détermine la morphologie et la localisation des gisements. Le contrôle structural a été effectué en vue d’une prospection détaillée et opérationnelle. 7. L a morphologie des gisements de Krivoyrog est extrêmement différente. Elle dépend du type, de la forme et des dimensions des structures à minerai. Les grandes colonnes de minerai prédominant. Leur section varie ; elles pénètrent jusqu’àplus de 2 km.On trouve aussi des dépôts du type à large strate et des dépôts articulés complexes (limités aux coudes des grands plis). Des gisements de moyenne et grande taille prennent la forme de lentilles,de masses à configurationextrêmementirrégulière et de poches dans lesquelles il n’est pas toujours possible de prédire les structures à minerais qui servirant de base pour déterminer les formations de minerai.

Iron deposits of Michigan (United States of America)' J. E. Gair U.S.Geological Survey, Washington D.C.20242 (United States of America)

The iron deposits of Michigan are principally in four areas, the Gogebic, Iron River-Crystal Falls, Marquette and Menominee districts, all in the northern peninsula of Michigan (Fig. 1). Iron-bearing beds form part of a sequence of middle Precambrian metasedimentary rocks, perhaps 1,900-2,500 m.y. old. The metasedimentary sequence (Fig. 2) is generally considered to have been deposited in a marine environment. Geologic structure in the Gogebic and Menominee districts is essentially 'one-sided'-monoclines or the dragfolded flanks of large regional uplifts; the Marquette district is in a narrow synclinorium and the Iron RiverCrystal Falls district occupies a broad three-cornered structuralbasin. The trend of synclinal and basin axes and of monoclinal iron-formationis generally eastward. The Marquette and Menominee depositional basins of Precambrian time probably were elongate, with long axes being roughly equivalent to the present tectonic axes. Clastic sediments in the iron-formation along the south side of the Marquette synclinorium indicate that the south side was closer to a shoreline than the north side. Little can be determined about the outlines of the Gogebic and Iron River-CrystalFalls depositional basins in Precambrian time. The iron-formationof the Gogebic, Marquette and Menominee districts is thought to be correlative, and is in the middle part of the middle Precambrian sequence. The iron-formationin the Iron River-CrystalFalls district is younger,in the upper part of that sequence.The correlative rock formations of the first three districts are correlated principally because of similarity of the rock sequence containing the iron-formation (Fig. 2). Iron-formation facies may change along strike in a given district and detailed iron-formationstratigraphy is markedly different in the three districts; whether the iron-formation or any of the associated rock units ever were entirely continuous from one district to the other is unknown. Basement rock for the middle Precambrian rock sequence in the Gogebic, Marquette and Menominee districts is gneissic and/or intrusive granite, amphibolite and/or volcanic greenstone,

all of lower Precambrian age, 2,600 m.y. old or more. The stratigraphic sequence containing the iron-formation in the Iron River-Crystal Falls district is underlain by middle Precambrian volcanic greenstone (Fig. 2). In the Gogebic district, the thickness of the ironformation is between 600 and 1,000ft (180-300 m), and is 800-900 ft (244-274 m)in most places. In the Marquette district the thickness ranges from 450 to 3,500 ft (1351,060m)or more,and commonly is about 1,000ft (300 m). In the Menominee district, thicknesses range from 300 to 600 ft (91-180 m) and average about 450 ft (135 m). In the Iron River-Crystal Falls district, thicknesses range from 150 to 600ft (46-180 m)at the west end of the district and from 500 to 800 ft (150-130 m) at the east end. The iron-formations nearly everywhere have been recrystallizedduring regional metamorphism,and minerals possibly of diagenetic or low-grade metamorphic origin generally cannot be distinguished from recrystallized primary minerals. Dominant primary minerals in the Gogebic and Marquette districts are siderite-chertand, locally in the stratigraphic section,hematite-chert (Fig. 3). Most magnetite is probably primary or diagenetic. The primary nature of chert and siderite is indicated by the widespread uniformity of beds, compositions and textures,which are not consistent with a replacement origin. Also, stylolites and preconsolidation slump structures that invqlve chert and siderite indicate that these minerals are primary or very early (Figs. 4 and 5). The draping of ferruginous laminae over chert beds and slump fragments is another indication of the presence of chert early in the history of the iron-formation (Fig, 6). Hematite is deduced to be primary where it occurs in oolites and granules and, in granules of probably organic origin, in relatively thick wavy or pod-shaped layers. Oolites or granules, particularly where they form lenticular beds, are interpreted as deposits that originated in shallow agitated water; the expected primary iron mineral is a ferric oxide. In places, 1. Publication authorized by the Director of the U.S.Geological Survey.

Unesco,1973. Genesis ofPrecambrian iron and manganese deposits. Proc. Kiev S y m p , ,1970.(Earth sciences, 9.)

365

J. E.Gair

90o

I

89'

88"

I

I

47

46

FIG. 1. Geologic sketch map of western part of northern peninsula of Michigan (United States), showing location of major iron-producingdistricts. unoxidized interbeds of siderite, magnetite, iron silicate or greywacke in hematitic iron-formationindicate a lack of oxidation since deposition of the rock and virtually prove the primary nature of the adjacent hematite. Magnetite is important in all districts except Iron River-Crystal Falls and may be primary, diagenetic or metamorphic. A primary or diagenetic origin is deduced for large amounts that are widely distributed in thin uniform laminations in iron-formationof low metamorphic grade. Uniformly alternating thin layers of magnetite and siderite or magnetite and hematitic chert are more readily explained by fluctuations in conditions during sedimentation than by post-depositionalprocesses, but generally it has not been possible to distinguish primary from diagenetic magnetite by direct evidence. Evidence for the 366

diagenetic origin of magnetite by the reduction of ferric oxide has been shown by severalworkers in other regions. In the eastern part of the Marquette district, some magnetite, possibly a large amount,has formed diagenetically (Han, 1962), mainly by the oxidation or decarbonation of siderite.Small amounts also have formed by the oxidation of iron silicate. At the Empire taconite mine, small relict 'islands' of siderite iron-formation occur sporadically within a unit of magnetite-richiron-formationfor a strike distance of about f mile (about 850 m) and through a thickness of 300400 ft (about 125 m). Commonly, bedding is continuous from sideritic relicts into the magnetiterich rock (Fig.7). In a few places,marginal concentrations of magnetite occur in granules that consist dominantly of carbonate,iron silicate or chert (Fig. 8). The magnetite

EX PLA N AT I O N

pz4

Greywacke

\

MAROUETTE

Greenstone

J p

......_. : ;: ... Quartzite

0 Iron-Formation IRO-N RIVER-CRYSTAL FALLS DISTRICT

Dolomite

Gneiss, granite

FEET

Columns broken where part of stratigraphicsection omitted

FIG.2.Correlation of major lithologies in Michigan iron-producing districts. 3U7

J. E. Gair

IRON RIVER-CRYSTAL FALLS DISTRICT -500 FEET

H-Hematite Mt-Magnetite Py-Pyrite Sid-Siderite Sil-Silicate Stilp -Stilpnomelane Subordinate minerals shown in parenthesis

I I Sid (Stilp)

IRON-FORMATION

O

-

MARQUETTE DISTRICT _/A.

GOGEBIC DISTRICT

H-Mt

/-A--/

NEGAUNEE IRON-' FORMATION

\ \ \ \ \ \ \ \,MENOMINEE DISTRICT

\ Cid-Sil-Mt

IRON-

\

---_-__

FORMATION

Sid (H-Mt) IRONFORMATION

Cid-Sil Cid (H-Mt)

________--.

FIG.3, Primary-diagenetic iron minerals in Michigan ironproducing districts.

I

-

N C

H - O

2 rnrn

E

-

S

-

368

Iron deposits of Michigan (United States of America)

1

,

FIG.5. Photograph, drill core; slump structure in cherty and carbonate layers.

.

J

FIG. 6.Photograph,drillcore;laminae rich in silicate and magnetite draped over chert-richfragment.

FIG.7.Photograph,polished surfaces;replacement of sideritic layers by magnetite;relict sideritic ‘islands’commonly bordered by reaction rim of secondary carbonate.

FIG.8, Photomicrograph;marginal to completereplacement of minnesotaite granules by magnetite. Plane light. 369

J. E.Gair

is attributed to oxidation of the carbonate or silicate or to replacement of the chert during diagenesis. Evidently, magnetite replaced iron carbonate or iron silicate in newly deposited sediment in response to a change in the original neutral or reducing conditions to moderately oxidizing conditions,or possibly because of a change in p H from near neutral to alkaline (see results of experimental studies dealing with influence of E h and p H on the deposition of iron minerals, reported by Garrels, 1960; Huber, 1958; and Krauskopf, 1957). Shallowing of the sea bottom could have increased the oxygen content of sea-water and adjacent interstitial water in bottom sediments, or by improving near-bottom circulation,may have lowered the acidity of sea-water. The experimental work cited above shows that siderite-stableconditions can change to magnetite-stable conditions by an increase in pH, with no change in Eh, or even with a decrease in Eh, although the actual geologic conditions that could produce a simultaneous increase in p H and drop in E h are difficult to visualize. Iron silicates, particularly minnesotaite and stilpnomelane, are abundant in parts of the Gogebic and Marquette districts, and stilpnomelane and iron chlorite are locally abundant in the Iron River-Crystal Falls district, but the absence of these minerals in unmetamorphosed iron-formationor in post-Precambrian ironstone of other regions suggests that they are not primary or diagenetic, but of low-grade metamorphic origin. On the basis of chemical composition (Deer, Howie and Zussmann,1963, Winchell, 1951), minnesotaite probably does not require a silicate parent and may have been derived solely by diagenetic or metamorphicreactionsbetween primary chert and siderite. The significant aluminium content of stilpnomelane, on the other hand, indicates a substantial increment $ofaluminous silicate in the primary sediment from which that mineral was derived. The widespread lack of a siderite-chertreaction at low metamorphic grade has been cited as evidence that both minnesotaite and stilpnomelane developed from primary silicate material (James, 1954).

TABLE 1. Modes of typical silicate iron-formation,eastern part of Marquette district (in volume per cent)

Chert Siderite Magnetite Stilpnomelane Minnesotaite Mixed magnetite and iron silicate Gruneritel Secondary hematite

1

2

Trace

6.2 0.5 1.o 22.0 49.5

1.2

18.4 54.4 20.9 5.1

20.6

1, Attributed to contact metamorphism by intrusion of mafic sill.

370

3

4.0 2.5 90.0 3.5

FIG.9. Photomicrograph;granules of minnesotaite and minnesotaite-magnetite(granulesmarked M)surrounded by chert-rich matrix. Note marginal concentrations of secondary magnetite. Cross nicols.

In the eastern part of the Marquette district, some thinly laminated iron-formationrich in iron silicate has a low chert content, less than 10 per cent (Table 1); other varieties of thinly laminated iron-formation typically contain 15-50 per cent chert. The silica content of cherty iron-formation varies widely depending mainly on the amount of chert.Pure chert-sideriteiron-formation,having about 61 per cent chert, contains about 45 per cent silica by weight, comparablewith the percentage of silicain some of the silicate-rich,chert-pooriron-formation.The silicate iron-formationtherefore differs from other thinly laminated iron-formation in the vicinity mainly in lacking chert laminae. This may be a result of the incorporation of original chert into iron silicate minerals formed after sedimentation. Some layers of iron-formationconsist largely of iron silicate granules.Granules and matrix commonly are similar in composition,as would be expected if granules formed by agitation of the original sediment. However, in some layers,iron silicategranules are surrounded by silicate-poor material,generally chert or siderite (Fig.9), or silicate-poor granules may be surrounded by silicate. Such silicate granules or matrix seem to be best explained by selective replacementof cherty or sideriticmaterial during diagenesis or by concretionary growth during diagenesis. A chert matrix for closely packed granules of silicate can be explained by infilling by silica,but this explanation does not seem adequatefor widely scatteredsilicategranules in chert. An alternative explanation, that granules differentiated during sedimentation or agitation of bottom sediments, seems unlikely without an accompanying segregationof the minerals into layers.Such differentiated granules,therefore,

Iron deposits of Michigan (United States of America)

FIG.10.Photomicrograph;riebeckite in chert-magnetite-carbonate iron-formation.

FIG.11. Photomicrograph;jaspilite; thin finer-grained layer is of jasper (hematitic chert); coarser-grainedlayers are of martite and chert.Note flattened granules in jasper layer and clear chert granules in thicker martite layer.

seem to provide clear evidence of the diagenetic or metamorphic growth of iron silicate. Primary iron minerals in the Iron River-CrystalFalls district are principally siderite and pyrite. The association of bedded pyrite both with siderite and carbon-rich sediment is a strong indication that it originated as a primary sediment. Stilpnomelane is common in the east part of the district but, as in the Gogebic and Marquette districts, is considered to be of low-grademetamorphic origin. In the Menominee district,hematite and possibly magnetite, were important primary minerals. Riebeckite and aegirinaugite are present in thin zones in the iron-formationin the eastern part of the Marquette district through a stratigraphicinterval of 300-400ft (about 125 m) and a distance along strike of about 2 miles (3 km) and down dip for at least mile (about 850 m) (Fig. 10). The soda content of such iron-formation ranges from 0.5per cent to 6 per cent. Some of the riebeckite-bearing iron-formationcontains,or is associated with,clastic sediment.Iinterpret the sedimentationof the riebeckite-bearing iron-formation as having taken place locally in shallow water under evaporiteconditions.Soda-bearingparts .ofthe Wabush Iron Formation of Labrador are also considered to have originated in an environment both of high E h and high salinity (Klein, 1966). In the Iron River-CrystalFalls,Marquette and Menominee districts there is little or no evidence of contemporaneous volcanism in the iron-formationor in conformable rock below.Significantvolcanism is known to have occurred during iron-formationdeposition only in the eastern part of the Gogebic,district,but even there only for a limited part of the entire period of ferruginous sedimentation.

Middle Precambrian sedimentation was brought to a close,or was followed closely by, the regionwidePenokean orogeny about 1,900m.y.ago.Metamorphism causedrecrystallization of iron-formationalmosteverywhere in the area. Although primary or diagenetic chert, hematite and magnetite commonly persist from lowest grades of metamorphism to the sillimanite grade, they are recrystallized to increasingly larger grains at higher gradesofmetamorphism. Jaspilite is a recrystallized variety of hematite-chert ironformation of low to moderate metamorphic grade. M u c h jaspilitein the Marquette district,however,contains a large percentage of magnetite or martite; commonly the ironrich laminae are mainly magnetite-martite,and the jasper laminae are hematitic chert with minor magnetite (Fig. 11). The sizes of chert and hematite grains,in particular, correspond closely to metamorphic grade.Siderite,on the other hand,recrystallized early at low metamorphicgrade but, at higher metamorphic grades, reacted with chert to form grunerite, and some possibly was altered to magnetite. The characteristic siderite in three of the four Michigan districts suggests deposition in basins isolated from the circulationof the open sea.The normal oxidizing condition of open seas was eliminated (changed to negative E h or to higher than normal acidity) by stagnation,except possibly in the shallower parts of depositional basins, such as near shore, or more widely after uplift or infilling of basins of sedimentation. In shallow areas, fully oxidizing conditions and normal slightly alkaline conditions persisted or recurred,marked by positive Eh and the depositionofhematite or ferric hydroxide-likeprecipitates, Magnetite may have precipitated in such an environment under marginal oxidizing conditions,at Eh near or a little less than zero. The

371

J. E.Gair

abrupt small-scale alternation of layers rich in hematite and magnetite, in jaspilite, suggests relatively rapid local fluctuations in oxygen activity during sedimentation, a condition most likely to be realized in shallow water near the base ofwave action.In the eastern part of the Marquette district,however, magnetite that formed by the diagenetic replacement of siderite indicates a change toward positive E h or greater alkalinity after sedimentation,as cited already. In the Iron River-CrystalFalls district, a combination of stagnation in a depositional basin and sufficiently deep water locally to prevent ‘contamination’by atmospheric oxygen permitted organic carbon to accumulate with chemically precipitated iron sulphide. Between the time of deposition and diagenesis of the iron-formations and the time of their regional metamorphism at the close of the middle Precambrian,they were partly weathered and eroded in places, shortly after they were deposited, and intruded by mafic igneous rock. In the Marquette district,at least,mafic intrusionstook place both before and after the middle Precambrian episode of weathering and erosion. Secondary iron oxide produced from siderite and magnetite during that episode in the Marquette district, and possibly elsewhere, was recrystallized during the Penokean metamorphism.This iron oxide in its recrystallized form is generally indistinguishablefrom recrystallized primary hematite. Thersfore, the principal recrystallized hematitic rock, jaspilite, may be either primary hematitic rock, or may have been derived by secondary oxidation prior to regionalmetamorphism.Criteria for distinguishing these two types are few and obscure. The widespread spatial association of jaspilite with the erosion

FIG. 12.Photomicrograph;retrograded porphyroblasts of grunerite in minnesotaite-magnetite-carbonateiron-formation. Pseudomorphs of grunerite are mainly of quartz, plus minor siderite and magnetite.

372

surface cutting into the iron-formationto different stratigraphic levels in different parts of the Marquette district is the strongest evidence for the derivation of jaspilite by premetamorphic weathering.The rare gradation ofjaspilite into small, apparently relict ‘islands’ of sideritic ironformation also is evidence for the secondary origin of jaspilite. The principal direct evidence of jaspilite of primary origin is the presence in places of hematitic oolites or granules. Jaspilite that appears to grade into sideritic ironformation is thinly laminated,with most layers less than 0.5inch thick (about 1.25 cm), whereas layers of granular jaspilite commonly are 1-3 inches thick (about 2.5-7.5 cm) and are pod-shaped. In the eastern part of the Marquette district,the mafic intrusions prior to Penokean metamorphism locally modified the iron-formationby converting some siderite to magnetite or pyrite near dyke contacts,and by forming grunerite porphyroblasts in siderite-chertlayers or in layers that are now mainly minnesotaite. During ensuing low-graderegional metamorphism in the area, the grunerite porphyroblasts commonly were altered retrogressively and replaced by quartz, quartz-siderite, or quartz-siderite-magnetite (Fig. 12). Ore-gradeconcentrationsof hematite that accumulated during the middle Precambrianweathering episode recrystallized during Penokean metamorphism to form what is called hard ore. Because the hard ore deposits typically average several per cent richer in iron than ore-grade concentrationsof ferric oxide (soft ore) that developed after metamorphic recrystallization,it is likely that secondary oxidation alone cannot explain the concentration and that hydrothermal solutions aided the concentrationofhard ore. Hydrothermal solutions alone have been invoked as the concentrating agent by some workers, but there is virtually no evidencefor such solutionsin iron-formationunderlying hard ore bodies. The only known igneous source of such solutions (the mafic intrusive bodies) cannot explain the localization of many of the most important orebodies at the middle Precambrian erosion suiface.Small amounts of autogenous hydrothermal solutions might have been derived from the heating ofconnatewater and the dehydration of chert during regional metamorphism and supplemented the concentration of hard ore. After the Penokean metamorphism the recrystallized iron-formation evidently remained largely unchanged for hundreds of millions of years until the Keweenawan-early Palaeozoic interval about 600to 900m.y. ago. Then,parts of the iron-formation were oxidized and leached (weathered) by supergene or mixed supergene and hypogene ground-water solutions (James et al., 1968), producing sporadic concentrations of earthy hematite aiid goethite in the iron-formation-soft ore-localized to a large degree along the axes of synclines and in other upward-opening structural traps. Typical occurrences of hard and soft ore are shown in Figure 13.

Iron deposits of Michigan (United States of America)

_____----. ._____._-----.___I_-Iron-formation

Sideritic iron-formation

__-.'

,.I-------______ ..I

.--.

,__,

.-_--I

iron-formation

'

\

\

SOFT ORE

u //

iron-formation Sideritic

/

HA R D...OR E

FIG.13. Cross-sections showing typical occurrences of iron ore, Michigan.

Acknowledgement Preparation of this report was aided by published work on the Michigan iron districts by James, 1954 James et al., 1968; Bayley et al., 1966; Huber, 1959, and Prinz, 1967; and unpublished data for the Gogebic district supplied by R.G.Schmidt of the U.S.Geological Survey.

373

J. E.Gair

Résumé Gisements de feu du Michiguii, aux États-Unis d'Amérique

(J. E.Gair) L'auteur présente un résumé des faciès de formation de fer, de l'environnement dans lequel les dépôts corrélatifs se sont formés ainsi que les modifications qui sont intervenues après le dépôt,y compriscelles qui se rapportent à la genèse du minerai. Ces différents aspects sont comparés dans les quatre zones principales d'industrie minière du fer de l'État de Michigan aux États-Unis-les districts Gogebic,IronRiver Crystal Falls, Marquette et Menominee. Dans les districts Gogebic et Marquette les minerais primaires dominants sont du silex à sidérite, peut-êtrede la magnétite et, dans de petites parties de la zone stratigraphique,du silex à hématite ; dans le district $Iron-River Crystal Falls, le silex à siderite et du matériel pyritique ; dans le district Menominee,du silex à hématiteet peut-êtrede lamagnétite. L a nature primaire du silex et de la sidérite est indiquée par la régularité des lits, leur composition et leur texture, et par des stylotiteset des structuresconsistant en éboulements consolidés antérieurement.Le silex primaire est indiqué de plus par le drapage de feuillets ferrugineux superposés à lentilles de silex et des fragments d'éboulis. L'hématite primaire est suggérée par son association avec des granules et des colites d'eaux peu profondes et des granules ayant peut-Ctreune origine organique dans des couches épaisses onduleuses ou lenticulaires,et par l'absence d'une oxydation secondaire dans des lits intermédiairesde sidérite,magnétite,silicate de fer et de sédiments clastiques.L a magnétite est importante dans tous les districts sauf dans celui #Iron-River Crystal Falls. Elle y est peut-être primaire, diagénétique ou les deux. Dans la partie est du district de Marquette la plus grande partie de la magnétite est formée par voie diagénétique à partir de sidérite à des profondeurs faibles, à peu près à l'époque où les lits intermédiaires minces de graywacke,de quartzite feldspathique et de formations de fer riches en soude se sont déposés immédiatement au-dessus.L'origine diagénétique est indiquée par de nombreux flots rémanents de formation de fer sidéritique à l'intérieur d'une formation de fer magnétitique. L a sidérite et la pyrite sont attribuées à des conditions presque stagnantes dans les parties les plus profondes de lagunes ou de bassins isolés de la haute mer et l'hématite aux bordures peu profondes de bassins au voisinage du rivage et à la diminution de la profondeur du fait de la sédimentation ou des subsidences. Des formations de fer riches en soude sont associées à des formations d'évaporite.

374

Dans le district de Marquette certaines jaspilites peuvent être associées à des formations de fer à faciès d'oxyde primaire, et certaines à des formations de fer oxydéespendant la subsidence du Précambrien.moyen, la détérioration météorologique et l'érosion, mais beaucoup de jaspilites dans la partie est du district ne présentent aucune structure liée à des eaux peu profondes ni à une relation évidente avec une formation de fer sidéritique. La recristallisation des formations de fer s'est produite pendant une orogénie régionale et au cours du niétamorphisme à la fin du Précambrien moyen, c'est-à-dire il y a 1,7 milliard d'années, la dimension des grains de quartz (silex) variant en relation avec le degré de métamorphisme. L a minnesotaite et (ou) le stilpnomelane se sont formés soit durant la déformation aux degrés inférieurs du métamorphisme ou pendant une diagenèse plus ancienne. Des effets spécifiques diagénétiques ou métamorphiques ne peuvent généralement pas être reconnus. L a présence dans la partie est du district de Marquette de formations de fer et de silicates pauvres en silex et de formations de fer sidéritique riches en silex, chacune avec à peu près 45 % de silice,fait penser que le contenu en silice a été fixé au cours du dépôt en grande partie sous la forme de silex,et que les minéraux silicatés postérieurs se sont développés par l'incorporation de silex. Les granules riches en silicate dans une matrice pauvre en silicate peuvent être plus facilement expliqués par le remplacement sélectif ou le développement de concrétions durant la diagenèse plutôt que par la différenciation pendant la sédimentation. D e la riebeckite et de l'augite aegyrinique dans la partie est du district de Marquette furent à l'origine d'une formation de fer contenant du silex, du carbonate et de la magnétite riche en soude durant un métamorphisme régional peu avancé. L a gruneriteest unproduit du métamorphismerégionalavancé dans la partie ouest du district de Marquette et un produit de métamorphisme de contact local à l'est. D u minerai d'hématite dure et tendre peut être différencié en concentrations prémétamorphiques (recristallisées)et postmétamorphiques.Leminerai tendres'est concentré dans des structures s'ouvrant vers le haut dans des régions à faible degré de métamorphismeet en relation avec la circulation d'eau souterraine d'une surface d'érosion datant du Précambrien récent. Une origine essentiellement analogue pour le minerai dur du district de Marquette est indiquée par l'association du minerai dur avec une surface d'érosion datant du milieu du Précambrien, malgré l'évidence d'effets locaux hydrothermaux.

Iron deposits of Michigan (United States of America)

Bibliography /Bibliographie BAYLEY, R.W.; DUTTON, C. E.;LAMEY, C. A. 1966. Geology of the Menominee iron-bearingdistrict, Dickinson County, Michigan, and Florence and Marinette Counties,Wisconsin. Prof. Pap. US.geol. Surv. 513. DEER,W.A.;HOWIE, R. A.;ZUSSMAN, J. 1962. Rock-forming minerals. New York,Wiley. 4 vols. GARRELS, R. M.1960.Mineral equilibria at low temperature and pressure. New York,Harper &Bros. 254 p. HAN, T.-M. 1962 Diagenetic replacement of ore of the Empire mine of northern Michigan and its effects on metallurgical concentration (abs.). 8th Annual Meeting Institute on Lake Superior Geology, Houghton, Mich.,Michigan Coll. Mining and Technology, p. 7.

HUBER, N.K.1958.The environmental control of sedimentary iron minerals. Econ. Geol., vol. 53, p. 123-40. __ . 1959. Some aspects of the origin of the Ironwood ironformation of Michigan and Wisconsin.Econ. Geol., vol. 54, no. 1, p. 82-118.

JAMES, H.L.1954. Sedimentary facies of iron-formation.Econ. Geol., vol. 49,no. 3, p. 235-90. JAMES, H.L.;DUTTON, C.E.; PETTIJOHN,F.J.; WIER, K.L.1968. Geology and ore deposits of the IronRiver-CrystalFalls district,Iron County,Michigan.Prof. Pap. U.S.geol. Surv. 570. KLEIN, Jr. 1966. Mineralogy and petrology of the metamorphosed Wabush Iron Formation, south-western Labrador. J. Petrol., vol. 7,no. 2,p. 246-305. KRAUSKOPP, K. B. 1957. Separation of manganese from iron in sedimentaryprocesses.Geochim. et cosmoch. Acta, vol. 12, p. 61-84. PRINZ, W.C.1967.Pre-Quaternarygeologic and magnetic map and sections of part of the eastern Gobegic Iron Range, Michigan. Misc. geol. inv. M a p U S . geol. Surv., 1-497. WINCHELL, A.N.1951.Elements of optical mineralogy, 4th ed. N e w York, Wiley.

375

Problems of nomenclature for banded ferruginous-cherty sedimentary rocks and their metamorphic equivalents

On an international basis, the nomenclature of banded ferruginous-cherty sedimentary rocks is unfortunately imprecise. In different countries,different terms are used for the same rocks and identical terms are also used for different rocks. In order to come to a common understanding throughout the world as to exactly what particular type of iron-richrock a particular name is meant to describe, clear concise definitions of terms should be presented in papers of more than local significance.In the published papers of the InternationalSymposium on the Geology and Genesis of Precambrian Iron-ManganeseFormations and Ore Deposits,held in Kiev,U.S.S.R.,in1970,thefollowing terms were used by Russian contributors:taconite (talconit), itabirite (itabivit),jaspilite (dzhespilit),ferruginousquartzite (zhelezisty Jcvartsit j, iron hornfels (zhelezisty rogovik), ferruginous chert (zhelezisfy Jcremen’),ferruginousjasper (zhelezistuya yashma), iron ore (zheleznaya rida). Usage in the U.S.S.R. differs considerably from that in much of the Western world;usage in the Western world is also not consistent. In the Western world a greater emphasis is placed on conditions of sedimentation of ironformations.The effects of widely differing degrees and types of metamorphism further complicate nomenclature. In the United States of America, Canada, Australia and South America, the generic term for banded ferruginous-cherty rocks of sedimentary origin has come to be ‘iron-formation’ (zhelezìstayajovrnafsiuj.Most geologistsin those countries accept James’ definition (1954, p. 239) or that of Gross (1966, p. 41). Iron-formation, generally believed to be a dominantly chemical (or biochemical) precipitate, typically consists of chert (kremen‘) or jasper (yashma) interbanded with one or more iron-richminerals: oxide,carbonate,silicate or sulphide.Very rarely,the rock does not contain chert. On both empirical and theoretical (Krumbein and Garrels, 1952;Kiauskopf, 1967) grounds, the primary iron-rich mineral is an indicator of the p H and Eh of the environment of deposition of the ironformation. Therefore the dominant type of iron mineral was used by James to name primary ‘facies’ of ironformation,Oxide-facies(hematitic)iron-formationindicates

a positive Eh, sulphide-faciesa strongly negative Eh, and carbonate- and silicate-facies are intermediate. The facies are intergradational;the hematitic oxide-and the sulphidefacies are incompatible. The term iron-formationin Western usage is strictly parallel to limestone, a generic lithologic name. Just as there are many different types of limestone, there are also different types of iron-formation.Thus, in naming formations in a stratigraphic sense,a formation in Australia may be called the ‘Brockman Iron Formation’for example, just as w e may call an American limestone formation the ‘NiobraraLimestone’. To avoid confusion, the U.S. Geological Survey hyphenates iron-formation when the words are used in a lithologic sense; the words are capitalized when used in a stratigraphic sense. This practice might be more widely adopted. The term ‘jaspilite’ was ñrst applied in the Lake Superior area to oxide-faciesiron-formation in which the silica is present as jasper (yashma).Subsequently,jaspilite has achieved wide internationalusage for other oxide-facies iron-formations.Unfortunately,in some areas it has been applied to rocks that would not be called jaspilite in the Lake Superior area. ‘Taconite’ is another term for iron-formation that originated in the United States and has achieved some international currency, particularly in the U.S.S.R.The word is a general term, now used primarily by mining engineers and metallurgists, and is without exact niineralogical or environmental implications. Therefore, many geologists feel it should be dropped from scientific literature in favour of more specific terminology. In South Africa, oxide-faciesiron-formationhas been called banded ironstone, although on the American continents and elsewhere ‘ironstone’is reserved for the minettetype ores, generally not cherty or banded, commonly in part clastic and fossiliferous,and almost everywhere postCambrian in age. The distinction between iron-formation and ironstone seems worth preserving, because the environment and processes of deposition are different for the two rock types.

Unesco, 1973. Genesis of Precambrian iron and tnanganese deposits. Proc. Kiev Synrp., 1970. (Earth sciences, 9.)

377

Problems of nomenclature for banded ferruginous-chertysedimentary rocks and their metamorphic equivalents

Most, but not all, Precambrian iron-formationshave been metamorphosed. James (1955) has shown that the grain size of the recrystallized chert or jasper varies directly with the degree of metamorphism. Susceptibility of oxidefacies iron-formation to supergene enrichment is closely controlled by grain size (Dorr, 1964). ‘Itabirite’is a Brazilian term that has achieved wide usage in South America, West Africa, and elsewhere for oxide-faciesiron-formation that has been metamorphosed to a degree that makes the individual crystals of the rock megascopically distinguishable (Dorr and Barbosa, 1963). The term has a specific and restricted meaning in both field and economic applications and may be worth preserving in the international nomenclature. ‘Banded hematite-quartzite’(BHQ)is a term widely used in India and to a lesser extent in Australia and elsewhere for oxide-faciesiron-formation.Some of thismaterial has been highly enough metamorphosed to make it the equivalent of the itabirite of Brazil;much is of lower metamorphic grade and cannot be considered the equivalent of that rock type. The latter is, in part, an equivalent of the jaspilite of the Lake Superior region. Although iron-formationmay locally contain considerable amounts of detrital material such as interbedded or intermixed shale, tuff and even sand or pebbles, it is dominantly a chemical or biochemical precipitate. ‘Ferruginous quartzite’is,in Western usage, reserved for rocks of dominantly detrital origin.Although the rock may have essentiallythesamechemicalcompositionas iron-formation, the quartz,and in many cases the iron minerals,are clastic in origin. It may or may not be grossly banded. In the U.S.S.R.,however,ferruginous quartziteis used,according to Semenenko (1956,1959, 1967), in three different senses: ferruginous clastic quartzose rock, coarse-grained metamorphosed iron-formationof either oxide-or silicate-facies, and all ferruginous cherty rocks. Hornfels’is a term used with widely differentmeanings in the West and the U.S.S.R. In the West, hornfels is ‘a fine-grainednonschistose metamorphic rock resulting from contact metamorphism. Large crystals may be present and may represent either porphyroblasts or relictphenocrysts’.l To our knowledge, hornfels has never been applied to iron-formationin the West. In the U.S.S.R., hornfels is commonly used for fine-grainedrocks including, but not restricted to,silicate-and oxide-faciesiron-formation,that need have no relation to contact metamorphism.The material may be somewhat schistose or foliated by regional or dynamicmetamorphism.‘Ironhornfels’in the U.S.S.R. literature is a coarse-banded iron silicate-chertrock with

378

fine-grained quartz; some authors also consider it as a synonym for ferruginous jasper. ‘Jaspilite’in the U.S.S.R. is a banded rock with iron present as hematite, magnetite, or martite and silica as ‘fine-grainedquartz-jasperor hornfels’.The term ‘itabirite’ has no usage in the U.S.S.R.,but is implied for metamorphosed jaspilite. ‘Iron ore’ is used very loosely in the U.S.S.R., as it is by some authors in the West. In some cases the term has specific economic implication, in others it has no implication of economic viability. Similarly, the word ‘iron’ is very loosely used in the literature of the U.S.S.R.;it merely indicates the presence of the element in some form, without any implication as to quantity or oxidation state or chemical composition of the iron-bearingmineral. The usage is the Sam= as that in the term iron-formation;this can be disconcerting to Western readers when applied to ‘ironhornfels’,‘iron quartzite’ or ‘iron chert’. Western readers commonly approach geologicalliterature of the U.S.S.R. via a translation.One often wonders how much of the terminological difficulty is caused by inept translation rather than original usage; certainly our colleagues in the U.S.S.R. must have the same problem with our literature. Eventually it will be to the advantage of our science to adopt a nomenclature that can be used on a world-wide basis to describe these distinctive ferruginous rocks that are very common in the Precambrian sedimentary column and are also known in the early Palaeozoic. The more specific the meaning of the words used, the easier will be international scientific communication. It would be presumptuousof the small group taking part in the symposium at Kiev to attempt to set up such international standards; for this season we asked each author in the symposium to define his terms. It is to be hoped that from this small beginning a coherent and internationally acceptable nomenclature for these rocks will eventually evolve. Until it does, clear definitions of rock terms used in papers for international audiences, if only by reference to standard accessible publications, will prevent obscurities and misunderstandings. The Ad Hoc Committee on Nomenclature was composed of the following geologists participating in the Kiev symposium: R . T. Brandt (Australia); J. Van N.Dorr II (United States of America); G. A. Gross (Canada); H.Grüss (Federal Republic of Germany); and N.P.Semenenko (U.S.S.R.). 1. American Geological Institute, Glossary, 2nd ed., p. 140.

Problems of nomenclaturefor banded ferruginous-chertysedimentary rocks and their metamorphic equivalents

Russian terms

English equivalent

General terms

Ferruginous-chertyforination

Iron-formation(geological term)

(Z~ielezisto-kremnistaya formatsia) Iron-formation= ferruginousformation (Zhelezistaya formaisia)

Iron-formation(geological term)

Iron ore

Iron-formation(economicterm)

(Zheleznaya ruda) Sedimentary facies terms

Iron-cherty-slate-keratophyrel

Iron-formation,Algoma type, associated with keratophyres

(Zhelezisto-kremnisto-slantsevokeratofivo

Jaspilite-leptiteiron-cherty-metabasitel

Iron-formation, Algoma type,associatedwith leptites-metabasites

(Dzhespilito-leptitovayazhelezisto-kremnisfo-metabazitovaya) Iron-cherty-ultrabasitel (Zhelezisto-lcremnisto-ul'trabazifovaya)

Iron-formation,Algoma type,associated with ultrabasites

Iron-cherty-slatel

Iron-formation,Lake Superior type, oxide facies

(Zhelezisto-kremnisto-slantsevaya)

Ferric ferrous (oxide-protoxide)-iron-cherty ( Okisno-zalcisnaya zhelezisto-lcremnistaya) Iron-chertycarbonatel

Iron-formation,Lake Superior type, ferric ferrous facies Iron-formation,Lake Superior type,carbonate facies

(Zhelezisto-kremnisto-karbonatnaya)

Iron-chertysilicate1

Iron-formation,Lake Superior type, silicate facies

(Zhelezisto-kremnisto-silika friaya) Metamorphic facies terms

Slate stagel

Epizonal metamorphic iron-formation(pumpellyite facies)

(Stupen' aspidnykh slantsev (pumpellitovayafatsia))

Phyllite stagel (Filitovaya stupen' (zelenoslantsevayafatsia))

Hornfels stagel

Mesozonal metamorphic iron-formation Greenschist facies Hornfels facies

(Rogovikovaya stupen')

Gneiss stage' (Gneisovayastupen' (amnfibolitovayai granulifovayafatsia))

Katazonal metamorphic Amphibolite facies Granulite facies

Petroguaphic terms

Hornfels2 (Rogoviki)

Iron hornfels (Zhelezisfye rogoviki)

Ferruginous quartzite (Zhelezisty kvartsit)

Fine-grainedmetamorphic quartzite, including iron-formation rocks Iron silicate chert rock coarse bandedz Ferruginous chertz Ferruginous clastic quartzose rock1 Coarse-grained,metamorphosed iron-formation* Ali ferruginous cherty rocks' Itabirit2

Taconite (Takonit)

Itabirite (Ztabirit)

Jaspilite (Dzhespilit)

Iron chert

Silicate ferruginous quartzite2 Silicate iron 'hornfeW2 Silicate 'itabirW2 Non-silicateferruginous quartzite' Non-silicate iron 'hornfels' Ferruginous quartzite' Metamorphosed jaspilitel Ferruginous jasper Non-silicateiron 'hornfels',fine banded2 Chert

(Zhebzisty /cremen')

Jasper

Jasper3

(Yashma) 1. Used or defined by N.P. Semenenko. 2. Used or defined by R. R. Petrov. 3. Used or defined by V. M. Chernov.

379

Problems of nomenclaturefor banded ferruginous-chertysedimentary rocks and their metamorphic equivalents

Bibliography /Bibliographie DORR, J. V.N., II. 1964. Supergene iron ores of Minas Gerais, Brazil. Econ. Geol., vol. 59, p. 1203-40. DORR, J. V. N.; BARBOSA, A. L. M., 1963. Geology and ore deposits of the Itabira district, Minas Gerais, Brazil. Prof. Pap. U.S. geol. Scirv.341-c, 110 p. GROSS, G . A. 1966. Principal types of iron-formation and derived ores.BuII. Canad. Inst.Min. vol.59,no.648,p. 150-3. JAMES, H.L. 1954. Sedimentary facies of iron-formation.Econ. Geol., vol. 49, p. 235-93. . 1955.Zones ofregionalmetamorphism in the Precambrian ofnorthernMichigan.Bull.geol.Soc.Amer.,vol.66,p. 14-56-87, KRUMBEIN, W. C.;CARRELS, R. M.1952. Origin and classification of chemical sediments in terms of p H and oxidationreduction potentials. J. Geol.,vol. 60, p. 1-33.

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KRAUSKOPF, K. B. 1967. Introduction to geochemistry. New York, McGraw-Hill.721 p. SEMENENKO, N.P. et al.,1946-1953. Struktura rudnykh poky Krivorozhskikh zhelezorudnykhmestoruzhdeniy [Structureof ore fields of Krivoy-rogiron ore deposits]. BuIl. Acad. Sei. U.R.S.S., Kiev, vol. 1, 1946; vol. II, 1953.

-. 1956. Petrography of iron-cherty formations of Ukrainian S.S.X.Kiev, Ukrainian Academy of Sciences. (In Russian.) -.1959.Geologiyazhelezisto-kremnistykhformatsiyUkrainy [Geology of iron-cherty formations of Ukrainian S.S.R.]. BuII. Acad. Sci. U.R.S.S., Kiev.

-. 1967. Geology of sedimentary-volcanogenic formations of Ukrainian Shield. Kiev, Naukova Dumka. (In Russian.)

List of participants/Liste des participants

BELEVTSEV, D r Y.N., Institut of Geochemistry and Physics o Metals,Academy of Sciences of the Ukrainian S.S.R.,Kiev (Ukrainian S.S.R.). BELYAEV, D r M.V.,Geological Service, Ministry of Ferrous Metais. Industry of the Ukrainian S.S.R.,Dniepropetrousk (Ukrainian S.S.R.). BEYGULENKO, D r I. P.,Geological Service, Ministry of Ferrous Metals Industry of the U.S.S.R.,Moscow (U.S.S.R.). BORISENKO, D r S. T., Geological Prospecting Service, Ministry of Geology of the Ukrainian S.S.R.,Kiev (Ukrainian S.S.R.). BRANDT, D r R. T., Goldsworthy Mining Limited,P.O.Box 84, Port Hedland,Western Australia 6721 (Australia). CAMBEL, D r B.,SlovakGeologicalInstitute,Obrancov mieru 41, Bratislava (Czechoslovakia). CHERNOV, D r V. M., Institut of Geochemistry, Karelian Branch of the Academy of Sciences of the U.S.S.R.,Petrozavodsk (US3.R.). CHOUBERT, D r Boris, Résidence Bernard-Palissy,77 AvonFontainebleau (France). CHOUBERT, D r Georges, Directeur de Recherches, Bureau de Cartographie Géologique Internationale, Muséum National d‘Histoire Naturelle, 36 Rue Geoffroy Saint-Hilaire, 75005 Paris (France). DORR, Dr JohnvanN.,II,U.S.GeologicalSurvey,Washington, D.C.20242 (United States of America). DZHEZDALOV, D r A. T., Trust ‘Lenruda’,Ministry of Ferrous Metals Industry of the Ukrainian S.S.R.,Krivoyrog (Ukrainian S.S.R.). EGOROV, D r E. V., Far East Geological Service, Ministry of Geology of the Russian Soviet Federated Socialist Republic, Khabarovsk (R.S.F.S.R.). FAURE-MURET, Miss A.,Muséum d‘HistoireNaturelle, 36 Rue Geoffroy Saint-Hilaire,75005 Paris (France). FRIETSCH, D r Rudyard, Geological Survey of Sweden, 10405 Stockholm 50 (Sweden). CAIR, D r Jacob E., U.S. Geological Survey, Washington, D.C.20242 (United States of America). GAVELYA, D r A. P.,Trust ‘Krivbassgeologiya’,Ministry of Geology of the Ukrainian S.S.R., Krivoyrog (Ukrainian S .S.R.). GOODWIN, Professor A. M., Department of Geology,University of Toronto,Toronto 5 (Canada). GORYAINOV, D r M. V., The Kola Branch of the Academy

of Science of the U.S.S.R.,Apatity, Murmansk Region (U3.S.R.). GROSS, D r G.A.,Head Geology of Mineral Deposits Section, Geological Survey of Canada, 601 Booth Street, Ottawa 4, Ontario (Canada). GROSSI SAD,Geol. J.H.,Dept. Engenharia de Minas,Universidade de Minas Gerais,Escola de Engenharia,Rua Espirito Santo,35-7”,Belo Horizonte,M.G.(Brazil). GRUSS, D r Hans, Gewerkschaft Exploration, Steinstrasse 20, Postfach 3526, Düsseldorf (Federal Republic of Germany). INGERSON,Professor Earl,Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 (United States of America). KALUGIN, D r A . S., Siberia Research Institute of Geology, Geophysics and Mineral Resources, Ministry of Geology Novosibirsk (U3.S.R.). of the US.S.R., KOBZAR, D r V. N.,Institute of Geochemistry and Physics of Minerals, Academy of Sciences of the Ukrainian S.S.R., Kiev (Ukrainian S.S.R.). KRAVCHENKO, D r V. M., Yakut Thematic Expedition,Ministry of Geology of the Russian Soviet Federated Socialist Republic, Yakutsk (R.S.F.S.R.). KRISHNAN, D r M.S., Hyderabad (India). MACLEOD, D r W. N.,6 Airlie Street, Peppermint Grove, Western Australia GOO5 (Australia). MALYLITIN, D r E.I.,Ministry of Ferrous Metals Industry of the U.S.S.R., Moscow (U.S.S.R.). MITKEEV, D r M. B., Trust ‘Dnieprogeologiya’,Ministry of Geology of the Ukrainian S.S.R.,Dniepropetrovsk (Ukrainian S.S.R.). MOMDZHI, D r Y.S.,All-Union Research Institute of Mineral Resources, Ministry of Geology of the U.S.S.R.,Moscow (U.S.S.R.). MOREY, D r G . B.,Minnesota Geological Survey, University of Minnesota, Minneapolis,Minnesota 55455 (United States of America). NIKIFOROV, D r M . S., Trust ‘Dzerzhinskruda’,Ministry of Ferrous Metal Industry of the Ukrainian S.S.R.,Krivoyrog (Ukrainian S.S.R.). NOVOKHATSKY, D r I. P.,Institute of Geological Sciences, Academy of Sciences of the Kazakh S.S.R.,Alma-Ata (Kazakh S.S.R.). PERCIVAL, D r F. G., Sadlers End, Haslemere,Surrey (United Kingdom). 381

List of participants

PERRY, D r Eugene C., Jr, Department of Geology,University of Minnesota, Minneapolis,Minnesota 55455 (United States of America). PLAKSENKO, Dr N.A.,Voronezh State University,Voronezh (U.S.S.R.). POLUNOVSKIY, Dr R.M., Azov Expedition,Ministry of Geology of the Ukrainian S.S.R.,Volnovakha (Ukrainian S.S.R.). RIBEIRO FILHO, Professor Evaristo, Instituto de Geociencias e Astronomia, Universidade de Sao Paulo, Cidade Universitaria,São Paulo (Brazil). ROY,D r Supriya, Department of Geological Sciences, Jadavpur University, Calcutta-32 (India). SCARPELLI, D r Wilson, c/o ICOMI, Caixa Postal 396, Belem do Para (Brazil). SEMENENKO, Academy ProfessorN.P.,Instituteof Geochemistry and Physics of Minerals, Academy of Sciences of the Ukrainian S.S.R.,Kiev (Ukrainian S.S.R.). SHKOLNIK, D r E. P., Far East Geological Service, Ministry of Geology ofthe Russian SovietFederated Socialist Republic, Khabarovsk (US.S.R.). SHKUTA, D r E. I., Geological Service, Ministry of Ferrous Metals Industry of the Ukrainian S.s.R.,Dniepropetrovsk (Ukrainian S.S.R.). SHTSHERBAK, D r V. M.,Institute of Geological Sciences, Academy of Sciences of the Kazakh S.S.R., Alma-Ata (Kazakh S.S.R.).

382

SHTSHERBAKOV,D r B.D.,Ministry of Geology of the U.S.S.R., Moscow (U.S.S.R.). SIMS, Dr Samuel J., Bethlehem Steel Corporation,Bethlehem, Pennsylvania 18016 (United States of America). SIROSHTAN, D r R. I.,Institute of Geochemistry and Physics of Minerals, Academy of Sciences of the Ukrainian S.S.R., Kiev (Ukrainian S.S.R.). STRUEV, Dr M . I.,Ministry of Geology of the Ukrainian S.S.R., Kiev (Ukrainian S.S.R.). TOKHTUEV, D r G.V., Institute of Geochemistry and Physics of Minerals,Academy of Sciences of the Ukrainian S.S.R., Kiev (Ukrainian S.S.R.). TOLBERT, Dr G.E.,Cia Vale do Rio Doce Div. de Desenvolvimento Av.Graça Aranha,26-8 andarRio de Janeiro(Brazi1). TRENDALL, Dr A. F.,Geological Survey of Western Australia, 26 Francis Street, Perth, Western Australia (Australia). TUGARINOV, Professor A. I.,Institute of Geochemistry and Analytical Chemistry,Academy of Sciences of the U.S.S.R., Moscow (U.S.S.R.). VERIGIN, Dr M.I.,Trust ‘Skrivbassgeologiya’,Ministry of Geology of the Ukrainian S.S.R.,Krivoyrog (UkrainianS.S.R.). WEBER, D r F., Laboratoire de géologie et de paléontologie, Université de Strasbourg,67 Strasbourg (France). ZAITSEV, Dr Y. S., Voronezh Geological Prospecting Expedition, Ministry of Geology of the Russian Soviet Federated Socialist Republic,Voronezh (U.S.S.R.).