Gold Deposits Related to Greenstone Belts in Brazil Deposit Modeling Workshop

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GOLD DEPOSITS RELATED TO

GREENSTONE BELTSIN BRAZI LDEPOSIT M OD ELINGWORKSHOP

Part A ExcursionsU.S. Geological Survey Bulletin1980-A

A workshop sponsored by the International Union of Geological Sciences and the UnitedNations Educational, Scientific, and Cultural Organization Deposit Modeling Program.The workshop held December 1-10,1986, in Belo Horizonte, Brazil, was hosted by theInstitute Brasileiro de Mineragao


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GOLD DEPOSITS RELATEDTOGREENSTONEBELTS IN BRAZIL-

DEPOSITMODELINGWORKSHOPPart A-Excursions

Edited by CHARLESH. THORMAN,

EDUARDOA. LADEIRA,and DIANEC. SCHNABEL

Field excursions of thesurface geology and toseveral gold mines in theQuadrilateroFerrffero,Minas Gerais,Brazil

Workshop:Belo Horizonte, Brazil, December1-10,1986

Sponsors:International Unionof GeologicalSciences (IUGS)United Nations Educational,Scientific, and Cultural Organization-

Deposit ModelingProgram (UNESCO)Host:

Institute Brasileiro deMineragao (IBRAM)

U.S. GEOLOGICALSURVEY BULLETIN 1980-A


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U.S. DEPARTMENTOF THE INTERIORMANUELLUJAN,JR., Secretary

U.S. GEOLOGICALSURVEY

Dallas L. Peck, Director

Any use of trade, product, or firm names in this publicationis for descriptivepurposes only and does not implyendorsement bythe U.S. Government.

UNITEDSTATES GOVERNMENTPRINTINGOFFICE: 1991

For sale by theBooks and Open-File Reports SectionU.S. GeologicalSurveyFederal CenterBox 25425Denver, CO 80225

Library of Congress Cataloging-in-Publicatlon Data

Gold deposits related to greenstone belts in Brazil Deposit modelingworkshop; Part A Excursions / edited byCharles H. Thorman, Eduardo A. Ladeira, andDiane C. Schnabel.

(U.S. GeologicalSurvey bulletin; 1980-A)91-3522

CIP


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U.S. GEOLOGICALSU RVEY BU LLETIN1980-A

GOLD DEPOSITS RELATED TO GREENSTONE BELTS IN BRAZIL-DEPOSIT MODELING WORKSHOP

Part A Excursions

CONTENTS

Introduction to a workshopon gold deposits related to greenstone belts in Brazil, BeloHorizonte,Brazil, 1986, by Charles H. Thorman and Eduardo A. Ladeira Al

Geological field excursionfrom Belo Horizonte to Ouro Preto,Minas Gerais,Brazil, byEduardo A. Ladeira A23

Field excursion from Belo Horizonte to Ouro Preto Side excursion along the road to Moeda,Minas Gerais, Brazil, by Eduardo A. Ladeira and Ronald Fleischer A39

Surface and underground geological excursionaround and in the Passagem de Mariana gold mine,Minas Gerais, Brazil, by Ronald F leischerand Diogenes Scipioni Vial A49

Excursion to the Morro Velho gold mine, Minas Gerais,Brazil, by Frederico Wallace Reis V ieira,Luiz Henrique do Amaral Lisboa, Jose LamasChaves, G eraldo A ntonioIbrahim de Oliveira,PaschoalLuiz Caiafa Clemente, andRafael Limade O liveira A63

Excursion to the Cuiaba gold mine, Minas Gerais, Brazil, by Frederico Wallace Reis V ieira,MarioCorbaniFilho, Jose Teotonio de Faria Fonseca, AguinaldoPereira, Geraldo Antonio IbrahimdeOliveira, and PaschoalLuiz Caiafa Clemente A7 5

Katherine Dorr AbreuIBRAM coordinator


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Introductionto a Workshop on

Gold Deposits Related toGreenstone Belts in Brazil-Belo Horizonte,Brazil, 1986

By CHARLESH. THORMANand

EDUARDOA.

LADEIRA

U.S. GEOLOGICAL SURVEYBULLETIN1980-A

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Structural complexities, Passagem de Marianagold mine

Mafic and ultramafic rocksat C6rrego dos Boiadeiros

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CONTENTS

Introduction ASRegionalgeologic setting A8Summary of the Precambrian history of the Quadrilatero Ferrifero A8Structural modifications A10Stratigraphicmodifications A14Summary of the concepts on the origin of the gold deposits in the Quadrilatero

Ferrifero A14References cited A19Appendix: Explanation of the deposit-modelform used to organize data A2 1

FIGURES

1. Geologicmap of the Quadrilatero Ferrifero (modified fromDorr, 1969) showingthe field trip routes for the workshop A6

2. Geologicmap of the Quadrilatero Ferrifero (modified fromDorr, 1969) showingthe location of major gold mines and occurrences in the area A7

3. Generalized Stratigraphic columnof Precambrian rocks of the QuadrilateroFerrifero (modifiedfrom Dorr, 1969) A9

4. Chart showing correlation of Precambrian deformationalevents in rocks in theQuadrilatero Ferrifero by various workers All

5. Simplified geologicmap of the Quadrilatero Ferrifero (modified fromDorr, 1969)showing interpretation of the Rio das Velhas and Minas Supergroups asallochthonouson the basement complex A13

6. Diagram showing gold deposits relativeto syngeneticversus epigeneticorigin A19

TABLES

1 . Criteria for gold deposits examinedor analyzed during the workshop and shownon figure 6 A16

2. Postulated criteria for overall setting of iron-formation-related gold deposits ingreenstone belts examined or analyzed during the workshop andshown onfigure 6 A17

3. Model for the origin of the deposits visited or discussed during the workshop A18

Contents A 3


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FIELD-TRIPLEADERS(fromleftto right):Skip Cunningham,John Kerswill,Chuck Thorman, Eduardo Ladeira, Stan Caddey, and Carl Anhaeusser

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Introduction toa Workshop on Gold DepositsRelated to Greenstone Belts in Brazil-Belo Horizonte, Brazil, 1986

By Charles H. Thorman1 and Eduardo A. Ladeira2

INTRODUCTION

This volume comprisestwo parts (Bulletins 1980-Aand 1980-B) and presents the results and scientificcontributionsof a workshop and field trips on "Golddeposits related to greenstone belts in Brazil," heldDecember 1-10,1986, in Belo Horizonte, Brazil (fig. 1).The workshopwas sponsored by the International Unionof GeologicalSciences IUGS) and United NationsEducational,Scientific and Cultural O rganization(UNESCO) Deposit Modeling Program,a programdesigned to advance geoscientificknowledgeandexpertise inmineral-deposit modeling for use inexploration,assessment, and developmentof resources.The purpose of the Deposit Modeling Programworkshops is to facilitate the transfer of knowledge andexpertise in these subject areas and to assist in trainingand educatinggeoscientistsso that they can moreeffectively explore and assess resources in their homecountries. BeloHorizonte was chosen as the workshopsite because of its location in the Quadrilatero Ferrifero("Iron Quadrangle"),famous forits world-class golddeposits (Morro Velho, Raposos, Cuiaba, Sao Bento)and iron deposits (Itabira, A guasClaras, Itabirito,Alegria).The goals of the workshop wereattainedthrough a series of lectures, discussion groups, an dseveral field trips and mine visits, beginningwith thegeneral geologic setting of the Quadrilatero Ferriferoand ending with the constructionof deposit models forthe gold mines visited. This m ixtureof formatsintroduced the participants togold occurrences in a

Manuscriptapproved for pub licationFebruary 22,1991.1U.S. GeologicalSurvey, Box 25046 M S 905, Denver, Colorado

80225, USA.Departamento de Geologia, Institute de Geociencias,

UniversidadeFederal de Minas Gerais, BeloHorizonte, Brazil.

major gold provinceand resulted in the development ofdeposit models forgreenstone belts that can be appliedin South American and other Precambrian shield areasin the exploration for goldand related commodities(silver, lead, zinc, copper). Figure 1 shows the field triproutes for the workshopand figure 2 shows the locationsof the major gold mines and occurrences in theQuadrilatero Ferrifero.

The Institute Brasileiro de Minerac.ao IBRAM) inBelo Horizonte was the host agency for the workshop.The organizing committee for the workshop consisted ofthe followingpeople:

Katherine Dorr Abreu, IBRAM.Aristides Eustquio Machado, IBRAM.Paulo Bahia Guimaraes,Minerac.ao Marex Ltda.Charles G. Cunningham, U.S. Geological Survey.Charles H. Thorman, U.S. GeologicalSurvey.Eduardo A. Ladeira, Departamento de Geologia,

Institute de Geociencias, UniversidadeFederal deMinas Gerais.

Ronald Fleischer, Andrade Gutierrez Minerac.aoLtda.

W orkshop leadersand coordinators were as follows:Charles H.Thorman, workshop coordinator and

leader.Charles G. Cunningham,IUGS/UNESCO

coordinator and leader.Katherine Dorr Abreu, coordinator for all logistical

functions and support for the workshop.Eduardo A. Ladeira, field trip coordinator, editor of

field trip guidebook,and leader.Carl R. Anhaeusser, Universityof Witwatersrand,

leader.Stanton W. Caddey, Homestake Mining Company,

leader.John A. Kerswill,Geological Survey of Canada,

leader.

Introduction to a Workshop on Gold Deposits Related to Greenstone Belts In Brazil AS


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The two parts ofthis bulletinare divided as follows:Part A-Excursions (1980-A) includes (1) a field tripfrom Belo Horizonte to OuroPreto (fig. 1); (2) asupplementaryside field trip; and (3) field-trip logs ofvisits to the Morro Velho, Passagem de Mariana,andCuiabgold mines. Part B Geology of deposits(1980-B) includes (1) the geologyof the QuadrilateroFerrifero; (2) the geologyof the Morro Velho, Passagemde Mariana, Sao Bento, andCuiab goldmines; (3)theHomestakeand Lupin gold mines in NorthAmerica;and (4) deposit models for the Brazilian and NorthAmericandeposits.

This introductory chaptercovers the followingtopics that relate to the QuadrilateroFerrifero: theregionalgeologic setting of the QuadrilateroFerrifero; asummaryof the Precambrianhistoryof the QuadrilateroFerrifero; structural a nd stratigraphic interpretationsofthe QuadrilateroFerrifero; and a summaryof theconceptson the origin of the gold deposits in theQuadrilatero Ferrifero.Some of the alternative regionalstratigraphicand structural interpretations presentedherein have been discussed for many yearsand aregenerallyaccepted bygeologistsworkingin theQuadrilateroFerrifero,but these interpretationsmaynot be widelyrecognized outsideof Brazil. Otherstratigraphic and structural interpretations are thesubject of much debate and are farfrom settled. Abyproductof the workshopwas the release of previouslyunpublisheddata by miningcompanieson the geology ofseveral of the ore depositsvisited in Brazil and on NorthAmerican deposits.

This publication differsfrom most in presentingamixture of new data in a variety of formats. Thepaperson the gold mines visited contain new data concerningboth the basic geology of the mines as well as productionand reserve information. Also, some of these papersdescribe trips through themines, as made by theparticipantsin the workshop. We have included thesedescriptionsbecause theyprovide important insights intomine geology.

REGIONAL GEOLOGIC SETTING

The Quadrilatero Ferriferohas been an area ofmineral exploration and geologic investigation forcenturies, datingback to the search for gold andprecious stones that began in the 16th century. Thediscovery in 1699-1701of black gold-goldcontaminatedwith iron and platinum-groupmetals inthe southeastern corner of the QuadrilateroFerriferogave rise to the name Serra de Ouro Preto (Black GoldRange) andthe name of thetown Ou ro Preto. TheQuadrilatero Ferriferocontainsworld-class goldand

iron ore deposits, manganese ore,bauxite, andprecious-stonelocales. Much data has been generatedabout themineral wealth and geology of the area. Thebest knownand most easily available work,which formsthe basic geologic frameworkof the QuadrilateroFerrifero, resultedfrom a cooperative study betw eentheDepartamentoNationalda Producao Mineral (DNPM)of the Ministe'rio das Minas e Energia (MM E)and theUnited States GeologicalSurvey(USGS) from1946-1962. This studywas primarilya geologicmappingprogram designed to develop a solid geologicdata base that would help in the future utilizationofBrazil's natural resources. Many reports andmaps onthe general geologyand economicgeologyof theQuadrilateroFerrtfero have been published in Braziland the UnitedStates. The summary report by Dorr(1969) includes a geologic map of the QuadrilateroFerrifero at a scale of 1:150,000,based on the detailedand reconnaissance mappingof forty-seven7}4-minutetopographicquadranglesat 1:25,000by DNPM andUSGS geologists.

The followingoutline of the geologic historyof theQuadrilateroFerriferowas established bytheDNPM/USGSprogram. Thedistributionof units, theirrelative positions in time, and the sequence of tectonicevents is little changed. Dorr's 1969 summarypaper ofthe geology of the QuadrilateroFerriferohas truly stoodthe test of time. Thisis an impressive statementfor thework of such a large group of geologists ina highlycomplex and, at the time, not easily accessible region.Work by manygeologists in the past two and one-halfdecades has indicatedthe need for only a few revisionsof Dorr's geologic outline. One revision, however, hasmajor implications on thestructuralevolution of theQuadrilatero Ferrifero andis presented in the followingsection. Figure 3 shows a generalizedstratigraphicsection of the Precambrianrocks in the QuadrilateroFerrifero modified fromDorr (1969).

SUMMARY OF PRECAMBRIAN HISTORYOF THE QUADRILATERO FERRIFERO

This summary is based on Dorr (1969); modificationsto it are discussed in the section after this one.

1. The oldest rockscomprise the Rio das VelhasSupergroup, a m etasedimentary andmetavolcanicsuccession dividedinto the Nova Lima and Maquin6Groups (fig. 3). Dorr (1969) considered theNova LimaGroup to be aeugeosynclinalflysch unit and theoverlyingMaquine" Group to be eugeosynclinal molasse.The Rio das Velhas Supergroup is now recognized asagreenstonebelt. The Nova Lima Grouphosts mostofthe large gold deposits in the QuadrilateroFerrifero.

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PROFOUND OROGENY W ITH REGIONAL METAM ORPfflSMItacolomiGroup

PROFOUND ANGULAR UNCO NFORMITY

Minas Supergroup

Piracicaba Group

Itabira Group

Caraca Group

Sabara FormationBarreiro FormationTabooes FormationFecho doFunil FormationCercadinhoFormation

GandarelaFormationCaue Itabirite

\/Batatal FormationMoeda Formation

Tamandua Group

MAJOR OROGENY WITH REGIONAL METAM ORPfflSM

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Maquin6 Group

Nova lima Group

Casa Forte FormationPalmital Formation

\/Upper unit*Middle unit* (includes Lapa Seca Form ation)Lower unit*

CONTACT CONSIDEREDINTRUSIVE BY DO RR(1969)

Granitic-gneissic basementrocks, includinggranitoids of various ages

Figure 3. Generalized stratigraphic columnof Precambrian rocksof the QuadrilateroFerrffero(modified fromDorr, 1969).Asterisk (*) indicates units not included in Dorr's paper but discussed in the text of the present paper.

2. The Rio das Velhas Supergroup rocks weredeformed andmetamorphosed during a major erogenicevent at about 2700 Ma (Herz, 1970), before depositionof the overlying Minas Supergroup. This orogenic eventmay have been accompaniedby igneous activity.

3. The Minas Supergroupincludes four groups. (1)The basal Tamandua Group contains a lower paralicprismatic clastic unit (Cambotas Formation) and anupper dolomiticphyllite, dolomitic oxide-faciesiron-formation, anda quartzose phyllite unnamed unit.A local unconformityseparates the overlying CaragaGroup from the Tamandua Group. (2) The CaracaGroup has a lower conglomerate, sandstone, andminorshale unit (Moeda Formation) and an upper phyllite,graphitic phyllite, metachert, and oxide-faciesiron-formation unit (Batatal Formation). These rocks

are conformably overlainby (3) the Itabira Group, whichcomprises the lower Caue Itabirite and upper GandarelaFormation. The Caue Itabirite is made up ofoxide-faciesiron-formation itabirite), dolomiticitabirite, amphibolitic itabirite, andminor phyllite anddolomite. The Gandarela Formation consists primarily

of dolomiticmarble, but includes dolomiticphyllite,dolomitic iron-formation,and phyllite. The world-classiron deposits in the Quadrilatero Ferrifero occur in theCaue Itabirite, and manganese is produced from boththe Caue and the Gandarela formations.

(4) The Piracicaba Group unconformablyoverliesthe Itabira Group. A local basal conglomerate(basalCercadinhoFormation) occurs at the base of the groupalong the unconformitywith minor relief. A concordant,continuous sequence ofdeposition is interpreted from

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the CercadinhoFormationthrough theFecho do FunilFormation,Tabooes Quartzite, andBarreiro Formation.These units are primarilyquartzitesto quartziticphyllites withminor dolomiticlenses. The Cercadinhoincludes ferruginous strata. The Barreiro,mainlyfine-grainedrocks, consists primarilyof phyllite andgraphitic phyllite. The uppermostunit, the SabarForm ation, restson all the older units in the Piraticaba

Group and locally has a basal conglomerate;beds aboveand below the contact are concordantstructurally. TheSabar consists of schist and phyllite, probablyderivedfrom tuffaceous andgraywackeprotoliths, andlocaltilloid-likeconglomerate. A stable shelf orblanketenvironment was interpretedby Dorr (1969) for thesiliciclasticCercadinho through Barreirosedimentarycycle and a eugeosynclinalflysch environment for theSabara.

4. An angular unconformityoccurs at the top of theMinas Supergroup;it was interpreted to indicatebroaduplift and erosion and tobe epeirogenic.

5. The Itacolomi Supergroup,a quartzitic toconglomeraticunit with varied amountsof sericite, wasconsideredby Dorr (1969) to be paralic (molasse?)inorigin.

6. A post-Itacolomiorogeny probablyformed thebasic geologic framework ofthe Quadrilatero Ferrifero.The age of this event, accordingto Herz (1970), wasabout 1000-1350Ma. This event caused all the complexlarge-scale folds and other major structures of theQuadrilatero Ferrifero,including the emplacement ofthe several doma l graniticstructures (Baclo Complexand others), considered intrusive igneousgranitic rocksby Dorr (1969).

7. The area was probably covered by carbonate andpelitic rocks of the Late ProterozoicBambui Group, andthe entire region was folded and faultedby the BrasilianCycle (about 600 Ma). Events post-dating theseactivities are not consideredhere.

Modificationsto the basic geologic framework of theQuadrilatero Ferrifero justoutlined that have beendevelopedsince the DNPM/USGSprogram and aregenerally acceptedare presented next. Thesemodifications includeboth stratigraphic and structuralcontributions.

STRUCTURAL MODIFICATIONS

The most fundamental and far-reachingmodificationto the geologic frameworkof the Quadrilatero Ferriferooutlined hi the summary ofDorr's work (1969) is theconcept that the granitic-gneissicbasement complex is asold or older than the Rio das Velhas Supergroup,whichoverlies it, and that the basementcomplex is cratonic.

This modification considersthe Archean Riodas VelhasSupergroup as part of a greenstone beltdepositedonoceanic crust somewhere east of the QuadrilateroFerrifero and thrust westward ontothe craton. In thismodel, the contact between the Riodas VelhasSupergroup andthe basementwas a major thrust. TheMinas Supergroupis interpretedas a Proterozoiccontinental margin assemblagethat was also thrust over

the basementcomplex and the Rio das VelhasSupergroup. Somemembersof the DNPM/USGSteamconsideredseveral major thruststo be present (Guild,1957;Barbosa, 1969; Maxwell, 1972) although theconcept of the Rio das Velhas and Minas Supergroupsas allochthons thrust over the granitic basementwas notincluded hi Dorr's geologic framework. These thrustsincluded one atthe contact of the Rio das Velhas-MinasSupergroup with the basement and anotherbetween theRio das Velhas Supergroup and the Minas Supergroupat many localities.

In addition toDorr's (1969) summaryof the region,Herz (1970), Drake and Morgan (1979), Ladeira andViveiros (1986; this volume,Part B Geology ofdeposits ) and Morritt (1988) presented summaries thathave many commonaspects as well as some significantdifferences. Herz, who was part of the USGS teaminvolved in the DNPM/USGSQuadrilatero Ferriferomapping project,based his outline of the QuadrilateroFerrifero geologichistory largely on Dorr's work, butHerz's major contributionwas to radiometricallydatemany of the rock units, thus giving substanceto the timefactor. Drake and Morgan (1979) added a newinterpretation basedon a plate tectonics model, hi whichthey considered ultramaficrocks at and near the base ofthe Archean Riodas Velhas Supergroup torepresentophiolite and the Rio das Velhas to be an allochthonousgreenstone belt thrustwestward ontoa cratonicbasementcomplex coeval withor older than the Rio dasVelhas. Ladeira and Viveiros (1986; this volume,PartB Geology of deposits) identifiedsix stages ofdeformation,but did not correlate them with anyregional tectonicor thermalevents. In contrast, M orritt(1988), in a more regional study, correlated theindividualdeformationalevents identifiedby theprevious workers and placedthem hi an order withregionallyestablishedevents. Figures 4 and 5 showthese summaries and the correlationsof Morritt.Although ideas and interpretationsdiffer among theworkers, the basic conceptsof Dorr (1969) remainunchanged except for the interpretation thatthe supra-crustal Rio das Velhas greenstone suite is allochthonous.

It is difficult to know who to credit with the conceptof the Rio das Velhas being allochthonous. The ideawas consideredby some of the DNPM/USGSgeologistsin the 1950's and 1960's (informal discussions byC.H.

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Thormanwith several mem bers of theDNPM/USGSteam); however, this interpretationis difficult to gleanfrom their reports. Grossi Sadand Ladeira(1968),Ladeira (1976,1980),and Schobbenhaus andothers(1981) all indicated a thrust relationshipbetween theArcheanbasement complex and the Rio das Velhas andMinas Supergroupsover much of the area. Thefollowingsequence ofevents indicates how one of us(Thorman) developed ideasregarding theallochthonousnature of the Rio das Velhas and Minas Supergroupsinthe Quadrilatero Ferrifero.

1 . In February, 1973, Thorman was teaching aphotogeologyreconnaissance mappingcourse aspart ofan MME/USAID (UnitedStates AgencyforInternationalDevelopment) program forMMEgeologists. One part of the course entailed constructinga geologic map ofpart of the Quadrilatero Ferrifero andthen field-checkingthe work. The first area checkedwasin the Serra da Moeda, on the west side of theQuadrilateroFerrifero, from the Minas Supergroupdown into the graniticbasement complex. The firsttraversewas on the dirt road from HighwayBR-040 toMoeda (see Ladeira andFleischer, this volume, PartA Excursions). Thorman and severalgeologists takingthe course recognizedthat the contact of the Rio dasVelhas-Minas Supergroupswith the basementshown onthe geologic map of Wallace (1965) was not intrusivebuta major fault dipping gently to the east beneath the Riodas Velhas and MinasSupergroups. The fault is markedby many myloniticzones withinthe basement (for morethan 100 m (meters) below the contact) that increase

upward to the contact and becomemore closely spacedand more intenselydeformed toward the contact.Tectonic brecciaoccurs locally along the contact.Indicationsof the deformationdisappear a shortdistance abovethe contact. A similar relation was notedfarther north,where the contact is exposed in severallocales. Thormanand the group looked at exposures ofthis contact inthe southern and western parts of theQuadrilateroFerrifero. At more thana dozen places,the contact was obviouslya low-angle fault. In otherplaces, however,the evidence was not as clear. Forexample, around themarg in of theBac.ao Complex, the

deformedrocks have been annealed by subsequentmetamorphism. However, both the map pattern, whichsuggests stronglythinned and pinched unitsagainst ornear the contact, and the strongly sheared rocks at thecontact indicate that the contact is a shear zone thatresulted from thrust faulting andwas later locallymetamorphosed.

The overall map pattern of units at the base ofthemetasedimentary sequence throughout theQuadrilateroFerrifero indicates either highly irregular stratigraphicrelationships and (or) major structuralcomplications

commonly observedat the base of allochthons. FromMarch, 1973 until July, 1974, Thorman examined severalother contacts of the basementwith the overlyingRiodas Velhas and MinasSupergroupsin the QuadrilateroFerrifero and became convinced that this contact istectonic and locallymodifiedby younger metamorphismor intrusion.

2. Later in the 1973 photogeology course,OscarBraun (thenwith Companhiade Pesquisa de RecursosMinerals (CPRM)) took the groupto visit severalcontactsof the Rio das Velhas Supergroupwith thebasement to show why he also thought thecontact wastectonic. He also pointed out that otherBraziliangeologistswere of the sameopinion.

3. Shortlybefore Thorman returnedto the UnitedStates in 1974, he took Avery Drake of the USGS on atour of the QuadrilateroFerrifero during the planningstages for a field geologycourse for MME geologists tobe run out of Belo Horizonte. During the tour,Thormanshowed Drake the exposures onthe west sideof the Quadrilatero Ferrifero,as well as others, aspotentialsites for detailedgeologicmapping fortrainingin structure andstratigraphy. Thorm an pointedout hisinterpretationof the allochthonous setting ofthe Rio dasVelhas and Minas Supergroups. Subsequently, Drakeand Morgan (1979) developed the concept that maficand ultramaficrocks at and near the base of the Rio dasVelhas Supergroupwere part of an ophiolite sequence,out ofplace on cratonic basem ent,that had been thrustwestward from an oceanic setting.

4. Charles Maxwell and SamuelMoore (part of the

USGS team that mapped theQuadrilateroFerrifero),pointed out to C.H.Thorman in the middle 1970's thatthe concept of the Rio das Velhas and M inasSupergroups being allochthonouswas not new and thatit had been discussed atgreat length earlierby manymembersof the team in the early 1960's. To present theidea in the context of the regional tectonicsasunderstood in the 1960's, however, was difficult; iftheconcept of plate tectonics had existed at that time, theoverall geologic framework establishedby theDNPM/USGS team would havebeen quite different.The idea that the Rio das Velhas-Minas/basement

contact might not be intrusive was alluded to in indirectterms by several individualworkers (including Johnson,1962; Pomerene, 1964; Wallace, 1965; Simmons,1968a,b;Moore, 1969), but it was not included in Dorr's(1969) summary of the geologyof the QuadrilateroFerrifero. Dorr believed all the basementrocksintruded,and thus were younger than,the metamorphicrocks.

5. It is now evidentthat many geologistswho haveworked or are working in the Quadrilatero Ferriferobelieve that the allochthon conceptis the only logical



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readily formed into two camps those with a restricteddefinition and those with a looser definition.Fortunately, considerable commonground existedbetween the two camps and a concensuswas not toodifficult to arrive at. The followingcriteria madeup thecommon ground. An elongate erogenic belt of supracrustalrocks

20-200 km wideand 100-800km long. Stratigraphic sequencecomm only consistsof the

following rock types from base to top:1 . A lower suite of m etavolcanic rockswith

ultramaficrocks atbase and calc-alkaline mafictofelsic rocks at top that contains somechemicalsedimentaryrocks.

2. An upper suite of clastic argillaceous deep-waterrocks that grade upward into shallow-water clasticrocks and chemicalprecipitates (carbonates, ironformation).

Supracrustalbelt rocks have been moderatelytointensely deformed; typicallymore than one regionalmetamorphicevent; complexstructuralsetting.

Metam orphic graderanges from greenschist togranulite facies; typicallygreenschist over large areas.

Belt is boundedby and intrudedby granitic plutonsthat form large massifs.

The supracrustalrocks comm only occur inmajorsynclinoriaseparated by graniticmassifs.

Marked increase in metamorphic gradeadjacent toplutons is common.

World-class m etallicore deposits related to plutons,volcanic rocks,and iron formation.

Rocks containearliest forms of life.The camp favoring arestricted definition ofa

greenstone beltrequired the presence of considerablymore ultramaficto mafic volcanicmaterial thandid theother camp. The am ount ofthis volcanicmaterial isextremelyimportant, becausethe restrictedview wasapplied to the Rio das Velhas greenstonebelt ourworkshop area. Ultramaficto mafic volcanic rocks inthe Rio das Velhas Supergroup in the Belo Horizontearea were consideredto be minimal by severalparticipants. And, therefore, they questioned referringto the Rio das Velhas sequenceas a greenstonebelt.

An upper suite of clastic and chemical rocks havelong been recognized in the area, but not until recentlyhas anyone emphasized the ultramaficto mafic volcanicrocks (Drake and Morgan, 1979). During our trip to theSao Bento mine we saw pillow structuresin mafic flowrocks in drill cores from the Rio das Velhas; thesestructuresconvinced participantsthat mafic volcanicrocks were more plentiful than previouslyrealized.More recentwork byLadeira and other workershavedocumented the presence of kom atiiteswith spinifextexture in the Quadrilatero Ferrifero(Ladeira and

others, 1985;Noca and others, 1990).General consensus was reached that the presence of

volcanic rocksis more im portant than therelativeamount ofsuch rocks. In addition, both camps agreedthat the amount ofultramafic to mafic rock recognizedcommonly maybe a function ofthe interpretationofprevious w orkersand that furtherwork will probablyreveal more of these rocks. From this common ground,we mutuallyagreed that the Riodas Velhas sequencedoes indeed constitutea greenstonebelt.

Some of the most productive time was spentdevelopingdeposit model forms for each ofthe minesvisited and for mostof the deposits discussed bytheworkshop leaders (the deposit-model formused for thistask is explained inthe appendix). In the discussiongroups assigned to fill out the forms for the variousmodels, a major part of the exercise was to separate factfrom interpretationso that the deposits could becomparedobjectively. The deposit-model formfor eachof the deposits are presentedin a separate chapter(Thorman, this volume,Part B Geology of deposits).The data for each deposit were compiledin tabular formin order to readilycompare similaritiesand differencesamong the deposits (tables1 and 2). A model for theorigin of the deposits, based on theseobservedsimilarities and differences,is shown in table 3.

Two extremes or ends ofthe spectrumwere takenwith regard to the source and transportationof the gold.At one end, the gold was considered tobe of strictlysyngenetic origin iniron-formation,carbon-richsedimentaryrocks, or mafic-felsic volcanicunits. Duringlater remobilization(metamorphism) thegold wasmoved laterallywithin the system, for some unknownbutprobably small distance, to favorable sites determinedbyhost-rock chemistryand structure.

At the other end of the spectrum, the gold wasconsideredto be of strictlyepigeneticorigin andintroducedto the system during deformation at thesametime as or just after metamorphism.The final site ofdepositionwas controlled by the rheology andchemistryof the host-rock environment andby favorablestructures,such as dilationalshear zones (see fig. 6).

Many of the workshop participants believed thatsome depositsvisited couldnot be readily categorizedaseither syngenetic or epigenetic. However,otherparticipantsargued heatedly that most of the depositswere clearly epigenetic,whereas a few pressed just asstrongly for a syngeneticorigin. Perhaps a realisticviewis that an environmentcharacterizedby volcanicactivity(greenstone)and chemicalsedimentation,includingoxide, carbonate, sulfide, and silica-rich iron-formationfacies and carbon-richpelites, was later metam orphosedto produce the contained gold deposits. In such anenvironment,gold derived from exhalative activity

Introduction to a Workshop on Gold Deposits Related to Greenstone Belts in Brazil A 15


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Table 1 . Criteriafor golddeposits examined oranalyzed during the workshopand shown on figure6

[D, diagnostic criteria;P, permissive criteria;P/D, not certain if criteria is diagnostic orpermissive;X, notpresent in deposit; ?, sufficientdata are not available to determine the role ofthat particularfactor or there is aquestion as to whether or not that particular itemis present at the deposit; asterisk (*) indicates the option oftwo interpretations;first letter refers to a facies or syngeneticmodel and the second letter refers to a structuralor epigeneticmodel]

Criteria

Greenstone belt ---------------------------Chemicalsediments (Algoma type) ------------

Oxide facies ---------------------------Iron-rich carbonate ---------------------Chert --------------------------------Sulfur-richchemical sediment ("sulfide facies")

Graphiticphyllite/rock ---------------------

Quartz-sericiteschist (+ /- intermediate/volcanicrocks)Itabirite (Superior type/Proterozoic/welldeveloped)Mafic volcanics ----------------------------

Orebodycontrolled bystructure --------------Stratabound .... .... .... .._ .. ..... ....Stratiform --------------------------------Metamorphismin greenschistor lower amphibolitefacies

Age of host rock --------------------------

Host-rock lithology:BIF (includes iron-richchemicalsediment; not itabirite)Graphiticphyllite -----------------------Mafic volcanic -------------------------Felsic volcanic -------------------------Itabirite -------------------------------

Quartz veins in ore body --------------------

Quartz-carbonateveins in ore body -------------Absence of base metals (exceptin trace amounts)Iron sulfides ------------------------------

Pyrite

Pyrrhotite -----------------------------Arsenopyrite ---------------------------

Tourmaline -------------------------------Scheelite --------------------------------Bismuth --------------------------------Antimony --------------------------------Silver (silvergold about 6:1) -------------------Mercury ---------------------------------Sphalerite -------------------------------

Galena ----------------------------------Chalcopyrite -----------------------------Alteration assemblages ----------------------

Sulfides -------------------------------Carbonates ----------------------------Sericite -------------------------------Chlorite -------------------------------Biotite -------------------------------Hornblende ----------------------------

Complexstructure -------------------------Shear zones ------------------------------

MorroVelho

DDXDPDP

P/DXP?

7DDD?

Archean

DXXXX

P7DD

P/DD

P/D

PP/D

XPDXP

PP7 7X77P?77XDD

Cuiaba

DDXDPDP

P/DX

P/D

PDDD?

Archean

DXPXX

P7DDD

P/DP

7777D7

P?

P?XP?PP?777XD

P/D

SaoBento

DDDDDXP

P/DX

P/D

DDXD?

Archean

DPXXX

DDDD

P/DDD

XD?7D

D(8:l)D?X

XXDDD7P7XDD

Passagem

P/D?*PXPPXP

P/DPX

P/D*P/D*

XD?

EarlyProterozoic?

XPXPP

DDDD

P/DP/D

D

PD?D?7D7X

7XDDD777XDD

Lupin

DDXXPDP

XXX

7DDD

Archean

DXXXX

PXDDPDP

XPXXDXX

XDPPXXPXDDD

Homestake

DDXDDPP

PXP

DPXD

EarlyProterozoic

DPXXX

DDDDPDD

PPXXDXP

PPDDDPDDPDD

A16 Gold Deposits Related to Greenstone Belts in Brazil Deposit Modeling W orkshop Part Excursions


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Table 2. Postulated criteriafor overall settingof iron-formation-relatedgold deposits in greenstone belts examinedoranalyzed duringthe workshopand shown on figure6

[Note that the generalclassificationof the deposits to a syngeneticor epigenetic originis highly controversial.An asterisk (*) indicates those deposits forwhich there is less agreement on their classificationand aretherefore listed under more than one heading; see figure6 and discussion in text]

Morro Cuiaba Sao Passagem Lupin Hom estakeCriteria Velho Bento

Relativeposition tovolcanic center1 ,proximal; 2, intermediate;3 , distal ------- 2-3 1-2 2 3? 2-3 3

Tectonic settingof host rockwhen formed:1 ,unstable; 2, intermediate;3, stable -------- 3 3 3 1-2 3 3

Tectonic settingat time of mineralization:1 ,strongdef.;2, intermediate;3 , no def. ------- 2? 2 1 1 2? 1?

Igneous intrusiveactivity:1 ,very active; 2, moderate; 3 , no activity ------ 3 3 3 3 3 3

Igneous extrusiveactivity:1 ,very active; 2 , moderate; 3 , no activity ------ 2 2 2 2 2 2

GENERAL GEOLOGICSETTING:Greenstonebelt.Periods of chemicalsedimentationwith BIF (includingcarbonate,oxide, sulfide, and minor silicate

facies) and carbonaceousrocks (graphitic associationimportant).Bimodal volcanismmay be important.Sulfur is needed hi the system. In a syngenetic model,the sulfur wouldhave to be depositedwith the

sediment,whereas in an epigenetic model the sulfur couldbe introducedlater.Greenschist-faciesmetamorphismappears to be im portantin the Quadrilatero Ferriferodeposits and

elsewhere. Thislevel of metamorphismmay be the minimum required; the upperlimit is probably

middle amphibolitefacies. In a syngenetic model,regionalmetamorphismis not needed,whereasin one variantof the epigenetic m odelthe metamorphismwould be the heatsource forhydrothermalactivity.

Moderatelycomplex deformationalhistory is common, typicallywith several episodicphases of structuraldevelopment.

Ductile shear zones are importantin some models.

MINERALOGICALAND CHEMICALSETTING:Sulfur occurs as abundant iron-sulfides.Arsenic and tungsten more common in structurally controlled deposits.Silica is common tothe overall system.Iron carbonateis common to the overall system.Iron importantas catalyst to drop sulfur, both during chemical sedimentationand subsequentsulfide

mineralization associatedwith deformation and metamorphism.Carbon may be im portantto the overall system, but its exact relationshipis not clear.

GENERALCLASSIFICATIONOF THESEDEPOSITSis considered tobe as follows:Syngenetic-prdbablyhas some subsequent movement ofgold;

Lupin, Morro Velho*, Homestake*.Syngenetic moderateamount ofmovement ofgold due to later events;

Cuiab^, Morro Velho*, Sao Bento*.Epigeneticmajor control of mineralizationdue to deformation;

Homestake*,Passagem,Sao Bento*.

Introduction to a Workshop on Gold Dep osits Related to G reenstone Belts in Brazil A17


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Table 3. Model for the originof the deposits visitedor discussed during theworkshop

GENERAL GEOLOGICSETTING: Greenstonebelt. Periods ofchemical sedimentation,with BIF (including carbonate, oxide, sulfide and minor

silicate facies) and carbonaceousrocks (graphitic associationimportant). Bimodal volcanismmay be important. Sulfur is needed hi the system. In a syngeneticmodel, the sulfur would haveto be depositedwith

the sediment,whereas hi an epigeneticmodel, the sulfur couldbe introduced later. Greenschist-faciesmetamorphism appearsto be important. This metamorphismmay be the

minimal grade required; the upperlimit is probably middle amphibolitefacies. In a syngeneticmodel, regional m etamorphismis not needed, whereashi one variant of the epigenetic model,themetamorphismwould be the heat source forhydrothermalactivity.

Moderatelycomplex deformationalhistory is common andtypicallyincludes several episodes orphases of structural development.

Ductile shear zonesare importanthi some models.

MINERALOGICAL AND CHEMICALSETTING:Sulfur occurs asabundantiron-sulfides.Arsenic and tungsten are morecommon hi structurally controlled orebodies.Silica is common to the overall system.Iron carbonate is common to the overall system.Iron is important as a catalystto drop sulfur, both during chemical sedimentationand subsequentsulfide mineralizationassociatedwith deformationand metamorphism(see earlier commentsregarding sulfur).

Carbonmay be importantto the overall system, but its exact relationship is not clear.

during sedimentationwas depositedas an extremelyfine,disseminatedstratiform component ofthe system andformed a gold occurrence,not an economicgold deposit.Whether the gold was originallytied up with the carbon,with sulfideminerals, orwith other minerals, isunknown. The gold was probably confined to aparticular stratigraphichorizon or interval, rather thandisseminatedthroughout a large verticalsection. Thetransformationof the gold from a disseminatedelementhi trace amounts to a majorgold deposit appears to havedepended on (1) regional m etamorphismto at least thegreenschistfacies, and (2) on strong deformation thatformed m oderate-to large-scalefolds and faults or shearzones. The metamorphismwas the catalyst thatprovided the heat to drive the fluid systemthatremobilizedgold, arsenic, sulfur, iron, boron, silica,calcium, and other elements. The folding and faultingprovided the plumbing system for the fluids, formingporosity and permeability.Dilated shearzones, axialzones of folds (some of which crossshear zones), andfault zones became sites for ore deposition frommineralizingfluids. The fluids probably did not movegreat distancesfrom original syngenetic source beds, forthe gold in all the deposits is restricted to the same typeof host rock oxide-, carbonate-,or sulfide-faciesrocks,

silica-rich iron-formation,and (or) graphiticrock.The favored explanation atthe workshop forthe

Quadrilatero Ferrtferogold deposits in the Rio dasVelhas Supergroupwas a combination of syngenetic andepigenetic processes. Figure6 shows syngenetic andepigenetic origins as end mem bers of asystem andshows twoalternatives to the interpretationof thegenesis of these deposits. Figure 64 represents thegeneral consensus ofthe participants at the end of theworkshopand figure 65 that of Thormanhi view ofrecent data (this interpretationis discussed in Thorman,this volume,Part B Geology of deposits). The intent isto emphasizethe generallystrong inclination to favor anepigenetic origin for most of the deposits. TheHomestake and Lupin deposits are included forcomparisonwith the Brazilian deposits. Positioningadeposit on the scale is highly subjective. It appears to usthat any originmust take into considerationa certainelement of both ends of the system. Such aconsiderationbecomes especiallypertinent hi thedevelopmentof a model for mineral exploration. Forinstance, continued detailed study of the Homestakedeposit led companygeologists to switch from asyngenetic to an epigeneticinterpretationhi 1987,though during the workshop ansyngenetic model was



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syngenetic syngenetic

LupinMorro VelhoHomestake

Cuiabei

Sao Bento

Lupin

Morro VelhoCuiab

HomestakeSao Bento

Figure 6. Diagram showing golddeposits (as classifiedintable 2) relative to syngeneticversus epigeneticorigin. Thepositioningof these deposits on this scale is highlysubjective and is not intendedto indicate that we are certainof any particularor exact relative position of one depositversus another. Rather, the intentis to show thatbothsyngenetic andepigenetic factors should be considered inthe genesis of these deposits. A represents consensus ofparticipantsat workshopand B that of Thorman (thisvolume, Part B Geology of deposits).

presented. An epigeneticmodel is presented forHomestake in Part B of this bulletinby Caddey andothers. S.W. Caddey (HomestakeMining Co., oralcommun.,1990) believes that as more detailed studiesare done elsewhere,there will be a strong shift to theepigenetic interpretationfor many gold depositsthroughout theworld. Thorman (this volume,PartB Geology of deposits) describesthe deposit modelsand discusses inmore detail the conclusions forgedduring the workshop.

REFERENCES CITED

[* indicates references cited in this paper that are not availableoutside of Brazil or are unpublished dissertations.These referencesare used in the interestof crediting workers,even though their workis not published,and letting the reader know whereto find thesereferences]

Barbosa,A.L.M., 1969, Geologic mapsof the Antonio Pereira,Sao Bartolomeu,Ouro Preto and SantaRita do OuroPreto, and Mariana and Rio das Bandeiras quadrangles,inDorr, J.V.N., 2d, 1969, Physiographic,stratigraphic,and

structural developmentof the Quadrilatero Ferrifero,Minas Gerais, Brazil: U.S. GeologicalSurveyProfessionalPaper 641-A, 110 p.

Dorr, J.V.N., 2d, 1969, Physiographic,stratigraphicandstructural development ofthe QuadrMteroFerrifero,Minas Gerais,Brazil: U.S. GeologicalSurveyProfessionalPaper 641-A, 110 p.

Drake, A A., and Morgan, BA., 1979, Precambrian platetectonicsin the Brazilianshield; evidencefrom thepre-Minas rocksof the Quadrildtero Ferrifero,MinasGerais: U.S. GeologicalSurveyProfessionalPaper1119-B, 1 9p.

*GrossiSad, J.H., and Ladeira, E A., 1968, Esbogo tect6nicodo estado de Minas Gerais: XXII Congresso Brasileiro deGeologia, Sociedade Brasileirode Geologia, and FundagaoGoreixde Ouro Preto,scale 1:1,000,000.

Guild, P.W., 1957, Geologyand mineral resources of theCongonhasdistrict, Minas Gerais, Brazil: U.S. GeologicalSurveyProfessionalPaper 290, 90p.

Herz, Norman,1970, Gneissic and igneous rocksof theQuadrilatero Ferrifero,Minas G erais,Brazil: U.S.GeologicalSurveyProfessionalPaper 641-B, 58 p.

Inda, HA.V., Schorscher,H.D., Dardenne, MA.,Schobbenhaus,Carlos, Haralyi, N.L.E., A zevedoBranco,P.C., Ramalho, Ronaldo,1984, O craton do Sao Franciscoe a faixa de dobramentosAraguai, in Schobbenhaus,Carlos, Almeida Campos, Diogenes de, Derze, G.R., andAsmus, H.E., Geologiado Brasil: Brasil DepartamentoNational da Produgao M ineral,Divisao de GeologiaeMineralogia,p. 193-248.

Johnson, R.F., 1962, Geologyand ore deposits of theCachoeirado Campo,Dom Bosco, and OuroBrancoquadrangles,Minas G erais,Brazil: U.S.GeologicalSurveyProfessionalPaper 341-B, 39 p.

Ladeira,E A., 1976, Principalsfeic,6es tectonicas do Escudo

Brasileiroin Loczy, Louis de and Ladeira, EA., Geologiaestructural e introducao a geotect6nica:Sao Paulo, EdgardBliicher Ltda;Rio de Janeiro, ConselhoNational deDesenvolvimento Cientificoe Tecnologico, p.433-462.

____ 1980, Metallogenesisof gold at the Morro Velhomine, and in the Nova Lima District,QuadrilateroFerrifero, Minas Gerais,Brazil: London; Ontario,Canada, Universityof Western OntariounpublishedPh.D.thesis, 272p.

*Ladeira,E A., Cardoso, R.N., Castro, S.D.C.,and FonsecaS.G., 1985, Relatorio dos trabalhos de pesquisasdeserpintinitose ouro nos locals denominadosfazendasGordura e Pau Branco,e de talco e serpentinito nolocale

denominadoIpe m unitipio de Brumandinho: internalreport to RodiminasMineradora IndustriaLtda., BeloHorizonte,vol. 1 ,72 p., vol. 2, sections and maps.

Ladeira, E A. and Fleiscner,Ronald, 1988, Field excursionfrom Belo Horizonte to OuroPrSto Side excursion alongthe road to Moeda,Minas G erais,Brazil: U.S. GeologicalSurvey Open-FileReport 88- 655, 4 p.

Ladeira,E A. and Viveiros, J.FM. de, 1984, Hipotese sobre aestruturagao do Quadrilatero Ferrifero,com base nosdado disponiveis: Sociedade Braseiro Geologia, NucleoMinas G erais, Boletim4 , 48 p.

Introduction to a Workshop on Gold Deposits Related to Greenstone Belts in Brazil A 19


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1986, Brief introductionto the geology ofQuadrildteroFerrifero-Hypothesisto its globalstructurewith basis on available data:Deposit modeling program,Institute Brasileiro de Mineragao, p.1 40.

Maxwell,C.H., 1972, Geology andore deposits of the A legriadistrict, Minas G erais,Brazil: U.S.GeologicalSurveyProfessionalPaper 341-J, 72 p.

Moore, S.L., 1969, Geologyand ore deposits of the Antonio

dos Santos, GongoS6co, and Conceigao doRio Acimaquadrangles,Minas Gerais, Brazil: U.S. GeologicalSurveyProfessionalPaper 341-1,50 p.

Morritt, R.F.C., 1988, The radar structure ofBrazil south ofthe AmazonBasin, with special emphasis on the structuralsetting of the QuadrMteroFerrifero, MinasGerais State:Kingston, Ontario, Canada, Queen's University Ph.D .thesis, v. 1 ,273p.

Noca, C.M., Pinheiro, S.O., Ladeira, EA., and Grossi Sad,J.H., 1990, Ocorrenciade metakomatiitoscom texturaspinifex no Groupo Nova Limaoeste do QuadrilateroFerrifero: XXXVICongresso Brasileirode Geologia,resumes das comunicagoes,Boletim 1 ,p. 215.

Pomerene,J.B., 1964, Geology andore deposits of the BeloHorizonte, Ibirite, and Macacos quadrangles, Minas

Gerais, Brazil: U.S.GeologicalSurvey ProfessionalPaper341-D, 84 p.

Schobbenhaus, Carlos,Almeida Campos,Diogenes de, Derze,G.R., and Asmus, H.E., 1981, Mapa geo 6gico do Brasil:Brasil DepartamentoNational da Produgao M ineral,Divisao de Geologiae Mineralogia, scale 1:2,500,000.

Schorscher, H.D., 1978,Komatiitosna estrutura "GreenstoneBelt" serie Rio das Velhas, Quadrilatero Ferrifero, Minas

Gerais, Brasil: X X X Congresso Brasileirode Geologia,resumes das comunicagoes,Boletim 1 ,p. 292-293.

Simmons, G.C.,1968a, Geologyand iron deposits of thewestern Serrado Curral,Minas Gerais, Brazil: U.S.GeologicalSurveyProfessionalPaper 341 G, 57 p.

____1968b, Geologyand mineralresources of the Barao deCocais area, Minas Gerais, Brazil: U.S.GeologicalSurveyProfessionalPaper 341-H, 46 p.

Wallace, R.M., 1965, Geologyand m ineralresources of th ePico de Itabirito District, Minas Gerais, Brazil: U.S.GeologicalSurveyProfessionalPaper 341-F, 68 p.

A20 Gold Deposits Related to Greenstone Belts in Brazil Deposit Modeling Workshop Part A- Excursions


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APPENDIX

Explanation of the Depos it-Mode lForm Used toOrganize Data

Introduction to a Workshop on Gold Deposits Related to Greenstone Belts in Brazil A21


28/94

EXPLANATION OF THE DEPOSIT MODEL FORM USED TO ORGANIZE DATA

Deposittype: (fill in your preferred name for the deposit type and the author ofmodel, if known, and the

reference)Sitename: (common name the area,mine, or districtis best known by)Synonymsfor site:Site type: (mine, prospect, mining district)County, state,province,country:Latitudeand longitude:Commodities:(include major and minor metals and (or) commoditiesproduced;may include important

nonmetallic byproducts)Production: (include productionhistory ofthe deposit, if any; include k nowndates of production,

commoditiesproduced, and data on tonnage and (or) gradeof ore produced)Depositdescription: (general geologicdescriptionof the deposit)Rock types: (rocks typical of the geologic terrane)Textures:(special textures of rock units)Host-rocklithology: (concise lithologicdescription of rockthat forms the principal host forthe deposit)Host-rockname: (formal and (or) informal name of thehost-rock unit)Host-rockage: (give the most specific age possible;include any radiometric datesavailable)Mineralizationage: (give the most specific age possible;include any radiometric datesavailable; if no

radiometric dates, then give line of reasoning that supports the inferred age)Lithotectonicsettingand or) depositionalsetting whe nhost rockformed:Tectonic settingat time of mineralization: (regional tectonicfeatures importantin the genesis of the

deposit)Associatediffieousrock: (briefly describe all igneous rocksgeneticallyassociatedwith the m ineralization)Associatedigneousrock age: (as above)Ore minerals: (give complete names ofall ore minerals known at the site being described,in decreasing

order of importance; list importantor guide minerals with + sign; list ore minerals hi assemblages;show zonal or temporalrelation betweenassemblages;group trace minerals separately;list byproductmetals (such as Cu, Ag) that may not form minerals)

Mineralogy: (describe completelybut briefly alteration, gangue, and other mineralassemblages associatedwith the m ineralization)

Alteration: (minerals producedby reaction of ore-formingfluids with rocks; list in assemblages;showzonal or temporalrelation betweenassemblages)

Textureand or) structure:(describe appearance o f ore)Ore controls: (describe factorscontrolling mineralization,includingstructure,stratigraphy, chemistry, or

any other type of control)Geochemicalsignature and or) metal concentrations: (regionalgeochemicalanomalies that might

indicate the deposit or related associateddeposits; elements expected tobe anomalous, eitherenriched or depleted, in and near the deposit; list element assemblages andshow zonal arrangementwhere possible)

Weathering:(optional; list any specialweathering characteristicsor secondaryminerals that might serveas prospectingguides)

Associateddeposits: (other deposit types associatedwith this deposit)Comments:(use this space forgeneral comments onthe deposit that don't seem to fit any other field; an

excellent place to include speculations concerningorigin that should be kept separate from the factsincluded in the other fields; a place to expound)

References:(include importantreferences on the deposit and the generalregion of the deposit)Diagrams: (include a well-labeled map and(or) cross section ofthe deposit and (or) a sketch of an

idealized map or cross sectionshowing ore controls, zoning, and approximate dimensions)



29/94

Geological Field Excursionfrom

Belo Horizonteto Ouro Preto,Minas Gerais, Brazil

By EDUARDO A . LADEIRA

Regional stratigraphyand tectonics of theArchean Rio das Velhas greenstone belt and theProterozoic Minas Supergroupas background for gold metailogenesis

U.S. GEOLOGICAL S U RV E YBULLETIN1980-A

A 23


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Town of Ouro Preto

A24


31/94


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Folded specular hemati teand quartz-m uscovite schistof Cau e Itabirite(stop 15 )

A 26


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Geological Field ExcursionfromBelo Horizonteto Ouro Preto,Minas Gerais, Brazil

By Eduardo A. Ladeira

INTRODUCTION

This geological field excursionshould be used inconnection with "BriefIntroduction to the Geology ofthe Quadrilatero Ferrifero" by Ladeira and Viveiros (thisvolume, Part B Geology of deposits). A companionpaper ("Field excursion from Belo Horizonte to OuroPreto Side excursion alongthe road to Moeda"; thisvolume, Part A Excursions) by Ladeira and Fleischerbegins at kilometer 27 of this trip. The side excursion isan integralpart of understanding the tectonics and thebasal Minas Supergroup.

SCOPE OF THE EXCURSION

The purpose of this field excursionis to provideinsight into the regional stratigraphyand tectonics of theArchean Rio das Velhas greenstone belt and theProterozoic Minas Supergroup,which will aid inunderstanding the geological setting andthemetallogenesis of the Passagem, Morro Velho, Cuiaba,and Sao Bento gold deposits. Papers on the geology ofthe Passagem, Cuiaba,and Sao Bento gold deposits areincluded in Part B of this volume.

Figure 1 shows the field trip route on a geologic mapof the Quadrilatero Ferrifero (Iron Quadrangle) that hasbeen simplified from Dorr (1969). Figure 2 shows thelocation of major gold mines and occurrences inthesame area. Figure 3 is a generalized stratigraphiccolumn of the area.

R O A D LOG

For the convenience of the user, the highway(BR-040) kilometer marks(kmM), which are on theright margin of the southbound lanewhen one travelssouth, are listed as reference points. Distance inkilometers (km).

Marks Distance(kmM) (km)

0 0

1.8

Manuscript approved for publicationFebruary22,1991.Departamentode Geologia, Institute de Geociencias

UniversidadeFederal de Minas Gerais, BeloHorizonte,Brazil

Intersection of Contorno Avenue andFederal HighwayBR-040, which links

Belo Horizonte to Rio de Janeiro. BeloHorizonte was built on a graniticmigmatitic terrane, but this part of thecity lies along the contact of this terraneand rocks of the Sabara Group.

To the right and left, the road intersectsschists and phyllites (some of volcanicorigin) of the Sabara Group. The rangeto the left is the Serra do Curral, whichwas carved in an inverted homoclinalstructure formed by metasedimentary

rocks of the Minas Supergroup in such away that the ridge is made up of CaueItabirite overlying the dolomites,phyllites, and dolomitic itabiritesof theGandarela Formation. The dolomiteshave been mined at several quarries tobe used as flux in metallurgyor for theproduction of refractory material. Thelower hills are formed by strata of thePiracicaba Group.

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PROFOUND OROGENY W ITH REGIONAL METAMO RPfflSMItacolomiGroup

PROFOUND ANGULARUNCONFORMITY

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Minas Supergroup

Piracicaba Group

Itabira Group

Caraga Group

Sahara FormationBarreiro Formation

Tabooes FormationFecho do Funil FormationCercadinhoFormation

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\/Batatal FormationMoeda Formation

v Tamandua Group

MAJOR OROGENYWITHREGIONAL M ETAMORPfflSM

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Maquin6 Group

Nova LimaGroup

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Casa Forte FormationPalmital Formation

\/Upper unit*Middleunit* (includes Lapa Seca Formation)Lower unit*

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< CONTACTCONSIDERED INTRUSIVEBY DORR (1969)Granitic-gneissic b asementrocks, including granitoidsof various ages

Figure 3. Generalizedstratigraphic columnof Precambrian rocksof the QuadrilateroFerrffero(modifiedfrom Dorr, 1969).Asterisk(*) indicates units not included in Dorr's pap er butdiscussed by Thorman and Ladeira in the introductorychapter ofthis volume, Part A Excursions. Thegranitic-gneissicbasement unit includesseveral informal units,including theBacaoComplexor Bacao GraniticComplex, whichis the oval basement body in the sou th-centralpart of figures 1 and 2.


2-

2.5

STOP1

Contact of Sabara G roup(the authorprefers raising the Sabara FormationtoGroup status). In an inverted sequencealong the road to the left, one finds theSabara Groupoverlainby the graphiticphyllite of the BarreiroFormation. The


Barreiro is overlainby very thin andbrecciated Tabooes Quartzite,which isoverlain by the weatheredphyllitesanddolomites ofthe Fecho do FunilFormation.

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3-4

6.5

STOP 2

The cuts to the left are in the CercadinhoFormation, a sequence ofinterbeddedsilvery phyllites and ferruginousquartzites thrown into variousasymmetricfolds. The quartzites aremedium to coarse grained anddisplaycross-bedding and graded bedding thatare used to determine the overturnednature of the beds. The roadcuts arenow coveredwith concrete.

STOPS

Gravity faulttransecting the CercadinhoFormation shows drag structures and

"fault horses" along the fault zone. Whenthe observer faces the structure, the dragstructures indicate that the left block hasmoved up relative to the right block.

Belo Horizonte Shopping Center to theleft, built on a filled-in karst lake thatformed in dolomitesof the GandarelaFormation. The hills to the right are onthe CercadinhoFormation (ferruginousquartzites and phyllites).

Weathered dolomitesof the GandarelaFormation with ochre as a weatheredproduct, overlain by the Caue Itabirite,which is covered by a thick lateritic crust("canga"). At the open pits, iron andiron-manganeseore has been mined.

Railway bridge to Aguas Clarasiron-oredeposits.

STOP 4

Outcrop of fresh dolomite of theGandarela Formation, weathers to apeculiar soil known as "coffee powder."So //Si: N. 55 E., 60 SE. Gash quartzveins have attitude N. 10 E., 50 NW.S2:N.55-60W.,55NE.Crenulationsplunge E. 35 or S. 85 E. at35.


Highway to the right is the road to SaoPaulo; it passesthrough CidadeIndustrial de Contagem,seen in thedistance.

STOPS

9 The highwaycuts asymmetricallyfoldedCaue Itabirite. To the right, iron-oremine of MannesmannCompany. Theprimarybedding-layeringof the itabiriteis strongly transposed by a foliationaveragingN. 45 E., 55-65SE., whichparallels the hinge surface of folds.Crenulations trend N. 75 E., 42(intersection ofSo with Si). Slip striae

trend S. 50 E., 60. To the left, on themountain top, are the Mutucairondeposits in Caue Itabirite of the MinasSupergroup.

10 10 New Viaduto da Mutuca (Mutucahighwaybridge). Drive over the oldMutuca bridge.

STOP 6

Both road cuts are on fragmentalrocksand metaconglomeratesof the NovaLima Group, which contains stretchedpebbles and is intersected by variousquartz veins that probably formed duringmovement onthe Mutuca thrust fault.

12 12 Quartz-chlorite schists with thininterlayered quartzitesand minorgraphitic phyllite of the Nova LimaGroup are exposed in the road cut to theleft.

13 13 Quartz-chlorite schists of the Nova LimaGroup, interbedded withmetaconglomerate(volcanicagglomerates?).

13.5 Highway Police station.

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14 14 Greenschists ofthe Nova Lima Group,interbeddedwith unweatheredagglomerate, locallypyritiferous.

14.3 Turnpike to Rio Verde MiningCompany.

STOP?

15.5 Drive about 0.5 km to the right on agravel road to a bauxite deposit. The oreprobablyis derived from aluminoussediments that accumulated ontheargillaceousbottom of a Tertiary lake.Later upgradingwas caused bysupergene enrichment. Two mines canbe visited. The first, and nearest one,belongs to MagnesitaSA., a producer ofrefractory tiles from the local clay. Theother is held by Alcan,which owns analuminum plant near Ouro Preto andextracts bauxiteat several sites in theQuadrilateroFerrffero, including thisone.

16.5 Gas station.

17.5 Jardim Canada, dry karst lake to theright.

20.2 Skol Brewery to the left.

22.1 Thick canga crustwith pebbles, cobbles,and fragments of hematite anditabirite.This crust covers both the GandarelaFormation (which doesn't crop out) andpart of the Caue Itabirite.

22.7 About 40 to the left, if the sky is no tcloudy, one can see the Pico do Itabirito(ItabiritoPeak), carved in hard hematiteore, which will be seen again a couple oftimes duringour trip; it is part of therange with the same name thatconstitutes the inverted limb of theMoeda synform, whosenormal limb is tothe right and is evidencedby the dipslopes to the right of the roadand fartheron. The inverted limb will be seenshortly.


27 27 Rodovia dos Inconfidentes(BR/MG-356, Inconfidente'sHighway).Drive to the left to Ouro Preto, formercapital of Minas GeraisState.

27.3 Highway Police station.

29 29 Exposure of ferruginousgrayishquartzites andsilvery phyllites of theCercadinhoFormationoverlain byweathereddolomites (which produce abrownish to black saprolite) and silveryphyllitesof the Fecho do FunilFormation. Tight asymmetricfolds haveenveloping surfaceswhose attitude is N.15 E., 58 NW. So //S i is N 10E.,45-50NW.

31 29.5- Cercadinho andFecho do Funil, as31 above.

STOPS

34 34 Folded and faulted metaconglomeratesmapped by Wallace (1965) as theItacolomi Group,with distorted pebblesand cobblesof quartz and quartzite.So //Si: N. 15 W., 58 W., 65 NE. Thecurrent author, Ladeira,believes thismay be a fragmentalvolcanic rock.

34.5 This area was a former Horto Florestal(forest station).

34.5 To the left, CodornasLake. Threehundred m eters before thebridge to theright, varicoloredphyllite interlayeredwith graphitic phyllite, both of BarreiroFormation,overlainby the younger, thin,saccharoidalTabooes Quartzite. Dipofbeds is vertical to near vertical. Strike isnortheast.

35.8 Bridge over the Codornas Creek.Roadcuts are on the varicoloredargillaceous phyllite and interlayereddolomites of the Fecho do FunilFormation.

36 36- Varicolored argillaceous phyllitesof the36.5 Fecho do Funil Formationinterlayered

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with "splash rock" (Guild, 1957),a namegiven to the saprolite of dolomite herethe saprolite of Fecho do FunilFormation. Reductionor bleached spotswith ellipsoidal shapeand light color areoriented parallel to fold hinges. So: N.30 W., vertical; N. 16 W., 30 NE.

STOP 9

37 37 Alluvial deposit ofa Tertiary lake, withquartzpebbles and cobbles inanarenaceous,argillaceousmatrix, covers"splash rock"and phyllites of the Fechodo Funil Formation.

38 38 CercadinhoFormation (ferruginousquartzitesand phyllites) overlainby theFecho do Funil Formation.So //S i: N.60-70 E., 60 SE. Con tact withlateriteand canga.

39 38.5- Roadcuts arecut successively deeper into39.5 the CercadinhoFormationdue to folding

with the underlyingweathereddolomitesof the G andarelaFormation. Theweathered productof the GandarelaFormationis commonlyreferredto as"splash rock."

39.8 HighwayPolice station and truckbalance.

40 40 As one approaches the police station, thehidden contact between the GandarelaFormationand the Caue Itabiritewill becrossed. The section is the inverted limbof the M oedasynform in which theyounger GandarelaFormationis overlainby the Caue Itabirite. To the right isItabirite Peak.

40.1 Folded Caue Itabiritewith kink folds.Canga crust derived from the itabiritecaps the roadcut. To the left,ITAMINAS MiningCo. iron-ore deposit,hosted in C aue Itabirite.

40.8 The road cuts the inverted sequenceconsisting of Caue Itabiriteoverlain byBatatalFormation,which in turn is


41 41

41.5

42

overlain by the older Moeda Form ation.Beds have a general trend ofN. 15-25W. and are overturned 60-70 NE .More incompetentBatatal Phyllite iscrumpled between the Caue andMoedaFormations. Notethick canga crust ontop of mountain to the right, carvedinCaue Itabirite.

Bonga strike-slip faultwith attitude N.70 E., 60-70NW. The northernblockmoved northeast relative to the southernblock; total strike-slip is about 250 m(meter). This movement causedrepetition ofthe invertedsequence: CaueItabirite,Batatal, and MoedaFormations,having steeper dipsof80SE.

Quartzite quarry to the left, from whichquartz sand is extracted, cleaned, andstockpiled(to the right).

42- From 42.0 to 43.5 km, the road swings43.5 through quartzitesand minor

interlayeredphyllites of the MoedaFormation,which crop out to the right,forming cuneiformcontrolled by a stronglineation ("pencilstructure"). Tothe leftis the Rio das Velhas Valley carved inthe Nova Lima Group (Riodas VelhasSupergroup)of the Rio das VelhasGreenstoneBelt (Ladeira, 1980).

STOP 10 S a n t a at theSerra do Itabirito.

43.5 Angular and erosionalunconformitybetween the variegatedschists of theNova Lima Group and theMoedaFormation. The contact zonetrendsabout N. 10 E. Imm ediately to thewestof the rest area, theNova Lima schistshave two schistosities.The older, Si (N.40 W., 65 NE.), locally parallelsbedding So of the schists; the younger,82 (N. 10-35E., 70 SE.), parallelsthecontact.Fracture cleavages are Ss (N.70 W., 30 NE.) andS4 (N. 70 E., 65SE.). The Moeda Formationbegins witha basal polymicticmetaconglomerate

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44 44

44-49

49 49

with pebbles ofquartz, quartzite andgreenschist (the latter similar toNovaLima Group greenschists). So in theMoeda quartzite trends N. 15 E., 80 SE,forming an angle of about 70 with So ofNova Lima Group schists. The 82schistosityin Nova Lima schists is alsocommon to the Moeda Formationquartzites. Fracture cleavages parallel toSs and 84 of the Nova Lima Group alsointersect the Moeda quartzite. Theangular pattern of So in both the NovaLima Group and Moeda quartziteindicates the unconformablerelation,which is proven by the presence of abasal conglomerate thatcontains clastsof schist from the Rio das VelhasSupergroup. However, the unconformitywas disrupted during the deformation ofthe Minas Supergroupand is now a faultsurface.

STOP 11

Cata Branca strike-slip fault. Thisstructure follows the valley to the right.Nova Limaschists crop out alongthehighway. The fault trendsnorthwest-southeast and dips 80 NE.and the hangingwall (northern block)has a total slip of 500 m towards thewest. Gold associatedwith stibnite in theMoeda quartzite and related to this faultwas mined nearby. The Cata Brancafault shifts the Bac.ao Complex-NovaLima Group contact near the Esperanc,aCo. steel plant to the east of this plant.The Bacjio Complex is the oval body inthe southern part of the QuadrilateroFerrffero (figs. 1 and 2).

The highway swings through weatheredNova Lim a variegatedschists derivedfrom interlayeredmetavolcanicandmetasedimentary rocks.

STOP 1 2

Near the head of highwaybridge overRFN (Rede Ferroviara National)


railway. To the right, Esperanga Co.blast furnace. Contactof the Nova LimaGroup metavolcanicandmetasedimentary rocks with the Ba^aoGranitic Com plex, whichthermallymetamorphosed the rocks and resultedin a garnet-staurolite metamorphicaureole. These rocks are cut bynumerous tourmaline-quartz veins, whichelsewhere contain littlegold. Metamaficrock and lean (low-grade) Archean typebanded iron formation (BIF), notreported earlier by the DNPM-USGS(Departamento National da Produc.aoMineral-United States GeologicalSurvey) team and similar toBIF strata inthe Nova Lima District, are interlayeredin the local succession. See side-roadroadcut (dirt road to the left to RioAcima).

50 After the highwaybridge, weatheredmetamafic dike cuts weathered Bacjiogranite. To the right, a view ofEsperanga Co. iron blast furnace.

51 51- Fresh exposure of Bac,ao Complex.52.5 Itabirito Country Club to the left; city of

Itabirito to the right. From here to

Cachoeira do Campo (20 km ahead) andback to STOP 12, the highway runsthrough and exposes thegranitic-migmatitic rocksof the BacjioComplex.

52 52 Bagao Com plex rocks.

54 54 Bac.ao Complex rocks.

54.5 Southeastern Itabirito turnoff.

57 CEMIG plant turnoff.

59 To the left, road to Acurui village (15km) and Cam ping Club do Brasil(22 km).

60 Alluvial flat.

62.5 Road to Minas Serra Geral (iron ore).Capanema mine.



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74

78

78.5

78.5

Town of Cachoeira do Campo, builtonBac,ao Complex.

Alluvialflat.

Outcrop of migmatites of the BacaoComplexshowing subverticalshear zonestransecting earlierfolded foliation.

STOP 13

Intersectionof Inconfidente'sHighwayand CVRD (CompanhiaVale do RioDoce) Railway. Funil railwaybridge. Atthe exposures tothe right of the highway,the Moeda quartziteis tectonicallyconcordantwith the underlyingNovaLima schists (mostlystaurolite-chlorite-garnetschists) derived fromvolcanic and sedimentaryrocks. Theserocks are intruded by the BacaoComplexin an intricate contact zoneinwhich NovaLima schists are penetratedor invaded by numerouspegmatiticandapliticbodies. The Bacao Complex isinterpreted to be a mantled gneiss domederived froma reworked basement(Ladeira, 1980). The M oeda quartziteistectonicallythinned, boudinaged, andcutby minor faults. Fresh Caue Itabiritecrops out at the bottom of thevalley,and, in addition to the normal itabirite, italso includes carbonate-amphibole(grunerite-cummingtonite)itabirite. Thediscordant relationscan be observed onlyin the railway cuts aboveyou, throughwhich we will be driving.

STOP 14

Drive back 500 m, turn right on the dirtroad, and drive up hill for 2 km to theCVRD railway station. Walk500 m tothe south along the railway tracks.

Faulted, originallydiscordantunconformablecontactbetween theMoeda quartzitesand freshNova Limachlorite-garnet stauroliteschist(greenschists). No conglomeratehas


been found at the base of Moedaquartzite at this site, but theunconformab le relation (establishedatmany localitiesin the region) canbededuced because the Nova Lima schistshave two penetrativeschistositiesthat aretruncatedat the contact, whereas theMoeda has just one schistosity;two setsof fracturecleavage intersect both units.In addition, the schists and Moedaquartzite have distinct structuralstylesthat cannot be explained solely by thedifference of competency. The schistsare asymmetrically foldedand refolded,and their axial surfaces trend differentlyfrom those in the quartzite. The presentcontact is clearly a tectonic one (thrust),which has been healed bymetamorphism. T he structural dataindicate that the units were deformedunder different conditions, lendingsupport to the statement that thetwo arestratigraphicallydistinct units.

Attitudeof S surfaces:50 (schist, folded): N. 60 E., 35 SE .51 (schist): N. 60 E.,35-40SE.82 (schist):N. 15-25E., 45 SE.Sa (common to schist and quartzite):

N. 15-25 E.,55 SE. refractsthrough the quartzite andchangestoN.25 0 E.,60-70SE.

Thrust faults withrelated minorstructuressuch as "mullions," slipstriae,and sheath folds that have beendismembered andnow appear as tectonicinclusions can be observed alongthesection. The Nova Lima schists locallyhave interlayers ofArchean-type BIF(200 m north of this contact) that werenot described at this locality bytheDNPM-USGS team nor by otherprevious workers. Along this section, theauthor recently found intrafolial tightrecumbentfolds in the Moeda quartzite,which are also associated withshear zoneand sigmoidalfolds. Apparently theseare the first reported recumbentfolds inthis formationat this site. Drive back toInconfidente'sHighwayand headtowards Ouro Preto.

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93 93- Road follows the strike of Caue Itabirite,94 which overlies the Moeda quartzites (not

seen).

95.0 Ouro Preto, Praca Tiradentes.

From here drive to Morro do Cruzeiro.

STOP 1 6

Bauxite deposit of Morro do Cruzeiro.This deposit is next to and partlyunderlies the campus of UniversidadeFederal de Ouro Preto. The Morro doCruzeiro (Hill of the Cross) is a smallplateau 1,230 m above sea level and issuperficiallycovered by a lateritic crust

("canga") and iateritic soil. The "canga"maintains theleveled surface andrangesin thicknessfrom 1 to 1.5 m. The bauxitelayer, which has been almost entirelymined out, overlies phyllite anddolomites of Fecho do Funil Formation.Before mining, the layer covered about130,000 square meters and had anaverage thicknessof 1.5 m. The bauxiteis light pink to intense red, which is afunction of variable proportions ofFe2 Os . It is compact, has a specific

gravity of 2.6, and occurs as disseminatedconcretions in laterite. Analyses of thebauxite ore in table 1 (facing page)indicate vertical migration ofiron andalumina.Exploration and mining workdemonstrated the presence of a largeclay deposit underlyingthe bauxite layer.

REFERENCES CITED

Dorr, J.V.N., 2d, 1969, Physiographic,stratigraphicandstructural developmentof the Quadrilatero Ferrifero,Minas Gerais, Brazil: U.S.GeologicalSurveyProfessionalPaper 641-A, 110 p.

Guild, P.W., 1957, Geologyand m ineralresources of theCongonhasdistrict, Minas Gerais,Brazil: U.S.GeologicalSurveyProfessionalPaper 290,90p.

Guimaraes, G.D.,and Coelho, I.S., 1945, Bauxita doMorro doCruzeiro enOuro Preto, MinasGerais: DepartamentoNational da Produc,aoMineral, Divisao de Fomento daProducjio Mineral, Boletim67,41 p.

Ladeira,E A., 1980, Metallogenesisof gold at the MorroVelho mine, and in the Nova Lima District,QuadrilateroFerrifero, MinasGerais, Brazil: London, Ontario,Canada,University of Western Ontariounpublished Ph.D.thesis,272p.

Wallace, R.M., 1965, Geologyand m ineralresources of thePico de ItabiritoDistrict,Minas Gerais, Brazil: U.S.GeologicalSurveyProfessionalPaper 341-F, 68 p.

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Bagao Complexalong CVRDrailroad (stop14)

A 38


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Field ExcursionfromBelo Horizonteto Ouro Preto-Side Excursion A longthe Road to Moeda,Minas Gerais, Brazil

By EDU ARDOA. LADEIRAand RONALD F LEIS CHER

U.S. GEOLOGICAL S U RVEY BULLETIN 1980-A

A39


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Sheared basement rock below Moeda Complex-Moeda Formation contact (stop1 )

A40


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CONTENTSIntroduction A43Scope of the excursion A43

Structuralsetting A43Stratigraphicsetting A43

Road log A46Stop 1 A46Stop 2 A46Stop 3 A47

References cited A47

FIGURES

1. Geologicmap ofthe QuadrilateroFerrifero (modified fromDorr, 1969) showingthe field trip routesfor the workshop A44

2. Geologicmap ofthe field excursionarea showing the locations of stops 1-3 A45

Contents A4 1


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Sheared basement rock of Moeda Complex(stop 1 )

A42


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Field ExcursionfromBelo Horizonteto Ouro Preto-Side ExcursionAlongthe Road

Gold Deposits Related to Greenstone Belts in Brazil Deposit Modeling Workshop

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Transcript of Gold Deposits Related to Greenstone Belts in Brazil Deposit Modeling Workshop