ribeiro-rodrigues&oliveira&friedrich2007 the archean bif-hosted cuiaba gold deposit qf mg...

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Ore Geology Reviews 3

The Archean BIF-hosted Cuiabá Gold deposit, QuadriláteroFerrífero, Minas Gerais, Brazil

Luiz Cláudio Ribeiro-Rodrigues a,⁎, Claudinei Gouveia de Oliveira b, Gunther Friedrich c

a Centro Universitário de Caratinga, Unidade III, Fazenda Experimental de Caratinga, BR-116, km 526, Caratinga-MG, 35 300-970, Brazilb Instituto de Geociências, Universidade de Brasília, Campus Asa Norte, Brasília-DF, 31270-901, Brazil

c Institute of Mineralogy and Economic Geology, Aachen University of Technology, Wüllnerstrasse 2, D-52056 Aachen, Germany

Received 9 June 2005; accepted 21 September 2006Available online 2 March 2007

Abstract

The Cuiabá Gold Deposit is located in the northern part of the Quadrilátero Ferrífero, Minas Gerais State, Brazil. The regionconstitutes an Archean granite–greenstone terrane composed of a basement complex (ca. 3.2 Ga), the Rio das Velhas Supergroupgreenstone sequence, and related granitoids (3.0–2.7 Ga), which are overlain by the Proterozoic supracrustal sequences of theMinas (<2.6–2.1 Ga) and Espinhaço (1.7 Ga) supergroups.

The stratigraphy of the Cuiabá area is part of the Nova Lima Group, which forms the lower part of the Rio das VelhasSupergroup. The lithological succession of the mine area comprises, from bottom to top, lower mafic metavolcanics intercalatedwith carbonaceous metasedimentary rocks, the gold-bearing Cuiabá-Banded Iron Formation (BIF), upper mafic metavolcanics andvolcanoclastics and metasedimentary rocks. The metamorphism reached the greenschist facies. Tectonic structures of the depositarea are genetically related to deformation phases D1, D2, D3, which took place under crustal compression representing oneprogressive deformational event (En).

The bulk of the economic-grade gold mineralization is related to six main ore shoots, contained within the Cuiabá BIF horizon,which range in thickness between 1 and 6 m. The BIF-hosted gold orebodies (>4 ppm Au) represent sulfide-rich segments of theCuiabá BIF, which grade laterally into non-economic mineralized or barren iron formation. Transitions from sulfide-rich to sulfide-poor BIF are indicated by decreasing gold grades from over 60 ppm to values below the fire assay detection limit in sulfide-poorportions. The deposit is “gold-only”, and shows a characteristic association of Au with Ag, As, Sb and low base-metal contents.The gold is fine grained (up to 60 μm), and is generally associated with sulfide layers, occurring as inclusions, in fractures or alonggrain boundaries of pyrite, the predominant sulfide mineral (>90 vol.%). Gold is characterized by an average fineness of 0.840 anda large range of fineness (0.759 to 0.941).

The country rocks to the mineralized BIF show strong sericite, carbonate and chlorite alteration, typical of greenschist faciesmetamorphic conditions. Textures observed on microscopic to mine scales indicate that the mineralized Cuiabá BIF is the result ofsulfidation involving pervasive replacement of Fe-carbonates (siderite–ankerite) by Fe-sulfides. Gold mineralization at Cuiabáshows various features reported for Archean gold–lode deposits including the: (1) association of gold mineralization with Fe-richhost rocks; (2) strong structural control of the gold orebodies, showing remarkable down-plunge continuity (>3 km) relative tostrike length and width (up to 20 m); (3) epigenetic nature of the mineralization, with sulfidation as the major wall–rock alteration

⁎ Corresponding author. Tel.: +55 31 8835 5609.E-mail address: [email protected] (L.C. Ribeiro-Rodrigues).

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544 L.C. Ribeiro-Rodrigues et al. / Ore Geology Reviews 32 (2007) 543–570

and directly associated with gold deposition; (4) geochemical signature, with mineralization showing consistent metal associations(Au–Ag–As–Sb and low base metal), which is compatible with metamorphic fluids.© 2007 Elsevier B.V. All rights reserved.

Keywords: Cuiabá; Orogenic gold; Banded iron formation; Quadrilátero Ferrífero; Brazil, Archean

1. Introduction

The Quadrilátero Ferrífero (QF) is one of the mostimportant gold provinces in the world, and comprisesgold deposits hosted mainly by banded iron formations(BIF), mafic volcanics and quartz veins associated withlithologies of the Nova Lima Group, the lower unit of

Fig. 1. Simplified geologic-structural map of the Quadrilátero Ferrífero showwith>5 t contained Au. Dotted lines in the Rio das Velhas Supergroup are Algfrom Dorr (1969), Marshak and Alkmim (1989) and Chemale et al. (1994).

the Rio das Velhas greenstone belt sequence (Fig. 1).At present, the Cuiabá Mine is the major gold producerin the QF and the largest underground mine in Brazil.The deposit currently produces 830,000 t ore per year,with an average grade of 8 g/t Au and by-product Ag.The cut-off grade is 2.7 g/t Au; proven reserves exceed200 t Au.

ing the location of the Cuiabá Mine and other BIF-hosted gold depositsoma-type, Archean banded iron formations. Geology and structures are

545L.C. Ribeiro-Rodrigues et al. / Ore Geology Reviews 32 (2007) 543–570

The earliest evidence of mining activity in theCuiabá Mine area dates back to 1740, when Portu-guese prospectors mined the surficial deposits.Between 1877, when it was acquired by the BritishSt. John Del Rey Mining Co., and 1940, the mineoperated only intermittently. In 1977, the Morro VelhoMineração S.A. (a joint venture of Anglo American,Bozano and Simonsen Bank) took control, re-eval-uated the deposit and began underground production in1985. During a period of two years (1987–1988), halfa million short tones of oxidized ore with estimated5 g/t Au were exploited by open-cut mining.Anglogold Ashanti became proprietor of the mine in1998.

This paper is based on the first author's Ph.D. thesis(Ribeiro-Rodrigues, 1998), which addresses the char-acter of the gold mineralization, its genesis and controlson ore formation. Most recent and/or importantpublications about the Cuiabá deposit include thecontributions of Costa (2000), Lehne (2000), Martins(2000), Ribeiro-Rodrigues et al. (2000) and Lobato et al.(2001a,b).

The main objectives of this study can be summarizedas follows: (i) determination of the mineralogical,petrographic and geochemical characteristics of theCuiabá BIF and its country rocks; (ii) mineralogical,petrographic and geochemical characterization of theBIF-hosted gold orebodies; (iii) understanding of thelocal and regional controls of mineralization; (iv)determination of mechanisms of gold transport anddeposition; (v) establishment of a genetic model for thegold mineralization within a regional context.

2. Geology of the Quadrilátero Ferrífero

Due to its mineral wealth, and in particular depositsof iron, gold and manganese, the QF is one of the bestsystematically investigated regions of Brazil. Researchhas been carried out by a large number of workers formore than one and a half centuries (e.g., Eschwege,1832, 1833; Gorceix, 1876; Eschwege, 1898; Derby,1906, 1911; Harder and Chamberlin, 1915; Rimann,1921; Moraes and Guimarães, 1930; Guimarães, 1931;Freyberg, 1932, 1934; Barbosa, 1949, 1954, 1961;Guimarães, 1966; Guimarães et al., 1967). The firstsystematic geological mapping was conducted by theDepartamento Nacional da Produção Mineral (DNPM)in cooperation with the United States Geological Survey(USGS) during 1946–1961. In this period, the entire QF(ca. 7000 km2) was mapped at a scale 1 :25000.Syntheses of earlier works are given by Dorr (1969) andHerz (1970, 1978).

The QF represents an Archean granite–greenstoneterrane overlain by Proterozoic supracrustal sequences.Its lithostratigraphy can be subdivided into four mainunits (Dorr, 1969; Marshak and Alkmim, 1989). Theseinclude, from bottom to top, the granite–gneiss terranes,the Rio das Velhas Supergroup greenstone belt sequenceand the Minas and the Espinhaço Supergroups (Figs. 1and 2). The shape of the QF (“Iron Quadrangle“) isdefined by the distribution of the Minas Supergroupmetasedimentary rocks. Recent data on the geologicaland geochronological framework of the QuadriláteroFerrífero, with emphasis on the age of gold mineraliza-tion hosted in Archean greenstone belts, are presentedby Noce et al. (2005), Balthasar and Zuchetti (2007-thisvolume) and Noce et al. (2007-this volume).

The granite–gneiss terranes comprise dome-likedomains, which either envelop or are surrounded bysupracrustal sequences. They consist of ca. 3.2 Gapolydeformed granitoid gneisses and minor metasedi-mentary rocks, which are intruded by 2.8–2.7 Ga oldmetatonalites, metandesites, metagranites, pegmatitesand Proterozoic mafic dikes (Carneiro, 1994; Carneiroet al., 1994; Noce, 1995).

The Rio das Velhas Supergroup (3.0–2.7 Ga;Machado et al., 1989; Machado and Carneiro, 1992)represents a greenstone belt sequence divided into theNova Lima and Maquiné Groups. The Nova LimaGroup is divided (cf. Oliveira et al., 1983, andmodifications by Vieira, 1987, 1991a,b) into: a lowerunit, composed of komatiitic to tholeiitic basicmetavolcanics with preserved primary structures (spi-nifex, pillow-lavas and variolites) interlayered withclastic metasedimentary rocks, acid metavolcanoclas-tics and exhalative chemical metasedimentary rocks(Algoma-type BIF and chert), and metaultrabasicintrusives with cumulate texture; a middle unit, withmafic and felsic metavolcanics (tuffs) interlayered withcarbonaceous epiclastic metasedimentary rocks, andchemical-exhalative metasedimentary rocks (Algoma-type BIF and chert), with felsic metavolcanoclasticsoccurring at the top of the unit; and an upper unitcomprising clastic metasedimentary rocks interlayeredwith mafic and felsic metavolcanoclastics. These rocksare overlain by quartzites and conglomerates of theMaquiné Group.

The Minas Supergroup (<2.6–2.1 Ga; Babinski etal., 1991) is characterized by a succession of Witwa-tersrand-type metaconglomerates, metarenites andmetapelites of the Caraça Group, Lake-Superior-typeBIF and dolomites of the Itabira Group, metarenites,metapelites and dolomites of the Piracicaba Group, aswell as metapelites and metatuffs of the Sabará Group.




Fig. 2. Stratigraphic column of the Quadrilátero Ferrífero. Modified after Marshak and Alkmim (1989).


The Espinhaço Supergroup (ca. 1.7 Ga; Machado etal., 1989), exposed only in the northeast of the QF, iscomposed of metaconglomerates, metarenites andintrusive metabasic rocks.

The QF lithological sequence is intruded by basicand intermediate dikes of Proterozoic and Phanerozoicages, and is locally covered by Tertiary and Quaternaryalluvial sediments as well as laterites (e.g., Gorceix,


1884; Maxwell, 1972; Teixeira, 1985; Carneiro, 1990;Chemale and Silva, 1990; Silva et al., 1991a,b).

3. Geology of the Cuiabá Mine Area

The Cuiabá deposit, like most of the gold deposits inthe QF, occurs in the metavolcano-sedimentarysequence of the Nova Lima Group. The lithostratigra-phy of the mine area was first established by Vial(1980). Subsequently, brief descriptions of the minelithologies were presented by Vieira and Oliveira(1988), Vial (1988), Ladeira (1988, 1991), Vieira etal. (1991a,b) and Vieira (1992). The lithologicalsuccession includes metavolcanic, metavolcanoclasticand metasedimentary rocks correlated with the lower,middle and upper units of the Nova Lima Group, asproposed by Vieira and Oliveira (1988) and Vieira(1992) (Figs. 3 and 4).

For purposes of clarity, the prefix meta is not beused in the present paper. The long-standing rockdesignation used by mine geologists, such as Fg, Xs,etc., is also used in the present paper. Rock typesinclude the lower and upper mafic metavolcanics (manand mba), the Cuiabá banded iron formation (BIF),metapelites (X1), carbonaceous metapelites (Fg),metavolcanoclastics (Xs) and metabasic dikes (d).Mappable hydrothermally altered rocks are representedby chloritized upper mafic metavolcanics (manx), andby sericitized (X2) and carbonatized (X2cl) lower andupper mafic metavolcanics.

The lower unit is characterized by a thick(>400 m) succession of mafic metavolcanics (man)interbedded with metapelites (X1), and lenses ofcarbonaceous metapelites (Fg) (Figs. 3 and 4). Maficmetavolcanics are characterized by the presence ofchlorite, epidote, quartz, plagioclase and actinolite.Information about these beds is mainly provided bydrill holes. The mafic volcanics are conformablyoverlain by the 15-m-thick Cuiabá BIF. The middleunit is a sequence of alternating carbonaceousmetapelites (Fg) and hydrothermally altered maficmetavolcanics (X2) with local intercalations ofmetapelites (X1), which overlies the Cuiabá BIF.The top of the middle unit is composed of non-alteredmafic metavolcanics (mba); estimated thickness is ca.150 m. The >600-m-thick upper unit is constituted ofmetapelites (X1) alternating with metavolcanoclastics(Xs). Intrusive rocks (d) are represented by basic dikes(up to 30 m in thickness), which crosscut all rocktypes. The lithological contacts with the middle unitare transitional and marked by the appearance ofmetavolcanoclastics. Radiometric ages (U–Pb-SHRIMP)

of detrital zircons yield a minimum age of 2.74 Ga forthe metavolcanoclastics (Schrank and Machado, 1996).

3.1. Mafic volcanics

The lower mafic volcanics (man) occur in the core ofthe Cuiabá structure and are known from drill holes andfrom gallery exposures at level 3 (Fig. 3). The light-green, fine-grained massive rocks are composed ofclinozoisite/epidote, plagioclase, amphibole, chloriteand quartz. The texture is characterized by randomrelicts of plagioclase replaced by clinozoisite, occurringtogether with acicular or prismatic tremolite/actinolite,chlorite and quartz. Vieira (1992) describes theoccurrence of pillows and variolites at level 3, whichis presently inaccessible. Both pillows and variolites aredeformed, showing lengths varying from 0.5 to 2.0 mand 0.1 to 2 cm, respectively. These structures indicate asubmarine origin for the lower mafic metavolcanics.

The upper mafic volcanics (mba) have a restrictedaereal distribution, occurring mainly in the eastern partof the mine area. The dark olive-green, massive andfine-grained lithotypes are locally foliated, with pillowstructures recorded near the Viana orebody (Fig. 3). Thethickness of the unit reaches >80 m. The rocks arecharacterized by a mineral assemblage similar to that ofthe lower mafic volcanics, comprising chlorite, epidote,plagioclase, quartz and minor amphibole. Plagioclaseshows an ophitic texture and has no preferredorientation.

Mafic metavolcanics exhibit a mineral assemblagethat is typical of the chlorite metamorphic zone:chlorite+albite+epidote+zoisite (clinozoisite)+quartz±actinolite±carbonate. This assemblage is characteristicof metamorphic temperatures from 350 to 450 °C, and isstable over a large pressure range (up to 4 kbar; Spear,1995). The absence of hornblende indicates that peakmetamorphic conditions did not exceed the greenschist–amphibolite facies transition (cf. Barker, 1994; Spear,1995). The absence of garnet is indicative of unfavorablebulk rock compositions and/or low-pressure conditions(cf. Spear, 1995).

3.2. The Cuiabá Banded Iron Formation (Cuiabá BIF)

The Cuiabá BIF represents an Algoma-type (cf.Gross, 1965), carbonate facies iron formation ranging inthickness from <1 to 15 m. Although carbonates are thedominant iron-bearing minerals of the Cuiabá BIF,below level 11 a magnetite-bearing BIF has beendisclosed. In the present work, both iron-formationsensu stricto (i.e., chemical sediment containing >15%

Fig. 3. Geological map of the Cuiabá Mine, at level 3. The geology is modified from Vial (1980) and Vieira (1992).

548L.C.Ribeiro-R

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Fig. 4. Stratigraphic column of the Cuiabá Mine area, modified after Vial (1980) and Vieira (1992).


Fe; cf. James, 1954; Gross, 1959; Brandt et al., 1972)and chert beds are referred to as the Cuiabá BIF. In itspresent position, the lateral extent of the BIF horizon canbe estimated to exceed 1 km (Fig. 3). A single maficmetavolcanic horizon, up to 15 cm thick, occurs withinthe Cuiabá BIF; this is commonly sheared andsulfidized.

The pronounced banding of the BIF is the result ofalternating dark carbonate–quartz, light quartz–carbo-nate and chert layers. In addition, sulfide layers occuronly in the mineralized areas. The distinct mesobandsvary in thickness from a few mm up to 1 m, and maycontain microbands at the thin-section scale. Late-stagequartz veins commonly crosscut the banding. Whereas

contacts between dark carbonate–quartz and lightquartz–carbonate or chert layers are generally abrupt,gradual contacts can be observed between light quartz–carbonate and chert layers.

Dark carbonate–quartz layers contain 50 to 95 vol.%carbonates (siderite, ankerite) and 5 to 45 vol.% quartz.Carbonaceous, “graphite-like” matter occurs in form ofvery fine-grained stringers (up to 3 vol.%) resulting inthe dark coloring of the layers; sulfides are also observed.Light quartz–carbonate layers consist of 50 to 70 vol.%quartz and 30 to 50 vol.% carbonates (siderite, Mn-richankerite, Fe-rich dolomite, calcite). Subordinate miner-als are muscovite and chlorite. Chert layers arecomposed of quartz (>95 vol.%) and minor carbonates.

Table 1Structural evolution of the Cuiabá Mine area

Event En

Nature CompressionalDeformation regime Ductile to brittle regimesTectonic transport E–SE to W–NWDeformation phase Main featuresD1 • Cuiabá fold, a kilometer-scale

south-east plunging tubular sheath fold• Penetrative, pervasive S1 foliation,axial planar of F1 folds, showing amineral lineation.• Development of a mylonitic foliationand a stretching lineation subparallelto the Cuiabá fold axis in ductile shearzones• NW-verging bedding and S1 foliationsubparallel thrust faults and shear zones

D2 • NE-trending, oblique-slip, sigmoidalthrust-fault/shear zone system whichenvelops the Cuiabá fold. ReactivationD1 faults/shear zones.• Folding, boudinage and rotation ofthe Cuiabá-BIF.• Quartz remobilization

D3 • Crenulation cleavage• Crenulation lineation• Dilational dark-colour quartz–calcitefilled veins.


3.3. Clastic sediments, volcanoclastics and dikes

The Cuiabá BIF is almost entirely enveloped by a1- to 3-m-thick layer of carbonaceous pelite (Fg). Therock is composed of muscovite, quartz, carbonate,chlorite and carbonaceous matter (whole rock organiccarbon content reaches up to 2 wt.%). Lenses of this rockoccur also intercalated within the altered, upper maficmetavolcanics and within the lower, mafic metavolcanics(Figs. 3 and 4). The carbonaceous metapelite lenses are 1to 2 m thick, and commonly occur within fault zones.

The pelites (X1) occur in the lower, middle and upperunits as lenses in the mafic volcanics or intercalated withmafic volcanoclastics and carbonaceous pelites. In areasof low strain, the bedded nature of the 1- to 5-m-thickmetapelites can be recognized. This is expressed byalternating mm- to cm-thick laminae of distinct miner-alogical composition. The pelites differ from thecarbonaceous pelites (Fg) by a lower content ofcarbonaceous matter and a higher modal compositionof carbonate. Contacts between these lithotypes aregradational, varying from carbon-rich pelites to typicalblack shales (>0.5 wt.% organic carbon).

Metavolcanoclastic rocks (Xs) of dacitic and rhyoliticcomposition form the most abundant rock type in thestudy area. The lithologies occur interlayered with, or aslenses within, metapelites (X1) forming an over 500-m-thick sequence. Rocks comprise a variety of typesranging from non-weathered light gray phyllites toreddish brown weathered types. Phenocrysts of quartzand plagioclase in a fine-grained matrix of sericite,chlorite, quartz and carbonate represent the mainmineral assemblages.

At level 17, drill holes intersected non-deformeddikes (d) which cut the upper sequence of metavolca-noclastics. They are up to 30 m in width and strike NE–SW. Their mineralogical composition is similar to thatof the mafic metavolcanics, and to other metabasalticand metadiabase dikes of the Nova Lima district (Vieira,pers. comm.).

3.4. Structural geology

Based on crosscutting and overprinting relationshipsof folds, foliations, lineations, faults/shear zones andfractures, at least three deformation phases (D1, D2, D3)can be recognized in the Cuiabá Mine area (Table 1).The identification of primary sedimentary structures islargely hampered due to the intense deformation andsubsequent static recrystallization. However, in places,primary structures are preserved as the BIF-banding(manifested by rhythmically alternating layers of chert,

sulfides and quartz–carbonate) and compositionalvariations in other metasedimentary rocks.

The most pervasive deformation phase recorded inthe area is D1, which is responsible for the formation ofthe prominent structures in the study area. The earlieststructures recognized include the Cuiabá fold, apenetrative axial-planar foliation (S1), a myloniticfoliation (Sm1), flexural-slip bedding-parallel foliation(Sf1), northeast-verging thrusts, mineral/stretchinglineations (Lmin1, Lstr1), shear fractures and quartzveins. Rocks were deformed under ductile to ductile–brittle conditions in the greenschist facies under regionalmetamorphic conditions.

The most prominent fold structure related to D1 isthe so-called Cuiabá fold (Figs. 5 and 6). This structureis represented by a kilometer-scale, cone-shapedtubular fold (cf. Skjernaa, 1989). The Cuiabá fold,marked by the contours of the Cuiabá BIF (Fig. 5c),has a hinge line angle ω close to 4°, locallyapproaching zero, and a relatively regular, closed,elliptical shape in cross sections (i.e., yz sections). Thedown-plunge extent of the cone axis (x axis) can betraced for over 3 km, showing progressively shallowerplunges with depth. Whereas at the surface the coneaxis plunges 116/35°, at levels 11 and 17 it plunges

Fig. 5. Characterization of geometrical elements of the Cuiabá fold compared with definitions of sheath and tubular folds. (a) geometrical elements oftubular and sheath folds: x = down-plunge direction axis, y = length axis, z = width axis, ω = hinge line angle; (b) geometrical definitions of theCuiabá fold compared with those of tubular and sheath folds; (c) Isometric diagram of the Cuiabá tubular fold, showing the contours of the CuiabáBIF at different elevations. Definitions after Skjernaa (1989).


116/27° and 116/24°, respectively. Dimensions of theintermediate y axis vary between 650 and 800 m. Thez axis is relatively constant showing a length of about350 m (Fig. 6a, b).

In the mafic metavolcanics, the minerals arealigned as a result of deformation phase D1, definingthe foliation S1. A prominent mineral lineation(Lmin1=126/34), which is expressed by the preferredorientation of sericite, carbonate and sulfides, isdeveloped on S1 and records a SE–NW tectonictransport direction. Northwest-verging, bedding- andS1-foliation, subparallel D1 shear zones were formedin response to local concentration of deformation. In

these high-strain zones, the most conspicuous struc-tures are a mylonitic foliation (Sm1=135/45) and amineral stretching lineation (Lstr1=122/33).

The D1 thrusts/shear zones were reactivated duringphase D2, developing a sigmoidal northeast-trending,thrust-to-reverse, oblique-slip fault system under duc-tile–brittle conditions in greenschist facies (Table 1).Structural elements of D2 are related to shearing, andrepresented by the mylonitic foliation S2, the southeast-plunging (horizontal to 35°) mineral stretching lineationLmin2, slickensides and D2 folds. Reactivation duringD2 is evidenced by (1) local transgressive relationshipbetween structures pertaining to D1 and D2, (2) different

Fig. 6. Cuiabá tubular fold. (a) Plan view projection of the 3, 11 and 17 levels of the Cuiabá BIF on the elevation of the No. 3 level. Dimensions of thegeometric axis x, y and z are presented. (b) Profile AB along the x axis direction (116=S64°E). Note the progressively shallower attitude of the xcone axis with depth.


orientations of slickenlines in the same slickensidesurface, indicating a complex history of surface move-ment with reactivation of planes and (3) development ofstructures under more brittle conditions (e.g., the D2

mesofaults).Refolding, boudinage and rotation of the Cuiabá-BIF

are associated with D2 shearing, and with quartz andsulfides remobilized to low-pressure areas in the form ofboudin necks, and formation of tension and shearfractures. Folds F2 are mostly b-type and no interferencefold pattern is observed, probably due to the different

amplitudes of folds F1 and F2. During phase D2, thetectonic transport direction was subparallel to that ofphase D1.

Phase D3 developed under a ductile–brittle to brittleregime. Structures related to this phase include a crenula-tion cleavage (S3=EW–NS/subvertical) and an NS- andEW-trending crenulation lineation (Lc3), which is parallelto the axes of small crenulations and F3 open folds. Thecrenulation cleavage S3 is the result of EW shorteningparallel to the foliation S1, and developed by microfoldingaccompanied by solution transfer processes (cf. Passchier


and Trouw, 1986). This latter mechanism indicatesconditions below 300 °C (cf. Passchier and Trouw, 1986;greenschist facies).

Fig. 7. Simplified geological maps of the Cuiabá Mine showing the gold orebodorebodies in normal letters. BIF-hosted and shear-related styles of mineralizationUnderlined dotted are shear-related, mafic-hosted quartz vein style mineralizati

The physical expression of foliations S1 and S3 variesbetween rock types, from a well-developed penetrativefoliation in the metavolcanics and metasedimentary

ies, at level 3 and (b) 11. Name of major orebodies in capitals and minor. Underlined names are ductile shear-zone style, mafic-hosted orebodies.on. Geology and structures by Vial (1980), Vieira (1992) and this study.


rocks to a poorly developed, discrete spaced cleavage inthe more competent BIF. Because of extensive deforma-tion and static recrystallization, it is difficult to identifyprimary structures that are preserved only as the bandingof the BIF and compositional variations in the

metasedimentary rocks. The development of dilationstructures such as strain shadows, quartz remobilization,fibrous veins and fringes in all deformation phases wasassociated with circulation of fluids in the rocks.Dilation structures indicate that fluid pressure was


relatively high (e.g., Passchier and Trouw, 1996). Fluidwith dissolved material migrated probably by solutiontransfer and was deposited as remobilization and strainshadows.

The deformation phases D1, D2 and D3 andcorrespond to a time interval during characterized by afamily of structures generated under steady meta-morphic conditions. Mineral assemblages (carbonate,chlorite) indicate conditions compatible with thegreenschist facies. Deformation progresses from ductileto brittle regimes, and the structures were reactivated orreoriented during the three phases. Kinematic indicatorsrecord an E–SE to W–NW tectonic transport directionfor all phases; these encompass the geometry of themovement planes of ductile shear zones, the plunge ofmineral stretching lineations and slickensides, S–C (C-type) fabrics, rotation of porphyroclasts, and strain-shadow porphyroblasts. As the orientation of the stressfield shows a relatively constant direction during thethree phases, they can be considered as representingphases of a single progressive deformation event (En)(Table 1).

4. Gold mineralization

The three main styles of greenstone-hosted goldmineralization identified in the Cuiabá Mine aretypical of other gold deposits in the QF. Thestratabound style dominates in mineralization hostedby BIF. Economic-grade gold mineralization is relatedto six main ore shoots, which are contained within theCuiabá BIF horizon (Fig. 7a, b). These include theFonte Grande Sul, Canta Galo, Balancão, Galinheiro,Galinheiro Extensão and Serrotinho orebodies. Sub-

Plate I. Gold mineralization at Cuiabá Mine

A. Polished slab of non-mineralized Cuiabá BIF. The light quartz–carbondefine the original compositional layering (Fonte Grande Sul orebod

B. Marked banded texture in ores. light quartz–carbonate bands (1) andC. Widespreadly recrystallization of pyrite over light and dark quartz caD. Mineralized Cuiabá-BIF showing isoclinal folding of sulfide mesobaE. Sheared ore showing the boudinage of more competent quartz–carbon

filled by quartz and sulfides (stope of Fonte Grande Sul orebody, levF. Large pyrite porphyroblast with inclusions of gold (Au), pyrrhotite (p

color) (Fonte Grande Sul orebody, level 5).G. Replacement structures in hand specimen. sulfide layers (3) alternate w

(2). Locally, late quartz vein crosscuts the layers (4). Note the replacorebody, level 5).

H. Fully developed euhedral pyrite grain with inclusions of carbonate. N(Canta Galo orebody, level 5).

I. Hand specimen of upper mafic metavolcanics showing sulfidation proJ. Quartz vein-type gold mineralization. Sulfides (with gold inclusions)

(Galinheiro Quartzo orebody, level 5).

ordinate mineralization can be found associated withductile shear zones, hosted by the upper maficmetavolcanics (e.g., Balancão Footwall and GalinheiroFootwall orebodies) and ductile–brittle, shear-related,quartz veins within the lower and upper maficmetavolcanics (e.g., Galinheiro Quartzo and Vianaorebodies). Furthermore, mafic metavolcanics andmetapelites are locally mineralized at sheared contactsadjacent to the Cuiabá BIF.

4.1. BIF-hosted mineralization

The BIF-hosted orebodies containing economic-grade gold mineralization (>4 ppm Au) representsegments of the Cuiabá BIF, which grade laterally intonon-economic, mineralized or barren iron formation(Plate IA). They consist of alternating sulfide andquartz–carbonate/chert layers varying in thickness froma few millimeters up to 1 m (Ribeiro-Rodrigues et al.,1996a,b, 1998). The gold-bearing BIF ranges from darkto light colors, and in places is marked by a well definedbanding (Plate IB). Widespread recrystallization ofsulfides is recognized (Plate IC), lending the ores amassive appearance. Transitions from sulfide-rich topoor BIF are indicated by decreasing sulfide abundancesfrom 30 to 70 vol.% to <1 vol.%, and decreasing goldgrades from over 60 ppm in sulfide-rich portions tovalues below the fire assay detection limit over shortdistances (1 to 3 m) (Fig. 8a). Barren and mineralizedareas are also delineated by faults/shear zones or quartzveins (Fig. 8b, c).

Orebodies show a typical pipe-like geometry.Orebody dimensions extend to over 3 km downplunge, ranging between 1 and 6 m in thickness and

ate bands (1) alternate with dark carbonate–quartz bands (2). The bandsy, level 5).sulfide bands (3) (Serrotinho orebody, level 4).rbonate layers (Balancão orebody, level 5).nds (Fonte Grande Sul orebody, level 5).ate, chert-rich layer. Sulfide is mostly pyrrhotite. Boudin neck areas areel 5).o), arsenopyrite (asp), and gangue minerals (quartz and carbonate, dark

ith light quartz–carbonate layers (1) and dark quartz–carbonate layersement of dark quartz–carbonate layer (1) by sulfides (3) (Canta Galo

ote that the replacement of ankerite takes place along grain boundaries

cesses along discrete shear zones (Balancão Footwall orebody, level 5).occur along the mylonitic foliation and around quartz remobilization

Fig. 8. Sketches of underground cross sections depicting outcrops that illustrate the relationship between mineralized and non-mineralized portions ofthe Cuiabá BIF: (a) Gradation, stope of the Galinheiro Sul orebody, level 6; (b) thrust-fault contact, stope of Fonte Grande orebody, level 4; and (c)limited by quartz vein, stope of the Fonte Grande Sul orebody, level 5.


between 100 to 4000 m2 in area. Average gold gradesreach 18 g/t Au.

4.1.1. BIF-hosted gold orebodiesThe bulk of the gold at Cuiabá comes from six main

orebodies (Fig. 7). The other orebodies have smalldimensions and/or low gold contents, largely sub-economic at present. The majority has been mined outto level 4. Oxidized portions of the Cuiabá BIF wereexploited by Portuguese prospectors in the early 20thCentury, and oxidized ores of the Balancão, GalinheiroExtensão and Surucucu bodies were mined as open-castoperations between 1986 and 1987.

The Fonte Grande Sul orebody, situated in the SEclosure of the Cuiabá-tubular fold, constitutes the richestgold ore of the mine. The orebody shows a fairlyconstant thickness, between 4 and 6 m, but both theorebody area and Au content increase progressivelywith depth. The mineralized BIF is intensively sulfi-dized, folded (Plate ID) and disrupted (Plate IE) by D1

thrust faults containing generally recrystallized andmassive ores that lack banding. Transitions of sulfide-

rich to poor portions of the BIF are gradational, markedby thrust faults or delineated by quartz veins.

The Canta Galo orebody is sited at the NE closure ofthe Cuiabá tubular fold, and truncated by a major D2

thrust fault at the extremity of the inverted limb,terminating against poorly mineralized BIF in itssouthern part. The orebody shows significant goldmineralization at levels 3, 4, 5 and 6. At level 11, thisportion of the Cuiabá BIF is weakly mineralized withAu contents of <0.5 ppm.

The Balancão orebody occurs in the inverted limb ofthe Cuiabá fold, with a stope-length of 200 m at level 3and 500 m at level 11. Strong deformation causeddisruption of the orebody and imparted pinch-and-swell,lenticular and rhomboidal geometries. For miningpurposes, the Balancão body was subdivided intothree main portions: Balancão Leste, Centro and Oeste.

TheGalinheiro orebody also shows significant miningareas (200 to 900 m2), with relatively high Au contents (7to 16 g/t Au) compared to other orebodies. LikeBalancão, the Galinheiro orebody has also been sub-divided into different portions, namely the Galinheiro


Norte and Sul. Average gold grades of the GalinheiroExtensão range from 6 to 11 g/t Au, and mining areasappear to be insignificant at lower depths (<150 m2 atlevel 11). The orebody has a complex structure, beingcharacterized by isoclinal and recumbent D2 folds. TheSerrotinho orebody has a mining area up to 1300 m2 andlow average gold contents, generally close to cut-offgrades. Sulfide-rich portions of this orebody areassociated with local selective replacement of BIFbands adjacent to foliation S3. At level 6, the sulfide-poor areas between the Fonte Grande Sul and Serrotinhoorebodies decrease, and mineralized, sulfide-rich layersjoin to form a single orebody.

4.1.2. BIF-hosted orebodies and the En deformationDetailed structural analysis of the BIF-hosted gold

ores shows that the most characteristic feature of theCuiabá mineralization is that the orebodies are con-cordant to phase D1 linear fabrics; all ore shoots plungeSE, parallel to the stretching direction Lstr1 andintersection lineations L1 (Fig. 9).

D2 shear zones were responsible for the formation ofrecrystallized, massive pyritic orebodies, which occurmainly in the NE limb of the Cuiabá BIF; thisrecrystallization is associated with boudinage geome-tries of the Cuiabá BIF. The spaces created between twoboudins as they move apart were filled by country rocksto BIF, by quartz veins, and locally also by sulfides andgold. Early formed boudins are rotated, giving rise to anen echelon arrangement of the orebodies. Final orebodygeometries are controlled by planes of D2 shear zones.

During phase D3, there was sulfidation of BIFquartz–carbonate layers, adjacent to fracture cleavage

Fig. 9. Relationship between the plunge of the gold orebod

S3. This sulfidation can be symmetric or asymmetricrelative to foliation S3 or occurs around quartz veins.Pressure shadows, extension fractures and crenulationhinges acted as important sinks for material (quartz andcarbonates), which was dissolved during deformation;these sites are, however, barren. Generation of quartzveins occurred during all phases of deformation.

4.1.3. Petrography, mineralogy and mineral chemistryof the gold orebodies

Pyrite is the predominant sulfide mineral (90 to 95vol.%) and occurs parallel to layering and/or to latefoliations, in discrete shear zones crosscutting the ore,and in quartz–carbonate veins. At least two pyritegenerations can be recognized in ore samples. The earlygeneration is represented by fine grained (<300 μm),massive aggregates of euhedral to subhedral cubes andporphyroblasts of pyrite up to 1.5 cm in size (cf.Ramdohr, 1969). The second pyrite generation consistsof coarse-grained porphyroblasts (up to 3 cm) withgrowth rims, which commonly overgrow a quartz–carbonate matrix. Both generations of pyrite porphyro-blasts were formed by the aggregation of numerousgrains. Many crystals are not true cubes, but areelongated or display rounded edges and apexes. Pyritecommonly displays a poikiloblastic texture, containingabundant, randomly oriented inclusions of gangueminerals (quartz and carbonates) and sulfides (pyrrho-tite, arsenopyrite, chalcopyrite, and sphalerite) (PlateIF). Locally, poikilitic porphyroblasts are zoned,displaying outer zones that are free of inclusions. Thistexture can be interpreted as the product of rapid initialgrowth, resulting in numerous inclusions followed by a

ies hosted by BIF and in the D1 phase linear fabric.


slower growth rate and consequently fewer inclusions(Barker, 1994).

Pyrrhotite represents 1 to 5 vol.% of all sulfides, andoccurs as inclusions in pyrite (Plate 1F), filling spacesbetween its grains or still intergrown with them.Individual grains are also observed in the quartz–carbonate matrix. The grains are typically anhedral andvary between 10 and 200 μm in size. Gangue mineralsand pyrite form inclusions in pyrrhotite. A secondgeneration of euhedral, elongated pyrrhotite crystalsoccurs locally, along and adjacent to shear zones thatcrosscut the BIF; in such ores, it is the dominant sulfide(>95 vol.%). Pyrrhotite, together with chlorite, is also acharacteristic component of the late quartz veins.

Arsenopyrite is a minor phase of the ore (1 to 3 vol.%), and forms euhedral to subhedral crystals up to500 μm in size; the mineral is chiefly included in, orintergrown with pyrite. Arsenopyrite can have inclu-sions of quartz, Fe-carbonates, chalcopyrite and pyrite.Chalcopyrite is a significant minor sulfide mineral insome orebodies (e.g., Fonte Grande, up to 3 vol.%),occurring mainly as inclusions in recrystallized pyrite.The crystals are anhedral, 10 to 500 μm in size, andcontain inclusions of quartz, Fe-carbonate and gold.Sphalerite and galena are minor phases (<1 vol.%) inthe sulfide layers. Both form tiny (<3 μm), anhedralinclusions in pyrite, accompanied by pyrrhotite, chalco-pyrite, arsenopyrite and minor gold. In places, theyoccur also in fractures in pyrite. Sphalerite is alsoobserved in the matrix or in pressure shadows of pyriteporphyroblasts, together with siderite and quartz. Thepresence of gersdorffite as free grains up to 40 μm andstibnite as small inclusions (2 μm) in pyrite werereported during mineralogical analysis of the Cuiabá oreconcentrate (Mintek, 1980).

Fig. 10. Geometry and sites of gold particles at the CuiabáMine. (a) Histogramsizes measured from grain diameters on polished sections.

Native gold is associated with sulfides. It typicallyforms discrete particles enclosed in pyrite together withother sulfides and gangue minerals (Plate 1F and Fig.10a). Gold inclusions are also observed in pyrrhotite andchalcopyrite; inclusions in arsenopyrite are reported byVial (1988). In addition, native gold may occur alonggrain boundaries or filling fractures in pyrite, as well asfree grains in the quartz–carbonate matrix of recrys-tallized ores. Dimensions of visible gold grains reach upto 60 μm in diameter, with an arithmetic mean of 14 μm(Fig. 10b). Nearly 56% of observed particles are<10 μm in diameter, indicating that the geometricmean (8 μm) better reflects average grain dimensions.Visible gold is preferentially associated with As-richpyrite crystals of both generations. Although higherconcentrations and larger size of gold grains tend to beassociated with recrystallized, coarse-grained pyrite,there is no strong relationship between pyrite morphol-ogy and/or size and gold grade. Both fine-grained andcoarse-grained pyritic ores show economic gold values.

Gangue minerals are dominantly carbonates andquartz (20 to 30 and 10 to 15 vol.%, respectively).Together with pyrrhotite, arsenopyrite and chalcopyrite,they are the most common inclusions in pyrite.

Identification of carbonate by X-ray diffraction andEPMA revealed that the bulk of the carbonate is siderite,ferroan dolomite and ankerite. Siderite is the mostabundant and occurs as fine-grained aggregate betweensulfide grains. Ankerite is generally intergrown withsiderite or forms poikilitic, recrystallized blasts withquartz inclusions. Chemical compositions range fromtypical ankerite to ferroan dolomite. Calcite is a minorphase in the ore, occurring as fine grains (<20 μm) incontact with siderite. In quartz–carbonate veins,secondary, coarse-grained (500 μm) calcite is the

of mode of occurrence of gold particles. (b) Histogram of gold particle


major component. A catholuminescence study con-firmed the presence of these two calcite types.

Quartz is the main mineral of stringers and veins,which are fairly common in all ores but also occur inthe matrix of sulfide layers. As in the quartz–carbonateand chert layers, quartz grains in sulfide layers show apolygonal arrangement with 120° triple junctions andlack of undulose extinction. Monazite was observed inone sample from a shear zone as a free grain about10 μm in size. Trace amounts of plagioclase (albite) andrutile were identified by X-ray diffraction. Scheelite(<3 μm), ilmenite and leucoxene are reported as grainsin the matrix from sulfide layers (Mintek, 1980;Ladeira, 1991).

4.1.4. Features of genetic significance

4.1.4.1. Replacement structures. A characteristic fea-ture commonly associated with the mineralization is thepervasive replacement of quartz–carbonate layers byFe-sulfides. This replacement is observed at all scales(Plate IG, H), and has imparted a pseudo-stratiformcharacter to the Cuiabá BIF. Hence, the alternation ofsulfide and quartz–carbonate layers does not represent aprimary texture, but formed during the extensive,pervasive or, in places, selective sulfidation of carbonatelayers. This gives the impression of a laterallycontinuous stratigraphic iron-formation grading fromsulfide into carbonate facies. Mineralogically, sulfida-tion involves the replacement of siderite and ankerite bypyrite, and appears to have taken place along grainboundaries. Following this pervasive replacement,sulfide layers show anhedral to euhedral pyrite grainscontaining inclusions of gold. Matrix minerals arepredominantly carbonate and minor quartz.

4.1.4.2. Remobilization of quartz. At least threegenerations of quartz veins can be recognized in theCuiabá BIF. The most abundant quartz vein type iscomposed of minor pyrrhotite, chlorite and carbonate,and occurs in the following three settings: (1) fillingneck areas of boudins in the Cuiabá BIF; (2) betweenboudinaged orebodies and country rocks; and (3)parallel to D2 shear zones, suggesting that they areassociated with D2 shearing. Their geometries areextremely variable and dimensions range from thecentimeter to meter scale. Where quartz veining wasincipient, banding of the BIF can still be recognized.Locally, veins are gold bearing, with pyrite envelopingthe vein margins.

Early generation quartz veins are milky white incolor, medium-to-coarse-grained and define the limits of

non-mineralized portions of the Cuiabá BIF and goldorebodies (Fig. 8c). They crosscut the BIF banding, aresteeply dipping, have strike lengths exceeding tens ofmeters and are 2 to 3 m wide. Another vein type iscommonly up to 50 cm thick and also entirely made upof barren, milky, coarse-grained quartz. These occuralong D1 shear zones or fill shear fractures, beingdeformed into pinch-and-swell boudins and/or arethemselves isoclinally folded.

4.1.4.3. Shearing structures and static recrystalliza-tion. “Durchbewegung” structures and boudinagesare important textures that result from the deformationof the BIF-hosted ore. “Durchbewegt” ore (e.g.,Marshall and Gilligan, 1989) is a result of intense,progressive shear strain in shear zones crosscuting theore. The “Durchbewegung” process involves disrup-tion, separation, milling and rotational movement ofthe incompetent carbonate–silicate matrix within thecompetent pyritic ore. Hand specimen-scale oreboudinages result from layer-parallel or layer-oblique(small angle) extension associated with shearing. Themore competent chert layers are stretched anddisrupted, forming rhomboidal boudins. Boudin necksare commonly filled with quartz and pyrrhotite. Brittledeformation features of pyrite are widespread and,locally, fractures are filled by galena, sphalerite andgold. In discrete D1 shear zones, pyrite is broken intofine-grained (<5 μm) crystals. Shearing has imparted amassive-sulfide-like aspect to these zones, and wasresponsible for pyrite milling. The development ofasymmetrical, δ-type, face-controlled pressure shadowsof quartz and carbonate around pyrite porphyroblasts isalso common.

4.1.4.4. Nucleation and growth of pyrite porphyro-blasts. The common development of euhedral pyriteporphyroblasts is a striking feature of the BIF-hostedore. Ore textures indicate that the first pyrite generationvaries from fine- to medium-grained. As proposed byRamdohr (1969) and Barker (1994), pyrite porphyro-blast nucleation and growth seem to be controlled by thepresence of suitable nucleation sites, which contain highenergy grain boundaries or previously strained crystals.At Cuiabá, such an explanation is supported since theabundance of pyrite crystals varies inversely with thegrain size of pyrite. Crystal growth commonly involvesthe consumption or inclusion of grains, eliminatingmany potential nucleation sites. Inert phases notinvolved in the porphyroblast growth, such as quartz,arsenopyrite, pyrrhotite and non-reacted carbonates, areenclosed by porphyroblasts as passive inclusions.


Skeletal shape is also commonly developed due to thedifficulty in replacing matrix quartz.

Syntectonic D1 pyrite porphyroblasts are quitecommon, probably due to the catalyzing effect ofdeformation on mineral nucleation and diffusion rates.Fine-grained pyrite crystals and sphalerite are com-monly adsorbed to the surface of porphyroblasts.Pyrite growth probably occurred continuously duringthe En event, with the most rapid growth at peakmetamorphic temperatures. The presence of polygonalquartz inclusions in the second generation pyrite,combined with foliation-porphyroblast relationships,indicate that some late porphyroblasts grew also afterdeformation and static recrystallization of the orematrix. In contrast to other deposits with pyriteporphyroblasts (e.g., Cherokee Mine; Brooker et al.,1987), no obvious relation is observed with respect tothe concentration of large porphyroblasts in areas ofrelatively low stress.

4.2. Shear-zone, mafic-hosted mineralization

The shear-zone mineralization style consists ofdisseminated sulfides occurring in zones of intenseshearing within the mafic metavolcanic and metasedi-mentary rocks (Plate II). Shear zones are ductile orductile–brittle, mostly subparallel to bedding, anddisplay an anastomozing pattern. Apart from thecommon occurrence of this type of gold mineralizationclose to the mineralized Cuiabá BIF, small economic-grade gold orebodies are represented by the BalancãoFootwall and Galinheiro Footwall orebodies (Fig. 7).Mineralization is hosted by the upper mafic metavolca-nics, with gold being associated with sulfides, whichoccur along discrete (0.2 to 1 mm) shear zones togetherwith sericite or around quartz stringers. Whereas thesheared portions of these rocks are sulfidized and,therefore, mineralized, weakly sulfidized areas displaylower gold contents. As in the BIF-hosted mineraliza-tion, pyrite is the most abundant sulfide (>80 vol.%).Pyrrhotite (up to 20 vol.%), sphalerite (5 vol.%) andchalcopyrite (3.5 vol.%) occur intergrown with pyrite;arsenopyrite and galena are minor. The composition ofthe major sulfides is similar to their counterparts in theBIF-hosted gold mineralization. The maximum Ascontent in pyrite (1.18 wt.%) occurs in grains thatcontain gold inclusions. Gangue minerals includecarbonates (Mg-rich ankerite), chlorite, plagioclaseand sericite. A few gold grains are present in fractureswithin pyrite. Gold contents in highly sulfidized areasreach 20 to 30 ppm, and average grades vary between 4and 6 ppm.

Shear-related quartz vein-type deposits are asso-ciated with quartz and/or carbonate veins withinmetavolcanic rocks (Plate IJ). Economic-grade miner-alization is represented by the Viana and GalinheiroQuartzo orebodies. The presently inaccessible Vianaorebody constitutes the former Viana Mine, which wasdiscovered in 1899 and was exploited until the late1940's. Past production amounts to over 1 t of goldand grades varied from 4 to 20 g/t Au (Mathias, 1964).The orebody consists of two parallel major quartz veinscontained within a NE-trending, 40–50° SE-dippingsigmoidal shear zone (Fig. 7). At a mine-scale, majorvein structures are composed of several interconnectedquartz veins with alteration haloes and disseminatedsulfides at the contact with wallrocks. Averagethickness of the ore varies from 0.3 to 0.6 m, showinga maximum of 1.8 m in folded areas (Vieira, 1991a).The Galinheiro Quartzo orebody was mined out tolevel 4, and has similar characteristics to Viana.Mineralization is associated with milky and smokyquartz veins related to shear zones within the uppermafic metavolcanics. Higher gold grades reach up to10 ppm; average contents vary from 2 to 4 g/t Au.Gold occurs as free grains finer than 12 μm in quartzfractures, or included within the sulfides that occuraround the veins. Sulfides are represented by pyritewith minor amounts of chalcopyrite, pyrrhotite andarsenopyrite. At the surface, the weathered ore containsalso goethite and limonite, the latter with goldinclusions (Vieira, 1988).

Field observations indicate that both the ductileshear-zone and vein-type mineralizations are end-members of a continuous spectrum and tend to betransitional to one another. This is supported by theirbroadly comparable mineralogical, structural and che-mical features. However, the lode–gold (quartz-veintype) of mineralization is largely restricted to shallowerlevels in all Archean gold mines in the QuadriláteroFerrifero (and indeed elsewhere; Robert and Poulsen,2001) and is associated with ductile–brittle shearingwith intense remobilization of quartz into dilation zones,due to the higher crustal level. Despite this, both types ofmineralization have a ‘stratabound’ character, in thatthey are confined chiefly to a single host lithology, i.e.,the upper metavolcanics.

4.3. Gold concentration and chemistry

The maximum gold content of typical ore sulfidemesobands is 23 ppm, but channel sample mapsprovided by Mineração Morro Velho show gold gradesup to 60 ppm. In places, gold concentration exceeds


100 ppm. The Ag content is <5 ppm, which is inaccordance with production data of 1 to 3 g/t Ag.Arsenic contents show a maximum of 0.74 wt.%, withan average concentration of 0.23 wt.%. The overallbase-metal content of the ore is low, with values rarelyabove 100 ppm; average abundances are 90 ppm Cu,65 ppm Zn and 42 ppm Pb. Maximum values (310 ppmCu, 102 ppm Zn, 148 ppm Pb) are from samples of theBalancão and Canta Galo orebodies. The meanabundances of Co, Ni, Sb, W are 25 ppm, 26 ppm,9 ppm and 5 ppm, respectively. Maximum values of anyof these elements do not exceed 45 ppm.

Gold grains associated with pyrite from differentorebodies were investigated by EPMA in order tocharacterize the Au–Ag ratios and the content of traceelements. Elements analyzed included Au, Ag, Cu, Fe,Te, Sb, Bi, Hg, Co and Ni. The average compositionin individual samples is grouped into five distinctclasses: gold grains of BIF-hosted ore occurring (1) asinclusions, (2) in fractures and between grainboundaries of pyrite, (3) as free gold in the quartz–carbonate matrix, (4) associated with pyrite of themafic horizon within the Cuiabá BIF (occurring asinclusions, in fractures and between grain boundaries),and (5) included in pyrite of the mafic metavolcaniccountry rocks.

Single gold grains are chemically homogeneous,with no core-rim Ag variations. All grains containmore than 6 wt.% of other metals. Silver contentsvary from 4 to 24 wt.%, averaging 14 wt.%; there isno apparent relation between a distinct orebody and

Fig. 11. Average and range of gold fineness of selected Archean BIF-hosted dVelho, Pari) andWestern Australia (Mt. Morgans). Data from Vielreicher et al.and Pari Mine by A. Germann, RWTH Aachen, Germany. n=number of ana

Au–Ag ratios. Distinct differences in Ag content are,however, observed according to the location of thegold grain and host rock. Gold inclusions in BIF-hosted ore show slightly lower Ag contents than thoseof gold occurring in the sheared mafic horizon (meanof 17.8 to 22.4 wt.% Ag). Electrum (i.e., Ag >20 wt.%) is detected only in samples of the sheared mafichorizon within the Cuiabá BIF, and the lowest Agcontents are observed in gold inclusions in recrys-tallized pyrite from BIF-hosted ore. Gold grainslocated along pyrite grain boundaries and free goldgrains of the BIF-hosted ore show consistent Agvalues in the range 15 to 16.5 wt.% and 12.5 wt.%,respectively. These values are within the field of goldgrains from the mafic horizon, suggesting formationunder similar chemical conditions. The three observedgold grains in the mafic country rocks showinconsistent values: two grains contain 15 wt.% Agand one grain only 5 wt.% Ag.

Fig. 11 exhibits the range and average of goldfineness [Au / (Au+Ag)×1000] in the Cuiabá Mine incomparison to other BIF-hosted gold deposits in the QF(e.g., São Bento and Pari) and in Western Australia (Mt.Morgans). Average values are in the characteristic rangefor Archean BIF-hosted deposits (average < .900).

5. Controls on gold mineralization

Gold mineralization at Cuiabá is marked by local andregional controls, which can assist mining operationsand further exploration in the mine area. The controls

eposits of the Quadrilátero Ferrífero, Brazil (Cuiabá, São Bento, Morro(1994), Abreu (1995) and this study. EPMA data from São Bento Minelyses.


can be grouped into three main types: lithological,mineralogical and structural.

5.1. Lithological and mineralogical controls

The stratabound mineralization at Cuiabá is char-acterized by confinement of sulfides and gold toparticular lithologies. Gold mineralization is associatedmainly with chemically favorable host rocks, i.e., Fe-rich units. Economic-grade gold mineralization ishosted by carbonate-facies BIF and subordinate miner-alization occurs in the mafic metavolcanics. The Fe-richnature of these rocks favored sulfidation and associatedgold precipitation. Within the Cuiabá BIF, sulfidationwas influenced by the amount of Fe-carbonate in thedifferent bands. The Fe-richer, dark carbonate layers arereplaced preferentially by sulfides relatively to the lightcarbonate and chert layers.

The bulk of the BIF-hosted gold mineralization isassociated with sulfide-rich portions of the BIF (Fig.12). As a consequence, gold grade is directly related tothe modal abundance of sulfides. There is a positivelinear correlation between sulfur content and gold gradeup to 10 ppm Au (Ribeiro-Rodrigues, 1998); higher

Fig. 12. Gold sulfide association at the Cuiabá Mine. The higher gold gradessulfidized BIF exihibt lower gold contents. Balancão orebody, level 11. Mod

gold grades occur in portions with over 40% of pyrite.Weakly or non-sulfidized portions of the Cuiabá BIF arebarren or show low Au contents (Fig. 13).

Gold is fine grained (up to 60 μm), and is alwaysassociated with pyrite, the most common sulfidemineral, occurring generally as inclusions in thatmineral. Gold-bearing pyrite grains are enriched in Asrelative to Au-free grains sulfides (Lobato et al., 2001a).Once precipitated, pyrite became also an active site forprecipitation of gold, as evidenced by the presence ofgold in fractures and along grain boundaries of pyrite.The strong association of gold with pyrite is a functionof surface characteristics (e.g., Mycroft et al., 1995;Maddox et al., 1998; Scaini et al., 1998). Huston andLarge (1989) suggest that decrease in H2S activity in themineralizing fluid is a possible cause of gold precipita-tion. As pyrite crystallizes, the hydrothermal fluidbecomes depleted in H2S, and causes gold to precipitateand to become included in the growing pyrite. Meyer etal. (1994) have shown evidence for electrochemicalprecipitation of gold, which is preferentially depositedon chemically heterogeneous pyrite surfaces. Combina-tions of alternating high- and low-As zones in pyriteresult in mixed np-type conductivity accompanied by

are associated with portions of high sulfide modal abundances. Weaklyified after Mineração Morro Velho S.A.

Fig. 13. Weakly sulfidized portion of the Cuiabá BIF showing non-economic gold grades. Canta Galo orebody, level 11. Modified after MineraçãoMorro Velho S.A.


electron exchange and the reduction-driven depositionof gold at the cathode.

Additional controls on gold deposition are physicaldefects (e.g., fractures), which create zones of increasedcharge density and/or areas of focused conductivity.

5.2. Structural control

As observed in other greenstone-hosted gold depositsof the Quadrilátero Ferrífero, the Cuiabá orebodiesdisplay a strong structural control. The most prominentfeature of the ore shoots is their consistent down-plungecontinuity (Figs. 5 and 6). Down-plunge dimensions aresignificantly greater than both strike and wide dimen-sions. The plunge of the orebodies is parallel to both thestretching lineation and to the intersection lineationbetween bedding (S0) and the foliation S1, correspond-ing to the regional tectonic transport of the En event.Although the orebodies are contained within the CuiabáBIF horizon, they are oblique to the down dip of the BIF.

On a more local scale, structural control is evidencedby the close association between the intensity ofmineralization and shearing. The development of pyrite-rich zones in the BIF, and associated gold mineralization,corresponds to zones of intense D1 and D2 shearing.Sulfide and gold remobilization also occur in fold hingezones and, furthermore, the ore shoots are locally locatedat intersections between faults/shear zones and the BIF.

Contrasts in competence between distinct lithologiesalso assert a significant control on deformation and golddeposition. Differences in rheological properties havecontrolled the emplacement of faults/shear zonesbetween lithological contacts of the Cuiabá BIF andcountry rocks. These portions are commonly sulfidizedand show economic gold grades. In addition, Fe-poorlithologies, such as the metapelites (X1) and carbonac-eous metapelites, are mineralized in discrete shear zonesat the contact with the BIF.

Deformation of the Cuiabá BIF was also controlledby primary differences in composition. The cherty, less

Table 2Mineralogy of the Cuiabá BIF before and after sulfidation

Cuiabá-BIF

BIF Layers Pre-sulfidation mineralogy Post-sulfidation mineralogy(1) Dark carbonate–quartz layers Siderite, ankerite, Quartz, carbonaceous matter. Sulfides; quartz, ankerite, siderite(2) Light quartz–carbonate layers Quartz, ankerite, calcite (muscovite, chlorite) Sulfides, quartz, ankerite, calcite.(3) Chert layers Quartz, minor carbonates (siderite and ankerite) Relatively unaltered (quartz and carbonates)


laminated, upper portion of the Cuiabá BIF horizon isweakly mineralized due to both Fe-poor characteristicsand the absence of primary discontinuities. In addition,shearing during deformation was concentrated in the lesscompetent, carbonaceous metapelites, which envelop theCuiabá BIF. This protected the more competent ironformation from pinch-and-swell and boudinage.

6. Gold mineralization and wallrock alteration

Investigation of the hydrothermal alteration style at agiven mineral deposit has both practical value forexploration purposes and for the understanding of thegenesis and nature of the mineralizing fluids. Since thealteration assemblages are the result of fluid interactionwith the original rocks, the evaluation of these assem-blages, combined with mass balance calculations, hasbeen utilized by several workers to reconstruct fluidcompositions (e.g., Neall, 1987; Neall and Phillips, 1987).

6.1. Alteration of the Cuiabá BIF

The mineral assemblages in the least-altered CuiabáBIF consist of: (1) siderite, ankerite and quartz in thedark carbonate–quartz layers; (2) ankerite, calcite andquartz in the light quartz–carbonate layers; and (3)quartz with minor Fe-carbonates in chert layers (Table2). Carbonaceous matter (up to 0.7 wt.% carbon in bulkrock analyses) occurs in the form of very fine-grainedstringers, resulting in the dark gray layers. With respectto the BIF ore, replacement textures indicate that theiralteration is dominated by sulfidation. The sulfidelayers are the result of sulfidation involving replace-

Table 3Typical mineral assemblages and possible metamorphic reactions involved i

Alteration Mineral assemblage

Least altered mafic metavolcanic chl–pl–ep/czo–qtz–actChloritization chl–ep/czo–ab–qtz–act–cal–Carbonatization cal–chl–qtz–pl–sulfSericitization chl–ser–calc–qtz–sulfSulfidation sulf–ser–cal–qtz–chl

ab = albite, act = actinolite, cal = calcite, chl = chlorite, ep = epidote, hbl = hosulf = sulfides (pyrite+pyrrhotite±chalcopyrite±sphalerite±galena).

ment of Fe-carbonates (siderite and ankerite) bysulfides. The least-altered BIF is separated from itsgold-bearing sulfidized portions by ankerite-rich altera-tion haloes (Lobato et al., 2001b). The typical, proximalalteration assemblage is pyrite+carbonate+pyrrhotite±arsenopyrite±chalcopyrite±sphalerite±galena.

The close association of iron formation, presence ofsulfides and economic Au grades support the principlethat BIF sulfidation is an efficient mechanism for goldprecipitation at Cuiabá. The strong gold–pyrite correla-tion indicates that fluid-BIF reactions that resulted indeposition of pyrite and gold must have been similar.Considering the nature of wallrock alteration, whichinvolved addition of sulfur and gold, it is most likelythat gold was transported as reduced sulfur complexes(e.g., Phillips and Groves, 1984). Typical reactionsduring sulfidation at Cuiabá probably included thefollowing (e.g., Colvine et al., 1988):

AuðHSÞ−2 þ FeCO3siderite

f FeS2pyrite

þAuþ H2CO3

AuðHSÞ−2 þ CaMgFeðCO3Þ3ankerite

f FeS2pyrite

þCaMgðCO3Þ2dolomite

þAuþ H2CO3

6.2. Alteration of mafic metavolcanics

From the Cuiabá BIF outwards, a zonal patternincluding sericite, carbonate and chlorite zones wasestablished earlier by Vieira (1988, 1992). The foliatedlower mafic metavolcanics (manx), in the core of the

n alteration of the upper mafic metavolcanics

Typical reaction

pl+hbl+qtz+Or+H2O⇔ep/czo+act+absulf ep/czo+act/tr+CO2+H2O⇔chl+qtz+cal+ser+ab

chl+cal+CO2⇔chl+ank/calc+qtz+H2Ochl+ank/cal+CO2+K2O ⇔ ser+ank/calc+qtzchl+H2S+O2⇔py+qtz+H2O

rnblende, Or = Orthoclase, pl = plagioclase, qtz = quartz, ser = sericite,

Table 4Chemical variation of major elements and selected trace elementsduring sulfidation of mafic volcanic rocks

Major elements

Sample A B Mass change

G 2.95 3.02

(wt. %) (wt. %) (%) g/100g

SiO2 50.20 39.20 −20.1 −10.07TiO2 1.89 1.55 −16.1 −0.30Al2O3 12.60 12.13 −1.4 −0.18Fe2O3 16.01 14.96 −4.3 −0.69Cr2O3 0.02 0.02 −0.6 0,00MnO 0.19 0.15 −19.5 −0.04MgO 4.85 3.33 −29.7 −1.44CaO 8.26 7.04 −12.8 −1.06Na2O 2.81 3.31 20.7 0.58K2O 0.20 1.75 813.2 1.59P2O5 0.26 0.07 −73.3 −0.19V2O5 0.09 0.04 −17.4 −0,01CO2 0.09 10.75 12127.9 10.92Corganic 3.00 10137.3 3.04S 0.31 8.92 2845.7 8.82

Trace elements

Sample A B Mass change

G 2.95 3.02

(ppm) (ppm) (%) g/100g

Au 0,01 38 775886.4 38.79Cr 120 110 −6.2 −7.39Co 49 81 69.2 33.92Ni 55 830 1444.9 794.69Cu 75 83 13.3 9.97Zn 118 238 106.5 125.65Sr 171 48 −71.3 −121.86Y 33 28 −13.1 −4.34Zr 136 171 28.7 39.06Sb 1.6 11 603.8 9.66Ba 305 657 120.5 367.59La⁎ 11.4 10 −9.3 −1.06Ce⁎ 33 28 −13.1 −4.34Nd⁎ 23 9 −59.9 −13.79Sm⁎ 4.3 3 −23.8 −1.02

Negative values indicate losses, positive values indicate gains.Sample A = least altered mafic volcanic (regional metamorphism),Viana Orebody No. 3 Level.Sample B = altered mafic volcanic (shear zone-hosted ore). Sulfide-rich carbonate–sericite–quartz schist (X2) Balancão Foootwall Ore-body, No. 6 Level.G = specific gravity.⁎Analysed by instrumental neutron activation analyses (INAA).


folded Cuiabá BIF, and the carbonate–sericite–quartzschist (X2), which occupies the core and outer parts,show an extensive aerial distribution and are shown inthe geological map of the Cuiabá Mine, level 3 (Fig. 3).

In order to characterize this alteration pattern, well-exposed profiles, from the sheared contact of the CuiabáBIF into the country rocks, were investigated in detail. Theinvestigations suggest the occurrence of three hydrother-mal alteration zones in the mafic metavolcanics, inagreement with Vieira's (1992) proposition. In places,sulfidation of the mafic volcanics is associated with thesericite zone. Whereas the sericite zone is easily recogniz-able, the carbonate and chlorite zones are less readilyrecognized. The alteration zoning is characterized bychanges in mineral abundance and in mineral assemblagesrelative to the least-altered mafic metavolcanics.

Alteration was contemporaneous with progressivedeformation during event En, and is spatially related tostructural features, more specifically to the proximity toD1

faults and/or shear zones. The thickness of alteration zonesdiffers from area to area, and ranges from 0.1 to 2 m in thesulfide zone, 1 to 15m in the sericite zone, 1 to 20m in thecarbonate zone and from 15 to 100 m in the chlorite zone.

Major lithological variations correspond broadly tochanges in the modal abundances of chlorite, carbonate,sericite, quartz, zoisite/clinozoisite and sulfides, reflect-ing, for the most part, CO2-consuming, i.e., carbonatiza-tion reactions. Typical reactions and modal abundanceschanges involved in fluid-wall rock alteration arepresented in Table 3. The mineral assemblage of theleast altered mafic volcanic rocks includes actinolite,albite, chlorite, epidote and quartz. The chlorite zone(distal zone) is characterized by an increase in modalchlorite and weak carbonatization of the least-alteredrock; its mineral assemblage comprises chlorite, epidote,quartz, actinolite, calcite, and albite. Chlorite and calcitewere possibly formed by the reaction of actinolite–epidote with a CO2–H2O fluid phase. The carbonate zoneis marked by the increase of carbonate (up to 30 wt.%)relative to the chlorite zone; epidote/clinozoisite iscompletely consumed and the mineral assemblageincludes calcite, chlorite, quartz, plagioclase and sulfides.The presence of ankerite is observed in other investigatedcross sections; this carbonate phase probably formed fromchlorite–calcite reacting with CO2. The typical paragen-esis of the sericite zone includes sericite, chlorite, calcite,quartz and sulfide. This zone is characterized by K2Oaddition, causing change from a chlorite–calcite-bearingmetavolcanic rock to a sericite-bearing schist.

The sulfidation, gold-rich zones of the metavolcanicsare characterized by an increase in the pyrite content(up to 20 wt.%) compared to the sericite zone.

Pyritization in the mafic metavolcanics is confined tosheared areas, specifically to discrete, millimeter-scaleductile shears. Pyrite is accompanied by sericite,chlorite, calcite, quartz, possibly minor K-feldspar andnative gold. Gold deposition possibly occurred due to


coupled reactions of the following type (cf. Phillips andGroves, 1984):

FeOðmineralÞchlorite

þ2H2S0f FeS2

pyriteþH2Oþ H2ðgÞ

AuðHSÞ−2þ þ Hþ þ 1=2H2ðgÞ⇔Au þ 2H2S0

Pyrite formed by the sulfidation of ferromagnesianphases (e.g., chlorite) present in the mafic volcanics.

Fig. 14. Genetic model for the Cuiabá Mine. (a) Development of the volcano-the chemically favorable Cuiabá BIF horizon and adjacent country rocks duehydrothermal fissure faults during the En event.

Similar to sulfidation of the BIF, the association of goldand sulfides indicates gold precipitation due to destabi-lization of reduced sulfur complexes.

Chemical changes during sulfidation were investi-gated by comparing ore samples with the least-alteredmafic metavolcanics. The least-altered sample waschosen using the following criteria: low sulfur contents,relative distance to shear zone/mineralized areas, andabsence of stringers or quartz–carbonate veins. Gains

sedimentary sequence of the Nova Lima Group. (b) (c) Replacement ofto pervasive and selective sulfidation by fluids introduced adjacent to


and losses of components are calculated assuming thatthe volume change is a factor common to allcomponents. For constant volume, sulfidation is chemi-cally expressed by addition of Au, S, CO2, K2O,Corganic, Sb, As and Ni (Table 4).

7. Discussion and conclusions

The exposed Late Archean crust (3.0 to 2.5 Ga) hasbeen long recognized as an extremely favorable terranefor hosting orogenic gold deposits (Groves and Ho,1990; Groves and Foster, 1991; Groves et al., 1998,Goldfarb et al., 2001). Late Archean gold ores areespecially well studied from the many world-classdeposits of the Yilgarn craton in Western Australia andthe Superior Province of central Canada. The goldmineralization in this context was associated withcollisional processes that resemble those characterizingCordilleran-type tectonics (Barley et al., 1989). Post-collisional reactivation of early formed shear zones mayhave been critical for the migration of gold vein-formingfluids. Gold veining in many greenstone belts occurredtoward the final 50 m.y.-long period of deformation andmagmatism.

With regards to the Quadrilátero Ferrífero, the Riodas Velhas orogeny is the best characterized deforma-tional event in which there was reworking of Archeansialic crust, intrusion of tonalitic bodies and felsicvolcanism, having taken place in the 2.78 to 2.70 Gatime interval (Carneiro, 1994). Although the mainmineralization event for the gold deposits has not beendated unequivocally, the results presented by Noce et al.(2007-this volume) point to an age of ca. 2.70 Ga.

Three styles of gold mineralization can be identifiedin the Cuiabá mine: (1) Economic-grade gold miner-alization is stratabound, BIF hosted and is associatedwith sulfide-rich 0BIF layers; (2) a subordinate type ofmineralization is stratabound, mafic-or sediment-hostedand is associated with ductile shear zones, or (3) shear-related quartz veins.

The gold mineralization at Cuiabá has features andcharacteristics that are in agreement with the epigeneticorogenic gold deposit model presented for Archeangold–lode deposits (e.g., Goldfarb et al., 2001). Thesecan be summarized as (Fig. 14): (i) mineralizationassociated with crustal compression; (ii) the lithologicalcontrol, i.e., gold mineralization is confined to Fe-richhost lithologies; (iii) strong structural control of theorebodies, showing remarkable down-plunge continuity(>3 km) relative to strike length and width (up to 20 m);(iv) the epigenetic nature of the mineralization, withsulfidation as the major process of wall–rock alteration

and directly associated with gold deposition; and (v) thegeochemical signature, i.e., mineralization shows con-sistent metal associations (Au–Ag–As–Sb–W and lowbase metals) which are compatible with epigenetic fluids(e.g., Fyfe and Kerrich, 1984; Groves et al., 1987, 1998;Goldfarb et al., 2001).

References

Abreu, G.C. 1995. Geologia e metalogênese do ouro da Mina do Pari,NE do Quadrilátero Ferrífero, MG. Ph.D. thesis, Universidade deSão Paulo, São Paulo, 120 pp.

Babinski, M., Chemale Jr., F., Van Schmus, W.R., 1991. Geocrono-logia Pb/Pb em rochas carbonáticas do Supergrupo Minas,Quadrilátero Ferrífero, Minas Gerais, Brasil. 3. CongressoBrasileiro de Geoquímica, São Paulo. Extended Abstract, vol. 2,pp. 628–631.

Balthasar, O.F., Zuchetti, M., 2007. Lithofacies associations and structuralevolution of the Archean Rio das Velhas greenstone belt, QuadriláteroFerrífero, Brazil: A reviewof the setting of gold deposits. OreGeologyReviews 32, 471–499 (this volume) , doi:10.1016/j.oregeorev.2005.03.021.

Barbosa, A.L.M., 1961. Tectônica do Quadrilátero Ferrífero de MinasGerais. Semana de estudos geológicos e econômicos do Quad-rilátero Ferrífero, Ouro Preto, 1961. Sociedade de IntercâmbioCultural e Estudos Geológicos, vol. 1, pp. 49–52.

Barbosa, O., 1949. Contribuição à geologia do centro de Minas Gerais.Mineração e Metalurgia 14 (79), 3–19.

Barbosa, O., 1954. Evolution du geosynclinal Espinhaço. 19thInternational Geological Congress, Alger. Extended Abstract,vol. 14, pp. 17–36.

Barker, A.J., 1994. Introduction to Metamorphic Textures andMicrostructures. Blackie Acamedic & Professional, London.162 pp.

Barley, M.E., Eisenlohr, B.N., Groves, D.I., Perring, C.S., Vearn-combe, J.R., 1989. Late Archean convergent margin tectonics andgold mineralization—a new look at the Norseman-Wiluna belt,Western Australia. Geology 17, 826–829.

Brandt, R.T., Gross, G.A., Gruss, H., Semenenko, N.P., Dorr, J.V.N.,1972. Problems of nomenclature for banded ferroginous–chertysedimentary rocks and their metamorphic equivalents. EconomicGeology 67, 682–684.

Brooker, D.D., Craig, J.R., Rimstidt, J.D., 1987. Ore metamorphismand pyrite porphyroblast development at the Cherokee Mine,Ducktown, Tennesse. Economic Geology 82, 72–86.

Carneiro, M.A., 1990. Enxames de dique máficos do ComplexoBonfim, borda setentrional do Quadrilátero Ferrífero. 2. Workshopde diques máficos, São Paulo, 1987. Extended Abstract Volume,pp. 32–37.

Carneiro, M.A., 1994. Geocronologia e Geoquímica do ComplexoBonfim. Unpublished Ph.D. thesis, Universidade de São Paulo,350 pp.

Carneiro, M.A., Teixeira, W., Machado, N., 1994. Geologicalevolution of a sialic fragment from the Quadrilátero Ferríferoin eastern-central Brazil, based on U–Pb, Sm–Nd, Rb–Srand K–Ar isotopic constraints. 14. GeowissenschaftlichesLateinamerika-Kolloquium, Tübingen. Terra Nostra, vol. 2,pp. 12–13.

Chemale Jr., F., Silva, A.M., 1990. Os diques máficos do QuadriláteroFerrífero e Espinhaço Meridional. 2. workshop de diques máficos,




São Paulo, 1987. Universidade do São Paulo, Extended AbstractVolume, pp. 26–31.

Chemale Jr., F., Rosière, C.A., Endo, I., 1994. The tectonic evolutionof the Quadrilátero Ferrífero, Minas Gerais, Brazil. PrecambrianResearch 65, 25–54.

Colvine, A.C., Fyon, J.A., Heather, K.B., Marmont, S., Smith, M.,Troop, D.G., 1988. Archean lode gold deposits in Ontario. OntarioGeological Survey Miscellaneous Paper, vol. 139. 136 pp.

Costa, M.N.S., 2000. Estudo de isótopos de carbono e oxigênio ecaracterização petrográfica do minério da Mina Cuiabá. M.Sc.Thesis, Instituto de Geociências, Universidade Federal de MinasGerais, Belo Horizonte, Brazil.

Derby, O.A., 1906. The Serra do Espinhaço, Brazil. Journal ofGeology 14, 374–401.

Derby, O.A., 1911. On the mineralization of the gold bearing ofPassagem, Minas Gerais, Brazil. American Journal of Science 3,185–190.

Dorr, J.V.N., 1969. Physiographic, stratigraphic and structuraldevelopment of the Quadrilátero Ferrífero, Minas Gerais, Brazil.United States Geological Survey Professional Paper 641-A109 pp.

Eschwege, W.L., 1832. Beitraege zur Gebirgskunde Brasiliens. VerlagG. Reimer, Berlin. 488 pp.

Eschwege, W.L., 1833. Pluto Brasiliensis. Verlag G. Reimer, Berlin.622 pp.

Eschwege, W.L., 1898. Ocorrências e jazidas de ouro. Revista doArchivo Público Mineiro, Ano II, 611–631, Ano III, 455–463,519–577.

Freyberg, B., 1932. Ergebnisse Geologischer Forschungen in MinasGerais, Brasilien. Neues Jahrbuch für Mineralogie, Geologie undPalaeontologie, Sonderband, vol. 2. 403 pp.

Freyberg, B., 1934. Die Bodenschatze des Staates Minas Geraes. Ed.Schweizerbart, Stuttgart, 453 pp.

Fyfe, W.S., Kerrich, R., 1984. Gold: natural concentrationprocesses. In: Foster, R.P. (Ed.), Gold '82. Balkema, Roterdam,pp. 99–127.

Goldfarb, R.J., Groves, D.I., Gardoll, S., 2001. Orogenic gold andgeologic time: a global synthesis. Ore Geology Reviews 18,1–75.

Gorceix, H.,, 1876. Les explorations de L' or dans la province deMinas Gerais, Brésil. Societè Geographic Paris, Bulletin 6, vol. 12.

Gorceix, H., 1884. Bacias terciárias de água doce nos arredores deOuro Preto (Gandarela e Fonseca), Minas Gerais, Brasil. Anais daEscola de Minas de Ouro Preto, vol. 3, pp. 95–114.

Gross, G.A., 1959. A classification of iron deposits of Canada.Canadian Mining Journal 80, 87–91.

Gross, G.A., 1965. Geology of iron deposits in Canada. Volume 1,General geology and evaluation of iron deposits. GeologicalSurvey of Canada Economic Report, vol. 22, pp. 24–27.

Groves, D.I., Foster, R.P., 1991. Archaean lode gold deposits. In:Foster, R.P. (Ed.), Gold Metallogeny and Exploration. Blackie andSon Ltd., London, pp. 63–103.

Groves, D.I., Ho, S.E., 1990. Archaean gold deposits in the YilgarnBlock, Western Australia. In: Hughes, F.E. (Ed.), EconomicGeology of Australia and Papua/New Guinea. AustralasianInstitute of Mining and Metallurgy, Melbourne, pp. 539–553.

Groves, D.I., Phillips, G.N., Falconer, L.J., Houstoun, S.M., Ho, S.E.,Browning, P., Dahl, N., Mc Naughton, N.J., 1987. Evidence foran epigenetic origin for BIF-hosted gold deposits in green-stones of the Yilgarn Block, Western Australia. In: Ho, S.E.,Groves, D.I. (Eds.), Recent Advances in UnderstandingPrecambrian Gold Deposits, Geology Department and Uni-

versity Extension, University of Western Australia. Publication,vol. 11, pp. 167–179.

Groves, D.I., Goldfarb, R.J., Gebre-Mariam, M., Hagemann, S.G.,Robert, F., 1998. Orogenic gold deposits: a proposed classificationin the context of their crustal distribution and relationship to othergold deposit types. Ore Geology Reviews 13, 7–27.

Guimarães, D., 1931. Contribuição à geologia do estado de MinasGerais. Serviço Geológico e Mineralógico do Brasil, Rio deJaneiro, Boletim, vol. 55. 36 pp.

Guimarães, D., 1966. Contribuição ao estudo do polimetamorfismo daSérie Minas. Departamento Nacional da ProduçãoMineral/Divisãode Fomento da ProduçãoMineral, Rio de Janeiro, Boletim, vol. 90.54 pp.

Guimarães, D., de Melo, S.M., Melo, E.A.V., 1967. O Complexo deBação. Boletim do Instituto Geológico Escola Federal de Minas deOuro Preto, vol. 2, pp. 1–12.

Harder, E.C., Chamberlin, R.T., 1915. The geology of central MinasGerais, Brazil. Journal of Geology 23, 341–378.

Herz, N., 1970. Gneissic and igneous rocks of the QuadriláteroFerrífero, Minas Gerais, Brazil. United States Geological SurveyProfessional Paper 641-B 58 pp.

Herz, N., 1978. Metamorphic rocks of the Quadrilátero Ferrífero,Minas Gerais, Brazil. United States Geological Survey Profes-sional Paper, 641-C. 78 pp.

Huston, D.L., Large, R.R., 1989. A chemical model for theconcentration of gold in volcanogenic massive sulfide deposits.Ore Geology Reviews 4, 171–200.

James, H.L., 1954. Sedimentary facies of iron-formation. EconomicGeology 49, 235–293.

Ladeira, E.A., 1988. Metalogenia dos depósitos de ouro doQuadrilátero Ferrífero, Minas Gerais. In: Schobenhaus, C., Coelho,C.E.S. (Eds.), Principais depósitos minerais do Brasil. Metaisbásicos não ferrosos, ouro e alumínio, vol. 3, pp. 301–375.

Ladeira, E.A., 1991. Genesis of gold in Quadrilátero Ferrífero: aremarkable case of permanency, recycling and inheritance—atribute to Djalma Guimarães, Pierre Routhier and HansRamberg. In: Ladeira, E.A. (Ed.), Brazil Gold’91. Balkema,Rotterdam, pp. 11–32.

Lehne, E., 2000. Struktur and Geometrie des Bancão Erzkoerpers,Cuiabá Mine, Minas Gerais, Brasilien- eine Strukturelle Kartier-ung. M.Sc. thesis, Institut of Mineralogy and Economic Geology,Aachen University of Technology, Germany.

Lobato, L.M., Ribeiro-Rodrigues, L.C., Vieira, F.W.R., 2001a. Brazil'spremier gold province. Part II: geology and genesis of golddeposits in the Archean Rio das Velhas greenstone belt,Quadrilátero Ferrífero. Mineralium Deposita 36, 249–277.

Lobato, L.M., Ribeiro-Rodrigues, L.C., Costa, M.N.S., Martins, R.,Lehne, E., Alves, J.V., Tassinari, C.C.G., Vieira, F.W.R., Biasi,E.E., Silva, R.C.F.E., Pereira, V.C.A., Noce, C.M., 2001b.Geologia do Depósito de ouro Cuiabá, Quadrilátero Ferrífero,Minas Gerais. In: Jost, H., Brod, J.A., de Queiroz, E.T. (Eds.),Caracterização de depósitos auríferos em distritos mineirosbrasileiros. Departamento Nacional da Produção Mineral(DNPM) and Agência para o Desenvolvimento Tecnológico daIndústria Mineral Brasileira (ADIMB), pp. 3–77.

Machado, N., Carneiro, M.A., 1992. U–Pb evidence of late Archaeantectono-thermal activity in the southern São Francisco shield,Brazil. Canadian Journal of Earth Sciences 29, 2341–2346.

Machado, N., Noce, C.M., De Oliveira, O.A.B., Ladeira, E.A., 1989.Evolução Geológica do Quadrilátero Ferrifero no Arqueano eProterozóico lnferior com base em geocronologia U/Pb. 5.Simpósio de Geologia de Minas Gerais, Belo Horizonte, 1989,


Sociedade Brasileira de Geologia, núcleo Minas Gerais, ExtendedAbstract Volume, pp. 1–5.

Maddox, L.M., Bancroft, G.M., Scaini, M.J., Lorime, J.W., 1998.Invisible gold; comparison of Au deposition on pyrite andarsenopyrite. American Mineralogist 83, 1240–1245.

Marshak, S., Alkmim, F.F., 1989. Proterozoic contraction/extensiontectonics of the Southern São Francisco Region, Minas Gerais,Brazil. Tectonics 3, 555–571.

Marshall, B., Gilligan, L.B., 1989. Durchbewegung structure, pierce-ment cusps, and piercement veins in massive sulfide deposits:formation and interpretation. Economic Geology 84, 2311–2319.

Martins, R., 2000. Caracterização petrográfica e geoquímica mineraldo minério da Mina de Ouro de Cuiabá, Quadrilátero Ferrífero,Minas Gerais. M.Sc. Thesis, Instituto de Geociências, Universi-dade Federal de Minas Gerais, Belo Horizonte, Brazil.

Mathias Jr., D.L., 1964. Review on the Outside Gold Properties of theSt. John del Rey Mining Co. Private Report to St. John del ReyMining Co. Ltd (Mineração Morro Velho S.A). 142 pp.

Maxwell, C.H., 1972. Geology and ore deposits of the Alegria District,Minas Gerais, Brazil. United States Geological Survey Profes-sional Paper 341-J 72 pp.

Meyer, F.M.,Möller, P., Bruin, D., Przybylowicz,W.J., Prozesky, V.M.,1994. The gold–pyrite association inWitwatersrand reefs: evidencefor electrochemical precipitation of gold. Exploration and MiningGeology 3, 201–217.

Mintek, 1980. Characterization of the Cuiabá Concentrate. MorroVelho Mineração Internal Report. 7 pp.

Moraes, L.J., Guimarães, D., 1930. Geologia da região diamantífera deMinas Gerais. Anais da Academia Brasileira de Ciências 2 (3),153–186.

Mycroft, J.R., Bancroft, G.M., McIntyre, N.S., Lorimer, J.W., 1995.Spontaneous deposition of gold on pyrite from solution containingAu(III) and Au(I) chlorides; Part I, A surface study. Geochimica etCosmochimica Acta 59, 3351–3365.

Neall, E.B., 1987. Sulphidation of iron-rich rocks as a precipitationmechanism for large Archaean gold deposits in Western Australia:thermodynamic confirmation. In: Ho, S.E., Groves, D.I. (Eds.),Recent Advances in Understanding Precambrian Gold Deposits,Geology Department and University Extension, University ofWestern Australia. Publication, vol. 11, pp. 265–269.

Neall, E.B., Phillips, G.N., 1987. Fluid-wall rock interaction in anArchaean hydrothermal gold deposit: a thermodynamic model forthe Hunt Mine, Kambalda. Economic Geology 82, 1679–1694.

Noce, C.M., 1995. Geocronologia dos eventos magmáticos, sedimen-tares e metamórficos na região do Quadrilátero Ferrífero, MinasGerais. Unpublished Ph.D. thesis, Universidade de São Paulo,Brazil, 128 pp.

Noce, C.M., Zuccheti, M., Baltazar, O.F., Armstrong, R., Dantas, E.,Renger, F.E., Lobato, L.M., 2005. Age of felsic volcanism and therole of ancient continental crust in the evolution of the NeoarcheanRio das Velhas Greenstone belt (Quadrilátero Ferrífero, Brazil):U–Pb zircon dating of volcaniclastic graywackes). PrecambrianResearch 141, 67–82.

Noce, C.M., Tassinari, C., Lobato, L.M., 2007. Geochronologicalframework of the Quadrilátero Ferrífero, with emphasis on the ageof gold mineralization hosted in Archean greenstone belts. OreGeology Reviews 32, 500–510 (this volume), doi:10.1016/j.oregeorev.2005.03.019.

Oliveira, G.A.I., Clemente, L.C., Vial, D.S., 1983. Excursão à Mina deOuro de Morro Velho. 2. Simpósio de Geologia de Minas Gerais,Belo Horizonte, 1983, Sociedade Brasileira de Geologia, núcleoMinas Gerais. Extended Abstract, vol. 3, pp. 497–505.

Passchier, C.W., Trouw, R.A.J., 1996. Microtectonics. Springer-Verlag, Berlin. 289 pp.

Phillips, G.N., Groves, D.I., 1984. Fluid access and fluid-wall rockinteraction in the genesis of the Archaean gold–quartz vein depositat Hunt Mine, Kambalda, Western Australia. In: Foster, R.P. (Ed.),Gold '82. Balkema, Rotterdam, pp. 389–416.

Ramdohr, P., 1969. The Ore Minerals and Their Intergowths.Pergamon Press, Oxford. 1174 pp.

Ribeiro-Rodrigues, L.C., 1998. Gold in Archaen banded iron-formation of the Quadrilátero Ferrífero, Minas Gerais, Brazil—The CuiabáMine. Ph.D. Thesis, Aachen University of Technology.Augustinus Verlag, Aachener Geowissenschaftliche Beiträge,Band 27, 264 pp.

Ribeiro-Rodrigues, L.C., Friedrich, G., Oliveira, C.G., Vieira, F.W.R.,Callegari, L.A., Biasi, E.E., 1996a. Ore textures and structures ofthe Archean banded iron formation Cuiabá gold deposit, IronQuadrangle, Minas Gerais, Brazil. Zentalblatt für Geologie undPaläontologie Teil I 1994, 627–642.

Ribeiro-Rodrigues, L.C., Friedrich, G., Oliveira, C.G., Vieira, F.W.R.,Biasi, E.E., Callegari, L.A., 1996b. The BIF-hosted Cuiabá Golddeposit, Iron Quadrangle, Minas Gerais, Brazil: characteristics,controls and genesis. Gold Deposits of South America Sympo-sium, 39. Congresso Brasileiro de Geologia, Salvador, 1996.Extended Abstract, vol. 7, pp. 224–228.

Ribeiro-Rodrigues, L.C., Friedrich, G., Lobato, L.M., Duquini Jr., J.,Vieira, F.W.R., 2000. Gold mineralization in the QuadriláteroFerrífero, Minas Gerais, Brazil. In: Miller, H., Hervé, F. (Eds.),Zeitschrift für Angewandte Geologie - Sonderheft (SpecialPublication 1 - SH1). Bundesanstalt für Geowissenschaften undRohstoffe, Hannover, pp. 143–151.

Rimann, E.T., 1921. Zur Kenntnis der Minas Series im StaateMinas Gerais, Brasilien. Zentralblatt fuer Mineralogie, Abtei-lung A, pp. 417–422.

Robert, F., Poulsen, K.H., 2001. Vein formation and deformation ingreenstone gold deposits. In: Richards, J.P., Tosdal, R.M. (Eds.),Structural Controls on Ore Genesis. Reviews in EconomicGeology, vol. 14, pp. 111–155.

Scaini, M.J., Bancroft, G.M., Knipe, S.W., 1998. Reactions of aqueousAu (super 1+) sulfide species with pyrite as a function of pH andtemperature. American Mineralogist 83, 316–322.

Schrank, A., Machado, N., 1996. Idades U–Pb em monazitas ezircões do distrito aurífero de Caeté, da Mina de Cuiabá e dodepósito de Carrapato, Quadrilátero Ferrífero. 39. CongressoBrasileiro de Geologia, Salvador, 1996. Extended Abstract, vol.64, pp. 473–475.

Silva, A.M., Kuymjian, R.M., Chemale Jr., F., 1991a. Mafic dikesswarms in the southern São Francisco Craton, Southeastern Brazil.3. International Symposium on Mafic Dykes, São Paulo, 1991.Extended Abstract Volume, pp. 90–92.

Silva, A.M., Kuymjiam, R.M., Chemale Jr., F., 1991b. Rochas básicasdo Quadrilátero Ferrífero e Espinhaço meridional. 6. Simpósio deGeologia de Minas Gerais, Belo Horizonte, 1991. Revista daEscola de Minas, vol. 45 (1/2), pp. 7–30.

Skjernaa, L., 1989. Tubular folds and sheath folds: definitions andconceptual models for their development, with examples from theGrapesvare area, northern Sweden. Journal of Structural Geology11, 689–703.

Spear, F.S., 1995. Metamorphic phase equilibria and pressure–temperature–time paths. Mineralogical Society of AmericaMonograph, vol. 1. 799 pp.

Teixeira, W., 1985. A evolução geotectônica da porção meridional doCráton do São Francisco, com base em interpretações


geocronológicas. Unpublished Ph.D. thesis, Universidade de SãoPaulo, 207 pp.

Vial, D.S., 1980. Mapeamento Geológico do nível 3 da Mina deCuiabá. Mineração Morro Velho Internal Report. 22 pp.

Vial, D.S., 1988. Mina de ouro de Cuiabá, Quadrilátero Ferrífero,Minas Gerais. In: Schobenhaus, C., Coelho, C.E.S. (Eds.),Principais depósitos minerais do Brasil. Metais básicos nãoferrosos, ouro e alumínio, vol. 3, pp. 413–419.

Vieira, F.W.R., 1987. Novo contexto geológico para a Mina deouro de Raposos. 4. Simposio de Geologia de MinasGerais, Belo Horizonte, 1987. Sociedade Brasileira deGeologia, núcleo Minas Gerais, Extended Abstract, vol. 7,pp. 343–357.

Vieira, F.W.R., 1988. Caracterização petrográfica e mineralógica dominério do corpo galinheiro extensão, open pit Cuiabá. MorroMineração Internal Report. 6 pp.

Vieira, F.W.R., 1991a. Textures and processes of hydrothermalalteration and mineralization in the Nova Lima Group, Minas

Gerais, Brazil. In: Ladeira, E.A. (Ed.), Brazil Gold'91. Balkema,Roterdam, pp. 319–327.

Vieira, F.W.R., 1991b. Petrologia e litogeoquímica do Setor W do“Greenstone belt” Rio das Velhas, MG. 1. Simpósio internacionalde Geologia do Grupo AMSA, Nova Lima, 1991, MineraçãoMorro Velho Internal Report. 24 pp.

Vieira, F.W.R., 1992. Geologia da Mina de Cuiabá, Níveis 3 e 4.Mineração Morro Velho Internal report. 23 pp.

Vieira, F.W.R., Oliveira, G.I., 1988. Geologia do Distrito aurífero deNova Lima, Minas Gerais. In: Schobenhaus, C., Coelho, C.E.S.(Eds.), Principais depósitos minerais do Brasil. Metais básicos nãoferrosos, ouro e alumínio, vol. 3, pp. 377–391.

Vielreicher, R.M., Groves, D.I., Ridley, J.R., Mc Naughton, N.J.,1994. A replacement origin for the BIF-hosted gold deposit atMt. Morgans, Yilgarn Block, W A. Ore Geology Reviews 9,325–347.

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