pereira&lobato&ferreira&jardim2007 nature and origin of the bif-hosted sao bento gold...

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Ore Geology Reviews 3

Nature and origin of the BIF-hosted São Bento gold deposit,Quadrilátero Ferrífero, Brazil, with special

emphasis on structural controls

Sérgio Luiz Martins Pereira a,⁎, Lydia Maria Lobato b,Juliano Efigênio Ferreira a, Eduardo César Jardim c

a São Bento Mineração S.A. (Eldorado Gold Corporation), Fazenda São Bento s/n, Santa Bárbara, Minas Gerais, 35960.000, Brazilb Departamento de Geologia, Instituto de Geociências, Universidade Federal de Minas Gerais, Av. Antônio Carlos, 6627,

Pampulha, Belo Horizonte, Minas Gerais, 31270.901, Brazilc Magnesita S.A., Vila Catiboaba s/n - GRM, Brumado, Bahia, 46100-000, Brazil

Received 22 July 2002; accepted 29 March 2005Available online 14 February 2007

Abstract

The orogenic banded iron formation (BIF)-hosted Au mineralization at São Bento is a structurally-controlled, hydrothermaldeposit hosted by Archean rocks of the Rio das Velhas greenstone belt, Quadrilátero Ferrífero region, Brazil. The deposit hasreserves of 14.3 t Au and historical (underground) production of 44.6 t Au between 1987 and 2001. The oxide-facies São BentoBIF is mineralized at its lower portion, where in contact with carbonaceous, pelitic schists, particularly in the proximity ofsulfide-bearing quartz veins. Shear-related Au deposition is associated with the pervasive, hydrothermal sulfidation (mainlyarsenopyrite) of the Fe-rich bands of the São Bento BIF. Auriferous, sulfide- and quartz-rich zones represent proximal alterationzones. They are enveloped by ankerite-dominated haloes, which reflect progressive substitution of siderite and magnetite withinthe BIF by ankerite and pyrrhotite, respectively. The São Bento BIF was intensely and extensively deformed, first into open,upright folds that evolved into tight, asymmetric, isoclinal folds. The inverse limb of these folds attenuated and gave way tosheath folds and the establishment of ductile thrusts. Mineralized horizons at São Bento result from early structuralmodifications imposed by major transcurrent and thrusts faults, comprising the Conceição, Barão de Cocais and São Bento shearzones. Dextral movement on the SW–NE-directed Conceição shear zone may have generated splays at a compressional side-stepping zone, such as the São Bento shear zone, which is the structural locus for the São Bento gold mineralization.Relaxation of the Conceição shear zone under more brittle conditions resulted in the development of dilatational zones wheregold–sulfide–quartz veins formed. These structures are considered to have been generated in the Archean. Geochronologicaldata are scarce, with Pb–Pb analyses of refractory arsenopyrite and pyrite from bedded and remobilized ore plotting on a single-stage growth curve at 2.65 Ga. A later compressional, ductile deformation of unknown age overprinted, rotated and flattened theoriginal, N60E-directed structure of the whole rock succession, with development of planar and linear fabrics that appear similarto Proterozoic-aged structures. Fluid inclusion studies indicate low salinity, aqueous fluids, with or without CO2 and/or CH4,with extremely variable CO2/CH4 ratios, of probable metamorphic origin. Fluid evolution shows a paragenetic decrease in the

⁎ Corresponding author. Tel.: +55 31 3 8377316; fax: +55 31 3 8377135.E-mail address: [email protected] (S.L. Martins Pereira).

0169-1368/$ - see front matter © 2007 Published by Elsevier B.V.doi:10.1016/j.oregeorev.2005.03.018

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572 S.L. Martins Pereira et al. / Ore Geology Reviews 32 (2007) 571–595

carbonic phase from 10–15% to 5%, and increase in the H2O/(CO2+CH4) and CO2/CH4 ratios, suggesting important interactionwith carbonaceous sediment. Trapping conditions indicate a temperature of 300 °C at 3.2 kbar.© 2007 Published by Elsevier B.V.

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

1. Introduction

The São Bento gold deposit is structurally-hosted(e.g., Godoy, 1995; Martins Pereira, 1995; Alves,1995; Lobato et al., 1998; Martins Pereira et al., 2000,2001a,b, and references therein), situated in the BarraFeliz district, Santa Bárbara municipality, Minas Gerais,Brazil (Figs. 1 and 2 of Baltazar and Zucchetti, 2007-this volume). Gold has been intermittently mined in thisarea since 1860, originally by open-pit operation. Theearliest evidence of underground activity dates back to1898, when the British group São Bento Gold StatesLtd. began work. Between 1898 and 1906, 211,000 t ofore were produced at an average recovery gradeof 9.27 g/t Au. The current owner, São Bento MineraçãoS.A. (Eldorado Gold Corporation), started activitiesin 1979 and produced the first bullion in 1987. As ofDecember 2002, the total reserves of the deposit(proven+probable) were 1.79 Mt at 9.22 g/t of Au,totaling some 16.5 t (531,193 oz) Au. From 1987 toDecember 2002, São Bento milled 6 Mt of ore andproduced 43.6 t (1.4 Moz Au) from underground miningof sulfide-bearing iron formation. Total cash costs for2002 were US$ 184/oz (Eldorado Gold Corporationannual report for 2002).

This paper describes the geological setting of the SãoBento deposit, emphasizing its structural and wall-rockalteration styles. The mineralogical, geochemical andfluid inclusion characteristics are also reported, as wellas different genetic aspects. It is shown that minerali-zation at São Bento is controlled by shear zonesassociated with major transcurrent and thrust faults,representing the conduits for the infiltration of ore fluidsinto the banded iron formation (BIF), which acted as achemical trap for epigenetic Au deposition, with pri-mary magnetite replaced by epigenetic pyrrhotite andarsenopyrite.

2. Regional geology

The São Bento deposit is located in the QuadriláteroFerrífero (QF, Fig. 1), a region containing an assemblyof Archean and Proterozoic rocks. A large amount ofgeological work has been performed in the region, e.g.,Harder and Chamberlin (1915), Dorr (1969), Herz

(1970), Schorscher (1976), Loczy and Ladeira (1976),Almeida (1977), Schorscher (1979) and Ladeira (1980).The basis of the stratigraphic subdivision of the QF wasestablished by the joint mapping program of theDepartamento Nacional da Produção Mineral – UnitedStates Geological Survey between 1946 and 1962, latersynthesized by Dorr (1969). The Precambrian rocks ofthe region were grouped into three major units, whichare, from oldest to youngest, the Rio das Velhas(Archean) and Minas (Paleoproterozoic) Supergroups,and the Mesoproterozoic Itacolomi Group. The pres-ently-known regional basement was loosely termed‘undivided granitic rocks’ or granite–gneissic complexand considered to be of intrusive origin.

Noce et al. (2007-this volume, and referencestherein) review the geochronological data available forthe QF region and indicate that (i) the granite–gneissiccomplexes derive from igneous protoliths older than2.9 Ga; (ii) three granitic magmatic episodes affected theregion during the Neoarchean, at ca. 2.78 to 2.760, 2.72to 2.70, and 2.60 Ga; two felsic magmatic eventsassociated with the greenstone belt sequence areseparated in time (ca. 3.03 and 2.772 Ga), the youngestconstraining a major magmatic and tectonic event; (iii)Pb-isotopic studies of lode-gold deposits indicate thatthe main mineralization episode occurred at about 2.8 to2.7 Ga; (iv) Proterozoic Lake-Superior-type banded ironformations were deposited at ca. 2.5 to 2.4 Ga; (v) theeastern part of the Quadrilátero Ferrífero was alsoaffected by the Brasiliano Orogeny (600 to 560 Ga).

The geological mapping by the Companhia dePesquisas de Recursos Minerais–CPRM (Zucchettiand Baltazar, 1998) provided a 1:25,000 detail on theRio das Velhas Supergroup, and the foundation forfurther research. In their work, the region is divided intofour main lithostructural domains, with the lowermostNova Lima Group rocks having been subdivided intofive related ‘lithofacies associations’; from bottom to topthe (i) mafic–ultramafic volcanic, (ii) volcanic-chemi-cal, (iii) clastic-chemical, (iv) volcaniclastic, and(v) resedimented associations. The authors subdividedthe top Maquiné Group into the coastal and non-marineassociations. Geological revisions of this region aregiven by Lobato et al. (2001b) and Baltazar andZucchetti (2007-this volume).






Fig. 1. Location and regional geological map of the NE border of the Quadrilátero Ferrífero (from Lobato et al., 2001a).

573S.L. Martins Pereira et al. / Ore Geology Reviews 32 (2007) 571–595


3. Lithostratigraphy of the São Bento deposit area

The geology of the São Bento deposit (Figs. 1 and 2)is described in detail in numerous public-domainpublications (e.g., Moseley, 1986; Abreu et al., 1988;Prado et al., 1991). Unpublished company reports

Fig. 2. Simplified geological map of the São Bento Ma

include those by Martins Pereira (1988, 1992), Fletcher(1989), Spencer (1989), and Ladeira and Ferreira(2000).

The dominant style of ore zones in the Santa Bárbararegion is BIF-hosted auriferous sulfide lodes similar toother gold deposits of the Nova Lima region (Fig. 2; see

nifesto (lease area), after Martins Pereira (1995).


also Fig. 2 of Baltazar and Zucchetti, 2007-this volume),as described by Lobato et al. (2001a) and Ribeiro-Rodrigues et al. (1996). The Santa Bárbara area is,however, characterized by tighter interference foldingand shearing compared with deformation elsewhere inthe Nova Lima region.

The understanding of the geology of the São Bentoarea is based primarily on the research ofMoseley (1986),Abreu et al. (1988), Prado et al. (1991), Martins Pereira(1995) and Martins Pereira et al. (2000). Clastic andchemical metasedimentary rocks dominate thearea, comprising a thick succession of carbonaceousquartz–chlorite schists, a major BIF (the São Bento BIF)and an overlying package of chloritic and micaceousschist with minor iron formations. In the area described inthis paper, Abreu et al. (1988) and Martins Pereira (1995)have identified four principal, informal lithostratigraphicunits that are mappable at a scale of 1:25,000. Fromstructural bottom to top, they are designated the Lowerbanded iron formation, the Basal carbonaceous unit, theSão Bento banded iron formation and the Carrapato unit(Figs. 1 and 2). There is insufficient evidence to determinewhether this represents the true stratigraphic relationships.The chosen column results from indications that thesequence is homoclinal, i.e., the structures at the SãoBento deposit have a relatively consistent NE-strike andSE-dip (Figs. 2 and 3). The mineral paragenesescharacteristic for this entire succession suggest conditionsof greenschist facies metamorphism.

The gold ore zones at São Bento are hosted bymineralized horizons of the lower portion of the SãoBento BIF, which directly lies on schists of the Basalcarbonaceous unit. The laterally disrupted horizons arecomposed of two main orebodies separated by a 150 to200 m gap, characterized by structural thinning of theiron formation. The No. 1 and 2 orebodies (Simmons,1968; Abreu et al., 1988) are similar. They have anaverage strike length of 250 m and unknown dipextent, the deepest drill intersection being at some1400 m vertical depth. Below level 21, No. 1 and 2orebodies converge (Martins Pereira, 1995; Figs. 2, 3and 4), hampering their distinction. As seen in Fig. 4,the NE part of the orebody extends to greater depththan the SW part, and the gap between No. 1 and 2orebodies is very indistinct.

3.1. Lower banded iron formation

The Lower BIF is well exposed in the westernportion of the lease area (Manifesto; Fig. 2) as oxide,silicate and carbonate facies BIF. The rock is finelybanded, and contains quartz, magnetite, chlorite and

carbonate. To date, no follow-up exploration has beenperformed, since mapping and soil geochemical surveyshave not shown any anomalous results.

3.2. Basal carbonaceous unit

The Basal carbonaceous unit consists of dark grey toblack, generally finely banded schists and phyllites, withquartz, sericite and minor chlorite as the essentialcomponents. The total thickness of this unit is approx-imately 650 m, considering an average dip of 55° and notaccounting for any possible structural complications. It isnow known that what was previously described byAbreu et al. (1988) as graphite represents amorphouscarbonaceous material. Non-auriferous pyrite nodulesoccur throughout the carbonaceous schists (Fig. 5A).They precede the development of the quartz–carbonate-rich intercalated laminae, which are common, and maybe Au-bearing.

3.3. São Bento banded iron formation

The São Bento BIF (Figs. 3 and 6) consists essentiallyof oxide-facies BIF rich in carbonate and clasticcomponents, which, in places, contains primary sulfideminerals. The rock contains mm- to cm-thick light- anddark-colored bands, due to varying proportions of quartz,carbonates, magnetite and stilpnomelane. The pale layersare generally cream-colored, and are dominated by quartzand carbonate minerals. The darker fissile layers containchlorite and stilpnomelane, whereas the massive, darkgrey layers are made up of magnetite or carbonaceousmaterial and sulfide minerals.

Fine-grained, terrigenous, pelitic metasedimentaryrocks form intercalations within the São Bento BIF,encompassing carbonaceous chlorite and mica schists.They predominate in the lower part of the iron formation,which, in underground workings, has an average thick-ness of 120 m. Both vertical and horizontal compositionalchanges are recorded in the São Bento BIF. Thus, on thebasis of combined distinctive, geological and geochem-ical characteristics, the São Bento BIF is subdivided intolower and upper formations (Abreu et al., 1988; MartinsPereira, 1995; Martins Pereira et al., 2000).

The Lower iron formation hosts the bulk of the goldmineralization at São Bento (Fig. 3). It is characterizedby the relative heterogeneity of its lithotypes. It containsoxide- and carbonate-facies BIF and ferruginous chert.Intercalations of fine, clastic metasedimentary rocksoccur in the form of carbonaceous mica schists, quartz–chlorite schists and phyllites. The non-mineralized BIFis characterized by a compositional layering with


Fig. 3. Vertical cross-section of the São Bento banded iron formation (BIF). DDH: underground diamond drill-holes. 11-1906 X/C and 23-1980 X/C:underground cross-cuts with local grid location.



alternating quartz–carbonate and magnetite–carbonatebands (Fig. 5B) The dominant carbonate is of sideriticcomposition (Godoy, 1995). This rock grades laterallyand vertically to a distinctive, cream-pinkish, carbonate-dominated iron formation, locally containing gold-bearing sulfides. It contains ankerite, calcite, ferroandolomite and siderite, with scheelite and albite, and canalso display streaks of variable proportions of magne-tite-carbonate rock (Fig. 5B–F). The transition fromnon-mineralized to mineralized BIF is sharp and markedby the presence of sulfides, quartz and carbonate. Wheremineralized, the Lower iron formation is characterizedby Au-bearing, sulfide-rich layers alternating withcarbonate–quartz-rich layers in mineralized horizons,with associated quartz veins (Fig. 5D). These horizons

Fig. 4. Block diagram of the mineralized horizons from levels 11 to 25 at theBento Horizon, E – East horizon (modified after Martins Pereira, 1995).

are NE–SW oriented and contain sulfide, carbonate(dominantly ankerite) and cherty bands (Figs. 5C–F and11B). The sulfide minerals include arsenopyrite, pyr-rhotite and pyrite; most of the gold is in solution. Theclose association of Au and sulfides is emphasized by apositive statistical correlation between Au values, S andAs contents (Fig. 7, B). Gold grades at São Bento areremarkably constant at all levels.

Upper iron formation: The upper portion of the SãoBento oxide-facies BIF (the Upper iron formation)contains less clastic contribution, has a lower arseniccontent (maximum of 200 ppm), and has a more homo-geneous appearance than the Lower iron formation. It hasa thickness of up to 100 m displaying sheared contactswith the overlying Carrapato unit and underlying Lower

São Bento mine. W – West horizon, M – Middle horizon, SB – São


iron formation. The Upper iron formation is composed ofindividual bands from 0.2 to 10 cm thick, made up ofcream-colored carbonate, light and dark grey quartz andmagnetite, and dark green chlorite and stilpnomelane.This BIF locally contains layers of quartz–sericite–chlorite schist, 1 to 2 m thick.

3.4. Carrapato unit

This unit, which has an apparent thickness of over500 m, overlies the São Bento BIF and consists of arelatively monotonous sequence of light and dark grey-coloredmica schists. Underground, these are referred to as

Fig. 5. (A) Fine-grained pyrite (Py) crystals form nodules in carbonaceous phyllite of the basal carbonaceous unit. The sample height is approximately3 cm; (B) Strongly folded, non-mineralized oxide-facies São Bento BIF. Note the intercalation between siderite- (Sid, light grey) and magnetite- (Mt,black) dominated layers. Quartz veins (Qz, white) cross-cut the compositional banding; (C) Rims (black dotted lines) of fine-grained, gold-bearingarsenopyrite (Apy) occur around quartz veins (Qz, white) and these cross-cut the compositional layering. Layering is defined by alternatingcarbonate- (Cb, mainly ankerite, Ank, light grey) and relict magnetite- (Mt, black, white dotted lines) rich layers. The former represent portions ofcarbonate-dominated alteration of BIF, locally containing sulfide-bearing gold; (D) mineralized Lower formation of the São Bento BIF, rich in gold-bearing arsenopyrite (Apy). Note that at the left hand side, above the scale bar, the enclosed portion of BIF is dominated by a carbonate (ankerite,Ank, medium grey)-rich zone with streaks of magnetite (Mt, dark grey, black dotted lines) and quartz veins (Qz, white). To the right hand side, quartzveins are parallel to the compositional layering, whereas veins at the center transect the banding; (E) carbonate- and magnetite-dominated BIF, locallycontaining sulfide minerals. Dark grey bands contain Mt, whereas light grey portions represent zones of carbonate alteration (ankerite, Ank). Hingezones are locally marked by the presence of gold-bearing arsenopyrite (Apy); (F) relics of magnetite-bearing BIF at the São Bento deposit, exhibitingcarbonate and sulfide alteration. Note that ankerite (Ank) and/or pyrrhotite (Po, shiny grey) laterally replace magnetite-rich bands and portions (Mt,very dark grey), which contains rare or no gold, associated with irregular quartz veins (Qz).

Fig. 6. Simplified geological map of level 25. W: west horizon; SB: São Bento horizon; E: east horizon.


Fig. 7. Diagrams showing correlation between gold and sulfur (A) and arsenic (B).


hanging wall schists (Figs. 2 and 3). Discontinuousportions of carbonaceous schists and intercalated carbo-naceous quartz–schists have been inferred to representmetagreywackes (Martins Pereira, 1995). The carbona-ceous portions may contain stretched nodules of pyrite(Fig. 5A), as well as carbonate-rich laminae, with whichgold may be associated.

The carbonaceous schists may host gold mineraliza-tion within discontinuous, smoky quartz veins. Theyhave a relative low (b 1%) sulfide content, mainlystibnite and arsenopyrite. This style of mineralization issimilar to that of the Córrego do Sítio deposit, some5 km southwest of São Bento (Lobato et al., 2001a),where auriferous quartz–carbonate–sulfide veins andveinlet systems occur in pelitic rocks.

3.5. Mafic dikes

Metamorphosed, mafic igneous rock occurs as scarceoutcrops and in diamond drill-hole cores at São Bento,normally in the form of dikes. These rocks also occur inother gold deposits hosted by theNovaLimaGroup rocks,such as Raposos (Ladeira, 1980, 1991; Junqueira et al.,2007-this volume; Fig. 2 of Baltazar and Zucchetti, 2007-this volume). The mafic dikes are green, with a fine-

grained texture, and, although foliated near the contactswith wall rocks, they grade to medium-grained over shortdistances towards the core of the intrusion, where thefoliation is not prominent. The dikes are composed ofrelics of green hornblende, actinolite–tremolite, epidote,carbonate, plagioclase and quartz. The matrix is com-posed of mica, chlorite, hornblende and albite. Accessoryminerals are titanite, apatite, sulfide mineralsand magnetite. Contact thermal metamorphism hascaused the growth of euhedral white mica and carbonateporphyroblasts, as well as veins with quartz, carbonateand recrystallized sulfide minerals in all enclosing rocktypes. The dikes cross-cut and displace the ore zonesbelow level 21 (Fig. 3), and therefore represent an im-portant time marker (Martins Pereira, 1995).

4. Textural, mineralogical and geochemical featuresof the mineralized Lower iron formation

Lithotype variations in the Lower iron formationinclude mm- to cm-thick, grey (magnetite-rich BIF),dark green (quartz–chlorite schist) and cream (quartz–carbonate–sericite schist) layers, which are well devel-oped and may be folded or pinched out due to shearing(Fig. 6). Monotonous intercalations of carbonate–



Fig. 8. Photomicrographs of mineralized BIF at the São Bento deposit. (A) Relict magnetite (Mt, medium grey) crystals are overgrown by pyrrhotite(Po, light grey) that is overgrown by arsenopyrite (Apy, white). Inclusions of carbonate (Cb) occur in Po; (B) synkinematic pyrrhotite (Po, mediumgrey) is overgrown by subhedral arsenopyrite (Apy, white). Note that the larger crystal in the center displays growth lines. The poikiloblastic aspect ofthis crystal, with inclusions of Po and carbonate, suggests its formation at the expense of both these phases. Scale bar=425 μm; mineralized SãoBento BIF; (C) Arsenic-rich pyrite (Py) overgrows arsenopyrite (Apy). The color of the pyrite crystals is slightly heterogeneous attesting to itscompositional variation; (D) elongated gold (Au) particle fills fracture in subhedral pyrite (Py). Pyrite contains relics of ill-formed pyrrhotite (Po, darkgrey), which displays an inclusion of chalcopyrite (Cpy). Scale bar=530 μm; (E) gold (Au) particle near the contact of pyrrhotite (Po, dark grey) andarsenopyrite (Apy, light grey). Note relict films of Po in arsenopyrite that replaces the latter.


sericite–quartz schists, carbonaceous–chlorite schistsand quartz–chlorite schists with minor amounts ofpyrite, pyrrhotite and arsenopyrite, but with no goldvalues, are also present. The occurrence of carbona-ceous chlorite schists is more common in the SW ex-tension of the underground ore zone.

4.1. Ore zone description

There are four mineralized ore horizons in the Loweriron formation, two of which are situated at its lower andupper contacts ((Figs. 3, 4 and 6)). The horizons varyfrom 0.5 to 8.0 m thick, with gold related to a quartz–


carbonate–sulfide assemblage. At the lower contactwith the Basal carbonaceous unit, the West horizon isgenerally present and, at the upper contact, the Easthorizon is developed. The other Au-bearing layers aretermed the Middle and São Bento horizons (Figs. 4 and6). The most important mineable horizons are West andSão Bento. Late-stage quartz veins with coarse and non-auriferous sulfides and carbonates also occur (Fig. 11C).

Along strike, these mineralized horizons are dis-rupted by shearing. Their lateral extensions have beennamed separately as orebodies No. 1, No. 2 and PintaBem (Simmons, 1968; Abreu et al., 1988; Figs. 4 and 6).Underground mapping indicates that the structuralfeatures, mineralogical composition, and even goldgrade distribution of the orebodies are the same, in-ferring that there are no significant variations betweenthe West, Middle, São Bento and East horizons. Withincreasing depth, the distinction between the four miner-alized horizons, or their laterally displaced orebodies, isno longer possible due to structural complications.Below level 21, these are merged to produce what ispresently mined as the Main and Secondary horizons(Martins Pereira et al., 2000; Figs. 4 and 6).

Fig. 9. Proposed hydrothermal alteration sequence of the least-altered (sulfideprovided by L.R.B. Miranda Sá and R.C.F. Silva), more readily applicable toferroan dolomite; musc = muscovite; sd = siderite; ser = sericite (fine-gracontinuous. Dashed lines indicate that the phase may be present or not.

In all horizons, alternating carbonate and quartzbands dominate the transition from non-mineralized tomineralized BIF. This rock is characterized by alternat-ing cream-pink quartz–carbonate and lighter-greymagnetite layers (Fig. 5C–F), which are interpreted asthe enclosing carbonate alteration halo (‘carbonate-facies iron formation’ of Ladeira and Ferreira, 2000).The gold–sulfide zones can be parallel to (Fig. 5D, E) ortransect (Fig. 5C) this layering. The layers can be foldedand pinched out due to shearing (Martins Pereira et al.,2000).

Thick, high-grade gold intercepts represent thickfolded zones resulting from intense strain. These zonesare the sheared, inverted and attenuated fold limbs thatbear the mineralized horizons.

4.1.1. Mineralogical aspectsIn the proximity to mineralized zones, magnetite is

replaced by ankerite and/or pyrrhotite (Fig. 8A) witharsenopyrite (Martins Pereira, 1995). In order of abun-dance, the composition of the host carbonate-dominatedhalo is ankerite, siderite, calcite, ferroan dolomite, mus-covite, quartz, chlorite, pyrite, pyrrhotite and arsenopyrite.

-poor) São Bento BIF (Lobato et al., 2001a and additional informationlevels 25 and 26. Abbreviations: ank = ankerite; cc = calcite; Fe-dol =ined muscovite). Solid lines indicate that the presence of a phase is

Table 1Average composition of rare earth element concentrations of the SãoBento magnetite–carbonate iron formation

(ppm) Orezone

STD QSV BIF STD Carbonaceousschist

STD

La 5.90 1.18 0.98 4.68 0.90 17.28 1.44Ce 12.12 2.55 2.24 9.92 2.47 36.12 1.05Nd 4.73 1.39 0.61 3.77 1.50 17.11 1.76Sm 0.95 0.33 0.12 0.87 0.42 2.99 0.10Eu 0.25 0.06 0.03 0.24 0.05 0.59 0.06Gd 0.68 016 0.12 0.70 0.22 1.67 0.12Dy 0.58 0.14 0.11 0.67 0.22 1.11 0.14Ho 0.11 0.01 0.02 0.13 0.04 0.23 0.03Er 0.30 0.04 0.07 0.36 0.13 0.67 0.07Yb 0.33 0.08 0.09 0.39 0.13 0.67 0.06Lu 0.06 0.16 0.02 0.08 0.02 0.11 0.01

Ore zone: quartz sulfide-bearing BIF (average of 8 samples; STD =standard deviation).QSV: quartz–sulfide-vein assemblage (1 sample).BIF: São Bento BIF (average of 5 samples; STD = standard deviation).Carbonaceous schist: carbonaceous, sericite-bearing schist (average of2 samples; STD = standard deviation).


Synkinematic pyrrhotite is locally the earliest sulfide, butsince it may carry fine-grained pyrite inclusions, an early-generation pyrite is deduced. Subhedral arsenopyrite isdeveloped at the expense of pyrrhotite (Fig. 8B), and bothare commonly replaced by arsenical pyrite (Fig. 8C).Microfractures in pyrite may be filled with pyrrhotite,gold (Fig. 8D, E) and rutile. Fine-grained chalcopyriteoccurs along pyrrhotite borders, or as inclusions in it(Fig. 8D). Carbonate minerals corrode sulfide borders,suggesting recurrent carbonate alteration. Siderite mayform inclusions in arsenopyrite, and together withankerite, they are in places included in pyrrhotite. Chloriteforms thin streaks and occurs as inclusions in sulfides.Stilplomelane and muscovite are invariably present,where sphalerite and galena are locally present. Otherminor phases are albitic plagioclase, magnetite, ilmenite,rutile, titanite, scheelite, sphalerite, covellite, bornite,chalcopyrite and galena (Fig. 9; Godoy, 1995; MartinsPereira, 1995; Lobato et al., 1998, 2001a).

The distribution of pyrite and pyrrhotite varythroughout the deposit, with arsenopyrite being themost abundant sulfide at a constant concentration ofabout 34% of all sulfides. Pyrrhotite and pyrite arenegatively correlated, with pyrrhotite increasing inzones poorer in pyrite. The amount of pyrrhotiteincreases at depths below level 21, where deformationbecomes more intense, with pyrrhotite ultimatelyoccupying hinge zones of tight folds (Martins Pereira,1995; Lobato et al., 1998, 2001a).

Gold particles are distributed as follows: (i) includedin arsenopyrite, pyrrhotite and pyrite (Fig. 8D, E);(ii) around the edges of sulfides, particularly near thecontacts of pyrrhotite and overgrown arsenopyrite;(iii) at the junction of gangue minerals; (iv) free in thegangue; and (v) in magnetite. Gold grain size variesfrom 2 to 125 μm. About 50% of visible gold is asinclusions within or along the margins of arsenopyrite.Gold is more rarely found along boundaries of gangueminerals (Godoy, 1995; Martins Pereira, 1995; Lobatoet al., 1998, 2001a).

4.1.2. Hydrothermal zoningThe above characteristics are interpreted to reflect

a hydrothermal-alteration zonal pattern of the mineral-ized BIF from (i) a proximal (central) gold-rich zonecontaining quartz+carbonate (ankerite)+muscovite+sulfides (pyrrhotite, arsenopyrite, pyrite); through(ii) an intermediate zone with quartz+muscovite±carbonate±sulfides±chlorite±magnetite; to (iii) a distal(outer) zone with quartz+chlorite+muscovite+ magne-tite+carbonate±pyrite (Godoy, 1995; Martins Pereira,1995; Lobato and Vieira, 1998; Fig. 9).

4.2. REE geochemistry of the São Bento banded ironformation and country rocks

Chemical analyses were performed in the LakefieldGeosol laboratories, Belo Horizonte, Brazil. Homoge-neous pulverized samples are weighed into tefloncrucibles. Samples are digested in a combination ofnitric, perchloric, and hydrofluoric acids. After beingtaken to dryness, samples are resumed with hydrochloricacid to redissolve the salts. The rare earth elements areconcentrated by ion exchange. All sample batches areprepared with a standard reference material (in-house)and a blank. Samples are analyzed using an ARL Model35000 ICP-OES; results are shown in Table 1.

The average compositions of the São Bento depositlithotypes are presented in Table 1 (Martins Pereira,1995). Rare earth element (REE) patterns are presentedfor schists of the Basal carbonaceous unit (Fig. 10), SãoBento, Raposos (Ladeira et al., 1991) and Isua (Dymekand Klein, 1988) BIFs, Au-bearing sulfidized São BentoBIF and mineralized, en énchelon quartz vein. Allsamples are normalized to the North American ShaleComposite (NASC; Haskin et al., 1968).

All samples have REE patterns depleted in relation toNASC. The pattern of the schist is slightly enriched inlight rare earth elements (LREE) and depleted in heavyrare earth elements (HREE). Both non-mineralized andmineralized São Bento BIF have almost flat patterns,close to that of NASC. In comparison to the Isua BIF,they are more enriched in LREE and poorer in theHREE. The typical negative cerium anomaly is lacking,

Fig. 10. Rare earth element pattern for the carbonaceous schist (onespecimen), for the average composition of non-mineralized (magne-tite–carbonate-bearing; average of five specimens, BIF) and mineral-ized (sulfide-bearing; average of ten specimens, ore) banded ironformation (orebodies nos. 1 and 2), and quartz–sulfide veins (onespecimen, quartz vein). Raposos BIF data from Ladeira et al. (1991).Issua data from Dymek and Klein (1988).


and the positive europium anomaly is relatively lesspronounced. Their pattern diverges strongly from theRaposos BIF.

The profile for sulfide-bearing quartz veins isrelatively parallel to both non-mineralized and mineral-ized BIF, but with a lower rare earth element concen-tration. This suggests its inheritance from the host ironformation (Martins Pereira, 1995). The similaritybetween the REE profile exhibited by the non-mineral-ized and mineralized São Bento BIF with NASC sug-gests an important clastic contribution. Both samples ofBIF and quartz–sulfide veins have very similar profiles.

5. Fluid inclusions

De Witt et al. (1994) showed that fluid inclusions areH2O-rich in non-mineralized quartz veins hosted by

Fig. 11. Lithostructural aspects of selected samples from São Bento. (A) Laternon-mineralized São Bento iron formation. Note that thin quartz veins cross-rock, representing a carbonate alteration zone. The sample displays alternatiand quartz (Qz) bands. Fine-grained, discrete arsenopyrite clusters are presenin white) vein (center of the photo). Quartz may carry non-auriferous, coarsetypical, folded termination of the São Bento banded iron formation. The foldfilled with quartz (stippled lines); these are parallel to the foliation Sn, coincid35) that is oblique to Sn. The left hand side is located at the southeast, whereasmarking the central portion of a sulfide alteration zone (stippled area), whichmainly ankerite) alteration zones develop. Note that the sulfide-rich zones arthat contain relics of magnetite-rich layers.

pelitic carbonaceous schists and H2O-poor in all otherquartz veins. A detailed study by Alves (1995) ofsamples collected from the same ore horizons as thosestudied by Martins Pereira (1995) shows a remarkablesimilarity in composition and thermodynamic condi-tions of fluids in inclusions in both mineralized andbarren quartz veins. Microthermometric and Ramanmicrospectroscopic results indicate that the fluids arecomposed of H2O+CH4+CO2(± N2±HS

−) and CH4+N2(± HS−), both of which were probably related to goldtransport and deposition. The main types of inclusionsare two-phase, aqueous-carbonic fluids. They maycontain CO2 and/or CH4, with extremely variableCO2/CH4 ratios, and minor N2, HS

− (± H2S). There isa decrease in the carbonic phase from 15 to 5%, andincrease in the H2O/(CO2+CH4) and CO2/CH4 ratios asthe fluid evolved. The shift from CH4- to CO2-dominated inclusions can be explained by the reactionCH4+2O2→CO2+2H2O (Alves, 1995). Trapping con-ditions evolved from about 300 °C at 3.2 kbar to 200 °Cat 1 kbar. Salinity is in the range of 3.4 to 5.0 wt.% NaClequiv. Rare CH4 (N2±HS

−) inclusions occur alongfractures and are typified by a salinity of 10 wt.% NaClequivalent.

6. Structure of the São Bento gold deposit

6.1. Structural data

The São Bento deposit is located approximately onthe axial trace of the Conceição anticline (Dorr, 1969),designated as the Conceição shear zone by MartinsPereira (1995, Fig. 2). The deposit lies within ahomoclinal sequence (Figs. 2, 3 and 4), with an averageattitude of N30 to 40E/45 to 55SE (Martins Pereira,1995). It is intensely deformed, as demonstrated byfoliation-parallel ductile shear zones, which causedstretching and attenuation of units of differing compe-tence (Martins Pereira, 1995).

Despite the intense deformation imposed on thedeposit, the compositional layers of the iron formationexhibit a relative degree of lateral and vertical continuity

al- and vertically semi-continuous compositional layering, typical of thecut the banding. The stick measures 1 m; (B) carbonate (ankerite)-richng carbonate (ankerite, ank), locally containing minor magnetite (Mt),t in the lower and upper portions; (C) late, non-mineralized quartz (Qz,pyrite and pyrrhotite in sulfide (Sulf) clusters; (D) cross-section of theis enclosed in carbonaceous schist and marked laterally by shear zonesing with the fold axial plane. The hinge orientation has a plunge (N60E/the right hand side is at the northwest; (E) en échelon array of the veinscontains gold-rich arsenopyrite (Apy), and out of which carbonate (Cb,e separated from magnetite (Mt)-rich BIF by carbonate alteration halos


(Fig. 11A). Although each horizon of the Au-sulfide-bearing BIF appears distinct, it is inferred that theyresult from intense folding and/or shearing of one or twolayers of mineralized BIF. This is demonstrated by theconvergence of the West and Middle horizons at depth,

behavior that is also similar for the São Bento and Easthorizons (Martins Pereira, 1995).

The internal planar structures at São Bento includecompositional banding, which tends to be parallel to thevarious structural elements, including the metamorphic-

Fig. 12. Longitudinal section of the São Bento ore zone.



deformational foliation Sn N37E/52SE, spaced cleavage(= fracture cleavage), joints, shear zones, axial surfacesof folds and all other fold elements. The most importantstructural elements are Sn, Lm, and Bn, analyzed in thestereograms (Fig. 13).

The principal linear element is the stretching and/ormineral lineation, Lm, plunging at S54E/52, defined bysulfides and sericite. Hinge lines are denoted by Bn; anolder hinge line, Bn−1, is recognized locally. The orehorizons are elongated along the linear elements thatcontrol them, which are themselves structurally con-strained. The linear structure Lm coincides with theplunge of the overall rock succession. Both the rocksand the ore can be described as B and or B/S tectonites(Turner and Weiss, 1963). The stretching lineation Lmand the hinge lines Bn that control the orebodies are inmost cases parallel; they have a consistent plunge ofS50E/55 down to level 15 (Martins Pereira, 1995). Atlevel 25, the plunge is S55E/52, suggesting that the orehorizons may flatten as the mine deepens (Fig. 3).

The deposit plunge is down dip and not necessarilycoincident with the closure of the mineralized horizons(Figs. 12 and 13A, B; Martins Pereira, 1988, 1992,1995). These characteristics reflect a sheath fold patternwith a variably oriented hinge line.

As pointed out above, the ore zones below level 21are restricted to the Main and Secondary horizons. Theformer is the structural junction of the West and Middlehorizons, whereas the Secondary horizon represents theSão Bento horizon. The economic East horizon of shal-lower levels is discontinuous and erratic at this depth.The ore horizons converge along strike and down dip(Figs. 3 and 4). These relationships confirm that eachhorizon is a result of a single, intensely folded layer ofiron formation. These folds have very tight hinges, withlong stretched limbs (Figs. 5B and 11D; Martins Pereira,1995).

Analyses of the linear-fabric array are presented inFig. 13A–C. The stereogram (Fig. 13C) shows twogirdles, viz. two families of Bn folds are inferred byjoining the two population maxima. This indicates thatthe fold hinge lines Bn are geometrically convergent.Such a distribution is interpreted to reflect the presenceof conical folds that could have resulted from sheathfolding, supporting the model of Spencer (1989) andMartins Pereira (1992, 1995). The latter concludes thatthe relative opening of the ore zone towards level 21 isprobably a consequence of the sheath–fold geometry.

Shear zones commonly deform and truncate the SãoBento BIF. Most of the terminations of the ore hori-zons are remarkably sheared. The distribution of thestretching lineations on the foliation planes and in the

shear zones is virtually coincident, suggesting thatboth features were generated by the same event ofsheath folding. The terminations of the ore zones arealso characterized by shear-related folding and the foldhinges may have variable orientations (Figs. 11C and13C; Martins Pereira, 1992, 1995).

The slight change in hinge lines towards the north-east, as well as the ore horizons, is considered to be theresult of the rotation/transposition of earlier shear struc-tures. Ladeira (1980, 1991) and Grossi Sad and Pinto(1986) have described the rotation/transposition of simi-lar early structures in other gold deposits hosted by theNova Lima rocks.

6.2. Structural control on gold ores

Two styles of sulfide distribution have been sug-gested for the São Bento deposit (Fletcher, in Spencer,1989), encompassing finely-laminated and bandedsulfides in iron formation, and sulfide-bearing quartzveins (Fig. 5C–E). Detailed underground mapping ofsome exposures of the ore horizons help establish thefollowing structural characteristics of these ore styles(Spencer, 1989; Martins Pereira, 1995).

1) The sulfide-bearing quartz veins are planar features.2) The veins are parallel and exhibit en échelon

arrangements (Figs. 11E and 14).3) Both quartz and sulfide minerals of the mineralized

horizons are generally deformed (Fig. 5C, D).4) Banded sulfides in mineralized iron formation are

best developed in the proximity of sulfide-bearingquartz veins. There is a decrease in the concentrationof Au-bearing sulfide minerals away from the veins(Fig. 5C, D, F).

5) The borders of gold–sulfide-bearing quartz veinsare marked by rims of sulfide (mainly arsenopyrite)crystals aligned parallel to the vein direction (Figs. 5Cand 11E). From the rims towards the host rock,sulfides are concentrated in layers that can be fol-lowed along strike and dip to portions of ankerite-richbanded rock and from there to barren magnetite-rich,oxide-facies laminae of BIF (Fig. 5E).

6) Sulfide-bearing quartz veins obliquely truncate foldlimbs in oxide-facies BIF. This feature is inter-preted as an indication that veins post-date foldingand that the banded sulfide in the iron formation isspatially associated with the veins (Fig. 5E).

7) At localities where detailed mapping was under-taken (Spencer, 1989), it has been shown that thesinistral, en échelon vein array does not affect thefoliation of the BIF (Fig. 5C), thereby indicating

Fig. 13. Lower-hemisphere stereographic projection of the São Bento BIF showing: (A) the distribution of the foliation, Sn,. Maximum: N37E/52SE;(B) the distribution of the mineral lineation, Lm, of the São Bento BIF. Maximum: S54E/52; (C) the distribution of the fold axis, Bn, of the São BentoBIF. Maxima: N77E/44; S67E/50; S10W/47.


Fig. 14. Schematic map of the distribution of quartz–sulfide veins at level 25. Note the en échelon array of the vein orientation in relation to thecompositional layering of the São Bento iron formation. WIDS: west horizon inter-drive south (underground local grid).


that this array is a primary structural feature and notthe result of disruption by planar sulfide-bearingquartz veins.

8) The ore minerals are hosted in heterogeneouslybanded, cream-colored quartz–carbonate (ankerite)-rich rock with variable proportions of magnetite(Fig. 11B). Notably, these rocks are the same in allore horizons, and interpreted to represent carbonatealteration haloes that enclose the ore.

A sinistral en échelon array of veins (Fig. 14) impliesa system of dextral, strike–slip, lateral shearing(Spencer, 1989; Martins Pereira, 1995). The low angle(∼ 5°), that exists between this en échelon system ofveins and the plane of shearing may be explained as the

result of attenuation of the en échelon system during alater compressional event, when the sulfide-bearingquartz veins were internally folded in sheaths (Spencer,1989; Martins Pereira, 1992, 1995: Figs. 5C, D and 14).The pervasive shearing of the Nova Lima successionsupports the conclusion as interpretation that compres-sional structural zones existed at the northeastern borderof the Quadrilátero Ferrífero.

Indications of zones of extreme ductile deformationare noted in road cuttings near the town of SantaBárbara. More competent units and quartz veins (chertybands?) in the schists are attenuated and boudinaged.Small-scale sheath-fold-like and hook-structures arecommon in the schists. The presence of crenulationcleavages indicates that these are oblique to Lm.


Evidence from exposures throughout the QF and the SãoBento deposit indicate that they transect Lm and aretherefore of late origin. There are also quartz–carbonateveins containing pyrrhotite and pyrite which areinterpreted to be a late stage. They are non-auriferousand are associated with both vertical and horizontalterminations of the ore horizons, cross-cutting thelayering (Fig. 11C).

Martins Pereira (1995) proposed a geological modelin which an original, N60E-directed structure wasrotated, generating sheath folds and associated dip–slip movements and late brittle structures. In the finalstages, mafic dikes were emplaced, displacing themineralized zones to the northeast.

6.3. Structural framework

The first structural event D1, described by Ladeiraand Viveiros (1984), is ductile in nature and producedrecumbent, isoclinal, reclined and inclined folds; theirhinge lines Bn−1 have a Sn−1 axial–planar schistositythat, along the limbs, is parallel to the compositionallayering of the BIF and carbonaceous schists. All foldhinges and linear fabrics plunge approximately N60E toS60E/35–45, and could be interpreted to represent eithera continual variation or a bimodal population. Thisdeformation affected the Rio das Velhas rocks and maybe interpreted as a result of a compression from NE toSW via overthrusts and low angle thrusts. At the SãoBento gold deposit, the fold hinges oriented at N77E/44(Fig. 13C) represent the Bn−1 folds. Probably during thesame event, D1, the Rio das Velhas rocks underwent latestrike–slip tectonics.

A later compressional, ductile deformation event(D2) overprinted and/or rotated any previous structuresand all the Nova Lima rocks at São Bento (Lm: S55E/52,Fig. 13B). The overall geometry identified in the SãoBento BIF was generated in this event. The D3 event isconsidered to be a more brittle stage of D2 and generatedfractures and crenulation cleavages in previouslyfoliated rocks (Martins Pereira, 1995).

7. Proposed evolution for the São Bento gold deposit

Any proposed evolution for the São Bento depositmust take into account the few geochronological dataavailable for this and other BIF-hosted gold deposits ofthe QF. The Pb–Pb analyses of refractory arsenopyriteand pyrite from bedded and remobilized ore plot on asingle-stage growth model curve at 2.65 Ga (De Wittet al., 1996). A single fraction of rutile from a shear-related, gold–sulfide-bearing zone within a felsic

metavolcanic rock, near the town of Caeté (15 kmfrom São Bento; Fig. 2 of Baltazar and Zucchetti, 2007-this volume), is highly discordant (24%) and yields a207Pb/Pb206 model age of 2.58 Ga (Noce, 1995), alsosuggesting a Late-Archean age for the Caeté golddeposit. Reviews of the geochronological data from theQuadrilátero Ferrífero region and its gold deposits aregiven by Noce (2000), Lobato et al. (2001a,b) and Noceet al. (2007-this volume).

The following scenario is proposed to explain theevolution of the São Bento gold deposit area (afterMartins Pereira, 1995; Ladeira and Ferreira, 2000).

1. Orebodies in the São Bento BIF-hosted golddeposit are stratabound and locally have astratiform appearance. Their development wasassociated with folding and shearing of BIF, butwere then folded and deformed with the enclosingrocks and their hydrothermal alteration products.

2. The São Bento BIF was intensely and extensivelydeformed, first into open upright folds thatsubsequently evolved into tight, asymmetricisoclinal folds. The overturned limb of thesefolds attenuated and gave way to sheath folds andthe establishment of ductile shear zones withthrust motion.

3. The São Bento, East and West mineralizedhorizons are the end result of a multi-stageprocess of ore genesis, which took place early inthe structural history of the São Bento golddeposit, in association with major transcurrent andthrusts faults. In other regions of the QuadriláteroFerrífero, one of these structures has been termedthe Rio das Velhas uplift (Dorr, 1969), which isequivalent to the Rio das Velhas anticlinorium ofLadeira (1980), or Paciência lineament (e.g.,Scarpelli, 1991; Baltazar and Zucchetti, 2007-this volume). In the region of the São BentoManifesto (lease area, Fig. 2), thrust and trans-current faults comprise the Conceição (MartinsPereira, 1995), Barão de Cocais and São Bento(Spencer, 1989) shear zones. Gold-bearing sul-fides and a pervasive foliation Sn developedduring this time.

4. Dextral movement on the SW–NE-directed Con-ceição shear zone (Fletcher, 1989; MartinsPereira, 1995) may have generated splays at acompressional side-stepping zone, such as the SãoBento shear zone— the structural loci for the SãoBento gold mineralization. This compressionalevent, D1, is considered to have resulted in uplift,such as the Rio das Velhas uplift of Dorr (1969),






and folding/deformation of the BIF in a ductileregime. At São Bento, the host rocks to themineralization were folded during D1. Thesulfide–quartz veins cut folded bands of the hostBIF suggesting that this is a structure that pre-dates the mineralization.

5. As a result of folding and shearing, sulfides mi-grated to the foliation interstices of Sn. The hingelines of these folds are thought to have an E–Worientation. The compression could have been fromN to S. Another alternative would be to postulatehinge lines with an approximate NS to NNE trendand shear compression from the SE or E.

6. Locally, pyrite is present as a very fine-grained,accessory phase in magnetite–siderite BIF, as wellas in carbonaceous schists (Fig. 5A). This pyritelies parallel to the compositional banding and isinterpreted to be the oldest sulfide generation inthe deposit (Martins Pereira, 1995). It probablypre-dates metamorphism and dynamic deforma-tion, and has a syngenetic origin (Ladeira, 1980).

7. The mineral associations of both wall rocks and theore suggest greenschist facies conditions of meta-morphism. The metamorphic mineral paragenesisprovides crustal temperatures from 250 to 350 °C,with depths of dynamo-metamorphism of about 10to 12 km (Ladeira, 1980, 1988). Fluid inclusion datasuggest 300 °C at a pressure of 3.2 kbar (Alves,1995).

8. Relaxation of the Conceição shear zone undermore brittle conditions resulted in the develop-ment of dilational features, such as tension gashes,in a relatively brittle–ductile regime. This mayhave facilitated access of auriferous fluids to thechemically and mechanically reactive BIF. Golddeposition followed, together with quartz andsulfide minerals in dilatational gashes fractures,forming an en échelon auriferous quartz–veinsystem. This system developed during the second,transcurrent-dominated stage of D1, related to therelaxation of the Conceição shear zone. Gold wasepigenetically concentrated in structural traps ofthe siderite-rich BIF through the action ofhydrothermal fluids, with development of gold–sulfide–quartz veins (Martins Pereira, 1995).

9. A later compressional (D2), ductile deformationoverprinted, rotated and flattened the whole rocksuccession. These structures, plus the geometricaland kinematic indicators, are described, forexample, by Guild (1957), Ladeira and Viveiros(1984), Grossi Sad and Pinto (1986), Belo deOliveira and Vieira (1987).

10. Despite restricted geochronology, the earlieststructures described in the area are interpreted tobe Archean (see also Lobato et al., 2001a,b).During the Archean, and the closure of the Rio dasVelhas Supergroup depository, these developed inassociation with the formation of the orebodies.

11. During the Paleoproterozoic, the TransAmazo-nian tectonic cycle affected rocks of the MinasSupergroup (e.g., Marshak and Alkmim, 1989;Alkmim and Marshak, 1998) that were uncon-formably deposited on the Rio das VelhasSupergroup. This tectonic regime caused severalmajor synformal folds on the Minas Supergroup,with inversion of limbs sliced by thrust zones. TheProterozoic deformation transported the MinasSupergroup as a piggy-back structural complex ontop of the Rio das Velhas Supergroup alongdécollement surfaces (detachment structures). Therelationship of the São Bento orebodies to thisdeformation is discussed in terms of opposingviews in 12 to 14 below.

12. Since D2 structures at São Bento are similar indirection to those affecting the Minas Supergrouprocks, Martins Pereira (1995) suggested thatthey were developed during the Proterozoic,TransAmazonian event (see also Belo-de-Oliveiraand Teixeira, 1990), including the sheath folds.This event in the Quadrilátero Ferrífero isconstrained by radiogenic data provided byMachado et al. (1992), Babinski et al. (1993),Noce (1995) and De Witt et al. (1994, 1996).Despite these evidences, it is not clear how and ifthis event really affected rocks at the deposit site,both in terms of structural and hydrothermal mod-ifications. Besides, no robust geochronologicaldata exist at São Bento to support any Proterozoicage-constrained evolutionary, structural history.

13. Isotopic resetting of the Rio das Velhas greenstonesuccession by the later TransAmazonian orogenyis depicted by the evolutionary picture presentedby Noce (1995, 2000) and Lobato et al. (2001b).There was generation of mantle-derived tonalitesand trondhjemites at 2162 to 2124 Ga andmetamorphic overprinting around 2020 Ga(Noce, 1995, 2000).

14. Other authors (e.g., Lobato et al., 2001a) refutethe interpretation that the present geometry at SãoBento is of Proterozoic age, and point to anentirely Archean evolution for the Nova–Lima-hosted gold deposits. In fact, the age relations ofthe various ductile structures and degree of oredisplacement are not yet well understood.


8. Concluding remarks

The lode-gold deposits of the Quadrilátero Ferrífero,including São Bento, share a number of geologicalcharacteristics with lode-gold deposits in other Archeancratons worldwide, particularly with those in greenschistfacies rocks (Lobato et al., 2001b). The deposits of theQuadrilátero Ferrífero resemble Late-Archean golddeposits elsewhere in the world, in terms of ore ele-ments, wall rock alteration and ore fluid composition.

There is an association between gold and BIFs inArchean greenstone belts in Canada (e.g., Fyon et al.,1983; Armitage et al., 1996), Australia (e.g., Vielreicheret al., 1994), Zimbabwe (e.g., Gilligan and Foster, 1987)and Tanzania (e.g., Borg, 1994). Most authors show thatmineralization is related to hydrothermal sulfidation oforiginal, metamorphic iron-bearing phases, mainlymagnetite. The closest analogues of the São Bentogold deposit in Brazil are deposits in the QF (Lobatoet al., 1998, 2001a), such as Raposos (Vial, 1980; Vieira,1987; Ladeira, 1991; Junqueira et al., 2007-this volume)and Cuiabá (Vial, 1988; Ladeira, 1991, Ribeiro-Rodri-gues et al., 1994, 1996, 2000). Global analogues includeLupin, NWT, Canada (Kerswill, 1993) and Vubachikwe,Gwanda greenstone belt, Zimbabwe (Fripp, 1976).

It has been suggested for other areas of theQuadrilátero Ferrífero that an Archean deformationalevent generated folds oriented 060–080, as describedfor example by Ladeira and Viveiros (1984), Grossi Sadand Pinto (1986). These are present in the São Bentoarea and are affected by strike–slip-dominated struc-tures that acted as conduits for gold-mineralizing fluidsresponsible for gold deposition. This set of D1 fabrics isconsidered to have been generated in the Archean, andto have been totally or partially transposed by a latershear compressional event, D2 (Martins Pereira, 1995).Structures pertaining to D2 are parallel to TransAmazo-nian-age structures and ascribed to this period by someauthors (e.g., Belo-de-Oliveira and Teixeira, 1990;Martins Pereira, 1995), despite the lack of precisegeochronological data.

Both carbonaceous schists and magnetite-dominantiron formation (of the Upper iron formation) behaved aszones of relatively low permeability in comparison withthe chert–carbonate-rich magnetite iron formations thattypify the São Bento host to gold. Whereas both schistsand magnetite-dominated iron formations responded ina more ductile manner, the host to gold had a morebrittle–ductile behavior. The carbonaceous schist alsoacted as a physical barrier to the fluids, enhancing fluidinteraction at the contact between the schist and theLower formation of the São Bento iron formation.

The proposed model by Martins Pereira (1995) andthe fluid inclusion studies (Alves, 1995) suggest ametamorphic origin for the mineralizing fluid. Fluidsignature is similar to that of other lode-gold deposits(e.g., Lobato et al., 2001a). A similar conclusion isproposed for other Archean banded-iron-formation-hosted gold deposits of the Quadrilátero Ferrífero(Lobato et al., 2001a). The absence of granitic rocks orporphyry stocks close to the deposit does not allow theimplication of fluids and metals of an igneous source.The nearest granitoids rocks are about 5 to 10 km fromthe gold deposits and are part of the basement complex.

The period 2.65±0.3 Ga was a time for golddeposition worldwide, including the Superior Province,the Zimbabwe craton and the Yilgarn block (Groves andFoster, 1991). The investigation of deposits in theseregions shows that the best gold provinces occur withinvolcano-sedimentary successions that formed duringepisodes of subduction and accretion (e.g., Herringtonet al., 1997), prior to the final stabilization of the ancientcratons.

The development of shear zones and related struc-tures, with tectonic transport from E to W and strongdevelopment of planar and linear fabrics, represents themost conspicuous structural event in the QuadriláteroFerrífero. Such transport direction is similar to thosementioned above for the São Bento deposit area(Martins Pereira, 1995). Although these structureswere originally interpreted by Martins Pereira (1995)as being of Paleoproterozoic age, other evidences tosupport this interpretation are lacking at this time.

The potential for further discoveries of São Bento-like gold deposits is high in the QF region. There areseveral poorly-evaluated gold prospects in BIF near SãoBento (e.g., São Jorge, Simmons, 1968), and, the recentre-evaluation of the nearby Córrego do Sítio deposit,with mining operations reopened in 2002, has providedmore incentive for exploration in this area. The strongstructural control of the mineralized iron formations andtheir better understanding by detailed mapping ofregional and district-scale discontinuities provides apotential opportunity to discover further resources(Martins Pereira, 1995).

Acknowledgements

We would like to thank Mr. Lincoln Silva (São BentoMineração S.A.) and Mr. Paul N. Wright (EldoradoGold Corporation) for permission to publish this paper.The help by other São Bento's staff personnel are alsoacknowledged: Geraldo Ferreira, Tarcísio Lima, EdmarLinhares, Paulo Santos, Luís Patrocínio and Ângela


Oliveira. The authors would like to express theirgratefulness to Dr. Eduardo A. Ladeira for his supportand discussions. Dr. David Groves kindly revised andsuggested pertinent modifications in the final manu-script. We are also thankful to Dr. J.H.D. Schorsher andF.B. Valladares for their geological discussions. Rosa-line Cristina Figueiredo e Silva and Laryssa RibeiroBarros Miranda de Sá undertook additional petrographicstudies. Luciana Costa helped with the final edition andFranciscus J. Baars revised the manuscript. LML isindebted to the Brazilian research foundation (ConselhoNacional de Desenvolvimento Científico e Tecnológico-CNPq) for personal grant and continuing financialsupport over the years. Undergraduate and graduatestudents benefit from scholarships by both CNPq andCoordenação de Aperfeiçoamento de Pessoal de NívelSuperior (CAPES).

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