Structural control and hydrothermal alteration at the BIF-hosted Raposos lode-gold deposit, Quadrilátero Ferrífero, Brazil - PDF

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Available online at Ore Geology Reviews 32 (2007) Structural control and hydrothermal alteration at the BIF-hosted Raposos lode-gold deposit,

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Available online at Ore Geology Reviews 32 (2007) Structural control and hydrothermal alteration at the BIF-hosted Raposos lode-gold deposit, Quadrilátero Ferrífero, Brazil P.A. Junqueira a,, L.M. Lobato b, E.A. Ladeira c, E.J.M. Simões d a CPRM Serviço Geológico do Brasil, Belo Horizonte, Av Brasil 1731, Funcionários, Belo Horizonte, Minas Gerais, , Brazil b Departamento de Geologia, IGC-CPMTC, Universidade Federal de Minas Gerais, Av Antônio Carlos 6627, Pampulha, Belo Horizonte, Minas Gerais, , Brazil c Geotecmin Geologia e Tecnologia Mineral Ltda., Rua dos Aimorés 462/606, Funcionários, Belo Horizonte, Minas Gerais, , Brazil d FIEMG Federação das Indústrias do Estado de Minas Gerais, Belo Horizonte, Av do Contorno, 4520, 8 andar, Funcionários, Belo Horizonte, Minas Gerais, , Brazil Received 25 April 2004; accepted 29 March 2006 Available online 15 February 2007 Abstract In the Raposos orogenic gold deposit, hosted by banded iron-formation (BIF) of the Archean Rio das Velhas greenstone belt, the hanging wall rocks to BIF are hydrothermally-altered ultramafic schists, whereas metamafic rocks and their hydrothermal schistose products represent the footwall. Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events (D 1,D 2 and D 3 ). A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D 3. The orebodies constitute sulfide-bearing D 1 -related shear s of BIF in association with quartz veins, and result from the sulfidation of magnetite and/or siderite. Pyrrhotite is the main sulfide mineral, followed by lesser arsenopyrite and pyrite. At level 28, the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping BIF define a gross zonal pattern surrounding the ore s. Metabasalt comprises albite, epidote, actinolite and lesser Mg/Fe chlorite, calcite and quartz. The incipient stage includes the chlorite and chlorite-muscovite alteration. The least-altered ultramafic schist contains Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. The incipient alteration stage is subdivided into the talc chlorite and chlorite carbonate. For both mafic and ultramafic wall rocks, the carbonate albite and carbonate muscovite s represent the advanced alteration stage. Rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock. In the case of the ultramafic schists, the precursor may have been peridotitic komatiite. The Eu anomaly of the Raposos BIF suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor. The ore fluid composition is inferred by hydrothermal alteration reactions, indicating it to having been H 2 O-rich containing CO 2 +Na + and S. Since the distal alteration halos are dominated by hydrated silicate phases (mainly chlorite), with minor carbonates, fixation of H 2 O is indicated. The CO 2 is consumed to form carbonates in the intermediate alteration stage, in halos around the chlorite-dominated s. These characteristics suggest variations in the H 2 OtoCO 2 -ratio of the sulfur-bearing, aqueous-carbonic ore fluid, which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration Published by Elsevier B.V. Keywords: BIF-hosted gold; Archean greenstone belt; Hydrothermal alteration; Structural control; Quadrilátero Ferrífero; Brazil Corresponding author. address: (P.A. Junqueira) /$ - see front matter 2007 Published by Elsevier B.V. doi: /j.oregeorev 630 P.A. Junqueira et al. / Ore Geology Reviews 32 (2007) Introduction The Quadrilátero Ferrífero (QF), Minas Gerais State, Brazil, was the most important gold-producing region in Brazil from the early 18th Century until the late 1970s, and represents a world-class gold province. The most important lode-gold deposits (orogenic deposits as defined by Groves et al., 1998) are hosted by rocks of the Archean Rio das Velhas greenstone belt, and are located in the northern portion of this region. These include Morro Velho, Cuiabá, São Bento, Raposos, Faria, Bicalho, and Bela Fama (Fig. 1). Papers dealing with Morro Velho, Cuiabá and São Bento can be found elsewhere in the present volume (Vial et al., 200X; Ribeiro-Rodrigues et al., 200X; Martins Pereira et al., 200X, respectively). The Raposos gold deposit, mined underground by Mineração Morro Velho Ltd., is situated in the northwestern portion of the QF, about 35 km SE of Belo Horizonte (Fig. 1). It was acquired in 1899 by the Fig. 1. Simplified geological map of the Quadrilátero Ferrífero region, showing the most important gold deposits (compiled from Dorr, 1969). The only operating mines at present are Cuiabá and São Bento. P.A. Junqueira et al. / Ore Geology Reviews 32 (2007) Fig. 2. Simplified geological map of the Raposos deposit, level 28 (modified from Junqueira, 1997). 632 P.A. Junqueira et al. / Ore Geology Reviews 32 (2007) British St. John Del Rey Mining Co., and sold in 1975 to Mineração Morro Velho S.A. The Raposos gold mine operated between 1920 and 1998, and when it was shut down over 10 Mt of gold ore had been extracted; in situ grades varied between 6.5 and 9.0 g/t Au. Gold mineralization is associated with sheared, hydrothermally altered oxide (± carbonate) facies BIF and is limited by envelopes of white mica-bearing schists in the foot- and hanging wall, respectively developed after ultramafic and mafic precursor schists (Vieira and Oliveira, 1988), and which represent proximal muscovite alteration s (Fig. 2). The deposit consists of 19 individual sulfide orebodies, with mineable areas from 100 to 1300 m 2, totaling 4000 m 2 that are distributed along the 1.6 km extent of banded iron-formation (BIF). Raposos was a deep (1423 m), mechanized mine, and exploitation took place by the cut-and-fill method with hydraulic fill. When it closed in May 1998, ore extraction was being carried out at levels 30 and 32 (1063 m depth), and mine development at levels 34 and 36 (1183 m depth). In 2002, the ore resource at Raposos stood at 3.39 Mt at an average grade of 6.97 g/t Au (Frederico W.R. Vieira, pers. comm., 2002). Fig. 3. Photographs of structural features of BIF. (A) Bedding (S 0 ) of BIF and S 1 schistosity parallel to S 0 (EW orebody, level 15). (B) D 1 shear band (the darkest portion), corresponding to the ore. Foliation S 2 is a spaced cleavage in BIF (EW orebody, level 15). (C) D 1 intrafolial folds. The ore is the darkest portion, where the bedding is completely destroyed (EW orebody, level 15). (D) Detailed view of a sheared/sulfidized (Espírito Santo orebody, level 24). (E) D 3 S-C foliations in a diabase dike indicating a clockwise movement (level 28). P.A. Junqueira et al. / Ore Geology Reviews 32 (2007) Fig. 4. Geological map of the Espírito West (EW) orebody, level 30 of the Raposos Mine (modified from Vieira and Simões, 1992). 633 634 P.A. Junqueira et al. / Ore Geology Reviews 32 (2007) Fig. 5. Detail of hand sample of carbonaceous metapelite showing D 2 micro folds and S 1 foliation parallel to S 0. The foliation S 2 has a transposition character (level 28). Vieira (1987a,b) gave pioneering detail about mineral assemblages related to the gold-associated hydrothermal alteration affecting rocks of the greenstone belt succession. Other notable contributions are those by Vieira (1988, 1991a) and Vieira and Simões (1992). Textural, mineralogical and chemical relationships have also been addressed by Ladeira (1980, 1985), Godoy (1994), Martins-Pereira (1995), Pereira (1996), Junqueira (1997) and Ribeiro-Rodrigues (1998), among others. The alteration and mineralization styles of selected gold deposits in the QF have been described and compiled by Lobato et al. (1998, 2001a). This paper presents a description of the BIF-hosted, orogenic Raposos gold deposit and a model for structural control of ore emplacement. The main focus of the paper is the mineralogical and geochemical aspects Fig. 6. Sketch geological map of the Raposos deposit, level 24, showing some of the main orebodies (modified from Vieira, 2000). P.A. Junqueira et al. / Ore Geology Reviews 32 (2007) Table 1 Estimated percentage of sulfide content in selected thin sections of BIF, Espírito Santo orebody at levels 28 and 30 of the Raposos Mine Thin section Gq1- S Gq3- S18 Gq13- S11 Gq2- S18 of the mafic and ultramafic wall schists, with special emphasis on the alteration zoning, based mainly on the work by Junqueira (1997). In light of this integrated information a genetic model for the deposit is proposed. 2. Geological setting of the Raposos gold deposit Pyrrhotite _ Arsenopyrite _ 1 10 b Pyrite _ b1 b Chalcopyrite b1 _ 1 b1 tr b1 1 Au (ppm) 0.41 _ The amount of sulfides increases from left to right. Note: in the thin section Gq13-S11, gold grains are included in pyrrhotite, carbonate and pyrite, whereas in Gq2-S18 finer gold grains are included in arsenopyrite. The Raposos gold deposit is associated with BIF of the Nova Lima Group, the basal section of the Rio das Velhas Supergroup (Vieira, 1987a), which hosts all the most important lode-gold deposits in the QF. The group comprises a classic greenstone succession, with ultramafic through mafic to intermediate volcanic rocks, and subordinate felsic volcanic rocks. Although there is no widely accepted stratigraphy for the Nova Lima Group as a whole (Dorr, 1969; Ladeira, 1980; Oliveira et al., 1983; Vieira and Oliveira, 1988; Ladeira, 1991; Baltazar and Pedreira, 1996), a review of earlier and newer data on the Rio das Velhas greenstone belt has been provided by Lobato et al. (2001b). Baltazar and Pedreira (1996) proposed a subdivision of the Nova Lima rocks based on lithofacies associations. The sequence displays mineral associations typical of the greenschist metamorphic facies. A new stratigraphic sequence of the Nova Lima Group is presented by Baltazar and Zucchetti (200X, this volume). The base of the Nova Lima Group encompasses mafic ultramafic rocks, with massive, amygdaloidal, variolitic and locally pillowed metabasalts, intercalated with oxide facies banded iron formation, metachert, and carbonaceous schist; spinifex-textured peridotitic komatiites and cumulate sills can also display minor banded iron formation. Volcanoclastic rocks are represented by pyroclastic dacitic tuffs and agglomeratic horizons with minor lava flows. Volcanogenic and metasedimentary rocks show intercalations of felsic pyroclastic and epiclastic horizons (graywackes) with minor carbonaceous schist and carbonate schists. Metasedimentary rocks of mixed sources include quartz mica feldspar chlorite, and carbonaceous schists (metapelites, metagraywackes and metapsammites). Recent reviews of the geochronology of the Quadrilátero Ferrífero are presented by Noce (2001) and Lobato et al. (2001a,b). With regards to the Nova Lima Group and its gold deposits, these authors conclude that: 1. The evolution of the Rio das Velhas greenstone belt was associated with a major tectono-metamorphic and magmatic period at Ma (Machado and Carneiro, 1992; Carneiro et al., 1998). 2. Mineralization must be older than the maximum age for the onset of Minas Supergroup sedimentation (2606±47 Ma; Machado et al., 1996), and was probably contemporaneous with or is older than minimum Table 2 Electron microprobe analyses of chlorites from the Raposos metabasalts (Junqueira, 1997) Sample GP-R901 GP-R901 ES1-S10 ES1-S10 number Pre-alteration Chlorite-muscovite Chlorite Ripidolite Picnochlorite Ripidolite Ripidolite SiO TiO 2 n.d Al 2 O FeO MnO MgO CaO Na 2 O K 2 O n.d Cr 2 O Total Cations on the basis of 36 bo,ohn Si Al Ti n.d Fe Mn Mg Ca Na K n.d. Cr Total Fe/(Fe+Mg) Note: The average estimated H 2 O content is 11 to 12%. n.d. not detected. Classification after Hey (1954). The degree of hydrothermal alteration increases from left to right. 636 P.A. Junqueira et al. / Ore Geology Reviews 32 (2007) Pb Pb model ages from the São Bento and Cuiabá deposits (2650 and 2670 Ma; DeWitt et al., 1994; Noce et al., 200X, this volume, respectively). The numerous geochronological data available for the QF region, Noce (2001) and Noce et al. (200X, this volume, and references therein) also indicate that: (i) the granite gneissic complexes derive from igneous protoliths older than 2900 Ma; (ii) three granitic magmatic episodes affected the region during the NeoArchean (at ca Ma, Ma, and 2600 Ma); two felsic magmatic events associated with the greenstone belt sequence are separated in time (ca Ma and 2772 Ma), the youngest constraining a major magmatic and tectonic event; (iii) although the main mineralization event for the gold deposits has not been dated unequivocally, the results known to date point to an age of 2700 Ma; (iv) Proterozoic Lake Superior-type banded iron formations were deposited at ca Ma; (v) the eastern part of the QF was also affected by the Brasiliano Orogeny (600 to 560 Ma). 3. Mine-scale structures Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events namely D 1,D 2 and D 3. A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D 3 (Ladeira et al., 1991; Junqueira, 1997). Structures assigned to D 1 include large-scale, ductile to ductile brittle shear s (SZ 1 ), mylonitic foliation, a stretching lineation and intrafolial folds and dislocated grains (Fig. 3A C), typical mainly in the BIF. SZ 1 are bounded by hydrothermally altered, blastomylonitic schists, which are 200 m thick and more than 3 km wide. These are the most important structures of the deposit, since they are interpreted to relate to both the hydrothermal alteration and the gold mineralization. The F 1 folds are tight, with E W hinge lines parallel to the stretching lineation (L 1 ), which plunges from 30 to 12 (at 1400 m depth) towards east. The folds form type-3 interference patterns of Ramsay (1967). At level 30, the geological map of the Espírito West orebody displays a relict F 1 fold in the western extremity (Fig. 4). The axial-planar foliation, S 1,is parallel to the bedding (S 0 ) of both BIF and metapelites, and dips from 035 to 120 at between 25 and 40 (Fig. 5). The foliation S 1 develops a mylonitic or transposition character in highly deformed s. Shear s SZ 1 are roughly parallel to S 1 and destroy the BIF S 0, which is preserved only in elongated relics within shear s (Fig. 3A). D 2 is defined by inclined similar folds verging towards north, which collectively form a major inclined and overturned fold that defines the shape of the deposit (Figs. 4 and 6). The axial-planar, NE-striking S 2 foliation dips towards 120 to 150 at between 45 at surface, and 12 at level 44 (1430 m depth); it is a Table 3 Electron microprobe analyses of carbonates from the Raposos metabasalts (Junqueira, 1997) Sample number GP-R901 GP-R901 2,3-10A 2,3-10A ES1-S10 GP17-S1 GP17-S1 GP17-S1 GP4-S18 Pre-alteration Chlorite muscovite Chlorite muscovite Carbonate-albite Carbonate Calcite Calcite Ankerite Ankerite Calcite Ankerite ferroan Ankerite ferroan Siderite magnesian Fe(CO 3 ) Mn(CO 3 ) Mg(CO 3 ) Ca(CO 3 ) Total Carbonatealbite Ankerite Cations on the basis of 6 bon Mg Fe Mn Ca Total Classification of carbonate after Deer et al. (1966). The degree of hydrothermal alteration increases from left to right. Note: values recalculated to 100% or to the total of the cations since some results of this element were overestimated during the analyses. Table 4 Electron microprobe analyses of chlorites from the Raposos ultramafic schists (Junqueira, 1997) Sample number 2,3-11 GP14-S11 GP14-S11 GP14-S11 GP14-S11 2,3-6 2,3-6 2,3-6 2,3-6 2,3-6 2,3-4B GP5-S18 GP5-S18 Prealteration Talc chlorite Talc chlorite Carbonate albite Carbonate muscovite Chlorite Ripidolite Picnochlorite Picnochlorite Picnochlorite Picnochlorite Picnochlorite Picnochlorite Picnochlorite Picnochlorite Ripidolite Ripidolite Ripidolite Ripidolite SiO TiO 2 n.d Al 2 O FeO MnO MgO CaO Na 2 O n.d. n.d K 2 O n.d n.d Cr 2 O NiO Total Cations on the basis of 36 bo,ohn Si Al Ti n.d. n.d n.d. n.d n.d Fe Mn 0.02 n.d n.d Mg Ca n.d n.d. n.d n.d n.d Na n.d. n.d n.d. n.d K n.d. n.d. n.d n.d. n.d. n.d. Cr n.d Ni Total Fe/(Fe+Mg) Classification of chlorite based on Hey (1954). The degree of hydrothermal alteration increases from left to right. Note: The average estimated H 2 O content is 11 to 12%. %. n.d. not detected. P.A. Junqueira et al. / Ore Geology Reviews 32 (2007) 638 P.A. Junqueira et al. / Ore Geology Reviews 32 (2007) transposition foliation in schists (Fig. 5), and a spaced cleavage in BIF (Fig. 3B). The hinge lines, b 2 and b 1, are grossly coaxial and parallel to mineral and stretching lineations. F 2 folds are generated by shearing as defined by drag folds at the extremities of the Espírito West and Espírito Santo orebodies (Fig. 2). These shear s are ductile (in schists) or ductile to ductile brittle (in BIF) and are roughly parallel to S 2. The trend of diabase dikes that crosscut the orebodies is also parallel to S 2. The dikes display S 2 schistosity along their borders, suggesting that their emplacement occurred during the late stages of D 2. The D 3 event occurred in the brittle ductile regime and generated shear s (SZ 3 ), spaced and crenulation cleavages (S 3 ), and lineations (L 3 ). The SZ 3 shear s trend N S/15 E (C foliation) and are common in both schists and diabase dikes (Fig. 3E). A decrease in S 2 and L 2 dip values down plunge with depth suggests a thrust ramp. The S-C foliations indicate tectonic transport from east to west. The S 3 foliation has an N S trend and dips between 70 and 90 to WSW. Gravity step faults that are parallel to S 3 are common and displace the orebodies, with small net slips. The crenulation of S 2 and the intersection lineations between S 3 and S 2 or S 1 generate L 3. North S and E W subvertical, spaced cleavages and joints overprint D 3 structures and are considered to represent a late structural phase. 4. Gold mineralization in the Raposos deposit Several publications address the geology and mineralization at the Raposos gold deposit, e.g., Tolbert (1964), Vial (1980), Ladeira (1980), Vieira (1987a,b, 1988, 1991a,b,c), Ladeira et al. (1991), Vieira and Simões (1992), Godoy (1994), Junqueira (1997) and Lobato et al. (1998). Gold mineralization is associated with sulfidized BIF, which is limited by envelopes of white micaceous schists in the foot- and hanging wall, respectively developed after ultramafic and mafic precursor schists (Vieira and Oliveira, 1988). The non-deformed and unaltered Raposos BIF studied at level 28 displays alternate layers of fine-grained granoblastic siderite, ankerite and rarely, magnetite and quartz. Where sulfide minerals are present, pyrrhotite is the dominant phase. Non-mineralized BIF, with segments in which S 0 is preserved, has an average gold grade of 2.5 g/t. All orebodies constitute sulfide-bearing SZ 1 shear s of BIF (Fig. 3B, D), except the Ouro Preto orebody which is associated with quartz boudins hosted by talc schist. Orebodies hosted in BIF result from the replacement of magnetite and/or siderite. Gold-bearing Table 5 Electron microprobe analyses of carbonates from the Raposos ultramafic schists (Junqueira, 1997) Sample number 2,3-11 2,3-11 2,3-11 GP14-S11 GP14-S11 GP14-S11 GP14-S11 2,3-6 2,3-6 2,3-6 2,3-4B 2,3-4B GP5-S18 GP5-S18 Pre-alteration Talc chlorite Talc chlorite Carbonate albite Carbonate muscovite Carbonate Calcite Calcite Calcite Dolomite Dolomite Dolomite Dolomite Calcite Calcite Calcite Ankerite Ankerite Ankerite ferroan Ankerite Fe(CO 3 ) Mn(CO 3 ) Mg(CO3) Ca(CO3) Total Cations on the basis of 6 bon Mg Fe Mn Ca Total Note values recalculated to 100% or to the total of the cations since some results of this element were overestimated during the analyses. Classification of carbonate based on Deer et al. (1966). The degree of hydrothermal alteration
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