Economic Geology; January 2005; v. 100; no. 1;
p. 7-28; DOI: 10.2113/100.1.0007
© 2005 Society of Economic Geologists
Geology and SHRIMP U-Pb Geochronology of the Igarapé Bahia Deposit, Carajás Copper-Gold Belt, Brazil: An Archean (2.57 Ga) Example of Iron-Oxide Cu-Au-(U-REE) Mineralization
Fernando H. B. Tallarico
Instituto de Geociências, Universidade Estadual de Campinas, Caixa Postal 6152, 13.083-970, Campinas, SP, Brazil, and Centre for Global Metallogeny, School of Earth and Geographical Sciences, University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia
Bernardino R. Figueiredo
Instituto de Geociências, Universidade Estadual de Campinas, Caixa Postal 6152, 13.083-970, Campinas, SP, Brazil
David I. Groves,
Natalie Kositcin,
Neal J. McNaughton and
Ian R. Fletcher
Centre for Global Metallogeny, School of Earth and Geographical Sciences, University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia
José L. Rego
Companhia Vale do Rio Doce, Caixa Postal 51, Serra dos Carajás, 68.516-000, Parauapebas, PA, Brazil
Corresponding author: e-mail, dgroves{at}segs.uwa.edu.au
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Abstract
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A striking feature of the Carajás region, Brazil, is the clustering of a variety of different types of Cu-Au deposits. The most abundant in the belt are the >200 million metric tons (Mt) of Fe oxide Cu-Au-(U-REE) deposits, which, despite the variety of host rocks and different orebody morphologies, share a number of diagnostic features, including (1) intense Fe metasomatism leading to the formation of grunerite, fayalite, and/or Fe oxides (magnetite and/or hematite); (2) intense carbonate alteration (mainly siderite); (3) sulfur-poor ore mineralogy (chalcopyrite and bornite); (4) quartz-deficient gangue; (5) extreme low REE enrichment, and (6) enrichment in U and Co. The Igarapé Bahia deposit is perhaps the best documented Fe oxide Cu-Au-(U-REE) deposit of the belt, containing about 219 Mt at 1.4 percent Cu and 0.86 g/t Au. The Cu-Au ore consists of steeply dipping breccia bodies that are hosted by hydrothermally altered metavolcano-sedimentary rocks.
SHRIMP II zircon dating of the host metavolcanic rocks gives a 207Pb/206Pb age of 2748 ± 34 Ma. This suggests a correlation between the Igarapé Bahia volcano-sedimentary sequence and the Grão Pará volcanic rocks, which have published ages of ca. 2.75 Ga. SHRIMP dating of monazite from the matrix of ore-bearing magnetite breccias gives a 207Pb/206Pb age of 2575 ± 12 Ma, confirming the epigenetic nature of the mineralization and placing it ~175 m.y. after accumulation of the host volcano-sedimentary sequence. The 2575 ± 12 Ma SHRIMP age of hydrothermal monazite from the Igarapé Bahia mineralization is indistinguishable from published conventional 207Pb/206Pb ages for zircons from the Archean A-type granites of the Carajás belt, indicating that mineralization processes at Igarapé Bahia were temporally related to these A-type Archean granites. The wide range of highly radiogenic 87Sr/86Sr ratios (0.714–0.755) of carbonates from the Igarapé Bahia deposit suggests multiple crustal sources, consistent with a magmatic-hydrothermal origin. SHRIMP dating of zircon xenocrysts recovered from crosscutting diabase dikes indicates a maximum 207Pb/206Pb age of ~2670 Ma, consistent with field evidence and the age of host rocks, but does not unequivocably constrain the age of the ores.
The styles of hydrothermal alteration, mineralogy, and geochemistry of the Igarapé Bahia ore, as well as published fluid inclusion and stable isotope data, support its classification as a member of the world-class Olympic Dam-type Fe oxide Cu-Au-(U-REE) group of deposits, as previously argued by several authors. The SHRIMP age of 2575 ± 12 Ma for hydrothermal monazite indicates that Igarapé Bahia is an Archean example of this deposit group.
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Introduction
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THE CARAJÁS bELT is a highly mineralized province, with world-class iron and manganese deposits (Tolbert et al., 1971; Valarelli et al., 1978; Gibbs and Wirth, 1990) and also a large number of Cu-Au deposits, known collectively as the Carajás Copper-Gold belt (Fig. 1). There appears to be two distinct types of Cu-Au deposits in the Carajás region. The first group includes Cu-Au-(W-Bi-Sn) deposits (e.g., Breves and Águas Claras), which contain quartz veins and may or may not have associated Fe oxides and are genetically related to the cooling of Paleoproterozoic (ca. 1.88 Ga) A-type granites, as discussed below. The second group includes several world-class (>200 Mt) Fe oxide Cu-Au-(U-REE) deposits (e.g., Salobo, Sossego, Cristalino, Cento e Dezoito, and Igarapé Bahia), which, despite the variety of host rocks and different orebody morphologies, share a number of diagnostic features.
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FIG. 1. Geologic map of the Carajás Copper-Gold belt, showing the major iron, manganese, Fe oxide Cu-Au-(U-REE) deposits and the Breves Cu-Au-(W-Bi-Sn) deposit. Modified from Tallarico et al. (2004). The Itacaiúnus shear zone is a broad zone centered on the Carajás fault and bounded by the two major faults (bold lines) shown to the north and south.
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Igarapé Bahia is the only gold deposit presently being mined in the region and is perhaps the best documented example of the Fe oxide Cu-Au-(U-REE)-type in the Carajás belt. The development of the deposit began in 1974 when Rio Doce Mineração e Geologia S.A. (Docegeo), the exploration branch of Companhia Vale do Rio Doce, located significant copper anomalies in stream-sediment samples from the Igarapé Bahia River. During the 1980s, several soil geochemistry surveys resulted in the discovery of three outcropping orebodies, which were subsequently drilled: Acampamento Norte, Acampamento Sul, and Furo Trinta. Mining began in 1991, with a processing capacity of 80,000 metric tons (t) of ore per month. In 1994, the capacity of the plant had doubled. In 1996, the Alemão orebody (ALM) was located at a depth of 250 m, through geological and geophysical modeling of an airborne magnetic anomaly followed by drilling (Barreira et al., 1999). The total resource of primary ore, including the four orebodies, is 219 Mt at 1.4 percent Cu and 0.86 g/t Au. The original reserves of oxidized gold ore were 15 Mt of high-grade ore (5 g/t Au) and 14 Mt of low-grade ore (1.4 g/t Au). To date, total production of gold is approximately 92 t, exclusively from supergene-enriched ore in the oxidized zone.
Previous genetic interpretations of the Igarapé Bahia deposit included syngenetic volcanic-hosted massive sulfide deposit (VHMS; Besshi-type) models (e.g., Ferreira Filho, 1985; Almada, 1998; Villas and Santos, 2001), epigenetic hydrothermal-magmatic models with ore related to Paleoproterozoic (ca. 1.88 Ga) granites (e.g., Lindenmayer et al., 1998), and multistage models that invoke an interplay of Archean and Proterozoic processes (e.g., Ribeiro, 1989; Mougeot et al., 1996). However, similarities with the Australian Olympic Dam deposit (Roberts and Hudson, 1983; Oreskes and Einaudi, 1990; Reeve et al., 1990) have encouraged several authors to propose an epigenetic hydrothermal model related to an alkaline magmatic component of unknown age (Oliveira et al., 1998; Tallarico et al., 1998; Soares et al., 1999; Tazava, 1999). Uncertainties in interpretation relate partly to poor exposure of fresh rocks but are mainly due to the absence of robust geochronologic data on the ore and host rocks.
The aim of this paper is to provide an overview of the geology and geochemistry of the Igarapé Bahia deposit and to constrain the age of the mineralization using SHRIMP geochronology. These data are used to test whether the deposit is contemporaneous with the Archean volcanic sequence or with alkaline granitoids of either Late Archean or Paleoproterozoic age.
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Regional Geology and Metallogeny
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The Carajás region (Fig. 1) lies on the eastern portion of the Archean Amazon craton, being truncated to the east by the Neoproterozoic Araguaia belt and to the west by Proterozoic sequences (Docegeo, 1988; Tassinari and Macambira, 1999). To the north, it is concealed by Proterozoic and Cenozoic sedimentary rocks of the Amazon basin (Pinheiro and Holdsworth, 1997). To the south, it is in contact with the Rio Maria block (Docegeo, 1988; Huhn et al., 1988), a relatively older, typical granitoid-greenstone terrane that hosts lode gold deposits, such as the Sapucaia and Cumaru deposits (Villas and Santos, 2001, and references therein).
Basement rocks consist of gneiss and migmatite of the Xingu Complex and orthogranulites of the Pium Complex that were metamorphosed at ca. 2.8 Ga (Machado et al., 1991; Rodrigues et al., 1992; Pidgeon et al., 2000). According to Araújo et al. (1988), the Pium Complex represents slabs of infracrustal rocks uplifted through deep shear zones and could possibly represent the suture zone between the Rio Maria block and the Carajás belt. In the Carajás belt, the basement assemblage defines a broad, steeply dipping, east-west–trending ductile shear zone (Itacaiúnas shear zone), which experienced multiple episodes of reactivation during the Archean and Paleoproterozoic (Pinheiro and Holdsworth, 1997; Holdsworth and Pinheiro, 2000).
The Carajás region includes one of the best preserved Archean volcano-sedimentary successions in the world. These rocks, assembled under the Itacaiúnas Supergroup, record different metamorphic grades, ranging from virtually unmetamorphosed and undeformed, to lower greenschist facies (Grão Pará Group) in the innermost portions of the belt, and intensely sheared amphibolite and/or granulite facies (Salobo Group) at the northern edge of the Itacaiúnas shear zone (Docegeo, 1988; Olszewski et al., 1989). Volcanism occurred at ca. 2.75 Ga (e.g., Machado et al., 1991; Trendall et al., 1998). The Grão Pará Group is the dominant volcano-sedimentary sequence in the Carajás basin and includes low-grade metavolcanic rocks that host significant banded iron formation (BIF). These contain the giant iron deposits, such as Serra Norte, Serra Sul, Serra Leste, and Serra de São Felix, that contain total resources of 17.3 Bt at 66 percent Fe (Tolbert et al., 1971; Gibbs and Wirth, 1990).
Sandstone and siltstone deposited in a shallow marine to fluvial environment (Águas Claras Formation, Nogueira et al., 1994, 2000) overlie the volcanic rocks of the Grão Pará Group. A minimum age of 2645 ± 12 Ma for the Águas Claras Formation was determined by Dias et al. (1996) by conventional U-Pb analysis of zircons from crosscutting gabbro dikes. A SHRIMP U-Pb age of 2681 ± 5 Ma was determined by Trendall et al. (1998) from a sandstone sample containing zircons apparently derived from syndepositional volcanic rocks. Based on this age, Trendall et al. (1998) inferred that the Águas Claras Formation could represent the uppermost unit of a continuous accumulation in the Grão Pará basin that lasted less than 100 m.y.
The Carajás region has been intruded by granitic magmas of distinct ages and compositions. Paleoproterozoic intrusions (ca. 1.88 Ga; Machado et al., 1991) include several anorogenic granitic plutons, such as the Central Carajás and Cigano granitoids. The latter belong to the most extensive A-type Proterozoic province of the world (e.g., Santos et al., 2001) that covers thousands of square kilometers of the Amazon craton. Archean intrusions include granitoids and diorites of the Plaquê Suite (ca. 2.74 Ga; Huhn et al., 1999b) and younger alkaline granitoids (ca. 2.57 Ga), such as the Estrela Complex (Barros et al., 1992, 1997, 2001; Barros and Barbey, 1998), the Old Salobo Granite (2573 ± 2 Ma; Machado et al., 1991), and the Itacaiúnas Granite (Souza et al., 1996). The Estrela Complex is the best documented example of the Archean alkaline granitoids in the Carajás belt. According to Barros et al. (1997), it consists of an east-west–trending, heterogeneously sheared monzogranitic batholith that has a characteristic A-type geochemical signature defined by high K2O + Na2O, extremely high FeOtotal/FeOtotal + MgO ratios, and elevated Zr, Y, and Nb. Fabric analysis of the Estrela Complex indicates that magmatic activity was syntectonic and coincident with the closure of the Carajás basin (Barros and Barbey, 1998; Barros et al., 2001).
A striking feature of the Carajás region is the clustering of a variety of different types of Cu-Au deposits. The most abundant in the belt are Fe oxide Cu-Au-(U-REE) deposits that are hosted by a variety of rocks and have different orebody morphologies, although most are near-vertical (>75°) breccia bodies. The most significant examples of this deposit type are Salobo, with resources of 994 Mt at 0.94 percent Cu and 0.52 g/t Au (Farias and Saueressig, 1982; Vieira et al., 1988; Lindenmayer, 1990), Igarapé Bahia, with resources of 219 Mt at 1.4 percent Cu and 0.86 g/t Au (Ferreira Filho, 1985; Almada, 1998; Tazava, 1999; Tallarico et al., 2000), Sossego, with resources of 355 Mt at 1.1 percent Cu and 0.28 g/t Au (Cordeiro, 1999; Lancaster et al., 2000), Cristalino, with resources of 500 Mt at 1.0 percent Cu and 0.3 g/t Au (Huhn et al., 1999a), and Cento e Dezoito with resources of 170 Mt at 1.0 percent Cu and 0.3 g/t Au (Rigon et al., 2000).
A second style of Cu-Au deposit, characterized by a Cu-Au-(W-Bi-Sn) association, includes a number of small- to medium-sized deposits that appear to be genetically related to Proterozoic A-type magmatism. Representative examples are the Águas Claras Cu-Au-(W-Sn) deposit, with resources of 9.5 Mt of oxidized ore at 2.43 g/t Au (Soares et al., 1994; Silva and Villas, 1998) and the Breves Cu-Au-(W-Bi-Sn) deposit, with resources of 50 Mt of primary ore at 1.22 percent Cu, 0.75 g/t Au, 2.4 g/t Ag, 1,200 g/t W, 70 g/t Sn, 175 g/t Mo, and 75 g/t Bi (Tallarico et al., 2004).
A major question to be resolved in the Carajás copper-gold belt is the relationship between the relatively small (<50 Mt) Paleoproterozoic (ca. 1.88 Ga) Cu-Au-(W-Bi-Sn) deposits (Tallarico et al., 2004) and the much larger (>200 Mt) Fe oxide Cu-Au-(U-REE) deposits. One possibility is that they are both related to the ca. 1.88 Ga A-type granites, but a number of important differences between the deposits make this unlikely. In particular, the Fe oxide Cu-Au-(U-REE) deposits are characterized by (1) intense Fe metasomatism leading to the formation of fayalite (e.g., Salobo; Lindenmayer, 1990), grunerite, and/or Fe oxides (magnetite and/or hematite); (2) extensive carbonate alteration (mainly siderite), at least in the lower temperature deposits; (3) S-deficient nature of the ore sulfides (chalcopyrite, bornite, and primary chalcocite); (4) quartz-deficient nature of the gangue, (5) extreme low REE enrichment (approximately 104 x chondritic values), and (6) enrichment in U and Co. Given the common alkaline association of Fe oxide Cu-Au-(U-REE) deposits elsewhere in the world (e.g., Hitzman et al., 1992; Oreskes and Hitzman, 1993; Campbell et al., 1998; Groves and Vielreicher, 2001), it is possible that these deposits of the Carajás belt are related to more alkaline rocks than the 1.88 Ga A-type (e.g., Kerrich et al., 2000) granites, including the ca. 2.57 Ga alkaline complexes of the Carajás belt (e.g., Estrela Complex, Old Salobo Granite). In this case, the deposits of the Carajás belt would represent one of the oldest known members of the iron-oxide Cu-Au-(U-REE) group worldwide. A genetic link between copper-gold mineralization and Archean magmatism has been established recently for the Salobo iron-oxide copper-gold deposit. Requia et al. (2003) determined Re-Os ages from molybdenite and a Pb-Pb age from bornite-chalcopyrite-magnetite of 2576 ± 8 Ma, which overlaps the previously published age of 2573 ± 2 Ma for the Old Salobo Granite (Machado et al., 1991). These data confirm that main-stage mineralization at Salobo was contemporaneous with Archean granitoid magmatism of alkaline affinity in the region (Requia et al., 2003).
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Geology of the Igarapé Bahia Fe Oxide Cu-Au-(U-REE) Deposit
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The Igarapé Bahia deposit is hosted by a hydrothermally altered metavolcano-sedimentary sequence, the Igarapé Bahia Group. The host sequence, together with the Furo Trinta, Acampamento Norte, and Acampamento Sul orebodies, crop out in a small structural window within the overlying sandstone of the Águas Claras Formation (Fig. 2A). The orebodies define a circular structure at surface, with a shape similar to that of an alkaline ring complex (Fig. 2B). The Alemão orebody is covered by a 250-m-thick discordant layer of sandstone and is a faulted part of the Acampamento Norte orebody (Soares et al., 1999).
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FIG. 2. A. Geologic map of the Igarapé Bahia Cu-Au deposit and geologic cross section A-A’ through the Acampamento Sul orebody (modified from Companhia Vale do Rio Doce/Docegeo, 1996, unpub.). Cpy = chalcopyrite; mgt = magnetite; sid = siderite. B. Structural map of the Igarapé Bahia copper-gold deposit. Coordinates and depth in hole for drill holes from which dated samples were taken are given in Table 2.
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The orebodies consist of steeply dipping (~75°) breccia bodies that dip outward in broad concordance with the strike of the bedding of the host sequence. The ore-bearing breccias are sited at or close to the contact between two distinct units of the host sequence. The lower unit is dominated by volcanic and pyroclastic rocks intercalated with BIF, whereas the upper unit includes mainly sedimentary and epiclastic rocks (Fig. 3A-B) with minor BIF and volcanic rocks. The mineralized breccias include fragments of all country rocks, cemented by variable amounts of hydrothermal matrix (Fig. 3C-F).
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FIG. 3. A. Photomicrograph of the primary sedimentary bedding that has been bent around a chalcopyrite (cpy) nodule and is continuous with discordant veins containing chalcopyrite (plane-polarized light, 384/141.50 m). B. Photomicrograph of a set of discordant and concordant chlorite + chalcopyrite veins crosscutting the sedimentary bedding (plane-polarized light, 384/141.20 m). C. Photomicrograph of a mineralized Fe chlorite breccia with a fragment of laminated sedimentary rock from the hanging wall in a chlorite ± magnetite ± chalcopyrite matrix (plane-polarized light, 353/168.70 m). D. Magnetite breccia with abundant chalcopyrite (cpy), crosscut by carbonate veins (BHMAG-003/407.25 m). E. Photomicrograph of the matrix of a magnetite breccia with abundant biotite (bi) showing pleochroic halos due to uraninite inclusions (plane-polarized light, BHMAG-003/407.25 m). F. Photomicrograph of the matrix of a magnetite breccia showing the textural relationship between chalcopyrite (cpy), bornite (bn), and magnetite (mgt; reflected light, BHMAG-001/476.00 m). All drill core is stored by Companhia Vale do Rio Doce at Igarapé Bahia.
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A set of radial fractures and faults controls the emplacement of quartz diorite and diabase dikes, which disrupt the orebodies, the host volcano-sedimentary sequence, and the Águas Claras sandstone. The quartz diorite dikes display a variety of textures ranging from unaltered granophyric intergrowths, to highly altered, veined and/or brecciated rocks, whereas the diabase dikes are texturally monotonous and unaltered. Geochronology of these diabase dikes potentially provides a younger age limit on the genesis of the deposits (see below).
Supergene alteration and ore types
At Igarapé Bahia, weathering was responsible for the development of a 200-m-thick supergene profile (Zang and Fyfe, 1993; Angélica, 1996) in which gold and copper were dissolved, separated, and reprecipitated in different ore types. The upper part of the profile includes a gold-bearing gossan that extends to a depth of approximately 150 m. The gossan is composed mainly of goethite, hematite, gibbsite, and kaolinite and has traces of secondary copper minerals and REE-rich phosphates (florencite, crandalite, and rhabdophane). From 150 to 200 m, there is a transitional zone where supergene copper was precipitated. This copper-gold ore is composed of goethite and hematite, together with abundant secondary copper minerals, including native copper, chalcocite, digenite, cuprite, malachite, azurite, and pseudomalachite.
The primary copper-gold mineralization occurs below 200 m and consists mainly of Fe chlorite, siderite, and magnetite-rich breccias.
Hypogene hydrothermal alteration and breccia types
The ore-bearing breccias of Igarapé Bahia are essentially polymictic. They are classified according to matrix mineralogy as Fe chlorite breccias, siderite breccias, and magnetite breccias. The rock fragments have angular to subrounded shapes and diameters that range from a few millimeters up to 20 cm. The fragment to matrix ratio is highly variable, possibly reflecting different fluid/rock ratios or different mechanical processes accompanying brecciation.
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TABLE 2 TABLE 2. Description of Samples from the Igarapé Bahia Fe
Oxide Cu-Au-(U-REE) Mineralization Selected for SHRIMP Analysis
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Magnetite breccias exhibit a granular matrix of euhedral magnetite cemented by Cu sulfides (chalcopyrite in equilibrium with bornite), together with minor grunerite, actinolite, minnesotaite, biotite, stilpnomelane, K-feldspar, tourmaline, fluorite, siderite, ankerite, and uraninite. This mineral assemblage defines a distinctive Fe-(K)–enriched zone in the deposit.
Iron chlorite breccias and siderite breccias possess virtually the same matrix mineralogy, which is typically fine grained and includes Fe chlorite, siderite, magnetite, chalcopyrite, and minor tourmaline. Siderite breccias contain a much higher percentage of siderite, clearly distinguishing them from the iron chlorite breccias (dominated by Fe chlorite) in thin section.
At the Acampamento Sul and Furo Trinta orebodies, mineralization is hosted preferentially by siderite breccias, whereas at the Acampamento Norte and Alemão orebodies, mineralization is associated with magnetite breccias. Mineralized iron chlorite breccias occur at the margins of all orebodies (Fig. 4).
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FIG. 4. Schematic distribution of the hydrothermal alteration zones of the Igarapé Bahia Cu-Au-(U-REE) deposit (not to scale).
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Chloritization is the most widespread hydrothermal alteration type in the deposit. Mg chlorite (50 > Mg no. >30) is typically associated with calcite and dolomite in barren and weakly altered host rocks outside the breccia pipe, whereas Fe chlorite (30 > Mg no. >12) is associated with siderite, chalcopyrite, and magnetite in the mineralized breccias (Tallarico et al., 2000).
Scanning electron microscopy (SEM) investigation of the breccias has revealed the presence of trace amounts of molybdenite, galena, cobaltite, hessite, altaite, cassiterite, ferberite, scheelite, uraninite, parisite, bastnäsite, allanite, monazite, apatite, and fluorite. The breccia matrix also is reported to include chloride-rich minerals such as ferropyrosmalite (Tazava, 1999) and scapolite (Althoff et al., 1994).
Several vein types crosscut the mineralized breccias and the host metavolcanic sequence. Veins are discordant to bedding and rarely display comb structures. The most common types are calcite + chalcopyrite ± fluorite ± stilpnomelane veins, ankerite ± chalcopyrite ± gold veins, siderite ± calcite ± chlorite ± chalcopyrite veins, and chalcopyrite ± biotite ± K-feldspar ± tourmaline ± REE mineral veins. The chronology of the veins is unclear due to poor exposure and equivocal relationships in drill core.
Primary copper-gold mineralization
Primary copper mineralization at Igarapé Bahia comprises xenomorphic and poikiloblastic chalcopyrite in the matrix of breccias. Rarely, chalcopyrite is rimmed by chalcocite, digenite, and covellite due to oxidation reactions along grain boundaries. In magnetite breccias, unlike siderite and Fe chlorite breccias, the chalcopyrite is intergrown with bornite. Rare pyrite occurs as inclusions in chalcopyrite in the more sulphur rich assemblages.
The major copper-rich ores contain gold in all breccia bodies. Additional subeconomic copper is disseminated or occurs in veins or nodules in distal, altered host rocks with negligible gold content.
Chalcopyrite nodules occur in the laminated upper unit of sedimentary rocks (Fig. 3A), where the primary bedding is deformed around the nodules. The nodules are mineralogically zoned, with rims enriched in inclusions of euhedral Fe chlorite in association with minor albite and quartz and cores consisting of massive chalcopyrite, with inclusions of K-feldspar, cobaltite, thorite, monazite, and apatite. The chalcopyrite nodules are interpreted to be epigenetic because they are linked to veins with the same mineralogy and zoning pattern, which are both discordant and concordant to the sedimentary bedding (Fig. 3A-B). Rarely, these sedimentary rocks also have strata-bound replacement textures with infilling of the more porous and permeable layers of the laminated rocks with chalcopyrite.
Native gold occurs in the matrix of the breccias as fine-grained (5–20-µm) inclusions in gangue minerals (quartz, siderite, and chlorite), chalcopyrite, and rarely magnetite. The gold grains contain up to 12 wt percent Ag, but the bulk Ag grades of the ore are subeconomic. Gold may also contain inclusions of tellurides (hessite, Ag2Te) and sulfides (argentite and/or acanthite, Ag2S).
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Geochemistry
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Major, trace, and rare earth element (REE) geochemical analyses of host volcanic rocks, BIF, and ore-bearing breccias of the Acampamento Sul orebody (DDH 340) are presented in Table 1. All samples from DDH 340 were analyzed for major elements, using X-ray fluorescence spectrometry (XRF). High-precision trace and REE analyses, using laser ablation inductively coupled plasma-mass spectrometry (ICP-MS), were also performed on all samples. Gold, Pt, and Pd (30-g samples) were analyzed, using fire assay with an atomic absorption spectrometry (AAS) finish. Fluorine was determined using an ion-specific electrode, As by hydride generation with AAS finish, Hg by the cold vapor method with an AAS finish, and CO2 and S by LECO analysis. All geochemical analyses were conducted at the Lakefield-Geosol laboratory in Brazil.
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TABLE 1 TABLE 1. Geochemical Data for Host Rocks and Ore-Bearing
Breccias of the Acampamento Sul Orebody, Diamond Drill Hole 340
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The mineralized breccias contain 25 to 64 wt percent FeOtotal, 0.5 to 11 wt percent Cu, 28 to 380 ppm U, 0.5 to 15 g/t Au, 4 to 52 g/t Ag, 26 to 200 ppm Ba, 390 to 31,000 ppm F, 900 to 6,200 ppm P, 260 to 2,300 ppm La, and 450 to 4,400 ppm Ce (Table 1). The ore-bearing breccias are also enriched in MnO (0.5–3 wt %), CaO (0.5–9 wt %), Mo (50–200 ppm), and Zn (150–450 ppm) relative to the host rocks (Tallarico et al., 2000). The positive correlation between these elements suggests a common metasomatic origin. The occurrence of barite, fluorite, galena, altaite, sphalerite, molybdenite, uraninite, apatite, monazite, xenotime, bastnasite, and parisite as inclusions in chalcopyrite and gangue minerals accounts for the high Ba, F, Pb, Zn, Mo, U, REE, and P (Table 1). The high manganese is related to siderite, which contains up to 7 wt percent MnO.
Iron chlorite, siderite, and magnetite breccias have similar REE patterns (Fig. 5) characterized by strong low REE enrichment (~104 chondritic values). Although the absolute concentrations of REE vary the patterns for altered metavolcanic rocks, quartz diorite dikes, and mineralized breccias show similar low REE enrichment. The REE pattern of BIF differs significantly from that of the ore-bearing breccias, arguing against a syngenetic sea-floor volcanic origin for the Igarapé Bahia mineralization, as previously suggested by Villas and Santos (2001). The contrasting La/Lu ratios of the host metavolcanic rocks (70–250) and the ore-bearing breccias (1,000–2,500) indicate that low REE were selectively concentrated during hydrothermal alteration and mineralization. The positive correlation between La, Ce, P, Cu, and Au (Fig. 6) supports the suggestion that the low REE minerals (e.g., monazite) are part of the ore paragenesis and, therefore, are suitable for dating mineralization.
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FIG. 5. Representative rare earth element (REE) distribution patterns for host rocks and mineralized breccias from the Igarapé Bahia Fe oxide Cu-Au-(U-REE) deposit, showing the extreme low REE enrichment of the breccia ores and surrounding volcanic rocks. Chondrite values are from Evensen et al. (1978).
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FIG. 6. Geochemical plots showing the relationship between Au and Cu enrichment and enrichment in Ce and La in the host volcanic rocks and the mineralized breccias. A. Log-log plots: A. Ce (ppm) vs. Au (ppm). B. La (ppm) vs. Cu (wt %). Plots illustrate the strong correlation of REE with Au and Cu and hence the likelihood that monazite is related to Cu-Au mineralization.
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Sampling and Analytical Procedures for SHRIMP Analysis
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The samples selected for SHRIMP analysis are listed in Table 2. Bulk samples of representative intersections of the host metavolcanic rocks from the lower unit (UWA-B18C and UWA-B18D) and diabase dikes (UWA-B21D and UWA-B36A) were collected, and zircons extracted, in order to determine their crystallization ages. Samples of hydrothermal monazite (UWA-B34) from the matrix of ore-bearing magnetite breccias also were analyzed to constrain the age of ore formation.
The host metavolcanic rock samples from the lower unit display an isotropic fabric defined by abundant fine-grained chlorite associated with minor albite and quartz. The primary mineralogy has been obliterated by intense chlorite alteration. The abundance of chlorite and the limited amount of quartz indicate a mafic composition, which is confirmed by previous geochemical investigations of the Igarapé Bahia metavolcanic rocks (e.g., Ferreira Filho, 1985).
The diabase dikes crop out as extremely weathered saprolite. Fresh samples are only obtained by drilling some 200 m below the surface. Field relationships indicate that the dikes crosscut both the host volcanic rocks and the ore (mineralized breccias). The samples collected for geochronology show no evidence of hydrothermal alteration or overprinting, display an ophitic texture, and are characterized by abundant plagioclase and clinopyroxene with accessory magnetite.
Magnetite-rich breccias crop out as extremely altered gossans. Fresh material was only reached through drilling below a 200-m-vertical depth. The breccia samples may include fragments of host metavolcanic rock and BIF that occur in the matrix of euhedral magnetite with minor chlorite, siderite, fluorite, grunerite, biotite, microcline, and sericite. Copper sulfides (mostly chalcopyrite with lesser bornite) are paragenetically late and surround other phases in the matrix. Gold is present as inclusions within copper sulfides.
The separation of zircons was carried out at the Universidade Federal de Ouro Preto, Brazil, with final handpicking at the University of Western Australia. The samples were crushed, and a <60-mesh fraction was collected and cleaned. The final selection of zircons was handpicked and mounted in epoxy resin disks together with chips of the 564 Ma CZ3 zircon standard (Pidgeon et al., 1994; Nelson, 1997). The disks were polished to expose the zircon grains in section. After detailed photography and SEM imaging, the disks were cleaned and coated with high-purity gold.
Uranium-Th-Pb isotope analyses of sectioned single zircons were carried out on the sensitive high-resolution ion microprobe (SHRIMP II) at the Curtin University of Technology, following the procedures documented by Compston et al. (1986), Williams and Claesson (1987), and Smith et al. (1998). Data were processed using the software package SQUID 1.00 (Ludwig, 2001a).
Monazite analyses were performed in situ on fragments of samples that were split or drilled from polished thin sections and mounted in epoxy disks. The monazite Pb/U standard VK1 (488 Ma) was cast in a separate mount. Analytical and data reduction methods for monazite are described by Foster et al. (2000) and Rasmussen et al. (2001).
The uncertainties in the radiogenic Pb isotope ratios are governed principally by ion-counting precision in the isotopes of direct interest and also in 204Pb, which provides a measure of common Pb. Because the 204Pb content indicated by the analyses of most of the sample zircons is similar to that obtained from the standard zircon analyses, it is assumed that all the Pb came from the gold coat of the sample surface. The data used for age determinations all required very low common Pb corrections and are therefore insensitive to the choice of common Pb composition. All analyses with higher levels of common Pb have other indications of isotopic disturbance, and the common Pb cannot be assumed to be original, so the choice of composition is necessarily an estimate. The total Pb in each analysis was corrected from common Pb by stripping initial 206Pb, 207Pb, and 208Pb from the measured amounts, using the observed 204Pb and Pb with the composition of Broken Hill galena.
Ages were calculated using U decay constants from Jaffey et al. (1971). Analytical uncertainties given in the tables and shown in plots are 1
. The ages from pooled data are weighted means, with uncertainties given as 95 percent confidence limits. Plots were prepared using ISOPLOT 2.49 (Ludwig, 2001b).
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SHRIMP U-Pb Results
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Representative cathodoluminescence (CL)-SEM images of zircon types from the host metavolcanic rocks (samples B18C and B18D) and the diabase dikes (samples B21D and B36A) are presented in Figure 7. Zircon morphologies vary from euhedral grains with distinct crystal faces to well-rounded grains and grain fragments. Cathodoluminescence imaging reveals that most zircon grains have intense oscillatory zoning (e.g., Fig. 7A, D, G, H), whereas relatively few have limited to no visible internal structure (e.g., Fig. 7B, I). Zircon grains with distinctively older cores and younger overgrowths are common (Fig. 7E, F, H), and analyses reveal that there is some correlation between morphology and/or internal features (e.g., compositional zoning) and age. This is evident where placement of the SHRIMP spot resulted in the analysis of older cores (e.g., Fig. 7E), where SHRIMP spots overlapped older cores and younger rims resulting in mixed ages (Fig. 7A), and where two analyses on one grain gave ages not within error of each other (Fig. 7A).
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FIG. 7. Representative cathodoluminescence (CL)-SEM images of analyzed zircon grains from host metavolcanic rock samples B18C and B18D and diabase dike samples B36A and B21D. The locations of SHRIMP analyses for dated grains are shown as circles. All ages are 207Pb/206Pb ages. Discordant data are marked with a (d). A. B18C.1-1 and B18.C1-2. B. B18C.3-1. C. B18C.6-1. D. B18D.2-1 and B18D.2-2. E. B36A.6-1. F. B36A.3-1. G. B36A.5-1. H. B21D. 6-1. I. B21D.7-1. J. B21D.2-1 and B21D.2-2.
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Host metavolcanic rocks: Zircon samples UWA-B18C and UWA-B18D
Sample UWA-B18D yielded only three zircons out of approximately 8 kg of bulk rock. To test the efficiency of sample preparation techniques and to increase the number of zircons, a second bulk sample (UWA–B18C) was prepared. Again the number of zircons was very limited, with only six grains obtained from 24 kg of crushed rock. This is typical of mafic volcanic rocks, which have low Zr contents.
Zircons analyzed from the host metavolcanic rock (Fig. 7A-D) comprise euhedral whole grains with relatively well defined crystal faces and terminations (Fig. 7A), subhedral to rounded grains (Fig. 7C-D), and rounded zircon fragments with no clear internal structure (Fig. 7B). Cathodoluminescence is moderate to strong, and almost all grains within these samples display relatively clear, concentric, euhedral, oscillatory zoning patterns.
A total of 13 spots were analyzed in nine zircon grains (Table 3). Only three concordant analyses from two grains (C.1-1, C.1-2, C.6-1) possibly define the rock age (2748 ± 34 Ma, MSWD = 2.7; Fig. 8), with two discordant analyses from another grain (D.2-1, D.2-2) giving similar 207Pb/206Pb ages. Among the other data, two are unquestionably inherited, 207Pb/206Pb ages of ~2865 Ma (C.2-1, D.1-1), consistent with previously reported ages for the Xingu Complex (e.g., Machado et al., 1991). The remaining data are all >25 percent discordant, except for C3.1, which has an anomalously young 207Pb/206Pb age of 2003 ± 9 Ma (Fig. 7B) and may be a reset age.
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TABLE 3 TABLE 3. Common Pb Corrected SHRIMP U-Pb Data for Zircons from
the Host Metavolcanic Rock from the
Igarapé Bahia Deposit (samples UWA-B18C and UWA-B18D)
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FIG. 8. Concordia plot of SHRIMP data from zircons from the host metavolcanic rocks of the Igarapé Bahia Fe oxide Cu-Au-(U-REE) deposit. The weighted mean 207Pb/206Pb age from the concordant analyses (shown as unshaded) is 2748 ± 34 Ma (MSWD = 2.7). Samples UWA-B18C and UWA-B18D.
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Diabase dikes: Zircon samples UWA-B36A and UWA-B21D
Sample UWA-B36A contained few zircons and yielded only 22 grains from approximately 31 kg of bulk rock. A second sample (UWA-B21D) was processed to increase the material for analysis, but only 10 zircons were recovered from 40 kg of crushed rock.
Zircons analyzed from mounts B36A and B21D comprise euhedral whole grains and fragments with well-defined crystal faces and terminations (Fig. 7F-H). Moderately to well-rounded grains are noted (Fig. 7E, I), with one grain displaying an irregular, fragmented morphology (Fig. 7J). Cathodoluminescence is strong, and almost all grains display clear concentric euhedral zoning patterns. A few grains have extremely bright CL, especially along rims and within cores. Several grains have more complex and irregular zoning patterns, particularly in the central regions. There are clear structural discontinuities between core regions and rim zones in several grains (e.g., Fig. 7E, F, H), indicating xenocrystic cores (e.g., structurally discordant patterns in their central regions, overgrown by more systematic and concentric, zoned zircon rims; Fig. 7E). This was confirmed by the SHRIMP analyses.
A total of 39 spots were analyzed from 32 grains, but no consistent date emerged from the data set. Approximately half of the analyses, mostly from grains with higher U or high 204Pb, are >5 percent discordant (Table 4). The remaining 20 concordant analyses from 19 grains yielded 207Pb/206Pb ages that range between 3050 ± 9 and 2653 ± 48 Ma. It is evident from CL imaging (Fig. 7E) that the SHRIMP analysis that gave an age of 3050 ± 9 Ma was partially placed on an older core. All analyzed grains display corrosion textures, indicating that zircons were not in equilibrium with the host magma, and are likely inherited. Analyses B21D.2-1 and B21D.2-2 gave ages of 2691 ± 52 and 2653 ± 48 Ma, respectively, and correspond to the irregular grain depicted in Figure 7J. This grain is the youngest concordant zircon analyzed. Therefore, the maximum age of host dike intrusion is ~2670 Ma.
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TABLE 4 TABLE 4. Common Pb Corrected SHRIMP U-Pb Data for Zircons from
Diabase Dike from
the Igarapé Bahia Deposit (samples UWA-B21D and UWA-B36A)
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Copper-gold–bearing magnetite breccia: Monazite sample UWA-B34
The selected monazite crystals are fine grained (20–40 µm) and occur in apparent textural equilibrium with chalcopyrite and magnetite in the matrix of mineralized breccia. Representative backscattered electron images of analyzed monazite grains, illustrating their relationship to ore and silicate minerals, are presented in Figure 9.
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FIG. 9. Representative backscattered electron images of sample UWA-B34, showing analyzed monazite grains (white) in apparent textural equilibrium with chalcopyrite (cpy) and magnetite (mgt) in the matrix of ore-bearing breccias of the Igarapé Bahia Fe oxide Cu-Au-(U-REE) deposit. Other associated minerals are siderite (sid), apatite (ap), and chlorite (chl). The locations of SHRIMP analyses for dated grains are shown as circles. All ages are 207Pb/206Pb ages. Discordant data are marked with a (d). A. B34.5.3-1. B. B34.5.2-1. C. B34.4.2-1. D. B34.7.3-1. E. B34.4.1-1. F. B34.6.2-1.
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A total of 18 spots were analyzed in 18 monazite grains from sample UWA-B34 (Table 5). Most of the data fall in a tight cluster near to concordia, with the remainder trending toward the origin (Fig. 10). The scattered data could be explained by some combination of recent loss of radiogenic Pb or secondary ion production due to the presence of submicroscopic mineral inclusions in the samples. The main cluster of data appears to be significantly reversely discordant, but this is considered to be an artefact of analyzing small, low U samples in situ. In particular, there can be offsets in the Pb/U calibration because the standard was in a separate mount, and there are minor defects in the Th- and U-related matrix corrections (the corrections are quite large in this case) due to high U and Th contents (~6,000 ppm, 15 wt %) of the standard. It is also possible that there are other trace element differences between the samples and the standard, affecting recorded Pb+/U+. The tight grouping of the near-concordant data cluster is taken to indicate that the samples are actually concordant and, therefore, that the 207Pb/206Pb data are reliable. Although not very precise, the Th/Pb data are also well grouped, consistent with preservation of the original isotopic system.
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TABLE 5 TABLE 5. Common Pb Corrected SHRIMP U-Pb Data for Monazite
from Cu-Au-Bearing Magnetite Breccias of
the Igarapé Bahia Deposit (sample UWA-B34)
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FIG. 10. Concordia plot of SHRIMP data for monazite from the matrix of mineralized magnetite breccias (sample UWA-B34) of the Igarapé Bahia Fe oxide Cu-Au-(U-REE) deposit. The weighted mean 207Pb/206Pb age of the main concordant cluster (unshaded), shown in the inset, is 2575 ± 12 Ma (MSWD = 1.19).
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By omitting five analyses that are discordant relative to the main cluster (4.3-1, 4.3-2, 5.1-1, 5.1-2, and 7.1-1) and one obvious young outlier (6.1-1) that yields a 207Pb/206Pb age of 2519 ± 19 Ma, a weighted mean 207Pb/206Pb age of 2575 ± 12 Ma (MSWD = 1.19, n = 12) is obtained.
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Strontium Isotope Composition of Carbonate Minerals
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Eight carbonate samples from the Igarapé Bahia deposit were concentrated from the matrix of ore-bearing siderite breccias and crosscutting veins to investigate their strontium isotope composition (Table 6) as a means of constraining fluid source. Preparation of carbonate samples and subsequent isotopic analyses were carried out at the University of São Paulo, Brazil. Prior to isotopic analysis, X-ray powder diffraction analyses were performed at Companhia Vale do Rio Doce to determine the carbonate specie(s) in each sample.
As the rubidium content of siderite and calcite is negligible, their 87Sr/86Sr ratios are assumed to be a reasonable approximation of the initial ratio of the original hydrothermal system from which they crystallized.
The 87Sr/86Sr ratios of the Igarapé Bahia carbonates are highly radiogenic and indicate crustal derivation (Table 6). These values are inconsistent with the isotopic composition of seawater at ~2.575 Ga (Veizer and Compston, 1976) and with the initial 87Sr/86Sr ratios of the Grão Pará metavolcanic rocks (0.7057 ± 0.0010) determined by Gibbs et al. (1986). This evidence clearly precludes seawater as a major component of the hydrothermal system, as would be expected if the deposits were syngenetic (e.g., VHMS model of Ferreira Filho, 1985; Almada, 1998). The wide range of highly radiogenic 87Sr/86Sr ratios (0.714–0.755) of the Igarapé Bahia carbonate minerals suggests multiple crustal sources, consistent with an epigenetic magmatic-hydrothermal origin (Fig. 11). Hydrothermal calcite and tourmaline from the Salobo deposit have similar 87Sr/86Sr ratios (Mellito, 1998), suggesting that highly radiogenic strontium signatures are common to Fe oxide Cu-Au deposits of the Carajás belt.
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FIG. 11. 87Sr/86Sr vs. time diagram showing the isotopic evolution of seawater (Veizer and Compston, 1976), continental crust and mantle (Faure and Powell, 1972) relative to the carbonate minerals from the matrix of mineralized breccias and crosscutting veins of the Igarapé Bahia Fe oxide Cu-Au-(U-REE) deposit. The Igarapé Bahia samples clearly indicate a crustal derivation. ACPS = Acampamento Sul orebody.
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Model for the Igarapé Bahia Fe Oxide Cu-Au-(U-REE) Mineralization
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Zircons extracted from the host metavolcanic rocks yield a 207Pb/206Pb age of 2748 ± 34 Ma. Although of very low precision, this result is sufficient to correlate the Igarapé Bahia volcano-sedimentary sequence to the Grão Pará volcanic rocks (ca. 2.75 Ga, Gibbs et al., 1986; Wirth et al., 1986; Machado et al., 1991). This result is not unexpected as the so-called Igarapé Bahia Group occurs within the domains of the Grão Pará Group and possibly represents a facies of the Grão Pará basin.
The positive correlation between REE, P, Au, and Cu, and textural evidence of chalcopyrite in equilibrium with REE minerals in the mineralized breccias (Fig. 9), supports the suggestion that monazite is genetically related to the precipitation of copper and gold in the Igarapé Bahia deposit. Thus, the in situ dating of monazite, from the matrix of ore-bearing magnetite breccias, constrains the age of ore formation. The SHRIMP 207Pb/206Pb age of 2575 ± 12 Ma of hydrothermal monazite indicates that the mineralization occurred ~175 m.y. after accumulation of the host volcano-sedimentary sequence. The data obtained from zircon xenocrysts recovered from the diabase indicate a maximum 207Pb/206Pb age of ~2670 Ma for dike emplacement, consistent with field evidence and the SHRIMP ages of the ore and host rocks (Fig. 12).
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FIG. 12. Summary of the SHRIMP ages obtained in this study compared to conventional U-Pb zircon ages and Rb-Sr bulk-rock ages of the Estrela Granitic Complex, the Old Salobo Granite, the Itacaiúnas Granite, Re-Os ages of molybdenite, and Pb-Pb model ages of bornite-chalcopyrite-magnetite mineralization at Salobo, and the host metavolcanic rocks of the Igarapé Bahia deposit.
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The 2575 ± 12 Ma monazite age of the Igarapé Bahia mineralization is indistinguishable from the conventional 207Pb/206Pb ages determined on zircons from the Archean A-type granites of the Carajás belt, such as the Old Salobo Granite (2573 ± 2 Ma; Machado et al., 1991) and the Itacaiúnas Granite (2560 ± 37 Ma; Souza et al., 1996). Similar ages (of lower precision, Fig. 12) also have been obtained by the Rb-Sr method in bulk samples of the Itacaiúnas Granite (2480 ± 40 Ma; Montalvão et al., 1984) and the Estrela Complex (2527 ± 34 Ma; Barros and Barbey 1998) and are possibly related to cooling of these alkaline intrusions. This age is also indistinguishable from the Re-Os and Pb-Pb weighted mean age of 2576 ± 9 Ma for copper-gold mineralization at Salobo (Requia et al., 2003). These data indicate that the mineralization processes at Igarapé Bahia were temporally related to the A-type Archean granites of the Carajás belt (Fig. 12). The Rb-Sr bulk rock age of 2577 ± 72 Ma for the Igarapé Bahia host metavolcanic rocks, determined by Ferreira Filho (1985), also supports this model as it indicates thermal resetting of strontium isotopes ~170 m.y. after accumulation of the volcanic host rock, probably due to the emplacement of A-type Archean granites or due to the associated mineralization itself.
The epigenetic nature of the Igarapé Bahia Cu-Au mineralization is supported by field evidence that includes the presence of fragments of rocks from both the lower and upper unit in the mineralized breccias (Fig. 3C) and the presence of chalcopyrite-rich veins and nodules in the upper unit (Fig. 3A-B). The circular shape of the cluster of breccia bodies (Fig. 2B) is similar to that of ring complexes associated with the intrusion of alkaline magmas.
Chlorite thermometry of the mineralized breccia, determined by Tallarico et al. (2000), indicates relatively high temperatures (313°–375°C), consistent with the fluid inclusion data of Ribeiro (1989) and Lindenmayer et al. (1998), which indicate the presence of a high-temperature (150°–430°C) group of high-salinity (up to 40% NaCl equiv) inclusions. Assuming that the fluids depositing magnetite and gold were similar to those postulated for other magnetite-rich Cu-Au systems (e.g., Davidson and Large, 1994; Skirrow and Walshe, 2002), copper and gold transport was most likely by chloride complexing, which could also explain the high REE concentrations (e.g., Gieré, 1996).
The stable isotope data of Oliveira et al. (1998) and Tazava (1999), from calcite and siderite in the matrix of mineralized breccias and crosscutting veins from the Igarapé Bahia deposit, yield a narrow range of negative 
13C values (–9.3 to –5.8
) and a relatively wide range of 18O values (0.7–9.4; Fig. 13). The negative 13C values are consistent with a homogeneous deep-seated carbon source from which carbonate minerals were precipitated within a limited pH range. The wider range of 18O values may be the result of mixing or overprinting of deep-seated solutions (high 18O) with meteoric fluids (low 18O). Additional evidence for the latter is provided by a second, low-temperature (100°–150°C) group of low-salinity (~10% NaCl equiv) inclusions identified in the studies of Ribeiro (1989) and Lindenmayer et al. (1998).
The mineralogy, geochemistry, strontium, carbon and oxygen isotope compositions, and fluid inclusion data are all compatible with the geochronological evidence for epigenetic mineralization of iron-oxide Cu-Au-(U-REE) type from a deep source. The evidence, in combination, indicates that ore formation was related to an Archean magmatic event at ca. 2.57 Ga, which produced A-type granites. Whether mineralization was directly related to these A-type granites or to some deeper alkaline body at depth, as implied by radiogenic isotope data for the Olympic Dam iron-oxide Cu-Au-(U-REE) deposit (e.g., Campbell et al., 1998), is unclear, as there is no definitive spatial relationship between the Igarapé Bahia deposit and granitoids of the same age (Fig. 1). However, the ringlike nature of the breccia bodies (Fig. 2) is at least suggestive of a magmatic body at depth. Further research is required to define the precise nature of the fluid or fluids involved in ore genesis and the precise conditions and mechanisms of metal transport and deposition.
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Discussion
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The previously proposed VHMS models for Igarapé Bahia (e.g., Ferreira Filho, 1985; Almada, 1998; Villas and Santos, 2001) are negated by the new geochronologic data. Additionally, the grade-tonnage data of the Igarapé Bahia deposit (219 Mt of primary ore at 1.4% Cu and 0.86 g/t Au) contrasts markedly with that of most VHMS deposits, which contain <100 Mt ore (Large, 1992; Slack, 1993; Barrie and Hannington, 1999). The ore paragenesis of the Igarapé Bahia deposit, characterized by magnetite + chalcopyrite ± bornite with significant amounts of U (28–380 ppm), F (390–31,000 ppm), P (900–6,200 ppm), La (260–2,300 ppm), and Ce (450–4,400 ppm), contrasts dramatically with that of VHMS deposits, which normally lack significant concentrations of Fe oxides, typically contain pyrite and pyrrhotite, and do not have anomalous concentrations of U, F, P, or REE (Large, 1992; Slack, 1993; Barrie and Hannington, 1999). The SHRIMP monazite ages, together with the mineralogical, geochemical, and radiogenic and stable isotope data from the Igarapé Bahia ore, indicate an epigenetic hydrothermal-magmatic origin, temporally related to the emplacement of Archean (~2.57 Ga) A-type granites of the Carajás belt.
Iron-oxide Cu-Au-(U-REE) deposits have been recognized as a distinctive ore deposit type during the last decade (Hitzman et al., 1992; Oreskes and Hitzman, 1993). Representative examples of this group include the Olympic Dam deposit with resources of 2,000 Mt at 1.6 percent Cu and 0.6 g/t Au (Roberts and Hudson, 1983; Reeve et al., 1990; Hitzman et al., 1992; Cross et al., 1993; Oreskes and Hitzman, 1993) and the Ernest Henry deposit with resources of 167 Mt at 1.1 percent Cu and 0.5 g/t Au (Williams, 1998). These structurally controlled epigenetic deposits share a number of common features such as (1) high tonnage (>100 Mt) and low copper (<2.0 %) and gold (<0.8 g/t) grades, (2) abundance of magnetite and/or hematite, (3) oxidized and S-poor ore paragenesis (chalcopyrite, bornite, and/or chalcocite), (4) low SiO2 content of the ore, and (5) a metal association of Fe-Cu-Au with anomalous low REE, U, P, and F (Hitzman et al., 1992; Oreskes and Hitzman, 1993; Williams, 1998; Groves and Vielreicher, 2001). Mineralization at all deposits involved high-temperature (200°–600°C) ore fluids that were extremely saline (as much as 50 wt % NaCl equiv), acid, and oxidizing, and carried metals as Cl complexes (Hitzman et al., 1992; Oreskes and Hitzman, 1993; Davidson and Large, 1994), with involvement also of relatively low temperature and less saline fluids (Oreskes and Einaudi, 1992; Oreskes and Hitzman, 1993; Gow et al., 1994; Haynes et al., 1995; Barton and Johnson, 1996). The styles of hydrothermal alteration, the ore mineralogy and geochemistry, and the strontium isotope data presented here, together with previous stable isotope and fluid inclusion data, strongly support the classification of Igarapé Bahia as a member of the world-class Olympic Dam-type Fe oxide Cu-Au-(U-REE) deposit group, as previously argued by several authors (Oliveira et al., 1998; Tallarico et al., 1998; Soares et al., 1999; Tazava, 1999). The SHRIMP 207Pb/206
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Abstract
Introduction
