The Iron Oxide Copper-Gold Systems of the Carajás Mineral Province, Brazil
Roberto Perez Xavier, Lena Virgínia Soares Monteiro, Carolina Penteado N. Moreto, André Luiz Silva Pestilho, Gustavo Henrique Coelho de Melo, Marco Antônio Delinardo da Silva, Benevides Aires, Cleive Ribeiro, Flávio Henrique Freitas e Silva, 2012. "The Iron Oxide Copper-Gold Systems of the Carajás Mineral Province, Brazil", Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe, Jeffrey W. Hedenquist, Michael Harris, Francisco Camus
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The Carajás mineral province in the southeastern part of the Amazon craton, northern Brazil, represents an Archean block divided into two tectonic domains: (1) the Carajás domain in the north and (2) the Rio Maria domain in the south. The Carajás domain contains one of the world's largest known concentrations of large-tonnage (100–789 Mt at 0.77–1.4 wt % Cu and 0.28–0.86 g/t Au) iron oxide copper-gold (IOCG) deposits. These IOCG systems are mainly represented in the northern sector of the Carajás domain by Salobo (789 Mt at 0.96 wt % Cu, 0.52 g/t Au, 55 g/t Ag) and Igarapé Bahia/Alemão (219 Mt at 1.4 wt % Cu, 0.86 g/t Au) deposits, whereas Sossego (245 Mt at 1.1 wt % Cu, 0.28 g/t Au), Cristalino (500 Mt at 1.0 wt % Cu; 0.3 g/t Au), and Alvo 118 (170 Mt at 1.0 wt % Cu, 0.3 g/t Au) are the most important examples in its southern sector. In addition to several other IOCG prospects that are currently under exploration, these deposits collectively yield resources of approximately 2 billion metric tons of Cu-Au ore.
The IOCG deposits in the northern and southern sectors of the Carajás domain are structurally controlled by regional-scale W-NW–striking, brittle-ductile shear zones that define the contact between the metavolcano-sedimentary units of the Itacaiúnas Supergroup (ca. 2.73–2.76 Ga) and Mesoarchean basement rocks (ca. 3.0–2.83 Ga). The deposits are hosted by a variety of lithotypes, including metavolcano-sedimentary units of the Itacaiúnas Supergroup, gabbro/diorite, quartz-feldspar porphyry, granophyric granite intrusions, and basement granitoids.
In general, the Carajás IOCG deposits display early high-temperature (>500°C) sodic-calcic alteration controlled by ductile structures and mylonitic fabrics containing albite-scapolite-actinolite alteration (e.g., Sequeirinho orebody at Sossego). This early stage is generally followed by magnetite-(apatite) formation and potassic (K-feldspar and biotite) alteration, which were subsequently overprinted by lower temperature (<300°C) chlorite, carbonate-epidote, or sericite-hematite alteration and Cu-Au mineralization, all controlled by brittle structures (e.g., Sossego orebody at Sossego and Alvo 118). The development and amplitude of the hydrothermal alteration types in individual deposits are dependent upon fluid-rock interactions at different structural levels. Thus, the higher temperature alteration assemblages at Salobo, with fayalite and garnet, may represent emplacement at relatively deep crustal levels, whereas potassic, chlorite, silica, and carbonate alteration are important in deposits formed under brittle-ductile conditions at shallower levels (e.g., Igarapé Bahia, Cristalino, Sossego, and Alvo 118).
Extensive zones of scapolite alteration (>20 km2) are mainly developed in the Mesoarchean basement rocks and supracrustal units around the Sossego deposit. These sodic alteration zones suggest a fluid regime dominated by deeply sourced, hot (>500°C), hypersaline brines without significant contributions of surface-derived fluids prior to Cu-Au mineralization in distal portions of the hydrothermal system. Metal leaching from the host rocks was probably enhanced by the high salinity of the fluids, driven by heat provided by intrusive episodes recorded in the Carajás domain. As a consequence, the Fe-Cu-Au-REE association, together with variable concentrations of U, Y, Ni, Co, Pd, Sn, Bi, Pb, Ag, and Te generally present in these deposits, reflect strong dependence of the geochemical ore signatures on the composition of the leached host rocks.
Copper-gold mineralization generally forms lens-shaped and massive replacement bodies parallel to the mylonitic foliation at deeper crustal levels (e.g., Salobo), but breccia bodies (e.g., Sossego) and vein stockworks (e.g., Alvo 118) are the dominant styles in the shallower IOCG systems. Additionally, ore mineral assemblages were invariably introduced during the late stages of all of the IOCG systems of the Carajás domain and are indicative of different sulfidation states of the source fluids: chalcopyrite-chalcocite–bornite-magnetite at Salobo; chalcopyrite ± chalcocite-digenite-covellite-magnetite at Igarapé Bahia; chalcopyrite-pyrite-magnetite at Sossego and Cristalino; and chalcopyrite-bornite-hematite at Alvo 118.
Fluid inclusions in ore-related minerals point to a fluid regime in which hot brine (>30 wt % NaCl equiv) solutions were progressively diluted and cooled by lower temperature, low-salinity (<10 wt % NaCl equiv) aqueous fluids. The fluid inclusion data together with stable isotope compositions (O, D, S, B, and Cl) and Cl/Br-Na/Cl systematics suggest that mixing of hot hypersaline metalliferous fluids with an important magmatic component and modified seawater (e.g., bittern brines generated by seawater evaporation), plus meteoric water within shear zones, represents the main Cu-Au precipitation mechanism.
Available geochronologic data for IOCG deposits and their host rocks in the Carajás domain do not unequivocally show whether all the deposits are genetically linked to a single Archean metallogenic event (2.75 or 2.57 Ga) or represent distinctly different events that may have extended into the Paleoproterozoic (e.g., Alvo 118). Additionally, despite the importance of magmatism for providing heat and fluids for development of the extensive hydrothermal systems, temporal relationships between intrusions and IOCG orebodies have still not been clearly defined in the Carajás domain.
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Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe
It has been recognized for the past century that copper deposits, in common with those of many other metals, are heterogeneously concentrated in Earth’s upper crust, resulting in areally restricted copper provinces that were generated during several discrete metallogenic epochs over time intervals of up to several hundred million years. Various segments of circum-Pacific magmatic arcs, for example, have total contained copper contents that differ by two orders of magnitude. Each metallogenic epoch introduced its own deposit type(s), of which porphyry copper (and related skarn), followed by sediment-hosted stratiform copper and then iron oxide copper-gold (IOCG), are globally preeminent. Nonetheless, genesis of the copper provinces remains somewhat enigmatic and a topic of ongoing debate.
A variety of deposit-scale geometric and geologic features and factors strongly influence the size and/or grade of porphyry copper, sediment-hosted stratiform copper, and/or IOCG deposits. For example, development of major porphyry copper deposits/districts is favored by the presence of clustered alteration-mineralization centers, mafic or massive carbonate host rocks, voluminous magmatic-hydrothermal breccias, low sulfidation-state core zones conducive to copper deposition as bornite ± digenite, hypogene and supergene sulfide enrichment, and mineralized skarn formation, coupled with lack of serious dilution by late, low-grade porphyry intrusions and breccias. Furthermore, the copper endowment of all deposit types undoubtedly benefits from optimization of the ore-forming processes involved.
Tectonic setting also plays a fundamental role in copper metallogeny. Contractional tectonomagmatic belts, created by flat-slab subduction or, less commonly, arc-continent collision and characterized by crustal thickening and high rates of uplift and exhumation, appear to host most large, high-grade hypogene porphyry copper deposits. Such mature arc crust also undergoes mafic magma input during porphyry copper formation. The premier sediment-hosted stratiform copper provinces were formed in cratonic or hinterland extensional sedimentary basins that subsequently underwent tectonic inversion. The IOCG deposits were generated in association with extension/transtension and felsic intrusions, the latter apparently triggered by deep-seated mafic magmas in either intracratonic or subduction settings. The radically different exhumation rates characteristic of these various tectonic settings account well for the secular distribution of copper deposit types, in particular the youthfulness of most porphyry relative to sediment-hosted stratiform and IOCG deposits. Notwithstanding the importance of these deposit-scale geologic, regional tectonic, and erosion-rate criteria for effective copper deposit formation and preservation, they seem inadequate to explain the localization of premier copper provinces, such as the central Andes, southwestern North America, and Central African Copperbelt, in which different deposit types were generated during several discrete epochs. By the same token, the paucity of copper mineralization in some apparently similar geologic settings elsewhere also remains unexplained.
It is proposed here that major copper provinces occur where restricted segments of the lithosphere were predisposed to upper-crustal copper concentration throughout long intervals of Earth history. This predisposition was most likely gained during oxidation and copper introduction by subduction-derived fluids, containing metals and volatiles extracted from hydrated basalts and sediments in downgoing slabs. As a result, superjacent lithospheric mantle and lowermost crust were metasomatized as well as gaining cupriferous sulfide-bearing cumulates during magmatic differentiation—processes that rendered them fertile for tapping during subsequent subduction-or, uncommonly, intraplate extension-related magmatic events to generate porphyry copper and IOCG districts or belts. The fertile lithosphere beneath some accretionary orogens became incorporated during earlier collisional events, commonly during Precambrian times. Relatively oxidized crustal profiles—as opposed to those dominated by reduced, sedimentary material—are also required for effective formation of all major copper deposits. Large sedimentary basins underlain by or adjoining oxidized and potentially copper-anomalous crust and filled initially by immature redbed strata containing magmatic arc-derived detritus provide optimal sites for large-scale, sediment-hosted stratiform copper mineralization. Translithospheric fault zones, acting as giant plumbing systems, commonly played a key role in localizing all types of major copper deposits, districts, and belts. These proposals address the long-debated concept of metal inheritance in terms of the fundamental role played by subduction-metasomatized mantle lithosphere and lowermost crust in global copper metallogeny.