Xavier, RP; Soares Monteiro, LV; Moreto, CPN; Silva Pestilho, AL; Coelho de Melo, GH; Delinardo da Silva, MA; Aires, B; Ribeiro, C; Freitas e Silva, FH; ,
The Carajas mineral province in the southeastern part of the Amazon craton, northern Brazil, represents an Archean block divided into two tectonic domains: (1) the Carajas domain in the north and (2) the Rio Maria domain in the south. The Carajas 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 Carajas domain by Salobo (789 Mt at 0.96 wt % Cu, 0.52 g/t Au, 55 g/t Ag) and Igarape Bahia/Alemao (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 Carajas 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 Itacaiunas 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 Itacaiunas Supergroup, gabbro/diorite, quartz-feldspar porphyry, granophyric granite intrusions, and basement granitoids.In general, the Carajas IOCG deposits display early high-temperature (> 500 degrees 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 degrees 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., Igarape Bahia, Cristalino, Sossego, and Alvo 118).Extensive zones of scapolite alteration (> 20 km(2)) 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 degrees 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 Carajas 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 Carajas domain and are indicative of different sulfidation states of the source fluids: chalcopyrite-chalcocite-bornite-magnetite at Salobo; chalcopyrite +/- chalcocite-digenite-covellite-magnetite at Igarape 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 Carajas 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 Carajas domain.