ABSTRACT

   The Carajás Mineral Province, northern Brazil, represents an Archaean cratonic block that contains the world's largest known concentration of large-tonnage IOCG deposits (e.g. Sossego, Salobo, Igarapé Bahia/Alemão, Cristalino, Alvo 118, Igarapé Cinzento/Alvo GT46). These deposits are hosted by 2.76 to 2.73 Ga metavolcano-sedimentary units, 2.70 to 2.65 Ga gabbro/diorite, granitoids, and porphyry dykes within brittle-ductile and ductile shear zones. Geochronologic data suggest that formation of the Carajás IOCG deposits may possibly be linked to three metallogenic events: ~2.74, 2.57 and 1.8 Ga.
  In general, the Carajás IOCG deposits display a hydrothermal alteration sequence characterised by early sodic and sodic-calcic assemblages, followed by potassic alteration, magnetite-(apatite) formation, chloritisation, copper-gold mineralisation and hydrolytic alteration. Tourmalinisation is particularly common in deposits hosted by metavolcanic-sedimentary units (e.g., Salobo and Igarapé Bahia/Alemão). The development of fayalite, garnet and sillimanite represents higher temperature alteration assemblages of some deposits hosted in ductile shear zones, such as Salobo and Igarapé Cincento/Alvo GT46. Silicification and carbonatisation are important in deposits formed in brittle-ductile conditions (e.g. Sossego and Alvo 118).
  Extensive zones of scapolitisation (>20km²) represent sodic alteration around IOCG deposits (e.g., Sossego), reflecting high salinity and buffered activity gradients in Cl in the early regional hydrothermal fluids. Metal leaching from the host rocks was probably enhanced by the high salinity of fluids, driven by heat from the intrusive episodes recorded in the CarajásMineral Province. As a consequence, geochemical ore signatures defined by the Fe-Cu-Au-REE-(U-Y-Ni-Co-Pd-Sn-Bi-Pb-Ag-Te) association is variably developed in the Carajás IOCG deposits, and strongly dependent upon the chemistry of the leached host rocks.
  Fluid inclusions in ore-related minerals point to a fluid regime in which hot brine (>30 wt% NaCl equiv.) solutions, represented by salt-bearing aqueous inclusions, were progressively cooled and diluted by lower temperature, low-salinity (<10 wt% NaCl equiv. aqueous fluids. This mixing process was likely responsible for a trend of salinity and temperature decrease (>550 to <300°C), accompanied by an f O2 increase towards the mineralisation stages. This process tends to favour the predominance of hematite-bornite in more oxidised deposits (e.g., Alvo 118) over magnetite-chalcopyrite (e.g., Sossego).
  Extensive fluid-rock interactions, possibly involving basinal/evaporite and magmatic fluid components, result in ¹⁸O-enriched fluids (δ¹⁸O fluid = 5 to 15 ‰) typical of most Carajás IOCG deposits. In addition, calculated fluid isotopic compositions for shallow-emplaced deposits, such Sossego and Alvo 118 (δ¹⁸O fluid = -5.2 ‰, δD fluid = -35 ‰, at 300°C), also reinforce the importance of the significant, structurally-controlled influx of meteoric fluids for ore deposition related to high fluid pressure release and brecciation. Chlorine and boron isotopes, combined with Cl/Br - Na/Cl systematics strongly suggest that fluid regimes responsible for the formation of the Carajás IOCG deposits involved a significant contribution from residual evaporative fluids (e.g., bittern fluids generated by seawater evaporation) that may have mixed with magma-derived brines.
  Sulphur isotope compositions for the Carajás IOCG deposits vary from values close to that expected for a mantle source (e.g., δ³⁴S = 0±1‰ at Salobo) to ³⁴S enriched values (>7 ‰) in deposits in which contribution of meteoric fluids was significant, reflecting distinct physic-chemical conditions or input of heavy sulphur from surficial reservoirs.

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