|
Lena Virgínia Soares Monteiro, Geoscience Institute, UNICAMP (Brazil)
Roberto Xavier, Geoscience Institute, UNICAMP (Brazil)
Carlos Roberto Souza Filho, Geoscience Institute, UNICAMP (Brazil)
Ignacio Torresi, Geoscience Institute, UNICAMP (Brazil)
Diego Bortholoto, Geoscience Institute, UNICAMP (Brazil)
Rafael Augusto, Geoscience Institute, UNICAMP (Brazil)
|
|
The Carajás Mineral Province (CMP), northern Brazil, represents an Archean cratonic block that contains the world's largest known concentration of large-tonnage IOCG deposits (e.g. Sossego, Salobo, Igarapé Bahia, Cristalino, Alvo 118, Alvo GT46). These deposits are hosted by 2.73-2.76 Ga metavolcano-sedimentary units, 2.65-2.70 Ga gabbro/diorite, granitoids, and porphyry dikes within regional ductile shear zones or along subsidiary brittle-ductile structures. Geochronological data from the Carajás IOCG deposits point to three possible metallogenetic events (∼2.7 Ga, 2.57 Ga, and 1.8 Ga), which are closely associated with known episodes of plume-related alkaline granitic magmatism.
The Carajás IOCG deposits display paragenetic evolution characterized by sodic, sodic-calcic, and potassic alterations, magnetite-(apatite) formation, chloritization, Cu-Au mineralization, and hydrolytic alteration. Tourmalinization is common in deposits hosted by metavolcanic-sedimentary units. Higher temperature alteration in ductile shear zones developed fayalite, garnet, and sillimanite (e.g. Salobo, Alvo GT46). Silicification and carbonatization are important in deposits formed in brittle-ductile conditions (e.g. Sossego, Alvo 118).
Extensive scapolitization zones (>20km) represent the 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 high salinity of the fluids driven by heat from the intrusive episodes recorded in the CMP. Due to this, ore geochemical signatures, Fe-Cu-Au-REE-(U-Y-Ni-Co-Pd-Sn-Bi-Pb-Ag-Te), reflect the chemistry of the leached host rocks.
Extensive fluid-rock interaction, involving possibly basinal/evaporite and magmatic fluid components, result in 18O-enriched fluids (δ18Ofluid = 5-15‰) characterized in most Carajás IOCG deposits. In addition, stable isotope data (δ18Ofluid=-5.2‰, δDfluid =-35‰, at 300 oC at Sossego and Alvo 118) also reinforce the importance of significant structurally-controlled episodic influx of meteoric fluids for ore deposition related with fluid pressure release brecciation. Mixing process was likely responsible for a trend of salinity and temperature decrease (>550 to <300 oC) accompanied of fO2 increase towards the mineralization stages, favoring in more oxidized deposits predominance of hematite-bornite (e.g. Alvo 118) over magnetite-chalcopyrite (e.g. Sossego).
Sulfur isotope compositions in Carajás IOCG vary from values close to that expected for a mantle source (δ34S = 0±1‰) in deposits associated with deeper crustal levels (e.g. Salobo) to 34S enriched values (>6‰) in deposits in which contribution of meteoric fluids was significant, reflecting distinct physic-chemical conditions or input of isotopically heavy sulfur from surficial reservoirs.
Research Grants FAPESP 03/09584-3, FAPESP/PRONEX 03/09916-6, and CNPq/MCT (Processo 555065/2006-5).
|
