Fluid regimes related to the formation of lode-gold deposits the rio itapicuru greenstone belt, Bahia: A fluid inclusion review

Abstract

The shear zone-hosted gold deposits of the Rio Itapicuru Greenstone Belt, northeastern Brazil, are located in its southern and northern sectors, the former including Fazenda Brasileiro Mine and Fazenda Canto deposit and the latter Fazenda Maria Preta, Mari and Ambrósio deposits. Fluid inclusion studies by microthermometry and laser Raman microspectroscopy in mineralized quartz veins of these deposits reveal two main populations of inclusions: primary, CO2 - (± CH ± N2) inclusions (type I), and low salinity (< 6 wt% NaCleq.) H20 - CO2 - (± CH4 ± N ) inclusions (type II), representing fluid regimes, which were active during the gold mineralizing events. Both types of ore fluids are interpreted as part of a deep hydrothermal system, in which the fluid components may have been derived by devolatilization reactions during regional metamorphism and/or from mantle-magmatic sources (e.g. CO2 H2S). The nature of the CO2-rich fluid regime, atypical in mesothermal lode-gold deposits, still remains an open question. Gold deposition from bisulfide complexes took place at different crustal levels, at temperatures varying from 280 up to 500°C, and pressures from 1 up to 4.4 kb, probably closely linked with and dependent upon redox changes that accompanied the interaction of the fluids with mafic and carbonaceous host rocks.