Characterization and evolution of ore-forming fluids at Fazenda Brasileiro Gold Mine, Rio Itapicuru Greenstone Belt, BA

Abstract

Fluid inclusion studies by microthermometry and Raman spectroscopy carried out on the main hosts of the gold mineralizations at Fazenda Brasileiro Mine, southern part of Rio Itapicuru greenstone belt define the mineralizing fluid s as relatively dense (0.85-0.95 g/cm3 ) and hot (> 400°C) solutions composed essentially of CO2 (89.7-85.3 mole%) and minor amounts of CH4 and N2, which gradually evolved to more a90ueous fluids (40 to 62.5 moles% H20) of low salinity(< 10 eq. wt% NaCl) at lower temperatures (250-300°C). The isochores of the carbonic and H20-C02 fluids and the temperature of total homogeneization combined with geothermometric data on the are paragenesis, point to at least two periods of gold deposition: 1. 380-419° C and 2.2-3.2 kb in the quartzofeldspathic and arsenopyrite. Pyrite quartz veins and 2. 270-300°C and 1.2-1.4 kb in the massive quartz vein. Based on experimental work on the solubility of gold complexes, chemical characteristics of mineralizing fluids, ore paragenesis and Au/Ag ratio, it was possible to speculate that the transport of gold occurred mainly by reducing (high ΣH2S/ ΣS04), slightly neutral to alkaline solutions, as thio-complexes such as Au(HS)2, HAu (HS)2 or AU2(HS)2S-2. In the earliest stage of mineralization conditions of gold deposition were probably attained due to fluid/ ock reactions which led to a decrease in the activity of S-2 by precipitating sulphides (arsenopyrite, pyrite, pyrrhotite, etc). The predominance of the H20 regime over the C02 regime in the latest stage might have been the cause of CO2 dilution, decrease in pH and oxidation and, as a consequence, the precipitation of gold. The fluid inclusion data also suggest a metamorphic origin for the mineralizing fluids, most likely through the devolatilization of the basal sequences of the volcano-sedimentary pile. The devolatilization process would be able produce low salinity, H20-C02 fluids which would later migrate through favourable structural sites and deposit their metal content.