The gold-bearing quartz-carbonate vein deposits of the Cuiabá Mine, northern sector of the Quadrilátero Ferrífero, State of Minas Gerais, SE Brazil, are hosted by a sequence of banded carbonaceous metasedimentary rocks within tholeiitic basalt flows of the Archean Rio das Velhas Greenstone Belt. A regional greenschist metamorphic grade characterizes rocks in this section of the Belt. Episodes of hydraulic fracturing enhanced the focussing of fluids percolating along the host primary layering, which promoted pervasive carbonatization and sulfidation associated with crosscutting fractures and quartz-carbonate veins/breccias. The gold mineralization is closely related to the sulfiderich zones. Fluid inclusion investigations show that the mineralizing fluids were dominantly aqueous, of low salinity (3-6 wt.% NaCleq.), with variable concentrations of CO2 (0 – 11.4 mol%), CH4 (1.5 – 10.2 mol%), subordinate N2 (0 – 0.7 mol%), and traces of H2S/HS- . Carbonates in the carbonaceous host rock display heavier δ13C compositions (-4.3 to –0.2o/oo) than carbonates in the gold-related, carbonate-sulfide altered zones and associated veins and breccias (-8.1 to –5.5o /oo). The range of carbonate δ18O values in the carbonaceous host rock (12.8 to 16o /oo) is broadly similar to the carbonates in the mineralized zones (10.6 to 15.2o /oo), with a slightly larger variation in the latter. Collectively, fluid inclusion and carbon isotope data indicate that: (1) the mineralizing fluid was originally composed of a H2O-CO2 solution, with significant concentrations of CH4 added via hydrolysis of the carbonaceous matter during vein-fluid/wallrock interaction; (2) sedimentary carbonates, probably of marine origin, are readily distinguished from those carbonates formed by reaction between CO2 -bearing vein fluids and wallrocks; (3) gold-related vein-fluid CO2 was relatively homogeneous in isotopic composition, suggesting equilibration with carbonate and reduced carbon reservoir (e.g. mantle-derived magma); (4) gold deposition may have been triggered by a sharp decrease in fO2 caused by the CH4 enrichment of the fluid.The gold-bearing quartz-carbonate vein deposits of the Cuiabá Mine, northern sector of the Quadrilátero Ferrífero, State of Minas Gerais, SE Brazil, are hosted by a sequence of banded carbonaceous metasedimentary rocks within tholeiitic basalt flows of the Archean Rio das Velhas Greenstone Belt. A regional greenschist metamorphic grade characterizes rocks in this section of the Belt. Episodes of hydraulic fracturing enhanced the focussing of fluids percolating along the host primary layering, which promoted pervasive carbonatization and sulfidation associated with crosscutting fractures and quartz-carbonate veins/breccias. The gold mineralization is closely related to the sulfiderich zones. Fluid inclusion investigations show that the mineralizing fluids were dominantly aqueous, of low salinity (3-6 wt.% NaCleq.), with variable concentrations of CO2 (0 – 11.4 mol%), CH4 (1.5 – 10.2 mol%), subordinate N2 (0 – 0.7 mol%), and traces of H2S/HS- . Carbonates in the carbonaceous host rock display heavier δ13C compositions (-4.3 to –0.2o/oo) than carbonates in the gold-related, carbonate-sulfide altered zones and associated veins and breccias (-8.1 to –5.5o /oo). The range of carbonate δ18O values in the carbonaceous host rock (12.8 to 16o /oo) is broadly similar to the carbonates in the mineralized zones (10.6 to 15.2o /oo), with a slightly larger variation in the latter. Collectively, fluid inclusion and carbon isotope data indicate that: (1) the mineralizing fluid was originally composed of a H2O-CO2 solution, with significant concentrations of CH4 added via hydrolysis of the carbonaceous matter during vein-fluid/wallrock interaction; (2) sedimentary carbonates, probably of marine origin, are readily distinguished from those carbonates formed by reaction between CO2 -bearing vein fluids and wallrocks; (3) gold-related vein-fluid CO2 was relatively homogeneous in isotopic composition, suggesting equilibration with carbonate and reduced carbon reservoir (e.g. mantle-derived magma); (4) gold deposition may have been triggered by a sharp decrease in fO2 caused by the CH4 enrichment of the fluid.