The Caxias deposit is situated in the Gurupi Auriferous Province of the Paleoproterozoic São Luís Craton, northern Brazil. It is a lode-type gold mineralization associated with a narrow, steeply dipping, NE-trending, shear zone crosscutting a hydrothermalized microtonalite (northern sector) and schists (southern sector). Fluid inclusion studies on vein quartz crosscutting the microtonalite (northern sector) have identified early carbonic and aqueous-carbonic inclusions and late aqueous inclusions, unrelated to the mineralizing event. The CO2 / H2O ratio and the other microthermometric properties show a wide range of values, which are interpreted as product of heterogeneous trapping of two (partially) immiscible fluids and to deformation-related post-formational processes. The resulting mineralizing fluid has XCO2 : 6-45 mol %; XN2 : < 2.5 mol %; XH2O: 55-95 mol %; mean salinity of 4.5 wt % NaCl equiv., and moderate density (0.7-1.0 g/cm3 ). Bulk isochores coupled with chlorite geothermometry constrained the P-T entrapment conditions between 262-307ºC and 1.6-3.7 kb. Log ¦O2 for this P-T-X range was estimated between –29.8 and –34.2. Geological characteristics and fluid properties found in the northern sector of Caxias gold mineralization are similar to those described for metamorphic fluids of mesothermal gold deposits.The Caxias deposit is situated in the Gurupi Auriferous Province of the Paleoproterozoic São Luís Craton, northern Brazil. It is a lode-type gold mineralization associated with a narrow, steeply dipping, NE-trending, shear zone crosscutting a hydrothermalized microtonalite (northern sector) and schists (southern sector). Fluid inclusion studies on vein quartz crosscutting the microtonalite (northern sector) have identified early carbonic and aqueous-carbonic inclusions and late aqueous inclusions, unrelated to the mineralizing event. The CO2 / H2O ratio and the other microthermometric properties show a wide range of values, which are interpreted as product of heterogeneous trapping of two (partially) immiscible fluids and to deformation-related post-formational processes. The resulting mineralizing fluid has XCO2 : 6-45 mol %; XN2 : < 2.5 mol %; XH2O: 55-95 mol %; mean salinity of 4.5 wt % NaCl equiv., and moderate density (0.7-1.0 g/cm3 ). Bulk isochores coupled with chlorite geothermometry constrained the P-T entrapment conditions between 262-307ºC and 1.6-3.7 kb. Log ¦O2 for this P-T-X range was estimated between –29.8 and –34.2. Geological characteristics and fluid properties found in the northern sector of Caxias gold mineralization are similar to those described for metamorphic fluids of mesothermal gold deposits.