The Cachoeira gold deposit is one of several large (N20 tonnes Au), shear-zone hosted hydrothermal gold deposits of the Proterozoic Gurupi Belt in northern Brazil. O, H, C, and S isotope compositions of carbonate, silicate, and sulfide minerals, and|from graphite and inclusion fluids from this deposit have been measured in order to infer the composition of ore-forming fluids. The data show small variation at the deposit scale, irrespective of host rock. Calculated y18O values of H2O in the fluid in|equilibrium with quartz, hydrous silicates and carbonates are, respectively, +6.2 to +12.4x, +5.0 to +9.0x, and +5.9 to +11.0x (SMOW). The yDH2O of inclusion fluids is 22x, whereas values for chlorite and white mica are 16x and 20 to 28x (SMOW), respectively. These values are compatible with a metamorphic origin, which is in agreement with the postmetamorphic timing of the mineralization. The calculated y13C value of CO2 of the fluid in equilibrium with the carbonates is|from 9.5x to 12.7x (PDB), whereas the graphite-bearing host rocks have y13C value of graphite carbon of 29.7x. The calculated y34S of H2S of the fluid varies mainly between +0.7x and +5.5x(CDT), and is interpreted as reflecting a magmatic or average crustal source for sulfur. The restricted isotopic compositions indicate a relatively homogeneous fluid and deposition under restricted temperature conditions. Nonetheless, a relatively wide temperature range of 272–383 8C is estimated for the ore-bearing fluid using the quartz–dolomite oxygen isotope thermometer. Collectively, the data indicate that gold was transported as a reduced sulfur complex, by a metamorphic fluid having a near neutral pH, at relatively low CO2. Furthermore,|the data suggest that gold was deposited during the structural evolution of the associated shear zone probably as a result of sulfidation and carbonatization reactions with the Fe- and C-rich host rocks, which resulted in changes of the pH and in the redox state of the ore fluid