Hydrothermal fluid evolution and structural control of the Guarim gold mineralisation, Tapajos Province, Amazonian Craton, Brazil

Fluid inclusion and structural studies were carried out at the Guarim gold deposit in the Palaeo-proterozoic Tapajos province of the Amazonian craton. Guarim is a fault-hosted gold deposit cutting basement granitoids. It cons ists of a quartz vein, which is massive in its inner portions, grading late rally either to a massive or to cavity-bearing quartz vein associated with hydrothermal breccias. The wallrock alteration comprises chlorite, carbonat e, white mica and sulphide minerals, with free gold occurring within quartz grains and spatially associated with sulphide mineral grains. Petrographic , microthermometric and Laser Raman investigations recognised CO2-rich, mix ed H2O-CO2, and H2O fluid inclusions. The coexisting CO2 and H2O-CO2 inclus ions were interpreted as primary immiscible fluids that formed the gold-bea ring vein. The H2O inclusions were considered a product of later infiltrati on of fluids unrelated to the mineralising episode. The mineralising fluid has CO2 ranging typically from 5-10 mol%, contains traces of N-2, has salin ities of similar to5 wt% NaCl equiv., and densities varying between 0.85 an d 0.95 g/cm(3). The P-T estimations bracket gold deposition between 270-320 degreesC and 0.86-2.9 kb; f(O2)-f(S2)-pH estimates suggest a reduced, near -neutral character for the fluid. Variations in the physico-chemical proper ties, as demonstrated by the fluid inclusion study, resulted from a combina tion of fluid immiscibility and pressure fluctuation. This interpretation, combined with textural and structural evidence, suggests the emplacement of the mineralised vein in an active fault and at a rather shallow level (4-7 km). The geological and structural setting, deposit-scale textures and str uctures, wallrock alteration and physico-chemical fluid properties are comp atible with those of epizonal to mesozonal orogenic lode gold deposits.