Fluids and epigenetic gold mineralisation at Shamva Mine, Zimbabwe: a combined structural and fluid inclusion study

The Shamva gold mine is hosted within the Shamva greenstone belt and is related to steeply dipping reverse-oblique crustal shear zones. Fluid infiltration resulted in widespread sulphidisation, K alteration, chloritisation, silicification and carbonatisation. Fluid inclusions from two types of quartz Veins were used to estimate the composition and pressure-temperature conditions of gold mineralisation: (1) Au mineralised sulphide +/- carbonate bearing veins; (2) late tensional barren veins. The vein types contain aqueous, mixed H2O-CO2(+/-CH4) and CO2(+/-CH4) rich inclusions. Fluid inclusions in the mineralised and barren veins are similar in composition and resulted from trapping of an immiscible aqueous and CO2 fluid. The pressure-temperature conditions of Au mineralisation are constrained to 250-450 degrees C and 1-3 kbar. The similar pressure conditions found for Au mineralisation and the peak of metamorphism implies that the retrograde pressure-temperature path followed isobaric cooling. Thermodynamic modelling of the fluid shows that: (1) XCO2/(XCO2 + XCH4) of the metamorphic fluid ranged between 0.75 and 0.85; (2) XH2O of the Au mineralising fluid ranged between 0.85 and 0.90; and (3) metamorphic temperatures ranged between 500 and 530 degrees C, assuming an oxygen fugacity buffered by quartz-fayalite-magnetite. (C) 1998 Elsevier Science Limited.