FLUIDS AND EPIGENETIC GOLD MINERALIZATION AT SHAMVA MINE, ZIMBABWE - A COMBINED STRUCTURAL AND FLUID INCLUSION STUDY

The Shamva gold mine is hosted within the Shamva greenstone belt and i s related to steeply dipping reverse-oblique crustal shear zones. Flui d infiltration resulted in widespread sulphidisation, K alteration, ch loritisation, silicification and carbonatisation. Fluid inclusions fro m two types of quartz Veins were used to estimate the composition and pressure-temperature conditions of gold mineralisation: (1) Au mineral ised sulphide +/- carbonate bearing veins; (2) late tensional barren v eins. 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 immi scible aqueous and CO2 fluid. The pressure-temperature conditions of A u mineralisation are constrained to 250-450 degrees C and 1-3 kbar. Th e 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 5 30 degrees C, assuming an oxygen fugacity buffered by quartz-fayalite- magnetite. (C) 1998 Elsevier Science Limited.