Fluid inclusion re-equilibration and P-T-X constraints on fluid evolution in the Ashanti gold deposit, Ghana

The gold-quartz veins at Ashanti, Ghana, are emplaced in strongly deformed Lower Proterozoic metasediments and metavolcanics (greenschist facies). The veins underwent ductile and brittle post-crystallization deformation. More than 90% of the fluid inclusions in quartz only contain visible CO 2 ± minor amounts of a contaminant gas (averaging <10 mol.% N 2 ). The density of the gas inclusions varies between 0.77 and 1.11 g/cm 3 (median=0.96 g/cm 3 ). The H 2 O-CO 2 (-salt) inclusions (5-80 mol.% CO 2 ) and aqueous inclusions without a visible CO 2 phase are subordinate. The aqueous inclusions which wre trapped late with respect to the CO 2 inclusions homogenize in the range 215-405 o C (median=325 o C). The salinities vary between 0.4 and 5.2 equiv. wt.% NaCl. The H 2 O and CO 2 isochores intersect at unrealistically high temperatures; i.e. the median H 2 O isochore corresponding to a homogenization temperature of 325 o C) intersects the lowest-density CO 2 isochore (0.77 g/cm 3 ) at 620 o C. Maximum temperatures are constrained to 370-470 o C by the stability of the mineral assemblages siderite + graphite and calcite + muscovite + quartz or by the presence of rutile. According to f O2 constraints derived from fluid-mineral equilibria, the original fluid was a CO 2 -H 2 O mixture. Compatible f O2 , P and T conditions imply that the fluid inclusions have changed after entrapment. A model is presented which postulates leakage of H 2 O(± salt) from mixed CO 2 -H 2 O(± salt) inclusions, PT re-equilibration of the residual CO 2 inclusions anbd retrapping of the leaked H 2 O (± salt) fluid