Fluid inclusion re-equilibration and P-T-X constraints on fluid evolution in the Ashanti gold deposit, Ghana
Schwartz, Michael O.; Oberthür, Thomas; Amanor, Joe; Gyapong, Will A.
European Journal of Mineralogy Volume 4 Number 5 (1992), p. 1017 - 1034
published: Oct 14, 1992
manuscript accepted: Feb 5, 1992
ArtNo. ESP147050405018, Price: 29.00 €
Abstract 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 CO2 ± minor amounts of a contaminant gas (averaging < 10 mol.% N2). The density of the gas inclusions varies between 0.77 and 1.11 g/cm3 (median = 0.96 g/cm3). The H2O-CO2-salt) inclusions (5-80 mol.% CO2) and aqueous inclusions without a visible CO2 phase are subordinate. The aqueous inclusions which were trapped late with respect to the CO2 inclusions homogenize in the range 215-405 °C (median = 325 °C). The salinities vary between 0.4 and 5.2 equiv. wt.% NaCl. The H2O and CO2 isochores intersect at unrealistically high temperatures; i.e. the median H2O isochore (corresponding to a homogenization temperature of 325 °C) intersects the lowest-density CO2 isochore (0.77 g/cm3) at 620 °C. Maximum temperatures are constrained to 370-470 °C by the stability of the mineral assemblages siderite + graphite and calcite + muscovite + quartz or by the presence of rutile. According to fO2 constraints derived from fluid-mineral equilibria, the original fluid was a CO2-H2O mixture. Compatible fO2∙P and T conditions imply that the fluid inclusions have changed after entrapment. A model is presented which postulates leakage of H2O(± salt) from mixed CO2-H2O(± salt) inclusions, PT re-equilibration of the residual CO2 inclusions and remapping of the leaked H2O(± salt) fluid.