Evolution of hydrothermal fluids in the Ashanti gold belt, Ghana; stable isotope geochemistry of carbonates, graphite, and quartz

Abstract

The oxygen and carbon isotope systematics of ankerite, siderite, calcite, quartz, and graphite from the Bogosu and Prestea mining districts of the Ashanti gold belt, Ghana are used to speculate on the processes leading to hydrothermal alteration and gold deposition. The geologic environment of this turbidite- and graywacke-hosted gold system was a sediment-dominated accretionary complex. The gold ores are localized within the Ashanti structural belt which we suggest became a conduit for deep-seated fluids during uplift and dilatancy.Ankerite and siderite in mineralized sedimentary rocks are broadly similar in oxygen and carbon isotope compositions to corresponding minerals in the metasedimentary country-rocks. Evidence of alteration that preceded gold mineralization is best preserved in spatially associated altered mafic dikes. The compositions of fluids responsible for the gold mineralization and associated alteration have been reconstructed using temperatures from carbonate-mineral geothermometry.Alteration of country rocks occurred under rock-dominant, greenschist facies conditions. The isotopic composition of the least altered mafic dikes was influenced by interaction with fluids generated from Birimian greenschist facies metasedimentary rocks. Later, extensive carbonate alteration of mafic dikes adjacent to the structural conduit occurred during slow upward migration of fluids. Lateral diffusion of hydrogen into the structural conduit permitted partial conversion of CO 2 to methane, resulting in 13 C enrichment of both the residual CO 2 and the deposited carbonate minerals.Rapid, even explosive, expulsion of deep-seated fluids and CO 2 -H 2 O phase separation occurred episodically throughout the evolution of the hydrothermal system. This caused precipitation of sulfides, arsenides, and gold at relatively high crustal levels and low ambient pressure and temperature. During mineralization, fluid ascent was too rapid for isotopic and chemical reequilibration between the deep-seated ore fluid and adjacent country rocks. The ore fluid had chemical and isotopic compositions consistent with deep-seated metamorphic fluids that were subsequently modified by preferential partitioning of 18 O and 13 C into a gas phase. The composition of the deep-seated metamorphic fluid determined using chemical equilibria (CO 2 -H 2 O rich; 7-25 mole % CO 2 ) is in approximate agreement with that required to produce the isotopic depletion of the residual fluid via phase separation.