Aspects of the Chemical Composition of the Birimian Gold Fluid.

Abstract

Fluid inclusion data, EPMA and geochemical studies of metamorphic chlorites from altered phyllites and graphitic phyllites were used to constrain the possible nature of the ore fluid involved in gold mineralization in the Birimian of Ghana. The results showed that the metavolcanics and metasediments rock groups in the Birimian gold belt were transformed by metamorphism and hydrothermal activity. The alteration processes involved in the transformation were sulphidation, sericitization, rutile formation, silicification, carbonation, carbonitization and of chloritization. Some of these processes were related to major gold mineralization in the country rocks. Minor alterations caused by the hydrothermal activity, which resulted in the mineralization are sulphidation, carbonation, tourmalization and chloritisation. It is inferred that gold was deposited under similar conditions as those of the hydrothermal activity. Calculated physico-chemical depositional conditions of the ore fluid, based on chlorite-pyrrhotite-pyrite and chlorite-albite relationships yielded fO2 = -33 to -32; fH2S = 1.5 to - 2.0; and pH= 5.8. Precipitation of gold in the country rocks occurred as gold-bearing sulphides, dominated by arsenopyrite, through interaction of the host country rocks with the mineralising fluid. Deposition of the gold occurred within major late-orogenic shear zones when the original mineralizing fluid decompressed, cooled and separated into CO2-rich and H2O-rich fluids.