Characterization of fluids associated with gold mineralization and with regional high-temperature retrogression of granulites in the Limpopo belt, South Africa

Abstract

The Hout River Shear Zone forms the terrane boundary between the granite-greenstone terrane of the Kaapvaal craton and the granulite terrane of the Limpopo belt. It dips steeply northward and flattens to the south and was responsible for allowing hot granulites to spread up and over the cooler greenstone terrane. This shear zone acted as a conduit for deep-seated infiltrating fluids into still hot granulites in the hangingwall producing a regional retrograde orthoamphibole isograd and an associated zone of rehydrated granulites. Thermodynamic constraints on the composition of fluids involved in retrograde anthophyllite formation in pelitic granulites on the isograd, supported by the results of a fluid inclusion study and by direct textural evidence in which olivine in closely associated ultramafic granulites has been partially replaced by hypersthene and magnesite, suggest that the hydrating fluid contained at least 70 mol% CO 2 and that this fluid infiltrated at P-Tconditions of about 620°C and 6 kbar pressure. The δS 13 C values of CO 2 extracted from magnesite vary between −5.5 and 6.0%. and suggests a deep-seated (possibly mantle) source for the CO 2-rich retrograde fluid. Fluid inclusion studies from two lode-gold deposits (Osprey and Louis Moore) in the same high-grade area, and three deposits from the immediately adjacent Sutherland greenstone belt (Fumani, Klein Letaba, and Birthday) at the northern edge of the Kaapvaal Craton, show that CO 2-rich fluids with similar characteristics were also associated with epigenetic gold mineralization. All of the ore bodies are located within EW-trending northward dipping, ductile shear zones with an oblique to reverse sense of movement. These relationships suggest that there is a direct link between the source of fluids involved in gold mineralization, the fluids that formed the zone of rehydration in the Southern Marginal Zone of the Limpopo belt, and southward verging faulting during the exhumation of the granulite terrane at about 2670 Ma.