Wall-rock alteration as a guide to gold-bearing fracture zones in the Zwartkoppie Section, Sheba Gold Mine, South Africa

摘要

Gold at the Sheba Gold Mine is found in a variety of rocks, the most important being the schists and chert of the Zwartkoppie Formation and the overlying metagreywacke and shale of the Fig Tree Group. Gold and sulphides occur in association with quartz and quartz-carbonate vein-filled fracture zones, either in the vein material itself or disseminated in the wall rock. Wall-rock alteration in different rocks follows a similar pattern, with the most important mineralogical changes being sulphidation, replacement of chlorite by muscovite (in the schists, metagreywacke, and shale), hydrolysis of feldspar (in metagreywacke and shale), and carbonatization. Chemical alteration is characterized by an increase in S and As (sulphidation), K_2O and Rb (owing to the replacement of chlorite and feldspar by muscovite), and CO_2 (carbonatization), and a decrease in Na_2O and MgO (owing to the replacement of feldspar and chlorite by muscovite). The presence of alteration haloes round the gold-bearing fracture zones at the Sheba Gold Mine can be used to define potential ore zones for both underground and surface exploration.