Petrographic and Mineralogical Factors Affecting Gold Recovery at Macraes Flat, Eastern Otago, New Zealand

摘要

Gold mineralisation at Macraes occurs as late quartz-carbonate veins within a body of sheared and hydrothermally altered Otago Scist, the Intrashear Schist. The alteration is characterised by the presence of rutile and the absence of epidote and titanite, indicative of an alteration fluid containing>10 mol per cent CO_2. In areas of high strain, a fabic of black microshears develop[s, gradational to mylonitic cataclasite in the high strain zone adjacent to the hanging wall of the duplex shear zone. The microshears contain hydrothermal graphite, inferred to have been deposited from a methane-rich fluid. The mylonitic cataclasite is characterised by abundant pyrite, the absence of alibite and extreme grain size reduction.