Hydrothermal alteration geochemistry of nugget gold vein system, shotover valley, Northwest Otago, New Zealand

Abstract

Ankeritic alteration zones are associated with gold mineralisation in the Nugget vein system, Shotover valley, northwest Otago. These zones consist of sheared and brecciated schist in Miocene normal faults. The zones are also extensively silicified. Mineralisation resulted from passage of low‐salinity, low‐CO2 water at 200–250°C within a few kilometres of the surface. Alteration has involved replacement and veining of hostpelitic schist in the mineralised zone. Most elements have been diluted by quartz and carbonate, with insignificant gains or losses. Ba has been leached as it became more soluble with decreasing fluid temperature. Considerable addition of Sr has occurred, as the fluid had higher Sr/Ca than most Otago mineralising fluids. Enrichment in Sr may be a distinctive feature of the Shotover style of mineralisation. Metallic mineralisation is due to addition of S, As, and Au during alteration. In contrast to many Otago gold vein systems, there has been no scheelite mineralisation. Carbonates have δ18O = +15.4 to +18.6, and δ13C = ‐8.2 to +0.5‰. The oxygen isotopic signature is similar to other Otago gold veins. Carbon isotopes in aqueous CO2 which deposited Nugget ankerites are generally heavier (by 2–4‰) than those in other Otago gold vein systems. This is caused by changes in dissolved carbonate species at the lower temperature, and fluid/rock interaction during ankerite mineralisation. © 1991 Taylor & Francis Group, LLC.