WHITE-ISLAND, NEW-ZEALAND, VOLCANIC-HYDROTHERMAL SYSTEM REPRESENTS THE GEOCHEMICAL ENVIRONMENT OF HIGH-SULFIDATION CU AND AU ORE DEPOSITION

The White Island volcanic-hydrothermal system, New Zealand, is thought to closely represent the chemical conditions that lead to the formado n of high-sulfidation Cu-Au ore deposits. The amounts of Cu and Au pro duced over a 10 ka period of activity, largely from degassing magma, a re calculated to be 10(6) and 45 t, respectively. Altered andesite blo cks ejected from recent vents contain alunite, anhydrite, and pyrite. Their S isotopic composition indicates vein filling at approximately 3 80-degrees-C. At this temperature, Cu and Au are highly soluble in aci d solutions, which may explain the depletion of Cu and absence of Au i n the ejecta. Mass-balance calculations, however, suggest that Cu and Au are precipitated in cooler zones before the acid solutions discharg e at the surface.