The White Island volcanic-hydrothermal system, New Zealand, is thought to closely represent the chemical conditions that lead to the formation of high- sulfidation Cu-Au ore deposits. The amounts of Cu and Au produced over a 10 ka period of activity, largely from degassing magma, are calculated to be 106 and 45 t, respectively. Altered andesite blocks ejected from recent vents contain alunite, anhydrite, and pyrite. Their S isotopic composition indicates vein filling at ∼380 °C. At this temperature, Cu and Au are highly soluble in acid solutions, which may explain the depletion of Cu and absence of Au in the ejecta. Mass-balance calculations, however, suggest that Cu and Au are precipitated in cooler zones before the acid solutions discharge at the surface.