Volatile transport of metals and the Cu budget of the active White Island magmatic-hydrothermal system, New Zealand

Abstract

Evidence of metal transport by volcanic gases is found, amongst others, in volcanic plumes and sublimates growing around fumaroles. The transport mechanisms (i.e. volatile metal compounds) are, however, not well constrained. Here, we present results on the volatile transport of metals at the scale of a magmatic-hydrothermal system, White Island volcano. We studied metal emissions from the main vent through the crater lake covering the conduit. Volcanic gases injected in the lake are responsible for the waters and sediments enrichment in various metals, while elemental sulfur forming at the bottom appears as the main sink for chalcophile metals. Metal chloride complexes are the dominant species in the hot and acidic crater lake waters. Nearby low-temperature fumarolic gases are lower in metals than higher temperature gases reported in the 1970s, emphasizing precipitation at depth and sequestration by condensing elemental sulfur. Efficient gaseous transport of metals by HCl and HBr is revealed by thermodynamic modeling, whereas the role of sulfur is restrained to controlling redox conditions. A copper budget for White Island magmatic-hydrothermal system is assessed, taking into consideration all the potentials sinks. About 3900 tons of copper are estimated to be retained in the subsurface environment every year. Over the 10,000 years lifetime of the magmatic-hydrothermal system, this adds up to ~40 megatons, depicting White Island as a potential Cu porphyry forming deposit.