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Abstract:

Magmas are an important source of noble metals. Metals are transported by magma from deep within the mantle to the shallow crust, where they form subsurface ore deposits1-6. The concentration of noble metals in silicate melts has been thought to be controlled by the stability of sulphide minerals in the upper mantle and crust, with only sulphide-undersaturated magmas capable of extracting significant amounts of metals from the mantle 1-6. Here we present a series of experiments carried out on basaltic and andesitic glasses, melted in the presence of water and sulphur in gold capsules at a pressure of 200 MPa and a temperature of 1,050 C, under a wide range of redox conditions. We show that gold solubility in silicate melts is highest within a narrow window of redox conditions, characterized by the transformation of sulphide to sulphate species in the magma. Within this redox range, we found that gold was particularly mobile and became dissolved in the silicate melts as a sulphide-bearing component. We suggest that gold-rich magmas can be generated in mantle systems that are sulphide-saturated, as long as they are relatively oxidized. Conditions that are favourable to the mobilization and transport of gold (and presumably other noble metals) may prevail in magmatic systems above subduction zones and in the environments beneath hotspot volcanoes.