Gold solubility and partitioning between sulfide liquid, monosulfide solid solution and hydrous mantle melts: Implications for the formation of Au-rich magmas and crust–mantle differentiation

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The solubility of Au in sulfur-free vs. sulfide-saturated melts and its partitioning behavior between sulfide liquid (SL), monosulfide solid solution (MSS) and hydrous basanite melt at variable Au activities was investigated in a fO2 range of FMQ−2 to FMQ+1.6 at 1200°C/1.5GPa using piston cylinder apparatus. Gold solubility in sulfur-free (<100μg/g S) melt is low (0.6–1.6μg/g) and increases with fO2 in a manner consistent with Au dissolution as AuO1/2, whereas in sulfide-saturated melts it is high (13.6±1.7μg/g) and independent of fO2. Variations in the chlorine content of sulfide-saturated melts (0.2–1.2wt% Cl) had no measurable effect on Au solubility. Gold partition coefficients between sulfide liquid and silicate melt (DAuSL/SM) are very high, ∼10,000±3000, which is at the upper end of values reported in previous studies. Gold partition coefficients between MSS and silicate melt (DAuMSS/SM) are much lower, 60±10, which is at the lower end of previous values. Both DAuSL/SM and DAuMSS/SM are independent of fO2. The new Au partition coefficients were used in conjunction with previously published Cu and Ag partition coefficients to investigate the role of MSS versus SL during partial melting in the source region of primitive potassic magmas and during crust–mantle differentiation.The high Au content of ore deposits associated with potassic magmas has commonly been explained by the dissolution of Au-rich sulfide liquid, either during partial melting in the mantle source or during partial re-melting of sulfide-bearing cumulates at the crust–mantle boundary. We argue that MSS is the dominant sulfide phase in the mantle source region of these magmas, and thus that their high Au content is a consequence of low MSS–silicate melt partition coefficients rather than of sulfide exhaustion or partial re-melting of sulfide-bearing cumulates.Continental crust is depleted in Au, Ag and Cu relative to mantle melts, which was thought to be due to removal of sulfide liquid at the crust–mantle boundary. However, based on sulfide phase relations at high-pressure and high-temperature and corresponding sulfide–silicate melt partition coefficients we come to the conclusion that sulfides precipitated at the crust–mantle boundary are dominated by MSS rather than sulfide liquid, and thus have a much lower capability to sequester Au, Ag and Pb than in the case of sulfide liquid.