Gold solubility in arc magmas: Experimental determination of the effect of sulfur at 1000 °C and 0.4 GPa

To investigate the behaviour of gold in sulfur-bearing hydrous intermediate calc-alkaline melts under different redox states typical of subduction-zone settings, we have determined the solubility of Au at 0.4 GPa and 1000 °C for three dacitic magmas (two adakites and one calc-alkaline composition) from the North-Luzon Arc (Philippines). The experiments were performed over an oxygen fugacity (fO2) range corresponding to reducing (∼NNO−1), moderately oxidizing (∼NNO+1.5) and strongly oxidizing (⩾NNO+3) conditions as measured by solid Ni–Pd–O sensors. They were carried out in gold containers, serving also as the source of gold, in presence of variable amounts of H2O and ∼1 wt.% of elemental sulfur (S). Concentrations of Au in glasses were determined by laser-ablation inductively-coupled plasma mass spectrometry (LA-ICPMS). Gold solubility in S-bearing melts is drastically enhanced compared to S-free melts, by up to two orders of magnitude. In addition, very high gold solubilities are reached under reducing conditions (