The solubility of Pd and Au in hydrous intermediate silicate melts: The effect of oxygen fugacity and the addition of Cl and S

The solubilities of Pd and Au in a hydrous trachyandesitic melt were experimentally determined at 1000 degrees C and 200 MPa at oxygen fugacity (fO(2)) from 0.45 log units below to 6.55 log units above the Ni-NiO buffer (NNO). The effect of adding metal-binding ligands (i.e. Cl and S) to the silicate melt was also studied. The solubility of Au increases from 0.15 perpendicular to 0.1 to 3.85 perpendicular to 1.48 ppm in Cl- and S-free melts with fO(2) increasing from NNO-0.45 to NNO+6.55 with a slope that suggests that it is present in 1+ oxidation state over the entire studied fO(2) range. On the other hand, Pd solubility, shows a more moderate increase with fO(2), especially in the lower half of the studied range, increasing from 2.66 +/- 0.25 ppm at NNO-0.45 to only 3.62 +/- 0.38 ppm at NNO+1.72 in Cl-and S-free melts. Overall, the variation in Pd solubility as a function of fO(2) indicates Pd being dissolved in the silicate melt in both zero and 1+ oxidation state, with the former being dominant below NNO+4.5. At NNO-0.45 to +3.48, the addition of 3170-4060 ppm Cl to the silicate melt increased the solubility of Au by an average factor of 1.5, in comparison to Cl-free melts. However, at NNO+6.55, Au solubility increased by a factor of 2.5. The addition of Cl had a negligible effect on the solubility of Pd except for a large increase (factor of 2.4) at NNO+6.55. At reducing conditions (NNO-0.45), the addition of 170 ppm S to the silicate melt increased the solubility of Au by a factor of similar to 4 but did not change the solubility of Pd in comparison to S-free melts. The observation that Pd is dominantly present as Pd-0 at NNO < +4.5 may explain the lack of complexation with potential reduced sulfur-bearing and chloride ligands. As one may expect similar behavior in fluids degassing from magmas at depth, the lack of oxidized Pd species could be an important factor behind the scarcity of economically viable Pd-rich magmatic-hydrothermal deposits observed in nature. (C) 2018 Elsevier Ltd. All rights reserved.