A predictive model for metal transport of silver chloride by aqueous vapor in ore-forming magmatic-hydrothermal systems

New experimental data demonstrate that the solubility of metals in water vapor has been greatly underestimated. We demonstrate here that the solubility of AgCl increases exponentially with H2O fugacity rather than linearly, as previously assumed, leading to much greater hydration and correspondingly higher solubility. Our data suggest that the solubility of silver chloride in low-density aqueous fluids is related to the formation of hydrated clusters, i.e., AgCl:(H2O)n. The hydration number of the predominant cluster increases systematically with increasing pressure, and each of the gaseous solutions investigated in our experiments contains a mixture of clusters with different hydration numbers that predominate at different pressures. Extrapolation of the data to magmatic conditions, based on an observed linear relationship between the Gibbs free energy of formation of the hydrated species and reciprocal temperature, predicts a solubility silver chloride comparable to the concentration of silver observed in vapor inclusions, thereby supporting a model of metal transport by vapor.