Shallow magmatic-hydrothermal systems are characterized by steep gradients in temperature and pressure, and because the fluid is of low density and highly compressible, the solubility of ore minerals in these systems varies considerably as a function of both temperature and pressure. We use novel pressure- and temperature-dependent experimentally derived thermodynamic data to geochemically model the transport and deposition of Au, Ag, and Mo by vapor and low- to intermediate-density supercritical fluids in the context of Au and Mo porphyry and Au-Ag epithermal ore formation. The results show that there is a strong compositional control on the Au/Mo ratio of the parental ore fluid, which can explain Au-Mo zoning in porphyry ore deposits and the formation of Au-rich and Mo-rich subtypes. Gold solubility reaches a maximum between 320 °C and 500 °C, depending on the fluid density, whereas Mo and Ag concentrations decrease with decreasing temperature and pressure. These differences in mineral solubility help explain the fractionation of Au from Ag and Mo and the preferential mobilization of Au into sites of epithermal ore deposition. Application of this modeling of metal solubility in vapor-like fluids offers an important opportunity for understanding individual ore-forming hydrothermal systems and determining the limiting factors for metal enrichment.

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