The effect of pressure and temperature on the fluid/melt partitioning of porphyry ore metals

A major fraction of the total Cu, Mo, and Au mined worldwide is derived from porphyry ore deposits, with Ag representing an abundant by-product from the exploitation of these deposits. The relative endowment of these metals is linked to deposit depth, with Au-rich deposits forming in the shallowest levels of the crust while Mo-rich deposits form deeper. These deposits are fed by magmatic-hydrothermal fluids derived from magma chambers which likely sit 5–15 km below the surface, but attainment of fluid saturation at greater depths has also been proposed. These deposits display a general depth-dependent metal endowment. To determine the effect of pressure ( P ) and temperature ( T ) on the ability of the fluid phase to extract these metals from the melt, experiments were conducted in cold-seal pressure vessels and a piston cylinder apparatus covering a range of P (150–700 MPa) and T (760–950 °C) conditions at which fluid exsolution is most likely to occur in crustal magma reservoir systems. Experimental capsules were composed of a Au-Ag-Cu alloy as the source of metals and contained a synthetic rhyolite glass, fluid of 1 or 12 mol Cl/kg H 2 O, and natural quartz chip for the entrapment of synthetic fluid inclusions. Metal solubilities in the melt and fluid phases increase roughly linearly with increasing T ; Cl concentration only affects the solubility of metals in the fluid phase. Fluid/melt partition coefficients of Cu, Ag, and Au decrease monotonically with increasing T and are generally unaffected by P at a fixed fluid salinity. Chlorine and Mo partitioning into the fluid increase by a factor of 4 with P increasing from 150 to 500 MPa at low fluid salinities but decrease slightly at high fluid salinities. Empirical functions were fit which describe the solubility, fluid/melt partitioning, and metal/Na exchange of Cl, Cu, Ag, Au, and Mo as functions of T and P . A simplified magma degassing model based on these equations reveals that ore metal extraction from magmas is the most effective at upper crustal emplacement depths and relatively high initial water concentrations.