Magma emplacement depth and magmatic-hydrothermal ore fertility: The effects of pressure and temperature on the fluid/melt partitioning of porphyry ore metals

Porphyry-type ore deposits are the most important source of Cu and Mo for our society, while also providing significant amounts of Au and Ag, making the understanding of their formation crucial to the targeted exploration of these metals. Recent studies emphasize the importance of the timing and depth of fluid exsolution for controlling magmatic sulphide saturation and fluid-assisted ore metal and sulphur extraction from the magma, and thus the overall ore fertility of arc magmas. As emplacement depth is related to the pressure (P) and temperature (T) of the magmatic system, accurate constraints on the influence of P and T on the fluid/melt partition coefficients (D^f/m) of Cu, Ag, Au and Mo are vital in understanding the genesis of porphyry deposits. To this end, experiments were conducted by equilibrating a synthetic rhyolite starting glass with S-free fluids containing 5.5 and 37 wt.% NaCl_­Eq­ chlorides at pressures of 150–700 MPa (corresponding to ~5–25 km depth) and temperatures of 750–950 °C. Experiments were performed using Au-Ag-Cu alloy capsules as a source of metals and the equilibrium fluid was entrapped as synthetic fluid inclusions in natural quartz fractured in-situ during experimental runs. Externally heated molybdenum-hafnium carbide pressure vessels were used for experiments up to P = 300 MPa, above which a piston cylinder apparatus was used. Results indicate a moderate decrease in D^f/m_­Cl­ with increasing T and a partitioning maximum at a P of about 400 MPa. Considering the ore metals, Cu, Ag, and Au partition coefficients decrease with increasing temperature, with the effect being greater at higher fluid salinity. Pressure has a weak effect on the partitioning of these metals at constant Cl concentration in the fluid phase and D^f/m may display a maximum at ~400 MPa. As for Mo­, temperature has a negligible effect on D^f/m_­Mo­ while increasing pressure increases Mo partitioning into the fluid, but only at low salinities. The results suggest that lower temperatures and moderate pressures (~400 MPa or ~12 km depth) are the most conducive to the generation of magmatic fluids with a high potential for porphyry ore generation. The new results serve as an important building block of semi-empirical models currently in development to predict the fluid/melt partition coefficients of porphyry ore metals in P-T-compositional space.