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Emplacement depth and ore metal extraction: The effect of pressure and temperature on the fluid/melt partitioning of ore metals in magmatic-hydrothermal systems

Ivano Gennaro , Alexandra Tsay, Zoltan Zajacz

Abstract

Porphyry-type ore deposits supply most Cu and Mo, and significant amounts of Au and Ag for our society, making the understanding of their formation essential to the targeted exploration for these metals, some of which are critical for the green energy transition. Several recent studies proposed the onset of fluid exsolution in porphyry ore-related magmas to take place at mid to lower crustal depths, affecting the metal, Cl, and S concentrations of ascending magmas; therefore, requiring accurate constraints on the influence of P and T on the fluid/melt partition coefficients (Df/m) of Cu, Ag, Au, and Mo in these systems. To this end, experiments were conducted at P = 150--700 MPa (~5--25 km depth) and T = 750--950 °C. Synthetic rhyolite melts were equilibrated with S-free fluids containing 5.5 and 37 wt.% NaCl­Eq­ chlorides in Au-Ag-Cu alloy capsules and the equilibrium fluid was trapped in quartz cylinders in the form of synthetic fluid inclusions. Experiments were run using cold-seal molybdenum-hafnium carbide pressure vessels (P ≤ 300 MPa) or a piston cylinder apparatus (P > 300 MPa). Df/m values of Cu, Au, and Ag decrease with increasing T; P shows negligible effects on Df/m­Ag­ and Df/m­Au­. Df/m­Mo­ is unaffected by T and increases with P only at low fluid salinity. Solubility data indicate that for Cu, Au, and Ag, solubility in the fluid imposes the T-dependence of fluid/melt partitioning, and that Au becomes highly insoluble in both fluid and melt phases in S-free systems below 750 °C.