Physical transport of magmatic sulfides promotes copper enrichment in hydrothermal ore fluids

Abstract

AbstractLoss of magmatic sulfides to the mantle is posited to explain the copper deficit of evolved arc magmas and the depleted Cu/Ag ratio of the continental crust. We address the question of whether saturating sulfides may instead be mechanically entrained with rising magmas, and how this would affect their geochemical fate in the upper crust. Entrainment is plausible considering sulfide wetting properties and settling velocities relative to magma ascent velocities. Entrained sulfide increases the pressure at which magmas become saturated with respect to H-O-S fluids in the upper crust by 10–100 MPa, with the pressure difference increasing with temperature, water content, and oxidation. Bubbles are likely to nucleate on sulfide particles, allowing transfer of S and Cu from the sulfide to the fluid over a small crystallization interval without limitations by diffusion through the silicate melt. This sequence of processes gives magmatic sulfides an active role in ore metal transport and enrichment to form porphyry copper deposits, and may have global implications for crustal Cu budgets.