MASS TRANSFER OF SULFUR AND CHALCOPHILE METALS DURING MAGMA MIXING: IMPLICATIONS FOR PORPHYRY ORE DEPOSITS

Cu-Au-Mo-Ag-porphyry-type ore deposits are fundamentally sulfur anomalies, containing anywhere from ten to hundreds of times more sulfur than average continental crust. Mass balance calculations indicate that the sulfur (S) and chalcophile metal (e.g., Cu, Au) budgets of porphyry deposits cannot be sourced solely from the intermediate to felsic magmatic rocks that host mineralization. Evidence from natural systems is consistent with the S and chalcophile metals being sourced from mafic magma(s) that underplates (without physical mixing or mingling) or intrudes into and physicochemically mixes with overlying intermediate to felsic magma. Mass transfer is then controlled by diffusion of S and chalcophile metals from mafic to felsic melt, and/or via aqueous fluid that exsolves from the mafic melt, scavenges S and metals and ascends into the felsic magma. Here, we explore this model directly by performing experiments to simulate the underplating process. Magma-magma diffusion couples were run by using natural andesite and dacite to synthesize, at 150 MPa and fO2 ~FMQ+3, two initial glass compositions: 1) andesite with ~1 wt. % H2O, ~700 ppm S, ~500 ppm Cl at 1030°C; and 2) dacite with ~3 wt. % H2O, ~100 ppm S, ~1500 ppm Cl at 900°C. The diffusion-couples were run at 950°C and 1000°C, 150 MPa, and FMQ+3, for 0.1 to 100 h. At 950°C and 1000°C the andesite crystallized significantly at the interface beween the two magmas, and became volatile saturated within the solidification front. EPMA traverses across the interface indicate that H2O and Cl show only a short diffusion profile. At 950°C, the concentration of S in the andesitic melt within the solidification front increased from ~700 ppm to ~1.2 wt. % within one to ten hours, and the sulfur concentration of the dacitic melt remained unchanged. At 1000°C, the concentration of S in the andesitic melt also increased near the interface, and a diffusion profile for S indicates slow mass transfer of S from andesite to dacite. The slow diffusion of S is likely related to the exchange of Fe from the andesite to the dacite. These results are consistent with observations from natural systems, wherein the degree of crystallization at the magma-magma interface kinetically hinders the mass transfer of S and chalcophile metals. The implications of these data for porphyry systems will be discussed.