THE EFFECT OF SULFUR ON MELT-VOLATILE PHASE PARTITIONING OF COPPER

Porphyry-copper ore deposits are spatially and temporally associated with felsic igneous rocks, suggesting that copper is transported from magma into evolving vapor and/or brine during ore genesis. Controversy has surrounded whether Cu is transported as a sulfide or chloride species, or both, in these volatile phases. Phase equilibria in the sodium chloride-water system suggest that the concentration of chloride-complexed metals should be higher in the brine relative to the vapor. However, fluid inclusion data from natural vapor-brine boiling assemblages demonstrate that Cu sometimes partitions preferentially into the vapor relative to the brine. Thus, partitioning of supposedly chloride-complexed solutes in favor of the vapor in natural boiling assemblages has led to the hypothesis that metal-bisulfide complexes in the vapor phase are responsible for the increased vapor-phase fractionation of these metals. As a first step toward testing this hypothesis, we have performed experiments to quantify the effect of sulfur on the partitioning behavior of Cu in a chloride-bearing silicate melt-magnetite-vapor (18 wt % NaCl eq.) -brine (37 wt % NaCl eq.) +/- sulfide assemblage at 800 C, 145 MPa. Aqueous fluids were trapped in quartz, as synthetic fluid inclusions, as well as in glass vesicles. Oxygen fugacity was buffered at Ni-NiO and sulfur fugacity was controlled by the magnetite-pyrrhotite assemblage in sulfide-saturated experiments. LA-ICPMS data demonstrate that the concentration of Cu in the brine exceeds that of vapor. However, the vapor/brine partition coefficient increases from 0.2 (sulfur-free) to 0.65 (sulfur-bearing). The vapor/melt and brine/melt partition coefficients increase by factors of five and nearly two, respectively, relative to the sulfur-free system. These data evince that, at the oxygen, sulfur, and water fugacities of our experiments: 1. sulfur has a measurable effect on the partitioning behavior of Cu at magmatic conditions; 2. the brine is still enriched in Cu relative to the vapor; and 3. even a sulfur-free vapor or brine can supply significant Cu to the porphyry environment and, thus, variations in sulfur fugacity are not likely to produce significant variations in the mass transfer of Cu from melt to vapor and/or brine during the formation of porphyry-Cu ore deposits.