GOLD AND COPPER PARTITIONING IN SULFUR-BEARING VAPOR-BRINE-MELT SYSTEMS AT 800 °C AND 100 MPA

Fluid inclusion data from porphyry-type ore deposits indicate that Au and Cu sometimes partition preferentially into vapor relative to coexisting brine (e.g., Heinrich et al., 1999, Geology, 27, 755). However, preferential partitioning of Cu and Au into vapor at magmatic conditions has not been reproduced experimentally. Therefore, in an attempt to determine under what conditions Cu and Au partition preferentially into vapor relative to brine, we performed experiments in the brine-vapor-haplogranitic melt-intermediate solid solution-pyrrhotite-quartz±bornite±magnetite system at 800°C, 100 MPa and oxygen fugacity buffered by Ni-NiO. The coexisting brine (~68wt.% NaCl eq.) and vapor (~3wt.% NaCl eq.) were composed initially of NaCl+KCl+HCl+H₂O, with starting HCl set to <2000 ppm in the aqueous mixture. Synthetic vapor and brine fluid inclusions were trapped in quartz microfractures during the experiment and analyzed by LA-ICP-MS. The resulting Nernst style partition coefficients for Cu and Au between coexisting brine (b) and vapor (v), D^b/v (±1σ), range from 7±1 to 163±24 and 1.7±0.2 to 26±8 for Cu and Au, respectively. Although Cu and Au were observed to partition preferentially into brine relative to vapor over the entire range experimental conditions, D^b/v at the lowest sulfur fugacity were elevated, 112 and 18 for Cu and Au, respectively, relative to those at higher sulfur fugacity, 9 for Cu and 7 for Au. These data suggest that D^b/v decreases with increasing sulfur fugacity; however, we note that a D^b/v less than one at magmatic conditions remains unobserved experimentally. We hypothesize that Cu and Au partition preferentially into the brine above the wet solidus, but, in response to changes in temperature, oxygen fugacity, sulfur fugacity, and acidity of the MVP, vapor transport of Cu and Au may become preferred at hydrothermal (i.e., subsolidus) conditions in select systems.