Gold, Copper, and Iron Partitioning in Sulfur-Bearing Vapor-Brine-Melt Systems at 800°C and 100 MPa.

Studies of boiling assemblage fluid inclusions from porphyry-type ore deposits indicate that Au and Cu may partition preferentially into the vapor relative to a coexisting brine (Heinrich et al., 1999, Geology, 27, 755). Recent experimental studies of Au (Frank et al., 2002, GCA, 66, 3719; Simon et al., 2005, GCA, 69, 3321) and Cu (Williams et al., 1995, CMP, 121, 388) have indicated that those elements will partition strongly into a high-salinity brine relative to a low-salinity vapor in a sulfur-poor environment. The conditions under which Au and Cu partition preferentially into vapor relative to brine have not been reproduced experimentally. Thus, there is a disparity between select field observations and experimental data. Therefore, in an attempt to determine under which conditions Cu and Au will partition preferentially into vapor relative to brine, we performed experiments in the brine-vapor-haplogranitic melt-intermediate solid solution-pyrrhotite-magnetite-quartz (log a(S2) = -1.5) and brine-vapor-haplogranitic melt-intermediate solid solution-pyrrhotite-bornite-quartz (log a(S2) = -3.9) systems at 800°C, 100 MPa and oxygen fugacity buffered by Ni-NiO. The coexisting brine (~68 wt.% NaCl equivalent) and vapor (~3 wt.% NaCl equivalent) were composed initially of NaCl+KCl+HCl+H2O, with starting HCl set to <5000 ppm in the aqueous mixture. Vapor and brine bearing fluid inclusions were trapped in quartz during the experiment. Select fluid inclusions (n = 100) were analyzed by laser ablation ICPMS, using a 193nm ArF Excimer laser coupled with an Elan 6100 ICPMS. Counts of Au, Cu, Fe, and K were related to those of Na (an internal standard that was derived from the microthermometric NaCl equivalent salinity) to calculate concentrations of those elements in the fluid inclusions. Analyses revealed that at low HCl concentrations in the aqueous mixtures, vapor and brine, Au and Cu partitioned preferentially into the brine over a range of sulfur activities with partition coefficients, D^{b/v}, ranging from 2 to 18 (Au) and 7 to 102 (Cu). Fe partition coefficients, D^{b/v}, range from 3 to 15. These data indicate that Au, Cu and Fe will partition into a brine relative to a coexisting vapor over the entire range of sulfur activities of our experiments.