GOLD, COPPER AND IRON PARTITIONING BETWEEN VAPOR, BRINE AND MELT

Studies of boiling assemblage fluid inclusions from some porphyry-type ore deposits indicate that Au and Cu may partition preferentially into the vapor relative to a coexisting brine. Whereas 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. Thus, there is a disparity between select field observations and experimental data. More recent studies have attempted to reconcile these observations (Frank et al., 2006, AGU abstracts with programs). This study will present a model detailing some of the most-likely factors controlling the partitioning of gold, copper and iron between vapor, brine and melt. Analyses of pertinent equilibria suggest that the HCl/H₂S ratios of the vapor and brine have a great influence on the concentration and speciation of ore metals in a magmatic volatile phase. At high HCl concentrations in the aqueous mixtures, vapor and brine, Au and Cu partitioned preferentially into the brine over a large range of sulfur activities. These data indicate that for Au, Cu and Fe to partition preferentially into a vapor relative a brine requires a very low HCl/H₂S value.