GOLD AND COPPER IN MAGMATIC VOLATILE PHASES: IMPLICATIONS FOR PORPHYRY AND EPITHERMAL ORE DEPOSITS

Gold and copper-bearing porphyry and epithermal ore deposits are associated spatially and temporally with porphyritic igneous (granitic, s.l.) intrusions. The distribution of minerals between the porphyry and epithermal regimes is controlled by the ability of the supercritical fluid, low-salinity vapor, and/or high-salinity brine to carry dissolved metals to the ore zone. Experiments were conducted in the Au + chalcopyrite + vapor + brine ± pyrrhotite ± bornite system at 500 to 700°C and 50 to 100 MPa to ascertain the concentration of Au and Cu in vapor and brine as a function of total salinity (1.3 to 66 wt. NaCl eq.), f O₂ (8.1x10^-14 to 1.4x10^-23 bars), and f S₂ (1.6x10^-11 to 1 bars). Additional experiments were conducted in the Au + chalcopyrite + supercritical fluid ± pyrrhotite ± bornite system at 500 to 600°C and 100 MPa. Fluids were trapped in synthetic fluid inclusions in quartz and analyzed by using laser ablation inductively coupled plasma mass spectrometry. Cu concentrations in the brine (35-66 wt.% NaCl_eq.) ranged from 800 to 8200 μg/g and were strongly correlated to the total chloride content of the fluid; whereas Cu in coexisting vapor (1-3 wt.% NaCl_eq.) ranged from ~40 to 1000 μg/g and were never greater than the brine in any vapor + brine experiment. Cu concentrations in a 5 wt.% NaCl_eq. supercritical fluid were determined to be 80-1200 μg/g and correlated strongly with the fluid’s total chloride content. The total salinity of the fluid also correlated with Au; maximum Au concentrations of 150, 10, and 16 μg/g were found in brine, vapor, and supercritical fluid inclusions, respectively. Temperature was also important: Au concentrations in supercritical fluid were 2 and 10 μg/g at 500 and 600°C, respectively. Variations in O₂ and S₂ fugacities produced only small variations in observed Cu concentrations, but elevated H₂S increased the total amount of Au in both the brine and vapor with a slightly more pronounced impact on metals in the vapor. Our data suggest that a rising moderate salinity supercritical fluid could supply sufficient Cu and Au to the porphyry environment with substantial Cu and Au being stored in the brine. Some portion of these metals could be transferred by the vapor to the epithermal regime through boiling of the brine, linking the porphyry and epithermal ore zones.