GOLD AND COPPER PARTITIONING BETWEEN VAPOR AND BRINE AT MODERATE TO HIGH TEMPERATURES

Gold and copper-bearing porphyry ore deposits are associated spatially and temporally with porphyritic igneous (granite, s.l.) intrusions. These deposits require magmatic and hydrothermal components to form and are thus called magmatic-hydrothermal deposits. The metals are derived predominantly from the magma whereas the hydrothermal fluid transports the metals away from the magma and to sites where gold and copper-bearing minerals can precipitate directly from the fluid. The PTX properties of the fluid most capable of transporting gold and copper are not well constrained. In these systems, the hydrothermal fluid can exist as a supercritical fluid, a low-salinity vapor, and/or a high-salinity brine. Experiments were conducted in the gold – chalcopyrite ± pyrrhotite ± bornite – vapor – brine system at 500 to 700 °C to ascertain the concentration of gold and copper in vapors and brines as a function of O₂ and S₂ fugacities. Fluids were trapped in synthetic fluid inclusions in quartz and analyzed by using laser ablation inductively coupled plasma mass spectrometry. Gold and copper were found to be 3-7 and 5-9 times more concentrated in the high-salinity brine relative to the coexisting low-salinity vapor, respectively. Thus, the total salinity of the hydrothermal fluid seemed to have a first order control on metal concentration. A second order control was the S₂ and H₂S imposed on the system. Higher S₂ conditions, and especially reduced S as H₂S, increased the total amount of gold and copper able to be transported by the brine and vapor, but seemed to have a slightly more pronounced impact on metals in the vapor than the brine. These data will be used to determine the conditions under which copper and gold porphyry deposits are most likely to form.