The partitioning of Cu, Au and Mo between liquid and vapor at magmatic temperatures and its implications for the genesis of magmatic-hydrothermal ore deposits

The partition coefficients of Cu, Au and Mo between liquid and vapor were determined at P = 130 MPa and T = 900 °C, and P = 90 MPa and T = 650 °C and redox conditions favoring the dominance of reduced S species in the fluid. The experiments at 900 °C were conducted in rapid-quench Molybdenum-Hafnium Carbide externally-heated pressure vessel assemblies, whereas those at 650 °C were run in René41 pressure vessels. The fluids were sampled at run conditions using the synthetic fluid inclusion technique. The host quartz was fractured in situ during the experiments ensuring the entrapment of equilibrium fluids. A new method was developed to quantify the composition of the vapor inclusions from LA-ICPMS analyses relying on the use of boron as an internal standard, an element that fractionates between vapor and liquid to a very small degree. The bulk starting fluid compositions closely represented those expected to exsolve from felsic silicate melts in upper crustal magma reservoirs (0.64 m NaCl, 0.32 m KCl, ±0.2 m HCl and/or 4 wt% S). The experiments were conducted in Au97Cu3 alloy capsules allowing the simultaneous determination of apparent Au and Cu solubilities in the liquid and the vapor phase. Though the apparent metal solubilities were strongly affected by the addition of HCl and S in both phases, all three elements were found to preferentially partition to a liquid phase at all studied conditions with an increasing degree of preference for the liquid in the following order Au < Cu < Mo. The presence of HCl and S did not have a significant effect on the liquid/vapor partition coefficients of either Au or Cu, whereas the presence of HCl slightly shifted the partitioning of Mo in favor of the vapor. Ore metal partition coefficients normalized to that of Na (Ki-Naliq/vap=Diliq/vap/DNaliq/vap) fall in the following ranges respectively for each studied metal: KAu-Naliq/vap = 0.20 ± 0.07-0.50 ± 0.19 (1σ); KCu-Naliq/vap = 0.36 ± 0.12-0.76 ± 0.22; KMo-Naliq/vap = 0.67 ± 0.15-2.5 ± 0.8. Decreasing T from 900 °C to 650 °C slightly shifted KAu-Naliq/vap and KCu-Naliq/vap to the lower end of the reported ranges. A consequence of KAu-Naliq/vap and KCu-Naliq/vap being significantly smaller than 1 is that much of the Au and a significant fraction of Cu may be carried to shallower levels of magmatic-hydrothermal systems by residual vapors despite potentially extensive brine condensation.