The solubility of silver in magmatic fluids: Implications for silver transfer to the magmatic-hydrothermal ore-forming environment

Silver is a common and economically important constituent of many magmatic-hydrothermal ore deposits, yet little is known about its mobility during magma degassing. We performed experiments to determine the effect of various chemical components (NaCl, KCl, LiCl, HCl, CaCl2, H2S) on the solubility and speciation of Ag in high-temperature, low-density fluids (≈0.38-0.54 g/cm3). The experiments were conducted at T = 900 °C, P = 2000 bar and oxygen fugacity (fO2) of 0.5 log units below Ni-NiO buffer (NNO-0.5) in rapid-quench Molybdenum-Hafnium Carbide externally-heated pressure vessel assemblies. The fluid phase was sampled at run conditions by the entrapment of synthetic fluid inclusions (SFI) in in situ fractured quartz chips. As capsule material, Au97Ag2Cu1 alloy was used, which imposed an Ag activity of 0.005. The apparent Ag solubility, defined as the equilibrium concentration of Ag in the fluid phase at an Ag activity of 0.005, exponentially increases as a function of total chloride concentration in the NaCl-H2O and NaCl-HCl-H2O systems. In the mixed NaCl-HCl system, at a fixed total Cl concentration of 1 mol/kg (1 m) H2O and varying NaCl/HCl ratios, the apparent solubility of Ag reaches a maximum (432 ± 123 μg/g) at equal NaCl and HCl concentration (0.5 m each) and then decreases towards both end-members (1 m for NaCl or HCl) following a parabolic function. Apparent silver solubilities predicted using the HKF model and thermodynamic properties for traditional Ag-chloride and bisulfide complexes greatly underestimate the measured values. Model calculations suggest that instead of charged AgCl2−, a neutral NaAgCl2 species is the dominant dissolved Ag species in S-free, chloride-bearing fluids with the additional presence of NaAgHSCl in S-bearing fluids. Comparison of the solubility data in high-temperature fluids to measured Ag solubilities in silicate melts suggests that efficient Ag partitioning into magmatic fluids will only take place when the composition of the silicate melt is felsic enough (rhyodacitic to rhyolitic), similarly to the behavior of Cu but in contrast to that of Au.