VAPOR TRANSPORT OF ORE METALS IN HYDROTHERMAL SYSTEMS

Recent analyses of the trace-element composition of vapor-rich fluid inclusions (e.g., Heinrich et al., 1992, 1999) suggest that the vapor phase may play a far more important role in transporting the metals of ore-forming hydrothermal systems than is currently recognized. By contrast, theoretical models based on volatility calculations suggest that the solubility of most metals in aqueous vapours is negligible. However, these models ignore the potentially important but unknown contributions of solvation processes, i.e., interactions between the volatilized species and water vapor, to the transport properties of the latter. In view of this, we have conducted experiments designed to determine the solubility of a suite of metals in chloride-bearing water vapor (Cu, Ag, Au, Sn) or pure water vapor (Mo) at elevated temperatures. These experiments showed that the solubility of the metal in the vapor is orders of magnitude greater than that predicted theoretically from volatility calculations. As has been proposed previously for sodium chloride, we attribute this enhanced solubility to the ability of water to hydrate the metal in the vapor phase. In the case of Cu and Au, the experiments also showed that metal concentrations typical of ore-forming aqueous liquids are possible in water vapor. Significantly, the solubility of these metals was retrograde over the ranges of temperatures investigated (280-320 °C; 300-360 °C) and tin solubility reached a maximum at ~320 °C. Interestingly, the hydration number for the three corresponding metal species decreased with increasing temperature; that of Ag remained constant. We therefore speculate that, in general, hydration number will decrease with increasing temperature and, as a result, the solubility of most metals in water vapor will reach a maximum at temperatures below 400°C. It should be noted, however, that solubility increases with increasing vapor pressure and that the critical pressure of aqueous fluids increases sharply with increasing salinity. Consequently, we predict that in highly saline systems solubility will be appreciable even at temperatures > 600 °C. In conclusion, transport of metals by water vapor may be important in the formation of a variety of ore deposit types where vapor is a major or dominant phase in the hydrothermal system.