The solubility and speciation of molybdenum in water vapour at elevated temperatures and pressures: Implications for ore genesis

The hydration number, n, is interpreted to have a value of 2.0 ± 1.0 at 300 °C, 2.4 ± 0.6 at 320 °C, and 3.1 ± 0.3 at 360 °C. Values of log K for this reaction are 18 ± 5 at 300 °C, 16 ± 3 at 320 °C, and 12 ± 1 at 360 °C. Comparison with data from the literature shows that the solubility of MoO3·nH2O increases non-linearly with increasing fH2O, and that the hydration number is equal to the slope of the tangent to a function inferred from a plot of logfMoO3·nH2O versus logfH2O.The predominant species in water vapour at fH2O≈1 bar is MoO3·H2O, whereas at the conditions of the present experiments it is MoO3·2-3H2O. Calculations based on the solubility of MoO3 in equilibrium with molybdenite at 600 °C and 500 bars, using average H2O and total S fluxes of actively degassing volcanoes, with fO2 and fS2 controlled by the assemblage hematite-magnetite-pyrite, indicate that the vapour phase can transport sufficient Mo in about 115,000 years (within the life of geothermal systems) to form a deposit of 336 Mt, with an average grade of 0.087% Mo (e.g., the Endako Mo-porphyry deposit, Canada). This suggests that vapour-phase transport of Mo is far more important than previously thought and should be given further consideration in modelling the formation of porphyry molybdenum deposits.