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

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The solubility of molybdenum trioxide in liquid undersaturated water vapour has been investigated experimentally at 300, 320, and 360 °C and 39 154 bars. Results of these experiments show that the solubility of MoO3 in water vapour is between 1 and 29 ppm, which is 19 20 orders of magnitude higher than the vapour pressure of MoO3(g). Molybdenum solubility increases exponentially with f, suggesting the formation of a gaseous hydrated complex of the type MoO3·nH2O by the reaction: MoO3(g)+nH2O⇔MoO3·nH2O(g) 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 f, and that the hydration number is equal to the slope of the tangent to a function inferred from a plot of logf versus logf.The predominant species in water vapour at f≈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 f and f 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.

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