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

The solubility of molybdenum trioxide in liquid-undersaturated water vapour has been investigated experimentally at 300, 320, and 360 degrees C and 39-154 bars. Results of these experiments show that the solubility Of MoO3 in water vapour is between I and 29 ppm, which is 19-20 orders of magnitude higher than the vapour pressure Of MoO3(g). Molybdenum solubility increases exponentially with f(H2O), suggesting the formation of a gaseous hydrated complex of the type MoO3.nH(2)O by the reaction: MoO3(g) + nH(2)O double left right arrow MoO3 . nH(2)O(g) (A.1) The hydration number, n, is interpreted to have a value of 2.0 +/- 1.0 at 300 degrees C, 2.4 +/- 0.6 at 320 degrees C, and 3.1 +/- 0.3 at 360 degrees C. Values of log K for this reaction are 18 +/- 5 at 300 degrees C, 16 +/- 3 at 320 degrees C, and 12 +/- 1 at 360 degrees C. Comparison with data from the literature shows that the solubility Of MoO3.nH(2)O increases non-linearly with increasing f(H2O), and that the hydration number is equal to the slope of the tangent to a function inferred from a plot of log f(MoO3.nH2O) versus log f(H2O). The predominant species in water vapour at f(H2O) approximate to 1 bar is MoO3.H2O, whereas at the conditions of the present experiments it is MoO3.2-3H(2)O. Calculations based on the solubility Of MoO3 in equilibrium with molybdenite at 600 degrees C and 500 bars, using average H2O and total S fluxes of actively degassing volcanoes, with f(O2) and f(S2) 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. (c) 2005 Elsevier Inc. All rights reserved.