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

• Published in: Geochimica et cosmochimica acta 2006-02, Vol.70 (3), p.687-696
• Main Authors: Rempel, K.U., Migdisov, A.A., Williams-Jones, A.E.
• Format: Article
• Language: English
• Subjects: Bars; Deposits; Hydration; Mathematical models; Molybdenum; Solubility; Vapor pressure; Vapour
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2005.09.013

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 MoO 3 in water vapour is between 1 and 29 ppm, which is 19–20 orders of magnitude higher than the vapour pressure of MoO 3(g). Molybdenum solubility increases exponentially with f H 2 O , suggesting the formation of a gaseous hydrated complex of the type MoO 3· nH 2O by the reaction: (A.1) MoO 3 ( g ) + n H 2 O ⇔ MoO 3 · n H 2 O ( 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 MoO 3· nH 2O increases non-linearly with increasing f H 2 O , and that the hydration number is equal to the slope of the tangent to a function inferred from a plot of log f MoO 3 · n H 2 O versus log f H 2 O .The predominant species in water vapour at f H 2 O ≈ 1 bar is MoO 3·H 2O, whereas at the conditions of the present experiments it is MoO 3·2–3H 2O. Calculations based on the solubility of MoO 3 in equilibrium with molybdenite at 600 °C and 500 bars, using average H 2O and total S fluxes of actively degassing volcanoes, with f O 2 and f S 2 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.