An experimental study of the solubility of MoO3 in aqueous vapour and low to intermediate density supercritical fluids

• Published in: Geochimica et cosmochimica acta 2014-07, Vol.136, p.169-193
• Main Authors: Hurtig, Nicole C., Williams-Jones, Anthony E.
• Format: Article
• Language: English
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2014.03.043

The solubility of MoO3 in HCl-bearing water vapour and vapour-like aqueous fluids having a density between 0.005 and 0.343gcm−3 was determined experimentally for temperatures between 300 and 500°C. Molybdenum concentration in the quenched experimental condensates ranged from 3 to 481ppm. The corresponding molybdenum fugacity increased exponentially with increasing water fugacity, indicating that the dissolution of MoO3 proceeded via the reaction: MoO3,s+yH2Og=MoO3(H2O)yKs1,y and that the hydration number (y) increases with increasing water fugacity. A set of equilibrium constants (Ks1,y) was calculated from the experimental results and fitted to a linear function involving the reciprocal of temperature for values of y between two and eight, and to a polynomial function for a hydration number of one. Modelling of an H–Cl–S–O-bearing intermediate density supercritical fluid shows that molybdenite solubility is increasingly sensitive with decreasing temperature to changes in oxygen fugacity due to fluid–rock interaction. In a H2S–SO2 gas-buffered, intermediate density fluid containing 1wt% S, the molybdenum concentration decreases with decreasing temperature and pressure from 89 to 0.07ppm. Values of MoO3 solubility calculated using the thermodynamic data reported in this study for temperatures and pressures above the range considered in our experiments agree well with those determined experimentally by other researchers. Moreover, the predicted Mo concentrations are similar to those measured in vapour and intermediate density fluid inclusions from Cu–Mo and Mo porphyry deposits.