Different metal coordination in sub- and super-critical fluids: Do molybdenum(IV) chloride complexes contribute to mass transfer in magmatic systems?

Abstract

Molybdenum (Mo) is extracted predominantly from magmatic-hydrothermal ore deposits, and knowledge of the thermodynamic properties of aqueous Mo complexes over a wide range in pressures (P) and temperatures (T) is required to predict the transport and deposition of Mo in ore-forming environments. These properties are usually derived from experimental studies conducted over a limited P-T range, with uncertainties increasing as temperature and pressure increase, because of inherent technical difficulties and decreasing number of studies. There is a discrepancy between numerous experimental studies at T < 400 °C and a handful of studies at magmatic-hydrothermal conditions. The low T (<400 °C) experiments suggested that Mo(VI) chloride complexes are unlikely to play a significant role in nature, whereas some studies in magmatic-hydrothermal conditions showed the predominance of Mo(VI) chloride complexes. In an attempt to resolve these inconsistencies, we investigated Mo(VI)-Cl complexations from 350 to 750 °C (500 to 2000 bar) using ab initio molecular dynamics (MD) simulations. The simulations reveal that in high-temperature magmatic-hydrothermal fluids (750 °C, 2000 bar), a tetrahedral Mo(VI) chloride complex, MoO2(OH)Cl(aq), becomes stable; this coordination differs from the octahedral Mo(VI)-chloride complexes identified in highly acidic sub-critical fluids. The ab initio thermodynamic integration method was employed to calculate the equilibrium constants of the reaction.MoO2(OH)Cl(aq) + H2O = MoO2(OH)2(aq) + HCl(aq) Thermodynamic modelling of the speciation using the calculated equilibrium constants shows that at lower temperatures (350 °C, 500 bar), Mo(VI)-chloro complexes are only important at highly acidic conditions (pH < 1), in accordance with previous studies. However, the MD results show that at higher temperatures, Mo-chloro-complexes may be important in transporting Mo in mildly acidic conditions up to the quartz-muscovite-K-feldspar buffered pH. Hence, Mo(VI) chloro-complexes could play an important role in the formation of magmatic-hydrothermal Mo deposits (e.g., Climax-type Mo deposits). We conclude that the mode of metal transport can change dramatically at high temperatures as a result of coordination changes caused by changes in the solvation properties of water; and that quantitative MD is an important tool to support the interpretation of scant experimental results available for magmatic-hydrothermal systems.

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