The effect of the trisulfur radical ion on molybdenum transport by hydrothermal fluids
Résumé
Knowledge of molybdenum (Mo) speciation under hydrothermal conditions is a key for understanding the formation of porphyry deposits which are the primary source of Mo. Existing experimental and theoretical studies have revealed a complex speciation, solubility and partitioning behavior of Mo in fluid-vapor-melt systems, depending on conditions, with the (hydrogen)molybdate (HMoO4-, MoO42-) ions and their ion pairs with alkalis in S and Cl-poor fluids [1-3], mixed oxy-chloride species in strongly acidic saline fluids [4, 5], and (hydrogen)sulfide complexes (especially, MoS42-) in reduced H2S-bearing fluids and vapors [6-8]. However, these available data yet remain discrepant and are unable to account for the observed massive transport of Mo in porphyry-related fluids revealed by fluid inclusion analyses demonstrating 100s ppm of Mo (e.g., [9]). A potential missing ligand for Mo may be the recently discovered trisulfur radical ion (S3•-), which is predicted to be abundant in sulfate-sulfide rich acidic-to-neutral porphyry-like fluids [10]. We performed exploratory experiments of MoS2 solubility in model sulfate-sulfide-S3•--bearing aqueous solutions at 300°C and 450 bar. We demonstrate that Mo can be efficiently transported by S3•--bearing fluids at concentrations ranging from several 10s ppm to 100s ppm, depending on the fluid pH and redox, whereas the available data on OH-Cl-S complexes cited above predict negligibly small (<100 ppb) Mo concentrations at our conditions. Work is in progress to extend the experiments to wider T-P-composition range of porphyry fluids and to quantitatively assess the role of S3•- in Mo transport by geological fluids.1. Kudrin A.V. (1989) Geochem. Int. 26, 87-99. 2. Minubayeva Z. and Seward T.M. (2010) Geochim. Cosmochim. Acta 74, 4365-4374. 3. Shang L.B. et al. (2020) Econ. Geol. 115, 661-669. 4. Ulrich T. and Mavrogenes J. (2008) Geochim. Cosmochim. Acta 72, 2316-2330. 5. Borg S. et al. (2012) Geochim. Cosmochim. Acta 92, 292-307. 6. Zhang L. et al. (2012) Geochim. Cosmochim. Acta 77, 175-185. 7. Kokh M.A. et al. (2016) Geochim. Cosmochim. Acta 187, 311-333. 8. Liu W. et al. (2020) Geochim. Cosmochim. Acta 290, 162-179. 9. Kouzmanov K. and Pokrovski G.S. (2012) Soc. Econ. Geol. Spec. Pub. 16, 573-618. 10. Pokrovski G.S. and Dubessy J. (2015) Earth Planet. Sci. Lett. 411, 298-309.