The role of hydrogen sulfide and trisulfur radical ion in molybdenum transport by hydrothermal fluids: implications for porphyry-epithermal Cu-Au-Mo deposits

Description

Knowledge of the chemical speciation of molybdenum in fluids under hydrothermal conditions is key to understanding the formation of porphyry Cu-Au-Mo deposits, which are the primary economic source of copper, molybdenum and rhenium. However, the Mo-speciation yet remains inconsistent and incomplete for sulfur-bearing fluids typical of such environments. We have experimentally studied the role of hydrogen sulfide (H2S and HS‒) and the trisulfur radical ion (S3•‒) in the transport of molybdenum by hydrothermal fluids at 300 °C and 500 bar as a function of pH, redox conditions and sulfur concentration. We combined solubility measurements of molybdenite in hydrothermal reactors using fluid quenching or sampling, with in situ synchrotron X-ray absorption spectroscopy experiments and thermodynamic and molecular modeling. Our solubility and spectroscopic dataset is consistent with the formation of tetrathiomolybdate complex, MoS42‒, in reduced, H2S/HS‒-dominated neutral-to-alkaline pH fluids. A mixed complex with both sulfide and trisulfur radical ion as ligands, MoS3(S3)‒, prevails in more oxidized, acidic-to-neutral pH fluids at the sulfide-sulfate coexistence where S3•‒ is abundant. In both complexes, Mo is nominally hexavalent and in a first-shell tetrahedral coordination with sulfur atoms. The derived equilibrium constants of the formal solubility reactions (log10K): MoS2(s) + 2 H2S0(aq) + 0.5 O2(g) = MoS42‒ + 2 H+ + H2O(liq) , MoS2(s) + H2S0(aq) + S3•‒ + 0.5 O2(g) = MoS3(S3)‒ + H2O(liq) , at 300 °C and 500 bar are 0.5±0.3 and 14.6±0.5, respectively. The solubility of MoS2(s) predicted using these constants aligns well with Mo concentrations measured in natural fluid inclusions in quartz that record S-rich fluids from porphyry-epithermal systems. In contrast, other types of Mo complexes invoked so far (molybdates, alkali ion pairs, oxy-chlorides or oxysulfides) are negligible at such conditions. Thus, trisulfur radical ion complexes may be important carriers of Mo in hydrothermal fluids and would require further systematic investigation across a wide range of temperature and pressure. Digital database contains: 1) Appendix_A including supplementary text, supplementary figures, supplementary tables, data for figure 1 of the main text (Tables S6-S9) and description of digital database; 2) digital data for figures 2-7 of the main text; 3) digital data for figures presented in Appendix_A

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Experimental and analytical methods: in situ X-ray Absorption Spectroscopy, autoclave experiments in hydrothermal reactor with flexible cell and ultrafast sampling, quench autoclaves, ICP-MS, ICP-AES, HPLC, AAS, titration. Modeling methods: FDMNES, thermodynamic modeling with a code of Gibbs free energy minimization

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