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

Kokh, MA; Pokrovski, GS; Klemme, S; Schmidt, C; Laskar, C; Hasemann, JL; Testemale, D; Kieffer, I; Proux, O; Lahera, E; Müller, H; Bazarkina, EF; Moeller, C, Ryaboshapka, D; Wilke, M

Abstract

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 chemical identity and thermodynamic stability of aqueous complexes of molybdenum and the role of different ligands on Mo metal transport yet remain inconsistent and incomplete, in particular 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 as well as sulfur speciation and 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 the tetrathiomolybdate complex, MoS42‒, in reduced, H2S/HS‒- dominated fluids of neutral-to-alkaline pH. In contrast, a mixed-ligand complex with three sulfide ions and one trisulfur radical ion, MoS3(S3)‒, prevails in more oxidized and more acidic fluids at the sulfide-sulfate transition where S3•‒ is far more 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) and MoS2(s) + H2S0(aq) + S3•‒ + 0.5 O2(g) = MoS3(S3)‒ + H2O(liq) , at 300 °C and 500 bar are 0.5±0.4 and 14.6±0.4, 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.