Database of Experimental and Modeling Results on Mo Transport in Sulfur-Rich Hydrothermal Fluids: Implications for Porphyry-Epithermal Cu-Au-Mo Deposits

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.