Molybdenum speciation in magmatic-hydrothermal fluids : Constraints from molybdenite solubility experiments and thermodynamic modeling

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Fang, Jing ; Audétat, Andreas ; Dolejš, David:
Molybdenum speciation in magmatic-hydrothermal fluids : Constraints from molybdenite solubility experiments and thermodynamic modeling.
In: Geochimica et Cosmochimica Acta. Bd. 395 (2025) . - S. 95-111.
ISSN 0016-7037
DOI: https://doi.org/10.1016/j.gca.2025.01.032

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Abstract

Magmatic-hydrothermal ore deposits are the world’s major source of molybdenum (Mo), yet the speciation of Mo in magmatic-hydrothermal fluids is still uncertain. We experimentally measured the solubility of molybdenite (MoS2) in H2O–S–NaCl ± HCl fluids at 800 °C, 200 MPa, with oxygen fugacity (fO2) controlled at ∼ Ni–NiO + 1 (∼NNO + 1), Re–ReO2 (RRO), MnO–Mn3O4 (MMO) and Fe3O4–Fe2O3 (HM) buffers. A relatively simple fluid system was chosen to facilitate thermodynamic interpretation of the experimental results. The fluids were first allowed to equilibrate with MoS2 for 48 − 72 h before they were sampled in-situ by means of synthetic fluid inclusions in quartz and finally analyzed by laser ablation ICP–MS. In the HM-, MMO- and sulfur-free RRO-buffered experiments, Mo oxides were identified in the run products. Molybdenum concentrations in the fluids of these experiments are very high, reaching up to 47,000 μg g−1, and they are independent of fO2 and the sulfur content of the fluid. In contrast, Mo concentrations in the fluids of the Mo oxide-free experiments (i.e., all ∼ NNO + 1-buffered experiments and the sulfur-bearing RRO-buffered experiments) vary from 33 to 11,000 μg g−1 and increase with decreasing sulfur content of the fluid (5.0 to 0.5 wt sulfur), increasing fO2 (∼NNO + 1 to RRO buffer), decreasing HCl content (1.0 to 0.02 molality of HCl), and increasing NaCl content (0.1 to 5 molality of NaCl), except for the most saline and two HCl-enriched fluids investigated. Thermodynamic calculations and interpretations were performed for all Mo oxide-free experiments to check whether the observed MoS2 solubility trends fit with any of the Mo species that have been previously proposed to be relevant in magmatic-hydrothermal fluids. The results suggest that NaHMoO40, NaHMoO3S0, H2MoO2S20 and/or NaHMoO2S20 species may dominate in low-salinity (≤1 molality of NaCl), HCl-poor (≤0.13 molality of HCl) fluids, NaMoO3Cl0 in more saline and/or HCl-rich fluids, and NaHMoO2S20 in extremely sulfur-rich (≥20 wt sulfur) fluids. For natural fluids exsolving from metaluminous to peraluminous magmas NaMoO3Cl0 is the most relevant Mo-bearing species. An empirical equation that predicts the Mo content of molybdenite-saturated fluids as function of temperature (T), pressure (P), fluid salinity, fO2, sulfur fugacity (fS2) and pH is provided. Mo concentrations measured in natural fluid inclusions agree well with predicted MoS2 solubilities. In natural Climax-type porphyry Mo systems, fluid cooling and decompression are probably the most important driving forces for MoS2 precipitation, as fluid–rock interaction (leading to lower HCl in the fluid) and mixing with meteoric water (leading to lower NaCl, HCl and sulfur content and to higher fO2) enhance the MoS2 solubility rather than decrease it.