Solubility of molybdenite (MoS2) in aqueous fluids at 600–800 °C, 200 MPa: A synthetic fluid inclusion study

Abstract

Solubility experiments of molybdenite in single-phase, NaCl (±HCl)-bearing aqueous fluids were conducted at 600–800°C, 200MPa and various fO2–fS2 conditions imposed by mineral buffers. Small aliquots of fluids were trapped after 1–7days of equilibration as synthetic fluid inclusions in quartz and subsequently analyzed by laser-ablation ICP MS. Measured Mo concentrations range from 20 to 3000ppm by weight and increase with increasing temperature, NaCl concentration and oxygen fugacity, but decrease with increasing sulfur fugacity. Our solubility data can be fitted by the following equation: logCMo=0.458×logfO2-0.463×logfS2+0.731×logCNaCl-1.57×(1000/T)+6.37, where CMo and CNaCl are the concentrations of Mo and NaCl in ppm by weight, fO2 and fS2 are fugacities of oxygen and sulfur in bar, and T is temperature in Kelvin. This equation reproduces our data and those of a previous study conducted at 400°C within 0.24log units, with the exception of two points with excess HCl that are reproduced within ⩽0.87log units. The observed dependencies on fO2, fS2, NaCl concentration and pH are compatible with MoS2 dissolution as NaHMoO2S2.If combined with Mo concentrations measured in natural fluid inclusions, our data provide important constraints on the fO2 and fS2 conditions of ore formation. While Mo contents of fluids trapped at magmatic conditions agree well with predicted MoS2 solubilities along the magnetite–pyrrhotite fO2–fS2 buffer, Mo concentrations in hydrothermal brines trapped at 400–500°C require orders of magnitude higher fO2 or lower fS2 conditions relative to this buffer. The fact that fS2 in porphyry systems usually increases relative to the magnetite–pyrrhotite buffer during cooling from >700°C to 400°C suggests that the observed high Mo contents are due to high fO2, which is in agreement with the general trend of increasing fO2 during subsolidus cooling of granitic plutons.