Molybdenum solubility and partitioning in H2O-CO2-NaCl fluids at 600 °C and 200 MPa

The Mo deposits from the Qinling-Dabie area, central China, which are characteristic for a new type of porphyry Mo deposits named the Collision-type, were formed in CO2-rich magmatic-hydrothermal environments. Yet the effects of CO2 on molybdenum transport and precipitation are still poorly known. To fill this gap and provide insight into the formation of Collision-type porphyry Mo deposit, we performed high pressure experiments to systematically quantify the role of CO2 on the solubility of molybdenum-bearing phases. Molybdenite was placed together with a single-phase H2O-CO2-NaCl fluid (8 wt% NaCl Eq.) at 600 degrees C and 200 MPa. The experiments were buffered by the pyrite-pyrrhotite-magnetite assemblage. At such conditions, combined microthermometric and LA-ICP-MS analysis of synthetic fluid inclusions reveals that the molybdenite solubility in the fluids coexisting with molybdenite decreases slightly (from 87 +/- 17 ppm to 38 +/- 13 ppm Mo) with increasing CO2 (from X-CO2 = 0.10 to 0.25 M fraction). Such a Mo solubility is comparable to that determined in CO2-free fluids (61 +/- 14 ppm) with the same salinity. At X-CO(2) = 0.33, fluid immiscibility is( )observed and Mo would partition preferentially into the brine phase, with a D-Mo(liq/vap )(=C-Mo(liquid)/C(Mo)(vapor )value of 2.9 +/- 1.0 (1 sigma). The evolution of molybdenite solubility with increasing CO2 in the fluid may be explained by changes of the dielectric constant of the solvent. Our results demonstrate that at the studied experimental temperature and pressure, CO2-rich fluids can transport comparable amounts of molybdenum as in H2O-dominated solutions. Combined with existing literature data over a broad range of pressure, temperature, and oxygen fugacity conditions, our data indicate that decreasing temperature and oxygen fugacity facilitate molybdenite precipitation. Albeit the nil-to-negative effect of CO2 on molybdenum solubility, the presence of CO2 would affect significantly fluid saturation of silicate melts. Notably, boiling is expected to occur at higher pressure in the presence of CO2, which may explain the deeper mineralization depth observed for the Collision-type porphyry molybdenum deposit.

Automated summary

This study systematically quantifies the effect of CO2 on the transport and precipitation of molybdenum through high pressure experiments, providing insights into the formation of Collision-type porphyry Mo deposits. The results show that at certain conditions, CO2 can slightly decrease the solubility of molybdenum in the fluid coexisting with molybdenite, and it significantly affects fluid saturation of silicate melts.