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 CO 2 -rich magmatic-hydrothermal environments. Yet the effects of CO 2 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 CO 2 on the solubility of molybdenum-bearing phases. Molybdenite was placed together with a single-phase H 2 O-CO 2 -NaCl fluid (8 wt% NaCl Eq.) at 600 °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 CO 2 (from X CO2 = 0.10 to 0.25 M fraction). Such a Mo solubility is comparable to that determined in CO 2 -free fluids (61 ± 14 ppm) with the same salinity. At X CO2 = 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 (1σ). The evolution of molybdenite solubility with increasing CO 2 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, CO 2 -rich fluids can transport comparable amounts of molybdenum as in H 2 O-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 CO 2 on molybdenum solubility, the presence of CO 2 would affect significantly fluid saturation of silicate melts. Notably, boiling is expected to occur at higher pressure in the presence of CO 2 , which may explain the deeper mineralization depth observed for the Collision-type porphyry molybdenum deposit. [ABSTRACT FROM AUTHOR]