Metal transfer in CO2-rich geological fluids

CO₂ is one of the most ubiquitous volatiles in crustal fluids and vapors. The role of CO₂ in the transport of metals has been ignored owing to the fundamental assumption that most base and chalcophile metals cannot directly bind with the non-polar CO₂ and thus metals are not soluble enough in the CO₂-rich phases. On the other hand, the supercritical CO₂ (>31 °C) is extensively used as solvent in industry for purification of organics and metal extraction from aqueous solution. Experimental study also verifies the capability of CO₂ in dissolving and mobilizing metals at low temperature and pressure (60 °C, and up to 160 bar). It remains unknown, however, whether a similar solvation phenomenon may operate in natural fluid systems for metal forming sulfide complexes. Recent advances in fluid inclusion studies of hydrothermal Mo(±Cu±Au±W±Fe±REE) deposits from the Qinling-Dabie Mo belt, Central China, reveal enriched CO₂ in the ore-forming fluids, with exceptional concentration attaining 50 to >95 mol% (Fig. 1). This finding evokes the possibility of enhanced metal (e.g. Mo, Au, Cu, W, Fe, Pb and REE) transfer in CO₂-rich fluids or vapors. To present rigorous interpretation of the geological phenomenon and hence better understanding the role of CO₂ on metal transfer and precipitation, a combined study based on LA-ICP-MS analysis of fluid inclusions and solubility experiments would be employed. The present study seeks to: (a) quantify the metal concentrations in natural CO₂-rich inclusions from typical hydrothermal Mo(±Cu±Au±W±Fe±REE) deposits; (b) determine experimentally the metal solubility in CO₂-rich fluid systems typical of hydrothermal ore deposit; (c) re-evaluate the role of CO₂ in the transport of metals.