Combined effect of carbon dioxide and sulfur on vapor–liquid partitioning of metals in hydrothermal systems

Abstract

Although CO2 is a ubiquitous volatile in geological fluids typically ranging from a few to more than 50wt%, its effect on metal vapor–liquid fractionation during fluid boiling and immiscibility phenomena in the Earth’s crust remains virtually unknown. Here we conducted first experiments to quantify the influence of CO2 on the partition of different metals in model water+salt+sulfur+CO2 systems at 350°C and CO2 pressures up to 100bar, which are typical conditions of formation of many hydrothermal ore deposits. In addition, we performed in situ Raman spectroscopy measurements on these two-phase systems, to determine sulfur and carbon speciation in the liquid and vapor phases. Results show that, in S-free systems and across a CO2 concentration range of 0–50wt% in the vapor phase, the absolute vapor–liquid partitioning coefficients of metals (Kvap/liq=Cvap/Cliq, where C is the mass concentration of the metal in the corresponding vapor and liquid phase) are in the range 10−6–10−5 for Mo; 10−4–10−3 for Na, K, Cu, Fe, Zn, Au; 10−3–10−2 for Si; and 10−4–10−1 for Pt. With increasing CO2 from 0 to 50wt%, Kvap/liq values decrease for Fe, Cu and Si by less than one order of magnitude, remain constant within errors (±0.2 log unit) for Na, K and Zn, and increase by 0.5 and 2 orders of magnitude, respectively for Au and Pt. The negative effect of CO2 on the partitioning of some metals is due to weakening of hydration of chloride complexes of some metals (Cu, Fe) in the vapor phase and/or salting-in effects in the liquid phase (Si), whereas both phenomena are negligible for complexes of other metals (Na, K, Zn, Mo). The only exception is Pt (and in a lesser extent Au), which partitions significantly more to the vapor of S-free systems in the presence of CO2, likely due to formation of volatile carbonyl (CO) complexes. In the S-bearing system, with H2S content of 0.1–1.0wt% in the vapor, Kvap/liq values of Cu, Fe, Mo, and Au are in the range 0.01–0.1, those of Pt 0.5–2.0, those of alkali metals are similar to the S-free system, and the partitioning of none of the studied metals is influenced by the presence of CO2 (up to 50wt% in the vapor). Our data thus confirm the large enhancement of volatility in the presence of reduced sulfur (H2S) due to formation of sulfide complexes for chalcophile metals such as Au, Pt, Mo and, to a lesser extent, Cu and Fe, as reported in previous studies of CO2-free water-salt systems. The negligible effect of CO2 on vapor–liquid partitioning of the studied metals in S-bearing systems is due to the lack of hydration of metal sulfide species making them little sensitive to changes in water activity and solvation power of CO2–H2O vapor. Our findings, combined with existing data over a wide range of temperature on vapor–liquid partitioning of metals in H2O-dominated systems, suggest that CO2 exerts mostly an indirect impact on metal fractionation, by extending vapor–liquid immiscibility to higher temperatures and pressures or depth compared to a CO2-free H2O-S-salt system. The deeper vapor–liquid separation, in particular in S-bearing systems, is expected to cause more significant partitioning of precious metals and molybdenum (Au, Pt, Mo) into the vapor phase while base metals (Fe, Zn, Cu) remain concentrated in the salt-rich (NaCl, KCl) liquid phase. In addition, irrespective of the presence of sulfur, an expansion of the immiscibility domain to higher temperature and pressure conditions in the presence of CO2 will also increase the depth of ore deposition and affect the vertical metal zonation in hydrothermal systems.