The transport of bismuth in HCl-bearing aqueous vapour and low-density aqueous supercritical fluids: Implications for natural systems

Abstract

Hydration by H2O clusters has been shown to be an important means of transporting metal complexes in hydrothermal vapours and low-density supercritical aqueous fluids. However, the effect of hydration on the transport of relatively volatile metal complexes such as those involving bismuth and chloride has not been evaluated. This effect is important to evaluate because bismuth is a key pathfinder element for gold in many intrusion-related gold ore-forming systems, where low-density supercritical aqueous fluids and vapour may play an important role in metal transport. Here, we present the results of experiments investigating the solubility of BiOCl(s) and the speciation of bismuth in HCl-bearing aqueous vapour and low-density supercritical aqueous fluids at temperatures between 250 and 400 °C and pressures of 5 to 296 bar. Two gaseous bismuth species formed in the HCl-bearing fluids (X(HCl) > 0.0005), namely BiCl3(g) and BiCl3(H2O)n(g). Our data clearly show that BiCl3(g) is highly volatile at low water fugacity (between 30 and 110 bar) and acts as an unhydrated gas molecule. At higher water fugacity, the gaseous BiCl3(g) reacts with water to form hydrated BiCl3(H2O)n(g) species. Significantly, the stability of hydrated bismuth chloride species, BiCl3(H2O)n(g), with low hydration numbers is lower than that of the unhydrated species, BiCl3(g), resulting in a decrease in bismuth solubility with increasing f(H2O). However, at a water fugacity great than 30–110 bar, that increases with increasing temperature, the hydration number and the solubility of hydrated species BiCl3(H2O)n(g) increase with increasing water fugacity or the density of the fluid. Consequently, unhydrated gaseous bismuth species increase in importance with increasing temperature and become dominant at temperatures > 450 °C in fluids that have low density (<0.10 g/cm3). Hydrated bismuth chloride species are dominant in fluids having higher density (0.10–0.34 g/cm3). The overarching conclusion of this study is that aqueous low-density supercritical fluids, which exsolve from magmas to form intrusion-related gold deposits, can transport bismuth in concentrations (up to hundreds ppm) sufficient to form economic deposits.