Speciation of gold in hydrosulphide-rich ore-forming fluids: Insights from first-principles molecular dynamics simulations

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To shed light on gold speciation in sulfur-containing ore-forming fluids, we perform first principles molecular dynamics (FPMD) simulations to investigate gold–hydrosulphide complexing under representative geological conditions. With this advanced technique, the electronic structures of solutes and solvents are calculated with density functional theory and the thermal motions are sampled with molecular dynamics. The molecular structures, solvated structures and stabilities of possible complexes are characterized in detail and the following insights have been gained. (1) The previously hypothesized species Au(HS)(H 2S) 3 and Au(HS) are found unstable under ore-forming conditions. Au(HS)(H 2S) 3 would dissociate to LAu(HS) (L = H 2S or H 2O) and free H 2S molecules spontaneously. Au(HS) is highly reactive and tends to capture a second ligand to form a double-coordinated complex. (2) In the thin vapor-like phases of low pressures, the stable complexes include Au(HS)(H 2O), Au(HS)(H 2S) and Au(HS) 2 − and their relative stability is Au(HS) 2 − > Au(HS)(H 2S) > Au(HS)(H 2O). In dense aqueous phases of high pressures, Au(HS)(H 2S) would spontaneously deprotonate to Au(HS) 2 − and thus Au(HS)(H 2O) and Au(HS) 2 − are the stable forms. All of these complexes can retain to the upper-limit of ore-forming temperatures. (3) The gold ions in the complexes do not favor coordinating more molecules and therefore the solvations happen mainly through H-bonding interactions between the ligands and environmental waters. H-bonds are found in vapor, liquid, and dense supercritical phases, whereas in the thin supercritical phase the hydration is very weak. These results provide quantitative and microscopic basis for understanding the speciation of gold in hydrothermal fluids.