Speciation of gold in hydrosulphide-rich ore-forming fluids: Insights from first-principles molecular dynamics simulations Xiandong Liu ⇑ , Xiancai Lu, Rucheng Wang, Huiqun Zhou, Shijin Xu State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, PR China Received 31 March 2010; accepted in revised form 12 October 2010; available online 20 October 2010 Abstract 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 pos- sible complexes are characterized in detail and the following insights have been gained. (1) The previously hypothesized spe- cies Au(HS)(H 2 S) 3 and Au(HS) are found unstable under ore-forming conditions. Au(HS)(H 2 S) 3 would dissociate to LAu(HS) (L = H 2 S or H 2 O) and free H 2 S 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 2 O), Au(HS)(H 2 S) and Au(HS) 2  and their relative stability is Au(HS) 2  > Au(HS)(H 2 S) > Au(HS)(H 2 O). In dense aqueous phases of high pressures, Au(HS)(H 2 S) would spontaneously deprotonate to Au(HS) 2  and thus Au(HS)(H 2 O) 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 super- critical 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. Ó 2010 Elsevier Ltd. All rights reserved. 1. INTRODUCTION In various geofluids, monovalent gold ions generally form complexes by binding inorganic/organic ligands. Understanding the species, structures and stability of these complexes is critical for realizing the dissolution, transport, distribution, and ore-formation of gold. Sulfur distributes ubiquitously in geological fluids and can exist in many forms, e.g., sulfides, sulfites and thiosulfates, all of which can form complexes with gold (Lengke and Southam, 2006; Williams-Jones et al., 2009). Thus, sulfur-bearing complexes have been recognized as the major carriers of gold in both the ore-forming fluids that formed most por- phyry and epithermal ores in the world and the hydrother- mal fluids from active volcanic areas (e.g., Akinfiev and Zotov, 2001; Berndt et al., 1994; Borisova et al., 2005, 2006; Migdisov and Bychkov, 1998; Stefa ´nsson and Sew- ard, 2004; Tagirov et al., 2006). The speciation and stability of S-bearing complexes have been under controversy because lots of difficulties are im- posed to the quantitative investigations by many factors, e.g., the high-temperature nature of ore-forming fluids, complex chemistry of sulfur, low solubility of gold, and fast reduction kinetics of gold ions. Until now, most available data come from the solubility measurements. Based on those experiments, it has been accepted that Au(HS) 2  is a dominant species in neutral and weakly-basic environ- ments (Seward, 1973; Pokrovski et al., 2009a,b and refer- ences therein). However, the exacted information from 0016-7037/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2010.10.008 ⇑ Corresponding author. Tel.: +86 25 83594664; fax: +86 25 83686016. E-mail address: xiandongliu@gmail.com (X. Liu). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 75 (2011) 185–194