Complexation of gold in S3 --rich hydrothermal fluids: Evidence from ab-initio molecular dynamics simulations

Recent Raman spectroscopic studies suggest that S₃ ^- is an important sulfur species in magmatic hydrothermal and metamorphic fluids at P>0.5GPa and T>250°C, and may be an important ligand for metal transport (Pokrovski and Dubrovinsky, 2011). Based on static Density Functional Theory calculations, Tossell (2012) confirmed the stability of the S₃ ^- ion, and suggested some possible Cu-S₃ complexes in the ideal gas phase and in aqueous solution. We investigated the complexation of Au and S₃ ^- in aqueous fluids by ab-initio molecular dynamics (MD) simulations. We performed ab-initio MD simulations in aqueous solution at 300°C, 0.5 and 2GPa to investigate the competition among the S₃ ^-, HS^-, H₂S(aq), OH^- and H₂O ligands for Au⁺, aiming at evaluating the significance of the 'exotic' S₃ ^- ligand for Au metallogenesis. The results indicate that, in contrast to results of static calculations that show a symmetric Au-S₃ complex with bidentate structure, Au⁺ forms linear complexes with S₃ ^-. The stoichiometry of these complexes depends on pH and fluid composition (e.g., Au(H₂O)S₃(aq); Au(HS)S₃ ^-; Au(OH)S₃ ^-; Au(S₃)₂ ^-). The S₃ ^- and bisulfide (HS^-) ions are ligands of similar strength for Au⁺; this confirms Pokrovski and Dubrovinsky's (2011) assumption that such 'exotic' ligands may play a major role in promoting Au mobility in magmatic and metamorphic environments.