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 > 0.5 GPa 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 2 GPa 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 Dubrovinskyʹs (2011) assumption that such ‘exotic’ ligands may play a major role in promoting Au mobility in magmatic and metamorphic environments.