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

Recent Raman spectroscopic studies suggest that S(3)(-)is an important sulfur species in magmatic hydrothermal and metamorphic fluids at P> 0.5 GPa and T> 250 degrees 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 S-3(-) ion, and suggested some possible Cu-S-3 complexes in the ideal gas phase and in aqueous solution. We investigated the complexation of Au and Si in aqueous fluids by ab-initio molecular dynamics (MD) simulations. We performed ab-initio MD simulations in aqueous solution at 300 degrees C, 0.5 and 2 GPa to investigate the competition among theS(3)(-), HS-, H2S(aq), OH- and H2O ligands for Au, aiming at evaluating the significance of the 'exotic' S-3(-) ligand for Au metallogenesis. The results indicate that, in contrast to results of static calculations that show a symmetric Au-S-3 complex with bidentate structure, Au+ forms linear complexes with S-3(-)-. The stoichiometry of these complexes depends on pH and fluid composition (e.g., Au(H2O)S-3(aq); Au(HS)S-3(-) Au(OH)S-3(-) Au(S-3)2(-). The S-3(-) 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. 2013 Elsevier B.V. All rights reserved.