Gold speciation in hydrothermal fluids revealed by in situ high energy resolution X-ray absorption spectroscopy

Abstract Gold mobilization, transfer, and concentration in the Earth’s crust are controlled by hydrothermal sulfur- chloride-bearing fluids. Yet exact chemical identity, structure, stability of Au-bearing species and, particular, respective contributions sulfide (HS−) trisulfur ion (S3⋅−) ligands to Au transport lack direct situ evidence. Here we employed high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) on aqueous sulfate/sulfide/S3⋅−-bearing solutions at typical temperatures pressures (T = 350 °C, P 600 bar) reveal differences dissolved spectral signatures indicative contrasting fluid-phase speciation as a function acidity redox conditions. Combined with solubility measurements quantum-chemical thermodynamic modeling, our spectroscopic data provide evidence for Au(HS)S3− Au(HS)2− complexes predominant acidic-to-neutral alkaline conditions, respectively. Our findings thus directly confirm recent scheme S-bearing fluids established using less methods, highlight an important role gold mobilization hydrothermal-magmatic deposits associated subduction zones. More generally, results show that HERFD-XAS enables identification structural coordination features metal virtually unresolvable classical XAS techniques. By avoiding limitations techniques, integrated high-resolution approach opens perspectives studies other metals T-P fluids, potentially silicate melts, inaccessible observation nature.