Gold transport in hydrothermal chloride-bearing fluids: insights from in situ X-ray absorption spectroscopy and Ab initio molecular dynamics

Chloride-bearing fluids are widespread in the Earth’s interior from low-temperature subsurface conditions to deep lithosphere. The concentration of chloride salts varies from diluted aqueous solutions to concentrated brines and anhydrous (dry) chloride melts beneath volcanoes. Here we report an investigation of the state of Au in hydrothermal chloride fluids and anhydrous melts by means of in situ X-ray absorption spectroscopy combined with ab initio molecular dynamics simulations and thermodynamic modeling. The experiments included registration of Au L3-edge X-ray absorption near edge structure/extended X-ray absorption fine structure spectra of Au-bearing fluids in the temperature range from 350 to 575 °C at pressures of 150–4500 bar. Spectra of Au dissolved in dry CsCl/NaCl/KCl + K2S2O8 melt were recorded at 650 °C. It was found that Au is coordinated by two Cl atoms (RAu–Cl = 2.25–2.28 A). The alkali metal atoms (Me) were detected in the distant coordination sphere of Au at RAu-Me = 3.3–4.1 A. The alk...