Gold transport in hydrothermal chloride-bearing fluids: Insights from in situ x-ray absorption spectroscopy and ab initio molecular dynamics

© 2018 American Chemical Society. 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 L 3 -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 + K 2 S 2 O 8 melt were recorded at 650 °C. It was found that Au is coordinated by two Cl atoms (R Au-Cl = 2.25-2.28 Å). The alkali metal atoms (Me) were detected in the distant coordination sphere of Au at R Au-Me = 3.3-4.1 Å. The alkali metal cations in the vicinity of Au-Cl complex partly compensate the positive charge located on Au and, by this way, affect the Au-Cl distance. An increase of the fluid pressure causes expansion of the second coordination sphere composed of the alkali metal cations, which leads to the increase of the positive Au charge and results in slight contraction of the first coordination sphere of Au. Accordingly, the transport of Au in high-temperature chloride-bearing natural ore-forming fluids of moderate to high densities (>0.3 g·cm -3 ) can be explicitly described by the formation of the AuCl 2- at any salt concentration from low-salinity fluids to hydrosaline liquids and anhydrous melts. In general, this means that the hydrothermal fluid chemistry simplifies with increasing temperature