High solubility of gold in H2S-H2O +/- NaCl fluids at 100-200 MPa and 600-800 degrees C: A synthetic fluid inclusion study

Numerous previous experimental and theoretical studies of gold deposits in the Earth's crust have shown that chloride and hydrosulfide are the most abundant ligands in the relevant hydrothermal fluids. However, most of these studies were performed with solutions under relatively oxidizing conditions in the presence of SO42-, HSO4-, and S-3(-). Reduced sulfur with a -2 valence (e.g., H2S, HS-, etc.) may play a key role in controlling redox conditions and promoting the migration of gold during the formation of epithermal and porphyry gold-copper deposits. To examine this possibility, we performed experiments in reduced H2S-bearing fluids to constrain the effects of temperature, pressure and NaCl concentration on Au solubility through the synthetic fluid inclusion method. Measured amounts of H2S and aqueous solutions (H2O +/- NaCl) were loaded in an Au capsule containing a fractured quartz column, and fluid inclusions were formed through healing of these fractures at specified pressures (100 or 200 MPa) and temperatures (600, 700 or 800 degrees C) for 20 days. Thin sections with both sides polished were prepared from quenched quartz columns, and conventional microthermometric measurements were performed. Raman spectra were collected from all phases in the fluid inclusions from room temperature to 300 degrees C to identify sulfur species and potential Au-bearing complexes. In all fluid inclusions, S and H2Sn (n >= 1) species were identified, but SO2 and SO42-, HSO4 and S-3(-) ions were not observed. Au concentrations in these fluid inclusions, which were analyzed using LA-ICP-MS with NaCl or RbCl reference standards, ranged from 91 to 3599 ppm; they were greatly affected by pressure and NaCl concentration but not obviously by temperature. H2Sn (n > 1) analogs have chemical properties similar to those of H2S, and their deprotonation products (HSn- and S-n(2-)) exhibit high affinities for Au in aqueous solution. Therefore, HSn-Au (or AuSn-) and Au-Cl could be the dominant species in H2S-bearing hydrothermal fluids and may play important roles during the dissolution and transport stages of Au in ore formation processes. (c) 2022 Elsevier Ltd. All rights reserved.