Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

The oxidation state of hydrothermal fluids, which form economic deposits of noble metals, varies in wide limits – from oxidized ones typical for porphyry mineralization to reduced which formed volcanogenic massive sulfide deposits. Sulfur-bearing species, along with chloride, are the most important ligands that form stable aqueous complexes with Au and determine Au concentration in natural ore-generating fluids. Depending on the f(O2) value, in high-temperature fluids (t > 300 °C) the dominant forms of sulfur are sulfides (H2S, HS-), sulfites (SO2, HSO3-, SO32-), sulfates (HSO4-, SO42-), and the radical species (S3-). Here we report an investigation of Au complexation in high-temperature sulfide-bearing fluids of contrasting oxidation states. The solubility of Au was measured in “oxidized” sulfide fluid (H2S/SO42- buffer controls the Red/Ox state) at 450 °C, 1000 bar, and compared with the Au solubility in “reduced” sulfide systems (H2S/HS- predominate) reported in the literature. The measured values of the Au solubility matches best the model of the formation of Au(HS)2- at near-neutral to weakly acidic pH, and AuHS° in acidic solutions. The solubility constants have been determined for the reactions,
Au(cr) + H2S°(aq) + HS- = Au(HS)2- + 0.5 H2(g) log KAu(HS)2- = -0.9 ± 0.1 ,
Au(cr) + H2S°(aq) = AuHS°(aq) + 0.5 H2(g) log KAuHS = -6.5 ± 0.1 .
The average value of log KAu(HS)2- = -1.3 ± 0.5 was calculated for 450 °C (P = 500 – 1500 bar) using all the available Au solubility constants obtained in both “reduced” and “oxidized” sulfide systems. The local atomic environment of Au in high-temperature hydrothermal fluids has been studied using X-ray absorption fine structure spectroscopy (XAFS) in high energy resolution fluorescence detection (HERFD) mode in combination with ab initio molecular dynamics (AIMD) and Reverse Monte Carlo (RMC) simulations. Interpretation of Au L3-edge EXAFS spectra showed that, independently of the Red/Ox (sulfide or sulfide/sulfate systems) and PT – conditions (350 – 450 °C, 500 bar) two S atoms are located in the first coordination shell of Au at 2.29±0.02Å. Comparison of the experimental spectra with those simulated by means of AIMD revealed that EXAFS spectroscopy is not sensitive to the presence of light atoms like S in a distant coordination shell of Au. However, theoretical calculations indicated that the shape of Au L3-edge HERFD-XANES spectra depends upon the composition of the distant coordination shell and, therefore, can be used to discriminate between Au(HS)2-, Au-(HS)-S3- and, probably, other complexes with distant-coordination-shell anions. Experimental Au L3-edge HERFD-XANES spectra are identical for all studied PT- and Red/Ox-parameters. These results allowed us to conclude that Au(HS)2- complex predominates Au speciation in weakly acidic to weakly alkaline pH independently from the oxidation state of the fluid. Besides that, XAFS experiment demonstrated that the formation of mixed Au-HS-Cl complex can be neglected. With increasing pressure (to nkbar) or decreasing temperature (to < 300 °C), due to increasing of concentration of S species in intermediate oxidation states, formation of the Au-HS complexes can be accompanied by the formation of other species with (hydro)sulfite, thiosulfate, (hydro)polysulfide, and sulfur radicals, which would enhance the hydrothermal Au mobility. Stability of these complexes needs further experimental and theoretical examination.