The solubility of cooperite PtS(cr) at 25 – 450 °C, Psat – 1000 bar and hydrosulfide complexing of platinum in hydrothermal fluids

Abstract

The solubility of cooperite PtS(cr) was measured in aqueous sulfide solutions at 25 °C/1 bar, 75 °C/1 bar, and 450 °C/1000 bar. The concentration of Pt increases with an increase of temperature from 10–10.2 to 10–7.4m (mol⋅(kg H2O)−1) in solutions which contain 0.06–0.07 m of total reduced sulfur. The experimental solubility data are accurately described by the reactionsPtScr+H2S˚aq=PtHS2˚aqKs˚PtHS2.PtScr+2H2S˚aq=PtHS3−+H+Ks˚PtHS3−..The charged complex Pt(HS)3− dominates in low-temperature near-neutral solutions but was not detected at 450 °C where the only hydrosulfide complex was Pt(HS)2°(aq). The effect of NaCl on the PtS(cr) solubility is negligible which implies that mixed Pt-HS-Cl complexes can be neglected. The PtS(cr) solubility constant, Ks°(Pt(HS)2), was determined as log Ks° = −9.09 ± 0.17 (25 °C/1 bar), −9.50 ± 0.35 (75 °C/1 bar), and − 6.68 ± 0.10 (450 °C/1000 bar). The PtS(cr) solubility constant, Ks°(Pt(HS)3−), was determined as log Ks° = −14.43 ± 0.31 (25 °C/1 bar), and − 13.15 ± 0.23 (75 °C/1 bar). The PtS(cr) solubility constants together with the literature data were fitted to a simple density modellogKSoPtHS2o=−7.30−638.9∙TK−1−5.98∙logdwlogKSoPtHS3−=0.633−4522.6∙TK−1+43.03∙logdwwhere d(w) is the pure water density. The Ks°(Pt(HS)2) increases with an increase of temperature, but Ks°(Pt(HS)3−) has a maximum at ca. 200 °C. Accordingly, at higher temperatures the role of the latter complex decreases and its contribution to the dissolved Pt concentration becomes negligible at t > 300 °C. Thermodynamic calculations show that in natural hydrothermal fluids, which contain up to 0.1 m of total reduced sulfur, the concentration of Pt-HS complexes can reach a few ppb. Higher solubility of Pt-bearing minerals can be attained in chloride-bearing fluids due to the formation of PtCl42−, which plays the dominant role in high-temperature hydrothermal transport of Pt.