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 increased with an increase of temperature from 10 to 10.2 to 10–7.4 m (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 reactions. PtS(cr) + H2S°(aq) = Pt(HS)2°(aq) Ks°(Pt(HS)2). PtS(cr) + 2 H2S°(aq) = Pt(HS)3− + H+ Ks°(Pt(HS)3−) . 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 what 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 model log K S o Pt HS 2 o = − 7.30 − 638.9 ∙ T K − 1 − 5.98 ∙ log d w log K S o Pt HS 3 − = 0.633 − 4522.6 ∙ T K − 1 + 43.03 ∙ log d w where 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 PtCl4−, which plays the dominant role in high-temperature hydrothermal transport of Pt.