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

The solubility of cooperite PtS(cr) was measured in aqueous sulfide solutions at 25 degrees C/1 bar, 75 degrees C/1 bar, and 450 degrees C/1000 bar. The concentration of Pt increases with an increase of temperature from 10(-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 degrees((aq)) = Pt(HS)(2)degrees((aq)) K-S degrees(Pt(HS)(2)) PtS(cr) + 2 H2S degrees((aq)) = Pt(HS)(3)(-) + H+ K-S degrees (Pt(HS)(3)(-)) The charged complex Pt(HS)(3)(-) dominates in low-temperature near-neutral solutions but was not detected at 450 degrees C where the only hydrosulfide complex was Pt(HS)(2)degrees((aq)). The effect of NaCl on the PtS(sr) solubility is negligible which implies that mixed Pt-HS-Cl complexes can be neglected. The PtS(cr) solubility constant, K-s degrees (Pt (HS)(2)), was determined as log K-s degrees = -9.09 + 0.17 (25 degrees C/1 bar), -9.50 + 0.35 (75 degrees C/1 bar), and - 6.68 + 0.10 (450 degrees C/1000 bar). The PtS(cr) solubility constant, K-s degrees(Pt(HS)(3)(-)), was determined as log K-s degrees = -14.43 + 0.31 (25 degrees C/1 bar), and - 13.15 + 0.23 (75 degrees C/1 bar). The PtS(cr) solubility constants together with the literature data were fitted to a simple density model logK(s)degrees(Pt(HS)(2)degrees) = -7.30 - 638.9.T(K)(-1) -5.98.logd(w) logK(s)degrees(Pt(HS)(3)(-)) = 0.633 - 4522.6.T(K)(-1) + 43.03.logd(w) where d(w) is the pure water density. The K-s degrees(Pt(HS)(2)) increases with an increase of temperature, but K-s degrees (Pt (HS)(3)(-)) has a maximum at ca. 200 degrees 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 degrees 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.

Automated summary

The solubility of cooperite PtS(cr) in aqueous sulfide solutions was measured at various temperatures and pressures. The study found that the concentration of Pt increases with temperature, with different complexes dominating at low and high temperatures. The influence of NaCl on PtS(cr) solubility was negligible.