The speciation and transport of palladium in hydrothermal fluids: Experimental modeling and thermodynamic constraints

The solubility of PdO(cr) was measured in NaOH (to 0.1m, mol/kg H2O) solutions at 400 °C, 1 kbar, and the solubility of Pd(cr) was determined at 400–500 °C, 1 kbar in acidic chloride solutions (to 1.5m   NaCl) buffered with respect to hydrogen. The Pd electrode potential Eo(PdCl42-)/Pd(cr) for the reaction PdCl42- + 2e− = Pd(cr) + 4 Cl− was determined at 50 and 70 °C in 1m   chloride solutions. These data, together with reliable literature values, were used for calculation of the standard thermodynamic properties and the formation constants for Pd–OH, Pd–Cl, and Pd–S–HS complexes within the framework of the revised Helgeson–Kirkham–Flowers model. It was found that PdCl3- and PdCl42- become the most important Pd complexes in high temperature (t > 300 °C), chloride-rich fluids, and PdCl42- predominates at m(Cl) > 0.1. The stability of Pd–Cl complexes increases sharply with increase in temperature. The near-neutral chloride–sulfide solutions (1m NaCl, <0.1m Stot) can transport Pd at ppm concentration levels at t ⩾ 600 °C, whereas decrease in temperature and increase in pH can lead to effective deposition of Pd minerals. The stability of Pd–S–HS complexes (Pd(HS)2°Pd(HS)2°, Pd(HS)3- and PdS(HS)2−) decreases with increase in temperature. Therefore, the role of these complexes in hydrothermal transport of palladium is restricted to the low temperature solutions (t < 100 °C) and sulfur can be considered an efficient depositing agent for Pd. The calculated HKF Equation of State parameters were used to predict thermodynamic properties of Pd2+, Pd–OH, Pd–Cl, and Pd–S–HS complexes to 700 °C, 2 kbar. These parameters are incorporated into the FreeGs web-enabled database (http://www-b.ga.gov.au/minerals/research/methodology/geofluids/thermo/calculator/search.jsp) that can be used for geochemical application of thermodynamic data obtained in the present study.