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Solubility and Transport of Platinum-Group Elements in Hydrothermal Solutions: Thermodynamic and Physical Chemical Constraints

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Geo-Platinum 87

Abstract

Thermodynamic calculations and physical chemical considerations suggest that at least some of the platinum-group elements (PGE) may be mobile in geological fluids such as chloride, hydroxide, bisulphide, polysulphide, thiosulphate or ammonia complexes, depending on pH, fO2, temperature and lignad concentration.

In saline fluids (i. e. ΣCl- > 1·0 m) at 25°C, Pt and Pd are soluble as \(PtCl^{2-}_4\) and \(PdCl^{2-}_4\), at a level of 10 ppb or greater, only in very acidic and oxidizing fluids. The complexes \(OsCl^{3-}_6\), \(IrCl^{3-}_6\), \(RuCl^{2-}_5\) and \(RhCl^{2-}_5\) and their hydration and hydrolysis products may also be responsible for solubilities of the order of l0 ppb under more restricted conditions. The thermodynamically predicted order of solubility as chlorides at 25°C is Pd > Pt > Os > Ir, Au. It was possible to extrapolate the data for Pt and Pd chloride complexes to 300°C where significant Pt and Pd solubility are attained only in highly acidic and oxidizing solutions.

Under mildly oxidizing, near neutral to basic conditions, hydroxy species such as \(Pt(\!OH\!)^{2-}_4\) or \(Pd(\!OH\!)^{2-}_4\) appear to give solubilities as high as 10 ppb. For the other PGE, oxyanionic species such as \(OsO^{2-}_4\) and \(RuO^{2-}_4\) can become important in very basic and oxidizing solutions. At low temperatures, thiosulphate and polysulphide species may be important. For example, at 25°C and ΣS = 0·1 m, 10 ppb of Pt and Pd can be attained as \(Pt(S_2O_3)^{6-}_4\) and \(Pd(S_2O_3)^{6-}_4\) at near neutral to basic pH near the \( HS^-\!/S_{2}O^{2-}_3\) boundary. Although no stability constant data are available, it is expected that the soft (in the Pearson sense) PGE metal ions would form strong complexes with HS-. It was possible to estimate the stability constants of \(Pt(\!HS\!)^{2-}_4\) and \(Pd(\!HS\!)^{2-}_4\) at 300° C. These result in calculated solubilities of 1–10 ppt (parts per trillion) in the region near the pyrite—pyrrhotite—magnetite triple point. Higher solubilities may be expected if additional bisulphide species or mixed bisulphidehydroxide species exist. Preliminary experimental data suggest that this is the case.

Ammonia may also be effective in mobilizing the PGE, however, only under near neutral to basic, oxidizing conditions. The hydroxide complexes may dominate over the ammonia complexes in this region. The effect of As, Se and Te appears to be to decrease the solubility of the PGE although the possibility of complexing by ligands containing these elements cannot be completely ignored.

Calculations also indicate that at temperatures greater than 1200K, 10 ppt Pd and 0·1 ppt Ru may be transported as volatile halide species in an aqueous vapour phase at log f hcl = 100 bars and log fO2 of the quart—fayalite—magnetite buffer.

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  1. Department of Geological Sciences, McGill University, 3450 University Street, Montreal, PQ, H3A 2A7, Canada

    Bruce W. Mountain & Scott A. Wood

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  1. Department of Earth Sciences, The Open University, Milton Keynes, UK

    H. M. Prichard  & P. J. Potts  & 

  2. Mineral Industry Research Organisation, Lichfield, UK

    J. F. W. Bowles  & S. J. Cribb  & 

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Mountain, B.W., Wood, S.A. (1988). Solubility and Transport of Platinum-Group Elements in Hydrothermal Solutions: Thermodynamic and Physical Chemical Constraints. In: Prichard, H.M., Potts, P.J., Bowles, J.F.W., Cribb, S.J. (eds) Geo-Platinum 87. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-1353-0_8

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  • DOI: https://doi.org/10.1007/978-94-009-1353-0_8

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