The aqueous geochemistry of platinum, palladium and gold; recent experimental constraints and a re-evaluation of theoretical predictions

ABsrRAcr Re-evaluation of theoretical calculations reaffirms that, at temperatures up to 300C, significant solubility (>10 ppb) of Pt and Pd as chloride complexes is restricted to highly oxidizing and acidic conditions. This conclusion is supported by recently,reported experimental solubility measurement!. Experiments on the solubility of ft and Pd in sodium hydroxide solutiors at 25'C confirm that a change in speciei from PdIOU)! to p'O(OfDt occurs at a pH of approximately 12, but apparcntly no such change in speciation occurs for Pt. The measured log p2 for R(OH)9 agrees very well with the previously predicted value, whereas- predicted stability-constants for ft(OH)J unO ntOUlo2- stroutO Ue considered only as prwisional- estimates. Hydroxide complexation is unlikely to contribute to the hidrothermal iransport of Pt and Pd in mosi geological environments. Nevertheless, ft, Pd and Au hydroxide complexes may be the dominant inoiganic species of these metals in oxidized surficial waters such as seawater, lake water, and rivei water. Ammonia complexes are predicGd to contnbute substantially to the solubility of Pd and Au, bulProbablY not pr, ar temperarures near 300'C, with a maximum in solubility at the N2-NH3-NHf triple_ point. P:;st-l9jtidggf.!' Pd and Au as ammonia complexes decrease very rapidly with decreasing temperatue, owing to the shift of the N2-NHJ-NHf triple point to successively lowei fugacities of o*ygen anA iigner pH. Preliminarry results of an experimental investigation of the solubility of I,t, pd and Au in hydrothermA bisulfide solutioni from 200 to 500'i yield between 2 and 300 ppm Au, benveen 4 and 800 ppb ft, and between 1 and 600 ppb Pd at0.3-2.2mol^|xs,5.91 -N > -O.

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