Title:

Mobilization of Platinum, Palladium and Gold by Magmatic Brines: An Experimental Study

Department: Earth Sciences
Issue Date: Nov-2020
Abstract (summary): Magmatic ore deposits enriched in platinum-group elements (PGE) often show evidence of previous hydrothermal activity. However, the exact role of hydrothermal processes in PGE ore formation has been uncertain due, in part, to a lack of systematic experimental data on the solubility and speciation of the PGE in hydrothermal fluids. To fill this knowledge gap, this thesis presents results using the synthetic fluid inclusion method to determine the solubility and speciation of Pt, Pd and Au in Cl-bearing aqueous fluids at conditions relevant to the emplacement of magmas in the crustal environment. The data presented here show that high temperature aqueous brines can transport well above 1 μg/g Pt, Pd and Au, as determined by fluid concentrations at metal saturation. Platinum and Pd are predicted to be in the 2+ oxidation state, whereas Au is predicted to be in the 1+ oxidation state in the fluids investigated. The solubilities of Pt, Pd and Au show a positive correlation with oxygen fugacity (ƒO2), temperature (T), fluid acidity (pH) and total chloride concentration (Cltotal). Trends in solubility with pH and total chlorinity indicate that Pt, Pd and Au are transported as chloride complexes. Results from thermodynamic modeling suggest that Pt and Pd are primarily transported as PtCl3– and PdCl3–, with minor contributions from PtCl2 and PdCl2, in high-temperature aqueous brines. Thermodynamic modeling predictions regarding the complexing species of Au are less certain, although HAuCl2 is expected to be important at high chloride concentrations and at low pH, with AuCl predominant at low chloride concentrations and at high pH. The relatively high metal solubilities determined in this work indicate hydrothermal fluids may be important in the processes that govern the formation of PGE enriched ore deposits. Additional work was done to apply the fluid-trap method with a modified cryogenic cell to obtain accurate metal solubilities at the sub-μg/g level. Despite several advancements being made in the method development, metal-fluid equilibrium could not be demonstrated in the experimental runs. Therefore, additional work is required to validate the fluid-trap method as a reliable technique to measure precious metal solubilities in upper crustal environments.
Content Type: Thesis

Permanent link

https://hdl.handle.net/1807/103367

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