The solubility of gold and palladium in magmatic brines: Implications for PGE enrichment in mafic-ultramafic and porphyry environments

We performed experiments to determine the solubility of Au and Pd in magmatic aqueous fluids as a function of oxygen fugacity (fO(2)), temperature (T), pH and total chloride concentration (Cl-total). Experiments were conducted at 800-1000 degrees C and 200 MPa in an externally-heated rapid-quench Molybdenum-Hafnium Carbide (MHC) cold-seal pressure vessel assembly. We employed a synthetic fluid inclusion (SFI) technique to entrap equilibrated, hydrothermal fluids in response to in situ fracturing of quartz cylinders at experimental run conditions. The solubility of Au and Pd both have positive relationships with fO(2), temperature, acidity and chlorinity. Concentrated aqueous brines containing 63 wt.% NaCl can dissolve wt.% levels of Au (similar to 1.2 wt.%) and Pd (similar to 1.7 wt.%) at metal saturation in relatively oxidized conditions, 1.44 log units above the Ni-NiO oxygen buffer (NNO+1.44), and mildly acidic pH at 900 degrees C and 200 MPa. Thermodynamic modeling of experimental results suggests that Au is mainly transported as AuCl(aq) at high pH and low Cl-total conditions, whereas HAuCl2(aq) and potentially AuCl2(aq)- predominates at low pH and high Cltotal conditions. Results from thermodynamic modeling also suggest Pd is mobilized in significant contributions by both PdCl2(aq) and PdCl3(aq)- with the latter gaining predominance in response to increasing Cl-total. Calculated fluid/melt partition coefficients for Au and Pd in low-density, magmatic vapors at 1000 degrees C and 200 MPa suggest that Pd may experience fractionation from Au in porphyry Au-Cu (+/- Pd, Pt) systems due to the restricted compatibility of Pd in the fluid phase (requiring strongly acidic and substantially high fO(2) conditions). Moreover, high-density, concentrated aqueous brines facilitate the compatibility of Pd in the fluid phase which may be important with respect to the formation of platinum-group element (PGE)-enriched horizons in layered mafic intrusions (e.g., J-M Reef, Stillwater Complex, U.S.A.). The potential for magmatic, near-neutral pH, high-salinity brines to dissolve significant amounts of Pd as Pd(II)-chloride complexes (similar to 400 to similar to 900 mu g/g) well below the NNO buffer suggests that such fluids may be responsible for late-stage hydrothermal remobilization of Pd within mafic-ultramafic igneous environments (e.g., Cu-Ni-PGE footwall deposits and low-sulfide PGE deposits in the Sudbury Igneous Complex, Canada). Crown Copyright (C) 2021 Published by Elsevier Ltd. All rights reserved.

Automated summary

Experimental results demonstrate that the solubility of Au and Pd is positively correlated with oxygen fugacity, temperature, acidity, and chlorinity. Concentrated aqueous brines with high NaCl content can dissolve significant amounts of Au and Pd at high temperatures and pressures.