The solubility of platinum and gold in NaCl brines at 1.5 kbar, 600 to 800°C: A laser ablation ICP-MS pilot study of synthetic fluid inclusions

The concentration and distribution of Pt and Au in a fluid-melt system has been investigated by reacting the metals with S-free, single-phase aqueous brines (20, 50, 70 wt% eq. NaCl) ± peraluminous melt at a confining pressure of 1.5 kbar and temperatures of 600 to 800 °C, trapping the fluid in synthetic fluid inclusions (quartz-hosted) and vesicles (silicate melt-hosted), and quantifying the metal content of the trapped fluid and glass by laser ablation ICP-MS. HCl concentration was buffered using the assemblage albite-andalusite-quartz and fO2 was buffered using the assemblage Ni-NiO. Over the range of experimental conditions, measured concentrations of Pt and Au in the brines (CPt fluid, CAu fluid) are on on the order of 1-103 ppm. Concentrations of Pt and Au in the melt (CPt melt, CAu melt) are ∼35-100 ppb and ∼400-1200 ppb, respectively. Nernst partition coefficients (DPt fluid/melt, DAu fluid/melt) are on the order of 102-103 and vary as a function of Cmetal fluid (non-Henry's Law behavior). Trapped fluids show a significant range of metal concentrations within populations of inclusions from single experiments (∼ 1 log unit variability for Au; ∼2-3 log unit variability for Pt). Variability in metal concentration within single inclusion groups is attributed to premature brine entrapment (prior to metal-fluid-melt equilibrium being reached); this allows us to make only minimum estimates of metal solubility using metal concentrations from primary inclusions. The data show two trends: (i) maximum and average values of CAu fluid and CPt fluid in inclusions decrease ∼2 orders of magnitude as fluid salinity (m∑Cl fluid) increases from ∼4 to 40 molal (20 to 70 wt % eq. NaCl) at a constant temperature; (ii) maximum and average values of CAu fluid increase approximately 1 order of magnitude for every 100°C increase temperature at a fixed m∑Cl fluid. The observed behavior may be described by the general expression: log (mmetal fluid,T,1.5kbar) = x · log (m∑Cl fluid) - y where x (slope of the solubility curve) is similar for both Au and Pt (-2), and y increases wi