PLATINUM AND GOLD SOLUBILITY AND PARTITIONING BEHAVIOR IN SYNTHETIC MAGMATIC MELT – BRINE MIXTURES

The behavior of Pt and Au in brines associated with silicate melt (relevant to Pt-bearing layered intrusions, and Au-bearing porphyry systems) was investigated by reacting the metals with high-T (500-850ºC), S-free brine (20-70 wt% NaCl) ± peraluminous melt, trapping the brine in synthetic fluid inclusions (quartz-hosted) and vesicles (melt-hosted), and quantifying the metal content of the trapped brine and glass by LA-ICPMS. HCl activity was buffered using albite-andalusite-qtz; fO₂ using Ni-NiO and MnO-Mn₃O₄. The following observations were made: (i) transient ablation signals for the inclusions demonstrate coincidence between metal and Na signals, suggesting that the metals are dissolved in solution and not present as accidentally trapped particles; (ii) C^Pt_brine and C^Au_brine are very high, with ranges in conc. over all experimental conditions of 10-10³ ppm Pt, and 10²-10⁵ ppm Au (wt% Au contents indicated by visible Au daughter crystals); (iii) trapped brines exhibit a wide range of C^metal (due to premature fracture healing and slow metal-brine-melt equilibration) allowing for only minimum estimates of metal solubility; (iv) C^Pt_melt and C^Au_melt are ~500 ppb and 2 ppm, respectively, and are invariant over the range of experimental conditions, but exhibit some heterogeneity within a single run (formation of micronuggets during quench?); (v) apparent brine-melt partition coefficients vary only as a function of C^metal_brine, independent of C^metal_melt; D*_Pt^brine/meltbrine/melt range from 3x10²-5.2x10⁴, D*_Au ^brine/melt range from 1.4x10⁴-4.9x10⁵; (vi) at fixed T and NaCl_tot, maximum and mean C^metal_brine increases with increasing fO₂ in accordance with oxidation states Pt²⁺ and Au¹⁺; (vii) the maximum and mean C^metal_brine decreases with increasing NaCl_tot at fixed T, but increases with increasing T at fixed NaCl_tot. The observed behavior is consistent with hydroxide complexing; the following mass action expressions are proposed: (i) NaAlSi₃O₈ + Au(s) + ¼O₂ + HCl_aq=AuOH + ½Al₂SiO₅ + 2.5SiO₂ + NaCl_aq, (ii) 2NaAlSi₃O₈ + Pt(s) + ½O₂ + 2HCl_aq=Pt(OH)₂ + Al₂SiO₅ + 5SiO₂ + 2NaCl_aq. The results demonstrate that S-free, aqueous brines which exsolve from/interact with residual magmatic liquids have the ability to carry large amounts of both Pt and Au across the magmatic-hydrothermal interface, even at moderately oxidizing conditions (Ni-NiO).