Platinum transport in chloride-bearing fluids and melts: Insights from in situ X-ray absorption spectroscopy and thermodynamic modeling

Abstract

Abstract Hydrothermal chloride-rich fluids are identified at the late stages of the formation of platinum group element (PGE) deposits in giant layered intrusions, and are considered as the PGEs transport media in Cu(-Mo,Au) porphyry systems. In order to quantify the hydrothermal mobility of Pt we performed an investigation of the speciation of Pt in hydrothermal chloride-bearing fluids and dry melt by means of X-ray absorption spectroscopy (XAS). The experiments consisted in recording the Pt L3-edge X-ray absorption near edge structure/extended X-ray absorption fine structure (XANES/EXAFS) spectra of Pt-bearing fluids obtained by dissolution of Pt metal in KCl/HCl and CsCl/HCl fluids in the temperature range from 450 to 575 °C at pressures from 0.5 to 5 kbar. A spectrum of Pt dissolved in dry CsCl/NaCl/KCl + K2S2O8 melt was recorded at 650 °C. The capillary method, when the experimental solution together with Pt(cr) is sealed inside a silica glass capillary, was used. As was determined from the XANES spectra, in all the experimental systems Pt existed in the +2 oxidation state. Analysis of EXAFS spectra showed that Pt is coordinated by four Cl atoms with RPt-Cl = 2.31 ± 0.01 A independently of the T-P-compositional parameters. No evidence of the formation of complex with alkali metal cations in the second coordination sphere of Pt was found by the analysis of the EXAFS spectra of concentrated CsCl brines and melt. Our results imply that PtCl42− is the main Pt-Cl complex which predominates in hydrothermal fluids at t > 400 °C and fluid density d > 0.3 g·cm−3. Experimental data obtained for dry melt of alkali metal chlorides suggest that Pt-Cl complexes can dominate Pt speciation in chloride-bearing aluminosilicate melts where Cl exhibits a salt-like atomic arrangement and ionic bonding. The literature data on the Pt solubility constant, P t ( c r ) + 2 H C l ( a q ) o + 2 C l - = P t C l 4 2 - + H 2 ( a q ) , are compiled and fitted to the simple density model equation log K s o ( P t C l 4 2 - ) = 0.973 - 8202 · T ( K ) - 1 - 5.505 · log d w + 2223 · log d ( w ) · T ( K ) - 1 , where d(w) is the pure water density in g·cm−3. The equation, combined with the extended Debye-Huckel equation for activity coefficients, can be used to calculate the solubility of Pt up to 1000 °C/5 kbar. It accurately predicts the solubility of Pt in concentrated chloride brine (up to 50 wt% NaCl) at parameters of magmatic-hydrothermal transition (800 °C/1.4 kbar). At fluid/vapor density below 0.3 g·cm−3 a neutral complex PtCl2°(aq) is suggested as the dominant Pt species. Our data demonstrate that Pt is highly mobile in high-temperature oxidized chloride-rich hydrothermal fluids. For example, at 800 °C/2 kbar the concentration of Pt can reach a few wt% in the 1 wt% HCl/50 wt% NaCl fluid which is in equilibrium with magnetite-hematite buffer. Once a Cl-reach fluid exsolves from alumuinosilicate melt, Pt follows Cl and enriches the fluid phase where it exists mostly in the form of PtCl42−. Decrease of temperature, acidity, and fluid chlorinity results in precipitation of Pt from the fluid phase.