Platinum Speciation and Transport in Sulfur-Rich Hydrothermal Fluids

Abstract

Understanding PGE deposit formation requires knowledge of the chemical state and mobility of platinum group elements (PGE) in magmatic-hydrothermal fluids. Yet, existing data on aqueous chloride, sulfate, and hydroxide complexes of PGE predict far too small metal contents (<ppt to ppb) in fluids from most geological settings, thus calling upon an important role of sulfide complexes [1-3], and, potentially, trisulfur ion ligands in PGE transport [4]. In an attempt to quantify the effect of sulfur on PGE mobility, here we combined solubility measurements, in-situ X-ray absorption spectroscopy (XAS), thermodynamic (TD) and molecular dynamics (MD) modeling, based on recent advances of our knowledge of sulfur speciation in crustal fluids [5]. Our new data at 300°C and 500 bar suggest formation of two main complexes transporting Pt in the fluid at concentrations of 10s ppm over a wide pH range (4-8): Pt(HS)42- in hydrogen sulfide H2S/HS- solutions, and Pt(HS)2(S3)22- in sulfide-sulfate H2S/SO42-/S3•− solutions. Notably, the obtained concentrations of Pt(HS)42- in H2S/HS- solutions, using an improved protocol for accurate fluid sampling from a flexible-cell reactor, are 1000 times higher than those reported in previous quench-based studies [6]. TD and MD simulations, based on combined solubility and in-situ XAS data confirm the large stability of the Pt(HS)2(S3)22- complex, analogous to that proposed for gold [4], with Pt(II) as the major oxidation state. Thus, our new data highlight an important role of previously ignored sulfur complexes that significantly enhance PGE transport by ore-forming fluids in the Earth’s crust.