A XANES study of Cu speciation in high-temperature brines using synthetic fluid inclusions

Cu K-edge X-ray absorption near edge structure (XANES) spectra were recorded from individual synthetic brine fluid inclusions as a function of temperature up to 500 °C. The inclusions serve as sample cells for high-temperature spectroscopic studies of aqueous Cu-Cl speciation. Cu⁺ and Cu²⁺ can both be identified from characteristic pre-edge features. Mixed oxidation states can be deconvoluted using linear combinations of Cu⁺ and Cu²⁺ spectra. This work illustrates how complex Cu XANES spectra can be interpreted successfully. Cu²⁺ is the stable oxidation state in solution at room temperature and Cu⁺ at high temperatures. The change in oxidation state with temperature was completely reversible. Cu⁺ was found to occur exclusively as the linear species [CuCl₂]⁻ in solutions containing KCl with Cu:Cl ratios up to 1:6. In the absence of K⁺, there is evidence for higher order coordination of Cu⁺, in particular the tetrahedral complex [CuCl₄]³⁻. The importance of such complexes in natural ore-forming fluids is yet to be determined, but may explain the vapor-phase partitioning of Cu as a Cl complex from a Cl-rich brine.