X-RAY ABSORPTION SPECTROSCOPY UP TO 700°C: DECOUPLING OF CU FROM OTHER METAL-CHLORIDE COMPLEXES

Experimental studies show that significant amounts of metal and ligands (such as Cl and S) can be sourced during degassing and volatile exsolution from hot hydrous silicic magmas. Although the inherent capacity of magmatic fluids to carry and precipitate metals is well established, little is known about metal speciation at near-magmatic conditions (above 600°C). Speciation controls metal solubility, melt-fluid partitioning and is linked to when and where metals precipitate to form economic ore deposits, including those rich in Cu, Au, Mo, Sn and/or W. In addition, knowledge of metal speciation is critical to thermodynamic modelling of hydrothermal processes. We report on in situ non-destructive high-temperature spectroscopy experiments of natural fluid inclusions heated to 700°C.

Our XANES spectroscopic study reveals that Cu in a saline sulfur-poor ore solution was found exclusively as the linear [CuCl₂]^- species at all temperatures above ~200°C. Modelling of EXAFS spectra acquired at 530°C reveals that the Cu-Cl bond length of 2.11(2) Å is consistent with that reported elsewhere for [CuCl₂]^- at lower temperature (100-325°C) conditions. New high-temperature XANES and EXAFS spectroscopy also show that [ZnCl₄]^2-, [FeCl₄]^2- and [MnCl₄]^2- coexist with [CuCl₂]^- at high temperature.

Our results extend the temperature range for speciation data of each of the elements studied beyond that determined by previous studies of natural and synthetic systems (which typically investigate metal complexes to 350°C). The recognition of contrasting metal-chloride species implies that Cu, with its simple linear Cl complexation, will behave differently from the tetrahedral Cl complexes of Zn and Mn. The differing stoichiometry of the Cu chloride and other metal chloride complexes may help to explain the distribution of metals in zoned mineral deposits and districts. The recognition of simple Cu-Cl complexes implies that Cu deposition will be less influenced by variations in chlorinity than will other highly coordinated metals. The difference in speciation could also reflect general differences in bonding, which could make Cu more susceptible to forming Cu-S complexes, which may prove to be the actual transporting agent of Cu.