An in situ X-ray absorption spectroscopy study of gold-chloride complexing in hydrothermal fluids

Abstract

Despite the growing body of experimental data on gold solubility in hydrothermal fluids, the identity, structure and stoichiometry of Au-bearing aqueous complexes remain poorly known. Here we present the first in situ measurements, using X-ray absorption fine structure (XAFS) spectroscopy, of the stability and structures of Au III and Au I chloride complexes at elevated temperatures and pressures ( T–P) typical of natural hydrothermal conditions. The HAuCl 4–NaCl–HCl–Au (s) and NaCl–H 2SO 4–Au (s) systems were investigated to 500 °C and 600 bar using a recently designed X-ray cell which allows simultaneous determination of the absolute concentration of the absorbing atom (Au) and its local atomic environment in the fluid phase. XAFS data combined with Density Functional Theory quantum-chemical calculations of species structures and ab-initio modeling of XANES spectra show that the Au IIICl 4 − species is rapidly reduced to Au ICl 2 − at temperatures above 100–150 °C in acidic NaCl–HCl solutions. In the latter complex, two chlorine atoms are aligned in a linear geometry around Au at an average distance of 2.267 ± 0.004 Å. Our data provide the first direct structural evidence for AuCl 2 −, which is the major Au-bearing species in acidic Cl-rich hydrothermal fluids over a wide T–P range, in agreement with previous solubility and Raman spectroscopy data. Total aqueous Au concentrations measured by XAFS in HAuCl 4–HCl–NaCl and NaCl–H 2SO 4 solutions in the presence of Au (s) are, however, one to two orders of magnitude lower than those predicted by equilibrium thermodynamic calculations. This discrepancy is believed to be due to the combined effects of the cell properties, X-ray beam induced phenomena, and kinetic factors which may complicate the interpretation of high T–P spectroscopic data in redox-sensitive systems.