“Invisible” gold in covellite (CuS): Synthesis and studies by EPMA, LA-ICP-MS, and XPS techniques

Samples of covellite CuS(cr) containing up to 0.3 wt % of gold in “invisible” form were obtained by means of hydrothermal synthesis (450–475°C, 1 kbar) and synthesis in eutectic chloride melts (495°C). The studies using EPMA and LA-ICP-MS techniques showed that gold was uniformly dispersed within the volume of covellite grains and Au content increased with increasing of sulfur fugacity. The XPS-analysis of the samples obtained showed no chemical shift of the Au-4f 7/2 line compared to Au and Au2S, whereas a positive chemical shift of this line was revealed for mixed Au-Ag sulfides and tellurides. The obtained data allow one to assume that gold does not occur in covellite as an isomorphous admixture (solid solution) but forms nanoparticles of Au or gold sulfide.