CHEMICAL-STATE OF GOLD DEPOSITED FROM QUENCHED MG-S-H-O FLUIDS BY X-RAY PHOTOELECTRON-SPECTROSCOPY

Reaction of MgS, H2O, and Au at 100 to 900 degrees C and 0.15 GPa yiel ds quenched products of brucite or periclase (or both), S-Au-bearing f luid, and H2S gas. The composition and chemical states in surface laye rs of dried solid and evaporated fluid products have been determined b y X-ray photoelectron spectroscopy (XPS). Gold and S occur in variable proportions, but minimum S:Au ratios are similar to 1.A significant t o dominant component in all Au 4f XPS spectra exists, with a 4f,,, pea k shifted positive in binding energy relative to Au degrees at 84.0 eV . In solid products dried in air, this peak is at 85.3 eV (+/-0.2 eV), and is assigned to Au+. The accompanying S 2p spectra are composite, but have a main peak at similar to 162.4 eV corresponding to hydrosulf ide (HS-). Re-analysis of these solid products after exposure to air a nd daylight for several weeks revealed Au-degrees only, and sulfate (S 2p peak at >168.0 eV) became the dominant S species. The Au 4f XPS sp ectra of solids from capsules opened in an inert atmosphere and dried in vacuum and of both air-and vacuum-dried fluids are dominated by a A u species with a 4f(1/2) peak at 84.5 and 84.3 eV, respectively, which is attributed to either nanoscale clusters of Au atoms or mononuclear Au degrees. These results support the predominance of Au+ in aqueous sulfidic ore fluids, may have bearing for a Au-cluster component of in visible Au in arsenian pyrite and arsenopyrite, and demonstrate the de position of Au by loss of H,S and reduction in the absence of metal su lfides.