Contrasting Surficial Composition of Native Gold from Two Different Types of Gold Ore Deposits

• Published in: Minerals (Basel) 2017-08, Vol.7 (8), p.142
• Main Authors: Tauson, Vladimir, Kravtsova, Raisa, Makshakov, Artem, Lipko, Sergey, Arsent’ev, Kirill
• Format: Article
• Language: English
• Subjects: AES; Agglomerates; Analytical methods; Auger spectroscopy; chemical composition; Composition; Electron microprobe; Electron microscopy; Electron probe microanalysis; Electron spectroscopy; electrum; EMPA; Exploration; Fineness; Geochemistry; Geological surveys; Gold; Grains; High temperature; Mineral deposits; Mineralization; native gold; Oxidation; Particle physics; Particulate composites; Photoelectron spectroscopy; Protective coatings; Quartz; Scanning electron microscopy; SEM-EDX; Silica; Silicon dioxide; Silver; Solid solutions; Solutions; Spectrometry; Spectroscopy; Stoichiometry; sulfidation; Sulfides; Sulfur; Sulphides; Sulphur; surface; X ray photoelectron spectroscopy; XPS
• ISSN: 2075-163X; 2075-163X
• DOI: 10.3390/min7080142

Native gold grains sampled at two different gold ore deposits in Eastern Russia have been studied by the techniques of electron spectroscopy (X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES)), electron microprobe analysis (EMPA), and scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDX). The high-fineness gold of the deposit hosted by relatively high temperature gold-quartz-sulfide mesothermal ore formation contains no oxidized Au constituents on grain surfaces, whereas the less fine gold of the epithermal Au-Ag deposit contains gold oxidized to the Au (I) state, or higher, in half of the cases. At this deposit the surface of native Au consists of a thin layer (~15 nm) with elevated Ag and S contents and an underlying SiO2-containing layer ~30-60 nm thick. Such a composite coating can perform a protective function and prevent the gold-silver sulfides in surficial parts of AuAg grains from oxidation. The sulfur-enriched marginal parts of native gold particles do not always correlate with the stoichiometry of well-known binary AuAg-sulfides and have a variable composition. This may be due to the existence of solid solutions, Ag2−xAuxS, if there is enough S or S adsorption-stabilized cluster agglomerates, AgnAumS, under sulfur deficit. The effect of the formation of nano-scale surficial zonality on the surface of native gold is quite common in nature and applicable to geological exploration and technology of gold-ore processing. It can facilitate establishing the geochemical environment and genetic type of Au mineralization.