Gold-bearing pyrite from ore metasomatites of an epithermal Au-Ag deposit was studied by Auger and X-ray photoelectron spectroscopy. The study showed the presence of a thin (∼0.5 µm) continuous film on the pyrite surface, which contains lithophile elements (Si, Al, K). Its composition varies with depth. The outer layer of the film is compositionally intermediate between sericite and adularia, which are present among the products of hydrothermal alteration of the quartz-feldspar groundmass. Comparison of the Auger spectra of natural and synthetic (hydrothermal) pyrites showed that the initial (juvenile) surface of hydrothermal or metasomatic pyrite is of oxide-sulfide composition and can contain ∼10−20 at.% O. The high concentrations of evenly distributed Au on the crystal surface are due to the deposition of gold under the interaction of ore-bearing hydrothermal solution with the “acidic” pyrite surface containing SiO2 and iron oxides. The chemical composition of the surface is a typomorphic indicator of its interaction with an ore-bearing hydrothermal solution. Along with the high content of “invisible” gold bound with the surface, it serves as a prospecting criterion. The experimental data confirm the presence of stable forms (thin-film precipitates, quasi-phases, or nonautonomous phases) on the mineral surfaces. These forms can be preserved throughout the geologic time and thus bear information about the consequence of physicochemical processes that occurred in the environment.

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