Epithermal Au‒Ag mineralization in terrigenous strata of the basement of the Pechalninsk volcanic dome structure (North-East of Russia)

Epithermal Au–Ag mineralization of the Pechalnoe deposit is of considerable interest, since it was formed in carbonaceous terrigenous strata of the basement of a volcanic dome structure, at a distance of about 200 km from the border of the Okhotsk-Chukchi marginal continental volcanic belt.The geological structure of the Pechalnoye deposit is two–tiered: quartz-adularic and quartz Au–Agveins are localized in keratinized terrigenous rocks of the lower tier, and quartz rhyolites and komendites of the Pechalninsky strata of the upper tier contain potentially industrial REE mineralization. Productive veins form three zones of sublatitudinal strike, the length of the veins in which is 200‒300 m,sometimes 640, 840 m; average thickness 0.1–3 m, rarely up to 6.2 m, average contents: Ag — 266 g/t,Au — 4.4 g/t. The following mineralogical features of ores have been established: low sulfidity (1–2%);native Ag, low-grade Au, polybasite, and highly selenic acanthite act as productive minerals, in addition,arsenic pyrite, arsenopyrite, pyrrhotite, ferruginous sphalerite, chalcopyrite and marcasite are quite widely developed in ores. The geochemical features of the ores are in good agreement with the mineral composition. The ores are enriched with a fairly wide range of trace elements (according to the rating): Ag, Au, As, Sb, Se, W, Tl, Li, Be, Bi, Cs, Mo, the predominance of light lanthanides over heavy ones has been established, very low Eu/Sm ratios  (<<1), slightly inclined nearchondrite spectra (without distinct Europian minima or maxima); the ratio of Ce/Ce* and Eu/Eu* values indicate oxidative conditions during ore formation; the REE spectra are dominated by light “hydrophilic” lanthanides of the “cerium” group; the REE varies widely. The obtained mineralogical and geochemical data allow us to attribute the mineralization of the Pechalnoye deposit to the selenium subtype of the lowsulfidized class of epithermal deposits. According to geological and mineralogical-geochemical data, the deposit can be classified as poorly eroded, which makes it possible to predict the identification of new ore bodies that do not come to the surface.