Epithermal Au–Ag Mineralization in Terrigenous Rock Sequences in the Basement of the Pechalninskaya Volcanic Dome Structure, Northeast Russia

Abstract

The epithermal Au–Ag mineralization at the Pechalnoe deposit is of considerable interest, since it was formed in carbonaceous terrigenous rock sequences making the basement of a volcanic dome structure at a distance of about 200 km from the boundary of the Okhotsk–Chukchi marginal continental volcanic belt. The geological structure of the Pechalnoe deposit consists of two levels, quartz–adularia and quartz Au–Ag veins are localized in keratinized terrigenous rocks of the lower level, while the quartz rhyolites and comendites of the Pechalninskaya rock sequence in the upper level harbor potential industrially valuable REE mineralization. The productive veins make three zones striking nearly east–west; the veins in these zones are 200–300 m long, occasionally reaching 640, 840 m; the mean thickness is 0.1–3 m, rarely reaching 6.2 m; the mean concentrations are Ag 266 g/t, Au 4.4 g/t. The following mineralogical features of the ores have been identified: low sulfides (1‒2%); the productive minerals are native Ag, low fineness Au, polybasite, and high-selenium acanthite. In addition, the ores have abundant enough arseniferous pyrite, arsenopyrite, pyrrhotine, ferruginous sphalerite, chalcopyrite, and marcasite. The geochemical features of the ores are in good agreement with their mineral composition. The ores are enriched in a wide range of trace elements (arranged in decreasing concentration): Ag, Au, As, Sb, Sе, W, Tl, Li, Be, Bi, Cs, and Mo; light lanthanoids prevail over heavy ones; very low Eu/Sm ratios (\( \ll 1\)), little inclined near-chondrite distributions (without distinct europium maxima or minima); the relationship between Ce/Ce* and Eu/Eu* indicates oxidizing conditions during the mineralization; the REE spectra are dominated by light “hydrophile” lanthanoids of the “cerium” group; ΣREE varies in a wide range. The mineralogical and geochemical data obtained enable us to classify the Pechalninskaya mineralization as belonging to the selenium subtype of the low sulfide class of epithermal deposits. According to geological and mineralogic-geochemical evidence, the deposit can be classified as poorly eroded, which suggests future identification of new ore bodies not at the ground surface.