The Bereznyakovsk gold deposit (South Urals, Russia)

The Bereznyakovsk deposit is located within the Birguildinsk-Tominsk copper -porphyry ore field and paragenetically relates to a subvolcanic intrusive body of diorite-porphyry (D-3-C-1) Veinlet-disseminated ores are situated i n the inner zone of hydrothermally altered rocks. The last ones are chlorit e-mica-quartzalbite and locally mica-quartz or pyrophyllite-quartz rocks re placing diorite-porphyry. In the outer zone of altered intrusive rocks, car bonate and chlorite content strongly increases. Farther, at the periphery o f the ore field carbonate-chlorite propylitic rocks occur. Primary mica par agenesis contains varying proportions of sericite, paragonite, and K-Na whi te mica. It was formed at 320-380 degrees C and 0.8-1.2 kb (data of calcite -dolomite geothermobarometer). Micas underwent hypogenic hydration in the z ones of tectonic dislocations; hydromica and mixlayered paragonite-smecthit e contain 5-30% smecthite layers, and their isotope composition varies delt a D from -54 to -75 parts per thousand, and delta(18)O from +11 to +13 part s per thousand. The ore is associated with quartz-carbonate and carbonate m ineralizations that were formed under 260-360 degrees C and 0.2-0.4 kb. The following ore mineral assemblages were distinguished: gold-base metal (fah lores and high-fineness gold), gold-telluride-base metal (low-fineness gold , mercury-containing electrum, and Ag-, Pb-, and Bi-tellurides), and tellur ide-base metal (native Te, silvanite, etc.). The geological setting of the deposit corresponds to the upper (subvolcanic) level of the copper-porphyry vertical zonation pattern. The deeper (mezoabissal) level of it occurs in the tectonic block of the basement (rocks of the Ordovician age) containing the Tominsk copper-porphyry ore zone. Probably, the ascending flow of init ially Na-rich fluids intensely interacted with host rocks, and, as a result of this, the K/Na ratio, CO2 concentration, delta(18)O, delta(34)S, and de lta(13)C increased.