The Berezovsk giant intrusion-related gold-quartz deposit, Urals, Russia: Evidence for multiple magmatic and metamorphic fluid reservoirs

The Berezovsk gold deposit in the Middle Urals has been mined for 270 years. Its endowment (past production and gold reserves) is estimated to be 490 t of gold. The deposit is located in the greenschist metamorphosed Silurian volcanogenic-sedimentary rocks intruded by granitoid dykes to the north-east of Late Carboniferous Shartash granite massif. Mineralisation is represented by sulphide-quartz veins in the granitoid dykes (ladder veins) and in the host rocks (krassyk veins) formed in the following four stages: ankerite-quartz, quartz-pyrite, gold-polymetallic and carbonate. Ore veins are accompanied by halos of gumbeite (quartz + orthoclase + carbonate), beresite (quartz + sericite + ankerite + pyrite) and listvenite (quartz + Fe-Mg carbonate + fuchsite + pyrite). The veins mainly consist of quartz with sulphide minerals (commonly 3-5 vol%). About 180 minerals have been identified in ores, but the most abundant minerals are quartz, calcite, ankerite, pyrite, galena, tennantite, chalcopyrite, aikinite, native gold, and sphalerite. Native gold was deposited during quartz pyrite (Au I) and gold-polymetallic (Au II) stages. Fineness of gold ranges from 863 to 984 and from 723 to 848 for Au I and Au II, respectively. The mineral and metal zoning was identified relative to the roof of the Shartash granite massif. The fluid inclusion study revealed that the gold mineralisation at the Berezovsk deposit was formed at 300-230 degrees C and 0.3-2.3 kbar (mostly 0.5-1.2 kbar), from a H2O-CO2-NaCl fluid with salinity of 7.3-18.2 wt% NaCl equiv. The fluid was separated into H2O-CO2-NaCl and CO2-rich fluids due to temperature and/or pressure drop at the deposition site. Calculated 5180 and delta D values are 5.2-8.1%0 and -39 to -63 parts per thousand, respectively, for the fluid in equilibrium with alteration assemblages. The average delta C-13 value for the fluid equilibrated with carbonates from the inner zones of metasomatic halos is -5.3 parts per thousand. The calculated 5180 and delta C-13 values are 3.0-9.6 parts per thousand and -3 to -9 parts per thousand, respectively, for ore-forming fluids. The delta S-34 values are 1.4-12.9 parts per thousand and -1.6 to 11.7 parts per thousand for the fluid in equilibrium with early and late sulphides, respectively. In addition to the isotopic data, the geological, mineralogical and fluid inclusion data confirmed the predominant contribution of the magmatic fluid to formation of the Berezovsk hydrothermal system. The light C, O, and S isotope enrichment of the fluid was mainly caused by fluid phase separation. Fluids generated by decarbonation and dehydration reactions due to the contact metamorphism of the host rocks during the Shartash massif emplacement were responsible for additional S-34 input. The ore-forming fluid was enriched in the light O-16 isotope on the deposit flanks indicating the mixing with heated meteoric water.