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

Abstract

The Berezovsk gold deposit in the Middle Urals has been mined for 270years. Its endowment (past production and gold reserves) is estimated to be 490t 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 3–5vol%). 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°C and 0.3–2.3kbar (mostly 0.5–1.2kbar), from a H2O-CO2-NaCl fluid with salinity of 7.3–18.2wt% 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 δ18O and δD values are 5.2–8.1‰ and −39 to −63‰, respectively, for the fluid in equilibrium with alteration assemblages. The average δ13C value for the fluid equilibrated with carbonates from the inner zones of metasomatic halos is −5.3‰. The calculated δ18O and δ13C values are 3.0–9.6‰ and −3 to −9‰, respectively, for ore-forming fluids. The δ34S values are 1.4–12.9‰ and −1.6 to 11.7‰ 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 34S input. The ore-forming fluid was enriched in the light 16O isotope on the deposit flanks indicating the mixing with heated meteoric water.