Carbonate–Alkaline Metasomatites of the Peschanoe Uranium Occur (Western Baikal Region): Mineral-Geochemical and Isotope-Geochemical Characteristics

—Metasomatic alterations in cataclased granites of the Primorsky Complex PR1 in the Southern Siberian Craton are represented by albitization and riebeckitization, accompanied by microclinization, dolomitization and completed by the formation of hydrothermal quartz–carbonate veins. Albitization, epidotization, and carbonatization are occurred in the host metavolcanics. Mineral parageneses and mineral thermometry data indicate the occurrence of hydrothermal–metasomatic processes in the temperature range from 400–500 to 200‒250°С. Calcite deposited at an early stage has values of δ13С –(3.2–3.3)‰ and δ18O +(8.3–9.4)‰, ferruginous dolomite δ13С –(1.6–1.9)‰ and δ18O +(10.6–11.1)‰. The calculated values of δ13C in CO2 contained in the fluid from which calcite was deposited at 450°C (about –0.7 to 0.8‰) indicate marine carbonates as the source of CO2; the calculated values of δ18O in a fluid in equilibrium with calcite (from +5.4 to +6.5‰) show that the fluid underwent isotopic exchange with magmatic silicate rocks at elevated temperatures and a low fluid-rock ratio. The initial (87Sr/86Sr)t ratio in newly formed calcite (0.718087) and dolomite (0.712033) and eNdT = –(7.3–8.2) indicate the leading role of an upper crustal source of material. The maximum U concentration (52 ppm on average) contains albite–biotite–magnetite schists at the contact with altered granites. The U concentrators in metasomatically altered rocks are the crichtonite group minerals, zircon, and thorite.