Isotope (δ18О, δ13С, δD) Characteristics of Biotite–Carbonate–Quartz Associations of Hydrothermal Veins in Metabasites of North Karelia

On the White Sea islands and coast of North Karelia, in the Chupa sector of the Belomorian mobile belt (BMB), carbonate–silicate and quartz veins are localized in bodies of Early Proterozoic metabasites and their contacts with host gneisses. The isotopic characteristics of coarse crystal vein carbonates (δ 18 O and δ 13 C), as well as associated quartz (δ 18 O) and biotite (δ 18 O and δD), were studied. The isotopic composition of oxygen suggests biotite–quartz–carbonate equilibria during the formation of veins at temperatures of 520–450°C. The O and H isotopic characteristics of biotite correspond to the equilibrium of the fluid with metamorphic rocks. At the same time, the isotopic compositions of O and C carbonates indicate different CO 2 sources in fluids. There are three groups of carbonates according to δ 18 О and δ 13 С. The first group shows “magmatic” isotopic characteristics of the CO 2 source (δ 18 О VSMOW = 7.1…12.1‰; δ 13 С VPDB = –6.3…–4.5‰). Carbonates of the second group have high δ 18 О and δ 13 С, characteristic of Precambrian sedimentary carbonates (δ 18 О VSMOW = 21.9…24.7‰; δ 13 С VPDB = –2.2…–2‰), similar to those described earlier in the neighboring Engozero sector of the BMB. In the third group, the δ 13 С values are similar to the carbonates of the first group, and δ 18 О, to the second group (δ 13 С VPDB = –1.7…0.9‰, δ 18 О VSMOW = 6…12‰). The O and C isotopic compositions of carbonates of the third group can be obtained from the action of a putative magmatic fluid on both the primary sedimentary source of metamorphic fluids and redeposited vein carbonates.