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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 (δ18O and δ13C), as well as associated quartz (δ18O) and biotite (δ18O 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 CO2 sources in fluids. There are three groups of carbonates according to δ18О and δ13С. The first group shows "magmatic" isotopic characteristics of the CO2 source (δ18ОVSMOW = 7.1...12.1‰; δ13СVPDB = –6.3...–4.5‰). Carbonates of the second group have high δ18О and δ13С, characteristic of Precambrian sedimentary carbonates (δ18ОVSMOW = 21.9...24.7‰; δ13СVPDB = –2.2...–2‰), similar to those described earlier in the neighboring Engozero sector of the BMB. In the third group, the δ13С values are similar to the carbonates of the first group, and δ18О, to the second group (δ13СVPDB = –1.7...0.9‰, δ18ОVSMOW = 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. [ABSTRACT FROM AUTHOR]