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

Abstract

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 (δ¹⁸О_VSMOW = 7.1…12.1‰; δ¹³С_VPDB = –6.3…–4.5‰). Carbonates of the second group have high δ¹⁸О and δ¹³С, characteristic of Precambrian sedimentary carbonates (δ¹⁸О_VSMOW = 21.9…24.7‰; δ¹³С_VPDB = –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 (δ¹³С_VPDB = –1.7…0.9‰, δ¹⁸О_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.