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 Loukhi region, carbonate-silicate and quartz veins are widespread, confined to bodies of Early Proterozoic metabasites and their contacts with host gneisses. The isotopic characteristics of coarsely crystalline 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 sources of CO₂ in fluids. There are three groups of carbonates according to δ¹⁸О and δ¹³С. The first group has “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 Belomorian mobile belt. 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.