Carbon and oxygen stable isotope ratios of vein carbonates from hydrothermal-metasomatically altered Mesoarchaean metakomatites of the Kostomuksha greenstone structure were studied, the isotopic composition, probable sources, and the main hypotheses of fluid origin, that influenced the formation of soapstone Ozerki deposit and Pentinsuo prospect, were established. The mineral forming fluid was characterized by narrow ranges of carbon isotopic composition δ¹³С_CО2 from -0.6 to + 1.1‰ and oxygen isotopic composition δ¹⁸О_Н2О from +5.6 to + 9.1‰. The obtained isotopic data indicate that the fluid was generated by the processes of dehydration, decarbonation and dissolution of carbonates during progressive metamorphism of hydrothermally altered (seafloor metamorphism) greenstone rocks at deep levels of the komatiite-basalt sequence of the Kostomuksha structure and / or as a result of isotopic exchange of late orogenic granite intrusions primary magmatic fluid with rocks of komatiite-basalt association.