<p>The oxygen isotopic composition of dolomite-type nephrite, metasomatic and host rocks of the Kavokta, Voimakan and Nizhnii Ollomi deposits of the Vitim Province was studied. The nephrite is characterized by an extremely light oxygen isotope composition δ¹⁸O –21.5 ÷ –15.8‰. The metasomatic rocks showed wider variations, including also extremely light isotope ratios –21.5 ÷ +9.2‰. The host rocks of the deposit are characterized by the heavier oxygen isotope compositions: granite and granodiorite –7.51 ÷ –0.71‰, amphibolite –8.38 ÷ +9.60‰, dolomitic marble +20.8 ÷ +26.1‰. The extremely light isotopic composition of nephrite is explained by the meteoric origin of the fluid from meltwater and the removal of isotopically heavy oxygen by carbon dioxide through decarbonatization of dolomitic marble. The granite mainly provided the regional heating that activated the fluid. The analyzed dolomite is characterized by a heavier carbon isotopic composition +3.2 ÷ +5.2‰ δ¹³C explained by the fermentation of organic matter and methane formation in the basin in which the dolomite was formed. Calcite from calcite–tremolite skarn is characterized by the lighter carbon isotopic composition –6.4 ÷ +0.7‰ δ¹³C as a result of metasomatic process.</p>