We present results of isotope-geochemical study of the Ermakovka F–Be deposit, including data on the oxygen and carbon isotope compositions in dolomite and calcite marbles and in carbonates accompanying skarns, of early and late stages of ore formation and of post-ore parageneses. To elucidate the sources of fluids participated in the ore formation, we calculated the oxygen isotope composition in water and the hydrogen isotope composition in hydroxyl-containing minerals. Phlogopite in marbleized dolomites, vesuvianite and amphibole in skarns, eudidimite and bertrandite in ore parageneses, and bavenite formed during post-ore processes are analyzed. Most of the ore-stage minerals are depleted in heavy oxygen. Their δ18O values are lower than 5–6‰ (SMOW). Oxygen in carbonate minerals of the initial stage (dolomite and bastnaesite) is heavier (1.3–4.9‰) than that in calcite (+ 2 to –3.7‰). The δ18O values of water in equilibrium both with carbonate and with silicate minerals (–4 to –14‰) suggest the contribution of meteoric water to the mineral formation. A magmatic fluid (δ18O from + 6 to + 9‰) participated in the skarn formation at the initial stage, and a meteoric fluid, at the final stage (δ18O from –1 to –9‰). A meteoric source is confirmed by the depleted hydrogen isotope composition in minerals (δD from –119 to –192‰).

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