Results of .sup.40Ar/.sup.39Ar Dating of Ores of the Ermakovo F-Be Deposit, West Transbaikalia, Russia

The Ermakovo F-Be deposit, the largest Be deposit in Russia and one of the world's largest deposits, is located in West Transbaikalia. The ores of the deposit are diverse in their morphology and composition; mostly there are phenakite-bertrandite-fluorite metasomatites after limestones. The origin of Be mineralization is related to the stock of leucocratic subalkali aegirine-bearing granites, the age of which corresponds to 224-226 Ma. The available age of ores of 225.5-219 Ma is close to the age of ore-generating granites. To specify the age of the ores, we determined the .sup.40Ar/.sup.39Ar age of K-feldspar and, for the first time, of the Be mineral milarite, a K- and H.sub.2O-bearing Be silicate (KCa.sub.2(Be.sub.2AlSi.sub.12O.sub.30)·H.sub.2O), from different ore bodies of the Ermakovo deposit. The age spectrum of milarite (sample Er-17) has a stable plateau in the high-temperature part, which consists of four consecutive steps composing ~50% of the released .sup.39Ar with an age of 182.5 ± 3.0 Ma. The age of K-feldspars is similar (169.8-170.6 Ma). The results of .sup.40Ar/.sup.39Ar dating of minerals from ores of the Ermakovo F-Be deposit indicate that the processes of mineral formation occurred over a long period from 225 to 170 Ma.