THE STRELTSOVSKOE DEPOSIT, EASTERN TRANSB AIKALIA - ISOTOPE DATING OFMINERALIZATION IN RUSSIA LARGEST URANIUM DEPOSIT

The chronology of formation processes and the prehistory of Russia's l argest Strel'tsovskoe uranium deposit, located in the Tulukaevskii vol cano-tectonic structure (TVTS) in the Argun River region-Mongolian vol canic belt, is discussed based on an examination of the U-Pb system in the hydrothermal uranium ore, as well as on the data of K-Ar and Rb-S r studies. The uranium ores, differing in both structure and compositi on, also have distinctive radiometric U-Pb ages. The Pb-206/U-238 and Pb-207/U-235 ages of massive pitchblende are concordant and fall withi n a narrow range of 134-136 Ma., while finely impregnated pitchblende and pitchblende with sulfide (galena, Fe-molybdenite) inclusions showe d discordant age values. An age calculation using the discordia diagra m (L. Stieff's method) yielded a value of 133 +/- 4 Ma. The age of rad iogenic lead from a pitchblend-molybdenite aggregate, from computation based on a two-stage model, is 250-260 Ma. Interpretation of this age as the age of protore, which had been the source of uranium for the S trel'tsovskoe deposit, is in accord with K-Ar ages of granites from th e basement of the Tulukaevskii volcano-tectonic structure. Dispersed u ranium mineralization in Precambrian rocks of the Tulukaevskii VTS bas ement may be another possible, still more ancient, source of uranium f or the Strel'tsovskoe deposit. From U-Pb dating, the age of this early mineralization is Ordovician (457-459 Ma.), while the age of uranium redeposition is Cretaceous (ca. 150 Ma.). Isotopic data on common lead from galena of the Strel'tsovskoe ore deposit are in accord with geoc hronology based on an examination of uranium minerals: common lead is a two-component mixture consisting of lead with a model age of 460 Ma and lead similar in composition to radiogenic lead from the uranium or es.