The Age of Nb Rare-Metal Mineralization of the Ilmeny–Vishnevogorsky Alkaline Complex (South Urals)

The results of local U–Th–Pb-SIMS-SHRIMP-II analysis of the collection of pyrochlores from Nb rare-metal deposits of the Ilmeny–Vishnevogorsky alkaline complex are discussed. Independently of the chemical composition, the host rocks, and the stage of carbonatite formation, the studied pyrochlores are characterized by an almost undisturbed U–Pb isotope system. This allowed to estimate the age of ore mineralization on three deposits (Vishnevogorsky, Buldymsky, and Potaninsky) with an accuracy of ±1–6 m. y. The U–Pb system of the studied pyrochlore samples suggests three epochs of the mineral formation: 380, 230–240, and 216–218 Ma. Convergence of the results obtained by the different methods of U–Pb dating for one of the samples from our collection (±0.5–1.0 Ma) allows to use it as an in-house standard for local dating.