New Evidence of the Polycyclic Genesis of Platinum Placer-Forming Formations of the Kondyor Alkaline-Ultramafic Massif: Results of ~(190)Pt-~4He Dating

摘要

This article presents the~(190)Pt–~(4)He dating results of native platinum group minerals from a unique platinum-metals placer deposit in the basin of the Kondyor and Uorgalan rivers and dunite fragments from the Kondyor alkaline-ultramafic massif. The results of~(190)Pt–~(4)He dating of 75 samples determine the age of the minerals and also date the ore-forming processes. It confirms the previously developed model of polycyclic genesis of placer-forming formations of platinum group minerals (PGMs) and, thus, sheds light on the duration of the formation of the Kondyor alkaline-ultramafic massif. The following~(190)Pt–~(4)He age stages are distinguished: (a) 143 ± 7 Ma, the formation time of the massif itself and early formation stages of PGMs of magmatogenic platinum (Pt) and the magmatogenic–fluid–metasomatic Pt and Os–Pt (Pt Os) types, as well as PGMs of fluid-metamorphogenic iridium–platinum (Pt Ir) type; (b) 128 ± 6 Ma, the early formation stages of the magmatogenic–fluid–metasomatic Pd–Pt (Pt Pd) type and the late stages of the Pt, Pt??Os, and Pt Ir types; (c) 115 ± 6 Ma, the late formation stages of the Pt Pd type. The good agreement of these dates with the geological and mineralogical data allows us to conclude that the formation of placer-forming PGMs and their formations in the Kondyor alkaline-ultramafic massif was a polycyclic process for about 30 Ma.