Platinum-group minerals from zoned clinopyroxenite-dunite massifs, Aldan Shield, Russia: genetic constraints inferred from osmium-isotope study

Negative thermal ionization mass spectrometry was used to firstly determine the osmium isotopic composition of platinum-group minerals (PGM) from chromitites and placers associated with the Kondyor and Inagli zoned clinopyroxenite-dunite massifs, located in the southern part of the Siberian Craton. The ¹⁸⁷Os/¹⁸⁸Os ratios in single PGM grains of different composition range from 0.1248 to 0.1252. Low ¹⁸⁷Os/¹⁸⁸Os values and a very narrow range of ¹⁸⁷Os/¹⁸⁸Os ratios within the PGM investigated are indicative for a common mantle source of the platinum-group elements (PGE). The Os-isotope results further show that the Re-Os system remained unchanged from the time of formation of the PGM until recent. This suggests one single event for PGE concentration and PGM formation during the evolution of the zoned ultramafic massifs. Os-isotope model ages indicate that this major event took place at the Devonian Carboniferous boundary, which represents a tectonically active time in the development of the Siberian Craton.