Model Re-Os Isotopic Age of the PGE Mineralization at the Gulinsk Massif (at the Northern Siberian Platform, Russia)

The isotopic age of the platinum group minerals (PGM) was studied in some placers associated with the Gulinsk massif (Maimecha-Kotui province) at the northern part of the Siberian Platform. The isotopic ratio ¹⁸⁷Os/¹⁸⁸Os in individual PGM grains was estimated using the mass-spectrometry measurements of the electronegative ions (NTIMS) with the modernized device MI-1320. The measured Os-isotopic ratios are rather low in native osmium, iridosmine, and some other PGM, and correspond to the ratio values common in mantle sources. Two isotopically different PGM groups were statistically distinguished independent of their chemical composition. The ¹⁸⁷Os/¹⁸⁸Os ratio value is 0.1248 ± 0.0003 for the first group and it is 0.1258 ± 0.0002 for the second group. The estimated values are lower than these in the modern mantle (0.12736 ± 0.00016), so the difference allowed us to calculate the mean model Re-Os age of the PGM. The age of these groups is 370 and 230 Ma, respectively. The mean model PGM Re-Os age for the first group (370 Ma) is close to the age of the PGE mineralization associated with some well known clinopyroxenite-dunite intrusive complexes at the Siberian Platform (Konder and Inagli massifs). So the PGM studied are in paragenetic relations with dunites, chromitites, and clinopyroxenites being formed at the same time as these rocks. Therefore, the formation of the Gulinsk clinopyroxenite-dunite complex relates to a critical stage of the Siberian Platform cover evolution: to the final stage of the Middle Paleozoic tectonic cycle (D₃-C₁). The model Re-Os PGM dating of the second group (230 Ma) probably correspond to the mantle metasomatism at the final stage of the Late Paleozoic-Early Mesozoic cycle of the Siberian Platform cover evolution (P₂-T₂).