Generation of Polyphase Assemblage of the Platinum - Group Minerals in the Inagli Dunite - Shonkinite Massif of the Aldan Shield of the Siberian Platform

The Inagli massif is a concentric - zonal ring massif consisting of a dunite core bordered by a sequential series of peridotites, pyroxenites and shonkinites. In dunites, there are densely disseminated accumulations of chromspinelides, as well as schlieren and veined particles of massive chromitites, to which polyphase growths of platinum - group minerals (PGM) are confined. The Inagli intrusive, like the well - known Konder massif, belongs to an independent "Aldan" type of platinum - bearing deposits. The Aldan type of ring intrusions is a platform analogue of the "Ural - Alaskan" zonal dunite - gabbro massifs of orogenic regions. In placers, dunites and chromitites of the Inagli massif, PGM are mainly represented by isoferroplatinum (Pt3Fe) with an admixture of iridium up to 8 wt %. In isoferroplatinum, symplektitic iridium particles and small inclusions of osmium, laurite, ehrlichmanite, malanite, as well as other sulfides and arsenides of platinum - group elements (PGE) are often observed. The bulk composition of such polymineral aggregates can be calculated based on the volume ratios and chemical composition of individual phases. The results obtained in this way show that the compositions of the initial polycomponent solid solutions vary from Pt - Ir - Fe to Ir - Os - Ru - Rh - Pt - Pd - Fe alloys. Polycomponent homogeneous solid solutions, which composition gradually changes from Ru - Rh - Ir - Os minerals to Fe - Pt alloys, are known in the Witwatersrand placers. A similar series of solid solutions of PGE is identified in the placers of the Guli massif on the Siberian platform. Unlike the placers of the Witwatersrand and Guli massif, where PGM are mainly represented by Os - Ir alloys, Inagli minerals have mainly a Pt - Ir - Fe composition with a low proportion of Os. The structures of most natural polyphase PGM aggregates are similar to those of artificial alloys, therefore, the former are also products of crystallization of multicomponent metal melts and their subsequent solid - phase transformations. The limits of solubility between PGE differ significantly, therefore, depending on the initial composition of metal alloys, both polycomponent solid solutions and polymineral aggregates can be formed. Based on the analysis of combined double and triple diagrams of PGE systems, the author considers possible ways of evolution of phase transformations of alloys of different composition.