MINERALOGY, TYPES, AND ORIGIN OF THE PLATINUM-BEARING PLACER DEPOSITSOF THE SIBERIAN PLATFORM

On the basis of detailed mineralogical studies carried out by the auth or and published data, a general description of platinum-bearing place rs of the Siberian Platform is given. Typomorphic features of platinum -group minerals (PGM) allow us to distinguish the genetic types of PGM associations in accordance with a set of major minerals and/or basic components of these minerals. Within the Siberian Platform, the follow ing types of PGM placers have been recognized-rhodian ferroan platinum (Vilyuyan), iridian isoferroplatinum (Aldanian), iridian ferroan plat inum (Yakokutian), sperrylite-platinum? ferroan platinum-palladium (No ril'skian), and iridian osmium (Gulian) types. In the eastern part of the Siberian Platform (Lena Pt-bearing province:), placer occurrences consisting of a unique association of high-Rh ferroan platinum and sma ll amounts of Os-Ir-Ru alloys (rhodian ferroan platinum type) are wide spread. These placers are located mainly in channels that drain Mesozo ic terrigenous deposits within buried basement block uplifts. Pt-beari ng sources are likely to have been Archean-Proterozoic ultramafic-mafi c complexes covered by Phanerozoic flat-lying strata. In the southern part of the Aldan Shield and the Stanovoy region, gold-bearing placer deposits typically contain sperrylite and platinum alloys (sperrylite- platinum type). These placers are widespread in regions where Proteroz oic stratiformly layered ultramafic-mafic complexes with Ti-magnetite and Cu-Ni sulfide mineralization are developed (Aldan-Stanovoy Pt-hear ing province). In the Aldan Shield, Pt-bearing placer deposits associa ted with alkaline-ultramafic massifs (In-agli, Kondyor, etc.) are the best known. Iridian isoferroplatinum is a major mineral of these place rs (iridian isoferroplatinum type) Isoferroplatinum is characteristica lly intergrown with Os-Is and Ir-Os-Pt alloys. PGM were derived from c hromitite bodies; dunites, and magnetite-pyroxenites of zoned ring mas sifs. Besides gains of ferroan platinum, iridium, rarely osmium, and r utheniridosmine (iridian ferroan platinum type) occur in gold-bearing placers of the central Aldan region. Chemical features of the phases i ntergrown with PGM are compatible with a mineral composition of potass ic picrites developed in the central Aldan Shield, indicating; a proba ble consanguinity between PGM in these placers and alkaline-ultramafic magmas. Thus, during Mesozoic activization of the Aldan Shield, alkal ine-ultramafic massifs were formed, giving rise to the iridian isoferr oplatinum and iridian ferroan platinum types of placer deposits (Aldan Pt-bearing province). In the northern part of the Siberian Platform, small placers were formed not far from the Noril'sk Cu-Ni deposits, wh ich ale related to Permian-Triassic differentiated gabbro-dolerite int rusions. Ferroan platinum, native platinum, tetraferroplatinum, pallad ian platinum, stannoplatinite, stannopalladinite, and other varieties were established in these placers (ferroan platinum-palladium type). A long the southern margin of the Khatanga trough, within the Gull dunit e massif, alluvial placers were discovered. In placers, PGM consist of native osmium, iridian osmium, and rarely isoferroplatinum (iridian o smium type). These magmatic complexes were emplaced in the Early Mesoz oic, and were related to the Yenisey-Khatanga rift zone (Yenisey-Khata nga Pt-bearing province).