Mineralogy, Types, and Origin of the Platinum-Bearing Placer Deposits of the Siberian Platform

Abstract

On the basis of detailed mineralogical studies carried out by the author and published data, a general description of platinum-bearing placers 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 following types of PGM placers have been recognized—rhodian ferroan platinum (Vilyuyan), iridian isoferroplatinum (Aldanian), iridian ferroan platinum (Yakokutian), sperrylite-platinum, ferroan platinum-palladium (Noril'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 small amounts of Os-Ir-Ru alloys (rhodian ferroan platinum type) are widespread. These placers are located mainly in channels that drain Mesozoic terrigenous deposits within buried basement block uplifts. Pt-bearing sources are likely to have been Archean-Proterozoic ultramafic-mafic 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 Proterozoic stratiformly layered ultramafic-mafic complexes with Ti-magnetite and Cu-Ni sulfide mineralization are developed (Aldan-Stanovoy Pt-bearing province). In the Aldan Shield, Pt-bearing placer deposits associated with alkaline-ultramafic massifs (Inagli, Kondyor, etc.) are the best known. Iridian isoferroplatinum is a major mineral of these placers (iridian isoferroplatinum type). Isoferroplatinum is characteristically intergrown with Os-Ir and Ir-Os-Pt alloys. PGM were derived from chromitite bodies, dunites, and magnetite-pyroxenites of zoned ring massifs. Besides grains of ferroan platinum, iridium, rarely osmium, and rutheniridosmine (iridian ferroan platinum type) occur in gold-bearing placers of the central Aldan region. Chemical features of the phases intergrown with PGM are compatible with a mineral composition of potassic picrites developed in the central Aldan Shield, indicating a probable consanguinity between PGM in these placers and alkaline-ultramafic magmas. Thus, during Mesozoic activization of the Aldan Shield, alkaline-ultramafic massifs were formed, giving rise to the iridian isoferroplatinum 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, which are related to Permian-Triassic differentiated gabbro-dolerite intrusions. Ferroan platinum, native platinum, tetraferroplatinum, palladian platinum, stannoplatinite, stannopalladinite, and other varieties were established in these placers (ferroan platinum-palladium type). Along the southern margin of the Khatanga trough, within the Guli dunite massif, alluvial placers were discovered. In placers, PGM consist of native osmium, iridian osmium, and rarely isoferroplatinum (iridian osmium type). These magmatic complexes were emplaced in the Early Mesozoic, and were related to the Yenisey-Khatanga rift zone (Yenisey-Khatanga Pt-bearing province).