MINERALOGY AND GENESIS OF "ALLUVIAL PLATINUM" FROM PLACERS OF THE SOUTHERN KORYAK HIGHLAND (RUSSIA)

Abstract

Analogues of Uralian platinum-metal placers were discovered in the southern Koryak Highland of Russia in the last decade. Until recently, the most promising are valley and terrace alluvial placers of platinum metals located in the Vetvei-Levtyrinyvayam-Tapel'vayam river basins. These tributaries of the Vyvenka River intersect distribution areas of multiphase zoned Gal'moenan and Seinav gabbro-pyroxenite-dunite massifs of the Olyutor structural zone of the Koryak-Kamchatka active margin. In this area, reserves of "alluvial platinum" in placer deposits of the Levtyrinyvayam River and Ledyanoi, Sentyabr', and Penistyi creeks are estimated and several promising placer occurrences are discovered. The "alluvial platinum" from placers is represented by xenomorphic aggregates of Pt and Fe intermetallic compounds with inclusions of more than 20 minerals of platinum-group elements (PGE), chromespinelides, olivine, pyroxene (diopside-hedenbergite), amphiboles, and other minerals. Some mineral-forming PGE of the "alluvial platinum" (Pt > Ir > Rh ≈ Os > Pd > Ru), the stable composition of isoferroplatinum with dominant iridium as an admixture, the prevalence of inclusions of Os and Ir phases amid minerals, and other features allow most of the reserves of placer deposits to be referred to the Ir-Pt mineralogical-geochemical type. The structural relationships and compositions of chromespinelides, olivines, pyroxenes, amphiboles, serpentines, and other minerals in intergrowths with platinum-group minerals (PGM) are noteworthy. Dunites represent the main primary source of placer-forming PGM, which is evidenced by most mineral intergrowths-olivine and chromespinelides. Formation of chromitite schlieren, accompanying PGE, and placer-forming xenomorphic segregation of Pt and Fe intermetallic compounds are closely related to the process of synmagmatic recrystallization of dunites. The earliest separation of ore components from the dunite substrate was observed as thin chromespinelide lamellae in primarily recrystallized olivine. These areas are characterized by an elevated or background content of PGE. The subsequent dunite recrystallization resulted in the formation of large chromespinelide metacrystals and chromitite schlieren associated with a most significant segregation of PGM. Dunite recrystallization processes played an important role in the distribution of PGM, their accumulation, and concentration. It can be assumed that PGE and their minerals in primary magmatic dunities occurred in the form of dispersed aggregates including micron-sized intergrowths of Fe and Ni sulfides with PGE. The Gal'moenan and Seinav gabbro-pyroxenite-dunite massifs intrude rocks of the Vatyna Series forming with the latter deformational structures of allochthon represented by the Tapel'vayam nappe of the Vatyna-Vyvenka thrust. Being a part of the allochthon, these massifs represent rootless bodies whereas serpentinite tectonites of these massifs are distributed along the base of the Tapel'vayam nappe. A peculiar feature of placers in the Olyutor structural zone is that they formed during a relatively short geological time (contrary to placers of the Aldan Shield or the Platinum-bearing belt of the Urals, which are characterized by significant inherited cycles of placer formation during a period lasting from the Paleozoic to Cenozoic). Nevertheless, the formation of placers located substantially far from bedrock sources may be explained by the existence of intermediate accumulating rocks. They were probably represented by sedimentary rocks that formed as a result of inherited selective denudation of serpentinite melange bodies many kilometers long and developed after multiphase zoned gabbro-pyroxenite-dunite massifs at the base of the nappe. This is evidenced by fragments of sedimentary rocks (clasts of quartz, feldspars, magnetite, chromespinelides, and other varieties cemented by the quartz-carbonate cement) located around grains of "alluvial platinum.