VARIATIONS IN THE COMPOSITION OF "ALLUVIAL PLATINUM" FROM PLACERS OF THE SOUTHERN KORYAK HIGHLAND (RUSSIA)

Abstract

In the previous work (Mochalov et al., 2002), the "alluvial platinum" from the largest placers (Levtyrinyvayam River, Ledyanoi Creek, and Penistyi Creek) of the southern Koryak Highland was characterized. Alluvial platinum is mainly referred to the Ir-Pt mineralogical-geochemical type. Native sources of placer-forming Ir-Pt mineralization-coarse-porphyroclastic dunites with relics of polygonal and pegmatoid varieties-were also described in detail. The genesis of placer-forming platinum-group minerals (PGM) was substantiated, which was related to repeated symnagmatic recrystallization of dunites and redistribution of platinum-group elements (PGE) from primary magmatic dunites into large mineral aggregates. The idea that transitional accumulating sediments took part in the formation of placers was suggest on the basis of fragments of sedimentary rocks around alluvial platinum grains. This work comprises the characteristics of the mineral composition of alluvial platinum from all large, medium, and small placers and transitional accumulating sediments developed amid the Gal'moenan and Seinav multiphase zoned gabbro-pyroxenite-dunite massifs. Variations in the chemical composition of PGM from placers make it possible, along with alluvial platinum of the Ir-Pt mineralogical-geochemical type, to distinguish alluvial platinum of the Pt mineralogical-geochemical type. The main peculiar features of the latter are (1) the low Fe content of the key mineral-isoferroplatinum; (2) isoferroplatinum is characterized by a low content of PGE admixtures, among which Pd and Rh dominate, whereas Ir is found in insignificant amounts; (3) the development of individual native Pt; (4) the presence of native Os with low Ir concentrations; (5) the absence of native Ir; and (6) sperrylite and cooperite dominate amid PGE sulfides. "Alluvial platinum" of the Pt mineralogical-geochemical type is widespread in all placers and the transitional accumulating sediments, although its economic potential is insignificant. Alluvial platinum of the Ir-Pt mineralogical-geochemical type amounts to 90% of the commercial value of placers, but it is confined to a limited number of large and medium placers. Large and medium placers represent products of denudation and fluvial reworking of the transitional accumulating sediments of Pt metals. The formation of small placers is related to direct denudation of the bedrock source of Pt metals. The bedrock source of placers-multiphase zoned gabbro-pyroxenite-dunite massifs-comprises two mineral assemblages: (1) the assemblage of the Pt mineralogical-geochemical type, which is mostly developed in fine-porphyroclastic dunites, pyroxenites, and gabbro and which is genetically related to the cumulative history of host rocks; (2) the assemblage of the Ir-Pt mineralogical-geochemical type in blocks of coarse-porphyroclastic, polygonal, and pegmatoid dunites, the formation of which is likely to be associated with symnagmatic recrystallization of earlier dunite cumulates during the crystallization of later dunites from a poorly differentiated melt.