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Nadezhda Tolstykh, Geology and Mineralogy SB RAS (Russian Federation)
Eugeniy Sidorov, Institute of vulcanology and seismology SB RAS (Russian Federation)
Andrey Kozlov, Institute of problems of complex development of natural resources (Russian Federation)
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Mineral association from the placer of the Olkhovaya-1 River, connected with the ophiolite ultramafite of Kmachatskiy Mys Peninsula (Russia), provides genetic information on the evolution of mineral formation in these ultramafite. Os-Ir-Ru alloys are predominant in this association. Spots of compositions of hexagonal alloys form a ruthenium trend. Pt-Fe alloys amount about 20 % from the sampling. Three mineral magmatic parageneses are typical of the association: early osmium-iridium, intermediate ismium-iridium-isoferroplatinum and later isoferroplatinum-ruthenium. All three ones are accompanied by laurite, and solid solution laurite-irarsite (Ru,Ir)(AsS)2.
Formation of PGM was going in some stages. At the magamtic stage Ir-rich osmium was the earliest and Os-rich iridium was the successive crystallized. Both of them were formed the early osmium-iridium paragenesis. During of their crystallization the residual melt and new portions of solid solutions became saturated with Pt and Ru. Distribution of Ru (ĘDRu) between equilibrium osmium and iridium remained constant, about 1.4. When the concentration of Ru in hexagonal alloys and content of Pt in the melt reach their critical values, in that case the isoferroplatinum-ruthenium paragenesis begins to form. Its formation is also durable in some interval of falling temperatures with the consistent enrichment of solid Os-Ir-Ru alloys with Ru and the formation of ruthenium trend of the compositions for the hexagonal alloys. Simultaneously, the unstable multicomponent Pt-Ru-Ir-Os-Fe solid solutions are being formed from the melt, which under the lower temperature decompose into ruthenium and ferrous platinum. Herewith, it is possible to observe fine Pt-Ru-Ir-Os-Fe-misc or grains with "layered" structure. The residual liquid of the magamatic melt enriched with Pt, Fe, and Ir fills the voids in the earlier osmium and iridium. The negative crystals of Ir-rich isoferroplatium, sometimes with sperrylite in the intergrowths are crystallized from them.
The insignificant share (10 %) of laurite and ruthenium trend with concentration of Ru in Os-Ir-Ru alloys more than 60 at.% are the evidences of moderate fugitiveness So at the magamtic stage in comparison with the known ophiolite complexes of the world (Garuti et al., 1999), where laurite is a predominant mineral. Further evolution of platinum mineralization developed at the post-magamtic stage, one of the processes of which could be serpentinization of the ultrabasic rocks. Gas-saturated serpentenizing fluids, enriched with Fe and Cu, had transformed the Pt-Fe alloys to tetraferroplatinum, tulameenite and Os-Ir-Ru-Fe compounds (ap to 18.5 mas.% Fe), like hexaferrum. Participation of S and As in these solutions resulted in formation of the late sulfides and sulfoarsenides, which were localized along the fractures in the primary minerals and in their intergranular space.
We acknowledge the financial support from grant of RFBR # 06-05-65055
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