Abstract

The Itchayvayamsky Ural-Alaskan-type massif is located in the northernmost part of the Koryak-Kamchatka Platinum Belt (Russia) and is surrounded by two rivers (Kamenistaya and Itchayvayam) that both contain specific PGM (platinum group minerals) assemblages. The PGMs of the Kamenistaya River are represented by isoferroplatinum with minor admixtures of Pd and Cu (up to 1.97 and 1.37 wt.%, respectively) and numerous inclusions of other PGMs such as laurite, erlichmanite, and boweite. The Itchayvayam River assemblage is different, as it contains minor amounts of isoferroplatinum and other minerals identical to those of the Kamenistaya River, as well as abundant native platinum rich in Cu and Pd (up to 4.30 and 2.34 wt.%, respectively) and rimmed by secondary cooperite and malanite. Grains of gold intergrown with unnamed Au-Hg-Pd minerals, cooperite, vysotskite, and malanite were also found exclusively in the Itchayvayam River. Three mineral assemblages are spatially related to the Itchayvayam massif and are distinguished by: (1) isoferroplatinum, (2) native platinum, and (3) gold-mercury-palladium. Isoferroplatinum assemblages are typical for placers derived from Ural-Alaskan-type massifs and lodes (e.g., the Galmoenan massif in the Koryak Highlands). Simultaneously, the native platinum assemblages are rare and have been described as lodes in clinopyroxenites, and in sub-economic placers derived from clinopyroxenite-dominant massifs. Therefore, it is assumed that the origin of the first assemblage is from lodes in dunite and the second is from lodes in clinopyroxenite. In addition, gold-mercury-palladium assemblages apparently originated from mineralized zones, analogous to those intruding gabbro and monzonite at the nearby Barany massif, which contains native gold and Pd-sulfides. Our study of the structure and compositions of Pt-Fe minerals showed that in the structure of isoferroplatinum Cu and Ni substitute for Fe, whereas in native platinum they substitute for both Pt and Fe.

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