The platinum-group minerals of the Baimka placer deposits, Aluchin horst, Russian Far East

An Alpine-type mafic-ultramafic complex and several Alaskan-Ural type (AUT) plutons are exposed in the Aluchin horst, Chukotka, Russian Far East. Plat inum-group element (PGE) mineralization is observed in placer deposits asso ciated with the occurrence of a local stratigraphic unit (Volga) of Late Ju rassic sediments. The platinum-group minerals (PGM) in these placer deposit s consist mainly of native platinum and grains of Pt-Fe alloy, with Fe cont ents ranging from 0.3 to 36.15 at.% and, rarely, iridium, cooperite and spe rrylite. Minerals present as inclusions in the grains of Pt-Fe alloy are: b owieite, cooperite, kashinite, erlichmanite, hollingworthire, irarsite, cup rorhodsite, laurite, Tr oxide (or hydroxide), osmium, prassoite, guanglinit e, rhodarsenide, magnetite, plagioclase, silicate glass inclusions and unna med PGE phases. Some PGM inclusions occur in assemblages of two or three ph ases indicative of a high-temperature origin for these nuggets. The data on PGM and other mineral inclusions in the grains of Pt-Fe alloy as well as t heir distribution and occurrence suggest that the primary source for the ma jority of PGM placer grains studied are AUT mafic-ultramafic complexes.