Chemical composition and origin of platinum-group minerals from placers of the Aunik River, Buryatia, Russia

Abstract

Platinum group minerals (PGMs) are occasionally encountered in gold placers around the world. Most research related to the gold placer deposits focuses on the origin, migration and concentration of gold, the primary commercial mineral in the placers. Both origin and fate of PGM are rarely studied. The present study of PGM grains reveals complex relationships between the mineral phases within the grains, conforms to the published results on immiscibility within certain PGM solid solutions, presents evidence of alteration of PGMs, and shows the presence of unusual secondary minerals within the grains. The studied PGM grains are Os-Ir-Ru alloys and can be divided into two types based on their chemical composition. The first type is solid solutions enriched in Os and Ru bearing inclusions of silicates such as olivine, pyroxene and amphibole. The second type is enriched in Ir and Pt, with inclusions of sulfides, tellurides and selenides such as cuproiridsite, iridisite, malanite, cooperite, laurite, erlichmanite and tolovkite. These two types of PGM have been described in other regions and the first type was attributed to ophiolitic origin of the mineralization. The authors confirm this theory, and find that the PGM of the Aunik River placers likely originated from ophiolites of the Shamanskaya spreading zone, with a possible contribution from ultrabasic rocks of Ural-Alaskan type. The secondary mineralization of the PGM, especially prominent in the peripheral parts of many studied samples, is most likely the result of post-magmatic metasomatic alteration. Interestingly, an unnamed, new mineral phase (Ir,Os)Se2 was detected in a hydrothermally altered grain.