Platinum-group minerals (PGM) nuggets from alluvial-eluvial placer deposits in the concentrically zoned mafic-ultramafic Uktus complex (Central Urals, Russia)

摘要

We report the results of a detailed mineralogical investigation of platinum-group minerals (PGM) and copper-gold nuggets from the Uktus Ural-Alaskan type complex in the Central Urals (Russia). The studied nuggets were sampled in alluvial-eluvial deposits from three small valleys, with temporary water flows, cutting across the Uktus massif. The volume of the washed samples varies from 0.03 to 0.08 m_3 and a few tens of PGM nuggets, ranging in size from about 100 mm to about 2 mm, were collected. According to their chemical composition, the most abundant PGM are native Ir-Os and alloys in the Pt-Fe-Cu-Ni system. The following less abundant PGM were also recognised: sulfarsenides of the irarsite-hollingworthite-platariste series, sulfides such as laurite, cuproiridsite, kashinite and the sulfantimonide tolovkite. One alloy corresponding to the formula Cu_3Au_2 was found, and proved to be Cu-rich tetraauricupride. The nuggets of Uktus have, in some cases, a polygonal shape. However, most of them have an irregular morphology and are characterised by a porous rim and zoning. The investigated nuggets occur as single-phase crystals or as polyphase grains, composed of different PGM. One nugget displays a very complex texture, being composed of a Pt-Fe alloy associated with osmium and Cu-rich tetraauricupride. These minerals are in contact with quartz that contains minute inclusions of hollingworthite and platarsite. The mineralogical similarity with the PGM inclusions in the Uktus chromitites indicates these rocks as a possible source for the PGM nuggets. The presence of faceted morphology in some nuggets suggests that they were mechanically liberated and transported for a relatively short distance from their lode deposits. The nuggets characterized by a rounded shape and occurring in association with quartz and Cu-rich tetraauricupride have probably been reworked in the placer environment. Therefore, in the Uktus placers deposits, two types of PGM nuggets can coexist: (i) primary with a magmatic origin, i.e., only mechanically liberated from their source rock, and (ii) secondary, i.e., reworked and grown in the placers. The mineralogical assemblage of the Uktus PGM nuggets, the fact that the Uktus PGM placers have never been mined and the recent exponential increase in demand for noble metals make the placer deposits associated with the Uktus complex potentially important for the economic recovery of these rare metals, at least on a small scale.