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Minerals 2016, 6(4), 108;

Platinum-Group Minerals and Other Accessory Phases in Chromite Deposits of the Alapaevsk Ophiolite, Central Urals, Russia

Department of Applied Geosciences and Geophysics, University of Leoben, Peter Tunner Str.5, A 8700 Leoben, Austria
Institute of Geology and Geochemistry, Ural Division of Russian Academy of Sciences, Str. Pochtovy per. 7, 620151 Ekaterinburg, Russia
Joint Stock Company Ural Geological Survey Expedition, str. Vaynera 55, 620014 Ekaterinburg, Russia
Author to whom correspondence should be addressed.
Academic Editor: Maria Economou-Eliopoulos
Received: 1 August 2016 / Revised: 2 October 2016 / Accepted: 10 October 2016 / Published: 19 October 2016
(This article belongs to the Special Issue Mineral Deposit Genesis and Exploration)
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An electron microprobe study has been carried out on platinum-group minerals, accessory phases, and chromite in several chromite deposits of the Alapaevsk ophiolite (Central Urals, Russia) namely the Bakanov Kluch, Kurmanovskoe, Lesnoe, 3-d Podyony Rudnik, Bol’shaya Kruglyshka, and Krest deposits. These deposits occur in partially to totally serpentinized peridotites. The microprobe data shows that the chromite composition varies from Cr-rich to Al-rich. Tiny platinum-group minerals (PGM), 1–10 µm in size, have been found in the chromitites. The most abundant PGM is laurite, accompanied by minor cuproiridsite and alloys in the system Os–Ir–Ru. A small grain (about 20 μm) was found in the interstitial serpentine of the Bakanov Kluch chromitite, and its calculated stoichiometry corresponds to (Ni,Fe)5P. Olivine, occurring in the silicate matrix or included in fresh chromite, has a mantle-compatible composition in terms of major and minor elements. Several inclusions of amphibole, Na-rich phlogopite, and clinopyroxene have been identified. The bimodal Cr–Al composition of chromite probably corresponds to a vertical distribution in the ophiolite sequence, implying formation of Cr-rich chromitites in the deep mantle, and Al-rich chromitites close to the Moho-transition zone, in a supra-subduction setting. The presence of abundant hydrous silicate inclusions, such as amphibole and phlogopite, suggests that the Alapaevsk chromitites crystallized as a result of the interaction between a melt enriched in fluids and peridotites. Laurite and cuproiridsite are considered to be magmatic in origin, i.e., entrapped as solid phases during the crystallization of chromite at high temperatures. The sulfur fugacity was relatively high to allow the precipitation of Ir-bearing sulfides, but below the Os–OsS2 buffer. The alloys in the system Os–Ir–Ru are classified as secondary PGM, i.e., formed at low temperature during the serpentinization process. The (Ni,Fe)5P phase is the first occurrence of a Ni-phosphide in terrestrial samples. Its composition indicates that it may be a new mineral. However, the small size has, so far, prevented a crystallographic study to support this conclusion. View Full-Text
Keywords: chromitite; platinum group minerals; silicate inclusions; Ni-phosphide; ophiolite; Alapaevsk; Urals; Russia chromitite; platinum group minerals; silicate inclusions; Ni-phosphide; ophiolite; Alapaevsk; Urals; Russia

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Zaccarini, F.; Pushkarev, E.; Garuti, G.; Kazakov, I. Platinum-Group Minerals and Other Accessory Phases in Chromite Deposits of the Alapaevsk Ophiolite, Central Urals, Russia. Minerals 2016, 6, 108.

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