Abstract
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.
Highlights
The Uralian orogenic belt, which extends 2500 km from the Polar Urals to the Aral Lake, is considered the boundary between Europe and Asia
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
That form tiny phases, generally less than 10 μm in size and unevenly disseminated in the chromitite. In this contribution we present the results of a new, detailed study of chromite composition and the first data on accessory minerals such as silicates, platinum-group minerals (PGM), and a rare NiFeP compound found in certain chromite deposits that occur in the Alapaevsk ophiolite, located in the Central Urals
Summary
The Uralian orogenic belt, which extends 2500 km from the Polar Urals to the Aral Lake, is considered the boundary between Europe and Asia. That form tiny phases, generally less than 10 μm in size and unevenly disseminated in the chromitite In this contribution we present the results of a new, detailed study of chromite composition and the first data on accessory minerals such as silicates, PGM, and a rare NiFeP compound found in certain chromite deposits that occur in the Alapaevsk ophiolite, located in the Central Urals. We use this data to model the tectonic setting and genesis of the chromitites and their host ophiolite and to better understand the mechanisms of the precipitation of PGM and other rare accessory phases
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