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

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) 5 P. 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-OsS 2 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) 5 P 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.

Tsikourasite, Mo3Ni2P1+x (x < 0.25), a New Phosphide from the Chromitite of the Othrys Ophiolite, Greece

“…Reducing conditions have been described during the serpentinization of peridotites [34]. These observations allowed us to postulate that the phosphides and the associated minerals found in the ophiolitic chromitites of Othrys and Gerakini-Ormylia (Greece), and Alapaevsk (Russia) may have crystallized during the serpentinization process at a low temperature [14][15][16]. This assumption was supported by the discovery of the Ni equivalent of tsikourasite in situ and was associated with the serpentine rich matrix of the Alapaevsk chromitite of Russia [14].…”

“…Tsikourasite does not correspond to any valid or invalid unnamed mineral [29]. According to the calculated formula, tsikourasite could represent the Mo equivalent of the grains recently found in chromitites of the Alapaevsk and Gerakini-Ormylia ophiolites, located in Russia and Greece, respectively, characterized by the formula (Ni,Fe) 5 P [14,15]. Although tsikourasite is similar in composition to monipite (MoNiP) [7], to the recently discovered polekhovskyite (MoNiP 2 ) [11], and to the synthetic material MoNiP 2 [30], these phases are hexagonal and not cubic like tsikourasite.…”

“…These observations allowed us to postulate that the phosphides and the associated minerals found in the ophiolitic chromitites of Othrys and Gerakini-Ormylia (Greece), and Alapaevsk (Russia) may have crystallized during the serpentinization process at a low temperature [14][15][16]. This assumption was supported by the discovery of the Ni equivalent of tsikourasite in situ and was associated with the serpentine rich matrix of the Alapaevsk chromitite of Russia [14]. In a recent paper, Etiope et al [35] demonstrated, among several rocks of ophiolites in Greece, i.e., serpentinite, peridotite, chromitite, gabbro, rodingite, and basalt, that only chromitites, including those of the Othrys complex, hosted considerable amounts of abiotic methane.…”

“…Natural phosphides are very uncommon minerals and only 15 phases, listed below in alphabetic order, have been officially approved by the IMA: allabogdanite (Fe,Ni) 2 P [1], andreyivanovite Fe(Cr,Fe)P [2], barringerite (Fe,Ni) 2 P [3], florenskyite Fe(Ti,Ni)P [4], halamishite Ni 5 P 4 [5], melliniite (Ni,Fe) 4 P [6], monipite MoNiP [7], murashkoite FeP [8,9], negevite NiP 2 [5], nickelphosphide (Ni,Fe) 3 P [10], polekhovskyite MoNiP 2 [11], schreibersite (Fe,Ni) 3 P [12] and references therein, transjordanite Ni 2 P [5], and zuktamrurite FeP 2 [13]. Recently, Zaccarini et al [14] and Sideridis et al [15], based on microprobe analyses, reported the presence of a new phosphide, characterized by the stoichiometry (Ni,Fe) 5 P, in the ophiolitic chromitites of Gerakini-Ormylia, Greece and Alapaevsk, Russia. Ifandi et al [16] and Zaccarini ei al.…”

“…They were tentatively classified as either Ni-allabogdanite or Ni-barringerite and either V-allabogdanite or V-barringerite and V-sulphide. All the described phosphides in the recent papers [14][15][16][17] were too small to be studied by X-ray diffraction.…”