Vol 148, No 2 (2019)

The article represents results of geological, geochronological and isotope-geochemical studies of the alkaline-ultrabasic massif located in the lower reaches of the Iokanga River in the Karelian-Kola Alkaline Province. The alkaline-ultramafic massif is composed of nepheline clinopyroxenites, nepheline gabbroids, malignites, and nepheline syenites. These rocks are of Paleoproterozoic age. The age of crystallization determined for nepheline clinopyroxenites by the ⁴⁰Ar/³⁹Ar phlogopite method is 1955 ± 10.8 Ma, and Rb-Sr isochron over the entire pool of samples of the intrusive rocks corresponds to an age of 1937 Ma, at I_Sr = 0.70316. According to their isotope-geochemical Sm-Nd and Rb-Sr characteristics, rocks of the alkaline-ultrabasic massif in the lower reaches of the Iokanga River have the mantle origin, and the source of their melts was apparently a mantle of the BSE type.

Tiny chromian spinel precipitations associated with plastic deformed enstatite crystals from spinel peridotite were studied. It is defined that there are several localizations of the precipitations: 1) in recrystallization zones where they coexist with olivine and enstatite neoblasts, 2) in the volume of polygonized primary enstatite crystals as lamellae and holly leaf grains, 3) in lamellae and neoblasts of diopside and pargasite which were formed during a plastic deformation of primary orthopyroxene. In the first two cases, we suggest the solid-state mechanism of their formation. First, it is a result of an impurities segregation on the structure defects (dislocation → subgrain boundary → grain boundary). Second, it is a result of the nucleation and growth of the new-formed mineral phases (spinel, forsterite, enstatite II) in most distorted places of porphyroclasts. In the third case, it is possible that chromian spinel were appears either as a result of the solid-state crystallization or as a cumulate of «diopside» or «pargasite» melts formed by frictional melting of orthopyroxene porphyroclasts in micro-chambers.

Dalnegorskite — the new pyroxenoid with the crystal-chemical formula Ca₂Ca₂MnCa(Si₃O₉)₂, and the simplified formula Ca₅Mn(Si₃O₉)₂, is a rock-forming mineral in the boron-bearing calcareous skarns of the Dalnegorskoe boron-silicate deposit (Dalnegorsk, Primorsky Krai, Russia). It belongs to the structural type of bustamite and forms a continuous solid-solution series with the isostructural mineral ferrobustamite Ca₂Ca₂FeCa[Si₃O₉]₂. These pyroxenoids form thinly-radiated banded beige, pinkish-white and milky-white aggregates typically consisting of split thin acicular to fiber-like individuals and are associated with hedenbergite, datolite, andradite, galena, sphalerite, and pyrrhotite. D_meas. = 3.02(2), D_calc. = 3.035 g·cm^–3. Dalnegorskite is optically biaxial, negative, α = 1.640 (3), β = 1.647 (3), γ = 1.650 (3)°, 2V_meas. = 75(10)º. The average chemical composition of the holotype (electron microprobe data) is: MgO 0.23, CaO 40.02, MnO 5.02, FeO 3.64, SiO₂ 50.65, total 99.56 wt.%. The empirical formula calculated on 18 O atoms is Ca_5.03Mn_0.51Fe_0.36Mg_0.04Si_6.03O₁₈. The crystal structure of the new mineral was refined by powder X-ray diffraction data using the Rietveld method, R_p = 0.0345, R_wp = 0.0444, R₁ = 0.0790, wR₂ = 0.0802. Dalnegorskite is triclinic, P-1, a = 7.2588(11), b = 7.8574(15), c = 7.8765(6) Å, α = 88.550(15), β = 62.582(15), γ = 76.621(6)º, V = 386.23(11) ų, Z = 1. Dalnegorskite is distinctly different from the related mineral wollastonite in the infrared spectrum. The wave-numbers of maxima of strong bands in the characteristic region of Si—O stretching vibrations in the IR spectrum of dalnegorskite are (cm^–1): 905, 937, 1025, 1070. The type specimen of dalnegorskite is deposited in the collection of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia (No. 96201).

Kempirsai is the largest massif of ultramafites in the South Urals, it is well known for its unique chromite ore deposits. Grains of chromespinelides in massive chromitites of the «Diamond-Pearl» deposit contain small isolated inclusions of PGE minerals represented by solid solutions of Os—Ir—Ru and Rh—Zn—Pt (later system has been revealed in the massif for the first time), disulfides of the isomorphic laurite-erlichmanite series, and sulfides of the Ir—Ni—Cu system. Some grains of the metal Os—Ir—Ru solid solutions contain up to 45 wt % of ruthenium, extending previously known boundaries of the mixing range in this triad of elements. The main trend in the Os, Ru and Ir fractionation in sulfides is enrichment of disulfides in osmium and ruthenium; iridium is mostly concentrated together with Ni and Cu in sulfides with different metal-sulfur ratios. The process of mineral-forming of these PGE associations was developing in several stages. The earliest among them are metallic solid solutions of Os—Ir—Ru and disulfides of laurite-erlichmanite series; minerals of Rh—Zn—Pt and Ir—Ni—Cu systems are related to later events connected with repeated partial melting and injection of the newly formed melt into already formed restites, or processes of the mantle metasomatism.

The article displays data on formation conditions and features of unique corundum-spinel aggregates from the Thurein-Taung deposit in Myanmar (Burma). Chemical composition of corundum and spinel is given, and there is traced the effect of iron admixture on the coloring of spinel and parameters of its crystal lattice on the base of X-ray diffraction analysis. The surface of corundum on contact with spinel was studied in detail with aid of the color cathodoluminescence; it is shown that the process of overgrowing by spinel was preceded by resorption of corundum surface. Mineral and fluid inclusions in these minerals have been studied with use of microprobe analysis and the Raman spectroscopy. The temperatures of homogenization have been determined for liquid carbon dioxide in fluid inclusions inside corundum. On the base of mineral equilibria analysis in the Al—Mg—O—CO₂—H₂S—C system there were determined the optimal activities of magnesium and calcium and the fugacity of carbon dioxide. They are equal to а_Mg = 10^–5; а_Ca = 10^–1 and f_CO2 = 10^–4. It was found that corundum-spinel aggregates formed at temperature about 400 ºС as a result of partial dissolution of corundum protocrystal by high-temperature solutions containing Mg and Ca. Crystallization of the spinel took place with increasing pH (>3.5) and a decrease of redox potential (Eh ≤ –0.3 V).

Results of ontogenetic studies of the early paragenesis of ore minerals in zonal clinopyroxenite-dunite massifes of the Middle Urals are discussed. It is shown that the formation of these minerals occurs in the following order: iridosmium → chrome-spinel → laurite-erlichmanite → оsmiridium → isoferroplatinum. Revealed ontogenic features of ore phases are in good agreement with models suggesting the concentration of platinum group metals at the magmatic stage of evolution of the clinopyroxenite-dunite massif before display of the superimposed high-temperature plastic deformation.