CC BY
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Research article
UDC 552.111:550.4 (470.21)
Magma feeding paleochannel in the Monchegorsk ore region: geochemistry, isotope U-Pb and Sm-Nd analysis (Kola region, Russia)
Valeriy F. SMOLKIN'H, Artem V. MOKRUSHIN2, Tamara B. BAYANOVA2, Pavel A. SEROV2, Aleksey A. ARISKIN3
1 Vernadsky State Geological Museum, Russian Academy of Sciences, Moscow, Russia
2 Geological Institute of the Kola Science Centre, Russian Academy of Sciences, Apatity, Russia
3 Lomonosov Moscow State University, Moscow, Russia
How to cite this article: Smolkin V.F., Mokrushin A.V., Bayanova T.B., Serov P.A., Ariskin A.A. Magma feeding paleochannel in the Monchegorsk ore region: geochemistry, isotope U-Pb and Sm-Nd analysis (Kola region, Russia). Journal of Mining Institute. 2022. Vol. 255, p. 405-418. DOI: 10.31897/PMI.2022.48
Abstract. A comprehensive study of a 340 m thick lenticular-sheet body of ultramafic composition penetrated by structural well M-1 at a depth of about 2.2 km was accomplished. Its main volume is composed of plagioharzburgite; finegrained rocks of norite and orthopyroxenite chilling zones are preserved on endocontacts. The rocks of the body are similar in composition to the rocks near the underlying ore-bearing layered intrusion - the Monchepluton. The age of intrusion of the ultramafic body is 2510 ± 9 Ma (U-Pb, ID-TIMS, zircon) and, taking into account analytical errors, is comparable with the formation period of the Monchepluton (2507-2498 Ma). According to the study of the Sm-Nd system in rocks and minerals, a positive value of the gNd (+1.1) parameter was established, similar to that in dunites and chromitites of the Monchepluton. Based on these results, the ultramafic body penetrated at depth was assigned to the magma feeding paleochannel through which the ultramafic, weakly contaminated magma entered the overlying magma chamber. This body is a unique example of a magma-feeding system for the ore-bearing layered intrusion of Precambrian age.
Keywords: Kola region; Paleoproterozoic; layered intrusions; gabbro-anorthosites; Monchepluton; Monchetundra massif; geochemical analysis; isotopic U-Pb and Sm-Nd systems
Acknowledgments. The research was carried out under the research programs 0226-2019-0053 and the grant of the Russian Science Foundation N 21-17-00161.
Received: 11.04.2022 Accepted: 15.06.2022 Online: 26.07.2022 Published: 26.07.2022
Introduction. Paleoproterozoic layered intrusions composed of mafic and ultramafic rocks are widespread in the Kola-Lapland-Karelian province, the most ancient part of the Fennoscandian Shield [1, 2]. One of the test sites for the study of layered intrusions was the Monchegorsk ore region lying in the central Kola region (Fig. 1).
Layered ore-bearing intrusions of two age groups occur in the Monchegorsk ore region - the Monchepluton (2.50 Ga), Imandra-Umbarechinsky complex (2.44 Ga), coeval massifs of the largest gabbro-anorthosite complex of the Main Range as well as the deposits and ore occurrences of sulfide Cu-Ni-PGE, low-sulfide platinum-metal, chromite and titanomagnetite ores confined to them. A good exposure and accessibility of the territory are also important.
In the central part of the Monchegorsk ore region, a 2.5 km deep structural well M-1 was drilled for detec-ting ore bodies (Fig.1). At a depth of 2037-2377 m, the well crossed the ultrabasic body which is considered to be a magma feeding channel. Earlier, such channels were virtually unknown in the Precambrian layered intrusions. However, they are very important for solving the problem of magma intrusion mechanism and its intrachamber crystallization. This paper presents the results of the studies aimed at solving the problem of channels through which magma could enter the above-lying magma chambers. The studies included geochemical, mineralogical, and isotope geochemical
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Fig.1. Scheme of geological structure of the Monchegorsk ore region (a) and section of structural well M-1 with indication of sampling sites and U-Pb zircon datings (b) a: 1 - sedimentary volcanic rocks of Imandra-Varzuga zone (<.2.44 Ga); 2 - norite-gabbro-norites of Imandra-Umbarechensky complex (2.44 Ga); 3 - gabbroids of Nyark-tundra; 4 - gabbro-anorthosites of the Main Range (2.51-2.47 Ga); 5 - basic-ultrabasic rock of the Monchepluton (2.50 Ga); 6 - amphibolites of the Late Archean greenstone belt; 7 - tonalitic gneisses, amphibolites and migmatites of Belomorian megablock (>2.80 Ga); 8 - granites, diorites, aluminous gneisses, ferruginous quartzites of the Central Kola megablock (2.83-2.70 Ga); 9 - tectonic faults; 10 - location of wells M-1, C-765; b: 1 - plagioharzburgites; 2 - orthopyroxenites; 3 - melanonorites; 4 - gabbronorites, gabbro-anorthosites, gabbro-pegmatites; 5 - dikes of harrisites, gabbro-norites, microgabbro, granophyres; 6 - dolerite dikes; 7, 8 - complexes of plagiogneisses (7) and hypersthene dio-
rites (8) and blastocataclasites after them in the fault zone; 2504 ± 6 is the U-Pb isotope age of zircon (Ma): 1 [3], 2 [4], 3 the present work
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(U-Pb zircon, Sm-Nd) analyses performed at the laboratories of IGEM RAS (Moscow), GI KSC RAS (Apatity) and CRPG-CNRS (National Centre for Scientific Research of France, Nancy).
Geological structure. The Monchegorsk ore region is confined to the junction of the Belomorian and Central Kola megablocks and the Imandra-Varzuga zones of Karelides [2]. The Belomorian megablock lying in the southwestern part of the region comprises tonalite gneisses, amphibolites, migmatites and pegmatites crushed into complex folds as a result of several deformation stages including viscous flow as well as plagiogranites with an age of 2.81 Ga [5]. The Central Kola megablock is composed of rocks of three complexes with an age of 2.83-2.70 Ga [2]. The first complex is represented by oligoclase granites, plagiogranites, and diorites forming dome structures; the second one, by biotite, biotite-amphibole, and sillimanite-garnet gneisses and ferruginous quartzites occurring in inter-dome spaces; the third one is fragmentarily composed of mafic and felsic metavolcanics of the Neoarchean greenstone belt. The Imandra-Varzuga zone of Karelides is represented by a trough-like structure filled with three metavolcanic rock sequences of mafic and intermediate composition separated by layers of tuffaceous sedimentary rocks and quartzites with a total thickness of 2.5-3.0 km. They rest with an angular unconformity on the Archean rocks as well as the eroded surface of the Monchepluton in the area of the Vurechuaivench foothills.
Two large intrusions occur in the central Monchegorsk ore region - the Monchepluton and the Monchetundra massif [3]. The Monchepluton is a typical layered multiphase intrusion composed of mafic-ultramafic rocks. The Monchetundra massif is part of the gabbro-anorthosite complex of the Main Range which separates the Belomorsk and the Central Kola megablocks. In the section of the
Monchetundra massif, gabbro-anorthosites predominate; orthopyroxenites, norites, gabbronorites, anorthosites as well as gabbro-pegmatites occur in subsidiary amounts. For a long time, the assumption of the Archean age of this complex prevailed [6, 7], but the results of U-Pb dating of zircon and baddeleyite indicate the intrusion of gabbro-anorthosite massifs in the Paleoproterozoic period [3, 8]. In the southern part of the region, chromite-bearing norite-gabbronorite intrusions of the Imandra-Umbarechensky complex occur, which were emplaced after the start of the Imandra-Varzuga zone formation.
The Monchepluton has an arcuate shape in plan and is composed of two magma chambers [3]. The northern chamber, 7 km long, is oriented to the northeast and shows up as peaks of the Nittis, Kumuzh'ya and Travyanaya mountains (NKT). The southern chamber, 9 km long, extends eastwards from the top of the Sopcha Mountain to Nyud and Poaz mountains (SNP) and further southeastwards to the Vurechuayvench foothills. Each of the chambers has a shape of a symmetrical trough with wings falling at angles of 30-40° (NKT) and from 40-45 to 20-25° with flattening to the axial parts (SNP). Both chambers are tilted to the southwest.
Composite section of the Monchepluton is made up of quartz norites and gabbronorites of the Basal Zone (BZ) and rocks of five megacycles (macrorhythms), each of them starting with rocks enriched in MgO. Within the Northern Chamber, the following megacycles are distinguished in ascending order: I - harzburgite-orthopyroxenite; II - dunite-harzburgite-orthopyroxenite; within the Southern Chamber: III - dunite-orthopyroxenite-norite; IV - harzburgite-norite-gabbronorite; V - gabbronorite-anorthosite. According to the authors of [9], megacycles formed as a result of pulsating intrusion of new portions of magma even prior to complete crystallization of the earlier megacycle. The intrusion of the Monchepluton occurred according to U-Pb dating of zircon in the period of 2507-2498 Ma [3, 8, 10, 11].
In the postintrusive period, during the emplacement of the Imandra-Varzuga zone, the internal structure of the Monchepluton was disturbed by tectonic movements, and rocks of the II megacycle (together with the Sopcheozero chromite ore deposit) were lowered relative to the Northern and Southern chambers. The main volume of rocks composing the Monchepluton is not metamorphosed, with the exception of the rocks of the Vurechuaivench foothills, which occur in the western part of the South Chamber and were metamorphosed under conditions of the epidote-amphibolite facies.
For the Monchepluton, two clearly defined trends in the change of contents of the principal rock-forming components have been established [9, 12]. The first trend comprises the series dunites -harzburgites - orthopyroxenites of the I, II and III megacycles which are characterized by a significant increase in SiO2 content with decreasing MgO. The second trend is the series norites - gabbronorites -anorthosites of the IV and V megacycles with a significant increase in the Al2O3, CaO contents at a relatively stable SiO2 as MgO decreases. For rocks of all megacycles, the same type of the REE distribution spectra normalized to chondrite were revealed with a slight excess of the LREE over the HREE, but with different degrees of differentiation. A characteristic feature of the rocks of all megacycles is a negative Nb and Ta anomaly and a positive Sr anomaly relative to the depleted mantle (DM).
Monchetundra massif is separated from the Monchepluton by a steeply dipping tectonic zone (Fig. 1). This zone is a tectonic mixture of blastocataclasites and blastomylonites after the Archean plagiogneisses and diorites, vein gabbroids and plagiogranites as well as metadolerite dikes cutting them. According to the data on Sm-Nd and Rb-Sr systems, in garnet-amphibole blastocataclasite after gabbro-anorthosite (well M-1, int. 1088.3 m), the fault was emplaced 2.0-1.9 Ga; the temperature reached 620-640 °C, pressure 6.9-7.6 kbar, which corresponds to the amphibolite facies conditions [13].
A significant part of the Monchetundra massif section, deep fault zone and rocks of the Archean basement were penetrated by structural well M-1 with a depth of 2472 m (Fig.1). Rocks of the Lower and Middle zones are also partially penetrated in the Loipishnyun gorge by shallow new wells MT-3, MT-25, MT-69, MT-70 and MT-79 with a depth of 250-355 m [4].
The following zones were distinguished within the central Monchetundra massif: the Lower (well M-1, int. 750-1020 m), the Middle (well M-1, int. 0-750 m) and the Upper (from the mouth of well M-1 to the peaks of the Monchetundra and Hipiknyunchorr, thickness is over 500 m) [3]. The Lower zone has the most heterogeneous composition. Its lower part (908-1020 m) is composed mainly of interbedded gabbronorites and norites, while the upper one is made up of orthopyroxenites, norites and gabbronorites. At a depth of 782 and 810 m, thin (about 10 m) harzburgite bodies occur grading at the contacts into melanocratic norites and orthopyroxenites. The Middle zone is composed of trachytoid, mesocratic, less frequently leucocratic gabbronorites; the Upper zone, of meso- and leucocratic gabbronorites and anorthosites. Later bodies of coarse-grained gabbro-norites and gabbro-pegmatites occur within all zones.
Composition points of the Monchetundra massif rocks are grouped in petrochemical diagrams in the form of segregated or partially overlapping fields, without forming, in contrast to the Monche-pluton, clearly defined trends [9]. Rocks of the Lower zone differ markedly from rocks of the Middle and Upper zones by higher contents of MgO, Cr and low contents of AhO3, CaO; and rock compositions of the Middle and Upper zones largely overlap. All the rocks of the Monchetundra massif are characterized by the same type of flat distribution of the REE spectra normalized to chondrite, a slightly pronounced excess of the LREE over the HREE, and a clear positive Eu anomaly. For them as well as for the Monchepluton, negative Nb and Ta anomalies and a positive Sr anomaly relative to the DM were also established.
Based on generalization of the results of U-Pb isotope analysis of zircon and baddeleyite in rocks of the Monchetundra massif [3, 4, 8], four phases can be distinguished: I (early) - 2521-2516 Ma (distinguished conditionally due to the metamorphic nature of the sampled rocks); II (main), 2507-2496 Ma (mean age 2502±5 Ma, six analyses); III (late) - 2476-2471 Ma (2473 ± 8 Ma, three analyses) and IV (pegmatoid) - 2456-2445 Ma (2451 ± 4 Ma, three analyses). These data indicate that the start of the Monchetundra massif and the Monchepluton intrusion occurred synchronously, but the total duration of their formation differs significantly. According to reconstructions of the P-T conditions of the Monchetundra massif and the Monchepluton formation different intervals of rock crystallization temperatures and pressures were determined: T = 1190-1000 °C and P = 5.3-6.4 kbar for the Lower zone of the Monchetundra massif, T = 1300-1200 °C and P = 3.0 kbar for the Monche-pluton [4]. Thus, the formation of the above intrusions occurred at different depths: of the Monche-tundra massif at a depth of about 20 km, and of the Monchepluton, at about 10 km. During the Svecofennian period of orogeny they spatially converged as a result of tectonic movements along the system of deep faults [13].
Samples for isotope studies. For the analysis of U-Pb and Sm-Nd isotope systems, two samples were taken from the core of wells M-1: M-75 (interval 2101-2102 m) and M-77 (interval 911-912 m) (Fig. 1).
Sample M-75 weighing 4.9 kg is composed of slightly altered melanocratic olivine gabbronorite occurring in the upper part of the ultramafic body. Its mineral composition (vol.%) is: olivine 35-40, orthopyroxene 40-45, clinopyroxene 5-10, interstitial basic plagioclase 13-15 partially replaced by chlorite, Cr-spinel as inclusions in olivine and orthopyroxene 2, amphibole 2-3. Rock structure is medium-grained, hypidiomorphic granular, poikiloophytic, kelyphitic.
Sample M-77 weighing 8.2 kg was taken from the Lower zone of the Monchetundra massif. It is composed of massive coarse-grained gabbronorite. Rock structure is poikiloophytic, characterized by inclusions of orthopyroxene grains in large prismatic crystals of mafic plagioclase. Clinopyroxene forms grains of subidiomorphic shape to 3-5 mm in size. Small grains of apatite are present. Ore minerals are represented by magnetite and ilmenite. Pyroxene grains are locally replaced by an amphibole aggregate with sulfide microinclusions. At the boundary of orthopyroxene and plagioclase grains, there is a kelyphitic rim of amphibole composition. In its outer part, elongated clinocoesite grains occur. The chemical composition of the sample (wt.%) is: SiO2 51.31, TiO2 0.28, AhOs 15.33, Fe2O3 10.94, MgO 9.60, CaO 8.44, Na2O 2.17, K2O 0.50, H2O 1.66, S 0.04.
Analytical methods. Concentration of the main and trace elements in rock samples was determined using the X-ray spectral fluorescence analysis (XRF) on a sequential vacuum spectrometer (with wavelength dispersion), model Axios mAX-Advanced manufactured by PANalytical (Netherlands). The spectrometer is equipped with a 4 kW X-ray tube with an Rh anode. The maximum voltage on the tube is 60 kV, the maximum anode current is 160 mA. When calibrating the spectrometer, branch and national standards, samples of chemical composition of rocks and mineral raw materials were used. Reference samples from the United States Geological Survey (USGS) were used as control samples. The analysis was performed according to the SIMS NSAM procedures, which ensure obtaining results of the III category of accuracy of quantitative analysis according to OST RF 41-08205-04. The analyses were carried out at the Mineral Substance Analysis Laboratory of the IGEM RAS, analyst A.I.Yakushev. Additionally, the analyses of rocks obtained by ICP-AES and ICP-MS methods (Nancy, France) were made.
Compositions of mineral phases were studied by X-ray spectroscopy using MS-46 CAMECA (GI KSC RAS, Apatity) and CAMECA SX 50 (National Research Centre CRPG-CNRS, Nancy, France).
Zircon populations and rock-forming minerals for U-Pb and Sm-Nd isotope analyses were isolated at the Laboratory of Substance Separation and Primary Sample Processing at the Geological Institute of the KSC RAS under the supervision of L.I.Koval using the standard separation procedure with electromagnets and heavy liquids. The anatomy of zircon crystals in reflected electrons (BSE) and cathodoluminescence (CL) was studied using LEO 1450 scanning electron microscope (Carl Zeiss, Germany) with a Quantax 200 energy dispersive analyzer (Bruker, Germany).
All isotope studies were carried out at the Laboratory of Geochronology and Isotope Geoche-mis-try of the Geological Institute of the KSC RAS. U-Pb zircon dating was performed using the ID-TIMS procedure. Zircon populations subsamples were preliminarily subjected to hydrothermal decomposition in concentrated (48 %) HF acid at a temperature of 205-210 °C for 1-10 days following the method of T.Krogh [14]. Isotope measurements were taken on a Finnigan MAT 262 seven-collector thermal ionization mass spectrometer. Isotope ratios were corrected for mass discrimination obtained by studying parallel analyses of SRM-981 and -982 standards and equal to 0.12 ± 0.04 %. The error in U-Pb ratios was calculated during the statistical calculation of parallel analyses of the IGFM-87 standards and was taken as 0.5 %; if the errors are higher, the real values are given in the table of isotope data. Point coordinates and isochrone parameters were calculated using the ISOPLOT software [15, 16]. Ages were calculated from the accepted values of the decay constants of uranium [17]; errors are given at the level of 2b. A correction for the admixture of ordinary Pb was made according to the model [18]. A correction for isotope composition of cogenetic plagioclases was also carried out in cases when the admixture of ordinary Pb exceeded 10 % of the total amount of Pb and 206Pb/204Pb isotope ratios were less than 1,000. The methodology was described in detail in the work [8].
Measurements of Nd isotope composition and Sm and Nd concentrations were carried out on a Finnigan MAT 262 (RPQ) seven-collector solid-phase mass spectrometer in a static double-filament mode using rhenium and tantalum filaments. The average value of 143Nd/144Nd ratio in the JNdr1 standard over the measurement period was 0.512081 ± 13 (N = 11). The error in 147Sm/144Nd ratios is 0.3 % (2o) - the average value of seven measurements in the BCR-2 standard [19]. Measurement error of Nd isotope composition in an individual analysis is to 0.01 % for minerals with low concentrations of neodymium and samarium. Blank intralaboratory contamination by Nd is 0.3 and Sm -0.06 ng. Determination accuracy of Sm and Nd concentrations is ± 0.5 %. Isotope ratios were normalized to 146Nd/144Nd = 0.7219 and then recalculated to the table-indicated ratio in the JNdr1 standard = 0.512115 [20]. Isochron parameters were calculated using ISOPLOT software [15]. When calculating the SNd(T) values and model TDm ages modern CHUR values were used after [21] (143Nd/144Nd = 0.512630, 147Sm/144Nd = 0.1960) and DM after [22, 23] (143Nd/144Nd = 0.513151, 147Sm/144Nd = 0.2136).
Results of geological, mineralogical, and geochemical studies. Well M-1 crossed at a depth of 2037-2383 m a 340 m thick intrusive body of ultramafic rocks with preserved endocontact chilling zones. Previously, similar rocks were drilled by the well 765 in the interval 1380-1600 m southwest of well M-1 (Fig. 1). Proceeding from these data, a conclusion can be drawn on the displacement (lowering) of the ultramafic body penetrated by the well M-1 relative to the fragment exposed by the well C-765 as well as its wedging out in the southwestern direction and a sheet-lenticular shape. The body is separated from the lower contact of the Monchetundra massif by a fault zone over one kilometer thick (Fig. 1). It has a clear intrusive relationship with the enclosing rocks of the Archean diorite complex exerting a pronounced thermal effect on them with formation of hornfels. At contacts with the enclosing rocks, rocks of endocontact chilling zones were found, their grain size ranging from small to fine when approaching the contacts. The upper zone about 10 m thick (of which 6.6 m are fine-grained) is composed of melanocratic norite; and the lower zone about 37 m thick (of which 2.5 m are fine-grained) is composed of orthopyroxenite.
Composition of minerals is given in Table 1; rock composition including the contents of the principal components, ore, trace, and rare earth elements, in Tables 2, 3.
The main volume of the intrusive body is filled with plagioharzburgite. It differs from harzburgite of the Monchepluton North Chamber by the presence of interstitial mafic plagioclase and a minor admixture of clinopyroxene. Rock structure is hypidiomorphic-granular, poikilitic. Mineral composition of plagioharzburgite (vol.%) is: 65-85 olivine (86.8-88.4 % Fo), 10-25 orthopyroxene (Fs = = 11.2-12.6 %), 6-15 plagioclase ( 43-53 % An), 3-5 augite (f = 10-13) and 1-3 Cr-spinel (Table 1). Olivine contains an admixture of NiO (0.09-0.62 %), Cr-spinel - TiO2 (0.22-0.71; 2.38 %), ZnO (0.10-0.73 %) and NiO (0.10-0.72 %). The composition of rock-forming minerals varies slightly through the section. One can note a slight increase in Fs in orthopyroxene when approaching the contacts of the body. At the olivine/plagioclase boundary, there is a single-layer kelyphitic reaction rim composed of a cross-fibrous amphibole aggregate. Plagioclase is partly replaced by chlorite.
Table 1
Composition of minerals from rocks of well M-1 based on microprobe analysis, wt.%
Depth, m SiO2 TiO2 &2O3 CaO Al2O3 FeO MnO MgO Na2O K2O ZnO NiO Sum Index
Olivine Fo, %
2145 40.04 0.30 - - - 12.33 0.68 45.83 - - - 0.17 99.34 86.9
2326.5 40.93 - 0.31 - - 12.39 0.09 46.34 - - 0.03 0.09 100.17 86.9
2326.5 41.06 - - - - 12.13 0.19 46.76 - - - 0.35 100.50 87.3
2326.5 41.13 - - - - 12.49 0.22 46.27 - - 0.04 0.39 100.54 86.8
2350.5 41.10 0.02 0.09 - - 11.03 0.09 47.13 - - 0.04 0.62 100.10 88.4
2350.5 41.04 0.04 - - - 11.03 0.11 46.95 - - - 0.60 99.77 83.5
Orthopyroxene Fs, %
2145 55.745 0.258 0.207 1.118 0.919 8.026 0.129 32.569 - - - 0.753 99.724 11.3
2326.5 54.726 - 0.327 0.963 1.422 8.682 0.164 32.581 - - - 0.091 98.956 12.0
2326.5 55.510 - 0.276 1.299 1.610 8.775 0.205 32.450 - - - 0.091 100.216 12.6
2350.5 56.238 0.222 0.621 0.302 1.339 8.120 0.114 32.845 - - 0.029 0.181 100.011 11.7
2350.5 56.311 0.094 0.397 0.452 1.484 7.927 0.095 32.595 - - 0.038 0.166 99.559 11.5
Plagioclase An, %
2145 53.490 - - 11.140 27.422 0.618 - - 6.345 0.054 - - 99.069 49.1
2145 54.010 - - 11.060 27.122 0.051 - - 6.707 0.043 - 98.993 47.6
Cr-spinel Mg#
2145 0.385 - 45.053 - 12.424 33.176 - 4.908 - - 0.381 0.11 96.437 25.94
2188.8 0.066 0.650 41.145 - 12.826 33.79 0.574 6.473 - - 0.295 0.095 95.914 33.67
2188.8 0.117 0.708 42.432 - 13.602 31.662 0.601 6.827 - - 0.219 0.047 96.215 35.11
2326.5 0.665 2.384 35.271 - 14.346 35.035 0.641 6.745 - - 0.162 0.111 95.360 33.58
2326.5 - 0.225 35.717 - 13.926 37.536 0.500 7.003 - - 0.477 0.095 95.479 36.64
2350.5 0.442 0.284 39.977 - 16.153 30.647 0.351 7.097 - - 0.409 0.330 95.690 36.91
2350.5 0.559 0.341 41.939 - 14.910 29.574 0.364 7.007 - - 0.143 0.362 95.199 36.63
2350.5 - 0.235 42.003 - 17.566 29.238 0.364 6.303 - - 0.257 0.723 96.689 32.65
Note. Fo - forsterite, mol.%, Fs - ferrosilite, mol.%, An - anorthite, mol.%, Mg# = 100xMg/Mg + Fe (a.p.f.u).
Table 2
Composition of rocks from well M-1 based on XRF data, wt.%, ppm
Depth, m
Components 2037.4 2096.7 2153.2 2167.3 2196.5 2216.2 2239 2285.5 2302 2318.8 2339.3 2351.3 2383.3
SiO2 52.89 41.62 41.39 40.83 40.9 40.67 40.56 40.5 40.96 42.3 43.85 49.53 49.33
AI2O3 7.72 4.11 3.63 3.32 3.30 2.78 3.14 2.55 3.34 3.22 3.03 4.57 8.77
TiO2 0.30 0.18 0.15 0.17 0.12 0.12 0.16 0.12 0.14 0.14 0.13 0.12 0.33
Fe2O3to( 8.50 11.18 10.88 11.05 10.78 11.09 11.14 10.64 10.45 10.24 10.05 9.67 10.85
MnO 0.15 0.15 0.14 0.15 0.14 0.15 0.15 0.14 0.14 0.14 0.145 0.15 0.17
MgO 22.94 36.87 38.92 39.55 39.93 41.05 40.08 41.85 39.9 39.3 37.87 31.45 19.83
CaO 5.04 2.75 2.20 2.28 1.72 1.53 2.09 1.38 2.02 1.98 2.29 2.45 6.68
Na2O 1.12 0.56 0.63 0.58 0.49 0.45 0.54 0.39 0.40 0.52 0.47 0.59 1.38
K2O 0.12 0.24 0.13 0.17 0.10 0.14 0.17 0.15 0.13 0.13 0.08 0.09 0.29
i.l. 0.37 0.61 0.01 0.01 0.44 0.01 0.01 0.24 0.63 0.17 0.35 0.01 1.60
Sum 99.15 98.27 98.08 98.11 97.92 97.99 98.04 97.96 98.11 98.14 98.26 98.63 99.23
Cr 3566 8722 9718 9390 10698 10246 9826 10266 9285 9260 8764 6997 2892
Ni 815 2274 2507 2585 2630 2729 2675 2733 2604 2465 2149 1469 778
Note. 2037.4, 2383.3 - rocks of chilling zones; the rest, plagioharzburgites; i.l. - ignition loss.
Table 3
Composition of rocks from well M-1 based on ICP-AES and ICP-MS, ppm
Components Depth, m Components Depth, m
2043.6 2052.2 2128.8 2272.0 2373.8 2043.6 2052.2 2128.8 2272.0 2373.8
SiO2 54.65 46.13 43.14 41.91 50.90 Y 4.77 4.58 3.46 3.20 7.27
AI2O3 5.41 5.52 4.25 3.00 8.96 Zr 13.41 11.91 10.00 12.99 23.69
TiO2 0.20 0.20 0.16 0.15 0.30 Nb 0.18 0.24 0.24 0.58 0.62
Fe2O3<0< 7.99 10.85 10.79 10.28 10.43 Ba 75.82 88.99 74.79 71.22 167.44
MnO 0.14 0.18 0.17 0.16 0.16 La 3.25 3.70 3.24 3.34 6.94
MgO 26.70 32.25 37.14 40.87 20.75 Ce 6.94 7.91 6.41 7.15 14.26
CaO 3.96 3.80 2.58 1.70 6.18 Pr 0.91 1.06 0.86 0.95 1.81
Na2O 0.75 0.84 0.67 0.31 1.44 Nd 3.95 4.35 3.49 3.90 7.43
K2O 0.06 0.11 0.08 0.24 0.37 Sm 0.85 1.00 0.74 0.76 1.52
P2O5 0.09 0.09 0.09 0.10 0.10 Eu 0.27 0.29 0.25 0.19 0.47
i.l. 0.01 0.01 0.28 0.74 0.48 Gd 0.75 0.86 0.61 0.60 1.32
Sum 99.96 99.98 99.35 99.46 100.07 Tb Dy 0.12 0.79 0.13 0.72 0.10 0.60 0.10 0.56 0.21 1.18
V 96.06 86.00 73.65 63.11 126.14 Ho 0.15 0.17 0.13 0.13 0.24
Co 70.24 109.95 124.92 133.21 89.59 Er 0.51 0.46 0.32 0.32 0.77
Ni 1045.0 1933.0 2479.0 2822.0 810.88 Tm 0.08 0.08 0.05 0.05 0.10
Cu 102.90 45.73 20.71 13.69 44.74 Yb 0.47 0.49 0.34 0.34 0.72
Pb 1.64 1.40 1.14 1.20 2.31 Lu 0.09 0.09 0.06 0.05 0.11
Zn 54.39 80.26 79.86 76.14 79.18 Hf 0.44 0.35 0.26 0.32 0.70
Rb 1.97 2.94 2.49 9.05 10.70 Ta 0.02 0.02 0.01 0.04 0.04
Sr 86.58 100.43 80.41 48.44 166.17 Th 0.18 0.21 0.16 0.49 0.35
Note. Fe2Ü3« = FeO + 0.9 Fe2Û3.
Melanocratic norite is the last member in the series of endocontact rocks: plagioharzburgite -orthopyroxenite - melanonorite connected by gradual transitions. Mineral composition of melanonorite (vol.%) is: 40-70 orthopyroxene (f = 12-14 %), 25-40 plagioclase (51-56 % An), 5-10 augite (f = 10-13 %), 0.5-1 Cr-spinel and <1 sulfides.
Vertical section of the investigated body (Fig.2) reflects variations in the principal (SiO2, AhO3, Fe2O3tot) and ore (Cr, Ni) components. The section clearly shows a symmetrical distribution of the contents of rock-forming components associated with a decrease in MgO, Cr and Ni and an increase in SiO2 and AhO3 towards both contacts. This feature is in good agreement with the distribution of crystallization temperature, which is stable in the main volume of the body and decreasing at the contacts. There is a slight increase in MgO content in the lower and of Al2O3 content in the upper parts of the massif, which is associated with minor shows of the accumulation of olivine and
H0
© Valeriy F. Smolkin, Artem V. Mokrushin, Tamara B. Bayanova, Pavel A. Serov, Aleksey A. Ariskin, 2022
2000 m-
M-1
2050
2100
2150
2200
2250
2300
2350
2400
M-75
i ■ i ■ i ■ i ■ i
20 30 40 7 8 9 10 11 12 40
->—r
% #
1 i 1
i 1 i 1 i
T—'-1-'-1
50 60 0 2 4 6 8 10 1000 2000 3000 4000 8000 12000
MgO, wt.%
Fe203/o/, wt.%
I ]1
Si02, wt.% AI2O3, wt.% ■ 2 H3 04 H5
Ni, ppm
Cr, ppm
6
Fig.2. Schematic section of the ultramafic rock body penetrated by well M-1 (int. 2,037-2,337 m) and variations of the contents
of rock-forming and ore components in rocks, wt.%, ppm
1 - host rocks of the hypersthene diorite complex; 2 - melanonorites and orthopyroxenites of chilling zones; 3 - plagioharzburgites; 4 - according to ICP-AES (CRPC-CMRS) data; 5 - according to XRF (IGEM RAS) data; 6 - location of M-75 sample
b
100
10
o
c o oR
0.1
1-1-1-1-1-1-1-1-1-1-1-1-1—
La Ce Pr Nd S111 Eu Gd Tb Dy Ho Er T111 Yb Lu
1000
100
Q
S3
c
coR
10
0.1
Rb Tli Nb La Pr Sr Zr Eu Tb Ho Er Yb Ba U Ta Ce Nd S111 Hf Gd Dy Y T111 Lu
Fig.3. REE distribution spectra in rocks of the ultramafic body (a) normalized to chondrite [24], and spider diagrams (b) normalized to DM after [22]
a
1
plagioclase, respectively. Fe2O3tot shows a slight increase up the section and a disturbance of the trends near the contacts. The first is associated with a gradual decrease of crystallization temperature, and the second - with a relatively abrupt cooling of melt at the contacts. For Nd, a uniform distribution was established over most of the section within 3.49-4.35 ppm and an increase in its content to 7.43 ppm near the lower contact (Table 2).
Rocks that make up the studied massif are characterized by the same type of the REE distribution spectra normalized to chondrite, with a slight excess of the LREE over the HREE and a minor degree of differentiation (Fig.3).
Their characteristic feature is a negative Nb- and Ta-anomaly and a positive Sr-relative to DM (Fig.4). Compared to the rocks of the Monchetundra massif, they lack a positive Eu-anomaly. Judging by the REE spectra and spider diagrams, rocks of the sheet body show the greatest similarity with rocks of the South Chamber of the Monchepluton [9].
Fig.4. Photomicrographs of zircon grains from samples M-75 (a-d) and M-77 (e-h) taken in reflected electron (BSE) and cathodo-
luminescence (CL) modes
Results of isotope studies. Sample M-75. Zircon monofraction (0.50 mg) was divided into four populations. The first of them is represented by fragments of large crystals. Their average size is 0.175 x 0.175 mm (Ec - 1). Light yellow colour, vitreous lustre, slightly corroded surface. Minor zoning is recorded in the marginal zone. Grains contain a fine network of microcracks; the marginal zone is fissured. Zircon population 2 - short prismatic crystals with well-defined pyramid and pina-coid faces and slightly rounded edges (Fig.4, a, c). Their average size is 0.175 x 0.175 mm (Ec - 1). Crystals are limpid, light yellow, with vitreous lustre. The surface is not corroded. Crystals contain a large nucleus with coarse zoning. Cracks are radiating from the nucleus. In the wide marginal part, a primary oscillation rhythmic zoning of magmatic type is recorded. Zircon population 3 is represented by small fragments of crystals of various shapes. Their average size is 0.170 x 0.170 mm (Ec - 1). Grains are limpid, light yellow with a vitreous lustre. Zircon population 4 contains elongated prismatic, intensely cracked crystals (Fig.4, b, d). Their average size is 0.24 x 0.105 mm (Ec - 2.2). Milky colour, vitreous lustre. In BSE and CL, intraphase heterogeneity was recorded due to the processes of post-intrusive metamorphic alterations. The results of U-Pb isotope analysis of zircon are presented in Table 4.
Table 4
Results of U-Pb isotope analysis of zircon from olivine melanocratic (M-75) and coarse-grained (M-77) gabbronorites
N Weight, mg Concentration, ppm Isotope composition of lead Isotope ratios and age, Ma Rho
Pb U 206Pb/204Pb 206Pb/207Pb 206Pb/208Pb 207Pb/235U 206Pb/238U 207Pb/206Pb
Magmatic zircon M-75
1 0.10 365.75 524.58 977 4.7720 1.7623 9.76278 0.431079 2500 0.99
2 0.20 95.98 168.93 6247 5.8814 2.2971 9.23281 0.409738 2508 0.92
3 0.10 211.83 335.50 828 5.3433 1.5995 8.84685 0.392795 2491 0.89
Xenogenic zircon M-75
4 0.10 28.19 35.16 1171 5.1455 2.1929 14.1301 0.536971 2789 0.97
Magmatic zircon M-77
1 0.40 153.45 478.32 4404 6.3914 5.5386 9.17510 0.420551 2436 0.93
2 0.10 435.73 797.9 1359 6.0709 1.6570 7.58804 0.354236 2406 0.95
3 0.10 209.05 400.5 1866 6.0811 1.5646 7.10140 0.332917 2431 0.83
4 0.40 374.48 926.2 3114 6.4656 1.8958 5.76009 0.277478 2352 0.96
a 0.56
0.52
0.48
C2.
P
0.44
0.40
0.36
10 12
207pb/235U
14 16
b 0.48
0.44
0.40
x>
P
0.32
0.28
0.24
6 8
207pb/235u
10
0.5128 0.5123
0.5118
d
| 0.5113 £
0.5108 0.5103
0.5098
0.5130
0.02 0.06 0.10 0.14
147Sm/144Nd
0.18
0.5120
0.5110
0.5100
M-77
2448±26 Ma sNd(T) = -0.3±0.5 MSWD = 1.9
Cpx+Opx Opx+Cpx
0.04
0.10 0.16
147Sm/144Nd
0.22
0
8
4
2
d
c
Fig.5. U-Pb diagrams with concordia for zircon (a, b) and mineral Sm-Nd isochrone (c, d) for melanocratic olivine (M-75) and coarse-grained (M-77) gabbronorites WR, M-75, M-75/1 - sample; Pl - plagioclase; Opx - orthopyroxene: Cpx - clinopyroxene; Mgt - magnetite; Ilm - ilmenite
On the U-Pb diagram with concordia (Fig.5, a), analytical points of zircon populations 1, 2, and 3 are approximated by discordia with an upper intersection at 2510 ± 9 Ma (MSWD = 1.4); the lower intersection equal to zero reflects the current Pb losses. Analytical point for population 4 lies on concordia with an age of 2780 ± 10 Ma. Based on the shape of grains, their internal structure and the results of U-Pb isotope analysis, populations 1, 2, and 3 are combined into a morphological type which characterizes zircon of magmatic genesis; population 4 should be assigned to the metamorphosed type. They differ markedly in U content (169-524 and 35 ppm). The result obtained, 2510 ± 9 Ma, coincides, within the analytical error, with the age of magmatic zircon and baddeleyite from intrusive rocks of the Monchepluton (2507-2498, average 2502 ± 5 Ma) [3, 10, 11, 8]. The age of 2780 ± 10 characterizes the metamorphism of xenogenic zircon which corresponds in age to one of the metamorphic stages of granite gneisses from the basement of the Monchegorsk ore region.
Sample M-77. Zircon monofraction weighing 1.0 mg is divided into four populations. Population 1 is represented by crystals of an elongated prismatic and dipyramidal habit (see Fig.4, e, g). Crystals are
transparent, light yellow with a vitreous lustre, the surface is not corroded. The average dimensions are 0.175 x 0.06 (Ec - 2.9). Weight of an average crystal is 25.2-10-6 g. Crystals contain a nucleus of di-pyramidal prismatic habit with a weakly pronounced intraphase inhomogeneity. The main part of the crystal has a well-defined primary oscillation magmatic zoning parallel to the outer faces. Population 2 contains lamellar fragments of crystals. Grains are limpid, light yellow with a greasy lustre; the surface is slightly corroded. Their average dimensions are 0.175 x 0.105 (Ec - 1.6). Weight of an average crystal is 7.7 10-6 g. Grains contain an internal seed of an oval irregular shape with fine rhythmic zoning. In the outer thin rim, a thin rhythmic growth zoning was also recorded. Population 3 is represented by fragments of semi-oval crystals. The average grain size is 0.140 x 0.105 (Ec - 1.3). Weight of an average crystal is 6.1 • 10-6 g. Grains are transparent, yellowish-brown with a greasy lustre; the surface is corroded. Outer zone is cracked. Grains contain a polygonal nucleus slightly expressed in BSE and CL with an intraphase inhomogeneity. There are relics of a thin rim. Population 4 contains fragments of zircon crystals of indefinite shape with minor inclusions (baddeleyite?). Grains are limpid, light yellow with a vitreous lustre. The surface is not corroded. The average grain size is 0.140 x 0.105 (Ec - 1.3). Weight of an average crystal is 6.1-10-6 g.
Grain shapes, their internal structure reflecting the growth processes in magmatic melt and the results of U-Pb isotope analysis of populations 1-4 characterize zircon of magmatic genesis. U content in this zircon varies from 400 to 926 ppm (Table 4), which is comparable with zircon from gabbro-norites in the Middle zone of the Monchetundra massif [3].
On the U-Pb diagram with concordia, the analytical points for four populations of zircon from sample M-77 are approximated by discordia with the upper intersection at 2451 ± 9 Ma (MSWD = = 0.76) (Fig.5, b). The lower intersection at 326 ± 20 Ma corresponds to the emplacement stage of major alkaline syenite intrusions (Khibiny, etc.). The age of 2451 ± 9 Ma does not coincide with the age of zircon from norites (2500 ± 2 Ma) and orthopyroxenites (2,496 ± 3 Ma) occurring within the Lower zone of the Monchetundra massif [4] but corresponds to the age of zircon from rocks of the IV Postintrusive pegmatoid phase that are cutting rocks of the Middle and Upper zones of the Monchetundra massif (2456-2445 Ma). These results testify to a heterogeneous structure of the Lower zone of the Monchetundra massif which comprises rocks aged from 2500 to 2450 Ma.
The results of Sm-Nd studies of samples M-75 and M-77 are presented in Table 5. For M-75, the sample, plagioclase and pyroxene subsamples were measured; for M-77, the sample, pyroxene, magnetite, and ilmenite subsamples were measured. The samples differ in Nd content in silicates and rock - 3.70-4.77 and 5.13-6.83 ppm, respectively.
Table 5
Results of isotope Sm-Nd analysis of samples M-75 and M-77
Subsample Concentration, ppm Isotope ratios Tdm, Ma ENd(T)
Sm Nd 147Sm/144Nd 143Nd/144Nd
M-75
WR-1 0.858 4.14 0.1253 0.511542±18 2762 +0.9
WR-2 0.747 3.70 0.1222 0.511493±8 2749 +0.9
Pl 0.391 4.77 0.0495 0.510332±12
Opx 0.536 1.914 0.1692 0.512259±13
Opx+Cpx 1.685 6.40 0.1591 M-77 0.512085±11
WR 1.099 5.13 0.1296 0.511529±28 2925 -0.3
Pl 0.752 6.46 0.0704 0.510585±12
Cpx+Opx 2.149 6.83 0.1902 0.512507±11
Opx+Cpx 1.893 6.17 0.1854 0.512451±14
Mgt 0.725 3.05 0.1437 0.511791±18
Ilm 1.930 7.41 0.1574 0.511985±15
Gr 1.053 3.38 0.1886 0.512221±9
For olivine gabbronorite (sample M-75). an isochron age of 2435 ± 25 Ma was obtained, MSWD = 0.5 (Fig.5, c). It differs from the U-Pb age of zircon, apparently, due to local alteration of plagioclase. For coarsegrained gabbronorite (sample M-77), the isochron age is 2448 ± 26 Ma, MSWD = 1.9 (Fig.5, d). It coincides within the analytical error with the U-Pb age of zircon. For sample M-75, a positive value of SNd (+1.1) parameter was determined; for sample M-77, it is negative (-0.3). Samples also differ in model age of the original magma protolith: 2.75 and 2.92 Ga respectively.
Discussion. The studied intrusive body of ultramafic rocks intersected by well M-1 is separated from the Monchetundra massif by a thick zone of blastocataclasites and blastomylonites and, therefore, cannot be included in its composition. The body is also not a younger intrusion, since the age of its emplacement coincides with the age of the Monche-pluton and the beginning of the Monchetundra massif formation. By its structure, the investigated body is an independent intrusive massif with preserved chilling zones and a slightly manifested intrachamber differentiation. These data allow assigning it to the fragment of a magma feeding system in the form of a paleochannel through which ultramafic magma could enter the overlying magma chamber.
MgO-SiO2 diagram (Fig.6) displays the composition fields of plagioharzburgites of the magma paleochannel and rocks of the Monchepluton (I-V megacycles), the Monchetundra massif (three zones) as well as komatiitie-basalts from the Vetreny Belt with an age of 2.41 Ga. In this diagram, plagioharzburgites of the paleochannel fall into the field of the Monche-pluton harzburgites differing from them by higher contents of AhO3 and CaO (twofold) at the same MgO content [9]. The diagram also reflects clear differences in differentiation in komatiitic basalts volcanics which are assigned to comagmatic formations of layered intrusions [25, 26] and in the Monchepluton. The differences in differentiation trends are due to the fact that crystallization of the olivine phase played the main role in volcanic rocks, while in the Monchepluton and other layered intrusions of the Paleoproterozoic age in the region, a more complex grading of olivine-chromite and olivine-orthopy-roxene paragenesis into orthopyroxene-plagioclase paragenesis occurred. A complicating factor is the repeated flow of melts differing in composition and minera-lization into the magma chamber.
SNd-r (Ma) diagram (Fig.7) plotted from the results of Sm-Nd analysis displays the data on the rocks of the paleochannel as well as the Monchepluton and Monchetundra massif with correction of the age of rocks according to the U-Pb analysis of zircon and baddeleyite. Most of analytical points lie in the area of negative values of the SNd parameter, which is typical for most of the layered ore-bearing intrusions of Paleoproterozoic age in the Kola-Lapland-Karelian province [8, 27, 28]. This feature, according to the authors of [3, 29], is due to the processes of assimilation and contamination of highly metamorphosed host rocks in deep and intermediate chambers within the lower and middle crust.
MgO, wt.%
Fig.6. Petrochemical SiO2-MgO diagram with fields of rock composition of ultramafic body (well M-1), the Monchepluton
and Monchetundra massif 1-3 - rocks I-III (1), IV (2) and V (3) of the Monchepluton megacycles; 4-5 - rocks of the Lower (4), Middle and Upper (5) zones of the Monchetundra massif; 6 - ultramafic body (UB); 7 - komatiitic basalts of the Vetreny Belt after [25, 26]
Positive values of the SNd parameter established for rocks of the paleochannel (+1.1), dunite and chromitite of the Sopcheozero deposit of the Monchepluton (+2.5 and +2.9) and orthopyroxe-nite from the Lower zone of the Monchetundra massif (+1.7), whose age is within 2510-2496 Ma (average 2501 ± 6 Ma), deserve special attention. These data point to the occurrence of rocks with mantle marks, which represent slightly contaminated mantle matter in the investigated rock massifs. From this an assumption about the supply of magmas from different deep or intermediate chambers and about an incomplete homogenization of magmas in the magma chamber follows. This conclusion allows to account for the occurrence in the Monchepluton of deposits of different metallogeny - chromite and sulfide Cu-Ni ores. The data obtained also indicate that thin bodies of ultramafic rocks occurring within the Lower zone of the Monchetundra massif [3, 4] are independent magma injections. Apparently, they are also the product of the common magma feeding system, which was in the junction area of the Monchepluton and the Monchetundra massif.
Conclusions. The 340 m thick lenticular-sheet body of plagioharzburgites occurring at a depth of about 2.2 km is a fragment of a magma feeding system in the form of a paleochannel, through which high-magnesium magma ascended into the overlying magma chamber. Rocks of the investigated body are quite similar in mineral composition, geochemical and isotope features to rocks of the Monchepluton, which is a classic example of a layered ore-bearing intrusion. The intrusion age determined from U-Pb isotope analysis of zircon (ID-TIMS) is 2510 ± 9 Ma, which is comparable to the period of the Monchepluton formation. The studied geological body is a unique example of a magma feeding system for an ore-bearing layered intrusion of Precambrian age.
The authors are grateful to Daniel Ohnenstetter and Evgeniy Sharkov for their assistance in the analytical work and to the reviewers for their constructive comments.
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sNd 3 -i
2 1
0
2450
2500
2550 -1
T, Ma
1 2
3
4
-1 -2
-3 -4
Fig.7. Primary SNd(T) ratio in rocks of the ultramafic body of Monchepluton and Monchetundra massif relative to age (T, Ma) 1 - the Monchepluton; 2 - dunite and chromitite of Sopcheozero deposit; 3 - ultramafic rocks; 4 - Monche-, Chuna-and Volchjatundra massifs. Data used: 1 - [3, 8, 27, 28]; 2 - [10]; 3 - present work; 4 - [3, 4, 8] Age is corrected from the results of U-Pb isotope analysis of zircon [3, 7, 8, present work]
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Authors: Valeriy F. Smolkin, Doctor of Geological and Mineralogical Sciences, v.smolkin@sgm, https://orcid.org/0000-0002-1400-6310 (Vernadsky State Geological Museum, Russian Academy of Sciences, Moscow, Russia), Artem V. Mokrushin, Candidate of Geological and Mineralogical Sciences, Senior Researcher, https://orcid.org/0000-0002-5586-8902 (Geological Institute of the Kola Science Centre, Russian Academy of Sciences, Apatity, Russia), Tamara B. Bayanova, Doctor of Geological and Mineralogical Sciences, Chief Researcher, https://orcid.org/0000-0001-7281-8707 (Geological Institute of the Kola Science Centre, Russian Academy of Sciences, Apatity, Russia), Pavel A. Serov, Candidate of Geological and Mineralogical Sciences, Leading Researcher, https://orcid.org/0000-0003-0930-0301 (Geological Institute of the Kola Science Centre, Russian Academy of Sciences, Apatity, Russia), Aleksey A. Ariskin, Doctor of Geological and Mineralogical Sciences, Professor, https://orcid.org/0000-0002-5081-2731 (Lomonosov Moscow State University, Moscow, Russia).
The authors declare no conflict of interests.
