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Article

Monchegorsk Mafic–Ultramafic Layered PGE-Bearing Complex (2.5 Ga, Kola Region, Russia): On the Problem of Relationships between Magmatic Phases Based on the Study of Cr-Spinels

by
Pavel Pripachkin
*,
Tatiana Rundkvist
,
Artem Mokrushin
and
Aiya Bazai
Geological Institute—Subdivision of the Federal Research Centre, Kola Science Centre of the Russian Academy of Sciences, Fersman St. 14, Apatity 184209, Russia
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(9), 856; https://doi.org/10.3390/min14090856
Submission received: 26 July 2024 / Revised: 16 August 2024 / Accepted: 22 August 2024 / Published: 24 August 2024
(This article belongs to the Section Mineral Deposits)
Browse Figures
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Abstract

:
The composition of Cr-spinels from rocks of the Monchegorsk layered complex (2.5 Ga) basically corresponds to the evolutionary trend that is typical for layered mafic–ultramafic intrusions (late magmatic phases contain Cr-spinels enriched in Fe and depleted in Mg, Cr, and Al). Cr-spinels within the Dunite Body of the Sopcha massif are almost identical to those within the Dunite Block rocks and are close to those from harzburgite of the NKT massif. Cr-spinels within the satellite bodies of the Ore Layer 330 are shown to have zonal structure, which confirms their origin from a new portion of melt, which may have been injected with several pulses. The composition of accessory Cr-spinels may indicate that the layered complex of rocks of the South Sopcha massif was formed from the most evolved portion of magmatic melt (linked with the Monchetundra intrusion), and its vein complex may be considered the one formed at the final stages of the magmatic system evolution. The composition of Cr-spinels from the Pentlandite Gorge mafic–ultramafic rocks may indicate that they are fragments of the NKT massif and not of the Monchetundra massif, as it was believed earlier.

1. Introduction

The Monchegorsk Paleoproterozoic layered mafic–ultramafic complex has been studied for almost a century. During this period, the very notion of the Monchegorsk complex was variously interpreted by different researchers. It has long been assumed that the Monchegorsk ore district located in the central part of the Kola Peninsula consists of two 2.5 Ga layered intrusions, i.e., mostly mafic Main Ridge complex (Monchetundra, Chunatundra, Volchetundra, Losevo–Medvezhya Tundra) and mostly ultramafic Monchegorsk pluton (or Monchepluton) (Figure 1a). Shown in the same figure, the Imandra–Umbarechka norite–gabbronorite complex has never been considered to be a part of the Monchegorsk complex because of its younger age (ca. 2.4 Ga).
Initially, the Main Ridge complex (MRC) was thought to be of the Archean age and more ancient than the Monchepluton [1,2]. This suggestion was based on an observation that the MRC rocks were highly metamorphosed, while most of the Monchepluton rocks seemed unaltered. Further geochronological studies have shown that both complexes are of the Early Proterozoic age (Table 1). Nonetheless, Table 1 demonstrates that despite the close ages of formation, the MRC and Monchepluton do have some peculiar features. First, the MRC formation lasted longer; second, the MRC datings are younger than those of Monchepluton.
The issue of relationships between the MRC and Monchepluton is clearly reflected in the history of the notion of ‘Monchegorsk complex’. As mentioned in [4], many geologists in the 1960s treated the notions of the Monchegorsk complex and Monchepluton as identical. Many authors have shared this opinion over the years [2,4,18,19,20,21] and some of them follow the point at present [22].
A different approach to the notion of the Monchegorsk complex appeared in the very same years. First, it was E.V. Sharkov [23,24] who united the MRC and Monchepluton in a single Monchetundra massif. Later, he and his colleagues began calling the MRC and Monchepluton a single ‘Monchegorsk layered mafic–ultramafic complex’ [25]. The same approach was shared by a number of other scientists [26,27].
Finally, probably because of the close proximity of the objects, the third version of interpretation of the concept of the Monchegorsk complex (MC) emerged. Some authors began using the ‘MC’ term meaning two objects, i.e., the Monchepluton and Monchetundra intrusion (part of the MRC contacting with the Monchepluton) (Figure 1a,b) [6,28]. Rather, due to the above-mentioned than petrological reasons, such an interpretation has become very convenient and has found support from other geologists [29,30].
It is worth noting that those who had initiated this approach avoided the term later and treated the Monchepluton and Monchetundra intrusions as independent formations [7,14]. Nevertheless, we will further discuss the MC using exactly this meaning (the Monchetundra intrusion/massif + Monchepluton) not only because of the spatial proximity of the objects but also because of their petrological and genetic links (in particular, there are data that some phases of the Monchetundra massif have intruded the Monchepluton).
The problem of interrelations between different units of the MC section (including the Monchepluton and Monchetundra intrusion) remains relevant currently [31,32,33,34], especially when it comes to our understanding of the sequence of magmatic phase injection, including ore phases (the most important ones are phases with sulfide Cu-Ni-PGE and chrome mineralization). Besides direct geological and isotope-geochemical data, there are recognized petrogenetic indicators that may effectively assist in such kind of research, i.e., Cr-spinels [35,36]. This article presents data on locational, morphological, and compositional features of Cr-spinels from different units of the MC. Most of the geological observations and chemical analyses of Cr-spinels were obtained from the objects of quite a disputable genesis. These objects are situated in a conjunction zone between the Monchetundra intrusion and Monchepluton (or a zone of the Monchetundra fault), i.e., the sites of Pentlandite Gorge and South Sopcha and the site of North-East Sopcha (Figure 1b), whose rocks contain fragments that may be considered to be the Dunite Block xenoliths (an issue of association of the xenoliths with the Monchetundra intrusion or Monchepluton is also disputable). Moreover, we have also studied Cr-spinels from the rocks of the Mine No. 5 site (harzburgite zone of the NKT massif) and Ore Layer 330 (the Sopcha massif) (Figure 1b), which rocks may belong to the different MC phases.

2. Geological Structure of the MC

2.1. General Information

As mentioned above, a number of authors [6,28,29,30] suppose that the MC consists of two large fragments, i.e., the Monchetundra intrusion (or massif) and Monchepluton (Figure 1a,b). Despite the close age (2497 ± 3–2501 ± 8 Ma) [4,37], the Monchetundra intrusion and the Monchepluton have complex relationships, reflecting the different sequence of their main magmatic phases [29,33] and were subject to intense post-magmatic tectonic transformation.
According to the deep drilling results (drill holes 765 and M-1), the Monchetundra massif section comprises either two [4] or three [16] zones. After [16], these zones contain the following rocks: the lower zone—plagiopyroxenites, plagioperidotites, norites, olivine norites, peridotites, and olivinites (ultramafic body); the middle zone—mostly medium-grained trachytoid gabbronorites; the upper zone—massive coarse-grained gabbronorites, leucogabbro, and gabbro-anorthosites. The lower zone is associated with the PGE deposit of Loipishnyun [28,38].
The Monchepluton consists of two branches, i.e., (1) a north–north–east (NNE) trending branch including Mts. Nittis, Kumuzhya, and Travyanaya, which form the so-called NKT massif (harzburgites/peridotites, pyroxenites, and melanocratic norites); and (2) an east–north–east (ENE) trending branch including the Sopcha, Nude-Poaz, and Vurechuaivench (pyroxenites, norites, gabbronorites, and anorthosites) massifs. As shown in Figure 1b, the so-called Dunite Block with the Sopcheozero chromite deposit is located between the NKT and Sopcha massifs. The Dunite Block position within the MC section remains disputable. Many geologists treated it as a giant xenolith, i.e., an early phase that preceded formation of the majority of the Monchepluton rocks; they suggested the presence of dunite xenoliths in underlying norites and plagioclase orthopyroxenites [2,39,40]. However, it was proved in [4,41] that the Dunite Block was a permanent member of the Monchepluton section.
The generalized stratigraphic column of the Monchepluton demonstrates a regular bottom–top changes in rocks from harzburgites to gabbro-anorthosites [22]. The repetition of horizons with olivine rocks in the Monchepluton section (the Ore Layer 330 of the Sopcha massif and rocks hosting the Critical Horizon of the Nude-Poaz massif) is thought to be connected with additional pulses of magmatic matter [4,6,7,22,25,42,43,44]. Different parts of the Monchepluton section contain different types of sulfide Cu-Ni-PGE mineralization, i.e., contact, vein, and reef ones. The richest Cu-Ni vein ores of the NKT massif had been mined out by the beginning of the 1970s. The main PGE-bearing object (the reef-type Vurechuaivench deposit) (Figure 1b) was included in the State Balance Sheet of Mineral Reserves in 2009 [45]; it has not been mined yet. Alongside a commercially non-valuable Fe-Ti-V, mineralization is associated with the Gabbro-10 massif (Figure 1b). An issue of association of the Gabbro-10 massif with the Monchepluton is also disputable [4,13,29,33].

2.2. The Pentlandite Gorge Site

The Pentlandite Gorge site is located at the upper part of a stream of the same name on the eastern slope of Mt. Monchetundra. In terms of geology, it is located in the Monchetundra fault zone, i.e., the intensively tectonized conjunction zone between the Monchetundra intrusion and Monchepluton (Square 1 on Figure 1b and Figure 2a). The site is composed of various rocks; interrelations between the rocks allow suggesting the presence of several intrusive phases as well as eruptive breccia with fragments (wedges) of basement formations and adjacent intrusive complexes [46,47]. The mafic–ultramafic (magmatic) part of the Pentlandite Gorge site section was referred to different geological formations, i.e., some scientists associated these rocks with the Monchetundra intrusion [46], the others suggested the relation of these rocks to the Monchepluton [47].
The eastern part of the site (Figure 2b) contains metamorphic formations, i.e., biotite-amphibole gneisses, amphibole-biotite gneisses, and carbonate-amphibole schists with graphite, presumably related to the Archean frame rocks. An anorthosite body occurs between the basement rocks and magmatic rocks, limited by the steeply dipped ruptures. The anorthosite xenoliths are revealed within the fine-to-medium-grained gabbronorites. The mafic–ultramafic block is represented (from bottom to top) by the following rocks: (1) harzburgites; (2) plagiopyroxenites; (3) layered units of norites, pyroxenites, and thin interlayers of gabbronorites (also including pegmatoid segregations of quite a fancy form); (4) norites and gabbronorites with the small-sized (up to 1 m) anorthosite xenoliths [46].
In the western part of the site (Figure 2b), predominantly amphibolized mesocratic massive gabbros are developed; near tectonic zones they are sheared and contain thin gabbrodolerite dikes. This block has a compositional feature, i.e., an orthopyroxenite body of a pear-shaped form in plan, 17 × 25 m size, which is probably a fragment of the eastern part of orthopyroxenites [46]. Age definitions for the rocks of the site are absent.

2.3. The South Sopcha Site

The site is situated within the massif of the same name (Square 2 on Figure 1b) and a zone of its contacts with the Verkhniy Nude block norites (Square 3 on Figure 1b). As well as the Pentlandite Gorge site, the South Sopcha massif is situated in the conjunction zone between the Monchetundra intrusion and Monchepluton, marked by the Monchetundra fault (Figure 1b and Figure 3). The massif stretches at a distance of about 10 km north–westward and dips south–westward at angles from 5° to 45°, according to the drilling data. In the south–west, the South Sopcha massif is adjacent to the rocks of the Early Proterozoic complex of Mt. Arvarench; in the north–east, it contacts with diorite gneisses of the Vezhetundra complex; in the north, it contacts with the Monchepluton rocks; and in the north–west, it probably connects with the Monchetundra intrusion through the fault zone (Figure 1b and Figure 3).
The internal structure of the South Sopcha massif (Square 2 on Figure 1b) has two distinct zones (Figure 3), i.e., a lower layered norite-orthopyroxenite and an upper gabbro one [46]. The lower zone with a thickness of 250–300 m is represented by an irregular alternation of layers of norites and pyroxenites with a thickness of 1–20 m, with schlierens and irregular areas of pegmatoid variations and a subordinate amount of harzburgites [6]. The upper zone, with a thickness of up to 80 m, is composed of leuco-mesocratic coarse-grained spotted gabbro and gabbronorites similar to gabbroids of the upper zone of the Monchetundra massif in terms of chemical composition [27]. The age of the South Sopcha massif rocks was determined by the U-Pb zircon method. The age of the lower zone norites is 2504 ± 1 Ma, and the age of medium-grained mesocratic gabbro of the upper zone is 2478 ± 20 Ma [6].
In the contact zone between the rocks of the lower and upper parts of the South Sopcha massif (Figure 4a), eruptive breccia is observed (Figure 4b). The breccia comprises fragments of norites and pyroxenites of the lower zone, which are cemented by gabbroids of the upper zone [19,20,33]. In addition, the contact zone contains numerous vein bodies (Figure 4c) with a thickness of up to 2 m, composed of pyroxenes, amphiboles, magnetite, and sulfides of Fe, Ni, and Cu. These veins are also developed in the norites of the Verkhniy Nude block (Square 3 on Figure 1b) adjacent to the South Sopcha massif. The Verkhniy Nude block norites contain Cr-spinels veins too (Figure 4d), with a small admixture of silicates [48]. The massif lower zone rocks and veins contain PGE minerals, and the site is considered to be a promising PGE object [19,27,33,45,46].

2.4. The Ore Layer 330 and North-East Sopcha Sites

The sites of Ore Layer 330 and North-East Sopcha are constituents of the Sopcha massif (Figure 1b, Figure 5 and Figure 6). The Sopcha massif belongs to the ENE-trending branch of the Monchepluton and is essentially a layered intrusion. Peridotites predominate in its lower part, and the upper part contains thick orthopyroxenite sequence. A thinly layered body (4–5 m thick) disturbs the sequence monotonous structure; it is composed of interrupted layers of dunites, harzburgites, olivine pyroxenites, and feldspar olivine pyroxenites containing Cu-Ni-PGE mineralization (the Ore Layer 330) (Square 4 on Figure 1b and Figure 5a–f). Besides the Ore Layer 330 itself, there are independent lenses of thinly layered rocks that are situated above and under it along the section. They were assumed to be tectonically displaced fragments of the main body [49]. In this article, we will further refer to them as satellites of the Ore Layer 330.
Of special interest is the lower contact between the rocks of the Ore Layer 330 and underlying rocks (Figure 5f). The contact is associated with the so-called pseudo-breccias, which were considered by some geologists [2] to be dunite xenoliths cemented by orthopyroxenites, and the orthopyroxenites were therefore treated as more recent formations in relation to the Ore Layer 330 rocks. A detailed study has shown that these formations have various forms (from rounded to micro-folded ones with complex dislocation in the result of a plastic viscous flow) and thus cannot be assigned to xenoliths. Judging by their morphology, they represent fragments (“drops”) of the first layer of olivine cumulate from below, subjected to magmatic erosion, which began to be deposited on orthopyroxene cumulate [4].
In the west, the Sopcha massif is adjacent to the Dunite Block, within which there is the Sopcheozero chromite deposit. In the east, the Sopcha massif borders with the mostly norite intrusion of Nude-Poaz; in the south, it borders with the South Sopcha massif (Figure 1b).
In the 1950s, small bodies of ultramafic rocks consisting mainly of olivine were discovered among the pyroxenites of the Sopcha massif. The characteristics of these bodies are given in [50]. E.K. Kozlov pointed out that these are lens-shaped bodies, ranging in size from 0.6 × 0.2 m to 8 × 50 m, the largest of which are located on the northeastern slope of Mount Sopcha (Square 5 on Figure 1b and Figure 6a–e). E.K. Kozlov showed that in the host pyroxenites in the exocontact zone, linearity is observed in the arrangement of orthopyroxene crystals, enveloping the boundaries of the Dunite Body. On this basis, E.K. Kozlov [2,50] suggested the xenolithic origin of these bodies, which in subsequent years became the subject of discussion [4,18,24]. An alternative point of view was to recognize the Sopcha ultramafic bodies as fragments of Ore Layer 330 [24].
Further research showed that these rocks were dunites, with the chromite content varying from 1 to 5% of the rock volume. Based on these data, the JSC Central Kola Expedition (JSC CKE) geologists suggested that the revealed bodies could be the so-called ‘dunite pipes’ similar to those in the Bushveld massif (SAR), where they contained large PGE deposits. The hypothesis was verified in 1999–2001 during the cooperative exploration works performed by the JSC CKE and JSC PANA. The studies showed that the dunites contained a thin chromite dissemination (up to 5% of the rock volume) with Cr2O3 content of up to 1.76 wt.%. No sulfides were revealed in the rocks, whereas the content of PGE and gold was not more than 0.1 ppm [51]. The idea of a pipe-like form of the dunite bodies was not confirmed as well. The chromite-bearing dunites were also established not to belong to the Ore Layer 330, as they differed from the latter in terms of geological structure and rock composition [18]. Age definitions were made by the Sm-Nd method for harzburgites from the Ore Layer 330, with the values being within the range of 2451 ± 64 Ma [6]; the dunite bodies were not dated.

2.5. The Mine No. 5 Site

Mine No. 5 site is located near a mouth of the mine with the same name at the bottom of the western slope of Mount Travyanaya within the harzburgite unit of the NKT massif (Square 6 on Figure 1b and Figure 7a,b). The sections of NKT and Sopcha massifs comprise (from bottom to top): quartz-bearing norites and gabbronorites of the near-bottom zone with a thickness of 10–100 m; harzburgites (100–200 m); alteration of harzburgites and orthopyroxenites (250–400 m); and orthopyroxenites (300–700 m). The NKT massif total thickness increases southwards from 200–300 m to 800–1000 m [4].
This kind of zoning in the Ore Layer 330 rocks was explained by new melt portions and further mixing of magmas. For example, previous researchers of the Ore Layer 330 suggested a magmatic origin of zoning in Cr-spinels, caused by the changes in physical and chemical conditions of magma crystallization [4,42,52,53]. In their opinion, the results of research on the Ore Layer 330 rocks and minerals indicated the process of melt crystallization to have been disturbed. It was suggested that a new portion of a high-temperature magmatic melt compositionally close to peridotite had been intruded in several pulses at a certain stage of the orthopyroxenite crystallization before the complete consolidation of orthopyroxenites [4].
Minerals 14 00856 g007
Figure 7. (a) Dumps of Mine No. 5 at the western foot of Mount Travyanaya. (b) Schematic geological section along the excavation line of Mine No. 5 [54]. Vein 4 and Vein 12 are essentially Cu-PGE-bearing sulfides.
Figure 7. (a) Dumps of Mine No. 5 at the western foot of Mount Travyanaya. (b) Schematic geological section along the excavation line of Mine No. 5 [54]. Vein 4 and Vein 12 are essentially Cu-PGE-bearing sulfides.
Minerals 14 00856 g007
According to [28], there are three levels of the complex PGE-Cu-Ni mineralization defined within the NKT massif. Discovered in the 1930s, the upper PGE-Cu-Ni ore field served as the main ore base of the Severonickel Mining Plant for 40 years until its exhaustion in the middle of the 1970s. These sulfide veins are exposed to the surface and occur to a depth of about 400 m. The PGE-Cu ores (significantly chalcopyrite ones) of the NKT massif compose its middle ore-bearing horizon, which is located 200–250 m lower than the upper vein field. Significantly, Ni injection ores of the NKT massif form a lower ore-bearing horizon, which is developed lower than the PGE-Cu vein ores [28]. The Mine No. 5 dumps contain samples of hosting gneisses of the Kola series, contact gabbronorites, disseminated and nest-disseminated ores in peridotites, pyroxenites, norites, mafic pegmatites, and also Cu veins with PGE mineralization (Figure 7b). Ages of the ophitic gabbronorite and quartz norite of the margin zone of the NKT massif are 2484.5 ± 7.9 Ma and 2507 ± 9 Ma, respectively [3,4].

3. Previous Studies of the MC Cr-Spinel Mineralization

A quite comprehensive analysis of the issues of ore chromite and vein and accessory Cr-spinels of the MC is given in [30]. In the present article, we shall briefly mention the studies that have significant petrological (and, in some cases, applied) results.
An article by V.S. Dokuchaeva [39] became a milestone in the studies of chromite mineralization; the author predicted the industrial feasibility of the MC resources of that kind (chrome ores). In 1994, the prediction was confirmed in the course of exploration works performed by the Norilsk Nickel Mining and Metallurgical Company. The exploration works led to a discovery of the Sopcheozero stratiform chromite deposit within the Dunite Block [4,28,41,55]. Further studies of ore chromites from the Sopcheozero deposit resulted in a rather unusual hypothesis of ‘chromitite dikes’ that may have served as feeders in the process of chromite deposit formation [56] and a very convincing criticism of this hypothesis [55].
When studying the Cr-spinel mineralization in the Ore Layer 330 rocks of the Sopcha massif, the scientists noticed such an Cr-spinel feature as zoning. The composition of accessory Cr-spinels in the form of inclusions in olivine and orthopyroxene was studied alongside the composition of accumulations of grains in interstices of the silicate grains. The scientists observed widely varying concentrations of chrome, iron, magnesium, and aluminum even in a single thin section and in optically homogenous grains. Compositional variations become even wider in the Cr-spinel zoned grains [4,57]. D.A. Orsoev studied zoned Cr-spinels of the Ore Layer 330 and showed that the cores of grains contained more Al, Mg, and Cr but less Ti and Fe in comparison with the rims [52].
This kind of zoning in the Ore Layer 330 rocks was explained by new melt portions and further mixing of magmas. For example, previous researchers of the Ore Layer 330 suggested a magmatic origin of zoning in Cr-spinels, caused by the changes in physical and chemical conditions of magma crystallization [4,42,52,53]. In their opinion, the results of research on the Ore Layer 330 rocks and minerals indicated the process of melt crystallization to have been disturbed. It was suggested that a new portion of a high-temperature magmatic melt compositionally close to peridotite had been intruded in several pulses at a certain stage of the orthopyroxenite crystallization before the complete consolidation of orthopyroxenites [4].
Later, V.V. Chashchin and Y.E. Savchenko thoroughly studied accessory Cr-spinels from the Sopcha Ore Layer 330, which were also divided into two groups. The first group of Cr-spinels formed homogenous grains as well as cores of zoned crystals and were referred to as Al-chromite. The second group Cr-spinels corresponded to ferrochromite and were developed in the form of homogenous grains; they also formed rims in zoned crystals [58]. According to the authors opinion, the Cr-spinel composition evolution was determined by cooling of the initial melt as the result of magmatic crystallization and was of polygenetic character [58]. It was noticed that Cr-spinels from the Ore Layer 330 rocks were close to those from the Monchepluton in terms of chemical composition, and their variability was caused by variations of rock composition and by crystallization of their cumulus predecessor. The authors suggestion was that this observation indicated a common mantle source for the rocks of the Ore Layer 330 and Monchepluton to have been formed from, yet different scripts of such formation [58].
Among metanorites of the Verkhniy Nude block (Figure 1b), vein Cr-spinels were first discovered as two minor (up to 10–20 cm thick, up to 1 m long) schlieren-like bodies [33,48]. These works indicate that vein Cr-spinels differ compositionally from ore chromites of the Sopcheozero deposit and accessory Cr-spinels from other massifs of the Monchepluton [33], being closer to Cr-spinels of the Imandra–Umbarechka complex [59].
The Nude-Poaz massif Cr-spinels were also studied, and that helped specifying some details of the massif genesis [30].

4. Materials and Methods

The article uses analytical data from previous studies presented in [22,30,52,53,56,58,60,61,62] and unpublished reports [63]. These data are accumulated in Supplementary Materials Table S1. The authors supplemented these data by studying Cr-spinels from five MC sites (Pentlandite Gorge, South Sopcha, Ore Layer 330, North-East Sopcha, Mine No. 5). Table 2 shows a list of the authors’ samples used in this article. New data are given in Supplementary Materials Tables S2–S7.
Microphotographs in transmitted and reflected light were taken using an Axioplan microscope (Carl Zeiss, Oberkochen, Germany) with a TopCam 16.0 MP digital camera. The chemical composition of minerals was studied by the X-ray spectral method, using an electron probe microanalyzer (Cameca MS-46 (EDS), Paris, France) with the accelerating tension of 22 kV and the probe current of 30–40 nA. The following standards were used: diopside (Si, Ca), pyrope (Mg, Al), lorenzenite (Ti), hematite (Fe), chromite (Cr), synthetic MnCO3 (Mn), synthetic vanadium (V), synthetic nickel (Ni), synthetic ZnO (Zn), and synthetic cobalt (Co). Detection limits of electron microprobe analyses (wt.%): Ca—0.03, Mg—0.1, Ni—0.01, Al—0.05, Si—0.05, Mn—0.01, Fe—0.01, Zn—0.01, Ti—0.02, Cr—0.02, and V—0.02.
The study of the mineral morphology, imaging in back-scattered electron BSEs, and preliminary chemical analyses were executed on artificial polished sections and monomineral fractions with the use of a scanning electron microscope (SEM), LEO-1450 (Carl Zeiss, Oberkochen, Germany), with an energy dispersive X-ray analytical device (EDS)—the Bruker Quantax-200 energy dispersive detector (Bruker, Bremen, Germany) and the ULTIM MAX-100 (OXFORD Instruments, Oxford, UK) with software Aztec 4.4 at the Laboratory of Physical Methods for studying rocks, ores, and minerals of the Geological Institute of the Kola Science Centre of the Russian Academy of Sciences (GI KSC RAS).
Once Cr-spinels grains were saturated in ilmenite or magnetite inclusions produced by the decomposition of the solid melt, the analysis was provided with a wide beam (beam diameter up to 40 µm) to measure the average mineral composition. BSE images were obtained using an LEO-1450 scanning electron microscope (Carl Zeiss, Oberkochen, Germany). In all cases, FeOtotal was recalculated for formula values of Fe+2 and Fe+3 according to the stoichiometric formula (except for sample M-8, in which a chemical analysis of the monomineral fraction was performed).

5. Results

5.1. Morphology and Mineral Composition of the MC Cr-Spinels

Cr-spinels from the different units of the MC have their specific features, i.e., different size, form (degree of idiomorphism), and internal homogeneity of grains.
Cr-spinels from harzburgite of the eastern block of the Pentlandite Gorge site (Supplementary Materials Table S2) are represented by a rare dissemination in olivine and orthopyroxene (Figure 8a–c). The grain size reaches 150 µm; the grain shape is mostly oval or irregular; the partial octahedral shape of grains is rare. The Cr-spinel grains are unzoned; some grains contain ilmenite ingrowths that form lattice structure. If the Cr-spinel crystals are confined inside the central parts of grains of orthopyroxene or olivine, they have smooth and clear margins. If the Cr-spinel crystals occur near the serpentinized olivine grain margins (Figure 8c), they are surrounded by an irregular magnetite rim (Figure 8d).
Rocks of the South Sopcha site layered complex (melanocratic norites, harzburgites) contain homogenous Cr-spinels as well as zoned ones (Figure 9a–d). Cr-spinels are usually included in the crystals of olivine or orthopyroxene, where they form grains with a size of 50–200 µm and groups of grains. The grains have isometric or irregular shapes. Many grains are distinctively zoned (Figure 9c). It is worth noting that the central parts of grains are composed by a Cr-spinel with the highest chrome, magnesium, and zinc contents, whereas the peripheral parts of grains (rims) are more ferruginous (Supplementary Materials Table S3). Fine structures of decay of a solid solution may be observed in the central parts of many grains (Figure 9d); elongated needle crystals are represented by ilmenite.
Cr-spinels form schlieren-like segregations within vein bodies amid the pyroxenites of the layered part of the South Sopcha site and norites from the Verkhniy Nude block. A silicate part of the vein matter is represented by amphiboles, chlorite, and epidote group minerals. Cr-spinels are represented by oval grains without firm crystallographic shapes, grain margins being resorbed (Figure 9e,f). A typical grain size varies from 100 to 400 µm. Judging by their internal structure, the Cr-spinel grains have appeared as the result of the decomposition of solid solutions.
The grains contain ilmenite, magnetite, chrome-magnetite, and ulvospinel. Ilmenite is represented by plate, rounded, or irregular segregations of various size and orientation (Figure 9g). Most grains are composed of thin, regular intergrowths of chrome-magnetite and ulvospinel (Figure 9h). The sample also contains skeletal crystals of magnetite.
Cr-spinels in veins contain high concentrations of Fe and Ti and low contents of Mg, Al, and Cr (Supplementary Materials Table S4). The ilmenite ingrowths in Cr-spinel grains generally correspond to stoichiometric composition and contain a minor amount of impurities. Magnetite composing skeletal crystals contains mechanical impurities of chalcopyrite and silicates; the titanium impurity has not been revealed. Thin ingrowths in chrome-magnetite are specified as ulvospinel.
The most homogenous chromite grains are observed in the Dunite Body of the North-East Sopcha site (Figure 10a–d). The grain size varies from 50 to 300 µm. Small chromite grains of an early generation are included in the olivine crystals (Figure 10b). Larger chromite grains of a late generation are confined in interstices between the olivine crystals (Figure 10c). The early generation chromite is more chromous but less ferruginous and titaniferous than the late generation chromite (Supplementary Materials Table S5). Grains have regular as well as irregular shapes. Octahedral crystals of chromite are well observed in a monomineral fraction (Figure 10d). Zoned grains (with magnetite rims) occur only in the most metamorphosed parts of the rock.
As opposed to the Dunite Body chromites, Cr-spinels from the Ore Layer 330 site (Figure 11a–f) feature a distinctly expressed zoning (Figure 11b–f). The core parts of the grains are formed by Cr-spinel, which is more aluminous and magnesian, less ferruginous, and less titanium compared to the Cr-spinel of the marginal parts of the grains (Supplementary Materials Table S6).
Concentration of Cr-spinels reaches 5%–7% in lower and central parts of the Ore Layer 330. The Cr-spinel grains have a size of up to 400 µm. Small grains have irregular shapes, whereas the large ones often have regular crystallographic outlines. The Cr-spinel zoning is typical of the Ore Layer 330 section at all levels; zoning is also extended to satellite lenses located higher up the section (Figure 11a,b) and under the bottom of the layer. The Cr-spinel concentration varies at different levels of the Ore Layer 330, though the zoning type remains the same.
Some Cr-spinels from the NKT massif rocks at the Mine No. 5 site (Figure 12a–d) also feature a pronounced zoning (Figure 12b). Another detail worth mentioning is that marginal parts of the zoned grains are enriched in iron and titanium (Supplementary Materials Table S7). The NKT Cr-spinels are often heterogeneous, i.e., they contain numerous inclusions (Figure 12c). In many cases, the grains are both zoned and heterogeneous (Figure 12d).
Internal heterogeneity of the Cr-spinel grains is most often indicated by the ilmenite inclusions that form lattice structure (Figure 13a–h). The NKT Cr-spinels most commonly form irregular-shaped grains with a size rarely exceeding 150 µm. There also occur zoned Cr-spinel grains (Figure 13i–l), which have small inclusions of ilmenite in the center and marginal parts enriched in Fe (Figure 13k).

5.2. Geochemistry of the MC Cr-Spinels

According to the classification [64], most of the studied Cr-spinels correspond to chromite (Figure 14a, Supplementary Materials Tables S1–S3 and S5–S7). Exceptions are samples from the veins of the South Sopcha site, where most varieties are represented by magnetite with a Cr content of 0.049–0.752 apfu (Cr-magnetite) (Supplementary Materials Table S4). Magnetite also forms rims around chromite grains in metamorphosed rocks.
The graph Fe2+/(Mg + Fe2+) vs. Cr/(Cr + Al) (Figure 14b) shows the general trend in the evolution of the composition of Cr-spinels of the MC. It has a sharply curved shape. The flatter low-iron branch of the trend includes Cr-spinels of both types of ores and host rocks of the Sopcheozero deposit, partially Cr-spinels of the South Sopcha massif (lower zone), as well as Cr-spinels of the Kumuzhya Mountain and North-East Sopcha site (Dunite Body). The vertical, more ferruginous branch of the trend includes Cr-spinels from the NKT and Nude-Poaz massifs, partially Cr-spinels from the South Sopcha massif (lower zone), Pentlandite Gorge site, Sopcha Ore Layer 330 site (both varieties), and Cr-spinels from the veins of the South Sopcha site.
A plot of Fe2+/(Mg + Fe2+) vs. Fe3+/(Cr + Al + Fe3+) (Figure 14c) also shows a clear trend depicting the composition of Cr-spinels varying from low-ferruginous chromites of the Sopcheozero deposit to chrome-magnetites and magnetites of the South Sopcha veins.
A plot of Fe3+/(Cr + Al + Fe3+) vs. TiO2 (Figure 14d) shows that most of the MC Cr-spinels contain low amounts of Ti, except for the chrome-magnetites and magnetites of the South Sopcha veins that may contain more than 10% of TiO2. Maximal TiO2 content is revealed in one analysis of the South Sopcha massif Cr-spinel (lower zone), thus indicating spatial proximity and genetic link between these rocks and veins of the South Sopcha site.
A variation diagram of Mg (apfu) vs. Fe2+ (apfu) (Figure 15a) shows that contents of these elements within the MC Cr-spinels are in a clearly defined inverse proportion. The reason for this is that other bivalent elements composing the tetrahedral position of the Cr-spinel structure are presented in very small amounts. A diagram of Mg (apfu) vs. Fe3+ (apfu) (Figure 15b) shows that low-magnesian Cr-spinels also have the biggest amount of Fe3+ in their composition. A diagram of Cr2O3 (%) vs. MgO (%) (Figure 15c) generally shows a direct proportion between these components.
Diagrams of Cr2O3 (%) vs. Al2O3 (%) (Figure 15d) and Al2O3 (%) vs. MgO (%) (Figure 15e) demonstrate a more complicated pattern. An inverse proportion is clearly observed between Cr2O3 and Al2O3 for Cr-spinels from the Monchetundra intrusion (ultramafic body as well as lower zone), for both types of ores, and for the majority of host rocks of the Sopcheozero deposit. At the same time, there is no indication of such proportion for Cr-spinels from the NKT and Nude-Poaz massifs. In the Al2O3 (%) vs. MgO (%) diagram (Figure 15e), a direct proportional relationship between Al2O3 and MgO is visible for Cr-spinels from the NKT and Nude-Poaz massifs, but for Cr-spinels from other MC units, such a relationship is not observed.
The most distinct separation of two Cr-spinel groups is shown on a diagram of Al (apfu) vs. Fe2+ (apfu) (Figure 15f). The first group (more chromous and magnesian) includes Cr-spinels from the Monchetundra intrusion (ultramafic body as well as lower zone), Cr-spinels of both types of ores, and Cr-spinels of most of the host rocks from the Sopcheozero deposit. The second group (more ferruginous) includes Cr-spinels from the rocks of the NKT and Nude-Poaz massifs. Some points denoting Cr-spinels from the Sopcheozero deposit host rocks are located in the second group area of the plots; a possible explanation here is that the Sopcheozero deposit host rocks may have obtained some tectonically displaced fragments of the NKT massif.
The contents of impurities in Cr-spinels from the large MC units are shown in Figure 15g–k. Most of the Cr-spinels contain no more than 0.4% of NiO, 0.5% of ZnO, and 0.8% of MnO. There is also no more than 2.0% of TiO2 and 1.0% of V2O5; these are the lowest contents of titanium and vanadium in the Sopcheozero deposit Cr-spinels.
Figure 16 shows compositional features of Cr-spinels of the following sites: the Pentlandite Gorge, South Sopcha massif (lower zone), South Sopcha site (veins), Ore Layer 330, North-East Sopcha (Dunite Body), and Mount Kumuzhya.
In terms of composition (ratios of MgO vs. Al2O3 (Figure 16a) and MgO vs. Cr2O3 (Figure 16b), Fe2+ content (Figure 16c)), Cr-spinels from the Pentlandite Gorge site belong to the second (more ferruginous in 15f) group (the site, however, is situated within the Monchetundra intrusion contact zone). These features are shown in Figure 14b,c as well.
The South Sopcha massif Cr-spinels (lower zone) are also clearly distinguished into two groups on diagrams of MgO vs. Al2O3 (Figure 16a), MgO vs. Cr2O3 (Figure 16b), and Al vs. Fe2+ (Figure 16c). Some Cr-spinels are referred to as the first group (more chromous and magnesian). The other Cr-spinels are referred to as the second group (more ferruginous). An interesting feature of the more ferruginous Cr-spinels is an unusually high content of ZnO (Figure 16d).
Cr-spinels from the South Sopcha site veins feature a very high level of ferruginosity (Figure 14c and Figure 16c), high contents of Ti and V, and low contents of Al and Ni; some samples feature a high content of Mn (Figure 16e–i).
Cr-spinels from the Ore Layer 330 are mostly zoned. Central and marginal parts of the grains have sharply different compositions. However, the diagrams show that both variations (Ore Layer 330 (1) and Ore Layer 330 (2)) belong to the second (more ferruginous) group of Cr-spinels. Formed by Cr-spinels from the rocks of the NKT and Nude-Poaz massifs, this group features a higher content of iron and a widely varying content of aluminum (Figure 14b and Figure 16a,c). On the diagrams, points denoting Cr-spinels from the North-East Sopcha site (Dunite Body) and Mount Kumuzhya are located in an area of the first (more chromous and magnesian) group of Cr-spinels (Figure 14b,c and Figure 16a–c).

6. Discussion

6.1. Mineralogical and Geochemical Features of MC Cr-Spinels

As a result of studying the morphology, mineralogy, and geochemistry of Cr-spinels in the studied sites of the MC, the following can be noted:
Cr-spinels in rocks and ores most often form oval and irregularly shaped grains; regular crystallographic outlines or individual faces in irregular grains are less often observed. According to the internal structure, Cr-spinel grains can be divided into: (1) homogeneous, (2) zonal, and (3) with a heterogeneous internal structure of grains (moreover, the same sample can contain homogeneous and heterogeneous grains).
The presence of homogeneous Cr-spinel grains in the rock may indicate that the crystallization process of the magmatic melt took place under stable conditions without additional portions of magma. Such grains are typical for chromites of the Sopcheozerso deposit, as well as for Cr-spinels of the Dunite Body of the North-East Sopcha site.
The zonal structure of Cr-spinels (the clearly defined presence of core and peripheral parts of grains) is a distinctive feature of the Sopcha Ore Layer 330 and its satellites. Some Cr-spinels of the NKT massif are also zoned (Mine No. 5 site). Such features of the morphology of Cr-spinels are often characteristic of changing conditions of melt crystallization (for example, the presence of additional magmatic injections).
The internal heterogeneity of Cr-spinel grains is most often expressed in the presence of ilmenite inclusions, creating a lattice structure. High-iron and high-titanium Cr-spinels are complex intergrowths of two or more phases with the participation of ilmenite, ulvo-spinel, and chromium-magnetite. Such features of the internal structure of Cr-spinels characterize the most recent, nonequilibrium phases of the magmatic melt [35,36]. They are observed in Cr-spinels in the vein rocks of the South Sopcha site.
In addition, in almost all areas around the Cr-spinel grains, magnetite rims are observed, which usually arise during rock metamorphism [36].
Thus, the morphological features and mineral composition of the studied Cr-spinels indicate some features of magmatic (various degrees of homogeneity and zoning) and metamorphic (magnetite rim) processes. However, more important petrological conclusions can be drawn, taking into account data on the chemical composition of Cr-spinels.
Based on their chemical composition, Cr-spinels are divided into two groups (Figure 14, Figure 15 and Figure 16). The first group is represented by more chromium and more magnesium varieties (Monchetundra intrusion, ores, and host rocks of the Sopcheozero deposit). The second group consists of more ferruginous Cr-spinels with greatly varying Al content from the rocks of the NKT (Mine No. 5 site) and Nude-Poaz massifs. It also includes Cr-spinels from the Pentlandite Gorge, Ore Layer 330, North-East, and South Sopcha sites. In general, for the MC, this corresponds to the evolutionary trend in layered mafic–ultramafic intrusions. This trend shows that the most magnesian and chromium-rich Cr-spinels formed from the most primitive igneous melts, while the ferruginous varieties are associated with evolved portions [36].
Below we will try to synthesize the data obtained, taking into account the geological specifics of each studied area.

6.2. Relationship with the Geological Structure of Various MC Units

The Pentlandite Gorge site is a tectonic collage of blocks of various geological formations—Archean basement rocks, as well as the MC massifs—Monchetundra and Monchepluton. Determining the affiliation of such blocks to one or another geological formation seems important from the point of view of the evolution of MC, as well as applied consequences, since significant PGE concentrations are noted within the mafic–ultramafic part of the site [46].
Judging by the geochemical features of the composition of the studied Cr-spinels from harzburgite and norite of the Pentlandite Gorge (the ratio of MgO vs. Al2O3 (Figure 16a), MgO vs. Cr2O3 (Figure 16b), and the Fe2+ content (Figure 16c), we can say that they are quite close to those from the rocks of the NKT massif, being more ferruginous and highly variable in Al content. Since this massif directly borders the Pentlandite Gorge site, it can be assumed that the block of mafic–ultramafic rocks in the eastern part of the site is part of the NKT, i.e., a fragment of Monchepluton. At the same time, gabbro from the western part of the Pentlandite Gorge site (no Cr-spinels were found in these rocks), as well as anorthosites developed among the rocks of the eastern part in the form of xenoliths, may belong to the Monchetundra intrusion [47,65].
The South Sopcha site (primarily the massif of the same name) is interesting from a petrological point of view because its rocks (especially the more leucocratic upper part) may belong to the latest phase of the Monchetundra intrusion, which was assumed by many authors [20,27,33]. A number of researchers believed that not only the upper but also the lower part of the section of the South Sopcha massif may belong to the Monchetundra intrusion [6]. It was from these rocks (the lower zone of the South Sopcha massif) that we studied Cr-spinels. In the diagrams MgO vs. Al2O3 (Figure 16a), MgO vs. Cr2O3 (Figure 16b), and Al vs. Fe2+ (Figure 16c), they are clearly divided into two groups. Some of the Cr-spinels belong to the first group previously identified for MC (more chromium and more magnesian); the other part belongs to the second group (more ferruginous). This heterogeneity in the composition of Cr-spinels can apparently be explained by the fact that tectonic fragments of the Nude-Poaz massif are present in the lower zone of the South Sopcha massif. The heterogeneity of this zone was emphasized earlier—for example, on the geological map given in [22], the entire South Sopcha massif is shown as a zone of blastocataclasites.
An interesting feature of the more ferruginous Cr-spinels of the South Sopcha site is the unusually high ZnO content (Figure 16d). With a high probability, this fact is caused by processes of contamination of later phases with host rocks (in particular, gneisses of the Archean basement).
Cr-spinels from the veins of the South Sopcha site are distinguished by very high iron content (Figure 14c and Figure 16c), high contents of Ti and V, and low contents of Al and Ni; some samples contain elevated Mn content (Figure 16e–i). These data may indicate that veins with the most evolved composition of Cr-spinels could have formed at the final stages of the formation of the MC during the intrusion into the Monchepluton of phases that formed the upper part of the Monchetundra intrusion. A number of authors believe that these veins could be carriers of PGE mineralization from the host rocks of earlier magmatic phases [33,66]. Thus, data on the morphology and geochemical features of Cr-spinels do not contradict the assumption that coarse-grained leucocratic gabbronorites of the South Sopcha site belong to the upper part of the Monchetundra massif, the intrusion of which is associated with the formation of ore veins.
The Ore Layer 330 and North-East Sopcha sites are also very interesting from a petrological point of view. According to our data, Cr-spinels from the Dunite Body of the Sopcha massif are identical in their morphological and geochemical characteristics to those of similar rocks of the Dunite Block and, therefore, can be considered xenoliths. In addition, the points for Cr-spinels of the North-East Sopcha site (Dunite Body) and rocks of the Kumuzhya mountain of the NKT massif [62] on the diagrams fall into the area of Cr-spinels of the first group (more chrome and more magnesian, Figure 14b,c and Figure 16a–c). This fact may indirectly testify in favor of the version of the presence of xenoliths of the Dunite block not only in the Sopcha massif but also in the NKT massif.
In contrast to the Dunite Body, Cr-spinels from the Ore Layer 330 site are characterized by pronounced zoning. The cores of the grains are formed by Cr-spinel, which is more aluminous and magnesian, less ferrous, and less titanium compared to the Cr-spinel of the marginal parts of the grains (Supplementary Materials Table S6). The data obtained confirm the observations of previous researchers about the additional injection of magma that formed Ore Layer 330 [4,42,43]. Perhaps this new impulse had a pulsation mechanism, which, in addition to the main body, also formed several of its satellite lenses (studied by us above and below the section), in which similar morphology, internal structure, and composition of Cr-spinels are observed. Cr-spinels of Ore Layer 330 are in most cases zoned, i.e., the central and marginal parts of the grains differ quite sharply in composition. Such zoning may indicate the mixing of different portions of magma [42,52] and its contamination with host rocks [6,7,22,25,44].
It is worth noting that in the diagrams, varieties from different zones of Cr-spinels (Ore Layer 330 (1) and Ore Layer 330 (2)) show belonging to the second group of more ferruginous Cr-spinels that vary greatly in Al content (Supplementary Materials Table S1, Figure 14b and Figure 16a,c), formed by chrome spinels from rocks of the NKT and Nude-Poaz massifs.
Our studies of Cr-spinels from the Mine No. 5 site (NKT massif) showed that they differ from those in the rocks of the Dunite block and the Sopcheozero chromite deposit. According to the geochemical characteristics, Cr-spinels from the rocks of the Mine No. 5 site are close to those from Ore Layer 330 of the Sopcha massif (Supplementary Materials Tables S5 and S7). Since the formation of the latter is associated with additional injection of melt, such an observation may indicate that the composition of the melt was close to that which formed the NKT massif at one time.

6.3. On the Problem of Relationships between Magmatic Phases of the MC

Summarizing the above observations, we can draw some conclusions related to the history of the formation of units of the MC section.
Therefore, for example, it can be assumed that the rocks of the Dunite Block could have formed earlier than all the phases that formed the MC. This is indicated by their xenoliths developed in the Sopcha massif (North-East Sopcha site), which was confirmed by the similarity of the compositions of Cr-spinels. In addition, the possibility of the presence of such xenoliths also exists for the NKT massif [62,67,68]. Note that previously, Cr-spinels from the rocks on the northern slope of Mount Kumuzhya (called “nodular chromitites” by the authors) were considered different in composition from the chromites of the Sopcheozero deposit [41]. However, the data obtained in [62,67,68] rather indicate the proximity of the Cr-spinels of the NKT massif and the Dunite Block.
Thus, in contrast to [22], the rocks of the Dunite Block most likely belong not to the second but to an earlier phase of MC formation. If the presence of their xenoliths in the NKT massif is confirmed, then the identity of the dunites comprising the Sopcheozero chromite deposit as Monchepluton will raise questions. Perhaps the point of view of E.V. Sharkov [25] about the belonging of these dunites to the MRC was not without reason. However, apparently, E.V. Sharkov mistakenly considered them to be younger relatives to Monchepluton. In our opinion, chromite-bearing dunites could belong to earlier MC phases that formed the lower part of the Monchetundra intrusion. Age determinations do not yet provide clear confirmation of this version—the age of dunites and chrome ore was 2500 ± 10 and 2500 ± 2 Ma, respectively [5].
After the formation of the main Monchepluton massifs, an additional injection of essentially ultrabasic (according to our data from the study of Cr-spinels—close to the one that formed the NKT massif) melt came from the same chamber, which formed Ore Layer-330 of the Sopcha massif. A younger age of the rocks of Ore Layer 330 was obtained by the Sm-Nd method and amounted to 2451 ± 64 Ma [6]. According to other researchers, it is 2492.5 ± 4.1 Ma [22].
The most recent phase in the MC was the one that formed the upper part of the Monchetundra intrusion and, probably, the rocks developed within the South Sopcha site (to a greater extent, the upper zone of the massif of the same name). Judging by the assumptions of some researchers, the gabbroids of the Gabbro-10 massif should also be attributed to this phase [33]. As for the rocks of the lower zone of the South Sopcha massif, the data from the study of Cr-spinels can only speak of a complex history of its formation (perhaps it consists of rocks of the Monchetundra intrusion and tectonic fragments of the Monchepluton). The injection of this MC phase resulted in the formation of amphibole-plagioclase-magnetite (with sulfide and PGE mineralization) veins with the most evolved composition of Cr-spinels (high-titanium and high-vanadite). The ages of various rocks of the South Sopcha massif vary widely—from 2504 ± 1 to 2453 ± 4 Ma (Table 1).
After solidification and consolidation, at the turn of about 1.9–1.8 Ga, within the framework of the processes of the Svecofennian orogeny [69,70], the MC was subjected to intense tectonic processing, most contrastingly manifested in the modern erosion truncating in the area Monchetundra fault. Some authors propose a slightly different interval for the formation of the Monchetundra fault (2.0–1.9 Ga) within the Lapland-Cola orogen [32]. Within the Pentlandite Gorge site, we can observe both the results of the processes of formation of a multiphase MC (in particular, the phases that formed the NKT massif—which is confirmed by the study of Cr-spinels and, possibly, the upper part of the Monchetundra intrusion) and its subsequent tectonic processing (including that affecting and Archean basement rocks). During the same period, the MC rocks underwent metamorphic transformations, which were reflected in the formation of magnetite rims around Cr-spinel grains in all studied areas.

7. Conclusions

  • The composition of Cr-spinels of the MC varies widely from aluminochromite to chrome-magnetite, generally corresponding to the evolutionary trend characteristic of layered mafic–ultramafic intrusions: the most magnesian and chromium-bearing Cr-spinels were formed from the most primitive melts, and ferruginous varieties are associated with evolved portions of magmatic melts.
  • Findings of xenoliths of rocks of the Dunite Block in outcrops of the Sopcha and, possibly, NKT massifs allow us to conclude that the Dunite Block (hosting the Sopcheozero chromite deposit) was formed earlier than the NKT and Sopcha massifs. This is confirmed by data on the morphology and composition of Cr-spinels: in the Dunite Body of the Sopcha massif, they are almost identical to those in the rocks of the Dunite Block and are close to those from the harzburgite of the NKT massif. This observation may cast doubt on the scheme for the formation of MC units outlined in Smol’kin and Mokrushin (2022), where the formation of the NKT massif is associated with the first stage of MC formation.
  • Zoning of Cr-spinels from rocks of Ore Layer 330 of the Sopcha massif probably indicates changing conditions for crystallization of an additional portion of the melt (for example, mixing of two magmas of different compositions). We have shown for the first time that in the satellite bodies of Ore Layer 330 (above and below it in the section), Cr-spinels are also zoned. The similarity of the composition and internal structure (zoning) of the Cr-spinels of Ore Layer 330 and its satellite bodies confirms their origin from a single portion of the melt, the injection of which probably had a pulsating character.
  • The composition of Cr-spinels, and, in particular, the high Zn content, allows us to support the point of view that the layered complex of rocks of the South Sopcha massif was formed from the most evolved portion of magmatic melt, genetically related to the Monchetundra intrusion, and, probably, significantly contaminated with the substance of the host rocks. The South Sopcha vein complex (containing sulfide and PGE mineralization) can be considered to have arisen in the final stages of the evolution of the magmatic system.
  • Tectonic factors played a significant role in the formation of the MC in its modern form. For example, the composition of Cr-spinels in the rocks of the Pentlandite Gorge clearly indicates that individual tectonic blocks are fragments of the NKT massif and not the Monchetundra intrusion, as previously thought.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min14090856/s1. Table S1: Cr-spinel analyses from the Monchegorsk complex used to construct the diagrams, excluding the authors’ analyses. Tables S2–S7: Representative chromian spinel and chromian magnetite analyses from the studied sites in the Monchegorsk complex.

Author Contributions

Conceptualization, P.P. and T.R.; methodology, T.R., A.M. and A.B.; validation, P.P., T.R. and A.M.; investigation, P.P., T.R., A.M. and A.B.; resources, T.R. and A.M.; data curation, P.P., T.R., A.M. and A.B.; writing—original draft preparation, P.P., T.R. and A.M.; writing—review and editing, P.P. and T.R.; supervision, P.P. and T.R.; project administration, P.P. and T.R. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by the state budget research (FMEZ-2024-0004).

Data Availability Statement

The data presented in this study are openly available online at Supplementary Materials.

Acknowledgments

We are very grateful to Valery Smol’kin for valuable comments and clarification of data, as well as Vladimir Anatsky for his help in translating the manuscript into English.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Scheme of the location of the main mafic–ultramafic intrusions in the central part of the Kola Region; (b) Geological map of the Monchegorsk layered complex. Squares with numbers are sampling sites: 1—Pentlandite Gorge; 2—South Sopcha (massif); 3—South Sopcha (Verkhniy Nude block); 4—Ore Layer 330; 5—North-East Sopcha; 6—Mine No. 5. The inset shows the position of the Monchegorsk complex on the map of the Kola Peninsula.
Figure 1. (a) Scheme of the location of the main mafic–ultramafic intrusions in the central part of the Kola Region; (b) Geological map of the Monchegorsk layered complex. Squares with numbers are sampling sites: 1—Pentlandite Gorge; 2—South Sopcha (massif); 3—South Sopcha (Verkhniy Nude block); 4—Ore Layer 330; 5—North-East Sopcha; 6—Mine No. 5. The inset shows the position of the Monchegorsk complex on the map of the Kola Peninsula.
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Figure 2. (a) Photograph of the Pentlandite Gorge—a fragment of the Monchetundra fault expressed in the relief. (b) Geological map of the Pentlandite Gorge site and (c) section along the line A–B. White circles with signatures—sample and its number.
Figure 2. (a) Photograph of the Pentlandite Gorge—a fragment of the Monchetundra fault expressed in the relief. (b) Geological map of the Pentlandite Gorge site and (c) section along the line A–B. White circles with signatures—sample and its number.
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Figure 3. (a) Geological map of the South Sopcha site. (b) Section along the line A–B. White circles with signatures—sample and its number.
Figure 3. (a) Geological map of the South Sopcha site. (b) Section along the line A–B. White circles with signatures—sample and its number.
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Figure 4. (a,b) Relationships between rocks in the South Sopcha massif. (c,d) Rocks of the vein complex of the South Sopcha site.
Figure 4. (a,b) Relationships between rocks in the South Sopcha massif. (c,d) Rocks of the vein complex of the South Sopcha site.
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Figure 5. (a) Outcrops of the rhythmically layered part of the Ore Layer 330. (b,c) Satellite bodies of the Ore Layer 330. (d) Geological map of the Sopcha massif. (e) Section through the Sopcha massif along the line A–B. (f) Geological column of the western fragment of the Ore Layer 330 (see Square 4 in Figure 1b) with sampling points.
Figure 5. (a) Outcrops of the rhythmically layered part of the Ore Layer 330. (b,c) Satellite bodies of the Ore Layer 330. (d) Geological map of the Sopcha massif. (e) Section through the Sopcha massif along the line A–B. (f) Geological column of the western fragment of the Ore Layer 330 (see Square 4 in Figure 1b) with sampling points.
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Figure 6. (a) Position of the Dunite Body on the northeastern slope of Mount Sopcha. (b) Geological scheme of the Dunite Body in pyroxenites of the Sopcha massif. (c) Outcrops of dunites and gruss after them. (d) Fragment of the Dunite Body outcrop. (e) Serpentine vein in dunite. White circles with signatures—sample and its number.
Figure 6. (a) Position of the Dunite Body on the northeastern slope of Mount Sopcha. (b) Geological scheme of the Dunite Body in pyroxenites of the Sopcha massif. (c) Outcrops of dunites and gruss after them. (d) Fragment of the Dunite Body outcrop. (e) Serpentine vein in dunite. White circles with signatures—sample and its number.
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Figure 8. Cr-spinels from harzburgite of the Pentlandite Gorge site (Sample 9427-1700). (a) Cr-spinel grains in olivine and orthopyroxene. (b) Cr-spinel grain with thin ilmenite ingrowths (enlarged fragment of (a)). (c) Unzoned chromite grains in the center of the olivine grain and a Cr-spinel grain with a magnetite rim on the right edge of the olivine grain. (d) Cr-spinel grain with magnetite rim (enlarged fragment of (c)). BSE images.
Figure 8. Cr-spinels from harzburgite of the Pentlandite Gorge site (Sample 9427-1700). (a) Cr-spinel grains in olivine and orthopyroxene. (b) Cr-spinel grain with thin ilmenite ingrowths (enlarged fragment of (a)). (c) Unzoned chromite grains in the center of the olivine grain and a Cr-spinel grain with a magnetite rim on the right edge of the olivine grain. (d) Cr-spinel grain with magnetite rim (enlarged fragment of (c)). BSE images.
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Figure 9. Cr-spinels from rocks of the South Sopcha site. (a) (Sample 1826/21.2) Groups of Cr-spinel grains in harzburgite of the South Sopcha massif. (b) (Sample Jus-8) Groups of Cr-spinel grains in harzburgite of the South Sopcha massif. (c) (Sample 1826/21.2) Zoned grains of Cr-spinel with magnetite rims in harzburgite of the South Sopcha massif. (d) (Sample 1826/144.9) Unzoned heterogeneous grain with thin ilmenite intergrowths in melanocratic norite of the South Sopcha massif; (e) (Sample 1a) Cr-spinel from the vein of the Verkhniy Nude block. (f) (Sample 2b) Cr-spinel from the vein of the Verkhniy Nude block. (g) (Sample 1a) Fragments of the Cr-spinel grain with intergrowths of ilmenite and ulvospinel. (h) (Sample 2b) Thin intergrowths of chromium magnetite (white) and ulvospinel (gray). (a)—polished section, (b–h)—BSE images.
Figure 9. Cr-spinels from rocks of the South Sopcha site. (a) (Sample 1826/21.2) Groups of Cr-spinel grains in harzburgite of the South Sopcha massif. (b) (Sample Jus-8) Groups of Cr-spinel grains in harzburgite of the South Sopcha massif. (c) (Sample 1826/21.2) Zoned grains of Cr-spinel with magnetite rims in harzburgite of the South Sopcha massif. (d) (Sample 1826/144.9) Unzoned heterogeneous grain with thin ilmenite intergrowths in melanocratic norite of the South Sopcha massif; (e) (Sample 1a) Cr-spinel from the vein of the Verkhniy Nude block. (f) (Sample 2b) Cr-spinel from the vein of the Verkhniy Nude block. (g) (Sample 1a) Fragments of the Cr-spinel grain with intergrowths of ilmenite and ulvospinel. (h) (Sample 2b) Thin intergrowths of chromium magnetite (white) and ulvospinel (gray). (a)—polished section, (b–h)—BSE images.
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Figure 10. Chromites from the Dunite Body of the North-East Sopcha site. (a) Chromite grains (white) in olivine. (b) Early-generation chromite grains (Chr-1) in olivine. (c) Late-generation chromite grains (Chr-2) in the interstices between olivine grains. (d) Chromite crystals. (a–c)—sample M-9, (d)—sample M-8. BSE images.
Figure 10. Chromites from the Dunite Body of the North-East Sopcha site. (a) Chromite grains (white) in olivine. (b) Early-generation chromite grains (Chr-1) in olivine. (c) Late-generation chromite grains (Chr-2) in the interstices between olivine grains. (d) Chromite crystals. (a–c)—sample M-9, (d)—sample M-8. BSE images.
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Figure 11. Cr-spinels of the Ore Layer 330 site. (a,b) (Sample S-2-2) Cr-spinels in the satellite lens of the Ore Layer 330. (c,d) (Sample Sop-Up) Cr-spinel in the upper part of the Ore Layer 330. (e,f) (Sample Sop-330) Cr-spinel in the central part of the Ore Layer 330. BSE images.
Figure 11. Cr-spinels of the Ore Layer 330 site. (a,b) (Sample S-2-2) Cr-spinels in the satellite lens of the Ore Layer 330. (c,d) (Sample Sop-Up) Cr-spinel in the upper part of the Ore Layer 330. (e,f) (Sample Sop-330) Cr-spinel in the central part of the Ore Layer 330. BSE images.
Minerals 14 00856 g011
Minerals 14 00856 g012
Figure 12. Cr-spinels from rocks of Mine No. 5 (NKT massif). (a) (Sample 1-2-21) Dissemination of Cr-spinel in plagiopyroxenite. (b) (Sample 3-3-21) Zoned Cr-spinels in harzburgite. (c) (Sample 2-2-21) Heterogeneous grain of Cr-spinel in harzburgite. (d) (Sample 3-3-21) Zoned grains of Cr-spinel with heterogeneity of the marginal parts in harzburgite. BSE images.
Figure 12. Cr-spinels from rocks of Mine No. 5 (NKT massif). (a) (Sample 1-2-21) Dissemination of Cr-spinel in plagiopyroxenite. (b) (Sample 3-3-21) Zoned Cr-spinels in harzburgite. (c) (Sample 2-2-21) Heterogeneous grain of Cr-spinel in harzburgite. (d) (Sample 3-3-21) Zoned grains of Cr-spinel with heterogeneity of the marginal parts in harzburgite. BSE images.
Minerals 14 00856 g012
Minerals 14 00856 g013
Figure 13. Back-scattered electron (BSE) images and element maps detailing the characteristics of Cr-spinels in the NKT massif (Mine No. 5 site). (a–d) (Sample 1-1-21) Cr-spinel grain (gray in photo (a)) in plagiopyroxenite with small magnetite inclusions (white in photo (a)) and ilmenite lattice (dark gray in photo (a)). (e–h) (Sample 1-1-21) Grain of Cr-spinel in plagiopyroxenite with an ilmenite lattice and a rim of high-titanium mineral phase. (i–l) (Sample 3-1-21) Zoned grain of Cr-spinel in harzburgite; the center contains small inclusions of ilmenite; and the margin is enriched in Fe.
Figure 13. Back-scattered electron (BSE) images and element maps detailing the characteristics of Cr-spinels in the NKT massif (Mine No. 5 site). (a–d) (Sample 1-1-21) Cr-spinel grain (gray in photo (a)) in plagiopyroxenite with small magnetite inclusions (white in photo (a)) and ilmenite lattice (dark gray in photo (a)). (e–h) (Sample 1-1-21) Grain of Cr-spinel in plagiopyroxenite with an ilmenite lattice and a rim of high-titanium mineral phase. (i–l) (Sample 3-1-21) Zoned grain of Cr-spinel in harzburgite; the center contains small inclusions of ilmenite; and the margin is enriched in Fe.
Minerals 14 00856 g013
Minerals 14 00856 g014
Figure 14. Composition of Cr-spinels of the Monchegorsk complex. (a) Ternary diagram Al3+-Cr3+-Fe3+. (b) Variation diagrams Fe2+/(Mg + Fe2+) vs. Cr/(Cr + Al). (c) Fe2+/(Mg + Fe2+) vs. Fe3+/(Cr + Al + Fe3+). (d) Fe3+/(Cr + Al + Fe3+) vs. TiO2. Red outline—layered intrusions, chromitites excluded; black outline—layered intrusions, chromitites only [36]. The red dotted arrow shows the main trend in the evolution of the MC Cr-spinels composition.
Figure 14. Composition of Cr-spinels of the Monchegorsk complex. (a) Ternary diagram Al3+-Cr3+-Fe3+. (b) Variation diagrams Fe2+/(Mg + Fe2+) vs. Cr/(Cr + Al). (c) Fe2+/(Mg + Fe2+) vs. Fe3+/(Cr + Al + Fe3+). (d) Fe3+/(Cr + Al + Fe3+) vs. TiO2. Red outline—layered intrusions, chromitites excluded; black outline—layered intrusions, chromitites only [36]. The red dotted arrow shows the main trend in the evolution of the MC Cr-spinels composition.
Minerals 14 00856 g014
Minerals 14 00856 g015
Figure 15. Variation diagrams of the composition of Cr-spinels of the Monchegorsk complex: Monchetundra intrusion, Dunite Block (Sopcheozero chromite deposit), NKT, and Nude-Poaz massifs. (a) Mg apfu vs. Fe2+ apfu. (b) Mg apfu vs. Fe3+ apfu. (c) Cr2O3% vs. MgO%. (d) Cr2O3% vs. Al2O3%. (e) Al2O3% vs. MgO%. (f) Al apfu vs. Fe2+ apfu. (g) Al2O3% vs. NiO%. (h) Al2O3% vs. ZnO%. (i) Al2O3% vs. MnO%. (j) Al2O3% vs. TiO2%. (k) Al2O3% vs. V2O5%.
Figure 15. Variation diagrams of the composition of Cr-spinels of the Monchegorsk complex: Monchetundra intrusion, Dunite Block (Sopcheozero chromite deposit), NKT, and Nude-Poaz massifs. (a) Mg apfu vs. Fe2+ apfu. (b) Mg apfu vs. Fe3+ apfu. (c) Cr2O3% vs. MgO%. (d) Cr2O3% vs. Al2O3%. (e) Al2O3% vs. MgO%. (f) Al apfu vs. Fe2+ apfu. (g) Al2O3% vs. NiO%. (h) Al2O3% vs. ZnO%. (i) Al2O3% vs. MnO%. (j) Al2O3% vs. TiO2%. (k) Al2O3% vs. V2O5%.
Minerals 14 00856 g015
Minerals 14 00856 g016
Figure 16. Chemical composition of Cr-spinels of the Monchegorsk complex. The contours of the composition fields are from Figure 15. (a) Al2O3% vs. MgO%. (b) Cr2O3% vs. MgO%. (c) Al apfu vs. Fe2+ apfu. (d) Al2O3% vs. ZnO%. (e) Al2O3% vs. TiO2%. (f) Al2O3% vs. V2O5%. (g) V2O5% vs. TiO2%. (h) Al2O3% vs. MnO%. (i) Al2O3% vs. NiO%.
Figure 16. Chemical composition of Cr-spinels of the Monchegorsk complex. The contours of the composition fields are from Figure 15. (a) Al2O3% vs. MgO%. (b) Cr2O3% vs. MgO%. (c) Al apfu vs. Fe2+ apfu. (d) Al2O3% vs. ZnO%. (e) Al2O3% vs. TiO2%. (f) Al2O3% vs. V2O5%. (g) V2O5% vs. TiO2%. (h) Al2O3% vs. MnO%. (i) Al2O3% vs. NiO%.
Minerals 14 00856 g016
Table 1. Age of the Monchegorsk complex rocks.
Table 1. Age of the Monchegorsk complex rocks.
Massif (Site)RockAge (Ma)Method/MineralReference
Monchepluton
NKTOphitic gabbronorite2484.5 ± 7.9U-Pb/zrn[3]
NKTQuartz norite, marginal zone2507 ± 9U-Pb/zrn[4]
Dunite BlockDunite2500 ± 10U-Pb/zrn[5]
Dunite BlockChrome ore2500 ± 2U-Pb/zrn[5]
Sopcha (Ore Layer 330)Harzburgite2451 ± 64Sm-Nd/rfm[6]
Sopcha (Ore Layer 330)Orthopyroxenite2492.5 ± 4.1U-Pb/zrn[7]
Nude-PoazGabbro-pegmatite2504.4 ± 1.5U-Pb/zrn[8]
Nude-PoazGabbro-pegmatite2500 ± 5U-Pb/zrn[4]
Nude-PoazGabbronorite-pegmatite2503 ± 4.6U-Pb/zrn[9]
Nude-PoazOlivine orthopyroxenite2484.3 ± 5.6U-Pb/zrn[10]
Nude-PoazNorite2493 ± 7U-Pb/zrn[11]
Nude-PoazGabbronorite2493 ± 5U-Pb/zrn[5]
Nude-Poaz (Nude-2)Norite ore-bearing2503 ± 8U-Pb/zrn[6]
Nude-Poaz (Nude-2)Orthopyroxenite2506 ± 3U-Pb/zrn[6]
Verkhniy Nude Block (Moroshkovoe Lake)Metanorite2463.1 ± 2.7U-Pb/zrn[6]
VurechuaivenchMetagabbronorite2497 ± 21U-Pb/zrn[4]
VurechuaivenchMetagabbronorite2498.2 ± 6.7U-Pb/bdy[4]
VurechuaivenchMetagabbronorite2504.2 ± 8.4U-Pb/zrn[12]
VurechuaivenchMetaanorthosite2507.9 ± 6.6U-Pb/zrn[12]
VurechuaivenchMetagabbronorite2504.3 ± 2.2U-Pb/zrn[6]
VurechuaivenchMetaplagioclasite2494 ± 4U-Pb/zrn[6]
Gabbro-10Metadiorite2498 ± 6U-Pb/bdy[13]
Monchetundra intrusion
LoipishnyunOrthopyroxenite2496.3 ± 2.7U-Pb/zrn[14]
LoipishnyunNorite2500 ± 2U-Pb/zrn[14]
LoipishnyunTrachytoid gabbronorite2501 ± 8U-Pb/zrn[15]
HipiknurchorTrachytoid gabbronorite2505 ± 6U-Pb/zrn[15]
HipiknurchorTrachytoid gabbronorite2504 ± 7.4U-Pb/zrn[9]
South SopchaMetagabbro2478 ± 20U-Pb/zrn[6]
South SopchaMetanorite ore-bearing2504 ± 1U-Pb/zrn[6]
South Sopcha (Upper zone)Gabbro-anorthosite2456 ± 5U-Pb/bdy[16]
South Sopcha (Upper zone)Gabbro-anorthosite2453 ± 4U-Pb/zrn[16]
South Sopcha (Upper zone)Metagabbronorite2471 ± 9U-Pb/zrn[17]
South Sopcha (Upper zone)Metagabbronorite2476 ± 17U-Pb/zrn[17]
Note: Whether the massifs belong to the Monchepluton or the Monchetundra intrusion is given in the interpretation of the authors in the references. Zrn—zircon, bdy—baddeleyite, rfm—rock-forming minerals.
Table 2. List of samples.
Table 2. List of samples.
No.LocationSample No.Rock
1Pentlandite Gorge site9427-1700Harzburgite
2Pentlandite Gorge sitePen 4 *Mesocratic norite
3South Sopcha massif, Lower layered zoneAYu-1Melanocratic amphibolized norite
4South Sopcha massif, Lower layered zone1826-21.2Harzburgite
5South Sopcha massif, Lower layered zone1826-144.9Melanocratic norite
6South Sopcha massif, Lower layered zoneHost-US-3Melanocratic norite
7South Sopcha massif, Lower layered zoneJus-8Harzburgite
8South Sopcha massif, Lower layered zoneV-SS-1 *Plagioclase-amphibole vein
9South Sopcha site, Verkniy Nude block1-160611Chromite vein
10South Sopcha site, Verkniy Nude block2-160611Chromite vein
11South Sopcha site, Verkniy Nude block1aChromite vein
12South Sopcha site, Verkniy Nude block2aChromite vein
13South Sopcha site, Verkniy Nude block2bChromite vein
14Sopcha massif (17 m upper from the bottom of Ore Layer 330). Satellite of Ore Layer 330. Ore Layer 330 siteS-2-2Orthopyroxenite
15Sopcha massif (16 m upper from the bottom of Ore Layer 330). Satellite of Ore Layer 330. Ore Layer 330 siteS-3-1Olivine orthopyroxenite
16Sopcha massif (10 m upper from the bottom of Ore Layer 330). Satellite of Ore Layer 330. Ore Layer 330 siteS-7b-2Olivine orthopyroxenite
17Sopcha massif (~ 6 m upper from the bottom of Ore Layer 330). Satellite of Ore Layer 330. Ore Layer 330 siteS-Up *Orthopyroxenite
18Sopcha massif (~ 6 m upper from the bottom of Ore Layer 330). Satellite of Ore Layer 330. Ore Layer 330 siteSop-UpOrthopyroxenite
19Sopcha massif (central part of Ore Layer 330). Ore Layer 330 siteSop-330 Olivine orthopyroxenite
20Sopcha massif (1 m lower from the bottom of Ore Layer 330). Ore Layer 330 siteS-Low *Orthopyroxenite
21Sopcha massif (1 m lower from the bottom of Ore Layer 330). Ore Layer 330 siteSop-LowOrthopyroxenite
22Dunite Body, North-East Sopcha siteM-8 **Dunite
23Dunite Body, North-East Sopcha siteM-9Dunite
24NKT massif, Mount Travyanaya, Mine No. 5 site1-1-21Plagiopyroxenite
25NKT massif, Mount Travyanaya, Mine No. 5 site1-2-21Plagiopyroxenite
26NKT massif, Mount Travyanaya, Mine No. 5 site1-3-21Plagiopyroxenite
27NKT massif, Mount Travyanaya, Mine No. 5 site2-1-21Harzburgite
28NKT massif, Mount Travyanaya, Mine No. 5 site2-2-21Harzburgite
29NKT massif, Mount Travyanaya, Mine No. 5 site3-1-21Harzburgite
30NKT massif, Mount Travyanaya, Mine No. 5 site3-2-21Harzburgite
Note: Indices in sample numbers: * artificial polished section, ** monomineral fraction. The remaining samples are represented by ordinary polished sections.
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Pripachkin, P.; Rundkvist, T.; Mokrushin, A.; Bazai, A. Monchegorsk Mafic–Ultramafic Layered PGE-Bearing Complex (2.5 Ga, Kola Region, Russia): On the Problem of Relationships between Magmatic Phases Based on the Study of Cr-Spinels. Minerals 2024, 14, 856. https://doi.org/10.3390/min14090856

AMA Style

Pripachkin P, Rundkvist T, Mokrushin A, Bazai A. Monchegorsk Mafic–Ultramafic Layered PGE-Bearing Complex (2.5 Ga, Kola Region, Russia): On the Problem of Relationships between Magmatic Phases Based on the Study of Cr-Spinels. Minerals. 2024; 14(9):856. https://doi.org/10.3390/min14090856

Chicago/Turabian Style

Pripachkin, Pavel, Tatiana Rundkvist, Artem Mokrushin, and Aiya Bazai. 2024. "Monchegorsk Mafic–Ultramafic Layered PGE-Bearing Complex (2.5 Ga, Kola Region, Russia): On the Problem of Relationships between Magmatic Phases Based on the Study of Cr-Spinels" Minerals 14, no. 9: 856. https://doi.org/10.3390/min14090856

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