minerals Article Accessory Cr-Spinels in the Section of the Nude-Poaz Massif in the Monchegorsk (2.5 Ga) Mafic-Ultramafic Layered Complex (Kola Peninsula, Russia): Comparison with Ore-Forming Chromites Tatiana Rundkvist and Pavel Pripachkin * Geological Institute—Subdivision of the Federal Research Centre, Kola Science Centre of the Russian Academy of Sciences, 184209 Apatity, Russia; [email protected] * Correspondence: [email protected]; Tel.: +7-8155579288 Abstract: The paper studies accessory Cr-spinels from deep drill holes crossing the Nude-Poaz massif, which is a part of the Monchegorsk mafic-ultramafic layered complex (2.5 Ga, Kola Peninsula, Russia). Cr-spinels occur as two morphological types that differ in their chemical composition, i.e., Cr-spinels of the first type are more aluminous, while Cr-spinels of the second type are more ferruginous and titaniferous. Cr-spinels of the Nude-Poaz massif are characterized by a Fe-Ti trend known for layered intrusions in the world. Cr-spinels of the Nude-Poaz massif quite clearly differ in composition from chromites of the Sopcheozero deposit: they are more ferruginous and less chromous. The specific composition of Cr-spinels in rocks of the Nude-Poaz massif can be correlated with the sequence of Citation: Rundkvist, T.; Pripachkin, the magmatic phases intrusion. P. Accessory Cr-Spinels in the Section of the Nude-Poaz Massif in the Keywords: Cr-spinels; Monchegorsk layered complex; Nude-Poaz massif; Kola Peninsula Monchegorsk (2.5 Ga) Mafic-Ultramafic Layered Complex (Kola Peninsula, Russia): Comparison with Ore-Forming Chromites. 1. Introduction Minerals 2021, 11, 602. https:// Chromite occurs in various forms, like a number of other minerals concentrating doi.org/10.3390/min11060602 commercially valued metals. It is mainly associated with ultramafic rock massifs, where chromite can be observed as disseminated or individual bodies (beds and lenses) with Academic Editors: Evgenii Pushkarev some of them producing major deposits. In petrological studies, Cr-spinels are considered and Giorgio Garuti a clue to understanding the evolutionary processes of magmatic melts [1,2]. Received: 27 April 2021 The study of Cr-spinel composition correlated with varied pressure and temperature Accepted: 1 June 2021 (PT) parameters provided the basis for modeling magmatic processes. As noted by G.S. Published: 3 June 2021 Nikolaev et al. [3], approaches to this issue advanced from simple empiric correlations between certain compositional parameters of Cr-spinel and intensive factors [1,4,5] to Publisher’s Note: MDPI stays neutral complicated thermodynamic models, linking compositional variations of Cr-spinel to the with regard to jurisdictional claims in temperature and composition of coexisting phases [6,7]. The mentioned authors have been published maps and institutional affil- successfully working at such modeling as well [3,8]. iations. These investigations revealed that compositions of Cr-spinels should be studied in detail not only in different intrusions but also within the same layered complex. Thus, comparing compositions of Cr-spinels from various horizons helps to understand whether they belong to the same or different magmatic phases. Copyright: © 2021 by the authors. In the rocks of the Monchegorsk layered mafic-ultramafic complex (hereinafter re- Licensee MDPI, Basel, Switzerland. ferred to as the Monchegorsk Complex or MC, Figure1) in the northeastern part of the This article is an open access article Fennoscandian Shield, Cr-spinels occur as several types, i.e., ore, vein, and accessory distributed under the terms and minerals. Ore chromites of the Sopcheozero deposit are best studied due to their indus- conditions of the Creative Commons trial value [9]. Vein chromites have been found in MC quite recently and described for Attribution (CC BY) license (https:// the Moroshkovoye Lake site [10]. Accessory Cr-spinel is present in almost all types of creativecommons.org/licenses/by/ MC rocks. 4.0/). Minerals 2021, 11, 602. https://doi.org/10.3390/min11060602 https://www.mdpi.com/journal/minerals Minerals 2021, 11, x FOR PEER REVIEW 2 of 24 Minerals 2021, 11, 602 2 of 21 the Moroshkovoye Lake site [10]. Accessory Cr-spinel is present in almost all types of MC rocks. Figure 1. 1. GeologicalGeological map map of the of Monchegorsk the Monchegorsk complex. complex. InIn the the early early 2000s, 2000s, four fourboreholes boreholes (1880, 1881, (1880, 1882, 1881, 1883, 1882, see Figure 1883, 1) seewere Figure drilled1 at) were drilled theat thedepth depth of 502.5 of to 502.5 740 m to during 740 mprospecting during prospectingfor platinum group for platinummetals (PGM) group within metals (PGM) the east-north-easttrending branch of MC. The works were carried out by JSC “Central within the east-north-east (ENE)-trending branch of MC. The works were carried out Kola Expedition” (CKE), JSC “Pana”, MMC “Norilsk Nickel”, and LLC “Pechengagelogiya”by JSC “Central Kola within Expedition” the Nude-Poaz (CKE), and Vurechuaivench JSC “Pana”, MMC massifs. “Norilsk The obtained Nickel”, and LLC “Pechengagelogiya”data on these deep drill holes within provided the Nude-Poaz a far clearer andview Vurechuaivench at both features of the massifs. geolog- The obtained icaldata structure on these of deep the Nude-Poaz drill holes and provided Vurechuaivench a far clearer massifs, view and at bothlocalization features of ofthe the geological platinumstructure group of the elements Nude-Poaz (PGE) and sulfide Vurechuaivench mineralization massifs,in them and[11]. localizationThe Nude-Poaz of the platinum massifgroup itself elements was crossed (PGE) by sulfidethe drill mineralizationhole 1882 in its central in them part and [11 ].by Thethe drill Nude-Poaz hole 1880 massif itself wasin the crossed eastern bypart. the During drill holethe study 1882 of in cores its central from these part anddrill byholes, the horizons drill hole with 1880 ac- in the eastern cessory Cr-spinels were discovered. The current article is dedicated to the results of part. During the study of cores from these drill holes, horizons with accessory Cr-spinels studying these Cr-spinels and their comparison with Cr-spinels from other MC massifs were(first of discovered. all, ore chromites The currentof the Sopcheozero article isdedicated deposit). to the results of studying these Cr-spinels and their comparison with Cr-spinels from other MC massifs (first of all, ore chromites of the Sopcheozero deposit). 2. Geological Structure of the Monchegorsk Complex and Former Research Several authors [12–14] believe that MC consists of two large fragments, i.e., the Monchetundra intrusion (or massif) and Monchepluton (Figure1). Despite their close age (2497 ± 3–2501 ± 8 Ma) [15,16], the Monchetundra intrusion and Monchepluton have complicated interrelations that reflect different sequences of intrusion of their major magmatic phases [10,14] and got shaded by intense postmagmatic tectonic processing. Thus, compiling the consolidated geological column of MC is quite challenging. Notably, considering the Monchetundra intrusion and Monchepluton as one complex is reasoned by Minerals 2021, 11, 602 3 of 21 their spatial proximity, in addition to geochronological data. The former is very important as we believe the Monchetundra intrusion includes, apart from itself, the South Sopchinsky and Gabbro-10 massifs [14]. From two [17] to three [18] zones are defined in the Monchetundra intrusion section based on deep drilling results (drill holes 765 and M-1). According to [18], these zones include the following rocks: lower zone—plagio-pyroxenites, plagio-peridotites, norites, olivine norites, peridotites, olivinites (ultramafic bodies); middle zone—mainly medium- grained trachytoid gabbronorites; upper zone—massive coarse-grained gabbronorites, leucogabbro, and gabbro-anorthosites. Structurally, the Monchepluton comprises two branches, as follows: (1) an north- north-east-trending branch, including Mts. Nittis, Kumuzhya, and Travyanaya, i.e., the so-called NKT massif, and (2) an east-north-east-trending branch, including the Sopcha, Nude-Poaz, and Vurechuaivench massifs. As shown in Figure1, the so-called Dunite block with the Sopcheozero chromite deposit is located between the NKT and Sopcha massifs. The position of the Dunite block in the MC section was debated. Many geologists considered it to be a giant xenolith, i.e., an early phase that preceded the formation of most rocks in the Monchepluton, based on the presence of dunite xenoliths in underlying norites and plagioclase orthopyroxenites [19–21]. However, it was quite convincingly proved in [15] that the Dunite block was a regular member of the Monchepluton section. The repetition of horizons with olivine rocks in the Monchepluton section (Ore Layer 330 of the Sopcha massif and rocks enclosing the Critical Horizon of the Nude-Poaz massif) is associated with additional impulses of magmatic matter [15]. Different parts of the Monchepluton section contain different types of Cu-Ni-PGE sulfide mineralization (contact, vein, reef). The richest Cu-Ni vein ores had been mined out in the NKT massif by the early 1970s. The main PGE-bearing object, i.e., the Vurechuaivench reef deposit, was put on the state balance in 2009 but has never been mined yet. In addition, non-industrial Fe-Ti-V mineralization is associated with the Gabbro-10 massif. The presence of chromite mineralization in MC rocks is another clearly crucial factor. However, it was poorly studied until quite recently. Only in the 1940–50s, prospecting for