Open AccessFeature PaperArticle

The Tepsi Ultrabasic Intrusion, the Northern Part of the Lapland–Belomorian Belt, Kola Peninsula, Russia

Andrei Y. Barkov

^1,*,

Andrey A. Nikiforov

¹,

Robert F. Martin

Sergey A. Silyanov

and

Boris M. Lobastov

Research Laboratory of Industrial and Ore Mineralogy, Cherepovets State University, 5 Lunacharsky Avenue, 162600 Cherepovets, Russia

Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, QC H3A 0E8, Canada

Institute of Non-Ferrous Metals, Siberian Federal University, 95 Krasnoyarskiy Rabochiy pr., 660025 Krasnoyarsk, Russia

Author to whom correspondence should be addressed.

Minerals 2024, 14(7), 685; https://doi.org/10.3390/min14070685

Submission received: 30 April 2024 / Revised: 26 June 2024 / Accepted: 28 June 2024 / Published: 29 June 2024

(This article belongs to the Special Issue From Mantle to Market: Platinum Group Elements and Minerals and Their Geological Significance)

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Abstract

The Tepsi ultrabasic body is located in the northeastern Fennoscandian Shield close to the junction of the Serpentinite Belt–Tulppio Belt (SB–TB) with suites of the Lapland–Belomorian Belt (LBB) of Paleoproterozoic age. The body is a deformed laccolith that has tectonic contacts with Archean rocks. Its primary textures and magmatic parageneses are widely preserved. Fine-grained olivine varies continuously from Fo_90.5 to Fo_65.4. The whole-rock variations in MgO, Fe₂O₃, SiO₂, and other geochemical data are also indicative of a significant extent of differentiation. Compositional variations were examined in the grains of calcic and Mg-Fe amphiboles, clinochlore, micas, plagioclase, members of the chromite–magnetite series, ilmenite, apatite, pentlandite, and a number of other minor mineral species. Low-sulfide disseminated Ni-Cu-Co mineralization occurred sporadically, with the presence of species enriched in As or Bi, submicrometric grains rich in Pt and Ir, or diffuse zones in pentlandite enriched in (Pd + Bi). We recognize two series: the pentlandite series (up to 2.5–3 wt.% Co) and the cobaltpentlandite series (~1 to ~8 apfu Co). The latter accompanied serpentinization. The two series display differences in their substitutions: Ni ↔ Fe and Co → (Ni + Fe), respectively. Relative enrichments in H₂O, Cl, and F, observed in grains of apatite (plus high contents of Cl in hibbingite or parahibbingite), point to the abundance of volatiles accumulated during differentiation. We provide the first documentation of scheelite grains in ultrabasic rocks, found in evolved olivine-rich rocks (Fo_77–72). We also describe unusual occurrences of hypermagnesian clinopyroxene associated with tremolite and serpentine. Abundant clusters of crystallites of diopside display a microspinifex texture. They likely predated serpentinization and formed owning to rapid crystallization in a differentiated portion of a supercooled oxidized melt or, less likely, fluid, after bulk crystallization of the olivine. We infer that the laccolithic Tepsi body crystallized rapidly, in a shallow setting, and could thus not form megacycles in a layered series or produce a well-organized structure. Our findings point to the existence of elevated PGE-Au-Ag potential in numerous ultrabasic–basic complexes of the SB–TB–LBB megastructure.

Keywords:

Tepsi complex; Lapland–Belomorian Belt; Serpentinite Belt; komatiite-related complexes; ultrabasic–basic rocks; spinifex-textured hypermagnesian clinopyroxene; ore assemblages; Kola Peninsula; Russia; Fennoscandian Shield

1. Introduction

The Tepsi complex of ultrabasic rocks, exposed in the Urochishche Tepsi area, Kola Peninsula, belongs to the northern portion of the extensive Lapland–Belomorian Belt (LBB) of Paleoproterozoic age [1,2,3,4,5,6]. It is closely related to the Yanisvaara body and located close to the junction of the LBB with the Serpentinite Belt–Tullpio Belt (SB–TB) in the Fennoscandian Shield [7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23].

We evaluate the geological structure here for the first time and examine mineral associations and compositional variations in the entire Tepsi body, an important and little-known member of the LBB structure in the Shield. We believe that our findings provide petrogenetically significant insights into the crystallization of highly magnesian melts as potential sources for platinum-group elements (PGE) and Cr. We describe Ni-Cu-Co-(As) assemblages, submicrometric grains of minerals enriched in the PGE, diffuse zones of Pd-Bi enrichment in pentlandite, and grains of Au-Ag alloy. These features imply the likely potential for PGE and Au-Ag mineralization in ultramafic–mafic complexes of the so-called “drusite” suites widely distributed in the LBB structure. This is the second report of platinum-group minerals (PGM) in the LBB; Pt-Pd-based minerals were earlier reported [6] in the Kovdozero (Kovdozerskiy) complex, also a member of the giant LBB structure. Occurrences of Ru-Os-Ir-(Rh) mineralization were also reported in the Pados–Tundra complex in the SB–TB structure [11].

2. Regional Geology

As noted, the Tepsi complex is associated with the two megastructures of global importance in the Fennoscandian Shield: the SB–TB and LBB structures, which host numerous “drusite” suites (Figure 1 and Figure 2). Interestingly, coronitic textures, which form according to a combination of magmatic and autometasomatic processes in shallow settings, were also documented in the Serpentinite Belt [20]. On this basis, close petrogenetic relationships exist between the subvolcanic–plutonic associations of the SB and the “drusites” of the LBB. Related and synchronous suites of komatiites and komatiitic basalts occur in the Windy Belt (cf. [24,25]). Many layered intrusions formed during Paleoproterozoic continental rifting in the eastern Fennoscandian Shield, at ~2.45–2.5 Ga (e.g., [26]). The Pados–Tundra subplutonic complex (2485 ± 38 Ma: [13]) and the associated Chapesvaara zoned sills were produced above a large-scaled plume of komatiitic magma undepleted in Al [12,15,20,21].

Peak conditions of regional metamorphism, corresponding to the granulite facies, were attained at the age of 1.92–1.93 Ga in rocks of the Lapland Belt, which are overlain by rocks of the Belomorian province, metamorphosed under the conditions of the amphibolite facies ([37] and references therein).

3. Materials, Samples, and Methods

A total of eighty-three outcrops, composed of ultrabasic and, subordinately, basic rocks, were examined and sampled (Figure 3). The rock exposures reveal platy or foliated blocks (Figure 4a,b) that display various degrees of tectonic stresses and alteration during events of regional metamorphism. Massive exposures are prominent (Figure 4c,d). Typically, the rocks look dense, dark-colored, and fine-grained or aphanitic (Figure 5). In general, these rocks are hard and brittle, so it is difficult to break off fragments from the outcrops.

About five-thousand analytical data points on various minerals were acquired at the R&D center of “Norilsk Nickel” at the Siberian Federal University, Krasnoyarsk, by means of scanning electron microscopy and energy-dispersive analysis (SEM/EDS), carried out using a Tescan Vega III SBH system (Tescan Orsay Holding, Brno, Czech Republic) equipped with an Oxford X-Act spectrometer (Oxford Instruments Nanoanalysis, High Wycombe, UK). The operating conditions were held at an accelerating voltage of 20 kV and a beam current of 1.2 nA. The commonly used combinations of X-ray lines were used, along with a set of standards provided by Micro-Analysis Consultants Ltd. (MAC, Cambridgeshire, UK; registration no. 11192). The K line was used for oxygen, Si (quartz standard), Ca (wollastonite), K (orthoclase), Na (albite), Cu (synthetic chalcopyrite), Fe, S (pyrite and pyrrhotite), Ni, Co, Ti, V, and Cr; specimens of Al₂O₃ were used for Al, MgO for Mg, pure Mn for Mn, sphalerite for Zn, and synthetic GaP for P. F- and Cl-bearing minerals were also analyzed using the K line, with specimens of fluorite and halite as the standards. Furthermore, the L line and standards of pure elements were used for Y, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb. The L line and synthetic CeO₂ and LaB₆ were used for Ce and La. The L line was also used for Nb, Ag, Se, and Sb (pure elements and stibnite as standards), as well as for Zr, Cd (pure Zr and Cd), Sn (pure Sn), Te (PbTe), As (arsenopyrite), and Pd and Rh (pure Pd and Rh). The M line was used for Au (pure Au), Ir (Ir), Os (Os), Pt (Pt), Bi (pure Bi), Pb (PbTe), Th (ThO₂), U (pure U), and also for Hf (pure Hf). The spectrum accumulation times were at least 120 s. The beam current was measured every 60 min using the MAC cobalt standard (registration no. 9941).

The whole-rock abundances of trace elements were established at the same center by inductively coupled plasma–mass spectrometry (ICP–MS) using a high-resolution Finnigan MAT mass spectrometer (the model ELEMENT). Analytical details are provided in [38]. The contents of major oxides in the rocks were established at the same center by X-ray fluorescence (XRF). The XRF analysis was carried out using molten tablets. First, the analyzed sample was dried at 105 °C for 1.5 h, then calcined at 960 °C for 2.5 h, and mixed with a flux (66.67% lithium tetraborate, plus 32.83% lithium metaborate and 0.5% lithium bromide) at a ratio of 1:9. The total weight of the mixture was 5 g. The mixture was melted in platinum crucibles in the induction furnace Lifumat 2.0 Ox. Measurements were performed on an ARL-9900XP X-ray fluorescence spectrometer (Thermo Fisher Scientific Ltd., Waltham, MA, USA). The following set of state-approved standards were used to build calibration relationships and control the correctness of the analysis—MU-1, MU-3, MU-4, SA-1, SChT-1, SChT-2, SDO-1, SDU-1, SG-1A, SG-2, SG-3, SGD-1, SGD-2, SGKh-1, SGKh-5, SGKhM-2, SGKhM-3, SI-1, SI-2, SNS-1, SNS-2, SOP-1, ST-1—plus synthetic mixtures based on components of MgO, SiO₂, Al₂O₃, TiO₂, CaO, CaSO₄, Cr₂O₃, and Fe₂O₃. The analytical accuracy corresponds to the category OST 41-08-205-99. The analytical data are provided in the form of Supplementary Tables S1–S20.

4. Results

4.1. Geological Investigation

Our measurements at opposite contacts (Figure 4a,b) show that the ultrabasic body dips southwest at an azimuth of ~200–220° with a dip of 40–45°. The body lies concordantly in tectonic contact with the Archean basement as host rocks. We present the results of our mapping in Figure 2b and Figure 6. We suggest a revision of the body’s shape, from a dike-like linear body, as in Figure 2a [31], to a deformed laccolith.

4.2. Compositional Variations

The dominant ultrabasic rock in the Tepsi intrusion is peridotite. Modal amounts of fine-grained olivine, serpentinized to some extent, vary usually between ~60 and 80 vol.%, in association with magnesian varieties of calcic and Mg-Fe amphiboles and clinochlore. Large domains of fresh olivine (Figure 7a–f) are commonly present in spite of an overprint of deuteric alteration and the results of regional deformation. A total of 808 data points, gathered on olivine grains from different portions of the body (Figure 8), yield an extensive range of compositions (Mg# 90.5 → 65.4: Table S1). The greatest magnesium content of Ol was recorded close to the southern contact. Note that the observed series is nearly continuous (Figure 9).

As noted, the hydrous silicates are notably magnesian (Figure 10a–c), consistent with the Mg-enriched compositions of precursor silicates formed at the magmatic stage of crystallization. The compositions of the calcic amphiboles (Table S2) typically correspond to tremolite varying to actinolite in the series (Figure 10a). Magnesio-hornblende is the next most abundant species among calcic amphiboles, whereas ferro-hornblende, edenite, ferro-edenite, and ferro-pargasite all occur here and there (Table S2). Ferro-tschermakite is only present close to the external contact, in sample TPST1, collected in a narrow “plagioclase-bearing zone” (Figure 8). This sample is composed of quartz-bearing mafic rocks in which accessory grains of titanite appear as a constituent of the ore assemblages. Grains of Mg-Fe amphibole (Table S3) correspond to anthophyllite (Figure 10b). Members of the chlorite group (Figure 10c) are typically represented by clinochlore (Table S4). The compositions of the grains of plagioclase (Table S5), some of which host grains of minor K-feldspar, range from An₆₅ to nearly pure albite (Figure 11).

Spinifex-Textured Hypermagnesian Clinopyroxene

Clusters of hypermagnesian crystallites of clinopyroxene (Figure 12 and Figure 13a–f), acicular, fan-shaped, and partly skeletal, are of particular interest (sample TPST65). These crystallites correspond to diopside (Table 1). They are hosted by a member of the serpentine subgroup, which contains 42.98–43.13 wt.% SiO₂, 36.05–41.26 wt.% MgO, and 3.05–6.95 wt.% FeO (total). The association shown in Figure 12 and Figure 13a–f involves grains of tremolite with a composition of (Ca_1.87–1.92Fe²⁺_0.06–0.11Mn_0.02–0.03)_Σ2.00(Mg_4.77–4.81Fe²⁺_0.15–0.21Fe³⁺_0.01–0.07Al_0.01)_Σ5.00(Si_7.93–7.98Al_0.02–0.08)_Σ8.00O₂₂(OH_1.77–1.79), along with accessory grains of chromian magnetite (4.34–6.88 wt.% Cr₂O₃). A Cr-enriched grain of magnetite has the composition (Fe²⁺_0.97Mg_0.03Ni_0.01)_Σ1.01(Fe³⁺_1.73Cr_0.22Ti_0.02V_0.01)_Σ1.98O₄. In the other samples (Table S6), veinlet-like, platy, or elongate grains of clinopyroxene, which correspond to diopside or augite, are present in close association with grains of tremolite and likely formed at the expense of tremolite itself. The observed series of clinopyroxenes is shown in Figure 14.

In addition, the compositions of grains of micas are presented in Table S7. Accessory grains of members of the garnet group are represented by grossular and andradite (Table S8). Almandine appears rarely, in relation to more strongly metamorphosed areas located close to the external boundary of the intrusion (TPST1, 8). Grains of talc, dolomite, calcite, and magnesite were also analyzed and have the expected compositions.

4.3. Accessory Minerals, Ore Assemblages, and Rare Species

Ultrabasic rocks are fairly enriched in accessory and ore minerals, especially chromian magnetite or chromite, ilmenite, and apatite, and show late veinlets of hematite; intergrowths of base-metal sulfides that usually involve pentlandite are common (Figure 15a–f). Chain-like aggregates composed of mutual intergrowths of several grains (up to 5–7) of these minerals are present locally (Figure 15c,d). Disseminated low-sulfide ores exhibit an irregular or heterogeneous distribution. In ore-bearing specimens, a large variety of ore and rare minerals were documented (Table 2, Figure 16a–f); their SEM/EDS compositions are listed in Supplementary Tables S9–S18.

Accessory grains of chromian magnetite, ilmenite, and apatite are especially common and attain 0.3–0.4 mm in size. Grains of magnetite with low to moderate levels of Cr prevail in the body. The continuous chromite–magnetite series is observed, which progressively extends toward more Fe³⁺-enriched compositions (low in Cr) (Figure 17). The Cr-rich varieties of the series tend to occur at the center or close to margin of the body (Figure 18). Late veinlets of hematite (Cr-free) are common. Ilmenite forms two series of solid solutions that extend towards geikielite or pyrophanite components (Figure 19). Members enriched in geikelite are attributed to early stages of magmatic crystallization, whereas the pyrophanite-enriched series, which involves the Mn-enriched phase (Fe_0.54Mn_0.42)_Σ0.96Ti_1.02O₃ with a 19.99 wt.% MnO phase (TPST8), likely formed in a fluid-saturated medium during events of late alteration. The compositions of chromite, Cr-bearing magnetite (Table S9), ilmenite, rutile, and titanite (Table S10) are presented in the Supplementary Tables.

The majority of the compositions of accessory grains of apatite (Table S11) occupy the field of hydroxylapatite. Sparse compositions correspond to chlorapatite and fluorapatite (Figure 20).

The compositions of sulfide minerals are provided in Table S12 (pentlandite, cobaltiferous pentlandite, cobaltpentlandite), Table S13 (pyrrhotite, troilite), Table S14 (chalcopyrite, cubanite), Table S15 (bornite, digenite), Table S16 (heazlewoodite, millerite), and Table S17 (sphalerite or wurtzite). The compositions of the other species listed in Table 2 are presented in Table S18.

The mineralized specimens generally display high levels of Ni, a reflection of pentlandite as the major species, with heazlewoodite and minor millerite. A characteristic enrichment in Co is expressed in the form of cobaltiferous pentlandite, cobaltpentlandite, cobaltiferous awaruite, and an arsenide compound Ni₈Co₃As₈ (maucherite type). Some mineralized specimens display elevated contents of Cu, manifested as occurrences of chalcopyrite, cubanite, bornite, and digenite. Fe-based sulfide is a pyrrhotite-type monosulfide; in many cases, it corresponds to troilite. A rim of magnetite is locally developed around grains of intergrown sulfides (Figure 15b,f).

Two series of compositions are recognized for members of the pentlandite family. The first pertains to normal pentlandite, which contains up to 2.5–3 wt.% Co present in solid solution (Table S12). The second series involves Co-enriched varieties of pentlandite and cobaltpentlandite, attributed to deposition at the late stage of serpentinization. These series with contrasting trends are well recognized on the plot of Fe vs. Ni (Figure 21a,b). Interestingly, small zones which are bright and diffuse in the BSE, up about 5 µm in diameter, occur in some of pentlandite grains. The results of the SEM/EDS analyses suggest that (Pd + Bi) are mutually incorporated into these zones at the level of 0.61–3.59 wt.% Pd and 5.77–10.67 wt.% Bi.

In addition, the ore minerals are enriched in As and Bi in the association (Table 2 and Table S18). Separate grains of arsenides of Ni and Co, corresponding to maucherite or orcelite, attain 0.2–0.3 mm in size (Figure 16b). Bismuth is expressed as parkerite and as native bismuth in some grains, oxidized to various extents.

Minute grains of Au-Ag alloy and a few micrometer-sized grains of platinum-group minerals close to (Ir,Pt)(As,S)₂ (iridarsenite?), and PtCu₃ (tomamaeite?) were found (Table S18). Accessory grains of zircon, monazite-(Ce), and xenotime-(Y) were also recognized (Table S18).

We document occurrences of hibbingite (or parahibbingite) recognized in two samples collected at Tepsi (Table S18). In sample TPST50 (Figure 22), hibbingite is hosted by one of the hematite veinlets (98.53 wt.% Fe₂O₃) associated with grains of clinochlore (28.75–32.13 wt.% SiO₂, 14.42–18.27 wt.% Al₂O₃, 1.65–1.80 wt.% Cr₂O₃, 30.40–32.17 wt.% MgO, 5.03–6.21 wt.% FeO), serpentine (42.23 wt.% SiO₂, 39.47 wt.% MgO, 3.42 wt.% FeO), and tremolite (Ca_1.73–1.93Fe²⁺_0.04–0.24Mg_0–0.04Mn_0–0.05)_{Σ1.97–2.00}(Mg_4.66–4.73Fe²⁺_0–0.29Fe³⁺_0–0.29Al_0–0.07Cr_0.01–0.03)_Σ5.00(Si_7.81–8.00Al_0–0.06)_{Σ7.87–8.00}O₂₂(OH_1.81–1.85).

Grains of chromite correspond to (Fe²⁺_1.01–1.03Mg_0.05Mn_0.02Zn_0–0.02)(Cr_0.96–0.98Fe³⁺_0.81–0.83Ti_0.09–0.10V_0.01–0.02Al_0–0.02)O₄. In the second sample (TPST68), hibbingite also occurs as an inclusion in hematite.

The results of the SEM/EDS analyses of hibbingite in these samples (#50 and 68, respectively), are 0.14 and 0.23 wt.% Si, 42.91 and 45.77 wt.% Fe, 5.99 and 5.51 wt.% Mn, 3.00 and 2.83 wt.% Mg, and 16.76 and 13.54 wt.% Cl, for a total of 100 wt.% each, which includes 31.20 and 32.12 wt.% (OH) (estimated by difference). The formulae, (Fe_1.39Mg_0.22Mn_0.20)_Σ1.81(Cl_0.86OH_0.14)(OH)_3.18 and (Fe_1.48Mg_0.21Mn_0.18)_Σ1.88(Cl_0.69OH_0.31)(OH)_3.11, are consistent with Fe₂Cl(OH)₃. The type specimen of hibbingite has a similar formula, (Fe_1.61Mg_0.20Mn_0.05)_Σ1.86(Cl_0.90OH_0.10)(OH)_3.13, in the Duluth complex, Minnesota, USA [40]. Hibbingite in the Strathcona mine, Sudbury, Canada, has the composition Fe_1.86Cl_0.99(OH)_3.16 [41]. Parahibbingite (Fe_1.98Mn_0.01Ca_0.01)Cl_0.92(OH)_3.08 was recently discovered in the Bushveld complex, South Africa [42].

It is of interest to record an unusual occurrence of scheelite in ultramafic rocks. In sample TPST20, inclusions of scheelite (Ca_0.88Fe²⁺_0.26)W_0.95O₄ (Table S18) are hosted by ilmenite as a separate grain or intergrowth with zircon. Another sample (TPST23) exhibits a close association of grains of scheelite Ca_0.97–1.02W_0.99–1.01O₄ (Figure 23a,b) with olivine, which is relatively poor in Mg, Fo_72.2–72.5 (a) and Fo_71.9–72.7 (b), magnetite (8.10–13.15 wt.% Cr₂O₃), ilmenite (0.83–0.94 wt.% MnO and 1.69–2.11 wt.% MgO), and a Cr-bearing clinochlore (with 0.85–1.17 wt.% Cr₂O₃). The calcic amphibole grain (Figure 23b) is magnesio-hornblende, of the composition Na_0.40(Ca_1.89Fe²⁺_0.12)_Σ2.00(Mg_3.97Fe²⁺_0.37Al_0.28Fe³⁺_0.26Cr _0.08Ti_0.04)_Σ5.00(Si_6.90Al_1.10)_Σ8.00O₂₂(OH_1.79).

4.4. Whole-Rock Chemistry

A total of 28 samples of rocks (peridotitic to mafic), collected in the Tepsi intrusion, were analyzed by means of X-ray fluorescence (XRF) (Table S19) and inductively coupled plasma–mass spectrometry (ICP–MS) (Table S20). The contents of major and minor oxides vary within the following ranges (with mean values expressed in weight %): SiO₂ 36.64–47.67 (40.47), TiO₂ 0.09–0.92 (0.32), Al₂O₃ 1.99–8.12 (3.88), Fe₂O₃ (total) 9.44–16.68 (12.17), MnO 0.11–0.27 (0.18), MgO 20.37–34.66 (29.53), CaO 0.89–8.18 (3.73), Na₂O < 0.05–0.89 (~0.12), K₂O 0.01–0.08 (0.03), P₂O₅ 0.01–0.08 (0.03), SO₃ < 0.03–0.39 (0.14), V₂O₅ 0.01–0.03 (0.02), Cr₂O₃ 0.26–0.73 (0.49), and NiO 0.06–0.28 (0.19) wt.%. Geochemical data are compared in selected plots with those published previously for some suites of the Serpentinite Belt (Figure 24a–d).

5. Discussion

On the basis of our new field observations (Figure 2b and Figure 6), we infer that the Tepsi body represents a deformed laccolith, formed by the shallow emplacement of magma of an ultrabasic composition and of komatiitic origin. It was emplaced at the periphery of the giant plume responsible for the Serpentinite Belt–Tulppio Belt suite (SB–TB) [12,15,16,17,19,20,21]. Because of its rapid cooling, the intrusion could neither build a well-organized internal structure nor develop layering and megastructural zoning. Lateral variations in grains of olivine and chromian magnetite–chromite (Figure 8 and Figure 18) display irregular or chaotic patterns. Early crystallization produced the most strongly magnesian olivine (Fo₉₁) close to the southern contact. This inference is consistent with observations from the Pados–Tundra layered complex, in which the most strongly magnesian phase of olivine appeared close to the external contact in the Dunite Block [15]. Thus, crystallization at Tepsi began in a bath of hot undifferentiated magma, losing heat at the boundary of the laccolith. The observed compositions of olivine vary continuously and are extensive, from Fo₉₁ to Fo₆₅ (Figure 9). Such maximum and minimum values are similar to those known in large layered intrusions, in which olivine poor in Fo is characteristic of the upper structural levels [43].

The crystallization of olivine caused a relative enrichment in H₂O and other volatiles initially present in the parental magma. Hydrous variants of highly magnesian melts are, indeed, known [44,45]. In our case, this characteristic was likely crucial in changing the rheological properties of the melt. This decreased its density and viscosity so that the melt was able to intrude up to a shallow level in a hypabyssal setting. The expected accumulation of dissolved H₂O during crystallization led to the deposition of abundant grains of hydroxylapatite (Figure 20).

In the regional context, conclusive lines of evidence point to elevated abundances of H₂O in high-Mg products of the crystallization of komatiitic melts. (1) Crystals of clinochlore occur commonly in intimate intergrowths with grains of laurite–erlichmanite and selenolaurite, hosted by members of the chromite–magnesiochromite series in chromitites and mineralized dunites of the Pados–Tundra layered intrusion [11]. (2) “Oriented inclusions” of clinochlore are present in grains of chromite hosted by primary silicate minerals in the Chapesvara and Lyavaraka complexes [46]. (3) A rim of calcic amphibole is developed around grains of porphyritic orthopyroxene in the Lotmvara II ultrabasic body [19]. (4) Pegmatitic spheroids of orthopyroxene (up to 10 cm) are partially replaced with Ca amphibole, as recorded at Lyavaraka, in which two generations of olivine, Mg# 89.1–90.3 (first) and 74.5–75.8 (second), were recognized [17]. (5) The association of tremolite and talc forms pseudomorphs after the pre-existing spheroids of orthopyroxene, hosted by the matrix of fresh grains of olivine, in dunite and harzburgite in the Pados–Tundra intrusion [9].

At Tepsi, the early-stage massive crystallization of olivine caused a progressive accumulation of Ca, Ti, P, REE, and other incompatible components in portions of melt enriched in H₂O and halogens. Thus, grains of calcic amphibole (probably associated with Mg-Fe amphibole) can be expected to have formed during the deuteric alteration of pre-existing grains of orthopyroxene, which largely reacted with the remaining portions of the late melt coexisting with hydrous fluid. Hornblende gabbros, associated with ultrabasic rocks in the Urals, are attributed to products of “hydrous magmatism” [47]. The overlapping events of regional metamorphism (cf. [37] and references therein) did affect the Tepsi body and presumably resulted in recrystallization, with the development of two-amphibole associations, followed by the appearance of garnet in zones of intense deformation.

Despite these changes, domains of fresh olivine are largely preserved in the Tepsi body (Figure 7a–f). Forsterite is known to exhibit the lowest resistance to chemical weathering in the susceptibility series Ol → Pl → Prx → opaque minerals [48]. Thus, we believe that the Ol-based parageneses of igneous minerals retained their primary characteristics.

We document the association of a calcic amphibole with clinopyroxene, with exceptionally high-Mg compositions, which also occurs in the neighboring Yanisvaara complex [49]. The extreme enrichment in Mg in these grains of Cpx at both Yanisvaara and Tepsi was likely promoted by high values of fO₂, likely close to the magnetite–hematite buffer, causing a shortage of Fe²⁺ during the oxidation-induced conversion of Fe²⁺ into Fe³⁺. These circumstances are consistent with other examples of hypermagnesian minerals in komatiite-derived suites that we have investigated [15,18]. As was postulated, such conditions could be related to the escape of H₂ as a consequence of the vesiculation and dissociation of H₂O during crystallization in shallow settings [46]; cf. [50].

Figure 12 and Figure 13a–f display examples of intriguing textures of a spinifex type, in which abundant clusters of needle-like, skeletal grains or crystallites of diopside have a hypermagnesian composition (Table 1). Those are hosted by serpentine in association with tremolite and chromian magnetite. Note that fresh olivine has a high magnesium content in these rocks (e.g., Mg# 85.6 in TPST66). These clusters of clinopyroxene likely formed in a highly oxidizing medium as a result of the quenching of a differentiated portion of supercooled melt or, less likely, fluid. A regional metamorphic origin seems an unlikely alternative. A “pyroxene spinifex” involving needle-like skeletal grains of augite does occur in the variolitic komatiitic lavas of the Windy (Vetrenyi) Belt [51]. The latter grains of Cpx are not so strongly magnesian, however. Veinlets or lamellar grains of augite are also present in some of the analyzed specimens associated with tremolite (Figure 14). The Windy Belt (VB), composed of sequences of komatiite and komatiite basalts, likely forms part of the megastructure (SB—TB → LBB → VB) of Paleoproterozoic age (2.4–2.5 Ga). The degree of magnesium enrichment generally decreases from the center (the Pados–Tundra–Chapesvara zone) to the periphery of the large-scale plume of komatiite undepleted in Al. In addition, the crystallization of clinopyroxene crystallites at Tepsi, as well as the appearance of clinopyroxene shells in the coronite textures [20], likely bears a petrogenetic relation to the formation of concentrically zoned complexes, like the Rogomu complex (Barkov et al., unpubl. data) or the Nota intrusion [52] in the LBB, which are composed of a peridotite core surrounded by gabbroic rocks.

The overall series of chromian magnetite–chromite at Tepsi (Figure 17 and Figure 18) is dominated by members that are relatively poor in Mg and Al, i.e., magnetite, mainly. This series also points to a more differentiated composition of the intruding melt compared to the more primitive melts in the complexes of the Serpentinite Belt: Pados–Tundra, Chapesvara, and Lyavaraka, which feature occurrences of chromian spinel rich in Mg and Al of the magnesiochromite–chromite series [12,15,18]. Interestingly, the other generation of spinel-group minerals is represented by hercynite in rare inclusions, with a magnesian –chromian composition: (Fe²⁺_0.65Mg_0.24Zn_0.11)_Σ1.00(Al_1.39Cr_0.46Fe³⁺_0.15)_Σ2.00O₄ (Figure 17); as it is hosted by olivine Fo_71.5, it likely crystallized from droplets of trapped melt. This specimen (TPST23) also contains unexpected grains of scheelite, as discussed below.

The chromite progressively developed Fe³⁺-enriched compositions (Figure 17); this trend points to a progressive rise in fO₂ during crystallization. Consequently, a rim of magnetite formed around grains of base-metal sulfides (Figure 15b,f). At the maximum values of fO₂, veinlets of hematite appeared in abundance during late alteration at low temperatures in a metasomatic medium enriched in volatiles. Inclusions of hibbingite or parahibbingite, (Fe,Mg,Mn) (Cl,OH)(OH)₃ (Figure 22), account for the presence of Cl. The chlorine content of these inclusions, 13.54 and 16.76 wt.%, is much greater than those in most Cl-bearing minerals in mafic–ultramafic complexes according to a recent review [53]. Members of the hibbingite–kempite solid solution were reported in the Duluth layered complex, Minnesota, USA; the Strathcona deep copper zone, Sudbury, Canada; the Norilsk complex of northern Russia; and metamorphosed kimberlite in the Udachnaya–Vostochnaya pipe, northern Yakutia, Russia. Parahibbingite, the β-polymorph of Fe₂(OH)₃Cl, formed during an event of hydrothermal alteration of the host pyroxenite in the Karee platinum mine in the Bushveld complex, South Africa [40,41,42,54,55,56,57].

Interestingly, both pentlandite (Pn) and cobaltpentlandite (Copn) are present at Tepsi (Figure 21a,b). Both also exist in the nearby Yanisvaara ultrabasic suite [49]. The Pn series is not unusual here. These compositions, which formed from droplets of sulfide melt, contain up to 2.5–3 wt.% Co, as found in pentlandite grains in other suites of the region. The Copn series is a consequence of serpentinization at low temperatures. The values of the upper stability of pentlandite (and its Co-bearing variety), heazlewoodite, and parkerite are 610–630 °C, 556 °C, and ~400 °C, respectively [58]. An unlimited field of solid solution is formed between (Fe,Ni)_9±xS₈ and Co_9±xS₈ over a temperature interval of ~600 °C to 300 °C. This solid solution can be divided into two fields that extend toward (Fe, Ni)₉S₈ and Co₉S₈ at 200 °C [59].

The two series, Pn and Copn, are well distinguished (Figure 21b) because of differences in their mechanisms of element substitution, (1) Ni ↔ Fe for Pn vs. (2) Co → (Ni + Fe), that governs the incorporation of Co into the Copn series, which contains from ~1 to ~8 apfu Co (Figure 21a; Table S12). The values of the correlation coefficient, calculated for a total of 473 data points, are R = −0.88 for the couple of Co vs. Fe and R = −0.66 for that of Co vs. Ni. We thus infer that the Fe-for-Co substitution dominates over the Co-for-Ni scheme in the Copn series at Tepsi. The same schemes of substitutions are recognized in the Yanisvaara samples, in which the maximum level of Co attained is 7.2 apfu in Copn [49], which is notably lower than the value (7.7 apfu Co) at Tepsi. In addition, members of the Copn series were previously reported from massive ores of the Norilsk complex, Russia; the Bushveld complex, South Africa; and an area of the Mid-Atlantic ridge, among other localities [60,61,62].

As noted, another oddity is the presence of minor scheelite at Tepsi (Figure 23a,b), associated with ilmenite, as well as minute crystals of zircon surrounded by olivine grains (Fo₇₇), or with more abundant scheelite and Fo₇₂ (TPST20, 23). This occurrence could well be the first record of scheelite in ultrabasic rocks; this species would be expected in contact metamorphic tactites, greisens, granite pegmatites, or high-temperature veins. A contact metasomatic mode of origin is unlikely in this case because these samples were collected far enough from the internal boundary (Figure 3). We suggest that trace levels of W likely followed Ti and Zr in their geochemical behavior to accumulate relatively late in the most evolved portions of the crystallizing melt, from which the low-Fo grains formed. The late appearance of scheelite with ilmenite could perhaps be an indication of the crystallization of an immiscible Fe-Ti melt. Tungsten is presumed to have been incompatible during the crystallization of droplets of the melt, from which small grains of scheelite were deposited to form the observed paragenesis with ilmenite.

6. Concluding Statements

The Tepsi ultrabasic intrusion is a fairly large member of the giant Serpentinite Belt–Tulppio Belt (SB—TB) → Lapland–Belomorian Belt (LBB) → Vetrenyi (Windy) Belt (VB) structure of Paleoproterozoic age (2.4–2.5 Ga). Both the Tepsi complex and the related Yanisvaara intrusion display much less primitive characteristics with respect to the Pados–Tundra layered intrusion, inferred to be located nearer the center of a large-scale plume of Al-undepleted komatiitic magma rising in the northeastern Fennoscandian Shield.

We infer that the Tepsi body is a deformed laccolith dipping southwest at an azimuth of ~200–220° and at an angle of 40–45°. The body has tectonic contacts with the host Archean basement. Its primary magmatic textures and parageneses are widely preserved in spite of tectonic stresses, which caused zones of foliation and deformation (likely at 1.9 Ga, coinciding with peak conditions of regional metamorphism). Fine-grained olivine in fresh domains, analyzed throughout the intrusion, gave the continuous range of Fo 90.5 → Fo 65.4. The corresponding variations in bulk-rock chemistry are also indicative of a significant extent of differentiation. The laccolithic body crystallized rapidly and could thus not form megacycles or a layered structure in such a shallow setting.

For the first time in the LBB—SB region, we document the development of spinifex-textured crystallites of hypermagnesian clinopyroxene (diopside) associated with serpentine and tremolite. They provide conclusive evidence of the komatiitic origin of the Tepsi complex, as well as the related complexes of subvolcanic or subplutonic suites in the LBB–SB structure. The spinifex-textured clinopyroxene likely predated the process of serpentinization and formed as a consequence of rapid crystallization in a differentiated portion of supercooled and oxidized melt or fluid, remaining after the bulk crystallization of olivine. Such conditions of petrogenesis could resemble those known in ultrabasic lavas or in the chill and quench zones of layered intrusions.

The compositions of hypermagnesian diopside, documented at Tepsi (and Yanisvaara), attain maximums of magnesium enrichment that are greater than those in samples of “normal” igneous clinopyroxene crystallizing in mafic–ultramafic complexes. The observed nucleation of clinopyroxene associated with calcic amphibole may be related to the development of gabbroic amphibole-bearing zones around a peridotite core in concentrically zoned intrusions of the LBB. The “opposite sequence” of crystallization, i.e., clinopyroxene after calcic amphibole, is ascribed at Tepsi to the effects of metastable and disequilibrium crystallization in response to rapid cooling of the injected melt.

Relative enrichments in H₂O, Cl, and F, observed in apatite (plus high contents of Cl in hibbingite or parahibbingite), indicate the abundance of volatiles accumulated during the differentiation of the Tepsi intrusion. As for the related suites of komatiite-derived rocks in the region, an intrinsic enrichment in volatiles was presumably important to affect the rheological characteristics of the intruding melt.

Low-sulfide Ni-Cu-Co mineralization occurs sporadically at Tepsi, with the presence of species enriched in As and Bi and submicrometric grains rich in Pt-Ir and Au-Ag. The first documentation of scheelite is reported here in relation to evolved olivine-based rocks with Fo_77–72. Diffuse zones, recorded in pentlandite, are enriched in (Pd + Bi). These findings point to the existence of PGE potential for Tepsi and the other complexes of the SB–TB–LBB–VB megastructure.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/min14070685/s1. Table S1. The composition of olivine in the Tepsi ultrabasic intrusion, Kola Peninsula, Russia; Table S2. The composition of calcic amphiboles in the Tepsi ultrabasic intrusion, Kola Peninsula, Russia, Table S3. The composition of Mg–Fe amphiboles in the Tepsi ultrabasic intrusion, Kola Peninsula, Russia, Table S4. The composition of clinochlore in the Tepsi ultrabasic intrusion, Kola Peninsula, Russia, Table S5. The composition of plagioclase and potassium feldspar in the Tepsi ultrabasic intrusion, Kola Peninsula, Russia, Table S6. The composition of clinopyroxene in the Tepsi ultrabasic intrusion, Kola Peninsula, Russia, Table S7. The composition of mica-group minerals in the Tepsi ultrabasic intrusion, Kola Peninsula, Russia, Table S8. The composition of garnet-group minerals in the Tepsi ultrabasic intrusion, Kola Peninsula, Russia, Table S9. The composition of minerals of the chromite–magnetite series in the Tepsi ultrabasic intrusion, Kola Peninsula, Russia, Table S10. The composition of ilmenite, rutile, and titanite in the Tepsi ultrabasic intrusion, Kola Peninsula, Russia, Table S11. The composition of apatite in the Tepsi ultrabasic intrusion, Kola Peninsula, Table S12. The composition of members of the pentlandite and cobaltpentlandite series in the Tepsi ultrabasic intrusion, Kola Peninsula, Table S13. The composition of pyrrhotite and troilite in the Tepsi ultrabasic intrusion, Kola Peninsula, Table S14. The composition of chalcopyrite and cubanite in the Tepsi ultrabasic intrusion, Kola Peninsula, Table S15. The composition of bornite and digenite in the Tepsi ultrabasic intrusion, Kola Peninsula, Table S16. The composition of heazlewoodite and millerite in the Tepsi ultrabasic intrusion, Kola Peninsula, Table S17. The composition of sphalerite or wurtzite in the Tepsi ultrabasic complex, Kola Peninsula, Table S18. The composition of minor or rare minerals in the Tepsi ultrabasic intrusion, Kola Peninsula, Table S19. Whole-rock contents of major oxides in the Tepsi ultrabasic intrusion, Kola Peninsula, Table S20. Whole-rock contents of minor and trace elements in the Tepsi ultrabasic intrusion, Kola Peninsula.

Author Contributions

A.Y.B. (investigation; project administration; funding acquisition; writing—original draft preparation); A.A.N. (investigation; writing—original draft preparation); R.F.M. (writing—original draft preparation; writing—review and editing); S.A.S. (formal analysis; methodology; data curation); B.M.L. (formal analysis; methodology; data curation). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Russian Science Foundation (grant #22-27-00419).

Data Availability Statement

The analytical data are provided in Tables S1–S20.

Acknowledgments

We thank the three reviewers, Academic Editors, and editorial staff for their comments and suggestions. We appreciate the professional help of the staff at the Analytical Center for multi-elemental and isotope studies, SB RAS, Novosibirsk, Russia. We also thank the staff of the Russian Geological Research Institute (VSEGEI) and the Federal Subsoil Resources Management Agency (Rosnedra) for providing access to the sets of geological maps. Egor Barkov is thanked for his assistance during the field investigation.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Geological setting of the Tepsi complex relative to Yanisvaara and related complexes of dunite–harzburgite–orthopyroxenite sequences in the Serpentinite Belt–Tulppio Belt (SB–TB) and of the “drusite” associations in the Lapland–Belomorian Belt (LBB) of Paleoproterozoic age. The locations and boundaries [21] are based on mapping by [27,28,29,30,31,32,33,34,35,36]. Peak conditions of metamorphism are estimated at 1.9 Ga in the region (e.g., [37]). Suites of the complementary Tulppio Belt (TB) are represented based on [22,23], for which a Neoarchean age was previously proposed.

Figure 2. Simplified geological map (a) shows the Paleoproterozoic Tepsi and Yanisvaara groups in the regional context [31]. In (b), we present a revised shape of the Tepsi intrusion based on results of our investigations.

Figure 3. The geologic sampling scheme of the Tepsi intrusion, which is based on GPS coordinates taken for a total of eighty-three outcrops examined (Figure 2b).

Figure 4. (a–d) Examples of outcrops examined in the Tepsi intrusion. Tectonic contacts, observed at the southwestern and northeastern contacts of the body, are shown in (a,b), respectively. The length of the sledgehammer is ~0.8 m.

Figure 5. Surface of a Tepsi specimen of fine-grained to aphanitic rock of ultrabasic composition.

Figure 6. Three-dimensional visualization of the geological structure (a) and a schematic cross-section (b) of the Tepsi intrusion along the line a-b shown in (a).

Figure 7. (a–f) Back-scattered electron images (BEI) showing mineral associations in ultrabasic rocks of the Tepsi complex. The following symbols are used: olivine (Ol), chromite (Chr), chromian magnetite (Mag), ilmenite (Ilm), hematite (Hem), apatite (Ap), anthophyllite (Ath), actinolite (Act), magnesio-hornblende (Mhbl), clinochlore (Clc), serpentine (Srp), dolomite (Dol), pentlandite (Pn), and troilite (Tro).

Figure 8. Lateral variations in compositions of olivine in the Tepsi complex, expressed in values of Mg#. The dashed line shows boundaries of the sampling area in which outcrops of ultrabasic rocks were examined. A small zone of plagioclase-bearing rocks is shown in blue.

Figure 9. The overall compositional variations in grains of olivine in the Tepsi complex are shown in a plot of FeO vs. MgO (both in weight %). Results of a total of 808 data points (n = 808) are plotted.

Figure 10. (a–c) Plot of FeO vs. MgO, expressed in weight %, showing variations at Tepsi in the composition of calcic amphiboles, based on a total of 1960 data points, n = 1960 (a), and of grains of Mg-Fe amphibole (b), for n = 157. A plot of MgO vs. Cr₂O₃ (both in weight %) in Figure 9c displays variations in the composition of members of the chlorite group (n = 712), mainly grains of clinochlore. These contain Cr in solid solution.

Figure 11. Compositions of grains of plagioclase and associated minor K-feldspar (Kfs), plotted on an Ab (albite)–An (anorthite)–Or (orthoclase) diagram (n = 57).

Figure 12. Clusters of micro-aggregates composed of crystallites of clinopyroxene, corresponding to hypermagnesian diopside (Di) hosted by a mineral of the serpentine subgroup (Srp) and associated with grains of tremolite (Tr) and chromian magnetite (Mag).

Figure 13. (a–f) BEI showing characteristic examples of spinifex-textured crystallites of hypermagnesian clinopyroxene (Cpx) associated with serpentine (Srp), tremolite (Tr), and chromian magnetite (Mag).

Figure 14. Compositional variation in grains of clinopyroxene, including crystallites of diopside of hypermagnesian composition, plotted on an En (enstatite)–Wo (wollastonite)–Fs (ferrosilite) diagram. The nomenclature used is after [39]. The symbol Di is diopside, Hd is hedenbergite, Aug is augite, Pgt is pigeonite, Cen is clinoenstatite, and Cfs is clinoferrosilite.

Figure 15. (a–f) BEI images showing associations of accessory and ore minerals developed in ultrabasic rocks of the Tepsi complex. The following symbols are used: apatite (Ap), Cr-bearing magnetite (Mag), ilmenite (Ilm), pentlandite (Pn), chalcopyrite (Ccp), troilite (Tro), bornite (Bn), sphalerite or wurtzite (Sp), as well as olivine (Ol), serpentine (Srp), tremolite (Tr), magnesio-hornblende (Mhbl), clinochlore (Clc), and dolomite (Dol). A micrometer-scaled mixture of Ni-Fe arsenides is labeled Ni-Fe-As.

Figure 16. (a–f) Accessory and ore minerals in ultrabasic rocks of the Tepsi complex. The following symbols are used, in addition to those shown in Figure 15: cobaltpentlandite (Copn), heazlewoodite (Hzl), orcelite (Orc), cobaltiferous maucherite (Muc), awaruite (Awr), monazite-Ce (Mnz-Ce), parkerite (Prk), Au-Ag alloy (Au); Ath is anthophyllite.

Figure 17. Compositional variation in accessory grains of members of the series chromite (Chr)–magnetite (Mag) from the Tepsi intrusion, plotted on a Cr–Fe³⁺–Al diagram. The symbol Hc stands for hercynite (sample TPST23). Results of a total of 908 data points are plotted. The miscibility gap is not shown for simplicity.

Figure 18. Lateral variations in Cr₂O₃ (expressed in weight %) observed in compositions of accessory grains of chromite–magnetite (Chr–Mag) in the Tepsi intrusion. The dashed line shows boundaries of the sampling area in which outcrops of ultrabasic rocks were examined.

Figure 19. Compositional variation in grains of ilmenite in the Tepsi intrusion, plotted on a Mg–Fe –Mn diagram. The symbols are ilmenite (Ilm), geikielite (Gk), and pyrophanite (Pph). Results of a total of 375 data points are plotted.

Figure 20. Compositional variation in accessory grains of apatite in the Tepsi intrusion, plotted on a Cl–F–OH diagram. The symbols are chlorapatite (Clap), fluorapatite (Fap), and hydroxylapatite (Hap). Results of a total of 318 data points are plotted.

Figure 21. (a,b) Compositional variations in grains of pentlandite (Pn) and cobaltpentlandite (Copn), expressed as values of atoms per formula unit (apfu) based on a total of 17 apfu. Results of a total of 473 data points are presented in plots of (Fe + Ni) vs. Co (a) and Ni vs. Fe (b).

Figure 22. The BSE image shows a small inclusion of hibbingite (Hib) or parahibbingite hosted in a late veinlet of hematite (Hem) developed among domains of serpentine (Srp) and clinochlore (Clc). Also present are tremolite (Tr) and subhedral grains of chromite (Chr).

Figure 23. (a,b) Grains of scheelite (Sch) in association with olivine (Ol), Cr-bearing magnetite (Mag), ilmenite (Ilm), clinochlore (Clc), and magnesio-hornblende (Mhbl).

Figure 24. (a–d) Plots of whole-rock contents of NiO vs. MgO (a) and CaO vs. Al₂O₃ (b), both expressed in weight %, and Nb vs. Zr (c) and Yb vs. Gd (d), plotted in ppm, for ultrabasic and basic rocks of the Tepsi intrusion (this study) compared with related rocks of the Chapesvara, Khanlauta, Lyavaraka, and Lotmvara II complexes, Kola Peninsula, based on [12,16,17,19].

Table 1. The composition of crystallites of hypermagnesian clinopyxene in the Tepsi ultrabasic intrusion, Kola Peninsula.

#	SiO₂	TiO₂	Al₂O₃	Cr₂O₃	FeO	MnO	MgO	CaO	NiO	Total
1	55.30				1.24	0.04	17.98	26.03		100.59
2	55.26				1.18	0.15	17.82	25.66		100.07
3	54.83				1.00	0.21	17.73	26.01		99.78
4	53.55			0.09	1.12		18.02	25.29		98.07
5	54.07			0.09	1.71	0.16	18.16	24.94	0.05	99.13
6	54.15	0.12			1.18	0.18	17.52	25.19		98.34
7	55.23	0.07			1.85	0.21	17.73	26.01		101.10
8	53.98				2.98	0.45	20.50	22.32		100.23
9	54.07	0.07			2.08	0.24	19.47	23.15		99.08
10	53.78				2.38	0.14	21.38	21.14	0.15	98.82
11	54.97	0.05			1.02	0.09	18.04	26.27		100.44
12	55.79			0.03	1.55	0.19	18.31	25.96		101.83
13	55.63				1.09	0.04	18.81	25.41	0.10	100.98
14	55.13				1.57	0.93	18.19	24.88	0.42	100.70
15	55.29			0.07	1.26	0.04	19.67	23.89		100.22
16	55.15	0.04			1.71	0.34	17.95	25.77	0.05	100.96
17	54.77	0.09			1.81	0.12	19.71	23.62	0.23	100.12
18	55.20				1.71	0.15	19.25	24.75		101.06
19	56.00				1.23	0.09	18.73	26.11	0.22	102.16
20	54.67	0.07			1.76	0.29	19.35	24.32	0.06	100.46
21	55.41				1.48	0.08	20.18	22.83	0.11	99.98
22	56.14				0.79	0.12	18.47	26.02	0.14	101.54
23	54.50			0.05	1.48	0.29	19.97	24.02		100.31
24	54.48				1.88	0.13	20.49	24.00	0.16	100.98
25	55.33	0.04			2.15	0.39	19.15	23.88		100.94
26	54.73			0.15	2.30	0.22	19.85	23.35		100.60
27	54.98				1.24	0.10	18.67	25.53		100.52
28	55.57				1.36	0.10	18.28	25.74	0.05	101.05
29	55.00			0.05	1.70	0.15	17.11	25.91		99.92
30	54.82	0.08			1.75	0.19	18.00	25.55	0.04	100.39
31	54.79	0.09			1.49	0.10	18.28	25.26	0.19	100.01
32	55.01				1.58		17.65	25.89	0.06	100.13
33	55.40			0.20	2.19	0.16	20.32	23.16	0.21	101.43
34	54.17	0.03		0.02	2.74	0.40	16.88	25.49	0.10	99.73
35	54.56			0.05	2.44	0.41	17.35	25.51	0.07	100.32
36	55.29	0.04		0.08	2.07	0.58	17.27	25.77	0.08	101.10
37	55.31	0.17			1.25	0.15	18.24	25.76		100.88
38	55.44	0.10		0.02	1.67	0.21	17.63	25.83	0.11	100.90
39	54.48	0.14	0.22	0.11	2.24	0.35	18.30	24.98		100.82
40	54.60	0.14		0.08	2.42		18.10	25.37	0.25	100.71
#	Si	Ti	Al	Cr	Fe	Mn	Mg	Ca	Ni	Wo	En	Fs	Mg#
1	1.99	0.000	0.000	0.000	0.04	0.001	0.97	1.00	0.000	50.0	48.1	1.9	96.2
2	2.00	0.000	0.000	0.000	0.04	0.005	0.96	1.00	0.000	49.8	48.2	2.0	96.0
3	1.99	0.000	0.000	0.000	0.03	0.006	0.96	1.01	0.000	50.4	47.8	1.8	96.3
4	1.97	0.000	0.000	0.003	0.03	0.000	0.99	1.00	0.000	49.4	48.9	1.7	96.6
5	1.97	0.000	0.000	0.003	0.05	0.005	0.99	0.98	0.001	48.3	48.9	2.8	94.5
6	2.00	0.003	0.000	0.000	0.04	0.006	0.96	1.00	0.000	49.7	48.2	2.1	95.8
7	1.98	0.002	0.000	0.000	0.06	0.006	0.95	1.00	0.000	49.7	47.2	3.1	93.9
8	1.94	0.000	0.000	0.000	0.09	0.014	1.10	0.86	0.000	41.7	53.3	5.0	91.4
9	1.97	0.002	0.000	0.000	0.06	0.007	1.06	0.90	0.000	44.5	52.0	3.5	93.7
10	1.95	0.000	0.000	0.000	0.07	0.004	1.15	0.82	0.004	40.0	56.3	3.7	93.8
11	1.98	0.001	0.000	0.000	0.03	0.003	0.97	1.01	0.000	50.3	48.1	1.7	96.7
12	1.99	0.000	0.000	0.001	0.05	0.006	0.97	0.99	0.000	49.2	48.3	2.6	94.9
13	1.99	0.000	0.000	0.000	0.03	0.001	1.00	0.97	0.003	48.4	49.9	1.7	96.7
14	1.98	0.000	0.000	0.000	0.05	0.028	0.97	0.96	0.012	47.7	48.5	3.8	92.8
15	1.99	0.000	0.000	0.002	0.04	0.001	1.05	0.92	0.000	45.7	52.4	1.9	96.4
16	1.98	0.001	0.000	0.000	0.05	0.010	0.96	0.99	0.001	49.2	47.7	3.1	94.0
17	1.97	0.002	0.000	0.000	0.05	0.004	1.06	0.91	0.007	44.9	52.2	2.9	94.8
18	1.97	0.000	0.000	0.000	0.05	0.005	1.03	0.95	0.000	46.7	50.6	2.7	94.9
19	1.98	0.000	0.000	0.000	0.04	0.003	0.99	0.99	0.006	49.1	49.0	1.9	96.2
20	1.96	0.002	0.000	0.000	0.05	0.009	1.04	0.94	0.002	46.0	50.9	3.0	94.4
21	1.99	0.000	0.000	0.000	0.04	0.002	1.08	0.88	0.003	43.8	53.9	2.3	95.8
22	2.00	0.000	0.000	0.000	0.02	0.004	0.98	0.99	0.004	49.6	49.0	1.4	97.3
23	1.95	0.000	0.000	0.001	0.04	0.009	1.07	0.92	0.000	45.2	52.2	2.6	95.3
24	1.94	0.000	0.000	0.000	0.06	0.004	1.09	0.91	0.005	44.4	52.7	2.9	94.8
25	1.98	0.001	0.000	0.000	0.06	0.012	1.02	0.92	0.000	45.5	50.7	3.8	93.1
26	1.96	0.000	0.000	0.004	0.07	0.007	1.06	0.90	0.000	44.1	52.2	3.7	93.3
27	1.98	0.000	0.000	0.000	0.04	0.003	1.00	0.98	0.000	48.6	49.4	2.0	96.1
28	1.99	0.000	0.000	0.000	0.04	0.003	0.98	0.99	0.001	49.2	48.6	2.2	95.7
29	2.00	0.000	0.000	0.001	0.05	0.005	0.93	1.01	0.000	50.6	46.5	2.8	94.3
30	1.98	0.002	0.000	0.000	0.05	0.006	0.97	0.99	0.001	49.0	48.1	2.9	94.3
31	1.98	0.002	0.000	0.000	0.05	0.003	0.99	0.98	0.006	48.6	49.0	2.4	95.3
32	1.99	0.000	0.000	0.000	0.05	0.000	0.95	1.00	0.002	50.1	47.5	2.4	95.2
33	1.96	0.000	0.000	0.006	0.06	0.005	1.07	0.88	0.006	43.5	53.1	3.4	93.9
34	1.98	0.001	0.000	0.001	0.08	0.012	0.92	1.00	0.003	49.6	45.7	4.8	90.5
35	1.98	0.000	0.000	0.001	0.07	0.013	0.94	0.99	0.002	49.2	46.5	4.3	91.5
36	1.99	0.001	0.000	0.002	0.06	0.018	0.93	0.99	0.002	49.7	46.3	4.0	92.1
37	1.99	0.005	0.000	0.000	0.04	0.005	0.98	0.99	0.000	49.3	48.6	2.1	95.9
38	2.00	0.003	0.000	0.001	0.05	0.006	0.95	1.00	0.003	49.8	47.3	2.8	94.3
39	1.96	0.004	0.009	0.003	0.07	0.011	0.98	0.96	0.000	47.6	48.5	3.9	92.6
40	1.96	0.004	0.000	0.002	0.07	0.000	0.97	0.98	0.007	48.4	48.0	3.6	93.0

Note. The results of SEM/EDS analyses are listed in weight %. Atom proportions were calculated on the basis of six atoms of oxygen per formula unit. Wo is wollastonite, En is enstatite, Fs is ferrosilite, and Mg# is defined as 100 Mg/(Mg + Fe + Mn).

Table 2. List of ore species and rare minerals recorded in the Tepsi intrusion.

#	Mineral	RA *	#	Mineral	RA *	#	Mineral	RA *
1	Magnetite (chromian)	M	14	Chalcopyrite	M	28	Westerveldite	R
2	Chromite	M	15	Pentlandite (With diffuse zones of Pd-Bi–enrichment)	M	29	Orcelite	R
3	Hercynite	R	16	Cobaltpentlandite	S	30	Arsenopyrite	R
4	Hematite	M	17	Pyrrhotite	M	31	Molybdenite	R
5	Ilmenite (magnesian and manganoan)	M	18	Troilite	M	32	Altaite	R
6	Rutile	R	19, 20	Heazlewoodite, Millerite	S	33	Hessite	R
7	Titanite	R	21	Cubanite	R	34	Ni-Fe-(Co) alloy (awaruite)	R
8	Hydroxylapatite	M	22	Bornite	S	35	Au-Ag alloy	R
9	Fluorapatite	S	23	Digenite	R	36	Bismuth (oxidized)	R
10	Chlorapatite	S	24	Sphalerite (or wurtzite)	S	37	Scheelite	R
11	Monazite-(Ce)	R	25	Parkerite	S	38	Hibbingite (or parahibbingite)	R
12	Xenotime-(Y)	R	26	Maucherite (and cobaltiferous variety: Ni₈Co₃As₈)	S	39	Iridarsenite? (Ir,Pt)(As,S)₂	R
13	Zircon	R	27	Baryte	S	40	Tomamaeite? PtCu₃	R

* RA: relative abundances estimated provisionally as main (M), subordinate (S), or rare (R) on the basis of frequency of occurrences observed in this study. Compositions of these minerals are represented in Supplementary Tables S9–S18.

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Barkov, A.Y.; Nikiforov, A.A.; Martin, R.F.; Silyanov, S.A.; Lobastov, B.M. The Tepsi Ultrabasic Intrusion, the Northern Part of the Lapland–Belomorian Belt, Kola Peninsula, Russia. Minerals 2024, 14, 685. https://doi.org/10.3390/min14070685

AMA Style

Barkov AY, Nikiforov AA, Martin RF, Silyanov SA, Lobastov BM. The Tepsi Ultrabasic Intrusion, the Northern Part of the Lapland–Belomorian Belt, Kola Peninsula, Russia. Minerals. 2024; 14(7):685. https://doi.org/10.3390/min14070685

Chicago/Turabian Style

Barkov, Andrei Y., Andrey A. Nikiforov, Robert F. Martin, Sergey A. Silyanov, and Boris M. Lobastov. 2024. "The Tepsi Ultrabasic Intrusion, the Northern Part of the Lapland–Belomorian Belt, Kola Peninsula, Russia" Minerals 14, no. 7: 685. https://doi.org/10.3390/min14070685

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