Critical Metals Mineralization in the Late-Stage Intrusions of Salmi Batholith, Ladoga Karelia, Russia

Abstract

The paper reports the results of studies on critical metal mineralization genetically related to the late-stage intrusions of Salmi anorthosite-rapakivi granite batholith (SARGB) in the Riphean age. In, Bi, and Be mineralization in skarn-greisen deposits and occurrences at the SARGB endocontact, as well as REE and Nb-Ta mineralization in Li-F granites, understood as the late intrusive phases of the batholith, were studied. It is the first report on columbite-group minerals, as well as REE-Ta-Nb and REE mineralization in SARGB granites. Optical and scanning electron microscopy, EDS and LA ICP MS microanalysis, X-ray fluorescence spectrometry, Raman spectroscopy, and inductively coupled plasma mass spectrometry (ICP-MS) were used. The data obtained show that roquesite formation was mainly triggered by the decay of In-bearing solid sphalerite and chalcopyrite solutions. Zavaritskite, associated with unoxidized sulphides, was derived hypogenically and seldom occurs in ores. A helvine-group mineral association with zinc-enriched spinel (ZnO 22%–25%) seems to have been one of the factors preventing genthelvite formation. The Muzilampi, Hepaoja and Avtodor ore occurrences in Li-F granites display similar REE and Nb-Ta mineralization. They are associated with Y-fluorite and Li-siderophyllite, which contain exceptionally high Nb concentrations (0.25%–0.78%) in Muzilampi granites. Additionally, fluorite-1 is commonly overfilled (to >50%) with micron-sized synchisite and parisite inclusions. Columbite-tantalite-group minerals, present at all the occurrences studied, occur solely as ferricolumbites with a dominant Mn/(Mn + Fe) ratio of <0.2. Biotite and Li-siderophyllite, associated with columbite, have an extremely high iron index Fe/(Fe + Mg) > 0.9 approaching the maximum values (~1.0) in the most differentiated granites.

1. Introduction

Modern high-tech production and science-intensive technologies are largely based on the use of rare metals generally recognized as critical minerals [1,2,3,4,5,6]. Progress in the modern global economy is impossible without critical metals. Their application range is very wide [1,6,7,8,9]. Russia has no government-endorsed list of critical minerals, but there is a list of the basic types of strategic minerals [10], which includes metals critical for Russia [11]. The deposits and occurrences of some of the metals are located in the Ladoga region of Russian Karelia [12]. They are concentrated mainly in the Pitkäranta Mining District (PMD). Their formation is believed to be associated with the SARGB [13,14,15,16,17,18] (Figure 1). The mineralogy of rare and critical metals in skarns from this area has been studied earlier [16,19,20,21]. The first results of the studies were based solely on optical and X-ray structural methods, but no chemical analyses were performed [16,19], resulting in some inaccuracies in identification of the minerals (particularly isomorphic series) and in the determination of rare-metal distribution in ore and skarn minerals. It was assumed, for example [16], that Be in skarn ores is almost entirely present isomorphically in vesuvianite, but the assumption was not quite correct. The main Be-concentrating minerals in greisenized skarns from the study area are danalite, bertrandite and chrysoberyl [19,20]. Recent studies on rare-metal mineralization in PMD dealt mainly with In, Bi and Be mineralogy in skarn-greisen associations [20,21]. However, ore localities in the late-stage intrusions of SARGB, consisting of rare-metal Li-F granites, have not been studied mineralogically.

The present paper reports the results of mineralogo-geochemical studies on ore mineralization (Ta, Nb, REE) in these granites. It is the first attempt to describe columbite-group minerals (CGM), REE-Ta-Nb and REE mineralization in SARGB granites. Earlier evidence for rare-metal mineralization in PMD skarns [20,21] is presented together with new data on their Be, Bi, and In mineralogy. The genesis of critical metal mineralization (CMM) in PMD is discussed and its industrial potential is appraised.

2. Geological Setting of the Salmi Batholith

SARGB is located at the eastern flank of the Ladoga Structure (Figure 1) recognized as part of the Mesoproterozoic Svecofennian Fold Belt, which evolved for over 1 Ga, witnessing continental and ocean-margin rifting and the opening of the Svecofennian Ocean followed by the convergent interaction of newly-formed oceanic crust with an Archean craton, the closure of the ocean and the formation of an accretion-collision orogen [22,23]. Post-collisional uplift, cratonization, and discrete intraplate magmatism took place at the final stage (SARGB—1.55–1.53 Ga) [16,23].

The Archean basement rocks (2.7–2.66 Ga gneiss, granite gneiss, and amphibolite) of the Ladoga Structure persist only as ”rimmed granite-gneiss domes” after P. Eskola [24]. Their formation mechanism has been the subject of long discussion [23,25,26]. The domes are rimmed by Pitkäranta volcanic-sedimentary rocks (Ludicovian 2.1–1.92 Ga) overlain by Ladoga turbidites (Kalevian 1.92–1.8 Ga) (Figure 1 and Figure 2). Pitkäranta carbonate rocks were metamorphosed to marbles, calciphyres, and skarns, with which complex (Sn-Cu-Zn-Fe-Be-In) mineralization at the SARGB exocontact is associated [13,14,15,16]. Ludicovian and Kalevian rocks are cut by Svecofennian (syn- and post-orogenic) granites and pegmatites, as well as SARGB of Riphean age. In the south of the area, all the above structural-mineralogical complexes are overlain through their weathering crust by Jotnian volcanic-sedimentary rocks (1.48 Ga Salmi suite) [16,23] (Figure 1 and Figure 2).

SARGB is part of the Ladoga-Dalecarlian anorthosite-rapakivi granite pluton belt stretching for almost 2000 km along the southern Fennoscandian Shield boundary (Figure 1) [16]. Similarly to most plutons of this belt, it intruded across long-lived weakened zones controlled locally by Late Svecofennian collision sutures. SARGB consists of the Salmi Massif proper, which intruded into Lower Proterozoic (Jatulian, Ludicovian, and Kalevian) supracrustal complexes, and its satellite, the Ulälega Massif, which cuts the Archean Karelian Craton (Figure 1). SARGB was produced by six discrete mafic and felsic magma intrusion impulses during ~17 Ma (1546.7 ± 1.7–1529.9 ± 0.6) [16,27]. The southwestern portion of the batholite is cut by subvolcanic basic rock bodies, comagmates of 1459–1457 Ma Salmi basalts [28], and the northeastern portion is cut by Enäjoki mildly alkaline medium-basic rocks of unknown age.

Geophysical data show that SARGB is a subhorizontal plate-like body varying in thickness from 2 km in its northwestern portion to 10 km in its southeastern portion [17,29]. As the rocks are succeeded locally in the same direction, their basicity and age increase (Figure 1).

The granitoids of the batholith are subdivided arbitrarily into three groups [16,30] recognized earlier [14,15] as intrusion phases: (1) biotite-amphibole granites, (2) biotite granites, (3) inequigranular Li-siderophyllitic and topaz-bearing zinnwalditic Li-F granites. In this series of granites, Ta and Nb concentrations increase gradually and Nb/Ta ratio decreases [30]. Li-F granites are widespread on the northwestern Salmi Massif margin in the southern PMD (Figure 2) and in the northern Ulälega Massif (Figure 1), making up small morphologically complex stock-like bodies and dikes. Their apical and endocontact zones are typically layered, forming striated textures, fluorite mineralization, (Figure 3) and stockscheider. The dark-colored minerals (biotite-lepidomelane, protolithionite, siderophyllite, zinnwaldite, ferrohastingsite, ferrohedenbergite and fayalite) of granitoids in all three groups are rich in iron [Fe/(Fe + Mg)] = 0.8–1.0. Fluorite, zircon, apatite, ilmenite, magnetite, anatase, allanite, uraninite, and thorite commonly occur as accessories. Granites of the second and third groups also contain monazite, bastnäsite, topaz, cassiterite, rutile, ilmenorutile, columbite, pyrochlore, and thorianite.

The REE distribution spectra of highly differentiated rare-metal siderophyllitic and lithium-fluorine (SARGB) granites exhibit a tetrad-effect of M-type [31]. This and other geochemical characteristics systematized by A.M. Larin [16], together with a low Zr/Hf ratio [12,21] (Figure 4), indicate a high rare-metal potential of SARGB’s Li-F granites.

In spite of the Sn-base metal specialization of SARGB granitoids [14], F concentration in the fluid equiponderous with the Li-F granite melt of this batholith is similar to that of Ta deposits in Li-F granites known in other regions [31].

3. Materials and Methods

Critical metal minerals and parent rocks were examined using optical microscopy, scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), LA ICP MS, X-ray fluorescence (XRF), Raman spectroscopy, inductively coupled plasma mass spectrometry (ICP-MS), and wet chemistry at the Institute of Geology, Karelian Research Centre, Russian Academy of Sciences (IG KRC RAS, Petrozavodsk, Russia).

Samples taken from natural rock exposures, exploration workings, ore spoil heaps in old mines, drill cores, as well as thin sections and polished sections kept at the Territorial Geological Fund for the Republic of Karelia, Petrozavodsk were studied.

A total of over 250 samples from the Old and New Ore Fields of the Pitkäranta deposit, the Hopunvaara and Lupikko ore fields, the Kitelä deposit, as well as the Kulismajoki, Muzilampi, Hepaoja and Avtodor occurrences, were analyzed. Specimens for ICP MS and XRF analysis were prepared from all samples and polished sections were created. The polished sections were studied by optical microscopy (an Axiolab pol-u optical microscope equipped with a digital photographic camera and a computer).

The composition of the minerals was analyzed in polished thin sections with a VEGA II LSH scanning electron microscope (Tescan, Brno, Czech Republic). The instrument was equipped with an energy dispersive spectrometry (EDS) Energy 350 system and an SDD X-Act3 detector (Oxford Inca Energy, Oxford, UK). Operating conditions were: 20 kV accelerating voltage, 5 nA probe current, 1 µs EDS process time, 105 cnts/s, 30 s counting time. The spectral lines for each element are CuKα, FeKα, ZnKα, MnKα, SKα, FKα, InLα, AgLα, AuLα, TeLα, SeLα, SnLα, AsLα, BiMα, PbMα, WMα. The standards used were: CaCO₃, CaF₂, FeS₂, PbTe, HgTe, TlSbSe₂, InAs, NaCl, Cu, Co, Ni, Zn, Mn, As, Se, Ag, Au, Sn, Te, W, and Bi. SEM-EDS quantitative data were obtained and processed using a Microanalysis Suite Issue 12, INCA Suite version 4.01; standard deviation (S) for In—0.6%–1.8%, Pb—1.3%–4.4%, Bi—0.8%–4.2%, Te—0.4%–2.0%, Se—0.4%–1.3%, Cu—0.7%–2.3%, Zn—1.0%–2.4%, Ag—0.3%–2.3%, S—0.4%–0.7%.

Trace elements in ore minerals were identified by the LA-ICP-MS method on an X-SERIES-2 quadrupole mass-spectrometer (Thermo Scientific, Waltham, MA, USA) equipped with a UP-266 Macro Laser Ablation attachment (New Wave Research, MODEL UP266 MACRO AT, Fermont, CA, USA) in the IG KSC RAS following the method described in [33]. The Nd:Yag laser operates at a wavelength of 266 nm and an energy output of 0.133 mJ (scan speed 70 µm/s, impulse frequency 10 Hz). All measurements were performed with identical parameters. Standard NIST 612 (National Institute of Standards and Technology USA) was used for calibration procedures. The trace element concentration values measured have the following relative standard deviation (RSD) parameters: for transition metals (Co, Mn, Cr, V, Cu, Zn) < 15%, for As < 20%, for In, Cd < 25%.

Major element concentrations in the granites were estimated by wet chemistry and XRF. XRF analysis was performed using an ARL ADVANT’X-2331 (Thermo Fisher Scientific, Ecublens, Switzerland) wavelength-dispersive spectrometer with a rhodium tube, working voltage of 60 kV, working current of 50 mA, and resolution of 0.01. Preliminarily, 2 g of each powdered sample was heated in ceramic crucibles, at 1000 °C, in a muffle furnace for 30 min. The loss of ignition was determined by a change in sample mass upon heating. For XRF measurements, 1 g of heated sample was mixed with Li-tetraborate flux and heated in an Au-Pt crucible to 1100 °C to form a fused bead.

Trace and rare-earth element concentrations were calculated by ICP-MS using an X Series 2 (Thermo Scientific, Bremen, Germany) mass spectrometer. Powdered samples were digested in acid mixture following a standard procedure [33]. Analytical accuracy was monitored by analyzing the USGS standard BHVO-2.

Raman spectroscopy was used to identify beryllium mineral phases. Raman spectroscopy analysis was conducted on a dispersive Nicolet Almega XR Raman spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with a green laser (532 nm, Nd-YAG (Thermo Fisher Scientific, Waltham, MA, USA). The spectra were collected at a spectral resolution of 2 cm⁻¹. A confocal microscope with a 50× lens was used to focus an excitation laser beam on the sample and to collect a Raman signal from a 2 µm diameter area. Raman spectra were obtained in an 85–4000 cm⁻¹ spectral region, with exposition time of 30 to 100 s for each scan, depending on signal intensity and a laser power of 2–10 mW to prevent sample disintegration.

4. Results

4.1. Critical Metallic Mineral Occurrence

SARGB-associated critical metal occurrences are divided into three types: skarn, skarn-greisen, and rare-metal Li-F granites.

4.1.1. Skarn and Skarn-Greisen Deposits and Occurrences

The first two types were described in detail earlier [20,21]. Their geology can be briefly described as follows. Skarn and skarn-greisen deposits and occurrences are located at the gently dipping southwestern SARGB exocontact, where the most differentiated rapakivi granites occur (Figure 2). The skarns were derived after Pitkäranta carbonate rocks, rimming remobilized Archean gneissose granite domes (Figure 2). Both lens-shaped and sheeted ore bodies occur. Skarn-greisen occurrences are either spatiously confined to Li-F-granite distribution areas or occur above their domes unexposed by erosion. Fe-Zn-Sn-Be mineralization is associated with them. The bulk of Be (up to 0.85%) is concentrated in vesuvianite [16]. The skarn deposits host Fe-Zn-Cu-Sn mineralization, in which tin of isomorphic form is abundant in garnet [14,15,34], reaching a concentration of about 3% [35]. The results of the Pb-isotopic systematics of PMD’s ores and igneous rocks [27,36,37] have shown that skarn deposits and SARGB granites have a common source of ore matter. Ore formation seems to have been split up into several stages. SARGB-related two-stage ore formation is supported by Sm-Nd mineral isochrones, 1558 ± 30 Ma and 1546 ± 28 Ma, obtained for tin-bearing skarns from the Kitelä deposit [16]. The formation of critical metallic minerals, mainly In, Be, and Bi, seems to be related to a second stage, at which rare-metal siderophyllitic and Li-F granites were involved.

In Mineralization

Indium mineralization is demonstrated by roquesite and In-bearing sphalerite [20,21]. In addition, unusual roquesite-galena association (intergrowths), micron-sized roquesite aggregates at the boundary of chalcopyrite and sphalerite grains and relatively high In concentrations in fluorite, magnetite and wittichenite, where In does not commonly occur, are noteworthy (see Supplementary Materials, Figures S1 and S2).

Bi Mineralization

Bismuth mineralization is demonstrated by a large group of minerals, in which native bismuth and wittichenite are most common and zavaritskite is least common [21]. Zavaritskite occurs in greisenized diopside skarns at W. Lupikko deposit (Figure 2). Xenomorphic zavaritskite aggregates measuring up to 60–80 µm are present in serpentinized diopside with pyrite-sphalerite mineralization (Figure 5a) and scarce fluorite grains (Figure 5b). Thread-like veinlets sometimes extend from its grains (Figure 5c). Zavaritskite-chalcopyrite intergrowths are occasionally encountered (Figure 5d). Zavaritskite has been found earlier [21] only as micron-sized rims evolving on hedleyite. In contrast to stechiometric composition, zavaritskite was found to contain Te and Pb impurities (

Table 1. Representative electron microanalyses and atomic proportions of zavaritskite.

wt.%	Lu1	Lu2	Lu3	Lu4	Lu5	Lu6	Lu7
Bi2O3	95.90	95.83	95.10	81.57	90.31	94.19	87.98
TeO2	bd	bd	bd	12.13	bd	bd	bd
PbO	bd	bd	bd	bd	4.73	bd	6.02
F	7.18	7.16	7.35	9.52	7.58	9.92	8.76
Total	103.08	102.99	102.44	103.22	102.62	104.11	102.76
-O=F2	3.02	3.01	3.09	4.01	3.19	4.18	3.69
Total	100.06	99.98	99.35	99.21	99.43	99.93	99.07
Chemical formula
Bi	0.984	0.985	0.976	0.724	0.930	0.890	0.878
Te				0.157
Pb					0.051		0.063
O	1.024	1.047	1.002	0.882	0.968	0.761	0.844
F	0.904	0.902	0.925	1.037	0.957	1.149	1.072

).

Be Mineralization

Optical microscopy and X-ray diffraction methods have revealed over ten beryllium minerals in PMD’s greisenized skarns [19], of which helvite-group minerals, e.g., bertrandite and chrysoberyl, are most common. So far, only helvite and danalite have been subjected to chemical analysis [38]. Danalite and helvite often occur in W. Lupikko and Herbertz-1 ores (Figure 2), chrysoberyl at Arsenic Mine, bertrandite in mushketowitic ores from the Bek mine.

Helvite and danalite at W. Lupikko deposit occurs in fluoritized gahnite-phlogopite skarns together with magnetite and micron-sized uraninite and thorianite aggregates in phlogopite (Figure 6). Their reddish-brown crystals measuring 3 cm [39], are variably corroded by phlogopite, biotite, and fluorite (Figure 6 and Figure 7a,c) and contain an abundance of gahnite, biotite, and other minerals present as inclusions (Figure 7b). Helvite-group minerals from W. Lupikko and Herbertz-1 ores are mainly high-Fe varieties of danalite, while the rest of the ore is a helvite variety, as indicated by their chemical composition (see Supplementary Materials, Table S1).

Bertrandite in mushketowitic ores from the Bek mine occurs as idiomorphic long-prismatic and colorless to pale-greyish spear-like crystals making up radiate fan-shaped aggregates, only several millimeters in size (Figure 8). The crystals are present in quartz, which also contains iron-free sphalerite, chlorite, and magnetite. In addition to optical methods and microprobe analysis, Raman spectroscopy was used to identify bertrandite (see Supplementary Materials, Figure S3).

4.1.2. Li-F Granites

A.M. Larin [16] was the first to reliably document the presence of Li-F granites in SARGB. Three rare-metal occurrences are now known. Two of them, Muzilampi and Hepaoja, were discovered by the Karelian Geological Survey [40] and one, Avtodor, by the Institute of Geology, KarRC, RAS.

Muzilampi

The Muzilampi ore occurrence lies in the northwestern mildly eroded portion of the Ulälega Massif (Figure 1). It is located in a mineralized Li-F granite dome (600 × 800 m) controlled by ring and radial faults [40]. The occurrence was studied only in natural rock exposures, because cores from two holes drilled in the 1980s were lost. One core is described briefly below [40] along with Muzilampi granites.

The domal structure of the Muzilampi occurrence resembles a 5–7 m-high ring bank towering over the surrounding terrain and Muzisuo Mire. The dome consists of inequigranular (porphyraceous, pegmatoid) topaz-bearing siderophyllitic granites carrying elevated Li (0.01%–0.03%) and F (up to 0.2%–0.4%) contents. Microcline-perthite is replaced by albite. Albite-oligoclase is sericitiized and albitized. Two varieties of quartz, dark dipyramidal and light xenomorphic, occur. Biotite is scattered, occurring as glomerogranular aggregates, and is variably replaced by muscovite and chlorite. Associated with it are orthite, pyrochlore, rutile, apatite, tourmaline, titanite, magnetite, monazite and fluorite. Striated texture, provoked mainly by the variable abundance of quartz, is occasionally encountered. Albitized zones locally contain fluoritic breccia (fluorite makes up as much as 30% of rock volume) and kaolinite. In mineralized granites with elevated trace element contents (see Supplementary Materials, Table S2), rare-metal mineralization, often with coffinite and zircon, is confined to glomerogranular biotite-siderophyllite and fluorite clusters (Figure 9).

It occurs as a group of REE-minerals, such as bastnesite, parisitite, monazite, euxenite, aeshynite, haleniusite, fluocerite, pyrochlore (Figure 10), as well as Nb and Ta minerals (Figure 11). Most of the above mineral aggregates are less than 100 µm in size. Bastnesite is commonly concentrated in biotite, forming clusters of fine, closely-spaced grains (Figure 10a,f) or making up bigger aggregates in fluorite (Figure 10h). Fluorite is commonly full of micron-sized parisite inclusions (Figure 10e,g), and its pure varieties (Figure 10i) are rich in Y (1.2%–2.8%) and Sc (0.37%–0.93%). Haleniusite-parisite intergrowths in biotite aggregates occur (Figure 10d,e). Pyrochlore, occurring as xenomorphic poorly-defined grains, often together with columbite, bastnesite, and zircon, also occurs there (Figure 10b). Columbite is more common in other mineral associations with: (a) quartz and biotite (Figure 11a), (b) with fluorite and biotite (Figure 11b), and (c) aeschynite, parisite and fluorite (Figure 11c,d) with bastnesite and biotite (Figure 11d). Spongy-textured columbite is genetically interesting and important (Figure 11b). The chemical composition of the above minerals, primarily euxenite-, aeschynite-, columbite- and tantalite-group minerals, is also essential.

Euxenite- and aeschynite-group minerals are difficult to identify due to their variable composition, isomorphism, and metasomatic alterations [41,42]. The composition of these minerals from the Muzilampi occurrence displays considerable variations in Ti, Fe, Ta, and Th concentrations (

Table 2. Representative electron microanalyses and atomic proportions of aeschynite. (Mu1, Mu4, Mu5) and euxenite (Mu2, Mu3, Mu6, Mu7) occurrence Muzilampi.

wt.%	Mu1	Mu2	Mu3	Mu4	Mu5	Mu6	Mu7
CaO	3.06	2.20	2.02	1.91	2.70	1.89	1.92
TiO2	7.64	13.00	11.71	11.51	12.30	11.21	12.57
FeO	7.16	4.45	6.94	4.42	6.17	7.14	8.33
Y2O3	bd	8.35	7.62	8.52	7.90	8.82	6.24
Nb2O5	36.15	40.94	43.38	38.84	42.79	40.99	42.12
Ta2O5	23.20	10.44	8.75	9.76	8.80	9.38	9.33
Ce2O3	3.72	3.37	2.05	3.14	4.80	3.04	3.05
Nd2O3	2.34	bd	1.92	1.43	bd	bd	0.48
WO3	bd	6.03	6.08	6.21	4.61	6.04	4.77
ThO2	8.65	4.76	2.24	3.49	2.51	3.43	4.38
UO3	6.95	5.53	7.33	9.66	6.38	7.29	6.49
Total	98.87	99.09	100.04	100.08	98.96	99.24	99.67
Chemical formula
Ca	0.227	0.151	0.135	0.133	0.180	0.128	0.128
Ti	0.398	0.628	0.552	0.563	0.577	0.536	0.588
Fe	0.415	0.239	0.364	0.241	0.322	0.380	0.433
Y		0.286	0.254	0.294	0.263	0.298	0.206
Nb	1.132	1.189	1.229	1.141	1.207	1.177	1.184
Ta	0.437	0.182	0.149	0.172	0.149	0.162	0.158
Ce	0.095	0.079	0.047	0.084	0.110	0.071	0.069
Nd	0.058		0.043	0.033			0.011
W		0.100	0.099	0.105	0.074	0.100	0.077
Th	0.137	0.070	0.032	0.052	0.036	0.050	0.062
U	0.101	0.074	0.097	0.132	0.083	0.097	0.085
O	6.066	6.209	6.119	6.082	6.025	6.075	6.046
CV1	−1.421	0.608	−0.005	0.528	0.324	−0.232	−0.356
CV2	−0.284	−1.031	−1.738	−1.361	−1.494	−1.867	−1.565

CV1, CV2 [41]; CV1 = 0.245 Na + 0.106 Ca − 0.077 Fe* + 0.425 Pb + 0.220 Y + 0.280 LREE + 0.137 HREE + 0.100 U* + 0.304 Ti + 0.097 Nb + 0.109 Ta* − 12.81 (oxide wt.%); CV2 = 0.102 Na − 0.113 Ca − 0.371 Fe* − 0.167 Pb − 0.395 Y − 0.280 LREE − 0.265 HREE − 0.182 U* − 0.085 Ti − 0.166 Nb − 0.146 Ta* + 17.29 (oxide wt.%) Fe* = FeO + Fe₂O₃ + MnO, U* = UO₂ + UO₃ + U₃O₈ + ThO₂, and Ta* = Ta₂O₅ + WO_3.

), making them hard to identify reliably.

Columbite- and tantalite-group minerals are more persistent in composition (

Table 3. Representative electron microanalyses and atomic proportions of columbite occurrence Muzilampi.

wt.%	Mu1	Mu2	Mu3	Mu4	Mu5	Mu6	Mu7	Mu8	Mu9	Mu10	Mu11	Mu12	Mu13	Mu14	Mu15	Mu16
Ta2O5	bd	bd	1.52	2.01	2.32	2.83	3.21	3.45	4.03	4.30	4.31	4.93	5.25	6.06	6.17	14.55
Nb2O5	76.18	76.41	72.82	73.01	72.28	73.84	73.04	72.91	73.11	68.79	74.66	71.81	74.56	67.61	70.95	59.28
TiO2	1.71	1.73	2.50	1.46	2.06	1.41	1.59	1.47	1.07	2.17	1.63	1.48	0.82	1.68	1.52	0.70
WO3	bd	bd	2.64	3.16	2.45	1.26	0.74	bd	2.36	1.98	bd	1.66	bd	2.16	2.56	4.12
Y2O3	bd	bd	1.71	2.10	bd	bd	bd	bd	bd	bd	bd	bd	bd	bd	bd	2.57
Yb2O3	bd	bd	bd	bd	bd	bd	bd	bd	bd	2.16	bd	bd	bd	bd	bd	bd
FeO	19.41	18.69	17.05	18.13	18.24	18.44	19.11	19.18	17.53	17.88	17.66	18.80	17.32	18.24	17.56	17.18
MnO	2.27	2.52	1.30	1.46	1.99	2.23	1.75	2.99	1.47	1.99	1.74	1.95	1.78	1.99	2.32	0.89
Total	99.57	99.36	99.55	101.34	99.33	100	99.44	100	99.57	99.27	100	100.63	99.72	97.95	101.10	99.28
Chemical formula
Ta			0.024	0.031	0.036	0.044	0.049	0.053	0.064	0.068	0.067	0.077	0.083	0.097	0.97	0.243
Nb	1.918	1.932	1.893	1.869	1.870	1.897	1.878	1.846	1.926	1.804	1.933	1.848	1.954	1.792	1.843	1.646
Ti	0.072	0.073	0.108	0.062	0.088	0.060	0.068	0.061	0.046	0.094	0.07	0.063	0.035	0.074	0.066	0.033
W			0.039	0.046	0.036	0.019	0.049		0.035	0.030		0.024		0.033	0.038	0.066
Y			0.053	0.063												0.084
Yb										0.039
Fe	0.904	0.875	0.820	0.859	0.872	0.876	0.909	0.898	0.854	0.867	0.845	0.895	0.840	0.904	0.843	0.882
Mn	0.107	0.119	0.064	0.071	0.097	0.107	0.084	0.142	0.073	0.098	0.084	0.094	0.087	0.099	0.113	0.046
O	5.950	5.970	6.089	6.036	6.018	6.013	5.980	5.910	6.099	5.982	6.069	6.00	6.089	5.973	6.052	6.041
Ta/(Ta + Nb)	0	0	0.013	0.016	0.019	0.023	0.025	0.028	0.032	0.036	0.034	0.040	0.041	0.051	0.050	0.129
Mn/(Mn + Fe)	0.106	0.120	0.072	0.076	0.100	0.109	0.085	0.137	0.079	0.102	0.090	0.095	0.094	0.099	0.118	0.050

). All analyses are consistent with ferricolumbite carrying low tantalum and manganese contents, presumably because the samples analyzed were all taken from natural rock exposures typical of only one level of upward differentiation in granites.

Muzilampi granites locally contain accessory ilmenorutile (up to 8% Nb, 0.37%–0.72% In). Forecast resources for the Muzilampi deposit to a depth of 100 m are estimated at 30,000 t Ta₂O₅, and the average Ta₂O₅ is 0.03% [40].

Hepaoja

The occurrence of Hepaoja ore appears in the western endocontact zone of the Salmi Massif, near the southeastern end of the Lupikko gneissose granite dome (Figure 1 and Figure 2). At Hepaoja, the Karelian Geological Survey has revealed elevated Nb₂O₅ (0.008%–0.05%) and Ta₂O₅ (0.003%–0.015%) contents in albitized and greisenized granites [40]. No drilling has been performed there.

Field work at Hepaoja natural exposures has exposed the apical portion of a stock-like rare-metal granite body made up of greisenized topaz-bearing siderophyllitic granites and stockscheider with elevated trace element contents (see Supplementary Materials, Table S3).

Rare-earth mineralization (bastnesite, parisite, monazite, aeschynite, euxinite, samarskite, and sahamalite) is associated with quartz, muscovite, and albite, less commonly with fluorite, biotite, kaolinite, and K-feldspar (Figure 12). Parisite occurs as laminae, up to 1 mm in size, in quartz-muscovite aggregates (Figure 12b) and as micron-sized inclusions together with bastnesite in fluorite (Figure 12a). The fluorite is less saturated with inclusions than Muzilampi fluorite. Aeschynite, euxenite, and samarskite occur as micron-sized inclusions in biotite. Their chemical composition is too complex (see Supplementary Materials, Table S4) to reliably identify these minerals using the methods proposed [41,42,43]. Bastnesite as the most common rare-earth mineral in Hepaoja granites contains minor Ca and Y impurities, and some analyses show the presence of Pm, Eu, Gd, and Th (

Table 4. Representative electron microanalyses and atomic proportions of the bastnäsite occurrence of Hepaoja.

wt.%	H1	H2	H3	H4	H5	H6
CaO	bd	1.36	bd	1.73	3.54	2.41
Y2O3	bd	bd	bd	bd	1.31	4.66
La2O3	354.80	37.75	16.73	20.22	13.34	12.21
Ce2O3	8.46	14.82	40.72	36.95	36.82	35.41
Pr2O3	7.51	3.78	3.39	3.56	4.26	4.00
Nd2O3	18.36	14.48	14.91	11.85	14.87	14.76
Pm2O3	bd	2.41	bd	bd	bd	bd
Eu2O3	bd	2.42	bd	bd	bd	bd
Gd2O3	3.13	2.04	bd	bd	bd	bd
ThO2	1.45	bd	bd	bd	2.20	4.21
CO2*	19.92	18.87	20.05	21.24	19.80	18.67
F	9.27	5.93	7.25	7.69	6.66	6.33
Total	103.90	102.50	103.05	103.24	102.80	102.66
-O=F2	3.90	2.50	3.05	3.24	2.80	2.66
Total	100.00	100.00	100.00	100.00	100.00	100.00
Chemical formula
Ca		0.055		0.065	0.138	0.096
Y					0.025	0.092
La	0.472	0.524	0.225	0.262	0.179	0.168
Ce	0.111	0.204	0.545	0.476	0.490	0.484
Pr	0.099	0.052	0.045	0.046	0.056	0.054
Nd	0.234	0.195	0.194	0.149	0.193	0.197
Gd	0.037	0.032
Th	0.024	0.031			0.036	0.071
		0.025
CO3	0.971	0.970	1.000	1.020	0.983	0.952
F	1.047	0.706	0.838	0.855	0.766	0.748

All analyses were normalized to 100%. CO₂*—calculated data.

).

Prismatic columbite aggregates, measuring up to 200 µm, are present in biotite, quartz (Figure 13a,b), and feldspars (Figure 13d). Zonal crystals, in which Ta concentration is highly variable, occur (Figure 13c). Compositionally, both Hepaoja and Muzilampi columbites are interpreted as ferricolumbites, but Hepaoja columbite has higher Ta contents and, hence, higher Ta/(Ta + Nb) ratios (

Table 5. Representative electron microanalyses and atomic proportions of the columbite occurrence of Hepaoja.

wt. %	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17
Ta2O5	3.40	4.60	5.93	7.07	7.44	8.01	11.49	12.11	12.56	12.60	12.94	14.82	17.07	18.04	21.75	25.00	38.16
Nb2O5	72.78	73.49	70.38	69.04	69.63	68.38	61.32	65.25	65.86	64.46	63.60	63.60	60.58	58.47	55.27	50.33	38.08
TiO2	1.46	1.67	1.17	1.07	1.58	1.57	1.68	0.65	bd	0.61	bd	0.71	0.84	1.18	1.41	1.28	2.33
WO3	1.64	bd	1.88	2.39	bd	1.75	6.24	1.81	2.81	2.56	2.62	2.52	2.35	bd	3.03	5.68	4.40
Sc2O3	bd	bd	0.38	bd	bd	bd	bd	bd	bd	bd	0.40	bd	bd	bd	0.41	bd	bd
FeO	18.58	18.38	17.88	18.52	19.39	19.03	17.69	18.11	17.39	17.88	18.39	16.73	16.92	17.06	16.46	15.93	15.38
MnO	2.14	1.86	2.38	1.93	1.96	1.26	1.59	2.08	1.38	1.89	2.06	1.63	2.35	2.17	1.68	1.78	1.64
Chemical formula
Ta	0.052	0.071	0.093	0.112	0.115	0.126	0.188	0.195	0.207	0.205	0.209	0.245	0.282	0.299	0.368	0.435	0.696
Nb	1.873	1.892	1.832	1.812	1.798	1.794	1.669	1.748	1.800	1.739	1.711	1.748	1.667	1.614	1.553	1.458	1.155
Ti	0.063	0.071	0.050	0.047	0.068	0.068	0.076	0.029		0.027		0.033	0.038	0.054	0.066	0.062	0.118
W	0.024		0.028	0.036		0.026	0.097	0.028	0.044	0.040	0.040	0.040	0.037		0.049	0.094	0.077
Sc			0.019								0.021				0.023
Fe	0.885	0.876	0.860	0.889	0.926	0.923	0.891	0.897	0.879	0.892	0.915	0.851	0.862	0.871	0.855	0.854	0.862
Mn	0.104	0.089	0.116	0.095	0.095	0.062	0.081	0.104	0.071	0.096	0.104	0.084	0.116	0.113	0.089	0.096	0.093
O	6.000	5.944	6.001	5.996	5.940	5.999	6.057	6.001	6.100	6.022	5.971	6.104	6.038	5.868	6.060	6.089	6.050
Ta/(Ta + Nb)	0.027	0.036	0.048	0.058	0.060	0.066	0.101	0.100	0.103	0.105	0.109	0.123	0.145	0.156	0.192	0.230	0.376
Mn/(Mn + Fe)	0.105	0.092	0.119	0.097	0.093	0.063	0.083	0.104	0.075	0.097	0.102	0.90	0.119	0.115	0.094	0.101	0.097

All analyses were normalized to 100%.

). It typically displays greater variations in W and occasionally high Sc₂O₃ contents (up to 0.4%).

Avtodor

Avtodor ore occurrence lies in the northeastern Lupikko gneissose granite dome (Figure 2). It is confined to a morphologically complex stock-like Li-F granite body exposed by a crushed stone quarry. The quarry walls show the following zonal structure of the body (from the base upwards): fine-grained leucogranites—striated-textured topaz-bearing leucogranites—greisenized topaz-bearing granites—a fluoritic breccia zone—stockscheider. Zones from greisenized granites upwards are most markedly mineralized (rare-earth minerals, columbite, ilmenorutile, Nb-rutile, molybdenite, wolframite and bismuth). The zones vary greatly in thickness (0–3 to 20–30 m) and morphology. The lateral contacts of the granite body are eruptive, forming compositionally complex breccias containing fluorite-rare-earth mineralization (Figure 14).

Monazite, xenotime, bastnesite, and synchysite are major rare-earth minerals at the Avtodor occurrence. In the greisenized topaz-bearing granite zone, monazite and xenotime are associated with topaz, fluorite, biotite, and molybdenite (Figure 15a–e), often forming intergrowths. Stetendite is scarce (Figure 15i). Bastnesite and synchysite occur jointly in the above zone (Figure 15f–h) and in fluoritic breccias and stockscheider zones (Figure 16c,f). Synchysite commonly evolves after bastnesite. In the fluoritic breccias and stockscheider zones, monazite and xenotime forms both idiomorphic zonal crystals (Figure 16b,d) and xenomorphic grains (Figure 16a). Monazite is overgrown with synchysite and hollandite rims (

Table 6. Representative electron microanalyses and atomic proportions of the monazite occurrence of Avtodor.

Wt.%	Av1	Av2	Av3	Av4	Av5	Av6	Av7	Av8	Av9	Av10
La2O3	18.80	12.53	11.12	22.46	10.67	13.84	13.64	12.77	13.00	14.59
Ce2O3	35.46	35.25	27.66	37.52	34.58	35.61	37.55	39.23	34.40	34.55
Pr2O3	bd	bd	bd	bd	4.01	3.08	bd	bd	bd	2.80
Nd2O3	12.35	13.23	22.00	8.66	16.19	10.55	9.86	10.93	14.58	12.37
ThO2	bd	5.60	6,60	bd	3.87	10.02	8.55	5.28	4.37	6.24
P2O5	32.05	33.38	32.86	30.78	30.29	27.28	29.34	32.33	32.29	29.32
Total	98.66	99.99	100.23	99.41	99.60	100.37	98.94	100.54	98.64	99.87
Chemical formula
La	0.269	0.183	0.157	0.320	0.164	0.211	0.201	0.170	0.173	0.216
Ce	0.504	0.503	0.398	0.540	0.506	0.538	0.560	0.530	0.451	0.509
Pr					0.060	0.046				0.041
Nd	0.172	0.183	0.304	0.120	0.230	0.155	0.140	0.140	0.191	0.178
Th		0.046	0.063		0.030	0.094	0.080	0.050	0.040	0.057
P	1.055	1.085	1.078	1.020	1.010	0.956	1.001	1.110	1.100	0.999
O	4.055	4.108	4.052	4.020	4.025	4.003	4.014	4.135	4.053	4.005

; Figure 16e). Avtodor rare-earth minerals are similar in chemistry to those of the Hepaoja occurrence (see Supplementary Materials, Tables S5 and S6). Hepaoja fluorite from the breccia zone is comparable in REE (up to 0.7% Y, up to 2.4% Nd) and Sc (дo 0.6%) contents to Muzilampi fluorite.

Ta-Nb mineralization is poorly developed and occurs mainly as micron-sized (<10 µm) ferricolumbite, ilmenorutile, and Nb-bearing rutile grains. These minerals are sometimes intergrown in topaz replaced by micron-sized kaolinite veinlets (Figure 17a,b). Ilmenorutile and Nb-rutile in the intergrowths are, in turn, closely intergrown. Isolated Nb-rutile aggregates occur together with pyrite in chlorite (Figure 17c). The main difference of Avtodor ferricolumbite from ferricolumbite in other occurrences is its higher Mn contents and higher Mn/(Mn + Fe) ratios (

Table 7. Representative electron microanalyses and atomic proportions of the columbite (Av1, Av2), ilmenorutile (Av3-Av8) and rutile (Av9) occurrence Avtodor.

wt. %	Av1	Av2	Av3	Av4	Av5	Av6	Av7	Av8	Av9
TiO2	1.79	3.13	31.06	68.27	61.52	74.10	71.55	78.14	95.24
FeO	13.94	13.53	9.52	6.90	7.20	5.48	5.76	5.08	2.10
MnO	4.58	5.35	3.68	bd	bd	bd	bd	bd	bd
Nb2O5	61.35	65.26	39.45	19.65	18.02	15.84	14.24	11.36	2.21
Ta2O5	14.79	8.66	9.33	4.61	12.08	4.21	5.98	4.99	bd
WO3	4.29	2.80	6.68	bd	1.34	bd	2.86	bd	bd
Sc2O3	bd	0.47	bd	bd	bd	bd	bd	bd	bd
Total	100.74	99.20	99.73	99.43	100.16	99.64	100.37	99.57	99.55
Chemical formula
Ti	0.081	0.138	0.413	0.764	0.722	0.812	0.798	0.846	0.963
Fe	0.704	0.664	0.141	0.086	0.094	0.067	0.071	0.061	0.024
Mn	0.235	0.266	0.550
Nb	1.673	1.729	0.315	0.132	0.127	0.104	0.095	0.074	0.013
Ta	0.243	0.161	0.450	0.019	0.051	0.017	0.024	0.019
W	0.067	0.042	0.031		0.005		0.011
O	6.092	6.057
Ta/(Ta + Nb)	0.127	0.085
Mn/(Mn + Fe)	0.250	0.286

). It is similar in Ta₂O₅ contents (<15%) to ilmenorutile with which it is intergrown (Table 7).

5. Discussion

5.1. Formation of Roquesite

The analysis of data on In contents in the various minerals of PMD’s skarn ores and roquesite distribution in them (see Supplementary Materials, Figures S1 and S2) [20,21] reveals something new in the indium formation pattern. Roquesite seems to have been formed due to the release of indium upon the disintegration of its solid solutions in chalcopyrite, sphalerite and, presumably, galena. Micron-sized roquesite aggregates, formed along the boundaries of sphalerite grains present in chalcopyrite (see Supplementary Materials, Figure S2), seem to provide evidence for its formation by the diffusion of In from sphalerite upon a hydrothermal-temperature effect on the ores triggered by SARGB’s late intrusive phases. This formation pattern of roquesite is similar to the mechanism of removal of invisible gold from arsenopyrite’s crystalline lattice, which provoked the formation of independent aggregates upon a rise in temperature [44]. Experimental data [45] and the study of In-bearing base-metal mineralization in Kumi greisens from Vyborg rapakivi batholiths in southern Finland [46] suggest that In could have penetrated isomorphically into galena. Upon further alteration of galena, In may have been released from it, binding in its own mineral—roquesite. This is how roquesite, intergrown with galena, seems to have been derived from propylitized skarns from Valkealampi Mine and the Kitelä deposit (see Supplementary Materials, Figure S2).

5.2. Formation of Zavaritskite

To better understand the origin of zavaritskite, more zavaritskite samples from PMD’s skarn ores are needed. Zavaritskite BiOF is a rare mineral discovered in the Sherlovaya Gora rare-metal deposit in quartz-siderophyllitic greisen located in the eastern Trans-Baikal region [47]. It is now known to occur in skarns [48,49,50], hydrothermal veins in granites [51,52], pegmatites [53,54], and greisens [55,56]. Zavaritskite is assumed to have been formed either hypogenically [12,51,57] or hypergenically [47,56,58]. Geochemical bonding between Bi and F has been described from many ore fields associated with granitoid magmatism [48,51,53]. The main argument in favor of the hypergenic formation of zavaritskite is provided by its late evolution, the replacement of native bismuth, and the formation of zonal (from the center towards the margin) bismuth-zavaritskite-bismite aggregates [58]. This replacement is simultaneous with the hypergenic alteration of other minerals. In PMD’s ores, zavaritskite occurs only in greisenized skarns, forming micron-sized rims around native bismuth and hedleyite grains [35] or independent aggregates in highly altered (serpentine) phlogopite-diopside skarns often intergrown with chalcopyrite (Figure 5). Associated with it in both cases are sulphides (pyrite, sphalerite) unaffected by oxidation. Thus, zavaritskite in PMD skarn ores was most probably derived hypogenically at the final stages of the greysen alteration of skarns. Microprobe chemical analyses of zavaritskite are readily recalculated using its formula, in spite of relatively high Te and Pb concentrations in some of them (Table 1). Hence, further studies are needed, because such impurity contents in zavaritskite have not been reported earlier in scientific publications.

5.3. Formation of Helvite-Group Minerals

Helvite-group minerals in PMD’s skarn ores occur selectively only at W. Lupikko and Herbertz-1 in greisenized skarns with abundant fluorite and magnetite. The unusual composition of minerals in this group (Mn,Fe,Zn)₄(Be₃Si₃O₁₂)S, combining metal orthosilicate and sulphide, is responsible for their high sensitivity to changes in the fugacities of O₂ and S₂, the activity of phenacite, and the alkalinity of the system [59,60,61,62]. These factors are responsible for the narrow stability field of helvite-group minerals within the above parameters. This applies particularly to danalite [59]. The composition of helvite-group minerals is also strongly controlled by the composition of the host medium [60,63]. Hence, considerable variations in the composition of helvite-group minerals PMD’s ores (Figure 18) indicate a difference in the conditions of their formation.

The prevalence of danalite over helvite and the absence of genthelvite in the samples analyzed suggest the limited volatility of oxygen and sulphur, low alkalinity and the external supply of Fe into the hydrothermal system conducive to danalite stability [64,65]. Furthermore, a helvite-group mineral association in W. Lupikko skarns with zinc-enriched spinel (ZnO 22%–25%) (Figure 6) seems to have been one of the factors preventing genthelvite formation.

5.4. Formation of Critical Metals Mineralization in Li-F Granites

The Muzilampi, Hepaoja, and Avtodor rare-metal occurrences display both similarities and differences. They are all genetically related to well-developed SARGB rare-metal granites, which were the last to intrude. Avtodor granites exhibit some geological (distinctive vertical structure), petrographic (layering, striated textures, and stockscheider) and mineralogo-geochemical (abundant topaz, fluorite and protolithionite) features of Li-F granites, while Muzilampi and Hepaoja granites show none of these features. Therefore, they cannot be studied over a great depth. However, some of the characteristics of these granites, described in Section 4 of the present paper, make it possible to tentatively interpret them as Li-F granites, although stronger arguments are needed.

The biotite of Muzilampi granites, similar in composition to Li-siderophyllite and protolithionite, contains high Nb, Ta and REE contents (

Table 8. Trace elements composition of biotite occurrence Muzilampi (LA ICP MS).

ppm	1	2	3	4	5	6
Be	22.35	44.7	22.35	bd	18.63	bd
Sc	39.89	44.22	45.86	46.68	40.12	49.25
Zn	264	253.2	390.3	593.5	131	199.4
Ga	125	166.3	192.2	222.9	103.5	137.3
Ge	29.49	26.22	30.95	32.04	19.66	39.32
As	35.91	39.57	58.26	32.98	38.47	64.12
Rb	2497	3756	5805	5037	1811	2932
Sr	183.5	134.9	125.9	135.8	182.3	335.9
Y	1442	941	1258	346.4	198.9	639.2
Zr	378.4	681.2	349.4	143.2	454.1	574
Nb	2517	5117	7779	7487	2645	3087
Cd	bd	23.57	bd	47.13	28.28	bd
In	10.17	7.557	8.196	7.128	6.628	9.077
Sn	43.37	64.32	68.69	98.09	25.61	48.02
Ba	692.4	1005	1110	1341	882.2	977.7
Hf	20.15	33.58	20.15	20.15	16.79	6.716
Ta	307.6	611.6	908.9	939.3	323.9	517.7
Pb	121.7	125.5	212.8	101.7	82.91	90.81
Th	43.5	48	79.5	25.5	36	29.25
U	46.41	65.25	33.82	16.42	17.47	8.84
REE	4756.85	2829.52	4016.57	1272.6	477.93	1786.21

). Such high Nb contents in biotite mica, in comparison with the data of Z. Zhu et al. [66], are the highest of those ever reported for mica from granites. Another distinctive feature is the high saturation (>50%) of fluorite in Muzilampi granites with micron-sized synchysite and parisite inclusions (Figure 10e,g). The same features are displayed by fluorite from Avtodor granites (Figure 14) and, to a lesser extent, by Hepaoja fluorite (Figure 12a). Petrographic data show that in all of the above cases fluorite is the latest primary mineral in granites, forming interstices between their major minerals: K-feldspar, oligoclase-albite, biotite, and quartz.

Hence, at the final stage of granitic melt crystallization, it was probably oversaturated with fluorine, including REE’s fluorine complexes. High F activity continued to dominate at the postmagmatic stage of mineral formation, as indicated by the formation of high-fluorine mineral phases (haleniusite, fluocerite) (Figure 10d,e), the F-bearing minerals of REE (parisite, synchysite ) and the overgrowing of primary Y-bearing fluorite with sterile fluorite rims (Figure 10i). Synchysite is formed even at the latest low-temperature stages of ore mineral formation, encrusting the walls of hollandite-filled micromiaroles (Figure 16a) and, together with the same mineral, evolving as micron-sized rims on monazite (Figure 16e). Cerianite and stetendite seem to have been formed almost simultaneously (Figure 15i), indicating highly oxidative conditions in the mineral-forming system [67].

Muzilampi and Hepaoja granites contain an abundance of aeschynite- and euxenite-group minerals, which are difficult to identify and interpret genetically. These problems were discussed in detail in [41,42]. The grain morphology of these minerals, present in veinlet-like fluorite aggregates (Figure 10a and Figure 12c) and their composition variation trends on the diagram (Figure 19), proposed by T.S Ercit [41], suggest their postmagmatic formation and substantial alterations at a hydrothermal stage. This seems to be the reason for the ambiguous identification of the above minerals based on the diagram (Figure 19) and the criteria proposed by Ewing [43].

Columbite-tantalite-group minerals in all the above occurrences are ferricolumbites (Figure 20). Interestingly, Muzilampi and Hepaoja columbites are rich in Fe. Their Mn/(Mn + Fe) ratio is <0.2 (Figure 20). Ta/(Ta + Nb) ratio in Muzilampi and Hepaoja ferricolumbites varies from 0 to 0.4. In the diagram (Figure 20), it forms an almost vertical trend closely coincident with the columbite composition field of the carbonatite complex. Biotite and siderophyllite in the granites contain maximum Fe concentrations: Fe/(Fe + Mg) = 1. Other dark-colored minerals (amphibole, pyroxene, and fayalite) occurring in SARGB granitoids are also rich in Fe. This distinctive feature of rapakivi magmatism seems to persist at a postmagmatic stage upon the formation of ore associations.

Avtodor columbites contain less Fe, but the number of their analyses is too small to draw any conclusions about the evolution of their composition. The morphology of ferricolumbite aggregates, the degree of idiomorphism, zoning, and intergrowing with various minerals do not provide sufficient evidence for their magmatic or hydrothermal origin. However, the presence of spongy-textured columbite in Muzilampi granites due to regularly distributed micron-sized fluorite inclusions in it (Figure 11b) suggests that it was derived from fluorine-enriched fluids or fluidized melts. The possible transportation of Ta and Nb by F-rich solutions is supported by experimental data and the results of the study of molten and liquid inclusions [69,70,71,72].

The formation of columbite-tantalite-group minerals is the subject of long debate. Some workers support their magmatic origin [73,74], others advocate a hydrothermal origin [75,76], and some [77,78,79] find both options agreeable. It has been shown [80] that columbite of magmatic origin contains more Nb, Sc, Fe, Ti, Y, Zr, U, Th, and W. Hence, an abundance of ferricolumbites in SARGB’s rare-metal granites has been derived magmatically. Few ferricolumbites with spongy structure in the studied granites are hydrothermal, others are probably of igneous origin.

6. Conclusions

1. Critical metal mineralization in the late-stage intrusions of the Salmi Batholith consists of In-Bi-Be in greisenized skarns and REE-Ta-Nb minerals in Li-F granites.

2. In addition to Zn, Cu, and Fe sulphides, magnetite and fluorite are In-concentrating minerals in greisenized skarns. Roquesite, an In mineral, was produced by the exsolution of its solid solutions in chalcopyrite, sphalerite, and, presumably, galena. In diffusion from sphalerite upon a thermal effect seems to have triggered the formation of its micron-sized aggregates at the boundary of contacting sphalerite and chalcopyrite grains.

3. Bismuth mineralization occurs as a large group of minerals, in which native bismuth and wittichenite are most common and zavaritskite least common. In PMD ores, hypogenically derived zavaritskite is associated with sulphides, which have not been oxidized.

4. PMD’s greisenized skarns display over ten beryllium mineral phases; danalite, bertrandite, and chrysoberyl are most common. The predominant occurrence of danalite above helvite and the absence of genthelvite in the samples analyzed suggest limited oxygen and sulphur fugacity, low alkalinity and the external supply of Fe upon its formation. A helvite-group mineral association with zinc-enriched spinel (ZnO 22%–25%) seems to have been one of the factors preventing genthelvite formation.

5. The Muzilampi, Hepaoja, and Avtodor ore occurrences in SARGB’s well-developed rare-metal granites, which were the last to intrude, display similar REE and Nb-Ta mineralization. The granites cannot be studied at depth, hence more arguments are needed for interpreting Muzilampi and Hepaoja granites as Li-F-type granites.

6. Major REE minerals in the above occurrences are bastnesite, synchisite, parisite, monazite, xenotime, aeschynite, euxenite, Nb-Ta—ferricolumbite, ilmenorutile, and Nb-rutile. They are commonly associated with fluorite and Li-siderophyllite, which contains high Nb contents (0.25%–0.78%) in Musilampi granites.

7. Fluorite-1 (interstitional), present in all three occurrences and overfilled (to >50%) with synchisite and parisite micron-sized inclusions, indicates the oversaturation of REE granitic melt with F and F-complexes at the final stage of its crystallization. High F activity persisted at the postmagmatic stage of mineral formation, triggering the formation of fluorite-2 and the high-F mineral phases of REE (håleniusite, fluocerite).

8. Columbite-tantalite-group minerals at all the occurrences discussed are all ferricolumbites with dominant Mn/(Mn + Fe) ratio of <0.2. Biotite and siderophyllite associated with them commonly exhibit a maximum Fe concentration, Fe/(Fe + Mg) = 1. This distinctive feature (high Fe concentration), typical of rapakivi magmatism, persists at a postmagmatic stage upon the formation of ore associations.

9. Unambiguous conclusions regarding their magmatic or hydrothermal origin cannot be drawn on the basis of the morphology of ferricolumbite aggregates, the degree of idiomorphism, zoning, and encrusting on various minerals. However, the presence of spongy-textured columbite in Muzilampi granites due to its evenly distributed fluorite micron-sized inclusions suggests that it was derived from fluorine-enriched fluids.

10. Only indium mineralization, associated with bismuth and beryllium mineralization, for which forecast resources have been estimated, could be of economic significance, based on our knowledge of the mineralization of critical metals [21].