Geology and Mineralogy of Rare Earth Elements Deposits and Occurrences in Finland

Abstract

Rare earth elements (REE) have critical importance in the manufacturing of many electronic products in the high-tech and green-tech industries. Currently, mining and processing of REE is strongly concentrated in China. A substantial growth in global exploration for REE deposits has taken place in the recent years and has resulted in considerable advances in defining new resources. This study provides an overview of the mineralogical and petrological peculiarities of the most important REE prospects and metallogeny of REE in Finland. There is a particularly good potential for future discoveries of carbonatite hosted REE deposits in the Paleozoic Sokli carbonatite complex, as well as in the Paleoproterozoic Korsnäs and Kortejärvi Laivajoki areas. This review also provides information about the highest known REE concentration in the alkaline intrusions of Finland in the Tana Belt and other alkaline rock hosted occurrences (e.g., Otanmäki and Katajakangas). Significant REE enrichments in hydrothermal alteration zones are also known in the Kuusamo Belt (Uuniniemi and Honkilehto), and occurrences of REE-rich mineralisation are also present in granite pegmatite bodies and greisens in central and southern Finland (Kovela monazite granite and the Rapakivi Granite batholith at Vyborg, respectively). REE minerals in all of the localities listed above were identified and analyzed by scanning electron microscopy (SEM) and electron microprobes (EMPs). In localities of northern and central Finland, both primary rock forming and epigenetic-hydrothermal REE minerals were found, namely phosphates (monazite-Ce, xenotime-Y), fluorcarbonates (bastnäsite-Ce, synchysite), and hydrated carbonates (ancylite-Ce), hydrated aluminium silicates (allanite-Ce, Fe-allanite, cerite, chevkinite), oxides (fergusonite, euxenite) and U-Pb rich minerals. The chondrite normalized REE concentrations, the La/Nd ratios and the REE vs. major element contents in several types of REE bearing minerals from prospects in Finland can be used to identify and define variable REE fractionation processes (carbonatites), as well as to discriminate deposits of different origins.

1. Introduction

The demand for rare earth elements has spiked in recent years due to their increasing usage in numerous high-technology applications that touch many aspects of modern life and culture. Specific REEs are used individually or in combination to make phosphors—substances that emit luminescence—for many types of ray tubes and flat panel displays, in screens that range in size from smart phone displays to stadium scoreboards. Some REEs are used in fluorescent and LED lighting. Yttrium, europium, and terbium phosphors are the red-green-blue phosphors used in many light bulbs, panels, and televisions.

Enrichment of the REE may occur through primary processes such as magmatic processes and hydrothermal fluid mobilization and precipitation, or through secondary processes that move REE minerals from where they originally formed, such as sedimentary concentration and weathering. Natural rare earth element deposits and occurrences may thus be divided into primary (high-temperature) and secondary (low-temperature) deposit types. The most important primary deposits with high grade and tonnage are typically associated with alkaline-peralkaline igneous rocks and carbonatites formed in extensional intracontinental rifts [1,2].

Today, almost all (~98%) of the world’s REE supply comes from China, with 40%–50% of this production contributed by the giant Fe-REE-Nb deposit at Bayan Obo [3,4]. These rich ores are dominated by light rare earth elements (e.g., [2,3,4,5,6]). Reserves are estimated at more than 40 million tons of REE minerals grading at 3–5.4 wt % REE (70% of world’s known REE reserves), 1 million tons of Nb₂O₅ and 470 million tons of iron. The deposit also contains an estimated 130 million tons of fluorite [2,6]. REE deposits associated with alkaline igneous rocks are typically lower grade but with larger tonnage and a higher content of HREE [2]. The other sources are minor and, in addition to the Mountain Pass bastnäsite and Russian loparite eudialyte deposits, include placers, where monazite and xenotime are extracted as by-products from ilmenite-zircon sands (India, Brazil, Malaysia). Since 2003, none of these sources have contributed more than 3.5 thousand metric tons (kt) rare earth oxides (REO) (i.e., <3% of global output).

The main REE metallogenetic provinces in Europe are those areas where extensional tectonics and introduction of enriched mantle melts into shallow crustal levels have produced alkaline silicate and carbonatite intrusions. The major REE deposits are currently known in those areas where those intrusions have been exposed by erosion [7]. The most notable REE deposits are found in the Mesoproterozoic Gardar Province of south-west Greenland [8], and the Protogine Zone, a major, multiply reactivated, in part extensional structure in southern Sweden [9]. The majority of the Kola Alkaline Province lies in Russia, where it contains significant REE deposits in the Lovozero and Khibiny intrusive complexes [10]. Mineral deposits of REE in the nepheline syenites and foidolites of the Lovozero plutons have 7.1 million tons (Mt) resources with 1.12 wt % average REO content, whereas in the apatite-nepheline rocks of Khibiny complex, 5.5 Mt REO resources with 0.40 wt %, average REO content are known [10,11]. The westernmost part of the province falls within the Finnish border, with two main intrusions: the Sokli phoscorite-carbonatite complex, and the Iivaara alkaline complex (Figure 1). The approximately 360–380 Ma old Sokli carbonatite complex in north eastern Finland hosts a deeply weathered and unexploited phosphate deposit, which is enriched in niobium (Nb), tantalum (Ta), zirconium (Zr), REE and uranium (U). The Sokli complex shows many important similarities to other alkaline complexes of the Devonian Kola alkaline province, especially to the Kovdor and to the Vuorijärvi complexes (in Russia). These similarities include occurrences of early stage ultramafic cumulate rocks, well-developed phoscorite-carbonatite associations and late stage carbonatites that represent evolution of carbonate magmas from dolomite carbonatites to final stage light rare earth element (LREE)-Sr-Ba dolomite carbonatite pulses. However, a critically important difference at Sokli is the vastly greater abundance of carbonatite relative to the cumulate ultramafic rocks. Exloration for REE by the Geological Survey of Finland in the Sokli complex has focused on the fenite aureole and associated late-stage, crosscutting carbonatite dikes that seem to have the highest potential for REE mineralization [12]. The ore reserve in the “soft-rock” phosphate-rich materials is about 114 Mt with 15 wt % P₂O₅ content with an additional, approximately 75 Mt resources in the weathered bedrock containing about 5.6 wt % P₂O₅. Iivaara is the type locality of ijolite, which is a common rock type in carbonatite-bearing alkaline complexes. The phosphor potential of the Iivaara intrusion is very high, but the REE potential is still under study. The Iivaara intrusion shows many similarities with the Lovozero alkaline massif in Russia. Results of the previous mineralogical studies in samples from historic drill cores indicate that the Iivaara nepheline-syenite contains apatite and a small amount of allanite. The samples contain 1%–5% P₂O₅ and 200 ppm REE only [13]. Other examples of alkaline magmatic rock occurrences include the c. 2600 Ma Siilinjärvi carbonatite complex [14,15]; and the c. 2050 Ma Katajakangas alkaline gneiss [12,16]. Apatite is currently mined at Siilinjärvi as a phosphate resource. The main rock types at Siilinjärvi are enriched in REE, and REE-hosting minerals in the carbonatite and associated “glimmerite” include monazite-(Ce), pyrochlore-group minerals, LREE-bearing strontianite and REE-bearing Ti, Nb-phases [17]. The Katajakangas gneiss contains mineralised layers, which are rich in zircon, bastnäsite, columbite, and thorite, and an informal resource estimate indicate 0.46 Mt ore with 2.4% average total rare earth oxides [16].

This paper presents an overview of the main REE occurrences and prospects in Finland, and identifies areas with the most significant potential for future exploration and development, on the basis of their geological suitability. In addition to a detailed geological description of the potential areas, mineralogy of occurrences and prospects are also fully characterized as beneficiation methods have to be tailor-made for each deposit, and are dependent on properties such as mineralogy, textures, and grain size of the ore. Results of geochemical and mineralogical studies on samples from new drillings completed by the Geological Survey of Finland between 2009 and 2016 are also included in this review. Mineralogical studies of samples from the new drill cores were focused on the following exploration targets: (1) the fenitic zone surrounding the Sokli carbonatite intrusions at Jammi and Kaulus; (2) the Pb-REE occurrence hosted by calcsilicate rocks and dikes of carbonatite at Korsnäs; (3) the Kortejärvi carbonatite and Laivajoki silicocarbonatite complexes; (4) the alkaline gneissic granite in the Otanmäki area; (5) the REE-Au mineralization at Mäkärä and Vaulo in the Tana Belt; (6) the metasomatic-hydrothermal Au-Co-Cu-Fe-U and REE deposits with intense albitization in the Kuusamo belt; (7) the Kovela monazite granite in southern Finland and (8) the Vyborg rapakivi granite batholith in south eastern Finland. Here we classify these deposits and occurrences on the basis of their origin and geological settings (Figure 1), and summarize their characteristics into four REE deposit types on the basis of the age, host rock type and mineral associations (

Table 1. Types of significant REE occurrences in Finland.

Deposit Examples	Ore-Bearing Rocks	Age(Ga)	Major REE Bearing Minerals
Carbonatite rocks
Sokli Jammi/ Kaulus	Carbonatites	0.38–0.36	Ancylite-(Ce), monazite, bastnäsite, allanite, xenotime-(Y), Sr-apatite, strontianite, barite
Korsnäs	Calcsilicate rocks (skarn) next to carbonatite	1.90–1.83	Apatite, monazite, Ca-ancylite, bastnäsite, barite, calcite
Kortejärvi-Laivajoki	Carbonatites	1.88–1.85	Apatite, allanite, chevkinite-(Ce) monazite, bastnäsite, columbite
Alkaline rocks
Otanmäki and Katajakangas	Alkaline gneiss	2.30–1.90	Fergusonite(Y), fergusonite (U), allanite, Ca-REE fluorocarbonates (bastnäsite and synchysite), columbite
Tana Belt (Vaulo and Mäkära)	Arkose gneiss	3.10–2.60	Euxenite, columbite, zircon
Hydrothermal alteration zones
Kuusamo_Uuniniemi	Epigenetic hydrothermal	1.90–1.80	Apatite, monazite, allanite euxenite, Fe-columbite, thorite
Kuusamo_Honkilehto	Albitite	2.45–2.44	Apatite, bastnäsite, ancylite, allanite, chevkinite-(Ce) davidite, uraninite
Granitoids/greisens
Kovela	Pegmatite in monzogranite	1.85–1.79	Monazite, huttonite/thorite, xenotime
Kymi granite	Greisen in Rapakivi Granite	1.64–1.63	Apatite, monazite, bastnäsite, allanite, xenotime, thorite

). An integral part of the study was the textural and mineral-chemical comparison of REE, Y, Th, U-rich mineral assemblages in different geological environments.

2. Materials and Methods

The first step of this study was the extension of the previous geological, mineralogical, petrological and geochemical databases with new observations on the recently completed new drill cores. On the basis of the results of the revision, drill cores were selected for re-logging and re-sampling for the purpose of detailed petrography and mineralogical studies. Polished thin sections were prepared from representative samples in order to investigate the mineralogical-textural characteristics of REE minerals and their host rocks by transmitted and reflected light polarizing microscopy and by scanning electron microscopy (SEM). For the purpose of high resolution SEM studies, the polished thin sections were carbon coated to a thickness of 25 nm using an EMITECH 960 L evaporation-coating unit. Backscattered electron imaging and qualitative compositional characterization of the samples was performed using an JEOL JSM 5900 LV high-resolution scanning electron microscope (GTK Electron optics and microanalysis Laboratory, Espoo, Finland) fitted with an Oxford Instruments X-MAX large area (50 mm²) silicon drift detector (SDD) energy-dispersive X-ray microanalysis (EDXA) system, run with Oxford Instruments’ INCA v.4 software. SEM analyses were particularly suited to the complexity and very fine grained nature of the mineral assemblages.

Compositions of REE minerals were analyzed using a CAMECA SX100/LKP type electron microprobe equipment (GTK Electron Optics Laboratory). During the analyses, the accelerating voltage was 15 keV with a beam current of 20 nA. The beam diameter was 1 μm in the analyses without fluorine and 5 μm when fluorine was included in the analyzed set of elements. A detailed analytical method is presented in Appendix A and electron probe microanalyzer (EMPA) data are presented in the tables of the Supplementary Material.

3. Geological and Mineralogical Characteristics of Major REE Deposits and Occurrences in Finland

3.1. REE Deposits in Carbonatites

3.1.1. The Jammi and the Kaulus Carbonatite Dikes in the Sokli Complex

The Sokli carbonatite complex (c. 360–380 Ma) in north eastern Finland is a part of the Kola alkaline magmatic province [20], and hosts an unexploited phosphate deposit enriched in REE, Nb, Ta, Zr and U [21,22]. The complex has a concentrically zoned structure that can be divided into two major zones (Figure 2). The inner zone is built up by multiple intrusions of carbonatites and phoscorites whereas the outer zone mainly consists of ultramafic rocks which were largely transformed into carbonate-rich metasomatites by CO₂-rich fluids derived from the late stage injections of carbonatite melts. The relict minerals and bulk compositions indicate that the ultramafic rocks were mostly pyroxenites [23,24]. The internal zones of carbonatite are surrounded by syenite and a fenite aureoles. The outer boundary of the fenitic zone is up to 2 km away from the central part of the complex. The late stage carbonatite dikes penetrate not only the complex, but also the fenitic aureole and country rocks up to 1.3 km from the contact. The country rocks consist of ultramafic and mafic volcanic units and tonalitic gneiss (Figure 2).

There is a strong connection between the complexity of fenite textures and associated mineralization due to the enrichment of REE in the intermediate and late stage carbonatite magma generations which produced multiple pulses of fenitizing fluids [21,25]. The presence of brecciated zones within the carbonatite and alkaline complex is the result of an explosive release of fluids and volatiles, thus brecciation indicates development of more evolved magma generations and increased potential of Nb and REE enrichments in the source intrusion [25,26].

Geochemical data from the drill cores R301 and R302 drilled by GTK in 2006 in the Jammi area (Figure 2) show that the carbonatite dikes in the fenitic aureole of the Sokli Complex are enriched in incompatible elements, such as P₂O₅ (19.9 wt % max), Sr (1.9 wt %), Ba (6.8 wt %) and Zn (0.3 wt %). The total REE content of samples are from 0.11 to 1.83 wt %, with dominance of LREE (0.11–1.81 wt % LREE and 0.002–0.041 wt % HREE contents [25]. Similar dikes also occur within the carbonatite intrusion in the Kaulus area (Figure 2). According to the results of whole rock geochemical analyses, most of dikes are varieties of carbonatite ranging from silico-carbonatite to ferro- and calico-carbonatite. Calcio-carbonatite dikes are rich in calcite (15–84 vol %) with subordinate amounts of dolomite, but locally contain up to 50 vol % apatite.

The REE minerals in the carbonatite dikes at Jammi and Kaulus (Figure 2) are usually LREE-rich and their assemblages consist of ancylite-(Ce), monazite-(Ce), bastnäsite-(Ce) and allanite. In the main mass of the Sokli Carbonatite, HREE rich minerals such as xenotime-(Y) and pyrochlore are more common. In general, ancylite is the most widespread REE mineral in the whole complex. This mineral occurs in close association with strontianite, barite and bastnäsite. These minerals commonly form complex intergrowths in the carbonate matrix of dikes, but also fill up veinlets and they can also be found as euhedral crystals in cavities of the carbonatite (Figure 3a).

Ancylite-(Ce) [CeSr(CO₃)₂(OH)·H₂O] from the Jammi carbonatite dyke consists of 31–34 wt % Ce₂O₃, 11–14.5 wt % La₂O₃, 8.1–9.3 wt % Nd₂O₃ and 18.2–24 wt % SrO with relatively high Ca (0.5–2.5 wt % CaO) and Th contents (1.8–4.9 wt % ThO₂) whereas Ba has low concentrations (0.5 wt % BaO) (Supplementary Materials Table S1). The atomic proportion of Sr and REE in the formula is close to 1:2, with a small excess of REE.

Monazite-(Ce) [(Ce,La,Nd,Th)PO₄] is the second important REE mineral in the carbonatite dikes of the Sokli complex. This mineral most commonly occurs in the form of microcrystalline, sporadic, and isolated equidimensional crystals in the carbonate matrix. Monazite also often occurs in a round, sunflower-like form. In the centre of latter precipitations, strontium-apatite is often present (Figure 3b,c). The total RE₂O₃ contents are in the range of 40–50 wt % with systematic differences in chemical compositions according to the different stages of monazite crystallization in different assemblages. The monazite crystals have higher in Ce₂O₃ contents (29.5–45.0 wt %) than the sum of La₂O₃ and Nd₂O₃ (e.g., La₂O₃ = 21.10–24.66 wt %, Nd₂O₃ = 5.81–7.56 wt %).

Bastnäsite-(Ce) [(Ce,La)(CO₃)F] has been reported as a relatively common product of alteration of allanite and Sr- or F-apatite [25] in the Sokli complex. Individual crystals of bastnäsite and allanite in the carbonatite dikes appear to be acicular or needle-like and they form either radial accumulations or intricate cross-cutting grids within a variety of minerals such as albite and dolomite (Figure 3d). Allanite-(Ce) [(Ce,Ca,Y)₂(Al,Fe³⁺)₃(SiO₄)₃(OH)] also occurs as both stubby and acicular crystals, generally in contact with bastnäsite and, at some places, with xenotime (Figure 3e). Representative electron-microprobe results of bastnäsite, allanite and xenotime are shown in the Supplementary Materials Table S1. Pyrochlore [(Na,Ca)₂Nb₂O₆(OH,F)] is one of the tantalum/niobium oxides (commonly comprising a U-Ta-rich and Nb-bearing rutile) that typically occurs as a rare, sector-zoned and prismatic crystals with up to 50–100 μm sizes (Figure 3f).

3.1.2. Korsnäs Pb-REE Deposit

The Korsnäs Pb-REE deposit in western Finland was operated by the Outokumpu Oy mining company between 1961 and 1972. The deposit comprises a network of narrow carbonatite veins and dikes in an area of 10 km² and one larger carbonatite dyke (Svartören) that hosts the main ore body measuring 5–30 m in width, up to 1.5 km in length and extending down to a depth of around 350 m (Figure 4). The Korsnäs dyke swarm is hosted by a north-south trending fracture zone dipping to east at an angle of 40–60°. The host rock of the dikes is the c. 1.9 Ga migmatitic biotite paragneiss of the South Ostrobothnian Schist Belt and the carbonatite dikes are dated at c. 1.83 Ga [27]. The average total REE oxide grade is 0.91 wt % [28]. The ore bodies consist of highly altered intrusive phases, including coarse-grained pegmatite and carbonatite dikes or calcareous scapolite-diopside-barite veins that may contain significant REE grades. The REE contents are correlated with the abundance of apatite (more than 6 wt %) with a slight excess of HREE with respect to presence of monazite and allanite [29,30].

Samples for the current geochemical and mineralogical studies were selected from the drill holes SÖ-66 and SÖ-104 which were drilled by the Geological Survey of Finland in 1955 (Figure 4). The total REE content of samples ranges from 0.7 to 2.2 wt % with LREE dominating the REE budget. The Eu content is high, from 66 to 242 ppm, whereas the Th contents range from 107 to 604 ppm. Ba and Sr are also enriched (3600–3800 ppm and 2200–3400 ppm respectively) mainly in the carbonatite dikes [13,17].

The accessory mineral assemblages in the carbonatite dikes of the Korsnäs zone are virtually identical, and include REE-bearing apatite, monazite, bastnäsite, ancylite, britholite, calcite and barite. Representative microprobe analyses of ancylite, bastnäsite and monazite are presented in the Supplementary Materials Table S2. The polycrystalline aggregates of ancylite-(Ce) occurring in some samples as disseminations in the carbonate matrix (Figure 5a) are characterized by a high LREE content: ~45 wt % Ce₂O₃, ~25 wt % La₂O₃) and ~12 wt % Nd₂O₃. Ancylite also forms fine-grained aggregates with bastnäsite. Figure 5b shows one example of their typical intergrowths in which bastnäsite cores are overgrown by ancylite. Bastnäsite contains 55.52–59.27 wt % REE₂O₃, 3.40–5.63 wt % F and 6.31–6.90 wt % CaO. Monazite is the main REE mineral with up to 60 wt % REE₂O₃. This mineral occurs as small, discrete crystals or as fine-grained inclusions in apatite within the zones of the weathered rocks (Figure 5d–f). Based on the EMPA data (Supplementary Materials Table S2), monazite grains contain an average of 29 ± 1.4 wt % Ce₂O₃, 11 ± 1.2 wt % La₂O₃ and 8 ± 0.5 wt % Nd₂O₃, 2 ± 0.5 wt % Pr₂O₃ with considerable amounts of ThO₂ (6.5 ± 0.5 wt %) and UO₂ (3.2 ± 0.6 wt %). Barite occurs as a secondary mineral, fracture and vug-filling phase together with the REE mineralization (Figure 5e,f). Barite composed mainly of BaO (54.7–62.0 wt %), SO₃ (19.6–33.1 wt %) and SrO (0.4–4.5 wt %). The composition of barite can be seen to be dominated by REE (0.3–18.0 wt % Ce₂O₃) and P (0.1–3.9 wt % P₂O₅), maybe due to partially replaced by various assemblages of REE-Sr-Ba minerals (Supplementary Materials Table S1).

3.1.3. Kortejärvi and Laivajoki Carbonatites

The 1.88–1.85 Ga [30] Kortejärvi and Laivajoki carbonatite dikes in the Kuusamo belt, north central Finland (Figure 1 and Figure 6), were emplaced into early Paleoproterozoic mafic volcanic rocks along a crustal-scale fault zone. The Kortejärvi carbonatite dyke is approximately 60 m thick and 2 km long and dips steeply to the east, whereas the Laivajoki carbonatite dyke is about 20 m thick and 4 km long and dips 60° to the southeast [31]. Both dikes consist of calcite-carbonatite and dolomite-carbonatite. The Kortejärvi dyke also contains glimmerite and olivine magnetite rocks, whereas silicocarbonatite and glimmerite occur in the Laivajoki dyke. Total REE contents are 210–1644 ppm and 443–892 ppm in the Laivajoki and in the Kortejärvi carbonatite, respectively [13].

Three samples were selected from the available drill cores of Kortejärvi carbonatite dyke for the purpose of mineralogical studies. The major rock forming minerals are calcite, dolomite and apatite. Apatite is interstitial between calcite and dolomite crystals, and commonly rimmed by overgrowths of monazite (Figure 7a–c). This kind of monazite apatite association is also quite common in carbonatite related deposits. Similar rim of monazite also occurs around calcite and dolomite (Figure 7b). Apatite has 53–54 wt % CaO content without significant concentrations of those trace elements (e.g., REEs, Y, Sr, Th, and U) that may substitute Ca in the crystal lattice. Apatite crystals are very rich in halogens with higher than 2 wt % F and over 1 wt % Cl concentrations (Supplementary Materials Table S3).

The most important REE-bearing accessories are monazite, allanite, synchysite and bastnäsite. These minerals associate with calcite, fluorapatite and quartz in fracture infillings within carbonate minerals and in aggregates of radiating individual crystals with bastnäsite (Figure 7c). Allanite grains are commonly altered into REE fluorcarbonate minerals, most often into bastnäsite (Figure 7c), suggesting to the F-rich nature of overprinting fluids. Bastnäsite forms radial or irregular aggregates of thin tabular crystals with up to ~50 μm sizes in cavities and cracks of altered allanite crystals. Monazite has low (from 0.03 to 3.05 wt %) CaO content with P₂O₅ concentrations between 27.92 and 29.91 wt % (Supplementary Materials Table S3). It contains Ce₂O₃ as the most abundant REE oxide with concentrations ranging from 35.44 to 38.53 wt %, followed by La₂O₃ from 14.43 to 20.98 wt %, and Nd₂O₃ from 9.61 to 13.46 wt %. ThO₂ displays low concentrations, from 0.02 to 1.07 wt %, and it is under the detection limit of microprobe analyses. Allanite-type minerals are characterized by high REE (43.43–47.70 REE₂O₃ wt %), Fe (8.03–11.05 FeO wt %), and Ti (2.93–3.65 TiO₂ wt %) contents, together with low contents of Mn (from 0.07 to 0.13 MnO wt %) and Mg (from 0.66 to 0.93 MgO wt %). The majority of bastnäsite grains in the studied samples are strongly enriched in LREE with approximately 70 wt % total REE. The average Ce₂O₃ is 33.35 wt %, whereas the average La₂O₃ is 19.1 wt % and the average Nd₂O₃ is 8.3 wt %.

Six representative samples of the silicocarbonatite rocks from cores of drillings conducted by the Rautaruukki Corporation in the 1970s were chosen for mineralogical studies from the Laivajoki dyke. Mineral assemblages in the samples consist primarily of quartz, calcite, dolomite, plagioclase, biotite and iron oxide minerals. Allanite and chevkinite are the most abundant REE-bearing minerals. Allanite occurs in the form of coarse-grained aggregates associated with silicate and iron oxide minerals. Allanite also contains inclusions of magnetite and chevkinite (Figure 7d). The grain size of allanite varies between 300 μm × 500 μm and 1000 μm (Figure 7e). Many crystals display a network of fractures with narrow strips of epidote (Figure 7d,e). Chevkinite fills fractures in iron oxide minerals and titanite (Figure 7f). REE-rich fluorocarbonate minerals (bastnäsite and synchysite) are commonly associated with each other but bastnäsite is more abundant in most of the studied samples. Bastnäsite also occurs as tiny anhedral inclusions up to 20 μm across in synchysite. Synchysite also forms radial or irregular aggregates of thin tabular crystals with up to ~50 μm sizes in cavities and cracks of altered allanite crystals, or in association with calcite, dolomite and Ti-bearing phases in some places (Figure 7f). The secondary carbonate minerals (mainly synchysite and bastnäsite) replace substantial parts of former allanite crystals (Figure 7f).

Results of electron microprobe analyses in polycrystalline allanite (Aln) grains from Laivajoki are given in Supplementary Materials Table S3. Allanite is characterized by relatively uniform composition with REE₂O₃ contents between 24.67 and 27.09 wt %, small Y contents (0.08–0.13 wt % Y₂O₃), and from 11.05 to 12.36 wt % Al₂O₃, from 11.26 to 13.52 wt % FeO total, from 10.41 to 12.22 wt % CaO and from 3.98 to 4.99 wt % MgO contents. ThO₂ and MnO₂ contents are very small, not exceeding 0.5 wt %. In places, a phase with intermediate allanite-epidote composition, showing elevated Al (~20 wt % Al₂O₃), Ca (~15 wt % CaO) and smaller Fe (~12 wt % FeO_total) and (REE,Y)₂O₃ contents of 15 wt % were also found in fractures of allanite.

In the samples from the Laivajoki carbonatite dyke, the majority of bastnäsite grains are strongly enriched in LREE (from 67.29 to 77.98 wt % REE₂O₃) with up to 33 wt % Ce₂O₃, 12 wt % La₂O₃, 18 wt % Nd₂O₃, and smaller contents of the remaining REE. Fluorine is the dominant anion (from 3.1 to 4.7 wt % F) and is partly substituted by hydroxyl. Synchysite-(Ce) [Ca(REE)(CO₃)₂F] reveals a significantly higher Ca content (from 11.74 to 17.32 wt % CaO) in comparison to bastnäsite but LREE still predominate in the composition with Ce³⁺ being the dominant REE cation (from 23.15 to 30.94 wt % Ce₂O₃). The F content shows small variation from 3.1 to 4.8 wt % and the ThO₂ content is just over 1 wt %.

Chevkinite-(Ce) has the general formula of A₄BC₂D₂(Si₂O₇)₂O₈, where the predominant cations in each site are: A = Ca, REE, Th; B = Fe²⁺; C = Fe²⁺, Fe³⁺, Ti; and D = Ti. Oxide totals of microprobe analyses from this phase are rather low (from 94.9 to 96.3 wt %) presumably due to presenting total Fe as Fe²⁺ (see below), and perhaps due to secondary hydration during metamictization. Chevkinite from Laivajoki has notably low Ca contents (~1 wt % CaO) and correspondingly high REE contents (48.62–50.74 wt % REE₂O₃). Thorium contents are very low (<0.10 wt % ThO₂). Thus, REE occupy more than 98% of the A-site. Divalent Fe is the dominant cation at the C-site, with values of FeO concentrations ranging from 9.27 to 9.48 wt %, whilst MgO values are slightly elevated (from 1.38 to 2.13 wt %). The SiO₂, TiO₂ and Ce₂O₃ are the main oxides with 18.19–19.52 wt % SiO₂, 15.26–15.74 wt % TiO₂ and 26.56–28.68 wt % Ce₂O₃ concentrations.

3.2. REE Deposits Related to Alkaline Intrusions

3.2.1. The Otanmäki Katajakangas Nb-REE Deposit

The Otanmäki area consists mainly of Archean granitic gneisses intruded by alkali-granite and gabbro-anorthosite of c. 2.05 Ga age (Figure 8) [32,33,34]. The gabbro-anorthosite intrusions host Fe-Ti-V deposits which were mined from 1950’s to 1980’s by the Otanmäki Oy mining company. In total, 30 Mt of ore grading of 32–34% Fe, 5.5–7.6% Ti and 0.26% V was mined [35]. The processing plant in Otanmäki produced 7.6 Mt magnetite, 3.8 Mt ilmenite and 0.2 Mt sulphur concentrates, as well as 55,454 t vanadium pentoxide [36]. The alkaline gneiss contains 0.7–1.5 wt % Zr and 0.1–0.2 wt % Th [37].

To the west of the old Otanmäki mine, the Nb-REE mineralisation at Katajakangas occurs in narrow lenses or layers with a few metres width only (Figure 8). These ore bodies are mostly composed of pervasively sheared and foliated, fine-grained, reddish-grey quartz-feldspar gneiss with riebeckite and alkaline pyroxene with some hydrothermal veins. The zones with Nb-REE mineralization in the alkaline gneiss contain high concentrations of Nb, Zr, Y, Th and REE, with an estimated Nb + Y + REE resource of 0.46 Mt at 2.4 wt % RE₂O₃, 0.31 wt % Y₂O₃ and 0.76 wt % Nb₂O₅ contents [7].

Representative EPMA analyses of the REE-bearing minerals from the Otanmäki-Katajakangas deposit are listed in the Supplementary Materials Table S4. Fergusonite and columbite are the major host for Nb, Th, U and Y, whereas REE is mainly hosted by allanite. Fergusonite-(Y) [YNbO₄] occurs commonly as rounded to irregular discrete crystals varying in size from 100 μm up to more than 500 μm, in associations with allanite, columbite and zircon. Fergusonite is chemically heterogeneous: many crystals display growth zoning with enrichments in U and Th in the cores of crystals (Figure 9a,b). The bright domains on SEM images of these cores are characterized by high content of U (up to 9.5 wt % UO₂), Th (up to 3.1 wt % ThO₂), Y (up to 26 wt % Y₂O₃) and Nb (46.3 wt % Nb₂O₅) (Supplementary Materials Table S4). Textures observed in SEM and EPMA data suggest to the presence of sub-micrometre intergrowths between fergusonite and allanite in most of the studied samples (Figure 9a,b). They are interpreted as early magmatic REE phases. Columbite (Fe,Mn) (Nb,Ta)₂O₆ is the most common niobium-bearing mineral in many of the studied samples. Columbite occurs in the form of irregular-sub round micro phenocrysts with up to ~50 μm sizes in associations with allanite (Figure 9c). Columbite is also present as elongated (250 µm), subhedral to euhedral crystals, which are commonly intergrown with euhedral to anhedral rutile. Columbite and rutile grains are dispersed throughout the matrix of the host rock. The columbite crystals are typically poorly zoned but some parts of the crystals are usually richer in uranium (Figure 9f).

The primary magmatic allanite forms euhedral to subhedral crystals, ~100 to ~300 μm across. It is associated with albite, biotite, quartz, zircon, fluorapatite, and titanite (Figure 9c). Allanite crystals are also partly to fully replace and overgrown by secondary REE carbonates (bastnäsite and synchysite). The Katajakangas allanite has a total REE-content between 23.6 and 55.7 wt % (average 39.6 wt %). Cerium (Ce) is dominant (with average Ce₂O₃ content of 20.7 wt %), followed by Nd (10.7 wt %) and La (7.7 wt %). Concentrations of HREE are commonly below the detection limit of the electron microprobe (Supplementary Materials Table S4). The allanite- bastnäsite-synchysite association form aggregates with more or less hexagonal outlines within the surrounding matrix (Figure 9d). The aggregates consist of radiating individual crystals.

In the hydrothermal veins, synchysite and bastnäsite form well-developed fibrous crystals and aggregates of radiating acicular crystals. The REE-carbonates replace and/or encompass allanite crystals in these veins (Figure 9e).

3.2.2. The Mäkärä Vaulo Area in the Tana Belt

The Tana Belt is located on the southern side of the Lapland Granulite Belt in northern Finland (Figure 1). It is characterized by prominent REE anomalies in till, but lithogeochemical data for various types of rocks also display enrichments in REE [38,39,40,41]. High La and Y concentrations in both till and arkose gneisses of the bedrock occur in an elongated, 200 × 200 km² area (Figure 10).

The Mäkärä region within the Tana Belt consists of Archean gneiss (3.1–2.6 Ga) and Paleoproterozoic supracrustal rocks (2.3–2.0 Ga), which were thrusted together with the Lapland Granulite Belt onto the Central Lapland Greenstone Belt at c. 1.9 Ga during the Svecofennian orogeny [42,43,44]. The bedrock of the study area is poorly exposed, and largely covered by saprolite, weathered regolith and Quaternary till with variable thicknesses, as well as with dense vegetation. This area is characterized by Au and REE anomalies in regional till and bedrock samples. Au anomalies can be related to occurrences of hydrothermal hematite-pyrite-quartz veins, whereas weathering of arkose gneiss has played a role in the formation of the REE-bearing kaolinitic saprolite. The REE enrichment in kaolinitic saprolite ranges from 0.04 to 0.1 wt % REE₂O₃, with LREE/HREE ratio of 2.7 to 15 in the kaolinitic saprolite sediment samples.

The arkose gneiss is mainly composed of quartz, K-feldspar, plagioclase, amphibole and mica (muscovite and biotite) phenocryst embedded in fine-grained groundmass. Lamprophyre dikes of 1775 ± 10 Ma [44] intrude the gneissic rocks at Mäkärä. The dikes are mainly composed of plagioclases, amphiboles, biotite, K-feldspars. Remnants/fragments of lamprophyre also occur in the kaolinitic saprolite. The average total REE content of dikes is 0.05 wt % (max 0.1 wt %).

Detailed electron-microprobe studies of the arkose gneisses from the Tana Belt revealed extensive sub solidus alteration of primary magmatic allanite to ferriallanite. Moreover, the alteration of ferriallanite resulted in development of complex pseudomorphs and overgrowths containing mainly REE carbonate phases such as synchysite and bastnäsite. Allanite and REE carbonate phases often occur as large anhedral and spheroidal aggregates with up to 200 μm in diameter (Figure 11a–c). Occasionally, these aggregates also fill up vugs. Ferriallanite is often altered to allanite in the rims and to bastnäsite in the centres of the aggregates (Figure 11a–c). Allanite does not show significant zoning by SEM imaging but contains REE-rich rims due to sub-micronsized intergrowths with allanite plus a bastnäsite-type mineral phase. At some places, the secondary carbonate minerals (mainly synchysite) occur in cavities of feldspar and in association with monazite and apatite (Figure 11e).

In the lamprophyre dikes, enrichments of REE are hosted by apatite, allanite, bastnäsite, cerite and xenotime. Rutile contains inclusions of xenotime and iron oxides. Allanite crystals are partly or fully replaced and overgrown by secondary minerals, mainly REE carbonate phases (bastnäsite and synchysite), chlorite, albite, quartz, and more rarely by xenotime, titanite and apatite (Figure 11a–d). Chemical compositions of selected allanite and associated accessory minerals from the lamprophyre dikes in the Mäkärä are presented in the Supplementary Materials Table S5. The oxide totals are in the range 96 to 99 wt %. Part of the short fall can be attributed to calculation of the total iron as FeO. The Ca content ranges between 0.89 and 6.17 wt % CaO and the sum of Y + REE varies from 21.44 to 55.16 wt % with the dominance of Ce (11.38–47.37 wt % Ce₂O₃). The concentration of Th in allanite ranges from 0.020 to 0.80 wt % ThO₂ and usually predominates over U (0.17 wt % UO₂). Generally, M sites of allanite are dominated by Al (14.37–15.83 wt % Al₂O₃) and Fe (8.79–15.16 wt % FeO). The Mg contents (0.12–2.46 wt % MgO) as well as the Si and F concentrations (19.36–31.24 wt % and 0.23–1.10 wt %, respectively) together with the high-end of Fe contents (up to 15.16 wt % FeO) can be attributed to the presence of significant amount of ferriallanite molecule in the composition of allanite. The bastnäsite compositions in lamprophyre dikes show dominance of light REE (LREE; La to Nd) over other REE, with Ce > La > Nd > Pr abundances, and 33.5 to 38.5 wt % Ce₂O₃, while calcium reaches only around 1.3 to 2.9 wt % CaO (Supplementary Materials Table S5). Synchysite reveals a significantly higher Ca content (5.62 to 9.14 wt % CaO) in comparison to bastnäsite. Again, LREE predominates here, with Ce³⁺ the dominant REE cation (21.79 to 30.67 wt % Ce₂O₃). The F content of bastnäsite is highly variable (from 3.40 to 6.36 wt % F) whereas it is ranging between 0.62 and 1.26 wt % F in synchysite.

Transformation of allanite to secondary REE carbonate minerals (mainly bastnäsite and synchysite) has been described in numerous cases in various magmatic and metamorphic rocks e.g., [45,46,47,48,49]. However, such alteration of allanite is usually associated with clay minerals (kaolinite, montmorillonite), and also with thorite, fluorite, and magnetite without carbonate minerals in some cases [45,50]. In contrast, allanite in the samples from the Tana Belt was replaced mainly by bastnäsite and synchysite, and rarely also by ferriallanite and chlorite. This type of alteration of allanite suggest to highly alkaline and oxidizing nature of the overprinting fluids [51,52,53,54].

Cerite [Ce₉Fe³⁺(SiO₄)₆(SiO₃)(OH)₄] occurs as fine-grained crystals filling fracture-veins in quartz and albite (50 µm in length and 5–15 µm in width) as seen in the studied lamprophyre dikes (Figure 11f). Cerite is strongly enriched in the LREE and also contains relatively low concentrations of Y and HREE (Supplementary Materials Table S5). The sum of the oxides of La, Ce, Pr, Nd, Sm, Gd, Dy and Y ranges from 71.15 to 81.60 wt %. Cerium-oxide contents are from 67.79 to 77.49 wt%. Cerite also contains from 12.19 to 17.86 wt % SiO₂, from 1.18 to 2.53 wt % Al₂O₃, from 0.02 to 2.41 wt % FeO and minor amounts of Ca (0.06–0.39 wt % CaO). Fluorine is invariably present in concentrations ranging from 1.21 to 1.73 wt %. Analytical totals between 94.3 and 99.6 wt % suggest the presence of minor amount of H₂O and other elements below their detection limits.

A few disseminated anhedral xenotime grains (diameter = 15 × 40 μm and 20 × 25 μm) have also been found in the lamprophyre dikes of the Mäkära area. Xenotime also forms inclusions within ilmenite and surrounded by quartz and K-feldspar (Figure 11f). Xenotime [YPO₄] grains contain significant amounts of Y₂O₃ (41.08–47.33 wt %) and extremely high HREE concentration. The Gd₂O₃ contents are up to 4.5 wt % and the Dy₂O₃ content are up to 4.8 wt %.

3.3. Hydrothermal REE Deposits

3.3.1. Uuniemi (Kuusamo Belt)

The Kuusamo schist belt (KSB) is a fold and thrust belt which is considered as a part of the Central Lapland greenstone belt, the most significant Paleoproterozoic orogenic gold province in northern Europe. The KSB consists of an epiclastic-volcanic sequence deposited during multiple rifting events affecting the Archean basement between 2.4 and 2.0 Ga. The lithological units (Figure 6) were metamorphosed and folded during the Svecofennian orogeny (1.9–1.8 Ga). Syenite and carbonatite dikes with strong albitization are widespread in the KSB. Albitization are related to late magmatic and metamorphic hydrothermal processes [55,56,57,58].

The zone of REE enrichment at Uuniniemi is composed of metasediments, syenite and metasomatic carbonatite dikes [57], which were subjected to intense albitization, Fe-metasomatism, carbonatization and brecciation. The metasomatic carbonatite dyke sampled in outcrops contains 2.8 wt % P₂O₅, 0.43 wt % REE₂O₃ and 256 ppm Nb. In albitite and albite carbonate rocks, the average total REE content is 0.1 wt % [13]. The albite carbonate rock samples are composed of sodic plagioclase with minor carbonate (calcite and dolomite), quartz and mica (biotite), but are also highly enriched with apatite and REE minerals such as monazite, allanite, euxenite, Fe-columbite and thorite.

Monazite in the albite carbonate rocks (albitite) mostly occurs as grains in composite mineral inclusions in apatite. In such inclusions, the irregular shaped monazite grains are typically 5–20 µm large and occupy 50–70 wt % of the total volume of the inclusions (Figure 12a). Monazite also occurs as relatively large (50−150 μm), oval or anhedral individual grains that are strongly porous and fractured in the matrix of the rock (Figure 12b). Monazite contains much more, ~66 wt % total REE₂O₃ compared to allanite (20.4 wt %), and lower Th and Ca contents (0.22 wt % ThO₂ and 0.33 wt % CaO).

Euxenite occurs as subhedral to anhedral, commonly 200–400μm large, but occasionally up to 500 μm large grains, (Figure 12c). Electron-microprobe analyses (Supplementary Materials Table S6) of euxenite grains show lower amounts of total REE (from 0.96 to 1.21 wt % REE₂O₃) and higher amounts Nb (36.10–37.56 wt % Nb₂O₅), Y (19.49–19.93 wt % Y₂O₃), and Ca (4.16–4.25 wt % CaO), compared with analyses of monazite and bastnäsite. Small amounts of W (2.50–3.61 wt % WO₃) and U (4.15–4.62 wt % UO₂) and trace amounts of Th (0.49–0.63 wt % ThO₂) were also detected. Thorite commonly occur not only as isolated grains but also as inclusions within monazite and apatite. Thorite is characterized by large agglomerations of anhedral to round crystals with up to 200 μm in diameter (Figure 12d). In addition to ThO₂ = 53.19–64.05 wt %, thorite contains 16.02–17.57 wt %, SiO₂, 4.32–16.38 wt % FeO and 2.94–3.06 wt % UO₂ with some Ce₂O₃, Y₂O₃, PbO and P₂O₅. Ferrocolumbite [(Fe,Mn)(Nb,Ta)₂O₆] crystals that occur in the metasomatic carbonatite dikes of Uuniniemi in the form of subhedral to anhedral crystals and range in size from 50 to 200 μm (Figure 12d). The major oxides in ferrocolumbite are FeO (19.36 wt %) and MnO (1.68 wt %) at the A sites of the crystal structure, whereas Nb₂O₅ (67.26 wt %) and Ta₂O₅ (1.73 wt %) occupies the B-sites. Also, minor amounts of Ti, Th, U, Y and LREE were observed as substitutions (Figure 12d). Apatite occurs as discrete subhedral grains in association with calcite and dolomite. Davidite and rutile are the principal opaque minerals. Davidite is present as large (up to 2 mm in diameter) zoned grains (Figure 12e).

3.3.2. Honkilehto (Kuusamo Belt)

The hydrothermal Au-Co-Cu-U-REE occurrence at Honkilehto in the central part of KSB is hosted by albitised, carbonatised and sulphidised sericite quartzite [57], which are intruded by numerous sills of albite diabase and by some minor metasomatic carbonatite dikes [59]. The carbonatite dikes are characterized by high content of P (2.8 wt % P₂O₅) and REE (0.45 wt % total REE₂O₃) and host several REE-bearing minerals such as monazite-(Ce), allanite-(Ce), ancylite-(Ce), bastnäsite-(Ce) and xenotime-(Y). The albite diabase dikes are characterized by high content of U-rich minerals (uraninite, davidite), all of which are associated with bastnäsite and allanite [27]. Coexisting bastnäsite and uraninite with up to 100 and 50 μm in size, respectively (Figure 12e), occur as accessory minerals in the host albitised rocks. Uraninite is partly or wholly replaced by bastnäsite and allanite. Bastnäsite is furthermore common in fractures, indicating that it is a paragenetically late mineral (Figure 12e). The pristine parts of the uraninite grains (Supplementary Materials Table S6) contain 71.8–77.5 wt % UO₂ and the Pb content varies from 1.3 to 15.8 wt % PbO. Concentrations of Si are from 0.03 to 6.1 wt % SiO₂, and the Fe content is from 0.5 to 2.5 wt % FeO. The Y₂O₃ and REE₂O₃ are from 0.5 to 5.5 wt % and from 0.05 to 4.5 wt %, respectively.

Details of REE mineralogy were checked in two drill core samples of albitised and carbonatised sericite-quartzite at Honkilehto. These samples are composed mainly of albite, calcite, biotite, quartz and sericite. Apatite occurs as discrete subhedral grains in association with calcite and dolomite. Davidite is isostructural with the crichtonite-group minerals, its association with rutile and both as principal opaque minerals. Davidite is present as up to 2 mm large zoned grains (Figure 12f). About 20 microprobe analyses have been carried out of davidite (Supplementary Materials Table S6). Its chemical composition is governed by Ti, V, Cr, and Fe-oxide accompanied by smaller amounts of REE, U and Pb. The major chemical components are TiO₂ (41.55–47.36 wt %) and FeO (15.38–19.66 wt %), and the U content ranges from 2.56 to 9.41 wt % UO₂. The Th content is much lower, ranging from <0.01 to 0.19 wt % ThO₂. Maximum amounts of other elements that were consistently detected include 0.92 wt % Sc₂O₃, 3.31 wt % V₂O₃, 12.06 wt % Cr₂O₃, 0.68 wt % Y₂O₃, 3.42 wt % La₂O₃, 3.13 wt % Ce₂O₃, 0.42 wt % CaO, 0.26 wt % MgO, 1.06 wt % SrO and 2.70 wt % PbO. Backscattered electron images of davidite display the presence of patchy compositional zoning and intense cracking of grains (Figure 13). It is not known whether the cracking is related to radiation damage induced volume expansion or to fracturing by tectonic processes. Reconnaissance EPMA traverses show that the compositional zoning, at least in part, reflects the distribution of Ti, Y, REE, U and Pb. Uranium, Cr, Ti and Pb tend to be relatively concentrated toward the central (more altered) portion of grains at the expense of Pb. Lighter areas thus have maximum U, Pb and minimum Ti, Y and REEs (Figure 13). REE appear to co-exist or substitute, respectively, for Pb and U, as indicated by the corresponding decreasing trends of Ti, Y, REE with increasing trend of Pb + U.

3.4. REE Occurrences in Pegmatite and Greisen (Magmatic-Hydrothermal Systems)

3.4.1. Rapakivi Granite

Rapakivi granite occurs in four large batholiths (e.g., Vyborg, Aland, Vehmaa, Laitila) and several smaller satellite intrusions (e.g., Suomenniemi, Ahvenisto, Onas, Bodom, Obbnäs, Kökar, Fjälskär, Eurajoki, Kokemäki) in southern Finland [60]. In this study, we report comprehensive mineralogical and petrographic data from the Kymi Granite Complex within the Proterozoic Vyborg Rapakivi Batholith in south eastern Finland (Figure 14).

The rapakivi granite in the Kymi Complex hosts several greisen zones which are characterized by silicification with fine grained masses of sericite, muscovite and chlorite. Fluorite, topaz, zircon, epidote, apatite, genthelvite, iron-titanium oxides and sulphide minerals (sphalerite, galena, and chalcopyrite) are the most abundant accessory minerals. Geochemically, high F, Li, Rb, Ga, Sn and Nb, concentrations and depleted Mg, Ti, Zr, Ba, Sr and Eu contents distinct zones of greisen from the fresh granite [61]. Some of the greisens in rapakivi granite are also enriched in indium and rare earth elements, with roquesite (CuInS₂) being the major indium bearing mineral (Figure 15a), whereas monazite, allanite, bastnäsite, xenotime and thorite are the main REE rich minerals (Figure 15b–f).

The total REE content of greisens range from 100 to 1025 ppm with an average of 500 ppm [62], whereas the indium content is commonly from 70 to 200 ppm, but it may reach values of up to 800 ppm [54]. Previous studies concluded that enrichments of REE is due to a combination of magmatic and hydrothermal processes [60,63,64,65].

The 1.64–1.63 Ga Kymi Granite Complex can be divided into three varieties of rapakivi granite: (1) granite with ovoid alkali feldspar phenocrysts larger than 3 cm in diameter, mantled by sodic plagioclase. In this variety, the feldspar crystals have been partially re-melted, and subsequently altered; the alteration produced the rim of greenish sodic plagioclase; (2) granite with quartz-feldspar porphyritic texture and angular phenocrysts of potassium feldspar (microcline), plagioclase and quartz; (3) hornblende bearing granite with porphyritic rapakivi texture.

Detailed mineralogical observations on REE-bearing minerals were completed in 12 samples from the greisenised Kymi granite stocks and the surrounding country rocks (Figure 14). Results of electron microprobe analyses in those minerals are reported in the Supplementary Materials Table S7. Monazite occurs as euhedral to subhedral accessory crystals (20–50 μm diameter) and it is often intergrown with zircon, xenotime, apatite and fluorite (Figure 15b) and also fills fractures in apatite (Figure 15c). Monazite grains usually coexist with xenotime (Figure 15b). Contents of the major elements in monazite are as follows: P₂O₅ from 27.7 to 29.3 wt %, F from 1.0 to 1.6 wt %, Ce₂O₃ from 31.4 to 34.4 wt %, La₂O₃ from 11.7 to 15.4 wt %, Nd₂O₃ from 11.3 to 13.5 wt %, Sm₂O₃ from 1.3 to 2.7 wt %, Gd₂O₃ from 1 to 2.2 wt % and Pr₂O₃ from 3.0 to 3.6 wt %. The ThO₂ contents vary from 0.53 to 3.1 wt % whereas UO₂ has low concentrations from 0.01 to 0.24 wt % and SiO₂ varies between 0.5 and 3.2 wt % (Supplementary Materials Table S7).

Bastnäsite is mostly present as disseminations of acicular and needle-shaped crystals and flakes (100 μm × 300 μm). It also occurs in thin fractures and vugs of fluorite, biotite and quartz as round-hexagonal crystals (Figure 15c–e). Bastnäsite is commonly associated with allanite and thorite. At some places, bastnäsite appears to replace allanite as it forms rims around corroded allanite cores (Figure 15f). Bastnäsite is rich in LREE (~65 wt %), with Ce₂O₃ from 30.6 to 33.7 wt %, La₂O₃ from 12.9 to 17.9 wt %, Nd₂O₃ from 7.3 to 11.6 wt %, Sm₂O₃ from 0.8 to 2.0 wt %, Gd₂O₃ from 0.7 to 1.7 wt % and Pr₂O₃ from 2.5 to 3.2 wt % (Supplementary Materials Table S7). The flour contents are between 6.7 and 9.3 wt %. Bastnäsite grains also contain significant amounts of total HREE (from 0.5 to 1.5 wt %), as well as ThO₂ (from 0.3 to 1.3 wt %) and Y₂O₃ (from 1.7 to 3.4 wt %).

3.4.2. Kovela Granitoid Complex

The Kovela Granitoid Complex is located in the Uusimaa Belt (1.85–1.79 Ga) in southern Finland (Figure 1). The complex has a well-developed zonal structure: the central part consists of coarse-grained equigranular to K-feldspar porphyritic biotite monzogranite and small granodiorite bodies, whereas the marginal zones of the complex consists of pyroxene gneiss. Garnet-cordierite gneiss as well as marginal pegmatite occur along the outer contact of the complex (Figure 16).

The strongly radioactive granitic pegmatite bodies in the central part of the complex have predominantly peraluminous and S-type character. Most of the granitic pegmatite dikes run roughly in NW-SE direction and they are named according to their locations: S dikes in the southern part and N dikes in the northern part of the complex. These dikes are usually 5–10 m wide and 60–70 m long and gently dip to the West. Among them, the S dikes are more radioactive than the N dikes. They consist of perthitic K-feldspar, quartz, plagioclase, biotite, garnet, sillimanite, cordierite and staurolite. Monazite, zircon, xenotime, magnetite (oxidized), titanite, ilmenite and chlorite are the most common accessories. The complex underwent granulite facies metamorphism at 700–820 °C temperatures and 2.6–5.8 kbar pressures [66].

Monazite and Th-rich monazite are the most abundant accessories in the pegmatite dikes in addition to the commonly observed zircon, xenotime and huttonite/thorite. Monazite grains (100–1000 μm) are usually elongated or sometimes rounded and fractured. Their colours range from light yellow to yellowish brown. Exceptionally, some euhedral to subhedral prismatic crystals are as long as 3 mm (Figure 17a). Monazite is mostly enclosed in quartz and K-feldspar and attached to garnet (Figure 17b). Backscattered SEM imaging revealed complex sector-, concentric- and patchy-zoning, as well as normal growth zoning in monazite and presence of abundant thorite inclusions in some grains (Figure 17e–f). Growth zoning only rarely occurs, whereas concentric and patchy zoning have been recognized on monazites from most of the studied samples. Representative microprobe analyses of monazite compositions from the Kovela monzogranite and granite samples are given in Supplementary Materials Table S8. The zoning reflects the variations between the abundances of LREE and Th (+U + Ca and Si). Dark zones on SEM images of monazite predominantly occur as outer rims and as rare inner cores (Figure 17e). These zones have lower Th (+Ca and Si) and higher REE contents compared to the brighter zones. This negative correlation between Th (+Ca and Si) and REE occurs on all monazite grains and reflects the typical coupled substitution mechanisms in monazite: (Th, U)⁴⁺ + Si⁴⁺ ⇋ REE³⁺ + P⁵⁺ e.g., [67] and (Th, U)⁴⁺ + Ca²⁺ ⇋ 2REE³⁺ [68]. The A-site cation sites in the crystal structure of monazite are principally occupied by Ce as the dominant REE (from 24.90 to 28.56 wt % Ce₂O₃). The La-content varies from 8.30 to 10.56 wt % La₂O₃ and the Nd₂O₃ contents are from 8.98 to 10.54 wt %. The amount of Th, representing the huttonite molecule as a solid solution in monazite varies from 14.9 to 20.1 wt % ThO₂. The U content is low and ranges from 0.47 to 1.47 wt % UO₂. The Ca content, representing the cheralite molecule as a solid solution in monazite, varies from 0.71 to 1.69 wt % CaO. Skeletal and porous intergrowths of 10–20 µm large crystals of thorium-silicate compositions (probably huttonite or thorite) with monazite were also observed by SEM (Figure 17f). The huttonite grains contain from 52.7 to 66.0 wt % ThO₂ as main component, with several weight percent of LREE (1.3–7.4 wt % Ce₂O₃, 0–1.6 wt % La₂O₃ and 0.7–2.1 wt % Nd₂O₃; Supplementary Materials Table S8). The P and U contents in huttonite grains vary from 3.2 to 6.5 wt % P₂O₅ and from 0.9 to 2.9 wt % UO₂, respectively. The percentage of monazite, cheralite and huttonite components in these thorium-silicate grains range between 5.0–18.2 wt %, 13.0–27.0 wt %, and 62.1–74.4 wt %, respectively.

Thorite forms inclusions within monazite and zircon (Figure 17a,b). Two types of zircons were found: (1) small (<50 µm) grains with round to subhedral habits as inclusions in garnet and monazite (Figure 17a–c); and (2) coarse, from 100 to 400 μm large crystals with mostly euhedral to subhedral prismatic shapes. Type 2 zircons often show oscillatory zoning on BSE images (Figure 17c). The elevated Th contents in the Kovela complex was most probably resulted from a complex, multi-stage process which involved primary magmatic crystallization and superimposed metamorphic-hydrothermal alteration. Förster and Harlov [69], Overstreet [70], Mohr [71] and Kelly et al. [72] suggested that metamorphism may generate the breakdown of primary monazite to Th-rich mineral in the monazite-huttonite series by depletion of the Y, HREE, Ca and P content parallel with the relatively enrichment of Th, Si and LREE.

4. Compositions of REE Minerals with Different Origins in Finland

Box-plot diagrams of REE concentration data according to the main REE-bearing phases from the studied deposits and occurrences in Finland are shown in (Figure 18). As it is expected, and also demonstrated by many other studies [63,64], LREE are generally enriched in monazite, allanite and carbonate minerals, whereas xenotime concentrates HREE regardless the conditions of crystallization. The highest average LREE concentrations at around 60 wt % occur in monazite, bastnäsite and synchysite, but the range of these values is highly variable. However, comparison of mantle-normalized [73] REE distributions in different types of REE-rich minerals from carbonatite and alkaline rocks, hydrothermal alteration zones and granitoids/greisens with elevated REE contents reveals significant and systematic variation in both absolute and relative abundances (Figure 19). The REE distribution patterns of bastnäsite, allanite and ancylite are similar to those of monazite. They are characterized by higher LREE over HREE, and negative Y anomalies. In each types of minerals, except of xenotime, strong enrichment in LREE relative to MREE and HREE can be clearly seen. The normalized plots for most monazite exhibit similar patterns in each types of deposits/occurrences, but monazite from granitoids-related magmatic-hydrothermal (e.g., pegmatite and greisen) systems have higher REE contents in comparison to the metasomatic-hydrothermal and carbonatite related environments (Figure 19a). Allanite and ancylite show similar REE patterns to monazite, with the lowest concentrations of REE in alkaline-rocks and carbonatite in comparison to the other types of geological environments (Figure 19b). Similarly to monazite and allanite, bastnäsite and synchysite also display steeply right-inclined normalized REE profiles with depleted Y contents (Figure 19c,e), but bastnäsite from pegmatite/greisen shows lower REE concentrations in comparison to the other geological environments.

HREE concentrations above the detection limits of the EPMA analyses characterize the pegmatite/greisen hosted monazite, allanite and bastnäsite only. However, the absolute HRRE concentrations are just around or even lower than those in the average of the mantle [72]. Strong enrichment of HREE over LREE characterize xenotime (Figure 19f). This reflects that xenotime prefers to incorporate HREEs into its structure [74].

The differences in REE-enrichments and CaO-contents according to the origin of the REE rich phosphate minerals are also evident in the LREE–HREE + Y–CaO ternary plots (Figure 20). Monazite from the carbonatite has a relatively wide range in the proportions of the different HREE+Y and LREE, as well as total REE and CaO contents (Figure 20a). This variability in composition probably reflects the cumulative effect of fractionation of REE during crystallization of carbonatite melts with different composition, as well as the effect of segregation of magmatic fluids. Under magmatic conditions, dolomite has lower partition coefficients for REE than calcite [3], and precipitation of rock forming carbonate minerals results in enrichment of LREE in the residual liquid. La-Ce and Sm-Nd also fractionate into magmatic fluids in different proportions [75]. In contrast, monazite from hydrothermal and granite-related magmatic-hydrothermal environments appear to show less variability in composition with dominance of LREE (Figure 20a).

Xenotime in carbonatites, alkaline rocks and granite related magmatic-hydrothermal systems also exhibits limited, although specific variations in the relative concentration of HREE + Y to LREE (Figure 20b). The source of elements for the formation of xenotime-(Y) in the studied granitic rocks is the leaching of P, REE and Y mainly from zircon and apatite. Monazite and apatite associated with xenotime related to the breakdown of primary (Fe, U, Y, REE, Ca, Si)-(Nb, Ta) oxide phases both tend to have slightly higher LREE compared to most xenotime grains. The latter type is also clearly higher in the HREE and lower in Y.

In the REE deposits of occurrences of Finland, bastnäsite and synchysite are the most common Ca-REE fluorocarbonates and they also show compositional variation according to the conditions of their crystallization (Figure 21a). Based on the EPMA data, bastnäsite with low Ca + Sr contents, ranging from 0.1 to 1.1 wt % CaO + SrO (hydrothermal), 2.0 to 7.3 wt % CaO + SrO (carbonatite), 0.1 to 6.2 wt % CaO + SrO (granitic), 3.6 to 6.1 wt % CaO + SrO (alkaline) and synchysite, with higher Ca, varying between 11.7 and 18.0 wt % CaO + SrO (hydrothermal) 11.8 and 17.6 wt % CaO + SrO (carbonatite), 8.6 and 13.5 wt % CaO + SrO (alkaline) respectively. Both minerals are classified as fluorocarbonates with F dominant in the F–OH–Cl site. Compositions of the Ca-REE fluorocarbonates (bastnäsite and synchysite) also discriminate formation conditions in the LREE–HREE–CaO ternary diagram, but in a different and less straightforward way (Figure 21a). Hydrothermal and carbonatite bastnäsite are richer in LREE in comparison to synchysite of similar origin, whereas the compositional differences considering the LREE and CaO contents are less pronounced in the pegmatite/greisen and alkaline rocks hosted occurrences. The less pronounced differences among REE-carbonates from different geological environments most probably reflect the complex effect of temperature-pressure and CO₂ concentration parameters [48]. On the basis of our investigations at the REE occurrences and prospects in Finland, we suggest that high mole fractions of CO₂ in a hydrothermal fluid also enhanced such fractionations, again due to the greater solubility of neutral complexes in fluids of low dielectric constants.

Figure 21b shows the compositions of REE silicate minerals (allanite-ferroallanite, and chevkinite) of different origin in the (Y + REE)–(Al + Fe)–(Ti) ternary. Most of the allanites in studied REE occurrences and prospects belongs to the allanite-ferriallanite series (up to 40% of allanite-Ce), and a smaller part, to the ferriallanite end-member (Figure 21b). The CaO content of allanite-(Ce) is unusually high and appears to be a characteristic feature of allanite from arkose gneiss in Tana Belt.

Chevkinite, which has the same crystal system as allanite contains more titanium than allanite and ferroallanite, and this Ti-enrichment of allanite-type silicates characterize chevkinite grains in carbonatites and albitite alteration zones in hydrothermal systems. The distribution of allanite and chevkinite in the studied carbonatites dikes suggests that the composition of the magmas is an important control on the stability fields of these minerals, a fact that is supported by the presence of primary allanite in rocks formed by processes of mixing magmatic fluids and a later hydrothermal fluid. Bagiński et al. [76] and Macdonald et al. [77] documented the alteration of chevkinite-(Ce) to a phase strongly enriched in Ti and depleted in Si, REE and Fe compared to the unaltered phase. In contrast to studies chevkinite did not alter to a member of the epidote-group, possibly because REE + Y abundance in chevkinite (43–50 wt %) is almost twice that observed in allanite.

5. Comparison of Results from Finland with Published Compositional Data of Carbonates, REE-Phosphates and Silicates from Other Areas

The (La/Nd)_cn ratio in REE-fluorocarbonates is used to identify and define variable hydrothermal systems, as an indicator of the paragenetic type of occurrence and geological environment and REE fractionation within, or between deposits [3,78,79,80,81,82]. The mean values of (La + Ce + Pr)/(ΣREE) and (La/Nd)_cn obtained from the present datasets (a total of several thousand analyses presented in the Supplementary Materials Tables), are summarized in

Table 2. The ratios of (La/Nd)_cn and (La + Ce + Pr)/(ΣREE) in REE minerals from the studied areas compared to measured ratios from other world’s REE deposits.

Mineral	(La + Ce + Pr)/(ΣREE)	(La/Nd)cn	Mineral	(La + Ce + Pr)/(ΣREE)	(La/Nd)cn
Monazite			Allanite
Carbonatite rocks	0.80	0.80–4.55	Carbonatite rocks	0.75	2.17–2.98
Hydrothermal metasomatic rocks	0.85	2.05–4.17	Hydrothermal metasomatic rocks	0.87	6.87–9.26
Granitoids/greisens	0.75	1.57–2.27	Granitoids/greisens	0.77	2.09–2.71
Alkaline rocks Nechalacho	0.77	2.03–2.26	Alkaline arkosic rocks	0.71	1.05–4.23
Hydrothermal ore, Olserum-Djupedal	0.70	0.60–2.50	Alkaline rocks, Nechalacho	0.83	2.8–2.9
Carbonatite, Bayan Obo	0.85	4.50–6.20	Carbonatite, Bayan Obo	0.86	3.8
Pegmatite, Creek Pass	0.63	1.48–1.51	Pegmatite Creek Pass	0.72	1.70–3.40
			Hydrothermal ore, Olserum-Djupedal	0.74	1.67–2.48
			Upper Crust	0.68	1.2
Bastnäsite			Synchysite
Carbonatite rocks	0.78	0.50–3.66	Carbonatite rocks	0.78	1.49–3.43
Hydrothermal metasomatic rocks	0.89	8.50–12.84	Hydrothermal metasomatic rocks	0.80	1.94–3.43
Granitoids/greisens	0.70	0.80–2.00	Alkaline arkosic rocks	0.71	1.56–2.37
Alkaline arkosic rocks	0.73	1.71–2.21	Alkaline rocks, Nechalacho	0.76	1.73–2.26
Alkaline rocks Nechalacho	0.79	2.22–2.28	Carbonatite, Bayan Obo	0.79	0.60–2.20
Hydrothermal ore, Olserum-Djupedal	0.72	1.70–2.38	Hydrothermal ore, Olserum-Djupedal	0.84	2.0–2.3
Upper Crust	0.68	1.2	Upper Crust	0.68	1.2

, and are compared with measured ratios from published compositional data. However, the wide ranges in the (La/Nd)_cn of REE minerals from the studied areas, indicate substantial fractionation of these elements in the most important metallogenic provinces and prospecting target of REE deposits in Finland. The wide variations also occur for specific minerals, even within a single sample or grain, indicating changing compositions of the fluid during crystallization.

Smith et al. [3] have discussed fractionation of La and Nd during hydrothermal mineral-forming processes, using data from the Bayan Obo REE deposit in China. According to their model, variations in the relative proportions of these elements are primarily related to REE speciation in the fluids and temperature conditions. At high temperatures (>250 °C) and in a CO₂-rich fluid, La-bearing fluoride complexes have a higher association-constant than, for example, for Nd, promoting the observed fractionation.

Result of our studies in Finland indicate origin-dependent distribution of REE minerals composition on the plots presenting (La/Nd)_cn ratios against the (La + Ce + Pr)/(ΣREE) ratios (Figure 22; Table 2). These results compare favorably with published data from the carbonatite dikes at Bayan Obo in China [3], alkaline/peralkaline igneous rocks hosted REE deposit at Nechalacho in NW Canada [78], the Olserum Djupedal REE-phosphate mineralization of hydrothermal origin in SE Sweden [79], and granite related pegmatite hosted deposit at Creek Pass, Colorado [80]. The highest ratios occur in REE minerals of hydrothermal origin and similarly high ratios was found in monazite and synchysite from carbonatites only. Monazite from the Finnish carbonatite deposits is characterized by a wide range of the proportions of lanthanides (La + Ce + Pr/ΣREE At %) and (La/Nd)_cn ratios compared to other monazite samples from different occurrences (Figure 22a). This probably reflects fractionation of REE between melts, fluids and monazite during the crystallization of the parent carbonatite melts, similarly to the fractionation trend observed by Smith et al. [3] at Bayan Obo. Monazite from granite-related environments (e.g., pegmatite and greisen zones) in Finland is characterized by a relatively narrow (La/Nd)_cn and (La + Ce + Pr)/(∑REE) ratios (from about 2 to 1.5 and less than 0.75, respectively), close to the values of the average upper crust (e.g., 1 and 0.68, respectively). These ratios are similar to those for monazite from REE deposits occurring in alkaline rocks, pegmatite and hydrothermal veins outside of Finland (Figure 22a).

Allanite from hydrothermal occurrences in Finland is lanthanum-enriched, relative to other allanite of different origin (Figure 22b). Compositions of allanite from carbonatites, alkaline rocks and greisens are characterized by relatively low (La/Nd)_cn ratios (between 3 and 1) and low La + Ce + Pr/ΣREE ratios (less than 0.85) and fall into the compositional trend defined by the average data from the Nechalacho (alkaline rock hosted), Olserum Djupedal (hydrothermal) and Creek Pass (pegmatite) deposits.

Bastnäsite from different types of REE deposits and occurrences in Finland shows the widest ranges in (La/Nd)_cn starting from about crustal values, as shown in granitoids/greisens rocks((La/Nd)_cn = 0.80 to 2.0) and going to the maximum ratios encountered, as in hydrothermal metasomatic rocks ((La/Nd)_cn = 8.5 to 12.84). The rapidly changing conditions that would prevail in high temperature metamorphic hydrothermal fluids with highly variable CO₂-content provide an explanation for the large compositional variations found on the bastnäsite compositions from different REE deposits. The REE distribution in bastnäsite-(Ce) from the carbonatite deposits, granitoids/greisens and alkaline arkosic rocks in Finland have similar patterns of variation compared to alkaline rocks at Nechalacho and hydrothermal deposits at Olserum-Djupeda, but plotting below the fractionation trend observed in the Bayan Obo carbonatite (Figure 22c; Table 2).

Synchysite from Finnish carbonatite, alkaline rock hosted and hydrothermal deposits have relatively wide ranges of La/Nd_cn ratios (from around 3.4 to 1.5) and La + Ce + Pr/ΣREE ratios (between 0.85 and 0.6) and these ratios largely corresponds to to those from different deposits from Finland (Figure 22d).

6. Disscusion

Despite of the existence of several areas with high REE potential and many ongoing exploration projects, the territory of Finland is still underexplored for REE. However, if demand is to be met in the future, continued research into, and further exploration of Finnish REE resources will be needed. The REE exploration potential in Finland may be evaluated by comparing the geochemical databases and known REE occurrences with global deposit types. In terrains which are largely covered by glacial or other young unconsolidated sediments, such as the area of Finland, mostly geochemical and geophysical methods are used for localization of areas with REE potential. The most probable pathway to REE production in Finland is via the by-production alongside with extraction of other commodities such as P, Nb, Ta and Au. Enrichment of the REE may occur through primary processes such as magmatic processes and hydrothermal fluid mobilization and precipitation, or through secondary processes that move REE minerals from where they originally formed, such as sedimentary concentration and weathering.

Several REE deposits and occurrences in various geological settings across Finland have been subjected to detailed geological and mineralogical studies. These REE resources are intimately associated with a variety of rock types such as late-stage carbonatite dikes, epigenetic-hydrothermal systems and granite-pegmatites and greisens. The main features of the major REE-bearing minerals from the REE-bearing deposits obtained in this study have been summarized in Table 1. Primary magmatic REE enrichments occur in late-stage carbonatite dikes in the Sokli carbonatite complex (1–10 wt % RE₂O₃), with carbonatite veins at the Korsnäs deposit (0.9 wt % RE₂O₃), alkaline gneiss at the Otanmäki-Katajakangas deposit (2.4 wt % RE₂O₃) and monazite granite in Kovela (0.5–4.3 wt % RE₂O₃). Hydrothermal REE deposits are also of interest, although typically low-grade; they include Fe-oxide-Cu-Au class deposits in the Paleoproterozoic Kuusamo Belt (Au, Cu, Mo, Ni, REE and U), and Tana Belt in Northern Finland. These deposits could be potential sources of REE as a by-product of Au and other metals. Higher concentrations of the REE are found in the Kortejarvi and Laivajoki carbonatite intrusions, whereas A-type granite intrusions within Vyborg rapakivi batholith display high average levels of Sn, Ga, Nb, Be, In and REE [12,60]. According to the existing mineralogical and geochemical data, the Eurajoki and Kymi stocks seem to have the most potential for REE mineralisation among the rapakivi granites.

The late stage carbonatite dikes in the Sokli complex (360–380 Ma), located in the Finnish part of the Devonian Kola alkaline province, contain one of the most diverse assemblages of REE minerals described so far from carbonatites and provide an excellent opportunity to track the evolution of late-stage carbonatites and their sub-solidus (secondary) changes. More than ten More than ten different types of rare earth minerals have been analysed in detail and compared with analyses published from other deposits. Most of these minerals common in carbonatites (e.g., Ca-REE fluocarbonates and ancylite-(Ce)) plus monazite, xenotime and Sr, REE-apatite and some of them are very rare Ba, REE fluocarbonates. Mineralogical and mineral chemical evidence demonstrates that hydrothermal and metasomatic reworking processes were responsible for the REE mineralization in Sokli carbonatite veins and confirms that such processes are predominant in the formation of REE minerals in carbonatites. During late-stage processes, apatite and carbonate minerals were replaced by various assemblages of REE-Sr-Ba minerals. Other example is the Korsnäs Pb-REE-bearing carbonatite dyke which intrudes Palaeoproterozoic mica-gneisses of the Pohjanmaa schist belt, and hosts REE-bearing apatite, monazite, allanite, calcio-ancylite, and bastnäsite. The deposit consists of mineralised zones in pegmatite and carbonatite or calcareous scapolite-diopside-barite-bearing skarn rocks. The wall rock of the dyke has been kaolinised in the strongly sheared ore zone. Apatite and monazite are heterogeneously distributed in the ore but occur together with galena. Monazite forms either anhedral grains or larger grain clusters that occur as inclusions within apatite phenocrysts.

In contrast to carbonatites, other notable examples of REE-rich products of the peralkaline rocks include REE-P deposits in Otanmäki Katajakangas deposit south of the Lake Oulujärvi in central Finland. It is not surprising, that in addition to monazite and allanite, which are typical LREE hosts in the studied rocks, the peralkaline varieties contain such HREE minerals as fergusonite-Y and fergusonite-U. Other notable heavy rare-earth element (REE) hosts include primary zircon and columbite. Hydrothermal reworking and autometasomatic processes have been reported to produce HREE mineralization (e.g., fergusonite-Y, and euxenite-Y) arising from the decomposition of allanite and other primary Nb, Ti minerals [81,82]. On the other hand, rare-metal minerals are hosted by the granites of both the older and younger phases and produced by a combination of anatexis (principally silicate minerals, as allanite) and later hydrothermal processes (mostly oxide minerals, as fergusonite). Furthermore, fractionation of rare-earth elements in allanite-(Ce) and monazite-Ce led to magma enrichment in middle and heavy rare earths, leading to the crystallization of fergusonite-Y, and euxenite-Y during the late magmatic stages.

Hydrothermal alteration zones associated with Au-Co-Cu ± U mineralization in the central part of the Kuusamo belt is characterized by REE and U-rich minerals. There are two stages in the evolution of Au-Co-Cu ± U deposits within the Kuusamo Belt. The first conforms to a classic magmatic scheme, the second is postmagamatic and is characterized of multiphase metasomatism and metamorphic hydrothermal processes. In the postmagmatic stage, albitization, sericitization, chloritization, carbonation, and silicification have been traced [16,57]. Albitisation is the most extensive alteration type and is, apparently, preceeded the establishment of peak metamorphic conditions of the Svecofennian orogeny. Several phases of mineralization appeared in conjunction with albitization, resulting the enrichments in niobium, yttrium, uranium, thorium and REE. Niobium mainly situated in pyrochlore, fergusonite, and titanite. The uranium content is high in some minerals (uraninite, with 78.6 to 80.5 wt % UO₂, davidite, euxenite, cloumbite), while thorium concentrated in other ones (thorite, monazite allanite). Albitization is followed by the chloritization, which is closely related to gold mineralization and indicated by the formation of chlorite, tremolite-actinolite, magnetite, talc and Fe sulphides. Chloritized rocks exhibit enrichments in heavy REE, and are relatively strongly enriched in Mg, strongly depleted in Si and large ion lithophile (such as U, Th, Nb, F) elements, and slightly depleted in light REE. The next stage is phyllic alteration, indicated by biotite and sericite ± pyrite and additional gold mineralisation and ductile deformation. Sericitized rocks were previously silicified; they are very strongly enriched in K-Rb-Cs-Ba, very strongly depleted in Na-Ca-Sr-Eu, and slightly depleted in light REE relative to albitization stage. This is followed by a stage of carbonation, silicification, where calcite, allanite-Ce, ancylite-Ce, and bastnäsite-Ce replace monazite-(Ce) after apatite. Alterations are localized on permeable zones such as fractures, flow tops, discordant breccia dikes, and conformable breccia horizons.

In addition to peralkaline granites (see above), their late-orogenic peraluminous counterparts in post- or anorogenic settings may be associated with Vyborg granites and granite pegmatite (Kovela) with high Th-LREE content. The Vyborg rapakivi batholith (1.64 Ga) in south-eastern Finland (Kymi) is one of a few regions in the Fennoscandian Shield with several documented occurrences of REE, indium and Zn-Cu-Pb sulphide mineralization. The late-orogenic peraluminous Kovela monazite-granite shows up as a strong positive aeroradiometric gamma radiation anomaly in the Svecofennian Uusimaa Belt. Monazite is the dominant REE-mineral; accessory minerals include zircon, xenotime, and thorite.

7. Conclusions

The most significant REE deposits and occurrences in Finland are associated with highly fractionated carbonatite intrusions and dikes, alkaline rocks, hydrothermal alteration zones in metamorphic rocks and greisens and pegmatites in granite intrusions. The most common REE minerals and REE-bearing minerals in these environments are phosphates (monazite, xenotime and fluorapatite), fluorcarbonates (bastnäsite and synchysite), hydrated carbonates (ancylite), hydrated aluminium silicates (allanite, Fe-allanite, cerite, chevkinite), oxides (fergusonite, euxenite) and U-Pb rich minerals.

Late-stage carbonatite dikes in the Sokli carbonatites are important sources of REE in Finland. Less pronounced that LREE enrichment is found in the metasomatic carbonatite bodies at Sokli and calcsilicate rocks (skarn) from Korsnäs. On the basis of variations in REE distributions and mineral chemistry, strong differentiation of REE contents in monazite can be distinguished in the carbonatite complexes in Finland and elsewhere. Other REE rich minerals from the magmatic and post-magmatic stages of carbonatite complexes show less pronounced variations in REE contents.

Alkaline rocks (Otanmäki) have potential for occurrences of significant REE deposits. The host alkaline granitic rocks are typically formed by progressive fractional crystallization of a parental magma and the deposits typically represent two periods of mineralization. The first primary magmatic period is associated with crystallization of highly fractionated magma rich in REE. The minerals of this period are commonly overprinted during the second period by late magmatic to hydrothermal fluids enriched the primary mineralization and remobilized REE in to hydrothermal veins.

The epigenetic hydrothermal alteration and associated REE enrichments are documented in the Kuusamo Belt. Several phases of REE mineralization can be distinguished. Subtle compositional differences among REE-fluorocarbonates (bastnäsite, synchysite), monazite and allanite define a spectrum from relatively La-enriched to (Ce + Nd)-enriched phases. The La-rich character of the mineralization in the Kuusamo Belt may be related to the high CO₂-contents of high temperature metamorphic–hydrothermal fluids as high coordination number complexes of La are predicted to be more strongly associated than for the other REE [3].

The pegmatite dikes within Kovela granitic complex, southern Finland are characterized by the presence of Th enriched monazite and its specific compositional variation (huttonite/thorite monazite) appears to represent multi-stage process which involved primary magmatic crystallization and late-stage hydrothermal alteration. The recrystallization during both the high-T and lower-T magmatic-hydrothermal events, led to monazite becoming depleted in Y, HREE, Ca and P, and relatively enriched in Th, Si and LREE to form the Th-rich minerals of the monazite huttonite series.

In the greisens of rapakivi granite in the Kymi stock, LREE are carried by monazite, bastnäsite and allanite, and the HREE by xenotime and zircon. The REE-mineralization was also formed by the combination of magmatic and post magmatic processes.

The fractionation processes leading to high variation in lanthanum contents of monazite in carbonatite hosted REE-rich deposits (e.g., Bayan Obo) was also observed in Finnish deposits. Results of studies in Finnish deposits and occurrences suggest that origin of REE-bearing minerals can be distinguished on the basis of the chondrite-normalized REE patterns, as well as on the basis of relative variation of LREE and HREE concentrations in relation to some major elements (Al, Ti, Fe, Ca). The exceptional high enrichment of LREE, HREE, U, Th and Y in various minerals is restricted to the REE occurrences in the metamorphic hydrothermal systems of the Kuusamo Belt.