Ore and Geochemical Specialization and Substance Sources of the Ural and Timan Carbonatite Complexes (Russia): Insights from Trace Element, Rb–Sr, and Sm–Nd Isotope Data

: The Ilmeno–Vishnevogorsk (IVC), Buldym, and Chetlassky carbonatite complexes are localized in the folded regions of the Urals and Timan. These complexes differ in geochemical signatures and ore specialization: Nb-deposits of pyrochlore carbonatites are associated with the IVC, while Nb–REE-deposits with the Buldym complex and REE-deposits of bastnäsite carbonatites with the Chetlassky complex. A comparative study of these carbonatite complexes has been conducted in order to establish the reasons for their ore specialization and their sources. The IVC is characterized by low 87 Sr/ 86 Sr i (0.70336–0.70399) and ε Nd (+2 to +6), suggesting a single moderately depleted mantle source for rocks and pyrochlore mineralization. The Buldym complex has a higher 87 Sr/ 86 Sr i (0.70440–0.70513) with negative ε Nd ( − 0.2 to − 3), which corresponds to enriched mantle source EMI-type. The REE carbonatites of the Chetlassky complex show low 87 Sr/ 86 Sr i (0.70336–0.70369) and a high ε Nd (+5–+6), which is close to the DM mantle source with ~5% marine sedimentary component. Based on Sr–Nd isotope signatures, major, and trace element data, we assume that the different ore specialization of Urals and Timan carbonatites may be caused not only by crustal evolution of alkaline-carbonatite magmas, but also by the heterogeneity of their mantle sources associated with different degrees of enrichment in recycled components.


Introduction
Complexes of carbonatites and alkaline rocks are known to host economically significant deposits of critical metals, for example, Nb and rare earth elements (REE) [1]. Carbonatite deposits comprise about 90% of the world's Nb reserves. Three operating carbonatite deposits-Araxá and Catalão-II (Brazil) and St. Honoré (Canada)-account for about 99% of the total worldwide production of ferroniobium and other sources make up 1% [2]. The largest REE-deposits are associated with carbonatites: Mountain Pass (USA) and Bayun-Obo (China) provide more than half of the world production of rare earth elements of the cerium group.

Chetlassky Carbonatite Complex (Middle Timan)
The Chetlassky complex of dyke K-alkaline ultrabasites and carbonatites is located in the Middle Timan, occupying an area of about 1000 km 2 , in the southeastern part of the Chetlassky Kamen, which is a ledge of Riphean rocks in the pericratonic trough of the East European platform (Figure 2). The host rocks for the Chetlassky complex are terrigenous and terrigenous-carbonate strata of the Chetlassky (Rf2) suite and the Bystrinsky (Rf3) series. Ultramafic dyke bodies trace northeast-trending faults, forming dyke fields (Kosyu, Mezenskoe, Bobrovskoe, and Oktyabrskoe, etc.) in which there are several thousand dykes. Alkaline metasomatites (fenites, phlogopite, and feldspar metasomatites) and carbonatites as well as goethite-feldspar and quartz-goethite-hematite hydrothermal rocks are found in close spatial, structural, and temporal connection with ultrabasic dykes.  The Chetlassky dyke complex is composed of picrite-lamprophyric rock series. The most magnesian varieties are represented by subalkaline picrites and are comparable to veined "kimberlite-picrites" [26,77,78]. The lamprophyres of the alneite-polzenite [25] and spessartite-kersantite series [60,79] have been identified. It should be noted that a significant part of the dyke ultramafics is composed of carbonate-bearing lamprophyres, containing, along with phlogopite and pyroxene, carbonate (5-50%), apatite, amphibole, and garnet.
The sections and grain surface morphology of pyrochlores and aeshinites of the Ural carbonatite complexes as well as rare-metal carbonates and phosphates from Kosyu carbonatites were studied using scanning electron microscopy (SEM). The images of micro-objects were obtained and their composition was analyzed using a Jeol JSM-6390LV (JEOL) scanning electron microscope with INCA Energy 450 X-Max 80 (Oxford Instruments) energydispersive spectrometer ("Geoanalyst", IGG Ud RAS, S.P. Glavatskikh and I.A. Gottman).
Major elements for rocks and ores (19 samples from Ural carbonatite complexes, Table 3, and 13 samples from Chetlassky carbonatite complex of Timan, Table 4) were analyzed by wet chemistry and XRF at the Institute of Geology and Geochemistry UD RAS in Ekaterinburg and at the Institute of Geology Komy Science Center UD RAS in Syktyvkar. The trace and rare element concentrations were determined using acid decomposition of the samples and subsequent mass-spectrometric analysis on a high-resolution tandemanalyzer with ionization in an inductively coupled plasma "HR-ICP-MS Element 2" (IGG UD RAS, Ekaterinburg). The error of multielement analysis is no more than 8-10% if the content of an element is 10-20 times higher than its detection limit.Sr and Nd isotope compositions and concentrations in carbonates, apatites, amphiboles of carbonatites and whole-rock samples (miaskites, syenites, carbonatites, fenites) of the Ilmeno-Vishnevogorsk and Buldym (olivinites and peridotites) complexes as well as host rocks (calciphyre and plagiogneiss) of the Vishnevogorsk and Ilmenogorsk series (South Urals) were determined at the IGG UD RAS (Ekaterinburg) and GI KSC RAS (Apatity) on a Finnigan MAT-262 (RPQ) seven-channel mass spectrometer in static mode (Table 5). Sm-Nd and Rb-Sr isotope systems of rare-metal minerals (pyrochlore and eshinite groups) of the Urals' carbonatite complexes were studied by isotope dilution and mass spectrometry using high-resolution mass spectrometers-TRITON, ICP-MS NEPTUNE Plus, Finnigan MAT-262 (Apatity, St. Petersburg, Ekaterinburg) ( Table 5). The analytical details of measurements are described in [28].
Measurements of the isotopic composition and concentrations of Sr and Nd in carbonates and apatites from carbonatites of the Chetlassky complex and whole-rock samples (carbonatites and carbonate-bearing lamprophyres) as well as host rocks (dolomite) of the Bystrinskaya Group (Middle Timan) were fulfilled at the IGG UD RAS (Ekaterinburg) by the isotope dilution and mass spectrometric method using a Finnigan MAT-262 (RPQ) seven-channel mass spectrometer in static mode ( Table 5). The analytical details of the measurements are described in [28].
Since the article is devoted to the ore specialization of carbonatite complexes, we describe precisely only the Nb and REE ore minerals that provide economic interest of ore components. We studied 25 samples of Nb minerals of the pyrochlore and aeshinite groups from the main Nb-deposits and ore occurrences of the Urals' carbonatite complexes (IVC and Buldym) and 10 samples of REE minerals-monazites and REE-carbonates from Kosye REE-occurrence of the Chetlassky complex, Middle Timan. From the IVC, two pyrochlore samples of pegmatoid miaskites, three pyrochlore samples of miaskite-pegmatites (Vishnevogorskoe deposit; Uvildinskoe occurrence), three samples of syenite-pegmatites (Vishnevogorskoe deposit, ore zone 125; Potaninskoe deposit), three samples of sövites I (Potaninskoe deposit, Uvildinskoe occurrence), and eight samples of sövites II (Vishnevogorskoe deposit, ore zone 147, 140, 125) have been studied. In the Buldym complex, pyrochlores from the dolomite-calcite carbonatites (sövites III) and associated phlogopiterichterite metasomatites as well as pyrochlores and aeshinites from beforsite, glimmeritelike rocks, and metasomatites (six samples) were studied. From the Chetlassky complex, monazites, REE-fluorocarbonates and REE-Ca-carbonates (10 samples) were investigated.
In the IVC miaskite-pegmatites, pyrochlore is present in the form of scattered dissemination of black and dark brown (Pcl I, uranpyrochlore). Light brown pyrochlore (Pcl II) occurs as grains and octahedral crystals up to 0.5 cm in size (in miaskite-pegmatites-up to 10 cm (Figure 3a) as well as of 1-10 µm pyrochlore inclusions in nepheline grains, feldspars, and zircon.
In silicocarbonatites (sövites I) and in glimmerite-like carbonate-silicate rocks of the IVC (Potaninskoe deposit, Uvildinskoe ore occurrence, CAB), pyrochlore is also represented by a U-(Ta)-enriched variety (Pcl I, uranpyrochlore) and occurs in the form of small crystals and rounded grains of black and greenish-black color 0.05-1 mm in size. The surface of uranpyrochlore grains often has spherical cavities, which are likely to represent destruction from the alpha particle path as a result of U and Th radioactive decay ( Figure 3d). Often, uranpyrochlore grains and crystals from the edges and along the cracks underwent secondary changes of varying degrees with the formation of concentric textures ( Figure 3e). Sövites I also contains dark-brown pyrochlore grains (Pcl II) with multiphase inclusions (apatite, calcite, potassium feldspar, chlorite, titanomagnetite) ( Figure 3f).
IVAC carbonatites are enriched in both HFSE (especially Nb, less in Ta, Zr, Hf, V, and Ti) and LILE (Sr, Ba, and total REE with high La/Yb ratio , similar to the averaged compositions of calciocarbonatites of the world (Table 3, Figure 6) [101]. The Nb/Ta ratio  in early IVC silicocarbonatites (sövite I) is close to the ratio in magmatic carbonatites [103]; the Eu/Eu* ratio (0.96-0.91) is high and close to associated miaskite ones, which confirms their belonging to the early high-temperature differentiates of miaskite magmas [46]. High enough Nb/Ta ratio (230-1400) and some decrease in Eu/Eu* (down to 0.75) in late IVC carbonatites (sövite II) with a maximum in pyrochlore-bearing varieties are typical for the later high-temperature members of the carbonatite series and fluidhydrothermal carbonate systems [28].
Buldym complex. Buldym carbonatites have a wide range of CaO (17.9-50.3%) and MgO (5.1-25.5%), thus belonging to calciocarbonatites (sövites III) and magnesiocarbonatites (beforsites) ( Figure 5). Sövites III has similar mantle-normalized trace element patterns and chondrite-normalized REE patterns with high REE-contents (Figure 6c,d) compared to the IVC sövites (Figure 6a,b) and differ by higher contents of Nb (up to 5800 ppm) that are controlled by the distribution of the pyrochlore. Buldym beforsites have extremely high contents of REE (up to 48,000 ppm) and Th (up to 1400 ppm) in the form of single mineral phases-monazite and eshinite, and low Sr, Ba, and Nb. The high ratio La/Yb (to 3050) and Nb/Ta (to 1100) as well as lower Eu/Eu* (0.65) in Buldym beforsites are typical for low-temperature hydrothermal carbonatite facies [28,105].

Rb-Sr and Sm-Nd Isotope Data
The results obtained on the Rb-Sr and Sm-Nd isotope composition of rocks and minerals of carbonatite complexes of the Urals and Timan fold regions are presented in Table 5 and Figure 8. The initial isotope ratios of neodymium and strontium for the Ilmeno-Vishnevogorsk and Buldym complexes of the Southern Urals were recalculated for a time of 440 m.y. ago [63,69,112], but for the Chetlassky complex at 590 Ma [26,56]. The εSr and εNd values were calculated relative to the composition of the model reservoirs UR ( 87 Rb/ 86 Sr = 0.0816, 87 Sr/ 86 Sr = 0.7045) and CHUR ( 147 Sm/ 144 Nd = 0.1967, 143 Nd/ 144 Nd = 0.512636) of the corresponding age.

Composition, Evolution, and Genesis of Ore Rare-Metal Mineralization
Pyrochlore group mineral species of the IVC and Buldym carbonatite complexes are determined according to the latest nomenclature of the pyrochlore group (pyrochlore supergroup) based on the predominant cation or anion in the positions B = Nb, Ti, Ta; A = Ca, Na, REE, Y, Sr, Ba, Mn, Mg, U, Th, and Y = O, OH, F [100]. According to [100], pyrochlores of the Urals and carbonatite complexes are represented by U-(Ta)-rich hydroxyland oxycalciopyrochlores (or uranopyrochlores, according to the classification [124]) and fluorocalciopyrochlores (including Ta-, REE Ce -, and Sr-containing varieties). Hydrothermally altered and supergene pyrochlores are represented by hydroxylcalciopyrochlores and hydropyrochlores. The pyrochlore population compositions are given in Table 1 and are illustrated in ternary diagrams characterizing the cation filling of the A and B positions ( Figure 9).
The IVC and Buldym pyrochlore varieties (Pyrochlore I-V, see Table 1, Figure 9) are associated with certain types of rocks and a certain stage of the alkaline-magmatic system evolution. Thus, U-(Ta)-rich oxycalciopyrochlores (uranpyrochlore I according to [124]) are found in the pegmatoid varieties of miaskites, in miaskite-pegmatites, and sövites I of the IVC (Potaninskoe deposit, Uvildinskoe ore occurrence), and glimmerite-like rocks (Buldym deposit) (Figure 3a,d,e). This type of pyrochlore is enriched in UO 2 (17-24 wt%) and Ta 2 O 5 (1-4 wt%), and has low Nb/Ta ratios   (Table 1), which is typical for primary magmatic pyrochlore [125,126] (see Figure 9b). This type of pyrochlore is formed earlier than other pyrochlores at the late magmatic crystallization stage, as evidenced by relics of U-containing pyrochlore in later generations of pyrochlore from late carbonatites (sövites II) (Figure 3g) [27].
Hydroxylcalciopyrochlore and hydropyrochlore (Pcl V) are quite rare in the IVC and Buldym complex compared to primary pyrochlores (I-IV). These varieties of pyrochlore are enriched by SrO (2-5 wt%), LREE2О3 (2-4 wt%), BaO (0.7-2.4 wt%), Fe2O3 (1.5-1.7 wt%), SiO2 (1.6 wt%), and in some cases Ta2O5 (to 13 wt%) [136,137]. They are naturally substituted by varieties, whereas in the A-site, Na contents decrease and as a result, vacancies in the А-site are formed (from 35 to 70%, to 1 a.p.f.u), and Sr, LREE, and Ba become the significant cations. In the B-site, a decrease in Nb content isomorphically substituted by Si and Fe occurs. In the Y-site, F decreases until it disappears and replaced by OHgroups. This feature of pyrochlores is usually associated with subsolidus [138], hydrothermal [139][140][141][142][143], or supergene [144] processes. Pcl V occurs in pegmatites and early siliciocarbonatites and is most developed in late carbonatites. We assumed that these pyrochlores are formed as a result of subsolidus hydrothermal alteration of early pyrochlore generations (hydrothermal trend, see Figure 9b) at the final stages of the IVC complex evolution.  Table 1) during transitional (hydrothermal) and secondary (supergene) alteration of pyrochlore.
Hydroxylcalciopyrochlore and hydropyrochlore (Pcl V) are quite rare in the IVC and Buldym complex compared to primary pyrochlores (I-IV). These varieties of pyrochlore are enriched by SrO (2-5 wt%), LREE 2 O 3 (2-4 wt%), BaO (0.7-2.4 wt%), Fe 2 O 3 (1.5-1.7 wt%), SiO 2 (1.6 wt%), and in some cases Ta 2 O 5 (to 13 wt%) [136,137]. They are naturally substituted by varieties, whereas in the A-site, Na contents decrease and as a result, vacancies in the A-site are formed (from 35 to 70%, to 1 a.p.f.u), and Sr, LREE, and Ba become the significant cations. In the B-site, a decrease in Nb content isomorphically substituted by Si and Fe occurs. In the Y-site, F decreases until it disappears and replaced by OH-groups. This feature of pyrochlores is usually associated with subsolidus [138], hydrothermal [139][140][141][142][143], or supergene [144] processes. Pcl V occurs in pegmatites and early siliciocarbonatites and is most developed in late carbonatites. We assumed that these pyrochlores are formed as a result of subsolidus hydrothermal alteration of early pyrochlore generations (hydrothermal trend, see Figure 9b) at the final stages of the IVC complex evolution.
It is known that variations in the composition of HFSE pyrochlores are usually associated with the crystallization stage of alkaline rocks and carbonatites. For example, the highest concentrations U and Ta were measured at the pyrochlore crystal cores in early carbonatites [145]. Early population of U-Ta-enriched pyrochlore, which is commonly resorbed and surrounded by late pyrochlore have been described as a multi-stage magmatic evolution in several carbonatites worldwide [124,128,[145][146][147][148]. At the Kaiserstuhl, resorbed U-and Ta-rich cores in pyrochlore have been interpreted as originally crystallized from a silicate alkaline magma and subsequently entrained in the carbonatite magma during emplacement [126]. In addition, early-crystallizing uranpyrochlore is a common accessory in nepheline syenites at the Lovozero alkaline complex, Russia [149].
Late generations of Sr-REE-Ba-enrich hydroxylcalciopyrochlore in IVC replace early generations of pyrochlore (Pcl I, Pcl II), both in pegmatites and carbonatites, but are most developed in late carbonatites. Their formation is probably associated with subsolidus and hydrothermal processes at the final stage of the evolution of miaskite and carbonatite magmas, as in other carbonatite complexes. The insignificant scale of development and lower contents of Sr, REE, and Ba in these pyrochlores distinguishes them from the Sr-Ba pyrochlores of late low-temperature magnesio-and ferrocarbonatites, which complete the carbonatite series in alkaline-ultramafic complexes of intraplate settings [141].
Thus, IVC pyrochlore, as in other alkaline rock and carbonatite complexes, is a product of residual crystallization of carbonated alkaline magma and crystallizes at the pegmatite and carbonatite stages of magma evolution. Evolution of IVC pyrochlore composition from early generations, with high U and Ta and low (<70) Nb/Ta ratio (in miaskite-pegmatites and silicocarbonatites) to later generations, with low U and Ta and high Sr, REE and F and Nb/Ta ratio >300 (in late carbonatites) is a well-known feature of the pyrochlore evolution in carbonatite complexes throughout the world [124,128,[145][146][147]. At the same time, hydrothermal varieties represented by Sr-REE-Ba-enrich hydroxylcalciopyrochlore associated with subsolidus processes of primary pyrochlore transformation are weakly manifested in the IVC.
Monazites of the Chetlassky complex are represented by Ce-rich and Nd-rich varietiesmonazite-(Ce) and monazite-(Nd). It exists as early crystals and late xenomorphic (or needle-like) generations with different La/Ce and La/Nd ratios: 0.38-0.47 or 3.12-3.34 and 0.8-1.29 or 0.64-0.83, respectively. The early generations are characterized by a high ThO 2 content (to 9.27 wt%), which is typical for high-temperature monazites [50]. In later generations, a higher PbO content is noted (up to 2.79 wt%). La contents as well as La/Ce and La/Nd ratio in the early monazites of the Chetlassky complex are lower than those in monazites of the Buldym complex (Table 2, Figure 10). It is known that La-depleted monazites are found in ferrocarbonatites of the Fen Massif (Norway) transformed into hematite-rich rock (rodbergite) [151]. The La-depleted trend with an increase in the Ce content relative to La and Nd is also characteristic for compositions of monazite from the Tomtor rare-metal deposit of a highly differentiated alkaline-ultramafic carbonatite complex in Siberia (Figure 10b). This trend is likely to mean precipitation from hydrothermal fluids altering previous phase [50]. Monazite compositions (Mz I) of Chetlassky carbonatites are at the beginning of this trend, and are close to the compositions of monazite in carbonatite of the Qinling and Mianning-Dechang orogenic belt (giant Bayan Obo REE deposit, Miaoya, Maoniuping, and other largest REE-deposits) (Figure 10b). Siberia (Figure 10b). This trend is likely to mean precipitation from hydrothermal fluids altering previous phase [50]. Моnazite compositions (Mz I) of Chetlassky carbonatites are at the beginning of this trend, and are close to the compositions of monazite in carbonatite of the Qinling and Mianning-Dechang orogenic belt (giant Bayan Obo REE deposit, Miaoya, Maoniuping, and other largest REE-deposits) (Figure 10b). Bastnäsites from the Сhetlassky complex can be classified as bastnäsite-(Ce), with a formula (Ce,La,Nd)(CO3) (F,OH) and hydroxylbastnäsite-(Сe) (Ce,Nd,La)(CO3) (OH,F) ( Figure 10a; Table 2). They are characterized by relatively uniform compositions with molar La/Ce ratios of 0.74-0.96, and La/Nd ratios of 4.66-8.96. All bastnäsites contain much higher contents of LREEs compared with CaO. Although the CaO content (1.18-2.36 wt%) is not high, it is a substitute for La and Ce in all studied bastnäsite varieties. The Ce and Nd concentrations in the bastnäsites of the Сhetlassky complex are lower than those in the Bayan Obo REE-deposit ( Figure 10a) and are close to the bastnäsite of the Mianning-Dechang REE belt with the largest REE carbonatite deposits such as Maoniuping ( Figure  10a) [15,52,157].
Although carbonatite complex REE-minerals can form under magmatic environments (for example, Mountain Pass, California [157,158]), they occur mainly within carbonatite emplacement at the later sequences of carbonatite complexes, forming at the final stages of the evolution of the alkaline-carbonatite magmatic system. Various mechanisms of REE accumulation in the late facies of carbonatites of alkaline-ultramafic complexes have been proposed: (1) magmatic REE concentrations in the orthomagmatic fluid created by fractional crystallization [159], (2) remobilization of REEs leached from primary minerals, such as carbonate or apatite [14,51,160,161], and (3) remobilization of REEs from the early magmatic REE minerals.
Our studies of carbonates from the Kosyu carbonatites of the Сhetlassky complex evidenced that early calcites of Chetlssky carbonatites are enriched in Ba, TR, and Sr (SrO 5.44 wt%, BaO 0.44 wt%, La2O3 0.23 wt%) (Figure 4a  Bastnäsites from the Chetlassky complex can be classified as bastnäsite-(Ce), with a formula (Ce,La,Nd)(CO 3 ) (F,OH) and hydroxylbastnäsite-(Ce) (Ce,Nd,La)(CO 3 ) (OH,F) ( Figure 10a; Table 2). They are characterized by relatively uniform compositions with molar La/Ce ratios of 0.74-0.96, and La/Nd ratios of 4.66-8.96. All bastnäsites contain much higher contents of LREEs compared with CaO. Although the CaO content (1.18-2.36 wt%) is not high, it is a substitute for La and Ce in all studied bastnäsite varieties. The Ce and Nd concentrations in the bastnäsites of the Chetlassky complex are lower than those in the Bayan Obo REE-deposit ( Figure 10a) and are close to the bastnäsite of the Mianning-Dechang REE belt with the largest REE carbonatite deposits such as Maoniuping (Figure 10a) [15,52,157].
Although carbonatite complex REE-minerals can form under magmatic environments (for example, Mountain Pass, California [157,158]), they occur mainly within carbonatite emplacement at the later sequences of carbonatite complexes, forming at the final stages of the evolution of the alkaline-carbonatite magmatic system. Various mechanisms of REE accumulation in the late facies of carbonatites of alkaline-ultramafic complexes have been proposed: (1) magmatic REE concentrations in the orthomagmatic fluid created by fractional crystallization [159], (2) remobilization of REEs leached from primary minerals, such as carbonate or apatite [14,51,160,161], and (3) remobilization of REEs from the early magmatic REE minerals.
Our studies of carbonates from the Kosyu carbonatites of the Chetlassky complex evidenced that early calcites of Chetlssky carbonatites are enriched in Ba, TR, and Sr (SrO 5.44 wt%, BaO 0.44 wt%, La 2 O 3 0.23 wt%) (Figure 4a,b,d,e), and later calcites have low contents of isomorphic addition of these elements (SrO 1.0 wt%, BaO 0.69 wt%, La 2 O 3 0.01 wt%), but at the same time contain ultrafine (µm) ingrowths of REE-Sr-  (Figure 4b), less often parisite as well as apatite, barite, hematite, ilmenorutile, quartz, and fluorite. These data support the model of formation of REEdeposits of the Chetlassky complex from primary REE-enriched carbonatite melt, from which REE-rich rock-forming minerals crystallize on liquidus with subsequent remobilization of REEs from primary minerals (calcite and apatite) at the final (hydrothermal) stages of the carbonatite genesis.

Evolution of Alkaline-Carbonatite Magmas as a Factor of Ore Specialization
Carbonatite complexes are enriched in HFSE (Nb, Ta, Zr, Hf, V, Ti) and LILE (Sr, Ba, LREE, Th) and often form economically significant deposits of these elements. Nb deposits are associated with carbonatites, accounting for 99% of the world's niobium (Nb) [2,162]. In most carbonatite deposits, the main niobium concentrators are minerals of the pyrochlore group and less frequently, the perovskite, columbite, and euxenite group minerals [2]. Nb carbonatite deposits with pyrochlore-type ores are the main manufacturing type of niobium deposits. REE deposits, related with carbonatites, account for more than 50% of global rare-earth element (REE) resources [163]. The main concentrators of REE are the fluorocarbonates (bastnäsite, parisite) and monazite group minerals [16]. Bastnäsite carbonatites are the main manufacturing type of REE deposits.
It is well known that carbonatite complexes have different ore specialization (a set of ore-forming and associated components and the mineral type of ores, which determines the manufacturing type of deposits). Multicomponent Nb-(Ta)-REE deposits with pyrochlore, pyrochlore-gattchetolite, and pyrochlore-columbite-monazite ores are associated with alkaline-ultrabasic carbonatite complexes of intraplate settings (for example, Nb-REE Tomtor and Nb-REE Chuktukonskoe deposits (Siberian platform, Russia), Nb-Ta-P Neske-Vaara deposit (Kola carbonatite province) and some others). REE specialization is typical for alkaline-mafic carbonatite complexes of folded areas. LREE-deposits of bastnäsite carbonatites with bastnäsite-parisite-monazite ores are associated with these complexes (REE Mountain Pass deposit, USA).
One of the important issues of ore formation associated with carbonatite complexes is the reason for their enrichment with various ore components. In alkaline-ultrabasic carbonatite complexes, which are usually highly differentiated, this is associated with the temperature evolution of carbonatite melts, when early calciocarbonatites were replaced by magnesiocarbonatites, and later ferrocarbonatites [164]. It is well known that carbonatites of different stages of formation are enriched in HFSE (Nb, Ta, Zr, Hf, Ti) to varying degrees as well as Sr, Ba, LREE and P, F. HFSE accumulation with crystallization of ore minerals in early carbonatites is confined to the magmatic stage of the carbonatite genesis [110]. Decrease in HFSE content occurs from early to late carbonatites [165]. Unlike HFSE, REE, Sr, and Ba enrichment is related to the latest low temperature facies of carbonatites [16,166]. Along with this, abnormally the REE-enriched metasomatic mantle source (SCLM with subducted component) and liquid immiscibility in the carbonatite-syenite magma is discussed as the reason for the formation of large REE-deposits related to bastnäsite carbonatites (Mianning-Dechang, China) [17,44,167]. In both cases, models involving fluids derived from carbonatite or alkaline magmatism, and ore-forming hydrothermal fluids released from carbonatite magmas are key.
The Buldym complex of the Southern Urals is associated with the Nb-REE type of deposits with the pyrochlore-monazite type of ores (Buldym and Spirikhinskoe deposits). The average contents of the main ore components in the Buldym ores are Nb 2 O 5 -0.164 wt% (Buldym deposit), Nb 2 O 5 -0.22 wt%, and TR 2 O 3 -0.71 wt% (Spirikhinskoe deposit) [27]. The ores of the Buldymskoe deposit are represented by the REE-Nb (pyrochlore-monazite) type of ore, and the Spirikhinskoe deposit by the REE-Nb (monazite-aeshinite) type of ore [27]. These indicate the Nb-REE specialization of the deposits of the Buldym complex.
The IVC pyrochlores, as in other complexes of alkaline rocks and carbonatites, are a product of the residual crystallization of carbonated alkaline magma and crystallizes at the pegmatite and carbonatites (see Section 5.1). At the same time, IVC carbonatites are represented by high-temperature mono-facies varieties of calciocarbonatites (see Figure 5), enriched in HFSE and LILE (Figure 6a,b). The HFSE and REE contents in the IVC carbonatites are comparable to the averaged compositions of calciocarbonatites of the world (Table 3, Figure 6) [101]. However, IVC carbonatites are significantly enriched in Sr (to 22,000 ppm) in comparison with the early facies of carbonatites from the intraplate alkalineultrabasic complexes (on average, 5800 ppm). Low Nb/Ta (<35) and La/Yb (<60) ratios as well as high Sr content and Eu/Eu* ratio (near 1) in early IVC silicocarbonatites indicate an insignificant degree of differentiation of IVC carbonatite magmas. High Nb/Ta ratio (230-1400) and decreasing Zr/Hf (to 18), Y/Ho (to 13), and Eu/Eu* (to 0.75) [28]) as well as a high F content in IVC calciocarbonatites II are typical for the fluid-hydrothermal carbonate systems [169].
Thus, in contrast to the intraplate alkaline-ultramafic carbonatite complexes, late low-temperature ferrocarbonatites with Sr-Ba-REE-mineralization are absent in the IVC. IVC carbonatites contain only HFSE accessory mineralization (pyrochlore, zircon, ilmenite, titanite, ilmenorutile), while the proper minerals of LILE (Sr, REE, Ba) are absent in these carbonatites. REE-Sr-Ba mineralization in IVC is very poorly developed and occurs only in fenite halos in late feldspar, calcite, and quartz-arfvedsonite veinlets.
In contrast to the IVC, in the Buldym complex, along with high-temperature calciocarbonatites, lower-temperature varieties are widespread-beforsites (magnesiocarbornatites according to [101] (see Figure 5)) are enriched in LREE and Th relative to the early facies of Buldym and IVC carbonatites (Figure 6c,d). REE enrichment of befosites is associated with REE and REE-Nb mineralization, represented by REE-phosphates (monazite, rarely rhabdophanite) and REE-tantaloniobates (aeshinite, less often chevkinite, polyakovite, ortite, fergusonite, fersmite), which are formed at lower temperatures 315-230 • C and P = 0.9-0.36 kbar [46] in paragenesis with amphibole, apatite, columbite, ilmenite, zircon, and phlogopite (replacing chlorite). Buldym beforsites are depleted in Ba and Sr with respect to the average compositions of magnesiocarbonatites (Figure 6c,d). Accordingly, Ba-Srmineralization is very poorly developed in them (strontianite, ankylite, barite are found, but very rare). It should be noted that, in contrast to the intraplate alkaline-ultramafic carbonatite complexes in the Buldym complex as well as in the IVC, the latest ferrocarbonatites with Sr-Ba-REE-mineralization are absent.
Chetlassky complex of ultramafic-mafic dykes and carbonatites of the Middle Timan has a REE specialization and is a representative of cerium-earth carbonatite deposits with the bastnäsite-monazite ore type. The average TR 2 O 5 content in carbonatite ores of the Kosyu occurrence is 1.84% in concentration, according to the estimated prognoses resource [82], with variations from 1 to 4%, which corresponds to the ore grade of rareearth deposits. At the same time, the average Nb 2 O 5 content is 0.04%, which is lower than the cut-off grade for niobium deposits, with a high Ta 2 O 5 content of 0.007%. The ores of the Kosyu deposit are represented by the rare-earth (bastnäsite-monazite) carbonatite manufacturing type (with associated Fe, Th, Ba, and F components as coproducts), similar to those in the bastnäsite carbonatite deposits of the alkaline-mafic complexes. The ore component content (TR 2 O 5 1-4 wt%) in them is slightly lower than in rich ores (5-10 wt%) of the largest deposits of this type (e.g., 7.98% TR 2 O 5 -Mountain Pass, USA and 6.8% Bayan-Obo).
Unlike IVC and Buldym, hypabyssal medium-and low-temperature facies (T = 500−150 • C) [26] of carbonatites are widely developed in the Chetlassky complex, represented by magnesio-and ferrocarbonatites ( Figure 5). Carbonatites of the Chetlassky complex are enriched in LREE and depleted in HFSE (Nb, Zr, Ti) relative to the world average compositions of magnesio-and ferrocarbonatites [101] (see Figure 7b) as well as to IVC calciocarbonatites (see Figure 6a,b). Having similar compositions of HFSEs with beforsites of the Buldym massif, Chetlassky carbonatites are distinguished by high fractionation of REE (with maximum La/Yb ratio and pronounced "tetrahedral effect" [169]) (Figure 7a), which is typical for fluid-hydrothermal carbonate systems.
Summarizing the above-mentioned, the different ore specialization of the Urals and Timan carbonatite complexes can be related, first of all, with the intracrustal processes of alkaline and carbonatite magma evolution as well as with the prevailing specific facies of carbonatites in each of the complexes. Thus, the Nb specialization and formation of the IVC Nb-deposit in the Southern Urals is associated with the late magmatic (pegmatite and high-temperature calciocarbonatite) facies of rocks with pyrochlore mineralization. According to thermobarometric data, crystallization of alkaline rocks and IVC carbonatites occurred at high pressures (5-2.5 kbar) and temperatures (850-490 • C) [27,46,170,171]. Uranpyrochlores and fluorocalciopyrochlores (with a low Nb/Ta, less than 70) crystallized at the late magmatic stage in pegmatites and siliciocarbonatites. Fluorocalciopyrochlores of late generations (with a high Nb/Ta value, > 300, and high F 4-5 wt%) were formed in evolved calciocarbonatites and in fenites from high-temperature fluid carbonate systems with high F.
The Nb-REE specialization of the deposits of the Buldym complex is due to the simultaneous development of high-and medium-temperature facies of calcio-and magnesiocarbonatites with pyrochlore and monazite-eshinite-columbite mineralization, respectively. According to the thermobarometric data, their formation took place in a wider temperature range (575-230 • C) and at lower pressures (1.6-0.56 kbar) [105]. Fluorocalciopyrochlores (with a low content of impurities and high F~4.5 wt%) crystallize in early high-temperature facies of calciocarbonatites at subsolidus temperatures from fluid-saturated carbonate systems. Aeshinite, columbite (replacing pyrochlore), and monazite (Ce) are formed in the mid-temperature facies of carbonatites (beforsites).

Mantle Source Characteristics: Rb-Sr and Sm-Nd Isotope Signatures
Nd and Sr isotopic composition is widely used to interpret the origin of carbonatite complexes and mantle sources of alkaline and carbonatite magmatism [31,38]. The magma source, depleted to varying degrees, suggests that the origin of intraplate ultramaficalkaline carbonatite complexes, which are widespread on the shields and along the edges of platforms, is associated with a deep mantle source, possibly with a mantle plume (HIMU, FOZO), but does not exclude a mixing of plume substances with an enriched component of the EMI type [31,33,34,37]. At the same time, mixed mantle-crustal sources, usually enriched in radiogenic strontium isotopes and non-radiogenic neodymium, have been identified for many carbonatite complexes of folded areas [9,[20][21][22]. The enrichment of carbonatite complexes in radiogenic Sr isotopes can be caused by recycling and enrichment of mantle reservoirs with components of the oceanic and continental crust [17,44,45,48,167].
The IVC rock composition data-points are shown on the 87 Sr/ 86 Sr i -εNd diagram within the mantle trend, on the line connecting the depleted (DM) and enriched (EM1) mantle ( Figures 8A and 11). A similar line of the isotope system evolution is characteristic of carbonatite complexes of the Kola province located within Baltic craton. According to Kramm [33] and Kogarko [38], the Kola carbonatite line reflects the mixing of the mantle reservoirs DM (or plume-like component FOZO) and EMI during magma generation. A similar isotopic composition was also found in carbonatite complexes framing the Siberian platform (Maymecha-Kotui and East Aldan provinces) [39,121,173].
The carbonatites of the Buldym complex (Buldym Nb-REE deposit) form a field in the diagram with higher 87 Sr/ 86 Sr I values from 0.70421 to 0.70470 (εSr(t) from +3.2 to +10.2) and low 143 Nd/ 144 Nd i values with negative εNd(t) from −1.4 to −3.4, corresponding to a more enriched source EMI type. The calciocarbonatite pyrochlore has the same isotopic composition (see Table 5), which indicates a single source of carbonatites and ore. The REE-Nb ore mineralization has a more radiogenic Sr isotopic composition ( 87 Sr/ 86 Sr i from 0.70617 to 0.70715) and negative εNd(t) from −0.7 to −5.8, which may be associated with the participation of crustal fluids in the alkaline metasomatism and ore formation within the Buldym complex. The Sr-Nd isotopic compositions of the Buldym carbonatites are also on the DM-EMI mixing line, but closer to the enriched mantle compositions of the EMI type compared to the IVC. It should be noted that similar isotopic compositions of the EMI type are also found in carbonatite complexes of rift zones of shields with the deepest mantle sources (e.g., the East African Rift, Aldan Shield, Eastern Siberia), in the formation of which the possible participation of the plume HIMU component is assumed [32,173].
The carbonatites of the Chetlassky complex of Middle Timan (Kosyu REE ore-occurrence) show a narrow variation: 87 Sr/ 86 Sr i from 0.70336 to 0.70369 and εNd(t) from +5.1 to +5.7, which are close to the signatures of the moderately depleted mantle array, but show a slight deviation toward high εNd ( Figure 11). This isotopic composition is close to those associated with Kosyu lamprophyres ( 87 Sr/ 86 Sr i from 0.7037 to 0.7043, εNd from +5.4 to +6.2) (see Table 5). These isotopic data indicate a rather common mantle source for the Kosyu carbonatite and lamprophyre substances. However, the lamprophyres of the Chetlassky complex are characterized by more significant variations in the initial isotope ratios and a more enriched isotopic composition ( 87 Sr/ 86 Sr I from 0.70365 to 0.70589 and ε Nd from +1.8 to +6.2) (see Table 5), which are close to those for the Proterozoic (V) diamondiferous aillikite-carbonatite dike complexes (e.g., Aillik-Bay, Labrador) [120] (see Figure 8B). It should be mentioned that the most radiogenic Sr isotopic composition ( 87 Sr/ 86 Sr i up to 0.711) in Chetlassky lamprophyres [60,113] is the same as previously noted for Italian lamprophyres and its origin was associated with recycling and enrichment of mantle reservoirs with oceanic and continental crust components [45].
Crustal assimilation [8,9,[20][21][22], sedimentary carbonate contamination [180] and heterogeneous mantle sources [17,174,179] are discussed as the cause of Sr and REE enrichment and deviation from the typical Sr-Nd carbonatite isotopic composition similar to OIBs. Ying [179] and How [17] substantiated that carbonatite-associated REE deposits (CARDs), highly enriched in radiogenic Sr and non-radiogenic Nd, were derived by the recycling of marine sediments. This two-stage model assumes that REE carbonatites are formed "by melting of the sub-continental lithospheric mantle (SCLM), which have been previously metasomatized by high-flux REE-and CO 2 -rich fluids derived from subducted marine sediments" [17].
To assess the possibility of contamination of mantle magmas of the Urals carbonatite complexes with crustal substances, we calculated the Nd-Sr isotope mixing lines of a DM-type mantle source with lower crustal [177] and upper crustal components (i.e., IVC and Buldym complex host rocks are gneisses of the Vishnevogorskaya Formation) (see Table 5). The calculation was carried out according to the equation of mixing the Sr and Nd isotopic ratios taking into account the concentrations of elements in the mantle and crustal components [181]. Calculations have shown the possibility of the presence of an insignificant amount of lower crustal material (<3%) in the isotopic composition of IVC alkaline feldspar syenites, while the possibility of the mixing of IVC and Buldym carbonatite magmas with upper crustal material is extremely unlikely (see Figure 11).
Calculation of the Nd-Sr isotope mixing-lines of the DM-type mantle source with the marine sedimentary-carbonate rocks showed that the mantle component reached more than 95% in lamprophyres of the Chetlassky complex and the marine sedimentary component did not exceed 5% (see Figure 8B). Contamination by marine sediments with a high 87 Sr/ 86 Sr (>0.712) can lead to a synchronous increase in 87 Sr/ 86 Sr, δ 18 O, and δ 13 C values in carbonatites [17]. However, such contamination cannot explain why the REE-carbonatites of the Chetlassky complex had low 87 Sr/ 86 Sri ( Figure 8A), the lowest δ 13 C values (−3.4‰), and the highest δ 18 O (15.2‰) among the Chetlassky rocks [112]. Therefore, we can assume that these Sr-Nd isotopic signatures in the REE-carbonatites of the Chetlassky complex reflect the heterogeneity of the mantle source, probably associated with different degrees of enrichment of the depleted mantle in components of subducted marine sediments. The Buldym REE-Nb carbonatites also support this hypothesis, showing binary mixing between the DM and EM1 mantle reservoirs (since "enriched mantle EMI is caused by the recycling of continental crust or lithosphere" [182]).
Thus, heterogeneous mantle sources slightly enriched in subducted oceanic crust (EMI type [182]) and marine sediments are likely to have been the melting substrate for the magmas of Ural and Timan carbonatite complexes. The IVC carbonatites with Nb specialization had the least contaminated Sr-Nd isotopic compositions, while REE-Nb and REE carbonatites of the Buldym and Chetlassky complexes showed different degrees of contamination. Based on the Sr-Nd isotope data, major composition of rocks and minerals and trace element pattern, we suggest that the ore specialization of carbonatite complexes in the Urals and Timan may be related (associated) not only with the evolution of carbonatite magmas, but also with the heterogeneity of mantle sources, which were probably produced by the mixture of a mantle component with subducted oceanic crust and marine sediments.

Conclusions
(1) The Ilmeno-Vishnevogorsk (IVC), Buldym, and Chetlassky carbonatite complexes are representatives of the off-cratonic carbonatite complexes with different ore specialization (Nb, Nb-REE, and REE, respectively). Nb specialization of the IVC deposits is associated with magmatic abyssal facies of miaskite-pegmatite and calciocarbonatites with pyrochlore ore mineralization. The Nb-REE specialization of the Buldym deposits is due to the presence of both facies of calcio-and magnesiocarbonatites, with pyrochlore and monazite-aeshinite-columbite mineralization, respectively. The REE specialization of the Chetlassky dyke complex (M. Timan) is associated with hypabyssal facies of magnesio-and ferrocarbonatites and late hydrothermal quartz-goethite-hematite veins with monazitebastnäsite ore mineralization.
(3) Sr-Nd composition of Urals and Timan carbonatites is close to those of intraplate carbonatite complexes located in the Baltic craton (Kola Province) and at the edges of the Siberian platform (Maymecha-Kotuiskaya, East-Sayan, Udzhinskaya, Sette-Dabanskaya, East-Aldan Alkaline provinces). Urals and Timan carbonatites differ by less contaminated mantle Sr-Nd isotopic signatures from collisional carbonatite complexes of Altai-Sayan, Transbaikalia, Tien Shan, and Himalayan fold regions, which are often highly contaminated by crustal recycled components and, as a consequence, enriched in radiogenic Sr and nonradiogenic Nd.
(4) The Sr-Nd isotopic compositions of Urals and Timan carbonatite complexes suggest that their different ore specialization can be caused not only by crustal evolution of alkaline and carbonatite magmas, but also by the heterogeneity of mantle sources associated with varying degrees of enrichment in subducted components of oceanic crust and marine sediments. To address these issues, further research with the use of various isotopic systems is expected.