Fluorite Mineralization Related to Carbonatitic Magmatism in the Western Transbaikalia: Insights from Fluid Inclusions and Trace Element Composition

Fluorite mineralization associated with different types of magmatism is common in the Western Transbaikalia. This study deals with Arshan, Yuzhnoe, and Ulan-Ude fluorite occurrences, which are the most significant examples of carbonatite-related fluorite mineralization in the region. The present paper focused on new fluorite geochemistry and fluid inclusion data, is aimed at revealing conditions of the fluorite mineralization formation, highlighting its genetic relationship with magmatism, compared to other deposits of this type. All the three locations belong to the Late Mesozoic Central Asian carbonatite province. Fluorites here are characterized by high rare earth elements (REE), Sr, and elevated La/Yb values. Fluid inclusions data imply that the formation of fluorite mineralization is a long process extending from late magmatic to the hydrothermal stage. Early fluorite crystallized from sulfate-carbonate orthomagmatic fluids at temperatures up to 500 °C. True hydrothermal fluorite was formed from the same fluids that were probably mixed with meteoric waters, which caused the temperature to drop to below 420 °C and led to an increase in the chloride component. The REE compositions of fluorite from the studied locations are similar to compositions of REE-rich fluorites from carbonatite-related deposits around the world.


Introduction
Carbonatite melts generally have anomalously high contents of F, rare earth elements (REE), Sr, P, U and other incompatible lithophile elements. One of the most common minerals in carbonatite-related REE deposits worldwide is fluorite. Fluorite mineralization has been reported to form both at the magmatic (Mato Preto) [1] and the hydrothermal stages (Amba Dongar, Songwe Hill) [2,3]. In some cases, magmatic and hydrothermal fluorite occurs within the same deposit (e.g., Bayan Obo, Mushgai-Khudag) [4][5][6][7]. Fluorine, together with chlorine and sulfate, is an important ligand that controls the transport and deposition of REE [8]. That is why trace element composition of fluorite and information obtained from studies of fluid inclusions in this mineral provide valuable insights for sources of fluid (e.g., magmatic versus formation waters), fluid composition, the deposition conditions, and make inferences about the ore potential of individual deposits.
In the Western Transbaikalia, fluorite occurrences are ubiquitous and belong to the Central Asian fluorite-bearing province, which comprises various formations: low-temperature (50-200 • C) hydrothermal quartz-fluorite, fluorine-beryllium, molybdenum-tungsten, REE carbonatites [8,9]. Fluorite mineralization has been extensively studied in the region [9][10][11][12][13][14][15], but most attention has been given to fluorite occurrences associated with granites. Here we present an overview and new information on the Arshan, Yuzhnoe and Ulan-Ude locations that are the key examples of fluorite mineralization related to the Late Mesozoic carbonatite-bearing magmatic complexes. We summarize previous studies that provided petrographic and mineralogical descriptions, and also age determinations for these occurrences [16][17][18]. In addition, we reviewed the available information on fluid inclusions. The data on fluid inclusions from the Arshan and Yuzhnoe occurrences have been limited to estimations of homogenization temperatures [16]. Fluid inclusions from the Ulan-Ude occurrence have been considered more broadly in a previous publication [18], which contains information about the homogenization temperatures and composition of the inclusions.
Here we present first data on REE concentrations in fluorites, which help to establish a genetic link between fluorite mineralization and carbonatite magmatism, and conditions for the formation of fluorite mineralization. Investigations of rare earth elements have been widely used as a good indicator to describe the geochemical processes in fluorite mineralization. Rare earth elements' evaluation of ore-forming fluids in a wide range of fluorite deposits can provide insights into the evolution of fluids through various geological and geochemical processes involved. Also, we present Raman spectroscopy data on the composition of the gaseous phase in the inclusions, daughter mineral assemblages, revised data on the salinities and salt composition of aqueous solutions, and homogenization temperatures of the inclusions.

Geological Setting
The carbonatite-related fluorite occurrences are located within the Central part of the Sayan-Baikal fold belt [19,20]. The tectonic structure started to form in the late Caledonian era. Later, the structure of the Caledonides was almost completely reworked in the late Paleozoic by intraplate magmatism and in the Mesozoic by continental rifting. The last stage is associated with the formation of carbonatite-bearing complexes [17].
The tectonic control of the fluorite-bearing intrusive complexes is evidenced by their close proximity to deep faults that form the system of Gusino-Uda rift basins. According to Vorontsov et al. [21], the graben system and the late Mesozoic magmatism in the area belongs to the structures of the Tugnuisko-Khiloksky sector of the Western Transbaikalia rift region. The Arshan and Yuzhnoe occurrences are located within the Slyudinsky horst ( Figure 1), which is composed mainly of Paleozoic gneisses, granites and schists. The horst is located in the northern part of a riftogenic depression filled with Cretaceous terrigenoussedimentary rocks (conglomerates, sandstones with interlayers of gravels, mudstones and siltstones). The contact of Cretaceous deposits with the rocks of the Slyudinsky horst is tectonic.
Arshan carbonatites form three low sloping flow-like bodies preserved as erosional remnants with dimensions: 250 m × 75 m, 80 m × 50 m and 75 m × 50 m. Thickness of the bodies does not exceed 6 m. The age of the carbonatite was determined at 126 ± 16 Ma by the Rb-Sr method [22]. Arshan carbonatites host more than 30 minerals. The main minerals include calcite, bastn‚ asite, Ba and Sr sulfates, and fluorite (as much as 15%), which have been interpreted to form at the magmatic stage. The fluorite grains up to 2 mm in size are purple in color and have an isometric appearance. There are also veins and lenses composed of calcite, barite, fluorite, parisite and other minerals that are believed to have a hydrothermal origin. This fluorite occurs as relatively homogeneous and light-colored isometric grains up to 3 mm in size, which occupy 5-10% of the rock [23].
The Yuzhnoe occurrence is associated with early Cretaceous dikes of alkaline syenites, and more than 20 carbonatite bodies. Carbonatites and syenites were dated by the Rb-Sr method at 122 ± 4 Ma and at 130 ± 5 Ma respectively [22]. Carbonatites form breccia pipe-like bodies and veins cropping out over an area of about 2 km 2 . The pipe-like bodies are up to 15-20 m in diameter and are represented by fine-grained rock of massive or weakly banded texture composed of calcite and minor fluorite (less than 5%). They contain numerous fragments of host rocks constituting up to 60-70% of the rock volume. Fluorite forms colorless at the rims and intensely colored (vivid or dark purple) octahedral crystals (up to 0.1 mm in size) or their nest-like clusters in the central part. The veins of carbonatites range in thickness from 1 to 10 m and extend lengthwise to 100-300 m. Fine-grained groundmass of the veins is composed of calcite (80-90%), biotite, fluorite (up to 10%) and barite, and encloses phenocrysts of calcite and barite. Vein fluorite is colorless or slightly colored isometric grains and cubic crystals up to 2.5 mm in size, which are located in the carbonate matrix [24]. Detailed descriptions of the geological settings and mineral composition of the Arshan and Yuzhnoe occurrences have been published by Doroshkevich and Ripp [17,23,24].
The Ulan-Ude fluorite occurrence is located in the northern part of the Mesozoic riftogenic Ivolga-Ude depression ( Figure 1). Late Cretaceous sediments lying on the eroded surface of the crystalline basement are widespread in this area and represented by sandstones, siltstones, shales and conglomerates. Gneisses, migmatites, granite gneisses, crystalline schists, and limestones compose the crystalline basement. Bastn‚ asite-fluorite and calcitebearing rocks are found along the Selenga coastline, in bedrock outcrops located in the center of Ulan-Ude city. The age of the fluorite-bearing rocks of the Ulan-Ude occurrence was established from phlogopite by the Ar-Ar method and is equal to 134.2 ± 2.6 Ma [18]. In composition, the rocks are close to bastn‚ asite-fluorite aggregates in carbonatites of the Arshan occurrence. However, the fluorite content reaches 30-55%. Ulan-Ude fluorite forms phenocrysts up to 0.5-1.5 cm in size and grains (0.2-1.5 mm) in a carbonatite matrix. Bastnesite, phlogopite, and feldspar are associated minerals. The outer zone (<1 mm) of Ulan-Ude fluorite has a dark purple or black color. Ripp et al. [25] presented a comprehensive description of the Ulan-Ude occurrence. pipe-like bodies and veins cropping out over an area of about 2 km 2 . The pipe-like bodies are up to 15-20 m in diameter and are represented by fine-grained rock of massive or weakly banded texture composed of calcite and minor fluorite (less than 5%). They contain numerous fragments of host rocks constituting up to 60-70% of the rock volume.
Fluorite forms colorless at the rims and intensely colored (vivid or dark purple) octahedral crystals (up to 0.1 mm in size) or their nest-like clusters in the central part. The veins of carbonatites range in thickness from 1 to 10 m and extend lengthwise to 100-300 m. Finegrained groundmass of the veins is composed of calcite (80-90%), biotite, fluorite (up to 10%) and barite, and encloses phenocrysts of calcite and barite. Vein fluorite is colorless or slightly colored isometric grains and cubic crystals up to 2.5 mm in size, which are located in the carbonate matrix [24]. Detailed descriptions of the geological settings and mineral composition of the Arshan and Yuzhnoe occurrences have been published by Doroshkevich and Ripp [17,23,24]. The Ulan-Ude fluorite occurrence is located in the northern part of the Mesozoic riftogenic Ivolga-Ude depression ( Figure 1). Late Cretaceous sediments lying on the eroded surface of the crystalline basement are widespread in this area and represented by sandstones, siltstones, shales and conglomerates. Gneisses, migmatites, granite gneisses, crystalline schists, and limestones compose the crystalline basement. Bastnȁsite-fluorite and calcite-bearing rocks are found along the Selenga coastline, in bedrock outcrops located in the center of Ulan-Ude city. The age of the fluorite-bearing rocks of the Ulan-Ude occurrence was established from phlogopite by the Ar-Ar method and is equal to 134.2 ± 2.6 Ma [18]. In composition, the rocks are close to bastnȁsite-fluorite aggregates in carbonatites of the Arshan occurrence. However, the fluorite content reaches 30-55%. Ulan-Ude fluorite forms phenocrysts up to 0.5-1.5 cm in size and grains (0.2-1.5 mm) in a carbonatite matrix. Bastnesite, phlogopite, and feldspar are associated minerals. The outer zone (<1 mm) of Ulan-Ude fluorite has a dark purple or black color. Ripp et al. [25] presented a comprehensive description of the Ulan-Ude occurrence.

Analytical Methods
Trace element data for fluorites were obtained by laser ablation ICP-MS analyses of individual fluorite grains mounted in epoxy resin. The analyses were carried out using Geolas Compex Pro 193 nm excimer laser coupled to a Thermo iCAP TQ mass spectrometer at GFZ Potsdam, Germany. Analytical conditions comprised a spot size of 24 µm,

Analytical Methods
Trace element data for fluorites were obtained by laser ablation ICP-MS analyses of individual fluorite grains mounted in epoxy resin. The analyses were carried out using Geolas Compex Pro 193 nm excimer laser coupled to a Thermo iCAP TQ mass spectrometer at GFZ Potsdam, Germany. Analytical conditions comprised a spot size of 24 µm, repetition rate of 8 Hz, and laser energy density of 5 J/cm 2 . NIST SRM 610 was used as external standard. Calcium contents in fluorite determined by microprobe analysis were in the range of 60.9-67.3 wt.% CaO and they were used as an internal standard in calculations. Detection limits: REE, Y 0.001 ppm; Sr, Zr, Nb, Ta 0.01 ppm; Ca 0.5 wt.%. Constructed REE plots are normalized to chondrite [26].
The fluid inclusion study was performed in the Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia. For fluid inclusion studies, double-polished sections (0.3-0.5 mm) were prepared. Initial studies were carried out using an optical microscope Zeiss. The Roedder's criteria [27] were used to classify the fluid inclusions. Primary inclusions are observed as individual inclusions or in small groups within the core of crystals. Pseudo-secondary inclusions mark fissures within a single mineral grain. Secondary inclusions trace cracks across several grains or are in the recrystallization rims. Based on the phase composition at room temperature and phase transitions during heating and cooling, the fluid inclusions (FI) can be distinguished into three types: vapor-liquid double-phase (VL-type); multi-phase inclusions with vaporliquid-solid phases (VLS-type); and multi-phase inclusions containing predominantly solid phases (CF-type). Microthermometric tests were carried out at Linkam THMSG-600, which allows the ability to perform real-time observations of phase transitions and temperature measurements in the temperature range from −196 to 600 • C. Microthermometric measurements were calibrated using synthetic fluid inclusion standards for the freezing points of pure CO 2 (−56.6 • C) and pure H 2 O (0 • C), Reproducibility of calibrations are ±0.6 • C for heating and ±0.2 • C for freezing. Salt concentration in the inclusions was calculated from the ice melting temperature (Tm ice) or solid phases melting temperature using calibration data from [28]. The salt composition of the solutions was determined from eutectic temperatures (Te) [29]. Solid and gaseous phase composition was identified by Horiba Jobin Yvon LabRAM HR800 Raman microspectrometer (HORIBA France SAS) coupled with a CCD detector and an Olympus BX40 microscope, using a 532-nm Nd:YAG laser excitation at 50 mW, to a spectral resolution of 2-2.5 cm −1 . The monochromator was calibrated against the silica band (520.7 cm −1 ). Selected integration time and number of accumulations equal 5 and 10 respectively. Mineral phases were identified using RRUFF database and CrystalSleuth software [30].

Arshan
Fluorites of the Arshan occurrence can be divided into three groups according to mineral associations and the trace element composition (median, minimum and maximum values for each type of fluorite are given in Table 1). The first group is represented by fluorites related to carbonatites that form dissemination in calcite matrix. These fluorites have relatively flat chondrite-normalized REE spectra ( Figure 2). They are characterized by depletion of light rare earth elements (LREE), as well as the absence of any obvious element anomalies. Fluorites of the first group have the highest REE and Sr contents. Total content of high field strength elements (HFSE) calculated as a sum of Zr, Nb and Ta concentrations (note that Hf and Ti values are usually below the detection limits) is at or below 0.5 ppm. Fluorites of the first group do not contain fluid inclusions.
Fluorite from veins and lenses constitutes the second and the third groups. The second group is characterized by chondrite-normalized REE spectra with a steep negative slope, a weak positive Eu anomaly, and an apparent Y negative anomaly. Total REE and Sr contents are the lowest for the Arshan fluorites. Total HFSE do not exceed 1 ppm. The third group shows moderate negatively sloped chondrite-normalized REE spectra with a weak Eu anomaly and a distinct positive Y anomaly. Strontium contents vary broadly. Total HFSE content is below 2 ppm. Note: med-average value, min-minimum, max-maximum; "-"-below the detection limit; Eu*-Eu anomaly; Y*-Y anomaly.   Fluorite of the second and the third groups host multiphase crystal-fluid or CF-type (Figure 3a,b) and double-phase vapor-liquid (VL-type) inclusions (Figure 3f,g). Data on fluid inclusions are accumulated in Table 2. The study of primary CF and secondary VL inclusions allowed us to trace the evolution from initial orthomagmatic to hydrothermal mineralized fluid. Primary CF inclusions occur in small clusters in the central parts of the grains. They are isometric or elongated and up to 20 µm in size. Daughter mineral phases occupy 50-90 vol.% of the inclusions, and are represented by Na-Ca-Sr sulfates (sharp peaks at 995-1010 cm −1 , 1132 cm −1 , 1155-1160 cm −1 and triple peak at (610-)621-635-650 cm −1 ) and Na-Sr carbonates (peak at 700-707 cm −1 and double peak at 1075-1078 cm −1 ). Thermometric tests of the primary CF inclusions indicate homogenization temperatures (T h ) of 420-560 • C and salinities of 50-67 wt.% NaCl-equiv. Gaseous phase (~8 vol.%) consists of CO 2 ± N 2 ± H 2 . Secondary VL inclusions trace cracks thought several quartz grains and have irregular shapes and sizes of 2-8 µm. Gaseous phase occupies about 30-50 vol.% and is close to pure CO 2 (±N 2 ). Ice melting temperatures for the secondary VL inclusions vary between −8 and −10 • C. Homogenization temperatures and salinity values range between 90 and 150 • C, and 11.7-13.9 wt.% NaCl-equiv. respectively. These results indicate that Arshan fluorites were formed from a homogeneous high-temperature sulfate-carbonate fluid (main cations Na, Ca, Sr). This was subsequently diluted by meteoric waters with a decrease in temperature and salinity. CO2 ± H2. The VLS inclusions yield Th of 360-470 °C and salinities of 43-56 wt.% NaClequiv. The secondary VL inclusions are isometric up to 10 µm. Assemblages of the secondary FI occur in recrystallized rims. The gaseous phase (occupying ~40 vol.%) composed of CO2. The Th fit into the interval 110-165 °C. The fluorite of the Yuzhnoe occurrence was deposited from a homogeneous moderately hot chloride-sulfate-carbonate fluid. The main cations in the fluid are Na, Ca, Sr. During the development of the system, the fluid was diluted, and its temperature decreased. Fluorite from the Ulan-Ude occurrence contains primary CF (Figure 3c,d) and pseudo-secondary and secondary VL inclusions (Figure 3i). The primary CF inclusions occur as negative crystal shape and up to 16 µm in size in unaltered parts of fluorite crystals. Daughter mineral assemblages of the CF inclusions include Ca-Na-REE carbonates (peaks at 686 cm −1 , 713 cm −1 , triple peak at 1082-1092-1099 cm −1 ), Na-Sr sulfates (sharp peak at 992-998 cm −1 , double peak at 1132-1157 cm −1 and triple peak at 624-636-652 cm −1 ) and chlorides (no Raman spectra, optically isotropic cubic crystal) ( Table 2). Mineral phases occupy up to 80 vol. % of the inclusion. Carbon dioxide is occurred as gaseous and liquid phases (bubble up to 12 vol.%). The Th and salinities values for the CF inclusions are in  Note: FI-fluid inclusion, pr-primary, ps-pseudo-secondary, s-secondary. The number of studied inclusions is indicated in parentheses.

Yuzhnoe
By mineral associations, color, and trace element composition fluorites were divided in two groups. The first one is confined to areas of bastn‚ asite and albite accumulation in carbonatite. Here fluorite forms octahedral crystals (0.5-2 mm in size), which are colorless along the edges and dark purple in the center, and their clusters. Fluorites of this group have chondrite-normalized REE spectra with a strong negative slope (Figure 2). They have no Eu anomaly and a slight negative Y anomaly. Strontium concentrations are similar to those of the second and the third groups at Arshan. The total HFSE contents vary broadly from 0.1 to 303 ppm (Table 1).
Fluorite crystals from fluorite-calcite matrix comprise the second group. This fluorite forms weakly purple or lilac isometric grains and cubic crystals 0.1-3 mm in size. In comparison to the first group, the second group is characterized by lower REE and Sr concentrations, a weakly positive Eu anomaly, and a clear positive Y anomaly. Total HFSE are up to 6 ppm. Fluorites from the second group of Yuzhnoe occurrence contain primary multi-phase VLS-type (vapor-liquid-solid) (Figure 3e) and secondary VL inclusions (Figure 3h). The VLS inclusions are negative crystal shape and 5-12 µm in size and localized randomly in fluorite's cores in groups. Daughter mineral assemblages (60-80 vol.%) of the VLS inclusions include Na-Sr sulfates (sharp peaks at 997-1000 cm −1 , 1152-1157 cm −1 and triple peak at 624-636-655 cm −1 ), Na ± Ca carbonates (sharp peaks at 712 cm −1 , 1075 cm −1 , 1086 cm −1 ) and chlorides (no Raman spectra, optically isotropic cubic crystal) ( Table 2). Liquid phase is a mixture of solution and liquid CO 2 . Gaseous phase (~10 vol.%) composition is CO 2 ± H 2 . The VLS inclusions yield T h of 360-470 • C and salinities of 43-56 wt.% NaCl-equiv. The secondary VL inclusions are isometric up to 10 µm. Assemblages of the secondary FI occur in recrystallized rims. The gaseous phase (occupying~40 vol.%) composed of CO 2 . The T h fit into the interval 110-165 • C. The fluorite of the Yuzhnoe occurrence was deposited from a homogeneous moderately hot chloride-sulfate-carbonate fluid. The main cations in the fluid are Na, Ca, Sr. During the development of the system, the fluid was diluted, and its temperature decreased.
Fluorite from the Ulan-Ude occurrence contains primary CF (Figure 3c,d) and pseudosecondary and secondary VL inclusions (Figure 3i). The primary CF inclusions occur as negative crystal shape and up to 16 µm in size in unaltered parts of fluorite crystals. Daughter mineral assemblages of the CF inclusions include Ca-Na-REE carbonates (peaks at 686 cm −1 , 713 cm −1 , triple peak at 1082-1092-1099 cm −1 ), Na-Sr sulfates (sharp peak at 992-998 cm −1 , double peak at 1132-1157 cm −1 and triple peak at 624-636-652 cm −1 ) and chlorides (no Raman spectra, optically isotropic cubic crystal) ( Table 2). Mineral phases occupy up to 80 vol. % of the inclusion. Carbon dioxide is occurred as gaseous and liquid phases (bubble up to 12 vol.%). The T h and salinities values for the CF inclusions are in the range of 440-520 • C and 52-62 wt.% NaCl-equiv respectively. In some cases, it is possible to distinguish a boundary between liquid carbon dioxide and the gas bubble. Pseudosecondary and secondary inclusions are irregular or isometric and up to 15 µm in size. They mark small cracks and occur in recrystallized rims of fluorite grains. Gaseous phase of the pseudo-secondary and secondary VL inclusions comprises CO 2 ± N 2 ± H 2 and occupies~15 vol.%. The T h are close to each other for pseudo-secondary and secondary inclusions and values are equal 135-250 • C. The eutectic temperatures of the pseudosecondary VL inclusions are between −19 and −15 • C, implying a KCl-NaCl solution. The ice melting temperatures and salinities for these inclusions are −12 and −10.5 • C and 14.5-16 wt.% NaCl-equiv. respectively. Thus, the primary fluid inclusions from Ulan-Ude fluorite trapped homogeneous high-temperature chloride-sulfate-carbonate fluid (main cations are Ca, Na, REE, Sr). The later fluid contained in pseudo-secondary and secondary inclusions is moderately concentrated low-temperature chloride.

Ulan-Ude
Here fluorite forms phenocrysts up to 0.5-1.5 cm in size and smaller grains (from 0.2 to 3 mm) in carbonatite matrix. Bastn‚ asite, phlogopite, and feldspars are associating minerals. The outer zone (<1 mm) of the fluorite grains has a dark violet to black color, presumably due to radioactive irradiation. Fluorite mineralization from the Ulan-Ude occurrence is the richest in REE. The chondrite-normalized REE spectra are characterized by a negative slope flattening in the region of heavy REE, a lack of Eu anomaly, and a positive Y anomaly ( Figure 2). The Sr content is similar to the average values in the other two occurrences that are described above. The HFSE content varies from 0.5 to 26 ppm (Table 1).

Fluorite Geochemistry
The described features of the Arshan fluorites ( Figure 4) suggest that fluorites of the first group are probably the earliest at the Arshan occurrence. They are characterized by the highest REE content and depletion in light REE, possibly due to co-crystallization with bastn‚ asite at the magmatic stage [31]. The second group fluorites apparently precede fluorites of the third group as the latter are distinguished by a lower (La/Yb)n ratio and the appearance of a positive Y anomaly instead of a negative one, implying a greater degree of fractionation of the parental fluid [2,4,32]. A constant content of Sr, a gradual change in REE content and the ratio of light to heavy REE may indicate successive formation of the entire fluorite mineralization from a single source [33].  The Möller Tb/Ca vs. Tb/La diagram [35] has been used to classify fluorites by mineral-forming processes, based on the features of REE. Our data for the Ulan-Ude, Yuzhnoe, and the first and second groups of the Arshan occurrence fall within the range of pegmatitic fluorite area ( Figure 5). The fluorites of the Arshan third group lie below the pegmatitc/hydrothermal boundary in the area of hydrothermal fluorite, which confirms our intrepretation of them as the latest in formation. The Arshan occurrence is characterized by long-term multistage formation of fluorite mineralization. A similar situation was revealed in the Mushugai-Khudag complex, which contains several generations of fluorite from orthomagmatic to hydrothermal [7]. Fluorites of the Yuzhnoe occurrence are comparable to fluorites of the second and third groups from Arshan ( Figure 4). Therefore, taking into account the above-listed criteria, we assume that the fluorites of the first group were formed earlier than the fluorites of the second one.
Fluorites of the Ulan-Ude occurrence belong to a single generation and are distinguished by the highest REE content and an apparent predominance of light REE.
The chondrite-normalized ratios and absolute REE concentrations in the fluorites allow to define rocks associated with fluorite mineralization. Generally, high REE concentrations, LREE enrichment and lack of a significant Eu anomaly are typical features of fluorites related to carbonatitic magmatism [34]. This confirms the direct genetic relationship between the studied fluorites and carbonatite magmatism.
The Möller Tb/Ca vs. Tb/La diagram [35] has been used to classify fluorites by mineral-forming processes, based on the features of REE. Our data for the Ulan-Ude, Yuzhnoe, and the first and second groups of the Arshan occurrence fall within the range of pegmatitic fluorite area ( Figure 5). The fluorites of the Arshan third group lie below the pegmatitc/hydrothermal boundary in the area of hydrothermal fluorite, which confirms our intrepretation of them as the latest in formation. The Arshan occurrence is characterized by long-term multistage formation of fluorite mineralization. A similar situation was revealed in the Mushugai-Khudag complex, which contains several generations of fluorite from orthomagmatic to hydrothermal [7].

Fluid Inclusions
Fluid inclusion studies of carbonatite-related fluorite mineralization worldwide have demonstrated wide ranges of homogenization temperatures and fluid salinities. The temperatures vary greatly from more than 500 to 100 °С. Salinities of fluids vary broadly from high to low; daughter minerals may be totally absent or may occupy more than 50 vol.% of an inclusion. Common daughter minerals are carbonates, sulfates, and chlorides of alkaline, alkaline-earth and rare-earth metals. Carbon dioxide is the most essential component of the vapor phase [2,4,7,[36][37][38][39][40][41].
Our studies have revealed broad similarities between fluid inclusions at all the three fluorite occurrences. The fluid inclusions data demonstrate the involvement of sulfatecarbonate, hypersaline, high temperature orthomagmatic fluid at the start of fluorite formation. This fluid may belong to a trend originating directly from crystallizing carbonatitic magma. The fluid temperature and its salinity were apparently decreasing during the evolution of the magmatic-hydrothermal system. Fluid cooling probably was the main reason for the decreasing of REE, content in fluorites, especially strong for the light REE. The role of chloride ion in the hydrothermal mineralizing fluid apparently increased during the general cooling and chemical evolution. These features are common and characteristic for the Late Mesozoic fluorite mineralization in the Central Asian alkaline province (for example, Mushugai-Khudag) and REE fluorite mineralization related to carbonatites worldwide.

Fluid Inclusions
Fluid inclusion studies of carbonatite-related fluorite mineralization worldwide have demonstrated wide ranges of homogenization temperatures and fluid salinities. The temperatures vary greatly from more than 500 to 100 • C. Salinities of fluids vary broadly from high to low; daughter minerals may be totally absent or may occupy more than 50 vol.% of an inclusion. Common daughter minerals are carbonates, sulfates, and chlorides of alkaline, alkaline-earth and rare-earth metals. Carbon dioxide is the most essential component of the vapor phase [2,4,7,[36][37][38][39][40][41].
Our studies have revealed broad similarities between fluid inclusions at all the three fluorite occurrences. The fluid inclusions data demonstrate the involvement of sulfatecarbonate, hypersaline, high temperature orthomagmatic fluid at the start of fluorite formation. This fluid may belong to a trend originating directly from crystallizing carbonatitic magma. The fluid temperature and its salinity were apparently decreasing during the evolution of the magmatic-hydrothermal system. Fluid cooling probably was the main reason for the decreasing of REE, content in fluorites, especially strong for the light REE. The role of chloride ion in the hydrothermal mineralizing fluid apparently increased during the general cooling and chemical evolution. These features are common and characteristic for the Late Mesozoic fluorite mineralization in the Central Asian alkaline province (for example, Mushugai-Khudag) and REE fluorite mineralization related to carbonatites worldwide.

Comparisons with Fluorites from Other Deposits
Carbonatites are generally enriched in REE (~2000 ppm), Nb (~1951 ppm), Zr (~1120 ppm), Mo, Y, Th and U, and impoverished in Ni, Cu, Cr, Au and other elements. This correlates quite well with the empirical data on carbonatite-related occurrences of the Western Transbaikalia and, in particular, fluorite mineralization.
The fluorites of the Ulan-Ude occurrence and the first group at Yuzhnoe (Figure 6) are probably products of the most extensive fractionation evidenced by the maximum REE contents and the highest (La/Yb)n ratio. They are similar to orthomagmatic fluorites of carbonatite complexes [1]. Fluorites from Arshan and the second group fluorites of the Yuzhnoe occurrence are geochemically similar to hydrothermal fluorites from REE deposits associated with carbonatite magmatism [2][3][4][5][6]42]. The high strontium content and the REE enrichment of fluorites are also consistent with the settings of fluorite mineralization precipitated from carbonatitic fluids, which were dominated by Ca and carbonate ions, but also contained high concentration of F and REE, and generally resembled a carbothermal residue [31,43]. correlates quite well with the empirical data on carbonatite-related occurrences of the Western Transbaikalia and, in particular, fluorite mineralization. The fluorites of the Ulan-Ude occurrence and the first group at Yuzhnoe ( Figure 6) are probably products of the most extensive fractionation evidenced by the maximum REE contents and the highest (La/Yb)n ratio. They are similar to orthomagmatic fluorites of carbonatite complexes [1]. Fluorites from Arshan and the second group fluorites of the Yuzhnoe occurrence are geochemically similar to hydrothermal fluorites from REE deposits associated with carbonatite magmatism [2][3][4][5][6]42]. The high strontium content and the REE enrichment of fluorites are also consistent with the settings of fluorite mineralization precipitated from carbonatitic fluids, which were dominated by Ca and carbonate ions, but also contained high concentration of F and REE, and generally resembled a carbothermal residue [31,43] Our data compare well with key geochemical characteristics of fluorite mineralization genetically related to carbonatite magmatism and the largest REE deposits in the world ( Figure 6). The fluorite mineralization studied here is clearly distinguished from late-stage, low temperature fluorite, e.g., at the Amba Dongar deposit or from the epithermal fluorite deposits of the Western Transbaikalia [2,10].

Conclusion
Fluorite mineralization related to carbonatite magmatism of the Central Asian carbonatite province was formed from highly concentrated orthomagmatic high temperature (400-500 °C) fluid of a sulfate-carbonate composition. The fluorite compositions are char- Our data compare well with key geochemical characteristics of fluorite mineralization genetically related to carbonatite magmatism and the largest REE deposits in the world ( Figure 6). The fluorite mineralization studied here is clearly distinguished from late-stage, low temperature fluorite, e.g., at the Amba Dongar deposit or from the epithermal fluorite deposits of the Western Transbaikalia [2,10].

Conclusions
Fluorite mineralization related to carbonatite magmatism of the Central Asian carbonatite province was formed from highly concentrated orthomagmatic high temperature (400-500 • C) fluid of a sulfate-carbonate composition. The fluorite compositions are characterized by high contents of REE and Sr, and LREE predominance over HREE. Fluid inclusions data and the trace element compositions are typical for fluorite mineralization associated with carbonatites.