Fluorite Composition Constraints on the Genesis of the Weishan REE Deposit, Luxi Terrane

Abstract

Fluorite, a key accessory mineral associated with rare earth element (REE) deposits, exerts a significant influence on REE migration and precipitation through complexation, adsorption, and lattice substitution within fluorine-bearing fluid systems. It therefore provides a valuable archive for constraining REE enrichment processes. The Weishan alkaline–carbonatite-related REE deposit, the third-largest LREE deposit in China, is formed through a multistage magmatic–hydrothermal evolution of the carbonatite system. However, limited mineralogical constraints on REE enrichment and precipitation have hindered a comprehensive understanding of its metallogenic processes and exploration potential. Here, cathodoluminescence imaging and LA-ICP-MS trace element analyses were conducted on fluorite of multiple generations from the Weishan deposit to constrain the physicochemical conditions of mobility and precipitation mechanisms of this REE deposit. Four generations of fluorite are recognized, recording progressive evolution of the ore-forming fluids. Type I fluorite, which coexists with bastnäsite and calcite, is LREE-enriched and exhibits negative Eu anomalies, indicating precipitation from high-temperature, weakly acidic, and reducing fluids. Type II fluorite occurs as overgrowths on Type I, while Type III fluorite replaces Type II fluorite, with both displaying LREE depletion and MREE-Y enrichment, consistent with cooling during continued hydrothermal evolution. Type IV fluorite, which is interstitial between calcite grains and associated with mica, is formed under low-temperature, oxidizing conditions, reflecting REE exhaustion and the terminal stage of fluorite precipitation. Systematic shifts in REE patterns among the four generations track progressive cooling of the system. The decreasing trend in La/Ho and Tb/La further suggests that these fluorites record dissolution–reprecipitation events and associated element remobilization during fluid evolution.

1. Introduction

Rare earth elements (REEs) constitute a strategic class of critical metals, and elucidating the controls on their origin, spatial distribution, and ore-forming processes remains a foremost frontier in contemporary geological research [1,2,3,4,5,6]. REE deposits form in a wide variety of geological settings, including carbonatites, alkaline–peralkaline igneous complexes, hydrothermal systems, and weathering-related ion-adsorption deposits [3,6,7,8,9,10,11,12,13]. These deposits commonly contain substantial coexisting resources of fluorite, apatite, and other mineral [14,15,16,17]. Fluorite is a ubiquitous vein-forming mineral that occurs widely in magmatic–hydrothermal, sedimentary–diagenetic, and metamorphic systems, and it is also frequently developed in critical metal deposits overprinted by hydrothermal activity. As a key geochemical carrier of REEs during mineralization, fluorite exerts strong control on fluid–mineral interactions and on the enrichment and transport of rare earth elements. Globally, fluorite-bearing REE deposits are diverse. Typical examples include (i) carbonatite-type REE deposits, such as the Bayan Obo deposit in Inner Mongolia, the Mianning-Dechang REE belt in Sichuan, and the Weishan REE deposit in Shan-dong, China, as well as the Kızılcaören REE deposit in Turkey and the Okorusu REE deposit in Namibia [16,18,19,20,21,22], and (ii) alkaline–peralkaline granite-related REE deposits, as exemplified by the deposit of the Gallinas Mountains in the United States and the Strange Lake deposit in Canada [23,24,25,26,27].

REE deposits may originate from magmatic, hydrothermal, or supergene processes [13,28,29,30,31,32], and the fluorite commonly associated with such deposits is typically closely linked to hydrothermal fluids. These fluids may derive directly from magmatic systems or be related to sedimentary architectures [23,29,33,34,35,36]. It is known that REEs in the Earth’s crust predominantly occur as trivalent ions (REE³⁺), which are characterized by large ionic radii and lithophile behavior. Their mobility in diverse geological media is significantly influenced by coordinating ligands, temperature, pH, and redox conditions. Within magmatic and hydrothermal systems, F-rich fluids are capable of effectively complexing, transporting, and concentrating REEs to economically significant levels [37,38,39,40]. Consequently, fluorite is not only an important mineral in REE mineralization but also a valuable geochemical indicator for deciphering the evolution and metallogenic mechanisms of magmatic–hydrothermal systems, and it is widely employed as a geochemical exploration tool. In F-bearing fluid systems, fluorite influences the migration and precipitation of REEs through complexation, adsorption, and lattice substitution [33,41,42,43,44]. Its coupled behavior with REEs is reflected not only in elemental abundances and distribution patterns but also in the thermodynamic and kinetic evolution of ore-forming systems. As ore-forming fluids evolve from high-temperature magmatic regimes to low-temperature hydrothermal environments, the precipitation of fluorite frequently marks a critical stage during which REEs transition from the fluid phase to the solid phase [18,45,46]. The Ca²⁺ in the fluorite lattice can be substituted by REEs, such as Sr²⁺, Na⁺ Y³⁺, and other ions, rendering fluorite highly sensitive to REE uptake and release under varying geological conditions [33,47,48]. In settings characterized by multiphase hydrothermal activity, fluorite may serve as both a reservoir for REEs and as a medium for REE redistribution during recrystallization or reprecipitation. Therefore, elucidating the role of fluorite in modulating REE mineralization is of fundamental significance for reconstructing the evolutionary pathways of fluid-mediated ore-forming systems, understanding the mechanisms of elemental migration, and constraining the physicochemical conditions of deposit formation.

The Weishan REE deposit is the third-largest light rare earth deposit in China [22], with an average grade of 3.25 wt% and total resources exceeding 1.0 Mt. Its genesis is closely associated with an alkaline–carbonatite complex [13,29]. Although previous studies have made significant progress in elucidating the petrogenesis of the alkaline rocks, the isotopic characteristics, the geochronological framework of magmatism and mineralization, and the sources of ore-forming fluids, a systematic understanding of the mechanisms governing REE enrichment and migration remains lacking. As one of the pervasive minerals in the system, fluorite represents an ideal carrier for investigating the geochemical behavior of REEs in carbonatite. Based on detailed field investigations and comprehensive laboratory analyses, this study focuses on fluorite–bastnäsite assemblages developed within the Weishan carbonatite. By integrating petrographic observations with geochemical data from fluorite, we aim to elucidate the precipitation mechanisms of fluorite, the physicochemical conditions governing REE mineralization, and the implications of ore-forming processes. The results are intended to provide new mineralogical constraints on the metallogenic mechanism of the Weishan REE deposit in the Luxi Terrane and to advance the understanding of REE mineralization.

2. Regional Geology

The North China Block (NCB) is one of the oldest and most well-preserved cratonic blocks in the world (Figure 1A). Its geological evolution involved the sequential development of an early continental core, crustal growth, amalgamation of micro-continental blocks, and extensive magmatic, metamorphic, and cratonization processes [49,50,51,52,53,54,55,56]. Subsequently, the NCB underwent multiple episodes of subduction-related metasomatism and lithospheric thinning, accompanied by multiphase, large-scale metallogenic events that produced a wide range of strategic mineral resources, including iron, copper, gold, and rare metals [13,29,57,58,59,60,61,62,63,64,65]. The Luxi Terrane, situated along the southeastern margin of the NCB (Figure 1B), is structurally bounded by four major fault zones: the Fengpei, Qihe–Guangrao, Liaocheng–Lankao, and Yishui fault zones. The area is characterized by multiple phases of deformation and repeated magmatic activity. Exposed strata include the Archean Yishui Group, the Neoarchean Taishan Group, Cambrian–Ordovician carbonate and clastic sequences, Carboniferous–Permian units, Triassic strata, Jurassic–Cretaceous clastic and volcanic rocks, and Quaternary deposits. Intense tectonic activity in the Luxi Terrane has produced a well-developed array of planar and linear structures, including numerous ductile shear zones. Multiple sets of faults with varying orientations occur, among which NW- and NE-trending systems are the most prominent. These regional faults exert primary control over metallogenesis and Mesozoic (Yanshanian) magmatism. The Neoarchean magmatic suite comprises gabbro, diorite, and granodiorite, whereas Mesozoic magmatism is dominated by syenite porphyry, monzonitic porphyry, granite, and carbonatite. These intrusive bodies display diverse morphological characteristics and occur as plutons, stocks, and dikes. Their spatial and temporal distribution is closely linked to the regional fault architecture, reflecting long-standing and vigorous deep-seated tectono-magmatic processes beneath the Luxi Terrane.

3. Deposit Geology

The Weishan REE deposit is located in the southern margin of the Luxi Terrane (Figure 1B) and represents the largest REE deposit in the region, as well as the third-largest light rare earth production base in China. The exposed basement rocks within the mining area consist of biotite plagioclase gneisses of the Archean Taishan Group (Shancaoyu Formation). Apart from limited outcrops along local topographic highs, most of the basement is overlain by Quaternary sediments (Figure 2). The distributions of Mesozoic intrusive rocks and carbonatite are predominantly controlled by NW- and NE-trending faults. Magmatic activity in the area is well developed and dominated by Yanshanian syenite (quartz syenite and aegirine–augite quartz syenite) and alkaline granites, collectively forming the Weishan alkaline complex (Figure 2). The exposed area of the complex is approximately 0.5 km². It extends in a NE-SW orientation and intrudes the Archean gneisses along sharp contacts, with local occurrences of variable degrees of alkaline metasomatism along the contacts. The carbonatite intrudes along structures within both the Archean metamorphic basement and the Mesozoic alkaline intrusions. Their contacts with the host rocks are well defined. Individual orebodies typically range from 20 to 540 m in length and from 10 cm to 10 m in width, exhibiting relatively continuous mineralization. Based on mineral assemblages, earlier studies subdivided the carbonatite of the Weishan deposit into REE-bearing quartz–barite–carbonatite veins, REE-bearing aegirine granite porphyry, aegirine-bearing veins, and monazite-bearing apatite veins [21,22]. The dominant REE minerals are bastnäsite-(Ce) and synchysite-(Ce), with minor occurrences of parisite-(Ce), strontianite phases, alkali REE-bearing phase, ancylite-(Ce), monazite, and apatite-(Ce). These REE minerals commonly coexist with quartz, barite, fluorite, calcite, and mica [67]. The REE ore is characterized by diverse and spatially heterogeneous mineral assemblages. In addition to the principal REE minerals and associated gangue minerals, metallic minerals such as pyrite, galena, and magnetite also occur. Other minerals present include chlorite, arfvedsonite, ankerite, titanite, and rutile.

4. Samples and Analytical Methods

4.1. Sample Descriptions

The fluorite samples investigated in this study were collected from the REE-bearing barite–carbonatite veins of the Weishan REE deposit (Figure 3). Sample 20CS01, consisting of calcite–bastnäsite–fluorite assemblages, contains the REE minerals bastnäsite-(Ce) and synchysite-(Ce), accompanied by gangue minerals such as calcite and barite. The bastnäsite-(Ce) mineral exists as prismatic, dark brown crystals with grain sizes of 1–5 mm, whereas synchysite-(Ce) appears as dark green grains that replace bastnäsite-(Ce). The fluorite mineral is purple, displays a pegmatitic texture, and coexists with calcite, with grain sizes ranging from 1 to 3 mm. In sample 20CS06, which is composed of calcite–bastnäsite–mica assemblages, bastnäsite-(Ce) exists as fine-grained, lath-like crystals intimately intergrown with calcite and mica.

4.2. Analytical Methods

The samples were crushed to a size range of 40–80 mesh and separated by magnetic and heavy-liquid separation. Fluorite grains were selected using a binocular microscope before epoxy mounting. In order to obtain the inner structure and relative surface, the grains were polished down to half of the section prior to cleaning in the ultrasonic washing machine. Microscopic petrographic observations, cathodoluminescence (CL) imaging, and backscattered-electron (BSE) imaging of the fluorite samples were conducted at the China University of Geosciences (Beijing) and the Research Institute of Petroleum Exploration and Development, PetroChina (Beijing, China). The analyses were performed using a CLF-2 cathodoluminescence system (BII, Pickering, ON, Canada) coupled with a Zeiss Axio Imager multifunctional microscope (Carl Zeiss AG, Jena, Germany). Operating conditions were 12.5 V accelerating voltage and 900 mA beam current. Grain microtextures including cracks, zonation, and inclusion were checked by the CL and BSE images, which were produced by the JXA-8800 electron microprobe (EPMA, JEOL Ltd., Tokyo, Japan) at operating conditions of 20 kV and 20 nA. Suitable sites were chosen within crystal grains for fluorite geochemistry analysis.

Major elements of fluorite were obtained with a JXA-iHP200F electron microprobe (EPMA, JEOL Ltd., Tokyo, Japan) installed at the Institute of Mineral Resources at the Chinese Academy of Geological Sciences. The working conditions of the test are as follows: The acceleration voltage is 15 kV. The acceleration current is 20 nA, and the beam spot diameter is 5 μm. All test data were processed by ZAF correction. Natural minerals or synthetic oxides are used as standards for testing elements. The trace element analyses were performed using the LA-ICP-MS (NWR193 laser-ablation microprobe from Elemental Scientific Lasers LLC, Bozeman, MT, USA; and attached to a Analytik Jena M90, Jena, Germany) apparatus at Yanduzhongshi Geological Analysis Laboratories Co., Ltd. (Beijing, China). Fluorite was sampled on 30 μm spots using the laser at 8 Hz and a density of approximately 10 J/cm². A flow of He carrier gas at a rate of 0.55 L/min carried particles ablated by the laser out of the chamber, allowing them to be mixed with Ar gas and then carried to the plasma torch. The unknown elemental contents in the fluorite grains were identified by the external standard (NIST610 glass standard; see Supplementary Table S2). The Ca concentrations determined via EPMA analyses were used as the internal standard for correcting the LA-ICP-MS data. The signal intensity of the ⁴⁴Ca measured at each spot was used to correct for matrix effects and to calculate the absolute concentrations of all trace elements. Measured trace elements include ³⁹K, ⁴³Ca, ⁸⁸Sr, ⁸⁹Y, ¹³⁷Ba, ¹³⁹La, ¹⁴⁰Ce, ¹⁴¹Pr, ¹⁴⁶Nd, ¹⁴⁷Sm, ¹⁵³Eu, ¹⁵⁷Gd, ¹⁵⁹Tb, ¹⁶³Dy, ¹⁶⁵Ho, ¹⁶⁶Er, ¹⁶⁹Tm, ¹⁷²Yb, ¹⁷⁵Lu, ⁸⁵Rb, ²³²Th, and ²³⁸U, with a measuring frequency of 0.18 s for each element.

5. Results

5.1. Petrography of Fluorite

The REE-bearing quartz–barite–carbonatite veins of the Weishan deposit are com-posed primarily of calcite, bastnäsite-(Ce), and synchysite-(Ce), which are commonly associated with fluorite and barite. The ores exhibit pegmatitic textures or disseminated structures (Figure 3). Fluorite displays moderate to high vitreous luster; is generally translucent, brittle; and is characterized by uneven fracture surfaces. Fluorite crystals typically occur as cubic forms, with octahedral crystals being rare. The crystal surfaces are relatively rough, and both cleavage and fractures are well developed. In some cases, fluorite occurs as irregular granular fillings between bastnäsite-(Ce), calcite, and barite (Figure 4). The color of fluorite is heterogeneous, ranging from purple to green; among these, purple fluorite is the most widespread and appears in nearly all fluorite-bearing veins. Based on morphological features and mineral assemblages, fluorite in the Weishan deposit can be classified into four generations.

Type 1 fluorite shows anhedral morphologies with highly variable grain sizes (200–1000 μm). This generation of fluorite coexists with calcite and bastnäsite-(Ce) and is partially replaced by late-stage barite and calcite, producing distinct replacement textures (Figure 4A–C,E,G-H). Some grains exhibit pronounced dissolution features, including irregularly corroded crystal outlines, internal dissolution pits, and dissolution channels subsequently filled by later minerals (Figure 5A,B,D). Cathodoluminescence (CL) imaging reveals blue luminescence, with a few irregular internal growth zones preserved (Figure 5D). Type 2 fluorite shows subhedral forms (Figure 4F) and locally occurs as overgrowths along the margins of Type 1 fluorite. In CL images, it appears bright blue, with relatively regular oscillatory zoning. Calcite commonly fills cleavage planes within fluorite crystals. The contact boundaries between Type 1 and Type 2 fluorites are irregular (Figure 5B,C). Irregular triangular dissolution pits occur within the grains, accompanied by elongated etch traces along cleavage surfaces (Figure 5F). Type 3 fluorite exhibits euhedral–subhedral forms and develops along the outer rims of earlier fluorite grains. It replaces the margins of earlier fluorite through metasomatic processes. Its distinct CL characteristics (Figure 5E,F) reflect compositional differences relative to the earlier fluorite generations. Type 4 fluorite occurs near the contact zone between the carbonatite bodies and the Archean metamorphic basement, and it is associated with mica and calcite (Figure 5G,H). This generation consists of euhedral granular crystals adjacent to calcite, along with clean, transparent interiors. In CL images, Type 4 fluorite displays pale blue luminescence, exhibits relatively homogeneous internal features, and shows well-developed oscillatory zoning.

5.2. Geochemical Composition

The present study conducts a systematic in situ geochemical investigation of four generations of fluorites (Types I–IV) from the Weishan REE deposit, with analytical results summarized in Supplementary Table S1. Major element analyses on representative fluorite samples were conducted by systematic EPMA. The results indicate consistently high fluorite purity across all spots, with CaO contents of 68.40–73.39 wt% and F contents of 46.37–54.55 wt%. Such compositions indicate that the investigated fluorite samples are of exceptionally high purity and have experienced only minimal late-stage alteration or incorporation of impurities. Consequently, their trace element signatures can be regarded as robust proxies for the composition and physicochemical characteristics of the original precipitating fluids. Type I fluorite is characterized by a distinctive “high-Th, low-Sr” signature. Th contents are relatively elevated (~16 ppm), whereas Sr contents are comparatively low (~3959 ppm), resulting in low Sr/Th ratios. Si contents are notably high (~5150 ppm), while Na and K occur at moderate levels. Type II fluorite differs markedly from Type I fluorite. Th contents decrease significantly (average < 5 ppm), whereas Sr contents increase sharply (average > 5000 ppm), defining a “low-Th, high-Sr” pattern. Na and K concentrations display large fluctuations, which may reflect the presence of alkali-rich fluid inclusions or K-Na mineral inclusions. Type III fluorite is the most widely developed type in the Weishan deposit and shows relatively homogeneous trace element compositions at the hand specimen scale, although local internal variability is present. Sr contents are generally high (commonly 5000–8000 ppm), while Th contents remain low (mostly < 1 ppm). Na and K contents are overall lower than in Type II fluorite, but Si exhibits considerable variation. Locally elevated Ba concentrations in some spots indicate transient increases in Ba activity in the coexisting fluid. Type IV fluorite is distinguished from Type III fluorite by its behavior in high-field-strength elements (HFSEs). Elements such as Zr, Hf, Nb, and Ta, which are detectable in some Type III fluorite analyses, occur at very low levels or below detection limits in Type IV fluorite. Sr contents remain relatively high, but Y contents drop sharply to ~100 ppm.

The analytical results and characteristic parameters of rare earth elements in fluorite are presented in Supplementary Table S1. Considerable variations in total REE contents are observed among the four fluorite generations. Type 1 and Type 2 fluorites show markedly higher REE contents than Type 3 and Type 4 fluorites. There is also a clear correlation between fluorite color and REE abundance: darker purple fluorite generally contains lower total REE concentrations. The patterns of REE distributions differ significantly among the four types. Type 1 and Type 2 fluorites display pronounced LREE–HREE fractionation and strong LREE enrichment; among these, Type 1 fluorite contains higher concentrations of La, Ce, Pr, and Nd compared to Type 2 fluorite. In contrast, Type 3 fluorite is depleted in both LREE and HREE but exhibits relative enrichment in MREE. Type 4 fluorite shows the lowest total REE content and displays only slight MREE enrichment.

The REE distribution patterns of the four generations of fluorites exhibit pronounced distinctions. Type 1 fluorite displays a strongly right-sloping chondrite-normalized REE pattern, which is characterized by significant LREE enrichment (Figure 6A). In contrast, Type 2 fluorite shows a relatively flat pattern with only a slight rightward inclination, indicating weaker LREE enrichment (Figure 6B). Type 3 fluorite is marked by a left-sloping pattern with pronounced MREE enrichment (Figure 6C), whereas Type 4 fluorite exhibits an overall flat REE distribution (Figure 6D). Eu anomalies also vary among the fluorite types. Type 1 fluorite generally exhibits weak to moderate negative Eu anomalies (Figure 6A). In the Y/Ho–La/Ho diagram (Figure 7A), all analyses define a broadly consistent Y/Ho trend. Type I fluorite plots are seen at the far right due to its exceptionally high La contents. The Y/Ho ratios of all fluorite types are similar and slightly higher than the chondritic value. In the Tb/Ca-Tb/La diagram, Type I fluorite falls within the pegmatitic–magmatic field. From Type II to Type IV, increasing Tb/La ratios accompanied by variations in Tb/Ca concentrations produces a band-like distribution within the hydrothermal field, outlining an evolutionary trajectory from early, REE-rich, and strongly fractionated fluids to late, REE-poor, and weakly fractionated fluids. Both positive and negative Eu anomalies occur in Type 2 and Type 3 fluorites, while Type 4 fluorite is characterized by a distinct positive Eu anomaly. Ce anomalies show similarly variable behavior: Type 1 and Type 3 fluorites display both positive and negative Ce anomalies, whereas Type 2 and Type 4 fluorites consistently show positive Ce anomalies (Figure 8).

6. Discussion

6.1. Precipitation Mechanisms of Fluorite

In general, the co-precipitating mineral assemblages exert a significant influence on the distribution of rare earth elements (REEs) in fluorite, particularly in the presence of bastnäsite-(Ce), monazite, and calcite [9,42,69,70]. In the Weishan REE deposit, minerals co-precipitating with Type 1 fluorite include bastnäsite-(Ce) and calcite (Figure 4F), and the REE distribution patterns of fluorite exhibits pronounced LREE enrichment. This feature is consistent with the REE patterns reported for magmatic-stage calcite and bastnäsite-(Ce) [71,72], indicating that Type 1 fluorite may have formed during the early stages of magmatic–hydrothermal evolution. The low Tb/La and high Tb/Ca ratios further support a magmatic–pegmatitic origin for this fluorite generation (Figure 7). Petrographically, Type 1 fluorite locally preserves replacement textures attributable to later hydrothermal overprinting, supporting the interpretation as an early-crystallized phase (Figure 5A). Type 2 fluorite displays generally lower total REE contents and is characterized by slight LREE depletion, modest MREE enrichment, and HREE depletion. Compared with Type 1 fluorite, Type 2 fluorite shows markedly decreased LREE concentrations and relatively elevated MREE and HREE concentrations (Figure 8). In the Tb/La-Tb/Ca diagram, Type 2 fluorite plots in a transitional field between pegmatitic and hydrothermal origins (Figure 7), indicating crystallization later than Type 1 fluorite. Type 2 fluorite crystals contain irregular dissolution pits, consistent with dissolution–reprecipitation processes. Within the study area, Type 3 and Type 4 fluorites commonly coexist with quartz, barite, and pyrite. As these associated minerals lack stoichiometric calcium, their influence on REE partitioning in fluorite is limited. Type 3 and Type 4 fluorites display broadly similar REE patterns, along with LREE and HREE depletion and relative enrichment in MREE, likely reflecting precipitation from evolved hydrothermal fluids in the later stages of the system. Petrographically, Type 3 fluorite occurs as euhedral crystals that display relatively homogeneous cathodoluminescence, whereas Type 4 fluorite occurs as fine-grained, anhedral aggregates filling interstices between calcite grains, suggesting that Type 4 fluorite postdates Type 3 fluorite. Taken together, mineralogical and geochemical evidence indicates a well-defined paragenetic sequence for fluorite in the Weishan REE deposit: Type 1 → Type 2 → Type 3 → Type 4. These successive generations document the progressive transition and evolution of the ore-forming system from high-temperature magmatic conditions to low-temperature hydrothermal regimes, reflecting systematic changes in fluid composition, temperature, and pH conditions over time.

According to the classical framework [46], fluorite precipitation can occur under three principal geological conditions: (i) fluid–rock interaction, (ii) fluid mixing accompanied by changes in chemical composition, and (iii) cooling and/or pressure reduction in Ca-F-saturated fluids. Magmatic fluorite commonly forms in highly evolved, F- and REE-enriched peraluminous granites, ongonites, pegmatites; fluorite-rich rhyolites; or selected alkaline magmatic systems [73,74,75,76,77]. Fluorite can crystallize in fluoride–calcium or Ca- and F-rich mafic fluorosilicate melts that result from magmatic fluoride–silicate liquid immiscibility [73,78]. Its crystallization requires fluorite saturation in the melt, a threshold controlled primarily by the melt composition and prevailing physicochemical conditions [75,76]. In this study, Type I fluorite is distinguished by anomalously high ΣREE contents (>2500 ppm) and a distinctive “high-Th, low-Sr” signature, in sharp contrast to the low-ΣREE, Sr-rich compositions typical of Types II–IV hydrothermal fluorite. This contrast raises the possibility that Type I fluorite represents a product most closely linked to a magmatic end-member fluid and may therefore preserve attributes that are transitional toward magmatic fluorite. The exceptionally high ΣREE and pronounced LREE enrichment of Type I fluorite are consistent with precipitation from a highly evolved, F-rich granitic silicate melt or from an incipient supercritical fluid exsolved from such a melt. Thorium, a strongly incompatible element, becomes concentrated in highly fractionated magmas; the elevated Th concentration in Type I fluorite may thus reflect inheritance from an evolved magmatic reservoir. Based on global REE-deposit models, mineralization in this district may involve silicate–carbonate melt immiscibility and silicate melt immiscibility for fluoride-silicate and fluoride-calcium (F-Ca or fluoritic) melts [75,76,79]. Because fluoride melts can dissolve and complex LREEs and Th efficiently, the high ΣREE and Th concentrations in Type I fluorite are best explained by precipitation from an early, high-temperature, hypersaline fluid exsolved from fluoride–calcium (F-Ca) and other fluoride-rich melts produced by such immiscibility.

During fluid–rock interaction, adsorption of REEs onto mineral surfaces commonly manifests as an ion exchange or substitution process [80,81]. However, the fluorite generations (Type 1–Type 3) developed in the Weishan REE deposit predominantly occur in pegmatitic habits, reflecting low-reactivity surface areas. Consequently, adsorption is unlikely to have exerted a major influence on REE enrichment. The widespread presence of calcite, barite, and REE fluorocarbonates in the deposit suggests that ligands such as F⁻, CO₃²⁻, and SO₄²⁻ were enriched within the fluid phase, implying that complexation played a more dominant role than adsorption in governing fluorite precipitation. The transition from Type 1 fluorite to Type 2 fluorite is marked by systematic changes in REE fractionation. Type 2 fluorite is characterized by LREE depletion coupled with relative enrichment of MREE and HREE (Figure 8D–F). LREE depletion in fluorite is commonly associated with element remobilization and recrystallization, processes controlled by differences in complex stability, evolving physicochemical conditions, and fluid compositions [47,68].

As LREEs are preferentially adsorbed on mineral surfaces during fluid migration, tectonically enhanced remobilization and recrystallization can therefore further deplete LREEs in fluorite [68,82]. Experimental petrology has demonstrated that fluorite often contains submicron REE-rich microphases that are more soluble than fluorite itself, producing heterogeneous REE distributions [47,83]. Selective dissolution of these microphases can preferentially reduce LREE concentrations while largely preserving HREE. Combined with the La/Ho-Y/Ho discriminant diagram, these observations suggest that coupled dissolution and recrystallization contributed to the formation of Type 2 fluorite. From Type 2 fluorite to Type 3 fluorite, MREE and HREE concentrations increase further, whereas LREE concentrations remain low (Figure 8D–F). Previous work has shown that, in F-rich hydrothermal systems, chloride and sulfate complexes act as effective ligands for REE transport [84]. In the Weishan deposit, however, the widespread precipitation of fluorite and bastnäsite-(Ce) suggests that REE mobility during the hydrothermal stage was dominated by fluoride complexation. At temperatures of 100–250 °C, REEs occur mainly as monofluoride complexes [REEF]²⁺, whose stability increases with atomic number [80,85]. Fractionation between Y and Ho is primarily controlled by fluid geochemistry rather than the fluid source: Y forms relatively stable difluoride complexes [YF₂], whereas Dy and Ho are dominated by monofluoride complexes [86], accounting for the positive Y anomalies in Type 2 and Type 3 fluorites. Because HREEs and Y form more stable fluoride complexes, they are preferentially retained and enriched in the fluid. Where complexation dominates, progressive cooling coupled with continued fluorite precipitation amplifies LREE–HREE fractionation and yields increasingly distinct REE patterns [47,87]. As shown in Figure 7, the marked decrease in La/Ho from Type 2 fluorite to Type 3 fluorite, together with near-constant Y/Ho, constitutes a characteristic signature of element reactivation and mineral reprecipitation [80,81]. The reactivation process is also reflected in Tb/La ratios: increased ligand to metal ratios during remobilization preferentially deplete La, whereas Tb remains comparatively stable [68]. Collectively, these observations suggest that Type 3 fluorite formed via partial dissolution and subsequent reprecipitation of earlier fluorite generations. Type 4 fluorite exhibits the lowest REE concentrations among all generations, implying diminished stability of LREE–F complexes and near-exhaustion of REEs in the fluid during its formation. Under these conditions, the fluid became supersaturated with respect to CaF₂, and fluorite precipitated mainly as late-stage infill between calcite grains. This generation therefore records the terminal phase of hydrothermal fluid evolution in the Weishan REE deposit.

6.2. Physicochemical Conditions of Mineralization

Variations in ore–fluid composition and physicochemical conditions (e.g., temperature and pH) during fluorite precipitation are commonly recorded by fluorite REE systematics, providing key constraints on the depositional environment [47,48,80]. The REE-Y signatures of hydrothermal fluorite are therefore widely used to trace its fluid composition and evolutionary history, with distinct evolution pathways expressed as characteristic partitioning patterns [33,47,48]. Precipitation temperature can be assessed, in part, from the presence or absence of Eu anomalies [47,80,87]. In hydrothermal fluids, the Eu²⁺/Eu³⁺ ratio is controlled by redox equilibrium and decreases sharply below ~250 °C under oxygen fugacity typical of the shallow lithosphere [88,89]. At higher temperatures, Eu is dominated by Eu²⁺; owing to its relatively large ionic radius, Eu²⁺ substitutes poorly for Ca²⁺ in the fluorite lattice and thus tends to remain in the fluid or be adsorbed onto mineral surfaces [88]. With cooling, Eu²⁺ is oxidized to Eu³⁺, whose smaller ionic radius allows more efficient incorporation into fluorite [44,47,87]. Where hydrothermal fluids are sourced from mildly reducing, carbonatite-related systems and inherit positive Eu anomalies from the protolith, fluorite with positive Eu anomalies is expected to precipitate only at low temperatures (<250 °C). Previous studies further indicate that fluorite displaying positive Eu anomalies commonly forms at temperatures below ~200 °C [47,48].

Both Eu and Ce anomalies (Eu* and Ce*) are widely used as indicators of the redox state during fluorite precipitation. In the Weishan REE deposit, Eu* and Ce* values vary considerably among fluorite generations, suggesting fluctuations in the physicochemical conditions of the ore-forming fluids. Type 1 fluorite consistently exhibits negative Eu anomalies, indicating formation temperatures above 200 °C. Its range of Ce* values, from positive to negative, primarily reflects fluctuations in the redox conditions (fO₂) of the parental fluid during its evolution. Type 2 fluorite shows mixed Eu anomalies but consistently positive Ce anomalies. Type 3 fluorite shows weakly positive Eu anomalies and variable Ce anomalies, whereas Type 4 fluorite is characterized by uniformly strong positive Eu and Ce anomalies. Overall, these trends suggest sustained fluorite precipitation during progressive cooling, accompanied by a shift from reducing to increasingly oxidizing conditions. The presence of CO₂-rich or sulfate-bearing fluid inclusions in later fluorite generations, compared to earlier CH₄- or H₂S-bearing inclusions, would chemically attest to an increase in fluid oxidation state [90]. Cerium anomalies in fluorite that coexists with bastnäsite-(Ce) are best interpreted in terms of mineral–fluid fractionation rather than redox reactions alone. Preferential partitioning of Ce relative to La and Pr into fluorocarbonates depletes the residual fluid in Ce such that later fluorite progressively develops negative Ce anomalies [33,91]. This framework accounts for the lower LREE contents of Types II–III relative to Type I and for the locally negative Ce anomalies in Types II–III fluorite, consistent with main- to late-stage mineralization. By contrast, the combination of comparatively stable Na-K, low Sr, high Th, and elevated Si in Type I fluorite is most compatible with an early, less-evolved magmatic fluid signature that predates large-scale fluorocarbonate precipitation. Additional trace element systematics support this interpretation. High Sr and Ba, low Th, and strongly variable Na/K levels in Types II–III point to changing sulfate activity and episodic increases in fO₂ during the principal mineralizing interval [17,23]. The coupled co-variation in REE, Na, and Ce* levels is consistent with lattice incorporation of Ce³⁺ via charge-balanced substitution (e.g., REE³⁺ + Na⁺ ↔ 2Ca²⁺, [14]), rather than submicroscopic inclusions. Type IV fluorite, which is restricted to the contact with Neoarchean plagioclase-hornblende gneiss, is characterized by pronounced positive Eu and Ce anomalies together with low total REE levels, indicating a late overprint by a compositionally distinct fluid. The positive Eu anomaly is best explained by efficient leaching of the metamorphic basement by an alkaline, reducing fluid, which mobilizes Eu preferentially as Eu²⁺ under low-fO₂ conditions [16,23]. Under the same reducing conditions, Ce is stabilized and transported predominantly as Ce³⁺ (rather than oxidized to relatively immobile Ce⁴⁺), allowing Ce to be effectively remobilized and carried by the fluid. A fenitizing, alkali-, and volatile-rich fluid interacting with wall rocks, followed by cooling and mixing with residual acidic fluids and/or CO₂ loss, would decrease pH, drive CaF₂ supersaturation, and precipitate Type IV fluorite. Its positive Eu-Ce signature is consistent with a wall-rock Eu contribution superimposed on a carbonatite-related Ce imprint.

Experimental petrology further demonstrates that fluid pH and salinity are key controls of fluorite dissolution–precipitation equilibria and REE solubility. Under constant-salinity conditions, REE solubility increases with decreasing pH; at a constant pH, the addition of NaCl dramatically enhances REE concentrations in the solution by several orders of magnitude [83]. In acidic to weakly acidic, high-salinity fluids, REE fluorides readily approach saturation, consistent with the relatively high solubility of fluorite under these conditions. Consequently, F⁻ exhibits higher chemical activity in acidic fluids relative to alkaline fluids. Under weakly acidic to alkaline conditions, shifts in REE/Ca and REE/F ratios can promote precipitation of LREE-enriched fluorides. At high temperatures and weakly acidic pH values, REE/Ca and REE/F ratios are positively correlated, and fluorite typically shows HREE depletion—consistent with the Type 1 fluorite signature. In acidic-fluid conditions, fluorite tends to exhibit LREE depletion and Y enrichment, matching the REE patterns of Type 2 and Type 3 fluorites. These observations suggest that Type 1 fluorite in the Weishan deposit precipitated from relatively high-temperature, weakly acidic, and moderately reducing fluids, whereas Type 2 and Type 3 fluorites crystallized from lower-temperature, more acidic fluids under redox conditions evolving from reducing to weakly oxidizing.

6.3. Implications for REE Mineralization

Previous studies on the Weishan REE deposit have focused on the petrogenesis of the alkaline–carbonatite complex, the source of ore-forming materials, the timing of magmatic and hydrothermal processes, and the mechanisms of REE precipitation, leading to several contrasting genetic models. Based on C-O isotopic compositions of carbonatites, some scholars demonstrated that the Weishan carbonatites are of igneous origin and were derived from CO₂-rich silicate magmas through liquid immiscibility [71]. Combined with the geochemical characteristics of apatite, these authors further proposed that the ore-forming carbonatites originated from a metasomatized lithospheric mantle and that REE minerals precipitated during the magmatic–hydrothermal transition. In contrast, the other authors, using in situ Sr-Nd isotopes of apatite, argued that the REE-rich syenite–carbonatite complex in Weishan resulted from crust–mantle interactions, with ore-forming materials sourced predominantly from the asthenosphere [92]. Zhao et al. (2023) suggested, based on the trace element composition of calcite, that the ore-forming fluid was derived from carbonate-rich melts exsolved from an alkaline magma, emphasizing that alkali-rich components played a crucial role in the liberation and transport of REEs [72]. They further proposed that REE mineralization was controlled principally by alkaline magmatism rather than carbonatite magmatism and that extensive REE mineral precipitation was triggered by fluid boiling.

Despite divergent interpretations of source characteristics and metallogenic processes at Weishan, there is broad agreement that the ore-forming fluids were F-rich. However, existing models are unable to explain the pervasive association of fluorite with REE minerals. Although previous study inferred extensive fluorite precipitation during the hydrothermal stage from mineral paragenesis, the source and genetic mechanism of fluorite remained unresolved [72]. Assuming that F was sourced from fluids exsolved from an alkaline magma, such fluids would be expected to be alkali-rich but calcium-poor. This expectation is inconsistent with fluid-inclusion evidence from quartz and barite that recorded Ca-bearing daughter minerals. Furthermore, if magmatic fluids carried both Ca and F, the very low solubility of fluorite would favor immediate precipitation upon fluid exsolution, which conflicts with field relationships at Weishan, where abundant pegmatitic fluorite coexists with late-stage hydrothermal minerals such as barite and celestine. These inconsistencies suggest that the F-rich fluids responsible for widespread fluorite precipitation were more likely derived from Ca-rich carbonatite magmas, from fluid–rock interactions with Ca-rich wall rocks, or from mixing with external fluids. In this regard, some scholars argued that volatile-rich carbonatite magmas at high temperatures can sustain high activities of Ca and F and that the high-salinity fluids exsolved from such magmas can simultaneously concentrate F⁻ and Ca²⁺, thereby promoting fluorite precipitation [93]. Some researchers proposed that the interaction between F-rich carbonatite fluids and carbonaceous wall rocks can generate abundant REE- and Sr-rich fluorites [18]. Therefore, the F-rich ore-forming fluids in the Weishan REE deposit were most likely derived from Ca-rich carbonatite magmas. During magmatic–hydrothermal evolution, fluorine and calcium activities were dynamically modulated by changing physicochemical conditions, producing successive generations of fluorite. This interpretation is consistent with published fluid-inclusion constraints: CO₂-bearing inclusions in quartz–fluorite veins have salinities of ~5 wt% and homogenization temperatures of 210–248 °C, indicating a medium- to low-temperature hydrothermal system of relatively low salinity [90]. Petrography further supports these trends (Figure 4 and Figure 5). Early Type 1 fluorite, interpreted to have formed at higher temperatures under weakly acidic conditions, commonly displays etch pits and dissolution channels along cleavage planes, features conducive to element remobilization. In contrast, late-stage fluorite precipitated at lower temperatures under more acidic conditions typically preserves fine-scale dissolution textures along crystal margins, recording shifts in fluid chemistry and precipitation kinetics.

Fluorine is a key volatile in REE mineralization, exerting first-order control on mineral assemblages during REE deposition and buffering the chemistry of ore-forming fluids. Fluoride may occur as ore and gangue minerals and as daughter minerals in fluid inclusions, thereby influencing REE mobility and precipitation. Fluid inclusion studies and experimental petrology demonstrate that granitic magmas can undergo fluoride–silicate immiscibility during emplacement, promoting preferential partitioning of REEs into fluoride melts. In hydrothermal systems, F promotes the formation of REE complexes, enabling REEs to be transported as complex ions and markedly increasing their solubility [23,40,83,84]. Experimental studies on hydrothermal fluorite further show that at high temperatures, LREE-F₂⁺ complexes are more stable than MREE-F₂⁺ complexes [52]. Accordingly, during early high-temperature fluid evolution, REEs are transported chiefly as LREE–F₂⁺ complexes, producing LREE-enriched early-stage fluorite. With cooling, the stability of LREE–F₂⁺ complexes decrease, whereas MREE– and HREE–fluoride complexes become relatively more stable, leading to progressive MREE and HREE enrichment in late-stage fluorite. Thermodynamic modeling additionally suggests that, in natural F-rich systems, REEs are transported mainly as chloride and sulfate complexes in acidic to weakly acidic fluids, whereas fluoride and phosphate, given the extremely low solubility of REE fluorides and phosphates, function primarily as precipitation ligands. In near-neutral to alkaline fluids, hydroxyl and/or carbonate complexes likely play a greater role in REE precipitation [84,94,95]. In the Weishan REE deposit, the coexistence of bastnäsite-(Ce), barite, celestine, fluorite, and calcite indicates that the ore-forming system is enriched in F⁻, CO₃²⁻, and SO₄²⁻, which likely served as the principal ligands governing REE transport in the hydrothermal fluids. REE mobility in hydrothermal solutions is controlled by both the stability of aqueous REE complexes and the solubility of REE-bearing minerals. Experimental studies on fluorite dissolution reveal that REE activation and mobility are also influenced by fluorite solubility. Between 100 and 250 °C, salinity and pH exert stronger controls on fluorite solubility than temperature [83]. Fluorite solubility, in turn, regulates REE release to the solution and the formation of fluoride phases. Given the extremely low solubility of fluorite, once fluid conditions cross the threshold for CaF₂ precipitation, F⁻ activity declines rapidly. This decrease destabilizes REE–fluoride complexes and promotes bastnäsite precipitation via the following reactions:

REEF₃⁰ + CO₃²⁻ = bastnäsite + 2F⁻

(1)

REEF₃⁰ + HCO₃⁻ = bastnäsite+ 2F⁻+ H⁺

(2)

F⁻+ H⁺ = HF⁰

(3)

In Reactions (1) and (2), consumption of F⁻ drives the system toward bastnäsite precipitation. In Reactions (2) and (3), generation of H⁺ further lowers the activities of Ca²⁺ and F⁻, thereby promoting fluorite dissolution and explaining the enhanced solubility of fluorite under acidic conditions. In summary, the fluorite and REE minerals developed in the Weishan REE deposit were most likely precipitated from residual carbonatite-derived fluids rich in Ca and CO₂, along with subordinate F and H₂O and appreciable dissolved REEs. As the temperature and pressure decreased, fluorite began to precipitate, leading to a rapid decline in F activity. The ensuing increase in acidity partially dissolved early-stage fluorite, enhanced REE mobilization, and destabilized REE-F complexes, thereby triggering extensive precipitation of REE fluorocarbonates. Subsequent re-enrichment of the residual fluid in Ca²⁺ and F⁻ promoted renewed, large-scale fluorite precipitation during the late stage, producing the multi-episodic fluorite–REE mineral assemblages observed in the deposit. This study aims to reconstruct the complete evolution of the Weishan REE–fluorite system, from deep magmatic processes to shallow hydrothermal activity, using fluorite and its geochemical record as primary tracers. Within a carbonatite-related mineral system ultimately rooted in a mantle source, fluorite-bearing REE orebodies represent one expression of this evolution under a specific range of physicochemical conditions. By contrast, coeval REE-rich (or other element-enriched) orebodies that lack fluorite may reflect the same ore-forming fluid operating under different temperature–pressure–pH–ligand regimes. Together, these mineralization styles are interpreted to constitute a single, integrated, and spatially zoned carbonatite ore-forming system.

7. Conclusions

(1) Four generations of fluorite are distinguished in the Weishan REE deposit, which occur in a temporal sequence of Type I → Type II → Type III → Type IV, corresponding to the early magmatic–hydrothermal stage, the hydrothermal stage, and the late, low-temperature stage.

(2) Paragenetic progression reflects a decrease in temperature, an increase in fluid pH, a gradual increase in oxygen fugacity, and extensive mineral dissolution–reprecipitation processes that facilitated REE redistribution within the system.

(3) Precipitation of fluorite is a process likely governed by variations in Ca²⁺ and F⁻ activities within a fluorine-rich carbonatite fluid, as well as by dissolution–reprecipitation mechanisms. By regulating the activity of fluorine, buffering the chemical conditions of the hydrothermal system, and potentially controlling co-precipitation with REE fluorocarbonates, fluorite is inferred to play a key, and possibly indispensable role in REE enrichment. These features provide essential mineralogical constraints for establishing genetic models of carbonatite REE deposits and for guiding future exploration strategies.