REE Enrichment during Magmatic–Hydrothermal Processes in Carbonatite-Related REE Deposits: A Case Study of the Weishan REE Deposit, China

: The Weishan carbonatite-related rare earth element (REE) deposit in China contains both high- and low-grade REE mineralization and is an informative case study for the investigation of magmatic–hydrothermal REE enrichment processes in such deposits. The main REE-bearing mineral is bastnäsite, with lesser parisite and monazite. REE mineralization occurred at a late stage of hydrothermal evolution and was followed by a sulfide stage. Barite, calcite, and strontianite appear homogeneous in back-scattered electron images and have high REE contents of 103–217, 146–13,120, and 194–16,412 ppm in their mineral lattices, respectively. Two enrichment processes were necessary for the formation of the Weishan deposit: Production of mineralized carbonatite and subsequent enrichment by magmatic–hydrothermal processes. The geological setting and petrographic characteristics of the Weishan deposit indicate that two main factors facilitated REE enrichment: (1) fractures that facilitated circulation of ore-forming ﬂuids and provided space for REE precipitation and (2) high ore ﬂuorite and barite contents resulting in high F − and SO 42 − concentrations in the ore-forming ﬂuids that promoted REE transport and deposition.


Introduction
More than 500 carbonatite intrusions are known worldwide [1], but there are few known large or giant carbonatite-related rare earth element (REE) deposits (CARDs). Most currently mined large or giant deposits are in China [2] [4]. Here the term REE is used to include the 14 naturally occurring lanthanides and Y, but not Sc. Giant deposits with reserves in excess of 1 Mt REO are exceptionally rare. REEs have become vital in manufacturing numerous high-technology products. Between 2011 and 2017, China produced~84% of the world supply of REEs, with the USA only producing REEs from 2012-2015. The USA production came entirely from the Mountain Pass mine (California), which provided~4% of the world's REE supply [5]. A wide variety of REE deposits occurs in China [6,7], including CARDs formed from REE-rich fluids exsolved from carbonatitic melts [8], which is the most common type of REE mineralization in China, accounting for~65% of total reserves, including the Bayan Obo deposit [9,10], Mianning-Dechang REE belt [11][12][13], and Weishan deposit of Shandong Province [14,15]. All the deposits are located along cratonic margins and have several similar features (Table 1). Other CARDs include Mountain Pass [3], Oka [4], Canada, and Mt. Weld in Australia [16]; Araxa and Catalao I in Brazil [17,18]; Tomtor in Russia [19]; Mrima Hill in Kenya [20]; Mabouni in Gabon; and several other carbonatitic laterites in the Amazon region of Brazil [21].
These deposits all occur along cratonic margins and have similar geological settings, ore sources, and mineral paragenetic sequences [12]. The known CARDs in China have been well studied, with their petrogenetic processes being well constrained. Carbonatites may be considered as either mineralized or barren in terms of their REE contents. However, some carbonatites do not form REE deposits. For example, a mineralized carbonatitic melt can have low REE contents in response to limited fractional crystallization [22,23]. Moreover, carbonatites rarely produce high-grade REE ores, as in the Qieganbulake deposit of Xinjiang [24] and Miaoya deposit of Hubei [25,26]. Therefore, a second magmatic-hydrothermal process appears to be vital for the formation of high-grade REE deposits.
Several studies have shown that magmatic processes can be responsible for high-grade REE mineralization, especially that hosted in pegmatitic granite [27,28]. However, recent studies of carbonatite-hosted REE deposits found that a second magmatic-hydrothermal process was required to form high-grade REE deposits [13,[29][30][31][32][33]. Recent exploration of the Weishan deposit has identified large volumes of various types of ore with different grades. The Weishan alkaline intrusive complex lies on the southeastern margin of the North China Craton (NCC). The REE deposit is 18 km southeast of Zaozhuang City in Weishan County, Shandong Province, within the Luxi tectonic block on the southeastern margin of the NCC (Figure 1). It covers~4 km 2 and >24 carbonatite-related ore veins have been found, which are each up to 500 m long ( Figure 2). The REO grade of these ore veins is 1.76-14.62% (average = 5.67%) [15]. These well-exposed ores are ideal for the study of the formation of this deposit, as they comprise both REE and gangue minerals, and display a clear, traceable sequence of formation. In this study, petrological and analytical methods, such as X-ray diffraction (XRD), electron microprobe (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) have been applied to elucidate the mechanisms of REE transport and mineralization. The main objectives of the study were to characterize REE mineralization in the Weishan deposit, and to identify the factors that facilitated the magmatic-hydrothermal enrichment of REEs.   [14,45,46]). (b) Map showing the distribution of Triassic alkaline igneous rocks on the northern margin of the Sino-Korean craton (location in inset map, which shows the main tectonic units of China; modified from reference [47]). (c) Geological map of the Luxi Block (modified from references [14,48]).

Figure 2.
Geological sketch map of the Weishan rare earth element (REE) deposit in Shandong Province (modified from reference [49]).
Host rocks of the Weishan REE deposit are mainly biotite-plagioclase gneiss of the Shancaoyu Formation of the Archean Taishan Group [44]. Quaternary strata are also well exposed. Igneous rocks in the mining area comprise mainly Early Cretaceous quartz syenite, aegirine-augite quartz syenite porphyry, and alkaline granite, as well as dikes of dioritic porphyry, lamprophyre, and aplite ( Figure 2). Peak magmatism in western Shandong Province occurred during the Mesozoic, when a range of mantle-derived igneous rocks was formed, including intrusive rocks, K-rich volcanic rocks, and lamprophyres. The Yanshanian subalkaline complex, which is also in this district, comprises dioritic porphyry, syenite, syenitic porphyry, diabase, and lamprophyre, which commonly occur as batholiths, stocks, and dikes [14,50].
Lan [14] and Liang [50] determined formation ages of syenite and related REE deposits in Weishan as follows: zircon LA-ICP-MS U-Pb ages of ore-bearing quartz syenite and aegirine-augite syenite are 122.4 ± 2.0 and 130.1 ± 1.4 Ma, respectively; 253.6 ± 6.1 Ma inherited zircons were assimilated during magma ascent and/or emplacement. Based on a Rb-Sr isochron age of 119.5 ± 1.7 Ma for muscovite in REE ore veins [16] and the timing of syenite formation, Liang [50] concluded that the Weishan alkaline magma was derived from enriched asthenospheric mantle in an extensional setting after the transformation from a compressional to an extensional tectonic regime in the NCC during the Mesozoic.
Geochemical analyses indicate that syenite of the Weishan alkaline igneous complex is of the metaluminous-alkaline series. The syenites have high total REE (ΣREE) contents of 633-4418 ppm, distinct fractionation of heavy and light REE (HREE and LREE), depletion in HREE, Nb, Ta, and Zr, enrichment in LREE, Rb, Ba, and Sr, and both positive and negative Eu anomalies (Eu/Eu* = 0.90-1.05), consistent with characteristics of enriched asthenospheric mantle-derived melts [50].
Disseminated ore samples from networks of REE-bearing carbonate veinlets in the Weishan REE deposit are mainly massive and comprise calcite, barite, quartz, and REE minerals, such as bastnäsite, parisite, monazite, and ancylite. Several further typical ore samples from the ore veins were also selected for comparison ( Figure 3). The characteristics of all the samples in this study are listed in Table 2. The sample is massive, mainly composed of calcite, which drops violently when hydrochloric acid is applied on it, and the xenomorphic, granular, grey, oily luster quartz scattered within it, 3-5 mm in diameter; a little melanic mineral distributed in the form of ribbon-like, with the width of about 1-3 mm and good continuity. The other side of the hand specimen is mainly pink calcite, containing some quartz and dark mineral (5% to 8%). A small mineral of yellowish brown; sparsely disseminated in green minerals and pale pink calcite. The whole is light pale, bearing some white or red calcite and dark mineral, forming clear taxitic structure. The main mineral is light pale barite. Yellow brown bastnäsite are flaky and unevenly distributed in pale barite, with different sizes. Steel gray galena aggregates, content of 5% to 8%; a little red calcite distributed in pale barite; in addition, the visible part of anhedral granular grey quartz, diameter 2-4 mm, inclusion distribution in barite, content in 5% to 8%. A small amount of pyrite, granular, particle size 1-2 mm, 1% to 2%, a very small amount of Muscovite distribution in barite, particle size <3 mm.
Sample SDW16-6 comprises mainly pale-yellow barite and lesser brown bastnäsite, interspersed with unknown pale green crystals. Granular, xenomorphic, smoky gray quartz crystals (1-4 mm) are also randomly distributed. Some granular pyrite (~1 vol. %) occurs on bastnäsite surfaces (Figure 4f).  Previous petrological studies [14] and observation of the above typical disseminated ore specimens from the No. 1, No. 2, and No. 4 orebodies have allowed us to determine the paragenetic sequence of minerals within the Weishan deposits ( Figure 6), with distinct sequences being associated with an early magmatic stage and a later hydrothermal stage.

XRD Analyses
The XRD analyses of ore samples were performed at the Xi'an Geological Survey Center, Xi'an, using Rigaku D/Mac-RC and Cu Kα1 radiation with a graphite monochromator and continuous scanning under the following operating conditions: voltage 40 kV; beam current 80 mA; scanning speed 8 • min -1 ; slit DS = SS = 1 • ; ambient temperature 18 • C; humidity 30%. The contents (wt. %) of the primary mineral phases identified by the XRD were quantified using an internal standard, with corundum chosen as the reference material. The basic principles and procedures of quantitative analysis using the internal-standard method have been described in detail by [52,53] and the XRD result was listed in Table 3.

EPMA Analyses
Major elements compositions of minerals were determined using a JXA-8230 (Japan Electron Optics Laboratory Co.Lt, Akishima-shi, Japan) electron microprobe at the Institute of Mineral Resources, Chinese Academy of Geological Sciences. Mineralized and barren syenite and carbonatite and typical ore samples were selected from the Weishan REE deposit. A total of 11 samples were selected for systematic chemical analyses. Backscattered electron (BSE) images and mineral compositions were acquired at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), Beijing, China. The operating conditions were accelerating voltage of 15 kV for silicate and oxide, and 20 kV for sulfide, beam current of 20 nA, and beam size of 5 µm. Natural minerals and synthetic oxides were used as standards. Matrix corrections were carried out using the ZAF correction program supplied by the manufacturer [54].
For EPMA, the following standards were used: Jadetite, Na  Tables 4 and 5.

In Situ LA-ICP-MS Analyses
In situ trace element compositions of individual minerals were determined by an excimer 193 nm ArF Analyte Excite Laser ablation ICP-MS system, coupled to an Agilent 7700x at the FocuMS Technology Co. Ltd., Nanjing, China. This was carried out on the same spots which had been analyzed by EPMA, and the analyzed trace elements include Rb, Sr, Ba, Ca, Y, Th, U, Nb, Ta, Zr, Ce, La, Pr, Nd, Sm, Eu. Gd. Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf. The analyses' conditions involved a 7 Hz repetition rate and a beam diameter of 25−40 µm. In addition, BCR-2, BHVO-2, AVG-2, and RGM-2 glasses referred in United States Geological Survey were used as external calibration standards, and Chinese Geological Standard Glasses (CGSG)-1, -2, -4, and -5 (prepared by National Research Center for Geoanalysis, Beijing, China) were treated as quality control [55]. Raw data reduction was performed off-line by ICPMSDataCal software using 100%-normalization strategy without applying internal standard [56], and the results of the analyses are listed in Table 6.

Whole-Rock Geochemistry Analyses
Major and trace elements compositions of typical ores of Weishan REE deposit were analyzed by wet chemistry and X-ray fluorescence (XRF), ICP-MS, and ICP-atomic emission spectrometry, at the National Research Center of Geoanalysis, CAGS (Beijing, China). Whole-rock geochemical analyses were performed at the National Research Center of Geoanalysis, Chinese Academy of Geological Sciences (CAGS), Beijing, China. Whole-rock powder samples (0.7 g) were mixed with 5.3 g Li 2 B 4 O 7 , 0.4 g LiF, and 0.3 g NH 4 NO 3 in a 25 mL porcelain crucible. The powder mixture was transferred to a platinum alloy crucible, 1 mL LiBr solution was added to the crucible, and the sample was then dried. The sample was then melted in an automatic flame fusion machine and the resulting cooled glass was used for XRF analyses. Analytical errors were <2%. For trace-element analyses, whole-rock powder samples (50 mg) were dissolved in 1 mL HF and 0.5 mL HNO 3 in 15 mL Savillex Teflon screw-cap capsules at 190 • C for 1 day, dried, digested with 0.5 mL HNO 3 , and then dried again. The capsule content was digested with 0.5 mL HNO 3 and dried again to ensure complete digestion. Then the sample was digested with 5 mL HNO 3 and sealed at 130 • C in the oven for 3 h. After cooling, the solution was transferred to a plastic bottle and diluted to 50 mL before analysis. The sample solutions were analyzed by ICP-MS according to the procedure of State Standard of the Peoples Republic of China (GB): Methods for chemical analysis of silicate rocks-Part 30: Determination of 44 elements (2010). The analytical precision for most elements was better than 5%. To verify the accuracy of the procedure in this study, several standard samples of GBW07120, GBW 07103, GBW 07105, and GBW 07187 were also analyzed together with other samples. As a limestone sample GBW07120 that has low Zr (11 ppm) and Hf (0.22 ppm) content. The whole-rock major and trace element analyses results were listed in Table 7.

XRD and EPMA Analyses
XRD and EPMA analyses of typical disseminated ores from veinlets in the Weishan REE deposit indicate that the REE minerals are bastnäsite and parisite (Table 3) Table 3). The different ores have similar gangue and REE mineral assemblages, although the mineral proportions vary. All the REE and gangue mineral compositions of samples SDW16-1-6 were determined by EPMA, with results given in Tables 4 and 5.          (Table 4). In these samples, the gangue minerals are mainly calcite, barite, quartz, muscovite, dolomite, pyrite, sphalerite, galena, chalcopyrite, K-feldspar, and albite. Accessory minerals include rutile and titanite. Sulfides are widely distributed in the ores, and include pyrite, galena, pyrrhotite, chalcopyrite, and molybdenum sulfide ( Table 5).

Whole-Rock Geochemical Analyses
Major and trace element compositions of disseminated ores in veinlets are reported in Table 7 [50]; normalizing values are after [57]).

Mineralized Carbonatite in the Weishan Deposit
Carbonatite-related REE deposits with high REE grades are favored targets for exploration [7,12,[58][59][60][61][62]. Mineralized carbonatites that host REE deposits commonly have high REE, Ba, and Sr contents, but their fertility is not correlated with CaO/MgO or FeO/MgO ratios ( Figure 9). Ore contents of REE, Ba, and Sr in the Weishan deposit are higher than those of large REE deposits elsewhere, with Weishan data plotting in the mineralized field in REE-(CaO/MgO), REE-(FeO/MgO), REE-Ba, and REE-(Sr/Ba) diagrams ( Figure 9). The higher REE contents than those of Bayan Obo and Maoniuping carbonatites suggest that the Weishan carbonatite-syenite complex has the potential to host large or giant REE deposits, with mineralized carbonatites hosting REE deposits with both high and low REE grades.
Although the Weishan REE deposit has various types of wall rock (including gneiss), the main REE-bearing material is considered as originating directly from the carbonatite-syenite complex. Liang and Ying [50,63] studied the syenite and carbonatite separately. Weishan quartz syenite and aegirine-quartz syenite are characterized by high alkali contents, metaluminous composition, and relatively low Ti, Fe, Mg, and Mn contents, with alkaline affinities. There is strong fractionation of LREE and HREE, with REE patterns displaying enrichment in LREE and depletion in HREE, with small positive and negative Eu anomalies (Eu/Eu* = 0.90-1.05). The ores are rich in LILEs such as Rb and other elements such as Th and U and depleted in high-field-strength elements (HFSEs), such as Nb, Ta, Zr, and Hf (Figure 8a,b).  Table 4 in [12,63] [12,63]).
Fresh carbonatite in the Weishan REE deposit has not yet been studied, although the Laiwu-Zibo carbonatites (LZCs) from around the Weishan deposit have been studied [63]. The Weishan REE ores are characterized by low Cr contents and Zr and Hf anomalies, with enrichment in LILEs and HFSEs. They are enriched in LREEs (20,292 ppm) and depleted in HREEs (431-1003 ppm). Considering that the carbonatite-syenite complex and REE ores have similar REE patterns and trace element compositions, it is concluded that the carbonatite-syenite complex is the main contributor of disseminated REE ores, or that it crystallized in equilibrium with other types of ore. The Weishan REE ores also have high Sr (9235-97,760 ppm) and Ba (25,820-167,900 ppm) contents, consistent with other REE mineralized carbonatites [11,12].
Bastnäsite is the most important economic mineral in the deposit and occurs in a wide variety of mineral assemblages, with crystal lengths of 20-200 µm. Bastnäsite and parisite crystallized in equilibrium with calcite. Euhedral bastnäsite crystals formed coevally with parasite, and barite formed in a subsequent phase or in equilibrium with bastnäsite and parisite (Figure 5a). Parisite crystals have lengths of 10-100 µm, with most being in equilibrium with calcite, barite, and bastnäsite (Figure 5b). In general, parisite contains less amount than bastnäsite. Monazite occurs mainly as isolated elongate grains that are 50-150 µm diameter, with some being massive or occurring as columnar aggregates (Figure 5c). Biotite, quartz, calcite, phlogopite, and calcite were overprinted by or formed in equilibrium with monazite and celestite. K-feldspar and albite in ores occur as relicts overprinted by monazite.
Gangue minerals in the deposit include apatite, Ca-strontianite, calcite, quartz, barite, phlogopite, K-feldspar, albite and dolomite (Table 5; Figure 5). Apatite crystals (20-50 µm length) are euhedral and elongate and occur as aggregates with calcite and as overgrowths on other gangue minerals. Apatite coexists with Ca-strontianite, which is scarce, fractured, and occurs as overgrowths on calcite and other gangue minerals (Figure 5g). Ca-strontianite also occurs with other minerals, with lower La 2 O 3 and Ce 2 O 3 contents than those of parisite or bastnäsite and high CaO and SrO contents of 9.2-16.2 and 24.2-59.7 wt. %, respectively (Figure 5h).
Calcite is the most common ganue mineral, accounting for at least 30% of REE ores. It is translucent and white or pale pink and commonly occurs with euhedral bastnäsite. We infer that calcite formed after other minerals, such as barite and parasite, because it overgrows these minerals (Figure 5a,b,f). Barite (0.2-1.5 mm) is hypidiomorphic to euhedral and occurs with bastnäsite (Figure 5a,b,d,f). Quartz is common in all types of ore and occurs as hypidiomorphic, xenomorphic, and granular grains (Figure 5d-g).
K-feldspar and albite occur in some ores, mainly as relict grains (Figure 5c), indicating that gneiss, syenite, and other wall rocks underwent alteration. Petrographic studies and ore occurrences indicate that REE mineralization occurred through alteration of syenite or gneiss or other wall rocks, mainly in the later hydrothermal stage.
Sphalerite is the most widely distributed sulfide mineral, was generally formed in the later hydrothermal stage, and in most cases is associated with galena. It is irregular in shape and 200-900 µm in diameter. Galena occurs along sphalerite grain boundaries and phlogopite occurs within sphalerite-galena intergrowths (Figure 5g). Pyrite is often associated with other sulfide minerals and formed during late-stage hydrothermal alteration. Idiomorphic pyrite (200-800 µm) is intergrown with phlogopite and occurs with quartz and galena (Figure 5i). Petrographic studies indicate that pyrite and other sulfides may have precipitated after the gangue minerals.
Based on petrographic studies and the geological setting, REE mineralization occurred through the magmatic to hydrothermal stages. During the magmatic stage, quartz, zircon, apatite, K-feldspar, albite, arfvedsonite, aegirine, and magnetite formed in the syenite-carbonatite complex. In the hydrothermal stage, barite, calcite, quartz, muscovite, strontianite, titanite, and thorite formed as the main gangue minerals. REE minerals such as britholite, parisite, and bastnäsite formed during late-stage hydrothermal alteration. These minerals were overgrown by sphalerite, pyrite, and galena. Based on previous studies [15], the general sequence of mineral formation is shown in Figure 6, which indicates that all carbonatite-syenite-related REE deposits have similar formation processes. The magmatic, hydrothermal, and sulfide stages can be separated on the basis of the results of this study, and those of the Maoniuping, Dalucao, Lizhuang, and Muluozhai REE deposits [13,29,35]. For the magmatic stage, the deposits have mineral compositions similar to those of a carbonatite-syenite complex. During the later hydrothermal stage, gangue minerals such as fluorite, barite, calcite, quartz, muscovite, strontianite, titanite, thorite, arfvedsonite, and aegirine-augite formed. Due to the high degree of alteration, arfvedsonite and aegirine-augite are found only in the Maoniuping and Bayan Obo REE deposits [29,30,64]. The occurrence of arfvedsonite and aegirine-augite in the Weishan ores thus suggests a high degree of alteration in carbonatite-related REE deposits.
Hand-specimen examination, BSE images, and previous studies [14] indicate two main stages: a carbonatite-syenite complex stage, and a hydrothermal stage ( Figure 6). Relict K-feldspar, quartz, and other minerals in ores suggest the existence of a carbonatite-syenite complex stage, with barite, celestite, and aegirine-augite minerals forming in the hydrothermal stage.
Based on the relative timing of the veins and stages of mineralization, barite and fluorite commonly formed earlier than or in equilibrium with REE minerals, which supports the interpretation that both minerals may be important for REE mineral formation. This could explain why, in disseminated ores, less fluorite and barite occur together with minor bastnäsite (0.4% to 2.9%). Similar carbonatite-related REE ores occur along the Mianning-Dechang REE belt, such as weathered and brecciated ores of the Dalucao deposit, ore veins of the Maoniuping deposit, and disseminated ores of the Lizhuang deposit (Table 1) [13,29]. REE ores in veins including barite, fluorite, calcite, and bastnäsite commonly have high REE grades of >10%, whereas individual barite, fluorite, calcite, and bastnäsite grades may be <2%, further indicating the importance of fluoride and sulfate in REE mineral formation (Table 1).

Magmatic-Hydrothermal REE Enrichment
Two factors are required to form CARDs: REE enrichment of the mantle by subduction processes [12], and liberation and enrichment of REEs during magmatism and hydrothermal alteration. The carbonatite-syenite complex at Weishan appears to be mineralized, based on its REE content being similar to those of the Mianning-Dechang REE belt and Laiwu-Zibo carbonatites [12,63,66]. Furthermore, the Weishan complex has been regarded as the host rock for the REE deposits. The enrichment of REEs during hydrothermal alteration is also important, and some mineralized carbonatite-syenite complexes have limited grades. The occurrence of disseminated ores may explain the low grades in veinlets of such deposits. Based on petrographic studies of disseminated ores and mineral assemblages in various types of ore, it is likely that several factors cause the low grades.
Many studies have focused on factors controlling the transport and precipitation of REEs, which can be transported in a diverse range of high-temperature fluid systems that are rich in sulfate, carbonate, fluoride, chloride, and hydroxyl ions [60,67]. Experimental studies have shown that sulfate ions can form stable complexes with REEs and could be important for REE transport [58,68]. Sulfate complexes are particularly important for REE transport under high-temperature and mildly acidic to near-neutral pH conditions [69]. However, in disseminated ores, the low barite content indicates that sulfate ion and REE activity is very low in the ore-forming fluids. Carbonate ions are also thought to form strong complexes with REEs [68,70,71], and REE enrichment shows a positive correlation with CO 2 activity in some hydrothermal REE deposits such as the Bayan Obo and Maoniuping deposits in China, the Wicheeda deposit in Canada, and the Sin Quyen deposit in NW Vietnam [2,13,67,72].
Hydrothermal carbonate minerals such as calcite occur in ores in the Weishan deposit, indicating high CO 2 activity in the ore-forming fluids. Among the ligands mentioned above, REEs form their strongest complexes with fluoride ions [58,68,71]. Furthermore, in many carbonatite-related hydrothermal REE deposits, F-rich hydrothermal minerals, such as fluorite, fluorapatite, and other F-bearing minerals, indicate abundant fluoride in the ore-forming fluids [7,9,73]. In the disseminated ores of the Weishan deposit, only fluorapatite and phlogopite contain detectable F (but at low concentrations), indicating that the ore-forming fluids had low Fconcentrations. Moreover, fluorite and REE fluorocarbonates are rare, despite the high REE and Ca activities of the ore-forming fluids. This indicates that although F − was present in the fluids, its activity was relatively low, resulting in reduced REE transport in the disseminated ores of the Weishan deposit.
Mineralized carbonatites that host REE deposits commonly have high REE, Ba, and Sr contents [12]. Concentrations of REEs, Ba, and Sr at Weishan are higher than those of giant or large REE deposits elsewhere in the world, with data for Weishan plotting in the mineralized field in REE-(CaO/MgO), REE-(FeO/MgO), REE-Ba, and REE-(Sr/Ba) diagrams [12,13]. The high REE contents suggest that the carbonatite-syenite complex at Weishan has the potential to host large or giant REE deposits.
In the ore area, four groups of fractures controlled ore formation, with NW-SE-and NE-SW-trending fractures being associated with REE mineralization [15]. During early stages of mineralization, local tectonic activity produced fractures or fissures in carbonatite-syenite complexes, facilitating fluid cycling and the modification of fluid chemistry by water-rock interactions. Circulation of ore fluids in fractures drove alteration within the carbonatite and led to high concentrations of REEs, F − , Cl − , CO 2 , SO 4 2− , and volatiles in the fluids. Deposits within the Mianning-Dechang REE belt indicate structural controls and exhibit a range of ore types associated with fluid infiltration, such as brecciated ores and ore veins. In contrast, in the Lizhuang deposits, where weaker tectonic activity is recorded, only disseminated ores have been found. Weak tectonic activity thus inhibits hydrothermal alteration and restricts large-scale REE mineralization. Tectonic activity also drives the infiltration of meteoric water, which reduces temperature, thereby facilitating the precipitation of REE-bearing minerals [60,74]. The occurrence and mineral assemblages of the ore veins and veinlets indicate that disseminated ores in veinlets experienced less tectonic activity, which would have hindered the circulation of hydrothermal fluids (Figure 3c,d).
Certain ligands in solution are necessary for REE transport or precipitation, and mechanisms of REE enrichment by transportation and precipitation are a major focus of current research into REE deposits. Laboratory experiments, theoretical studies, and analyses of natural examples suggest that REEs can be transported in a diverse range of high-temperature fluid systems that are rich in sulfate, carbonate, fluoride, chloride, and hydroxyl ions [32,33,60,75]. Sulfate and fluoride ions form stable complexes with REEs and could be important for REE transport and precipitation, respectively [60]. Carbonate ions are also known to form strong complexes with REEs [58,71], and the positive correlation between REE enrichment and CO 2 activity has been well established in hydrothermal carbonatite-related deposits such as at Bayan Obo and Maoniuping in China, the Wicheeda deposit in Canada, and the Sin Quyen deposit in NW Vietnam [2,13,67,72]. Moreover, REE sulfate and carbonate complexes are particularly important in high-temperature and mildly acidic to near-neutral pH conditions [60,69,76]. Large and giant REE deposits generally contain large amounts of fluorine-and sulfate-bearing minerals such as fluorite, fluorapatite, barite, and celestite as the main gangue minerals. The abundance of these minerals implies high fluoride and sulfate contents in the ore-forming fluids [7,13]. However, fluorite and barite are rare in disseminated ores ( Table 5), suggesting that fluorine and sulfate contents were low in the ore-forming fluids, with a consequent reduction in transport and precipitation of REEs. We conclude that low concentrations of suitable ligands for REEs is another factor that inhibited REE mineralization at Weishan.

Conclusions
(1) Based on petrographic studies of disseminated ores in the veinlets in the Weishan deposit, REE mineral formation is inferred to have occurred during the late stage of hydrothermal evolution, as REE minerals occur in equilibrium with or formed after gangue minerals such as barite, calcite, and quartz, which were overprinted by REE minerals.
(2) Petrographic, XRD, and geochemical data indicate that the disseminated ores have lower grades, with less barite and fluorite than the vein and veinlet ores in the same deposit. In contrast, ore veins with high grades have high barite, fluorite, calcite, and bastnäsite contents.
(3) Several factors controlled the low grades of the disseminated ores in the veinlets. Relatively limited tectonic activity reduced the circulation of fluids, and Fand SO 4 2-, which facilitated the transport and precipitation of REE, were present in only minor amounts in the ore-forming fluids. Our examination of the massive ores explains not only the reasons for their low grades but also the reasons for their differences from other REE deposits.