Genetic Relationship between Granite and Fluorite Mineralization in the Shuanghuajiang Fluorite Deposit, Northern Guangxi, South China: Evidence from Geochronology, REE, and Fluid Geochemistry

Hydrothermal vein-type fluorite deposits are the most important metallogenic type of fluorite deposits in South China, most of which are closely related to granitoid in space; however, the genetic relationship between granitoid and fluorite mineralization remains controversial. The Shuanghuajiang fluorite deposit in northern Guangxi of South China is a typical vein-type fluorite deposit hosted in a granite pluton, with the orebodies occurring within brittle faults. Zircon UPb dating of the hosting Xiangcaoping granite yields an emplacement age of 228.04 ± 0.76 Ma (MSWD = 0.072). Fluorite Sm-Nd dating yields an isochron age of 185 ± 18 Ma. The new age data indicate that the fluorite deposit was precipitated significantly later than the emplacement of the hosting Xiangcaoping granite pluton. The fluorite and granite exhibit similar rare earth element (REE) patterns with negative Eu anomalies, suggesting that fluorine (F) was derived from the granite. The fluorite fluid inclusions show a homogeneous temperature mainly ranging between 165 ◦C and 180 ◦C. Salinity is typically less than 1% NaCl eqv, while the δOV-SMOW and δDV-SMOW values are between −5.2‰–−6.1‰ and −17.35‰–−23.9‰, respectively. These indicate that the ore-forming fluids were a NaCl-H2O system with medium-low temperature and low salinity, which is typical for meteoric water. Given the combined evidence of geochronology, REE, and fluid geochemistry, the mineralization of the Shuanghuajiang fluorite deposit is unrelated to magmatic-hydrothermal activity but achieved via hydrothermal circulation and leaching mechanisms. Our study presents a genetic relationship between the fluorite deposit and granitoids based on an example of northern Guangxi, providing a better understanding of the genesis of hydrothermal vein-type fluorite deposits in granitoids outcropping areas.


Introduction
As an important mineral raw material for modern industry, fluorite is widely used in aerospace engineering, chemical engineering, and metallurgy [1][2][3]. Much attention has been paid to fluorite in recent years, and countries and organizations such as China, the United States, Australia, and the European Union, classify it as a strategic and critical mineral [4][5][6]. South China is tectonically formed by the amalgamation of the Yangtze Block and Cathaysia Block [7,8] (Figure 1a) and is rich in fluorite resources, accounting for more than 80% of the total known fluorite reserve in China [9]. Fluorite deposits in South China can be classified into three major types: (1) hydrothermal fluid filling type;  [28,30]); (c) Geological map of Miaoershan-Yuechengling district (modified after [28]).  [28,30]); (c) Geological map of Miaoershan-Yuechengling district (modified after [28]).

Regional Geologic Setting
The Miaoershan region of northern Guangxi, where the Shuanghuajiang fluorite deposit is located, is tectonically a part of the southeastern margin of the Yangtze block [31]. Stratified rocks mainly include the Neoproterozoic Gongdong Formation of low-grade metamorphic sandstone and slate with a small amount of tuff, the Nanhua System of conglomerate-bearing argillaceous sandstones, shales, and siliceous rocks, the Sinian System of carbonaceous slate interbedded with microcrystalline dolomites, Cambrian quartz sandstones, Ordovician argillaceous siltstone and limestones, and Cretaceous pebbly sandstone ( Figure 2a).

Deposit Geology
The stratified rocks in the Shuanghuajiang deposit include the Neoproterozoic Gongdong Formation and the Nanhua System. The Nanhua System has a parallel unconformity above the Gongdong Formation. The Gongdong Formation can be separated into three parts: (1) the upper part is composed of low-grade meta-sandstone and shale with calcareous bands; (2) the middle part is composed of low-grade metamorphic shale and feldspathic quartz sandstone; (3) the lower part consists of low-grade metamorphic carbonaceous shale and locally calcareous dolomite. The Nanhua System can be separated into two parts: (1) the upper part consists of sandstone, dolomitic limestone, and argillaceous limestone; (2) the lower part is composed of argillaceous sandstone and feldspathic quartz sandstone (Figure 2b).
Major faults in the Shuanghuajiang area are the NE-striking faults, with F1, F2, F3, and appeared as silicified crushing dikes, in which fluorite ore bodies hosted. The late faults developed along the sides of the silicified crushing dikes formed earlier, and partially destroyed it.
The Miaoershan region has experienced several significant tectonic-magmatic events [28,32,33]. The NNE-trending Miaoershan batholith is the largest granitoid pluton in the region. The batholith was initially emplaced in the Caledonian time and expanded with additional granitoid intrusions during the Indosinian and Yanshanian orogenies [34][35][36]. The Caledonian granitoids are the main phase of the batholith and consist mainly of medium-grained porphyritic biotite granite, fine-to medium-grained, and coarse-to medium-grained biotite monzogranite [37]. The Indosinian granitoids occur as stocks within the Caledonian granitoids, represented by the Xiangcaoping granite [38,39]. The Indosinian granitoids are mainly medium-to coarse-grained porphyritic biotite granites and locally medium-to coarse-grained two-mica granite.

Deposit Geology
The stratified rocks in the Shuanghuajiang deposit include the Neoproterozoic Gongdong Formation and the Nanhua System. The Nanhua System has a parallel unconformity above the Gongdong Formation. The Gongdong Formation can be separated into three parts: (1) the upper part is composed of low-grade meta-sandstone and shale with calcareous bands; (2) the middle part is composed of low-grade metamorphic shale and feldspathic quartz sandstone; (3) the lower part consists of low-grade metamorphic carbonaceous shale and locally calcareous dolomite. The Nanhua System can be separated into two parts: (1) the upper part consists of sandstone, dolomitic limestone, and argillaceous limestone; (2) the lower part is composed of argillaceous sandstone and feldspathic quartz sandstone Two granitic plutons, the Miaoershan and Xiangcaoping plutons, are exposed in the deposit and vicinity (Figure 2b). The Shuanghuajiang fluorite orebodies are hosted by the Xiangcaoping pluton (Figure 2a,b). The Xiangcaoping pluton is mainly pale gray, medium-to coarse-grained biotite granite. It is composed of quartz (35%-40%), plagioclase (30%-35%), K-feldspar (20%-25%), and biotite (5%) (Figure 3a). The accessory minerals include zircon, apatite, tourmaline, and pyrite. Quartz shows a xenomorphic granular texture. Plagioclase and K-feldspar usually occur as subidiomorphic plate strips, showing polysynthetic twin and grid twin under microscope, respectively. Biotite is fine-grained and flaky, partly replaced with chlorite and muscovite (Figure 3b).   fault zone and the orebodies contain some fragments of granite that are cemented by silica (Figure 3d), indicating that the orebodies were formed after the granite. The orebodies are up to 720 m long along the strike, with thicknesses ranging between 1.5-3.5 m. The ore grade is 49.5%-74.5% fluorite. The thickness and ore grade of the orebodies vary considerably along the strike and dip, and include mainly fluorite, chalcedony, and quartz. Fluorite is mainly purple and green in color (Figure 3e,f), showing coarse-grained euhedral to subeuhedral textures or fine-grained subeuhedral to anhedral textures. Ore structures include massive, banded, brecciated, and stockwork structures. In places, the fluorite crystal fragments are cemented by chalcedony (Figure 3g), or chalcedony fragments are cemented by fluorite (Figure 3h). Wallrock alteration is dominated by silicification, followed by chloritization and sericitization. Generally, silicification is most closely related to fluorite mineralization, and as a matter of fact, all orebodies occur in the silicification zones.

Sampling and Analytical Methods
All the samples used for geochronological and geochemical analyses in this study were collected from the 520 m gallery (mean sea level) of orebody II in the Shuanghuajiang fluorite deposit. A total of six samples from the Xiangcaoping granite were selected for zircon U-Pb geochronological and REE geochemical analysis. Nine fluorite samples were selected for REE geochemical, Sm-Nd isopotic, fluid inclusion, and H-O isotopic analyses.
Zircon U-Pb dating was performed at Guangxi Key Laboratory of Hidden Metallic Ore Exploration, Guilin University of Technology, using an NWR-193 laser ablation system, and coupled with an Agilent 7500cx ICP-MS. High purity helium was used as the carrier gas for exfoliated material during the analysis. Laser beam spot diameter, pulse frequency, and energy density were 32 µm, 8 Hz, and 15 J/cm 2 , respectively. Zircon SRM610 and GJ-1 were used as standards. ICPMSDataCal software was used to process the data, and age calculation and Concordia diagrams were prepared using the ISOPLOT program [40].
Fluorite Sm-Nd analysis was carried out at the Beijing Institute of Uranium Geology, Beijing, using an ISOPROBE-T thermal ionization mass spectrometer (TIMS) following the procedure given by [41]. Sm and Nd contents have an analytical error within 0.5%, and 147 Sm/ 144 Nd (2σ) has an error of ±0.5%. The analytical result of the standard sample BCR-l is Sm = 6.571 µg/g, Nd = 28.753 µg/g, and 143 Nd/ 144 Nd = 0.512644 ± 0.000005 (2σ), consistent with the reported values of 6.58 ppm for Sm and 28.8 ppm for Nd [42]. The Sm-Nd isochron ages were calculated using the ISOPLOT program [40].
The REE geochemical analysis of granite and fluorite samples was performed at Qingdao Sparta Analysis and Test Co. Ltd., Qingdao, China. Following removal of the weathered rims, fresh biotite granite samples were crushed and grounded to~200 mesh in an agate ring mill. The fluorite mineral separates were handpicked under a microscope, with a purity of above 99%, washed with distilled water, dried at a low temperature, and powered to 200 mesh in an agate mortar. The REE analyses were performed by an Agilent 7500a ICP-MS after acid digestion of the sample powders in high-pressure Teflon bombs, with the detailed procedure described by [43].
Fluid inclusion micro-thermometry and in-situ Raman spectroscopic analysis were performed at Guangxi Key Laboratory of Hidden Metallic Ore Exploration, Guilin University of Technology. The Linkam THMSG-600 Cooling-Heating Stage coupled to a polarizing microscope was used for inclusion observation and the heating-cooling process. The temperature range was −196 • C to 600 • C and the freezing and heating rate was 0.1-130 • C/min with a precision of 0.1 • C. The heating rate near the phase transformation point was controlled between 0.5-1 • C/min. The precision is estimated as ±0.1 • C when it comes to the temperature measurement during the observation of the phase transformation. In situ Raman spectroscopic analysis of fluorite inclusions was performed using a Renishaw inVia microscopic confocal laser Raman spectrometer. The light source used in the instrument is the SpectraPhysics argon ion laser. The laser excitation wavelength was 514.5 nm, the laser power was 20 mW, the scanning range was 100-4500 cm −1 , the minimum diameter of the laser beam was 1 µm, and the spectral resolution was 1-2 cm −1 . Double polished wafers were used for the analyses.
Hydrogen and oxygen isotopic measurements of pure fluorites were performed at the Beijing Institute of Uranium Geology, Beijing. Hydrogen was measured for vapors re-leased from fluid inclusions in fluorite grains by thermal decrepitation and reaction with zinc powder. Oxygen isotope compositions from fluid inclusions in fluorite samples were analyzed based on the BrF5 extraction technique [44]. Hydrogen and oxygen isotopic were analyzed on a Finnigan MAT-253 isotope ratio mass spectrometer, with analytical precisions of ±3‰ for hydrogen isotope and ±0.2‰ for oxygen isotope. The Vienna standard mean ocean water (V-SMOW) was used as the standard.

Zircon U-Pb Dating
Cathodoluminescence (CL) images ( Figure 4) show that the zircon grains are 80-120 µm in diameter, with a smooth and flat surface; some of them have fractures. The zircon grains are mostly long prismatic or short stubby, with some equidimensional in shape. Their length/width ratios range from 1:1 to 2:1. They show clear oscillatory zones. A few zircon grains show a core-edge structure. Every effort was made to avoid ablating the inherited cores during the analytical process. analyzed on a Finnigan MAT-253 isotope ratio mass spectrometer, with analytical precisions of ±3‰ for hydrogen isotope and ±0.2‰ for oxygen isotope. The Vienna standard mean ocean water (V-SMOW) was used as the standard.

Zircon U-Pb Dating
Cathodoluminescence (CL) images ( Figure 4) show that the zircon grains are 80-120 μm in diameter, with a smooth and flat surface; some of them have fractures. The zircon grains are mostly long prismatic or short stubby, with some equidimensional in shape. Their length/width ratios range from 1:1 to 2:1. They show clear oscillatory zones. A few zircon grains show a core-edge structure. Every effort was made to avoid ablating the inherited cores during the analytical process. Table 1 shows the isotopic data obtained from the LA-ICP-MS U-Pb zircon analysis. A total of 27 zircon grains were analyzed. Their Th and U contents fall in the range of 337.4 ppm-1710.3 ppm and 1502.4 ppm-7678.5 ppm, respectively, with Th/U ratios in the range of 0.11-0.59, indicating a magmatic origin [45,46]. Twenty-seven spots yielded a coherent cluster with the 206 Pb/ 238 U weighted age of 228.04 ± 0. 76 Table 1 shows the isotopic data obtained from the LA-ICP-MS U-Pb zircon analysis. A total of 27 zircon grains were analyzed. Their Th and U contents fall in the range of 337.4-1710.3 ppm and 1502.4-7678.5 ppm, respectively, with Th/U ratios in the range of 0.11-0.59, indicating a magmatic origin [45,46]. Twenty-seven spots yielded a coherent cluster with the 206 Pb/ 238 U weighted age of 228.04 ± 0.76 Ma (MSWD = 0.072) ( Figure 5).

Fluorite Sm and Nd Isotopic Analysis
A total of four samples were used for Sm-Nd radiometric dating; all of the samples were collected from orebody II of the Shuanghuajiang fluorite deposit. The analytical data of Sm-Nd isotopes are given in Table 2, and the isochron plot is shown in Figure 6; Sm and Nd concentrations of fluorite range from 0.556 to 2.28 ppm and from 4.1 to 9.02 ppm, respectively. The 147 Sm/ 144 Nd and 143 Nd/ 144 Nd ratios vary from 0.0813 to 0.1526 and from 0.511887 to 0.511976, respectively. The 143 Nd/ 144 Nd initial ratio is 0.511787 ± 0.000015. Isoplot was utilized here for age calculation. The Sm-Nd isotopic data of the four samples define an isochron age of 185 ± 18 Ma (MSWD = 1.6). were collected from orebody II of the Shuanghuajiang fluorite deposit. The analytical data of Sm-Nd isotopes are given in Table 2, and the isochron plot is shown in Figure 6; Sm and Nd concentrations of fluorite range from 0.556 to 2.28 ppm and from 4.1 to 9.02 ppm, respectively. The 147 Sm/ 144 Nd and 143 Nd/ 144 Nd ratios vary from 0.0813 to 0.1526 and from 0.511887 to 0.511976, respectively. The 143 Nd/ 144 Nd initial ratio is 0.511787 ± 0.000015. Isoplot was utilized here for age calculation. The Sm-Nd isotopic data of the four samples define an isochron age of 185 ± 18 Ma (MSWD = 1.6).

Rare Earth Elements (REE)
The classification of REE in this paper adopts the dichotomous method. Light rare earth elements (LREE) include La~Eu, and heavy rare earth elements (HREE) include Gd~Lu. REE analytical results of the Xiangcaoping granite samples are provided in Table  3. Their chondrite normalized REE patterns are shown in Figure 7a. The REE distribution patterns are consistent. Further, all show a right-leaning feature and an obvious Eu negative anomaly. The total REE content in the granite samples ranges from 110.57 to 159.43 ppm (exclude Y), the LREE/HREE ratio is 8.17-11.89, and the (La/Yb)N ranges from 9.14 to 18.73, indicating a high degree of LREE and HREE fractionation and a relative enrichment of LREE. The (La/Sm)N ranges from 3.0 to 3.53, indicating fractionation of the LREE. The (Gd/Yb)N ranges from 1.83 to 3.09, indicating fractionation of the HREE. The δEu value ranges from 0.20 to 0.42, showing a strong negative Eu anomaly.

Rare Earth Elements (REE)
The classification of REE in this paper adopts the dichotomous method. Light rare earth elements (LREE) include La~Eu, and heavy rare earth elements (HREE) include Gd~Lu. REE analytical results of the Xiangcaoping granite samples are provided in Table 3. Their chondrite normalized REE patterns are shown in Figure 7a. The REE distribution patterns are consistent. Further, all show a right-leaning feature and an obvious Eu negative anomaly. The total REE content in the granite samples ranges from 110.57 to 159.43 ppm (exclude Y), the LREE/HREE ratio is 8.17-11.89, and the (La/Yb) N ranges from 9.14 to 18.73, indicating a high degree of LREE and HREE fractionation and a relative enrichment of LREE. The (La/Sm) N ranges from 3.0 to 3.53, indicating fractionation of the LREE. The (Gd/Yb) N ranges from 1.83 to 3.09, indicating fractionation of the HREE. The δEu value ranges from 0.20 to 0.42, showing a strong negative Eu anomaly.
The REE analytical results of fluorite samples are given in Table 3. Their chondrite normalized REE patterns are shown in Figure 7b. The distribution patterns of the REE and Eu negative anomalies are similar to that of the biotite granite samples. In comparison with the Xiangcaoping granite samples, the total REE content in the fluorite samples is substantially lower, ranging from 27. 55

Fluid Inclusions
Primary fluid inclusions contained within the fluorite samples (samples SHJ01-SHJ04) were analyzed in this study. Microscopic observation reveals that the fluid inclusions are all bi-phase types, containing liquid and vapor (L + V type) with a general size of 5-10 µm and a vapor-liquid ratio of 20%-35% (Table 4). Morphologically, the fluid inclusions are subcircular, elongated, square, or irregular. They are randomly distributed within the fluorite crystals or occur in groups or zones, typical for primary fluid inclusions (Figure 8). During the heating process, the fluid inclusions were homogenized to the liquid phase. The homogenization temperature ranges widely from 126 • C to 281 • C with a peak at around 165 • C to 180 • C (Figure 9). According to the revised equation of the H 2 O-NaCl system [47], the calculated salinities range from 0.18% to 7.86% NaCl eqv, with an average of 1.99% NaCl eqv, and a peak in the range of less than 1% NaCl eqv. Laser Raman spectroscopic analysis indicates that the vapor and liquid compositions of the inclusions are dominated by H 2 O, along with minor amounts of CO 2 present (Figure 10).
The REE analytical results of fluorite samples are given in Table 3. Their chondrite normalized REE patterns are shown in Figure 7b. The distribution patterns of the REE and Eu negative anomalies are similar to that of the biotite granite samples. In comparison with the Xiangcaoping granite samples, the total REE content in the fluorite samples is substantially lower, ranging from 27.55 to 123.48 ppm (exclude Y). The LREE/HREE ratio is 4.50-11.75. The (La/Yb)N ranges from 4.74 to 16.31, indicating a high degree of LREE and HREE fractionation and a relative enrichment of LREE. The (La/Sm)N value ranges from 2.15 to 4.49, indicating fractionation of the LREE. The (Gd/Yb)N value ranges from 1.77 to 2.52, indicating fractionation of the HREE. The δEu value ranges from 0.42 to 0.53, showing a strong negative Eu anomaly.

Fluid Inclusions
Primary fluid inclusions contained within the fluorite samples (samples SHJ01-SHJ04) were analyzed in this study. Microscopic observation reveals that the fluid inclusions are all bi-phase types, containing liquid and vapor (L + V type) with a general size of 5-10 μm and a vapor-liquid ratio of 20%-35% (Table 4). Morphologically, the fluid inclusions are subcircular, elongated, square, or irregular. They are randomly distributed within the fluorite crystals or occur in groups or zones, typical for primary fluid inclusions (Figure 8). During the heating process, the fluid inclusions were homogenized to the liquid phase. The homogenization temperature ranges widely from 126 °C to 281 °C with a peak at around 165 °C to 180 °C (Figure 9). According to the revised equation of the H2O-NaCl system [47], the calculated salinities range from 0.18% to 7.86% NaCl eqv, with an average of 1.99% NaCl eqv, and a peak in the range of less than 1% NaCl eqv. Laser Raman spectroscopic analysis indicates that the vapor and liquid compositions of the inclusions are dominated by H2O, along with minor amounts of CO2 present ( Figure 10).

H-O Isotopic Compositions of the Fluid Inclusions
The oxygen and hydrogen isotopic compositions of the liquids from the primary fluid inclusions (for samples SHJ01, SHJ03, SHJ04, and SHJ07) are presented in Table 5. The δDV-SMOW values range between −17.35‰ and −23.9‰. The δ 18 OV-SMOW value ranges between −5.2‰ and −6.1‰. Since fluorite itself does not contain hydrogen and oxygen, no O and H isotopic exchange exists between fluorite and the fluids. Thus, the isotopic composition of water in various geological environments can be measured directly in the water of fluid inclusions in minerals such as fluorite [48,49].

H-O Isotopic Compositions of the Fluid Inclusions
The oxygen and hydrogen isotopic compositions of the liquids from the primary fluid inclusions (for samples SHJ01, SHJ03, SHJ04, and SHJ07) are presented in Table 5. The δDV-SMOW values range between −17.35‰ and −23.9‰. The δ 18 OV-SMOW value ranges between −5.2‰ and −6.1‰. Since fluorite itself does not contain hydrogen and oxygen, no O and H isotopic exchange exists between fluorite and the fluids. Thus, the isotopic composition of water in various geological environments can be measured directly in the water of fluid inclusions in minerals such as fluorite [48,49].

H-O Isotopic Compositions of the Fluid Inclusions
The oxygen and hydrogen isotopic compositions of the liquids from the primary fluid inclusions (for samples SHJ01, SHJ03, SHJ04, and SHJ07) are presented in Table 5. The δD V-SMOW values range between −17.35‰ and −23.9‰. The δ 18 O V-SMOW value ranges between −5.2‰ and −6.1‰. Since fluorite itself does not contain hydrogen and oxygen, no O and H isotopic exchange exists between fluorite and the fluids. Thus, the isotopic composition of water in various geological environments can be measured directly in the water of fluid inclusions in minerals such as fluorite [48,49].

. Timing of Granitic Emplacement and Fluorite Mineralisation
The emplacement age of the Xiangcaoping granite pluton, which hosts the Shuanghuajiang fluorite deposit, has been previously dated. For example, Xie et al. [50] obtained a zircon SHRIMP U-Pb age of 228 ± 11 Ma. Hu et al. [31] performed U-Pb isotope analysis for accessory minerals monazite and xenotime contained in the Xiangcaoping granite and obtained ages of 220 ± 6 Ma and 211 ± 7 Ma. Wang [39] reported a U-Pb age of 223.9 ± 1.3 Ma. The new zircon U-Pb age obtained in this study is 228.04 ± 0.76 Ma, which is consistent with the reported ages within the acceptable error range, confirming that the Xiangcaoping granite was emplaced in the Indosinian time.
The fluorite deposits in northern Guangxi lack direct geochronological data. Using the fluorite Sm-Nd isotope dating method, for the first time, this study reports a mineralization age of 185 ± 18 Ma (MSWD = 1.6) for the Shuanghuajiang fluorite deposit, which is significantly later than the emplacement of the Xiangcaoping granite (228.04 ± 0. 76 Ma). This significant time lag is consistent with the geological fact that the orebodies are mainly present as veins in the granite (Figure 3c) and fragments of granite are cemented by silica or fluorite (Figure 3d). The time lag between fluorite mineralization and granite emplacement exists objectively in South China [51,52]. For example, at the Xiefang fluorite deposit of southern Jiangxi Province and the Daoji fluorite deposit of Guangdong Province, the Cretaceous Formation has a parallel unconformity above the early Yanshanian biotite granite and contains many fragments of granite, and the fluorite veins cut across the biotite granite and extend into the Cretaceous Formation [14,51,53]. At the Beishi largescale fluorite deposit, the largest fluorite deposit in Guangxi Province, the fluorite veins were found to cut across the early Indosinian granite and extend into the Cretaceous Formation, which represents unconformable contact with the granite [30]. The above geological findings further indicate that such vein-like and granite-related fluorite deposits were formed significantly later than the solidification of the intrusive granite magma.
In addition, the Miaoershan and Yuechengling batholith contains a number of polymetallic deposits (W, Sn, Mo, Pb, Zn, etc.), which are closely related to Caledonian and Indosinian magma intrusion (Figure 2b) [54][55][56]. For instance, Li et al. [54] obtained molybdenite Re-Os ages of 226.2 ± 4.1 Ma and 219.3 ± 4.0 Ma, and zircon U-Pb age of ore-related granites of 216.8 ± 4.9 Ma at the Yuntoujie W-Mo deposit. Yang et al. [35] reported two zircon U-Pb ages of 215.0 ± 1.5 Ma and 212.3 ± 1.2 Ma from the Youmaling W deposit. Zhang et al. [57] reported scheelite Sm-Nd isochron age of 212 ± 20 Ma and ore-related granites zircon U-Pb ages of 224.9 ± 1.4 Ma and 220.2 ± 1.6 Ma from the Gaoling W deposit. In Yuechengling batholith, Chen et al. [58] obtained scheelite Sm-Nd isochron age of 417 ± 35 Ma, and zircon U-Pb age of ore-related granites of 423~421 Ma at the Dushiling W-Cu deposit. Based on the published data, the mineralization of these metallic deposits and related magmatic activities are significantly earlier than the mineralization of the Shuanghuajiang fluorite deposits, which may also prove that the mineralization of the Shuanghuajiang fluorite deposits is different from the nearby metal deposits and is unrelated to magmatic activities. The reason can be that the magmatic activity and subsequent magma solidification have a higher surrounding temperature, while the chemical properties of the F are active and volatile. F is easy to migrate and disperse towards lower temperature and pressure; therefore, it is difficult to enrich and mineralize independently in magmatic-hydrothermal systems [51,59].
Based on the mineralization ages and regional tectonic setting, it has been shown that South China experienced the early Yanshanian continental extension and crustal thinning process [60][61][62][63]. The Miaoershan region in northern Guangxi was transformed into a postorogenic extensional tectonic environment in the early Jurassic time after experiencing a post-collision environment with weakened compression and stress relaxation [64,65]. With the extension and thinning of the lithosphere, a series of faults were formed in the region, and the faults facilitated fluorite mineralization.

Ore-Forming Fluids
The fluid inclusion data of fluorite collected from the Shuanghuajiang fluorite deposit show that the ore-forming fluids have low homogenization temperatures and low salinities, which can only be formed due to the introduction of a large amount of atmospheric meteoric water. Another possibility is that the fluids were heated meteoric water [66,67]. The results of laser Raman spectroscopy show that the chemical composition of the fluid inclusions is mainly H 2 O with small amounts of CO 2 (Figure 11), implying a single source of mineralizing fluids.
The geochemical behaviors of Eu and Ce in fluorite are closely related to pH, Eh, material composition, temperature, and other characteristics of the ore-forming fluids, which are often used to indicate the redox conditions and temperature of the mineralizing fluids [68][69][70]. Under the same redox conditions, Eu and Ce show different valence states. For example, under reducing conditions, Eu exists in a divalent state, whereas Ce exists in a trivalent state. Since the ion radius of Eu 2+ (1.33 Å) is larger than that of Ca 2+ (1.2 Å), it is difficult to replace Ca 2+ in fluorite, which increases the loss of Eu in fluorite and makes fluorite a strong negative Eu anomaly. The negative anomaly of Ce can only occur under oxidation conditions, due to the presence of Ce in the tetravalent form under oxidizing conditions; however, Ce 4+ has little solubility in the fluid and is easily adsorbed out of the fluid by hydroxide, thus forming a Ce-deficient fluid and causing the material crystallized from it to exhibit a Ce negative anomaly [12,71]. The fluorite samples of the Shuanghuajiang fluorite deposit show strong negative Eu anomalies and no or only slightly positive Ce anomalies (Figure 7b), indicating that the fluorite was formed under relatively reducing conditions. The Tb/Ca vs. Tb/La bivariate diagram can be used to effectively discriminate the genetic type of fluorite [72,73]. Based on the atomic ratios of Tb/Ca and Tb/La, the diagram is divided into three genetic fields: pegmatitic, hydrothermal, and sedimentary. The fluorite samples are all plotted in the hydrothermal field (Figure 11), indicating that the Shuanghuajiang fluorite deposit is a hydrothermal deposit, consistent with many granite-related fluorite deposits found in South China. The δ 18 OV−SMOW and δDV−SMOW values can be projected onto a δD-δ 18 O diagram to further understand the source characteristics of ore-forming fluids [75]. The δ 18 OV-SMOW value of the fluorite samples from the Shuanghuajiang fluorite deposit ranges from −5.2‰ to −6.1‰, and the δDV-SMOW value ranges from −17.35‰ to −23.9‰, with a narrow variation range, indicating a relatively singular source for the ore-forming fluids [76]. All the δ 18 OV-SMOW and δDV-SMOW values fall near the meteoric water line (Figure 12), implying that the ore-forming fluids were mainly meteoric water and not derived from the crystallizing granite, even though the hosting rock is granite. From published H-O isotope data, meteoric water was the source of ore-forming fluids for many granite-related fluorite deposits in South China [52,[77][78][79]. The δ 18 O V−SMOW and δD V−SMOW values can be projected onto a δD-δ 18 O diagram to further understand the source characteristics of ore-forming fluids [75]. The δ 18 O V-SMOW value of the fluorite samples from the Shuanghuajiang fluorite deposit ranges from −5.2‰ to −6.1‰, and the δD V-SMOW value ranges from −17.35‰ to −23.9‰, with a narrow variation range, indicating a relatively singular source for the ore-forming fluids [76].
All the δ 18 O V-SMOW and δD V-SMOW values fall near the meteoric water line (Figure 12), implying that the ore-forming fluids were mainly meteoric water and not derived from the crystallizing granite, even though the hosting rock is granite. From published H-O isotope data, meteoric water was the source of ore-forming fluids for many granite-related fluorite deposits in South China [52,[77][78][79].
of the fluorite samples from the Shuanghuajiang fluorite deposit ranges from −6.1‰, and the δDV-SMOW value ranges from −17.35‰ to −23.9‰, with a narrow range, indicating a relatively singular source for the ore-forming fluids [76]. All SMOW and δDV-SMOW values fall near the meteoric water line (Figure 12), implyin ore-forming fluids were mainly meteoric water and not derived from the cry granite, even though the hosting rock is granite. From published H-O isotope d oric water was the source of ore-forming fluids for many granite-related fluorit in South China [52,[77][78][79].  [75,80]. Data source: the Shuanghuajiang fluorite deposit is from this study; other depos [77,78].  [75,80]. Data source: the Shuanghuajiang fluorite deposit is from this study; other deposits are from [77,78].

Source of the Ore-Forming Materials
Among large-scale fluorite deposits around the world, F has two common sources: magmatic-hydrothermal fluid and basinal brine. For example, fluorite deposits are widespread in northern Mexico, and two different types of fluorites have been found: MVT type and skarns type fluorite deposits. The F of the MVT type fluorite deposits is related to the brines from Jurassic petroleum basins, whereas that of skarn type fluorite deposit is derived from Tertiary magmatic activity [81,82]. The F of the world-class Vergenoeg fluorite deposit in South Africa was derived from the granitic fluids of the Bushveld complex [83]. The mineralization age of the Shuanghuajiang fluorite deposit is significantly later than the emplacement of the hosting Xiangcaoping granite, which indicates that magmatic-hydrothermal fluid cannot provide F for fluorite mineralization. The Shuanghuajiang deposits and their vicinity do not develop marine sedimentary carbonate rocks, indicating that F is not derived from the basin brine; therefore, the source of F in the Shuanghuajiang fluorite deposit is different from the above fluorite deposits.
Different minerals have different REE abundance and geochemical behaviors. REE characterization is an important tool for tracing the origin of ore-forming materials and mineralization processes [2,84,85]. REE patterns of both the fluorite and the hosting Xiangcaoping granite are similar and show a right-leaning trend with a strong negative Eu anomaly. This feature suggests an ore-forming material exchange between ore-forming fluid and the hosting Xiangcaoping granite [86], indicating that the fluorite inherited the REE characteristics of the Xiangcaoping granite, and the F originated from the Xiangcaoping granite. This is because F and REE mainly exist in the form of the F-REE complex in the fluid [87,88]. When the ore-forming fluid leaches the ore-hosting granite and destroys minerals such as biotite, F and REE in the biotite are picked up by the ore-forming fluid in the form of F-REE complexes, resulting in the crystallized fluorite exhibiting the same distribution pattern of REE as the ore-hosting granite [12,14]. In addition, the F content of the Xiangcaoping granite is 0.12%-0.30% (Table 3), which is higher than the average F content of granites in the Nanling area (0.11%) [89]. It is suggested that the Xiangcaoping granite can provide sufficient F for fluorite mineralization. The stratified rocks in the fluorite deposit and vicinity were exposed incompletely and have been weathered; therefore, samples were not collected to measure the F content; however, the stratified rocks are mainly feldspathic quartz sandstone and shales, which generally do not contain F.
Ca may be sourced from sedimentary rocks and Xiangcaoping granite in the Shuanghuajiang fluorite deposit. As a matter of fact, Ca-bearing sedimentary rocks, such as limestone, tuffs, or Ca-bearing clastic rocks, always exist around most large-sized fluorite deposits in South China. Calcareous interlayers occur as part of the sedimentary rocks in some small and medium-sized fluorite deposits in South China. For example, at the Longping largescale fluorite deposit of southern Jiangxi Province, Sinian calcareous sandstones, tuffaceous sandstone, and calcareous siltstone are widespread in the ore district [19]. Ca-bearing sedimentary rocks such as tuff and limestone exist in large-scale fluorite deposits, including the Xinqiao, Yucun, and Gaowushan-Jiaokengwu deposits in northwestern Zhejiang Province. The 87 Sr/ 86 Sr ratios of these fluorite deposits also indicate that the Ca-bearing strata are an important source of Ca [24]. Fluorite deposits may be formed where a granite pluton was emplaced in Ca-rich sedimentary rocks; however, for the same granite pluton, fluorite deposits failed to develop where it intruded in Ca-poor sedimentary rocks. For example, the Shizhuyuan giant fluorite deposit of south Hunan Province occurs in the southern part of the Qianlishan granite, where the granite was emplaced in the Middle-Upper Devonian Ca-rich tuff; however, no fluorite deposits have ever been discovered in the northern part of the Qianlishan granite pluton that was emplaced in the Middle Devonian Ca-poor sandstone [52]. In short, Ca-rich stratigraphy is crucial for the formation of fluorite deposits. The Neoproterozoic Gongdong Formation and Nanhua System exist in the southwest of the Shuanghuajiang fluorite deposit. The Gongdong Formation locally contains calcareous dolomite and the Nanhua System contains dolomitic limestone and argillaceous limestone, both of which can provide a large amount of Ca for the formation of fluorite deposits. In addition, the CaO content of the Xiangcaoping granite is 0.87%-1.22%, with an average of 0.99% [39]. The CaO content of the granite near the orebodies measured in this study is only 0.30%-0.87%, with an average of 0.60% (Table 3). It is suggested that the decrease in Ca content found in granite near the orebodies may be caused by meteoric water leaching, and that the leached Ca is also involved in the formation of fluorite. The sericitization of granite commonly developed in fluorite deposits provides evidence to support this suggestion; therefore, both the Ca-bearing sedimentary rocks and the Xiangcaoping granite in the Shuanghuajiang fluorite deposit served as sources of Ca for the formation of fluorite, with the Ca-bearing sedimentary rocks may playing a more dominant role.

Model of Fluorite Mineralization
Based on previous discussions, the granite, Ca-bearing sedimentary rocks, and faults are the controlling factors in the formation of the Shuanghuajiang fluorite deposit. The Ca-bearing sedimentary rocks and the Xiangcaoping granite provide a source for oreforming materials. The post-orogenic extensional tectonic setting is favorable for the formation of fluorite deposits [90][91][92]. In the Shuanghuajing deposit, the NNE-NE striking faults provided pathways and room, and facilitated migration and precipitation of the oreforming fluids. The mineralization of the Shuanghuajiang fluorite deposit was achieved via hydrothermal circulation and leaching mechanisms. Besides the Shuanghuajiang fluorite deposit, there are some other fluorite deposits formed by this mechanism distributed in the Nanling metallogenic belt. The mineralization age of these fluorite deposits is not concentrated, which is the reason for the large age span of fluorite deposits in the Nanling metallogenic belt.
Based on the data obtained in this study and the discussions set out above, a comprehensive mineralization model for hydrothermal vein-type fluorite deposits is proposed and described as follows. (1) A granite pluton was emplaced in Ca-rich sedimentary rocks, followed by brittle faulting of the pluton (Figure 13a). (2) The deep-cutting faults provided pathways for meteoric water to reach deep into the crust (Figure 13b). Low-temperature meteoric water has a limited dissolution ability and could leach and pick up only a small number of ore-forming materials [93]. (3) However, the meteoric water that reached deep into the crust was heated to become hydrothermal fluids with enhanced leaching and dissolution capabilities. Due to the large time lag between diagenesis and mineralization in the Shuanghuajiang fluorite deposit, the heat source that heats up the meteoric water may be unrelated to magma. Granite is more concentrated with radioactive elements (U, Th, K) than other types of rocks, and the radioactive heat from it can be diffused outward through heat conduction or groundwater [94][95][96]; therefore, the heat source may be the granite batholith. (4) Driven by temperature and pressure gradients, the meteoric hydrothermal fluids circulated upward through the granite plutons and sedimentary rocks with enhanced dissolution and leaching. The contents of ore-forming ingredients such as Ca and F became enriched in the hydrothermal fluids ( Figure 13b). (5) The temperature and pH value of the ore-forming fluids decreased following mixing with meteoric water containing CO 2 at higher levels of the crust [93]. For example, a pH value < 4 means that the solubility of CaF 2 increases as the pH value decreases [97]. With a pH value = 4-7, the solubility of CaF 2 becomes lower and more stable, while the solubility of SiO2 increases as the pH value increases [67,98]. (6) Therefore, during the upward migration of the hydrothermal ore-forming fluids, SiO 2 of the acidic component reached its saturation point, which caused silicification. Subsequently, the ore-forming fluid temperature was further lowered, and the pH value of the fluids gradually increased due to SiO 2 precipitation, changing from acidic to weak acidic. At this time, CaF 2 precipitation began, and the fluorite veins were formed (Figure 13c). (7) Starting from the late Yanshanian period, a number of transient tectonic compression events occurred in parts of South China [99,100]. Under this tectonic setting, the Miaoershan region experienced compression, rapid uplift, and denudation, resulting in the exposure of some orebodies in the Shuanghuajiang fluorite deposit and eventually in the present-day topography (Figure 13d). and denudation, resulting in the exposure of some orebodies in the Shuanghuajiang fluorite deposit and eventually in the present-day topography (Figure 13d).  (a) the hosting Xiangcaoping granite was emplaced in Ca-rich sedimentary rocks, followed by brittle faulting of the pluton; (b) meteoric water percolated into the crust, which are heated by the radioactive heat from the hosting granite and ascended along the pathways provided by brittle faults, resulting in the fluids with low-T and low-salinity leaching the ore-forming materials (F and Ca) from the hosting granite and Ca-bearing sedimentary, and evolved into ore-forming fluids. (c) due to the changes in temperature, pressure, and pH conditions during the upward migration of the ore-forming fluids, the CaF 2 begins to precipitate and the fluorite veins were formed. (d) under transient tectonic compression setting, the Miaoershan region experienced compression, rapid uplift, and denudation, resulting in the exposure of some fluorite orebodies in the Shuanghuajiang fluorite deposit.

Conclusions
(1) A new zircon U-Pb age (228.04 ± 0. 76 Ma) for the Xiangcaoping granite pluton that hosts the Shuanghuajiang fluorite deposit is reported in this study. The fluorite Sm-Nd dating yielded an isochron age of 185 ± 18 Ma (MSWD = 1.6) for the fluorite mineralization. The new age data indicate that the fluorite deposit was precipitated significantly later than the emplacement of the hosting Xiangcaoping granite pluton.
(2) Primary fluid inclusions in the Shuanghuajiang fluorite deposit are liquid-rich H 2 O inclusions. The ore-forming fluids belong to a NaCl-H 2 O system with low temperature and low salinity, typical for meteoric water and not derived from the magmatic-hydrothermal activity.
(3) The fluorite and the hosting Xiangcaoping granite show similarities in REE distribution patterns and characteristics, indicating that the F is derived from the Xiangcaoping granite. The Ca is most likely derived from the Ca-rich sedimentary rocks of the Neoproterozoic Gongdong Formation, the Nanhua System, and the Ca-bearing Xiangcaoping granite. The Xiangcaoping granite plays a role in providing ore-forming materials.
(4) The mineralization of the Shuanghuajiang fluorite deposit was achieved via hydrothermal circulation and leaching mechanisms. This may reveal the genetic relationship between granite and fluorite mineralization in northern Guangxi. A metallogenic model is proposed here that faults developed in the granite pluton provided pathways for meteoric water to reach deep into the crust, where it evolved into hydrothermal fluids that circled back with enhanced dissolution and leaching capacities. Ore-forming ingredients Ca and F were leached out from the Xiangcaoping granite and Ca-bearing strata, picked up by the hydrothermal fluids, and later migrated and precipitated in the faults at shallower levels due to changes in temperature, pressure, and pH conditions.