The Source and Evolution of Ore-Forming Fluids in the Xiaobaihegou Fluorite Deposit, Altyn-Tagh Orogen, NW China: Constraints from Trace Element, Fluid Inclusion, and Isotope Studies

Abstract

The Xiaobaihegou fluorite deposit is located in the southwest of the Altyn-Tagh Orogen, NW China. However, the provenance, thermodynamic properties, and enrichment mechanisms of the ore-forming fluids in this deposit remain unclear. Fluorite mineralization primarily occurs in the vicinity of the contact zone between the granite and the wall rocks. The zircon U-Pb age of the alkali-feldspar granite in the Xiaobaihegou fluorite deposit is 482.3 ± 4.1 Ma. The ore-hosting lithologies are mainly calcareous rock series of the Altyn Group. The ore bodies are controlled by NE-trending faults and consist primarily of veined, brecciated, massive, and banded ores. The ore mineral assemblage is primarily composed of calcite and fluorite. The rare earth element (REE) patterns of fluorite and calcite in the Xiaobaihegou deposit exhibit right-dipping LREE enrichment with distinct negative Eu anomalies, which closely resemble those of the alkali-feldspar granite. This similarity suggests that the REE distribution patterns of fluorite and calcite were likely inherited from the pluton. The ore-forming process can be divided into an early stage and a late stage. The massive ores formed in the early stage contain mainly gas-rich two-phase fluid inclusions and CO₂-bearing three-phase inclusions, with homogenization temperatures ranging from 235 °C to 426 °C and salinities from 28.59% to 42.40% NaCl equivalent. In the late stage, brecciated and stockwork ores were formed. They host liquid-rich two-phase and gas-rich two-phase fluid inclusions, with homogenization temperatures ranging from 129 °C to 350 °C and salinities from 0.88% to 21.61% NaCl equivalent. The results of hydrogen and oxygen isotope studies indicate that the ore-forming fluids were derived from a mixture of magmatic–hydrothermal and meteoric water. Fluorite precipitation in the early stage was mainly due to the mixing of magmatic–hydrothermal solution and meteoric water, as well as a water–rock reaction. In the late stage, fluid mixing further occurred, resulting in a decrease in temperature and the formation of brecciated and stockwork ores. The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios of fluorite from the deposit range from 0.71033 to 0.71272 and 0.511946 to 0.512073, respectively, indicating that the ore-forming material originates from the crust. Based on the ore-forming characteristics, it is proposed that Ca may be primarily leached from the strata formation, while F may predominantly originate from magmatic–hydrothermal solutions. The formation of fluorite deposits is closely related to the transition of the Central Altyn-Tagh Block and Qaidam Block from a compressional orogenic environment to an extensional tectonic environment.

1. Introduction

Fluorite (CaF₂), serving as the primary natural source of fluorine, is predominantly utilized in the production of anhydrous hydrogen fluoride (AHF). Aqueous hydrogen fluoride derived from AHF constitutes the principal feedstock for synthesizing most fluorine-containing chemicals, notably refrigerants and fluoropolymers. Additional industrial applications of fluorite include its use as a flux in steelmaking, a raw material in cement production and glass manufacturing, a component in enamel formulations, an agent in iron and steel foundry operations, and a constituent in welding rod coatings [1,2,3,4,5,6]. The accelerated advancement of high-technology industries has generated escalating demand for fluorite resources, necessitating intensified exploration efforts to identify new fluorite deposits.

According to data published by the U.S. Geological Survey, by the end of 2024, the global fluorite reserves were estimated to be 320 × 10⁶ t, with nearly half of the reserves distributed in Mexico, China, and South Africa [6]. In China, the identified fluorite resources were mainly distributed in Inner Mongolia, Jiangxi, Zhejiang, Hunan, Fujian, and Henan provinces [2]. These fluorite deposits are predominantly situated in the central and eastern regions of China. In recent years, substantial advancements have been made in the exploration of fluorite mineral resources within the Altyn-Tagh Orogenic Belt of northwestern China, including the Kaerqiaer, Kumutashi, and Xiaobaihegou deposits [5,7,8,9,10].

This study presents the discovery and geological characteristics of the Xiaobaihegou fluorite deposit in the western segment of the Altyn-Tagh Orogen. Geochemical analyses, fluid inclusion studies, and C-H-O-Sr-Nd isotopic systematics were employed to characterize the deposit genesis, decipher the ore-forming processes in detail, and provide insights for exploring analogous large-scale fluorite deposits in China.

2. Geological Background

The Altyn-Tagh Orogen lies at the junction of Xinjiang, Qinghai, and Gansu provinces, bordering the Tarim, Kunlun, Qaidam, North Qaidam, and North Qilian regions (Figure 1a,b). The Altyn-Tagh Orogen preserves a complex tectonic history encompassing Neoproterozoic extensional and collisional events, Early Paleozoic oceanic plate subduction and continental collision, as well as Mesozoic to Cenozoic faulting activities [11,12,13,14,15,16,17].

Through comprehensive regional geological, geochemical, and geochronological investigations, the Altyn-Tagh Orogen has been categorized into the North Altyn Terrane, Hongliugou-Lapeiquan Ophiolitic Melange, Middle Altyn Terrane, Jianggasiyi-Bashwak High-Pressure Accretionary Complex, and Apa-Mangya Ophiolitic Melange [16,18,19] (Figure 1b). The Xiaobaihegou fluorite deposit is situated in the northwestern part of the Altyn-Tagh Orogen, approximately 75 km southeast of Ruoqiang Town, and falls within the Middle Altyn Terrane subdivision (Figure 1c).

The strata in the Middle Altyn Terrane primarily consist of the Altyn Group, Bashikuergan Group, Taxidaban Group, and Suoerkuli Group. The Altyn Group is composed of biotite–plagioclase gneiss, garnet–sillimanite–biotite gneiss, monzonitic quartz schist, and plagioclase amphibolite. The Bashikuergan Group primarily consists of gray biotite quartz schist, two-mica quartz schist interbedded with grayish-white marble, quartzite, plagioclase amphibolite, and phyllite. The Taxidaban Group is characterized by grayish-white medium-thick bedded quartzite, two-mica quartz schist, phyllite, albite quartz schist, siltstone, and slate. The Suoerkuli Group is predominantly composed of carbonate rocks, mainly including marble, dolomite, and limestone. The Xiaobaihegou area is characterized by several significant regional tectonic activities that are complex and long-lived. These activities involve various orogenic events dating back to the Archaean, accompanied by records of widespread Neoproterozoic and Paleozoic intermediate to felsic magmatism. In the northern part of the Xiaobaihegou fluorite deposit, the belt is bounded by the Kaerqiaer-Kuoshi Fault, with the Gaijile, Yuemakeqi-Kulangele, and Altyn-Tagh Faults occurring sequentially to the south. The Kaerqiaer-Kuoshi Fault, a structure genetically linked to fluorite mineralization, trends northeast for approximately 70 km. It cuts through Paleozoic intermediate-to-felsic plutons and displays a sinuous surface morphology, indicating its multi-phase activity history (Figure 1c). Extensional secondary faults are pervasively developed between these regional faults. The Xiaobaihegou fluorite deposit is spatially associated with granites. The Altyn-Tagh Terrane has undergone multiple tectonic events, accompanied by the emplacement of felsic-to-intermediate plutons during the Neoproterozoic, Palaeozoic, and Mesozoic eras [5,10].

3. Geology of the Xiaobaihegou Fluorite Deposit

In the Xiaobaihegou fluorite deposit, most fluorite veins are hosted in biotite–plagioclase gneiss, with alkali-feldspar granite dykes frequently associated in the vicinity of these fluorite veins. The alkali-feldspar granite dykes and fluorite veins intrude into NE- and ESE-trending faults that cut through Neoarchaean to Palaeoproterozoic gneisses within the Altyn Complex (Figure 2). These units are overlain by Quaternary eluvium and diluvium exposed in gullies. In the mining area, the exposed intrusive rocks are predominantly alkali-feldspar granite, which shows a close spatial association with fluorite veins.

The mining area contains three major faults: two striking in the NE direction and one striking in the NW direction. Fluorite veins are closely associated with these three faults. The southern zone extends approximately 2.5 km in length and 0.4 km in width, striking northeast. The northern mineralization zone is about 0.4 km wide and 1.7 km long, trending nearly east (Figure 2). The fluorite ore body strikes nearly east–west, with a northerly dip of 30–40°. The ore bodies generally occur in lenticular, vein-like, and lenticular shapes, exhibiting good continuity with minor variations in attitude. They contain a few internal interlayers, exhibit bulging and constriction phenomena, and locally display branching and recombination. The overall morphology of the ore bodies is moderately complex.

The deposit is characterized by the occurrence of high-grade ore, with CaF₂ grades exceeding 30% and locally reaching over 90%. The fluorite ore mainly occurs as veinlets filling the contact zone between alkali-feldspar granite veins (Figure 3a–e) and biotite–plagioclase gneiss. The ore types are dominated by massive and layered ores, with fluorite as the main mineral, accompanied by locally occurring calcite and minor quartz. Fluorite is white, green, purple, purple black, and so on. The ore exhibits a coarse-grained structure, characterized by euhedral-to-subhedral granular textures (Figure 3f–h). The main industrial types of ore are CaF₂ and CaF₂-CaCO₃.

Ore-forming processes can be divided into two stages based on the types of ore. Early stage: Ores are predominantly massive, mainly distributed in the central part of the ore veins (Figure 3f). They exhibit good crystallization with a coarse-grained texture. Late stage: Ores are dominated by brecciated and stockwork types, occurring in the marginal parts of the ore veins (Figure 3g). A gradual transition from massive to brecciated textures is observed from the center toward both sides (Figure 3e). The ores exhibit moderate crystallization, predominantly with euhedral–subhedral, anhedral granular, and cataclastic textures.

The country rocks of the fluorite ore bodies are biotite–plagioclase gneiss and alkali-feldspar granite (Figure 3i–l). The gneiss has a fine-grained lamellar granular structure, mainly composed of quartz, plagioclase, biotite, and microplagioclase, with the remainder comprising sphene and other minor minerals. The quartz particle size ranges from 0.06 to 2.80 mm; it is heteromorphic granular, with the long axis slightly oriented. The plagioclase particle size ranges from 0.10 to 1.60 mm; it is heteromorphic granular, with cluster twin crystals, sericitization on the surface, and a slightly oriented distribution along the long axis. The biotite particle size ranges from 0.04 to 1.00 mm, and it is scaly and sheet-like with a directional distribution. Microplagioclase has a particle size of 0.10–0.20 mm, with lattice twinned, and its long axis is slightly oriented among the plagioclase grains (Figure 3i,j).

The granite exhibits a medium-fine-grained subhedral granular texture and massive structure, predominantly composed of alkali feldspar, plagioclase, quartz, and biotite, with accessory minerals including apatite. The alkaline feldspar has a grain size of 0.9–5.0 mm, occurring as subhedral platy crystals with carlsbad twinning, grid twinning, or perthitic texture, and is predominantly microcline. The plagioclase has a grain size of 0.5–2.0 mm, occurs as subhedral platy crystals, and exhibits a reticulate edge texture. The quartz has a grain size of 0.5–4.5 mm, occurring as anhedral granular and elongated prismatic crystals. The biotite has a grain size of 0.4–3.5 mm, occurs as lamellar crystals and has undergone sericitization and chloritization (Figure 3k,l).

The alteration in biotite–plagioclase gneiss shows multiple-stage alteration superposition. Pre-ore alterations are primarily manifested as K-feldspathization and biotitization, while ore-stage alterations include epidotization, chloritization, carbonatization, fluoritization, followed by sericitization, pyritization, etc. K-feldspathization is the most widely distributed alteration type in the mining area, with K-metasomatism occurring during the emplacement of alkali-feldspar granite magma. It has indistinct boundaries and a diffuse distribution, with a bandwidth of 1–2 m, characterized by percolation–diffusion-dominated metasomatism, where plagioclase and other minerals in metamorphic rocks form K-feldspar through K-metasomatism. Silicification is mainly manifested as silicified quartz filling and cementing country rock breccia in structural fracture zones. During the fluorite mineralization stage, the country rock alterations in alkali-feldspar granite are primarily epidotization, chloritization, carbonatization, fluoritization, and kaolinization. In country rock marble, alteration phenomena caused by fluorite–calcite veins filling along fractures are commonly observed.

4. Sampling and Analytical Methods

All samples were obtained from field outcrops, drilling cores, and underground mine workings throughout the mining area. Over 100 doubly polished sections were prepared to facilitate microscopic observation and research on fluid inclusion. Fluorite and calcite separation was conducted at Xi’an Kuangpu Geological Exploration Technology Co., Ltd. (Xi’an, China). The separated fluorite and calcite grains were carefully selected for analyses of Sr-Nd isotopes, C-H-O isotopes, and trace elements.

4.1. Zircon U-Pb Geochronology

This study conducted zircon U-Pb dating on alkali-feldspar granites from the Xiaobaihegou fluorite deposit. Zircon grains were isolated via conventional electromagnetic and heavy fluid separation techniques at Xi’an Kuangpu Geological Exploration Technology Co., Ltd. Subsequent handpicking was performed under a binocular microscope. The selected zircon grains were embedded in epoxy resin, polished to expose their surfaces, and then photographed under transmitted and reflected light, followed by cathode luminescence (CL) imaging. CL images, integrated with transmission and reflected light photographs, were utilized to mark inclusion-free and fracture-free spots for laser ablation.

Zircon U-Pb isotopic dating was conducted via LA-ICP-MS at the Key Laboratory of Magmatism Mineralisation and Prospecting, Ministry of Natural Resources. A Geolas excimer solid sampling system served as the laser ablation device, coupled to a Thermo Fisher X Series II (Thermo Scientific, Bremen, Germany) quadrupole inductively coupled plasma mass spectrometer (ICP-MS). The laser parameters were set as a 32 μm beam diameter, 8 Hz frequency, and helium as the carrier gas. The American National Standard NIST610 was employed for equipment optimization and external standardization of trace element measurements [20]. Zircon standard 91,500 (²⁰⁶Pb/²³⁸U = 1065 Ma) was used for external calibration, with standard zircon GJ-1 serving as the monitoring sample [21]. The ICPMSDataCal 8.3 software was utilized for data processing [22], while age calculations and concordia diagrams were generated using Isoplot 3.0 [23]. Uncertainties for individual analyses are reported at the 1σ level, and errors on weighted mean ages are provided at 2σ (95% confidence level) [24].

4.2. Analysis of Major and Trace Elements

The major element analysis of the granite was conducted at the Xi’an Mineral Resources Survey, China Geological Survey, Xi’an, Shaanxi Province, China. Fresh samples were crushed and sieved through a 200-mesh sieve. After calcination, a fluxing agent was added to the samples, which were then melted to prepare glass disks. Major elements were analyzed using a pressed powder pellet method with an X-ray fluorescence spectrometer, with a relative error of less than 5%.

Trace element geochemical analyses of collected fluorite and calcite samples were completed at Nanjing Jupu Geological Services Ltd., Nanjing, China. Initially, the samples were manually crushed into smaller particles and then purified under a binocular microscope. Purified grains were further ground in an agate mortar until they reached a powder fineness of less than 200 mesh. Trace and rare earth elements were analyzed using inductively coupled plasma mass spectrometry (ICP-MS), with analytical errors of less than 10%.

4.3. Fluid Inclusion Analysis

Fluid inclusion petrography and microthermometric analyses were performed using a Linkam THMS 600 (Linkam, Godalming, Surrey, UK) cooling–heating stage mounted on an Olympus BX53 microscope (Olympus Corporation, Tokyo, Japan) at the Key Laboratory of Western Mineral Resources and Geological Engineering, Ministry of Education, Chang’an University, Xi’an, Shaanxi Province, China. The study samples are primarily composed of fluorite and calcite from the Xiaobaihegou fluorite deposit, encompassing representative samples from various ore-forming stages and exhibiting distinct spatial characteristics. Firstly, the mineral samples were ground into about 0.2 mm thick inclusions, and the large primary inclusions at each stage were observed under a microscope. The micro-temperature measurement experiment and the determination of gas-phase composition by laser Raman spectroscopy were carried out. Before the experiment, temperature correction was performed by pure H₂O inclusion (0 °C), pure CO₂ inclusion (−56.6 °C), and critical H₂O inclusion (374 °C). The measured temperature range is −195 ~ +600 °C, and the analysis accuracy is ±0.2 °C, <30 °C; ±1 °C, <300 °C; ±2 °C, <600 °C. In the freezing–heating process, the temperature rise and fall rates were set to not exceed 20 °C/min, and the rate was reduced to less than 1 °C/min near the phase transition point. Laser Raman probing for single fluid inclusion composition analysis was conducted at the Metallogenesis and Dynamics Laboratory of Chang’an University. The analysis was conducted using a LabRAM HR Evolution, a next-generation high-resolution Raman spectrometer manufactured by HORIBA (Kyoto, Japan). The experiment was conducted under controlled conditions of 23 °C and 65% relative humidity.

4.4. Isotope Analysis

Calcite single mineral with a purity of more than 99% was selected for carbon and oxygen isotopic measurement, and it was fully ground to below 200-mesh powder. The carbon and oxygen isotope analysis was completed at the State Key Laboratory of Continental Dynamics, Northwest University. The isotopic compositions of calcite were determined by a Delta V Advantage stable isotope mass spectrometer from Thermo Fisher. The brief analysis steps are as follows: Weigh an appropriate amount of dried to constant weight calcite sample powder (150–250 μg) into the reaction bottle, seal it with a lid with a silicone spacer, inject high-purity helium gas into the reaction bottle with a draining needle (the blowing time of each reaction bottle is about 30 s), and then empty the air in the reaction bottle successively. Following emptying, 2 to 3 drops of saturated phosphoric acid were added manually to the reaction vessel. The reaction was conducted at 70 °C for approximately 1 h. After a period of equilibration, CO₂ generated from the reaction of saturated phosphoric acid with carbonates was separated from impurity gases using a chromatographic column. The purified CO₂ was then directed for analysis. Using standard samples for quality control, the test accuracy of δ¹³C and δ¹⁸O was better than ±0.1‰ and ±0.2‰, respectively, as determined by repeated testing of standard substances.

H-O isotope analysis of fluorite inclusions was conducted at the State Key Laboratory of Continental Dynamics, Northwestern University, using a Thermo 253plus gas stable isotope mass spectrometer (Thermo Scientific, Bremen, Germany). Oxygen isotope analysis employed the BrF₅ method: oxygen was generated, collected in a vacuum using a sample tube packed with 5A molecular sieves, and its δ¹⁸O value was measured on the 253plus mass spectrometer. The external precision for standard samples was better than ±0.2‰, referenced to V-SMOW, with a single-sample internal precision of 0.05‰. For hydrogen isotope analysis, the explosive method was used to extract water and produce hydrogen. Samples were baked in a high-temperature (1420 °C) cracking furnace (Flash EA, Thermo) filled with glass carbon particles. Upon cracking and releasing the mineral inclusion water, H₂ and CO formed via instantaneous reaction with glass carbon, transported by high-purity helium (5N) through a chromatographic column to the mass spectrometer (253plus, Thermo) for H₂ isotope ratio (δD) determination. The testing precision for international standard materials (polyethylene, IAEA-CH-7, δD_VSMOW = −100.3‰) was better than 1‰.

Sr and Nd isotope chemical pretreatment and mass spectrometry analyses were conducted at Nanjing Poly Spectrum Detection Technology Co., Ltd. The contents of Sr and Nd, along with Sr isotope ratios, were calculated via the isotope dilution method. The ⁸⁷Sr/⁸⁶Sr and Nd isotope ratios were determined on a Nu Plasma II MC-ICP-MS. During the measurements, a ⁸⁶Sr/⁸⁸Sr ratio of 0.1194 was used for internal calibration of instrumental mass fractionation, with the Sr isotope standard NIST SRM 987 serving as the external standard to correct for instrumental drift [25]. For Nd isotopes, ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 was applied for internal calibration, and the international standard material JNdi-1 was used as the external standard to correct instrumental drift.

5. Results

5.1. Zircon U-Pb Ages

Zircon grains within the alkali-feldspar granite samples exhibit a columnar habit, featuring a long-axis diameter ranging from 50 to 100 μm and a length-to-width ratio varying between 1:1.2 and 1:3. Their cathodoluminescence (CL) imaging reveals typical oscillatory zoning patterns (Figure 4a). The results of the zircon U-Pb isotopic analyses are documented in

Table 1. LA-ICP-MS zircon U-Pb isotopic data of the alkali-feldspar granite in the Xiaobaihegou fluorite deposit.

Spot NO.	Th	U	Th/U	Corrected Isotopic Ratios						Age (Ma)
	(×10−6)			207Pb/206Pb	1σ	207Pb/235U	1σ	206Pb/238U	1σ	207Pb/206Pb	1σ	207Pb/235U	1σ	206Pb/238U	1σ
XBH-01	2050	4918	0.42	0.05047	0.00278	0.53884	0.02857	0.07779	0.00164	217	128	438	19	483	10
XBH-02	1617	3243	0.50	0.06439	0.00483	0.68846	0.04887	0.07804	0.00117	754	159	532	29	484	7
XBH-03	1188	3659	0.32	0.05176	0.00356	0.55384	0.03986	0.07750	0.00129	276	162	448	26	481	8
XBH-04	1548	3013	0.51	0.05265	0.00380	0.56364	0.04307	0.07781	0.00154	322	169	454	28	483	9
XBH-05	1914	4180	0.46	0.05365	0.00251	0.56999	0.02582	0.07746	0.00138	367	101	458	17	481	8
XBH-06	1818	2800	0.65	0.06047	0.00389	0.64252	0.03973	0.07755	0.00163	620	139	504	25	481	10
XBH-07	2068	4326	0.48	0.05367	0.00228	0.57639	0.02826	0.07761	0.00136	367	96	462	18	482	8
XBH-08	3354	6681	0.50	0.05978	0.00270	0.63917	0.02894	0.07752	0.00092	594	103	502	18	481	5
XBH-09	1538	3845	0.40	0.06053	0.00388	0.64604	0.04000	0.07791	0.00157	633	137	506	25	484	9
XBH-10	1918	3404	0.56	0.05995	0.00356	0.63968	0.03679	0.07778	0.00141	611	123	502	23	483	8
XBH-11	4182	8068	0.52	0.05540	0.00257	0.59081	0.02677	0.07777	0.00172	428	104	471	17	483	10
XBH-12	1724	3192	0.54	0.05486	0.00319	0.58504	0.03441	0.07761	0.00155	406	99	468	22	482	9
XBH-13	1010	2189	0.46	0.05594	0.00428	0.58865	0.04110	0.07773	0.00183	450	140	470	26	483	11
XBH-14	3744	5272	0.71	0.05791	0.00329	0.62097	0.03554	0.07774	0.00133	528	129	490	22	483	8
XBH-15	1367	2749	0.50	0.05758	0.00348	0.61788	0.03815	0.07786	0.00151	522	133	489	24	483	9
XBH-16	1357	2284	0.59	0.05722	0.00419	0.60899	0.04310	0.07759	0.00162	502	161	483	27	482	10

, indicating that the thorium (Th) and uranium (U) contents range from 1010 to 4182 ppm and 2189 to 8068 ppm, respectively. The Th/U ratios (0.32–0.65) all exceed 0.1, which is characteristic of a magmatic origin. Among the analytical data, 16 measurement points plot on or near the concordia curve (Figure 4b), with spot ages ranging from 481 to 484 million years (Ma). The weighted mean age is 482.3 ± 4.1 Ma (n = 16, MSWD = 0.016), which is interpreted to represent the emplacement time of the granite (Figure 4b,c).

5.2. Major and Trace Elements

Table 2 presents the geochemical results of rare earth elements (REE) for fluorite and calcite samples from the Xiaobaihegou deposit. The corresponding chondrite-normalized REE patterns are illustrated in Figure 5.

The total rare earth element (ΣREE) contents of early-stage fluorite range from 91.5 to 97.3 ppm, whereas those of late-stage fluorite range from 45.1 to 79.5 ppm. Early-stage fluorite from the Xiaobaiheigou deposit exhibits LREE/HREE ratios, (La/Yb)_N values, and δEu values of 2.42–7.49, 1.44–9.66, and 0.34–0.46, respectively. In contrast, late-stage fluorite from the deposit has LREE/HREE ratios, (La/Yb)_N values, and δEu values of 2.80–6.09, 1.77–7.14, and 0.36–0.43, respectively. The ΣREE concentrations in calcite range from 1465 to 1579 ppm. The calcite’s LREE/HREE ratios, (La/Yb)_N, and δEu in Xiaobaihegou deposit range from 12.5~13.5, 5.59~8.65, and 0.38~0.43, respectively.

The REE geochemical characteristics of fluorite in the Xiaobaihegou deposit reveal that the REE distribution patterns of fluorite and calcite are analogous to those of alkali-feldspar granite [9,10]. Both exhibit LREE-enriched patterns with distinct negative Eu anomalies (Figure 5), indicating that the REE signatures of fluorite and calcite in the fluorite ore belt likely inherit from the source rock.

Table 3 presents the results of major elements for granite samples from the Xiaobaihegou deposit. The Xiaobaihegou granites are characterized by high SiO₂ (73.08%–75.30%), high K₂O (3.91%–7.76%), and high alkali contents (K₂O + Na₂O = 8.33%–10.18%), coupled with low CaO (0.43%–1.2%) and low MgO (0.10%–0.69%). The ΣREE concentrations in granite range from 12.7 to 98.3 ppm. The granite’s LREE/HREE ratios, (La/Yb)_N, and δEu in Xiaobaihegou deposit range from 2.08~5.81, 0.63~4.83, and 0.18~0.68, respectively. They have an A/CNK ratio of 0.6–0.75 and are further distinguished by depletion in Sr, Eu, Ba, Ti, and P and enrichment in high-field-strength elements (HFSEs), thus exhibiting characteristics typical of A-type granites [26].

Table 2. Trace (ppm) element compositions of the fluorites and calcites at the Xiaobaihegou fluorite deposit.

Sample	XBH-1	XBH-2	XBH-3	XBH-4	XBH-5	XBH-6	XBH-7	XBH-8	XBH-9	XBH-10	XBH-11	XBH-12	XBH-13
	Early Fluorite	Early Fluorite	Early Fluorite	Early Fluorite	Early Fluorite	Late Fluorite	Late Fluorite	Late Fluorite	Late Fluorite	Late Fluorite	Late Fluorite	Late Fluorite	Late Fluorite
Cs	0.01	0.02	0.08	0.03	2.29	0.09	0.01	0.01	0.01	1.06	0.25	0.31	0.35
Pb	3.35	4.78	5.69	5.78	26.9	5.78	5.67	2.25	4.96	17.5	3.11	1.99	9.18
Rb	0.23	0.14	0.19	0.23	49.8	2.78	0.46	0.57	0.13	23.2	5.46	6.22	3.70
Ba	1.12	1.35	2.18	1.08	92.1	2.86	31.4	0.93	1.18	40.7	12.3	11.6	5.97
Th	0.24	0.19	0.10	0.09	2.20	0.65	0.28	0.22	0.07	1.29	0.59	0.51	2.70
U	0.25	0.26	0.19	0.33	1.51	0.71	0.64	0.69	0.53	2.13	1.10	0.61	1.85
Ta	0.15	0.14	0.08	0.09	0.04	0.20	0.12	0.09	0.18	0.04	0.01	0.01	0.01
Nb	0.19	0.18	0.16	0.18	3.48	0.55	0.59	0.25	0.28	4.82	1.29	0.47	3.79
Zr	4.04	2.97	1.79	1.57	8.75	10.9	10.8	4.83	1.27	4.20	0.73	0.84	0.79
Hf	0.36	0.32	0.27	0.26	0.26	0.38	0.38	0.25	0.15	0.12	0.02	0.03	0.02
Ti	0.00	0.00	0.00	22.3	95.0	0.00	918	23.4	0.00	53.4	16.1	15.4	8.08
Sr	358	354	323	371	348	405	402	414	394	384	419	388	425
La	7.46	8.26	8.40	7.97	18.7	4.47	5.12	4.00	5.16	12.0	12.4	7.83	8.78
Ce	26.0	27.1	26.3	26.4	37.8	14.1	16.2	13.3	16.9	25.6	29.1	22.5	20.4
Pr	4.13	4.24	3.91	4.22	4.33	2.18	2.47	2.10	2.57	3.14	3.78	3.48	2.66
Nd	21.9	22.2	20.4	22.1	16.7	10.9	12.3	10.5	12.9	12.9	16.3	17.5	11.4
Sm	6.33	6.57	5.99	6.43	3.30	3.07	3.35	2.99	3.69	2.78	3.70	4.80	2.68
Eu	0.82	0.83	0.79	0.83	0.51	0.40	0.43	0.39	0.47	0.40	0.53	0.68	0.39
Gd	8.10	8.30	7.61	8.21	3.40	3.67	3.98	3.55	4.39	2.83	4.01	5.88	2.92
Tb	1.14	1.15	1.06	1.16	0.48	0.50	0.56	0.49	0.61	0.42	0.60	0.86	0.43
Dy	7.46	7.65	6.91	7.53	2.85	3.22	3.49	3.20	3.94	2.49	3.59	5.47	2.58
Ho	1.60	1.63	1.48	1.61	0.59	0.69	0.72	0.66	0.84	0.52	0.75	1.15	0.54
Er	4.33	4.44	4.05	4.35	1.71	1.86	1.98	1.82	2.27	1.50	2.21	3.40	1.57
Tm	0.64	0.63	0.58	0.62	0.23	0.27	0.29	0.27	0.33	0.20	0.30	0.46	0.21
Yb	3.71	3.77	3.49	3.75	1.39	1.61	1.71	1.62	1.96	1.20	1.85	2.70	1.26
Lu	0.50	0.52	0.47	0.52	0.19	0.22	0.24	0.22	0.27	0.17	0.26	0.38	0.18
Y	133	133	124	133	41.6	54.5	55.1	52.8	65.1	38.1	57.8	98.8	42.4
ΣREE	94.1	97.3	91.5	95.7	92.2	47.2	52.8	45.1	56.3	66.2	79.5	77.1	56.0
LREEs	66.6	69.2	65.8	68.0	81.3	35.1	39.9	33.3	41.7	56.9	65.9	56.8	46.4
HREEs	27.5	28.1	25.7	27.8	10.9	12.0	13.0	11.8	14.6	9.33	13.6	20.3	9.68
LREE/HREE	2.42	2.46	2.56	2.45	7.49	2.92	3.08	2.81	2.85	6.09	4.86	2.80	4.79
(La/Yb)N	1.44	1.57	1.73	1.52	9.66	1.99	2.15	1.77	1.89	7.14	4.81	2.08	4.98
δEu	0.35	0.34	0.36	0.35	0.46	0.36	0.36	0.36	0.36	0.43	0.42	0.39	0.42
δCe	1.15	1.12	1.13	1.12	1.03	1.11	1.12	1.13	1.14	1.02	1.04	1.06	1.04
Sample			XBH-20	XBH-21	XBH-22	XBH-23	XBH-24	XBH-25	XBH-26	XBH-27	XBH-28	XBH-29	XBH-30
			Calcite	Calcite	Calcite	Calcite	Calcite	Calcite	Calcite	Calcite	Calcite	Calcite	Calcite
Cs			0.06	0.05	0.05	0.06	0.05	0.05	0.06	0.06	0.09	0.04	0.61
Pb			64.8	56.8	54.6	64.6	53.2	52.8	53.1	57.8	62.5	60.2	56.5
Rb			0.10	0.49	0.25	0.13	0.65	0.35	0.62	0.82	1.02	0.10	3.83
Ba			48.3	41.4	53.9	57.0	62.7	71.4	51.2	44.1	30.7	40.9	26.1
Th			0.10	0.10	0.08	0.08	0.17	0.08	0.09	0.03	0.03	0.05	0.04
U			0.05	0.04	0.04	0.04	0.05	0.03	0.04	0.03	0.03	0.03	0.07
Ta			0.02	0.01	0.05	0.02	0.02	0.01	0.01	0.01	0.01	0.01	0.01
Nb			0.02	0.01	0.01	0.02	0.01	0.01	0.01	0.02	0.01	0.01	0.01
Zr			0.11	0.00	0.22	0.23	0.03	0.10	0.20	0.20	0.26	0.06	0.12
Hf			0.06	0.09	0.07	0.04	0.05	0.07	0.04	0.09	0.06	0.03	0.07
Ti			0.54	0.54	0.60	0.66	0.47	0.70	0.71	0.53	0.41	0.48	0.54
Sr			1198	1191	1173	1211	1193	1240	1188	1204	1173	1233	1220
La			51.8	63.4	62.9	60.8	67.8	56.4	49.9	65.1	63.0	68.9	74.1
Ce			131	146	142	150	150	143	125	148	143	153	168
Pr			15.6	15.6	15.8	17.0	16.7	15.9	14.5	16.7	16.0	17.0	17.6
Nd			59.2	60.5	60.6	62.8	63.3	60.1	56.5	62.8	61.7	64.8	67.0
Sm			10.1	11.5	11.1	12.4	11.9	11.5	9.95	12.1	11.4	11.2	11.6
Eu			1.36	1.30	1.35	1.38	1.46	1.35	1.34	1.41	1.44	1.41	1.32
Gd			8.61	8.53	8.03	8.21	8.48	8.91	8.50	8.63	8.42	8.47	8.35
Tb			1.27	1.25	1.36	1.37	1.38	1.36	1.32	1.33	1.29	1.40	1.44
Dy			7.79	7.59	7.65	8.30	7.53	8.21	7.47	7.79	8.03	7.49	7.91
Ho			1.62	1.55	1.62	1.70	1.72	1.75	1.68	1.73	1.69	1.75	1.67
Er			4.82	4.69	5.15	4.84	5.07	4.74	4.91	4.95	4.88	4.89	4.89
Tm			0.74	0.73	0.73	0.71	0.73	0.67	0.75	0.77	0.70	0.75	0.77
Yb			5.42	5.65	5.73	6.03	5.86	5.30	6.06	5.85	5.50	5.81	5.82
Lu			0.85	0.86	0.81	0.85	0.88	0.88	0.93	0.82	0.84	0.88	0.86
Y			41.7	41.4	41.5	42.3	42.6	42.6	43.0	42.9	42.3	42.3	42.6
ΣREE			1487	1509	1486	1536	1523	1549	1465	1531	1489	1569	1579
LREEs			1381	1401	1378	1423	1411	1440	1364	1418	1379	1456	1462
HREEs			106	108	108	113	113	109	101	113	110	114	117
LREE/HREE			13.1	13.0	12.8	12.6	12.6	13.2	13.5	12.6	12.5	12.8	12.5
(La/Yb)N			6.49	7.63	7.46	6.85	7.86	7.23	5.59	7.56	7.78	8.05	8.65
δEu			0.43	0.38	0.42	0.39	0.42	0.39	0.43	0.40	0.43	0.42	0.39
δCe			1.10	1.09	1.06	1.11	1.05	1.14	1.11	1.06	1.06	1.05	1.09

Notes: La_N/Yb_N values are La/Yb ratios normalized to chondrite values after [27], Eu/Eu* = 2 × w(Eu)_N/[w(Sm)_N + w(Gd)_N], Ce/Ce* = 2 × w(Ce)_N/[w(La)_N + w(Pr)_N].

Table 2. Trace (ppm) element compositions of the fluorites and calcites at the Xiaobaihegou fluorite deposit.

Sample	XBH-1	XBH-2	XBH-3	XBH-4	XBH-5	XBH-6	XBH-7	XBH-8	XBH-9	XBH-10	XBH-11	XBH-12	XBH-13
	Early Fluorite	Early Fluorite	Early Fluorite	Early Fluorite	Early Fluorite	Late Fluorite	Late Fluorite	Late Fluorite	Late Fluorite	Late Fluorite	Late Fluorite	Late Fluorite	Late Fluorite
Cs	0.01	0.02	0.08	0.03	2.29	0.09	0.01	0.01	0.01	1.06	0.25	0.31	0.35
Pb	3.35	4.78	5.69	5.78	26.9	5.78	5.67	2.25	4.96	17.5	3.11	1.99	9.18
Rb	0.23	0.14	0.19	0.23	49.8	2.78	0.46	0.57	0.13	23.2	5.46	6.22	3.70
Ba	1.12	1.35	2.18	1.08	92.1	2.86	31.4	0.93	1.18	40.7	12.3	11.6	5.97
Th	0.24	0.19	0.10	0.09	2.20	0.65	0.28	0.22	0.07	1.29	0.59	0.51	2.70
U	0.25	0.26	0.19	0.33	1.51	0.71	0.64	0.69	0.53	2.13	1.10	0.61	1.85
Ta	0.15	0.14	0.08	0.09	0.04	0.20	0.12	0.09	0.18	0.04	0.01	0.01	0.01
Nb	0.19	0.18	0.16	0.18	3.48	0.55	0.59	0.25	0.28	4.82	1.29	0.47	3.79
Zr	4.04	2.97	1.79	1.57	8.75	10.9	10.8	4.83	1.27	4.20	0.73	0.84	0.79
Hf	0.36	0.32	0.27	0.26	0.26	0.38	0.38	0.25	0.15	0.12	0.02	0.03	0.02
Ti	0.00	0.00	0.00	22.3	95.0	0.00	918	23.4	0.00	53.4	16.1	15.4	8.08
Sr	358	354	323	371	348	405	402	414	394	384	419	388	425
La	7.46	8.26	8.40	7.97	18.7	4.47	5.12	4.00	5.16	12.0	12.4	7.83	8.78
Ce	26.0	27.1	26.3	26.4	37.8	14.1	16.2	13.3	16.9	25.6	29.1	22.5	20.4
Pr	4.13	4.24	3.91	4.22	4.33	2.18	2.47	2.10	2.57	3.14	3.78	3.48	2.66
Nd	21.9	22.2	20.4	22.1	16.7	10.9	12.3	10.5	12.9	12.9	16.3	17.5	11.4
Sm	6.33	6.57	5.99	6.43	3.30	3.07	3.35	2.99	3.69	2.78	3.70	4.80	2.68
Eu	0.82	0.83	0.79	0.83	0.51	0.40	0.43	0.39	0.47	0.40	0.53	0.68	0.39
Gd	8.10	8.30	7.61	8.21	3.40	3.67	3.98	3.55	4.39	2.83	4.01	5.88	2.92
Tb	1.14	1.15	1.06	1.16	0.48	0.50	0.56	0.49	0.61	0.42	0.60	0.86	0.43
Dy	7.46	7.65	6.91	7.53	2.85	3.22	3.49	3.20	3.94	2.49	3.59	5.47	2.58
Ho	1.60	1.63	1.48	1.61	0.59	0.69	0.72	0.66	0.84	0.52	0.75	1.15	0.54
Er	4.33	4.44	4.05	4.35	1.71	1.86	1.98	1.82	2.27	1.50	2.21	3.40	1.57
Tm	0.64	0.63	0.58	0.62	0.23	0.27	0.29	0.27	0.33	0.20	0.30	0.46	0.21
Yb	3.71	3.77	3.49	3.75	1.39	1.61	1.71	1.62	1.96	1.20	1.85	2.70	1.26
Lu	0.50	0.52	0.47	0.52	0.19	0.22	0.24	0.22	0.27	0.17	0.26	0.38	0.18
Y	133	133	124	133	41.6	54.5	55.1	52.8	65.1	38.1	57.8	98.8	42.4
ΣREE	94.1	97.3	91.5	95.7	92.2	47.2	52.8	45.1	56.3	66.2	79.5	77.1	56.0
LREEs	66.6	69.2	65.8	68.0	81.3	35.1	39.9	33.3	41.7	56.9	65.9	56.8	46.4
HREEs	27.5	28.1	25.7	27.8	10.9	12.0	13.0	11.8	14.6	9.33	13.6	20.3	9.68
LREE/HREE	2.42	2.46	2.56	2.45	7.49	2.92	3.08	2.81	2.85	6.09	4.86	2.80	4.79
(La/Yb)N	1.44	1.57	1.73	1.52	9.66	1.99	2.15	1.77	1.89	7.14	4.81	2.08	4.98
δEu	0.35	0.34	0.36	0.35	0.46	0.36	0.36	0.36	0.36	0.43	0.42	0.39	0.42
δCe	1.15	1.12	1.13	1.12	1.03	1.11	1.12	1.13	1.14	1.02	1.04	1.06	1.04
Sample			XBH-20	XBH-21	XBH-22	XBH-23	XBH-24	XBH-25	XBH-26	XBH-27	XBH-28	XBH-29	XBH-30
			Calcite	Calcite	Calcite	Calcite	Calcite	Calcite	Calcite	Calcite	Calcite	Calcite	Calcite
Cs			0.06	0.05	0.05	0.06	0.05	0.05	0.06	0.06	0.09	0.04	0.61
Pb			64.8	56.8	54.6	64.6	53.2	52.8	53.1	57.8	62.5	60.2	56.5
Rb			0.10	0.49	0.25	0.13	0.65	0.35	0.62	0.82	1.02	0.10	3.83
Ba			48.3	41.4	53.9	57.0	62.7	71.4	51.2	44.1	30.7	40.9	26.1
Th			0.10	0.10	0.08	0.08	0.17	0.08	0.09	0.03	0.03	0.05	0.04
U			0.05	0.04	0.04	0.04	0.05	0.03	0.04	0.03	0.03	0.03	0.07
Ta			0.02	0.01	0.05	0.02	0.02	0.01	0.01	0.01	0.01	0.01	0.01
Nb			0.02	0.01	0.01	0.02	0.01	0.01	0.01	0.02	0.01	0.01	0.01
Zr			0.11	0.00	0.22	0.23	0.03	0.10	0.20	0.20	0.26	0.06	0.12
Hf			0.06	0.09	0.07	0.04	0.05	0.07	0.04	0.09	0.06	0.03	0.07
Ti			0.54	0.54	0.60	0.66	0.47	0.70	0.71	0.53	0.41	0.48	0.54
Sr			1198	1191	1173	1211	1193	1240	1188	1204	1173	1233	1220
La			51.8	63.4	62.9	60.8	67.8	56.4	49.9	65.1	63.0	68.9	74.1
Ce			131	146	142	150	150	143	125	148	143	153	168
Pr			15.6	15.6	15.8	17.0	16.7	15.9	14.5	16.7	16.0	17.0	17.6
Nd			59.2	60.5	60.6	62.8	63.3	60.1	56.5	62.8	61.7	64.8	67.0
Sm			10.1	11.5	11.1	12.4	11.9	11.5	9.95	12.1	11.4	11.2	11.6
Eu			1.36	1.30	1.35	1.38	1.46	1.35	1.34	1.41	1.44	1.41	1.32
Gd			8.61	8.53	8.03	8.21	8.48	8.91	8.50	8.63	8.42	8.47	8.35
Tb			1.27	1.25	1.36	1.37	1.38	1.36	1.32	1.33	1.29	1.40	1.44
Dy			7.79	7.59	7.65	8.30	7.53	8.21	7.47	7.79	8.03	7.49	7.91
Ho			1.62	1.55	1.62	1.70	1.72	1.75	1.68	1.73	1.69	1.75	1.67
Er			4.82	4.69	5.15	4.84	5.07	4.74	4.91	4.95	4.88	4.89	4.89
Tm			0.74	0.73	0.73	0.71	0.73	0.67	0.75	0.77	0.70	0.75	0.77
Yb			5.42	5.65	5.73	6.03	5.86	5.30	6.06	5.85	5.50	5.81	5.82
Lu			0.85	0.86	0.81	0.85	0.88	0.88	0.93	0.82	0.84	0.88	0.86
Y			41.7	41.4	41.5	42.3	42.6	42.6	43.0	42.9	42.3	42.3	42.6
ΣREE			1487	1509	1486	1536	1523	1549	1465	1531	1489	1569	1579
LREEs			1381	1401	1378	1423	1411	1440	1364	1418	1379	1456	1462
HREEs			106	108	108	113	113	109	101	113	110	114	117
LREE/HREE			13.1	13.0	12.8	12.6	12.6	13.2	13.5	12.6	12.5	12.8	12.5
(La/Yb)N			6.49	7.63	7.46	6.85	7.86	7.23	5.59	7.56	7.78	8.05	8.65
δEu			0.43	0.38	0.42	0.39	0.42	0.39	0.43	0.40	0.43	0.42	0.39
δCe			1.10	1.09	1.06	1.11	1.05	1.14	1.11	1.06	1.06	1.05	1.09

Notes: La_N/Yb_N values are La/Yb ratios normalized to chondrite values after [27], Eu/Eu* = 2 × w(Eu)_N/[w(Sm)_N + w(Gd)_N], Ce/Ce* = 2 × w(Ce)_N/[w(La)_N + w(Pr)_N].

Table 3. Major element (%), trace element (ppm), and REE (ppm) analysis results of granites at the Xiaobaihegou fluorite deposit.

Sample	XB1	XB2	XB3	XB4	XB5	XB6	XB7
Major element (wt.%)
SiO2	73.1	74.7	74.3	75.3	75.0	73.6	73.1
Al2O3	15.0	14.7	14.6	14.0	14.3	12.6	15.0
Fe2O3t	1.68	0.76	0.99	0.79	0.88	1.73	0.85
CaO	0.43	0.54	0.62	0.63	0.59	1.20	0.54
MgO	0.10	0.14	0.14	0.14	0.13	0.69	0.13
K2O	3.91	5.07	4.51	4.20	4.12	7.76	6.57
Na2O	5.54	3.79	4.33	4.13	4.50	1.28	3.61
P2O5	0.08	0.10	0.12	0.09	0.09	0.21	0.10
TiO2	0.02	0.02	0.03	0.02	0.03	0.26	0.02
LOI	0.62	0.56	0.43	0.50	0.60	1.08	0.40
Total	99.9	99.8	99.6	99.3	99.6	99.3	99.9
A/CNK	0.60	0.67	0.64	0.64	0.63	0.75	0.68
A/NK	1.13	1.25	1.22	1.23	1.21	1.20	1.15
ALK	9.45	8.86	8.84	8.33	8.62	9.04	10.2
Mg#	10.6	26.7	21.9	26.0	22.6	44.1	23.3
Trace element (ppm)
Bi	1.65	0.22	0.45	0.23	0.39	0.10	2.63
Li	9.07	10.8	15.3	16.0	12.2	10.9	9.00
Be	5.93	9.01	8.51	6.82	8.17	1.09	7.25
Sc	1.81	1.32	0.81	0.76	0.83	4.20	0.96
Co	1.60	2.61	2.02	1.92	2.01	5.45	1.96
Cu	3.48	6.08	3.48	2.77	3.89	4.58	2.33
Zn	8.04	34.8	42.5	29.7	30.8	31.7	43.2
Ga	16.7	20.6	18.0	19.3	20.5	13.5	23.4
Rb	279	475	427	372	396	297	498
Sr	57.7	21.1	16.6	20.8	24.9	80.6	19.0
Zr	23.8	17.1	17.8	16.1	18.8	105	10.8
Nb	12.1	17.5	16.0	11.9	15.3	5.11	21.1
Cs	9.56	22.9	14.1	11.5	12.2	6.97	18.6
Ba	77.9	76.8	61.5	91.4	84.4	670	75.7
Hf	1.22	1.29	1.09	0.97	1.16	3.48	0.62
Ta	2.94	1.73	1.59	1.11	1.44	0.35	2.01
W	20.0	1.17	1.13	0.83	1.12	0.75	1.26
Tl	1.55	2.01	1.74	1.53	1.64	1.42	2.18
Pb	36.5	37.9	35.1	31.1	31.0	27.1	32.2
Th	2.86	3.54	3.52	3.77	3.71	7.72	5.61
U	10.6	5.91	5.25	7.36	4.67	1.39	7.70
La	1.48	5.18	6.48	6.82	6.71	19.3	6.01
Ce	4.36	10.9	10.2	10.5	10.5	33.2	10.4
Pr	0.43	1.26	1.44	1.49	1.46	5.37	1.49
Nd	1.59	3.90	4.35	4.52	4.32	18.9	4.26
Sm	0.64	1.28	1.60	1.62	1.38	4.77	1.88
Eu	0.07	0.13	0.16	0.16	0.16	1.06	0.12
Gd	0.84	1.37	1.70	1.72	1.51	4.77	2.14
Tb	0.19	0.21	0.25	0.26	0.22	0.66	0.37
Dy	1.21	1.18	1.38	1.50	1.27	4.63	2.29
Ho	0.23	0.15	0.17	0.20	0.16	0.77	0.30
Er	0.67	0.45	0.47	0.52	0.48	2.43	0.78
Tm	0.11	0.06	0.06	0.07	0.06	0.30	0.10
Yb	0.76	0.42	0.45	0.50	0.45	1.86	0.67
Lu	0.10	0.06	0.07	0.07	0.07	0.26	0.09
Y	6.10	5.94	6.81	7.73	6.49	25.4	11.3
∑REE	12.7	26.6	28.8	30.0	28.8	98.3	30.9
LREEs	8.57	22.7	24.2	25.1	24.5	82.6	24.2
HREEs	4.11	3.90	4.55	4.84	4.22	15.7	6.74
LREE/HREE	2.08	5.81	5.33	5.19	5.81	5.27	3.58
(La/Yb)N	0.63	4.00	4.67	4.42	4.83	3.36	2.91
δEu	0.28	0.30	0.30	0.29	0.34	0.68	0.18
δCe	1.31	1.03	0.80	0.79	0.81	0.78	0.84

Notes: Mg# = Mg/(Mg + Fe²⁺) × 100.

Table 3. Major element (%), trace element (ppm), and REE (ppm) analysis results of granites at the Xiaobaihegou fluorite deposit.

Sample	XB1	XB2	XB3	XB4	XB5	XB6	XB7
Major element (wt.%)
SiO2	73.1	74.7	74.3	75.3	75.0	73.6	73.1
Al2O3	15.0	14.7	14.6	14.0	14.3	12.6	15.0
Fe2O3t	1.68	0.76	0.99	0.79	0.88	1.73	0.85
CaO	0.43	0.54	0.62	0.63	0.59	1.20	0.54
MgO	0.10	0.14	0.14	0.14	0.13	0.69	0.13
K2O	3.91	5.07	4.51	4.20	4.12	7.76	6.57
Na2O	5.54	3.79	4.33	4.13	4.50	1.28	3.61
P2O5	0.08	0.10	0.12	0.09	0.09	0.21	0.10
TiO2	0.02	0.02	0.03	0.02	0.03	0.26	0.02
LOI	0.62	0.56	0.43	0.50	0.60	1.08	0.40
Total	99.9	99.8	99.6	99.3	99.6	99.3	99.9
A/CNK	0.60	0.67	0.64	0.64	0.63	0.75	0.68
A/NK	1.13	1.25	1.22	1.23	1.21	1.20	1.15
ALK	9.45	8.86	8.84	8.33	8.62	9.04	10.2
Mg#	10.6	26.7	21.9	26.0	22.6	44.1	23.3
Trace element (ppm)
Bi	1.65	0.22	0.45	0.23	0.39	0.10	2.63
Li	9.07	10.8	15.3	16.0	12.2	10.9	9.00
Be	5.93	9.01	8.51	6.82	8.17	1.09	7.25
Sc	1.81	1.32	0.81	0.76	0.83	4.20	0.96
Co	1.60	2.61	2.02	1.92	2.01	5.45	1.96
Cu	3.48	6.08	3.48	2.77	3.89	4.58	2.33
Zn	8.04	34.8	42.5	29.7	30.8	31.7	43.2
Ga	16.7	20.6	18.0	19.3	20.5	13.5	23.4
Rb	279	475	427	372	396	297	498
Sr	57.7	21.1	16.6	20.8	24.9	80.6	19.0
Zr	23.8	17.1	17.8	16.1	18.8	105	10.8
Nb	12.1	17.5	16.0	11.9	15.3	5.11	21.1
Cs	9.56	22.9	14.1	11.5	12.2	6.97	18.6
Ba	77.9	76.8	61.5	91.4	84.4	670	75.7
Hf	1.22	1.29	1.09	0.97	1.16	3.48	0.62
Ta	2.94	1.73	1.59	1.11	1.44	0.35	2.01
W	20.0	1.17	1.13	0.83	1.12	0.75	1.26
Tl	1.55	2.01	1.74	1.53	1.64	1.42	2.18
Pb	36.5	37.9	35.1	31.1	31.0	27.1	32.2
Th	2.86	3.54	3.52	3.77	3.71	7.72	5.61
U	10.6	5.91	5.25	7.36	4.67	1.39	7.70
La	1.48	5.18	6.48	6.82	6.71	19.3	6.01
Ce	4.36	10.9	10.2	10.5	10.5	33.2	10.4
Pr	0.43	1.26	1.44	1.49	1.46	5.37	1.49
Nd	1.59	3.90	4.35	4.52	4.32	18.9	4.26
Sm	0.64	1.28	1.60	1.62	1.38	4.77	1.88
Eu	0.07	0.13	0.16	0.16	0.16	1.06	0.12
Gd	0.84	1.37	1.70	1.72	1.51	4.77	2.14
Tb	0.19	0.21	0.25	0.26	0.22	0.66	0.37
Dy	1.21	1.18	1.38	1.50	1.27	4.63	2.29
Ho	0.23	0.15	0.17	0.20	0.16	0.77	0.30
Er	0.67	0.45	0.47	0.52	0.48	2.43	0.78
Tm	0.11	0.06	0.06	0.07	0.06	0.30	0.10
Yb	0.76	0.42	0.45	0.50	0.45	1.86	0.67
Lu	0.10	0.06	0.07	0.07	0.07	0.26	0.09
Y	6.10	5.94	6.81	7.73	6.49	25.4	11.3
∑REE	12.7	26.6	28.8	30.0	28.8	98.3	30.9
LREEs	8.57	22.7	24.2	25.1	24.5	82.6	24.2
HREEs	4.11	3.90	4.55	4.84	4.22	15.7	6.74
LREE/HREE	2.08	5.81	5.33	5.19	5.81	5.27	3.58
(La/Yb)N	0.63	4.00	4.67	4.42	4.83	3.36	2.91
δEu	0.28	0.30	0.30	0.29	0.34	0.68	0.18
δCe	1.31	1.03	0.80	0.79	0.81	0.78	0.84

Notes: Mg# = Mg/(Mg + Fe²⁺) × 100.

5.3. Fluid Inclusions

The ore samples from the mining area are abundant in fluid inclusions (FIs), making them ideal for petrographic and microthermometric studies of FIs. Based on phase relationships at 25 °C, optical observations of more than 300 inclusions identified three major types of FIs in Xiaobaihegou. Liquid-rich (L-type) inclusions are dominant (up to 90%) in host minerals across all mineralization stages. These two-phase L-type inclusions, with a vapor phase comprising 5–20 vol% of the total inclusion volume, are significantly more abundant than other inclusion types observed in the samples. They vary in size from 5 to 60 μm and commonly exhibit elongated, triangular, elliptical, negative crystal, or irregular shapes (Figure 6a,b).

Vapor-rich two-phase (V-type) inclusions mostly occur as isolated entities in early-stage fluorite and calcite crystals, exhibiting rounded or regular morphologies (Figure 6c). They are less than 40 μm in size, with a vapor volume percentage ranging from 55% to 90%. Solid-bearing inclusions (S-type) are three-phase FIs that contain daughter minerals (Figure 6d–i).

Multiphase fluid inclusions are mostly found in purple–black and purple fluorite minerals, and a small amount can be seen in calcite and quartz. They are mostly elliptical or long strips, ranging in size from 2 to 120 μm, and can be roughly divided into two categories: those measuring 2 to 5 μm in size, grouped together, and those measuring 20 to 120 μm in dispersion.

Laser Raman microspectroscopy was employed to characterize the chemical composition of fluid inclusions. Multiphase daughter-mineral-bearing inclusions and CO₂-rich inclusions were specifically targeted for analysis. Raman spectral signatures reveal the following diagnostic peaks: The characteristic Raman peak of fluorite is at 315.8 cm⁻¹ (Figure 7a); for CH₄, the characteristic peak is at 2914 cm⁻¹ (Figure 7a); for H₂O, the characteristic peak is at 3441 cm⁻¹ (Figure 7b); for CO₂, the characteristic peaks are at 1279 and 1382 cm⁻¹ (Figure 7c); the daughter minerals contain gypsum, with a characteristic Raman peak at 1018 cm⁻¹ (Figure 7d). Due to the lack of distinctive Raman scattering signals for halide salts, the presence of NaCl and KCl was inferred primarily from petrographic observations. NaCl daughter minerals are colorless, cubic, and homogeneous, with a particle size of about 5–10 μm. KCl daughter mineral is colorless or light green, homogeneous, cubic crystal, but because of corrosion, the edge of the daughter mineral is often ground into a rounded shape, with a particle size of about 5–50 μm; gypsum daughter mineral is colorless, plate, with a particle size of about 2–10 μm. In general, the appearance of the above daughter minerals indicates the existence of a high-salinity fluid.

Based on petrological characteristics, these inclusions formed synchronously with the host mineral, distributed regularly along crystal growth textures, and they displayed homogeneous compositional and physicochemical properties. A total of over 100 primary FIs were identified via microscopic observation and freezing–heating experiments [28] (Figure 8). Microthermometric data are presented in

Table 4. Microthermometric data for primary fluid inclusions of the fluorite deposits in the Kaerqiaer district.

Deposit	Host Mineral	Numbers	Th (°C) (Main Range)	Salinity (wt.%) (Main Range)	Density (g/cm3)	Date Sources
Xiaobaihegou	Early-stage Fluorite	9	235~426	28.59~42.40	0.93~1.03	this study
	Late-stage Fluorite	23	129~350	0.88~21.61	0.61~0.93
Kumutashi	Early-stage Fluorite	13	225~390	5.20~8.91	0.63~0.88	[29]
	Late-stage Fluorite	16	117~215	0.53~12.73	0.89~1.02
Kaerqiaer	Fluorite	37	135~359	2.07~3.87	/	[8]

. Notably, the homogenization temperatures and salinities of FIs in different fluorite deposits within the Kaerqiaer district exhibit consistent characteristics. Liquid-rich (L-type) FIs were homogenized to the liquid phase, whereas vapor-rich (V-type) FIs were homogenized to the vapor phase.

As mentioned above, the ore-forming process is subdivided into two stages: an early stage and a late stage. Early-stage massive ores are characterized by gas-rich two-phase inclusions and three-phase inclusions containing daughter crystals, with homogenization temperatures ranging from 235 to 426 °C, salinities of 28.59–42.40 wt.% NaCl eq., and densities of 0.93–1.03 g/cm³. In the late stage, brecciated and stockwork ores host predominantly liquid-rich two-phase and gas-rich two-phase fluid inclusions. These inclusions exhibit homogenization temperatures ranging from 129 to 350 °C, salinities of 0.88 to 21.61 wt% NaCl eq., and densities of 0.61–0.93 g/cm³. For individual deposits in the Kaerqiaer district, both homogenization temperatures and salinities show a gradual decrease from the early to late stages.

5.4. C-H-O Isotopes Geochemistry

The carbon and oxygen isotopic compositions of calcite samples obtained from the Xiaobaihegou are compiled in

Table 5. Carbon and oxygen isotopic compositions of calcites from the Xiaobaihegou fluorite deposit.

Sample	δ13CV-PDB (‰)	δ18OV-PDB (‰)	δ18OSMOW (‰)
XB-01	−6	−15.13	15.31
XB-02	−6.01	−14.99	15.46
XB-03	−6.04	−14.88	15.57
XB-04	−5.01	−18.6	11.74
XB-05	−4.98	−18.8	11.53
XB-06	−4.93	−18.36	11.99
XB-07	−4.31	−18.89	11.44
XB-08	−4.31	−18.89	11.44
XB-09	−4.36	−18.75	11.58
XB-10	−5.36	−11.6	18.95
XB-11	−5.57	−16.14	14.27
XB-12	−5.23	−20.09	10.2
XB-13	−4.06	−20.01	10.28
XB-14	−4.73	−20.41	9.87
XB-15	−9.53	−13.56	16.93
XB-16	−5.19	−20.22	10.07
XB-17	−5.21	−20.51	9.76
XB-18	−6.38	−13.06	17.45
XB-19	−3.75	−19.01	11.31

. The δ¹³C_V-PDB and δ¹⁸O_V-SMOW values of the calcite in Xiaobaihegou vary from −9.53‰ to −3.75‰ (mean −5.31‰) and from 9.76 to 17.45 (mean 12.90), respectively.

The obtained hydrogen and oxygen isotopic compositions of fluorite from the Xiaobaihegou are presented in

Table 6. Oxygen and hydrogen isotopic compositions of fluorites from the Xiaobaihegou fluorite deposit.

Sample	δDV-SMOW (‰)	δ18OV-SMOW (‰)
XB-01	−91.1	0.5
XB-02	−88.2	0.4
XB-03	−85.4	−1.4
XB-04	−85.8	−1.1
XB-05	−72.6	−3.6
XB-06	−70.8	−3.2
XB-07	−81.9	−9.3
XB-08	−83.2	−9.4
XB-09	−85.1	−2.2
XB-10	−47.9	−3.6
XB-11	−49.7	−3.1

. The δ¹⁸O_V-SMOW and δD_V-SMOW values of the fluorite in Xiaobaihegou vary from −9.4‰ to 0.5‰ (mean −3.3‰) and from −91.1‰ to −47.9‰ (mean −76.5‰), respectively.

5.5. Sr-Nd Isotopic Compositions

The Rb-Sr isotopic data of fluorite from the Xiaobaihegou deposit are presented in

Table 7. Sr-Nd isotopic composition of fluorites from Xiaobaihegou deposit.

Sample	Mineral	Rb/10−6	Sr/10−6	Rb/Sr	87Sr/86Sr	Sm/10−6	Nd/10−6	147Sm/144Nd	143Nd/144Nd
XB-01	Fluorite	49.8	348	0.14	0.712717	3.61	19.3	0.1129	0.511946
XB-02	Fluorite	23.2	384	0.06	0.711282	2.78	12.9	0.1299	0.511988
XB-03	Fluorite	5.46	419	0.01	0.710491	3.70	16.3	0.1370	0.512009
XB-04	Fluorite	6.22	388	0.02	0.710756	4.80	17.5	0.1654	0.512073
XB-05	Fluorite	3.70	425	0.01	0.710332	2.68	11.4	0.1421	0.512005

Note: εNd(t) is calculated with ¹⁴³Nd/¹⁴⁴Nd = 0.512638 and ¹⁴⁷Sm/¹⁴⁴Nd = 0.1967 for present-day CHUR [30]; T_DM is calculated with ¹⁴³Nd/¹⁴⁴Nd = 0.51315 and ¹⁴⁷Sm/¹⁴⁴Nd = 0.2137 for present-day depleted mantle [31].

. The ⁸⁷Sr/⁸⁶Sr ratios of fluorite from this deposit range from 0.71033 to 0.71272, with a mean value of 0.71112. The ¹⁴³Nd/¹⁴⁴Nd ratios vary from 0.511946 to 0.512073, with a mean of 0.512004.

6. Discussion

6.1. Timing of Fluorite Mineralization and Geodynamic Setting

LA-ICP-MS zircon U-Pb dating of the alkali-feldspar granite in the Xiaobaihegou fluorite mining area in this study shows that the rock-forming age is 482.3 ± 4.1 Ma, indicating that the rock was formed in the early Paleozoic Ordovician. Generally, the metallogenic age of magmatic–hydrothermal fluorite ore is later than the formation age of the ore-forming rock body. Flesh-red alkali-feldspar granite vein bodies are found in all the fluorite mining areas in the Kaerqiaer area. Fluorite mineralization mainly occurs near the contact zone between the inner and outer rock bodies, and fluorite–calcite veins are interspersed in alkali-feldspar granite vein bodies (Figure 3b–d). The alkali-feldspar granite exhibits intense hydrothermal alteration, characterized by carbonatization, fluoritization, silicification, and other associated alteration types. These results indicate a close genetic relationship between the alkali-feldspar granite and fluorite mineralization in this area. Meanwhile, studies on the Kumutashi fluorite deposit indicate that the alkali-feldspar granite in the mining area is a fluorine-rich rock with an average w(F) > 0.1%. Its magmatic emplacement age is constrained to 450 ± 2.7 Ma [10], which is consistent with the LA-ICP-MS U-Pb age of apatite from fluorite ore bodies (448 ± 27 Ma) [5]. However, no magmatic rocks or wall-rock alterations associated with ~450 Ma magmatism (analogous to that in the Kumutashi area) have been identified in the study area, nor have late-stage dikes, hydrothermal veins with zircon ages of 450 Ma, or isotopic signatures of mantle/crustal melting during this stage been detected in our sampling and analysis. There is a 30 Ma age discrepancy between the study area and the Kumutashi region. Although the formation age of fluorite deposits is typically slightly postdated that of their host granites, a 30 Ma interval exceeds the expected temporal offset. We hypothesize that the mineralization timing of the Xiaobaihegou fluorite deposit is distinct from that of the Kumutashi deposit. Specifically, the Xiaobaihegou deposit likely formed at ~480 Ma, whereas the Kumutashi deposit formed at ~450 Ma. However, the granites from the Xiaobaihegou and Kumutashi fluorite deposits exhibit similar geochemical characteristics. They are characterized by high SiO₂, high K₂O, and high alkali contents, coupled with low CaO and low MgO; they are enriched in large ion lithophile elements (LILEs), relatively depleted in high field strength elements (HFSEs), with a high total rare earth element content (ΣREE), enriched in light rare earth elements (LREEs), relatively depleted in heavy rare earth elements (HREEs), and generally exhibit a strong negative Eu anomaly, thus exhibiting characteristics typical of A-type granites. Therefore, their similar granite properties may be one of the important reasons why both can form fluorite deposits.

Regionally, large-scale early Paleozoic magmatic rocks developed along the southwest margin of the Altyn-Tagh Orogen are the products of magmatic activity during the oceanic to continental transition between the Middle Altyn Block and the Qaidam Block [13,18,32,33]. Studies of regional ultra-high-pressure metamorphic rocks indicate that the peak metamorphic age is concentrated between 504 and 486 Ma, and the retrograde metamorphic age is ~450 Ma [13,18]. Studies of granites near Kaerqiaer show that monzonitic granites in the Paxialayidang area yield a zircon U-Pb age of 460 ± 4 Ma. These rocks formed in a tectonic environment transitioning from a compressive to an extensional system [32]. The zircon U-Pb age of the granitic rocks in the Qingshuiquan area is 451 ± 4 Ma, indicating that they were formed in an extensional tectonic regime [33]. In contrast, the mafic and ultramafic intrusions, dated at 468–454 Ma, suggest that the collisional orogeny had shifted to the extensional stage by this time [34]. All the above studies indicate that during the Middle and Late Ordovician, the Middle Altyn Block and Qaidam Block transitioned from a compressional orogenic regime to an extensional tectonic setting. The Kaerqiaer super-large fluorite belt thus represents the product of magmatic activity during this tectonic transition. In addition, large-scale pegmatite dyke swarms formed during the Early and Middle Ordovician collisional orogenic stage occur in the region. For example, the zircon U-Pb age of the ore-forming biotite monzonite in the Tugeman Li-Be rare metal deposit is 475–482 Ma. The U-Pb age of niobium–tantalite in ore-bearing pegmatite vein is 472 ± 8 Ma, and the U-Pb age of kainite is 468 ± 8.7 Ma [17,34,35]. In summary, the Early Paleozoic Caledonian orogeny represents a critical period for the mineralization of regional fluorite and Li-Be rare metal deposits.

6.2. Source of Ore-Forming Fluids

FIs within fluorite (the ore mineral) provide more direct and reliable constraints on ore-forming fluid characteristics while minimizing uncertainties [36,37]. Thus, this FIs study focused primarily on fluorite-hosted inclusions, with minor analysis of calcite-hosted inclusions. In the Kaerqiaer district, FIs are predominantly L-type, with minor V-type and S-type inclusions.

The homogenization temperatures and salinities of FIs in the early ore-forming stage of the Xiaobaiheigou deposit range from 235 to 426 °C and 28.59 to 42.40 wt.% NaCl eq., respectively, indicating a medium–high-temperature and medium–high-salinity fluid system. In the late stage, these parameters decrease to 129–350 °C and 0.88–21.61 wt.% NaCl eq., reflecting a medium–low-temperature and medium–low-salinity fluid regime [4].

For the Kaerqiaer deposit, liquid-rich FIs exhibit homogenization temperatures of 135–237 °C and salinities of 2.07–7.59 wt.% NaCl eq., indicating low-temperature and low-salinity hydrothermal fluids. Minor CO₂-bearing three-phase fluid inclusions in fluorite exhibit homogenization temperatures of 240–359 °C and salinities of 2.58–3.39 wt.% NaCl eq., suggesting the presence of a moderate-temperature and low-salinity fluid end-member [7,8].

In the Kumutashi fluorite deposit, early-stage FIs have homogenization temperatures of 225.1–410.8 °C and salinities of 5.20–11.00 wt.% NaCl eq., indicating a medium–high-temperature and medium–low-salinity fluid. Late-stage FIs show lower homogenization temperatures (117.2–291.2 °C) and salinities (0.53–12.73 wt.% NaCl eq.), consistent with medium–low-temperature and low-salinity fluids [29].

Regionally, both homogenization temperature and salinity decrease from the early to late ore-forming stages. The early-stage fluids were NaCl-H₂O-CO₂ systems with medium-to-high temperatures and salinities, while the late-stage fluids evolved into NaCl-H₂O-CO₂ systems with medium-to-low temperatures and salinities.

In the Xiaobaihegou deposit, calcite occurs in coexistence with fluorite, both forming cross-cutting bands or veins. Hydrothermal-derived calcite is typically enriched in LREEs [38]. The calcite analyzed in this study exhibits LREE enrichment (Figure 5). Oxygen, carbon, and hydrogen isotope compositions are used to constrain fluid sources [39,40]. Carbon–oxygen isotopes in calcite indicate that the ore-forming fluids were predominantly of magmatic origin (Figure 9). For hydrogen–oxygen isotopes, δ¹⁸O_V-SMOW and δD_V-SMOW values of samples from the Kaerqiaer district (encompassing Xiaobaiheigou, Kumutashi, and Kaerqiaer deposits) plot between magmatic fluids and atmospheric precipitations (Figure 10), suggesting that the ore-forming fluid represents a mixture of magmatic water and atmospheric precipitation.

Regional fluorite mineralization is closely associated with alkali-feldspar granite, with ore bodies primarily developed in fault tectonic belts of the rock mass. Early-stage ore-forming fluids were characterized by medium-to-high temperatures, indicating they were predominantly magmatic water. The late-stage ore-forming fluids exhibited medium-to-low temperature characteristics due to the incorporation of substantial atmospheric precipitation, thus forming a mixed-fluid system of magmatic water and atmospheric precipitation.

In the Xiaobaihegou fluorite deposit, fluorite formed during the early mineralization stage has higher total rare earth element (REE) contents, whereas that formed during the late mineralization stage has relatively lower total REE contents (Table 2). The REE distribution patterns of fluorite from the early and late stages are similar, with both stages of fluorite precipitated from fluids evolved from the same parent magmatic–hydrothermal system. Consistent REE distribution patterns indicate a common fluid source, which is likely derived from the exsolution of the same alkali-feldspar granite magma. However, differences in ore textures and fluid inclusions between the early and late stages reflect the progressive evolution of the fluid over time, such as decreasing temperature and salinity. The Tb/La vs. Tb/Ca bivariate diagram, established based on studies of over 150 fluorite deposits worldwide, is used to determine the genetic origins of fluorite and identify water–rock reactions between ore-forming fluids and host rocks [41]. This diagram classifies fluorite deposits into three genetic types: (1) pegmatite (gas–liquid) origin; (2) hydrothermal origin; and (3) sedimentary genesis. Here, the Tb/Ca atomic ratio reflects the chemical environment during fluorite crystallization, carrying genetic information, while the Tb/La ratio indicates the degree of REE fractionation in ore-forming fluids, implying assimilative mixing of the liquids with surrounding rocks during mineralization.

Fluorite samples from the Xiaobaihegou deposit plot within the hydrothermal origin field (Figure 11), demonstrating that the fluorite deposits in this area formed through magmatic–hydrothermal processes. The significant variation in Tb/Ca ratios suggests that water–rock interactions occurred between ore-forming fluids and wall rocks. The fluorite samples plotted in Figure 11 include both early- and late-stage varieties from the Xiaobaihegou district. As expected, their data points are distributed along both the primary crystallization and remobilization trends. This is well illustrated in Figure 11: those of early fluorites lie along the primary crystallization trend, while late-stage fluorite data points align with the remobilization trend. Additionally, these fluorites are compared with samples from Iran’s Bozijan deposit and the United States’ Sierra deposit; both are hydrothermal fluorite deposits with analogous geochemical characteristics [42,43].

Sr-Nd isotopic analyses of the Xiaobaihegou fluorite deposit reveal ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.71033 to 0.71272 and ¹⁴³Nd/¹⁴⁴Nd ratios from 0.511946 to 0.512073, indicating crustal origin characteristics [44,45]. This is consistent with findings from the Kumutashi and Kaerqiaer fluorite deposits (Figure 12). Data in Table 7 reveal the Sr and Nd isotopic compositions of the ore veins. The measured radiogenic isotope characteristics closely reflect the source and are unlikely to have been significantly modified by wall-rock assimilation or hydrothermal overprinting. Initial isotopic values of the ore vein material show little variation and form a limited data cluster on conventional diagrams (Figure 12). These similarities indicate that the Xiaobaihegou, Kumutashi, and Kaerqiaer fluorite deposits were formed in the same tectonic setting and share a generally consistent source of ore-forming material.

6.3. Fluid Evolution and Precipitation Mechanisms

The principal mechanisms governing fluorite precipitation from ore-bearing hydrothermal fluids can be categorized as follows: (1) water–rock interaction between hydrothermal fluids and host lithologies; (2) thermobaric changes within the hydrothermal system; and (3) mixing of chemically distinct hydrothermal fluid populations [46]. Among these, water–rock interaction is proposed to be the dominant precipitation mechanism [47], as evidenced by typical fluorite deposits, including, but not limited to, the Xiaobeigou deposit in Inner Mongolia, the Yixian deposit in Liaoning Province, and the Wuyi deposit in Zhejiang Province. These deposits are characterized by medium–low-temperature, low-salinity, and low-density fluid regimes, where water–rock interaction plays a pivotal role in fluorite deposition [48].

Chinese fluorite deposits predominantly occur in association with medium- to low-temperature (homogenization temperatures generally <300 °C) and low-salinity hydrothermal systems [2,47,48,49,50,51,52,53,54,55,56,57,58,59]. For instance, the fluorite metallogenic belt in southeastern Sichuan and the Shuanghe barite–fluorite deposit in northeastern Guizhou exhibit narrow ranges of fluid inclusion homogenization temperatures, collectively indicative of low-temperature fluid characteristics. Upwelling ore-forming fluids, after enriching ore-forming elements, migrate to favorable stratigraphic horizons, where subsequent cooling induces fluorite precipitation [2,47,48,49,50,51].

Fluid mixing typically involves two end-member fluids: high-temperature/high-salinity and low-temperature/low-salinity, as documented in the Sumochagan Aobao fluorite deposit [2]. The presence of these dual-fluid end-members in the Xiaobaihegou fluorite deposit suggests a plausible role of fluid mixing in promoting fluorite precipitation.

Fluid inclusion studies reveal that, in the Xiaobaihegou fluorite deposit, primary fluid inclusions from the early mineralization stage have homogenization temperatures (Th) ranging from 235 to 426 °C, whereas those from the late stage exhibit lower Th values (129–350 °C), indicating a progressive cooling trend. The early ore-forming fluids are characterized by medium-to-high salinities (28.59–42.40 wt.% NaCl equivalent), which decrease to 0.88–21.61 wt.% NaCl equivalent in the late stage. Fluorite (CaF₂) precipitates when the ion activity product (aCa²⁺ × aF⁻) exceeds its solubility product (Ksp) in the fluid, and in this study area, this process is primarily triggered by the following changes in fluid conditions: (1) Cooling of the ore-forming fluids. Fluorite solubility decreases significantly with decreasing temperature. During the early stage, fluids migrate at 235–426 °C, where Ca²⁺ and F⁻ remain in solution due to high solubility; as these fluids ascend to shallower crustal levels (lower pressure) and cool to 129–350 °C via heat loss to wall rocks, their capacity to retain Ca²⁺ and F⁻ diminishes. This cooling-driven reduction in solubility causes aCa²⁺ × aF⁻ to exceed Ksp, thereby triggering fluorite precipitation. (2) Fluid mixing with meteoric water. Late-stage fluids exhibit lower salinities and Th values compared to the early stage, accompanied by shifts in H-O isotopes, which indicate mixing with meteoric water (with regional δ¹⁸O values of −9.4 to 0.5‰ and δD values of −91.1 to −47.9‰). The input of meteoric water dilutes the fluid, reduces salinity, and lowers temperature, further decreasing fluorite solubility; additionally, meteoric water introduces minor Ca²⁺ from altered wall rocks (e.g., carbonated country rocks), which increases aCa²⁺ and pushes aCa²⁺ × aF⁻ above Ksp.

6.4. Genetic Model of the Xiaobaihegou Fluorite Deposit

Based on the above observations, the genetic model of fluorite mineralization in the Xiaobaihegou deposit is synthesized as follows. The fluorite ore-forming process represents a complex geochemical evolution. According to the metallogenic background, ore-forming characteristics, and specific conditions of the Altyn fluorite belt, the ore-forming model is established in Figure 13, summarizing the process as follows.

In the Xiaobaihegou area, magma migrated upward and mixed with crustal materials under strong extensional tectonics, forming A-type granitic magma that ascended and emplaced along regional deep faults to the shallow crust [5,12,13,14,15]. During upward emplacement, metallogenic elements and volatile components were highly enriched in magmatic–hydrothermal solutions. As the F-rich magmatic–hydrothermal fluids migrated along tectonic fractures, they continuously reacted with Ca-rich strata (e.g., biotite plagioclase gneiss and marble of the Altyn Group), activating and extracting Ca, Mg, Na, and other elements to form F-bearing ore-forming hydrothermal fluids [5,7,8,9,10]. The escape of F-rich fluids from magma involves several key processes, including the enrichment of F in residual magma during fractional crystallization, fluid exsolution triggered by decreasing pressure and temperature, and the role of F in reducing magma viscosity to facilitate fluid migration.

Since different types of inclusions can be found coexisting within the same fluorite grain (Figure 6j), and as further illustrated in Figure 8b, all data points are not distributed along a single cooling/dilution trend line; these observations collectively strongly indicate that phase separation (immiscibility) occurred within a specific temperature interval, thereby trapping inclusions of different phases. The ore-forming fluids underwent fluid unmixing, where F played a dual role: acting as a catalyst for immiscibility and as a complexing agent with metallogenic elements to facilitate their migration, and as the physico-chemical conditions of the hydrothermal system (e.g., acid–base properties, redox state, pH) changed, the ore-bearing fluids migrated to suitable ore-hosting environments, interacting with host rocks to induce carbonatization, which, in turn, triggered the decomposition of F-containing complexes, releasing F⁻ and Ca²⁺ ions that combined to form CaF₂, precipitating and filling favorable fractures [57,58,59].

Fluid inclusion studies indicate that the early-stage ore-forming fluids belong to a medium–high-temperature and -salinity fluid system, whereas the late-stage ore-forming fluids are characterized as medium–low-temperature and -salinity fluids. For the early-stage, stable isotope compositions and fluid inclusion component analysis indicate the fluids were weakly acidic and existed in a relatively reducing environment. These conditions favored the transport of fluorine as F⁻ and metal–fluoride complexes (e.g., CaF⁺). For the late-stage, influx of meteoric water and fluid mixing caused a shift to near-neutral to weakly alkaline conditions. The redox state became more oxidizing, as indicated by the appearance of calcite in late veins. This pH increase reduced the solubility of fluorite by decreasing F⁻ activity, while the oxidizing shift destabilized metal–fluoride complexes: both are critical triggers for late-stage fluorite precipitation. These physicochemical parameters are closely related to the migration and precipitation of fluorine in the fluids [52,53,54,55,56,57,58,59].

7. Conclusions

(1)

The Xiaobaihegou fluorite deposit is controlled by NE-trending secondary faults, which provide space for the orebodies. Fluorite mineralization is closely related to alkali-feldspar granite, and fluorite veins occur in fractures within or near the fracture zones of alkali-feldspar granite dykes. Most ore bodies extend >10 km in a discontinuous N-S trend. Mineralogy is simple, dominated by fluorite and calcite.

(2)

The formation age of the alkali-feldspar granite related to mineralization in Xiaobaihegou fluorite mine is 482.3 ± 4.1 Ma (MSWD = 0.016), which was formed in the Ordovician era.

(3)

The REE distribution pattern of the Xiaobaihegou deposit is of a right-leaning, LREE-enriched type with a significant negative Eu anomaly. This pattern is highly similar to that of the ore-forming alkali-feldspar granite, suggesting that the REE characteristics of fluorite and calcite may have been inherited from the granite.

(4)

The ore-forming process can be divided into an early and a late stage. Fluid inclusion studies indicate that the early-stage ore-forming fluids belong to a medium–high-temperature and -salinity fluid system, whereas the late-stage ore-forming fluids are characterized as medium–low-temperature and -salinity fluids. The ore-forming fluid primarily originates from magmatic–hydrothermal fluids and atmospheric precipitation.

(5)

The Sr-Nd isotopic composition of fluorite in the Xiaobaiheogou area shows that the ore-forming material originates from the crust. It is suggested that Ca may mainly comes from the leaching and extraction of the strata formation by the magmatic–hydrothermal solution, while F may mainly comes from the magmatic–hydrothermal solution.