Geochemical and Isotopic Compositions of Fluorites from the Yama Fluorite Deposit in the Qilian Orogen in Northwest China, and Their Metallogenic Implications

Abstract

The Yama area is characterized by numerous large-scale fluorite–quartz veins that are located along faults within the widespread Late Devonian–Late Silurian syenogranites in the Tataleng granitic batholith, Qilian Orogen, Northwest China. These fluorite–quartz veins contribute to an important fluorite reserve, but their ore genesis remains unresolved so far. In this study, trace elements, rare earth elements (REEs), and hydrogen, oxygen, and strontium isotopic compositions of fluorites are analyzed. The studied fluorite samples have similar chondrite-normalized REEs, including Y patterns, with relatively strong enrichment in heavy REEs, negative Eu anomalies, strongly positive Y anomalies, and comparably invariable Y/Ho ratios of 41.43–73.79, suggesting a unique hydrothermal genesis. The relatively variable values of δD and δ¹⁸O are −77.4‰ to −102.4‰ and −12.7‰ to −4.3‰, respectively, close to the meteoric water line. These fluorites yield relatively invariable analytical ⁸⁷Sr/⁸⁶Sr ratios of 0.749089−0.756628 (except for an anomalously high ratio), and their calculated initial ⁸⁷Sr/⁸⁶Sr ratios, based on the ore-forming ages provided, are apparently higher than the calculated initial ⁸⁷Sr/⁸⁶Sr ratios of syenogranite wall rocks. Collectively, the geochemistry of trace elements, REEs, and stable isotopes (H, O, and Sr) suggests that the ore-forming fluids were of meteoric origin and that the Sr sources were directly derived from the ore-forming fluids themselves rather than syenogranite wall rocks. Finally, it was considered that the Yama fluorite deposit is a fault-controlled hydrothermal vein-type deposit which was possibly related to the evolution of the Paleo-Tethys Ocean in the Permian–Triassic.

1. Introduction

The Qilian Orogen (QLO) is traditionally subdivided from north to south into the North Qilian Belt (NQB), the Central Qilian Block (CQB), and the South Qilian Belt (SQB) (Figure 1a) [1,2,3,4,5,6]. It is generally believed that the QLO has developed as a result of the closure of the Proto-Tethys Ocean [1,2,6,7,8,9,10]. Because numerous granitic rocks in the QLO are thought to be associated with subducted, collisional, or post-collisional magmatism that was connected to the Proto-Tethys Ocean, these rocks have been studied in the past [1,4,6,10,11,12,13,14,15,16,17,18,19,20]. Following the widespread Early Paleozoic granitic magmatism, numerous hydrothermal mineralizations of W, Mo, Nb, Ta, Cu, Pb, Zn, Au, and F have been developed throughout the NQB and CQB [6,21,22,23]. However, in the SQB, there are apparently less granitic-magmatism-related mineralizations than those in the NQB and CQB. Currently, the SQB has only a limited number of significant mineral deposits, including a few gold deposits in the Danghe Nanshan mountain in the northern margin of the SQB [6,21], a few W deposits in the Tianjun area in the central segment of the SQB [6,23], and the Yama fluorite deposit and the Yansenhada (i.e., Yamatu) quartz vein deposit in the Tataleng granitic batholith in the western segment of the SQB [6,10,23,24]. Any detailed reports of geological and geochemical research on the mineralization in the SQB are lacking, leaving the ore genesis unresolved for the time being. Specifically, there are no reports on the geological and geochemical characteristics, the ore-forming age, and the ore genesis of the Yama fluorite deposit. Furthermore, in order to assess more accurately the potential for mineralization in the granitic rocks located in and around the Yama fluorite deposit within the Tataleng granitic batholith, it is necessary to conduct detailed geological and geochemical research on this fluorite mineralization.

This paper presents the trace elements and REE abundance, as well as the H, O, and Sr isotopic compositions of fluorites from various types of fluorite ores in the Yama fluorite deposit within the Tataleng granitic batholith in the SQB. The aim is to investigate the ore-forming fluid origin, metallogenic source, and ore genesis of the fluorite–quartz veins in the Yama area as well as the potential fluorite mineralization of syenogranites within the Tataleng granitic batholith.

2. Geological Background and Geology of the Yama Fluorite Deposit

2.1. Geological Background

The Qinghai–Tibet Plateau’s northeastern edge hosts the QLO, which is bordered by the Alxa Block to the north and the Quanji massif (QM) to the south (Figure 1a) [4,6]. The QLO, the northernmost orogenic collage of the Tethyan domain, has an excellent record of the Early to Middle Paleozoic subduction–accretion that resulted from the convergence of the Alax and Qaidam blocks [1,6,9,25]. The North Qilian and South Qilian suture zones normally split the OLO into three tectonic belts, i.e., the NQB, CQB, and SQB (Figure 1a) [1,3,4,6,7,9,10,26].

The NQB is widely recognized as a typical cold oceanic suture zone resulting from the closure of the Early Paleozoic North Qilian Ocean along the North Qilian suture zone [1,2,4,6,18,27]. It hosts Early Paleozoic ophiolite sequences (560–450 Ma), high-pressure metamorphic belts, island-arc-related volcanic rocks (510–450 Ma), and granitic intrusions or plutons (510–420 Ma), as well as Silurian flysch formations, Devonian molasses, and Carboniferous to Triassic sedimentary cover sequences [2,3,4,6,20,27,28]. The CQB has been previously considered as either a detached fragment of the Yangtze Craton [3,6,20,29], a fragment rifted from the Alxa Block [2,4,6,30,31], or a fragment of an accretionary orogen [1,6]. Within the CQB, numerous Precambrian rocks, Cambrian to Ordovician volcanic sequences, Paleozoic sedimentary sequences, and Early Paleozoic granites have been identified [4,6,28,31]. The SQB is generally considered to be an accretionary belt comprising microcontinents, ophiolite complexes, and Early Paleozoic arc-related magmatic rocks [1,5,6,20,25]. The Early Paleozoic arc-related magmatic rocks in the SQB are mainly composed of dominant granitic rocks including several large-scale granitic batholiths, such as the Tataleng granitic batholith [6,10,11,20,32,33,34,35,36,37], the Delingha pluton [18], and the Huangyuan pluton [38]. These granitic rocks from the Early Paleozoic era are typically believed to have formed due to subduction and collisional events related to the South Qilian Ocean, which was a southern branch of the Proto-Tethys Ocean in the Qilian Orogen [14,17,18,19,20]. The Yama fluorite deposit is situated in the eastern section of the extensive Tataleng granitic batholith (Figure 2).

Figure 1. (a) Regional geological map of the Qilian Orogen (revised after [4]). NQ–UHPM (North Qaidam–ultrahigh-pressure metamorphic belt). (b) Schematic map showing major tectonic units in China (simplified after [39]).

Figure 1. (a) Regional geological map of the Qilian Orogen (revised after [4]). NQ–UHPM (North Qaidam–ultrahigh-pressure metamorphic belt). (b) Schematic map showing major tectonic units in China (simplified after [39]).

2.2. Geology of the Yama Fluorite Deposit

The Tataleng granitic batholith is situated in the northwestern part of the SQB (Figure 2) [10,20]. Quaternary sediments cover the northern, eastern, and southeastern regions of the Tataleng granitic batholith, while its southwest edge is adjacent to northwest-striking ductile faults with the Carboniferous to Permian sedimentary rocks (Figure 2). The Tataleng granitic batholith underwent a long crystallization period, ranging from the Late Ordovician to the Early Silurian (O₃–S₁) [10,20,24,32,34,35,37,40,41]. The central part of the Tataleng granitic batholith is covered by Triassic sedimentary rocks (Figure 2) [20].

The Yama fluorite deposit is situated in the eastern section of the Tataleng granitic batholith. Throughout the study region, the O₃–S₁ granitic rock outcrops are widely exposed, with the exception of the southeast part which is extensively covered by massive Quaternary sediments (Figure 3) [10,24]. Numerous northeast-striking faults and minor northwest-striking faults have developed in the O₃–S₁ syenogranites. Numerous fluorite-free quartz veins and fluorite–quartz veins were emplaced along these faults within the O₃–S₁ syenogranite in the Yama area (Figure 3) [10,24]. Fluorites are of high economic significance due to their abundance in numerous large-scale fluorite–quartz, of which 81 fluorite ore bodies or fluorite mineralized occurrences, ranging from No. M1 to No. M81, has been delineated in the Yama area. Among 81 fluorite ore bodies or fluorite mineralized occurrences, 76 fluorite ore bodies contribute to an important, inferred 3,555,500 tons fluorite reserve at 34.53% grade in the Yama fluorite deposit [24], in which the No. M23 fluorite ore body yield inferred a 1,844,400 tons fluorite reserve at 37.38% grade. Most of the fluorite ore bodies are located in the Yama ore district (Figure 4a), except ore body M33 which is located in the Wulandawu ore district (Figure 4b). The Yama porphyritic syenogranite, located in the central part of the Yama area, and the Wulandawu syenogranite, located in the northwest of the Yama area, make up the wall rocks of the fluorite–quartz veins in the Yama fluorite deposit (Figure 3) [10].

Based on the geological relationship (Figure 5), it can be observed that generally the fluorite-free quartz veins were formed earlier than the fluorite–quartz veins, and then a few quartz–calcite veinlets can be seen cutting the fluorite–quartz veins. Additionally, the paragenetic sequences for the Yama fluorite mineralization can be divided into two major stages, i.e., the pre-ore quartz stage and the ore-forming hydrothermal stage, and the latter stage could be subdivided into the quartz–fluorite epoch and the quartz–calcite epoch (Figure 6). Fluorite-free quartz veins, fluorite–quartz veins, and a few quartz–calcite veins were also formed in the pre-ore stage, the quartz–fluorite epoch, and the quartz–calcite epoch, respectively.

Fluorite ore bodies are commonly located within fluorite–quartz veins, which strike predominantly north–northwest (NNW), with minor northeast (NE) or north (Figure 4), and they dip predominantly in the NE (Figure 5). Rare alterations can be observed close to fluorite–quartz veins within the syenogranite wall rocks. The fluorite ore bodies are usually continuous in striking length from 67 m to 1200 m and are generally invariable in width from 0.72 to 7.3 m. A few of them have branches and variable widths in single veins (Figure 4 and Figure 5). Fluorite–quartz veins are generally characterized by vein structures (Figure 7a,d,g), and some narrow quartz–fluorite veins intruded into brecciated syenogranite wall rocks, showing veinlets, stringer structures (Figure 7d), and/or pectinate (where the quartz is vertical to the margin of veins, Figure 7g–i); herein, we named these fluorite ores as Type 1. Type 1 ores commonly consist of fluorite and quartz, with minor quantities of sericite and calcite (Figure 7). However, some important fluorite–quartz veins may also enclose certain variable sizes of wall rock breccias, showing breccia structures within the veins (Figure 7j,m,n); herein, we named these fluorite ores as Type 2. Breccias in Type 2 ores should be derived from variable numbers and sizes of syenogranite wall rocks (Figure 7j,m), and the fine-grained fluorites and quartz which commonly grow around syenogranite breccias show crusty or colloform structures (Figure 7l,n,o), indicating such fluorites might be crystalized earlier than those in Type 2 ores during a low-temperature mineralization process.

3. Sampling and Analytical Methods

3.1. Sampling

Seven samples from two types of typical fluorite–quartz veins were collected from drilling holes (22ZK0802, 22ZK0803, and 22ZK0004) in the Yama area (Figure 4a and Figure 5). The other eleven fluorite–quartz vein samples were collected from outcrops (locations YM05, YM09–YM11, YM13, and YM15) in the Yama fluorite deposit (Figure 4a).

Table 1. Sample list in the Yama fluorite deposit.

No.	Sample Name	Rock Type	Ore Type	Mineral Assembly	Location
1	22ZK0801-B2	fluorite–quartz vein	Type 1	quartz, fluorite	90.5 m at the core of drilling hole 22ZK0801
2	22ZK0801-B6	breccia-bearing fluorite–quartz vein	Type 2	quartz, fluorite, syenogranite breccia	161.7 m at the core of drilling hole 22ZK0801
3	22ZK0801-B17	fluorite–quartz vein	Type 1	quartz, fluorite	149.4 m at the core of drilling hole 22ZK0801
4	22ZK0802-B21	fluorite–quartz vein	Type 1	quartz, fluorite	253.6 m at the core of drilling hole 22ZK0802
5	22ZK0803-B1	breccia-bearing fluorite–quartz vein	Type 2	quartz, fluorite, syenogranite breccia	202.8 m at the core of drilling hole 22ZK0803
6	22ZK0803-B3	pectinal fluorite–quartz vein	Type 1	quartz, fluorite, minor syenogranite breccia	251.0 m at the core of drilling hole 22ZK0803
7	22ZK0004-B6	pectinal fluorite–quartz vein	Type 2	quartz, fluorite	171.6 m at the core of drilling hole 22ZK0004
8	YM05-B3	fluorite–quartz vein	Type 1	quartz, fluorite, minor sericite	96°01′03″ E 37°58′50″ N
9	YM05-B7	fluorite–quartz vein	Type 1	quartz, fluorite, minor sericite	96°01′03″ E 37°58′50″ N
10	YM09-B2	breccia-bearing banded fluorite–quartz vein	Type 2	quartz, fluorite	96°03′58″ E 37°57′27″ N
11	YM09-B6	breccia-bearing banded fluorite–quartz vein	Type 2	quartz, fluorite, minor sericite	96°03′58″ E 37°57′27″ N
12	YM10-B2	fluorite–quartz vein	Type 2	quartz, fluorite	96°03′57″ E 37°57′31″ N
13	YM11-B1	breccia-bearing fluorite–quartz vein	Type 2	quartz, fluorite, syenogranite breccia	96°03′57″ E 37°57′31″ N
14	YM11-B2	breccia-bearing crusty fluorite–quartz vein	Type 2	quartz, fluorite, minor sericite, syenogranite breccia	96°03′57″ E 37°57′31″ N
15	YM11-B4	breccia-bearing crusty fluorite–quartz vein	Type 2	quartz, fluorite, minor sericite, syenogranite breccia	96°03′57″ E 37°57′31″ N
16	YM11-B6	breccia-bearing crusty fluorite–quartz vein	Type 2	quartz, fluorite, minor sericite, syenogranite breccia	96°03′57″ E 37°57′31″ N
17	YM13-B2	pectinal fluorite–quartz vein	Type 1	quartz, fluorite, minor sericite	96°04′00″ E 37°57′40″ N
18	YM15-B1	pectinal fluorite–quartz vein	Type 1	quartz, fluorite	96°04′02″ E 37°57′43″ N
19	22ZK0802-B16	Yama porphyritic syenogranite	wall rock	alkali feldspar (perthite), plagioclase, quartz, muscovite, and biotite, with a few accessory minerals including zircon and epidote	58.6 m at the core of drilling hole 22ZK0802
20	22ZK0802-B18	Yama porphyritic syenogranite			138.0 m at the core of drilling hole 22ZK0802
21	22ZK0803-B8	Yama porphyritic syenogranite			36.3 m at the core of drilling hole 22ZK0803
22	YM03-B1	Wulandawu syenogranite	wall rock	alkali feldspar (perthite), plagioclase, quartz, biotite, and muscovite, with a few accessory minerals including tourmaline, zircon, and epidote	96°01′29″ E 37°58′03″ N
23	YM06-B1	Wulandawu syenogranite			96°01′04″ E 37°58′52″ N
24	M33-ZK0001-B1	Wulandawu syenogranite			18.0 m at the core of drilling hole 22ZK0803

provides detailed descriptions of the samples. In the processing of the selected samples, crushing, conventional magnetic, and heavy liquid separation methods were used to extract quartz for H and O isotopes, fluorites for H, O, and Sr isotopes, as well as for trace element analyses.

To compare the Sr isotope of syenogranite wall rocks, six syenogranite samples were collected from drilling holes (22ZK0802, 22ZK0803, and M33-ZK0001-B1) and outcrops (locations YM03 and YM06) (Figure 4). Sample descriptions for the selected syenogranite wall rocks are listed in Table 1 and for detailed descriptions of the samples we refer to our previously published paper [10].

3.2. Trace Element and REE Analyses of Fluorites

Trace element contents (excluding REEs) for samples were determined using inductively coupled plasma–atomic emission spectroscopy (ICP-AES) (Agilent 5110 ICP–AES, Santa Clara, CA, USA) and inductively coupled plasma–mass spectrometry (ICP–MS) (Agilent 7900 ICP–MS, Santa Clara, CA, USA) at ALS Chemex (Guangzhou, China) Co., Ltd. Perchloric, nitric, hydrofluoric, and hydrochloric acids were used to digest a prepared sample. The residue was leached with dilute hydrochloric acid and diluted to volume. After that, ICP–AES and ICP–MS were used to analyze it. REEs for samples were also determined using ICP–MS (Agilent 7900 ICP–MS, Santa Clara, CA, USA) at ALS Chemex (Guangzhou, China) Co., Ltd. A prepared sample was added to lithium borate flux, mixed well, and fused in a furnace at 1025 °C. The resulting melt was then cooled and dissolved in an acidic mixture containing nitric, hydrochloric, and hydrofluoric acids. Finally, this solution was analyzed via ICP–MS. The precision for trace elements was better than 10%, which was controlled by the system settings, and the detailed analytical procedure was also referred to in [6].

3.3. H, O Isotope Analyses of Fluorites and Quartz

H and O isotope analyses were carried out at the Analytical Laboratory of the Beijing Research Institute of Uranium Geology, China National Nuclear Corporation (BRIUG). The O isotope analysis of quartz was carried out with the bromine pentafluoride method [42]. Water was liberated from 5 to 10 mg of quartz by reaction with BrF₅ and converted to CO₂ on a platinum-coated carbon rod. The δ¹⁸O determinations were made with a Thermo Scientific Delta V Advantage mass spectrometer. Because quartz is an O-bearing mineral, the O isotope of fluid inclusions in quartz, i.e., the δ¹⁸O_H2O values, should be calculated by using the isotopic fractionation equation of O between quartz and water as shown in Equation (1) as follows [43]:

1000lnα = 3.38 × 10⁶/T² − 3.4

(1)

1000lnα is commonly equal to the value of δ¹⁸O_quartz minus δ¹⁸O_H2O.

By mechanically crushing 5–10 mg of quartz, the H isotopic ratios of decrepitated quartz-hosted fluid inclusions were measured. Samples were heated to 1400 °C in order to release water from fluid inclusions after being degassed for 4 h under vacuum at 105 °C. After that, the released water was captured, reduced to H₂ by glassy carbon, and then analyzed for δD values using a Thermo Scientific MAT-253 mass spectrometer.

Through the mechanical crushing of 0.5 g of fluorites, H and O isotopic ratios of decrepitated fluorite-hosted fluid inclusions were measured. Water in fluid inclusions within fluorite samples was first released by heating at 550 °C; then, the released water was trapped, reduced to H₂ by glassy carbon at 1400 °C, and then analyzed for δD values using a Thermo Scientific MAT-253 mass spectrometer. Water was liberated from fluorites by reaction with BrF₅ and converted to CO₂ on a platinum-coated carbon rod. The δ¹⁸O determinations were performed on a Thermo Scientific MAT-253 mass spectrometer.

The results are reported relative to the Vienna Standard Mean Ocean Water (V-SMOW), and the analytical precisions are ±0.1‰ for δ¹⁸O and ±1‰ for δD. The detailed analytical procedures were described in [44].

3.4. Sr Isotope Analyses of Fluorites

At Beijing Createch Testing Technology Co., Ltd., (Beijing, China) Sr isotope analyses were performed. Every chemical preparation was carried out on Class 100 work benches within a Class 1000 clean laboratory. A 15 mL Savillex^TM PFA screw-top beaker was filled with about 150 mg of sample powder. After adding concentrated HNO₃ and HF (1 mL and 2 mL, respectively) to the samples, the sealed beakers were heated for one week at 150 °C on a hotplate. Following digestion, acids were evaporated on the hotplate and residues were dissolved in 1 mL of concentrated HCl. After that, the same procedures were repeated 3 times. Finally, the samples were dissolved in 1.5 mL of 3.5 M HNO₃ for Sr purification. Sr-Spec (Triskem, 100–150 μm) resin was used to separate and purify Sr from samples. Isotopic compositions of Sr analyses were carried out using a Thermo Fisher Scientific Neptune Plus MC-ICP-MS. ⁸⁷Sr/⁸⁶Sr ratios were corrected for instrumental mass fractionation using the exponential fractionation law and assuming ⁸⁸Sr/⁸⁶Sr = 8.375209. The analytical methods were modified from those in [45].

Fifteen fluorite and six syenogranite samples were analyzed for Sr isotopic compositions; furthermore, the Sr isotope international standard NBS 987 was repeatedly tested for accuracy monitoring, yielding an average ⁸⁷Sr/⁸⁶Sr ratio of 0.710249 ± 14 (2SD, n = 21) during the analytical procedure of fluorites and ⁸⁷Sr/⁸⁶Sr ratios = 0.710247 ± 13 (2SD, n = 24) during the analytical procedure of syenogranites. Initial ⁸⁷Sr/⁸⁶Sr ratios (i.e., (⁸⁷Sr/⁸⁶Sr)_i) were calculated based on rock-forming ages for syenogranite wall rocks dated by [10], and on the possible ore-forming ages of fluorites.

4. Results

4.1. Trace Elements and REEs of Fluorites

The trace element and REE data for fluorites are listed in

Table 2. Trace elements and REEs of fluorites in the Yama fluorite deposit.

Sample Name	22ZK0801-B2	22ZK0802-B21	22ZK0803-B3	YM05-B3	YM05-B7	22ZK0004-B6	YM09-B2	YM09-B6	YM10-B2	YM11-B2	YM11-B4	YM11-B6	YM13-B2	YM15-B1
Types	Type 1	Type 1	Type 1	Type 1	Type 1	Type 2	Type 2	Type 2	Type 2	Type 2	Type 2	Type 2	Type 1	Type 1
Li	2.60	3.60	1.90	5.20	5.20	2.80	0.50	2.30	7.00	0.60	0.70	0.60	1.80	1.70
Be	0.1	0.4	0.1	0.4	0.5	0.2	0.8	0.6	7.5	<0.05	<0.05	0.1	0.21	0.13
Mg	<0.01	0.01	0.01	0.01	0.01	<0.01	<0.01	0.01	0.01	<0.01	<0.01	<0.01	0.02	0.01
Sc	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1
Ti	<0.005	<0.005	<0.005	<0.005	<0.005	<0.005	<0.005	<0.005	<0.005	<0.005	<0.005	<0.005	<0.005	<0.005
V	1	1	1	1	2	1	1	1	1	<1	1	<1	1	1
Cr	1	1	1	1	1	<1	<1	<1	1	1	<1	1	<1	<1
Mn	<5	<5	<5	<5	<5	<5	<5	<5	<5	<5	<5	<5	<5	<5
Fe	0.01	0.01	0.01	0.04	0.04	0.01	0.01	<0.01	0.01	0.01	<0.01	0.01	0.02	0.01
Co	0.7	0.6	0.7	0.6	0.7	0.7	0.7	0.7	0.6	0.8	0.8	0.8	0.8	0.8
Ni	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2
Cu	0.8	0.5	0.6	0.6	1.2	0.8	0.8	0.8	0.6	0.8	0.6	0.8	0.8	0.5
Zn	<2	<2	<2	<2	<2	<2	<2	<2	<2	<2	<2	<2	<2	<2
Ga	0.2	0.3	0.2	1.3	1.6	0.2	0.3	0.2	0.5	0.2	0.1	0.2	0.25	0.23
Rb	2.6	4.6	4.4	20.0	21.5	5.3	3.5	2.8	6.9	2.3	1.8	3.3	4.1	3
Sr	20.1	18.3	17.8	64.5	54.3	21.2	15.2	15.2	17	23.1	22.2	22	21.9	26.7
Y	11.2	14.0	29.5	107.0	64.1	5.6	7.5	3.2	8.2	2.9	2.2	2.7	53.9	5.9
Zr	3.00	<2	<2	3.00	<2	<2	3.00	4.00	6.00	3.00	<2	<2	<2	3
Nb	<0.1	<0.1	<0.1	0.10	0.10	<0.1	0.10	0.10	<0.1	0.10	<0.1	<0.1	<0.1	<0.1
Mo	<0.05	<0.05	0.09	0.06	<0.05	<0.05	<0.05	0.05	0.42	<0.05	<0.05	<0.05	0.07	0.42
Cd	<0.02	<0.02	<0.02	<0.02	<0.02	<0.02	<0.02	<0.02	<0.02	<0.02	<0.02	<0.02	<0.02	<0.02
Sn	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	0.2
Sb	<0.05	<0.05	<0.05	0.07	0.06	<0.05	0.3	<0.05	0.08	<0.05	<0.05	<0.05	<0.05	<0.05
Cs	0.2	0.3	0.2	1.0	1.1	0.2	0.1	0.1	0.3	0.1	0.1	0.1	0.2	0.21
Ba	2.3	7.0	2.1	2.1	2.4	5.6	2.6	3.5	1.8	4.3	13.3	8.1	6.9	6.3
Ca	47.00	46.70	>50	46.20	46.70	>50	49.10	48.50	45.50	>50	>50	>50	>50	49.8
La	0.8	1.4	7.6	1.4	1.1	0.4	0.6	0.3	0.2	1.2	0.3	0.4	1.2	1.2
Ce	1.60	2.70	16.20	3.10	2.40	0.90	1.00	0.40	0.20	2.00	1.30	1.20	3	1.7
Pr	0.18	0.30	1.86	0.51	0.39	0.10	0.15	0.06	0.03	0.19	0.06	0.09	0.37	0.3
Nd	0.80	1.20	6.80	2.30	1.80	0.40	0.60	0.20	0.20	0.70	0.20	0.30	1.5	1.1
Sm	0.32	0.32	1.77	1.21	0.93	0.13	0.19	0.08	0.08	0.10	0.09	0.10	0.66	0.25
Eu	0.03	0.05	0.14	0.10	0.08	<0.02	0.02	<0.02	<0.02	0.02	<0.02	<0.02	0.09	0.04
Gd	0.63	0.62	2.30	3.10	2.07	0.24	0.35	0.15	0.23	0.23	0.15	0.17	1.65	0.41
Tb	0.13	0.13	0.42	0.71	0.49	0.06	0.08	0.03	0.05	0.05	0.03	0.04	0.37	0.07
Dy	1.02	1.04	2.91	5.88	3.96	0.44	0.63	0.24	0.44	0.31	0.24	0.29	3.39	0.5
Ho	0.24	0.26	0.66	1.45	1.00	0.11	0.15	0.06	0.14	0.07	0.05	0.06	0.94	0.12
Er	0.77	0.90	2.12	4.33	3.00	0.32	0.47	0.19	0.47	0.19	0.16	0.19	3.54	0.34
Tm	0.1	0.2	0.4	0.7	0.5	0.1	0.1	0.0	0.1	0.0	0.0	0.1	0.59	0.04
Yb	0.78	1.09	2.33	4.80	3.58	0.32	0.51	0.28	0.62	0.21	0.23	0.23	4.39	0.33
Lu	0.1	0.2	0.4	0.8	0.7	0.1	0.1	0.1	0.1	0.1	0.1	0.0	0.75	0.05
Hf	0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1	<0.1
Ta	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05
W	0.1	0.2	0.1	0.2	0.4	0.1	0.1	0.1	0.1	<0.1	<0.1	<0.1	0.3	0.1
Pb	1.00	<0.5	<0.5	0.70	0.70	<0.5	<0.5	1.30	6.00	<0.5	0.60	<0.5	26.3	<0.5
Bi	2.35	0.02	0.10	0.25	0.11	0.74	0.05	8.31	15.15	2.64	1.87	1.55	0.16	0.01
Th	0.31	0.16	0.34	0.54	0.30	0.21	0.43	0.27	<0.05	0.11	0.06	0.08	2.77	1.13
U	0.15	0.21	1.04	0.47	0.63	0.23	0.14	0.21	0.29	0.28	0.19	0.19	0.30	0.28
∑REEs	7.56	10.36	45.86	30.42	21.96	3.53	4.91	2.07	2.87	5.35	2.89	3.16	22.44	6.45
LREEs/HREEs	0.97	1.36	2.99	0.40	0.44		1.09			3.69			0.44	2.47
(La/Yb)N	0.69	3.34	1.89	8.43	4.24	1.29	3.03	0.77	1.02	0.75	0.85	1.03	7.62	1.20
(La/Sm)N	0.82	0.60	0.79	0.77	0.70	0.86	0.78	0.54	0.45	0.93	0.66	0.79	0.49	0.95
Eu/Eu*	0.20	0.34	0.21	0.16	0.18		0.24			0.40			0.26	0.38
Y/Y*	1.64	1.95	1.54	2.65	2.33	1.84	1.77	1.93	2.39	1.43	1.45	1.48	2.19	1.74

Note: Units for Ca, Mg, Fe, and Ti are wt. %, and units for other trace elements are ppm. ∑REEs—total REE contents, LREEs—light rare earth elements, including La, Ce, Pr, Nd, Sm, and Eu, HREEs—heavy rare earth elements, including Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, Eu/Eu* = Eu_N/√[Sm_N × Gd_N], Y/Y* = (Y_N)/√[Dy_N × Ho_N]. REE normalization values were obtained from [46]. The Y normalization value was obtained from [47].

. Most trace and REE elements show concentrations of <10 ppm. Relatively high concentrations (a few to several tens of ppm) were observed for Rb (1.8–21.5 ppm), Sr (15.2–64.5 ppm), Y (2.2–107 ppm), and Ba (1.8–13.3 ppm), whereas very low concentrations (usually <1 ppm) were found for Fe, Co, W, Pb, Th, U and some of REEs, such as Eu, Tb, Tm, and Lu, and some of them were even below the detection limits, such as Mg, Mn, Ni, Zn, Nb, Cd, Mo, Sn, Sb, Hf, and Ta.

The total REE contents (∑REEs) in studied fluorites are relatively low (<2.09–45.86 ppm), i.e., 2.89–30.42 ppm for the fluorites in Type 1 ores, and 2.09–45.89 ppm for the fluorites in Type 2 ores. The Y concentrations for all types of fluorites are relatively high, i.e., 5.90–107 ppm for the fluorites in Type 1 ores and 2.20–29.50 ppm for the fluorites in Type 2 ores. In general, Type 1 ores have slightly higher total REE contents and Y contents than Type 2 ores (Table 2). In chondrite-normalized REE, including Y, pattern (namely a REY pattern) diagrams (Y inserted between Dy and Ho), the fluorite samples have roughly similar chondrite-normalized REY patterns, basically indicating a similar source and ore genesis. They are also characterized by relatively strong enrichment in heavy rare earth elements (HREEs), showing negative Eu anomalies (Eu/Eu* = 0.16–0.40, Table 2), strongly positive Y anomalies (Y/Y* = 1.43–2.39, Table 2), and relatively high (La/Yb)_N values of 0.69–8.43 (Figure 8), although fluorites in Type 1 ores show higher total REE contents (average = 14.58 ppm) than those in Type 2 ores (average = 9.69 ppm). However, both types of fluorite ores show slightly different chondrite-normalized REY patterns compared to the average of syenogranite wall rocks (which show a flat HREE pattern and are slightly enriched in LREEs) relative to HREEs as described in our previously published paper [10].

4.2. H and O Isotopic Compositions of Quartz

The Yama fluorite deposit contains fluorites with varying δD and δ¹⁸O values from −77.4‰ to −102.4‰ (average = −90.1‰) and from −12.7‰ to −4.3‰ (average = −8.0‰), respectively (

Table 3. H and O isotope data for quartz and fluorites in the Yama fluorite deposit.

No.	Sample Name	Rock Type	Mineral	δDV-SMOW (‰)	δ18OV-SMOW (‰)	δ18OH2O (‰) *
1	22ZK0801-B2	Type 1	quartz	−82.1	−3.6	−17.2
2	22ZK0801-B17	Type 1	quartz	−85.3	−2.8	−16.4
3	22ZK0802-B21	Type 1	quartz	−80.6	−3.6	−17.2
4	22ZK0803-B3	Type 1	quartz	−87.3	−4.2	−17.8
5	22ZK0004-B6	Type 2	quartz	−83.6	−1.4	−15.0
6	YM09-B2	Type 2	quartz	−86.0	−3.4	−17.0
7	YM10-B2	Type 2	quartz	−83.6	−2.7	−16.3
8	YM11-B1	Type 2	quartz	−91.2	−4.5	−18.1
9	YM11-B4	Type 2	quartz	−86.1	−4.2	−17.8
10	YM11-B6	Type 2	quartz	−82.9	−4.4	−18.0
11	YM13-B2	Type 1	quartz	−81.0	−2.3	−15.9
12	YM15-B1	Type 1	quartz	−79.6	−2.6	−16.2
13	22ZK0801-B2	Type 1	fluorite	−84.7	−12.7
14	22ZK0801-B6	Type 2	fluorite	−88.8	−9.1
15	22ZK0802-B21	Type 1	fluorite	−89.3	−10.2
16	22ZK0803-B1	Type 2	fluorite	−92.1	−11.7
17	22ZK0803-B3	Type 1	fluorite	−87.0	−9.8
18	YM05-B3	Type 1	fluorite	−77.4	−9.1
19	22ZK0004-B6	Type 2	fluorite	−83.3	−10.7
20	YM09-B2	Type 2	fluorite	−83.5	−4.6
21	YM09-B6	Type 2	fluorite	−92.6	−4.4
22	YM10-B2	Type 2	fluorite	−97.1	−4.3
23	YM11-B2	Type 2	fluorite	−98.2	−5.0
24	YM11-B4	Type 2	fluorite	−94.6	−5.7
25	YM11-B6	Type 2	fluorite	−102.4	−7.8
26	YM13-B2	Type 1	fluorite	−94.6	−6.5
27	YM15-B1	Type 1	fluorite	−86.5	−8.8

* The calculated δ¹⁸O_H2O values of the fluids are based on an average homogenization temperature of quartz of 173 °C using Equation (1) [43].

). These values indicate the initial isotope compositions of the ore-forming fluids in the Yama fluorite deposit. The δD value ranges of two types of fluorite ores overlap each other, but Type 2 samples have slightly heavier δ¹⁸O values than Type 1 (Figure 9).

The Yama fluorite deposit contains quartz in different types of fluorite ores with slightly invariable δD values ranging from −91.2‰ to −79.6‰ (average = −84.1‰). The values are within the range of δD values of fluorite (Figure 9). The quartz in different types of fluorite ores in this deposit also has invariable δ¹⁸O values ranging from −4.5‰ to −1.4‰ (average = −3.3‰). Calculating the precise δ¹⁸O_H2O values of fluids that were in equilibrium with the quartz is challenging due to the large variation in temperatures obtained from fluid inclusions (e.g., [48]). However, for a temperature range of 200–500 °C, Equation (1) provides a good approximation of the isotopic fractionation of O between quartz and water [43]. However, the homogenization temperatures of quartz in fluorite–quartz veins ranged from 135 °C to 209 °C (average = 173 °C) in the Yama fluorite deposit (unpublished data). Therefore, if using 173 °C as the trapping temperature, the calculated δ¹⁸O_H2O values of the fluids for fluid inclusions from the quartz in fluorite ores were from −18.0‰ to −15.0‰ (Table 3), which are totally different from the analytical O isotopic data from −12.7‰ to −4.3‰ of fluorite (Figure 9).

Figure 9. Plot of δD versus δ¹⁸O diagram in the Yama fluorite deposit. Magmatic and metamorphic water fields after [49].

Figure 9. Plot of δD versus δ¹⁸O diagram in the Yama fluorite deposit. Magmatic and metamorphic water fields after [49].

4.3. Sr Isotopic Compositions of Fluorites

Table 4. Sr isotopic compositions of fluorites and syenogranites from the Yama fluorite deposit.

Sample Name	Rock/Mineral Types	Rb a (ppm)	Sr a (ppm)	87Rb/86Sr	87Sr/86Sr	±2σ	(87Sr/86Sr)I b	(87Sr/86Sr)I c
22ZK0801-B2	Type 1	2.6	20.1	0.374295	0.753393	0.000007	0.751031
22ZK0801-B6	Type 2	2.6	20.1	0.374295	0.753517	0.000008	0.751155
22ZK0802-B21	Type 1	4.6	18.3	0.727351	0.755463	0.000008	0.750873
22ZK0803-B1	Type 2	4.4	17.8	0.715269	0.754075	0.000007	0.749562
22ZK0803-B3	Type 1	4.4	17.8	0.715269	0.755551	0.000007	0.751037
YM05-B3	Type 1	20	64.5	0.897237	0.828185	0.000008	0.822523
22ZK0004-B6	Type 2	5.3	21.2	0.723398	0.756299	0.000006	0.751734
YM09-B2	Type 2	3.5	15.2	0.666287	0.756628	0.000006	0.752424
YM09-B6	Type 1	2.8	15.2	0.533030	0.753935	0.000009	0.750571
YM10-B2	Type 2	6.9	17	1.174457	0.754550	0.000007	0.747139
YM11-B2	Type 2	2.3	23.1	0.288106	0.752564	0.000006	0.750746
YM11-B4	Type 2	1.8	22.2	0.234615	0.751325	0.000008	0.749845
YM11-B6	Type 2	3.3	22	0.434039	0.753245	0.000006	0.750506
YM13-B2	Type 1	4.1	21.9	0.541722	0.749089	0.000007	0.745671
YM15-B1	Type 1	3	26.7	0.325122	0.753748	0.000006	0.751696
22ZK0802-B16	Yama syenogranite	374	37.6	28.781987	0.854448	0.000009		0.672287
22ZK0802-B18	Yama syenogranite	439	46.7	27.200986	0.832625	0.000008		0.660469
22ZK0803-B8	Yama syenogranite	377	34.2	31.897178	0.880566	0.000007		0.678689
YM03-B1	Wulandawu syenogranite	477	20.5	67.328902	1.091983	0.000010		0.668841
YM06-B1	Wulandawu syenogranite	443	23.7	54.086938	1.056637	0.000009		0.716716
M33-ZK0001-B1	Wulandawu syenogranite	522	19.2	78.669482	1.191950	0.000013		0.697535

^a Contents determined by ICP-MS (fluorites from Table 2, and syenogranites from [10]). ^b Recalculation of (⁸⁷Sr/⁸⁶Sr)i ratios to 443 Ma for fluorites (ages obtained from [10]). ^c Recalculation of (⁸⁷Sr/⁸⁶Sr)i ratios to 444.3 Ma for the Yama porphyritic syenogranite, and to 441.2 Ma for the Wulandawu syenogranite (ages obtained from [10]), using λ⁸⁷Rb = 1.42 × 10⁻¹¹ year⁻¹ [50].

lists the Sr isotopic compositions of fluorites. The samples for the Yama porphyritic syenogranite and the Wulandawu syenogranite have variable analytical ⁸⁷Sr/⁸⁶Sr ratios of 0.832625−0.880566 and 1.056637−1.191950, respectively. Based on the average age of their rock-forming ages of 444.3 Ma and 441.2 Ma [10], their calculated initial ⁸⁷Sr/⁸⁶Sr ratios (i.e., (⁸⁷Sr/⁸⁶Sr)_i) are 0.660469−0.678689 and 0.668841−0.697535, respectively.

Both types of fluorites have variable ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.749089 to 0.828185, with an average of 0.758771. However, except for an anomalously high ratio of 0.828185 for the sample YM05-B3, the other analytical ⁸⁷Sr/⁸⁶Sr ratios are focused on a relatively invariable range of 0.749089−0.756628, with an average of 0.753813. As we do not know the exact ore-forming age of fluorite ore bodies in the Yama fluorite deposit, it should be considered to be either the same or later than the rock-forming ages of the syenogranite wall rocks, as provided by [10]. If the ore-forming age is equal to the average formation age of 443 Ma of syenogranite wall rocks provided in [10], the initial ⁸⁷Sr/⁸⁶Sr ratios for fluorites can be calculated as ranging from 0.745671 to 0.822523, with an average of 0.755101 (Figure 10). If we discarded the anomalously high ratio of 0.822523 for the sample YM05-B3, the calculated (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.745671 to 0.751696 for fluorites in Type 1 ores are very similar to the calculated (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.747139 to 0.752424 for fluorites in Type 2 ores, and both (⁸⁷Sr/⁸⁶Sr)_i ratios of fluorites are relatively invariable, ranging from 0.745671 to 0.752424.

5. Discussion

5.1. Origin of Ore-Forming Fluids for Fluorite Mineralization

A latitude effect on δD values has been proposed for fluorite deposits in Eastern China [51]. The δD value of these deposits decreases with increasing latitude, with implications of a significant origin from meteoric water. Figure 9 shows the hydrogen and oxygen isotopic compositions of fluorites in the Yama fluorite deposit. Both types of fluorites display relatively unvaried δD values and variable δ¹⁸O_H2O values, and they roughly overlap with each other. The plot of the data is close to the meteoric water line with relatively lower δD values than the typical composition of magmatic or metamorphic water, indicating that the ore-forming fluids might predominantly be meteoric water. Their relatively constant δD values might indicate there is no apparent H isotope exchange between the ore-forming water and wall rocks.

The δ¹⁸O_H2O values of different types of fluorites have a more variable range; thus, samples show a horizontal trend in δ¹⁸O_H2O values (“O isotope shift”) extending from the meteoric water line, which might result from possible fluid–rock interactions either before or during fluorite mineralization (e.g., [52]). Certainly, mixing with minor other deep water also cannot be totally precluded. Because quartz–fluorite veins were dominantly filled in the brittle faults within syenogranite wall rocks, rare fluid–rock interactions happened during fluorite mineralization, which is proven by the different initial Sr isotopic compositions between fluorites and syenogranite wall rocks and rare alterations close to fluorite–quartz veins within the syenogranite wall rocks. Therefore, the δ¹⁸O_H2O values of fluorite samples showing an O isotope shift extending from the meteoric water line may have resulted from possible deep fluid–rock interactions before fluorite mineralization. The fluorites in Type 2 ores have slightly heavier and variable δ¹⁸O_H2O values than those in Type 1 ores, possibly because they contain some syenogranite breccias within such breccia-bearing fluorite–quartz veins. There would also possibly be a slight exchange between ore-forming fluids and syenogranite breccias during fluorite mineralization.

Compared to fluorites, the quartz in different fluorite ores has a similar δD value range for fluid inclusions to those of fluorites (Figure 9); however, their calculated δ¹⁸O_VSMOW values from −18.0‰ to −15.0‰ for fluid inclusions from the quartz in the Yama fluorite deposit are totally different from the analytical δ¹⁸O_H2O values from −12.7‰ to −4.3‰ of different types of fluorites. We have argued that the reasons might be as follows: firstly, the trapping temperature (173 °C) is lower than the temperature range of 200–500 °C for Equation (1), which is used as a good approximation of isotopic fractionation of O between quartz and water [43]; secondly, bulk fluids extracted from quartz, which are a mixture of primary, pseudosecondary, and secondary inclusions, usually reflect H and O isotopic compositions of important secondary fluid inclusions formed in the presence of meteoric water during the uplift of the deposits (e.g., [53,54,55]). Therefore, the H and O isotopic compositions for the ore-forming fluids should be represented by the data obtained from studied fluorites rather than quartz, in particular from those fluorites which usually contain more cleavages than quartz and cannot trap the considerable secondary fluid inclusions. These are consistent with our microscopic observations.

In conclusion, the origin of ore-forming fluids for the Yama fluorite mineralization might predominantly originate from paleo-meteoric water, but a mixture with certain deep-water sources (e.g., magmatic water or metamorphic water) cannot be totally precluded.

5.2. Source of Ore-Forming Materials for Fluorite Mineralization

Because strontium and calcium have similar geochemical natures, fluorites are therefore usually characterized by enriched Sr and depleted Nd and could keep their initial Sr isotopic compositions when they are formed. Two types of fluorites typically have invariable analytical ⁸⁷Sr/⁸⁶Sr ratios of 0.749089−0.756628 with an average of 0.753813, except for the anomalously high ratio. In addition, the corresponding calculated (⁸⁷Sr/⁸⁶Sr)_i ratios for both types of fluorites, which range from 0.745671 to 0.752424 with an average of 0.750285 based on the average formation age of 443 Ma of syenogranite wall rocks [10], are apparently higher than the calculated (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.660469−0.697535 of the syenogranite wall rocks (Figure 10). If a younger ore-forming age than the 443 Ma is used, then the calculated (⁸⁷Sr/⁸⁶Sr)_i ratios should be larger than the range of 0.745671−0.752424 with the corrected age of 443 Ma. Furthermore, they will be larger than the calculated (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.660469−0.697535 of the syenogranite wall rocks. Therefore, whatever the ore-forming age which was the same or younger than 443 Ma, the calculated (⁸⁷Sr/⁸⁶Sr)_i ratios of fluorites should be much greater than the calculated (⁸⁷Sr/⁸⁶Sr)_i ratios of the syenogranite wall rocks. This indicates that the Sr source should be predominantly derived from the ore-forming fluids themselves rather than from the syenogranite wall rocks, as do other ore-forming materials. This argument has further been proven by the geological fact that the fluorite–quartz veins were formed by the low-temperature-filling mineralization rather than by an apparent water–rock reaction between the ore-forming fluids and syenogranite wall rocks.

5.3. Implication for the Genesis of Fluorite Mineralization

The Tb/Ca ratio can reveal the assimilation of Ca from the surrounding rocks or the adsorption of REEs during the crystallization of fluorite [56]. The Tb/La ratio can be employed to evaluate both the fractionation degree of REEs in fluorite and the reactivation of fluorite [57,58]. Therefore, the Tb/Ca versus Tb/La diagram was used to discriminate fluorite genetic types, i.e., pegmatitic, hydrothermal, and sedimentary genesis [56]. In this study, four fluorites in Type 2 and two samples in Type 1 have a Ca content of >50% (the detection limit) by LA-ICP-MS trace element analyses; therefore, it is approximated that n(Ca) is equal to the theoretical value (51.3328%) in CaF₂ (e.g., [59]). Meanwhile, the Tb/Ca ratios are calculated for such samples. In this way, fluorites in Type 1 ores in the Yama fluorite deposit predominantly plot in the hydrothermal field on the Tb/Ca versus the Tb/La diagram (Figure 11a); meanwhile, fluorites in Type 2 ores predominantly plot in the sedimentary field in the Tb/Ca versus the Tb/La diagram (Figure 11a). Moreover, as the partitioning behavior of Dy into fluorite is similar to that of Tb [56], Dy can be substituted for Tb in the Tb/Ca versus the Tb/La discrimination diagram [60]. Similarly, in the Dy/Ca vs. Dy/La plot (Figure 11b), fluorites in Type 1 ores also predominantly plot in the hydrothermal field, and fluorites in Type 2 ores predominantly plot in the sedimentary field. Therefore, it is reasonably considered that the crusty structures for fluorite in Type 2 ores might not only indicate the low-temperature structure but also the slow speed of crystallization of ore-forming fluids while they infilled in the ore-controlling faults. The process of such low-temperature and slow-speed crystallization of the ore-forming fluids might be similar to a chemical sedimentary process; therefore, they plot in the sedimentary fields in the Tb/Ca vs. Tb/La plot (Figure 11a) or the Dy/Ca vs. Dy/La plot (Figure 11b). However, with geological observations in the field, fluorite−quartz veins in the study area were all infilled along the faults within the syenogranite wall rocks; thus, indicating that all types of fluorites within fluorite−quartz veins should be a hydrothermal filling genesis. In the Tb/Ca vs. Tb/La plot (Figure 11a), fluorites in the Type 1 ore also show a crystallization trend, indicating normal hydrothermal crystallization. The slightly increased Tb/La atom ratios (Figure 11) and the increased ∑REE contents (Table 2) in fluorites from Type 2 to Type 1 ores suggest that REEs in the ore-forming fluid were gradually fractionated. This is consistent with the fact that fine-grained fluorites showing crusty or colloform structures in Type 2 ores were crystallized earlier than other fluorites within fluorite−quartz veins (Figure 6 and Figure 7), and the more perfect crystal structure of fluorite might be more suitable for the entry of REEs (e.g., [61]).

Moreover, the Y/Ho ratios of fluorite can be used to determine the origin of the fluid because the Y and Ho have comparable ionic radii and potentials [58], and the Y/Ho ratios of fluorite from the same origin are usually constant [62]. In this study, the Y contents of fluorite in the Yama fluorite deposit are basically positively correlated with ∑REE contents (Figure 12a), and the Y/Ho ratios of fluorite from both types of fluorite ores have a comparably invariable range of 41.43–73.79. Apparently, fluorites from the Yama fluorite deposit or their origins of ore-forming fluids follow a recrystallization trend, indicating an identical fluid origin (Figure 12b; e.g., [58]). Furthermore, hydrothermal fluorites are identified by their high Y/Ho ratios (average between 35 and 250; e.g., [58]). The Y/Ho ratio range of 41.43–73.79 for the studied fluorite samples is much higher than the chondrite Y/Ho ratio [63], PAAS Y/Ho ratio [47], and most igneous rocks [58], and is located within the Y/Ho value range of hydrothermal fluorites (Figure 12c) [58], suggesting that the fluorites in the Yama fluorite deposit might be derived from a hydrothermal source rather than the surrounding syenogranites. Moreover, fluorites in the study area showing positive Y anomalies also suggested that Y enrichment depends on the presence of Y–F complexes (e.g., [64]), which have a higher stability when compared with Ho–F complexes [65]. Therefore, Y preferentially stays within the fluorite formed from F-rich fluids.

The valence state of Eu in fluorite and the Eu anomalies relative to other REEs can reveal the redox state of the ore-forming fluid [58]. In the reducing environment, some Eu takes on the form of Eu²⁺, and the Eu²⁺ ionic radius is large and cannot easily replace Ca²⁺, thus forming a negative Eu anomaly [66]. There were weak to moderate negative Eu anomalies (Eu/Eu* = 0.16–0.40) of fluorite in the Yama fluorite deposit; therefore, Eu²⁺ should exist in the fluorite in the Yama fluorite deposit and indicate a reducing environment. These negative Eu anomalies indicate the low O fugacity conditions of the hydrothermal fluids (e.g., [67,68,69]).

To obtain a precise ore-forming age for fluorite mineralization in the Yama fluorite deposit, we attempted to carry out the Sm-Nd isotope analysis of five fluorite samples. However, we did not obtain reliable Sm-Nd isotope data for our samples, as well as an isochron age. Fluorite mineralization is a type of hydrothermal filling of fluorite–quartz veins in the Yama fluorite deposit, and no coeval hydrothermal alterations can be observed close to the quartz–fluorite veins within the syenogranite wall rocks. According to the above geological fact, it was indicated that there is very weak fluid–rock interactions between ore-forming meteoric fluid and surrounding syenogranites during fluorite and quartz filling. Therefore, the ore-forming age for fluorite mineralization should be later than the forming age of syenogranite wall rocks (i.e., mean age = 443 Ma [10]).

Commonly, the Late Ordovician–Early Silurian syenogranites in the study area were considered to be emplaced in a post-collisional setting which was related to the closure of the South Qilian Ocean (a branch of the Proto-Tethys Ocean) between the Qaidam Block and SQB [10]. Since the occurrence of such numerous Early Paleozoic granitic emplacements, subsequent dominant tectonic and magmatic activities within the SQB had happened predominantly in the Permian–Triassic and were considered to be closely related to the evolution of the Paleo-Tethys Ocean between the QM and the SQB [6]. Therefore, we assume that the meteoric water under the Yama area was possibly heated and convected in the Permian–Triassic, and then might be turned into a type of reduced, low-temperature, F-bearing hydrothermal fluid which might extract F from the deep surrounding rocks. The late, large-scale brittle faults within the Late Ordovician–Early Silurian syenogranites in the Yama area would provide a wide space for migration pathways and the rapid filling of F-bearing ore-forming fluids. Finally, quartz veins and quartz–fluorite veins filled along faults within the syenogranite wall rocks in the Yama area. In essence, the Yama fluorite deposit is a fault-controlled hydrothermal vein-type deposit (e.g., [70,71]), and might be also classified as a Qiquanba type of meso-epithermal fluorite deposit in China, as proposed by [72]. The Late Ordovician–Early Silurian syenogranites as well as other granites in the Tataleng granitic batholith are just wall rocks for fluorite mineralization; thus, potential fluorite mineralization should be searched for within the subsidiary brittle faults within granites rather than in the granitic bodies themselves.

6. Conclusions

Petrographic observation; H, O, and Sr isotope analyses; and trace element and REE analyses were conducted on fluorites within the quartz–fluorite veins in the Yama fluorite deposit in the Tataleng granitic batholith in the SQB. Based on these analyses, the following conclusions can be drawn:

(1)

The paragenetic sequences for the Yama fluorite mineralization can be divided into two major stages, i.e., the pre-ore quartz stage, and the hydrothermal stage, and the latter stage can be subdivided into the quartz–fluorite epoch and the quartz–calcite epoch. Fluorite-free quartz veins, fluorite–quartz veins, and a few quartz–calcite veins were formed in the pre-ore stage, the quartz–fluorite epoch, and the quartz–calcite epoch, respectively. Typical fluorite–quartz veins consist of Type 1 fluorite ores showing vein structures, veinlet, stringer structures, and/or pectinate, and Type 2 ores showing breccia structures within veins.

(2)

Fluorite samples from fluorite–quartz veins have roughly similar REY patterns with relatively strong enrichment in HREEs, negative Eu anomalies (Eu/Eu* = 0.16–0.40), strongly positive Y anomalies (Y/Y* = 1.43–2.39), relatively high (La/Yb)_N values of 0.69–8.43, and comparably invariable range Y/Ho ratios (41.43–73.79), suggesting a reduced hydrothermal genesis.

(3)

Fluorites in two types of fluorite ore have relatively variable δD and δ¹⁸O values of −77.4‰ to −102.4‰ and −12.7‰ to −4.3‰, respectively, and the δD value ranges of two types of fluorite ores roughly overlap each other; however, Type 2 samples show slightly heavier δ¹⁸O values than Type 1, suggesting the origin of the ore-forming fluid for the Yama fluorite mineralization might predominantly originate from paleo-meteoric water, and a mixture with certain deep water sources (e.g., magmatic water or metamorphic water) cannot also be totally precluded.

(4)

Two types of fluorites have a relatively invariable range of 0.749089−0.756628 except an anomalously high ratio of 0.828185, and the syenogranite wall rocks have variable analytical ⁸⁷Sr/⁸⁶Sr ratios of 0.832625−1.191950. The calculated initial ⁸⁷Sr/⁸⁶Sr ratios of fluorites based on given ore-forming ages less than 443 Ma are apparently higher than the initial ⁸⁷Sr/⁸⁶Sr ratios of syenogranites, indicating the Sr source should be predominantly derived from the ore-forming fluids themselves rather than from the syenogranite wall rocks.

(5)

Combining the geological characteristics, trace element and REE geochemistry, and the isotope (H, O, and Sr) geochemistry, it was suggested that the Yama fluorite deposit can be classified as a fault-controlled hydrothermal vein-type deposit, and ore-forming water and materials might be directly derived from the evolutionary meteoric water rather than syenogranite wall rocks. The late large-scale faults within the Late Ordovician–Early Silurian syenogranites in the Yama area would provide a wide space for migration pathways and rapid filling of F-bearing ore-forming fluids. Finally, quartz veins and quartz–fluorite veins are filled alongside them, and weak hydrothermal alterations can be observed close to the veins within syenogranite wall rocks.