Occurrence and Favorable Enrichment Environment of Lithium in Gaoping Coal Measures: Evidence from Mineralogy and Geochemistry

Abstract

The Carboniferous-Permian coal measure strata in the Qinshui Basin exhibit highly lithium (Li) enrichment, with substantial exploitation potential. To further explore the enrichment mechanism of lithium in coal measure strata, the No. 15 coal of the Taiyuan Formation from the Gaoping mine is taken as the research object, and its mineralogical and geochemistry characteristics are evaluated using optical microscopy, X-ray diffraction, scanning electron microscopy, inductively coupled plasma mass spectrometry, X-ray fluorescence, and infrared spectral. The results show that the No. 15 coal is semi-anthracite coal with low moisture, low ash, low volatility, and high sulfur. Organic macerals are primarily vitrinite, followed by inertinite, and liptinite is rare; the inorganic macerals (ash) are dominated by clay minerals (predominantly kaolinite, cookeite, illite, and NH₄-illite), calcite, pyrite, quartz, siderite, gypsum, and zircon. The average Li content in the coal is 66.59 μg/g, with higher content in the coal parting (566.00 μg/g) and floor (396.00 μg/g). Lithium in coal occurs primarily in kaolinite, illite, cookeite, and is closely related to titanium-bearing minerals. In addition, Li in organic maceral may occur in liptinite. The No. 15 coal was formed in the coastal depositional system, and the deposition palaeoenvironment is primarily a wet–shallow water covered environment in open swamp facies; the plant tissue preservation index is poor, and aquatic or herbaceous plants dominate the plant type. The reducing environment with more terrestrial detritus, an arid climate, and strong hydrodynamic effects is favorable for Li enrichment in coal. The results have important theoretical significance for exploring the enrichment and metallogenic mechanisms of Li in coal.

1. Introduction

Strategic critical metals generally refer to metallic elements with significant applications in new materials, information technology, new energy, aerospace, military defense, and other emerging industries [1]. The demand for strategic mineral resources is steadily increasing in tandem with the sustainable development of the global economy. Lithium (Li) is one of the most critical and commonly used rare metals in civil and industrial production and military applications. In recent years, research on co-associated Li in coal has been strengthened to meet the national strategic demand for Li resources and find alternative sources of Li. Many researchers have conducted extensive research on Li in coal and found that a large amount of associated Li is enriched in coal measures in some regions [2,3,4], and coal may be an ideal alternative source of Li resources [5]. Currently, research on Li in coal mainly focuses on its occurrence, distribution, and depositional palaeoenvironment [6,7,8,9,10,11,12,13]. Conversely, the favorable enrichment environment of Li in coal has not yet been clarified, and research on the source rocks and provenance characteristics of Li is lacking.

Ketris and Yudovich ever estimated the world average amount of lithium in coal in 2009, and the result showed that the world average amount of Li in coal was 14 μg/g [14]. However, many scholars found that the Li content of coal in different coal fields in China was much higher than the world average. It is considered that Li in the coal of some coalfields may have potential utilization value. Dai et al. found that the No. 6 coal of the Har’erwusu open-pit mine was significantly enriched in Li, with average and maximum Li contents of 116.00 μg/g and 566.00 μg/g, respectively [15]. Similarly, Sun et al. found that the average Li content of the Antabao coal reached 172.00 μg/g, the highest Li content reached 657.00 μg/g [3], and they believed that the Li content in coal reached 120.00 μg/g, which is reasonable for industrial comprehensive mining and utilization [4]. The occurrence of Li in coal has not been fully investigated because of its low content and relative atomic mass, which cannot typically be detected by microbeam analysis [2]. Therefore, the occurrence of Li in coal is typically inferred using indirect methods such as correlation. Dai et al. concluded that Li occurs in chlorite, kaolinite, and possibly illite in the Guanbanwusu coal [2]. Sun et al. performed sequential chemical extraction of Li from the Pingshuo No. 9 coal and demonstrated that the Li concentration is mainly related to minerals, with only approximately 5.50% of Li having organic affinity [16]. Zhang believed that the Li in the Guanbanwusu coal occurred in a silica-aluminum oxide binding state [17]. Yi and Wang believed that the Li in the Anjialing No. 9 coal was closely related to silicates [18]. Zuo et al. proposed that Li carriers in the Xuangang No. 2 coal were mainly silicate minerals [19]. Wang found that the Li in the Ningwu coal mainly occurred as clay minerals in a silicate state and believed that Li was easily enriched in the strongly reducing salty water environment [20]. Zhou et al. believed that Caotang lithium-rich coal measures were formed by the interaction between kaolinite and Li-rich solutions under the influence of multistage hydrothermal injection [21]. Dai et al. (2021) summarized the occurrence modes of Li in coal as clay minerals (kaolinite and illite), tourmaline, silicate minerals, cookeite, etc. [22].

In this study, the No. 15 lithium-rich coal of the Taiyuan Formation from the Gaoping mine in the Qinshui Basin was collected as the research object to investigate its mineralogical and geochemical characteristics, provenance, and depositional palaeoenvironment. According to these research results, we need to understand the occurrence modes and favorable enrichment environment of Li in Gaoping No. 15 coal. The findings will provide a reference for the exploitation and utilization of Li resources in lithium-rich coal measures and the exploration of enrichment and metallogenic mechanisms.

2. Geological Background

The Qinshui Basin is a super-large coal-bearing basin and an important coal base in North China (Figure 1A). The Qinshui Basin was separated into large-scale independent multiple synclines after the Triassic Indosinian orogeny and the Jurassic–Cretaceous Yanshan orogeny in the North China Craton (NCC), with an axial direction from NNE to SSW.

Many small faults and folds have developed in the interior of the basin. The directions are NS in the southern and northern parts of the basin and NNE in the central part of the basin. The faults are mainly normal faults in NE, NNE, and NEE directions, and the major faults include the Sitou and Jinhuo Faults (Figure 1B). The Silurian to Lower Carboniferous strata are absent, whereas the Upper Carboniferous Benxi and Taiyuan Formations, Permian Shanxi Formation, Lower and Upper Shihezi Formations, Shiqianfeng Formation, and Triassic strata are continuously deposited. The Taiyuan Formation mainly consists of fine sandstone, siltstone, gritstone, mudstone, limestone, and coal seams (Figure 1C). The Taiyuan and Shanxi Formations are the primary coal-bearing strata, of which the Taiyuan Formation has total and average thicknesses of 62.70–103.52 m and 79.11 m, respectively. This formation contains approximately 8–11 layers of coal seams with an average total thickness of 6.34 m, and the coal-bearing coefficient is 8.01%.

As a part of the NCC, the study area mainly presents the paleogeographic background of marine continental transition facies in the Late Paleozoic. The northern part of Qinshui Basin is the lower delta plain depositional system, the central and southern part is the barrier island-lagoon depositional system, and the southeastern part is the offshore shelf depositional system [23,24,25,26]. The Gaoping mine is located in the southeast of the Qinshui Basin, belonging to the offshore carbonate shelf depositional facies. Its geographical coordinates are 35°40’–36°0’ N, 112°40’–113°10’ E (Figure 1B). The No. 15 coal from the Taiyuan Formation are the main minable coal seams with the thickness of 0.55–3.89 m, and an average of 2.54 m (Figure 1C).

Coalification in the southern Qinshui Basin was mainly influenced by high heat flow from magmatic activity due to a tectonic thermal event during the Yanshanian Orogeny, which occurred during the Late Jurassic-Early Cretaceous [27,28]. The geothermal gradient was up to 6 °C/100 m [29], which led to the formation of these abnormally high-rank coals.

Figure 1. (A) Map of coal-bearing regions in China (modified from reference [30]); (B) Regional geographical map of research region; (C) Stratigraphic column section of the Gaoping coal-bearing strata.

Figure 1. (A) Map of coal-bearing regions in China (modified from reference [30]); (B) Regional geographical map of research region; (C) Stratigraphic column section of the Gaoping coal-bearing strata.

3. Samples and Analytical Methods

Twelve samples were collected following the American Society for Testing and Materials Standard ASTM D4596-22 (2022) [31] via slot sampling at the Gaoping Coal Mine (Figure 1), including one roof, one floor, one parting (15R, 15F, and 15P), and nine coal samples (C1–C9). After sample collection, the samples were immediately placed into sealing bags to prevent contamination and oxidation. According to the Chinese National Standard GB/T 16773-2008 [32], polished coal sections can be prepared to observe macerals under a microscope and determine the vitrinite reflectance. Powdered samples with sizes less than 74 μm, were prepared for coal quality analysis, geochemistry analysis, and X-ray diffraction (XRD) analysis. Vitrinite reflectance was determined according to the American Society for Testing and Materials Standard ASTM D2798-21 (2021) [33] using a Leica DM4P microscope.

Low-temperature oxygenated plasma ashing of the coal powder samples was performed using K1050X plasma. The low-temperature ash and non-coal mineral components were analyzed by XRD (Bruker D8 Discovery). The test 2θ angle was 5°–70° with a step of 0.02° and a count time of 0.75 s/step. These tests and experiments were conducted at the Shanxi Key Laboratory of Fine Exploration of Coal-based Critical Mineral Resources. In quantitative analysis, the major diffraction peaks (d-values and intensities) were compared with PDF standard cards to determine the major mineral compositions. The calculations were based on the formula [34,35,36]:

$$W_A=(I_A/K_A)/(I_A/K_A+I_B/K_B+I_C/K_C+\dots \dots )\times 100\%$$

where W_A, I_A, and K_A are the weight percentage, the intensity of the most intense peak, and the reference intensity ratio (K) of the mineral phase to be determined, respectively. Symbols such as A, B, and C represent different minerals.

The major element test was conducted according to the Chinese National Standard GB/T 14506.28-2010 [37], and each powdered sample was weighed to 1 g and prepared using the melting method, with anhydrous lithium tetraborate and lithium chloride as the mixing solvents, ammonium nitrate as the oxidant, and a small amount of lithium bromide as the flux. The mass ratio of sample to flux was 1:8, and the sample was melted at 1200 °C for 10–15 min to prepare a glass sheet. The glass sheet was evaluated using an X-ray fluorescence spectrometer (Axios–mAX). The trace element test was conducted according to the Chinese National Standard GB/T 14506.30-2010 [38], and each powder sample was dissolved in a mixture of hydrofluoric acid (HF) and nitric acid (HNO₃) (approximately 0.05 g) and then diluted with HNO₃ in a sealed tube up to 50 mL. The trace elements in solutions were analyzed by inductively coupled plasma mass spectrometry (NexION300D plasma mass spectrometer). Major and trace element tests were completed at the Analytical Laboratory of Beijing Research Institute of Uranium Geology.

Scanning electron microscopy (SEM) images were obtained using a JEOL JSM-5610LV scanning electron microscope coupled with an energy-dispersive X-ray spectrometer (EDS) at the Chinese Academy of Geological Sciences. The operating conditions included an accelerating voltage of 20 kV and a spot size of 38 μm.

Infrared spectroscopy (IR) was performed using a Nicolet 6700 FTIR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The dried potassium bromide and powder samples were mixed at a ratio of 200:1, thoroughly ground, and compacted into tablets under a pressure of ~30 MPa for 1 min. Spectral scanning resolution was set at 4 cm⁻¹, spanning the wave number range of 4000–400 cm⁻¹.

4. Results

4.1. Coal Petrology

The macrolithotype of the No. 15 coal is dominated by semi-bright and semi-dull coal, containing a layer of parting. In the macro-petrographic units, vitrain and clarain are primarily present as bands (Figure 2). In addition, calcite films and pyrite nodules can be observed. Pyrite content is higher in the upper and lower coal seams than in the middle seams.

The dominant maceral in the No. 15 coal is vitrinite at a content of 60.20%–71.57% (

Table 1. Maceral contents of No. 15 coal (%).

Samples	Vitrinite (V)					Inertinite (I)				Liptinite (L)				Mineral (M)
	T	TC	DC	CC	VD	Sf	Ma	ID	F	Cu	LD	Re	Ba	CM	CaM	SM
C1	0.00	26.31	29.40	0.00	15.86	3.68	3.29	1.93	11.99	0.00	1.16	0.00	1.55	1.55	0.58	2.71
C2	0.00	24.50	29.48	0.60	15.34	3.98	3.78	1.59	8.96	0.00	1.79	0.00	1.00	3.39	1.59	3.78
C3	0.00	25.83	14.37	1.36	18.64	2.72	3.30	4.08	8.35	2.91	1.36	0.97	1.75	8.35	1.94	2.91
C4	0.39	22.18	19.65	2.33	15.76	7.39	3.50	4.86	9.92	0.78	0.00	0.39	0.78	5.64	1.95	3.89
C5	0.19	18.93	23.47	2.56	16.57	9.47	4.34	2.96	11.83	0.19	0.79	0.59	1.38	3.75	1.78	0.99
C6	0.00	20.46	30.40	1.15	17.40	3.44	3.25	2.68	9.56	0.76	1.15	0.00	0.78	5.93	0.19	2.29
C7	0.39	29.73	27.99	0.19	11.39	3.86	1.93	4.25	8.49	0.39	0.97	0.19	2.12	5.21	0.58	2.12
C8	0.00	18.04	34.55	0.58	9.02	4.80	4.22	1.73	9.60	0.00	0.38	0.00	3.07	5.76	0.77	7.29
C9	0.00	16.73	33.47	0.00	14.69	2.04	5.31	2.86	8.57	0.00	2.04	0.00	1.67	4.90	0.41	7.35
Average	65.54					20.95				3.43				9.73

Note: T–Telinite; TC–Telocollinite; DC–Desmocollinite; CC–Corpocollinite; VD–Vitrodetrinite; Sf–Semifusinite; Ma–Macrinite; ID–Inertodetrinite; F–Fusinite; Cu–Cutinite; LD–Liptodetrinite; Re–Resinite; Ba–Barkinite; CM–Clay mineral; CaM–Carbonate mineral; SM–Sulfide minerals; unlabeled macerals include gelocollinite, funginite, secretinite, micrinite, sporinite, suberinite, bituminite, exsudatinite, fluorinite, alginite, silicon oxide, and other minerals.

), which mainly comprises desmocollinite, telocollinite, and vitrodetrinite, with a minor amount of corpocollinite. Telinite has a well-preserved cellular structure, with the cell cavities filled with clay or other minerals (Figure 3D). Alternatively, the cellular structure may be crushed and broken, resulting in incomplete cell cavities (Figure 3A,B,E).

The inertinite primarily comprises fusinite, semifusinite (Figure 3F,H), macrinite, and inertodetrinite, with minor amounts of micrinite and funginite. The cellular structure of the fusinite is fragmentary, mostly occurring as aggregates of fragments (Figure 3C,E,F). Macrinite exhibits prominent protrusions and variable morphology (Figure 3E). The average content of liptinite is 3.43%, with liptodetrinite accounting for the majority (0.40%–2.00%). Resinite and exsudatinite are observed under reflection fluorescence (Figure 3A,D,G).

The maceral distribution indicates that the plant tissue preservation of the No. 15 coal is poor. This may be related to the relatively turbulent peat swamp environment and the high intensity of physicochemical effects during coal diagenesis. The widespread presence of pyrite in coal suggests the effect of seawater on depositional environments [39,40].

4.2. Coal Quality

The results of the proximate analysis and vitrinite reflectance (

Table 2. Results of proximate analysis and vitrinite reflectance test of Qinshui No. 15 coal.

Samples	Mad/%	Ad/%	Vdaf/%	St,d/%	Sp,d/%	Ss,d/%	So,d/%	Ro, max/%
C1	3.13	19.48	12.86	6.13	2.48	1.03	2.62	2.26
C2	2.34	17.40	15.77	4.21	1.20	0.65	2.37	2.22
C3	3.41	28.75	16.96	3.98	0.75	1.12	2.11	2.12
C4	4.16	16.78	12.01	3.37	0.28	0.59	2.49	2.40
C5	3.22	17.34	11.78	2.79	0.09	0.28	2.42	2.51
C6	3.47	23.71	12.81	2.80	0.10	0.44	2.26	2.46
C7	4.11	16.78	13.15	4.21	0.65	1.19	2.38	2.28
C8	3.40	17.78	14.71	6.89	2.98	1.35	2.57	2.58
C9	4.65	21.08	14.41	9.82	6.40	0.67	2.75	2.30
Max	4.65	28.75	16.96	9.82	6.40	1.35	2.75	2.58
Min	2.34	16.78	11.78	2.79	0.09	0.28	2.11	2.12
Average	3.54	19.9	13.83	4.91	1.66	0.81	2.44	2.38

Note: ad–air dry basis; d–dry basis; daf–dry ash free basis; M–moisture; A–ash yield; V–volatile yield; S_t–total sulfur; S_p–sulfide sulfur; S_s–sulfate sulfur; S_o–organic sulfur; R_{o, max}%–Maximum vitrinite reflectance with oil–immersion microscope.

) show that the No. 15 coal is a low moisture, low ash, and low volatility coal, which is based on the American Society for Testing and Materials Standard ASTM D3173/D3173M-17a (2017), ASTM D388-23 (2023), Chinese National Standard GB/T 15224.1-2018, and GB/T 15224.2-2021 [41,42,43,44]. The total sulfur content varies significantly from 2.79% to 9.82%, with an average of 4.91%, with organic sulfur accounting for the majority (2.44%), followed by sulfide sulfur (1.66%) and sulfate sulfur (0.81%). The maximum vitrinite reflectance (R_{o, max}%) is 2.12%–2.57%, with an average maximum reflectance of 2.35%. Combining the two indexes of V_daf and R_{o, max}, the No. 15 coal is classified as semi-anthracite coal according to ASTM D2798-21 (2021) and ASTM D388-23 (2023) [33,42].

4.3. Mineralogy

Different depositional environments and geological processes affect the mineral composition and characteristics of coal. The XRD results (Figure 4) show that the main minerals in the No. 15 coal are kaolinite, NH₄-illite, cookeite, quartz, calcite, pyrite, anhydrite, anatase, and rutile. Microscopy further confirms the presence of pyrite, calcite, and clay minerals (Figure 5).

The distribution of pyrite in the coal seams varies. It is concentrated in the upper C1 and C2 layers and lower C8 and C9 layers. The pyrite morphologies in C1 and C2 are mainly framboidal (Figure 5A), veined (Figure 5B,F), and disseminated (Figure 5C), whereas in C8 and C9, the pyrite morphology is mainly massive (Figure 5H,I). In C8, pyrite filled the cell cavities (Figure 5G). Framboidal and massive pyrites typically form during peat accumulation or early diagenesis under the influence of seawater [40]. Conversely, disseminated and veined pyrites are generally associated with later epigenetic stages [39]. Interestingly, XRD analysis (Figure 4) did not detect pyrite in the C5 and C6 layers. The overall prevalence of pyrite in the coal indicated that the depositional environment was significantly influenced by reducing environment.

The XRD analysis (

Table 3. Quantitative XRD analysis results (%) (the results are normalized to 100% ash in coal).

Samples	Qz	Kf	Pl	Cal	Arg	Sd	Mgn	Py	Hem	Anl	Br	Anh	Trd	Ant	Ank	CM
15R	19.40	–	–	–	–	–	–	31.50	–	–	3.40	1.50	–	–	–	44.20
C1	31.40	1.00	–	2.60	–	–	–	37.30	–	–	–	2.60	–	–	–	25.10
C2	4.10	–	–	18.40	–	–	–	17.90	–	3.70	–	4.30	1.80	–	–	49.80
C3	1.80	–	–	4.00	–	–	1.30	1.90	–	2.80	–	2.30	25.40	–	–	60.50
C4	–	–	–	3.50	–	–	–	5.20	–	4.40	4.90	–	–	2.60	–	79.40
C5	–	0.20	–	2.30	–	–	1.70	–	–	2.20	–	–	27.00	1.80	–	64.80
C6	–	0.60	–	1.00	8.30	–	1.30	–	–	4.10	4.50	–	–	3.10	–	77.10
C7	–	–	–	3.00	–	–	–	9.90	–	4.90	5.50	3.70	–	–	–	73.00
C8	0.50	–	–	4.10	–	0.50	–	–	–	4.80	–	3.50	–	–	26.00	60.60
15P	0.90	0.50	–		–	–	–	–	–	–	–	–	–	–	–	98.60
C9	–	2.00	1.90	1.30	–	–	1.80	–	14.20	1.90	2.20	–	7.00	–	–	67.70
15F	–	–	–		–	–	0.80	12.80	–	–	–	–	–	–	–	86.40

Note: Qz–Quartz; Kf–Kalifeldspar; Pl–Plagioclase; Cal–Calcite; Arg–Aragonite; Sd–Siderite; Mgn–Magnesite; Py–Pyrite; Hem–Hematite; Anl–Analcite; Br–Barite; Anh–Anhydrite; Trd–Tridymite; Ant–Anatase; Ank–Ankerite; CM–Clay Minerals.

) shows that the clay minerals in the coal include kaolinite, illite, NH₄-illite, and cookeite (Figure 4E), among which kaolinite and NH₄-illite occurred more frequently (Figure 4). The SEM results show that the kaolinite is vermicular (Figure 6C), and clay minerals are visible under the microscope, mainly filling the fissures and cell cavities.

The calcite in the coal occurred in C2, C3, and C8; the calcite was primarily veined under the microscope and was of epigenetic origin according to the relationship between calcite and pyrite. Organic maceral fragments were also observed on the surface of the calcite veins (Figure 5E), and two group cleavages of the calcite were observed under the scanning electron microscope (Figure 6A).

Fourier transform infrared spectroscopy (FTIR) analysis results (Figure 7) show that the infrared absorption peaks of the samples mainly include OH stretching vibration peak, ${\mathrm{NH}}_4^{+}$ stretching vibration peak, benzene ring skeleton vibration peak, ${\mathrm{NH}}_4^{+}$ bending vibration peak, Si–O stretching vibration peak, OH bending vibration peak, Si(Al)–O stretching vibration peak, and Si(Al)–O bending vibration peak. The three absorption bands 3692, 3651, and 3620 cm⁻¹ belong to OH stretching vibration peaks, which may be caused by OH stretching vibration inside kaolinite and NH₄-illite [45,46,47,48,49]. The absorption peak at 1580 cm⁻¹ is attributed to the skeleton vibration of the benzene ring. The presence of NH₄-illite is further confirmed by the bending vibration peaks of ${\mathrm{NH}}_4^{+}$ at 1432 cm⁻¹ and the weaker stretching vibration peaks of ${\mathrm{NH}}_4^{+}$ at 3308 and 3040 cm⁻¹ [45,46,47,48,49,50,51]. The absorption peaks of 1095, 1033, and 1008 cm⁻¹ belong to Si–O stretching vibration, which may be caused by Si–O bond vibration in the tetrahedron of kaolinite, while the vibration peaks of 936 and 915 cm⁻¹ belong to in-plane and out-of-plane bending vibration of OH in octahedron [45,49]. The absorption peaks of 790, 750, and 690 cm⁻¹ are caused by Si(Al)–O stretching vibration [49]. The absorption peaks at 537, 471, and 430 cm⁻¹ are caused by Si(Al)–O bending vibration [49]. In summary, kaolinite, NH₄-illite, and other aluminosilicate minerals were present in the samples. The absorption peaks in 3692–3620 cm⁻¹ in C2 and C7 samples showed weak peaks, suggesting less kaolinite content [45]. The absorption peaks of 15P and 15F samples at 3692–3620 cm⁻¹ and 1095–915 cm⁻¹, respectively were obvious, indicating that there was more kaolinite in the samples [45], while the absorption peak at 1432 cm⁻¹ showed not obvious peak, indicating that there might be no ${\mathrm{NH}}_4^{+}$ in the samples [46,47,48,49]. However, the absorption peaks of C2, C5, C6, and C7 at 1432 cm⁻¹ are obvious, indicating the presence of ${\mathrm{NH}}_4^{+}$, which further confirms the existence of ammonium in illite.

4.4. Elemental Geochemical Analysis

4.4.1. Major Elements

The results (

Table 4. Test results for major elements (wt.%) in coal and non-coal samples.

Samples	SiO2	Al2O3	CaO	MgO	K2O	MnO	Na2O	TiO2	TFe2O3	P2O5
15R	43.20	16.15	0.39	0.50	0.82	0.05	0.07	0.47	19.96	0.02
C1	3.05	0.63	0.45	0.12	0.06	0.00	0.02	0.07	3.02	0.01
C2	1.69	1.00	4.58	0.14	0.07	0.00	0.04	0.14	1.06	0.01
C3	14.72	10.22	1.16	0.19	0.17	0.00	0.04	0.43	1.37	0.01
C4	7.70	5.02	0.54	0.17	0.13	0.00	0.04	0.20	1.49	0.01
C5	8.21	5.63	0.51	0.16	0.11	0.00	0.03	0.46	0.24	0.01
C6	13.68	9.72	0.38	0.23	0.28	0.00	0.07	0.85	0.39	0.02
C7	3.74	2.70	0.57	0.13	0.09	0.00	0.05	0.10	1.36	0.01
C8	2.24	1.65	0.50	0.12	0.07	0.00	0.03	0.04	2.63	0.01
15P	39.85	33.84	0.42	0.17	0.16	0.02	0.05	1.59	1.02	0.02
C9	7.14	5.20	0.34	0.13	0.07	0.00	0.03	0.15	10.09	0.01
15F	27.63	21.75	0.30	0.16	0.12	0.01	0.10	0.68	4.74	0.03
WA–C	6.91	4.64	1.00	0.15	0.12	0.00	0.04	0.27	2.40	0.01
Late-Paleozoic coals of North China	8.14	6.78	1.2	0.28	0.17	0.01	0.15	0.38	1.31	0.13
Chinese coals [52]	8.47	5.98	1.23	0.22	0.19	0.02	0.16	0.33	4.85	0.09

Note: WA–C, weighted average for coal samples.

) show that SiO₂ has the highest content (6.91%) and MnO has the lowest content among the major elements. The average contents, from high to low, are SiO₂, Al₂O₃, Fe₂O₃, CaO, TiO₂, MgO, K₂O, Na₂O, P₂O₅, and MnO. Compared with the average element content of the Late Paleozoic coals of North China, the content of Fe₂O₃ (2.40%) is high, whereas those of the other elements are low. However, its order is consistent with the content of major elements in the Late Paleozoic coals of North China and Chinese coals.

The trends of SiO₂, TiO₂, Al₂O₃, and K₂O are similar in the vertical section (Figure 8), with lower contents in C1, C2, C7, and C8, and higher contents in C3–C6. The changes in the MgO, Na₂O, and P₂O₅ contents are stable, and the content of CaO in the layers is approximately 0.50%, except for C2 (4.58%) and C3, which have higher contents; therefore, more calcite is observed to be distributed in C2 (Figure 4B and Figure 5C). The Fe₂O₃ content in C9 is high (Table 4, 10.09%), while the massive pyrites (Figure 5H,I) were mostly found in C9, which forms a good correspondence between them.

4.4.2. Trace Elements

The trace element concentration coefficient (CC) is the ratio of the element concentration in the investigated coals to that in world hard coals [53]. The result (Figure 9) shows that Li, Mo, In, Pb, U, Ta, Zr, and Hf are slightly enriched in the coal (2 < CC < 5); Be, Sc, Cr, Cu, Zn, Ga, W, Th, Nb, and REY are normally enriched in the coal (1 < CC < 2); V, Co, Sr, and Cd are slightly depleted in the coal (0.5 < CC < 1); the remaining elements of Ni, Rb, Sb, Cs, Ba, Tl, and Bi are significantly depleted in the coal (CC < 0.5). The average abundance of Li in the coal was 66.59 μg/g, higher than in world hard coals (14.00 μg/g) and Chinese coals (31.8 μg/g).

The trace elements in the roof, parting, and floor of the coal seam are more concentrated than those in the coal samples. The CC values of Li in the parting and floor samples are 40.40 and 26.40 (Figure 9), which is highly enriched (10 < CC < 100), and the Li contents in the parting and floor samples are 566.00 μg/g and 396.00 μg/g, respectively (

Table 5. Trace element contents of No. 15 coal and non-coal samples (μg/g).

Element	15R	C1	C2	C3	C4	C5	C6	C7	C8	15P	C9	15F	WA–C	World 1	Chinese 2
Li	54.00	6.14	6.60	113.00	44.40	60.70	120.00	107.00	35.50	566.00	106.00	369.00	66.59	14.00	31.80
Be	1.64	0.96	0.84	1.95	0.96	0.97	2.46	1.10	4.69	6.13	5.46	7.65	2.15	2.00	2.11
Sc	10.90	2.63	1.35	2.82	3.77	4.79	9.44	3.75	2.16	15.60	6.51	60.50	4.14	3.70	4.38
V	48.60	14.00	13.70	14.00	22.70	30.70	56.40	19.80	13.80	65.00	45.80	179.00	25.66	28.00	35.10
Cr	72.10	12.60	8.96	8.17	12.20	17.70	56.60	12.40	7.57	45.80	18.50	66.20	17.19	17.00	15.40
Co	10.70	1.29	0.99	2.56	2.69	2.61	7.07	2.67	1.70	7.15	11.40	4.00	3.66	6.00	7.08
Ni	41.80	7.13	2.31	3.51	3.87	3.88	11.80	4.76	4.40	20.80	13.30	15.20	6.11	17.00	13.70
Cu	49.50	8.32	7.48	19.40	16.80	13.30	25.50	11.90	13.60	22.80	49.70	22.30	18.44	16.00	17.50
Zn	28.00	22.70	37.40	33.50	30.50	36.70	45.40	26.70	24.90	43.20	27.20	40.30	31.67	28.00	41.40
Ga	16.70	3.02	1.87	14.90	9.40	10.80	15.80	6.04	3.13	37.70	36.40	23.50	11.26	6.00	6.55
Rb	37.50	0.72	0.84	4.20	2.28	1.97	10.50	0.98	0.61	6.13	1.11	4.06	2.58	18.00	9.25
Sr	63.80	112.00	114.00	109.00	52.40	53.00	57.50	105.00	114.00	57.30	76.80	49.10	88.19	100.00	140.00
Mo	3.19	9.84	2.77	4.34	4.53	5.66	16.00	8.41	5.40	14.50	7.86	6.69	7.20	2.10	3.08
Cd	0.21	0.13	0.14	0.21	0.14	0.16	0.21	0.17	0.22	0.07	0.20	0.09	0.18	0.20	0.25
In	0.06	0.08	0.10	0.08	0.09	0.09	0.09	0.10	0.10	0.09	0.11	0.18	0.09	0.04	0.05
Sb	0.51	0.08	0.07	0.11	0.21	0.12	0.44	0.20	0.22	0.31	0.76	0.45	0.25	1.00	0.84
Cs	5.32	0.11	0.12	0.86	0.38	0.26	2.59	0.10	0.07	2.10	0.12	0.93	0.51	1.10	1.13
Ba	97.00	6.36	7.02	15.90	13.20	14.40	23.40	9.34	8.41	23.70	14.40	19.20	12.49	150.00	159.00
W	1.25	0.63	0.60	1.02	0.65	1.15	1.86	2.34	1.87	4.88	2.42	2.61	1.39	0.99	1.08
Tl	1.11	0.11	0.08	0.22	0.17	0.11	0.21	0.16	0.32	0.17	1.19	0.51	0.28	0.58	0.47
Pb	71.40	5.40	2.21	14.70	30.80	3.17	8.02	9.40	18.00	23.60	98.20	79.10	21.10	9.00	15.10
Bi	0.55	0.15	0.22	0.45	0.36	0.34	0.63	0.26	0.10	1.82	0.22	1.57	0.30	1.10	0.79
Th	9.26	1.14	1.59	14.00	5.73	6.97	13.80	3.73	1.22	34.40	4.06	38.90	5.80	3.20	5.84
U	3.52	6.82	0.69	5.18	2.87	3.25	9.67	2.16	0.70	12.50	5.89	35.60	4.14	1.90	2.43
Nb	14.60	1.53	2.01	17.30	6.82	12.90	21.10	3.60	0.81	24.20	5.56	20.00	7.96	4.00	9.44
Ta	1.07	0.06	0.14	2.03	0.39	0.92	1.60	0.16	0.07	0.06	0.36	1.23	0.64	0.30	0.62
Zr	125.00	20.60	19.00	105.00	84.20	107.00	154.00	73.80	42.40	287.00	214.00	379.00	91.11	36.00	89.50
Hf	5.03	0.45	0.62	3.80	2.60	3.25	5.21	2.27	1.33	10.90	6.73	11.40	2.92	1.20	3.71
REY	127.33	79.84	68.97	96.17	93.64	86.50	143.22	70.48	104.09	267.48	176.66	386.99	102.18	68.41	135.89

Note: WA–C, the weighted average for all coal samples; ¹ World coals, data from Ketris and Yudovich (2009) [14]; ² Chinese coals, data from Dai et al. (2012) [52].

). The parting is highly enriched in Th, whereas the floor is highly enriched in Sc, Th, U, and Zr.

4.4.3. Rare Earth Elements and Yttrium (REY)

Rare earth elements are stable during deposition and their distribution pattern can be used to judge the changes in paleoclimate, water salinity, redox conditions, and supply of material sources in coal depositional environments to study the depositional mechanisms of abnormal enrichment or depletion of the elements in the coal [54].

The REY content varies from 68.97 μg/g to 386.99 μg/g (Table 5). The average REY content in the coal samples is 102.77 μg/g, which is higher than that in world hard coal (68.41 μg/g) and Late Paleozoic coals of the Qinshui Basin (80.30 μg/g) [55], but lower than that in China coals (135.89 μg/g). The REY content is higher at the bottom and lower at the top seam, and it is the highest in C9 coal. Furthermore, the REY content in the roof, parting, and floor is higher than that in the coal.

Seredin and Dai (2012) classified REY into three types [56]: light rare earth elements (LREY: La, Ce, Pr, Nd, and Sm), medium rare earth elements (MREY: Eu, Gd, Tb, Dy, and Y), and heavy rare earth elements (HREY: Ho, Er, Tm, Yb, and Lu). The REY data were normalized by the Upper Continental Crust (UCC) [57] to generate the REY distribution pattern of the Gaoping coal (Figure 10). The REY parameters (

Table 6. REY parameters of Gaoping No. 15 coal and non-coal samples.

Samples	EuN/EuN *	CeN/CeN *	(La/Lu)N	(La/Sm)N	(Gd/Lu)N	Type
15R	1.16	1.05	0.67	1.03	0.68	H
C1	1.37	1.05	0.14	0.87	0.20	H
C2	1.29	1.01	0.21	0.45	0.52	H
C3	0.95	1.06	0.76	1.27	0.77	H
C4	1.00	1.05	0.61	0.93	0.70	H
C5	1.06	1.04	0.41	0.72	0.57	H
C6	0.88	0.85	0.66	1.40	0.62	H
C7	1.09	1.01	0.22	0.44	0.50	H
C8	1.18	0.98	0.17	0.30	0.56	H
15P	1.04	0.90	0.51	0.99	0.55	H
C9	1.25	0.91	0.19	0.52	0.37	H
15F	0.99	0.98	0.38	0.68	0.58	H

Note: Eu_N/Eu_N* = Eu_N/(0.5 × Sm_N + 0.5 × Gd_N); Ce_N/Ce_N * = Ce_N/(0.5 × La_N + 0.5 × Pr_N).

) show that δEu (Eu_N/Eu_N*) is 0.88–1.37, with an average value of 1.11. The coal sample C6 is an Eu-negative anomaly (0.88); the C3, C4, 15P, and 15F samples have no anomalies; the C1, C2, and C9 samples are positive anomalies. δCe (Ce_N/Ce_N*) is 0.85–1.06, with an average of 0.99, and there is no anomaly, excluding C6, 15P, and C9, which are less than 0.95, which is a negative anomaly. C3 is higher than 1.05 (1.06), which is a slightly positive anomaly. The others have no anomalies.

Seredin and Dai classified the distribution patterns of REY [56] based on the UCC-normalized La_N, Lu_N, Sm_N, and Gd_N, including the following:

• Light rare earth-enriched type (L-type): La_N/Lu_N > 1.0;
• Medium rare earth-enriched type (M-type): La_N/Sm_N < 1.0 and Gd_N/Lu_N > 1.0;
• Heavy rare earth-enriched type (H-type): La_N/Lu_N < 1.0.

In addition, there are mixed enriched types such as light and medium rare earth-enriched types (L-M-type) and medium and heavy rare earth-enriched types (M-H-type). The (La/Lu)_N values of the No. 15 coal are less than 1.0, with an average of 0.41 (Table 6), and the (La/Lu)_N values of C1, C2, and C7–C9 are lower than those of the other samples; the (La/Sm)_N is 0.30–1.40, with an average of 0.80, and the (Gd/Lu)_N is 0.20–0.77, with an average value of 0.51. All samples belong to the H-type.

The REY distribution patterns of the different strata samples differ (Figure 10). The REY distribution curves of the C3–C6 layers, roof, floor, and parting are gentle, and the degree of differentiation of LREY, MREY, and HREY is not obvious (Figure 10A,C). In contrast, the distribution curves of C1–C2 and C7–C9 are similar, and exhibit obvious left tendency (Figure 10B), indicating that the HREY is more enriched. In addition, the Gd-negative anomalies are obvious (Figure 10B), whereas the roof, floor, and parting are Y-negative anomalies (Figure 10A).

5. Discussion

5.1. Provenance Analysis

Al, Ti, Nb, Zr, and REY are resistant to alterations during weathering, transportation, and diagenesis [58,59]. This makes them valuable geochemical indicators for determining the source of detrital materials in coal-bearing strata [6,53,60] and the composition of source rocks [61,62,63]. Dai et al. (2016) demonstrated Al₂O₃/TiO₂ and Zr/TiO₂ vs. Nb/Y ratios [64], and believed that the distribution pattern of REY changed minimally during the weathering–sedimentation–diagenesis process, which can be used as reliable geochemical indicators for determining the source of detrital materials [64]. The Al₂O₃/TiO₂ ratio is particularly useful for identifying the source rocks of coal seams [58,65] and for inferring the source rock composition of coal seams and interbeded volcanic ash. Hayashi et al. (1997) classified the Al₂O₃/TiO₂ ratios of different source rocks into three numerical intervals ranging from 3–8, 8–21, and 21–70, which represent mafic, intermediate, and felsic igneous rocks, respectively [58,66,67]. The sample points of the C1 and C2 layers of the No. 15 coal fall within the mafic rock region, the C5 and C6 sample points fall within the intermediate rock region, and the remaining sample points fall within the felsic igneous rock region (Figure 11A).

The La/Yb–ΣREE source rock discrimination diagram [68] by Allègre and Minster (1978) provides further insights (Figure 11B). Five coal samples (C1–C2 and C7–C9) fall within the central part of the basalt area, indicating that continental tholeiitic basalts are the source rock. In contrast, samples C3–C6 and the roof sample fall within the sedimentary rock region. The parting and floor samples fall near the intersection of alkali basalts and sedimentary rocks.

Winchester and Floyd (1977) suggested that the source rock types in the provenance region could be traced based on the relationship between Zr/TiO₂ and Nb/Y [7,62,69,70]. However, Yttrium (Y) is mobile in low-temperature aqueous solutions, and its easy migration properties may affect the reliability of this method [71]. Therefore, we employed the Al₂O₃/TiO₂ vs. Zr/TiO₂ and Al₂O₃/TiO₂ vs. Nb/Yb discriminant diagrams optimized by Zheng et al. to analyze the material sources [72]. The potential provenance [73,74,75,76,77,78] of the coal measure strata (Figure 12) shows that the sample points mainly fall in the region of Yinshan diorite and granite and partly fall in the region of Zhong-tiao Mountain. No sample point falls in the Paleozoic granite region of the North Qinling Mountains, indicating that the sediments in this region mainly originate from the Yinshan diorite and granite and the Zhongtiao Mountain. Hou et al. suggested that bauxite is likely one of the main sediment provenances of the No. 15 coal seam [9]. Figure 12 shows that some sample points fall within the bauxite region from the NCC, supporting the possibility of the supply of material sources from underlying bauxite to coal seams in this area.

Previous studies have demonstrated that the sediment sources in this area are primarily the northern Zhongtiao Mountains, Inner Mongolia Mountains uplift, and Yinshan Paleoland uplift, etc., and the felsic igneous rocks are widely developed [6,8,9,79], which is consistent with our study.

5.2. Depositional Palaeoenvironment

5.2.1. Redox Conditions

The V/(V+Ni) and Cu/Zn indexes were selected to analyze the redox conditions of the palaeoenvironment, because vanadium (V) in the water can more effectively sediment as an organic complex in a reducing environment, i.e., the reducing property gradually improves with increasing V/(V+Ni) ratio. On the other hand, copper (Cu) and zinc (Zn) are separated in different redox environments. Thus, the Cu/Zn ratio can also be used to discriminate redox conditions.

Jones et al. considered a V/(V+Ni) ratio of 0–0.60 to be an oxygen-rich environment; a V/(V+Ni) ratio of 0.60–0.77 to be an oxygen-poor environment; and V/(V+Ni) > 0.77 to be an oxygen-deficient environment [80]. Li et al. considered V/(V+Ni) > 0.50 to be a reducing environment; V/(V+Ni) < 0.50 to be an oxidizing environment; and Cu/Zn > 0.30 to be a reducing environment [54]. Murry et al. (1991) found that in marginal sea, shallow sea, or land-enclosed sea, the change in δCe is extremely weak. The V/(V+Ni) in the No. 15 coal exceeds 0.50 with an average of 0.79. The Cu/Zn ratio of all the samples is greater than 0.30, with an average of 0.69, and the change in δCe in the coal is small, with no obvious anomaly (Table 6). These results indicate that the coal-forming environment was an oxygen-poor reducing environment in marginal or shallow seas. The vitrinite/inertinite ratio (V/I) of the No. 15 coal shows that it formed in an extremely wet overlying water environment (1 < V/I < 4,

Table 7. Depositional palaeoenvironmental parameters of the No. 15 coal and non-coal samples.

Samples	Sr/Ba	Th/U	V/(V+Ni)	Cu/Zn	Sr/Cu	Al2O3/TiO2	C	GI	TPI	GWI	VI	V/I
15R	0.66	2.63	0.54	1.77	1.29	34.51	0.35	–	–	–	–	–
C1	17.61	0.17	0.66	0.37	13.46	9.04	0.97	4.25	0.83	0.37	2.22	3.43
C2	16.24	2.30	0.86	0.20	15.24	7.37	2.15	5.07	0.75	0.46	2.00	3.78
C3	6.86	2.70	0.80	0.58	5.62	23.99	0.11	4.19	0.91	0.83	1.42	3.23
C4	3.97	2.00	0.85	0.55	3.12	25.35	0.17	2.88	0.91	0.70	1.88	2.30
C5	3.68	2.15	0.89	0.36	3.99	12.27	0.07	2.72	0.85	0.60	2.00	2.14
C6	2.46	1.43	0.83	0.56	2.26	11.40	0.04	4.63	0.62	0.53	1.51	3.59
C7	11.24	1.73	0.81	0.45	8.82	26.73	0.32	4.31	0.93	0.34	2.51	3.72
C8	13.56	1.74	0.76	0.55	8.38	41.25	0.84	4.12	0.66	0.45	2.91	3.06
15P	2.42	2.75	0.76	0.53	2.51	21.28	0.02	–	–	–	–	–
C9	5.33	0.69	0.78	1.83	1.55	34.90	0.86	5.21	0.49	0.54	1.40	3.46
15F	2.56	1.09	0.92	0.55	2.20	31.89	0.11	–	–	–	–	–
Average	7.22	1.78	0.79	0.69	5.70	23.33	0.50	4.15	0.77	0.54	1.98	3.19

C = (Fe₂O₃+CaO+MgO)/(SiO₂+Al₂O₃); TPI = (telinite+telocollinite + fusinite + semifusinite)/(desmocollinite + vitrodetrinite + macrinite + inertodetrinite); GI = (telinite + macrinite)/(fusinite + semifusinite + inertodetrinite); GWI = (desmocollinite + corpocollinite + gelocollonite + minerals)/(telinite + telocollinite + desmocollinite); VI = (telinite + telocollinite + fusinite + semifusinite + suberinite + resinite)/(vitrodetrinite + inertodetrinite + liptodetrinite + alginite + sporinite + cutinite); V/I = vitrinite/inertinite.

), reflecting that the coal-forming plants were in a wet reducing environment.

5.2.2. Palaeosalinity

Strontium (Sr) is easily dissolved in water and can migrate with water, whereas barium (Ba) can be adsorbed and precipitated by clay minerals or other ions, reducing its content in water [81]. Thus, the Sr/Ba ratio can indicate whether the depositional environment is terrestrial or marine. Sr/Ba < 0.6 indicates slightly brackish water deposition; Sr/Ba = 0.6–1.0 indicates brackish water deposition; Sr/Ba > 1.0 indicates salt lake or marine deposition [54]. The Sr/Ba ratios of the No. 15 coal samples are greater than 1, except for the roof sample, with an average value of 7.20, indicating that the coal is deposited in a marine environment.

The Th/U ratio can also determine marine and terrestrial depositional environments. Th/U > 7.0 indicates a terrestrial freshwater environment; Th/U = 2.0–7.0 indicates slightly brackish water to the semi-saltwater environment; Th/U < 2.0 indicates a marine saltwater environment [82]. The average Th/U ratio of the No. 15 coal is 1.78, which is less than 2.0, indicating a marine saltwater environment.

5.2.3. Palaeoclimate and Peat Swamp Facies

Concentrated precipitation of lake water or sea intrusion in hot arid climates may lead to the precipitation of Sr as SrSO₄, resulting in higher Sr content in sediments and lower Sr content in humid climates. Lerman found that Sr/Cu can be used to analyze the palaeoclimate. Sr/Cu = 1.0–10.0 indicates a humid climate and Sr/Cu > 10.0 indicates an arid climate [83]. The Sr/Cu ratios of the No. 15 coal samples are in the range of 1.0–10.0, except for C1 and C2 with Sr/Cu >10.0, indicating that the palaeoclimate is humid. The Sr/Cu ratios of the C1 and C2 samples are greater than 10, which is probably due to transgression, and the resulting increase in Sr content is a potential cause of this high ratio.

The tissue preservation index–gelification index (TPI–GI), groundwater index–vegetation index (GWI–VI), vitrinite/inertinite ratio (V/I), and the (CaO+MgO+Fe₂O₃)/(SiO₂+Al₂O₃) ratio of coal (C-value) can be used to characterize peat swamp environments [84,85,86,87,88,89]. The TPI–GI diagram in Figure 13 shows that the coal samples fall in the transgressive area of the Back Barrier region, indicating that the peat swamp environment is mainly affected by transgressive action. The GWI–VI diagram shows that the coal samples fall within the bog area (Figure 13).

The (CaO+MgO+Fe₂O₃)/(SiO₂+Al₂O₃) ratio of coal (C-value) is typically used to indicate the media conditions during peat accumulation, where a C-value of 0.23 is the boundary between marine and terrestrial facies [11,90,91]. The C-values of the No. 15 coal samples are greater than 0.23, except for C3–C6 with C-values less than 0.23 (Table 7), suggesting that C3–C6 belong to terrestrial facies peat swamp, whereas the remaining samples are influenced by seawater. However, the total sulfur content (S_t,d) of C3–C6 ranges from 2.80% to 3.98%, with an average of 3.24% (Table 2), and the organic sulfur contents of all samples are higher than 2.00%, indicating that they had been affected by seawater during peat swamp accumulation.

According to research results of depositional facies in the southeastern Qinshui Basin [92,93,94], the No. 15 coal is assumed to belong to the coastal zone depositional system. Combined with the above analyses, the depositional palaeoenvironment is believed to be mainly wet, with weak groundwater dynamics, and a peat swamp environment with aquatic or herbaceous plants, and is significantly influenced by seawater.

5.2.4. Seawater and Hydrothermal Influences

The total sulfur (S_t,d) content of the No. 15 coal is 2.79%–9.82% (Table 2), and the higher sulfur content may be due to the high ${\mathrm{SO}}_4^{2-}$ content in seawater, which affects bacterial activity, and the large distribution of pyrite in the C1–C2 and C8–C9 layers, indicating that the coal seams are affected by seawater [9,40,95]. The Sr/Ba ratios of all coal samples are greater than 1.0, with those of C1–C2 and C7–C8 being significantly higher than those of other coal samples (Table 7), indicating that seawater affects the formation of coal seams [96] and that the upper and lower coal seams layers are more affected by seawater than the central layers. Pyrite in coal mainly occurs as massive (Figure 5H,I), which are typically formed during peat accumulation or early diagenetic processes [40]. Most of these syngenetic or early diagenetic pyrites are due to seawater intrusion into coal seams, which provide sulfur for sulfide formation [6,8,9,12,97,98]. Epigenetic pyrite is geologically younger, therefore it acts as a filling material in the joints and fissures [99], and therefore it mainly occurs as veined and filled pyrite (Figure 5B,F,G). The multiple forms of pyrite in coal suggest that seawater influenced the coal formation. The correlation analysis between Li and C-value, and Sr/Ba ratios showed a strong negative correlation (r_Li–C = −0.65, Figure 14A; r_Li–Sr/Ba = −0.69, Figure 14B), indicating that the deposition was influenced by terrestrial detritus, thereby enriching the No. 15 coal with Li. However, the upper part of the coal seams (C1–C2) is low in Li, which may be due to the leaching effect of seawater.

Previous studies have shown that the higher vitrinite reflectance of the Qinshui Basin coals may be related to hydrothermal action due to magmatic intrusion during the Yanshan period (Late Jurassic-Early Cretaceous) [27,100]. In addition, the No. 15 coal is primarily is HREY enriched type (Table 6), which is different from the LREY enrichment modes of most coals and may be related to the infiltration of Fe-rich fluids into the coal seams or the leaching effect of infiltrating overlying marine sediments [9,56]. In a high-temperature strongly reducing environment (hydrothermal fluids), Eu³⁺ can be reduced to Eu²⁺, resulting in Eu-positive anomalies [101]. The average δEu of the No. 15 coal samples is 1.11, with those of C1, C2, and C9 being significantly greater than 1.05 (1.37, 1.29, and 1.25, respectively), indicating a high-temperature effect, leading to Eu-positive anomalies. Veined, filled, and disseminated pyrites are typically formed by low-temperature hydrothermal fluid alteration of pyrite in the epigenetic stage [11,102,103] (Figure 5), suggesting that the coal seams had also been affected by hydrothermal fluids, whereas calcite veins discovered in C1, C2, and C4 may be the products of low-temperature hydrothermal fluid migration processes (Figure 5). Zheng et al. (2017) identified NH₄-illite in the No. 15 coal seam of the Yangquan Mine and believed that the mineral was produced by the transformation of kaolinite (>150 °C). The prevalence of NH₄-illite in the XRD patterns of the No. 15 coal (Figure 4) corroborates the presence of high-temperature environmental influences on the coal seam [6,10,104,105].

5.3. Occurrence Modes of Li in Coal

Although the content of lithium in coal in this paper is low (average 66.59 μg/g), and does not reach the minimum mining grade (80 μg/g) and industrial grade (120 μg/g) of Chinese coals determined by Sun et al. [4], the study of the occurrence modes of lithium in coal has important theoretical significance for exploring the enrichment and metallogenic mechanism of Li in coal.

Many studies have demonstrated that lithium is an active element. During surface weathering, elements can easily migrate from the parent rock into clay minerals and be adsorbed, and lithium in coal occurs typically in clay minerals. For example, Li⁺ in hectorite and cookeite can enter into Si-O tetrahedron or Al-O octahedron by replacement, whereas kaolinite cannot only replace Li⁺ but also adsorb part of Li via ion adsorption [12,17,18,19,106].

Lithium in coal is generally associated with aluminosilicate minerals (e.g., clay minerals or mica) as well as with organic matter or acid-insoluble substances [2,16,107,108,109]. To study the occurrence modes of Li in Gaoping No. 15 coal, we used SPSS 27.0 software to calculate the pearson correlation coefficient between the content of lithium and ash, major element, maceral, clay mineral content, and SiO₂/Al₂O₃ molar ratio. Correlation analysis between Li content and ash yield and clay minerals shows that Li is well correlated with them (r_Li–Ad = 0.58, r_{Li–Clay minerals} = 0.66), indicating that Li exhibits a good inorganic affinity and mainly occurs in clay minerals. This result (Figure 15) shows that Li is highly correlated with Al₂O₃, TiO₂, and SiO₂ (r_Li–SiO2 = 0.72, r_Li–Al2O3 = 0.78, and r_Li–TiO2 = 0.56), suggesting that Li has an aluminosilicate affinity, especially in Ti-bearing minerals, because Ti may be leached out of detrital components under low-pH conditions [9,64] and resettle in minerals, resulting in a positive correlation between Ti and Li content. The occurrence of rutile and anatase was found in the XRD and SEM analyses (Figure 4C,D, and Figure 6D) and the positive correlation between TiO₂ and Li content(Figure 15D), indicating that Li may occur in titanium-bearing minerals such as rutile and anatase. The simultaneous occurrence of rutile and anatase suggests that the geochemical composition is significantly influenced by the depositional environment. The content of Li in coal has a good correlation with K₂O and P₂O₅, indicating that Li also occurs in illite and phosphate minerals. When Li shows a strong negative correlation with the SiO₂/Al₂O₃ molar ratio, it may be associated with one or more Al-bearing mineral phases, such as kaolinite and chlorite group minerals [110,111,112]. The absorption peaks of 15P and 15F samples at 3692–3620 cm⁻¹ and 1095–915 cm⁻¹, respectively were obvious (Figure 7), indicating that there was more kaolinite in the samples. Table 5 shows that the Li content of 15P and 15F samples is higher than that of other samples, indicating that Li occurs in kaolinite. The moderate negative correlation between Li and the SiO₂/Al₂O₃ molar ratio (r = −0.55,

Table 8. Correlations between Li and maceral, clay minerals, and SiO₂/Al₂O₃ molar ratio.

	Vitrinite	Inertinite	Liptinite	Clay Minerals	SiO2/Al2O3 Molar Ratio
Li	−0.17	−0.26	0.50	0.66	−0.55

) confirms that Li occurs in different Al-bearing mineral phases. Some studies have shown that the main Li-bearing mineral in the coal of the Qinshui Coalfield is cookeite [9,12,113]. The XRD patterns for the C7 sample also confirmed the discovery of cookeite (Figure 4E), suggesting that cookeite is a Li-carrier mineral in the No. 15 coal.

Although Li exhibits a high inorganic affinity, part of Li can also enter the structure of organic matter by analog homology while having a low organic affinity [112]. The results of correlation analysis between Li and organic macerals in the coal samples showed that Li is negatively correlated with both the vitrinite and inertinite while positively correlated with the liptinite (r = 0.50, Table 8), indicating that Li in the organic matter mainly occurs in the liptinite.

5.4. Favorable Enrichment Environment of Li in Coal

The Li content in coal is positively correlated with SiO₂, Al₂O₃, and K₂O, indicating that Li mainly occurs in aluminosilicate clay minerals (such as kaolinite, illite, etc.). The higher Li content in the parting, roof, and floor samples and the positive correlation between Li and clay minerals indicate that the parting, roof, and floor dominated by clay minerals may be more favorable to the occurrence and enrichment of lithium (Table 5). The positive correlation between Li content and ash yield in coal indicates that Li is mainly enriched in the syndepositional stage, whereas the significant difference in the Li content in coal seams indicates that the changes in the deposition environment affect Li enrichment.

The concentration of Li shows an entirely different distribution in coal samples and non-coal samples (roof, parting, and floor). The parting exhibits the highest Li content (566 μg/g), followed by Li content on the floor (369 μg/g). The content of Li in coal samples is on the whole decreasing trend from bottom to top, and the content of Li in the roof is relatively low (Table 5). The distribution of Li content in Jincheng samples was similar to that in Gaoping samples (Figure 16). Based on the above analysis, it is shown that Li originated from the underlying strata of coal measures, and the parting is a good lithium enrichment site. In addition, coal acted as a sealing layer for Li, which hindered the upward migration of Li [114].

The correlation between Li and deposition environmental parameters shows that Li is negatively correlated with Sr/Ba and C-values (Figure 17), indicating that Li enrichment is related to terrigenous detritus, i.e., the terrestrial deposition environment is conducive to Li enrichment. The positive correlation of Li with V/(V+Ni), Cu/Zn, and V/I (Figure 17) suggests that a reduced sedimentary medium is favorable for Li enrichment. There is a significant negative correlation between Li and Sr/Cu (Figure 17), indicating that a stable and arid climate is conducive to Li enrichment. The Li and GWI show a positive correlation (Figure 17), implying that Li in coal is influenced by water flow transport and deposition, indicating its terrigenous origin. The terrigenous detrital quartz, anatase, and layered clay minerals (Figure 4, Figure 6, and Figure 7) provide strong support for terrigenous clastic input in the study area.

6. Conclusions

(1) The No. 15 coal is semi-anthracite coal with low moisture, low ash, low volatility, and high sulfur. Minerals in the coal mainly include kaolinite, illite, NH₄-illite, cookeite, quartz, calcite, pyrite, anhydrite, anatase, rutile, and phosphate.

(2) The Li content in the No. 15 coal is 6.14–120.00 μg/g, with an average of 66.59 μg/g. The coal roof, floor, and parting have higher Li contents of 54.00–566.00 μg/g. The depositional system is coastal facies, and the deposition palaeoenvironment is primarily open swamp facies with wet–shallow water covered, which is dominated by aquatic or herbaceous plants, and significantly influenced by seawater.

(3) Lithium in coal occurs mainly in aluminosilicate minerals, such as kaolinite, cookeite, illite, and is closely related to titanium-bearing minerals. Li also may occur in liptinite with organic macerals. Most of the Li in coal is derived from minerals in the syndepositional stage. The concentration of Li is affected by terrigenous clastic input and marine transgression.

(4) The roof, parting, and floor dominated by clay minerals may be more favorable for Li occurrence and enrichment, and the reducing environment with more terrestrial detritus, arid climate, and strong hydrodynamic effect are conducive to Li enrichment in coal.