Enrichment mechanisms of rare earth elements in Early Permian coal from the Kaiping coalfield and indicative significance for coal-forming environments

Abstract

The Early Permian witnessed significant shifts in both the paleogeographic location and paleoclimatic conditions of the North China Craton, leading to the formation of extensive coal deposits. This study investigates the No. 7 and No. 8 coal seams of the Lower Permian Damiaozhuang Formation in the Kaiping Coalfield, located on the northeastern margin of the North China Craton. Through integrated analyses using Gas Chromatography-Mass Spectrometry, X-ray Diffraction, Inductively Coupled Plasma Mass Spectrometry, and Electron Probe Microanalysis, this study elucidates the enrichment mechanisms of rare earth elements (REEs) and reconstructs the paleoenvironmental conditions of coal formation. The results indicate that the No. 7 and No. 8 coal seams exhibit significant concentration in REEs, with average concentrations of 319.38 ppm and 690.27 ppm, respectively. The ratios of (La/Lu)_N, (La/Sm)_N, and (Gd/Lu)_N indicate a light RRE (LREE)-enriched pattern. Monazite, identified as discrete grains disseminated within clay minerals, serves as the host mineral for REEs in coal deposits. The enrichment of REEs in coal is attributed to the combined influence of synsedimentary processes, REE-bearing minerals, organic matter, and clay minerals. Geochemical proxies, including Sr/Ba, Cu/Zn, Ni/Co, and δEu values, together with the ternary diagram of fluorene compounds and cross-plots of alkyl dibenzothiophenes/alkyl dibenzofurans versus Pr/Ph ratios, indicate an oxidizing freshwater depositional environment. Furthermore, Sr/Cu ratios exceeding 5 suggest a warm to hot paleoclimate during the coal formation. The bimodal distribution pattern observed in the mass chromatograms of n-alkanes, combined with parameters such as Pr/C₁₇, Ph/C₁₈, Carbon Preference Index, and Odd-to-Even Predominance, indicates that the organic matter was derived predominantly from higher plants, with a minor contribution from aquatic organisms. The sedimentary provenance is primarily identified as felsic rocks. This study elucidates the enrichment mechanisms of REEs in coal and reconstructs the associated coal-forming environment, providing a theoretical basis for the exploration and identification of REE-enriched coals.

Keywords

Kaiping coalfield Early Permian rare earth elements coal-forming environment monazite

Introduction

Rare earth elements (REEs), due to their unique physicochemical properties, have become indispensable strategic critical minerals for modern technological industries and national security (Banerjee et al., 2024; Chakladar et al., 2024; Kumar et al., 2022; Kumari et al., 2023). They play a core role in fields such as advanced materials, defense industries, and aerospace, directly influencing the competitiveness of high-end manufacturing and the level of national defense modernization. Recognizing the criticality of REEs, China's “National Mineral Resources Planning (2021–2025)” has classified REEs alongside petroleum, natural gas, gallium, copper, and lithium as strategic minerals. Coal seams, characterized by their function as geochemical barriers—particularly reduction and adsorption barriers—can facilitate the enrichment of REEs under favorable geological conditions. Consequently, certain coal deposits exhibit REE concentrations that meet or exceed the economic cutoff grades for independent or associated mineralization, presenting the potential for the development of large- to super-large-scale rare earth deposits (Li et al., 2022; Rajak et al., 2024). REEs are enriched in coals from several localities within the North China Craton, including the Jungar, Ningwu, and Hanxing coalfields (Ding et al., 2009; Zhao, 2015). In some mining areas, the concentration of rare earth element oxides in fly ash exceeds 1000 ppm. Fly ash has potential for industrial exploitation when the total rare earth element (∑La-Lu) oxide (REO) content in coal ash exceeds 1000 ppm (Dai et al., 2024). To date, extensive research has been conducted on REEs in coals from the North China Craton. For instance, trace elements in Early Permian coals from the southern margin of the North China Basin were investigated, and it was proposed that the enrichment of trace elements in coal is influenced by adsorption onto clay minerals (Yuan, 2023). Trace elements in coals from the Huaibei Coalfield were analyzed, and it was suggested that REE enrichment is jointly controlled by marine transgression and volcanic activity (Li and Li, 2023). However, systematic studies on the sources, occurrence modes, enrichment mechanisms, and distribution characteristics of REEs in coals from the northeastern margin of the North China Basin remain to be conducted. During the Permian, the North China Craton gradually drifted northward into the arid climatic belt, a period of extensive coal accumulation. The Carboniferous–Permian transgression in the North China Craton occurred predominantly from the southeastern and northeastern margins (Shen et al., 2022).

The deposition process of coal is influenced by multiple factors, such as paleoclimatic conditions (Zhao et al., 2023), redox conditions (Singh et al., 2020), tectonic setting (Hatem et al., 2023) and organic matter sources (Liu et al., 2024). Biomarker and elemental geochemical parameters and proxies are frequently employed to infer palaeoenvironmental conditions (Hakimi et al., 2026a; Kumar et al., 2024b; Xu et al., 2025). Based on the analysis of Pr/Ph, Pr/C₁₇, Ph/C₁₈, isoprenoid, hopanoid, and steroid biomarker parameters in coals, it can be inferred that the type of organic matter which was deposited in various settings under oxic to suboxic conditions (Hakimi et al., 2026b; Sidik et al., 2024). Analysis of elemental geochemical parameters (e.g. Sr/Ba, Sr/Cu, δY, and Al₂O₃/TiO₂ ratios) could indicate that redox and paleoclimatic conditions, and tectonic setting (Kumar et al., 2020, 2022; Liu et al., 2023).

Whether the Early Permian coal accumulation in the Kaiping Coalfield was influenced by this transgression remains to be further investigated. This study investigates the No. 7 and No. 8 coal seams of the Lvjiatuo Coal Mine in the Kaiping Coalfield. Through trace element analysis, mineralogical characterization, and organic geochemical analyses of coal seams and their roof/floor samples, this research elucidates the enrichment mechanisms of REEs in coal and discusses the associated coal-forming environment.

Geological setting

The Kaiping Coalfield is located in the northeastern part of the North China Craton (Zhang et al., 2017). The Lvjiatuo Mine, which is the study area, is situated in the eastern part of the Kaiping Coalfield (Figure 1). Since the Middle Carboniferous, the study area has developed marine and marine–terrigenous transitional facies deposits; the Early Permian is dominated by continental facies deposits, while the development of fluvial facies in the Late Permian. During the Triassic, crustal activity has been significant, with intense tectonic deformation. The coal-bearing strata in this region include the Upper Carboniferous Kaiping, Tangshan, and Zhaogezhuang Formations, as well as the Lower Permian Damiaozhuang Formation. The Zhaogezhuang Formation consists of marine–terrigenous transitional coal-bearing deposits, with lithologies dominated by gray siltstone, mudstone, and medium-to-coarse grained sandstone. It contains 3–4 coal seams, of which the No. 12 coal seam is the main mineable coal. The Damiaozhuang Formation is a continental coal-bearing deposit, with lithologies mainly of gray sandstone, grayish-green sedimentary tuff, lithic sandstone, etc. interbedded with mudstone and carbonaceous mudstone. It contains 3–9 coal seams, of which the No. 7 and No. 8 coal seams are the main mineable seams in this formation (Figure 1).

Figure 1.

Location and structural simplified map of Kaiping coalfield mining area.

Samples and methods

Samples

The subjects of this study are the No. 7 and No. 8 coal seams of the Lower Permian Damiaozhuang Formation. Samples for this study were collected from the Lvjiatuo Mine, following the Method of Sampling Coal Seams (GB/T 482-2008). A total of 31 samples were obtained, including six roof and floor samples and 25 coal samples strictly (Figure 1).

Methods

Sample preparation was strictly conducted following the Method for Preparation of Coal Samples (GB/T 474-2008). All analyses were performed at the Key Laboratory of Resource Exploration Research in Hebei Province. The mineralogical composition of the low-temperature ash residues from the coal samples was determined using a ZJ207 Bruker D8 Advance X-ray diffractometer operating at a 2θ range of 5° to 70°. Qualitative and quantitative analyses of the minerals were conducted using MDI Jade 5.0 and Xpert HighScore Plus software. Based on these analyses, the occurrence modes of minerals in coal samples were further investigated using a JXA-8230 electron probe microanalyzer. For trace element analysis, samples were first digested, and then analyzed using a high-resolution inductively coupled plasma mass spectrometer (Element I). For organic geochemical analysis, coal samples ground to 200 mesh were continuously Soxhlet-extracted for 72 h. The extracts were concentrated by rotary evaporation, dried to constant weight, and then separated by column chromatography. The separated aromatic and saturated hydrocarbon fractions, spiked with squalane as an internal standard and dissolved in dichloromethane, were analyzed using an Agilent 7290GC-MS5973 gas chromatography-mass spectrometry system.

Results

Coal quality characteristics

According to the proximate analysis of coal, the moisture content ranges from 0.22% to 2.06%, with an average of 0.69%, indicating ultralow moisture coal, according to the Classification of Total Moisture in Coal (MT/T 850-2000). The ash yield (dry basis) varies significantly, with an average of 18.92% for the No. 7 coal seam and 25.32% for the No. 8 coal seam, both classified as medium-ash coal. The volatile matter (dry ash-free basis) ranges from 9.85% to 23.41%, with an average of 17.50% for the No. 7 coal and 16.1% for the No. 8 coal, both classified as medium-volatile coal. The total sulfur content of the coal samples (dry basis) ranges from 0.05% to 0.49%, indicating low-sulfur coal, consistent with previous studies (Xiao, 1990).

Major element geochemical characteristics

The main oxide elements in the coal are primarily SiO₂, Al₂O₃, and Fe₂O₃ (Table 1). The SiO₂ content ranges from 38.15% to 51.63%, with an average of 43.84%; Al₂O₃ ranges from 18.43% to 37.75%, with an average of 31.50%; Fe₂O₃ ranges from 2.00% to 19.93%, with an average of 6.25%.

Table 1.

Major element composition of Lvjiatuo coal samples.

	Major element oxide content (%)
	Na₂O	MgO	Al₂O₃	SiO₂	P₂O₅	SO₃	K₂O	CaO	TiO₂	MnO	Fe₂O₃
Maximum	0.64	2.12	37.75	51.63	5.33	4.68	3.49	8.94	3.67	3.88	19.93
Minimum	0.25	0.50	18.43	38.15	0.15	0.99	0.16	1.76	1.38	0.03	2.00
Average	0.36	1.31	31.50	43.84	0.85	3.11	0.85	6.12	2.69	0.58	6.25

Trace element geochemical characteristics

Rare earth element content and enrichment characteristics

The rare earth element content in the No. 7 coal of the Lvjiatuo Mine ranges from 11.08 to 819.24 ppm, with an average of 319.38 ppm. Among the 31 samples, seven samples have REE content exceeding 300 ppm, and the REE content in the roof samples is significantly higher than that in the floor samples. For the No. 8 coal in Lvjiatuo, the REE content ranges from 133.15 to 1365.71 ppm, with an average of 690.27 ppm, showing significant enrichment in the middle part of the coal seam (Figure 1). The REE content in the roof and floor samples is generally consistent.

The enrichment coefficient (CC) of REEs in the No. 7 and No. 8 coals of the Lvjiatuo Mine was calculated (Seredin and Dai, 2012). For the No. 7 coal, the average CC of Tm is 0.28, indicating depletion; the average CC of Sr and Eu is 3.4 and 2.15, respectively, indicating slight enrichment; the average CC of Ce is 12, indicating significant enrichment. For the No. 8 coal, the average CC of Ho is 0.34, indicating depletion; the average CC of Gd and Eu is 2.1 and 3.05, respectively, indicating slight enrichment; the average CC of Ce is 27.90, indicating significant enrichment. The Eu and Ce elements in the No. 7 and No. 8 coals of the Lvjiatuo Mine show enrichment, while Tm and Ho show depletion (Figure 2).

Figure 2.

Enrichment coefficient of REEs in coal No. 7 and No. 8 of Lvjiatuo mine.

Geochemical parameters of trace elements

Using UCC normalization for REEs (Lin et al., 2018; Prachiti et al., 2011), we obtained standardized values for Sm_N, Eu_N, Gd_N, La_N, Ce_N, Pr_N, Lu_N and Tb_N. The parameter ratios La_N/Lu_N, La_N/Sm_N and Gd_N/Lu_N reflect the enrichment type of REEs. δCe, δEu, δGd and δY are the anomaly values of Ce, Eu, Gd and Y, respectively, which are significant in geochemical research. The calculation formulas are: as follows

δ Ce = {Ce}_{N} / C e_{N}^{*} = {Ce}_{N} / (0.67 \times {La}_{N} + 0.33 \times \Pr_{N})

(1)

δ Eu = {Eu}_{N} / E u_{N}^{*} = {Eu}_{N} / [({Sm}_{N} \times 0 .67) + ({Tb}_{N} \times 0 .33)]

(2)

δ Gd = {Gd}_{N} / G d_{N}^{*} = {Gd}_{N} / [({Sm}_{N} \times 0 .33) + ({Tb}_{N} \times 0 .67)]

(3)

δ Y = Y_{N} / H o_{N} (Bau and Dulski, 199 6)

(4)

The trace element geochemical parameters of the No. 7 and No. 8 coal samples from Lvjiatuo were calculated and are listed in Table 2.

Table 2.

Geochemical parameters of trace elements in coal samples from the Lvjiatuo mine.

Coal seam	Sample No.	Sr/Ba	Cu/Zn	Sr/Cu	Ni/Co	δCe	δEu	δGd	δY	La_N/Lu_N	La_N/Sm_N	Gd_N/Lu_N	C _outl
No. 7 Coal	KL7-R1	0.04	0.43	0.22	1.74	159.51	1.40	3.97	0.51	0.04	0.30	1.52	0.5
	KL7-R2	0.09	0.27	4.63	1.13	5.01	1.49	1.70	0.81	1.09	1.05	2.11	1.5
	KL7-1	0.25	0.43	49.28	1.25	10.28	3.07	2.81	0.86	1.80	2.11	3.62	0.9
	KL7-2	0.11	0.69	35.79	0.90	16.84	14.32	4.91	0.82	0.69	2.44	2.11	1.3
	KL7-3	0.19	0.63	33.57	1.58	11.57	2.23	2.37	0.87	1.26	1.05	3.33	1.0
	KL7-4	0.14	0.67	23.85	1.26	18.99	6.40	2.67	1.04	0.50	1.41	2.11	1.4
	KL7-5	0.52	0.65	37.93	1.11	6.05	1.88	2.15	1.01	1.96	1.97	2.86	1.2
	KL7-6	0.16	0.38	5.43	7.00	4.61	1.39	1.23	0.85	0.28	0.21	2.19	2.9
	KL7-7	0.21	0.47	101.30	2.00	9.82	6.45	2.83	0.96	1.16	1.36	2.69	1.5
	KL7-8	0.05	0.92	5.58	3.00	71.14	20.52	3.99	1.02	0.11	1.50	1.43	1.4
	KL7-9	0.13	0.49	36.26	1.16	18.21	5.12	3.81	0.95	1.67	2.62	4.63	0.7
	KL7-10	0.07	0.42	23.09	0.84	23.45	6.05	2.43	0.81	0.66	1.55	2.78	1.1
	KL7-11	0.21	0.24	86.40	1.03	11.09	3.24	2.65	0.88	3.53	1.60	7.24	0.9
	KL7-12	0.03	1.22	1.43	1.54	4.95	5.41	2.79	0.91	0.95	2.67	1.26	1.6
	KL7-13	0.03	0.70	6.40	5.50	35.33	53.43	25.12	0.40	0.18	1.28	1.18	1.5
	KL7-14	0.29	0.29	101.02	0.82	13.68	2.67	2.76	0.74	2.90	1.63	7.16	0.8
	KL7-15	0.32	1.46	32.63	0.13	4.70	1.70	1.78	0.86	2.42	1.28	3.49	1.5
	KL7-16	0.06	0.64	3.81	2.50	35.61	6.14	3.93	0.76	0.17	0.60	1.68	1.0
	KL7-D1	0.27	0.76	0.18	1.70	4.63	3.82	5.38	0.62	0.04	0.30	0.25	1.4
	KL7-D2	0.03	0.55	0.04	1.68	5.79	1.53	1.47	0.69	0.13	0.47	0.63	1.6
	Average	0.16	0.62	29.44	1.89	23.56	7.41	4.04	0.82	1.08	1.37	2.71	1.3
No. 8 Coal	KL8-R1	1.06	0.27	2.74	1.77	1.51	1.38	1.28	0.95	0.62	0.69	1.09	2.5
	KL8-1	0.09	0.12	54.08	7.11	30.67	4.42	4.91	3.96	0.49	0.36	2.19	1.0
	KL8-2	0.05	0.07	147.32	3.57	24.53	4.90	4.14	2.73	1.30	0.80	3.62	0.7
	KL8-3	0.07	0.01	1037.80	5.46	63.24	5.80	5.61	2.45	0.53	0.68	3.33	0.5
	KL8-4	0.06	0.21	80.20	4.80	19.75	3.69	3.84	2.33	3.67	1.23	8.93	0.6
	KL8-5	0.06	0.07	236.37	3.81	40.32	5.10	5.26	3.96	1.54	0.83	6.74	0.5
	KL8-6	0.06	0.01	985.50	2.71	41.46	5.93	6.10	4.25	1.11	0.74	5.05	0.6
	KL8-7	0.62	0.12	639.89	3.00	97.27	3.14	12.24	1.24	0.54	0.45	4.88	0.2
	KL8-8	0.13	0.13	100.85	4.23	16.55	3.40	3.73	3.82	1.63	0.82	3.71	0.8
	KL8-9	0.08	0.14	85.83	5.09	43.76	4.50	4.75	2.80	0.77	0.49	4.04	0.6
	KL8-D2	0.35	0.52	0.30	1.45	7.91	1.31	1.74	0.90	0.42	0.91	1.14	1.3
	Average	0.22	0.15	306.44	3.91	35.18	3.96	4.87	2.67	1.15	0.73	4.07	0.9

Distribution patterns of REEs

After UCC normalization of REE content in coal (Lin et al., 2018), the distribution curves of REEs in the two coal seams are similar. As shown in Figure 3(a), the REE distribution curve of the No. 7 coal exhibits a broad “L” shape with high levels on the left and low levels on the right, consistent with the typical characteristics of Paleozoic medium- to high-rank coals in the North China Craton, indicating higher enrichment of light REEs compared to heavy REEs.

Figure 3.

Distribution patterns of REEs in coal No. 7 and No. 8 of Lvjiatuo mine.

As shown in Figure 3(b), the REE distribution curve of the No. 8 coal in the Lvjiatuo Mine generally shows a left-high, right-low L-shaped bimodal pattern. The two peaks are located in the La-Pr segment and the Sm-Tb segment, with a gentle distribution in the Tb-Y segment, showing negative anomalies. Light REEs such as La and Ce exhibit significant positive anomalies, with light REEs being more enriched than heavy REEs.

From Table 2, the (La/Lu)_N ratio of the No. 7 coal seam ranges from 0.04 to 3.53, with an average of 1.08; (La/Sm)_N ranges from 0.21 to 2.67, with an average of 1.37; (Gd/Lu)_N ranges from 0.25 to 7.24, with an average of 2.71, indicating a dominance of light REE enrichment. For the No. 8 coal seam, the (La/Lu)_N ratio ranges from 0.42 to 3.67, with an average of 1.15; (La/Sm)_N ranges from 0.36 to 1.23, with an average of 0.73; (Gd/Lu)_N ranges from 1.09 to 8.93, with an average of 4.06, also indicating a dominance of light REE enrichment, consistent with previous studies (Li et al., 2024; Lin et al., 2018).

Mineralogical characteristics

Through X-ray diffraction and electron probe analysis, the mineral compositions of the No. 7 and No. 8 coals are generally consistent. They mainly consist of kaolinite, dolomite, siderite, and monazite, with minor amounts of pyrite and rutile (Figure 4). In the No. 7 coal seam, the kaolinite content ranges from 61.9% to 95.7%, siderite content from 1.9% to 13.4%, dolomite content from 1.4% to 11%, and pyrite content from 0.2% to 1.1%. In the No. 8 coal seam, the kaolinite content ranges from 83% to 98%, siderite content from 1.6% to 7.5%, dolomite content from 1.4% to 4.9%, and pyrite content from 0.6% to 0.9%.

Figure 4.

Diffraction analysis spectrum of coal sample from Lvjiatuo coal No. 7 and No. 8.

In sample KL8-3, siderite was found to replace dolomite in a disseminated manner, with dolomite and siderite occurring together (Figure 5(a)). Small amounts of pyrite are present in each coal sample, occurring mostly as granular grains encapsulated by kaolinite (Figure 5(b)). The phosphate mineral monazite was found in the coal, occurring as irregularly granular distributed within kaolinite and commonly surrounded by pyrite (Figure 5(c)–5(e)). The spectra of monazite and pyrite are shown in Figure 5(e)-1, and 5(e)-2, respectively.

Figure 5.

Electron probe microanalyzer images of the No. 7 and No. 8 coal seams. (a) Disseminated siderite replacing dolomite; (b) Granular pyrite encapsulated in kaolinite; (a-1) Siderite energy spectrum; (a-2) Dolomite energy spectrum; (c) Granular monazite dispersed in kaolinite; (d) Star-shaped monazite dispersed in kaolinite; (e) Monazite and pyrite codistributed in kaolinite; (e-1) Monazite energy spectrum; (e-2) Pyrite energy spectrum; ⊕: Energy spectrum spot location.

Organic geochemical characteristics

The coal contains saturated hydrocarbon ranging from 2.90% to 44.28%, and aromatic hydrocarbons ranging from 11.10% to 49.25%. Resulting in a saturated-to-aromatic ratio of 0.09 to 1.04. The aromatic hydrocarbon content is generally higher than the saturated hydrocarbon content.

Geochemical characteristics of saturated hydrocarbons

As shown in Table 3, the carbon number distribution of n-alkanes in the coal ranges from C₁₄ to C₃₃, showing either a bimodal front-peak type or a bimodal rear-peak type (Figure 6). The front-peak type is dominated by C₁₆ as the main peak, while the rear-peak type is dominated by C₂₅ and C₂₆ as the main peaks. The ΣC₂₁₋/ΣC₂₂₊ ratio ranges from 0.13 to 1.38, with an average of 0.63, indicating an abundance of heavy hydrocarbon components. The Carbon Preference Index (CPI) and Odd-to-Even Predominance (OEP) can indicate the ratio of odd- to even-carbon molecules and reflect the thermal maturity of the sample (Wang et al., 2024; Zhao et al., 2025). For the No. 7 coal seam, OEP ranges from 0.82 to 1.02 (average 0.97), and CPI ranges from 0.89 to 1.12 (average 1.04). For the No. 8 coal seam, OEP ranges from 0.59 to 1.06 (average 0.89), and CPI ranges from 0.77 to 1.11 (average 0.97). The CPI and OEP values for both seams are stable around 1, indicating relatively high coal maturity.

Figure 6.

Gas chromatogram of n-alkanes in coal samples No. 7 and No. 8 from Lvjiatuo (a) Chromatogram of front-peak type saturated hydrocarbons (b) Chromatogram of rear-peak type saturated hydrocarbons.

Table 3.

Parameters of saturated hydrocarbons of coal samples from Lvjiatuo mine.

Coal seam	Sample	Carbon number	n-alkane max.	Pr/Ph	Pr/C₁₇	Ph/C₁₈	CPI	OEP	∑C₂₁₋/∑C₂₂₊
No. 7 Coal	KL7-3	C₁₄∼C₃₃	C₁₆	1.99	0.66	0.24	0.89	0.83	0.71
	KL7-6	C₁₄∼C₃₄	C₂₆	1.59	0.56	0.25	1.06	1.02	0.27
	KL7-7	C₁₄∼C₃₃	C₁₆	1.57	0.44	0.17	1.01	0.97	1.28
	KL7-11	C₁₄∼C₃₃	C₂₅	2.66	0.55	0.15	1.10	1.00	0.45
	KL7-15	C₁₄∼C₃₃	C₂₅	2.31	0.45	0.16	1.12	1.02	0.24
	Average			2.03	0.53	0.19	1.04	0.97	0.59
No. 8 Coal	KL8-R1	C₁₄∼C₃₃	C₂₅	1.35	0.84	0.41	1.11	1.06	0.31
	KL8-1	C₁₄∼C₃₃	C₂₅	1.60	0.48	0.14	1.02	0.91	0.30
	KL8-3	C₁₄∼C₃₃	C₁₆	1.73	0.68	0.33	0.96	0.72	0.80
	KL8-4	C₁₄∼C₃₄	C₁₆	1.98	0.44	0.23	0.77	1.04	1.38
	KL8-5	C₁₄∼C₃₃	C₁₆	2.06	0.48	0.17	0.84	0.97	0.99
	KL8-6	C₁₄∼C₃₃	C₂₅	2.22	0.46	0.16	1.06	0.99	0.46
	KL8-8	C₁₄∼C₃₄	C₂₆	2.28	0.49	0.19	1.06	1.02	0.13
	KL8-D2	C₁₄∼C₃₃	C₁₆	1.56	0.68	0.28	1.07	0.59	1.21
	Average			1.92	0.53	0.21	0.97	0.89	0.65

Note: CPI: Carbon Preference Index; OEP: Odd-to-Even Predominance.

CPI = ((n − C₂₅ + n − C₂₇ + n − C₂₉ + n − C₃₁ + n − C₃₃)/(n − C₂₄ + n − C₂₆ + n − C₂₈ + n − C₃₀ + n − C₃₂) + (n − C₂₅ + n − C₂₇ + n − C₂₉ + n − C₃₁ + n − C₃₃)/(n − C₂₆ + n − C₂₈ + n − C₃₀ + n − C₃₂ + n − C₃₄))/2; OEP = ((C _i ₋₂ + 6Ci + C _i ₊ ₂)/(4C _i ₋₁ + 4C _i ₊ ₁)) $(- 1)^{i + 1}$ , where C_i is the main carbon peak; ΣC₂₁₋/ΣC₂₂₊ is the ratio of the sum of n-alkanes before C₂₁ to the sum after C₂₂.

Pristane (Pr) and phytane (Ph) are commonly used indicators of the depositional environment, thermal maturity, and organic matter source. In the No. 7 coal seam, the Pr/Ph ratio ranges from 1.57 to 2.66, with an average of 2.02. In the No. 8 coal seam, the Pr/Ph ratio ranges from 1.35 to 2.27, with an average of 1.84. The Pr/C₁₇ and Ph/C₁₈ ratios indicate the degree of biodegradation of organic matter; significant changes in these ratios suggest strong biodegradation (Meng et al., 2004). For the Lvjiatuo No. 7 and No. 8 coal seams, Pr/C₁₇ ranges from 0.44 to 0.84, and Ph/C₁₈ ranges from 0.13 to 0.40, with no significant changes observed, indicating that the samples did not undergo strong biodegradation during deposition.

Geochemical characteristics of aromatic hydrocarbons

The aromatic hydrocarbon compounds detected in the No. 7 and No. 8 coals of Lvjiatuo mainly include naphthalenes, biphenyls, fluorenes, dibenzofurans (DBF), dibenzothiophenes (DBTs), phenanthrenes, anthracenes, pyrenes, benzofluorenes, benzotriphenylenes, benzanthracenes, chrysenes, benzofluoranthenes, benzopyrenes, and benzyls, along with their derivatives. The aromatic hydrocarbons in the Lvjiatuo No. 7 and No. 8 coals are mainly benzene-polycyclic aromatic hydrocarbons (naphthalenes, phenanthrenes, biphenyls, etc.) and sulfur-polycyclic aromatic hydrocarbons (DBTs, naphthothiophenes, etc.), with oxygen-polycyclic aromatic hydrocarbons (DBFs) accounting for a smaller proportion. Among them, naphthalenes, phenanthrenes, trifluorenes, and biphenyls are relatively abundant.

As shown in Figure 7, the naphthalene series compounds detected in the Lvjiatuo No. 7 and No. 8 coals include methylnaphthalenes (MNs), dimethylnaphthalenes (DMNs), trimethylnaphthalenes (TMNs), tetramethylnaphthalenes (TeMNs), phenylnaphthalenes (PhNs), and ethylnaphthalene (ENs). Among these, phenylnaphthalene has the highest relative content with PhNs > TMNs > DMNs > MNs > TeMNs > ENs (Figure 8). The source of phenylnaphthalene is not yet clear; Killops et al. inferred that phenylnaphthalene is a product of gymnosperm combustion (Killops and Massoud, 1992).

Figure 7.

Partial aromatic hydrocarbon chromatogram of Lvjiatuo sample.

Figure 8.

Composition distribution diagram of naphthalene components.

The phenanthrene compounds in coal include phenanthrenes (Ps), methylphenanthrenes (MPs), dimethylphenanthrenes (DMPs), and trimethylphenanthrenes (TMPs), with relative abundance of MPs > Ps > DMPs > TMPs. TMPs tend to demethylate with increasing vitrinite reflectance (R_o), converting to MPs and DMPs. The coal samples have the lowest TMP content among the phenanthrene series, indicating relatively high maturity. The methylphenanthrene content follows the order 2-MP > 3-MP > 1-MP > 9-MP (Figure 9), with 9-MP having the lowest content, ranging from 0.72 to 2.68.

Figure 9.

Distribution characteristics of methyl phenanthrene isomers.

In the No. 7 and No. 8 coals, the relative contents of fluorenes, DBTs, and DBFs are in the order of fluorenes > DBTs > DBFs. The relative proportions of fluorenes, DBTs, and DBFs in the total trifluorene compounds range from 45.08% to 71.79%, 26.03% to 46.60%, and 1.36% to 14.56%, respectively.

Discussion

Coal-forming environment and material source

Coal-forming environment

(1)

Water salinity

The Sr/Ba ratio and Ce anomaly can be used to assess water salinity. Generally, Sr/Ba < 0.6 indicates freshwater deposition, Sr/Ba > 1 indicates marine deposition, and 0.6 < Sr/Ba < 1.0 indicates brackish water deposition (Yu et al., 2024). The Sr/Ba method is applicable when Sr and Ba are derived from external input rather than endogenous deposition (Zhang et al., 2022). In this study, most samples from the No. 7 and No. 8 coals have Sr/Ba values less than 0.6, with an average of 0.18, indicating a continental depositional environment. Typically, in shallow marine and enclosed sea areas, Ce concentrations are near normal, while in open seas and outer seas, Ce shows significant negative anomalies. Positive Ce anomalies may be related to weak reducing conditions in coal-forming water (Dai et al., 2016). The δCe values of the studied samples show positive anomalies, with an average of 27.67. However, various geochemical parameters, such as Cu/Zn and Ni/Co, indicate an oxidizing to weakly reducing depositional environment, and the Ce anomalies vary significantly among different samples. Therefore, it is inferred that the positive Ce anomaly is not only caused by the redox conditions of the depositional environment but may also be influenced by terrigenous input. There are various factors that cause Y anomalies in coal, including pregeochemical processes within the source rocks, depositional environment, and hydrothermal fluids (Wang et al., 2021). Coal with felsic source rocks usually shows weak or no Y anomalies, while positive Y anomalies in coal may be related to seawater activity or hydrothermal fluids (Dai et al., 2016). The δY of the No. 7 coal ranges from 0.40 to 1.04, with an average of 0.82, indicating that the sediment source for the No. 7 coal is felsic rocks. The δY of the No. 8 coal ranges from 0.90 to 4.25, with an average of 2.67, showing a strong positive Y anomaly. Combined with other geochemical parameters, it is concluded that the No. 8 coal seam was not influenced by marine activity but may have been affected by hydrothermal fluids.

The cross-plot of Pr/C₁₇ and Ph/C₁₈ can reflect the sedimentary environment and kerogen type (Yang et al., 2025). As shown in Figure 10, the kerogen of the Lower Permian Damiaozhuang Formation coal measures in the Kaiping Coalfield is mainly Type III (humic type), with a predominantly continental sedimentary environment. Among them, KL8-R1 falls into the mixed group. The content and distribution of aromatic hydrocarbons such as fluorene (F), DBF, and DBT are significant indicators for distinguishing sedimentary environments. Swamp facies source rocks are dominated by DBF, marine and saline lake facies source rocks by DBT, and freshwater lake and brackish lake facies source rocks by fluorene (Zhang and Philp, 2010). As shown in Figure 11, the trifluorene distribution in the samples is relatively concentrated, with fluorene being dominant, indicating that the sedimentary environments of the two coal seams are similar, belonging to freshwater or brackish lake facies. The alkyl DBTs/alkyl DBFs ratio (ADBTs/ADBFs), combined with Pr/Ph values, can be used to classify sedimentary environments (Radke et al., 2000). As shown in Figure 12, the No. 7 and No. 8 coals are rich in alkyl DBTs, while methyl DBFs are less abundant. The ADBTs/ADBFs values are all above 1.0, ranging from 1.95 to 6.87, and the Pr/Ph values range from 1.35 to 2.66. The coal samples are located in Zone 1C and 1D in the figure, indicating that the No. 7 and No. 8 samples originate from mature mudstones and high-rank coals formed in freshwater lacustrine depositional environments, consistent with the trifluorene diagram. (2)

Redox conditions

Figure 10.

Cross-plot of Ph/C₁₈ versus Pr/C₁₇ of coal samples from the Lvjiatuo mine (Yang et al., 2025).

Figure 11.

Trifluorene diagram of coal samples from the Lvjiatuo mine (Zhang and Philp, 2010).

Figure 12.

Distribution of ADBTs/ADBFs and Pr/Ph ratio of coal samples from the Lvjiatuo mine (Radke et al., 2000).

During sedimentation, trace elements in sediments can be influenced by the redox conditions of the depositional environment due to their variable valences. Relevant geochemical parameters can indicate the redox characteristics of the sedimentary environment. The Cu/Zn ratio can indicate the redox conditions of the depositional environment. Cu gradually transitions to Zn during sedimentation, and the intensity of this transition is influenced by the oxygen fugacity of the medium, leading to changes in the Cu/Zn value. Typically, Cu/Zn > 0.63 indicates an oxidizing environment, 0.50 < Cu/Zn < 0.63 indicates a weakly oxidizing environment, 0.38 < Cu/Zn < 0.50 indicates a semireducing to semioxidizing environment, 0.21 < Cu/Zn < 0.38 indicates a weakly reducing environment, and Cu/Zn < 0.21 indicates a reducing environment (Ma et al., 2024). For the No. 7 coal, Cu/Zn ranges from 0.24 to 1.46, with an average of 0.61, and most samples have Cu/Zn > 0.63. For the No. 8 coal, Cu/Zn ranges from 0.27 to 0.52, with an average of 0.15, indicating that the depositional environments of the No. 7 and No. 8 coals fluctuated between weakly reducing and oxidizing conditions, with oxidizing conditions being dominant. The Ni/Co ratio can also indicate the depositional environment: Ni/Co > 7 indicates a reducing environment, 5 < Ni/Co < 7 indicates a weakly oxidizing environment, and Ni/Co < 5 indicates an oxidizing environment (Xuying et al., 2023). For the No. 7 coal, Ni/Co ranges from 0.13 to 7, with an average of 1.89. For the No. 8 coal, Ni/Co ranges from 1.45 to 7.11, with an average of 3.91, indicating that the depositional environments of the No. 7 and No. 8 coals were oxidizing. Eu is an important indicator for determining the redox conditions of the depositional environment. In an oxidizing environment, Eu shows a negative anomaly, while in a reducing environment, it shows a positive anomaly (Lin et al., 2023). Strong positive Eu anomalies are related to hydrothermal fluids (Dai et al., 2016). The average δEu of the samples is 6.27, showing a strong positive anomaly, which is inconsistent with the oxidizing depositional environment and may be related to the influence of hydrothermal fluids during sedimentation. (3)

Paleoclimate

Due to differences in elemental geochemical behavior, elements can become enriched under different climatic conditions. This characteristic allows elements to be classified into dry-preferring and wet-preferring types. Sr is a dry-preferring element, while Cu is a wet-preferring element. Sr/Cu can be used as an indicator of climate change, with 1.3 < Sr/Cu < 5 indicating a warm and humid climate, and Sr/Cu > 5 indicating a hot climate (Ma et al., 2025). Most coal samples have Sr/Cu values greater than 5, with only the roof and floor samples having values below 5. The average Sr/Cu is 127, indicating that the climate during the coal deposition period was hot, while the climate during the deposition of the roof and floor strata was warm and humid.

Material source

The Al₂O₃/TiO₂ ratio can be used to determine the type of sediment source. When 3 < Al₂O₃/TiO₂ < 8, the sediment source is mafic rocks; when 8 < Al₂O₃/TiO₂ < 21, the source is intermediate rocks; and when Al₂O₃/TiO₂ > 21, the source is felsic rocks (Hayashi et al., 1997). The Al₂O₃/TiO₂ ratio of the coal samples in the study area ranges from 8.97 to 24.31, with an average of 12.97, indicating that the sediment source is felsic and intermediate rocks (Figure 13 ).

Figure 13.

The Al₂O₃/TiO₂ ratio of coal samples from Lvjiatuo mine.

Figure 14.

Correlation of parameters in coal from Lvjiatuo mine (a) correlation between rare earth content and ash (b) correlation between rare earth content and total sulfur content (c) correlation between rare earth content and saturated hydrocarbons (d) correlation between rare earth content and aromatic hydrocarbons.

The distribution patterns of saturated hydrocarbons in the Lvjiatuo coal exhibit bimodal front-peak and bimodal rear-peak types. The bimodal front-peak distribution indicates the input of lacustrine aquatic organisms, while the bimodal rear-peak distribution suggests that the organic matter was derived from terrestrial higher plants (Qin et al., 2018), indicating that the coal-forming materials during the Early Permian were derived from both terrestrial higher plants and lacustrine aquatic organisms. Compounds such as 1,2,5-TMN originate from the structural degradation of oleanane-type triterpenoids in gymnosperms, which are considered as markers of gymnosperms (Alexander et al., 1986). In the Lvjiatuo coal, the detected TMNs are predominantly dominated by the stable isomers 1,2,7 + 1,2,6 + 1,6,7-TMN, with relatively low content of 1,2,5-TMN. This is attributed to the fact that with increasing thermal maturity of coal, the α-methyl groups on naphthalene with higher reactivity may be substituted by more stable β-positions, and compounds such as 1,2,5-TMN are converted into more stable 1,2,7 + 1,2,6 + 1,6,7-TMN through structural reorganization (Zhou, 2008). This also indicates that the coal samples have relatively high maturity, and the organic matter is primarily derived from terrestrial higher plants. Among the phenanthrene series, retene was not detected in the No. 7 and No. 8 coals of the Lvjiatuo Mine. As an indicator of terrestrial organic matter, retene is mainly derived from coniferous plants. The absence of retene may suggest that the coal-forming environment of the Lvjiatuo coal was confined (Jing et al., 2021), with limited input from coniferous plants. The methylphenanthrene isomers in the coal samples are of great significance for indicating the sources of sedimentary organic matter. A higher abundance of 1,7-dimethylphenanthrene (1,7-DMP) and a lower abundance of 9-methylphenanthrene (9-MP) were observed in the coal. 1,7-DMP can serve as an indicator of terrigenous organic matter input, while 9-MP can be used as an indicator of marine organic matter or aquatic algae contribution (Radke et al., 1982). This indicates that the No. 7 and No. 8 coals are primarily dominated by terrigenous higher plant input, with a limited contribution from aquatic organisms.

Enrichment mechanisms of REEs

Influenced by coal-forming environments and diagenesis, the occurrence modes of REEs in coal are diverse. Previous studies have shown that REEs in the Pingshuo mining area mainly originate from terrigenous clastics and are enriched in mineral form (Qin et al., 2005). The trace elements in the Jungar coal mainly originate from the Yinshan Old Land and occur in inorganic, organic, and sulfide-bound forms (Liu et al., 2018). The Kaiping Coalfield is located on the northwestern margin of the North China Block. During the Carboniferous–Permian period, the sediment sources were mainly felsic rocks from the Yinshan Old Land in the north (Zhao and Meng, 1987). The correlation coefficient between REE content and ash yield at 0.28, with total sulfur content at 0.17, saturated hydrocarbon components at 0.29, and with aromatic hydrocarbons at 0.20 (Figure 14). The correlation between total REE content and ash yield, total sulfur content, and organic matter components is not significant, but a weak positive correlation still exists. The samples are dominated by light REE enrichment, which is easily adsorbed by organic matter in coal. Monazite and abundant kaolinite clay minerals were found in the coal (Figure 5(c)–(e)). Monazite occurs as star-shaped grains dispersed in kaolinite, with small particle sizes, typically less than 10 µm, and serves as one of the carriers for REE occurrence (Chelgani et al., 2015). This indicates that the occurrence forms of REEs in coal include organic and inorganic forms. Inorganic REEs are mainly hosted in monazite or adsorbed by clay minerals. The organic-bound REEs are more complex; REEs may be adsorbed in organic matter, occur as water-soluble species in organic pores, or be chemically bonded to organic molecules in coal (Yu et al., 2024; Zhang et al., 2022). The mode of organic-bound REEs requires further study.

During the Permian period, the North China region experienced a series of intense tectonic activities influenced by the Variscan tectonic movement, including extension–compression cycles. Surface uplift led to prolonged weathering and erosion of felsic rocks in the Yinshan Old Land, transporting clastic minerals containing REE components into peat swamps (Gao et al., 2024). Based on geochemical parameters such as Sr/Ba, the depositional environment of the coal samples is characterized by low-salinity freshwater. The REEs in the coal of the Kaiping Coalfield originated from clastic minerals in the source area during the syngenetic sedimentation stage. Stable REE-bearing minerals like monazite were preserved during coal formation. In the low-salinity freshwater depositional environment, free REEs were easily adsorbed by kaolinite clay minerals and organic matter in the coal, leading to enrichment (Dai et al., 2014).

The variation in volatile matter content in coal is closely related to organic matter maturity and type. The lower the volatile matter, the higher the metamorphic grade, the tighter the organic matter structure, and the greater the specific surface area and micropore volume, enhancing adsorption capacity. Based on the correlations between ash, volatile matter, δY, δEu, and total REE content, the No. 7 and No. 8 coals were vertically divided into three zones (Figure 15). In Zone 1, REE content shows an overall negative correlation with volatile matter and a weak correlation with ash content, suggesting that REE enrichment was mainly influenced by organic matter adsorption. In Zone 2, REE content shows a positive correlation with ash content. Changes in ash content are related to the degree of terrigenous clastic input, indicating that REE occurrence in this period was mainly influenced by terrigenous input and kaolinite adsorption. In Zone 3, REE content shows a negative correlation with ash content and a positive correlation with volatile matter. During this period, both δY and δEu values are greater than 1, indicating hydrothermal fluid activity. REE occurrence may have been influenced by hydrothermal fluids. The majority of samples from the No. 7 and No. 8 coal seams fall within Zone II (Figure 15), indicating that the concentrations of REEs are mainly related to terrigenous detrital input and adsorption by kaolinite. Meanwhile, the ratio of critical REEs to excessive REEs in a given material is defined as the outlook coefficient (C_outl), calculated as (Kumar et al., 2024a; Seredin and Dai, 2012):

C_{outl} = [(Nd + Eu + Tb + Dy + Er + Y) / \sum REY] / [(Ce + Ho + Tm + Yb + Lu) / \sum REY]

Figure 15.

Vertical distribution of ash, volatile matter, δY, δEu, and REEs in coal.

The C_outl values of No. 7 and No. 8 coals range from 0.82 to 2.59, suggesting that they could be a promising source of REEs (Kumar et al., 2024a; Seredin and Dai, 2012). The REEs are primarily hosted in inorganic phases, and they can be extracted synchronously by acid or alkali leaching processes. Monazite, an REE-bearing mineral with relatively high specific gravity, which could be extracted from coal ash by gravity separation.

Conclusions

The average REE contents in the No. 7 and No. 8 coals of the Lvjiatuo Mine are 319.38 ppm and 690.27 ppm, respectively, with a dominance of light rare earth enrichment. The REE distribution curves generally exhibit an L-shape with higher values on the left and lower values on the right. The saturated hydrocarbons in the coal samples show bimodal front-peak and rear-peak distributions, with a clear even-carbon predominance. The aromatic hydrocarbon components are mainly benzene-polycyclic aromatic hydrocarbons and sulfur-polycyclic aromatic hydrocarbons, with the naphthalene, biphenyl, trifluorene, and phenanthrene series being the main types.

The sedimentary source of REEs in the No. 7 and No. 8 coals of the Lvjiatuo Mine is felsic and intermediate rocks. The main mineral components in the coal are kaolinite, siderite, dolomite, pyrite, monazite, and rutile. The relative mineral contents of the No. 7 and No. 8 coal seams are generally consistent. Irregular granular monazite dispersed in kaolinite was found in the coal. The REEs originate from terrigenous clastic input, and the low-salinity freshwater environment in the coal-forming swamp facilitated the adsorption and enrichment of REEs by kaolinite clay minerals and organic matter. This study reveals the enrichment mechanism of REEs in the Early Permian coals of the Kaiping Coalfield, and the REEs are mainly hosted in monazite and kaolinite with inorganic phases. According to the C_outl values, suggesting that the REEs in the coals have great potential for extraction.

The coal-forming environment of the No. 7 and No. 8 coals in the Lvjiatuo Mine is mainly characterized by continental freshwater deposition, with oxidizing to weakly reducing conditions. The overall climate was hot, while the climate during the deposition of the roof and floor strata was warm and humid. The primary sources of the coal-forming materials were terrestrial higher plants, with a limited contribution from aquatic organisms.

Footnotes

Acknowledgments

We sincerely appreciate Lvjiatuo Coal Mine for providing essential data support for this study.

ORCID iDs

Zikai Li

Shiming Liu

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: this work was supported by the Central Guide Local Fund for Scientific and Technological Development of Hebei Province and Natural Science Foundation of Inner Mongolia Autonomous Region (Grant Nos. 246Z4103G and 2024LHMS04003).

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data availability

The datasets supporting the findings of this study are available from the corresponding author on reasonable request.

References

Alexander

Cumbers

Kagi

(1986) Alkylbiphenyls in ancient sediments and petroleums. Organic Geochemistry 10(4–6): 841–845.

Banerjee

Chakladar

Kumar

, et al. (2024) Petrology and association of rare earth elements in magmatically altered high-ash coal of Indian origin. International Journal of Coal Science & Technology 11(1): 117.

Chakladar

Kumari

Kumar

, et al. (2024) Distribution of trace elements and rare earth elements in coal from the Bhalukasba Surni coal block, Rajmahal coalfield, Eastern India. International Journal of Coal Science & Technology 11(1): 259.

Chelgani

Rudolph

Leistner

, et al. (2015) A review of rare earth minerals flotation: Monazite and xenotime. International Journal of Mining Science and Technology 25(6): 877–883.

Dai

Graham

Ward

(2016) A review of anomalous rare earth elements and yttrium in coal. International Journal of Coal Geology 159: 82–95.

Dai

Ren

Wang

, et al. (2014) Coal-hosted rare metal deposits: Genetic types, modes of occurrence, and utilization evaluation. Journal of China Coal Society 39(8): 1707–1715.

Dai

Zhao

Wang

, et al. (2024) Advance and prospect of researches on the mineralization of critical elements in coal-bearing sequences. Bulletin of Mineralogy, Petrology and Geochemistry 43(1): 49–63.

Ding

Sun

, et al. (2009) The rare earth element (REE) geochemical characteristics of illite clay rocks in Hanxing mining area and its geological significance. Journal of China Coal Society 34(5): 583–589.

Gao

Zhang

Fang

, et al. (2024) Discussion on the significant geological events during the Carboniferous-Permian transition and the main controlling factors of shale gas enrichment in the eastern margin of the Ordos basin. Earth Science 49(12): 4501.

10.

Hakimi

Haque

Lathbl

, et al. (2026b) Multi-scale organic geochemical dataset and 1D-basin modelling analysis for gas generation potential of the Mid Cretaceous Hoiho Formation in the Offshore Great South Basin, New Zealand. Geological Journal 104: 9.

11.

Hakimi

Kumar

Lashin

, et al. (2026a) Molecular structure and thermal decomposition kinetics of kerogen from the Paleocene oil-shale facies in the Bikaner–Nagaur basin, western India. Scientific Reports 16(1): 12645.

12.

Hatem

Mustapha

Hakimi

, et al. (2023) Preliminary study of crude oils from Shabwah depression in the central Sab’atayn basin (Yemen): Revealing oil-oil correlation with organic matter input and maturity based on geochemical properties of maltene and asphaltene fractions. Geoenergy Science and Engineering 226: 211754.

13.

Hayashi

K-I

Fujisawa

Holland

, et al. (1997) Geochemistry of∼ 1.9 Ga sedimentary rocks from northeastern Labrador, Canada. Geochimica et Cosmochimica Acta 61(19): 4115–4137.

14.

Jing

Zhang

Peng-Peng

(2021) Distribution characteristics and significance of the aromatic hydrocarbons molecular biomarker in crude oil from the northwestern Qaidam basin. Natural Gas Geoscience 32(5): 738–753.

15.

Killops

Massoud

(1992) Polycyclic aromatic hydrocarbons of pyrolytic origin in ancient sediments: Evidence for Jurassic vegetation fires. Organic geochemistry 18(1): 1–7.

16.

Kumar

Banerjee

Mustapha

, et al. (2024a) Mineralogical compositions and distributions of trace and rare earth elements in Eocene carbonaceous sediments of western India: Implications for paleoenvironment during peat accumulation. Environmental Earth Sciences 83(23): 49.

17.

Kumar

Hakimi

Singh

, et al. (2022) Geochemical and petrological characterization of the Early Eocene Carbonaceous shales: Implications for oil and gas exploration in the Barmer basin, northwest India. ACS Omega 7(47): 42960–42974.

18.

Kumar

Singh

Christanis

(2024b) Paleodepositional environment and hydrocarbon generation potential of the Paleogene lignite in the Barmer basin, Rajasthan, India. Journal of Asian Earth Sciences 259: 105892.

19.

Kumar

Singh

Paul

, et al. (2020) Evaluation of hydrocarbon potential with insight into climate and environment present during deposition of the Sonari lignite, Barmer basin Rajasthan. Energy and Climate Change 1: 100006.

20.

Kumari

Roy

Chakladar

, et al. (2023) Distribution, mode of occurrence, and significance of rare-earth elements in coal from Samaleswari open cast coal blocks, Odisha, India with their provenance and paleodepositional environment. Environmental Earth Sciences 82(5): 259.

21.

(2023) Geochemical characteristics of trace elements in Zhuzhuang coal mine of Huaibei coalfield. Coal Science and Technology 51(8): 178–191.

22.

Zhang

, et al. (2024) Geochemical characteristics of rare earth elements in coal from Kaiping coalfield. Coal Technology 43(10): 118–123.

23.

Pan

Ning

, et al. (2022) Coal measure metallogeny: Metallogenic system and implication for resource and environment. Science China Earth Sciences 65(7): 1211–1228.

24.

Lin

Sun

, et al. (2023) Geochemical characteristics of rare earth elements in late Permian coals in western Henan and indicative meaning. Coal Science and Technology 51(5): 184–192.

25.

Lin

Soong

Granite

(2018) Evaluation of trace elements in US coals using the USGS COALQUAL database version 3.0. Part I: Rare earth elements and yttrium (REY). International Journal of Coal Geology 192: 1–13.

26.

Liu

Bechtel

Stojanović

, et al. (2024) Organic petrography, biomarkers, and stable isotope (δ13C, δD, δ15N, δ18O) compositions of liptinite-rich coals. International Journal of Coal Geology 290: 104561.

27.

Liu

Gao

Chi

, et al. (2018) Distribution rule of rare earth and trace elements in the Heidaigou openpit coal mine in the Junggar coal field. Acta Geologica Sinica 92(11): 2368–2375.

28.

Liu

Jiang

Liu

, et al. (2023) Investigation of organic matter sources and depositional environment changes for terrestrial shale succession from the Yuka depression: Implications from organic geochemistry and petrological analyses. Journal of Earth Science 34(5): 1577–1595.

29.

Wang

, et al. (2025) Geochemical characteristics and geological significance of major and trace elements in the Permo-carboniferous Taiyuan formation coal from Ningdong coalfield at the western margin of Ordos basin. Science Technology & Engineering 25(5.

30.

Meng

Wang

, et al. (2024) Trace element geochemistry of different fine-grained rocks of Upper Paleozoic in the northeastern Ordos basin and the geological significance. Mineralogy and Petrology 44(4): 30–40.

31.

Meng

Fang

, et al. (2004) Biomarkers and geochemical significance of carboniferous source rocks and coals from Qaidam basin. Acta Sedimentologica Sinica 22(4): 729–736.

32.

Prachiti

Manikyamba

Singh

, et al. (2011) Geochemical systematics and precious metal content of the sedimentary horizons of lower Gondwanas from the Sattupalli coal field, Godavari valley, India. International Journal of Coal Geology 88(2): 83–100.

33.

Qin

, et al. (2018) Organic geochemistry of the Late Permian coal from the Zhongliangshan mine, Chongqing. J China Coal Soc 43(7): 1973–1982.

34.

Qin

Wang

Song

, et al. (2005) Geochemistry characteristics and sedimentary micro-environments of No. 11 coal seam of the Taiyuan formation of upper carboniferous in Pingshuo mining district, Shanxi province. Journal of Palaeogeography 7(2): 249–260.

35.

Radke

Vriend

Ramanampisoa

(2000) Alkyldibenzofurans in terrestrial rocks: Influence of organic facies and maturation. Geochimica et Cosmochimica Acta 64(2): 275–286.

36.

Radke

Willsch

Leythaeuser

, et al. (1982) Aromatic components of coal: Relation of distribution pattern to rank. Geochimica et Cosmochimica Acta 46(10): 1831–1848.

37.

Rajak

Gopinathan

Kumar

, et al. (2024) Geochemical and mineralogical assessment of environmentally sensitive elements in Neyveli lignite deposits, Cauvery Basin, India. Environmental Geochemistry and Health 46(11): 202.

38.

Seredin

Dai

(2012) Coal deposits as potential alternative sources for lanthanides and yttrium. International Journal of Coal Geology 94: 67–93.

39.

Shen

, et al. (2022) Carboniferous and Permian integrative stratigraphy and timescale of north China block. Science China Earth Sciences 65(6): 983–1011.

40.

Sidik

Mustapha

Kumar

, et al. (2024) Geochemical characteristics of crude oils and source rocks in the Malaysia-Thailand joint development area (MTJDA), north Malay basin. Marine and Petroleum Geology 170: 107135.

41.

Singh

Hakimi

Kumar

, et al. (2020) Geochemical and organic petrographic characteristics of high bituminous shales from Gurha mine in Rajasthan, NW India. Scientific Reports 10(1): 22108.

42.

Wang

, et al. (2024) Geochemical characteristics and sedimentary environment of Chang 7 source rocks, southern Ordos basin. Journal of China University of Petroleum (Edition of Natural Science) 48(1): 91–103.

43.

Wang

Zhang

, et al. (2021) Geochemical characteristics of rare earth elements and the indicative significance in the Ximing coal mine, Shanxi province. Geoscience 35(04): 1009.

44.

Xiao

(1990) Sulfur content and sedimentary environment of coal in Kaiping coalfield, Hebei province. China Coalfield Geology 2(4): 39–40.

45.

Meng

Bechtel

, et al. (2025) Palaeoenvironmental and palaeoclimatic changes during the Cretaceous (early Albian) oceanic anoxic event 1b: A record from coal from northeast China. Palaeogeography, Palaeoclimatology, Palaeoecology 676: 113181.

46.

Xuying

Fengjie

Pengfei

(2023) Geochemical characteristics and paleoenvironmental significance of the Paleogene Xiaganchaigou formation mudstone in the Eboliang area, northwest margin of Qaidam basin, China. Journal of Chengdu University of Technology (Science & Technology Edition) 50(2): 172–186.

47.

Yang

Hai

, et al. (2025) Characteristics of oil shale development and paleoenvironment restoration of Jurassic Yan’an formation in southwestern Ordos basin. Bulletin of Geological Science and Technology 44(3): 182–196.

48.

Zheng

, et al. (2024) Paleoenvironmental controls on organic-rich lithofacies of Eocene saline lacustrine in the Chentuokou depression, Jianghan basin. Bulletin of Geological Science and Technology 43(5): 70–80.

49.

Yuan

(2023) Occurrence characteristics and main control mechanism of trace elements in early Permian coal in the southern margin of north China plate. Bulletin of Geological Science and Technology 42(5): 138–149.

50.

Zhang

Chenjun

Huang

, et al. (2022) Geochemical characteristics and sedimentary environment of Carboniferous Benxi formation in eastern Ordos basin. Natural Gas Geoscience 33(9): 1485–1498.

51.

Zhang

Philp

(2010) Geochemical characterization of aromatic hydrocarbons in crude oils from the Tarim, Qaidam and Turpan basins, NW China. Petroleum Science 7(4): 448–457.

52.

Zhang

Liu

(2017) Regional geology of China Hebei chronicles. Beijing: Geological Press. Epub ahead of print 2017.

53.

Zhao

Guo

Bechtel

, et al. (2025) Tectonic-driven thermal alteration of organic matter in the Permian high-rank coals in the southern north China basin. Fuel 390: 134715.

54.

Zhao

Zhang

Xiao

, et al. (2023) Paleoclimate-induced wildfires in a paleomire in the Ordos basin, northern China during the Middle Jurassic greenhouse period. Chemical Geology 637: 121677.

55.

Zhao

(2015) Distribution and enrichment mechanism of multi-metallic-elements associated with coal in Ordos basin . China University of Mining and Technology, pp. 30–128(in Chinese with English abstract).

56.

Zhao

Meng

(1987) Paleogeographic and lithological characteristics, as well as paleoecological features, of coal bearing sediments in the Carboniferous Permian period of the Kaiping coalfield. Journal of Tangshan Institute of Engineering and Technology 1: 10–19.

57.

Zhou

(2008) Formation mechanism of alkylated naphthalene in petroleum hydrocarbon and its geochemistry significance. Geological Science and Technology Information 5: 92–96.