Geochemistry and Mineralogy of Upper Paleozoic Coal in the Renjiazhuang Mining District, Northwest Ordos Basin, China: Evidence for Sediment Sources, Depositional Environment, and Elemental Occurrence

Abstract

This study aims to investigate the depositional environment, sediment sources, and elemental occurrence of Upper Paleozoic coal in the Renjiazhuang Mining District, Western Ordos Basin. Furthermore, SEM-EDX, optical microscope (OM), ICP-AES, ICP-MS, and AAS were used. Compared with hard coal of the world, M3 coals were enriched in Ga, Li, Zr, Be, Ta, Hf, Nb, Pb, and Th, M5 coals were enriched in Li (CC = 10.21), Ta (CC = 6.96), Nb (CC = 6.95), Be, Sc, Ga, Hf, Th, Pb, Zr, In, and REY, while M9 coals were enriched in Li (CC = 14.79), Ta (CC = 5.41), Ga, W, Hf, Nb, Zr, Pb, and Th. In addition, minerals were mainly composed of kaolinite, dolomite, pyrite, feldspar, calcite, and quartz, locally visible minor amounts of monazite, zircon, clausthalite, chalcopyrite, iron dolomite, albite, fluorite, siderite, galena, barite, boehmite, and rutile. In addition, maceral compositions of M3 coals and M9 coals were dominated by vitrinite (up to 78.50%), while M5 coals were the main inertite (up to 76.26%), and minor amounts of liptinite. REY distribution patterns of all samples exhibited light REY enrichment and negative Eu anomalies. The geochemistry of samples (TiO₂ and Al₂O₃, Nb/Y and Zr × 0.0001/TiO₂ ratios, and REY enrichment types) indicates that the sediment sources of samples originated from felsic igneous rocks. Indicator parameters (TPI, GI, VI, GWI, V/I, Sr/Ba, Th/U, and Ce_N/Ce_N*) suggest that these coals were formed in different paleopeat swamp environments: M3 coal was formed in a lower delta plain and terrestrial (lacustrine) facies with weak oxidation and reduction, and M5 coal was formed in a terrestrial and dry forest swamp environment with weak oxidation–oxidation, while M9 coal was formed in a seawater environment of humid forest swamps and the transition from the lower delta plain to continental sedimentation with weak oxidation and reduction. Statistical methods were used to study the elemental occurrence. Moreover, Li, Ta, Hf, Nb, Zr, Pb, and Th elements were associated with aluminosilicates, and Ga occurred as silicate.

1. Introduction

In recent years, the abundance, occurrence mechanism, enrichment source, and distribution pattern of key elements in coals have drawn widespread concern [1,2,3,4,5,6,7,8,9]. On the one hand, the trace elements contain geological information that reflects coal-bearing strata, regional structures, and evolutionary history [8,10,11,12,13,14]. On the other hand, these key metal elements (Ga, Li, and REE) may also be important sources of materials for industrial use and have great economic value [15,16,17,18,19].

The Ordos Basin in China ranks as the second largest amount of coal [20]. Many scholars have conducted geochemical and mineralogical studies on the Ordos Basin [15,21,22,23,24,25,26,27]. It has been found that the content of Ga, Al, and REY in coal is relatively high, and elements (such as Al and Ga) are successfully extracted from coal ash, triggering direct economic value [28,29].

The geochemistry on the coal-bearing strata of the Upper Paleozoic in northern China and the Ordos Basin was studied by Fu and Zhao [30,31], respectively. In addition, Liu et al. found that variations in the sedimentary environment of the Ningdong Coalfield controlled differences in the geochemistry of the coal seam in the Yan’an Formation [23]. Wu et al. conducted geochemical research on M9 coal of the Taiyuan Formation in Ningdong Coalfield, and claimed that trace elements in coal are mainly carried by pyrite and coal ash [32]. Although attention has been paid to some elements in coal from the western part of the Ordos Basin, there is a lack of in-depth discussion on the enrichment carriers, sedimentary environment, and sediment sources of elements in Upper Paleozoic coal [24,27,33,34].

In this paper, the results of mineralogy, geochemistry, and petrology of the Upper Paleozoic coal in Renjiazhuang Mining District, northwest Ordos Basin, were reported. Furthermore, the characteristics of organic petrology, geochemistry, and mineralogy were analyzed. Beyond that, the sedimentary environment, provenance, and element occurrence of the Upper Paleozoic coal in the Renjiazhuang Mining District, the northwest edge of the Ordos Basin, were discussed.

2. Geological Setting

Renjiazhuang Mining District, located at the northwest edge of the Ordos Basin (Figure 1a), is about 32 km away from the northeast of Lingwu City in the middle east of Ningxia Province (Figure 1b). The fault strikes, triggered by multistage tectonic events in the Renjiazhuang Mining District, are SN–NNW, SE–NNE, and NNE, including Huangcaogou anticline, Sandaogou anticline, Shagou syncline, and Huangcaogou syncline (Figure 1c) [35]. The coal seams in the Renjiazhuang Mining District are almost unaffected by magmatic heat [23].

The coal-bearing formations of Renjiazhuang Mining District are mainly the Taiyuan Formation of the Pennsylvanian and the Shanxi Formation of the Early Permian, with 24 coal layers (Figure 2) [13]. The lithology of the Shanxi Formation includes grayish-white fine sandstone, mudstone, dark-gray medium-coarse sandstone, grayish-black siltstone, coal seam as well as a small amount of clay rock and asphaltene mudstone. The Shanxi Formation contains 13 coal layers in total, with minable coal seams of M1, M3, M4, M5, and M6. The total thickness is 3.98–24.69 m and the average coal thickness is 15.30 m. Moreover, the Shanxi Formation is deposited in a typical delta environment [25,36].

The lithology of the Taiyuan Formation includes fine sandstone, grayish-black mudstone, siltstone, limestone, coal seam, grayish-white medium-coarse sandstone, clay rock, and asphaltene mudstone. The Taiyuan Formation contains 11 coal seams in total, with minable coal seams of M8, M9, and M10. The thickness of the minable coal seams is 3.25–13.55 m (8.21 m on average). It is formed in sedimentary environments of tidal deltas, barrier sands, and lagoons [25,32,36,37].

Figure 1. Geological map of the northwest Ordos Basin and structural sketch-map of Renjiazhuang Mining District, modified after Wu et al. [32], Zhao [38], and Zhang [39]. (a) locations of the Ordos Basin in China, (b). geological map of the northwest Ordos Basin (c) structural sketch map of the Renjiazhuang Mining District.

Figure 1. Geological map of the northwest Ordos Basin and structural sketch-map of Renjiazhuang Mining District, modified after Wu et al. [32], Zhao [38], and Zhang [39]. (a) locations of the Ordos Basin in China, (b). geological map of the northwest Ordos Basin (c) structural sketch map of the Renjiazhuang Mining District.

Figure 2. Lithological sequence and sample sections of the Renjiazhuang Mining District. The sample numbers from D-R-1 to D-R-15 are from Wu et al. [32].

Figure 2. Lithological sequence and sample sections of the Renjiazhuang Mining District. The sample numbers from D-R-1 to D-R-15 are from Wu et al. [32].

3. Sampling and Methods

3.1. Sampling and Preparation

In this study, a series of samples (including 7 coal samples from T-H-1, T-H-M, T-H-2, F-H-1, F-H-2, N-H-1, and N-H-2, and 6 non-coal samples from the roof samples 3G-DN, 5G-DN, 9G-DN, and bottom samples 3G-DE, 5G-DE, and 9G-DE) from 13 different lithological units had been collected layer by layer from the working surfaces at the Renjiazhuang Mining District (Figure 2). Moreover, samples preparation with particle size less than 75 μm for geochemical analysis were based on ASTM standard D2013/D2013M-12 [40].

3.2. Experiments and Methods

Approximate analysis, involving the quantification of moisture, ash yield, and volatile matter of the samples, was measured with reference to the ASTM Standard D3175-17 (2017), D3173M-17a (2017), and D3174-12 (2012) [41,42,43]. The maximum vitrinite reflectance (R_o,max), total sulfur, and forms of sulfur were tested following the ASTM standards D2798–11a (2019), D2492-02 (2012), and D3177-02 (2011) [44,45,46]. The pyro-hydrolysis and fluoride ion-selective electrode (ISE) method was used to determine the fluoride content in coal following the Chinese standard (GB/T 4633-2014) [47]. Fe, Al, Mg, and Ca in coal were analyzed using inductively coupled atomic emission spectrometer (ICP-AES). Additionally, K, Na, P, Mn, and Ti were determined by atomic absorption spectrophotometer (AAS), and spectral photometer was analyzed for Si. Furthermore, Sc, V, Li, Ni, Cu, Be, Cr, Co, Zn, Ba, Nb, Mo, Ga, Rb, Sr, Cd, In, Ta, Zr, Pb, W, Tl, Hf, U, Sb, Cs, Bi, Th, REY (La, Pr, Nd, Ce, Sm, Ho, Er, Tm, Yb, Gd, Eu, Tb, Dy, Y, Lu) were measured with inductively coupled plasma mass spectrometry (ICP-MS).

In order to determine the minerals in coal, the coal samples were first treated with a low-temperature asher (LTA), and then determined using an X-ray diffractometer (XRD). Meanwhile, optical microscopes with ultraviolet light and white light reflections, X-ray analyzers, and scanning electron microscopes (SEM-EDX) were used to observe the mineral morphology in coal samples.

Coal facies parameters, such as TPI, GI, GWI, and VI, were calculated from maceral data [48,49,50,51]. The calculation formula is shown in Equations (1)–(4):

$$\mathrm{TPI}={\displaystyle \frac{\mathrm{telinite}+\mathrm{collotelinite}+\mathrm{fusinite}+\mathrm{semifusinite}}{\mathrm{collodetrinite}+\mathrm{vitrodetrinite}+\mathrm{gelinite}+\mathrm{corpogelinite}+\mathrm{inertodertrinite}+\mathrm{macrinite}}}$$

(1)

$$\mathrm{GI}={\displaystyle \frac{\mathrm{macrinite}+\mathrm{vitrinite}}{\mathrm{fusinite}+\mathrm{semifusinite}+\mathrm{inertodetrinite}}}$$

(2)

$$\mathrm{GWI}={\displaystyle \frac{\mathrm{vitrodetrinite}+\mathrm{gelinite}+\mathrm{corpogelinite}+\mathrm{quartz}+\mathrm{clay} \mathrm{mineral}}{\mathrm{telinite}+\mathrm{collotelinite}+\mathrm{collodetrinite}}}$$

(3)

$$\mathrm{VI}={\displaystyle \frac{\mathrm{telinite}+\mathrm{collotelinite}+\mathrm{corpogelinite}+\mathrm{fusinite}+\mathrm{semifusinite}+\mathrm{resinite}+\mathrm{suberinite}}{\mathrm{collodetrinite}+\mathrm{vitrodetrinite}+\mathrm{liptodetrinite}+\mathrm{sporinite}+\mathrm{inertodertrinite}+\mathrm{cutinite}}}$$

(4)

The anomalies in redox-sensitive Eu, Gd, Ce, and Y were listed as Eu_N/Eu_N*, Gd_N/Gd_N*, Ce_N/Ce_N*, and Y_N/Y_N*, respectively [8,23,52,53]. The REE parameters (i.e., REY, LREE/MREE (L/M), LREE/HREE (L/H), MREE/HREE (M/H), Ce_N/Ce_N*, Gd_N/Gd_N*, Eu_N/Eu_N*, and Y_N/Y_N*) could be calculated. The calculation formula is shown in Equations (5)–(12):

$$\mathrm{REY} =\mathrm{La}+\mathrm{Ce}+\mathrm{Pr}+\mathrm{Nd}+\mathrm{Sm}+\mathrm{Eu}+\mathrm{Gd}+\mathrm{Tb}+\mathrm{Dy}+\mathrm{Y}+\mathrm{Ho}+\mathrm{Er}+\mathrm{Tm}+\mathrm{Yb}+\mathrm{Lu}$$

(5)

$${\displaystyle \frac{\mathrm{L}}{\mathrm{M}}}={\displaystyle \frac{\mathrm{La}+\mathrm{Ce}+\mathrm{Pr}+\mathrm{Nd}+\mathrm{Sm}}{\mathrm{Eu}+\mathrm{Gd}+\mathrm{Tb}+\mathrm{Dy}+\mathrm{Y}}}$$

(6)

$${\displaystyle \frac{\mathrm{L}}{\mathrm{H}}}={\displaystyle \frac{\mathrm{La}+\mathrm{Ce}+\mathrm{Pr}+\mathrm{Nd}+\mathrm{Sm}}{\mathrm{Ho}+\mathrm{Er}+\mathrm{Tm}+\mathrm{Yb}+\mathrm{Lu}}}$$

(7)

$${\displaystyle \frac{\mathrm{M}}{\mathrm{H}}}={\displaystyle \frac{\mathrm{Eu}+\mathrm{Gd}+\mathrm{Tb}+\mathrm{Dy}+\mathrm{Y}}{\mathrm{Ho}+\mathrm{Er}+\mathrm{Tm}+\mathrm{Yb}+\mathrm{Lu}}}$$

(8)

$${\displaystyle \frac{{\mathrm{Eu}}_N}{{{\mathrm{Eu}}_N}^{*}}}={\displaystyle \frac{{\mathrm{Eu}}_N}{0.67 \times {\mathrm{Sm}}_N+0.33 \times {\mathrm{Tb}}_N}}$$

(9)

$${\displaystyle \frac{{\mathrm{Ce}}_N}{{{\mathrm{Ce}}_N}^{*}}}={\displaystyle \frac{{2 \times \mathrm{Ce}}_N}{{\mathrm{La}}_N+{ \mathrm{Tb}}_N}}$$

(10)

$${\displaystyle \frac{{\mathrm{Gd}}_N}{{{\mathrm{Gd}}_N}^{*}}}={\displaystyle \frac{{\mathrm{Gd}}_N}{0.33 \times {\mathrm{Sm}}_N+0.67 \times {\mathrm{Tb}}_N}}$$

(11)

$${\displaystyle \frac{{\mathrm{Y}}_N}{{{\mathrm{Y}}_N}^{*}}}={\displaystyle \frac{{\mathrm{Y}}_N}{{\mathrm{Ho}}_N}}$$

(12)

4. Results

4.1. Coal Quality

The approximate analysis, including vitrinite reflectance, forms of sulfur, and total sulfur of all samples in the Renjiazhuang Mining District in the northwest Ordos Basin, China are listed in

Table 1. Basic properties of all samples (%).

Samples	Ro,max	Mad	Ad	Vdaf	St,d	Sp,d	Ss,d	So,d
3G-DN	nd	0.78	94.01	97.98	0.77	0.75	0.01	0.01
T-H-1	0.66	1.52	24.99	38.69	0.67	0.35	0.00	0.32
T-H-M	0.73	1.54	13.95	39.15	0.61	0.24	0.00	0.37
T-H-2	0.79	1.34	26.27	37.23	0.47	0.13	0.01	0.32
Av	0.73	1.47	21.74	38.36	0.58	0.24	0.00	0.34
3G-DE	nd	0.60	93.41	99.39	0.08	0.03	0.02	0.03
5G-DN	nd	0.40	92.72	93.24	0.12	0.02	0.07	0.03
F-H-M	0.85	1.69	21.08	32.14	0.57	0.00	0.03	0.54
F-H-2	0.82	1.60	33.07	34.39	0.53	0.02	0.02	0.49
Av	0.84	1.65	27.08	33.27	0.55	0.01	0.03	0.52
5G-DE	nd	0.66	91.21	96.68	0.06	0.00	0.04	0.02
9G-DN	nd	0.60	89.21	84.25	0.42	0.40	0.01	0.01
N-H-1	0.74	0.94	38.82	42.33	2.33	0.69	0.01	1.64
N-H-2	0.78	0.84	36.48	41.77	2.20	0.37	0.00	1.83
Av-D-R*	0.71	0.90	13.21	36.99	3.07	0.36	0.01	2.70
Av	0.76	0.89	37.65	42.05	2.27	0.53	0.01	1.74
9G-DE	nd	0.55	94.26	77.93	0.84	0.66	0.03	0.15

R_o,max, maximum vitrinite reflectance; A, ash; M, moisture; V, volatile matter; S_s, sulfate sulfur; S_t; total sulfur; S_p, pyritic sulfur; S_o, organic sulfur; d, dry basis; ad, air basis; daf, dry and ash-free basis; nd, no date. Av-D-R*, weighted average of the M9 coals (from R-D-1 to R-D-15) are from Wu et al. [32].

. The average values of maximum vitrinite reflectance (R_o,max) of M3, M5, and M9 coals were 0.73%, 0.84%, and 0.76%, respectively, which were medium-rank coal (0.60% < R_o,max < 1.00%). The volatile substances in the coal samples of the Shanxi Formation in the Renjiazhuang Mining District (38.36% on average in the M3 coals, and 33.27% on average in the M5 coals) indicated medium- to high-volatile coal following the Chinese standard (MT/T849-2000) [54], while the coal samples of the Taiyuan Formation (42.05% on average in the M9 coals) were high-volatile coal. The average ash yield of M3 coals and M5 coals in the Shanxi Formation of the Renjiazhuang Mining District was 21.74% and 27.08%, respectively, indicating a medium-ash coal following the Chinese Standard (GB/T 15224.1-2018) [55]. The average ash yield of M9 coals in the Taiyuan Formation was 37.65%, indicating a high-ash coal from 30% to 40%. M3 coals (0.58% on average) and M5 coals (0.55% on average) of the Shanxi Formation were low-sulfur coal following the Chinese standard (GB15224.2-2010) [56], whereas M9 coal (2.27% on average) was medium-high sulfur coal. From the Early Permian to the Pennsylvanian, the ash yield and organic sulfur content in coals had increased. As pointed out by Wu et al. and Ji et al. [27,32], the volatile matter in the M5 coals of the Renjiazhuang Mining District ranges from 33% to 41% (36.36% on average), the ash yield varies from 12% to 41% (26.09% on average), the volatile matter yield in the M9 coals ranges from 32.1% to 42.3% (37.2% on average), and R_o,max is from 0.67% to 0.75% (0.71% on average), which is consistent.

4.2. Organic Petrography

The proportion of microscopic components in the liptinite, vitrinite, and inertinite varied among the three coal seams (

Table 2. The results of coal petrography in the research area.

Samples	T	CT	CD	CG	VD	T-V	F	SF	Ma	Mi	ID	T-I	Sp	Cu	Re	Sub	Alg	Ba	Bi	LD	T-L	TPI	GI	GWI	VI	V/I
T-H-1	2.34	11.70	38.01	0.00	4.09	56.14	6.43	23.98	0.00	0.00	5.85	36.26	4.09	1.76	0.58	0.00	0.00	0.00	1.17	0.00	7.60	0.93	1.55	0.55	0.84	1.55
T-H-M	3.02	11.58	55.16	0.00	3.53	73.29	5.54	11.09	1.00	0.00	3.53	21.16	3.53	0.51	0.51	0.00	0.00	1.00	0.00	0.00	5.55	0.49	3.69	0.17	0.48	3.46
T-H-2	4.49	3.84	38.45	0.00	3.21	49.99	1.93	32.69	0.00	0.00	5.13	39.74	5.13	1.28	0.65	0.00	0.00	1.93	1.28	0.00	10.27	0.92	1.26	0.66	0.82	1.26
F-H-M	1.13	0.57	14.68	0.00	0.00	16.38	0.57	68.90	0.57	0.57	5.65	76.26	5.09	1.70	0.57	0.00	0.00	0.00	0.00	0.00	7.36	3.41	0.23	1.24	2.65	0.21
F-H-2	0.69	4.16	19.45	0.00	1.39	25.69	1.39	58.34	0.00	0.69	8.34	68.76	4.16	1.39	0.00	0.00	0.00	0.00	0.00	0.00	5.55	2.21	0.38	1.71	1.86	0.37
N-H-1	2.04	12.92	42.86	0.00	8.84	66.66	2.72	21.76	0.68	0.00	4.09	29.25	3.40	0.68	0.00	0.00	0.00	0.00	0.00	0.00	4.09	0.70	2.36	0.86	0.66	2.28
N-H-2	0.65	8.44	41.56	0.00	6.50	57.14	5.20	29.88	0.65	0.65	3.89	40.26	1.94	0.00	0.65	0.00	0.00	0.00	0.00	0.00	2.59	0.84	1.48	0.90	0.83	1.42
D-R-1	20.90	4.40	13.60	0.60	0.00	39.50	8.10	37.60	2.40	0.60	5.50	54.30	0.00	3.70	2.40	0.00	0.00	0.00	0.00	0.00	6.20	3.20	0.80	0.70	3.20	0.73
D-R-2	9.90	1.60	18.60	1.10	0.00	31.20	13.50	36.80	3.60	3.60	5.20	62.60	0.00	3.60	2.60	0.00	0.00	0.00	0.00	0.00	6.20	2.20	0.60	0.20	2.40	0.50
D-R-3	10.80	21.20	37.40	2.50	0.00	71.90	5.40	11.90	0.00	0.50	7.40	25.20	0.90	0.50	1.50	0.00	0.00	0.00	0.00	0.00	2.90	1.00	2.90	0.10	1.20	2.85
D-R-4	8.00	15.40	46.80	0.60	0.00	70.80	7.50	11.50	0.00	1.70	3.40	24.10	1.70	1.10	1.70	0.00	0.60	0.00	0.00	0.00	5.20	0.80	3.20	0.10	0.80	2.94
D-R-5	11.70	9.20	35.10	0.50	0.00	56.50	13.20	18.00	0.50	3.40	3.90	39.00	1.00	1.90	1.00	0.00	0.00	0.00	0.00	0.50	4.50	1.30	1.60	0.00	1.30	1.45
D-R-6	7.70	27.30	40.20	2.60	0.00	77.80	8.80	3.60	0.50	3.60	0.50	17.10	2.60	0.00	2.60	0.00	0.00	0.00	0.00	0.00	5.10	1.10	6.10	0.10	1.20	4.55
D-R-7	3.40	14.90	42.80	2.00	0.00	63.10	13.00	9.60	0.90	1.40	5.80	30.70	2.40	3.40	0.50	0.00	0.00	0.00	0.00	0.00	6.30	0.80	2.30	0.10	0.80	2.06
D-R-8	6.20	9.10	38.30	0.50	0.00	54.00	15.30	22.50	0.00	0.50	3.80	42.20	2.40	1.40	0.00	0.00	0.00	0.00	0.00	0.00	3.80	1.20	1.30	0.00	1.20	1.28
D-R-9	5.30	7.80	19.40	1.00	0.00	33.50	10.00	36.10	1.60	3.10	3.10	53.90	0.60	4.10	5.70	0.00	0.00	0.60	1.60	0.00	12.60	2.40	0.70	0.30	2.40	0.62
D-R-10	10.60	19.70	29.80	0.50	0.00	60.60	12.50	16.40	0.50	3.80	2.90	36.00	0.50	1.00	1.90	0.00	0.00	0.00	0.00	0.00	3.40	1.80	1.90	0.10	1.80	1.68
D-R-11	6.90	5.20	22.00	1.50	0.00	35.70	10.50	34.10	2.60	1.50	4.70	53.40	4.70	4.10	2.10	0.00	0.00	0.00	0.00	0.00	10.90	1.80	0.80	0.30	1.70	0.67
D-R-12	5.70	10.00	33.40	0.50	0.00	49.70	8.90	27.20	1.10	1.60	4.70	43.40	3.20	2.10	0.50	0.00	0.00	0.00	1.10	0.00	6.90	1.30	1.20	0.10	1.20	1.15
D-R-13	2.90	19.20	48.50	0.50	0.00	71.20	7.20	9.20	1.50	1.90	5.70	25.50	2.40	0.90	0.00	0.00	0.00	0.00	0.00	0.00	3.30	0.70	3.30	0.10	0.70	2.79
D-R-14	8.00	13.40	25.70	1.00	0.00	48.10	4.80	34.30	0.00	1.00	4.30	44.50	1.00	3.20	1.00	0.60	0.00	0.00	1.00	0.60	7.40	1.90	1.10	0.30	1.80	1.08
D-R-15	5.20	33.80	37.90	1.50	0.00	78.50	4.70	7.70	0.50	1.10	2.10	16.10	3.20	1.10	1.10	0.00	0.00	0.00	0.00	0.00	5.30	1.20	5.50	0.10	1.20	4.88

T, telinite; CD, collodetrinite; CT, collotelinite; CG, corpogelinite; VD, vitrodetrinite; T-V, total vitrinites; SF, semifusinite; F, fusinite; Mi, micrinite; ID, inertodetrinite; Ma, macrinite; T-I, total inertinites; Cu, cutinite; Sp, sporinite; Re, resinite; Alg, alginite; Sub, suberinite; Ba, barkinite; Bi, bituminite; L-D, liptodetrinite; T-L, total liptinites; TPI, tissue preservation index; GI, gelification index; GWI, groundwater index; VI, vegetation index.

). M3 coals and M9 coals were dominated by vitrinite, in which the content ranged from 16.38% to 78.50% (53.97% on average). Among them, the characteristics of M3 coals, M5 coals, and M9 coals were high collodetrinite content (Figure 3a), in which the highest value was recorded in the middle of the M3 coal seams (55.16%). Collotelinite (Figure 3b) was the second highly enriched submaceral of the M3 coals, M5 coals, and M9 coals (except for T-H-2, F-H-M, D-R-1, D-R-2, D-R-5, and D-R-11), while telinite (Figure 3c) was the submaceral with less enrichment in each coal seam.

Inertite was the main maceral of the M5 coals, with the maximum value of 76.26% (sample F-H-M). The proportion of inertinite in the M3 coals (39.74% on maximum value) and M9 coals (62.60% on maximum value) was relatively high. The inertinite components were composed of semifusinite (Figure 3d), fusinite (Figure 3f), and inertodetrinite (Figure 3e), as well as minor amounts of macrinite (Figure 3g) and micrinite (Figure 3h).

Liptinite was the minimum maceral among the three coal seams, and its contents ranged from 2.59% to 12.60%. The proportion of liptinite in the M5 coals and M9 coals was lower than that in the M3 coals. Sporinite (Figure 3i) was the first highly enriched submaceral of the coal seams, followed by the presence of resinite (Figure 3j) and cutinite (Figure 3k). M3 coals also contained barkinite (Figure 3l) and bituminite.

4.3. Geochemistry

4.3.1. Main Elemental Oxides

The results of major elemental oxides in coal and non-coal samples from the Renjiazhuang Mining District are shown in Table 3. Specifically, MgO was slightly enriched in the M3 coals, while SiO₂, Al₂O₃, CaO, TiO₂, and MnO₂ were close to Chinese hard coal, and the remaining elements were lower than Chinese coal. The average concentrations of MgO, TiO₂, Al₂O₃, and SiO₂ in the M5 coals were similar to those of Chinese coal, and were relatively depleted in K₂O, Na₂O, Fe₂O₃, CaO, MnO₂, and P₂O₅ (CC < 0.5). M9 coals were weakly enriched in SiO₂, Al₂O₃, and MgO. TiO₂ and P₂O₅ were similar to those of Chinese coal. K₂O, Na₂O, Fe₂O₃, CaO, and MnO₂ were depleted.

The average SiO₂/Al₂O₃ ratio in coals from the Renjiazhuang Mining District (1.17 on average in the M3 coals, 1.14 in the M5 coals, and 1.10 in the M9 coals) was lower than the average SiO₂/Al₂O₃ ratio (1.42) of Chinese coal [57], and lower than the theoretical SiO₂/Al₂O₃ ratio of kaolinite (1.18). The average concentrations of K₂O, SiO₂, TiO₂, MnO₂, P₂O₅, and Al₂O₃ in non-coal samples from the Renjiazhuang Mining District were similar to the average values of North American shale, and Na₂O, Fe₂O₃, CaO, and MgO were relatively depleted. The average SiO₂/Al₂O₃ ratio in non-coal samples from the Renjiazhuang Mining District (4.34) was higher than that in North American shale (3.83) [57]. Notably, the SiO₂/Al₂O₃ ratio in 3G-DE (6.02), 5G-DN (4.00), and 9G-DE (6.92) samples was significantly higher than that in the North American shale.

Table 3. Results of the main elemental oxides in the samples (%).

Samples	K2O	Na2O	SiO2	Al2O3	Fe2O3	CaO	MgO	TiO2	MnO2	P2O5	SiO2/Al2O3	Al2O3/TiO2
3G-DN	2.68	0.14	67.27	17.78	1.88	0.23	0.71	0.75	0.00	0.05	3.78	23.71
T-H-1	0.07	0.05	11.85	10.12	0.46	0.54	0.61	0.19	0.01	0.01	1.17	53.26
T-H-M	0.03	0.02	3.72	3.12	0.33	4.29	0.90	0.11	0.04	0.01	1.19	28.36
T-H-2	0.05	0.06	13.13	11.36	0.23	0.22	0.31	0.22	0.00	0.02	1.16	51.64
Av	0.05	0.05	9.56	8.20	0.34	1.68	0.61	0.18	0.02	0.01	1.17	45.56
3G-DE	2.20	0.10	66.88	11.11	0.87	0.50	0.25	0.92	0.00	0.03	6.02	12.08
5G-DN	2.33	0.15	67.24	16.80	2.72	0.45	0.62	1.12	0.03	0.12	4.00	15.00
F-H-M	0.01	0.04	10.33	9.25	0.08	0.19	0.24	0.28	0.00	0.01	1.12	33.04
F-H-2	0.03	0.05	16.56	14.35	0.16	0.13	0.34	0.50	0.00	0.02	1.15	28.70
Av	0.02	0.05	13.45	11.80	0.12	0.16	0.29	0.39	0.00	0.02	1.14	30.26
5G-DE	2.91	0.18	57.17	18.89	2.25	0.40	0.64	0.80	0.04	0.07	3.03	23.61
9G-DN	2.38	0.11	56.55	24.72	2.03	0.34	0.89	0.96	0.11	0.15	2.29	25.75
N-H-1	0.03	0.07	18.20	16.62	0.95	0.63	0.69	0.33	0.00	0.05	1.10	50.36
N-H-2	0.04	0.07	17.66	15.91	0.59	0.26	0.43	0.37	0.00	0.07	1.11	43.00
Av-D-R*	0.01	0.03	5.67	5.14	0.57	0.52	0.23	0.12	0.00	0.08	0.90	46.73
Av	0.04	0.07	17.93	16.27	0.77	0.45	0.56	0.35	0.00	0.06	1.10	46.49
9G-DE	1.39	0.02	77.82	11.25	1.28	0.17	0.40	0.94	0.01	0.03	6.92	11.97
Av-non-coal	2.32	0.12	65.49	16.76	1.84	0.35	0.59	0.92	0.03	0.08	4.34	18.69
Chinese coala	0.19	0.16	8.47	5.98	4.85	1.23	0.22	0.33	0.02	0.09	1.42	18.12
NASCb	3.99	1.15	64.80	16.90	5.70	3.56	2.85	0.78	0.06	0.11	3.83	21.67

Av, average percentage of coal samples; Av-non-coal, average percentage of roof and floor samples; Av-D-R*, weighted average of the M9 coals (from R-D-1 to R-D-15) were from Wu et al. [32]; Chinese coal^a, from Dai et al. [15]; NASC^b, from Gromet et al. [58].

Table 3. Results of the main elemental oxides in the samples (%).

Samples	K2O	Na2O	SiO2	Al2O3	Fe2O3	CaO	MgO	TiO2	MnO2	P2O5	SiO2/Al2O3	Al2O3/TiO2
3G-DN	2.68	0.14	67.27	17.78	1.88	0.23	0.71	0.75	0.00	0.05	3.78	23.71
T-H-1	0.07	0.05	11.85	10.12	0.46	0.54	0.61	0.19	0.01	0.01	1.17	53.26
T-H-M	0.03	0.02	3.72	3.12	0.33	4.29	0.90	0.11	0.04	0.01	1.19	28.36
T-H-2	0.05	0.06	13.13	11.36	0.23	0.22	0.31	0.22	0.00	0.02	1.16	51.64
Av	0.05	0.05	9.56	8.20	0.34	1.68	0.61	0.18	0.02	0.01	1.17	45.56
3G-DE	2.20	0.10	66.88	11.11	0.87	0.50	0.25	0.92	0.00	0.03	6.02	12.08
5G-DN	2.33	0.15	67.24	16.80	2.72	0.45	0.62	1.12	0.03	0.12	4.00	15.00
F-H-M	0.01	0.04	10.33	9.25	0.08	0.19	0.24	0.28	0.00	0.01	1.12	33.04
F-H-2	0.03	0.05	16.56	14.35	0.16	0.13	0.34	0.50	0.00	0.02	1.15	28.70
Av	0.02	0.05	13.45	11.80	0.12	0.16	0.29	0.39	0.00	0.02	1.14	30.26
5G-DE	2.91	0.18	57.17	18.89	2.25	0.40	0.64	0.80	0.04	0.07	3.03	23.61
9G-DN	2.38	0.11	56.55	24.72	2.03	0.34	0.89	0.96	0.11	0.15	2.29	25.75
N-H-1	0.03	0.07	18.20	16.62	0.95	0.63	0.69	0.33	0.00	0.05	1.10	50.36
N-H-2	0.04	0.07	17.66	15.91	0.59	0.26	0.43	0.37	0.00	0.07	1.11	43.00
Av-D-R*	0.01	0.03	5.67	5.14	0.57	0.52	0.23	0.12	0.00	0.08	0.90	46.73
Av	0.04	0.07	17.93	16.27	0.77	0.45	0.56	0.35	0.00	0.06	1.10	46.49
9G-DE	1.39	0.02	77.82	11.25	1.28	0.17	0.40	0.94	0.01	0.03	6.92	11.97
Av-non-coal	2.32	0.12	65.49	16.76	1.84	0.35	0.59	0.92	0.03	0.08	4.34	18.69
Chinese coala	0.19	0.16	8.47	5.98	4.85	1.23	0.22	0.33	0.02	0.09	1.42	18.12
NASCb	3.99	1.15	64.80	16.90	5.70	3.56	2.85	0.78	0.06	0.11	3.83	21.67

Av, average percentage of coal samples; Av-non-coal, average percentage of roof and floor samples; Av-D-R*, weighted average of the M9 coals (from R-D-1 to R-D-15) were from Wu et al. [32]; Chinese coal^a, from Dai et al. [15]; NASC^b, from Gromet et al. [58].

4.3.2. Trace Elements

The comparison results of elemental abundances between samples from the Renjiazhuang Mining District and world hard coal are shown in Figure 4 and

Table 4. Concentration results of trace elements and REY in the samples (μg/g).

Samples	Li	Be	Sc	V	Cr	Co	Ni	Cu	Zn	Ga	Rb	Sr	Ta	W	Tl	Hf
3G-DN	42.40	3.67	12.90	78.00	88.10	5.29	12.80	16.00	132.00	26.70	133.00	71.00	2.36	2.41	0.97	11.10
T-H-1	49.30	2.43	3.66	18.80	8.86	3.20	6.29	7.48	7.22	13.68	3.46	68.39	0.78	1.11	0.14	4.97
T-H-M	23.60	6.92	3.99	12.00	5.70	17.60	19.10	7.57	7.00	13.40	2.77	107.00	0.32	0.74	0.15	2.00
T-H-2	73.70	4.80	8.78	24.00	9.31	3.04	6.47	8.49	7.38	16.40	3.56	51.00	0.97	1.00	0.085	6.71
3G-DE	71.20	4.20	13.40	89.80	71.50	3.47	7.01	17.80	81.30	28.57	112.00	87.99	1.87	2.09	0.76	10.60
5G-DN	28.30	1.87	12.50	80.90	76.00	8.70	11.30	14.40	97.10	21.40	92.40	96.00	1.48	1.37	0.57	14.90
F-H-M	94.10	4.67	8.06	28.30	9.37	1.56	3.66	14.00	10.90	15.90	0.85	35.80	1.5	1.00	0.057	4.61
F-H-2	151.00	5.17	10.70	34.20	12.60	1.20	3.68	15.80	13.60	25.20	2.37	44.40	2.4	1.67	0.053	6.12
5G-DE	66.40	4.64	16.30	94.90	60.80	6.82	9.59	22.80	121.00	31.00	169.00	124.00	2.02	2.70	0.95	9.90
9G-DN	84.20	2.90	19.50	157.00	84.90	19.00	37.00	29.50	126.00	31.90	130.00	169.00	1.57	1.90	0.67	7.45
N-H-1	181.00	1.70	5.59	17.30	7.56	1.62	2.19	10.70	13.20	19.60	0.57	124.50	1.57	2.97	0.12	4.38
N-H-2	174.00	1.34	7.36	18.50	10.30	1.24	2.68	11.50	9.86	15.20	2.55	247.00	1.46	2.31	0.067	5.62
Av-D-R*	98.47	1.45	4.35	10.97	3.66	1.03	1.58	12.27	3.89	9.57	0.84	171.67	0.43	0.65	nd	nd
9G-DE	47.24	0.72	6.28	34.60	89.10	9.94	21.80	9.59	60.60	12.70	46.20	86.50	1.4	0.89	0.35	15.00
AV-C	106.67	3.86	6.88	21.87	9.10	4.21	6.30	10.79	9.88	17.05	2.30	96.87	1.3	1.50	0.10	4.90
AV-P	56.62	3.00	13.48	89.20	78.40	8.87	16.58	18.35	103.00	25.38	113.77	105.75	1.80	1.90	0.70	11.50
*China coala	31.80	2.11	4.38	35.10	15.40	7.08	13.70	17.50	41.40	6.55	9.25	140.00	0.62	1.08	0.50	3.71
*World coalb	12.00	1.60	3.70	29.00	16.00	5.10	13.00	16.00	23.00	5.80	8.30	110.00	0.28	1.10	0.63	1.20
*World claysc	54.00	3.00	15.00	120.00	110.00	19.00	49.00	36.00	89.00	16.00	133.00	240.00	110.00	2.60	0.47	120.00
Samples	Nb	Mo	Cd	In	U	Sb	Cs	Zr	Pb	F	As	Bi	Th	Ba	REY
3G-DN	29.40	1.83	0.220	0.074	4.59	0.21	8.32	351.00	29.50	201.57	0.00	0.30	15.40	428.00	343.53
T-H-1	12.20	0.96	0.020	0.049	2.37	0.30	0.26	163.00	17.40	203.09	1.02	0.21	8.86	75.90	101.89
T-H-M	4.58	1.18	0.030	0.020	1.17	0.33	0.12	70.40	8.22	78.20	0.00	0.12	3.18	164.00	61.06
T-H-2	14.60	0.71	0.023	0.060	2.71	0.30	0.26	234.00	21.20	184.47	0.00	0.30	12.80	37.60	125.36
3G-DE	21.50	1.11	0.110	0.071	7.10	0.21	7.32	384.00	25.40	370.22	0.00	0.32	17.20	986.92	404.41
5G-DN	21.10	1.54	0.150	0.050	3.05	0.14	2.00	558.00	20.54	514.06	1.00	0.09	17.00	524.00	305.40
F-H-M	17.40	1.51	0.048	0.051	3.70	0.17	0.05	155.00	20.30	105.79	0.00	0.28	12.40	12.90	133.61
F-H-2	34.00	2.02	0.052	0.074	4.37	0.19	0.05	194.00	26.30	127.03	0.00	0.40	18.80	20.90	144.55
5G-DE	22.40	0.59	0.190	0.085	6.47	0.16	8.30	333.00	30.90	738.88	1.01	0.52	22.60	661.00	377.44
9G-DN	18.90	1.11	0.180	0.084	4.64	0.45	9.40	244.00	30.20	293.76	12.07	0.64	19.20	341.00	334.71
N-H-1	17.40	3.82	0.068	0.048	4.19	0.31	0.07	162.00	26.80	151.42	3.03	0.56	12.98	77.26	112.91
N-H-2	16.70	3.41	0.061	0.053	3.57	0.23	0.08	169.00	23.80	135.14	1.01	0.58	16.10	71.20	110.64
Av-D-R*	4.13	1.66	0.04	nd	1.98	nd	0.24	nd	17.64	nd	nd	0.30	7.61	62.27	100.3
9G-DE	16.50	1.91	0.039	0.032	2.53	0.11	1.34	550.00	12.80	203.12	1.01	0.11	11.40	269.00	184.92
AV-C	16.70	1.94	0.043	0.051	3.15	0.26	0.13	163.91	20.57	140.73	0.72	0.35	12.16	65.68	112.86
AV-P	21.63	1.35	0.148	0.066	4.73	0.21	6.11	403.33	24.89	386.93	2.51	0.33	17.13	534.99	325.07
*China coala	9.44	3.08	0.25	0.05	2.43	0.84	1.13	89.50	15.10	130.00	3.79	0.79	5.84	159.00	135.89
*World coalb	3.70	2.20	0.22	0.03	2.40	0.92	1.10	36.00	7.80	88.00	8.30	0.97	3.30	150.00	68.47
*World claysc	11.00	1.60	0.91	0.063	4.30	1.30	13.00	190.00	14.00	610.00	9.30	0.38	4.30	460.00	169.00

*China coal^a, from Dai et al. [57]; *World coal^b, from Ketris and Yudovich [60]; *World clays^c, from Grigoriev [61]; Av-D-R*, weighted average of the M9 coals (from R-D-1 to R-D-15) are from Wu et al. [32].

. Moreover, Dai et al. proposed the classification level of CC value (CC, concentration coefficient, refers to the concentration ratio between the trace elements in the collected samples and the average value of world hard coal) [59].

The M3 coals of the Renjiazhuang Mining District (2.0 < CC < 5.0) were slightly enriched in lithium, Be, Ga, Ta, Hf, Nb, Zr, Pb, and Th. Zn, Sb, Cr, Cu, Cd, Rb, Tl, Cs, As, Mo, and Bi were relatively depleted compared to world hard coal, and the concentrations of Sc, Ni, Sr, W, In, V, U, F, Ba, Co, and REY (0.5 < CC < 2.0) were close to the average concentration of world hard coal (Figure 4a).

The M5 coals were significantly enriched in Lithium (CC = 10.21), enriched in Ta (CC = 6.96) and Nb (CC = 6.95) compared with world hard coal, and weakly enriched in Hf, Sc, Ga, Be, In, Zr, Pb, Th, and REY (2.0 < CC < 5.0). The other elements were normal (0.5 < CC < 2.0) or lacking (CC < 0.5) (Figure 4b).

The M9 coals were significantly enriched in Li (CC = 14.79), enriched in Ta (CC = 5.41) compared with world hard coal, and weakly enriched in Ga, W, Hf, Nb, Zr, Pb, and Th (2.0 < CC < 5.0). Co, Ni, Rb, Tl, Cd, Sb, Cs, As, and Ba were depleted (CC < 0.5). The other elements were normal (0.5 < CC < 2) (Figure 4c).

In the roof and floor samples, Th (CC = 3.98) was slightly enriched. Co, Ni, Sr, Ta, Hf, Cd, and Cs (CC < 0.5) were deficient, and the remaining elements were normal (Figure 4d).

4.3.3. Distribution Pattern of Rare Earth Elements

A three-fold geochemical classification of REY was used [62]: heavy REY (Er, Ho, Yb, Tm, and Lu), medium REY (Gd, Eu, Dy, Tb, and Y) and light REY (Ce, La, Nd, Pr, and Sm). To classify the enrichment types of REY distribution, the concentration of REY was normalized to the upper continental crust [8,63]. The REY enrichment type was divided into three enrichment types: heavy REY enrichment (La_N/Lu_N < 1), medium REY enrichment (Gd_N/Lu_N > 1, La_N/Sm_N < 1), and light REY enrichment (La_N/Lu_N > 1) [62].

The concentration of total REY in the Upper Paleozoic coal samples varied from 61.06 μg/g to 144.55 μg/g (112.86 μg/g on average) (Table 4). It was lower than the average value of Chinese coals (135.89 μg/g), but higher than the average value of world hard coal (68.47 μg/g). Among them, the concentration of rare earth elements in the M3 coal samples varied from 61.06 μg/g to 125.36 μg/g (96.10 μg/g on average), higher than the world’s average hard coal, but lower than Chinese coal. M5 coals ranged from 133.61 μg/g to 144.55 μg/g (139.08 μg/g on average), and their REY concentration was higher than that of world hard coal and the average Chinese coal. M9 coals were from 110.64 μg/g to 112.91 μg/g (111.78 μg/g on average). The REY concentration was higher than the world’s average hard coal and lower than that of Chinese coal. The concentration of REY in the floor and roof samples increased from 184.92 μg/g to 404.41 μg/g (325.07 μg/g on average) (Table 4).

Previous studies have revealed that when the Ba/Eu ratio is lower than 1000, the results of Eu content in the samples are relatively accurate [52,64]. In this paper, the Ba/Eu ratio in the coal samples was 19.25–512.5, and the Ba/Eu ratio in the non-coal samples was 165.53–387.03. Meanwhile, the Eu_N/Eu_N* of all samples was lower than 1.00 (0.41–0.64,

Table 5. Concentration results of yttrium and rare earth elements in samples from Renjiazhuang Mining District (μg/g).

Samples	REY	LREE	MREE	HREE	L/M	L/H	M/H	LaN/LuN	LaN/SmN	GdN/LuN	EuN/EuN*	CeN/CeN*	GdN/GdN*	YN/YN*
3G-DN	343.53	263.10	66.39	14.04	3.96	18.74	4.73	9.01	4.00	1.47	0.41	0.80	0.92	0.94
T-H-1	101.89	72.77	23.98	5.13	3.03	14.18	4.67	6.05	3.50	1.16	0.46	0.94	0.86	0.95
T-H-M	61.06	40.34	17.46	3.26	2.31	12.37	5.36	5.40	3.39	1.25	0.52	0.91	0.90	1.04
T-H-2	125.36	83.23	34.87	7.26	2.39	11.46	4.80	4.94	3.48	0.99	0.43	0.92	0.84	1.01
Av	96.10	65.45	25.44	5.22	2.58	12.67	4.94	5.46	3.46	1.13	0.47	0.92	0.87	1.00
3G-DE	404.41	327.59	66.68	10.15	4.91	32.27	6.57	12.63	3.17	2.42	0.58	0.99	0.90	1.18
5G-DN	305.40	251.85	45.13	8.42	5.58	29.91	5.36	14.42	5.04	1.92	0.64	0.89	0.97	1.13
F-H-M	133.61	95.27	32.40	5.94	2.94	16.04	5.45	7.46	4.06	1.33	0.55	0.94	0.90	1.07
F-H-2	144.55	99.67	37.99	6.89	2.62	14.47	5.51	6.93	4.37	1.18	0.54	0.93	0.89	1.11
Av	139.08	97.47	35.20	6.42	2.78	15.26	5.48	7.20	4.22	1.26	0.55	0.94	0.90	1.09
5G-DE	377.44	298.50	66.52	12.42	4.49	24.03	5.36	10.73	3.90	1.82	0.58	0.91	0.94	1.05
9G-DN	334.71	260.26	61.14	13.31	4.26	19.55	4.59	8.09	3.79	1.32	0.64	0.96	0.86	0.92
N-H-1	112.91	82.50	25.52	4.88	3.23	16.91	5.23	7.26	3.93	1.26	0.53	0.92	0.86	1.08
N-H-2	110.64	79.46	25.52	5.66	3.11	14.04	4.51	6.90	4.61	1.10	0.54	0.90	0.87	0.90
Av-D-R*	100.30	68.96	26.06	5.28	2.82	14.11	4.92	6.37	4.41	0.87	0.55	0.95	0.87	1.20
Av	111.78	80.98	25.52	5.27	3.17	15.48	4.87	7.08	4.27	1.18	0.54	0.91	0.87	0.99
9G-DE	184.92	158.14	22.41	4.37	7.06	36.19	5.13	14.80	4.61	2.06	0.48	0.89	0.99	1.06
Av-C	112.86	79.04	28.25	5.57	2.80	14.18	5.07	6.42	3.93	1.17	0.51	0.92	0.87	1.03
Av-P	325.07	259.91	54.71	10.45	4.75	24.87	5.23	10.89	3.95	1.77	0.56	0.91	0.92	1.03

Av-D-R*, weighted average of the M9 coals (from R-D-1 to R-D-15) are from Wu et al. [32].

). Therefore, the Eu content in this study was not disturbed by the Ba concentration.

The La_N/Lu_N of coal samples from the Renjiazhuang Mining District varied from 4.94 to 7.46 (6.42 on average), while the La_N/Lu_N of non-coal samples ranged from 8.09 to 14.80 (11.61 on average). In comparison to the REY enrichment in the upper crust, it can be observed from distribution patterns of REY and anomalies in all samples that non-coal and coal samples exhibit light REY enrichment types and negative Eu anomalies. The concentration of light rare earth enrichment in the M5 coals was greater than that in the M3 coals and the M9 coals (Figure 5a). The enrichment concentration of L-REY in the roof and floor samples of the M5 coals and M9 coals was less than that in the non-coal samples of the M3 coals (Figure 5b).

4.4. Minerals and Occurrence Modes

The mineralogy of coal was observed using optical microscopy and SEM-EDX.

Table 6. The results of mineralogical composition in coal samples.

Sample	Mineral Compositions (wt%)
T-H-1	Quartz 3.5; Dolomite 9.3; Pyrite 7.3; Kaolinite 79.9
T-H-M	Quartz 1.4; Feldspar 0.6; Calcite 33.3; Dolomite 21.9; Pyrite 1.8; Kaolinite 41.0
T-H-2	Quartz 2.2; Dolomite 3.7; Kaolinite 94.1
F-H-M	Dolomite 3.1; Kaolinite 96.9
F-H-2	Dolomite 1.1; Kaolinite 98.9
N-H-1	Dolomite 8.4; Pyrite 1.8; Kaolinite 89.8
N-H-2	Dolomite 14.0; Pyrite 1.1; Kaolinite 84.9

shows the main mineral components from coal samples quantitatively analyzed by XRD. The coal samples were mainly composed of kaolinite, dolomite, quartz, feldspar, pyrite, and calcite, but clay minerals are the main components (Figure 6). Small amounts of monazite, zircon, iron dolomite, chalcopyrite, albite, fluorite, siderite, galena, barite, boehmite, and rutile are locally visible.

4.4.1. Clay Minerals

Clay minerals in coal were always associated with organic matter, mostly filling in the fractures and cell lumens (Figure 3a,c,d and Figure 6g), indicating an authigenic origin [13]. From the SEM images, it can be seen that kaolinite occurs in the form of strip-shaped (Figure 7a), cell-filling (Figure 7b), flaky (Figure 7c), dispersed (Figure 7d), lens-like (Figure 7e), and irregular block-like kaolinite (Figure 7f), which may indicate its origin in terrestrial environments [65,66]. Kaolinite also appeared in the form of flocculent (Figure 8c,f), and flocculation of clay minerals may be caused by acidity or saline water in swampy environments [67].

4.4.2. Al-Oxyhydroxide/Hydroxide Minerals

The SEM-EDX analysis indicates that diaspore and boehmite are the major aluminum hydroxide minerals in coal. The latter mainly appears in block-form in a kaolinite matrix (Figure 8c,d,f). The diaspore (Figure 8a,b), boehmite (Figure 8c,d) and brannerite (Figure 8c,e) in the coal samples are analyzed to be formed from hydrothermal fluids (low-temperature) or colloidal aluminous gels produced by weathered bauxite in the depositional region [22,66,67,68,69].

4.4.3. Pyrite

The pyrite appeared in the forms of fissure-filling (Figure 6a), cell-filling (Figure 6b), blocky (Figure 6c), framboidal (Figure 6e), and discrete granular pyrite in coals (Figure 6d,f). The framboidal and disseminated pyrite were associated with authigenic and syngenetic origins [32,65,70]. The fracture filled with pyrite indicates the origin of epigenetics, which may be influenced by rich sulfate solutions during the epigenetic or late diagenesis stages [32,71,72].

4.4.4. Calcite

The calcite mainly appeared in fracture-filling (Figure 3a and Figure 6d), irregular plate (block) (Figure 6h and Figure 9a,d), and cell-filling calcite (Figure 9c), indicating authigenic origins [13,67]. The calcite in coal may have originated from carbonate solutions rich in Ca [71].

4.4.5. Dolomite

The dolomite in the coal seams was associated with calcite occurring parallel to the massive forms (Figure 9a). The fractures and cell lumens of coal were filled with layered dolomite and blocky forms of dolomite (Figure 9a,b), which were mainly influenced by sedimentary microfacies and diagenesis epigenesis [65,67,73,74].

4.4.6. Quartz

The quartz occurred as irregular massive (Figure 6i) and granular quartz (Figure 9d), mostly surrounded by kaolinite, indicating its syngenetic clastic origin. In addition, SiO₂ may originate from the water infiltration overlying the peat, or from the water flowing into the peat mires [24,75].

4.4.7. Other Minerals

The ankerite appeared in aggregation (Figure 10a,b), and the process of microbial-mediated formation of primary dolomite helped iron enter the dolomite lattice, increasing the iron content [15,24]. The euhedral zircon (Figure 10c,d) and monazite (Figure 10i,j) were wrapped in kaolinite crystals in granular form, indicating an authigenic origin or pyroclastic source [68,76]. The cell lumens were filled with agglomerated fluorite (Figure 10e,f) and flaky albite arranged in different directions (Figure 10g,h), revealing an authigenic origin and the influence of multistage tectonic activities [20,21,77].

The irregular massive chalcopyrite (2–10 μm) (Figure 10k,l) and barite (2–15 μm) (Figure 10m,n) were embedded in the coal, indicating that these minerals were replaced by epigenesis and developed in syngenesis [1,32].

The siderite appeared in the form of fine-grained crystalline aggregates (Figure 10o,p), while the irregular granular galena (Figure 10q,r) and agglomerated clausthalite (Figure 10s,t) were dispersed and embedded in the coal samples. Additionally, the veined kaolinite, granular pyrite, and galena were mixed and filled in the pores and fractures of the coal that would indicate an epigenetic origin [68,76].

5. Discussion

The elemental enrichment in coal is influenced by various factors including the marine environment, source rock, hydrothermal fluid, volcanic ash and groundwater, etc. [20,57]. According to the mineralogy and geochemistry of the coals, there are no late magmatic intrusions in the Carboniferous-Permian system in the study area [24]. The two major controlling factors for the elemental enrichment in the coal and non-coal samples in the Renjiazhuang Mining District are the paleopeat-forming environments and the sediment source region.

5.1. Sedimental Source

The Al₂O₃/TiO₂ ratio can serve as important evidence for identifying the source area of sedimentary rocks and indicating whether a specific element has an organic matter affinity [59,78,79,80]. In addition, due to the possibility of changes or the leaching of Al and Ti during both deep and epigenetic processes, it is essential to cautiously use this ratio for coal. For example, Permana et al. and Qin et al. demonstrated that fractures are filled with kaolinite [20,81]. Meanwhile, the aluminum-rich solution invades the South Walker Creek coal seams after sedimentation, so that aluminum can be introduced into the coal from external sources such as a hydrothermal solution. Sometimes, re-precipitation of the Ti element in Chinese coal seams and the authigenic kaolinite in some coal indicates remobilized Al in some coals [82]. However, deposited kaolinite and re-precipitated titanium-bearing minerals were not found in the samples of the study area. Therefore, the Al₂O₃/TiO₂ ratio may be taken as a source indicator for these coals [20].

The Al₂O₃/TiO₂ ratios of sedimentary rocks derived from mafic, medium, and the range of felsic igneous rocks was 3–8, 8–21, and 21–70, respectively [20,78]. The Al₂O₃/TiO₂ ratios of all the coals in the Renjiazhuang Mining District ranged from 27.30 to 53.99 (40.98 on average), indicating that the source rocks of these samples are felsic igneous rocks. The Al₂O₃/TiO₂ ratio of the roof and floor samples ranged from 11.97 to 25.75, indicating that the source rocks of the non-coal samples are felsic and intermediate igneous rocks. Meanwhile, the testing results of the coal samples provided by Ji et al. show that the Al₂O₃/TiO₂ ratio of the M5 coals is from 9.18 to 59.01 (27.55 on average), while the M9 coals are from 20.61 to 57.33 (36.60 on average). The average Al₂O₃/TiO₂ values of the coal seams are mainly from 21 to 70, verifying that the material source of the coal samples is mainly felsic igneous rocks (Figure 11a) [27]. The relationship between Nb/Y and Zr × 0.0001/TiO₂ also supports the above results (Figure 11b), indicating that the source rocks of igneous rocks in the Renjiazhuang Mining District are mainly trachyandesite and rhyolite/dacite. As pointed out by Sun et al. [16], between the Late Permian and the end of the Early Permian, the Central Asian Orogenic Belt erupted with medium- to high-grade felsic lava, which may have provided an elemental source for coal seams. Based on the Al₂O₃/SiO₂ ratios of the samples (representing felsic igneous rocks) in the Renjiazhuang Mining District, there was a weak difference in the chemical composition of the parent rocks, which may be due to differences in the weathering degree of the sedimentary source rocks.

The sediment source area of the Renjiazhuang Mining District can be further inferred from REY and trace element combinations [1,51,62]. Meanwhile, the higher concentrations of REY in the roof and floor samples compared to the coal samples reflected that REY mainly existed in inorganic minerals [24,32]. The REY maps of M3 coals, M5 coals, and M9 coals in the Renjiazhuang Mining District were different from those in the Hedong and Ningwu coalfields [13,83], but similar to those in the Datong coalfield [18], exhibiting L-REY enrichment and smaller Eu content. REY went through differentiation during weathering and migration, with HREE preferentially entering the aqueous phase and LREE enriched in the residual material [52]. Moreover, the trace element combinations of M3 coals, M5 coals, and M9 coals in the Renjiazhuang Mining District were basically consistent with previous studies on the Ningdong and Longdong coalfields [66], indicating their similar provenance. The Moyite in the Yinshan Ancient Land and the Alxa Block in the northwest controlled the terrestrial input of coal-bearing stratum the northwest edge of the Ordos Basin [36].

5.2. Paleopeat-Forming Environments

5.2.1. Depositional Environment

Information about the paleoenvironment can be indicated by geochemistry in coal, combined with the petrological, mineralogical, and maceral dates [83]. In addition, macerals and coal facies parameters, such as the GI, TPI, VI, and GWI, can indicate hydrodynamic conditions, depositional environment, and coal-forming plants [83,84,85,86].

TPI, as a proportion of woody plants during coal-forming plants, indicates the degree of fragmentation in plant tissues [87]. Due to weak microbial activity, a low pH environment can be beneficial for plant preservation, so that a high TPI value represents weak mechanical fragmentation of plant tissue, rapid peat accumulation, and good plant tissue maintenance [26,48]. A low GI value (GI < 2) presents the dry basin margin, while sediments from the wet mire have a high GI value (GI > 2). In addition, both TPI and GI can reflect the equilibrium ratio of peat accumulation and plant growth versus the rise in groundwater level [49]. The content of ash yield is an essential factor that determines the paleoenvironment using this model. Low TPI and high GI values indicate that peat accumulates in largely herbaceous, constantly humid highland swamps with low ash content, indicating that peat has a higher ash content from telmatic to limnitelmatic. High TPI and low GI values indicate that peat originates from dry periods with high ash content or occasionally experiences forest swamps due to low ash content [88].

The sedimentary environment was determined using the crossplot of GI and TPI in coal [50]. The TPI and GI of the Upper Paleozoic coal were 0.49–3.41 (1.46 on average) and 0.23–6.10 (2.01 on average), respectively (Table 1). In the intersection diagram of TPI and GI (Figure 12a), M3 coals and M5 coals of the Early Permian in the Renjiazhuang Mining District were basically located in the mainland region. Among them, M3 coals were distributed in the lower delta plain and terrestrial (lacustrine) facies, while M5 coals were distributed in arid forest swamp areas. M9 coals of the Taiyuan Formation (the Pennsylvanian) were mainly distributed in the lower delta plain, upper delta plain, and arid forest swamp areas, reflecting that the coal samples of the Pennsylvanian are gradually formed in the humid forest swamps and the transition from continental sedimentation to the lower delta plain.

The GWI can indicate mineral content and the water level and the degree of groundwater during peat accumulament control over the peat swamp [49,50]. A GWI value of >5 reveals that the mire surface is flooded or prone to siliceous debris flooding, whereas a GWI value of 3–5 suggests rheological mires in a lacustrine environment. Values of 0.5–3 are described as mesotrophy, and <0.5 signifies ombrotrophic from mesotrophic fens to elevated mires [49]. The VI can reflect the types of different coal-forming plants. To be specific, a high VI value (VI > 3) indicates a large amount of structured tree-derived materials and more terrestrial environments. In addition, a low VI value (VI < 3) reveals a smaller amount of preserved structured materials and is dominated by herbaceous or marginal aquatic plants in the more limnic environments [88].

The VI in the Upper Paleozoic coal was 0.48–3.20 (1.41 on average). Among them, the VI of the M3 coal ranged from 0.48 to 0.84 (0.71 on average). The VI of the M5 coal varied from 1.86 to 2.65 (2.26 on average). The VI of the M9 coal ranged from 0.66 to 3.20 (1.43 on average). The plants of coal-forming in the M5 coal were mainly woody plants, while the plants of coal-forming in the M3 and M9 coal were mainly herbaceous plants. The GWI of the Upper Paleozoic coal ranged from 0 to 1.71 (0.40 on average). Among them, the GWI of the M3 coal varied from 0.17 to 0.66 (0.46 on average). The GWI of the M5 coal was 1.24–1.71 (1.48 on average). The GWI of the M9 coal ranged from 0 to 0.90 (0.26 on average). The GWI of the M5 coal was greater than 1, indicating that the peat swamps are continuously stable in terms of groundwater supply. That is to say, the tissue of coal-forming plants in the M5 coal has been well preserved. The coal-forming swamps of the M3 coal and M9 coal are unstable in terms of groundwater supply. The slow accumulation of peat and the activity of bacteria in coal make the gelatinization of plant residues low.

5.2.2. Oxidation–Reduction Environment

The vertical deposition process of the sedimentary environment can be indicated by variations in A_d, S_t,d, Ce_N/Ce_N*, Sr/Ba, and Th/U. High Th/U and Ce_N/Ce_N* values represent the oxidizing environment, high Sr/Ba value reflects marine or salt-lake sedimentation, and high A_d, and S_t,d values represent strong hydrodynamic conditions and reduction environment, respectively [71,89,90].

The A_d value of the M3 coal in Renjiazhuang Mining District varied from 21.08% to 33.07%, identifying that the M3 coal is formed in a weak hydrodynamic environment. The Ce_N/Ce_N* value ranged from 0.91 to 0.94, indicating the depth of cover water fluctuations relatively little and terrestrial supply is continuous. The Th/U value ranged from 2.72 to 4.72, indicating that the M3 coal is in a hypoxic environment. Moreover, the M3 coal was formed in a brackish water to saltwater environment (Sr/Ba = 0.65–1.36), with ash yield ranging from 13.95% to 26.27% (Figure 13). The V/I of the M3 coal ranged from 1.26 to 3.46 (2.09 on average), and the S_t,d value was between 0.47% and 0.67% (0.58 on average), and the high vitrinite content in the coal indicates that the coal seam is between weak oxidation and reduction (Table 1). As claimed by Murray et al. [91], in the high seas or open seas, there is a serious shortage of Ce, while in marginal seas, shallow seas, or land-enclosed sea areas, Ce is basically normal with minimal changes. For coal affected by seawater intrusion, the content of Ce in coal may be significantly associated with the removal process involving organic matter and metallic oxide (Fe, Mn) [92]. As mentioned earlier, the M3 coal seam is formed under the continental (lacustrine) and lower delta plain sedimentary systems, which are not open in the sea and have shallow depths. There is no significant negative anomaly in Ce in M3 coal, further reflecting that the sedimentary environment in M3 coal is between weak oxidation and reduction.

The M5 coal of the Renjiazhuang Mining District was formed in low oxygen (Ce_N/Ce_N* = 0.93–0.94, Th/U = 3.35–4.30), weak hydrodynamic conditions (A_d = 21.08%–33.07%), and saline water environments (Sr/Ba = 2.12–2.78) (Figure 13). The V/I of the M5 coal ranged from 0.21 to 0.37, and the low content of vitrinite in coal is the reason for the significant impact of sea-level decline compared with other coal seams, reflecting that the coal seam is in an oxidizing weakly–oxidizing dry environment. Ji et al. tested 14 coal samples from the M5 coal and found that the Sr/Ba ratio varies from 0.02 to 29.92 (4.94 on average) [27]. The Th/U value ranged from 2.14 to 4.76 (3.62 on average); the S_t,d value varied from 0.53% to 0.57% (0.55% on average); Ce_N/Ce_N* = 0.82–1.05 (0.95 on average), further reflecting that the M5 coal is formed in a dry environment with weak oxidation and reduction.

The M9 coal was formed in low oxygen (Ce_N/Ce_N* = 0.90–0.92, Th/U = 3.10–4.51), moderate hydrodynamic conditions (A_d = 36.48%–38.82%), and a saline water environment (Sr/Ba = 1.61–3.47) (Figure 13). The V/I of the M9 coal ranged from 0.50 to 4.88 (1.94 on average), and the content of vitrinite in coal indicates that the M9 coal is between weak oxidation and reduction. The change in Eu_N/Eu*_N was relatively small, reflecting that terrestrial supply is relatively stable and the depth of coverage water remains basically unchanged. According to Ji et al. [27], the Sr/Ba value is 0.77–19.07 (4.63 on average), the Th/U value is 0.35–5.13 (3.16 on average), S_t,d = 2.20%–2.33% (2.27 on average), and Ce_N/Ce_N* = 0.86–1.06 (0.98 on average), further reflecting that the M9 coal is formed in a seawater environment with reduction and weak oxidation.

5.3. Occurrence Mode of Trace Elements

The existence and occurrence modes of trace elements in coal have been systematically analyzed [59,93,94]. Moreover, several methods have been used to identify the occurrence status of elements, in which the statistical method is the most common. When combined with direct methods such as SEM-EDX, statistical methods are more effective in exploring their existence form in coal [13,60,67,94,95,96].

The correlation coefficient (r) was used to conduct a preliminary study on the occurrence patterns of trace elements in M3 and M5 and M9 coal samples. The element affinity obtained based on the correlation coefficient between the concentration of various elements in coal and ash yield indicates their mode of existence in the coal samples. Many elements, such as SiO₂, TiO₂, Al₂O₃, Li, Ta, Hf, Nb, Zr, Pb, and Th, are closely associated with ash yield (

Table 7. Identification of element affinity using correlation coefficients.

Correlation with Ash Yield
Rash: from 0.70 to 1.00 SiO2 (0.97), TiO2 (0.78), Al2O3 (0.94), Li (0.82), Ta (0.82), Hf (0.81), Nb (0.81), Zr (0.77), Pb (0.77), Th (0.87)
Rash: from 0.40 to 0.69
Sc (0.69), V (0.45), Cu (0.60), Ga (0.66), U (0.54),
Rash: from −0.50 to +0.40
Fe2O3 (0.11), MnO2 (−0.26), MgO (−0.14), CaO (−0.29), Na2O (0.09), K2O (0.23), P2O5 (0.17), Be (0.08), Cr (0.20), Co (−0.05), Ni (0.05), Zn (0.19), Rb (0.23), Sr (0.06), Tl (0.04), Cs (0.26), Ba (0.27)
Correlation coefficients between selected pairs of elements Li-Al2O3 (0.90), Li-SiO2 (0.75), Ta-Al2O3 (0.85), Ta-SiO2 (0.79), Hf-Al2O3 (0.80), Hf-SiO2 (0.77), Nb-Al2O3 (0.81), Nb-SiO2 (0.76), Zr-Al2O3 (0.72), Zr-SiO2 (0.74), Pb-Al2O3 (0.75), Pb-SiO2 (0.71), Th-Al2O3 (0.90), Th-SiO2 (0.83), Ga-SiO2 (0.74), Ga-TiO2 (0.57), Li-TiO2 (0.64)

). Their correlation coefficient with ash yield (r > 0.70) indicates inorganic affinity. Li, Ta, Hf, Nb, Zr, Pb, and Th elements are positively correlated with SiO₂ and Al₂O₃, indicating their presence in aluminosilicates in coal samples. For example, zirconium and Pb could be observed through SEM-DEX to mainly occur in zircon and galena, and coexist with kaolinite (Figure 10d,r). Furthermore, the higher concentrations of their presence in the floor and roof samples compared to the coal samples reflects that these elements may not only be influenced by material sources, pH values, and fluid dynamics conditions, but also by weathering and epigenetic leaching [18,26,27,32,57,83]. There is a good positive correlation between gallium and SiO₂ (r = 0.74), indicating that Ga is existent in silicate mining areas. Other elements (Co, Be, Cr, Rb, Sr, Ni, Tl, Cs, Zn, and Ba) have inorganic-organic affinity (−0.50 < r < 0.40).

6. Conclusions

(1)

The M3, M5, and M9 coal samples were medium-rank bituminous coals (0.66%–0.85%). The M3 and M5 coal samples were classified as medium-ash (13.95%–33.07%), medium-high volatile (32.14%–39.15%), and low-sulfur (0.47%–0.67%) coals. The M9 coal samples were classified as high-ash (36.48%–38.82%), high-volatile (41.77%–42.33%), and medium-high sulfur (2.20%–2.33%) coals. Inertite was the major maceral with a content of up to 76.26% in M5 coal, and the main macerals of the M3 and M9 coal were vitrinite, with concentrations up to 78.50%. The main minerals of the three coal seams included dolomite, pyrite, feldspar, quartz, kaolinite, and calcite.

(2)

Compared to hard coal in the world, M3 coals were enriched in Be, Ta, Li, Hf, Ga, Nb, Zr, Th, and Pb, M5 coals were enriched in Li, Ta, Nb, Be, Sc, Ga, Hf, In, Zr, Pb, Th, and REY, whereas M9 coals were enriched in Ta, Li, Ga, Pb, W, Hf, Nb, Zr, and Th. The high correlation between Li, Ta, Hf, Nb, Zr, Pb, Th, and ash yield indicates the presence of inorganic affinity. Meanwhile, SiO₂ was positively correlated with Al₂O₃, indicating their presence in aluminosilicates.

(3)

Indicator parameters (GI, TPI, GWI, VI, V/I, Sr/Ba, Th/U, and Ce_N/Ce_N*) suggest that three coal seams are formed in different paleopeat environments: M3 is formed in a terrestrial and lower delta plain facies with weak oxidization–reduction, and M5 coal is formed in a dry forest swamp and terrestrial environment with weak oxidation–reduction, while M9 coal is formed in a seawater environment of humid forest swamps and the transition from the lower delta plain to continental sedimentation with weak oxidization–reduction. The geochemical indicators (TiO₂ and Al₂O₃, Nb/Y and Zr × 0.0001/TiO₂ ratios, and REY distribution patterns) reflect that the sediments of coal and non-coal samples are mainly sourced from felsic igneous rocks.