Major and Rare Earth Element Characteristics of Late Paleozoic Coal in the Southeastern Qinshui Basin: Implications for Depositional Environments and Provenance.

To understand the geochemical characteristics of late Paleozoic coal in the Changzhi and Jincheng mining areas in the southeastern Qinshui Basin, major and rare earth element analyses were conducted through inductively coupled plasma-mass spectrometry (ICP-MS), X-ray fluorescence spectrometry (XRF), and proximate analysis. The results show that the study coals are bituminous A rank and anthracite C rank (R o,ran: 1.6-3.24%) with low-ash, low-moisture, low-volatile, and low- to medium-sulfur characteristics. The main forms of sulfur in the study coals are organic sulfur, followed by pyritic sulfur, only some coals with high sulfur contents in the Taiyuan Formation (SGJ, WTP, FHS) are mainly pyritic sulfur, and the contents of sulfate sulfur are extremely low. The major elements of the late Paleozoic coal in the southeastern Qinshui Basin are mainly SiO2 (4.77%) and Al2O3 (3.64%), followed by Fe2O3 (1.22%), CaO (1.53%), FeO (0.48%), MgO (0.25%), Na2O (0.21%), P2O5 (0.18%), TiO2 (0.15%), and minor K2O (0.04%) (on a whole-coal basis). Through correlation analysis and cluster analysis, the occurrence states of major elements in the Shanxi and Taiyuan Formations are different. The average rare earth elements and yttrium (REY) value in the study area is 88.68 μg/g (on a whole-coal basis). The mean light REY (LREY)-to-heavy REY (HREY) ratio is 26.33. The mean values of δEu, δCe, Y, and Gd are 0.60, 0.99, 1.07, and 1.02, respectively. The Shanxi Formation is dominated by the L-type REY enrichment, while the Taiyuan Formation is dominated by the M-H-type REY enrichment. The fractionation degree of REY in the Taiyuan Formation is lower than that in the Shanxi Formation. Rare earth elements in Shanxi coal mainly occur in clay minerals, and some rare earth elements are adsorbed and enriched by vitrinite. Rare earth elements in Taiyuan coal mainly occur in clay minerals and pyrite, and some rare earth elements occur in inertinite. A warm, humid, low-salinity, oxidizing, and acidic environment was favorable for REY enrichment. The coal-forming environment was weakly oxidizing and reducing, and the paleosalinity of the water was relatively high during late Paleozoic coal deposition in the southeastern Qinshui Basin. The paleotemperature of the Shanxi Formation is higher than that of the Taiyuan Formation. The provenance is mainly from an upper crustal felsic source region, the source rocks are mainly post-Archean sedimentary and calcareous mudstones mixed with some granite and alkaline basalt from the Yinshan Upland, and the tectonic setting of the source area mainly includes island arcs and active continental margins.

Introduction

Coal is a special sedimentary organic rock with unique characteristics of reducibility, adsorption, and high heterogeneity.[1] The study of elemental geochemistry in coal is of great significance for the clean utilization of coal resources, the assessment of the coal accumulation environment, and the comprehensive development of beneficial associated minerals.[2−5] The major elements in coal can be used to calculate different ash compositional parameters, which are often used as indicators of the coal-accumulating paleoenvironment.[6] Rare earth elements in coal have special geochemical characteristics, such as a high degree of homogenization and stable chemical properties, and they are not easily affected by metamorphism, diagenesis, and alteration, which can more accurately reflect the provenance composition and play an important role in determining the tectonic environment of the source rocks.[7,8] On the other hand, when rare earth elements are enriched to a certain extent to reach industrial grade, they have economic benefits.[9−11] The Qinshui basin is one of the most important areas of Carboniferous-Permian coal resources in North China, and it is also an important base of coalbed methane exploration and development in China.[12] Coal-bearing strata are mainly distributed in the Upper Carboniferous Taiyuan Formation and Lower Permian Shanxi Formation, containing mainly anthracite with relatively low ash content.[13] Compared with other coals at home and abroad, most elements in this coal are obviously depleted, mainly due to the low ash yield.[14,15] However, some toxic trace elements significantly enriched after coal mining, combustion, and utilization dominated by chalcophile elements can cause considerable damage to the environment and human health.[16,17] Some critical elements (Ga, Li, and REE) in the Jincheng No. 15 coal seam are considered promising materials for industrial recovery.[17] Although a great number of previous studies have been carried out mainly on the geology of coal seams in the southeastern Qinshui Basin,[18−22] only a few studies have focused on the composition, distribution, and occurrence mode of elements in coal,[23−25] with limited published literature explaining the coal-forming environment and material source based on the sensitivity of the geochemical characteristics of some elements to environmental change.[26,27] This paper took late Paleozoic coal in the Changzhi and Jincheng mining areas in the southeastern Qinshui Basin as an example to analyze the contents, distribution, and occurrence state of major and rare earth elements and discuss the sedimentary environment and provenance characteristics of coal formation from the perspective of coal geochemistry to provide an initial theoretical basis for the clean utilization of coal resources.

Geological Background

The study area is located in Jincheng and Changzhi cities in southeastern Shanxi Province in the southeastern slope zone of the Qinshui Basin. The Qinshui Basin is located in southeastern Shanxi Province; the basin’s shape generally has a long axis that extends in an NNE direction, and the shape shrinks into an ellipse in the middle. The basin is a large-scale north–south synclinal structural basin developed on the Paleozoic basement with undeveloped internal faults and strong marginal structural deformation (Figure ).[28] The Qinshui Basin, which has a coal-bearing area of 29 500 km2 and coal reserves of 510 Gt, is the primary super-large coal-bearing basin in China.[29] Coal-bearing strata include a set of offshore coal-bearing transitional facies deposits developed on the Ordovician weathered crust and are mainly distributed in the upper Carboniferous Taiyuan Formation and the lower Permian Shanxi Formation.[19] There are 10–16 coal seams, of which the No. 1–4 coal seams are in the Shanxi Formation and No. 5–16 coal seams are in the Taiyuan Formation.[30] The No. 3 coal seam is stable and recoverable in the entire area, the No. 15 coal seam is mostly recoverable, and the No. 9 coal seam is partially recoverable.[12] Other coal seams have small thicknesses and unstable development.[12] Among them, the Qinshui Basin of the No. 4 coal seam is missing or extremely thin in most areas, generally less than 0.5 m thick.[12] The No. 12 coal seam is distributed stably in the Qinshui Basin, except Qinshui, Jincheng, Yangcheng, Changzhi, and other mining areas, which can be seen in other areas.[31] The coal seam has a simple structure, and its thickness is not more than 1 m.[31] The No. 3, No. 10, and No. 15 coal seams are the sampling coal seams (Figure C). According to the paleogeographic pattern, coal seams of the Taiyuan Formation mainly formed in barrier lagoon and lower delta plain environments, while coal seams of the Shanxi Formation mainly formed in distributary bay environments on a lower delta plain[32] (Figure ).

Figure 1

(A) Location of the Qinshui Basin in China; (B) locations of sample points in the southeastern Qinshui Basin; and (C) overview of upper Paleozoic strata in the southeastern Qinshui Basin (modified after Zhu[33] and Yan[31]).

Sample and Methods

A total of 26 samples were collected from the Permo-Carboniferous No. 3, No. 10, and No. 15 coal seams in the Jincheng and Changzhi mining areas in the southern Qinshui Basin (Table , Figure A,B); the samples were collected layer by layer according to the section of coal seam from top to bottom with a grooving depth of 5 cm. The coal samples were collected in strict accordance with the Chinese Standard GB/T 482-2008,[34] and the samples were quickly placed into plastic bags to prevent pollution and oxidation; the samples were crushed to prepare composite samples and were then subjected to various analyses.

Table 1

Proximate Analytical Values of Late Paleozoic Coal in the Southeastern Qinshui Basina

sample	horizon	coal seam	coal lithotypes	Mad (%)	Ad (%)	Vdaf (%)	St,d (%)	Sp,d (%)	SS,d (%)	So,d (%)	Ro,ran (%)	V (%)	I (%)	L (%)
DS	Shanxi	3	semibright	0.77	8.57	11.32	0.31	0.04	0.01	0.26	2.27	84.6	8.4	bdl
WZ	Shanxi	3	semibright	0.64	12.64	13.57	0.27	0.04	0.00	0.23	2.01	76.4	17.3	bdl
CP	Shanxi	3	semibright	0.89	8.06	13.10	0.34	0.03	0.00	0.31	2.47	78.4	15.3	bdl
NY	Shanxi	3	bright	0.96	8.10	12.32	0.30	0.04	0.01	0.25	2.27	78.7	15.6	bdl
TA	Shanxi	3	bright	0.95	12.88	11.15	0.30	0.04	0.00	0.26	2.49	84.3	14.2	bdl
JF	Shanxi	3	semidull	0.68	7.95	13.96	0.38	0.08	0.00	0.30	1.85	82.2	12.7	bdl
WY	Shanxi	3	bright	0.87	7.18	9.02	0.35	0.03	0.00	0.32	2.21	88.1	8.7	bdl
GH	Shanxi	3	semibright	0.53	17.30	15.50	0.32	0.03	0.01	0.28	1.60	85.6	13.2	bdl
ZZ	Shanxi	3	semibright	0.71	12.26	13.31	0.40	0.07	0.01	0.32	2.42	78.3	15.6	bdl
HEXH	Shanxi	3	semibright	0.60	17.20	13.57	0.34	0.04	0.01	0.29	1.89	85.5	8.7	bdl
LC	Shanxi	3	bright	0.65	7.49	12.94	0.37	0.03	0.00	0.34	2.33	74.5	16.3	bdl
BF	Shanxi	3	semibright	1.08	6.28	7.60	0.39	0.08	0.01	0.30	2.32	77.5	17.2	bdl
SH	Shanxi	3	semibright	0.53	9.64	5.59	0.34	0.01	0.00	0.33	2.80	84.6	11.8	bdl
NZZ	Shanxi	3	semibright	0.53	9.26	8.08	0.45	0.00	0.00	0.45	2.90	81.3	9.13	0.2
CZ	Shanxi	3	semibright	1.62	10.40	6.82	0.38	0.00	0.00	0.38	1.98	86.5	11.5	bdl
Av. Shanxi				0.80	10.35	11.19	0.35	0.04	0.00	0.31	2.25	81.8	13.0
NH	Taiyuan	10	bright	0.50	18.03	11.13	3.26	0.37	0.02	2.87	2.37	75.3	10.4	bdl
SM	Taiyuan	15	semibright	0.56	9.90	15.17	2.29	1.09	0.00	1.20	2.57	79.5	9.3	bdl
SGJ	Taiyuan	15	semibright	1.40	15.79	13.44	4.40	3.77	0.45	0.18	2.51	78.3	13.2	bdl
TC	Taiyuan	15	bright	0.54	10.63	10.32	3.41	0.02	0.00	3.39	2.87	83.2	7.5	bdl
CLS	Taiyuan	15	semibright	0.77	14.62	14.27	1.30	0.02	0.00	1.28	2.38	82.5	10.7	bdl
WTP	Taiyuan	15	semibright	4.24	3.09	3.96	2.29	1.40	0.11	0.78	2.99	81.8	12.4	bdl
FHS	Taiyuan	15	semibright	3.45	14.23	7.84	1.42	0.69	0.13	0.60	3.24	85.1	9.5	bdl
LDS	Taiyuan	15	bright	0.50	10.61	7.52	2.50	0.30	0.00	2.20	1.97	83.9	13.1	bdl
BC	Taiyuan	15	bright	0.92	5.74	4.82	nd	nd	nd	nd	2.68	89.3	8.6	bdl
SC	Taiyuan	15	semidull	nd	nd	nd	1.17	0.11	0.01	1.05	2.47	78.3	9.5	bdl
Av. Taiyuan				1.43	11.40	9.83	2.45	0.86	0.08	1.51	2.61	81.7	10.4

Mad: moisture (air dry basis); Ad: ash content (dry basis); Vdaf: volatile matter (dry ash-free basis); St,d: total sulfur (dry basis); Sp,d: pyritic sulfur; So,d: organic sulfur; Ss,d: sulfate sulfur; Ro,ran: mean random vitrinite reflectance; V: vitrinite; I: inertinite; L: liptinite; Av.: average; bdl: below detection limit; nd: no data.

Proximate analyses (moisture, ash, and volatiles) of the coal samples were conducted in accordance with ASTM standards (ASTM D3173-11,[35] ASTM D3174-11,[36] and ASTM D3175-11,[37] respectively). The total sulfur analysis was performed according to the ASTM standard of D3177-02,[38] and the sulfur form was analyzed according to ASTM D2492-02.[39]

X-ray fluorescence (XRF) spectrometry (ARL Advant’ XP+, Thermo Fisher, Waltham, MA) was used to determine the major element oxides in the coal ash (burned at 815°C), including SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, and K2O, as described by Dai et al.[40] Rare earth elements of the samples were analyzed by inductively coupled plasma–mass spectrometry (ICP–MS). The samples, which were 50 mg and less than 200 mesh, were weighed and placed into poly(tetrafluoroethylene) dissolution sample bottles; 6 mL of HNO3, 0.5 mL of HClO4, and 2 mL of HF were successively added. After sealing, the samples were placed on a 200 °C hot plate for 48 h and then unpacked and evaporated in the bottle to a colloidal state. Next, 2 mL of aqua regia was added, and all of the samples were again placed on a 200 °C hot plate for 24 h. The final prepared samples were diluted with 50 mL of 5% HNO3 solution without precipitation. The obtained sample solutions were detected by ICP–MS. The experiment was carried out in the State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology (Beijing), and the analysis accuracy was better than 5%.

The analyze the maceral and vitrinite reflectance of coal, the samples were pulverized to 0.1–1 mm to make coal flake. According to ASTM standard D2797/D2797 M-11a (2011),[41] using a ZEISS Axio Scope A1 optical microscope produced by Carl Zeiss, Germany, the microscopic component morphological characteristics of fly ash flakes were observed and quantitatively counted under oil-immersed reflected light, and a Zeiss photometer (220 V, 50 Hz) was used to measure the vitrinite reflectance in coal according to ASTM standard D388-12(2012).[42]

X-ray diffraction analysis (XRD) was performed using an X’Pert PRO MPD automatic Powder X-ray diffractometer (voltage: 40 kV, current: 40 mA, X-ray target: Cu target, measuring angle range: 5–70°, measuring angle accuracy: ±0.003°) to analyze the mineral types and content of coal samples after low-temperature ashing (LTA).

Results

Coal Petrology and Quality Characteristics

The coal rocks in the Qinshui Basin can be divided into bright, semibright, semidull, and dull types. The Shanxi Formation is dominated by semibright types, while the Taiyuan Formation is dominated by semibright and bright types. The ash yields of Shanxi Formation coal range from 6.28 to 17.30% (average of 10.34%), and those of Taiyuan Formation coal are 3.09–18.03% (average of 11.40%), indicating low-ash coal according to Chinese National Standards (GB/T15224.1-2004).[43] The moisture contents of Shanxi Formation coal range from 0.53 to 1.62% (average of 0.80%), and those of Taiyuan Formation coal range from 0.5 to 4.24% (average of 1.43%), indicating low-moisture coal, following MT/T 850-2000.[44] The volatile contents of Shanxi Formation coal are 5.59–15.50% (average of 11.19%), and those of Taiyuan Formation coal are 3.96–15.17% (average of 9.83%) (Table ), indicating that it is a low-volatile bituminous coal to semianthracite according to the ASTM classification (D388-18a, 2018).[45] The contents of random vitrinite reflectance (Ro,ran) in Shanxi Formation and Taiyuan Formation coals are 1.6–3.24% (average of 2.4%), which places the samples in bituminous A rank and anthracite C rank following ISO 11760 (2005).[46]

Total sulfur consists of organic sulfur and inorganic sulfur (including pyritic sulfur and sulfate sulfur), and organic sulfur is a dominant form (except SGJ, WTP, and FHS) (Table ). The sulfur contents of coal in the Shanxi Formation range from 0.27 to 0.45% (average of 0.35%), indicating low-sulfur coal (Table ), and those of Taiyuan Formation coal range from 1.17 to 4.4% (average of 2.45%) (Table ), indicating medium-sulfur coal according to Chinese National Standards (GB/T 15224.2-2004).[47] Although the sulfur content of coal in the Shanxi Formation is low, the organic sulfur content is high, and the distribution is relatively stable, accounting for 73–100% of total sulfur (Table , Figure A). The sulfur in the Taiyuan Formation coals is mainly organic sulfur, accounting for 4–99%, and pyritic sulfur is dominant in some coals with high sulfur contents (SGJ, WTP, FHS) (Table , Figure B). Statistics show that pyritic sulfur in coal tends to increase with increasing total sulfur content, and the correlation coefficient is 0.60 (Table ). On the whole, sulfur in the studied coals in the southeastern Qinshui Basin is mainly organic sulfur (79.50%), followed by pyritic sulfur (18.75%), only some coals with high sulfur content in the Taiyuan Formation are mainly pyritic sulfur (SGJ, WTP, FHS), and the content of sulfate sulfur is extremely low (1.75%), which is consistent with the knowledge obtained by Liu et al.,[48] who studied the distribution law of sulfur in late Paleozoic coal in North China. Low-sulfur coal (St,d < 1%) contains mainly organic sulfur, which mainly comes from original coal-forming plants.[49] Medium- and high-sulfur coal (St,d > 1%) are mostly affected by seawater, are related to iron sulfide, and are mainly dependent on the iron supply in terrigenous areas and sulfate ions brought by seawater.[49] The Taiyuan Formation is more affected by seawater than the Shanxi Formation, and sulfur ions more easily accumulate in the coal seam of the Taiyuan Formation to form sulfide.

Figure 2

Percentage distribution of morphologic sulfur to total sulfur in coal samples from the Qinshui Basin: (A) Shanxi Formation and (B) Taiyuan Formation.

Table 8

Correlation Coefficients of REY with Major Oxides and Proximate Analytical Values in the Taiyuan Formation

	Ad	Vdaf	St,d	Sp,d	SS,d	So,d	REY	LREY	MREY	HREY	V	I
Ad	1.00
Vdaf	0.57	1.00
St,d	0.21	0.16	1.00
Sp,d	0.02	0.09	0.60	1.00
SS,d	0.19	0.00	0.56	0.94	1.00
So,d	0.15	0.06	0.21	–0.66	–0.63	1.00
REY	0.34	0.25	0.55	0.89	0.93	–0.58	1.00
LREY	0.36	0.28	0.56	0.89	0.93	–0.57	1.00	1.00
MREY	0.15	–0.04	0.45	0.83	0.85	–0.59	0.92	0.90	1.00
HREY	–0.03	–0.29	0.49	0.66	0.68	–0.35	0.73	0.70	0.92	1.00
V	–0.39	–0.37	–0.60	–0.33	–0.18	–0.18	–0.36	–0.35	–0.39	–0.42	1.00
I	–0.04	–0.13	0.24	0.75	0.71	–0.69	0.78	0.76	0.95	0.89	–0.36	1.00
LOI	–0.39	–0.52	–0.62	–0.83	–0.84	0.44	–0.90	–0.92	–0.66	–0.41	0.28	–0.47
SiO2	0.37	0.58	0.67	0.67	0.69	–0.20	0.78	0.80	0.51	0.31	–0.23	0.29
Al2O3	0.31	0.78	0.30	0.62	0.54	–0.47	0.68	0.70	0.41	0.09	–0.20	0.31
Fe2O3	0.29	0.04	0.51	0.91	0.99	–0.64	0.92	0.93	0.82	0.62	–0.16	0.67
MgO	–0.12	0.58	–0.23	–0.14	–0.43	0.00	–0.29	–0.28	–0.39	–0.51	–0.26	–0.23
CaO	0.23	0.09	–0.18	–0.54	–0.62	0.50	–0.44	–0.45	–0.26	–0.10	–0.56	–0.14
Na2O	–0.11	0.58	–0.49	–0.24	–0.46	–0.13	–0.33	–0.32	–0.46	–0.63	0.03	–0.29
K2O	–0.28	–0.65	–0.16	–0.04	0.03	–0.12	0.07	0.03	0.44	0.67	–0.02	0.56
TiO2	0.01	0.70	0.44	0.53	0.32	–0.21	0.39	0.42	0.13	–0.09	–0.31	0.07
P2O5	0.39	0.24	–0.57	0.06	0.21	–0.63	0.27	0.28	0.18	–0.11	0.38	0.20
MnO	0.06	–0.47	0.23	0.54	0.56	–0.45	0.41	0.39	0.57	0.56	–0.34	0.60
FeO	0.62	0.85	–0.03	0.27	0.24	–0.35	0.51	0.53	0.32	0.01	–0.32	0.28

All studied coal seams are rich in vitrinite (V) (74.5–89.33%, with an average value of 81.75%), followed by inertinite (I) (7.5–17.3%, with an average value of 11.97%). Due to the high degree of metamorphism, it is difficult to identify liptinite (L) under the microscope (Table ).

Characteristics of Major Elements

Major elements in late Paleozoic coal in the southeastern Qinshui Basin are dominated by SiO2 (4.77%) and Al2O3 (3.64%), followed by Fe2O3 (1.22%), CaO (1.53%), FeO (0.48%), MgO (0.25%), Na2O (0.21%), P2O5 (0.18%), TiO2 (0.15%), and minor K2O (0.04%) (on a whole-coal basis). Compared with the contents of major element oxides in Chinese coals,[40] except for the slight enrichment of P2O5 (CC = 2.01) and the loss of Fe2O3 (CC = 0.25), K2O (CC = 0.24), TiO2 (CC = 0.46), MnO (CC = 0.42), and FeO (CC = 0.42), the remaining MgO (CC = 1.14), CaO (CC = 1.24), and Na2O (CC = 1.29) are within the normal range (Table , Figure ).

Table 2

Content of Major Element Oxides in Late Paleozoic Coals in the Southeastern Qinshui Basin and Analytical Resultsa (On a Whole-Coal Basis)

sample	LOI (%)	SiO2 (%)	Al2O3 (%)	Fe2O3 (%)	MgO (%)	CaO (%)	Na2O (%)	K2O (%)	TiO2 (%)	P2O5 (%)	MnO (%)	FeO (%)	C
DS	90.45	4.21	3.58	0.31	0.14	0.27	0.27	0.04	0.23	0.21	0.00	0.25	0.09
WZ	86.25	3.80	3.16	1.15	0.65	4.25	0.23	0.03	0.14	0.02	0.01	0.88	0.87
CP	89.73	3.12	2.45	0.75	0.26	2.62	0.24	0.03	0.11	0.67	0.01	0.60	0.65
NY	89.37	3.73	3.21	0.81	0.24	1.69	0.22	0.04	0.14	0.28	0.01	0.62	0.40
TA	88.19	4.92	4.21	0.76	0.23	1.05	0.22	0.05	0.19	0.08	0.01	0.49	0.22
JF	86.37	3.29	2.56	2.15	0.49	4.40	0.32	0.03	0.12	0.03	0.01	1.52	1.20
WY	91.61	3.11	2.59	0.38	0.23	1.24	0.20	0.03	0.13	0.04	0.01	0.31	0.32
GH	85.43	5.53	4.63	0.59	0.31	2.95	0.22	0.03	0.17	0.02	0.01	0.47	0.38
ZZ	86.78	4.39	3.60	0.71	0.20	2.40	0.25	0.03	0.11	1.47	0.00	0.51	0.41
HEXH	85.50	6.55	5.84	0.68	0.18	0.36	0.21	0.03	0.20	0.25	<0.004	0.50	0.10
LC	91.49	3.79	3.20	0.52	0.18	0.17	0.25	0.02	0.24	0.04	0.00	0.37	0.13
BF	90.73	3.30	2.81	0.59	0.25	1.46	0.29	0.03	0.16	0.07	0.01	0.42	0.38
average value of the Shanxi group	88.49167	4.145	3.4867	0.7828	0.2802	1.9046	0.243	0.03	0.16	0.27	0.01	0.58	0.43
NH	89.53	4.34	2.94	1.01	0.19	1.70	0.09	0.08	0.07	0.02	0.01	0.31	0.40
SM	86.61	5.62	4.76	0.52	0.43	0.96	0.33	0.03	0.27	0.03	<0.004	0.40	0.18
SGJ	75.84	10.70	5.64	6.85	0.13	0.26	0.07	0.06	0.24	0.04	0.01	0.46	0.44
TC	87.99	6.69	3.38	0.36	0.14	0.58	0.09	0.05	0.14	0.01	0.00	0.14	0.11
CLS	87.10	6.03	4.46	0.65	0.22	0.98	0.21	0.08	0.11	0.06	0.00	0.51	0.18
WTP	90.86	3.37	2.76	1.61	0.15	0.81	0.09	0.11	0.07	0.02	0.01	0.10	0.42
FHS	88.65	4.08	3.29	2.88	0.12	0.55	0.12	0.06	0.05	0.06	0.01	0.26	0.48
average value of the Taiyuan group	86.65	5.83	3.89	1.98	0.19	0.83	0.14	0.07	0.13	0.04	0.01	0.31	0.32
average value in the study area	87.81	4.77	3.64	1.20	0.25	1.53	0.21	0.04	0.15	0.18	0.01	0.48	0.39
Chinese coalb	nd	8.47	5.98	4.85	0.22	1.23	0.16	0.19	0.33	0.09	0.02
concentration coefficient (CC)	nd	0.56	0.61	0.25	1.14	1.24	1.29	0.24	0.46	2.01	0.42

The experiment was carried out in the State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology (Beijing); CC = Average value of samples/Chinese coal; LOI: loss on ignition.

Dai et al.;[40]C is the ash component index: C = (Fe2O3 + CaO + MgO)/(SiO2 + Al2O3).

Figure 3

Concentration coefficients of major elements in late Paleozoic coals in the southeastern Qinshui Basin.

Characteristics of Rare Earth Elements

Rare Earth Element Contents

According to the classification levels of the concentration coefficient (CC), as proposed by Dai et al.,[50] most of the rare earth elements have similar concentrations to the average values for Chinese coals (0.5 < CC < 2) (Table , Figure ), Gd, Ho, Tm, Yb, and Lu are depleted (CC < 0.5).

Table 3

Concentrations of Rare Earth Elements (μg/g, on a Whole-Coal Basis) and Their Comparison with Averages for World Hard Coals in Late Paleozoic Coals in the Southeastern Qinshui Basina

sample	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Y	Ho	Er	Tm	Yb	Lu
DS	33.00	48.60	4.71	15.20	2.39	0.48	2.00	0.39	1.75	9.69	0.29	0.97	0.16	0.75	0.13
WZ	4.81	11.00	1.30	4.92	0.96	0.20	0.78	0.21	0.93	5.35	0.21	0.55	0.07	0.56	0.08
CP	4.86	9.74	1.24	5.35	1.12	0.23	1.14	0.21	0.96	7.27	0.19	0.64	0.09	0.54	0.07
NY	10.50	20.40	2.37	8.79	1.58	0.29	1.31	0.26	1.41	8.04	0.24	0.88	0.15	0.87	0.11
TA	23.00	37.30	3.71	11.90	1.69	0.25	1.49	0.22	1.51	6.89	0.22	0.66	0.10	0.59	0.11
JF	5.80	12.50	1.48	5.73	1.12	0.23	0.75	0.18	0.97	5.93	0.17	0.47	0.08	0.43	0.07
WY	6.82	12.40	1.34	4.86	0.89	0.18	0.65	0.15	0.75	4.60	0.16	0.48	0.09	0.50	0.06
GH	5.87	13.00	1.57	5.96	1.32	0.22	1.08	0.32	1.50	7.57	0.27	0.81	0.16	0.88	0.12
ZZ	10.70	20.50	2.60	10.70	2.02	0.39	1.88	0.37	1.77	11.50	0.34	0.95	0.14	0.83	0.11
HEXH	65.60	84.90	7.66	24.40	3.94	0.66	3.43	0.57	2.78	12.60	0.40	1.20	0.14	0.77	0.12
LC	7.19	16.40	2.05	7.80	1.45	0.32	1.16	0.22	1.09	6.57	0.22	0.72	0.11	0.86	0.10
BF	16.40	24.60	2.39	7.77	1.27	0.25	1.12	0.18	1.13	6.04	0.21	0.60	0.09	0.54	0.07
SH	13.40	27.70	3.12	11.30	2.32	0.43	2.63	0.37	2.39	11.00	0.44	1.35	0.18	1.27	0.18
NZZ	43.60	79.60	8.17	28.20	4.54	0.80	4.89	0.48	2.54	10.60	0.42	1.17	0.15	1.04	0.14
CZ	9.23	17.00	1.92	7.01	1.27	0.23	1.40	0.18	1.16	5.97	0.22	0.74	0.10	0.73	0.11
JPS	13.20	26.00	2.66	9.09	1.75	0.32	1.92	0.27	1.53	7.26	0.26	0.72	0.09	0.57	0.08
NH	7.45	16.70	1.88	7.62	1.62	0.30	1.71	0.29	1.56	9.22	0.28	0.96	0.18	1.10	0.16
SM	6.37	17.10	2.16	8.13	1.50	0.28	1.19	0.27	1.19	6.70	0.22	0.70	0.11	0.71	0.10
SGJ	71.10	130.00	13.50	45.50	7.29	1.22	5.85	0.87	4.28	18.70	0.65	1.59	0.26	1.74	0.22
TC	2.03	4.57	0.56	2.40	0.61	0.14	0.58	0.15	1.05	5.72	0.21	0.69	0.14	1.08	0.16
CLS	18.80	35.50	4.35	16.50	3.07	0.56	2.70	0.44	2.14	10.80	0.40	1.09	0.19	1.09	0.13
WTP	10.70	25.90	3.12	12.80	2.93	0.48	2.65	0.49	2.57	14.80	0.49	1.51	0.22	1.70	0.23
FHS	15.50	24.30	2.51	8.41	1.59	0.33	1.43	0.20	1.76	9.19	0.29	0.84	0.15	0.87	0.12
LDS	23.50	43.80	5.29	22.20	4.92	0.86	5.01	0.64	3.98	23.70	0.76	2.27	0.31	2.13	0.31
BC	32.30	95.30	11.60	46.00	9.13	1.18	8.70	1.03	5.96	19.00	1.05	3.25	0.48	3.59	0.51
SC	8.44	25.20	2.53	8.89	1.67	0.24	1.49	0.18	1.07	4.83	0.21	0.67	0.11	0.82	0.12
average	18.08	33.85	3.68	13.36	2.46	0.43	2.27	0.35	1.91	9.60	0.34	1.02	0.16	1.02	0.14
world coalb	11.00	23.00	3.50	12.00	2.00	0.47	2.70	0.32	2.10	8.40	0.54	0.93	0.31	1.00	0.20
Chinac	22.50	46.70	6.42	22.30	4.07	0.84	4.65	0.62	3.74	18.20	0.96	1.79	0.64	2.08	0.38
CC	0.80	0.72	0.57	0.60	0.60	0.51	0.49	0.56	0.51	0.53	0.35	0.57	0.25	0.49	0.37

The experiment was carried out in the State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology (Beijing).

Average concentrations of rare earth elements for world hard coals, data from Ketris and Yudovich.[51]

Chinese coals from Dai et al.;[40] CC: ratio of rare earth element concentration in coal samples investigated vs averages for Chinese coals.

Figure 4

CCs of rare earth elements in late Paleozoic coals in the southeastern Qinshui Basin. CC: ratio of rare earth element concentration in coal samples investigated vs averages for Chinese coals.

Based on Seredin and Dai’s classification,[52] rare earth elements and yttrium (REY) were divided into three types: low, medium, and high (LREY, MREY, and HREY, respectively). The ∑REY values in the Shanxi Group No. 3 coal seam and Taiyuan Group No. 10 and No. 15 coal seams in the southeastern Qinshui Basin range from 20.90 to 302.77 μg/g (average of 88.68 μg/g) (on a whole-coal basis), which are significantly lower than the average values of 168.37 μg/g in the upper crust and 135.89 μg/g in Chinese coals[50] and slightly higher than the average values of 68.61 μg/g globally and 62.19 μg/g in the United States coals.[51] The contents of LREY (10.17–267.39 μg/g, mean 71.44 μg/g) are much higher than those of MREY (6.33–35.87 μg/g, mean 14.55 μg/g) and HREY (1.22–8.88 μg/g, mean 2.68 μg/g) (Table ) (on a whole-coal basis).

Table 4

Geochemical Parameters of REY of Late Paleozoic Coal in the Southeastern Qinshui Basina

sample	horizon	∑REY (μg/g)	∑LREY (μg/g)	∑MREY (μg/g)	∑HREY (μg/g)	L/H	L/M	M/H	LaS/SmS	Gds/YbS
DS	Shanxi	120.51	103.90	14.31	2.30	45.17	7.26	6.22	2.07	1.54
WZ	Shanxi	31.93	22.99	7.47	1.47	15.64	3.08	5.08	0.75	0.81
CP	Shanxi	33.65	22.31	9.81	1.53	14.58	2.27	6.41	0.65	1.22
NY	Shanxi	57.20	43.64	11.31	2.25	19.40	3.86	5.03	1.00	0.87
TA	Shanxi	89.64	77.60	10.36	1.68	46.19	7.49	6.17	2.04	1.46
JF	Shanxi	35.91	26.63	8.06	1.22	21.83	3.30	6.61	0.78	1.01
WY	Shanxi	33.93	26.31	6.33	1.29	20.40	4.16	4.91	1.15	0.75
GH	Shanxi	40.65	27.72	10.69	2.24	12.38	2.59	4.77	0.67	0.71
ZZ	Shanxi	64.80	46.52	15.91	2.37	19.63	2.92	6.71	0.79	1.31
HEXH	Shanxi	209.17	186.50	20.04	2.63	70.91	9.31	7.62	2.50	2.58
LC	Shanxi	46.26	34.89	9.36	2.01	17.36	3.73	4.66	0.74	0.78
BF	Shanxi	62.66	52.43	8.72	1.51	34.72	6.01	5.77	1.94	1.20
SH	Shanxi	78.08	57.84	16.82	3.42	16.91	3.44	4.92	0.87	1.20
NZZ	Shanxi	186.34	164.11	19.31	2.92	56.20	8.50	6.61	1.44	2.72
CZ	Shanxi	47.27	36.43	8.94	1.90	19.17	4.07	4.71	1.09	1.11
JPS	Shanxi	65.72	52.70	11.30	1.72	30.64	4.66	6.57	1.13	1.95
NH	Taiyuan	51.03	35.27	13.08	2.68	13.16	2.70	4.88	0.69	0.90
SM	Taiyuan	46.73	35.26	9.63	1.84	19.16	3.66	5.23	0.64	0.97
SGJ	Taiyuan	302.77	267.39	30.92	4.46	59.95	8.65	6.93	1.46	1.95
TC	Taiyuan	20.09	10.17	7.64	2.28	4.46	1.33	3.35	0.50	0.31
CLS	Taiyuan	97.76	78.22	16.64	2.90	26.97	4.70	5.74	0.92	1.43
WTP	Taiyuan	80.59	55.45	20.99	4.15	13.36	2.64	5.06	0.55	0.90
FHS	Taiyuan	67.49	52.31	12.91	2.27	23.04	4.05	5.69	1.46	0.95
LDS	Taiyuan	139.68	99.71	34.19	5.78	17.25	2.92	5.92	0.72	1.36
BC	Taiyuan	239.08	194.33	35.87	8.88	21.88	5.42	4.04	0.53	1.40
SC	Taiyuan	56.47	46.73	7.81	1.93	24.21	5.98	4.05	0.76	1.05

Meteorite data according to Evensen;[53] total contents of ∑REY: ∑REY = La + Ce + Pr + Nd + Sm + Eu + Gd + Tb + Dy + Y + Ho + Er + Tm + Yb + Lu (μg/g, on a whole-coal basis); ∑LREY = La + Ce + Pr + Nd + Sm (μg/g, on a whole-coal basis); ∑MREY = Eu + Gd + Tb + Dy + Y (μg/g, on a whole-coal basis); ∑HREY = Ho + Er + Tm + Yb + Lu (μg/g, on a whole-coal basis); L/H = LREY/HREY; δEu: abnormal degree of Eu element, which is δEu = Eu/Eu* = Eus/(0.67 × Sms + 0.33 × Tbs);[54] δCe: abnormal degree of Ce element, which is δCe = Ce/Ce* = CeS/(LaS × PrS)1/2; GdS* = SmS × 0. 33 + TbS × 0.67;[55] Ceanom = lg(3CeN/(2LaN + NdN));[54] L: light rare earth enrichment type; L–M: light-medium rare earth enrichment type; M: medium rare earth enrichment type; M–H: medium-heavy rare earth enrichment type; H: heavy rare earth enrichment type. Subscript S values normalized by the average REY content of Upper Continental Crust, Subscript N values normalized by the average REY content of chondrite.

The average values of REY in the Shanxi Group and Taiyuan Group are 75.24 and 110.18 μg/g, respectively (on a whole-coal basis). The REY contents in the Shanxi Group are significantly lower than those in the Taiyuan Group, which is consistent with the understanding of Wang et al.[56] regarding the distribution pattern of rare earth elements in medium- and high-sulfur coal in northern Shanxi.

Geochemical Parameters of REY

Table lists the geochemical parameters of REY of late Paleozoic coal in the southeastern Qinshui Basin. The average LREY/HREY ratios have an average value of 26.33 (Table ), showing the characteristics of LREY enrichment and HREY depletion, which is consistent with previous studies on Paleozoic rare earth elements in North China.[57,58] This value is higher than the arithmetic average LREY/HREY values of Chinese coal and North China late Paleozoic coal (20.50 and 20.81, respectively),[10,40] indicating that the fractionation degrees of light and heavy rare earth elements in late Paleozoic coal in southeastern Qinshui basin are obvious.

The average LREY/HREY ratios of the Shanxi and Taiyuan Formations are 28.82 and 22.35 (Table ), respectively, indicating that the fractionation degrees of light and heavy rare earth elements in the Taiyuan Group are lower than those in the Shanxi Group. The average LaS/SmS values in the Shanxi and Taiyuan Formations are 1.23 and 0.82 (Table ), respectively, indicating that the LREY fractionation degree in the Shanxi Formation is higher than that in the Taiyuan Formation. The mean GdS/YbS values in the Shanxi and Taiyuan Formations are 1.33 and 1.12 (Table ), respectively, indicating that the HREY fractionation degree in the Shanxi Formation is higher than that in the Taiyuan Formation, and the REY contents in the Shanxi Group coal (75.24 μg/g, on a whole-coal basis)are significantly lower than those in the Taiyuan Group coal (110.18 μg/g, on a whole- coal basis), which is consistent with the results of Wu et al.,[59] by studying 371 Permian coal samples taken from 11 coal seams of three depositional formations in Zhuji Coal Mine, Huainan coalfield, showed that the fractionation degree of light and heavy rare earth elements is approximately negatively correlated with the total amount of rare earth elements.

The δEu values range from 0.41 to 0.78 (average of 0.60), and the average δEu values of the Shanxi and Taiyuan Formations are 0.62 and 0.58 (Table ), respectively, with obvious negative anomalies, indicating that the samples in the study area were greatly affected by terrestrial materials.[60,61] The δCe values are between 0.89 and 1.28 (average of 0.99), and the average δCe values of the Shanxi and Taiyuan Formations are 0.96 and 1.03 (Table ), respectively. The weak negative Ce anomaly indicates that it is slightly affected by seawater.[62]

Distribution Pattern of REY

The distribution pattern of REY can reflect their provenance supply and the evolution of the sedimentary environment.[63,64] Previous scholars used chondrites to standardize REY, but chondrites do not show REY differentiation during deposition (Figure A,B). The REY distribution patterns in coal seam samples from the Shanxi and Taiyuan Formations are approximately the same, with LREY enrichment and HREY deficit characteristics (Figure A,B). The REY data are standardized by the upper continental crust (UCC) to draw the pattern distribution map (Figure C,D). The REY distribution patterns in each coal seam sample in the Shanxi and Taiyuan Formations are different. The Shanxi Formation shows mainly a light rare earth element enrichment pattern, and the Taiyuan Formation shows mainly a medium-heavy rare earth element enrichment pattern (Figure C,D). Drawing the pattern distribution map with the UCC standardized REY can better indicate the distribution and fractionation characteristics of REY.[65,66]

Figure 5

(A) Chondrite-normalized REY patterns of the Shanxi Formation coal samples; (B) chondrite-normalized REY patterns of the Taiyuan Formation coal samples; (C) UCC-normalized REY patterns of the Shanxi Formation coal samples; and (D) UCC-normalized REY patterns of the Taiyuan Formation coal samples.

Based on the threefold classification,[52] three enrichment types are identified in the coal samples present in this study, including L-type (LaS/LuS > 1), M-type (LaS/SmS < 1 and GdS/LuS > 1), and H-type (LaS/LuS < 1).[63] L-type REY includes DS, TA, WY, ZZ, HEXH, BF, NZZ, JPS, SGJ, and FHS (Table , Figure C,D), with weak negative Eu anomalies, weak negative Ce anomalies, and weak positive Gd and Y anomalies (Table ). The weak positive Gd anomalies in the L-type REY may be the result of seawater influence, as Gd can show slight positive anomalies in coal deposited within marine carbonate sequences,[55,67,68] while the weak negative Ce anomalies may be due to the combined influence of seawater and the source area of intermediate-acid volcanic rocks.[55,69,68] H-type REY includes WZ, JF, GH, LC, CZ, NH, TC, and WTP (Table , Figure C,D), with weak negative Eu anomalies, weak positive Ce anomalies, weak negative Gd anomalies, and weak positive Y anomalies (Table , Figure C,D). The H-type REY and positive Y anomaly may be the result of the combined action of seawater and hydrothermal solution into the original peat.[55,68,70] M-type REY includes CP, NY, ZZ, CLS, SH, SM, LDS, BC, and SC, which can be divided into light-medium (L–M REY) (NY, ZZ, and CLS) and medium-heavy (M–H REY) (SH, SM, LDS, BC, and SC) rare earth element enrichment types.[63]

Minerals in Coals

X-ray diffraction (XRD) data show that the minerals in the coal of the Shanxi Formation in the southeastern Qinshui Basin are mainly clay minerals (71.02%), followed by calcite (24.83%), dolomite (13.07%), ankerite (7.48%), siderite (4.64%), and quartz (2.20%) (Table ). The minerals in the Taiyuan Formation coal of the southeastern Qinshui Basin are mainly clay minerals (71.14%), followed by calcite (19.42%), quartz (10.70%), and pyrite (4.40%) (Table ).

Table 5

Mineral Contents in Coal Samples Determined by XRD in the Southeastern Qinshui Basin (%)a

sample	CM	quartz	pyrite	calcite	dolomite	ankerite	siderite
DS	97.70	bdl	bdl	bdl	bdl	bdl	2.30
WZ	65.30	bdl	bdl	bdl	19.90	14.80	bdl
CP	nd	nd	nd	nd	nd	nd	nd
NY	70.20	1.20	bdl	14.30	bdl	9.10	5.20
TA	78.60	3.20	bdl	8.30	bdl	5.90	4.00
JF	48.10	bdl	bdl	32.70	bdl	8.90	10.20
WY	42.30	bdl	bdl	45.40	12.30	bdl	bdl
GH	81.30	bdl	bdl	14.50	bdl	4.20	bdl
ZZ	nd	nd	nd	nd	nd	nd	nd
HEXH	96.50	bdl	bdl	bdl	bdl	2.00	1.50
LC	nd	nd	nd	nd	nd	nd	nd
BF	59.20	bdl	bdl	33.80	7.00	bdl	bdl
average value of the Shanxi group	71.02	2.20		24.83	13.07	7.48	4.64
SM	86.40	bdl	bdl	0.10	bdl	13.50	bdl
SGJ	67.30	6.30	7.30	19.10	bdl	bdl	bdl
TC	82.00	15.10	0.20	2.70	bdl	bdl	bdl
CLS	nd	nd	nd	nd	nd	nd	nd
WTP	84.90	bdl	bdl	15.10	bdl	bdl	bdl
FHS	35.10	bdl	5.70	59.20	bdl	bdl	bdl
average value of the Taiyuan group	71.14	10.70	4.40	19.24	bdl	bdl	bdl

CM is clay minerals; bdl is below detectable level; nd is no data.

Discussion

Major and Rare Earth Element Geochemical Characteristics

Major Element Geochemical Characteristics

Al2O3: The supply of Al in coal mostly comes from terrigenous detritus, which is mainly enriched in hydroxide, kaolinite, and silicate minerals.[63,71] SiO2: The main source of Si in coal is the input of terrigenous debris during peat accumulation, which is usually enriched in quartz, clay minerals, and silicate minerals.[72] The average SiO2/Al2O3 value in the study area is 1.31, which is lower than the Chinese coal average of 1.42 but higher than the theoretical SiO2/Al2O3 value of 1.18 in a kaolinite crystal.[73,74] Generally, the high SiO2/Al2O3 ratio in coal reflects the existence of silicon-rich minerals (such as quartz),[75] suggesting that SiO2 and Al2O3 of late Paleozoic coal in the southeastern Qinshui Basin mainly occur in clay minerals and silicon-rich minerals (quartz) are present.[28] The average SiO2/Al2O3 value of the Shanxi Formation is 1.20, and SiO2 has a good correlation with Al2O3 (r = 0.98) and Ad (r = 0.90) (Table ), suggesting that SiO2 in the Shanxi Formation mainly exists in clay minerals and silicate minerals and a small amount of quartz, which is consistent with the mineral compositions (Table ). The average SiO2/Al2O3 value of the Taiyuan Formation is 1.48, and SiO2 has a good correlation with Al2O3 (r = 0.84) and a poor correlation with Ad (r = 0.37) (Table ), suggesting that SiO2 in the Taiyuan Formation mainly exists in quartz and partly in clay minerals.

Table 6

Correlation Coefficients of REY with Major Oxides and Proximate Analytical Values in the Shanxi Formation

	Ad	Vdaf	St,d	Sp,d	Ss,d	So,d	∑REY	∑LREY	∑MREY	∑HREY	V	I
Ad	1.00
Vdaf	0.60	1.00
St,d	–0.32	–0.20	1.00
Sp,d	–0.24	–0.22	0.61	1.00
Ss,d	0.30	0.00	0.11	0.23	1.00
So,d	–0.31	–0.12	0.86	0.13	–0.16	1.00
∑REY	0.47	0.01	–0.07	–0.04	0.48	–0.14	1.00
∑LREY	0.45	–0.02	–0.08	–0.05	0.45	–0.15	1.00	1.00
∑MREY	0.55	0.29	0.08	0.02	0.66	–0.02	0.85	0.82	1.00
∑HREY	0.54	0.37	–0.07	–0.21	0.75	–0.08	0.65	0.62	0.87	1.00
V	0.36	–0.06	–0.17	–0.23	0.12	–0.09	0.38	0.39	0.19	0.09	1.00
I	–0.19	0.05	0.07	0.25	–0.15	–0.04	–0.55	–0.56	–0.37	–0.23	–0.86	1.00
LOI	–0.82	–0.72	0.17	–0.18	–0.20	0.35	–0.24	–0.23	–0.39	–0.27	–0.17	0.01
SiO2	0.90	0.44	–0.22	–0.22	0.47	–0.21	0.75	0.73	0.75	0.73	0.42	–0.35
Al2O3	0.87	0.37	–0.23	–0.24	0.48	–0.22	0.78	0.77	0.76	0.74	0.41	–0.37
Fe2O3	–0.05	0.38	0.10	0.53	–0.37	–0.13	–0.26	–0.25	–0.25	–0.45	–0.15	0.20
MgO	0.09	0.32	–0.31	0.20	–0.43	–0.43	–0.46	–0.45	–0.50	–0.55	–0.24	0.36
CaO	0.09	0.44	–0.07	0.34	–0.28	–0.24	–0.58	–0.59	–0.42	–0.48	–0.19	0.38
Na2O	–0.47	–0.09	0.51	0.77	0.00	0.18	–0.19	–0.18	–0.17	–0.31	–0.33	0.20
K2O	0.11	–0.21	–0.53	–0.05	0.12	–0.64	0.24	0.25	0.12	0.07	0.38	–0.17
TiO2	0.14	–0.05	–0.19	–0.35	0.16	–0.05	0.51	0.52	0.30	0.45	0.09	–0.27
P2O5	0.07	0.18	0.40	0.24	0.29	0.30	0.04	0.00	0.48	0.36	–0.24	0.17
MnO	–0.04	–0.14	–0.31	0.01	–0.31	–0.34	–0.42	–0.40	–0.60	–0.63	0.16	0.21
FeO	–0.05	0.41	0.06	0.49	–0.36	–0.15	–0.29	–0.28	–0.27	–0.45	–0.16	0.20

Iron in coal mainly exists in sulfide (pyrite, pyrrhotite, etc.), carbonate mineral (siderite), sulfates, hydroxides, and oxides have also been organically bound (oxalate iron ore).[74] Fe2O3 in the Shanxi Group has good correlations with MgO (r = 0.71) and CaO (r = 0.75) (Table ), suggesting that ankerite and siderite may be their carriers which is consistent with the mineral compositions (Table ), while Fe2O3 in the Taiyuan Group has poor correlations with MgO (r = −0.29) and CaO (r = −0.43) (Table ), suggesting that the occurrence state of Fe2O3 in the Shanxi Formation is different from that in the Taiyuan Formation. Fe2O3 in the Taiyuan Group has good correlations with pyritic sulfur (Sp,d) (r = 0.91) and sulfate sulfur (Ss,d) (r = 0.99), and the correlation between Fe2O3 and organic sulfur (So,d) is poor (r = −0.18) (Table ), suggesting that Fe2O3 in the Taiyuan coal is mainly contained in pyrite and sulfate minerals. Sulfate minerals were too small to be detected by XRD in the Taiyuan Group (Table ). Fe2O3 in the Taiyuan coal mainly occurs in pyrite. Pyrite in most high sulfur coal is the product of seawater intrusion during peat accumulation, and pyrite in a considerable part of coals is formed by epigenetic filling in fissures. Sometimes epigenetic low-temperature hydrothermal fluids rich in iron siliceous or rich in iron calcareous can also cause iron enrichment in coal.[56] Fe2O3 in the Taiyuan Group has a correlation with I (r = 0.67), suggesting that part of Fe2O3 may occur in organic matter.

Table 7

Correlation Coefficients between the Characteristic Parameters and Rare Earth Elements in the Shanxi Formation

	∑REY	∑LREY	∑MREY	∑HREY	(La/Yb)N	Ce/La	Y/Ho	CaO/(CaO + Fe2O3)	CaO/(MgO × Al2O3)	AAI	SI	RI
∑REY	1.00
∑LREY	1.00	1.00
∑MREY	0.85	0.82	1.00
∑HREY	0.65	0.62	0.87	1.00
(La/Yb)N	0.97	0.98	0.72	0.46	1.00
Ce/La	–0.79	–0.80	–0.57	–0.32	–0.86	1.00
Y/Ho	0.09	0.07	0.29	0.10	0.04	–0.14	1.00
CaO/(CaO + Fe2O3)	–0.61	–0.61	–0.43	–0.42	–0.56	0.32	–0.05	1.00
CaO/(MgO × Al2O3)	–0.60	–0.62	–0.33	–0.46	–0.58	0.40	0.45	0.78	1.00
AAIa	0.75	0.75	0.60	0.59	0.68	–0.46	–0.01	–0.93	–0.83	1.00
SIb	–0.63	–0.63	–0.52	–0.62	–0.57	0.60	0.05	0.71	0.79	–0.79	1.00
RIc	–0.50	–0.49	–0.49	–0.48	–0.48	0.62	0.33	–0.03	0.37	–0.19	0.41	1.00

AI = (Al2O3 + SiO2)/(CaO + MgO).[28]

SI = (CaO + MgO)/(Al2O3 + SiO2 + Fe2O3).[28]

RI = (Fe2O3*(SO/SP))/(Al2O3 + SiO2 + CaO + MgO).[28]

Magnesium in coal mainly exists in clay minerals and carbonate minerals.[74] MgO in the Shanxi Group has good correlations with Fe2O3 (r = 0.71), FeO (r = 0.75), and CaO (r = 0.87) (Table ), suggesting that MgO may occur in dolomite or ankerite. MgO in the Taiyuan Group has a negative correlation with Fe2O3 (r = −0.6) and poor positive correlations with FeO (r = 0.38), and CaO (r = 0.34), while MgO in the Taiyuan Group has good correlations with Na2O (r = 0.94) and TiO2 (r = 0.59) (Table ), suggesting that MgO mainly exists in clay minerals.

The CaO mainly exists in the form of sulfate (gypsum), carbonate (calcite), phosphate, and silicate.[74,76] CaO in the Shanxi Group has a poor correlation with Ad (r = 0.09) and good correlations with Fe2O3 (r = 0.75) and MgO (r = 0.87) (Table ), suggesting that during the formation of coal, most Ca elements are released from organic matter and precipitate with the solution into the crack, thus forming calcite and ankerite. The CaO in the Taiyuan Group has a good correlation with SO (r = 0.5) and a poor correlation with Ad (r = 0.23) (Table ), suggesting that Ca mainly exists in the form of an organic combination.

The MnO in the Shanxi Group has good correlations with MgO (r = 0.53) and CaO (r = 0.52) (Table ), suggesting that MnO occurs in carbonate minerals. MnO in the Taiyuan Group has a good correlation with Fe2O3 (r = 0.57) (Table ), indicating that MnO has a similar occurrence state to that of Fe2O3. Mn is mostly replaced in pyrite as isomorphisms of Fe and Mg.[77,78] The MnO in the Taiyuan Group has a correlation with I (r = 0.60), suggesting that part of MnO may occur in organic matter.

Titanium can replace Al in clay minerals such as kaolinite in the form of isomorphism or exist in kaolinite in the form of anatase.[79] TiO2 in the Shanxi Group has correlations with SiO2 (r = 0.44) and Al2O3 (r = 0.48) (Table ), suggesting that TiO2 in the rock may occur in anatase or silicoaluminate. The TiO2 in the Taiyuan Group has good correlations with SiO2 (r = 0.69) and Al2O3 (r = 0.84) (Table ), suggesting that TiO2 in the rock may occur in kaolinite and other clay minerals.

The reasons for the different occurrence states of oxides between the Shanxi Formation and Taiyuan Formation are as follows: (1) The Shanxi Formation is mainly a delta sedimentary system.[6,72] The coal sedimentary environment of the Taiyuan Formation belongs to coastal carbonate platform deposition, which was frequently affected by seawater during the coal-forming period. The roof of the No. 15 coal seam of the Taiyuan Formation is limestone, and the seeping seawater provides a sufficient sulfur source for the formation of pyrite and anaerobic conditions conducive to bacterial activity.[80] Therefore, the content of sulfur and pyrite in the Taiyuan Formation coal is higher than that in the Shanxi Formation. (2) During the diagenesis of the Taiyuan Formation coal, a part of the calcite was metasomatized by quartz. Thin-film calcite is generally a secondary mineral formed in the later stage. In the process of diagenesis, the phenomenon of quartz replacing calcite is very common.[81] (3) The groundwater brings the dissolved Ca2 + and Mg2+ in the limestone of the Taiyuan Formation into the overlying coal seam (Shanxi Formation) and precipitates into calcite veins in the cleats and fractures of the coal seam.[80]

Rare Earth Element Geochemical Characteristics

The modes of occurrence of REY of late Paleozoic coal in the southeastern Qinshui Basin were preliminarily investigated using cluster analysis and correlation analysis, which are effective indirect methods for coal geochemistry.[40] Previous studies have shown that REY may be hosted in clay minerals (e.g., kaolinite, illite), organic matter, and independent minerals (rare earth elements exist in the crystal structure as essential elements for minerals, such as monazite, xenotime, and bastnaesite, etc.)[57,74]

The rare earth element contents in the Shanxi Formation coal have good correlations with SiO2 (r = 0.75) and Al2O3 (r = 0.78) (Table ), indicating that rare earth elements in the Shanxi Formation coal are mainly contained in clay minerals. The rare earth element contents in the Shanxi Formation coal have a negative correlation with CaO (r = −0.58), MgO (r = −0.46), and MnO (r = −0.42), which may be due to the leaching of groundwater and migration of carbonate minerals, or under certain conditions, carbonate minerals are metasomatized by quartz during burial diagenesis, and quartz has a certain “dilution” effect on rare earth elements, resulting in a reduction of rare earth elements.[82] Rare earth elements mainly have a certain correlation with major elements related to terrigenous sediments, while major elements related to seawater do not correlate, indicating that rare earth elements in the Shanxi Formation coal mainly come from terrigenous sediments. The rare earth element contents in the Shanxi Formation coal have a good correlation with TiO2 (r = 0.51) (Table ), indicating that rare earth elements may occur in anatase or silicoaluminate.

The REY contents in coal have less significant positive correlations with Ad (r = 0.47) and V (r = 0.38) and a negative correlation with I (r = −0.55) (Table ), indicating that rare earth elements in coal may mainly exist in inorganic minerals; however, some rare earth elements were adsorbed and enriched by vitrinite during the gelation stage of anoxic reduction. Ad has correlations with light rare earth elements (r = 0.45), medium rare earth elements (r = 0.55), and heavy rare earth elements (r = 0.54) (Table , Figure ), indicating that REY enrichment in coal is complicated and that the inorganic affinity of medium and heavy rare earth elements is slightly stronger than that of light rare earth elements. There are samples with low ash yield but high content of rare earth elements and samples with high ash yield but low content of rare earth elements, indicating that rare earth elements mainly occur in inorganic minerals, but a small amount also occur in organic matter (Figure C,D).

Figure 6

(A) CaO/(CaO + Fe2O3) vs ∑LREY plot; (B) CaO/(MgO*Al2O3) vs ∑LREY plot; (C) Ad vs ∑LREY plot; and (D) Ad vs ∑MREY plot.

The rare earth element contents in the Shanxi Formation coal have negative correlations with CaO/(CaO + Fe2O3) (r = −0.61), CaO/(MgO × Al2O3) (r = −0.60), and Ce/La (r = −0.79) (Table , Figure A,B), indicating that the warm, humid, low-salinity (fresh water), and oxidized environments are favorable for REY enrichment. Compared with medium rare earth elements and heavy rare earth elements, warm, humid, low-salinity (fresh water), and oxidized environments are favorable for LREY enrichment (Table , Figure A,B). The rare earth element contents in the Shanxi Formation coal have a good correlation with AAI (r = 0.75), indicating that the acid environment is favorable for REY enrichment. The rare earth element contents in the Shanxi Formation coal have negative correlations with SI (r = −0.63) and RI (r = −0.50) (Table ), indicating that low-salinity (fresh water) and strong hydrodynamic force environments are favorable for REY enrichment.

The Y/Ho ratio is used to evaluate the relative strength of siliciclastic (Y/Ho = 25–30) and seawater (Y/Ho = 60–70) inputs. The Y/Ho ratio of the Shanxi formation is 25.48–38.26, with an average value of 31.47. The correlation between REY and Y/Ho is not obvious (r = 0.09), indicating that terrigenous debris is the main factor leading to the distribution pattern of rare earth elements in coal.[83]

The rare earth element contents in the Taiyuan Formation coal have good correlations with SiO2 (r = 0.75), Al2O3 (r = 0.78), and Fe2O3 (r = 0.92) (Table ), indicating that rare earth elements in coal may occur in pyrite and clay minerals. The content of pyrite in sample SGJ was the highest (7.3%, Table ), and the content of rare earth elements in sample SGJ was the highest (302.77 μg/g, Table ). Liu et al.[80] by single-component tests of pyritic veins in coal samples, mudstone, limestone, and coal, found that the content of rare earth elements in pyritic veins in Qinshui Basin was far greater than that in coal, and the content of rare earth elements also increased with the increase of pyritic content. Veined pyrite in coal is generally a secondary mineral of hydrothermal origin, and hydrothermal fluids can lead to the formation of rare earth enriched phosphor and bastnaesite.[52] Mao et al.[84] studied gold-bearing pyrite with high rare earth content in Jinshan, Jiangxi Province, and found that pyrite captures some silicate microminerals, such as zircon, rutile, spinel, ilmenite, etc., during the crystallization process. These minerals control the content and distribution of rare earth elements in pyrite, so pyritic veins in coal are very likely to be enriched in rare earth elements. Due to the relative enrichment of rare earth elements in clay minerals and pyritic veins, the REY contents in Taiyuan Group coal are higher than those in Shanxi Group coal seams, and the rare earth element contents in coal, which are affected by seawater, are not necessarily lower than those in coal seams that are affected by terrigenous debris.[80]

The rare earth element contents in the Taiyuan Formation coal have good correlations with Sp (r = 0.89) and Ss (r = 0.93) but a poor correlation with organic sulfur (r = −0.26) (Table ), suggesting that the REY contents in sulfur- and iron-rich minerals may be very high. The REY contents in coal have a less significant positive correlation with Ad (r = 0.34) but a significant positive correlation with I (r = 0.78) and a negative correlation with V (r = −0.36), while the macerals of the Paleogene coals in the southeastern Qinshui Basin are dominated by vitrinite, indicating that the rare earth elements in organic matter are not abundant, so the rare earth elements in the Taiyuan Formation mainly occur in pyrite and clay minerals (Table ). I has significant correlations with light rare earth elements (r = 0.76), medium rare earth elements (r = 0.95), and heavy rare earth elements (r = 0.89) (Table , Figure C,D), indicating that medium rare earth elements are more easily adsorbed by inertinite than light and heavy rare earth elements. The inertinite is formed by carbonization of lignocellulose under relatively dry oxidation conditions, which suggests that wood fiber may be the adsorbent of REY in the Taiyuan Formation.

Figure 7

(A) CaO/(CaO + Fe2O3) vs ∑LREY plot; (B) CaO/(MgO*Al2O3) vs ∑LREY plot; (C) Ad vs ∑LREY plot; and (D) Ad vs ∑MREY.

The paleoclimate and paleoredox of the Taiyuan Formation have little influence on REY enrichment, while the rare earth element contents in the Taiyuan Formation coal have negative correlations with CaO/(CaO + Fe2O3) (r = −0.71), SI (r = −0.48), and RI (r = −0.39) (Table , Figure B), indicating that low-salinity (fresh water) and strong hydrodynamic force environments are favorable for REY enrichment. The rare earth element contents in the Taiyuan Formation coal have good correlations with (La/Yb)N (r = 0.95) and AAI (r = 0.89) (Table , Figure A), indicating that the strong hydrodynamic force and acidic environments are favorable for REY enrichment. The Y/Ho ratio of the Taiyuan Formation is 27–32.93, with an average value of 29.75. The negative correlation between REY and Y/Ho (r = −0.14) shows that terrigenous debris is the main factor leading to the distribution pattern of rare earth elements in coal.[83]

Table 9

Correlation Coefficients between the Characteristic Parameters and Rare Earth Elements in the Taiyuan Formationa

	∑REY	∑LREY	∑MREY	∑HREY	(La/Yb)N	Ce/La	Y/Ho	CaO/(CaO + Fe2O3)	CaO/(MgO × Al2O3)	AAI	SI	RI
∑REY	1.00
∑LREY	1.00	1.00
∑MREY	0.92	0.90	1.00
∑HREY	0.73	0.70	0.92	1.00
(La/Yb)N	0.95	0.95	0.80	0.52	1.00
Ce/La	–0.40	–0.41	–0.33	–0.18	–0.58	1.00
Y/Ho	–0.14	–0.15	–0.01	0.02	–0.09	–0.03	1.00
CaO/(CaO + Fe2O3)	–0.71	–0.70	–0.69	–0.53	–0.74	0.57	–0.10	1.00
CaO/(MgO × Al2O3)	–0.38	–0.40	–0.10	0.19	–0.45	–0.04	0.60	0.27	1.00
AAI	0.89	0.90	0.69	0.49	0.85	–0.36	–0.34	–0.69	–0.58	1.00
SI	–0.48	–0.50	–0.27	–0.07	–0.54	0.35	0.57	0.65	0.78	–0.75	1.00
RI	–0.39	–0.37	–0.48	–0.35	–0.40	–0.01	–0.70	0.51	–0.04	–0.13	–0.12	1.00

AI = (Al2O3 + SiO2)/(CaO + MgO);[28] SI = (CaO + MgO)/(Al2O3 + SiO2 + Fe2O3);[28] RI = (Fe2O3*(SO/SP))/(Al2O3 + SiO2 + CaO + MgO).[28]

Coal-Accumulating Environment

The poor correlation between δCe and (La/Sm)S (r2 = 0.25) indicates that the Ce anomalies in the samples can yield information regarding the original sediments, and the interpretation of the paleosedimentary environment with rare earth elements has high reliability.[85] Tribovilllard et al.[86] believed that if sedimentary rocks are heavily influenced by terrigenous detritus, trace elements are not suitable for environmental analysis. Therefore, trace elements alone should not be used to assess the paleoenvironment of coal seam deposition in the southeastern Qinshui Basin.

Paleosalinity

Paleosalinity refers to the salinity in the sediments in a certain geological period, which can be analyzed by major element contents and major element ratios.[87] The SiO2 and Al2O3 in coal are mainly supplied by terrigenous debris during peat accumulation, while Fe2O3, CaO, and MgO are mostly products of seawater intrusion during peat accumulation. (Fe2O3 + CaO + MgO)/(SiO2 + Al2O3) is also known as the ash component index (C). The C refers to the medium conditions during the accumulation of peat bogs, and usually, 0.23 is used as the boundary value between sea and land peat bogs. The change in C values depends on the coal-forming environment.[88]C values ranging from 0.03 to 0.22 indicate a weak reducing type. C values ranging from 0.23 to 1.23 indicate a strong reducing model.[89] The C values in the study area range from 0.09 to 1.20 (average of 0.39) (Table ). In addition to the C values of individual samples plotting in the continental swamp area, most C values show that the sedimentary environment during the late Paleozoic in the southeastern Qinshui Basin was a marine-continental environment, the coal-forming environment was mainly a reducing environment, and a few coal samples formed in weakly reducing environments (Figure ).

Figure 8

Coal ash index distribution map of the Shanxi and Taiyuan Formations.

The CaO/(CaO + Fe2O3) ratio reflects the salinity of the sedimentary water medium, and the higher the ratio is, the higher the salinity.[90] CaO/(CaO + Fe2O3) values less than 0.2 represent low salinity; values from 0.2–0.5 represent medium salinity; and values greater than 0.5 represent high salinity.[90] The CaO/(CaO + Fe2O3) ratios in the study area range from 0.036 to 0.833 (average of 0.54); only two samples are less than 0.2, 3 samples are between 0.2 and 0.5, and 14 samples are greater than 0.5, indicating high salinity. The mean values of the Shanxi and Taiyuan Formations are 0.64 and 0.43, respectively, indicating that the paleosalinity of the Shanxi Formation was higher than that of the Taiyuan Formation. CaO/(CaO + Fe2O3) fluctuates in a certain range, which may have been caused by the rise and fall of paleosalinity caused by a dry climate and high evaporation, and it is also closely related to the oscillation and periodic changes in sea level within a small range.

Paleoredox Conditions

Rare earth elements are often used as indicators of the sedimentary environment and accumulation processes due to their stable chemical properties, and they are not easily disturbed by various geological processes. Ce and Eu are more sensitive to changes in the redox environment and are widely used in studies of the sedimentary paleoenvironments of coal seams.[91−94] Dai et al.[10] used the δCe/δEu ratio to reflect the redox nature of the sedimentary environment. When δCe/δEu > 1, the surface coal deposits were dominated by a reducing environment, and vice versa. The δCe/δEu ratios of coal seams in the study area range from 1.29 to 2.81 (average of 1.69). The average δCe/δEu ratios of coal seams in the Shanxi and Taiyuan Formations are 1.03 and 1.16, respectively. The correlation between δCe/δEu and ∑REE is shown in Figure A, and the point at which coal samples decrease is greater than 1, indicating that the environment during the coal depositional period was inclined to reduction,[59] and the Taiyuan Formation was more reducing than the Shanxi Formation.

Figure 9

(A) Correlation between δCe/δEu and ∑REE and (B) scatter distribution of Ce/La and Ceanom.

When Ce/La < 1.5, it represents an oxygen-rich environment; when 1.5 < Ce/La < 2, it represents an anaerobic environment; and when Ce/La > 2, it represents an anaerobic environment.[95] The Ce/La ratios in the study area range from 1.29 to 2.99 (average of 2.03). In Figure B, most points plot in the oxygen-poor reducing environment, and only a few points in the Shanxi Group plot in the oxygen-rich environment.

According to the Ce anomaly calculation proposed by Elderfield and Greaves,[96] Ceanom = lg(Ce/(2LaN + NdN)), Ceanom > −0.1 indicates Ce enrichment and an anoxic reducing environment. Ceanom < −0.1 represents a loss of Ce, indicating that the water body was in an oxidized environment. Ceanom values in the Shanxi Group were −0.16–0.02 (average of −0.05), and Ceanom values in the Taiyuan Group were −0.10–0.13 (average of 0.01). According to Figure B, the environment was mainly reducing, except for a few oxidizing environments in the Shanxi Group. This is consistent with the sedimentary environmental analysis based on other facies markers.

Paleoclimate

Based on paleomagnetic measurements of Carboniferous and Permian strata in North China, the study area was located in tropical and subtropical regions 13.9° north of the equator,[19] and it had a humid and rainy climate that was favorable for coal accumulation.[19] CaO/(MgO × Al2O3) and Mg/Ca also have significance in regard to climate change. They can indicate the temperature, and the higher the value is, the higher the temperature.[97−99] According to the calculation, the mean CaO/(MgO × Al2O3) and Ca/Mg values of the Shanxi Formation are higher than those of the Taiyuan Formation (Figure ), indicating that the temperature during the depositional period of the Shanxi Formation was higher than that of the Taiyuan Formation. During the Shanxi Formation deposition, the climate was arid and hot, and evaporation was high, while during the Taiyuan Formation deposition, the climate was warm and humid.

Figure 10

Variations in the Sr/Cu ratios of Carboniferous and Permian samples from the southeastern Qinshui Basin.

Provenance Analysis

Rare earth elements have a strong inheritance and can more accurately reflect the provenance composition.[100] The REY distribution patterns in coal seam samples from the Shanxi and Taiyuan Formations are (Figure A,B) consistent with the REY distribution model of the upper crust (Figure ) with LREY enrichment and HREY deficit characteristics, indicating that the original material should come from the upper crust, which may have been related to terrigenous detrital sediment during coal seam formation.[57,58] Because HREY dissolves and migrates more easily under the action of seawater than LREY, the relative loss of HREY in the coal formed by Carboniferous-Permian sea–land interactions in the southeastern Qinshui Basin is significant. According to the principle of “similarity and homology”, the REY distribution curves of the Proterozoic granite of Yinshan in the northern margin of the basin (Figure A)[67] are more similar to those of coal seam samples from the Shanxi and Taiyuan Formations (Figure A,B) than those of the Paleoproterozoic potassic granitoids in Zhongtiao Mountain (Figure B),[69] suggesting that the provenances of coal rocks in the Shanxi and Taiyuan Formations are related to the paleoproterozoic granite in the Yinshan Upland.[101]

Figure 15

Rare earth element distribution patterns under different tectonic backgrounds (after Zhang et al.[97]).

Figure 11

(A) Chondrite-normalized REY patterns of Paleoproterozoic granites in the Yinshan Upland; (B) chondrite-normalized REY patterns of Paleoproterozoic potassic granitoids in Zhongtiao Mountain.

Combined with the ∑REY-(La/Yb)N source rock discrimination diagram (Figure ), most data points plot in the intersection area between sedimentary rock and calcareous mudstone, and fewer plot in granite and basalt areas. The coal samples from the Shanxi Formation mainly plot in areas with sedimentary rocks and calcareous mudstones. The coal samples of the Taiyuan Formation mainly plot in calcareous mudstone areas; one sample plots in the intersection area between calcareous mudstone and basalt, and one sample plots in the intersection area between sedimentary rocks and granite. The provenances of coal rocks in the Shanxi and Taiyuan Formations are different to some extent; negative Eu anomalies are obvious in the Taiyuan Formation, and more granite is mixed in its provenance.

Figure 12

Diagram of mudstone ∑REY–(La/Yb)N values in the late Paleozoic in the southeastern Qinshui Basin (after Allègre et al.[102]).

Major element Al2O3/TiO2 ratios can be used to indicate the source of deposited materials. When 3 < Al2O3/TiO2 < 8, it represents basic magmatic rocks; when 8 < Al2O3/TiO2 < 21, it represents neutral magmatic rocks; and when 21 < Al2O3/TiO2 < 70, it represents acidic magmatic rocks.[103] The Al2O3/TiO2 values in the study area are 13.45–68.54 (average of 27.97). Most samples have values greater than Al2O3/TiO2 = 21, which generally indicates that the study area contains mainly acidic magmatic rocks mixed with some neutral magmatic rocks (Figure ). Wu et al.[104] concluded that there is a significant positive correlation between ∑REE and CaO values in Kaili coal, indicating that the relative enrichment of rare earth elements is related to the water quality conditions of alkaline swamps in coal-bearing basins. However, there is a negative correlation between ∑REY and CaO values in the study area (r2 = 0.25). While ∑REY values are positively correlated with acid minerals (SiO2 and Al2O3) (r2 = 0.32, r2 = 0.05), they indicate that rare earth elements in coal are related to the conditions of acidic swamps.[105] Girty et al.[106] pointed out that when Al2O3/TiO2 < 14, the sediments originated from mafic rocks. When 19 < Al2O3/TiO2 < 28 in the sediments, the sediments originated from felsic rocks. Except for one sample, most of the samples are in the range of 19 < Al2O3/TiO2 < 28, and the study area is dominated by a felsic source mixed with some mafic rocks. It is mainly acidic magmatic rocks mixed with some neutral magmatic rocks. Dai et al.[10] believed that REY in late Paleozoic coal in North China mainly occurred in carbonate, which may be related to the terrigenous supply of acidic granite.

Figure 13

Al2O3/TiO2 diagram in the southeastern Qinshui Basin (modified after Wang et al.[56]).

The Eu/Eu* and GdN/YbN ratios of rare earth elements are sensitive parameters for assessing the properties of source rocks and the geological formation ages.[63] In the Eu/Eu*–GdN/YbN diagram (Figure ), the GdN/YbN ratios of most samples are between 1 and 2, and the Eu/Eu* values of the samples are less than 0.85, which indicates that the late Paleozoic parent rocks in the southeastern Qinshui Basin were mainly formed after the Archean, and the rocks are mixed with some old strata that have been stripped from the provenance area, indicating the characteristics of multiple sources, which is consistent with the conclusion shown in the ∑REY–(La/Yb)N diagram.

Figure 14

Scatter diagram of mudstone Eu/Eu*–GdN/YbN values in the late Paleozoic in the southeastern Qinshui Basin (after McLennan et al.[7]).

The average values of GdN/YbN in the Taiyuan and Shanxi Formations in the study area are 1.46 and 1.84, respectively. The GdN/YbN value of the Shanxi Formation is larger than that of the Taiyuan Formation, indicating that the provenance area was gradually uplifted and that the old strata were continuously exposed. Chen et al.[107] showed that the uplift and denudation state of the southern provenance began to intensify in the late sedimentary period of the Shanxi Formation in the Ordos Basin; this state occurred later than the uplift of the northern provenance, which is consistent with the regional tectonic evolution background; that is, the uplift of the northern provenance of the North China Plate occurred earlier than that of the southern provenance. Zhao et al.[108] pointed out that the transformation of the provenance area began at the bottom of the Shanxi Formation during the sedimentary period, mainly from the Inner Mongolia paleouplift in the North China Craton and northern North China Craton.

Analysis of the Tectonic Environment in the Source Area

The composition of REY is relatively weakly affected by late weathering, diagenesis, and alteration. It is mainly controlled by the rock composition in its source area and can represent the rare earth characteristics of the source rocks in the source area.[7,8] Bhatia et al.[109] summarized the characteristic REY values and model curve characteristics of graywackes under different tectonic backgrounds (Figure ). The comparison between the chondrite-normalized REY distribution curve in Figure and the typical tectonic background shows that the provenance in late Paleozoic coal in the southeastern Qinshui Basin has affinities with the upper crust, continental island arc, and continental margin. Bhatia and Crook[100] summarized the REY compositional parameters of graywackes under different tectonic backgrounds (Table ). Lanthanum, Ce, and REY values are between the values of oceanic island arcs and continent arcs and closer to the values of continental island arcs. LREY/HREY values are close to those of oceanic island arcs, and Eu/Eu* ratios in the study area are close to those of an active continental margin background. The La/Yb and LaN/YbN ratios of the coal are within the values of the passive continental margin tectonic background. The tectonic setting of the late Paleozoic provenance area in the southeastern Qinshui Basin is mainly a complex tectonic setting composed of continental island arcs, oceanic island arcs, active continental margins, and passive continental margins. In the late Paleozoic, the Paleo-Asian Ocean Plate subducted beneath the North China Plate, and the northern margin of North China changed from a passive continental margin to an active continental margin, resulting in a large amount of magmatic activity.[110,111] Through zircon U-Pb dating and Lu-Hf isotope analysis of the upper Paleozoic tuff interlayer in Beijing, Zhang et al.[112] found that these tuffs mainly originated from the Inner Mongolia uplift on the northern edge of North China, which proved that there was volcanic activity on the northern edge of North China at that time, but they were completely denuded in the later stage. Ancient sediments and felsic volcanic rocks mainly developed in the tectonic setting of continental island arc and active continental margin, mixed with a very small amount of oceanic island arc and passive continental margin materials.

Table 10

REY Characteristics of the Sample Mean in the Study Area and Graywackes from Different Tectonic Settings[100]a

structural setting	oceanic island arc	continental island arc	active continental margin	passive continental margin	late Paleozoic in the southeastern Qinshui Basin
source rock type	complete magmatic arc	partial magmatic arc	basement uplift	intracratonic tectonic highland	average
La/(μg/g)	8 ± 1.7	27 ± 4.5	37	39	18.08
Ce/(μg/g)	19 ± 3.7	59 ± 8.2	78	85	33.85
REY/(μg/g)	58 ± 10	146 ± 20	186	210	88.68
LREY/HREY*	3.8 ± 0.9	7.7 ± 1.7	9.1	8.5	4
La/Yb	4.2 ± 1.3	11 ± 3.6	12.5	15.9	19.3
LaN/YbN	2.8 ± 0.9	7.5 ± 2.5	8.5	10.8	12.74
Eu/Eu*	1.04 ± 0.11	0.79 ± 0.13	0.6	0.56	0.6

∑LREY = La + Ce + Pr + Nd + Sm + Eu; ∑HREY = Gd + Tb + Dy + Y + Ho + Er + Tm + Yb + Lu; LREY/HREY: ratio of LREY to HREY content.

There are significant differences in the major element compositions in coals from different tectonic backgrounds. Crook et al.[113] summarized the metasomatic method to distinguish between the compositional variation in sandstone and the tectonic environment (SiO2 < 58%, K2O/Na2O ≪ 1) as symbols of magmatic island arcs. The SiO2 contents are approximately 68–71%, and K2O/Na2O < 1 is a symbol of the continental margin of the Andes and the upper continental crust. SiO2 > 89% and K2O/Na2O > 1 are symbols of Atlantic passive continental margins.[114] The average SiO2 content in the study area is 38.92% (after loss on ignition (LOI) correction), and K2O/Na2O values range from 0.09 to 1.23, with an average of 0.33. The environment in the study area was closer to a magmatic island arc, the Andean continental margin, and the upper continental crust. Only the K2O/Na2O ratio of one sample from the Taiyuan Formation indicates an Atlantic passive continental margin.

A structural environmental discrimination diagram composed of different principal elements is considered, and individual errors in the detection and analysis of principal elements are eliminated. Roser et al.[115] and Maynard et al.[116] proposed discrimination diagrams of K2O/Na2O-SiO2 and SiO2/Al2O3-K2O/Na2O to distinguish tectonic environments (Figure A,B), respectively. Most Shanxi Formation samples plot in island arc areas (Figure A,B), Taiyuan Formation samples mainly plot in island arc and active continental margin areas, and fewer samples plot in passive continental margin areas, indicating that the provenance of the study area mainly came from island arcs and active continental margins. The provenance is an active continental margin with a gully/arc/basin system and a passive continental margin with a collisional orogenic belt. This tectonic setting was the result of the collision between the North China Plate and the Siberian Plate in the region.[107,117]

Figure 16

(A) SiO2-K2O/Na2O structural environment discrimination diagram (the data in this chart have been corrected by LOI and modified after Roser and Korsch[115]). (B) SiO2/Al2O3-Na2O/K2O structural environment discrimination diagram (the data in this chart have been corrected by LOI and modified after Roser and Korsch[115]).

Conclusions

The coal seams of the Shanxi Formation in the study area mainly contain bituminous A rank and anthracite C rank (Ro,ran: 1.6–2.9%) with semibright, low-ash, low-moisture, low-volatile, and low-sulfur characteristics. The coal seams of the Taiyuan Formation mainly contain bituminous A rank and anthracite C rank (Ro,ran: 1.97–3.24%) with semibright and bright, low-ash, low-moisture, low-volatile, and medium-sulfur. The main forms of sulfur in the study coals are organic sulfur (79.50%), followed by pyritic sulfur (18.75%), only some coals with high sulfur content in the Taiyuan Formation are mainly pyritic sulfur (SGJ, WTP, FHS), and the content of sulfate sulfur is extremely low (1.75%).

Major elements in late Paleozoic coal in the southeastern Qinshui Basin are mainly SiO2 (4.77%) and Al2O3 (3.64%), followed by Fe2O3 (1.22%), CaO (1.53%), FeO (0.48%), MgO (0.25%), Na2O (0.21%), P2O5 (0.18%), TiO2 (0.15%), and minor K2O (0.04%). Compared with the contents of major element oxides in Chinese coals, except for the slight enrichment of P2O5 (CC = 2.01) and the loss of Fe2O3 (CC = 0.25), K2O (CC = 0.24), TiO2 (CC = 0.46), MnO (CC = 0.42), and FeO (CC = 0.42), the remaining MgO (CC = 1.14), CaO (CC = 1.24), and Na2O (CC = 1.29) are within the normal range. The SiO2/Al2O3 average value is 1.31. SiO2 in the Shanxi Formation mainly exists in clay minerals and silicate minerals, and a small amount of quartz, and that in the Taiyuan Formation mainly occurs in quartz and partly in clay minerals. Fe2O3 in the Shanxi Group mainly occurs in ankerite and siderite, but that in the Taiyuan Group is mainly contained in pyrite, and part of Fe2O3 may be adsorbed by inertinite. CaO in the Shanxi Group is mainly in the form of calcite and dolomite, followed by ankerite, while CaO in the Taiyuan Group mainly exists in the form of organic combinations. MnO in the Shanxi Group occurs in carbonate minerals, while MnO in the Taiyuan Group mainly occurs in pyrite, and part of MnO may be adsorbed by inertinite. TiO2 in the Shanxi Group may occur in anatase or silicoaluminate, but that in the Taiyuan Group may occur in kaolinite and other clay minerals.

The average content of rare earth elements in the study coals is 88.68 μg/g (on a whole-coal basis) which is higher than the average ∑REY in U.S. coal and world coal but lower than that of Chinese coal. The REY content in the Taiyuan Formation coal (110.18 μg/g) (on a whole-coal basis) is slightly higher than that in the Shanxi Formation coal (75.24 μg/g) (on a whole-coal basis). The mean LREY/HREY value is 26.33. The distribution patterns of UCC-normalized REY in each coal seam sample in the Shanxi and Taiyuan Formations are different. The Shanxi Formation is dominated by L-type REY, while the Taiyuan Formation is dominated by M–H-type REY. The fractionation degree of REY in the Taiyuan Formation is lower than that in the Shanxi Formation. The mean values of δEu, δCe, Y, and Gd are 0.91, 0.96, 1.07, and 1.02, respectively. Rare earth elements in coal in the Shanxi Formation are mainly contained in clay minerals, some rare earth elements may exist in clay minerals containing titanium or associated with rutile minerals, and some of rare earth elements are adsorbed and enriched by vitrinite. The rare earth elements in coal in the Taiyuan Formation mainly occur in pyrite and clay minerals, some rare earth elements are adsorbed by inertinite, and medium rare earth elements are more easily adsorbed by inertinite than light and heavy rare earth elements. Warm, humid, low-salinity (fresh water), and oxidized environments are favorable for REY enrichment. The rare earth element contents in the Shanxi Formation coal have good correlations with AAI (r = 0.75) and (La/Yb)N (r = 0.97), indicating that strong hydrodynamic forces and acidic environments are favorable for REY enrichment.

The paleosalinity indices (Fe2O3 + CaO + MgO)/(SiO2 + Al2O3) and CaO/(CaO + Fe2O3) show that the paleosalinity of the southeastern Qinshui Basin is high and that of the Shanxi Formation is higher than that of the Taiyuan Formation. The redox indices, namely, δCe/δEu, Ce/La, and Ceanom, indicate that the study area is mainly a weakly oxidized and reducing environment. The paleotemperature indices CaO/(MgO × Al2O3) and Mg/Ca indicate that the paleotemperature of the Shanxi Formation is higher than that of the Taiyuan Formation.

Combined with the REY distribution pattern and ∑REY–(La/Yb)N diagram, the parent rocks in the study area were mainly derived from sedimentary rocks and calcareous mudstones mixed with granite and basalt from the Yinshan Upland. The provenance of coal rocks in the Shanxi and Taiyuan Formations is different to some extent, and the provenance of the Taiyuan Formation is mixed with more granite than that of the Shanxi Formation. Al2O3/TiO2 ratios indicate that the main provenance is acid magmatic rocks, which are mainly felsic volcanic rocks mixed with some mafic rocks. The Eu/Eu*–GdN/YbN diagram shows that the parent rocks mainly formed in the late Archean and mixed with the old Archean strata that are exposed in some provenance areas. Based on the comprehensive analysis of the structural discrimination maps of the related rare earth elements and major elements, it is concluded that the tectonic setting of the source area was mainly composed of island arcs and active continental margins.