Mineralogical and Geochemical Compositions of Ammonian Illite-Enriched High-Rank Coals of the Xingying Mine, Northeastern Chongqing, China.

Ammonian illite (NH4-illite)-rich late Permian coals of high rank were discovered in southwestern China. This research reports new mineralogical and geochemical data of 11 bench samples from the adjacent Xingying mine, northeastern Chongqing Coalfield, southwestern China, with an emphasis on the modes of occurrence and origin of NH4-illite. The Xingying coals, with low ash yields and medium sulfur, have a high rank (semianthrite, R o,ran = 3.67%), owing to the plutonic metamorphism. Minerals in the coal consist of NH4-illite and pyrite and, to a lesser extent, jarosite, albite, and anatase, with traces of chamosite, quartz, bassanite, apatite, fluorapatite, florencite, and rhabdophane. Compared with world hard coals, vanadium is significantly enriched with a concentration coefficient (CC) higher than 10; Mo and Pb are enriched (5 < CC < 10); F, Co, Ni, Cu, Ge, Se, Y, Zr, Nb, Ag, Cd, In, Sn, Cs, Sm, Eu, Tb, Dy, Er, Yb, Hf, Bi, and U are slightly enriched (2 < CC < 5) in the Xingying coals. Fluorine in host rocks, including roof, floor, and parting, is significantly enriched. Fluorine concentration in the coal may be increased greatly if the coal is mixed with host rocks during mining activity. Hence, the Xingying coals should be subjected to beneficiation before utilization for the environment and human health. The Al2O3/TiO2 and Eu anomalies demonstrated that the terrigenous materials come from the mafic basalts of the Kangdian Upland. NH4-illite is formed by interaction of pre-existing kaolinite or K-illite with NH4 + released from organic matter under high temperatures during the process of hydrothermal alteration. The authigenic chamosite, albite, quartz, anatase, apatite, fluorapatite, and rhabdophane are also deposited from the hydrothermal solutions. In addition, the Xingying coals are subjected to marine influences. Based on the preliminary evaluation, the Xingying coals cannot be a potential source for critical elements such as rare earth elements and yttrium. This indicates that not all the late Permian coals in southwestern China have economic significance for critical elements.

Introduction

In addition to organic matter, mineral matter is the other component of coal. Mineral matter encompasses crystalline minerals, noncrystalline minerals, and nonmineral elements.[1−4] Coal or coal-bearing strata are enriched significantly in critical elements such as rare earth elements and yttrium (REY), Li, Nb, Ta, Zr, Hf, Ga, Ge, etc. and have the potential to recover these critical elements.[5−11] Although organic matter is a main carrier for some critical elements (e.g., Ge) in the coal,[12,13] critical elements mainly occur in minerals.[2,4,14] Meanwhile, minerals are also main carriers for some environmentally sensitive elements, e.g., F, As, and Hg.[15−17] Moreover, modes of occurrence of minerals could provide useful information on the coal-forming process and even the regional geological background or evolution.[15,18]

Ammonian illite (NH4-illite), with a similar structure with tobelite, is uncommon in coal.[1] However, it has been identified in some high-rank coals, varying from low volatile bituminous coal to semianthracite.[19−23] NH4-illite is considered as an interaction product of pre-existing kaolinite or K-illite with NH4+ originated from organic matter decomposition during hydrothermal solution influx.[1] Thus, the presence of NH4-illite indicates a hydrothermal alteration origin.

The late Permian coals in southwestern China (including Yunnan, Guizhou, Sichuan Provinces, and Chongqing Municipality) have intrigued scientists for more than three decades, due to their geochemical and mineralogical anomalies. The Kangdian Upland, mainly composed of mafic basalts, provided the dominant terrigenous materials for the late Permian coals in southwestern China.[24] Moreover, other geological factors, e.g., hydrothermal fluid injection, volcanic ash input, and marine influences, made significant contributions to the enrichment of trace elements.[15] Anomalous critical elements, especially REY, Li, Nb, Ta, Zr, Hf, and Ga, were discovered in a few late Permian coal deposits, including Moxinpo,[23,25,26] Zhongliangshan,[27] and Songzao[28] in Chongqing, southwestern China.

The Xingying Mine, located in northeastern Chongqing, which is adjacent to the Moxinpo, Zhongliangshan, and Songzao Mines, is an important coal base for power generation (Figure ). However, whether the late Permian coals in the Xingying Mine contain such critical elements is unclear. In addition, there is lack of mineralogical and geochemical data to discriminate the origin of terreginous materials. The purpose of this paper is to study the geochemical and mineralogical compositions, especially significantly enriched NH4-illite, of the Xingying coals and to understand the geological factors of their formation. It also makes an preliminary evaluation of critical elements of the Xingying coals.

Figure 1

Location of the Xingying Mine.

Geological Setting

The coal-accumulating basin in southwestern China is located in the western Yangtze Plate (Figure ). From west to east, the late Permian coal-bearing sequences in southwestern China vary from terristrial to transitional to marine environments.[24] During the middle Permian period, the Dongwu Movement leads to the Yangtze Plate uplift and differential erosion of the Maokou Formation. Hence, the formation of residual plains provides favorable terrian conditions for depositon of coal-bearing sequences. The Maokou Formaiton disconformably underlies with the coal-bearing sequences, which is an important sedimentary interface. Subsequently, Emeishan basalts erupt on the eroded surface of the Maokou Formation. The continuous eruption of basalts accumulates and produces the wellknown Kangdian Upland (Figure ).

The coal-bearing stratum in the present study is the late Permian Wujiaping Formation (P3w) with a thickness from 60.9 to 143 m (Figure A). The Wujiaping Formation consists of mudstone, carbonaceous mudstone, tuff layer, marl, limestone, cherty limestone, and coal seam, which is numbered as K2. The K2 is the exclusive minable coal seam in the Xingying Mine and has a thickness varying from 0.52 to 2.52 m, with an average of 1.57 m.

Figure 2

(A) Sedimentary sequences and the (B) collected samples of the Xingying Mine.

The Changxing Formation (P3c) conformably overlies the Wujiaping Formation and is made up of limestone, cherty limestone, silty limestone, and limstone with banded chert. It has a thickness from 60.9 to 143 m.

The middle Permian Maokou Formation (P2m) underlies the Wujiaping Formation inconformably. It is composed of thick-layered bioclastic limestone, cherty limestone, and limestone, with a thickness from 95.1 to 161 m.

Samples and Analytical Procedures

The studied samples, including one roof, one parting, one roof, and eight coal samples, were collected at the Xingying underground Mine in the northeastern Chongqing Coalfield, southwestern China (Figure B). In order to discriminate the noncoal samples, suffixes of r, p, and f were added in the sample number to represent roof, parting, and floor samples, respectively. All the samples were preserved in plastic bags in case of pollution and oxidation.

The blocked samples were prepared to make the polished sections for vitrinite reflectance and scanning electron microscope analysis. Then, the individual samples were crushed and ground to <1 mm. Parts of <1 mm samples were further gound to <0.076 mm (200 mesh) for proximate, ultimate, mineralogical, and geochemical analyses.

The moisture, ash yield, and volatile matter were tested based on the ASTM standards D3173-11, D3174-11, and D3175-11, respectively.[29−31] The total sulfur and forms of sulfur were analyzed according to ASTM standards D3177-02 and D2492-02, respectively.[32,33] The ultimate analyses including carbon, hydrogen, and nitrogen were conducted using an element analyzer. The vitrinite random reflectances of coal polished samples were measured using a spectrophotometer equipped on an optical microscope at a magnification of 500× in which the test standard is gadolinium gallium garnet with reflectance 1.72% made in China.

Prior to X-ray diffraction (XRD), coal samples were ashed at a temperature lower than 120 °C. Then, the coal low-temperature ashes (LTAs) and roof, parting, and floor samples were subjected to mineral composition analysis. The analysis condition of XRD was reported in detail in previous studies.[23,34] Based on the XRD diffractograms, the minerals were quantitatively determined using the Siroquant technique. Five representative samples were analyzed for modes of occurrence of minerals using scanning electron microscopy in conjunction with energy-dispersive spectrometry (SEM-EDS). The SEM-EDS analysis procedure and conditions were also reported by Zou et al.[34]

The major element oxides including SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O, and P2O5 were determined by X-ray fluorescence (XRF) spectrometry. Prior to XRF analysis, all the coal and host rock samples were ashed at 815 °C and the resultant ashes were made into tableting samples with lithium borate.

Except for F and Hg, the trace elements were analyzed using inductively coupled plasma mass spectrometry (ICP-MS). Dai et al. described the ICP-MS analysis procedures in detail.[23] In order to avoid the interference of polyatomic ions, the collision/reaction cell technology was employed to test the concentrations of As and Se.[35]

Fluorine in the samples was tested using the pyrohydrolysis ion-selective electrode method in accordance with the ASTM D5987-96.[36] Mercury was determined using a Milestone DMA-80 analyzer with a detection limit of 0.005 ng.

Results

Coal Characteristics

The coal quality data, including proximate and ultimate analyses, total sulfur, forms of sulfur, and vitrinite random reflectance, are listed in Table . The average values of volatile matter and vitrinite random reflectance are 9.15 and 3.67%, respectively, indicating a semianthracite based on the ASTM D388-12,[37] due to the plutonic metamorphism.[38] The Xingying coals have low ash yields (16.86%) and medium sulfur contents (1.66%) according to classifications of the Chinese standards GB/T 15224.1-2010 (coals with ash yields varying from 10 to 20% are low ash coals)[39] and GB/T 15224.2-2010 (coals with sulfur contents varying from 1.01 to 2% are medium sulfur coals).[40] Organic sulfur is the dominant form of total sulfur, followed by sulfate and pyritic sulfur. However, the discrepancies among organic, sulfate, and pyritic sulfur are insignificant. The average contents of carbon, hydrogen, and nitrogen are 89.86, 2.56, and 1.52%, respectively.

Table 1

Proximate and Ultimate Analyses, Forms of Sulfur, and Random Vitrinite Reflectance for Coals from the Xingying Mine (%)a

samples	proximate analyses			ultimate analyses				forms of sulfur			Ro,ran
	Mad	Ad	Vdaf	Cdaf	Hdaf	Ndaf	St,d	Sp,d	Ss,d	So,d
WSXY-2	3.68	37.75	11.23	88.44	2.79	1.41	1.61	0.50	0.48	0.63	3.86
WSXY-4	3.48	11.61	10.46	87.95	2.62	1.50	1.55	0.28	0.69	0.58	3.73
WSXY-5	4.17	16.29	9.21	90.01	2.58	1.62	1.95	0.58	0.62	0.75	3.64
WSXY-6	5.01	15.17	6.56	91.55	2.42	1.65	1.33	0.44	0.15	0.74	3.68
WSXY-7	3.85	12.52	7.13	91.73	2.44	1.54	1.32	0.35	0.29	0.68	3.70
WSXY-8	4.71	11.65	11.71	87.42	2.52	1.40	2.98	0.51	1.31	1.16	3.47
WSXY-9	3.84	16.92	8.78	90.91	2.55	1.57	1.37	0.52	0.47	0.38	3.59
WSXY-10	3.04	12.98	8.10	90.87	2.54	1.49	1.16	0.41	0.38	0.38	3.70
average	3.97	16.86	9.15	89.86	2.56	1.52	1.66	0.45	0.55	0.66	3.67

M, moisture; A, ash yield; V, volatile matter; C, carbon; H, hydrogen; N, nitrogen; St, total sulfur; Sp, pyritic sulfur; Ss, sulfate sulfur; So, organic sulfur; ad, air-dried basis; d, dry basis; daf, dry and ash-free basis; Ro,ran, vitrinite random reflectance.

Minerals

Mineral Compositions in Coal LTAs, Roof, Floor, and Parting Samples

Figure shows that the LTAs have significant positive correlations with the high-temperature ash (HTA) yields in the Xingying coals (R2 = 0.99). However, the LTAs are sligtly higher than the HTAs, which is not an uncommon phenomenon, confirmed by previous studies,[34,41,42] owing to the complete decomposition of minerals under high temperature conditions and the formation of new minerals under low temperature conditions. The mineralogical compositions of the Xingying coal LTAs and host rock samples are listed in Table . Minerals of the Xingying coal LTAs are dominated by NH4-illite (55–74.6%) and pyrite (5.9–15.5%), followed by jarosite (1.8–23.8%), albite (1.9–32.44%), and anatase (bdl-10%) with traces of chamosite. Quartz is only present in sample WSXY-2. Bassanite is detected in samples WSXY-4 and WSXY-5. Florencite only occurs in sample WSXY-10. Minerals of the roof mainly comprise NH4-illite, quartz, pyrite, and albite and, to a lesser extent, anatse, chamosite, and jarosite, which is similar to those of coal LTAs. However, the content of quartz in roof (13%) is much higher than that in coal LTAs. The floor and parting have similar mineral assemblages, whereas NH4-illite is abnormally enriched (higher than 90%) with traces of anatase, chamosite, pyrite, and jarosite. In addtion, trace minerals below the XRD detection limit, such as apatite, fluorapatite, and rhabdophane, are comfirmed using SEM–EDX.

Figure 3

Comparison between high-temperature ash (HTA) and low-temperature ash (LTA).

Table 2

Mineralogical Compositions of Coal LTAs and Noncoal Samples by XRD and Siroquant Analysis (wt %)a

samples	LTA/HTAb	Qz	Chm	NH4-illite	Py	Ant	Ab	Jrs	bassanite	florencite
WSXY-1-r	88.37b	13.0	1.7	66.3	10.1	2.7	5.7	0.6
WSXY-2	40.1	1.1	2.2	55.0	5.9	1.6	32.4	1.8
WSXY-3-p	84.23b		1.3	94.3	0.3	3.7		0.4
WSXY-4	14.36		1.6	71.4	10.2	1.8	2.9	10.8	1.4
WSXY-5	18.65		0.1	73.9	11.8	4.4	3.6	6.0	0.3
WSXY-6	17.17		2.4	74.6	10.8	5.4	2.8	4.0
WSXY-7	13.68		0.4	70.3	11.4	5.6	2.9	9.3
WSXY-8	15			55.9	15.5		4.8	23.8
WSXY-9	19.62		0.3	67.4	11.3	10.0	1.9	9.1
WSXY-10	15.27		0.1	71.8	12.9	4.7	2.9	7.3		0.2
WSXY-11-f	90.43b		2.6	91.7	1.5	4.0		0.1

Qz, quartz; Chm, chamosite; Py, pyrite; Ant, anatase; Ab, albite; Jrs, jarosite. Blank represents below detection limits.

HTA.

Ammonian Illite and Chamosite

Owing to the XRD characteristics with a d(001) crystal spacing of around 10.35 Å (Figure ),[19,43] the illite in the Xingying coals is confirmed as ammonian illite (NH4-illite or tobelite) rather than K-illite. NH4-illite is considered as an interaction product between kaolinite or K-illite already present in the coal and NH4+ derived from decompostion of the organic matter during hydrothermal alteration at a relatively high temperature.[1,20,23] NH4-illite is usually present in the high-rank coal seams, such as the low volatile bituminous coal of the South Walker Creek area, Australia,[20] low volatile bituminous coal of the Adaohai Mine,[19] anthracite of the Wangtaipu Mine,[21] and semianthracite of the Tianjia Mine,[22] China. NH4-illite occurs as a bedding plane (Figure A–C) or cell-filling (Figure D–F). A total of 48 spots of NH4-illite have been determined under SEM–EDX (Table ). The content of potassium varying from the below detection limit to 3.96%, with an average of 2.54%, is lower than that of K-illite (7.5%, K1.5Al4(Si6.5Al1.5)O20(OH)4).[1] Thus, the SEM-EDS data further confirmed the existence of ammonian illite. Chamosite is generally rare in coal.[44,45] However, it is not uncommon in the late Permian coals in southwestern China.[22,44,46−48] In the present study, chamosite fills in the cell cavity (Figure A–C), coexisting with albite or quartz. Chamosite also occurs as a colloidal form in the matrix of NH4-illite (Figure D).

Figure 4

X-ray diffractogram of sample WSXY-7, showing ammonian illite and other phases present.

Figure 5

Modes of occurrence of NH4-illite and other mineral phases. (A) NH4-illite occurred as bed planes in sample WSXY-7; (B) NH4-illite occurred as bed planes in sample WSXY-2; (C) NH4-illite in collodetrinite in sample WSXY-7; (D) cell-filling NH4-illite in sample WSXY-7; (E) cell-filling NH4-illite in sample WSXY-2. (F) Fracture-filling NH4-illite and chamosite in sample WSXY-7. Images A–F: SEM and back-scattered electron images.

Table 3

SEM–EDX Semiquantitative Analysis of Some Minerals in the Coal and Rock Samples (%; on Carbon-Free Basis)a

		O	F	Na	Mg	Al	Si	P	S	K	Ca	Ti	V	Fe	Cu	Mn
NH4-illite (n = 48)	min	53.05	bdl	bdl	bdl	5.36	9.84	bdl	bdl	bdl	bdl	bdl	bdl	bdl	bdl	bdl
	max	80.40	bdl	bdl	4.01	16.50	25.72	bdl	0.77	3.96	0.98	6.31	0.46	5.97	bdl	bdl
	ave	59.88	bdl	bdl	1.80	12.62	20.81	bdl	0.06	2.54	0.03	0.65	0.01	1.63	bdl	bdl
chamosite (n = 27)	min	49.82	bdl	bdl	4.31	5.07	8.24	bdl	bdl	bdl	bdl	bdl	bdl	7.91	bdl	bdl
	max	73.43	bdl	bdl	11.23	12.89	18.62	bdl	0.60	1.78	0.50	1.61	bdl	18.27	bdl	bdl
	ave	56.54	bdl	bdl	7.25	9.41	12.81	bdl	0.05	0.25	0.03	0.15	bdl	13.51	bdl	bdl
albite (n = 13)	min	46.27	bdl	4.85	bdl	7.39	22.56	bdl	bdl	bdl	bdl	bdl	bdl	bdl	bdl	bdl
	max	62.24	bdl	8.96	bdl	13.43	33.72	bdl	bdl	bdl	bdl	bdl	bdl	1.59	bdl	bdl
	ave	55.12	bdl	7.38	bdl	9.08	28.21	bdl	bdl	bdl	bdl	bdl	bdl	0.21	bdl	bdl
pyrite (n = 20)	min	bdl	bdl	bdl	bdl	bdl	bdl	bdl	5.19	bdl	bdl	bdl	bdl	8.51	bdl	bdl
	max	66.04	7.05	bdl	4.75	10.25	15.47	2.48	57.91	2.11	5.98	1.25	bdl	45.59	3.16	0.62
	ave	26.58	0.35	bdl	0.50	2.62	3.88	0.12	34.64	0.39	0.30	0.06	bdl	30.23	0.30	0.03
anatase (n = 9)	min	50.18	bdl	bdl	bdl	0.57	1.02	bdl	bdl	bdl	bdl	12.23	bdl	bdl	bdl	bdl
	max	60.26	bdl	bdl	3.20	9.57	14.45	bdl	bdl	2.08	bdl	46.57	bdl	4.58	bdl	bdl
	ave	54.57	bdl	bdl	1.02	4.28	7.07	bdl	bdl	0.79	bdl	30.75	bdl	1.53	bdl	bdl
quartz (n = 10)	min	48.88	bdl	bdl	bdl	bdl	34.41	bdl	bdl	bdl	bdl	bdl	bdl	bdl	bdl	bdl
	max	62.85	bdl	bdl	2.07	4.05	43.04	bdl	0.62	0.85	2.74	bdl	bdl	3.29	bdl	bdl
	ave	58.15	bdl	bdl	0.37	1.27	39.38	bdl	0.06	0.16	0.27	bdl	bdl	0.33	bdl	bdl
apatite (n = 3)	min	47.03	bdl	bdl	1.36	1.34	1.94	5.32	bdl	bdl	11.92	bdl	bdl	2.89	bdl	bdl
	max	53.67	bdl	bdl	5.28	5.51	8.70	13.19	bdl	bdl	27.83	bdl	bdl	9.60	bdl	bdl
	ave	50.72	bdl	bdl	3.54	3.56	5.67	9.72	bdl	bdl	20.37	bdl	bdl	6.43	bdl	bdl
fluroapatite (n = 3)	min	43.72	5.41	bdl	bdl	bdl	bdl	13.66	bdl	bdl	28.29	bdl	bdl	bdl	bdl	bdl
	max	46.95	14.16	bdl	bdl	1.67	3.04	15.26	bdl	0.47	31.83	bdl	bdl	bdl	bdl	bdl
	ave	45.40	8.98	bdl	bdl	0.56	1.01	14.25	bdl	0.16	29.64	bdl	bdl	bdl	bdl	bdl

bdl, below detection limit.

Figure 6

Modes of occurrence of chamosite and other mineral phases. (A) Cell-filling chamosite and albite in sample WSXY-2; (B) cell-filling chamosite and albite in sample WSXY-2; (C) cell-filling chamosite and quartz in sample WSXY-2. (D) Colloidal chamosite in sample WSXY-3-p. Images A–D: SEM and back-scattered electron images.

Albite and Pyrite

The Na-bearing mineral in the semianthracite of Tianjia Mine, adjacent to the Xingying Mine, is paragonite.[22] However, the Na-bearing mineral in the present study is albite. This can be confirmed not only by the XRD + Siroquant data (Table ) but also by the SEM–EDX data (Table ). Table shows that albite is present in most coal samples, especially sample WSXY-2 with a proportion of 32.4%. The chemical compostions of albite are relatively pure and are composed of O, Al, Si, Na, and in some cases traces of Fe, with Na varying from 4.85 to 8.96% and averaging 7.38% (Table ). In addition, the intensity of Al is approximately half that of Si in albite under EDX spectra, which is different from the paragonite EDX spectral characteristics with almost equal intensities of Al and Si. Albite is uncommon in the coal[4] and has been observed in the Songzao coals and Donglin coals in Chongqing.[49,50] Albite is presented in the matrix of mixed layers of illite/smectite,[49] but it occurs as cell- or fractrue-fillings (Figure A–D) in the present study. Pyrite is a common mineral in coals, especially in coals formed in marine environments.[51] Pyrite is relatively abundant in the Xingying coals vayring from 5.9% to 15.5% (Table ) and occurs as a framboid and pyritohedron (Figure A–D).

Figure 7

Modes of occurrence of albite and other mineral phases. (A) Cell-filling albite and euhedral pyrite in sample WSXY-2; (B) cell-filling albite in sample WSXY-2; (C) fracture-filling albite and apatite in sample WSXY-1-r; (D) fracture-filling albite in sample WSXY-2. Images A–D: SEM and back-scattered electron images.

Figure 8

SEM backscattered images of pyrite and other mineral phases. (A) Framboidal pyrite and disseminated quartz in sample WSXY-1-r; (B) framboidal pyrite in sample WSXY-2; (C) cell-filling pyrite in sample WSXY-3-p; (D) pyritohedron pyrite in sample WSXY-7.

Anatase and Quartz

Anatase distributes in the matrix of NH4-illite and occurs as subhedral (Figure A) or colloidal form (Figure B). Compared to the other late Permian coals in southwestern China,[52] quartz is not enriched in the Xingying coals. Quartz fills in the cell cavity (Figure C) or occurs as disseminations (Figure A).

Figure 9

SEM backscattered images of anatase and other mineral phases. (A) Subhedral anatase in sample WSXY-3-p; (B) colloidal anatase in sample WSXY-11-f.

Apatite, Fluorapatite, and Rhabdophane

Apatite is observed in the roof sample and occurs as fracture-filling (Figure C), coexisting with albite. Fluroapatite is common in the host rocks and present as a matrix or subhedral texture (Figure A,B), which is the cause of the fluorine enrichement. The content of fluorine in the fluroapatite varies from 5.41 to 14.16%, with an average of 8.98% (Table ). Rhabdophane is detected in the roof, coal, and floor samples, occurring in the granular form in the matrix of NH4-illite (Figure A–F).

Figure 10

SEM backscattered images of fluorapatite and other mineral phases. (A) Fluorapatite and quartz in sample WSXY-1-r; (B) fluorapatite in sample WSXY-11-f.

Figure 11

SEM back-scattered images and EDX spectra of rhabdophane and other mineral phases. (A) Rhabdophane occurred in the matrix of NH4-illite in sample WSXY-3-p; (B) EDX spectrum of spot 1; (C) rhabdophane occurred in the matrix of NH4-illite in sample WSXY-7; (D) EDX spectrum of spot 2; (E) rhabdophane occurred in the matrix of NH4-illite in sample WSXY-11-f; (F) EDX spectrum of spot 3.

Geochemistry

Major Elements

The major elements in the Xingying coals are mainly composed of SiO2, Al2O3, and Fe2O3 (Table ). However, the contents of SiO2, Al2O3, and Fe2O3 (7.29, 3.94, and 2.64%, respectively) in the present study are all lower than those in the Chinese coals.[15] The ratio of SiO2/Al2O3 (1.78) is slightly higher than that in the Chinese coals (1.44)[15] and NH4-illite (1.18), indicating quartz present, which is mentioned above.

Table 4

Loss on Ignition (LOI, %), Percentages of Major Element Oxides (%), and Concentrations of Trace Elements (μg/g) for Coal and Host Rock Samples in the Xingying Mine (on Coal or Rock Basis)a

	WSXY-
element	1-r	2	3-p	4	5	6	7	8	9	10	11-f	avg	world coal[57]	CC
LOI	11.63	63.64	15.77	88.80	84.39	85.59	87.96	88.90	83.73	87.41	9.57	83.8	nd
SiO2	48.36	18.68	44.59	4.66	6.68	6.34	5.38	4.16	6.73	5.71	48.42	7.29	8.47b
TiO2	2.3	1.3	4.23	0.43	1.15	1.2	0.77	0.11	1.9	0.75	4.54	0.95	0.33b
Al2O3	17.82	8.43	22.18	2.73	3.86	3.74	3.12	2.33	3.98	3.35	23.66	3.94	5.98b
Fe2O3	9.72	4.29	5.17	2.48	2.72	1.99	1.72	3.72	2.45	1.73	4.02	2.64	4.85b
MnO	0.051	0.006	0.022	0.003	0.002	0.002	0.003	0.002	0.003	0.002	0.008	0	0.02b
MgO	4.29	1.44	2.99	0.25	0.34	0.32	0.29	0.25	0.34	0.29	4.18	0.44	0.22b
CaO	0.79	0.14	0.31	0.14	0.15	0.17	0.17	0.08	0.16	0.17	0.4	0.15	1.23b
Na2O	0.384	0.527	0	0.011	0.03	0.021	0.022	0.015	0.014	0.021	0	0.08	0.16b
K2O	3.44	1.03	3.82	0.29	0.41	0.37	0.31	0.29	0.41	0.33	4.49	0.43	0.19b
P2O5	0.344	0.029	0.048	0.009	0.014	0.015	0.011	0.008	0.018	0.012	0.039	0.01	0.09b
Al2O3/SiO2	2.71	2.22	2.01	1.71	1.73	1.7	1.72	1.79	1.69	1.7	2.05	1.78	1.42b
Li	27.4	9.45	16.9	2.08	2.99	1.56	2.14	1.95	2.04	1.52	16.6	2.97	12	0.25
Be	5.83	1.78	5.64	1.78	2.29	2.36	2.48	1.2	2.37	2.32	2.5	2.07	1.6	1.30
B	193	88.2	309	51.2	66.6	68.1	55.8	37.8	65.4	57	350	61.2	52	1.18
F	1889	551	2382	265	393	390	589	228	327	278	3385	378	88	4.29
Sc	12.5	11.8	26.5	3.15	6.44	5.97	4.69	2.44	6.89	4.76	19.8	5.77	3.9	1.48
V	156	412	482	310	364	316	155	107	243	186	641	262	25	10.46
Cr	153	36.2	191	22.9	42.8	38.1	26.9	19.7	40.8	27.1	73.2	31.8	16	1.99
Co	26.4	13.5	21.1	9.88	15.1	14.7	14.7	6.9	13.7	14.6	4.66	12.9	5.1	2.53
Ni	71.3	30.6	53.1	43.7	55.2	49.3	36.8	16.1	39.9	37	34.4	38.6	13	2.97
Cu	142	96.2	211	44.1	93.9	77.2	48.4	27.9	123	63.8	130	71.7	16	4.48
Zn	183	35.1	117	23.6	40.2	27.5	26.8	25.4	27.6	22	37.4	28.5	23	1.24
Ga	27.6	12	40.6	8.48	10.9	11.5	10.1	6.99	9.99	9.2	38.1	9.88	5.8	1.70
Ge	2.24	1.64	5.43	4.63	5.7	5.77	5.84	3.26	4.3	4.23	4.11	4.42	2.2	2.01
As	4.64	2.45	3.79	6.07	6.68	4.39	4.3	8.27	6.39	4.52	1.1	5.38	8.3	0.65
Se	3.01	5.08	9.03	4.82	4.83	3.93	4.14	4.64	5.97	4.78	1.31	4.77	1.3	3.67
Rb	50.8	14.5	70.7	4.76	6.63	5.73	4.52	5.27	6.73	4.75	93.3	6.61	14	0.47
Sr	426	161	435	132	146	157	161	101	154	152	424	145	110	1.32
Y	41.8	39	52.3	22.5	26.3	27.9	24.9	25.7	31.1	28.9	22	28.3	8.4	3.37
Zr	580	379	627	56.2	121	111	76.3	42.3	122	82.5	618	124	36	3.44
Nb	70.1	10.3	108	3.53	9.63	8.12	7.36	3.38	19.2	10.1	87.3	8.96	3.7	2.42
Mo	1.51	2.74	5.3	14.3	22.9	22.5	20.2	11	15.9	15.6	11.1	15.64	2.2	7.11
Ag	2.17	1.23	2.36	0.19	0.53	0.38	0.26	0.16	0.44	0.28	2.23	0.44	0.095	4.58
Cd	1.85	0.79	2.19	0.38	0.50	0.44	0.39	0.46	0.42	0.37	0.83	0.47	0.22	2.13
In	0.2	0.12	0.2	0.05	0.11	0.1	0.07	0.04	0.13	0.08	0.27	0.09	0.031	2.83
Sn	5.47	2.73	4.93	1.59	3.34	3.44	3.14	1.44	3.06	2.45	5.51	2.65	1.1	2.41
Sb	0.12	0.08	0.53	0.09	0.42	0.37	0.33	0.23	0.31	0.32	0.11	0.27	0.92	0.29
Cs	15.2	2.14	15.3	bdl	bdl	bdl	bdl	bdl	bdl	bdl	13.1	2.14	1	2.14
Ba	115	47.9	197	29.9	45.5	46.5	33.9	18.9	49.2	34.9	207	38.3	150	0.26
La	58.5	50.1	75.1	9.41	17.2	14.9	10.1	8.83	19.8	12.2	18.9	17.8	11	1.62
Ce	133	93.8	149	19.1	32	29.8	20.8	18.6	42.5	26.4	40.1	35.4	23	1.54
Pr	14.8	12.6	16.9	2.75	4.32	4.1	2.97	2.8	5.96	3.81	4.26	4.91	3.5	1.40
Nd	59.8	51.3	66.1	12.4	18.3	17.6	13.3	13	25.8	17	17.2	21.1	12	1.76
Sm	12.1	9.68	12.8	3	4.08	3.9	3.21	3.28	5.56	3.9	5.14	4.58	2	2.29
Eu	2.77	2.38	3.17	0.77	1.08	1.05	0.87	0.85	1.41	1.03	1.6	1.18	0.47	2.51
Gd	12.8	9.38	13.5	3.57	4.65	4.67	3.96	3.98	6.18	4.63	6.19	5.13	2.7	1.90
Tb	1.86	1.27	1.95	0.56	0.72	0.73	0.64	0.63	0.88	0.71	0.99	0.77	0.32	2.40
Dy	10.7	7.48	11.3	3.66	4.61	4.81	4.23	4.13	5.32	4.67	5.83	4.86	2.1	2.32
Ho	1.99	1.48	2.06	0.72	0.92	0.96	0.88	0.79	1.03	0.92	0.98	0.97	0.54	1.79
Er	5.73	4.64	5.51	2.15	2.76	2.88	2.68	2.25	2.99	2.83	2.66	2.9	0.93	3.12
Tm	0.75	0.67	0.7	0.28	0.36	0.39	0.36	0.29	0.38	0.35	0.34	0.38	0.31	1.24
Yb	5.02	4.87	4.41	1.84	2.47	2.47	2.43	1.84	2.41	2.26	2.33	2.57	1	2.57
Lu	0.7	0.71	0.59	0.25	0.34	0.35	0.35	0.26	0.33	0.32	0.31	0.36	0.2	1.81
Hf	14.5	7.45	17.3	1.56	3.11	2.9	2.05	1.22	3.11	2.24	15.2	2.96	1.2	2.46
Ta	3.15	0.49	8.16	bdl	0.34	0.23	0.23	0.06	1.24	0.47	6.25	0.44	0.28	1.56
W	1.97	0.26	3.66	0.44	0.52	0.53	0.53	0.29	1.95	0.65	3.47	0.65	1.1	0.59
Bi	0.5	0.4	0.31	0.12	0.31	0.32	0.24	0.23	0.39	0.28	0.63	0.29	0.1	2.86
Hg	0.09	0.15	0.11	0.13	0.12	0.1	0.09	0.16	0.13	0.11	0.05	0.12	0.63	0.19
Tl	0.32	0.03	0.28	0.05	0.08	0.03	0.01	0.1	0.04	0.02	0.14	0.04	7.8	0.01
Pb	17.8	5	10.4	7.64	9.43	6.74	5.69	11.8	7.93	5.77	5.15	7.5	0.97	7.73
Th	9.59	4.9	15.9	1.68	3.1	2.78	2.08	1.51	2.99	1.99	14.7	2.63	3.3	0.80
U	3.7	11.1	18.5	7.54	7.81	5.22	4.32	8.62	8.14	4.05	13.8	7.1	2.4	2.96

CC, concentration coefficient, ratio between the average value of individual elements in studied coals and the mean value in world hard coals; bdl, below detection limit; nd, no data.

The data are cited from Dai et al.[15]

Trace Elements

The trace element concentrations in the Xingying coals are also listed in Table . In order to evaluate the enrichement degree of trace elements in coal, the concentration coefficient (CC, ratio of trace element concentration in studying coals vs that in world hard coals) was proposed.[53] CC > 100, 10 < CC < 100, 10 < CC < 100, 5 < CC < 10, 2 < CC < 5, 0.5 < CC < 2, and CC < 0.5 represent unusually enriched, significantly enriched, enriched, slightly enriched, normal, and depleted, respectively.[53] Based on the classifications, vanadium is significantly enriched; Mo and Pb are enriched; F, Co, Ni, Cu, Ge, Se, Y, Zr, Nb, Ag, Cd, In, Sn, Cs, Sm, Eu, Tb, Dy, Er, Yb, Hf, Bi, and U are slightly enriched; Li, Rb, Sb, Ba, Hg, and Tl are depleted; while the remaining elements are close to the average values for world hard coals (Figure ).

Figure 12

Concentration coefficients of trace elements in the Xingying coals.

Fluorine

The concentration of fluorine in the Xingying coals varies from 228 to 589 μg/g, with an average of 378 μg/g, which is higher than the average values for Chinese coals (130 μg/g)[53] and world hard coals (82 μg/g).[54] Note that fluorine concentrations of roof, floor, and parting in the Xingying Mine are enriched significantly, where fluorine of roof, floor, and paring is up to 1889, 3385, and 2382 μg/g, respectively (Figure ), much higher than the average values for the upper continental crust (UCC, 611 μg/g)[55] and world clay (610 μg/g).[56]

Figure 13

Variations of F, V, Cr, Co, Ni, and REY along the coal seam profile.

Fluorine is an environmentally sensitive element, and endemic fluorosis happened severely in western Guizhou province, where approximately 10 million people suffered from dental fluorosis and 1 million people suffered from skeletal fluorosis.[57] Previous studies indicated that coal-fired fluorosis in western Guizhou province is caused by combustion of the high-F coal.[58,59] However, more and more pieces of evidence indicate that F content is within the usual range of Chinese and world coals.[57,60] Thus, it is generally considered that the endemic fluorosis is due to the combustion of high-F clay used as a briquette binder for fine coal.[15,60] The F content in the host rocks (including roof, floor, and parting) is also unusally enriched, so if the Xingying coals are mixed with these rocks when mining, the mixed coals may enrich F significantly. The local residents may suffer from endemic fluorosis if the mixed coal is burned directly without beneficiation.[61]

The correlation coefficient between F and ash yield in the Xingying coals is 0.94, indicating that F mainly occurs as mineral matter. The correlation coefficients of rF-SiO2 and rF-Al2O3 are 0.94 and 0.97, implying that F may associated with clay minerals. Fluorine also correlated significantly with CaO (rF-CaO, 0.67), and the Ca-bearing minerals (e.g., fluroapatite) may be carriers of fluorine, especially in the noncoal samples (Figure ). Fluorine is also detected in pyrite under SEM–EDX (Table ).

Vanadium, Cr, Co, and Ni

Vanadium is enriched significantly in the Xingying coals with CC being 10.5. Cobalt (CC, 2.53) and Ni (CC, 2.97) are enriched sligtly. The content of Cr is close to the world hard coals, and the CC is 1.99. Along the coal profile, the concentration of V, Cr, Co, and Ni gradually decreases from top to botom (Figure ).

The correlation coefficients between ash yield and V, Cr, Co, and Ni are 0.54, 0.85, 0.34, and 0.42, respectively, indicating that V and Cr have significant positive correlations with ash yield. Thus, it is inferred that V and Cr mainly occurred in the mineral matter. The correlation coefficients of V and Cr with SiO2 (rV-SiO2, 0.53, rV-SiO2, 0.85) and Al2O3 (rCr-SiO2, 0.64, rV-SiO2: 0.82) further indicate that V and Cr are associated with clay minerals. However, cobalt and Ni have both inorganic and organic affinities.

Rare Earth Elements and Yttrium (REY)

The content of REY in the Xingying coals is from 82.9 to 289 μg/g and averages 131 μg/g, higher than those of the world hard coals (68 μg/g)[54] but close to those of the Chinese coals (136 μg/g).[15] Along the K2 coal profile, concentration variations of REY are insignificant except for sample WSXY-2 (Figure ). On the basis of coal ash, only two samples (WSXY-9 and WSXY-10) have higher REY than the cutoff grade of coal ash proposed by Seredin and Dai (REO, 1000 μg/g).[11]

Based on the threefold geochemical classifications, ie., LREY, MREY, and HREY, and three enrichement types (L-type, M-type, and H-type) proposed by Seredin and Dai[11] and the upper continental crust normalization,[55] the Xingying coals are all M-REY enrichment types except for sample WSXY-7 (H-type) (Figure ). The Xingying coals and parting all exhibit slightly CeN/CeN* negative anomalies; however, the roof and floor show a slightly CeN/CeN* positive anomalies. EuN/EuN* positive anomalies occur in all the coal and host rock (including roof, floor, and parting) samples. Some studies considered that Eu may be interfered by Ba in ICP-MS when Ba/Eu is higher than 1000.[62,63] The Ba/Eu ratio of the Xingying coal and host rock samples varies from 20.2 to 130, much lower than 1000, indicating that Eu is free of Ba interference. REY in the Xingying coals correlated significantly with ash yield, with a correlation coefficient of 0.99, indicating that REY mainly occur in the inorganic matter.

Figure 14

UCC-normalized REY distribution patterns of coal and host rock samples in the Xingying Mine. REY data of upper continental crust (UCC) are from Taylor and Mclennan.[55]

Discussion

Source of Terrigenous Materials

Kangdian Upland, derived from the Emeishan mantle plume,[50,64−66] is mainly composed of flood basalts and considered as the dominant source region providing terrigenous materials for late Permian coals in southwestern China.[42,67−69] For most late Permian coals in southwestern China, the clastic materials are the tholeiitic basalt of the Kangdian Upland.[24] However, some studies have shown that the clastic materials of late Permian coals in southwestern China are the felsic-intermediate rocks at the top of the Kangdian Upland[70] or mafic tuff[23] or other provenances.[41,50,71] The Al2O3/TiO2 ratio is a reliable geochemical index for sediment source discrimination of sedimentary rocks including coal deposits,[9,72,73] which has been extensively used. The Al2O3/TiO2 ratio of 3–8, 8–21, 21–70 indicates that the parent rock composition is mafic, intermediate, and felsic igneous rocks, respectively.

All the Xingying coal and host rock samples fall into the category of mafic rocks except sample WSXY-8 (Figure ), implying that the terrigenous materials come from the mafic basalts of the Kangdian Upland. Moreover, the Eu anomaly is another parameter for indicative of the sediment source region of coal deposits.[62] If the UCC-normalized Eu anomaly occurs positive or negative in coals, it is suggested that the inorganic materials derived from mafic or felsic compositions, respectively.[62] The Eu anomaly has been used in the Huayingshan,[71] Nantong,[50] and southeastern Chongqing Coalfields[34] for indicative of the sediment source region being not the mafic basalts of the Kangdian Upland. The REY distribution patterns indicate that the inorganic material of the Xingying coals is from the mafic basalts of the Kangdian Upland, owing to the significant positive Eu anomalies, which further confirmed the results mentioned above.

Figure 15

Relationships between Al2O3 and TiO2 for coal and host rock samples in the Xingying Mine.

Injection of Hydrothermal Fluids

Injection of hydrothermal fluids plays an important role in the enrichment of trace elements especially the critical elements in the coal seams.[2,18,22,74] Previous studies have proved that geochemical and mineralogical anomalies of coals in southwestern China are attributed to the hydrothermal solutions.[23,75−77] Zhou et al. demonstrated that the Tianjia coals, adjacent to the present study area, were significantly affected by the hydrothermal solutions on the basis of Sr. isotope signature and mineral assemblages.[22] Similarly, the Xingying coals are also affected by the hydrothermal fluids, leading to the mineralogical and geochemical anomalies. There are some mineralogical and geochemical evidence.

NH4-illite is considered an interaction product between pre-existing kaolinite or K-illite and NH4+ decomposed from organic matter at a relatively high temperature during hydrothermal alteration.[20,21,23,78] Due to the high formation temperature (>250 °C),[43] NH4-illite is usually present in the high rank coals and associated rocks.[1,19,20,78−81] Based on the nitrogen isotope discrepancy between inorganic and organic matter, Xie et al. confirmed the hydrothermal origin of NH4-illite.[82] Thus, the presence of NH4-illite is indicative of precipitates derived from hydrothermal fluids that passed through the coal or subjected to increased geothermal gradients.[1,22] NH4-illite is enriched and distributed in the cell cavities or fractures (Table ; Figure D–F) in the Xingying coals, providing evidence for the hydrothermal origin. During the hydrothermal alteration process, NH4-illite is formed by interaction of pre-existing kaolinite or K-illite with NH4+ released from organic matter under high temperatures. Owing to the kaolinite or K-illite absence under SEM, the pre-existent kaolinite or K-illite may be completely altered by the hydrothermal solutions.

Chamosite is also a typical mineral in relation to the hydrothermal origin. It is formed by interaction of pre-existing kaolinite and Fe–Mg-rich fluids[20,44,48] or directly deposited from the siliceous solutions containing Fe–Mg.[46] In the present study, chamosite, independent of kaolinite, occurs in the cell-fillings (Figure A–C), indicating its hydrothermal origin. Albite of detrital origin was found in the roof sample from borehole 11,424 of Bowen Basin, Australia.[20] However, the occurrence of albite with cell- or fracture-filling (Figure A–D) in the present study indicates its authigenic origin. Another Na-bearing mineral, paragonite, is considered to be the result of hydrothermal alteration,[20,22] and this may also be the mechanism of albite in the present study. The cell-filling quartz was also identified (Figure C), indicating an authigenic origin. Additionally, chamosite, albite, and quartz occur in the same cell or fracture (Figure A–C), suggesting that the Xingying coals were subjected at least three injections of hydrothermal fluids.[1,22] In the hydrothermal fluid injections, the compositions of each fluid changed, leading to the formation of different minerals.[83] Based on the occurrence of these three minerals, chamosite formed earlier than quartz but later than albite.

The subhedral and colloidal anatase and fracture-filling apatite also suggest a hydrothermal origin. Rhabdophane, a secondary mineral containing REY, is rare in coal but is not uncommon in late Permian coals from southwestern China and considered as a product in relation to the hydrothermal origin.[23,34] Rhabdophane was confirmed in the roof, coal, and floor samples and occured in the granular form in the matrix of NH4-illite (Figure A–F), indicating a hydrothermal origin.

Gadolinium generally shows very weak negative anomalies in Chinese and US coals.[62] However, the coals influenced by hydrothermal solutions or other waters exhibit positive or weakly positive anomalies.[62,84] The Xingying coals occur weakly Gd positive anomalies varying from 0.96 to 1.19 and averaging 1.08 (Table ) and are characterized by M-type REY patterns (Figure ), which are typical of acid waters, including high pCO2-waters in coal basins.[84] Another evidence for the hydrothermal fluid injection in the present study is the redistribution of elements such as Nb/Ta, Zr/Hf, and U/Th in the coal, parting, roof, and floor samples. It is generally acknowledged that ratios of Nb/Ta, Zr/Hf, and U/Th in the coal altered by hydrothermal fluids are higher than those in the parting, roof, and floor samples.[71,77] Ratios of Nb/Ta, Zr/Hf, and U/Th in sample WSXY-2 are higher than those in the underlying parting (WSXY-3-p) (Figure ). These element pairs in sample WSXY-10 are also higher than those in the underlying floor except for Zr/Hf (Figure ). The geochemical evidence further confirmed that the Xingying coals are influenced by the injection of hydrothermal fluids.

Table 5

REY Geochemical Parameters of Coal and Host Rock Samples in the Xingying Minea

sample no.	REY (μg/g)	LaN/LuN	LaN/SmN	GdN/LuN	enrichment type	CeN/CeN*	EuN/EuN*	YN/HoN	GdN/GdN*	LaN/LaN*
WSXY-1-r	363	0.83	0.73	1.44	M–H	1.04	1.03	0.76	1.13	1.19
WSXY-2	289	0.71	0.78	1.04	M–H	0.85	1.15	0.96	1.16	1.23
WSXY-3-p	415	1.28	0.88	1.81	L–M	0.95	1.11	0.92	1.14	1.21
WSXY-4	82.9	0.38	0.47	1.14	M–H	0.86	1.08	1.13	1.11	1.49
WSXY-5	120	0.51	0.63	1.08	M–H	0.85	1.14	1.04	1.11	1.37
WSXY-6	116	0.43	0.57	1.06	M–H	0.87	1.13	1.05	1.12	1.30
WSXY-7	91.6	0.29	0.47	0.90	H	0.86	1.12	1.02	1.10	1.42
WSXY-8	87.2	0.34	0.40	1.22	M–H	0.85	1.08	1.18	1.11	1.62
WSXY-9	152	0.60	0.53	1.47	M–H	0.89	1.10	1.09	1.17	1.24
WSXY-10	110	0.38	0.47	1.13	M–H	0.88	1.11	1.14	1.13	1.35
WSXY-11-f	129	0.62	0.55	1.60	M–H	1.02	1.30	0.82	1.10	1.32
Average	131	0.45	0.54	1.13		0.86	1.11	1.08	1.13	1.38

LaN/LuN, ratio between LaN and LuN; LaN/SmN, ratio between LaN and SmN; GdN/LuN, ratio between GdN and LuN; YN/HoN, ratio between YN and HoN; CeN/CeN* = CeN/(0.5LaN + 0.5PrN); EuN/EuN* = EuN/(0.5SmN + 0.5GdN); GdN/GdN* = GdN/[(SmN × 0.33) + (TbN × 0.67)]; LaN/LaN* = LaN/(3PrN – 2NdN); N, upper continental crust normalized; L, light rare earth element enrichment; M, medium rare earth element enrichment; H, heavy rare earth element enrichment.

Figure 16

Variations of Nb/Ta, Zr/Hf, and U/Th along the coal seam profile.

Marine Influence

Marine sediment environments also make contribution to the trace element enrichment. Being sensitive to the sedimentary environment, B/Ga and Sr/Ba ratios are usually used as indices for sedimentary environment indicatives.[85] Boron and Ga are two different elements. Borate has high solubility and could migrate over long distances. The inactive gallium, however, is easy to be precipitated during migration processes. The B/Ga ratios being <1.5, 1.5–3, and >4–5 represent fresh, brackish, and saline water facies, respectively.[86] Strontium and Ba usually combine with SO42– to form SrSO4 and BaSO4, respectively. Owing to the different solubility, BaSO4 could be precipitated adjacent to provenance area and SrSO4 could be transported over long distances and precipitated in the ocean.[85] Hence, Sr in marine sediments has high Sr content and Sr/Ba ratios. The Sr/Ba ratios higher than 1 and lower than 1 indicate marine and continental environments, respectively.[85] The B/Ga and Sr/Ba ratios in the Xingying coals are all higher than 4 and 1, respectively, indicating that the Xingying coals are deposited in marine environments. As mentioned above, the total sulfur in the Xingying coals varies from 1.16 to 2.98% and averages 1.66%, also indicating a seawater-influenced deposition environment. Additionally, seawater is characterized by Y positive anomalies, so coals subjected to the seawater influence would expect to have positive Y anomalies.[62,85] Yttrium in the Xingying coals shows significant positive anomalies (Figure ), further demonstrating the marine influence.

Critical Elements

Due to their irreplaceable applications in modern technologies, REY are considered as critical elements.[11,62,87] In order to meet the increasing growth of REY demand, discovering new deposits is necessary.[5,87] Fortunately, coal and coal byproducts have the potential to provide REY in the foreseeable future.[8,11,88−92] The late Permian coals in southwestern China have been reported repeatedly for enrichment of REY.[9,23,27,42,68,93] Seredin and Dai proposed the cut-off grade of oxides of REY in coal ash (1000 μg/g) and an evaluation index, outlook coefficient (Coutl).[11] REY with the Coutl > 2.4, 0.7 ≤ Coutl < 1.9, and Coutl < 0.7 represent highly promising, promising, and unpromising, respectively.[23] Based on the relationship between cut-off grade and Coutl, most coal benches, the roof, parting, and the floor fall within the unpromising area (Figure ). Only two coal benches (WSXY-9 and WSXY-10) are within the promising area. However, the thickness of these two coal benches is thin (20 cm). Therefore, unlike other late Permian coals, the Xingying coals have no potential for extracting REY. The location of the present study is far away from the sediment source region, which may lead to a relatively lesser REY input. In addition, the hydrothermal fluids may contain lesser REY compositions, compared with other later Permian coals enriched with REY.

Figure 17

Evaluation of REY in the coal ashes and host rocks in the Xingying Mine.

Conclusions

The Xingying coals are classified as semianthracite owing to the plutonic metamorphism, with low ash and medium sulfur. The sulfur in coal is dominated by organic sulfur, followed by sulfate and pyritic sulfur. NH4-illite, which accounts for more than 50% in minerals, is significantly enriched in the Xingying coals. The other minerals include pyrite and, to a lesser extent, jarosite, albite and anatase, with traces of chamosite, quartz, bassanite, apatite, fluorapatite, florencite, and rhabdophane. NH4-illite is formed by the interaction of pre-existed kaolinite or K-illite with NH4+ decomposed from organic matter at high temperatures during hydrothermal solution ingress. The hydrothermal solution ingress also resulted in deposition of chamosite, albite, quartz, anatase, apatite, fluorapatite, and rhabdophane. The Xingying coals enrich V, Mo, Pb, F, Co, Ni, Cu, Ge, Se, Y, Zr, Nb, Ag, Cd, In, Sn, Cs, Sm, Eu, Tb, Dy, Er, Yb, Hf, Bi, and U, compared with world coals. The F content in the host rocks (including roof, floor, and parting) is unusally enriched, so if the Xingying coals are mixed with these rocks when mining, the mixed coals may enrich F significantly. The local residents may suffer from endemic fluorosis if the mixed coal is burned directly without beneficiation, which requires attention. The inorganic material of the Xingying coals comes from the mafic basalts of the Kangdian upland. Additionally, marine environments also make contribution to the inorganic material formation in the Xingying coals. Unlike other late Permian coals in southwestern China, the Xingying coals cannot be considered as alternative sources for extraction of REY.