Mineralogical and Geochemical Characteristics of Lithium and Rare Earth Elements in High-Sulfur Coal from the Donggou Mine, Chongqing, Southwestern China

: Coal and coal by-products are considered as the potential raw materials for critical elements (e.g., rare earth elements, Li, Ga, Ge, etc.), which have attracted much attention in recent years. The purpose of this study is to investigate the mineralogical and geochemical characteristics, and controlling geological factors of lithium and rare earth elements in the Lopingian (Wujiaping Formation) coal from the Donggou Mine, southeastern Chongqing Coalﬁeld, China. Results indicate that lithium and rare earth elements are signiﬁcantly enriched in the Donggou coals, which could be new potential alternative sources for critical elements. Concentrations of lithium and rare earth elements in the Donggou coals gradually increase from top to bottom. Lithium is mainly associated with kaolinite, while rhabdophane, ﬂorencite, goyazite, and xenotime are the main hosts of rare earth elements. The controlling geological factor is the groundwater leaching of underlying tu ﬀ , and to a lesser extent, the terrigenous clastic materials input from the top layer of the Kangdian Upland. This study provides mineralization information for lithium and rare earth elements exploration in coal measures.


Introduction
With the development of new energy and energy storage technologies, the consumption of critical elements such as lithium and rare earth elements (REY, or REE if yttrium is excluded) is increasing rapidly [1][2][3][4][5]. In recent years, coal and coal utilization by-products (e.g., coal gangue, coal combustion products, and acid mine drainage) have been considered as alternative sources for these critical elements [6][7][8][9][10][11].
Anomalies of lithium are uncommon in coals worldwide. High-Li coals have been discovered only in the Krylovsk and Verkhne-Bikinsk coal basins in the Russian Far East [7], Jungar Coalfield [12][13][14], Ningwu Coalfield [15], and Jincheng Coalfield [16,17] in Northern China.  indicated that the Appalachian bituminous coals have relatively high probabilities of being promising sources of Li in the United States [18]. It is found that Li was evenly distributed in coal fly ash with different particles [19,20] and most of the Li was found to be concentrated in the amorphous glass portion of fly ash from some coal-fired power plants in Shanxi and Inner Mongolia provinces, northern China [20].
The overlying stratum of the Wujiaping Formation is the Changxing Formation (P 3 c), the thickness of which varies from 144 to 173 m. The upper portion is mainly composed of dark gray thick layered bioclastic limestone, with massive or layered chert. The lower part consists of dark gray limestone with siliceous dolomite.
The lowermost stratum of the Wujiaping Formation is the Maokou Formation (P 2 m) which is in disconformable contact with the Wujiaping Formation and consists of light, dark gray bioclastic limestone. The upper part is composed of dark grey to grayish black thick layered limestone, mixed with dark grey thin layered siliceous limestone. The lower part includes dark grey to black grey thin-layer marl and thick layered limestone.
The overlying stratum of the Wujiaping Formation is the Changxing Formation (P3c), the thickness of which varies from 144 to 173 m. The upper portion is mainly composed of dark gray thick layered bioclastic limestone, with massive or layered chert. The lower part consists of dark gray limestone with siliceous dolomite.
The lowermost stratum of the Wujiaping Formation is the Maokou Formation (P2m) which is in disconformable contact with the Wujiaping Formation and consists of light, dark gray bioclastic limestone. The upper part is composed of dark grey to grayish black thick layered limestone, mixed with dark grey thin layered siliceous limestone. The lower part includes dark grey to black grey thin-layer marl and thick layered limestone.

Sampling and Analytical Methods
According to the Chinese standard for coal sampling (GB/T 482-2008) [38], 11 samples were collected from a working surface at the Donggou mine in the southeastern Chongqing Coalfield. These include one roof, two floor (tuff), and eight coal samples. All samples were numbered in an increasing order from top to bottom ( Figure 2B). Samples collected in the field were immediately stored in plastic bags to prevent potential contamination and oxidation, and sent to the laboratory for sample preparation.
Polished blocks were prepared for vitrinite random reflectance and scanning electron microscope (SEM) using particles 1 cm. Then all samples were ground to fine powders (<200 mesh) for coal chemistry, mineralogical, and geocdhemical tests. The proximate analysis (including moisture, ash, and volatile matter) of coal samples was conducted in accordance with ASTM Standards D3173-11 [39], D3174-11 [40], and D3175-11 [41]. The total sulfur was determined using the Eschka method according to ASTM D3177-02 [42] and forms of sulfur in coal were determined according to ASTM D2492-02 [43]. The carbon, hydrogen, and oxygen were measured using an elemental analyzer (VarioMACRO, Elementar, Langenselbold, Germany). The vitrinite random reflectance of coal samples was determined using the Craic QDI 302 TM spectrophotometer (Craic

Sampling and Analytical Methods
According to the Chinese standard for coal sampling (GB/T 482-2008) [38], 11 samples were collected from a working surface at the Donggou mine in the southeastern Chongqing Coalfield. These include one roof, two floor (tuff), and eight coal samples. All samples were numbered in an increasing order from top to bottom ( Figure 2B). Samples collected in the field were immediately stored in plastic bags to prevent potential contamination and oxidation, and sent to the laboratory for sample preparation.
Polished blocks were prepared for vitrinite random reflectance and scanning electron microscope (SEM) using particles 1 cm. Then all samples were ground to fine powders (<200 mesh) for coal chemistry, mineralogical, and geocdhemical tests. The proximate analysis (including moisture, ash, and volatile matter) of coal samples was conducted in accordance with ASTM Standards D3173-11 [39], D3174-11 [40], and D3175-11 [41]. The total sulfur was determined using the Eschka method according to ASTM D3177-02 [42] and forms of sulfur in coal were determined according to ASTM D2492-02 [43]. The carbon, hydrogen, and oxygen were measured using an elemental analyzer (VarioMACRO, Elementar, Langenselbold, Germany). The vitrinite random reflectance of coal samples was determined using the Craic QDI 302 TM spectrophotometer (Craic Technologies, San Dimas, CA, USA) installed on the Leica DM4500P microscope (Leica Inc., Wetzlar, Germany).
The oxides of major elements were determined by X-ray fluorescence spectrometry (XRF). Prior to XRF analysis, all the samples were ashed at 815 • C. Abundances of trace elements were tested by the inductively coupled plasma mass spectrometry (ICP-MS). Before ICP-MS analysis, the powdered coal and non-coal samples were subjected to microwave digestion using an UltraClave Microwave High Pressure Reactor (Milestone, Sorisole, BG, Italy). The digestion reagents were mixed acid composed of 65% HNO 3 and 40% HF. For coal samples digestion, the volumes of HNO 3 and HF were 5 mL and 2 mL, respectively. For non-coal samples digestion, the volumes of HNO 3 and HF were 2 mL and 5 mL, respectively. HNO 3 and HF were both Guaranteed reagent (GR). The basic loads of the digestion tank consisted of 330 mL ultrapure water, 30 mL H 2 O 2 , and 2 mL H 2 SO 4 . Inorganic Ventures standard references (CCS-1, CCS-4, CCS-5, and CCS-6) were used for calibration of trace element concentrations. In order to ensure the stability of ICP-MS, Rh was used as an internal standard.
Coal samples were subjected to low-temperature ashing (LTA) under the condition of <150 • C using the plasma low-temperature ashing instrument (Quorum/Emitech K1050X, Quorum Technologies, Lewes, UK). Then the mineral compositions of coal LTAs and non-coal samples were analyzed by X-ray powder diffraction (XRD). The instrument for the XRD analysis is D/max-2500/PC powder diffractometer (Rigaku Corporation, Tokyo, Japan) with Ni-filtered Cu-Kα radiation and a scintillation detector. The XRD pattern was recorded over a 2θ interval of 2.5~70 • , with a step size of 0.02 • . After the X-ray diffraction patterns were obtained, the minerals were quantitatively analyzed using Siroquant TM 3.0 software (Siroquant, Mitchell, Australia), which is set out by Rietveld [44] and developed by Taylor [45].
Modes of occurrence of minerals in representative samples were determined by a scanning electron microscope (SEM, JSM-6610LV, JEOL, Tokyo, Japan) equipped with energy-dispersive spectrometer (EDS, OXFORD X-max, Oxford Instruments, Abingdon-on-Thames, UK). Samples were platinum-coated using the JEOL JFC-1600 auto fine coater (JEOL, Tokyo, Japan) prior to SEM-EDS analysis. The analysis conditions of SEM-EDS are: accelerating voltage 20 kV, working distance 10 mm, and high vacuum mode. Table 1 lists data of the proximate and ultimate analysis, forms of sulfur, and vitrinite random reflectance of K1 coal seam in the Donggou Mine, southeastern Chongqing Coalfield. The ash yield of the Donggou coals varies from 15.41% to 43.74% (24.52% on average). Based on the Chinese standard GB/T 15224.1-2010 [46], the Donggou coals are low-medium ash coals. The average values of carbon, hydrogen, and nitrogen are 84.3%, 3.64%, and 1.43%, respectively. Based on Chinese standard GB/T 15224.2-2010 [47], all coals apart from PSDZ-4 are high sulfur coals, with an average of 6.65%. The sulfur in the Donggou coals is mainly pyritic sulfur, followed by organic sulfur and sulfate sulfur. The average values of volatile matter and vitrinite random reflectance are 16.02% and 2.19%, respectively. According to ASTM D388-12 [48], the Donggou coals are classified as low volatile bituminous coal.

Mineralogical Compositions
The low temperature ash yields (LTAs) of coal samples correlated significantly positively with the high temperature ash yields (HTAs) (R 2 = 0.99, Figure 3). However, the LTAs are slightly higher than the HTAs, especially sample PSDZ-8 and PSDZ-9, with LTAs of more than 50%, because minerals could be decomposed under high temperature conditions and new minerals could be formed under low temperature environments [49]. The quantitative mineralogical compositions of the Donggou coal LTAs and non-coal samples are presented in Table 2. The dominant minerals of the Donggou coal LTAs are clay minerals (including illite, kaolinite, illite/smectite mixed layer (I/S), and trace amounts of chamosite), and pyrite, followed by jarosite, quartz, and orthoclase. The minerals in the roof are mainly pyrite, clay minerals (including I/S and kaolinite), and quartz, with small amounts of aluminum hydroxide minerals (gibbsite and diaspore) and gypsum. The main minerals in the floor (tuff) are clay minerals (including kaolinite, illite, and a small portion of chlorite and chamosite) and pyrite, followed by anatase and gypsum. In addition to the minerals mentioned above, other trace minerals such as rhabdophane, florencite, goyazite, xenotime, and zircon are also identified under SEM-EDS observation. 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.

Mineralogical Compositions
The low temperature ash yields (LTAs) of coal samples correlated significantly positively with the high temperature ash yields (HTAs) (R 2 = 0.99, Figure 3). However, the LTAs are slightly higher than the HTAs, especially sample PSDZ-8 and PSDZ-9, with LTAs of more than 50%, because minerals could be decomposed under high temperature conditions and new minerals could be formed under low temperature environments [49]. The quantitative mineralogical compositions of the Donggou coal LTAs and non-coal samples are presented in Table 2. The dominant minerals of the Donggou coal LTAs are clay minerals (including illite, kaolinite, illite/smectite mixed layer (I/S), and trace amounts of chamosite), and pyrite, followed by jarosite, quartz, and orthoclase. The minerals in the roof are mainly pyrite, clay minerals (including I/S and kaolinite), and quartz, with small amounts of aluminum hydroxide minerals (gibbsite and diaspore) and gypsum. The main minerals in the floor (tuff) are clay minerals (including kaolinite, illite, and a small portion of chlorite and chamosite) and pyrite, followed by anatase and gypsum. In addition to the minerals mentioned above, other trace minerals such as rhabdophane, florencite, goyazite, xenotime, and zircon are also identified under SEM-EDS observation.     Table 3 shows the data of major and trace elements in the Donggou coals. Taking the Chinese common coals as comparative standard [24], concentrations of Fe 2 O 3 (7.11%) and K 2 O (0.68%) in the Donggou coals are relatively higher. By contrast, concentrations of MnO (0.002%), CaO (0.012%), Na 2 O (0.02%), and P 2 O 5 (0.013%) are lower than the Chinese common coals [24]. The remaining major elements and ratios of SiO 2 /Al 2 O 3 are close to the Chinese common coals [24]. Dai et al. (2015) proposed an index of concentration coefficient (CC) to characterize the concentration levels of trace elements in coal [50]. CC was defined as the ratio of elemental concentration of studying coals versus that of world hard coals [50,51]. Compared with world hard coals [51], lithium is enriched in the Donggou coals, the CC of which is up to 8.04. The Li concentration of the Donggou coals varies from 6.5 µg/g to 244 µg/g, with an average of 96.5 µg/g, which is significantly higher than the average value of world hard coals (12 µg/g) [51]. This value is slightly lower than that in the Haerwusu coals, Inner Mongolia, China (116 µg/g) [12], and that in the Jincheng coals, Qinshui Basin of China (132 µg/g) [16]. Note that the Li concentrations of two floor samples (tuff) are up to 898 µg/g and 531 µg/g. The Li concentration in the Donggou coals gradually increases from top to bottom (Figure 4). The closer to the tuff, the higher the Li concentration in the Donggou coals ( Figure 4).

Elemental Concentrations
Minerals 2020, 10, x FOR PEER REVIEW 6 of 18 Table 3 shows the data of major and trace elements in the Donggou coals. Taking the Chinese common coals as comparative standard [24], concentrations of Fe2O3 (7.11%) and K2O (0.68%) in the Donggou coals are relatively higher. By contrast, concentrations of MnO (0.002%), CaO (0.012%), Na2O (0.02%), and P2O5 (0.013%) are lower than the Chinese common coals [24]. The remaining major elements and ratios of SiO2/Al2O3 are close to the Chinese common coals [24]. Dai et al. (2015) proposed an index of concentration coefficient (CC) to characterize the concentration levels of trace elements in coal [50]. CC was defined as the ratio of elemental concentration of studying coals versus that of world hard coals [50,51]. Compared with world hard coals [51], lithium is enriched in the Donggou coals, the CC of which is up to 8.04. The Li concentration of the Donggou coals varies from 6.5 μg/g to 244 μg/g, with an average of 96.5 μg/g, which is significantly higher than the average value of world hard coals (12 μg/g) [51]. This value is slightly lower than that in the Haerwusu coals, Inner Mongolia, China (116 μg/g) [12], and that in the Jincheng coals, Qinshui Basin of China (132 μg/g) [16]. Note that the Li concentrations of two floor samples (tuff) are up to 898 μg/g and 531 μg/g. The Li concentration in the Donggou coals gradually increases from top to bottom (Figure 4). The closer to the tuff, the higher the Li concentration in the Donggou coals ( Figure 4). The concentration of REY in the Donggou coals is between 92.9 μg/g and 405 μg/g, with an average of 213 μg/g, higher than the average value of world hard coals (68 μg/g) [51]. The average oxides of REY (REO) concentration of the Donggou coal ashes is 1026 μg/g, which is higher than the cut-off grade proposed by Seredin and Dai (REO, 1000 μg/g in coal ash) [6]. Similar to the Li distribution in coals, REY concentration also gradually increases from top to bottom of the coal seam ( Figure 4).

Elemental Concentrations
All the samples do not show distinctive fractionation among light, medium, and heavy REY (  The concentration of REY in the Donggou coals is between 92.9 µg/g and 405 µg/g, with an average of 213 µg/g, higher than the average value of world hard coals (68 µg/g) [51]. The average oxides of REY (REO) concentration of the Donggou coal ashes is 1026 µg/g, which is higher than the cut-off grade proposed by Seredin and Dai (REO, 1000 µg/g in coal ash) [6]. Similar to the Li distribution in coals, REY concentration also gradually increases from top to bottom of the coal seam ( Figure 4).
All the samples do not show distinctive fractionation among light, medium, and heavy REY ( Figure 5). Based on the classifications proposed by Seredin and Dai (2012), the REY enrichments in coal are classified as three types: L-type (La N /Lu N > 1), M-type (La N /Sm N < 1, Gd N /Lu N > 1), and H-type (La N /Lu N < 1) [6]. The roof PSDZ-1 exhibits relatively L-type REY enrichment, with weak Eu negative anomaly. The coal bench samples display M-type and H-type REY enrichments, along with weak negative anomalies. The floor sample PSDZ-10-f shows an M-type REY enrichment, with a weak positive Eu anomaly. The lowermost floor sample PSDZ-11-f has a H-type REY enrichment, along with a relatively weak negative Eu anomaly.

Modes of Occurrence of Li and REY
Although the modes of occurrence of elements include minerals, organic, and intimate organic associations [53], minerals are generally considered as the major hosts for most elements present in coal [54]. Due to its low atomic number (No.3), lithium cannot be detected by the SEM-EDS. Some studies have shown that Li is mainly present in clay minerals [12,55,56]. There is also an opinion that Li in coal has both organic and inorganic affinities [57]. Finkelman et al. (2018) suggested that Li in most coals is dominantly (90%) associated with clays and micas [58]. Quartz, chalcedony, cristobalite, zircon, and tourmaline are also the hosts of lithium in coal [54]. Zhao (2015) conducted a stepwise chemical extraction method to study the modes of occurrence of Li in coals from the Jungar Coalfield [56]. It was found that Li in coal was bound predominantly to the aluminosilicate minerals [56]. Hu et al. (2018) indicated that lithium in the coal ash was strongly correlated with Al and Si [20]. Cookeite, a Li-bearing chlorite, which is uncommon in coal, was found to be the host of the abundant Li in the No. 15 coal of the Jincheng Coalfield, northern China [16,17].  identified chlorite phase containing lithium in the Guanbanwusu coals [13].
The correlation coefficient between Li and ash yield in the Donggou coals is 0.74, indicating that Li has an inorganic affinity ( Figure 6A). The correlation coefficients of Li-SiO 2 and Li-Al 2 O 3 are 0.75 ( Figure 6B) and 0.90 ( Figure 6C), respectively, further demonstrating that Li mainly occurs as aluminosilicates. As mentioned above, the aluminosilicate minerals in the Donggou coals include kaolinite, illite, and I/S. Moreover, the correlation coefficients of Li-kaolinite ( Figure 6D), Li-illite ( Figure 6E), and Li-I/S ( Figure 6F) are 0.91, 0.13, and 0.10, respectively, indicating that Li in the Donggou coals is mainly associated with kaolinite. The correlation coefficients of Li-sulfur and P 2 O 5 are 0.66 and 0.66, respectively, showing that the sulfur and phosphorous-containing minerals may host partial lithium. Eskenazy (1987) found that REY in Bulgaria coals have an inorganic affinity and are mainly in the form of aluminosilicates [59]. Finkelman et al. (2018) demonstrated that phosphate minerals are the main hosts of REY in bituminous coals, while clay minerals are the dominant hosts of REY in the low-rank coals [58]. However, Seredin (1996) indicated that REY are mainly associated with organic matter, while minerals play an insignificant role in total REY concentration in the Far East, Russia [21]. Arbuzov et al. (2019) concluded that the modes of occurrence of REY in coal change with rank advance [60]. In low rank coals such as brown coals, the dominant hosts of REY are organic matter, while in hard coals or anthracites, the main hosts of REY are authigenic minerals [60].  suggested that Light REY in the Jincheng No. 15 coal, northern China, are predominantly associated with phosphate minerals, while heavy REY show a mixed phosphate and organic affinity [16]. Although REY in coals, especially in low-rank coals, have an association with organic matter, it is generally considered that minerals host REY significantly in coal [53]. Compared with LREY, HREY are often more apt to have an organic affinity, probably because the HREY are easier to be captured by organic matter to form organometallic complexes [53].
aluminosilicates. As mentioned above, the aluminosilicate minerals in the Donggou coals include kaolinite, illite, and I/S. Moreover, the correlation coefficients of Li-kaolinite ( Figure 6D), Li-illite ( Figure 6E), and Li-I/S ( Figure 6F) are 0.91, 0.13, and 0.10, respectively, indicating that Li in the Donggou coals is mainly associated with kaolinite. The correlation coefficients of Li-sulfur and P2O5 are 0.66 and 0.66, respectively, showing that the sulfur and phosphorous-containing minerals may host partial lithium.  Minerals 2020, 10, x FOR PEER REVIEW 1 of 18 Eskenazy (1987) found that REY in Bulgaria coals have an inorganic affinity and are mainly in the form of aluminosilicates [59]. Finkelman et al. (2018) demonstrated that phosphate minerals are the main hosts of REY in bituminous coals, while clay minerals are the dominant hosts of REY in the low-rank coals [58]. However, Seredin (1996) indicated that REY are mainly associated with organic matter, while minerals play an insignificant role in total REY concentration in the Far East, Russia [21]. Arbuzov et al. (2019) concluded that the modes of occurrence of REY in coal change with rank advance [60]. In low rank coals such as brown coals, the dominant hosts of REY are organic matter, while in hard coals or anthracites, the main hosts of REY are authigenic minerals [60].  suggested that Light REY in the Jincheng No. 15 coal, northern China, are predominantly associated with phosphate minerals, while heavy REY show a mixed phosphate and organic affinity [16]. Although REY in coals, especially in low-rank coals, have an association with organic matter, it is generally considered that minerals host REY significantly in coal [53]. Compared with LREY, HREY are often more apt to have an organic affinity, probably because the HREY are easier to be captured by organic matter to form organometallic complexes [53].
The correlation coefficient between REY and ash yield in the Donggou coals is 0.84 ( Figure 7A), indicating that REY are mainly present in inorganic matter. The correlation coefficients of REY-Al2O3, REY-SiO2 and REY-P2O5 are 0.63 ( Figure 7B), 0.76 ( Figure 7C), and 0.88 ( Figure 7D), respectively, indicating that aluminosilicates and phosphrous-containing minerals host REY in the Donggou coals. Under SEM-EDS observation, some REY-bearing phosphate minerals, including rhabdophane ( Figure 8A

Controlling Geological Origin
The enrichment and depletion of trace elements in coal are mainly affected by geological factors such as terrigenous clastic material input, volcanic ash eruption, hydrothermal leaching and sedimentary environment [49,[61][62][63]. To a specific coalfield, there is always one or more geological factors influencing the enrichment and depletion of trace elements in coal. In the present study, the terrigenous materials of the Donggou coals originated from the felsic-intermediate rocks at the top of the Kangdian Upland. Anomalous concentrations of Li and REY in overlying coals are mainly a than 21, implying that the sediment source region may have, more or less, felsic compositions. The Kangdian Upland, a well-known sediment source region for most late Permian coals in southwestern China, is composed of tholeiitic and high-Ti basalts with trachytes and/or rhyolites at the top of the sequence. On the basis of the chemical compositions, the terrigenous materials were mainly from the felsic-intermediate rocks at the top of the Kangdian Upland, which is similar to the Dahebian coals [49].

Controlling Geological Origin
The enrichment and depletion of trace elements in coal are mainly affected by geological factors such as terrigenous clastic material input, volcanic ash eruption, hydrothermal leaching and sedimentary environment [49,[61][62][63]. To a specific coalfield, there is always one or more geological factors influencing the enrichment and depletion of trace elements in coal. In the present study, the terrigenous materials of the Donggou coals originated from the felsic-intermediate rocks at the top of the Kangdian Upland. Anomalous concentrations of Li and REY in overlying coals are mainly a result of the groundwater leaching from underlying tuff.

Indications of Al 2 O 3 /TiO 2
Owing to the stable nature of Al 2 O 3 and TiO 2 , the Al 2 O 3 /TiO 2 ratio in igneous-derived sedimentary materials is usually invariable during the alteration of parent magma [64]. Thus, the Al 2 O 3 /TiO 2 ratio is considered as a reliable index to discriminate the source region in sedimentary rocks including coal [49].   Figure 4). The underlying tuff may act as a dominant donor of elements including Li and REY, while the overlying coal operates as a recipient [49]. In addition, the groundwater leaching process may have led to redistribution of some element pairs such as Nb/Ta, Zr/Hf, and U/Th in coal, parting, roof, and floor [65,66]. As the former element is relatively easier to be leached, the ratios of Nb/Ta, Zr/Hf, and U/Th of coal are generally higher than those of the adjacent roof and floor ( Figure 10) [65,66]. Therefore, anomalies of Li and REY in the Donggou coals may mainly be a result of the groundwater leaching from underlying tuff, which is similar to the REY redistribution mechanism in the Heidaigou [67] and Dahebian [49] coals. Moreover, the hypothesis that Li and REY anomalies were subjected by groundwater leaching  Figure 4). The underlying tuff may act as a dominant donor of elements including Li and REY, while the overlying coal operates as a recipient [49]. In addition, the groundwater leaching process may have led to redistribution of some element pairs such as Nb/Ta, Zr/Hf, and U/Th in coal, parting, roof, and floor [65,66]. As the former element is relatively easier to be leached, the ratios of Nb/Ta, Zr/Hf, and U/Th of coal are generally higher than those of the adjacent roof and floor ( Figure 10) [65,66]. Therefore, anomalies of Li and REY in the Donggou coals may mainly be a result of the groundwater leaching from underlying tuff, which is similar to the REY redistribution mechanism in the Heidaigou [67] and Dahebian [49] coals.
Moreover, the hypothesis that Li and REY anomalies were subjected by groundwater leaching is supported by modes of occurrence of rhabdophane ( Figure 8A), florencite ( Figure 8B), goyazite ( Figure 8C), xenotime ( Figure 8D), and Zircon ( Figure 11). These authigenic minerals could be redeposited as secondary minerals in the Donggou coals after the REY and Zr were leached out from the underlying tuff [68].
may have led to redistribution of some element pairs such as Nb/Ta, Zr/Hf, and U/Th in coal, parting, roof, and floor [65,66]. As the former element is relatively easier to be leached, the ratios of Nb/Ta, Zr/Hf, and U/Th of coal are generally higher than those of the adjacent roof and floor ( Figure 10) [65,66]. Therefore, anomalies of Li and REY in the Donggou coals may mainly be a result of the groundwater leaching from underlying tuff, which is similar to the REY redistribution mechanism in the Heidaigou [67] and Dahebian [49] coals.  Figure 11). These authigenic minerals could be redeposited as secondary minerals in the Donggou coals after the REY and Zr were leached out from the underlying tuff [68].

Prelimiary Economic Evaluation of REY and Li
Seredin and Dai (2012) proposed the cut-off grade of REY in the coal ash (REO in coal ash is 1000 g/g) [6]. In addition, in order to further evaluate the economic significance of REY, a three-fold classification was used based on the REY supply and demand relationships [6]. REY were divided into critical (including Nd, Eu, Tb, Dy, Y, and Er), uncritical (including La, Sm, Pr, and Gd), and excessive (including Ce, Ho, Tm, Yb, and Lu) groups [6,69]. An index called outlook coefficient (Coutl) was defined as the ratio of the proportion of critical REY in total REY to the proportion of excessive REY in total REY [6]. The Coutl>1 represents REY in coal ash being highly promising, 0.7 < Coutl < 1.9 represents it as promising, while Coutl < 0.7 is unpromising [70]. According to the classifications based on   [70], a graph of Coutl-REY was plotted ( Figure 12). It can be seen that there is one coal sample (PSDZ-9) located in the highly promising area, and three coal samples (PSDZ-5, PSDZ-6, and PSDZ-7) located in the promising area. As mentioned above, the average concentration of REY oxides in the Donggou coal ashes is 1026 g/g, higher than the cut-off grade of REY in coal ash [6]. Furthermore, REY types in the Donggou coals are all M-type or H-type enrichments, which have greater economic significance.
Although there is a lack of lithium evaluation criteria like REY in coal, lithium could be co-extracted with REY considering the comprehensive utilization. Thus, the four Donggou coal samples (lower part of K1 coal seam) could be considered as potential alternative sources for extracting REY and Li.

Prelimiary Economic Evaluation of REY and Li
Seredin and Dai (2012) proposed the cut-off grade of REY in the coal ash (REO in coal ash is 1000 µg/g) [6]. In addition, in order to further evaluate the economic significance of REY, a three-fold classification was used based on the REY supply and demand relationships [6]. REY were divided into critical (including Nd, Eu, Tb, Dy, Y, and Er), uncritical (including La, Sm, Pr, and Gd), and excessive (including Ce, Ho, Tm, Yb, and Lu) groups [6,69]. An index called outlook coefficient (C outl ) was defined as the ratio of the proportion of critical REY in total REY to the proportion of excessive REY in total REY [6]. The C outl > 1 represents REY in coal ash being highly promising, 0.7 < C outl < 1.9 represents it as promising, while C outl < 0.7 is unpromising [70]. According to the classifications based on   [70], a graph of C outl -REY was plotted ( Figure 12). It can be seen that there is one coal sample (PSDZ-9) located in the highly promising area, and three coal samples (PSDZ-5, PSDZ-6, and PSDZ-7) located in the promising area. As mentioned above, the average concentration of REY oxides in the Donggou coal ashes is 1026 µg/g, higher than the cut-off grade of REY in coal ash [6]. Furthermore, REY types in the Donggou coals are all M-type or H-type enrichments, which have greater economic significance.
Although there is a lack of lithium evaluation criteria like REY in coal, lithium could be co-extracted with REY considering the comprehensive utilization. Thus, the four Donggou coal samples (lower part of K1 coal seam) could be considered as potential alternative sources for extracting REY and Li.
average concentration of REY oxides in the Donggou coal ashes is 1026 g/g, higher than the cut-off grade of REY in coal ash [6]. Furthermore, REY types in the Donggou coals are all M-type or H-type enrichments, which have greater economic significance.
Although there is a lack of lithium evaluation criteria like REY in coal, lithium could be co-extracted with REY considering the comprehensive utilization. Thus, the four Donggou coal samples (lower part of K1 coal seam) could be considered as potential alternative sources for extracting REY and Li.

Conclusions
The average concentrations of Li and REY in the Donggou coals are 96.5 µg/g and 213 µg/g, which are significantly higher than the average values of world hard coals. Li is mainly associated with kaolinite. REY mainly occurred as rhabdophane, florencite, goyazite, and xenotime. Anomalies of Li and REY in coal are mainly a result of the groundwater leaching from underlying tuff during coalification (either syngenic, diagenetic or epigenetic), plus a small proportion from the terrigenous materials input from the felsic-intermediate rocks at the top of the Kangdian Upland. The Donggou coal ashes could be considered as potential alternative sources of extracting Li and REY. Coals from the surrounding area that are further enriched in Li and REY might be discovered, which are being studied.