Geological Controls on Enrichment of Rare Earth Elements and Yttrium (REY) in Late Permian Coals and Non-Coal Rocks in the Xian’an Coalfield, Guangxi Province

The highly elevated concentrations of the rare earth elements and yttrium (REY), U, Mo, Se, and Pb in late Permian coals in some coalfields in Guangxi Province, South China, have been widely investigated; however, the mode of occurrence and enrichment mechanism of these critical elements are still under debate. This study investigates the mineralogical and geochemical compositions of coals and non-coal rocks from the Xian’an Coalfield in Guangxi Province to discuss the geological factors influencing the distribution of critical elements. The mineral in the studied coals consists mainly of quartz, and to a lesser extent, muscovite and kaolinite, with a trace amount of anatase. The coals are significantly enriched in REY, Pb, Se, Mo, and U and display the REY–U–Se–Mo–Pb-enrichment horizon (Horizon I) and U–Mo-enrichment horizon (Horizon II) adjacent to the host rocks or partings. The REY, U, Se, and Pb show organic association while Mo is primarily hosted by Fe-sulfides within Horizon I. The U and Mo have a phosphate affinity within Horizon II. Both the input of pyroclastic and epiclastic materials and the leaching of acidic solutions jointly govern the distribution of the REY–U–Se–Mo–Pb-enrichment horizon (Horizon I) and the U–Mo-enrichment horizon (Horizon II). The concentrations of REY in Horizon I exceed the cutoff grade of REY, and, therefore, the coals in the Horizon I can be regarded as promising raw materials of REY.


Introduction
Rare earth elements and yttrium (REY) in coals have attracted increasing attention in recent years [1][2][3][4][5], mainly due to the concentrations of REY in some coals close to or exceeding the cutoff grade of REY in coal ashes [6][7][8] and the supply shortage of conventional types of REY ore since 2009 [7]. Moreover, the REY-rich coals are mostly enriched in critical REY (Nd, Eu, Tb, Dy, Y, and Er), which could alleviate the raw materials crisis of critical REY in the near future [7]. Therefore, the REY-rich coals can be regarded as alternative REY raw materials [1,7].

Methodology
Eighteen samples, including eight coal samples and ten non-coal samples, were collected from the outcrop section in the Qianxian exploration region of Xian'an Coalfield in Guangxi Province, SW China ( Figures 1B, C and 2B). The weathered layer of the outcrop section was first removed before sampling to reduce the influence of chemical weathering on mineralogical and geochemical constituents. The studied samples were cut over an area 10 cm wide and 10 cm deep, and then, were put in plastic bags to avoid contamination and oxidation.
The studied samples were ground to ≤0.2 mm and split into two representative portions. A portion of the sample (<0.2 mm) was directly used for proximate analysis based on the ASTM Standards D3173-11 (2011), D3174-11 (2011), and D3175-11 (2011) [39][40][41], while another representative portion was ground further to ≤0.076 mm (200 mesh) using an agate mortar and pestle for mineralogical and geochemical analysis. The total sulfur content in the studied samples was determined following ASTM Standard D3177-02 (2002) [42]. Mineralogical analyses of the sample powders were performed by powder Xray diffraction (XRD) using a Bruker D8 A25 Advance (Bruker D8 A25 Advance, Leipzig, Germany) at the Institute of Environmental Assessment and Water Research (Spain). The detailed XRD analysis procedure and semiquantitative analysis were reported in a previous study [32]. The diffractograms were obtained at 40 kV and 40 mA by scanning at a 2θ interval of 4-60°, with a step size of 0.019° and a counting time of 0.1 s/step and the sample

Methodology
Eighteen samples, including eight coal samples and ten non-coal samples, were collected from the outcrop section in the Qianxian exploration region of Xian'an Coalfield in Guangxi Province, SW China ( Figure 1B,C and Figure 2B). The weathered layer of the outcrop section was first removed before sampling to reduce the influence of chemical weathering on mineralogical and geochemical constituents. The studied samples were cut over an area 10 cm wide and 10 cm deep, and then, were put in plastic bags to avoid contamination and oxidation.
The studied samples were ground to ≤0.2 mm and split into two representative portions. A portion of the sample (<0.2 mm) was directly used for proximate analysis based on the ASTM Standards D3173-11 (2011), D3174-11 (2011), and D3175-11 (2011) [39][40][41], while another representative portion was ground further to ≤0.076 mm (200 mesh) using an agate mortar and pestle for mineralogical and geochemical analysis. The total sulfur content in the studied samples was determined following ASTM Standard D3177-02 (2002) [42]. Mineralogical analyses of the sample powders were performed by powder X-ray diffraction (XRD) using a Bruker D8 A25 Advance (Bruker D8 A25 Advance, Leipzig, Germany) at the Institute of Environmental Assessment and Water Research (Spain). The detailed XRD analysis procedure and semiquantitative analysis were reported in a previous study [32]. The diffractograms were obtained at 40 kV and 40 mA by scanning at a 2θ interval of 4-60 • , with a step size of 0.019 • and a counting time of 0.1 s/step and the sample under rotation of 15/min. Semiquantitative XRD analysis was performed by using the matrix-flushing method devised by Chung [43] and calculating the integrated area of the main peak for the different mineral phases in each sample. This method first requires the calculation of the XRD constants for each crystalline phase by applying the Klug and Alexander equation [44]. Subsequently, the material under study is spiked with the internal standard and analyzed by XRD to obtain the proportions of each crystalline phase of the sample.
Approximately 0.1 g of coal sample was weighed and digested based on the method proposed by Querol et al. [45] for geochemical analysis. Major elements (Al, Ti, Fe, Mg, Ca, Na, K, and P) and trace elements were identified by inductively coupled plasma-atomicemission spectrometry (ICP-AES, Iris Advantage TJA Solutions, Thermo Fisher Scientific, Waltham, MA, USA) and inductively coupled plasma-mass spectrometry (ICP-MS, X-Series II Thermo, Thermo Fisher Scientific, Waltham, MA, USA). Silicon content was measured by wavelength-dispersive X-ray fluorescence spectrometry (XRF; ZSX Primus II) following the methods for chemical analysis of silicate rocks [46].
A small portion of representative block samples was used to prepare the polished sections for the SEM-EDS analysis. The modes of occurrence of minerals were studied using a field-emission scanning electron microscope (ZEISS Sigma300, Carl Zeiss AG, Jena, Germany), equipped with an energy-dispersive X-ray spectrometer (EDS) in the State Key Laboratory of Geological Processes and Mineral Resources (China).

Standard Coal Characteristics
The moisture contents, ash yields, volatile matter yields, and sulfur contents of the coal samples are tabulated in Table 1. The moisture contents range from 2.2% to 14.3% with an average of 10.0%. The ash yields of the coal samples vary between 36.4% and 79.7% with an average of 52.5%. The high ash yield of the coal samples is due to exposure and chemical weathering of the coals along the outcrop profile. The volatile matter yields of the coal samples range from 22.9% to 56.4%, with an average of 41.1%. The high volatile matter yields in the coal samples are ascribed to relatively high ash yields, which contribute to some proportion of volatile matter as indicated by the linear correlation of volatile matter and ash yield (r = 0.81) in coal samples except for sample SL-13. Previous coalfield exploration data show that the coals from the Xian'an Coalfield have 10-18% volatile matter yield, indicating low-volatile bituminous coals.
The total sulfur contents in the coal samples vary from 1.1% to 3.1% with an average of 1.9%, indicating medium sulfur coal (<1.0%, 1.0-3.0%, and >3.0% for low, medium, and high-S coals, respectively) [47]. The proportions of crystalline mineral phases in samples are summarized in Table 2. The minerals in coal samples mainly consist of quartz, and to a lesser extent, muscovite and kaolinite, with a trace amount of anatase ( Figure 3). Hematite is only identified in sample SL-13. The sandstone samples, including samples SL-18, SL-20, SL-24, and SL-26, are characterized by a high level of quartz, with minor proportions of kaolinite, dickite + kaolinite, and muscovite ( Figure 3). The claystone samples SL-6 and SL-7 are composed predominantly of kaolinite with minor amounts of anatase while claystone samples SL-9, SL-10, and SL-11 consist almost exclusively of dickite + kaolinite with minor amounts of anatase and pyrite ( Figure 3). The minerals in mudstone sample SL-19 consist primarily of quartz, rectorite, and muscovite, with minor kaolinite and trace of anatase.
Dickite is a polytype of kaolinite and has the same chemical composition as kaolinite, but they can be distinguished by the XRD pattern. Dickite can be distinguished from kaolinite by the presence of diffraction peaks of 3.95, 3.79, and 3.43 Å (sample SL-11; Figure 4). However, relatively low intensities of 3.95, 3.79, and 3.43 Å appear to indicate a complex of dickite and kaolinite. Thus, this mineral phase is reported as dickite + kaolinite in the XRD results. Most claystone and coal samples have poorly ordered kaolinite (e.g., samples SL-6, SL-8) (Figure 4), indicating a detrital origin [48,49]. However, sample SL-7 immediately adjacent to the coal bench (sample SL-8, Figure 3) shows an increasing kaolinite crystallinity, which is characterized by the weak diffraction of (020) as well as the presence of six diffraction peaks of kaolinite from 20 • to 25 • (2θ) and from 35 • to 40 • (2θ), respectively ( Figure 4); this relatively well-ordered kaolinite is most likely caused by acidic solutions from the degradation of organic matter, which allow inorganic materials to be degraded and recrystallized. Table 2. Mineralogical proportions of the coal and non-coal samples determined by X-ray diffraction (XRD) from the outcrop profile in the Xian'an Coalfield (on whole-coal basis; unit in %).

Mode of Occurrence of Minerals
Pyrite was identified in samples SL-9, SL-10, and SL-11 by XRD analysis. Pyrite primarily occurs as individual euhedral crystals, with eroded cavities within them ( Figure 5A); this form of pyrite is considered to be formed by direct interaction of Fe 2+ with HS − [47]. To a lesser extent, pyrite was found as massive form ( Figure 5B) and framboidal aggregates ( Figure 5B) within the kaolinite matrices.
Kaolinite primarily occurs as thin layers or lenses ( Figure 5C) and as a kaolinite matrix ( Figure 5D), all of which indicate that kaolinite was mechanically transported as a detrital input into paleomire. The detrital kaolinite is also confirmed by the XRD patterns, which show the abundant existence of poorly ordered kaolinite ( Figure 4). Moreover, vermiculate kaolinite was also observed in the studied coals ( Figure 5F).
Quartz is found as cavities and fracture infillings within the organic matter ( Figure 5E), and cavity infillings within the kaolinite matrix ( Figure 5D), all of which indicate authigenic precipitation of Si-rich fluids. Fine-grained disseminated anatase particles were identified within the kaolinite matrix ( Figure 5C), and the size of anatase was mostly <10 µm.

Mode of Occurrence of Minerals
Pyrite was identified in samples SL-9, SL-10, and SL-11 by XRD analysis. Pyrite primarily occurs as individual euhedral crystals, with eroded cavities within them ( Figure  5A); this form of pyrite is considered to be formed by direct interaction of Fe 2+ with HS − [47]. To a lesser extent, pyrite was found as massive form ( Figure 5B) and framboidal aggregates ( Figure 5B) within the kaolinite matrices.
Kaolinite primarily occurs as thin layers or lenses ( Figure 5C) and as a kaolinite matrix ( Figure 5D), all of which indicate that kaolinite was mechanically transported as a detrital input into paleomire. The detrital kaolinite is also confirmed by the XRD patterns, which show the abundant existence of poorly ordered kaolinite ( Figure 4). Moreover, vermiculate kaolinite was also observed in the studied coals ( Figure 5F).
Quartz is found as cavities and fracture infillings within the organic matter ( Figure  5E), and cavity infillings within the kaolinite matrix ( Figure 5D), all of which indicate authigenic precipitation of Si-rich fluids. Fine-grained disseminated anatase particles were identified within the kaolinite matrix ( Figure 5C), and the size of anatase was mostly <10 μm.

Geochemistry 4.3.1. Major Elements
The major elemental contents of the coal and non-coal samples collected from the outcrop profile are listed in Table 3. SiO 2 and to a lesser extent, Al 2 O 3 are the major inorganic components in the coal samples, as would be expected from mineral assemblage (quartz-muscovite-kaolinite) ( Table 2). Furthermore, the claystone samples (sample SL-6, SL-7, SL-9, SL-10, and SL-11) are dominated by SiO 2 and Al 2 O 3 due to kaolinite-and dickite-bearing mineral fractions. SiO 2 is almost exclusively an inorganic constituent in the sandstone samples (sample SL-18, SL-20, SL-24, and SL-26) due to the quartz-bearing mineral fraction. Compared with Chinese coals reported by Dai et al. [10], the proportions of SiO 2 and K 2 O in coal samples are evidently higher than those in Chinese coals due to the abundant occurrence of quartz and aluminosilicate minerals (e.g., muscovite); the remaining major elements are similar in abundance to those in Chinese coals.
The SiO 2 /Al 2 O 3 ratio in the coals (2.3-10.8, 5.7 on average) and sandstone samples (6.4-123.1, 40.2 on average) are significantly higher than that in claystone samples (1.2-1.4, 1.3 on average); this result is in accordance with the mineral assemblage in them. The K 2 O/Al 2 O 3 ratios in the coals (0.08-0.14, 0.11 on average) and sandstone (0.08-0.27, 0.11 on average) are higher than those in claystone (0.006-0.034, 0.018 on average) due to the presence of aluminosilicate minerals (e.g., muscovite) in the coals and sandstones.

Trace Elements
The trace element contents of the coal and non-coal samples collected from the outcrop profile are listed in Table 4. The concentration coefficient (CC) is used in the present study to conveniently describe the degree of enrichment or depletion of trace elements; the CC is a ratio of the coal samples versus world hard coals reported by [50], with CC <0.5, 0.5-2, 2-5, 5-10, and >10 indicative of depletion, similarity, slight enrichment, enrichment, and significant enrichment, respectively [51]. Compared with world hard coals [50], the studied coals are significantly enriched in V, Se, Mo, Ce, Sm, and U, enriched in As, Cs, La, Pr, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, and Pb, slightly enriched in Sc, Cu, Ga, Zr, Nb, Sn, Tm, Hf, and Th, and depleted in B, Mn, Co, Ni, Sr, and Ba; other trace elements are similar to the corresponding averages of world hard coals ( Figure 6). Therefore, the studied coals contain highly elevated concentrations of the REY-V-Se-Mo-U assemblage.

Rare Earth Elements and Y (REY)
A threefold geochemical classification (LREY, MREY, and HREY) and the three enrichment types (L-type, M-type, and H-type) of REY proposed by Seredin and Dai [7] as well as the upper continental crust [52]-normalized REY pattern were used in the present study to monitor the fractionation among REY. The Upper Continental Crust (UCC)-normalized REY patterns show an H-type REY distribution pattern in coal benches SL-8, SL-15, SL-16, and SL-13 ( Figure 7A) and an M-type REY pattern in coal benches SL-21, SL-22, SL-23, and SL-25 ( Figure 7B), possibly indicating the influence of aqueous solutions [5,53]. The coal samples exhibit slightly negative Eu anomalies ( Figure 7A,B), indicating contributions from felsic compositions [53]. The coal benches showing an H-type REY pattern display essentially no Ce anomalies ( Figure 7A) while coal benches showing an M-type REY pattern exhibit slightly positive Ce anomalies ( Figure 7B).

Rare Earth Elements and Y (REY)
A threefold geochemical classification (LREY, MREY, and HREY) and the three enrichment types (L-type, M-type, and H-type) of REY proposed by Seredin and Dai [7] as well as the upper continental crust [52]-normalized REY pattern were used in the present study to monitor the fractionation among REY. The Upper Continental Crust (UCC)-normalized REY patterns show an H-type REY distribution pattern in coal benches SL-8, SL-15, SL-16, and SL-13 ( Figure 7A) and an M-type REY pattern in coal benches SL-21, SL-22, SL-23, and SL-25 ( Figure 7B), possibly indicating the influence of aqueous solutions [5,53]. The coal samples exhibit slightly negative Eu anomalies ( Figure 7A,B), indicating contributions from felsic compositions [53]. The coal benches showing an H-type REY pattern display essentially no Ce anomalies ( Figure 7A) while coal benches showing an M-type REY pattern exhibit slightly positive Ce anomalies ( Figure 7B).

Rare Earth Elements and Y (REY)
A threefold geochemical classification (LREY, MREY, and HREY) and the three enrichment types (L-type, M-type, and H-type) of REY proposed by Seredin and Dai [7] as well as the upper continental crust [52]-normalized REY pattern were used in the present study to monitor the fractionation among REY. The Upper Continental Crust (UCC)-normalized REY patterns show an H-type REY distribution pattern in coal benches SL-8, SL-15, SL-16, and SL-13 ( Figure 7A) and an M-type REY pattern in coal benches SL-21, SL-22, SL-23, and SL-25 ( Figure 7B), possibly indicating the influence of aqueous solutions [5,53]. The coal samples exhibit slightly negative Eu anomalies ( Figure 7A,B), indicating contributions from felsic compositions [53]. The coal benches showing an H-type REY pattern display essentially no Ce anomalies ( Figure 7A) while coal benches showing an M-type REY pattern exhibit slightly positive Ce anomalies ( Figure 7B).

Source of Detrital Materials
The Al2O3/TiO2 ratio is widely utilized to infer the geochemical composition of detrital materials transported or blown into peat mires [5,18,28,54] due to little fractionation of both elements during geological process [55],

Source of Detrital Materials
The Al 2 O 3 /TiO 2 ratio is widely utilized to infer the geochemical composition of detrital materials transported or blown into peat mires [5,18,28,54] due to little fractionation of both elements during geological process [55],  Figure 7A,B) because the UCC-normalized negative Eu anomalies in coals are thought to represent contributions from felsic constituents [15].  Figure 7A,B) because the UCC-normalized negative Eu anomalies in coals are thought to represent contributions from felsic constituents [15]. The previous studies demonstrate that the Yunkai Upland is the dominant sedimentsource region and provides terrigenous felsic detrital materials into the late Permian coalbearing basin in Guangxi Province, China [1,2,33,36]. During the deposition of late Permian coal-bearing strata, the Xian'an Coalfield was located in the margin of the isolated carbonate platform and bounded to the south by a deep-water basin ( Figure 1D). This The previous studies demonstrate that the Yunkai Upland is the dominant sedimentsource region and provides terrigenous felsic detrital materials into the late Permian coalbearing basin in Guangxi Province, China [1,2,33,36]. During the deposition of late Permian coal-bearing strata, the Xian'an Coalfield was located in the margin of the isolated carbonate platform and bounded to the south by a deep-water basin ( Figure 1D). This appears to denote that detrital materials from the Yunkai Upland are rarely mechanically transported into the Xian'an Coalfield. Additionally, the detrital materials in the Xian'an Coalfield are possibly derived from the weathering residues (immediately above the Maokou Formation limestone) within the coastal zone, which were transported into the peat mire by seawater during the formation of the coal-bearing stratum. In this case, detrital calcite and dolomite grains are generally associated with the clay matrices [56]. However, the carbonate mineral phases cannot be identified in the coals by the XRD and SEM-EDS analysis, indicating a negligible detrital input from the adjacent weathering crust directly above the Maokou Formation limestone. Furthermore, the morphological characteristics of kaolinite matrices shown in Figure 5 are similar to those of the pyroclastic materials as reported in previous studies [57][58][59][60], appearing to indicate the pyroclastic input into the paleomires. The input of pyroclastic materials is also supported by the following evidence: (1) the presence of vermiculate kaolinite ( Figure 5F), which is usually originates from the transformation of volcanic ash [5,60], and (2) the abundant occurrence of synchronous pyroclastic materials, which are deposited in the late Permian strata close to the studied region [38]. Thus, the input of pyroclastic materials is a possible source for the inorganic constituents in the late Permian Heshan Formation.

Origin of Highly Elevated Concentrations of the REY-Se-Pb-Mo-U Assemblage
There is a REY-Se-Pb-Mo-U-enrichment horizon (Horizon I) immediately overlying the sandstone throughout the outcrop section ( Figure 9). In Horizon I, the REY, Se, Pb, Mo, and U contents are up to 1013, 38, 123, 72, and 81 µg/g, respectively; the REY negatively correlates with ash yield (r = −0.99) and show essentially no correlations with Al 2 O 3 ( Figure 9), P 2 O 5 , and Zr, thereby appearing to indicate an organic association; this result differs from previous studies showing that the REY in some REY-rich coals is mostly likely hosted by phosphates, heavy minerals, and clay minerals [5,7,56]. Moreover, Horizon I is characterized by a high degree of REY enrichment and normal levels of Zr, Hf, Nb, and Ta, appearing to exclude the possibility of the input of alkaline volcanic ash due to them being enriched in the REY-Zr (Hf)-Nb (Ta) assemblage [5,7,[60][61][62][63][64]. Furthermore, the highly elevated concentrations of REY in Horizon I, however, are accompanied by elevated contents of Se, U, Mo, and Pb; this assemblage most commonly represents the influx of metal-bearing solutions [7]. It is noteworthy that the highly elevated REY concentrations in Horizon I are jointly controlled by both the input of felsic epiclastic and pyroclastic detrital materials, and the subsequent leaching of acidic solutions based on the following evidence. As mentioned above, the terrigenous detrital materials from the Yunkai Upland are dominated by felsic compositions, which can provide relatively high background values of incompatible elements such as REY. This result is also further substantiated by the fact that the parting (sample SL-24) and roof (sample SL-26) contain REY contents (353 and 481 µg/g, respectively) distinctly higher than the upper crust composition (UCC) (168 µg/g) [52]. Moreover, the coals from the Yishan, Heshan, and Fusui coalfields that have the same sediment source region as the studied coals, are characterized by the relatively enhanced contents of REY [2,16,33], also supporting the input of REY-rich felsic terrigenous detrital materials. Moreover, the pyroclastic input from the adjacent regions also supplies abundant inorganic constituents into the coal-bearing strata as discussed previously. Subsequently, the pyroclastic and epiclastic materials that were deposited as parting and roof were subjected to the intense leaching of acidic solutions, which causes the breakdown of REY-Se-U-Mo-Pb-bearing detrital minerals. Consequently, the leachates derived from the host rocks or parting migrate upward or downward into the adjacent coal benches and were then re-deposited as authigenic minerals or adsorbed onto clay minerals or organic matter under anoxic conditions as reported elsewhere [56]; this process is supported by the Zr/Hf ratio, which shows a higher value in the coal benches than in the parting (sample SL-24) and roof (sample SL-26) (Figure 9), mainly because Zr is more easily mobilized than Hf during the activity of acidic solutions as reported in previous studies [5,33]. This is also supported by the cleat/fracture infilling of quartz, which most likely originates from the epigenetic precipitation of Si-rich solutions through leaching processes during coalification. Due to the low migration capacity of REY, the REY-rich leachates shortly migrate, and thus the REY-enrichment horizon is confined to Horizon I ( Figure 9); this result is consistent with previous studies [5,23]. The REY displays similar distribution to that of Se, U, and Pb, which is most likely because these elements are synchronously leached from the same detrital materials (roof and parting), and then REY-Se-U-Pb-bearing leachates simultaneously precipitate into the adjacent coal benches. There is a Mo-U-enrichment horizon (Horizon II) below the sandstone layer ( Figure  9). In Horizon II, the contents of Mo and U are up to 174 and 98 μg/g, respectively. The U and Mo display a similar distribution and an increasing, upward tendency toward the roof rocks (sample SL-18). The U (r = 0.99) and Mo (r = 0.99) significantly positively correlate with CaO, and CaO and P2O5 significantly correlate with each other. This appears to denote that the U and Mo are hosted by the phosphate phases (e.g., apatite), and their derivation is possibly from the leaching of pyroclastic materials within the roof rocks (sample SL-18).

Potential Economic Significance of REY in the Coals and Non-Coal Rocks
Total REY contents and individual REY compositions in coals are important for evaluating the economic recovery of REY. Seredin and Dai [7] proposed that the cutoff grade of REY oxides (REO) in coal ashes is 1000 μg/g. For REY-rich coal ashes, the outlook coefficient (Coutl) was used to determine the economic potential of REY, and its calculation follows Equation (1), with >2.4, 0.7-2.4, and <0.7 indicative of highly promising, promising, and unpromising REY raw materials, respectively [1,7,16]. Moreover, the Coutl-REO graph was also utilized to evaluate the REY economic potential [15].
The REO concentrations in coal ashes and non-coal rocks are plotted as a function of Coutl in Figure 10, where the REO contents of coal ashes in Horizon I (samples SL-21, SL- In Horizon I, U, Se, and Pb significantly positively correlate with each other, possibly reflecting that they have the same carrier; however, they show essentially no correlations with S and Fe 2 O 3 but strongly negative correlations with ash yield (Figure 9), apparently pointing to an organic association. As reported above, the U-Se-Pb-bearing solutions, which are leached from the host rocks and partings, flow and ultimately precipitate into coal benches due to the reduction of organic matter during coalification. This process can better explain why the U-enrichment horizons are closely adjacent to the host rocks and parting throughout the outcrop profile ( Figure 9). By contrast, the distribution of Mo differs from that of U, Se, and Pb, indicating various carriers; Mo is positively correlated with Fe 2 O 3 (r = 0.90), appearing to indicate that the Fe-sulfides are the major host of Mo. Fe-sulfides (e.g., pyrite) predominantly occur as euhedral crystals, indicating a syngenetic origin. Thus, Mo involved within pyrite is most likely hosted during peat accumulation or the early diagenetic stage under anoxic conditions. The contents of V and Cr are strikingly higher in the host rocks (samples SL-26 and SL-20) compared to that in the coal benches in Horizon I, suggesting a detrital input, which is consistent with the previous studies [56].
There is a Mo-U-enrichment horizon (Horizon II) below the sandstone layer ( Figure 9). In Horizon II, the contents of Mo and U are up to 174 and 98 µg/g, respectively. The U and Mo display a similar distribution and an increasing, upward tendency toward the roof rocks (sample SL-18). The U (r = 0.99) and Mo (r = 0.99) significantly positively correlate with CaO, and CaO and P 2 O 5 significantly correlate with each other. This appears to denote that the U and Mo are hosted by the phosphate phases (e.g., apatite), and their derivation is possibly from the leaching of pyroclastic materials within the roof rocks (sample SL-18).

Potential Economic Significance of REY in the Coals and Non-Coal Rocks
Total REY contents and individual REY compositions in coals are important for evaluating the economic recovery of REY. Seredin and Dai [7] proposed that the cutoff grade of REY oxides (REO) in coal ashes is 1000 µg/g. For REY-rich coal ashes, the outlook coefficient (C outl ) was used to determine the economic potential of REY, and its calculation follows Equation (1), with >2.4, 0.7-2.4, and <0.7 indicative of highly promising, promising, and unpromising REY raw materials, respectively [1,7,16]. Moreover, the C outl -REO graph was also utilized to evaluate the REY economic potential [15].
The REO concentrations in coal ashes and non-coal rocks are plotted as a function of C outl in Figure 10, where the REO contents of coal ashes in Horizon I (samples SL-21, SL-22, SL-23, and SL-25) range from 899 to 4503 µg/g and their average (2398 µg/g) exceeds the cutoff grade (1000 µg/g; Seredin and Dai, 2012), indicating that the coals in Horizon I can be regarded as promising raw materials of REY ( Figure 10). However, the REO concentrations of other coal seams (samples SL-8, SL-13, SL-15, and SL-16) (320-613 µg/g) and non-coal rocks (11-567 µg/g) are much lower than the cutoff grade of REY in coal ash, suggesting that these coals and non-coal rocks are not seen as raw materials of REY.

Conclusions
The coals from the outcrop profile in the Xian'an Coalfield, Guangxi Province, South China, show the REY (1013 µg/g)-Se (38 µg/g)-Pb (123 µg/g)-Mo (72 µg/g)-U (81 µg/g)enrichment horizon (Horizon I) and Mo (174 µg/g)-U (98 µg/g)-enrichment horizon (Horizon II). In Horizon I, REY, Se, Pb, and U are intimately associated with organic matter while Mo is hosted by Fe-sulfides (e.g., pyrite). In Horizon II, the Mo and U are hosted within phosphate phases. The highly elevated concentrations of REY, Se, Pb, Mo, and U are derived from the input of pyroclastic and epiclastic materials. However, the leaching of acidic solutions allows the inorganic materials to be degraded and soluble REY, Se, Pb, Mo, and U to be migrated and redistributed, leading to higher contents of REY, Se, Pb, Mo, and U in the coal benches immediately adjacent to the roof and partings. The coals in Horizon I can be regarded as promising raw materials of REY.