Mineralogical and Geochemical Compositions of the Lopingian Coals in the Zhongliangshan Coalfield, Southwestern China

The mineralogical and geochemical compositions of the Lopingian coals from an exploratory drill core (ZK4-1) in the Zhongliangshan Coalfield, southwestern China are reported in this paper. The Zhongliangshan coals are medium volatile bituminous in rank (random vitrinite reflectance, average 1.38%), characterized by a medium-ash yield (26.84%) and high sulfur content (3.38%). Minerals in the Zhongliangshan coals are mainly composed of clay assemblages (kaolinite, the illite/smectite mixed layer (I/S) and chamosite), pyrite, quartz, carbonate minerals (calcite, marcasite, ankerite, and dolomite), and anatase, followed by rutile, jarosite, natrojarosite, bassanite, gypsum and K-feldspar, with traces of apatite, rhabdophane and barite. Compared with the average concentrations of the world hard coals, some trace elements including Li, V, Co, Cu, Se, Y, Zr, Nb, rare earth elements (REE), Cd, Ta, Hf and Hg, are enriched in the Zhongliangshan coals. The modes of occurrence of chamosite, barite, rhabdophane, quartz and calcite in the Zhongliangshan coals indicate that the coals have probably been affected by the injection of low-temperature hydrothermal fluids. Based on the concentrations of Sc, V, Cr, Co, Ni, Cu and Zn, the ratios of Al2O3/TiO2 and the upper continental crust-normalized rare earth element and yttrium (REY) distribution patterns of the Zhongliangshan coals, the dominant sediment source regions are the Leshan–Longnvsi Uplift, Hannan Upland, and Dabashan Uplift, with a small proportion of terrigenous materials from the Kangdian Upland. The K7 and the upper portion of K1 coals have the potential as raw materials for the recovery of REY.

The Zhongliangshan Coalfield, located in the eastern Sichuan Basin, is one of the main coal resource bases in Chongqing Municipality (Figure 1).Previous studies have shown that the dominant terrigenous material of the Lopingian coals in the Sichuan Basin is basaltic Kangdian Upland [12].However, it has been reported recently that the dominant terrigenous materials for the Lopingian coals in the Huayingshan and Nantong coalfields, Sichuan basin are three Uplands/uplifts (TUUs), namely Leshan-Longnvsi Uplift, Dabashan Uplift and Hanan Upland rather than the Kangdian Upland [8,13].However, the sediment source region of the Lopingian coals in the Zhongliangshan Coalfield is not known.In addition, we have reported an anomalous enrichment of critical metals (Nb, Ga, and REY) in the tuff underlying the coal in the Zhongliangshan Coalfield, which can be regarded as a potential economically significant coal-bearing stratum hosting a polymetallic ore deposit [14,15].The concentrations of rare metals in the Lopingian coals overlying the tuff layer need further investigation.
namely Leshan-Longnvsi Uplift, Dabashan Uplift and Hanan Upland rather than the Kangdian Upland [8,13].However, the sediment source region of the Lopingian coals in the Zhongliangshan Coalfield is not known.In addition, we have reported an anomalous enrichment of critical metals (Nb, Ga, and REY) in the tuff underlying the coal in the Zhongliangshan Coalfield, which can be regarded as a potential economically significant coal-bearing stratum hosting a polymetallic ore deposit [14,15].The concentrations of rare metals in the Lopingian coals overlying the tuff layer need further investigation.The purpose of this paper is to address the sediment source region and to identify whether the Lopingian coals are enriched rare metals in the Zhongliangshan Coalfield based on the mineralogical and geochemical data.

Geological Setting
From bottom to top, the sedimentary sequences in the Zhongliangshan Coalfield are composed of the Middle Permian, Upper Permian, Lower Triassic, Middle Triassic, and Upper Triassic strata.The Middle Permian stratum is the Maokou Formation (P2m), while the Upper Permian strata include the Longtan (P3l) and Changxing Formations (P3c).The Lower Triassic strata consisted of the Feixianguan (T1f) and Jialingjiang Formations (T1j).The Middle and Upper Triassic Formations are the Leikoupo Formation (T2l) and Xujiahe Formation (T3xj), respectively.
The Maokou Formation occurs as bioclastic limestone and displays light gray to dark gray, which underlies the Longtan Formation in disconformity.It has a thickness from 80 to 250 m (166 m on average).
The Longtan Formation is the coal-bearing stratum in the study area, and consists of the sandstone, siltstone, sandy mudstone, mudstone, and ten coal seams (indexed as K10 to K1 from bottom to top, Figure 2A).It is precipitated in a marine-continental transitional environment and has a thickness varying from 26.5 to 105.02 m (71.08 m on average).Some fossils are abundant in this formation, including brachiopods, fern and cephalopods.
The tuff layer is distributed widely in the lowermost Longtan Formaion in Chongqing.It has been discussed in detail, not only because the tuff is one of the marker beds in the coalfields [16] and can provide new evidence for the origin of the end-Guadalupian mass extinction [17], but also because the tuff can be considered as the potential sources for the critical elements [7,14].The characteristics of the tuff layer in the study area have been described by Zou et al. [14].The purpose of this paper is to address the sediment source region and to identify whether the Lopingian coals are enriched rare metals in the Zhongliangshan Coalfield based on the mineralogical and geochemical data.

Geological Setting
From bottom to top, the sedimentary sequences in the Zhongliangshan Coalfield are composed of the Middle Permian, Upper Permian, Lower Triassic, Middle Triassic, and Upper Triassic strata.The Middle Permian stratum is the Maokou Formation (P 2 m), while the Upper Permian strata include the Longtan (P 3 l) and Changxing Formations (P 3 c).The Lower Triassic strata consisted of the Feixianguan (T 1 f) and Jialingjiang Formations (T 1 j).The Middle and Upper Triassic Formations are the Leikoupo Formation (T 2 l) and Xujiahe Formation (T 3 xj), respectively.
The Maokou Formation occurs as bioclastic limestone and displays light gray to dark gray, which underlies the Longtan Formation in disconformity.It has a thickness from 80 to 250 m (166 m on average).
The Longtan Formation is the coal-bearing stratum in the study area, and consists of the sandstone, siltstone, sandy mudstone, mudstone, and ten coal seams (indexed as K10 to K1 from bottom to top, Figure 2A).It is precipitated in a marine-continental transitional environment and has a thickness varying from 26.5 to 105.02 m (71.08 m on average).Some fossils are abundant in this formation, including brachiopods, fern and cephalopods.
The tuff layer is distributed widely in the lowermost Longtan Formaion in Chongqing.It has been discussed in detail, not only because the tuff is one of the marker beds in the coalfields [16] and can provide new evidence for the origin of the end-Guadalupian mass extinction [17], but also because the tuff can be considered as the potential sources for the critical elements [7,14].The characteristics of the tuff layer in the study area have been described by Zou et al. [14].The Changxing Formation, conformably overlying the Longtan Formation, consists of thick limestone intercalated with flint nodules and thin mudstone.It has a thickness of 102-114 m (108 m on average).This formation is enriched in brachiopods, spindle dragonflies, sponges, and other fossils.
Besides the Kangdian Upland, the Zhongliangshan Coalfield was surrounded by the Leshan-Longnvsi Uplift, Hannan Upland, and Dabashan Uplift (Figure 1) [8].The Leshan-Longnvsi Uplift, Hannan Upland, and Dabashan Uplift were the vital positive structural units during the Lopingian stage, mainly composed of mudstone, sandstone and carbonate [18], could also provide the terrigenous materials for the coalfield during the coal-forming processes.Besides the Kangdian Upland, the Zhongliangshan Coalfield was surrounded by the Leshan-Longnvsi Uplift, Hannan Upland, and Dabashan Uplift (Figure 1) [8].The Leshan-Longnvsi Uplift, Hannan Upland, and Dabashan Uplift were the vital positive structural units during the Lopingian stage, mainly composed of mudstone, sandstone and carbonate [18], could also provide the terrigenous materials for the coalfield during the coal-forming processes.

Sampling and Methods
A total of 40 samples of eight coal seams were taken from the exploratory drill core (No.ZK4-1) located in the Zhongliangshan Coalfield, Chongqing, southwestern China.These include 14 coal benches, nine partings, eight roofs, eight floors and one tuff samples (Figure 2B).Samples from each coal seam are numbered in an increasing order from top to bottom.The roof, partings, and floor strata were indexed with suffixes of r, p, and f, respectively.All samples were stored immediately in plastic bags to reduce oxidation and contamination.The Figure 2B shows the sample number, thickness, and lithology data in detail.
All the collected samples were treated through the procedures of air-drying, pulverizing, mixing and dividing using the method of coning and quartering.The coal samples were prepared with three types including <0.075 mm for geochemical analysis, <0.2 mm for proximate analysis, and <1 mm in size for petrographic analysis [19].
The mineralogical compositions of low-temperature ashing (LTA) of coal and non-coal samples were carried out using the powder X-ray diffraction (XRD).The instrument for the XRD analysis is D/max-2500/PC powder diffractometer (Rigaku Corporation, Tokyo, Japan).All the X-ray diffractograms of the coal LTAs and non-coal samples were subjected to quantitative mineralogical analysis using Siroquant™.The Siroquant™ is a commercial interpretation software, which is set out by Rietveld and developed by Taylor, respectively [25,26].Ward et al. and Ruan and Ward have provided the utilization of this technique for coal-related materials in detail [27][28][29].
In order to study the microstructure and morphology of minerals, a scanning electron microscope (SEM, JSM-6610LV, JEOL, Tokyo, Japan) equipped with an energy-dispersive X-ray spectrometer (EDX, OXFORD X-max, Oxford Instruments, Abingdon-on-Thames, Britain) was used under the conditions of 20 kV accelerating voltage of and high vacuum mode.
The contents of carbon, hydrogen, and nitrogen in coal were determined using the elemental analyzer (VarioMACRO, Elementar, Langenselbold, Germany).Concentrations of major elements in the ashes (815 • C) were measured by X-ray fluorescence spectrometry (XRF, ARL ADVANTXP+).
Abundances of trace elements except for fluorine and mercury in all the samples were determined using the inductively coupled plasma mass spectrometry (ICP-MS, X series II, Thermo Fisher, Waltham, MA, USA).Before ICP-MS determination, an ultraClave microwave High Pressure Reactor (Milestone, Sorisole, BG, Italy)) was applied to digest the samples.Note that arsenic and selium were analyzed by ICP-MS with collision-cell technology (CCT) based on the method proposed by Li et al. [30].The detailed ICP-MS procedures are discussed by Dai et al. [31,32].Fluorine was analyzed using the method of pyrohydrolysis with an ion-selective electrode described in ASTM Standard D 5987-96 [33].Mercury was determined using a Milestone DMA-80 Hg analyzer (Milestone, Sorisole, BG, Italy).

Coal Characteristics
The proximate and ultimate analyses, total sulfur, forms of sulfur, and random vitrinite reflectance data for the 14 coal samples from the ZK4-1 drill core in the Zhonglianshan Coalfield are listed in Table 1.The average volatile matter and vitrinite reflectance are 25.77% and 1.38%, respectively, indicating a medium volatile bituminous coal based on the ASTM classification [34].

Mineralogical Characteristics in the Coals
Table 2 lists the the proportion of minerals in the coal LTAs, partings, roof and floor samples identified by X-ray diffractograms plus Siroquant.The minerals in the coal samples are composed mainly by kaolinite, quartz, pyrite, and calcite, with small proportions of anatase.I/S (mixed minerals of illite and smectite), chamosite, and dolomite are present in most of the samples.Marcasite, ankerite and siderite also occur in a few samples.Either jarosite or natrojarosite occurs in the LTAs of coals throughout the seam.The bassanite can be observed only in samples K1-1, K4-1 and K10-1, and the gypsum can be identified only in sample K2-1.K-feldspar is relatively abundant in the studied coals especially in sample K10-2, the proportion of which is up to 11.5% (ash basis).In addition, apatite, rhabdophane and barite have been identified under the SEM-EDX although they are below the detection limit of the XRD.

Kaolinite
From the K1 to K10, the proportion of kaolinite varies from 18.6% to 52.1% and averages 31.4% (LTA basis).Kaolinite in the coal occurs as cell-fillings in telinite and fusinite (Figure 3A-D).This is common in many other coals, and indicates formation by authigenic processes [38].Vermicular kaolinite is also present (Figure 3B-D), a feature of which indicates an in-situ precipitation [39].

Chamosite
Two species of chlorite could occur in coal (chamosite and clinochlore), although it is not commonly observed in coal.Chamosite and clinochlore have the same diffraction peaks in the XRD patterns and can be identified by their different intensities of peaks in X-ray diffractograms [40,41].The chlorite present in this study was identified as chamosite rather than clinochlore, not only because of the weaker odd-order peaks (001 and 003) of XRD patterns but also because of high Fe and low Mg percentages determined by the energy dispersive X-ray (EDX) analysis.
XRD studies indicate that chamosite occurs in all of the coal LTA residues except for the coal samples K3, K5 and K8.The chamosite, usually coexisting with the kaolinite (Figure 3C,D), occurs as cell-fillings in the coal (Figure 3C-F), indicating an authigenic origin.

Illite/Smectite Mixed Layer
The Zhongliangshan coals have significant proportions of illite/smectite mixed layer (I/S) (Table 2).The I/S in the coal is filled in the plant matrix (Figure 3G) or in the cell cavity (Figure 3H), indicating that the I/S in present study is of authigenic origin.

Quartz
Quartz in the coal LTA residues varies from 6.8% to 45.3% and averages 20%.Quartz occurs as cell infillings and euhedral crystal (Figure 4D-F), indicating an authigenic origin.

Calcite, Ankerite, Dolomite, and Siderite
Calcite is abundant in the Zhongliangshan coals, varying from 2.2% to 21.1% and with an average of 10.4%.The dolomite was detected in all the coal LTA residues except samples K1-2, K3-1 and K10-2.However, the ankerite and siderite were identified only in samples K1-2 and K3-1.Calcite occurs as cell-fillings (Figure 4G,H) and fracture-fillings (Figure 5A), indicating an epigenetic origin.The mode of occurrence of ankerite is similar to that of calcite (Figure 5B,C), indicating that it has an epigenetic origin as well.

Anatase
Anatase is present commonly in the Lopingian coals in southwestern China, mainly derived from the sediment source region from high-Ti basalt in the Kangdian upland and injection of hydrothermal solutions [8,45].Anatase is distributed in all the coal LTA residues, and varies from 1% to 3.7% in abundance (2.1% on average).Anatase occurs as a replacement of glass shards or pumice (Figure 5D), similar to that in the K2 coal in the Songzao coalfield [39].

Apatite, Barite and Rhabdophane
Apatite in the Zhongliangshan coals is distributed in the collodetrinte (Figure 5E).Barite occurs as cavity-fillings, indicating an authigenic origin (Figure 5F).Rhabdophane appears to be as cell-fillings associating with kaolinite and chamosite (Figure 5G,H).

Major Elements
The major elements in the Zhongliangshan coals are dominated by SiO 2 and Al 2 O 3 , and to a lesser extent, Fe 2 O 3 and CaO (Table 3).In comparison with Chinese coals [2], the concentrations of SiO 2 , TiO 2 and MgO in studied coals are slightly higher.However, the abundances of other major element oxides are lower than those in Chinese coals.The SiO 2 /Al 2 O 3 ratio (2.24) of the Zhongliangshan coals is higher than the average Chinese coals (1.42) [2].

Comparison between Mineralogical and Chemical Compositions
Based on the calculation methods described by Ward et al. [27], the reliability of the quantitative XRD data was checked by the comparison with the observed ash chemistry determined by XRF.Before the comparison, data from the two methods were both normalized to allow for difference in LTA, CO 2 , H 2 O + , and SO 3 percentages [46].The correlations of SiO 2 , Al 2 O 3 , K 2 O, Fe 2 O 3 , TiO 2 , CaO and MgO are revealed in Figure 6.
For SiO 2 , Al 2 O 3 , Fe 2 O 3 and TiO 2 , the points fall very close to the diagonal equality line, indicating that the minerals indicated by the XRD analysis are highly consistent with the independently-determined chemical data.The plots for CaO and MgO show that the data points of CaO tend to fall above the equality line and that of MgO fall below the equality line, indicating that partial Ca 2+ was replaced by Mg 2+ in the calcite, this is also observed by the SEM-EDX analysis.The plot for Na 2 O also shows that the points are close to the equality line.However, the points of K 2 O fall below the diagonal equality line, implying the I/S may have a lesser K + .

Trace Elements
From the Table 3 and Figure 7, it can be obtained that some trace elements are enriched in comparison with the average concentrations of the world hard coals [47].The concentration coefficients (CC, the ratio of the trace-element concentrations in investigated samples vs. world hard coals) [32] of trace elements between 5 and 10 include Se, Zr, Nb, Cd and Ta.Many other elements in the coals (Li, V, Co, Cu, Hf and Hg) are slightly enriched (2<CC<5).Elements B, As, Rb, Sb, Tl and Bi are depleted, with a CC <0.5.The remaining trace elements have concentrations close to the world hard coals, with CC between 0.5 and 2.

Sc, V, Cr, Co, Ni, Cu and Zn
It is worthy to note that Sc, V, Cr, Co, Ni, Cu and Zn, which are significantly abundant in the basalt of Kangdian Upland [11,43,45,48,49], are not highly enriched in the Zhongliangshan coals.Compared with the Lopingian C2 and C3 coals in the Xinde Mine (Yunnan Province, China) [45], with the sediment-source region of the Kangdian Upland, concentrations of Sc, V, Cr, Co, Ni, Cu and Zn of present study are relatively lower (Figure 8).However, concentrations of Sc, V, Cr, Co, Ni, Cu and Zn of the Zhongliangshan coals are relatively higher than those of the Lopingian coals in the Donglin Mine and Lvshuidong Mine (Figure 8) [8,13], with the dominant sediment source regions of the Leshan-Longnvsi Uplift, Hannan Upland, and Dabashan Uplift rather than the Kangdian Upland.

Rare Earth Elements and Yttrium
Although there are some inconsistencies on the abbreviation of "REE" or "REY" in some geochemical literature [50], REY is adopted in this study to represent the lanthanides and Yttrium [51].Owing to the unique geochemical behaviors, REY have been widely used as geochemical parameters to identify sediment source region and clarify the evolution processes of coal basins [51][52][53][54][55][56][57][58][59].In order to describe the REY distribution in coals more conveniently, a three-fold geochemical classification and three enrichment types were proposed by Seredin and Dai [58].Accordingly, normalized to the upper continental crust (UCC) [60], three enrichment types of REY in coal were generally identified: L-type (light-REY; LaN/LuN >1), M-type (medum-REY; LaN/SmN <1, GdN/LuN >1), and H-type (heavy-REY; LaN/LuN <1) [58].In addition, several REY geochemical parameters (e.g., CeN/CeN*, EuN/EuN*, LaN/LaN*, GdN/GdN* and YN/HoN) are often used to rebuild the geochemical history [54,61], to recognize the sediment-source region [10,13,52,62], and to explain the tectonic evolution of coal deposits [3,63,64].The REY concentrations and parameters of the Zhongliangshan coals are listed in Tables 4 and 5.The content of REY varies from 112 to 396 μg/g and averages 171 μg/g, higher than that in common world hard coals [47].In order to describe the REY distribution in coals more conveniently, a three-fold geochemical classification and three enrichment types were proposed by Seredin and Dai [58].Accordingly, normalized to the upper continental crust (UCC) [60], three enrichment types of REY in coal were generally identified: L-type (light-REY; La N /Lu N >1), M-type (medum-REY; La N /Sm N <1, Gd N /Lu N >1), and H-type (heavy-REY; La N /Lu N <1) [58].In addition, several REY geochemical parameters (e.g., Ce N /Ce N *, Eu N /Eu N *, La N /La N *, Gd N /Gd N * and Y N /Ho N ) are often used to rebuild the geochemical history [54,61], to recognize the sediment-source region [10,13,52,62], and to explain the tectonic evolution of coal deposits [3,63,64].The REY concentrations and parameters of the Zhongliangshan coals are listed in Tables 4 and 5.The content of REY varies from 112 to 396 µg/g and averages 171 µg/g, higher than that in common world hard coals [47].Note: REY, sum of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, and Lu; La N /Lu N , ratio of La N and Lu N ; La N /Sm N , ratio of La N and Sm N ; Gd N /Lu N , ratio of Gd N and Lu N ; Y N /Ho N , ratio of Y N and Ho N ; Ce N /Ce N * = Ce N /(0.5LaN + 0.5Pr N ); Eu N /Eu N * = Eu N /(0.5SmN + 0.5Gd N ); Gd N /Gd N * = Gd N /[(Sm N × 0.33) + (Tb N × 0.67); La N /La N * = La N /(3Pr N -2Nd N ); N, REY are normalized by Upper Continental Crust (UCC) [60].
The Zhongliangshan coals, characterized by negative Ce, Eu anomalies and positive Gd, La anomalies, with no pronounced Y anomalies (Figure 9), are dominated by M-H type and, to a lesser extent, L-and L-M types, along with H-type (Table 5).The REY has a weakly positive correlation with ash yield in the Zhongliangshan coals (Figure 10A).This indicates that the modes of occurrences of REY may be not only associated with the mineral matter, but also with the organic matter in the coal.Some studies have shown that the correlation coefficients of REY with ash yields decrease along with atomic numbers [10,52,62,65].However, the correlation coefficients between REY and ash yields in present study increase from 0.28 (r La-Ash ) to 0.79 (r Lu-Ash ) (Figure 10B) with the increasing atomic number, similar to the trend of Haerwusu coals [66].This probably suggests that the light REY have a mixed inorganic-organic affinity and the heavy REY have an inorganic-dominated affinity.It can be inferred that the ability being absorbed on the organic matter of LREY is higher than that of HREY, leading to higher HREY-ash correlation coefficients than those of LREY-ash pair [66].
In present study, the concentrations of Sc, V, Cr, Co, Ni, Cu and Zn are not as high as those in the Xinde Mine, and not as low as those in the Huayingshan and Nantong coalfields, neither (Figure 8).This may indicate that the Kangdian Upland is not the dominant sediment source region for the Zhongliangshan coals.
Many studies have shown that the Al 2 O 3 /TiO 2 ratios are useful provenance indicators to determine the terrigenous materials not only for normal sedimentary rocks but also for coal seams [8,10,73,75,76].It is suggested that the Al 2 O 3 /TiO 2 ratios for sedimentary rocks with 3-8, 8-21, and 21-70 are considered to originate from mafic-, intermediate-, and felsic dominated sediment source regions, respectively [75].In the plot of Al 2 O 3 vs.TiO 2 for coal samples from the Zhongliangshan coalfield (Figure 11), almost all coal samples fall in the area between 8 and 21, indicating that the terrigenous source of the studied samples is of intermediate composition.This further indicates that the dominant sediment source region is not the mafic basalts of the Kangdian upland, where the typical Al 2 O 3 /TiO 2 ratios are between 3 and 8 [76].
In present study, the concentrations of Sc, V, Cr, Co, Ni, Cu and Zn are not as high as those in the Xinde Mine, and not as low as those in the Huayingshan and Nantong coalfields, neither (Figure 8).This may indicate that the Kangdian Upland is not the dominant sediment source region for the Zhongliangshan coals.
Many studies have shown that the Al2O3/TiO2 ratios are useful provenance indicators to determine the terrigenous materials not only for normal sedimentary rocks but also for coal seams [8,10,73,75,76].It is suggested that the Al2O3/TiO2 ratios for sedimentary rocks with 3-8, 8-21, and 21-70 are considered to originate from mafic-, intermediate-, and felsic dominated sediment source regions, respectively [75].In the plot of Al2O3 vs. TiO2 for coal samples from the Zhongliangshan coalfield (Figure 11), almost all coal samples fall in the area between 8 and 21, indicating that the terrigenous source of the studied samples is of intermediate composition.This further indicates that the dominant sediment source region is not the mafic basalts of the Kangdian upland, where the typical Al2O3/TiO2 ratios are between 3 and 8 [76].Eu anomalies in coal can also be used as an indicator to interpret the sediment source region [51].This is because Eu anomalies in coal are usually inherited from rocks within the sediment source region, and would not be affected between the weathering and transportation processes from the sediment source region to the peat swamp [51,52,77,78].However, Eu anomalies may be influenced under the conditions of high-temperature hydrothermal fluids (>200 °C) and extremely reducing conditions [79,80].Eu anomalies in coal can also be used as an indicator to interpret the sediment source region [51].This is because Eu anomalies in coal are usually inherited from rocks within the sediment source region, and would not be affected between the weathering and transportation processes from the sediment source region to the peat swamp [51,52,77,78].However, Eu anomalies may be influenced under the conditions of high-temperature hydrothermal fluids (>200 • C) and extremely reducing conditions [79,80].Generally, coals with input of felsic or felsic-intermediate terrigenous materials usually have pronounced negative Eu anomalies [51], with a few exceptions of coals characterized by positive Eu anomalies that are caused by high content of plagioclase and other feldspars [81].On the contrary, coals with input of mafic basalts display strongly positive Eu anomalies [45].Chen et al. consider that the dominant terrigenous material of the Lopingian coals in the Nantong Coalfield is not the Kangdian Upland using the Eu anomalies.Dai et al. used the Eu anomalies to identify that the Yishan coals have been influenced by high-temperature hydrothermal solutions [10].However, not all the coals influenced by hydrothermal solutions should have positive Eu anomalies [82].For example, in some cases, coals or stone coals affected by hydrothermal solutions have a weak negative anomaly because the felsic sediment input overlapped the hydrothermal solutions [82].
The Figure 9 shows that almost all coals present distinctive negative Eu anomalies, different from the Emeishan basalts.This further indicates that the dominant terrigenous material is not the basalt of Kangdian Upland.
Note that there are some other three uplands/uplifts (TUUs) including the Leshan-Longnvsi uplift, Hannan Upland, and Dabashan Uplift around the Zhongliangshan coalfield (Figure 1).Dai et al. have proposed that TUUs are the dominant terrigenous materials for the Huayingshan coalfield [8].Hence, the dominant sediment source region of present study may also be TUUs.However, based on the abundances of Sc, V, Cr, Co, Ni, Cu and Zn in the Zhongliangshan coals and the proximity to the Kangdian Upland, the Kangdian Upland may also provide a small proportion of terrigenous materials for the Zhongliangshan Coalfield.

Injection of Low-Temperature Hydrothermal Fluids
Hydrothermal fluid activity plays an important role in the enrichment of mineral matter in the coals not only from southwestern China [2,40,43,[83][84][85][86][87][88], but also from some coal deposits elsewhere [89][90][91].However, the characteristics and alteration mechanism of hydrothermal solution are not clear.Recently, Dai et al. illustrate the hydrothermal solution compositions, nature and mineralization of the alkali volcanic ash in Yunnan Province (southwestern China) based on the analysis of H-O isotope and petrological, mineralogical and geochemical assemblages [92].Firstly, the alkali volcanic ash was leached by the mixed high-temperature hydrothermal solution (including acidic waters and CO2 degassing from the Emeishan Plume) and resulted in the in situ enrichment of Al, Ti and the depletion of Nb, Zr, Ga and REY [92].Secondly, the leached Nb, Zr, Ga and REY were precipitated under the environment of cooler, neutral or alkaline hydrothermal fluid alteration [92], and in some cases, with injection of sea water [93].
Chamosite is not usually observed in the coal, however, it does commonly occur in the Lopingian coals in southwestern China [39,40,42,45,49].Two origins of chamosite have been proposed, which is either derived from the alteration of kaolinite by the injection of Fe-Mg-rich fluids during diagenesis process [40], or directly precipitated from hydrothermal fluids enriched Fe [49].
The coexistence of chamosite and kaolinite and their close relationship (Figure 3C,D) indicate that chamosite was the interaction product between the kaolinite and Fe-Mg-rich fluids, which is Generally, coals with input of felsic or felsic-intermediate terrigenous materials usually have pronounced negative Eu anomalies [51], with a few exceptions of coals characterized by positive Eu anomalies that are caused by high content of plagioclase and other feldspars [81].On the contrary, coals with input of mafic basalts display strongly positive Eu anomalies [45].Chen et al. consider that the dominant terrigenous material of the Lopingian coals in the Nantong Coalfield is not the Kangdian Upland using the Eu anomalies.Dai et al. used the Eu anomalies to identify that the Yishan coals have been influenced by high-temperature hydrothermal solutions [10].However, not all the coals influenced by hydrothermal solutions should have positive Eu anomalies [82].For example, in some cases, coals or stone coals affected by hydrothermal solutions have a weak negative anomaly because the felsic sediment input overlapped the hydrothermal solutions [82].
The Figure 9 shows that almost all coals present distinctive negative Eu anomalies, different from the Emeishan basalts.This further indicates that the dominant terrigenous material is not the basalt of Kangdian Upland.
Note that there are some other three uplands/uplifts (TUUs) including the Leshan-Longnvsi uplift, Hannan Upland, and Dabashan Uplift around the Zhongliangshan coalfield (Figure 1).Dai et al. have proposed that TUUs are the dominant terrigenous materials for the Huayingshan coalfield [8].Hence, the dominant sediment source region of present study may also be TUUs.However, based on the abundances of Sc, V, Cr, Co, Ni, Cu and Zn in the Zhongliangshan coals and the proximity to the Kangdian Upland, the Kangdian Upland may also provide a small proportion of terrigenous materials for the Zhongliangshan Coalfield.

Injection of Low-Temperature Hydrothermal Fluids
Hydrothermal fluid activity plays an important role in the enrichment of mineral matter in the coals not only from southwestern China [2,40,43,[83][84][85][86][87][88], but also from some coal deposits elsewhere [89][90][91].However, the characteristics and alteration mechanism of hydrothermal solution are not clear.Recently, Dai et al. illustrate the hydrothermal solution compositions, nature and mineralization of the alkali volcanic ash in Yunnan Province (southwestern China) based on the analysis of H-O isotope and petrological, mineralogical and geochemical assemblages [92].Firstly, the alkali volcanic ash was leached by the mixed high-temperature hydrothermal solution (including acidic waters and CO 2 degassing from the Emeishan Plume) and resulted in the in situ enrichment of Al, Ti and the depletion of Nb, Zr, Ga and REY [92].Secondly, the leached Nb, Zr, Ga and REY were precipitated under the environment of cooler, neutral or alkaline hydrothermal fluid alteration [92], and in some cases, with injection of sea water [93].
Chamosite is not usually observed in the coal, however, it does commonly occur in the Lopingian coals in southwestern China [39,40,42,45,49].Two origins of chamosite have been proposed, which is either derived from the alteration of kaolinite by the injection of Fe-Mg-rich fluids during diagenesis process [40], or directly precipitated from hydrothermal fluids enriched Fe [49].
The coexistence of chamosite and kaolinite and their close relationship (Figure 3C,D) indicate that chamosite was the interaction product between the kaolinite and Fe-Mg-rich fluids, which is similar to the mechanism reported by Dai and Chou [40].Chamosite of hydrothermal origin was also observed (Figure 3E), where chamosite was independent of kaolinite and occurred as cell-fillings.
Besides chamosite, the modes of occurrence of barite (Figure 5F) and rhabdophane (Figure 5G) in the coal also illustrate that these minerals were precipitated from hydrothermal fluids.Barite has been indicated to be formed by the injection of hydrothermal fluids [8].Rhabdophane is often observed in some REY-rich coals and derived from hydrothermal fluids in southwestern China [8,39,45,84].
The cell-filling quartz (Figure 4D,E) indicates an authigenic origin.In addition, calcite in the present study, which infills cell (Figure 4G,H) and fracture cavities (Figure 5A) indicate an epigenetic origin.
Based on the strong negative Eu anomalies of coals (Figure 9), the temperature of hydrothermal fluids may be all relatively low (<200 • C), although the multi-stage injection of hydrothermal fluids has been put forward.Otherwise, the Eu anomaly would be expected to be strong positive due to the ingress of hydrothermal fluids at >200 • C during the peatification, in that the reduction of Eu requires not only extremely reducing conditions but also high-temperature [10,51].

Evaluation of Rare Metals
Some Lopingian coals or non-coal strata in southwestern China are considered as potential economic sources for critical metals including Nb, Zr, Ga and REY [7,8,14,94].In order to assess the REY in coal ashes as economic raw materials, several criteria (including the REY cut-off grade and the individual elemental composition) were proposed by Seredin and Dai [58].Seredin and Dai have also proposed the cut-off grade for recovery from the coal ash (REY oxides (REO) ≥ 1000 µg/g) [58].Based on the classification reported by Dai et al. [9], the C outl -REO graph is plotted in Figure 12 to evaluate industrial potential of REY in present study (C outl = [(Nd + Eu + Tb + Dy + Er + Y)/ΣREY]/[(Ce + Ho + Tm + Yb + Lu)/ΣREY]) [58].It can be obtained that the REY concentrations in most samples of the Zhongliangshan coals are lower than the cut-off grade except for samples K1-1, K7 and K5-r.Thus, the K7 and the upper portion of K1 have the potential to be a source of raw material for REY recovery.However, the K2, K3, K4, K5, K8 and K10 coals cannot be considered as REY raw material sources.It is noted that some coal seams in other coalfields surrounding Zhongliangshan coalfield enrich REY (Figure 13), where the REO in coal ash are higher than 1000 ppm.However, the REY data in the Shuijiang, Nantong, Xishui, Junlian, and Furong Coalfields, are lower than the cut-off grade or absent, which needs further research.
In addition to factors as mentioned above that could lead to enrichment of REY in coal deposit, some other factors such as alkaline volcanic ashes [95] and to a lesser extent, ground water leaching [96], can also play an important roles in the Lopingian coals.Similar to the K7 and the upper portion of K1 coal seams, the coals subjected to alkaline volcanic ashes and in some cases, groundwater leaching, usually have a high potential of REY [95,96].similar to the mechanism reported by Dai and Chou [40].Chamosite of hydrothermal origin was also observed (Figure 3E), where chamosite was independent of kaolinite and occurred as cell-fillings.Besides chamosite, the modes of occurrence of barite (Figure 5F) and rhabdophane (Figure 5G) in the coal also illustrate that these minerals were precipitated from hydrothermal fluids.Barite has been indicated to be formed by the injection of hydrothermal fluids [8].Rhabdophane is often observed in some REY-rich coals and derived from hydrothermal fluids in southwestern China [8,39,45,84].
The cell-filling quartz (Figure 4D,E) indicates an authigenic origin.In addition, calcite in the present study, which infills cell (Figure 4G,H) and fracture cavities (Figure 5A) indicate an epigenetic origin.
Based on the strong negative Eu anomalies of coals (Figure 9), the temperature of hydrothermal fluids may be all relatively low (<200 °C), although the multi-stage injection of hydrothermal fluids has been put forward.Otherwise, the Eu anomaly would be expected to be strong positive due to the ingress of hydrothermal fluids at >200 °C during the peatification, in that the reduction of Eu requires not only extremely reducing conditions but also high-temperature [10,51].

Evaluation of Rare Metals
Some Lopingian coals or non-coal strata in southwestern China are considered as potential economic sources for critical metals including Nb, Zr, Ga and REY [7,8,14,94].In order to assess the REY in coal ashes as economic raw materials, several criteria (including the REY cut-off grade and the individual elemental composition) were proposed by Seredin and Dai [58].Seredin and Dai have also proposed the cut-off grade for recovery from the coal ash (REY oxides (REO) ≥ 1000 μg/g) [58].Based on the classification reported by Dai et al. [9], the Coutl-REO graph is plotted in Figure 12 to evaluate industrial potential of REY in present study (Coutl = [(Nd + Eu + Tb + Dy + Er + Y)/ΣREY]/ [(Ce + Ho + Tm + Yb + Lu)/ΣREY]) [58].It can be obtained that the REY concentrations in most samples of the Zhongliangshan coals are lower than the cut-off grade except for samples K1-1, K7 and K5-r.Thus, the K7 and the upper portion of K1 have the potential to be a source of raw material for REY recovery.However, the K2, K3, K4, K5, K8 and K10 coals cannot be considered as REY raw material sources.It is noted that some coal seams in other coalfields surrounding Zhongliangshan coalfield enrich REY (Figure 13), where the REO in coal ash are higher than 1000 ppm.However, the REY data in the Shuijiang, Nantong, Xishui, Junlian, and Furong Coalfields, are lower than the cut-off grade or absent, which needs further research.
In addition to factors as mentioned above that could lead to enrichment of REY in coal deposit, some other factors such as alkaline volcanic ashes [95] and to a lesser extent, ground water leaching [96], can also play an important roles in the Lopingian coals.Similar to the K7 and the upper portion of K1 coal seams, the coals subjected to alkaline volcanic ashes and in some cases, groundwater leaching, usually have a high potential of REY [95,96].[8].The data of the Songzao Coalfield is from Seredin and Dai [58].The data of the Guxu Coalfield is from Dai et al. [82].

Conclusions
The Lopingian coals from the ZK4-1 drill core in the Zhongliangshan Coalfield are medium volatile bituminous coal, which are characterized by a medium-ash yield (26.84%) and high sulfur content (3.38%).Minerals in the Zhongliangshan coals are mainly composed of kaolinite, pyrite, quartz and calcite, with small proportions of anatase.The illite/smectite mixed layer (I/S), chamosite, rutile, marcasite, ankerite, dolomite, jarosite, natrojarosite, bassanite, gypsum and K-feldspar are also present in various coal samples.In addition, apatite, rhabdophane and barite are observed under the SEM-EDX although they are below the detection limit of the XRD.
Compared with the average concentrations of the world hard coals [47], some trace elements including Li, V, Co, Cu, Se, Y, Zr, Nb, REE, Cd, Ta, Hf and Hg, are enriched in the Zhongliangshan coals.Based on the concentrations of Sc, V, Cr, Co, Ni, Cu and Zn, the ratios of Al2O3/TiO2 and the UCC-normalized REY distribution patterns of the Zhongliangshan coals, the dominant sediment source region are the Leshan-Longnvsi Uplift, Hannan Upland, and Dabashan, Uplift, with a small proportion of terrigenous materials from the Kangdian Upland.The modes of occurrence of chamosite, barite, rhabdophane, quartz and calcite indicate that the Zhongliangshan coals have probably been affected by the injection of different low-temperature hydrothermal fluids.The K7 and the upper portion of K1 have the potential to be a source of raw material for REY recovery.2014CB238902), National Natural Science Foundation of China (No. 41502162), the "111" Project (No. B17042) and Chongqing Performance Incentive Project (No. cstc2017jxjl90013).We are indebted to Zhen Wang for his assistance of geochemical analysis.We thank Lei Zhao for the suggestions during the manuscript preparing and revising.Thanks are given to the editor and two anonymous reviewers for their constructive suggestions.[8].The data of the Songzao Coalfield is from Seredin and Dai [58].The data of the Guxu Coalfield is from Dai et al. [82].

Conclusions
The Lopingian coals from the ZK4-1 drill core in the Zhongliangshan Coalfield are medium volatile bituminous coal, which are characterized by a medium-ash yield (26.84%) and high sulfur content (3.38%).Minerals in the Zhongliangshan coals are mainly composed of kaolinite, pyrite, quartz and calcite, with small proportions of anatase.The illite/smectite mixed layer (I/S), chamosite, rutile, marcasite, ankerite, dolomite, jarosite, natrojarosite, bassanite, gypsum and K-feldspar are also present in various coal samples.In addition, apatite, rhabdophane and barite are observed under the SEM-EDX although they are below the detection limit of the XRD.
Compared with the average concentrations of the world hard coals [47], some trace elements including Li, V, Co, Cu, Se, Y, Zr, Nb, REE, Cd, Ta, Hf and Hg, are enriched in the Zhongliangshan coals.Based on the concentrations of Sc, V, Cr, Co, Ni, Cu and Zn, the ratios of Al 2 O 3 /TiO 2 and the UCC-normalized REY distribution patterns of the Zhongliangshan coals, the dominant sediment source region are the Leshan-Longnvsi Uplift, Hannan Upland, and Dabashan, Uplift, with a small proportion of terrigenous materials from the Kangdian Upland.The modes of occurrence of chamosite, barite, rhabdophane, quartz and calcite indicate that the Zhongliangshan coals have probably been affected by the injection of different low-temperature hydrothermal fluids.The K7 and the upper portion of K1 have the potential to be a source of raw material for REY recovery.
and Chongqing Performance Incentive Project (No. cstc2017jxjl90013).We are indebted to Zhen Wang for his assistance of geochemical analysis.We thank Lei Zhao for the suggestions during the manuscript preparing and revising.Thanks are given to the editor and two anonymous reviewers for their constructive suggestions.

Figure 1 .
Figure 1.The location and tectonic map of the Zhongliangshan Coalfield.

Figure 1 .
Figure 1.The location and tectonic map of the Zhongliangshan Coalfield.

Minerals 2018, 8 , 26 Figure 2 .
Figure 2. Sedimentary sequences of the Zhongliangshan Coalfield (A) and the collected samples of the ZK4-1 drill core in present study (B).Partings in blue, host rocks (roof and floor strata) in orange, tuff in purple, and coal benches in black.The suffixes r, f, and p stand for roof, floor, and partings, respectively.

Figure 2 .
Figure 2. Sedimentary sequences of the Zhongliangshan Coalfield (A) and the collected samples of the ZK4-1 drill core in present study (B).Partings in blue, host rocks (roof and floor strata) in orange, tuff in purple, and coal benches in black.The suffixes r, f, and p stand for roof, floor, and partings, respectively.

Figure 6 .
Figure 6.Comparison of observed oxide percentages from chemical analysis (x-axis) to oxide percentages inferred from XRD data (y-axis) in the Zhongliangshan coal and non-coal samples.The diagonal line in each plot indicates equality.

Figure 7 .
Figure 7. Concentration coefficients (CC) of trace elements in the Zhongliangshan coals, normalized by average trace element concentrations in the word hard coals [44].

Figure 6 . 26 Figure 6 .
Figure 6.Comparison of observed oxide percentages from chemical analysis (x-axis) to oxide percentages inferred from XRD data (y-axis) in the Zhongliangshan coal and non-coal samples.The diagonal line in each plot indicates equality.

Figure 7 .
Figure 7. Concentration coefficients (CC) of trace elements in the Zhongliangshan coals, normalized by average trace element concentrations in the word hard coals [44].

Figure 7 .
Figure 7. Concentration coefficients (CC) of trace elements in the Zhongliangshan coals, normalized by average trace element concentrations in the word hard coals [44].

Figure 8 .
Figure 8. Concentration comparison of Sc, V, Cr, Co, Ni, Cu and Zn in the Xinde Mine, Zhongliangshan Coalfield, Lvshuidong Mine and Donglin Mine.The data of the Xinde, Lvshuidong and Donglin are from Dai et al. and Chen et al., respectively [8,14].

Figure 8 .
Figure 8. Concentration comparison of Sc, V, Cr, Co, Ni, Cu and Zn in the Xinde Mine, Zhongliangshan Coalfield, Lvshuidong Mine and Donglin Mine.The data of the Xinde, Lvshuidong and Donglin are from Dai et al. and Chen et al., respectively [8,14].

Figure 9 .
Figure 9. Upper Continental Crust-normalized REY patterns of samples from the Zhongliangshan coals (A-C); non-coal bands (D-I); and Emeishan high-Ti and low-Ti basalts (J).Note: the data of high-Ti and low-Ti are from Huang et al. [67].

Figure 10 .
Figure 10.Relations between REY and ash yield (A); and between REY-ash correlation coefficients and REY atomic number (B).

Figure 10 .
Figure 10.Relations between REY and ash yield (A); and between REY-ash correlation coefficients and REY atomic number (B).

Figure 12 .
Figure 12.Evaluation of REY in the coal ashes and host rocks in the Zhongliangshan Coalfield.The classification base map is based on Dai et al. [9].

Figure 12 .
Figure 12.Evaluation of REY in the coal ashes and host rocks in the Zhongliangshan Coalfield.The classification base map is based on Dai et al. [9].

Figure 13 .
Figure13.REY oxides (REO) concentrations in coal ash higher than 1000 ppm in Zhongliangshan and surrounding Coalfields.Tha data of Huayingshan Coalfield is from Dai et al.[8].The data of the Songzao Coalfield is from Seredin and Dai[58].The data of the Guxu Coalfield is from Dai et al.[82].

Figure 13 .
Figure13.REY oxides (REO) concentrations in coal ash higher than 1000 ppm in Zhongliangshan and surrounding Coalfields.Tha data of Huayingshan Coalfield is from Dai et al.[8].The data of the Songzao Coalfield is from Seredin and Dai[58].The data of the Guxu Coalfield is from Dai et al.[82].

Table 1 .
Proximate and ultimate analysis, forms of sulfur, and random vitrinite reflectance for coals from the ZK4-1 drill core in the Zhongliangshan Coalfield (%).

Table 2 .
Mineral compositions of coal LTAs and non-coal samples by X-ray diffraction (XRD) and Siroquant analysis (wt %).

Table 3 .
Major element oxides (%) and trace elements in the coal and non-coal samples from the ZK4-1 drill core in the Zhongliangshan Coalfield (µg/g).

Table 4 .
Concentrations of rare earth element and yttrium (REY) in the coal and non-coal samples from the ZK4-1 drill core in the Zhongliangshan Coalfield (μg/g).

Table 4 .
Concentrations of rare earth element and yttrium (REY) in the coal and non-coal samples from the ZK4-1 drill core in the Zhongliangshan Coalfield (µg/g).

Table 5 .
REY parameters in the coal and non-coal samples from the ZK4-1 drill core in the Zhongliangshan Coalfield.