Mineralogical Characteristics of Late Permian Coals from the Yueliangtian Coal Mine, Guizhou, Southwestern China

This paper reports the mineralogical compositions of super-low-sulfur (Yueliangtian 6-upper (YLT6U)) and high-sulfur (Yueliangtian 6-lower (YLT6L)) coals of the Late Permian No. 6 coal seam from the Yueliangtian coal mine, Guizhou, southwestern China. The mineral assemblages and morphology were detected and observed by X-ray diffractogram (XRD), optical microscopy and field-emission scanning electron microscope (FE-SEM) in conjunction with an energy-dispersive X-ray spectrometer. Major minerals in the coal samples, partings and host rocks (roof and floor strata) include calcite, quartz, kaolinite, mixed-layer illite/smectite, chlorite and pyrite and, to a lesser extent, chamosite, anatase and apatite. The Emeishan basalt and silicic rocks in the Kangdian Upland are the sediment source for the Yueliangtian coals. It was found that there are several modes of chamosite occurrence, and precursor minerals, such as anatase, had been corroded by Ti-rich hydrothermal solutions. The modes of occurrence of minerals present in the coal were controlled by the injection of different types of hydrothermal fluids during different deposition stages. The presence of abundant pyrite and extremely high total sulfur contents in the YLT6L coal are in sharp contrast to those in the YLT6U coal, suggesting that seawater invaded the peat swamp of the YLT6L coal and terminated at the YLT6U-9p sampling interval. High-temperature quartz, vermicular kaolinite and chloritized biotite were observed in the partings and roof strata. The three partings and floor strata of the No. 6 coal seam from the Yueliangtian coal mine appear to have been derived from felsic volcanic ash. Four factors, including sediment-source region, multi-stage injections of hydrothermal fluids, seawater influence and volcanic ash input, were responsible for the mineralogical characteristics of the Yueliangtian coals.


Introduction
Guizhou province in southwestern (SW) China contains abundant coal resources.The Late Permian coals from western Guizhou province have attracted much attention [1][2][3][4][5], not only because of the coal-hosted rare-metal ore deposits found in this area [6][7][8], but also due to their mineralogical and geochemical indications for the reginal geology evolution, such as the mantle plume formation located to the west of the coal basin [1,3].Practically, although Panxian county is closely located to the high-incidence area of the endemic arsenosis and fluorosis in SW China, the relation between mineralogical compositions of the coals in this area and the endemic disease occurring in the surrounding areas is unknown.The Yueliangtian coal mine in Panxian county provides large-scale energy resources for its region, and the coals are directly used for combustion, despite high mineral contents in these coals, as described below.Thus, it is necessary to determine the contents and modes Minerals 2016, 6, 29 2 of 21 of occurrence of minerals in coals for the environmental issues caused by coal combustion in the area.Theoretically, mineral matter in coal may indicate depositional environments during peat accumulation, as well as geological processes during diagenetic and epigenetic stages [9].Although the coals in Panxian county of southwestern Guizhou province have been reported by a few researchers [4,5], geochemical characteristics, modes of mineral occurrence and their controlling geological factors in these coals, however, have not been well addressed.In this paper, we report the mineralogical characteristics of the Late Permian coals from the No. 6 coal seam at the Yueliangtian coal mine, Guizhou, SW China.

Geological Setting
The Yueliangtian coal mine is located in Panxian county, western Guizhou, southwestern China, covering a total area of around 15 km 2 , 6 km N-S long and 2.5 km W-E wide (Figure 1).Tectonically, the Yueliangtian deposit belongs to a monocline structure with an approximate east dip [6].It is limited by well-developed normal faults within the epsilon-type structure in the Puan tectonic zone [6,10], covering the area between latitudes 25 ˝54 1 22" and 25 ˝57 1 44" N and longitudes 104 ˝30 1 36" and 104 ˝31 1 59" E. The Kangdian Upland is the dominant sediment-source region for this coal deposit in western Guizhou [11].
The sedimentary sequences in the Yueliangtian coal mine are the Late Permian and Early Triassic (Figure 2) strata.The Late Permian strata, with major coal resources in the area, consist of the Longtan and Emeishan Basalt Formations.The upper and lower portions of the Emeishan Basalt Formation are dominated by grey-lilac tuffs and grey-dark basalts, respectively, similar to those in the surrounding areas such as the Zhijing Coalfiled in western Guizhou [12].
accumulation, as well as geological processes during diagenetic and epigenetic stages [9].Although the coals in Panxian county of southwestern Guizhou province have been reported by a few researchers [4,5], geochemical characteristics, modes of mineral occurrence and their controlling geological factors in these coals, however, have not been well addressed.In this paper, we report the mineralogical characteristics of the Late Permian coals from the No. 6 coal seam at the Yueliangtian coal mine, Guizhou, SW China.

Geological Setting
The Yueliangtian coal mine is located in Panxian county, western Guizhou, southwestern China, covering a total area of around 15 km 2 , 6 km N-S long and 2.5 km W-E wide (Figure 1).Tectonically, the Yueliangtian deposit belongs to a monocline structure with an approximate east dip [6].It is limited by well-developed normal faults within the epsilon-type structure in the Puan tectonic zone [6,10], covering the area between latitudes 25°54′22″ and 25°57′44″ N and longitudes 104°30′36″ and 104°31′59″ E. The Kangdian Upland is the dominant sediment-source region for this coal deposit in western Guizhou [11].
The sedimentary sequences in the Yueliangtian coal mine are the Late Permian and Early Triassic (Figure 2) strata.The Late Permian strata, with major coal resources in the area, consist of the Longtan and Emeishan Basalt Formations.The upper and lower portions of the Emeishan Basalt Formation are dominated by grey-lilac tuffs and grey-dark basalts, respectively, similar to those in the surrounding areas such as the Zhijing Coalfiled in western Guizhou [12].
The Longtan Formation (P2l) is the major coal-bearing strata of the coal mine.The sedimentary environment of the Longtan Formation varies greatly from lower delta plains, through tidal flats, to carbonate subtidal flats [11][12][13][14].As shown in Figure 2, the upper portion of the Longtan Formation (81.97 m) is made up of siltstone, siderite layers and twelve coal seams.Its middle portion (89.75 m), intercalated with siderite layers and coal beds, is mainly composed of siltstone and silty mudstone.A total of 16 coal beds occur in the middle portion.The lower portion is composed of siltstone, silty mudstone, pelitic siltstone, siderite and ten coal seams.The Yueliangtian 6-upper (YLT6U) and 6-lower (YLT6L) coals, separated by a gray siltstone (2.94 m) that contains siderite layers and plenty of plant-root fossils, occur in the upper portion of the Longtan Formation.Currently, the YLT6U coal seam is the only mineable seam in this area (Figure 2).The Longtan Formation (P 2l ) is the major coal-bearing strata of the coal mine.The sedimentary environment of the Longtan Formation varies greatly from lower delta plains, through tidal flats, to carbonate subtidal flats [11][12][13][14].As shown in Figure 2, the upper portion of the Longtan Formation (81.97 m) is made up of siltstone, siderite layers and twelve coal seams.Its middle portion (89.75 m), intercalated with siderite layers and coal beds, is mainly composed of siltstone and silty mudstone.A total of 16 coal beds occur in the middle portion.The lower portion is composed of siltstone, silty mudstone, pelitic siltstone, siderite and ten coal seams.The Yueliangtian 6-upper (YLT6U) and 6-lower (YLT6L) coals, separated by a gray siltstone (2.94 m) that contains siderite layers and plenty

Sample Collection and Analytical Procedures
According to Chinese Standard Method GB/T 482-2008 [16], a total of nineteen bench samples were collected from the No. 6 coal seam (YLT6) in the underground workings of the Yueliangtian coal mine in Panxian county, western Guizhou.The floor of the YLT6U coal and the roof of the YLT6L coal are unavailable, due to the complex structure, as well as limited capacity in sample collection.From top to bottom, the roof strata (with suffix-r), coal benches, partings (with suffix-p) and floor-stratum samples (with suffix-f) are identified, with the coal seams numbered in increasing order from top to bottom (Figure 3).Each bench sample was immediately stored in a clean and uncontaminated plastic bag, to ensure as little contamination and oxidation as possible.The thickness of each sample is given in Table 1.

Sample Collection and Analytical Procedures
According to Chinese Standard Method GB/T 482-2008 [16], a total of nineteen bench samples were collected from the No. 6 coal seam (YLT6) in the underground workings of the Yueliangtian coal mine in Panxian county, western Guizhou.The floor of the YLT6U coal and the roof of the YLT6L coal are unavailable, due to the complex structure, as well as limited capacity in sample collection.From top to bottom, the roof strata (with suffix-r), coal benches, partings (with suffix-p) and floor-stratum samples (with suffix-f) are identified, with the coal seams numbered in increasing order from top to bottom (Figure 3).Each bench sample was immediately stored in a clean and uncontaminated plastic bag, to ensure as little contamination and oxidation as possible.The thickness of each sample is given in Table 1.The samples were crushed and ground to pass the 75-μm sieve prior to analysis.In accordance with ASTM Standards D3173-11, D3175-11 and D3174-11 [17][18][19], proximate analysis, covering moisture and volatile matter percentages and ash yield, was conducted.Total sulfur was determined based on ASTM Standard D3177-02 [20].The percentages of C, H and N in the coals were determined by an elemental analyzer (vario MACRO).Coarse-crushed samples of each coal were prepared as grain mounts and examined with 50× oil immersion for microscopic analysis.Following ASTM Standards D2797/D2797M-11a and D2798M-11a [21,22], vitrinite random reflectance (Rr, %) was determined using a Leica DM-4500P microscope (at a magnification of 500×) equipped with a Craic QDI 302™ spectrophotometer (Leica Inc., Wetzlar, Germany).An X-ray diffractogram (XRD) was used to determine the mineralogical compositions.Prior to XRD analysis, the low-temperature ashing (LTA) of coal was performed using an EMITECH K1050X plasma asher (Quorum Inc., Lewes, UK).A commercial interpretation software Siroquant™ [23,24] was used to obtain mineral contents in the LTAs and non-coal samples based on XRD.More information illustrating the use of this technique for coal-related materials was given by Ward et al. [25,26], Ruan and Ward [27] and Dai et al. [28,29].Following the procedures described by Dai et al. [15,30], a field-emission scanning electron microscope (FE-SEM) in conjunction with an energy-dispersive X-ray spectrometer (EDAX Inc., Mahwah, NJ, USA) was used to observe modes of mineral occurrence, and also to determine the distribution of some selected elements.Samples were carbon-coated using a Quorum Q150T ES sputtering coater (Quorum Inc.) or were not coated for The samples were crushed and ground to pass the 75-µm sieve prior to analysis.In accordance with ASTM Standards D3173-11, D3175-11 and D3174-11 [17][18][19], proximate analysis, covering moisture and volatile matter percentages and ash yield, was conducted.Total sulfur was determined based on ASTM Standard D3177-02 [20].The percentages of C, H and N in the coals were determined by an elemental analyzer (vario MACRO).Coarse-crushed samples of each coal were prepared as grain mounts and examined with 50ˆoil immersion for microscopic analysis.Following ASTM Standards D2797/D2797M-11a and D2798M-11a [21,22], vitrinite random reflectance (Rr, %) was determined using a Leica DM-4500P microscope (at a magnification of 500ˆ) equipped with a Craic QDI 302™ spectrophotometer (Leica Inc., Wetzlar, Germany).An X-ray diffractogram (XRD) was used to determine the mineralogical compositions.Prior to XRD analysis, the low-temperature ashing (LTA) of coal was performed using an EMITECH K1050X plasma asher (Quorum Inc., Lewes, UK).A commercial interpretation software Siroquant™ [23,24] was used to obtain mineral contents in the LTAs and non-coal samples based on XRD.More information illustrating the use of this technique for coal-related materials was given by Ward et al. [25,26], Ruan and Ward [27] and Dai et al. [28,29].Following the procedures described by Dai et al. [15,30], a field-emission scanning electron microscope (FE-SEM) in conjunction with an energy-dispersive X-ray spectrometer (EDAX Inc., Mahwah, NJ, USA) was used to observe modes of mineral occurrence, and also to determine the distribution of some selected elements.Samples were carbon-coated using a Quorum Q150T ES sputtering coater (Quorum Inc.) or were not coated for low-vacuum SEM working conditions (60 bar).The working distance, beam voltage, aperture and spot size of the FE-SEM-EDS was 10 mm, 20.0 kV, 6 and 4.5-5.0,respectively.The images were captured by a back-scattered electron detector (BSE).The clay (<2 µm) fraction of rock samples was conducted based on the Chinese Industry Standard Method [31].The sample was dissolved in 80 mL ultrapure water and isolated by ultrasonic dispersion.After 4h standing, the natural-oriented aggregate was analyzed in an air-dried state.The glycol-saturated (50 ˝C, 8 h) and heated (450 ˝C, 2.5 h) oriented aggregates were analyzed subsequently.The mineralogy of this fraction was also analyzed by XRD.

Coal Chemistry and Vitrinite Reflectance
The vitrinite random reflectance values and the volatile matter yields of the YLT6U and YLT6L coals (Table 1) indicate a high volatile A bituminous coal in rank [32].The ash yield of the YLT6U coal samples varies from 10.07%-42.36%, with the weighted average much lower than that of the YLT6L coal.Total sulfur contents vary greatly between the YLT6U and YLT6L coal seams (Table 1).As shown in Tables 1 and 2 the high-sulfur samples also have high percentages of pyrite (described in more detail below).Based on Chinese Standards GB/T 15224.1/2-2010[33], the YLT6U coal is classified as a medium-ash and super-low-sulfur coal; the YLT6L coal, with its total sulfur content higher than other coals in Guizhou province [14,34], is a medium-high-ash and high-sulfur coal.

Minerals in Coal Benches
The proportions of each crystalline phase in the coal-LTA and non-coal samples identified from XRD and Siroquant are given in Table 2.The vertical concentration (%) variations of minerals, and low-temperature ash yields (%) through the No. 6 coals in the Yueliangtian coal mine are described in Figure 4. Like other Late Permian coals in western Guizhou described by Dai et al. [14,34,35] and Zhuang et al. [36], the major minerals of the YLT6U coal LTAs are mainly calcite, kaolinite and quartz, with trace amounts of chlorite and anatase.Various contents of illite, mixed-layer illite/smectite, pyrite, bassanite and ankerite also occur in a few coal benches (Table 2).The minerals in the LTA of the YLT6L coal are mainly represented by calcite, quartz and pyrite, with a lesser proportion of kaolinite, chlorite, rutile and bassanite.The LTA of sample YLT6L-2 also contains a small proportion of xenotime.The proportions of each crystalline phase in the coal-LTA and non-coal samples identified from XRD and Siroquant are given in Table 2.The vertical concentration (%) variations of minerals, and low-temperature ash yields (%) through the No. 6 coals in the Yueliangtian coal mine are described in Figure 4. Like other Late Permian coals in western Guizhou described by Dai et al. [14,34,35] and Zhuang et al. [36], the major minerals of the YLT6U coal LTAs are mainly calcite, kaolinite and quartz, with trace amounts of chlorite and anatase.Various contents of illite, mixed-layer illite/smectite, pyrite, bassanite and ankerite also occur in a few coal benches (Table 2).The minerals in the LTA of the YLT6L coal are mainly represented by calcite, quartz and pyrite, with a lesser proportion of kaolinite, chlorite, rutile and bassanite.The LTA of sample YLT6L-2 also contains a small proportion of xenotime.Quartz in the YLT6U and YLT6L coals occurs in three forms: (1) as cell-fillings of structured inertinite macerals, suggesting an authigenic origin (Figure 5A); (2) as discrete particles embedded in collodetrinite (Figure 5B,C); and (3) coexisting with clay minerals (Figure 5D).Quartz with the latter two forms was probably derived from detrital materials of terrigenous origin.
As shown in Table 2 and Figure 4, the host rocks (roof and floor strata) and partings (YLT6U-9p and YLT6L-4p) of the YLT6U and YLT6L coals contain minor calcite.By contrast, the YLT6U coal contains abundant calcite, accounting for 76.8% of the total minerals in the LTA sample.Calcite layering has been macroscopically observed in sample YLT6U-5l during sampling in the field.Microscopically, it occurs mainly as cell-fillings (Figure 6A), as isolated particles with various Quartz in the YLT6U and YLT6L coals occurs in three forms: (1) as cell-fillings of structured inertinite macerals, suggesting an authigenic origin (Figure 5A); (2) as discrete particles embedded in collodetrinite (Figure 5B,C); and (3) coexisting with clay minerals (Figure 5D).Quartz with the latter two forms was probably derived from detrital materials of terrigenous origin.
As shown in Table 2 and Figure 4, the host rocks (roof and floor strata) and partings (YLT6U-9p and YLT6L-4p) of the YLT6U and YLT6L coals contain minor calcite.By contrast, the YLT6U coal contains abundant calcite, accounting for 76.8% of the total minerals in the LTA sample.Calcite layering has been macroscopically observed in sample YLT6U-5l during sampling in the field.Microscopically, it occurs mainly as cell-fillings (Figure 6A), as isolated particles with various forms (Figure 6B,C) and as vein-fillings in the vitrinite (Figure 6D).A platy calcite (Figure 6B) with a large size up to 570 µm in length and 100 µm in width has distinct edges and angles.Kaolinite in the YLT6 coal has two modes of occurrence: as cell fillings (Figure 7A,B) and as detrital particles (Figure 5D).The former is of authigenic origin, and the latter is probably of terrigenous origin.In contrast to the minerals described above, pyrite is distributed completely differently between the YLT6U and YLT6L coals.Although below the detection limit of the XRD technique, cell-filling pyrite was observed under the optical microscope in sample YLT6U-8 (Figure 8A).Pyrites of framboidal (Figure 8B), cell-fillings (Figures 7A,B and 8C), cubic (Figure 8D) and nodular (Figure 8E) forms occur in the YLT6L coal.It is probably of syngenetic or early diagenetic origin [37][38][39][40][41]. Pyrite also occurs as cleat-and fracture-fillings (Figures 7A and 8C), suggesting an epigenetic origin of hydrothermal solutions with a formation temperature around   Kaolinite in the YLT6 coal has two modes of occurrence: as cell fillings (Figure 7A,B) and as detrital particles (Figure 5D).The former is of authigenic origin, and the latter is probably of terrigenous origin.In contrast to the minerals described above, pyrite is distributed completely differently between the YLT6U and YLT6L coals.Although below the detection limit of the XRD technique, cell-filling pyrite was observed under the optical microscope in sample YLT6U-8 (Figure 8A).Pyrites of framboidal (Figure 8B), cell-fillings (Figures 7A,B and 8C), cubic (Figure 8D) and nodular (Figure 8E) forms occur in the YLT6L coal.It is probably of syngenetic or early diagenetic origin [37][38][39][40][41]. Pyrite also occurs as cleat-and fracture-fillings (Figures 7A and 8C), suggesting an epigenetic origin of hydrothermal solutions with a formation temperature around Kaolinite in the YLT6 coal has two modes of occurrence: as cell fillings (Figure 7A,B) and as detrital particles (Figure 5D).The former is of authigenic origin, and the latter is probably of terrigenous origin.In contrast to the minerals described above, pyrite is distributed completely differently between the YLT6U and YLT6L coals.Although below the detection limit of the XRD technique, cell-filling pyrite was observed under the optical microscope in sample YLT6U-8 (Figure 8A).Pyrites of framboidal (Figure 8B), cell-fillings (Figures 7A,B and 8C), cubic (Figure 8D) and nodular (Figure 8E) forms occur in the YLT6L coal.It is probably of syngenetic or early diagenetic origin [37][38][39][40][41]. Pyrite also occurs Minerals 2016, 6, 29 9 of 21 as cleat-and fracture-fillings (Figures 7A and 8C), suggesting an epigenetic origin of hydrothermal solutions with a formation temperature around 150-200 ˝C [5,41,42].Fe-S compounds with different levels of brightness (Figure 8F) deserve a special note: the atomic ratios of Fe against S of Spots 1 and 2 (Figure 8G,H) are 1:2.4 and 1:2, respectively, probably suggesting an oxidation product of pyrite in Spot 1.

Minerals in the Partings
In comparison with the mineral matter in the coal benches, the four partings (YLT6U-2p, YLT6U-9p, YLT6L-3p and YLT6L-4p) have lower calcite and higher kaolinite and mixed-layer illite/smectite contents.In addition, the partings within the YLT6U coal have higher anatase and lower quartz contents.The partings within the YLT6L coal have higher chlorite and lower pyrite contents relative to those in the respective coal benches (Table 2, Figure 4).XRD analysis of the fractions of the partings shows relatively more abundant kaolinite and I/S, with small proportion of quartz, chamosite and berthierine in the clay mineral assemblage (Figure 9A-D).The chamosite is represented by a peak at 14.4 Å under air-dried conditions in sample YLT6L-4p, and its peak of 7.1 Å appears on heating (Figure 9D).Authigenic kaolinite and mixed-layer illite/smectite in the partings occur as fusinite-and semifusinite-cell fillings (Figure 10A), as vermicular form (Figure 10B,C) and as a matrix (Figure 10D).Kaolinite and mixed-layer illite/smectite (I/S) also occur as discrete particles (Figure 10E,F) and, in some cases, are distributed along bedding planes (Figure 10G).
Chlorite in the partings occurs generally as a cryptocrystalline matrix coexisting with anatase and pyrite (Figure 11A,C).Anatase provides Ti peaks in some of the chlorite EDS spectrum (Figure 10K).A small proportion of epigenetic chlorite occurs as fracture-filling in sample YLT6L-4p (Figure 11B).Chlorite with sub-angular forms (Figure10F,J) formed because earlier formed apatite was chloritized by Fe-Mg fluids.

Minerals in the Partings
In comparison with the mineral matter in the coal benches, the four partings (YLT6U-2p, YLT6U-9p, YLT6L-3p and YLT6L-4p) have lower calcite and higher kaolinite and mixed-layer illite/smectite contents.In addition, the partings within the YLT6U coal have higher anatase and lower quartz contents.The partings within the YLT6L coal have higher chlorite and lower pyrite contents relative to those in the respective coal benches (Table 2, Figure 4).XRD analysis of the fractions of the partings shows relatively more abundant kaolinite and I/S, with small proportion of quartz, chamosite and berthierine in the clay mineral assemblage (Figure 9A-D).The chamosite is represented by a peak at 14.4 Å under air-dried conditions in sample YLT6L-4p, and its peak of 7.1 Å appears on heating (Figure 9D).Authigenic kaolinite and mixed-layer illite/smectite in the partings occur as fusinite-and semifusinite-cell fillings (Figure 10A), as vermicular form (Figure 10B,C) and as a matrix (Figure 10D).Kaolinite and mixed-layer illite/smectite (I/S) also occur as discrete particles (Figure 10E,F) and, in some cases, are distributed along bedding planes (Figure 10G).
Chlorite in the partings occurs generally as a cryptocrystalline matrix coexisting with anatase and pyrite (Figure 11A,C).Anatase provides Ti peaks in some of the chlorite EDS spectrum (Figure 10K).A small proportion of epigenetic chlorite occurs as fracture-filling in sample YLT6L-4p (Figure 11B).Chlorite with sub-angular forms (Figure 10F,J) formed because earlier formed apatite was chloritized by Fe-Mg fluids.Anatase in the partings occurs as coarse-crystalline (Figure 11A,B) and beaded form (Figures 10D and 11D).They were closely associated with clay minerals and were probably precipitated from Ti-rich fluids with Ti leaching from volcanic ash as described below.The blocky and framboidal pyrite, associated with some fragments of organic matter in sample YLT6U-9p (Figure 10G), suggests that the pyrite is a syngenetic sedimentary material rather than a derivation of volcanic ash.Euhedral pyrite of syngenetic origin also occurs as cubes and octahedrons (Figure 11E).Cell-and cavity-filling pyrite (Figures 10A and 11F) in the YLT6L coal is of authigenic origin.
A number of accessory minerals have also been found in the partings.Epigenetic barite occurs as fracture fillings (Figure 11G), and in some cases, it also coexists with mixed-layer I/S (Figure 11H).Lathlike sphalerite is embedded in mixed-layer I/S (Figure 11I), suggestive of epigenetic origin.Anatase in the partings occurs as coarse-crystalline (Figure 11A,B) and beaded form (Figures 10D and 11D).They were closely associated with clay minerals and were probably precipitated from Ti-rich fluids with Ti leaching from volcanic ash as described below.The blocky and framboidal pyrite, associated with some fragments of organic matter in sample YLT6U-9p (Figure 10G), suggests that the pyrite is a syngenetic sedimentary material rather than a derivation of volcanic ash.Euhedral pyrite of syngenetic origin also occurs as cubes and octahedrons (Figure 11E).Cell-and cavity-filling pyrite (Figures 10A and 11F) in the YLT6L coal is of authigenic origin.
A number of accessory minerals have also been found in the partings.Epigenetic barite occurs as fracture fillings (Figure 11G), and in some cases, it also coexists with mixed-layer I/S (Figure 11H).Lathlike sphalerite is embedded in mixed-layer I/S (Figure 11I), suggestive of epigenetic origin.Monazites are present in YLT6L-4p and YLT6U-9p samples (Figure 10B,D).The Energy Dispersive X-ray Spectroscopy (EDS) spectra of minerals in Figure 11 are shown in Figure 12.The roof of the YLT6U coal (YLT6U-r) and the floor of the YLT6L coal (YLT6L-f) are macroscopically and microscopically different.Macroscopically, sample YLT6L-f shows a well-developed bedding plane compared to sample YLT6U-r.Microscopically, samples YLT6U-r and YLT6L-f are mainly composed of mixed-layer illite/smectite and quartz, with a lesser proportion of chlorite, anatase and calcite (Table 2, Figure 4).Siderite and muscovite are also present in sample YLT6U-r, and kaolinite, pyrite and apatite are present in the floor of the YLT6L coal.The clay minerals identified in the host rocks by the clay-fraction studies are kaolinite, chamosite and mixed-layer I/S (Figure 9E,F).The mixed-layer I/S in sample YLT6L-f has a peak of 11.0 Å on the natural-oriented aggregate.The sharp peak of 10.1 Å in sample YLT6L-f indicates the presence of illite on heated aggregate (Figure 9F).The roof of the YLT6U coal (YLT6U-r) and the floor of the YLT6L coal (YLT6L-f) are macroscopically and microscopically different.Macroscopically, sample YLT6L-f shows a well-developed bedding plane compared to sample YLT6U-r.Microscopically, samples YLT6U-r and YLT6L-f are mainly composed of mixed-layer illite/smectite and quartz, with a lesser proportion of chlorite, anatase and calcite (Table 2, Figure 4).Siderite and muscovite are also present in sample YLT6U-r, and kaolinite, pyrite and apatite are present in the floor of the YLT6L coal.The clay minerals identified in the host rocks by the clay-fraction studies are kaolinite, chamosite and mixed-layer I/S (Figure 9E,F).The mixed-layer I/S in sample YLT6L-f has a peak of 11.0 Å on the natural-oriented aggregate.The sharp peak of 10.1 Å in sample YLT6L-f indicates the presence of illite on heated aggregate (Figure 9F).Quartz in the host rocks occurs mainly as discrete particles with irregular forms (Figure 13A,B).Kaolinite in sample YLT6L-f occurs as discrete particles (Figure 13B,C), and some kaolinite has flocculent shapes (Figure 13D,E).Authigenic anatase in the host rocks is ring shaped (Figure 13A) or occurs as discrete particles (Figure 13B), coarse-crystalline (Figure 13D), platy (Figure 13E), linear (Figure 13F) and fracture-infilling (Figure 13G,H) forms.In addition, some anatases coexist with stripped apatite (Figure 13H).Chamosite coexisting with kaolinite in flocculent forms (Figure 13D,E,H) occurs in the roof strata.Euhedral pyrite is present in the floor (Figure 13I), suggestive of syngenetic origin.Quartz in the host rocks occurs mainly as discrete particles with irregular forms (Figure 13A,B).Kaolinite in sample YLT6L-f occurs as discrete particles (Figure 13B,C), and some kaolinite has flocculent shapes (Figure 13D,E).Authigenic anatase in the host rocks is ring shaped (Figure 13A) or occurs as discrete particles (Figure 13B), coarse-crystalline (Figure 13D), platy (Figure 13E), linear (Figure 13F) and fracture-infilling (Figure 13G,H) forms.In addition, some anatases coexist with stripped apatite (Figure 13H).Chamosite coexisting with kaolinite in flocculent forms (Figure 13D,E,H) occurs in the roof strata.Euhedral pyrite is present in the floor (Figure 13I), suggestive of syngenetic origin.Quartz in the host rocks occurs mainly as discrete particles with irregular forms (Figure 13A,B).Kaolinite in sample YLT6L-f occurs as discrete particles (Figure 13B,C), and some kaolinite has flocculent shapes (Figure 13D,E).Authigenic anatase in the host rocks is ring shaped (Figure 13A) or occurs as discrete particles (Figure 13B), coarse-crystalline (Figure 13D), platy (Figure 13E), linear (Figure 13F) and fracture-infilling (Figure 13G,H) forms.In addition, some anatases coexist with stripped apatite (Figure 13H).Chamosite coexisting with kaolinite in flocculent forms (Figure 13D,E,H) occurs in the roof strata.Euhedral pyrite is present in the floor (Figure 13I), suggestive of syngenetic origin.

Discussion
The assemblage of minerals in the No. 6 coal seam of the Yueliangtian coals and associated non-coal samples is attributed to four factors, including sediment-source region, multi-stage injections of hydrothermal fluids, seawater influence and volcanic ash input.

Sediment-Source Region Influence
Similar to other Late Permian coals from southwestern China [36,[43][44][45], the Kangdian Upland (Figure 1) is the sediment-source region for the YLT6 coal [11].Not only the Emeishan mafic basalts, but also the overlying felsic-intermediate rocks could have been the terrigenous source materials for the Late Permian coals present in this study [46].Quartz in the YLT6 coal is slightly higher than that in coals from the Dafang (0.8%-11.4% on average) [14] and Zhijin (0.5%-9.4% on average) mines [35] (Figure 1), consistent with Dai et al. [15] and indicating that the closer the coals are located to the sediment-source Kangdian Upland region, the higher the quartz in the coal.The abundant authigenic quartz (Figure 5A) is precipitated from the silica-bearing solutions from the weathering of basalt in the Kangdian Upland [15,47,48].
Although the thick lava sequence from the Emeishan large igneous province in southwestern China is basalt, silicic rocks also occur in the uppermost part of the Kangdian upland [47].There is no quartz and apatite in Emeishan basalt of the Kangdian Upland [49]; the abundant detrital quartz (Figure 5B-D) and authigenic apatite (Figure 13H) probably indicate that the parent rock is also granite or other silicic rocks [47].

Discussion
The assemblage of minerals in the No. 6 coal seam of the Yueliangtian coals and associated non-coal samples is attributed to four factors, including sediment-source region, multi-stage injections of hydrothermal fluids, seawater influence and volcanic ash input.

Sediment-Source Region Influence
Similar to other Late Permian coals from southwestern China [36,[43][44][45], the Kangdian Upland (Figure 1) is the sediment-source region for the YLT6 coal [11].Not only the Emeishan mafic basalts, but also the overlying felsic-intermediate rocks could have been the terrigenous source materials for the Late Permian coals present in this study [46].Quartz in the YLT6 coal is slightly higher than that in coals from the Dafang (0.8%-11.4% on average) [14] and Zhijin (0.5%-9.4% on average) mines [35] (Figure 1), consistent with Dai et al. [15] and indicating that the closer the coals are located to the sediment-source Kangdian Upland region, the higher the quartz in the coal.The abundant authigenic quartz (Figure 5A) is precipitated from the silica-bearing solutions from the weathering of basalt in the Kangdian Upland [15,47,48].
Although the thick lava sequence from the Emeishan large igneous province in southwestern China is basalt, silicic rocks also occur in the uppermost part of the Kangdian upland [47].There is no quartz and apatite in Emeishan basalt of the Kangdian Upland [49]; the abundant detrital quartz (Figure 5B-D) and authigenic apatite (Figure 13H) probably indicate that the parent rock is also granite or other silicic rocks [47].

Multi-Stage Injections of Hydrothermal Fluids
Hydrothermal fluids play an important role in the enrichment of minerals and trace elements in the coals of Guizhou province [5,12,14,41].Multi-stage hydrothermal fluids also influenced the mineralogical characteristics of the Yueliangtian coals, partings, roof and floor.
The cell-filling kaolinite (Figure 7A,B) suggests an authigenic origin in coal.Al-and Si-bearing solutions from the sediment-source Kangdian Upland region precipitated in the cells of coal-forming plants during coal formation.Although chlorite (e.g., chamosite) is rare in coals, if present, it is usually observed in high-rank coals or in coals influenced by epigenetic hydrothermal solutions [44,50,51].Chamosite is present in the YLT6 coal and coexists with kaolinite and quartz (Figure 13D,E) in the roof strata.As suggested by Equation ( 1), the earlier-precipitated kaolinite was invaded by Fe-Mg-rich hydrothermal fluids and generated kaolinite and chamosite early diagenesis at temperatures around 165-200 ˝C [52].Then, kaolinite and chamosite desilicated and generated quartz, as suggested by Equation (2).Dispersed chamosite (Figure 13B) independent of kaolinite and quartz may have precipitated from Fe-rich hydrothermal solutions, with Fe probably coming from siderite layers in the sedimentary sequence (Figure 2) [53].
Kaolinite `Fe `Mg Ñ Kaolinite `Chamosite p165 ´200 ˝Cq Kaolinite `Chamosite Ñ Chamosite `Quartz pDesilicationq The abundant authigenic quartz (Figure 5A) is probably precipitated from the silicious solution of the weathering product of Emeishan basalt from the Kangdian Upland [47,54].Epigenetic vein-like calcite (Figure 6D) is probably precipitated by the circulation of Ca-bearing meteoric fluids or by Ca-rich solutions during the coal-formation process [55].The average formation temperature of vein calcite is about 190 ˝C [14].
The crystal-shaped cavities commonly occurred in the samples YLT6L-4p (Figures 10D,E  and 11C,D) and YLT6L-f (Figure 13E), suggesting that the earlier formed mineral crystals were corroded by hydrothermal solutions [15].

Seawater Influence
Although the vertical distance between the floor of the YLT6U and the roof of the YLT6L coals is only 2.94 m, the deposit environments of the two coals are strikingly different.
The total sulfur contents (ranging from 3.61%-13.34%) in the YLT6L coal are higher than those in the YLT6U coal (0.27%-0.75%).Except for the YLT6U-8 and YLT6U-9p samples, no pyrite was present in the YLT6U coal.However, pyrite of syngenetic origin is common in the YLT6L coal, non-coal partings and floor strata (Figure 8B,D,E, Figures 10G, 11E and 13I).It can be inferred that seawater provided a sulfur source both during peat forming and in the diagenetic process of the YLT6L coal.The abundant calcite and mixed-layer I/S in the YLT6L coal also signifies the alkaline, medium environment of seawater (Table 2), which are also beneficial for calcite formation and the transformation from kaolinite to mixed-layer I/S [4].The paleoenvironment of the Longtan Formation varies from lagoons and tidal flats [11][12][13][14].Thus, seawater had a tremendous influence on YLT6L coal and terminated at the YLT6U-9p sampling interval.
The three partings (samples YLT6U-2p, YLT6U-9p and YLT6L-4p) have a lateral continuity the Yueliangtian coal mine.As shown in Table 2, the mineralogy of the three non-coal partings is dominated by kaolinite and mixed-layer I/S, accounting for 80%-89.1% of the mineral compositions of the respective parting.The mixed-layer I/S in the partings mostly occurs as cryptocrystalline matrix (Figures 10D and 11H,I).Kaolinite in samples YLT6U-2p and YLT6U-9p occurs as large crystals with a well-developed vermicular texture (Figure 10B,C).Vermicular kaolinite is thought to indicate air-fall volcanic ash layers altered and deposited in a non-marine, coal-forming environment [61].Biotite pseudomorphs with chlorite laminae occur in the sample YLT6L-4p (Figure 10E).Such an occurrence of chloritized biotite suggests an in situ crystallized origin [15].
High-temperature quartz has been identified in the sample YLT6L-f.The β-quartz (Figure 13B) present in this study shows triangle and irregular forms and is considered to have originated from autochthonous syngenetic felsic to intermediate volcanic ashes.Dai et al. [59] and Zhou et al. [60] suggested that tonsteins in the lower and upper portions of the Late Permian were mainly of alkali and felsic composition, respectively.The No. 6 coal seam in the present study is located in the upper portion of the Late Permian strata (Figure 2).
The modes of anatase occurrence in the partings (Figure 11D) and floor strata (Figure 13A,F) suggest that it probably either was altered by pyroxene crystals in the original volcanic ash or reprecipitated after labile components was chemically leached in the original volcanic ash [15].
Zircon was not observed by optical microscope or SEM in the tonsteins of the present study, probably either because the relevant volcanic ashes did not contain this mineral phases or because multi-stage hydrothermal fluids as mentioned above altered it.
The mineral assemblages and occurrences of mineral of partings and roof strata in the No. 6 coal seam of the Yueliangtian coal mine suggest that the three partings (samples YLT6U-2p, YLT6U-9p and YLT6L-4p) and roof strata appear to have been derived from felsic volcanic ash.

Conclusions
(1) The major mineral phases in the YLT6U and YLT6L coals are calcite, quartz, kaolinite and pyrite and, to a lesser extent, chlorite, anatase, illite, mixed-layer illite/smectite, rutile, bassanite and ankerite.The Emeishan basalt and silicic rocks in the Kangdian Upland are the parent rocks of the Yueliangtian coals.(2) Different modes of occurrence of chamosite are present in the YLT6 coal.This, accompanied with cell-filling quartz, pyrite, and calcite veins, suggests that multi-stage hydrothermal fluids influenced the Yueliangtian coals.(3) The sedimentary environment is different between the YLT6U and YLT6L coals.Seawater had a tremendous influence on the YLT6L coal and terminated at the YLT6U-9p sampling interval.(4) Three tonstein (samples YLT6U-2p, YLT6U-9p and YLT6L-4p) layers identified in the coal are probably derived from felsic volcanic ash.These tonsteins are characterized by the occurrence of vermicular kaolinite and chloritized biotite.The roof strata of the YLT6L coal also appear to have been derived from felsic volcanic ash.

Figure 1 .
Figure 1.Location of the Yueliangtian coal mine.(a) China map and the location of study area; (b) Depositional environments during the Late Permian in Guizhou province, China.I, Kangdian Upland.II, Northern Vietnam Upland.(b) The enlargement of the red area in (a), modified from Dai et al. [15].

Figure 1 .
Figure 1.Location of the Yueliangtian coal mine.(a) China map and the location of study area; (b) Depositional environments during the Late Permian in Guizhou province, China.I, Kangdian Upland.II, Northern Vietnam Upland.(b) The enlargement of the red area in (a), modified from Dai et al. [15].

of 20 Figure 2 .
Figure 2. Sedimentary sequences and coal seams (red area) in the Yueliangtian coal mine.

Figure 2 .
Figure 2. Sedimentary sequences and coal seams (red area) in the Yueliangtian coal mine.
Minerals 2016, 6,29 8 of 20 forms (Figure6B,C) and as vein-fillings in the vitrinite (Figure6D).A platy calcite (Figure6B) with a large size up to 570 μm in length and 100 μm in width has distinct edges and angles.
°C[5,41,42].Fe-S compounds with different levels of brightness (Figure8F) deserve a special note: the atomic ratios of Fe against S of Spots 1 and 2 (Figure8G,H) are 1:2.4 and 1:2, respectively, probably suggesting an oxidation product of pyrite in Spot 1.
M, moisture; A, ash yield; V, volatile matter; S t , total sulfur; C, carbon; H, hydrogen; N, nitrogen; ad, air-dried basis; d, dry basis; daf, dry and ash-free basis; R r , random reflectance of vitrinite; Wa, weighted average for bench sample (weighted by thickness of sample interval); * total thickness.
Wa, weighted average for bench sample (weighted by thickness of sample interval).