Mineralogical and Geochemical Compositions of the Lopingian Coals and Carbonaceous Rocks in the Shugentian Coalfield , Yunnan , China : with Emphasis on Fe-Bearing Minerals in a Continental-Marine Transitional Environment

This paper presents the mineralogical and geochemical compositions of coal benches and non-coal (carbonaceous rock benches, parting, roof and floor) samples from the No. 1 Coal in the Longtan Formation of the Permian-Lopingian epoch from the Shugentian Coalfield, eastern Yunnan Province, southwestern China. The coal is rich in Nb, Ta, Zr, and Hf, which were derived from the Kangdian Upland with the dominant compositions of the Emeishan basalt. The minerals identified in the samples include mixed-layer illite-smectite, kaolinite, quartz, siderite, and minor calcite, pyrite, anatase and ankerite. Albite and chamosite occur in the roof and floor samples. The parting sample (SGT1-2p) is characterized by abundant siderite (64.9%) and calcite (20.1%), and one carbonaceous rock sample SGT1-11 contained a large amount of pyrite (26.1%). Four factors were responsible for the geochemical and mineralogical compositions in the samples; namely, the terrigenous detrital materials transported from the Kangdian Upland, direct volcanic ash inputs, multi-stage inputs of hydrothermal fluids, and marine influences. The co-existence of siderite and pyrite was attributed to a continental-marine transitional environment.


Introduction
The mineralogical and geochemical compositions of coal seams are influenced by many factors and geological processes that may have taken place over several periods [1][2][3][4][5][6][7].Ren et al. [8] proposed five genetic types of enrichment for trace elements in Chinese coals, including source rock controlled, sedimentation-organism controlled, magma hydrothermalism controlled, deep and large fault-hydrothermalism controlled, and groundwater controlled types.Later, Ren et al. [9] and Dai et al. [10] suggested that volcanic activity and submarine exhalation are also factors affecting coal geochemistry and mineralogy.Subsequently, Dai et al. [11] reviewed the geochemistry of Chinese coals and proposed five types of genetic controls: source rock, marine environment, volcanic ash, hydrothermal fluids (including magmatic hydrothermal fluid, low temperature hydrothermal fluid, and submarine exhalation), and groundwater.
The Shugentian Coalfield is the one of the major coal producers in eastern Yunnan Province and No. 1 Coal is the major minable coal seam in the coalfield, although its thickness is generally 0.3 m, as revealed by the 201 drill-hoe.The strata overlying and underlying the No. 1 Coal contains high organic matter, and thus they are mined together with the coal as feed coals for local power plants, although the ash yield in these strata are over than 50% (from 54% to 72% and 64.5% on average) and are not generally considered to be coal.These strata are named as carbonaceous rock in this study.The elemental and mineralogical compositions in these strata, and their comparison with those in the coal, have not been addressed in detail in the Shugentian Coalfield.
This paper presents new data and discusses the mineralogical and geochemical compositions in No. 1 Coal, and in its underlying and overlying strata (carbonaceous rocks) in the Shugentian Coalfield.Particularly, we emphasized the formation of pyrite and siderite that co-exists in the samples, as well as factors responsible for mineralogical and elemental compositions.

Geological Setting
The Shugentian Coalfield is located in the Luoping County of eastern Yunnan Province, southwestern China (Figure 1).The sedimentary sequences in the Shugentian Coalfield include Devonian, Carboniferous, Permian, Triassic, Paleogene, Neogene, and Quaternary.The Carboniferous, Permian, and Triassic strata are widely distributed in the coalfield.No. 201 drill hole covers the Lopingian Longtan and Changxing formations, the Lower Triassic Kayitou and Feixianguan formations, and the Quaternary as well.
The coal-bearing sequences in the Shugentian Coalfield are the Lopingian Longtan Formation and the overlying Changxing Formation, containing seven to 12 coal seams (include four minable seams).The Longtan Formation, with a thickness of 124 to 209 m (average 166 m), is distributed in the western coalfield and is mainly composed of sandstone and mudstone, interbedded with thin layers of limestone (in 201 drill-hole, limestone is in the bottom of Longtan Formation, with a depth of ~650 m) and coal seams [23].This formation was deposited in a continental-marine transitional environment [23] (Figure 1).
No. 1 Coal, the major minable coal seam in the coalfield, is located in the uppermost portion of the Longtan Formation (Figure 2) and has a thickness ranging from 0.47 to 3.94 m (2.02 m on average).The carbonaceous rock (ash yields vary from 50% to 72%, on air-dry basis), with macroscopic characteristics similar to coal, is calculated in the coal thickness.The roof strata of No. 1 Coal is sideritized siltstone and muddy siltstone, and the floor strata is dominated by mudstone.
The source region of the sediments in the Shugentian Coalfield is the Kangdian Upland, which is located to the west of the coalfield, and is major part of the Emeishan Large Igneous Province (ELIP) [21].The ELIP is the only Large Igneous Province discovered in China, and has been widely investigated [44,[50][51][52][53][54].Also, the topography of the area to the east of the ELIP (high in west and low in east) that resulted from the ELIP, provided favorable sites for the peat development of eastern Yunnan, western Guizhou, and southern Sichuan Provinces [21].

Samples and Analytical Methods
A total of 18 drill core bench samples including coal, carbonaceous rock, and host rocks (roof and floor) were collected from the No. 1 Coal in the 201 drill-hole in the Shugentian Coalfield.The samples were at a depth ranging from 552.75 to 555.49 m.The roof and floor samples were numbered as SGT1-R and SGT1-F1, SGT1-F2 respectively, while other coal and carbonaceous rock samples were named SGT1-1 to SGT1-16 from top to bottom (Figure 2).Sample SGT1-16 was not able to be collected in the present study because it has all been used for gas testing.The thickness of the collected floor sample is 20 cm, and that of the two roof samples, SGT1-F1 and SGT1-F2, are 13

Samples and Analytical Methods
A total of 18 drill core bench samples including coal, carbonaceous rock, and host rocks (roof and floor) were collected from the No. 1 Coal in the 201 drill-hole in the Shugentian Coalfield.The samples were at a depth ranging from 552.75 to 555.49 m.The roof and floor samples were numbered as SGT1-R and SGT1-F1, SGT1-F2 respectively, while other coal and carbonaceous rock samples were named SGT1-1 to SGT1-16 from top to bottom (Figure 2).Sample SGT1-16 was not able to be collected in the present study because it has all been used for gas testing.The thickness of the collected floor sample is 20 cm, and that of the two roof samples, SGT1-F1 and SGT1-F2, are 13

Samples and Analytical Methods
A total of 18 drill core bench samples including coal, carbonaceous rock, and host rocks (roof and floor) were collected from the No. 1 Coal in the 201 drill-hole in the Shugentian Coalfield.The samples were at a depth ranging from 552.75 to 555.49 m.The roof and floor samples were numbered as SGT1-R and SGT1-F1, SGT1-F2 respectively, while other coal and carbonaceous rock samples were named SGT1-1 to SGT1-16 from top to bottom (Figure 2).Sample SGT1-16 was not able to be collected in the present study because it has all been used for gas testing.The thickness of the collected floor sample is 20 cm, and that of the two roof samples, SGT1-F1 and SGT1-F2, are 13 and 20 cm, respectively.
The thicknesses of samples SGT1-2p and SGT1-15 are 5 cm and 6 cm, respectively; the thicknesses of other bench samples are all 15 cm.All the samples were crushed and ground to pass 200-mesh for geochemical analysis.Proximate analysis was conducted following ASTM (American Society for Testing and Materials) Standards D3173-11 [55], D3175-11 [56], and D3174-11 [57].The total sulfur and forms of sulfur in each coal were analyzed according to ASTM Standards D3177-02 [58] and ASTM Standard D2492-02 [59], respectively.Percentages of C, H, and N in coal benches were determined by an elemental analyzer (Vario MACRO, Elementar, Langenselbold, Germany).Mean random reflectance of vitrinite (percent Ro) in coal was determined according to ASTM Standard D2798-11a [60], using a Leica DM4500P microscope (at a magnification of 500×) equipped with a Craic QDI 302™ spectrophotometer (Leica Inc., Wetzlar, Germany).
The mineral composition was determined by X-ray powder diffraction (XRD).Low-temperature ashing of the coal benches was performed using an EMITECH K1050X plasma asher (Quorum Inc., Lewes, UK), prior to XRD analysis.XRD analysis of the low-temperature ashes (LTAs), as well as the non-coal samples, was performed on a powder diffractometer (D/max-2500/PC XRD, 40 kV, 100 mA) with Ni-filtered Cu-Kα radiation and a scintillation detector (Rigaku, Tokyo, Japan).X-ray diffractograms of the LTAs and non-coal samples were subjected to quantitative mineralogical analysis using the Siroquant™ software system.
Modes of occurrence and some selected elements in some minerals in samples are determined by a field emission scanning electron microscope (SEM), equipped with an energy dispersive spectroscopy (EDS) (FE SEM-EDS).Samples were carbon-coated using a Quorum Q150T ES sputtering coater(Quorum Inc., Lewes, UK), or were not coated for low-vacuum SEM working conditions (60 bar), and were then mounted on standard aluminum SEM stubs using sticky conductive carbon tabs.The working distance of the FE SEM-EDS was 10 mm, beam voltage 20.0 kV, aperture 6, and spot size 5.The images were captured via a retractable solid state back-scattered electron detector.
The oxides of major elements, including SiO 2 , TiO 2 , Al 2 O 3 , Fe 2 O 3 , MgO, CaO, MnO, Na 2 O, K 2 O, and P 2 O 5 , were determined by X-ray fluorescence spectrometry (XRF; ARL ADVANT XP+).Prior to XRF analysis, samples were ashed at a temperature of 815 • C, and the loss on ignition (LOI) was also determined at this temperature.
The concentrations of trace elements, except for Hg and F, were determined by inductively coupled plasma mass spectrometry (ICP-MS, Thermo Fisher (Waltham, MA, USA), X series II).The preparation of samples for ICP-MS was as follows: ~Fifty mg of the powdered samples (without ashing) of all coal and non-coal samples were digested using an UltraClave Microwave High Pressure Reactor (Milestone, Sorisole, BG, Italy).The digestion reagents were 5 mL 65% HNO 3 , and 2 mL 40% HF for coal samples and 2 mL 65% HNO 3 and 5 mL 40% HF for carbonaceous rock and host rock samples.The HNO 3 and HF for sample digestion were further purified through sub-boiling distillation before use.Inorganic Ventures standard references (CCS-1, CCS-4, CCS-5, and CCS-6) were used for calibration of trace element concentrations.In order to avoid interference by polyatomic ions, the collision/reaction cell technology of ICP-MS was performed to determine concentrations of As and Se in the samples, as outlined by Li et al. [61].
The concentration of Hg was determined on ~100 mg fresh samples (not ashed) in a Milestone DMA-80 analyzer (Milestone, Sorisole, BG, Italy), with a detection limit of 0.005 ng, and the linearity of the calibration was in the range 0-1000 ng.The concentration of fluorine was determined on ~500 mg raw samples (not ashed) by pyrohydrolysis using an ion selective electrode according to the method of ASTM D5987-96 [62].

Proximate and Ultimate Analyses and Coal Rank
The proximate and ultimate analyses of coal and carbonaceous rock benches from No. 1 Coal are listed in Table 1.The ash yield in air dry basis was 34.1% on average, indicating media-high ash coal according to Chinese standard GB/T 15224.1-2010[63].The total sulfur content of the coal benches Minerals 2017, 7, 170 5 of 25 was below 1%, and thus the coal could be classified as a low-sulfur coal [64].The carbonaceous rock benches had ash yields ranging from 54.9% to 72.0% (64.5% on average, on air dry basis), volatile matter from 30.3% to 67.50% (44.4% on average, on air dry basis), and total sulfur content from 0.42% to 12.12% (2.83% on average).
The vitrinite reflectance values of two coal benches (1.8% for SGT1-8; 1.9% for SGT1-9), and the weighted average volatile matter (23.1%, dry and ash free basis) indicated a medium volatile bituminous coal according to the ASTM D 388-12 Standard [65].2).The roof and floor samples were significantly enriched in MnO, and to a lesser extent, Fe 2 O 3 , and had higher content of CaO and MgO than coal and carbonaceous rock benches.

Trace Elements
The concentrations of trace elements in the coal benches and the averages for the world hard coals [66] are listed in Table 3.Compared with the averages for world hard coals, tantalum in the coal benches in this study is significantly enriched (CC = 10.2), while other lithophile elements including V, Zr, Nb, and Hf are enriched (CC = 5.2-7.4).Lithium, Sc, Cr, Co, Cu, Zn, Ga, and Th are also slightly enriched in the studied samples.Arsenic, Rb, Tl, and Bi are depleted (CC < 0.5), and the concentrations of the rest of the elements are close to those of the global average values for the world hard coals (Figure 3A).The concentrations of trace elements in the carbonaceous rock benches are listed in Table 3.Compared to world clay and shale values [67], selenium is significantly enriched; elements V, Cu, Zr, Nb, Ta, and Hg were slightly enriched; B, Rb, Cs, Bi were depleted; other elements were close to the average of world clay and shale values in abundance (Figure 3B).Compared with the averages for world hard coals [66], tantalum, Nb and Zr in the carbonaceous rock benches in this study were significantly enriched (CC > 10); Sc, V, Co, Cu, Zn, La and Hf were enriched (5 < CC ≤ 10); Be, B, Ge, As, Cs, Pb and U were in a normal range; Tl and Bi were depleted; other trace elements were slightly enriched (Figure 3C).The rows with gray shading are of carbonaceous rock (C R) benches.LOI, loss on ignition; C-Wa, weighted average for coal and carbonaceous rock, based on thickness of bench interval; Chinese coals, date from Dai et al. [11].
Minerals 2017, 7, 170 6 of 25 Zr, Nb, Ta, and Hg were slightly enriched; B, Rb, Cs, Bi were depleted; other elements were close to the average of world clay and shale values in abundance (Figure 3B).Compared with the averages for world hard coals [66], tantalum, Nb and Zr in the carbonaceous rock benches in this study were significantly enriched (CC > 10); Sc, V, Co, Cu, Zn, La and Hf were enriched (5 < CC ≤ 10); Be, B, Ge, As, Cs, Pb and U were in a normal range; Tl and Bi were depleted; other trace elements were slightly enriched (Figure 3C).The rows with gray shading are of carbonaceous rock (C R) benches.LOI, loss on ignition; C-Wa, weighted average for coal and carbonaceous rock, based on thickness of bench interval; Chinese coals, date from Dai et al. [11].Based on the classifications by Seredin and Dai [68], the rare earth elements and Yttrium (REY, or REE if Y is not included) are divided into three groups: light rare earth elements and yttrium (LREY: La, Ce, Pr, Nd, and Sm), medium REY (MREY: Eu, Gd, Tb, Dy, and Y), and heavy REY (HREY: Ho, Er, Tm, Yb, and Lu).Compared to the upper continental crust [69], three REY enrichment types were identified: L-type (La N /Lu N > 1), M-type (La N /Sm N < 1, Gd N /Lu N > 1), and H-type (La N /Lu N < 1) Seredin and Dai [68].
The average concentration of REY in coal benches was 199 µg/g, higher than the global average values of hard coals (68.6 µg/g, [66]).The average concentration of REY in the carbonaceous rock benches was 391 µg/g, higher than that of the global average value for clay and shale values (226 µg/g, [67]).The REY in coal benches had L-(La N /Lu N > 1) and M-type enrichment (La N /Sm N < 1, Gd N /Lu N > 1) and were characterized by positive Eu anomalies (Figure 4B).Samples SGT1-R and SGT1-2p were characterized by H-(La N /Lu N < 1) and M-types (La N /Sm N < 1, Gd N /Lu N > 1), and had distinct positive Eu anomalies (Figure 4A).Sample SGT1-11 was characterized by L-type (La N /Lu N > 1) and a strongly negative Eu anomaly (Figure 4C).The Eu anomaly is calculated as [70]: Eu N is the ratio of Eu concentration in the investigated samples vs. the Eu concentration in the UCC [70].Floor and other carbonaceous rock benches are of L-type (La N /Lu N > 1) and M-type (La N /Sm N < 1, Gd N /Lu N > 1), and are characterized by positive Eu anomalies (Figure 4A,C).Based on the classifications by Seredin and Dai [68], the rare earth elements and Yttrium (REY, or REE if Y is not included) are divided into three groups: light rare earth elements and yttrium (LREY: La, Ce, Pr, Nd, and Sm), medium REY (MREY: Eu, Gd, Tb, Dy, and Y), and heavy REY (HREY: Ho, Er, Tm, Yb, and Lu).Compared to the upper continental crust [69], three REY enrichment types were identified: L-type (LaN/LuN > 1), M-type (LaN/SmN < 1, GdN/LuN > 1), and H-type (LaN/LuN < 1) Seredin and Dai [68].

Comparison between the Mineralogical and Chemical Compositions
The comparison between the percentages of major element oxides (SiO 2 , Fe 2 O 3 , Al 2 O 3 , CaO, MgO, and TiO 2 ) calculated from the XRD-Siroquant, and XRF results are presented as X-Y plots (Figure 6), based on the methods based on Ward et al. [72] and Dai et al. [73,74].Both coal benches and non-coal samples data sets were calculated on a SO 3 -free basis.The points for SiO 2 , Fe 2 O 3 , Al 2 O 3 and CaO in the plots were close to the equal lines.This indicates that XRD-Siroquant produced reliable results for the major minerals (kaolinite, quartz, siderite and calcite) in coal benches and non-coal samples.However, the majority of the points in the plots of SiO The SEM-EDS data, the Si/Al ratio of kaolinite in the No. 1 Coal of Longtan Formation coal benches and non-coal samples higher than the standard values (1.18) (Figure 7), agree with this; The siderite in the coal benches and non-coal samples of the present study contained some Ca (Figure 8); however, the present study calculated siderite as FeCO 3 .Most of the points in the plot for MgO fell below the equality line, this was because Mg occurs in siderite as indicated by SEM-EDS data (Figure 8).

Comparison between the Mineralogical and Chemical Compositions
The comparison between the percentages of major element oxides (SiO2, Fe2O3, Al2O3, CaO, MgO, and TiO2) calculated from the XRD-Siroquant, and XRF results are presented as X-Y plots (Figure 6), based on the methods based on Ward et al. [72] and Dai et al. [73,74].Both coal benches and non-coal samples data sets were calculated on a SO3-free basis.The points for SiO2, Fe2O3, Al2O3 and CaO in the plots were close to the equal lines.This indicates that XRD-Siroquant produced reliable results for the major minerals (kaolinite, quartz, siderite and calcite) in coal benches and non-coal samples.However, the majority of the points in the plots of SiO2 and Fe2O3 fell slightly above the equality line, indicating an over-estimation of SiO2 and Fe2O3 calculated from XRD-Siroquant relative to XRF results.The majority of the points for the plots of Al2O3 and CaO fell slightly below the equality line, indicating an under-estimation of Al2O3 and CaO as calculated from XRD-Siroquant, relative to XRF results.The SEM-EDS data, the Si/Al ratio of kaolinite in the No. 1 Coal of Longtan Formation coal benches and non-coal samples higher than the standard values (1.18) (Figure 7), agree with this; The siderite in the coal benches and non-coal samples of the present study contained some Ca (Figure 8); however, the present study calculated siderite as FeCO3.Most of the points in the plot for MgO fell below the equality line, this was because Mg occurs in siderite as indicated by SEM-EDS data (Figure 8).
Carbonate minerals identified in almost all the samples investigated in this study (Table 4) included siderite, calcite, and ankerite.Siderite was present in all samples, with the highest content in SGT1-2p.Siderite mainly occurs as massive, syngenetic nodules, cell-infillings, and epigenetic cleat-infillings (Figure 9).The cleat-infilling siderite cuts across the cell-infilling kaolinite, indicating that the former formed later than the latter.Siderite contains traces of Ca, Mn, and Mg (Figure 8), which substitutes for Fe.Calcite mainly occurs as vein-fillings (Figures 9A,B and 10A,C), and massive aggregates of crystals together with pyrite (Figure 11C).The euhedral calcite crystals show two groups of approximately orthogonal cleavages (Figure 10D).Ankerite was identified in several samples (SGT1-R, 1-8, 1-9, 1-11, 1-F1, 1-F2) under SEM-EDS and by XRD, and co-exists with siderite in the cavities of the organic matter (Figure 9E,F).
Carbonate minerals identified in almost all the samples investigated in this study (Table 4) included siderite, calcite, and ankerite.Siderite was present in all samples, with the highest content in SGT1-2p.Siderite mainly occurs as massive, syngenetic nodules, cell-infillings, and epigenetic cleat-infillings (Figure 9).The cleat-infilling siderite cuts across the cell-infilling kaolinite, indicating that the former formed later than the latter.Siderite contains traces of Ca, Mn, and Mg (Figure 8), which substitutes for Fe.Calcite mainly occurs as vein-fillings (Figures 9A,B and 10A,C), and massive aggregates of crystals together with pyrite (Figure 11C).The euhedral calcite crystals show two groups of approximately orthogonal cleavages (Figure 10D).Ankerite was identified in several samples (SGT1-R, 1-8, 1-9, 1-11, 1-F1, 1-F2) under SEM-EDS and by XRD, and co-exists with siderite in the cavities of the organic matter (Figure 9E,F).
Carbonate minerals identified in almost all the samples investigated in this study (Table 4) included siderite, calcite, and ankerite.Siderite was present in all samples, with the highest content in SGT1-2p.Siderite mainly occurs as massive, syngenetic nodules, cell-infillings, and epigenetic cleat-infillings (Figure 9).The cleat-infilling siderite cuts across the cell-infilling kaolinite, indicating that the former formed later than the latter.Siderite contains traces of Ca, Mn, and Mg (Figure 8), which substitutes for Fe.Calcite mainly occurs as vein-fillings (Figure 9A,B and Figure 10A,C), and massive aggregates of crystals together with pyrite (Figure 11C).The euhedral calcite crystals show two groups of approximately orthogonal cleavages (Figure 10D).Ankerite was identified in several samples (SGT1-R, 1-8, 1-9, 1-11, 1-F1, 1-F2) under SEM-EDS and by XRD, and co-exists with siderite in the cavities of the organic matter (Figure 9E,F).The content of pyrite varied greatly in different bench samples of No. 1 Coal, and the highest pyrite content occurred in carbonaceous rock bench SGT1-11 (26.1%).It occurred as framboidal, massive, and euhedral crystal forms (Figure 11).The spherical framboids were aggregates of euhedral discrete crystals, which were uniformly distributed within the framboids (Figure 11B).The content of pyrite varied greatly in different bench samples of No. 1 Coal, and the highest pyrite content occurred in carbonaceous rock bench SGT1-11 (26.1%).It occurred as framboidal, massive, and euhedral crystal forms (Figure 11).The spherical framboids were aggregates of euhedral discrete crystals, which were uniformly distributed within the framboids (Figure 11B).The content of pyrite varied greatly in different bench samples of No. 1 Coal, and the highest pyrite content occurred in carbonaceous rock bench SGT1-11 (26.1%).It occurred as framboidal, massive, and euhedral crystal forms (Figure 11).The spherical framboids were aggregates of euhedral discrete crystals, which were uniformly distributed within the framboids (Figure 11B).
The framboidal pyrite in the present study, which is associated with clay minerals and euhedral pyrite, was of inorganic origin, and had a similar formation mechanism with euhedral pyrite [75].The framboidal pyrite was formed during peat development or in early diagenesis [31,64,[75][76][77].Rickard et al. [78] and Luther [79] suggested that the single crystal pyrite of framboids formed at low temperature (25 • C).
Quartz was identified in all coal and carbonaceous rock benches in the present study, and its amount seemed to be similar in all samples.Massive quartz and/or clay mineral distributes in organic matter (Figure 12A,B).To a lesser extent, quartz occurred as individual rounded grains, with the major axis being longer than 100 µm, surrounded by syngenetic clay minerals, indicating a detrital origin (Figure 12C,D).The framboidal pyrite in the present study, which is associated with clay minerals and euhedral pyrite, was of inorganic origin, and had a similar formation mechanism with euhedral pyrite [75].The framboidal pyrite was formed during peat development or in early diagenesis [31,64,[75][76][77].Rickard et al. [78] and Luther [79] suggested that the single crystal pyrite of framboids formed at low temperature (25 °C).
Quartz was identified in all coal and carbonaceous rock benches in the present study, and its amount seemed to be similar in all samples.Massive quartz and/or clay mineral distributes in organic matter (Figure 12A,B).To a lesser extent, quartz occurred as individual rounded grains, with the major axis being longer than 100 μm, surrounded by syngenetic clay minerals, indicating a detrital origin (Figure 12C,D).Clay minerals in the present study included kaolinite, mixed-layer illite and smectite (I/S), and chamosite.Kaolinite was the most common clay mineral, and occurred as cell-infillings and fissure-infillings (Figure 13A,B), a book-like form, and as a matrix of other minerals (Figure 13C,D).Mixed-layer I/S clay was identified by XRD in most samples while chamosite was found only in the floor samples (SGT1-F1 and SGT1-F2) (Figure 5).
Anatase occurs as fine crystal grains and is scattered in the kaolinite matrix (Figures 9C, 11B and 13C).Apatite occurred as a detrital form and as fine grains, both of which were coated by kaolinite (Figure 14A,B).Detrital apatite was derived from terrestrial sources or volcanic ash [22,76,[80][81][82].Albite occurred as euhedral crystals, and was distributed in the matrix of siderite (Figure 14C).Florencite was found in the matrix of organic matter (Figure 14D).Clay minerals in the present study included kaolinite, mixed-layer illite and smectite (I/S), and chamosite.Kaolinite was the most common clay mineral, and occurred as cell-infillings and fissure-infillings (Figure 13A,B), a book-like form, and as a matrix of other minerals (Figure 13C,D).Mixed-layer I/S clay was identified by XRD in most samples while chamosite was found only in the floor samples (SGT1-F1 and SGT1-F2) (Figure 5).
Anatase occurs as fine crystal grains and is scattered in the kaolinite matrix (Figures 9C, 11B and 13C).Apatite occurred as a detrital form and as fine grains, both of which were coated by kaolinite (Figure 14A,B).Detrital apatite was derived from terrestrial sources or volcanic ash [22,76,[80][81][82].Albite occurred as euhedral crystals, and was distributed in the matrix of siderite (Figure 14C).Florencite was found in the matrix of organic matter (Figure 14D).

Influencing Factors on Geological and Mineralogical Compositions
The geochemical and mineralogical characteristics of No. 1 Coal in the Longtan Formation indicated that they were probably influenced by four factors: input from the sediment-source region, sea water influence, volcanic ash input, and injection of hydrothermal fluids.

Sediment-Source Region
As with other Lopingian coalfields in western Guizhou, eastern Yunnan, southern Sichuan, and Chongqing [11,21,23], the sediment-source region for the Shugentian Coalfield was the Kangdian Upland, based on the following evidence: The ratios of Al 2 O 3 /TiO 2 of the coal and carbonaceous rock benches exclusively fall in the area between 3 to 8 (Figure 15), indicating that the terrigenous source of the studied samples is of mafic composition [83].A number of studies showed that the ratio Al 2 O 3 /TiO 2 is a reliable indicator for identifying the sediment-source regions not only for normal sediments but also for coal [32,80,[83][84][85][86][87]].

3.
It has been reported that quartz derived from the Kangdian Upland is abundant in the Lopingian coals in southwestern China [16].The high authigenic quartz identified in the coal and carbonaceous rock benches (Figure 12A,B) was probably deposited from silica-bearing solutions originating from the weathering of basaltic rocks in the Kangdian Upland [48].4.
For coals, Eu anomalies inherited from rocks within the sediment source region or affected by high-temperature hydrothermal fluids [70].In Shugentian Coalfield, with no evidence indicating high temperature hydrothermals, the positive Eu anomalies are derived from tholeiitic basalts of the Kangdian Upland.Furthermore, The REY distribution patterns of coal, carbonaceous rock (except SGT1-11), parting, and roof and floor in the Shugentian Coalfield, characterized by M-type enrichment and positive Eu anomalies (Figure 15), are similar to those for low-Ti and high-Ti basalts from the western Emeishan Large Igneous Province as reported by Xiao et al. [71].

Influencing Factors on Geological and Mineralogical Compositions
The geochemical and mineralogical characteristics of No. 1 Coal in the Longtan Formation indicated that they were probably influenced by four factors: input from the sediment-source region, sea water influence, volcanic ash input, and injection of hydrothermal fluids.

Sediment-Source Region
As with other Lopingian coalfields in western Guizhou, eastern Yunnan, southern Sichuan, and Chongqing [11,21,23], the sediment-source region for the Shugentian Coalfield was the Kangdian Upland, based on the following evidence: 1.The elevated concentrations of Sc, V, Cr, Co, Ni and Zn in the No. 1 Coal and carbonaceous rocks (Figure 3A,B), which were derived from basalts in the Kangdian Upland.2. The ratios of Al2O3/TiO2 of the coal and carbonaceous rock benches exclusively fall in the area between 3 to 8 (Figure 15), indicating that the terrigenous source of the studied samples is of mafic composition [83].A number of studies showed that the ratio Al2O3/TiO2 is a reliable indicator for identifying the sediment-source regions not only for normal sediments but also for coal [32,80,[83][84][85][86][87]].3. It has been reported that quartz derived from the Kangdian Upland is abundant in the Lopingian coals in southwestern China [16].The high authigenic quartz identified in the coal and carbonaceous rock benches (Figure 12A

Volcanic Ash Input
Elemental and mineralogical compositions in carbonaceous rock bench SGT1-11 are distinctive in comparison with other coal and carbonaceous rock benches.The depleted concentrations of Sc, V, Cr, and Co in sample SGT1-11 (Table 3) indicate that sediments in this bench were not derived from the Kangdian Upland, which however is rich in these elements.The sample was characterized by a strong negative Eu anomaly (δEu = 0.5) (Figure 4D), quite different to those in other coal and carbonaceous rock benches.Although sediments derived from the Emeishan intermediate-felsic rocks, which are located on the top of the Emeishan mantle plume, have strong negative Eu anomalies [86], these sediments are always in the lowermost portions of the Longtan Formation [21,49,86].The coal and carbonaceous rock horizons in this study, however, were in the uppermost portion of the Longtan Formation.Apatite, euhedral albite, and sharp-edged quartz (Figure 14A-C), kaolinite associated with quartz, and apatite and anatase (Figures 13C and 14A,B), can also be indicators for volcanic ash inputs into palaeomire and/or detrital inputs from the ELIP.[39,76,[90][91][92].

Injection of Multi-Stage Hydrothermal Fluids
A number of studies showed that hydrothermal fluids have significantly influenced the geochemical and mineralogical composition of Lopingian coals in southwestern China (e.g., [26,48,86,[93][94][95]).The evidence for the No. 1 Coal and carbonaceous rock benches subjected to hydrothermal fluids includes: (1) Chamosite identified in samples SGT1-F1 and SGT1-F2 by XRD.Previously studies [29,30,48] showed that the formation of chamosite in eastern Yunnan coals was related to hydrothermal fluids, either deposited from Fe-rich hydrothermal fluids [29] or formed via interaction between kaolinite and Fe-Mg-rich fluids during early diagenesis [30]; (2) Florencite identified in the samples (Figure 14C).This is a secondary REE-bearing mineral related to hydrothermal fluid [38,86,96]; (3) Fracture-filling siderite (Figure 9C,D) and fracture-filling calcite in the samples.These carbonate minerals indicate epigenetic solution precipitation; (4) Abundant siderite (65%) and calcite (20%) in sample SGT1-2p.Their occurrence may indicate Ca-and Fe-rich solutions injected into the coal.This horizon had a low REY value, while its underlying SGT1-3 had a higher REY concentration compared to its adjacent seams, indicating a hydrothermal leaching occurred to the parting, and the leached REY was ultimately deposited in the underlying horizons; (5) SGT1-11 showed a Gd-maximum, which may have resulted from the injections of acid solutions, including high pCO 2 -water in coal basin [97].

Sea Water
Previous studies indicated that the Longtan Formation in eastern Yunnan and western Guizhou was deposited in a continental-marine transitional environment [4,15,20,85,86,98,99].The high concentration of some elements (e.g., K, Mg, Li, F, Sr) in the samples, which were higher in seawater than in freshwater [100], may also indicate a marine influence.In addition, the high S and pyrite content of the carbonaceous rock (Table 1) also suggests an influence by seawater, sulfate-rich lake waters, or groundwater [63,[101][102][103].
The high ratio of Sr/Ba in carbonaceous rocks also indicates the influence of seawater.Sr/Ba < 1 usually represents continental sediments while Sr/Ba > 1 usually suggest marine sediments [104].Coal benches in No. 1 Coal in the coalfield had a ratio of Sr/Ba < 1, and most carbonaceous rock benches, however, had Sr/Ba > 1, indicating that coal benches of the coalfield were deposited during the regression stage.It should be noted that the roof, parting and floor samples (SGT1-R, SGT1-2p, SGT1-F1, SGT1-F2) contained abundant calcite, which in most cases is of epigenetic origin and may contain Sr as an isomorphism with Ca, and thus, the ratio of Sr/Ba could not be used as an indicator of marine influence.
Gadolinium in common Chinese coals generally shows very weak negative anomalies [70].However, all the studied coal benches and non-coal samples had positive Gd anomalies (from 1.16 to 1.26, with an average of 1.19).In addition to low-Ti and high-Ti basalt of the Kangdian Upland, which have an average Gd anomaly of 1.07 [71], seawater and/or hydrothermal fluids also be responsible for positive Gd anomalies [70].It was reported that seawater generally shows positive Gd anomalies [105,106].

Major Fe-Bearing Minerals
The major Fe-bearing minerals in No. 1 Coal are pyrite and siderite, both of which coexist in all coal and carbonaceous rock benches (Table 4).Framboidal pyrite, which generally indicate a reducing condition and an authigenic origin [64,75,[107][108][109][110][111], was identified in some samples investigated in this study (Figure 11A,B).The pyrite with irregular shape was scattered in the matrix of siderite (Figure 16).Syngenetic pyrite is generally indicative of deposition in marine influenced environment or sulfate-rich lake waters or groundwaters [63,[101][102][103], and abundant siderite indicates non-marine, at least waters with a low sulfate content [103].However, siderite has been observed in many cases to coexist with syngenetic pyrite [72,103,112].When SO 4 2− is totally reduced to S 2− , and iron occurs in surplus, minor siderite will be formed in marine-influenced environments.In sulfate-containing freshwater, pyrite can form together with siderite in a reductive condition [113].Pyrite and siderite could also form sequentially in freshwater environments due to the change of from neutral to acidic conditions.In neutral to slightly alkaline environments, siderite can form, whereas pyrite is typical for slightly acidic settings (which are optimal for the sulfate-reducing bacteria) [76,114].

Major Fe-Bearing Minerals
The major Fe-bearing minerals in No. 1 Coal are pyrite and siderite, both of which coexist in all coal and carbonaceous rock benches (Table 4).Framboidal pyrite, which generally indicate a reducing condition and an authigenic origin [64,75,[107][108][109][110][111], was identified in some samples investigated in this study (Figure 11A,B).The pyrite with irregular shape was scattered in the matrix of siderite (Figure 16).Syngenetic pyrite is generally indicative of deposition in marine influenced environment or sulfate-rich lake waters or groundwaters [63,[101][102][103], and abundant siderite indicates non-marine, at least waters with a low sulfate content [103].However, siderite has been observed in many cases to coexist with syngenetic pyrite [72,103,112].When SO4 2− is totally reduced to S 2− , and iron occurs in surplus, minor siderite will be formed in marine-influenced environments.In sulfate-containing freshwater, pyrite can form together with siderite in a reductive condition [113].Pyrite and siderite could also form sequentially in freshwater environments due to the change of from neutral to acidic conditions.In neutral to slightly alkaline environments, siderite can form, whereas pyrite is typical for slightly acidic settings (which are optimal for the sulfate-reducing bacteria) [76,114].Contents of siderite and pyrite showed a negative correlation in No. 1 coal and carbonaceous rock.The abundant pyrite and less abundant siderite formed when sea water was injected into the coalfield, and vice versa.

Conclusions
Minerals in the Shugentian coal and carbonaceous rock samples consisted of carbonates (siderite and calcite), kaolinite, mixed layer I/S, quartz, pyrite, and minor amounts of anatase, ankerite, albite, and chamosite.The coal benches were significantly enriched in Nb, Ta, Zr, Hf, and V (CC > 5), and slightly enriched in REY.
The sediment-source region (the Kangdian Upland), volcanic ash inputs, multi-stage hydrothermal fluid activities, and marine influence were responsible for the geochemical and mineralogical anomalies in the Shugentian coal and carbonaceous rock samples.
Minerals in sample SGT1-2p were dominated by carbonates (64.9% siderite and 15.9% calcite) that may have originated from hydrothermal fluid, which was enriched in iron and calcium.SGT1-11 had a high total sulfur content (12.03%) and a high organic sulfur content (6.79%), as well as a strong negative δEu value (0.46), probably due to the input of felsic volcanic ash to peat swamp during peat accumulation.
The common occurrence of both siderite and pyrite in the same sample is attributed to the continental-marine transitional environment.Furthermore, sufficient iron in a marine environment and changing acidity in a freshwater environment are also essential.Contents of siderite and pyrite showed a negative correlation in No. 1 coal and carbonaceous rock.The abundant pyrite and less abundant siderite formed when sea water was injected into the coalfield, and vice versa.

Conclusions
Minerals in the Shugentian coal and carbonaceous rock samples consisted of carbonates (siderite and calcite), kaolinite, mixed layer I/S, quartz, pyrite, and minor amounts of anatase, ankerite, albite, and chamosite.The coal benches were significantly enriched in Nb, Ta, Zr, Hf, and V (CC > 5), and slightly enriched in REY.
The sediment-source region (the Kangdian Upland), volcanic ash inputs, multi-stage hydrothermal fluid activities, and marine influence were responsible for the geochemical and mineralogical anomalies in the Shugentian coal and carbonaceous rock samples.
Minerals in sample SGT1-2p were dominated by carbonates (64.9% siderite and 15.9% calcite) that may have originated from hydrothermal fluid, which was enriched in iron and calcium.SGT1-11 had a high total sulfur content (12.03%) and a high organic sulfur content (6.79%), as well as a strong negative δEu value (0.46), probably due to the input of felsic volcanic ash to peat swamp during peat accumulation.
The common occurrence of both siderite and pyrite in the same sample is attributed to the continental-marine transitional environment.Furthermore, sufficient iron in a marine environment and changing acidity in a freshwater environment are also essential.

Figure 1 .
Figure 1.Location and sedimentary environments of the Lopingian Longtan Formation in the Shugentian Coalfield (modified from China Coal Geology Bureau, [23]).

Figure 3 .
Figure 3. Concentration coefficient (CC) of trace elements in average Shugentian coal (A) and carbonaceous rock benches (B,C), Normalized by trace element concentrations in the world hard coals [66] and world clay [67].

Figure 3 .
Figure 3. Concentration coefficient (CC) of trace elements in average Shugentian coal (A) and carbonaceous rock benches (B,C), Normalized by trace element concentrations in the world hard coals [66] and world clay [67].

Figure 4 .
Figure 4. Distribution patterns of rare earth elements and yttrium (REY) of samples in Shugentian coalfield (A-C).REY are normalized by the Upper Continental Crust (UCC) [67].Data for high-and low-Ti alkali basalts (D) are from Xiao et al. [71].

Figure 4 .
Figure 4. Distribution patterns of rare earth elements and yttrium (REY) of samples in Shugentian coalfield (A-C).REY are normalized by the Upper Continental Crust (UCC) [67].Data for high-and low-Ti alkali basalts (D) are from Xiao et al. [71].
2 and Fe 2 O 3 fell slightly above the equality line, indicating an over-estimation of SiO 2 and Fe 2 O 3 calculated from XRD-Siroquant relative to XRF results.The majority of the points for the plots of Al 2 O 3 and CaO fell slightly below the equality line, indicating an under-estimation of Al 2 O 3 and CaO as calculated from XRD-Siroquant, relative to XRF results.

Figure 6 .
Figure 6.Comparison of observed normalized oxide percentages from chemical analysis (x-axis) to oxide percentages for sample ash inferred from XRD analysis data (y-axis).The diagonal line represents equality in each plot.

Figure 6 .
Figure 6.Comparison of observed normalized oxide percentages from chemical analysis (x-axis) to oxide percentages for sample ash inferred from XRD analysis data (y-axis).The diagonal line represents equality in each plot.

1 .
The elevated concentrations of Sc, V, Cr, Co, Ni and Zn in the No. 1 Coal and carbonaceous rocks (Figure 3A,B), which were derived from basalts in the Kangdian Upland.2.
,B) was probably deposited from silica-bearing solutions originating from the weathering of basaltic rocks in the Kangdian Upland [48].4. All coal benches and non-coal samples (carbonaceous rock benches, parting, and roof and floor samples) have strong positive Eu anomalies (1.37 on average), except SGT1-11 (Figure 4).Eu anomalies require both high-temperature (>250 °C) and extreme reducing conditions [70,88,89].For coals, Eu anomalies inherited from rocks within the sediment source region or affected by high-temperature hydrothermal fluids [70].In Shugentian Coalfield, with no evidence indicating high temperature hydrothermals, the positive Eu anomalies are derived from tholeiitic basalts of the Kangdian Upland.Furthermore, The REY distribution patterns of coal, carbonaceous rock (except SGT1-11), parting, and roof and floor in the Shugentian Coalfield, characterized by M-type enrichment and positive Eu anomalies (Figure 15), are similar to those for low-Ti and high-Ti basalts from the western Emeishan Large Igneous Province as reported by Xiao et al. [71].

Figure 15 .
Figure 15.Bivariate plot for Al2O3 and TiO2 in No 1 coal seam of the Shugentian coalfield.

Figure 15 .
Figure 15.Bivariate plot for Al 2 O 3 and TiO 2 in No. 1 coal seam of the Shugentian coalfield.

Table 3 .
Trace elements (ppm) in the Shugentian coal, carbonaceous rock, roof and floor strata.
[67]: weighted average for coal, based on thickness of coal bench interval; Coal: the world hard coals, data from Ketris and Youdovich[66].CC1: C-wa/Coal.C R-wa, weighted average for carbonaceous rock(C R) benches, based on thickness of coal bench interval; clay, the world clay, date from Grigoriev[67].CC2: C R-wa/Clay; CC3: C R-wa/Coal.

Table 4 .
Minerals in coal low-temperature ashes (LTAs) and carbonaceous rock, roof and floor of the Shugentian Coalfield (%).

Table 4 .
Minerals in coal low-temperature ashes (LTAs) and carbonaceous rock, roof and floor of the Shugentian Coalfield (%).