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Article

Origin of Minerals and Elements in the Late Permian Coal Seams of the Shiping Mine, Sichuan, Southwestern China

by
Yangbing Luo
* and
Mianping Zheng
MLR Key Laboratory of Saline Lake Resources and Environments, Institute of Mineral Resources, Chinese Academy of Geological Sciences (CAGS), Beijing 100037, China
*
Author to whom correspondence should be addressed.
Minerals 2016, 6(3), 74; https://doi.org/10.3390/min6030074
Submission received: 27 January 2016 / Revised: 13 July 2016 / Accepted: 13 July 2016 / Published: 19 July 2016
(This article belongs to the Special Issue Minerals in Coal)
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Abstract

:
Volcanic layers in coal seams in southwestern China coalfields have received much attention given their significance in coal geology studies and their potential economic value. In this study, the mineralogical and geochemical compositions of C19 and C25 coal seams were examined, and the following findings were obtained. (1) Clay minerals in sample C19-r are argillized, and sedimentary layering is not observed. The acicular idiomorphic crystals of apatite and the phenocrysts of Ti-augite coexisting with magnetite in roof sample C19-r are common minerals in basaltic rock. The rare earth elements (REE) distribution pattern of C19-r, which is characterized by positive Eu anomalies and M-REE enrichment, is the same as that of high-Ti basalt. The concentrations of Ti, V, Co, Cr, Ni, Cu, Zn, Nb, Ta, Zr, and Hf in C19-r are closer to those of high-Ti basalt. In conclusion, roof sample C19-r consists of tuffaceous clay, probably with a high-Ti mafic magma source. (2) The geochemical characteristics of the C25 coals are same as those reported for coal affected by alkali volcanic ash, enrichment in Nb, Ta, Zr, Hf, and REE, causing the C25 minable coal seams to have higher potential value. Such a vertical study of coals and host rocks could provide more information for coal-forming depositional environment analysis, for identification of volcanic eruption time and magma intrusion, and for facilitating stratigraphic subdivision and correlation.

Graphical Abstract

1. Introduction

Volcanic ash ejected from volcanoes falls into peat swamps and subsequently can form a thin and stable stratum in coal seams, which is usually “tonstein” and in some cases, occurs as roof and floor strata of coal seams [1,2]. These volcanic layers in coal seams get much attention as they can help to identify times of volcanic eruption and magma intrusion in this area, assess the coal-forming environments, and facilitate stratigraphic subdivision and correlation [1,2,3,4,5,6,7,8]. Moreover, some volcanic layers in the coal seams may contain valuable trace elements (such as rare earth elements, Nb, Ta, Zr, Hf, and Ga) that could increase the potential value of the coal [4,5]. Consequently, host rocks (tonsteins, floor and roof) in coal seams related to volcanic ash are of great importance in coal geology [2,3,6,7].
Volcanic activity frequently occurred in the Late Permian age in southwestern China [1,2,9,10,11,12,13], and volcanic ashes deposited in the peat-forming environment have been found in some coal seams in this area [1,2,3,4,5]. These volcanic layers found in southwest China can be divided into four types: felsic [2,14], alkali [2,4,5,15,16], mafic [1,3], and dacitic [3]. It was reported that felsic and dacitic volcanic layers are common in Late Permian coal seams in southwestern China [1,2,6,7]. Alkali volcanic layer in the coal seams in southwestern China have attracted much attention given that coal seams affected by alkali volcanic ash are enriched in Nb, Ta, Zr, Hf, and REE [2,4,5,15,16]. However, because mafic eruptions generally do not form tuffs, mafic volcanic layers occur rarely in coal-bearing strata around the world and only a few have been found in the Late Permian coals from southwestern China [1,3].
More attention has been paid to tonsteins in coal seams [5,16]; however, research on the roof and floor strata with a volcanic source was scarcely reported [3]. In this paper, geochemical and mineralogical characteristics of the roof sample in the C19 coal seam with a high-Ti mafic magma source were investigated. Variation in the element geochemistry and mineralogy of the C19 and C25 coals and floor sample in Late Permian from the Shiping mine, Sichuan, southwestern China was also described, with an emphasis on elements and minerals of volcanic origin in these coals and hosts rocks. Such a vertical study of coals and host rocks could provide more information for coal-forming depositional environments analysis, for identification of volcanic eruption time and magma intrusion, and for facilitating stratigraphic subdivision and correlation.

2. Materials and Methods

The Shiping mine is located in the southeastern part of Sichuan Province, southwestern China (Figure 1).
The sedimentary sequences in the Shiping mine include the Quaternary, Upper Triassic Xujiahe Formation, Middle Triassic Leikoupo Formation, Lower Triassic Jialingjiang and Feixianguan Formations, Upper Permian Changxing and Longtan Formations, Middle Permian Maokou and Xixia Formation, Lower Permian Liangshan Formation, and Silurian.
The coal-bearing sequence of the Shiping mine is the Late Permian Longtan Formation, which is composed mainly of mudstone, siltstone, sandstone, flint-bearing limestone, muddy sandstone, claystone, and six coal seams (Figure 2).
The Longtan Formation contains two major minable coal seams, namely the C19 and the C25. The C25 coal is the lowermost coal seam in the Late Permian strata of southwestern China. The thickness of the C25 coal seam is 0.80–1.30 m. The C19 coal, with a thickness of 1.50–2.00 m, is the lower-middle coal seam in the Late Permian strata of southwestern China.

3. Samples and Analytical Procedures

Six samples, including four coal bench samples (C19-1, C25-1, C25-2, and C25-3), one roof sample (C19-r), and one floor sample (C25-f), were taken from C19 and C25 coal seams mined at the coal working face of the Shiping mine (Figure 2). The identifications of all samples were shown in Figure 2. Each sample was cut over an area 10 cm wide and 10 cm deep. All samples were stored immediately in plastic bags to minimize contamination and oxidation.
Proximate analysis was conducted with ASTM Standards D3173-11 [17], D3174-11 [18], and D3175-11 [19]. Total sulfur and forms of sulfur were determined under ASTM Standards D3177-02 [20] and D2492-02 [21], respectively. Petrographic examination of the coals was performed under optical microscope following ASTM Standard D2797/D2797M-11a [22]. Mean random reflectance of vitrinite (percent Ro,ran) was determined by using a Leica DM-4500P microscope (Leica Camera AG, Wetzlar, Germany). Maceral constituents were identified under white-light reflectance oil immersion microscopy.
A field emission-scanning electron microscope (FE-SEM, FEI Quanta™ 650 FEG, FEI, Hillsboro, OR, USA), in conjunction with an EDAX energy-dispersive X-ray spectrometer (Genesis Apex 4, EDAX Inc., Mahwah, NJ, USA), was used to study morphology and microstructure and also to determine the distribution of some elements in the coal and rock samples. Low-temperature (oxygen-plasma) ashing (LTA) was performed, using an Emitech K1050X plasma asher (Quorum Inc., Lewes, UK), to remove organic matter in the coal prior to XRD analysis. The residues of this process were then analyzed by X-ray diffraction (XRD) using a D/max-2500/PC powder diffractometer (Rigaku, Tokyo, Japan) with Ni-filtered Cu-Kα radiation and a scintillation detector. The XRD pattern was recorded over a 2θ interval of 2.6°–70°, with a step size of 0.01°.
Concentrations of major element oxides in the samples (on ash basis; ashing temperature of 815 °C) were obtained by X-ray fluorescence (XRF) spectrometry. Mercury was determined by a Milestone DMA-80 Hg analyzer (Milestone, Sorisole, Italy). Fluorine analysis was conducted using ASTM Standard D5987-96 [23]. Inductively coupled plasma mass spectrometry (Thermo Fisher, Edmonton, AB, Canada, X series II ICP-MS) was used to determine the trace elements in the samples. All samples were digested using an UltraClave Microwave High Pressure Reactor (Milestone Inc., Shelton, CT, USA). Details for these coal-related sample digestion and ICP-MS analysis techniques are given by Dai et al. [1]. Arsenic and selenium were determined by ICP-MS, using collision cell technology (CCT), in order to avoid disturbance of polyatomic ions [24].

4. Results

4.1. Coal Chemistry and Coal Petrology

The vitrinite random reflectance (Ro,ran average 2.21%) and the weighted average volatile matter (Vdaf average 15.98%) of the coal bench samples (Table 1) indicate a bituminous coal according to the ASTM classification D388-12, 2012 [25]. The total sulfur content varies considerably between the C19 (0.59%) and C25 (average 3.52%) coals, and the coals can be classified as low-sulfur coal and high-sulfur coal, respectively, according to Chinese standard GB 15224.1-2004 [26].
The C19 and C25 coals contain abundant vitrinite (Table 2), with collodetrinite (Figure 3A) being the most abundant maceral, followed by collotelinite (Figure 3A), along with small proportions of telinite (Figure 3B) and vitrodetrinite (Figure 3C).
Inertinite macerals occur in lesser proportions (Table 2) and are dominated by semifusinite (Figure 3A), followed by macrinite (Figure 3D) and inertodetrinite (Figure 3A), with trace amounts of micrinite (Figure 3E) and fusinite (Figure 3F). The cell structures of the semifusinite and fusinite are better preserved and have swelled and deformed form (Figure 3A,F).

4.2. Modes of Occurrence of Minerals

The mineral compositions of the C19 and C25 coal low-temperature ashes (LTA), roof, and floor samples, as determined by XRD and Siroquant software, are listed in Table 3.
All samples contain clay minerals and pyrite (Table 3). The clay minerals in the C19 coal seams are mainly kaolinite, followed by illite and illite-smectite mixed layer clays. Kaolinite is the only clay mineral in the C25 coal seams. Kaolinite in the coals occurs as discrete particles (Figure 4A), cell-fillings (Figure 4B), and fracture-fillings (Figure 4A). Pyrite occurs as discrete particle aggregates (Figure 5A) and as cell fillings (Figure 5B), but it is present mainly as framboidal (Figure 5C) and needle-like forms together with marcasite (Figure 5D).
Quartz and marcasite are mainly contained in the C19 coal (Table 3). Quartz in coals occurs as discrete particles (Figure 6A), cell-fillings (Figure 4B), and fracture-fillings (Figure 6B), and marcasite occurs as needle-like forms together with pyrite (Figure 5D).
Calcite, Ti-oxide minerals, and fluocerite are mainly contained in the C25 coals. The calcite in the C25 coals occurs as fracture fillings (Figure 7). As observed by SEM-EDS analysis of C25 coals, anatase, rutile (Figure 8A), and fluocerite (Figure 8B) are distributed in the kaolinite matrix.
Chalcopyrite, titaniferous magnetite, apatite, and Ti-augite are present only in roof sample C19-r. Chalcopyrite occurs as fracture fillings (Figure 9A). Barite occurs in the form of discrete particles (Figure 9B). Titaniferous magnetite occurs as irregular granular. Apatite occurs as acicular idiomorphic crystals (Figure 9C). Ti-augite and rare magnetite occur as phenocrysts (Figure 9D). Marcasite was observed by SEM-EDS analysis in sample C25-f (Figure 10).

4.3. Concentration and Distribution of Major and Trace Elements

4.3.1. Major Element Oxides

The element compositions of the C19 and C25 coals, roof, and floor are listed in Table 4.
The percentages of Al2O3 and CaO in the C19 coal (C19-1) are lower than those in the more normal Chinese coals reported by Dai et al. [27]. The SiO2/Al2O3 ratio of sample C19-1 (7.19 on average) is much higher than that of other Chinese coals (1.42) [27] and also than the theoretical ratio for kaolinite (1.18), indicating free SiO2 in the coal. The percentages of other major element oxides, however, are either lower than or close to those in common Chinese coals.
The percentages of Al2O3 and CaO in the C25 coals are higher than those in the more normal Chinese coals reported by Dai et al. [27]. The percentages of other major element oxides, however, are either lower than or close to those in common Chinese coals. The SiO2/Al2O3 ratio of the C25 coals (1.06 on average) is lower than that of other Chinese coals (1.42) [27], and also than the theoretical ratio for kaolinite (1.18), indicating no free SiO2 in the coal.
It should be noted that the percentages of TiO2 are high in roof sample C19-r (3.88%) but low in the C19 coal, C25 coals, and the floor sample (sample C25-f) (0.23–0.93%). Oxides SiO2, TiO2, Al2O3, MgO, Na2O, K2O, and P2O5 show a pattern of vertical variation the same as that of the ash yield through the seam section (Figure 11).

4.3.2. Trace Elements

Trace elements in most of the coal samples of this study are lower than or close to those of hard coals of the world [28]. However, a large number of trace elements are enriched in sample C25-3 (Figure 12).
Compared to the average for hard coals of the world [28] and based on the trace-element enrichment classification [29], a large number of trace elements are depleted in samples C19-1, C25-1, and C25-2. The trace elements with a concentration coefficient (CC = ratio of element concentration in Shiping coals/concentration hard coals of the world [28]) <0.5 include As, Rb, Sb, Cs, Ba, Tl, and Bi. Only Zr in sample C25-1 (5 < CC < 10) and Be in sample C25-2 (5 < CC < 10) are enriched.
Compared to the average for hard coals of the world [28], a large number of trace elements, including Li, V, Cr, Se, Zr, Nb, Cd, REE, Hf, Ta, and U, are enriched in C25-3 (CC > 10). Trace elements with a CC of 5–10 include Ga, In, Sn, and Th. Many other elements, including Be, F, Sc, Ni, Zn, Ge, Sr, and Hg, are slightly enriched (CC = 2–5) in the coal. The concentrations of the remaining elements are depleted or close to the average for hard coals of the world [28].
Compared to the average for the world clay [30] (Figure 13), trace elements Se and Mo are enriched in roof sample C19-r (CC > 10). Trace elements with a CC of 5–10 include Cu, Nb, and Eu. Many other elements, including Rb, Cs, Ba, Tl, and Bi are depleted (CC < 0.5) in C19-r. The concentrations of the remaining elements are slightly enriched (CC = 2–5) or close to (CC= 0.5–2) the average for world clay [30].
Compared to the average for the world clay [30] (Figure 13), a large number of trace elements, including V, Cr, Se, Nb, Mo, Cd, and U, are enriched in floor sample C25-f (CC > 10). Trace elements with a CC of 5–10 include Li, Zr, In, Sb, REE, Hf, Ta, and Hg. Some elements, including Rb, Cs, Ba, and Tl, are depleted (CC < 0.5) in sample C25-f. The concentrations of the remaining elements are slightly enriched (CC = 2–5) or close to (CC = 0.5–2) the average for world clay [30].

4.3.3. Rare Earth Elements

A threefold classification of rare earth elements [31] was used in this study. By comparison with the upper continental crust (UCC), three enrichment types were identified: L-REE (light REE; LaN/LuN > 1), M-REE (medium REE; LaN/SmN < 1, GdN/LuN > 1), and H-REE (heavy REE; LaN/LuN < 1) [31].
The REE enrichment patterns in the C19-1 and C25-1 coal benches are characterized by weak Eu anomalies and H- and M-REE enrichment (Figure 14A). The REE enrichment patterns in sample C25-2 and C25-3 coal benches are characterized by strong negative Eu anomalies and M-REE enrichment (Figure 14B,C).
The REE enrichment patterns in the roof sample C19-r (Figure 15A) are characterized by positive Eu anomalies and M-REE enrichment. Floor sample C25-f has the same REE enrichment patterns as the C25 coals (Figure 15B), which are characterized by strong negative Eu anomalies and M-REE enrichment.

5. Discussion

A number of volcanic layers [33,34,35] and dispersed volcanic ashes in organic matter of coal seams [36,37,38,39,40] have been observed in the southwestern China coalfield, and these volcanic layers can be divided into four types: felsic, alkali, mafic, and dacitic [1,2,10]. In this paper, we have discussed the characteristics of the elements and minerals in the C19 and C25 coal seams, which are affected by different types of volcanic activity.

5.1. C19 Coal Seam and Basalt

The petrological characteristics of roof sample C19-r are different from those of other normal roof deposits. The clay minerals in C19-r are argillized (Figure 9A,D), and sedimentary layering is not observed (Figure 16), indicating that roof sample C19-r probably is a tuffaceous clay.
The contents of kaolinite, Illite, I/S, etc. are the same as in the floors of the C2 and C3 Coals in Xinde Mine, Xuanwei, eastern Yunnan, China [3]. These floor strata are identified as fully argillized, fine-grained, tuffaceous clays with high-Ti mafic magma source [3].
A vermicular texture in the kaolinite of roof sample C19-r (Figure 9B) is often used as an indicator of a volcanic origin [2,3,41,42,43]. The acicular idiomorphic crystals of apatite and the phenocrysts of Ti-augite coexisting with magnetite in roof sample C19-r (Figure 9C,D) are common minerals in basaltic rock [2,12,44,45,46]. Irregular granular titaniferous magnetite in C19-r (Figure 9B) is a typical mineral in basalt [47,48,49,50]. All the crystal modes of minerals indicate that roof sample C19-r has a high-Ti basalt origin.
The major elements TiO2/Al2O3 ratio can indicate the acidic/basic/intermediate property of deposits, including normal sedimentary rocks, coal seam, and tonsteins [5,6,14,51,52,53,54,55,56,57,58,59,60]. The TiO2/Al2O3 ratios are >0.08 for mafic, 0.08–0.02 for intermediate, and <0.02 for silicic rocks [1,2,3]. The TiO2/Al2O3 ratio for C19-r is 0.17, indicating a mafic origin. Trace elements, including V, Co, Cr, Ni, Cu, Zn, Nb, Ta, Zr, Hf, and TiO2 in C19-r, are closer to those of mafic tuff rather than alkalic and silicic tuff [1].
The concentration of TiO2 in C19-r is high (3.88%). Concentrations of TiO2 in basalt higher than 2.8% are indicative of high-Ti basalt [10,11]. The concentrations of trace elements, including V, Co, Cr, Ni, Cu, Zn, Nb, Ta, Zr, and Hf, in sample C19-r are closer to those of high-Ti basalt than to those of low-Ti basalt (Table 5). The elemental compositions indicate that roof sample C19-r has a high-Ti basaltic volcanic source.
The REE distribution patterns of C19-r are characterized by positive Eu anomalies and M-REE-type enrichment. This is different from those of normal deposits of Late Permian clay rocks in the Emeishan large igneous province, which are generally characterized by weakly negative or no Eu anomalies [1,51], and are the same as those of high-Ti [10] (Figure 17A). To summarize, roof sample C19-r consists of tuffaceous clays and probably has a high-Ti mafic magma source.
Coal sample C19-1 has trace element and REE distribution patterns similar to those of normal coal deposits derived from the Kangdian Upland source region [6,7], indicating that sample C19-r is a normal sedimentary rock.

5.2. C25 Coals and Volcanic Ashes of Alkali Rhyolites

In the early part of the Late Permian, the volcanic ash had mainly alkalic composition [1,2]. The C25 coals examined in this study are the lowermost coal seam of the Late Permian strata, and these coals are in the same layer as other coals that were affected by alkali volcanics in southwestern China [4,5,33]. Moreover, the Shiping mine is within the area of alkali tonstein distribution [16].
High field strength elements, including Nb, Ta, Zr, Hf, and REE, are significantly enriched in C25 coals, and other elements such as Sc, Ti, V, Cr, Co, Ni, Cu, and Zn are depleted in C25 coals [4,13]. This characteristic is the same as that of alkali tonsteins and other coals affected by alkali volcanics in southwestern China [4,5,61]. This indicates that the C25 coal in the Shiping mine had been subjected to alkalic volcanic ash.
The REE distribution patterns of the C25 coals are characterized by strongly negative Eu anomalies and M-REE-type enrichment, the same as some Chinese alkali granites [62,63], some alkali tonsteins [1] reported in the south of China, and some coals affected by alkali volcanics [5] (Figure 17B). Overall, the C25 coals have high concentrations of Nb, Ta, Zr, Hf, and REE, which came mainly from alkali volcanics. The origin of these rare metals in the C25 coal present in this study is similar to those in the coals or coal-bearing sedimentary sequences in the surrounding areas [63,64,65,66,67].
The floor sample (C25-f) has high concentrations of Nb, Ta, Zr, Hf, and REE and has an REE distribution pattern similar to that of the C25 coal. It is the same as some alkali tonsteins reported in the south of China [1]. These results indicate that floor sample C25-f probably has alkali volcanic origin.

6. Conclusions

Sample C19-r does not have distinct stratification, indicating that the roof stratum is not a normal rock. The acicular idiomorphic crystals of apatite and the phenocrysts of Ti-augite coexisting with magnetite in roof sample C19-r are common minerals in basaltic volcanic rock. The REE distribution patterns of C19-r are characterized by positive Eu anomalies and M-REE-type enrichment. All of the above indicates that the roof sample C19-r probably has a basaltic origin.
The concentration of TiO2 in C19-r is high (3.88%). Concentrations of TiO2 in basalt higher than 2.8% are indicative of high-Ti basalt. The concentrations of trace elements, including V, Co, Cr, Ni, Cu, Zn, Nb, Ta, Zr, and Hf, in C19-r are closer to those of high-Ti basalt instead of those of low-Ti basalt. To summarize, roof sample C19-r probably has a high-Ti basaltic origin.
Nb, Ta, Zr, Hf, and REE are significantly enriched in C25 coals, and other elements such as Sc, Ti, V, Cr, Co, Ni, Cu, and Zn are depleted in C25 coals. The REE distribution patterns of the C25 coals are characterized by strongly negative Eu anomalies and M-REE-type enrichment. This indicates that the C25 coals in the Shiping mine were probably affected by alkalic volcanic ashes.

Acknowledgments

This research was supported by the National Key Basic Research Program of China (No. 2014CB238902), the National Natural Science Foundation of China (Nos. 41420104001, 41272182, and U1407207), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13099). Many thanks are given to Shifeng Dai for his constructive comments, which greatly improved the manuscript quality. We are grateful to the three anonymous reviewers for their careful comments.

Author Contributions

All co-authors participated in the work of this study. Yangbing Luo carried out the mineralogy and geochemistry analyses data. Mianping Zheng helped to design the research and structure of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Distribution of Late Permian Emeishan basalts in southwestern China [13].
Figure 1. Distribution of Late Permian Emeishan basalts in southwestern China [13].
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Figure 2. The sedimentary sequences of the Shiping mine.
Figure 2. The sedimentary sequences of the Shiping mine.
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Figure 3. Macerals in the coal samples, reflected light, and oil immersion. (A) collodetrinite, collotelinite, semifusinite, and inertodetrinite in sample C25-1; (B) telinite in sample C25-2; (C) vitrodetrinite in sample C25-1; (D) fusinite with swelling cells in sample C25-3; (E) micrinite in sample C25-3; (F) macrinite in sample C25-3.
Figure 3. Macerals in the coal samples, reflected light, and oil immersion. (A) collodetrinite, collotelinite, semifusinite, and inertodetrinite in sample C25-1; (B) telinite in sample C25-2; (C) vitrodetrinite in sample C25-1; (D) fusinite with swelling cells in sample C25-3; (E) micrinite in sample C25-3; (F) macrinite in sample C25-3.
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Figure 4. SEM back-scattered electron images of discrete particles and fracture-filling kaolinite in sample C19-1 (A); and cell-filling kaolinite and quartz in sample C25-2 (B).
Figure 4. SEM back-scattered electron images of discrete particles and fracture-filling kaolinite in sample C19-1 (A); and cell-filling kaolinite and quartz in sample C25-2 (B).
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Figure 5. Pyrite in the C25 coal. (A) Particles of pyrite in sample C25-2; (B) cell-filling pyrite in sample C25-3; (C) framboidal pyrite in sample C25-3; (D) needle-like forms combined with marcasite in sample C25-3. Optical microscope, reflected light.
Figure 5. Pyrite in the C25 coal. (A) Particles of pyrite in sample C25-2; (B) cell-filling pyrite in sample C25-3; (C) framboidal pyrite in sample C25-3; (D) needle-like forms combined with marcasite in sample C25-3. Optical microscope, reflected light.
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Figure 6. Quartz in sample C19-1. (A) Particles of quartz; (B) fracture-filling quartz. Optical microscope, reflected light.
Figure 6. Quartz in sample C19-1. (A) Particles of quartz; (B) fracture-filling quartz. Optical microscope, reflected light.
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Figure 7. Calcite in C25-2. Optical microscope, reflected light. (A) Fracture-filling calcite and; (B) Fracture-filling calcite.
Figure 7. Calcite in C25-2. Optical microscope, reflected light. (A) Fracture-filling calcite and; (B) Fracture-filling calcite.
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Figure 8. Back-scattered electron images of (A) Ti-oxide and (B) fluocerite in C25-3.
Figure 8. Back-scattered electron images of (A) Ti-oxide and (B) fluocerite in C25-3.
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Figure 9. SEM back-scattered electron images of (A) fracture-filling chalcopyrite; (B) irregular granular of titaniferous magnetite and a vermicular texture in the kaolinite; (C) acicular idiomorphic crystals of apatite; and (D) phenocrysts of Ti-augite in C19-r.
Figure 9. SEM back-scattered electron images of (A) fracture-filling chalcopyrite; (B) irregular granular of titaniferous magnetite and a vermicular texture in the kaolinite; (C) acicular idiomorphic crystals of apatite; and (D) phenocrysts of Ti-augite in C19-r.
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Figure 10. SEM back-scattered electron images of marcasite in C25-f. (A) Marcasite and claystone; (B) Marcasite.
Figure 10. SEM back-scattered electron images of marcasite in C25-f. (A) Marcasite and claystone; (B) Marcasite.
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Figure 11. Variations of ash yield and selected major elements (%) through the roof, coal seam, and floor section of the Shiping C19 and C25 coal.
Figure 11. Variations of ash yield and selected major elements (%) through the roof, coal seam, and floor section of the Shiping C19 and C25 coal.
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Figure 12. Concentration coefficients (CCs) of trace elements in the Shiping coals, normalized by average trace element concentrations in hard coals of the world [28] and based on the trace-element enrichment classification [29].
Figure 12. Concentration coefficients (CCs) of trace elements in the Shiping coals, normalized by average trace element concentrations in hard coals of the world [28] and based on the trace-element enrichment classification [29].
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Figure 13. Concentration coefficients (CCs) of trace elements in the Shiping roof and floor samples, normalized by average trace element concentrations in clay of the world [30].
Figure 13. Concentration coefficients (CCs) of trace elements in the Shiping roof and floor samples, normalized by average trace element concentrations in clay of the world [30].
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Figure 14. Distribution patterns of REE in the coal samples from the Shiping mine. REE are normalized by Upper Continental Crust (UCC) [32]. (A) Distribution patterns of REE in sample C19-1 and C25-1; (B) Distribution patterns of REE in sample C25-2; (C) Distribution patterns of REE in sample C25-3.
Figure 14. Distribution patterns of REE in the coal samples from the Shiping mine. REE are normalized by Upper Continental Crust (UCC) [32]. (A) Distribution patterns of REE in sample C19-1 and C25-1; (B) Distribution patterns of REE in sample C25-2; (C) Distribution patterns of REE in sample C25-3.
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Figure 15. Distribution patterns of REE in the roof and floor samples from the Shiping mine. REE are normalized by Upper Continental Crust (UCC) [32]. (A) Distribution patterns of REE in sample C19-r; (B) Distribution patterns of REE in sample C25-f.
Figure 15. Distribution patterns of REE in the roof and floor samples from the Shiping mine. REE are normalized by Upper Continental Crust (UCC) [32]. (A) Distribution patterns of REE in sample C19-r; (B) Distribution patterns of REE in sample C25-f.
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Figure 16. Images of C19-r and C25-f samples collected from the Shiping mine.
Figure 16. Images of C19-r and C25-f samples collected from the Shiping mine.
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Figure 17. Distribution patterns of REE in the (A) high-Ti basalts [10] and C19-r; and (B) the coal in Lvshuidong [5] and C25-3. REE are normalized by Upper Continental Crust (UCC) [32].
Figure 17. Distribution patterns of REE in the (A) high-Ti basalts [10] and C19-r; and (B) the coal in Lvshuidong [5] and C25-3. REE are normalized by Upper Continental Crust (UCC) [32].
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Table
Table 1. Bench thickness (cm), proximate analysis (%), vitrinite random reflectance (%), and gross calorific values (MJ/kg) of coal benches from the Shiping mine.
Table 1. Bench thickness (cm), proximate analysis (%), vitrinite random reflectance (%), and gross calorific values (MJ/kg) of coal benches from the Shiping mine.
SamplesThicknessMadAdVdafSt,dSs,dSp,dSo,dRo,ranQgr,d
C19-1501.7517.429.140.59ndndnd2.3029.35
C25-1402.7016.5213.906.532.113.580.842.2128.13
C25-2401.3216.6613.041.450.331.080.052.2328.99
C25-3501.5844.5427.842.590.811.400.382.1015.45
M, moisture; A, ash yield; V, volatile matter; St, total sulfur; ad, air-dry basis; d, dry basis; daf, dry and ash-free basis; Ro,ran, random reflectance; Qgr,d, gross calorific value, on a dry basis; nd, not detected.
Table
Table 2. Maceral composition (vol. %; on mineral-free basis) of the Shiping coals.
Table 2. Maceral composition (vol. %; on mineral-free basis) of the Shiping coals.
SamplesCdCtTCgVdT-VFSfMaMiScIdT-I
C19-147.932.70.4bdl2.783.70.48.61.60.4bdl5.416.3
C25-154.521.8bdlbdl2.378.6bdl14.44.3bdlbdl2.721.4
C25-260.825.20.4bdl0.787.1bdl7.22.9bdlbdl2.912.9
C25-351.119.60.9bdl2.374.0bdl10.08.70.5bdl6.826.0
Cd, collodetrinite; Ct, collotelinite; T, telinite; Cg, corpogelinite; Vd, vitrodetrinite; T-V, total vitrinite; F, fusinite; Sf, semifusinite; Ma, macrinite; Mi, micrinite; Id, inertodetrinite; T-I, total inertinite. bdl, below detection limit.
Table
Table 3. LTA yields of coal samples and mineral compositions (%) of coal LTAs, partings, roofs, and floors determined by XRD and Siroquant.
Table 3. LTA yields of coal samples and mineral compositions (%) of coal LTAs, partings, roofs, and floors determined by XRD and Siroquant.
MineralsC19-rC19-1C25-1C25-2C25-3C25-f
LTAs-17.5824.9420.2149.35-
Kaolinite34.534.851.227.665.580.3
Illite9.815.5----
I/S mixed-layer26.61.6----
Quartz19.826.61.4---
Pyrite0.47.427.933.45.4
Marcasite-10.3----
Anatase4.9-3.1---
Rutile--6.8---
Calcite0.4-7.36628.44.1
Siderite3-----
Jarosite0.6----2
Bassanite-3.72.23.42.7-
Gypsum-----8.2
Table
Table 4. Bench thickness (cm), major element oxides (%), loss on ignition (%), and trace elements (μg/g) in the Shiping coals and host rocks.
Table 4. Bench thickness (cm), major element oxides (%), loss on ignition (%), and trace elements (μg/g) in the Shiping coals and host rocks.
ElementsC19-rC19-1C25-1C25-2C25-3C25-f
Thickness-50404050-
SiO243.413.54.754.2016.125.5
TiO23.880.2790.2910.2290.4820.934
Al2O322.51.884.514.1214.522.4
Fe2O31.900.3375.501.402.704.49
MnO0.0070.0030.0040.0370.0170.004
MgO0.3630.0960.0660.0640.1760.192
CaO0.7080.4260.3893.785.752.63
Na2O0.4800.0530.0500.0220.0550.057
K2O1.150.0170.0660.0310.1220.228
P2O50.4030.0070.0090.0060.0160.026
LOI24.682.983.983.656.239.8
SiO2/Al2O31.937.191.051.021.111.14
Li49.19.9721.733.2178343
Be3.382.897.7316.29.479.29
F78736.877.252.4342480
Sc22.65.006.794.078.9615.3
V33138.561.026.36462904
Cr11812.724.113.82821837
Co16.217.92.612.353.8320.0
Ni36.934.614.89.6546.2166
Cu27820.614.89.4229.295.8
Zn30013.920.414.780.671.5
Ga38.04.0612.89.0136.575.3
Ge2.470.933.798.6210.34.80
As11.41.663.292.677.9825.2
Se22.24.025.094.0922.863.2
Rb33.70.4961.530.5543.436.02
Sr75977.9164325301177
Y69.210.620.217.1197137
Zr61382.722812517441648
Nb91.311.417.019.1192131
Mo20.00.8681.310.7073.8224.2
Cd1.630.2100.6660.43214.617.9
In0.1830.0460.0200.0450.2260.563
Sn4.880.8311.121.187.8912.6
Sb1.260.0970.2510.1770.9667.63
Cs4.070.0710.2060.0691.061.82
Ba22313.518.39.4746.378.0
REE70340.773.812410371323
La1275.4811.425.4173221
Ce27411.719.544.3322533
Pr29.81.292.135.2142.554.6
Nd1174.727.7218.1159224
Sm26.01.081.773.1530.639.8
Eu6.450.2540.4160.4362.823.26
Gd22.51.342.323.4432.236.5
Tb2.760.2220.4380.4934.875.07
Dy13.11.392.972.6728.627.3
Ho2.460.3130.6360.5356.015.58
Er6.350.9691.921.4617.515.6
Tm0.8780.1490.2820.2032.502.37
Yb5.480.9941.851.2616.115.6
Lu0.8040.1590.2800.1892.352.39
Hf13.71.783.513.0138.228.0
Ta5.430.6190.8350.9305.165.93
Hg (ppb)59.819.5396197372621
Tl0.0770.0040.1880.0450.0830.115
Pb66.66.1812.52.1912.629.1
Bi0.1640.2120.1460.2050.4270.598
Th17.83.204.194.6116.928.0
U4.340.7532.331.30155505
Table
Table 5. The trace element (μg/g) in sample C19-r compared with high/low Ti basalts [10].
Table 5. The trace element (μg/g) in sample C19-r compared with high/low Ti basalts [10].
SamplesVCrCoNiCuZnGaZrNbHfTa
C19-r33111816.236.927830038.061391.313.75.43
High-Ti Basalt37471.040.363.524813123.939148.58.512.97
Low-Ti Basalt28923044.512410990.719.912913.43.050.773

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Luo, Y.; Zheng, M. Origin of Minerals and Elements in the Late Permian Coal Seams of the Shiping Mine, Sichuan, Southwestern China. Minerals 2016, 6, 74. https://doi.org/10.3390/min6030074

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Luo Y, Zheng M. Origin of Minerals and Elements in the Late Permian Coal Seams of the Shiping Mine, Sichuan, Southwestern China. Minerals. 2016; 6(3):74. https://doi.org/10.3390/min6030074

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Luo, Yangbing, and Mianping Zheng. 2016. "Origin of Minerals and Elements in the Late Permian Coal Seams of the Shiping Mine, Sichuan, Southwestern China" Minerals 6, no. 3: 74. https://doi.org/10.3390/min6030074

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