Revisiting the late Permian coal from the Huayingshan, Sichuan, southwestern China: Enrichment and occurrence modes of minerals and trace elements

International Journal of Coal Geology 122 (2014) 110–128

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Revisiting the late Permian coal from the Huayingshan, Sichuan, southwestern China: Enrichment and occurrence modes of minerals and trace elements Shifeng Dai a,⁎, Yangbing Luo a, Vladimir V. Seredin b, Colin R. Ward c, James C. Hower d, Lei Zhao a, Shande Liu a, Cunliang Zhao a, Heming Tian e, Jianhua Zou e a

State Key Laboratory of Coal Resource s and Safe Mining, China University of Mining and Technology, Beijing 100083, China Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Staromonetny per. 35, Moscow 119017, Russia c School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia d University of Kentucky, Center for Applied Energy Research, 2540 Research Park Drive, Lexington, KY 40511, United States e Chongqing Institute of Geology and Mineral Resources, Chongqing 400042, China b

a r t i c l e

i n f o

Article history: Received 25 October 2013 Received in revised form 21 December 2013 Accepted 22 December 2013 Available online 27 December 2013 Keywords: Minerals in coal Trace elements in coal Late Permian coal Hydrothermal fluid Terrigenous material Volcanic ash

a b s t r a c t The Late Permian coals from the Huayingshan Coalfield of southwestern China are significantly enriched in Zr (695 μg/g), Nb (75.9 μg/g), Se (6.99 μg/g), Hf (10.1 μg/g), and rare earth elements and Y (1423 μg/g). Previous studies showed that the sediment-source region for these coals was the Kangdian Upland, which was formed at an early stage of the late Permian Period. The source rocks have a basalt composition, and those studies attributed the enrichment of the above high field strength elements (HFSEs) to derivation from the Kangdian Upland. Geochemical and mineralogical data presented in this study show that the dominant sediment-source regions for the coal and roof strata of the Huayingshan Coalfields are the Dabashan Uplift, Hannan Upland, and Leshan– Longnvsi Uplift. The highly-elevated concentrations of HFSEs in the coals are due to hydrothermal fluids. Three tonstein layers derived from alkali rhyolite were identified. These tonsteins are characterized by highlyenriched HFSEs and by strong negative Eu anomalies in the rare earth element distribution patterns. The major carriers of the rare earth elements in the coal are rhabdophane and silicorhabdophane, the latter of which is also enriched in Zr. Zirconium, however, mainly occurs in zircon. Rhabdophane and silicorhabdophane in the coal are mainly distributed along the bedding planes and occur as cell-fillings. Zircon in the coal occurs as cell-fillings and is of authigenic origin. Anatase in the partings and coals contains Nb, and occurs as fracture-filling and colloidal forms. The modes of occurrence of the above minerals indicate that they were derived from hydrothermal fluids. Mercury and Se mainly occur in sulfide minerals (pyrite and marcasite). © 2013 Elsevier B.V. All rights reserved.

1. Introduction The trace elements and minerals in the coals of southwestern China have attracted much attention, not only because this area is associated with endemic arsenosis, fluorosis, and lung cancer related to indoor coal combustion (Dai et al., 2012; Finkelman et al., 2002; Large et al., 2009; Tian et al., 2008; Zheng et al., 1999), but also because the area is associated with elevated concentrations of rare metals in the coals and in coal-bearing rocks (Dai et al., 2010; Hu et al., 2009; Qi et al., 2004; Seredin and Dai, 2012; Zhuang et al., 2012). The Huayingshan Coalfield, located in the eastern part of Sichuan Province, southwestern China (Fig. 1A), provides an important energy resource base for its region. Studies by Zhang (1993) and the China National Administration of Coal Geology (1996) showed that the dominant

⁎ Corresponding author. E-mail address: [email protected] (S. Dai). 0166-5162/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.coal.2013.12.016

sediment source region for the Huayingshan coals is the Kangdian Upland (Fig. 1B), which was formed at an early stage of the late Permian Period and has a basalt composition. Zhuang et al. (2012) have provided geochemical data of the Huayingshan Late Permian coals and concluded that: (1) many elements in the coal, including Al, Ti, Li, Ta, Th, Ga, U, Sn, Sc, Cr, Cu, Rb, Co, and Se, probably have an aluminosilicate affinity; Zr, Nb, Hf, Y, REEs, and U probably occur in zircon; and Sr and Mn mainly have a carbonate affinity; and (2) the high-enrichment of Zr, Nb, Hf, REEs, Ta, Ga, and Th in the coal was derived from the sediment-source of the Kangdian Upland (and probably also by input of volcanic ash). The purpose of this paper is to re-investigate the enrichment, origin and modes of occurrence of elevated trace elements and minerals in the coals from the Huayingshan Coalfield, and thus to address the following questions: (1) Was the Kangdian Upland the dominant sediment-source region for the Huayingshan coals and the major cause of the above elevated element concentrations in the coal? (2) What are the modes of occurrence of Zr, Nb, rare earth elements, and Y in the coal?

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111

Fig. 1. Location (A) and tectonic map (B) of the Huayingshan Coalfield. (B) is the enlargement of the red area in (A). Y–L, Leshan–Longnvsi. (A) is modified from Dai et al. (2012).

2. Geological setting The sedimentary sequences in the coalfield include the Lower Permian Maokou Formation, Upper Permian Longtan and Changxing Formations, Lower Triassic Feixianguan and Jialingjiang Formations, Middle Triassic Leikoupo Formation, Upper Triassic Xujiahe Formation, and Jurassic strata. The Maokou Formation is 180-m thick and is composed of limestone and flint-bearing limestone (Fig. 2A). It underlies the Longtan Formation with an unconformable contact. The major coal-bearing unit (Fig. 2A) is the Longtan Formation, which conformably underlies the limestones of the Changxing Formation. The Longtan Formation is mainly composed of flint-bearing limestone, mudstone, siltstone, sandstone, and two to three coal seams. The K1 Coal is the lowermost coal seam in the late Permian strata in southwestern China. It is not only the major minable seam, with a thickness 0.8–3.57 m in the Huayingshan Coalfield, but also stretches over an area 300-km long (W-E) and 250-km wide (N-S), with an areal extent of N 70,000 km2. The K1 Coal was formed in a tidal flat environment (China National Administration of Coal Geology, 1996; Zhang, 1993). Note that the K1 Coal in the Huayingshan Coalfield sometimes splits in two layers, K1-1 and K1-2 (Fig. 2B). Sample K1-1r in the present study can be considered both as the roof of the K1-1 layer and as the floor of K1-2 layer (Fig. 2B). A mafic tuff layer with a thickness mostly of 2–5 m, light-gray or light-gray–white in color, with a conchoidal fracture and a soapy feel, underlies the lowermost Longtan Formation and has a disconformable contact with the underlying limestone of the Maokou Formation (Early Permian age) (Fig. 2A). The tuff is enriched in siderite and shows massive bedding characteristics. The Changxing Formation has a thickness of 120–260 m (188 m on average) and consists of flint-bearing limestone, interlayered with thin layers of sandstone and mudstone. The Lower Triassic Feixianguan Formation, with a thickness of 440–590 m, consists of mudstone, marly limestone, limestone, and dolomitic limestone. The Kangdian Upland (Fig. 1B), which formed at early stage of the Late Permian and is mainly composed of Emeishan basalts, supplied terrigenous materials for most Late Permian coal-bearing areas in southwestern China (China National Administration of Coal Geology, 1996; Dai et al., 2012, 2013b; Zhang, 1993). However, three uplands/uplifts around the coal basin (the TUUs), including the Dabashan Uplift, the Hannan Upland, and the Leshan–Longnvsi (Y–L) Uplift in the northern part of southwestern China (Fig. 1), as well as the Jiangnan Upland

and Qianzhong Uplift in the southern part, were also important positive tectonic elements during the late Permian and Triassic periods (Chen et al., 1990; Jiang et al., 2007; Xie et al., 2006; Zhang et al., 2010). The TUUs had been formed (Fig. 1B) by the end of the Caledonian orogeny and were composed of carbonate, sandstone and mudstone of the Sinian to Silurian systems (Chen et al., 1990; Jiang et al., 2007; Xie et al., 2006; Zhang et al., 2010); they became the dominant sedimentsource region for the coalfield in the present study.

3. Samples and analytical procedures Five coal bench samples, three partings, and four host rocks (roof and floor) were taken from the mined coal face at the Lvshuidong Mine in the Huayingshan Coalfield. From top to bottom, the coal bench samples, partings, and host rocks are identified as shown in Fig. 2B. Each coal bench sample was cut over an area 10-cm wide and 10-cm deep. All samples were immediately stored in plastic bags to minimize contamination and oxidation. Proximate analysis was conducted using ASTM Standards D3173-11, D3175-11, and D3174-11 (2011). The total sulfur and forms of sulfur were determined following ASTM Standard D3177-02 (2002) and D2492-02 (2002), respectively. Samples were prepared for microscopic analysis by reflected light following ASTM Standard D2797/D2797M− 11a (2011). Mean random reflectance of vitrinite (percent Ro, ran) was determined using a Leica DM-4500P microscope (at a magnification of 500 ×) equipped with a Craic QDI 302™ spectrophotometer. Maceral constituents were identified using white-light reflectance microscopy under oil immersion, and more than 500 counts were measured for each polished pellet. Concentrations of major element oxides in the samples (on ash basis; 815 °C ashing temperature) were determined by X-ray fluorescence spectrometry. Mercury was determined by using a Milestone DMA-80 Hg analyzer. Fluorine was determined by pyrohydrolysis with an ion-selective electrode, following the methods described in ASTM Standard D 5987–96 (2002). Inductively coupled plasma mass spectrometry (ICP-MS) was used to determine other trace elements in the coal and rock samples. For ICP-MS analysis, the samples were digested by using an UltraClave Microwave High Pressure Reactor. More details for these coal-related sample digestion and ICP-MS analysis techniques are given by Dai et al. (2011). Arsenic and Se were determined by ICP-MS by using collision-cell technology (CCT) in order to avoid disturbance of polyatomic ions.

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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°. X-ray diffractograms of the coal LTAs and rock samples were subjected to quantitative mineralogical analysis using Siroquant™, commercial interpretation software developed by Taylor (1991) based on the principles for diffractogram profiling set out by Rietveld (1969). Further details indicating the use of this technique for coal-related materials are given by Ward et al. (1999, 2001) and Ruan and Ward (2002). A Field Emission-Scanning Electron Microscope (FE-SEM, FEI Quanta™ 650 FEG), in conjunction with an EDAX energy-dispersive X-ray spectrometer (Genesis Apex 4), was used to study morphology and microstructure, and also to determine the distribution of some elements in the coal and rock samples. Samples were made into pellets, polished, coated with carbon using a Quorum Q150T ES sputtering coater, and then mounted on standard aluminum SEM stubs using sticky electron-conductive carbon tabs. The working distance of the FE-SEMEDS was 10 mm, beam voltage 20.0 kV, aperture 6, and spot size 5–6. Images were captured via a retractable solid state backscatter electron detector. 4. Results 4.1. Coal chemistry and vitrinite reflectance The vitrinite reflectance (1.46%) and the weighted average volatile matter (22.55%, dry and ash free basis) of the coal bench samples (Table 1) indicate a low volatile bituminous coal according to the ASTM classification (ASTM D388-12, 2012). The coal is a medium-ash and high-sulfur coal according to Chinese Standards GB 15224.1-2004 (coals with ash yield 20.01–30.00% are high-ash coal) and GB/T 15224.2-2004 (coals with total sulfur content N3% are high-sulfur coal). The sulfur is mainly pyritic (Table 1). However, sample K1-1c has a higher content of sulfate sulfur than other coal benches. 4.2. Maceral composition The vitrinite in the Lvshuidong coals is dominated by collodetrinite and, to a lesser extent, collotelinite and vitrodetrinite, along with small proportions of telinite and corpogelinite (Table 2). The collodetrinite generally contains embedded quartz and pyrite (Fig. 3A,B,C). Fractures, which are generally filled with calcite and quartz, cut through different vitrinite macerals (Fig. 3A,B,C). The inertinite in the coals is mainly composed of semifusinite, macrinite, and inertodetrinite, with trace amounts of fusinite, micrinite, and funginite (Table 2). Fusinite is represented by forms indicative of wood oxidation and shows varying levels of preservation (Fig. 3D,E,F). In most cases, the cell structures of semifusinite and fusinite are not well-preserved, and have a swelled and deformed form (Fig. 3D, E). Sample K1-1r, which is considered as either the roof of K1-1 or the floor of K1-2, has a higher loss on ignition (38.03%) due to a higher organic matter content than the other host rocks examined in this study. The vitrinite and inertinite macerals are distributed along the bedding

Table 1 Coal bench thickness (m), proximate analysis (%), sulfur (%), and vitrinite random reflectance (%) of the Lvshuidong coals. Fig. 2. Sedimentary sequences of the Huayingshan Coalfield (A) and the collected bench samples through the section in present study (B). Tonsteins in green, host rocks (roof and floor strata) in orange, tuff in blue, and coal benches in black. The suffixes f, p and r stand for floor, parting, and roof, respectively.

Low-temperature (oxygen-plasma) ashing was performed to remove the organic matter from the coal, using an EMITECH K1050X plasma asher. The residues of this process were then analyzed by X-ray diffraction (XRD) using a D/max-2500/PC powder diffractometer with

Sample

Thickness

Mad

Ad

Vdaf

St,d

Ss,d

Sp,d

So,d

Ro,ran

K1-2b K1-2a K1-1c K1-1b K1-1a WA

5 90 40 38 30 203*

0.75 1.29 1.71 1.30 1.73 1.43

32.89 13.18 35.46 14.26 30.88 20.87

27.80 23.24 24.13 19.60 21.22 22.55

2.38 2.57 4.12 3.19 3.73 3.16

0.12 0.59 1.14 0.70 0.28 0.66

1.76 1.04 1.80 1.68 2.77 1.58

0.50 0.94 1.17 0.81 0.68 0.91

1.31 1.44 1.41 1.60 1.46 1.46

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; WA, weighted average for bench samples; *, the total thickness of the coal, partings excluded.

S. Dai et al. / International Journal of Coal Geology 122 (2014) 110–128

Liptinite was not observed, owing to the difficulty of distinguishing liptinites in low volatile bituminous coals.

Table 2 Maceral composition of the K1 coal samples (vol.%; mineral-free basis). Sample CD

CT

T

CG

VD T-V

F

K1-2b K1-2a K1-1c K1-1b K1-1a WA

15.2 15.5 16.1 7.6 22.3 15.1

0.5 0.6 2.6 0.4 0.5 0.9

bdl 0.3 bdl 0.4 1.0 0.4

5.8 1.6 2.1 2.9 3.6 2.3

0.9 16.1 8.5 bdl 0.3 11.7 5.1 1.9 1.0 5.2 6.8 bdl 0.4 8.4 5.5 bdl 3.1 2.6 10.4 bdl 0.9 8.6 6.4 0.9

44.6 51.7 53.6 55.5 41.5 51.1

66.1 69.7 74.5 66.8 68.9 69.9

SF

Mac Mic Fun ID 0.5 1.0 0.5 0.8 1.6 0.9

113

8.0 10.4 12.0 18.1 13.5 12.6

T-I 33.9 30.3 25.5 33.2 31.1 30.1

CD, collodetrinite; CT, collotelinite; T, telinite; CG, corpogelinite; VD, vitrodetrinite; T-V, total vitrinite; F, fusinite; SF, semifusinite; Mac, macrinite; Mic, micrinite; Fun, funginite; ID, inertodetrinite; T-I, total inertinite.

planes (Fig. 4A–D), and clay minerals are embedded in the inertodetrinite and vitrodetrinite (Fig. 4A,B). In some cases, the funginite is pyritized (Fig. 4C).

4.3. Concentration and distribution of major and trace elements 4.3.1. Major element oxides The major element oxides are mainly represented by SiO2 and Al2O3, and, to a lesser extent, Fe2O3 and CaO (Table 3). The percentages of SiO2 and TiO2 are slightly higher than those in more normal Chinese coals reported by Dai et al. (2012). The percentages of other major element oxides, however, are either lower or close to those in common Chinese coals. The SiO 2 /Al2 O3 ratio of the coal (1.69 on average) is higher than that of other Chinese coals (1.42) (Dai et al., 2012) and also than the theoretical ratio for kaolinite (1.18), indicating free SiO2 in the coal.

Fig. 3. Vitrinite and inertinite macerals in the coal. (A) collodetrinite embedding quartz and pyrite, calcite in fractures; sample K1-2b; reflected light. (B), collodetrinite embedding quartz and veinlet-filling quartz; sample K1-1b; reflected light. (C), quartz embedded in collodetrinite and occurring as fracture-fillings; sample K1-2b; reflected light. (D), fusinite with deformed structures; sample K1-2a; reflected light, oil immersion. (E), semifusinite with swelling cells; sample K1-2a; reflected light, oil immersion. (F), fusinite with cell-filling quartz and pyrite; sample K1-2a; reflected light. CD, collodetrinite; CT, collotelinite; F, fusinite; SF, semifusinite; Ca, Calcite; Py pyrite; Q, quartz.

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Fig. 4. Maceral and minerals in sample K1-1r, reflected light. (A) Macerals and pyrite distributing along bedding planes. (B) macrinite, as well as macerals and framboidal pyrite distributing along bedding planes. (C) pyritized funginite. (D) Needle-like pyrite of bacterial origin. F, fusinite; Ma, macrinite; Py, pyrite.

4.3.2. Trace elements Compared to the average for world hard coals (Ketris and Yudovich, 2009), a large number of trace elements are enriched in the Lvshuidong coals (Table 3; Fig. 5). The trace elements with a concentration coefficient (CC = ratio of element concentration in Lvshuidong coals vs. world hard coals) N 10 only include Zr and Nb. Trace elements with a CC of 5–10 include Se and Hf. Many other elements, including Li, Be, V, Ga, Y, Cd, In, Sn, La, Yb, Ta, Hg, Th, and U, are slightly enriched in the coal (CC = 2–5). However, Co, Ni, As, Rb, Sb, Ba, Tl, and Bi are depleted in the coal (CC b0.5). The concentrations of the remaining elements (0.5 b CC b 2), are close to the average for world hard coals. Note that some trace elements, including Sc, V, Cr, Co, Ni, and Cu, which are significantly abundant in the basalt of the Kangdian Upland (Dai et al., 2012, 2014; Zhou et al., 2000), are not highly enriched and are even depleted (e.g., Co, Ni) in the Lvshuidong coals (Figs. 5, 6). Concentrations of these trace elements, however, are much higher in the Late Permian coals influenced by the sediment-source region of the Kangdian Upland in southwestern China, as reported by Dai et al. (2014), Wang et al. (2012), and Zhuang et al. (2012). These elements have higher concentrations in the three host rocks (K1-1f, K1-1r, and K1-2r), but are significantly depleted in the three partings (K1-1p1, K1-1p2, and K1-2p) relative to the average for world clays (Grigoriev, 2009; Figs. 6, 7). All these elements show a similar pattern of vertical variation through the section covered by the present study (e.g., V, Cr, Co, and Ni in Fig. 6). In contrast, a number of elements, including Li, Ga, Nb, Zr, Yb, La, Th, and U, are not only abundant in the Lvshuidong coals relative to the average for world hard coals (Figs. 5, 8) as reported by Ketris and Yudovich (2009), but also significantly enriched in the three partings (K1-1p1, K1-1p2, and K1-2r) in comparison with the average for world clays (Figs. 7, 8). Elements Zr, Nb, La, and Yb reach their maximum concentrations in the upmost coal bench K1-2b.

The anomalous geochemistry of the Lvshuidong coals, partings and host rocks is also indicated by the ratios of some trace elements. On average the Lvshuidong coals have higher Nb/Ta, Zr/Hf, and U/Th ratios than the partings and host rocks (roof and floor), with the exception of U/Th in sample K1-1r (Fig. 9). These geochemical anomalies are probably attributed to re-deposition of the first elements (Nb, Zr, and U) of each element pair, because of their relatively more active leaching from the partings, and then deposition in the underlying organic matter (Dai et al., 2013b; Seredin, 2004). The similar vertical distribution of Se and Hg, enriched in samples K1-2r, K1-1r, and K1-1a, but depleted in samples of the three partings and sample K1-1f (Fig. 6), indicates similar modes of occurrence. Mercury and Se are positively correlated with pyrite, indicating that the two elements occur in pyrite. However, the higher correlation coefficients of Hg–sulfide (pyrite + marcasite) (r = 0.92) and Se–sulfide (pyrite + marcasite) (r = 0.75), respectively, compared to those of Hg–pyrite (r = 0.90) and Se–pyrite (r = 0.58), suggest that Hg and Se occur not only in pyrite but also in marcasite (Fig. 10). 4.3.3. Rare earth elements and yttrium A threefold classification of rare earth elements and yttrium (REY) was used for this study: light (LREY: La, Ce, Pr, Nd, and Sm), medium (MREY: Eu, Gd, Tb, Dy, and Y), and heavy (HREY: Ho, Er, Tm, Yb, and Lu) REY (Seredin and Dai, 2012). Accordingly, in comparison with the upper continental crust (UCC; Taylor and McLennan, 1985), three enrichment types are identified (Seredin and Dai, 2012): L-type (light-REY; LaN/LuN N 1), M-type (medium-REY; LaN/SmN b 1, GdN/ LuN N 1), and H-type (heavy REY; LaN/LuN b 1). The REY enrichment patterns in the coal benches are characterized by clear negative Eu anomalies and H- and M-REY enrichment (Fig. 11A,B). In particular, sample K1-2b has the highest REY concentration relative to other coal and rock samples, and the strongest negative Eu anomaly among the coal bench samples (Fig. 11).

S. Dai et al. / International Journal of Coal Geology 122 (2014) 110–128

115

Table 3 Major element oxides (%) and trace elements in the coals, partings, and host rocks (μg/g unless indicated) from Lvshuidong Mine. Sample

K1-2r

K1-2b

K1-2p

K1-2a

K1-1r

K1-1c

K1-1p2

K1-1b

K1-1p1

K1-1a

K1-1f

Tuff

WAa

World b

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI SiO2/Al2O3 Li Be F Sc V Cr Co Ni Cu Zn Ga Ge As Se Rb Sr Y Zr Nb Mo Cd In Sn Sb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Hg (ng/g) Tl Pb Bi Th U

39.36 1.86 20.95 12.11 0.177 1.112 2.27 0.423 4.101 0.126 15.10 1.88 24.2 4.76 230 17.9 341 134 38.4 101 80.0 72.1 27.8 1.89 51.1 8.32 47.9 946 36.3 462 65.1 15.8 0.84 0.127 5.36 1.35 3.55 71.5 87.7 182 18.6 65.0 11.2 1.29 10.8 1.38 7.72 1.58 4.41 0.64 4.51 0.65 12.5 4.84 20.0 437 0.268 23.4 0.26 15.1 9.92

16.09 0.15 4.16 1.89 0.024 0.225 4.53 0.263 0.372 0.143 67.36 3.87 25.0 7.14 202 4.78 16.9 6.67 1.70 10.3 11.0 202 17.6 20.8 1.12 5.21 7.26 258 274 7288 682 9.02 7.63 0.487 8.13 0.26 1.09 15.6 310 610 69.8 277 49.6 2.98 56.8 7.69 44.9 8.22 24.5 3.41 23.9 3.44 50.0 0.50 3.28 111 0.299 9.81 0.14 10.9 14.9

46.56 0.83 28.29 2.51 0.020 1.296 0.76 0.256 3.775 0.094 14.20 1.65 98.6 5.69 940 2.87 13.5 15.0 1.10 5.99 23.2 69.3 78.5 5.72 6.36 3.81 39.2 2956 155 1577 235 7.54 2.11 0.497 21.6 1.64 3.66 41.3 240 569 63.8 221 39.9 2.15 37.9 5.27 31.1 6.26 16.9 2.33 15.6 2.03 76.3 29.3 5.02 67 0.399 46.5 0.80 95.0 17.6

4.97 0.22 3.31 1.69 0.009 0.069 1.28 0.039 0.077 0.013 86.99 1.50 30.9 6.04 82.1 3.30 46.3 14.2 1.99 6.21 16.4 20.3 6.19 3.52 2.02 6.09 1.16 93.4 25.1 187 20.4 2.15 0.30 0.050 1.50 0.14 0.19 7.7 28.5 64.1 7.07 27.1 5.16 0.72 5.33 0.77 4.19 0.89 2.64 0.40 2.67 0.42 2.74 0.36 1.25 163 0.090 5.67 0.17 4.54 1.92

27.53 2.17 21.28 8.91 0.008 0.293 0.28 0.096 0.700 0.048 38.03 1.29 164 3.39 460 14.7 249 89.2 26.5 40.5 110 44.7 31.9 2.30 17.5 15.6 12.1 287 57.8 879 124 15.2 1.43 0.166 5.37 0.70 2.32 71.4 95.0 172 16.9 54.9 9.85 1.62 11.1 1.62 9.79 2.00 5.37 0.73 4.75 0.65 18.3 7.35 6.47 262 0.504 21.3 0.35 19.0 10.3

17.56 0.70 10.54 4.09 0.003 0.176 0.43 0.066 0.429 0.062 65.15 1.67 79.6 4.43 256 2.79 68.7 24.6 3.42 6.42 26.3 51.8 29.2 2.01 4.36 6.48 8.90 76.7 57.3 1583 202 5.92 1.72 0.212 9.15 0.57 3.05 26.3 90.9 165 21.9 79.8 14.1 1.64 14.0 2.10 11.6 2.40 6.77 1.02 6.59 0.99 27.7 0.62 2.09 210 0.261 24.5 0.37 18.9 8.51

36.49 0.80 27.34 3.97 0.005 0.292 0.76 0.150 1.084 0.120 27.74 1.33 262 6.31 403 3.25 6.11 3.00 1.86 4.22 16.3 21.9 63.3 3.52 8.19 4.59 17.7 475 100 1338 210 6.98 1.86 0.299 13.8 0.59 2.47 68.1 322 703 79.6 286 49.1 4.56 42.5 4.60 21.3 3.88 10.0 1.36 9.08 1.21 40.8 19.2 18.7 77 0.389 19.0 0.66 70.4 9.30

6.04 0.15 3.28 2.41 0.006 0.096 0.89 0.036 0.175 0.011 85.93 1.84 19.3 3.60 106 2.06 27.2 10.7 2.05 6.29 24.2 21.0 6.50 4.04 3.05 8.13 6.50 63.8 28.7 228 16.9 4.26 0.35 0.053 1.59 0.29 1.59 10.8 37.3 80.7 8.97 33.6 6.07 0.70 6.18 0.89 4.85 1.04 3.05 0.46 2.98 0.44 3.95 0.59 5.91 167 0.049 13.1 0.14 4.74 1.93

37.44 1.30 27.65 2.38 0.005 0.310 0.49 0.010 1.177 0.159 28.25 1.35 209 7.83 439 0.00 7.09 3.14 1.11 3.13 23.8 42.9 63.5 10.3 3.74 5.18 18.4 247 97.0 1232 291 3.16 2.45 0.251 12.5 0.44 2.92 134 279 547 63.9 252 40.9 4.64 37.1 3.90 19.0 3.19 9.02 1.23 8.06 1.06 34.3 22.3 6.67 163 0.038 6.63 0.84 56.8 6.41

15.42 0.65 9.01 2.96 0.009 0.166 0.91 0.047 0.313 0.021 69.65 1.71 70.8 5.85 230 6.93 195 37.0 7.12 17.0 81.0 23.2 16.2 9.06 5.41 9.22 9.23 87.4 48.9 528 48.3 6.46 0.74 0.118 3.44 0.57 2.35 53.2 42.9 94.9 10.7 40.8 7.98 1.45 8.52 1.46 8.78 1.87 5.42 0.82 5.30 0.80 9.69 1.89 20.0 377 0.066 10.8 0.36 11.2 5.85

42.07 3.59 29.48 3.93 0.023 0.628 0.40 0.083 1.709 0.112 17.46 1.43 192 6.69 890 28.6 300 173 21.8 59.2 230 270 54.0 3.72 3.52 2.19 26.3 274 109 1266 174 1.78 3.16 0.364 9.44 0.34 6.96 250 151 315 35.9 132 24.0 4.68 24.5 3.52 19.6 3.86 10.5 1.48 9.29 1.32 25.3 8.89 4.74 47 0.111 17.6 0.45 21.1 6.61

34.05 3.03 32.58 9.93 0.526 0.681 1.11 0.058 0.306 0.181 15.91 1.05 684 6.26 349 9.90 246 177 44.9 91.6 238 50.1 43.3 3.12 5.87 3.19 5.19 934 108 832 106 0.8 1.50 0.37 8.79 0.75 2.29 146 142 281 33.3 129 23.3 4.91 22.1 2.96 16.9 3.53 9.57 1.31 8.63 1.25 18.9 6.59 1.96 35.3 0.080 9.65 0.78 21.7 5.44

9.47 0.36 5.59 2.49 0.008 0.113 1.06 0.050 0.207 0.027 79.44 1.69 44.1 5.26 145 3.54 68.4 18.8 3.03 7.96 29.2 31.5 12.5 4.57 3.15 6.99 5.03 87.7 41.8 695 75.9 4.09 0.83 0.104 3.47 0.32 1.36 18.9 51.5 105 12.4 46.8 8.61 1.06 8.94 1.33 7.46 1.54 4.48 0.67 4.41 0.67 10.1 0.68 5.11 204 0.118 11.6 0.23 8.54 4.12

8.47c 0.33c 5.98c 4.85c 0.015c 0.22c 1.23c 0.16c 0.19c 0.092c nd 1.42 14 2 82 3.7 28 17 6 17 16 28 6 2.4 8.3 1.3 18 100 8.4 36 4 2.1 0.2 0.04 1.4 1 1.1 150 11 23 3.4 12 2.2 0.43 2.7 0.31 2.1 0.57 1 0.3 1 0.2 1.2 0.3 0.99 100 0.58 9 1.1 3.2 1.9

a b c

Weighted averages for the Lvshuidong coal benches. From Ketris and Yudovich (2009). From Dai et al. (2012). nd, no data.

The REY distribution patterns for the three partings (K1-1p1, K11p2, and K1-2p) are characterized by an L-REY enrichment type and very strong negative Eu anomalies (Fig. 11C). However, the floor samples (K1-1f and tuff) have similar REY distribution patterns, with weak Eu anomalies, and L- and M-REY enrichment types (Fig. 11D). Samples K1-1r and K1-2r also have similar REY distribution patterns, characterized by negative Eu anomalies and an L-REY enrichment type, distinctly different to those of the floor samples.

4.4. Abundance and modes of occurrence of minerals 4.4.1. Mineral phases identified The proportion of each crystalline phase identified from the X-ray diffractograms of the coal LTA, partings, roof, and floor samples is given in Table 4. The phases identified in the coal LTAs include quartz, kaolinite, illite, mixed-layer illite/smectite (I/S), pyrite, calcite, and trace amounts of anatase and jarosite. Small

S. Dai et al. / International Journal of Coal Geology 122 (2014) 110–128

Concentration Coefficient

116

100

10

1

0.1

Li Be F Sc V Cr Co Ni Cu Zn Ga Ge As Se Rb Sr Y Zr Nb Mo Cd In Sn Sb Cs Ba La Yb Hf Ta Hg Tl Pb Bi Th U CC>10

5
2
0.5
CC<0.5

Fig. 5. Concentration coefficients (CC) of trace elements in the Lvshuidong coals, normalized by average trace element concentrations in the world hard coals (Ketris and Yudovich, 2009).

proportions of marcasite, albite, dolomite, and zircon were identified in sample K1-2b. Minerals identified by XRD + Siroquant in the partings (K1-1p1, K1-1p2, and K1-2p) include kaolinite, mixed layer I/S, illite, quartz, and trace amounts of pyrite, jarosite, and anatase. Small proportions of gypsum and albite were identified in sample K1-2p. With the exception of kaolinite and illite, which are absent in sample K1-2r and K1-1f, the minerals in the host rocks (K1-1f, K1-1r, and K12r) are mainly composed of kaolinite, illite, mixed layer I/S, quartz, pyrite, and anatase. Note that the proportions of calcite and albite are relatively high (6.8%) in sample K1-2r compared to other host rocks. The tuff is mainly composed of kaolinite and siderite, with traces of talc, anatase, pyrite, calcite, florencite, rutile and probably quartz. The abundance of siderite is thought to indicate the deposition of the mafic volcanic ash mainly under non-marine or at least weakly-marine conditions. Iron in solution that would otherwise combine with bacterially produced H2S appears instead to have combined with dissolved CO2 to form the siderite component (Ward, 2002). Sulfate minerals, including bassanite (CaSO4·1/2H2O), anhydrite (CaSO4), and alunogen (Al2(SO4)3·17H2O), were identified in some coal benches, partings, and host rocks. These minerals may represent artifacts produced in the plasma ashing process, formed by interaction between the organic sulfur in the coal, and Ca and Al occurring as inorganic components in the organic matter (López and Ward, 2008; Ward et al., 2001; Zhao et al., 2012). Bassanite and anhydrite may also have been derived from dehydration of gypsum in the raw coal samples. Gypsum (CaSO4·2H2O) was only identified in roof and partings, where it appears to have been produced by reactions between calcite and sulphuric acid, with the acid being produced by oxidation of pyrite in the coal (cf. Pearson and Kwong, 1979; Rao and Gluskoter, 1973).

0

5

500 1

10

100 1000 1

Additionally, rhabdophane (or silicorhabdophane) (Ce,Al,Fe)(P, Si)(O,OH)4, water-bearing Fe-oxysulfates that contain Si or Si and Al, and florencite, were identified in some of the coal samples by SEMEDS, but these phases were below the detection limit of the XRD and Siroquant system (both ~0.1%). 4.4.2. Modes of mineral occurrence 4.4.2.1. Kaolinite and illite (mixed-layer I/S). Kaolinite in the coal occurs mainly as cell-fillings (Fig. 12A,B,C) and, to a lesser extent, as fracturefillings (Fig. 12D). It occurs as a matrix material, and does not follow the bedding planes in the parting samples K1-1p1, K1-1p2, and K1-2p (Fig. 13). However, kaolinite also occurs as individual lumps embedded in the illite (or mixed-layer I/S) in sample K1-1f (Fig. 14A) and shows distinct bedding plane layering in sample K1-1r (Figs. 4A,B; 14B,C). Mixed-layer I/S (illite) in K1-2p and K1-2r contains embedded albite and calcite, respectively (Figs. 15, 16). 4.4.2.2. Quartz. Quartz in the coal occurs as cell- (Figs. 12B, 17A) and fracture-fillings (Figs. 3B, 17B, C–F) and as discrete particles in collodetrinite (Fig. 3A, B). In the partings, K1-1p and K1-2p, it occurs as irregular massive and elongated columnar forms (Fig. 13B,E,F). In the roof sample (K1-1r) quartz occurs as a cavity-filling of the corroded calcite (Fig. 16D) and as fracture-fillings (Fig. 16E). 4.4.2.3. Pyrite. Pyrite occurs as discrete particles (Fig. 3A,C) and cellfillings (Fig. 3F) in the coal, but is mainly present as framboidal pyrite (Fig. 4B), discrete anhedral crystals (Fig. 4A), and needle-like (Fig. 4D) forms of probable bacterial origin in the parting (K1-1r). In some cases, euhedral and fracture-filling pyrite in the coal has been subjected

10

100 1

10

100 1000 0

10

20 0

200

400

50

100

150

200

250

300 Fig. 6. Vertical variations of V, Cr, Co, Ni, Se, and Hg through the Lvshuidong coal section. The vertical lines indicate values for world hard coals (full lines) reported by Ketris and Yudovich (2009) and world clays (dotted lines; Grigoriev, 2009).

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117

Fig. 7. Vertical variations of Li, Ga, Zr, Nb, Cd, Sn, La, Yb, Th, and U through the seam section. The vertical lines represent values for world hard coals (full lines) reported by Ketris and Yudovich (2009) and world clays (dotted lines; Grigoriev, 2009).

to oxidization and is altered to jarosite or water-bearing Fe-oxysulfates (Fig. 18A,B); this is the reason for the high proportion of sulfate sulfur in sample K1-1c. In the tonsteins pyrite occurs as anhedral crystals, and, in some cases, has been substituted for anatase and subjected to corrosion.

4.4.2.4. Calcite. Calcite in the coal occurs as fracture- and cell-fillings (Fig. 19A), indicating an epigenetic origin. However, in the roof sample K1-2r, calcite occurs as irregular lumps embedded in the mixed-layer I/S (illite) (Fig. 16A–D), or as fine-grained particles (b 0.5 μm) distributed in quartz (Fig. 16F)

4.4.2.5. REE-bearing minerals. Although they are at concentrations below the detection limit of the XRD and Siroquant analysis, a number of REEbearing minerals, including rhabdophane, silicorhabdophane, and florencite, were observed by SEM-EDS analysis in the coals of the present study. Rhabdophane and silicorhabdophane are distributed along the bedding planes (Figs. 17C,D, E; 20A,B), as cell-fillings coexisting with kaolinite (Fig. 12C) and quartz (Fig. 17E,F), and as fracture (?)fillings (Fig. 18B). Silicorhabdophane in the tonstein samples, K1-1p and K1-2p, occurs on the surface of the secondary anatase (Figs. 13A, 15C). Florencite in tonstein K1-1p is embedded in the kaolinite matrix (Figs. 13E, 14C). SEM-EDS data show that the silicorhabdophane

(A)

(B)

(C)

Fig. 8. Concentration coefficients (CC) of trace elements in the Lvshuidong partings and host rocks, normalized by average trace element concentrations in world clays (Grigoriev, 2009).

118

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Fig. 9. Variations of Nb/Ta, Zr/Hf, Yb/La, and U/Th through the seam section. The vertical lines represents the values for world hard coals.

contains Zr (2.1–12.6 wt.% with an average of 5.1 wt.%, based on seven test spots), but the rhabdophane does not contain this element.

matrix as a colloidal form (Figs. 13A,B; 15C,D); in the roof and floor samples it occurs as discrete particles (Figs. 14A, 16C).

4.4.2.6. Zircon and anatase. Zircon occurs as cell-fillings coexisting with both kaolinite (Fig. 12B) and calcite (Fig. 19B). Anatase in the tonsteins either fills the fractures (Fig. 13C,D) or is distributed in the kaolinite

4.4.2.7. Albite, K-feldspar, and fluorapatite. In sample K1-2p albite occurs as discrete particles and has been subjected to corrosion (Fig. 15A, B). It fills in the cavities of the corroded calcite in sample K1-2r (Fig. 16B).

(A)

(B)

500

Se (µg/g)

Hg (ng/g)

16

r = 0.92

400 300 200

r = 0.75

12 8 4

100

0

0 0

5

10

15

20

25

0

5

10

Pyrite + Marcasite (%)

15

20

25

Pyrite + Marcasite (%)

(C)

(D)

500 r = 0.58

Se (µg/g)

Hg (ng/g)

16

r = 0.90

400 300 200

12 8 4

100 0

0 0

5

10

Pyrite (%)

15

20

0

5

10

Pyrite (%)

Fig. 10. Relations of Hg–pyrite + marcasite (A), Se–pyrite + marcasite (B), Hg–pyrite (C), and Se–pyrite (D).

15

20

S. Dai et al. / International Journal of Coal Geology 122 (2014) 110–128

(A)

(B) 16

3

Sample / UCC

Sample / UCC

4

2

1

12

0

8

4

0 La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu K1-1a

K1-1b

K1-1c

La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu

K1-2a

(C)

K1-2b

(D)

12

7 6

Sample / UCC

10

Sample / UCC

119

8 6 4 2

5 4 3 2 1

0

0 La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu K1-2p

K1-1p2

La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu K1-2r

K1-1p1

K1-1r

K1-1f

Tuff

Fig. 11. Distribution patterns of rare earth elements and Y (REY) in the coal bench (A and B), parting (C), and floor and roof samples (D) from Lvshuidong Mine. REY are normalized by Upper Continental Crust (Taylor and McLennan, 1985).

Additionally, traces of K-feldspar and fluorapatite fill in the cavities of corroded calcite in sample K1-2r (Fig. 16B,C). 5. Discussion Ren et al. (2006) and Dai et al. (2012) have discussed seven factors that control the enrichment of elements and minerals in Chinese coals: the sediment-source rocks, low-temperature hydrothermal fluids, volcanic ash, marine environments, magmatic fluids, submarine exhalation, and groundwater. The first three factors are likely to be responsible for the geochemical and mineralogical anomalies of the

Lvshuidong coals. As described above, the assemblage and vertical variation of minerals and high-concentration elements in the coal benches, partings, roofs and floors are quite different, indicating different sources of these inorganic materials.

5.1. Input from sediment-source region Unlike other Late Permian coals from southwestern China, for which the dominant source-region is the basaltic Kangdian Upland (Fig. 1B) (China National Administration of Coal Geology, 1996; Zhang, 1993)

Table 4 LTA yields of coal samples and mineral compositions (%) of coal LTAs, partings, roofs, and floors determined by XRD and Siroquant. Sample LTA Quartz Kaolinite Illite Mixed layer I/S Pyrite Marcasite Jarosite Calcite Bassanite Anhydrite Anatase Alunogen Gypsum Albite Siderite Dolomite Zircon Talc Rutile Florencite

K1-2r 10.9 25.5 30.8 17.8 2.4 0.7 6.8 0.3

K1-2b 34.20 29.7 5.0 6.5 20.6 1.0 4.0 0.3 24.4 2.5

1.6 0.4 2.9

K1-2p

K1-2a

6.6 17.0 11.3 55.6 1.8

12.95 9.5 26.9 10.5 10.7 8.2

2.1 0.2 0.4 1.0

4.9

2.1 2.0

0.2 16.4 14.4 1.1 2.2

K1-1r 2.2 39.5 5.7 27.8 8.2 5.7 5.2

3.0 2.7

K1-1c

K1-1p2

K1-1b

43.70 10.5 27.8 13.2 25.7 10.0

3.6 43.5 5.2 35.3 2.0

17.20 14.4 14.9 15.5 22.1 11.7

5.2

4.6

0.2 0.7 1.6 5.0

1.2

1.0 1.6 5.8 11.9 1.1

K1-1p1

K1-1a

5.2 51.4 14.9 22.5 3.8

34.85 14.7 36.9 8.6 20.0 12.5

0.3 0.7

1.3

5.0 0.4 0.3 1.6

K1-1f

Tuff

3.7 46.6

0.1 65.0

42.0 2.7

1.9

0.7 1.5 3.5

2.3

4.7 26.2

0.6 0.4

LTA, low temperature ash; I/S, illite–smectite.

2.8 0.3 0.7

120

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Fig. 12. SEM back scattering images of minerals in sample K1-1c. (A), cell-filling kaolinite; (B) cell-filling kaolinite, quartz, and zircon; (C), cell-filling kaolinite, anatase, and rhabdophane; (D), fracture-filling kaolinite and jarosite.

and which are thus generally enriched in Cr, Co, Ni, and Cu (Dai et al., 2013a; Wang et al., 2012; Zhuang et al., 2012), the Lvshuidong coals are not significantly enriched in these transition elements but instead are enriched in lithophile elements, including Zr, Nb, Hf, Li, Be, Ga, REY, In, Sn, Ta, Th, and U. This indicates that the sediment-source region for the Lvshuidong coals was probably not the Kangdian Upland, although previous studies have shown that the Kangdian Upland basalt controlled the input of terrigenous materials to the coals of the present study (China National Administration of Coal Geology, 1996; Zhang, 1993; Zhuang et al., 2012). The distinct negative Eu anomaly and H-REY type found in the Lvshuidong coals (Fig. 11A,B) are different to characteristics found in many Late Permian coals from southwestern China, which are generally characterized by weak or no Eu anomalies and by L- and M-REY enrichment types (Dai et al., 2011). The kaolinite in sample K1-1r is distributed along bedding layers and along the pyrite edges (Fig. 14B,C), indicating that both the kaolinite and pyrite are of syngenetic origin, and that the kaolinite was also deposited from detrital materials of terrigenous origin. The distorted bedding layers of the kaolinite (Fig. 14C) suggest strong hydrodynamic conditions or compaction features. The mode of occurrence of calcite in sample K1-2r (Fig. 16A–D), not filling in the cavities or fractures but occurring as individual particles in the illite (or mixed-layer I/S) matrix, indicates a terrigenous material of detrital origin. Synsedimentary calcite of detrital region is seldom observed in coal because calcite can be easily-decomposed under the acid conditions of a peat bog. However, the deposition of roof strata did not necessarily occur in acid

conditions, and thus detrital calcite of terrigenous origin could have been preserved. The similar REY distribution patterns of samples K1-1r and K1-2r (negative Eu anomalies and L-REY enrichment type; Fig. 11D) indicate that the two roof materials were derived from the same sediment source-region, but that they differ from the sedimentary roof strata of many Late Permian coals in southwestern China, which have dominant characteristics derived from the Kangdian Upland source-region and very weak Eu anomalies (Dai et al., 2014). The three uplands/uplifts (TUUs), including the Hannan Upland, and the Dabashan, and Leshan–Longnvsi Uplifts (Fig. 1B), were the dominant sediment-source regions during peat accumulation in the Late Permian, not only for the coal benches, but also for the roof materials (samples K1-1r and K1-2r). The detrital calcite in roof sample K1-2r was probably also derived from the TUUs. However, the high concentrations of Cr, Co, Ni, and Cu in the host rocks (K1-1r and K1-2r) show that, in addition to the TUUs, a proportion of terrigenous materials were also derived from the Kangdian Upland.

5.2. Volcanic ashes of alkali rhyolites Tonstein deposits derived from air-borne material of pyroclastic origin in the peat-forming environment have been widely developed in the coal seams of southwestern China. Four types of tonsteins have been identified in the coals of southwestern China, including felsic (Burger et al., 2002; Zhou et al., 1982, 2000), alkali (Dai et al., 2011;

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Fig. 13. SEM back scattering images of minerals in the parting K1-1p1. (A), kaolinite as matrix and secondary anatase, silicorhabdophane, and florencite. (B), quartz, kaolinite as matrix, and secondary anatase. (C) and (D), kaolinite and fracture-filling anatase. (E) and (F), kaolinite as matrix, quartz.

Zhao et al., 2013; Zhou, 1999; Zhou et al., 2000), mafic (Dai et al., 2011), and dacitic (Dai et al., 2014) tonsteins. The three partings (K1-1p1, K1-1p2, and K1-2p) in the present study appear to have been derived from volcanic ash of alkali rhyolites, and are thus identified as intra-seam tonsteins, based on the following evidence: (1) Like tonsteins in other coals of southwestern China, the three partings have a relatively continuous lateral extent in the

Huayingshan Coalfield, although they are thin (3, 7, and 15 cm, respectively) in thickness. (2) A number of elements, including Sc, Ti, V, Cr, Co, Ni, Cu, and Zn, are not enriched in these three partings (Table 3), in contrast to the host rocks (tuff, K1-1f, K1-1r, and K1-2r) described in this study, mafic tonsteins and tuffs described by Dai et al. (2011), and other epiclastic sedimentary clays in southwestern China. This indicates that the three partings were neither derived from the Kangdian Upland sediment-source region nor from mafic

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(3)

(4)

(5)

(6)

volcanic ashes; otherwise, they would be expected to be enriched in these elements. Gallium and the high field strength elements (HFSEs), including Nb, Ta, Zr, Hf, Th, and rare earth elements (lanthanides plus Y), are significantly enriched in the three tonsteins of this study. These elements also highly enriched in the alkali tonsteins from southwestern China reported by Zhou et al. (2000), Dai et al. (2011), and Zhao et al. (2013). This further indicates that the three tonsteins in the Lvshuidong mine were derived from volcanic ashes with an alkali composition. However, the concentrations of these elements are much lower in mafic, felsic, and dacitic tonsteins (Dai et al., 2011, 2014; Zhou et al., 2000). The albites in sample K1-2p, some with sharp edges (Fig. 15A, lower left), occurring as phenocrysts in the mixed-layer I/S matrix rather than as fracture-/cavity-fillings, were derived from an alkali rhyolite. However, in addition to syngenetic albites of pyrogenic origin present in this study and in other cases (e.g., Brownfield et al., 2005), albites of terrigenous (Bouška et al., 2000; Dai et al., 2013a; Moore and Esmaeili, 2012; Ruppert et al., 1991; Ward, 2002) and authigenic origin (Golab et al., 2006; Zhao et al., 2012) have been reported as well. Unlike felsic volcanic ashes in coal that have been widely reported and often contain sanidine, high-temperature quartz, biotite, or psudedomorphs of these diagnostic minerals (Burger et al., 2002; Dai et al., 2014; Spears, 2012; Ward, 2002; Zhou et al., 2000), the phenocrysts in alkali rhyolite often include albite (e.g., identified in the partings of this study), sodian sanidine, or anorthoclase. However, high-temperature quartz and biotite do not necessarily exist in alkali rhyolite. Sodian sanidine or anorthoclase was not observed in the tonsteins of this study, probably either because these phases did not exist in the volcanic ashes or because they were altered by hydrothermal fluids, in the same way as the albite partly altered as shown in Fig. 15A,B. The REY distribution patterns of the three partings are characterized by strong negative Eu anomalies and an L-REY enrichment type. This is not only different from the sedimentary host rocks derived from the Kangdian Upland source-region, which are characterized by weak Eu anomalies (He et al., 2010), but also completely different from both the high-Ti and low-Ti alkali basalts of the Kangdian Upland itself, which are characterized by an Eu maximum and an M-REY enrichment type (cf. Xiao et al., 2004). However, the REY distribution patterns of the three partings are similar to those of some Chinese alkali granites (Zhao and Zhou, 1994), and to those of some alkali tonsteins reported in the Songzao Coalfield (Dai et al., 2011). Relative to the tonsteins derived from alkaline rhyolites present in this study, the felsic and mafic tonsteins have weaker negative Eu anomalies (Dai et al., 2011) and the tuff is characterized by a weak Eu anomaly (Fig. 11). Zhou et al. (2000) and Dai et al. (2011) suggested that tonsteins formed early in the Late Permian were mainly of alkali composition, and the tonsteins formed in the middle to late stages of the Late Permian were of felsic origin. The K1 coal in the present study is the lowermost coal seam of the Late Permian strata. Moreover, the Lvshuidong mine is within the area of alkali tonstein distribution outlined by Zhou (1999).

The floor (sample K1-1f) is a tuffaceous clay, as evidenced by the high concentrations of Cr, Co, Ni, Cu, and the similar REY distribution pattern with that of the tuff (Fig. 11D). The REY distribution patterns of samples K1-1f and tuff are both similar to those of the high-Ti and low-Ti basalts of the Kangdian Upland (Xiao et al., 2004). Fig. 14. SEM back scattering images of minerals in samples K1-1f and K1-1r. (A) Kaolinite lumps and discrete particles of pyrite and anatase in sample K1-1f; (B) kaolinite in sample K1-1r showing bedding planes and distributing along the framboidal pyrite edge, indicating that both kaolinite and pyrite are of syngenetic origin. (C) Kaolinite in K1-1r occurs as distorted bedding planes.

5.3. Multi-stage injection of hydrothermal fluids Previous studies (Dai et al., 2012, 2013a, 2013b; Ren et al., 2006; Wang et al., 2012; Zhang et al., 2004; Zhou and Ren, 1992) show that

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Fig. 15. SEM back scattering images of quartz, albite, mixed layer I/S, anatase, and silicorhabdophane in sample K1-2p. Si-rha, silicorhabdophane.

hydrothermal overprinting plays an important role in the local enrichment of trace elements and minerals in the Late Permian coals of southwestern China. It has also significantly influenced the mineralogical and elemental compositions not only of the coals, but also of the partings and host rocks from the Lvshuidong mine. The existence and modes of occurrence of rhabdophane and silicorhabdophane in the coal and partings indicate that these minerals were deposited from hydrothermal fluids at a syngenetic or at early diagenetic stage. Rhabdophane and silicorhabdophane are the major carriers of rare earth elements in the samples of the present study. These two minerals in coal and coal-bearing strata are generally derived from hydrothermal fluids, and are the major REE carriers in some other REY-rich coals as well (Dai et al., 2013b; Seredin and Dai, 2012). In contrast to the rhabdophane, the silicorhabdophane in the present study contains Zr, indicating that Si and Zr may have been leached from the tonsteins by ground water and then precipitated together with REYrich hydrothermal fluids to form silicorhabdophane. The same REY-rich hydrothermal fluids, without input of Si-, Zr- rich leachates from tonsteins, would be expected to form rhabdophane. Spears (2012) has demonstrated the loss of SiO2 in the transformation of ash with a rhyolitic composition to a tonstein, which was probably due to the groundwater leaching. Hower et al. (1999) showed that volcanic components in the tonstein in the Fire Clay coal bed, Eastern Kentucky, were leached by groundwater, and that authigenic minerals precipitated in the underlying coal from leachates are rich in Zr and REY. The cell-filling mode of zircon occurrence (Figs. 12B, 19B) indicates an authigenic or hydrothermal origin. However, zircon in coals and tonsteins

is generally regarded as representing detrital material of terrigenous or pyroclastic origin (Dawson et al., 2012; Spears, 2012; Ward, 2002; Zhou et al., 1994). Rare earth elements and Nb were not detected in the authigenic zircon in the present study. Studies by Finkelman (1981) and Seredin (2004) also showed that authigenic zircons are not enriched in Hf, Th, U, Y, and HREE, but volcanogenic zircons are enriched in these elements. Nb-bearing zircon was found in the Late Permian coal seams in the Songzao Coalfield, southwestern China (Zhao et al., 2013). Quartz in the present study, with cell-filling, fracture-filling and colloidal modes of occurrence (Figs. 3B, 12, 16E, 17A–F), may also have an authigenic origin, derived from at least two stages hydrothermal solutions. Authigenic quartz is common in Late Permian coals of southwestern China, either deposited from silica-bearing solutions originating from the weathering of basaltic rocks in the Kangdian Upland (e.g., Ren, 1996) or derived from hydrothermal solutions (Dai et al., 2014). However, the authigenic quartz in the coals in present study was not likely to have originated from the weathering of basaltic rocks, because the sediment-source region for the coals was the TUUs rather than the Kangdian Upland as described above. A small proportion of the anatase, occurring as discrete particles, was of detrital origin; however, anatase largely occurs as fracture-fillings (Fig. 13C,D), and as colloidal forms in the partings (Figs. 13A,B; 15C, D), indicating an epigenetic hydrothermal origin. Anatase in sample K1-2p, with a colloidal form, has different layers (Fig. 15A,B), further indicating authigenic deposition from solutions. SEM-EDS data show that the anatase is of authigenic origin and contains Nb and Zr. Additionally, the cell-filling calcite in the coal (Fig. 19) indicates an epigenetic and calcium-rich solution origin. The corrosion

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Fig. 16. SEM back scattering images of Illite (mixed-layer I/S), calcite, pyrite, quartz, K-feldspar, albite and anatase in sample K1-2r. Ca, calcite; Py, pyrite, Q, quartz; K-F, K-feldspar; Alb-albite; Ana, anatase; Flap, fluorapatite.

of calcite in the roof (K1-2r; Fig. 16 B–E), and of the albite in parting K1-2p (Fig. 15A,B), may also have been caused by hydrothermal solutions. 6. Preliminary evaluation of rare metals recovery As rare earth elements and yttrium (REY) are crucial metals for alternative power and energy-efficient technologies, and coal deposits are regarded as promising targets for recovery of REY as economic by-

products of coal mining and combustion (Seredin and Dai, 2012; Seredin et al., 2013), it is important to evaluate these potential resources during coal exploitation and utilization studies. High REY concentrations (N0.1% in ash) have only been found in a couple of tens of coal deposits worldwide (Seredin et al., 2013). The average REY concentration in the Lvshuidong coal ash is 1423 μg/g (or 0.17% REY oxides, REO); this is much higher than the typical REY cut-off-grade (0.1% REO) in coal combustion wastes for byproduct recovery (Seredin and Dai, 2012).

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Fig. 17. SEM back scattering images of minerals in sample K1-2b. (A) Quartz fills in the distorted cell cavities; (B) fracture-filling quartz and calcite; (C)–(E) quartz in the fractures and rhabdophane distributing along bedding planes; (E) and (F), cell-filling rhabdophane and cell-/fracture-filling quartz.

A REYdef, rel–Coutl graph is proposed for comparative evaluation of high-REY ash in terms of potential industrial value (Fig. 21; Seredin and Dai, 2012). The y-axis is the percentage of critical elements (Nd, Eu, Tb, Dy, Y, and Er) in the total REY (REYdef, rel) and the x-axis represents the outlook coefficient, which is the ratio of the relative amount of critical REY metals in the REY sum to the relative amount of excessive REY (Ce, Ho, Tm, Yb, and Lu). High-REY coal ashes can be grouped into three areas (Fig. 21). The Lvshuidong

coal ashes fall mostly in the area II and thus can be regarded as promising REY raw materials. Additionally, based on the Chinese industry standards (Specifications for Rare Metal Mineral Exploration, DZ/T 0203–2002, 2002), the required (Nb,Ta)2O5 concentrations for marginal and industrial grade Nb(Ta) ore deposits of weathered crust type are 80–100 μg/g and 160–200 μg/g, respectively; equivalent concentrations are 40–60 μg/g and 100–120 μg/g for Nb(Ta) ore deposits of river placer type. The concentration of (Nb,

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Fig. 18. SEM back scattering images of jarosite (A), Fe(Si)-oxysulfate and rhabdophane (B) in sample K1-1c.

Fig. 19. SEM back scattering images of calcite, kaolinite, and zircon in sample K1-2b.

Ta)2O5 in the Lvshuiding coals (Table 5) is therefore much higher than that of marginal and industrial grade weathered crust and river placer deposit types. Additionally, the concentrations of (Zr, Hf)O2 in the Lvshuidong coals are also up to the standards for industrial utilization of

Coast Sand Deposit types (1600–2400 μg/g) and of marginal grade for Weathered Crust Deposit types (3000 μg/g) (DZ/T 0203–2002, 2002). The sum of rare metal oxides including (Zr,Hf)2O5, (Nb,Ta)2O5, and REO, which can reach up to 3.985% and has an average value of 0.573%

Fig. 20. SEM back scattering images of silicorhabdophane in sample K1-1c. The gray background in (B) is kaolinite.

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rhabdophane, silicorhabdophane, zircon, and anatase, in the coal, host rocks, and partings were predominantly derived from hydrothermal fluids. Three tonstein layers identified in the coal are deduced to have been derived from alkali rhyolite. These tonsteins are characterized by highly-enriched HFSEs and by strong negative Eu anomalies in their rare earth element distribution patterns. Acknowledgments This research was supported by the National Key Basic Research Program of China (no. 2014CB238902), the National Natural Science Foundation of China (nos. 41272182 and 40930420), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT 13099). Many thanks are given to Mr. Michael Trippi from USGS who helped with generation of the GIS image for the coalfield distribution in China. References

Fig. 21. The REYdef, rel–Coutl plot for Lvshuidong coal ashes, partings, and host rocks. Area I, unpromising, Area II, promising, and Area III, highly promising.

Table 5 Concentration of rare metal oxides (μg/g unless indicated as %), estimation parameters (REYdef and Outl) in coal ash. Sample

(Zr,Hf)2O5

(Nb,Ta)2O5

REO

Sum-RM (%)

REYdef (%)

Outl

K1-2r K1-2b K1-2p K1-2a K1-1r K1-1c K1-1p2 K1-1b K1-1p1 K1-1a K1-1f Tuff WA

752 30,358 2589 1966 1951 6231 2569 2224 2378 2390 2109 1363 3617

117 2990 433 228 301 830 448 177 618 235 314 190 406

614 6499 1972 1618 861 1640 2724 1843 2289 1111 1231 1124 1710

0.148 3.985 0.499 0.381 0.311 0.870 0.574 0.424 0.529 0.374 0.365 0.268 0.573

26.8 35.7 30.6 34.6 29.5 33.4 26.0 33.2 28.2 38.1 33.1 34.4 34.6

0.61 0.97 0.73 0.88 0.73 0.91 0.59 0.84 0.69 1.03 0.85 0.92 0.90

Sum-RM, sum of (Zr,Hf)2O5, (Nb,Ta)2O5, and REO. REYdef, the percentage of critical elements (Nd, Eu, Tb, Dy, Y, and Er) in total REY; Outl, outlook efficient, the ratio of the relative amount of critical REY metals in the REY sum to the relative amount of excessive REY (Ce, Ho, Tm, Yb, and Lu).

(Table 5), should be a subject for further attention due to the potential economic significance of these elements.

7. Conclusions The dominant sediment-source regions for the coal and roof strata of the Huayingshan Coalfield are the Dabashan Uplift, the Hannan Upland, and the Leshan–Longnvsi Uplift, which surrounded the coal basin during peat accumulation, although a small amount of terrigenous material was also derived from the Kangdian Upland. The Late Permian coals of the Huayingshan Coalfield are significantly enriched in Zr, Nb, Se, Hf, rare earth elements, and Y. The rare earth elements mainly occur in rhabdophane and silicorhabdophane, with the latter also being enriched in Zr. Zirconium mainly occurs in zircon, but a small proportion may occur in authigenic anatase. Mercury and Se mainly occur in both pyrite and marcasite. The highly-elevated concentrations of HFSEs in the coals are due to hydrothermal fluids. For example, the HFSE-bearing minerals, including

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