Mineralogical and geochemical compositions of the Pennsylvanian coal in the Hailiushu Mine, Daqingshan Coalfield, Inner Mongolia, China: Implications of sediment-source region and acid hydrothermal solutions

International Journal of Coal Geology 137 (2015) 92–110

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Mineralogical and geochemical compositions of the Pennsylvanian coal in the Hailiushu Mine, Daqingshan Coalfield, Inner Mongolia, China: Implications of sediment-source region and acid hydrothermal solutions Shifeng Dai a,⁎, Tianjiao Li a, Yaofa Jiang b, Colin R. Ward c, James C. Hower d, Jihua Sun a, Jingjing Liu a, Hongjian Song a, Jianpeng Wei a, Qingqian Li a, Panpan Xie a, Qing Huang a a

State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing 100083, China Jiangsu Institute of Architectural Technology, Xuzhou 221116, China 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 b

a r t i c l e

i n f o

Article history: Received 19 September 2014 Received in revised form 24 November 2014 Accepted 26 November 2014 Available online 3 December 2014 Keywords: Mineral in coal Trace element in coal Sediment-source region Acid hydrothermal solution Daqingshan Coalfield

a b s t r a c t This paper investigates the mineralogical and geochemical compositions of the Pennsylvanian coal in the Hailiushu Mine, Daqingshan Coalfield, neighboring previously-reported Al (Ga, REE) ore deposits (including the Adaohai Mine in the same coalfield and deposits in the Jungar Coalfield), using optical microscopy, field emission-scanning electron microscopy, X-ray fluorescence, and inductively coupled plasma mass spectrometry. The mineralogical and geochemical compositions in the coal were primarily controlled by the sediment-source region during peat accumulation, and by epigenetic acid hydrothermal solutions. The Hailiushu coal was deposited in a sub-depression (intermontane basin) in the inner part of the orogenic belt, with a sediment-source region composed mainly of Cambrian–Ordovician strata and Archaean metamorphic rocks. The minerals in the coal from the Hailiushu Mine dominantly consist of kaolinite, with minor amounts of quartz, sulfide and selenide minerals (including chalcopyrite, selenian galena, galena, sphalerite, clausthalite, and siegenite), aluminophosphates, and rhabdophane. The coal is enriched in SiO2 (17.05% on average), TiO2 (0.60%), Al2O3 (13.71%), Zr (289 μg/g), Hf (7.09 μg/g), and to a lesser extent, F, Sc, V, Cu, Ga, Se, Y, Nb, Mo, Cd, Sn, La, Ta, W, Hg, Pb, Bi, and Th. Titanium largely occurs in the kaolinite. Elements such as Cu, Se, Sn, Hg, Pb, and Bi in the coal mainly occur in sulfide and/or selenide minerals. Zirconium, Hf, and Nb were largely derived from the sediment source region. The substitution of Ti for Al in kaolinite, the corrosion of previously-formed zircon, anatase, and quartz, and the enrichment of middle rare earth elements in the coal were caused by the epigenetic acid hydrothermal solutions. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Trace elements and minerals in the coals of North China, particularly in the Jungar, Daqingshan (Fig. 1A), and Shengli Coalfields, have attracted much attention, because these areas are associated with elevated concentrations of rare metals in the coals. For example, the early Cretaceous Wulantuga high-Ge coal deposit in the Shengli Coalfield, Inner Mongolia, is one of the major coal-hosted Ge deposits in the world and is currently being used as a raw material for Ge recovery (Dai et al., 2012b; Du et al., 2009; Qi et al., 2007; Zhuang et al., 2006). A number of studies have shown that the coals of the Jungar Coalfield are highly enriched in Ga, Al, rare earth elements and Y (REY, or REE if

⁎ Corresponding author at: State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, D11, Xueyuan Road, Haidian District, Beijing 100083, China. Tel./fax: +86 10 62341868. E-mail address: [email protected] (S. Dai).

http://dx.doi.org/10.1016/j.coal.2014.11.010 0166-5162/© 2014 Elsevier B.V. All rights reserved.

Y is not included) (Dai et al., 2012a; Seredin, 2012; Wang et al., 2011; Wu et al., 2009; Zhang and Wang, 2009; Zou et al., 2012), and the coal has therefore been considered as a coal-hosted Al (Ga, REY) ore deposit (Dai et al., 2012a). As with the Jungar Coalfield, the coals from the Adaohai Mine in the Daqingshan Coalfield (Fig. 1B), are also enriched in elements including Al, Ga, and REE; in addition, minerals such as diaspore, boehmite, and gorceixite are present in the coal (Dai et al., 2012c; Zou et al., 2012). Understanding the concentrations, modes of occurrence, and origins of trace elements and minerals in the coals neighboring the above raremetal ore deposits is significant from both academic and practical perspectives, not only because it can provide further evidence on the formation mechanisms for these rare-metal ore deposits, but also because it can enhance the economic value of the coals if they are enriched in rare metal components. The purpose of this paper is to investigate the mineralogical and geochemical compositions of the coals from the Hailiushu mine (located in the southwestern Daqingshan Coalfield, which also has the Adaohai Mine in the southeast, as reported by Dai

S. Dai et al. / International Journal of Coal Geology 137 (2015) 92–110

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Fig. 1. Paleogeographic map of the Late Paleozoic in North China and location of the Daqingshan and Jungar Coalfields (A), and coal-rank distribution in the different mines of the Daqingshan coalfield (B). (A), modified from Han and Yang (1980); (B), after Dai et al. (2012c).

et al., 2012c; Fig. 1B) and to understand the geological factors controlling their formation.

2. Geological setting The Daqingshan Coalfield is located in Inner Mongolia of northern China (Fig. 1A), covering the area between latitudes 40°35′ and 40°44′ N and longitudes 110°07′ and 110°31′ E. It includes 16 mines distributed from west to east (Fig. 1B), with the Hailiushu Surface Mine and Adaohai Underground Mine being located respectively in the southwest and southeast of the coalfield (Fig. 1B). The strata in the Daqingshan Coalfield include the Archaean Wulanshan Group, Sinian System, Cambrian–Ordovician strata, the Pennsylvanian coal-bearing Shuanmazhuang Formation, and the Permian Zahuaigou and Shiyewan Formations. More details of the coal-bearing sedimentary sequences of the Daqingshan Coalfield, as well as a column section of the coal-bearing strata, have been presented by Dai et al. (2012c).

The Pennsylvanian Shuanmazhuang Formation and the Early Permian Zahuaigou Formation, which were deposited in a continental environment, are the coal-bearing sequences in the Daqingshan Coalfield (Jia and Wu, 1995; Qimu, 1980; Wang and Ge, 2007; Zhang et al., 2000a; Zhong et al., 1995; Zhou and Jia, 2000). The major coalbed (Cu2 coal) in the Hailiushu Mine of the Daqingshan Coalfield is located in the upper portion of the Shuanmazhuang Formation. The thickness of the coalbed in the whole coalfield varies from 4.72 to 42.79 m and averages 22.58 m. The Cu2 coal is called the CP coal in the Adaohai Mine (Dai et al., 2012c). The major coalbed in the coalfield contains 3 to 42 parting layers, with thickness from 0.02 to 3.4 m. As a result of epigenetic tectonic movements, the coal in most coal mines of the coalfield was strongly brecciated and has a dip of more than 50°. The Shuanmazhuang Formation is overlain by Permian strata, which consist of the Zahuaigou and Shiyewan Formations. The upper portion of the Zahuaigou Formation is composed of mudstone and sandstone, and the lower portion is made up of white quartz–pebble conglomerate and locally intercalated mudstone beds (Dai et al., 2012c). The

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Table 1 Bench thickness (cm), proximate and ultimate analyses (%), and random vitrinite reflectance (%) of the Cu2 coal in the Hailiushu Mine. Sample

Thickness

Mad

Ad

Vdaf

St,d

Cdaf

Hdaf

Ndaf

Rr

HLS-18-R HLS-17 HLS-16 HLS-15 HLS-14-P HLS-13-P HLS-12-P HLS-11-P HLS-10 HLS-9-P HLS-8 HLS-7-P HLS-6 HLS-5-P HLS-4 HLS-3 HLS-2-P HLS-1 HLS-0-F Av Coal Adaohai**

N50 30 40 40 30 23 40 40 40 16 45 10 30 10 35 50 15 40 N50 350* 2800*

0.60 0.82 1.08 0.96 0.62 0.59 0.52 0.53 1.07 0.41 1.09 0.66 1.06 0.43 0.84 0.97 1.02 0.82 nd 0.97 0.38

83.05 50.42 12.67 33.43 76.36 80.74 67.40 87.69 32.57 85.22 30.65 85.43 26.00 83.79 30.81 39.06 85.47 38.35 89.20 32.53 25.1

nd 55.09 41.34 40.65 nd nd nd nd 43.32 nd 43.08 nd 43.88 nd 42.15 48.12 nd 45.55 nd 44.64 21.65

0.09 0.35 0.55 0.39 0.02 0.02 0.25 0.77 0.44 0.03 0.57 0.03 0.87 0.04 0.57 0.55 0.05 0.60 0.04 0.54 0.78

nd 73.98 84.87 82.67 nd nd nd nd 80.29 nd 80.89 nd 82.64 nd 81.94 78.94 nd 79.08 nd 80.66 86.79

nd 6.44 5.63 5.57 nd nd nd nd 5.76 nd 5.59 nd 5.77 nd 5.50 5.85 nd 5.83 nd 5.76 4.46

nd 1.20 1.37 1.26 nd nd nd nd 1.24 nd 1.25 nd 1.41 nd 1.40 1.26 nd 1.29 nd 1.29 1.48

nd 0.83 0.81 0.90 nd nd nd nd 0.8 nd 0.83 nd 0.85 nd 0.83 0.85 nd 0.89 nd 0.84 1.58

M, moisture; A, ash yield; V, volatile matter; C, carbon; H, hydrogen; N, nitrogen; St, total sulfur; ad, air-dry basis; d, dry basis; daf, dry and ash-free basis; Rr, random reflectance of vitrinite; Av, weighted average (weighted by thickness of sample interval); *, total thickness of coal benches; nd, not detected. **, data from Dai et al. (2012c).

Shiyewan Formation consists mainly of thick layers of sandstone, interbedded with mudstone (Dai et al., 2012c). The Cambrian–Ordovician strata underlying the coal-bearing sequences are dominated by limestone and are intercalated with silty mudstone in the lower portion; the uppermost portion is composed of calcareous shales. The Sinian System, with a thickness varying from 5 to 15 m (7.03 m on average), consists mainly of thick white quartzite and gravel-bearing quartz sandstone. The Cambrian System has an unconformable contact with the underlying Sinian System. The Archaean Wulanshan Group is mainly composed of red compound gneiss, and to a lesser extent, gray garnet–gneiss, dark-green amphibole–gneiss, and white quartzite. Pegmatitic and aplitic granite veins are generally observed in the gneiss and quartzite. The thickness of the Wulanshan Group is unknown. 3. Samples and analytical procedures A total of 19 bench samples were taken from the face of the Cu2 coal at the Hailiushu Mine; these include nine coal benches, eight partings, and one roof and one floor samples. Each coal bench sample was cut over a volume 10-cm wide and 10-cm deep. From bottom to top, the

nine coal bench samples and the eight partings (with a suffix -P) are identified as HLS-1 to HLS-18 (Table 1). The floor and roof samples are numbered as HLS-0-F and HLS-18-R, respectively. The cumulative thickness of the Cu2 coal in this section is 5.34 m, of which partings amount to 34.5% (total thickness of partings is 1.84 m). Proximate analysis was conducted following ASTM Standards D3173-11, D3175-11, and D3174-11 (2011). Total sulfur was determined following ASTM Standard D3177-02 (2002). An elemental analyzer (Vario MACRO) was used to determine the percentages of C, H, and N in the coals. Coarse-crushed samples of each coal were prepared as grain mounts for microscopic analysis by reflected light following ASTM Standard D2797/D2797M-11a (2011). Mean random reflectance of vitrinite (percent Rr) was determined using a Leica DM-4500P microscope equipped with a Craic QDI 302™ spectrophotometer. The maceral classification and terminology used in the study are based on Taylor et al. (1998) and the ICCP System 1994 (ICCP, 1998, 2001). A field emission-scanning electron microscope (FE-SEM, FEI Quanta™ 650 FEG), in conjunction with an energy-dispersive X-ray spectrometer (EDAX; Genesis Apex 4), was used to study the morphology of the minerals, and also to determine the distribution of some elements. The mineral composition was determined by X-ray powder diffraction. Low-temperature ashing (LTA) of the coal samples was performed using an EMITECH K1050X plasma asher, prior to XRD analysis. XRD analysis of the low-temperature ashes, and also of the noncoal samples, was performed on a powder diffractometer (D/max2500/PC XRD) with Ni-filtered Cu-Kα radiation and a scintillation detector. X-ray diffractograms of the LTAs and non-coal samples were subjected to quantitative mineralogical analysis using the Siroquant™ interpretation software system. X-ray fluorescence (XRF) spectrometry (ARL ADVANT'XP +) was used to determine the major element oxides for each sample ash (815 °C). The ash samples were prepared by borate fusion in an automated fusion furnace (CLAISSE TheBee-10). Each ash sample (1 g) was mixed and homogenized with lithium borate flux (10 g; CLAISSE, pure, 50% Li2B4O7 + 50% LiBO2). The mixture was melted in a Pt– Au crucible (25 ml; 95% Pt + 5% Au) and after fusion the melt was cast to a Pt–Au mold flat disk. The chemical composition of the (high-temperature) coal ash expected to be derived from the mineral assemblage indicated by the XRD and Siroquant analyses of each coal or rock sample was also calculated, using methodology described by Ward et al. (1999). XRF data for the samples were recalculated to provide normalized percentages of the major element oxides in the inorganic fraction (loss-on-ignition free basis). The inferred percentages of major element oxides in the coals and rocks, as calculated from the XRD data, were then compared to the normalized percentages of the same oxides in the SO3-free ash as calculated from the geochemical data obtained by separate XRF analysis. Inductively coupled plasma mass spectrometry (X series II ICP-MS), in pulse counting mode, was used to determine trace elements in

Table 2 Maceral contents determined under the optical microscope for coals from the Hailiushu Mine (vol.%; on mineral-free basis). Sample HLS-17 HLS-16 HLS-15 HLS-10 HLS-8 HLS-6 HLS-4 HLS-3 HLS-1 Av Adaohai*

T

CT

CD

CG

VD

T-V

SF

F

41.0 60.1 49.8 43.9 40.4 48.8 28.2 42.7 42.9 44.3 28.9

0.6 0.3 1.6 0.8 2.3

3.1 0.0 0.6 6.8 1.8 3.6 1.5 0.4 2.4 2.1 1.3

95.7 81.3 71.7 80.2 84.2 76.6 44.2 69.0 60.7 73.4 64.7

1.2 1.8 10.6 3.0 2.9 4.4 12.1 7.8 7.9 5.8 12.5

0.6

0.4 27.5

50.9 20.9 19.3 27.4 39.8 24.2 12.6 25.9 15.1 26.0 6.9

0.3 1.3

1.9

0.4 0.7 Tr

0.3 0.4 1.2 0.5 0.9 1.2 0.6 6.3

Mic

Mac

ID

T-I

Sp

Cu

T-L

0.6 0.6 0.4 0.6

2.1 3.4 1.7 0.6 2.0 8.7 5.2 7.5 3.6 8.7

1.2 5.2 8.7 6.3 6.4 5.2 13.1 6.0 7.1 6.7 7.6

3.1 9.8 23.7 11.8 11.7 11.7 34.5 20.3 23.8 17.0 35.3

4.6 1.9 0.4 0.6 1.6 2.4 2.2 2.0 1.8

1.2 4.3 2.8 7.6 3.5 10.1 18.9 8.6 13.5 7.8

1.2 8.9 4.7 8.0 4.1 11.7 21.4 10.8 15.5 9.5

0.4 0.3 Tr

T, telinite; CT, collotelinite; CD, collodetrinite; CG, corpogelinite; VD, vitrodetrinite; T-V, total vitrinites; SF, semifusinite; F, fusinite; Mic, micrinite; Mac, macrinite; ID, inertodetrinite; T-I, total inertinites; Sp, sporinite; Cu, cutinite; T-L, total liptinites. Av, weighted average based on thickness of sample interval. *, data from Dai et al. (2012c).

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Fig. 2. Macerals in the Hailiushu coals. (A), Semifusinite with cell-filling clay minerals in sample HLS-4; (B), fusinite and clay minerals in collodetrinite in sample HLS-4; (C), shards/ fragments of fusinite or secretinite in sample HSL-1; (D), fusinite and semifusinite in sample HLS-4; (E), thick-walled cutinite in sample HLS-4; (F), sporinite, inertodetrinite, and collodetrinite in sample HLS-4. Sf, semifusinite; F, fusinite; Mac, macrinite; CD, collodetrinite; Sp, sporinite; ID, inertodetrinite; Sh, shards/fragments of fusinite or secretinite.

Table 3 Mineralogical compositions of coal LTA samples, partings, roof and floor strata samples by XRD and Siroquant (wt.%), as well as minor minerals identified by SEM-EDS. Sample

LTA/HTA yield (%)

Quartz

Kaolinite

Illite

Pyrite

HLS-18-R HLS-17 HLS-16 HLS-15 HLS-14-P HLS-13-P HLS-12-P HLS-11-P HLS-10 HLS-9-P HLS-8 HLS-7-P HLS-6 HLS-5-P HLS-4 HLS-3 HLS-2-P HLS-1 HLS-0-F

82.55b 59.52 15.34 37.78 75.89b 80.26b 67.05b 87.23b 39.16 84.87b 35.15 84.87b 29.91 83.43b 38.72 48.22 84.6b 46.8 85.73b

0.9 0.3 0.3 0.2 0.3 0.2 0.2 23.9 0.5 0.6 1.7 0.4 1.5 1.8 9.6 2.1 0.4 1.7 8

97.3 98.5 96.5 97.4 98.9 99.6 99.1 73.5 98.2 99.4 96.2 99.5 97.1 97.6 85 96.9 99.4 94.6 87.8

1.3 1 1

0.5 0.2 0.1 0.2 0.1 0.2 0.3 1.3 0.5

0.7 0.4 0.3 0.3 1.5 0.3 5.1 0.5 0.1 2.7 3.8

0.4 0.1 1.1 0.6 0.4 0.5 0.1 0.5

Calcite

Siderite

Goyazite

1.8 2.2

Bassanite

0.2

1.0 0.4 0.2

0.4 0.3

Minora nd nd nd nd Alu Sph, Alu, zircon Anatase, selenian galena Anatase, Rha, zircon, gypsum, Sph, Flo nd Sph, florencite nd Galena nd Sph, chalcopyrite, anatase Rha, Clau, zircon, pyrite, Sph nd Sph Pyrite, anatase, flo, siegenite, Sph, Rha, (ZnFe0.5Cu0.5)S2 nd

LTA, low temperature ash. HTA, high temperature ash. a, Minerals found by SEM-EDS but below detection limit of XRD. b, HTA. Sph, sphalerite; Alu, (Sr, Ba, Ca)-aluminophosphate. Rha, rhabdophane. Flo, florencite. Clau, clausthalite. nd, not detected by SEM-EDS.

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samples, except for Hg and F. For ICP-MS analysis, samples were digested using an UltraClave Microwave High Pressure Reactor (Milestone). The digestion reagents for each 50-mg coal sample were 5-ml 65% HNO3 and 2-ml 40% HF. The reagents for each 50-mg noncoal sample were 2-ml 65% HNO3 and 5-ml 40% HF. The GuaranteedReagent HNO3 and HF for sample digestion were further purified by sub-boiling distillation. Multi-element standards (Inorganic Ventures: CCS-1, CCS-4, CCS-5, and CCS-6) were used for calibration of trace element concentrations. Arsenic and Se were determined by ICP-MS, using collision cell technology (CCT) in order to avoid disturbance of polyatomic ions (see Li et al., 2014). For boron determination, addition of H3PO4 to the HNO3 and HF was used in the sample digestion process to reduce boron volatilization during acid-drying after sample

digestion. A 2% ammonia solution was injected into the ICP-MS spray chamber to eliminate the memory effect of boron (Dai et al., 2014a). Mercury was determined using a Milestone DMA-80 Hg analyzer. Fluorine was determined by pyrohydrolysis in conjunction with an ion-selective electrode, following ASTM Method D 5987-96 (2002). 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 (lightREY; 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).

Fig. 3. SEM backscattered images of kaolinite, quartz, silicorhabdophane in the Hailiushu coals. (A), massive and fracture-filling kaolinite; fracture-filling quartz; sample HLS-1. (B), cellfilling kaolinite and silicorhabdophane. (C), massive kaolinite and rounded quartz. (D), irregular quartz particles in collodetrinite and silicorhabdophane in kaolinite. (E), silicorhabdophane in kaolinite; veinlet-filling kaolinite; quartz in collodetrinite. (F), silicorhabdophane in kaolinite; kaolinite distributing along bedding planes. (B)–(F), sample HLS-4. Kaol, kaolinite; Qua, quartz; Rhab, silicorhabdophane.

S. Dai et al. / International Journal of Coal Geology 137 (2015) 92–110

4. Results 4.1. Coal chemistry and vitrinite reflectance The volatile matter (44.64% on average) and vitrinite random reflectance (0.84% on average) indicate a high volatile bituminous A coal according to the ASTM classification (ASTM D388-12, 2012). Ash yield varies from 12.67 to 50.42%, with an average of 32.53%, and overall the coal is classified as high-ash coal according to Chinese Standard GB 15224.1-2004 (coals with ash yield N 29% are high-ash coal). The coal is characterized by a low sulfur content (0.54% on average) and, thus, is classified as a low-sulfur coal (Chou, 2012).

4.2. Maceral content of the CP2 coal The total proportion of vitrinite-group macerals (73.4% on average, mineral-free) in the Hailiushu coals is much higher than that of the inertinite-group (17.0% on average; Table 2). This is quite different to the relative proportions found in other Late Paleozoic coals in the northern part of North China (Fig. 1A), and to that in coals from the Jungar Coalfield, located immediately to the south of the Daqingshan Coalfield (Fig. 1A), both of which usually have slightly higher proportions of inertinite than vitrinite (Dai et al., 2012a; Han et al., 1996). The maceral compositions of the Hailiushu coals are also different to those of the Adaohai coals, which typically have 64.7% vitrinite and 35.3% inertinite (on a mineral-free basis) (Dai et al., 2012c). The vitrinite in the Hailiushu coals is mainly composed of collodetrinite and collotelinite, along with traces of vitrodetrinite, corpogelinite, and telinite (Table 2). The inertinite in the coals is dominated by semifusinite, inertodetrinite, macrinite and, to a lesser extent, macrinite, with trace amounts of fusinite and micrinite. The liptinite is mainly represented by sporinite and cutinite. Fusinite and semifusinite are represented by forms indicative of wood oxidation (Figs. 2A–D). Generally, the cell structures of the semifusinite and fusinite are not well-preserved (Figs. 2B–D), and in some cases have a swelled and degraded form (Fig. 2B), suggesting

97

that they were on a path to degradation, possibly to macrinite, before oxidation (Hower et al., 2013; O'Keefe et al., 2013). Semifusinite and fusinite cells are generally filled by clay minerals (Fig. 2A). In some cases, shards or fragments of fusinite or secretinite are observed (Fig. 2D). Cutinite is represented by thick-walled cutinite in collodetrinite (Fig. 2E) and sporinite is distributed along the bedding planes (Fig. 2F). Collodetrinite occurs as a matrix with embedded clay minerals, sporinite, and inertodetrinite (Figs. 2B, F).

4.3. Minerals 4.3.1. Minerals in coal The proportion of each crystalline phase identified from the diffractograms of the coal LTAs, partings, and floor and roof samples is given in Table 3. The major phase identified in the coal LTA samples is kaolinite (85.0–98.5%, with a weighted average of 95.7%), along with traces of quartz (1.9% on weighted average), pyrite (0.4%), and illite (1.3%). Minor amounts of calcite, goyazite, and bassanite were detected in a few coal benches (Table 3). Traces of florencite, zircon, rhabdophane, clausthalite, siegenite, sphalerite, (ZnFe0.5Cu0.5)S2, and anatase were identified by SEM-EDS (Table 3), but are below the detection limit of the XRD techniques. Kaolinite in the coals occurs as disseminated fine particles, lensshaped, thin-layered, and massive forms (Fig. 3) along the bedding planes, as fracture- and cell-fillings (Figs. 3A, B), and, in a few cases, as vermicular forms (Figs. 4A, B). Similar modes of kaolinite occurrence are common in many other coals and closely associated strata, and may indicate formation by authigenic processes (Ward, 1989). Quartz occurs as both rounded and irregular discrete particles in collodetrinite (Figs. 3C, D, E), suggesting detrital materials of terrigenous origin. In a few cases, quartz occurs as fracture-fillings (Fig. 3A). Silicorhabdophane is mainly distributed in the kaolinite matrix (Figs. 3D, E, F), and in some cases, fills in the cells of fusinite (Fig. 3B). SEM study shows that it typically contains a trace of Ca (~0.67–0.70%). Silicorhabdophane and rhabdophane are commonly observed in REYrich coals and are considered to be of hydrothermal origin (Seredin

Fig. 4. Minerals observed under optical microscope (reflected light). (A), vermicular kaolinite in sample HLS-15; (B), vermicular kaolinite in sample HLS-8; (C), fracture-filling pyrite in sample HLS-1; (D), pyrite in the broken fusinite cells of sample HLS-4.

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Table 4 Contents of major-element oxides (%) and trace elements (μg/g) in the coal benches, partings, and roof and floor strata of the Hailiushu Mine. Thickness (cm)

SiO2

TiO2

Al2O3

Fe2O3

MgO

CaO

MnO

Na2O

K2O

P2O5

SiO2/Al2O3

LOI

Li

Be

B

F

Sc

V

Cr

Co

Ni

Cu

Zn

N50 30 40 40 30 23 40 40 40 16 45 10 30 10 35 50 15 40 N50 350* 184*

44.32 26.55 6.47 17.18 39.45 41.94 35.9 57.15 17.11 45.75 15.92 45.88 13.77 43.07 17.30 20.23 43.92 19.89 48.74 17.05 44.3 8.47 2.01

1.60 0.55 0.27 0.76 1.09 0.42 0.99 1.33 0.60 0.64 0.49 0.55 0.27 1.19 0.86 0.63 0.85 0.93 0.99 0.60 0.95 0.33 1.82

35.58 22.51 5.50 14.51 33.16 35.23 29.74 26.46 14.29 38.05 12.76 38.04 11.07 35.04 11.90 16.10 36.47 15.60 34.73 13.71 32.28 5.98 2.29

0.52 0.13 0.04 0.14 0.13 0.10 0.12 1.77 0.08 0.15 0.09 0.07 0.40 0.11 0.16 0.32 0.15 0.28 0.59 0.18 0.48 4.85 0.04

bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 0.01 0.02 0.01 bdl bdl 0.02 bdl 0.03 0.05 bdl bdl 0.22 na

0.112 0.051 0.110 0.237 0.083 0.055 0.048 0.051 0.040 0.044 0.045 0.068 0.038 0.046 0.037 0.038 0.056 0.139 0.066 0.083 0.057 1.23 0.07

0.003 0.001 0.001 0.003 0.003 bdl bdl 0.016 0.001 bdl 0.001 bdl 0.001 bdl bdl 0.002 0.002 0.002 bdl 0.001 0.004 0.015 0.07

bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 0.16 na

0.110 0.077 0.008 0.015 0.040 0.048 0.063 0.140 0.018 0.033 0.036 0.036 0.016 0.039 0.068 0.039 0.046 0.073 0.330 0.038 0.067 0.19 0.20

0.070 0.024 0.004 0.023 0.046 0.057 0.035 0.065 0.012 0.036 0.070 0.038 0.021 0.037 0.029 0.022 0.023 0.035 0.079 0.027 0.045 0.092 0.29

1.25 1.18 1.18 1.18 1.19 1.19 1.21 2.16 1.20 1.20 1.25 1.21 1.24 1.23 1.45 1.26 1.20 1.28 1.40 1.25 1.41 1.42 na

16.95 49.58 87.33 66.57 23.64 19.26 32.60 12.31 67.43 14.78 69.35 14.57 74.00 16.21 69.19 60.94 14.53 61.65 10.80 67.47 20.17 nd na

16.1 15.7 3.36 11.0 19.5 14.4 18.4 29.6 28.3 18.7 10.9 28.7 9.46 45.6 27.1 41.3 77.7 45.6 104 22.3 27.4 14 1.59

2.33 2.48 3.79 4.04 3.26 2.72 2.64 2.38 2.34 2.70 2.91 3.90 3.36 3.94 5.51 2.85 2.87 2.30 2.81 3.26 2.86 2 1.63

3.97 0.47 0.85 10.2 3.11 1.73 3.46 4.24 bdl 1.24 7.73 3.42 0.71 bdl 5.34 0.24 2.96 8.66 11.8 3.92 2.93 47 0.08

280 262 78.2 166 363 373 320 228 198 476 200 368 169 390 162 190 330 195 244 179 335 82 2.18

12.3 9.20 5.77 7.81 14.7 9.94 12.0 15.5 5.71 5.82 5.51 4.16 4.80 11.6 14.6 7.86 5.52 10.2 13.2 7.86 11.4 3.7 2.13

112 33.8 32.4 42.8 42.6 21.5 50.0 130 72.0 13.9 62.6 5.91 49.6 64.2 131 87.7 32.8 177 92.6 77.9 56.4 28 2.78

41.6 7.05 10.1 8.85 11.3 2.65 6.94 60.4 9.44 0.86 7.93 0.54 8.93 9.95 44.3 23.5 5.54 42.8 50.1 18.3 17.9 17 1.08

9.81 2.87 6.77 4.79 1.04 0.67 1.24 17.7 4.43 0.28 4.09 0.44 6.39 0.99 4.81 10.2 1.23 11.3 7.10 6.37 4.57 6 1.06

19.5 5.88 7.98 8.91 5.44 2.37 3.42 37.3 6.82 0.20 6.48 0.86 9.70 2.45 8.61 21.6 3.88 30.6 25.5 12.3 10.5 17 0.72

29.1 14.7 15.4 15.6 17.4 6.24 14.2 32.1 20.7 7.75 18.4 4.06 27.5 32.7 69.5 43.1 10.2 78.5 37.5 34.0 17.2 16 2.12

113 18.4 49.1 38.4 9.13 110 179 117 38.8 64.8 30.9 99.4 53.6 82.9 86.0 51.8 58.2 132 50.7 55.6 99.8 28 1.99

Sample

Ga

Ge

As

Se

Rb

Sr

Zr

Nb

Mo

Cd

In

Sn

Sb

Cs

Ba

Hf

Ta

W

Hg

Tl

Pb

Bi

Th

U

HLS-18-R HLS-17 HLS-16 HLS-15 HLS-14-P HLS-13-P HLS-12-P HLS-11-P HLS-10 HLS-9-P HLS-8 HLS-7-P HLS-6 HLS-5-P HLS-4 HLS-3 HLS-2-P HLS-1 HLS-0-F Av Coal Av Partings World CC

38.5 21.4 19.8 16.0 40.3 40.4 35.5 31.3 13.4 37.2 18.2 26.1 19.8 38.2 21.1 22.6 39.0 21.1 28.2 19.2 36.1 6 3.21

1.68 1.30 1.51 1.66 0.83 0.85 1.05 1.34 1.57 0.95 2.80 1.33 2.82 0.76 1.52 2.29 1.17 2.22 1.79 1.99 1.05 2.4 0.83

1.76 0.41 0.16 0.20 0.20 0.31 0.53 2.06 0.47 0.33 1.26 0.22 4.15 0.49 0.41 1.44 0.07 0.69 0.15 0.97 0.71 8.3 0.12

0.80 1.32 1.40 1.13 1.36 0.93 0.84 1.04 2.22 0.42 1.82 0.64 2.54 1.61 7.45 3.72 0.64 6.16 0.51 3.09 0.96 1.3 2.38

4.18 3.85 0.36 0.62 1.71 2.04 3.67 3.51 0.74 1.96 1.79 2.10 0.82 1.14 4.62 1.78 2.28 2.95 12.13 1.88 2.63 18 0.1

29.6 4.78 1.79 13.7 15.0 17.0 9.77 15.5 2.58 1.60 58.1 7.78 6.30 2.99 10.9 10.2 5.19 21.6 44.0 15.5 11.2 100 0.15

581 317 152 362 490 339 422 456 213 347 321 411 475 538 373 241 275 217 200 289 417 36 8.02

29.6 24.6 7.13 20.0 27.7 19.9 23.7 16.6 8.11 46.2 16.0 46.1 26.1 40.4 15.9 11.5 19.6 8.05 12.6 14.6 26.0 4 3.65

2.35 3.48 3.78 2.87 1.09 0.97 2.67 1.47 3.90 1.54 11.8 1.24 13.0 2.60 6.10 7.26 0.64 8.40 0.22 6.74 1.59 2.1 3.21

1.02 0.44 0.23 0.42 0.51 0.67 0.67 0.70 0.26 0.54 0.41 0.72 0.61 0.69 0.77 0.44 0.55 0.65 0.31 0.46 0.63 0.2 2.3

0.18 0.08 0.05 0.07 0.17 0.11 0.10 0.10 0.05 0.06 0.07 0.08 0.08 0.15 0.14 0.06 0.08 0.09 0.07 0.08 0.11 0.04 1.88

10.2 4.48 3.52 3.10 5.15 6.01 3.91 2.63 1.44 3.80 2.37 6.97 3.74 7.13 3.95 2.71 10.0 7.50 2.17 3.57 4.93 1.4 2.55

0.26 0.20 0.34 0.34 0.10 0.19 0.54 0.12 0.24 0.10 0.35 0.07 0.33 0.06 0.21 0.78 0.26 0.60 0.05 0.40 0.22 1 0.4

0.49 0.57 0.06 0.12 0.32 0.40 0.64 0.43 0.14 0.40 0.22 0.36 0.12 0.21 0.53 0.29 0.41 0.41 1.46 0.27 0.43 1.1 0.24

49.5 18.5 6.27 32.4 62.4 69.5 28.4 38.7 6.68 0.70 104 16.8 9.53 2.14 36.0 19.7 15.3 36.2 58.2 31.5 35.8 150 0.21

15.0 9.03 4.10 8.75 14.2 10.8 11.2 11.5 5.15 12.6 7.41 15.0 11.7 16.8 8.51 6.16 8.89 5.01 5.14 7.09 12.1 1.2 5.91

1.81 1.80 0.32 1.27 1.86 1.32 1.48 0.54 0.47 3.37 0.95 3.30 1.59 2.51 1.04 0.81 1.65 0.28 0.59 0.90 1.65 0.3 3

25.1 1.37 0.9 6.45 4.05 4.75 4.56 91.7 0.83 2.48 1.12 4.26 0.60 7.11 1.21 14.4 2.75 0.51 14.6 3.48 23.2 0.99 3.52

229 127 220 798 32 112 221 219 150 854 173 149 382 174 463 559 90 433 57 375 214 100 3.75

0.25 0.10 0.03 0.08 0.04 0.06 0.11 0.41 0.10 0.07 0.11 0.05 0.33 0.08 0.16 0.38 0.03 0.21 0.15 0.17 0.14 0.58 0.29

33.5 39.3 19.9 31.6 24.8 54.0 41.3 23.6 12.8 51.0 22.2 53.8 31.1 35.0 76.1 23.0 10.7 30.6 14.9 30.6 35.0 9 3.4

0.73 0.31 10.7 0.15 bdl 0.99 0.33 0.01 bdl 0.08 bdl 0.07 0.70 0.11 0.42 0.70 20.1 21.1 bdl 3.88 1.86 1.1 3.53

15.5 6.36 6.11 3.20 23.4 12.2 37.9 10.9 4.46 42.6 7.44 5.78 17.5 18.6 12.8 7.58 3.80 3.23 5.51 7.31 21.3 3.2 2.28

2.64 2.83 2.98 2.26 5.70 3.83 8.57 1.34 1.07 8.67 3.35 4.34 3.25 4.73 2.92 3.02 1.64 0.99 1.05 2.51 4.94 1.9 1.32

LOI, loss on ignition. Av, weighted average based on thickness of coal bench interval. *, total thickness. **, world hard coal, data from Ketris and Yudovich (2009). CC, concentration coefficient, ration of studied samples vs. Chinese or world hard coals. bdl, below detection limit. na, not available.

S. Dai et al. / International Journal of Coal Geology 137 (2015) 92–110

Sample HLS-18-R HLS-17 HLS-16 HLS-15 HLS-14-P HLS-13-P HLS-12-P HLS-11-P HLS-10 HLS-9-P HLS-8 HLS-7-P HLS-6 HLS-5-P HLS-4 HLS-3 HLS-2-P HLS-1 HLS-0-F Av Coal Av Partings World** CC

S. Dai et al. / International Journal of Coal Geology 137 (2015) 92–110

and Dai, 2012). Pyrite mainly occurs as fracture- and cell-fillings (Figs. 4C, D). Other minor sulfides, including siegenite, sphalerite, and (ZnFe0.5Cu0.5)S2, are distributed in collodetrinite (Figs. 5A, B, C) or occur as cell-fillings (Fig. 5C). The siegenite ((Co,Ni)3S4) contains traces of Fe and Pb, and the (ZnFe0.5Cu0.5)S2 phase contains a trace of Pb. Minor amounts of Cu and Fe are also detected in the related mineral clausthalite (PbSe), which has a similar mode of occurrence. Zircon occurs as discrete particles and has a well-rounded shape, indicating it to be a detrital material of terrigenous origin (Fig. 5D). Two modes of occurrence were observed for anatase, discrete particles in collodetrinite (Fig. 5E) of terrigenous origin and cell-fillings (Fig. 5F) of authigenic origin. 4.3.2. Minerals in partings, roof, and floor strata With the exceptions two samples with high quartz percentages (8% in floor sample HLS-0-F, 23.9% in parting sample HLS-11-P), the

99

partings, roof, and floor samples are mainly composed of kaolinite with traces of quartz, illite, and pyrite (Table 3), representing a similar assemblage to that in the coal LTAs. Quartz in partings HSL-5-P and HSL-11-P shows sharp edges (Figs. 6) and, in some cases, contains inclusions (Fig. 6D). The matrix kaolinite in the two partings, which do not show sedimentary bedding planes, contains cavities with sharp edges (Figs. 6A–D). The modes of quartz occurrence and the sharped edges of the cavities indicate that the two partings were derived from air-borne material of pyroclastic origin. The sharp-edged cavities (Figs. 6A–D) were probably the remains of decomposition of crystal fragments in volcanic ash. Such partings of pyroclastic origin, deposited in the peat-forming environment, have also been found in some coal seams from other areas and are called “tonsteins” (Burger et al., 1990; Guerra-Sommer et al., 2008; Lyons et al., 2006; Yudovich and Ketris, 2002; Zhao et al., 2013; Zhou et al., 2000). Zhang et al. (2000a,b), Zhou et al. (2001), and Wang and Ge

Fig. 5. SEM backscattered images of sulfide minerals, zircon, anatase, and kaolinite in the Hailiushu coals. (A) siegenite and veinlet-filling kaolinite, sample HLS-1; (B) clausthalite in collodetrinite, sample HLS-4; (C) sphalerite, (ZnFe0.5Cu0.5)S2 and anatase in sample HLS-1; (D) zircon in collodetrinite, sample HLS-4; (E) anatase in collodetrinite, sample HLS-4; (F) cell-filling anatase, sample HLS-1. Kaol, kaolinite; Spha, sphalerite; Qua, quartz.

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Fig. 6. SEM backscattered images of quartz, kaolinite, zircon, Ti-oxide, and florencite in the partings. (A), kaolinite matrix and cavities derived from the completely corroded crystal fragment. (B), quartz with sharp edges and cavities derived from the completely corroded crystal fragment. (C), quartz, florencite, and cavities. (D), quartz with inclusions, zircon, discrete anatase, and pyrite. (E), Ti-oxide with sharp shape and kaolinite matrix. (F), hydrothermally-corroded zircon, discrete anatase, authigenic quartz, and kaolinite. (A) and (B), sample HLS-5-P. (C)–(F), sample HLS-11-P.

(2007) also found intra-seam kaolinite layers derived from felsic to intermediate volcanic ash in the coal from this coalfield. The cavities derived from corrosion of zircon and quartz are filled by the authigenic quartz (Fig. 6F). Ti-oxide (probably anatase) occurs as discrete particles showing sharp angular forms (Figs. 6E, F), which were probably due to corrosion by hydrothermal solutions. In the other partings, neither quartz with sharp edges and inclusions nor sharp-edged cavities were observed (Fig. 7). The zircon particles in these partings were found to be broken (Fig. 7B) or well-rounded (Fig. 7C), possibly due to transportation from the sediment source region to the peat swamp. These partings are thought to be epiclastic sedimentary layers, although no distinct bedding planes were observed. The lack of layering could be a consequence of humic acid (from the peat) altering/leaching the clay material (cf. Staub and Cohen, 1978).

Particularly, the tonstein HLS-11-P was not directly overlain by a coal bench but by normal sedimentary clay partings, which is not the same as for tonsteins found in other areas (Burger et al., 1990; Guerra-Sommer et al., 2008; Lyons et al., 2006; Zhao et al., 2013; Zhou et al., 2000). This indicates that the terrigenous materials (corresponding to partings HLS-12-P, HLS-13-P, and HLS-14-P) were input into the basin immediately after the volcanic ash eruption (corresponding to tonstein HSL-11-P). Minor occurrences of some additional minerals, with abundance below the detection limit for the XRD system, were observed in the partings by SEM-EDS. These include sulfide minerals such as galena, selenian galena (Fig. 7A), pyrite (Fig. 6D), sphalerite (Fig. 7B), Pbcontaining sphalerite (composed of 20.6% Pb, 24.1% S, and 55.3% Zn), chalcopyrite, Pb-containing chalcopyrite; REE-bearing minerals such

S. Dai et al. / International Journal of Coal Geology 137 (2015) 92–110

101

Fig. 7. SEM backscattered images and selected EDS data of minor minerals in partings. (A), selenian galena in the kaolinite matrix in sample HLS-12-P. (B), zircon, (Sr, Ba, Ca, Ce)aluminophosphate, and sphalerite in sample HLS-13-P. (C), discrete zircon in sample HLS-7-P. (D), (Sr, Ba, Ca, Ce)-aluminophosphate in the kaolinite in sample HLS-13-P. (E) and (F), EDS data for selenian galena and aluminophosphate. Alu-pho, (Sr, Ba, Ca, Ce)-aluminophosphate.

as florencite (Fig. 6C), rhabdophane, (Sr, Ba, Ca, Ce)-aluminophosphate (Figs. 7B, D); and anatase (Table 3). 4.4. Major-element oxides and trace elements in coal 4.4.1. Major-element oxides and their comparison to mineralogical compositions Table 4 lists the concentrations of major-element oxides and trace elements in the samples from the Hailiushu Mine. Compared to average values for Chinese coals (Table 4; Dai et al., 2012b), SiO2, TiO2, and Al2O3 are enriched in the Hailiushu coal; the remaining major-element oxides, including Fe2O3, MgO, CaO, MnO, Na2O, K2O, and P2O5, are at much lower concentrations. The LTA and HTA yields for the coal samples in the present study are close to the equality line but the former is a bit higher than the latter (Fig. 8A), due to dehydration of the clay minerals and oxidation of the sulfides during the high-temperature ashing process. Figs. 8B–F show the relationship of the percentages of major element oxides (SiO2, Al2O3, Fe2O3, CaO, K2O; all on SO3-free basis) inferred from the XRD analysis to the percentages of the same oxides (normalized, SO3-free) derived from XRF analysis data. The points in the plots of each major element oxides, even considering the low proportions of most nonkaolinite components (Table 3) and the associated errors associated with the Siroquant determinations, are very close to the equality line, suggesting that the inferred percentages of these elements from the XRD results are compatible with the chemical analyses and adding confidence to the quantifications derived from the XRD data. The inferred Al2O3 is a little higher than the observed values (Fig. 8C), possibly because a large proportion of Ti has been substituted for Al in the kaolinite as described below. As well as the relationship in

Fig. 8F, the positive correlation coefficients between K and ash yield (r = 0.7), K–SiO2 (r = 0.74), and K–Al2O3 (r = 0.64), suggest that K in the Hailiushu coals mainly occurs in illite. The average concentration of TiO2 in the Hailiushu coal is 0.6% (Table 4), much higher than the average for Chinese coals (0.33%, Dai et al., 2012b). The correlation coefficient of TiO2 with ash yield (r = 0.51) for the coal benches indicates that Ti largely occurs in the mineral phases. However, no crystalline Ti-minerals (e.g., anatase, rutile, ilmenite) were identified in either the coal LTAs or the host rocks (partings, roof, and floor strata) by the XRD analyses, although minor anatase was identified in the coal benches and partings by SEM-EDS (Figs. 5E, 6D, E). Experience with other coals suggests that the detection limit for anatase/rutile by XRD and Siroquant is ~0.1%; in the absence of such phases, the high proportion of TiO2 in the coal LTAs (1.56% on average) and the host rocks suggests that a large proportion of the Ti may not occur in Ti minerals (e.g., anatase), but in some other forms. The correlation coefficients of TiO2–Al2O3 (r = 0.42), TiO2–SiO2 (r = 0.53), and TiO2–K2O (r = 0.61) suggest that Ti largely occurs in kaolinite and a small proportion may also occur in illite, although the illite percentages in both coal and host rocks are very low (Table 3). Two modes of Ti occurrence were observed by SEM in the kaolinite in the coals and partings: (1) Ti closely associated with kaolinite but not occurring as discrete particles (Figs. 9A, B, C), and (2) Ti as discrete minerals, probably anatase (Figs. 9D, 10A, 11). Titanium is thus thought to both substitute for Al in the crystal lattice of the kaolinite (and also probably of illite) and occur as fine-grained anatase within the kaolinite. The first mode of Ti occurrence in kaolinite in the Hailiushu coals is similar to that observed for coals in the Gunnedah Basin (Australia), where around 1.5% Ti appears to be present in the “pure” kaolinite phases (Ward et al., 1999), probably as a combination of Ti replacement for Al in the

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S. Dai et al. / International Journal of Coal Geology 137 (2015) 92–110

(A)

SiO2

70

60

Inferred Oxide in Ash (%)

Low temperature ash yield (%)

(B)

Relaon between HTA and LTA

70

50 40 30 20 10

65

60

55

50

0 0

10

20

30

40

50

60

50

70

55

(C) 45

40

35

30

70

2.0 1.5 1.0 0.5 0.0

30

35

40

45

50

0.0

0.5

Observed Oxide in Ash (%)

(E)

1.0

1.5

2.0

2.5

0.8

1.0

Observed Oxide in Ash (%)

(F)

CaO

K2O

1.0

Inferred Oxide in Ash (%)

2.0

Inferred Oxide in Ash (%)

65

Fe2O3

2.5

Inferred Oxide in Ash (%)

Inferred Oxide in Ash (%)

(D)

Al2O3

50

60

Observed Oxide in Ash (%)

High temperature ash yield (%)

1.5

1.0

0.5

0.8 0.6 0.4 0.2 0.0

0.0 0.0

0.5

1.0

1.5

2.0

Observed Oxide in Ash (%)

0.0

0.2

0.4

0.6

Observed Oxide in Ash (%)

Fig. 8. Comparison of HTA to LTA, and observed normalized oxide percentages from chemical analysis (x-axis) to oxide percentages for sample ash inferred from XRD data (y-axis). The diagonal line in each plot indicates equality.

crystal lattice and a separate Ti-bearing phase (e.g. anatase) along the kaolinite cleavages. Shoval et al. (2008) also showed that titanium occurs in kaolinites both as a substitute for aluminum in the octahedral sheet of the kaolinite and as the free titania mineral anatase. The study by Dolcater et al. (1970) showed that in Ti-containing kaolinite, 86% of the total Ti was in the TiO2 form, primarily as anatase, or anatase with smaller amounts of rutile. It is probable that the acidic hydrothermal solutions were favorable not only for decomposition of previouslyformed Ti minerals but also for the subsequent substitution of Ti for Al in the developing kaolinite crystals. 4.4.2. Trace elements Compared to average values for world hard coals reported by Ketris and Yudovich (2009), Zr and Hf are enriched (5 b CC b 10); many other elements, F, Sc, V, Cu, Ga, Se, Y, Nb, Mo, Cd, Sn, La, Ta, W, Hg, Pb, Bi, and Th, are slightly enriched (2 b CC b 5); some elements, including B, As, Rb, Sr, Sb, Cs, Ba, and Tl, are depleted (CC b 0.5); and the remaining elements (0.5 b CC b 2) are close to the average values for world hard coals (Fig. 11; Tables 4, 5).

The high correlation coefficients, e.g., rZr-Hf = 0.97, rZr-Nb = 0.88, rZr-Th = 0.71, and rZr-Ta = 0.81, suggest that these lithophile trace elements in the coals (Hf, Nb, Th, and Ta) mainly occur in zircon. In addition, minor amounts of Zr and Y were also detected in cellfilling authigenic anatase in sample HLS-1. Authigenic rhabdophane contains minor Th (0.72–2.08%; sample HLS-1). Also, elements Zr, Nb, and Th, as well as the REY in samples HLS-8 to HLS-18R, show similar variations through the seam section (Fig. 12), indicating the same carriers of these elements. The similar variation of F and ash yield through the coal benches (rF-ash = 0.93, Fig. 12) and the high correlation coefficients for F-SiO2 (r = 0.93) and F-Al2O3 (r = 0.93) suggest that the F mainly occurs in the kaolinite. Although both Ga and F are highly enriched in the partings relative to the coal benches (Fig. 12), Ga is weakly correlated to ash yield (rGa-ash = 0.22), indicating a mixed organic–inorganic affinity for Ga in the coals. Elements Bi and Sn in the coals are positively correlated to Ni, Cu, Zn (e.g., rBi-Cu = 0.47; rBi-Ni = 0.68; rBi-Zn = 0.74; rSn-Cu = 0.65; rSn-Ni = 0.66; rSn-Zn = 0.76) and the correlation coefficient for Bi to Sn is high

S. Dai et al. / International Journal of Coal Geology 137 (2015) 92–110

103

Fig. 9. SEM backscattered images and selected EDS data of kaolinite and Ti-bearing kaolinite. (A) and (B), vermicular Ti-bearing kaolinite and matrix kaolinite. (C) kaolinite and Ti-bearing kaolinite. (D) Ti-bearing kaolinite fills in the cavities of kaolinite. (E) and (F), EDS data for Ti-kaolinite and pure kaolinite. (A), sample HLS-2-P. (B)–(D), sample HLS-9-P.

(r = 0.81), suggesting that Bi and Sn occur mainly in sulfide minerals, such as the chalcopyrite, sphalerite, and siegenite found in the coals. Selenium and Pb mainly occur in clausthalite, selenian galena, and to a lesser extent, galena, where Se could substitute for S in the lattice. Galena has also been found in the coals from the adjacent Jungar Coalfield (Dai et al., 2006; Li and Zhao, 2007). Overall, Cu, Se, Hg, Pb, and total S show similar patterns of variation through the seam section (Fig. 13), suggesting a sulfide and/or selenide affinity for the Cu, Hg, and Pb; this is also shown by the identification of clausthalite, (ZnFe0.5Cu0.5)S2, and pyrite by SEM-EDS (Fig. 5) and under the optical microscope (Fig. 4). A sulfide and/or selenide affinity for Cu, Hg, and Pb has also been reported in a number of studies (e.g., Hower and Robertson, 2003; Kolker, 2012; Mastalerz and Drobniak, 2007; Yudovich and Ketris, 2006). The Mo concentration is high in the coal benches but is low in the partings, roof, and floor strata (Figs. 11, 13). The correlation coefficient of Mo-ash for the coal benches is −0.13, suggesting that Mo is largely associated with the coals' organic matter. A recent study by Dai et al. (2014b) showed that Mo in a coal-hosted U–Se–Mo–Re–V ore deposit mainly shows an organic affinity. Seredin and Finkelman (2008), however, found that Mo mainly occurs as molybdenite in U-bearing coal deposits. Although concentrations of P2O5 and several trace elements (Sr, Ba, and Ce) in the present coals are much lower than the average for Chinese (Dai et al., 2012b) and world hard coals (Ketris and Yudovich, 2009), the P2O5 concentration shows a strong correlation to the latter trace elements (rP-Sr = 0.96, rP-Ba = 0.97, and rP-Ce = 0.82), suggesting that Sr, Ba, and Ce in the Hailiushu coals mainly occur as aluminophosphate minerals of the goyazite–gorceixite–crandallite– florencite group. The aluminophosphate mineral identified in the Hailiushu coals has a structure between that of gorceixite, goyazite, crandallite, and florencite. It is thus probably an intermediate phase, representing a solid solution between these four end members.

Aluminophosphate minerals were also identified in the partings (Figs. 7D,F) in the present study. Similar aluminophosphate minerals have also been reported in some Australian coals (Ward et al., 1996). The correlation coefficients of Rb–K (r = 0.95) and Cs–K (r = 0.97) in the coals indicate that Rb and Cs are intimately related to the K; additionally, both Rb and Cs have positive correlation coefficients with Si and Al (rRb-Al = 0.50, rRb-Si = 0.63; r Cs-Al = 0.65, rCs-Si = 0.75), suggesting that the Rb and Cs mainly occur in illite. Compared to average values for world clays (Grigoriev, 2009), the partings are only enriched in W; Ga, Se, Zr, Nb, Hf, Hg, Pb, and Bi in the partings are slightly enriched; the remaining elements are either close to (0.5 b CC b 2) or depleted. Selenium and Pb in the partings mainly occur in selenian galena (Fig. 7A). As with coals, zircon is the primary carrier for Zr, Hf, and Nb (Figs. 6D, 6F, 7B, 7C). The REY in the coal benches are mainly characterized by an M-type enrichment (Figs. 14A, B), with the exceptions of samples HLS-8 and HLS-16, which have L- and H-type enrichment, respectively (Fig. 14C). Additionally, the concentrations of LREE in some samples are slightly higher than those of the HREE (Fig. 14A). Samples HLS-10 and HLS-15 are more enriched in HREE than LREE (Fig. 14B). The M-type of REY enrichment in coal is generally caused by three factors, (1) acid natural waters (Johanneson and Zhou, 1997) that circulate in coal basins, including those of acid hydrothermal solutions with especially high REY concentrations (McLennan, 1989; Michard, 1989); (2) higher sorption of MREY by humic matter in comparison with LREE and HREE (Seredin and Shpirt, 1999a, b); and (3) sedimentsource regions composed of high-Ti and low-Ti alkali basalts. Because of the modes of Ti occurrence and the presence of corroded zircons (Fig. 6F), both of which were caused by acid hydrothermal solutions, the M-type of REY enrichment in the Hailiushu coal is also attributed to such epigenetic solutions. The REY in the roof and floor strata and the normal sedimentary partings (HLS-2-P, -7-P, -9-P, and -13-P) are characterized by L-type

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Fig. 10. SEM backscattered images of kaolinite and Ti-bearing kaolinite. (A) Ti-bearing kaolinite (gray part) and discrete anatase particles (whitest part). (B)–(E), Ti-bearing kaolinite in kaolinite matrix. (F), Ti-bearing kaolinite in the cavities. (A) and (E), sample HLS-12-P. (B), (C), and (D), sample HLS-14-P. (F), sample HLS-7-P.

enrichment (Fig. 15), indicating a felsic sediment-source region. This is also supported by the Al2O3/TiO2 ratio in both the coals and the host rocks, which is a useful provenance indicator of sedimentary rocks (Hayashi et al., 1997; He et al., 2010), because of the similar ratio of these elements in mudstones/sandstones to that in their parent rocks (Hayashi et al., 1997). Typical Al2O3/TiO2 ratios are 3–8, 8–21, and 21–70 for sediments derived from mafic, intermediate, and felsic igneous rocks, respectively (Hayashi et al., 1997). The Al2O3/TiO2 ratios of the roof (22.2), floor (35.1), and partings (30–83.9), as well as the coal benches (24.6 on average), further support their derivation from a felsic sediment-source region. Although samples HLS-12-P and HLS-14-P are considered as normal sedimentary clay layers, they are respectively characterized by M- and H-type enrichments. The volcanic-ash-derived partings represented by samples HLS-5-P and HLS-11-P, respectively, have M- and L-type distribution patterns (Figs. 15B, C), and sample HLS-11-P also has a Gd

anomaly (Fig. 15B), even though they were derived from felsic volcanic ashes. Both H- and M-type enrichment of REY in coals may be caused by hydrothermal solutions (Seredin and Dai, 2012). 5. Discussion Both local geological reports from the coal company and previous studies (e.g., Dai et al., 2012c; Zou et al., 2012) have shown that the Hailiushu and Adaohai Mines are both located in the Daqingshan Coalfield (Fig. 1B), and that the CP2 Coal in the Adaohai Mine and the Cu2 Coal in the present study both represent the same coal seam. The low sulfur contents in both the Hailiushu and Adaohai coals were the result of a continental environment during peat accumulation. A continental environment is also indicated by the geochemical compositions of the coal and host rocks. The boron concentrations in the coal, which are 3.92 μg/g on weighted average, are much lower

S. Dai et al. / International Journal of Coal Geology 137 (2015) 92–110

105

Hailiushu/world

(A) 10

1

0.1

0.01 Li

Be

B

F

Li

Be

F

Sc

Li

Be

B

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 W Hg Tl Pb Bi Th

U

Hailiushu/Adaohai

(B) 10

1

0.1 V

Cr

Co

Ni

Cu Zn Ga Ge Se Rb

Sr

Y

Zr Nb Mo Cd

In

Sn Sb

Cs

Ba

La

Yb

Hf

Ta

W Hg

Tl

Pb

Bi

Th

U

Zr Nb Mo Cd In Sn Sb Cs Ba La Yb Hf Ta W Hg Tl Pb Bi Th

U

Parng/world clay

(C) 10

1

0.1

0.01 Sc

V

Cr Co Ni Cu Zn Ga Ge As Se Rb Sr CC < 0.5

Y

2
0.5
5
Fig. 11. Concentration coefficient (CC) of coals and parting clays in the Hailiushu coals. (A) CC of Hailiushu coals vs. world hard coals. (B) CC of Hailiushu coals vs. Adaohai coals. (C) CC of Hailiushu partings vs. world clay. Data of Adaihai coals and world hard coals from Dai et al. (2012b) and Ketris and Yudovich (2009). World clay data from Grigoriev (2009).

than the average for world hard coals (47 μg/g; Ketris and Yudovich, 2009). Although use of boron concentration in coal as a paleosalinity indicator for the original sedimentary environment remains controversial

(Bouska and Pesek, 1983; Eskenazy et al., 1994; Lyons et al., 1989), this extraordinarily low B concentration indicates a fresh-water-influenced coal-forming sedimentary environment (Goodarzi and Swaine, 1994).

Table 5 Concentrations of rare earth elements (μg/g) in the coal benches, partings, and roof and floor strata of the Hailiushu Mine. Sample no

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Y

Ho

Er

Tm

Yb

Lu

REY

REO*

HLS-18-R HLS-17 HLS-16 HLS-15 HLS-14-P HLS-13-P HLS-12-P HLS-11-P HLS-10 HLS-9-P HLS-8 HLS-7-P HLS-6 HLS-5-P HLS-4 HLS-3 HLS-2-P HLS-1 HLS-0-F Av Coal Av Partings World coal* CC

91.2 20.7 2.48 15.1 15.3 48.7 25.1 83.8 10.1 73.0 71.0 29.7 46.9 41.0 73.6 27.2 11.8 42.2 75.7 34.1 43.4 11 3.10

185 56.4 6.80 41.9 33.9 98.6 56.6 168 25.8 166 180 39.8 113 90.4 161 64.0 25.5 97.8 140 82.5 90.3 23 3.59

21.4 6.57 0.89 4.83 3.91 9.48 6.57 19.2 2.89 15.6 15.3 3.58 11.6 9.46 16.7 6.76 2.79 9.66 16.2 8.25 9.71 3.4 2.43

74.1 26.8 4.09 19.7 14.3 31.8 25.3 67.4 12.1 46.5 48.8 11.0 41.4 33.0 60.0 26.2 9.93 34.2 58.1 29.9 33.7 12 2.49

11.0 6.02 1.18 4.15 3.62 5.13 4.98 9.57 2.48 6.63 7.75 1.93 7.22 6.03 11.5 4.86 1.7 6.06 9.91 5.56 5.54 2.2 2.53

1.79 1.29 0.28 0.85 0.88 0.98 0.98 1.58 0.55 1.02 1.36 0.38 1.06 0.86 1.72 0.92 0.37 1.34 2.02 1.02 1.01 0.43 2.38

9.72 5.49 1.64 4.11 4.81 5.49 5.16 8.95 2.60 6.55 8.09 2.87 7.34 6.26 11.5 4.78 1.95 5.90 8.84 5.60 5.76 2.7 2.08

0.98 0.72 0.36 0.60 0.85 0.75 0.81 0.98 0.36 0.73 0.83 0.44 0.88 0.94 1.53 0.59 0.26 0.69 0.87 0.71 0.78 0.31 2.29

4.34 3.55 3.02 3.5 5.38 4.27 5.01 4.78 2.03 3.75 3.84 2.74 4.57 5.77 8.30 3.05 1.36 3.33 3.57 3.81 4.44 2.1 1.82

15.8 14.1 18.8 16.6 27.4 22.5 25.9 18.0 10.6 17.2 15.3 17.4 21.3 32.4 36.0 13.9 6.31 12.7 13.2 17.3 21.6 8.4 2.06

0.67 0.58 0.67 0.62 0.98 0.77 0.94 0.76 0.38 0.61 0.61 0.54 0.79 1.07 1.38 0.51 0.22 0.51 0.54 0.66 0.78 0.57 1.15

1.64 1.61 2.28 1.82 2.77 2.15 2.71 1.89 1.17 1.70 1.68 1.60 2.24 3.09 3.80 1.46 0.58 1.31 1.34 1.89 2.17 1 1.89

0.20 0.20 0.33 0.24 0.38 0.29 0.38 0.23 0.16 0.23 0.21 0.23 0.30 0.44 0.49 0.19 0.08 0.16 0.16 0.25 0.29 0.3 0.83

1.25 1.37 2.43 1.67 2.58 1.89 2.62 1.43 1.19 1.56 1.54 1.59 2.08 2.98 3.31 1.27 0.52 1.05 1.01 1.73 1.96 1 1.73

0.16 0.18 0.35 0.22 0.36 0.25 0.35 0.17 0.16 0.20 0.20 0.23 0.29 0.42 0.44 0.18 0.07 0.13 0.13 0.23 0.26 0.2 1.17

419 146 46 116 117 233 163 387 73 341 357 114 261 234 391 156 63 217 332 194 222 69 2.82

606 346 432 416 185 346 291 530 267 480 1396 160 1203 335 1522 478 89 679 446 738 328 na na

Av, weighted average based on thickness of coal bench interval. *, World hard coal, data from Ketris and Yudovich (2009). CC, concentration coefficient, ration of studied samples vs. world hard coals. REO*, oxides of REY, on an ash basis. na, not available.

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Thickness (cm)

Benches Thinkness

Ash yield (%) 0

Zr (μg/g)

20

40

60

0

REY (μg/g)

200

400

600

0

F (μg/g)

250

500

0

Ga (μg/g) 250

500

0

Nb (μg/g) 25

50

0

Th (μg/g) 25

50

0

25

50

0

HLS-18-R HLS-17 100

HLS-16 HLS-15

200

HLS-14-P HLS-13-P HLS-12-P HLS-11-P

300

400

500

HLS-10 HLS-9-P HLS-8 HLS-7-P HLS-6 HLS-5-P HLS-4 HLS-3 HLS-2-P HLS-1

600

HLS-0-F

Parng

Coal bench

Roof & Floor

Fig. 12. Variations in ash yield, Zr, REY, F, Ga, Nb, and Th through the coal seam section.

The low concentrations of some other elements (e.g., Mg, Ca, Na, Sr, and Rb; Table 3), which are 2–4 orders-of-magnitude higher in sea water than in fresh water (Reimann and de Caritat, 1998), further indicate a continental environment of coal formation. Moreover, the paleoclimate in the Adaohai Mine (Daqingshan Coalfield) during peat accumulation was similar to that in the Jungar Coalfield, as indicated by the short distance between the two areas and their similar mineral assemblages. Based on the values of δ18O and δ13C in the Pennsylvanian limestones in the Jungar Coalfield, Liu et al. (1991) and Cheng et al. (2001) deduced that the paleotemperature of the seawater was 29–32 °C, a hot and moist climate. Chen et al. (1984), Lin (1984), and Cheng et al. (2001) have shown that the Pennsylvanian latitude of the Jungar Coalfield was about 14° north. However, the coal rank, mineralogical, and geochemical compositions of the CP2 and Cu2 Coals in the two mines of the Daqingshan Coalfield are quite different.

Thickness (cm)

Benches Thinkness

Ash yield (%) 0

20

40

St (%) 60

0

Compared to the Hailiushu coals in the present study, the Adaohai coals reported by Dai et al. (2012c) display lower volatile matter (21.65%) and a higher vitrinite random reflectance (1.58%), and, thus, are classified as low volatile bituminous coals according to ASTM D388-12 (2012). The differences in rank for the same coal seam are mainly related to igneous intrusions, which were associated with the Yanshan Movement of the Late Jurassic and Early Cretaceous Epochs (Zhong and Chen, 1988). The coal rank increases from the northwest to the southeast, from high volatile through medium volatile to low volatile bituminous (Fig. 1B), with increasing proximity to the intrusive heat source. A number of minerals, including diaspore, gorceixite, ammonian illite, and dolomite (or ankerite), which are present in the Adaohai coals (Dai et al., 2012c), were not observed in the samples from the Hailiushu Mine. Boehmite is also absent from the coals of the present study, but is highly enriched in the coals of the Jungar Coalfield (Dai

Se (μg/g) 0.5

1

0

Cu (μg/g) 5

10

0

Pb (μg/g) 50

100

0

50

Hg (ng/g) 100

0

HLS-18-R HLS-17 100

HLS-16 HLS-15

200

HLS-14-P HLS-13-P HLS-12-P HLS-11-P

300

400

500

HLS-10 HLS-9-P HLS-8 HLS-7-P HLS-6 HLS-5-P HLS-4 HLS-3 HLS-2-P HLS-1

600

HLS-0-F

Coal bench

Coal bench

Roof & Floor

Fig. 13. Variations in ash yield, total S, Se, Cu, Pb, Hg, and Mo through the coal seam section.

0

Mo (μg/g) 500

1000

0

5

10

15

S. Dai et al. / International Journal of Coal Geology 137 (2015) 92–110

(A)

3.5 3 2.5

Coal / UCC

HLS-17 2

HLS-6 HLS-4

1.5

HLS-3 1

HLS-1

0.5 0 La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu

(B)

1.2

Coal / UCC

1 0.8 HLS-15

0.6

HLS-10 0.4 0.2 0 La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu

(C)

3

Coal / UCC

2.5 2 HLS-16

1.5

HLS-8

1 0.5 0 La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu Fig. 14. REY distribution patterns in coal benches of the Hailiushu Mine. (A), MREE-type for samples HSL-1, HSL-3, HSL-4, HSL-6, and HSL-17. (B), MREE-type for samples HLS-10 and HLS-15. (C), LREE-type and H-REE type for samples HLS-8 and HLS-16 respectively. REY are normalized to Upper Continental Crust (UCC) (Taylor and McLennan, 1985).

et al., 2012a; Wang et al., 2011), located a short distance to the south of the Daqingshan Coalfield (Fig. 1A). Additionally, the Hailiushu coals are characterized by high percentages of kaolinite and some minor sulfide and selenide minerals, as described above (Table 3). Relative to the Adaohai coals, the Hailiushu coals are lower in Fe2O3, MgO, CaO, MnO, P2O5 (Fig. 16), Sr, and Ba, but are higher in Li, Se, Cd, Cs, W, and Bi (Fig. 11B). The differences in mineralogical and geochemical compositions between the coals from Hailiushu and Adaohai mines are due to different sediment-source regions and different hydrothermal solutions of epigenetic origin, both of which have significantly influenced the inorganic components. The Daqingshan Coalfield is located in the inner part of an orogenic belt, the Yinshan Upland (Li, 1954; Li et al., 2004), which is situated to the north of the North China Plate (Fig. 1A) and served as the dominant sediment-source region for coals on the North China Plate itself (Han and Yang, 1980). The Daqingshan Coalfield was developed in a subdepression (or intermontane basin) within the orogenic belt, and the sub-uplifts around the sub-depression served as the sediment-source region (Qimu, 1980). The different sub-uplifts between the Adaohai

107

and Hailiushu Mines led to different terrigenous regions for coal deposition during the same peat-accumulation stage. The sediment-source region for the Adaohai Mine was oxidized bauxite in the weathered crust of the Benxi Formation (Dai et al., 2012c), but that for the Hailiushu Mine was mainly composed of Cambrian–Ordovician strata and Archaean metamorphic rocks (Zhou and Jia, 2000; Zhou et al., 2010). The minerals in the Adaohai Mine, including diaspore- and gorceixite-forming materials, were derived from the oxidized bauxite in the weathered terrigenous region during peat accumulation (Dai et al., 2012c). The diaspore was derived from gibbsite that was subjected to dehydration by the heat of the igneous intrusions. The ammonian illite may have been formed at a relatively high temperature by interaction of kaolinite with nitrogen released from the organic matter in the coal during metamorphism caused by the igneous intrusion. The high contents of calcite and dolomite were also derived from igneous intrusion activity (Dai et al., 2012c). The coals of the Hailiushu mine had not been subjected to igneous intrusion but had been significantly influenced by acid hydrothermal solutions, leading to corrosion of previously-formed minerals in the partings (Fig. 6F); formation of sulfide and selenide minerals (Figs. 4C, D, 5A–C, 7A–B); enrichment of MREY (Figs. 14A, B), Se, and Bi; and decomposition of Ti minerals and subsequent substitution of Ti for Al within the kaolinite lattice (Figs. 9, 10), as well as the decomposition of crystal fragments in volcanic-ash-derived intraseam kaolinite layers (Figs. 6A–D). Titanium, as well as Zr, are normally resistant to leaching but are relatively soluble in highly acid conditions (Ward et al., 1999). The close associations between Ti and kaolinite (Figs. 9, 10) indicate a reprecipitation of Ti leached from previously-formed Ti-minerals by acid hydrothermal solutions during the epigenetic stage, probably during the intense tectonic activity of the late Jurassic to early Cretaceous (He et al., 1998; Yang, 1999; Zhong and Chen, 1988). However, the anatase percentages (0.3–4.1%, on organic matter-free basis) in the Adaohai coal benches correspond to the TiO2 percentage (0.55% on average) indicated by XRF analysis (Dai et al., 2012c). In addition to occurring in a thick coal seam (16.5 m), the Adaohai coal ash contains high concentrations of valuable metals such as Al2O3 (44.46%), Ga (72.9 μg/g), and oxides of REY (976 μg/g), and is thus considered as a potential source for Al, Ga, and REE recovery (Dai et al., 2012a,c). However, the Hailiushu coal ash contains 738 μg/g oxides of REY and the thickness of the coal seam is only 3.5 m. Based on the criteria suggested by Dai et al. (2012a,b,c) and Seredin and Dai (2012) (e.g., Ga N 50 μg/g, REY oxides N 800–900 μg/g, SiO2/Al2O3 b 1, on ash basis; and coal seam thickness N 5 m), the Hailiushu coals cannot be considered as meta-sources for Al, Ga, and REY recovery, although they have concentrations of 67.2 μg/g Ga and 42.2% Al2O3, higher than those of common Chinese and world coals.

6. Conclusions The Cu2 coal in the Hailiushu Mine of the Daqingshan Coalfield was deposited in a sub-depression of the orogenic belt (Yinshan Upland) and has a lower rank (high volatile bituminous) than other deposits in the basin. The coal has a high-ash yield (32.53%) and a low sulfur content (0.54% on average). Two factors, the sediment-source region (mainly composed of Cambrian–Ordovician strata and Archaean metamorphic rocks) and the activity of acid hydrothermal solutions, have significantly influenced the mineralogical and geochemical compositions of the coal. Compared to the previously-reported coals from Adaohai mine of the same coalfield, the minerals in the Hailiushu coals dominantly consist of kaolinite, with minor amounts selenide minerals and rhabdophane. The coal is enriched in Ti, which largely occurs in kaolinite. The modes of Ti occurrence, the M-type enrichment of the REY, and the corrosion of previously-formed zircon, anatase, and quartz, were caused by the epigenetic acid hydrothermal solutions.

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(A)

3.5 3

2.5

2 HLS-18-R 1.5

HLS-0-F

1

Sample / UCC

Sample / UCC

2.5

2 HLS-13-P HLS-11-P

1.5

HLS-9-P 1

HLS-2-P

0.5

0.5

0

0

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

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

(C)

1.8

1.6

1.4

1.4

1.2

1.2

1

HLS-7-P

0.8

HLS-5-P

0.6

1

HLS-14-P

0.8

HLS-12-P

0.6

0.4

0.4

0.2

0.2

0

(D)

1.8

1.6

Sample / UCC

Sample / UCC

(B)

3

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

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

Fig. 15. REY distribution patterns for partings, roof and floor strata samples from the Hailiushu Mine. (A), LREE-type for the floor and roof strata samples. (B), LREE-type for some parting samples. (C), M-REE type for samples HLS-5-P and HLS-7-P. (D) HREE-type for samples HLS-12-P and HLS-14-P. REY are normalized to Upper Continental Crust (UCC) (Taylor and McLennan, 1985).

Adaohai/Hailiushu

100

10

1

0.1 SiO2

TiO2

Al2 O3

Fe2 O3

MgO

CaO

MnO

Na2 O

K2 O

P2 O5

Fig. 16. Comparison of major-element oxides in coals from the Adaohai and Hailiushu mines of the Daqingshan Coalfield.

Although the previously-reported Adaohai coal has been considered as a potential source for Al, Ga, and REE recovery, the same coal seam in the Hailiushu Mine has much less economic significance for rare metals and Al. 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 and 41272182), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13099). Thanks are given to Editor-in-Chief Dr. Ralf Littke and reviewer Dr. Yudovich for their careful comments, which improved the quality of the paper. References ASTM Standard D2797/D2797M-11a, 2011. Standard Practice for Preparing Coal Samples for Microscopical Analysis by Reflected Light. ASTM International, West Conshohocken, PA.

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