Mineralogy of the Pennsylvanian coal seam in the Datanhao mine, Daqingshan Coalfield, Inner Mongolia, China: Genetic implications for mineral matter in coal deposited in an intermontane basin

    Mineralogy of the Pennsylvanian coal seam in the Datanhao mine, Daqingshan Coalfield, Inner Mongolia, China: Genetic implications for mineral matter in coal deposited in an intermontane basin Lei Zhao, Jihua Sun, Wenmu Guo, Peipei Wang, Dongping Ji PII: DOI: Reference:

S0166-5162(16)30540-7 doi: 10.1016/j.coal.2016.10.006 COGEL 2737

To appear in:

International Journal of Coal Geology

Received date: Revised date: Accepted date:

14 September 2016 17 October 2016 18 October 2016

Please cite this article as: Zhao, Lei, Sun, Jihua, Guo, Wenmu, Wang, Peipei, Ji, Dongping, Mineralogy of the Pennsylvanian coal seam in the Datanhao mine, Daqingshan Coalfield, Inner Mongolia, China: Genetic implications for mineral matter in coal deposited in an intermontane basin, International Journal of Coal Geology (2016), doi: 10.1016/j.coal.2016.10.006

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Mineralogy of the Pennsylvanian coal seam in the Datanhao Mine,

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Daqingshan Coalfield, Inner Mongolia, China: Genetic Implications for

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mineral matter in coal deposited in an intermontane basin

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Lei Zhao 1,2, Jihua Sun 3, Wenmu Guo 1,2, Peipei Wang 1,2, Dongping Ji 1,2 1

State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, China

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College of Geoscience and Survey Engineering, China University of Mining and Technology (Beijing),

Beijing 100083, China

Inner Mongolia Minerals Experiment Research Institute, Hohhot 010031, China

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Abstract: This paper deals with the mineral matter in the coal and associated strata from the Datanhao mine, Daqingshan Coalfield, which is closely located to the previously-reported Adaohai and Hailiushu mines of the same coalfield. The coal from the Datanhao mine is mainly high-ash and low-sulphur bituminous. The mineral assemblage in the coal is dominated by quartz, kaolinite, with varying proportions of carbonates (calcite, ankerite and siderite), small proportions of muscovite, illite, pyrite and anatase. The associated non-coal (claystone) samples have similar mineral assemblage, except that most non-coal samples contains less quartz than the adjacent coal plies. Although deposited in the same intermontane basin of the orogenic belt, the Yinshan Upland, the Datanhao coal seam shows different mineralogical characteristics from those of the nearby Adaohai and Hailiushu mines. This may reflect the different sediment-source regions during the peat accumulation stage. The sediment source for the Datanhao coal was probably the Ordovician quartz-rich sandstone or the Sinan quartzite in the surrounding sub-uplifts. High proportions of quartz in the Datanhao coal suggest intense terrigenous input into the paleomire and a short distance from the sediment source region. Different mineralogical characteristics of the coals from the three mines (Datanhao, Hailiushu, and Adaohai) of the Daqingshan Coalfield are probably due to the different source material in their respective surrounding subuplifts within the intermontane basin. Kaolinite displaying graupen, tabular and vermicular textures, and the common presence of β-quartz in the intra-seam non-coal bands from the Datanhao coal seam indicate that most of these bands are tonsteins (altered airborne volcanic ash). The original magma is probably high level volatile-rich silicic in composition. Keywords: Coal; Mineral matter; Tonstein; Intermontane basin; Inner Mongolia

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1. Introduction

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Coal-bearing sequences that have been influenced by air-borne volcanic ash falls have been reported in many coal basins in the world (e.g. Triplehorn et al., 1991; Bieg and Burger, 1992; Crowley et al., 1993; Hower et al., 1999; Burger et al., 2000; Knight et al., 2000; Zhao et al., 2012), and such cases have been particularly studied in southwestern China (e.g. Zhou et al., 1982, 1992, 2000; Burger et al., 1990; Zhou and Ren, 1994; Zhou, 1999; Dai et al., 2008b, 2011, 2014, 2016a; Wang, 2009; Zhao et al., 2013, 2015). On the other hand, some coal deposits in North China, including the Shengli Coalfield (Zhuang et al., 2006; Qi et al., 2007; Dai et al., 2015b) and Jungar Coalfield (Dai et al., 2006, 2008a, 2012a), have been known as potential raw source of valuable metals including Ga, Ge, rare earth elements, and Al (Seredin, 2012; Seredin and Finkelman, 2008; Seredin and Dai, 2012; Dai et al., 2016b).

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The Adaohai and Hailiushu coal mines of the Daqingshan Coalfield, Inner Mongolia, although not far from each other, have different mineralogical and geochemical compositions (Dai et al., 2012b, 2015a). The different mineralogical and geochemical characteristics between the coals from both coalfields are due to different sediment-source material and different degree of influence of igneous intrusion (Dai et al., 2012b, 2015a). The mineralogical data of coals from other mines in the Daqingshan Coalfield, other than the Adaohai and Hailiushu coal mines, could provide not only the new insights into the origin of mineral matter in coals deposited in the intermontane basin, but also the information on coal-hosted Al-Ga-REE ore deposits that could be considered as raw sources for the these metals.

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The present paper reports the mineralogical and geochemical compositions of the Pennsylvanian coal seam from the Datanhao coal mine, around 10 km west of the Adaohai coal mine, intended to evaluate different geological factors that may have controlled the abundance and distribution of mineral matter in the coal seam developed in an intermontane basin of Northern China, with an emphasis on the volcanic influence on the coal seam.

2. Geological setting The Daqingshan Coalfield is located in Inner Mongolia, northern China (Fig. 1A), and includes 16 mines (Fig. 1B). The Coalfield was developed in an intermontane basin within an orogenic belt, the Yinshan Upland (Li, 1954; Li et al., 2004), which is the main sediment source region for coal basins of Northern China (Han and Yang, 1980). The coal-bearing sequence in the Daqingshan coalfield is the Pennsylvanian Shuanmazhuang Formation, which was deposited in a continental environment (Zhong et al., 1995). The Shuanmazhuang Formation in the Datanhao mine is divided 2

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into two parts, the CP1 (lower) and CP2 (upper) members. The lower part of the CP1 member consists of mostly gravel which is fining upward. The upper part of CP1 member mainly consists of coarse sandstones and gravel-bearing sandstones. The total thickness of the CP1 member is 30-100 m.

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The CP2 member mostly consists of coal, claystones, siltstones and sandstones, with a thickness varying from 15 to 90 m (Zhong et al., 1995). A number of studies (e.g. Jia and Wu, 1995; Zhong et al., 1995; Zhang et al., 2000; Zhou and Jia, 2000) indicated that both the upper and lower parts of the Shuangmazhuang Formation in the Daqingshan Coalfield contains a number of pyroclastic sediments.

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The Shuanmazhuang Formation is overlain by Permian strata, which consist of the Zahuaigou and Shiyewan Formations. The Zahuaigou Formation is made up of white quartz-pebble conglomerate and intercalated mudstone beds in the lower portion, and mudstone and sandstone in the upper portion. The Shiyewan Formation consists mainly of thick layers of sandstone and conglomerate, interbedded with mudstone.

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The Shuanmazhuang Formation is disconformably underlain by the Cambrian-Ordovician strata, with an average thickness of 50 m. The strata are dominated by limestone and are intercalated with silty mudstone in the lower part (Dai et al., 2012b). The Cambrian system is disconformably underlain by the Sinian strata, which consist mainly of thick quartzite (Zhou and Jia, 2000).

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The coal rank in the coalfield varies from high volatile bituminous in the northwest, through medium volatile bituminous, to low volatile bituminous in the southeast (Fig. 1B). This is due to the igneous intrusions in the east of the coalfield (Dai et al., 2012b, 2015a), which were associated with the Yanshan Movement of the Late Jurassic and Early Cretaceous Epochs (Zhong and Chen, 1988), which is one of the signficant tectonic events in China occurring in the Late Jurassic and Early Cretaceous Epochs.

3. Samples and analytical techniques A total of 62 coal and associated non-coal samples (channel samples) were collected from the CP2 seam at the Datanhao mine, Daqingshan Coalfield. The individual samples from these sections were differentiated from each other on the basis of their lithotype. The Datanhao CP2 coals are dominated by banded bright and dull lithotypes. The immediate floor and roof strata are carbonaceous claystones, with a light grey mudstone overlying the immediate roof. The intra-seam non-coal samples are also mostly carbonaceous claystones. The contact between coal and non-coal bands is usually sharp. Epoxy-mounted blocks were made from chips representing each coal and non-coal sample for petrographic and electron microscope analyses. Mean random vitrinite 3

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reflectance was measured on the polished surfaces of epoxy-mounted blocks , according to ASTM D2798-11 (2011). Maceral analysis were carried out for samples with ash yield< 50%, according to ASTM D2799-11 (2011). Maceral classification and terminology are after Taylor et al. (1998) and ICCP (1998, 2001).

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Polished thin-sections of the intra-seam claystone samples were subjected to petrographic analysis using a Leica DM4500P polarizing microscope equipped with Leica LAS digital imaging software. Selected coal and rock polished sections were also studied using a Field Emission-Scanning Electron Microscope (FE-SEM, FEI Quanta™ 650 FEG), equipped with an EDAX energy-dispersive X-ray spectrometer.

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Each sample was also ground to fine powder and split into representative portions for proximate, ultimate and geochemical analyses. Proximate analysis was carried out based on ASTM Standards D3173-11 (2011), D3174-11 (2011), and D3175-11 (2011). The total sulfur content was analyzed following ASTM Standard D3177-11 (2011). Major element concentrations in the ashes (815°C) of coal and rock samples were determined by X-ray fluorescence techniques.

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4. Results

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Low-temperature oxygen-plasma ashing was also carried out on the powdered coal samples, using an EMITECH K1050X plasma asher. The resultant low-temperature ashes (LTAs) were subjected to X-ray diffraction (XRD) analysis, using a Rigaka D/max-2500/PC powder diffractometer with Cu-Kα radiation and a scintillation detector.

4.1. Coal characteristics Data of proximate and ultimate analyses, total sulphur, and random reflectance of vitrinite for the individual coal plies from the Datanhao coal mine are listed in Table 1. Table 1 Proximate analysis and mean random vitrinite reflectance value of the coal samples from the Datanhao mine, Daqingshan Coalfield (%) Coal sample

Mad

Vdaf

Ad

Cdaf

Hdaf

Ndaf

St,d

Ro,ran

D59 D58

0.78 0.74

36.92 38.57

48.36 48.36

78.56 79.98

5.74 5.53

0.93 0.93

0.44 0.33

1.15 1.06

D49

0.77

30.72

32.28

84.78

5.33

1.08

0.47

1.13

D47

0.90

31.06

22.73

85.99

5.31

1.19

0.57

1.08

D43

0.72

27.03

28.77

86.02

5.15

1.18

0.48

1.16

D42

0.80

29.13

27.54

82.50

5.17

1.16

0.48

1.19

D40

0.87

32.86

41.58

79.50

5.55

1.13

0.44

1.20

4

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30.66

45.87

79.61

5.42

1.06

0.35

1.20

D36

0.70

36.51

39.51

81.12

5.39

1.11

0.51

1.20

D33

0.77

34.08

48.94

77.21

5.61

0.82

0.37

1.23

D32

0.55

27.85

30.08

83.80

5.22

1.08

0.62

1.22

D20

0.98

30.52

21.70

84.82

5.16

1.31

0.59

1.15

D18

0.81

33.88

49.31

78.71

5.64

1.00

0.45

1.13

D17

0.99

36.48

49.61

75.97

5.80

0.98

0.38

1.19

D16

0.87

31.43

18.89

84.41

5.05

1.35

1.23

D14

0.94

29.52

22.82

86.01

4.68

1.00

0.63

1.05

D12

1.01

29.35

20.25

90.95

5.10

D11

0.68

56.48

42.77

68.42

D10

0.92

27.85

15.64

80.44

D1

0.51

43.49

41.27

74.77

0.83

1.40

0.75

1.06

3.65

0.80

0.40

1.20

4.97

1.21

0.60

1.17

5.08

1.06

0.60

1.15

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D38

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ad, air-dried basis; d, dry basis; daf, dry, ash-free basis; M, inherent moisture; V, volatile matter; A, ash yield; C, carbon; H, hydrogen; N, nitrogen. St, total sulphur; Ro,ran, mean random vitrinite reflectance.

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The ash yield of the Datanhao coal varies between 15.64% and 49.61% (34.66% on average). According to Chinese Standards GB/T 15224.1-2010 (2010), coals with 30.01-40% ash yield are medium- to high-ash coals and coals with 40.01-50% ash are high-ash coals; the Datanhao coals are mainly medium- to high-ash coals. The total sulfur ranges between 0.33% and 0.83%, with an average value of 0.51%. The Datanhao coal is thus a low sulfur coal (Coals with total suflur <1% is considered as low sulfur coal; Chou, 2012).

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The moisture content of the coal samples ranges between 0.51 and 1.01% (air-dried basis). The volatile matter ranges between 27.03% and 56.48%, with 33.97% on average (dry, ash-free basis). The high volatile matter value of most coal samples, however, is due to high content of carbonate minerals to be discussed later. The high volatile yield is thus associated with the loss of carbon dioxide from the carbonates during the analysis process. The random reflectance of vitrinite is 1.17% on average. According to the relation between the rank of U.S. coals and vitrinite reflectance (ASTM Standard D388, 2012), the Datanhao coal ranks mostly medium volatile bituminous. 4.2. Maceral Table 2 presents the maceral compositions of selected coal samples from the Datanhao mine. On a mineral-free basis, the vitrinite is dominated by collodetrinite (49.6% on average), followed by collotelinite (23.5% on average) and vitrodetrinite (6.9% on average), with telinite being a minor maceral. The inertinite group macerals occurring in the Datanhao coals are mainly semifusinite (7.5% on average) and inertodetrinite (5.5% on average) and, to a lesser extent, macrinite, fusinite and micrinite. Sporinite is the only liptinite maceral that presents in the coal, and its content is below the detection limit. 5

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On a mineral-free basis, the total proportion of vitrinite group macerals (69.8-89.6%, 80.5% on average) is higher than that of the Hailiushu coal (73.4% on average) (Dai et al., 2015a), and much higher than that of the Adaohai coal (64.7% on average) (Dai et al., 2012b). The coals from the Jungar Coalfield (e.g. Guanbanwusu, Haerwusu and Heidaigou mines; Fig. 1A) which is adjacent to the Daqingshan Coalfield, however, have higher inertinite than vitrinite percentages (Dai et al., 2006, 2008a, 2012a). Table 2 Maceral contents of selected coal from the Datanhao mine (vol.%, mineral-free basis). T

CT

CD

VD

TV

F

SF

Mac

Mic

ID

TI

D59

2.1

22

58.9

4.3

87.2

5.0

2.8

4.3

bdl

0.7

12.8

D58

1.6

1.1

66.7

9.8

79.2

1.6

11.5

3.8

bdl

3.8

20.8

D49

0.4

15.6

57.2

6.2

79.4

3.9

5.8

3.1

0.4

7.4

20.6

D47

bdl

18.1

57.5

4.0

79.6

0.4

4.9

1.8

5.3

8.0

20.4

D43

bdl

7.9

63.8

4.2

75.8

0.4

15.8

0.8

1.7

5.4

24.2

D42

bdl

17.1

49.5

3.2

D40

bdl

19.3

57.8

8.9

D38

bdl

11.3

61.5

5.2

D36

bdl

36.9

45.1

D33

1.4

11.7

52.6

D32

0.3

6.0

D20

0.4

32.7

D18

0.4

D17

2.4

D16

0.4

D14

bdl

D12

bdl

D1 WA

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9.9

3.2

4.5

9.5

30.2

85.9

0.5

4.7

1.0

1.0

6.8

14.1

77.9

0.9

10.8

7.4

bdl

3.0

22.1

7.5

89.6

1.5

6.0

2.2

bdl

0.7

10.4

8.5

74.2

0.5

16

1.9

bdl

7.5

25.8

18.4

D

3.2

65.6

0.7

72.6

5.4

15.4

2.3

bdl

4.3

27.4

38.8

1.9

73.8

0.4

3.8

5.8

6.2

10.0

26.2

37.3

18.4

74.6

0.8

8.2

2.9

0.8

12.7

25.4

54.9

17.5

10.2

85.0

1.2

4.9

2.0

bdl

6.9

15.0

25.9

48.3

3.9

78.4

0.4

4.7

2.2

7.8

6.5

21.6

42.9

38.0

4.4

85.4

2.0

8.8

1.0

Bdl

2.9

14.6

17.2

66.2

1.0

84.3

1.0

4.0

1.5

3.5

5.6

15.7

bdl

35.1

22.6

28.6

86.3

5.4

3.6

0.6

2.4

1.8

13.7

bdl

41.1

34.7

3.5

79.2

bdl

5.9

5.0

1.0

8.9

20.8

bdl

20.2

62.6

6.7

89.6

bdl

3.7

5.5

bdl

1.2

10.4

23.5

49.6

6.9

0.5

80.5

7.5

1.9

2.9

1.7

5.5

19.5

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TE

D10

69.8

AC

D11

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Sample

T, telinite; CT, collotelinite; CD, collodetrinite; VD, vitrodetrinite; TV, total vitrinite; F, fusinite; SF, semifusinite; Mac, macrinite; Mic, micrinite; ID, inertodetrinite; TI, total inertinite; bdl, below detection limit; WA, weighted average.

4.3. Mineralogical and ash chemical data The high temperature ashes (HTAs) of all the samples, proportion of minerals in each coal LTA and non-coal sample, as well as the LTA percentages of the coals, are given in Table 3. Although some samples have ash yield in the range of 50-60%, they are included in local mining operations. Indeed, those samples have mineralogical characteristics different from the adjacent claystones that have a relatively high ash yield, as to be discussed. Those samples, as well as those with ash yield <50%, are 6

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The relationship between ash chemistry and mineralogy was studied to check the reliability of the quantitative XRD data determined by XRD and Siroquant analysis. The chemical composition of the mineral assemblage in the coal LTAs and rock samples was calculated and compared with the actual ash composition of the 815 °C coal HTAs and rocks as determined by XRF (Table 4), following the calculation procedure described by Ward et al. (1999). The chemical composition was also modified for each sample by deducting the CO2 and H2O+ to derive an equivalent to an ash analysis. The actual chemical composition determined by XRF was also normalized to an SO3-free basis. The correlations of major oxides are shown in Fig. 3.

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The plots for SiO2 and Al2O3 of the Datanhao coal and non-coal samples both show a high correlation between the respective proportions from the Siroquant data and those indicated by the ash chemistry. This indicates that Siroquant gives consistent results for those major minerals (quartz and kaolinite) which contribute to the major element oxides in the analysed rocks and coal ashes.

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A relatively strong correlation is observed for the CaO plot, with all the points being close to the 1:1 diagonal line. The plots for MgO and Fe2O3 also show a high correlation between their respective proportions inferred from the Siroquant data and those indicated by the ash chemistry. However, the majority of the points for MgO and Fe2O3 fall slightly below the equality line, indicating an under-estimation of MgO and Fe2O3 inferred from Siroquant, relative to the observed values. As indicated by the SEM-EDS data, the ankerite in the coals of the present study generally contains some Mg, due to element substitution. However, this was not allowed for in the stoichiometric calculations in this study. The plot for Ti2O shows a broad scatter, with most of the points falling below the equality line. In some volcanic-influenced coals and associated strata, anatase occurs as fine-grained inclusion possibly with poor crystallinity within the kaolinite (Zhao et al., 2012, 2013), or Ti replaces Al in the kaolinite (Ward et al., 1999; Dai et al., 2015a). Such poorly-crystalized anatase and Ti in the kaolinite is difficult traced through XRD and Siroquant analysis. Table 3 Mineralogy of the Datanhao LTA and associated non-coal samples by XRD and Siroquant (wt. %). Thickness Sample

LTA

HTA

Q

K

M

I

I/S

Cal

Sid

Ank

7.9

7.3

0.2

1.3

0.8

Py

sph

Ana

goy

(cm) D61

＞70

-

88.4

19.4

62.9

D60

90

-

76.5

14.0

82.5

D59-c

26

58.2

48.0

20.2

76.0

2.7

0.9 1.6

7

0.3

2.0

0.3

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15

58.0

48.0

27.2

61.6

D57

26

-

86.1

12.6

72.0

4.6

D56

13

-

77.0

15.1

80.5

4.0

D55

75

-

85.5

10.6

84.8

D54

10

-

79.7

15.0

82.7

0.3

D53

25

-

75.5

14.2

84.6

0.5

D52

6

-

79.2

8.6

87.2

D51

28

-

71.3

36.8

61.2

D50-c

30

-

67.4

5.4

93.0

D49-c

7

39.7

32.0

32.7

64.6

D48

15

-

79.6

4.0

95.5

D47-c

20

25.6

22.5

24.6

70.3

D46

15

-

85.1

1.8

94.9

2.6

D45-c

10

-

63.8

3.1

43.4

7.7

D44

7

-

81.8

2.5

96.8

D43-c

15

37.6

28.6

38.2

60.2

D42-c

28

30.6

27.3

18.5

80.3

D41-c

7

-

66.9

10.2

89.8

D40-c

26

48.2

41.2

16.0

80.0

D39

6

-

74.6

9.6

D38-c

18

52.5

45.5

D37

8

-

78.4

D36-c

20

46.6

39.2

D35

35

D34-c

30

D33-c

20

D32-c

25

3.9 9.3

0.3

T

2.0 0.3

IP

0.4

0.2

1.9

SC R

4.0

0.6

0.3

0.5

0.5

2.3 0.4

NU

3.5

MA

31

1.1 0.7

8.3

2.9

3.4

0.3 0.6 1.2

0.4

0.4

0.5

0.3

0.3

0.9

0.6

89.3

1.1

TE

D

1.9

0.5

0.1

0.3

73.1

0.7

8.9

90.1

1.1

9.8

73.7

CE P

26.1

5.2

4.8

5.9

0.6

74.2

2.1

97.5

-

64.3

9.3

90.7

56.6

48.6

15.0

83.4

0.3

1.2

0.1

33.4

29.9

24.7

73.6

0.5

0.7

0.5

0.2

0.1

AC

-

0.4

55.4

12.6

87.1

12

-

80.3

4.5

92.7

20

-

66.1

3.3

96.7

D-28

6

-

76.0

3.5

96.3

D27-c

25

-

51.7

7.9

89.4

2.2

D26

40

-

82.7

2.9

96.1

0.4

D25-c

18

-

55.5

5.4

81.5

D24

13

-

85.5

1.3

98.7

D23-c

3

-

52.1

7.6

D22

10

-

85.5

D21-c

30

-

D20-c

17

D19

D29-c

0.2

1.4

4.3

-

D30

6.7

0.5

22

D31-c

0.4

2.4

0.4

0.1

2.1

0.5

0.1 0.6

2.9

4.3

3.8

89.0

0.7

2.4

0.3

3.4

96.2

0.4

0

58.8

8.8

89.6

24.6

21.5

8.9

88.8

17

-

86.4

6.7

93.3

D18-c

5

55.6

48.9

24.8

71.2

D17-c

20

59.4

49.1

7.0

93.0

1.0 1.0

3.6

0.6 1.3

0.4

8

D16-c

13

21.0

18.7

3.5

86.2

D15

4

-

66.4

11.4

80.6

3.7

D14-c

11

28.4

22.6

9.9

57.4

5.9

D13

12

-

82.3

2.8

97.2

D12-c

15

23.2

20.0

3.8

85.4

5.1

3.2

2.0

D11-c

25

63.9

42.5

13.0

55.2

26.7

5.1

D10-c

30

18.9

15.5

17.9

59.9

20.8

1.4

D9

25

-

78.1

50.9

31.5

6.4

9.3

0.5

0.3

D8

30

-

76.5

38.6

51.9

0.8

5.4

D7

15

-

82.3

52.4

33.4

2.0

10.0

D6

15

-

82.4

19.9

76.9

2.7

D5

25

-

71.6

20.1

75.4

3.2

D4

25

-

78.8

11.7

83.9

0.1

D3

25

-

79.5

13.3

81.2

D2-c

30

-

68.0

2.3

95.9

NU

ACCEPTED MANUSCRIPT

D1-c

15

53.2

41.1

0.7

62.0

4.2

4.4

D0

30

-

84.6

3.1

3.5

0.7

17.9

3.6

5.0

0.3

4.3 0.1

0.1

T

IP 1.1

SC R

3.9

2.5

0.5

2.6

0.7

0.6

1.6 0.6

0.6

0.8

0.4 1.3

0.3

MA

96.6

3.1

0.6

18.6

1.7 0.3

0.6

10.1

3.4

Q, quartz; K, kaolinite; M, mica; I, illite; I/S, mixed-layer illite/smectite; Cal, calcite; Sid, siderite; Ank, ankerite; Py,

D

pyrite; Sph, sphalerite; Ana, anatase; Goy, goyazite; -, not detected; blank place, mineral below detection limit; -c,

TE

coal samples.

CE P

A broad scatter is also shown in the K2O plot. XRD and Siroquant may have underestimated the proportion of non-kaolinite clay minerals (illite and I/S) in some samples. On the other hand, the occurrence of NH4-illite in some samples causes an overestimate of K by Siroquant, in comparison with the actual values.

AC

Table 4 Major element analyses of HTAs of the Datanhao coal and non-coal samples (%), as determined by XRF analysis. Sample

SiO2

Al2O3

TiO2

Fe2O3

MgO

CaO

Na2O

K2O

MnO

P2O5

SO3

61.32

31.90

0.54

2.37

0.76

0.98

bdl

1.58

0.006

0.17

0.08

DTH-60

60.34

36.88

1.32

0.43

0.09

0.18

bdl

0.37

bdl

0.07

0.02

DTH-59

59.42

32.50

0.99

0.91

0.50

2.11

bdl

0.28

0.010

0.11

0.26

DTH-58

57.95

25.47

0.45

2.77

1.60

5.96

bdl

0.25

0.058

0.07

0.54

DTH-57

59.10

34.31

0.45

1.19

0.53

3.63

bdl

0.31

0.006

0.06

0.11

DTH-56

59.80

34.33

0.55

0.69

0.25

0.51

bdl

0.55

0.007

0.08

0.07

DTH-55

57.75

37.43

0.59

0.57

0.22

0.38

bdl

0.66

bdl

0.04

0.03

DTH-54

60.16

35.82

2.15

0.30

0.16

0.32

bdl

0.54

bdl

0.08

0.03

DTH-53

59.09

36.15

0.70

0.32

0.12

0.26

bdl

0.31

bdl

0.05

0.04

DTH-52

58.17

39.76

0.62

0.36

0.09

0.26

bdl

0.37

bdl

0.06

0.04

DTH-51

68.96

23.38

2.05

0.33

0.11

0.17

bdl

0.34

bdl

0.13

0.03

DTH-50

55.16

40.77

1.00

1.43

0.31

0.38

bdl

0.44

0.003

0.05

0.08

DTH-49

67.30

25.81

0.82

1.23

0.60

1.69

bdl

0.19

0.026

0.09

0.63

DTH-61

9

DTH-48

55.65

42.37

0.71

0.27

0.13

0.25

bdl

0.28

bdl

0.05

0.03

DTH-47

61.22

29.32

1.17

1.72

0.31

3.06

bdl

0.18

0.062

0.19

0.82

DTH-46

54.65

43.77

0.56

0.32

0.04

0.17

bdl

0.15

bdl

0.10

0.02

DTH-45

29.92

21.80

0.28

22.52

1.28

18.37

bdl

0.39

0.735

0.06

1.75

DTH-44

54.82

43.45

0.89

0.21

0.05

0.17

bdl

0.19

0.004

0.04

0.02

DTH-43

70.37

24.79

2.61

0.72

0.07

0.36

bdl

T

ACCEPTED MANUSCRIPT

0.14

0.08

0.11

DTH-42

60.79

35.84

0.99

0.82

0.12

0.44

bdl

DTH-41

58.00

39.84

1.08

0.40

0.09

0.15

bdl

DTH-40

59.24

36.84

1.63

0.77

0.15

0.48

bdl

DTH-39

57.79

39.71

1.64

0.25

0.03

0.12

DTH-38

64.63

32.38

1.61

0.48

0.08

DTH-37

56.75

40.50

1.79

0.33

0.03

DTH-36

48.45

33.19

1.53

5.65

1.40

DTH-35

55.01

42.65

1.44

0.30

DTH-34

57.69

39.60

1.09

0.80

DTH-33

59.26

37.03

1.04

0.84

0.20

DTH-32

62.10

31.88

1.42

0.67

DTH-31

58.62

39.15

1.04

DTH-30

55.42

40.90

0.77

DTH-29

53.11

41.73

0.49

DTH-28

53.18

41.24

DTH-27

56.05

39.89

DTH-26

54.41

DTH-25

42.04

DTH-24

54.40

DTH-23

52.99

DTH-22

52.91

0.06

0.15

0.19

bdl

0.05

0.02

0.13

0.015

0.08

0.17

bdl

0.16

bdl

0.07

0.01

0.17

bdl

0.13

0.005

0.09

0.04

0.13

bdl

0.14

bdl

0.10

0.02

NU

IP

0.026

6.02

bdl

0.14

0.070

0.34

1.10

0.07

0.13

bdl

0.15

bdl

0.05

0.02

0.13

0.17

bdl

0.18

0.003

0.06

0.05

0.59

bdl

0.14

0.008

0.06

0.23

0.11

0.36

bdl

0.14

0.011

0.07

0.14

0.31

0.05

0.17

bdl

0.16

bdl

0.08

0.04

0.20

0.04

0.09

bdl

0.29

bdl

0.05

0.01

0.17

0.07

0.13

bdl

0.19

bdl

0.04

0.02

42.94

0.65

0.19

0.07

0.13

bdl

0.20

bdl

0.04

0.02

1.16

0.82

0.11

0.84

bdl

0.19

0.025

0.08

0.22

1.31

0.36

0.03

0.28

bdl

0.18

0.009

0.16

0.03

30.95

0.66

13.67

1.09

5.07

bdl

0.14

0.554

0.10

0.63

44.41

0.56

0.23

bdl

0.09

bdl

0.08

bdl

0.05

0.01

37.13

0.83

5.73

0.26

0.97

bdl

0.14

0.240

0.13

0.41

40.98

0.87

0.32

bdl

0.41

bdl

0.08

0.010

0.17

0.06

CE P

TE

D

MA

SC R

0.11

AC

DTH-21

0.023

56.17

39.25

1.62

1.92

0.09

0.21

bdl

0.22

0.076

0.05

0.07

56.38

38.70

1.13

0.91

0.22

1.06

bdl

0.14

0.024

0.05

0.41

57.59

41.40

0.31

0.14

0.04

0.11

bdl

0.16

0.003

0.02

0.01

DTH-18

63.33

31.22

0.56

0.69

0.09

0.14

bdl

0.27

0.006

0.07

0.04

DTH-17

57.10

41.34

0.43

0.30

0.04

0.17

bdl

0.15

0.003

0.05

0.04

DTH-16

46.32

35.55

1.16

7.74

0.72

3.80

bdl

0.11

0.293

0.05

1.55

DTH-15

57.99

36.21

0.69

1.06

0.13

0.20

bdl

0.64

0.027

0.07

0.04

DTH-14

41.34

25.72

0.87

9.02

1.32

15.47

bdl

0.15

0.373

0.05

2.16

DTH-13

55.31

43.58

0.54

0.13

0.04

0.10

bdl

0.16

0.002

0.02

0.01

DTH-12

44.72

35.14

0.46

6.14

0.57

5.18

bdl

0.08

0.272

0.08

1.20

DTH-11

6.01

4.99

0.10

44.95

2.54

36.69

bdl

bdl

1.410

0.10

1.22

DTH-10

53.67

29.04

0.80

2.96

0.13

7.85

bdl

0.12

0.125

0.11

0.96

DTH-9

74.05

17.92

0.54

1.83

0.39

0.81

bdl

1.17

0.042

0.04

0.22

DTH-8

67.33

24.94

0.56

2.55

0.29

0.29

bdl

0.62

0.056

0.07

0.13

DTH-7

75.72

18.93

0.54

0.95

0.19

0.09

bdl

1.06

0.019

0.05

0.02

DTH-20 DTH-19

10

DTH-6

63.26

34.69

1.07

0.22

0.06

0.10

bdl

0.35

0.002

0.05

0.01

DTH-5

62.89

33.93

1.09

0.57

0.16

0.41

0.10

0.25

0.007

0.09

0.08

DTH-4

63.10

34.94

0.67

0.33

0.06

0.20

bdl

0.29

bdl

0.05

0.04

DTH-3

61.15

35.55

0.87

0.78

0.19

0.66

bdl

0.21

0.006

0.04

0.11

DTH-2

52.22

41.16

1.31

0.51

0.12

0.27

0.06

0.21

0.004

0.08

0.04

DTH-1

32.61

26.76

1.29

14.22

2.50

17.82

bdl

T

ACCEPTED MANUSCRIPT

0.14

0.12

1.41

DTH-0

52.65

43.37

0.41

0.20

0.04

0.09

bdl

0.04

0.01

IP

0.351 0.002

SC R

0.26

4.4. Minerals in coal samples

MA

NU

The mineral assemblage in the coal samples is dominated by kaolinite, quartz, varying proportions of carbonates (calcite, ankerite and siderite), with small proportions of muscovite, illite, pyrite and anatase (Table 3). The vertical trends in abundance of the different minerals through the seam are not distinct. 4.4.1 Quartz

CE P

TE

D

With the exception of coal plies from the lower portions of the seam, quartz typically comprises 20-30% of the mineral assemblage in the Datanhao coal LTAs. The proportions of quartz in most coal LTAs would be even higher, if allowance is made for dilution by relatively abundant carbonates in the coals. Quartz in the coals largely occurs as discrete grains (Fig. 4A), and to a lesser extent, as cell-infillings. Such two forms indicates detrital and authigenic origins, respectively.

AC

4.4.2 Clay minerals (kaolinite and NH4-illite) As indicated by the XRD patterns, the kaolinite in all the Datanhao coal LTAs is well crystallized. The kaolinite mainly occurs as cell-infillings, and in a few cases, in vermicular form (Fig. 4B). Such occurrence along with the well-ordered structure reflected on the XRD patterns, indicate that the kaolinite in the coal was formed mainly by authigenic precipitation. Illite is only present in a few coal LTAs in small proportions. Where it is present, it occurs as NH4-bearing illite, which has an [001] peak around 10.3 Å on the XRD patterns. NH4-bearing illite has been observed insome high rank coals, and has been suggested to have formed by interaction of kaolinite with nitrogen, released from the organic matter (e.g. Daniels and Altaner, 1990; Dai et al., 2012b; Permana et al., 2013; Zheng et al., 2016). Clay minerals also occur as laminae form in high ash coals (Fig. 4C). Such clay minerals are probably illite or mixed I/S, with a detrital origin.

11

ACCEPTED MANUSCRIPT 4.4.3 Carbonate minerals

SC R

IP

T

Carbonate minerals, including calcite, ankerite and siderite, are common in the Datanhao coals, although they appear to have a scattered distribution through the vertical coal-seam section. Calcite and ankerite occur as cleat- and cell-fillings, andalso as infillings of the cleats around siderite nodules (Fig. 4D). The ankerite in the Datanhao coal, as indicated by the EDS data, has chemical composition between dolomite and ankerite. Siderite occurs essentially as spheroidal nodules in the tellocolinite matrix (Fig. 4D). The bedding surrounding the nodules shows clear compaction effects, indicating the formation of siderite at a syngenetic or early diagenetic stage.

NU

4.4.4 Other minerals

CE P

TE

D

MA

Pyrite is minor, generally lower than 2% in the coal LTAs. It occurs as framboids (Fig. 5A) and euhedral crystals (Fig. 5B). The association between pyrite framboids and siderite nodules indicates that the pyrite postdates the siderite (Fig. 5C). Epigenetic pyrite also occurs, sometimes enclosing early-formed siderite (Fig. 5D). Anatase persistently occurs in most coal plies, although in small proportions. Sphalerite (Fig. 5E) and goyazite were detected in two high-ash coals by Siroquant analysis, respectively (Table 3). Minerals that below XRD and Siroquant detection limits, and are traced under SEM include Ca-bearing REE-phosphate (Fig. 5E) and galena (Fig. 5F), which have been observed in other coals (Hower and Dai, 2016; Hower et al., 2015, 2016; Johnston et al., 2015). 4.5. Mineral in non-coal samples

AC

4.5.1 Quartz

In contrast to most coal samples, quartz is a relatively minor component of non-coal samples of the coal seam, especially those from the middle part of the seam. Under the optical and scanning electron microscopes, the quartz in the Datanhao non-coal samples typically occurs as angular fragments or elongated flakes (Fig. 6A, B, C), and in some cases, shows magmatic resorption (Fig. 6B). Quartz with such modes of occurrence is of clear pyroclastic origin. Some quartz grains in Fig. 6A-B are also subrounded, perhaps suggesting marginal reworking or rounding due to abrasion in a pyroclastic flow environment. Euhedral quartz is also relatively common in the non-coal samples through the seam section (Fig. 6D-H). Some euhedral crystals appear to have syngenetic glass inclusions (Fig. 6D). Figure 6E shows an euhedral quartz particle that has landed in soft organic matter and is undoubtedly fragmental. Some hexagon crystals appear to have been broken or corroded (Fig. 6G-H). β-quartz has been reported in many tonsteins 12

ACCEPTED MANUSCRIPT worldwide (e.g. Bohor and Triplehorn, 1993; Hower et al., 1999; Dai et al., 2008b; Zhao et al., 2012; Zhou et al., 2000), as well as in thick tuff layers in coal-bearing sequences (Zhao et al., 2016, 2017).

SC R

IP

T

Euhedral quartz in coal may also be formed by authigenic processes (Sykes and Lindqvist, 1993; Ward, 2002, 2016). Those crystals typically have prism faces as well as bipyramids. The quartz crystals in Fig. 6D-H, however, are all β-quartz with no prism development, and thus are clearly of pyroclastic silicic origin. The quartz in the Datanhao non-coal samples therefore is suggested to be largely of pyroclastic volcanic origin, most likely high-temperature and high level volatile-rich silicic volcanics, given the common presence of β-quartz in the non-coal layers.

MA

NU

A number of non-coal samples (e.g. samples D48, D44-46, D39, D37, D22, and D19) have quartz content much lower than that in the adjacent coal plies. The difference is even greater if allowance is made for dilution by the relatively abundant carbonates in the coals. This may indicate that the quartz in these non-coal samples has source different from that in the adjacent coal samples.

TE

D

Feng (1989) identified some tonstein layers developed in Carboniferous-Permian Coal measures in North China, according to the occurrence of volcanogenic minerals including β-quartz, sanidine and zircon. Zircon has also been observed under the optical microscope (Fig. 7A) as a volcanogenic mineral in the present study.

CE P

4.5.2 Clay minerals and muscovite

AC

The XRD patterns show that the kaolinite in the associated claystone bands is essentially well-ordered. Some non-coal samples show a graupen texture (Fig. 7B) of spheroidal aggregates of cryptocrystalline to microcrystalline kaolinite. Tabular to vermicular kaolinite is also frequently observed in some non-coal samples under the optical and electron microscopes (Fig. 7C-F). Vermicular kaolinite, although not restricted to tonsteins, is often used as evidence that the sediment is a tonstein (Spears, 1971; Ruppert and Moore, 1993; Bohor and Triplehorn, 1993; Dai et al., 2014). Kaolinite of graupen, vermicular and tabular forms has been reported in intra-seam tonsteins in many coal-bearing sequences (e.g. Zhou et al., 1982; Spears et al., 1988; Knight et al., 2000; Zhao et al., 2012, 2013; Dai et al., 2014). Like some coal plies, most claystone bands also contain NH4-bearing illite. Some of them (DTH-7 and DTH-9) have both muscovite and NH4-bearing illite, reflected by two separate peaks between 10.0-10.3 Å. The NH4-bearing illite in claystones has a similar origin as that in coal samples. Muscovite, however, is most likely to be detrital in origin. The lowermost claystone band has mixed layer illite/smectite, which 13

ACCEPTED MANUSCRIPT probably also has a detrital origin.

IP

T

Anatase (Fig. 8A, B) and pyrite are minor minerals occurring in the non-coal samples. Some anatase has a diameter <2 μm and contains EDS-detectable Nb and Zr (Fig 8B). Traces of zircon (Fig. 8A), barite (Fig. 8C), galena (Fig. 8D), and Ca,Th,U-bearing REE-phosphate (Fig. 8E) were detected by EDS analysis, although their abundance is below the detection limit of the XRD system.

SC R

5. Discussion 5.1. Carbonate minerals

D

MA

NU

Syngenetic siderite formation represents the interaction of iron and dissolved CO2 (Ward, 2016). The CO2 was probably produced by fermentation of the organic matter and dissolved in pore water. Siderite is typically formed when the supply of sulfate ions for bacterial reduction is limited (e.g. Gould and Smith, 1979; Ward, 1984; Spears, 1987). As indicated by Ward (2002, 2016), abundant syngenetic siderite usually indicates deposition of the coal mainly under non-marine conditions, or under the influence of low sulphate content water in the paleomire.

AC

CE P

TE

Comparison of carbonates for both coal and non-coal samples shows some correlation between the percentages of siderite and the sum of calcite and ankerite (Fig. 9), with a coefficient of determination (R2) of 0.77. The correlation is elevated by a high-siderite sample (26.7% siderite on LTA basis). However, the relationship persists (R2=0.56), even if the high-siderite sample is omitted from the calculation. This may reflect the close association between these carbonates. As described above, siderite essentially occurs as syngenetic nodules. The later compression and dehydration of organic matter around the nodules caused fractures surrounding the nodules, which subsequently were filled by calcite and ankerite at a relatively late diagenetic stage. 5.2. Source materials for the coal seam in the Datanhao mine Quartz is generally abundant in most of the coal LTA samples (20-30%, LTA basis), but it is only a minor component in both the Adaohai and Hailiushu coals, being <0.5% and <2% (LTA basis), respectively (Dai et al., 2012b, 2015a). This probably points to different sources of inorganic material flashed into the respective paleomires from the margins in different locations within the coal basin. Because the Daqingshan Coalfield was developed in an intermontane basin of the Yinshan Upland (Dai et al., 2015a), the sediment source region is probably the surrounding sub-uplifts. High proportions of quartz also suggest intense terrigenous input during peat accumulation from the sediment source region. According to 14

ACCEPTED MANUSCRIPT

IP

T

previous studies by Zhou and Jia (2000) and Zhou et al. (2010), the source rocks for the Daqingshan Coalfield in the late Carbonifeous to Early Permian were the carbonate and clastic rocks of the lower Paleozoic system, as well as Sinian quartzite. The source rock for the CP2 coal seam at the Datanhao mine may be the Ordovician quartz-rich sandstone or the Sinan quartzite in the surrounding sub-uplifts within the Orogen.

D

5.3. Formation of clay minerals

MA

NU

SC R

A number of intra-seam non-coal samples from the Datanhao coal seam, on the other hand, especially those from the middle part of the seam, contain much less amount of quartz than the LTAs of the adjacent coal plies. Contrasting abundance of quartz in the coals and adjacent non-coal plies may tentatively indicate that some non-coal samples were derived from sources other than terrigenous sediments. Furthermore, the occurrence of β-quartz, and kaolinite of graupen, tabular and vermicular forms in most of the non-coal samples indicate that these samples were derived from airborne volcanic ash. Such intra-seam bands are thus regarded as tonsteins, according to some publications (e.g. Spears, 1971; Ruppert and Moore, 1993).

AC

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The Clay minerals in both the coal and non-coal samples are dominated by well-ordered kaolinite. Well-ordered kaolinite in the coal is consistent with a continental environment during peat accumulation and coal formation. A marine-influenced environment, in contrast, would favor the formation of non-kaolinite clay minerals, such as illite and mixed I/S (Ward, 1989; Zhao et al., 2014), although the non-kaolinite clay minerals may still be minor components. The well-ordered structure of kaolinite indicates that the kaolinite was formed mainly by authigenic precipitation (Ward, 1989, 2002, 2016). Well-ordered kaolinite crystals in the non-coal samples are also typical for tonsteins, derived from the devitrification of volcanic glass and by alteration of primary minerals (e.g., biotite and feldspar) originally present in the volcanic ash. Minor proportions of NH4-bearing illite occur in some coal and non-coal samples. NH4-bearing illite is relatively common in many high rank coals and associated rocks, and has been suggested to have formed by interaction of kaolinite with nitrogen, released from the organic matter during metamorphism at relatively high temperature (e.g. Daniels and Altaner, 1990; Ward and Christie, 1994; Dai et al., 2012b; Permana et al., 2013). The proportion of NH4-bearing illite in the Adaohai coals is much higher than the Datanhao coals. This is consistent with an increasing degree of coal metamorphism from the east to the west of the Daqingshan Coalfield due to the igneous intrusion. 15

ACCEPTED MANUSCRIPT 5.4. Change in the mineral assemblage within the Daqingshan Coalfield

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Previous studies (Dai et al., 2012b, 2015a) have suggested that CP2 coal of the Datanhao mine is laterally correlated with the CP2 coal of the Adaohai mine to the east and the Cu2 coal of the Hailiushu mine to the west (Fig. 1B). However, the mineralogical composition of the Datanhao coal is quite different from that of both neighboring areas.

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The bauxite minerals (e.g., diaspore and boehmite) and aluminophosphate mineral (e.g., gorceixite), which are relatively abundant in the Adaohai coal (Dai et al., 2012b), are not present in the Datanhao coal. This may be because of the difference in the sediment source regions. As indicated by Dai et al. (2012b), the sediment-source region for the Adaohai Mine was in part the oxidized bauxite in the weathered crust of the Benxi Formation to the N and E. However, such material was not transported into the original peat mire of the Datanhao coal, which was developed in an intermontane basin of the orogenic belt.

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High proportions of calcite and dolomite in the Adaohai coal were suggested by Dai et al. (2012b) to derive probably from igneous fluids. Carbonate minerals in the Datanhao coal, although very common, are not very common as those in the Adaohai coal. This is also consistent with an increasing degree of coal metamorphism from the east to the west of the Daqingshan Coalfield.

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Dai et al. (2012b) also noticed some correlation between the presence of high illite and carbonate proportions in the Adaohai CP2 coal, and thus assumed that the illite and diaspore were formed due to the heat supplied by the igneous intrusion. However, there is no distinct correlation between carbonates and illite in the coal and non-coal samples of the Datanhao mine. The coals from both the Datanhao and Hailiushu mines are also very different in the mineralogical composition. Quartz, which is relatively abundant in the Datanhao coal, is only a minor mineral component in the Hailiushu coal. Both the Datanhao and the Hailiushu coals were deposited in intermontane basins of the orogenic belt. This may be due to the difference in the source rocks in their respective material source region, namely the surrounding sub-uplifts.

6. Summary and conclusions The Pennsylvanian coal seam in the Datanhao mine, which is around 10 km west of the Adaohai mine, has different mineralogical characteristic from both the Adaohai and the Hailiushu mines. The Datanhao coal is mainly high-ash and low-sulphur bituminous coal. The minerals in the coal samples are mainly well-ordered kaolinite and quartz, and varying proportions of carbonates (calcite, ankerite and siderite), as 16

ACCEPTED MANUSCRIPT well as minor muscovite, illite, pyrite and anatase. The intra-seam claystone bands have similar mineral assemblage, except that most non-coal samples have relatively less abundant quartz than the adjacent coal plies.

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The presence of abundance syngenetic siderite indicates deposition of the coal mainly under non-marine conditions. The siderite content and the sum of calcite and ankerite moderately correlate. Compression of organic matter around the syngenetic siderite nodules probably caused fractures in the surrounding area, which subsequently were filled in by epigenetic calcite and ankerite at a relatively late diagenetic stage.

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The relatively high proportions of quartz in the coal plies indicate a short distance from the material source region, which is consistent with the deposition of the original peat in an intermontane basin of the orogenic belt. The sediment-source region was probably the Ordovician quartz-rich sandstone or the Sinan quartzite in the surrounding sub-uplifts within the orogenic belt. The common presence of β-quartz, and graupen, tabular and vermicular kaolinite in the intra-seam non-coal samples from the Datanhao coal seam indicates that most of these samples were tonsteins, derived from high level volatile-rich silicic volcanic ash.

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This research was supported by the National Key Basic Research Program of China (No. 2014CB238900), the National Natural Science Foundation of China (Nos. 41672152 and 41302128), and the Fundamental Research Funds for the Central Universities of China (2014QM01). The authors would like to thank Prof. Yaofa Jiang, Mrs. Tianjiao Li and Cunliang Zhao for their assistance in sample collection. Prof. Colin Ward, Drs. Ian Graham, and David French are thanked for their assistance in mineral identification. Thanks are also expressed to two anonymous reviewers for their constructive comments on the manuscript.

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ACCEPTED MANUSCRIPT Fig.1. Paleogeographic map of the Late Paleozoic in North China and location of the Daqingshan and Jungar Coalfields, modified from Han and Yang (1980) and Dai et al. (2015a) (A), and coal-rank distribution in the different mines of the Daqingshan coalfield, after Dai et al. (2012b)

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Fig. 2. Sedimentary sequences of the Daqingshan Coalfield, (Modified from Dai et al., 2012b).

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Fig. 3. Comparison between proportions of major-element oxides in coal LTAs and non-coal samples from the Datanhao mine, inferred from Siroquant and determined by XRF. The diagonal line represents equality in each plot.

Fig. 4. Minerals in the Datanhao non-coal samples under the optical microscope (reflected light).

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(A) detrital quartz grains (grey) in collodetrinite, in air; (B) vermicular kaolinite, oil immersion; (C) clay minerals occurring as laminae, oil immersion; (D) siderite nodules surrounded by

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Fig. 5. Minerals in the Datanhao non-coal samples. (A) framboidal pyrite; (B) euhedral pyrite crystals; (C) pyrite framboids (py) and siderite (S) nodules; D siderite nodules (S) and epigenetic

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pyite (py); (E) calcite (Cal), sphalerite and Ca-bearing REE-phosphate and as cell-infillings; (F)

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galena and calcite (Cal) as cell-infillings. (A), (B), (C) and (D) are images under the optical microscope, in air with reflected light; (E) and (F) are SEM backscattered electron images.

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Fig. 6. Quartz in clay groundmass of non-coal samples under the optical microscope. (A), (B) and (C) are angular quartz grains, note a quartz crystal in the center of (B) shows textbook magmatic resorption; quartz in (D), (E), the center of (F), (G) and (H) are β-quartz crystals. Note the β-quartz in (D) has a syngenetic inclusion. The quartz crystals in the center of (G) and (H) have probably

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been broken or corroded. (A) is observed under transmitted, cross polarized light; (B)-(H) are observed under reflected, plane polarized light. Fig. 7. Kaolinite in different intra-seam non-coal samples. (A) homogenous kaolinite with a euhedral zircon grain in the center, under transmitted, cross polarized light; (B) graupen kaolinite in sample D-39-P, under transmitted, cross polarized light; (C) SEM backscattered electron image of vermicular kaolinite with white inclusions being sulfide phase(s); (D), (E), (F), vermicular and tabular kaolinite after biotite, under reflected, plane polarized light. Fig. 8. SEM backscattered electron images of minerals in non-coal samples. (A) anatase and zircon; (B) anatase and Nb,Zr-bearing anatase; (C) epigenetic barite; (D) galena and quartz (Q); (E); Ca,Th,U-bearing REE phosphate (possibly monazite); (F) EDS spectrum of spot 1 in (E). Fig. 9. Comparison of the proportions of siderite and the sum of calcite and ankerite, on LTA (coal samples) or HTA (rock samples) basis.

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