Mineralogy and Geochemistry of the Lower Cretaceous Coals in the Junde Mine, Hegang Coalﬁeld, Northeastern China

: Hegang coalﬁeld is one of the areas with abundant coal resources in Heilongjiang Province. Characteristics of minerals and geochemistry of No. 26 coal (lower Cretaceous coals) from Junde mine, province, China, were reported. The results showed that No. 26 coal of Junde mine is slightly enriched in Cs, Pb, and Zr compared with world coals. The minerals in No. 26 coal of Junde mine primarily include clay minerals and quartz, followed by calcite, siderite, pyrite, monazite, and zircon. The diagrams of Al 2 O 3 –TiO 2 , Zr/Sc–Th/Sc, Al 2 O 3 /TiO 2 –Sr/Y, and Al 2 O 3 /TiO 2 –La/Yb indicate that the enriched elements in No. 26 coal were mainly sourced from the Late Paleozoic meta-igneous rocks in Jiamusi block. The volcanic ash contribution to No. 26 coal seems very low. Sulfate sulfur indicating oxidation/evaporation gradually decreases during No. 26 coal formation.

Heilongjiang Province, Northeastern (EN) China, has a vast territory and rich resources. In terms of coal resources, the Hegang coalfield is rich [20,21]. Although there has been much literature focusing on coal geochemical and mineralogical characteristics in China, these works are mainly concentrated in the coals in southwestern and northern China [13,[22][23][24][25][26][27][28][29]. In Heilongjiang province, a few studies [30,31] have been carried out about coal-bearing sequences in the Hegang coalfield. Previous investigations have shown that the Hegang coalfield was affected by three periods of tectonic stress and accompanied by multiple periods of volcanic activity, which had an impact on coal quality in the coal basin [32]. There are still two problems to be solved about coal geochemistry in the Hegang coalfield: (1) did the volcanic activity influence the element enrichment in the coals? (2) The occurrence modes and the provenance of elements in the Palaeogene coal-bearing seams have been deeply studied recently in Jilin province [33][34][35][36], neighboring the Heilongjiang province. Is the provenance of trace elements in Hegang coals the same as in the coals from Jilin Province [33,35]? Further investigation of these issues has important theoretical and economic value for improving the comprehensive utilization efficiency of coal measures mineral resources. The major coal-bearing formation in the Hegang coalfield is the Lower Cretaceous Chengzihe Formation. The Lower Cretaceous Chengzihe Formation consists of graywhite conglomerate, coarse and fine sandstone, gray-yellow medium sandstone, dark gray siltstone, mudstone, coal seams, and tuff. In Hegang coalfield, 36 coal seams are minable or partially minable, which are divided into four coal bearing groups from top to bottom. The first coal seam group includes coal seams No. 1 and No. 2, which are unstable. The second coal seam group includes coal seams No. 3~22. The third coal seam group, including coal seams No. 23~26, is mainly developed in the south of the coalfield, and its thickness gradually thins or even pinches out to the north. The fourth coal seam group includes coal seams No. 27~32.
No. 26 coal, as the main mining seam in Junde mine from Hegang coalfield, is the main research object of this paper. The thickness of No. 26 coal is about 2.9 m, deposited in continental facies [37,38].

Sampling and Analytical Techniques
Ten samples were obtained in No. 26 coal in Junde mine, Hegang Coalfield, which include one roof sample, eight coal samples, and one floor sample ( Figure 2). From top down, eight coal samples are identified as JD-26-01 to JD-26-08 ( Figure 2B). The roof and floor are identified as JD-26-R and JD-26-F, respectively ( Figure 2B). All samples are fresh, without pollution and oxidation.  [37,38].

Sampling and Analytical Techniques
Ten samples were obtained in No. 26 coal in Junde mine, Hegang Coalfield, which include one roof sample, eight coal samples, and one floor sample ( Figure 2). From top down, eight coal samples are identified as JD-26-01 to JD-26-08 ( Figure 2B). The roof and floor are identified as JD-26-R and JD-26-F, respectively ( Figure 2B). All samples are fresh, without pollution and oxidation.   According to ASTM D3173-11, D3174-11 and D3175-11(2011) [39][40][41], the coal samples collected from Junde mine were tested for the proximate analysis. The macerals were obtained based on the ICCP System 1994 (ICCP, 1998(ICCP, , 2001 [42,43] and Pickel, Kus [44]. The total sulfur content was obtained by ZCL-3 automatic sulfur analyzer, and the contents of various sulfur forms in the samples were obtained according to ASTM D3177-02(2011) and ASTM D2492-02(2012) [45,46]. These analyses were made at the Société Générale de Surveillance S.A-China Stand Science and Technology Group Corporation (SGS-CSTC) Standards Technology Service Corporation.
Based on SY/T 5163-2010, the minerals were identified by X-ray diffractometer. The morphology of minerals was observed by the scanning electron microscopy (SEM) with an energy dispersive X-ray spectrometer (EDS). The working distance was 8.4-11.4 mm, and beam voltage is 15.0 kV during the SEM operation. These experiments were conducted at the Beijing Center for Physical and Chemical Analysis (BCPCA).
The cleaned samples used for the geochemical analysis were crushed and ground to less than 200-mesh size using a tungsten carbide ball mill. The Major Oxides contents were obtained using X-ray fluorescence spectrometry (XRF; PANalytical Axios). The trace elements were obtained by the inductively coupled plasma mass spectrometry (ICP-MS, ELEMENT XR). Before ICP-MS analysis, samples were digested by using an UltraClave Microwave High Pressure Reactor (Milestone, Milan, Italy). Multi-element standards (Inorganic Ventures: CCS-1, CCS-4, CCS-5, and CCS-6; GBW07103, GBW07104) were used for trace element concentrations calibration. More method details can be seen in [47]. XRF analyses and the ICP-MS analyses were performed at the China National Nuclear Corporation (CNNC) Beijing Research Institute of Uranium Geology.

Coal Characteristics and Coal Petrology
As shown in Table 1, the ash yield and total sulfur of Junde coals is 8.6-17.6% (13.0% on average) and 0.54-0.72%, (0.62% on average). The contents above show that No. 26 coals are characterized by low ash and low sulfur based on GB15224.1-2010 and GB15224.2-2010 [49,50]. In addition, sulfur in coal occurs mainly in organic and pyritic sulfur. Sulfate sulfur increases with depth.  Table 2). It is composed mainly of collotelinite ( Figure 3A-C,E,I) and collodetrinite ( Figure 3A,D,I), and to a lesser extent, by telinite ( Figure 3H), with a few corpogelinite and virtrodetrinite. The collodetrinite is usually in matrix form, embedded in quartz ( Figure 3B), clay minerals ( Figure 3D,I), calcite ( Figure 3E), or fracture-filling pyrite ( Figure 3G).      Table 3 shows the MEO percentages of studied coals and the average value of Chinese coals. The MEO in studied coals are primarily composed of SiO 2 (average 7.63%), Al 2 O 3 (average 2.95%) and Fe 2 O 3 (average 1.62%). By comparison with the average values of Chinese coals [22], the percentages of MnO and K 2 O are a bit higher, while the content of other MEO is near or below ( Figure 4). Value of SiO 2 /Al 2 O 3 of No. 26 coals (average 2.65) exceeds that of Chinese coals [22] and the theoretical figure of SiO 2 /Al 2 O 3 of kaolinite, probably because a high content of quartz existed in the studied coals ( Figure 3B,D). Notes: LOI, loss on ignition; a , concentration of MEO in Chinese coals [22].
In addition, from the vertical section of No. 26 coal seams, the contents of Cs, Pb, and Zr in coal near the host rocks (roof and floor) outclass those in the middle coal seams ( Figure 6).

Rare Earth Elements and Yttrium (REY)
REY geochemical classification adopted in our study is according to [55]. Previous studies have shown that because of interference of BaO or BaOH, the Eu content measured in coal by ICP-MS based on quadrupole should be carefully used to identify the positive Eu anomaly [56,57]. In our study, the Ba/Eu values (56 to 370, average of 185), and the relation degree of Ba and Eu in coal samples are low (Figure 7), indicating that Ba concentration has no interference with Eu content.  Figure 5C). In addition, from the vertical section of No. 26 coal seams, the contents of Cs, Pb, and Zr in coal near the host rocks (roof and floor) outclass those in the middle coal seams ( Figure 6). Average concentration of trace elements of world hard coals is used to normalize the corresponding element concentration in studied coal samples [53]. Average concentration of trace element of world clays is used to normalize the corresponding element concentration in the studied roof and floor samples [54].  [53]. Average concentration of trace element of world clays is used to normalize the corresponding element concentration in the studied roof and floor samples [54].     REY geochemical classification adopted in our study is according to [55]. Previous studies have shown that because of interference of BaO or BaOH, the Eu content measured in coal by ICP-MS based on quadrupole should be carefully used to identify the positive Eu anomaly [56,57]. In our study, the Ba/Eu values (56 to 370, average of 185), and the relation degree of Ba and Eu in coal samples are low (Figure 7), indicating that Ba concentration has no interference with Eu content.  The contents of REY of No. 26 coal in the Junde mine (from 34.1 µg/g to 174.6 µg/g, average 80.6 µg/g; Table 5) slightly exceed those in the world's hard coal [53], but below those in Chinese coals [22] and the upper continental crust [58]. The REY contents in host rocks (roof and floor) are about four times that of coals (Table 5). Additionally, REY contents of the roof (340.9 µg/g) and the floor (327.8 µg/g) are well above those of average world clays [54] and the upper continental crust [58].
According to [55], the features of REY in No. 26 coals from Junde mine are LREY enrichment, with obviously negative Eu ( Figure 8A), and, in the upper coal bench only, by HREY enrichment with obviously negative Eu, and positive Y anomalies. Correspondingly, the feature of REY of the roof is HREY enrichment like the top coal sample, while the feature of REY of the floor is LREY enrichment, like other coal samples ( Figure 8B).
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Mineralogy
Based on X-ray diffraction experiment (XRD) results (Figure 9), clay minerals and quartz are the chief minerals in the Junde coals, and after them calcite, siderite, and pyrite. Some monazite and zircon were detected in No. 26 coal by SEM-EDS. The minerals in the floor and roof are primarily clay minerals, quartz, potassium feldspar, and plagioclase.

Mineralogy
Based on X-ray diffraction experiment (XRD) results (Figure 9), clay minerals and quartz are the chief minerals in the Junde coals, and after them calcite, siderite, and pyrite. Some monazite and zircon were detected in No. 26 coal by SEM-EDS. The minerals in the floor and roof are primarily clay minerals, quartz, potassium feldspar, and plagioclase.

Quartz
In No. 26 coals, some quartz occurs as fine-grained particles ( Figure 10C,D), indicating a syngenetic detrital origin. A lot of quartz in the studied coals has sharp edges Figures 3B,D and 10A) and indicates the influence of high temperature and volcanic activity [35,59,60].

Pyrite
Some pyrite in No. 26 coal is in the form of discrete crystals ( Figures 3B,D and 10E), suggesting an authigenic origin. In addition, fracture fillings pyrite has also been found ( Figures 3A,G and 10F), suggestive of an epigenetic origin [61].

Carbonate Minerals
Calcite and siderite are the major carbonate minerals. Calcite generally occurs as plates ( Figure 10G) and crack fillings ( Figure 10H), suggesting an authigenic origin and epigenetic origin, respectively. Additionally, the siderite occurs as fracture-fillings cutting through the kaolinite ( Figure 10I), indicating an epigenetic origin.

Quartz
In No. 26 coals, some quartz occurs as fine-grained particles ( Figure 10C,D), indicating a syngenetic detrital origin. A lot of quartz in the studied coals has sharp edges (Figures 3B,D and 10A) and indicates the influence of high temperature and volcanic activity [35,59,60].

Pyrite
Some pyrite in No. 26 coal is in the form of discrete crystals ( Figures 3B,D and 10E), suggesting an authigenic origin. In addition, fracture fillings pyrite has also been found ( Figures 3A,G and 10F), suggestive of an epigenetic origin [61].

Carbonate Minerals
Calcite and siderite are the major carbonate minerals. Calcite generally occurs as plates ( Figure 10G) and crack fillings ( Figure 10H), suggesting an authigenic origin and epigenetic origin, respectively. Additionally, the siderite occurs as fracture-fillings cutting through the kaolinite ( Figure 10I), indicating an epigenetic origin.

Sediment Source
The value of Al2O3/TiO2 is commonly used to infer the sediment source (including coal deposits) [62,63]. The value of Al2O3/TiO2 in Junde coals varies from 26 to 45 (average 22.6) ( Figure 13A), suggesting source rocks during No. 26 coal formation in Junde mine are primarily felsic rocks. This conclusion can also be evidenced by the diagrams of Zr/Sc-Th/Sc ( Figure 13B), which have been proved a useful provenance indicator for coal deposits [64,65].
The No. 26 coals in the study area and the Fuqiang coals from the Hunchun Coalfield [33] have similar Cs-Pb-Zr enrichment characteristics. The provenance in Fuqiang coals is

Sediment Source
The value of Al 2 O 3 /TiO 2 is commonly used to infer the sediment source (including coal deposits) [62,63]. The value of Al 2 O 3 /TiO 2 in Junde coals varies from 26 to 45 (average 22.6) ( Figure 13A), suggesting source rocks during No. 26 coal formation in Junde mine are primarily felsic rocks. This conclusion can also be evidenced by the diagrams of Zr/Sc-Th/Sc ( Figure 13B), which have been proved a useful provenance indicator for coal deposits [64,65].
The No. 26 coals in the study area and the Fuqiang coals from the Hunchun Coalfield [33] have similar Cs-Pb-Zr enrichment characteristics. The provenance in Fuqiang coals is Mesozoic and Paleozoic igneous and metamorphic rocks [33]. To further explore the terrigenous supply of No. 26 coal, a comparison about Sr/Y, La/Yb, and Al 2 O 3 /TiO 2 values of Junde coals with the local (the Jiamusi block) rocks of Neoproterozoic, Late Paleozoic, Mesozoic and Cretaceous ages [66][67][68][69] is shown in Figure 14. Figure 14 indicates that the source material of the Junde mine mainly comes from the Late Paleozoic igneous rocks from Jiamusi block. In addition, the REY patterns of No. 26 coal are akin to those of the Late Paleozoic igneous rocks with negative Eu anomalies ( Figure 15). Sun et al. have also determined that detrital zircons of Mesozoic and Paleozoic ages predominate in sandstones of the Chengzihe Formation [70]. Those zircons are shaped similarly to the zircons described in this study ( Figure 11A-C). Therefore, the terrigenous components in No. 26 coal from Junde mine were mainly derived from the Late Paleozoic meta-igneous rocks in local block.

Sediment Source
The value of Al2O3/TiO2 is commonly used to infer the sediment source (including coal deposits) [62,63]. The value of Al2O3/TiO2 in Junde coals varies from 26 to 45 (average 22.6) ( Figure 13A), suggesting source rocks during No. 26 coal formation in Junde mine are primarily felsic rocks. This conclusion can also be evidenced by the diagrams of Zr/Sc-Th/Sc ( Figure 13B), which have been proved a useful provenance indicator for coal deposits [64,65].
The No. 26 coals in the study area and the Fuqiang coals from the Hunchun Coalfield [33] have similar Cs-Pb-Zr enrichment characteristics. The provenance in Fuqiang coals is Mesozoic and Paleozoic igneous and metamorphic rocks [33]. To further explore the terrigenous supply of No. 26 coal, a comparison about Sr/Y, La/Yb, and Al2O3/TiO2 values of Junde coals with the local (the Jiamusi block) rocks of Neoproterozoic, Late Paleozoic, Mesozoic and Cretaceous ages [66][67][68][69] is shown in Figure 14. Figure 14 indicates that the source material of the Junde mine mainly comes from the Late Paleozoic igneous rocks from Jiamusi block. In addition, the REY patterns of No. 26 coal are akin to those of the Late Paleozoic igneous rocks with negative Eu anomalies ( Figure 15). Sun et al. have also determined that detrital zircons of Mesozoic and Paleozoic ages predominate in sandstones of the Chengzihe Formation [70]. Those zircons are shaped similarly to the zircons described in this study ( Figure 11A-C). Therefore, the terrigenous components in No. 26 coal from Junde mine were mainly derived from the Late Paleozoic meta-igneous rocks in local block.

Sedimentary Environment
The value of Sr/Ba has been widely used to identify the depositional environment of sedimentary rocks and coals [71,72]. Generally, coals with Sr/Ba ratio greater than 1 and less than 1 was formed in the environment of seawater and freshwater intrusion, respectively. However, we should be cautious when using this indicator, because the content of Sr and Ba in terrigenous clastic minerals (especially feldspar) may cause misjudgment of the sedimentary environment [73,74]. In the present study, feldspar content is so low that it is not detected by XRD. Thus, Sr/Ba ratio is used to roughly distinguish sedimentary environment in this study. Sr/Ba value of No. 26 coal seam from Junde mine varies from 0.48 to 1.01, and the average Sr/Ba value of the coal seam is 0.81, suggestive of a primarily fresh-water affected process of No. 26 coal.
Because NE China is abundant with Cu porphyry ores [75] that may be eroded and consequently changing the ratio in terrigenous sediments, Sr/Cu ratio is not suitable to infer the sedimentary environment. The relationship of sulfur and the environment has been widely recognized [72]. In the Junde mine, sulfate sulfur increases with depth ( Figure 16), which indicate oxidation/evaporation gradually decreases during No.26 coal formation.

Sedimentary Environment
The value of Sr/Ba has been widely used to identify the depositional environment of sedimentary rocks and coals [71,72]. Generally, coals with Sr/Ba ratio greater than 1 and less than 1 was formed in the environment of seawater and freshwater intrusion, respectively. However, we should be cautious when using this indicator, because the content of Sr and Ba in terrigenous clastic minerals (especially feldspar) may cause misjudgment of the sedimentary environment [73,74]. In the present study, feldspar content is so low that it is not detected by XRD. Thus, Sr/Ba ratio is used to roughly distinguish sedimentary environment in this study. Sr/Ba value of No. 26 coal seam from Junde mine varies from 0.48 to 1.01, and the average Sr/Ba value of the coal seam is 0.81, suggestive of a primarily fresh-water affected process of No. 26 coal.
Because NE China is abundant with Cu porphyry ores [75] that may be eroded and consequently changing the ratio in terrigenous sediments, Sr/Cu ratio is not suitable to infer the sedimentary environment. The relationship of sulfur and the environment has been widely recognized [72]. In the Junde mine, sulfate sulfur increases with depth ( Figure  16), which indicate oxidation/evaporation gradually decreases during No.26 coal formation.

Influence of Volcanic Ash
It is generally believed that the Eu anomaly in coal does not originate from the weathering process during the transportation of metals from provenance to coal forming peat, but is inherited from rocks in the source areas [56]. Eu anomalies characteristics of No. 26 coal from Junde mine (including roof and floor) resembles those of felsic volcanic rocks.
Previous studies suggested that the crystalline habit and morphology of terrigenous detrital zircons are quite different from those of igneous detrital zircons [76]. The

Influence of Volcanic Ash
It is generally believed that the Eu anomaly in coal does not originate from the weathering process during the transportation of metals from provenance to coal forming peat, but is inherited from rocks in the source areas [56]. Eu anomalies characteristics of No. 26 coal from Junde mine (including roof and floor) resembles those of felsic volcanic rocks.
Previous studies suggested that the crystalline habit and morphology of terrigenous detrital zircons are quite different from those of igneous detrital zircons [76]. The characteristics of the former are tetragonal bipyramids with relatively short prisms, and the length width ratio (c/a values) is about two [77], while the feature of the latter is long and well-developed, with a c/a value of more than 2.5 [77]. In the present study, the ratio of c/a in the zircon found in No. 26 coal ( Figure 11C) is > 2.5, indictive of a pyroclastic origin. Another possibility that its angular and elongated shapes may evidence just short transportation by relatively quiet water streams cannot be ruled out.
A large number of sharp-edged quartz particles were observed in No. 26 coal ( Figures 3B,D and 10A), indicating a volcanic origin rather than a terrigenous clastic origin [35,59,60].
Although zircon and some high-T quartzs have been found in the No. 26 coals, the vermicular kaolinite/chlorite particles that commonly indicate volcanic ash have not been found. Thus, the volcanic ash contribution to No. 26 coal seems very low, if occurred. This conclusion is consistent with [70].

Conclusions
The characteristics of minerals and elements of No. 26 coal from Junde mine, Hegang coalfield, northeastern China, were studied. The main conclusions are summarized as follows.