Occurrence of some valuable elements in the unique ‘high-aluminium coals’ from the Jungar coalfield, China

Ore Geology Reviews 72 (2016) 659–668

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Occurrence of some valuable elements in the unique ‘high-aluminium coals’ from the Jungar coalfield, China Yuzhuang Sun a, Cunliang Zhao a,⁎, Shenjun Qin b, Lin Xiao b, Zhongsheng Li c, Mingyue Lin a a b c

Key Laboratory of Resource Exploration Research of Hebei Province, Hebei University of Engineering, Guangming Nan Dajie 199, Handan, Hebei 056038, China Collaborative Innovation Center of Coal Exploitation, Hebei Province, Hebei University of Engineering, Handan 056038, Hebei, China CSIRO Energy, Onshore Gas, 11 Julius Ave, North Ryde, New South Wales 2113, Australia

a r t i c l e

i n f o

Article history: Received 20 June 2015 Received in revised form 13 September 2015 Accepted 15 September 2015 Available online 20 September 2015 Keywords: Al and rare elements High-Al coal Occurrence Origin Jungar coalfield

a b s t r a c t In China, a unique type of coal with extremely high aluminium (Al) content commonly occurs in the southern region of the Yinshan Oldland, Jungar Basin. The typical Al2O3 content in the coal ash is ~45%, and exceeds 60% in some cases. This type of coal forms a league of its own and is named as ‘high-Al coal’. This high-Al coal, not only contains unusually high level of Al but also has high concentrations of Li, Ga, and REY (REE + Y). All these elements (i.e. Al, Li, Ga, REY) with high concentrations in this type of coal are very valuable source to various industries and therefor make perfect economic sense and add extra benefits into coal mines if they can be recovered by beneficiation processes. The mineral compositions in these coals mainly consist of kaolinite, boehmite, chlorite-group minerals, along with minor amounts of quartz, calcite, pyrite, siderite and amorphous clay materials. The Al and rare elements show different patterns of distribution and affinity relationships with organic and inorganic matter. The strong peraluminous granites and moyite from the Yinshan Oldland may be the main source of the Al and rare elements in the coals. Bauxite from the Benxi Formation may also be another source for the Al and rare elements in the coal of the Jungar coalfield. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Coal is the most abundant fossil fuel in China and it can meet China's energy requirements for next few hundreds of years (Zhao et al., 2014). In recent years, metallic elements (e.g. Ge, Ga, Li) with high concentrations in coal which may be economically recovered through beneficiation process have been reported in literature (Xu et al., 2011; Sun et al., 2010a,b,c,d, 2012a, 2012b, 2013a, 2013b; Seredin et al., 2013; Dai et al., 2012a; Qin et al., 2015a, b). For example, the presence of Ge (Zhuang et al., 2006), Ga (Dai et al., 2006, 2010), Li (Sun, 2015), Nb (Dai et al., 2014) and REY (Seredin and Dai, 2012) in high concentrations in coals has been reported. Coal with high Al content has been identified in some Chinese coals. Yao et al. (2014) reported an averaged 45% Al 2 O 3 content, and in some cases exceeds 60% of in the Chinese coal ashes they studied. This type of coal has been named as ‘high-Al coal’ and commonly occurs in the southern region of the Yinshan Oldland, China (Qi et al., 2006). High Al concentration along with high Li, Ga, and REY concentrations have been found recently in the coal seams from the coalfields in southern Yinshan Oldland, NW China (Dai

⁎ Corresponding author at: Guangming Nan Dajie 199, Handan, Hebei 056038, China. E-mail addresses: [email protected] (Y. Sun), [email protected] (C. Zhao).

http://dx.doi.org/10.1016/j.oregeorev.2015.09.015 0169-1368/© 2015 Elsevier B.V. All rights reserved.

et al., 2006; Sun et al., 2012b, 2013b). The concentrations of Al and rare elements (i.e. Li, Ga and REY) are very high in the Jungar coalfield. The mechanisms of the Al and rare element enrichment in the Jungar coalfield have been discussed in previous studies (Dai et al., 2006, 2012a; Sun et al., 2013b). The Jungar coalfield has 31 coal mines and exploration blocks (Shi, 2014). The purpose of this study is to: (1) take ‘high-Al coal” samples from all coal mines in the Jungar coalfield, (2) study the distribution patterns of Al, Li, Ga and REY in all those samples collected in order to develop a regional trend, and then (3) discuss and offer the possible enrichment mechanisms of these elements in coal.

2. Geological settings 2.1. Occurrence of ‘high-Al coals’ in the southern Yinshan Oldland ‘High-Al coals’ occur mainly in the southern regions of the Yinshan Oldland, including Jungar, Shendong, Datong and Ningwu coalfields (Fig. 1). These coals commonly occur in the PermoCarboniferous sequence and usually occur as 1–5 coal seams in different coalfields (Sun et al., 2013a, b; Xu et al., 2011; Dai et al., 2006, 2010). The total coal resources in this region reach around 100 Gt (ONDRC, 2011).

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Fig. 1. Paleogeographic map of the Late Paleozoic in the study area. After Sun. et al., 2013a.

2.2. Jungar coalfield The Jungar coalfield is located in the southern Yinshan Oldland (Fig. 1). The Jungar coalfield occurs as one of several Late Palaeozoic coal-bearing basins in this region (Dai et al., 2006). The Jungar coalfield is ~ 64 km long (N–S) by ~ 26 km wide (W–E), with a total area of 1700 km2. This coalfield was sustained dynamic tectonic activities, and the formation, sedimentation and evolution of the Jungar coalfield was controlled by the tectonic processes of the Central Asian Orogenic Belt (CAOB) (Xiao et al., 2009; Yang et al., 2011). The Permo-Carboniferous denudation processes of the basin development started from around the Cambrian–Ordovician periods, then gradually progressed to the Middle and Late Palaeozoic (Jian et al., 2012). During the period from the end of the Early Permian to Late Permian, intermediate-felsic lavas erupted in the CAOB and this lava sequence might have provided an important source of Al, Li Ga and REE and Y to the coal formations in this region. The detailed lithology, petro chemistry and REE (rare earth element) analyses of the volcanic rocks of the Late Palaeozoic basins reveal their close signature and point to derivation from volcanics in the CAOB. The volcanic rocks are formed from intermediatefelsic materials that erupted in the Late Carboniferous and Early Permian. The eroded volcanic materials were possibly transported and feeded into the basin by wind (Hanson, et al., 2007; Yang et al., 2011). In the Late Carboniferous, the ancient ‘Yinshan’ continental block was uplifted and became a land-source area due to a compressional regime formed by the strong subduction of the North China Block underneath the South Mongolia micro-plate (Peng et al., 2010). With the strong collision and uplifting from the end of the Early Permian-to Early Triassic, the ‘Yinshan’ block gradually became a foreland folded thrust belt and was strongly overlapped and transformed by the Mesozoic–Cenozoic or orogenic movements, eventually turning into the present denudation area characterized by an ancient metamorphic and crystallised basement rocks (Peng et al., 2010; Jian et al., 2012).

The CAOB and the Late Paleozoic coal-bearing basins developed within the same structural framework and dynamic regime (Hanson et al., 2007). Their formation and evolution were controlled by the tectonic processes between the Siberian plate and North China plate. The Late Paleozoic coal-bearing basin in the northern margin of North China gradually transformed along with the formation and evolution of the orogenic belt to the north. Meanwhile, the coal-measures in the basin were also transformed under the evolution of the CAOB. Its depressional and sedimentary rate and variation in the sandstone and conglomerate clast compositions were mainly controlled by the plate processes of the CAOB across different time periods (Yuan et al., 2007). The coal-bearing sequences in the Jungar coalfield essentially present as the Benxi Formation and the Taiyuan Formation (both Pennsylvanian). The Benxi Formation, with a thickness of 15–35 m, lies unconformably over the Middle Ordovician Majiagou Formation and was deposited in a shallow marine environment. The sediments are mainly composed of bauxite, sandstone, mudstone, and siltstone (Yao et al., 2009). The Taiyuan Formation, with a total thickness of 21–95 m, is mainly composed of grey and greyish-white quartzose sandstone, mudstone, siltstone and coal which is interbedded with dark-grey mudstone, siltstone, limestone, and thin-bedded quartzose sandstone. The Taiyuan Formation was formed in paralic delta and tidal flat-barrier complex environments. The No. 6 coal seam is located at the uppermost Taiyuan Formation and has a thickness between ~12 and ~18 m (with an average 15 m). There are 9 partings present sandwiched in the No. 6 coal seam with a cumulative thickness of 2 m (Dai et al., 2006, 2010; Sun et al., 2013b). The Shanxi Formation, with a total thickness of 45–90 m, is composed of mainly terrigenous coal-bearing clastic rocks dominated by sandstones. It was formed in fluvial and delta environments. The Shanxi Formation includes five coal seams (Nos. 1, 2, 3, 4 and 5), but only Nos. 3 and 5 are locally minable. The strata overlying the coal-bearing sequences are the non-coal-bearing Upper Shihezi Formation, Lower Shihezi Formation, and Shiqianfeng Formation (Dai et al., 2006, 2010; Sun et al., 2012b).

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3. Al and rare element in the ‘high-Al coal’ of southern Yinshan Oldland 3.1. Al2O3 The coal ash from the coals of the Jungar coalfield are rich in alumina, making it a potential substitute for bauxite (Blissett and Rowson, 2012; Yao et al., 2014, 2015). The Al2O3 contents in the coal ashes are shown in Table 1, and most of the Al2O3 values (19 out of 21 mines) are higher than 41%. The higher Al2O3 contents in coal ash (≥50%) exist in the middle of the Jungar coalfield (Fig. 2). The coal reserve in the study area is approximately 36.7 Gt (Sun et al., 2013b). Out of this coal reserves, about 25% would be ash potentially generated from the total coal resources and thus, calculated total potential Al2O3 content is up to 3.7 Gt (approximately 2 Gt Al). According to the Geology and Ore Deposit Standard Specifications for Bauxite Exploration of the People's Republic of China (DZ/T 02032002, 2003), the deemed economic grade for Al2O3 deposits is taken as N40% of the rock. There is no minimum economic mining grade assigned for Al content in coal seams at present. Referring to the standard DZ/T 0203-2002 (2003), we suggest setting the economic grade for Al2O3 deposits for coal ash the same as 40% in rocks and thus assume that this coal has attained the status of an Al2O3 deposit.

3.2. Concentrations of Li Lithium (Li) is an important element widely used for making batteries for energy storage with great significance to the global economy and national security (Tatascon and Armand, 2001). Li enrichment in coal has been studied by many authors (Finkelman, 1993; Dai et al., 2012a; Sun et al., 2010b, c; Lewińska-Preis et al., 2009; Ketris and Yudovich, 2009; Qin et al., 2015b). Except for China, the average Li contents in coals are generally less than 20 mg/kg in other countries (Franceschelli et al., 1998; Zivotic et al., 2008; Dale and Lavrencic, 1993; Hu et al., 2006; Kara-Gulbay and Korkmaz, 2009). The Li contents of coal ashes of the Jungar coalfield coals are shown in Table 1. The average Li content is 229 mg/kg in the coal samples and 654 mg/kg in the parting samples of the Guanbanwusu Mine. The highest Li values are 710 mg/kg in the GB-A17 bench coal sample and 1592 mg/kg in the PB-5 parting sample, respectively (see details in

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Sun et al., 2012b). The Li has been concentrated to 1603 mg/kg or 3435 mg/kg Li2O in the ash of the Guanbanwusu coal samples. The average Li content is 143 mg/kg in the coal samples of the Heidaigou Surface Mine, with the highest Li content of 379 mg/kg. The average ash yield of this coal is 16% in these samples. The Li is concentrated to 1013 mg/kg or 2170 mg/kg Li2O in the ash derived from the Heidaigou coal samples. The average Li content is 119 mg/kg in the coal samples of the Haerwusu Surface Mine, with the highest Li content of 498 mg/kg. The average ash yield is 15% for these coal samples. The Li is concentrated to 987 mg/kg or 2115 mg/kg Li2O in the ash of the Haerwusu coal samples. Yudovich and Ketris (2006) suggested a minimum 100 mg/kg as being the economic grade of Li deposits. Sun et al. (2012a, 2014) have studied Li concentrations in many Chinese coals and suggested that, it was reasonable to take 80 mg/kg Li as the minimum mining grade and 120 mg/kg as the economic grade or industrial grade for potentially economic Li beneficiation from Chinese coals. According to their demarcation, the Jungar coalfield has reached the level of economic deposits for Li beneficiation. The coal reserves of the No. 6 Seam in the Jungar coalfield is 1.91 × 1010 tons. Given the minimum Li content of 126 mg/kg in the No 6 seam, the total Li resources could reach as much as 2,406,600 tons, or 5,157,000 tons of Li2O in the No. 6 Seam alone in the Jungar coalfield. According to the classification of Sun et al. (2012a, 2014), this amounts of Li content may make this No 6 coal seam as a giant Li deposit. A Li content distribution map of the Jungar coalfield is given in Figure 3. As illustrated in Figure 3, the Li distribution pattern is different from that of Al (Fig. 2). The northwestern area is most enriched in Li with the middle portion among the least enriched area (Fig. 3). 3.3. Concentrations of gallium (Ga) Gallium is a widely used element with strategic value for both civilian and military purposes. It is a critical component for the electronics industry and can also be used in the energy storage field (Moskalyk, 2003). In recent years, the occurrence of gallium has been reported from several Chinese Permo-Carboniferous coalfields (Dai et al., 2006; Shi, 2014; Sun et al., 2015). In some of these coalfields, gallium was distributed widely enough to reach the industrial scale and has commercial value. A giant gallium deposit in the Jungar coalfield has been reported

Table 1 Al (ash basis) and rare metal (coal basis) contents of the coal samples from the Jungar coalfield. Mine Jungar Coalfield Xiaoyugou Dongkongdui Sunjiahao Niuliangou Tanggongta Longwanggou Tingziyan Yaogou Guanbanwusu

Heidaigou

Haerwusu South Exploration Area Guanzigou Chuancaogedan Huangshuliang

Sample quantity

9 192 5 31 65 11 66 6 1 29 36 19 7 32 54 34 55 30 14 22 4

Li (mg/kg)

Ga (mg/kg)

REY (mg/kg)

Al2O3 (%)

Reference

147

24.3

196

51

Dai et al. (2010)

8.9 18.7 8.6 17.4 17.4 21.1 20.1 6 18 18 15 16.5 45

216.2

Zhao (2015) Wang et al. (2011) Zhao (2015) Wang et al. (2011) Wang et al. (2011) Zhao (2015) Wang et al. (2011) Zhao (2015) Wang et al. (2011) Dai et al. 2008 Sun et al. 2012a, 2012b Zhao (2015) Dai et al. (2006) Zhao (2015) Wang et al. (2011) Dai et al. (2008) Zhao (2015) Wang et al. (2011) Zhao (2015) Zhao (2015) Zhao (2015)

94.6 35.5

403.1 34.9 116 263.6 229 37.8 138 116 126 177.2 61.3 23.8

20.5 11.8

189

44 43 46 50 41 38 46 44 47 62 41 43 53 52 54 53

13.7 23.8 31.1 6.3

166.2 276.7 121.1

31 44 41 43

52.2

166.6 58.4 154 178 214

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Fig. 2. Distribution of Al2O3 (%, ash basis) in the main coal seam of the Jungar coalfield.

in Inner Mongolia (Dai et al., 2006). In other cases, the Ga concentrations in some coals are much higher than the industrial grade. However, because Ga is only locally enriched as pockets in these coal seams (Zhou and Ren, 1981; Dai et al., 2012b), it may not be a valuable deposit. The Ga contents are higher than 10 mg/kg in most samples from the Jungar coalfields (Table 1). Although the values are much higher than the average Ga content in most Chinese coal, they do not reach the minimum mining grade (Dai et al., 2006). However, the Ga content exceeds 30 mg/kg in two mines, the Heidaigou and Chuancaogedan (Fig. 4). The minimum mining grade for a Ga deposit in coals is 30 mg/kg according to the Geology and Ore Deposit Standard Specifications for Rare Metal Mineral Exploration of the People's Republic of China (DZ/T 02032002, 2003). Based on this Standard, the Ga content in these two coal

mines has been highly enriched so as to be designated as a Ga deposit in the Heidaigou and Chuancaogedan mines. The distribution pattern of Ga is similar to that of Al (Figs. 2 and 4). The most enriched pocket is located in the middle of the Jungar coalfield. 3.4. Concentrations of REY Lanthanides and Yttrium (REY) play a key role in the manufacture of semi-conductive materials as well as in alternative energy technologies (Seredin and Dai, 2012). The average REY contents, based on the average individual element contents, are 68.5 mg/kg for world coals and 404 mg/kg for world coal ash (Ketris and Yudovich, 2009). The sum of REY oxides (REY2O3), which is commonly used to estimate the

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Fig. 3. Distribution of Li (mg/kg, coal basis) in the main coal seam of the Jungar coalfield.

abundance of elements in ores, is 483 mg/kg in the coal ash. Given such a high concentrations of REY in these coals, REY recovery as a byproduct from coal deposits may be considered a promising way to obtain these critical elements to meet expected global demand (Seredin et al., 2013). Sun et al. (2014) and Ketris and Yudovich (2009) suggested that REY contents of 300 mg/kg in coal could be used as the minimum mining grade guideline. REY contents are higher than 100 mg/kg in the coal samples across the Jungar coalfield, except for the samples from the Longwanggou and southern areas (Table 1). In several coal mines, REY contents even exceed 200 mg/kg (Fig. 5).

The distribution pattern of REY is different from those of Al, Li and Ga (Figs. 2–5). Three enriched pockets appear evident in the northwestern, middle and southeastern areas of the Jungar coalfield (Fig. 5). 4. Occurrence of Al and rare elements (Li, Ga, and REY) 4.1. Affinity of Al and rare elements Dai et al. (2006, 2012a) reported that Al and REY concentrations are mainly related to minerals in coal. The Al mainly occurs in kaolinite and boehmite (Dai et al., 2012a). REY was transported into the peat bog

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Fig. 4. Distribution of Ga (mg/kg, coal basis) in the main coal seam of the Jungar coalfield.

from two sources (bauxite and surface waterways) in both the lower and upper parts of the No. 6 coal seam sections, and at some point the two sources overlapped. The source of REY in the upper section of No 6 seam was dominated by weathered bauxite and that of the lower part was mainly associated by paleo-waterways (Dai et al., 2012b). The occurrence and enrichment mechanisms of Li in coals has been studied by Ward (2002). Karayigit et al. (2006) attributes the affinity of Li to aluminosilicates in coal. Research on trace element distributions in the Kaffioyra and Longyearbyen coals of Spitsbergen, Norway (Lewińska-Preis et al., 2009) has indicated that Li in the former coal is bound to minerals; in contrast, Li shows significant affinity with organic matter in the latter coal, with 72% of the Li associated with organic

matter. Thus, the occurrence of Li in coals may be related to both inorganic and organic fractions and it requires further study. To investigate the organic or inorganic affinities of Li in the Jungar coals, sequential chemical extraction experiments were carried out (Sun et al., 2013a). The results showed that Li contents are much higher in the inorganic fraction than those in the organic fraction (Sun et al., 2013b). This sequential extraction result is also in good agreement with X-ray powder diffraction analysis which shows that the main Li-bearing minerals are silicates (Sun et al., 2012b). The Li content in the carbonate-associated, ion exchangeable and water soluble fractions is below 2 mg/kg, suggesting least association with these fractions (Sun et al., 2013b). Therefore, Li is mainly

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Fig. 5. Distribution of REY (mg/kg, coal basis) in the main coal seam of the Jungar coalfield.

associated with inorganic matter, especially silicates, than with organic matter in the Jungar coals studied. Gallium is generally related to clay minerals in coal (Chou, 1997). Because of its similar geochemical characteristics to Al, Ga could isomorphously substitute for Al in Al-bearing minerals in the right environment. The modes of Ga occurrence in the Guanbanwusu, Haerwusu and Heidaigou coals are slightly different. Ga in the Haerwusu coals occurs mainly in boehmite and organic matter, and boehmite is the main carrier of Ga (Dai et al., 2006). The sequential chemical extraction experiment (SCEE) results for samples from the Guanbanwusu coal mine are given in Table 2. The SCEE results show that ~90% of Ga contents are associated with inorganic fraction, in particular with silicate, and only 6% of Ga are bonded with

Table 2 Ga contents in different fractions from the SCEE. Fraction

Ga (%)

Silicate Sulphide Ion exchangeable Water soluble Carbonate Organic bonded

89.2 3.3 bdl bdl 1.2 6.3

bdl: below detection limit.

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the organic fraction. The Ga contents in the carbonate, ion exchangeable and water soluble fractions are only 1%, 0.2% and 0.1%, respectively (Table 2). This result is consistent with X-ray powder diffraction results which show that the main minerals containing Ga are boehmite and kaolinite (Sun et al., 2012b). 4.2. Mineral compositions of the ‘high-Al coals’ Mineralogical characteristics may provide some clues regarding the modes of occurrence Al and rare elements formation (Prachiti et al., 2011). The main minerals in the coals from the Jungar coalfield are kaolinite, boehmite, chlorite, pyrite, quartz, and calcite. These minerals have been described by Dai et al. (2012a) and Sun et al. (2012b). Clay minerals are dominated by kaolinite with minor illite and amorphous clay minerals (Dai et al., 2012a; Sun et al., 2012b). The clay minerals, in general, occur as disseminated fine particles intimately mixed with organic matter, along with various lens-shaped, thin-layered and massive forms within collodetrinite bands and/or as cell-fillings in fusinite masses of syngenetic origin (Singh et al., 2012). The clay minerals are often accompanied by inertodetrinites, vitrodetrinites, and small sporinites in these coals. In the Guanbanwusu mine, kaolinite content exceeds 80% of the total minerals, whereas they are around 26% and 28% in most samples from the Heidaigou and Haerwusu mines, respectively. Based on the Li content in kaolinite analysed by ICP-MS, it is can be calculated that a total of 100% Li in kaolinite alone is around 80% of total Li in the coal, so it clearly indicates that the majority of Li is associated with clay in Guanbanwusu coal, as reported by Swaine and Goodarzi (1995). Boehmite occurs in all the coal samples studied and is the secondmost common mineral in Jungar coals (Dai et al., 2006; Sun et al., 2012b). Boehmite content is particularly high in the coals from Heidaigou and Haerwusu mines. Detailed description and mode of occurrence of the boehmite were reported by Dai et al. (2006). The boehmite in the Jungar coalfield is cryptocrystalline, occurring mainly as lumps in collodetrinite as well as cell-fillings in fusinite. In some cases, boehmite also occurs as finely disseminated particles in collodetrinite. The boehmite lumps present as different shapes and variable sizes, from two micrometres to larger than 1 cm. The Al contents are closely correlated to the amounts of kaolinite and boehmite in the coals. In a previous study, Ga contents are found to be closely related to boehmite content (Dai et al., 2006). However, no clear distribution regularity between the Li content and boehmite ratios was observed in this study. Pyrite is commonly seen in all the mines, and its content is very low. The predominant mode of pyrite occurrence in the coal is as massive, fracture-filling and cell-filling and widely distributed among various macerals and in some cases as finely dispersed grains. The morphology of the pyrite suggests that some of them are epigenetic (Sun et al., 2010d; Singh et al., 2013). The pyrite content in the top and bottom sections of the No. 6 seam is much higher than that in the middle section. To study the relationship between pyrite and Li content, one pure pyrite sample (100% pyrite) separated from Guanbanwusu coal was analysed by ICP-MS, in which the Li content is only 4.5 mg/kg (Sun et al., 2012b). This result indicates that the Li content is not related to pyrite. The contents of calcite and gypsum in the coal samples studied are very low and mainly occur as secondary fracture- or cleat-fillings, indicating that they are of epigenetic origin. According to the regional geology (Fig. 1), the possible source of the calcite and gypsum appears to be from uncrystallised carbonate and sulphates, which are derived from brackish water during peat accumulation (Liu et al., 1991). Sulphates in the percolating fluids also could react with Ca and recrystallize in coal fractures. No relationship between both minerals and Li content is confirmed. Quartz is very rare and occurs as very fine particles with size in b10 μm. Most quartz is observed as round grains, suggesting a terrigenous detrital origin and might have been transported in long distance.

Siderite is also found in coals of all the studied mines, but only in trace amounts. 4.3. The associations between the elements of the ‘high-Al coals’ Statistical correlation analysis and cluster analyses can provide quantifiable indicators to measure the similarity of distribution of the elements. Some researchers have used these indicators to understand the relationships among the elements in the No.6 coal seam of Jungar coalfield (Xu et al., 2011; Sun et al., 2012b, 2013b; Dai et al., 2012a). Lithium, commonly occurs together with Rb and Cs as one group (Seredin, 2003). They have obvious lithophile affinity and correlate well with clay minerals. The relationship between Li concentrations and SiO2 and Al2O3 contents has been studied by Sun et al. (2012b). A plot of the Li concentrations in ash against the Al2O3 in ash shows a scattered positive correlation. Another plot of the Li in ash against the SiO2/Al2O3 ratio shows a relatively strong negative correlation, suggesting that the Li may be related to one or more of the Al-bearing phases, such as kaolinite and chlorite-group minerals (Sun et al., 2012b; Dai et al., 2012a). The Li distribution pattern is different from that of Al2 O 3 because boehmite is one of main carriers of Al but not Li (Zhao, 2015). Gallium, In, V, U, Pb, Bi, Cr, Se, Cu and Be also commonly occur together as one group. Generally speaking, all of them can be associated with organic and inorganic phases in coal (Finkelman, 1993, 1995; Dai et al., 2012b; Tian et al., 2013; Seredin et al., 2013). However, Pearson's correlation coefficients (r) of Ga-ash, Ga-Al2O3, Ga-SiO2 are high (r N 0.5) so it is further confirmed that the dominant carriers of Ga are silicate and Al-bearing minerals such as kaolinite, boehmite, goyazite in the coal of Jungar coalfield. REY have positive correlation but with low correlation coefficients with ash yields and SiO2 which indicates that the REY have a mixed (organic and inorganic) affinity (Xu et al., 2011; Dai et al., 2012a). Seredin et al. (2013) found out that the origin of high-REY accumulation was mainly terrigenous in most of Jungar coalfield. In addition, exfiltration of groundwater percolating through the coal seams can result in localised enrichment of REY (Sun et al., 2012b). This is why the distribution of REY is different from that of Li, Al and Ga (Figs. 2–5). 5. Source of the Al and Li, Ga and REY Al, Li, Ga, and REY in the ‘high-Al coals’ mainly occur in minerals such as clay minerals, boehmite and chlorite that originated from a sediment-source region (Sun et al., 2010a, 2012b, 2013a, b; Dai et al., 2012a, b; Prachiti et al., 2011). Previous studies showed that the Yinshan Oldland, located in the northern and northeastern Ordos Basin, was one of the main sedimentary regions of the Jungar coalfield during the Late Carboniferous and Middle Permian (Ritts et al., 2004; Li et al., 2009; Sun et al., 2013a, b). The eastern uplift areas of the Ordos Basin were denuded from the Late Ordovician to Early Carboniferous, with the uplift area subsequently subsiding since Middle Carboniferous (Benxi Formation; Yang et al., 2008). The weathered sediments, which contained abundant aluminium, were transported to the marginal areas of the basin and deposited under conditions that allowed monohydrallite (Benxi Formation bauxite ore) to be crystallised (Liu et al., 2013). However, some of these areas were uplifted and denuded again during coal accumulation period in Early Permian, so the bauxite of the Benxi Formation was also an important sediment-source (Yang et al., 2008; Sun et al., 2012b; Dai et al., 2012a). 5.1. Strong peraluminous granitoids in the Yinshan Oldland Wang (1996) concluded that terrigenous clastic materials were possibly sourced from the Yinshan Oldland during the early periods of the 6 coal seams' accumulation. The Jungar Basin was in a littoral swamp environment as the coal measures being formed (Fig. 1). The south was

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sea and the north was Yinshan Oldland (Liu et al., 1991; Yang et al., 2008; Sun et al., 2013b). Strongly peraluminous (SP) granites in the central segment of the North China craton were reported to have an Al saturated index (ASI) of 1.1 (ASI = n (Al2O3) / [n(CaO) + n(Na2O) + n(K2O)]) ( Liu et al., 2004). Strongly peraluminous granites mainly occur in the southern region along the Yinshan Oldland. According to Dai et al. (2008), abundant moyite also exists in this area and is rich in Li (26 mg/kg). The moyite is probably one major source of Li in the No. 6 coal in the Jungar coalfield. Therefore, the Yinshan Oldland could be one of the major sources of Al, Li, Ga, and REY. 5.2. Monohydrallite The Benxi Formation, with an average thickness of 18.1 m, unconformably overlies Middle Ordovician limestones in the Ordos Basin. It is mainly composed of mudstones and marls and is partly intercalated with coal seams (Liu et al., 1991). The lowest portion of the Benxi Formation is a layer of greyish bauxite, formed as weathered surface material. Yang et al. (2004) studied the minerals and occurrence of bauxite in the Benxi Formation and reported that the Al, rare and rare-earth elements were mainly associated with diaspore and kaolinite. According to Liu et al. (2013), the highest contents of Ai2O3, Li, Ga, and REE in the bauxite form the Benxi Formation are 79.07%, 630.61 mg/kg, 56.8 mg/kg, and 1173.17 mg/kg, respectively. The uplift of the Lüliang Peninsula could have exposed the layer of greyish bauxite to the surface, and its subsequent weathering and erosion products could have served as a source of Al and rare elements for the peat moor of the No. 6 coal seam (Fig. 1). The high concentrations of Al, Ga and Li in the bauxite layer were possibly resulted from the formation of weathered crust from Middle Ordovician to Early Carboniferous. The source of the bauxite was likely from the Middle Proterozoic moyite (Dai et al., 2006). Therefore, Al and rare element sources could have been originated from the Middle Proterozoic moyite and strongly peraluminous granites of the Yinshan Oldland. According to Zhang and Wang (2009), Al, Ga and REE could have been derived from four sources: I — igneous rocks and metamorphic rocks with high K and Al contents in the erosion area; II — claystones and bauxite in the Upper Carboniferous Benxi Formation overlying the Middle Ordovician sequence; III — volcanic crystal fragments and volcanic ash; and IV — paleo-lowland paleogeography serving as an advantageous depositional environment for coal and bauxite strata accumulation and for enrichment of Al and rare elements. The above hypotheses I, II, and IV have been well described and established by the abovementioned researchers. However, no obvious evidence has been found so far to support the hypothesis III that the volcanic ash has been a source for Al and rare elements. 6. Regional geological and tectonic processes control the enrichment of Al, Li, Ga, and REY As mentioned above (Dai et al., 2006; Shi, 2014), Al and rare element enrichment in the ‘high-Al coals’ were mainly controlled by the regional geological activities. At first, the CAOB and the Late Paleozoic coalbearing basins were developed in the same structural frameworks. The CAOB could have been one of the major the source for Al and rare elements (Dai et al., 2006; Sun et al., 2012b; Shi, 2014). The basin depression and sedimentary deposition rate and the compositional variations in the sandstone and conglomerate clasts were all controlled by the tectonic processes of the CAOB over the Late Paleozoic (Shi, 2014). Second, the Lüliang Peninsula might have been another source of Al and rare elements. The paleogeography and sequence stratigraphy of the Late Paleozoic coal-bearing measures in the Northeastern Ordos Basin were studied by Yang et al. (2008). They reported that not only did the Yinshan Oldland exist to the north of Ordos Basin, but the Lüliang Peninsula also uplifted to become a paleo-highland in the eastern area of the Jungar Basin during the Middle Carboniferous. The

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Lüliang Peninsula could have also influenced the sedimentation of the surrounding areas (Fig. 1). 7. Conclusions Al and rare element concentrations are enriched to abnormally high concentrations in this unique type ‘high-Al coals’ from the Jungar coalfield. The average content of Al2O3 is further enriched to 46% in the ashes of these coals. And the average contents of Li, Ga, and REY are also further concentrated to 133.6 mg/kg, 18.1 mg/kg, and 165.7 mg/kg in these coals, respectively. The estimated total resources of Al2O3 and Li2O reach 3.7 Gt and 5,157,000 tons in these coals if fully recovered by beneficiation. The enrichment of the valuable elements Al, Ga, Li, REY are mainly controlled by the tectonic processes and regional geological settings. According to the paleogeographic environments and mineral compositions, the strongly peraluminous granites and moyite of the Yinshan Oldland are the most probable sources of Al and rare elements in these coals. The minerals in these coals mainly consist of kaolinite, boehmite, chlorite-group minerals, along with minor amounts of quartz, calcite, pyrite, siderite and amorphous clay material. The silicates including kaolinite, chlorite-group minerals, quartz and amorphous clay material were most likely derived from the erosion and weathering of the strongly peraluminous granites and moyite. The boehmite formation in the No. 6 coal seam of the Jungar coalfield formed from a bauxite source region, which was transported to the peat swamp. Therefore, the bauxite from the Benxi formation could possibly be the source of Al and rare elements in the coals of the Jungar coalfield. The bauxite in this area was possibly derived from the Yinshan Oldland, which is the most probable source of Al and rare elements in these coals. Al and rare elements show different affinity relationships with organic matter and minerals because these elements have different chemical characteristics and micro-depositional environments within the coal basin. Acknowledgements This work was supported by the National Science Foundation of China under grant numbers: 41330317, 51174262, and 41440024. We thank Dr. Rachel Walker for revision suggestions and English polishing. We would also like to thank Editor-in-Chief Dr Franco Pirajno and anonymous reviewers for their careful reviews and constructive comments, which greatly improved the manuscript. References Blissett, R.S., Rowson, N.A., 2012. A review of the multi-component utilisation of coal fly ash. Fuel 97, 1–23. Chou, C.L., 1997. Abundances of sulfur, chlorine, and trace elements in Illinois Basin coals. USA, Proceedings of the 14th Annual International Pittsburgh Coal Conference, Taiyuan, Shanxi, China, p. 76 (Abstracts). Dai, S.F., Jiang, Y.F., Ward, C.R., Gu, L.D., Seredin, V.V., Liu, H., Zhou, D., Wang, X.B., Sun, Y.Z., Zou, J.H., Ren, D.Y., 2012a. Mineralogical and geochemical compositions of the coal in the Guanbanwusu Mine, Inner Mongolia, China: Further evidence for the existence of an Al (Ga and REE) ore deposit in the Jungar Coalfield. Int. J. Coal Geol. 98, 10–40. Dai, S.F., Li, D., Chou, C.L., Zhao, L., Zhang, Y., Ren, D.Y., Ma, Y.W., Sun, Y.Y., 2008. Mineralogy and geochemistry of boehmite-rich coals: new insights from the Haerwusu Surface Mine, Jungar Coalfield, Inner Mongolia, China. Int. J. Coal Geol. 74, 185–202. Dai, S.F., Luo, Y.B., Seredin, V.V., Ward, C.R., Hower, J.C., Zhao, L., Liu, S.D., Zhao, C.L., Tian, H.T., Zou, J.H., 2014. Revisiting the late Permian coal from the Huayingshan, Sichuan, southwestern China: enrichment and occurrence modes of minerals and trace elements. Int. J. Coal Geol. 122, 110–128. Dai, S.F., Ren, D.Y., Chou, C.L., Finkelman, R.B., Seredin, V.V., Zhou, Y.P., 2012b. Geochemistry of trace elements in Chinese coals: a review of abundances, genetic types, impacts on human health, and industrial utilization. Int. J. Coal Geol. 94, 3–21. Dai, S.F., Ren, D.Y., Chou, C.L., Li, S.S., Jiang, Y.F., 2006. Mineralogy and geochemistry of the No. 6 coal (Pennsylvanian) in the Jungar Coalfield, Ordos Basin, China. Int. J. Coal Geol 66, 253–270. Dai, S.F., Zhao, L., Peng, S.P., Chou, C.L., Wang, X.B., Zhang, Y., Li, D., Sun, Y.Y., 2010. Abundances and distribution of minerals and elements in high-alumina coal fly ash from the Jungar Power Plant, Inner Mongolia, China. Int. J. Coal Geol. 81, 320–332.

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