Concentration and mode of occurrence of trace elements in marine oil shale from the Bilong Co area, northern Tibet, China

International Journal of Coal Geology 85 (2011) 112–122

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International Journal of Coal Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j c o a l g e o

Concentration and mode of occurrence of trace elements in marine oil shale from the Bilong Co area, northern Tibet, China Xiugen Fu a,⁎, Jian Wang a, Yuhong Zeng a,b, Fuwen Tan a, Xinglei Feng a a b

Chengdu Institute of Geology and Mineral Resources, Chengdu 610081, China College of Chemistry, Sichuan University, Chengdu 610065, China

a r t i c l e

i n f o

Article history: Received 8 April 2010 Received in revised form 1 September 2010 Accepted 9 October 2010 Available online 20 October 2010 Keywords: Marine oil shale Trace elements Qiangtang basin China

a b s t r a c t The Bilong Co oil shale zone is located in the South Qiangtang depression. This zone, together with the Shengli River-Changshe Mountain oil shale zone in the North Qiangtang depression, northern Tibet plateau, represents a potentially large marine oil shale resource in China. With the aim of better understanding geochemistry of marine oil shale, 18 samples from the Bilong Co area are studied and 56 elements in them are determined. The contents of Mo, Se, Cd, Cs, As, Bi, U Rb, Pb, Th, Li, Sr, and Zn are enriched from 1.3 to 35.0 times as compared with the average concentration in the crust (Clarke values), whereas the other elements are slightly higher/lower than the respective Clarke values. Compared to common Australian oil shales, the contents of Ce, Co, Cs, F, Ga, Hf, Nb, Nd, Pb, Pr, Rb, Sc, Sn, Th, and Zr in the Bilong Co oil shale samples exhibit relatively high levels, whereas As, Br, Cd, Cr, Cu, Dy, Er, Eu, Ho, Lu, Mo, Ni, Se, Tb, Tm, U, V, Y, Yb, and Zn are relatively depleted. However, the distribution patterns of trace elements are quite similar indicating that trace-element speciation in oil shale is governed by general processes rather than by individual geochemical mechanisms within one particular sample. The elements in the Bilong Co oil shale may be classified into three groups of association according to their modes of occurrence, i.e. Groups A, B, and C. Group A (Si–Al–K–Na–Ti–Zr–Sc–Th–Sn–Nb–Ga–Ta–Be–Hf–Cs– Li–Rb–ash–REEs) is strongly correlated with ash yield and mainly has an inorganic affinity. Group B (Cr–W– Mg–Mn–Br–F–Sr–Ba) has weakly or negatively positive correlation coefficients (with an exception of F) with ash yield. Group C (Zn–Bi–Cd–Ca–Fe–Pb–Hg–Co–Cu–Ni–P–Mo–Se–U–As–V) shows negative correlation coefficients with ash yield (with an exception of Bi) and possibly has an organic affinity. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Oil shale, as an alternative resource, has received much attention (Dyni, 2006; Kök, 2006; Liu et al., 2009). In China, oil shale was formed mainly in lacustrine environments, such as Tertiary oil shale in the Maoming, Huadian and Fushun areas (Liu et al., 2009), and Cretaceous oil shale in the Songliao (Wang et al., 1996) and Minhe basins (Liu et al., 2009). Marine oil shale was mainly found in the Qiangtang basin, northern Tibet, China (Fu et al., 2008, 2009a,b, 2010a; Wang and Zhang, 1987), including the Bilong Co oil shale zone and the Shengli River-Changshe Mountain oil shale zone. These zones represent a large marine oil shale resource in China. The Bilong Co oil shale is located in the southern part of the Qiangtang basin (Fig. 1a). Proved reserves are estimated to be 90.6 million tonnes (Liu et al., 2009). In the early studies of the oil shale, the focus was on the organic geochemistry (Lin et al., 2001) and

⁎ Corresponding author. Tel.: + 86 28 83231651. E-mail address: [email protected] (X. Fu). 0166-5162/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2010.10.004

anoxic event (Chen et al., 2005; Yi et al., 2003). However, few studies reported trace-element data from this oil shale. The knowledge of the distribution and concentration of trace elements in oil shale is very important not only for geochemistry but also for studies related to mobilization of these elements into the environment. The object of the present study is to investigate in details the content, vertical distribution and mode of occurrence of the trace elements in the Bilong Co oil shale. 2. Geological setting On a large scale, the Tibetan Plateau constitutes a tectonic collage of continental blocks (terranes). From north to south, Tibet is comprised of the Kunlun-Qaidam, Songpan-Ganzi flysch complex, Qiangtang, and Lhasa terranes, which are separated by the eaststriking Anyimaqen-Kunlun-Muztagh, Hoh Xil-Jinsha River and Bangong Lake-Nujiang River suture zones, respectively (Fig. 1a). It is generally accepted that the paleo-Tethys represented by the present Jinsha River suture opened possibly in Early Carboniferous time (Yin and Harrison, 2000) and closed by Permian to latest Triassic time (Kapp et al., 2003). The mid-Tethys branch between the Lhasa and

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Fig. 1. (a) Map of the Tibetan plateau showing major terranes. (b) Generalized map, showing location of study area. (c) Simplified geological map of the Bilong Co area, showing location of oil shale section. TR, Tarim basin; QD, Qaidam; AKMS, Anyimaqen-Kunlun-Muztagh suture; HJS, Hoh Xil-Jinsha River suture; SP, Songpan-Ganzi flysch complex; HXP, Hoh Xili piedmont zone; QT, Qiagtang basin; BNS, Bangong Lake-Nujiang River suture; LS, Lhasa terrane; YTS, Yarlung Tsangpo suture; HMLY, Himalayas.

Qiangtang terranes was open by Early Jurassic time (Kapp et al., 2003) and closed along the Bangong Lake-Nujiang River suture during the Late Jurassic time (Yin and Harrison, 2000). The Qiangtang block, bounded by Hoh Xil-Jinsha River suture zone to the north and Bangong Lake-Nujiang River suture zone to the south, respectively, consists of the South Qiangtang depression, the central uplift and the North Qiangtang depression (Fig. 1b). During the Permo-Trassic time, the Paleo-Tethys Ocean closed by northern subduction beneath the Kunlun terranes and southward subduction beneath the Qiangtang terranes (Dewey et al., 1988; Kapp et al., 2003; Nie et al., 1994; Pearce and Mei, 1988), resulting in a large-scale regression in the Qiangtang basin. During this interval, most parts of the Qiangtang basin were uplifted and exposed to erosion. Meanwhile paleo-weathering crusts occurred widely in the Qiangtang Basin (Fu et al., 2010b; Wang et al., 2007). Subsequently, these weathering crusts were overlain unconformably by a succession of volcanic–

volcaniclastic strata that mark the onset of the Mesozoic Qiangtang basin (Fu et al., 2010b). As a result, the sediments are almost exclusively Mesozoic marine deposits that crop out in the South Qiangtang depression and in the North Qiangtang depression. Paleozoic marine sedimentary sequences are locally preserved in the central uplift. The Bilong Co area is located in the South Qiangtang depression, northern Tibet plateau, China (Fig. 1b), where Jurassic marine deposits are widely spread including lower Jurassic Quse Formation, Middle Jurassic Sewa Formation, Buqu Formation and Xiali Formation, and upper Jurassic Suowa Formation (Fig. 1c). The oil shale, about 35.5 m in thickness, is exposed for a distance of more than 4 km in an east–west direction. Abundant ammonites (Harpoceras sp.), occurring at the top of the Bilong Co oil shale section (Yi et al., 2003), indicate that the Bilong Co oil shale is of Early Jurassic age (i.e. Quse Formation strata).

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Table 1 Content of TOC, ash, total sulphur, organic sulphur, and major elements in samples from the Bilong Co oil shale (unit in %). Sample nos.

Lithology

TOC

Ad

St,

BP-6 BP-7-1 BP-7-2 BP-7-3 BP-8 BP-9 BP-10-1 BP-10-2 BP-10-3 BP-10-4 BP-11 BP-12-1 BP-12-2 BP-12-3 BP-13 BP-14-1 BP-14-2 BP-14-3

Micritic limestone Oil shale Oil shale Oil shale Micritic limestone Micritic limestone Oil shale Oil shale Oil shale Oil shale Micritic limestone Oil shale Oil shale Oil shale Micritic limestone Oil shale Oil shale Oil shale

1.67 6.75 7.23 6.91 1.36 0.36 19.20 7.95 7.66 9.11 2.10 10.27 10.66 7.13 1.24 10.96 8.80 13.50

67.09 65.96 65.32 64.05 66.08 61.61 62.99 67.76 69.59 71.96 60.12 62.06 63.63 71.44 60.27 64.58 64.91 58.88

1.88 1.14 1.13 1.12 1.77 0.87 2.00 1.05 1.37 1.42 0.88 1.41 1.32 1.46 0.95 1.19 1.34 1.52

d

So,

d

0.28 0.42 0.43 0.46 0.28 0.25 0.47 0.43 0.42 0.37 0.26 0.46 0.45 0.43 0.27 0.43 0.48 0.45

Si

Al

Ca

Mg

K

Na

Ti

P

Mn

Fe

10.84 9.40 8.96 9.01 9.17 3.80 10.11 10.2 11.84 14.55 2.59 9.36 9.30 13.34 3.65 9.54 9.17 8.18

4.07 3.65 3.42 3.48 3.24 1.48 4.17 4.14 4.57 5.75 0.88 3.81 3.84 5.43 1.40 3.75 3.80 3.47

20.66 23.08 23.75 23.36 23.84 33.84 16.17 22.91 19.61 14.79 34.57 19.29 20.71 16.79 28.72 21.97 21.71 19.00

0.46 0.51 0.51 0.49 0.47 0.46 0.44 0.59 0.46 0.49 0.63 0.51 0.46 0.53 3.91 0.54 0.67 0.43

1.2 0.98 0.93 0.98 0.97 0.54 1.13 1.21 1.29 1.73 0.37 1.05 1.09 1.58 0.37 1.03 1.04 0.9

0.24 0.14 0.15 0.15 0.19 0.08 0.22 0.19 0.27 0.25 0.06 0.25 0.2 0.27 0.1 0.16 0.23 0.22

0.174 0.156 0.150 0.150 0.138 0.072 0.186 0.186 0.21 0.276 0.048 0.174 0.174 0.252 0.066 0.174 0.174 0.168

0.052 0.043 0.044 0.044 0.052 0.021 0.148 0.052 0.096 0.087 0.061 0.105 0.131 0.092 0.038 0.052 0.074 0.153

0.027 0.029 0.029 0.030 0.028 0.029 0.026 0.033 0.026 0.028 0.041 0.031 0.029 0.033 0.067 0.029 0.036 0.026

2.81 2.31 2.24 2.21 2.6 0.92 4.17 1.96 2.68 3.27 1.09 3.22 2.94 3.47 1.19 2.49 2.57 3.04

TOC, total organic carbon; A, ash; St, total sulphur; So, organic sulphur; d, dry basis.

3. Samples and analytical methods The study area and sample locations are presented in Fig. 1. A total of 18 samples were collected from the Bilong Co oil shale section. Thirteen of them were collected from oil shale seams with a vertical sampling interval of 1 m on average, and the other five samples were collected from micritic limestone layers.

Samples for geochemical analysis were all crushed and ground to less than 200 mesh. X-ray fluorescence spectrometry (XRF) was used to determine the oxides of major elements, including Si, Al, Ca, K, Na, Fe, Mn, Mg, Ti and P. The analytical procedures are similar to those described by Kimura (1998). The analytical uncertainty is usually b5%. Inductively coupled plasma mass spectrometer (ICP-MS) was used to determine trace-element contents in samples, following the method

Fig. 2. XRD patterns for selected oil shale samples from the Bilong Co oil shale.

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4. Results and discussion 4.1. Oil shale characterization The Bilong Co oil shale samples show higher concentrations of organic matter (6.75–19.20%) compared to micritic limestone samples (0.36–2.10%) (Table 1). The organic sulphur (So, d) contents of oil shale samples from the Bilong Co area range between 0.37% and 0.48%, whereas micritic limestone samples contain 0.25–0.28% organic sulphur contents (Table 1). The Bilong Co oil shale samples are characterized by high ash yields ranging from 58.88% to 71.96% (Table 1) with low or moderate total sulphur (St, d) contents (1.05–2.00%) (Table 1) and intermediate shale oil contents (average 9.18%; Liu et al., 2009). 4.2. Minerals in the Bilong Co oil shale Fig. 3. Selected microscopic photograph showing major petrographical constituents in the Bilong Co oil shale samples. Calcite occurring as disseminated fine particles, and clay minerals occurring in massive form, and quartz occurring mainly as cell-fillings.

described in Chinese National Standard DZ/T0223-2001. The samples were digested in microwave furnace using distilled HF + HNO3. The level of detection limit for elements is 0.n − n × 10− 12 (Liu et al., 1996). Mercury, Se and As were determined by atomic fluorescence spectrometry, using chemical method according to Chinese Standard DZG20.10-1990. Fluorine was determined by pyrohydrolysis in conjunction with ion-selective electrode, following the method described in Chinese National Standard GB/T 4633-1997. Ash yield and the content of total sulphur were determined according to Chinese Standard methods GB/T212-2008 and GB/T2142007, respectively. Total organic carbon (TOC) and organic sulphur were determined at the Geological Laboratory of Exploration and Development Research Institute of PetroChina Southwest Oil and Gas Field Company, using chemical method according to Chinese Standard GB/T19145-2003 and GB/T215-2003, respectively. The mineralogical phases were determined by optical microscopic observation and powder X-ray diffraction spectrometry (XRD). The XRD measurements were carried out at Tianjin Institute of Geology and Mineral Resources using a D8 ADVANCE diffractometer equipped with a Cu-target tube and a curved graphite monochromator, operating at 35 kV and 40 mA. Samples were step-scanned from 5° to 70° with a step size of 0.02° (2θ). The analytical procedures follow the method described by Chinese National Standard SY/T6210-1996. The results were statistically treated using the SPSS statistical program. Pearson's correlation coefficients between the concentration of the trace elements and ash, main element and sulphur contents were obtained. The elemental associations were studied by cluster analysis.

Fig. 4. Fossil shell material including nearly pure calcite.

Microscopic observation reveals that the mineral component of oil shale samples is normally higher than 45% by volume, ranging between 41.6% and 68.7%. These comprise mainly of carbonates (19.8–42.3%), quartz (7.0–16.3%), clay minerals (21.2–36.5%) and pyrite (0.3–5.3%). The minerals identified by X-ray diffraction in the oil shale samples are abundant in calcite, clay minerals, and quartz, and minor quantities of dolomite, feldspar and pyrite (Fig. 2), and anhydrite and haematite are also detected in several oil shale samples. The calcite content is high in the Bilong Co oil shale samples, occurring mainly as disseminated fine particles (Fig. 3). Abundant fossil shells were also found in the oil shale samples. The shell material was found to be nearly pure calcite (Fig. 4). Clay minerals are common in the oil shale samples. They occur in thin-layered and massive forms (Fig. 3). Quartz occurs mainly as cell-fillings (Fig. 3). Pyrite is also determined in each oil shale sample. They occur mainly as disseminated fine particles (Fig. 5), and minor quantities of pyrite crystals were also found in field survey. The carbonate mineral dolomite, and silicate minerals plagioclase and K-feldspar are also detected by X-ray diffraction in the oil shale samples (Fig. 2). 4.3. Elements in oil shale 4.3.1. Major element geochemistry As for most oil shale samples, the most abundant elements are Si, Al and Ca (Table 1). Aluminum and Si are enriched in the Bilong Co oil shale samples suggesting that the mineral component of these oil shale samples is rich in clay minerals and quartz, which is consistent with the abundant occurrence of kaolinite, illite, and quartz identified by the XRD analysis. High Ca content may

Fig. 5. Disseminated pyrite particles in the Bilong Co oil shale samples.

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Table 2 Concentrations of trace elements in samples from the Bilong Co oil shale (F in %, others in μg/g). Element

BP-6

BP-7-1

BP-7-2

BP-7-3

BP-8

BP-9

BP-10-1

BP-10-2

BP-10-3

BP-10-4

BP-11

As Ba Be Bi Br Cd Ce Co Cr Cs Cu Dy Er Eu F Ga Gd Hf Hg Ho La Li Lu Mo Nb Nd Ni Pb Pr Rb Sc Se Sm Sn Sr Ta Tb Th Tm U V W Y Yb Zn Zr

3.73 202 0.88 0.27 1.44 0.5 31 6.84 40.8 7.97 27.7 1.74 1.06 0.45 0.094 10.2 2.14 1.53 0.032 0.34 15.3 22.3 0.14 11.4 4.99 12.5 39 15.8 3.38 87.6 6.97 0.5 2.35 1.46 309 0.41 0.29 5.61 0.15 3.23 58.5 0.8 10.6 0.97 266 48.6

4.66 171 0.89 0.22 1.91 0.19 29.2 6.82 60.1 6.99 23.3 1.73 0.99 0.42 0.083 9.5 1.96 1.3 0.026 0.34 14.5 20.9 0.13 11.5 5.04 11.9 42.8 13.4 3.16 79.2 6.59 0.11 2.07 1.51 338 0.38 0.28 5.37 0.14 2.78 52.1 0.92 10 0.9 74.3 48.3

3.95 162 0.85 0.21 1.67 0.21 27.7 6.75 59.4 6.66 22.1 1.5 0.94 0.4 0.12 9.08 1.87 1.31 0.032 0.3 13.7 20.2 0.13 10.3 4.67 11.2 43 12.4 3.04 74.9 6.12 0.07 1.95 1.44 345 0.36 0.26 4.92 0.13 2.74 48.9 0.9 9.64 0.84 77.1 45.6

4.17 152 0.87 0.21 2.03 0.18 27.5 6.72 48.8 6.57 21.7 1.58 0.92 0.41 0.098 8.77 1.87 1.27 0.025 0.31 13.8 19.2 0.13 9.13 4.44 11.2 37.5 12.2 2.99 72.8 6.11 0.06 2.05 1.38 318 0.34 0.26 4.97 0.14 2.65 47.7 0.89 9.19 0.88 63.6 42.1

4.61 579 0.74 0.28 1.45 0.33 26.1 9.57 47.4 6.06 28.7 1.51 0.86 0.42 0.08 8.76 1.78 1.16 0.022 0.29 12.9 18 0.11 8.62 4.4 10.4 42.4 14.4 2.84 72.9 6.01 0.16 1.86 1.37 413 0.33 0.25 4.51 0.13 3.36 52.2 0.87 8.97 0.8 183 42.1

3.16 155 0.39 0.08 1.05 b 0.05 14.7 3.2 52.8 2.43 10.7 0.97 0.54 0.23 0.041 3.84 1.19 0.57 0.023 0.18 7.37 7.41 0.07 1.39 2.32 5.83 30.1 3.95 1.57 34.3 3.26 0.05 1.19 0.83 505 0.18 0.16 2.39 0.07 1.13 21.6 0.6 5.32 0.5 17.1 20.9

12.9 215 0.99 0.7 1.33 0.67 38.6 22.6 68.2 8.38 77.4 2.36 1.22 0.64 0.1 10.7 2.71 1.52 0.11 0.44 18.2 23.4 0.18 89 5.36 15.2 85.5 40.9 4.07 83.9 7.84 2.18 2.86 1.62 343 0.42 0.38 6.51 0.18 6.52 134 1.29 13 1.17 202 53.5

4.58 200 0.87 0.23 2.44 0.07 38.2 9.51 48.7 8.07 29.9 2.16 1.25 0.55 0.081 11.3 2.47 1.5 0.031 0.44 18.8 22.5 0.17 3.53 5.91 15.2 35.3 13.6 4.04 93.6 8.13 0.06 2.68 1.68 600 0.44 0.38 6.48 0.18 2.06 62.5 0.96 12.7 1.18 20 55.8

4.51 367 1.12 0.41 1.45 0.12 41.6 12.1 106 8.82 51.8 2.6 1.47 0.73 0.083 11.9 3.34 1.76 0.031 0.5 20 24.4 0.18 13.5 6.03 16.6 86.6 22.3 4.48 96.1 8.59 0.42 3.61 1.91 457 0.48 0.49 6.93 0.2 3.54 74.9 1.46 15.6 1.29 42.3 59.3

6.81 329 1.3 0.46 1.93 0.43 50.5 15.1 75.6 11.1 59.4 2.98 1.79 0.79 0.099 15.4 3.71 2.25 0.039 0.61 25 30.3 0.24 26.3 8.32 20.4 70.6 25.9 5.47 127 10.7 1.08 3.87 2.23 381 0.63 0.54 8.98 0.25 4.19 124 1.39 18.1 1.7 49.8 80.7

2.41 139 0.35 0.07 0.68 0.11 10.8 3.04 30.2 2.06 14 0.82 0.45 0.23 0.035 2.76 0.98 0.55 0.012 0.17 5.31 5.33 0.06 8.16 1.59 4.65 30.1 6.86 1.2 22.9 2.32 0.32 0.93 0.54 639 0.12 0.14 1.6 0.07 1 87.9 0.38 5.16 0.44 12.9 23.4

WM in ML, weighted mean in micritic limestone samples. WM in OS, weighted mean in oil shale samples. BP-12-2 BP-12-3in oil BP-13 EF,BP-12-1 the ratio of element content shale to BP-14-1 the crust. BP-14-2 a Tayor and McKennan (1995). b Fu et al. (in press). c Patterson et al. (1986).

BP-14-3

WM in ML

correspond to abundant calcite and bivalve and gastropod fossil remains in oil shale samples. Fe and K are the second most abundant elements (Table 2), while all other major elements (Mg, Na, Ti, P, and Mn) almost have a concentration of b1.0%. In contrast, major elements from micritic limestone samples exhibit a higher Ca concentration (20.66–34.57%), and slightly lower Si (2.59–10.84%), Al (0.88–4.07%), and Fe (0.92–2.81%) concentrations, while all other oxides (Mg, K, Na, Ti, P and Mn) almost have a concentration of b1.0%. 4.3.2. Concentration of trace elements in the Bilong Co oil shale The concentrations of trace elements of 18 samples from the Bilong Co oil shale are presented in Table 2. Table 2 also contains the concentrations of trace elements in Chinese oil shale, Australian oil shale, the Clarke value and enrichment factor (EF) of some elements. The arithmetic mean for each element from oil shale samples and from micritic limestone samples was calculated.

WM in OS

Clarke valuea

EF

Chinese oil shaleb

Australian oil shalec

On average, the most abundant trace elements are F (810– 1200 μg/g), Sr (318–600 μg/g), Ba (152–603 μg/g), Zn (20–396 μg/g), V (47.7–148 μg/g), Rb (71.5–127 μg/g), and Ni (35.3–112 μg/g), whereas all the other elements occur in amounts smaller than 100 μg/g. Compared with oil shale samples, the trace-element concentrations in the micritic limestone samples from the Bilong Co area are a little lower, with an exception of Sr. The pioneering work of Patterson et al. (1986) established that some elements are enriched in oil shale. Enrichment of an element in oil shale may be described by an EF which is the ratio of the concentration of an element in oil shale to the average concentration in the crust (Clarke values, Taylor and McLennan, 1995). The trace elements Mo, Se, Cd, Cs, As, Bi, and U are enriched with enrichment values of 35.0, 15.5, 8.7, 8.0, 7.9, 6.1, and 4.4, respectively. The enrichment of these elements is consistent with high organic matter content and abundant pyrite crystal found in oil shale seams because they are hosted in sulphides and/ or organic associated (Spear and Zheng, 1999). The elements Rb,

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Table 2 (continued) BP-12-1

BP-12-2

BP-12-3

BP-13

BP-14-1

BP-14-2

BP-14-3

WM in ML

WM in OS

Clarke valuea

11.6 325 1.04 0.44 2.29 3.26 36.1 20.4 61.6 7.89 85.9 2.55 1.38 0.65 0.09 10.2 3.15 1.45 0.046 0.51 17.5 21 0.2 54.2 5.3 14.6 102 26.8 3.81 81.5 7.91 1.58 2.99 1.49 442 0.39 0.42 6.09 0.21 5.45 139 1 15 1.25 396 55.6

11.5 450 1.15 0.38 1.53 1.27 34.9 13.8 53.4 8.02 54.2 2.16 1.23 0.57 0.098 9.98 2.74 1.64 0.025 0.45 16.8 19.7 0.17 35.5 5.45 13.5 80.5 22.7 3.73 83.6 7.15 0.89 2.61 1.6 491 0.41 0.38 6.39 0.19 4.19 129 1.04 12.9 1.15 114 55.8

9.27 364 1.41 0.45 1.45 0.69 48.7 16.4 64.7 9.89 62.8 2.94 1.68 0.73 0.09 14.5 3.5 2.14 0.042 0.58 23.5 25.5 0.23 38.3 7.79 19.3 78.7 26.7 5.14 112 10 0.97 3.77 2.06 476 0.59 0.47 8.6 0.24 5.14 148 1.33 17.6 1.43 51.9 76.3

2.69 75.7 0.3 0.09 0.81 0.24 14.8 3.74 23.2 1.86 13.1 1.16 0.68 0.31 0.096 3.63 1.42 0.68 0.009 0.22 7.23 6.29 0.08 8.82 2.16 6.41 25.7 6.77 1.66 25.2 3.02 0.06 1.33 0.72 415 0.16 0.19 2.16 0.09 1.01 63.4 0.51 7.23 0.56 17.3 25.8

7.69 603 0.87 0.32 1.68 0.75 37.1 14.3 41.5 6.69 37.8 2.4 1.33 0.64 0.082 10.7 3.08 1.5 0.031 0.47 17.9 19.6 0.2 40.9 5.52 15.5 52.4 20.9 4.1 75.3 9.39 0.47 3.04 1.51 449 0.4 0.42 7.64 0.19 3.72 94.5 0.94 13.7 1.23 54.8 53.3

10.9 506 0.99 0.35 1.9 0.45 35.5 18.7 158 7.46 79.3 2.34 1.24 0.63 0.09 9.94 2.97 1.45 0.034 0.46 17.3 20.2 0.17 27.7 5.18 15.2 112 20.5 3.89 79.3 7.28 0.81 2.96 1.86 434 0.39 0.4 6.09 0.17 4.48 123 1.7 13.9 1.17 30.4 51.2

10.5 302 0.91 0.38 1.49 2.8 35.2 16.1 58.8 6.96 49.2 2.57 1.29 0.67 0.093 8.71 3.23 1.53 0.047 0.49 16.5 20 0.17 95.5 4.92 14.8 102 22.8 3.8 71.5 6.62 1.39 2.96 1.22 472 0.36 0.44 5.78 0.18 4.76 123 1.13 14.1 1.11 250 51.7

3.32 230.14 0.53 0.16 1.09 0.24 19.48 5.29 38.88 4.08 18.84 1.24 0.72 0.33 0.069 5.84 1.50 0.90 0.02 0.24 9.62 11.87 0.09 7.68 3.09 7.96 33.46 9.56 2.13 48.58 4.32 0.22 1.53 0.98 456 0.24 0.21 3.25 0.10 1.95 56.7 0.63 7.46 0.65 99.3 32.2

7.93 318.92 1.02 0.37 1.78 0.85 36.98 13.79 69.6 7.96 50.37 2.30 1.29 0.60 0.093 10.82 2.82 1.59 0.04 0.45 17.96 22.07 0.18 35.03 5.69 14.97 71.45 21.62 3.98 86.98 7.88 0.78 2.88 1.65 427 0.43 0.39 6.52 0.18 4.02 100.0 1.15 13.49 1.18 109.7 56.1

1.0 250 1.5 0.06

7.9 1.3 0.68 6.1

0.098 33 29 185 1.0 75 3.7 2.2 1.1

8.7 1.1 0.48 0.38 8.0 0.67 0.62 0.58 0.55

18 3.3 3.0

0.60 0.85 0.53

0.78 16 13 0.30 1.0 11 16 105 8 3.9 32 30 0.05 3.5 2.5 260 1.0 0.63 3.5 0.32 0.91 230 1.0 20 2.2 80 100

0.58 1.1 1.7 0.59 35.0 0.52 0.94 0.68 2.7 1.0 2.7 0.26 15.5 0.82 0.66 1.6 0.43 0.63 1.9 0.58 4.4 0.43 1.2 0.67 0.53 1.4 0.56

Pb, Th, Li, Sr and Zn have enrichment values N1.3 which is understandable because Rb, Pb, Th, Li and Zn are possibly controlled by sulphides and/or organic matter, and Sr may substitute for Ca in calcite (Eskenazy, 2009). Cobalt, Cr, Sc, Ta, and V have an EF less than 0.5 and are therefore considered to be depleted. The depletion of these elements is probably attributed to weathering source rocks because they are mainly or partly present in clay minerals. All other elements studied show more or less the same concentration as the Clarke values with the EF between 1.3 and 0.5. Clearly, the enrichment or deletion of an element may be a function of that association and the origins of the various oil shale fractions. 4.3.3. Geochemical associations Three groups of elements association were identified (Fig. 6), referred to as Groups A, B and C. Group A: This group includes Si, Al, K, Na, Ti, Zr, Sc, Th, Sn, Nb, Ga, Ta, Be, Hf, Cs, Li, Rb, ash, and rare earth elements (REEs) (Fig. 6).

EF

Chinese oil shaleb 9.91 203.2 0.97 0.31 1.44 1.71 29.6 14.56 81.83 8.04 72.41 1.74 1.04 0.41 0.14 11.99 1.96 1.83 0.03 0.35 14.63 19.22 0.15 34.71 7.09 11.56 77.57 13.89 3.19 98.14 8.6 1.72 2.09 1.77 371.31 0.53 0.28 6.55 0.15 3.73 87.43 1.2 10.21 1.04 138.78 65.41

Australian oil shalec 50 300 1 b1.0 6 25 26 9 90 2.2 110 3.8 2.3 1 0.065 6 3 1 0.6 18 0.6 270 5 12 160 6 3 22 6 30 3 1 400 0.8 2.3 0.3 30 2000 28 3 800 50

The correlation coefficients of Ce–Pr (1.00), La–Nd (0.98), Ti–Zr (0.98), Sc–Th (0.98), Er–Tm (0.98), Lu–Yb (0.96), Eu–Gd (0.99), Sm–Tb (0.97), Ho–Y (0.99), K–Rb (0.99), Si–Li (0.95), Al–Ta (0.99), and Ga–Nb (0.99) are all higher than 0.95. Elements from this group have relatively high positive correlation coefficients with ash yield, ranging from 0.34 to 0.88 (Table 3). Group B: This group represents the Cr–W–Mg–Mn–Br–F–Sr–Ba association (Fig. 6). With exceptions of the high correlation coefficients of Cr–W (0.87) and Mg–Mn (0.90), the correlation coefficients of other pairs of elements in this association are lower than 0.53. Elements from this group have weakly or negatively positive correlation coefficients (with except F) with ash yield, ranging from 0.03 to 0.36 (Table 3). Group C: Zinc, Cd, Ca, Fe, Pb, Hg, Co, Cu, Ni, P, Mo, Se, U, As, and V have negative correlation coefficients with ash yield (Table 3). These elements, together with Bi, are clustered in the third association (Fig 6). The correlation coefficient between Bi and Pb (0.99) is the highest within this association.

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4.3.4. Affinity of the elements The correlation of the element concentrations with ash yield may provide preliminary information for their organic or inorganic affinity (Dai et al., 2008). Five group elements are classified according to their correlation coefficients with ash yield (Table 3). The first group includes elements Al, Cs, Ga, Hf, K, La, Li, Nb, Rb, Si, Sn, Ta, Ti, and Zr (Table 3). They have a very high positive correlation coefficient (N0.7) with ash yield indicating mainly inorganic affinity. All of these elements normally relate with aluminosilicate minerals demonstrating that they mainly originate from a mixed clay assemblage, which is consistent with the occurrence of kaolinite and illite identified by the XRD analysis. The correlation coefficient (0.88) between ash yield and Si is the highest and the Al/Si ratios of oil shale samples are low (0.38–0.42), suggesting that Si has another source besides clay minerals. The abundant quartz identified by the XRD analysis suggests that the extra Si is present in the form of quartz. For a few samples, Al, K and Si are also present as feldspar, determined by the XRD analysis (Fig. 2a). The second group includes elements Be, Ce, Er, Nd, Pr, Sc, Sm, Th, and Yb (Table 3). Elements from this association have positive correlation coefficients with ash yield, which vary from 0.51 to 0.70 showing prevailing inorganic affinity. All of these elements are highly positively correlated with Al content. The correlation coefficients range from 0.87 to 0.94, indicating that these elements occur mainly in clay minerals. Additionally, elements from this group have positive correlation coefficients with total sulphur content ranging from 0.23 to 0.43. Chatziapostolou et al. (2006) proposed that authigenic sedimentation of pyrite and, to some extent of sulphates, in the palaeomire was proportional to the accumulation of organic matter; hence statistical tools cannot discriminate between elements that are exclusively correlated to organic matter and those to Fe and Stotal. In the present study, the high correlation coefficients between sulphate sulphur and Be (0.71), Ce (0.55), Er (0.58), Nd (0.50), Pr (0.52), Sc (0.49), Sm (0.50), Th (0.54), and Yb (0.56) indicate that these elements are also present in sulphates. The correlation coefficients of the elements with ash yield in the third group vary from 0.20 to 0.50, including elements Dy, Eu, Gd, Ho, Lu, Mg, Na, Tb, Tm, W, and Y. Dysprosium, Eu, Gd, Ho, Lu, Na, Tb, Tm and Y are highly positively correlated with Al content with correlation coefficients of 0.75, 0.72, 0.68, 0.78, 0.81, 0.74, 0.69, 0.85 and 0.81, respectively, suggesting that these elements occur mainly in clay minerals. Sodium is also present as plagioclass identified by the XRD analysis (Fig. 2b, c). In the Bilong Co oil shale, positive relationships have recorded between total sulphur and Dy (0.51), Eu (0.55), Gd (0.46), Ho (0.43), Lu (0.41), Na (0.56), Tb (0.41), Tm (0.38), and Y (0.42) indicating that these elements are also present in sulphates. The high correlation coefficients between sulphate sulphur and Dy (0.59), Eu (0.53), Gd (0.54), Ho (0.60), Lu (0.68), Na (0.53), Tb (0.45), Tm (0.69), and Y (0.58) further support the above mentioned observations. In addition, a weakly positive correlation between TOC and Dy (0.25), Eu (0.30) and Gd (0.22) indicates that Dy, Eu and Gd associate partly with organic matter. The affinity of elements Mg and W from the Bilong Co oil shale samples is some different from the data reported by Kortenski and Sotirov (2002) and Querol et al. (1997), whose studies showed that some elements such as Mg and W, mainly have organic affinity. In the present study, a positive correlation between W and Al (0.53), pyritic sulphur (0.52), and Mg (0.28) indicated that W is mainly controlled by clay minerals and sulphide, to a lesser extent, by dolomite. Magnesium occurs mainly in Ca-bearing minerals such as dolomite identified by the XRD analysis (Fig. 2). A positive relationship (r = 0.35) between Mg and Ca further supports the above mentioned recognition. The fourth group includes elements with correlation coefficients below the statically significant value (±0.2): Ba, Bi, Br, Cr, Cu, F, Fe, Mn, Pb, Sr and V (Table 3). With the exceptions of Ba, Br, Cr, Mn and Sr, elements in this group exhibit aluminosilicate and organic affinity

indicating intermediate (organic and inorganic) affinity. Depending on the type of the organic or mineral form they are found in various associations. Barium, Br, Cr, Mn and Sr are characterized by carbonate affinity (Table 3). Dai et al. (2005) also reported data for similar affinity for some of these elements. The elements of the fifth group (Table 3) have negative correlation coefficients with ash yield, which vary between −0.20 and −0.65 (As, Ca, Cd, Co, Hg, Mo, Ni, P, Se, U, and Zn). With an exception of Ca, elements in this group have positive correlation coefficients with TOC, ranging from 0.39 to 0.87. The significantly positive correlations between TOC and As (0.72), Co (0.70), Hg (0.87), Mo (0.87), P (0.75), Se (0.82), and U (0.72) indicate that these elements are mainly controlled by organic matter. In the Bilong Co oil shale, positive relationships have recorded between TOC and Cd (r = 0.39), Ni (r = 0.43), and Zn (r = 0.52) indicating that these elements are also present in organic matter. However, the correlation coefficients between TOC and Cd, Ni and Zn are low, suggesting that these elements have another source besides organic association. The close relationships between total sulphur and Cd (r = 0.32), Ni (r = 0.64) and Zn (r = 0.46) suggest sulphur affinity. Calcium displays a positive correlation with Mg (r = 0.35) and appears to be related to the Ca-bearing minerals such as calcite identified by the XRD analysis. Calcium is believed to be present in more than one form such as carbonates and organic association, as inferred by Mukhopadhyay et al. (1998). Note that oil shale samples BP-7-1, BP-7-2, BP-7-3, BP-10-2, BP-14-1, and BP-14-2 have high contents of Ca (Table 1) corresponding to abundant bivalve and gastropod fossil remains, which suggests that Ca is also related to the fossil remains.

4.3.5. Vertical variation of elements The vertical distributions of the investigated elements in the Bilong Co oil shale section are shown in Fig. 7.

(1) Although not a consistent pattern, most trace-element contents generally increase from micritic limestone layer to oil shale seam (e.g., from ply BP-9 to ply BP-10-1 or from ply BP-11 to ply BP-12-1; Table 2, Fig. 7). This trend may attribute to a change of deposition conditions. Influenced by regional tectonics, sea level fluctuations in the Qiangtang basin were frequent during the Early Jurassic time (Wang et al., 2004), leading to different depositional microenvironments from micritic limestone to oil shale. The depositional environment controlled the lithology of the samples (micritic limestone or oil shale), which in turn controlled trace-element distributions. The oil shale samples exhibit high clay mineral and organic matter contents as discussed earlier. Therefore, the elements controlled by organic matter such as As (Fig. 7), Co, Hg, Mo (Fig. 7), P, Se, and U, and those associated with clay minerals such as Al (Fig. 7), Be (Fig. 7), Ce (Fig. 7), Cs, Ga, Hf, K, La, Li, Nb, Rb, Si (Fig. 7), Sn, Ta, Ti, and Zr show higher content in oil shale seams compared with micritic limestone layers. (2) Sr is the only element whose content generally decreases from micritic limestone layer to oil shale seam (Fig. 7). Strontium has no correlation with the ash yield and organic sulphur content in any of the oil shale samples in the Bilong Co oil shale. Eskenazy and Minčeva (1989) showed that the host minerals of Sr in several Bulgarian coals are aragonite, gypsum, calcite and feldspar. Karayigit et al. (2001) showed that Sr appears be related to barite and fossil shells in the Kalburcayiri coals. A study by Dai et al. (2006) indicated that Sr occurs mainly as goyazite in coals from Junger Coalfield, Ordos Basin of China. In the present study, micritic limestone layers exhibit higher content of calcite than oil shale seams, corresponding to higher Sr content. This indicates that calcite (or fossil shells) is probably a Sr carrier in the Bilong Co oil shale.

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Fig. 6. Dendrogram produced by cluster analysis of analytical results on 13 oil shale samples (cluster method, centroid clustering; interval, Person correlation; transform values, maximum magnitude of 1).

(3) The highest values for individual elements are not uniformly distributed among the seams studied. Very often, maximum element concentrations are observed on the top (e.g., BP-104, BP-12-3 and BP-14-3) or at the bottom of the seam (e.g., BP-10-1, BP-12-1 and BP-14-1). The BP-10-4 seam is characterized by the highest concentrations of Ce (Fig. 7), Cs, Dy, Er, Eu, Ga, Gd, Hf, Ho, Li, Lu, Nb, Nd, Pr, Rb, Sc, Sm, Sn, Ta, Tb, Th, Tm,Y, Yb, and Zr; the BP-12-3 seam is characterized by the highest concentrations of Be (Fig. 7), La and V; the BP-14-3 seam is characterized by the highest concentrations of Mo (Fig. 7). In contrast to the near-top seam, the near-

bottom seam BP-10-1 is characterized by the highest concentrations of As, Bi, Co, F, Hg, Pb, and U; the BP-12-1 seam is characterized by the highest concentrations of Cd (Table 2), Cu, Se, and Zn; the BP-14-1 seam is characterized by the highest concentrations of Ba (Table 2). Only four elements (Br, Cr, Ni and W) (Table 2) have maximum contents in the middle part of the seam, while Sr has a maximum content in the micritic limestone layer (BP-11). Clearly, the distribution of elements in oil shale seams and micritic limestone layers is mainly the function of ash and organic matter contents.

Table 3 Element affinities deduced from the calculation of Pearson's correlation coefficients between the content of trace elements in oil shale and ash yield or selected major elements. Correlation with ash yield Group 1: rash = 0.70–1.00 Al (0.83), Cs (0.73), Ga (0.84), Hf (0.70), K (0.85), La (0.71), Li (0.77), Nb (0.80), Rb (0.85), Si (0.88), Sn (0.87), Ta (0.84), Ti (0.77), Zr (0.71) Group 2: rash = 0.50–0.70 Be (0.63), Ce (0.65), Er (0.57), Nd (0.62), Pr (0.67), Sc (0.69), Sm (0.52), Th (0.67), Yb (0.57) Group 3: rash = 0.21–0.50 Dy (0.35), Eu (0.34), Gd (0.31), Ho (0.40), Lu (0.46), Mg (0.23), Na (0.34), Tb (0.40), Tm (0.50), W (0.36), Y (0.47) Group 4: rash = − 0.20 to 0.20 Ba (0.03), Bi (0.001), Br (0.04), Cr (0.18), Cu (− 0.07), F (− 0.14), Fe (− 0.03), Mn (0.20), Pb (− 0.06), Sr (0.1), V (− 0.07) Group 5: rash ≤ − 0.20 As (− 0.45), Ca (− 0.29), Cd (− 0.63), Co (− 0.20), Hg (− 0.24), Mo (− 0.58), Ni (− 0.26), P (− 0.37), Se (− 0.31), U (− 0.23), Zn (− 0.65) Aluminosilicate affinity rAl–Si N 0.7 Be, Ce, Cs, Dy, Eu, Er, Ga, Hf, Ho, K, La, Li, Lu, Nb, Nd, Pr, Rb, Sc, Sm, Sn, Ta, Tb, Th, Ti, Tm, Y, Yb, Zr rAl–Si = 0.5–0.7 Gd, Na, W rAl–Si = 0.2–0.5 Bi, Cu, Fe, Pb, V Carbonate affinity rCa–Mg = 0.70–1.00 no elements rCa–Mg = 0.50–0.70 no elements rCa–Mg = 0.20–0.50 Ba, Br, Cr, Mn, Sn, Sr, W Organic affinity rOrganic = 0.70–1.00 As, Bi, Co, Fe, Hg, Mo, P, Pb, Se, U rOrganic = 0.50–0.70 V, Zn rOrganic = 0.20–0.50 Cd, Cu, Dy, Eu, Gd, Ni Sulphur affinity rS–Fe = 0.70–1.00 As, Bi, Co, Cu, Hg, Mo, Pb, Se, U, V rS–Fe = 0.50–0.70 Dy, Eu, Na, Ni rS–Fe = 0.20–0.50 Be, Cd, Ce, Cs, Er, Ga, Gd, Hf, Ho, La, Li, Lu, Nd, Pr, Sc, Sm, Ta, Tb, Th, Tm, W, Y, Yb, Zn, Zr Correlation coefficients between selected major elements Si–Al = 0.99; Ca–Mg = 0.35; Ca–Fe = − 0.89; Mg–Fe = − 0.42; S–Fe = 0.94

Fig. 7. Vertical variations of ash, organic sulphur (So, d) selected major and trace elements in the Bilong Co oil shale section (ash, organic sulphur and major elements in %, trace elements in μg/g).

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4.4. Comparison with other oil shale Twenty trace elements (As, Br, Cd, Cr, Cu, Dy, Er, Eu, Ho, Lu, Mo, Ni, Se, Tb, Tm, U, V, Y, Yb, and Zn) in the Bilong Co oil shale seams are present in low concentrations (Table 2) in comparison with common Australian oil shale, as reported by Patterson et al. (1986). Cerium, Co, Cs, F, Ga, Hf, Nb, Nd, Pb, Pr, Rb, Sc, Sn, Th, and Zr in the Bilong Co oil shale samples are higher than arithmetic means for the corresponding elements in Australian oil shale, and the remainder elements have arithmetic means that are about equal. If compared with common Chinese oil shale (Fu et al., in press), the elements Ba, Bi, Br, Hg, Li, Pb, Sr, V, and REEs have relatively high levels, whereas As, Cd, Cr, Cu, F, Nb, Se, Ta, and Zn are relatively depleted. In the previous discussion, we proposed that high enrichment elements in the Bilong Co oil shale include As, Bi, Cd, Cs, Mo, Se, and U. In fact, these elements were also enriched in the Julia Creek oil shale, Australia and in the Changshe Mountain oil shale, China. Elements with the highest EFs are thought to occur in organic matter and/or sulphides in the Julia Creek oil shale, Australia (Patterson et al. 1986). This is especially true for the present case. High concentrations of As, Cd, Mo, Se and U in the Julia Creek oil shale may be attributed to high organic matter (10.8–19%) and/or sulphur contents ( 0.6–1.5%). Cesium is the exception because Cs occurs mainly in clay minerals and sulphates in the Bilong Co oil shale, whereas Cs is mainly present in clay minerals in the Julia Creek oil shale. Elements with high correlation coefficients with ash and Al contents, such as Ga, Hf, Li, Rb, Sc, Sn, Th, Zr and REEs, are thought to occur mainly in clay minerals. Therefore, the deletion or enrichment of these elements may be attributed to weathering source rocks. Although different oil shale deposits revealed significant variations in trace-element concentrations, the distribution patterns of trace elements are quite similar, indicating that trace-element speciation in oil shale is governed by general processes rather than by individual geochemical mechanisms within one particular sample (Hirner and Xu, 1991). 5. Conclusions (1) The trace elements Mo, Se, Cd, Cs, As, Bi, and U are enriched as compared with Clarke values, with enrichment values of 35.0, 15.5, 8.7, 8.0, 7.9, 6.1 and 4.4, respectively. The elements Rb, Pb, Th, Li, Sr, and Zn have enrichment values N1.3, while Co, Cr, Sc, Ta, and V have an EF less than 0.5 and are therefore considered to be depleted. All other elements studied show more or less the same concentration as the Clarke values with the EF between 1.3 and 0.5. (2) The highest values for individual elements are not uniformly distributed among the seams studied. Very often, maximum element concentration is observed on the top or at the bottom of the seam. (3) The elements in the Bilong Co oil shale may be classified into three associations by cluster analysis. The first group (Si–Al–K– Na–Ti–Zr–Sc–Th–Sn–Nb–Ga–Ta–Be–Hf–Cs–Li–Rb–ash–REEs) comprises elements with positive correlation with the ash yield. The second group (Cr–W–Mg–Mn–Br–F–Sr–Ba) has weakly or negatively positive correlation coefficients (with an exception of F) with ash yield, while the third group (Zn–Bi– Cd–Ca–Fe–Pb–Hg–Co–Cu–Ni–P–Mo–Se–U–As–V) shows negative correlation coefficients (with an exception of Bi) with ash yield. (4) Comparison with common Australian oil shales, the contents of Ce, Co, Cs, F, Ga, Hf, Nb, Nd, Pb, Pr, Rb, Sc, Sn, Th, and Zr in the Bilong Co oil shale samples have relatively high levels, whereas As, Br, Cd, Cr, Cu, Dy, Er, Eu, Ho, Lu, Mo, Ni, Se, Tb, Tm, U, V, Y, Yb, and Zn are relatively depleted. However, the distribution patterns of trace elements are quite similar. These indicate

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