Ge distribution in the Wulantuga high-germanium coal deposit in the Shengli coalfield, Inner Mongolia, northeastern China

International Journal of Coal Geology 78 (2009) 16–26

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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

Ge distribution in the Wulantuga high-germanium coal deposit in the Shengli coalfield, Inner Mongolia, northeastern China Gang Du a,b, Xinguo Zhuang c,d,⁎, Xavier Querol e, Maria Izquierdo e, Andrés Alastuey e, Teresa Moreno e, Oriol Font e a

Key Laboratory of Marginal Sea Geology, Chinese Academy of Sciences, People's Republic of China Coal Geology Bureau of Inner Mongolia, Hohhot, 010051, People's Republic of China Institute of Sedimentary Basin and Mineral, Faculty of Earth Resources, China University of Geosciences, Hubei, 430074, People's Republic of China d State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Hubei, 430074, People's Republic of China e Institute of Earth Science ‘Jaume Almera’, CSIC, C/ LLuis Solé Sabarís s/n, 08028 Barcelona, Spain b c

a r t i c l e

i n f o

Article history: Received 14 March 2008 Received in revised form 14 October 2008 Accepted 17 October 2008 Available online 28 October 2008 Keywords: Coal Germanium Wulantuga Inner Mongolia China

a b s t r a c t The geological and geochemical controls of the Ge distribution in the Cretaceous Wulantuga highGermanium coal deposit in the Shengli coal field, Inner Mongolia are investigated. This paper focuses mainly on the spatial distribution of the Ge contents in coal. The high-Ge coals mainly occur in three splits of the #6 coal in the southwestern part of the Shengli coal field. Mean germanium contents in the coal range from 32 to 820 μg/g, with a mean value of 137 μg/g, on a bulk coal basis (mean of 939 coal samples from 75 boreholes in the #6 coal seam) in an area of 2.2 km2. The highest Ge content occurs SW of #6 coal seam, close to the margins of the coal basin, decreasing with a fan-shaped trend towards NW, the direction of the coal basin. There is an negative correlation between the mean Ge content and the thickness of the coal seam. Different distribution patterns of Ge content were found in vertical profiles. High Ge concentrations may occur in the middle parts of coal seams, at the bottom and/or the top of thick coal seams and inconspicuous variation. A major organic affinity was determined for Ge, with a special enriched in the banded bright and semibright coal. The high-Ge coals and the coalified wood in the sandstone overlaying the #6-1 coal highly enriched in Ge, As, Sb, W, Cs, Tl, Be, and Hg. The Late Jurassic silicified volcanic rocks in the NW of the Ge coal deposit relatively high enriched in Ge, Ga, Sb, As, Cs, Be, Ge and Hg. The correlation coefficients among the elements enriched showed marked variations at close sites in this deposit, suggesting a possible diagenetic origin of the geochemical anomaly. The main Ge anomaly was attributed to early Cretaceous hydrothermal fluids circulating through the fault systems and porous volcanic rocs, probably from the subjacent granitoid rocks. The fault systems, the porous coarse clastic rocks overlying coal seam and the lithotype of coal played an important role in the transport and trapping of Ge. A main diagenetic origin (overlapping a slight synsedimentary accumulation) is deduced for the Ge anomaly. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Ge in coal has attracted considerable attention in the last 50 years. Ge is probably the trace element with the largest number of references as regards content, distribution, mode of occurrence and origin in coal (Swaine, 1990). According to many studies summarized by Swaine (1990), there is a general agreement about that Ge in coal occurs organically bound to the coal matrix. The high enrichment of Ge in lignitic inclusions and coalified wood demonstrates that such

⁎ Corresponding author. Institute of Sedimentary Basin and Mineral, Faculty of Earth Resources, China University of Geosciences, Hubei, 430074, People's Republic of China. E-mail address: [email protected] (X. Zhuang). 0166-5162/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2008.10.004

enrichment occurs early in the coalification stage. Ge could form bonds with OH groups of humic acid during peat formation. Minor amounts of Ge could also occur in sphalerite (present in some coals) and in very few cases could be associated with clay minerals. The typical range of coal concentration for Ge is 0.5 to 50 μg/g (on a coal basis) with a mean value of 5 μg/g (Swaine, 1990) or 2 μg/g (Yudovich and Ketris, 2005, 2006). Most of the largest Ge hosted coal deposits occur in the Eastern Asia. A number of studies focused on the enrichment, occurrence and distribution and genesis of Ge in coal seams in the Pavlovsk (Primorye, Russia), Lincang (Yunnan, China) and Shengli (Inner Mongolia, China) Ge coal deposits (Hu et al., 1996; Zhuang et al., 1998; Wang, 1999; Su et al., 1999; Qing, 2001; Seredin and Danilcheva, 2001; Qi and Hu, 2002; Seredin, 2003; Du et al., 2003, 2004; Qi et al., 2004; Seredin,

G. Du et al. / International Journal of Coal Geology 78 (2009) 16–26

2004, 2005; Seredin et al., 2006; Hu et al., 2006; Zhuang et al., 2006; Qi et al, 2007a,b). In the case of the Russian deposits, other elements such as Au and the PGE are also enriched. Seredin (2003, 2004, 2005), Seredin and Danilcheva (2001), and Seredin et al. (2006) reported the general characteristics and genesis of the Ge, Sb, REEs and Au-PGE epigenetic mineralization at the highGe Cenozoic brown coal deposit at Pavlovsk (Primorye, Russia). The Pavlovsk deposit is enriched in As, Sb, W, Mo, Be, U, Cs and Hg. The high enrichment of these elements was attributed to volcanogenic chloride-sulphate vapor hydrothermal waters percolating coal beds in early diagenetic stages with subsequent accumulation of Ge, fixed by the organic matter. The precious metal mineralization in the Pavlovsk deposit is polygenic and polychromic, and resulted from the fluid-hydrothermal activity related to two impulses of a rift-genetic Late Cenozoic volcanism (Seredin, 2004; Seredin and Finkelman, in press). Hu et al. (1996, 2006), Su et al. (1999), Qi and Hu (2002), Qi et al. (2004) reported the enrichment of Ge in the high-Ge coal at Lincang (Yunnan, China) was generally attributed to ascending Ge-rich hydrothermal fluids, which circulated through the fault systems leaching Ge and other trace elements from the subjacent granitoid rocks. The Ge-rich hydrothermal fluids entered the swamp and accumulated in the coal in early diagenetic stages. The Early Cretaceous high-Ge Wulantuga coal deposit in the Shengli coal field, Inner Mongolia, was initially discovered by the Coal Geology Bureau of Inner Mongolia (CGBIM) in 1974. A detailed exploration within an area of 0.72 km2 near the margin of this deposit was conducted by the CGBIM in 1998. The corresponding coal reserves were estimated at 6 Mt of brown coal with Ge content N30 μg/g, and 1600 t of metallic Ge extractable from coal with Ge content N100 μg/g (Wang, 1999). Another extended exploration of an area of 6.54 km2 adjoining the previous area was performed by the CGBIM in 2005. The boundary of this high-Ge coal deposit was delineated and it was estimated that there were 12.3 Mt of brown coal with Ge content N30 μg/g and 1700 t of metallic Ge extractable from coal with Ge content N100 μg/g in the new explored area. The geological characteristics, the Ge content and its relationship with volatile matter, the ash yield, sulfur content and the geochemistry and mineralogy of the Shengli coal deposit have been reported by Wang (1999), Qing (2001), Du et al. (2003, 2004), Zhuang et al. (2006), and Qi et al. (2007a,b). In the light of new exploration and chemical analyses, the present study deals with Ge distribution in coal seams and with the geological controls of the enrichment of Ge in this geochemically interesting coal deposit. 2. Geological setting The Shengli coal field is located in northeastern China within a faulted basin along NE–SW trend, with a length of 45 km, mean width of 7.6 km, and a total area of 342 km2 (Fig. 1). The sedimentary sequence in the coal-bearing basin consists of the Xin'anling Group (Late Jurassic), the Baiyanhua Group (Early Cretaceous) and discordant Pliocene deposits, with a basement composed of low grade Devonian and Silurian metasediments (Fig. 1). The Wulantuga high-Ge coal deposit is located in the southwestern part of the Shengli coal field and has proven workable high-Ge coals distributed in an area of about 2.2 km2 (Figs. 1 and 2). The deposit consists of a monocline structure (with a NW slope) limited by normal faults on the northeastern and southwestern boundaries. The coal seams extend NW from the eroded southeastern boundary of the deposit across a north-dipping structure (Fig. 3, see location of cross section in Fig. 2).The simple geological structure makes open pit mining possible, and three open pits (No.1, No.6, and No.7) are mined at the southern part of this deposit (Fig. 2). No.1 open pit produces Ge coal for the Tongli Ge plant, No.6 and No.7 open pits produce Ge-barren coal.

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In this area the Baiyanhua Group is divided into the Tengeer and Saihantala Formations (Fig. 1) and the lithostratigraphy of the Tengeer and Saihantala Formations is shown in Figs. 1 and 3. The Tengeer Formation comprises mainly mudstone and siltstone, with one thin coal seam (#12) and sandy conglomeratic rocks distributing at the basin margin. The Saihantala Formation comprises a series of siliciclastic sediments including mudstone, siltstone, sandstone, and conglomerate, and three coal seams (#6, #7, and #9). The #6 coal seam is workable and locally split into three seams (upper 6, #6-1 and #6-2). Both the upper 6 (thickness ranging from 0.8 to 2.1 m, 1.6 m on average) and #6-2 (thickness is usually b3 m) seams are only locally developed. The #6-1 coal seam is distributed throughout the No.1 Wulantuga mine, with a mean thickness of 16.1 m (ranging from 0.8 to 36.2 m), and sandwiched in between the overlying sandstone and conglomerate, and the underlying mudstone and siltstone (Fig. 3). This area was influenced by different magmatic events. Thus, intrusive activity gave rise to Palaeozoic granitic rocks to the NW, Palaeozoic dioritic rocks to the SW and SE, and Mesozoic granites to the W (Indosinian) and SE (Yanshan). Furthermore, subaerial extrusive activity generated late Early Permian andesites and Middle Jurassic basalts and tuffs, and Late Jurassic basalts, volcanic breccias, rhyolitic and andesitic tuffs. The most recent volcanic events triggered subaerial eruptions of Quaternary basalts. 3. Methodology A total of 939 coal samples and 81 rock samples (partings, roofs, and floors of coal seam) were collected from 75 boreholes in the Wulantuga high-Ge #6 coal seam (locations of the boreholes are shown in Fig. 2) by the CGBIM. Samples collected were either 0.5 to 2 m or N3 m in length depending on coal seam thickness. The whole coal seam was collected as a single sample in the boreholes of coal thickness b2 m. Furthermore, in this study 29 channel samples were collected from three different profiles (M2,3,4, M6, and M7) in different open pit mines (Nos.1, 6, and 7) of Wulantuga deposit. A coalified wood inclusion sample (M5-1) in the overlying sandstone of # 6-1 coal seam and a high-Ge coal sample (channel profile) from Lincang, Yunnan (China) were also collected for comparison. Finally, five volcanic and plutonic rocks were sampled in the surrounding area to determine the possible enrichment of Ge and associated elements. These rock samples consisted of quartz diorite (Y1-1), granodiorite (Y1-2) and quartz diorite-porphyrites (Y1-3) from intrusive veins (Late Jurassic), silicified rhyolitic tuff breccia (Y2-1) and silicified rhyolitic breccia lava (Y3-1) from volcanic units (Late Jurassic). These samples were collected in two locations in the NW of the coal deposit (Fig. 1). Samples Y1-1, Y1-2, Y1-3, and Y2-1 were obtained from the same open pit, while Y3-1 sample came from an outcrop close to the mine. The location of sampling sites is shown in Figs. 1 and 2. Published data on geochemistry of 13 samples from a channel profile (MY) obtained only a few hundred of meters from the above M2,3,4 profile (Zhuang et al., 2006) are also used in this paper. Furthermore, the results of the analysis of the above sample from the Cenozoic Lincang deposit (Yunnan Province, China) kindly supplied by Prof. X. Wang, are also used for comparison purposes. Proximate analysis of coal samples was performed following ISO recommendations (ISO-11722-1999, ISO-1171-1997, and ISO-5621998). The total sulfur content of these samples was determined by high temperature combustion (ISO-351-1984), and the Ge contents were determined by distillation separating-phenylfluorone colorimetry (Chinese national standard GB 8207-87) by the CGBIM. Coal samples of three channel profiles, the coal inclusion and five rock samples were acid digested following a two-step digestion method devised to retain volatile elements in dissolution (Querol et al., 1997). The resulting solutions were analyzed at the Institute of Earth Sciences of CSIC by Inductively Coupled Plasma Atomic-Emission

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G. Du et al. / International Journal of Coal Geology 78 (2009) 16–26

Fig. 1. Right: Geological map of Wulantuga Ge-coal deposit (modified after Wang, 1999, 1:2,000,000 and 1:50,000 geological map). 1, unconformity; 2, Ge-coal deposit; 3, Quaternary; 4, Bogedawula Fm. of Tertiary; 5, Erlian Fm of upper Cretaceous; 6, Beiyanhua Fm. of lower Cretaceous; 7, Manitu Fm. of upper Jurassic; 8, Beiyingaolao Fm. of upper Jurassic; 9, Gegenaobao Fm. of lower Permian; 10, Zesi Fm. of lower Permian; 11, Benbatu Fm. of upper Carboniferous; 12, Baoyintu Fm. of Proterozoic; 13, Quaternary basalt; 14, Hercynian diorite; 15, Hercynian granodiorite; 16, Mesozoic granite; 17, clastic rocks of Beiyanhua Fm.; 18, volcanic clastic and rhyolitic lava of Manitu Fm.; 19, clastic rocks and tuff of Gegenaobao Fm.; 20, andesite and volcanic breccias of Benbatu Fm.; 21, metamorphic rocks of Baoyintu Fm.; 22, fault; 23, location of sampling sites. Left: Stratigraphic column of the Wulantuga coal deposit.

Spectrometry (ICP-AES) for major and selected trace elements and by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for most trace elements. Hg analyses were carried out, also at CSIC, on solid samples using an AMA-LECO gold amalgam atomic absorption spectrometer. The accuracy of the analytical procedures used was tested using South African coal reference materials SARM19 and 20. The results indicated a relative error of b10% for most of the elements analyzed, with slightly higher errors for P, Se and Cd. Mineralogical analyses of the coal samples were performed by means of X-ray diffraction (XRD, at CSIC). The occurrence of mineral

species was also investigated by means of a JEOL 6400 scanning electron microscope (SEM) with a LINK LZ5 EDX analyzer at CSIC. 4. Results and discussions 4.1. Coal characterization Proximate analysis and total sulfur determinations of coals from the 22 boreholes show that Wulantuga high-Ge coal is a medium- to highash (6–30%, mean 17%) and medium sulfur (St,d, 1–2.8%, mean 1.53%) coal. The sulfur in this coal occurs mainly as organic sulfur (So,d, 0.72–

G. Du et al. / International Journal of Coal Geology 78 (2009) 16–26

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Fig. 2. Distribution of Ge contents (in μg/g on bulk dry coal basis) of the Wulantuga Ge coal deposit in the Shengli coal basin.

1.14%, accounting for 57–88% of total sulfur), with minor proportions of pyritic sulfur (Sp,d, 0.13–0.91%) and very low sulfur occurring in sulfate. Based on the volatile matter yields (VMdaf, 39–45%, mean 42%), moisture contents (Mad, 8–16%, mean 11%) and calorific value

(Qgr,ad, 20.8–23.47 MJ/kg, mean 21.74 MJ/kg), the coal may be classified as subbituminous coal. The highest ash and sulfur contents usually occur at the bottom of the coal seam, although in a few cases they are present at the top and/or in the middle of the coal seam. The

Fig. 3. Stratigraphic section of the Wulantuga Ge-coal deposit. See location in Fig. 2.

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G. Du et al. / International Journal of Coal Geology 78 (2009) 16–26

ash and sulfur contents are correlated when both of them are enriched at the bottom of the coal seam, but a weak correlation was obtained at the top and middle sections, indicating a major organic affinity for sulfur with a partial sulfide association at the bottom (in a few cases at the top) margins of the seams. The lithology of the #6-1 coal consists mainly of banded bright and semibright coal and fusodurain. Maceral contents of selected samples from the #6-1 coal seam comprise vitrinite (54–98%), inertinite (b1– 30%) and liptinite (2–17.5%) (Zhuang et al., 2006). XRD analysis showed that major minerals in the #6-1 coal seam are quartz, kaolinite, montmorillonite, clinochlore, gypsum, pyrite (mainly at the top) and traces of calcite and dolomite. This mineral paragenesis is similar to that described by Zhuang et al. (2006) for the same open pit and coal seam. However, Zhuang et al. (2006) reported the occurrence of calcium oxalates, whereas these were not identified in this study. This provides evidence of the heterogeneity of coal characteristics in this coal seam. The SEM study of the Ge-rich coals from the profile M2,3,4 revealed abundant FeS2 particles, typically 0.5–5 μm in size and ranging in shape from anhedral to euhedral with well developed cubic and octagonal crystal faces. Arsenic is commonly present and is usually associated with relatively large (around 5 μm), diffuse, corroded pyrite grains. Calcite, barite, and the trace metals Zn and Cu are occasionally detectable in these samples. Neither Ge nor Sb were detected as individual grains despite their relatively high concentrations in samples such as the coalified wood, which has 1626 μg/g of Ge and 2700 μg/g of Sb. This lack of prominent Ge-bearing mineral nuggets supports the earlier conclusions of Zhuang et al. (2006) that Ge occurs in extremely small particles (0.5 μm) associated with the carbon in these coals. 4.2. Distribution of germanium in coal A total of 939 coal samples and 81 rock samples (partings, roofs and floors of coal seam) collected from 75 boreholes showed a wide range of Ge contents. The upper #6 coal seam showed the highest Ge concentrations, ranging from 105 to 603 μg/g (298 μg/g on average) across an area of about 0.21 km2. The #6-1 coal seam showed a wider range of content (from 30 to 820 μg/g, 137 μg/g on average), in an area of about 2.2 km2. The #6-2 coal seam is relatively depleted in Ge, with a content ranging from 1.5 to 157 μg/g (26 μg/g on average), and Ge contents over 100 μg/g were only recorded in one borehole (Zx4) of this coal seam. Ge contents in the partings and roofs and floors were relatively low as compared with those in coal, but still high when compared with sedimentary rocks. The Ge contents of nine samples collected from the parting, roof, and floor rocks of the upper #6 coal seam were b19 μg/g (relatively high concentration for sedimentary rocks) except one parting sample (borehole 404) which yielded 231 μg/g of Ge. Similarly, 65 rock samples collected from the #6-1 coal seam showed b10 μg/g Ge in the roof and floor rocks, except one roof rock sample (mudstone, borehole 202) with 101 μg/g Ge and one floor rock sample (mudstone, borehole 203) with 107 μg/g Ge, and slightly higher but still b30 μg/g in the partings with one exception (163 μg/g, borehole 8). The Ge contents of seven samples of parting, roof and floor rocks of the coal seam #6-2 were b3 μg/g. The distribution of the mean Ge contents in the #6-1 coal followed a fan-shaped trend decreasing from the SW to the NE part of the coal field, with a sharp gradient of Ge content at the southern margin of the deposit (Fig. 2). Furthermore, there is a negative correlation between the mean Ge content and the thickness of the coal seam (Fig. 4). Thin coals along the margin of basin have higher Ge contents (374–820 μg/g) than thick coal seams (32–341 μg/g) in the northern and eastern part of the deposit, and Ge contents diminished down to 2–10 μg/g in the centre of the basin. The relatively high contents of Ge in the areas far from the intense Ge anomaly may be due to a possible

Fig. 4. Cross-correlation plot of Ge content and thickness of coal seams.

slight synsedimentary enrichment. The observation of the higher Ge content in the thin coal seams along the margin of the basin is consistent with earlier reports on high-Ge coals. Such a distribution has been attributed to Ge reaching the peat though the roof and floor rocks (Kulinenko, 1976; Hower et al., 2002), but also the accumulation of Ge due to the peat weathering may occur. The vertical distribution of Ge contents in the #6-1 coal seam in 58 different boreholes is displayed in Fig. 5. The distribution patterns can be classified into three main types according to sample location, the variations of Ge content and the absolute Ge concentrations reached: 1) Type A involves boreholes drilled between the 100–150 μg/g isolines, and their Ge distribution patterns are characterized by, either similar values throughout the seam (A1: 002, 101, 102, 201, Zx2, Zx4, Zx6, Zx7, 403, 203) or by slight variation across the coal (A2: Zx3, 302, 303, 405, 202, 401, 404, Zx1, 001, K16). This type also includes boreholes 5, 24, 25, 26, Sh6, located between the 200– 300 μg/g isolines in the SW corner of the Ge anomaly (Fig. 2). These specific boreholes are rich in Ge throughout the seam and show higher Ge contents than other boreholes of this type. 2) Type B (K1, K3, K7, K8, K6, K9, K13, K11, K12, K10, K4,), with low Ge contents, includes boreholes drilled in the N and E part of the study area (Fig. 2). These samples register only sporadically elevated Ge concentrations usually in the middle of the coal seam rather than in the borders. Therefore, a clear Ge distribution cannot be drawn. 3) Type C includes samples showing variable distributions of Ge contents. According to the variations of Ge content and the absolute Ge concentrations reached, this type is classified into five groups: Group C1 (406, 504, Zx8, 503, Zx5, 502, 501) includes samples with a lower in Ge content than Rows A and B collected from sites lying between the 30–100 μg/g isolines (Fig. 2). All samples display a considerable variation in Ge content throughout the seam, although Ge depletion at seam boundaries is not frequent. In particular, there are marked concentration troughs, usually two, separating peaks at the top, middle and base of the seam. Group C2 (14, 18, 19, 20) samples lie between the 150–300 μg/g Ge isolines and all show a considerable variation in Ge content throughout the seam. There is also a clear trend towards a “2trough” pattern, with the highest Ge concentrations at the top and in the middle of the seam. Group C3 (13, K18, Zk9) samples lie around the NE–SW oriented isoline, high centred on borehole 13 (Fig. 2). These are grouped together given that all show a distinctive Ge enrichment in the upper middle part of the borehole, diminishing sharply at the top and base of the coal seam.

G. Du et al. / International Journal of Coal Geology 78 (2009) 16–26

Fig. 5. Vertical distributions of Ge content in the #6 coal seam (in μg/g on bulk dry coal basis).

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Group C4 (3, 7, 8, 6, 12, 2) samples are all characterized by the highest Ge contents not only in the upper middle part of the seam but also towards the base, with Ge contents at the top of the seam varying from enriched (e.g. borehole 3) to strongly depleted (e.g. boreholes 6, 8 and 12). Finally group C5 (9, 22) samples all lie above the 300 μg/g Ge isoline and display the highest concentrations in boreholes drilled in the

Ge anomaly (Fig. 2). The Ge content throughout the seam is also variable. Considering the data summarized above, it goes without saying that the high-Ge Wulantuga coals do not observe the behavior of “Zilbermints Law”, whereby Ge enrichment occurs near the bottom, roof and partings of the coal seam. This observation was first made by Zilbermints et al. (1936) in Donetsk coal Basin (Pavlov, 1966; Yudovich,

Table 1 Thickness, ash yields, coal lithotype and contents of major elements (%) and trace elements (μg/g) in the coal seam profile M2,3,4 from Wulantuga Bottom

Top

M2-1

M2-2

M2-3

M2-4

M2-5

M2-6

M2-7

M3-1

M3-2

M3-3

M3-4

M3-5

M4-1

M4-2

M4-3

M4-4

M4-5

M4-6

M4-7

Thickness (m)

0.8

0.15

0.2

0.35

0.5

0.2

0.85

0.8

0.4

0.15

0.2

0.7

1.2

0.4

0.4

0.1

0.4

0.7

0.7

Coal lithology

Fd

B

Fd

B

Fd

Sb

Fd

Fd

B

Fd

B

Fd

B

Sb

Sb

Fd

Sb

Fd

Fd

Ash(%) Al Ca K Na Fe S Mg Li (μg/g) Be B Sc P Ti V Cr Mn Co Ni Cu Zn Ga Ge As Se Rb Sr Y Zr Nb Mo Cd Sn Sb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Tl Pb Bi Th U Hg

35 3.23 0.56 0.44 0.10 2.12 2.07 0.46 20 15 40 4 51 1110 41 28 43 3 5 26 bdl 9 187 580 5 48 61 9 50 4 1 bdl 2 15 43 73 12 29 3 12 3 bdl 2 bdl 2 bdl bdl bdl bdl bdl 2 1 309 1 10 bdl 6 3 2.4

15 0.76 0.57 0.13 0.14 1.23 1.87 0.17 5 15 77 1 29 452 11 18 29 1 2 8 bdl 2 422 560 2 8 53 5 13 2 bdl bdl bdl 27 9 33 4 11 1 4 1 bdl 1 bdl bdl bdl bdl bdl bdl bdl bdl bdl 588 bdl 3 bdl 2 1 bdl

16 0.73 0.54 0.08 0.11 1.75 2.34 0.17 5 20 57 bdl 23 389 9 13 32 1 1 7 bdl 2 266 651 2 5 48 4 11 1 bdl bdl bdl 23 6 26 4 9 1 3 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 359 bdl 2 bdl 1 bdl 2.2

5 0.25 0.41 0.02 0.06 0.81 1.33 0.10 1 15 85 bdl 13 150 4 4 22 1 2 6 bdl 1 777 525 2 1 37 3 5 bdl bdl bdl bdl 45 4 12 2 4 bdl 2 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 627 bdl bdl bdl bdl bdl bdl

9 0.34 0.60 0.04 0.14 1.64 2.34 0.17 3 22 64 bdl 24 175 6 7 34 bdl 1 4 bdl 1 282 513 2 2 52 2 6 bdl bdl bdl bdl 19 4 23 2 5 bdl 2 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 337 1 2 bdl bdl bdl 1.9

13 0.79 0.63 0.06 0.09 1.12 1.63 0.16 6 26 60 bdl 16 485 10 10 40 bdl 1 7 bdl 2 171 369 2 3 56 3 14 2 bdl bdl bdl 5 4 26 4 9 bdl 3 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 197 bdl 2 bdl 1 bdl bdl

14 0.99 0.68 0.05 0.12 0.75 1.13 0.17 7 28 48 bdl 30 390 7 8 41 bdl 1 5 bdl 2 67 202 1 3 55 3 13 1 bdl bdl bdl 3 4 35 4 8 bdl 3 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 89 bdl 3 bdl 2 bdl 0.46

20 1.15 0.50 0.17 0.05 1.58 1.85 0.14 8 31 34 1 37 886 13 14 40 bdl 1 9 bdl 3 98 259 2 10 45 4 21 3 bdl bdl bdl 41 8 141 6 15 1 5 1 bdl 1 bdl bdl bdl bdl bdl bdl bdl bdl bdl 107 2 3 bdl 2 bdl 2.3

16 0.81 0.37 0.13 0.07 1.31 1.85 0.12 5 16 61 bdl 23 594 14 13 29 1 2 8 bdl 3 407 821 2 8 36 3 14 2 2 bdl bdl 185 7 34 4 9 1 3 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 296 2 2 bdl 2 bdl bdl

7 0.20 0.33 0.01 0.05 1.47 2.05 0.08 1 16 62 bdl 17 95 5 7 25 bdl 2 3 bdl 9 626 706 3 bdl 29 2 3 bdl 2 bdl bdl 455 3 16 1 3 bdl 1 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 366 4 1 bdl bdl bdl bdl

5 0.18 0.33 0.01 0.07 1.01 1.60 0.09 1 15 70 bdl 7 64 3 5 24 1 4 3 bdl 2 641 712 1 bdl 29 1 2 bdl 2 bdl bdl 132 3 139 1 3 bdl 1 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 388 2 bdl bdl bdl bdl 4.1

7 0.27 0.42 0.03 0.08 1.38 1.91 0.12 2 23 60 bdl 18 144 5 10 31 1 2 10 bdl 2 254 566 3 1 38 2 6 bdl 1 bdl bdl 131 3 27 2 5 bdl 2 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 204 3 1 bdl bdl bdl bdl

10 0.37 0.33 0.04 0.05 1.51 2.06 0.08 2 14 58 bdl 24 264 7 7 25 1 4 4 bdl 24 558 786 4 2 31 3 8 1 3 bdl bdl 1039 4 22 2 5 bdl 2 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 281 6 1 bdl bdl bdl 14

6 0.17 0.36 0.01 0.04 1.16 1.73 0.07 1 18 54 bdl 17 67 3 5 24 bdl 2 3 bdl 20 443 713 3 bdl 28 2 2 bdl 3 bdl bdl 863 3 18 1 3 bdl 1 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 215 3 1 bdl bdl bdl bdl

8 0.32 0.33 0.03 0.05 1.18 1.80 0.07 2 17 53 bdl 18 322 7 9 24 2 5 5 4 41 490 644 4 2 28 3 10 2 4 bdl bdl 1595 3 23 2 5 bdl 2 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 187 6 2 bdl bdl bdl 25

13 0.67 0.31 0.04 0.05 0.92 1.47 0.08 4 17 52 bdl 22 489 9 12 24 2 4 5 bdl 29 285 431 4 2 29 3 13 1 3 bdl bdl 1141 3 24 2 7 bdl 3 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 121 4 1 bdl 1 bdl bdl

8 0.15 0.31 0.01 0.04 1.69 2.45 0.06 bdl 12 56 bdl 23 107 5 7 20 4 11 3 bdl 57 754 546 5 bdl 28 2 6 1 4 bdl bdl 2382 3 21 1 3 bdl 1 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 240 9 bdl bdl bdl bdl 36

16 0.95 0.46 0.06 0.06 1.79 2.28 0.13 6 27 40 1 103 667 8 14 34 3 4 7 20 12 51 470 3 3 63 10 74 4 2 bdl 1 353 4 83 10 23 2 8 2 bdl 2 bdl 2 bdl bdl bdl bdl bdl 2 bdl 35 5 5 bdl 3 bdl bdl

12 0.75 0.48 0.03 0.05 1.45 1.94 0.12 5 25 40 2 29 398 9 14 34 2 3 5 45 3 47 396 2 1 36 15 24 2 1 bdl bdl 242 3 20 7 20 2 9 2 bdl 2 bdl 2 bdl 1 bdl 1 bdl bdl bdl 36 4 2 bdl 1 bdl 6.5

B—Bright coal, Sb—Semibright coal, Fd—Fusodurain, bdl—blow determined limit.

G. Du et al. / International Journal of Coal Geology 78 (2009) 16–26

2003), and has been corroborated by a number of authors (Breger and Schopf, 1955; Admakin, 1970; Kulinenko, 1976; Eskenazy, 1996; Hower et al., 2002, among others). In our study there are different patterns of Ge distribution (and often no obvious pattern at all), with high-Ge levels more frequently occurring towards the middle of the seam than along the margins of the seam. Any explanation of the genesis of the Wulantuga coals should bear in mind this disagreement with Zilbermints Law.

23

4.3. Coal geochemistry 4.3.1. Trace element contents Table 1 shows the trace element contents of M2,3,4 coal profile samples from Wulantuga Ge coal mine (No.1). It can be concluded that the M2,3,4 coal profile samples are highly enriched in Ge, As, Sb, W, and Hg (47–777, 202–821, 3–2382, 35–627, and 0.46–36 μg/g, respectively) and relatively high in Be, Cs, and Tl (12–31, 3–43, and

Table 2 Average ash yields and contents of major elements (%) and trace elements (μg/g) in the Wulantuga samples and comparison with the contents in the Lincang coal, and with the worldwide contents of coal reported by Swaine (1990) and Yudovich and Ketris (2005, 2006), and the mean Chinese coal contents reported by Dai et al. (2007, 2008)

Ash (%) Al Ca K Na Fe S Mg Li (μg/g) Be B Sc P Ti V Cr Mn Co Ni Cu Zn Ga Ge As Se Rb Sr Y Zr Nb Mo Cd Sn Sb Cs Ba La Ce Pr Nd Sm Gd Tb Dy Er Yb Hf Ta W Tl Pb Bi Th U Hg

Lincang Chinese coals

C-wood

Wulantuga

LB-301

M5-1

MY

M2,3,4

M6

M7

30 2.7 0.3 0.6 0.03 1.7 2.7 0.1 19 198 67 3 61 480 11 8 79 6 8 18 166 8 889 152 0 69 40 14 16 28 11 2 7 31 30 103 4 9 1 4 1 1 0 2 1 2 0 1 533 7 35 3 4 81 b1

11 0.6 0.3 0.1 0.1 1.5 3.0 0.1 2 2 63 0 38 716 106 31 34 18 14 5 0 23 1626 391 4 7 38 5 89 16 6 0 2 2719 11 28 2 4 0 2 0 0 0 0 0 0 1 0 43 13 1 0 1 1 17

19 1.4 2.2 0.1 0.1 1.1 1.0 0.4 10 33 96 2 29 663 17 11 42 6 18 12 60 4 472 377 4 10 266 7 31 2 3 0 0 75 10 163 8 14 2 7 1 1 0 1 1 1 1 0 498 2 5 0 2 17 4

12 0.7 0.4 0.1 0.1 1.4 1.9 0.1 5 20 56 0 28 382 9 11 30 1 3 7 4 12 359 550 3 5 41 4 15 1 1 0 0 458 6 42 4 9 1 4 0 0 0 0 0 0 0 0 262 3 2 0 1 0 9

9 0.3 0.9 0.0 0.0 0.4 1.5 0.2 1 3 59 0 28 167 5 5 173 4 2 5 0 1 10 14 2 1 76 5 5 0 0 0 0 3 1 12 2 4 0 2 0 0 0 0 0 0 0 0 7 0 1 0 0 0 0.17

13 1.2 1.0 0.2 0.4 0.6 1.1 0.3 4 0 91 0 302 281 7 6 185 1 1 5 0 1 1 3 2 1 180 3 9 1 1 0 0 0 0 281 3 6 0 2 0 0 0 0 0 0 0 0 4 0 2 0 1 0 0.05

C-wood (coalified wood).

Chinese coals

Worldwide coal range

Dai et al. (2007, 2008)

Swaine (1990)

Yudovich and Ketris (2005, 2006)

Mean

Max

Min

Brown

Hard

80 15 400 10 3000 2000 100 60 300 30 50 50 300 20 50 50 10 50 500 50 200 20 10 3 10 10 5 300 40 70 10 30 6 4 1 4 3 3 5 2 5 1 80 0.5 10 10 1

1 0.1 5 1 10 10 2 1 5 1 0.5 1 5 1 1 2 0 2 10 2 5 1 0.1 0.1 1 0.1 0.1 70 1 2 1 3 1 0 0 1 1 0 0.5 0.1 0.5 0.2 2 0.1 0.5 0.5 0.02

10 ± 1.0 1.2 ± 0.1 56 ± 3 4.1 ± 0.2 200 ± 30 720 ± 40 22 ± 2 15 ± 1 100 ± 6 4.2 ± 0.3 9.0 ± 0.9 15 ± 1 18 ± 1 5.5 ± 0.3 2.0 ± 0.1 7.6 ± 1.3 1.0 ± 0.15 10 ± 0.9 120 ± 10 8.6 ± 0.4 35 ± 2 3.3 ± 0.3 2.2 ± 0.2 0.24 ± 0.04 0.79 ± 0.09 0.84 ± 0.09 0.98 ± 0.10 150 ± 20 10 ± 0.5 22 ± 1 3.5 ± 0.3 11 ± 1 1.9 ± 0.1 2.6 ± 0.2 0.32 ± 0.03 2.0 ± 0.1 0.85 ± 0.08 1.0 ± 0.05 1.2 ± 0.1 0.26 ± 0.03 1.2 ± 0.2 0.68 ± 0.07 6.6 ± 0.4 0.84 ± 0.09 3.3 ± 0.2 2.9 ± 0.3 0.10 ± 0.01

14 ± 1 2.0 ± 0.1 47 ± 3 3.7 ± 0.2 250 ± 10 890 ± 40 28 ± 1 17 ± 1 71 ± 5 6.0 ± 0.2 17 ± 1 16 ± 1 28 ± 2 6.0 ± 0.2 2.4 ± 0.2 9.0 ± 0.7 1.6 ± 0.1 18 ± 1 100 ± 7 8.2 ± 0.5 36 ± 3 4.0 ± 0.4 2.1 ± 0.1 0.20 ± 0.04 1.4 ± 0.1 1.00 ± 0.09 1.1 ± 0.12 150 ± 10 11 ± 1 23 ± 1 3.4 ± 0.2 12 ± 1 2.2 ± 0.1 2.7 ± 0.2 0.31 ± 0.02 2.1 ± 0.1 1.00 ± 0.07 1.0 ± 0.06 1.2 ± 0.1 0.30 ± 0.02 0.99 ± 0.11 0.58 ± 0.04 9.0 ± 0.7 1.1 ± 0.1 3.2 ± 0.1 1.9 ± 0.1 0.10 ± 0.01

3.23 1.0 0.17 0.144 1.2 0.15 32 2.1 53 4.7 424 2220 35 15 124 7.1 14 18 42 6.6 3.0 3.8 2.5 9.2 140 18 89 9.5 3.2 0.25 2.1 0.8 1.1 159 26 0.9 5.5 22 4.3 3.7 0.7 3.1 1.9 2.1 3.8 0.7 1.0 0.47 15 0.8 5.8 2.4 0.19

24

G. Du et al. / International Journal of Coal Geology 78 (2009) 16–26

b1–9 μg/g, respectively), as compared with the mean contents of worldwide coals reported by Swaine (1990) and Yudovich and Ketris (2005, 2006), and with the mean contents of common Chinese coals reported by Dai et al. (2007, 2008) (Table 2). Table 3 presents the trace element contents of the M6 and M7 coal profile samples from the Nos.6 and 7 coal mines. In these profile samples, contents of almost all trace elements are in the range of the mean contents of worldwide coals reported by Swaine (1990) and

Table 3 Thickness, ash yields and contents of major elements (%) and trace elements (μg/g) in the coal seam profiles M6 and M7 from Wulantuga Bottom

Top

Bottom

M6-2

M6-3

M6-4

M7-1

M7-2

M7-3

M7-4

Thickness (m) 0.65

0.25

0.2

0.5

0.95

1.1

1.2

1.1

0.9

Ash (%) Al Ca K Na Fe S Mg Li (μg/g) Be B Sc P Ti V Cr Mn Co Ni Cu Zn Ga Ge As Se Rb Sr Y Zr Nb Mo Cd Sn Sb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Tl Pb Bi Th U Hg

5 0.14 1.03 0.01 0.04 0.41 1.48 0.24 bdl 3 60 bdl 31 67 3 4 203 3 4 4 bdl bdl 7 11 2 bdl 83 3 3 bdl bdl bdl bdl 1 1 11 2 4 bdl 2 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 4 bdl bdl bdl bdl bdl bdl

9 0.51 0.96 0.02 0.04 0.36 1.53 0.23 2 3 65 bdl 24 193 5 4 174 4 2 4 bdl bdl 13 13 2 1 80 5 6 bdl bdl bdl bdl 1 1 13 2 5 bdl 2 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 8 bdl 1 bdl bdl bdl 0.08

11 0.45 0.88 0.03 0.04 0.43 1.46 0.21 2 4 50 1 27 257 7 9 173 5 2 6 bdl 1 14 11 2 1 72 9 8 1 bdl bdl bdl bdl 1 11 2 5 bdl 2 bdl bdl bdl bdl 1 bdl bdl bdl bdl bdl bdl bdl 9 bdl 2 bdl bdl bdl 0.06

15 3.77 0.47 0.79 1.59 0.64 0.00 0.22 6 bdl 81 bdl 197 300 8 6 161 bdl 1 4 bdl 2 1 4 3 1 126 2 11 1 bdl bdl bdl bdl bdl 149 2 5 bdl 2 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 2 bdl 2 bdl 0.04

12 0.60 1.21 0.02 0.06 0.70 1.51 0.34 5 bdl 81 bdl 380 240 6 5 161 bdl bdl 4 bdl 1 bdl 3 2 bdl 214 2 8 1 bdl bdl bdl bdl bdl 384 5 10 bdl 3 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 2 bdl 1 bdl bdl

14 0.64 1.25 0.07 0.08 0.42 1.18 0.36 5 bdl 92 bdl 375 467 10 8 193 bdl bdl 7 bdl 2 bdl 3 2 3 214 2 13 2 bdl bdl bdl bdl bdl 322 3 7 bdl 3 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 2 bdl 2 bdl 1 bdl bdl

13 0.38 1.19 0.03 0.11 0.76 1.58 0.34 3 bdl 108 bdl 298 170 4 6 231 3 1 4 bdl 1 bdl 3 2 1 184 2 5 bdl 1 bdl bdl bdl bdl 448 2 4 bdl 2 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 5 bdl 2 bdl bdl bdl 0.05

11 0.50 1.06 0.04 0.08 0.34 1.08 0.31 3 2 91 bdl 257 231 6 5 181 4 4 4 bdl 1 3 4 1 1 160 7 8 bdl 3 bdl bdl 1 bdl 101 3 6 bdl 3 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 12 bdl 1 bdl bdl bdl 0.05

M6-1 9 0.29 0.83 0.03 0.04 0.50 1.64 0.20 1 1 62 bdl 31 153 5 6 140 2 1 4 bdl 1 8 20 1 1 68 2 5 bdl bdl bdl bdl 8 2 12 2 4 bdl 2 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 6 bdl 1 bdl bdl bdl 0.36

Top M7-5

Yudovich and Ketris (2005, 2006) and the mean contents of Chinese coals reported by Dai et al. (2007, 2008) (Table 2). The results of the M2,3,4 coal profile samples are similar to the results (MY) reported by Zhuang et al. (2006) and Qi et al. (2007a), but a relatively high content of U in the MY coal profile was found (Zhuang et al., 2006; Table 2). The coalified wood in the sandstone overlaying the #6-1 coal contains high contents of Ge, As, Sb, Cs, W, Tl, and Hg (1626, 391, 2719, 11, 43, 13, and 17 μg/g, respectively) (Table 2). As compared with the LB301 sample from Lincang, both Wulantuga and Lincang Ge coals are enriched in Ge, As, Sb, Cs, W, Tl, and Be, but relatively high contents of Nb, Mo, and U are found in the LB301 sample (Table 2). However, the lateral and vertical contents of the above elements significantly varied. For example, the content of Ge, As, W and Sb decreased from hundreds of μg/g in the No.1 mine to b14 μg/g in the No.6 mine and b4 μg/g in No.7 mine in a distance not exceeding 1 km (Table 2) and Be, Ga, Cs, Tl, and Hg also decreased clearly from the No.1 to No.7 mines. In contrast to the decrease in these elements towards the East (Nos.6 and 7 mines), there is an increase in Mn, Sr and Ba. This could be attributed to the higher contents of carbonate minerals and barite (identified by SEM) in the relatively low Ge coal. The fact that there are groups of elements with different vertical distributions is probably due to a heterogeneous enrichment of the main Ge diagenetic pulses overlapping the slight synsedimentary accumulation of Ge. This possibility is also supported by the extremely high content of most of the aforementioned elements in the coalified wood in the sandstone and high content of Ge close to the outcrop of coal in the southern of the deposit. The analyses of the plutonic and volcanic rocks in the NW of the Ge coal deposit are shown in Table 4. The contents of Ge and their above associated elements are only relatively high in the silicified volcanic rock (Y3-1 and Y2-1). The contents of Ga, Sb, As, Cs, Be, Ge and Hg reach 102, 31, 17, 13, 12, 5, and 0.02 μg/g, respectively in Y3-1, and Ga, Sb, As, Cs, Be, and Hg reach 34, 31, 25, 15, 9, and 0.05, respectively in Y2-1, whereas the contents are much lower, often below the detection limit, in the other intrusive rocks. This shows that the possible volcanic origin of the high Ge and metallic mineralization anomaly existed before the early Cretaceous coal accumulation in the Shengli coal. The erosion of volcanic rocks and subsequent accumulation of Ge in the upper sandstone of #6 coal and/or hydrothermal fluids circulating through the volcanic structures after coal accumulation could account for this Ge anomaly. 4.3.2. Trace element affinities Fig. 6 depicts variations in the concentrations of 8 trace elements through the two high-Ge #6-1 coal channel profiles MY and M2,3,4 100 m distant (Fig. 2). Most elements revealed higher concentrations towards the top of the coal seam (e.g. Ga, Ni, Sb, Tl, Hg, this is also true for S, Mn, Sr and Ba, not shown in Fig. 6) at the two profiles, indicating a lateral geochemical continuity between the two #6-1 coal profiles. There are, however, some differences between the #6-1 coal profiles: V, Cr, Co, Cu and Ga showed elevated levels at the top and base of the coal seam in the profile MY, but not in M2,3,4. Likewise, comparison between patterns of As, W and Ge enrichment revealed little similarity between MY and M2,3,4 (Fig. 6) despite the fact that the Pearson correlation coefficients of these elements indicate an association. This association is especially clear in profile M2,3,4 where As, W and Ge concentrations are inversely correlated with mineral matter. This provides evidence of the high heterogeneity of the geochemical enrichment that may vary within hundreds of meters, corroborating the major diagenetic origin of the anomaly. Five groups of elements were identified according to the results of the cross correlation analysis performed on the geochemical data from M2, 3, and 4: Group 1 includes Al, K, Mg, Li, Sc, Ti, V, Cr, Cu, Rb, Nb, Cs, Pb, La, Ce, and Th. These elements have high correlation coefficients (r = 0.6– 0.98) with Al. If M4-6 and M4-7 samples (top of #6-1 coal) are not

G. Du et al. / International Journal of Coal Geology 78 (2009) 16–26 Table 4 Contents of major elements (%) and trace elements (μg/g) in the plutonic and volcanic rocks surrounding the Shengli coal deposit

Al (%) Ca K Na Fe S Mg Li (μg/g) Be B Sc P Ti V Cr Mn Co Ni Cu Zn Ga Ge As Se Rb Sr Y Zr Nb Mo Cd Sn Sb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Tl Pb Bi Th U Hg

Y1-1

Y1-2

Y1-3

Y2-1

Y3-1

1.49 2.27 0.06 0.12 1.71 3.30 0.65 10 bdl 4 2 100 1580 36 12 312 4 3 8 17 16 bdl 4 7 44 229 9 38 2 bdl bdl 2 bdl 4 286 9 24 2 10 2 bdl 2 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 9 bdl 5 bdl 0.002

7.72 1.86 1.40 3.50 1.50 0.01 0.53 17 bdl 5 bdl 428 1630 32 9 197 2 bdl 9 2 17 bdl 6 7 63 90 8 50 3 bdl bdl 2 bdl 7 207 11 26 3 10 2 bdl 2 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 5 bdl 5 bdl 0.002

7.31 3.11 0.81 3.02 3.32 0.01 1.58 35 bdl 11 6 740 3525 81 44 696 12 25 10 38 17 bdl 3 5 22 234 20 125 3 bdl bdl 3 bdl 3 227 11 31 3 14 4 bdl 4 bdl 3 bdl bdl bdl 2 bdl 4 bdl bdl bdl 4 bdl 6 2 0.003

1.7 4.11 0.25 0.10 0.92 0.03 0.18 46 9 9 bdl 145 278 49 17 166 bdl 6 12 bdl 34 bdl 25 4 21 170 3 13 bdl bdl bdl bdl 31 15 433 7 12 bdl 7 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 7 bdl 2 bdl 0.050

0.42 0.11 0.09 0.05 0.28 0.01 0.05 35 12 21 bdl 31 82 8 7 117 bdl 2 3 bdl 102 5 17 bdl 12 48 bdl 3 bdl bdl bdl bdl 31 13 197 2 3 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 3 bdl bdl bdl 0.020

included, P, Zr and Y also have high correlation coefficients (r = 0.8– 0.97) with Al. Mg, Be and Cs have a relatively low (r = 0.3–0.5) correlation coefficients with Al. This group is similar to that identified by Zhuang et al. (2006) and is positively correlated with ash yield as well. Consequently, a main clastic origin, dominated by an aluminosilicate affinity, is interpreted for the elements included here. Group 2 includes Na, Mn, Sr, and Ca, with high correlation coefficients (r = 0.6–0.7) with Ca. If M2-1 sample (with a very high ash yield in the bottom of #6-1 coal) is not included in the analysis, Mg and Ca have a high correlation coefficient (r = 0.9). These elements are enriched mainly in the lower part of the seam, and probably represent a carbonate mineral assemblage.

25

Group 3 includes Ni, Ga, Se, Mo, Sb, Tl, and Hg, with high correlation coefficients among their concentrations (r = 0.6–0.98). They were enriched mainly in the upper part of the seam, probably by diagenetic pulses.

Fig. 6. Vertical distributions of selected trace elements in the M2-4 and MY profiles sampled at the Wulantuga coal mine No.1 (#6 coal seam, in μg/g on bulk dry coal basis).

26

G. Du et al. / International Journal of Coal Geology 78 (2009) 16–26

Group 4 includes W, B, Ge, with correlation coefficients (r) between these elements ranging from 0.4 to 0.7. If M2-1 sample (with high ash yield in the bottom of #6-1 coal) is not included in the analysis, Ge is negatively correlated with ash yield and the contents of elements from Groups 1 and 2. The negative correlation of the Ge contents with ash yields indicates its major organic affinity. Furthermore, a relatively high Ge content in the banded bright and semibright coal was evident (Table 1); this also supporting to the affinity of Ge with huminite (Zhuang et al., 1998). The study of Zhuang et al. (2006) and Qi et al. (2007a) showed that Ge is also positively correlated with Mo, usually present also with an organic affinity in many coals. Group 5 consists of S and Fe (r = 0.8). Arsenic is positively correlated with S and Fe as reported by Zhuang et al. (2006). This association is attributed to the presence of sulfide minerals. 5. Conclusions 1) High-Ge coals in Shengli coal field mainly occur in three splits of the #6 coal in an area of 2.2 km2. Germanium contents range from around 100 to 600 μg/g (about 300 μg/g on average) for the upper #6 coal in an area of 0.2 km2, from 30 to 820 μg/g (average 140 μg/g) for #6-1 coal in a area of 2.2. km2 and from 1 to 160 μg/g (average 26 μg/g) for #6-2 coal. 2) The content distribution of Ge in #6-1 coal showed a pronounced fanshaped trend decreasing from the SW margin to the NE part of the coal field. Thin coals along the margins have higher Ge contents (374– 820 μg/g) than thick coal seams (32–341 μg/g) in the northern part of the deposit and Ge contents diminished down to 2–10 μg/g in the centre of the basin. The relatively high Ge content, even in the centre of the basin may be caused by a slight synsedimentary Ge enrichment. 3) The distribution of Ge in the vertical profile of #6-1 coal and the correlation coefficients between elements enriched showed very marked variations at very close places in this deposit. This suggests major diagenetic origin (overlapping a slight synsedimentary accumulation) for the Ge anomaly. 4) The main Ge anomaly was attributed to early Cretaceous hydrothermal fluids circulating through the fault systems and porous volcanic rocks, probably from the subjacent granitoid rocks. 5) The fault systems, the porous coarse clastic rocks overlying coal seam and the lithotype of coal played an important role in the transport and trapping of Ge. Acknowledgements The authors would like to express their gratitude to the Spanish Ministry of Foreign Affairs and the Department of Science. We would like to thank Mrs. S. Rico for her help in the analytical tasks. This research was supported by the National Natural Science Foundation of China (No. 40572089), Specialized Research Fund for the Doctoral Program of Higher Education (No. 20050491053) and the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (No. GPMR0515). We are indebted to Mrs. Li Lirong from the Coal Geology Bureau of Inner Mongolia for providing the data, and to Qing Shengli, senior engineer from Coal Geology Bureau of Inner Mongolia for his help. We are grateful to Prof. X. Wang from the China University of Geosciences for supplying the Lincang coal sample. References Admakin, L.A., 1970. Relationship between ash and germanium content of coal and its genetic significance. Doklady Akademii Nauk SSSR 192, 1353–1355. Breger, I.A., Schopf, J.M., 1955. Germanium and uranium in coalified wood from upper Devonian black shale. Geochimica et Cosmochimica Acta 7, 287–293. Dai, S.F., Zhou, Y.P., Ren, D.Y., Wang, X.B., Li, D., Zhao, L., 2007. Geochemistry and mineralogy of the Late Permian coals from the Songzao Coalfield, Chongqing, southwestern China. Science in China Series D: Earth Science 50, 678–688.

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