factors controlling geochemical and mineralogical compositions of coals preserved within marine...

International Journal of Coal Geology 109–110 (2013) 77–100

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International Journal of Coal Geology

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Factors controlling geochemical and mineralogical compositions ofcoals preserved within marine carbonate successions: A case studyfrom the Heshan Coalfield, southern China

Shifeng Dai a,⁎, Weiguo Zhang b,a, Vladimir V. Seredin c, Colin R. Ward d, James C. Hower e, Weijiao Song a,Xibo Wang a, Xiao Li a, Lixin Zhao a, Huan Kang a, Licai Zheng a, Peipei Wang a, Dao Zhou a

a State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing 100083, Chinab Xi'an University of Science and Technology, Xi'an 710054, Chinac Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Staromonetny per. 35, Moscow 119017, Russiad School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australiae University of Kentucky, Center for Applied Energy Research, 2540 Research Park Drive, Lexington, KY 40511, United States

⁎ Corresponding author. Tel./fax: +86 10 62341868.E-mail address: [email protected] (S. Dai).

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a b s t r a c t
a r t i c l e i n f o
Article history:Received 21 December 2012Received in revised form 2 February 2013Accepted 4 February 2013Available online 16 February 2013

Keywords:Minerals in coalTrace elements in coalLate Permian coalHydrothermal fluidsHeshan of southern China

The Late Permian coals in the Heshan Coalfield of southern China are preserved within marine carbonate suc-cessions and characterized by super-high organic sulfur (5.13–10.82%). Minerals identified in the coals in-clude quartz, kaolinite, illite, mixed layer illite/smectite, albite, pyrite, marcasite, calcite, and dolomite,along with trace amounts of smectite, fluorite, strontianite, REY-bearing carbonate minerals, jarosite, andwater-bearing Fe-oxysulfate. The coals are very rich in trace elements including F (up to 3362 μg/g), V (upto 270 μg/g), Se (up to 24.4 μg/g), Mo (up to 142 μg/g), U (up to 111 μg/g), and, to a lesser extent, Sr, Y, Zr,Nb, Cd, Cs, heavy rare earth elements, Hf, Ta, W, Hg, and Th.Previous studies attributed the high organic sulfur and elevated trace elements to the seawater influence orthe formation of soil horizons before the accumulation of peat in the basin. However, mineralogical and geo-chemical data presented in this study have shown that the sediment-source region and multi-stage hydro-thermal fluids are the dominant influences on the mineralogical composition and elevated trace elementsin the coal, although seawater influence also contributed to the composition of the mineral matter.For example, a large proportion of the quartz and clay minerals, as well as almost all the albite, in both thecoal benches and the parting mudstones were derived from detrital materials of terrigenous origin in theYunkai Upland. High concentrations of lithophile trace elements were also derived from the sediment sourceregion. Minerals including fluorite, calcite, dolomite, strontianite, and REY-bearing carbonate minerals werederived frommulti-stage hydrothermal activities. High concentrations of V, Mo, and U that occur through thecoal seam sections were probably derived from hydrothermal solutions during peat accumulation or at theearly diagenetic stages. The hydrothermal fluids also corroded the syngenetically-formed minerals (quartz,albite, and pyrite) and caused re-distribution of lithophile elements from partings to the underlying coalbenches, resulting in higher key element ratios (Yb/La, Nb/Ta, and Zr/Hf) and more abundant heavy rareearth elements in the coal benches than in the immediately overlying partings.

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1. Introduction

Coals preserved within marine carbonate successions have beenreported from Guiding and Ziyun of Guizhou Province, Yanshan ofYunnan Province, and Heshan of Guangxi Province in southern China.These coals usually have a thickness of 1–2 mandhigh-sulfur (especiallyorganic sulfur) contents, and are intercalated with marine carbonaterocks (Hou et al., 1995; Lei et al., 1994; Shao et al., 2003; Zeng et al.,2005).

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Some previous studies have focused on the depositional environ-ment (Chen, 1987; Huang et al., 1994; Jin and Li, 1987), the geochemis-try of the high-organic sulfur (which was considered as being derivedfrom marine influence; Chou, 1997, 2004; Hou et al., 1995; Lei et al.,1994), and the trace-element geochemistry andmineralogical composi-tions (Dai et al., 2008; Shao et al., 2003; Zeng et al., 2005).

The Yanshan coals are significantly enriched in B, F, V, Cr, Ni, Mo, Se,and U. High-temperature quartz, sanidine, muscovite, illite, albite, anddawsonite were identified in the coal by Dai et al. (2008) and Ren etal. (2006). The geochemical andmineralogical anomalies were attribut-ed to volcanic ash, submarine exhalation, and seawater input duringpeat accumulation (Dai et al., 2008).

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78 S. Dai et al. / International Journal of Coal Geology 109–110 (2013) 77–100

Two articles have described the geochemical andmineralogical com-positions of the Heshan coals, and have shown the Heshan coals to behighly enriched in U, V, and Mo, with lesser amounts of Cr, Cs, Ga, Nb,Sc, Sr, Th, W, Y, Ta, and Zr (Shao et al., 2003; Zeng et al., 2005). Zeng etal. (2005) suggested that the enrichment of V, Cr, Zn, Mo, Ni, and Rb, aswell as total S and Fe in the lower part of most of the coal seams, mightbe associated with the formation of soil horizons before the accumula-tion of peat in the basin, but they did not explain how the soil horizoncould influence the peat geochemistry. Chlorine, F, Sr, and Ca are locallyconcentrated in the top of specific coal seams as a consequence of theleaching of the overlying carbonate (Zeng et al., 2005). Shao et al.(2003) attributed the enrichment of the elements with elevated concen-trations tomarine transgression duringpeat accumulation. However, theorigin andmodes of occurrence of the trace elements in the Heshan coalswere deduced from statistical analysis (Shao et al., 2003; Zeng et al.,2005). Statistical methods are among the more commonly used indirectapproaches for interpreting element modes of occurrence. However,these procedures have some problems and are risky, particularly whenbulk samples or multiple samples are used (Eskenazy et al., 2010;Finkelman, 1980). Additionally, the sediment-source region and epige-netic processes associated with the coals were not addressed in the arti-cles by Shao et al. (2003) and Zeng et al. (2005).

In this paper, we report new data and shed light on the factors, in-cluding sediment source region, marine influence, and multi-stagehydrothermal solutions, controlling the mineralogical and geochemi-cal anomalies in the Heshan coals.

Fig. 1. Location of the Heshan Coalfield (A)

2. Geological setting

The Heshan Coalfield is located in the center of Guangxi Province,southern China (Fig. 1A). The coalfield covers an area of 360 km2 andcontains 13mines (Fig. 1B). The sedimentary sequences in the coalfield(Fig. 2) include the Lower Permian Maokou Formation, Upper PermianHeshan and Dalong Formations, Lower Triassic Luolou Formation, Mid-dle Triassic Gaoling Formation, and the Quaternary system.

The coal-bearing stratum is the Heshan Formation, which overlieslimestones of the Maokou Formation with an unconformable contactand conformably underlies limestones of the Luolou Formation. TheHeshan Formation is also mainly composed of limestones interlayeredwith coal and thin layers of mudstone and calcareous mudstone (Lu,1996). Its thickness varies from 107 to 207 m. The unit contains thenos. 2, 3U, 3M, 3L, 4U, 4L, and 5 coal beds, of which the 3U, 3L, 4U,and 4L areminable. Theminable coal seams vary from 1 to 3 m in thick-ness and contain several siliceous mudstone partings.

The roof of the no. 3U coal is made up of gray, medium to thick silic-ified limestone layers with fossil debris (3U-R). The floor is a thick lime-stone, usually overlain by a thin flint layer (3U-F). The roof of the no. 3Lcoal is represented by medium to thick limestones with foraminiferaand algal fossil debris (3L-R). These are silicified and intercalated withbanded flint. The floor of this coal is a thick silicified limestone with fo-raminifera and algal debris (3L-F). The no. 4L coal overlies a thick lime-stone and is overlain by a calcareous mudstone. The roof of the no. 4Ucoal is limestone (Wang et al., 1995). The lithological characteristics of

and the coal mines in the coalfield (B).

Fig. 2. Sedimentary sequences of the Heshan Coalfield and bench samples present inthis study.

79S. Dai et al. / International Journal of Coal Geology 109–110 (2013) 77–100

the roof and floor materials indicate that the coal-bearing strata, likethose of the Guiding Coalfield in Guizhou and the Yanshan Coalfield inYunnan (Fig. 1A), were formed in tidal flat environments on a restrictedcarbonate platform (Shao et al., 2003; Wang et al., 1995, 1997).

The Dalong Formation is made up of silty mudstone interlayeredwith tuff layers and, locally, limestone. The Gaoling Formation consistsof sandy mudstone, fine-grained sandstone, and mudstone. The LuolouFormation is mainly composed of limestone interlayered with tufflayers and thin argillaceous limestone beds.

The sediment-source region for the Heshan coal basin was theYunkai Upland (Dai et al., 2013; Feng et al., 1994; Fig. 3), rather thanthe Kangdian Upland that supplied terrigenous materials to most ofthe Late Permian coal-bearing areas in southwestern China.

3. Samples and analytical procedures

The samples evaluated in this study were collected from the facesof the mined coal seams in the Zhongxu Mine (nos. 3U and 3L coal)

and the Shicun Mine (nos. 4U and 4L coal) of the Heshan Coalfield(Fig. 1B). From top to bottom, the bench samples and the partingswere numbered as indicated in Fig. 2. Each coal bench sample wascut over an area 10-cm wide and 10-cm deep.

Proximate analysiswas conducted usingASTMStandards D3173-03,D3175-02, and D3174-04 (2005). The total sulfur and forms of sulfurwere determined following ASTM Standards D3177-02 and D2492-02(2005), respectively. Coarse-crushed samples of each coal were pre-pared as grainmounts formicroscopic analysis by reflected light follow-ing ASTM Standard D2797-04 (2005). Mean random reflectance ofvitrinite (percent Ro, ran) was determined using a Leica DM-4500Pmicroscope (at a magnification of 500×) equipped with a Craic QDI302™ spectrophotometer. The standard reference for vitrinite reflec-tance determination was gadolinium gallium garnet (manufacturer:Klein & Becker) with a calculated standard reflectance of 1.722% forλ=546 nm under oil immersion. Maceral constituents were identifiedusing white-light reflectance microscopy under oil immersion andmore than 500 counts were measured for each polished pellet. Themaceral classification and terminology applied in the current studyare based on Taylor et al. (1998) and the ICCP System 1994 (ICCP,1998, 2001).

A field emission-scanning electron microscope (FE-SEM, FEIQuanta™ 650 FEG), in conjunction with an EDAX energy-dispersiveX-ray spectrometer (Genesis Apex 4), was used to study themorpholo-gy of the minerals, and also to determine the distribution of some ele-ments. Samples were either chromium/carbon-coated using a QuorumQ150T ES sputtering coater or not coated at all for working underlow-vacuum SEM conditions. Samples were mounted on standard alu-minum SEM stubs using sticky electronic-conductive carbon tabs. Theworking distance of the FE-SEM-EDS was 10 mm, the beam voltagewas 20.0 kV, the aperture was 6, and the micron spot size was 5 or5.5. The images were captured via a retractable solid state backscatterelectron detector.

The mineralogy was determined by optical microscopic observa-tion and X-ray powder diffraction. Low-temperature ashing of coalwas performed using an EMITECH K1050X plasma asher, prior toXRD analysis. XRD analysis of the low-temperature ashes, and alsoof the non-coal samples, was performed on a powder diffractometer(D/max-2500/PC XRD) with Ni-filtered Cu-Kα radiation and a scintil-lation detector. The XRD pattern was recorded over a 2θ interval of2.6–70°, with a step size of 0.01°. X-ray diffractograms of the LTAsand partings were subjected to quantitative mineralogical analysisusing Siroquant™, a commercial interpretation software developedby Taylor (1991) based on the principles of diffractogram profilingset out by Rietveld (1969). Further details indicating the use of thistechnique for coal-related materials are given by Ward et al. (1999,2001) and Ruan and Ward (2002).

Samples were crushed and ground to pass 200 mesh (75 μm) forgeochemical analysis. An elemental analyzer (vario MACRO) was usedto determine the content of C, H, and N in the coals. X-ray fluorescence(XRF) spectrometry (ARL ADVANT'XP+) was used to determine themajor oxides as outlined by Dai et al. (2012a).

Inductively coupled plasma mass spectrometry (X series II ICP-MS),in pulse counting mode (three points per peak), was used to determinetrace elements in the coal samples, except for Hg and F. The ICP-MSanalysis and sample microwave digestion programs, related to coaland coal-related materials, are outlined by Dai et al. (2011). For boronICP-MS determination, coal samples were digested using 3 ml 65%HNO3, 1 ml 40%HF, and 0.5 ml 85%H3PO4, but for the non-coal samples,the reagents include 3 ml 40% HF, 1 ml 65% HNO3, and 0.5 ml 85%H3PO4. Arsenic and Se were determined by ICP-MS using collision celltechnology (CCT) in order to avoid disturbance of polyatomic ions. ForICP-MS analysis, samples were digested using an UltraClaveMicrowaveHigh Pressure Reactor (Milestone). Multi-element standards (InorganicVentures: CCS-1, CCS-4, CCS-5, and CCS-6; NIST 2685b and Chinesestandard reference GBW 07114) were used for calibration of trace

image of Fig.�2

Fig. 3. The Late Permian sedimentary environments of southern China.


element concentrations. The detection limit (dl=3σ) of each traceelement in the samples present in this study is shown in Table 1.

Mercury was determined using a Milestone DMA-80 Hg analyzer.Fluorine and Cl were determined by pyrohydrolysis in conjunctionwith an ion-selective electrode, following the method described inChinese National Standard GB/T 4633–1997 (1997) and GB/T 3558–1996 (1996).

The chemical composition of the (high-temperature) coal ashexpected to be derived from the mineral assemblage indicated bythe XRD and Siroquant analyses of each LTA or parting sample wascalculated, using the methodology described by Ward et al. (1999).This process includes calculations to allow for the loss of hydroxylwater from the clay and bauxite minerals and CO2 from the carbonates,at the temperatures associated with high-temperature (815 °C) ashingand combustion processes. Chemical analysis data for the samples wererecalculated to provide normalized percentages of themajor element ox-ides in the inorganic fraction. These represent the chemical compositionof the (high-temperature) ash derived from each sample. The inferredpercentages ofmajor element oxides in the coal LTAs and partings, as cal-culated from the XRD data, were then compared to the normalized per-centages of the same oxides in the SO3-free ash as calculated from thegeochemical data obtained from the XRF analysis.

Table 1ICP-MS detection limit (dl) for trace element in coal (μg/L).

Element dl Element dl Element dl Element dl

Li 0.00530 Se 0.51201 Ba 0.02478 Er 0.00018Be 0.00136 Rb 0.01325 La 0.00105 Tm 0.00004B 0.34496 Sr 0.00979 Ce 0.00012 Yb 0.00001V 0.04104 Y 0.06114 Pr 0.00028 Lu 0.00008Cr 0.04236 Zr 0.00185 Nd 0.00018 Hf 0.00029Co 0.00197 Nb 0.00140 Nd 0.00001 Ta 0.00068Ni 0.01434 Mo 0.00288 Sm 0.00012 W 0.00049Cu 0.00830 Cd 0.00468 Eu 0.00017 Tl 0.00123Zn 0.01304 In 0.00241 Gd 0.00315 Pb 0.00796Ga 0.00027 Sn 0.03168 Tb 0.00475 Bi 0.00026Ge 0.00390 Sb 0.00108 Dy 0.00001 Th 0.00020As 0.05502 Cs 0.02665 Ho 0.00004 U 0.00033

4. Results

4.1. Coal chemistry and vitrinite reflectance

Table 2 summarizes the proximate and ultimate analyses, total S,and S forms for the 18 bench and five coal channel samples from theHeshan Coalfield. The volatile matter yields (Table 2) and the vitrinitereflectance values of the channel samples (1.92% for 3U-C, 1.90% for3L-C, 1.88% for 4U-C1, 1.90% for 4U-C2, and 1.74% for 4L-C) indicate alow volatile bituminous (lvb) coal according to the ASTM classification(ASTM D 388–99, 2005).

The coals display high ash yields and are classified as medium-ash(3L) and high-ash coals (3U, 4U, and 4L) according to Chinese Stan-dards GB/T 15224.1-2004 (2004); coals with ash yield 16.01–29% aremedium-ash coals and >29.00% are high-ash coals.

The coal is a superhigh-organic-sulfur (SHOS) coal (Chou, 2012;Table 2), especially the 3L coal, for which organic sulfur ranges from7.51 to 10.82%with an average of 9.24%. These SHOS coals are character-istically low in pyritic S. However, the host rocks (roofs and floors) arelow in organic S.

Such SHOS coals with 4–11% organic S (Chou, 1997) are rare in theworld, but some were reported from Guiding of Guizhou Province (Leiet al., 1994); Wuyi Mine, Anxian, Sichuan (Ren et al., 2006); Yanshanof Yunnan Province (Dai et al., 2008); as well as coals of Tertiary agealong the on-shore margin of the Gippsland Basin, Victoria, Australia(Smith and Batts, 1974). High organic sulfur (around 6%) has alsobeen observed in the Permian Tangorin seam of the Cranky CornerBasin, eastern Australia (Marshall and Draycott, 1954; Ward et al.,2007); the Upper Palaeocene Raša coal from Istria (Slovenia) containsup to 11% organic sulfur (Damste et al., 1999).

4.2. Maceral content

The Heshan coals sampled for the present study are generally dom-inated by collotelinite and vitrodetrinite (Table 3). Samples 3L-1 and3L-9 have >20% inertinite. Samples 3L-4, 4U-1, and 4U-C2 exceed 30%inertinite (59.5% inertinite in 4U-C2). Liptinite macerals were not

image of Fig.�3

Table 2Bench thickness (cm), proximate and ultimate analyses (%), forms of sulfur (%) in the coals of the Heshan Coalfield.

Sample Thickness(cm)

Mad Ad Vdaf Cdaf Hdaf Ndaf St,d Ss,d Sp,d So,d

3U benches 3U-R 0.55 84.03 1.67 bdl 1.14 0.533U-1 16 0.78 41.91 15.42 85.84 3.36 0.71 7.16 0.30 0.53 6.333U-2 10 0.72 31.33 15.17 85.97 3.25 0.64 8.57 0.04 0.32 8.213U-3P 40 2.64 69.39 5.18 0.45 1.39 3.343U-4 12 1.34 46.11 16.96 85.73 3.55 0.74 6.64 0.19 0.77 5.683U-5P 7 2.61 66.74 5.88 0.47 1.48 3.923U-F 0.16 99.09 0.25 bdl 0.03 0.213U-WA 38 0.94 40.45 15.84 85.84 3.39 0.70 7.37 0.19 0.55 6.62

3L benches 3L-R 0.15 69.44 1.90 bdl 0.41 1.493L-1 4 0.70 30.79 16.87 87.07 2.48 0.92 8.51 0.06 0.38 8.083L-2P 20 0.75 51.67 8.72 0.06 1.77 6.893L-3 14 0.27 14.10 12.54 82.58 2.20 0.74 11.20 bdl 0.38 10.823L-4 5 0.19 39.32 17.92 89.34 3.87 0.61 8.22 bdl 0.70 7.513L-5 10 0.35 23.31 25.29 86.40 3.31 0.07 10.21 0.11 0.46 9.643L-6P 20 1.69 75.59 5.32 0.16 1.63 3.523L-7 15 0.44 24.46 13.57 85.22 4.07 0.71 10.47 bdl 0.61 9.873L-8P 13 1.03 52.78 7.27 0.28 1.33 5.663L-9 12 0.41 31.30 13.26 86.97 3.65 0.71 9.41 bdl 0.75 8.663L-F 0.08 68.44 0.34 0.18 0.09 0.073L-WA 60 0.38 24.88 15.80 85.62 3.30 0.62 10.07 0.19 0.55 9.49

Channel 3U-C 83 0.80 36.13 17.79 81.55 4.13 0.77 7.44 0.11 0.74 6.583L-C 113 0.45 29.30 14.43 81.99 4.22 0.87 9.64 bdl 0.65 8.994U-C1 227 1.31 34.01 16.71 80.03 5.03 1.06 6.53 0.11 bdl 6.414U-C2 155 2.63 45.46 25.77 77.66 4.90 1.26 6.68 0.69 0.87 5.134L-C 106 1.66 41.65 15.41 80.44 4.62 1.41 7.10 0.34 0.07 6.69

AV 3U 0.87 38.29 16.82 83.70 3.76 0.73 7.40 0.15 0.65 6.603L 0.41 27.09 15.12 83.81 3.76 0.74 9.85 bdl 0.60 9.244U 1.97 39.73 21.24 78.84 4.96 1.16 6.61 0.40 0.44 5.77

M, moisture; A, ash yield; V, volatile matter; C, carbon; H, hydrogen; N, nitrogen; St, total sulfur; Ss, sulfate sulfur; Sp, pyritic sulfur; So, organic sulfur; ad, air-dry basis; d, dry basisdaf, dry and ash-free basis; WA, weighted average for bench samples; WA, weighted average for bench samples of seam sections; bdl, below detection limit.


observed, owing to the difficulty of distinguishing liptinites in the lowvolatile bituminous rank range.

Discussions of the origins of inertinite macerals can be found inHower et al. (2009, 2011), Scott (1989, 2000, 2002), Scott andGlasspool (2005, 2006, 2007), Scott and Jones (1994), and Scott et al.(2000). The inertinite macerals in the Heshan coals appear as distinct,fire-derived fusinite and semifusinite forms (Fig. 4, inertinite maceralsA–C, and, to a lesser extent, D–F). The fusinite and semifusinite inFig. 4D–F show some signs of degradation. The inertinites in Fig. 4G–Hsuggest a degradation of woody material followed by burning. Themaceral textures resemble those found in severely brecciated and oxi-dized coals described in western Kentucky (de Wet et al., 1997;Hower and Williams, 2001; Hower et al., 1987; O'Keefe et al., 2008).Macrinite associated with semifusinite and fusinite is seen in Fig. 4I–K.

Table 3Maceral composition of the Heshan coals (vol.%; mineral-free basis).

Sample T CT VD CD CG G TV F SF Mic Mac Sec Fg ID TI

3U-1 0 11.6 79.8 0 0 0 91.4 7.1 1.5 0 0 0 0 0 8.63U-2 0 52.2 43.3 0 0 0 95.5 3.8 0.6 0 0 0 0 0 4.53U-4 0 44.8 50.6 0 0.6 0 96.1 0.6 3.2 0 0 0 0 0 3.93L-1 0.2 28 45.4 0 0.7 0.7 75.1 12.2 12.4 0 0 0 0 0.2 24.93L-3 1 57.3 31.2 0 0.8 0 90.4 3.3 6.1 0.2 0 0 0 0 9.63L-4 0.2 48 11.7 0 0.2 0.7 60.9 27.4 11 0 0.5 0.2 0 0 39.13L-5 0.2 61.7 20 0 0 0.2 82.2 7.6 10.2 0 0 0 0 0 17.83L-7 0 59.9 28.7 0 0.3 0 88.9 4.9 5.4 0.3 0.3 0.3 0 0 11.13L-9 3.3 31.5 39.9 0 0.5 0.5 75.6 18.8 5.2 0 0.5 0 0 0 24.43U-C 0 20.9 73.2 0 0.3 0.3 94.7 1 3.3 0 1 0 0 0 5.33L-C 0.2 49.4 34.5 0 0.2 0.2 84.6 6 9.4 0 0 0 0 0 15.44U-C1 14 35.8 39.4 0 0.6 0 89.7 8.4 1.4 0 0 0.6 0 0 10.34U-C2 3.9 4.7 30.5 0 1.2 0.4 40.6 52.3 6.3 0 0 0 0.8 0 59.44L-C 10.2 24.1 14.7 0 1.1 0 50.1 45.7 2.8 0 0.3 0.8 0 0.3 49.9

T, telinite; CT, collotelinite; VD, vitrodetrinite; CD, collodetrinite; CG, corpogelinite; G, gelinite; TV, total vitrinite; F, fusinite; SF, semifusinite; Mic, micrinite; Mac, macrinite; Secsecretinite; Fg, funginite; ID, inertodetrinite; TI, total inertinite.

;

4.3. Minerals

4.3.1. Mineral phases identifiedThe proportion of each crystalline phase identified from the X-ray

diffractograms of the coal LTA, partings, roof, and floor samples isgiven in Table 4. The phases identified in the samples include quartz;kaolinite; illite; mixed layer illite/smectite (I/S); feldspar (albite); py-rite; marcasite; calcite; and dolomite; with trace amounts of smectite,anatase, bassanite, and gypsum in some samples. Additionally, fluorite,strontianite, REY-bearing carbonate minerals (containing F and U),jarosite, and water-bearing Fe-oxysulfates that contain Si or Si and Alwere identified in some coal samples by SEM-EDS, but were belowthe detection limit of the XRD and Siroquant system. Other Ca and Fesulfates (e.g., szomolnokite and anhydrite) may also be present in the

,

Fig. 4. Inertinite and vitrinite in the coal. Reflected light, oil immersion. (A) Fusinite (f) in vitrinite matrix, photo 3L 1 08; (B) fusinite (f), photo 3L 4 04; (C) semifusinite (sf) and fusinite(f), photo 4U C2 13; cross-polars; wavelength plate; (D) fusinite (f) in vitrinite matrix, Photo 3L 1 09; (E) fusinite (f) with gelinite (g) in cell lumens, photo 3L 4 07; (F) semifusinite (sf)and fusinite (f), photo 4L 1 05; cross-polar; wavelength plate; (G) degraded vitrinite as fusinite (f), photo 3L 4 13; (H) degraded vitrinite as fusinite (f), photo 4U C2 12; cross-polars;wavelength plate; (I) fusinite and macrinite (m), photo 3L 4 15; (J) macrinite/fusinite mix, photo 3L 4 16; (K) semifusinite (sf) in macrinite (m) matrix, photo 3U C 04.


3U-3P and 3U-5P parting samples, but if so, they are below the level forreliable quantification.

With the exception of the 4U coal, in which the minerals includeonly minor proportions of quartz and feldspar and are dominatedby kaolinite and illite, the minerals in the coal samples and the

partings are mainly represented by quartz and mixed-layer illite/smectite, with lesser but still significant proportions of feldspar (al-bite), illite, and kaolinite. Calcite also occurs in the LTA of the 3L and3U coals, but is only a very minor component of the 3U partings andis not present at all in the partings of the 3L seam section. Calcite is,

image of Fig.�4

image of Fig.�4

Table 4Low temperature ash (LTA) yields of coal samples and mineral compositions (%) of coal LTAs, partings, roof and floor determined by XRD and Siroquant.

Sample LTA Quartz Kaolinite Illite I/S Smectite Feldspar Pyrite Marcasite Calcite Dolomite Anatase Bassanite Gypsum

3U benches 3U-R 48.4 4.4 2.2 5.6 1.4 2.5 1.6 33.5 0.3

3U-1 42.6 51.9 5.7 8.9 16.0 2.5 3.7 9.0 0.4 1.9

3U-2 32.3 46.3 7.0 8.8 17.1 4.2 2.5 10.8 1.3 2.1

3U-3P 22.7 10.3 8.9 43.3 2.8 4.5 1.5 1.2 0.9 1.0 3.0

3U-4 52.0 34.2 8.6 12.8 25.8 2.1 2.6 1.7 8.2 2.1 1.9

3U-5P 25.5 12.0 4.8 36.8 2.0 1.4 6.3 1.2 0.4 5.3 1.3 2.9

3U-F 94.2 1.5 1.7 0.8 0.3 0.8 0.7

3L benches 3L-R 17.4 2.7 2.4 5.0 10.9 0.9 1.7 57.4 1.6

3L-1 31.6 24.6 5.6 8.2 43.3 4.6 1.6 1.8 10.3

3L-2P 28.8 3.6 29.7 14.5 12.0 6.8 3.0 0.8 0.8

3L-3 13.7 20.6 1.9 11.1 35.7 12.2 4.7 2.7 7.9 3.0 0.2

3L-4 38.5 60.0 1.5 4.8 4.5 2.2 4.3 0.5 15.6 5.5 1.0

3L-5 23.2 38.4 1.2 8.7 24.0 11.9 5.3 0.8 3.5 5.2 1.0

3L-6P 24.8 0.7 11.0 45.0 8.3 5.5 1.8 2.9

3L-7 27.3 30.1 2.5 10.9 35.7 10.5 5.1 2.0 1.4 1.4 0.4

3L-8P 22.5 0.5 8.8 48.2 7.4 4.9 1.2 6.6

3L-9 34.4 28.6 2.2 26.1 24.0 5.7 6.5 0.2 5.1 1.6

3L-F 10.7 1.6 0.4 0.4 1.4 0.4 0.7 83.7 0.6

Channel 3U-C 42.18 41.6 11.8 17.1 6.2 1.4 3.0 0.8 15.6 2.5

3L-C 32.07 26.6 2.8 23.4 23.1 11.0 4.2 1.1 4.6 3.2

4U-C1 37.33 0.5 61.4 33.8 2.6 0.2 1.6

4U-C2 49.68 1.6 31.4 51.6 9.1 0.6 3.3 0.3 1.4 0.8

4L-C 47.5 15.5 6.4 50.8 14.2 12.2 0.3 0.6

The rows with gray shading are of roof, floor, and parting samples.

50

60


however, very abundant in the roof and floor of the 3L coal and in theroof of the 3U coal; indeed, the floor of the 3L coal has the mineralogyof a relatively pure limestone. The floor of the 3U coal, by contrast,consists almost entirely of quartz, consistent with its silicified lime-stone lithology.

Small proportions of marcasite and dolomite, and in some cases,small proportions of bassanite or gypsum, also occur in the LTA derivedfrom many of the 3L and 3U coal samples. As discussed for other coalsby Ward (2002), the bassanite may represent partial dehydration ofgypsum in the coals, or it may represent a product of interactionbetween organically-associated Ca and S during the low-temperatureashing process.

The LTA yield for the samples studied is very close to butmostly a lit-tle higher than the high-temperature ash yield from the same samples(Fig. 5). The difference is partly due to dehydration of the clayminerals,oxidation of the pyrite, and/or CO2 release from the carbonate mineralsduring the (high-temperature) ashing process.

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Fig. 5. Relation between low-temperature ash and high-temperature ash yields.

4.3.2. Modes of occurrence of minerals

4.3.2.1. Modes of occurrence of minerals in coal. Quartz occurs asfine-grained crystals (generally b1 μm) restricted to the organic matter(Fig. 6A) or as larger well-developed crystals (subhedral to euhedral) incollodetrinite (Fig. 6B). The latter are generally corroded (Fig. 6B, C).Quartz also occurs as cell- and fracture-fillings (Fig. 6D–F).

Kaolinite, illite, and mixed-layer illite/smectite generally occur ascell- and fracture-fillings (Fig. 6D, E), and, to a lesser extent, are distrib-uted along the bedding planes. In places, illite occurs in the organic

matter matrix with needle- and lath-like shapes (Fig. 7A), andmixed-layer clay minerals with a colloform texture are distributed inthe organic matrix as well (Fig. 7B).

Well-developed but corroded crystals of albite occur in collodetrinite(Fig. 8A–D). Some of the corroded cavities are filled by other minerals(e.g., fluorite, Fig. 8B).

Carbonate minerals, including calcite, dolomite, and strontianite, aswell as fluorite, typically occur as fusinite-cell- and fracture-fillings(Fig. 9A–D).

Jarosite (KFe3+3(OH)6(SO4)2) andwater-bearing Fe(Si, Al)-oxysulfateminerals are distributed close to the fractures (Fig. 10A, B). Jarosite, prob-ably derived from the oxidation of iron sulfides (cf. Rao and Gluskoter,

Unlabelled image

Fig. 6. SEM and back-scattered electron images of quartz, albite, clay minerals, fluorite, and carbonate minerals in the coal. (A) Fine-grained quartz, sample 3L-1; (B) corrodedquartz crystal; sample 3L-5; (C) corroded quartz and albite; sample 3L-5; (D) cell-filling quartz and mixed mineral of illite and smectite; sample 3L-5; (E) fracture-filling quartz,kaolinite, and calcite; sample 3L-1; (F) fracture-filling quartz, fluorite, and strontianite; sample 3L-5.


1973), occurs aswell-developed ca. 1 μmcrystals (Fig. 10A). Additionally,water-bearing Fe(Si)-oxysulfate minerals also substituted for the outerpart of the corroded pyrite (Fig. 10C, D) or are distributed in the areasurrounding the corroded pyrite (Fig. 10C).

REY-bearing phases, occurring as Sr(Ca)CO3 and Ca(Mg)CO3(F)minerals, are found as infillings of cells and fractures (Fig. 11).

4.3.2.2. Modes of occurrence of minerals in the partings. A large propor-tion of the quartz in the partings occurs as independent subhedral toeuhedral grains, with long-axis parallel to the bedding planes. Particlesizes range from less than 1 μm to more than several tens of μm,although a minor proportion of the quartz occur as fine-veined fillingsthat cut across the clay minerals (Fig. 12). Almost all the albites in the

3L partings occur as subhedral grains, and no cell- or fracture-fillingswere observed (Fig. 12).

Mixed-layer illite/smectite shows fine lath- or needle-like shapesin the partings (Fig. 12C). Dolomite occurs as fracture- and cell-fillings,and as columnar forms in the clay matrix (Fig. 12D). Framboidal pyriteor fine-grained pyrite crystals are distributed in the clay matrix(Fig. 12B, F).

4.4. Geochemistry

4.4.1. Major element oxidesWhen considered on a whole-coal basis (Table 5), the Heshan

coals contain higher proportions of Al2O3, K2O, Na2O, MgO, and

Fig. 7. SEM and back-scattered electron images of lath-like illite (A; sample 3L-5) and mixed-layer clay minerals with a colloform texture (B; sample 3L-5) in the coal.


especially SiO2, than the average values for Chinese coals reported byDai et al. (2012b); this could be due to the high proportions of miner-al matter (ash yield) associated with the coal samples. Other major el-ement oxides (CaO, TiO2, Fe2O3, and P2O5), however, are either lowerthan or close to those average values. The SiO2/Al2O3 ratios for thecoals are much higher than those of other Chinese coals (1.42) (Daiet al., 2012b) and also than the theoretical SiO2/Al2O3 ratio of kaolin-ite (1.18), indicating the existence of free SiO2 in the coal and consis-tent with the relatively high proportions of quartz in the mineralmatter (Table 4).

Compared to the 3L coal, the 3U coal is higher in SiO2 and CaO but islower in Na2O. The proportions of SiO2, MgO, and Na2O are higher insample 4L-C than in the 4U coal samples. With the exception of sample

Fig. 8. SEM and back-scattered electron images of albite in the coal. (A) Corroded albite in c3L-5 (C) corroded albite; sample 3L-5; (D) cell-filling albite and fracture-filling fluorite; sam

3U-F, the roofs and floors of the coals are characterized by a limestonecompositionwith high CaO, but are also rich in SiO2. Sample 3U-F is par-ticularly rich in SiO2 (94.3%), again consistent with the high quartz con-tent (Table 4).

4.4.2. Comparison between mineralogical and chemical compositionsThe chemical composition of the (high-temperature) coal ash calcu-

lated from the XRD and Siroquant analyses of each LTA or parting sam-ple is listed in Table 6. These data were compared to the normalizeddata for the major inorganic oxides (Table 5), and presented as x–yplots (Fig. 13), with a diagonal line on each plot indicating where thepoints would fall if the estimates from the two different techniques(XRD and XRF) were equal. The points for SiO2, Al2O3, CaO, and Na2O

ollodetrinite; sample 3L-1; (B) cavities of the corroded albite filled by fluorite; sampleple 3L-5.

image of Fig.�7

image of Fig.�8

Fig. 9. SEM and back-scattered electron images of carbonate minerals and fluorite in the coal. (A) Cell-filling dolomite, fluorite, and calcite; sample 3L-5; (B) fracture-filling stron-tianite, calcite, and fluorite; sample 3L-1; (C) fracture-filling dolomite and strontianite; sample 3L-5; (D) fracture-filling calcite, strontianite, and fluorite; sample 3L-1.

Fig. 10. SEM and back-scattered electron images of natural sulfate minerals in the 4U-C2 sample. (A) and (B), jarosite and water-bearing Fe(Si)-oxysulfate mineral distribute alongthe fractures; (C) and (D), water-bearing Fe(Si)-oxysulfate mineral and pyrite.


image of Fig.�9

image of Fig.�10

Fig. 11. SEM and back-scattered electron images of REY-bearing carbonate minerals. (A) REY-bearing Sr(Ca)CO3 mineral, dolomite and strontianite in the cells (3L-5 sample);(B) REY-bearing Ca(Mg)CO3(F) mineral and calcite in the fractures (3L-5 sample).


(Fig. 13) plot close to the equality line, suggesting that the XRD resultsare generally compatible with the chemical analysis data and the min-eralogical variations accord with the major element compositionsthrough the section. The points for K2O are more scattered, possiblyreflecting variation in the nature of the illite and I/S, but also plot aroundthe equality line.

The proportions of MgO, Fe2O3, and CaO are close to the equalityline, although the values calculated from the XRF analyses are higherthan those inferred from the XRD data. This may be because: 1) fluorite,strontianite (containing minor Ca), and water-bearing Fe-oxysulfatesoccurred in some coal samples but were below the detection limit ofXRD and Siroquant; 2) SEM-EDS data indicate that the illite containsminor Mg and the dolomite contains minor Fe. These components ofthe respective minerals were not included in the calculation of MgO,Fe2O3, and CaO from the XRD data.

4.4.3. Trace elementsCompared to average values for world hard coals (Ketris and

Yudovich, 2009), a large number of trace elements are enriched in theHeshan coals (Table 7; Fig. 14). The Heshan coals are all enriched in F,V, Se, Mo, U, and, to a lesser extent, Sr, Y, Zr, Nb, Cd, Cs, heavy rareearth elements, Hf, Ta, W, Hg, and Th. In addition, the 4U and 4L coalsare rich in Li, Sc, and Ga. Tin is enriched in the 3U and 4U coals. However,other elements, including Co, Ba, and Bi, are depleted in the coals. Withthe exception of the 3U coal, the coals are also depleted in As and Sb.The 3U and 3L coals are depleted in Ge.

Based on the variations of trace element concentrations through theseam section, two subdivisions can be identified for each of the 3U and3L seam sections. For the 3L coal, the upper subdivision is from the roofto bench 3L-5 and the lower part from bench 3L-6P to bench 3L-9. Theupper portion of the 3U coal is represented by benches 3U-1 and3U-2; benches 3U-3P, 3U-4, and 3U-5P comprise the lower portion.The trace elements with elevated concentrations, including V, Mo, andU, are more abundant in the upper portion than in the lower portionof the both seam sections (Fig. 15). The variations of V, Mo, and U aresimilar through each of the sections. However, both Se and organic sul-fur show a saw-like variation, having an inverse trend through both the3U and 3L seam sections (Fig. 15).

The trace-elements with elevated concentrations in the partingshave a similar association to the average values for world clays(Grigoriev, 2009), being high in F, Mo, and U, and to a lesser extent inSr and Pb. However, the concentration of V in the partings is lowerthan the average world clay and shale values.

4.4.4. Rare earth elements and yttrium (REY)A three-fold geochemical classification of REYwas used in the present

study, including light (LREY: La, Ce, Pr, Nd, and Sm),medium (MREY: Eu,Gd, Tb, Dy, and Y), and heavy (HREY: Ho, Er, Tm, Yb, and Lu) REY(Seredin and Dai, 2012). Accordingly, the three enrichment types areidentified as L-type (light-REY; LaN/LuN>1), M-type (medium-REY;LaN/SmNb1, GdN/LuN>1), and H-type (heavy REY; LaN/LuNb1), in com-parison with the upper continental crust (UCC) (Seredin and Dai, 2012).

The concentrations of REY in the 3U, 3L, 4U, and 4L coals are, respec-tively, 189, 111, 377, and 261 μg/g. These are higher than the averagevalues for world hard coals (68.6 μg/g; Ketris and Yudovich, 2009).

With the exceptions of the 3U and 3L roof samples (bothL-enrichment type) and the 3L floor sample (M-enrichment type), theREY enrichment patterns in the coal benches, partings, and coal channelsamples are of the H-type (LaN/LuNb1), and are characterized by nega-tive Eu anomalies (Fig. 16). Except for the floor samples (3U-F, 0.67;3L-F, 0.69), the coal benches and partings and the roofs have weak Ceanomalies (Fig. 16), varying from 0.90 to 1.03.

The concentration of LREE (e.g., La) in the coal bench is lower thanthat in the overlying partings of the 3L coal (e.g., 3L-3 vs. 3L-2P; 3L-7vs. 3L-6P; 3L-9 vs. 3L-8P; Fig. 17A). In spite of the HREE (e.g., Yb)concentrations being higher in 3L-6P and 3L-8P than in the underlyingcoal bench, the Yb/La ratio is higher in the coal bench than in partings(Fig. 17A).

The LREE concentration (e.g., La) in the 3U-3P parting of the 3U coalis close to that of the underlying coal bench (3U-4) but the HREE con-centrations (e.g., Yb) are much lower (Fig. 17B). As a result, the Yb/Laratio is higher in the coal bench.

The Yb/La ratio is higher in the upper portions than in the lower por-tions of the 3U and 3L seams through the two sections. Both La and Ybdecrease from the top to bottom of the 3U section, but have the highestconcentration in the middle part of the 3L section.

Other ratios including Nb/Ta, Zr/Hf, and U/Th, are higher in themiddle part of the 3L coal (Fig. 18). With the exception of Nb/Ta inthe 3U coal, these ratios are higher in the coal than in the adjacentoverlying benches through the two seam sections (Fig. 18A, B).

5. Discussion

Three processes were probably responsible for the geochemical andmineralogical anomalies of the Heshan coals: (1) input from thesediment-source region, (2) marine influence during deposition, and(3) multi-stage injection of hydrothermal fluids into the coal beds.

image of Fig.�11

Fig. 12. SEM and back-scattered electron images of minerals in the partings. (A) Detrital quartz and albite in the 3L-2P sample; (B) detrital quartz and albite, andmicro-veinlet-filling quartz in the 3L-2P sample; (C) detrital quartz and lath/needle-like mixed layer illite/smectite in the 3L-2P sample; (D) fracture-filling dolomite and quartzin the 3L-2P sample; (E) and (F), detrital subhedral and euhedral quartz, and detrital albite in the 2U-3P sample; (G) detrital and micro-veinlet-filling quartz in the 2U-3P sample;(H) fracture filling kaolinite in the 2U-3P sample.


image of Fig.�12

Table 5Major oxide elements and loss of ignition of the samples from the Heshan Coalfield (%).

Sample SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 SiO2/Al2O3 LOI

3U benches 3U-R 48.4 0.19 5.26 1.7 0.015 0.44 19.5 0.04 0.84 0.017 9.17 15.973U-1 30.0 0.22 5.28 1.55 0.008 0.32 1.42 0.16 0.91 0.011 5.68 58.093U-2 20.6 0.14 4.71 1.00 0.006 0.31 1.60 0.16 0.80 0.008 4.37 68.673U-3P 41.3 0.43 16.6 3.32 0.020 0.93 1.22 0.13 2.32 0.015 2.49 30.613U-4 28.7 0.31 8.63 2.02 0.010 0.66 1.79 0.15 1.24 0.015 3.32 53.893U-5P 40.4 0.40 13.9 3.95 0.027 1.62 1.64 0.18 1.15 0.032 2.91 33.263U-F 94.3 0.03 1.06 0.66 0.006 0.27 0.99 0.03 0.13 0.027 90.9 0.913U-WA 27.1 0.23 6.19 1.55 0.080 0.42 1.58 0.16 0.99 0.010 4.59 59.55

3L benches 3L-R 20.5 0.20 3.43 0.92 0.007 1.01 33.9 0.67 0.33 0.013 5.99 30.563L-1 17.4 0.16 5.97 0.72 0.002 0.51 2.52 0.29 0.40 0.005 2.92 69.213L-2P 33.9 0.32 9.31 3.88 0.019 0.71 0.45 0.90 1.09 0.015 3.64 48.333L-3 7.64 0.14 2.96 0.78 0.002 0.27 0.70 0.25 0.43 0.004 2.58 85.903L-4 23.9 0.09 2.60 1.64 0.006 0.60 4.20 0.18 0.41 0.009 9.17 60.683L-5 14.6 0.17 3.69 1.10 0.006 0.44 0.96 0.37 0.49 0.008 3.95 76.693L-6P 47.8 0.47 16.0 3.82 0.046 1.33 0.93 1.01 1.52 0.028 2.99 24.413L-7 15.1 0.23 5.32 1.18 0.004 0.34 0.46 0.35 0.75 0.006 2.84 75.543L-8P 31.3 0.65 12.9 2.80 0.011 0.94 0.42 0.66 1.79 0.016 2.43 47.223L-9 19.0 0.22 6.32 1.67 0.005 0.51 0.99 0.31 1.06 0.013 3.00 68.703L-F 9.17 0.02 0.55 0.15 0.003 0.95 51.2 0.02 0.05 0.007 16.7 31.563L-WA 14.9 0.18 4.51 1.18 0.004 0.41 1.15 0.30 0.64 0.010 3.53 75.12

Coal channel 3U-C 23.1 0.20 6.43 1.35 0.007 0.44 2.29 0.15 0.87 0.013 3.58 63.873L-C 18.1 0.23 6.24 1.23 0.003 0.49 0.69 0.40 0.87 0.008 2.90 70.704U-C1 17.3 0.37 13.3 0.46 0.001 0.25 0.43 0.10 0.39 0.009 1.30 65.994U-C2 22.5 0.55 15.0 2.46 0.003 0.70 0.57 0.22 1.15 0.015 1.50 54.544L-C 24.5 0.60 10.9 1.14 0.006 0.90 0.47 0.51 1.16 0.013 2.24 58.35

AV 3U 25.1 0.21 6.31 1.45 0.008 0.43 1.94 0.15 0.93 0.01 4.09 61.713L 16.5 0.21 5.38 1.20 0.004 0.45 0.92 0.35 0.76 0.01 3.21 72.914U 19.9 0.46 14.2 1.46 0.002 0.48 0.50 0.16 0.77 0.012 1.40 60.27

Chinese coala 8.47 0.33 5.98 4.85 0.015 0.22 1.23 0.16 0.19 0.092 1.42

WA, weighted average for bench samples of seam sections; AV, average for seam sections; LOI, loss of ignition.a From Dai et al. (2012b).


5.1. Sediment-source region

The modes of occurrence of quartz and albite in the partings andcoals (Figs. 6, 12), occurring as grains (some with well-developedshapes and long-axis closely parallel to the bedding planes), indicatethat they were from detrital materials of terrigenous origin, althoughthe fine-vein- and fracture-filling modes of occurrence for a small

Table 6The chemical composition of the (high-temperature) coal ash calculated from the XRD and

Sample SiO2 TiO2 Al2O3 Fe2O3 MgO

3U-R 67.35 0.00 5.56 3.31 0.143U-1 75.41 0.00 11.95 2.67 0.253U-2 72.65 0.00 13.34 1.82 0.483U-3P 63.06 0.00 23.39 4.33 0.643U-4 66.78 0.00 18.34 3.16 0.773U-5P 62.60 0.00 21.11 5.54 1.703U-F 97.46 0.00 1.31 0.20 0.173L-R 41.68 0.00 7.81 2.40 0.553L-1 64.70 0.00 21.83 2.49 0.443L-2P 65.93 0.85 20.37 6.97 0.333L-3 61.69 0.22 20.56 5.48 1.093L-4 76.23 0.00 4.75 3.63 1.413L-5 70.59 0.00 14.75 4.42 1.483L-6P 66.12 0.00 21.65 5.21 1.123L-7 68.38 0.00 19.58 5.05 0.683L-8P 64.54 0.00 21.92 4.42 2.053L-9 65.37 0.00 20.69 4.85 0.623L-F 20.65 0.00 1.89 1.18 0.223U-C 68.39 0.00 14.87 2.88 0.693L-C 65.43 0.00 20.70 3.84 0.994U-C1 53.34 0.00 41.86 0.15 0.034U-C2 52.04 1.53 37.15 2.40 0.094L-C 63.12 0.00 29.01 0.21 0.27

proportion of quartz and clay minerals indicate a hydrothermal origin(Figs. 6, 12) in some partings (e.g., 3U-3P). Very fine-grained quartzparticles (usually less than 5–10 μm) have generally been consideredas being of authigenic origin (Dai et al., 2012B), whereas detrital quartzgenerally has a silt- to sand-size (Kemezys and Taylor, 1964; Ruppertet al., 1991). Ren (1996) has also shown that the detrital quartz inmany Chinese coals usually has silt size particles (0.0625–0.0039 mm).

Siroquant analyses (%).

CaO Na2O K2O P2O5 SO3 Total

22.85 0.25 0.55 0.00 0.00 100.006.48 0.44 1.66 0.00 1.13 100.008.04 0.67 1.72 0.00 1.27 100.002.79 0.68 3.00 0.00 2.10 100.006.75 0.47 2.57 0.00 1.15 100.003.92 0.50 2.33 0.00 2.29 100.000.68 0.11 0.08 0.00 0.00 100.00

45.12 1.83 0.61 0.00 0.00 100.006.63 0.93 2.97 0.00 0.00 100.000.36 1.62 3.58 0.00 0.00 100.006.17 1.87 2.92 0.00 0.00 100.00

12.28 0.33 0.73 0.00 0.63 100.004.43 1.71 2.04 0.00 0.60 100.001.25 1.39 3.26 0.00 0.00 100.001.70 1.59 2.78 0.00 0.24 100.002.51 1.31 3.24 0.00 0.00 100.003.80 0.91 3.76 0.00 0.00 100.00

75.71 0.27 0.09 0.00 0.00 100.0010.84 0.24 2.09 0.00 0.00 100.004.01 1.58 3.45 0.00 0.00 100.001.02 0.02 3.58 0.00 0.00 100.000.58 0.15 5.57 0.00 0.48 100.000.29 1.61 5.49 0.00 0.00 100.00

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Fig. 13. Comparison of observed normalized oxide percentages from chemical analysis (x-axis) to oxide percentages for sample ash inferred from XRD+Siroquant analysis data(y-axis). The diagonal line in each plot indicates equality.


However, the particle size of the detrital quartz in this study indicatesthat such fine-grained detrital quartz derived from sediment source re-gion is also possible.

Feldspars (e.g., K-feldspar, albite and anorthite) are rare in coals,but where present are mostly detrital minerals of terrigenous origin(e.g., Bouška et al., 2000; Kemezys and Taylor, 1964; Moore andEsmaeili, 2012; Ruppert et al., 1991;Ward, 1989, 2002). Feldspar appar-ently derived from epigenetic hydrothermal fluids, however, has been

found in a number of coal deposits (Golab and Carr, 2004; Yao andLiu, 2012; Zhao et al., 2012), mostly associated with igneous activity.Syngenetic albites of pyrogenic (Brownfield et al., 2005) and hydrother-mal origins (Dai et al., 2008), have also been reported. However, in thepresent study, the similarmineral assemblages in both the partings andthe coal LTAs, combined with the similar modes of albite occurrence,suggest that albite in the coals and partings came from the samedetritalsource material.

image of Fig.�13


Themodes of occurrence for the illite andmixed layer illite/smectite,with lath- and needle-shapes and long-axes parallel to the beddingplanes, may also indicate that these components are detrital materialsof terrigenous origin.

The high proportions of quartz, illite, and illite/smectite in themineral matter of the 3L and 3U coal benches (Table 4), the similarityof the mineral assemblage in the coal LTAs to that in the partings, andthe similar modes of occurrence of the main minerals (especiallyquartz, albite, illite and mixed-layer illite/smectite) suggest thatinput of detrital sediment played a significant role in accumulationof the Heshan coal deposit. Such input may have been less significant,however, for the 4L coal, the mineral matter of which contains almostno quartz or feldspar and is dominated by kaolinite and illitecomponents.

The sediment-source region for the Heshan Coalfield was theYunkai Upland, composed of felsic magmatic rocks (Feng et al.,1994). Input of sediment from this source may have led to high con-centrations of lithophile elements in the coal, similar to those in theFusui coals, which have the same sediment-source region and havethe same elevated lithophile element concentrations (Dai et al.,2013). Elements, including Co, Ni, and Cu, which are usually enrichedin mafic magmatic rocks, are depleted in the Heshan coals, indicatingdifferent sources to other coals in southwestern China, which had asediment source in the Kangdian Upland region, mainly composedof basalt.

The Al2O3/TiO2 ratio is a useful provenance indicator of sedimentaryrocks (Dai et al., 2011; Hayashi et al., 1997; He et al., 2010), because ofthe similar ratio of these elements in mudstones/sandstones to that intheir parent rocks (Hayashi et al., 1997). Typical Al2O3/TiO2 ratios are3–8, 8–21, and 21–70 for sediments derived from mafic, intermediate,and felsic igneous rocks, respectively (Hayashi et al., 1997). The Al2O3/TiO2 ratios for the Heshan partings (38.6 for 3U-3P, 29.14 for 3L-2P,34.0 for 3L-6P, and 19.8 for 3L-8P) are higher than those of two mud-stones derived from themafic Kangdian Upland (6.67 and 6.85, respec-tively) reported by He et al. (2010) and those of four mafic tuffs insouthwestern China (9.77 on average; Dai et al., 2011), also indicatingthat the sediment source region was not the mafic Kangdian Uplandbut the felsic Yunkai Upland.

The coals and partings in the Heshan Coalfield generally have neg-ative Eu anomalies compared to the upper continental crust (Fig. 16),a feature that is also typical for the REY geochemical characteristics offelsic igneous rocks. However, the mudstones derived from the maficKangdian Upland (Dai et al., 2013; He et al., 2010), as well as mafic ig-neous rocks generally (Cullers and Graf, 1984) show weak to no Euanomalies. The REY distribution patterns in the mudstones from theGuadalupian–Lopingia Boundary (G–LP) at Chaotian in SW China(He et al., 2010; Fig. 19A–D), derived from erosional deposits of theKangdian Upland (He et al., 2010), as well as the acidic tuffs at theP–T boundary (He et al., 2010), are all different from the parting mud-stones in the present study. The Late Permian coals from SW China(Fig. 19F), which have a sediment-source in the mafic KangdianUpland, also show different REY distribution patterns (no or weakpositive Eu anomalies) to those of the Heshan coals.

Although other minerals likely to be of detrital origin (e.g., apatiteand zircon) are seldom observed in the Heshan coals, larger propor-tions of detrital quartz, albite, and clay minerals of terrigenous originare found in the Heshan coals than in Fusui coals (Dai et al., 2013), in-dicating more input of detrital materials of terrigenous origin fromthe Yunkai Upland to the Heshan coals.

5.2. Marine influence during peat accumulation

As mentioned above, the Heshan coals are intercalated with car-bonate rocks, and thus the influence of marine conditions (Feng, etal., 1994; Shao et al., 2003; Zeng et al., 2005) on the geochemicalanomalies of the Heshan coals appears to have been significant. This

point of view is supported by the high concentration of some ele-ments (e.g., B, Mg, K, Sr, Rb), which are higher in seawater than infreshwater (Reimann and de Caritat, 1998). The higher ratio of Sr/Bathrough the seam section (Fig. 18) than the average for world coals(0.67) may also indicate a significant seawater influence. In addition,the coals have high boron concentrations (Table 7), comparable tothose of coals formed in mildly brackish to brackish environments.Goodarzi and Swaine (1994) suggested that coals formed in freshwa-ter, mildly brackish water, and brackish water have B concentrationsof b50 μg/g, 50–110 μg/g, and >110 μg/g, respectively. The boronconcentrations in both the coal benches and partings have an increas-ing trend from the top to bottom (Table 7), indicating that the uppersubdivision had a weaker seawater influence than the lower subdivi-sion. The boron concentration and vertical variation through theHeshan seam sections are in contrast to those in the nearby Fusuicoals (Dai et al., 2013), which have a lower boron concentration, com-parable to those of coals formed in a freshwater environment.

The high S content of the coal also seems to indicate the influenceof seawater on the coal (Chou, 2012); although (as described below)not all of the sulfur was derived from the seawater. The modes of sul-fur occurrence (organic sulfur, framboidal pyrite, and disseminatedfine pyrite crystals in the collodetrinite) seem to suggest that atleast part of the sulfur in the coal was derived from seawater.Fracture-filling epigenetic pyrite was rarely observed in the Heshancoal samples.

5.3. Multi-stage hydrothermal fluids

Compared to the sediment-source region and marine influences,the Heshan coals were significantly more influenced by multi-stagehydrothermal fluids. This is supported by both mineralogical and geo-chemical evidence.

5.3.1. Mineralogical evidenceAlthough the roof and floor strata consist of limestone or silicified

limestone, the partings within the 3U and 3L coals contain almost nocalcite. The mineral matter of the 3U and 3L coal benches, however,contains quite significant proportions of calcite, representing up toaround 15% of the LTA material (Table 4). Additionally, fluorite andstrontianite that were present in the coal benches were not observedin the partings under SEM-EDS. The difference in distribution ofcalcite-, fluorite- and strontianite-filling fractures between coalbenches and partings may indicate that post-depositional fluidshave passed through the coal benches (but not the partings) afterfracture formation in the coal had provided a permeability path.

The cell- and fracture-filling occurrences of quartz, clay minerals,carbonate minerals, fluorite, REE-bearing minerals, and water-bearingFe(Si)-oxysulfate minerals (Figs. 6, 8–11) indicate that the min-erals with such modes of occurrence were of authigenic origin orwere derived from hydrothermal fluids. The mixed layer clay min-erals with a colloform texture (Fig. 7B) may also have an authigenicorigin.

The relations of the carbonate mineral modes of occurrence(Fig. 9C, D) indicate that strontianite, where present in the Heshancoals, was formed later than the associated calcite and dolomite. Epi-genetic calcite and dolomite are widely distributed in coals (Ward,2002), but strontianite has rarely been reported in coal seams. Stron-tianite has, however, been found in Late Permian coals of the HunterValley, Australia by Golab et al. (2006) and Tarriba et al. (1995).Strontium may partially replace Ca in calcite to form strontianite(Sia and Abdullah, 2011). Ward et al. (1999) have also found an asso-ciation between Sr and carbonate minerals (calcite and dolomite) insome Australian coals. The strontianite in the Heshan coals was prob-ably derived from the reaction between Sr leached from the nearbylimestones and the acid hydrothermal fluids that were responsiblefor the Sr leaching as well.

Table 7Trace elements in the coals, partings, and host rocks from the Heshan Coalfield (μg/g unless indicated).

Sample 3U benches 3L benches

3U-R 3U-1 3U-2 3U-3P 3U-4 3U-5P 3U-F 3U-WA 3L-R 3L-1 3L-2P 3L-3 3L-4 3L-5

Li 5.80 10.6 8.12 35.7 17.5 39.3 4.71 12.1 6.67 17.6 23.7 9.40 11.3 10.9Be 0.43 1.56 2.23 2.99 3.36 2.08 0.11 2.30 0.34 1.44 1.28 1.84 0.92 1.64B 100 90.6 76.5 417 169 167 31.0 112 59.8 81.0 172 51.8 38.3 58.3F 895 1086 821 2554 1601 2791 249 1179 1226 3362 2752 1032 1993 1565Cl 710 260 690 370 640 130 290 493 150 1540 320 1280 2730 1100Sc nd nd nd nd nd nd nd nd nd nd nd nd nd ndV 42.1 270 127 102 166 85.9 23.6 200 148 42.1 95.2 65.8 41.9 171Cr 32.6 45.5 17.4 18.0 29.7 23.8 39.0 33.1 55.5 9.98 39.0 12.4 20.9 19.6Co 2.63 2.23 1.38 3.91 3.15 3.49 0.52 2.30 1.75 1.57 7.56 1.63 2.12 1.95Ni 18.1 22.7 11.4 10.9 21.4 16.9 8.2 19.3 33.5 5.5 21.9 9.5 19.6 12.0Cu 7.41 11.6 9.12 20.2 39.9 21.7 4.36 19.9 13.8 13.1 559 9.69 38 12.8Zn 43.6 56.3 52.6 68.7 159 125 27.6 87.8 43.6 18.1 89.2 32.5 122 24.2Ga 4.18 6.42 7.59 23.8 13.3 15.7 1.02 8.90 3.92 14.0 10.0 11.2 8.62 9.84Ge 0.16 0.32 0.37 0.55 0.52 0.55 0.26 0.40 0.39 0.30 0.83 0.72 0.66 0.70As 13.5 12.8 8.66 29.3 13.5 27.1 1.33 11.9 1.13 3.69 9.15 1.84 7.62 2.22Se 6.08 8.28 5.15 10.9 5.93 9.59 1.09 6.71 4.64 9.07 19.1 6.71 19.1 8.11Rb 24.2 26.8 21.3 59.5 32.0 38.4 2.33 27.0 8.96 9.68 27.6 9.96 11.3 11.0Sr 1089 164 215 676 327 777 98.4 229 1417 1548 349 293 402 624Y 9.71 18.4 37.8 38.3 60.9 56.8 0.80 36.9 10.3 14.2 12.2 42.4 20.0 35.3Zr 29.5 153 97.6 202 186 389 16.2 149 24.6 105 70.5 45.3 27.1 108Nb 3.05 8.98 10.6 10.8 13.4 25.9 0.84 10.8 3.19 13.0 5.78 4.44 3.09 9.82Mo 18.3 142 75.0 52.3 67.3 39.0 2.56 101 15.7 11.1 38.1 37.3 33.0 49.9Cd 0.47 1.22 0.76 0.81 0.77 1.04 0.23 0.96 0.77 0.482 1.09 0.58 1.39 0.53In bdl 0.068 0.103 0.116 0.049 0.141 0.021 0.07 0.063 0.068 0.132 0.031 0.141 0.019Sn bdl 8.94 11.8 12.2 bdl 9.79 3.48 6.87 8.42 3.56 15.6 bdl 16.3 bdlSb 3.92 5.19 2.04 4.26 1.61 4.17 0.893 3.23 0.60 0.04 1.38 0.16 1.95 0.19Cs 1.40 1.45 1.23 4.81 3.15 4.35 0.24 1.93 1.51 2.95 5.12 1.10 0.90 1.41Ba 74.6 38.7 29.2 68.3 59.7 67.0 30.2 42.8 28.7 79.6 54.3 28.9 32.2 59.9La 30.0 8.71 15.3 27.1 27.3 56.9 0.44 16.3 47.3 4.73 13.2 11.0 9.23 10.4Ce 59.9 19.5 37.3 60.6 62.6 125 0.92 37.8 111 12.1 28.8 27.2 20.5 22.9Pr 6.95 2.48 4.91 7.51 7.68 14.4 0.12 4.76 12.7 1.60 3.44 3.85 2.80 3.18Nd 24.4 9.26 19.1 28.2 29.2 50.6 0.48 18.1 41.9 6.19 12.1 16.3 11.1 13.2Sm 5.05 2.50 5.43 7.04 8.30 12.0 0.13 5.10 7.15 1.98 2.87 5.02 2.93 4.06Eu 0.76 0.43 0.78 1.10 1.23 1.44 0.03 0.77 0.65 0.30 0.47 1.01 0.61 0.87Gd 4.59 2.84 6.08 7.69 9.56 12.3 0.16 5.81 5.94 2.36 2.85 6.19 3.46 5.13Tb 0.57 0.50 1.04 1.19 1.67 1.98 0.03 1.01 0.67 0.43 0.44 1.10 0.59 0.94Dy 2.78 3.30 6.75 7.23 11.1 12.5 0.17 6.67 2.96 2.95 2.65 7.25 3.82 6.46Ho 0.49 0.73 1.45 1.51 2.38 2.64 0.04 1.44 0.52 0.64 0.56 1.56 0.83 1.42Er 1.26 2.25 4.34 4.40 7.15 7.82 0.11 4.35 1.34 1.96 1.64 4.55 2.40 4.28Tm 0.17 0.35 0.66 0.66 1.10 1.20 0.02 0.67 0.19 0.31 0.25 0.67 0.36 0.63Yb 1.10 2.41 4.47 4.23 7.28 8.16 0.10 4.49 1.18 2.09 1.70 4.32 2.29 4.23Lu 0.16 0.38 0.68 0.65 1.12 1.26 0.02 0.69 0.17 0.32 0.27 0.64 0.36 0.65Hf 1.37 2.63 2.17 6.13 4.43 8.87 0.42 3.08 0.89 3.28 2.31 0.83 0.86 1.60Ta 0.72 0.40 0.24 2.00 7.66 2.49 0.09 2.65 0.49 1.70 1.05 0.042 2.07 0.08W 7.21 10.2 7.23 2.86 18.5 7.58 27.3 12.0 2.82 3.77 175 2.10 2.47 4.39Hg (ng/g) 142 376 198 185 97.7 322 7.18 241 83.5 90.0 367 91.3 235 103Tl 1.57 3.7 2.14 5.26 3.39 7.15 0.22 3.19 0.42 0.20 0.78 0.55 1.32 0.63Pb 4.81 9.27 10.5 28.2 13.7 26.1 1.64 11.0 8.87 13.1 19.0 4.65 11.2 5.48Bi 0.33 0.27 0.30 0.50 0.32 0.50 0.04 0.29 0.34 0.71 0.58 0.21 0.54 0.19Th 6.34 4.51 6.92 14.2 8.28 27.1 0.41 6.33 6.27 21.1 8.68 2.09 2.51 2.25U 19.0 111 85.3 43.4 76.3 38.7 1.59 93.3 18.2 35.6 51.6 64.6 55.4 97.8


Fluorite is rare in coal (Bouška and Pešek, 1999; Dai et al., 2012b).However, a few reports have shown that fluorite may occur in thehost rocks of coal seams. Hower et al. (2001) noted fluorite of hydro-thermal origin in the non-coal portion of the coal-bearing strata inUnion County, Western Kentucky. Fluorite veins of hydrothermal or-igin also occur in the wall rocks near some high-rank Chinese coals(Yang, et al., 1982). Epigenetic hydrothermal-fluid-derived fluoritewas deposited in the unconformable contact between the MaokouLimestone (Early Permian) and Longtan Formation (Late Permian)(Zhou and Ren, 1992). The mode of occurrence of the fluorite in theHeshan coals (Fig. 9) also indicates an epigenetic hydrothermal ori-gin. The fluorite was probably derived from the reaction between Caleached from the nearby limestones and F-rich hydrothermal fluids.

The relation between fluorite and carbonate (Fig. 9D) suggests thatcalcite was formed earlier than the fluorite. The occurrence of fluoriteas fillings in the cavities of leached albite provides further evidencethat albite formed earlier than fluorite. As discussed above, the albite

was derived from detrital material of terrigenous origin during peat ac-cumulation, but the fluorite is of epigenetic origin.

The corroded appearance of some quartz, albite, and pyrite particles(Figs. 6B, C; 8A, C; 10C, D) indicates that they were subjected to thecorrosion by additional hydrothermal fluids, and that the leached cavi-ties were filled by other hydrothermal minerals (Fig. 8B).

Corrosion on minerals in coal by hydrothermal fluids has also beenfound in other coals. Albite in the Yanshan coals, for example, wasderived from hydrothermal solutions but subsequently was corrodedby additional hydrothermal activity (Dai et al., 2008). Detrital minerals,including quartz, apatite, zircon, and REY-bearing minerals in the Fusuicoals (Dai et al., 2013) were destroyed by hydrothermal solution.

5.3.2. Geochemical evidenceA large proportion of the sulfur in the Heshan coals probably orig-

inated from hydrothermal fluids which were carried into the peatswamp and then evenly distributed in the organic matter. Not all

3L benches Channel AV

3L-6P 3L-7 3L-8P 3L-9 3L-F 3L-WA 3U-C 3L-C 4U-C1 4U-C2 4L-C 3U 3L 4U World

29.2 14.6 37.1 20.7 2.97 13.9 13.1 15.7 152 67.3 52.1 12.6 14.8 110 141.21 1.74 1.79 1.27 0.17 1.56 0.85 0.22 1.61 3.08 2.69 1.58 0.89 2.35 2268 125 254 116 7.76 84.9 102 116 90.7 165 204 107 100 128 47861 2984 3549 2558 286 2149 1153 2103 991 2350 2692 1166 2126 1671 82850 1570 600 700 500 1345 650 1740 290 150 100 572 1542 220 340nd nd nd nd nd nd 7.31 8.14 13.5 16 11.7 3.66 4.07 14.75 3.779.6 23.1 82.2 111 40.2 78.1 106 77.6 119 238 150 152 77.9 178.5 2819.8 11.5 36.0 60.8 31.9 23.6 21.8 23.3 43.4 269 93.9 27.5 23.5 156 174.16 2.53 5.56 4.02 0.82 2.42 2.28 2.96 4.91 7.38 3.20 2.29 2.69 6.15 610.6 6.5 20.8 24.0 14.6 12.6 11.7 10.0 10.3 40.8 18.1 15.5 11.3 25.6 1712.9 10.1 19.4 15.3 2.03 14.0 14.1 13.7 25.9 33.0 32.3 17.0 13.9 29.5 1698.5 23.4 73.1 231 21.1 75.0 58.5 40.8 9.2 61.4 44.4 73.1 57.9 35.3 2812.8 11.5 12.0 9.38 0.56 10.7 9.48 11.6 19.2 19.4 20.8 9.19 11.1 19.3 60.86 0.58 0.77 0.69 0.08 0.64 0.49 0.64 2.11 3.35 3.44 0.44 0.64 2.73 2.417.7 3.32 9.48 5.64 0.38 3.64 11.6 3.10 2.92 7.69 3.11 11.8 3.37 5.31 8.317.2 7.25 24.0 12.9 1.37 9.51 5.97 8.21 13.8 24.4 17.8 6.34 8.86 19.1 1.336.5 18.8 48.1 27.0 1.66 15.8 25.4 21.4 9.79 40.5 46.0 26.2 18.6 25.1 18613 604 412 376 3042 535 237 531 199 345 363 233 533 272 10024.6 35.2 14.2 15.1 12.0 30.2 57.5 31.7 64.3 79.5 41.4 47.2 31.0 71.9 8.4263 90.5 107 91.0 7.72 78.7 127 82.0 242 255 225 138 80.3 249 3613.6 7.15 8.18 9.13 0.60 7.41 9.23 6.62 11.7 21.8 26.7 10.0 7.01 16.8 413.2 8.65 26.2 45.6 2.21 31.8 59.1 25.5 12.1 39.0 49.1 79.9 28.6 25.6 2.10.67 0.56 1.02 1.60 0.15 0.83 0.68 0.59 0.53 1.26 0.77 0.82 0.71 0.89 0.20.058 0.051 bdl 0.092 bdl 0.058 0.087 0.055 0.18 0.183 0.121 0.079 0.056 0.182 0.04bdl bdl bdl 3.03 bdl 2.20 4.14 0.88 5.83 4.2 2.24 5.50 1.54 5.02 1.43.36 0.46 1.84 1.33 0.27 0.61 1.62 bdl bdl 0.27 0.211 2.43 0.31 0.13 18.53 3.27 8.51 3.65 0.20 2.31 2.07 3.72 2.93 10.5 8.90 2.00 3.02 6.72 1.145.3 29.6 67.9 47.1 11.4 41.5 42.6 60.1 33.3 68.4 84.4 42.7 50.8 50.9 15042.6 13.9 13.5 8.41 13.7 10.5 28.0 12.7 49.9 58.2 45.1 22.2 11.6 54.1 1187.6 31.9 29.5 19.6 16.3 24.6 61.1 28.5 101 138 92.3 49.4 26.5 120 239.58 4.09 3.5 2.52 1.88 3.29 7.75 3.66 11.5 17.4 10.7 6.26 3.48 14.5 3.432.3 15.9 12.1 9.6 6.79 13.2 33.1 14.9 42.6 69.7 36.4 25.6 14.1 56.2 127.09 4.34 2.9 2.65 1.83 3.84 8.46 3.73 9.76 15.2 6.87 6.78 3.78 12.5 2.21.27 0.88 0.60 0.55 0.37 0.78 0.97 0.74 1.36 2.19 0.96 0.87 0.76 1.78 0.437.26 5.17 2.92 2.85 2.44 4.61 8.52 4.29 9.51 14 6.03 7.17 4.45 11.8 2.71.09 0.87 0.49 0.48 0.34 0.81 1.63 0.89 1.87 2.37 1.26 1.32 0.85 2.12 0.316.19 5.75 3.14 3.08 1.95 5.34 9.83 5.44 12.3 13.5 7.64 8.25 5.39 12.9 2.11.24 1.24 0.67 0.65 0.38 1.15 2.08 1.10 2.35 2.56 1.47 1.76 1.13 2.46 0.573.55 3.62 2.02 1.96 1.00 3.40 6.39 3.45 7.05 7.97 4.59 5.37 3.43 7.51 10.54 0.53 0.32 0.30 0.13 0.50 0.99 0.57 1.12 1.20 0.70 0.83 0.54 1.16 0.33.73 3.38 2.17 1.99 0.77 3.29 6.64 3.77 7.35 7.98 4.72 5.57 3.53 7.67 10.57 0.51 0.34 0.31 0.11 0.50 1.02 0.61 1.04 1.23 0.71 0.86 0.56 1.14 0.26.28 2.13 2.52 1.93 0.22 1.67 3.40 2.36 6.65 6.98 6.33 3.24 2.01 6.82 1.22.91 0.52 1.11 1.21 0.08 0.68 0.54 0.61 1.12 2.26 2.34 1.60 0.65 1.69 0.32.83 2.67 3.22 5.63 2.51 3.47 3.58 2.06 1.96 2.52 3.37 7.81 2.77 2.24 0.99404 108 242 167 4.94 124 182 120 188 401 226 212 122 295 1000.95 0.53 0.80 1.15 0.10 0.72 3.97 0.68 0.14 0.74 0.89 3.58 0.70 0.44 0.5836.0 9.92 26.3 13.7 0.63 9.03 12.9 10.3 31.7 33.9 20.0 11.9 9.66 32.8 90.70 0.28 0.99 0.47 0.05 0.34 0.40 0.46 0.80 1.11 0.68 0.35 0.40 0.95 1.123.5 5.81 11.6 4.50 0.77 4.83 10.4 6.83 15.2 18.9 16.1 8.37 5.83 17.1 3.212.7 12.4 36.0 54.5 10.3 52.4 47.7 43.3 21.4 48.7 31.6 70.5 47.8 35.1 1.9


the sulfur could be provided by seawater during peat accumulation,although, as mentioned above, the coals were strongly influenced byseawater. The concentration of SO4

2− in paleo-seawater was in therange of 5 mmol/kg–27.6 mmol/kg in the Phanerozoic (Lowenstein etal., 2003; Strauss, 2004). However, Shao et al. (2003) have suggestedthat the high sulfur in theHeshan coalswas entirely derived from seawa-ter. The coal in the Yanshan Coalfield, also preserved within marine car-bonate successions, has a superhigh-organic-sulfur (9.51% on average),which is thought to have been derived from hydrothermal solutions(Dai et al., 2008).

Despite the high organic sulfur in the coal, the proportion of pyriticsulfur is relatively low (Table 2). This is mainly attributed to the limitedsupply of Fe available in the waters during peat formation; if available,the Fewould have fixedmore of the sulfur in pyritic form (Chou, 1990).

In addition to high F (carrier fluorite) of hydrothermal fluid origin, thehigh concentrations of V, Mo, U, as well as Se, in the Heshan coals werealso derived from hydrothermal fluids rather than the sediment-source

region, marine influence (see Shao et al., 2003), or from the soil horizonunderneath the coal (see Zeng et al., 2005). The ratios of elements in ma-rinewater to elements in freshwater, calculated based onReimannanddeCaritat's (1998) data, are relatively low, 0.8 forV, 2.0 forMo, and0.2 for Se,indicating a non-marine origin, although the ratio for U is much higher(80). The sediment-source region was mainly made up felsic rocks(Feng et al., 1994), which are usually depleted in V relative to maficrocks (Grigoriev, 2009).

Very high concentrations of V,Mo, U, and Se have been found in othercoals, but are usually of epigenetic origin. The high V (1.06% on a wholecoal basis) in the 12.6-cm-thick upper part of the Western Kentuckyno. 9 coal bed was leached from adjacent shales and redeposited in thecoals owing to circulation of hydrothermal solutions (Hower et al.,2000). High Mo concentrations (up to a few thousand μg/g on a wholecoal basis and up to 1–2% in the ash) have been found in infiltration epi-genetic deposits of the USA and the former USSR (Ilger et al., 1987;Kislyakov and Shchetochkin, 2000; Seredin and Finkelman, 2008).

CC>10 5<CC<10 2<CC<5 0.5<CC<2 CC<0.5

0.1

1

10

100

0.1

1

10

100

0.1

1

10

100

0.1

1

10

100

3U Coal

3L Coal

4U Coal

4L Coal

CC

CC

CC

CC

0.1

1

10

100

Li Be B F Cl Sc V Cr Co Ni Cu Zn GaGe As Se Rb Y Sr Zr NbMo Cd In Sn Sb Cs Ba La Yb Hf Ta W Hg Tl Pb Bi Th U





Heshan All Seams

CC

Fig. 14. Concentration coefficients (CC) of trace elements in the Heshan coals, normalized by average trace element concentrations in the world hard coals (Ketris and Yudovich, 2009).


High concentrations of U and high U/Th ratios in coal usually indi-cate a strong influence of hydrothermal solutions (Bostrom, 1983;Bostrom et al., 1973; Dai et al., 2008; Seredin and Finkelman, 2008;Wu et al., 1999). Very high concentrations of U in coal are usually ac-companied by high Mo, Se, and sometimes V (Seredin and Finkelman,2008). Extensive high U coal deposits may be formed by epigenetic in-filtration processes (Kislyakov and Shchetochkin, 2000; Seredin andFinkelman, 2008). A syngenetic exfiltration process can also lead to Uenrichment, but both the concentration and resources of uranium areusually smaller.

The enrichment of U in the Heshan coals, with relatively thinseams (around 1 m and less than 2 m), was probably a result ofsyngenetic exfiltration processes; the concentration of U in thecoals, however, is lower than in other deposits of epigenetic infiltra-tion origin (c.f. Seredin and Finkelman, 2008). Such U enrichment isalso accompanied by Se, V, and Mo, and such U-rich coal seams aretypically intercalated among the impermeable rocks, as mentionedabove. The similar distribution of V, Mo, and U through both the 3U

and 3L coals in the Heshan section (Fig. 15) suggests that these ele-ments were probably derived from the same hydrothermal solutionsduring peat accumulation or at the early diagenetic stages. High con-centrations of Mo, V, and U in the Yanshan coals were also attributedto hydrothermal fluids (Dai et al., 2008). However, the similar distri-bution of Hg and Se in the Heshan materials (Fig. 15) suggests thatthese two epithermal elements have the same hydrothermal source.

The inverse vertical variations of U, V, and Mo through the coalsections, compared to the ash yield (Fig. 15), and the lack of any U-,V-, or Mo-bearing minerals identified in the coal, probably indicatea more organic mode of occurrence.

Although LREE-bearing minerals were observed in the coal underthe SEM (Fig. 11), the HREY enrichment observed in the seams maybe attributed not only to marine waters (Elderfield and Greaves,1982) but also to hydrothermal solutions (Michard and Albarède,1986; Seredin and Dai, 2012).

Multi-stage hydrothermal fluids not only led to the enrichment ofminerals and trace elements as mentioned above, and the corrosion

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Benches 10 20 30 40 0 10 0 100 200 0 20 40 60 0 50 100 0 5 10 15 20 25

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Thinckness Coal Ash (%) So,d (%) V (µg/g) Mo (µg/g) U (µg/g) Se (µg/g) Hg (ng/g)(cm) 0 200 400

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30 50 0 5 10 0 100 200 300 0 100 0 50 100 150 0 5 10 15 0 200 400Ash (%) So,d (%) V (µg/g) Mo (µg/g)Thinckness Coal U (µg/g) Se (µg/g) Hg (ng/g)

(cm)

A

B

Fig. 15. Vertical variations of ash yield, organic sulfur, V, Mo, U, Se, and Hg in the coal. (A) 3U coal; (B) 3L coal.

0.0

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3U-R 3L-R 3L-F 3U-F

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3U-1 3U-2 3U-4

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3L-1 3L-3 3L-43L-5 3L-7 3L-9

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La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu


A) All seams/UCC

3U-C 3L-C 4U-C1 4U-C2

4L-C 3U-WA 3L-WA Heshan-wa

Fig. 16. Distribution patterns of REY in the coals, partings, and host rocks from the Heshan Coalfield. REY are normalized by Upper Continental Crust (UCC) (Taylor andMcLennan, 1985).


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BLa (µg/g) Yb (µg/g) Yb/LaThinckness Coal

Thinckness Coal La (µg/g) Yb (µg/g) Yb/La

Fig. 17. Vertical variation of REY in the 3L (A) and 3U (B) coals.


or destruction of some minerals, but also caused some trace-elementre-distribution between coal benches and partings. The enrichment ofsome elements (Nb, Y, HREE, and Zr) and the higher element ratios(Yb/La, Nb/Ta, and Zr/Hf) in the coal benches compared to the overly-ing partings (Fig. 18) are attributed to the re-deposition of the first el-ement in each pair, because of the relatively more active leaching ofthat element from the partings, and then deposition in the underlyingorganic matter (Dai et al., 2013; Seredin, 2004). Such re-distribution ledto higher element ratios in the Heshan coal benches than seen on aver-age in world hard coals. For example, the average Yb/La value is 0.09for world hard coals (cf. Ketris and Yudovich, 2009) but is 0.24–0.44 inthe Heshan coal benches. The higher U/Th ratio in the Heshan coals isnot only due to leaching of U from the partings and subsequent accumu-lation in the underlying organicmatter, but also to the U-rich hydrother-mal fluids, as mentioned above.

The redistribution of trace elements between coal benches and theadjacent partings has also been reported in the nearby Fusui coals(Dai et al., 2013), the Jungar coals of northern China (Dai et al.,2012a), and some US coals (Crowley et al., 1989; Hower et al., 1999).

The vertical variations of Yb/La, Nb/Ta, Zr/Hf, and U/Th indicate thatthe upper portion of each section (Fig. 18A, B) was influenced more byhydrothermal fluids than the lower portion. The difference of these

ratios between the coal bench and the overlying parting in the upperportion is larger than that observed in the lower portion.

The possibility of hydrothermal activity is also supported by enrich-ment of other hydrothermally-associated elements, e.g., Hg and Cd.Syngenetic hydrothermal solutionswere input during peat accumulation;these were probably rich in Hg and thus led to high Hg concentrations inall the coal benches.

The hydrothermal solutions not only influenced coal and partingsbut also the host rocks (roof and floor strata). For example, the modesof occurrence of the abundant quartz, coating the edge of calcite(Fig. 20A, B) or filling the fractures of limestone (Fig. 20C, D, E) in thehost rocks, indicate a hydrothermal fluid origin. Sphalerite filling inthe limestone fractures also indicates an additional hydrothermal fluidinput (Fig. 20F).

5.4. Preliminary estimation of REY recovery from the 4U coal

The average REY concentration in the ash of the 4U coal is 950 μg/g(or about 0.11% REY2O3), higher than the REY cut-off-grade (0.1%REY2O3) in coal combustion wastes for economic by-product recovery(Seredin and Dai, 2012). The pattern of individual REY composition(coefficient of outlook of REY raw material=1.08) is better than that

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Thinckness Coal Sr/Ba Nb/Ta Rb/Cs

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Fig. 18. Vertical variations of selected ratios of trace-element pairs of the 3L (A) and 3U (B) coals.


of many coals with similar REY concentrations in their combustionwastes (Seredin and Dai, 2012).

The high ash yield (4U-C1, 34.01%; 4U-C2, 45.46%) and probableorganic mode of heavy REY occurrence in these coals suggest that con-ventional coal beneficiation may be an effective process to obtainlower-ash coal concentrated in higher REY (on ash basis) than foundin the ash of the raw feed coal materials. Moreover, the high S contentof these coals provides an opportunity for sulfuric acid production as aby-product during coal burning for REY leaching from the relevant com-bustion wastes. All these factors suggest potential prospects of theHeshan 4U coal as a source for REY by-product recovery, similar tothose of the Fusui coals reported by Dai et al. (2013).

6. Conclusions

The Late Permian low volatile bituminous coals in the Heshan Coal-field of southern China are intercalated with marine carbonate rocksand are characterized by super high-organic sulfur (5.13–10.82%).

Minerals identified in the Heshan coals include quartz, kaolinite, il-lite, mixed-layer illite/smectite, feldspar (albite), pyrite, marcasite, cal-cite, dolomite, and trace amounts of smectite, fluorite, strontianite,REY-bearing carbonate minerals, jarosite, and water-bearing Fe(Si)-oxysulfate. The similar assemblage andmodes of occurrence ofminerals

in the coal and parting benches indicate that a large proportion of thequartz and clayminerals, aswell asmost of the albite, are detrital mate-rials of terrigenous origin; authigenic quartz and clay minerals accountfor a lesser but still significant proportion in the mineral matter.

The Heshan coals are rich in trace elements including F (up to3362 μg/g), V (270 μg/g), Se (24.4 μg/g), Mo (142 μg/g), U (111 μg/g),and, to a lesser extent, Sr, Y, Zr, Nb, Cd, Cs, heavy rare earth elements,Hf, Ta, W, Hg, and Th. The elevated lithophile trace elements, however,came from the sediment source region of the Yunkai Upland, ratherthan the Kangdian Upland which is the sediment source region formost coals from southwestern China. The coals and partings in the pres-ent study have negative Eu anomalies, further indicating derivationfrom the felsic Yunkai Upland, and are different from the coals fromsouthwestern China which have weak or no Eu anomalies.

It seems that multi-stage hydrothermal activities influenced thegeochemical compositions of the Heshan coals. The high organic sulfurin the coalswas derived both from the seawater (the seawater influenceis supported by elevated B,Mg, K, Sr, and Rb in the coal) and hydrother-mal fluids. The presence of minerals, including fluorite, calcite, dolo-mite, strontianite, and REY-bearing carbonate minerals, was due tomulti-stage epigenetic hydrothermal activities. However, the V, Mo,U, and Se in the coals were derived from the hydrothermal solutionsduring peat accumulation or at the early diagenetic stage. The possibility

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CT57 CT58

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CT60 CT61

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La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu


La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu

C2-1c

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ple

/ UC

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ple

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Sam

ple

/ UC

C

Sam

ple

/ UC

C

Sam

ple

/ UC

C

Sam

ple

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C

F) Coals from Eastern Yunnan

Fig. 19. REY distribution patterns of mudstones from the Guadalupian-Lopingia Boundary (G–LP) and the acidic tuffs of P–T boundary at Chaotian of SW China (A–E; data from He etal., 2010), as well as the coals from eastern Yunnan (F; data from Dai et al., 2013). REY are normalized by Upper Continental Crust (UCC) (Taylor and McLennan, 1985).


of hydrothermal activity is also supported by hydrothermally-associatedelements, e.g. Hg, Cd, and Cs, in the coal.

The hydrothermal fluids corroded the early-formed minerals (quartz,albite, and pyrite) and caused some re-distribution of lithophile elementsfrom partings to the underlying coal benches, resulting in higher keyelement ratios (Yb/La, Nb/Ta, and Zr/Hf) and more abundant heavy rareearth elements in the coal benches than in their adjacent overlyingpartings.

Overall, factors controlling the mineralogical and geochemicalcomposition include the material input from the sediment-source re-gion (Yunkai Upland), the influence of seawater during deposition,and hydrothermal fluids, but not previously-reported factors such asthe formation of soil horizons before peat accumulation and marinetransgression during peat accumulation.

Acknowledgments

This researchwas supported by the Fundamental Research Funds forthe Central Universities (no. 2011YM02) and the National Natural Sci-ence Foundation of China (nos. 41272182 and 40930420). Thanks aregiven to two anonymous reviewers, and also to editor Dr. Ralf Littke,for their constructive comments, which improved the manuscript.

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