Energy & Fuels 2007, 21, 1663-1673
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Effects of Magmatic Intrusion on Mineralogy and Geochemistry of Coals from the Fengfeng-Handan Coalfield, Hebei, China Shifeng Dai*,†,‡ and Deyi Ren†,‡ State Key Laboratory of Coal Resources and Safe Mining, D11, Xueyuan Road, Beijing 100083, China, and Department of Resources and Earth Science, China UniVersity of Mining and Technology, D11, Xueyuan Road, Beijing 100083, China ReceiVed December 5, 2006. ReVised Manuscript ReceiVed February 6, 2007
This paper describes the effects of magmatic intrusions on petrology, mineralogy, and geochemistry of the late Palaeozoic coals from the Fengfeng-Handan coalfield, Hebei, China. The narrowly zoned coals of variable ranks, from high-volatile A bituminous (hvAb), through medium-volatile bituminous (mvb), low-volatile bituminous (lvb), semianthracite (sa), and anthracite (an), to meta-anthracite (ma) in the coalfield, were found to be best explained by magmatic inputs. The minerals derived from magmatic thermal alteration consist of pyrite, calcite, and ankerite, which mainly occur as fracture or vesicle fillings in the thermally altered highrank coals. The variation in element concentrations with coal ranks (enrichment, depletion, and no variation) and mineralogical affinity were used to classify elements in coals into six groups, groups A-F. Elements in group A (B, F, Cl, Br, and Hg), group B (As, Co, Cu, Ni, and Pb), group C (Sr, Mg, Ca, Mn, and Zn), and Group D (U) were enriched in the altered coals, indicating that the magmatic inputs are the source of these elements. Group A elements are volatile elements that probably came from the hydrothermal solutions, then deposited or were driven off from an organic component in coal by magmatic heat, and then redeposited in the coal. Group B elements mainly distribute in the fracture or vesicle fillings of pyrites. The dominant carriers of group C elements are thermally altered calcite and ankerite. Uranium in group D occurs in organic-bonded and silicate associations. Group E elements, including Sb, Sc, and V, have a depletion trend in the altered coals, and the remaining elements in group F do not clearly vary in the unaltered, slightly altered, or altered coals. The element concentrations independent of coal ranks in groups E and F may suggest that these elements are inherent to the coal.
Introduction Coal intruded by magmatic rocks is not an uncommon occurrence.1 Magmatic intrusions affect the safety, productivity, and economic viability of many coal mines.2 The effects of magmatic intrusions on the organic matter have been extensively studied by many authors.3-19 Information about the effects of * To whom correspondence should be addressed. Telephone/Fax: 86-10-62341868. E-mail:
[email protected]. † State Key Laboratory of Coal Resources and Safe Mining. ‡ China University of Mining and Technology. (1) Finkelman, R. B.; Bostick, N. H.; Dulong, F. T.; Senftle, F. E.; Thorpe, A. N. Int. J. Coal Geol. 1998, 36, 223-241. (2) Golab, A. N.; Carr, P. F. Int. J. Coal Geol. 2004, 57, 197-210. (3) Bogdanova, L. A. Microcomponents and types of coals from zones of contact metamorphism. In Petrograficheskiye tipy ugley SSSP, Izd (in Russian); Lyuber, A. A., Ed.; Nedra: Moscow, Russia, 1975; pp 116-129. (4) Bostick, N. H.; Collins, B. A. Petrography and programmed pyrolysis of coal and natural coke intruded by an igneous dike, Coal Basin, Pitkin County. Geol. Soc. Am. Prog. Abstr. 1987, 19, 262. (5) Crelling, J. C.; Dutcher, R. R. A petrologic study of a thermally altered coal from the Purgatoire River Valley of Colorado. Geol. Soc. Am. Bull. 1968, 79, 1375-1386. (6) Dutcher, R. R.; Campbell, D. A.; Thornton, C.P. Coal metamorphism and igneous intrusions in Colorado. In Coal Science; American Chemical Society Advances in Chemistry Series: Washington, D.C., 1968, Vol. 55, pp 708-723. (7) Meyers, P. A.; Simoneit, B. R. T. Org. Geochem. 1999, 30, 299305. (8) Goodarzi, F.; Cameron, A. R. Energy Resources 1990, 12, 315-343. (9) Gurba, L. W.; Ward, C. R. Int. J. Coal Geol. 2000, 44, 127-147. (10) Karayigit, A. I.; Whateley, M. K. G. Int. J. Coal Geol. 1997, 34, 131- 155. (11) Kisch, H. J. Econ. Geol. 1966, 61, 1043-1063. (12) Kisch, H. J.; Taylor, G. H. Econ. Geol. 1966, 61, 343-361.
magmatic intrusions on the inorganic constituents in coal may provide insights into the resource potential of coal and coke affected by the intrusion; however, only a few authors have investigated the effects of igneous intrusions on the inorganic constituents in the coal.1 Goodarzi and Cameron analyzed 36 elements in the thermally altered coal samples from Telkwa, British Columbia.8 Merritt has looked at the effect of magma on the thermally altered Alaskan coal.20 A study by Querol et al. showed that the high concentration of Mn (8600 µg/g) in a coal from the Fuxin Basin, Liaoning, China, was attributed to magmatic intrusions.21 Finkelman et al. reported the influence of an igneous intrusion on the inorganic geochemistry of a bituminous coal from Pitkin County, CO.1 Golab and Carr described impacts of two igneous intrusions on the Late Permian bituminous Upper Wynn seams at Dartbrook coal mine in the (13) Podwysocki, M. H.; Dutcher, R. R. Econ. Geol. 1971, 66, 267280. (14) Reinemund, J. A.; Baldwin, E. M.; Brill, K. J., Jr. Coalfields of the Republic of Korea: Part 2. U.S. Geological Survey Bulletin 1041-C, D, E11-99, 1957. (15) Sasaki, M. On the coal affected by the thermal metamorphism through the intrusion of the igneous rock in the Tagawa district, Chikuho coal field, Kyushu (in Japanese with English abstract). Jpn. Geol. SurV. Bull. 1959, 10, 103-110. (16) Taylor, G. H.; Teichmu¨ller, M.; Davis, A.; Diessel, C. F. K.; Littke, R.; Robert, P. Organic Petrology. Gebruder Borntraeger: Berlin, Germany, 1998; pp 704. (17) Thorpe, A. N.; Senftle, F. E.; Finkelman, R. B.; Dulong, F. T.; Bostick, N. H. Int. J. Coal Geol. 1998, 36, 243-258. (18) Varga, S. S.; Horvath, Z. T. I. Int. J. Coal Geol. 1986, 6, 381-391. (19) Ward, C. R.; Warbrooke, P. R.; Roberts, F. I. Int. J. Coal Geol. 1989, 11, 105-125. (20) Merritt, R. D. Int. J. Coal Geol. 1990, 14, 255-276.
10.1021/ef060618f CCC: $37.00 © 2007 American Chemical Society Published on Web 03/22/2007
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Figure 1. Sample location and coal-rank distribution of the Fengfeng-Handan coalfield (sample numbers 1-12 from C2 coal seam, numbers 13 and 14 from C4 coal seam, and number 15 from C6 coal seam).
Upper Hunter Valley, New South Wales, Australia, and discussed changes in the mineralogy and geochemistry of the inorganic constituents of the coal.2 In this paper, we describe the effect of intermediate magmatic intrusions on the inorganic geochemistry and mineralogy of the C2 (late Permian), C4, and C6 (Pennsylvanian) coal seams in the Fengfeng-Handan coalfield, Heibei, North China. Experimental Section Geological Setting. The Fengfeng-Handan coalfield is located in northern China (Figure 1) and consists of coal-bearing strata of (21) Querol, X.; Alastuey, A.; Lopez-Soler, A.; Plana, F.; FernandezTuriel, J. L.; Zeng, R. S.; Xu, W. D.; Zhuang, X. G.; Spiro, B. Int. J. Coal Geol. 1997, 34, 89-109.
Pennsylvanian and Permian ages. The late Palaeozoic coal-bearing strata in the Fengfeng-Handan coalfield include the Pennsylvanian Benxi Formation (C2b), the Pennsylvanian Taiyuan Formation (C2t), and the Early Permian Shanxi Formation (P1sh). The Taiyuan and Shanxi Formations are the major coal-bearing formations in the study area. The Taiyuan Formation consists of sandstone, limestone, mudstone, and seven coal seams (the C3-C9 coal seams), with a thickness between 100 and 130 m. The Shanxi Formation consists of sandstone, mudstone, and two coal seams (the C1 and C2 coal seams), with a thickness between 60 and 90 m (Figure 2). The Late Palaeozoic paleogeography in the study area ranged from the alluvial and fluvial plain, through the paralic delta and tidal flat, to the shallow marine (Figure 2). The C2, C4, and C6 coal seams are the main workable coalbeds in the coalfield, but the C7, C8, and C9 coal seams are not mined because of the danger of the water from the Ordovician limestone.
Magmatic Intrusion on Coals from China
Figure 2. Sedimentary sequence of coal-bearing strata of the Fengfeng-Handan coalfield.
The Late Permian C2 coal seam contains three partings and has a thickness of 2.5-7.0 m with an average of 5.5 m. The Pennsylvanian C4 coal seam with a thickness of 0.9-2.0 m (mean of 1.4 m) was overlain directly by a 2.0 m limestone. The Pennsylvanian C6 coal seam was overlain by a 1.2-2.0 m limestone and has a thickness of 0.97-2.4 m with an average of 1.4 m. The overlying limestone of the C4 and C6 coal seams showed that seawater invaded into the mire during peat accumulation.22 The magmatic intrusions, mainly occurring as diorites, diabases, and syenite, were widely distributed in the north rather than in the
Energy & Fuels, Vol. 21, No. 3, 2007 1665 south of the coalfield.23,24 The distribution of magmatic intrusions is up to half of the total coalfield area. The magmatic rocks, whose Rb/Sr isotopic age is about 38-171 Ma, intruded the strata several times during the Yanshan period.23 The strata intruded by magmatic rocks include the middle Ordovician, Pennsylvanian, Permian, and Triassic systems (Figure 2). Samples and Analytical Procedures. The sampling program was designed according to the coal-rank distribution in the coalfield. A total of 12 coal-seam channel samples of the C2 coal seam were collected from the mined faces to ensure as little contamination as possible from mine operations and to minimize oxidation effects on the samples. The number for each of the coal-rank samples (hvAb, mvb, lvb, sa, an, and ma) was 2. In addition, two samples (mvb and an, respectively) of the C4 coal seam and one anthracite sample of the C6 coal seam for a comparison study were collected from the coalfield (Figure 1). The collection of samples was in accordance with the Chinese Standard for Collecting Channel Samples GB482-1995. The channel sample was cut over an area with 10 cm wide and 10 cm deep, and partings thicker than 3 cm were excluded. Table 1 contains information of the standard coal characteristics of these samples. For microscopical analysis by reflected light, samples were prepared on the basis of the American Society for Testing and Materials (ASTM) standard D2797-04. Petrologic observations were taken under white light using a magnification of 500×. Mean maximum reflectance of vitrinite (percent Rmax) was determined using a Leitz MPV-III photometer system, following the ASTM standard D2798-05. The proximate analysis covering the determination of moisture, volatile matter, and ash yield was in accordance with ASTM standards D3173-03, D3175-02, and 3174-04, respectively. The total sulfur and forms of sulfur were determined according to the ASTM standards D3177-02 and D2492-02, respectively. X-ray fluorescence (XRF) analysis was used to determine the oxides of major elements, including SiO2, Al2O3, CaO, K2O, Na2O, Fe2O3, MnO, MgO, TiO2, and P2O5. The inductively coupled plasma mass spectrometry (ICP-MS) was used to analyze the concentrations of trace elements in coal, except that mercury was determined by cold vapor atomic absorption spectrometry (CVAAS), Sb and Se were determined by atomic fluorescence spectrometry (AFS), and B was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES). Fluorine was determined by the pyrohydrolysis/fluoride ion-selective electrode method, in accordance with the Chinese National Standard GB/T 4633-1997. For quality control in fluorine determination, the standard reference materials GB 11121 and GB 11123 (coal, China) were analyzed with each batch of samples as an internal control. Instrumental neutron activation analysis (INAA) was used to determined the concentration of Br in coal. The major and trace elements in this study are determined in the coal rather than by stoichiometric recalculation from the oxide content determined in ash from coal. Samples were all crushed and ground to less than 200 mesh for the geochemical study. The mineralogical phases were determined by optical microscopic observations and powder X-ray diffraction (XRD). The XRD analysis of the coal was performed on a powder diffractometer with a Ni-filtered Cu KR radiation and a scintillation detector. Diffraction patterns were registered in a 2θ interval 3-70° with a step size of 0.02°. The particle size of samples for XRD analysis was less than 75 µm. A scanning electron microscope in conjunction with an energy-dispersive X-ray spectrometer (SEM-EDX) was used to study the surface characteristics and determine the distribution of some elements in coal, using a 20 kV accelerating voltage and a (22) Chou, C. L. Geological factors affecting the abundance, distribution, and speciation of sulfur in coals. In Geology of Fossil FuelssCoal. Proceedings of the 30th International Geological Congress; Yang, Q., Ed.; VSP: Utrecht: The Netherlands, 1997; Vol. 18, part B, pp 47-57. (23) Yuan, S. W. Coal Quality ReViews of China (in Chinese); Coal Industry Press: Beijing, China, 1999. (24) Dai, S.; Ren, D.; Zhang, J.; Hou, X. Int. J. Coal Geol. 2003, 55, 59-70.
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Table 1. Proximate Analysis, Total Sulfur, Sulfur Forms, Vitrinite Reflectance, and XRD Data of Coals from the Fengfeng-Handan Coalfield (%)a XRD data sample number
coal seam
coal rank
number 1 number 2 number 3 number 4 number 5 number 6 number 7 number 8 number 9 number 10 number 11 number 12 number 13 number 14 number 15
C2 C2 C2 C2 C2 C2 C2 C2 C2 C2 C2 C2 C4 C4 C6
hvAb hvAb mvb mvb lvb lvb sa sa an an ma ma mvb an an
Rmax
proximate analysis Mad ashd VMdmf
St, d
Sp, d
So, d
Ss, d
quartz
0.95 0.89 0.98 1.39 1.58 1.79 2.14 2.25 4.51 6.15 6.54 7.41 1.25 6.02 5.98
4.3 4.2 3.9 4.3 3.2 1.9 2.1 2.1 1.2 2.5 3.2 1.3 3.6 1.5 2.1
0.46 0.52 0.48 0.48 0.67 0.48 0.34 0.56 0.39 1.82 3.27 0.36 1.52 3.46 4.24
nd nd nd nd nd nd nd nd nd 1.26 2.77 nd 0.83 2.57 3.12
nd nd nd nd nd nd nd nd nd 0.47 0.42 nd 0.60 0.78 1.02
nd nd nd nd nd nd nd nd nd 0.09 0.08 nd 0.09 0.11 0.10
8.3 7.3 9.6 6.6 8.6 9.6 10.5 10.6 9.4 12.4 8.6 7.5 5.2 5.4 2.6
18.5 16.2 19.5 16.1 18.6 20.6 22.4 29.7 23.4 26.6 30.3 22.1 15.2 25.3 26.7
32.2 33.4 29.4 24.7 19.7 15.8 11.7 9.7 7.5 6.5 1.9 1.5 26.4 7.1 7.3
pyrite
calcite
ankerite
0.8 1.0 0.4 0.6
2.2 4.2 1.9 3.9 5.2
4.5 1.2 0.8 5.9 4.9 1.2 4.5 2.9
0.9 1.5
clay minerals
organic matter
11.0 10.6 12.8 9.9 12.1 11.6 12.9 14.7 12.3 13.5 9.6 8.7 7.6 9.7 17.6
79.9 82.1 76.6 83.1 79.3 78.2 76.6 71.3 74.7 71.1 71.7 78.0 84.1 75.0 71.7
a hvAb, high-volatile A bituminous; mvb, medium-volatile bituminous; lvb, low-volatile bituminous; sa, semianthracite; an, anthracite; ma, meta-anthracite; M, moisture; VM, volatile matter; St, total sulfur; Sp, pyritic sulfur; Ss, sulfate sulfur; So, organic sulfur; Rmax, mean maximum vitrinite reflectance; ad, as determined; d, dry basis; dmf, dry, mineral free basis; nd, not detected.
10-10 A beam current. The standard sample for EDX is Co, and the detection limit is 0.01 wt %. For the study of modes of occurrence, the authors identified six types of elements in coal using a six-step sequential chemical extraction procedure (SCEP) as outlined by Dai et al.25 The detailed discussion of this method has appeared elsewhere.25 The six types of elemental association are water soluble, ion exchangeable, carbonate, organic-bonded, silicate, and sulfide.
Results and Discussion Coal-Rank Distribution. On the basis of proximate analysis data of 257 drill core samples collected from the Fengfeng and Handan Coal Bureaus, the coal-rank distribution of coals from the Fengfeng-Handan coalfield was outlined as shown in Figure 1. The coal rank was narrowly zoned and distributed from south to north, from high-volatile A bituminous, through mediumvolatile bituminous, low-volatile bituminous, semianthracite, and anthracite, to meta-anthracite within several kilometers. The narrowly zoned distribution of the coal rank is entirely related to magmatic hydrothermal intrusions during the Yanshan Movement of the Cretaceous period.23-25 The magmatic intrusions led the Palaeozoic coals to be metamorphosed to anthracite, meta-anthracite, or even coke. It should be noted that, although the coal rank varies greatly in a single coal seam within several kilometers, the different coal seams in the same location have a similar coal rank. The cover of the Late Palaeozoic coal-bearing strata, containing the Permian and Triassic sedimentary systems, is less than 2300 m. The average geothermal gradient of 2.5 °C/100 m indicates that the paleogeotemperature was about 70-80 °C, which should have only led to sub-bituminous coals. Coals of higher rank are therefore attributed to the proximity of intermediate magmas. Geochemical and Mineralogical Trends in Coal Seams. Proximate analysis and rank (mean maximum vitrinite reflectance) for the coal channel samples of the C2, C4, and C6 coal seams from the Fengfeng-Handan coalfield are given in Table 1. The C2 Coal Seam. The ash yield of the C2 coal seam is 16.1-30.3% (mean of 22%). The coal is classified as a mediumash coal on the basis of the Chinese Standard GB 15224.1(25) Dai, S.; Li, D.; Ren, D.; Tang, Y.; Shao, L.; Song, H. Appl. Geochem. 2004, 19, 1315-1330.
2004 (coals with ash yields of 16.0-29.0% are considered medium-ash coal). On the basis of the data of 257 drill core samples of the C2 coal seam, the sulfur content of this coal seam varies from 0.28 to 2.85%, with an average of 0.46%, designated as a low sulfur coal. There are only 16 among 257 samples that contain total sulfur higher than 1.5%. The remaining samples widely distributing in the area of unaltered and altered coals have a low sulfur content (
