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Effect of igneous intrusive on coal microconstituents: Study from an Indian Gondwana coalfield
International Journal of Coal Geology 85 (2011) 161–167
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International Journal of Coal Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j c o a l g e o
Effect of igneous intrusive on coal microconstituents: Study from an Indian Gondwana coalfield Shinjini Sarana a, Ratan Kar b,⁎ a b
Department of Botany, National P.G. College, Lucknow-226 007, India Birbal Sahni Institute of Palaeobotany, 53 University Road, Lucknow-226 007, India
a r t i c l e
i n f o
Article history: Received 30 January 2010 Received in revised form 2 November 2010 Accepted 2 November 2010 Available online 12 November 2010 Keywords: Igneous intrusive Coal petrology Palynology Gondwana India
a b s t r a c t Petrological and palynological analyses of coals from two bore-holes, intruded by an igneous body (dolerite dyke), reveal characteristic changes due to thermal alteration. The unaffected coals are high volatile bituminous (Rr: 0.50–0.61%). The coals recovered from samples located close to the dyke had vitrinite with devolatization microvacuoles, higher reflectance ranging from 0.9% to 3.0% and liptinite with none to very low fluorescence. There is an increasing trend in reflectance and a decreasing trend in fluorescence in the samples occurring closer to the dyke. Presence of injected mineral matter and pyrite in the macerals further indicate the impact of thermal metamorphism on the coals occurring near the igneous body. The palynological investigation of unaffected coals shows the presence of a number of well-identified spores and pollen. However, very few palynomorphs could be recovered from coals located close to the intrusive as the pollen/ spores were found to be charred beyond recognition. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Study area
During the process of coalification, heat is normally available to the sediments by the depth of burial. The effect of temperature on rank of coal is gradual and is also related to the duration of burial. But whenever the sediments, particularly coal, come in close contact with the igneous bodies, sudden changes in the rank and nature of organic matter have been observed (Chandra, 1963; Ghosh, 1967; Pareek, 1970). A similar effect is also produced due to the movement of rocks along major tectonic features. Coal present near the thrust zones bears the testimony of such heat-induced changes from several areas in the Himalaya (Anand Prakash, 1992; Ghosh, 1997). Though, a number of studies are available on thermally altered coals from India (Chandra and Srivastava, 1980; Chandra and Taylor, 1982; Pareek, 1988; Anand Prakash and Sarate, 1991; Mishra and Cook, 1992), most of these are related to the rank behavior alone, and not to the overall changes brought about in the organic microconstituents. However, comprehensive work has recently been done from Indian coalfields on the genesis of natural cokes due to magmatic intrusions (Singh et al., 2007; 2008) and on the behavior of burnt coal macerals and their reactivity (Choudhary et al., 2007; 2008). The present investigation is an attempt to understand such heat-induced changes in the coal macerals and palynodebris from two bore-holes TRDM-1 and TRDM-2, located at Tatapani–Ramkola Coalfield, India.
The area of the present study, Tatapani–Ramkola Coalfield, is situated in the Surguja District of Chhattisgarh State and is the westernmost extension of the Damodar–Koel Valley master Gondwana Basin (Fig. 1). The coalfield is located between latitudes 23°30′: 23°55′ and longitudes 83°00′: 83°40′ and is so named owing to the presence of a conspicuous hot spring (vern.: ‘tata’ — hot, ‘pani’ — water) emanating near Tatapani village (Raja Rao, 1983). The Gondwana Sequence in Tatapani–Ramkola Coalfield has been classified into Talchir, Karharbari, Barakar, Barren Measures, Raniganj, Panchet and Mahadeva Formations. The commercially viable coal seams occur within the Barakar Formation (Table 1). At some places the Gondwana sediments in the coalfield are intruded by the basic intrusives, possibly related to the Deccan volcanic activity (Raja Rao, 1983). During the recent years, the Geological Survey of India and Mineral Exploration Corporation Ltd. have taken up this coalfield as one of the thrust areas for coal exploration. During the course of prospecting of coal reserves in Dhamni Block of Tatapani– Ramkola Coalfield, dolerite intrusives were struck in two bore-holes — TRDM-1 and TRDM-2 (Figs. 1 and 2). In the first bore-hole — TRDM-1, the collection of samples was restricted to the sediments above the intrusive, as during our field work the bore-hole was drilled only up to that point. The samples were primarily collected for palynostratigraphical studies as the Birbal Sahni Institute of Palaeobotany, Lucknow, had undertaken a major project for the surface and subsurface correlation of coal and associated sediments in Tatapani–Ramkola Coalfield (Kar, 2001; Srivastava and Kar, 2001; Kar, 2003; Kar and Srivastava, 2003). Though, palynological studies were not fruitful, due to the presence of the intrusive, it gave us an opportunity to observe its
⁎ Corresponding author. Tel.: +91 9451088928; fax: +91 522 2740485. E-mail address: [email protected] (R. Kar). 0166-5162/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2010.11.006
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Fig. 1. Map of the locality showing area of study.
effect on the coal microconstituents and organic matter found in the sediments. Studies on these samples were thereby initiated and since interesting results were obtained, we again visited the locality in the next field season to collect the complete sequence from the second borehole — TRDM-2.
Table 1 Generalised stratigraphic sequence of Tatapani–Ramkola coalfield. Age
Formation
Recent Cretaceous? Upper Triassic
Basic intrusives Mahadeva
Lower Triassic
Panchet
Upper Permian
Raniganj
Middle Permian
Barren Measures
Lower Permian
Barakar
Karharbari
Talchir
Lithology Alluvium Dolerite dykes. Thick, cross-bedded, coarse-grained, ferruginous sandstone. Yellowish, fine-grained sandstone with alternating red and green siltstones, shales and clays. Micaceous, fine-grained, ripple laminated sandstones, grey and carbonaceous shales and shaly coal bands. Ironstone shales showing box structure, fine-grained sandstone, shales and argillaceous sandstone. Medium to coarse-grained pebbly arkosic sandstone, grey and carbonaceous shales and coal seams. Coarse-grained to granular arkosic sandstone with conglomerate lenses, shales, carbonaceous shales and intermittent coal seams. Diamictite, khaki-green needle shales, siltstone, fine-grained sandstone and varves.
Unconformity Archean
Granites, gneisses, micaceous green schists, phyllites and quartz veins.
3. Material and methods For petrological analysis only coal samples were chosen, while palynological studies were undertaken on both coal and shale samples. For petrological study the samples (Fig. 2) were crushed to a particle size of 2000 μm (±18 meshes) and embedded in epoxy resin. These embedded pellets were then ground and polished for counting of macerals. The rank was determined by measuring the reflectance (Rr) on polished vitrinite surfaces on Leitz Photometer Voltmeter (MPV-1) system. For palynological analysis, 150 to 200 g of the material from each sample was taken and crushed to 2–4 mm size. The samples were initially treated with 40% hydroflouric acid to remove the silica and thereafter, by nitric acid to release the palynomorphs from the rest of the organic debris. In the intrusive affected samples, palynomorphs remain black in colour and opaque even after sustained nitric acid treatment. To facilitate the clarity of palynomorphs, the macerates were, therefore, boiled for 6 to 8 h in concentrated nitric acid using water bath. However, even after such strong treatment, the transparency of spores and pollen improved only to a small extent as most of the palynomorphs were badly charred rendering them unidentifiable. The intrusive also had an indurating effect on the sediments making them much harder. 4. Results and discussion The vitrinite group is mainly represented by three macerals namely telocollinite, tellinite and desmocollinite (Fig. 3a). The affected vitrinite was marked by the presence of uneven cracks and dessication cracks, infilled with argillaceous mineral matter, pyrite or calcite (Fig. 3a–f). The presence of carbonate, pyrite and clay minerals has also been observed in the thermally metamorphosed coals of South Sumatra Basin, Indonesia (Amijaya and Littke, 2006). Numerous micropores are present (vesicles/escape pores/devolatization microvacuoles), formed due to devolatization of liptinite and vitrinite.
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Fig. 2. Litholog of bore-holes TRDM-1 and TRDM-2 showing position of intrusives and samples.
Occasionally, the rounding of the margins was observed in the vitrinite grains, which had developed due to the softening and plastic tendency of vitrinite (Goodarzi and Murchison, 1977; 1978). The Rr of the unaffected coals ranged from 0.50% to 0.61% (Table 2). Semifusinite was present filled with mineral matter (Fig. 3b). Desmocollinites were generally associated with inertodetrinite and liptinite macerals (Figs. 3b and 5a). In coaly-shale samples, the desmocollinite was present as streaks and lenses (Fig. 5a). They were of dark grey colour with mineral inclusions (Fig. 3b and e). In affected coals, the Rr varied between 2.95% to 3.4% in the only sample studied from bore-hole TRDM-1 (Table 3) and 0.95% to 1.4% in samples from bore-hole TRDM-2 (Table 2). The vitrinite reflectance, has been observed to range from 1.5% to N6% in the intrusive affected coals of Jharia Coalfield (Singh et al., 2007; 2008). Inertinite group was represented by the pyrofusinite, fusinite and semifusinite macerals characterized by the complete absence of macrinite and sclerotinite or resinosclerotinite, which are present in the unaffected coals of other areas of Tatapani–Ramkola Basin (Sarana, 2002; Sarana and Anand-Prakash, 2002). Rarity of macrinite has also been reported in the tectonically affected coals of Eastern Himalayas (Misra et al., 1987). No major change could be observed in the inertinite group of the altered coals (Fig. 3d and e). The first sample in bore-hole TRDM-2 located at 43 m depth and 47 m from the intrusive shows excellent preservation and abundance of palynofossils. The spores, pollen, cuticles and tracheids are clear and partly transparent in nature (Fig. 4a and b). The striate disaccates represented by Faunipollenites, Striatopodocarpites, Striatites and Strotersporites are the dominant forms followed by the subdominance
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of Densipollenites (mainly D. invisus). The palynoassemblage is comparable to the Striatopodocarpites–Densipollenites Assemblage Zone representing Barren Measures Palynozone (Middle Permian; Tiwari and Tripathi, 1992). As the sample was shaly, reflectance could not be measured due to the absence of vitrinite. At 53.00 m and 61.40 m depth (37.00 m and 28.60 m respectively from the intrusive) the organic matter recovered from the samples start showing the effects of thermal alteration with reflectance increasing to 0.95% Rr. The palynomorphs, tracheids and cuticles tend to become darker and more opaque than those recovered from the earlier sample and show a distinct charring effect (Fig. 4c and d). The Rr of the sample present 37.0 m from the intrusive ranged between 0.9% and 1.0%, the mean Rr being 0.95 (Table 2). The liptinite macerals present here were weakly fluorescing. The colour of the liptinite macerals ranges from orange-brown to pale yellow (Fig. 5a). At 213.00 m depth (18.25 m from the intrusive), the sample that is closest to the intrusive, the heat effect is most pronounced. It is marked by the complete absence of spores, pollen and cuticles. The tracheids have become completely opaque showing burning effect (Fig. 4e). The mean Rr is increased to 1.4%. However, some weakly fluorescing liptinitic macerals, which cannot be categorized to a definite type, still exist (Fig. 5b). 243.20 m depth onwards (48.25 m from the intrusive and below), the heat impact shows a marked decline. Palynomorphs and cuticles are clear and transparent. The tracheids are remarkably less dark than the earlier samples and also show transparency. Spores and pollen though not abundant, show a dominance of striate disaccates. Faunipollenites and Striatopodocarpites are the most common forms while non-striate disaccate Scheuringipollenites is sub-dominant. The palynofloral composition is comparable to Faunipollenites–Scheuringipollenites Assemblage Zone representing Upper Barakar Palynozone (Early Permian; Tiwari and Tripathi, 1992). Thereafter, all the samples at further depths, away from the intrusive, do not show any heat-induced changes (Table 2). Palynomorphs, cuticles and tracheids are clear without any charring effect. The Rr of the unaffected coals ranged between 0.50% and 0.61%, which is the normal range for high volatile bituminous coals of Indian Gondwana coalfields. The liptinite macerals were strongly fluorescing, having bright yellow colour (Fig. 5c). Thus, on the basis of palynological study, reflectance Rr, supported by the colour range of the liptinite macerals under fluorescence mode, the effect of the intrusive was manifested up to the distance of 48.25 m. The coal bands present between 18.25 m and 48.25 m from the intrusive in bore-hole TRDM-2 show definite evidences of temperature induced changes. The coal samples collected at a distance of 48.25 m or more were found to be unaffected from the impact of the intrusive and the nature of microconstituents from such samples is comparable to the Lower Gondwana coals of other basins (Anand-Prakash and Khare, 1974; Anand-Prakash and Sarate, 1993; Mishra and Singh, 1990; Mishra et al., 1990). In bore-hole TRDM-1, the palynomorphs recorded from all the samples from 15.00 to 51.00 m depth (37 m–1 m from the intrusive), were charred due to the impact of underlying intrusive (Fig. 4f). Even in some of the surface samples collected from a stream cutting, about 100 m east of this bore-hole, the palynomorphs were found to be charred beyond recognition. Petrologically, only one carbonaceous shale sample of bore-hole TRDM-1, having coal streaks (16.50 m from the intrusive; Pellet no. TRDM-1/6) could be studied and it showed the maximum effect of the intrusive. Its mean reflectance was 3.0% (Table 3). The sporinite maceral associated with vitrinite was arranged linearly along a definite bedding plane. However, it did not fluoresce at all; probably because the light hydrocarbons were expelled, due to the heat supply, as volatile matter. Mineral matter was thereby injected in the voids under high temperature and pressure. These appear as lensoid structures resembling sporinites (Fig. 3a).
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Fig. 3. Photomicrographs of intrusive affected coals (v = vitrinite, m = mineral matter, dc = dessication cracks, dv = devolatization microvacuoles, i = inertinite, p = pyrite, c = calcite); a, vitrinite (Rr = 3.0%) showing typical pattern of dessication cracks and devolatization microvacuoles; b, semifusinite and vitrinite (Rr 1.40%) showing characteristic micropores with infilled mineral matter; c, vitrinite (Rr 0.95% showing characteristic devolatization vacuoles and injected mineral matter; d, massive and infilterational pyrite associated with vitrinite (Rr 1.40%); e, vitrinite (Rr 1.40%) showing the micropores with infilled mineral matter and inertinite; f , calcite associated with vitrinite (Rr 1.4%).
Argillaceous mineral matter is generally present as ground mass (Fig. 3b–e). The pyrite present in these coals is infilterational and massive in nature (Fig. 3d). However, at places, euhedral pyrite has also been observed. Above 500 °C, pyrite (FeS2) changes to iron sulphide (FeS) and ultimately forms hematite (Chandra and Taylor, 1982). Thermal distortion of banding and removal of sporinite occurs between 300 °C and 500 °C (Taylor, 1961). This indicates that the temperature of the sediments did not exceed more than 500 °C as pyrite was found almost unaffected. It appears that the sandstone/ shale partings lying between the intrusive body and the coal layer acted as a barrier and did not allow the temperature to rise further. Calcite, which was rare in the unaffected coals, frequently occurred in the affected coals as crack fillings (Fig. 3f). It was probably formed due to the large amount of carbon dioxide generated during igneous activity (Chandra and Taylor, 1982).
the sudden heat supply during the injection of the intrusive. The salient results are summarized as under:
5. Conclusions
Acknowledgments
Certain well defined trends can be observed, which show the characteristic changes brought about in the coal microconstituents by
We thank the authorities of the Birbal Sahni Institute of Palaeobotany, for the award of Birbal Sahni Research Scholarship
i. The effect of intrusive was manifested up to a distance of 48 m, samples beyond that do not show any heat-induced changes. ii. Expulsion of volatile matter has produced large number of devolatization vacuoles which are infilled with mineral matter. iii. Spores, pollen, tracheids and other vegetative fragments show distinct burning/charring effect and most of the liptinite depolarized close to the intrusive. iv. Calcite is commonly present as crack fillings in the vitrinite, which has formed due to activities of mineralized water. v. Presence of unaltered pyrite indicates that the temperature did not rise more than 500 °C in the coal bands affected by the intrusive.
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Table 2 Results of maturation, fluorescence and palynological studies in bore-hole TRDM-2. Depth(m)
Sample nos.
43.00 53.00 61.40 90.00 to 194.75 213.00 243.20 258.00 273.00 283.00 292.00 300.00 303.00 312.00 323.00 325.00 342.00 344.00 352.00 356.00 365.00 375.00 380.00
Lithology
Pellet nos.
1 2 3
Shale Coal Shale
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Coal Coal Coal Coal Coal Coal Coal Coal Coal Coal Coal Coal Coal Coal Coal Coal Coal Coal
– TRDM-2/1− TRDM-2/2 Intrusive TRDM-2/3 TRDM-2/4 TRDM-2/5 TRDM-2/6 TRDM-2/7 TRDM-2/8 TRDM-2/9 TRDM-2/10 TRDM-2/11 TRDM-2/12 TRDM-2/13 TRDM-2/14 TRDM-2/15 TRDM-2/16 TRDM-2/17 TRDM-2/18 TRDM-2/19 TRDM-2/20
Distance from intrusive (m)
Rr
Rank
Fluorescence observations
Palynological observations
47.00 37.00 28.60
– 0.95 -
– High vol. Bit.A –
– Pale yellow –
Unaffected Affected Affected
18.25 48.25 63.25 78.25 88.25 97.25 105.25 108.25 117.25 128.25 130.25 147.25 149.25 157.25 161.25 170.25 180.25 185.25
1.40 0.53 0.60 0.61 0.61 0.57 0.53 0.53 0.53 0.52 0.56 0.53 0.50 0.51 0.57 0.59 0.55 0.52
Med. Vol. Bit. High vol. Bit.C " " " " " " " " "" " " " " " " "
Very pale yellow Bright yellow " " "" " " " " " " " " " " " " "
Affected Unaffected " " " " " " " " " " " " " " " "
during the tenure of which this work was done and to the present Director, Dr. N.C. Mehrotra, for continued support and permission to publish this paper. We are grateful to the Geological Survey of India, Coal Wing, for permission to collect the bore-hole samples and to Sri. D.K. Das for help during fieldwork. Special thanks to Dr. Anand Prakash for his able guidance and encouragement. The authors also thank the two anonymous reviewers for critically going through the manuscript and for their useful suggestions.
Table 3 Details of samples in bore-hole TRDM-1. Depth(m) Sample nos. Lithology
Pellet nos.
Distance from intrusive (m)
15.00 18.00 20.00 21.00 22.00 22.60 23.60 24.60 25.50 26.50 28.00 29.50 30.50 31.50 32.00 33.50 34.25 35.50 36.00 37.00 38.00 39.00 40.00 40.50 42.00 45.00 50.00 51.00 52.00
– – – TRDM-1/1 – – – – TRDM-1/2 – – TRDM-1/3 TRDM-1/4 – – TRDM-1/5 – TRDM-1/6⁎ – – – – – – – – – – –
37.00 34.00 32.00 31.00 30.00 29.40 28.40 27.40 26.50 25.50 24.00 22.50 21.50 20.50 20.00 19.00 17.75 16.50 16.00 15.00 14.00 13.00 12.00 11.50 10.00 7.00 2.00 1.00
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Grey shale Grey shale Grey shale Carb. shale Grey shale Grey shale Sandy shale Sandy shale Carb. shale Grey shale Grey shale Grey shale Grey shale Grey shale Sandy shale Grey shale Grey shale Carb. shale Grey shale Grey shale Grey shale Grey shale Sandy shale Grey shale Grey shale Shale Sandstone Sandy shale Intrusive
⁎ (Petrological study was done; mean Rr 3.0%, fluorescence nil).
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Fig. 4. Photomicrographs of palynomorphs (po = pollen, cu = cuticles, tr = trachieds); a,b, palynomorphs liberated from the unaffected coals, the spores/pollens are clear along with the cuticles and tracheids which show transparency (a is from the sample 47 m above the intrusive and b is from the sample 48.25 m below the intrusive in TRDM-2); c, d, organic matter from affected coals are more dark and less transparent (c 37 m from intrusive and d 28.60 m from intrusive in TRDM-2); e, represents sample closest to the intrusive (18.25 m, TRDM-2), tracheids are completely burnt, dark and opaque; f, represents sample from TRDM-1 (22.50 m from the intrusive), the palynomorphs show distinct charred effect. Pareek, H.S., 1988. Petrographic characteristics of solid fuels of India with particular reference to the coking coals. International Journal of Coal Geology 10, 285–306. Raja Rao, C.S., 1983. Coalfields of India — coal measures of Madhya Pradesh and Jammu & Kashmir. Bulletin Geological Survey of India 45, 75–80. Sarana, S., 2002. Characterisation of organic source material from Tatapani and Ramkola Coalfields, Chhattisgarh, India. Palaeobotanist 51, 81–92. Sarana, S., Anand-Prakash, 2002. Microconstituents and depositional environment of Lower Gondwana coals of Tatapani–Ramkola Coalfield, Surguja District, Chhattisgarh. Journal of the Geological Society of India 60, 663–676. Singh, A.K., Singh, M.P., Sharma, M., Srivastava, S.K., 2007. Microstructures and microtextures of natural cokes: a case study of heat-altered coking coals from the Jharia Coalfield, India. International Journal of Coal Geology 71, 153–175.
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Fig. 5. Photomicrographs of liptinite macerals showing colour variation in the coals of different ranks vis-à-vis the intrusive effect from bore-hole TRDM-2; a, liptinite macerals in affected coals (37 m from the intrusive) of High Volatile Bituminous A rank (Rr 0.95%) are weakly fluorescing showing orangish-brown to pale yellow colour; b, liptinite macerals in coals closest to the intrusive (18.25 m) of Medium Volatile Bituminous rank (Rr 1.4%) are of pale yellow colour showing very weak fluorescence; c, liptinite macerals in unaltered coals (N48.25 m from the intrusive) of High Volatile Bituminous C rank (Rr 0.50–0.61%) are strongly fluorescing showing bright yellow colour.
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Effect of igneous intrusive on coal microconstituents: Study from an Indian Gondwana coalfield Shinjini Sarana a, Ratan Kar b,⁎ a b
Department of Botany, National P.G. College, Lucknow-226 007, India Birbal Sahni Institute of Palaeobotany, 53 University Road, Lucknow-226 007, India
a r t i c l e
i n f o
Article history: Received 30 January 2010 Received in revised form 2 November 2010 Accepted 2 November 2010 Available online 12 November 2010 Keywords: Igneous intrusive Coal petrology Palynology Gondwana India
a b s t r a c t Petrological and palynological analyses of coals from two bore-holes, intruded by an igneous body (dolerite dyke), reveal characteristic changes due to thermal alteration. The unaffected coals are high volatile bituminous (Rr: 0.50–0.61%). The coals recovered from samples located close to the dyke had vitrinite with devolatization microvacuoles, higher reflectance ranging from 0.9% to 3.0% and liptinite with none to very low fluorescence. There is an increasing trend in reflectance and a decreasing trend in fluorescence in the samples occurring closer to the dyke. Presence of injected mineral matter and pyrite in the macerals further indicate the impact of thermal metamorphism on the coals occurring near the igneous body. The palynological investigation of unaffected coals shows the presence of a number of well-identified spores and pollen. However, very few palynomorphs could be recovered from coals located close to the intrusive as the pollen/ spores were found to be charred beyond recognition. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Study area
During the process of coalification, heat is normally available to the sediments by the depth of burial. The effect of temperature on rank of coal is gradual and is also related to the duration of burial. But whenever the sediments, particularly coal, come in close contact with the igneous bodies, sudden changes in the rank and nature of organic matter have been observed (Chandra, 1963; Ghosh, 1967; Pareek, 1970). A similar effect is also produced due to the movement of rocks along major tectonic features. Coal present near the thrust zones bears the testimony of such heat-induced changes from several areas in the Himalaya (Anand Prakash, 1992; Ghosh, 1997). Though, a number of studies are available on thermally altered coals from India (Chandra and Srivastava, 1980; Chandra and Taylor, 1982; Pareek, 1988; Anand Prakash and Sarate, 1991; Mishra and Cook, 1992), most of these are related to the rank behavior alone, and not to the overall changes brought about in the organic microconstituents. However, comprehensive work has recently been done from Indian coalfields on the genesis of natural cokes due to magmatic intrusions (Singh et al., 2007; 2008) and on the behavior of burnt coal macerals and their reactivity (Choudhary et al., 2007; 2008). The present investigation is an attempt to understand such heat-induced changes in the coal macerals and palynodebris from two bore-holes TRDM-1 and TRDM-2, located at Tatapani–Ramkola Coalfield, India.
The area of the present study, Tatapani–Ramkola Coalfield, is situated in the Surguja District of Chhattisgarh State and is the westernmost extension of the Damodar–Koel Valley master Gondwana Basin (Fig. 1). The coalfield is located between latitudes 23°30′: 23°55′ and longitudes 83°00′: 83°40′ and is so named owing to the presence of a conspicuous hot spring (vern.: ‘tata’ — hot, ‘pani’ — water) emanating near Tatapani village (Raja Rao, 1983). The Gondwana Sequence in Tatapani–Ramkola Coalfield has been classified into Talchir, Karharbari, Barakar, Barren Measures, Raniganj, Panchet and Mahadeva Formations. The commercially viable coal seams occur within the Barakar Formation (Table 1). At some places the Gondwana sediments in the coalfield are intruded by the basic intrusives, possibly related to the Deccan volcanic activity (Raja Rao, 1983). During the recent years, the Geological Survey of India and Mineral Exploration Corporation Ltd. have taken up this coalfield as one of the thrust areas for coal exploration. During the course of prospecting of coal reserves in Dhamni Block of Tatapani– Ramkola Coalfield, dolerite intrusives were struck in two bore-holes — TRDM-1 and TRDM-2 (Figs. 1 and 2). In the first bore-hole — TRDM-1, the collection of samples was restricted to the sediments above the intrusive, as during our field work the bore-hole was drilled only up to that point. The samples were primarily collected for palynostratigraphical studies as the Birbal Sahni Institute of Palaeobotany, Lucknow, had undertaken a major project for the surface and subsurface correlation of coal and associated sediments in Tatapani–Ramkola Coalfield (Kar, 2001; Srivastava and Kar, 2001; Kar, 2003; Kar and Srivastava, 2003). Though, palynological studies were not fruitful, due to the presence of the intrusive, it gave us an opportunity to observe its
⁎ Corresponding author. Tel.: +91 9451088928; fax: +91 522 2740485. E-mail address: [email protected] (R. Kar). 0166-5162/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2010.11.006
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Fig. 1. Map of the locality showing area of study.
effect on the coal microconstituents and organic matter found in the sediments. Studies on these samples were thereby initiated and since interesting results were obtained, we again visited the locality in the next field season to collect the complete sequence from the second borehole — TRDM-2.
Table 1 Generalised stratigraphic sequence of Tatapani–Ramkola coalfield. Age
Formation
Recent Cretaceous? Upper Triassic
Basic intrusives Mahadeva
Lower Triassic
Panchet
Upper Permian
Raniganj
Middle Permian
Barren Measures
Lower Permian
Barakar
Karharbari
Talchir
Lithology Alluvium Dolerite dykes. Thick, cross-bedded, coarse-grained, ferruginous sandstone. Yellowish, fine-grained sandstone with alternating red and green siltstones, shales and clays. Micaceous, fine-grained, ripple laminated sandstones, grey and carbonaceous shales and shaly coal bands. Ironstone shales showing box structure, fine-grained sandstone, shales and argillaceous sandstone. Medium to coarse-grained pebbly arkosic sandstone, grey and carbonaceous shales and coal seams. Coarse-grained to granular arkosic sandstone with conglomerate lenses, shales, carbonaceous shales and intermittent coal seams. Diamictite, khaki-green needle shales, siltstone, fine-grained sandstone and varves.
Unconformity Archean
Granites, gneisses, micaceous green schists, phyllites and quartz veins.
3. Material and methods For petrological analysis only coal samples were chosen, while palynological studies were undertaken on both coal and shale samples. For petrological study the samples (Fig. 2) were crushed to a particle size of 2000 μm (±18 meshes) and embedded in epoxy resin. These embedded pellets were then ground and polished for counting of macerals. The rank was determined by measuring the reflectance (Rr) on polished vitrinite surfaces on Leitz Photometer Voltmeter (MPV-1) system. For palynological analysis, 150 to 200 g of the material from each sample was taken and crushed to 2–4 mm size. The samples were initially treated with 40% hydroflouric acid to remove the silica and thereafter, by nitric acid to release the palynomorphs from the rest of the organic debris. In the intrusive affected samples, palynomorphs remain black in colour and opaque even after sustained nitric acid treatment. To facilitate the clarity of palynomorphs, the macerates were, therefore, boiled for 6 to 8 h in concentrated nitric acid using water bath. However, even after such strong treatment, the transparency of spores and pollen improved only to a small extent as most of the palynomorphs were badly charred rendering them unidentifiable. The intrusive also had an indurating effect on the sediments making them much harder. 4. Results and discussion The vitrinite group is mainly represented by three macerals namely telocollinite, tellinite and desmocollinite (Fig. 3a). The affected vitrinite was marked by the presence of uneven cracks and dessication cracks, infilled with argillaceous mineral matter, pyrite or calcite (Fig. 3a–f). The presence of carbonate, pyrite and clay minerals has also been observed in the thermally metamorphosed coals of South Sumatra Basin, Indonesia (Amijaya and Littke, 2006). Numerous micropores are present (vesicles/escape pores/devolatization microvacuoles), formed due to devolatization of liptinite and vitrinite.
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Fig. 2. Litholog of bore-holes TRDM-1 and TRDM-2 showing position of intrusives and samples.
Occasionally, the rounding of the margins was observed in the vitrinite grains, which had developed due to the softening and plastic tendency of vitrinite (Goodarzi and Murchison, 1977; 1978). The Rr of the unaffected coals ranged from 0.50% to 0.61% (Table 2). Semifusinite was present filled with mineral matter (Fig. 3b). Desmocollinites were generally associated with inertodetrinite and liptinite macerals (Figs. 3b and 5a). In coaly-shale samples, the desmocollinite was present as streaks and lenses (Fig. 5a). They were of dark grey colour with mineral inclusions (Fig. 3b and e). In affected coals, the Rr varied between 2.95% to 3.4% in the only sample studied from bore-hole TRDM-1 (Table 3) and 0.95% to 1.4% in samples from bore-hole TRDM-2 (Table 2). The vitrinite reflectance, has been observed to range from 1.5% to N6% in the intrusive affected coals of Jharia Coalfield (Singh et al., 2007; 2008). Inertinite group was represented by the pyrofusinite, fusinite and semifusinite macerals characterized by the complete absence of macrinite and sclerotinite or resinosclerotinite, which are present in the unaffected coals of other areas of Tatapani–Ramkola Basin (Sarana, 2002; Sarana and Anand-Prakash, 2002). Rarity of macrinite has also been reported in the tectonically affected coals of Eastern Himalayas (Misra et al., 1987). No major change could be observed in the inertinite group of the altered coals (Fig. 3d and e). The first sample in bore-hole TRDM-2 located at 43 m depth and 47 m from the intrusive shows excellent preservation and abundance of palynofossils. The spores, pollen, cuticles and tracheids are clear and partly transparent in nature (Fig. 4a and b). The striate disaccates represented by Faunipollenites, Striatopodocarpites, Striatites and Strotersporites are the dominant forms followed by the subdominance
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of Densipollenites (mainly D. invisus). The palynoassemblage is comparable to the Striatopodocarpites–Densipollenites Assemblage Zone representing Barren Measures Palynozone (Middle Permian; Tiwari and Tripathi, 1992). As the sample was shaly, reflectance could not be measured due to the absence of vitrinite. At 53.00 m and 61.40 m depth (37.00 m and 28.60 m respectively from the intrusive) the organic matter recovered from the samples start showing the effects of thermal alteration with reflectance increasing to 0.95% Rr. The palynomorphs, tracheids and cuticles tend to become darker and more opaque than those recovered from the earlier sample and show a distinct charring effect (Fig. 4c and d). The Rr of the sample present 37.0 m from the intrusive ranged between 0.9% and 1.0%, the mean Rr being 0.95 (Table 2). The liptinite macerals present here were weakly fluorescing. The colour of the liptinite macerals ranges from orange-brown to pale yellow (Fig. 5a). At 213.00 m depth (18.25 m from the intrusive), the sample that is closest to the intrusive, the heat effect is most pronounced. It is marked by the complete absence of spores, pollen and cuticles. The tracheids have become completely opaque showing burning effect (Fig. 4e). The mean Rr is increased to 1.4%. However, some weakly fluorescing liptinitic macerals, which cannot be categorized to a definite type, still exist (Fig. 5b). 243.20 m depth onwards (48.25 m from the intrusive and below), the heat impact shows a marked decline. Palynomorphs and cuticles are clear and transparent. The tracheids are remarkably less dark than the earlier samples and also show transparency. Spores and pollen though not abundant, show a dominance of striate disaccates. Faunipollenites and Striatopodocarpites are the most common forms while non-striate disaccate Scheuringipollenites is sub-dominant. The palynofloral composition is comparable to Faunipollenites–Scheuringipollenites Assemblage Zone representing Upper Barakar Palynozone (Early Permian; Tiwari and Tripathi, 1992). Thereafter, all the samples at further depths, away from the intrusive, do not show any heat-induced changes (Table 2). Palynomorphs, cuticles and tracheids are clear without any charring effect. The Rr of the unaffected coals ranged between 0.50% and 0.61%, which is the normal range for high volatile bituminous coals of Indian Gondwana coalfields. The liptinite macerals were strongly fluorescing, having bright yellow colour (Fig. 5c). Thus, on the basis of palynological study, reflectance Rr, supported by the colour range of the liptinite macerals under fluorescence mode, the effect of the intrusive was manifested up to the distance of 48.25 m. The coal bands present between 18.25 m and 48.25 m from the intrusive in bore-hole TRDM-2 show definite evidences of temperature induced changes. The coal samples collected at a distance of 48.25 m or more were found to be unaffected from the impact of the intrusive and the nature of microconstituents from such samples is comparable to the Lower Gondwana coals of other basins (Anand-Prakash and Khare, 1974; Anand-Prakash and Sarate, 1993; Mishra and Singh, 1990; Mishra et al., 1990). In bore-hole TRDM-1, the palynomorphs recorded from all the samples from 15.00 to 51.00 m depth (37 m–1 m from the intrusive), were charred due to the impact of underlying intrusive (Fig. 4f). Even in some of the surface samples collected from a stream cutting, about 100 m east of this bore-hole, the palynomorphs were found to be charred beyond recognition. Petrologically, only one carbonaceous shale sample of bore-hole TRDM-1, having coal streaks (16.50 m from the intrusive; Pellet no. TRDM-1/6) could be studied and it showed the maximum effect of the intrusive. Its mean reflectance was 3.0% (Table 3). The sporinite maceral associated with vitrinite was arranged linearly along a definite bedding plane. However, it did not fluoresce at all; probably because the light hydrocarbons were expelled, due to the heat supply, as volatile matter. Mineral matter was thereby injected in the voids under high temperature and pressure. These appear as lensoid structures resembling sporinites (Fig. 3a).
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Fig. 3. Photomicrographs of intrusive affected coals (v = vitrinite, m = mineral matter, dc = dessication cracks, dv = devolatization microvacuoles, i = inertinite, p = pyrite, c = calcite); a, vitrinite (Rr = 3.0%) showing typical pattern of dessication cracks and devolatization microvacuoles; b, semifusinite and vitrinite (Rr 1.40%) showing characteristic micropores with infilled mineral matter; c, vitrinite (Rr 0.95% showing characteristic devolatization vacuoles and injected mineral matter; d, massive and infilterational pyrite associated with vitrinite (Rr 1.40%); e, vitrinite (Rr 1.40%) showing the micropores with infilled mineral matter and inertinite; f , calcite associated with vitrinite (Rr 1.4%).
Argillaceous mineral matter is generally present as ground mass (Fig. 3b–e). The pyrite present in these coals is infilterational and massive in nature (Fig. 3d). However, at places, euhedral pyrite has also been observed. Above 500 °C, pyrite (FeS2) changes to iron sulphide (FeS) and ultimately forms hematite (Chandra and Taylor, 1982). Thermal distortion of banding and removal of sporinite occurs between 300 °C and 500 °C (Taylor, 1961). This indicates that the temperature of the sediments did not exceed more than 500 °C as pyrite was found almost unaffected. It appears that the sandstone/ shale partings lying between the intrusive body and the coal layer acted as a barrier and did not allow the temperature to rise further. Calcite, which was rare in the unaffected coals, frequently occurred in the affected coals as crack fillings (Fig. 3f). It was probably formed due to the large amount of carbon dioxide generated during igneous activity (Chandra and Taylor, 1982).
the sudden heat supply during the injection of the intrusive. The salient results are summarized as under:
5. Conclusions
Acknowledgments
Certain well defined trends can be observed, which show the characteristic changes brought about in the coal microconstituents by
We thank the authorities of the Birbal Sahni Institute of Palaeobotany, for the award of Birbal Sahni Research Scholarship
i. The effect of intrusive was manifested up to a distance of 48 m, samples beyond that do not show any heat-induced changes. ii. Expulsion of volatile matter has produced large number of devolatization vacuoles which are infilled with mineral matter. iii. Spores, pollen, tracheids and other vegetative fragments show distinct burning/charring effect and most of the liptinite depolarized close to the intrusive. iv. Calcite is commonly present as crack fillings in the vitrinite, which has formed due to activities of mineralized water. v. Presence of unaltered pyrite indicates that the temperature did not rise more than 500 °C in the coal bands affected by the intrusive.
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Table 2 Results of maturation, fluorescence and palynological studies in bore-hole TRDM-2. Depth(m)
Sample nos.
43.00 53.00 61.40 90.00 to 194.75 213.00 243.20 258.00 273.00 283.00 292.00 300.00 303.00 312.00 323.00 325.00 342.00 344.00 352.00 356.00 365.00 375.00 380.00
Lithology
Pellet nos.
1 2 3
Shale Coal Shale
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Coal Coal Coal Coal Coal Coal Coal Coal Coal Coal Coal Coal Coal Coal Coal Coal Coal Coal
– TRDM-2/1− TRDM-2/2 Intrusive TRDM-2/3 TRDM-2/4 TRDM-2/5 TRDM-2/6 TRDM-2/7 TRDM-2/8 TRDM-2/9 TRDM-2/10 TRDM-2/11 TRDM-2/12 TRDM-2/13 TRDM-2/14 TRDM-2/15 TRDM-2/16 TRDM-2/17 TRDM-2/18 TRDM-2/19 TRDM-2/20
Distance from intrusive (m)
Rr
Rank
Fluorescence observations
Palynological observations
47.00 37.00 28.60
– 0.95 -
– High vol. Bit.A –
– Pale yellow –
Unaffected Affected Affected
18.25 48.25 63.25 78.25 88.25 97.25 105.25 108.25 117.25 128.25 130.25 147.25 149.25 157.25 161.25 170.25 180.25 185.25
1.40 0.53 0.60 0.61 0.61 0.57 0.53 0.53 0.53 0.52 0.56 0.53 0.50 0.51 0.57 0.59 0.55 0.52
Med. Vol. Bit. High vol. Bit.C " " " " " " " " "" " " " " " " "
Very pale yellow Bright yellow " " "" " " " " " " " " " " " " "
Affected Unaffected " " " " " " " " " " " " " " " "
during the tenure of which this work was done and to the present Director, Dr. N.C. Mehrotra, for continued support and permission to publish this paper. We are grateful to the Geological Survey of India, Coal Wing, for permission to collect the bore-hole samples and to Sri. D.K. Das for help during fieldwork. Special thanks to Dr. Anand Prakash for his able guidance and encouragement. The authors also thank the two anonymous reviewers for critically going through the manuscript and for their useful suggestions.
Table 3 Details of samples in bore-hole TRDM-1. Depth(m) Sample nos. Lithology
Pellet nos.
Distance from intrusive (m)
15.00 18.00 20.00 21.00 22.00 22.60 23.60 24.60 25.50 26.50 28.00 29.50 30.50 31.50 32.00 33.50 34.25 35.50 36.00 37.00 38.00 39.00 40.00 40.50 42.00 45.00 50.00 51.00 52.00
– – – TRDM-1/1 – – – – TRDM-1/2 – – TRDM-1/3 TRDM-1/4 – – TRDM-1/5 – TRDM-1/6⁎ – – – – – – – – – – –
37.00 34.00 32.00 31.00 30.00 29.40 28.40 27.40 26.50 25.50 24.00 22.50 21.50 20.50 20.00 19.00 17.75 16.50 16.00 15.00 14.00 13.00 12.00 11.50 10.00 7.00 2.00 1.00
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Grey shale Grey shale Grey shale Carb. shale Grey shale Grey shale Sandy shale Sandy shale Carb. shale Grey shale Grey shale Grey shale Grey shale Grey shale Sandy shale Grey shale Grey shale Carb. shale Grey shale Grey shale Grey shale Grey shale Sandy shale Grey shale Grey shale Shale Sandstone Sandy shale Intrusive
⁎ (Petrological study was done; mean Rr 3.0%, fluorescence nil).
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Fig. 4. Photomicrographs of palynomorphs (po = pollen, cu = cuticles, tr = trachieds); a,b, palynomorphs liberated from the unaffected coals, the spores/pollens are clear along with the cuticles and tracheids which show transparency (a is from the sample 47 m above the intrusive and b is from the sample 48.25 m below the intrusive in TRDM-2); c, d, organic matter from affected coals are more dark and less transparent (c 37 m from intrusive and d 28.60 m from intrusive in TRDM-2); e, represents sample closest to the intrusive (18.25 m, TRDM-2), tracheids are completely burnt, dark and opaque; f, represents sample from TRDM-1 (22.50 m from the intrusive), the palynomorphs show distinct charred effect. Pareek, H.S., 1988. Petrographic characteristics of solid fuels of India with particular reference to the coking coals. International Journal of Coal Geology 10, 285–306. Raja Rao, C.S., 1983. Coalfields of India — coal measures of Madhya Pradesh and Jammu & Kashmir. Bulletin Geological Survey of India 45, 75–80. Sarana, S., 2002. Characterisation of organic source material from Tatapani and Ramkola Coalfields, Chhattisgarh, India. Palaeobotanist 51, 81–92. Sarana, S., Anand-Prakash, 2002. Microconstituents and depositional environment of Lower Gondwana coals of Tatapani–Ramkola Coalfield, Surguja District, Chhattisgarh. Journal of the Geological Society of India 60, 663–676. Singh, A.K., Singh, M.P., Sharma, M., Srivastava, S.K., 2007. Microstructures and microtextures of natural cokes: a case study of heat-altered coking coals from the Jharia Coalfield, India. International Journal of Coal Geology 71, 153–175.
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Fig. 5. Photomicrographs of liptinite macerals showing colour variation in the coals of different ranks vis-à-vis the intrusive effect from bore-hole TRDM-2; a, liptinite macerals in affected coals (37 m from the intrusive) of High Volatile Bituminous A rank (Rr 0.95%) are weakly fluorescing showing orangish-brown to pale yellow colour; b, liptinite macerals in coals closest to the intrusive (18.25 m) of Medium Volatile Bituminous rank (Rr 1.4%) are of pale yellow colour showing very weak fluorescence; c, liptinite macerals in unaltered coals (N48.25 m from the intrusive) of High Volatile Bituminous C rank (Rr 0.50–0.61%) are strongly fluorescing showing bright yellow colour.
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