Petrological Characteristics of Barakar Coal Seams, Metamorphosed by Lamprophyre Sill in the Jharia Coalfield, Bihar*

PETROLOGICAL CHARACTERISTICS OF BARAKAR COAL SEAMS, METAMORPHOSED BY LAMPROPHYRE SILL IN THE JHARIA COALFIELD, BIHAR*

BY H. S. PAREEK (Geological Survey of India, Lucknow) Received June 15, 1965 (Comunicated by Mr. B. Rama Ran, F.A.S¢.)

ABSTRACT In the Jhatia Coalfield, Bihar, dykes and sills of mica lamprophyre are common and have thermally metamorphosed the coal seams in contact into natural coke. Petrological study of the metamorphosed zones of the coal seams in the south-eastern extremity of the coalfield has been carried out. Four petrologically distinct zones designated as natural coke, coked coal, affected coal and unaffected coal are traced, when studied away from the contact. Chemically, a marked increase of volatiles and decrease of ash is recorded away from the seam-igneous contact. Phosphorus content is quite high in the natural coke ash. The natural coke is characterised by numerous mineral-filled or empty vacuoles and strongly anisotropic, highly-reflecting nature of the fused matrix having physically unaltered fusinite embedded. The fused matrix is of mosaic appearance and consists of ill-defined units varying from one to fifty microns size. In the coked coal, the size and number of vacuoles has considerably reduced, and so also, the anisotropism and mosaic appearance of the matrix, accompanied by an increase in size of the units around fifty microns. The affected coal is distinctly folded, though minutely, and is micro-laminated, the lamination being due to occurrence of vitrinite layers and sheets with interbedoed fusinite, semifusinite, resinite and micri- nitc. The vesicles occur here and there. Resinite is seen in cleats and as coalesced films migrated along the bedding. The unaffected coal is a typical banded bituminous coal. The natural coke formation is discussed. Graphite is concluded to be absent.

INTRODUCTION PETROLOGICA~L investigations on natural coke of Japan (Ueji, 1932, 1936), U,K. (Marshall, 1936, 1945), U.S.A. (Clegg, 1955), Germany (Stach, 1951,

* Published with the kind permission of the Director-General, Geological Survey of India. 261 262 H.S. PAREEK

1952; Hoehne, 1962) and Australia (C.S.I.R.O., 1965) have brought out significant details about their nature, mode of formation and the extent of temperatures reached. In India, the natural coke petrology has dIawn increased interest within the past few years (Ganju and Pant, 1962; 2Vlukherjee, 1962; Pareek, 1964; Sanyal, 1964). This study is of particular interest in this country since natural coke occurrences are confined to the Darnodar valley coalfields, which are the only reserves of coking coals. The author studied the altered zones of the affected coal seams in the Jharia coalti~ld, Bihar. Its south-eastern portion was geologically mapped, the coal seams affected by the igneous sills were studied and representative samples of altered zones and unaffected coal collected for detailed studies. The petrological studies were made in the Coal Petrology Laboratory, Depart- ment of G0ology, Aligarh Muslim University, while the chemical analyses were carried out in the Central Fuel Research Institute, Jealgora (Pareek, 1964). The results are described and discussed here.

GEOLOGY OF THE AREA

The area studied falls between the latitude~ 23 ° 38' and 23 ° 41' N. and longitudes 86 ° 27' and 86 ° 30' E. (see Fig. 1). The lower Gondwana forma- tions form a triangular portion and are separated on the northerrL eastern and southern sides by boundary faults. They comprise the Barakars with a small patch of Barren Measures in the somh-western portion of 1he alea. The coal seams strike NE-SW and dip 450-60 ° towards south. After Tisra (23 ° 38' : 86 ° 28') their strike is changed to NNE-SSW with high diFs. The seams thus acquire a sickle-shaped character in this area, which is indicative of the Gondwana sedimentation being in a basin. Dykes and sills of mica lamprophyre are common in the Barakars. The intruding rock when fresh is characterised by dark brown colour, fine-grained nature and compact appearance. No difference in its general nature has been observed at different places. Besides thick bodies, it also occurs as thin stringers, streaks and veins in the coal seams in contact. The seams affected severely by the igneous sills and studied for the present work are It, v, IX/X and XI/XI1/XIII/XIV.

CHEMICAL COMPOSITION Proximate analyses of samples representing natural coke from the different seams and of the metamorphosed zone of a seam located at regular intervals were carried out and the results appear in Tables I and II, respectively. Petrological Characteristics of Barakar Coal Seams 2a3

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TABLE 1 Proxhnate analyses of natural coke

Seam Moisture Ash Volatiles Fixed carbon Phosporus (~o) (%) (%) (~o by diff.) (%)

I1 0.4 36.27 5.34 57.99 .. V 0.2 39.00 6.34 54.46 .. IX/X 0.6 38.00 5.55 55.85 8.2 XI-XIV 0.8 38.91 5.35 54~84 10.0

Unaffected Coal: XI-XIV 3.2 11-07 23.0 62.73 Nil

TABLE II Proximate analyses of altered zone of XI-XIV seam

Distance from the igneous contact in metres Proximate analysis 0 0.15 0.30 0.45 0-60 0.75 0.90 1.05

Moisture (~) .. 0.25 0.60 0.80 1.00 1.25 1.70 2.10 2"83 Ash(yo) .... 39.00 35.88 30.24 27.75 25.50 23.20 20.74 17.20 Volatiles (Yo) 6.34 6.60 7.08 7.98 8.00 8.98 9-16 10.25 Fixed Carbon(~o).. 54.41 56.92 61.88 63.27 64.65 66.12 68-0 69.72

The chemical na:.u:c of the natural cokes will be cleal from Table I. The phosphorus content in natural coke ash is quite high and it can be useful in fertiliser industry, if recoverable economically. Table II indicates that effect on the coal seam was severe near the sill and retarded gradually away from it; volatiles increase and ash decreases progressively with increase in distance. The ash is related to the mineral-filled vacuoles and cracks, which decrease away from the contact.

PETROLOGICAL STUDY

The natural coke occurs as a broad zone of more or less tmifoim thickness in contact with the sill. It is black, hard, granular and vesicular, lacking the Petrological Characteristics of Barakar Coal Seams 265 banding so characteristic of the Damodar valley coals. It invariably shows columnar structure near the igneous contact and is probably due to contraction caused by abrupt dcvolatilization of the coal. Veins of the inorganic material are common in the natural coke and fill the joints and fractures; some of them are of lamprophyric material, while others are of apatite and calcite. Holes of varying dimensions form a characteristic feature of ihe natural coke in which none of the lithological ingredients of coal are discernible. Further away from the contact, the characteristics of the natural coke tend to decline and the altered zone is neither natural coke nor coal. This altered zone has been petrologically differentiated into three distinct st~tges. The description of the four zones is as follows:

First Zone (Natural Coke) Attempts to reduce the natural coke to sections thin enough for transmitted light study proved futile, and hence polished sections were studied under oil immersion in reflected light. Microscopically, it is characterised by highly reflecting mass with a large number of mineral-filled or empty vacuoles of diameter around or up to 4130 microns. Fusinised woody tissues are the only recognisable maceral. No other microstructural details could be revealed even on prolonged etching. The micro-constituents of the natural coke are as follows: (1) Fused material.--It comprises high reflectance, strongly anisotropic, ill-defind units showing mosaic structure. It shows undulose extinction and the units differ from each other in optical orientation varying in size between one and five microns across. (2) Fusinite.--Folded fragments of fusinised woody tissues are very common and exhibit moderately high reflectance. They are frequently angular and show corroded margins and cracked, often shrunken, natuic of the tracheid walls. The thickest fragments recorded average 1,1300 micrer.s across their transverse directions. They occur in a haphazald mar, ner lackirg the systematic orientation as in coals. (3) Inorganic material.--Apatitc and calcite occur as laths and angular bodies in the general coke matrix, and as vacuole and fissure-fillings. Clay particles have also beert observed. Second Zone (Coked Coal) Away from the contact, it is r~ext to the natural coke zone and is differ, entiated physically by harder, closely compressed and more lustrous natures 266 H.S. P/d~EK banding being discernible at places. The thickness of this zone does not exceed that of the natural coke. In an effort to prepare its thin sections, the edges were found to gain slight translucency, and with extreme eme in final thinning of the section, a dark grey colour was obtained. The study in plane polarized light does not show anything more than vacuoles, but some phenomenon can be observed with crossed nicols. On the whole, the thin section study does not reveal details of any advantage and the polished sections were, therefore, depended upon.

The sections, when examined under low power dry objective, show as if the layers have melted, flowed and undergone selective cooling. The vacuoles are reduced to a maximum diameter of 200 microns. The micro- constituents are as follows: (1) Fused material.--Interpenetrating, ill-defined material of high refleetivity transforms loeaUy into a fused mass. It is chalacterised by the anisotropic nature. The size of the units varies between 5 ar.d 15 microns. (2) Spherical grains.--They appear as segregated patches of high refleetivity and anisotropic nature. They are greater than 50 microns in size and show persistence of mosaic structure. (3) Fusinite.-~,Fusinised woody tissues are of moderately high reflectivity and appear embedded in the fused material. (4) Inorganic material.--Apatite, calcite and clay. In this zone there is an overall 1eduction comparatively of the shrinkage cracks, proportion of inorganic material, and size and number of vacuoles. It seems to represent a stage which occurred within a very narrow iange of temperature within 10° to 20 ° C. of the formation of natural coke. The material on solidification retained the original banding partially, depending upon selective cooling. This zone has accordingly been named as coked coal. Third Zone (Affected Coal) The study of the altered zone further away from the co~atact indicates that the degree of.metamorphism is not so intense although the material is neither coked coal nor precisely unaffected coal. The banding is distir~ct but closely compressed, as if squeezed out, and minutely folded. The shrinkage arccks are uncommon. With distance from the contact, the folding dies away. The thickness of this zone is generally small and rarely exceeds more than a few cms. Petrological Characteristics of Barakar Coal Seams 267 Translucent sections can be prepared for transmitted light study. In polished sections, the matrix composed mai dy of vitrinite is of moderate reflectivity, being lower than that of fusinitc, and shows micro-lamination. The vacuoles are practically absent. The other macerals distinguished are semifusinite and micrinite. Resinite is the only maceral of exinite readily detectable and is seen to occur as discrete bodies, as layers along the micro- lamination, a~ad in some cleats. It is likely that the last two mode~ of occur- fence are coalesced resin in situ and migrated respectively. Fourth Zone (Unaffected Coal) It varies widely in width with thickness of the seam. It is a typical banded bituminous coal in which the distinction between the different lithological ingredients, vitrain, durain and fusain is well marked. Vitrain bands are quite thick, persistent and of brilliant black colour showing cleats quite commonly. Fusain is recognizable only when coals are split along the bedding and is generally scarce. Microscopically, it cart be studied satisfactorily both in thin and polished sections. Telinite (derived from wood with resin-filled cells), collinite, spori.. nite (microspores), resinite (discrete oval bodies), micrinite (mostly massive), semifusinite and fusinite form the macerals composing these coals. The general petrographic composition of the coals appears in Table III. TABLE I|I Petrographic composition of coal

Proportion Maceral Microlithotype

Dominant Vitrinite Vitrite, Clarite Abundant Fusinite and Semifusinite ,. Very common Miorinite Durite Common _ Fusite and "Intermediates" Rare Sporinite and Resinite Shale Absent Fusinised resins ...

DISCUSSION In the process of intrusive activity, the highly molten igneous material was forced upwards in fluid state along the fault planes, rupture zones and 268 H.S. PAREF,K

bedding planes of the strata. It thermally altered the coal seams in contact, without changing by metamorphism the surrounding rocks. The metamorphism involved obliteration of volatile-rich organic layers, destruction of banding, rapid evolutioll of gases, development of vacuoles, vesicles and fissures, and assimilation of inorganic malter from the apatite-pervading mica-rich lamprophyric, intrusive. Similarly, the imruding rock assimilated streaks of the melted coal. The coal thus flowed and tar was distilled during the process and per- meated as far into the zon~ of unaffected coal fractures and joint planes. The existence of tar is strengh~ned by the observation that permeations resembling it exist in the "coal" away from the seam-lamprophyre contact and are seen to follow the bedding, present network and occur along the fault plane. Their occurrence is related to the structural relationship of seam and intrusive; the frequency of occurrence will be more in beds affected by sills tkan in dykes. On cessation of the igneous activity, tl~,e molten igneous mass and tl'.e melted coal underwent rapid cooling, which is confirmed by their fine granular nature. The evolution of gases in the organic melt led to development of vacuoles at the contact and of vesicles away from it, and these were mostly occupied by the inorganic material derived from the intrusive rock. The flattened nature of the vacuoles is suggestive of the existence of tn'essme, more so vertical rather than lateral. Vitrinite is the main maoeral of the Jharia coals, which thus underwent maximum alteration and is represented by the fused mass showing marked anisotropism, mosaic structure and high reflcctivity. These optical characteristics decreased and diminished with distance. Fusinite remained inert throughout the metamorphic process. Sporinite and resinite dis- appeared as gases. Resinite is traceable in the third zone, which did not undergo the metamorphism that may lead to alteration. Resinite was, how- ever, affected and it underwent melting and migration to planes of weakness. Micrinite behaved in a way similar to semifusinile. During the process, a gradual transition from natural coke to unaffected coal existed and was related to the extent of thermal heat received by the seam. In natural coalifieation, all macerals have a practically identical composition at the end of the process (Dormans et aL, 1957). "First the differences between exinites and vitrinitcs disappear, next tho~e bc,ween vitrmites mid micrinites and finally those between fusinites and vitrinites. The composition of fminite is almost constant throughout the coalification range ", In artificial coke formation, according to Taylor (1957), "vitrinite is replesented Petrological Characteristics of Barakar Coal Seams 269 in coke by fused material to the extent of about 0,55 of the former vitrinite volume," semifusinite does not become plastic, and fusiniteand sclelotinite do not undergo contraction or physical alteration on a microscopic scale. It is thought that micrinite behaves like other inertinite macerals. The process involved alteration of substance and development of vcsicles. The natural coke formation is thus not too dissimilar wilh that of artificial coke formation. The natural coke resembles the artificially-prepared metallurgical coke, the size and nature of the vacuoles/vesicles varying signifi- cantly since volatiles could not escape in thermal metamorphism. Graphite is not found in the present study. The question arises about the rank of coal at the time of intrusion. It will appear that the "affected coal" zone is very thick and with acute folds in the metamorphosed portion of Dishergarh seam, Raniganj coalfield (Gartju and Pant, 1962). The seam volatilcs are 34 to 40 per cent. on a pure coal basis in the Dishergarh seam, while the variation is from 20 to 28 per cent. on pure coal basis in Jharia coal (see Whitaker, 1949). Thus ihe Jhafia coal was more matured than the Dishergarh coal at the time of intiusion. That the Jharia coal fused on thermal heat indicates its bituminous rank reached prior to intrusion.

CONCLUSIONS The thermally metamorphosed portions of the coal seams comprise three petrologically distinct, laterally persisting zones, i.e., natural coke, eoked coal, and. affected coal, away from the contact. These zones gradually merge into each other. Graphite is absent. The coal maeerals become distinct in the affected coal, which also shows evidence of resinite migration. The studies of the metamorphosdu por'tion indicate that, away frem the contact, there is (i) a gradual increase in the size of the anisolropic units from 1 to 50 microns, with decrease in anisotropism, reflectivity ard mo,~aie structure, and (ii) a decrease in the size and number of vacuoles and the proportion of mineral matter.

ACKNOWLEDGEMENTS The author expresses his deep gratitude to late P. N. Ganju, the then Professor and Head of th= Department of Geology at Aligarh, for rendering very valuable guidanee throughout; to Prof. N. L. Sharma, Professor of Geology, Irdian School of Mines and Applied Geology, Dhanbad, for examining certain micro-sections and offering useful suggestions; and to Dr. Gilbert H, Cady, Illinois State Geological S~rvey~ 270 H.S. PAREEK Urbana, for critical study of the typescript and expressing valuable eornments.

REFERENCES

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