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 appear- ance 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 g -- ,Ii ~ >~ % ,~ • ," , I, x ~-~ I :!':: $/S..~C~ , . ~ . Ii ~i ~ : <"",, ~ ~ ~i:::~:~7~ ,< i ,< ,//~',:/ ~ ~;, ,J " <.>'~= :.<~:~ ~ "--'~o.4,. ' i,, .I x *' o~ o~l , .'- ii|~i x : .. i O ~ .~ ~ o u; .~ = o c ~11 .I ...... #.. ...... ~ " - "~ ,- < : ~ m ~ n r~ r:i ', , ,t 7717 : : : /J: : : : : :M \.,<. I d ~ Ill I~I I_3 ~ i,l , l : :; 'l ,i ..... ,., ....... .~ ~ i: .I...,,~ (;51..~ .......... .# ~ ,< • ~' ,~ ,,-4, ~/!~ ;'.7//.~ : :: :1 1" " F, 7 • l~.h ~ ~ " =.,I,,,"~_,," • • 4 ~.'l, " .-i • # * . , ~! :S = % ,, ' ~ " " ~k ri .:I ~ " < < "~-,'1!, ~71~;,~ ." ./." "#~'L" ""~'~¢- o .~.: ~:~, ,, * "~ ~ ~ :~.. ~///,x.,-/. : .... 1" " # "~l" ..... ~ _~ /,~ .... 1,,< ,< x I V ~ • - it • I. tr/~. ..... il#" "1 ....... " ...... ~ ,,, . 7.: .,.'~,.,,~ ..... ,,.../. ~ I~~:: : :: :l./z °~ ,, ,<,<,::,:'//,,'...~,::...,f//_:,,,,..,:::~.,:#: :,'¢:..::.,:. I ;x'-,'~.~.f~'::.".::~/'~: ./.~t, ........ /,<., ,t;,'.,.';~,'~,:- r..:'?,'.':~, 7:\ ,~.::.:~',;" ..r;/.I"." "l/~~ ""~- '~ "~'~ Z,............ "l.',~,~i,.~, ",/~'..~;,'.,,-- ~''. ......-.:. "l:" ~ , • .# .. ,, ........ ~.~1~,'. 7 ,,.-,, ....... ,, ,, ,< ,~ , f I ~ "> . # i . L. iillii ~:. , ¢ ::%-':'-11~ ~I: :_/~ Jl~_ ~1 :y .1/I ~ ~<:~ ..'/////~/~..:.. ... :,: ;;- ;'//,G : :~ h~t'/t • k. :Y:;"~:/:/ ! i<~ "////.d"/////J'/'~:::: .. ,~ ~1- . ~1 ,. , 264 H. S. PAREEK 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 indi- cates 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 contrac- tion 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 trans- mitted 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.