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CHAPTER-I INTRODUCTION: ,Pakkanadu and its environs are comprised of rocks of late Pre-cambrian age that are characterised by migmatites, zoned pegmatites, syenites, ultramafics, pyroxenites and carbonatites. Till now, a few causal references and a report on radioactive carbonatite has been made by earlier workers and its detailed geology has been neglected. Therefore, the present investigation has been undertaken to present a detailed account of the geology around Pakkanadu in order to decipher its geological history. LOCATION: Pakkanadu (Lat.11 °41'55" and Long.77°49'30") is a small tillage in the Sankari taluk of Salem district of Tamil Nadu. The area under investigation is 20 square miles in extent and falls within the toposheet 58E/14 of the one inch to one mile of the Survey of India published in the year 1926-27. Its precise location is between Lat.11°41' and 11°45' N and Long. 770 48' and 77° 51' 30" E. and it is shown in Fig.1. GENERAL STATEMENT: The study area was mapped during the summer seasons from 1985 and 1986. The northern region was mapped from the field camps set up at Aduvapatti. The Southern region was mapped from the field camps set up at Pakkanadu. Due to the Fig.I SKETCH MAP SOING THE LOCATIONOFTHE STUDY AREA kt,'451 "PAKKANADU VILLAGE" SALEY DISTRICT, ....77.---........—.........................—............ (ii / Mulakkadu 1 1697, ' kr I \," „tranavakur N Q, It .a ‘■:, ) A 1, 1 Sangarpatiftlii 1 1 if I 1 1 1 \ \ \ ( I 1!)94 \ II 1 I li kkanadu R,F. . Aduvanatti/ 1115 I 1 1 / /' / Oaaall nagattul 10 ,,‘.. -,,,,..„....., A..., Kaniyampa. . ....,..... ), al (4.1 Kummiyahui 1 215 V up p a i a t t y / N‘ Palanalai R,R, 4? il \ it 4. 1 t; ,11ama'reMiyur i ALAYANDAFRA! Avadattur / / v., / •\' , 144 o' ‘ ...-,, Savaevur \N .1 fr...,..., 4 , ■■ .. 1, .......,:f ....," . . Ramakevandanur r47 , 730 14115 Kalarpatti presence of talus on the hill slopes and the soil cover on the plains, contacts between the rock typos are visible only in a few places. To trace the contacts between different lithologies, outcrop mapping technique was employed. When the outcrops between two different lithologies were within 5 metres, the boundary drawn between them was considered as an observed contact. In cases where outcrops of two different lithologies were more than 50 metres apart, the contact between them was regarded as inferred. Inspite,of soil cover and vegetation, the outcrops encountered in the study area provide wealth of information about the rock types. The geological map of the terrain around Pakkanadu was prepared on one inch to a mile scale by utilizing one inch to one mile toposheet as the base map. The geological map of the terrain around Pakkanadu is depicted in Fig.2 The conclusions put forth in the study area are the result of intensive field observations of several outcrops and the laboratory investigation of 80 thin sections. Chemical analysis reports from other sources have been used. PREVIOUS WORK: In 1971, Grady (U.N.Photogeologist, Project for Mineral Development in Tamil Nadu) dealt on "Deep Main Faults in South India". The study comprised photogeological investigations combined with the results of an airborne geophysical MAP SHOWING 1HE GEOLOGY AROUND PArKANADU AND MUL. AK KADU , SALEM DIST . TAMIL NADU 58[/1I NO TOI-OSNE ET ., P. SCALE 1 0 4, ■ . .■ ro.• 4 ) 2 1 1 , t..,4,.... .4 . Ot , 4 P ...) II. < Lk . I • .1 ' '..-: 1 .. • / ' 2.;7•, . ' ' IVA\ •, . '• • / .,, • A .. / •, • . :-....,..;SA•4 A R P A 1 i I /1 ' );'• '," -••••':-'3.'/[--- • -',.._. 1 . / . '.„ • _ ' . ! 1• . • •••, . 3.2, . •0 ,, \ , • . . •„f45 • / / . ILI ' 1 I- ' 14 • < 2 .... / ...0, 4.//,. t;' / i MN A NC-"AT T UP - - P11/65-A IC 7 1"'''' . '42( ANNIT• • PAT TI , . - . .7 . /I , ' fra, "I PAKKANADU u i., , ,i '' .'" ' .1 'JAL Ax•NTAPuRA m nil 1 r . .. . ,..!A0IYUly 1 1044 . rII ( II ' r , / II , ....- . .... • _KU QUMA_FIUR --f ''' '' -r4 ' / P . Flk.M.AKAVANOANUP • • I27 S . • yr: , • / ' t• ..c) . -J - Pur.c....PA L h• '.- . . . • II., F I/ 4 .../ :17 .,.. ' ' ' .. ' / / , • . . „ , ... iv. . - • ' . k r.'-, If _ •'? ..,2 ...7 /\_, 40' 77. SO' : --.V-- 1777-7, •••• ROxiNilt D: I. " ■.! .1 Ir /47. ! '-4-4 A.. C•1390NA1IT( C OMPL E X ; Ul. TPA i BASICS FOLIATION I DUNITE) r 1 i ---1 _4_2J w• li II 0 l■ i ' ‘,... .. II i i) ■(•,. . ' o • 20 ,2 • ,„,,to„,,,,,,I, ,., ... , ich . .... .,i. . yilli 10 ‘: , ,,,,,, 16'.... /.' 11 ii :% 7 -...-..)-7T - , . / Lk A.P.P AOU I A , // 1 In -,_ . • l• (-7 / , / .- • " •.' • • '.1599 . '''l • 2(2 Z / / , „t_....,• --T , ----, , ... 21 i':\i.—\ , ."— 4VANiv_, A.1)IYUR{ . / • / . / A ' s._ ... ' / .: * • ...--, I 4 / / A111 •\ Pf Fiv•v•NAVASi c . , , , ... " 5 / / CONTACTS SlENITE ,- --' ."....,,,E „ ...,,„ I DE EP ( APPROX OBAWN) CRUSTAL FAULTS ..II CM•711 E 5 Fig.2. (After Suryanarayana Rao et al, 1978) survey and field work(Fig.3). Ho has described a system of d OD main faults containing carbonatite complexes and ultrabasics in South India. He regards that the Main Fault at 45°E is made evident by a northeast-southwest alignment of carbonatite complexes, ultrabasic to alkaline intrusions, zones of metasomatic alteration, pegmatite, quartz veins, brecciation and recent seismic activity, Regarding Pakkanadu carbonatite he opines that 1. It is associated with zones of alteration of gneisses and ultrabasic intrusions. 2. The intrusions occur near the junction of the N 45° E faults with the strong Evil. Godumalai shear zone. 3. Between Pakkanadu and Salem occur 20 ultrabasic intrusions including dunite, one of them is at Chalk hills near Salem and contains magnesite. Vemban et al (1971) points out that 1. The main fault at 45°E compares well with those of G.S.I. maps. Fault 'C' of Grady compares well with Fault No.20 as shown in G.S.I. map. 2. Godumalai shear zone compares with that of the SalemAttur Fault, F-3 as shown in G.S.I. map (Fig.4). Dealing on Pre-cambrian carbonatites of Tamil Nadu, South India, Borodin et al (1971) states that both geological setting and petrological features support the view that the Tamil Nadu carbonatite province differs in many respects from Narmada valley province in India as from other carbonatito provinces all over the world. Li • • 7k; 6•Ca./. 00. • 'ILLY •••:;;- c•■••aamt (SP 30. • law.. • ■•••••• ikevy•mc.. l WORATTI IARBONATITE 001•1•AY.0 —J ■••• no.% 46160Z:7. • •••••• •zt ••••••% • I •••••••• OVA ON■l 10•••• •••••• •••• v•■■••■1% ••••••••11 ••••••••11 SHFAR ZONE / •AL.' / / .1•••••it Va.. twaw.•• lot ovum. soy es •••■•••••• / • ., .„ _ . 4, / •:. •••••• / 10•11101....1 ........... { . r .1 .11.... I... .1101”.• ........ oary 00 0.••■•••• I J0•1 E RV FAULT l014,600 03•0•al; •• tang.. 00•C, CO•rnE3 BETWEEN NE SW FAULT ZONES INTRUSIONS AND MINERAL DEPOSITS SOUTH INDIA RELATIONSHIP • el ay ed 7§1d - Figure • .• 3 "037,F,P r.A.IN FAULTS IN SOUTTT TNDIA" ( After Grady, 1971) • / MAJOR FAULT SYSTEMS IN TAM IL NAuU (i,f Lei Vernb44n ct al, 1171) • I\ 0 c" SCALE In AjkeOETAI1 10 0 ,•e; y, 4. 3, I" , 0.1 Y1/4 P. / -• ......• (.....-- • MAD3iAb. f•••1,••• . .1 i4/ 7 ,,%://s/v E Aoloo.7 '. 1 / 14/ 4 col;L I if•7/, A ! • *44 /4 / ,. / / ,1 „,v fI „lA- ” .d ic // c., %Ilk / s`rf -%' k /I . / /Y / 11 kf / d".. .J • ■/ / / , / .1/ , c,2 /.1•••1i.... ." j... 1 4i f./ // % •• 5 Vit , ; MET131)/ •••••••1 V I ' 4..1ALE101 ri4.0. / ••■ 1 1 . / , .....M.;..., I '''''' K sat ri • ■1"/ / U9 4116, lii 4‘k• I 1 .ru// A 1-----I \ ..../ 4 / :....4/1 1 ./ 4.7 „I II / / 7.6•• 44.1C. aN ..............„ •••■•• ...... • 4 t r -,S contioofic ,. Q _R, . / 46 ,0 ” _,7• ---- ,__ G a vva.t o .I s; .••••...N -.... 401 1,41,17,.t-V' .A . - • herr--(,1-o ire r I„k/1.i / %' lir,\ . .• . 11PUNCHIRAppAL 7i ,/ .. // • oil// 1A‘ • • 1 v1 4 .• - 7 ) s •VI. ..... 1 i 04‘ ,4.9A. Al 0. ‘..,,,,,, / w,..39- .....$%, - •••■• .1- .e.■•••• • \i N * / 4 -• .1 • • d / 1 ily il'i i f / fls.. fil i 4$/.., led • •'. . :...,'• • 41,•■ • (1,...s .i ii , 4 411 • 1 r•-.1.or'lf • --Ar ..... I,. „A. ,14 • • Z. I cc co •/.:" .-,..1.4 4, 4411-1,k4;;,•,,,• I- ■ . PottP ruth..,NO-f- • ' .. '' • ,._ 4./ kt\ ' • ,447 • NO; / v . •0/ i • ' 47 /..4* ..-- '4 1 \ \ S• + 1% 4' • 1, t .• . rI ••• •• ••••• •r• 64,\ .■ N. . • • LEGEND •••••••■•■■• ■■■•■••■■•••■•■■••••■••■■■••■•■•■••01yay■■•■• • • The location of the Pre-Cambrian carbonatites occuring in Tamil Nadu are shown in Fig.5. The carbonatites at Koratti was initially considered as a band of crystalline limestone with plenty of magnetite and apatite crystals, adjacent to the vermiculite bearing pyroxenites, which were prospected for vermiculite in 1965-66. In 1967, Deans inspected the carbonate rocks of Koratti and confirmed them as really belonging to the carbonatite group. The samples collected by him were analysed and trace element contents in the carbonatites were found to be readily distinguished and different from the local limestones. It was pointed out by Deans and Powell (1968) that all the elements reported are much more abundant in the carbonatites; butcertain of them, notably barium and niobium, are rather capricious in their abundance. The high abundance of strontium and phosphorous is perhaps more striking but the persistence of lanthenum and cerium is equally significant. The strontium isotope ratios of Koratti carbonatites (Sevathur carbonatites according to Deans and Powell) were determined to be lower than of Tamil Nadu crystalline limestones and hence to be characteristic of carbonatites. K/Ar dating of biotite in biotite pyroxenites adjacent to Koratti carbonatites was carried out by Deans and Powell (1968). An age of 720± 30 million years (Upper Pre-cambrian) for Koratti carbonatites has been suggested by them. During 1968, Semenov, U.N.D.P. petrologist, discovered the 7 0° 76° 0° ANDRA PRADESH MYSORE MADRAS . tANCALIME 0 OVELLORE .1 '2.4 3••5 12 •64 ' *SALEM .TAMIL N A DU V` . . 0 MIOURAI — TRIVANDRUM CEY LON 7 lees. r • I 7 as 7 s° MLLE 0 tOO 200 Figure, 5 Location map of carbonatitc occurrences in Tamil Nadu. L Koratti, 2. Jogipatti, 3. Reddipatti, 4. Karapattu, 5. Pallasulakkarai, 6. Paklcanadu. (After Borodin et al, 1971) presence of pyrochlore minoralization in the Koratti carbonatites (Borodin 1971). In 1969, zones of pyrochlore mineralization were identified by Borodin. The various types of analytical work on minerals were carried out by Borodin in the Institute of Mineralogy, Geochemistry and Crystal chemistry of Rare Elements, Moscow under U.N.D.P. programme. Borodin classified the carbonatites into several types (Table. I) and determined them as para-ankeritic (also known as beforsites) and calcitic (known as sovites). Taking into consideration all possible variations and occurrences of carbonatites,,Boradin has proposed a genetic classification of the carbonatites in general. The carbonatites of Tamil Nadu can be assigned to the IV type associated with alkali syenite and granosyenites. However the dolomitic nature and presence of uraniferous pyrochlore are rather strong in carbonatites of Koratti, (Borodin 1971). Though Borodin 1971 has not done detailed study on carbonatites of Pakkanadu, from his preliminery field and laboratory investigations he observes that, 1. The carbonatite located at Pakkanadu is away from all the other five occurrences which form a definite carbonatite area (Refer Fig.5). 2. Even though the carbonate rocks of Pakkanadu are very different from these of Koratti and Jogipatti, the possibility of their being some kind of carbonatite cannot be ruled out. 3. The carbonate rock and associated rocks have suffered regional tectonic activity and minor folding can be seen. 4. Carbonatite rocks are rich in monazite and barite. 5. The associated albitite or oligoclasite rocks contain plenty of sphene and allanite which are not common to carbonatite complexes. From the above observations he concludes that the carbonate rocks of Pakkanadu can represent some late phase of hydrothermal or carbonatite activity. Reporting on the major and trace element compositions of the Sevathur carbonatites, Krishnamurthy (1977) shows that there is an enrichment of total iron, MgO, MnO, P205 and Na20 with successively' younger phases within the main carbonatite body and such a feature is comparable with the liquid lines of descent observed in synthetic carbonatite systems. Further fenitisation of the country rocks (granitic gneisses) along the southern border of the carbonatite body is indicted from the enrichment of K20, Na20, A1203 and impoverishment in Si02, total iron and MgO in the fenites when compared with the country rocks, Krishnamurthy further observes that 1. Sr (4109-9375, mean 6427) exceeds Ba (1000-2420 mean 1663) in all types of carbonatites of Sevathur. 2. La and Ce are enriched in the calcite types when compared to the beforsites. 3. Beforsites are generally rich in Ba, Nb, Ta and Se. 4. Early calcites are much enriched in Sr (5500-6200 ppm). TABLE . (After 9orodin et a', 1 077: VI GENETIC CLASSIFICATION OF CARBONATITES (SUBVOLCANIC AND DEEP-SEATED COMPLEXES) I. Alkali-ultrabasic rocks and nepheline-syenites Genetic features of carbonatites typomorphic elements rare-metal composition minerals and of carbona- examples of tites deposits 11. Alkali-gabbroids and nepheline-syenites rare-metal composition minerals and of carbonaexamples of tiles deposits III. Nepheline—and feldspathoid- syenites rare-metal composition minerals and of carbona- examples of tites deposits Stage mode of formation High temperature Intrusive bodies (stocks, dikes) and related metasomatic zones in rocks of any composition Ca, Mg Ti, Nb Ta, Zr U, Th CO2 P205 Calcite, Dolomite Uranpyrochlore (` hatchettolite') pyrochlore, dysanalyte, baddeleyite, niobozirconolite, calzirtite, thorianite —Sove (Norway) Alno (Sweden), Iron Hill (USA) Shava, Dorovo, Spitskop (Africa), Siberia (USSR) Calcite, Pyrochlore, dysanalyteKaiserstuhl (W. Germany) Stjorna (Norway), East Siberia (USSR) Mediumtemperature ( (-390 500) Injection metasomatic zones and vein fillings in rocks of any composition Ca, Mg Ti, Nb REE, TO U CO2 P20, SiO2 Calcite, dolomite, ankerite, feldsparcarbonate Pyrochlore, dysanalyte, rutile, monazite, zircon— Magnet Cove, Iron Hill (USA), Sove (Norway), Kola Peninsula (USSR) Calcite, dolomite, feldsparcarbonate Pyrochlore, monazite burbankite— Kaiserstuhl (W. Germany) Rocky Boy, Mountain Pass (USA) Calcite apatitecalcite, feldsparcalcite, dolomite Pyrochlore, zircon, cerianiteLueshe (Africa) Nemegosenda (Canada), Urals (USSR) Lowtemperature Metasomatic zones in early carbonatites and vein fillings in rocks of any composition Mg, Fe Ca, Ba Sr, REE Th CO / SO4 Si0/ F, S Ankerite, dolomite, siderite baryte carbonate, zeolitecarbonate Burbankite, ancylite, monazite, REE Fluorocarbonates, columbite, fersmite, zircon—Iron Hill (USA) Kola Peninsula (USSR) Dolomite, ankerite, siderite, barytecarbonate, fluorite barytecarbonate Bastnasitc and other REE carbonates, thorite, cerite, orthite, zircon—Mountain Pass (USA) Barytecarbonate fluoritecarbonate siderite REE carbonates and fluorocarbonates—Bearpaw Mountain (USA); Kalkfeld (Africa); Urals (USSR) 500700) 100 300) Calcite IV. Alkali syenites and granosyenites composition of carbonatites rare-metal minerals and examples of deposits Pyrochlore, bad deleyite, lueshiteLueshe, Kalkfeld (Africa) Gronnedal (Greenland) Feldsparcarbonate, fluoritecarbonate fluoritebarytecarbonate (siderite and others), hematitesiderite Columbite. ilmenite, rutile, thorite, xenotime, monazite, REE fluorocarbonates— Siberia, Ukraine (USSR) Karonga (Africa) C 14; Srinivasan (1977) while comparing the carbonatite complex (pyroxenite-syenite-carbonatite) of Hogenakal area of Tamil Nadu with the adjoining parts of Karnataka states that both of them were emplrced along rim-ssw fracture zones within the Pre-cambrian gneissic complex. He compares the carbonatite of Hogenakal with Carbonatites of Koratti, Pakkanadu and Strangways Range of Australia (Table-II). He points out that the Hogenakal carbonatite can be distinguished from metamorphosed limestone by its genetic connection with crustal fractures having been emplaced along deep fracture zones. The events relating to the emplacement of the carbonatite are: i) Faulting. ii) Emplacements of a) pyroxenite, b) syenite and c) carbonatites and iii) Metasomatism (Fenitisation) (Ramakrishnan et al 1973). According to Krishnamurthy (1971) carbonatites of Hogenakal differs from carbonatites of Pakkanadu in many respect. He observes that the carbonatites of Hogenal:al has certain similarity with that of Strangways Range area of central Australia and differs in certain respects from other carbonatite occurrences of Tamil Nadu. While surveying the previous work conducted at Pakkanadu and Mullakkadu(5 Km.North of Pakkanadu) Suryanarayana Rao et al (1978) states that TABLE TI. CHARACTERISTICS OF HOGENAKAL CARBONAT1TES COMPARED WITH CARBONATITES OF KORATTI, PAKKANADU AND STR A.NGWAYS RANGE Tamil Nadu (India) Carbonatite Province Hogenakal Koratti • Australia Pakkanadu • Strangways Range •• I. Only calcite sovite Sovite, para ankeritic and dolomitic carbonatite (beforsite) Dolomite and calcite sovite Calcite and dolomitic carbonatites 2. Long sinuous lenses within the linear bands (dykes) of mixed rock pyroxenite and syenite Arcuate (wider) outcrops associated with syenites and pyroxenites Arcuate outcrops associated with albitite or oligoclasite Lenses within linear bands of mixed rock of pyroxenite and syenite 3. Rich in apatite, less so in magnetite (apatite fluoroapatite ; magnetite—rich in Ti, V & P) Rich in apatite, magnetite, pyrochlore (apatite= fluorapatite ; magnetite rich in Ti, V & P) Rich in monazite and baryte Rich in apatite, magnetite, zircon and pseudomorphs of columbite after pyroch lore 4. Little or no fenitization (development of biotite seen from pyroxenite due to syenite intrusion and limited further formation of phlogopite/ vermiculite due to carbonatite activity) Little fenitization but strong development of biotite, phlogopite/ vermiculite in pyroxenite due to the intrusion of both syenite and carbonatite Not studied Little or no fenitization 5. Associated with NNESSW faults. Associated with NE-SW faults Associated with NE-SW faults Data not available (however, the alignment of outcrops is reported to be in a north-easterly (NE-sw) direction) • after Borodin, etal.. (1971); ** " after Crohn and Gellately (1968) (After Srinivasan, 1977) 1. Radiometric surveys were carried out by Rama Rao, in 1959 and allanitc and monazite bearing pyroxenites and calcsilicate rocks are associated with ferruginous stained limestone to the ':lest of Pakkanadu. 2. In 1970, examination of the ,microsections of the cores of calcite-biotite rocks from depths of 40 to 60 metres from bore holes Nos.BXN-3,5 and 7 brought to light accessory minerals like ppatite, barite, strontianite, celestite, ankerite, siderite, sphene and biotite, besides minute opaque rectangular grains enclosed in biotite flakes. 3. Paper chromatographic tests on these biotites indicated the presence of Nb. 4. During 1972-73 Krishnaiah Setty and Suryanarayana Rao while looking for niobium and tantalum bearing minerals in this region came across outcrops of radioactive pyroxenite carbonatite bands emplaced along a fault zone within syenitic rocks one kilometre to the West of Mullakkadu. 5. Though the carbonatites occur near the N 450 E major crustal fault of Grady (1971) on a closer examination of Pakkanadu and Mullakkadu areas, the pyroxenite and carbonatite complex is found emplaced along a .N 30° E trending subsidiary fault to the west of the main fault (Fig,'). 6. Carbonatites of Pakkanadu and Mullakkadu are associated with pyroxenites and occur as discontinous lenticular bodies intruding the syenites. 7. They strike north east to southwest with easterly dips. 8. Large outcrops of serpentinised dunites carrying accessory chromite, but by asbestos and magnesite veins are also found along the same fault zone near Pakkanadu. 9. The carbonatites are of the pure calcite rich sovite type with biotitic and ankeritic variants, 10. Apatite, magnetite, allanite, barite, monazite, zircon and cerianite are the accessory minerals. 11. Spectrographic data suggest that the carbonatite pyroxenite complex is Ce-La rich. 12. Nickel is not found in economic concentration. 13. The general radioactivity ranges from 5 to 20 x 8G. 14. Whole rock sample 1 assay e U308 from 0.02% to 0.054% in carbonatites and from 0.02% to 0.06% in pyroxenites. 15. Allanites assay e U308 with chemical U308 from 0.007% to 0.034% thereby indicating the predominance of thorium over uranium. 16. Monazite is mostly concentrated in the biotite-rich portions of the carbonatites to the west of Pakkanadu. 17. Allanite predominates in the pyroxenites west of Mullakkadu. Intensive study on the rare earth beaing minerals in the carbonatites of Koratti, Samalpatti and Pakkanadu was done by Semenov at al 1978 at the Institute of Mineralogy, Geochemistry and crystal chemistry of Rare earths, Moscow. They conclude that, 47 1. The alkaline complexes with carbonatite activity are enriched in niobium bearing minerals like pyrochlore, fergusonite, eschynite and Fe/Nb rutile. 2. Eschynite (CaCeNyi012) occurs as brown grains with pitchy lustre in association with baryte and allanite in carbonatites of Pakkanadu. 3. Generally the light lanthonaus (LREE) are predominent over heavier ones (HREE). 4. The pyroxenites contain relatively very small portion of rare earth minerals. The other relevant reference on rocks similar to the study area will be cited in the proper context. METHODS OF STUDY: References to Air-Photos on 1:50,000 scale were made at P.W.D. Water Institute Building, Taramani, Madras. to decipher the major structures and to trace the contacts between different lithologies. The geomorphic units, vegetation and water bodies of the area under investtgation were deciphered by examining the Landsat imageries of different spectral bands (available at P.W.D. Water Institute Building, Taramani, Madras.) The rock slices were examined using polarizing microscopes, Carl Zeiss and Getner model. Refractive Index of minerals was determined on grains of -80 mesh size, seperated in bromoform and Clerci's solution and the refractive index of matching liauids were read on Leitz Refractometer. The modal composition of rocks was determined the swift point counter with a spacing of third of a millimetre following the method of Chayes (1949). About 6000 points uere counted to approximte the actual volumetric composition. Published analytical reports of important rock types of the area under invest&gation have been used for interpretation.