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Miscellaneous Publication

No. 30, Part XIX – SIKKIM

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Published by order of the Government of India2012

© Govt. of India

Controller of Publication

ISSN 0579-4706

PGSI. 340

700-2011 (DSK-II)

© INDIA, GEOLOGICAL SURVEY (2012)

Compiled bythe officers of Sikkim Unit, Operation: WSA

Processing and editing of the manuscript byS/Sri Subhra Suchi Sarkar, Md. Amjad Ali and Smt. Gargi Bhattacharya

Senior Geologists

under the supervision ofS/Sri Pradip De , Bikash Chandra Roy and K.V.Nambiar

Directors, Publication Division,Geological Survey of India, Eastern Region

andunder the overall supervision of

Shri I. K. Khan,Deputy Director General

Geological Survey of India, Eastern Region

Price:

Rs. l30 $ 7 £ 4

Published by the Director General, GSI, 27 J. L. Nehru Rd. Kolkata 700016 and printed atM/s Arunima Printing Works, 81 Simla Street, Kolkata 700006, Phone: 91-33-22411006, E-mail: [email protected]

iii

Foreword

Adetailed account of Geology and Mineral resources of Sikkim has beenprepared in order to publish the volume in the form of MiscellaneousPublication 30 series for the first time.

The present write up gives a summarized account of information on the geology,structure and mineral resources to be used by earth science fraternity, administrators,entrepreneurs and also common man.

The climate of Sikkim is favourable for medicinal herbs and plants. The temperatureof the state ranges between 4°-35°C in lower altitudes and 1°-25° C in higher altitudes.The state houses one National Park and five wild life sanctuaries. All these factorscontribute to the setting up of tourism and pharmaceutical industries in years tocome.

The Himalayas in Sikkim has been divided into linear geotectonic belts with distinctgeological characteristics. Beginning from south these are Sub-Himalayas, LesserHimalayas, Trans Himalayas and Higher Himalayas. The Main Boundary Thrustseparates the Siwaliks of the sub-Himalayas domain from the Lesser HimalayanBelt and the Main Central Thrust separates the lesser Himalayas from HigherHimalayas.

The rocks of the area ranges in age from Proterozoic (undifferentiated & Meso) toOrdovician followed by Permo-Carboniferous Gondowana rocks overlain by Triassicsequence of sandstone, shale and limestone and culminating in Upper Pleistocene-Holocene (variegated clay, fine to medium sand and pebble bed) sediments. Due tohighly deformed nature of rocks the area is susceptible to landslides and the recentearthquake of September, 2011 has warranted a detailed study of structural data ofthe terrain for taking up remedial measures and also for handling various geotechnicalprojects.

The state is endowed with rich mineral resources. This includes asbestos, basemetals, coal, dolomite, graphite, limestone, marble, sillimanite, talc, topaz, tungsten &vermiculite. The details of individual mineral regarding the mode of occurrences,grade and reserves has been dealt in depth in the manuscript.

KolkataDated, 14th December, 2011

vii

CONTENTS

I INTRODUCTION 1

Climate 1Agriculture 1Fauna and flora 1Physiography and drainage 2Lakes of Sikkim 3Glaciers of Sikkim 4Hot Springs 5

II GENERAL GEOLOGY 6

Stratigraphic classification 6CENTRAL CRYSTALLINES 6

Chungthang FormationKanchenjunga Gneiss/Darjeeling Gneiss (Undifferenciated) 7

Banded / Streaky gneisses / Migmatites 7Augen bearing biotite gneiss with//without Garnet, Kyanite, Sillimanite 8Sillimanite Granite Gneiss 8

DALING GROUP 11Gorubathan Formation 12Reyang Formation 13Buxa Formation 13

LINGTSE GRANITE-GNEISS 15MOUNT EVEREST PELITIC FORMATION 16MOUNT EVEREST LIMESTONE FORMATION 16GONDWANA SUPERGROUP 17

Rangit pebble slate 17Damuda Formation 18Lachi Formation 19Chho Lhamo (TSO Lhamo) Formation 21Intrusive Granites 22Syenitic Rocks 23Sesela Formation 24

viii

STRUCTURE AND TECTONICS 25METAMORPHISM 28SEISMICITY AND EARTHQUAKES 30

III MINERAL RESOURCES 32

Asbestos 32Base metals 32Lead 40Calcareous Tuffa 40Coal 40Dolomite 42Graphite 43Kyanite 44Limestone 44Marble 45Silica and silica sand 46Sillimanite 47Talc 47Topaz 47Tungsten 47Uranium mineralization 47Vermiculite 47

IV LOCALITY INDEX 48

V REFERENCE 48

MISC. PUB. NO. 30(XIX) 1

I. Introduction

Sikkim or Sikhim is a name derived from theSanskrit word Shikhim meaning ‘Crested’. In all ofSikkim there is not a single kilometer of ‘flat land’. Withan area of 7096 sq km, measuring 113 km from north tosouth and 64 km from west to east, the state haselevations ranging from 240 to 8484 meters above meansea level. The State of Sikkim is located in the southernmountain ranges of Eastern Himalayas between NorthernLatitudes 27°05and 28°08 and Eastern Longitudes88°10 and 88° 55. Sikkim is the 22nd State of the IndianUnion, which came into existence with effect from 16th

May; 1975. Sikkim is bounded by Nepal in the west,Bhutan in the east, Tibet in the north and state of WestBengal in the south. The state has been divided into fourdistricts, viz. East, West, North and South districts andeach district has further been subdivided into two sub-divisions for administrative purposes.

Climate :

The climate of the State has been roughly dividedinto the tropical, temperate and alpine zone. For mostpart of the year, the climate is cold and humid. Mostpart of the state receives heavy rainfall throughout theyear. It is only in the month of October-March that theState remains comparatively drier. The extreme northernparts, adjoining Tibet, however, receive very littlerainfall. The mean annual rainfall is minimum at Thangu(82 mm) and maximum at Gangtok (3494 mm). Anisohyatal analysis of these data reveals that there aretwo maximum rainfall areas (i) South-east quadrantincluding Mangan, Singhik, Dikchu, Gangtok, Rongli,etc. (ii) South-west corner including hilly terrain. Inbetween these two regions, there is a low rainfall area(around Namchi). There is an area in north-west Sikkimwhich receives less than 4.9 mm of rainfall. Rainfall isheavy and well distributed during the months from Mayto early October. July is the wettest month in most ofthe places.

Places with an altitude of 6065 m and above aresnowbound and places as low as 3002m come within thesnowline in the winter. The temperature in the loweraltitudes fluctuates between 4°-35°C and places withmoderate height (around 1829 m) such as Gangtoktemperature varies between 1°C and 25°C. In the highaltitude area (above 3993 m), the temperature never risesabove 15°C and slides down to the freezing point in winter.

Agriculture

Agriculture accounts for one third of Sikkim’s grossdomestic product. More than 64% of the populationdepends on agriculture for livelihood. Products such asrice, wheat, maize, finger-millet, barley, buckwheat, andpulses like ricebean, rajmah, fieldpea, cowpea andclusterbeans and oil seeds like rapseed, mustard,soyabean and sunflower are grown in the area. Of latethere is a thrust on planting medicinal herbs, whichabound the natural vegetation. The State is reported tohave more than 424 species of the medicinal plants.

Fauna and flora

Sikkim is enriched in forest resources. The total areaunder the administrative control of Forest, Environmentand Wild Life Department is 5765.10 sq km whichaccounts for 81.24% of the total geographical area of thestate. The State has one national park (KangchenjungaNational Park) and five wildlife sanctuaries. Names ofprincipal endangered species of Sikkim are bharal,clouded leopard, fishing cat, golden cat, himalayan thar,leopard cat, red panda, marbled cat, musk deer, nayan ofgreat Tibetan sheep, pangolin, serow, snow leopard,spotted lingsang, Tibetan antelope, Tebetan Fox, Tibetangazzelle, Tibetan wild ass, tiger, Tibetan wolf. Importantbirds are black necked crane (migratory), blood pheasant,peafowl, Tibetan snow cock, tragopan pheasant, snowpartridge, and siberian crane (migratory).

1

2 GEOL. SURV. IND

The dominant flora in temperate zone is oak, cuherry,alurel, chestnut, maple, birch and rhododendron and thedominant flora in the southern part of Sikkim is plantains,bamboos, tree ferns, walnut, sal and oak. Cimbidiums,vanda, cattaleya, hookeriana, farmeri, dendrobiun-amoenum and noble orchid are the most popular flora.

Physiography and drainage

The State encompasses parts of Lesser Himalaya,Higher Himalaya, and the Trans Himalaya and hostssome of the highest mountain peaks of the Himalaya.The elevation ranges from 300 to 8586 m with increasingelevation from south to north. In Sikkim-DarjeelingHimalaya, the Lesser Himalaya starts from Kalijhora(in Darjeeling) in south/west and continues up to Singhik(in Sikkim) in north/east. The low grade metamorphicrocks of Lesser Himalaya start at as low as 300 m andcontinue up to 3050 m (Mount Mainak).

In Sikkim exposures of Higher Himalayan rocks startat about 650 m and continue upto 8586 m (MountKangchenjunga). The outcrops of gneisses with graniticintrusions exhibit sharp, rugged, snow bound mountainswith steep in accessible scarp faces. Extending in theform of an east-west trending ridge, it forms a barrierbetween the Trans-Himalayan zone and the LesserHimalayas with Kanchenjunga as its western limit andLama Angdang as the eastern limit and a series of highpeaks, deep valleys, gorges and numerous glaciers inbetween. Langbo Chhu, Naku Chhu and Chhombo Chhuform main drainage flow through flat bottomed ‘U’shaped valleys and finally meet with Tista River.

The Trans Himalaya starts from 5300 m and continueupto 7000 m. The snow capped jagged ridges in thenorthern portion of the state are feeders to the glacierswhich come down to about 4000 m. The topography ofthe northern part, where rocks are of sedimentary origin,is characterized by barren, reddish-brown mounds andhillocks, with broad, flat bottomed valleys so typical ofTibetan plateau. These hillocks are seen protruding outas fingers, from the main Himalayan ranges, gentlymerging with broad plains of Tibetan plateau. The rivervalleys are open towards the top and attain a steep gorgelike character towards the beds of river.

In Sikkim-Darjeeling Himalaya the first hill rangesstarting from south are having summit between 1158 mto 732 m and runs east west. In the western part, theheight increases very sharply to 2592 m within 20 km

crow fly distance towards north. Siwaliks starts at 153m and ends at 823 m. Dalings continues up to 1220 mand then Darjeeling Gneiss starts. On the eastern sideSiwalik ends at 732 m, Dalings at 1376 m. At the centralpart of the area, i.e. in Tista Dome the height is in generalbetween 1200 to 2500 m though the summit point is on3233 m. As soon as the Daling rocks are crossed theheight increases sharply from average 1800 m to 3000m in the lower reaches, 3000 m to 4500 m in the middlereaches and 4500 m to 6700 m in the upper reaches ofthe dome. The rocks north of South Tibetan DetachmentSystem(STDS) are in general occurring above 4200 mto 7300 m.

The Tista Rangit water divide is the main N-S waterdivide within the Sikkim-Darjeeling Himalaya. Thiswater divide takes an E-W swing after crossing theDaling Dome. The N-S stretch culminates at 3700 mand the E-W trend shows sharp rise in altitude from 3700to 7300 m. The west bound water divide is having thesummit point at famous Kangchenjunga peak. AnotherN-S water divide is between Lachen Chu and LachungChu which starts at about 3000 m and slowly increasesup to 6700 m.

The other water divide in general trends E W. Theheight of the water dividing ridges increases slowly from1100 m to 1500 m, 2100 m, 3000 m, 3600 m, 6700 mtowards north. These ridges are lower in height in centralportion and height increases both towards eastern andwestern side. Sharp increase in height has been observedafter crossing the Daling Dome. On the eastern side ofTista valley water divide of Rilli, Rongpo, Rani Chu,and Dickchu are spectacular. The ranges south of Dikchubeing the longest one known as Gangtok ridge as theGangtok city is on this ridge. The most important featureis that these ridges continue crossing the Tista River tothe Rangit - Tista water divide. The ridge portions whichcrosses the Tista River are very steep.

North of Dickchu, steep ridges on the eastern sideare there but of smaller extent as the Tista valley aresubdivided by the water divide between Lachen Chu andLachung Chu. On the western part long ridges arepresent. These are Talang Chu and Zemu Chu waterdivide.

Thus, the total relief picture of Sikkim-DarjeelingHimalaya is such that the northern part is always above3300 m to 7000 m forming a part of Tibetan Plateau.


The middle part slowly decreases its height towardssouth and central part giving rise to an amphitheater typerelief feature. The amphitheatre follows the trend ofMCT indicating that the Higher Himalaya is upliftedrelative to the Lower Himalaya. The important Mountainpeaks of Sikkim are: Mt. Kanchenjunga (8559.42 m),Mt. Kabru (7361.36 m), Mt. Talung (7356.8m), Mt.Siniolchu (6870.4 m), Mt. Simovo (6832.7 m), Mt.Pandim (6718.4 m), Mt. Rathong (6718.4 m), Mt.Paunhri (6688m), Mt. Kokthang (6129.2 m), Mt.Lamaongden (5887.26 m) and Mt. Masunyange (5867.2m).

Salient feature of Tista Drainage Basin:

Tista is the trunk river in Sikkim-DarjeelingHimalaya. Like all other major rivers of Himalaya, Tistais also an antecedent river. It starts from Tso Lamo lakeof Trans Himalayan region of north Sikkim. Near itsorigin it initially flows for some distance in E-Wdirection, and then takes a sudden N-S trend. NearChungthang it takes an ENE – WSW trend from Mongantakes a N – S trend. This trend continues for its entireroute through Himalaya.

At the upper reaches, before Zemu Chu and LachenChu confluence, i.e. in the north of Higher Himalayathe valley wall is 1520 m high. The valley wall rises upto 3344m from Lachen to Dickchu as it enters the HigherHimalaya. After Dickchu, the valley wall slowlydecreases to 915 m near Rangpo and further downwardin the outer hill ranges the valley wall is 457 m high.Thus the gorge forming tendency is mostly noticed inthe Higher Himalayan region. It is a case of rapid incisionwith higher rate of uplift.

Gradient of longitudinal profile of the Tista River isvery low in lesser Himalaya and slowly increasestowards north. The change in the gradient is very sharpnorth of Dickchu, near MCT and highest gradient wasobserved near Zemu. After Zemu, towards north thegradient is again low. The Longitudinal profile of Tistais concave from lesser to Higher Himalaya but changesto convex in the higher reaches. It has been observedthat the longitudinal profiles of the Trans HimalayanRivers are convexo-concave as they pass from theTibetan Plateau through the Higher Himalaya to theLesser Himalaya and Sub-Himalaya.

The Tista drainage basin is an asymmetric drainagebasin. The western side is wider than the eastern side.

The sub basins of the western side tributaries are biggercompared to the eastern side. The migration of the rivertowards east may be considered to be a result of tilt ofthe rocks towards east. A spectacular observation is thatthe western bounding water divide of the Tista basin isalways higher than the eastern bounding water divideacross the same latitude. This feature continues all alongthe east west trend of the bounding water divides.

Rangit sub basin, the biggest sub basin in the Tistavalley on its western bank, also shows an asymmetricnature. The southern block is more constricted than thenorthern block indicating a tilt towards south (Cox 1994).The Talang Chu, another sub basin in the western partof the Tista valley just north of Rangit also shows asouthern block constriction indicating a southern tilt.However, the river north of Talang Chu, the Zemu Chuis a symmetric sub basin. The Lachung Chu, the easterntributary in the similar latitude also shows a more orless symmetric sub basin. The Tista basin in this latitudeis symmetric upto the confluence of Lachen Chu andZemu. Then the eastern constriction starts. The Tistabasin is again symmetric after Tista Rangit confluence.The Rongpo Chu, the biggest eastern tributary issymmetric barring a small part constricted towards north.The other eastern tributaries are symmetric. The lengthsof the eastern tributaries are smaller compared to thewestern tributaries.

The drainage pattern in Tista basin is mostly amixture of sub dendritic and sub parallel. However, inthe northern part directional trellis type drainage patternis observed. Drainage density is high in the lower reachesbut in the higher reaches the drainage density ismoderately high.

Lakes of Sikkim:

The state is endowed with some of the mostpicturesque lakes of Himalayan terrain although theyare not very large in size. Most of the lakes haveoriginated as depression scooped by the glaciers andsubsequently filled by glacial melt. Others have formedby the damming of glacial water by the terminalmoraines. The lakes are mostly confined to the northernpart of Sikkim but a few are also found in the westernand eastern part.

Gurudongmar is the largest lake in Sikkim, locatedprobably at the highest point among the lakes. The wellknown Thang Chho Lake which is located in the northern

4 GEOL. SURV. IND

part of the state, very near to the Tibet border, is an in-filled glacial lake which is bounded on all sides bymoraine ridges. Choka is another spectacular lake ofnorth Sikkim.

In south-western Sikkim, a number of small lakeslike Lam Pokhri, Kali Pokhri, Kanahailal Pokhri,Deoningale Pokhri, Torepool pokhri are most prominentones. Now these lakes are located in the sub-tropical totemperate region but they show that once upon a timethe area was covered by heavy snow and glaciers.Kechopari Lake is another lake that lies on way toYoksum from Peling. Chhojo Lake is located SW ofYoksam on the opposite side of Rathang Chhu. This lakerepresents original ‘neve’ region of an ancient hangingglacier, the depression having developed due to thescooping action of the glacier. A moraine ridge is seenlower down forming southern bank of Lethang Valley.

In the eastern part of the state, lakes like ChhanguLake, Syeberuka chho, Serabthang chho and many othershave all been produced by the erosive action of theglaciers moraine which carved depressions in the bottomof the valley. Such lakes are geologically known as‘Tarn’. Chhangu (Tsomo) lake is located at an altitudeof 3693 meters. Two nearby lakes are Bidang chho andMenme chho.

Glaciers of Sikkim :

In Sikkim Himalayas the glaciers are restricted tothe western and northern part of the state. The water ofthe glaciers drain into Tista River and hence, aregenerally known as Tista Basin glaciers. Puri et al. (1999)have given detailed account of various glaciers.

The area of four glacierised basins of SikkimHimalaya is 7172.21 sq km. These fifth order basins areEast Rathong basin, Talung basin, Changme Khangpubasin and Zemu basin.

East Rathong basin :

This is the southern most among the basins of SikkimHimalayas. It is located west of the Tista River andextends in that direction towards the Nepal border,occupying an area of 2351.12 sq km. It supports 36glaciers and almost all the glaciers are restricted to thenorthern part of the basin. Obsequent drainage is mostprominent feature of the basin. The major channel isRathong Chhu, which joins the Rajal River at Legship.The East Rathong basin has preserved multi-cyclic

geomorphic history in which both proglacial andperiglacial regimes are found. Epigenetic gorges are verycommon from Bakkim northwards. Distinct U-shapedvalley profile is discernible from south of Dzongri.

Talung basin:

The basin lies towards north of the East-Rathongbasin and is west of Tista River and extends towards theeastern border of Nepal. It occupies an area of 1270.74sq km and supports 61 glaciers. Most of the glaciers arerestricted to the northwestern portion of the basin. TheTalung glacier, which is the largest one in the basincovers a sizable part of northwestern portion of the basin.Subsequent type of drainage is observed to be common.This basin has also preserved multi-cycle geomorphichistory in which proglacial and periglacial environmentsare discernible. West of Umrang Chhu and Rukal Chhuconfluence, distinct U-shaped profile of the valley andcolonized moraine debris are found which appears to bethe lower limit of the palaeo-glaciation in the valley.

Changme Khangpu basin:

This is the eastern most glacier bearing basin of theSikkim Himalayas that lies between the Zemu basin inwest and Chumbi valley (Tibet) in the east. The basinoccupies an area of 1158.75 sq km and supports 102glaciers. The glacierised area is restricted to the northof Lachung. In the longitudinal profile of the valley fromKhadom to Yomesomdong, three glacial terraces areidentified. The subsequent fluvial action has resulted ina few cascades. All along the valley up to Khandomglacier smoothened valley walls are discernible.

Zemu Basin:

It occupies an area of 2391.60 sq km and is supportedby 250 glaciers. The basin is drained by two majortributaries, the Zema Chhu and Hema Chhu. The ZemaChhu sector has an area of 1358.88 sq km. The largestglacier in this sector is Zemu glacier, which is also thelargest glacier in Sikkim Himalaya. Different cycles ofglacier and periglacial regimes are preserved. Thedistinct U-shaped valley profile noticed near Telimindicates the probable lowest limit of the glaciation inthe basin.

The Hema Chhu sector is located east of Zemu Chhuand occupies an area of 1032.72 sq km. The largestglacier in the Hema Chhu sector is Tista Khangse glacierwhich is the source of Tista River. Various phases ofgeomorphic history have been preserved in this sector


and about two km south of Thangu, the evidences ofpast glaciation are discernible.

Hot Springs:

The thermal springs of Sikkim are included withinthe Himalayan Geothermal Province. More than 1000hot springs are situated in this collision zone. Sikkim isblessed with a number of hot springs; out of which fivehot springs in Sikkim are studied in detail. These areYumthang hot spring, Yumesamdong hot spring, Boronghot spring, Polot hot spring and Rishi hot spring.

The Yumthang hot spring, also known as Chhachhahot spring, is located at about 1.5 km SSW of Yumthang,North Sikkim district at an altitude of 3570 m on the leftbank slope about 200m above the Yumthang Chhu. TheYumesamdong hot spring is located at an altitude of 4700m. It is situated at about 1.5 km NW of Yumesamdong,about 40 m above the Sebu Chhu river bed. The Boronghot spring, also known as Mangnam Cha Chu is situatedon the left and right banks of Rangit River at about 1 kmNW of Brang village, West Sikkim district at an altitudeof 1000 m. Polot hot spring also known as Ralong ChaChu, is situated on the left and right banks of Rangit

River about 1.5 km west of Polot village, West Sikkimdistrict at an altitude of 920 m. The Rishi hot spring isalso called as Phur Cha Chu hot spring and Tatopani. Itis situated on the left bank of Rangit River about 1.8 kmnorth of Rishi at about 500 m altitude.

A comparative analysis of the chemical data of thethermal waters of Borong, Polot and Rishi indicatesless concentration of Na, SiO2 (except Rishi hot spring)and Cl from Borong hot spring. This may be attributedto the greater admixture of surface / subsurface waterwith original hot fluid in this area. The weight ratiosand constituents concentrations in the three (Borong,Polot, Rishi) hot springs indicates that these springscompletely differ from volcanic type of thermal waterwith the exception of K / Na (for all the springs) andSO4 / Cl (for Polot and Rishi hot springs) values. Highervalues of HCO3 / Cl and lower to moderateconcentration of TDS are attributed to the admixtureof cold water at shallower levels. On geochemicalconsiderations, it can be concluded that the thermalwater of all the springs receive contribution from deepmagmatic source and yet diluted by surface andsubsurface water.

6 GEOL. SURV. IND

II. GENERAL GEOLOGY

The Himalayas are traditionally been divided intolinear geotectonic belts supposedly with distinctgeological characteristics. Beginning from the souththese are Sub Himalayas, Lesser Himalayas, HigherHimalayas and Trans Himalayas. Like other parts of theHimalaya, in Sikkim-Darjeeling Himalaya, the SubHimalayan domain comprises the molasses type depositsof the Siwaliks. It is followed northward successivelyby a thin strip of sandstone, carbonaceous shale and coal(Gondwana), stromatolitic dolomite and variegated slate(Buxa and Reyang Formation of Daling Group) and athick metasedimentary sequence of dominantly peliteswith subordinate psammite and wacke (GorubathanFormation of Daling Group), constituting the LesserHimalayan Belt. Towards the north, Daling sequence isoverlain by Higher Himalayan rocks of medium to highgrade dominantly pelitic schist with minor interbandedquartzite, calc-silicate and metabasites (commonlyknown as Chungthang / Paro Formation) and smallbodies of granites (Lingtse Gneiss). This sequence inturn towards north overlies a migmatitic terrain knownas Darjeeling Gneiss/Kangchenjunga Gneiss and thoughtto be equivalent of what is variously described as CentralCrystalline/ Greater Himalayan Crystalline /HigherHimalayan Crystalline (GHC/HHC). In the far north, athick pile of fossiliferous Cambrian to Eocene sediments,belonging to the Tethyan Belt (Tethyan SedimentarySequence) overlies the HHC.

The Main Boundary Thrust (MBT) separates theSiwaliks of the Sub Himalayan domain from theoverlying rocks of the Lesser Himalayan Belt and theMain Central Thrust (MCT) separates the LesserHimalaya from the Higher Himalaya. In the western aswell as in the eastern part of the Sikkim-DarjeelingHimalayas, the Lesser Himalayan Package is exposedas thin strip between MBT and MCT. However, in thecentral part of the Sikkim-Darjeeling Lesser Himalayas,a domal shaped culmination structure (known as Tista/

Daling Dome) has exposed a wide expanse of the LesserHimalayan rocks. The MBT, with the Mio-Pliocenesynorogenic Siwalik Group in the footwall and thePermo-carboniferous Gondwanas in the hanging wall,has not been affected by this culmination structure andhas a roughly E-W trace. The Gondwana rocks as wellas the Buxa and Rangit pebble slate are exposed in theRangit window zone where these are surrounded byDaling Group of rocks (Gansser, 1964; Acharyya, 1989,1992). The Tethyan Belt is exposed on the hanging wallside of a series of north-dipping normal faults constitutingthe South Tibetan Detachment System (STDS), HigherHimalayan Crystallines being the footwall.

Geological investigations in Sikkim and theadjoining Bengal Region began in the middle ofnineteenth century. Hooker (1854) in his famousHimalayan Journal reported the geological observationsfrom his extensive two years travel in many parts ofSikkim. He was able to trace the regional domal pictureof the gneisses. Darjeeling district and its foothills wereexamined by Mallet (1875). Auden (1936) discussed theproblems of the Daling and Darjeeling Gneiss. Heimand Gansser (1939) visited Tista region, Darjeeling andmade a traverse to Gangtok. A.M.N. Ghosh (1952) firstreported patches of Gondwana within the Daling area,i.e. Rangit window. Different workers of GSI worked inconnection with mapping and different investigations.

Stratigraphic classification schemes

In recent years Geological Survey of India developedunified legend for all the formations of India. Theformations exposed in Sikkim can be classified as givenin Table 1 in accordance with the unified legend scheme.

CENTRAL CRYSTALLINES

The Precambrian sequence of Sikkim State canbroadly be classified into two major groups. Low grade

6


Precambrian meta-sedimentary, generally known asDaling Group, is exposed in the central part of the DalingDome. The rock sequence in the outer part of the TistaDome have variously been classified as KanchenjungaGneiss, Darjeeling Gneiss, ‘Chhubakha Series’ (Rainaand Bhattacharya, 1962), etc. and by a number of otherlocal names by pervious workers. This sequencecomprising various types of gneisses, schists, quartzitesand calc-silicates have been given the name of CentralCrystallines in the western and central part of Himalayas.Keeping in view the necessity of retaining one name fora single litho pack, they have been hereto called asCentral Crystallines.

Mallet (1875) was one of the earliest workers in thearea who systematically classified the rocks of Nepal,Darjeeling and Sikkim area. After Mallet (1875), someof the earliest records of geological mapping andobservations pertain to those of Dutt (1948, 53, 54, 61),Ghosh (1956);Raina (1965, 66, 68); Raina andBhattacharya (1962 and 65); Bhattacharya and Pattanaik(1964);Bhattacharya (1966); Singh and Bhattacharya(1968). During this phase of mapping in fifties andsixties, a lot of proliferation of lithounit names was invogue, possibly because the mapping was done inisolated patches.During early eighties some of the areaswere remapped and some are mapped a fresh onexpedition by Neogi et.al (1983,84,86 and 98).The

Table: 1

Generalised stratigraphic succession of Sikkim Himalaya

(As per unified legend scheme of GSI)

LITHOLOGY FORMATION GROUPAGE

Variegated clay, fine and medium sand, Sesela Formation Upper Pleistocenepebble bed Holocene

Tourmaline / biotite leuco granite, schroll rock/pegmatite, aplite (Undifferentiated) Intrusive

Syenite / basic dyke/sill (Undifferentiated) Intrusive

Fossiliferous sandstone, limestone, shale Tso Lhamo Formation Triassic

Boulder bed, Fossiliferous limestone and Lachi FormationCarboniferous tosandstone.PermianSandstone, shale, carbonaceous shale with coal Damuda Group Gondwana

Pebble/boulder slate, conglomerate, phyllite Rangit pebble state Groups Supergroup

Fossiliferous limestone with quartzite Everest Limestone OrdovicianFormation

Granite gneiss (mylonite) Lingtse Granite Meso ProterozoicGneiss

PhyIlite, quartztte, biotite gneiss Everest Pelite Formation Meso Proterozoic

Amphibole schist / amphibolite Sill

Dolostone, ortho-quartzite, purple phyllite /slate, chert Buxa Formation

Ortho-quartzite, pyritiferous black slate,Proterozoicvariegated cherty phyllite, meta-greywacke Reyang Formation

Daling Group UndifferenciatedInterbanded chlorite-sericite schist / phyllite Gorubathan Formationand quartzite, meta-greywacke (quartzofeldspathic greywacke), pyritiferous black slate,biotite phyllite / mica schist, biotite quartzite,mica schist with garnet, with / without staurolite,chlorite quartzite

Banded / streaky migmatite, augen bearing Kanchenjunga Gneiss/(garnet) biotite gneiss with/ without kyanite, Darjeeling Gneiss Centralsillimanlte with palaeosomes of staurolite, (Undifferentiated) Crystalline Proterozoickyanite, mica schist, biotite gneiss, sillimanite Gneissic Undifferenciatedgranite gneiss Complex

(CCGC)

1. Quartzite 2. Garnet kyanite sillimanitebiotite schist / Garnetiferous mica schist Chungthang3. Calc-silicate, carbonaceous schist FormationChungthang Formation

8 GEOL. SURV. IND

central crystalline gneiss of higher Himalayan crystallineof the Darjeeling –Sikkim Himalaya has been classifiedby Ray(1989) into the Sikkim Group comprisingKanchenjunga Gneiss , Darjeeling Gneiss andChungthang Sub-Group.

In addition to the gneisses, a prominent sequence ofcalc-silicates, calc-gneisses, quartzites and schists whichappeared to be different from the gneissic sequence tothe earlier mappers, was separately identified andmapped as Chungthang Group of rocks (named after thevillage of Chungthang, where this unit crops outprominently).

In this write-up, in the proposed scheme ofclassification, the Central Crystallines have beenclassified into Kanchenjunga Gneiss / Darjeeling Gneissand Chungthang Formation.

A) Chungthang Formation :

Raina and Bhattacharya (1961-62) while workingin North Sikkim marked a formation which was tracedover a width of 19 km between Myang (27°10:88°22)in south and Lema (27°39:88°43) in north and has beendesignated as Chunthang Formation. This name has beengiven after Chungthang (27°36:88°39), a village inNorth Sikkim where the rocks of this formation areprominently exposed. The main rock types of thisformation are quartzites, garnet-kyanite-staurolitebearing biotite schist, calc silicate rock, graphitic schistand amphibolite. The rocks of this formation are alsoexposed in the districts of east Sikkim and west Sikkim.

1) Quartzite:- The quartzite which forms animportant constituent of this formation is well exposedin different parts of Sikkim. Raina and Bhattacharya(1961-62) reported quartzite at Tong (27°33:88°39),Chungthang (27°36:88°39), near Lema (27°39:88°43)and in south of Lachung (27°41:88°45). Singh andBhattacharya (1968) reported quartzite exposed as thinbands within calc-gneiss and granulite, graduallyattaining sizeable thickness to be mapped as a separateunit near Chungthang in north Sikkim and Kyagnosa La(27o22:88o43) on Gangtok - Nathu La road. Severalbands of felspathic quartzites have been found north ofMenshithang (27o38:88o37) near Rabong Chhu.

The quartzite is greyish and show development ofmica along the bedding planes. Under microscope theyare seen to be quartz-feldspar-schist. Larger grains of

quartz show strain shadows and occasionally containinclusions of biotite. Smaller grains of quartz occur in agranular aggregate. At places biotite grains enclosequartz crystals. Some times the quartzite shows thepresence of chlorite. Along the contact with gneisses,garnet is seen to have developed in the quartzite.

2) Calc- silicate rocks and graphitic schist:- Singhand Bhattacharya (1968) while mapping in North Sikkimand East Sikkim mapped calc-gneisses and granulite withbands of marble and quartzite associated with sillimanitegarnet gneiss and graphite schist as the most dominantrock association which could be traced near Naga(27°3215:88°3730), Theng (27°34:88°39) andRangma in north Sikkim and near 5th and 10th mile postson Gangtok - Nathu La road sections. Several bands ofmarble are noted at Bop and Theng in north Sikkim andat 5th and 24th milestone on Gangtok - Nathu La road.Bands are milky white in colour and vary in thicknessfrom 30 m to 60 metres, and are traceable for considerabledistance along the strike. The marble band, west of Bopvillage, shows graphite concentration along a shear zone.Traces of graphite are also seen near Theng(27°24:88°39) and Chhangu. Between Myong(27°3440:88°2640) in the South and Kishong(27°4315:88°27’45) in the north three bands of calc-silicates and associated quartzite bands have beenrecorded within gneisses. First band is near Mayong,second band is near Tolung Gompa(27°3830:88°2755). The bands trend in NE-SWdirection and are locally folded (Neogi et al. 1986). InWest Sikkim district the limestones and marbles areobserved only in one section at Meguthang(27°25:88°03) (Raina, 1966).

Under microscope, the calc-silicate containsdiopside, quartz, feldspar, actinolite, tremolite andcalcite. Diopside and feldspars occur as porphyroblastswhile quartz occurs as large plates and also as mosaicof recrystallised grains. Feldspars are mostly alteredorthoclase, fresh labradorite and few grains of microclineshowing wavy extinction. Diopside occasionally altersto actinolite-tremolite. Olive green hornblende is alsoseen. Sphene and magnetite are accessory minerals. NearMyang, lenses of wollastonite were found to havedeveloped within these rocks.

Under microscope; the marble shows idioblasticcalcite and subordinate diopside, tremolite, phlogopiteand a few grains of scapolite. Sphene is a common


accessory mineral with inclusions of carbonate. Ingraphite bearing marble, graphite occurs as flakes,sometimes encircling sphene. In Tolung - Kishong areathe calc silicate units show a granular mosaic of equantgrains of calcite, quartz with wollastonite and diopsidetypical of high temperature marble. Lamellar twinningin calcite may be due to twin gliding during plasticdeformation. Visuvianite alters to calcite. Feldspars aresericitized (Neogi et al., 1986).

3) Biotite Schist:- Biotitic schist with garnet andstaurolite was demarcated as Rongli schist . It has beennamed after Rongli, a village in East Sikkiin, where therocks of these types were first observed. However, laterworkers ignored the Rongli schist in the context that theRongli schist represents the higher metamorphic gradeequivalent (garnet , staurollite, etc) of Daling metapelitesand has been variously included within the LesserHimalayan or the Higher Himalayan package.Bhattacharya and Pattanayak (1963 – 64) while mappingin the North and West Sikkim districts informed that therocks comprising Rongli schist are seen in the areabetween the Tista Valley and Talung Valley. Theschistose rocks exposed between Sangklang and Namok(27°26:88°32), between east of Rhamthang(27°2430:88°3345) and Lebrong (27°25:88°35) andbetween Lindok (27°23:88°3515) and Nabhe(27°23:88°36) have been described by Singh andBhattacharya (1968). This schist contains abundantgarnet and occasionally shows bands of flaggy quartzite.The schistose rocks exposed in West Sikkim district havebeen described by Raina (1966) as schist zone. Schistswith interbedded thin quartzite bands are the main rocktypes of this zone.

This rock contains abundant garnet and mica.Garnet, staurolite, biotite and muscovite constitute themain minerals. Garnet porphyroblasts show sievestructure with inclusions of quartz and chlorite alongthe cracks. Staurolite shows well developed cruciformtwinning and some grains show inclusion of quartz.Biotite and muscovite show simultaneous crystallisation.Helicitic inclusions in staurolite and garnet are found tobe arranged in curved lines, across the schistosity of therocks suggesting inclusion of minerals during growth.Alteration of staurolite to chlorite and biotite indicatesretrogression. Sometimes biotite occurs as coronasurrounding quartz crystals.The schistose rocks exposedin Sangklang, Lindok, Nabhe, Namok, Rhamthang andLebrong areas contains abundant garnet with absence

of staurolite. Thin section show it to be fine grainedquartz-sericite-muscovite-biotitic schist with prolificalmandine garnet (subhedral grains), a few grains offeldspar (plagioclase) occur as recrystallised aggregate.

4) Amphibolite:- Amphibole schist andamphibolites as lenticular sills have been observed tooccur in all rock types of Chungthang Formation. In someof the cases they are sulphide mineral bearing. They havebeen found profusely north-west of Penlong La (27o22:88o37). These are also common between Mangang andSinghik and at Kahior (27o31: 88o32).

Under the microscope the rock shows characters ofhornblende-quartz-schist with hornblende, oligoclasefeldspar, biotite (developed from hornblende), quartz andminor rutile as its main constituents.

B) Kanchenjunga Gneiss / Darjeeling Gneiss

Kanchenjunga / Darjeeling Gneiss are extensivelyexposed in the northern part of the Sikkim, to the southof the Tethyan sequence. In the west; MountKanchenjunga and adjoining hills comprise these rockswhich pass into Nepal territory further west. To the eastthese rocks are well exposed around Yum Samdong,Yumthang, Lachung, Chhangu and Nathu La areas andfurther east they continue into Tibet and Bhutan.

The gneisses, dominantly comprising quartz,feldspar and biotite (with minor amounts of otherminerals) have been classified into three types, ie.1)banded / streaky gneisses / migmatites, 2) augen bearingbiotite gneiss with/without garnet, kyanite, sillimaniteand 3) sillimanite granite gneisses. Mapping of theserocks as individual units is very difficult because theyare characterized by frequent interchanging andgradational features among themselves. However acouple of zones have been demarcated and shown in themap.

1) Banded Gneiss/ Streaky Gneiss/ Migmatite:-In the northern most part of the Central Crystallines ofSikkim, the gneisses of various types form the majorrock unit (Sinha Roy and Roy, 1972) and of them thebanded gneiss is the dominant type. Hyden, Fermor andAuden reported ortho gneisses from different parts ofSikkim which are characteristicaly similar to the bandedgneiss described by the later workers. The peliticmigmatite of Neogi also indicates this type of rock. Rainaand Bhattacharya (1962 &65), reported banded gneisses

10 GEOL. SURV. IND

(named as Chhubakha Gneiss by them) between Lema(27°39 :88°43) in the south and Yome Samdong(27°54:88°45) in the north Sikkim. These are biotitegneisses whose impregnation by the granitic materialhave completely obliterated their original characters.Bhattacharya and Pattanaik (1964) reported highlybrecciated and folded banded gneiss near Chhateng.These gneisses are profusely intruded by granites andpegmatites and have been affected by a series of faults.Thin graphite schists are observed along shear plane orfault plane. It is found from Yukti Chu 3 km north ofMenshithang (27°4000:88°3530) to Lachen(27°44:88°33). Coarse to medium grained, grayishwhite to dark grey banded gneiss covers a major portionof area between Chittre (27°15:88°03) and Phedi(27°22:88°03) in the south and Zongri (27°27:88°10)and Yaksam (27°22:88°13) in the north Sikkim (Raina,1965). Dutt (1955) reported garnetiferous banded gneissat Donkung (28°02:88°36) in north-central part ofSikkim. Singh & Bhattacharya (1968) reported thestreaky banded gneiss at Serabthang on Gangtok - NathuLa road and migmatised banded gneiss between Singhikand Manul, near B 2, between B 3 and Penlang in northSikkim and between the 1st and 5.5 milepost and at 7th

milepost on Gangtok - Nathu La road.

The banded gneiss consists of alternating bands richin quartzo-feldspathic material and mafic schistosecomponents, imparting a strong s-tectonite fabric.

Under the microscope the rock consists of biotite,quartz, muscovite, sericitized microcline and plagioclasefeldspars. The quartz is of two types (i) small fracturedgrains, showing strain-shadows in the groundmass and(ii) relatively strain-free patchy grains. Feldspar is albite-oligoclase with minor amount of orthoclase togetherforming more than 35 percent of the mineral assemblage.Anhedral large andesine/oligoclase with deformed twinlamellae had also been observed. Secondary albite hasdeveloped along andesine orthoclase contact.Occasionally highly altered, fine to coarse myrmekitedeveloped at the interface of quartz and anhedral K-feldspar. Deep brown to greenish brown biotite is a majorconstituent. Biotite in some sections is seen to havealtered to chlorite. Muscovite flakes are interwoven withbiotite and are twisted and bent enclosing grains ofquartz. Some of the sections showed the presence ofsillimanite and almandine garnet. Anhedral garnetoccasionally showing incipient alteration to chlorite withinclusions of quartz, biotite and sillimanite is abundance.

Apatite, zircon, sphene, epidote, rutile, graphite andopaques are main accessories.

2) Augen bearing biotite gneiss with/withoutgarnet, kyanite, sillimanite:- Raina and Bhattacharya(1965) described reddish brown to grayish black augengneisses showing well foliated nature with prominentaugens of feldspar. To the west of Lungme nala(27°52:88°21), these gneisses show lateral variationinto a coarse grained porphyritic variety which showsan almost unfoliated nature with subrounded feldspargrains (10-12 mm across) lying scattered within the bodyof the rock. Similar lateral variation is observed to thewest of Khora Chachen (27°56:88°20). In some areaslarge blocks of garnet sillimanite gneisses are seenembedded within the augen gneisses. These xenolithsare rusty brown in colour and composed of biotite flakesalternating with crushed quartz stringers. In the north-central part of Sikkim porphyritic augen-gneissescontaining streaks of biotite has been recorded by Dutt(1955) to the north of Lachen (27°44:88°33). Half amile north of Zemu - Lachen confluence several bandsof calc-gneisses are also observed as inter-stratified withaugen gneiss. Near the Choptha Chhu (27°54:88°32)yellow ferruginous mica schist bands are recorded withingneisses. The augen gneisses along Zemu river valleychange from the horizontally banded and foliated rockon the southern side to the much more knotted andcontorted gneisses on the north. They extend throughthe entire Zemu valley from very near to the confluenceof Zemu river with Tista, as well as from Tungaphu-phiak (27°39:88°28) to Tolung on the Tolung valley(Bhattacharya and Pattanaik, 1964). Augen gneisses alsooccur south of Menshithang (27°40:88°3530) on Tistavalley. Sinha Roy and Roy (1972) reported augen gneissin the upper reaches of the Tista River. Raina (1966)recorded augen gneisses around Singrangpung(27°21:88°08) in the valley of Rimbi Chhu. Singh andBhattacharya (1968) reported augen gneiss at Bop innorth Sikkim and at Karponang, Chhanggu andSerabthang on Gangtok - Nathu La Road.

The augens mostly of felspar and rarely of quartz,show extreme idioblastic habit and vary in length fromone cm to 7-5 cm. At isolated locations, the quartzo-feldspathic bands often show selective bulges in the formof pinches and swells as a precursor to the formation ofaugen gneiss. The gneissosity defined by biotite andsillimanite wraps round the bulges. At Tolung valleysometimes there are imperfect gradations from streaky


granite gneiss to the augen gneiss with gradual increasein the size and numbers of augen and the amount offerromagnesian minerals. Augen gneisses occuring southof Menshithang (27040’:88035’30") on Tista valley(Bhattacharya and Pattanaik, 1964) have been intenselycriss-crossed by tourmaline rich muscovite bearinggranite and pegmatite.

Under the microscope, the rock showsporphyroblasts of orthoclase and quartz, mostly arrangedparallel to the foliation-planes, embedded in matrix ofbiotite, chlorite, quartz and plagioclase. These gneissescontain quartz which is of two types, (i) highly strainedsmall fractured grains and (ii) strips and ribbons showingincipient polygonization, and shapeless patchesintergrown with K-feldspar. In the larger grains of quartz,the cracks and fractures are all aligned across thedirection of banding, though the elongated direction ofthe crystal itself is oriented along the banding of therocks. Biotite is highly corroded and dispersed. K-feldspar occurs as anhedral grains and as largeporphyroblasts, which constitute the augens, showingslight strain effects and sericitization along cleavage andtwin planes. Another constituent of the augen is lamellarperthite with crystallographically oriented thin albitewithin K-feldspar. Secondary albite also occurs as thinrims at the margin of K-feldspar. Coarse to very finemyrmekite occurs at the contact of quartz and K-feldspar.A few grains of tourmaline, apatite, zircon garnet,hornblende, scapolite and suspected monazite are notedas the accessory. In Jemu valley augen gneisses containonly porphyroblasts of potash feldspar (Bhattacharya andPattanaik, 1964).

3) Sillimanite Granite Gneiss:- Bhattacharya andPattanaik (1964) reported that sillimanite bearinggneisses are exposed in Tolung valley from Laben(27°33:88°2730) to Tolung Monastery and nearChungthang in the Tista valley. Singh & Bhattacharya(1968) reported sillimanite-garnet-biotite gneiss nearBop, Theng, Naga and Singhik in north Sikkim and fromthe 6th to 9th, 11th to 13th and 15th to 18th milepostson Gangtok - Nathu La road. Kyanite-biotite-muscovitegneiss with bands of flaggy quartzites are exposedbetween Mangan and Sangklang Bridge(27°3030:88°3230), in Rangrang (27°27:88°3430), near Chawang (27°2550:88°3530) andwest of Penlang (27°3022:88°3730) in north Sikkim.To the west of Lachung; sillimanite-biotite gneisses arereported by Dutt (1955). Raina and Bhattacharya (1961-

1962) also reported banded and augen gneisses showingeffects of high grade metamorphism around the Dalingdome in its eastern and northern part. Here the gneissesstarts as kyanite bearing gneiss at the lowermoststructural level near the contact with ChungthangFormation and become sillimanite bearing gneiss in theupper level. Raina (1966) while mapping the westernpart of Sikkim reported sillimanite-garnet-biotitegneisses forming mountain ranges west of Dentam(27°16:88°09) and around Uttare (27°16:88°05).

Under microscope the rock reveals gneissic texturedefined by quartz and feldspar (both as individual grainsand as intergrowth), sillimanite, biotite, muscovite andgarnet as main constituents. Larger crystals of quartzshow strain shadow whereas smaller grains occur ingranular aggregates. Perthitic growth between orthoclaseand plagioclase, replacement growth between quartz andfelspar, and sericitisation of feldspar are quite common.Biotite is of two generations. First generation biotite isproduced during progressive metamorphism, followedby garnet and sillimanite. Second generation biotite isdeveloped due to retrogression and occurs as coronasand patchy aggregates around garnet and sillimanitecrystals. Sillimanite is present as acicular aggregatesarranged in sheaf, parallel to the foliation and sometimeswithin the granular aggregates of quartz and feldspar.Curved patches of sillimanite (fibrolite) are also seen.Garnet is mostly pyrope almandine type and showszoning indicating probable growth of garnet in more thanone stage. Large porphyroblasts of recrystallised garnetare seen to be surrounded by lamellae of biotite. Thesegarnets are highly shattered and show numerous fracturesfilled in by megacrysts of quartz and biotite. Presenceof rims of biotite around garnet and formation ofphlogopite from biotite are also noticed. Accessoryminerals are apatite, sphene, epidote and zircon. In somesections kyanite forms the major constituent and isarranged haphazardly in the country rock. Kyanite ismostly broken and contains inclusions of quartz.Alteration of biotite to chlorite, garnet to biotite andkyanite to muscovite has been observed.

DALING GROUP

Unfossiliferous low grade metasediments of theDarjeeling-Western Duars area were subdivided intodominantly greenish argillaceous assemblage comprisingthe Daling “Series” and dolostone, quartzite andvariegated slate assemblage making up the Buxa “Series”by Mallet (1875). The terms ‘Daling’ and ‘Buxa’ have

12 GEOL. SURV. IND

been extended later to cover wider areas of the EasternHimalaya but much confusion exists in theirclassification and nomenclature even in the Darjeeling-Sikkim-Duars area. The Daling Group (sensu lato) ofDarjeeling-Sikkim Himalaya is characterized by distincttectono-stratigraphic position being placed below highgrade gneissic rocks limited by the sheared belt ofmylonitic granite gneiss and above its own cover bythe Lower Gondwana equivalent sediments of theHimalaya.

Three distinct mappable and regional lithotectonicassemblages have been recognized within the DalingGroup (sensu lato) by Acharyya (1989) which aredescribed below:

❑ Gorubathan Formation (Daling sensu stricto)❑ Reyang Formation❑ Buxa Formation (sensu stricto)

Gorubathan Formation :

It is best exposed around Daling Fort (27o01:88o43)in Darjeeling Himalaya from where the term ‘Daling’(sensu stricto) started. The assemblage is renamedGorubathan from the same type area by Ray (1976). Therocks are exposed extensively in the southern part ofSikkim from east of Renok to west of Geizing and northof Dikchu to Rongpo in the south having a dome shapedoutcrop loosely known as Tista Dome.

The formation consists of mappable, monotonoussequence of inter banded chlorite sericite schist / phyllite,quartzite, meta greywacke, pyritiferrous black slate/carbon phylllite, basic meta volcanics. Chlorite phylliteis dark green to light green whereas the quartz chloritephyllite is only light green in color. It shows fine quartzveins in Tarku, Rongpo, Vasme, Duga and Pachekhaniarea. Quartzites are occurring as thin partings tomappable bands within the chlorite schist and quartzchlorite schist. Quartz chlorite schist with quartzitepartings is observed to the west of Temi, Barmek, andChamthang area. Intercalation of quartzite and chloriteschist is observed in Rongpo, Duga, Pandem, Singtam,Manka area. Quartzites are mainly white to light greendepending upon the percentage of chlorite in them. It isalso grey to buff at places. Mostly these are fine grainedin size and massive in nature. Bhattacharyya andPattanayak (1963 - 64) reported that the quartzite atplaces, form prominent horizon (as at Bhale Dunga andPati Dunga near Menam Dunra (27°20:88°50). They

vary in colour from lighter grey to milky white. Severaloccurrences of dark grey quartzite and beds of flakyquartzite are also seen. Quartzites are found in theRangpo and Dikchu areas. Most outcrops at Rangpo areless than 50 feet while the main band just east of Dikchuis about 200 feet. Intercalations of quartzite appear inphyllites on the right bank of the river north of DikchuBazar. There after upto the 16 mile stone north of Dikchuthese quartzites continue. Raina and Bhattacharyya(1961 – 62) reported that the quartzites are preservingprimary sedimentary features such as graded and currentbedding. The carbon phyllite was encountered at KaliKhola section and Kartok in the east of Pakiyang. Thecarbon phyllite usually soils hand but in case of fewbands near Pachekhani this soiling characteristic wasabsent. Medium grained massive feldspathic wackepartings have been observed near Vasme and Tarkuwithin chlorite schist. Raina (1966) reported schistosegrits from Rinchingpong area. Gritty rocks, light greento greenish grey in colour, have been observed associatedwith the phyllites. These grits show a well bedded nature,with grains of greasy quartz present along the beddingplanes. Together with this, conformable bands of basicintrusive occur with the units of the GorubathanFormation. They are 2 to 15 m long and 1.5 cm to 2mthick and observed within the chloritic phyllite nearSang, Temi, and near Tashiding. Recrystallised quartzveins and stringers are quite common. Near Rabong(27o18:88o2030) Ben Monastery, several micapegmatite intusions have been noticed within slatyphyllites and quartzite.

Chlorite phyllite and quartz chlorite phyllite containsfollowing assemblage:- quartz + chlorite + muscovite +feldspar + opaque. Quartz shows two distinct mode ofoccurrence. Matrix quartz are fine and recrystallisedgiving rise to mosaic of rectangular and polygonal grains,the long axis of these grains being in a preferredorientation parallel to the planar fabric of the rock.Coarser quartz grains are detrital in nature and mostlysub-angular to rounded in shape, with the onset ofrecrystallisation along the borders. Some grains showfaint undulose extinction but detritus nature is evidentin spite of recrystallisation. At least in two localities,volcanogenic parentage of the Daling phyllite is noticed.These are (a) chloritic phyllite from the top of theabandoned adit of IBM near Rorathang adjacent to thePachekhani copper mines. The former shows embayedquartz grains indicating volcanic characteristic and (b)Chloritic phyllite adjacent to the pebbly carbon phyllite


of Kali Khola near Rongpo. In later case, chlorite grainsare generally elongated shapeless to prismatic in form.Some grains show faint pleochroism. White mica occursas very fine flakes. However, some moderately bigmuscovite flakes could be identified. Feldspar is alsodetrital in nature. They are angular grains with someshowing twin lamellae of plagioclase. Opaque grainsare elongated/ flattened and generally aligned parallelto fabric. Other types of opaque grains are subroundedto rectangular mainly associated with chlorite. Chloriteand white mica together with finer grained quartzshowing preferred orientation define the planer fabricof the rock. However, as the clear-cut Q domain and Mdomain are not defined it could be a foliation, and notschistosity. The carbon phyllite microscopically is afoliated rock with elongated opaque grains. Thefeldspathic wacke are matrix supported rock though theclast matrix ratio is as high as 70:30. The matrix is madeup of very fine aggregate of slightly recrystallised quartz,chlorite and white mica. The framework grains containabout 20 percent feldspar grains of microcline andplagioclase, quartz and chloritic rock fragments. Theframework grains are sub-angular to sub-rounded inshape. There is evidence of onset of recrystallisationaround the framework grains; however, no preferredorientation has been observed in these rocks.

Reyang Formation:

Ray (1976), Acharyya and Ray (1977) described adistinct sequence of thick bedded, ortho to protoquartzite, variegated phyllite/slate with minorimpersistent beds of crystalline carbonates, conformablemetabasites, from the type section around Reyang in theTista River valley in Darjeeling district, transitionalbetween the Gorubathan and the Buxa Subgroups. Thislithoassociation is underlain and overlain by units ofGorubathan and Buxa Subgroups, respectively, in theRangit Window Zone.

The white, brown or purplish quartzites aretexturally mature with up to 90-97% quartz in a mediumto coarse grained mosaic along with chert or jasper,minor feldspar, rock fragments, sericite, chlorite and ironoxides. These show various stages of mineralogical andtextural maturity. They often show ripple marks and crossbedding are locally associated with lenticular horizonsof oligomictic quartz-chert conglomerate (Acharyya,1989).

Slates and phyllites of the Reyang Subgroup are

variegated in shades of purple, maroon, steel-grey or greenwith other colour blotches. Pyritiferous black slates andminor carbon phyllites are also present. Compared tothose of the Gorubathan Formation the slates and phyllitesare more siliceous and with ubiquitous iron ores. Commonoccurrence of chloritoids in the higher grade, restrictedto this unit, corroborates their aluminous nature. Minorbeds and lenses of brown, pink or yellow crystallinelimestone on dolomitic limestones are also present. Veinsof crystalline magnesite are also recorded from ReyangRiver section, Darjeeling Hills.

Conformable bands of mafic meta-volcanic rocksand meta-dolerite have been reported from the ReyangSubgroup which contains relict igneous textures andprimary pyroxene and plagioclase, grading into foliatedamphibolites in more altered regions (Acharyya, 1989).

Buxa Formation:

In the normal stratigraphic section of the RangitWindow Zone, the rocks of the Buxa Formation occurabove the Reyang Formation and below the horizon ofGondwana diamictite (Rangit Pebble Slate). Elsewherein the foothill belt, it occurs as a discontinuous narrowstrip of variable thickness between the Gondwanasediments on the bottom and the units of the ReyangSubgroup on the top. The Buxa formation essentiallycomprises an alternating sequence of thin, grey chertymature quartzite, chert, fine grained, and finely laminatedto massive grey dolostone, pyritous sericitic andvariegated slates. The type area for the Buxa is the JaintiRiver section in western Duars, where the dolostoneattains considerable thickness and is intimatelyassociated with pyritous and carbonaceous slates. Thedolostone units are locally fossiliferous, as in the RangitRiver section, near Tatapani in West Sikkim, wherestromatolites have been reported from the upper horizonsof the dolostone beds (Ray, 1976; Acharyya, 1989).

The best exposures of the Buxa Formation in Sikkimare recorded along the course of Rangit River on NayaBazar to Legship road where Dolomite, which is themain rock type of the Buxa Formation outcrops. Thelimestone/dolomite is exposed around Naya Bazar,Reshi, Rahu, Tatapani, Tinkitam, Mangalbaria, Phali,Dhara, Wak and Kamigaon. Unlike Gorubathan andReyong Formations, the Buxa Formation has only localdistribution. It is characterized by fine grained, finelaminated to massive, grey and pink dolostone andlimestone beds, often with a weathered, pitted surface.

14 GEOL. SURV. IND

It is associated with pyritiferous, sericitic and variegatedslates, chert, and cherty and mature quartzite (Acharyya,1989).

There are three different litho units whichcharacterizes the Buxa Formation

1. Mainly dolomite with purple quartzites andphyllites.

2. Pink limestone, Grey limestone with calcareousslates and phyllites.

3. Calcareous purple and green slates withintraformational conglomerate.

Purple dolomites are recorded along Rishi-Mangalbaria road section. They are thinly bedded andsiliceous in nature. Wherever the siliceous bands areassociated, differential weathering is prominent. Thegrey dolomites which constitute a major part of BuxaFormation exhibit cross lamination and ripple marks.Under microscope, the rock shows a fine grainedcrystalline mosaic of dolomite crystals with a few grainsof cryptocrystalline quartz, some chert and also algalimpressions (Raina and Ray, 1967). In the Rangit Riversection around Tatapani, the dolostone beds arestromatolitic in the upper most levels and the dolostone-chert-slate alternations are para conformably succeededby Rangit pebble-slate. Near Khandosangphu(27o15:88o18) and some of the stream sections; theformation is represented by compact, massive looking,light brown to cherty-grey, often with bluish tingeddolomites with minor bands of calcareous slates andphyllites. Dolomite outcrops are seen as cliff and scarpsalong the western bank of Rangit River near Rishi. Theyare highly jointed and sheared and exhibit currentbedding, algal structure and bubble prints. Some of theshear planes are the locale of sulphide mineralization,mainly galena.

Pink limestones and slates are exposed at Nayabazar-Damthang-Tinkitam ridge and Chimchi spur. Limestonesare present in the form of thin intercalations, often shearedinto nodular shape. Grey limestones interbedded withgreen phyllitic slates are observed only in Rishi khola.These limestones, almost crystalline, show a well beddednature and are fine to medium grained. Limestones arefound associated with purple slate at the bridge abutmentnear Nayabazar (27o08:88o17). On the left bank of Rangitthe limestones are well stratified with purple slates, buton the other bank it occurs as a massive outcrop. Purpleslates are also found south of Somdong (27o09:88o19),though it is not calcareous here (Dutt, 1954).

Calcareous, purple and green slates form the basalportion and are exposed in Ranji khola (stream) section.Calcareous and non-calcareous slates/phyllites are bestobserved along Reshi-Sikkip road section, Mangalbaria,Chakung and Chumbung. They are purple coloured andsubstantially thick at places.

Near Rangpo the main rock types of this formationare carbon phyllites, quartz-chlorite-sericite-phyllite, andquartz-sericite phyllites, often with a purple tinge, whiteorthoquartzite and calcareous-quartzite. The carbonphyllites comprises alternating thin layers of highlyrecrystallised polygonal quartz with small dispersedgrains of carbonate and small stumpy sericite felt withdispersed carbonaceous material defining the bedding.In some cases, there are a few subrounded grains ofgarnet, arranged in zones, parallel to the bedding, whichsuggests that they could be detrital (Sinha Roy andMukhopadhyay, 1974).

The black slates underlying the Rangit pebble-slateare lithologically identical to similar beds within thelatter. The dolostone beds are conformably underlainby pink and variegated limestone-dolostone beds andvariegated slates, which represent transitional beds withthe adjacent section of Reyang Formation.

Sedimentary structures:

Current Bedding: It is exhibited by Buxa exposures,along the west bank of Rangit River. Current beddingindicates normal sequence of strata. This is alsoconfirmed by the fact that Gondwanas overly Buxa.Ripple marks: They were observed on the underside ofa dolomite band just south of Khandosangphu (27°15:88°18). Ripple marks also show a normal sequence.Graded bedding: Graded bedding was observed insiltstone bands associated with the phyllites of DalingGroup. One such section outcrops along the western bankof Rangit River, north of Sosing bridge. Graded beddingindicates a reverse sequence. Similar reverse sequenceon the basis of graded bedding was also observed eastof Yangthang (27°17:88°20).

Age of Buxa Formation:

Stromatolitic assemblage from the dolostone bedsfrom Tatapani area, representing top-most unit of Buxadolostone, indicates Lower to Middle Riphean age. Butthose occurring within the lithologically identicaldolostone from Bhutan foothills indicate late upperRiphean age (Raha, 1980). Stromatolite biostratigraphy,


if valid, would mean diachronous nature of BuxaFormation.

While discussing the age of Everest LimestoneFormation, Wager (1939) correlated it with the BuxaFormation of Mallet (1875) and placed it into Permo-Carboniferous. However, it should be remembered thatEverest limestone is crinoidal in nature and no crinoidshave been reported from Buxa. Both Buxa limestone/dolomite and Everest Limestone contain stromatolitesand algal mat.

LINGTSE GRANITE-GNEISS

Granite gneiss, exposed around Martam and LingtseGompa area of East Sikkim, has been designated as“Lingtse Gneiss” by Bose (1891). The gneisses are sheetlike bodies of coarse to medium grained, foliated tostrongly lineated granite mylonite. These are streaky,banded, augen gneisses or porphyroblastic gneisses andare traversed by concordant and discordant pegmatiteveins. Amphibolite intrusives with sharp contacts arealso recorded within gneisses. The most characteristicfeature of the Lingtse granite is the presence of astretching lineation. This is defined by stretched quartzand feldspar grains. Biotite flakes are aligned parallelto the stretching direction. Similar granite gneiss isexposed all along the Himalayan belt in the sametectonostratigraphic level. It start from west havingBesham gneiss, Iskere gneiss, Kotla Indress, Shashergneiss (in Pakistan) to Rameshwar granite, Kulu-Bajuragneiss, Bandal Granite, Wangtu Granitic Complex,Naitwar, Hanuman Chatti, Bhatwari, Namik, Gwalda,Chailli, Ghuttu, Chirpatiya, Rihee-Ganga, Ramgarh,Tawaghat, Almora-Askot-Dhramgarh gneiss (in India)to Ulleri augen gneiss, Mellung augen gneiss (in Nepal)through Lingtse, Darjeeling-Sikkim, Kangpur, Ari UriGranite in Bhutan, Kalaktang and Bomdila granite gneiss(in Arunachal Pradesh).

This granitoid gneiss occur in two modes; as anarrow belt occurring at the base of the HigherHimalayan Crystalline, along the trace of the MainCentral Thrust (MCT) or as detached sheets within theLesser Himalayan low grade rocks. The Lingtse Gneissof the first mode in the Darjeeling-Sikkim area extendsdiscontinuously along the southern fringe of theKalimpong hills through Lingtse Gompha (the typelocality), to Gangtok in the eastern part of the DalingDome. In western part of the Daling Dome, LingtseGneiss is exposed in Sikkim from Barmek to Heegaon

and in the Kurseong-Darjeeling hills from Mahanandathrough Mongphu and Peshoke. The Lingtse Gneiss ofthe second mode, i.e. within the Lesser Himalayan lowgrade rocks which in Darjeeling Sikkim Himalaya is theGorubathan Formation of the Daling Group, occur asmajor elongated north south body. This central Lingtsegneiss body is exposed from Ramthang - Phodong sectorin the north to Pendum in the south via Martam. Inaddition to these larger bodies a number of small granitebodies have also been recorded from within the DalingGroup of rocks.

The granite is two-feldspar granite where K-feldsparpredominates over plagioclase. The main constituentminerals are quartz, K-feldspar, plagioclase (mainlyoligoclase), biotite, muscovite and opaque ores.Accessory minerals are apatite, sphene, epidote, zirconand tourmaline. At places tremolite is also seen.Microscopic study reveals albite-oligoclase with minoramount of orthoclase forming more than 35% of theminerals assemblage. Secondary albite has developedalong andesine-orthoclase contact.

The minerals occur as subhedral and anhedral grains.Augen to porphyroblastic texture is usually veryprominent with clear evidence of post crystallinedeformation and recrystallization of feldspar and quartz.Feldspars occur as porphyroblasts and are surroundedby recrystallised biotite and muscovite. Largeporphyroblasts of quartz having wavy extinction andeven granulation have also been observed at places. Ingeneral quartz shows evidences of strain. K-feldsparshows evidences of fracturing and marginal granulation.The schistosity is seen to bend around feldspar grains.

Dutt (1955) observed that the contact of phyllitesand gneiss is very disturbed near 10th milestone onGangtok-Singhik road. This granite-gneiss appears tohave been intruded through a fault plane (op. cit). Inparts of toposheet nos. 78 A/7, 8, 11 & 12 both of thesetwo modes of Lingtse Gneiss have been mapped by S.Sinha Roy and S. K. Mukhopadhyay (1974). Accordingto them these bodies were previously considered to beintrusive granites, but from the adjacent area to the east,it was shown that they are allochthonous masses withinthe Daling rocks (Sinha Roy, 1969). The granite bodiesin the area are also thrust bound and not intrusive, asevidenced by their clear-cut contact relations and by theoccurrence of granite mylonite at the contact zone. Inthese zones the granite is highly sheeted, with well

16 GEOL. SURV. IND

developed mylonite banding. But away from the contact,the degree of crushing decreases appreciably, so that thegranitoid texture is retained. These granites are typifiedby the well developed mineral lineation, marked by feltof biotite.

In major element composition, these granites aresimilar to alkaline granites (Paul et al. 1981). Chakrabarti(1989) on the basis of CaO-Na2O-K2O diagram ofGlikson (1979) classified them as granulites, with aminor spread to trondhjemite field and adamellite field.On the basis of the above mentioned chemical andnormative data they opined that Lingtse granite gneissindicates mixed S-type and I-type character.

Two different ages have been proposed for thegranite-gneiss. Paul et al. (1982) advocated age of 1075±28 Ma which Paul et. al. (1996) revised to 1678 Ma onthe basis of Rb-Sr whole rock isochron.

There have been many opinions regarding the natureand origin of Lingtse Gneiss. Wager (1934) consideredLingtse Gneiss as an intrusive sheet similar to that foundwithin the Mt. Everest Pelitic Series north of Mt. Everest.Auden (1935) considered it as granite gneiss. Ray (1935)recorded the granitic nature of “Lingtse granite” distinctfrom the granodioritic character of Darjeeling Gneiss.Ghosh (1956) considered them as lit-par-lit interfolialinjection gneisses through concentrated igneous activityalong shear zones. Gansser (1964) considered them asdiapthoretic rocks and considered them as foreign toDaling environment. Sinha Roy (1977, 80) opined thatthis gneiss represents wedge sliced up from the basementof the Daling Group. Acharyya (1971, 1980) andAcharyya and Ray (1977) opined that the gneissrepresents partly recrystallised Daling volcanogenicsediments and partly metasomatic granite. Similargranites from adjacent sections of western Nepal areregarded as recrystallised volcano-sediments by Le Fort(1975). Sinha Roy and Sengupta (1986) suggested thatthe Lingtse granite represent a part of the Precambrianbasement (> 1000 Ma) over which deposition of theyounger Daling and Buxa sediments took place in theeastern Himalayas.

In view of the variable mode of occurrence of variousgranite-gneiss exposures at or very near to the contactof Dalings and Central Crystallines, and also within theDaling Group of rocks in the form of smaller bodies(which, at present, have been put under the basket term

of Lingtse granite-gneiss), it is necessary to carry outdetailed petro-chemical and geo-chronological studiesof individual occurrences.

MOUNT EVEREST PELITE FORMATION

Mount Everest Pelitic Formation is exposed in thenorth-western part of Sikkim only and is absent in thenorth-eastern part where younger Tethyan sequence iswell exposed. This formation is well exposed north ofChorten Labsang (27056´:88014´) and continues oneither side; east and west, constituting an inaccessibleridge separating Sikkim from Tibet. The sentinel peakand Chorten Nyima peak (27057´:88011´) are areas wherethis formation is well exposed. Raina and Bhattacharya(1965) carried out geological mapping of this sequence.

Quartz-biotite schist, which forms the base of thisseries, overlies the lime-silicate rocks of ChungthangFormation. The schist is characterized by lit-par-litinjection of granitic material. Under microscope, it iscomposed of biotite, quartz, chlorite and a few grains ofplagioclase. The latter are highly sericitized and showmyrmekitic growth with quartz. A few crystals of highlysheared and fractured sillimanite are also observed alongthe junction with intrusive granites. The sillimanitesshow development of biotite along the boundary.

Quartzites and phyllitic-quartzites are next insuccession. They are very much deformed and fractured;and like the underlying schists, have been affected bygranitisation but only along the base. These are darkbrown in colour and are marked by the presence ofelongated quartz stringers, at times forming boudinage,along the foliation. Microscopic study reveals these tobe essentially recrystallised arkosic sandstonescomposed of quartz with few grains of feldspar and abiotite rich matrix. Some sections show incipient growthof sericite and exhibit very low grade metamorphism.

Phyllites which become prominent towards top ofthis formation are dark in colour and are composed ofquartz, sericite and chlorite with traces of carbonaceous(graphitic) material and exhibit a very low grade ofmetamorphism.

MOUNT EVEREST LIMESTONE FORMATION

Wager (1939) adopted this name for a typicalarenaceous limestone formation outcropping along thetop of Mt. Everest which was subsequently traced


eastward into Sikkim. In Sikkim the formation is exposedin two widely separated patches. The Limestone seriesin the north-east Sikkim is defined from CentralCrystallines by the presence of a fault. In the north-western part, however, the formation overlies EverestPelitic Formation. The limestone was mapped by Rainaand Bhattacharya (1965) and subsequently byRoychoudhury et al. (1998). Raina and Bhattacharya (op.cit.) have extended this name to include a thick formationwhich consists of brown argillaceous limestone, chertyand flaggy quartzite, highly contorted ferruginoussandstone and limestone and typical arenaceouslimestone in ascending order. This series forms the baseof the Tethyan sedimentary sequence in Sikkim.

❑ Mount Everest Limestone Formation containsfollowing litho units

❑ Cherty quartzite❑ Ferruginous sandstones and limestone❑ Arenaceous limestone

Argillaceous limestone is massive andunfossiliferous and form vertical cliffs on the westernsector of the series of Chorten Nyima ridge. Thoughmassive, closer examination reveals a finely laminatednature with calcite stringers both along and across thelamination. Thin sections reveal the rock to beconstituted mainly of calcite with clay matter.Roychaudhuri et al. (1998) have recorded algal mat inthis limestone. The argillaceous limestone is overlainby cherty (flaggy) quartzites which are fragmentary innature and show contortions. Individual beds show anaverage thickness of 2 cm. Quartzites are overlain byinterbedded ferruginous sandstone and limestone, thelatter often showing a brecciated nature. The sandstoneis reddish in colour and show a fine granular surface.Under microscope they reveal the presence of calcareousmatter within the matrix. Iron ore occurs as opaquemineral. Associated limestone bands, on microscopicexamination, were found to be calcareous marlscomposed of fine grained calcite and clay matter.

The topmost formation of this series is a highlyfractured, thinly bedded arenaceous limestone, thetypical Mt. Everest limestone. It outcrops along the topof sentinel and Lhonak peaks. The dark grey limestone,characterized by the presence of thin yellow bands orstringers, weather into a reddish-brown to buff colouredrock. They are practically unfossiliferous and are seamedthroughout by calcite stringers. Though well bedded,individual beds show compact character and exhibit a

thickness of five to six centimeters. The yellow beds,representing arenaceous impurities, stand outprominently in weathered sections. Thin sections showthe rock to be essentially a recrystallised calcite rockwith grains of quartz and feldspar.

GONDWANA SUPERGROUP

In Sikkim, the Gondwana Supergroup is representedby two formations - Rangit pebble slate overlain byDamuda Formation.

Rangit Pebble Slate:

Rangit pebble slate is a very distinct horizon whichoccurs in the ‘Rangit Window’ at the contact of Buxadolomite and Damuda Formation of Gondwana age. Dutt(1954) felt that these beds are important on account ofthe association of marine brachiopod fossils in one ofthe outcrops which indicates the age of the beds of thecoal measures as Upper Palaeozoic.

The pebble slates are well exposed in the roadsection towards Singtep, west of Naya Bazar; on theRangit River and adjoining areas near Tatapani Templeand on the Rishi Khola, north of Rishi. In addition tothese the localities the formation is also observedbetween Tatapani Temple and Rishi as well as north ofNaya Bazar. Marine fossils have been reported fromGondwana sequence near Khemgaon (Ghosh, 1952) andnear Wak (Dutt, 1954). Similar marine fossil bearingsequence is suspected to occur in the diamictite bearingsection exposed on the Rangit River near TatapaniTemple (De, 1982).

The pebble slate is represented by diamictite, darkslate, dark grey claystones, bands and lenses ofsandstone. The pebbles essentially comprise quartzite,calcareous quartzite, sandstone, limestone, stromatoliticdolomite, slaty phyllite and granite gneiss (De, 1982).The matrix of the rock may enclose rock fragmentsranging in size from microscopic to 3 feet in diameter.The enclosed pebbles may be made of diverse rocks suchas porphyritic granite, mica-schist, quartzite, vein quartz,limestone and black slate. The irregular nature in whichthese unsorted fragments occur suggests that they couldhave been deposited by glacial or fluvio-glacial action.The matrix is usually a black clay-slate very much likea glacial till, but it is white in colour and siliceous ontop of the Salebong (27°9:88°23) peak.

At an altitude of 1494 m south-west of Khemgaon

18 GEOL. SURV. IND

(27°11:88°22) a conglomerate in which well roundedelliptical pebbles of quartzite, mica-schist, vein quartzand granite, measuring up to 20 cm across are present,contains distorted invertebrate fossils including that ofspirifer. The thickness of fossiliferous horizon is about100 feet. Between Mangble (27°11´:88°21´) andRanguthang (27°12´:88°22´) four bands of limestonealternate with boulder slate. Some of these, at the contactof the limestone, contain casts resembling invertebratefossils. These casts are almost totally replaced bylimonite.

In the Rangi Khola (27°12´:88°22´) section, the slatecontains nodules and lenses of calcareous matter someof which appears to have been formed syngenetically.

Boulders slate is found as continuous outcropsextending WNW from Manpur Khola section to theSalphuk (27°09:88°21) peak. They are alsointerstratified with sandstone and carbonaceous shalein all the nala sections south of the Baisalu(27°08´:88°18) – Asangthang (27°09:88°20) ridge,between Kitam (27°07:88°21) and Sorokpani(27°08:88°21), at Dumra (27°11:88°20), in Khemgaonarea and on either side of the Pabong stream.

Outcrops of pebbly slate are recorded on the descentto Tarkhola from Sangser, about 1/2 km east of Tarkholaand in the Rishi village opposite Rishi-Rangli confluence(here this appears to be grading upwards in togreywacke). Dutt (1955) found black slate, similar tothe Gondwana slate in Namchi area in western Sikkim,on the northern slopes of the hills south-east of YumChho (27°03:88°42). Besides blue calcareoussandstone, he found a conglomerate with brown clayeymatrix in which pebbles of chert, quartzite, limestoneand slate were embedded. In appearance they resembledcertain pebble beds of western Sikkim.

When the size of the pebbles varies between 1 mmand 30 mm, the rock exhibits nature of a greyish-yellowsplintery slate. With the increase in size of the pebbles(5 cm to 30 cm) the rock almost looses its slaty characterand gives conglomeratic appearance. Pebbles showelongation which is parallel to dip direction. Quartziteis the main pebble constituent, followed by granite,dolomite and shale. Acharyya (1971) published detailedaccount of the Rangit Pebble slate.

Das et al. (1984) located small exposure of

Gondwana rocks in Kali Khola section, about 3 km southof Pakyong in east Sikkim, just to the east of Bhasme.The exposure is in the form of a small window zone andis surrounded by Daling phyllite. The pebble-phyllite islight grey in colour and contains pebbles of quartzite,phyllite, grey slate, granite and quartz vein embeddedin argillaceous matrix. The pebbles are stretched andaligned parallel to the foliation plane. The Gondwanashave been affected by F2 and F3 folding.

Damuda Formation:

Damuda Formation is one of the most prominentunits of the Rangit Window, and prominently exposedon Naya Bazar-Legship road, Jorthang-Namchi road andsurrounding areas. The exposures of this unit are alsorecorded in a small window about 6 km south of Pakyongon the northern bank of Rangpo Chu (stream). Mallet(1875), Dutt (1954, 1955, 1961), Raina (1966) havemade observations on Gondwanas of Rangit Window.

The formation comprises sandstone, calcareoussandstone, shale, carbonaceous shale with thin coal beds.The sandstone and siltstone at places exhibit crossstratificaton and channel structures. The sandstoneoccurring in this group are coarse grained, micaceous,ferruginous, and of either brown or blue colour.Sandstone of grey colour is found in the Rangit riversection north of Nayabazar (27°08´: 88°17´) and betweenKamrang (27°11:88°21) and Denchong(27°10:88°20). Hard, greyish-black, felspathicsandstones and arkose grit are located on the right bankof Rangit, one mile north of Nayabazar. A rockresembling grit is found in the stream east of Burntar(27°9:88°22), at Namchi (27°10: 88°22) and in theBhondi Khola (27°08:88°19) section (Dutt, 1954).

The sandstone is usually intercalated with blackfissile slate, carbonaceous slate and occasionally withthin impure coal horizons. Sandstone is far in excess ofargillaceous rocks south of the Salgari (27°08´:88°18´),Asangthang ridge, but north of it they are subordinateto slate. The sandstone so imperceptibly grades into slatethat it is sometimes difficult to draw a stratigraphicalboundary between the two. At places, such as in smallstream west of Bhondi Khola, the sandstone is twicerepeated in a bed of slate.

In the Rangit section at Nayabazar, the sandstonegroup occurs adjacent to the magnesian limestones.Sandstone is developed just above the purple and green


slates at Samdong (27°09´:88°19´). Such arenaceousbeds are also found in contact with the limestone bedsnear Salebong (27°09´:88°23´).

Under microscope the sandstone comprises of sub-angular to rounded grains of quartz, plagioclase,muscovite, biotite and opaque minerals. Larger grainsshow strain effects. Some of the feldspar and quartzgrains have completely sericitized. Muscovite and biotiteshow alteration to clay minerals. Finer variety ofsandstone has similar mineral composition but has morerounded grains.

Frequently the sandstone has been converted intoquartzite, the shale into splintery slate and carbonaceousshale into carbonaceous or even graphitic schist; whilethe coal has lost a large proportion of its volatile matter,so as to approach to anthracite in composition. Theamount of metamorphism in the Damuda is by no meansconstant, generally the beds are more or less altered andthe rocks closely resemble typical ones of the Raniganjfield.

Small structural windows of Gondwanas also occurnorth of Majhitar, in Ratu Khola, Tarkhola and at KaliKhola in eastern Sikkim (Dutt, 1961). Plant fossils sofar have not been recorded from last three localities.

Fossils in Gondwanas:

In the geological succession of the area the coalseams contain plant fossils such as Glossopteris,Gangmopteris and Vertebraria. This assembelage is alsoseen in carbonaceous shale and slate. Recently Pal andWanjarwadkar (2002) have recorded Calamites,Palaeopteridium, and Lesleya etc. from Damudasandstones of Rangit Valley. On the basis of this fossilfinds the age of the coal bearing sandstone becomesLower Carboniferous.

The sandstone bed is underlain by pebble andboulder slate in grey argillaceous matrix and Brachiopodand Lamellibranch bearing beds of Wak (27°13:88°21)by Dutt (1954) and Khemgaon (27°11:88°23) by Ghosh(1952).

Dykes in Gondwanas:

The sandstone-slate sequence is intersected bysills of lamprophyre at Dumra (27°11:88°20),

Kamrang (27°11:88°21) and in the section of theRangit River. When in the contact with thecarbonaceous beds the lamprophyre has produced arock resembling coal. Under microscope the rockshows sericitized feldspar, pale coloured biotite assmall flakes, carbonate and long corroded streaks ofa dark opaque mineral. Apatite, orthoclase, spheneand epidote are also observed.

Some 300 feet above Salebong, a 30 feet thick dykeof orthoclase-porphyre (orthophyre) intersects thedolomites. The minerals are Orthoclase (80%), biotite(7%), haematite (8%) apatite, quartz etc. make up theremainder (5%).

LACHI FORMATION

Sediments of the Lachi Formation are exposed inthe north-eastern part of Sikkim only. The Tethyansediments start with a basal conglomerate. TheFormation is represented by a sequence of quartzite andshale with some pebble beds. This rock sequence isexposed to the north of Dongkya Range, in the vicinityof Chho Lhamo Lake (28°01:88°46) and the hillsconsisting of these sediments is named as Lachi hill.The sequence is exposed in the form of small roundedhillocks with smooth topography. Lachi hill, a continuousridge of small hillocks trending roughly north-south witha height of about 300 metres or so above the surroundingplains, forms a barrier between Gurudongmar Chhu(stream) in west and Chho Lhamo lake in the east. Thisridge is in contact with the Central Himalayan range onthe southern side and is cut off from the Kampa DzongSeries of Tibet, in the north, by the valley of the TistaRiver.

Wager (1939) first studied this formation and namedit as Lachi Series, allotting a Permo-Carboniferous ageon the basis of fossil evidences. This was furthersupported by A.M. N. Ghosh (1952 and 1956) bycorrelating pebble bed of Lachi Series with the boulderslate bed of Rangit Valley. Auden discovered Triassicfossils towards the top of this formation and came to theconclusion that the top of Lachi Series is Triassic inage. He renamed the top of this formation as ChhoLhamo series. G. N. Dutt (1955) examined in the ChhoLhamo- Lachi hill area and indicated presence of theLachi sequence. Raina and Bhattacharya (1962, 1965)carried out systematic mapping of this sequence and laterRaychoudhury et. al. (1998) worked in this area.

20 GEOL. SURV. IND

Lachi Formation comprise the following lithologicalunits

◆ Fossil bearing calcareous sandstone, slate andquartzite

◆ Fossil bearing argillaceous limestone andcalcareous sandstone

◆ Pebble beds and conglomerate◆ Crinoidal limestone, infolded shale and

quartzite

The base of Lachi Series consists of infoldedquartzite and shale with a thin band (4 m. thick) of fossilbearing crinoidal limestone (Raina and Bhattacharya,1965). The quartzite is hard and cherty and stands outas ridges whereas the shale forms depression. Quartziteis light to greyish brown in colour and show a coarsematrix. Brachiopod fossil- Syringothyris of Lydekkeri(Dien) of Upper Permian age was found in one of thequartzitic sandstone boulders covering the quartzite(Raina and Bhattacharya, 1962).The quartzite showswell bedded nature while the shale is fragmentary andshows the tendency of breaking into thin rectangularfragments. This shale is devoid of any plant fossils. Shaleis found to be harder in nature and is almost slaty incharacter. Thin dark shale bands which appear to becarbonaceous are also seen to be interbedded with theshale. The limestone band occurs in the upper parts ofthe shales. It is dark crinoidal limestone showing pittedsurface and fragmentary nature which has been causedby the intersection of two prominent joint systems. Undermicroscope; it proved to be a bryozoan limestoneconsisting of bryozoan fragments in calcareous matrix(Raina and Bhattacharya, 1962). Fossils collected fromthis zone could not be properly identified as they arenot properly preserved.

The pebble bed forms the most prominent zone ofthe Lachi ridge. Its outcrop width is about one kilometerthough it is only 200 m thick. Auden (1935) hascorrelated them with the Talchir boulder bed. Angulargrains (pebbles?) of quartz, shale and sandstone arefound in siliceous matrix. In general it looks like a pebblebearing slaty grey coloured siltstone. Actually it showsa regular variation in grain size of the matrix as well asthat of the pebbles from bottom to top. At the bottomthe matrix is silty with pebble size between one to twomillimeters. This is followed by coarse sandstone withpebbles ranging in size from 5 to 10 mm. It becomes aboulder bed formation with boulders ranging in size from30 to 100 cm towards top. The pebbles are mostly of

quartzite and show a subangular, subrounded or ellipticalshape. Larger boulders show a well rounded and polishedsurface. In addition to quartzite, pebbles of granite, shaleand limestone are also encountered. These generallyshow a flattened (bladed) or disc shape. Undermicroscope, the matrix is formed of clay associated withsmall grains of quartz and feldspar. Within this matrix,rounded and angular grains of quartz, quartzite, calcite,shale and mica-gneiss are seen. Veins of calcite cut acrossrocks.

The pebble beds are overlain by a zone of highlyfossiliferous calcareous sandstone and arenaceouslimestone of about 100 m thickness. In some parts thepebble beds are overlain by carbonaceous shale (inferiorcoal seams) and splintery shale. The carbonaceous shaleis very much puckered. The splintery shale is similar tothe shale forming the base of the sequence. Thesandstone and arenaceous limestone are reddish-brownin colour, show a sandy surface and resemble quartzites.It weathers into irregular fragments. Raina andBhattacharyya (1965) recorded a large number of fossilsfrom this calcareous sandstone and arenaceous limestoneof Lachi Formation. These are Productus(Linoproductus) cora d’Orb., Productus cf.semireticulatus, Productus semireticularus Martin,Productus (Dictyoclostus) cf . spiralia Waagen,Productus (Dictyoclostus) cf. gratiosus, Productus(Linoproductus) cf. kulikid Freedericks, Productus(Waagenoconcha) purdoni (Davidson), Productus (Sp.Unidentifiable), . Spirifer (Spiriferella) rajah Salter.Spiriferina (Spriferellina) zewanensis Diener, ChonetesWageri Muir Wood, Chonetes (Sp. Unidentifiable),Marginifera himalyensis Diener, Marginifera sp,Derbyia cf. dorsoplna, Protocetapora ampla Lonsdale,Ingelarella pelicanensis. The fossil assemblage indicatesa definite Permian age (Raina and Bhattacharya, 1965).Some of the fossils like Marginifera Himalayensis,Diener Spirifer, (Spiriferella) rajah Salter are thecharacteristic fossils of Permian horizons in other partsof Himalayas.

This highly fossiliferous horizon is overlain by 120m thick horizon of quartzite and slate. Towards top, fossilbearing calcareous sandstone bands are seen infoldedwith the slate. The quartzite which shows a yellowishcolour often reveals a brecciated character. Associatedslate beds, characterized by a cleavage, show a highlyjointed nature. Under microscope the slate is entirelyformed of clay matter with few arenaceous patches.


Associated fossil bearing calcareous sandstone, showinga weathered pitted surface, do not outcrop so commonly.These are encountered in Chhothina (28°04:88°42)section only and are missing in Lachi ridge. Among thefossils Productus (Linoproductus) cora d’Orb, Spirifertrigonalis Mart, Spirifer Lydekkeri Diener, and Spirifercurzoni, Chonetes Wageri Muie wood, Pleurotomaria(Trepospira) cf. Chitralensis Reed, Pleurotomariasp.,Lima sp. are most prominent ones. Fossilidentification indicates an upper Permian age for thishorizon.

The topmost formation of this series is a thickquartzite which may represent a different zone altogether.The quartzite is exposed with a width of 1000 m or so.They are light grey in colour and are associated withcherty white bands. Towards the contact with theunderlying formation, they show brecciated nature. Athin section of white band revealed recrystallised quartzrock metamorphosed at very low temperature. Thestratigraphic position of this quartzite band is ratherdoubtful. Although at present it is placed within Lachis,it could, however, represent the base of Triassics whichis missing in this area.

Roychoudhury et al (1998) have recorded sprifer,Neophricadothyris, Lissella, Daviecella, Echinaria,Muirwoodia, Squamaria, Reticulatia, Tschernyschewia,Spiriferellina, Syringothyris (s.l.), Stenoscisma,Notospirifer, Eospiriferina, Probolionia, Sinuatella,Productus, Taeniothaerus and Planolites. Fossils werecollected from different levels at Dorjila hill section andhave been grouped into four assemblages. Lower oneoccurs close to the contact of Everest LimestoneFormation and is identified by Eospiriferina - Daviecella-Reticulatia assemblage. Middle of the succession ischaracterized by Syringothyris-Spirifer assemblage. Thisassemblage is correlated with Syringothyris Limestonefossil assemblage of northwestern Tethyan Himalayawhere Lower Carboniferous age has been assigned tothis assemblage (Middlemiss 1910). Next assemblageis the Productus-Sinuatella assemblage. This assemblageis correlated with those from Po Series (Lr.Carboniferous) of Spiti and with lower part ofFennestella Shale (Up Carboniferous) of Kashmir.Towards the top, most richly fossiliferous association isrepresented by Muirwoodia-Athyris-Spririferellaassemblage having close faunal similarity withbrachiopods of Nihal, Nabi and Girthigal Formations ofKumaon-Garwal Tethyan sequence. Such a rich and

diversified brachiapods of Lachi Formation match withZewan Formation (Mid-Up Permian) of Kashmir whichcontains numerous colonies of sessile benthonics ofsimilar characters. Thus, Lachi Formation isprovisionally dated Upper Devonian to UpperCarboniferous (Middle Permian).

CHHO LHAMO (TSO LHAMO) FORMATION

This sequence of rocks is exposed only in the formof a small patch in north-east Sikkim. The name “ChhoLhamo” was given by Auden (1935) to an ammonitebearing horizon of limestone, sandstone and shale onthe eastern side of the Lachi ridge. This was subsequentlyexamined by Raina and Bhattacharya (1962, 1965), andRoychoudhury et al. (1998).

This Formation is represented by sandstone at thebase, overlain by limestone and shale. They show afaulted contact with the rocks of underlying LachiFormation.The entire Formation is tectonically disturbedresulting in a faulted contact between the limestone andoverlying shale. On the eastern bank of Chho Lhamo,limestone is seen overriding the sandstone and shale ofLachi Series.

Basal sandstone is hard and slightly coarse in textureand grey to light brown in colour. The sandstone containswell rounded nodules of hardened clay, a few of themhaving a limestone core. Towards its junction with theoverlying limestone this sandstone become calcareousand exhibits a flaggy character. They are quite rich ininvertebrate fossils especially brachiopods and corals.A few ammonite fossils were also collected. This horizonwas found to be rich in Rhynchonella Thinodosi showingMuschelkalk (middle Triassic) affinity. The fossilsidentified are as follows: Pseudosageceras. Sp,Spiriferina sp, Rhynchonella cf. trinodosi. Bilt,Rhynchonella sp, Orthoceras sp., Chonotes sp.(Rainaand Bhattacharyya 1963, 1965)

Black coral bearing limestone, which overly thesandstone shows a weathered wrinkled surface due tothe presence of corals. The rocks are black and wellbedded and jointed. They also reveal Muschelkalk agefrom their fossil assemblage among which Meekocerasis the most prominent. Towards top they alternate withshale.

The shale is from the top of Triassic exposures. This

22 GEOL. SURV. IND

horizon is formed of shale and quartzite with interbeddedthin bands of limestone. The shale is very rich inammonite fauna and reveals a Muschelkalk age.Cephalopods form the major proportion of the fossilscollected from this horizon, though brachiopods andlamellibranches are also present. The fossils identifiedin this horizon are as follows: Ceratites trinodosus Mojs,Ceratites dimorphus, Ceratites sp., Meekocers cf.nalikanta Diener, Meekoceras khanikofi Oppel,Meekoceras kesva Diener, Proptychites plicatus Waagen,Pleurophors cf. curonii Hau Lam , Dielasma cf.himalayana Bittner, Athyris sp., Euomphalus sp.,Pleorotomaria sp., Lima sp. ind. Aff. Subpunctatae D‘Orb., Gervillia cf. praecursob Quen, Meekoceras cf.affine E.V. Mojs, Meekoceras cf. Rhanikofi opal,Ptychites Sp., Ceratites ef. Brinodocus Mojs, Ptychitescf. Sumitrs Diener, Ceratires sp. (op. cit.)

Roychoudhury et. al. (1998) reported that 740 mthick Chho Lahmo Formation has yielded ninefossiliferous horizons where cephalopod is representedby Genus Salterites Diener 1907 and TibetitesMojsisovics 1896. Salterites belongs to similar affinitywith Otoceras zone (Lower Triassic) of western TethysHimalaya and Tibetites being marker genera of UpperTriassic (Lower Noric) beds of Himalaya, is correctablewith ammonoid assemblage of Holorites beds ofKumaon Himalaya(op. cit.).

INTRUSIVE GRANITES

Throughout the Himalaya, two main variants of lateintrusive granites have been documented, viz. a) Twomica granites and b) tourmaline granite. This is true ofDarjeeling Sikkim also. Intrusive granites are generallyrecorded in Central Crystalline Gneisses. These occuras sills and discordant intrusives. Apart from these thereare local products of partial melting such as aplites andpegmatites.

These intrusive granites such as biotite granite,tourmaline granite and pegmatite veins had been reportedin north and west districts of Sikkim by Bhattacharyaand Pattanaik (1964), Raina (1966), Raina andBhattacharya (1965), Sinha Roy and Ray (1972), andNeogi (2004).

Heron (1922) was the first to notice presence of thesetourmaline granites (termed as schorl granite) in theEverest Region. He states “the schorl granite is the latest

in the age of igneous rocks and occurs practicallyeverywhere in the crystallines examined”. Hayden(1907) regarded unfoliated granites of Chumbi Valleyas equivalent to the schorl granite of Everest. Since theChumbi granite has intruded Jurassic beds, both it andthe related granite of Sikkim and Everest were presumedto be of Tertiary age.

Biotite granite

It has been observed in the north-eastern part ofSikkim Raina and Bhattacharya (1965) intruding intofoliated biotite granite and gneisses in the form of lit-par- lit injections, as thin veins along the foliation planeof country rock and also as small bosses. The bossescontain xenoliths of older gneisses. It is also observedto have intruded into the basal rocks of the Everest PeliticSeries as lit-par-lit injections. Sinha Roy and Ray (1972),while working in parts of toposheet 77 D/12 and 78 A/9, observed following modes of occurrence:

(i) Small pods at the core of small scale folds and atthe construction zone of boudins in banded gneiss.

(ii) Along strain-slip cleavages within banded gneiss.(iii) As thin veins cross-cutting the gneissosity in which

case a relic foliation is preserved within the granitethrough the orientation of recrystallised biotite.

(iv) As concordant bodies within biotite-rich bandedgneiss where there is selective crystallization ofgarnet in the host rock around the biotite granite.

It is a medium grained rock with quartz, feldsparand biotite as the main mineral constituents. Thisbiotite granite, like tourmaline granite, is exposedfrom Chungthang to Jakthang on Tista and Zemuvalley and from Phentong to Phyaggu on the TolungValley. It is best developed between Menshithang(27°40: 88°3530) and Lachen (27°44:88°33) inthe Tista Valley. South of Menshithang, a zone ofaugen gneisses is seen which has been formed bythe massive intrusion of biotite granite. A thin sectionfrom Lungme (27°52:88°21) stream shows quartz,biotite, muscovite, minor chlorite, zoned plagioclaseand microcline. Plagioclase is earl ier thanmicrocline. Another thin section of the rock collectedfrom Khora Phu (27°57:88°18) showed presence ofoligoclase and a few almandine garnets. The biotitecrystals vary from small flakes to big books. Thebooks are generally seen near the contact with thecountry rock.


Tourmaline granite:

The part of North Sikkim, especially fromChungthang to in Tista Valley to Jakthang (27°4730:88°29) in the Zemu Valley and from Phentong(27°3446:88°25) to Phyaggu (27°42:88°2630) inthe Tolung Valley has been criss-crossed by numerousgranitic masses and aplitic veins. Granite and pegmatitesveins of varying thickness of 1 cm to nearly one kmhave been met with (Bhattacharya and Pattanaik, 1964).They are seen mainly within the area betweenChungthang (27°36:88°38) and Lachen (27°44:88°33). At some places, these intrusions have takenplace along the fault and joint plane. It occurs as smalllenticular bodies (3 to 6 m thick) disposed athwart thegneissosity. When they are located near the mesoscopicfolds in banded gneiss, they invariably occur along theaxial plane. Near the contact with the wall rock,tourmaline is aligned parallel to the contact plane. Buttowards the centre of the body, this alignment is lostand tourmaline shows crude dimensional orientation atright angles to the contact. The granite varies from finegrained homogenous granite to a coarse pegmatite intexture. They vary in colour and texture. Colour variesfrom milky white to light brown to light grey. Thesegranites have been found to be very rarely affected byfaulting and folding of the Himalayan orogeny. Thesegranites are medium grained with different sizes ofprismatic crystals of tourmaline in a ground mass ofplagioclase and quartz. Under microscope, it reveals acoarse grained nature with tourmaline, quartz, biotiteand feldspar as main constituents.

In Tista valley, very near to Jorepul (27°42:88°34),a tourmaline bearing granite containing Topaz has beenlocated. Auden (1935) has reported the occurrence oftopaz bearing granite from a place on the Tista Valleywhich is nearly 48 km north of this occurrence. The topazcrystals are 1.5 cm to 2 cm in length; flesh coloured andirregularly arranged. The zone is 30 cm thick and nearly8 metres long.

Near Jorepul, a little south of the locality referredto the above section, a massive well jointed, unfoliatedzone of tourmaline granite is seen. The tourmaline occursas clusters of radiating fine needles occupying an areawhich is rectangular in shape. Auden (1935) has alsomentioned the presence of clustered tourmaline granite,well jointed and massive in nature, on the Sibu ChuValley in northeast Sikkim.

Pegmatite and Aplitic granite:

In north district, Sikkim the pegmatites, except forthe coarse nature of its minerals and its occurrence asveins and stringers, it shows the same mineralassemblage as the biotite granite. In field they areobserved intruding along well developed joint planes,crack and fissures.

In West District, Sikkim the pegmatites intrude intogneisses and schists along well developed joints andfissures. They do not appear to be intruding in to thefoliated granites.

Thin section shows zoned coarse grains ofplagioclase and oligoclase and inclusions of quartz andbiotite. Garnet and apatite are accessories withoccasional tourmaline.

Aplitic granite is a fine grained, homogenous massof grey and black clots in which biotite grains are easilydiscernable, and occurs as thin bands varying inthickness from 1 cm to 10 cm. These intrude the countryrock along N75W-S75E joint planes. Under microscopeit is composed of fine grains of quartz, feldspar andbiotite with flakes of muscovite and chlorite. Feldsparis oligoclase. In one section garnet and epidote are seen.They intrude into earlier granite.

SYENITIC ROCKS

The Ultrapotassic syenitic rocks of Sikkim occur asintrusive within the “Dalings” of Sikkim Himalaya.Syenitic rocks are reported from many localities. Vermaet al, (1983) presented an account of these rocks. GhoshRoy (1996A, 1996B, 1997) studied these rocks in greatdetails. Small bouldery outcrops of syenitic rocks occurat many localities within the Daling Formation in SikkimHimalayas near Legship, Ben, Damthang, Rishi, andManiram. Although their intrusive nature can berecognized at places, their disposition is not clear dueto paucity of outcrops and small dimension of the bodies.

They are of two varieties. One is coarse grainedmesocratic rocks having dark greenish grey color. Coarsefeldspars and fibrous blue riebeckite are often discerniblein hand specimen. Anastomising veins of riebeckite ofvariable width are also common. The rock is composedof orthoclase, biotite, aegirine and riebeckite. Orthoclaseis mostly subhedral. Anorthoclase is a minor phase. The

24 GEOL. SURV. IND

biotite has a deep reddish brown colour, and distinctcolour zoning with dark reddish brown cores and lightercoloured rims. The aegirine grains are often bleached.Riebeckite is distinctly a late phase and replaces micaand feldspars. Brown katophoritic amphibole also occursas minor phase in the syenitic rocks. Sphene is a commonaccessory mineral in these rocks. This variety of syeniticrocks shows occasional porphyritic and crudepanidiomorphic texture. The rocks have very high K2O(4.87 - 10.31%) compared to Na2O (1.76 - 2.49%) andthey also contain high TiO2 (2.27 - 4.47%) which impliestheir ultrapotassic nature. Presence of acmite andpotassium metasilicate in norms of some samples andpresence of modal aegirine and riebeckite indicate theirperalkaline nature. Trace element data for these rocksreveal that they contain high amount of Rb (40 - 160ppm), Ba (300 - 3900 ppm) and Sr. (210 - 3600 ppm).

The other variety is comparatively finer grainedhaving dark ash-grey colour. These are generallymassive. This finer-grained variety is composed oforthoclase and katophoritic amphibole as essentialphases with minor amounts of riebeckite and phlogopiticmica but does not contain aegirine. These rocks exhibitpanidiomorphic texture. They contain high SiO2, Al2O3and total alkali and less TiO2, MgO, CaO and total ironand also higher Zr, La and Y, lower Ba and Sr comparedto the former group.

Although the rocks are apparently of syeniticcomposition, these are in many respects dissimilar totypical alkali syenites. Absence of microcline/microclineperthite, occurrence of colour zoned phlogopitic micaand dominance of aegirine-riebeckite and katophoriticamphibole exhibiting panidiomorphic texture withevidences of autometasomatism and typicallyultrapotassic chemical character indicates that theserocks are more akin to lamprophyres rather than typicalalkali syenites.

Sodic-calcic (katophorite/anophorite) and alkaliamphiboles (arfvedesonite / riebeckite) are reported fromlamprophyres and primary alkali amphiboles are mostlyreported from minettes but may occur as a secondaryphase in all calc-alkaline lamprophyres (minettes,kersantite, vogesite, spessartite etc) (Rock, 1991).Besides, Ca-Na pyroxenes (sodian augite and aegirineaugite) have been reported from all lamprophyres (Rock,op cit). Thus, the mineralogy coupled with chemicalcharacters of these syenitic rocks suggests them to belong

to the lamprophvre clan and are likely to be soda minettesrather than ‘syenitic’ composition.

The two groups of syenitic rocks have been foundto occur in close proximity. Close to them, neither typicalminettes nor leucite or olivine lamproites have beenfound. However, a coarse grained rock comprising fresholivine, diopsidic augite, phlogopite, richterite as majorphases have been found near Rishi. This rock showsdistinct evidences of ‘modal metasomatism’. Theinvading veins had given rise to secondary phlogopiteand apatite and the veins also contain minor amount ofcarbonate. The rock having higher 100 Mg/ (Mg + FeT)is fairly primitive and it appears that the original rockhas been considerably modified by the metasomatisingfluids. This rock is likely to be a richterite-peridotide ofdeep seated origin and the textural features observedare comparable with the features considered to represent‘mantle metasomatism’. It is probable that the originalrock was a mantle rock of lherzolite or garnet lherzolitecomposition which has been substantially modified bymetasomatism. In that case, apart from olivine, diopsideand some phlogopite, richterite, phlogopite and apatitehave possibly resulted from the invading metasomaticfluids. The metasomatising fluid was partially rich inCO2 as is evidenced by presence of carbonates in theveins. For the syenitic rocks, this rock may representthe source material from which the magma for theultrapotassic syenitic rocks had formed by partial meltingof the source material (Ghosh Roy, 1997).

SESELA FORMATION

Rocks of Sesela Formation are exposed in the northeastern corner of Sikkim state bordering Tibet. They areexposed along Bomchho area in the north to 5554 mpeak in southeast through 5456 m peak and Sesela peak.They are best developed along Kareng-Sesela axis andrepresented by 200m thick sediments and are named asSesela Formation (Roychoudhury et. al. 1998).

Sesela Formation comprises well rounded, disc andball shaped pebbles (few millimeters to 10 cm diameter)of shale, sandstone, limestone, argillite, quartzite,granites and tilloids embedded in a sandy matrix. Rocksare crudely banded, unconsolidated to semi-consolidatedin nature.

Alternation of variegated clay, fine and medium sandand pebble beds indicate bedding which is mostly


horizontal or very low dip (2-5°). The more accessibleKerang-Sesela section has been studied in detail whereconglomerate, grey, yellow, brown sand, sandy clay,pink, black-brown clay and pebbles layers have beenfound in irregular alternations.

Most of the pebbles of para-conglomerate andconglomerate are well polished of different size andshape and are made-up of shale, quartzite, limestone,sandstone and granite of older horizons. Sediments wereapparently laid down in shallow to moderately deepdepressions or lakes and transported and rearranged bystreams. No fossil has been recorded from these rocks.Sesela Formation is provisionally correlated onlithological criteria with Upper Karewa Formation ofKashmir, Potwar Silts of Potwar plateau and similardeposits of Trans-Himalaya, and assigned pliostoceneage.

Sesela Formation, presumably of Pliestocene agedeposited in shallow to moderately deep depression orlake, is of fluvio-glacial nature, but well polished,pebbles of various size and shape are result of long termrolling, dumping and transportation. Restricted outcropof this Formation suggests that present sectarian ChhoLahmo Lake is a closed remnant of large sized watermass, thought to be of tectonic origin or obstructeddrainage channel by glacial moraines.

STRUCTURE AND TECTONICS

The present tectonic framework of the Himalaya andadjacent orogenic belts is the result of tectonic collageof autochthonous to allochthonous linear crustal blocks,geologically unrelated to each other and sutured togetherduring different periods of its evolutionary history. Suchunrelated crustal blocks of regional extent having theirdistinctive tectono-stratigraphic characters, which aregenerally fault bounded, are being termed “terranes”following Howell (1985). The evolution of the accretedallochthonous terranes of different ages, forming theHimalaya and adjacent orogenic belts, by fragmentationand reassembly of the north-eastern margin of the greaterGondwana super continent has been advocated byMolnar and Tapponier, 1975; 1977; Bassoulet et al.,1980; Nur and Ban-Avraham, 1982; Allegre et al, 1984;Valdiya, 1988; Thakur, 1996; Acharyya, 1996; Ghose,2000a, 2000b and many others in recent years.

The Himalaya and adjacent parts of the Trans-

Himalayan regions are generally subdivided into thefollowing morpho-tectonic units from north to south.

Karakoram/Changtang Belt—Shyok (SS)/Bangong-Nujian (BNS) Suture Zone —

Kohistan/Ladakh/Lhasa Terrane—Indus Tsangpo Suture Zone (ITS)—

Tethyan ZoneSouth Tibetian Detachment System (STDS)

Higher Himalayan Zone—Main Central Thrust (MCT)—

Lesser Himalayan Zone—Main Boundary Thrust (MBT)—

Sub-Himalayan (Foothill) Zone—Main Frontal Thrust (MFT)—

Indus-Ganga-Brahmaputra foreland basins

These morpho-tectonic units also representdistinctive tectono-stratigraphic terranes, bounded bymajor thrusts or sutures and constitute the main tectonicframework of the Himalaya and adjacent regions. TheHimalayan region, sensu stricto, comprising Higher,Lesser and Sub Himalaya (Foothill) is bounded by theIndus-Tsangpo Suture (ITS) in the north and the MainFrontal Thrust (MFT) in the south. A substantial part ofthe Himalayan region constitutes recently elevatedIndian platform (Wadia, 1966) and the main Himalayanregion upto the Indus-Tsangpo Suture is represented bythe reactivated northern part of the Indian shield(Gansser, 1980 and1981).

The State of Sikkim encompasses parts of LesserHimalayas, Higher Himalayas and Tethyian Zone. Mainboundary thrust (MBT) just passes through the areaslightly south of West Bengal-Sikkim boundary. MainFrontal Thrust (MFT) between Siwaliks and Neogenesediments is located further south. Main Central Thrust(MCT) passes through the Central part of the State. Thenormal fault (STDS) between Central Crystallines andTethyan Sequence passes through the northern part ofthe State. There is one tectonic window named as Rangitwindow which exposes subthrust rocks of the LesserHimalayan Package.

Tethyan zone lying between the Indus-Tsangposuture in the north and Higher Himalayancrystallines(HHC) in the south is represented by nearlycomplete stratigraphic sequences of the northerncontinental margin of India over the LateNeoproterozoic-Eocene interval with some breaks in theform of unconformitics / diastems.

The base of this Tethyan sedimentary sequence has

26 GEOL. SURV. IND

long been regarded as a pronounced unconformityoverlying the high grade metamorphic rocks of theHigher Himalayan sequence (Gansser, 1964: Stocklin.1980). A major fault system has, however, been recordedat many places along the southern boundary of theTethyan zone in the last two decades (Burg et al., 1984;Burchfiel et al., 1992; Searle, 1986; and Valdiya, 1987).Valdiya (1987) delineated Trans-Himadri Thruststretching for 1600 km between Ladakh and Sikkim thatseparates the Higher Himalayan Crystalline Zone fromthe Tethyan sedimentary zone. The existence of such afault is however, not seen at all the places and the LateNeoproterozoic sequence commences unconformablyover the crystalline with diamictite and conglomerate atthe base.

However, in Sikkim the presence of Fault is reportedby Raina and Bhattacharyya (1965) and Roychoudhuryet. al. (1998). It has been observed that all along thecontact of Tethyan Sedimentary Sequence withunderlying Central Crystalline Gneissic Complex,bedding trend of Tethyan are abuted against the foliationsof rocks of CCGC. There is tectonic pinching andswelling, tear apart and scattered occurrence of Tethyansequence all along the north eastern part of Sikkim.Slices of Lachi Formation within Chholahmo Formationare additional references to such disturbances(Roychoudhury et. al. (1998).

The Tethyan Sedimentary sequence is in generalunmetamorphosed and most of the rocks of TethyanSedimentary Sequence shows prominent bedding andis defined by colour banding, lithological variation andgrain size variation. Among Tethyan successions,lineations and at places kinks are present in EverestLimestone Formation.

The Higher Himalayan zone, defined by the MCTin the south and arbitrarily demarcated in the north bythe appearance of marine Tethyan sequence, representsthe metamorphic core of the Himalaya. A continuousbelt of high grade metasedimentary and meta-igneousrocks and associated granitoids commonly known as theCentral Crystallines are exposed all along the HigherHimalayan zone.

Higher Himalayan rocks preserve imprints of poly-phase deformation and related fabric elements (Nahaand Roy, 1970; Sinha Roy, 1977). In the micro-scale,the dominant structural surface in the pelitic rocks has

been correlated with the penetrative foliation S2, axialplanar to a set of, locally preserved, tight isoclinal folds,F2 formed during the D2 deformation. A relict earlyfabric, S1, related to the earliest deformation D1, isoccasionally recorded in the pelitic rocks, defining microfolds within the S2-related layering, or as internal trailswithin syn-S2 garnet prophyroblasts. The pelitic unitsalso show a D2 mylonitic microfabric, SD2m,characterised by narrow bands of fine grainedrecrystallised quartz-feldspar aggregates whichanastomose around porphyroclasts of feldspars andquartz. The SD2m fabric is generally conformable withS2 and often mutually inseparable. The most commonfolds (F3) in the Higher Himalayan pelites, however, arethose developed on the S2 folia during a later deformationD3. A set of weakly developed crenulation cleavage, S3,have locally developed parallel to axial planes of thesemeso and macro scale folds and are marked by incipientgrowth of biotite and chlotite. The high grade HHC isconsidered here as a dominantly gneissic thrust sheetoverlying the metasedimentary mica schist-quartzite ofthe Lesser Himalayan stack along the Main CentralThrust (Neogi et.al.1996).

Main Central Thrust:

Two distinct tectono-stratigraphic terranescomprising Higher and Lesser Himalayan zones,corresponding to two heterogeneous morphotectonicdomains, constitute the main Himalayan region and areseparated by a major intracrustal thrust zone termed asthe Main Central Thrust (MCT). The thrust contactbetween the Central Crystallines and equivalent rocksof the Higher Himalaya in the north and weaklymetamorphosed to unmetamorphosed volcano-sedimentary sequences in the south in Garhwal-Kumaonarea was termed as the Main Central Thrust (MCT) byHeim and Gansser (1939) to designate the surface alongwhich the crystalline rocks of the Great (Higher)Himalaya had moved southward over the LesserHimalayan sedimentary sequences.The MCT representsa major structural zone extending for at least 2500 kmalong strike all along the Himalaya from Jammu andKashmir in the west to Arunachal Pradesh in the eastand is one of the most distinctive structural features ofthe Himalayan range. Recently Stephenson et al (2000,2001) defined the MCT as a crustal scale high strainductile shear zone, between 1.5 and 3 km. wide,commonly coincident with an inverted metamorphicfield gradient from biotite to kyanite grade, which places


the metamorphic rocks of the High Himalayan zone overthe unmetamorphosed to weakly metamorphosed rocksof the Lesser Himalaya.

In Sikkim-Darjeeling area, Central Crystallines arerepresented by Darjeeling Gneiss, Kanchenjunga Gneissand Chungthang Gneiss. According to Sinha Roy andBhargava (1989), the greenschist facies assemblagesrepresented by the Lesser Himalayan Daling sequence,is delimited in the north by a dislocation zone at thebase of the tectonised slivers of the Lingtse Gneiss andhe termed this thrust as MCT-1. The rocks on the northof MCT-1, represented by the Darjeeling Gneiss, aremetamorphosed to amphibolite facies. The tectoniccontact of the Dalings with the Gondwanas in thesouthern front of the Lesser Himalaya has been termedas MCT-II.

The Lesser Himalayan zone, bounded by the MainBoundary Thrust (MBT) in the south and the MainCentral Thrust (MCT) in the north, are represented bythe areno-argillaceous formations with inter layeredvolcanics of Late Palaeoproterozoic age at the basefollowed by carbonate-orthoquartzite sequence ofMesoproterozoic age. However the sequence in nottotally represented in Sikkim Himalaya barring theCarbonate – ortho-quartzite sequence ofMesoproterozoic age which in Sikkim Himalaya isrepresented by Buxa Formation exposed only in theRangit window. The Gorubathan Formation, areno-argillaceous lithofacies of the Daling Group comprise athick assemblage of green slate, epidiorite and chloriticgreywacke and carbon phyllites (Acharyya and Sastry,1979).

The different structural elements and tectonics ofDaling rocks have been stated by different workers(Sinha Roy, 1974; Sinha Roy, 1977; Bhattacharya, 1989;Acharyya, 1989). At least, three main phases ofdeformation (D1, D2, D3) may be recognized. The firstdeformation (D1) produced schistosity, mostly parallelto the bedding, folds (F1) and linear structures. MinorF1 folds, rarely recognized within the thin intercalatedquartzite bands, are overturning to recumbent and tightto isoclinals folds. The main penetrative and pervasivefoliation (S1) is the axial planar foliation of these folds.Microscopically the S1 foliation is defined by parallelalignment of chlorite, sericite, muscovite and lenticularquartz and opaque grains. The second deformation (D2)is responsible for the generation of recumbent / reclined

folds (F2) on all scales with pervasive schistosity andoccasional crenulation cleavage. These minor F2crenulations and folds are asymmetrical (sinistral) andclose to tight in nature. The marker units in Dalings viz.,graywacke, psammite etc. define such folds onmesoscopic scale. In the late stages of seconddeformation, mylonitic zones developed almost parallelto the axial planes of second generation folds (F2). Thethird deformation (D3) is manifested in conjugate folds,kinks and monoclines and on regional scale incontemporaneous cross folds (E-W, N-S) at right angle(F3, F4) and that may form the Sikkim domal structure(Sinha Roy, 1977; Acharyya and Ray, 1977, Acharyyaand Sastry, 1979). The structures of both the second andthird deformations might have been generated byindependent N – S directed principal stress. The planarstructures related to first and second deformation, thrustplanes and regional metamorphic isograds were foldedon steep to upright axial planes with axes plunging east-west or north to form in contemporaneous cross folds.

In eastern Himalaya the younger Phanerozoic beltis represented as a linear belt of Permo-Carboniferoussequence equivalent to Gondowana all along the frontalzone of the Himalaya just adjacent to the MBT fromcentral Nepal to Arunachal Pradesh. In SikkimDarjeeling, these are represented by the Rangit pebbleslate and Damuda Formation. In Sikkim though DamudaFormation is exposed only in Rangit Window, the Pebbleslate beds are exposed in the surroundings of the windowwithin Dalings as slices.

From the map pattern of the Sikkim DarjeelingHimalaya it has been observed that in the western aswell as in the eastern part MBT and MCT run parallelto one another defining a narrow E-W trending zone,where as in the central part the MCT recedes towardsnorth defining deep recesses along the N - S flowingTista. Strike of the dominant foliation of rocks in thehanging wall as well as footwall of the MCT in the recesszone runs parallel to the thrust trace there by indicatingnortherly plunging antiformal folding of the MCT inthe recesses zone. In other words, the MCT has laterallyarched up in the shape of isolated northerly plungingantiform. Such lateral arches with axes trending at highangles to the mountain front can be termed asCulmination (McClay, 1992). This culmination zoneexposes a vast tract of Lesser Himalayan sequencestogether with the presence of a window where youngerrocks of a lower level thrust sheet are exposed. Within

28 GEOL. SURV. IND

Teesta Culmination Zone in the Darjeeling-SikkimHimalayas, the Rangit window exposes ProterozoicBuxa Formation and Mesozoic Gondwanas within theProterozoic Lesser Himalayan Daling Group of rocks.

Upper level thrust surfaces along with the thrustsheets on their hanging walls arch upwards because ofinsertion of imbricate thrust wedges at the lower levels(Elliot and Johnson, 1980, Medewedeff, 1992). Thesewedges may be arranged (Boyer and Elliot, 1982) asimbricate fan, duplex or imbricate antiformal stack. Butsuch imbricate wedges are parallel to the thrust transportdirection and the axes of the resulting roof arch trendparallel to the orogenic front (frontal antiforms,Schirmer, 1988). The culmination antiforms, on the otherhand, are lateral arches trending at high angle (almost atright angles) to the orogenic front. Hence theirdevelopment can not be explained by insertion of thrustwedges.

Ray (2000) explained the development ofculmination in Sikkim Darjeeling Himalaya. Accordingto him the sole thrust shifts to a lower level to bypassthe zone of high frictional resistance forming arejoining splay of the sole thrust. By this process apart of the autochthonous foreland below the solethrust, usually a slice of the crystalline basement, getsaccreted to the allochthon and moves forward as a horseand finally occur as a far-traveled horse. A large partof the autochthonous foreland below the sole thrustmay get tectonically mobilized and accreted to theoverlying allochthonous zone. Because of insertion ofthis additional material within the allochthonous zone,the roof sequence of the stack of allochthons archesup in the form of antiforms. Arches with two separateorientations develop simultaneously. Because of listriccurvature of the rejoining splay, the horse (or the bundleof horses) cut from the foreland, has a spindle-likeshape in transverse section (Ray, 2000), and as aconsequence, the roof sequence arches up on an axisperpendicular to the movement direction (a frontalantiformal arch). Moreover, because of concave-up-ward lateral curvature of the rejoining splay, anotherarch with axis parallel to the movement direction(lateral antiformal arch) forms. Interference of thesetwo sets of antiformal arches with their respective axesat high angles to one another, would build up a domalstructure which is characteristic of the culminationsformed because of insertion of basement horse(s). TheTeesta culmination zone has this characteristic domal

structure. Moreover, a complementary synform shouldform in front of the frontal antiform. Such acomplementary frontal synform is also very prominentin the Teesta culmination zone. The resultant mappattern of the culmination zones is a mushroom-shapedpattern, which is seen in the Sikkim-DarjeelingHimalaya.

METAMORPHISM

The metapelitic rocks of the Sikkim LesserHimalayas show an inverted metamorphic sequence(IMS) of the complete Barrovian zones from chlorite tosillimanite + K-feldspar, with the higher grade rocksappearing at progressively higher structural levels. Ray1947 first studied in detail this inverted metamorphism.However Neogi et.al.1998, Ganguly 2000, Neogi 2004carried out detail studies on metamorphism of SikkimHimalaya.

Metamorphic Character of Daling and Centralcrystallines

Chlorite biotite zones:

In these zones, millimetre to centimetre scaledomains of phyllosilicate-rich (muscovite + chlorite) andquartzose bands define a compositional layering (S0).Muscovite in the chlorite zone is rich in paragonite (d”12mol %) and celadonite (d”16 mol %). The latter wasfound to decrease in the biotite zone. In a rare outcropwithin the biotite zone, idiomorphic (50–75 lm) Mn richgarnet porphyroblasts were observed, which show stronggrowth zoning. Since Mn fractionates strongly intogarnet, the stability limits of garnet are expanded in Mn-rich bulk compositions. These rocks contain significantamount of plagioclase, which shows compositionalzoning from around Ab75An25 at the core to aroundAb84An16 at the rim.

Garnet zone:

This zone starts with appearance of small garnetcrystals on foliation and bedding planes of schists andquartzites. Schistosity becomes prominent and garnetcrystals become bigger further away from the marginsof the zone. The rocks are quartz – garnet-biotite schistsand phyllite, quartz-sericite-garnet schist and garnetbearing quartzite. The minerals are quartz, garnet(almandine), biotite, sericite and vermiculite.


Both garnet and chloritoid are present in some thinsections of aluminous pelites, but are segregated in 1–5mm thick layers, with the garnet having a relatively Mn-rich composition and a well-preserved growth zoning.The chloritoid bearing layers are commonly rich inmuscovite and have minor chlorite. The formation ofchloritoid and garnet in Mn-poor and Mn-rich bands,respectively, is due to the effect of Mn in expanding thefield of stability of garnet.

There is a marked decrease in the modal abundanceof chlorite, particularly in garnet-bearing domains.Garnet shows preservation of growth zoning. Biotite incontact with garnet has slightly different compositionsfrom those away from it. Ilmenite contains 6 mol %pyrophanite. The compositions of garnet and ilmeniteindicate Mn-rich bulk composition. The observeddepletion of chlorite in garnet-rich layers suggests thatgarnet formed from the breakdown of chlorite, such as

Chl + Qtz = Grt +H2O

Staurolite zone :

This zone is demarked on the basis of the presenceof staurolite with relatively Mn-rich garnet. Garnet grainsshow partly preserved growth zoning, and an increaseof Mn content in the rim. The modal abundance of garnetis distinctly less than that in similar rocks at lower grades.There is total disappearance of prograde chlorite,whereas biotite is intergrown with staurolite. Theseobservations suggest formation of staurolite plus biotiteat the expense of chlorite, for example,

Grt + Chl + Ms = St + Bt +Qtz + H2O

Staurolite-kyanite zone:

The rocks are characterized by the first appearanceof staurolite and kyanite. There are minor amounts ofretrograde chlorite replacing staurolite and garnet.Garnet is distinctly poor in Mn compared with that ofthe previous assemblages, and shows considerablerelaxation of the growth zoning. Biotite shows onlyminor compositional variation regardless of spatialdistribution and grain size. Plagioclase is sodic (XAnaround 0.2) and shows no zoning even when it is incontact with garnet. The latter observation implies thatgarnet did not suffer a resorption reaction to produce aplagioclase bearing assemblage in these rocks. Presenceof rare chloritoid inclusions in garnet and of staurolitein the same thin section suggest a reaction of the type

Cld + Qtz = St + Grt (±Chl) + H2O

Kyanite zone:

Two groups of metapelites were recognized in thiszone. One group has high-modal abundance of kyanite(>25% by volume) with very little garnet, while the otheris characterized by high-modal garnet and very fewkyanite grains. The former possibly represents aluminousand the latter sub-aluminous pelites. Staurolite is presentonly as rare inclusions in outer massive portions ofgarnet. The rock is locally migmatitic with quartz plusplagioclase-bearing leucosomes alternating with biotite-rich melanosome. Garnet shows poorly preserved growthzoning and retrograde diffusion zoning (increase of Feand Mn and decrease of Mg) near the rim at the contactwith biotite. Biotite grains in the matrix do not showany appreciable variation of composition irrespectiveof whether these are in contact with garnet or not.Plagioclase shows a flat profile with XAn around 0.43.Petrographic features, namely the preservation of rarestaurolite inclusions in the outer rims of garnet andpresence of kyanite in the same thin section, suggest thereaction

St +Qtz = Grt + Ky + H2Oand/orSt + Bt + Qtz = Grt +Ky + Ms + H2O

Sillimanite-muscovite zone:

In this zone, pelitic rocks contain prismaticsillimanite and rarely relict kyanite, along withmuscovite. K-feldspar is present at places and locallyoccurs with quartz and plagioclase in the leucosomes.Garnet shows retrograde diffusion zoning of Fe and Mgnear the rim, especially when it is in contact with matrixbiotite, as expected from the change of equilibriumcomposition at the interface during exhumation(Ganguly et al., 2000). There is also a small increase ofMn concentration near the rim. Muscovite containsaround 14 mol % paragonite identical to that in theprevious zone. Plagioclase is sodic (XAn around 0.22)and shows no significant zoning.

Sillimanite + K-feldspar zone:

With the total elimination of primary muscovite, K-feldspar appears as a dominant constituent along withgarnet, prismatic sillimanite and biotite. The peliticmigmatites are by far the most dominant constituent ofthe Higher Himalaya in this zone, and consists of Qtz +Kfs (locally perthitic) + minor Pl bearing leucosomes

30 GEOL. SURV. IND

dotted with small euhedral garnet, and mesosomes withporphyroblasts of garnet, biotite and sillimanite. Garnetbecomes considerably Mg-rich at this grade and showswell developed retrograde zoning of Fe and Mg nearthe rim when these are in contact with biotite.

The results of detailed petrographic andthermobarometric studies of the metapelites along aroughly E–W transect by Neogi 2004 shows progressiveincrease of both pressure and temperature withincreasing structural levels in the entire IMS. This iscontrary to all models that call for thermal inversion asa possible reason for the origin of the IMS. Also, theobservation of the temporal relation betweencrystallization and S2 structures is problematic formodels of post-/late-metamorphic tectonic inversion byrecumbent folding or thrusting.

Prolonged and complex polymetamorphic historywith two episodes of prograde metamorphism (Ml andM2) followed by retrograde metamorphism (M3) havebeen recorded from the Higher Himalayan CrystallineComplex from Sikkim (Neogi et al., 1998). Ml mayrepresent pre-Himalayan metamorphic episode.Prograde metamorphism M2 is related to renewed burialof the crystalline complex in the early stages ofcollisional events. Geothermo-barometric estimates forpeak M2 metamorphism indicate a pressure of 10-12kbar and temperature of 800°-850°C. Subsequentmetamorphic event (M3) took place as a result of ~5kbar of decompression. The inverted Barrovianmetamorphic zonation has been attributed to probablelarge scale structural inversion and/or tectonicjuxtaposition.

The exhumation history of pelitic migmatites of theHigher Himalayan Crystalline from Sikkim-Darjeelingsection has been determined on the basis of thermo-barometric analysis, retrograde breakdown reaction,compositional zoning of garnet and numerical modelling,suggesting initial rapid exhumation (~15mm/year) from~34km upto a depth of-15 km followed by a much slowerprocess (~2mm/ year) upto at least, ~5km depth. Thedramatic change in the rate of exhumation might reflecta process of tectonic thinning followed by erosion and/or horizontal flow at shallow depth (Ganguly et al. 2000).

SEISMICITY AND EARTHQUAKES

The State of Sikkim encompasses parts of LesserHimalayas, Higher Himalayas and Tibetan plateau. Mainboundary thrust (MBT) just passes through the area

slightly south of West Bengal-Sikkim boundary; MainFrontal Thrust (MFT) between Siwaliks and Neogenesediments is further south. Main Central Thrust (MCT)passes through the Central part of the State and thenormal fault between Central Crystallines and TethyanSequence passes through the northern part of the State.A number of smaller thrusts and NW and NE trendingoblique faults are also recorded in Sikkim Himalayas,some of which appear to be active faults.

In this area most of the earthquakes are shallow focus(<40 km) and commonly of 4.5 to 5.5 magnitude range.The state along with adjoining regions have been dividedinto three blocks. The central main Himalayan Blockwith considerable Seismicity separates the northernTethyan block and southern fore deep block withsubdued seismic activity.

A look at the regional distribution of earthquakesand lineament/fault patterns in Sikkim and adjoiningregion indicates that a number of high (5.0 to 5.9) andmedium magnitude earthquakes are clustered around andrelated to Tista Lineament along NW-SE direction. Onemore NW-SE trending lineament, and marked as a faultalong Tista River between Lachen and Chungthang (andfurther south-east) appears to have been the cause ofhigh magnitude earthquakes. There also appears to beyet another NW-SE trending lineament in north-eastSikkim along which the earthquakes have been recorded.Similarly a NE-SW alignment of earthquakes is alsoobserved.

The map published by the India MeteorologicalDepartment indicates that the state of Sikkim comesunder seismic zone IV- a zone of considerablevulnerability. The nearby regions come under zone Vand are highly susceptible to earthquakes.

Table indicates the list of recorded earthquakes ofmagnitude >4 in Sikkim state. It is interesting to notethat a majority of the earthquakes originated at a depthof ~30 km. Apart from a few earthquakes which appearto have been controlled by aforesaid lineaments, otherearthquakes do not appear to have structural controlswhich are visible on surface.

Deep seismic reflection profiling beneath southernTibet by Zhao et al. (1993) indicates MFT and MBTmerging at the depth of around 5 km and having a slopeof about 3° towards north. The thickness of the crustbetween MBT and MCT varies from 2 km to 4 km inthis region and then MFD, MBT and MCT finally mergewith the Detachment Zone or Main Himalayan Thrust


at the depth varying from 28 km in the south to about 40km in the north. The depth of Moho discontinuity in theentire region is of the order of ~80 km. From the data onthe depth of origin of the earthquakes it is found thatmajority of the earthquakes have originated at depths ofabout 30 km. This is the surface of detachment. Theprogressive drift of the Indian subcontinent beneathTibetan plate at a rate of ~3 cm per year (op. cit) may betaking place along this zone of detachment and theaccumulated strain must be reflected in periodicearthquakes of this region with a depth of about 30 km.It is also observed from the table that the earthquakeactivity is periodical in nature, followed by a period ofquiescence.

Site response studies in Sikkim using strong motion

network by Nath et al. (2000) reveal a much higherfrequency of earthquakes of smaller magnitude. Thestudies indicate that small magnitude earthquakes aremore frequent in number and the depth of their originvaries from 3.2 km to maximum 8.0 km. Theseearthquakes could be the result of small structuraladjustments within Himalayas, possibly due toreactivation of faults.

The seismically vulnerable state of Sikkim, whichfalls in seismic zone IV, comprises of a variety of earthmaterial (various types of soils, weak and strong rocks)with complicated structure etc., and on which multi-storied buildings are being constructed. In view of thevulnerability of the region, seismic-microzonation oftowns is essential for hazard prevention and mitigation.

Year Month Date Lat. Long. Magnitude Depth Source of the(°N) (0E) (Km) Earthquake

mS mb

1959 12 15 27 88 - - - -1960 08 21 27.00 88.50 5.5 5.5 29 CGS1961 09 11 28.2 88.3 231964 08 30 27.36 88.21 - 5.1 21 ISC1964 08 30 27.9 88.52 - - 33 -1964 10 25 27.9 88.6 4.8 0 ISC1966 12 28 28.00 89.00 - 5.2 - ISC1972 08 21 27.33 88.01 - 4.5 33 ISC1972 08 21 27.23 88.02 - 5.1 33 ISC1974 03 24 27.63 88.01 - 4.7 ? ISC1974 01 22 28.1 88.7 331975 01 23 27.44 88.37 - 4.5 33 ISC1979 11 16 27.90 88.70 - 4.6 39 ISC1980 11 19 27.40 88.80 6.1 6.0 47 ISC1982 04 05 27.38 88.83 4.6 5.0 09 ISC1985 05 25 27.60 88.48 - 4.6 33 ISC1986 01 07 27.40 88.43 - 4.7 41 ISC1987 12 06 27 88.52 - - 42 ISC1988 01 19 27.8 88.8 - 4.3 33 ISC1988 03 27 27.1 88.42 - 4.1 70 ISC1988 05 26 27.45 88.61 - 4.7 42 ISC1988 09 27 27.19 88.37 4.6 5.0 23 ISC1990 09 18 27.32 88.84 - 4.1 33 -1995 11 28 27.58 8.84 - - 67 ISC1995 12 24 27.54 88.26 - 4.1 26 ISC1996 02 25 27.6 88.8 331996 03 23 27.17 88.30 4.0 33 -1996 91 32 7.32 88.54 - 4.1 33 ISC1996 09 25 27.43 88.55 5.0 33 -1996 12 30 27.28 8.6 - 3.8 33 ISC1998 03 18 27.37 88.33 4.0 33 -1998 09 10 27.20 88.34 4.7 33 -1998 11 27 27.5 88.8 - 3.5 45 ISC1999 09 20 28.18 8.1 3.8 842000 01 25 27.7 88.36 - 1.8 33 ISC2001 04 08 27.34 88.09 3.8 33 -2001 10 03 27.01 88.10 4.8 - 77 -2001 12 02 27.15 88.17 5.1 33 -

32 GEOL. SURV. IND

In spite of low accessibility, thick vegetal and soil/debris/moraine cover, the state of Sikkim is representedby a number of base metal occurrences. Most of theoccurrences are in Daling Group of rocks in parts ofwest, south and east districts of Sikkim. Base metaloccurrences at Bhotang, Pachekhani and Dikchu havebeen explored in details. The Bhotang and Pachekhanilodes which are mined presently are possibly the onlyworking mines in Himalayas. Among non metallicminerals coal, graphite, dolomite, limestone, marble,wollastonite, talc, sillimanite and asbestos etc. also occur.

ASBESTOS:

Sosing Hill:

In Legship area an amphibolite sill, intruded intophyllites at the southern foot of Tashiding ridge and verynear the confluence of Rathong Chhu with Rangit river,and another in the western face of Sosing hill(27°17:88°20) are asbestos bearing. Asbestos which isdeveloped along cracks and fissures is bluish-grey incolour and of short, matted, harsh fiber type. Themaximum length of fibers is 10 cm. The fibers areassociated with acicular tremolite and actinolite crystals.Both of these occurrences are not of much economicimportance.

Crocidolite bearing highly kaolinised rocks havebeen traced near Rabong (27°18:88°2030). They occurin area of 30m x 10m. The country rock is most probablymica pegmatite.

BASE METALS:

Most of the base metal occurrences in Sikkim arerecorded in Gorubathan Formation of Daling Group ofrocks. Systematic geological mapping and mineralexploration programmes launched by Geological Survey

Mineral Resources of Sikkim

of India revealed presence of base metals at Rangpo,Dikchu, Pachekhani, Temi, Reshi, Jungdum, Sumbuk,Rothak, Mangreng, Rinchingpong, Chongbong, Keorani,Sirbong, Sisni, Dong Busty, Uttare, Siging, Bum, Sipik,Tukhani, Namthang, Pamphuk Khani, Parbing andRatopani.

Out of all these occurrences, the Bhotang (Rangpo),Pachekhani and Dikchu are most important and havebeen proved in the form of deposits. Rangpo andPachekhani are currently been mined.

RANGPO (Bhotang) (27o11:88o32 ):Basemetal occurrences of East Sikkim are most

important in the sense that they constitute the onlyworking base metal mines of Himalayas. In Rangpo-Pachekhani area the main rock units are phyllites schistsand quartzites, associated with ellipsoidal lenses ofamphibolites and quartz veins and stringers. These rockunits are repeated and intricately intermixed with eachother and can be traced at different places and altitudes.Numerous intra formational faults, thrusts and folds haveaffected the country rocks considerably (Raina andBhattacharya, 1965).

Phyllites are exposed in the area between the Tistain the west and Kalikhola-Pachekhola in the east andconsists of chlorite-sericite-phyllite, sericite-chlorite-phyllite, quartz-biotite-phyllite, slaty-carbonaceous-phyllite and quartz-sericite-phyllite. The first two rocktypes are soft, grey to deep grey in colour and arerepeatedly exposed in the western part of the area. Thechlorite-sericite-phyllite contains specks of pyrite,pyrrhotite and chalcopyrite, which at places like Bhotangand Mangreng, show good concentration. Quartz-biotite-phyllite is fine grained, light green in colour and showsthe presence of thin blebs of biotite along the foliationplanes. Slaty-carbonaceous-phyllite is dark charcoal-

32


grey in colour, soft and friable in nature, contains specksand occasional good concentration of sulphide mineralsand graphitic carbon at places like Pache, Rorathang andKalikhola.

The schists are generally exposed in the eastern partof the area, between Rorathang-Damlakha to Mamring-Taza. The gritty, quartz-biotite-schist is highly sheared,shows the presence of irregular shaped (0.1 cm to 0.65cm in length) quartz grains and contains disseminationof sulphide minerals like chalcopyrite, pyrrhotite, pyriteand galena. Good concentration of sulphide minerals isrecorded at Rorathang and Pachekhani. Sericite-biotite-schists and quartz-biotite-schists are repeatedly exposedin this area. These two rock types show gooddevelopment of biotite, whereas garnet though presentsparsely in the sericite-biotite-schist, is well developedonly in the quartz-biotite-schist.

Quartzites are generally phyllitic, cherty andcalcareous in nature. The phyllitic quartzite which isrepeatedly exposed in the entire area is grey to greyish-brown in colour and fine grained in nature. It containsgood concentration of sulphide minerals in the Bhotangand Mangreng areas. Cherty quartzite bands are exposedat Lusing and along Rangpo-Rorathang road. This rockis highly sheared and brecciated and locally has givenrise to autoclastic conglomerate, consisting of elongatedto sub-elongated quartz grains arranged closely alongthe foliation planes. Calcareous quartzite is exposed onlyat Pachekhani. This rock is medium grained, hard, lightbrown to buff in colour and dolomitic in nature. It showsgood concentration of sulphide minerals likechalcopyrite, pyrrhotite, pyrite, and galena.

Basic intrusives occur as amphibolite andamphibolite schists. These are lenticular and elongatedsills of irregular length, show concordant relation withthe country rocks. These intrusives invariably containsulphide minerals as disseminations, where from pyritehas commonly leached out leaving voids.

Two types of quartz veins i.e. pre-tectonic and post-tectonic are identified in this area. The pre-tectonic veinis only traceable at the junction of Baghe khola andNH31A, where a band could be traced for more than100 m and is tapered off at its southwestern side. It showsconformable relation with the country rocks. The post-tectonic veins and stringers are seen both along andacross the foliation planes and have filled up the

fractures, faults and thrust planes and have criss-crossedthe country rocks in a net-work fashion. Presence ofsulphide minerals is frequent in these post tectonic veins.

Sulphide minerals like chalcopyrite, pyrrhotite,pyrite and galena show four modes of occurrence: (i) asdisseminations within the country rocks (strata bound),(ii) as lensoid replacements within vein quartz whichvary from 3 cm to 6 cm in thickness and 10 cm to 10 min length,(iii) within basic intrusives (iv) as joint andfracture fillings localized along fault, thrust and shearzones.

This is the only polymetallic base metal depositunder active exploitation in the entire Himalayaproducing Cu-Zn-Pb concentrates. Several old workingspresent in the area were formerly opened by local Nepaliworkers, and were later on prospected by M/S Burn andCo. and the Indian Bureau of Mines. The Cu-Zn-Pbsulphide deposit of Rangpo is exposed on the left bankof Tista River, just north of Rangpo town. The host rocksof mineralization are rocks of Daling Group. The orebody is stratabound and folded. The mineralized quartzveins are post-tectonic and are of hydrothermal origin.Disseminations of sulphide minerals are confined mostlyto the basic sills (amphibolite and amphibolite schists),chlorite sericite phyllites and phyllitic quartzite (Rainaand Bhattacharya, 1965)

In Rangpo-Bhotang Block sulphide minerals of allthree types occur, but the predominant one is thereplacement type. Two lodes- Bhotang lode or Lode-Iand Subsidiary lode or Lode-II – have been delineatedat Rangpo. Bhotang lode is the important lode and isbeing mined in Bhotang Mines. The ore body is 280 mlong and average width is 5 metres. The principal oreminerals are Chalcopyrite, sphalerite and galena withpyrrhotite, pyrite and marcasite. Minor phases comprisearsenopyrite, tetrahedrite, pyrartyrite, jordanite – Silverand gold are present in traces in the ore with the formerin greater amount. The copper content varies from 0.02%to 1.57%, lead from 0.64% to 1.2% and zinc from 2.10%to 3.15 percent.

Indian Bureau of Mines(Mukherjee and Rao, 1974)conducted drilling and exploratory mining operationsand estimated 0.6 m. tones of ore with 2% copper andZn. GSI’s efforts between 1978 to 1982 extended thewestern extension of Rhotang lode by more than 60metres, so that till date, more than 400 metres of strike

34 GEOL. SURV. IND

length of Bhotang lode has been established. The lodeincreases in width and copper content at depth. Morethan 0.1 m tones of additional 2-3 Cu ore have beenestimated due to additional 60 m strike extensionestablished by G.S.I. It is expected that with furtherdevelopment of lower levels, nearly 0.3 m tonnes of Cu-Zn-Pb ores could be established. A noteworthy featureis the silver metal occurrences in the lead concentrate.The amount of silver is estimated to be around 700-800gm per tonne of lead concentrate. Traces of bismuth,cobalt, cadmium, chromium and manganese are alsorecorded.

Studies/exploration have also been carried out toprove the western extension of Bhotang lode across TistaRiver by Mitra and Das (1984), Das (1985), Williamet.al. (1984), Mukhopadhyay and Banerjee (1978) andothers.

Pachekhani-Rorathang block (27°12:88°32 ):There are three old workings in the Rorathang area

and five in the Pachekhani area. These workings werebeing exploited at the time of visit of P. N. Bose (1891).The Indian Bureau of Mines also carried out detailedexploration in this block from 1957 to 1961 (Mukherjeeand Jog, 1963). The sulphide mineralization occurs asdisseminations in gritty, quartz-biotite-phyllites, slaty-carbonaceous-phyllites and calcareous quartzites and istraceable for a distance of 300 m up the dip and for about150 m along the strike. Some occurrences are also seenin post-tectonic hydrothermal quartz-veins and alongfault and shear zones. Vein-quartz type of concentration,occurring mostly along the foliation, joint and fractureplanes, is prominently seen in an old working atRorathang and is traceable for a distance of about 30 m.In the Pachekhani area, a highly mineralized, calcareousquartzite was traced, within which mineralization wasfound to be pinching both up dip and down dip. Sulphideminerals are mostly chalcopyrite, pyrite, pyrrhotite andgalena. This area has been found to contain several smallisolated highly mineralized pockets, which do not showany systematic extension. Investigation carried out inthis block by the Indian Bureau of Mines indicated noworkable lode (Raina and Bhattacharya, 1965).

Old working of adits and incidence of copper bearingquartz veins in both the upper and lower reaches of Pacheand Rorathang villages led Indian Bureau ofMines(1958-61) & Geological Survey of India(1970-71)

to explore the two areas separately. In all, three blocks-upper and lower Pachekhani Blocks and RorathangBlock – were delineated by IBM. IBM drilled 10boreholes (1047m) and carried out exploratory miningof about 1500m. length in Upper Pachekhani andRorathang Blocks only. Later GSI carried outgeophysical survey and test drilling in Lower PachekhaniBlock. Sulphide mineralization occurs as lenticularbodies arranged in “en echelon” fashion. The countryrock hosting the base metal mineralization is chloriteschist, phyllites and quartzite of Gorubathan Formationof Daling Group.

Copper mineralization is confined to small, narrowimpersistent lenses having limited strike and depthextensions and analyze in general 1.5% copper overwidths of 1-1.20 metres. Hardly a single lode could befound which showed persistence in strike for more than30 metres and in depth for more than 70 metres.

Detailed study on Pachekhani area was again carriedout from 1990 to 1995. During this period 1.20 sq kmarea was covered on 1:2000 scale and an area of 212 sqkm was covered on 1:25,000. A total of 7 boreholestotaling 705.70m were drilled and a possible ore reservewas estimated to 0.15 m tonnes of 0.75% Cu and 636.55tonnes at 1% Cu.

Ore-Microscopic studies from Bhotang andPachekhani deposits reveal that the ore mainly containspyrrhotite, chalcopyrite, sphalerite and a minor amountof galena. In some samples concentric bands of pyrrhotiteand chalcopyrite with quartz at the centre are observed.Pyrrhotites are usually rimmed and veined by marcasitewhich is seemingly the alteration product of pyrrhotitewhich in turn is altering to pyrite. A few grains ofinterstitial tennantite set in chalcopyrite are also noted.Ilmenite intimately associated with pyrrhotite is alsonoticed. Disseminated allotriomorphic aggregates ofchalcopyrite with minor amounts of pyrite and pyrrhotiteare seen in some of the samples. Ilmenite grains are alsonoted. Coarse idiomorphic to hypidiomorphic granularaggregates of pyrite with interstitial grains of highlyaltered pyrrhotite are dispersed into gangue, which alsooccur as discrete dispersed grains along with a few grainsof chalcopyrite. Dispersed chalcopyrite containinginterstitial grains of galena and pyrrhotite with minoramount of pyrite is also recorded. Discrete aggregatesof ilmenite are conspicuous. Pyrrhotite alters to pyrite.Pyrite shows a wide range of size variation from coarse


single idiomorphic to micro spherules of tiny pyriteaggregates. Hypidiomorphic granular aggregates ofpyrite, occurring in bands, contain interlocked grains ofilmenite.

Duglakha Block (27°14:88°36 ) :The location is along Pache Khola stream; about 2

km upstream of Pachekhani copper deposit. Exactlocation of the lode is 27°14:88°37. The Daling Groupof rocks comprises a sequence of Garnetiferous-biotite-chlorite phyllite, biotite-chlorite phyllite, chloritephyllite, quartzites and amphibolite schists. AmphiboliteSchist occurs as sill within the metasediments. TheQuartz veins follow foliation planes but at times havecross-cutting relations. The general trend of the rocks isnorth-south to WNW-ESE with persistent easterly tonortherly direction. Raina and Bhattacharya (1963)carried out detailed study of this occurrence.

At Dugalakha three lenses of varying dimensionsshow on an average 2-5% lead in the surface exposures.Lode ‘A’ is exposed over a strike length of 66 m andaverage width of about 1 metre. Average grade is 2.5%lead. Lode ‘B’ could be traced over a strike length of 58metres on the hanging wall of the amphibolite sill;average thickness of lode is 1 metre. Average grade oflode ‘B’ is 5.8% lead. Lode ‘C’ is traced over a strikelength of 160 m and thickness of 15 m. But in lode ‘C’the mineralization is in disseminated mode. Theanalytical results are also very poor. Here mineralizationis mostly of galena and pyrrhotite with subordinatechalcopyrite and sphalerite concentrated generally inhydrothermal vein quartz, and disseminated along thefoliation of the country rock. Several such veins are seenalong the fault or shear zones. This area does not showmuch of promise.

Geophysical investigation (1965-66) was carried outcovering an area of about 0.5 sq km at Dugalakha whichindicated poor copper mineralization with limitedextents. Test drilling by GSI confirmed the findings ofgeophysical investigation.

Dikchu (27o24:88o31 : 78 A/11):

The base metal occurrence of Dikchu was visitedby P. N. Bose during 1891 and 1894 (Bose, 1891, 1928).

During the period 1908-1911 M/s Burn & Co. initiatedexploration work and carried out underground aditing(60.96 m in 4 adits). In the year 1909 Pillow Harveg, aLondon based Mining Engineer examined the depositand expressed favourable opinion. In the year 1948 theGeological Survey of India carried out sampling of themineralized zone. Indian Bureau of Mines carried outexploration during the period November, 1959 to June,1963 by drilling and exploratory mining (undergrounddevelopment 1544.20 m; drilling-4604.21 metres). It wasfurther investigated by Mineral Exploration Corporationduring the period May, 1977 to May, 1983.

The Cu-Zn- mineralization occurs as conformablevein in the garnet-staurolite bearing quartz-mica-schistof Daling Group near its contact with the gneisses ofCentral Crystalline Gneissic Complex. The hanging wallcomprises of hard compact quartz-mica schist gradinginto quartzite. The footwall comprises quartz-biotite-muscovite-sericite schist.

Two lodes have been delineated at Dikchu. TheDikchu main lode trends north-south and dips at 30° to35° towards east. Ore appears to be stratabound. It isfound to be persistent over a length of about 600m up toa depth of 100 metres and has a thickness of about 1.25metres. The Lingdok lode is thin and impersistent. TheDikchu lode is essentially a massive sulphide lode ofcopper and zinc. It is composed of chalcopyrite,pyrrhotite, sphalerite and traces of galena and covelite.Magnetite and garnet are common gangue minerals.Exploration by IBM (1959-63) earlier established aprobable reserve of 0.3 million tones of 2.96% Cu and1.5% Zn. Tests carried out by IBM on bulk sample ofDikchu ore yielded 90-95% recoveries of copperconcentrate, 3.4 ppm gold and 92 ppm of silver. Thecommon ore minerals associated with the mineralizationare chalcopyrite, pyrite, pyrrhotite, sphalerite and galena.Magnetite is the common oxide. Presence of gold andsilver has also been recorded in this deposit.

The work by MECL included 1891.8 cum ofexploratory mining, 290.50 m of underground drillingand 3 numbers (500 kg each) of bulk sampling. The workwas confined to the southern part of Dikchu River.MECL proved 0.45 m tones of ore with 2.82% Cu and0.9% of zinc under following categories:

36 GEOL. SURV. IND

1. Developed category 1.66 lakh tonnes 3.07% Cu1.01% Zn

2. Proved category 1.58 lakh tonnes 3.12% Cu0.98% Zn

3. Probable category 0.99 lakh tonnes 1.48% Cu0.58% Zn

4. IBM’s proved and probable category to the north of the river 0.27 lakh tonnes 4.08% Cu0.40% Zn

Mamreng-Duga Belt:

Dominant lithounits are phyllite and quartzite-talcose phyllite. Epidiorite also occurs as subordinateunit. Sulphide mineralization is found withinepidiorite, talcose phyllite and also in the associatedquartz veins.

Danak Khola:

The mineralization is located in Danak Khola(stream), about 5 km east of Rorathang village. Twomineralized veins have been recorded (Das et al., 1984).The first mineralized vein, in the northern part, has awidth of 1.5 m and continues for about 30 m alongapparent dip direction. The second vein is 20 m south ofthe first vein and has a maximum width of 2.5 metres andcontinues up o 35 m along apparent dip direction. Thesulphide mineralized part consists of chalcopyrite, pyrite,pyrrhotite, magnetite and a few specks and stringers ofgalena. Mineralisation is sporadic and discontinuous innature. Nine samples from these veins indicated coppervalues ranging from 0.14% to 9.15 percent.

Rungdu-Sodunlakha area:

Recent investigation by Geological Survey of Indiahas brought to light a promising sulphide occurrence inthe Rungdu-Sodunlakha Belt. The copper mineralizationis associated with the quartz-biotite-chloritoid schistband (within chlorite schist). It is confined to theschistosity plane and more so along the silica richportion. Schistosity of the rocks varies from N25°E-S25°W to N70°E-S70°W with a south-easterly dip of20°-65°. Groove samples indicate maximum values of1.81% Cu in Rungdu and 1.45% Cu in Sodunlakha area.Mineralisation is structurally controlled and betterconcentrated at the closure of Rongpo Chhu antiform,near Sodunlakha than the limb portion. Gold values fromthis area vary from 50 ppb to 255 ppb. The investigationis in progress.

Rolep area:

This area is about 15 km from Rongli by road and

occurs by the side of a newly constructed bridge. Thehost rock is a amphibolite intruding calcareous quartziteenclave within banded biotite gneiss (Mullick andChattopadhyay, 1991). The mineralization is in the formof specks and big crystals of chalcopyrite, pyrrhotite andbornite. Near Rolep village and slightly north of it, onthe opposite bank of Rangpo Khola, there are calc-silicates showing richer portions of sulphidemineralization in the form of disseminations of pyrrhotiteand chalcopyrite with rare bornite.

Rongli:

Near Rongli Chu-Sewa Khola confluence, a onemetre garnetiferous sericite-quartzite with quartz veinsshow mineralization in the form of pyrite,bornite+pyrrhotite and chalcopyrite. The thickness ofmineralized zone is more than 2 metres and apparentlength is 25 metres (Mullick and Chattopadhyay, 1991).

Sumbuk (27°07:88°23):Indication of copper mineralization is observed on

both the banks of right tributary of the Khani Khola, abranch of Manpur Khola. Two adits and two minorexcavations are located on the left bank of the stream.

Sulphide mineralization is noted in quartz veintraversing a dark grey phyllite of the Daling Group.These quartz veins are mostly of pinch and swell type.Sulphide minerals include chalcopyrite and occasionalpyrite and occur as nests and stringers in quartz veinand as dissemination in the country rock in the proximityof the veins. The mineralized lode has maximum widthof 1.00m and can be traced along the strike for 30m.Chemical analysis of the groove samples collected fromthe mineralized zone indicates copper (0.98% to 1.86%)while Pb, Zn, Ag are present as traces.

Pampukhani (27°07:88°29) :In Pampuk area, base metal mineralization has been

observed in Pampuk R.F., located 5 km. south of theNamthang village. The mineralization is exposed on ascarp face at a height of 200 m. from the Tista river bed.


Sulphide mineralization is noted in quartz veinstraversing slaty phyllite of Gorubathan Formation ofDaling Group. The veins, which have a brown stainedspongy appearance on the surface, are exposed in a seriesof old inclines. Overall strike length of the mineralizedzone appears to be 30m and maximum width ofmineralized zone is about 20 cm. Mineralization mainlycontains pyrite, chalcopyrite, galena and sphalerite.Chemical analysis of the mineralized zone indicates0.05-1.64% Cu, 0.15-0.52% Pb and 0.16-0.96% Zn.

Besides the above two locations, there are someminor base metal sulphide occurrences at Temi(27o14’:88o26’, 0.06%-1.25% Cu), Dong Basti(27o12’:88o20’, 1.32% Cu) and Parbing (27o11’:88o25’,0.78% Cu) areas in South Sikkim.

Temi area:

The Dalings comprise chlorite phyllite and quartzitewith quartz veins. Both bedding and schistosity trendwest to NW with moderate dips towards south-east.Basic sills within phyllite have also been reported withasbestos bearing quartz veins near the contact.Chalcopyrite and pyrite bearing quartz veins areemplaced along schistosity, shear plane and fracturecleavages in the phyllite. The mineralized veins areconfined along a zone which is about 370 m long and 4m wide. The channel samples yielded traces to 7.93%copper.

Dong Basti:

Phyllite and quartzite dominant lithounits of the areaare traversed by quartz veins. Mineralization asdisseminations and granular aggregates of chalcopyriteoccurs in quartz veins (3 m thick zone). Other associatesare pyrite and covelite. Analytical data of channelsamples show 1.32% copper. This occurrence needsgeophysical and geochemical appraisal before a finalverdict.

Sikkip:

The mineralized quartz veins traverse slate anddolomite, north of Sikkip Thrust. The mineralized quartzveins occupy north-south trending fractures. The bedsstrike N70°E-S70°W with 40° dip towards north. Dueto the proximity of the thrust, these rocks are highlycontorted and secondary foliation (N10°W-S10°E) has

been developed (Nautiyal et al, 1961). The mineralizedquartz veins showing pinch and swell structures plungeat 25°-30° towards north. A few small 3 to 6 metres longadits occur in these zones. The copper values range from10.7% to 3.40% over 30-40 cm width. Themineralization is structurally controlled by thecomplementary shears. The deposit is fracture-fill typewith little wall rock alteration except for some sericitebordering the mineralized quartz veins. The predominantsulphide minerals in order of abundance are chalcopyrite,pyrrhotite and pyrite.

Tukhani:

Copper mineralization is in the form of old workings,disseminations of chalcopyrite, stains of malachite whichare seen in the quartz veins traversing the phyllites ofthe Daling Group.

Namthang:

The area comprises slaty phyllite and quartz-chloriteschist traversed by quartz veins. Sporadic pyrite andchalcopyrite is found in quartz veins in Shenti Kholaover 0.5 m thick and 30 m long zone.

Parbing:

Mineralised quartz veins, emplaced along thefoliation planes of quartz-chlorite phyllite and are spreaddiscontinuously over 70 m long and 0.4 m wide zone.Pyrite and chalcopyrite are found as disseminations.Analysis of ore indicated 0.78% copper.

Ratopani:

Traces of pyrite and chalcopyrite have been noticedin quartz veins traversing the phyllite and quartzite. Soilsamples assayed less than 50 ppm copper.

Rothak Khola:

Rothak Khola area is dotted with a number of smallbase metal sulphide mineralization of which mostimportant ones are Rothak Khani (27°0950:88°15180.15% Cu), Jagdum (27°11:88°1415, 0.64% Cu),Sirbong (27o1035:88°1530, 0.05% Cu), Chungbong(27°0730:88°15, 0.41% Cu), Sisni (27°1018:88°1420, 0.35% Cu) and Siging (27°1040:88°1510,0.30% Cu). During 1960-62, G.S.I. worked in this area,0.645 sq km area was mapped on 1:2000 scale, 600

38 GEOL. SURV. IND

geochemical samples were collected and 700m weredrilled in all the mineralization ore associated with quartzveins as stringers and fracture filling within host ofchlorite phyllite/sericite chlorite phyllite of DalingGroup. Main ore minerals are chalcopyrite and pyritewith minor amount of sphalerite and galena.

A revaluation of the above deposits by GSI’ during1996-97 also indicates that the above occurrences arenot of economic importance in the present day standard.

Jugdam (27°11:88o14°15):The area consists of chlorite phyllite, dark slaty

phyllite, quartz sericite phyllite and epidiorite-quartzveins. Copper mineralization occurs in quartz veins asstringers, foliation and fracture filling (Nautiyal et al,1962). The mineralized quartz veins with strike extensionof 15 m to 107 m and width of 1.50 m to 3.96 m havebeen reported. Groove samples indicated 0.51% copperwhere as drill core samples indicated 0.21% to 0.64%copper.

Rothak Khani (27°0950:88°1518):

This occurrence is located at the confluence of KhaniKhola and Rothak Khola. Dominant lithounits are greyphyllite, sericite-chlorite phyllite and crumpledcarbonaceous slate with thin bands of quartzite. Generaltrend of foliation is north-south to NE-SW with 60° diptowards WNW to NW. Chalcopyrite with subordinatepyrite has been observed within quartz veins traversingslaty phyllite. Main zone with 120 m length and 1 m to1.5 m width is located on right bank of Khani Khola. Afew mineralized quartz veins are present on the left bank.Soil samples from these areas indicated 1500 to 3000ppm and 500 to 2000 ppm copper respectively. Thisoccurrence needs to be evaluated by geophysical andgeochemical surveys.

Rinchinpong:

The area comprises greenish-grey chlorite schistwith opaque quartz and chlorite-sericite schistinterbedded with flaggy quartzite, containing occasionalsheared limonitised bands (Kurien and Pattanaik, 1963).A sheared limonitised 15 m long zone containingprimarily pyrite and chalcopyrite has been recorded.

Chonbong (Chungbong- 27°0730:88°15):Greyish-green phyllitic slate, interbedded with

quartzite is traversed by quartz veins. Schistosity trendsN67°E-S67°W with 35° to 45° dip towards north-west(Kurien and Pattanaik, 1963). Chalocopyrite andmalachite with occasional crysocola associated withquartz veins has been observed in phyllite along foliationplanes and joints. Samples analysed 0.41% copper. Afurther geochemical appraisal is called for.

Keorani:

Carbonaceous and chloritic slates with quartz veinshost the mineralization in this area. The foliation trendsN20°E-S20°W with 40 to 45° dip towards WNW. Quartzveins varying in thickness from 2.5 to 5 cm contain poormineralization along a 20 metres long and 0.6 m widezone.

Sirbong (27°1035:88°1530):Chalcopyrite and pyrite has been observed in a 6 m

wide and 40-50 m long zone, traversed by thin quartzveins within the grey-slaty phyllite and quartzite.Foliation plane of the host rock trends N30°E-S30°Wwith 53° dip towards N60°W. Analytical data indicatescopper values from traces to 0.5 per cent.

Sisni (27°1018:88°1420):Carbonaceous slates with chlorite and sericite

intercalations, traversed by quartz veins have foliationwhich in general trends N20°E-S20°W (Nautiyal et al,1962). The main mineralized band, 0.77 m to 2.44 mwide occures towards upslope over a slope distance of44 metres. Total mineralized zone is 213 m long. A partof the mineralized zone is across the Rothak Khola.Groove samples from this zone indicated 0.15% to0.35% copper. The minerals are mainly chalcopyrite andminor pyrite.

Chitre:

The country rock of this area belongs tometamorphites of Darjeeling Group of rocks with calcsilicate inrusive, along with granite and pegmatite(Nautiyal et al, 1962). Foliation in calc-silicate bandstrends north-south with moderate to high dips towardseast. Pyrrhotite is sporadically distributed.


Siging (27°1040:88°1510):

There are a few old adits at the confluence of SigningKhola and Santolo Khola. The mineralization is in thevein quartz traversing the highly sheared and fracturedquartzite. Geochemical samples gave values rangingfrom 50-3000 ppm copper. The anomaly trends inroughly NE-SW, making an angle of 15° to 20° with thestrike of the beds. Old adits have been driven in N50°Wdirection for a distance of 10-15 metres. This occurrenceneeds appraisal by structural analysis, geophysical andgeochemical surveys.

Bum:

The mineral occurrence is observed in phyllite andquartzite which are exposed on either bank of RishiKhola near Bum village (Nautiyal et al, 1961). Pyrite,chalcopyrite, bornite, galena and pyrrhotite stringershaving thickness of less than 1 cm to 13 cm are seenassociated with quartz veins. Samples from a 15 cm thickquartz vein have analysed 3.41% and 4.5% copper.However, the mineralized zone does not show strikepersistence.

Legship:

In Legship area the base metal mineralization isobserved in the northern bank of the Baniya Nala, nearits confluence with the Rangit River. Pyrite, pyrrhotite,chalcopyrite, bornite and galena occur as disseminationsand as thin hairline fracture-fillings in quartz veins.Groove samples from the area indicated 1200 ppm toless than 100 ppm copper.

Sontale:

Copper mineralization within the sericitic phyllite,slate and chlorite schist of Daling Group occurs inassociation with quartz veins. Three mineralized quartzveins were identified having thickness ranging from 0.3m to 0.60 metres.

Bhattacharya and Pattanaik (1964), in the course oftheir systematic mapping recorded a number of basemetal occurrences, the details of which are producedbelow. :

Sada (27°24:88°22):At this place several thin, highly ferruginous, quartz

veins and stringers have been seen on the Rangit Riverbed, within hard and well bedded quartzite. These quartzveins contain some concentration of galena associatedwith chalcopyrite and pyrite. The zone is 5 metres thickand nearly 10 metres long are exposed along strikedirection. The quartz veins are sheared.

Rangpo Chu:

On this stream concentration of sulphide mineralshas been detected at four places.

Near the confluence of Ranken Chu and RangpoChu- Nearly 300 m downstream from the said confluenceon the southern bank two metres thick quartzite zonesrich in pyrite separated by a 25 m thick phyllitic band,have been met with. Mineralization in a disseminatedpattern has taken place within the quartzite itself. Thequartzites are light brownish grey in colour, fine grained,hard and compact. Along with pyrite a certain amountof chalcopyrite and pyrrhotite has also been detected.The zone is exposed along the strike for 100 metres.

Pading (27°18:88°2330): Very near to this village,on the southern side of Rangpo Chu, within a highlysheared and brecciated zone of quartzite, a some whatgood concentration of chalcopyrite and pyrrhotite hasbeen found. The qurtzites are light grey in colour and atplaces, are highly limonitic. The zone is 10 m in widthand 15 m in length.

On Rangpo Chu near the log bridge on the mule trackfor Aitabarey. Very near to the bridge, at several placeswithin a distance of 150 m. sulphide minerals likechalcopyrite, galena and pyrite have been seendisseminated within the quartz veins and stringers. Thecountry rock is chloritic phyllites and quartzites. Thedissemination is concentrated only within quartz veins andstringers. The zone is exposed on both sides of the river.

Upstream of Momain Chu near Thurphyak(27°18:88°23) nearly 800 m. upstream of Momain Chu,a tributary to Rangpo Chu, on the western bank,concentration of chalcopyrite has been found withinthree quartz veins which are approximately 2 to 3 metresthick and are exposed for about 25 metres. They aremilky white to brownish in colour.

40 GEOL. SURV. IND

Brom Chu (Brom: 27°1830:88°2650):A little upstream, nearly 300 m. within highly

sheared sericite phyllite and quartzites, disseminationsof chalcopyrite and pyrrhotite has been observed. Thezone is 16 m. long and 5 m wide.

Run Chu:

From the confluence of Run Chu and Tista Rivernearly 800 m. upstream along Run Chu, a pocket ofsillimanite bearing schists is located. They are highlysheared and brecciated. The zone is exposed on a verticalcliff nearly 10 m. in width and exposed for about 70 m.The zone is highly intruded by quartz veins and stringers.This area shows best concentration of sulphide mineralsof all the localities. Sulphide minerals are mostlychalcopyrite along with pyrrhotite and pyrite. Theexposure is seen on both sides of the river.

Bartag (27°30:88°30):At this place, very near the contact between the

quartzose schist and garnet rich biotite schist, somedissemination of pyrite has been located, in a zone 3 m.in width. The zone is exposed for a distance of 7 m.

Section between Chungthang (27°36:88°38) toJorepul (27°42:88°34):

In this area, along the new road section, at severalplaces, dissemination of sulphide minerals have beenseen restricted within amphibolites and amphiboleschists. Chalcopyrite is the most common mineral.

Phentong (27°3446:88°25):Near this village on road to Sakyong some pyrite

disseminations are observed within sheared and fracturedquartzite bed.

Traces of sulphide mineralization in the form ofdisseminations, occur in the basal rocks of the EverestPelitic Series exposed along the western wall of thequarry below Chorten Nyima Peak (27°57:88°11) andsome malachite and azurite encrustations in lime-silicates in the eastern valley wall of Lungme nala(27°52:88°21). These are so meager that they do notpossess any economic interest (Raina and Bhattacharya,1965).

LEAD MINERALIZATION:

Pawde (1965) reported lead mineralization betweenIngming khola (local name) and Chunbhatti khola (localname, toposheet 78 A/8) on the hill slope to the west ofRangit River at an altitude of 740 metres. The host rock,Buxa dolomite, strikes N30°E-S30°W and dip 50° dueN60°W. Mineralization of galena is recorded along jointplanes trending NW-SE and NNE-SSW. Some of thesejoints have been traversed by quartz stringers. Themineralization is observed in quartz veins when theyare present; otherwise joint planes of dolomites are host.The thickness of mineralized zone varies from 10 cm to25 cm and it is observed over a distance of 7 meters.

Dutt (1954) reported that dissemination of galenaoccurs in the vein quartz, which is 10 inches thickemplaced in flaggy quartzite 300 yards south ofDamthang and also between 32 and 33 milestone nearJaubari (27°12:88°24).

CALCAREOUS TUFFA:

Calcareous tuffa is located in the northern bank ofRangit River. This occurs in pockets within the Buxaslate. The tuffa is creamy white to earthy looking, softand spongy. Six channel samples indicated followingchemical analytical results.

Maximum Minimum

CaO (%) 51.45 43.40MgO (%) 1.75 1.00R2O3 (%) 3.93 1.30Insol (%) 14.03 2.51

COAL:

Coal beds associated with Gondwanas are foldedand faulted, and show pinching and swelling. Coal,varying from carbonaceous shale to semi-anthraciticvariety occurs as crushed, often nodular variety. Itsexposures can be seen at a number of places withinGondwanas. Linking and correlation of individualoutcrop is very difficult due to the complexity of thestructure.

Around Namchi:

Regional geological exploration by GSI in Namchiarea of south district has established a reserve of 1.4lakh tonnes of bituminous to semi-anthracitic coalanalyzing 40% to 60% fixed carbon, 4% to 22% ash,


8% to 13% volatile matter and 3% moisture. Thin coalseams (1 m to 5 m) are found to occur on the road linkingLegship with Nayabazar and to the south of Putkhola. Agood account of coal seams of this area is provided byRaina and Ray (1967).

Regional geological exploration in and aroundNamchi (27o10’:88o24’) area of south district of Sikkimwas carried out during the period from 1968 to 1971.The investigation led to the estimation of a total reserveof 1.4 lakh tones of coal in the area. This is bituminousto semi-anthracite variety of coal, occasionallypulverized due to intensive folding and shearing. Onanalysis, these varieties of coal indicate 40% to 60%fixed carbon, 22%-40% ash, 3%-8% volatile matter and3%-6% moisture. These coals have not been foundsuitable for domestic and industrial use in their presentstate.

Coal also occurs as thin seams within the Gondwanasandstone shale sequence in Nayabazar Legship area ofWest district, in the valley of Little Rangit River. Anumber of coal seams are exposed on the road linking

Legship-Nayabazar, with varying thickness. Other seamsare exposed south of Put Khola; in between 14 and 15km. milestones. However, these coal seams are 1m-5mthick and are not of economic importance.

Only one coal seam, located near Rishi Bazar, hasbeen investigated by pitting and drilling during F.S.1990-92 and 1992-93. The total reserve potential of this CoalSeam is approximately 0.58 million tones (Sharma,1994).

There are numerous coal outcrops of variousthicknesses in road sections, stream cuttings, and hilland scarp sections. Majority of these occurrences are oflimited strike extension. The coal occurrences are asfollows:

Near Kamling School in new road cutting ofKamling approach road:

An outcrop of coal of about 5 m thickness with shale/sandstone partings of approximately 2 metres thicknessis exposed. To the SSE of Kamling village, followingsuccession (Table no. 1) is noted by De (1982):

Table No. 1 : Coal bed succession near Kamling village.

Coal 0.6 m.

Parting 12.30 m

Coal 1.20 m (with numerous shale/ carb. Shale bands/lenses.

Parting 8.00 m

Coal 1.00 m

Parting 110.00 m

coal 4.50 m to 6.00 m (with some slate lenses)

About 2 km from Rothak Khola towards Reshi onNayabazar Legship road section:

Two coal seams are exposed with parting ofsandstones. The continuity of the seams is seen onopposite side of Rangit River.

Near Rothak Power Station:

Two coal seams (0.8 m and 1.10-1.50 m thick) areseparated by 3.4 m parting of sandstones near theconfluence of Rothak Khola and River. Further west ofthis section, a few thin coal outcrops are seen. South ofSamsing, near Roong doong Khola and Rangit Riverconfluence, a few thin coal exposures are seen varyingin thickness from 0.20 m to 0.90 metres.

About 3 km from Nayabazar on Nayabazar-Sombaria road section:

Coal seam of approx. 4 metres thickness with shale/sandstone partings is exposed in the stream section.

About 30 metres below Nayabazar Forest Bunglow:

In toe cutting section of Ramvang River, coal seamsare observed but are covered with thick overburden. NearSise village below Nayabazar-Legship road section,opposite to Karfektar a coal seam of about 5 metres istraced out but its thickness and lateral continuity couldnot be traced. On the NW slope of Gumpagaon aboutNamchi Bazar, two coal seams of approximately 2 mand 1 metre thickness are exposed. They show continuity

42 GEOL. SURV. IND

along NE direction and one of the seams is exposed onNamchi road section about 1 km above Namchi.

Tinher area:

Along old road section connecting Denchong andNamchi there are number of coal exposures. One of themis 2.5 metres thick and is exposed about 1 km fromNamchi bazar towards Denchong.

On Kalikhola section below Bumtar:

There are a few coal seams but only three showthickness of more than 1 metre.

On Tiri Khola section:

Along Tiri Khola section, SE of Wak village, numberof coal seams are exposed. They are of limited strikeextension and covered with thick overburden.

Coal exposures of Put Khola section:

About 200 m above the road along Put Kholasection, about 2 m thick coal seam is exposed havingstrike extension of 30 m. along NE direction.

West of Nandugaon four thin coal outcrops are seenwhich appear to abut against an ENE-WSW trendingfault. The coal beds are intercalated with carbonaceousshale and vary in thickness from 0.20 m to 0.90 metres.Two highly weathered coal outcrops are seen about akm east of Jorthang dipping 44° towards north-west.

The coal of Rangit River is flaky in nature and highlyfriable. They are bituminous to Semi-anthracitic havinghigh carbon-ash content and low volatile matter. Theinvestigation carried out so far reveals that the coal seamsof Rishi extends about 1500 m to 1700 m along strikealong Rishi Khola section in east-west direction withlittle variation in thickness (Sharma, 1992). The coalseam on the road section is approximately 6 metres inthickness, and runs parallel to Reshi Khola for about1700 metres on the northern bank. The seam crossesover to the southern bank with more or less samethickness. However, in between these two places the coalseam is not traceable.

The coal occurrence of the southern bank iscontinuous up to Reshi-Mangalbari road section and

continues over Rothok Khola. It is divided into threedifferent seams with partings of shale and sandstone of5 to 10 metres. The results of analysis of coal byDirectorate of Mining & Geology, Sikkim are as follows:

Max. Min.

Ash percent 51.7 26.6

Moisture content percent 17.5 12.2

Volatile matter percent 3.15 1.4

Fixed carbon percent 60 38

The utility studies of CMPDIL, Ranchi indicate thatthis coal can be used for room heating and cookingpurpose after making briquettes/pellets by addingsuitable additives which burn well and lasts for 3 to 4hours non-stop.

DOLOMITE:

Essentially the dolomite has been recorded in BuxaFormation and the best exposures are found along NayaBazar-Legship road (along Rangit River) andsurrounding hills. Both high grade massive and low gradeflaggy types of dolomites have been located in Rishiarea, west of the Jorethang-Legship state highway. Oneither banks of Rishi Khola, adjacent to Rishi village(27o13’N:88o46’E), four dolomite bearing blocks havebeen delineated by geological mapping, pitting andtrenching by Geological Survey of India. Massivedolomites are light grey in colour, fine grained with highpercentage MgO (18% to 22%), CaO being around 30%and insoluble 1.5 to 3%. A total reserve of more thanone million tones has been estimated by GSI down to adepth of 30m from the surface. The low grade flaggytype has high silica content and is not suitable for blastfurnace. However, this type may find selective usefulnessas powder for neutralization of acidic soil. Dolomite hasalso been recorded at Phong, Kaluk, Rishi area west ofJorethang-Legship Highway. Preliminary assessment ofoccurrences along Rangit River and along Rishi kholahas been done by Sarkar (1978).

Rangit River:

The dolomite is mostly flaggy in nature althoughmassive dolomite is also recorded. It is intercalated withquartzite. The occurrence is divided into four blocks;three on the northern bank of Rangit River and one onthe south of the river. The dolomite indicates CaOvarying between 17.85% and 29.50%, MgO varying


between 9.05% and 20.70%, R2O3 between 1.28% and8.00% and insolubles between 0.86% and 45.98%. Assuch this is not a good quality dolomite. Rough estimateshave also been made for this dolomite.

Rishi khola occurrence:

The dolomite occurs as a thick horizon forming aridge and strikes N30°E-S30°W to almost east-west withmoderate dips towards north. This dolomite underliesthe Gondwanas in the west along a thrust boundary.Reserves of approximately 1.1 million tonnes have beenestimated (down to 30 m depth) on the basis of channelsampling. The CaO varies from 28.37% to 40.32%, MgOfrom 12.06% to 21.50%, R2O3 from 0.95% to 2.95%and insolubles from 0.15% to 3.22 percent. This is agood quality dolomite.

GRAPHITE:

Graphite, both lumpy and flaky type with graphiteschist, marble and limonitised pegmatite of ChungthangFormation of Darjeeling Group, has been located atChitre (27°1620:88°0210) and Dareli (27°15:88°03) of west district. The graphite bands occur evenat depths of 2-3m below the surface and the thicknessvaries from 30 cm to 80 cm. The graphite bearing zoneextended in depth and can be traced over a considerablelength in a very irregular manner. Preliminaryexploration, including limited drilling, was conductedby GSI in Chitre area. An estimated reserve of about6000 tonnes of graphite has been computed from Chitresector.

Impersistent and pockety nature of graphiteoccurrences, their inaccessibility and high elevation(3000-4000m) and locations near the India-Nepal borderhave rendered these graphite occurrences uneconomicalin view of the high cost of the exploration, mining andtransportation of the materials.

Insignificant amount of amorphous to flaky varietyof graphite occurs as stringers, lenticles and small lensesalong the foliation plane of the high grade biotite gneissat some places around Dentam and Bega village.

Chitre and Dareli (west Sikkim):

Occurrences of graphite have been classified intothree groups. (a) In Kali Khola and Pache Khola, flakygraphite occurs as dissemination in the gritty quartz

biotite phyllite and slaty carbonaceous phyllite, mostlyoriented parallel to the foliation planes and are ofirregular thickness and length. Flaky graphiteconcentration has also been noted within the marblebands at Bop in north Sikkim and at the 18th milestoneon the Gangtok-Natu La road sections. Theconcentrations of graphite are of very small extent andhave sharp contacts with the host rocks. The graphite iscrystalline to amorphous type. (b) Amorphous graphiteconcentrations, associated with vein quartz and claymatter is seen mainly along shear and thrust zones andare exposed in upper Kali Khola and middle Kali Kholaarea. The zones vary in thickness from one to two metresin length up to 15 metres or more. At places, the cavitieswithin the graphite are filled with box-work of limonite,quartz, chert and kaolin. (c) Lump graphite is seen innorth Sikkim near Rangma, Chungthang and on theGangtok-Natu La road near the 15th and 18th mileposts.Here graphite is associated with pegmatite, occurring inthe shear zone between sillimanite-garnet-gneiss andcalcareous suite of rocks. Within the calc-silicate rocksgraphite fills scattered pods and fractures.

Considering the different types of distribution it canbe postulated from occurrence (a) that the originalsediments must have been contaminated withcarbonaceous material. The crystallization of graphitehas taken place by dynamothermal metamorphism dueto heat generated by tectonic movement during thrusting,faulting and folding as is evidenced by the occurrence(b) Migration of this graphite and its subsequentconcentration has been helped by quartz and pegmatiteveins as revealed by occurrences (b) and (c). Occurrencesof graphite within the marble might have probably takenplace due to denudation.

Prospecting for graphite was carried out in KaliKhola and Pache Khola area, where some surficialexposures of graphite could be traced. As the geologicaland structural setting of the graphite zone was known,exploratory trenches were dug along the strike wiseextensions of the different graphitic horizons and fromthe trenches 73 groove samples were collected coveringa length of 105 m. The chemical analysis reveals poorconcentration of graphitic carbon.

The graphite mineralization of insignificant naturehas also been recorded about 13.3 km NW ofChungthang-Lachen road and 600 m south of Tarun Chucutting across the highway (Kumar and Mitra, 1985).

44 GEOL. SURV. IND

The graphite mineralization occurs along some shearplanes which are at an acute angle to regional schistosity.The marble band, west of Bop village, shows graphiteconcentration along a shear zone. Traces of graphite arealso seen near Theng (27°24:88°39) and Chhanggu.

KYANITE BEARING PEGMATITE:

Kyanite bearing pegmatites were observed by Raina(1966) to the west of Gerethang (27°21:88°15) whileworking in west Sikkim (toposheet nos. 78 A/3, 4, 7, 8).The pegmatites cut across kyanite bearing schists.Kyanite blades are 15 cm × 2 cm × 2 cm in size.

LIMESTONE:

The Buxa Formation comprises dolomite andlimestone with interbeds of slates and phyllites. TheGeological Survey of India and State Department ofMining & Geology have done preliminary explorationwork. Grey limestones, interbanded with green phyllites,are observed in Rishi Khola, south of Namgaon. Anexposure of limestone, about 30m thick, is traceable overa strike length of 60m near Rishi Khola. Selectedportions of this limestone band indicate 42 to 46% CaO,1.22 to 2.20% MgO and 12 to 14% insolubles. Thislimestone almost crystalline, shows well bedded natureand are fine to medium grained in texture. Pink limestonewith shales is exposed at Nayabazar. Selected portions

of the limestone horizon have 42 to 44% CaO, 1.22 to1.6% MgO and 11 to 18% insolubles.

Mangalbaria and Namgaon area:

Two exposures of limestones are recorded atNamgaon and four bands of limestones recorded atMangalbaria area. The CaO percentage in limestonevaries from 30% to 42%, MgO from 0.5% to 4%, R2O3from 1.3% to 7% and insolubles from 15% to 30%.

Seiging area:

The limestone is interbedded with phyllite and slateand is about 35 m thick. The limestone contains 34% to43% CaO, 0.23% to 2.3% MgO, 1.1% to 1.9% R2O3and 16% to 25% insolubles.

North of Nayabazar:

Raina and Ray (1967) have recorded a thicksequence of highly folded limestone and lime bearingpurple and green slates in the area north of the NayabazarBridge. The sequence is traceable from the west bankof Rangit River up to about 1 km east of it. This formationoutcrops only along the road section, being coveredotherwise by soil and forest cover. The sequence alongthe road section (Nayabazar-Namchi road) and thechemical analysis of samples is indicated in the table 5.

Table no. 5: Chemical analysis of limestones exposed along Namchi-Nayabazar road.

Rock type Outcrop Analyses (%)Width (m) Ins R2O3 CaO MgO L.I.

Slumped cherty limestone 60 40.42 9.06 26.29 0.93 22.24No exposures 300Massive limestone 13 13.27 2.87 44.42 1.65 31.00Slates with nodules of limestone 6Massive limestone 16 15.54 3.15 42.98 1.24 35.29Greyish-green slates 45Massive limestone 16 10.43 7.30 44.45 1.34 34.46Slates with thin intercalation of limestone 9No exposure 150Massive limestone with intercalation of shale 25 16.66 4.72 42.21 1.03 33.00Slates with thin limestone bands and nodules 13Massive limestone with shale intercalations 30 18.57 3.25 40.67 1.51 33.85Slates with occasional nodules of limestone 45Greenish-grey shales 13Green slates with thin pink limestone bands 10Slumped greenish-grey slatesPebble slates


The outcrop is 30 m thick and traceable over a strikedistance of 60 m across the stream; the surrounding areais covered by soil and forest. Representative analysis oflimestone band is produced in the table no. 6.

Table no. 6: Chemical analysis of the limestones of Rishi khola area.

Rock type Outcrop Analyses (%)Width (m) Ins R2O3 CaO MgO L.I.

Massive limestone 1.5 13.69 2.80 42.69 1.66 36.77Calcareous slates and phyllitesMassive limestone 3 13.17 2.04 44.71 1.24 37.00Calcareous slatesMassive limestone 3 12.94 1.44 46.44 1.03 37.54Limestone with phylliteMassive limestone 3 12.25 2.60 43.00 2.17 33.15Phyllites with thin bands of limestoneGreen phyllite

Rishi Khola south of Namgaon:

Greyish-black to steel grey, hard, compact, partlycrystalline limestone interbanded with green calcareousslates outcrop in the Rishi stream just south of Namgaon.

Table no. 7: Chemical analysis of limestones from Chho Lhamo and Everest Limestone Formations.

Sm Lat. Long. SiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O TiO2 P2O5 LOI.No.

46 28°0440 88°4240 10.78 2.26 0.65 0.45 0.03 0.63 47.25 0.06 0.51 0.04 0.05 37.0246A 28°0435 88°4250 5.73 2.07 0.85 0.73 0.12 2.81 48.09 0.04 0.19 0.03 0.05 38.5977A 28°0130 88°4445 4.26 1.98 2.36 0.82 0.09 1.56 49.60 0.04 0.16 0.07 0.06 38.59

On the northern bank of Rishi khola Raina (1968) did preliminary prospecting for limestone in three blocks andestimated reserves of 0.81 million tonnes. On the southern bank five small blocks were delineated with the totalreserves of 0.263 million tonnes. The limestone is cement grade. This report is supported by sufficient analyticaldata.

Chhomdo hill & eastern slopes of Dorjila hill:

Roychoudhury et al. (1998) indicate presence of good quality limestone bands in Chholahmo Formation andEverest Limestone Formation of north Sikkim, particularly in Chhomdo hill and eastern slopes of Dorjila hill. Thelimestone bands are up to 55 to 60 m thick and traced over a strike length of 500 m to 750 m. Following is thechemical analysis of three representative samples of limestones (table no. 7).

MARBLE:

Several bands of marble of variable thickness wereseen near Chungthang, Theng and Naga in north Sikkimand near the 6th & 16th mile posts and near Changgu onthe Gangtok-Natu La road.

Chhanggu area :

Greyish white to creamy white, coarse grainedmarble bands are reported from about 1.5 kilometer westof Chhangu Lake in East Sikkim. The bands vary inthickness from 12m to 42m with average CaO varyingfrom 39% to 73%.

A. M. N. Ghosh (1952) collected a few samples ofmarble from Chhangu area, which shows SiO2- 16.34%to 14.31%, Al2O3- 3.82% to 3.21%, Fe2O3- 1.19% to1.24%, CaO- 42.98% to 44.85, MgO- 2.50% to 1.97%.He concluded that the marble can be utilized for low-heat cement.

Raina and Vohra (1969) mapped four marble bandswithin a horizontal distance of 600 m (east-west) andvertical distance of 100 m (between 3600 m and 3700m) between the 6th and the 10th loop about 1 km west ofChhanggu. The marble bands are closely associated withsillimanite-garnet-biotite- gneiss and show an almostnorth-south trend with steep dips.

46 GEOL. SURV. IND

The marble is greyish-white to creamy white, coarsegrained and almost granulitic in appearance. It showscharacteristic differential weathering. Microscopicexamination reveals presence of quartz, diopside,scapolite, feldspar, sphene, zircon and biotite besidescalcite. Individually the eastern most band is 42 m wide,followed westwards by 20 m thick zone of banded gneiss

underlain by second band. The second band is 12 m wideand shows synformal folding towards the base. Again a200 m thick biotite-sillimanite gneiss band separates 2nd

band from 3rd band. The third marble band is 18 m thick;the western-most fourth band is 30 m wide. Betweenthird and fourth band there is 180 m thick zone of gneiss.Analysis of chip samples from four bands is indicatedin table no. 8.

Table No. 8: Chemical analysis of marble samples from Chhanggu area.

Band No. Insol. (%) R2O3 (%) CaO (%) MgO (%)

1. 18.40 2.10 42.73 0.282. 17.19 1.59 43.25 1.113. 29.52 1.79 36.80 0.624. 32.55 1.73 34.96 0.84

Area around Chungthang:

A very preliminary assessment of marble bands ofthis area was carried out by Kumar and Mitra (1985).Prior to that NEITCO, a consulting agency of SIDICO,Sikkim State had made assessment of these marbleoccurrences.

4.5 km NW of Chungthang on Chungthang-Lachenroad:

The occurrence of marble is very much limited andonly some boulders of marble are seen.

Chubinbin nala occurrence:

This occurrence is 1 km NW of the above mentionedoccurrence. It has a length of about 300 m along theroad upto the point where the Chubinbin nala cuts acrossthe highway.

South of Tarun Chu:

The occurrence is located about 13.3 km north-westof Chungthang-Lachen road and 600 m south of TarunChu cutting across the highway. Crystalline marble isassociated with calc-silicate, quartzite and calc-granulite(Kumar et.al., 1986).

Malten Shiva Temple:

This occurrence is near Malten Shiva Temple 6 kmnorth-east of Chungthang on Chungthang-Lachung road.The occurrence covers a distance of about 50 m along

the road. Good exposures are found on the scarp faceonly.

Pegong occurrence:

This occurrence is located about 5 km south ofChungthang on Chungthang-Mangan road near Pegongvillage. It is a very minor occurrence.

Other minor occurrences of calc-silicate/marble arerecorded on Singhik-Chungthang road, particularlyaround Myang village and near police check post at Tongvillage. Marble is also located at Selep, Mensithang,Malten, Theng, Tong, Myong, Chhangu, and Chakung.

G. N. Dutt (1955) located small calc-tufa occurrencesat an altitude of 8400 ft above the confluence of Benshoiand Lachen streams. They appear to have been originatedfrom the nearby calc-gneisses as seen in one of tributariesto Benshoi Chhu. In Chungthang-Lachen area of NorthSikkim several bands of calc silicate/marble areintimately associated with high grade metasedimentariesof Chungthang Formation. These calc silicate may beused for mini cement factories.

SILICA AND SILICA SAND:

Upper middle part of the Everest LimestoneFormation in north-east Sikkim is represented by pinkand white ortho-quartzite which is 45 metres thick, ofgood quality and considerable strike length (300 m).Following is the chemical analysis of the quartzite. Thisquartzite can be used in ferro-silicon, glass and otherrelated industries Roychoudhury et al., 1998).


SILLIMANITE:

Sillimanite occurs as needles (Fibrolite) or asaggregates in the high grade gneisses in Chitre, Uttare,Dentam, Chhange Khola and Sordung area of WestSikkim. Sillimanite occurs as fibrous aggregates mainlyalong the closely spaced slip planes within the gneiss oralong the contact of the pegmatite vein cutting acrossthe foliation plane of the gneissic country rock. Kyaniteis also found to be associated with sillimanite. Thekyanite and sillimanite occurrences in the area are atpresent not of economic significance.

TALC:

The talc hosted by Mansari quartzite unit at andaround Mansari village (27°0841:88°1210) is ofeconomic importance and was mined by M/s SikkimMinerals Pvt. Ltd. through three crude rat hole typeexcavations. The talc is lumpy and occurs along fracturesin the host quartzite. The quartzite itself is reportedly ofoptical grade and was also quarried. A crushing unit wasalso set up. A preliminary estimate of quartzite and talcreserves up to 25m depth is a round 15 million tones,maximum true thickness of the quartzite being around65-80m.

TOPAZ:

In Tista valley, very near to Jorepul (27°42:88°34),a tourmaline bearing granite containing Topaz has beenlocated (Bhattacharya and Pattanaik, 1964). Auden(1935) has reported the occurrence of topaz bearinggranite from a place on the Tista Valley which is nearly48 km north of this occurrence. But in this area, theoccurrence of this mineral is found for the first time.The topaz crystals are 1.5 cm to 2 cm in length; fleshcoloured and irregularly arranged. The zone is 30 cmthick and nearly 8 metres long.

Sm.No. Lat. Long. SiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O TiO2 P2O5 LOI

54-1 28°0030 88°4550 96.38 0.77 0.53 0.18 0.01 0.62 <0.01 0.02 0.11 0.23 0.03 0.53

TUNGSTEN:

Wolfram has been observed at Chitre. Sills and dykesof tourmaline granite and garnetiferous pegmatites areintruded in to banded migmatised biotite psammitegneiss and granite gneiss of Central Crystallines. Atplaces, these granites are found to contain sheelite whichis concentrated along the joint and fracture planes(Roychudhury et al., 1998)

URANIUM MINERALIZATION:

Udas (1986) indicated association of uraniummineralization with copper-lead-zinc ores in low gradeschists of lesser Himalayas and in Daling Group of rocksof Darjeeling and Sikkim. The significant concentrationof uranium have been met with along a major zone ofintense shearing within the Daling phyllites, parallel andclose to the Gondwana-Daling thrust. The shear zone istraceable from Nepal border in the west to the Bhutanborder in the east. The well known copper-lead-zinc atGorubathan falls in this shear zone. Significant uraniumconcentrations have also been found in the shearedDaling phyllites on the foot wall and hanging wall ofthe Cu-Pb-Zn lode in the Bhotang mine near Rangpo.

VERMICULITE:

Quartz vermiculite veins either occur independentlyin a close proximity of pegmatites in the rocks ofsillimanite and kyanite zone. The rock type associatedis granitised mica schist. The thickness of the vein variesfrom one cm. to about 30 cm. Conspicuous deposits arefound on the north Sikkim road south of Labi, at Tingda,about 4th milestone east of Gangtok and between 8th &9th milestone.

In addition building stones and sand are alsoavailable.

48 GEOL. SURV. IND

LATITUDE LONGITUDE

Degree Min. Sec. Degree Min. Sec.

Bartag 27 30 88 30Bhondikhola 27 08 88 18Bop 27 37 25 88 39 10Borong (Hot spring) 27 21 48 88 19 22Brom 27 18 30 88 26 50Bumtar 27 09 88 22Chakung 27 09 88 12Chawang 27 25 50 88 35 30Chejima 27 50 88 46Chhangu 27 23 88 46Chho Lhamo Lake 28 01 45 88 45 30Chhothina 28 04 88 42Chittre 27 16 20 88 02 10Chongbong 27 07 30 88 15 00Chopta Chhu 27 54 88 32Chorten Nyima La 27 58 88 13Chorten Labsang 27 56 88 14Chorten Nyima peak 27 57 88 11Chungbong 27 07 30 88 15Chungthang 27 36 20 88 39 00Danak Khola 27 13 00 88 39 00Dareli 27 15 88 30Denchong 27 10 88 20Dentam 27 16 88 09Dholepchan 27 11 20 88 40 48Dhond 27 20 88 05Dikchu 27 16 48 88 36 47Dong Busty 27 12 30 88 29 00Dongkung 28 01 15 88 35 30Dongkung 28 02 88 36Duga 27 11 40 88 32 10Duglakha 27 14 88 36Dumra 27 11 88 20Gangtok 27 20 00 88 37 15Geyzing 27 16 45 88 15 30Gogong 27 58 88 35Gurudongmar Lake 28 02 15 88 42 45Jagdum 27 11 88 14Jorepul 27 42 88 34Jorthang 27 07 30 88 17Kamling 27 12 88 17Kamrang 27 11 88 21Kanchendzonga 27 42 88 08Keorani 27 10 15 88 14 52Khandosangphu 27 15 88 18Kyagnosa La 27 22 88 43Labing 27 21 30 88 14 30

Locality Index

48


LATITUDE LONGITUDE


Lachen 27 43 50 88 33 15Lachung 27 41 30 88 44 50Legship 27 17 00 88 13 00Lema 27 39 15 88 43 35Lukrap 28 00 88 36Malten 27 37 30 88 40 45Mamring (Mamreng) 27 13 30 88 37 45Mangalbaria 27 16 15 88 26 28Mangan 27 30 00 88 32 30Mangreng 27 10 88 31Mansari 27 08 41 88 12 10Mayang 27 15 88 18Menshithang 27 38 40 88 36 50Myang (Myong) 27 34 40 88 26 40Naga 27 32 15 88 37 30Namchi 27 10 88 22Namgoan 27 12 88 17Namok 27 26 05 88 32 10Namthang 27 10 88 29Naya Bazar 27 08 88 16Pachekhani 27 12 05 88 36 50Pamphuk Khani 27 07 88 29Parbing 27 11 88 29 30Penlong 27 30 22 88 37 30Phedi 27 22 88 03Polot 27 21 00 88 19 46Rabong La 27 18 20 88 22 00Rangma 27 36 45 88 38 15Rangpo 27 10 00 88 31 00Rangrang 27 28 45 88 32 00Ratopani 27 11 88 25Rinchingpong 27 14 88 16Rishi 27 14 20 88 18 15Rolep 27 16 15 88 43 10Rongli 27 12 12 88 42 05Rorathang 27 11 35 88 37 05Rothak (Roathak) 27 10 88 18Rungdu 27 10 15 88 38 45Runglo 27 32 00 88 29 00Sada 27 24 88 22Salebong 27 09 88 23Sang 27 15 24 88 30 29Sanklang 27 30 30 88 30 30Sardung 27 16 20 88 11 30

Selep

Siging 27 10 40 88 15 10Sikkip Bridge 27 12 45 88 19 50Singrangpung 27 21 88 08

Sipik

Sirbong 27 10 35 88 15 30Siringyong 27 20 88 09Sisni 27 10 18 88 14 20Sodunlakha 27 12 00 88 40 00Somdong 27 09 88 19Sosing Hill 27 17 88 20Sumbuk 27 07 88 23Suntole (Sontale) 27 10 88 16

50 GEOL. SURV. IND

LATITUDE LONGITUDE


Temi 27 14 00 88 26 00Thanggu 27 54 88 32Theng 27 24 88 39

Tigda (6 km of Gangtok)

Tolung 27 40 40 88 26 30Tong 27 33 00 88 38 55Tukhani 27 08 88 26Tunga-phu-phiak 27 39 88 28Uttare 27 16 88 05Yome Samdong 27 55 00 88 42 00Yaksam (Yoksum) 27 22 00 88 13 00Yangdi 27 56 88 31Yumthang 27 47 30 88 42 00Zongri 27 27 88 10


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