ISSN 0579-4706 © Govt. of PGSI. 340 Controller of Publication 700-2011 (DSK-II)

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GEOLOGICAL SURVEY OF INDIA Miscellaneous Publication No. 30, Part XIX –

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Published by order of the Government of India 2012 © INDIA, GEOLOGICAL SURVEY (2012)

Compiled by the officers of Sikkim Unit, Operation: WSA

Processing and editing of the manuscript by S/Sri Subhra Suchi Sarkar, Md. Amjad Ali and Smt. Gargi Bhattacharya Senior Geologists

under the supervision of S/Sri Pradip De , Bikash Chandra Roy and K.V.Nambiar Directors, Publication Division, Geological Survey of India, Eastern Region and under 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 at M/s Arunima Printing Works, 81 Simla Street, Kolkata 700006, Phone: 91-33-22411006, E-mail: [email protected] Foreword

detailed account of Geology and Mineral resources of Sikkim has been prepared in order to publish the volume in the form of Miscellaneous APublication 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 temperature of 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 factors contribute to the setting up of tourism and pharmaceutical industries in years to come.

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

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

The state is endowed with rich mineral resources. This includes asbestos, base metals, 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.

Kolkata Dated, 14th December, 2011

iii iv vi CONTENTS

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

II GENERAL GEOLOGY 6 Stratigraphic classification 6 CENTRAL CRYSTALLINES 6 Formation Kanchenjunga Gneiss/ Gneiss (Undifferenciated) 7 Banded / Streaky gneisses / Migmatites 7 Augen bearing biotite gneiss with//without Garnet, Kyanite, Sillimanite 8 Sillimanite Granite Gneiss 8 DALING GROUP 11 Formation 12 Reyang Formation 13 Buxa Formation 13 LINGTSE GRANITE-GNEISS 15 MOUNT EVEREST PELITIC FORMATION 16 MOUNT EVEREST LIMESTONE FORMATION 16 GONDWANA SUPERGROUP 17 Rangit pebble slate 17 Damuda Formation 18 Lachi Formation 19 Chho Lhamo (TSO Lhamo) Formation 21 Intrusive Granites 22 Syenitic Rocks 23 Sesela Formation 24

vii STRUCTURE AND TECTONICS 25 METAMORPHISM 28 SEISMICITY AND EARTHQUAKES 30

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

IV LOCALITY INDEX 48

V REFERENCE 48

viii MISC. PUB. NO. 30(XIX) 1

I. Introduction

Sikkim or Sikhim is a name derived from the Places with an altitude of 6065 m and above are Sanskrit word Shikhim meaning ‘Crested’. In all of snowbound and places as low as 3002m come within the Sikkim there is not a single kilometer of ‘flat land’. With snowline in the winter. The temperature in the lower an area of 7096 sq km, measuring 113 km from north to altitudes fluctuates between 4°-35°C and places with south and 64 km from west to east, the state has moderate height (around 1829 m) such as elevations ranging from 240 to 8484 meters above mean temperature varies between 1°C and 25°C. In the high sea level. The State of Sikkim is located in the southern altitude area (above 3993 m), the temperature never rises mountain ranges of Eastern Himalayas between Northern above 15°C and slides down to the freezing point in winter. Latitudes 27°05and 28°08 and Eastern Longitudes Agriculture 88°10 and 88° 55. Sikkim is the 22nd State of the Indian Union, which came into existence with effect from 16th Agriculture accounts for one third of Sikkim’s gross May; 1975. Sikkim is bounded by in the west, domestic product. More than 64% of the population in the east, Tibet in the north and state of West depends on agriculture for livelihood. Products such as Bengal in the south. The state has been divided into four rice, wheat, maize, finger-millet, barley, buckwheat, and districts, viz. East, West, North and South districts and pulses like ricebean, rajmah, fieldpea, cowpea and each district has further been subdivided into two sub- clusterbeans and oil seeds like rapseed, mustard, divisions for administrative purposes. soyabean and sunflower are grown in the area. Of late there is a thrust on planting medicinal herbs, which Climate : abound the natural vegetation. The State is reported to The climate of the State has been roughly divided have more than 424 species of the medicinal plants. into the tropical, temperate and alpine zone. For most part of the year, the climate is cold and humid. Most part of the state receives heavy rainfall throughout the Fauna and flora year. It is only in the month of October-March that the Sikkim is enriched in forest resources. The total area State remains comparatively drier. The extreme northern under the administrative control of Forest, Environment parts, adjoining Tibet, however, receive very little and Wild Life Department is 5765.10 sq km which rainfall. The mean annual rainfall is minimum at Thangu accounts for 81.24% of the total geographical area of the (82 mm) and maximum at Gangtok (3494 mm). An state. The State has one national park ( isohyatal analysis of these data reveals that there are National Park) and five wildlife sanctuaries. Names of two maximum rainfall areas (i) South-east quadrant principal endangered species of Sikkim are bharal, including Mangan, Singhik, , Gangtok, Rongli, clouded leopard, fishing cat, golden cat, himalayan thar, etc. (ii) South-west corner including hilly terrain. In leopard cat, , marbled cat, musk deer, nayan of between these two regions, there is a low rainfall area great Tibetan sheep, pangolin, serow, snow leopard, (around ). There is an area in north-west Sikkim spotted lingsang, Tibetan antelope, Tebetan Fox, Tibetan which receives less than 4.9 mm of rainfall. Rainfall is gazzelle, Tibetan wild ass, tiger, Tibetan wolf. Important heavy and well distributed during the months from May birds are black necked crane (migratory), blood pheasant, to early October. July is the wettest month in most of peafowl, Tibetan snow cock, tragopan pheasant, snow the places. partridge, and siberian crane (migratory).

1 2 GEOL. SURV. IND

The dominant flora in temperate zone is oak, cuherry, crow fly distance towards north. Siwaliks starts at 153 alurel, chestnut, maple, birch and and the m and ends at 823 m. Dalings continues up to 1220 m dominant flora in the southern part of Sikkim is plantains, and then Darjeeling Gneiss starts. On the eastern side bamboos, tree ferns, walnut, sal and oak. Cimbidiums, Siwalik ends at 732 m, Dalings at 1376 m. At the central vanda, cattaleya, hookeriana, farmeri, dendrobiun- part of the area, i.e. in Tista Dome the height is in general amoenum and noble orchid are the most popular flora. between 1200 to 2500 m though the summit point is on 3233 m. As soon as the Daling rocks are crossed the Physiography and drainage height increases sharply from average 1800 m to 3000 The State encompasses parts of Lesser Himalaya, m in the lower reaches, 3000 m to 4500 m in the middle Higher Himalaya, and the Trans Himalaya and hosts reaches and 4500 m to 6700 m in the upper reaches of some of the highest mountain peaks of the Himalaya. the dome. The rocks north of South Tibetan Detachment The elevation ranges from 300 to 8586 m with increasing System(STDS) are in general occurring above 4200 m elevation from south to north. In Sikkim-Darjeeling to 7300 m. Himalaya, the Lesser Himalaya starts from Kalijhora (in Darjeeling) in south/west and continues up to Singhik The Tista Rangit water divide is the main N-S water (in Sikkim) in north/east. The low grade metamorphic divide within the Sikkim-Darjeeling Himalaya. This rocks of Lesser Himalaya start at as low as 300 m and water divide takes an E-W swing after crossing the continue up to 3050 m (Mount Mainak). Daling Dome. The N-S stretch culminates at 3700 m and the E-W trend shows sharp rise in altitude from 3700 In Sikkim exposures of Higher Himalayan rocks start to 7300 m. The west bound water divide is having the at about 650 m and continue upto 8586 m (Mount summit point at famous Kangchenjunga peak. Another Kangchenjunga). The outcrops of gneisses with granitic N-S water divide is between Lachen Chu and intrusions exhibit sharp, rugged, snow bound mountains Chu which starts at about 3000 m and slowly increases with steep in accessible scarp faces. Extending in the up to 6700 m. form of an east-west trending ridge, it forms a barrier between the Trans-Himalayan zone and the Lesser The other water divide in general trends E W. The Himalayas with Kanchenjunga as its western limit and height of the water dividing ridges increases slowly from Lama Angdang as the eastern limit and a series of high 1100 m to 1500 m, 2100 m, 3000 m, 3600 m, 6700 m peaks, deep valleys, gorges and numerous glaciers in towards north. These ridges are lower in height in central between. Langbo Chhu, Naku Chhu and Chhombo Chhu portion and height increases both towards eastern and form main drainage flow through flat bottomed ‘U’ western side. Sharp increase in height has been observed shaped valleys and finally meet with Tista River. after crossing the Daling Dome. On the eastern side of Tista valley water divide of Rilli, Rongpo, Rani Chu, The Trans Himalaya starts from 5300 m and continue and Dickchu are spectacular. The ranges south of Dikchu upto 7000 m. The snow capped jagged ridges in the being the longest one known as Gangtok ridge as the northern portion of the state are feeders to the glaciers Gangtok city is on this ridge. The most important feature which come down to about 4000 m. The topography of is that these ridges continue crossing the Tista River to the northern part, where rocks are of sedimentary origin, the Rangit - Tista water divide. The ridge portions which is characterized by barren, reddish-brown mounds and crosses the Tista River are very steep. hillocks, with broad, flat bottomed valleys so typical of Tibetan plateau. These hillocks are seen protruding out North of Dickchu, steep ridges on the eastern side as fingers, from the main Himalayan ranges, gently are there but of smaller extent as the Tista valley are merging with broad plains of Tibetan plateau. The river subdivided by the water divide between Lachen Chu and valleys are open towards the top and attain a steep gorge Lachung Chu. On the western part long ridges are like character towards the beds of river. present. These are Talang Chu and Zemu Chu water divide. In Sikkim-Darjeeling Himalaya the first hill ranges starting from south are having summit between 1158 m Thus, the total relief picture of Sikkim-Darjeeling to 732 m and runs east west. In the western part, the Himalaya is such that the northern part is always above height increases very sharply to 2592 m within 20 km 3300 m to 7000 m forming a part of Tibetan Plateau. MISC. PUB. NO. 30(XIX) 3

The middle part slowly decreases its height towards The sub basins of the western side tributaries are bigger south and central part giving rise to an amphitheater type compared to the eastern side. The migration of the river relief feature. The amphitheatre follows the trend of towards east may be considered to be a result of tilt of MCT indicating that the Higher Himalaya is uplifted the rocks towards east. A spectacular observation is that relative to the Lower Himalaya. The important Mountain the western bounding water divide of the Tista basin is peaks of Sikkim are: Mt. Kanchenjunga (8559.42 m), always higher than the eastern bounding water divide Mt. Kabru (7361.36 m), Mt. Talung (7356.8m), Mt. across the same latitude. This feature continues all along Siniolchu (6870.4 m), Mt. Simovo (6832.7 m), Mt. the east west trend of the bounding water divides. Pandim (6718.4 m), Mt. Rathong (6718.4 m), Mt. Paunhri (6688m), Mt. Kokthang (6129.2 m), Mt. Rangit sub basin, the biggest sub basin in the Tista Lamaongden (5887.26 m) and Mt. Masunyange (5867.2 valley on its western bank, also shows an asymmetric m). nature. The southern block is more constricted than the northern block indicating a tilt towards south (Cox 1994). Salient feature of Tista Drainage Basin: The Talang Chu, another sub basin in the western part Tista is the trunk river in Sikkim-Darjeeling of the Tista valley just north of Rangit also shows a Himalaya. Like all other major rivers of Himalaya, Tista southern block constriction indicating a southern tilt. is also an antecedent river. It starts from Tso Lamo lake However, the river north of Talang Chu, the Zemu Chu of Trans Himalayan region of north Sikkim. Near its is a symmetric sub basin. The Lachung Chu, the eastern origin it initially flows for some distance in E-W tributary in the similar latitude also shows a more or direction, and then takes a sudden N-S trend. Near less symmetric sub basin. The Tista basin in this latitude Chungthang it takes an ENE – WSW trend from Mongan is symmetric upto the confluence of Lachen Chu and takes a N – S trend. This trend continues for its entire Zemu. Then the eastern constriction starts. The Tista route through Himalaya. basin is again symmetric after Tista Rangit confluence. The Rongpo Chu, the biggest eastern tributary is At the upper reaches, before Zemu Chu and Lachen symmetric barring a small part constricted towards north. Chu confluence, i.e. in the north of Higher Himalaya The other eastern tributaries are symmetric. The lengths the valley wall is 1520 m high. The valley wall rises up of the eastern tributaries are smaller compared to the to 3344m from Lachen to Dickchu as it enters the Higher western tributaries. Himalaya. After Dickchu, the valley wall slowly decreases to 915 m near and further downward The drainage pattern in Tista basin is mostly a in the outer hill ranges the valley wall is 457 m high. mixture of sub dendritic and sub parallel. However, in Thus the gorge forming tendency is mostly noticed in the northern part directional trellis type drainage pattern the Higher Himalayan region. It is a case of rapid incision is observed. Drainage density is high in the lower reaches with higher rate of uplift. but in the higher reaches the drainage density is moderately high. Gradient of longitudinal profile of the Tista River is very low in lesser Himalaya and slowly increases Lakes of Sikkim: towards north. The change in the gradient is very sharp The state is endowed with some of the most north of Dickchu, near MCT and highest gradient was picturesque lakes of Himalayan terrain although they observed near Zemu. After Zemu, towards north the are not very large in size. Most of the lakes have gradient is again low. The Longitudinal profile of Tista originated as depression scooped by the glaciers and is concave from lesser to Higher Himalaya but changes subsequently filled by glacial melt. Others have formed to convex in the higher reaches. It has been observed by the damming of glacial water by the terminal that the longitudinal profiles of the Trans Himalayan moraines. The lakes are mostly confined to the northern Rivers are convexo-concave as they pass from the part of Sikkim but a few are also found in the western Tibetan Plateau through the Higher Himalaya to the and eastern part. Lesser Himalaya and Sub-Himalaya. Gurudongmar is the largest lake in Sikkim, located The Tista drainage basin is an asymmetric drainage probably at the highest point among the lakes. The well basin. The western side is wider than the eastern side. known 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- geomorphic history in which both proglacial and filled glacial lake which is bounded on all sides by periglacial regimes are found. Epigenetic gorges are very moraine ridges. Choka is another spectacular lake of common from Bakkim northwards. Distinct U-shaped north Sikkim. valley profile is discernible from south of Dzongri.

In south-western Sikkim, a number of small lakes Talung basin: like Lam Pokhri, Kali Pokhri, Kanahailal Pokhri, The basin lies towards north of the East-Rathong Deoningale Pokhri, Torepool pokhri are most prominent basin and is west of Tista River and extends towards the ones. Now these lakes are located in the sub-tropical to eastern border of Nepal. It occupies an area of 1270.74 temperate region but they show that once upon a time sq km and supports 61 glaciers. Most of the glaciers are the area was covered by heavy snow and glaciers. restricted to the northwestern portion of the basin. The Kechopari Lake is another lake that lies on way to Talung glacier, which is the largest one in the basin Yoksum from Peling. Chhojo Lake is located SW of covers a sizable part of northwestern portion of the basin. Yoksam on the opposite side of Rathang Chhu. This lake Subsequent type of drainage is observed to be common. represents original ‘neve’ region of an ancient hanging This basin has also preserved multi-cycle geomorphic glacier, the depression having developed due to the history in which proglacial and periglacial environments scooping action of the glacier. A moraine ridge is seen are discernible. West of Umrang Chhu and Rukal Chhu lower down forming southern bank of Lethang Valley. confluence, distinct U-shaped profile of the valley and colonized moraine debris are found which appears to be In the eastern part of the state, lakes like Chhangu the lower limit of the palaeo-glaciation in the valley. Lake, Syeberuka chho, Serabthang chho and many others have all been produced by the erosive action of the Changme Khangpu basin: glaciers moraine which carved depressions in the bottom This is the eastern most glacier bearing basin of the of the valley. Such lakes are geologically known as Sikkim Himalayas that lies between the Zemu basin in ‘Tarn’. Chhangu (Tsomo) lake is located at an altitude west and Chumbi valley (Tibet) in the east. The basin of 3693 meters. Two nearby lakes are Bidang chho and occupies an area of 1158.75 sq km and supports 102 Menme chho. glaciers. The glacierised area is restricted to the north of Lachung. In the longitudinal profile of the valley from Glaciers of Sikkim : Khadom to Yomesomdong, three glacial terraces are In Sikkim Himalayas the glaciers are restricted to identified. The subsequent fluvial action has resulted in the western and northern part of the state. The water of a few cascades. All along the valley up to Khandom the glaciers drain into Tista River and hence, are glacier smoothened valley walls are discernible. generally known as Tista Basin glaciers. Puri et al. (1999) have given detailed account of various glaciers. Zemu Basin: It occupies an area of 2391.60 sq km and is supported The area of four glacierised basins of Sikkim by 250 glaciers. The basin is drained by two major Himalaya is 7172.21 sq km. These fifth order basins are tributaries, the Zema Chhu and Hema Chhu. The Zema East Rathong basin, Talung basin, Changme Khangpu Chhu sector has an area of 1358.88 sq km. The largest basin and Zemu basin. glacier in this sector is , which is also the largest glacier in Sikkim Himalaya. Different cycles of East Rathong basin : glacier and periglacial regimes are preserved. The This is the southern most among the basins of Sikkim distinct U-shaped valley profile noticed near Telim Himalayas. It is located west of the Tista River and indicates the probable lowest limit of the glaciation in extends in that direction towards the Nepal border, the basin. occupying an area of 2351.12 sq km. It supports 36 glaciers and almost all the glaciers are restricted to the The Hema Chhu sector is located east of Zemu Chhu northern part of the basin. Obsequent drainage is most and occupies an area of 1032.72 sq km. The largest prominent feature of the basin. The major channel is glacier in the Hema Chhu sector is Tista Khangse glacier Rathong Chhu, which joins the Rajal River at . which is the source of Tista River. Various phases of The East Rathong basin has preserved multi-cyclic geomorphic history have been preserved in this sector MISC. PUB. NO. 30(XIX) 5 and about two km south of Thangu, the evidences of River about 1.5 km west of Polot village, West Sikkim past glaciation are discernible. district at an altitude of 920 m. The Rishi hot spring is also called as Phur Cha Chu hot spring and Tatopani. It Hot Springs: is situated on the left bank of Rangit River about 1.8 km The thermal springs of Sikkim are included within north of Rishi at about 500 m altitude. the Himalayan Geothermal Province. More than 1000 hot springs are situated in this collision zone. Sikkim is A comparative analysis of the chemical data of the blessed with a number of hot springs; out of which five thermal waters of Borong, Polot and Rishi indicates hot springs in Sikkim are studied in detail. These are less concentration of Na, SiO2 (except Rishi hot spring) Yumthang hot spring, Yumesamdong hot spring, Borong and Cl from Borong hot spring. This may be attributed hot spring, Polot hot spring and Rishi hot spring. to the greater admixture of surface / subsurface water with original hot fluid in this area. The weight ratios The Yumthang hot spring, also known as Chhachha and constituents concentrations in the three (Borong, hot spring, is located at about 1.5 km SSW of Yumthang, Polot, Rishi) hot springs indicates that these springs at an altitude of 3570 m on the left completely differ from volcanic type of thermal water bank slope about 200m above the Yumthang Chhu. The with the exception of K / Na (for all the springs) and

Yumesamdong hot spring is located at an altitude of 4700 SO4 / Cl (for Polot and Rishi hot springs) values. Higher m. It is situated at about 1.5 km NW of Yumesamdong, values of HCO3 / Cl and lower to moderate about 40 m above the Sebu Chhu river bed. The Borong concentration of TDS are attributed to the admixture hot spring, also known as Mangnam Cha Chu is situated of cold water at shallower levels. On geochemical on the left and right banks of Rangit River at about 1 km considerations, it can be concluded that the thermal NW of Brang village, at an altitude water of all the springs receive contribution from deep of 1000 m. Polot hot spring also known as Ralong Cha magmatic source and yet diluted by surface and Chu, is situated on the left and right banks of Rangit subsurface water. 6 GEOL. SURV. IND

II. GENERAL GEOLOGY

The Himalayas are traditionally been divided into Daling Dome) has exposed a wide expanse of the Lesser linear geotectonic belts supposedly with distinct Himalayan rocks. The MBT, with the Mio-Pliocene geological characteristics. Beginning from the south synorogenic Siwalik Group in the footwall and the these are Sub Himalayas, Lesser Himalayas, Higher Permo-carboniferous Gondwanas in the hanging wall, Himalayas and Trans Himalayas. Like other parts of the has not been affected by this culmination structure and Himalaya, in Sikkim-Darjeeling Himalaya, the Sub has a roughly E-W trace. The Gondwana rocks as well Himalayan domain comprises the molasses type deposits as the Buxa and Rangit pebble slate are exposed in the of the Siwaliks. It is followed northward successively Rangit window zone where these are surrounded by by a thin strip of sandstone, carbonaceous shale and coal Daling Group of rocks (Gansser, 1964; Acharyya, 1989, (Gondwana), stromatolitic dolomite and variegated slate 1992). The Tethyan Belt is exposed on the hanging wall (Buxa and Reyang Formation of Daling Group) and a side of a series of north-dipping normal faults constituting thick metasedimentary sequence of dominantly pelites the South Tibetan Detachment System (STDS), Higher with subordinate psammite and wacke (Gorubathan Himalayan Crystallines being the footwall. Formation of Daling Group), constituting the Lesser Himalayan Belt. Towards the north, Daling sequence is Geological investigations in Sikkim and the overlain by Higher Himalayan rocks of medium to high adjoining Bengal Region began in the middle of grade dominantly pelitic schist with minor interbanded nineteenth century. Hooker (1854) in his famous quartzite, calc-silicate and metabasites (commonly Himalayan Journal reported the geological observations known as Chungthang / Paro Formation) and small from his extensive two years travel in many parts of bodies of granites (Lingtse Gneiss). This sequence in Sikkim. He was able to trace the regional domal picture turn towards north overlies a migmatitic terrain known of the gneisses. and its foothills were as Darjeeling Gneiss/Kangchenjunga Gneiss and thought examined by Mallet (1875). Auden (1936) discussed the to be equivalent of what is variously described as Central problems of the Daling and Darjeeling Gneiss. Heim Crystalline/ Greater Himalayan Crystalline /Higher and Gansser (1939) visited Tista region, Darjeeling and Himalayan Crystalline (GHC/HHC). In the far north, a made a traverse to Gangtok. A.M.N. Ghosh (1952) first thick pile of fossiliferous Cambrian to Eocene sediments, reported patches of Gondwana within the Daling area, belonging to the Tethyan Belt (Tethyan Sedimentary i.e. Rangit window. Different workers of GSI worked in Sequence) overlies the HHC. connection with mapping and different investigations.

The Main Boundary Thrust (MBT) separates the Stratigraphic classification schemes Siwaliks of the Sub Himalayan domain from the In recent years Geological Survey of India developed overlying rocks of the Lesser Himalayan Belt and the unified legend for all the formations of India. The Main Central Thrust (MCT) separates the Lesser formations exposed in Sikkim can be classified as given Himalaya from the Higher Himalaya. In the western as in Table 1 in accordance with the unified legend scheme. well as in the eastern part of the Sikkim-Darjeeling Himalayas, the Lesser Himalayan Package is exposed CENTRAL CRYSTALLINES as thin strip between MBT and MCT. However, in the central part of the Sikkim-Darjeeling Lesser Himalayas, The Precambrian sequence of Sikkim State can a domal shaped culmination structure (known as Tista/ broadly be classified into two major groups. Low grade

6 MISC. PUB. NO. 30(XIX) 7

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 Pleistocene pebble 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 Formation sandstone. Carboniferous to Sandstone, shale, carbonaceous shale with coal Damuda Group Gondwana Permian Pebble/boulder slate, conglomerate, phyllite Rangit pebble state Groups Supergroup Fossiliferous limestone with quartzite Everest Limestone Ordovician Formation Granite gneiss (mylonite) Lingtse Granite Meso Proterozoic Gneiss 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, variegated cherty phyllite, meta-greywacke Reyang Formation Proterozoic Interbanded chlorite-sericite schist / phyllite Gorubathan Formation Daling Group Undifferenciated and quartzite, meta-greywacke (quartzo feldspathic 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 Central sillimanlte with palaeosomes of staurolite, (Undifferentiated) Crystalline Proterozoic kyanite, mica schist, biotite gneiss, sillimanite Gneissic Undifferenciated granite gneiss Complex (CCGC) 1. Quartzite 2. Garnet kyanite sillimanite biotite schist / Garnetiferous mica schist Chungthang 3. Calc-silicate, carbonaceous schist Formation Chungthang Formation Precambrian meta-sedimentary, generally known as Mallet (1875) was one of the earliest workers in the Daling Group, is exposed in the central part of the Daling area who systematically classified the rocks of Nepal, Dome. The rock sequence in the outer part of the Tista Darjeeling and Sikkim area. After Mallet (1875), some Dome have variously been classified as Kanchenjunga of the earliest records of geological mapping and Gneiss, Darjeeling Gneiss, ‘Chhubakha Series’ (Raina observations pertain to those of Dutt (1948, 53, 54, 61), and Bhattacharya, 1962), etc. and by a number of other Ghosh (1956);Raina (1965, 66, 68); Raina and local names by pervious workers. This sequence Bhattacharya (1962 and 65); Bhattacharya and Pattanaik comprising various types of gneisses, schists, quartzites (1964);Bhattacharya (1966); Singh and Bhattacharya and calc-silicates have been given the name of Central (1968). During this phase of mapping in fifties and Crystallines in the western and central part of Himalayas. sixties, a lot of proliferation of lithounit names was in Keeping in view the necessity of retaining one name for vogue, possibly because the mapping was done in a single litho pack, they have been hereto called as isolated patches.During early eighties some of the areas Central Crystallines. were remapped and some are mapped a fresh on expedition by Neogi et.al (1983,84,86 and 98).The 8 GEOL. SURV. IND

central crystalline gneiss of higher Himalayan crystalline quartz show strain shadows and occasionally contain of the Darjeeling –Sikkim Himalaya has been classified inclusions of biotite. Smaller grains of quartz occur in a by Ray(1989) into the Sikkim Group comprising granular aggregate. At places biotite grains enclose Kanchenjunga Gneiss , Darjeeling Gneiss and quartz crystals. Some times the quartzite shows the Chungthang Sub-Group. presence of chlorite. Along the contact with gneisses, garnet is seen to have developed in the quartzite. In addition to the gneisses, a prominent sequence of calc-silicates, calc-gneisses, quartzites and schists which 2) Calc- silicate rocks and graphitic schist:- Singh appeared to be different from the gneissic sequence to and Bhattacharya (1968) while mapping in North Sikkim the earlier mappers, was separately identified and and East Sikkim mapped calc-gneisses and granulite with mapped as Chungthang Group of rocks (named after the bands of marble and quartzite associated with sillimanite village of Chungthang, where this unit crops out garnet gneiss and graphite schist as the most dominant prominently). rock association which could be traced near Naga (27°3215:88°3730), Theng (27°34:88°39) and In this write-up, in the proposed scheme of Rangma in north Sikkim and near 5th and 10th mile posts classification, the Central Crystallines have been on Gangtok - Nathu La road sections. Several bands of classified into Kanchenjunga Gneiss / Darjeeling Gneiss marble are noted at Bop and Theng in north Sikkim and and Chungthang Formation. at 5th and 24th milestone on Gangtok - Nathu La road. Bands are milky white in colour and vary in thickness A) Chungthang Formation : from 30 m to 60 metres, and are traceable for considerable Raina and Bhattacharya (1961-62) while working distance along the strike. The marble band, west of Bop in North Sikkim marked a formation which was traced village, shows graphite concentration along a shear zone. over a width of 19 km between Myang (27°10:88°22) Traces of graphite are also seen near Theng in south and Lema (27°39:88°43) in north and has been (27°24:88°39) and Chhangu. Between Myong designated as Chunthang Formation. This name has been (27°3440:88°2640) in the South and Kishong given after Chungthang (27°36:88°39), a village in (27°4315:88°27’45) in the north three bands of calc- North Sikkim where the rocks of this formation are silicates and associated quartzite bands have been prominently exposed. The main rock types of this recorded within gneisses. First band is near Mayong, formation are quartzites, garnet-kyanite-staurolite second band is near Tolung Gompa bearing biotite schist, calc silicate rock, graphitic schist (27°3830:88°2755). The bands trend in NE-SW and amphibolite. The rocks of this formation are also direction and are locally folded (Neogi et al. 1986). In exposed in the districts of east Sikkim and west Sikkim. West Sikkim district the limestones and marbles are observed only in one section at Meguthang 1) Quartzite:- The quartzite which forms an (27°25:88°03) (Raina, 1966). important constituent of this formation is well exposed in different parts of Sikkim. Raina and Bhattacharya Under microscope, the calc-silicate contains (1961-62) reported quartzite at Tong (27°33:88°39), diopside, quartz, feldspar, actinolite, tremolite and Chungthang (27°36:88°39), near Lema (27°39:88°43) calcite. Diopside and feldspars occur as porphyroblasts and in south of Lachung (27°41:88°45). Singh and while quartz occurs as large plates and also as mosaic Bhattacharya (1968) reported quartzite exposed as thin of recrystallised grains. Feldspars are mostly altered bands within calc-gneiss and granulite, gradually orthoclase, fresh labradorite and few grains of microcline attaining sizeable thickness to be mapped as a separate showing wavy extinction. Diopside occasionally alters unit near Chungthang in north Sikkim and Kyagnosa La to actinolite-tremolite. Olive green hornblende is also (27o22:88o43) on Gangtok - Nathu La road. Several seen. Sphene and magnetite are accessory minerals. Near bands of felspathic quartzites have been found north of Myang, lenses of wollastonite were found to have Menshithang (27o38:88o37) near Rabong Chhu. developed within these rocks.

The quartzite is greyish and show development of Under microscope; the marble shows idioblastic mica along the bedding planes. Under microscope they calcite and subordinate diopside, tremolite, phlogopite are seen to be quartz-feldspar-schist. Larger grains of and a few grains of scapolite. Sphene is a common MISC. PUB. NO. 30(XIX) 9

accessory mineral with inclusions of carbonate. In of staurolite. Thin section show it to be fine grained graphite bearing marble, graphite occurs as flakes, quartz-sericite-muscovite-biotitic schist with prolific sometimes encircling sphene. In Tolung - Kishong area almandine garnet (subhedral grains), a few grains of the calc silicate units show a granular mosaic of equant feldspar (plagioclase) occur as recrystallised aggregate. grains of calcite, quartz with wollastonite and diopside typical of high temperature marble. Lamellar twinning 4) Amphibolite:- Amphibole schist and in calcite may be due to twin gliding during plastic amphibolites as lenticular sills have been observed to deformation. Visuvianite alters to calcite. Feldspars are occur in all rock types of Chungthang Formation. In some sericitized (Neogi et al., 1986). of the cases they are sulphide mineral bearing. They have been found profusely north-west of Penlong La (27o22: 3) Biotite Schist:- Biotitic schist with garnet and 88o37). These are also common between Mangang and staurolite was demarcated as Rongli schist . It has been Singhik and at Kahior (27o31: 88o32). named after Rongli, a village in East Sikkiin, where the rocks of these types were first observed. However, later Under the microscope the rock shows characters of workers ignored the Rongli schist in the context that the hornblende-quartz-schist with hornblende, oligoclase Rongli schist represents the higher metamorphic grade feldspar, biotite (developed from hornblende), quartz and equivalent (garnet , staurollite, etc) of Daling metapelites minor rutile as its main constituents. and has been variously included within the Lesser Himalayan or the Higher Himalayan package. B) Kanchenjunga Gneiss / Darjeeling Gneiss Bhattacharya and Pattanayak (1963 – 64) while mapping Kanchenjunga / Darjeeling Gneiss are extensively in the North and West Sikkim districts informed that the exposed in the northern part of the Sikkim, to the south rocks comprising Rongli schist are seen in the area of the Tethyan sequence. In the west; Mount between the Tista Valley and Talung Valley. The Kanchenjunga and adjoining hills comprise these rocks schistose rocks exposed between Sangklang and Namok which pass into Nepal territory further west. To the east (27°26:88°32), between east of Rhamthang these rocks are well exposed around Yum Samdong, (27°2430:88°3345) and Lebrong (27°25:88°35) and Yumthang, Lachung, Chhangu and Nathu La areas and between Lindok (27°23:88°3515) and Nabhe further east they continue into Tibet and Bhutan. (27°23:88°36) have been described by Singh and Bhattacharya (1968). This schist contains abundant The gneisses, dominantly comprising quartz, garnet and occasionally shows bands of flaggy quartzite. feldspar and biotite (with minor amounts of other The schistose rocks exposed in West Sikkim district have minerals) have been classified into three types, ie.1) been described by Raina (1966) as schist zone. Schists banded / streaky gneisses / migmatites, 2) augen bearing with interbedded thin quartzite bands are the main rock biotite gneiss with/without garnet, kyanite, sillimanite types of this zone. and 3) sillimanite granite gneisses. Mapping of these rocks as individual units is very difficult because they This rock contains abundant garnet and mica. are characterized by frequent interchanging and Garnet, staurolite, biotite and muscovite constitute the gradational features among themselves. However a main minerals. Garnet porphyroblasts show sieve couple of zones have been demarcated and shown in the structure with inclusions of quartz and chlorite along map. the cracks. Staurolite shows well developed cruciform twinning and some grains show inclusion of quartz. 1) Banded Gneiss/ Streaky Gneiss/ Migmatite:- Biotite and muscovite show simultaneous crystallisation. In the northern most part of the Central Crystallines of Helicitic inclusions in staurolite and garnet are found to Sikkim, the gneisses of various types form the major be arranged in curved lines, across the schistosity of the rock unit (Sinha Roy and Roy, 1972) and of them the rocks suggesting inclusion of minerals during growth. banded gneiss is the dominant type. Hyden, Fermor and Alteration of staurolite to chlorite and biotite indicates Auden reported ortho gneisses from different parts of retrogression. Sometimes biotite occurs as corona Sikkim which are characteristicaly similar to the banded surrounding quartz crystals.The schistose rocks exposed gneiss described by the later workers. The pelitic in Sangklang, Lindok, Nabhe, Namok, Rhamthang and migmatite of Neogi also indicates this type of rock. Raina Lebrong areas contains abundant garnet with absence and Bhattacharya (1962 &65), reported banded gneisses 10 GEOL. SURV. IND

(named as Chhubakha Gneiss by them) between Lema Apatite, zircon, sphene, epidote, rutile, graphite and (27°39 :88°43) in the south and Yome Samdong opaques are main accessories. (27°54:88°45) in the north Sikkim. These are biotite gneisses whose impregnation by the granitic material 2) Augen bearing biotite gneiss with/without have completely obliterated their original characters. garnet, kyanite, sillimanite:- Raina and Bhattacharya Bhattacharya and Pattanaik (1964) reported highly (1965) described reddish brown to grayish black augen brecciated and folded banded gneiss near Chhateng. gneisses showing well foliated nature with prominent These gneisses are profusely intruded by granites and augens of feldspar. To the west of Lungme nala pegmatites and have been affected by a series of faults. (27°52:88°21), these gneisses show lateral variation Thin graphite schists are observed along shear plane or into a coarse grained porphyritic variety which shows fault plane. It is found from Yukti Chu 3 km north of an almost unfoliated nature with subrounded feldspar Menshithang (27°4000:88°3530) to Lachen grains (10-12 mm across) lying scattered within the body (27°44:88°33). Coarse to medium grained, grayish of the rock. Similar lateral variation is observed to the white to dark grey banded gneiss covers a major portion west of Khora Chachen (27°56:88°20). In some areas of area between Chittre (27°15:88°03) and Phedi large blocks of garnet sillimanite gneisses are seen (27°22:88°03) in the south and Zongri (27°27:88°10) embedded within the augen gneisses. These xenoliths and Yaksam (27°22:88°13) in the north Sikkim (Raina, are rusty brown in colour and composed of biotite flakes 1965). Dutt (1955) reported garnetiferous banded gneiss alternating with crushed quartz stringers. In the north- at Donkung (28°02:88°36) in north-central part of central part of Sikkim porphyritic augen-gneisses Sikkim. Singh & Bhattacharya (1968) reported the containing streaks of biotite has been recorded by Dutt streaky banded gneiss at Serabthang on Gangtok - Nathu (1955) to the north of Lachen (27°44:88°33). Half a La road and migmatised banded gneiss between Singhik mile north of Zemu - Lachen confluence several bands and Manul, near B 2, between B 3 and Penlang in north of calc-gneisses are also observed as inter-stratified with Sikkim and between the 1st and 5.5 milepost and at 7th augen gneiss. Near the Choptha Chhu (27°54:88°32) milepost on Gangtok - Nathu La road. yellow ferruginous mica schist bands are recorded within gneisses. The augen gneisses along Zemu river valley The banded gneiss consists of alternating bands rich change from the horizontally banded and foliated rock in quartzo-feldspathic material and mafic schistose on the southern side to the much more knotted and components, imparting a strong s-tectonite fabric. contorted gneisses on the north. They extend through the entire Zemu valley from very near to the confluence Under the microscope the rock consists of biotite, of Zemu river with Tista, as well as from Tungaphu- quartz, muscovite, sericitized microcline and plagioclase phiak (27°39:88°28) to Tolung on the Tolung valley feldspars. The quartz is of two types (i) small fractured (Bhattacharya and Pattanaik, 1964). Augen gneisses also grains, showing strain-shadows in the groundmass and occur south of Menshithang (27°40:88°3530) on Tista (ii) relatively strain-free patchy grains. Feldspar is albite- valley. Sinha Roy and Roy (1972) reported augen gneiss oligoclase with minor amount of orthoclase together in the upper reaches of the Tista River. Raina (1966) forming more than 35 percent of the mineral assemblage. recorded augen gneisses around Singrangpung Anhedral large andesine/oligoclase with deformed twin (27°21:88°08) in the valley of Rimbi Chhu. Singh and lamellae had also been observed. Secondary albite has Bhattacharya (1968) reported augen gneiss at Bop in developed along andesine orthoclase contact. north Sikkim and at Karponang, Chhanggu and Occasionally highly altered, fine to coarse myrmekite Serabthang on Gangtok - Nathu La Road. developed at the interface of quartz and anhedral K- feldspar. Deep brown to greenish brown biotite is a major The augens mostly of felspar and rarely of quartz, constituent. Biotite in some sections is seen to have show extreme idioblastic habit and vary in length from altered to chlorite. Muscovite flakes are interwoven with one cm to 7-5 cm. At isolated locations, the quartzo- biotite and are twisted and bent enclosing grains of feldspathic bands often show selective bulges in the form quartz. Some of the sections showed the presence of of pinches and swells as a precursor to the formation of sillimanite and almandine garnet. Anhedral garnet augen gneiss. The gneissosity defined by biotite and occasionally showing incipient alteration to chlorite with sillimanite wraps round the bulges. At Tolung valley inclusions of quartz, biotite and sillimanite is abundance. sometimes there are imperfect gradations from streaky MISC. PUB. NO. 30(XIX) 11

granite gneiss to the augen gneiss with gradual increase 1962) also reported banded and augen gneisses showing in the size and numbers of augen and the amount of effects of high grade metamorphism around the Daling ferromagnesian minerals. Augen gneisses occuring south dome in its eastern and northern part. Here the gneisses of Menshithang (27040’:88035’30") on Tista valley starts as kyanite bearing gneiss at the lowermost (Bhattacharya and Pattanaik, 1964) have been intensely structural level near the contact with Chungthang criss-crossed by tourmaline rich muscovite bearing Formation and become sillimanite bearing gneiss in the granite and pegmatite. upper level. Raina (1966) while mapping the western part of Sikkim reported sillimanite-garnet-biotite Under the microscope, the rock shows gneisses forming mountain ranges west of Dentam porphyroblasts of orthoclase and quartz, mostly arranged (27°16:88°09) and around Uttare (27°16:88°05). parallel to the foliation-planes, embedded in matrix of biotite, chlorite, quartz and plagioclase. These gneisses Under microscope the rock reveals gneissic texture contain quartz which is of two types, (i) highly strained defined by quartz and feldspar (both as individual grains small fractured grains and (ii) strips and ribbons showing and as intergrowth), sillimanite, biotite, muscovite and incipient polygonization, and shapeless patches garnet as main constituents. Larger crystals of quartz intergrown with K-feldspar. In the larger grains of quartz, show strain shadow whereas smaller grains occur in the cracks and fractures are all aligned across the granular aggregates. Perthitic growth between orthoclase direction of banding, though the elongated direction of and plagioclase, replacement growth between quartz and the crystal itself is oriented along the banding of the felspar, and sericitisation of feldspar are quite common. rocks. Biotite is highly corroded and dispersed. K- Biotite is of two generations. First generation biotite is feldspar occurs as anhedral grains and as large produced during progressive metamorphism, followed porphyroblasts, which constitute the augens, showing by garnet and sillimanite. Second generation biotite is slight strain effects and sericitization along cleavage and developed due to retrogression and occurs as coronas twin planes. Another constituent of the augen is lamellar and patchy aggregates around garnet and sillimanite perthite with crystallographically oriented thin albite crystals. Sillimanite is present as acicular aggregates within K-feldspar. Secondary albite also occurs as thin arranged in sheaf, parallel to the foliation and sometimes rims at the margin of K-feldspar. Coarse to very fine within the granular aggregates of quartz and feldspar. myrmekite occurs at the contact of quartz and K-feldspar. Curved patches of sillimanite (fibrolite) are also seen. A few grains of tourmaline, apatite, zircon garnet, Garnet is mostly pyrope almandine type and shows hornblende, scapolite and suspected monazite are noted zoning indicating probable growth of garnet in more than as the accessory. In Jemu valley augen gneisses contain one stage. Large porphyroblasts of recrystallised garnet only porphyroblasts of potash feldspar (Bhattacharya and are seen to be surrounded by lamellae of biotite. These Pattanaik, 1964). garnets are highly shattered and show numerous fractures filled in by megacrysts of quartz and biotite. Presence 3) Sillimanite Granite Gneiss:- Bhattacharya and of rims of biotite around garnet and formation of Pattanaik (1964) reported that sillimanite bearing phlogopite from biotite are also noticed. Accessory gneisses are exposed in Tolung valley from Laben minerals are apatite, sphene, epidote and zircon. In some (27°33:88°2730) to Tolung Monastery and near sections kyanite forms the major constituent and is Chungthang in the Tista valley. Singh & Bhattacharya arranged haphazardly in the country rock. Kyanite is (1968) reported sillimanite-garnet-biotite gneiss near mostly broken and contains inclusions of quartz. Bop, Theng, Naga and Singhik in north Sikkim and from Alteration of biotite to chlorite, garnet to biotite and the 6th to 9th, 11th to 13th and 15th to 18th mileposts kyanite to muscovite has been observed. on Gangtok - Nathu La road. Kyanite-biotite-muscovite gneiss with bands of flaggy quartzites are exposed DALING GROUP between Mangan and Sangklang Bridge Unfossiliferous low grade metasediments of the (27°3030:88°3230), in Rangrang (27°27: Darjeeling-Western Duars area were subdivided into 88°3430), near Chawang (27°2550:88°3530) and dominantly greenish argillaceous assemblage comprising west of Penlang (27°3022:88°3730) in north Sikkim. the Daling “Series” and dolostone, quartzite and To the west of Lachung; sillimanite-biotite gneisses are variegated slate assemblage making up the Buxa “Series” reported by Dutt (1955). Raina and Bhattacharya (1961- by Mallet (1875). The terms ‘Daling’ and ‘Buxa’ have 12 GEOL. SURV. IND

been extended later to cover wider areas of the Eastern vary in colour from lighter grey to milky white. Several Himalaya but much confusion exists in their occurrences of dark grey quartzite and beds of flaky classification and nomenclature even in the Darjeeling- quartzite are also seen. Quartzites are found in the Sikkim-Duars area. The Daling Group (sensu lato) of Rangpo and Dikchu areas. Most outcrops at Rangpo are Darjeeling-Sikkim Himalaya is characterized by distinct less than 50 feet while the main band just east of Dikchu tectono-stratigraphic position being placed below high is about 200 feet. Intercalations of quartzite appear in grade gneissic rocks limited by the sheared belt of phyllites on the right bank of the river north of Dikchu mylonitic granite gneiss and above its own cover by Bazar. There after upto the 16 mile stone north of Dikchu the Lower Gondwana equivalent sediments of the these quartzites continue. Raina and Bhattacharyya Himalaya. (1961 – 62) reported that the quartzites are preserving primary sedimentary features such as graded and current Three distinct mappable and regional lithotectonic bedding. The carbon phyllite was encountered at Kali assemblages have been recognized within the Daling Khola section and Kartok in the east of Pakiyang. The Group (sensu lato) by Acharyya (1989) which are carbon phyllite usually soils hand but in case of few described below: bands near Pachekhani this soiling characteristic was absent. Medium grained massive feldspathic wacke ❑ Gorubathan Formation (Daling sensu stricto) partings have been observed near Vasme and Tarku ❑ Reyang Formation within chlorite schist. Raina (1966) reported schistose ❑ Buxa Formation (sensu stricto) grits from Rinchingpong area. Gritty rocks, light green to greenish grey in colour, have been observed associated Gorubathan Formation : with the phyllites. These grits show a well bedded nature, It is best exposed around Daling Fort (27o01:88o43) with grains of greasy quartz present along the bedding in Darjeeling Himalaya from where the term ‘Daling’ planes. Together with this, conformable bands of basic (sensu stricto) started. The assemblage is renamed intrusive occur with the units of the Gorubathan Gorubathan from the same type area by Ray (1976). The Formation. They are 2 to 15 m long and 1.5 cm to 2m rocks are exposed extensively in the southern part of thick and observed within the chloritic phyllite near Sikkim from east of Renok to west of Geizing and north Sang, Temi, and near . Recrystallised quartz of Dikchu to Rongpo in the south having a dome shaped veins and stringers are quite common. Near Rabong outcrop loosely known as Tista Dome. (27o18:88o2030) Ben Monastery, several mica pegmatite intusions have been noticed within slaty The formation consists of mappable, monotonous phyllites and quartzite. sequence of inter banded chlorite sericite schist / phyllite, quartzite, meta greywacke, pyritiferrous black slate/ Chlorite phyllite and quartz chlorite phyllite contains carbon phylllite, basic meta volcanics. Chlorite phyllite following assemblage:- quartz + chlorite + muscovite + is dark green to light green whereas the quartz chlorite feldspar + opaque. Quartz shows two distinct mode of phyllite is only light green in color. It shows fine quartz occurrence. Matrix quartz are fine and recrystallised veins in Tarku, Rongpo, Vasme, Duga and Pachekhani giving rise to mosaic of rectangular and polygonal grains, area. Quartzites are occurring as thin partings to the long axis of these grains being in a preferred mappable bands within the chlorite schist and quartz orientation parallel to the planar fabric of the rock. chlorite schist. Quartz chlorite schist with quartzite Coarser quartz grains are detrital in nature and mostly partings is observed to the west of Temi, Barmek, and sub-angular to rounded in shape, with the onset of Chamthang area. Intercalation of quartzite and chlorite recrystallisation along the borders. Some grains show schist is observed in Rongpo, Duga, Pandem, , faint undulose extinction but detritus nature is evident Manka area. Quartzites are mainly white to light green in spite of recrystallisation. At least in two localities, depending upon the percentage of chlorite in them. It is volcanogenic parentage of the Daling phyllite is noticed. also grey to buff at places. Mostly these are fine grained These are (a) chloritic phyllite from the top of the in size and massive in nature. Bhattacharyya and abandoned adit of IBM near Rorathang adjacent to the Pattanayak (1963 - 64) reported that the quartzite at Pachekhani copper mines. The former shows embayed places, form prominent horizon (as at Bhale Dunga and quartz grains indicating volcanic characteristic and (b) Pati Dunga near Menam Dunra (27°20:88°50). They Chloritic phyllite adjacent to the pebbly carbon phyllite MISC. PUB. NO. 30(XIX) 13 of Kali Khola near Rongpo. In later case, chlorite grains variegated in shades of purple, maroon, steel-grey or green are generally elongated shapeless to prismatic in form. with other colour blotches. Pyritiferous black slates and Some grains show faint pleochroism. White mica occurs minor carbon phyllites are also present. Compared to as very fine flakes. However, some moderately big those of the Gorubathan Formation the slates and phyllites muscovite flakes could be identified. Feldspar is also are more siliceous and with ubiquitous iron ores. Common detrital in nature. They are angular grains with some occurrence of chloritoids in the higher grade, restricted showing twin lamellae of plagioclase. Opaque grains to this unit, corroborates their aluminous nature. Minor are elongated/ flattened and generally aligned parallel beds and lenses of brown, pink or yellow crystalline to fabric. Other types of opaque grains are subrounded limestone on dolomitic limestones are also present. Veins to rectangular mainly associated with chlorite. Chlorite of crystalline magnesite are also recorded from Reyang and white mica together with finer grained quartz River section, Darjeeling Hills. showing preferred orientation define the planer fabric of the rock. However, as the clear-cut Q domain and M Conformable bands of mafic meta-volcanic rocks domain are not defined it could be a foliation, and not and meta-dolerite have been reported from the Reyang schistosity. The carbon phyllite microscopically is a Subgroup which contains relict igneous textures and foliated rock with elongated opaque grains. The primary pyroxene and plagioclase, grading into foliated feldspathic wacke are matrix supported rock though the amphibolites in more altered regions (Acharyya, 1989). clast matrix ratio is as high as 70:30. The matrix is made Buxa Formation: up of very fine aggregate of slightly recrystallised quartz, chlorite and white mica. The framework grains contain In the normal stratigraphic section of the Rangit about 20 percent feldspar grains of microcline and Window Zone, the rocks of the Buxa Formation occur plagioclase, quartz and chloritic rock fragments. The above the Reyang Formation and below the horizon of framework grains are sub-angular to sub-rounded in Gondwana diamictite (Rangit Pebble Slate). Elsewhere shape. There is evidence of onset of recrystallisation in the foothill belt, it occurs as a discontinuous narrow around the framework grains; however, no preferred strip of variable thickness between the Gondwana orientation has been observed in these rocks. sediments on the bottom and the units of the Reyang Subgroup on the top. The Buxa formation essentially Reyang Formation: comprises an alternating sequence of thin, grey cherty Ray (1976), Acharyya and Ray (1977) described a mature quartzite, chert, fine grained, and finely laminated distinct sequence of thick bedded, ortho to proto to massive grey dolostone, pyritous sericitic and quartzite, variegated phyllite/slate with minor variegated slates. The type area for the Buxa is the Jainti impersistent beds of crystalline carbonates, conformable River section in western Duars, where the dolostone metabasites, from the type section around Reyang in the attains considerable thickness and is intimately Tista River valley in Darjeeling district, transitional associated with pyritous and carbonaceous slates. The between the Gorubathan and the Buxa Subgroups. This dolostone units are locally fossiliferous, as in the Rangit lithoassociation is underlain and overlain by units of River section, near Tatapani in West Sikkim, where Gorubathan and Buxa Subgroups, respectively, in the stromatolites have been reported from the upper horizons Rangit Window Zone. of the dolostone beds (Ray, 1976; Acharyya, 1989).

The white, brown or purplish quartzites are The best exposures of the Buxa Formation in Sikkim texturally mature with up to 90-97% quartz in a medium are recorded along the course of Rangit River on Naya to coarse grained mosaic along with chert or jasper, Bazar to Legship road where Dolomite, which is the minor feldspar, rock fragments, sericite, chlorite and iron main rock type of the Buxa Formation outcrops. The oxides. These show various stages of mineralogical and limestone/dolomite is exposed around Naya Bazar, textural maturity. They often show ripple marks and cross Reshi, Rahu, Tatapani, , Mangalbaria, Phali, bedding are locally associated with lenticular horizons Dhara, Wak and Kamigaon. Unlike Gorubathan and of oligomictic quartz-chert conglomerate (Acharyya, Reyong Formations, the Buxa Formation has only local 1989). distribution. It is characterized by fine grained, fine laminated to massive, grey and pink dolostone and Slates and phyllites of the Reyang Subgroup are limestone beds, often with a weathered, pitted surface. 14 GEOL. SURV. IND

It is associated with pyritiferous, sericitic and variegated Calcareous, purple and green slates form the basal slates, chert, and cherty and mature quartzite (Acharyya, portion and are exposed in Ranji khola (stream) section. 1989). Calcareous and non-calcareous slates/phyllites are best observed along Reshi-Sikkip road section, Mangalbaria, There are three different litho units which Chakung and Chumbung. They are purple coloured and characterizes the Buxa Formation substantially thick at places. 1. Mainly dolomite with purple quartzites and phyllites. Near Rangpo the main rock types of this formation 2. Pink limestone, Grey limestone with calcareous are carbon phyllites, quartz-chlorite-sericite-phyllite, and slates and phyllites. quartz-sericite phyllites, often with a purple tinge, white 3. Calcareous purple and green slates with orthoquartzite and calcareous-quartzite. The carbon intraformational conglomerate. phyllites comprises alternating thin layers of highly Purple dolomites are recorded along Rishi- recrystallised polygonal quartz with small dispersed Mangalbaria road section. They are thinly bedded and grains of carbonate and small stumpy sericite felt with siliceous in nature. Wherever the siliceous bands are dispersed carbonaceous material defining the bedding. associated, differential weathering is prominent. The In some cases, there are a few subrounded grains of grey dolomites which constitute a major part of Buxa garnet, arranged in zones, parallel to the bedding, which Formation exhibit cross lamination and ripple marks. suggests that they could be detrital (Sinha Roy and Under microscope, the rock shows a fine grained Mukhopadhyay, 1974). crystalline mosaic of dolomite crystals with a few grains of cryptocrystalline quartz, some chert and also algal The black slates underlying the Rangit pebble-slate impressions (Raina and Ray, 1967). In the Rangit River are lithologically identical to similar beds within the section around Tatapani, the dolostone beds are latter. The dolostone beds are conformably underlain stromatolitic in the upper most levels and the dolostone- by pink and variegated limestone-dolostone beds and chert-slate alternations are para conformably succeeded variegated slates, which represent transitional beds with by Rangit pebble-slate. Near Khandosangphu the adjacent section of Reyang Formation. (27o15:88o18) and some of the stream sections; the formation is represented by compact, massive looking, Sedimentary structures: light brown to cherty-grey, often with bluish tinged Current Bedding: It is exhibited by Buxa exposures, dolomites with minor bands of calcareous slates and along the west bank of Rangit River. Current bedding phyllites. Dolomite outcrops are seen as cliff and scarps indicates normal sequence of strata. This is also along the western bank of Rangit River near Rishi. They confirmed by the fact that Gondwanas overly Buxa. are highly jointed and sheared and exhibit current Ripple marks: They were observed on the underside of bedding, algal structure and bubble prints. Some of the a dolomite band just south of Khandosangphu (27°15: shear planes are the locale of sulphide mineralization, 88°18). Ripple marks also show a normal sequence. mainly galena. Graded bedding: Graded bedding was observed in siltstone bands associated with the phyllites of Daling Pink limestones and slates are exposed at Nayabazar- Group. One such section outcrops along the western bank Damthang-Tinkitam ridge and Chimchi spur. Limestones of Rangit River, north of Sosing bridge. Graded bedding are present in the form of thin intercalations, often sheared indicates a reverse sequence. Similar reverse sequence into nodular shape. Grey limestones interbedded with on the basis of graded bedding was also observed east green phyllitic slates are observed only in Rishi khola. of Yangthang (27°17:88°20). These limestones, almost crystalline, show a well bedded nature and are fine to medium grained. Limestones are Age of Buxa Formation: found associated with purple slate at the bridge abutment Stromatolitic assemblage from the dolostone beds near Nayabazar (27o08:88o17). On the left bank of Rangit from Tatapani area, representing top-most unit of Buxa the limestones are well stratified with purple slates, but dolostone, indicates Lower to Middle Riphean age. But on the other bank it occurs as a massive outcrop. Purple those occurring within the lithologically identical slates are also found south of Somdong (27o09:88o19), dolostone from Bhutan foothills indicate late upper though it is not calcareous here (Dutt, 1954). Riphean age (Raha, 1980). Stromatolite biostratigraphy, MISC. PUB. NO. 30(XIX) 15 if valid, would mean diachronous nature of Buxa and in the Kurseong-Darjeeling hills from Mahananda Formation. through Mongphu and Peshoke. The Lingtse Gneiss of the second mode, i.e. within the Lesser Himalayan low While discussing the age of Everest Limestone grade rocks which in Darjeeling Sikkim Himalaya is the Formation, Wager (1939) correlated it with the Buxa Gorubathan Formation of the Daling Group, occur as Formation of Mallet (1875) and placed it into Permo- major elongated north south body. This central Lingtse Carboniferous. However, it should be remembered that gneiss body is exposed from Ramthang - sector Everest limestone is crinoidal in nature and no crinoids in the north to Pendum in the south via Martam. In have been reported from Buxa. Both Buxa limestone/ addition to these larger bodies a number of small granite dolomite and Everest Limestone contain stromatolites bodies have also been recorded from within the Daling and algal mat. Group of rocks.

LINGTSE GRANITE-GNEISS The granite is two-feldspar granite where K-feldspar Granite gneiss, exposed around Martam and Lingtse predominates over plagioclase. The main constituent Gompa area of East Sikkim, has been designated as minerals are quartz, K-feldspar, plagioclase (mainly “Lingtse Gneiss” by Bose (1891). The gneisses are sheet oligoclase), biotite, muscovite and opaque ores. like bodies of coarse to medium grained, foliated to Accessory minerals are apatite, sphene, epidote, zircon strongly lineated granite mylonite. These are streaky, and tourmaline. At places tremolite is also seen. banded, augen gneisses or porphyroblastic gneisses and Microscopic study reveals albite-oligoclase with minor are traversed by concordant and discordant pegmatite amount of orthoclase forming more than 35% of the veins. Amphibolite intrusives with sharp contacts are minerals assemblage. Secondary albite has developed also recorded within gneisses. The most characteristic along andesine-orthoclase contact. feature of the Lingtse granite is the presence of a stretching lineation. This is defined by stretched quartz The minerals occur as subhedral and anhedral grains. and feldspar grains. Biotite flakes are aligned parallel Augen to porphyroblastic texture is usually very to the stretching direction. Similar granite gneiss is prominent with clear evidence of post crystalline exposed all along the Himalayan belt in the same deformation and recrystallization of feldspar and quartz. tectonostratigraphic level. It start from west having Feldspars occur as porphyroblasts and are surrounded Besham gneiss, Iskere gneiss, Kotla Indress, Shasher by recrystallised biotite and muscovite. Large gneiss (in Pakistan) to Rameshwar granite, Kulu-Bajura porphyroblasts of quartz having wavy extinction and gneiss, Bandal Granite, Wangtu Granitic Complex, even granulation have also been observed at places. In Naitwar, Hanuman Chatti, Bhatwari, Namik, Gwalda, general quartz shows evidences of strain. K-feldspar Chailli, Ghuttu, Chirpatiya, Rihee-Ganga, Ramgarh, shows evidences of fracturing and marginal granulation. Tawaghat, Almora-Askot-Dhramgarh gneiss (in India) The schistosity is seen to bend around feldspar grains. to Ulleri augen gneiss, Mellung augen gneiss (in Nepal) through Lingtse, Darjeeling-Sikkim, Kangpur, Ari Uri Dutt (1955) observed that the contact of phyllites Granite in Bhutan, Kalaktang and Bomdila granite gneiss and gneiss is very disturbed near 10th milestone on (in Arunachal Pradesh). Gangtok-Singhik road. This granite-gneiss appears to have been intruded through a fault plane (op. cit). In This granitoid gneiss occur in two modes; as a parts of toposheet nos. 78 A/7, 8, 11 & 12 both of these narrow belt occurring at the base of the Higher two modes of Lingtse Gneiss have been mapped by S. Himalayan Crystalline, along the trace of the Main Sinha Roy and S. K. Mukhopadhyay (1974). According Central Thrust (MCT) or as detached sheets within the to them these bodies were previously considered to be Lesser Himalayan low grade rocks. The Lingtse Gneiss intrusive granites, but from the adjacent area to the east, of the first mode in the Darjeeling-Sikkim area extends it was shown that they are allochthonous masses within discontinuously along the southern fringe of the the Daling rocks (Sinha Roy, 1969). The granite bodies hills through Lingtse Gompha (the type in the area are also thrust bound and not intrusive, as locality), to Gangtok in the eastern part of the Daling evidenced by their clear-cut contact relations and by the Dome. In western part of the Daling Dome, Lingtse occurrence of granite mylonite at the contact zone. In Gneiss is exposed in Sikkim from Barmek to Heegaon these zones the granite is highly sheeted, with well 16 GEOL. SURV. IND

developed mylonite banding. But away from the contact, of Lingtse granite-gneiss), it is necessary to carry out the degree of crushing decreases appreciably, so that the detailed petro-chemical and geo-chronological studies granitoid texture is retained. These granites are typified of individual occurrences. by the well developed mineral lineation, marked by felt of biotite. MOUNT EVEREST PELITE FORMATION Mount Everest Pelitic Formation is exposed in the In major element composition, these granites are north-western part of Sikkim only and is absent in the similar to alkaline granites (Paul et al. 1981). Chakrabarti north-eastern part where younger Tethyan sequence is (1989) on the basis of CaO-Na2O-K2O diagram of well exposed. This formation is well exposed north of Glikson (1979) classified them as granulites, with a Chorten Labsang (27056´:88014´) and continues on minor spread to trondhjemite field and adamellite field. either side; east and west, constituting an inaccessible On the basis of the above mentioned chemical and ridge separating Sikkim from Tibet. The sentinel peak normative data they opined that Lingtse granite gneiss and Chorten Nyima peak (27057´:88011´) are areas where indicates mixed S-type and I-type character. this formation is well exposed. Raina and Bhattacharya (1965) carried out geological mapping of this sequence. Two different ages have been proposed for the granite-gneiss. Paul et al. (1982) advocated age of 1075 Quartz-biotite schist, which forms the base of this ±28 Ma which Paul et. al. (1996) revised to 1678 Ma on series, overlies the lime-silicate rocks of Chungthang the basis of Rb-Sr whole rock isochron. Formation. The schist is characterized by lit-par-lit injection of granitic material. Under microscope, it is There have been many opinions regarding the nature composed of biotite, quartz, chlorite and a few grains of and origin of Lingtse Gneiss. Wager (1934) considered plagioclase. The latter are highly sericitized and show Lingtse Gneiss as an intrusive sheet similar to that found myrmekitic growth with quartz. A few crystals of highly within the Mt. Everest Pelitic Series north of Mt. Everest. sheared and fractured sillimanite are also observed along Auden (1935) considered it as granite gneiss. Ray (1935) the junction with intrusive granites. The sillimanites recorded the granitic nature of “Lingtse granite” distinct show development of biotite along the boundary. from the granodioritic character of Darjeeling Gneiss. Ghosh (1956) considered them as lit-par-lit interfolial Quartzites and phyllitic-quartzites are next in injection gneisses through concentrated igneous activity succession. They are very much deformed and fractured; along shear zones. Gansser (1964) considered them as and like the underlying schists, have been affected by diapthoretic rocks and considered them as foreign to granitisation but only along the base. These are dark Daling environment. Sinha Roy (1977, 80) opined that brown in colour and are marked by the presence of this gneiss represents wedge sliced up from the basement elongated quartz stringers, at times forming boudinage, of the Daling Group. Acharyya (1971, 1980) and along the foliation. Microscopic study reveals these to Acharyya and Ray (1977) opined that the gneiss be essentially recrystallised arkosic sandstones represents partly recrystallised Daling volcanogenic composed of quartz with few grains of feldspar and a sediments and partly metasomatic granite. Similar biotite rich matrix. Some sections show incipient growth granites from adjacent sections of western Nepal are of sericite and exhibit very low grade metamorphism. regarded as recrystallised volcano-sediments by Le Fort (1975). Sinha Roy and Sengupta (1986) suggested that Phyllites which become prominent towards top of the Lingtse granite represent a part of the Precambrian this formation are dark in colour and are composed of basement (> 1000 Ma) over which deposition of the quartz, sericite and chlorite with traces of carbonaceous younger Daling and Buxa sediments took place in the (graphitic) material and exhibit a very low grade of eastern Himalayas. metamorphism.

In view of the variable mode of occurrence of various granite-gneiss exposures at or very near to the contact MOUNT EVEREST LIMESTONE FORMATION of Dalings and Central Crystallines, and also within the Wager (1939) adopted this name for a typical Daling Group of rocks in the form of smaller bodies arenaceous limestone formation outcropping along the (which, at present, have been put under the basket term top of Mt. Everest which was subsequently traced MISC. PUB. NO. 30(XIX) 17 eastward into Sikkim. In Sikkim the formation is exposed thickness of five to six centimeters. The yellow beds, in two widely separated patches. The Limestone series representing arenaceous impurities, stand out in the north-east Sikkim is defined from Central prominently in weathered sections. Thin sections show Crystallines by the presence of a fault. In the north- the rock to be essentially a recrystallised calcite rock western part, however, the formation overlies Everest with grains of quartz and feldspar. Pelitic Formation. The limestone was mapped by Raina and Bhattacharya (1965) and subsequently by GONDWANA SUPERGROUP Roychoudhury et al. (1998). Raina and Bhattacharya (op. In Sikkim, the Gondwana Supergroup is represented cit.) have extended this name to include a thick formation by two formations - Rangit pebble slate overlain by which consists of brown argillaceous limestone, cherty Damuda Formation. and flaggy quartzite, highly contorted ferruginous sandstone and limestone and typical arenaceous Rangit Pebble Slate: limestone in ascending order. This series forms the base Rangit pebble slate is a very distinct horizon which of the Tethyan sedimentary sequence in Sikkim. occurs in the ‘Rangit Window’ at the contact of Buxa dolomite and Damuda Formation of Gondwana age. Dutt ❑ Mount Everest Limestone Formation contains (1954) felt that these beds are important on account of following litho units the association of marine brachiopod fossils in one of ❑ Cherty quartzite the outcrops which indicates the age of the beds of the ❑ Ferruginous sandstones and limestone coal measures as Upper Palaeozoic. ❑ Arenaceous limestone Argillaceous limestone is massive and The pebble slates are well exposed in the road unfossiliferous and form vertical cliffs on the western section towards Singtep, west of Naya Bazar; on the sector of the series of Chorten Nyima ridge. Though Rangit River and adjoining areas near Tatapani Temple massive, closer examination reveals a finely laminated and on the Rishi Khola, north of Rishi. In addition to nature with calcite stringers both along and across the these the localities the formation is also observed lamination. Thin sections reveal the rock to be between Tatapani Temple and Rishi as well as north of constituted mainly of calcite with clay matter. Naya Bazar. Marine fossils have been reported from Roychaudhuri et al. (1998) have recorded algal mat in Gondwana sequence near Khemgaon (Ghosh, 1952) and this limestone. The argillaceous limestone is overlain near Wak (Dutt, 1954). Similar marine fossil bearing by cherty (flaggy) quartzites which are fragmentary in sequence is suspected to occur in the diamictite bearing nature and show contortions. Individual beds show an section exposed on the Rangit River near Tatapani average thickness of 2 cm. Quartzites are overlain by Temple (De, 1982). interbedded ferruginous sandstone and limestone, the latter often showing a brecciated nature. The sandstone The pebble slate is represented by diamictite, dark is reddish in colour and show a fine granular surface. slate, dark grey claystones, bands and lenses of Under microscope they reveal the presence of calcareous sandstone. The pebbles essentially comprise quartzite, matter within the matrix. Iron ore occurs as opaque calcareous quartzite, sandstone, limestone, stromatolitic mineral. Associated limestone bands, on microscopic dolomite, slaty phyllite and granite gneiss (De, 1982). examination, were found to be calcareous marls The matrix of the rock may enclose rock fragments composed of fine grained calcite and clay matter. ranging in size from microscopic to 3 feet in diameter. The enclosed pebbles may be made of diverse rocks such The topmost formation of this series is a highly as porphyritic granite, mica-schist, quartzite, vein quartz, fractured, thinly bedded arenaceous limestone, the limestone and black slate. The irregular nature in which typical Mt. Everest limestone. It outcrops along the top these unsorted fragments occur suggests that they could of sentinel and Lhonak peaks. The dark grey limestone, have been deposited by glacial or fluvio-glacial action. characterized by the presence of thin yellow bands or The matrix is usually a black clay-slate very much like stringers, weather into a reddish-brown to buff coloured a glacial till, but it is white in colour and siliceous on rock. They are practically unfossiliferous and are seamed top of the Salebong (27°9:88°23) peak. throughout by calcite stringers. Though well bedded, individual beds show compact character and exhibit a At an altitude of 1494 m south-west of Khemgaon 18 GEOL. SURV. IND

(27°11:88°22) a conglomerate in which well rounded Gondwana rocks in Kali Khola section, about 3 km south elliptical pebbles of quartzite, mica-schist, vein quartz of in east Sikkim, just to the east of Bhasme. and granite, measuring up to 20 cm across are present, The exposure is in the form of a small window zone and contains distorted invertebrate fossils including that of is surrounded by Daling phyllite. The pebble-phyllite is spirifer. The thickness of fossiliferous horizon is about light grey in colour and contains pebbles of quartzite, 100 feet. Between Mangble (27°11´:88°21´) and phyllite, grey slate, granite and quartz vein embedded Ranguthang (27°12´:88°22´) four bands of limestone in argillaceous matrix. The pebbles are stretched and alternate with boulder slate. Some of these, at the contact aligned parallel to the foliation plane. The Gondwanas of the limestone, contain casts resembling invertebrate have been affected by F2 and F3 folding. fossils. These casts are almost totally replaced by limonite. Damuda Formation: Damuda Formation is one of the most prominent In the Rangi Khola (27°12´:88°22´) section, the slate units of the Rangit Window, and prominently exposed contains nodules and lenses of calcareous matter some on Naya Bazar-Legship road, Jorthang-Namchi road and of which appears to have been formed syngenetically. surrounding areas. The exposures of this unit are also recorded in a small window about 6 km south of Pakyong Boulders slate is found as continuous outcrops on the northern bank of (stream). Mallet extending WNW from Manpur Khola section to the (1875), Dutt (1954, 1955, 1961), Raina (1966) have Salphuk (27°09:88°21) peak. They are also made observations on Gondwanas of Rangit Window. interstratified with sandstone and carbonaceous shale in all the nala sections south of the Baisalu The formation comprises sandstone, calcareous (27°08´:88°18) – Asangthang (27°09:88°20) ridge, sandstone, shale, carbonaceous shale with thin coal beds. between Kitam (27°07:88°21) and Sorokpani The sandstone and siltstone at places exhibit cross (27°08:88°21), at Dumra (27°11:88°20), in Khemgaon stratificaton and channel structures. The sandstone area and on either side of the Pabong stream. occurring in this group are coarse grained, micaceous, ferruginous, and of either brown or blue colour. Outcrops of pebbly slate are recorded on the descent Sandstone of grey colour is found in the Rangit river 1 to Tarkhola from Sangser, about /2 km east of Tarkhola section north of Nayabazar (27°08´: 88°17´) and between and in the Rishi village opposite Rishi-Rangli confluence Kamrang (27°11:88°21) and Denchong (here this appears to be grading upwards in to (27°10:88°20). Hard, greyish-black, felspathic greywacke). Dutt (1955) found black slate, similar to sandstones and arkose grit are located on the right bank the Gondwana slate in Namchi area in western Sikkim, of Rangit, one mile north of Nayabazar. A rock on the northern slopes of the hills south-east of Yum resembling grit is found in the stream east of Burntar Chho (27°03:88°42). Besides blue calcareous (27°9:88°22), at Namchi (27°10: 88°22) and in the sandstone, he found a conglomerate with brown clayey Bhondi Khola (27°08:88°19) section (Dutt, 1954). matrix in which pebbles of chert, quartzite, limestone and slate were embedded. In appearance they resembled The sandstone is usually intercalated with black certain pebble beds of western Sikkim. fissile slate, carbonaceous slate and occasionally with thin impure coal horizons. Sandstone is far in excess of When the size of the pebbles varies between 1 mm argillaceous rocks south of the Salgari (27°08´:88°18´), and 30 mm, the rock exhibits nature of a greyish-yellow Asangthang ridge, but north of it they are subordinate splintery slate. With the increase in size of the pebbles to slate. The sandstone so imperceptibly grades into slate (5 cm to 30 cm) the rock almost looses its slaty character that it is sometimes difficult to draw a stratigraphical and gives conglomeratic appearance. Pebbles show boundary between the two. At places, such as in small elongation which is parallel to dip direction. Quartzite stream west of Bhondi Khola, the sandstone is twice is the main pebble constituent, followed by granite, repeated in a bed of slate. dolomite and shale. Acharyya (1971) published detailed account of the Rangit Pebble slate. In the Rangit section at Nayabazar, the sandstone group occurs adjacent to the magnesian limestones. Das et al. (1984) located small exposure of Sandstone is developed just above the purple and green MISC. PUB. NO. 30(XIX) 19

slates at Samdong (27°09´:88°19´). Such arenaceous Kamrang (27°11:88°21) and in the section of the beds are also found in contact with the limestone beds Rangit River. When in the contact with the near Salebong (27°09´:88°23´). carbonaceous beds the lamprophyre has produced a rock resembling coal. Under microscope the rock Under microscope the sandstone comprises of sub- shows sericitized feldspar, pale coloured biotite as angular to rounded grains of quartz, plagioclase, small flakes, carbonate and long corroded streaks of muscovite, biotite and opaque minerals. Larger grains a dark opaque mineral. Apatite, orthoclase, sphene show strain effects. Some of the feldspar and quartz and epidote are also observed. grains have completely sericitized. Muscovite and biotite show alteration to clay minerals. Finer variety of Some 300 feet above Salebong, a 30 feet thick dyke sandstone has similar mineral composition but has more of orthoclase-porphyre (orthophyre) intersects the rounded grains. dolomites. The minerals are Orthoclase (80%), biotite (7%), haematite (8%) apatite, quartz etc. make up the Frequently the sandstone has been converted into remainder (5%). quartzite, the shale into splintery slate and carbonaceous shale into carbonaceous or even graphitic schist; while the coal has lost a large proportion of its volatile matter, LACHI FORMATION so as to approach to anthracite in composition. The Sediments of the Lachi Formation are exposed in amount of metamorphism in the Damuda is by no means the north-eastern part of Sikkim only. The Tethyan constant, generally the beds are more or less altered and sediments start with a basal conglomerate. The the rocks closely resemble typical ones of the Raniganj Formation is represented by a sequence of quartzite and field. shale with some pebble beds. This rock sequence is exposed to the north of Dongkya Range, in the vicinity Small structural windows of Gondwanas also occur of Chho Lhamo Lake (28°01:88°46) and the hills north of Majhitar, in Ratu Khola, Tarkhola and at Kali consisting of these sediments is named as Lachi hill. Khola in eastern Sikkim (Dutt, 1961). Plant fossils so The sequence is exposed in the form of small rounded far have not been recorded from last three localities. hillocks with smooth topography. Lachi hill, a continuous ridge of small hillocks trending roughly north-south with a height of about 300 metres or so above the surrounding Fossils in Gondwanas: plains, forms a barrier between Gurudongmar Chhu In the geological succession of the area the coal (stream) in west and Chho Lhamo lake in the east. This seams contain plant fossils such as Glossopteris, ridge is in contact with the Central Himalayan range on Gangmopteris and Vertebraria. This assembelage is also the southern side and is cut off from the Kampa Dzong seen in carbonaceous shale and slate. Recently Pal and Series of Tibet, in the north, by the valley of the Tista Wanjarwadkar (2002) have recorded Calamites, River. Palaeopteridium, and Lesleya etc. from Damuda sandstones of Rangit Valley. On the basis of this fossil Wager (1939) first studied this formation and named finds the age of the coal bearing sandstone becomes it as Lachi Series, allotting a Permo-Carboniferous age Lower Carboniferous. on the basis of fossil evidences. This was further supported by A.M. N. Ghosh (1952 and 1956) by The sandstone bed is underlain by pebble and correlating pebble bed of Lachi Series with the boulder boulder slate in grey argillaceous matrix and Brachiopod slate bed of Rangit Valley. Auden discovered Triassic and Lamellibranch bearing beds of Wak (27°13:88°21) fossils towards the top of this formation and came to the by Dutt (1954) and Khemgaon (27°11:88°23) by Ghosh conclusion that the top of Lachi Series is Triassic in (1952). age. He renamed the top of this formation as Chho Lhamo series. G. N. Dutt (1955) examined in the Chho Lhamo- Lachi hill area and indicated presence of the Dykes in Gondwanas: Lachi sequence. Raina and Bhattacharya (1962, 1965) The sandstone-slate sequence is intersected by carried out systematic mapping of this sequence and later sills of lamprophyre at Dumra (27°11:88°20), Raychoudhury et. al. (1998) worked in this area. 20 GEOL. SURV. IND

Lachi Formation comprise the following lithological quartzite and show a subangular, subrounded or elliptical units shape. Larger boulders show a well rounded and polished surface. In addition to quartzite, pebbles of granite, shale ◆ Fossil bearing calcareous sandstone, slate and and limestone are also encountered. These generally quartzite show a flattened (bladed) or disc shape. Under ◆ Fossil bearing argillaceous limestone and microscope, the matrix is formed of clay associated with calcareous sandstone small grains of quartz and feldspar. Within this matrix, ◆ Pebble beds and conglomerate rounded and angular grains of quartz, quartzite, calcite, ◆ Crinoidal limestone, infolded shale and shale and mica-gneiss are seen. Veins of calcite cut across quartzite rocks. The base of Lachi Series consists of infolded quartzite and shale with a thin band (4 m. thick) of fossil The pebble beds are overlain by a zone of highly bearing crinoidal limestone (Raina and Bhattacharya, fossiliferous calcareous sandstone and arenaceous 1965). The quartzite is hard and cherty and stands out limestone of about 100 m thickness. In some parts the as ridges whereas the shale forms depression. Quartzite pebble beds are overlain by carbonaceous shale (inferior is light to greyish brown in colour and show a coarse coal seams) and splintery shale. The carbonaceous shale matrix. Brachiopod fossil- Syringothyris of Lydekkeri is very much puckered. The splintery shale is similar to (Dien) of Upper Permian age was found in one of the the shale forming the base of the sequence. The quartzitic sandstone boulders covering the quartzite sandstone and arenaceous limestone are reddish-brown (Raina and Bhattacharya, 1962).The quartzite shows in colour, show a sandy surface and resemble quartzites. well bedded nature while the shale is fragmentary and It weathers into irregular fragments. Raina and shows the tendency of breaking into thin rectangular Bhattacharyya (1965) recorded a large number of fossils fragments. This shale is devoid of any plant fossils. Shale from this calcareous sandstone and arenaceous limestone is found to be harder in nature and is almost slaty in of Lachi Formation. These are Productus character. Thin dark shale bands which appear to be (Linoproductus) cora d’Orb., Productus cf. carbonaceous are also seen to be interbedded with the semireticulatus, Productus semireticularus Martin, shale. The limestone band occurs in the upper parts of Productus (Dictyoclostus) cf . spiralia Waagen, the shales. It is dark crinoidal limestone showing pitted Productus (Dictyoclostus) cf. gratiosus, Productus surface and fragmentary nature which has been caused (Linoproductus) cf. kulikid Freedericks, Productus by the intersection of two prominent joint systems. Under (Waagenoconcha) purdoni (Davidson), Productus (Sp. microscope; it proved to be a bryozoan limestone Unidentifiable), . Spirifer (Spiriferella) rajah Salter. consisting of bryozoan fragments in calcareous matrix Spiriferina (Spriferellina) zewanensis Diener, Chonetes (Raina and Bhattacharya, 1962). Fossils collected from Wageri Muir Wood, Chonetes (Sp. Unidentifiable), this zone could not be properly identified as they are Marginifera himalyensis Diener, Marginifera sp, not properly preserved. Derbyia cf. dorsoplna, Protocetapora ampla Lonsdale, Ingelarella pelicanensis. The fossil assemblage indicates The pebble bed forms the most prominent zone of a definite Permian age (Raina and Bhattacharya, 1965). the Lachi ridge. Its outcrop width is about one kilometer Some of the fossils like Marginifera Himalayensis, though it is only 200 m thick. Auden (1935) has Diener Spirifer, (Spiriferella) rajah Salter are the correlated them with the Talchir boulder bed. Angular characteristic fossils of Permian horizons in other parts grains (pebbles?) of quartz, shale and sandstone are of Himalayas. found in siliceous matrix. In general it looks like a pebble bearing slaty grey coloured siltstone. Actually it shows This highly fossiliferous horizon is overlain by 120 a regular variation in grain size of the matrix as well as m thick horizon of quartzite and slate. Towards top, fossil that of the pebbles from bottom to top. At the bottom bearing calcareous sandstone bands are seen infolded the matrix is silty with pebble size between one to two with the slate. The quartzite which shows a yellowish millimeters. This is followed by coarse sandstone with colour often reveals a brecciated character. Associated pebbles ranging in size from 5 to 10 mm. It becomes a slate beds, characterized by a cleavage, show a highly boulder bed formation with boulders ranging in size from jointed nature. Under microscope the slate is entirely 30 to 100 cm towards top. The pebbles are mostly of formed of clay matter with few arenaceous patches. MISC. PUB. NO. 30(XIX) 21

Associated fossil bearing calcareous sandstone, showing diversified brachiapods of Lachi Formation match with a weathered pitted surface, do not outcrop so commonly. Zewan Formation (Mid-Up Permian) of Kashmir which These are encountered in Chhothina (28°04:88°42) contains numerous colonies of sessile benthonics of section only and are missing in Lachi ridge. Among the similar characters. Thus, Lachi Formation is fossils Productus (Linoproductus) cora d’Orb, Spirifer provisionally dated Upper Devonian to Upper trigonalis Mart, Spirifer Lydekkeri Diener, and Spirifer Carboniferous (Middle Permian). curzoni, Chonetes Wageri Muie wood, Pleurotomaria (Trepospira) cf. Chitralensis Reed, Pleurotomaria sp.,Lima sp. are most prominent ones. Fossil CHHO LHAMO (TSO LHAMO) FORMATION identification indicates an upper Permian age for this This sequence of rocks is exposed only in the form horizon. of a small patch in north-east Sikkim. The name “Chho Lhamo” was given by Auden (1935) to an ammonite The topmost formation of this series is a thick bearing horizon of limestone, sandstone and shale on quartzite which may represent a different zone altogether. the eastern side of the Lachi ridge. This was subsequently The quartzite is exposed with a width of 1000 m or so. examined by Raina and Bhattacharya (1962, 1965), and They are light grey in colour and are associated with Roychoudhury et al. (1998). cherty white bands. Towards the contact with the underlying formation, they show brecciated nature. A This Formation is represented by sandstone at the thin section of white band revealed recrystallised quartz base, overlain by limestone and shale. They show a rock metamorphosed at very low temperature. The faulted contact with the rocks of underlying Lachi stratigraphic position of this quartzite band is rather Formation.The entire Formation is tectonically disturbed doubtful. Although at present it is placed within Lachis, resulting in a faulted contact between the limestone and it could, however, represent the base of Triassics which overlying shale. On the eastern bank of Chho Lhamo, is missing in this area. limestone is seen overriding the sandstone and shale of Lachi Series. Roychoudhury et al (1998) have recorded sprifer, Neophricadothyris, Lissella, Daviecella, Echinaria, Basal sandstone is hard and slightly coarse in texture Muirwoodia, Squamaria, Reticulatia, Tschernyschewia, and grey to light brown in colour. The sandstone contains Spiriferellina, Syringothyris (s.l.), Stenoscisma, well rounded nodules of hardened clay, a few of them Notospirifer, Eospiriferina, Probolionia, Sinuatella, having a limestone core. Towards its junction with the Productus, Taeniothaerus and Planolites. Fossils were overlying limestone this sandstone become calcareous collected from different levels at Dorjila hill section and and exhibits a flaggy character. They are quite rich in have been grouped into four assemblages. Lower one invertebrate fossils especially brachiopods and corals. occurs close to the contact of Everest Limestone A few ammonite fossils were also collected. This horizon Formation and is identified by Eospiriferina - Daviecella- was found to be rich in Rhynchonella Thinodosi showing Reticulatia assemblage. Middle of the succession is Muschelkalk (middle Triassic) affinity. The fossils characterized by Syringothyris-Spirifer assemblage. This identified are as follows: Pseudosageceras. Sp, assemblage is correlated with Syringothyris Limestone Spiriferina sp, Rhynchonella cf. trinodosi. Bilt, fossil assemblage of northwestern Tethyan Himalaya Rhynchonella sp, Orthoceras sp., Chonotes sp.(Raina where Lower Carboniferous age has been assigned to and Bhattacharyya 1963, 1965) this assemblage (Middlemiss 1910). Next assemblage is the Productus-Sinuatella assemblage. This assemblage Black coral bearing limestone, which overly the is correlated with those from Po Series (Lr. sandstone shows a weathered wrinkled surface due to Carboniferous) of Spiti and with lower part of the presence of corals. The rocks are black and well Fennestella Shale (Up Carboniferous) of Kashmir. bedded and jointed. They also reveal Muschelkalk age Towards the top, most richly fossiliferous association is from their fossil assemblage among which Meekoceras represented by Muirwoodia-Athyris-Spririferella is the most prominent. Towards top they alternate with assemblage having close faunal similarity with shale. brachiopods of Nihal, Nabi and Girthigal Formations of Kumaon-Garwal Tethyan sequence. Such a rich and The shale is from the top of Triassic exposures. This 22 GEOL. SURV. IND

horizon is formed of shale and quartzite with interbedded in the age of igneous rocks and occurs practically thin bands of limestone. The shale is very rich in everywhere in the crystallines examined”. Hayden ammonite fauna and reveals a Muschelkalk age. (1907) regarded unfoliated granites of Chumbi Valley Cephalopods form the major proportion of the fossils as equivalent to the schorl granite of Everest. Since the collected from this horizon, though brachiopods and Chumbi granite has intruded Jurassic beds, both it and lamellibranches are also present. The fossils identified the related granite of Sikkim and Everest were presumed in this horizon are as follows: Ceratites trinodosus Mojs, to be of Tertiary age. Ceratites dimorphus, Ceratites sp., Meekocers cf. nalikanta Diener, Meekoceras khanikofi Oppel, Meekoceras kesva Diener, Proptychites plicatus Waagen, Biotite granite Pleurophors cf. curonii Hau Lam , Dielasma cf. It has been observed in the north-eastern part of himalayana Bittner, Athyris sp., Euomphalus sp., Sikkim Raina and Bhattacharya (1965) intruding into Pleorotomaria sp., Lima sp. ind. Aff. Subpunctatae D foliated biotite granite and gneisses in the form of lit- ‘Orb., Gervillia cf. praecursob Quen, Meekoceras cf. par- lit injections, as thin veins along the foliation plane affine E.V. Mojs, Meekoceras cf. Rhanikofi opal, of country rock and also as small bosses. The bosses Ptychites Sp., Ceratites ef. Brinodocus Mojs, Ptychites contain xenoliths of older gneisses. It is also observed cf. Sumitrs Diener, Ceratires sp. (op. cit.) to have intruded into the basal rocks of the Everest Pelitic Series as lit-par-lit injections. Sinha Roy and Ray (1972), Roychoudhury et. al. (1998) reported that 740 m while working in parts of toposheet 77 D/12 and 78 A/ thick Chho Lahmo Formation has yielded nine 9, observed following modes of occurrence: fossiliferous horizons where cephalopod is represented by Genus Salterites Diener 1907 and Tibetites (i) Small pods at the core of small scale folds and at Mojsisovics 1896. Salterites belongs to similar affinity the construction zone of boudins in banded gneiss. with Otoceras zone (Lower Triassic) of western Tethys (ii) Along strain-slip cleavages within banded gneiss. Himalaya and Tibetites being marker genera of Upper (iii) As thin veins cross-cutting the gneissosity in which Triassic (Lower Noric) beds of Himalaya, is correctable case a relic foliation is preserved within the granite with ammonoid assemblage of Holorites beds of through the orientation of recrystallised biotite. Kumaon Himalaya(op. cit.). (iv) As concordant bodies within biotite-rich banded gneiss where there is selective crystallization of garnet in the host rock around the biotite granite. INTRUSIVE GRANITES Throughout the Himalaya, two main variants of late It is a medium grained rock with quartz, feldspar intrusive granites have been documented, viz. a) Two and biotite as the main mineral constituents. This mica granites and b) tourmaline granite. This is true of biotite granite, like tourmaline granite, is exposed Darjeeling Sikkim also. Intrusive granites are generally from Chungthang to Jakthang on Tista and Zemu recorded in Central Crystalline Gneisses. These occur valley and from Phentong to Phyaggu on the Tolung as sills and discordant intrusives. Apart from these there Valley. It is best developed between Menshithang are local products of partial melting such as aplites and (27°40: 88°3530) and Lachen (27°44:88°33) in pegmatites. the Tista Valley. South of Menshithang, a zone of augen gneisses is seen which has been formed by These intrusive granites such as biotite granite, the massive intrusion of biotite granite. A thin section tourmaline granite and pegmatite veins had been reported from Lungme (27°52:88°21) stream shows quartz, in north and west districts of Sikkim by Bhattacharya biotite, muscovite, minor chlorite, zoned plagioclase and Pattanaik (1964), Raina (1966), Raina and and microcline. Plagioclase is earlier than Bhattacharya (1965), Sinha Roy and Ray (1972), and microcline. Another thin section of the rock collected Neogi (2004). from Khora Phu (27°57:88°18) showed presence of oligoclase and a few almandine garnets. The biotite Heron (1922) was the first to notice presence of these crystals vary from small flakes to big books. The tourmaline granites (termed as schorl granite) in the books are generally seen near the contact with the Everest Region. He states “the schorl granite is the latest country rock. MISC. PUB. NO. 30(XIX) 23

Tourmaline granite: Pegmatite and Aplitic granite:

The part of North Sikkim, especially from In north district, Sikkim the pegmatites, except for Chungthang to in Tista Valley to Jakthang (27°4730: the coarse nature of its minerals and its occurrence as 88°29) in the Zemu Valley and from Phentong veins and stringers, it shows the same mineral (27°3446:88°25) to Phyaggu (27°42:88°2630) in assemblage as the biotite granite. In field they are the Tolung Valley has been criss-crossed by numerous observed intruding along well developed joint planes, granitic masses and aplitic veins. Granite and pegmatites crack and fissures. veins of varying thickness of 1 cm to nearly one km have been met with (Bhattacharya and Pattanaik, 1964). In West District, Sikkim the pegmatites intrude into They are seen mainly within the area between gneisses and schists along well developed joints and Chungthang (27°36:88°38) and Lachen (27°44: fissures. They do not appear to be intruding in to the 88°33). At some places, these intrusions have taken foliated granites. place along the fault and joint plane. It occurs as small lenticular bodies (3 to 6 m thick) disposed athwart the Thin section shows zoned coarse grains of gneissosity. When they are located near the mesoscopic plagioclase and oligoclase and inclusions of quartz and folds in banded gneiss, they invariably occur along the biotite. Garnet and apatite are accessories with axial plane. Near the contact with the wall rock, occasional tourmaline. tourmaline is aligned parallel to the contact plane. But towards the centre of the body, this alignment is lost Aplitic granite is a fine grained, homogenous mass and tourmaline shows crude dimensional orientation at of grey and black clots in which biotite grains are easily right angles to the contact. The granite varies from fine discernable, and occurs as thin bands varying in grained homogenous granite to a coarse pegmatite in thickness from 1 cm to 10 cm. These intrude the country texture. They vary in colour and texture. Colour varies rock along N75W-S75E joint planes. Under microscope from milky white to light brown to light grey. These it is composed of fine grains of quartz, feldspar and granites have been found to be very rarely affected by biotite with flakes of muscovite and chlorite. Feldspar faulting and folding of the Himalayan orogeny. These is oligoclase. In one section garnet and epidote are seen. granites are medium grained with different sizes of They intrude into earlier granite. prismatic crystals of tourmaline in a ground mass of plagioclase and quartz. Under microscope, it reveals a SYENITIC ROCKS coarse grained nature with tourmaline, quartz, biotite and feldspar as main constituents. The Ultrapotassic syenitic rocks of Sikkim occur as intrusive within the “Dalings” of Sikkim Himalaya. In Tista valley, very near to Jorepul (27°42:88°34), Syenitic rocks are reported from many localities. Verma a tourmaline bearing granite containing Topaz has been et al, (1983) presented an account of these rocks. Ghosh located. Auden (1935) has reported the occurrence of Roy (1996A, 1996B, 1997) studied these rocks in great topaz bearing granite from a place on the Tista Valley details. Small bouldery outcrops of syenitic rocks occur which is nearly 48 km north of this occurrence. The topaz at many localities within the Daling Formation in Sikkim crystals are 1.5 cm to 2 cm in length; flesh coloured and Himalayas near Legship, Ben, Damthang, Rishi, and irregularly arranged. The zone is 30 cm thick and nearly Maniram. Although their intrusive nature can be 8 metres long. recognized at places, their disposition is not clear due to paucity of outcrops and small dimension of the bodies. Near Jorepul, a little south of the locality referred to the above section, a massive well jointed, unfoliated They are of two varieties. One is coarse grained zone of tourmaline granite is seen. The tourmaline occurs mesocratic rocks having dark greenish grey color. Coarse as clusters of radiating fine needles occupying an area feldspars and fibrous blue riebeckite are often discernible which is rectangular in shape. Auden (1935) has also in hand specimen. Anastomising veins of riebeckite of mentioned the presence of clustered tourmaline granite, variable width are also common. The rock is composed well jointed and massive in nature, on the Sibu Chu of orthoclase, biotite, aegirine and riebeckite. Orthoclase Valley in northeast Sikkim. is mostly subhedral. Anorthoclase is a minor phase. The 24 GEOL. SURV. IND biotite has a deep reddish brown colour, and distinct to the lamprophvre clan and are likely to be soda minettes colour zoning with dark reddish brown cores and lighter rather than ‘syenitic’ composition. coloured rims. The aegirine grains are often bleached. Riebeckite is distinctly a late phase and replaces mica The two groups of syenitic rocks have been found and feldspars. Brown katophoritic amphibole also occurs to occur in close proximity. Close to them, neither typical as minor phase in the syenitic rocks. Sphene is a common minettes nor leucite or olivine lamproites have been accessory mineral in these rocks. This variety of syenitic found. However, a coarse grained rock comprising fresh rocks shows occasional porphyritic and crude olivine, diopsidic augite, phlogopite, richterite as major panidiomorphic texture. The rocks have very high K2O phases have been found near Rishi. This rock shows (4.87 - 10.31%) compared to Na2O (1.76 - 2.49%) and distinct evidences of ‘modal metasomatism’. The they also contain high TiO2 (2.27 - 4.47%) which implies invading veins had given rise to secondary phlogopite their ultrapotassic nature. Presence of acmite and and apatite and the veins also contain minor amount of potassium metasilicate in norms of some samples and carbonate. The rock having higher 100 Mg/ (Mg + FeT) presence of modal aegirine and riebeckite indicate their is fairly primitive and it appears that the original rock peralkaline nature. Trace element data for these rocks has been considerably modified by the metasomatising reveal that they contain high amount of Rb (40 - 160 fluids. This rock is likely to be a richterite-peridotide of ppm), Ba (300 - 3900 ppm) and Sr. (210 - 3600 ppm). deep seated origin and the textural features observed are comparable with the features considered to represent The other variety is comparatively finer grained ‘mantle metasomatism’. It is probable that the original having dark ash-grey colour. These are generally rock was a mantle rock of lherzolite or garnet lherzolite massive. This finer-grained variety is composed of composition which has been substantially modified by orthoclase and katophoritic amphibole as essential metasomatism. In that case, apart from olivine, diopside phases with minor amounts of riebeckite and phlogopitic and some phlogopite, richterite, phlogopite and apatite mica but does not contain aegirine. These rocks exhibit have possibly resulted from the invading metasomatic panidiomorphic texture. They contain high SiO2, Al2O3 fluids. The metasomatising fluid was partially rich in and total alkali and less TiO2, MgO, CaO and total iron CO2 as is evidenced by presence of carbonates in the and also higher Zr, La and Y, lower Ba and Sr compared veins. For the syenitic rocks, this rock may represent to the former group. the source material from which the magma for the ultrapotassic syenitic rocks had formed by partial melting Although the rocks are apparently of syenitic of the source material (Ghosh Roy, 1997). composition, these are in many respects dissimilar to typical alkali syenites. Absence of microcline/microcline perthite, occurrence of colour zoned phlogopitic mica SESELA FORMATION and dominance of aegirine-riebeckite and katophoritic Rocks of Sesela Formation are exposed in the north amphibole exhibiting panidiomorphic texture with eastern corner of Sikkim state bordering Tibet. They are evidences of autometasomatism and typically exposed along Bomchho area in the north to 5554 m ultrapotassic chemical character indicates that these peak in southeast through 5456 m peak and Sesela peak. rocks are more akin to lamprophyres rather than typical They are best developed along Kareng-Sesela axis and alkali syenites. represented by 200m thick sediments and are named as Sesela Formation (Roychoudhury et. al. 1998). Sodic-calcic (katophorite/anophorite) and alkali amphiboles (arfvedesonite / riebeckite) are reported from Sesela Formation comprises well rounded, disc and lamprophyres and primary alkali amphiboles are mostly ball shaped pebbles (few millimeters to 10 cm diameter) reported from minettes but may occur as a secondary of shale, sandstone, limestone, argillite, quartzite, phase in all calc-alkaline lamprophyres (minettes, granites and tilloids embedded in a sandy matrix. Rocks kersantite, vogesite, spessartite etc) (Rock, 1991). are crudely banded, unconsolidated to semi-consolidated Besides, Ca-Na pyroxenes (sodian augite and aegirine in nature. augite) have been reported from all lamprophyres (Rock, op cit). Thus, the mineralogy coupled with chemical Alternation of variegated clay, fine and medium sand characters of these syenitic rocks suggests them to belong and pebble beds indicate bedding which is mostly MISC. PUB. NO. 30(XIX) 25 horizontal or very low dip (2-5°). The more accessible Himalayan regions are generally subdivided into the Kerang-Sesela section has been studied in detail where following morpho-tectonic units from north to south. conglomerate, grey, yellow, brown sand, sandy clay, pink, black-brown clay and pebbles layers have been Karakoram/Changtang Belt found in irregular alternations. —Shyok (SS)/Bangong-Nujian (BNS) Suture Zone — Kohistan/Ladakh/Lhasa Terrane —Indus Tsangpo Suture Zone (ITS)— Most of the pebbles of para-conglomerate and Tethyan Zone conglomerate are well polished of different size and South Tibetian Detachment System (STDS) shape and are made-up of shale, quartzite, limestone, Higher Himalayan Zone —Main Central Thrust (MCT)— sandstone and granite of older horizons. Sediments were Lesser Himalayan Zone apparently laid down in shallow to moderately deep —Main Boundary Thrust (MBT)— depressions or lakes and transported and rearranged by Sub-Himalayan (Foothill) Zone —Main Frontal Thrust (MFT)— streams. No fossil has been recorded from these rocks. Indus-Ganga-Brahmaputra foreland basins Sesela Formation is provisionally correlated on lithological criteria with Upper Karewa Formation of These morpho-tectonic units also represent Kashmir, Potwar Silts of Potwar plateau and similar distinctive tectono-stratigraphic terranes, bounded by deposits of Trans-Himalaya, and assigned pliostocene major thrusts or sutures and constitute the main tectonic age. framework of the Himalaya and adjacent regions. The Himalayan region, sensu stricto, comprising Higher, Sesela Formation, presumably of Pliestocene age Lesser and Sub Himalaya (Foothill) is bounded by the deposited in shallow to moderately deep depression or Indus-Tsangpo Suture (ITS) in the north and the Main lake, is of fluvio-glacial nature, but well polished, Frontal Thrust (MFT) in the south. A substantial part of pebbles of various size and shape are result of long term the Himalayan region constitutes recently elevated rolling, dumping and transportation. Restricted outcrop Indian platform (Wadia, 1966) and the main Himalayan of this Formation suggests that present sectarian Chho region upto the Indus-Tsangpo Suture is represented by Lahmo Lake is a closed remnant of large sized water the reactivated northern part of the Indian shield mass, thought to be of tectonic origin or obstructed (Gansser, 1980 and1981). drainage channel by glacial moraines. The State of Sikkim encompasses parts of Lesser STRUCTURE AND TECTONICS Himalayas, Higher Himalayas and Tethyian Zone. Main boundary thrust (MBT) just passes through the area The present tectonic framework of the Himalaya and slightly south of -Sikkim boundary. Main adjacent orogenic belts is the result of tectonic collage Frontal Thrust (MFT) between Siwaliks and Neogene of autochthonous to allochthonous linear crustal blocks, sediments is located further south. Main Central Thrust geologically unrelated to each other and sutured together (MCT) passes through the Central part of the State. The during different periods of its evolutionary history. Such normal fault (STDS) between Central Crystallines and unrelated crustal blocks of regional extent having their Tethyan Sequence passes through the northern part of distinctive tectono-stratigraphic characters, which are the State. There is one tectonic window named as Rangit generally fault bounded, are being termed “terranes” window which exposes subthrust rocks of the Lesser following Howell (1985). The evolution of the accreted Himalayan Package. allochthonous terranes of different ages, forming the Himalaya and adjacent orogenic belts, by fragmentation Tethyan zone lying between the Indus-Tsangpo and reassembly of the north-eastern margin of the greater suture in the north and Higher Himalayan Gondwana super continent has been advocated by crystallines(HHC) in the south is represented by nearly Molnar and Tapponier, 1975; 1977; Bassoulet et al., complete stratigraphic sequences of the northern 1980; Nur and Ban-Avraham, 1982; Allegre et al, 1984; continental margin of India over the Late Valdiya, 1988; Thakur, 1996; Acharyya, 1996; Ghose, Neoproterozoic-Eocene interval with some breaks in the 2000a, 2000b and many others in recent years. form of unconformitics / diastems.

The Himalaya and adjacent parts of the Trans- The base of this Tethyan sedimentary sequence has 26 GEOL. SURV. IND long been regarded as a pronounced unconformity been correlated with the penetrative foliation S2, axial overlying the high grade metamorphic rocks of the planar to a set of, locally preserved, tight isoclinal folds, Higher Himalayan sequence (Gansser, 1964: Stocklin. F2 formed during the D2 deformation. A relict early

1980). A major fault system has, however, been recorded fabric, S1, related to the earliest deformation D1, is at many places along the southern boundary of the occasionally recorded in the pelitic rocks, defining micro Tethyan zone in the last two decades (Burg et al., 1984; folds within the S2-related layering, or as internal trails

Burchfiel et al., 1992; Searle, 1986; and Valdiya, 1987). within syn-S2 garnet prophyroblasts. The pelitic units Valdiya (1987) delineated Trans-Himadri Thrust also show a D2 mylonitic microfabric, SD2m, stretching for 1600 km between Ladakh and Sikkim that characterised by narrow bands of fine grained separates the Higher Himalayan Crystalline Zone from recrystallised quartz-feldspar aggregates which the Tethyan sedimentary zone. The existence of such a anastomose around porphyroclasts of feldspars and fault is however, not seen at all the places and the Late quartz. The SD2m fabric is generally conformable with Neoproterozoic sequence commences unconformably S2 and often mutually inseparable. The most common over the crystalline with diamictite and conglomerate at folds (F3) in the Higher Himalayan pelites, however, are the base. those developed on the S2 folia during a later deformation D3. A set of weakly developed crenulation cleavage, S3, However, in Sikkim the presence of Fault is reported have locally developed parallel to axial planes of these by Raina and Bhattacharyya (1965) and Roychoudhury meso and macro scale folds and are marked by incipient et. al. (1998). It has been observed that all along the growth of biotite and chlotite. The high grade HHC is contact of Tethyan Sedimentary Sequence with considered here as a dominantly gneissic thrust sheet underlying Central Crystalline Gneissic Complex, overlying the metasedimentary mica schist-quartzite of bedding trend of Tethyan are abuted against the foliations the Lesser Himalayan stack along the Main Central of rocks of CCGC. There is tectonic pinching and Thrust (Neogi et.al.1996). swelling, tear apart and scattered occurrence of Tethyan sequence all along the north eastern part of Sikkim. Slices of Lachi Formation within Chholahmo Formation Main Central Thrust: are additional references to such disturbances Two distinct tectono-stratigraphic terranes (Roychoudhury et. al. (1998). comprising Higher and Lesser Himalayan zones, corresponding to two heterogeneous morphotectonic The Tethyan Sedimentary sequence is in general domains, constitute the main Himalayan region and are unmetamorphosed and most of the rocks of Tethyan separated by a major intracrustal thrust zone termed as Sedimentary Sequence shows prominent bedding and the Main Central Thrust (MCT). The thrust contact is defined by colour banding, lithological variation and between the Central Crystallines and equivalent rocks grain size variation. Among Tethyan successions, of the Higher Himalaya in the north and weakly lineations and at places kinks are present in Everest metamorphosed to unmetamorphosed volcano- Limestone Formation. sedimentary sequences in the south in Garhwal-Kumaon area was termed as the Main Central Thrust (MCT) by The Higher Himalayan zone, defined by the MCT Heim and Gansser (1939) to designate the surface along in the south and arbitrarily demarcated in the north by which the crystalline rocks of the Great (Higher) the appearance of marine Tethyan sequence, represents Himalaya had moved southward over the Lesser the metamorphic core of the Himalaya. A continuous Himalayan sedimentary sequences.The MCT represents belt of high grade metasedimentary and meta-igneous a major structural zone extending for at least 2500 km rocks and associated granitoids commonly known as the along strike all along the Himalaya from Jammu and Central Crystallines are exposed all along the Higher Kashmir in the west to Arunachal Pradesh in the east Himalayan zone. and is one of the most distinctive structural features of the Himalayan range. Recently Stephenson et al (2000, Higher Himalayan rocks preserve imprints of poly- 2001) defined the MCT as a crustal scale high strain phase deformation and related fabric elements (Naha ductile shear zone, between 1.5 and 3 km. wide, and Roy, 1970; Sinha Roy, 1977). In the micro-scale, commonly coincident with an inverted metamorphic the dominant structural surface in the pelitic rocks has field gradient from biotite to kyanite grade, which places MISC. PUB. NO. 30(XIX) 27

the metamorphic rocks of the High Himalayan zone over folds (F2) on all scales with pervasive schistosity and the unmetamorphosed to weakly metamorphosed rocks occasional crenulation cleavage. These minor F2 of the Lesser Himalaya. crenulations and folds are asymmetrical (sinistral) and close to tight in nature. The marker units in Dalings viz., In Sikkim-Darjeeling area, Central Crystallines are graywacke, psammite etc. define such folds on represented by Darjeeling Gneiss, Kanchenjunga Gneiss mesoscopic scale. In the late stages of second and Chungthang Gneiss. According to Sinha Roy and deformation, mylonitic zones developed almost parallel

Bhargava (1989), the greenschist facies assemblages to the axial planes of second generation folds (F2). The represented by the Lesser Himalayan Daling sequence, third deformation (D3) is manifested in conjugate folds, is delimited in the north by a dislocation zone at the kinks and monoclines and on regional scale in base of the tectonised slivers of the Lingtse Gneiss and contemporaneous cross folds (E-W, N-S) at right angle

he termed this thrust as MCT-1. The rocks on the north (F3, F4) and that may form the Sikkim domal structure of MCT-1, represented by the Darjeeling Gneiss, are (Sinha Roy, 1977; Acharyya and Ray, 1977, Acharyya metamorphosed to amphibolite facies. The tectonic and Sastry, 1979). The structures of both the second and contact of the Dalings with the Gondwanas in the third deformations might have been generated by southern front of the Lesser Himalaya has been termed independent N – S directed principal stress. The planar as MCT-II. structures related to first and second deformation, thrust planes and regional metamorphic isograds were folded The Lesser Himalayan zone, bounded by the Main on steep to upright axial planes with axes plunging east- Boundary Thrust (MBT) in the south and the Main west or north to form in contemporaneous cross folds. Central Thrust (MCT) in the north, are represented by the areno-argillaceous formations with inter layered In eastern Himalaya the younger Phanerozoic belt volcanics of Late Palaeoproterozoic age at the base is represented as a linear belt of Permo-Carboniferous followed by carbonate-orthoquartzite sequence of sequence equivalent to Gondowana all along the frontal Mesoproterozoic age. However the sequence in not zone of the Himalaya just adjacent to the MBT from totally represented in Sikkim Himalaya barring the central Nepal to Arunachal Pradesh. In Sikkim Carbonate – ortho-quartzite sequence of Darjeeling, these are represented by the Rangit pebble Mesoproterozoic age which in Sikkim Himalaya is slate and Damuda Formation. In Sikkim though Damuda represented by Buxa Formation exposed only in the Formation is exposed only in Rangit Window, the Pebble Rangit window. The Gorubathan Formation, areno- slate beds are exposed in the surroundings of the window argillaceous lithofacies of the Daling Group comprise a within Dalings as slices. thick assemblage of green slate, epidiorite and chloritic greywacke and carbon phyllites (Acharyya and Sastry, From the map pattern of the Sikkim Darjeeling 1979). Himalaya it has been observed that in the western as well as in the eastern part MBT and MCT run parallel The different structural elements and tectonics of to one another defining a narrow E-W trending zone, Daling rocks have been stated by different workers where as in the central part the MCT recedes towards (Sinha Roy, 1974; Sinha Roy, 1977; Bhattacharya, 1989; north defining deep recesses along the N - S flowing Acharyya, 1989). At least, three main phases of Tista. Strike of the dominant foliation of rocks in the

deformation (D1, D2, D3) may be recognized. The first hanging wall as well as footwall of the MCT in the recess deformation (D1) produced schistosity, mostly parallel zone runs parallel to the thrust trace there by indicating to the bedding, folds (F1) and linear structures. Minor northerly plunging antiformal folding of the MCT in F1 folds, rarely recognized within the thin intercalated the recesses zone. In other words, the MCT has laterally quartzite bands, are overturning to recumbent and tight arched up in the shape of isolated northerly plunging to isoclinals folds. The main penetrative and pervasive antiform. Such lateral arches with axes trending at high

foliation (S1) is the axial planar foliation of these folds. angles to the mountain front can be termed as Microscopically the S1 foliation is defined by parallel Culmination (McClay, 1992). This culmination zone alignment of chlorite, sericite, muscovite and lenticular exposes a vast tract of Lesser Himalayan sequences quartz and opaque grains. The second deformation (D2) together with the presence of a window where younger is responsible for the generation of recumbent / reclined rocks of a lower level thrust sheet are exposed. Within 28 GEOL. SURV. IND

Teesta Culmination Zone in the Darjeeling-Sikkim structure. Moreover, a complementary synform should Himalayas, the Rangit window exposes Proterozoic form in front of the frontal antiform. Such a Buxa Formation and Mesozoic Gondwanas within the complementary frontal synform is also very prominent Proterozoic Lesser Himalayan Daling Group of rocks. in the Teesta culmination zone. The resultant map pattern of the culmination zones is a mushroom-shaped Upper level thrust surfaces along with the thrust pattern, which is seen in the Sikkim-Darjeeling sheets on their hanging walls arch upwards because of Himalaya. insertion of imbricate thrust wedges at the lower levels (Elliot and Johnson, 1980, Medewedeff, 1992). These wedges may be arranged (Boyer and Elliot, 1982) as METAMORPHISM imbricate fan, duplex or imbricate antiformal stack. But The metapelitic rocks of the Sikkim Lesser such imbricate wedges are parallel to the thrust transport Himalayas show an inverted metamorphic sequence direction and the axes of the resulting roof arch trend (IMS) of the complete Barrovian zones from chlorite to parallel to the orogenic front (frontal antiforms, sillimanite + K-feldspar, with the higher grade rocks Schirmer, 1988). The culmination antiforms, on the other appearing at progressively higher structural levels. Ray hand, are lateral arches trending at high angle (almost at 1947 first studied in detail this inverted metamorphism. right angles) to the orogenic front. Hence their However Neogi et.al.1998, Ganguly 2000, Neogi 2004 development can not be explained by insertion of thrust carried out detail studies on metamorphism of Sikkim wedges. Himalaya.

Ray (2000) explained the development of Metamorphic Character of Daling and Central culmination in Sikkim Darjeeling Himalaya. According crystallines to him the sole thrust shifts to a lower level to bypass the zone of high frictional resistance forming a Chlorite biotite zones: rejoining splay of the sole thrust. By this process a part of the autochthonous foreland below the sole In these zones, millimetre to centimetre scale thrust, usually a slice of the crystalline basement, gets domains of phyllosilicate-rich (muscovite + chlorite) and accreted to the allochthon and moves forward as a horse quartzose bands define a compositional layering (S0). and finally occur as a far-traveled horse. A large part Muscovite in the chlorite zone is rich in paragonite (d”12 of the autochthonous foreland below the sole thrust mol %) and celadonite (d”16 mol %). The latter was may get tectonically mobilized and accreted to the found to decrease in the biotite zone. In a rare outcrop overlying allochthonous zone. Because of insertion of within the biotite zone, idiomorphic (50–75 lm) Mn rich this additional material within the allochthonous zone, garnet porphyroblasts were observed, which show strong the roof sequence of the stack of allochthons arches growth zoning. Since Mn fractionates strongly into up in the form of antiforms. Arches with two separate garnet, the stability limits of garnet are expanded in Mn- orientations develop simultaneously. Because of listric rich bulk compositions. These rocks contain significant curvature of the rejoining splay, the horse (or the bundle amount of plagioclase, which shows compositional of horses) cut from the foreland, has a spindle-like zoning from around Ab75An25 at the core to around shape in transverse section (Ray, 2000), and as a Ab84An16 at the rim. consequence, the roof sequence arches up on an axis perpendicular to the movement direction (a frontal Garnet zone: antiformal arch). Moreover, because of concave-up- ward lateral curvature of the rejoining splay, another This zone starts with appearance of small garnet arch with axis parallel to the movement direction crystals on foliation and bedding planes of schists and (lateral antiformal arch) forms. Interference of these quartzites. Schistosity becomes prominent and garnet two sets of antiformal arches with their respective axes crystals become bigger further away from the margins at high angles to one another, would build up a domal of the zone. The rocks are quartz – garnet-biotite schists structure which is characteristic of the culminations and phyllite, quartz-sericite-garnet schist and garnet formed because of insertion of basement horse(s). The bearing quartzite. The minerals are quartz, garnet Teesta culmination zone has this characteristic domal (almandine), biotite, sericite and vermiculite. MISC. PUB. NO. 30(XIX) 29

Both garnet and chloritoid are present in some thin Kyanite zone: sections of aluminous pelites, but are segregated in 1–5 mm thick layers, with the garnet having a relatively Mn- Two groups of metapelites were recognized in this rich composition and a well-preserved growth zoning. zone. One group has high-modal abundance of kyanite The chloritoid bearing layers are commonly rich in (>25% by volume) with very little garnet, while the other muscovite and have minor chlorite. The formation of is characterized by high-modal garnet and very few chloritoid and garnet in Mn-poor and Mn-rich bands, kyanite grains. The former possibly represents aluminous respectively, is due to the effect of Mn in expanding the and the latter sub-aluminous pelites. Staurolite is present field of stability of garnet. only as rare inclusions in outer massive portions of garnet. The rock is locally migmatitic with quartz plus There is a marked decrease in the modal abundance plagioclase-bearing leucosomes alternating with biotite- of chlorite, particularly in garnet-bearing domains. rich melanosome. Garnet shows poorly preserved growth Garnet shows preservation of growth zoning. Biotite in zoning and retrograde diffusion zoning (increase of Fe contact with garnet has slightly different compositions and Mn and decrease of Mg) near the rim at the contact from those away from it. Ilmenite contains 6 mol % with biotite. Biotite grains in the matrix do not show pyrophanite. The compositions of garnet and ilmenite any appreciable variation of composition irrespective indicate Mn-rich bulk composition. The observed of whether these are in contact with garnet or not. depletion of chlorite in garnet-rich layers suggests that Plagioclase shows a flat profile with XAn around 0.43. garnet formed from the breakdown of chlorite, such as Petrographic features, namely the preservation of rare staurolite inclusions in the outer rims of garnet and Chl + Qtz = Grt +H O 2 presence of kyanite in the same thin section, suggest the Staurolite zone : reaction This zone is demarked on the basis of the presence St +Qtz = Grt + Ky + H O of staurolite with relatively Mn-rich garnet. Garnet grains 2 and/or show partly preserved growth zoning, and an increase St + Bt + Qtz = Grt +Ky + Ms + H O of Mn content in the rim. The modal abundance of garnet 2 is distinctly less than that in similar rocks at lower grades. Sillimanite-muscovite zone: There is total disappearance of prograde chlorite, In this zone, pelitic rocks contain prismatic whereas biotite is intergrown with staurolite. These sillimanite and rarely relict kyanite, along with observations suggest formation of staurolite plus biotite muscovite. K-feldspar is present at places and locally at the expense of chlorite, for example, occurs with quartz and plagioclase in the leucosomes.

Grt + Chl + Ms = St + Bt +Qtz + H2O Garnet shows retrograde diffusion zoning of Fe and Mg near the rim, especially when it is in contact with matrix Staurolite-kyanite zone: biotite, as expected from the change of equilibrium The rocks are characterized by the first appearance composition at the interface during exhumation of staurolite and kyanite. There are minor amounts of (Ganguly et al., 2000). There is also a small increase of retrograde chlorite replacing staurolite and garnet. Mn concentration near the rim. Muscovite contains Garnet is distinctly poor in Mn compared with that of around 14 mol % paragonite identical to that in the the previous assemblages, and shows considerable previous zone. Plagioclase is sodic (XAn around 0.22) relaxation of the growth zoning. Biotite shows only and shows no significant zoning. minor compositional variation regardless of spatial distribution and grain size. Plagioclase is sodic (XAn Sillimanite + K-feldspar zone: around 0.2) and shows no zoning even when it is in contact with garnet. The latter observation implies that With the total elimination of primary muscovite, K- garnet did not suffer a resorption reaction to produce a feldspar appears as a dominant constituent along with plagioclase bearing assemblage in these rocks. Presence garnet, prismatic sillimanite and biotite. The pelitic of rare chloritoid inclusions in garnet and of staurolite migmatites are by far the most dominant constituent of in the same thin section suggest a reaction of the type the Higher Himalaya in this zone, and consists of Qtz + Kfs (locally perthitic) + minor Pl bearing leucosomes Cld + Qtz = St + Grt (±Chl) + H2O 30 GEOL. SURV. IND dotted with small euhedral garnet, and mesosomes with slightly south of West Bengal-Sikkim boundary; Main porphyroblasts of garnet, biotite and sillimanite. Garnet Frontal Thrust (MFT) between Siwaliks and Neogene becomes considerably Mg-rich at this grade and shows sediments is further south. Main Central Thrust (MCT) well developed retrograde zoning of Fe and Mg near passes through the Central part of the State and the the rim when these are in contact with biotite. normal fault between Central Crystallines and Tethyan Sequence passes through the northern part of the State. The results of detailed petrographic and A number of smaller thrusts and NW and NE trending thermobarometric studies of the metapelites along a oblique faults are also recorded in Sikkim Himalayas, roughly E–W transect by Neogi 2004 shows progressive some of which appear to be active faults. increase of both pressure and temperature with increasing structural levels in the entire IMS. This is In this area most of the earthquakes are shallow focus contrary to all models that call for thermal inversion as (<40 km) and commonly of 4.5 to 5.5 magnitude range. a possible reason for the origin of the IMS. Also, the The state along with adjoining regions have been divided observation of the temporal relation between into three blocks. The central main Himalayan Block crystallization and S2 structures is problematic for with considerable Seismicity separates the northern models of post-/late-metamorphic tectonic inversion by Tethyan block and southern fore deep block with recumbent folding or thrusting. subdued seismic activity. Prolonged and complex polymetamorphic history A look at the regional distribution of earthquakes with two episodes of prograde metamorphism (Ml and and lineament/fault patterns in Sikkim and adjoining M2) followed by retrograde metamorphism (M3) have region indicates that a number of high (5.0 to 5.9) and been recorded from the Higher Himalayan Crystalline medium magnitude earthquakes are clustered around and Complex from Sikkim (Neogi et al., 1998). Ml may related to Tista Lineament along NW-SE direction. One represent pre-Himalayan metamorphic episode. more NW-SE trending lineament, and marked as a fault Prograde metamorphism M2 is related to renewed burial along Tista River between Lachen and Chungthang (and of the crystalline complex in the early stages of further south-east) appears to have been the cause of collisional events. Geothermo-barometric estimates for high magnitude earthquakes. There also appears to be peak M2 metamorphism indicate a pressure of 10-12 yet another NW-SE trending lineament in north-east kbar and temperature of 800°-850°C. Subsequent Sikkim along which the earthquakes have been recorded. metamorphic event (M3) took place as a result of ~5 Similarly a NE-SW alignment of earthquakes is also kbar of decompression. The inverted Barrovian observed. metamorphic zonation has been attributed to probable The map published by the India Meteorological large scale structural inversion and/or tectonic Department indicates that the state of Sikkim comes juxtaposition. under seismic zone IV- a zone of considerable The exhumation history of pelitic migmatites of the vulnerability. The nearby regions come under zone V Higher Himalayan Crystalline from Sikkim-Darjeeling and are highly susceptible to earthquakes. section has been determined on the basis of thermo- Table indicates the list of recorded earthquakes of barometric analysis, retrograde breakdown reaction, magnitude >4 in Sikkim state. It is interesting to note compositional zoning of garnet and numerical modelling, that a majority of the earthquakes originated at a depth suggesting initial rapid exhumation (~15mm/year) from of ~30 km. Apart from a few earthquakes which appear ~34km upto a depth of-15 km followed by a much slower to have been controlled by aforesaid lineaments, other process (~2mm/ year) upto at least, ~5km depth. The earthquakes do not appear to have structural controls dramatic change in the rate of exhumation might reflect which are visible on surface. a process of tectonic thinning followed by erosion and/ or horizontal flow at shallow depth (Ganguly et al. 2000). Deep seismic reflection profiling beneath southern Tibet by Zhao et al. (1993) indicates MFT and MBT SEISMICITY AND EARTHQUAKES merging at the depth of around 5 km and having a slope of about 3° towards north. The thickness of the crust The State of Sikkim encompasses parts of Lesser between MBT and MCT varies from 2 km to 4 km in Himalayas, Higher Himalayas and Tibetan plateau. Main this region and then MFD, MBT and MCT finally merge boundary thrust (MBT) just passes through the area with the Detachment Zone or Main Himalayan Thrust MISC. PUB. NO. 30(XIX) 31

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 CGS 1961 09 11 28.2 88.3 23 1964 08 30 27.36 88.21 - 5.1 21 ISC 1964 08 30 27.9 88.52 - - 33 - 1964 10 25 27.9 88.6 4.8 0 ISC 1966 12 28 28.00 89.00 - 5.2 - ISC 1972 08 21 27.33 88.01 - 4.5 33 ISC 1972 08 21 27.23 88.02 - 5.1 33 ISC 1974 03 24 27.63 88.01 - 4.7 ? ISC 1974 01 22 28.1 88.7 33 1975 01 23 27.44 88.37 - 4.5 33 ISC 1979 11 16 27.90 88.70 - 4.6 39 ISC 1980 11 19 27.40 88.80 6.1 6.0 47 ISC 1982 04 05 27.38 88.83 4.6 5.0 09 ISC 1985 05 25 27.60 88.48 - 4.6 33 ISC 1986 01 07 27.40 88.43 - 4.7 41 ISC 1987 12 06 27 88.52 - - 42 ISC 1988 01 19 27.8 88.8 - 4.3 33 ISC 1988 03 27 27.1 88.42 - 4.1 70 ISC 1988 05 26 27.45 88.61 - 4.7 42 ISC 1988 09 27 27.19 88.37 4.6 5.0 23 ISC 1990 09 18 27.32 88.84 - 4.1 33 - 1995 11 28 27.58 8.84 - - 67 ISC 1995 12 24 27.54 88.26 - 4.1 26 ISC 1996 02 25 27.6 88.8 33 1996 03 23 27.17 88.30 4.0 33 - 1996 91 32 7.32 88.54 - 4.1 33 ISC 1996 09 25 27.43 88.55 5.0 33 - 1996 12 30 27.28 8.6 - 3.8 33 ISC 1998 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 ISC 1999 09 20 28.18 8.1 3.8 84 2000 01 25 27.7 88.36 - 1.8 33 ISC 2001 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 -

at the depth varying from 28 km in the south to about 40 network by Nath et al. (2000) reveal a much higher km in the north. The depth of Moho discontinuity in the frequency of earthquakes of smaller magnitude. The entire region is of the order of ~80 km. From the data on studies indicate that small magnitude earthquakes are the depth of origin of the earthquakes it is found that more frequent in number and the depth of their origin majority of the earthquakes have originated at depths of varies from 3.2 km to maximum 8.0 km. These about 30 km. This is the surface of detachment. The earthquakes could be the result of small structural progressive drift of the Indian subcontinent beneath adjustments within Himalayas, possibly due to Tibetan plate at a rate of ~3 cm per year (op. cit) may be reactivation of faults. taking place along this zone of detachment and the The seismically vulnerable state of Sikkim, which accumulated strain must be reflected in periodic falls in seismic zone IV, comprises of a variety of earth earthquakes of this region with a depth of about 30 km. material (various types of soils, weak and strong rocks) It is also observed from the table that the earthquake with complicated structure etc., and on which multi- activity is periodical in nature, followed by a period of storied buildings are being constructed. In view of the quiescence. vulnerability of the region, seismic-microzonation of Site response studies in Sikkim using strong motion towns is essential for hazard prevention and mitigation. 32 GEOL. SURV. IND

Mineral Resources of Sikkim

In spite of low accessibility, thick vegetal and soil/ of India revealed presence of base metals at Rangpo, debris/moraine cover, the state of Sikkim is represented Dikchu, Pachekhani, Temi, Reshi, Jungdum, , by a number of base metal occurrences. Most of the Rothak, Mangreng, Rinchingpong, Chongbong, Keorani, occurrences are in Daling Group of rocks in parts of Sirbong, Sisni, Dong Busty, Uttare, Siging, Bum, Sipik, west, south and east districts of Sikkim. Base metal Tukhani, Namthang, Pamphuk Khani, and occurrences at Bhotang, Pachekhani and Dikchu have Ratopani. been explored in details. The Bhotang and Pachekhani lodes which are mined presently are possibly the only Out of all these occurrences, the Bhotang (Rangpo), working mines in Himalayas. Among non metallic Pachekhani and Dikchu are most important and have minerals coal, graphite, dolomite, limestone, marble, been proved in the form of deposits. Rangpo and wollastonite, talc, sillimanite and asbestos etc. also occur. Pachekhani are currently been mined.

ASBESTOS: RANGPO (Bhotang) (27o11:88o32 ): Basemetal occurrences of East Sikkim are most Sosing Hill: important in the sense that they constitute the only In Legship area an amphibolite sill, intruded into working base metal mines of Himalayas. In Rangpo- phyllites at the southern foot of Tashiding ridge and very Pachekhani area the main rock units are phyllites schists near the confluence of Rathong Chhu with Rangit river, and quartzites, associated with ellipsoidal lenses of and another in the western face of Sosing hill amphibolites and quartz veins and stringers. These rock (27°17:88°20) are asbestos bearing. Asbestos which is units are repeated and intricately intermixed with each developed along cracks and fissures is bluish-grey in other and can be traced at different places and altitudes. colour and of short, matted, harsh fiber type. The Numerous intra formational faults, thrusts and folds have maximum length of fibers is 10 cm. The fibers are affected the country rocks considerably (Raina and associated with acicular tremolite and actinolite crystals. Bhattacharya, 1965). Both of these occurrences are not of much economic importance. Phyllites are exposed in the area between the Tista in the west and Kalikhola-Pachekhola in the east and Crocidolite bearing highly kaolinised rocks have consists of chlorite-sericite-phyllite, sericite-chlorite- been traced near Rabong (27°18:88°2030). They occur phyllite, quartz-biotite-phyllite, slaty-carbonaceous- in area of 30m x 10m. The country rock is most probably phyllite and quartz-sericite-phyllite. The first two rock mica pegmatite. types are soft, grey to deep grey in colour and are repeatedly exposed in the western part of the area. The chlorite-sericite-phyllite contains specks of pyrite, BASE METALS: pyrrhotite and chalcopyrite, which at places like Bhotang Most of the base metal occurrences in Sikkim are and Mangreng, show good concentration. Quartz-biotite- recorded in Gorubathan Formation of Daling Group of phyllite is fine grained, light green in colour and shows rocks. Systematic geological mapping and mineral the presence of thin blebs of biotite along the foliation exploration programmes launched by Geological Survey planes. Slaty-carbonaceous-phyllite is dark charcoal-

32 MISC. PUB. NO. 30(XIX) 33 grey in colour, soft and friable in nature, contains specks fractures, faults and thrust planes and have criss-crossed and occasional good concentration of sulphide minerals the country rocks in a net-work fashion. Presence of and graphitic carbon at places like Pache, Rorathang and sulphide minerals is frequent in these post tectonic veins. Kalikhola. Sulphide minerals like chalcopyrite, pyrrhotite, The schists are generally exposed in the eastern part pyrite and galena show four modes of occurrence: (i) as of the area, between Rorathang-Damlakha to Mamring- disseminations within the country rocks (strata bound), Taza. The gritty, quartz-biotite-schist is highly sheared, (ii) as lensoid replacements within vein quartz which shows the presence of irregular shaped (0.1 cm to 0.65 vary from 3 cm to 6 cm in thickness and 10 cm to 10 m cm in length) quartz grains and contains dissemination in length,(iii) within basic intrusives (iv) as joint and of sulphide minerals like chalcopyrite, pyrrhotite, pyrite fracture fillings localized along fault, thrust and shear and galena. Good concentration of sulphide minerals is zones. recorded at Rorathang and Pachekhani. Sericite-biotite- schists and quartz-biotite-schists are repeatedly exposed This is the only polymetallic base metal deposit in this area. These two rock types show good under active exploitation in the entire Himalaya development of biotite, whereas garnet though present producing Cu-Zn-Pb concentrates. Several old workings sparsely in the sericite-biotite-schist, is well developed present in the area were formerly opened by local Nepali only in the quartz-biotite-schist. workers, and were later on prospected by M/S Burn and Co. and the Indian Bureau of Mines. The Cu-Zn-Pb Quartzites are generally phyllitic, cherty and sulphide deposit of Rangpo is exposed on the left bank calcareous in nature. The phyllitic quartzite which is of Tista River, just north of Rangpo town. The host rocks repeatedly exposed in the entire area is grey to greyish- of mineralization are rocks of Daling Group. The ore brown in colour and fine grained in nature. It contains body is stratabound and folded. The mineralized quartz good concentration of sulphide minerals in the Bhotang veins are post-tectonic and are of hydrothermal origin. and Mangreng areas. Cherty quartzite bands are exposed Disseminations of sulphide minerals are confined mostly at Lusing and along Rangpo-Rorathang road. This rock to the basic sills (amphibolite and amphibolite schists), is highly sheared and brecciated and locally has given chlorite sericite phyllites and phyllitic quartzite (Raina rise to autoclastic conglomerate, consisting of elongated and Bhattacharya, 1965) to sub-elongated quartz grains arranged closely along the foliation planes. Calcareous quartzite is exposed only In Rangpo-Bhotang Block sulphide minerals of all at Pachekhani. This rock is medium grained, hard, light three types occur, but the predominant one is the brown to buff in colour and dolomitic in nature. It shows replacement type. Two lodes- Bhotang lode or Lode-I good concentration of sulphide minerals like and Subsidiary lode or Lode-II – have been delineated chalcopyrite, pyrrhotite, pyrite, and galena. at Rangpo. Bhotang lode is the important lode and is being mined in Bhotang Mines. The ore body is 280 m Basic intrusives occur as amphibolite and long and average width is 5 metres. The principal ore amphibolite schists. These are lenticular and elongated minerals are Chalcopyrite, sphalerite and galena with sills of irregular length, show concordant relation with pyrrhotite, pyrite and marcasite. Minor phases comprise the country rocks. These intrusives invariably contain arsenopyrite, tetrahedrite, pyrartyrite, jordanite – Silver sulphide minerals as disseminations, where from pyrite and gold are present in traces in the ore with the former has commonly leached out leaving voids. in greater amount. The copper content varies from 0.02% to 1.57%, lead from 0.64% to 1.2% and zinc from 2.10% Two types of quartz veins i.e. pre-tectonic and post- to 3.15 percent. tectonic are identified in this area. The pre-tectonic vein is only traceable at the junction of Baghe khola and Indian Bureau of Mines(Mukherjee and Rao, 1974) NH31A, where a band could be traced for more than conducted drilling and exploratory mining operations 100 m and is tapered off at its southwestern side. It shows and estimated 0.6 m. tones of ore with 2% copper and conformable relation with the country rocks. The post- Zn. GSI’s efforts between 1978 to 1982 extended the tectonic veins and stringers are seen both along and western extension of Rhotang lode by more than 60 across the foliation planes and have filled up the metres, so that till date, more than 400 metres of strike 34 GEOL. SURV. IND length of Bhotang lode has been established. The lode to explore the two areas separately. In all, three blocks- increases in width and copper content at depth. More upper and lower Pachekhani Blocks and Rorathang than 0.1 m tones of additional 2-3 Cu ore have been Block – were delineated by IBM. IBM drilled 10 estimated due to additional 60 m strike extension boreholes (1047m) and carried out exploratory mining established by G.S.I. It is expected that with further of about 1500m. length in Upper Pachekhani and development of lower levels, nearly 0.3 m tonnes of Cu- Rorathang Blocks only. Later GSI carried out Zn-Pb ores could be established. A noteworthy feature geophysical survey and test drilling in Lower Pachekhani is the silver metal occurrences in the lead concentrate. Block. Sulphide mineralization occurs as lenticular The amount of silver is estimated to be around 700-800 bodies arranged in “en echelon” fashion. The country gm per tonne of lead concentrate. Traces of bismuth, rock hosting the base metal mineralization is chlorite cobalt, cadmium, chromium and manganese are also schist, phyllites and quartzite of Gorubathan Formation recorded. of Daling Group.

Studies/exploration have also been carried out to Copper mineralization is confined to small, narrow prove the western extension of Bhotang lode across Tista impersistent lenses having limited strike and depth River by Mitra and Das (1984), Das (1985), William extensions and analyze in general 1.5% copper over et.al. (1984), Mukhopadhyay and Banerjee (1978) and widths of 1-1.20 metres. Hardly a single lode could be others. found which showed persistence in strike for more than 30 metres and in depth for more than 70 metres.

Pachekhani-Rorathang block (27°12:88°32): Detailed study on Pachekhani area was again carried There are three old workings in the Rorathang area out from 1990 to 1995. During this period 1.20 sq km and five in the Pachekhani area. These workings were area was covered on 1:2000 scale and an area of 212 sq being exploited at the time of visit of P. N. Bose (1891). km was covered on 1:25,000. A total of 7 boreholes The Indian Bureau of Mines also carried out detailed totaling 705.70m were drilled and a possible ore reserve exploration in this block from 1957 to 1961 (Mukherjee was estimated to 0.15 m tonnes of 0.75% Cu and 636.55 and Jog, 1963). The sulphide mineralization occurs as tonnes at 1% Cu. disseminations in gritty, quartz-biotite-phyllites, slaty- carbonaceous-phyllites and calcareous quartzites and is Ore-Microscopic studies from Bhotang and traceable for a distance of 300 m up the dip and for about Pachekhani deposits reveal that the ore mainly contains 150 m along the strike. Some occurrences are also seen pyrrhotite, chalcopyrite, sphalerite and a minor amount in post-tectonic hydrothermal quartz-veins and along of galena. In some samples concentric bands of pyrrhotite fault and shear zones. Vein-quartz type of concentration, and chalcopyrite with quartz at the centre are observed. occurring mostly along the foliation, joint and fracture Pyrrhotites are usually rimmed and veined by marcasite planes, is prominently seen in an old working at which is seemingly the alteration product of pyrrhotite Rorathang and is traceable for a distance of about 30 m. which in turn is altering to pyrite. A few grains of In the Pachekhani area, a highly mineralized, calcareous interstitial tennantite set in chalcopyrite are also noted. quartzite was traced, within which mineralization was Ilmenite intimately associated with pyrrhotite is also found to be pinching both up dip and down dip. Sulphide noticed. Disseminated allotriomorphic aggregates of minerals are mostly chalcopyrite, pyrite, pyrrhotite and chalcopyrite with minor amounts of pyrite and pyrrhotite galena. This area has been found to contain several small are seen in some of the samples. Ilmenite grains are also isolated highly mineralized pockets, which do not show noted. Coarse idiomorphic to hypidiomorphic granular any systematic extension. Investigation carried out in aggregates of pyrite with interstitial grains of highly this block by the Indian Bureau of Mines indicated no altered pyrrhotite are dispersed into gangue, which also workable lode (Raina and Bhattacharya, 1965). occur as discrete dispersed grains along with a few grains of chalcopyrite. Dispersed chalcopyrite containing Old working of adits and incidence of copper bearing interstitial grains of galena and pyrrhotite with minor quartz veins in both the upper and lower reaches of Pache amount of pyrite is also recorded. Discrete aggregates and Rorathang villages led Indian Bureau of of ilmenite are conspicuous. Pyrrhotite alters to pyrite. Mines(1958-61) & Geological Survey of India(1970-71) Pyrite shows a wide range of size variation from coarse MISC. PUB. NO. 30(XIX) 35 single idiomorphic to micro spherules of tiny pyrite During the period 1908-1911 M/s Burn & Co. initiated aggregates. Hypidiomorphic granular aggregates of exploration work and carried out underground aditing pyrite, occurring in bands, contain interlocked grains of (60.96 m in 4 adits). In the year 1909 Pillow Harveg, a ilmenite. London based Mining Engineer examined the deposit and expressed favourable opinion. In the year 1948 the Geological Survey of India carried out sampling of the Duglakha Block (27°14 :88°36 ) :   mineralized zone. Indian Bureau of Mines carried out The location is along Pache Khola stream; about 2 exploration during the period November, 1959 to June, km upstream of Pachekhani copper deposit. Exact 1963 by drilling and exploratory mining (underground location of the lode is 27°14:88°37. The Daling Group development 1544.20 m; drilling-4604.21 metres). It was of rocks comprises a sequence of Garnetiferous-biotite- further investigated by Mineral Exploration Corporation chlorite phyllite, biotite-chlorite phyllite, chlorite during the period May, 1977 to May, 1983. phyllite, quartzites and amphibolite schists. Amphibolite Schist occurs as sill within the metasediments. The The Cu-Zn- mineralization occurs as conformable Quartz veins follow foliation planes but at times have vein in the garnet-staurolite bearing quartz-mica-schist cross-cutting relations. The general trend of the rocks is of Daling Group near its contact with the gneisses of north-south to WNW-ESE with persistent easterly to Central Crystalline Gneissic Complex. The hanging wall northerly direction. Raina and Bhattacharya (1963) comprises of hard compact quartz-mica schist grading carried out detailed study of this occurrence. into quartzite. The footwall comprises quartz-biotite- muscovite-sericite schist. At Dugalakha three lenses of varying dimensions show on an average 2-5% lead in the surface exposures. Two lodes have been delineated at Dikchu. The Lode ‘A’ is exposed over a strike length of 66 m and Dikchu main lode trends north-south and dips at 30° to average width of about 1 metre. Average grade is 2.5% 35° towards east. Ore appears to be stratabound. It is lead. Lode ‘B’ could be traced over a strike length of 58 found to be persistent over a length of about 600m up to metres on the hanging wall of the amphibolite sill; a depth of 100 metres and has a thickness of about 1.25 average thickness of lode is 1 metre. Average grade of metres. The Lingdok lode is thin and impersistent. The lode ‘B’ is 5.8% lead. Lode ‘C’ is traced over a strike Dikchu lode is essentially a massive sulphide lode of length of 160 m and thickness of 15 m. But in lode ‘C’ copper and zinc. It is composed of chalcopyrite, the mineralization is in disseminated mode. The pyrrhotite, sphalerite and traces of galena and covelite. analytical results are also very poor. Here mineralization Magnetite and garnet are common gangue minerals. is mostly of galena and pyrrhotite with subordinate Exploration by IBM (1959-63) earlier established a chalcopyrite and sphalerite concentrated generally in probable reserve of 0.3 million tones of 2.96% Cu and hydrothermal vein quartz, and disseminated along the 1.5% Zn. Tests carried out by IBM on bulk sample of foliation of the country rock. Several such veins are seen Dikchu ore yielded 90-95% recoveries of copper along the fault or shear zones. This area does not show concentrate, 3.4 ppm gold and 92 ppm of silver. The much of promise. common ore minerals associated with the mineralization Geophysical investigation (1965-66) was carried out are chalcopyrite, pyrite, pyrrhotite, sphalerite and galena. covering an area of about 0.5 sq km at Dugalakha which Magnetite is the common oxide. Presence of gold and indicated poor copper mineralization with limited silver has also been recorded in this deposit. extents. Test drilling by GSI confirmed the findings of geophysical investigation. The work by MECL included 1891.8 cum of exploratory mining, 290.50 m of underground drilling

o o and 3 numbers (500 kg each) of bulk sampling. The work Dikchu (27 24:88 31: 78 A/11): was confined to the southern part of Dikchu River. The base metal occurrence of Dikchu was visited MECL proved 0.45 m tones of ore with 2.82% Cu and by P. N. Bose during 1891 and 1894 (Bose, 1891, 1928). 0.9% 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: occurs by the side of a newly constructed bridge. The host rock is a amphibolite intruding calcareous quartzite Dominant lithounits are phyllite and quartzite- enclave within banded biotite gneiss (Mullick and talcose phyllite. Epidiorite also occurs as subordinate Chattopadhyay, 1991). The mineralization is in the form unit. Sulphide mineralization is found within of specks and big crystals of chalcopyrite, pyrrhotite and epidiorite, talcose phyllite and also in the associated bornite. Near Rolep village and slightly north of it, on quartz veins. the opposite bank of Rangpo Khola, there are calc- silicates showing richer portions of sulphide mineralization in the form of disseminations of pyrrhotite Danak Khola: and chalcopyrite with rare bornite. The mineralization is located in Danak Khola (stream), about 5 km east of Rorathang village. Two mineralized veins have been recorded (Das et al., 1984). Rongli: The first mineralized vein, in the northern part, has a Near Rongli Chu-Sewa Khola confluence, a one width of 1.5 m and continues for about 30 m along metre garnetiferous sericite-quartzite with quartz veins apparent dip direction. The second vein is 20 m south of show mineralization in the form of pyrite, the first vein and has a maximum width of 2.5 metres and bornite+pyrrhotite and chalcopyrite. The thickness of continues up o 35 m along apparent dip direction. The mineralized zone is more than 2 metres and apparent sulphide mineralized part consists of chalcopyrite, pyrite, length is 25 metres (Mullick and Chattopadhyay, 1991). pyrrhotite, magnetite and a few specks and stringers of galena. Mineralisation is sporadic and discontinuous in Sumbuk (27°07 :88°23 ): nature. Nine samples from these veins indicated copper   values ranging from 0.14% to 9.15 percent. Indication of copper mineralization is observed on both the banks of right tributary of the Khani Khola, a branch of Manpur Khola. Two adits and two minor Rungdu-Sodunlakha area: excavations are located on the left bank of the stream. Recent investigation by Geological Survey of India has brought to light a promising sulphide occurrence in Sulphide mineralization is noted in quartz vein the Rungdu-Sodunlakha Belt. The copper mineralization traversing a dark grey phyllite of the Daling Group. is associated with the quartz-biotite-chloritoid schist These quartz veins are mostly of pinch and swell type. band (within chlorite schist). It is confined to the Sulphide minerals include chalcopyrite and occasional schistosity plane and more so along the silica rich pyrite and occur as nests and stringers in quartz vein portion. Schistosity of the rocks varies from N25°E- and as dissemination in the country rock in the proximity S25°W to N70°E-S70°W with a south-easterly dip of of the veins. The mineralized lode has maximum width 20°-65°. Groove samples indicate maximum values of of 1.00m and can be traced along the strike for 30m. 1.81% Cu in Rungdu and 1.45% Cu in Sodunlakha area. Chemical analysis of the groove samples collected from Mineralisation is structurally controlled and better the mineralized zone indicates copper (0.98% to 1.86%) concentrated at the closure of Rongpo Chhu antiform, while Pb, Zn, Ag are present as traces. near Sodunlakha than the limb portion. Gold values from this area vary from 50 ppb to 255 ppb. The investigation Pampukhani (27°07 :88°29 ) : is in progress.   In Pampuk area, base metal mineralization has been observed in Pampuk R.F., located 5 km. south of the Rolep area: Namthang village. The mineralization is exposed on a This area is about 15 km from Rongli by road and scarp face at a height of 200 m. from the Tista river bed. MISC. PUB. NO. 30(XIX) 37

Sulphide mineralization is noted in quartz veins been developed (Nautiyal et al, 1961). The mineralized traversing slaty phyllite of Gorubathan Formation of quartz veins showing pinch and swell structures plunge Daling Group. The veins, which have a brown stained at 25°-30° towards north. A few small 3 to 6 metres long spongy appearance on the surface, are exposed in a series adits occur in these zones. The copper values range from of old inclines. Overall strike length of the mineralized 10.7% to 3.40% over 30-40 cm width. The zone appears to be 30m and maximum width of mineralization is structurally controlled by the mineralized zone is about 20 cm. Mineralization mainly complementary shears. The deposit is fracture-fill type contains pyrite, chalcopyrite, galena and sphalerite. with little wall rock alteration except for some sericite Chemical analysis of the mineralized zone indicates bordering the mineralized quartz veins. The predominant 0.05-1.64% Cu, 0.15-0.52% Pb and 0.16-0.96% Zn. sulphide minerals in order of abundance are chalcopyrite, pyrrhotite and pyrite. Besides the above two locations, there are some minor base metal sulphide occurrences at Temi (27o14’:88o26’, 0.06%-1.25% Cu), Dong Basti Tukhani: (27o12’:88o20’, 1.32% Cu) and Parbing (27o11’:88o25’, Copper mineralization is in the form of old workings, 0.78% Cu) areas in South Sikkim. disseminations of chalcopyrite, stains of malachite which are seen in the quartz veins traversing the phyllites of the Daling Group. Temi area: The Dalings comprise chlorite phyllite and quartzite with quartz veins. Both bedding and schistosity trend Namthang: west to NW with moderate dips towards south-east. The area comprises slaty phyllite and quartz-chlorite Basic sills within phyllite have also been reported with schist traversed by quartz veins. Sporadic pyrite and asbestos bearing quartz veins near the contact. chalcopyrite is found in quartz veins in Shenti Khola Chalcopyrite and pyrite bearing quartz veins are over 0.5 m thick and 30 m long zone. emplaced along schistosity, shear plane and fracture cleavages in the phyllite. The mineralized veins are Parbing: confined along a zone which is about 370 m long and 4 Mineralised quartz veins, emplaced along the m wide. The channel samples yielded traces to 7.93% foliation planes of quartz-chlorite phyllite and are spread copper. discontinuously over 70 m long and 0.4 m wide zone. Pyrite and chalcopyrite are found as disseminations. Analysis of ore indicated 0.78% copper. Dong Basti: Phyllite and quartzite dominant lithounits of the area Ratopani: are traversed by quartz veins. Mineralization as disseminations and granular aggregates of chalcopyrite Traces of pyrite and chalcopyrite have been noticed occurs in quartz veins (3 m thick zone). Other associates in quartz veins traversing the phyllite and quartzite. Soil are pyrite and covelite. Analytical data of channel samples assayed less than 50 ppm copper. samples show 1.32% copper. This occurrence needs geophysical and geochemical appraisal before a final Rothak Khola: verdict. Rothak Khola area is dotted with a number of small base metal sulphide mineralization of which most Sikkip: important ones are Rothak Khani (27°0950:88°1518 The mineralized quartz veins traverse slate and 0.15% Cu), Jagdum (27°11:88°1415, 0.64% Cu), dolomite, north of Sikkip Thrust. The mineralized quartz Sirbong (27o1035:88°1530, 0.05% Cu), Chungbong veins occupy north-south trending fractures. The beds (27°0730:88°15, 0.41% Cu), Sisni (27°1018: strike N70°E-S70°W with 40° dip towards north. Due 88°1420, 0.35% Cu) and Siging (27°1040:88°1510, to the proximity of the thrust, these rocks are highly 0.30% Cu). During 1960-62, G.S.I. worked in this area, contorted and secondary foliation (N10°W-S10°E) has 0.645 sq km area was mapped on 1:2000 scale, 600 38 GEOL. SURV. IND

geochemical samples were collected and 700m were Chonbong (Chungbong- 27°0730:88°15): drilled in all the mineralization ore associated with quartz Greyish-green phyllitic slate, interbedded with veins as stringers and fracture filling within host of quartzite is traversed by quartz veins. Schistosity trends chlorite phyllite/sericite chlorite phyllite of Daling N67°E-S67°W with 35° to 45° dip towards north-west Group. Main ore minerals are chalcopyrite and pyrite (Kurien and Pattanaik, 1963). Chalocopyrite and with minor amount of sphalerite and galena. malachite with occasional crysocola associated with quartz veins has been observed in phyllite along foliation A revaluation of the above deposits by GSI’ during planes and joints. Samples analysed 0.41% copper. A 1996-97 also indicates that the above occurrences are further geochemical appraisal is called for. not of economic importance in the present day standard.

Keorani: Jugdam (27°11:88o14°15): Carbonaceous and chloritic slates with quartz veins The area consists of chlorite phyllite, dark slaty host the mineralization in this area. The foliation trends phyllite, quartz sericite phyllite and epidiorite-quartz N20°E-S20°W with 40 to 45° dip towards WNW. Quartz veins. Copper mineralization occurs in quartz veins as veins varying in thickness from 2.5 to 5 cm contain poor stringers, foliation and fracture filling (Nautiyal et al, mineralization along a 20 metres long and 0.6 m wide 1962). The mineralized quartz veins with strike extension zone. of 15 m to 107 m and width of 1.50 m to 3.96 m have been reported. Groove samples indicated 0.51% copper where as drill core samples indicated 0.21% to 0.64% Sirbong (27°1035:88°1530): copper. Chalcopyrite and pyrite has been observed in a 6 m wide and 40-50 m long zone, traversed by thin quartz Rothak Khani (27°0950:88°1518): veins within the grey-slaty phyllite and quartzite. Foliation plane of the host rock trends N30°E-S30°W This occurrence is located at the confluence of Khani with 53° dip towards N60°W. Analytical data indicates Khola and Rothak Khola. Dominant lithounits are grey copper values from traces to 0.5 per cent. phyllite, sericite-chlorite phyllite and crumpled carbonaceous slate with thin bands of quartzite. General Sisni (27°1018:88°1420): trend of foliation is north-south to NE-SW with 60° dip towards WNW to NW. Chalcopyrite with subordinate Carbonaceous slates with chlorite and sericite pyrite has been observed within quartz veins traversing intercalations, traversed by quartz veins have foliation slaty phyllite. Main zone with 120 m length and 1 m to which in general trends N20°E-S20°W (Nautiyal et al, 1.5 m width is located on right bank of Khani Khola. A 1962). The main mineralized band, 0.77 m to 2.44 m few mineralized quartz veins are present on the left bank. wide occures towards upslope over a slope distance of Soil samples from these areas indicated 1500 to 3000 44 metres. Total mineralized zone is 213 m long. A part ppm and 500 to 2000 ppm copper respectively. This of the mineralized zone is across the Rothak Khola. occurrence needs to be evaluated by geophysical and Groove samples from this zone indicated 0.15% to geochemical surveys. 0.35% copper. The minerals are mainly chalcopyrite and minor pyrite. Rinchinpong: Chitre: The area comprises greenish-grey chlorite schist with opaque quartz and chlorite-sericite schist The country rock of this area belongs to interbedded with flaggy quartzite, containing occasional metamorphites of Darjeeling Group of rocks with calc sheared limonitised bands (Kurien and Pattanaik, 1963). silicate inrusive, along with granite and pegmatite A sheared limonitised 15 m long zone containing (Nautiyal et al, 1962). Foliation in calc-silicate bands primarily pyrite and chalcopyrite has been recorded. trends north-south with moderate to high dips towards east. Pyrrhotite is sporadically distributed. MISC. PUB. NO. 30(XIX) 39

Siging (27°1040:88°1510): Sada (27°24:88°22): At this place several thin, highly ferruginous, quartz There are a few old adits at the confluence of Signing veins and stringers have been seen on the Rangit River Khola and Santolo Khola. The mineralization is in the bed, within hard and well bedded quartzite. These quartz vein quartz traversing the highly sheared and fractured veins contain some concentration of galena associated quartzite. Geochemical samples gave values ranging with chalcopyrite and pyrite. The zone is 5 metres thick from 50-3000 ppm copper. The anomaly trends in and nearly 10 metres long are exposed along strike roughly NE-SW, making an angle of 15° to 20° with the direction. The quartz veins are sheared. strike of the beds. Old adits have been driven in N50°W direction for a distance of 10-15 metres. This occurrence needs appraisal by structural analysis, geophysical and Rangpo Chu: geochemical surveys. On this stream concentration of sulphide minerals has been detected at four places. Bum: Near the confluence of Ranken Chu and Rangpo The mineral occurrence is observed in phyllite and Chu- Nearly 300 m downstream from the said confluence quartzite which are exposed on either bank of Rishi on the southern bank two metres thick quartzite zones Khola near Bum village (Nautiyal et al, 1961). Pyrite, rich in pyrite separated by a 25 m thick phyllitic band, chalcopyrite, bornite, galena and pyrrhotite stringers have been met with. Mineralization in a disseminated having thickness of less than 1 cm to 13 cm are seen pattern has taken place within the quartzite itself. The associated with quartz veins. Samples from a 15 cm thick quartzites are light brownish grey in colour, fine grained, quartz vein have analysed 3.41% and 4.5% copper. hard and compact. Along with pyrite a certain amount However, the mineralized zone does not show strike of chalcopyrite and pyrrhotite has also been detected. persistence. The zone is exposed along the strike for 100 metres.

Legship: Pading (27°18:88°2330): Very near to this village, on the southern side of Rangpo Chu, within a highly In Legship area the base metal mineralization is sheared and brecciated zone of quartzite, a some what observed in the northern bank of the Baniya Nala, near good concentration of chalcopyrite and pyrrhotite has its confluence with the Rangit River. Pyrite, pyrrhotite, been found. The qurtzites are light grey in colour and at chalcopyrite, bornite and galena occur as disseminations places, are highly limonitic. The zone is 10 m in width and as thin hairline fracture-fillings in quartz veins. and 15 m in length. Groove samples from the area indicated 1200 ppm to less than 100 ppm copper. On Rangpo Chu near the log bridge on the mule track for Aitabarey. Very near to the bridge, at several places Sontale: within a distance of 150 m. sulphide minerals like chalcopyrite, galena and pyrite have been seen Copper mineralization within the sericitic phyllite, disseminated within the quartz veins and stringers. The slate and chlorite schist of Daling Group occurs in country rock is chloritic phyllites and quartzites. The association with quartz veins. Three mineralized quartz dissemination is concentrated only within quartz veins and veins were identified having thickness ranging from 0.3 stringers. The zone is exposed on both sides of the river. m to 0.60 metres. Upstream of Momain Chu near Thurphyak Bhattacharya and Pattanaik (1964), in the course of (27°18:88°23) nearly 800 m. upstream of Momain Chu, their systematic mapping recorded a number of base a tributary to Rangpo Chu, on the western bank, metal occurrences, the details of which are produced concentration of chalcopyrite has been found within below. : three quartz veins which are approximately 2 to 3 metres thick and are exposed for about 25 metres. They are milky white to brownish in colour. 40 GEOL. SURV. IND

Brom Chu (Brom: 27°1830:88°2650): LEAD MINERALIZATION: A little upstream, nearly 300 m. within highly Pawde (1965) reported lead mineralization between sheared sericite phyllite and quartzites, disseminations Ingming khola (local name) and Chunbhatti khola (local of chalcopyrite and pyrrhotite has been observed. The name, toposheet 78 A/8) on the hill slope to the west of zone is 16 m. long and 5 m wide. Rangit River at an altitude of 740 metres. The host rock, Buxa dolomite, strikes N30°E-S30°W and dip 50° due N60°W. Mineralization of galena is recorded along joint Run Chu: planes trending NW-SE and NNE-SSW. Some of these From the confluence of Run Chu and Tista River joints have been traversed by quartz stringers. The nearly 800 m. upstream along Run Chu, a pocket of mineralization is observed in quartz veins when they sillimanite bearing schists is located. They are highly are present; otherwise joint planes of dolomites are host. sheared and brecciated. The zone is exposed on a vertical The thickness of mineralized zone varies from 10 cm to cliff nearly 10 m. in width and exposed for about 70 m. 25 cm and it is observed over a distance of 7 meters. The zone is highly intruded by quartz veins and stringers. This area shows best concentration of sulphide minerals Dutt (1954) reported that dissemination of galena of all the localities. Sulphide minerals are mostly occurs in the vein quartz, which is 10 inches thick chalcopyrite along with pyrrhotite and pyrite. The emplaced in flaggy quartzite 300 yards south of exposure is seen on both sides of the river. Damthang and also between 32 and 33 milestone near Jaubari (27°12:88°24). Bartag (27°30:88°30): At this place, very near the contact between the CALCAREOUS TUFFA: quartzose schist and garnet rich biotite schist, some Calcareous tuffa is located in the northern bank of dissemination of pyrite has been located, in a zone 3 m. Rangit River. This occurs in pockets within the Buxa in width. The zone is exposed for a distance of 7 m. slate. The tuffa is creamy white to earthy looking, soft and spongy. Six channel samples indicated following Section between Chungthang (27°36:88°38) to chemical analytical results. Jorepul (27°42:88°34): Maximum Minimum In this area, along the new road section, at several CaO (%) 51.45 43.40 places, dissemination of sulphide minerals have been MgO (%) 1.75 1.00 R O (%) 3.93 1.30 seen restricted within amphibolites and amphibole 2 3 Insol (%) 14.03 2.51 schists. Chalcopyrite is the most common mineral. COAL: Phentong (27°3446:88°25): Coal beds associated with Gondwanas are folded Near this village on road to Sakyong some pyrite and faulted, and show pinching and swelling. Coal, disseminations are observed within sheared and fractured varying from carbonaceous shale to semi-anthracitic quartzite bed. variety occurs as crushed, often nodular variety. Its exposures can be seen at a number of places within Traces of sulphide mineralization in the form of Gondwanas. Linking and correlation of individual disseminations, occur in the basal rocks of the Everest outcrop is very difficult due to the complexity of the Pelitic Series exposed along the western wall of the structure. quarry below Chorten Nyima Peak (27°57:88°11) and some malachite and azurite encrustations in lime- Around Namchi: silicates in the eastern valley wall of Lungme nala (27°52:88°21). These are so meager that they do not Regional geological exploration by GSI in Namchi possess any economic interest (Raina and Bhattacharya, area of south district has established a reserve of 1.4 1965). lakh tonnes of bituminous to semi-anthracitic coal analyzing 40% to 60% fixed carbon, 4% to 22% ash, MISC. PUB. NO. 30(XIX) 41

8% to 13% volatile matter and 3% moisture. Thin coal Legship-Nayabazar, with varying thickness. Other seams seams (1 m to 5 m) are found to occur on the road linking are exposed south of Put Khola; in between 14 and 15 Legship with Nayabazar and to the south of Putkhola. A km. milestones. However, these coal seams are 1m-5m good account of coal seams of this area is provided by thick and are not of economic importance. Raina and Ray (1967). Only one coal seam, located near Rishi Bazar, has been investigated by pitting and drilling during F.S.1990- Regional geological exploration in and around 92 and 1992-93. The total reserve potential of this Coal Namchi (27o10’:88o24’) area of south district of Sikkim Seam is approximately 0.58 million tones (Sharma, was carried out during the period from 1968 to 1971. 1994). The investigation led to the estimation of a total reserve of 1.4 lakh tones of coal in the area. This is bituminous There are numerous coal outcrops of various to semi-anthracite variety of coal, occasionally thicknesses in road sections, stream cuttings, and hill pulverized due to intensive folding and shearing. On and scarp sections. Majority of these occurrences are of analysis, these varieties of coal indicate 40% to 60% limited strike extension. The coal occurrences are as fixed carbon, 22%-40% ash, 3%-8% volatile matter and follows: 3%-6% moisture. These coals have not been found suitable for domestic and industrial use in their present Near Kamling School in new road cutting of state. Kamling approach road:

Coal also occurs as thin seams within the Gondwana An outcrop of coal of about 5 m thickness with shale/ sandstone shale sequence in Nayabazar Legship area of sandstone partings of approximately 2 metres thickness West district, in the valley of Little Rangit River. A is exposed. To the SSE of Kamling village, following number of coal seams are exposed on the road linking succession (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 on About 3 km from Nayabazar on Nayabazar- Nayabazar Legship road section: Sombaria road section: Two coal seams are exposed with parting of Coal seam of approx. 4 metres thickness with shale/ sandstones. The continuity of the seams is seen on sandstone partings is exposed in the stream section. opposite side of Rangit River. About 30 metres below Nayabazar Forest Bunglow: Near Rothak Power Station: In toe cutting section of Ramvang River, coal seams Two coal seams (0.8 m and 1.10-1.50 m thick) are are observed but are covered with thick overburden. Near separated by 3.4 m parting of sandstones near the Sise village below Nayabazar-Legship road section, confluence of Rothak Khola and River. Further west of opposite to Karfektar a coal seam of about 5 metres is this section, a few thin coal outcrops are seen. South of traced out but its thickness and lateral continuity could Samsing, near Roong doong Khola and Rangit River not be traced. On the NW slope of Gumpagaon about confluence, a few thin coal exposures are seen varying Namchi Bazar, two coal seams of approximately 2 m in thickness from 0.20 m to 0.90 metres. and 1 metre thickness are exposed. They show continuity 42 GEOL. SURV. IND

along NE direction and one of the seams is exposed on continues over Rothok Khola. It is divided into three Namchi road section about 1 km above Namchi. different seams with partings of shale and sandstone of 5 to 10 metres. The results of analysis of coal by Directorate of Mining & Geology, Sikkim are as follows: Tinher area:

Along old road section connecting Denchong and Max. Min. Namchi there are number of coal exposures. One of them Ash percent 51.7 26.6 is 2.5 metres thick and is exposed about 1 km from Moisture content percent 17.5 12.2 Namchi bazar towards Denchong. Volatile matter percent 3.15 1.4 Fixed carbon percent 60 38 On Kalikhola section below Bumtar: The utility studies of CMPDIL, Ranchi indicate that There are a few coal seams but only three show this coal can be used for room heating and cooking thickness of more than 1 metre. purpose after making briquettes/pellets by adding suitable additives which burn well and lasts for 3 to 4 hours non-stop. On Tiri Khola section: Along Tiri Khola section, SE of Wak village, number DOLOMITE: of coal seams are exposed. They are of limited strike extension and covered with thick overburden. Essentially the dolomite has been recorded in Buxa Formation and the best exposures are found along Naya Bazar-Legship road (along Rangit River) and Coal exposures of Put Khola section: surrounding hills. Both high grade massive and low grade About 200 m above the road along Put Khola flaggy types of dolomites have been located in Rishi section, about 2 m thick coal seam is exposed having area, west of the -Legship state highway. On strike extension of 30 m. along NE direction. either banks of Rishi Khola, adjacent to Rishi village (27o13’N:88o46’E), four dolomite bearing blocks have West of Nandugaon four thin coal outcrops are seen been delineated by geological mapping, pitting and which appear to abut against an ENE-WSW trending trenching by Geological Survey of India. Massive fault. The coal beds are intercalated with carbonaceous dolomites are light grey in colour, fine grained with high shale and vary in thickness from 0.20 m to 0.90 metres. percentage MgO (18% to 22%), CaO being around 30% Two highly weathered coal outcrops are seen about a and insoluble 1.5 to 3%. A total reserve of more than km east of Jorthang dipping 44° towards north-west. one million tones has been estimated by GSI down to a depth of 30m from the surface. The low grade flaggy The coal of Rangit River is flaky in nature and highly type has high silica content and is not suitable for blast friable. They are bituminous to Semi-anthracitic having furnace. However, this type may find selective usefulness high carbon-ash content and low volatile matter. The as powder for neutralization of acidic soil. Dolomite has investigation carried out so far reveals that the coal seams also been recorded at Phong, Kaluk, Rishi area west of of Rishi extends about 1500 m to 1700 m along strike Jorethang-Legship Highway. Preliminary assessment of along Rishi Khola section in east-west direction with occurrences along Rangit River and along Rishi khola little variation in thickness (Sharma, 1992). The coal has been done by Sarkar (1978). seam on the road section is approximately 6 metres in thickness, and runs parallel to Reshi Khola for about Rangit River: 1700 metres on the northern bank. The seam crosses over to the southern bank with more or less same The dolomite is mostly flaggy in nature although thickness. However, in between these two places the coal massive dolomite is also recorded. It is intercalated with seam is not traceable. quartzite. The occurrence is divided into four blocks; three on the northern bank of Rangit River and one on The coal occurrence of the southern bank is the south of the river. The dolomite indicates CaO continuous up to Reshi-Mangalbari road section and varying between 17.85% and 29.50%, MgO varying MISC. PUB. NO. 30(XIX) 43

between 9.05% and 20.70%, R2O3 between 1.28% and biotite phyllite and slaty carbonaceous phyllite, mostly 8.00% and insolubles between 0.86% and 45.98%. As oriented parallel to the foliation planes and are of such this is not a good quality dolomite. Rough estimates irregular thickness and length. Flaky graphite have also been made for this dolomite. concentration has also been noted within the marble bands at Bop in north Sikkim and at the 18th milestone on the Gangtok-Natu La road sections. The Rishi khola occurrence: concentrations of graphite are of very small extent and The dolomite occurs as a thick horizon forming a have sharp contacts with the host rocks. The graphite is ridge and strikes N30°E-S30°W to almost east-west with crystalline to amorphous type. (b) Amorphous graphite moderate dips towards north. This dolomite underlies concentrations, associated with vein quartz and clay the Gondwanas in the west along a thrust boundary. matter is seen mainly along shear and thrust zones and Reserves of approximately 1.1 million tonnes have been are exposed in upper Kali Khola and middle Kali Khola estimated (down to 30 m depth) on the basis of channel area. The zones vary in thickness from one to two metres sampling. The CaO varies from 28.37% to 40.32%, MgO in length up to 15 metres or more. At places, the cavities

from 12.06% to 21.50%, R2O3 from 0.95% to 2.95% within the graphite are filled with box-work of limonite, and insolubles from 0.15% to 3.22 percent. This is a quartz, chert and kaolin. (c) Lump graphite is seen in good quality dolomite. north Sikkim near Rangma, Chungthang and on the Gangtok-Natu La road near the 15th and 18th mileposts. Here graphite is associated with pegmatite, occurring in GRAPHITE: the shear zone between sillimanite-garnet-gneiss and Graphite, both lumpy and flaky type with graphite calcareous suite of rocks. Within the calc-silicate rocks schist, marble and limonitised pegmatite of Chungthang graphite fills scattered pods and fractures. Formation of Darjeeling Group, has been located at Chitre (27°1620:88°0210) and Dareli (27°15: Considering the different types of distribution it can 88°03) of west district. The graphite bands occur even be postulated from occurrence (a) that the original at depths of 2-3m below the surface and the thickness sediments must have been contaminated with varies from 30 cm to 80 cm. The graphite bearing zone carbonaceous material. The crystallization of graphite extended in depth and can be traced over a considerable has taken place by dynamothermal metamorphism due length in a very irregular manner. Preliminary to heat generated by tectonic movement during thrusting, exploration, including limited drilling, was conducted faulting and folding as is evidenced by the occurrence by GSI in Chitre area. An estimated reserve of about (b) Migration of this graphite and its subsequent 6000 tonnes of graphite has been computed from Chitre concentration has been helped by quartz and pegmatite sector. veins as revealed by occurrences (b) and (c). Occurrences of graphite within the marble might have probably taken Impersistent and pockety nature of graphite place due to denudation. occurrences, their inaccessibility and high elevation (3000-4000m) and locations near the India-Nepal border Prospecting for graphite was carried out in Kali have rendered these graphite occurrences uneconomical Khola and Pache Khola area, where some surficial in view of the high cost of the exploration, mining and exposures of graphite could be traced. As the geological transportation of the materials. and structural setting of the graphite zone was known, Insignificant amount of amorphous to flaky variety exploratory trenches were dug along the strike wise of graphite occurs as stringers, lenticles and small lenses extensions of the different graphitic horizons and from along the foliation plane of the high grade biotite gneiss the trenches 73 groove samples were collected covering at some places around Dentam and Bega village. a length of 105 m. The chemical analysis reveals poor concentration of graphitic carbon. Chitre and Dareli (west Sikkim): The graphite mineralization of insignificant nature Occurrences of graphite have been classified into has also been recorded about 13.3 km NW of three groups. (a) In Kali Khola and Pache Khola, flaky Chungthang-Lachen road and 600 m south of Tarun Chu graphite occurs as dissemination in the gritty quartz cutting across the highway (Kumar and Mitra, 1985). 44 GEOL. SURV. IND

The graphite mineralization occurs along some shear of the limestone horizon have 42 to 44% CaO, 1.22 to planes which are at an acute angle to regional schistosity. 1.6% MgO and 11 to 18% insolubles. The marble band, west of Bop village, shows graphite concentration along a shear zone. Traces of graphite are Mangalbaria and Namgaon area: also seen near Theng (27°24:88°39) and Chhanggu. Two exposures of limestones are recorded at Namgaon and four bands of limestones recorded at KYANITE BEARING PEGMATITE: Mangalbaria area. The CaO percentage in limestone

Kyanite bearing pegmatites were observed by Raina varies from 30% to 42%, MgO from 0.5% to 4%, R2O3 (1966) to the west of Gerethang (27°21:88°15) while from 1.3% to 7% and insolubles from 15% to 30%. working in west Sikkim (toposheet nos. 78 A/3, 4, 7, 8). The pegmatites cut across kyanite bearing schists. Seiging area: Kyanite blades are 15 cm × 2 cm × 2 cm in size. The limestone is interbedded with phyllite and slate and is about 35 m thick. The limestone contains 34% to LIMESTONE: 43% CaO, 0.23% to 2.3% MgO, 1.1% to 1.9% R2O3 The Buxa Formation comprises dolomite and and 16% to 25% insolubles. limestone with interbeds of slates and phyllites. The Geological Survey of India and State Department of Mining & Geology have done preliminary exploration North of Nayabazar: work. Grey limestones, interbanded with green phyllites, Raina and Ray (1967) have recorded a thick are observed in Rishi Khola, south of Namgaon. An sequence of highly folded limestone and lime bearing exposure of limestone, about 30m thick, is traceable over purple and green slates in the area north of the Nayabazar a strike length of 60m near Rishi Khola. Selected Bridge. The sequence is traceable from the west bank portions of this limestone band indicate 42 to 46% CaO, of Rangit River up to about 1 km east of it. This formation 1.22 to 2.20% MgO and 12 to 14% insolubles. This outcrops only along the road section, being covered limestone almost crystalline, shows well bedded nature otherwise by soil and forest cover. The sequence along and are fine to medium grained in texture. Pink limestone the road section (Nayabazar-Namchi road) and the with shales is exposed at Nayabazar. Selected portions chemical 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 22.24 No exposures 300 Massive limestone 13 13.27 2.87 44.42 1.65 31.00 Slates with nodules of limestone 6 Massive limestone 16 15.54 3.15 42.98 1.24 35.29 Greyish-green slates 45 Massive limestone 16 10.43 7.30 44.45 1.34 34.46 Slates with thin intercalation of limestone 9 No exposure 150 Massive limestone with intercalation of shale 25 16.66 4.72 42.21 1.03 33.00 Slates with thin limestone bands and nodules 13 Massive limestone with shale intercalations 30 18.57 3.25 40.67 1.51 33.85 Slates with occasional nodules of limestone 45 Greenish-grey shales 13 Green slates with thin pink limestone bands 10 Slumped greenish-grey slates Pebble slates MISC. PUB. NO. 30(XIX) 45

Rishi Khola south of Namgaon: The outcrop is 30 m thick and traceable over a strike distance of 60 m across the stream; the surrounding area Greyish-black to steel grey, hard, compact, partly is covered by soil and forest. Representative analysis of crystalline limestone interbanded with green calcareous limestone band is produced in the table no. 6. slates outcrop in the Rishi stream just south of Namgaon.

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 36.77 Calcareous slates and phyllites Massive limestone 3 13.17 2.04 44.71 1.24 37.00 Calcareous slates Massive limestone 3 12.94 1.44 46.44 1.03 37.54 Limestone with phyllite Massive limestone 3 12.25 2.60 43.00 2.17 33.15 Phyllites with thin bands of limestone Green phyllite

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

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

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

Sm Lat. Long. SiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2OK2OTiO2 P2O5 LOI. No. 46 28°0440 88°4240 10.78 2.26 0.65 0.45 0.03 0.63 47.25 0.06 0.51 0.04 0.05 37.02 46A 28°0435 88°4250 5.73 2.07 0.85 0.73 0.12 2.81 48.09 0.04 0.19 0.03 0.05 38.59 77A 28°0130 88°4445 4.26 1.98 2.36 0.82 0.09 1.56 49.60 0.04 0.16 0.07 0.06 38.59

MARBLE: A. M. N. Ghosh (1952) collected a few samples of marble from Chhangu area, which shows SiO - 16.34% Several bands of marble of variable thickness were 2 to 14.31%, Al O - 3.82% to 3.21%, Fe O - 1.19% to seen near Chungthang, Theng and Naga in north Sikkim 2 3 2 3 and near the 6th & 16th mile posts and near Changgu on 1.24%, CaO- 42.98% to 44.85, MgO- 2.50% to 1.97%. the Gangtok-Natu La road. He concluded that the marble can be utilized for low- heat cement.

Chhanggu area : Raina and Vohra (1969) mapped four marble bands Greyish white to creamy white, coarse grained within a horizontal distance of 600 m (east-west) and vertical distance of 100 m (between 3600 m and 3700 marble bands are reported from about 1.5 kilometer west th th of Chhangu Lake in East Sikkim. The bands vary in m) between the 6 and the 10 loop about 1 km west of thickness from 12m to 42m with average CaO varying Chhanggu. The marble bands are closely associated with from 39% to 73%. sillimanite-garnet-biotite- gneiss and show an almost north-south trend with steep dips. 46 GEOL. SURV. IND

The marble is greyish-white to creamy white, coarse underlain by second band. The second band is 12 m wide grained and almost granulitic in appearance. It shows and shows synformal folding towards the base. Again a characteristic differential weathering. Microscopic 200 m thick biotite-sillimanite gneiss band separates 2nd examination reveals presence of quartz, diopside, band from 3rd band. The third marble band is 18 m thick; scapolite, feldspar, sphene, zircon and biotite besides the western-most fourth band is 30 m wide. Between calcite. Individually the eastern most band is 42 m wide, third and fourth band there is 180 m thick zone of gneiss. followed westwards by 20 m thick zone of banded gneiss Analysis of chip samples from four bands is indicated in 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 0.28 2. 17.19 1.59 43.25 1.11 3. 29.52 1.79 36.80 0.62 4. 32.55 1.73 34.96 0.84

Area around Chungthang: the road. Good exposures are found on the scarp face only. A very preliminary assessment of marble bands of this area was carried out by Kumar and Mitra (1985). Prior to that NEITCO, a consulting agency of SIDICO, Pegong occurrence: Sikkim State had made assessment of these marble This occurrence is located about 5 km south of occurrences. Chungthang on Chungthang-Mangan road near Pegong village. It is a very minor occurrence.

4.5 km NW of Chungthang on Chungthang-Lachen Other minor occurrences of calc-silicate/marble are road: recorded on Singhik-Chungthang road, particularly The occurrence of marble is very much limited and around Myang village and near police check post at Tong only some boulders of marble are seen. village. Marble is also located at Selep, Mensithang, Malten, Theng, Tong, Myong, Chhangu, and Chakung.

Chubinbin nala occurrence: G. N. Dutt (1955) located small calc-tufa occurrences This occurrence is 1 km NW of the above mentioned at an altitude of 8400 ft above the confluence of Benshoi occurrence. It has a length of about 300 m along the and Lachen streams. They appear to have been originated road upto the point where the Chubinbin nala cuts across from the nearby calc-gneisses as seen in one of tributaries the highway. to Benshoi Chhu. In Chungthang-Lachen area of North Sikkim several bands of calc silicate/marble are intimately associated with high grade metasedimentaries South of Tarun Chu: of Chungthang Formation. These calc silicate may be The occurrence is located about 13.3 km north-west used for mini cement factories. of Chungthang-Lachen road and 600 m south of Tarun Chu cutting across the highway. Crystalline marble is associated with calc-silicate, quartzite and calc-granulite SILICA AND SILICA SAND: (Kumar et.al., 1986). Upper middle part of the Everest Limestone Formation in north-east Sikkim is represented by pink and white ortho-quartzite which is 45 metres thick, of Malten Shiva Temple: good quality and considerable strike length (300 m). This occurrence is near Malten Shiva Temple 6 km Following is the chemical analysis of the quartzite. This north-east of Chungthang on Chungthang-Lachung road. quartzite can be used in ferro-silicon, glass and other The occurrence covers a distance of about 50 m along related industries Roychoudhury et al., 1998). MISC. PUB. NO. 30(XIX) 47

Sm.No. Lat. Long. SiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2OK2OTiO2 P2O5 LOI 54-1 28°0030 88°4550 96.38 0.77 0.53 0.18 0.01 0.62 <0.01 0.02 0.11 0.23 0.03 0.53

SILLIMANITE: TUNGSTEN:

Sillimanite occurs as needles (Fibrolite) or as Wolfram has been observed at Chitre. Sills and dykes aggregates in the high grade gneisses in Chitre, Uttare, of tourmaline granite and garnetiferous pegmatites are Dentam, Chhange Khola and Sordung area of West intruded in to banded migmatised biotite psammite Sikkim. Sillimanite occurs as fibrous aggregates mainly gneiss and granite gneiss of Central Crystallines. At along the closely spaced slip planes within the gneiss or places, these granites are found to contain sheelite which along the contact of the pegmatite vein cutting across is concentrated along the joint and fracture planes the foliation plane of the gneissic country rock. Kyanite (Roychudhury et al., 1998) is also found to be associated with sillimanite. The kyanite and sillimanite occurrences in the area are at present not of economic significance. URANIUM MINERALIZATION: Udas (1986) indicated association of uranium mineralization with copper-lead-zinc ores in low grade TALC: schists of lesser Himalayas and in Daling Group of rocks The talc hosted by Mansari quartzite unit at and of Darjeeling and Sikkim. The significant concentration around Mansari village (27°0841:88°1210) is of of uranium have been met with along a major zone of economic importance and was mined by M/s Sikkim intense shearing within the Daling phyllites, parallel and Minerals Pvt. Ltd. through three crude rat hole type close to the Gondwana-Daling thrust. The shear zone is excavations. The talc is lumpy and occurs along fractures traceable from Nepal border in the west to the Bhutan in the host quartzite. The quartzite itself is reportedly of border in the east. The well known copper-lead-zinc at optical grade and was also quarried. A crushing unit was Gorubathan falls in this shear zone. Significant uranium also set up. A preliminary estimate of quartzite and talc concentrations have also been found in the sheared reserves up to 25m depth is a round 15 million tones, Daling phyllites on the foot wall and hanging wall of maximum true thickness of the quartzite being around the Cu-Pb-Zn lode in the Bhotang mine near Rangpo. 65-80m. VERMICULITE: TOPAZ: Quartz vermiculite veins either occur independently In Tista valley, very near to Jorepul (27°42:88°34), in a close proximity of pegmatites in the rocks of a tourmaline bearing granite containing Topaz has been sillimanite and kyanite zone. The rock type associated located (Bhattacharya and Pattanaik, 1964). Auden is granitised mica schist. The thickness of the vein varies (1935) has reported the occurrence of topaz bearing from one cm. to about 30 cm. Conspicuous deposits are granite from a place on the Tista Valley which is nearly found on the north Sikkim road south of Labi, at Tingda, 48 km north of this occurrence. But in this area, the about 4th milestone east of Gangtok and between 8th & occurrence of this mineral is found for the first time. 9th milestone. The topaz crystals are 1.5 cm to 2 cm in length; flesh coloured and irregularly arranged. The zone is 30 cm In addition building stones and sand are also thick and nearly 8 metres long. available. 48 GEOL. SURV. IND

Locality Index

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

48 MISC. PUB. NO. 30(XIX) 49

LATITUDE LONGITUDE Degree Min. Sec. Degree Min. Sec.

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

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

Sipik Sirbong 27 10 35 88 15 30 Siringyong 27 20 88 09 Sisni 27 10 18 88 14 20 Sodunlakha 27 12 00 88 40 00 Somdong 27 09 88 19 Sosing Hill 27 17 88 20 Sumbuk 27 07 88 23 Suntole (Sontale) 27 10 88 16 50 GEOL. SURV. IND

LATITUDE LONGITUDE Degree Min. Sec. Degree Min. Sec. Temi 27 14 00 88 26 00 Thanggu 27 54 88 32 Theng 27 24 88 39 Tigda (6 km of Gangtok) Tolung 27 40 40 88 26 30 Tong 27 33 00 88 38 55 Tukhani 27 08 88 26 Tunga-phu-phiak 27 39 88 28 Uttare 27 16 88 05 Yome Samdong 27 55 00 88 42 00 Yaksam (Yoksum) 27 22 00 88 13 00 Yangdi 27 56 88 31 Yumthang 27 47 30 88 42 00 Zongri 27 27 88 10 MISC. PUB. NO. 30(XIX) 51

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