Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

R. Islam ¹, K. S. Bist ¹, B. K. Chaudhuri ¹, B. S. Rawat ¹, K. K. Purohit ¹

Affiliations
1 Wadia Institute of Himalayan Geology, Dehra Dun-248001, IN

Abstract

A maiden attempt has been made to study the far-flung Hanuman Tibba Granites of Kullu valley, Himachal Pradesh. The granite has intruded the low grade Haimanta (-Lahaul) Formation which tectonically overlies the Rohtang Gneissic Complex. Integrated field, petrographic and geochemical studies have revealed that it is peraluminous S- type granite derived from sedimentary protolith. Compositional difference in terms of depletion of Fe, Mg, Ti, Ca, Sr, Zr and enhancement of Sit K and Rb from margin to core of the granite body is consistent with fractional crystallization process.

The emplacement of Hanuman Tibba Granite on the same tectonic level as Jispa and Ratilaman granite (500±ma) in adjacent Lahaul-Spiti region suggests a Cambro-Ordovician age for the Hanuman Tibba Granite.

Keywords

Petrology. Geochemistry, Hanuman Tibba Granite (HTG), Haimanta Formation, Kullu Valley, Himalaya.

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V. V. Sesha Sai ¹, B. K. Chaudhuri ², R. N. Ghosh ³

Affiliations
1 Petrology Division, Geological Survey of India, Southern Region, Bandlaguda, Hyderabad - 500068, IN
2 Operations West Bengal, Geological Survey of India Complex, Korunamayee, Salt lake, Kolkata - 700 091, IN
3 39-B, Govinda Auddy Road, Kolkata - 700027, IN

Abstract

The volcano-Sedimentary assemblage of rocks belonging to the Singhbhum Group of Proterozoic age is exposed around Babaikundi, in Ranchi district of Jharkhand state. The east-West trending Tamar-Porapahar shear zone passes north of Babaikundi area. The Chotanagpur Granite Gneissic Complex (CGGC) is to the north of the area, while the rocks of the Dalma Volcanic Belt (DVB) are exposed towards the south. The rocks of the area have undergone polyphase deformation and subjected to metamorphism ranging from greenschist to lower amphibolite facies.

Gold mineralisation is of epigenetic type and is mainly confined to the lensoidal sheared quartz vein traversing the litho-Contacts of the quartz-Mica schist and amphibolite of the Singhbhum Group. The attitude of the mineralised quartz vein is in conformity with the regional schistosity trending ENE-WSW. At the footwall contact of the mineralised quartz vein extensive wall rock alteration halos are observed in the amphibolite unit in the fOrm of pervasive biotitisation, carbonatisation, sericitisation, chloritisation, sulphidisation and Silicification. Ore microscopic studies of the mineralised quartz vein reveal that the native gold is associated, at places, with pyrrhotite and sphalerite. Pyrite and chalcopyrite along with magnetite are also present within the mineralised quartz vein. The paper presents the salient details on the occurrence of primary gold mineralisation in Babaikundi area.

Keywords

Gold Mineralisation, Singhbhum Group, Babaikundi, Jharkhand.

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A. Acharyya ¹, S. Ray ², B. K. Chaudhuri ², S. K. Basu ³, S. K. Bhaduri ⁴, A. K. Sanyal ⁴

Affiliations
1 Geological Survey of India, 27 Jawaharlal Nehru Road, Kolkata - 700 016, IN
2 Geological Survey of India, Op WB-SK-AN, Eastern Region, DK-6, Salt Lake, Kolkata - 700 091, IN
3 Geological Survey of India, Flat#4, H/31M, BP Township, Kolkata - 700 094, IN
4 Geological Survey of India, Chemical Division, Eastern Region, DK-6, Salt Lake, Kolkata - 700 091, IN

Abstract

The South Purulia Shear Zone (SPSZ) is a part of the conspicuous -E-W to -ESE-WNW trending Tamar-Porapahar lineament. This borders the Meso-Proterozoic greenschist facies lithopackage of the Singhbhum Group (SG) comprising metasedimentaries, felsic volcanics, mafites/Ultramafites, granitoids, and alkaline/Carbonatite suite of rocks bounded by the Chhotanagpur Gneissic Complex (CGC) attaining amphibolite to granulite facies in the north CGC presumably had certatn loci of crustal weakness where pre-Rift crustal sagging led to the deposition of supracrustal rocks. Intracratonic rifting had then initiated in those pre-Existing zones of crustal weakness and troughs of the resident crust. The intracratonic basin was covered by huge volume of the felsic volcanics represented by layered tuffaceous volcaniclastic, felsic ignimbrites, crystal-Vitric tuff and tuffaceous phyllite. Meta-Greywacke has been considered as epiclastic deposit derived from felsic volcanics. The nature of welding, absence of phenocrysts typical of lava flows, parallel aligned lenticular pumice fragments exhibiting pumice-Lenticle foliation, fiamme and eutaxitic fabric and petrochemistry could imply that the felsic igniimbrite was close to welded pumiceous ash-Flow tuff having dacitic to rhyodacitic composition. Mafic-ultramafic suites of rocks in close proximity to felsic volcanics suggest a bimodal character of magmatic episode in the area Bands and lenses of tourmalinite disposed along the contact of the CGC and the SG, argued to be of volcanogenic sedimentary origin, possibly demarcates the rupture locales of a rift. The alkali-Feldspar granite(± richterite, ± aegirine-augite), syenite and carbonatites suggest tapping of deeper fractures in the rifted basin. This was coupled with the presence of granite plutonism in the setting. Geochemical signatures imply that felsic volcanics are chiefly of dacitic composition with strong LREE enrichment and narrow range of fractionation while the metabasic rock chemistry scatter in basalt to basaltic andesite field. Trace element data further corroborate 'volcanic arc granite', ' Within plate granite' and 'continental arc granite' composition for the felsic volcanics. The basin was then subscribed to compressional regime where ductile shear zones developed syn to post kinematic to F1 with the northern block (CGC) thrusting over the southern block (SG) on northerly dipping planes. Thus, an entire sequence of rifting, tourmalinite emplacement, felsic volcanism, mafic extrusion, granite plutonism, syenite/Carbonatite emplacement and sedimentation of pelites/Epiclastics during and after opening of the basin in an intracratonic setting is demonstrated. Subsequently, development of the shear zone (SPSZ) took place over the area at the close of the basin.

Keywords

Ignimbrite, Dacite, Tourmalinite, Rift, Shear, Petrography, Geochemistry, Purulia, West Bengal.

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