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

S. K. Som ¹, K. C. Bandyopadhyay ², R. K. Prasad ³

Affiliations
1 Earthquake Geology Division, Eastern Region, DK-Block, Sector-11, Salt lake City, Kolkata - 700 091, IN
2 H R D Division, 27, J L Neheru Road, Kolkata - 700 016, IN
3 Ranchi Institute, Ranchi Centre, 433 A, Road No 5 , Ashoknagar, Ranchi - 834 002, IN

Abstract

The cesium bearing aplite and pegmatite in the southern slope of Belamu hill occur in association with calc-Silicate and granite gneiss country rocks of Chhotanagpur Gneissic Complex (CGC). The hydrothermal units are highly peraluminous and tectonically disturbed in which, cesium is mainly incorporated within pollucite and Li-Muscovite Rb/Sr shows sympathetic relationship with cesium having a strong correlation towards the higher end for both aplite and pegmatite. Cesium resource evaluation by ground survey and shallow drilling has been done for aplite and pegmatite. Reserves and grades established for both aplite and pegmatite have shown concoidance with the statistical laws of distribution, suggestive of productive output.

Keywords

Cesium, Aplite, Pegmatite, Chhotanagpur Gneissic Complex, Belamu, West Bengal.

Full Text

     

S. K. Som ¹, K. C. Bandyopadhyay ¹, S. K. Basu ¹, D. K. Santra ¹, R. N. Ghosh ²

Affiliations
1 Geological Survey of India, Op. WB-S-A&N, GSI Office Complex, DK-Block, Sector 11, Salt Lake City, Kolkata -700 091, IN
2 Geological Survey of India, Zorem building, Nongrim Hills, Shillong - 793 003, IN

Abstract

The Li-Cs-Rb bearing Beku pegmatite occurs in a calc-silicate rock enclave within the Proterozoic Chhotanagpur Gneissic Complex (CGC). Its internal structure shows zonation that comprises an outer tourmaline-rich pegmatite and inner rare metal-bearing pegmatite containing spodumene-pollucite-lepidolite. The pegmatite is strongly peraluminous with high SiO₂, low Fe₂O₃(t), CaO, TiO₂, P₂O, and variable Al₂O₃, K₂O and Na₂O. Its Cs/Rb and Li/Rb values increase towards the centre and decrease towards the boundary. K/Rb value increases from pollucite-bearing pegmatite to tourmaline-rich pegmatite, depicting the more evolved nature of the pollucite-bearing pegmatite. Petrography and geochemistry of the Beku pegmatite suggest fractionation from a peraluminous S-type granitic magma. It is synchronous with shear development in the area, under a minimum formational P-T of 4 kbar and 500°C.

Keywords

Rare Metal (Li-Cs-Rb) Pegmatite, Calc Silicate Rock, Proterozoic, Chhotanagpur Gneissic Complex, Beku, West Bengal.

Full Text