ZONING PATTERN OF JABOR RARE-METAL PEGMATITE BODY, PURULIA DISTRICT, WEST BENGAL

摘要

This paper documents mineralogical, textural and chemical zoning behaviour within Jabor rare-metal (RM)-bearing pegmatite of Purulia district, West Bengal. Among the reported five number of rare-metal-bearing pegmatite bodies along ENE-WSW-trending lineament zone within the Chotanagpur Gneissic Complex (CGC) (others are near Beku, Belamu and Khatanga villages), the Jabor RM pegmatite is the largest one and is fairly exposed on surface. This Precambrian pegmatite body is of LCT type enriched in Rb, Li, Cs rare-earth alkalis and contains minor amount of pollucite (by far the only ore of Cs metal). It is enriched with Rb component in comparison to Li & Cs. Like other LCT pegmatites of the world, it resides within a non-consanguineous granitoid pluton and form anastomising vein and fracture-filling dykes within a granitic host rock. Seven zonal sequences, represented by individual petrographic units were identified on surface outcrop Conspicuous grainsize and textural variations were noted within the pegmatite body from the outer zone to central quartz-rich core. Whole-rock chemistry of individual zonal pegmatite units more or less shows systematic variation of K/Rb from outer zone (=58.4) to inner zone (=28.5). The Jabor RM pegmatite is of peraluminous nature (P.A index varying from 1.12 to 1.92) and possibly evolved from the adjoining composite granite gneiss showing peraluminous chemistry.