Granitic Pegmatites of Koradi-Kolar Sector, Nagpur District, Central India: Field, Petrographic and Mineralogical Features

Abstract

Several bodies of pegmatites have been located within the rocks of Sausar Group (Precambrian) occurring in the Kolar valley, north of Koradi, in Nagpur district of Central India. The XRD and the chemical data have shown that these pegmatites are mostly potassic (K 2 O= 15.58 to 16.31 % and Na 2 O=2.50 to 3.12 %) and are barren of economic mineral deposits. The pegmatites range from a few cm to more than 50 m in width with a maximum extension of about one kilometre. Several mappable pegmatite bodies trend roughly EW and NS, and many good outcrops are encountered in the Kolar Valley and Suradevi Hills. Based on the field and petrographic studies, the pegmatites of this area are characterised in terms of size, shape and mode of emplacement. They are mostly tabular, lenticular, branching and irregular bodies, localised along bedding, foliation, fault planes, tension joints and fold axis. Structurally they can be subdivided into concordant and discordant types on the basis of their relationship with the country rocks. Mineralogically, pegmatites are grouped into (1) Simple Pegmatite, containing quartz, feldspar, biotite, apatite, garnet and (?) monazite, and (2) Complex Pegmatite comprising in addition muscovite, tourmaline and epidote. However, common mineral variation is observed at different places within the same body of pegmatite. In these pegmatites, muscovite, quartz, tourmaline and feldspar seem to have developed mainly in two distinct generations, for example, the muscovite of earlier generation forms tiny "books" while later crystals occur as small disseminated flakes in the groundmass of less coarser quartz. The earlier quartz is strongly deformed, while the later occurs as veins of variable dimensions. The black tourmaline of earlier generation is represented by schorlite and of later origin has been identified as dravite. It may be concluded that these pegmatites were formed by the influx of fluids rich in silica, Al and K that filled the fractures and other planes of weakness (foliation) in the host rocks. The alkali components while migrating towards the wall of the country rocks, crystallised as muscovite and feldspar. In larger pegmatite bodies of Suradevi Hills, the development and concentration of muscovite along their contact with the mica schist is probably due to the migration of Al and K and volatiles towards the border of the intrusion. This crystallization of muscovite in preference to potash-feldspar may be due to the presence of high contents of volatiles.