Occurrence of Spodumene in the Rare Metal-Bearing Pegmatites of Mariagalla-Allapatna Area, Mandya Dist., Karnataka

Abstract

Spodumene bearing rare metal pegmatites are reported mostly from Karnataka in contrast to lepidolite and amblygonite bearing pegmatites described from major mica-fields of India. Tantalite (Ta₂O₅ 50-70%) rich pegmatites intruding the Sargur metasediments of Nagamangala schist belt near Marlagalla- Allapatna are also rich in spodumene (10-15% by vol.) as well as beryl (10.6-11.4% BeO). Light green spodumene (Li₂O 6.55-7.35%) occurs randomly as prismatic crystals a few cm to 30 cm in length. Under the microscope, symplectic intergrowth between spodumene and quartz along the contact with albite and replacement of spodumene by quartz and muscovite are noticed. The 'd' spacings and intensities obtained from the X-ray diffraction patterns match well with the published values for Alphaspodumene from Pala San Diego County, California, U.S.A. Some variations observed in the intensities ofinterplanar spacings could be due to preferred orientations exhibited by the mineral. When heated to 1000° C for 3 hours, the Alpha-spodumene transformed to Beta- spodumene.

Chemically these spodumenes are "high iron non-chromian" types (FeO+Fe₂O₃ 1.17 to 1.29%, Cr47- 87 ppm). While Mn content is high (1237-1826 ppm), other trace elements such as V, Cu, Co, Pb and Be are less than 10 ppm, Cs 39-98 ppm and Sn 19-28 ppm. Sn values are considerably low as compared to specimens reported from elsewhere, possibly because these pagmatites are devoid of cassiterite. Mineralogical assemeblage ofspodumene-tnntalite-pollucite in these pegmatites places them in "LCT (Lithium-Caesium-Tantalum) family" of rare element pegmatites as per classification of Cerny. Petromineralogical and geochronological studies of Allapatna granites have been undertaken which will throw furthur light on the geochemistry and paragenesis of these pagmatites. Few hundred tonnes of spodumene are expected to be recovered commercially from shallow depths alone along with tantalites and beryl.