Chrysoberyl Pegmatites of South Kerala and their Metallogenic Implications

Abstract

Recent investigations prove that the southern part of Kerala and the adjoining areas further to the south in Tamil Nadu constitute a chrysoberyl-pegmatite field in the granulite terrain of southwestern India. Gem stones like chrysoberyl and its varieties, sapphire, topaz and zircon are mostly derived from pegmatites, though some come from the alluvial beds also. Chrysoberyl-pegmatites occur in a predominantly garnet-sillimanite ± graphite gneissic terrain with intermittent bands of garnet-biotite gneiss and enderbites. Geochronological data reveal that the pegmatites were emplaced 445-474 m.y. ago. Fluid inclusion studies show that the pegmatite minerals contain liquid carbon dioxide and give an entrapment temperature of 500-570°C. CO₂ densities permit a pressure estimate of 250 Mpa and more, which is also consistent with the sillimanite-andalusite and quartz-chrysoberyl associations in the pegmatites. The similarities in geology, the ages of pegmatites and granites and also the mineral composition of pegmatites of southwestern India and the alluvial gem gravels of Sri Lanka suggest that the al1uvial gem beds of Sri Lanka may also be of pegmatitic origin.