Gemstone Mineralization in Southern Kerala, India

Within the gem field in southern Kerala and adjacent Tamil Nadu region, extending over an area of 70 X 35 sq km, a variety of precious and semi-precious stones occur such as chrysoberyl (cymophane or "eat's eye", and alexandrite varieties), ruby, sapphire, emerald, topaz, zircon, and amethyst. The primary mineralization occurs within zoned, complex pegmatites of Pan-African age, emplaced within granulite facies metapelites (khonda1ites), and variably weathered and lateritised. Gemstone deposits of secondary type are associated with stream gravels and placers.

Preliminary fluid inclusion results provide evidence for the involvement of CO₂-enriched felsic melts which interacted with the aluminous supracrustals in the formation of the pegmatite-hosted gem mineralization. Thennal decrepitation offluid inclusions under high vacuum, extraction and quantification of the fluids from inclusions indicate the presence of variable amounts of CO₂ in quartz (72.7 ppm), garnet (141.2 ppm) and chrysoberyl (51.6 ppm). The carbon isotopic composition of CO₂ in quartz indicates a 'Juvenile" magmatic nature, with δ¹³C va1ue of -8 per mil. The gem-bearing pegmatites may therefore represent cryptic pathways through which large volumes of felsic melts migrated, a process which can be correlated to deep-seated extension in the Gondwana crustal fragments during the PanAfrican.

Gem recovery in Kerala, despite vast untapped potential, is largely done through illegal means. We emphasize the need for a gem industry in the State, and the requirement to bring the local prospectors under the fold of artisans so as to make use of their experience and expertise, in gem prospecting.