Illite Crystallinity Indices and Their Significance in Anchimetamorphism and Mineralisation in the Cumbum Formation (Cuddapah Supergroup), Andhra Pradesh

Phyllites from the Cumbum Formation are exclusively made up of 2 M polymorph of illite. The 2 M polymorph, the most stable phase in the temperature range of 200-300°C, is comidered to be due to its heritage from the source area, having been derived from high-grade metamorphic Eastern Ghat and Dharwarian provenances. The Illite Crystallinity Jndex shows progressive improvement with depth as well as nearness to the Pb-Zn mineralised zone. The crystallinity Index averaging 0.4°2θ is consistent with anchizone of metamorphism. The progressive increase in the crystallinity and size with depth and in the proximity of the mineralized zone suggest that the mineral suffered post-depositional recrystallisation effected by the depth of burial, uplift and folding in the eastern margin of the Cuddapah basin and subsequent hydrothermal activity concomitant with the Pb-Zn mineralisation at Bandlamotu.