Quartz-Pebble Conglomerates of Chikmagalur Area, Karnataka, India: Implications on Uranium Concentration

The Quartz - Pebble Conglomerates (QPC) represent the angular unconformity prevailed between the older group of Sargur rocks and younger Dharwar volcano-sedimentary association in Chikmagalur area of Karnataka state. The older group of rocks consist of amphibolites, Peninsular gneisses, granites, quartzites and quartz-sericite schists whereas the younger group has amphibolites, quartzites, phyllites, chlorite schists, differentiated volcanic rocks and banded iron formations. The QPCs have rutile, ilmenite, pyrite, pyrrhotite, chalcopyrite and rarely uraninite as the major opaque phases. Gamma ray spectrometry, autoradiography and ore petrographic studies have shown that the concentration of uranium in the QPCs is due to solution redeposition of uranium by adsorption or absorption on rutile generated by the decomposition of ilmenite with hydrous iron as another product. The genesis of pyrite is described through a polyplylitic process . The oxidising environments prevailed in Pre-cambrian might have promoted the transportation of U+6, S+6 and Cr+6 and were concentrated in reducing pockets of Bababudan as U+4, S−2 and Cr+3. Cr+3 is represented by fuchsite mica abundant in pyrite rich zones. The scarcity of uraninite is due to the destruction of these grains by oxidation during transportation.