Insight into Proterozoic organic activity and uranium mineralisation in Vempalle dolostone, Cuddapah Basin, Andhra Pradesh, India

摘要

Carbonate sedimentary rocks like limestones, dolomites are considered to contain less uranium among all the rocks of earth's crust due to the inhibitory effects of carbonate and bicarbonate ions and oxidizing conditions of precipitation. These are the factors responsible to keep uranium in solution. Dolostone of Vempalle Formation around Tummalapalle hosts large uranium deposit of more than llakh tonne with an average grade of 0.045%. This is a unique type of uranium deposit in the world and the largest deposit in India. The total organic carbon (TOC) content in the host dolostone is not high because of diagenetic effects but stromatolites, framboidal pyrite have given clue in favour of microbial activities. The early Proterozoic Vempalle rocks contain extensive accumulation of stromatolites form in carbonate platform. The signatures of such microbial activities and their relationship with uranium mineralisation is the main topic of discussion in present context. The organic matters (OM) derived from such microbes or blue green algae have played significant role in uranium mineralisation and diagenesis of host rock. Therefore, the detailed studies of the Proterozoic organisms have been done logically to establish the paleoclimatic conditions and genesis of uranium deposit of Tummalapalle area. The objectives are fulfilled on the basis of critical analysis and interpretation of both inorganic and organic geochemical data, stable isotope and microscopic observations along with field work. The studies established that the OM was derived from prokaryotic blue green algae and played one of the major roles in uranium mineralisation and diagenesis of host dolostone.