Nature of Mineralizing Fluid in Paleoproterozoic Dolomite Hosted Talc Deposits, Cuddapah Basin, India

Dolomite hosted talc deposits of the Vempalle Formation (VF), Cuddapah basin, Andhra Pradesh produces nearly 6% of total talc demand of India. The strata bound talc zones extending over kilometers along strike have tabular geometry and are concordant with the stratification and primary depositional contacts, though sometimes the talc bodies locally cross-cut the stratification. Field and petrographic evidences suggest that the strata-bound talc deposit, located at the flank of the basin, was formed by replacement of platformal carbonate sequences of the passive margin (stromatolitic dolomite), preserving the primary texture and structure of the host. Lack of anomalous concentrations of V, Fe, Ni, Cr and Co; depletion of Ta, Hf, Zr and Th and Ni, Cu, Pb, Zn in the talc and talc bearing dolomites may be indicative of the absence of magmatic and/or metamorphic source for the mineralization fluid. The igneous intrusions within the Vempalle Formation (VF) - earlier thought to be the source of mineralization fluid - thus are neither coeval with talc mineralization nor they acted as the heat source. Furthermore, depletion of REE, Al3+, K+, Na and Sr in the talc and talc bearing dolomites suggest that the mineralizing fluid was probably evolved from Paleoproterozoic ocean water that was super-saturated with respect to talc and had higher dissolved silica concentration. This evolved fluid source can also explain the supply and sustenance of large volume of water needed for the talcification of the host dolomite.