The crater-facies kimberlite system of Tokapal,Bastar District,Chhattisgarh,India

摘要

Discovery of diamondiferous kimberlites in the Mainpur Kimberlite Field,Raipur District,Chhattisgarh in central India,encouraged investigation of similar bodies in other parts of the Bastar craton.The earlier known Tokapal ultramafic intrusive body,located beyond the 19-km milestone in Tokapal village along the Jagdalpur-Geedam road,was reinterpreted as crater-facies kimberlite.Its stratigraphic position in the Meso-Neoproterozoic intracratonic sedimentary Indravati basin makes it one of the oldest preserved crater-facies kimberlite systems.Ground and limited subsurface data (dug-,tube-wells and exploratory boreholes) have outlined an extensive surface area (>550 ha) of the kimberlite.The morphological and surface color features of this body on enhanced satellite images suggest that there is a central feeder surrounded by a collar and wide pyroclastic apron.Exploration drilling indicates that the central zone probably corresponds to a vent overlain by resedimented volcaniclastic (epiclastic) rocks that are surrounded by a 2-km-wide spread of pyroclastic rocks (lapilli tuff,tuff/ash beds and volcaniclastic breccia).Drill-holes also reveal that kimberlitic lapilli tuffs and tuffs are sandwiched between the Kanger and Jagdalpur Formations and also form sills within the sedimentary sequence of the Indravati basin.The lapilli tuffs are commonly well stratified and display slumping.Base surges and lava flows occur in the southern part of the Tokapal system.The geochemistry and petrology of the rock correspond to average Group I kimberlite with a moderate degree of contamination.However,the exposed rock is intensely weathered and altered with strong leaching of mobile elements (Ba,Rb,Sr).Layers of vesicular fine-grained glassy material represent kimberlitic lava flows.Tuffs containing juvenile lapilli with pseudomorphed olivine macrocrysts are set in a talc-serpentine-carbonate matrix with locally abundant spinel and sphene.Garnet has not been observed,and phlogopite is very rare.Very limited microdiamond testing (two 18-kg samples) proved negative;however,the composition of chromite grains indicate crystallization in the diamond stability field.