The Tokapal Crater-Facies Kimberlite System, Chhattisgarh, India: Reconnaissance Petrography and Geochemistry

The crater-Facies kimberlite system at Tokapal occurs within the Late Proterozoic chemoclastic cover sequence of the Indravati Basin on the Meso-Proterozoic/Archaean Bastar craton in central India. The pyroclastic lapilli tuffs consist of pseudomorphs of olivine macrocrysts and juvenile lapilli set in a fine-Grained talc-Serpentine-Carbonate matrix with locally abundant spinel and titanite. No garnets or diamonds have been found so far. The inequigranular and locally bedded texture is largely preserved, although the olivine component has been completely destroyed. The multiple kimberlite system has about circular shape (2 5 km in diameter) and is probably the oldest and largest crater-Facies kimberlite system known in the world. The satellite Bhejripadar system of similar petrographic and chemical characteristics is situated about 4 km to the northwest and has a few hundred meters of diameter.

Samples from both systems have mostly moderate degree of contamination( 35-50 wt% SiO₂, <4 wt% A1₂O₃,) and Mg numbers of 82-89. The trace element patterns are typical of kimberlitic rocks (REE, Cr, Ni, Nb, Zr), with mobile elements strongly leached due to intense postmagmatic alteration and weathering (Ba, Sr, Rb, alkalis). The Nd isotope composition of E_Nd around 2 (T=1100 Ma) is diagnostic of archetypal (Group I) kimberlites and similar to the diamondiferous Majhgawan pipe with E_Nd 0 and the kimberlites of southern India with E_Nd2 (T=1080 Ma). The Early Paleozoic diamondiferous Kodomali kimberlite pipe from the nearby Mainpur kimberlite field also has E_Nd1 to 2 (T=480 Ma), suggestive of a slightly depleted to undifferentiated asthenospheric mantle source for all Indian kimberlitic rocks.