Diamond-facies chrome spinel from the Tokapal kimberlite, Indrāvati basin, central India and its petrological significance

Abstract

Widespread and abundant spinel is the only primary mineral of petrogenetic significance preserved in the hydrothermally altered, crater-facies, Neoproterozoic (≥620 Ma) Tokapal kimberlite pipe that intruded the Indrā-vati basin, Bastar craton, Central India. Two distinct spinel populations occur: (i) finer-grained (<50 μm) microcrysts which are zoned from titaniferous magnesiochromitechromite to magnetite; and (ii) larger macrocrysts (>400 μm) with cores having distinctly chromium-rich (Cr 2 O 3 up to 63.67 wt%), and TiO 2 -poor (<0.68 wt%) compositions. Based on their morphology and chemical composition the macrocrysts are inferred to be disaggregated mantle xenocrysts and their compositional range extends well into the diamond stability field. However, the reported absence of diamond and other indicator minerals (such as pyrope garnet, chrome diopside and magnesian ilmenite) in the Tokapal pipe is intriguing since diamondiferous cratonic roots are indeed preserved in the Bastar craton, and also the kimberlite itself was derived from a similar source region(s) as that of the well-known diamondiferous Mesoproterozoic (ca. 1,100 Ma) kimberlites from Wajrakarur, Dharwar craton, southern India. Given the large areal extent (>550 ha) of kimberlite, there is a possibility that it contains diamonds but they were not recovered during the sampling. Alternately, highly oxidising conditions imparted by the metasomatic fluids/melts derived from (i) asthenosphere-lithosphere interaction, or (ii) the kimberlite itself might have played an important role in the destruction of diamond, and other indicator minerals.