Petrology and geochemistry of the diamondiferous Jamnidih occurrence, Bastar Craton, Central India: Metabasalt and not a kimberlite

Abstract

We have re-visited the ‘diamondiferous kimberlite’ from the Jamnidih area of Mahasamund, Bastar Craton, Central India. This study presents detailed field observations, petrographic and geochemical studies of Jamnidih rocks in view of the reported occurrence of diamonds from it. Megascopically, the Jamnidih rocks are fine grained, yellowish green colour and fragile in nature and lack any visible phenocrysts or xenocrysts. Petrography reveals that chloritization and sericitization has altered much of the primary mineralogy. Feldspar and pyroxene are the identifiable major phases with imprints of alteration and development of feeble foliations at places. Lack of inequigranular texture (two generations of olivine) and kimberlitic indicator minerals viz. chromium- and pyrope-rich garnets, chrome-bearing diopsides, and magnesian-ilmenites are characteristic of Jamnidih rocks. This aspect is also reflected in the geochemistry of these rocks which is entirely different from that of the well-established kimberlites (Wajrakarur field, Eastern Dharwar Craton, southern India), orangeites (Behradih orangeites, Bastar Craton, central India) and transitional kimberlites (Tokapal kimberlite, Bastar Craton, central India). Geochemically the Jamnidih rocks display silica-saturated, and alumina-rich nature and depletion of Ba, Co, Ni, V, Nb, Hf and other metasomatically mobilised elements. Petrography (lack of inequigranular macrocrystic texture), mineralogy (paucity of kimberlite typomorphic phases) and bulk-rock geochemistry (extreme impoverishment of high field strength elements, presence of negative Eu anomaly in chondrite-normalized plot) testify to them to be meta-basalts and exclude their affinities to the bonafide Group I or II or even transitional varieties of kimberlites. In this context, the reported occurrence of microdiamonds in these rocks is clearly anomalous and assumes significance.