The Geology and Geochemistry of the Wadagera Kimberlite and the Characteristics of the Underlying Subcontinental Lithospheric Mantle, Dharwar Craton, India

Abstract

The Wadagera kimberlite is situated near the northern bank of the Krishna River in northern Karnataka. It is the largest pipe in a cluster discovered by De Beers during exploration in 2002. The pipe has been dated at 1,083.8 ± 5.3 Ma, which corresponds to published ages of kimberlites in the adjacent Narayanpet cluster. The pipe was discovered during follow-up of an airborne multispectral scanner survey. Ground magnetic and drilling data indicate that the pipe has an area of approximately 6 ha. The kimberlite intrudes the Peninsular Gneiss Complex, and limited petrographic analyses suggest that both coherent and volcaniclastic kimberlites are preserved. Kimberlite surface samples show extreme calcretisation and yield few mantle garnets. Garnet xenocrysts are more abundant in drill samples below 20 m depth. Differential destruction of silicate and oxide minerals is observed, which impacts on the abundance and distribution of indicator minerals in the secondary environment. KIMs recovered from the Wadagera kimberlite reveal the SCLM architecture sampled by the kimberlite. The major element garnet mineral chemistry represents a dominant lherzolitic population. Diamond proxy composition garnets are rare or absent. The well-developed CCGE trend, together with the spinel data, suggests that the kimberlite has mainly sampled fertile mantle at relatively shallow depth. The nature of the SCLM at Wadagera contrasts with that reported further south and south-east on the EDC within the diamondiferous Wajrakarur kimberlite and Banganapalle lamproite clusters. The geochemical data, as well as limited petrographic data, are consistent with the apparent absence of diamond at Wadagera.