Geology, petrology and geochemistry of Narayanpet kimberlites in Andhra Pradesh and Karnataka

Apart from the well known occurrence of kimberlites in south India at Wajra karur in Anantapur district, kimberlites' were located by Geological Survey of India (GSI), during 1984-85 around Maddur and Narayanpet areas in Mahab ubnagar district of Andhra Pradesh. Landsat imagery and photo-interpretatio n studies followed by intense geological traverses including stream sedimen t surveys, launched during 1994-96, led to the discovery of 20 additional k imberlites in the western part of Mahabubnagar district, Andhra Pradesh and in the adjoining Gulbarga district, Karnataka, thus reporting kimberlite f or the first time from the state of Karnataka. A total of 30 kimberlites ha ve been reported in this field named as "Narayanpet Kimberlite Field" (NKF) . The kimberlites are emplaced along E-W and NW-SE trending major faults and associated NE-SW tear faults in the Peninsular Gneissic Complex carrying en claves of Dharwar greenstones of Gadwal schist belt. These kimberlites occu r both as small bodies of a few metres across and also as dykes with a stri ke length of about 2 km. The characteristic mineral assemblages and textures indicate that these bod ies correspond to hypabyssal facies or root zone kimberlites. The indicator mineral suite present in these rocks are olivine, chrome diopside, magnesi an ilmenite, chrome spinel, and rare pyrope garnet. Mantle xenoliths are ra re, nevertheless spinel Iherzolite; abundantly found in two kimberlites. All the kimberlites of NKF are depleted in SiO2 (similar to 35%), potassic (K2O >Na2O) and magnesium rich (MgO >20%). The REE abundance of these kimbe rlites is consistent with their derivation by partial melting of mantle and subsequent fractional crystallisation of the melt. The similarity of petro logical and geochemical characters of these kimberlites indicate that all t he kimberlites of NKF are derived from similar source in the upper mantle. Preliminary processing and testing of some of the kimberlite bodies have no t yielded any diamonds so far. However, the reported shay diamond finds in the Krishna and Bhima River gravels indicate the presence of diamondiferous kimberlites further south and southwest of the area.