Petrography and Geochemistry of the Krishna Lamproite Field, Andhra Pradesh

Search for primary source rocks of diamond in close proximity of the well known alluvial gravel workings for diamonds along the palaeochannels of the River Krishna, has resulted in the discovery of a lamproite field comprising several lamproite clan rocks in parts of Krishna and Nalgonda districts of Andhra Pradesh (South India). These rare rock types are defined in this work as 'lamproites', on the basis of their occurrence in a unique tectonic setting and containing one or more of the typomorphic minerals and having the requisite rock geochemical characters of members of the suite as a whole. These bodies with an exotic and diagnostic mineralogy, composition and textures occur in a complex tectonic setting with unique intrusive history. The discovery of this lamproite field is considered very significant from the point of diamond exploration as it is located very close to the famous Golconda Mines along the banks of the River Krishna which yielded many of the celebrated diamonds in the past. This field is christened "Krishna Lamproite Field", in view of the proximity of the lamproite occurrences to the Krishna River having the alluvial placer diamonds. These rocks are emplaced along the eastern margin of the Precambrian Dharwar Craton, at the contact between the northeastern periphery of the Proterozoic Cuddapah Basin and the Archaean crystalline basement rocks, in the form of dykes (usually in clusters) and plugged conduits. The emplacement is controlled by major faultslfractures, trending in NNW-SSE and WNW-ESE directions and occur closely associated with mafic (dolerite) dykes and followed the same channels for their emplacement. The lamproites contain glass, numerous vesicles, and xenoliths of the basement granite-gneiss, dolerite and probably platformal sediments of the Cuddapah Basin, besides exhibiting fragmental and pyroclastic textures supgestive of near-surface emplacement. Radiometric age determinations suggests emplacement of these lamproites took place around 1224±14 Ma, probably during the reactivation of major crustal lineaments coinciding with the uplift of the mid-Proterozoic Nallamalai sediments of the Cuddapah Basin.

Diopside, phlogopite, tetra-ferri phlogopite, richterite, sanidine, apatite and pseudomorphs of leucite, enstatite and olivine are the major primary constituent minerals present. Diopside-leucite-lamproite, phlogopite-leucitelamproite, olivine-lamproite and richterite-phlogopite-lamproites are the variants. The bodies are ultrapotassic (molar K₂O/Na₂O >10), and contain high TiO₂, P₂O₅, K₂O, MgO, Ba, Sr, Zr, Cr, Ni, La and Ce, pointing to crystallisation from typical lamproite magma. The presence of mantle peridotite xenoliths and xenocrystic minerals such as chromediopside, spinel and garnet indicate deep seated mantle derivation of the Krishna lamproites.