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The coal bearing Gondwana and associated rock groups of the eastern India coalfields are intruded by innumerable dykes and sills of lamproitic and lamprophyric compositions. These early Cretaceous intrusives intrude mostly the Barakar Formation as sub-vertical dykes or sills trending E-W or ENE-WSW and at places display conspicuous thermal metamorphism by completely de-volatilising the coal seams.
The lamproites are considered to be the products of deepest levels of continental alkaline magmatism on the basis of the occurrence of diamond in some of these rocks and the presence of entrained mantle derived xenoliths. The geochemical characteristics of lamproites suggest that their mantle source regions must be enriched in K, light rare earth elements (LREE), Ti, Ba, P, F and other incompatible elements. The present study carried out on the samples from Bokaro, Raniganj, and Jharia coal fields reaffirms the mineralogical characteristics like i) silica under saturation ii) ultra potassic nature i.e high K2O content (Max- 7.5 %) iii) presence of phlogopite mica of possible mantle origin iv) high Mg # (MgO/ FeO + Fe2O3 ) v) presence of carbonates and hydrous minerals vi) absence of plagioclase feldspar vii) high TiO2 content and high abundance of Ba, Sr, Th and REE with pronounced fractionation trend. The petrographic studies, mineral chemistry, major and trace element data and experimental petrological studies on natural systems at upper mantle conditions demonstrate that the co-magmatic suite of lamproites, ultramafic lamprophyres and minettes have crystallized from magmas derived by partial melting of mica bearing harzburgite source rock of mantle origin and they are essentially related to lamproitic assemblages by heteromorphic reactions in presence of water under mantle conditions.
The lamproites are now considered as the only non kimberlitic rocks of proven economic value. Since the discovery of diamonds in these rocks from Argyle, Western Australia, the economic potentiality of these rocks needs to be critically examined .Geochemical prospecting, GM survey, airborne magnetic survey and EM survey along with geo-thermometric and geo barometric studies are the requisites to elucidate the P-T paths and to fully understand the genesis of potassic magmatism and thereby indirectly inferring about the economic importance of these intrusives,
Keywords : lamprophyres, lamproites, minettes, harzburgite
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