A Spectrum Of Potentially Diamondiferous Lamproites And Minettes From The Jharia Coalfield, Eastern India
Journal Article: A Spectrum Of Potentially Diamondiferous Lamproites And Minettes From The Jharia Coalfield, Eastern India
AbstractAbundant Cretaceous (approximately 110 Ma) lamprophyric sills in the Gondwana coalfields show a comagmatic continuum from olivine-lamproites to lamproites and minettes. Four sills in the Jharia coalfield have exotic mineralogy, dominated by olivine (Fo85-92) and phlogopite (5-10% TiO2, 5-13% Al2O3), with subordinate potassium (titanian) richterite and/or arfvedsonite, and spectacularly zoned Sr-rich apatite (up to 20% SrO). Accessory to minor phases include clinopyroxene (titanian diopside to aegirine-augite), K-feldspar (Or> 96), variable spinel (up to 47% Cr2O3), perovskite, niobian ilmenite, niobian rutile, sphene and Ba-poor priderite. Complex primary carbonate assemblages involve breunnerite, ferroan dolomite, witherite, alstonite and strontianite. The whole rocks show low Al2O3 (5-7%) combined with high K2O (4-5%), MgO (up to 30%) and Cr (up to 1500 ppm), plus extreme TiO2 (up to 8.5%), Sr, Zr (up to 6000 ppm each), Ba (up to 8000 ppm), and LREE (Ce up to 3000 ppm). In many rocks, K/Al (molecular) > 0.7, (Na + K)/Al > 1, and K/(Na + K) > 0.6. Radiogenic isotopes (ΕSr + 7 to + 21 and ΕNd - 1.5 to - 3.1) imply derivation from a metasomatized lithosphere mantle source with time-integrated Rb/Sr and Sm/Nd slightly higher than Bulk Earth. Intensive carbonation (up to 14% CO2) in some rocks may reflect fluids derived from the magmas themselves, from carbonatite-like fluids, and perhaps from coking of the invaded coal seams. Minor crystal fractionation within sills as little as approximately 2.5 m thick has yielded complementary olivine-lamproites and feldspathic lamproites; the latter grade ultimately into minettes via increase of Al2O3 and decrease of MgO. Positive indicators of diamond potential in these rocks include their bulk compositional similarity to diamondiferous olivine-lamproites elsewhere (e.g., Argyle), their Cr-rich spinels, and perhaps their tectonic position; negative factors include their quiet emplacement, the noted differentiation, and the associated coking of the coal seams.
- N. M. S. Rock, B. J. Griffin, A. D. Edgar, D. K. Paul and J. M. Hergt
- Published Journal
- Journal of Volcanology and Geothermal Research, 1992
- Not Provided
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N. M. S. Rock,B. J. Griffin,A. D. Edgar,D. K. Paul,J. M. Hergt. 1992. A Spectrum Of Potentially Diamondiferous Lamproites And Minettes From The Jharia Coalfield, Eastern India. Journal of Volcanology and Geothermal Research. (!) .