A Spectrum Of Potentially Diamondiferous Lamproites And Minettes From The Jharia Coalfield, Eastern India

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Journal Article: A Spectrum Of Potentially Diamondiferous Lamproites And Minettes From The Jharia Coalfield, Eastern India

Abstract
Abundant 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.

Authors 
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





DOI 
Not Provided
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Citation

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. (!) .