Magmatic versus hydrothermal origin of carbonatite-hosted rare earth element mineralisation

Speaker:  Professor Biswajit Mishra, Senior Professor, Indian Institute of Technology, Kharagpur

Abstract:  In order to assess the relative contributions of magmatic and hydrothermal processes towards mineralisation of rare earth elements (REE), this study focuses on two carbonatite complexes in western India, i.e., Kamthai and Amba Dongar.  The novel findings of this study include: (i) report of well-preserved pegmatitic carbonatite at Kamthai, which can be considered as an exploration target, (ii) extreme enrichment of Sr and REE in magmatic calcite, which could contribute significant amount of REE during secondary redistribution, (iii) calcite chemistry, coupled with calcite-fluid REE partition coefficients and lattice strain model is used to trace ore fluid evolution, (iv) secondary REE redistribution into REE-(fluor/hydroxyl) carbonates by post-magmatic fluids, (v) REE transport by alkali rich carbonatite-derived fluids, first as sulfate and then as hydroxyl-carbonate complexes, (vi) REE precipitation due to fluid’s temperature decrease promoted by mixing of the carbonatite-derived fluid with a relatively cooler and oxidised fluid, as evident from (a) negative Ce-anomaly in hydrothermal calcite, (b) δ¹³C/ δ¹⁸O, (c) ⁸⁷Sr/⁸⁶Sr values in calcites.

Bio:  Biswajit Mishra works as a senior professor at the Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur. He is an ore geologist with a teaching experience of thirty-five years. He specialises in mineralogy and geochemistry of hydrothermal ore deposits. Biswajit's research work includes base metal sulfide deposits of Rajasthan, orogenic gold deposits in the Dharwar Craton, uranium mineralisation in the Singhbhum Shear Zone and carbonatite-hosted rare earth element mineralisation. Biswajit teaches subjects such as Ore Genesis and Mineralogical Thermodynamics; and he has guided several PhD students. Since 2000, he has been a Fellow of the Society of Economic Geologists.