• Anubhooti Saxena

      Articles written in Journal of Earth System Science

    • Fluid–rock interaction across the South Tibetan Detachment, Garhwal Himalaya (India): Mineralogical and geochemical evidences

      Anubhooti Saxena Himanshu K Sachan Pulok K Mukherjee Dilip K Mukhopadhya

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      The Malari Leucogranite in the Garhwal Himalaya is cut across by a continental-scale normal fault system called the South Tibetan Detachment (STD). A mineralogical, geochemical and fluid inclusion study of samples from the fault zone of the Malari Granite was performed to reveal the imprints of fluid–rock interaction. Fluid inclusion assemblages observed in the alteration zone indicate the presence of NaCl-dominated aqueous fluids with varied salinity of 6 –16 wt.% of NaCl equivalent. Mineralogical changes include the alteration of feldspar to muscovite and muscovite to chlorite. This alteration took place at temperatures of 275°$–$335°C and pressures between 1.9 and 4.2 kbars as revealed by the application of chlorite thermometry, fluid isochores, and presence of K-feldspar+muscovite+chlorite+quartz mineral assemblage. Geochemical mass-balance estimates predict 32% volume loss during alteration. An estimated fluid/rock ratio of 82 is based on loss of silica during alteration, and reveals presence of a moderately low amount of fluid at the time of faulting.

      Results of fluid inclusion and alteration mineralogy indicate that the Malari Leucogranites were exhumed due to normal faulting along the STD and erosion from mid-crustal levels. Most of the leucogranites in the Himalayas occur along the STD and possibly a regional-scale fluid flow all along the STD might have caused similar alteration of leucogranites along this tectonic break. Regional fluid flow was probably concentrated along the STD and channelized through mesoscopic fractures, microcracks and grain boundaries.

    • Crystallochemical studies on davidite from Bichun, Jaipur District, Rajasthan, India

      Yamuna Singh Anubhooti Saxena A K Bhatt R Viswanathan T S Shaji L K Nanda

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      Crystallochemical data on metamict davidite from albitites and albitised rocks from the Bichun area (Jaipur district, Rajasthan, India) of Banded Gneissic Complex (BGC) are provided. Davidite occurs as euhedral, subhedral to anhedral crystals in the form of disseminated grains and also as fracture filled veins. The crystals of davidite are up to 8 cm in length and 6 cm in width. The powder X-ray diffraction (XRD) pattern of the heat-treated davidite (at 900◦C) reveals well-defined reflections of crystallographic planes. The calculated unit-cell parameters of the heat treated davidite are: a0 = b0 = 10.3556˚A and c0 = 20.9067˚A, with unit-cell volume (V) = 1941.6385˚A³; and α = β = 90◦ and γ = 120◦, which are in agreement with the values of davidite standard. Geochemical data reveals that the investigated davidite contains 51.5–52.6% TiO₂, 14.8–15.1% Fe₂O₃, 9.8–10.2% FeO, 6.97–7.12% U₃O₈, 6.72–6.92% RE₂O₃, 3.85–3.61% K₂O, 0.9–1.4% Al₂O₃, and 0.8–1.2% SiO₂. The calculated structural formulae of the two davidite crystals are: D-1: K0.0044/0.004Ba0.0044/0.005Ca0.20/0.20Na0.012/0.012Mn0.053/0.053Mg0.14/0.14Pb0.0076/0.008Fe2.675/2.675Fe1.59/1.59Y0.1175/0.118P0.053/0.053Nb0.008/0.008Sn0.001/0.001Zr0.033/0.033U0.468/0.468Th0.009/0.009REE0.6829/0.683)6.05/6.05(Ti12.15/12.15 Fe1.9022/1.903Si0.372/0.372 Al0.517/0.517 Cr0.018/0.018Co0.009/0.009Ni0.027/0.027)15/15O36/36(OH0.319/0.319[]1.681/1.681)2/2 and D-2: (K0.004/0.004Ba0.005/0.005Ca0.20/0.20Na0.012/0.012Mn0.05/0.05Mg0.094/0.094Pb0.007/0.007Fe2.58/2.58Fe1.71/1.71Y0.112/0.112P0.106/0.106Nb0.006/0.006Sn0.001/0.001Zr0.03/0.03U0.48/0.48Th0.009/0.009REE0.665/0.665)6.088/6.088(Ti12.48/12.48Fe1.87/1.87Si0.249/0.249Al0.334/0.334Cr0.019/0.019Co0.008/0.008Ni0.04/0.04)15/15O36/36(OH0.098/0.098[]1.90/1.90)2/2.The calculated structural formulae are not fully stoichiometric, which could be due to metamict nature of davidite. The characteristic feature of distribution pattern of REE in davidite is unusually high concentration of LREE and HREE and substantially low content of MREE. It may be due to the occupation of REEs in two distinct crystallographic sites in davidite structure, i.e., M(1) and M(O) sites. Chondrite-normalised plot of davidite reveals a pronounced negative Eu-anomaly (Eu/Eu∗ = 0.30−0.39), which suggests extremely fractionated nature of the metasomatising fluids from which davidite had crystallized. Metamict davidite-bearing U ores not only from Rajasthan, but also from other parts of India are likely to yield very high U leachability, thereby making them attractive sources of U, which otherwise are ignored by mineral engineers as uneconomic U ores.

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