Microstructural and compositional variations of allanite in hydrothermal veins: Implications for REE mobilization in Chhotanagpur Granite Gneiss Complex, Purulia district, West Bengal, India

This study employs a combination of texture and composition of allanite to investigate the chemical evolution of rare earth elements (REEs) in the hydrothermal veins within the Chhotanagpur Granite Gneiss Complex around Kalapathar area, Purulia district, West Bengal, India. Allanite occurs as pockets and lumps in the hydrothermal veins. Locally, the allanite pockets/lumps have high concentrations of REEs (∑REEs ~9.8 wt.%), Th (up to 1427.2 ppm) and U (up to 642.1 ppm). Three hydrothermal fluids and associated REE mobilization have been distinguished and characterized. The hydrothermal fluid₁ causes metasomatic addition of REEs that leads to the formation of early REE-enriched allanite. Hydrothermal fluid₂ imposed changes in the texture and composition of the allanite where the early allanite was replaced by relatively REE-depleted allanite following the substitution scheme REE⁺³ + (Fe⁺², Mg⁺²) → Ca⁺² + (Al, Fe⁺³). The hydrothermal fluid₃ is marked by the alteration of allanites to the bastnaesite and monazite. REEs have been transported mainly in aqueous chloro-complexes with high to moderate salinity and low homogenization temperature (257°–361°C). The timing of the hydrothermal event is established through U–Th–total Pb dating of monazite, yielding at around 875 ± 43 Ma.

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The allanite clots and lumps in hydrothermal veins within CGGC around Kalapathar, Purulia District, West Bengal display high ∑REEs up to 9.8 wt.% along with Th and U up to 1427.22 and 642.14 ppm, respectively.