The present study combines textures, compositions, and in situ dating of allanite and whole-rock geochemistry from the Bagjata uranium deposit in the Singhbhum shear zone, the most important uranium-producing belt in India, to investigate the chemical evolution of rare earth element (REE) mineralization. Allanite, volumetrically the most important sink of light rare earth elements (LREEs) in this deposit, occurs as disseminated grains, pockets, veins, and stringers in biotite schist and chlorite schist. Metasomatic addition of LREEs in the rock was associated with Ca-K-Fe ± B metasomatism, as evident from the vein mineralogy. The allanite-bearing metasomatized rocks are locally extremely enriched in REEs, reaching up to ~4.8 wt percent ∑REEs. The textures and compositions of allanite demonstrate that early REE-rich allanite was replaced by later, relatively less REE bearing allanite-epidote, and this alteration followed a substitution scheme of REE+3 + (Fe+2, Mg+2) → Ca+2 + (Al, Fe+3). The presence of halite-bearing fluid inclusions in associated tourmaline, high Cl content of biotite, and other circumstantial evidence indicate that a high-salinity, Cl-rich fluid was likely responsible for initial LREE metasomatism. A later hydrothermal event altered preexisting allanite and removed REEs. The more oxidized nature of altered allanite and the presence of unidentified secondary REE minerals in association with sulfide might indicate the involvement of oxidized, sulfate-rich fluid in the alteration of allanite and the removal of REEs. Alternatively, fluoride complexing of REEs and selective removal of HREEs during this alteration are possible.
In situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating of allanite and monazite indicates that LREE metasomatism took place at ~1.88 Ga. The next hydrothermal event, which altered existing allanite, removed some REEs, and perhaps was associated with U (+HREE) addition, is dated at ~1.66 Ga. The youngest age determined from the thin rim of allanite is ~1.02 Ga. These three different ages, determined from allanite, overlap with the known thermotectonic events in and around the Singhbhum shear zone.
Comparison between the calculated ∑REE content of allanite-rich rock prior to allanite alteration and whole-rock ∑REE composition of the same rock suggests that the addition of a large quantity of REEs was followed by significant REE removal. Strong variations in the modal abundances of the constituent minerals and in whole-rock compositions, including REEs, indicate open-system behavior. The whole-rock REE pattern suggests that decoupling between LREEs and HREEs might be possible during superimposed alteration. Therefore, petrogenetic modeling using an REE-based discrimination diagram and REE patterns should be done with caution for rocks that underwent multiple events of hydrothermal fluid flux.