Reaction textures in a suite of kyanite rich rocks from parts of Singhbhum Cu-U belt, East India: evidence of repeated fluid flux in a mid crustal ductile shear zone

The arcuate Singhbhum shear zone (SSZ) of East India separates Meso-to Palaeo-Proterozoic supracrustal rocks of North Singhbhum fold belt (NSFB) from the Archaen Singhbhum craton. The SSZ exposes diverse lithologies with high variant mineral assemblages that host economically viable polymetallic deposits (Cu, Fe & U) ore deposits. Detailed geological investigation in this narrow belt has identified superposed folding and accompanying ductile shearing. Laterally continuous to discontinuous outcrops of kyanite-rich rocks occur all along the SSZ close to its hanging wall side and also with in the immediate pelitic rocks of the adjoining NSFB. Field features together with low concentrations of Fe₂O₃ (0.03-0.61 wt%), TiO₂ (0.55+0.13 Wt%) and Ga (23+12 ppm),high Al₂O₃ (c.35+10 wt%) and lack of any correlation between the last two chemical constituents are inconsistent with the idea that these rocks represent metamorphosed bauxite deposits or sedimented residual clay. Base leaching and consequent enrichment of Al₂O₃ in the host rocks by acidic fluids presumably derived from magmatogenic hydrothermal fluids seem to be consistent with the geological and geochemical data and high δ¹⁸O values of kyanite rich rocks (>60 vol% kyanite, c. 4.7- 6.8 per mil) including δ¹⁸O values of two kyanite grains (5, 5.5 per mil) from an unaltered massive kyanite deposits. Extensive fluid-rock interaction during and subsequent to the intense shearing of this region converted the anhydrous kyanite quartzite to muscovite schist with variable proportion of chloritoid, chlorite, biotite, and tourmaline. Frequently, kyanite grains are pseudomorphed by muscovite and chloritoid without the development of any Al-rich phases nearby. These features indicate nearly uniform rates of kyanite dissolution and precipitation of less aluminous muscovite and chloritoid, which in turn, support Al mobility during this metasomatic event. Part of the scavenged Al was presumably re-deposited as kyanite (+quartz) bearing veins that dissected the kyanite-muscovite schists. Ramifying veins of fine-grained tourmaline and randomly oriented paragonite flakes are also present in muscovite schist indicating that the infiltrating fluid was locally rich in boron and Na. In eastern and south eastern parts of SSZ, physical conditions deduced from the conventional geothermobarometry and stability relation of the pseudomorph forming phases converged at 6.4+0.4 Kbar and 480+ 40^o C. Extant fluid inclusion data and thermodynamic modeling of aH₂O computed from mineral fluid equilibria indicate high salinity (>25 wt% NaCl equivalent) of the infiltrated fluids. These fluids were out of equilibrium with the host rocks and triggered the metasomatic alteration of the kyanite rich rocks, transport of Al³⁺ and formation of kyanite rich veins.