Hydrothermal alteration of accessory minerals (allanite and titanite) in the late Archean Closepet granitoid (Dharwar Craton, India): A TEM study

Allanite, a member of the epidote supergroup, is a widespread rare earth element (REE)-rich accessory mineral in the late Archean Closepet batholith (Dharwar craton, India). It is commonly associated with titanite. Previously recognized shear zones served as preferential paths for magma and later fluids. As a response to hydrothermal activity, allanite exhibits complex alteration textures, geochemical features, and breakdown products that vary across the batholith. In the central zone, allanite displays the largest variations. It has decomposed into secondary allanite, bastna<spacing diaeresis>site, chlorite, thorite, calcite, pyrite, and galena. In the southern zone, magmatic allanite core is preserved. The alteration products in the marginal regions are limited to secondary allanite, bastna<spacing diaeresis>site, chlorite, thorite, and synchysite. The breakdown products and textural features of allanite in the northern intrusions differ strongly from those in the other zones of the Closepet batholith and are limited to secondary allanite and chamosite. However, nanoscale element remobilization at the interface between allanite and titanite is evident. The observed texture in allanite indicates a complete dissolution-reprecipitation process. The chemical variations and differences in alteration products after allanite indicate that the fluid composition changed along the Closepet granitoid. The fluids that altered the allanite were most likely F-, Cl-, and CO2-rich and alkaline but eventually became acidic. When the chlorine-bearing fluids reached the northern zone, the concentrations or active contributions of CO2, F and H2S were very low. The alteration products (bastnasite, chlorite, and thorite) indicate a rather low-temperature fluid.