Allanite in Singhbhum Shear Zone, India and Its Implications on the Genesis of REE Mineralization

Abstract

Abstract Allanite, being one of the most common REE bearing mineral phases in the multimetallic mineralized domain of Singhbhum Shear Zone, has always attracted attention from researchers. The present paper outlines a detailed petromineralogical study on various textural types of allanites. The allanite is categorized into 6 different types falling under three major categories on the basis of deformation, metamictisation and darkening of pleochroic halos. On the basis of mineral assemblage and probe analysis of allanite it follows that the optical and textural variations are an implication of not only multiple generations of their formation but rather significant variation in nature of hydrothermal fluids as evidenced by the differences in co-crystallized assemblages. They hint at relatively reducing hydrothermal conditions in Singhbhum Shear Zone responsible for REE-U-Th mineralization overprinting earlier event of oxidizing hydrothermal mineralization in which the well-known Th-poor uraninite, crystallized. Majority of the allanite appeared post or at the most syn-tectonic in their formation. The available geochronological data of allanite suggests them to be formed around 1.8 Ga. Considering textural criteria as seen in the present study, it appears that the majority of allanite owing to their undeformed morphology must have formed later to or at the most during waning stages of shearing and hence the minimum age of shearing in SSZ could be considered around 1.8 Ga instead of 1.6 Ga.