Mineral chemistry and geothermometry of chlorites in relation to physico-chemical conditions of uranium mineralization in the central part of the Singhbhum Shear Zone, eastern India

The Singhbhum Shear Zone (SSZ) in eastern India is one of the major uranium repositories of the country. Some of the important uranium deposits viz., Bagjata, Jaduguda, Bhatin, Narwapahar and Banadungri-Singridungri are juxtaposed in the central part of SSZ. The uranium mineralization in central SSZ is mainly hosted by tourmaline-apatite-magnetite bearing sericite quartz chlorite schist with uraninite being the main radioactive mineral phase. A close spatial relationship exists between uraniferous minerals and chlorite occurrences in central SSZ. Three different types of chlorite assemblages were recognized and their relationship with uranium mineralization was studied in detail to unravel the physico-chemical conditions for precipitation of uranium vis-avis chlorite geothermometry and constrains on the chlorite chemistry. Detailed petrographic studies and mineral chemical studies of the host rocks indicate three distinct chlorite type viz. (1) metamorphic chlorite, forming a typical metamorphic assemblage with quartz, muscovite, sericite, biotite and K-feldspar, (2) hydrothermal alteration product of biotite and (3) hydrothermal vein chlorite. The three chlorite types were characterized based on: (1) the range of X-Fe and Al-IV, (2) correlation between Si (apfu) and X-Fe and (3) compositional diagram between X-Fe vs. Al-IV. Based on the mineral chemistry and thermometry of chlorite 3, the co-genetic uranium mineralization has been suggested to be precipitated from a hydrothermal fluid rich in Fe, Mg and Ti in a reducing environment within a temperature window of about 312 degrees C-355 degrees C. A fluid mixing model for the remobilization and reconstitution of this hydrothermal uranium mineralization vis-avis chlorite occurrence is postulated and its geothermometry suggest an epithermal mineralization in the study area. A comparative study of chlorites from four important uranium provinces of the world i.e. Jabiluka of Australia, Rozna of Czech Republic, Beaverlodge of Canada and central SSZ of India reveals a similarity in the mineral chemistry of the chlorites associated with uranium mineralization. Thus, textural study of chlorites corroborated with their mineral chemistry and geothermometry is a useful reconnoitre tool for uranium exploration.