Mineral Chemistry of Chlorites and Feldspars and Their Genetic Linkage to Uranium Mineralization: An Example from Polymetallic Uranium Deposit in Rohil, Rajasthan, Western India

A genetic linkage between U–Cu–Mo mineralization with feldspar and chlorite minerals and the discrimination of different mineralization events in the hydrothermal and metasomatic system in the Rohil polymetallic uranium deposit in India is presented on the basis of textural relationships and mineral chemistry. Field and EPMA studies reveal that the chlorite formed in two possible modes, viz. (a) replacement of ferromagnesian minerals of the host rock and (b) precipitated directly from hydrothermal solutions. Chlorites follow a distinctive composition from Al-saturated to Al-undersaturated and, in most cases, from Mg- to Fe-rich species as alteration progressed. The chlorites show a wide range of Fe content (1.86–5.06 apfu), high Mg content (3.96–6.28 apfu), and Si contents (5.99–6.90 apfu) with an Fe/(Fe + Mg) ratio (0.23–0.56), leading to their classification as Diabantite/Pycnochlorite. Empirical and thermodynamic geothermometers have been used to determine the temperature of chlorite formation based on chemical composition, which revealed a large variation in temperatures from 130 °C to 260 °C. The feldspar geothermometry reveals a temperature range of 158 to 236 °C, which is in congruence with that of chlorites. Geothermometry by two different methods provides the range of temperature that prevailed in the study area during and succeeding the crystallization of uraninite and associated ore minerals. Mineral chemistry vis-à-vis geothermometry of feldspars and chlorite can provide impetus to geochemical evolution in the North Delhi Fold Belt (NDFB) and similar geological setups in metasomatite-type uranium deposits.