Fluid Inclusion Characteristics of Unconformity-Related Uranium Mineralisation, Lambapur-Peddagattu Area, Andhra Pradesh

Aqueous bi-phase fluid inclusions in quartz from mineralised vein, granite and pebbly quartzite have been studied to decipher physico-chemical conditions of the unconformity-related uranium mineralisation at Lambapur- Peddagattu area in the northern fringe of Cuddapah basin. Fluid inclusions in vein- and granitic- quartz show significant similarity in phase ratios and homogenisation temperatures, though they differ in the mode of origin. main population of inclusions in vein quartz is of primary origin, while in granitic quartz it is of secondary origin. The size of fluid inclusions generally varies between lOpm to 20pm, with low gas to liquid ratios corresponding to high degree of fill (F=0.8 to 0.9). Inclusions in vein- and granitic- quartz homogenise to liquid phase between 140°C to 240°C (mode 150°C to 170°C) [T_H]. Similar temperature of homogenisation has been obtained in quartz of pebbly quartzite. The ice melting temperatures in vein quartz are in the range of -2°C to - 10°C [T_M] equivalent to salinity of 1-14 wt% eNaCl and in granitic quartz these are in the range of -6°C to -14"C[T_M] corresponding to salinity values of 10-18 wt% eNaC1. The low temperature fluid inclusions homogenising at similar range of temperatures but showing variable salinity suggest mixing of isothermal fluids of contrasting salinity. It is likely that uranium precipitated from the interaction of cooling low saline hydrothermal solution at the temperatures of 150-170°C with high saline mineralising fluid transported by recirculating meteoric water.