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 conditi ons of the unconformity-related uranium mineralisation at Lambapur-Peddagat tu area in the northern fringe of Cuddapah basin., Fluid inclusions in vein - and granitic- quartz show significant similarity in phase ratios and homo genisation temperatures, though they differ in the mode of origin. The main population of inclusions in vein quartz is of primary origin, while in gra nitic quartz it is of secondary origin. The size of fluid inclusions genera lly varies between 10 mum to 20 mum, with low gas to liquid ratios correspo nding to high degree of fill (F=0.8 to 0.9). Inclusions in vein- and granit ic- quartz homogenise to liquid phase between 140 degreesC to 240 degreesC (mode 150 degreesC to 170 degreesC) [T-H]. Similar temperature of homogenis ation has been obtained in quartz of pebbly quartzite. The ice melting temp eratures in vein quartz are in the range of -2 degreesC to -10 degreesC [T- M] equivalent to salinity of 1-14 wt% eNaCl and in granitic quartz these ar e in the range of -6 degreesC to -14 degreesC[T-M] corresponding to salinit y values of 10-18 wt% eNaCl. The low temperature fluid inclusions homogenis ing at similar range of temperatures but showing variable salinity suggest mixing of isothermal fluids of contrasting salinity. It is likely that uran ium precipitated from the interaction of cooling low saline hydrothermal so lution at the temperatures of 150-170 degreesC with high saline mineralisin g fluid transported by recirculating meteoric water.