Unique younger phreatic uranium mineralisation hosted by Neogene sediments from Taunsa area, Dera Ghazi Khan, eastern Sulaiman Range, Pakistan: New exploratory targets in deformed geological setting

The Taunsa uranium mineralisation is hosted by the upper Miocene–Pliocene Litra Formation belonging to molasse sediments of the Siwalik Group deposited in the Himalayan foreland basin of the eastern Sulaiman Range, Dera Ghazi Khan, Pakistan. The Litra Formation is the only rock unit hosting economic grade uranium occurrences in the stratigraphic record of Pakistan. Surface radioactivity is mainly associated with cemented layers of the host sandstone, known as hard-bands, whereas weakly cemented sandstone is unradioactive. The outcropping sandstone is dominantly whitish grey due to bleaching which is probably related to hydrocarbon migration from the underlying marine black shales. This sandstone-type uranium mineralisation is mainly phreatic due to its parallelism with water table, whereas the concordantly oriented tabular (roll-type) ore is subordinate. The host sandstone layers have high dips ranging from 70 to 85°E indicating a deformed geological setting which makes the Taunsa uranium ore unique. Uranium is trapped mostly by scant organic matter, probably related to petroleum, and also by Fe-Ti oxide phases, phyllosilicates (chlorite and biotite), graphitic schist and black shale clasts in the host sandstone. Uranium ore minerals are mainly coffinite, pitchblende and probably brannerite. Uranium in the ore phases is accompanied by the high contents of Ca, Fe, Ti, Si and Al. Radiometric disequilibrium in the phreatic ore is strongly positive, whereas that in the tabular (roll-front) is moderately positive, which in turn suggests a younger age of the former than that of the latter. The coexistence of the two ore-types, characterised by different intensities of radiometric disequilibrium occurring in the same uranium deposit, suggests that phreatic uranium ore has possibly resulted from the remobilisation of the tectonically uplifted tabular (roll-type) ore and its reprecipitation at the horizontally oriented redox interface following the water table in the host sandstone aquifers.