No abstract

Low grade uranium mineralisation associated with phosphorites/phosphatic sediments and carbonaceous shale has been well established at and/or close to Krol-Tal contact in some of sub-basins of inner Krol belt such as Nigalidhar, Mussoorie and Garhwal synclines in the realm of lesser Himalaya. Unlike the other sub-basins of the belt, though the Nainital syncline is sparsely phosphatic and carbonaceous, carbonate hosted uranium occurrence is reported for the first time from the syncline. The carbonate-hosted uranium occurrences in Krol D of Nainital syncline are of limited extent with values ranging from 200 ppm to 840 ppm eU3O8. Radioactivity is associated with fractured dolostone/dolomitic limestone and pelloidal limestone. Uranium phases mostly occur in association with carbonaceous, ferruginous matters and clays. However, ultrafine pitchblende grains do occur associated with idiotopic pyrite aggregates suggesting initial trapping of uranium by organic matters present in sediments which later remobilised and precipitated as fine pitchblende. Subsequently, second phase of remobilisation of uranium under oxidising condition gave rise uraniferous goethite, limonite and other iron oxyhydoxides. In contrast to other sub-basins, the Krol-Tal contact in Nainital syncline does not appear to hold any promising phosphate and uranium mineralisation due to less suitable basinal morphology and low Ca/Mg ratio.

The following work is a part of wider uranium prospecting programme of the Atomic Mineral Directorate for Exploration and Research (AMD) for mapping radioactive anomalies in mineralized regions of Siwalik, Himachal Pradesh. This study reports the analysis of average activity concentration, potential radiological hazards; radium equivalent activity (Raeq), external hazard index (Hex), internal hazard index (Hin) and excess cancer life time risk (ECLR) calculated due to naturally occurring radioactive materials (NORM); Radium (226Ra), Thorium (232Th) and Potassium (40K) in uranium mineralized zone of Siwaliks for health risk assessment. The geometric mean value of calculated indoor and outdoor annual effective dose in the Una, Hamirpur and Kangra region are 0.88 and 0.22; 1.77 and 0.44; 1.86 and 0.46 mSvy-1 respectively. The average air absorbed dose rate were measured to be 118 nGyh-1 in Una, 163 nGyh-1 in Hamirpur and 135 nGyh-1 in Kangra district respectively. Correlation study indicated good correlation of the measured gamma dose rate and the estimated gamma dose rate with a correlation coefficient of (R2 =0.62). This data provides important information on the radiation risk and spatial variability of the natural terrestrial gamma radiation in the Siwalik region.

The terminal Proterozoic Krol-Tal succession disposed in a doubly plunging syncline of Nainital in the domain of Lesser Himalaya is an allochthon and forms the southernmost sub-basin of ca. 280 km linear belt comprising of six sub-basins. The mixed siliciclastic-carbonate sediments of the sequence show a wide spectrum of stromatolitic morphology, such as stratiform, laterally linked hemispheroids (LLH), columnar, domal and branching columnar, and other microbialites, such as thrombolites. These various morphological characteristics are result of a subtle balanced interaction among biological, hydrodynamic and sedimentation processes and have high potential for inferring different environments of their accretion. Among the five members of Krol Formation, viz. Krol A, Krol B, Krol C, Krol D and Krol E, the lowermost member, i.e. Krol A stromatolites represent a supratidal to intertidal environment, whereas Krol B stromatolites favour a supratidal zone of deposition. The stromatolite characteristics of intertidal environments marked with channels have been recorded from the Krol C member. A peritidal realm of deposition for Krol D member and a partly protected intertidal to upper subtidal zone for Krol E have been inferred. The lower Tal sediments were laid down in a supratidal to lower intertidal/upper subtidal environment. No stromatolite was recorded from the siliciclastic dominated upper Tal member that was deposited in supratidal to intertidal zones. The entire Krol-Tal stratigraphic succession is characterised by a repetitive shallowing-upward succession and the environmental interpretation based on the stromatolite morphometry is well in accordance with various observed sedimentary structures and sedimentological characteristics of the succession.