URANIUM AND ASSOCIATED MINERALIZATION IN SONRAI BASIN, BUNDELKHAND CRATON, INDIA

Abstract

To meet the ever increasing demand for energy by the developed and developing countries and also to keep the environment clean, the use of nuclear energy is going to be a necessity for the society in near future. The production of conventional energy, mainly from coal and hydrocarbon, needs to be supplemented and the nuclear/atomic fuel is one of the viable options. In the present world, uranium has gained importance as a strategic commodity because of this India should be as per as possible self sufficient in the nuclear energy and uranium ore is the only resource for the same. Exploration of uranium by Atomic Minerals Directorate for Exploration and Research (AMD) and Uranium Corporation of India Ltd (UCIL) of Department of Atomic Energy (DAE), Government of India has been accelerated in last few decades in many unexplored/partly explored areas like Proterozoic Sonrai Basin, Lalitpur district, Uttar Pradesh. Present work is based on a research project funded by Board of Research in Nuclear Sciences (BRNS) to help in the investigation of the Sonrai Basin. Data presented in this thesis is produced during this project work. Uranium ore deposits are unevenly distributed through geologic time and also vary greatly from the Mesoarchean time to the Present. Global uranium deposits have been classified into twenty major types based on the host environment or geotectonic setting of the concerned deposit including some forty subtypes and classes of deposits by pioneer worker like Dahlkamp. However, the global attention has been focused in some deposits like unconformity type, Sub-unconformity-epimetamorphic type, sandstone-hosted-type, vein-type, Quartz- Pebble-Conglomerate type, polymetallic Iron Oxide Copper Gold deposits of Olympic Dam and volcanogenic deposits etc. In India, uranium ores are found in Jaduguda (Vein Type), Tummalapalle, Lambapur, Srisailam and Palnad sub-basin (unconformity type), Domiasiat (sandstone type) etc. The objectives of the present study are focused around to find out the possible source of uranium, to ascertain the role of fluid in transportation and accumulation of uranium in the Sonrai Basin and to work out a possible exploration model for uranium ore, if any. The methodology adopted for this includes detailed field work and systematic sampling and core logging, preparation of thin, polish and thin-cum polish sections and detailed petrographic studies selected rock samples under microscope followed by EPMA study, qualitative mineralogical studies of selected rock samples using XRD, whole rock geochemistry of trace elements of selected rock samples using XRF, elemental analyses of total nitrogen and carbon (and sulfur) to get the total organic carbon/total nitrogen (C/N) ratios and ii C% using CHNS Analyzer, fluid inclusion studies of hydrothermal quartz and carbonate samples to know the P-T-X conditions of the fluid and identification molecular species like CO2 and CH4 etc. using Laser Raman Spectroscopy and EPMA studies of selected polished and thin-cum polish sections of rock samples to know the mineral phases, internal zoning and inclusions etc. in minerals particularly from the BSE images and to find out the chemical compositions of the constituent minerals. The Sonrai Basin is located at Lalitpur District in the extreme SW corner of Uttar Pradesh bounded by latitude and longitude, 240 13’N to 240 20’N and 780 45’E to 780 55’E, respectively in the Survey of India Toposheet Nos.54L/15 and 16. This E-W trending intracratonic basin is 28 km long and 5-7 km wide and lies at the southern margin of the Bundelkhand Craton. Upliftment of the Bundelkhand Craton followed by lithospheric extension and rifting due to mantle plume activities led to the sagging of juvenile crust along the margins and consequently met with the formation of Paleoproterozoic pericratonic continental rift basins, over the rifted platform of the craton. After formation, sedimentation started in these riftogenic basins, where the clastic sediments were provided after doming up, rapid uplift and fast erosion of the BGC. The basement Archean Bundelkhand Granitic Complex (BGC) is overlain by the rocks of Paleoproterozoic Bijawar Group followed by the Vindhyan Supergroup of rocks of Neoproterozoic age. These three major stratigraphic horizons are further covered with the Deccan Traps of Cenozoic age. The overall geological set up is such that there exists a ‘Basement-Cover’ relationship between the ‘basement Bundelkhand Craton and the cover linear Bijawar-Vindhyan sequences. The rocks of the Bijawar Group are represented by meta-sedimentary rocks. Kaimur and Semri are the representative formation of the Vindhyan Supergroup mainly comprised of sandstone, black shale, carbonates, conglomerate and breccia etc. Rewa and Bhander have no outcrop in this area. The rocks of the Bijawar Group are divided into two formations – lower Sonrai and upper Solda Formations which are in turn further sub-divided into different members based on their distinctive characteristics. The Members of the Sonrai Formation from older to younger are Jamuni Carbonate (JCM), Gorakalan Black Shale (GBSM), Rohini Carbonate (RCM) and Bandai Sandstone (BSM) Member. The Members of Solda Formation from older to younger are Chloritic Shale (CSM), Dhorisagar Quartzite (DQM) and Solda Iron (SIM) Member. The Bijawars also contain one major basic volcanic unit i.e. Kurrat Metavolcanic (KMV) occurring between RCM and BSM. The overall sedimentation pattern shows that there is a continuous cycle of transgressive and regressive phases of sedimentation might have occurred during the deposition of sediments in the Sonrai Basin. The overall iii structure of the area shows presence of a very open type syncline. The beds are mostly E-W in strike with 20-300 dip towards south. In the eastern part the beds have NW-SE strike and southwesterly dip. Two Basement faults (F-1 and F-2) have affected both the Bijawars and the Vindhyans. Out of three lineaments L-1, L-2 and L-3, only L-3 is present only within the Vindhyans. Maximum numbers of faults/fractures/lineaments are present within the Bijawars. The interpretation of litho-logs of boreholes reveals presence of one more fault (F-3) between LKN-8/LKN-6-/LKN-27, unknown earlier. Uranium ores are mainly present in two forms - uraninite and coffinite, minor Mo, V bearing phases having epigenetic occurrences in the micro-fractures, micro-pits and inter granular spaces, veins and fractures. Along with uranium ores pyrite, chalcopyrite, chalcocite, galena, sphalerite and acanthite are other associated ore minerals. Gold, native silver, REE (La, Ce) minerals and zircon are present as fine to ultra fine disseminations in the matrix. Both syngenetic (disseminated, bedding parallel i.e. parallel to sedimentary beddings) and epigenetic (vein/fracture-filling) sulfide mineralization are found in the Bijwars and the Vindhyans. But, sulfide ores are more common in the Bijawars compared to the Vindhyans. The RCM and the CSM of the Bijawar Group are potentially more important for uranium and associated sulfide mineralization compared to other stratigraphic horizons of the Sonrai Basin. The precipitation of uranium form the ore bearing hydrothermal fluids takes place due to presence of sulfide minerals which provided reducing conditions. Besides in RCM, the uranium ore is found to be associated with a carbon rich zone. This carbon and other reduced S-species associated with uraninite is a direct evidence for precipitation of uranium ore by reduction reaction from U-ore bearing fluid. In case of CSM the hydrothermal alteration like chloritisation might have played a major role in U precipitation and the reduction of uranyl ion to uranous state is due to oxidation on ferrous iron to ferric state due to involvement of Fe+2 in chlorite. Data from trace elements analysis of selected elements of different rock types from all the stratigraphic horizons (both Bijawars and Vindhyans) shows the relative enrichment of U in ‘Rohini Carbonate’ and ‘Chloritic Shale’ Members compared to the rocks of other stratigraphic horizons of the Sonrai Basin. Concentration of uranium in the Sonrai Formation of the Bijawars varies from 1-12ppm and a highest concentration is recorded in the case of RCM. Similarly in case of Solda Formation of the Bijawars, U concentration ranges from 4-12ppm with a highest value observed in the CSM. In case of Vindhyan sediments the concentration of U ranges from 2-12ppm which is reported in few samples from trace element study and these might be detrital in nature. Interestingly, overall higher concentration of compatible elements (Sc, V, Cr, Ni) and lower concentration of incompatible ones (Ba, Pb, Rb, Sr etc.) compared to UCC indicates a iv strong possibility of influence of a volcanic source (i.e. Kurrat Metavolcanics) in the study area. The fluid inclusion studies were carried out in some selected samples quartz and carbonate vein samples collected from the mineralized zones occurring within different members. The vein-samples from the boreholes intersecting the mineralized zones were given maximum importance. Based on petrography, primary, secondary and pseudo-secondary inclusions are identified. According to number of phases present, the inclusions are classified into four types viz. aqueous (Type-I), aqueous carbonic with minor CH4 and N2 (Type-II), necked inclusions (Type-III) and liquid rich aqueous inclusions with presence of daughter crystals from microthermometry and Raman data. Out of these, only aqueous (Type-I; H2ONaCl) and aqueous carbonic (Type-II; H2O-CO2-NaCl) are of primary interest from the ore mineralizations point of view and studied in detail. The homogenization temperature of Type-I fluid varies between 98-2850C and the salinity of Type-I fluid vary from 10.4-18 wt% NaCl equiv. In case of Chloritic Shale Member, both Type-I and Type-II inclusions show evidence of boiling. The homogenization temperature of Type-II inclusions varies from 125-2830C and salinity varies 9.7-13.1 wt% NaCl equiv. In Kurrat Member both Type-I and Type-II fluids are present. Temperature of homogenization of Type-I fluid is between 108-1810C and that of Type-II fluid is 128-2090C. Salinity of Type-I fluid varies from 9-17 wt% NaCl equiv. and that of Type-II is 10.4-14.4 wt% NaCl equiv. The Type-II inclusions of Kurrat Member also show evidence of boiling. Raman study on some Type-II inclusions confirmed the presence of CO2, CH4 and very small quantity of N2 in the fluid. Based on occurrences of uranium ores below the unconformity surface between the Bijawars and the Vindhyans, type of occurrences, types of uranium ores and other associated ore minerals and gangue minerals, hydrothermal alteration like chloritisation and overall geological set up, it is inferred that uranium deposits in the Sonrai Basin is of Proterozoic Sub-unconformity Epimetamorphic Type or Alligator River Type Uranium Deposits.