POTENTIAL FOR UNCONFORMITY-RELATED URANIUM DEPOSITS IN THE NORTHERN PART OF THE CUDDAPAH BASIN, TELANGANA AND ,

M.B. VERMA, A.V. JEYAGOPAL, R.V. SINGH, H.S. RAJARAMAN Atomic Minerals Directorate for Exploration and Research, Hyderabad, India

1. INTRODUCTION

The intra-cratonic, Mesoproterozoic Cuddapah Basin in the Dharwar Craton hosts several types of uranium deposit in various stratigraphic levels. Uranium mineralization is recorded in the lower part of the Gulcheru and Vempalle Formations and along the unconformity between the granitic basement and the overlying sediments of the /Banganapalle Formation in the upper part of the Cuddapah sequence. The Srisailam and Palnad sub-basins, located in the northern part of the Cuddapah Basin, host Proterozoic unconformity-related uranium mineralization. Mineralization in these sub-basins occurs close to the unconformity between the basement complex containing granitoids, basic dykes of Palaeoproterozoic age, greenstone belts of Archaean age and arenaceous, argillaceous and calcareous sediments of Mesoproterozoic–Neoproterozoic age. Sub-surface exploration over two decades has established the presence of three small to medium tonnage uranium deposits at Lambapur (~1200 tU), Peddagattu (~6400 tU) and Chitrial (~8000 tU) which are located along the unconformity between the basement Mahabubnagar granite and the overlying Srisailam Formation in the Srisailam sub-basin; and one small deposit at Koppunuru (~2300 tU) close to the unconformity contact between basement granite and the Banganapalle Formation of the Group in the Palnad sub-basin. In all these deposits, uranium mineralization is concealed and lies below the cover rocks at a depth of <5–150 m. Uranium mineralization located in these sub-basins show dissimilarities with those unconformity-type uranium deposits known in Australia and Canada, especially in respect of their geometry, basement composition and the absence of palaeosol (regolith).

2. GEOLOGICAL SETTING AND URANIUM MINERALIZATION

The Cuddapah Basin, covering about 44 000 km2, is the second largest Proterozoic basin in India. It contains a thick sequence of sediments and subordinate volcanics and hosts Proterozoic unconformity-related uranium deposits in its northern part in Telangana and Andhra Pradesh states [1]. The Cuddapah Basin comprises four sub-basins, Papaghni, Srisailam, Palnad and Nallamalai. The Srisailam and Palnad sub- basins lie in the northern parts of the Cuddapah Basin and contain, respectively, the sediments of the Srisailam Formation and of the Kurnool Group [2]. The Srisailam, Palnad and part of the Nallamalai sub- basins cover an area of about 10 000 km2 and are developed over the basement Palaeoproterozoic granitoids.

The basement complex for the Srisailam and Palnad sub-basins comprises Archaean schists (Peddavoora Schist Belt), Palaeoproterozoic granites, basic dykes, pegmatites and quartz veins of Palaeoproterozoic age (2268 ± 32 Ma to 2482 ± 70 Ma) [3]. Well-developed fracture systems (trending N–S, NE–SW, NW–SE) along with basic dykes intruding both cover rocks and basement cross-cut the unconformity surface. The N–S to NE–SW trending dykes in the basement have played a major role by increasing the thermal gradient, permitting the release of uranyl ions into the solutions and effecting their remobilization along fractures and the unconformity plane. A grit to pebbly horizon immediately above the unconformity surface acts as a conduit system for uranium mineralization.

The Srisailam sub-basin, covering an area of around 3000 km2 forms a prominent plateau, exposing Neoproterozoic sediments of the Srisailam Formation, the youngest unit of the Cuddapah Supergroup. The

419 sediments have sub-horizontal dips to the south-east and attain a maximum thickness of 300 m. The Srisailam Formation comprises a sequence of feldspathic quartzite with intercalated shale, siltstone and grit. The sediments directly overlie the basement rocks in the northern margins of the sub-basin, whereas in the south-eastern margin the sediments are underlain by the Nallamalai Group metasediments with an angular unconformity. The northern fringes of the Srisailam sub-basin have a highly dissected topography with several isolated flat-topped outliers above the basement and rising 100–150 m above ground level. The Lambapur, Peddagattu and Chitrial uranium deposits are located in such individual outliers [4]. The outliers of the Srisailam Formation are characterized by a sequence of pebbly–gritty arenite horizons successively overlain by shale, shale/quartzite intercalations and massive quartzite with a thickness of 5–70 m and a gentle 3–5° dip towards the south-east.

The basement granitoids are sodic in nature, with a moderate to high Na2O/K2O ratio (0.09–1.62). Their composition varies from granite to granodiorite and they are strongly peraluminous, with the A/CNK ratio >1.1 [4]. The granitoids mineralogically correspond to biotite-granites and consist of an assemblage of albite–oligoclase, quartz, K-feldspar, with accessory hornblende, biotite, apatite, sphene, zircon, allanite and epidote. They are equivalents of the Closepet granite from the Eastern Dharwad Craton. Alteration such as chloritization, sericitization, calcitization and epdotization are pronounced in the basement, especially when close to the unconformity. Pyrite, chalcopyrite, galena, ilmenite, anatase and hydrated iron oxides are the opaque minerals. The study of granitoid core samples shows the presence of two to three sets of foliations, cross-cut by dolerite dykes and quartz veins. Granites are U-enriched (average 20 ppm U) with a U/Th ratio of 6.68 [5].

In the three deposits of the Srisailam sub-basin (Lambapur, Peddagattu and Chitrial), uranium mineralization occurs close to the unconformity, both in the granites, basic dykes and quartz veins of the basement and in the overlying pebbly arenites, with most of the mineralization (>85%) located in the basement [6]. Although the orebody appears to be blanket-shaped along the unconformity, scout drilling in various areas covering the entire outliers has indicated that the U-rich pods and ore shoots are confined to definite trends, i.e. NNE–SSW and NW–SE (Lambapur), WNW–ESE and N–S (Chitrial) and N–S (Peddagattu). The intensity of fracturing within the granite and their intersections with the unconformity, apparently controls the grade of mineralization [7]. The orebodies show gentle dips towards the south-east and follow the basement slopes. Mineralization comprises pitchblende, uraninite and coffinite as primary minerals and uranophane and autunite as secondary minerals. It is well exposed on the outcrops of granitoids at Lambapur and along road cuttings in the Peddagattu and Chitrial plateaux. Botryoidal and massive pitchblende occurs as thin veins lying sub-parallel to the unconformity surface, as massive pods, in the fracture planes of feldspars, as irregular segregations and also adsorbed onto globular organic matter. In places, the massive pitchblende is replaced by coffinite. Extensive hydrothermal activity, both in the basement and in the overlying sediments, is evidenced by high amounts of sulphides such as pyrite, chalcopyrite and galena. The pitchblende veins are found to cut across the basement granitoid and enter into the cover rocks.

EPMA studies of radioactive granitoids core samples from the Chitrial area have indicated that the UO2 content in pitchblende and coffinite lie in the range 71.37–88.14% and 59.97 –73.91%, respectively, and have also confirmed the presence of complexes such as U–Si, U–Si–Ti and U–Si–Al. X ray diffraction studies indicate that the unit cell dimension of uraninite in the Lambapur area is in the range of 5.3973A– 5.4285 Å in contrast with the uraninite of the Chitrial area which is 5.3959 Å. The oxygen content in the formula unit (UO2) is in the range of UO2.30 to UO2.69 in uraninite of the Srisailam sub-basin deposits. Evidence of remobilzation has been observed in the samples. At Lambapur, radiogenic lead gives ages of 480–500 Ma while the Sm–Nd data of uraninite yield an isochron age of 1327 ± 170 Ma, indicating two mineralizing events.

The Palnad sub-basin has a surface area of around 4500 km2 and hosts the Koppunuru uranium deposit in its western part [8]. The sub-basin exposes the Neoproterozoic Kurnool Group, which comprises a thick

420 sequence of clastic and calcareous sediments. In the northern part of the basin, basement granite and gneisses are unconformably overlain by sub-horizontal sediments. The Banganapalle Formation, the lowermost sequence of the Kurnool Group, comprises gritty arenite successively overlain by shale/siltstone intercalations and quartzites of high mineralogical maturity [9]. The fertile nature of the basement granite is indicated by high uranium values (average 32 ppm U, n = 16) and a high U/Th ratio (average 4.41, n = 16), compared with the average U/Th ratio of normal granite of 0.25. A granite inlier is exposed to the east of Koppunuru, along the upthrown block of the Kandlagunta fault, which trends WNW–ESE. Both the basement and the sediments are fractured and cross-cut by quartz veins trending N–S, NNE–SSW and WNW–ESE.

Uranium mineralization at the Koppunuru deposit occurs both in the cover sediments and in the basement. It is composed of three sub-horizontal ore lodes, two hosted in the arenites and one in the basal polymictic grit/conglomerate of the Banganapalle Formation, locally transgressing the basement granite [10]. Pitchblende and coffinite are identified as primary uranium minerals. Traces of carbonaceous matter are associated with uranium mineralization along with sulphide minerals. EPMA analysis has indicated that pitchblende and coffinite contain 73.47–78.58% UO2 and 63.45–71.53% UO2 respectively, while mixed phases contain lesser uranium oxide (42.14–47.90% UO2). In addition, uranophane, phosphuranylite, metazeunerite and U–Ti complexes occur as secondary uranium minerals. The radioactive minerals are epigenetic in nature and occur as thin veins, fracture/cavity fillings and grain boundary coatings. Other metalliferous minerals are galena, pyrite, chalcopyrite, pyrrohotite, marcasite and traces of pentlandite. X ray diffraction studies of uraninite indicate that the cell unit dimension for the Koppunuru deposit is in the range 5.4382–5.4534 Å. The oxygen content in the formula unit (UO2) is in the range UO2.15 to UO2.29 for uraninite in the Palnad sub-basin.

The radiometric age of mineralized granite samples is 1545 ± 140 Ma. In addition, dating of uraniferous quartzite samples by the Pb–Pb step leaching method has indicated mineralization ages of 576 ± 180 Ma, 891 ± 160 Ma and 936 ± 60 Ma. This confirms the episodic nature of mineralization, where uranium concentration/enrichment took place during several phases.

3. DISCUSSION AND CONCLUSION

The northern margins of the Cuddapah Basin represent favourable areas for hosting Proterozoic unconformity-related uranium mineralization. Litho-structural and metallogenic characteristics are well established for the Lambapur, Peddagattu, Chitrial and Koppunuru uranium deposits. In the Srisailam and Palnad sub-basins, two major mineralization events are suggested. The primary mineralization event took place at 1500–1300 Ma with a major mobilization/rejuvenation event around 950–450 Ma.

Comprehensive exploration efforts in the northern margin of Srisailam sub-basin has established a resource of about 20 000 t of uranium oxide at relatively shallow depths (75–120 m). Large areas of the Srisailam sub-basin, which have a considerable thickness of cover rocks, are still unexplored and are likely to increase resources at depths of 200–250 m. Extensive surficial exploration in other outliers and also in other areas of the main Srisailam sub-basin, i.e. Amrabad, Akkavaram, Udimilla, has resulted in the discovery of several surface uranium showings, indicating the persistence of metallogenic features observed in the Lambapur, Peddagattu and Chitrial deposits, in the entire Srisailam sub-basin.

Similarly, in the Palnad sub-basin, the Koppunuru uranium deposit is unique in respect to its mineralization pattern in the sediments, proximal to the unconformity contact with the basement granitoids. Structures have played a major role in controlling the mineralization. A 150 km long area along the northern margin of the Palnad sub-basin with litho-structural characteristics similar to those at Koppunuru may warrant exploration. Ground radiometrics and ‘heli-borne’ geophysical surveys along the northern margin of the sub-basin have delineated several surface uranium showings at R.V. Tanda, Mathamapalle, etc., and

421 geophysical anomalies at Durgi, Daida, Gurajala, etc. Intensive sub-surface exploration is envisaged in several sectors of the Srisailam and Palnad sub-basins.

REFERENCES

[1] RAMESH BABU, P.V., RAHUL BANERJEE, ACHAR, K.K., Srisailam and Palnad sub-basins: potential geological domains for unconformity-related uranium mineralization in India, Expl. Res. At. Miner. 22 (2012) 21–42. [2] NAGARAJA RAO, B., RAJURKAR, S., RAMAILINGASWAMY, G., RAVINDRA BABU, “Stratigraphy, structure and evolution of the Cuddapah Basin”, Purana Basins of Peninsular India, Geol. Soc. India Mem. 6 (1987) 33–86. [3] PANDEY, B.K., et al., “Rb-Sr whole rock ages for the granites from parts of Andhra Pradesh and Karnataka”, paper presented at Fourth Natl Sym. on Mass Spectrometry, Bangalore, 1988. [4] SINHA, R.M., PARTHASARATHY, T.N., DWIVEDI, K.K., “On the possibility of identifying low cost, medium grade uranium deposits close to the Proterozoic unconformity in the Cuddapah basin, Andhra Pradesh, India”, Innovations in Uranium Exploration, Mining and Processing Techniques, and New Exploration Target Areas, IAEA-TECDOC-868, IAEA, Vienna (1996) 35–55. [5] VERMA, M.B., SOM, A., LATHA, A., UMAMAHESWAR, K., MAITHANI, P.B., “Geochemistry of host granitoids of uranium deposit at Chitrial area, Srisailam sub-basin, Nalgonda District, Andhra Pradesh”, Contributions to Geochemistry, Geol. Soc. India Mem. 73 (2008) 37–54. [6] VERMA, M.B., MAITHANI, P.B., CHAKI, A., NAGESWARA RAO, P., KUMAR PRAKHAR, Srisailam sub-basin, a uranium province for unconformity-related uranium deposits in Andhra Pradesh — case study of Chitrial uranium exploration, Curr. Sci. 96 4 (2009) 588–591. [7] UMAMAHESWAR, K., ACHAR, K.K., MAITHANI, P.B., “Proterozoic unconformity-related uranium mineralisation in the Srisailam and Palnad sub-basins of Cuddapah basin, Andhra Pradesh, India,” Uranium Raw Material for the Nuclear Fuel Cycle: Exploration, Mining, Production, Supply and Demand, Economics and Environmental Issues (URAM-2009), IAEA-TECDOC-1739, IAEA, Vienna (2014). [8] JEYAGOPAL, A.V., PRAKHAR KUMAR, SINHA, R.M., Uranium mineralization in the Palnad sub-basin, Cuddapah Basin, Andhra Pradesh, India, Curr. Sci. 71 12 (1996) 957–959. [9] GUPTA SHEKHAR, BANERJEE RAHUL, RAMESH BABU, P.V., MAITHANI, P.B., Sedimentation pattern and depositional environment of Banganapalle Formation in southwestern part of Palnad sub-basin, Guntur district, Andhra Pradesh, Gondwana Geol. Mag. Spec. Pub. 12 (2010) 59–70. [10] VERMA, M.B., et al., Ore body characterisation of Koppunuru uranium deposit in Palnad sub-basin, Guntur district, Andhra Pradesh, Ind. Miner. 45 1 (2011) 51–61.

422