Vishnu Bhoopathi·S.Niranjan Kumar· R.S.N.Sastry·B.Srinivas

Surface radioactive anomalies at Srisailam sub basin，Cuddapah basin，India

Vishnu Bhoopathi·S.Niranjan Kumar· R.S.N.Sastry·B.Srinivas

Surfaceradioactivesurveyhasbeencarriedwith portable gamma ray spectrometer（PGRS）about the 45 km2around the Srisailam sub basin，the surface radioactive anomalies were found at Chennakesavula gutta in the Srisailam sub basin of Cuddapah basin in the age of meso-neo proterozoic，India.The integrated geochemical and radiometricsurveyswereconductedaspartofresearchworkinthe Srisailam basin，it has resulted the locating of surface radioactive anomalies near the Chennakesavula gutta.Secondary uranium minerals are observed along with the fracture planes in the Archean basement granite in the proximity of unconformity contact of Srisailam quartzite. The granite underlies the Srisailam formation and forms the source for possible unconformity related uranium mineralisation in the Srisailam sub-basin evidence by the significant radioactive anomalies.The radioactivity recorded by PGRS reflected uraniferous nature of the radioactivity（Average total counts=195 ppm；U=202 ppm；Th= 22 ppm；%K=4.1，n=15）.

Radioactivity·Unconformity·Srisailam sub basin·Cuddapah basin

1　Introduction

High-grade and large-tonnage unconformity-related uranium deposits were found in Canada and Australia（Dahlkamp 1993；Needham and Roarty 1991）.These vein-like uranium deposits occur as fracture/breccia-fillings in the Lower Proterozoic pelitic rocks were close to the unconformity，under the cover of dominantly arenaceous Middle Proterozoic strata（Roy et al.1991；Satyanarayana rao 1963）.With a view to locate similar deposits in India，renewed ground radiometric surveys were initiated in 1990 on the northwestern margin of the Middle Proterozoic Cuddapah basin（Seetharam Sarma 1959；Sibbald 1986；Viswanath et al.1990；Chakrabarty et al.2009；Sarkar 1984；Jayagopal et al.1996；Dhana Raju et al.1993；Senthil kumar and Srinivasan 2002；Sinha et al.1995；Verma et al.2009）.This led to the discovery of numerous uraniferous anomalies，close to the unconformity between the Archaean/Lower Proterozoic Mahaboobnagar granite and Srisailam Formation，in a number of dissected outliers. This discovery represents a significant break-through and may contribute substantially to the uranium resource of India.More significantly，the geological understanding of this unique mineralization may give definite clues in locating the classical unconformity-type deposits in the northern parts of the Cuddapah basin.

1.1Methodology

The surface radioactivity survey has been carried out 45 km2in the Srisailam sub basin，north east fringe of Cuddapah basin.The study area related to agriculture fields and natural biodiversity.The Portable Gamma Ray Spectrometer（PGRS）was used to carry out for measurement of U，Th counts in the field as per grid sampling，theinstrument was made by Electronic Corporation of India Limited，Hyderabad.Carrying the instrument in the field it shows the results on the display by the automatic system in the units of parts per million（PPM）.The PGRS was used in by the Atomic mineral directorate for Exploration and Research for the first time in India（1990）.

V.Bhoopathi（?）·R.S.N.Sastry·B.Srinivas

Department of Applied Geochemistry，Osmania University，Hyderabad 500007，India

e-mail:vishnu.bhupathi@gmail.com

S.Niranjan Kumar

Atomic Mineral Directorate for Exploration and Research，Begumpet，Hyderabad，India

1.2Geology

Srisailam Formation is the youngest member of the Cuddapah Super group，and located at Northern fringes of Cuddapah basin，forms a very prominent plateau，having an areal extent of over 5000 km2（Fig.1）.The Srisailam Formation is mainly an arenaceous unit alternating with shale，with sub-horizontal dips，and attains a maximum thickness of 300 m.It directly overlies，on its northwestern margin，the basement rocks comprising Archaean gneisses and younger granites（Mahaboobnagar granite）with ages ranging from 2268±32 to 2482±70 Ma.On the southern and eastern margins，the Srisailam Formation covers the Nallamalai Group with an angular unconformity.

The peninsular gneissic complex covers the most of the area，comprises granite，gneisses，and migmatite with the undigested patches of older metamorphic rocks associated with agglomerates and thin bands of banded iron formation， at some places were observed between the granite and quartzite kankar has been formed，major quartz veins have been identified.The rocks of peninsular gneissic complex and Dharwar Super Group intruded by the younger granitoids of tonalite trondhjemite granodiorite composition and diabase dykes.These dykes are thoielitic to sub alkalic in composition and intrude along EW，NW—SE and NE—SW directions.Radiometric dating by Sm—Nd，Rb—Sr，Pb—Pb methods indicated the age of the dyke systems to be around 2200 ma of this area（Pandey et al.1988）.The southern parts of the area belongs to the Cuddapah Super group.The Cuddapah Super group sediments deposited non—conformably over peninsular gneisses and Dharwar comprises conglomerate，quartzite，limestone，dolomite and shale.

The Srisailam sub-basin has a highly dissected topography on its north-western fringes，resulting into the development of numerous isolated flat-topped hills rising up to 100—150 m above the ground level（Nagaraja rao et al. 1987）.The basement granite is traversed by three sets of basic dykes，trending NNW，SSW，NW—SE and E—W as well as by NNE—SSW trending quartz veins.

Fig.1 Location and geological map of Srisailam sub basin

Prior to 1990 the emphasis was given to identify anomalies associated with granites and look for possible vein-type mineralization associated with them（Ramachandran et al.1992）.The ground radiometric surveys conductedsince the late 1950s，resulted in discovering numerous uraniferous anomalies associated with the Mahaboobnagar granite and intrusive basic dykes within them，on the northwestern fringes of the Cuddapah basin（Shrivastava et al. 1991）.With the advent of unconformity-type uranium deposit，renewed efforts were initiated in the form of ground radiometric surveys since 1990 to look for similar set-up available in the Cuddapah basin.This led to the discovery of numerous uraniferous anomalies close to the unconformity between the Archaean/Lower Proterozoic Mahaboobnagar granite and the Srisailam Formation in the Amrabad outlier. These anomalies were distinctly different from those discovered in the earlier phases of surveys.The most promising outcropanomalyatLambapur（Shrivastavaetal. 1993；Venkatakrishna and Dotiwala 1987；Viswanath et al. 1990；Sinha 1993）is under detailed exploration since March，1992.The adjoining areas like Yellapur and Chitrial outliers discovered subsequently，has also indicated thepresence of numerous uraniferous anomalies at the unconformity interface.The outcrop samples of the Srisailam Quartzite，granite and dolerite dyke from mineralized outcrops are close to the unconformity.

Fig.2 a Microcline，b orthoclase，c chlorite，d sericite，e epidote，and f quartz

1.3Nature of ore body and controls of uranium mineralization

In the Srisailam sub basin the secondary uranium mineralizationisfoundclosetotheunconformityinmanyareas，both in the basement granite and the overlying Srisailam pebbly arenite.Basic dykes and vein quartz within the basement granite are also mineralized close to the unconformity and quartz veins are associated with lead and copper mineralization.The main uranium ore body is confined to the unconformity between the basement granite and the overlying SrisailamFormation.Inaddition，thereareisolatedadditional mineralized zones much deeper within the basement granite. Theradiometricsurveyswerecarriedoutonbasementgranite through out the Srisailam sub basin.Among the entire NNW of Srisailam sub basin the gamma ray spectrometer shows average value is 10 ppm for fifteen measurements，but at Chennakesavula gutta and near the Padra village，shows highest readings respectively 195 and 202 ppm compared with remaining area.

Fig.3 a Secondary uranium mineral encrustations，b measurement of surface radio activity，c highly fractured and altered granite，d chlorite veins associated with granite，e secondary uranium mineral stains，and f secondary uranium mineral on surface

The morphological observations have also been corroborated in a shallow pit which exposes the unconformity，regolith and secondary uranium mineralization in the granite.Mineralization appears to be controlled by NNE-trending vertical fracture in the basement granite，filled with quartz associated with Chlorite veins.Such an association clearly points to open-space filling and hydrothermal nature of mineralization at Amrabad village.Thus，it is evident from the field observations and four-channel differential spectrometer reading data（Chakraborty 1981），prominent sets of fractures/faults trending NNE—SSW and NW—SE，in the basement granite，have proved the concentration of secondary uranium mineralization at Amrabad.In Amrabad the intensity of fractures within the granites their intersections with the unconformity，and the role of basic dyke acting like heat engines in generating necessary geotherms，as established are similar to the Athabasca basin，Canada.

Fig.4 a，b Thin sections of radioactive granite under cross nicols and the corresponding alpha tracks obtained by using the CN film，reflecting the presence of secondary uranium minerals.c Secondary uranium minerals along the fractures in the orthoclase

2　Petrography of the host rocks and mineralogy

The host rock for secondary uranium mineralization at Srisailam sub basin is mainly granite，which is medium to coarse-grained and composed of phenocrysts of microcline，orthoclase，chlorite，sericite，epidote and quartz（Fig.2a—f）. The morphological characters of granite shows fractures& hosting clusters of chlorites and epidote veins（Fig.3a—f）. The host rocks of Srisailam quartzite varies from feldspathic sandstone to arkose.Uraninite and secondary uranophane（Fig.4a—c）are hosted by the dolerite dykes and highly altered granites consisting of plagioclase and chlorite（altered from pyroxene/amphiboles）with minor calcite，epidote，apatite，biotite are studied with CN film.

2.1Comparison with classical unconformity-type deposits of Canada and Australia

The Archean unconformity-type deposits，identified in Canada and Australia，which are low-cost，high-grade and have large resources（Needham et al.1988）.These deposits are of two classes:（a）fracture bound，and（b）clay bound，and are defined by their setting with respect to the unconformity，host rock and ore grades（Sibbald 1988）.The principal recognition criteria for this class of deposits are defined on the basis of their salient geological features and are based on parameters such as host environment，wallrock alteration，and mineralization and age constraints（Fogwill 1981）.The Srisailam sub basin of secondary uraniumoccurrencehassimilarpropertiescompare（Table 1）to the world class Canada and Australia like（i）the presence of Lower Proterozoic pelitic carbonaceous sediments，（ii）the age of the unconformity，（iii）alterations，（iv）formation of secondary uranium minerals，（v）thermal events manifested by NW—SE，NNW&SSE trends dykes acting as heat engines for uranium mobilization，（vi）NE—SW faults，fractures，basement dykes.

Table 1 Showing the U，Th readings of the Srisailam sub basin

2.2Results and discussion

The following results are shown in the Table 1 as per PGRS recordingsaveragehighestvalues（8No）amongthe15counts in the granite and quartzite rocks of the Srisailam sub basin.

The count no.1 shows highest Uranium reading in the granite as well as above the permit limits in the quartzite. Alteration studies like sericitization，chloritization，epidotization shows in the Archean basement granite and low values are showing in the quartzite rock.

2.3Dissimilarities

The most striking dissimilarity（i）in contrast to the peliticcarbonaceous schist and gneisses in both the Athabasca basin，Canada and the Pine Creek Geosynclines，Australia the host rocks for uranium mineralization，（ii）is sheared/ fractured granite at Lambapur，Chitrial and Srisailam sub basin but not in Canada&Australia，（iii）the age of Canada and Australia deposits is 1350 ma but not estimated in the Lambapur，Chitrial and NNW of Srisailam sub basin.

The surface radioactivity surveys taken up in the northern parts of the Cuddapah basin，to locate Mineralization/deposits similar to the classical unconformity-type at the unconformity between the basement and the Middle-Proterozoic metasedimentary rocks.Which reveals that the occurrence of secondary uranium minerals in the Srisailam sub basin associated with granites，dykes and Srisailam quartzites.Srisailam Formations have opened-up new investigation scenario in the northern parts of the Middle Proterozoic Cuddapah basin.The influence of the unconformity and the fractures in the basement granite on controlling the secondary uranium mineralization is now well recognized.It makes the Srisailam sub basin is a unique deposit，compare to the classical unconformity-type deposits of Canada and Australia.

3　Conclusion

The surface radioactivity surveys carried out in the northern parts of the Srisailam sub basin in the Cuddapah basin. Radioactivity measured with PGRS and more significant values recorded about 202 and 195 ppm at Chennakesavula gutta in north western part of Srisailam sub basin.Secondary uranium mineralization is associated with granites，dykes and Srisailam Formations（Quartzite）as secondary uranium minerals along the unconformity.Comparison studies of uranium mineralization with world class unconformity related uranium mineralization in Canada& Australia like（1）Alterations such as Sericitization，chloritization&epidotization are making the Amrabad a unique deposit.（2）Thermal events manifested by NW—SE，NNW&SSE trends dykes acting as heat engines for uranium mobilization，and NE—SW faults，fractures，basement dykes with some dissimilarities.Efforts in the identical geological set-up of the Srisailam Sub-basin would result in establishing many such deposits are similar grades.The petrological&morphological studies reveals that the investigation has given new report and discovery of uranium occurrence along the unconformity of Srisailam sub basin.

AcknowledgmentsThe author thank to the Supervisor as well as to the Director，Atomic Mineral Directorate for Exploration&Research，Hyderabad.Thank to BRNS projects coordinator for given opportunity to utilize the data for Ph.D.，and cooperated for publishing of the article.

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18 July 2014/Revised:27 October 2014/Accepted:11 November 2014/Published online:10 March 2015 ?Science Press，Institute of Geochemistry，CAS and Springer-Verlag Berlin Heidelberg 2015