Inversion and modeling of heliborne ZTEM and magnetic data for uranium exploration over Papaghni, Chitravati, Kurnool and Nallamalai sub-basins around Kadapa, southern margin of Cuddapah basin

Audio Frequency Magneto-Telluric (AFMAG) is a natural source electromagnetic exploration technique which uses the EM natural fields in Audio Frequency range which are planar and propagate horizontally. The Heliborne ZTEM (Z-axis Tipper Electromagnetic) system comprises a horizontal EM receiver coil/sensor which measures the Z-component of magnetic fi eld as a continuous spectrum from 25 Hz to 1000 Hz. Vertical fi eld response is caused by the lateral conductivity/resistivity contrasts in the Earth. The vertical EM fi eld is remotely referenced to the horizontal base station coils positioned within the survey area in EW and NS direction. High resolution heliborne ZTEM, magnetic and Gamma Ray Spectrometric survey was conducted around Khajipeta, Kadapa and Rachapalli, along the southwestern margin of Cuddapah Basin to delineate the unconformity-proximal /sulphide related conductors at depth of upto 1500 m to target unconformity related uranium mineralization. The survey area exposes Nallamalai Group of rocks in NE, Kurnools in the NW, Chitravati and Papaghni Groups and the Archean granitoids in the SW. Interpretation of magnetic data indicates that the EW trending Idupulapaya fault was subjected to N-S faulting through Guvvalacheruve with the eastern block down thrown making a way for Nallamalai's to be deposited. 2D and 3D modeling of magnetic data confirms the basement low (~1 km) in the south and high (~600 m) in the north confirming the extension of Idupulupaya basement fault in NW-SE direction below the sediments. High frequency magnetic anomalies deciphered a number of NE-SW trending basement fractures which are cross-cutting the NW-SE trending Idupulupaya fault sympathetic to well-known Lakkireddipalli fracture. Uranium anomalies with peak values > 6 ppm and with eU/eTh ratios>0.6 are occurring in clusters along the south-western margin of the basin in association with Vempalle dolomite and Gulcheru quartzite of Papaghni Group. ZTEM 2D - 3D modeling and inversion results indicate both formational and discrete type of conductors less than 100 Ohm-m over background resistivity of ~5000 Ohm-m. Deep-seated flat-lying conductive feature occurring at depth of ~700 m is attributed to shale formations underlying more resistive quartzitic/dolomite and carbonate rocks, represents possible host for strata-bound carbonate hosted mineralization. This setup may be similar to well-known Tummalapalle uranium deposit. The other steeply dipping discrete conductors occurring below the sediments in NE SW faults sympathetic to Lakkireddipalli fracture could be good locales for uranium mineralization at various depths. Integrated interpretation of heliborne geophysical data has identified and delineated a number of potential targets for the exploration of carbonate hosted and fracture-related uranium mineralization. (author)