ABSTRACT

The Peddagattu, Lambapur, Chitrial and Koppunuru uranium deposits along the northern margins of the Cuddapah Basin are confined to the middle Proterozoic unconformity interface between Archean basement granites and the overlying resistive quartzites. Negative transients observed in the coincident loop heliborne time-domain electromagnetic (HTEM) data over these deposits (occurring in outliers) are believed to be due to thick polarizable conductive zones occurring along the unconformity. Similar negative HTEM responses are recorded over the Gorukunta Tanda outlier. A ground spectral induced polarization (SIP) survey conducted over the outlier and ground geologic observations indicated an altered basement/regolith with thickness up to 5 m below 20–30 m thick quartzite. Interpretation of Cole-Cole parameters computed from the SIP data indicated a change in the dispersion. These Cole-Cole parameters were used in modeling negative HTEM data assuming a polarizable plate placed in a layered earth at a depth of approximately 50 m using the CSIRO LeroiAir program. A negative dBz/dt response in the late channels indicated that the negatives can be explained in terms of inductive induced polarization effects. Modeling of HTEM dBz/dt data for the profile through the Lambapur uranium deposit and the Gorukunta Tanda reveals the presence of a polarizable lithologic unit at a depth of approximately 40 m. This unit is interpreted as an argillic alteration of basement, with the presence of clay and/or disseminated sulfides as evidenced from the core extracted from the boreholes at depths below the highly resistive quartzite. Uranium mineralization is closely associated with altered basement and sulfides along the unconformity where the distinct negative electromagnetic (EM) signature is recorded. Furthermore, there exists a good correlation between the uranium mineralization grade and the thickness versus the averaged late-channel negative HTEM response over the known deposits. The negative EM response helped in locating the new target areas for uranium exploration.

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