Geophysical Techniques for Uranium Exploration in Southwestern Margin of Caddapah Basin - A Case Study from Madyalabodu Area, Kadapa District, Andhra Pradesh, India.

摘要

Geophysics plays an important role in discovering the concealed atomic raw materials which are main ingredients for the generation of nuclear power. Reconnaissance and detailed geological, radiometric, hydro-geochemical surveys conducted in southwestern margin of Cuddapah basin has brought out a number of promising areas. In this part of the basin, uranium mineralization is mostly controlled by basement structural features - faults/fractures, basement shears and unconformity related strataform/stratabound types. Locally uranium anomalies have been delineated along the E-W trending fault associated with lower part of Gulcheru formation and close to the unconformity contact with basement. Sparse exploratory drilling resulted in understanding the subsurface mineralization associated with brecciated chloritized zones adjacent to the basic dykes emplaced along the re-activated fault/fracture and close to the unconformity contact between lower part of the Gulcheru sediments and basement granite. Detailed geophysical surveys comprising magnetic and very low frequency - electromagnetic (VLF-EM) surveys are conducted over an area of about 2.5 sq km for a strike length of 2 km in Madyalabodu area, southwestern margin of the basin. Magnetic surveys in the area recorded moderate to high amplitude linear magnetic anomalies representing the high susceptibility rocks. As the area is mostly traversed by non-magnetic sediments the magnetic anomalies reflects the linearly trending WNW-ESE and ENE-WSW subsurface intrusive basic dykes in strike direction. Interpretation of observed and processed magnetic image maps clearly brought out the structural fabric of the area - faults/fractures, intersection of magnetic anomaly trends indicating shear zones and alteration zones. Location of basic dykes and its strike extension were clearly demarcated from the derived image maps. Analysis of radially averaged power spectrum has yielded average depths to the magnetization contrasts at three levels. The shallow depths, 30m and 50m, has given the depth to top of intrusive basic dykes and depth of 120m represent the deepest part of the basin in the survey area. VLF-EM in-phase and out-of-phase induced vertical magnetic field components, apparent resistivity and phase parameters are recorded to identify the fault/fracture and alteration zones. VLF-EM responses also supported the magnetic anomalies in identifying the structural features and alteration zones. The moderate magnetic anomaly amplitudes associated with low resistivity and cross overs on in-phase and out-of-phase anomalies are interpreted due to the altered/brecciated zones adjacent to the concealed basic dykes in Gulcheru formation. The mineralization intercepted in one of the boreholes drilled in the area is well correlatable with VLF-EM anomaly trends and moderate high magnetic anomalies. This characterization of geophysical signatures can be extended in the adjoining areas for further exploration by drilling.