Genetic models for uranium mineralization found at Proterozoic unconformities between fluvial sandstones and crystalline lithologies include three general geologic features: (1) structural relief at the unconformity; (2) permeable zones controlled by lithology and faulting; and (3) graphitic and sulfide-bearing zones commonly associated with mantled gneiss domes. Though radiometric surveys have been successfully used where the fluvial rocks are thin or eroded away, non-radiometric geophysical methods are commonly used in the interior parts of the sedimentary basins to detect one or more of the above geologic features to define favorable areas for uranium. In the Athasbasca basin of Canada, airborne and ground electromagnetic (EM) methods as currently used are thought to h ve a depth of exploration of 200 m. Application of these non-radiometric geophysical methods to Precambrian sedimentary basins in the United States is not likely to produce as great a depth of exploration because the fluvial cover rocks are not as geophysically transparent as the Athabasca sandstone. We suggest that two approaches be used to improve the

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depth of exploration and resolution of non-radiometric geophysical methods. The first approach is to use EM methods that have not been commonly used such as the controlled-source or natural-field audiomagnetotelluric method which can have a depth of exploration of several kilometers. The second approach is to use advanced geophysical interpretation methods to define responses from the alteration halos around the uranium mineralization. Advanced Induced Polarization surveys may detect such halos as evidenced by applications in Tertiary sedimentary basins.

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