A stream sediment survey was conducted in the Pena Blanca uranium district, Chihuahua, Mexico, to determine the applicability of gamma ray spectrometry as an exploration method for volcanogenetic uranium deposits. The Pena Blanca district is 40 km northeast of Chihuahua City, in outflow facies of ash flow tuffs, which are 45 to 35 m.y. old. The minerlization in the district is confined to these ash flows and the underlying Cretaceous limestones.

Stream sediment samples were collected and 500-g, 100-mesh separates were analyzed for eU, eTh, and eK. The samples were also analyzed for both acid extractable uranium and total uranium.

The results of the study show that, with the use of two standard deviations from the mean as an anomalous value, the gramma ray data show anomalies, not over the deposits, but at the break in slope in the streams and at the proximal edge of alluvial fans. Other anomalies were detected by this method, but they were single-sample anomalies. The extractable uranium and the total uranium data show a high degree of correlation (0.92), and also correlate well with the eU values determined for the stream sediments. Uranium being mobilized from the Pena Blanca deposits produced geochemical anomalies at the tops of alluvial fans. This knowledge can be applied to exploration programs elsewhere.

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