Diagenesis of the Ogallala (upper Miocene-Pliocene?) and Pearlette (Pleistocene?) rhyolitic volcanic ash in western Oklahoma was studied. The Ogallala ash ranges from relatively fresh to highly altered. Chemical changes during its devitrification are characterized by a loss of silicon, sodium, potassium, and uranium, and addition of magnesium to the ash. The thorium to uranium ratios range from 4:1 to 10:1 for relatively unaltered ash to 16:1 to 64:1 for the highly altered ash. The average loss of uranium due to devitrification is greater than 3 ppm. Migration of the released uranium in the alteration system was made possible by carbonate complexing agents.

Study of the Ogallala Formation in the high plains and equivalent formations in the Gulf coastal plain indicates that these strata originally were widespread over most of Oklahoma but that the sediments were removed

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from all but the western part of the state during late Pliocene-Pleistocene erosion. The volcanic-ash beds that had existed in the eroded Ogallala are a potential source for some of the uranium occurrences.

Analyses of the Pearlette ash did not disclose any alteration trends or a downward decrease in the uranium concentration. The uranium content of the Pearlette is significantly higher than that of the altered Ogallala ash. The minimum ash of the Pearlette is 0.6 m.y. which suggests that efficient release of uranium from volcanic glass, at least in some examples, is not a geologically contemporaneous process.

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