The Pruett, Duff, and Tascotal Formations (Eocene-Oligocene) form a 1-km thick sequence of tuffaceous sediment composed, prior to diagenesis, of rock and mineral fragments and volcanic glass. Ground-water diagenesis dissolved glass and some mineral fragments to produce the following mineral assemblages from top to bottom: (1) hydrated glass, (2) clinoptilolite-opal, (3) clinoptilolite-quartz, and (4) analcime-quartz. Calcite and montmorillonite formed early and are present throughout the section. The presence of minor uranium mineralization in underlying Cretaceous rocks and in channel sandstones and lacustrine deposits in tuffaceous sediments and the presence of uranium concentrations up to 100 ppb in present ground water in tuffaceous sediments confirm that some uranium mobilization has occurred.

Average uranium concentration increases down section reflecting either (1) greater primary abundance in older sediment due either to greater abundance of glass or more U-rich

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glass or (2) minor enrichment in uranium resulting from diagenesis. Whole-rock Th/U ratios average about 2 whereas ratios in glass alone are about 3 to 4, implying that a significant fraction of uranium is contained in relatively Th-poor detrital components such as zircon. Th/U ratios in sediments do not show any correlation with either stratigraphic position or mineralogic zone, implying that no measureable uranium depletion has occurred.

Fission-track maps of glassy sediment show that uranium occurs in glass and in high concentrations in zircons and amorphous iron hydroxides. In altered rocks uranium is enriched in amorphous iron hydroxides and is unevenly distributed in moderate concentrations through the groundmass associated with no distinct mineral. Opal, calcite, clinoptilolite, and analcime contain little uranium. This shows that solution of glass, including minor etching during hydration, releases uranium but most is immediately absorbed by hydroxides. Later recrystallization of diagenetic minerals does not mobilize uranium. Diagenesis which affected amorphous iron hydroxide could mobilize uranium.

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