Galloway and Henry reported in 1981 that uranium can be released from volcanic glass shards by solution of the shards. Studies of the Tertiary Tascotal Formation of Trans-Pecos Texas suggest that total solution of shards during diagenesis in an open meteoric flow system does not release uranium for migration. Instead, uranium is only redistributed locally. In the Tertiary Catahoula Formation of the Texas Coastal Plain, uranium also was not mobilized for long-distance migration and ore formation during diagenesis of glass in the zone of saturation within an open meteoric flow system but was released during solution and argillation of glass by subaerial soil-forming processes and diagenesis in the vadose zone.
Tuffaceous facies are depleted in proportion to the intensity of vadose and pedogenic processes. It is unclear why uranium mobility is limited within meteoric flow systems. Release for long-distance migration can be documented by chemical evidence for depletion of uranium in potential source beds. Soil processes impart typical vadose microscopic textures and cause formation of distinctive outcrop features. Diagenesis in open meteoric flow systems occurs below the water table producing characteristic isopachous rims of secondary minerals. Together such field, textural, and geochemical criteria allow recognition of functional sources of uranium, which in turn provides a rapid, inexpensive valuation of regional resource and exploration potential.
The authors concluded that although four possible modes exist for release of uranium from volcanic glass, only total solution of glass has been shown to operate at a significant scale. Selective leaching of uranium without complete solution probably does not occur and evidence for slow release from alteration products of glass is equivocal. Uranium is not present in significant quantities on the surface of freshly erupted shards, so washing of absorbed ions will release little uranium. Further, geologically continuous, slow release is not compatible with evidence for a temporally restricted mineralization epoch typical of major sandstone uranium districts (Galloway et al., 1979).
Total solution of glass releases uranium to migrate only when it occurs in vadose environments very early in the history of the sediment. Complete solution of glass beneath the water table during diagenesis in open hydrologic systems causes only local redistribution of uranium. These observations permit the development of rapid, inexpensive prospecting methods aimed at the recognition of functioning source rocks. Such source rocks are reliably recognized by genetic facies relationships and by textural features visible in hand specimens and thin section. The release of uranium can be confirmed by chemical analysis. Use of these techniques early in the evaluation of regions can lead to a ranking according to the likelihood of epigenetic uranium occurrence. Conventional geologic, geophysical, and geochemical prospecting techniques and models can then be concentrated in geographic areas and geologic horizons where deposits are most likely.
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