Roll-type uranium deposits are found in the Tertiary fluvial sandstones of the Texas Gulf Coastal Plain including the Miocene Oakville Sandstone from Live Oak County, Texas. The original source of the uranium is volcanic ash beds predominantly occurring within the Miocene Catahoula Formation. Early weathering (pedogenic processes) remobilized the uranium during devitrification of the volcanic glass. Groundwater flow in shallow aquifers has transported uranium and other elements into permeable sandstone bodies in various units.

Mineralization in the immature lithic-rich sandstone host beds typically results from both oxidation and reduction. Meteoritic waters flowed from the surface and produced oxidation fronts in the sandstone that include zones of uranium, molybdenum and selenium enrichment. Deep basinal brines with a reducing chemistry flowed upward along fault zones and superimposed pyritization and iron enrichment on the previously formed oxidation zones. The resulting overlapping mineral zonation generates a complicated geochemical system.

The Oakville sandstones are litharenites and feldspathic litharenites. Carbonate and volcanic rock fragments are common constituents along with quartz and feldspar. Reworked Cretaceous fossils are also common. Many sands are only weakly cemented by early clay cements. Others have varying degrees of calcite cement in addition to clay. Zeolite cement is a rare constituent.

Optical and scanning electron microscope (SEM) observations have been undertaken to better understand the location and mineralogy of the uranium in these sandstones. Previous studies have reported that uranium occurs 1) as a discrete phase (pitchblende) which coats grains, 2) adsorbed on clay-mineral coats of sand grains, 3) associated with leucoxene cement around grains, and 4) adsorbed in clay-mineral-rich rock fragments. Preliminary results from new SEM observations suggest that uranium occurs in discrete micrometer- to nanometer-scale phases in clay coats and clay-rich grains in addition to absorption. Uranium-bearing phases previously identified include the hydrous U-silicate minerals, boltwoodite, weeksite, and coffinite as well as uraninite. This study has found in addition umohoite and abundant calcurmolite (U-bearing hydroxide minerals) and a U-Ti oxide phase (brannerite-thorutite solid-solution?). The combination of previous and new research shows the presence of complex U-systematics in the sandstone ores.