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Two contrasting types of roll-front uranium deposits occur in south Texas. In the barrier-bar sands of the Eocene Jackson Group, organic matter was essential to uranium reduction, whereas in the fluvial sands of the Miocene Oakville Formation, epigenetic pyrite was the reductant.
In a sample of reduced Oakville ore, a uranium phase with grains ranging in diameter from < 1 to 20µm was recognized by SEM back-scattered-electron imaging and wavelength-dispersive spectrometer (WDS) elemental-dot mapping. Quantitative microprobe analyses indicated that the phase is a uranium-calcium silicate-phosphate with molar Ca/P approximately equal to 1.0, U/P equal to 2.8 ± 0.4 (n = 27), and U/Si approaching 1.0 in samples uncontaminated with quartz, feldspar, or clay minerals. Highest uranium content is 59%. Oakville ore is typically easy to leach by in-situ methods.
Jackson ore contains 2 uranium phases. Sulfur-rich organic matter contains 4.1 ± 1.6% uranium (n = 27). Although individual grains of a possible uranium mineral within the organic matter are too small to be resolved by electron imaging, a consistent molar U/Fe (0.5 ± 0.1) suggests a uranium-iron oxide phase. Alternatively, uranium is adsorbed by or otherwise bound to the organic matter. The second phase is a uranium-calcium silicate-phosphate that differs from the Oakville ore. Molar Ca/P equals 0.8 ± 0.2 (n = 13), and U/P equals 4.7 ± 0.4. Small grain size (generally less than 1 µm) prevented analysis of samples uncontaminated with quartz and pyrite. The grain with highest uranium content (43%) has U/Si equal to 0.34. Jackson ore is less favorable for in-situ leaching than Oakville ore in part because the organic-associated uranium is difficult to extract.
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