RESIDENCE OF URANIUM IN ROLL-FRONT DEPOSITS: A CASE STUDY

Roll-front deposits represent one of the main sources of uranium worldwide. They occur in permeable fine- to coarse-grained sandstone aquifers interbedded with siltstone and mudstone within intramontane basins. Despite generally low uranium grades, roll-front uranium deposits are attractive exploration targets as these deposits are amenable to in-situ mining techniques. Previous studies have proposed that in-situ leaching mobilizes uranium through the dissolution of uraninite. However, due to the unfavorable kinetics of uraninite dissolution, the principal ore mineral must likely be a more soluble phase. To provide new constraints on the residence of uranium in roll-front deposits, ore samples from two deposits in Nebraska and Wyoming were analyzed using a combination of different microanalytical techniques.

Uranium phases in the sandstone samples were too fine-grained to be detected using optical microscopy and automated mineralogy analysis. For this reason, fission track mapping on thin sections was conducted to qualitatively map the distribution of grains emitting alpha radiation and to target areas for additional microanalytical research. The fission track mapping showed that radioactive phases are primarily present in the clay-rich groundmass of the sandstone and commonly enriched in areas containing abundant organic material and Fe sulfides. Quantitative trace element mapping of selected areas (0.25mm²) using EPMA was performed to confirm the presence of uranium and to identify the exact location of the grains emitting the alpha radiation. High-resolution BSE imaging on a FE-SEM revealed that the uranium is contained in small (<1 µm) wispy phases, forming grain coatings, fine disseminations within the clay-rich matrix, and rare irregularly shaped aggregates. EDX spectrometry showed peaks for uranium, calcium, silicon, and possibly aluminum. The spectra obtained suggest that uranophane (Ca(UO₂)₂(SiO₃OH)₂ · 5H₂O) is the main U-bearing mineral in the roll-front ore. However, due to the small size of the grains, quantitative chemical analysis could not be performed and contributions to the spectra from surrounding mineral grains cannot be entirely ruled out. To allow confident mineral identification, future research will involve TEM investigations on the U-bearing phases.