Uranium isotope composition of waters from South Texas uranium ore deposits

Redox conditions and associated changes in mobility of uranium (U) are tightly linked to a multitude of challenges connected with U mining in sandstone-hosted deposits and new methods that directly measure reduction or oxidation of U can inform on these questions. A novel proxy for understanding U redox chemistry has recently emerged, the volume dependent isotopic fractionation of uranium-238 (238U) from uranium-235 (235U). Novel measurements of 238U/235U isotopic ratio are combined with measurements of the more commonly utilized uranium-234/uranium-238 activity [(234U/238U)] ratio, as both isotopic ratios can be measured simultaneously. However, application of both U isotopic ratios in the contexts of exploration and environmental remediation of U ores requires characterization of these isotopic ratios across a variety of redox settings. Here, 238U/235U and (234U/238U) ratios are examined from eight transects in two U ore bodies (the Rosita and Kingsville Dome deposits) in South Texas; these sites are classic roll front deposits and exhibit a wide variety of both natural and altered redox conditions. Across all transects it is observed that (234U/238U) ratios decrease systematically towards the ore body from both the oxidizing and reducing sides, irrespective of whether the site has been mined or not. This pattern reflects geologically recent and significant U leaching and mobility and is characteristic of active roll fronts. Overall δ238U values in these transects decrease systematically towards the reducing zone. A simple Rayleigh fractionation model, where U ore is deposited from an increasingly isotopically depleted reservoir of dissolved U best explains the overall trend; very negative δ238U values likely reflect multiple cycles of U deposition and dissolution. The South Texas data set indicates that both (234U/238U) ratios and δ238U values can be variable at an individual mine site. However, overall low (234U/238U) ratios and negative δ238U values are characteristic of active roll front deposits. The comprehensive U isotopic composition of both ores and well waters represents a powerful new tool in prospecting of sandstone-hosted U ore and in environmental remediation following extraction of U ore.