H31I-1541
Characterization of U ore from a roll-front U deposit: Implications of dominant U-Ti mineral for ore genesis and post solution-mining U immobilization

Wednesday, 16 December 2015
Poster Hall (Moscone South)
Anirban Basu1, Shaun T Brown2, John Neil Christensen2, Paul W. Reimus3, Jeffrey M Heikoop3, Giday W WoldeGabriel4, Matt Hartmann5 and Donald J DePaolo1, (1)University of California Berkeley, Berkeley, CA, United States, (2)Lawrence Berkeley National Laboratory, Berkeley, CA, United States, (3)Los Alamos National Laboratory, Los Alamos, NM, United States, (4)Los Alamos Natl Lab, Los Alamos, NM, United States, (5)Uranium Resources, Inc, 6950 S. Potomac Street, Suite 300, Centennial, CO, United States
Abstract:
Reductive immobilization of dissolved U(VI) is an important process that gives rise to roll-front U deposits as well as offers a remediation strategy after in situ recovery (ISR) mining of roll-fronts by oxidative dissolution of the U ore. About 25% of the global and over 90% of all U resources in the United States consist of roll-front deposits. Accordingly, ~50% of global U mining and almost all current U mining in the United States is ISR mining. Therefore, it is important to identify the U immobilization pathways for an improved understanding of the U ore genesis and postmining U(VI) remediation. Here, we characterize (XRD, XRF, SEM/EDS, QEMSCAN) the U ore from a roll-front U deposit and sediments downgradient of the ore from an ISR site at Rosita, TX, USA.

The dominant U mineral in Rosita U ore is brannerite (nominally U4+Ti2O6, up to 0.032 wt%), followed by coffinite and U-oxides. The U mineralized sand is composed of quartz (41-53%), calcite (15-30%), plagioclase (11-19%), microcline (2-9%), clinoptilolite (0.5-7%) with minor amounts of pyrite/marcasite (2-7%) and clays/micas (1-4%), and very little organic C (<0.1%). Ore zone samples contain minor amounts (<2%) of hematite, V-oxides/V-Ti-Fe-oxides and sulfidized Fe-Ti oxides with variable Fe, Ti and S ratios locally hosting low levels of U. The dominant sulfide mineral is marcasite. We observe a complex relationship between U-Ti minerals and sulfide/silicate phases where U minerals occur as inclusions, irregularly developed veins or intergrowths. Except for the U concentrations, the downgradient sediments are compositionally similar to the ore and contain abundant smectite/illite (7-45%).

The predominance of brannerite implies direct reduction of U(VI) on surfaces of reduced Fe-Ti oxides as a major ore-forming mechanism. Our results reveal an as yet unidentified mechanism of ore genesis that differs from the current model that presupposes the sulfidized Fe-Ti oxides as the main reductant of U(VI). Furthermore, previous research demonstrated formation of U-Ti phases structurally similar to brannerite during reduction of U(VI) by titanomagnetites. Abundant Fe-Ti oxides in the downgradient sediments suggest that the interaction between postmining residual U(VI) and Fe-Ti oxides may play a major role in natural remediation of U at Rosita and similar ISR sites.