Because of its molecular structure and properties, uranium is most commonly found in specific rock formations. Uranium roll-front deposits typically form in relatively near-surface sandstone aquifers of fluvial or lacustrine origin. As noted above, deposition of uranium and other metals occurs in these deposits at redox interfaces where the change from oxidizing to reducing conditions causes uranium deposit formation. Groundwater travels from oxidizing conditions near the surface, where it solubilizes low concentrations of uranium and other metals from soils and rock into the deeper portions of the aquifer, where chemically reducing conditions are present. These reducing conditions are commonly attributed to the increased abundance of organic carbon from woody debris and plant material (kerogen), pyrite (FeS2) or hydrogen sulfide (H2S) gas. As oxidized groundwater moves into the reduced zone, the dissolved uranium is precipitated in its reduced mineral form and removed from solution. In addition to uranium, elements such as arsenic, molybdenum, iron, manganese, selenium, and vanadium, which generally are mobile in oxidized conditions, also are precipitated in the vicinity of uranium roll-front deposits because of the low solubility of their reduced forms. Ore-related minerals reported in uranium roll-fronts include the uranium minerals uraninite [UO2(s)] and coffinite [U(SiO4)1-x(OH)4x], the iron sulfides FeS2 and marcasite, hematite (Fe2O3), iron silicates, calcite (CaCO3), gypsum (CaSO4•2H2O), ferroselite (FeSe2), native selenium (Se), molybdenite [MoS2(c)], and jordisite [MoS2(am)].mineralized portion of the roll front appears black in the photograph. This black area corresponds to the yellow area in the diagram above. The boundary between oxidized and reduced conditions in the aquifer represents a redox interface at which metals become concentrated. Over time, the redox interface will migrate in the direction of groundwater flow, creating a redistributed ore body that can extend laterally across the groundwater gradient for hundreds or thousands of feet but only tens of feet in the downgradient direction. The characteristic “roll” shape of the redox interface is formed over time as groundwater moves more rapidly through the central, more permeable portion of the sandstone. Within the host aquifer, the oxidized and reduced zones can be distinguished by color and mineralogy. The oxidized portion of the aquifer is frequently yellow to red, containing iron oxides and ferric iron clay minerals that have replaced FeS2. In some cases, the oxidized portion of the aquifer has been reduced after ore zone formation by the influx of strongly reducing fluids containing H2S. A range of permeabilities commonly is observed within the host sedimentary unit because of spatial differences in depositional environments.

Generic Environmental Report in Support of the Nuclear Regulatory Commission’s Generic Environmental Impact Statement for In Situ Uranium Recovery Facilities

National Mining Association (NMA) - November 30, 2007

The image below is of an actual rollfront in a pit wall.