Isotopic Evidence for Reductive Immobilization of Uranium Across a Roll-Front Mineral Deposit

We use uranium (U)
isotope ratios to detect and quantify the extent
of natural U reduction in groundwater across a roll front redox gradient.
Our study was conducted at the Smith Ranch-Highland in situ recovery
(ISR) U mine in eastern Wyoming, USA, where economic U deposits occur
in the Paleocene Fort Union formation. To evaluate the fate of aqueous
U in and adjacent to the ore body, we investigated the chemical composition
and isotope ratios of groundwater samples from the roll-front type
ore body and surrounding monitoring wells of a previously mined area.
The 238U/235U of groundwater varies by approximately
3‰ and is correlated with U concentrations. Fluid samples down-gradient
of the ore zone are the most depleted in 238U and have
the lowest U concentrations. Activity ratios of 234U/238U are ∼5.5 up-gradient of the ore zone, ∼1.0
in the ore zone, and between 2.3 and 3.7 in the down-gradient monitoring
wells. High-precision measurements of 234U/238U and 238U/235U allow for development of a
conceptual model that evaluates both the migration of U from the ore
body and the extent of natural attenuation due to reduction. We find
that the premining migration of U down-gradient of the delineated
ore body is minimal along eight transects due to reduction in or adjacent
to the ore body, whereas two other transects show little or no sign
of reduction in the down-gradient region. These results suggest that
characterization of U isotopic ratios at the mine planning stage,
in conjunction with routine geochemical analyses, can be used to identify
where more or less postmining remediation will be necessary.