Title: Nebraska — The New Uranium Frontier
Authors: Steven S. Sibray and Dr. Marvin Carlson
Publication: The Outcrop, October 2009, p. 8, 11, 13
When people think of resources found in Nebraska, they might think of corn or cattle. Rarely do they think in terms of nuclear energy and new technology. Yet Nebraska has been a significant producer of uranium since 1991 and is arguably one of the most underexplored uranium provinces. The Crow Butte mine near Crawford, Nebraska has produced a little over 13 million pounds of uranium utilizing the innovative in situ recovery [ISR] technology. This low cost and high tech method of uranium mining has allowed Crow Butte Resources to develop this world class deposit during a period of record low uranium prices (around $10/lb). In recent years, uranium spot prices have been volatile, increasing to over $130/lb and then falling back down to around $48.00/lb. Despite the dramatic price increase, there is only one company actively exploring for uranium in the state.
The uranium mineralization at the Crow Butte mine occurs as a classic “roll front” deposit (see Fig. 1). Roll front deposits form when oxygen rich groundwater carrying high concentration of hexivalent uranium (U+6) encounters low oxygen groundwater and precipitates tetravalent uranium (U+4) minerals such as coffinite and [or uraninite. Iron minerals in the aquifer typically change from pyrite on the down gradient side of the oxidation front to hematite on the oxidized side of the interface. The change in iron mineral is very visible in drill cuttings and can be very useful as an exploration guide.
ISR technology utilizes the fact that uranium is very soluble in the presence of oxygen rich water. Oxygen enriched water is injected into the uranium bearing sand/sandstone to oxidize and dissolve the uranium minerals. The uranium rich solution is then pumped out and piped to an ion exchange plant where it is processed and the water is then recycled back to the well field.
The uranium roll front deposits at the Crow Butte mine occur in the basal sands of the Chamberlain Pass Formation (CPF) which is the oldest part of the White River Group (WRG) of Eocene age (see Fig. 2). The CPF consists of a basal white to greenish white sand or sandstone composed of coarse grains of quartz, quartzite, and chert. Extensive chemical weathering including the removal of iron and kaolinization give the sandstone a “bleached” appearance. The sandstone thickness varies from 0 to 350 feet and unconformably overlies the Cretaceous Pierre Shale or the Yellow Mounds Paleosol. The uppermost part of the CPF consists of the bright red Interior Paleosol. The Chadron Formation (CF) unconformably overlies these paleosols and consists of bluish green mudstones; thin, interbedded, lacustrine limestone beds; and localized channel sandstone deposits. In areas where the CF paleochannels have completely removed the CPF, the CF sands rest directly on top of the Cretaceous Pierre Shale. The CF sands are arkosic and contain more weatherable minerals than the CPF. Differentiation of the two sandstones is likely a critical factor in exploring for uranium.
The tuffaceous WRG rocks have long been considered the source of uranium in the Tertiary basins of Wyoming. Although the overlaying Chadron Formation bentonitic mudstones might be considered a source of uranium, evidence from paleohydrogeology (tufas and lacustrine limestone) suggests that groundwater table was high and groundwater discharge was largely local. In contrast, groundwater during the development of the Interior Paleosol Equivalent was largely oxidizing and groundwater flow was downward into the underlying aquifers (CPF and older units). The initial uranium mineralization at Crow Butte probably occurred during the development of the Interior Paleosol Equivalent at the CPF-CF unconformity. In Nebraska and Wyoming, the parent material for the Interior Paleosol Equivalent was the tuffaceous WRG. In South Dakota, the parent material appears to be the Yellow Mounds Paleosol which would be depleted in uranium in comparison to the volcanic glass found in the WRG sediments.
Exploration efforts should consider these hydrostratigraphic relationships and be directed to the CPF sands underlying the Interior Paleosol Equivalent in Nebraska and Wyoming rather than the CF sands. The distribution of the CPF sands in Nebraska is shown in Figure 3. The highest probability of finding uranium is along the margins of the main paleochannel which trends northwest to southeast from northern Sioux county to Cheyenne and Duel counties in the southern Nebraska Panhandle. The depth to the base of the CPF in most of this area is less than 1500 ft. When uranium prices increase due to projected future demand and supply constraints, Nebraska may become a significant uranium exploration frontier.
Swinehart, J.B., Souders, V.L., DeGraw, H.M., and Diffendal, R.F., Jr., 1985, Cenozoic paleogeography of Western Nebraska, in Flores, R.M., and Kaplan, S.eds., Cenozoic paleogeography of the West-Central United States, Special Publication, Rocky Mountain Section: Tulsa, Society of Economic Paleontologists and Mineralogists, p. 209-229.
Terry, D.O., 1998, Lithostratigraphic revision and correlation of the lower part of the White River Group, South Dakota to Nebraska in Geological Society of America Special Paper 325, Depositional Environments, Lithostratigraphy, and Arikaree Groups (Late Eocene to Early Miocene, North America).