Summary:
The deposits are epigenetic uranium roll-fronts. The Lance Project is located on the eastern periphery of the Powder River structural basin and on the western margin of the Black Hills Uplift. Geologic strata in the vicinity of the Lance Project comprise Cretaceous marine and marginal marine fluvial sediments belonging successively to the Pierre Shale Formation, Fox Hills Formation and Lance Formation within the Lance Project. Mineralization primarily occurs within the upper Fox Hills sandstone, although in localized areas there is some mineralization within the overlying Lower Lance sandstone. All the formations of interest (Lance, Fox Hills, Pierre) outcrop immediately east of the Lance Project at the Black Hills Monocline. The monoclinal feature caused a steepening of the dip (85°to 90°) to the east of the Lance Project, which resulted in the formations outcropping more abruptly than the 1° to 2° regional dip to the west.
Roll front deposits result from oxidizing groundwater migrating down gradient through regionally reduced sediments. Subsequent geochemical cells, produced by migrating groundwater, formed deposits through a dynamic process of oxidation, dissolution, transportation, reduction, and precipitation of uranium.
Previous geological modelling of the extensive downhole geophysical data has accurately defined the impermeable shales and mudstones that form the confining seals to the mineralised aquifers.
Geological interpretation
The sandstones that make up the various formations of the Lance uranium deposits were all deposited in a fluvialmarine environment as channel sand or overbank deposits. They are characterized by fining-upward sequences comprising thick, laterally persistent, tabular, sheet-like sandstones.
Uranium mineralization occurs preferentially in the sand units of the Fox Hills or lower Lance Formations, which were deposited under more reducing conditions. Within the sandstone, uranium distribution is controlled by basin- ward migration of chemical fronts that represent the interface between reduced and oxidized sandstone. The primary uranium-bearing minerals are uraninite, uranophane, autunite, or coffinite representing tetravalent and hexavalent forms in the reduced zone with H2S and organic carbon acting as the reducing agent to precipitate uranium.
Vanadium and, to a much lesser degree, selenium and arsenic are the main associated elements.
Dimensions
In plan-view, the deposits range from several hundred meters long to over 9,000 metres long with widths of between 10 metres and 80 metres wide. The high-grade cores of the roll fronts within the deposit range from about 2 metres to 10 meters wide and average 1.5 metres thick in section.
Mineralization occurs in several horizons with a total mineralized package of up to 60m in thickness.
Host sandstones dip at -1° to -2° towards the west and southwest.