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There are three main types of uranium deposits (more info) including unconformity-type deposits, paleoplacer deposits and sandstone-type deposits (Figure 1). Sandstone-type deposits are abundant in sedimentary rocks of the Colorado Plateau (Colorado - Utah, USA). These deposits formed when oxidized groundwater that had leached uranium from surface rocks flowed down into aquifers, where it was reduced to precipitate uraninite, the primary ore mineral of uranium. In some deposits reduction took place along curved zones known as roll-fronts, which represent the transition from oxidized to reduced conditions in the aquifer ([Kesler, 1994] ).

Sandstone deposits - Sandstone uranium deposits occur in medium to coarse-grained sandstones deposited in a continental fluvial or marginal marine sedimentary environment. Impermeable shale/mudstone units are interbedded in the sedimentary sequence and often occur immediately above and below the mineralised sandstone. Uranium precipitated under reducing conditions caused by a variety of reducing agents within the sandstone including: carbonaceous material (detrital plant debris, amorphous humate, marine algae), sulphides (pyrite, H₂S), hydrocarbons (petroleum), and interbedded basic volcanics with abundant ferro-magnesian minerals (eg chlorite). There are three main types of sandstone deposits:

• rollfront deposits - arcuate bodies of mineralisation that crosscut sandstone bedding;
• tabular deposits - irregular, elongate lenticular bodies parallel to the depositional trend, deposits commonly occur in palaeochannels incised into underlying basement rocks;
• tectonic/lithologic deposits - occur in sandstones adjacent to a permeable fault zone.

The United States has large resources in sandstone deposits in the Western Cordillera region, and most of its uranium production has been from these deposits, recently by in situ leach (ISL) mining. The Powder River Basin in Wyoming, the Colorado Plateau and the Gulf Coast Plain in south Texas are major sandstone uranium provinces. Other large sandstone deposits occur in Niger, Kazakhstan, Uzbekistan, Gabon (Franceville Basin), and South Africa (Karoo Basin). Kazakhstan has reported substantial reserves in sandstone deposits with average grades ranging from 0.02 to 0.07% U.