The basal-type uranium deposits in south central British Columbia occur within unconsolidated, late Miocene fluvial paleochannel sediments that overlie major fault zones within the Okanagan Highlands Intrusive Complex. Prior to the formation of mineralization, paleovalleys containing host sediments were flooded by Pliocene valley basalts of the Plateau Basalt Formation. Five uranium deposits have been outlined to date, of which the Blizzard (4,020 metric tons U) and Tyee (650 metric tons U) are the largest. The basement intrusive complex underlying the deposits varies in age from early Cretaceous to Eocene and is comprised of quartz monzonite, granodiorite, Coryell monzonite, porphyritic granite, and pegmatite. All these phases are fractured and altered due to repeated tectonic activity. Structural analysis of the complex leads to the conclusion that it is a fabric-loosened body of interconnecting fault and fracture systems capable of sustaining well-developed intermediate and regional ground-water flow systems.The mineralized host sediments comprise a complex sequence of interfingering conglomerates, arkosic sandstones, and mudstones. Organic material is abundant in many, but not all, mineralized beds. Uranium mineralization is present in the form of uranous (ningyoite) or uranyl (saleeite, autunite) phosphates coating clastic grains and filling voids. Because of very strong reducing conditions related to large concentrations of marcasite and organic material, ningyoite is the only uranium mineral in the Tyee deposit, whereas the Blizzard deposit contains a more complex assemblage of minerals (saleeite, autunite, ningyoite). The observed paragenetic sequence of mineral precipitation in the Blizzard deposit (autunite-saleeite-ningyoite) indicates that the uranyl minerals, saleeite and autunite, are primary. Small amounts of Zn, Ni, Co, and Mo have been introduced into the Blizzard deposit whereas Mo is the only nonore-forming element enriched in the Tyee deposit. Mn is the only depleted element in both deposits.The paleoclimate at the time of ore formation was characterized by a temperate climate accompanied by a moderate amount of uniformly distributed precipitation.Investigations of the source of the ore-forming elements (U, Ca, Mg, PO 4 ), including ground-water leaching of the intrusive basement rocks, volcanic rocks (Eocene), and host sediments, as well as emplacement of ore by hydrothermal activity, have led to the conclusion that these elements were derived by ground-water leaching of the Okanagan Highlands Intrusive Complex. The deposits are believed to have formed by the infiltration into fluvial sediments of deep-seated, structurally controlled, ground waters that migrated in a well-developed regional hydrologic system within the Okanagan Highlands Intrusive Complex. Research indicates that the ore-forming ground waters were cold, slightly bicarbonated (150-400 ppm), highly uraniferous (10-50 ppb), and slightly oxidizing (dissolved oxygen = 2-4 ppm). In addition, the ore-forming ground waters contained sufficient amounts of Ca, Mg, and PO 4 to account for the observed mineralogy. Upon entering the host sediments the ground waters were rapidly acidified as the result of the formation of humic acids and oxidation of iron sulfides.Mineralization formed after a period of extensional tectonism (late Miocene-early Pliocene) leading to optimum ground-water leaching of the intrusive basement complex and prior to a period of epeirogenesis (late Pliocene-Pleistocene uplift) which resulted in cessation of the ore-forming hydrologic regime. The maximum period of formation is estimated to be between 4 and 1 m.y.