IGNIMBRITE LAKE is a postglacial uranium concentration in lake- bottom sediments. The occurrence is recent, having formed from the interaction between uranium-rich groundwater and unconsolidated material containing organics or clay. In the case of a closed basin such as Ignimbrite Lake, concentration by evaporation may also be important. These processes are believed to still be active (Culbert and Leighton, 1988).

The area is underlain by granodiorite of the Jurassic Okanagan Intrusions which is unconformably overlain by the Nimpit Lake Member of the Marron Formation and the White Lake Formation all of the Penticton Group. The Nimpit Lake volcanics are recessive, reddish weathering, amygdaloidal, trachyandesites with minor intercalated pyroclastic deposits. The White Lake Formation consists of epiclastic sediments with minor volcanic breccia and pyroclastic rock.

The source of the uranium is thought to be the surrounding igneous and volcanic rocks, where groundwater rich in carbonate and alkali ions have leached labile uranium from fresh rock exposed after glaciation.

Ignimbrite Lake contains layered brine, with 90 parts per billion uranium in the upper part and 2800 parts per billion in the lower. The ratio of uranium to bicarbonate in these waters is moderately low (Assessment Report 6575). Auger sampling of the lake-bottom sediments has identified an area of uranium enrichment over 3,600 square metres (Culbert, 1979). A 1.5-metre thick layer averaged 0.0131 per cent uranium with a 0.5-metre section averaging 0.0193 per cent uranium (Culbert, 1979). The uraniferous layer lies 0.5 metre below the surface (Culbert, 1979).