Gneiss and augen gneiss of the Castlegar gneiss complex, of unknown age, are intruded by a Middle Eocene Coryell syenite stock. Greisen and pegmatite zones occur in a cupola of the syenite.

Two of the larger quartzose pegmatite-greisen zones, 300 metres apart, measure 20 by 30 metres and 8 by 30 metres. They consist predominantly of coarse-grained feldspar with minor quartz and muscovite and contain niobium-tantalum oxide minerals. The black radioactive minerals are likely samarskite and ilmenorutile or niobium rutile. They occur as disseminations in reddish feldspar and patches up to 5 centimetres across. A chunk of samarskite assayed 5.8 per cent uranium, 2.5 per cent thorium, 25.7 per cent niobium and 9.8 per cent tantalum (Minister of Mines Annual Report 1956, page 77). In 1967, a sample from the area assayed 2.2 per cent niobium, 0.14 per cent uranium and 0.32 per cent yttrium (Metcalfe, 1967). A more recent grab sample assayed 1.25 per cent Nb2O5 and 0.14 per cent Ta2O5 (Assessment Report 14652).

The niobium-tantalum minerals are formed from magmatic fluid by a process of differentiation and concentration of metal-rich volatile phases in the roof of the intrusive body (Assessment Report 14652).