The Cretaceous Horsethief Batholith intrudes Helikian Purcell sediments. The intrusion is zoned from fine-grained/medium-grained granodiorite to coarse-grained quartz monzonite. East trending aplite dykes cut the intrusion.

Radioactivity occurs along fractures with biotite alteration within the quartz monzonite. The radioactive veins, which are up to 65 centimetres wide contain rare earth elements. The radioactive zone is north trending, 200 metres wide and may be up to 1500 metres long.

A selected hand sample assayed 0.0075 per cent uranium, 0.063 per cent thorium, 0.0425 per cent vanadium, 0.32 per cent cerium, 0.25 per cent lanthanum and 0.118 per cent niobium. Another sample assayed 0.022 per cent uranium, 0.013 per cent thorium, 0.015 per cent lanth- anum, 0.165 per cent niobium (Assessment Report 4614). A represent- ative heavy concentrate sample from the quartz-monzonite contained the minerals: pyrochlore-microlite, euxenite-polycrase, uraninite, anatase, lepidocrocite, epidote, allanite, magnetite, ilmenite, rutile, sphene, apatite, fluorite, and zircon (GSC Memoir 369).