The Cretaceous Horsethief Batholith intrudes Helikian Purcell sediments. The intrusion is zoned from fine-grained - medium-grained granodiorite to coarse-grained quartz monzonite.
Black sand placer concentrations containing uranium-and columbium-bearing minerals occur in the outwash gravels from glacier action in the Horsethief Batholith. Minerals include euxenite- polycrase, pyrochlore, uraninite, uranothorite, allanite, andalusite, apatite, epidote, fluorite, garnet, hematite, ilmenite, magnetite, pyrite, sphene, zircon, lepidocrocite, cassiterite, and molybdenite.
The placer concentrations occur intermittently along Forster Creek and its tributaries for 12 kilometres. A heavy mineral fraction from the upper part of the creek assayed 0.11 per cent uranium (Assessment Report 6593).
See also East Creek (082KNE006), Vowell Creek (082KNE007), Malloy (082KNE008) and Upper Bugaboo (082KNE023).