The St Louis Crown grant is underlain by siltstone, argillite and hornfels of the Pennsylvanian and possibly Permian Mount Roberts Formation. To the north, the Mount Roberts rocks are intruded by the granodiorite of the Middle to Late Jurassic Trail pluton and to the south by rocks of the Early Jurassic Rossland monzonite. Augite porphyry (Rossland sill) of the Elise Formation, Rossland Group is also reported in the area. The country rock is highly metamorphosed with some skarn alteration as a result of the intrusions.

Weak sulphide mineralization consisting of disseminated pyrite, pyrrhotite and minor chalcopyrite occurs in the area. Lenses of mainly pyrrhotite also occur. Chalcopyrite is found in the skarn and other metamorphic rocks in the area of the St. Louis shaft.