The Gillis occurrence is underlain by the middle to Late Cretaceous Germansen batholith which is composed of coarse-grained granite (Fieldwork 1991, page 113). About 4 kilometres to the southwest, the batholith is in contact with sedimentary and volcanic rocks of the Upper Triassic Inzana Lake Formation, Takla Group.

Weakly serpentinized bodies of pyroxenite and gabbro within the batholith are reported to contain from 1 to 5 per cent disseminated pyrite, pyrrhotite, pentlandite and chalcopyrite (Assessment Report 19844). Malachite also occurs.