The Eureka Crown grant is underlain by the Rainy Day pluton of the Middle to Late Jurassic Nelson Intrusions comprised of quartz- monzonite to quartz-diorite. The monzonite is medium-grained and hosts epidote, apatite, sphene, with minor sericite and penninite.

On the property there is a quartz vein, outcropping on the east side of Little Sheep Creek, which hosts pyrite, molybdenite and chal- copyrite. The vein is within the monzonitic stock and hosts pyrite cubes. Pyritized fracture planes trend north to south.