The region in which the Jen 4 showing occurs is within the Intermontane Belt, underlain dominantly by Lower to Middle Jurassic volcanic and sedimentary rocks of the Hazelton Group. These as- semblages are overlain by the Upper Cretaceous to Lower Tertiary Ootsa Lake Group and Miocene plateau basalt. Intruding Lower Jurassic rocks of the Hazelton Group in the northeastern part of the map sheet is a belt of granodiorite, diorite and quartz diorite plutons of the Lower Jurassic Topley intrusive suite. Felsic plutons of probable Cretaceous age intrude both Lower and Middle Jurassic Hazelton strata.

The Topley intrusive suite ranges in composition from diorite to alaskite but consists mainly of quartz monzonite and diorite. In the area of the showings two main intrusive phases have been recognized, the Nithi quartz monzonite and the Casey quartz monzonite to alaskite.

The showing is within Casey alaskite which is surrounded by Nithi quartz monzonite. A set of narrow northeast striking quartz- molybdenite veins occur in the hanging wall of a fault. Alteration of the wallrock consists of weak to strong argillic and potassic alteration assemblages.