The Wilmac showing is 1.5 kilometres northwest of Twelve Mile (Corral) Creek, 6.0 kilometres west-northwest of the creek's confluence with Whipsaw Creek and 15 kilometres southwest of Princeton.

A mass of hornblende pyroxenite intrudes a sequence of mafic flows, pyroclastic breccias, tuffs and argillaceous rocks of the Upper Triassic Nicola Group in the headwaters of Twelve Mile Creek. This ultramafic stock is possibly related to the Early Jurassic Tulameen Ultramafic Complex. The body outcrops discontinuously over a width of 200 metres, and is comprised of four northwest-trending lenticular pods averaging 12 metres in thickness.

Small lenses of magnetite and narrow pegmatitic veins of hornblendite, with interstitial plagioclase (epidote-altered) and apatite, are associated with areas of coarse-grained, black hornblendite in the intrusion. Disseminated chalcopyrite and pyrite occur in the mafic pegmatites and in the sheared margins of the pods.