The Brass Tags showing is underlain by mainly sedimentary rocks of the Middle Jurassic to Lower Cretaceous Relay Mountain Group, intruded by a series of porphyritic latite and aphanitic rhyolite dykes. These dykes, which are probably of Eocene age, are northeast trending; their emplacement may be related to a steeply dipping fault recognized in the area.

Gold mineralization occurs within brecciated siltstone related to faulting. Alteration of the siltstone to a kaolinite-sericite- carbonate-pyrite- (pyrrhotite) assemblage is intense in places and is accompanied by a few chalcedonic veins. The felsic dykes are similarly altered but locally contain arsenopyrite.

There is a strong spatial correlation between the felsic dykes and anomalous gold in soil samples. In 1983, a 1.52-metre diamond- drill hole (83-7) interval graded 1.4 gram per tonne gold (Assessment Report 11847). Three metres below this another 1.52-metre intersection, within the fault, graded 3.0 grams per tonne gold.