The area is underlain by Lower-Middle Jurassic Dewdney Creek Formation (Ladner Group) tuffaceous sediments comprised of volcanic sandstone, siltstone, wacke, tuff and argillite with interlayered fossiliferous limestone. These rocks are separated from the Lower- Upper Cretaceous Pasayten Group sediments to the east, by the major northwest trending Chuwanten fault.

The Blackjack showing is hosted by interbedded tuff, quartzite and argillite which are crosscut by a coarse grained, black felsic dike trending between north and north-northeast. The main showing, located on the west side of the junction of Dewdney Creek, is exposed in an opencut about 6 metres from the creek. Disseminated pyrite, galena and sphalerite occurs in an oxidized band which ranges from 25 to 30 centimetres in width, and consists of sheared and altered wallrock.

Another showing, located about 500 metres south of the creek junction, occurs in coarser grained sediments. The same black, felsic dike crosscuts these bedded sediments nearly at right angles. On both sides of the dike, which averages 6 metres in width, mineralization consists of disseminated galena and sphalerite. The contacts are highly altered with oxidized and leached wallrock averaging 0.5 metre in width. In 1985, a 0.2-metre sample from this leached zone assayed 23.31 grams per tonne silver (Assessment Report 14714).