The Siwash occurrence is 100 to 200 metres east of Siwash Creek, 6 kilometres northwest of the creek's confluence with Galena Creek and 40.5 to 41 kilometres north-northeast of Princeton.

Two significant copper showings occur immediately north and south of an elongate, northwest-trending body of diorite up to 400 metres wide, in andesite of the Upper Triassic Nicola Group. This intrusion may be one of a number of monzonitic to dioritic bodies of Late Triassic to Early Jurassic age that are thought to be comagmatic with the Nicola Group.

The northernmost showing consists of chalcopyrite pods, veinlets and disseminations in skarn-altered andesite over an area 120 metres long and 50 metres wide. Some pyrite and malachite accompany this mineralization. Secondary minerals include epidote, albite, chlorite, garnet and actinolite. A channel sample assayed 0.42 per cent copper over 21.3 metres (Assessment Report 5547, page 8).

The second showing is 460 metres south-southwest, in albitized and slightly silicified andesite. Mineralization consists of chalcopyrite, mostly as disseminations, but also as veinlets, over a 36 by 20 metre area. The showing is intruded by a pinkish monzonite dike containing pyrite and sparse chalcopyrite. A channel sample assayed 0.43 per cent copper over 9.1 metres (Assessment Report 5547, page 8). A sample of a 0.3-metre section of black gouge in a northeast-striking shear zone assayed 0.4 per cent copper (Assessment Report 5547, page 9). Chalcocite and magnetite are reported to accompany mineralization in one or both showings.

Several other occurrences are found in the vicinity. Minor chalcopyrite is hosted in sheared andesite and is disseminated in weakly chloritized andesite.

This occurrence was geologically mapped, soil sampled and geophysically surveyed by Phelps Dodge Corporation, Utah Mines Ltd. and Brenda Mines Ltd. between 1972 and 1980.