The Stonewall adit is located on Mine Creek, an east flowing tributary of the Coldwater River, approximately 5 kilometres north of the Coquihalla Lakes and about 1 kilometre south of the the old Keystone mine (092HNW024).
The Stonewall adit is situated near the western margin of an Early Tertiary quartz diorite (Keystone) stock which has intruded the Late Jurassic and Early Cretaceous Eagle Plutonic Complex west of its contact with Late Triassic Nicola Group rocks. The stock has an estimated area of approximately 1300 by 2200 metres. The southern half of the stock is brecciated, as is the older granodiorite adjacent to it. The brecciation has been attributed to the emplacement of small stocks and dykes of porphyritic rhyolite. This event was apparently accompanied by pervasive alteration and the introduction of metallic mineralization.
Little is known about the vein(s) developed in the Stonewall adit. It is reported to be narrow (5 to 10 centimetres wide), strike north-northeast and dip steeply. Sampled numerous times between 1939 and 1954, precious metal values were found to be generally low. The only assays of significance included several silver values "in excess of an ounce" (Assessment Report 18485, page 8).
Refer to the Keystone mine for further details of area geology and typical mineralization style.