The Union showing is a north striking, steeply west dipping shear zone within argillites and quartzites of the Pennsylvanian and possibly Permian Mount Roberts Formation. This sequence is thrust over volcanic rocks of the Lower Jurassic Rossland Group, Elise Formation. The strata are intruded by Trail pluton granodiorite of the Middle to Late Jurassic Nelson Intrusions and by syenite of the Middle Eocene Coryell Intrusions

Mineralization consists of galena and sphalerite with gold and silver values hosted within a quartz gangue in the 30 centimetre wide shear zone. Small tonnages were shipped in 1937, 1946, and 1952 for a total of 31.74 tonnes containing approximately 187 grams gold, 37.9 kilograms silver, 4.8 kilograms lead, and 2558 kilograms zinc.

In 1967, Thorpe examined the mineralogy of the outer zone deposits in the Rossland area. The Union mine is considered part of the outer zone of mineralization and hosts arsenopyrite, pyrite, sphalerite and galena. Additional minerals in this deposit include boulangerite, ruby silver and native bismuth.

In 1983, samples were collected from the old Union workings. A sample of quartzite hosting disseminated galena was taken from pyritic mineralization along a north trending carbonaceous shear that contains epidote and rich quartz veining. The sample assayed 3.22 grams per tonne gold, 1310.0 grams per tonne silver, 6.0 per cent lead, 13.2 per cent zinc, and 0.11 per cent copper (Assessment Report 11618).