The Jackass showings consist of three pegmatite rich areas within the Middle to Late Jurassic Nelson Intrusions and Valhalla complex plutonic rocks on the southeast flank of the Passmore Dome.

The granitic intrusions have background radioactivity in the order of 30 to 50 counts per second by scintillometer (GRS 101A). Flat lying pegmatite sills, consisting of about 50 per cent plagioclase, 25 to 30 per cent quartz and lesser amounts of muscovite, biotite, garnet and opaques, contain erratic zones of radioactivity up to 5000 counts per second near the hanging wall contacts in relation to coarser-grained pegmatite sections. Disseminated uraninite is the primary uranium mineral, which is generally associated with biotite within the pegmatite.