The Iolanthe showing is located on the east side of Kootenay Lake, "near the lakeshore", about 0.5 kilometre north of Sanca Creek.

The showings are hosted in metasedimentary rocks tentatively assigned to the Purcell Supergroup of Middle Proterozoic age, but the formation is uncertain. These rocks, which comprise quartzose mica schist, talc schist and quartzite, form a roof pendant in the middle Cretaceous Bayonne granite batholith. Stringers and veinlets of quartz with pyrite, sphalerite and galena lie parallel to the foliation, which strikes southeast and dips east. A grab sample of the hangingwall mineralization (from the dump) assayed 16.5 per cent zinc, 3.4 per cent lead, 190 grams per tonne silver and 1.4 grams per tonne gold (Minister of Mines Annual Report 1926). Copper is mentioned in one Minister of Mines Annual Report (1927) but no copper minerals are described.