The area around Swakum Mountain consists of folded Upper Triassic Nicola Group volcanic rocks with interbedded sedimentary units. These rocks are intruded by large north trending felsic to intermediate intrusions (batholiths) east and west of the mountain. Nicola Group rocks on the mountain strike north to northeast with generally steep dips. For a large part they consist of andesitic flows and tuffs, agglomerates, and occasional basalts and rhyolites. A break occurs in the volcanic stratigraphy and is comprised of a mixed volcanic-sedimentary unit consisting of a thick sequence of felsic volcanic flows, lithic and crystal tuffs, limy sediments and a prominent limestone. This unit has a northeast strike and crosses the mountain for a 2.5 kilometre strike length. The unit has been historically used as a marker horizon in interpreting a large, asymmetrical, south plunging anticline with its north trending axis near Swakum Mountain summit. Narrow quartz porphyry dykes locally intrude the Nicola Group sequence. To the east of this marker unit are a thick, unconformable wedge of immature sediments, predominantly coarse polymictic conglomerates (fan-type) and grits with minor cherty units. Most of the old workings on the mountain occur in close proximity to or within this volcanic-sedimentary unit and consist of polymetallic skarn-type mineralization, lead-zinc-silver bearing quartz veins and replacements, and polymetallic quartz veins.
The property covers the contact zone between the volcanic and sedimentary sequences of the Upper Triassic Nicola Group. Limestone and conglomerate beds strike north and dip steeply to the east. At the 1615 metre elevation, a 30 metre wide band of dark grey limestone is exposed for 106 metres in a northerly direction. The limestone is massive and contains scattered thin stringers of white calcite, some chert nodules and vague shapes that may be fossils. A sample was taken across 30 metres at the centre of the exposure and analyzed 0.28 per cent Fe2O3, 0.07 per cent MnO, 0.44 per cent MgO, 51.22 per cent CaO, 0.026 per cent P2O5, 0.03 per cent S, 40.8 per cent Ig. Loss, 0.04 per cent H2O, 7.08 per cent Insol. and 0.46 per cent R2O3 (Minister of Mines Annual Report 1958).
The Thelma occurrence consists of one shaft, underground workings and a number of surface trenches, all of which have collapsed and filled in since work ceased in 1940. Silver-lead-zinc mineralization is exposed in tabular and lenticular garnet-epidote skarn zones up to 5 metres wide within the limestone. Pyrite, galena and sphalerite, with gold and silver values, occur as metasomatic replacements along bedding planes and as disseminations throughout the limestone. Minor copper values are also associated with the skarn. Quartz veins 10 to 15 centimetres wide are hosted by Nicola Group andesitic rocks near the volcanic-sedimentary contact. These veins carry galena and sphalerite with minor gold and silver values.