Copper mineralization at Tomnadashan is associated with granitic lenses intruded into a small Caledonian diorite body. The mineralization occurs as disseminations and irregular veinlets and mainly comprises pyrite, chalcopyrite, tetrahedrite-tennantite, calcite, and quartz. There are small inclusions of bismuthinite, galena, aikinite [(Cu,Pb,Fe,Bi)S₃], and a mineral with a chemistry similar to hodrushite (Cu₈Bi₁₂S₂₂). The tetrahedrite contains up to 6.1 wt. % bismuth.

Four km to the south, at Corrie Buie, are quartz veins, hosted by meta-limestones, containing galena with minor amounts of chalcopyrite, pyrrhotite, sphalerite, electrum, schirmerite, and lillianite homologues.

The mineralogy of both deposits is consistent with that associated with acid magmatism. There are mineralogical similarities between the two deposits but the style of mineralization at Tomnadashan suggests a limited hydrothermal system and therefore the Corrie Buie veins may be related to felsic intrusives to the south.