The Lillomer mercury prospect is at the headwaters of North Cinnabar Creek, 5 kilometres west-northwest of the north end of Tyaughton Lake, and adjacent to the informally named Castle Pass fault (Fieldwork 1988, page 115-143).

The showings are hosted in greenstones, argillites, schists, quartzites and cherts of the Mississippian to Jurassic Bridge River Complex (Group). The greenstone occurs as 6-metre thick discontinuous interbeds in the northwest trending metasediments. The metasediments also include occasional narrow bands of red and grey limestone and small bodies of carbonatized serpentinite.

Mineralization is concentrated in fractured greenstone and along the contact between greenstone and the underlying quartzite units. Small veinlets and stringers of calcite, dolomite, quartz and pyrite carry seams (up to 12 millimetres) and disseminations of cinnabar and occasional globules of native mercury. Solid masses of cinnabar assay up to 9.5 per cent mercury and samples taken over 2 metres of branching quartz-cinnabar veins average up to 0.4 per cent mercury (Minister of Mines Annual Report 1929, page 234).