The Mesabi taconite deposit occurs on the west slope of Mount Sullivan. The area is underlain by schistose metasediments of the Mississippian to Pennsylvanian Fourth Lake Formation, Buttle Lake Group. Bedding and schistosity in the rocks are essentially parallel; the strike is northwest and the dip 70 to 80 degrees southwest.

The mineralized zone strikes intermittently along a hillside for about 150 metres. It consists of lenticular bands of jasper, interlayered with schist containing streaks, bands and lenses of magnetite and smaller amounts hematite. Some of the ore masses are up to 45 metres long and 3 to 6 metres wide. The deposit is cut by irregular veins and stringers of white quartz. A sample of the ore assayed 30.0 per cent iron, 53.3 per cent silica and 0.2 per cent phosphorus (Minister of Mines Annual Report 1918, page 299).