South of Botts Lake on Dunbar Creek, magnesite-bearing dolomite occurs in the lower part of the Middle Proterozoic Mount Nelson Formation of the Purcell Supergroup.
Botts Lake magnesite outcrops were traced over a distance of 118 metres along strike. The magnesite-bearing unit is at least 10 metres thick, strikes 130 degrees and dips 47 degrees east. The footwall consists of hard, aphanitic to fine-grained, dark grey to black dolomite which weathers pale grey. It is cut by pale grey dolomite and milky white quartz veinlets (<5 millimetres thick). Light to medium grey dolomite which weathers pale fawn in colour overlies the dark dolomite. The magnesite-bearing rock is snow white and weathers white or light grey. Field estimates indicate that the rock consists of a mixture of dolomite (40 to 70 per cent) and magnesite (30 to 60%) and is expected to have lower magnesia content than other magnesite deposits of the Brisco area. Traces of enargite (Cu3AsS4) were found in hairline fractures within this horizon.