The Mudmain magnesite showing is approximately seven kilometres northeast of Tyaughton Lake and is exposed at the junction of the Noaxe and Mudmain Forest Service roads. The exposure is at least 50 by 30 metres of quartz-carbonate-fuchsite altered serpentinite (listwanite) within a strand of the Cretaceous to early Tertiary Marshall Creek fault system. The serpentinite is probably related to the Permian and older Shulaps Ultramafic Complex to the east.

Magnesite which occurs as crystalline masses in the outcrop contains anomalous mercury and antimony. A grab sample of listwanite with quartz stockwork assayed 0.009 gram per tonne gold and 1.05 per cent antimony (Open File 89-4, Sheet 2, Sample 8S002).