The Mugwump mercury prospect, 2.5 kilometers northwest of the confluence of Relay and Tyaughton creeks, is hosted in conglomerate of the informally named Dash Formation of the Cretaceous Taylor Creek Group, and adjacent quartz-carbonate altered serpentinite (listwanite). The serpentinite-listwanite occurs along a fault strand of a major northwest trending fault system and separates the Taylor Creek Group from rocks of the Mississippian to Jurassic Bridge River Complex (Group) to the east.

Cinnabar and stibnite occur as disseminated grains, smears on fractures, blebs, streaks and partly massive seams associated with quartz veinlets, calcite and hematite along fractures and joints within pebble conglomerate. Cinnabar is relatively abundant as disseminations within listwanite. Stibnite, as acicular needles, forms drusy clusters that occupy vugs, and also forms semimassive seams along quartz veinlets in conglomerate.

The main mineralized shear in the conglomerate trends 330 degrees and dips to the northeast, and has been traced for 457 meters. Faulting and shearing has enhanced permeability in the conglomerate. Cinnabar and stibnite were most likely deposited at relatively shallow depths from low temperature (or epithermal) hydrothermal solutions. The disseminated nature of deposition was coincident with quartz-carbonate alteration of the serpentinite.