The area of the Three Musketeers showing is underlain by basalt and andesite of the Upper Triassic Karmutsen Formation, Vancouver Group. Stocks of quartz diorite related to the Tertiary Late Eocene to Early Oligocene Mount Washington Intrusive Suite (formerly Catface Intrusions) intrude the strata.
Little is known of these deposits except that they appear on Carson's map of mineral deposits of Vancouver Island (Carson, 1968). The Brown occurrence, located on Carson's map within a few kilometres to the northeast of the Gem Lake prospect (092F 239), is classified as a gold-quartz vein or fissure zone. The Three Musketeers showing, located within a few kilometres to the northeast of the Brown showing, is classified as a copper-shear zone type deposit.