The Congress Extension (Primrose vein) is 0.5 kilometers north of the west end of Marshall Lake. The prospect is within argillite, chert, greenstone and minor limestone of the Mississippian to Jurassic Bridge River Complex (Group) and is adjacent to the Marshall Creek fault zone. The fault zone is intermittently occupied by serpentinite and listwanite, assigned to the Permian and older Shulaps Ultramafic Complex. The fault separates Bridge River rocks to the south from slate and argillite of the Upper Triassic Hurley Formation (Cadwallader Group), to the north.

Two parallel quartz veins, up to 2.0 metres in thickness and 240 metres in length, strike northwest and follow a chert-argillite- listwanite contact. The quartz veins contain minor disseminated pyrite and rare chalcopyrite, with little or no gold.