Revision geological mapping has shown that the Robert Service Thrust is imbricated and isoclinally folded in Mayo map area. On Fork Plateau, highly deformed Keno Hill quartzite and meta-diorite that occur above the hanging wall Hyland Group are inferred to be antiformal windows resulting from folding of the fault surface. The occurrence of vuggy meta-diorite on Fork Plateau, geochemically anomalous headwater creeks and placer mining downstream, indicates that the Fork Plateau may warrant further exploration.