The northeast corner of Grass Lakes map area is underlain by polydeformed metasedimentary, metavolcanic and metaplutonic rocks of Yukon-Tanana Terrane, and ultramafic rocks of unknown affinity. Layered metasedimentary and metavolcanic rocks, about half the area mapped, include a basal unit of quartz-rich schist, psammite, metapelitic schist, and grit with a laterally continuous sandy marble marker high in the unit; a unit made up of a variety of mafic metavolcanic (?) rock types interlayerd with carbonaceous phyllite, grey quartzite, quartzofeldspathic psammite and grit; Devono-Mississippian felsic metavolcanic (?) rocks interlayered with carbonaceous phyllite and grey quartzite, and an upper mafic metavolcanic (?) unit with quartzofeldspathic psammite and grit, and carbonaceous phyllite and quartzite. Metarhyolite of the third unit hosts the Kudz Ze Kayah massive sulphide deposit and underlies numerous gossans. The succession is thought to be upright based on an interpretation of the structure of metarhyolitic rocks at Kudz Ze Kayah. The remainder of the area is underlain by metaplutonic rocks. Quartz-feldspar metaporphyry near Kudz Ze Kayah is thought to be a hypabyssal intrusion coeval with the adjacent metavolcanic pile. Coarse-grained granitic to monozonitic Grass Lakes Orthogneiss of probably Early Mississippian age underlies much of the area; it and its apophyses intrude the layered units. The undated North Lakes Metadiorite is thought to intrude the Grass Lakes Orthogneiss. Three large undated bodies and numerous discordant dykes of weakly foliated biotite-muscovite granite intrude the layered succession. Ultramafic rocks occur in two settings in the eastern part of the area. Discontinuous 10 to 100 metre-scale bodies of ultra-mafic rock occur along the contact between actinolite-chlorite schist and calcareous quartzose psammite (above) near the Pack occurrence. A km-scale body of ultramafic rock also overlies the same calcareous quartzose psammite in the prominent peak along the east side of the map area. An axial surface trace of a recumbent north-closing fold lies between the two localities suggesting that the ultramafic rocks are folded. All rocks except biotite-muscovite granite and ultramafic rocks are strongly foliated and lineated. The prominent foliation transposes an older foliation and bedding and is axial-planar to south-vergent folds mappable at 1:50 000 scale. This deformation occurred after the intrusion of the Grass Lakes Orthogneiss (therefore post-Mississippian) and waned during the emplacement of biotite-muscovite granite. Two weak later phases of deformation comprise sporadically developed north-trending, east-vergent folds of the second-phase foliation and regional arching of the second phase foliation over a southwest-trending hinge in the central part of the map area.