The McQuesten River region in the northern part of the McQuesten and Mayo map areas (scale 1:250 000) is underlain by Upper Proterozoic to Mississippian rocks that were deposited in an offshelf setting during the formation of the northern Cordilleran continental margin, deformed during the Mesozoic, and intruded by pre and post-kinematic intrusions. The Selwyn Basin phase of evolution of the continental margin is represented by rock units that correlate with units defined in the eastern part of Selwyn Basin. Dark clastic and rare felsic metavolcanic rocks of the Deconian-Mississippian Earn Group unconformably overlie rocks of the Selwyn Basin phase and are overlain conformably by the Mississippian Keno Hill quartzite. Dark, fine-grained metaclastic rocks of unknown age locally overlie Keno Hill quartzite. Four episodes of plutonism can be distinguished in the area, the earliest probably Early Paleozoic in age, another mid-Triassic in age, and two phases of Cretaceous granitic magmatism. Early Paleozoic bodies are typically metre-scale, fine-grained diabasic dikes and sills intruding rocks of the Hyland Group. Mid-Triassic diorite to gabbro occurs in discontinuous pods of various sizes, primarily in the Tombstone Thrust sheet where they intrude Devonian and Mississippian rocks. The most voluminous and widespread granitic rocks are the early Late Cretaceous Tombstone intrusions (92 ± 2 Ma). Typical Tombstone intrusions are weakly porphyritic, medium-grained hornblende-biotite granite to granodiorite, but they range from syenite to granodiorite and are locally peraluminous. The latest episode of granitic magmatism, the 65 ± 3 Ma McQuesten intrustions, is not yet fully delineated but includes five stocks of peroluminous potassium feldspar megocrystic granite. Paleozoic and Mesozoic structures occur in the region. The Sprague Creek Fault, a pre-Late Cambrian normal fault, is inferred from stratigraphic relationships. A possibly Jurassic phase of shortening is represented by west-northwest-trending, south-vergent folds that pre-date Jura-Cretaceous structures. The most pervasive and important phase of deformation is Jura-Cretaceous in age and kinematically complex. The Robert Service and Tombstone thrusts and Tombstone Strain Zone formed between the Late Jurassic and early Late Cretaceous during northward and northwestward displacement of more southerly hanging wall rocks. The McQuesten River region has numerous mineral occurrences, a long history of mining and mineral exploration and good potential for further discoveries.