A geological reconnaissance of 4300 sq mi of the Bear-Slave Upland was carried out in 1964-65-66. The area is underlain primarily by Archean rocks of the Yellowknife Supergroup, divisable into three overlapping phases. The oldest phase (Point Lake Formation) consists of felsic calc-alkalic to mafic tholeiitic volcanic rocks and local late mugearitic flows. A transition al phase consists of iron-formationbearing greywacke-turbidites (Contwoyto Formation) and coarse conglomerates (Keskarrah Formation). The latest phase (ltchen Formation) consists of greywacke-turbidites alone. With early Kenoran plutonism (minimum age 2642 m.y.), which affected rocks of the Point Lake Formation, a mylonite zone developed and minor ultramafic bodies were emplaced along the west margin of the area. Later gabbroic to granitic plutonism (minimum age 2500 m.y.) affected the greywacke-turbidites and was accompanied by metamorphism reaching the si llimanitecordierite- orthoclase-almandine subfacies. Optic axial angles of cordierite. crystallized as a result of this metamorphism, increase with rising metamorphic grade. Late Aphebian argillites and quartzites of the Coronation Geosyncline extend into the area from the west (Epworth Group) and east (Goulburn Group) along its north margin. An outlier of the Goulburn Group is preserved in a half graben associated with emplacement of diabase sills. West-northwest striking porphyritic diabase dykes are related to similar flows locally preserved within the Epworth Group west of the area. Northwest trending dykes of the Mackenzie swarm (1200 m.y.) are concentrated in two zones some 35 mi apart along the east margin and through the central part of the area. The principal metal occurrences are Fe-Cu showings within early mafic volcanics, Fe-Zn-Cu showings associated with felsic volcanics, and oxide-silicate-sulphide iron-formation with local syngenetic aresenical gold deposits associated with late mafic volcanism. Examination of pyrrhotite in sulphide iron-formation suggests that hexagonal (sulphur-poor) pyrrhotite is predominant in the deposit of highest metamorphic grade and may be concentrated along fold axes at lower grades.