Molybdenum extraction from the western Cordillera of North America in the last half century has grown primarily by increased production from porphyry-type deposits. The Climax mine alone has contributed approximately half the total world production in past years. Response to industrial demand, aside from by-product molybdenum, has resulted in development of five mines of the Climax-type in the last half decade, and at least one similar major ore body will begin operation by 1975. The six producing stockwork deposits accounted for approximately 71 percent of free world molybdenum production in 1969.The stockwork deposits contain 0.2-0.5 percent molybdenite, distributed throughout or near contacts of associated siliceous intrusives. With the exception of the Climax deposit where tin, tungsten and pyrite are recovered, stockwork ores produce only molybdenum. Simple, composite, or multiple intrusions, dikes and breccia pipes localize these ores. Compositionally igneous phases vary from granodiorite to granite, whereas textures may be porphyritic, aplitic, or inequigranular.Hydrothermal alteration is primarily marked by K-feldspathization and silicification. Commonly, assemblages can be arranged in order of decreasing hydrogen metasomatism and include sericitic, argillic, propylitic and potassium silicate types. Oxidation products are often fixed at the surface and supergene enrichment is absent.Molybdenite in stockworks occurs in (a) veinlets associated with quartz, and lesser amounts of other sulfides, oxides and gangue; (b) fissure veins; (c) fine fractures containing molybdenite paint; (d) breccia filling; and more rarely (e) disseminated grains. Intrusion and mineralization commonly occur in the epizone of the crust. Temperatures of formation where reconstructed are less than 400 degrees C. Mineral and alteration assemblage zoning, while variable, are best displayed in ores associated with multiple intrusions. Paragenetically, molybdenum occurs early in a mineralizing stage in ores that are commonly near the center of igneous activity.The stockwork ores and some other classes of molybdenum deposits occur in igneoustectonic provinces, where intrusives are localized by crossing fractures, lineaments, or domes. These provinces, distributed in particular segments of the Cordillera, include (1) Alaska-Yukon, passing into (2) British Columbia-Washington, (3) (eastern) Oregon-Idaho-(western) Montana, (4) Colorado-New Mexico, (5) Nevada-southern California, and (6) Sonora-Sinaloa. To date the productive deposits are located in the southern Rocky Mountains (Urad-Henderson, Climax and Questa) and in the Interior Belt of British Columbia (Alice (B.C. Molybdenum), Endako and Boss Mountain).The geologic history of the productive provinces exhibits major sedimentational, igneous and tectonic differences. The western stockwork provinces occur in eugeosynclinal environments, whereas the eastern part of the Idaho belt and the southern Rocky Mountains area are old miogeosynclinal or shelf areas. Additionally, plutonic events associated with these deposits have different ages and reflect, in part, the geologic evolution of the western Cordillera. Correspondingly, the southern Rocky Mountain deposits were formed 30 to 23 m.y. ago, around the Paleogene-Neogene boundary, and are correlatable with a Mid-Tertiary phase of igneous activity in this and adjacent areas. Those deposits in British Columbia vary in age from 142 to 53 m.y., corresponding to an interval from Late Jurassic to Eocene. Apparent lack of a preferred depositional environment, together with preliminary sulfur and strontium isotope data, suggest that some fraction of the source materials is subcrustal.