The Silver Bell Mountains, 50 km northwest of Tucson, Arizona, are a mountain range in the transition from the higher-relief mountains of southeastern Arizona to the low-relief ranges of the western Arizona deserts. The Silver Bell Mountains include the area of the Silver Bell mining district (Richard and Courtright, 1966) which has produced over 100 million pounds of copper (Graybeal, 1982) since mining began in the late 19th century. Most of the mountain range and adjoining flanks are composed of Upper Cretaceous volcanic and plutonic rocks which are interpreted as parts of a ash-flow caldera; the intrusions which formed the porphyry copper deposits of the Silver Bell mining district were emplaced into the caldera margin. The latest Cretaceous and early Tertiary period of formation of most of Arizona's many copper deposits during was also a period of widespread continental silicic caldera volcanism, later fragmented into alternating mountain range exposures and buried beneath alluvial basins by late Tertiary extensional deformation. Lipman and Sawyer (1985) identified as many as seven Cretaceous caldera fragments in southern Arizona, as well as pieces of Cretaceous volcanoes in more than a half-dozen other mountain ranges in southern Arizona. Undoubtedly, much of the evidence about the character of Cretaeous volcanism is buried beneath alluvial valleys. This is the fourth detailed mapping study of these Cretaceous volcanoes: studies of the Tombstone caldera (Moore, 1994) and the Tucson Mountain caldera (Lipman, 1994) have recently been published, and a geologic map of Cretaceous volcanic rocks in the Sierrita Mountains (Fridrich and Lipman, 1991) is in preparation.
