COLORADO MINERAL BELT REVISITED: AN ANALYSIS OF NEW DATA

New geologic and geophysical data extend our knowledge of the complementary role of inherited zones of weakness in Precambrian basement and Late Cretaceous-Tertiary magmatism in development of the Colorado Mineral Belt (COMB). The new data indicate that the northeast-trending ductile shear zones that localized ore-related igneous activity in COMB are more abundant than known previously, extend outward laterally to the northwest and southeast, and can account for localizing valuable ore deposits in outlying regions such as Cripple Creek, Rosita Hills-Westcliff, and Summitville. The shear zones formed in the Mesoproterozoic (ca. 1.4 Ga) as a result of transpressional tectonics. Locally, they followed pre-existing Paleoproterozoic structures. A second set of Mesoproterozoic shears, of northwest orientation, which has been poorly understood, had a secondary control on emplacement of intrusions in the central part of COMB. This control is most evident in central Colorado where the mineral belt abruptly widens from a width of about 15 to 60 km to about 140 km.

Recent geochronologic data indicate that a minor late Tertiary ore-forming event was locally superposed on the two major periods of mineralization: Late Cretaceous-early Tertiary and mid-Tertiary. Accordingly, episodic magmatism in the region spanned an interval of about 70 m.y. Gravity data indicate that COMB overlies two distinct gravity lows (-300 mg) interpreted as long-lived magma chambers that spawned both Late-Cretaceous-early Tertiary and mid-Tertiary magmas. The gravity lows are en-echelon and separated by a prominent northwest-trending basement shear zone, exposed in the Gunnison River area.