ORIGIN OF THE COLORADO MINERAL BELT

The Colorado mineral belt (CMB) is a NE-trending 500-km-long narrow belt of plutons and mining districts within a 1200-km-wide Late Cretaceous-Paleogene magma gap overlying flat segments of the subducted Farallon plate. The CMB is unique among 24 known volcanic gaps overlying flat-slab segments around the Pacific Basin. The primary control of the CMB was a NE-trending segment boundary within the Farallon flat slab. The boundary was dilated during warping of slab segments by overriding thick (>200km) lithospheres of the Wyoming Archean craton and the continental interior craton during acceleration of Farallon-North American convergence beginning ca. 75 Ma (mid-Campanian). Because the primary control was not in the North American plate, the CMB cut indiscriminately across the geologic grain of Colorado, seemingly independent of the tectonic elements it crossed.

Geologic contrasts on opposite sides of the CMB reflect its relationship to a segment boundary. Laramide uplifts mostly trend northwesterly north of the CMB but northerly to the south. Upper Cretaceous and Paleogene sedimentary deposits are up to 7 km thick north of the CMB but only about 2 km maximum south of CMB. The Farallon segment south of CMB rolled back to the southwest beginning ca. 37 Ma with resultant voluminous ignimbrite eruptions from southwest-younging caldera clusters. Volcanism north of CMB was sparse; the fate of that segment is unknown. Superimposing of roll-back magmatism and Rio Grande rift extension on the CMB changed compositions of both plutons and ore deposits, resulting in World-class manto sulfide replacement ores in the Leadville area ca. 40-36 Ma and major porphyry molybdenum deposits ca. 33-24 Ma at Climax and Red Mountain.