Shape and extent of the Colorado Mineral Belt (CMB) as drawn by different authors. (A) Outline of the CMB as defined by principal mining districts of Laramide age versus maximum boundary when middle Cenozoic mining districts are included (modified from Tweto and Sims, 1963). (B) Inset showing CMB (COMB) as defined by only Laramide plutons (modified from Mutschler et al., 1987). Note different scale, and that there is no age progression of Laramide plutons along the CMB. Isotopic dating has determined a ca. 75 Ma age for the Jamestown stock at the northeast end of the CMB (McDowell, 1971; Gable, 1984) and confirmed the 70 Ma age for the Carrizo Mountains laccolith at the southwest end (Semken and McIntosh, 1997). A 75 has been added to B for the Jamestown stock. The 70 Ma Carrizo laccolith and 72–71 Ma Ute laccolith at the southwest end of the CMB are labeled C and U, respectively. Other laccoliths in the Four Corners area are mid-Tertiary in age (Mutschler et al., 1987).
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