Stratigraphy of the Mississippian Leadville Dolomite, Gilman to Leadville, Colorado: Redefinition of the Castle Butte and Red Cliff Members
David W. Beaty, Robert J. Johansing, Tommy B. Thompson, 1988. "Stratigraphy of the Mississippian Leadville Dolomite, Gilman to Leadville, Colorado: Redefinition of the Castle Butte and Red Cliff Members", Geology and Mineralization of the Gilman-Leadville Area, Colorado, T. B. Thompson, David W. Beaty
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The stratigraphy of the Leadville Dolomite has been much more completely characterized than previously possible through study of more than 50 drill cores in the area between Leadville and Gilman, Colorado. Data from these cores, supplemented with literature descriptions and limited outcrop observations, leads to three conclusions. First, there are three major unconformities associated with the Leadville (M-l through M-3 in order of decreasing age). M-l and M-3 are at the base and top of the Leadville, respectively; M-2 is within the Leadville. Two of these unconformities (M-2 and M-3) are very well developed throughout the Leadville-Gilman area, and have been recognized in the surrounding region; M-1 is less well developed and/or of more local extent. The M-2 surface is locally associated with distinctive lithologies (pink breccia at Gilman, black shale at Mt. Sherman) which form critical marker beds. Second, a well- defined lithostratigraphy can be identified by classifying the dolostones of the Leadville on the basis of mean dolomite grain size. The number of beds at each locality is typically 10-15, and eight of these beds can be consistently traced throughout the Gilman-Leadville area. Third, the definition of the Castle Butte and Red Cliff Members of the Leadville proposed by Nadeau (1971) is ambiguous, and has led to inconsistent results when applied away from his reference sections.
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The central Colorado mineral belt is endowed with an impressive wealth of mineral deposits, including the world-class deposits at Leadville, Gilman, and Climax, that formed in a variety of geologic environments. The geology of the area spans more than 1.8 Ga, commencing with the Early Proterozoic accretion of volcanic arc and back-arc complexes to the southern margin of the Archean craton. These rocks were complexly deformed and intruded by large Early and Middle Proterozoic batholiths. During Paleozoic and Mesozoic time, the Proterozoic basement complex was buried beneath several kilometers of marine and continental sediments, and it was partially exhumed during Pennsylvanian orogenic uplift. Subduction-related calc-alkalic magmatism and uplift affected the region during the Late Cretaceous-early Tertiary Laramide orogeny. Oligocene and younger extension generated the north-trending Rio Grande rift zone, which was accompanied by bimodal magmatic activity.
Most of the mineral deposits in the central Colorado mineral belt are associated with Oligocene calc-alkalic magmatism or to later bimodal activity. Deposits of demonstrably Laramide age are relatively small, and a few small carbonate-hosted deposits may have formed during the Mississippian.
The mountains of central Colorado contain some of the largest concentrations of mineral deposits, including those at Climax, Leadville, and fiilman, in the Rocky Mountain region. These ores are part of an elongate zone of hydrothermal deposits, known as the Col or ado mineral belt, that extends northeast from the San Juan Mountains to the Front Range north of Denver (Fig. 1). Although most of the deposits are the products of Cenozoic tectonic and hydrothermal processes, the geology of the central Colorado mineral belt represents more than 1.8 billion years of tectonism, plutonism, and mineralized region, world-class cratonic sedimentation. As with any heavily deposits such as those described in this volume are the culminations of numerous unrelated geologic events that occurred over hundreds of mi11ions of years.
The intent of this paper is to briefly summarize the geologic history of central Colorado and its relation to mineralization. In general, the region is underlain by a crystalline Proterozoic basement complex on which several kilometers of Phanerozoic sediments were deposited. Orogenic up 1ift occurred in the late Paleozoic and twice during the Cenozoic, and a major rifting event began in the middle Tertiary. Voluminous plutonic rocks were emplaced during several Late Cretaceous and Cenozoic magmatic events. Recurrent orogenic activity throughout the geologic history generated new structures and reactivited many preexisting faults.