Robert J. Johansing, 1988. "Sherman-type Mineralization: Variation in its Appearance Throughout the Mosquito Range", Geology and Mineralization of the Gilman-Leadville Area, Colorado, T. B. Thompson, David W. Beaty
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The term “Sherman-type ore” was initially applied by Behre (1953) to silver-base metal deposits in the Mosquito Range which share the following characteristics: 1)abundant, white bladed barite which was deposited prior to most of the sulfides and sulfosalts; 2)widespread galena and sphalerite which was deposited upon or replaces the barite; 3) light olive-green sphalerite; 4) quartz with minor pyrite may be present and preceded barite deposition; and 5) ore deposits occur as both tabular bodies and replacement veins in the Mississippian Leadville Dolomite. Behre (1953) believed that the above features were sufficiently characteristic to merit the use of a type name for such-ores the Sherman type of ore. Since Behre's definition, there have been numerous studies of this type of deposit both in the Mosquito Range and elsewhere in Colorado.
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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.