Genetic Model for the Leadville District: Constraints from Field Data Wall-Rock Alteration, Ore-Gangue Mineralogy, Metal-Mineral Zoning, Fluid Inclusions, Stable Isotopes and Fission-track Geochronology
Tommy B. Thompson, 1988. "Genetic Model for the Leadville District: Constraints from Field Data Wall-Rock Alteration, Ore-Gangue Mineralogy, Metal-Mineral Zoning, Fluid Inclusions, Stable Isotopes and Fission-track Geochronology", Geology and Mineralization of the Gilman-Leadville Area, Colorado, T. B. Thompson, David W. Beaty
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The world-class leadville district was discovered in 1859 and is still producing ore from the Black Cloud mine, a relatively new operation begun in 1971 (see Smith, 1988: this volume, for history of the mine) and from the Diamond mine. District cumulative production through 1987 is approximately 26.2 million short tons (23.8 million metric tons) of ore from veins and dolomite-hosted massive sulfide replacement deposits. Placer gold and oxidized replacement orehodles provided much, of the early district production (Emmons et al., 1927), but the unoxidized orebcdies found further down-dip provide the unambiguous exposures for understanding the district ore deposits. Leadville is of historical significance, also, as the U.S. Geological Survey in one of its first actions assigned S.F. Emmons the responsibility of detailed investigations of the district, culminating in U.S. Geological Survey Monograph 12 (1886). Several additional publications updated his continuing studies in the district (Emmons and Irving, 1907: Emmons et al., 1927).
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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.