Genetic model for the Gilman District, based on fluid inclusion, stable isotope, alteration/vein/replacement distribution, and fission-track geochronologic studies
David W. Beaty, C.W. Naeser, C.G. Cunningham, Gary P. Landis, 1988. "Genetic model for the Gilman District, based on fluid inclusion, stable isotope, alteration/vein/replacement distribution, and fission-track geochronologic studies", Geology and Mineralization of the Gilman-Leadville Area, Colorado, T. B. Thompson, David W. Beaty
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A large manto/chimney complex is located at Gilman, Colorado, about 120 km WSW of Denver. The district was discovered in 1879, was consolidated beginning in 1912 by New Jersey Zinc Company as the Eagle mine, and was closed in 1984. Cumulative production of all ore types has been about 11.7 million metric tons with an average grade of 8.5% Zn, 1.5% Pb, 0.7% Cu, 228 gpt (grams per metric ton) Ag, and 1.7 gpt Au (Beaty et al., 1989).
At Gilman, a sequence of Cambrian through Mississippian sedimentary rocks (total stratigraphic thickness-200 m) dip homoclinally eastward at 12-15°. These sedimentary rocks unconformably overlie Precambrian basement (predominantly the Cross Creek batholith), and are in turn overlain by 3000+ m of coarse elastics of the Pennsylvanian Minturn Formation. The sedimentary rocks in the Gilman area are intruded by one exposed igneous mass, the Pando Porphyry, which forms a sill 12–27 m thick in the Belden shale. The Pando Porphyry is a fine-grained quartz latite which crystallized at about 72 Ma (data of Pearson et al., 1962 recalculated by Beaty et al., 1987).
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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.