Paleokarst-Controlled Ore in Central Colorado
We propose that many Tertiary man to orebodies typical of Leadville and adjacent districts of Central Colorado (northern Sawatch range and Mosquito range) are modifications to pre-existing mineralized paleokarst cave systems (Tschauder and Landis, 1985; Landis and Tschauder, 1985). Major karst cave development occurred in this region during mid- to late- Mississippian time. Cave breccia mineralization (as an integral part of cave fill) is observed to dominate in Paleozoic caves located away from the influence of Tertiary intrusions (Breece Hill Center, Leadville; Buckskin stock, Alma district). Most data and many observations on the nature of this Paleozoic cave breccia mineralization are based upon access in the Sherman mine to the east of the inferred Leadville paleokarst system. However, the overprint of Laramide through mid-Tertiary ore deposition associated with intrusive centers has obscured much of the evidence for earlier cave-hosted mineralization. Tertiary mineralization in the Leadville district that we suggest exhibits and preserves features typical of cave systems include the orebodies of Fryer Hill, Downtown, Carbonate Hill, Iron Hill-Rock Hill, Julia Fiske, and Resurrection groups of mines. Additional mineralization occurs in cave systems of nine districts on the north and east slopes of the Sawatch Range from Redcliff-Eagle to Mount Lincoln in the Alma district, and then south 25 km to Weston Pass, and in a zone of discontinuous mineralization more than 50 km long on the west slope, from north of the Aspen district south into Gunnison County. Some Tertiary massive sulfide mantos typical of the Leadville district mineralization are better understood by recognizing their possible karstic cave control and Paleozoic cave breccia progenitor mineralization.
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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.