Arthur A. Bookstrom, 1990. "Igneous Rocks and Carbonate-Hosted Ore Deposits of the Central Colorado Mineral Belt", Carbonate-Hosted Sulfide Deposits of the Central Colorado Mineral Belt, David W. Beaty, Gary P. Landis, Tommy B. Thompson
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Many carbonate-hosted snlfide deposits of the central Colorado mineral belt are interpreted as products of hydrothermal systems associated with Late Cretaceons to Oligocene magmatism, Cordilleran magmatism migrated rapidly northeastward and inland during the Late Cretaceous-early Tertiary Laramide compressional orogeny. In Colorado, Laramide magmatism advanced northeastward, from La Pata to Jamestown, along en echelon segments of the Colorado mineral belt between about 75 and 65 Ma. Magmatism of the monzonite association generally preceded magmatism of the granodiorite association. Monzonitic magmatism was predominant in the northeastern Colorado mineral belt, but granodioritic magmatism was predominant in the central Colorado mineral belt. Some monzonitic intrnsive suites are associated with zoned polymetallic hydrothermal systems, characterized by distal and/or late gold-silver telluride veins. Some granodioritic intrusive suites are associated with zoned polymetallic hydrothermal systems, with internal copper-molybdenum occurrences, proximal gold-bearing pyritic veins, and/or distal silver-lead-zinc deposits.
Between about 45 and 35 Ma, monzonitic and granodioritic magmatism retreated to the southwest, toward the Cordilleran trench. Laramide compression waned and post-Laramide plutonism surged, giving rise to many large plutons of the monzogranite association in the central Colorado mineral belt. Silver-lead-zinc deposits, with by-product copper and gold and minor but characteristic antimony and bismuth, accompany many intrusions of the monzogranite association. Some monzogranitic suites are transitional to quartz monzonite and/or granodiorite, and their accompanying hydrothermal assemblages tend to be relatively enriched in gold and zinc (as at Breckenridge, 45-40 Ma). Other monzogranitic suites are transitional to granite, and their accompanying hydrothermal assemblages tend to be relatively enriched in silver, lead, and molybdenum (as at Montezuma, 40-36 Ma).
Fissure veins tended to form in silicate rocks, as at Montezuma-Argentine, whereas skarns formed locally in proximal carbonate rocks, as at Breckenridge and Leadville. Replacement veins and disseminations tended to form in fractured carbonate rocks, lateral and distal to monzogranitic intrusions, as in the Monarch, Tincup, and Tennessee Pass districts. Massive sulfide mantos tended to form in carbonate rocks above inferred monzogranitic intrusions (and beneath preore sills), as at Leadville (about 43-33 Ma), Tennessee Pass-Buckeye Gulch (42-40 Ma), and’ Gilman (34.5 Ma).
Composite volcanic suites were erupted from calderas along the axis of the Sawatch uplift between about 36 and 34 Ma. Minor hydrothermal deposits, with diverse characteristics, are spatially associated with these calderas, and/or with related, pre- and postcaldera intrusions.
Beginning at about 35 Ma, intrusions of the bimodal alkali-feldspar granite-lamprophyre association were emplaced in the central Colorado mineral belt, near its intersection with the then-incipient Rio Grande rift system. Porphyry molybdenum deposits formed above some cupolas of composite stocks of alkali-feldspar granite porphyry, as at Climax (33-24 Ma).
After about 28 Ma, basaltic volcanism became increasingly common and widespread in the expanding Rio Grande rift system. Basalts generally are unmineralized at present levels of exposure but locally are associated with sulfur-rich hot springs. Fluorine-rich hot springs and fluorspar deposits also occur locally along normal faults of the Rio Grande rift system.
Figures & Tables
Carbonate-Hosted Sulfide Deposits of the Central Colorado Mineral Belt
The carbonate-hosted ore deposits at Leadville, Gil-man, Red Cliff, Aspen, Alma, Tincup, Kokomo, and Mount Sherman have enjoyed a long and storied production history. These orebodies, as well as dozens of smaller deposits, are all located in the central Colorado mineral belt and together constitute an important metallogenic province (Figs. 1 and 2).
Recorded metal production of the major districts in this province to date has consisted of 1,630,000 metric tons of zinc, 1,500,000 metric tons of lead, 145,000 metric tons of copper, 15,600,000 kg of silver, and 110,000 kg of gold (Table 1). For several reasons these figures represent only a portion of the metal concentrated by nature in these deposits:
1. Early production records are probably incomplete.
2. Inefficient methods were used to process much of the ore mined during the 1800s, particnlarly for zinc and copper.
3. The ores in the principal mining districts were partially removed by erosion prior to mining.
4. Significant reserves remain in the Leadville district.
In comparison to other mining districts around the world, the carbonate-hosted sulfide deposits of the central Colorado mineral belt have produced relatively low tonnages of high-grade ore (Table 2). The largest of the districts is Leadville, which to date has produced aboul 24,000,000 metric tons of polymetallic ore. By contrast, the Aspen district has produced only an estimated 4,000,000 metric tons of ore (Table 2), but that ore averaged about 1,000 g/metric ton silver. Although all of the deposits in this metallogenic province are polymetallic, the economic significance of the various metals is not equal. The ores at Gilman, Aspen, and Leadville were valuable primarily for their contained Zn-Cu-Ag, Ag-Pb, and Ag-Au-Pb-Zn, respectively (Table 2).
The first discovery of gold in Colorado was made in July 1858, in a stream draining the eastern Rocky Mountains. This led to the “Pike's Peak” gold rush of 1859, during which an estimated 50,000 people moved into the area (Blair, 1980). These so-called “Fifty-Niners” established most of the mining districts in the northeast portion of the Colorado mineral belt during the summer of 1859. By late 1859 the prospectors had penetrated the Continental Divide, and in April 1860, the placer gold deposits at Leadville were discovered.
A rush to Leadville ensued, and as a result of heavy mining pressure, the Leadville placers were essentially depleted by 1868. The much larger and more valuable carbonate replacement ores at Leadville,