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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.

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