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The Leadville district has yielded gold, silver, and base metals for more than 125 years, principally from dolostone-hosted massive sulfide replacement bodies (mantos). The carbonate host rocks were deposited on a Paleozoic shallow marine shelf overlying a Proterozoic (1.4 Ga) granitic basement, and subsequent diagenetic processes converted the shelf rocks to do-lostones. Multiple stages of dissolution with infilling generated paleokarst units within the uppermost dolostone, the Leadville Dolomite. The ore host rocks lie in the Mosquito Range on the eastern flank of the Laramide Sawatch uplift. Southwest-directed, east-dipping, low-augle thrust faults offset by high-angle reverse faults characterized the compressional environment in the Leadville district during the Laramide orogeny.

The first Phanerozoic igneous eveut in the area, emplacement of the Pando Porphyry, occurred contemporaneously with low-angle thrusting. The Pando Porphyry was emplaced along thrust planes and along an unconformity at the top of the Leadville Dolomite. Six igneous events resulting in sills, dikes, and a small stock followed during Tertiary time, culminating with the emplacement of the Breece Hill stock in the eastern part of the Leadville district. All of the preore igneous rocks are latite to quartz monzonite in composition:

Four genetic types of ore deposits are present within the Leadville district: (1) contact-related magnetite-serpeutine bodies, (2) dolostone-hosted barite-silver-minor base metal replacement and open-space filliugs termed “Sherman-type ore,” (3) dolostone-hosted zinc-lead-silver-gold massive sulfide replacement bodies with associated veins, referred tg as “Leadville-type ore,” and (4) placer gold.

The contact-related magnetite-serpentine bodies replace dolostone aud formed during emplacement of the Breece Hill stock as well as locally around the margin of au older stock southeast of Breece Hill. These ores were used sporadically as smelting flux.

The main Leadville district ore deposits are principally massive sulfide replacement bodies in dolostone, but there are related quartz-pyrite-gold veins, disseminated quartz-pyrite-gold in porphyry, and quartz-base metal veins. These are zoned spatially about the Breece Hill stock with central-most quartz-pyrite-gold veins and local disseminations surrounded by quartz-base metal veins and mautos. Mantos closer to Breece Hill have higher gold coutent (Ag/Au = 20), as do those localized along an east-dipping low-angle thrust fault west of Breece Hill; with increasing distance from Breece Hill silver/gold ratios of ores within mantos increase to>75. Manganese in gangue and alteration products increases away from the stock as well.

Leadville orebodies in dolostone are dominated by pyrite with lesser marmatite, galena, chalcopyrite, tetrahedrite, pyrrhotite, marcasite, eleetrum, and late-stage veinlets with elevated silver, gold, bismuth, and tellurium contents. Late golden barite, dolomite, and rare fluorite are found in veinlets or vugs within ore. The sulfide coutent of ores in dolostone is>65 vol perceut. The ores are massive and commonly exhibit banding due to textural, size, or min-eralogical differences between bands. The bands parallel sedimentary layering or fracture ‘ planes cutting the dolostones. Diffusion-controlled replacement resulted in ores that locally exhibit concentric banding about unreplaced dolostone bodies, reflecting the stratigraphic and structural controls on ore fluid movement. Local sulfide replacement of paleokarst results in complete preservation of finely laminated cave fill with soft sediment impact features due to rock falls in the paleocaves; however, Leadville ores replaced principally massive dolostone with paleokarst localizing only miuor amouuts of ore.

Hydrothermal alteration of the Breece Hill stock is extensive with both sericitic and argillic products common. Adjacent to veins and mantos more intense sericitic alteration is friuged by argillic and propylitic assemblages, extending as much as 100 m from ore. All assemblages contain abundant siderite or manganosiderite. The carbonate host rocks exhibit limited visible alteration exteuding uo more than 5 m from ore. Thermal effects on dolostone insoluble residue phyllosilicates is more extensive.

Potassium-argon dates on sericite in Johnson Gulch Porphyry yield an average age on the Leadville ore event of 39.6 ± 1.7 Ma or early Oligocene. Fission-track dates on apatite and zircon from altered preore Johnson Gulch Porphyry indicate that cooling of the hydrothermal system occurred over a 6-m.y. period.

The Leadville orebodies are cut by rhyolite porphyry dikes and plugs; these, in turn, have been broken by fragmental porphyry, a fluidized breccia system of tabular to irregular bodies around the perimeter of the Breece Hill stock. Late-stage quartz-base metal sulfide veins cut the fragmental porphyry and yield similar fluid inclusion data as found in the main Leadville-type orebodies. The rhyolite and fragmental porphyries were apparently derived from an intrusive system centered beneath the Breece Hill stock. As the fragmental porphyry bodies were emplaced, the downdropped block, a large triangular-shaped graben in the southeastern part of the district, subsided.

Postmineral normal faulting (east side up) associated with development of tbe Rio Grande-Upper Arkansas rift exposed the ores in several blocks up the western flank of the Mosquito Range, leading to oxidation and supergene enrichment of the Leadville-type orebodies and the development of placer gold in California Gnlch, the site of initial discovery.

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