Geology and the Origin of Ore Deposits in the Leadville District, Colorado: Part I. Geologic Studies of Orebodies and Wall Rocks
Tommy B. Thompson, Greg B. Arehart, 1990. "Geology and the Origin of Ore Deposits in the Leadville District, Colorado: Part I. Geologic Studies of Orebodies and Wall Rocks", Carbonate-Hosted Sulfide Deposits of the Central Colorado Mineral Belt, David W. Beaty, Gary P. Landis, Tommy B. Thompson
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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.
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,