The Stratigraphic Position, Lithologic Character, and Preore Dissolution Features of Ore-Bearing Beds in the Leadville Dolomite, Leadville-Gilman Area
David W. Beaty, Tommy B. Thompson, Robert J. Johansing, 1990. "The Stratigraphic Position, Lithologic Character, and Preore Dissolution Features of Ore-Bearing Beds in the Leadville Dolomite, Leadville-Gilman Area", Carbonate-Hosted Sulfide Deposits of the Central Colorado Mineral Belt, David W. Beaty, Gary P. Landis, Tommy B. Thompson
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To constrain speculation about the character of the host rock at the site of manto ore deposition, we have studied the Leadville Dolomite peripheral to three similar systems of high-temperature, pyritic mantos in central Colorado. The deposits studied are at Leadville, Buckeye Gulch, and Gilman. Comparison of unmineralized sections shows that the Leadville Dolomite has systematic internal stratigraphy, consisting of 15 named beds (from base to top: 6G-8G, 1-12). Many of the mantos are located in one of four stratigraphic positions: the uppermost Leadville, beds 7-8, bed 3, and bed 1. Three of these mineralized beds (containing the largest orebodies) share the characteristics of relatively coarse grain size and porous texture. The fourth mineralized bed (7-8) is the locus of most of the paleocaves in the Leadville Dolomite. In addition, some beds in the Leadville (6G, 4) are selectively unmineralized even in regions of extensive replacement, and these beds are distinguished by their fine grain size and low porosity.
Limited data indicate that the fine-grained beds are six orders of magnitude less permeable than the coarse-grained zebra-textured beds, which in turn are as much as two orders of magnitude less permeable than the manto orebodies. This suggests that the principal control on manto formation was permeability contrast in the dolostone beds of the Leadville Dolomite and that paleocaves were only locally important. The permeability of initial fluid conduits was almost certainly increased by the ore-forming fluid, thereby focusing a higher fraction of the flow into the initial channel. This positive feedback mechanism would result in runaway permeability enhancement and the focusing of essentially all of the flow into narrow conduits. The characteristic shape of the mantos in central Colorado may therefore be an inevitable consequence of the mechanics of fluid flow within carbonate rocks. The general lack of pa-leokarst cave control for the high-temperature pyritic manto deposits is confirmed by comparison of orebody morphology to solution features of hydrothermal and paleokarst origin.
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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,