Geology and Origin of Sherman-Type Deposits, Central Colorado
Robert J. Johansing, Tommy B. Thompson, 1990. "Geology and Origin of Sherman-Type Deposits, Central Colorado", Carbonate-Hosted Sulfide Deposits of the Central Colorado Mineral Belt, David W. Beaty, Gary P. Landis, Tommy B. Thompson
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Dolomite-hosted silver-lead-zinc-barium deposits occur in the Mosquito Range of central Colorado and are locally termed Sherman-type deposits. A comparative study of these deposits shows that they are different from the sulfide deposits at Leadville and Gilman in geologic setting, mineralogy, and geochemistry.
Sherman-type mineral deposits are dominantly hosted by the Leadville Dolomite, which is typically only slightly altered. Coeval veins physically connected to the Sherman-type deposits crosscut older and younger rocks, the youngest being middle Tertiary in age. Deposit morphologies are influenced by a combination of geologic features: Tertiary faults and fractures, solution breccias, and stratigraphy. Pre-Pennsylvanian karst-related solution breccias serve as the dominant host and contain variable amounts of quartz, barite, ferroan dolomite, sphalerite, argentian tetrahedrite, and galena. Early barite, low iron sphalerite, high silver content, and abundant late galena are common to these deposits throughout the region.
Ore deposition occurred between 150° and 300°C by fluids with salinities less than 8 equiv wt percent NaCl. Sulfur and lead for sulfate and sulfide minerals were derived from sedimentary rocks (δ34S: early barite = 13.2-24.0‰; galena = -6.8 to -0.6‰; 206Pb/204Pb > 19.10).
The above data show that Sherman-type deposits are a regionally distinct deposit type and formed by regional processes which can be constrained to the Tertiary. Emplacement of the Colorado mineral belt batholith may have initiated regional warming which affected the basins of the central Colorado trough. In response to this early to mid-Tertiary geothermal gradient, dilute formational waters leached metal and sulfur from the basinal rocks and carried them up the flanks of the Sawatch anticline along faults and aquifers. Mineral deposition occurred in permeable breccias of karst and tectonic origin which flank the anticline.
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,