Robert J. Johansing, 1990. "A Reply to the Discussion of Sherman-Type Deposits by Brian J. Skinner", Carbonate-Hosted Sulfide Deposits of the Central Colorado Mineral Belt, David W. Beaty, Gary P. Landis, Tommy B. Thompson
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The criteria for the identification of Sherman-type deposits are based on field studies by Behre (1953). The presence and paragenetic position of white barite (prior to base and precious metals) is probably the most important field criterion used in the recognition of the Sherman-type assemblage because early white barite has not been found in other ore deposit types in the Mosquito Range. Additional criteria such as host rock (carbonate), sphalerite color (olive green), silver content (moderate to high), and quartz-pyrite content (typically low) are important, but by themselves are not diagnostic. More specifically:
A. The Sherman-type assemblage has been ob served in both clastic rocks and intrusive porphyries.
B. Olive green sphalerite occurs in many ore de posit types in central Colorado.
C. Base metal ore with high silver content occurs within the Leadville, Gilman, and Aspen districts,
D. A low qnartz-pyrite content, like the silver content, is not unique to the Sherman assemblage.
In the presence of early white barite, these features serve to support the identification of the assemblage as the Sherman type. However, in the absence of early white barite, the supporting field criteria do not unequivocally identify the deposit as Sherman type.
Behre’s description is based upon macroscopic features which can be readily applied in the field. In the time since his studies, the ores have been partially characterized isotopically and thermometrically. The two studies summarized in this volume have agreed that Sherman-type deposits contain J-type lead in the galena, contain sulfur from a sedimentary source, and have measured fluid inclusion homogenization temperatures between 150° and 300°C. These should also be considered as characteristics of Sherman-type deposits.
Up to this point, we have limited the definition to physical characteristics of the ores. However, Behre’s definition of the Sherman-type ore presumed a genetic relation to the-Leadville district. We conclude, in light of the two conflicting genetic models proposed in this volume for the origin of the Sherman-type ore, that it is premature to include a genetic model in the definition of Sherman-type deposits. We propose that Behre’s definition be modified to accommodate these uncertainties until adequate studies have been conducted.
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,