BRIAN J. SKINNER, EDITOR, Economic Geology, 1990. "A Discussion of Sherman-Type Deposits", Carbonate-Hosted Sulfide Deposits of the Central Colorado Mineral Belt, David W. Beaty, Gary P. Landis, Tommy B. Thompson
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To the reader: During review it became apparent that two groups of authors in this volume had drawn different and incompatible conclusions concerning the origin of Sherman-type deposits. R. J. Johansing and T. B. Thompson, in their paper “Geology and origin of Sherman-type deposits, central Colorado,” concluded that the deposits formed during the Tertiary from basinal brines moving along thermal gradients controlled by igneous intrusions. An entirely different conclusion was drawn in the two-part paper “Late Mississippian karst caves and Ba-Ag-Pb-Zn mineralization in central Colorado,” in which Part I on “Geologic framework, mineralogy, and cave morphology” is by R. J. Tschauder, G. P. Landis, and R. Npyes; and Part II on “Fluid inclusion, stable isotope, and rock geochemistry data and a model of ore deposition” is by G. P. Landis and R. J. Tschauder. These authors concluded that Sherman-type deposits formed during the Mississippian from regional brines moving under a hydraulic gradient.
The differences between the two conclusions are so great it seems likely that at least one of the genetic interpretations is based either on an incomplete set of facts or on erroneous data. It is not apparent to the reviewers, to the editors of this monograph, or to me where the resolution may lie. Therefore, in order to assist readers and to help those who may carry out research on the deposits in the future, a series of questions have been addressed to both groups of authors. The questions are designed to explore the key differences between the two papers, and both groups of authors graciously agreed to respond and to allow their responses to be published. In addition, because some of the observations that each group considers to be facts are in dispute, both groups of authors have been asked to identify the points of factual disagreement in the other’s work. Both groups responded with a list of data they consider questionable, and these follow their replies to my questions.
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,