Richard H. De Voto, 1990. "Paleozoic Stratigraphy, Tectonism, Thermal History, and Basin Evolution of 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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The Cambrian through Mississippian stratigraphic sequence of central Colorado comprises a thin seqnence (300-400 m thick) of interbedded quartz sandstones, carbonates, and minor shales separated by several unconformities. Extensive Paleozoic dolomitization of the carbonate units has occurred, increasing porosity and permeability in the Mississippian Leadville Formation. Karst solntion features, integrated cave systems with collection sinkholes and cutters, subhorizontal channels, and outlet chimneys, developed in the Leadville Formation and, to a lesser extent, in the Devonian Gilman and Dyer Formations beneath a regionally extensive karst erosion surface of Late Mississippian age. Although the sediments were deposited on a relatively stable cratonic shelf, several large-scale tectonic features, the Gore Range and ancestral Front Range-Wet Mountain uplift areas, were active along their bounding faults, as were the Homestake shear zone and other intrabasinal faults. These structures influenced sedimentary patterns and facies throughont the early and middle Paleozoic.
Pennsylvanian and Permian strata of central Colorado record a history of active basin subsidence along nnmerous faults and contemporaneous sedimentation of as much as 5,000 m of noumarine to marine strata. Central Colorado was broken into a mosaic of fault blocks, on which recurrent tectonic activity produced the sedimentary and structural basin of the northern Central Colorado trough, including many adjacent mountainous erosional areas, the ancestral Uncompahgre and Front Range uplifts, the basin-center ancestral Sawatch uplift, and many other uplifts within the basin. Abrupt thickness variations and facies changes, onlap and overlap relationships, and occasionally, angular discordances occur adjacent to faults and uplift blocks. Local volcanism apparently occurred in the Pennsylvanian using the intersection area of the Gore and Mosquito faults as a conduit.
Organic-rich source rocks occur abundantly in the Lower and Middle Pennsylvanian strata of the Belden, Gothic, Minturn, and Eagle Valley Formations. Thermal modeling of source-rock maturation data shows that the geothermal gradient locally within the rift basins in central Colorado during the Pennsylvanian and Early Permian was anomalously high (40°-48°C/km). Oil generation from Belden source rocks commenced in the Pennsylvanian and Early Permian and, in the local anhydrite-salt basin grabens of thickest late Paleozoic sediments, oil generation probably terminated in the late Paleozoic. Organic-rich mudstones, shales, and gypsum of the Minturn and Eagle Valley Formations were heated to the point of significant methane generation in the late Paleozoic. The maximum thermal maturity of Pennsylvanian strata was achieved during deepest burial near the end of the Cretaceous, prior to the uplift and erosion caused by Laramide and Tertiary tectonic activity and intrusion of the Pando Porphyry 72 Ma.
Major porosity reduction and water expulsion from the Pennsylvanian fine-grained sediments, source rocks, and gypsum and the associated development of impermeability occurred during the first 1,000 to 2,000 m of burial in the Pennsylvanian. With continued sedimentation and burial in the late Paleozoic, large volumes of organic acids, oil, and gas were generated from the solid kerogen of the organic-rich source rocks. It is likely that the significant volume expansion created overpressured hydraulic conditions. Fracturing of the impermeable source rocks probably provided hydrologic continuity with intrabasinal and basin margin faults and fluid migration along the faults to underlying and overlying permeable strata. The confining nature of the thick sequence of generally impermeable overlying strata and the excellent permeability of the underlying Leadville Formation with its karst solution features suggest that large volumes of basinal fluids, hydrothermal organic acid-rich waters and hydrocarbons, may have moved from the overpressured source rocks into and through the underlying Leadville strata.
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,