|First Published in the 1996 NSS Convention Guidebook
Copyright (c) 1996, 2002 by R. Mark Maslyn
Several mining districts surrounding the central Colorado Sawatch Range contain economic deposits hosted by late Mississippian paleokarst features. These are primarily developed in and along the upper surface of the early Mississippian Leadville Formation. Paleokarst features include isolated eaves and sinkholes as well as integrated cavern systems that are mineralized and can be traced from insurgence to outlet.
Luiszer [19961 provides a generalized stratigraphic column for the Sawatch Range area. Unconformably overlying the 1.4 Ga Precambrian Kevin Granite Batholith, the sedimentary sequence begins with the Cambrian Sawatch Quartzite, a brown, red and white transgressive deposit.
Gradationally overlying the Sawatch Quartzite is Cambrian Peerless Shale, a sandy unit that is unconforrnably overlain in turn by the light grey color Ordovician Manitou Dolomite. The latter is a major host of cavern development in the Williams Canyon area near Manitou Springs, Colorado, the type section. Included in the generalized stratigraphic section, but missing in the Leadville area are the Ordovician Fremont Dolomite and underlying Harding Quartzite.
Unconformably above the Manitou Dolomite are Devonian Parting Sandstone and Dyer Dolomite Members of the Chaffee Formation. There is limited cave development in the Dyer Dolomite in other areas of Colorado. Unconformably above the Dyer Dolomite is the early Mississippian Leadville Formation. This unit, first described by Emmons , is divided into three members, each bounded by upper and lower unconformities
The lowermost member is the Gilman Sand, a mixed assemblage of sand and thin dolomite. This unit may be observed in parts of the Cave of the Winds near Manitou Springs. In the cliffs of Williams Canyon the Gilman Sand is a slope former. The type section is at the mining town of Gilman, a short distance north of Leadville.
Above the Gilman Sand is the Redcliff Member, a thin bedded dolomite. The type section is near the town of Redeliff, in the vicinity of Gilman, north of Leadville. The upper surface of the Redcliff Member is a marked unconformity with local cave development and limestone breecia.
The uppermost unit of the Leadville is the Castle Butte Member. This massive cliff forming unit, where limestone, is the host for most of the cavern development on the White River Plateau and other areas of Colorado. The type section is located at Castle Butte, a few miles south of Aspen, Colorado. Castle Butte is prominent knob exposed at the edge of paleokarst sinkhole that was first described by Maslyn .
A note on nomenclature: the uppermost unit of the Leadville can be either a limestone or a dolomite. In the Aspen area, the Castle Butte Member is a limestone and the formation is commonly referred to as the Leadville Limestone. In most of the mineralized areas of the Sawatch Range, including Leadville however, the Castle Butte Member is a dolomite and the formation is commonly referred to as the Leadville Dolomite.
For a more detailed discussion of the Leadville Dolomite in the Leadville area, the reader is referred to Dorward .
Regionally, the Leadville district is located along the east flank of the Sawatch Range, a Laramide feature that was active in the late-Mississippian as well. The overall structural dip of the sedimentary rocks surrounding the range is away from the core. In the Leadville area this is to the east (Figure 1). Later tectonic activity associated with the northern end of the Rio Grande Rift, broke the section into a series of east dipping blocks, extending from Leadville to the Weston Fault on the east.
The historic portions of the Leadville district are located a short distance to the east of the town, while the Mt. Sherman or Horseshoe district, is located in the topographically highest fault blocks at the summit of the Mosquito Range.
In the Leadville district, a total of $512 Million worth of metal was mined from the first discovery in 1860 to 1963. Listing the metals by percentage of the total amount gives [Tweto, 1968]:
Although the geology of the Leadville district is complex, the majority of the base metal silver production came from manto deposits in the Leadville Formation that followed pre-existing paleokarst features.
|Paleokarst in the Historic Leadville District
An extensive erosional surface separates late Mississippian to Early Pennsylvanian rocks in North America and has been described at length by Palmer and Palmer . In the central Colorado area this paleokarst is typified by filled eaves, sinkholes, karst breecias and remnant karst towers [Maslyn 1976,1977].
Figure 2 is a map of the central portion of the historic Leadville district. Data for the map is from Emmonís, et al., 1927 report with interpretations added by DeVoto . The map shows a northwest-southeast trending paleovalley extending into the area, the Breece Hill intrusive, and several mineral deposits. In the center of the figure is the Iron Hill Group, one of the two most economically productive in the historic portion of the district [Emmons, et al., 1927] and one that shows mineralization along paleokarst features.
The geology of the area is further complicated by multiple north-south trending faults and intrusive porphories cutting the Paleozoic sedimentary section. Also note that ore bodies are found in three carbonate units, the Leadville and Dyer Dolomites and the Ordovician Manitou Dolomite.
The single most striking geographic feature of the Leadville district, the paleovalley, was described by DeVoto  based on the pre-Pennsylvanian subcrop map. It clearly shows the Leadville Formation entirely removed in an area 3 mi (4.8 km) long by 1 mile (1.6 km) wide, exposing the underlying Dyer Dolomite. In the western portion of the paleovalley, the Dyer Dolomite is removed as well, with the Pennsylvanian section unconformably overlying the Devonian Parting Sandstone. The total amount of section removed by erosion is more than 250 ft (75 m).
Note the strong southwest-northeast trends to the ore deposits in the Iron Hill Group south of the paleovalley. These features are interpreted from their morphology and from Emmons, et al.,  maps and descriptions to be mineral deposits filling caves formed in association with the paleovalley. In general, the ore bodies in the lowest stratigraphic units are closest to the paleovalley.
|Figure 3 is a cross-section through part of the Iron Hill ore deposits. Note the cave like shapes of the integrated
ore bodies. In the map view (Figure 2), the ore deposits in the Iron Hill Group show their lowest stratigraphic
development to the northeast, nearer the paleovalley. Similarly with the cross-section (Figure 3), the most extensive
development in the lowest stratigraphic horizon is at the northeast end, closest to the paleovalley.
Also note the multiple porphory intrusives dissecting the sedimentary rocks as well as the post-intrusive faulting. The presence of the porphories and the Breece Hill intrusive stock led early workers (e.g., [Emmonís, 1886] and [Emmons, et al., 1927] to conclude that the ore deposits were hydrothermally emplaced. Later workers (e.g., [Tschauder, et al, 1990] challenged that hypothesis.
Paleokarst Cave Systems in the Sherman Mine
Paleogeographic Reconstruction of the Leadville District in Late-Mississippian Time
Each of these factors will be discussed in turn.
The Leadville area in the late-Mississippian was dominated by a single valley, draining to the southeast off the ancestral Sawatch Range. Areas north and South of the valley tilted to the northeast.
The surface itself was gently rolling, intercepted by a number of sinkholes, whose location was controlled in part by small faults. Water entered the sinkholes and traversed multi-level caves, flowing from the southwest to the northeast. Cave Passages joined together in convergence passages at the northeast ends of the cave systems.
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Here's a link to an article on how to recognize many paleokarst features.