Our studies of geology, karst and cave morphology, mineralogy, geochemistry, and isotopes of the carbonate-hosted ore deposits of central Colorado have compeled us to two principal conclusions. First, some manto orebodies typical of the area are mineralized modifications of preexisting paleokarst cave systems. Second, some of the ores are geologically, mineralogically, and geochemically distinct from ores of the main Leadville district (Sherman-type defined below), are substantially older (Late Mississippian) than, and unrelated to, mid-Tertiary mag-matism and main Leadville district ore formation, and formed by processes similar to those that formed some Mississippi Valley-type deposits in the midcontinent region of the United States.
In response to Late Mississippian emergence, an extensive karst system developed on and in the Leadville Dolomite. The resultant integrated cave systems have a distinct and easily recognizable large-scale morphology in plan view: pipe-shaped conduits in the form of a distributary network are elongated downdip from a surface input zone and converge in a central conduit or room. The caves also include many small-scale features such as dolines, chimneys, and bypass tubes. On the eastern flank of the Sawatch uplift, strata dipped into a trough whose base level may have been sea level. To the west, strata dippedinto the incipieut Eagle basin. Long dimensions of the cave conduits parallel the paleodip while convergent zones are aligned along the paleostrike.
Prior to deposition of the Molas Formation (a Late Mississippian-Early Pennsylvanian karst-derived paleosol), the base level intermittently rose, enabling alternating phreatic and vadose conditions in the caves. The caves filled with iron oxide, carbonate speleothems, stratified dolomitic sand, silt, and clay, with splatter marks, dessication cracks, and mud drapes, collapse breccia, huntite mud, and angular to rounded grains of silver-rich Sherman-type minerals. The Molas Formation forms the uppermost unit of this sedimentary sequence and restricts the age of formation of the cave fill Sherman-type mineral assemblage to Late Mississippian. Cave fill textures are best preserved away from the destructive effects of later mid-Tertiary replacement (manto) ores in the central part of the main Leadville district. The best documented example of Sherman-type ores occurs in paleocaves of the Sherman mine, east of the Leadville district.
Characteristics of Sherman-type mineral assemblages include presulfide white barite, banded dolomite flowstone and dripstone, low iron sphalerite, a high silver content, a very low gold content, very low pyrite and chalcopyrite content, light stable isotope and lead isotope data indicating crustal sources, karst cave control on ore morphology, and occurrence of ore minerals as clasts within the cave sediments.
The Pennsylvanian Belden Formation marks the beginuing of a brief marine transgression into central Colorado. Sherman-type mineralization had ceased by this time. Following transgression, the area was buried under 2,500 to 5,500 m of mostly continental sediments.
Quartz-gold-pyrite and silver-base metal ores were emplaced in carbonate rocks in the Leadville and Gilman mining districts at depth iu the mid-Tertiary. The shapes of the mid-Tertiary orebodies are analogous to the shapes of fully integrated cave systems, though their mineralogic and isotopic data are significantly different from Sherman-type ores. Also preserved in these orebodies are several small-scale karst and cave fill features, including Molas Formation within the Leadville Dolomite and relict Sherman-type ore minerals. The mid-Tertiary orebodies therefore are controlled, at least in part, by the precursor paleokarst cave systems. We speculate that much of the silver in Leadville district orebodies is the result of redistribution of high amounts of silver from precursor Sherman-type ores.
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,