Geologic studies of the Leadville Dolomite
Geochemical studies of Leadville Formation dolomites provide information about the solutions from which they formed. Seven dolomite types can be grouped into six geochemical types on the basis of trace element contents and oxygen isotopes. Medium and coarse crystalline dolomite are indistinguishable on the basis of either trace element or oxygen isotope contents. Low trace element abundances and slightly negative δ18OPDB values are snggestive of diagenesis in solutions containing a significant meteoric component.
Fine crystalline dolomite formed penecontemporaneously with deposition and contains heavier oxygen than either medium or coarse crystalline dolomite or undolomitized Leadville Limestone. The oxygen isotope data supports dolomitization of fine crystalline dolomite by evaporatively concentrated sea water followed by stabilization by meteoric water.
Two varieties of zebra spar dolomite precipitated into cavities formed during karst dissolution. Trace element abundances and 18O composition of earlier formed “cloudy zebra spar” are similar to those of medium and coarse crystalline dolomite and suggest that this dolomite formed as cement during karst solution erosion. Later formed “clear zebra spar” contains lighter oxygen and higher Fe and other trace elements, and fluid inclusions in it indicate precipitation above 130°C from evolved brines. This dolomite is interpreted to have formed from sedimeutary brine fluids after burial by Pennsylvanian sediments. Baroque dolomite occurring as cement in karst breccia bodies contains very light oxygen (δ18OPDB = —20%) and is high in Fe, Mn, and base metals. This dolomite is intimately associated with sphalerite and galena in some ore deposits and formed by recrystallization of detrital dolomite sand in karst breccias during ore deposition.
Paleomagnetic studies reveal a well-defined late Paleozoic remanence in most dolomite types spatially removed from Laramide-Tertiary igneous activity. The remanences are carried in magnetite of possible diagenetic origin. Preservation of these remanences suggests that these rocks have not undergone significant aquatic alteration since late Paleozoic time.
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,