Abstract

Numerous small manto deposits occur in the vicinity of Tennessee Pass, located between the Gilman and Leadville mining districts in central Colorado. These deposits are hosted in the Mississippian Leadville Dolomite, crop out over 7 km of strike, and average about 10 to 15 ppm Au. The ores are now heavily oxidized but at one time consisted of semimassive pyrite, fine-grained quartz, zeolite, and native gold. Most primary fluid inclusions in quartz homogenize at temperatures of 200 degrees to 220 degrees C, suggesting deposition temperatures of about 250 degrees C. Sulfur isotopes (delta 34 S pyrite = 1-4ppm) suggest an igneous sulfur source. The trace element geochemistry (high As, Sb, Hg) is similar to that in many other low-temperature hydrothermal systems. Oxygen isotope ratios in quartz (delta 18 O = 14-17ppm) indicate that the ore fluid was relatively heavy (delta 18 O fluid = 4-8ppm). Alteration around the Tennessee Pass deposits is stratigraphically controlled and most intense near the Leadville Dolomite. Two major areas (each 2 km diam) of pervasive alteration and veining affect the rocks 3 to 8 km east (Cooper Hill) and southeast (Buckeye Gulch) of Tennessee Pass. Fission-track geochronology in apatite and zircon indicates that these alteration centers are part of a large (5-7 km diam) paleothermal anomaly which has reset ages of 38 to 42 m.y. This suggests the presence of a large, unexposed stock.These geologic relations suggest the following mineralization model. Within the two major alteration centers, the hydrothermal flux was dominantly vertical, similar to that in the central part of the Leadville district. On the western margin of the thermal center (i.e., the Tennessee Pass area), updip fluid flow was concentrated in the Leadville Dolomite, resulting in few veins and stratigraphically controlled alteration (similar to the Gilman district). The Tennessee Pass district thus may represent the peripheral portion of a much larger manto-type system. The recognition of this potential was based in large part on the isotope systematics, which therefore may have important exploration applications for this type of ore deposit.

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