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Abstract

Dolomite-hosted silver-lead-zinc-barium deposits occur in the Mosquito Range of central Colorado and are locally termed Sherman-type deposits. A comparative study of these deposits shows that they are different from the sulfide deposits at Leadville and Gilman in geologic setting, mineralogy, and geochemistry.

Sherman-type mineral deposits are dominantly hosted by the Leadville Dolomite, which is typically only slightly altered. Coeval veins physically connected to the Sherman-type deposits crosscut older and younger rocks, the youngest being middle Tertiary in age. Deposit morphologies are influenced by a combination of geologic features: Tertiary faults and fractures, solution breccias, and stratigraphy. Pre-Pennsylvanian karst-related solution breccias serve as the dominant host and contain variable amounts of quartz, barite, ferroan dolomite, sphalerite, argentian tetrahedrite, and galena. Early barite, low iron sphalerite, high silver content, and abundant late galena are common to these deposits throughout the region.

Ore deposition occurred between 150° and 300°C by fluids with salinities less than 8 equiv wt percent NaCl. Sulfur and lead for sulfate and sulfide minerals were derived from sedimentary rocks (δ34S: early barite = 13.2-24.0‰; galena = -6.8 to -0.6‰; 206Pb/204Pb > 19.10).

The above data show that Sherman-type deposits are a regionally distinct deposit type and formed by regional processes which can be constrained to the Tertiary. Emplacement of the Colorado mineral belt batholith may have initiated regional warming which affected the basins of the central Colorado trough. In response to this early to mid-Tertiary geothermal gradient, dilute formational waters leached metal and sulfur from the basinal rocks and carried them up the flanks of the Sawatch anticline along faults and aquifers. Mineral deposition occurred in permeable breccias of karst and tectonic origin which flank the anticline.

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