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Publication authorized by the Director, U. S. Geological Survey.

Abstract

The gold-and silver-rich base-metal sulfide ore deposits of the Central City district, Colorado, are arranged in well-defined concentric zones. A core of pyritequartz veins is surrounded by an outer zone of veins containing dominant galena and sphalerite in a gangue of quartz or, less commonly, rhombohedral carbonates, barite, or fluorite. In an intermediate area transitional veins contain pyrite, copper minerals, galena, and sphalerite. The transition from the core outward is marked also by systematic changes in gold and silver content, metal ratios, and mineral textures.

Temperatures of mineralization, as determined by sphalerite and fluid-inclusion thermometry, indicate that the veins in the inner parts of the district formed at temperatures on the order of 600° C, whereas those on the margins formed in the interval 150°–300° C. A steep geothermal gradient that shifted with in a relatively narrow zone separated the two areas of relatively uniform thermal regime. Heat-dissipating processes believed to be responsible for the thermal pattern are mainly expansion and cooling of the hot high-pressure magmatic fluids followed by mixing of the fluids with cooler meteoric solutions. Throttling (also known as Joule-Thomson expansion) probably was a major mechanism for cooli ng the hot magmatic solutions.

The activity of S2 decreased wi th temperature, but at a rate such that more sulfur-rich mineral assemblages could form toward the margins of the district. Most of the sulfide ore and the gold and silver were deposited in the intermediate and peripheral zones of the district, where changes in the ore-forming solutions were most rapid and drastic.

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