Abstract

Hydrothermal veins of Tertiary age in the Freeland-Lamartine district, Clear Creek County, Colo., fill long, cymoid-shaped fissures that cut granitic rocks and complexly folded Precambrian metasedimentary rocks. The fissures formed under a regional compressional stress that produced a fracture pattern composed of three elements that formed in a distinct sequence. Mineralization took place in two stages during fracturing.The first set of fractures developed approximately parallel to north-northeast-trending axial planes of major Precambrian folds; those fractures now form the middle segment of the cymoid fissures. Subsequent fractures formed at both ends of the early fractures; one set trends east-northeast and the final set trends east. The latter fractures cut through and locally displace the earlier fractures.In the first stage of mineralization, quartz and auriferous pyrite with some chalcopyrite and tetrahedrite filled the early fractures, chiefly the north-northeast set and adjacent parts of the east-northeast set. In the second stage, galena and sphalerite with minor amounts of quartz, pyrite, chalcopyrite, tetrahedrite, and carbonate filled the younger fractures near the ends of the cymoid structures. Veins formed during the second stage of mineralization locally cut the earlier veins in the east-northeast-trending segments of the fissures, forming composite ores.The veins were deposited in openings and show well-defined hypogene zoning--individual veins have a central segment of pyrite-gold ore bounded at both extremities by galena-sphalerite ore.

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