Geology, wall-rock alteration, and massive sulfide mineralization in a portion of the West Shasta District, California

Univ. Oreg., Dep. Geol., Eugene, OR, United States

Devonian ore deposits, lens-shaped bodies of massive pyrite with minor quartz, chalcopyrite, and sphalerite. Paleoenvironment of ore deposition, based on 26 diamond drill holes and surface mapping, indicates that sulfides accumulated in small (50-700 ft) basins that were enclosed by rhyolite domes. Part of the hydrothermal flow system included lateral solution migration in hot water aquifers with hot spring outlets in the ore basins. Beneath the main ore horizon, the rhyolites are pervasively altered to quartz, sericite, and pyrite with or without chlorite. Electron microprobe analyses of sphalerite and alteration chlorite, sericite, albite, and epidote, combined with thermodynamic calculations, indicate that the solutions responsible for alteration and mineralization were quite enriched in Mg and Na relative to Fe and K and had a pH in the range of 3 to 5.--Modified journal abstract.

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