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Hitzman, M.W., 1983

Geology of the Cosmos Hills and its relationship to the Ruby Creek copper-cobalt deposit

Bibliographic Reference

Hitzman, M.W., 1983, Geology of the Cosmos Hills and its relationship to the Ruby Creek copper-cobalt deposit: Palo Alto, California, Stanford University, Ph.D. dissertation, 266 p., charts, illust., maps.

Abstract

The Cosmos Hills in northwestern Alaska contain a metamorphosed sequence of mixed middle Devonian clastic and carbonate rocks unconformably overlying more highly metamorphosed sediments and basalts of the Proterozoic to early Paleozoic Kogoluktuk schist. The middle Devonian basal sequence (Anirak schist) consists of interlayered quartz-mica schists, quartzites, and greenstones conformably overlain by the Bornite carbonate sequence. The carbonate section represents a northward-thickening carbonate platform with a biohermal barrier complex separating it from time-equivalent phyllites to the north. Carbonates are overlain by the Beaver Creek phyllite. The Middle Devonian section thickens from 2,000 m in the Cosmos Hills to more than 6,000 m in the southern Brooks Range. The presence of syndepositional conglomerates adjacent to the edge of the carbonate bank, the variation in the thickness of the section, and the occurrence of Devonian bimodal volcanics in the southern Brooks Range suggest the area was an intracratonic rift basin. The section was folded and metamorphosed to lower greenschist facies during the late Jurassic Brooks Range orogeny. Complex deformational features formed in carbonate rocks due to pressure solution and competency differences between marble and dolostone. Latest Jurassic obduction emplaced the ophiolitic, Devonian to Jurassic Angayucham basalt over the metasedimentary rocks. Major deformation ceased following the emplacement of lower Cretaceous quartz-pebble conglomerate over the Angayucham basalt. The Ruby Creek Cu-Co-Zn deposit occurs in lagoonal and biohermal facies carbonate rocks. Two distinct stages of Devonian alteration and mineralization are recognized. Early alteration produced several large hydrothermal dolostone bodies that contain restricted zones of disseminated to massive sphalerite and pyrite. The second stage of alteration fractured these bodies and deposited copper sulfides in veins and replacement zones. Fluid inclusion, isotopic, and organic geochemical studies indicate 100-300 degrees C solutions were responsible for the alteration and mineralization. Copper deposition was probably related to a rapid temperature decline of the second stage solution. Ruby Creek is similar to the Irish base metal deposits and the discordant mineralization at McArthur River, Australia.

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