Geology and mineralization of the Seaman Gulch area, east Shasta mining district, Shasta County, California Public Deposited

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  • The Seaman Gulch area is located approximately 20 miles east of Redding, California and is part of the eastern Klamath lithotectonic belt. Rock units of the area are chiefly mafic and silicic volcanics of island arc affinity and shale that are Permian to Triassic in age. Volcanic rocks of Tertiary and Quaternary age unconformably overlie the pre-Tertiary rocks. All pre-Tertiary rocks have been altered by hydro-thermal and(or) metamorphic processes. Stratabound epigenetic mineralization is hosted by metadacitic pyroclastics of the Bully Hill Rhyolite. The hypogene sulfide minerals present are pyrite, sphalerite, chalcopyrite, tetrahedrite-tennantite and galena. Minor amounts of supergene chalcocite are found. These sulfides occur as disseminations, stringer veinlets, massive pods and an irregular network around pyroclastic fragments. Controls that localized the occurrences of sulfides are intrusive rock-country rock contacts, permeability and porosity of pyroclastic rocks, faults, and fractures. The gangue mineralogy of the stringer veinlets consists of quartz, carbonate, and anhydrite. The distribution of sulfide minerals is zoned vertically and from bottom to top as follows: pyrite - chalcopyrite as disseminations and stringer veinlets; pyrite - sphalerite - chalcopyrite as disseminations, stringer veinlets, and networks; and pyrite - sphalerite - chalcopyrite - tetrahedrite-tennantite - galena as disseminations, stringer veinlets, and massive pods. Hydrothermal alteration assemblages are chloritic alteration, quartz - white mica alteration, propylitic alteration, silicification, argillic alteration, and carbonate alteration. These alteration assemblages display both vertical and lateral zonations. A central zone of chloritic alteration grades laterally into propylitic alteration, and upward into quartz-white mica alteration. The quartz - white mica alteration in turn grades laterally into propylitic alteration. Argillic alteration and carbonate alteration are genetically related but slightly later events that overprint the earlier assemblages. The mineralization in the Seaman Gulch area is similar to the stockwork zone that underlies Kuroko-type volcano-genic massive sulfide deposits. The sulfides and their associated products of hydrothermal alteration were deposited during submarine geothermal activity that was operative during the latest Permian time. The economic potential of the Seaman Gulch area is considered to be low because of the paucity of base metalbearing sulfides, their occurrence as chiefly disseminations and veinlets, and the restricted distribution of economic assays.
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