Graduate Thesis Or Dissertation
 

Uranium mineralization of the metamorphic aureole of the Spirit pluton, Stevens County, Washington

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  • Several uranium anomalies, with concentrations of U₃0₈ that average below 0.003 percent and reach maximums of 0.069 percent, are associated with black, fossiliferous, pyritic parts of the Ordovician Ledbetter Slate in northern Stevens County, Washington. The most uraniferous parts of the slate occur in small (average of 5 by 30 cm), tabular-shaped bodies of black argillite within the crests of several small-scale folds (amplitudes and wavelengths less than 1 m) along the contact between the Ledbetter Slate and Metaline Limestone from within 30 m of the southern border of the Late Cretaceous Spirit pluton. The concentration of uranium in the slate varies in direct proportion to the amount of reduced organic matter and pyrite contained in the rocks as well as with the extent of contact metamorphism imposed on the slate during forceful intrusion of the Spirit pluton. Discrete phases of uranium-bearing minerals were not identifiable during the course of petrographic examinations, therefore, the uranium is believed to occur as dispersed ionic disseminations that are physically and(or) chemically bonded to the minute particles of reduced organic matter in the rocks. The uranium anomalies had a multi-stage genesis which may have included: 1) the extraction of uranium from the Ordovician sea by organic matter and the syngenetic accumulation of the uraniferous organics with muds in an euxinic depositional environment; 2) the localized remobilization and corresponding reconcentration of the uranium in the rocks adjacent to the sourthern border of the Spirit pluton in response to contact metamorphism; 3) the minor addition of uranium into the country rock from siliceous volatile-rich fluids may have originated from the late-stage differentiation of the Spirit pluton; and(or) 4) the possible supergene enrichment of uranium that may have been leached by meteoric waters from overlying igneous or metasedimentary rocks. The realtively low-grade and small volume of the uraniferous parts of the Ledbetter Slate in the Bruce Creek study area render the uranium anomalies uneconomic. Nonetheless, an understanding of their distribution, genesis, and petrographic characteristics may lead to the discovery of similar and economically more viable deposits of uranium elsewhere in northeast Washington.
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