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
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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