Death Valley

Site Name

Primary: Death Valley

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Commodity

Primary: Uranium

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Location

State: Alaska
District: Koyuk

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Land Status

Not available

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Holdings

Not available

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Workings

Not available

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Ownership

Not available

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Production

Not available

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Deposit

Record Type: Site
Operation Category: Prospect
Operation Type: Unknown
Years of Production:
Organization:
Significant:

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Physiography

Not available

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Mineral Deposit Model

Model Name: Sandstone U

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Orebody

Not available

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Structure

Not available

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Alterations

Alteration Type: L
Alteration Text: Various clays are developed in the host sedimentary rocks of the epigenetic deposit that may reflect alteration processes. The supergene enrichment accompanies alteration associated with weathering processes.

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Rocks

Not available

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Analytical Data

Not available

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Materials

Ore: Coffinite
Ore: Sphalerite
Ore: Meta-Autunite
Gangue: Pyrite

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Comments

Comment (Reference): Primary Reference = Dickinson and others, 1987

Comment (Exploration): Status = Active

Comment (Deposit): Model Name = Sandstone U (Cox and Singer, 1986; model 30c)

Comment (Workings): Workings / Exploration = Surface pits, surface mapping, and extensive core drilling have been completed on this deposit.

Comment (Geology): Age = Epigenetic mineralization is considered to be early Eocene in age, a time when the climate was temperate or subtropical (Dickinson and others, 1987). This is the age of the host sediments and mineralization must have occurred before Tertiary faulting disrupted groundwater flowing eastward from the Darby pluton. The supergene mineralization is Recent in age and may be ongoing today.

Comment (Geology): Geologic Description = The Death Valley sandstone-type uranium deposit (Dickinson and others, 1987) was discovered in 1977 and soon explored by the Houston International Minerals Corporation. It is in early Eocene continental sediments that are unconformably deposited on deeply weathered granitic rocks of the Darby pluton (Miller and Bunker, 1976; Johnson and others, 1979). The Eocene sediments were deposited in a graben between the uplifted Darby pluton to the west and lower Paleozoic metamorphic rocks to the east (this is probably an onshore, exposed equivalent of the deeper parts of the offshore Norton basin, see below). The proximity to the slightly uraniferous Darby pluton seems to be an important control on the development of this deposit. The continental sediments are conglomerate, arkosic sandstone, mudstone, and coal. The unconformable contact between the coarse, poorly sorted basal sediments and the granitic pluton is gradational and some sediments in this part of the section are interpreted to be mudflows within alluvial systems. The upper part of the sedimentary section contains mudstones deposited in lucustrine environments. Eocene basalt is interbedded with and caps the sedimentary section in this area. These basalts may have created a dam that led to lucustrine sedimentation. The lucustrine mudstones contain laminated siderite but all the sediments compositionally reflect the nearby granitic provenance of the Darby pluton; granitic clasts, quartz, and k-feldspar are common detrital components. Carbonized wood fragments are also common in the section and it contains bituminous coal beds up to 100 feet thick. ? Uranium mineralization is both epigenetic and supergene. Epigenetic mineralization consists of coffinite, small amounts of pyrite, and trace amounts of sphalerite; it extends vertically over a stratigraphic interval of 300 feet both above and below basalt layers. This primary mineralization, interpreted to be early Eocene in age, is formed by the reduction of oxidized groundwaters derived from areas of granitic bedrock by carbonized-wood in conglomerate and arkosic sandstone. The principal mineralized zone defined by drilling covers an area of 395 by 9,850 feet and averages 10 feet in thickness. With an average grade of 0.27 % U3O8, this deposit has a calculated resource of 1,000,000 pounds of U3O8 (Dickinson and others, 1987). The supergene mineralization, related to the present surface, consists of several varieties of meta-autunite in soil and weathered bedrock intervals less than 20 feet thick. The mineralized surficial materials include three zones: (1) a one-foot thick zone of organic-rich clay and sand containing basalt cobbles that may be a debris flow; (2) a zone up to 10-feet thick of arkosic sand containing carbonized wood fragments; and (3) a zone of granitic grus or semiconsolidated arkosic sandstone and mudstone. Some arkosic sandstone fragments contain 11 % U3O8 and some basalt fragments have uraniferous weathering rinds. Epigenetic mineralization is considered to be early Eocene in age, a time when the climate was temperate or subtropical (Dickinson and others, 1987). This is the age of the host sediments and mineralization must have occurred before later Tertiary faulting disrupted groundwater flow eastward from the Darby pluton. The supergene mineralization is Recent in age and may be ongoing today. The Death Valley sandstone-type uranium deposit is the farthest north deposit of its type in the world. At the time of its formation, it was probably at an even higher latitude than it is today, 64 degrees north.

Comment (Geology): Age = Host sediments are Early Eocene.

Comment (Deposit): Other Comments = the prospect is presently controlled by David Hedderly-Smith, P. O. Box 443, Park City, UT 84060. The non-mineralized, early Eocene geology exposed and documented at this prospect may be representative of the basal parts of Tertiary basins elsewhere in the region. This includes the offshore Norton basin to the south of Seward Peninsula that has been explored for its hydrocarbon potential (e. g. Fisher and others, 1982; Worrall, 1991).

Comment (Reserve-Resource): Reserves = the principal mineralized zone that has been defined by drilling covers an area of 395 by 9,850 feet and averages 10 feet in thickness. This deposit has a calculated resource of 1,000,000 pounds of U3O8 at an average grade of 0.27 % U3O8 (Dickinson and others, 1987). This is the largest presently known uranium deposit in Alaska.

Comment (Commodity): Ore Material = meta-autunite

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References

Reference (Deposit): Gamble, B.M., 1988, Non-placer mineral occurrences in the Solomon, Bendeleben, and southern part of the Kotzebue quadrangles, western Alaska: U.S. Geological Survey Map MF-1838-B, 13 p., 1 sheet, scale 1:250,000.

Reference (Deposit): Worrall, D.M., 1991, Tectonic history of the Bering Sea and the evolution of Tertiary strike-slip basins of the Bering Shelf: Geological Society of America Special Paper 257, 120 p.

Reference (Deposit): Johnson, B.R., Miller, T.P., and Karl, S., 1979, Uranium-thorium investigations of the Darby pluton, Seward Peninsula, Alaska: U.S. Geological Survey Circular 804-B, p. 68-70.

Reference (Deposit): Fisher, M.A., Patton, W.W., Jr., and Holmes, M.L., 1982, Geology of Norton Basin and continental shelf beneath northwestern Bering Sea, Alaska: American Association of Petroleum Geologists Bulletin, v. 66, p. 255-285.

Reference (Deposit): Dickinson, K.A., Cunningham, K.D., and Ager, T.A, 1987, Geology and origin of the Death Valley uranium deposit, Seward Peninsula, Alaska: Economic Geology, v. 82, p. 1558-1574.

Reference (Deposit): Miller, T.P., and Bunker, C.M., 1976, A reconnaissance study of the uranium and thorium contents of plutonic rocks of the southeastern Seward Peninsula, Alaska: U.S. Geological Survey Journal of Research, v. 4, p. 367-377.

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