Boulder Creek; Death Valley Prospect, Koyuk Mining District, Nome Census Area, Alaska, USAi

Regional Level Types
Boulder Creek; Death Valley Prospect	Prospect
Koyuk Mining District	Mining District
Nome Census Area	Census Area
Alaska	State
USA	Country

Location: The Boulder Creek or Death Valley (in the early literature) uranium deposit is located in the headwaters of Boulder Creek, a north tributary to the middle Tubutulik River. It is 3.5 miles south of the point where the Tubutulik River leaves Death Valley. The deposit has been explored on the east side of Boulder Creek and the discovery pits are 1.0 mile southeast of Hill 990 and 1.0 mile east of Tubutulik River. This is locality 60 of Gamble (1988).
Geology: The Boulder Creek or Death Valley (in the early literature) sandstone-type uranium deposit (Dickinson and others, 1987) was discovered in 1977 and soon explored and drilled by Houston International Minerals Corporation. The Death Valley deposit is in lower Eocene continental sedimentary rocks that unconformably overlie 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. The proximity to the slightly uraniferous Darby pluton seems to be an important control on the development of this deposit. The continental sedimentary rocks are conglomerate, arkosic sandstone, mudstone, and coal. The unconformable contact between the coarse, poorly sorted basal sedimentary strata and the granitic pluton is gradational and some strata in this part of the section are interpreted as mud flows in alluvial systems. The upper part of the sedimentary section contains mudstones deposited in a lucustrine environment. 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 lacustrine 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 which 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 thick. 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 mudstone and sandstone containing basalt cobbles that may be a debris flow; (2) a zone up to 10-feet thick of arkosic sandstone containing carbonized wood fragments; and (3) a zone of granitic grus or semi-consolidated arkosic sandstone and mudstone. Some arkosic sandstone fragments contain 11 percent 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 rocks 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. The Boulder Creek deposit is currently a joint venture between Full Metals Minerals (2007, Boulder Creek) and Triex Minerals (2007) with Triex as the operator. Triex completed a four-week reconnaissance program in 2005 and identified several geochemical anomalies in the surrounding area. In 2006, they drilled 14 core holes totaling 1,237 meters. Twelve of the holes were on the Boulder Creek deposit; they did not change the known dimensions of the deposit. They drilled two additional holes on geochemical anomalies about 6 kilometers north-northwest of the Boulder Creek deposit; they did not intersect significant mineralization but they did cut rocks favorable for uranium mineralization. Triex also staked a large block of claims about 25 kilometers to the southwest in the southeast portion of McCarthy Marsh based on airborne geophysical anomalies. They followed up these anomalies in 2006 by considerable soil sampling and biogeochemical surveys.
Workings: The Death Valley sandstone-type uranium deposit (Dickinson and others, 1987) was discovered in 1977 and soon explored and drilled by the Houston International Minerals Corporation. Houston Oil and Minerals completed 3,300 m of core drilling in 52 holes and about 60 m of near-surface split-tube sampling in 21 holes, focused on the Discovery Zone area. Surface grab samples reported by previous operators returned up to 34 percent U3O8, with intercepts up to 3.3 meters, averaging 0.58% U3O8. The Boulder Creek deposit is currently a joint venture between Full Metals Minerals and Triex Minerals with Triex as the operator. Triex completed a four-week reconnaissance program in 2005 and identified several geochemical anomalies in the surrounding area. In 2006, they drilled 14 core holes totaling 1,237 meters. Twelve of the holes were on the Boulder Creek deposit; they did not change the known dimensions of the deposit. They drilled two additional holes on geochemical anomalies about 6 kilometers north-northwest of the Boulder Creek deposit; they did not intersect significant mineralization but they did cut rocks favorable for uranium mineralization. Triex also staked a large block of claims about 25 kilometers to the southwest in the southeast portion of McCarthy Marsh based on airborne geophysical anomalies. They followed up these anomalies in 2006 by considerable soil sampling and biogeochemical surveys.
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 sedimentary host rocks 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.
Alteration: Various clays are developed in the sedimentary host rocks of the epigenetic deposit that may reflect alteration processes. The supergene enrichment accompanies alteration associated with weathering processes.
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.

Commodities (Major) - U
Development Status: No
Deposit Model: Sandstone U (Cox and Singer, 1986; model 30c)

Other Databases

Link to USGS - Alaska:	BN089

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References