| Basis for focus area |
Data-driven prospectivity ranking of watersheds (HUCs; Karl and others, 2016) shows that the focus area includes 16,660 km2 with a total of 23 high prospectivity HUCs and 66 medium prospectivity HUCs out of 402 HUCs. The belt contains the Bokan Mountain REE deposit and other associated mineral occurrences. Bokan Mountain contains an Indicated and Inferred resource of 5.8 Mt @ 0.36% LREE and 0.23% HREE. Bokan Mountain has the largest NI 43-101 compliant resource of HREE in the United States, and is a significant deposit in Alaska. The deposit is associated with a peralkaline, sodic intrusion spanning approximately 2 miles in diameter in current outcrop pattern. Intrusion is a ring-like dike complex with compositional zoning from border phases into the coarser-grained interior of the pluton. |
| Identified resources |
Identified resources for REE. Historical production of thorium and uranium. |
| Production |
Unknown. |
| Status |
Past mining. Current exploration and NI 43-101 resource at Bokan Mountain (Bentzen and others, 2013). Currently developing a processing plant for ores in Ketchikan with planned commencement of production in 2019. |
| Estimated resources |
Bokan Mountain: 5.8 Mt @ 0.36% LREE, 0.23% HREE, 0.6% ΣREO (Bentzen and others, 2013). |
| Geologic maps |
Approximately 20% of the area has been mapped at 1:63,360 scale or better. Multiple vintages of maps of Bokan Mountain, including MacKevett (1963), scale 1:24,000; and figures in publications, such as Thompson and others (1980, 1982); Saint-Andre and others (1983); Gehrels (1992); Thompson, 1997). |
| Geophysical data |
Inadequate aeromagnetic and aeroradiometric coverage. |
| Favorable rocks and structures |
Lithology scores range from 0-1 with an average of 0.70 for high prospectivity HUCS. Igneous scores range from 0-4 with an average of 0.88 for high prospectivity HUCs. Peralkaline intrusions occur through SE Alaska, and are known to be enriched in REE. Peralkaline, sodic, granites and pegmatites have a genetic association with the U, Th, and REE mineralization at the deposit. The core of the intrusion consists of a riebeckite granite porphyry with outer border zones composed of pegmatite and aplite and an intermediate zone of aegerine granite porphyry. |
| Deposits |
None. |
| Evidence from mineral occurrences |
ARDF occurrences are localized around Bokan Mountain, Dora Bay, and Salmon Bay. The latter two have not been the subject of significant exploration to date. Multiple mineral occurrences and past production of uranium-thorium are present across the Bokan Mountain district, including the Ross Adams Mine, I and L mines, the Carol Ann. ARDF scores range from 0-2 with an average of 0.15 in high prospectivity HUCs. USMIN sites AK00022 (niobium), AK00057 and AK00192 (REE). |
| Geochemical evidence |
Stream sediment scores range from 0-5 with an average of 1.26 for high prospectivity HUCs. |
| Geophysical evidence |
Aeroradiometric scores range from 0-1 with an average of .18 for high prospectivity HUCs. |
| Evidence from other sources |
No data. |
| Comments |
SE Alaska is perhaps one of the better studied, yet poorly understood areas of the state. The only significant known REE deposit in the state, and similar, poorly-understood rocks occur in the same geologic belt to the north. Prospectivity for other systems like Bokan is not known but is not expected to be significant based on existing mapping. Bokan Mountain is the largest known REE deposit in the state of Alaska. It is actively being explored by UCORE who hopes to bring the deposit into production in the next few years. Bokan Mountain is purported to have the greatest HREE contents in the United States. Commodities include LREE, HREE, allanite, bastnaesite, tantalite, euxenite, monazite, uranothorite, xenotime, zircon, uranium, and thorium. |
| Cover thickness and description |
Unknown. |
| Authors |
Douglas C. Kreiner, James V. Jones III, Melanie B. Werdon. |
| New data needs |
New mapping, geophysics, updated geochemical sampling. |
| Geologic mapping and modeling needs |
Framework geology-includes lithology, structure, and hydrothermal alteration. There is no detailed mapping in the region. Particular attention paid to characterizing key structures (i.e., Porcupine Shear Zone), which may play a role in fluid plumbing. Host rock mapping-Selwyn basin stratigraphy present, is there potential for Carlin-like gold systems as in the eastern portions of the Selwyn basin? Igneous phases identified and quantified geochemically-what are the key alkaline phases associated with REE mineralization at Old Crow, are there other magmatic hydrothermal systems? Isotopic data (hafnium, lead) to test crustal character and terrane affinity. |
| Geophysical survey and modeling needs |
Rank 1 or 2 aeromagnetic data over the entire belt. Targeted EM where relevant for lithologic and structural mapping under cover. |
| Digital elevation data needs |
IfSAR coverage is complete over entire focus area. |
