| Basis for focus area |
Data-driven prospectivity ranking of watersheds (HUCs; Karl and others, 2016) shows that the focus area includes 30,010 km2 with a total of 96 high prospectivity HUCs and 107 medium prospectivity HUCs out of 326 HUCs. This belt also contains the Tired Pup Tertiary alkaline intrusive complex with REE mineral occurrences localized around it, the Styx River granite which contains U-Th enrichment, and eudialyite-bearing dikes, and the peralkaline Windy Fork granite with elevated apatite, monazite and zircon. The northwestern part of the focus area includes the Eudialyite occurrence, a eudialyite-bearing, uranium-thorium-enriched diorite dike emplaced along the contact of the peralkaline Windy Fork pluton. The dike is cut by 3-10 cm thick uranium-thorium-REE veins. |
| Identified resources |
None. |
| Production |
None recorded. |
| Status |
Unknown. |
| Estimated resources |
No data. |
| Geologic maps |
Approximately 20% of the area has been mapped at 1:63,360 scale or better. In the area of the Eudialyite occurrence in the northwestern part of the focus area, a published 1:125:000 scale geologic map of eastern McGrath quadrangle and preliminary inch-to-mile map of McGrath A-3 quadrangle. Otherwise, no detailed published geologic mapping. |
| Geophysical data |
Inadequate aeromagnetic and aeroradiometric coverage. |
| Favorable rocks and structures |
Lithology scores range from 1-1 with an average of 1. Igneous scores range from 0-4 with an average of 1.66. Known peralkaline intrusions such as the Tired Pup, Styx River granite, and Windy Fork plutons are present in the belt. Windy Fork peralkaline granite complex is cut by alkaline to peralkaline diorite dikes. |
| Deposits |
None. |
| Evidence from mineral occurrences |
Three hard rock mineral occurrences including the Eudialyte occurrence, are found across the Western Alaska Range associated with peralkaline granite intrusions. Zirconium-enriched eudialyite is reported in ARDF record MG036. ARDF Eudialyte occurrence associated with the Oligocene Windy Fork peralkaline pluton. ARDF scores range from 0-2 and average 0.0625. |
| Geochemical evidence |
Sediment scores range from 0-6 with an average of 4.66. For grab samples at Eudialyte prospect: 33.3 ppm U and 46.3 ppm Th. |
| Geophysical evidence |
Aeroradiometric scores range from 0-2 with an average of 0.54. |
| Evidence from other sources |
No data. |
| Comments |
Western Alaska Range has received the most study of any of the other belts in Alaska through recent USGS and DGGS projects. There is still unknown critical element potential associated with many of the mineralized systems as USGS mapping and bedrock sampling produced substantial data but was not accompanied by systematic, detailed geologic mapping at scales better than 1:250:000. Eudialyte prospect is associated with the Oligocene Windy Fork pluton. Regional USGS mapping and sampling revealed other, previously unrecognized Oligocene plutons with similar geochemistry and geologic setting. But systematic, detailed geologic mapping of these bodies does not exist to characterize spatio-temporal patterns of igneous activity and relationship of REE-bearing systems to the igneous units. Commodities include allanite, xenotime, monazite, uranium, thorium, zircon, apatite, and eudialyte. |
| 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 |
Bokan Mountain is well-mapped and characterized through prior USGS, academic, and industry efforts. Published inch-to-mile maps are not available for the entire area. Occurrences at Dora Bay and Salmon Bay are less well-understood. The peralkaline belt extends throughout much of the Alexander terrane in southeast Alaska and needs to be better mapped and characterized. Systematic, detailed geologic mapping of quadrangles surrounding Bokan Mt. deposit and in surrounding areas of southeast Alaska is needed to identify broader geologic controls on formation of the deposit. Isotopic data (hafnium, lead) of all igneous phases is needed 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. |
