| Basis for focus area |
Data-driven prospectivity ranking of watersheds (HUCs; Karl and others, 2016) shows that the focus area includes 112,710 km2 with a total of 35 HUCs containing high prospectivity. The belt contains carbonatite sills, syenite, alkaline volcanic rocks and alkaline to calc-alkaline intrusive bodies associated with REE mineralization. Includes carbonatite-hosted REE mineralization located within a belt of placer Sn deposits, collectively called the Tofty tin belt. |
| Identified resources |
None. |
| Production |
None recorded. |
| Status |
Multiple previous exploration campaigns beginning in the 1980s for uranium-thorium, and most recently in the early 2000s for REE potential in the Tofty Ridge area. |
| Estimated resources |
Unknown. |
| Geologic maps |
Approximately 15-20% mapped at 1:63:360 scale or better. |
| Geophysical data |
Inadequate aeromagnetic and aeroradiometric coverage. |
| Favorable rocks and structures |
Within this belt are a swath of alkaline Late Cretaceous-Paleocene intrusive rocks consisting of carbonatite, syenite, and monzonites. The lithology scores for the prospectivity analysis range from 0-1 with an average of 0.91. The igneous scores range from 0 to 4 with an average of 1.29. In the Tofty Ridge area, carbonatite sills traceable over ~10 km, are 30 m thick. Outcrops are pale yellow and contain magnetite, apatite, and melanite garnet crystals. The carbonatite sills and bodies are emplaced into serpentinized mafic and ultramafic rocks. |
| Deposits |
None. |
| Evidence from mineral occurrences |
Known ARDF occurrences are associated with the Tofty carbonatite complex, Roy Creek alkaline granite and other small occurrences in the Ray Mountains area. Tofty Ridge and Tofty Gulch ARDF occurrences and other minor placer occurrences; ARDF Placer occurrences are scattered throughout the alkaline igneous province and contain zircon, xenotime, and monazite; ARDF: TN088; TN099; ARDF scores range from 0-3. |
| Geochemical evidence |
Tofty Ridge area: An Alaska Division of Geological and Geophysical Surveys (ADGGS) sample (97RN235B) of weathered carbonatite in Miller Gulch contained more than 15% Fe and 10,000 parts per million (ppm) P; 2,100 ppm Sr, 51 ppm Sc, 740 ppm La, 433 ppm V, and 40 ppm U (Liss and others, 1998). Several carbonatite samples from the same body in Harter Gulch contained maximums of 952 ppm As, 12 ppm Bi, 403 ppm Cr, 595 ppm La, 4,120 ppm Mn, 982 ppm Ni, 2,900 ppm Sr, 18 ppm Sc, 117 ppm V, and more than 10,000 ppm P (sample numbers 97RN227.2, 97RN227.3, 97RN232A, 96KC203). Reanalysis of magnetite-bearing carbonate (carbonatite) from U.S. Bureau of Mines drill core by ADGGS indicated the unusual nature of the carbonatite, with maximum values of 542 ppm Nb, 7,245 ppm Sr, 39 ppm Y, 281 ppm Zr, 26 ppm Ga, 3,416 ppm Ni, and 638 ppm Zn (ADGGS unpublished data). |
| Geophysical evidence |
Aeroradiometric scores range from 0-1 with an average of 0.83. Aeromagnetic data identified the carbonatite sill and associated ultramafic rocks in the Tofty Ridge area. |
| Evidence from other sources |
No data. |
| Comments |
This belt contains several clusters of known REE-uranium-thorium mineralization associated with both alkaline granite-hosted quartz-REE veins and carbonatite systems. There are many other poorly mapped intrusions with HUCs delineating medium to high prospectivity areas. Tofty is one of the only known carbonatites in the state, and has reported highly anomalous REE minerals. Cassiterite placer development in the vicinity shows regional enrichments in REE-bearing phases that are being weathered from the carbonatite-related mineralization. A complete commodity list includes: REE, uranium, monazite, zircon, uranothorite, fluorapatite, cerium, niobium, yttrium, aeschynite, arsenopyrite, chalcopyrite, and euxenite. |
| 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 mapping, including lithology, structure, and hydrothermal alteration. Mapping of all alkaline intrusions with updated geochemistry and petrogenetic studies. Identification of wall rock alteration assemblages and zoning patterns to better identify causative pluton phases. Better understanding of hydrothermal assemblages to characterize mineral systems and correctly place them into mineral systems framework. Isotopic data (Hf, Pb) needed to test crustal character and terrane affinity. Systematic, detailed geologic mapping of areas containing and surrounding ARDF records and potentially correlative igneous units throughout the region to understand petrogenesis and tectonic setting of the carbonatite. |
| 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. |
