| Quadrangle map, 1:250,000-scale | TN |
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| Quadrangle map, 1:63,360-scale | A-2 |
| Latitude | 65.0927 |
| Longitude | -150.93691 |
| Nearby scientific data | Find additional scientific data near this location |
| Location and accuracy | This lode occurrence is in Idaho Gulch at an elevation of about eight hundred feet. The site is in the northeast quarter of section 14, T. 3 N., R. 17 W., of the Fairbanks Meridian. The location is accurate within a half mile. The site corresponds to location 5 of Cobb (1972), and very roughly to the location of Sullivan Creek, U.S. Bureau of Land Management MAS number 0020480061. The MAS site is listed as a gold lode. |
Geologic descriptionThis lode occurrence is in an area of placer deposits, known as the Tofty tin belt, which contain significant amounts of cassiterite and gold (Thomas, 1957). These deposits are along a 12 mile trend that strikes approximately S70W on the north side of Patterson and Sullivan creeks, between Roughtop Mountain to the north and Hot Springs Dome to the south (Thomas, 1957). These two features are underlain by granitic plutons respectively of Cretaceous and Tertiary age.
The Tofty tin belt lies along a major thrust fault which juxtaposes Triassic argillite, sandstone and shale to the northwest over Cretaceous sandstone, siltstone and shale to the southeast (Reifenstuhl and others, 1998). Carbonatite bodies occur in the Triassic sedimentary rocks at the head of Idaho Gulch, as well as across the length of most of the Tofty tin belt (Reifenstuhl and others, 1998). Wayland (1961) describes an exposure of carbonatite as a 100-foot thick lens of pale yellow limestone containing minor magnetite and apatite crystals exposed near the head of Harter Gulch. Small exposures of serpentinized mafic rocks have been mapped around the town of Tofty.
At least two types of lode deposits occur in the Idaho Gulch area. One consists of gossan in quartzite. A sample from this gossan contained 1.34 ounces of silver per ton but no gold (Wayland, 1961). The other consists of radioactive- and rare-earth-bearing minerals in carbonatite bodies in Triassic sedimentary rocks (Warner and others, 1986; Reifenstuhl and others, 1998). The carbonatite forms two bodies of iron-rich regolith that are conformable to N60E-trending wall rocks (Warner and others, 1986).
Moxham (1954) reported aeschynite, zircon, and niobium-bearing minerals in the area of Idaho, Miller, and Harter gulches. In 1956 and again in 1984, the U.S. Bureau of Mines explored the area around Idaho Gulch with nine trenches and nine diamond drill holes, searching for a lode source of the gold and cassiterite in the Tofty tin belt. This work led to the discovery of the carbonatite.
Warner and others (1986) reported that concentrates from churn drill cuttings in Miller and Idaho gulches contained 0.2 to 7.0 percent niobium-bearing minerals and up to 0.6 percent CeO2 and 1.8 percent Nb2O5. The regolith contains hematite, limonite, magnetite, apatite, rutile, zircon, and trace amounts of monazite (high cerium-lanthanum and low yytrium-thorium variety), columbite, aeschynite, xenotime, and brewsterite(?) (Warner and others, 1986).
Warner and others (1986) identified two carbonatite bodies, 3 to 80 feet thick, dipping about 45NW. These bodies are characteristically dark red, sponge-like masses of siliceous hematite containing up to 40 percent finely crystalline apatite. This amount of apatite is distinct from other marble units in this area, and, along with trace element geochemistry, support the carbonatite model. Warner and others (1986), however, point out that the geologic setting differs significantly from that of classic carbonatite occurrences. A random U.S. Geological Survey rock sample from a small gossan in the Idaho Gulch area contained 1.34 ounces of silver per ton, with no gold detected (Eberlein and others, 1977). Analytical results from a rock sample of weathered carbonatite in Miller Gulch include more than 15 percent Fe and 10,000 parts per million (ppm) phosphorus; 2,100 ppm strontium, 51 ppm scandium, 740 ppm lanthanum, 433 ppm vanadium, and 40 ppm uranium (Liss and others, 1998). Several carbonatite samples from the same body in Harter Gulch had maximum anomalous analytical results of 952 ppm arsenic, 403 ppm chromium, 120 ppm lanthanum, 4,120 ppm manganese, more than 10,000 ppm phosphorus, 2,900 ppm strontium, 17 ppm scandium, and 117 ppm vanadium (Liss and others, 1998). Reanalysis of magnetite-bearing carbonate (carbonatite) from U.S. Bureau of Mines drill core by the Alaska Division of Geological and Geophysical Surveys (ADGGS) indicated the unusual nature of the carbonatite, with maximum values of 542 ppm niobium, 7,245 ppm strontium, 39 ppm yttrium, 281 ppm zirconium, 26 ppm gallium, 3416 ppm nickel, and 638 ppm zinc (ADGGS unpublished data). A whole-rock sample of altered phyllite adjacent to carbonatite in the Idaho Gulch area yielded an Ar40/Ar39 age of 193 +/- 15 Ma, with a reset age of about 55 Ma (Reifenstuhl and others, 1998). The reset age may represent the age of emplacement of the carbonatite. | |
| Geologic map unit | (-150.939388064658, 65.0922026504349) |
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| Mineral deposit model | Carbonatite deposits (Cox and Singer, 1986; model 10). |
| Mineral deposit model number | 10 |
| Age of mineralization | A whole-rock sample of altered phyllite adjacent to carbonatite in the Idaho Gulch area yielded an Ar40/Ar39 age of 193 +/- 15 Ma, with a reset age of about 55 Ma (Reifenstuhl and others, 1998). The reset age may represent the age of emplacement of the carbonatite. |
| Alteration of deposit |
Alteration consists of chemical weathering of carbonatite. This alteration extends 200 to 250 feet down dip, where unweathered magnetite-pyrite- apatite-zircon-bearing carbonatite was encountered in drill core (Warner and others, 1986). |
| Workings or exploration | Placer mining and prospecting has occurred throughout the Tofty area since around 1900. Later work focused on the rare-earth-bearing lode deposits in the area. In 1956, the U.S. Bureau of Mines trenched in upper Idaho Gulch and identified two bodies of radioactive, ferruginous regolith, subsequently interpreted as carbonatite. Reanalysis and work on the area continued in 1984, as part of a program to determine the availability of critical and strategic minerals in Alaska. Diamond drilling outlined the bodies (Warner and others, 1986). Mapping by Reifenstuhl and others (1998) extended the known outcrop area of the carbonatite. Geochemical sample results accompanying the geologic mapping are presented in Liss and others (1998). North Star Exploration, Inc., drilled 2,723 feet in 8 diamond drill holes at the Tofty Ridge prospect (TN099) during 2000 (North Star Exploration, Inc., 2001 [Alaska exploration opportunities]). Portions of the core were geochemically anomalous for niobium, rare-earth elements, and yttrium. |
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| Indication of production | None |
| Reserve estimates | Estimated reserves of approximately 100,000 pounds of niobium (as Nb2O5) are present in placer tailings in upper Idaho Gulch (Southworth, 1984). More recent work shows that the carbonatite body is much more extensive than previously thought; it thus is likely that the niobium resources of the area were underestimated by several orders of magnitude (Reifenstuhl and others, 1998). |
| MRDS Number | A015181 |
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ReferencesNorth Star Exploration, Inc., 2001, Alaska exploration opportunities: unpublished company brochure, 4 p.
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| Reporters | G.E. Graham (ADGGS), D.J. Szumigala (ADGGS) |
| Last report date | 7/21/2003 |