Tofty Ridge Prospect, Hot Springs Mining District, Yukon-Koyukuk Census Area, Alaska, USAi

Regional Level Types
Tofty Ridge Prospect	Prospect
Hot Springs Mining District	Mining District
Yukon-Koyukuk Census Area	Census Area
Alaska	State
USA	Country

Estimated reserves of approximately 100,000 pounds of niobium (as Nb2O5) are present in placer tailings in upper Idaho Gulch (TN083) (Southworth, 1984).
Location: The Tofty Ridge lode prospect is on a southeast-facing ridge between Cache Creek and Gold Basin Creek, 0.15 mile east of the center of section 4, T. 3 N., R. 16 W., of the Fairbanks Meridian. The site is the center of an area of recent lode exploration bounded on the west by Cache Creek and on the northeast and east by Irish Gulch and Killarney Creek. The location is accurate within 100 feet. This site corresponds approximately with site 'Tof' (Tof claims), U.S. Bureau of Land Management MAS number 0020480117. The ARDF site for the Tofty Ridge prospect, however, is different from that of Tof in the BLM record, and agrees instead with a location based on more recent work by North Star Exploration, Inc.
Geology: The Tofty Ridge prospect is near a group of cassiterite- and gold-bearing placer deposits known as the Tofty tin belt, a 12-mile-long area that trends east-northeast, between Roughtop Mountain to the north and Hot Springs Dome to the south (Thomas, 1957). Roughtop Mountain and Hot Springs Dome respectively are underlain by Cretaceous (K-Ar age date of 92 +/- 5 Ma) and Tertiary (K-Ar age date of 62 +/- 3 Ma) granitic plutons (Chapman and others, 1982). The plutons intrude and contact metamorphose Mesozoic marine sedimentary strata (Reifenstuhl and others, 1998). A carbonatite sill(?) is in the Triassic section of these strata and there are exposures of serpentinized, Cretaceous(?) mafic and ultramafic rock, mainly on Serpentine Ridge. The carbonatite sill has a strike length of at least 10 km (Reifenstuhl and others, 1998). The Tofty tin belt lies along a regional-scale, east-northeast-striking 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 (sills?) are in the Triassic sedimentary rocks at the head of Idaho Gulch, and these bodies occur 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. The Tofty Ridge prospect is at the intersection of an east-northeast-striking thrust fault that dips north, and a high-angle fault that dextrally displaces the thrust fault by about 1/4 mile. The local exposed bedrock is quartz-veined quartzite along the north side of Tofty Ridge. Loess, generally 4 feet thick, and colluvial gravel, usually 2 to 4 feet thick, blanket most of the area. Black shale, gray shale, and quartzite are the most common rocks in trenches and drill holes (North Star Exploration, Inc., 2000 [Drilling and trenching program, Tofty Ridge, Manley Hot Springs village block, Alaska]). Other rock types encountered in drilling at this prospect include sericitic schist, carbonatite, quartzite breccia, pyroxenite, and metacarbonate. The carbonatite is dominantly ferroan dolomite or calcite, along with tremolite, apatite, magnetite, and traces to varying amounts of pyrite, hematite, zircon, limonite, barite, arsenopyrite, aeschynite, euxenite, and chalcopyrite. The Tofty Ridge area was explored and over 120 claims were staked in 1978 by Resource Associates of Alaska, Inc. (RAA) for Houston Oil & Minerals (HOM) (Jim Adler, oral commun., 2003). The original exploration was to identify and evaluate the source of cassiterite ('tin') in the local placer deposits. Field work in 1979 and subsequent petrologic study of rock samples found minerals, including apatite, phlogopite, and melanite, indicative of carbonatite. In 1979, 3 geophysical traverses (one on the ridge between Harter Gulch and Sullivan Creek, one on the ridge between Dalton Gulch and Cache Creek, and one on the ridge between Cache Creek and Gold Basin Creek) consisted of 7,500 feet of ground electromagnetic survey, 7,500 feet of induced polarization, and 7,500 feet of resistivity survey (Alaska Kardex files). A geochemical survey in 1979 consisted of 135 samples collected on 100-foot spacing along the geophysical lines. A detailed, 474-line-mile, fixed-wing aeromagnetic survey was conducted by Jerry Hook of Manley Hot Springs for HOM. Large, very high positive magnetic anomalies outlined areas of known carbonatite and were subsequently used to locate buried carbonatite. In 1980, 3 angle core holes, totaling about 1,200 feet, were drilled along a north-south line using a hydrowink drill (Jim Adler, oral commun., 2003). The drilling found at least one large carbonatite dike. Core recovery was very good and consisted of limonite-stained, magnetite-rich, coarsely crystalline carbonatite containing about 10 percent magnetite and up to 10 percent apatite, as well as rare-earth elements (REE), but no significant tin, gold, uranium, or base metals. In 1981, ten backhoe trenches, up to twenty-five feet deep and 2,267 feet in total length, were excavated by Jack Neubauer for HOM. All trenches were backfilled after being sampled and mapped. Material in the trenches consisted of limonitic regolith (weathered carbonatite), which contained several times more REE than the unweathered iron-rich carbonatite bedrock. The near-surface, oxidized, lateritic residual material is enriched in REE, but still contained only about one-third of the values required for a commercial open-pit mine (Jim Adler, oral commun., 2003). In one trench, the north side of the carbonatite dike is in fault contact with serpentinite. The best sample from trenching of the carbonatite dike system produced 30 feet averaging more than one percent REE (as cesium and lanthium) and 0.15 percent niobium. Locally, niobium and tantalum are present at geochemically anomalous levels. In 1982, the HOM Tofty project data and claims were returned to RAA under prior agreement. North Star Exploration, Inc., cut 9 trenches through loess, gravel, colluvium, and bedrock in the Tofty area in 2000 (North Star Exploration, Inc., 2001 [Alaska exploration opportunities]; Szumigala and others, 2001). North Star also diamond drilled 8 holes totaling 2,723 feet. One hole cut 159 feet of magnetite-rich, niobium-bearing carbonatite. Portions of the core were geochemically anomalous for niobium, REE, and yttrium. The orientation of the carbonatite body could not be determined by the drilling, but the geophysical data best-fit a model of an overturned, V-shaped body dipping to the north. Plans for 2001 work included drilling an additional line of holes from Gold Basin Creek toward Irish Gulch, but this work was not implemented. An Alaska Division of Geological and Geophysical Surveys (ADGGS) sample (97RN235B) of weathered carbonatite in Miller Gulch contained more than15 percent iron and 10,000 parts per million (ppm) phosphorous; 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 contained maximums of 952 ppm arsenic, 12 ppm bismuth, 403 ppm chromium, 595 ppm lanthanum, 4,120 ppm manganese, 982 ppm nickel, 2,900 ppm strontium, 18 ppm scandium, 117 ppm vanadium, and more than 10,000 ppm phosphorous (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 niobium, 7,245 ppm strontium, 39 ppm yttrium, 281 ppm zirconium, 26 ppm gallium, 3,416 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 Ar/Ar age of 193 +/- 15 Ma, with a reset age of about 55 million years (Reifenstuhl and others, 1998). The reset age may represent the age of emplacement of the carbonatite.
Workings: The Tofty Ridge area was explored and over 120 claims were staked in 1978 by Resource Associates of Alaska, Inc. (RAA) for Houston Oil & Minerals (HOM) (Jim Adler, oral commun., 2003). The original exploration was to identify and evaluate the source of cassiterite ('tin') in the local placer deposits. Field work in 1979 and subsequent petrologic study of rock samples found minerals, including apatite, phlogopite, and melanite, indicative of carbonatite. In 1979, 3 geophysical traverses (one on the ridge between Harter Gulch and Sullivan Creek, one on the ridge between Dalton Gulch and Cache Creek, and one on the ridge between Cache Creek and Gold Basin Creek) consisted of 7,500 feet of ground electromagnetic survey, 7,500 feet of induced polarization, and 7,500 feet of resistivity survey (Alaska Kardex files). A geochemical survey in 1979 consisted of 135 samples collected on 100-foot spacing along the geophysical lines. A detailed, 474-line-mile, fixed-wing aeromagnetic survey was conducted by Jerry Hook of Manley Hot Springs for HOM. Large, very high positive magnetic anomalies outlined areas of known carbonatite and were subsequently used to locate buried carbonatite. In 1980, 3 angle core holes, totaling about 1,200 feet, were drilled along a north-south line using a hydrowink drill (Jim Adler, oral commun., 2003). The drilling found at least one large carbonatite dike. Core recovery was very good and consisted of limonite-stained, magnetite-rich, coarsely crystalline carbonatite containing about 10 percent magnetite and up to 10 percent apatite, as well as rare-earth elements (REE), but no significant tin, gold, uranium, or base metals. In 1981, ten backhoe trenches, up to twenty-five feet deep and 2,267 feet in total length, were excavated by Jack Neubauer for HOM. All trenches were backfilled after being sampled and mapped. Material in the trenches consisted of limonitic regolith (weathered carbonatite), which contained several times more REE than the unweathered iron-rich carbonatite bedrock. The near-surface, oxidized, lateritic residual material is enriched in REE, but still contained only about one-third of the values required for a commercial open-pit mine (Jim Adler, oral commun., 2003). In one trench, the north side of the carbonatite dike is in fault contact with serpentinite. The best sample from trenching of the carbonatite dike system produced 30 feet averaging more than one percent REE (as cesium and lanthium) and 0.15 percent niobium. Locally, niobium and tantalum are present at geochemically anomalous levels. In 1982, the HOM Tofty project data and claims were returned to RAA under prior agreement. North Star Exploration, Inc., cut 9 trenches through loess, gravel, colluvium, and bedrock in the Tofty area in 2000 (North Star Exploration, Inc., 2001 [Alaska exploration opportunities]; Szumigala and others, 2001). North Star also diamond drilled 8 holes totaling 2,723 feet. One hole cut 159 feet of magnetite-rich, niobium-bearing carbonatite. Portions of the core were geochemically anomalous for niobium, REE, and yttrium. The orientation of the carbonatite body could not be determined by the drilling, but the geophysical data best-fit a model of an overturned, V-shaped body dipping to the north. Plans for 2001 work included drilling an additional line of holes from Gold Basin Creek toward Irish Gulch, but this work was not implemented. An Alaska Division of Geological and Geophysical Surveys (ADGGS) sample (97RN235B) of weathered carbonatite in Miller Gulch contained more than15 percent iron and 10,000 parts per million (ppm) phosphorous; 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 contained maximums of 952 ppm arsenic, 12 ppm bismuth, 403 ppm chromium, 595 ppm lanthanum, 4,120 ppm manganese, 982 ppm nickel, 2,900 ppm strontium, 18 ppm scandium, 117 ppm vanadium, and more than 10,000 ppm phosphorous (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 niobium, 7,245 ppm strontium, 39 ppm yttrium, 281 ppm zirconium, 26 ppm gallium, 3,416 ppm nickel, and 638 ppm zinc (ADGGS unpublished data).
Age: A whole-rock sample of altered phyllite adjacent to carbonatite in the Idaho Gulch area yielded an Ar/Ar age of 193 +/- 15 Ma, with a reset age of about 55 million years (Reifenstuhl and others, 1998). The reset age may represent the age of emplacement of the carbonatite.

Commodities (Major) - Ce, Nb, Y; (Minor) - Ag, REE, U
Development Status: None
Deposit Model: Carbonatite deposits (Cox and Singer, 1986; model 10).

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Link to USGS - Alaska:	TN099

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References