Idaho Gulch

Mine, Inactive

Commodities and mineralogy

Main commodities Au
Other commodities Ag; REE; Sn
Ore minerals cassiterite; columbite; gold; monazite; unknown silver mineral

Geographic location

Quadrangle map, 1:250,000-scale TN
Quadrangle map, 1:63,360-scale A-2
Latitude 65.07848
Longitude -150.91881
Nearby scientific data Find additional scientific data near this location
Location and accuracy The site of the Idaho Gulch placer mine is in the gulch just downstream from a primitive road crossing, near the north boundary of section 24, T. 3 N., R. 17 W., of the Fairbanks Meridian. The site corresponds to location 28 of Cobb (1972), and roughly to the site for Idaho Gulch, U.S. Bureau of Land Management MAS number 0020480017.

Geologic setting

Geologic description

The Idaho Gulch mine is one of 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, which also are cut by diverse faults, including regional-scale, east-northeast-striking, thrust faults (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 gold- and tin-bearing creeks flow normal to the trend of the tin belt. They head in the plutonic, metamorphic, and mafic/ultramafic rocks of Roughtop Mountain and Serpentine Ridge, which probably are the source(s) of some of the metalliferous minerals in the placer deposits. Concentrations of gold diminish toward the south, probably due to dispersion (Thomas, 1957).
The Idaho Gulch deposit lies on or just south of the trace of an east-northeast-striking thrust fault. The bedrock surface of Idaho Gulch is marked by bold scarps that do not have any surface expression (Eakin, 1915). Well-rounded cobbles of brecciated quartz with tourmaline and cassiterite are common in the boulder piles (Wayland, 1961). Cassiterite placer deposits have been found as far as 1,500 feet upstream from the access road, and as much as 2,500 feet downstream from it. The cassiterite tends to occur as well-rounded to subangular particles that range in size from microscopic to several inches across. The larger sizes often are accompanied by vein quartz and tourmaline, as well as by fragments of sedimentary country rock. The smaller sizes generally are free of impurities (Thomas,1957).
The pay streaks in Idaho Gulch are discontinuous. Extensive drilling in 1912 led to the discovery of gold and the start of drift mining in 1912-1913 (Eakin, 1915). Since then, some drilling was done by the Alaska Gold Dredging (1929) and Cleary Hill Mines (1940-1941) companies (Wayland, 1961). The results of the latter work led to drift mining of a small pay streak. The tailings from this mining contained a higher proportion of light-colored gravels and boulders. McGee and Strandberg Mines, Inc., were active on Idaho Gulch in 1967 (Heiner and others, 1968).
The U.S. Bureau of Mines studied the placer tin deposits around Tofty in 1954-1956 (Thomas, 1957). Their channel samples of tailings in Idaho Gulch yielded an average of 1.00 pound of 'tin' (cassiterite) and 0.02 ounce of gold per cubic yard. They also reported sporadic tin in drill holes on both Idaho and Tofty (TN088) gulches . By 1956, production from Idaho Gulch was 61 ounces of gold and 300 pounds of cassiterite (Thomas, 1957).
In Idaho Gulch and Miller Gulch (TN085), pan concentrates contained 0.2-7.0 percent niobium-bearing minerals, and concentrates from churn-drill cuttings contained up to 0.6 percent CeO2 and 1.8 percent Nb2O5 (Warner and others, 1986). These minerals are often associated with carbonatite, and may be the result of weathering of the carbonatite sill(?) in the Triassic country rocks. Moxham (1954) reported that the eU content of the concentrates ranged from 0.015 percent to 0.035 percent, and that one sample contained 2.3 percent eU. Cobb (1977) speculated that these values implied the presence of aeschynite, columbite, monazite, and zircon.
Geologic map unit (-150.921285924548, 65.0779829795829)
Mineral deposit model Placer Au(-Sn) (Cox and Singer, 1986; model 39a).
Mineral deposit model number 39a
Age of mineralization Quaternary.

Production and reserves

Workings or exploration The first work in Idaho Gulch was reported in 1911 (Wayland, 1961). Drift mining appears to have been predominant. Drilling programs were conducted by the Alaska Gold Dredging Company in 1929, Cleary Hill Mining Company in 1940-41, and the U.S. Bureau of Mines in 1954-56 (Wayland, 1961; Thomas, 1957). McGee and Strandberg Mines, Inc., were active on Idaho Gulch in 1967 (Heiner and others, 1968).
Indication of production Yes; small
Production notes Thomas (1957) reported that 61 ounces of gold and 300 pounds of cassiterite concentrate (at 60% tin) were produced from Idaho Gulch through 1956.

References

MRDS Number A015200

References

Heiner, L.E., Wolff, E.N., and Lu, F.C.J., 1968, Mining regions and mineral commodities, in Heiner, L.E., and Wolff, E.N. eds., Final Report - Mineral Resources of Northern Alaska: Mineral Industry Research Laboratory, University of Alaska Report No. 16, p. 3-137.
Reporters G.E. Graham (ADGGS)
Last report date 1/15/2001