Tofty Gulch Mine, Hot Springs Mining District, Yukon-Koyukuk Census Area, Alaska, USAi

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

Location: The site of the Tofty Gulch placer mine is at the mine symbol on the gulch, about 0.2 mile upstream from its junction with Sullivan Creek, at the western edge of section 18, T. 3 N., R. 16 W., of the Fairbanks Meridian. The site corresponds to location 29 of Cobb (1972), and roughly to the site for Tofty Gulch, U.S. Bureau of Land Management MAS number 0020480004.
Geology: The Tofty 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 Tofty Gulch placer deposit 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 bedrock consists of phyllite and minor interbedded graywacke that strikes N85E and dips steeply to the north (Wayland, 1961). The rocks are sheared and contain barren, milky quartz-calcite-pyrite veinlets. According to Mertie (1934) and Ellsworth (1910), gold was discovered in Tofty Gulch in the winter of 1906-07. There is no well-defined pay zone. Some of the gold is in a bench placer approximately 1,000 feet from Sullivan Creek (Eakin, 1913). The gravels, which are about 4 to 6 feet thick (Eakin, 1913), consist chiefly of angular graywacke, phyllite, and quartz. Other cobbles include metadiorite, quartzite, and altered phyllite containing quartz-tourmaline veins (Wayland, 1961). Cassiterite particles in the gold-bearing gravels range from well-rounded (Hess, 1912) to subangular, and are up 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). In addition to gold and cassiterite, Wayland (1961) reported chromite. Moxham (1954) reported elevated radioactivity measurements, and suggested that columbite, aeschynite, monazite, and zircon may also be present. Newberry and Clautice (1997) analyzed a gold grain by electron microprobe that contained a tiny inclusion of native nickel. Five other gold grains had cores with silver contents varying from 10 to 17 percent, mercury contents of 1 percent, and strongly silver- and mercury-depleted rims. Gold was discovered in Tofty Gulch in the winter of 1907, and presumably mined by drifting or shaft-sinking. There was open-cut mining in 1909-12, 1917, 1929 and 1941 (Cobb, 1977). L. McGee mined the property in 1951 (Williams, 1951). The total production reported by Thomas (1957) was 8,855 ounces of gold, 1,376 ounces of silver, and 19,600 pounds of cassiterite concentrate. Drilling took place west of the gulch in 1941 (Wayland, 1961). Man Mining Company had a washing plant, three bulldozers, two draglines, and five men working a placer claim in 1975 (Carnes, 1976). In 1992, GHD Resources reacquired the Tofty (Gulch?) property and began to prepare the ground for the 1993 season. In September, the company sluiced 150,000 cubic yards of pay gravels, which yielded 130 ounces of gold and 1,500 pounds of cassiterite (Swainbank and others, 1993). In 1993, the company sluiced 89,000 cubic yards of pay, producing gold, silver and byproduct tin (Bundtzen and others, 1994). In January, 1994, the property was sold back to Cassiterite Placers, Inc., who mined that year and again in 1998 (Swainbank and others, 1995; Szumigala and Swainbank, 1999).
Workings: Mining in Tofty Gulch appears to have been somewhat sporadic. Gold was discovered in the winter of 1907, presumably by drifting. There was open-cut mining in 1909-12, 1917, 1929, and 1941 (Cobb, 1977). L. McGee mined the property in 1951 (Williams, 1951). Man Mining Company had a washing plant, three bulldozers, two draglines, and five men working a placer claim in 1975 (Carnes, 1976). In 1992, GHD Resources reacquired the Tofty (Gulch?) property and began to prepare the ground for the 1993 season. In September, the company sluiced 150,000 cubic yards of pay dirt, which yielded 130 ounces of gold and 1,500 pounds of cassiterite (Swainbank and others, 1993). In 1993, the company sluiced 89,000 cubic yards of pay, producing gold, silver and byproduct tin (Bundtzen and others, 1994). In January, 1994, the property was sold back to Cassiterite Placers, Inc., who mined that year and again in 1998 (Swainbank and others, 1995; Szumigala and Swainbank, 1999).
Age: Quaternary.
Production: Thomas (1957) reported that 8,855 ounces of gold, 1,376 ounces of silver, and 19,600 pounds of cassiterite concentrate (at 60 percent tin) were produced from Tofty Gulch through 1956.

Commodities (Major) - Au; (Minor) - Ag, Cr, Sn
Development Status: Yes; small
Deposit Model: Placer Au(-Sn) (Cox and Singer, 1986; model 39a).

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Link to USGS - Alaska:	TN088

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