|Quadrangle map, 1:250,000-scale||TN|
|Quadrangle map, 1:63,360-scale||A-2|
|Nearby scientific data||Find additional scientific data near this location|
|Location and accuracy||This record represents a generalized area of lode occurrences of cassiterite, gold, and silver in the Tofty tin belt. The site is at the old town of Tofty, the midpoint of this 12-mile-long area, on the north boundary of section 18, T. 3 N., R. 16 W., of the Fairbanks Meridian.|
The Tofty tin belt is a group of cassiterite ('tin')- and gold-bearing placer deposits in 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 country rocks exposed near the placer workings in this area are sheared and contorted phyllite, schist, quartzite, graywacke, and slate that are cut by numerous pyrite-bearing quartz veins and stringers, locally containing small amounts of cassiterite. The larger pieces of placer cassiterite (up to several inches in diameter) often are attached to quartz and tourmaline, as well as to fragments of sedimentary country rock (Thomas, 1957). Based on the chemical composition of typical tin-bearing granite, Waters (1934) proposed that the Hot Springs pluton is more likely to be associated with cassiterite mineralization than the Roughtop pluton.
The gold- and tin-bearing creeks flow southeasterly, 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 placers. Concentrations of placer gold diminish to the south, probably due to dispersion. Veins containing small amounts of gold and silver are associated with the granite of Hot Springs Dome (TN103).Eakin (1913) reported that the miners in Sullivan Creek (TN093) felt that the placer gold was more abundant in areas of more heavily concentrated quartz veins, and one miner reported tiny stringers of gold along cleavage planes in a fragment of quartzite. Both the tin and gold were believed to have the same source (quartz veins), and cassiterite, along with tourmaline and fluorite, cemented breccias of vein quartz.
|Geologic map unit||(-150.88174794553, 65.0925042352632)|
|Mineral deposit model||Sn-polymetallic veins? (Cox and Singer, 1986; model 20b?).|
|Mineral deposit model number||20b?|
|Indication of production||None|
|Production notes||As of 1956, placers in the Tofty tin belt had produced 127,528 ounces of gold, 14,356 ounces of silver, and 470,157 pounds of cassiterite (280,600 pounds of tin at approximately 60 percent tin) (Thomas, 1957).|
Chapman, R.M., Yeend, W.E., Brosge, W.P., and Reiser, H.N., 1982, Reconnaissance geologic map of the Tanana quadrangle: U.S. Geological Survey Open-File Report 82-734, 20 p., scale 1:250,000.
Eakin, H.M., 1913, A geologic reconnaissance of a part of the Rampart quadrangle, Alaska: U.S. Geological Survey Bulletin 535, 38 p.
Reifenstuhl, R.R., Dover, J.H., Newberry, R.J., Clautice, K.H., Pinney, D.S., Liss, S.A., Blodgett, R.B., and Weber, F.R., 1998, Geologic map of the Tanana A-1 and A-2 quadrangles, central Alaska: Alaska Division of Geological & Geophysical Surveys Public Data File 98-37A v. 1.1, 19 p., 1 sheet, scale 1:63,360.
Thomas, B.I., 1957, Tin-bearing placer deposits near Tofty, Hot Springs district, central Alaska: U.S. Bureau of Mines Report of Investigations 5373, 56 p.
|Reporters||J.E. Athey (ADGGS)|
|Last report date||3/16/2001|