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).