Alluvial cassiterite concentrations are widespread in river gravel and high level terraces within the Ray River watershed. The area lies in unglaciated terrain of Alaska's northern interior. Cassiterite originates from several calc-alkaline plutons of the ruby batholith. Extensional stresses resulted in Graben-like Tertiary basins that were flooded by a 200-ft-thick section of mid- to late-Tertiary basalt flows. Basalt blocked local drainages, and 50 to 100 ft of terrance gravel was eventually deposited on top of the flows. Further downwarping and eventual fluvial downcutting of the flows resulted in cycles of accelerated sediment transport, deposition, and reconcentration. Repeated erosional cycles concentrated heavy minerals and resulted in development of tin placers. Preliminary resource estimates of contained tin in recent alluvium range from 62 to as much as 172 million lb sn in 300 million yd3. The grade of about 90 pct of the gravels is estimated to range between 0.2 and 0.5 Lb sn/yd3. Associated gold and rare earth elements (ree) in monazite and xenotime may be recoverable. Most of the tin is considered a subeconomic inferred resource, though at least some of the gravels contain 2 to 6 lb sn/yd3. Estimates are projected on the basis of surface sampling and several auger drill holes and are provided for the purpose of land-use management planning.