Geologic descriptionAt the site, a bulk channel sample from a well-developed remnant channel exposed in the wall of a gravel pit contained 0.24 pounds of tin per cubic yard (Barker, 1991a). The channel is incised into granite bedrock of the Fort Hamlin Hills pluton. The channel is composed granitic and metasedimentary gravel and semi-rounded granitic boulders, and likely represents an ancient channel of the Fort Hamlin Hills Creek. The Fort Hamlin Hills pluton is considered part of the broad northeast-trending peraluminous Ruby Batholith of central Alaska (Chapman and others, 1982; Patton and Miller, 1973; Barker, 2012; Barker and Foley, 1986; Herreid, 1969). Generally the granitic rocks are coarse-grained equigranular to porphyritic orthoclase-biotite-quartz monzonite varying to granite with subordinate phases of aplite, biotite aplite, tourmaline aplite, and fine-grained quartz monzonite. Tourmaline pegmatite phases have been recognized locally. The granitic rocks cut Paleozoic schist, phyllite, and quartzite within a half-mile downhill to the west. |
| Geologic map unit |
(, ) |
| Mineral deposit model |
Placer Sn (Cox and Singer, 1986; model 39e). |
| Mineral deposit model number |
39e |
| Age of mineralization |
Quaternary; the occurrence represents ongoing erosional and mineral concentration processes that have concentrated valuable heavy minerals, a process that has apparently continued since the late Tertiary.|Multiple stages of fluvial activity and downcutting of the Ray River drainage and its tributaries has resulted in these streams further incised into local bedrock. The Prospect Pit #2 occurrence represents an ancient channel abandoned as the creek cut new channels to its present level several hundred feet lower than the site (Barker, 1991a,b). Sampling of the present channel of Fort Hamlin Hills Creek was not possible due to infilling of frozen silt and vegetation.|These remnant terraces and channels, as high as 300ft (100 m) above present drainages along the west of the Fort Hamlin Hills pluton, are evidence of ancient (late Tertiary(?)) alluvial gravel deposition and are now perched above present-day channels of the No Name Creek and the Ray River valleys (Barker, 1991a, b, 2012). |
| Alteration of deposit |
Thermal alteration from the batholith is widespread and silicification extends well into the Paleozoic host rocks. The source of the placer tin and rare earth element minerals appears to be unexposed greisen zones in the granite in the Fort Hamlin Hills Creek valley. Greisen samples from the Ruby Batholith region will generally contain 100 ppm to as much as 2500 ppm tin and elevated rare earth elements. Cassiterite has also been found associated with quartz veins at a few locations elsewhere. |
