Gravel Pit Prospect No. 1

Occurrence, Inactive

Commodities and mineralogy

Main commodities REE; Sn
Other commodities Nb; Ti; W; Zr
Ore minerals cassiterite; ilmenite; monazite; wolframite; xenotime; zircon

Geographic location

Quadrangle map, 1:250,000-scale BT
Quadrangle map, 1:63,360-scale A-1
Latitude 66.1212
Longitude -150.132
Nearby scientific data Find additional scientific data near this location
Location and accuracy The site is located adjacent to the Trans-Alaska Pipeline about 0.5 miles southeast of its crossing of No Name Creek and east from the Dalton Highway. Site for this record is along the north side of the gravel pit and accurate to within 500 feet. The occurrence is accessible from the Dalton Highway with permission of the pipeline security personnel. Two mineralized ancient channel exposures were mapped and sampled in the pit wall (Barker, 1991a,b). The site is located in the W1/2 SE1/4 Section 17, T. 14 N., R. 12 W., of the Fairbanks Meridian.

Geologic setting

Geologic description

At the site, two well-developed remnant channels are exposed in the wall and floor of a gravel pit perched on a prominent basalt bluff facing north toward the present No Name Creek. Channels are oriented to the southwest as is the modern No Name channel. These perched channels are ancestral No Name Creek, which is now located about 2000 feet north and about 150 feet lower. Samples contain trace to 0.12 pounds of tin per cubic yard (Barker, 1991a, b). The channels are filled with coarse gravel including cobbles of mostly granitic rock of the Fort Hamlin Hills pluton, schist, and quartz. Exposures were sampled where the channels are incised into basalt bedrock. 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, 1970, 1973; 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 in the nearby granite pluton.
Geologic map unit (, )
Mineral deposit model Placer tin (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 locally since the late Tertiary. An age-date from a nearby basalt flow indicates an age of 30.59 m.y. +/- 0.92 (Albanese, 1987). Multiple stages of fluvial activity and downcutting of the No Name Creek drainage has obviously occurred near this site.
Alteration of deposit Thermal alteration from the batholith is widespread, and silicification extends well into the Paleozoic host rocks. Source of the placer tin and rare earth element minerals appears to be unexposed greisen zones in the granite in the Fort Hamlin Hills pluton. Greisen samples will generally contain 100 ppm to as much as 2500 ppm tin and elevated levels of rare earth elements. Cassiterite has also been found associated with silicification and quartz veins at a few locations elsewhere.

Production and reserves

Workings or exploration
The Gravel Pit Prospect No. 1 occurrence has been mapped and sampled by the U.S. Bureau of Mines as part of the Critical and Strategic Minerals Program in the 1970s-1980s (Barker, 1983, 1991a,b, 2012). The occurrence serves as evidence of the evolving development of the accumulated placer minerals in the present active channels of No Name Creek.
All samples were processed by gravity methods for the analysis of the heavy mineral content and the channel samples contain trace to 0.12 pounds of tin per cubic yard (Barker, 1991a). Values for contained REE were not estimated.
Reworking the fluvial sediment from these elevated channels due to downcutting of the modern stream generally will generally increase the grade of the heavy mineral content. By comparison, gravel from a small bar in the modern channel due north of Gravel Pit Prospect #1 contained 0.17 pounds tin per cubic yard. Discontinuous exposures of the ancient channels in Gravel Pit Prospect No. 1 would be expected to be present at similar elevations to the northeast and southwest.
Indication of production None

Additional comments

The site is gated due to the close proximity to the Trans Alaska Pipeline.

References