Highway

Prospect, Inactive

Commodities and mineralogy

Main commodities Au; Cr; Cu
Other commodities Pd; Pt
Ore minerals chromite; gold; native copper

Geographic location

Quadrangle map, 1:250,000-scale AN
Quadrangle map, 1:63,360-scale B-6
Latitude 61.47
Longitude -149.2
Nearby scientific data Find additional scientific data near this location
Location and accuracy Prospect occurs where rocks outcrop on the south side of the Old Glenn highway, north of West Twin Peak. Location is accurate within 500 ft. This is the 'Highway Area' of Rose (1966), locality 42 of Cobb (1972), and locality 32 of MacKevett and Holloway (1977).

Geologic setting

Geologic description

Bjorklund and Wright (1948) and Rose (1966) report chromite deposits in Lower to Middle Jurassic dunite found near base of the Eklutna ultramafic body. Near the highway the chromite bands are oriented N 17 E, 85 NW. The chromite-bearing zone is 8 ft wide and contains discontinuous stringers and lenses of chromite from 1/4 to 1 inches wide. The bands are composed of euhedral chromite grains 1-2 mm in diameter in a matrix of olivine and minor clinopyroxene. The chromite bands are displaced as much as 5 ft by minor faults (Bjorklund and Wright, 1948). The dominant rock types are peridotite-dunite and hornblendite-pyroxenite (Clark and Greenwood, 1972). A banded chromite sample (Rose, 1966) was found to contain free gold and native copper.
The weighted average of samples collected in 10 trenches at the prospect had a grade of 5.7 percent chromite. The grade of the rock generally decreases going uphill. Rose (1966) reports that an 8 ft wide road-cut sample averaged 11.5 percent chromite; one 8 inch wide chromite band contained 25.8 percent chromite. Of 16 samples collected by Clark and Greenwood (1972), 12 contained an average value of 0.042 ppm Pt and 0.060 ppm Pd.
Geologic map unit (-149.202213069975, 61.4694608745396)
Mineral deposit model Podiform chromite or Alaskan PGE (Cox and Singer, 1986; models 8a or 9).
Mineral deposit model number 8a or 9
Age of mineralization Early to Middle Jurassic.
Alteration of deposit Slight serpentinization of olivine.

Production and reserves

Workings or exploration Exploration is limited to 10 trenches and 1 diamond-drill hole completed by U.S. Bureau of Mines in 1942 (Bjorklund and Wright, 1948). The weighted, average grade of samples collected in 10 trenches at the prospect was 5.7 percent chromite. The grade generally decreased uphill. Rose (1966) reported that an 8-foot-wide, road-cut sample averaged 11.5 percent chromite; one 8-inch chromite band contained 25.8 percent chromite. Of 16 samples collected by Clark and Greenwood (1972), twelve contained an average value of 0.042 ppm Pt and 0.060 ppm Pd.
Indication of production None

Additional comments

Deposit too small and too low grade to be mineable under conditions prevailing in 1940s (Bjorklund and Wright, 1948).

References

MRDS Number A011643

References

Reporters D.P. Bickerstaff (USGS contractor); S.W. Huss (USGS)
Last report date 7/30/1998