Mitchell and McPherson

Prospect, Inactive

Commodities and mineralogy

Main commodities Ag; Au
Other commodities Pb
Ore minerals galena; pyrite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale JU
Quadrangle map, 1:63,360-scale C-3
Latitude 58.5468
Longitude -134.7254
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Mitchell and McPherson prospect is 1.5 miles west-northwest of the terminus of Herbert Glacier. It is at an elevation of approximately 1,700 feet near the west end of the ridge between Eagle River and Herbert Glacier in the SW1/4SW1/4 section 29, T. 38 S., R. 65 E. of the Copper River Meridian. The location is accurate within 1/4 mile.

Geologic setting

Geologic description

The Mitchell and McPherson prospect was discovered in 1904 and consists of 2 adits and several trenches. The prospect is on a northeast-trending, vertical shear zone in diorite gneiss whose foliation trends northwest and dips 60 NE (Redman and others, 1989). A breccia zone 6 feet thick trends across the foliation and contains quartz carrying sparse pyrite and galena. Around the time of discovery, the shear zone was reported to contain gold valued up to $12 per ton (with gold at $20.67 per ounce )(Knopf, 1912). In 1939, the prospect was reported to have a reserve of 1.5 million tons that averages 0.2 ounce of gold per ton (Redman and others, 1989). The U.S. Bureau of Mines failed to locate the shear zone in the late 1980s but a sample of altered pyritic diorite contains 2.7 ppm gold, 39.0 ppm silver, and 0.14 percent lead (Redman and others, 1989).
This prospect is in the Juneau Gold Belt, which consists of more than 200 gold-quartz-vein deposits that have produced nearly 7 million ounces of gold. These gold-bearing mesothermal quartz vein systems form a zone 160 km long by 5 to 8 km wide along the western margin of the Coast Mountains. The vein systems are in or near shear zones adjacent to west-verging, mid-Cretaceous thrust faults. The veins are hosted by diverse, variably metamorphosed, sedimentary, volcanic, and intrusive rocks. From the Coast Mountains batholith westward, the host rocks include mixed metasedimentary and metavolcanic sequences of Carboniferous and older, Permian and Triassic, and Jurassic-Cretaceous age. The sequences are juxtaposed along mid-Cretaceous thrust faults (Miller and others, 1994). The sequences are intruded by mid-Cretaceous to middle Eocene plutons, mainly diorite, tonalite, granodiorite, quartz monzonite, and granite. Sheetlike tonalite plutons emplaced just east of the Juneau Gold Belt and undeformed granite and granodiorite bodies that are emplaced farther to the east are between 55 and 48 Ma (Gehrels and others, 1991). The structural grain of the belt is defined by northwest-striking, moderately to steeply northeast-dipping, penetrative foliation that developed between Cretaceous and Eocene time (Miller and others, 1994). The majority of the veins in the Juneau Gold Belt strike northwest. Isotopic dates indicate that the auriferous veins in the Juneau Gold Belt formed between 56 and 55 Ma (Miller and others, 1994; Goldfarb and others, 1997).
Geologic map unit (-134.7271895859, 58.5464744178092)
Mineral deposit model Low-sulfide Au-quartz vein (Cox and Singer, 1986; model 36a)
Mineral deposit model number 36a
Age of mineralization Isotopic dates indicate that the auriferous veins in the Juneau Gold Belt formed between 56 and 55 Ma (Miller and others, 1994; Goldfarb and others, 1997).

Production and reserves

Workings or exploration The Mitchell and McPherson prospect was discovered in 1904 and consists of 2 adits and several trenches.
Indication of production None
Reserve estimates In 1939, the prospect was reported to have a reserve of 1.5 million tons that averages 0.2 ounce of gold per ton (Redman and others, 1989).

References

MRDS Number A012069

References

Gehrels, G.E., McClelland, W.C., Samson, S.D., and Patchett, P.J., 1991, U-Pb geochronology of detrital zircons from a continental margin assemblage in the northern Coast Mountains, southeastern Alaska: Canadian Journal of Earth Sciences, v. 28, no. 8, p.1285-1300.
Goldfarb, R.J., Miller, L.D., Leach, D.L., and Snee, L.W, 1997, Gold deposits in metamorphic rocks in Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 151-190.
Miller, L.D., Goldfarb, R.J., Gehrels, G,E., and Snee, L.W., 1994, Genetic links among fluid cycling, vein formation, regional deformation, and plutonism in the Juneau gold belt, southeastern Alaska: Geology, v. 22, p. 203-206.
Reporters J.C. Barnett and L.D. Miller (Juneau, Alaska )
Last report date 12/15/2001