Falcon Arm

Prospect, Undetermined

Commodities and mineralogy

Main commodities Au
Ore minerals gold
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale SI
Quadrangle map, 1:63,360-scale C-6
Latitude 57.5521
Longitude -135.9281
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Falcon Arm prospect is at an elevation of about 180 feet on the valley wall east of the head of Falcon Arm. The prospect is 0.4 mile north-northwest of the center of sec. 10, T. 50 S., R. 59 E. It is location P-90 of Bittenbender and others (1999), location 42 of Cobb (1972, 1978), and MAS no. 0021140033 (U.S. Bureau of Land Management, 2002). The location probably is accurate within 0.2 mile.

Geologic setting

Geologic description

Johnson and Karl (1985) map the rocks in the area of this prospect as Cretaceous graywacke. The rocks are cut by numerous steeply-dipping, mainly northwest-striking faults. They include the extensions of the faults at the Hirst (SI087) and Chichagof (SI093) mines, which control the mineralization there and whose traces are about a mile northeast of this prospect.
Reed and Coats (1941) describe the prospect as fault-controlled, auriferous quartz veins up to 4 inches thick in graywacke. The graywacke is cut by numerous, 1- to 3-foot-thick dikes of light-colored, fine-grained, intrusive rock. The graywacke strikes about N50W and dips about 70SW; the principal faults and splits from them strike N35W-N30E and dip steeply west. Some of the dikes are similar in attitude to some of the faults. The principal set of joints strikes N45E and is nearly vertical. Workings include a 2,260-foot crosscut and a 750-foot drift off the crosscut.
Bittenbender and others (1999), citing Overbeck (1919) and Still and Weir (1981), report that the Falcon Arm prospect was staked in 1916 on fault-controlled, auriferous quartz veins in mafic dikes and graywacke. Workings consist of 4 adits totaling 3,130 feet, and a pit . The most significant mineralized rock is quartz rubblecrop; blocks of it up to 1.2 feet thick assayed 0.5 ounce to 2.15 ounces of gold per ton. A sample from the upper workings contained 2.16 ounces of gold per ton. There are no published records of production but the assay values and the extent of the workings indicate that some gold may have been produced.
Isotopic studies indicate that the gold-quartz veins in coastal southern and southeastern Alaska are Eocene, about 50 Ma in age (Haeussler, 1995; Goldfarb, 1997; Goldfarb and others, 1997).
Geologic map unit (-135.92990857479, 57.5517369448362)
Mineral deposit model Low-sulfide gold-quartz veins (Cox and Singer, 1986; model 36a).
Mineral deposit model number 36a
Age of mineralization Isotopic studies indicate that the gold-quartz veins in coastal southern and southeastern Alaska are Eocene, about 50 Ma in age (Haeussler and others, 1995; Goldfarb, 1997; Goldfarb and others, 1997).

Production and reserves

Workings or exploration Reed and Coats (1941) report a 2,260-foot crosscut and a 750-foot drift off the crosscut. Bittenbender and others (1999) report that the Falcon Arm prospect was staked in 1916 and developed by 4 adits totaling 3,130 feet and a pit.
Indication of production Undetermined
Production notes There are no published records of production but the assay values and the extent of the workings indicate that some gold may have been produced.

Additional comments

The prospect is in West Chichagof-Yakobi Wilderness.

References

MRDS Number A013287

References

Bittenbender, P., Still, J.C., Maas, K., and McDonald, M., Jr., 1999, Mineral resources of the Chichagof and Baranof Islands area, southeast Alaska: Bureau of Land Management, BLM-Alaska Technical Report 19, 222 p.
Goldfarb, R J., 1997, Metallogenic evolution of Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 4-34.
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.
Haeussler, P J., Bradley, D., Goldfarb, R., Snee, L., and Taylor, C., 1995, Link between ridge subduction and gold mineralization in southern Alaska: Geology, v. 23, no. 11, p. 995-998.
U.S. Bureau of Land Management, 2002, Alaska mineral locations database report (Sitka quadrangle), July 2, 2002, 205 p. [http://imcg.wr.usgs.gov/dem.html]
Reporters H.C. Berg (U.S. Geological Survey)
Last report date 10/16/2004