Northern Copper Company

Prospect, Inactive

Alternative names

Tower Arm Copper
Portage Bay Copper Company

Commodities and mineralogy

Main commodities Cu; Zn
Ore minerals chalcopyrite; magnetite; pyrite; pyrrhotite; sphalerite

Geographic location

Quadrangle map, 1:250,000-scale PE
Quadrangle map, 1:63,360-scale D-5
Latitude 56.8877
Longitude -133.3743
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Northern Copper prospect is at an elevation of about 1300 feet on the southern spur of Kupreanof Island. It is near the the center of the northern half of section 36, T. 57 S., R. 76 E. The area is heavily wooded and the prospect is is not easy to locate on the ground. Still and others (2002) have published detailed maps of the surface and underground workings.

Geologic setting

Geologic description

The Northern Copper prospect was first staked in 1900 and had several periods of activity, notably in 1900 and 1901 and between 1918 and 1921. Numerous workings were driven from 1900 to 1921 including a shaft, numerous trenches and open cuts, and three adits that are about 360 feet, 30 feet, and 285 feet long. A road was built part way to the prospect from the head of Duncan Canal and a tram was planned but not built from tidewater to the prospect. It has been active for short periods several times since, notably in 1978 and 1979 when Amoco Minerals Co. staked a large block of claims that covered the area around the Northern Copper prospect, and carried out extensive airborne and ground geophysics, geologic mapping, and soil and stream sediment geochemistry in the area. They also core drilled six holes to test geophysical anomalies near the Northern Copper prospect. There apparently has been no production. The prospect was covered by 4 claims patented in 1907 that were deeded to the Forest Service in 1995 and are now part of the Petersburg Creek-Duncan Salt Chuck Wilderness area. (More detailed information can be found in Wright and Wright (1908), Buddington, (1923); Roehm, 1945 [DGGS IR 195-37]; Twenhofel and others, 1949; Still and others (2002).
The rocks in the older upper workings workings consist of a thick layer of massive greenstone and chlorite schist, locally with garnet and pyroxene (Wright and Wright, 1908; Buddington, 1923; Roehm, 1945 [DGGS IR 195-37]; Twenhofel and others, 1949; Still and others, 2002). Karl and others (1999) consider the rocks to be Devonian in age. The greenstone also has layers or lenses of white crystalline marble. The mineralization consists of massive layers up to several feet thick, patches, and disseminations of sulfides, mainly pyrrhotite, pyrite, chalcopyrite, and minor sphalerite. The sulfide layers are generally oriented parallel to the layering in the greenstone. Still and others (2002) collected 36 samples in the massive greenstone. The highest copper values were 1.7 percent across 1.5 feet, 1.4 percent across 1.8 feet, and 3.5 percent in an outcrop sample. The highest zinc values were 1.2 percent across 3 feet and 2.4 percent across 1.5 feet. The highest silver value was 32.6 parts per million; the highest gold value was 165 parts per billion (ppb). A band of massive sulfides, mainly pyrrhotite, pyrite, and chalcopyrite, 0.2 to 2 feet thick is exposed in the 30-foot adit for about 20 feet. Samples contained up to 12.4 percent copper across 2 feet; the highest gold value was 440 ppb and the highest silver value was 37.7 ppm.
The work in the late 1970s by Amoco Minerals Co. identified a unit of gray argillite under the greenstone. The argillite contains layers 0.2 to 1.7 feet thick of massive pyrrhotite with chalcopyrite and minor sphalerite. Samples contained up to 2.5 percent copper across 1.4 feet; the maximum gold value was 29 ppb, the maximum silver value was 11.5 ppm.
The origin of the deposit is enigmatic. Most likely it is a metamorphosed volcanogenic massive sulfide deposit. But it may be a replacement deposit, or possibly a skarn-type deposit.
Geologic map unit (-133.376032208015, 56.8873614757821)
Mineral deposit model Probably a metamorphosed volcanogenic massive sulfide deposit.
Age of mineralization Devonian or later based on the age of the host rocks.

Production and reserves

Workings or exploration The Northern Copper prospect was first staked in 1900 and had several period of activity, notably in 1900 and 1901 and between 1918 and 1921. Numerous workings were driven from 1900 to 1921 including a shaft, numerous trenches and open cuts, and three adits that are about 360 feet, 30 feet, and 285 feet long. A road was built part way to the prospect from the head of Duncan Canal and a tram was planned but not built from tidewater to the prospect. It has been active for short periods several times since, notably in 1978 and 1979, when Amoco Minerals Co. staked a large block of claims that covered the area around the Northern Copper prospect, and carried out extensive airborne and ground geophysics, geologic mapping, and soil and stream sediment geochemistry in the area. They also core drilled six holes to test geophysical anomalies near the Northern Copper prospect.
Indication of production None

Additional comments

The prospect was covered by 4 claims patented in 1907 that were deeded to the Forest Service in 1995; they are now part of the Petersburg Creek-Duncan Salt Chuck Wilderness area.

References

MRDS Number A010323

References

Still, J.C., Bittenbender, P.E., Bean, K.W., and Gensler, E.G., 2002, Mineral assessment of the Stikine area, central Southeast Alaska: U.S. Bureau of Land Management Technical Report 51, 560 p.
Reporters D.J. Grybeck (Port Ludlow, WA)
Last report date 4/8/2007