Mirror Harbor

Prospects, Probably inactive

Alternative names

Davison Bay
Fleming Island

Commodities and mineralogy

Main commodities Co; Cu; Ni
Ore minerals chalcopyrite; niccolite; pentlandite; pyrrhotite

Geographic location

Quadrangle map, 1:250,000-scale SI
Quadrangle map, 1:63,360-scale D-7
Latitude 57.7855
Longitude -136.3082
Nearby scientific data Find additional scientific data near this location
Location and accuracy This site is near the center of a 2-mile by 3-mile area of geologically similar prospects along the west coast of northwest Chichagof Island. The area extends from Little Bay to Bertha Bay and includes Davison Bay and Fleming Island; most commonly the occurrences have been referred to under the name Mirror Harbor, which is not named on the D-7 USGS topographic map (1997 ed.). For this record, the site is plotted about at sea level at the northeast corner of sec. 22, T. 47 S., R. 56 E. It is location P-45 of Bittenbender and others (1999), location 16 of Cobb (1972, 1978), and MAS no. 0021140068 (U.S. Bureau of Land Management, 2002). The location is accurate.

Geologic setting

Geologic description

Johnson and Karl (1985) map the rocks in the Mirror Harbor area as a Tertiary, composite stock of gabbronorite and norite, enclosed by a discontinuous shell of tonalite. About 5 square miles of the stock are above sea level. The stock intrudes and thermally metamorphoses Cretaceous graywacke and melange. The stock postdates several high-angle, northwest-striking faults.
Cobb (1972, 1978), summarizing several early USGS reports (see References at the end of this record), describes the deposit as intergrown pyrrhotite, pentlandite, and chalcopyrite that form magmatic segregations in norite. The sulfides locally are concentrated into podlike masses and are accompanied by some secondary niccolite.
Cobb (1978) reports that the largest body of concentrated sulfides is on Fleming Island and is estimated to contain about 8,000 tons of material that averages 1.57 percent nickel, 0.88 percent copper, and probably a little cobalt. A disseminated deposit near Davison Bay contains an estimated several million tons of rock that averages about 0.2 percent nickel and 0.1 percent copper.
Bittenbender and others (1999) report that nickel-copper-cobalt mineralization was discovered near Mirror Harbor in 1911. In 1915, a 175-foot shaft (since flooded) was sunk, with crosscuts at 75 and 175 feet below the surface (Pecora, 1942). During WW II the US Bureau of Mines mapped, trenched, and drilled the property; private interests mapped, sampled, and conducted geochemical and geophysical surveys from 1972-1982.
Bittenbender and others (1999) estimate the resources of 3 deposits in the Mirror Harbor area; two are massive bodies of sulfides and the other is disseminated sulfides. A body of massive sulfides near the shaft contains 7,300 tons of rock that averages 1.60 percent nickel and 0.90 percent copper. A body of massive sulfides on the south side of Davison Bay contains a few tons of rock with slightly less grade than the one at the shaft. A disseminated deposit between Mirror Harbor and Davison Bay contains one million tons of rock that averages 0.32 percent nickel and 0.12 percent copper (Still, 1988). Bittenbender and others report that there has been no production and there are no active claims.
Geologic map unit (-136.310034182922, 57.7851249674169)
Mineral deposit model Duluth Cu-Ni-PGE (Cox and Singer, 1986; model 5a).
Mineral deposit model number 5a
Age of mineralization The magmatic segregation deposits are roughly contemporaneous with the crystallization of the Tertiary gabbronorite-norite host rock.

Production and reserves

Workings or exploration Bittenbender and others (1999) report that nickel-copper-cobalt mineralization was discovered near Mirror Harbor in 1911. In 1915, a 175-foot shaft (since flooded) was sunk, with crosscuts at 75 and 175 feet below the surface (Pecora, 1942). During WW II the US Bureau of Mines mapped, trenched, and drilled the property, and private interests mapped, sampled, and conducted geochemical and geophysical surveys from 1972-1982.
Indication of production None
Reserve estimates
Cobb (1978) reports that the largest body of concentrated sulfides is on Fleming Island and is estimated to contain about 8,000 tons of material that averages about 1.57 percent nickel, 0.88 percent copper, and probably a little cobalt. A disseminated deposit near Davison Bay contains an estimated several million tons grading about 0.2 percent nickel and 0.1 percent copper.
Bittenbender and others (1999) estimate the resources of 3 deposits in the Mirror Harbor area; two are massive bodies of sulfides and the other is disseminated sulfides. A body of massive sulfides near the shaft contains 7,300 tons of rock that averages 1.60 percent nickel and 0.90 percent copper. A body of massive sulfides on the south side of Davison Bay contains a few tons of rock with slightly less grade than the one at the shaft. A disseminated deposit between Mirror Harbor and Davison Bay contains one million tons of rock that averages 0.32 percent nickel and 0.12 percent copper (Still, 1988).

Additional comments

The prospects are in West Chichagof-Yakobi Wilderness.

References

MRDS Number A013252; A013253; A013254; W000378; W000379

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.
Sanford, R.S., 1942, Preliminary report and project setup, Mirror Harbor nickel deposits, Chichagof Island, Territory of Alaska: Unpublished report, 9 p. (Report held by the Mineral Information Center, Bureau of Land Management, Juneau, Alaska.)
Traver, W. M., 1944, Appendix to War Minerals Report, Mirror Harbor nickel deposits, Chichagof Island, Alaska: Unpublished report, 54 p. (Report held by the Mineral Information Center, Bureau of Land Management, Juneau, Alaska.)
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]
U. S. Bureau of Mines, 1944, Mirror Harbor, Chichagof Island, Alaska: War Minerals Report 333, 9 p. (Report held by the Mineral Information Center, Bureau of Land Management, Juneau, Alaska.)
Reporters H.C. Berg (U.S. Geological Survey)
Last report date 10/16/2004