Unnamed (Magoun Islands)

Occurrence, Inactive

Commodities and mineralogy

Main commodities Au; Cu; Mo
Ore minerals chalcopyrite; molybdenite; pyrite

Geographic location

Quadrangle map, 1:250,000-scale SI
Quadrangle map, 1:63,360-scale A-5
Latitude 57.1659
Longitude -135.5787
Nearby scientific data Find additional scientific data near this location
Location and accuracy The location of this occurrence is only known approximately; it is is at sea level on the westernmost of the Magoun Islands in Krestof Sound. For this record, the site is plotted at the southeast corner of sec. 20, T. 54 S., R. 62 E. It is location P-108 of Bittenbender and others (1999), location 50 of Cobb (1972, 1978), and MAS no. 0021140059 (U.S. Bureau of Land Management, 2002). The location probably is accurate within 0.5 mile.

Geologic setting

Geologic description

The rocks on the Magoun Islands are part of a Tertiary intrusive stock that consists mainly of hornblende-biotite tonalite with subordinate biotite tonalite and hornblende-biotite granodiorite (Loney and others, 1975). As described by Smith (1942 [B 926-C) the deposit consists of quartz veinlets up to 6 inches thick in the tonalite. The veinlets contain sparse chalcopyrite and pyrite, and near the walls of the veinlets, plates of molybdenite up to 1/2 inch in diameter. Bittenbender and others (1999) collected 7 samples of quartz veinlets in tonalite, with various combinations of up to 3 percent molybdenite, pyrite, and chalcopyrite. The samples contained from less than 5 to 125 parts per billion gold, 52 parts per million (ppm) to 1 percent copper, and up to 800 ppm molybdenum. There is no indication of exploration beyond surface sampling.
Geologic map unit (-135.580473838134, 57.1655438596468)
Mineral deposit model Porphyry copper-molybdenum deposit?
Age of mineralization Tertiary or younger based on the age of the host rock.

Production and reserves

Workings or exploration Apparently only surface sampling.
Indication of production None

Additional comments

The Magoun Islands have now been set aside as the Magoun State Marine Park.

References

MRDS Number A013297

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.
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 Donald Grybeck (U.S. Geological Survey)
Last report date 5/5/2005