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Unnamed (near Purple Lake)

Occurrence, Undetermined

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Commodities and mineralogy

Main commodities Ag; Au; Cu; Mo
Ore minerals unspecified sulfides

Geographic location

Quadrangle map, 1:250,000-scale KC
Quadrangle map, 1:63,360-scale A-5
Latitude 55.095
Longitude -131.535
Nearby scientific data Find additional scientific data near this location
Location and accuracy This occurrence is at an elevation of about 500 feet, midway between the northeast head of Tamgas Harbor and the foot of Purple Lake. The site is in section 22 T. 78 S., R. 92 E., of the Copper River Meridian. It corresponds to loc. 45 in Karl (1972). The location, which is erroneously marked as a mine on the Ketchikan A-5 topographic map (1955 ed., revised 1962), is accurate to within about a hundred feet.

Geologic setting

Geologic description

The country rock at this site is Silurian trondhjemite (Berg, 1972 [I 684]; Berg and others, 1988). It was regionally metamorphosed to greenschist grade in Late Cretaceous time, and locally is intensely sheared, sericitized, and permeated with hydrothermal hematite. The occurrence consists of unspecified disseminated sulfide minerals in aplite dikes that cut altered trondhjemite. A sample of the sulfide-bearing aplite contained 0.25 ppm Au, 1.5 ppm Ag, 1000 ppm Cu, and 50 ppm Mo (Karl, 1992, loc. 45). If the aplite dikes postdate the Late Cretaceous regional metamorphism, the mineralization may be Tertiary in age.
Geologic map unit (-131.536677679766, 55.0946583199245)
Mineral deposit model Disseminated sulfide mineralization of undetermined origin.
Age of mineralization If the aplite dikes postdate the Late Cretaceous regional metamorphism, the mineralization may be Tertiary in age.
Alteration of deposit The trondhjemite locally is sericitized and permeated with hydrothermal hematite. The aplite dikes may postdate this alteration.

Production and reserves

Workings or exploration A sample of the sulfide-bearing aplite contained 0.25 ppm Au, 1.5 ppm Ag, 1000 ppm Cu, and 50 ppm Mo (Karl, 1992, loc. 45).
Indication of production None

References

MRDS Number 10307694

References

Berg, H.C., 1972, Geologic map of Annette Island, Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-684, 8 p., 1 sheet, scale 1:63,360.
Berg, H.C., Elliott, R.L., and Koch, R.D., 1988, Geologic map of the Ketchikan and Prince Rupert quadrangles, southeastern Alaska: U.S. Geological Survey Mineral Investigations Series Map I-1807, 27 p., scale 1:250,000.
Reporters H.C. Berg, USGS
Last report date 7/7/1999