Sumdum

Prospect, Active

Commodities and mineralogy

Main commodities Ag; Cu; Zn
Other commodities Pb
Ore minerals azurite; bornite; chalcocite; chalcopyrite; galena; malachite; pyrite; pyrrhotite; sphalerite

Geographic location

Quadrangle map, 1:250,000-scale SD
Quadrangle map, 1:63,360-scale D-5
Latitude 57.78483
Longitude -133.45131
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Sumdum copper-zinc prospect consists of a belt of massive-sulfide mineralization that extends northwest for more than 2 miles. Much of the belt is under the Sumdum and other nearby glaciers. The center of the belt is near the northeast corner of section 20, T. 47 S., R. 74 E. (under the Sumdum Glacier).

Geologic setting

Geologic description

The Sumdum copper-zinc deposit consist of bands of disseminated and massive sulfide mineralization scattered along a belt 10,000 or more feet long and 2,000 feet wide (MacKevett and Blake, 1964; Kimble and others, 1984). The host rocks are interlayered biotite-quartz-plagioclase gneiss and hornblende-plagioclase gneiss. This belt of gneiss is bordered to the northeast by the contact of the Tertiary or Cretaceous tonalite sill that marks the edge of the Coast Plutonic complex in southeastern Alaska; it is bordered to the southwest by Cretaceous schist (Brew and Grybeck, 1984).
The mineralized bands generally strike northwest and dip steeply, and vary from 1 foot to 50 feet thick. The ore minerals consist mainly of disseminated and massive layers of pyrrhotite and pyrite with chalcopyrite and sphalerite and small amounts of bornite, chalcocite, malachite, azurite, and galena. The gneiss is isoclinally folded with numerous small folds on the limbs of the large folds and some of the mineralization is localized on the crests of the small folds.
The deposit was discovered in 1958; 62 lode claims were staked in 1958 and 1959 by City Services Minerals, who dug 300 feet of open cuts and drilled 14 diamond drill holes totaling 5,400 feet (Seraphim, 1959; MacKevett and Blake, 1964; Kimble and others, 1984). The deposit is on patented claims.
The bands of mineralization are only discontinuously exposed on the surface and much of the intervals between the outcrops of mineralization is covered by glacial ice and rubble. However, the diamond drilling indicates that at least several of the bands can be traced for several hundred feet in the subsurface.
Kimble and others (1984) identify a total inferred resource of 26,700,000 tons of material with an average width of 31.4 feet and an average grade of 0.57 percent copper, 0.37 percent zinc, and 0.30 ounce of silver per ton. However, there are smaller, higher grade portions. One block of indicated ore mineralization extends for 575 feet along strike, has an average width of 12.7 feet, and contains 1.18 percent copper, 0.15 percent zinc, and 0.25 ounce of silver per ton. Another block of inferred ore extends for 600 feet along strike, has an average width of 12.5 feet, and contains 1.15 percent copper and 0.37 percent zinc.
Geologic map unit (-133.453029392528, 57.7844961439531)
Mineral deposit model Metamorphosed, volcanogenic Ag-Cu-Zn massive sulfide deposit?
Age of mineralization Remobilized during Cretaceous metamorphism of the gneiss host rock; the age of the protolith uncertain but the deposit may have originally been syngenetic.

Production and reserves

Workings or exploration The deposit was discovered in 1958; 62 lode claims were staked in 1958 and 1959 by City Services Minerals who dug 300 feet of open cuts and drilled 14 diamond drill holes totaling 5,400 feet (Seraphim, 1959; MacKevett and Blake, 1964; Kimble and others, 1984). The deposit is on patented claims.
Indication of production None
Reserve estimates Kimble and others (1984) identify a total inferred resource of 26,700,000 tons of material with an average width of 31.4 feet and an average grade of 0.57 percent copper, 0.37 percent zinc, and 0.30 ounce of silver per ton. However, there are smaller, higher grade portions. One block of indicated ore extends for 575 feet along strike, has an average width of 12.7 feet, and contains 1.18 percent copper, 0.15 percent zinc, and 0.25 ounce of silver per ton. Another block of inferred ore extends for 600 feet along strike, has an average width of 12.5 feet, and contains 1.15 percent copper and 0.37 percent zinc.

Additional comments

The Sumdum deposit is covered by patented claims but the surrounding area is part of the Tracy Arm-Fords Terror Wilderness Area which is closed to prospecting and mining.

References

MRDS Number A010667; A013330; A013331

References

Newberry, R.J., Crafford, T.C., Newkirk, S.R., Young, L.E., Nelson, S.W., and Duke, N.A., 1997, Volcanogenic massive sulfide deposits of Alaska, in Goldfarb, R.J. and Miller, L. D., eds., Mineral deposits of Alaska: Economic Geology Monograph 9, p. 120-150.
Reporters Donald Grybeck (U.S. Geological Survey)
Last report date 10/8/2004