Duke Island

Prospects, Probably inactive

Commodities and mineralogy

Main commodities Cr; Fe
Other commodities Cu; Ni; PGE
Ore minerals chalcopyrite; chromite; magnetite; pentlandite; pyrite; pyrrhotite

Geographic location

Quadrangle map, 1:250,000-scale PR
Quadrangle map, 1:63,360-scale D-4
Latitude 54.923
Longitude -131.314
Nearby scientific data Find additional scientific data near this location
Location and accuracy
This site represents the locations of all prospects and occurrences of magnetite- and chromite-bearing ultramafic rocks on Duke and adjacent islands for which there either are U.S. Bureau of Mines (1974) claim records, or other published descriptions (Irvine, 1959; Taylor and Noble, 1960; Clark and Greenwood, 1972; Elliott and others, 1978, locations 159-174; Cobb and Elliott, 1980, p. 137). The coordinates are for the approximate center of the estimated 48-square-mile outcrop area of the ultramafic complex and are accurate within about 0.25 mile. (The 16 prospects and occurrences represented by this site are in both the Prince Rupert D-4 and D-5 quadrangles; the coordinates of the center of the site are in the D-4 quadrangle.)
Also see Additional Comments field.

Geologic setting

Geologic description

Duke Island and adjacent islands (Irvine, 1959; Taylor and Noble, 1960, p. 178-180; Berg and others, 1988) are underlain by metamorphosed undivided Silurian or Ordovician intermediate and mafic intrusive rocks; by Triassic gabbro, which intrudes the metamorphic rocks; and by a Cretaceous ultramafic complex, which intrudes the gabbro. The ultramafic complex is roughly concentrically zoned outward from a core of dunite and peridotite, succeeded by shells of olivine pyroxenite and hornblende pyroxenite. The complex is distinguished by locally conspicuous graded layering and other cumulate features, especially in the dunite, peridotite, and olivine pyroxenite.
Metalliferous minerals include chromite, which occurs in the dunite and peridotite as individual crystals and as small veins and clots; titaniferous magnetite, which locally accompanies pyroxene in graded layers in the hornblende pyroxenite; and rare specks of pyrrhotite, pyrite, pentlandite, and chalcopyrite (Irvine, 1959; Cobb and Elliott, 1980, p. 137).
Analyses of 22 samples (Clark and Greenwood, 1972, p. 159) showed averages of 0.037 ppm Pt (10 samples), 0.033 ppm Pd (16 samples), and 0.010 ppm Rh (6 samples). The acid-soluble iron content of 261 samples of pyroxenite (Berg and others, 1978, p. 25) ranged from 8 to 18% and averaged about 12%. Several of the prospects have been drilled by private interests, but no data on potential iron resources have been made public.
The site includes the 16 claims and prospects scattered over the ultramafic complex that have been described in the literature (and are located individually in the 'Additional comments' field). However, chromite, magnetite, and disseminated sulfides are pervasively distributed throughout the complex, rather than only at a few specific locations.
Geologic map unit (-131.315675992887, 54.9226663557147)
Mineral deposit model Alaska PGE (Cox and Singer, 1986; model 9).
Mineral deposit model number 9
Age of mineralization Cretaceous.

Production and reserves

Workings or exploration Analyses of 22 samples (Clark and Greenwood, 1972, p. 159) showed averages of 0.037 ppm Pt (10 samples), 0.033 ppm Pd (16 samples), and 0.010 ppm Rh (6 samples). The acid-soluble iron content of 261 samples of pyroxenite (Berg and others, 1978, p. 25) ranged from 8 percent to 18 percent and averaged about 12 percent. Several of the prospects have been drilled by private interests, but no data on potential iron resources have been made public.
Indication of production None

Additional comments

The following names, most probably names of claims, (Elliott and others, 1978) have been used for some of the prospects on Duke and adjacent islands (location numbers are from Elliott and others, 1978): Dud, loc. 163; Sec 14, T. 80 S., R. 93 E., of the Copper River Meridian; Peter?, loc. 164; Sec 13, T. 80 S., R. 93 E., of the Copper River Meridian; Red, loc. 165; Sec 13, T. 80 S., R. 93 E., of the Copper River Meridian; Ditto?, loc. 166; Sec 14, T. 80 S., R. 93 E., of the Copper River Meridian; Cave, loc. 167; Sec 23, T. 80 S., R. 93 E., of the Copper River Meridian; Camp, loc. 169; Sec 27, T. 80 S., R. 93 E., of the Copper River Meridian; Richard, loc. 171; Sec 31, T. 80 S., R. 94 E., of the Copper River Meridian; Creek?, loc. 173; Sec 7, T. 80 S., R. 95 E., of the Copper River Meridian; and Ted, loc. 174; Sec 3, T. 81 S., R. 95 E., of the Copper River Meridian.

References