Alaska Chief Prospect, Juneau District, Skagway-Hoonah-Angoon Borough, Alaska, USA
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Location: The Alaska Chief prospect is on a steep northeast-facing sidehill at about 1150 ft elevation. The prospect is about 0.7 mile southwest of Ripple Cove and about 0.85 mile slightly north of east from Hill 2146. Its general location is 5 miles north of the mouth of Glacier Bay into Icy Straits on the west side of the bay. The prospect is accurately located by elevation, but its location could range within about 0.15 mile along the contour. It was formerly accessible by trail from Ripple Cove, but the trail is badly overgrown.
Geology: Rocks exposed in the vicinity of the deposit are limestone and marble of Paleozoic age, and foliated granodiorite of probable Cretaceous age. About 0.1 mile west of the deposit, a lighter colored granitic rock of probable Tertiary age intrudes the granodiorite (MacKevett and others, 1971). Massive sulfides and tactite replace marble and hornfels in the contact zone of the granodiorite pluton. The major components of the tactite are grossularite-rich garnet, epidote and zoisite. The contact and relict bedding at the deposit strike about N 30 W and dip steeply to the southwest. Although the deposit is physically associated with the contact of foliated granodiorite of Cretaceous age, it is possible that the mineral deposit is genetically related to the younger intrusion and only physically controlled by the favorable older contact zone. The deposit consists of a massive-sulfide body grading out into a disseminated deposit in contact altered country rock (Reed, 1938). The massive sulfide replacement body occurs under a 150- by 55-foot stripped area. At the surface, the massive sulfide body was oxidized to a limonitic gossan, possibly manganiferous. Azurite and malachite are locally abundant in the oxidized part of the deposit. The underlying sulfide-rich deposit contains bornite, chalcopyrite, pyrite, pyrrhotite and sphalerite. Grab samples of ore stockpiled near old workings contained more than 10 percent copper. MacKevett and others (1971) reported as much as 0.232 ounce per ton gold, 4.4 ounces per ton silver, and 1.5 percent copper when they resampled the deposit in 1966.
Workings: The prospect was discovered before 1906 and was patented in 1924. It was explored by clearing vegetation off the outcrop and by an adit driven about 60 feet below the outcrop. Wright and Wright (1937, p. 221-222) reported the adit as 130 feet in length. Reed (1938, p. 37), and MacKevett and Cornwall (in MacKevett and others, 1971, p. 45-48, fig. 9) found an adit less than 40 feet long. It thus appears likely that the Wrights' footage measurement is in error or that the workings found by the Wrights are covered by surface debris. MacKevett and others (1971) reported as much as 1.5 percent copper, 700 ppm zinc, 0.232 ounce per ton gold and 4.4 ounce per ton silver and geochemically anomalous sample values molybdenum, bismuth, nickel and cobalt. A grab sample from an old ore pile contained more than 10 percent copper. A soil sample collected below the stripped area contained 50 ppm each of silver and gold, 15,000 ppm copper, 300 ppm each of bismuth and cobalt, 500 ppm nickel and 1500 ppm zinc. The deposit has not been drilled.
Age: Late Cretaceous or younger.
Alteration: Formation of contact-metasomatic calc-silicate minerals in limestone and contact zone of granitic intrusion; local oxidation of copper and iron minerals.
Reserves: Based on a surface dimension of 150 x 30 feet projected to a depth of 50 feet, and a tonnage factor of 8 cubic-feet / ton, MacKevett and others (1971, p. 48) propose an indicated reserve of about 28000 tons that would contain more than 1 percent copper, 2 ounces per ton silver, and 0.10 ounce per ton gold. This estimate, however, is probably too high because the adit driven below the deposit is in lower-grade ore. Kimball and others (1978, p. C353) concurred with the USGS's reserve estimate but it seems more realistic to call it inferred because the subsurface data are so scant.
Commodities (Major) - Ag, Au, Cu; (Minor) - Bi, Co, Ni, Zn
Development Status: No
Deposit Model: Copper skarn (Cox and Singer, 1986; model 18b). Contact metasomatic copper-gol
Mineral List
10 entries listed. 9 valid minerals.
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References
Cobb, E.H., 1972, Metallic mineral resources map of the Mount Fairweather quadrangle, AK: U.S. Geological Survey Miscellaneous Field Study Map MF-436, 1 sheet, scale 1:250,000. Kimball, A.L., Still, J.C., and Rataj, J.L., 1978, Mineral resources, in Brew, D. A., and others, Mineral resources of the Glacier Bay National Monument wilderness study area, Alaska: U.S. Geological Survey Open-File Report 78-494, p. C1-C375. MacKevett, E.M., Jr., Brew, D.A., Hawley, C.C., Huff, L.C., and Smith, J.G., 1971, Mineral resources of Glacier Bay National Monument, Alaska: U.S. Geological Survey Professional Paper 632, 90 p., 12 plates, scale 1:250,000. Reed, J.C., 1938, Some mineral deposits of Glacier Bay and vicinity, Alaska: Economic Geology, v. 33, p. 52-80. Wright, F.E., and Wright, C.W., 1937, The Glacier Bay National Monument in southeastern Alaska, its glaciers and geology: U.S. Geological Survey manuscript report, 224 p.
