Unnamed Mine (ARDF - MF067; Francis Island), Juneau District, Skagway-Hoonah-Angoon Borough, Alaska, USA
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Location: The Francis Island mine site is on the southwest side of the island. The original site, described in several older reports (Buddington, 1926; Smith, 1933; Reed, 1938; Rossman, 1963) is now, apparently, largely covered by landslide debris. The occurrence is number 28 of MacKevett and others (1971), number 47 of Cobb (1972), and number 94 of Kimball and others (1978). It is accurate within 0.1 mile.
Geology: The Francis Island occurrence is on the easternmost contact of a granitic intrusion of Cretaceous age. The intrusion is mostly underwater, but a substantial granitic mass, inferred to be part of the same pluton, is exposed on the south part of Marble Mountain across Whidbey Passage from Francis Island. The granitic rock is intruded into marble of Devonian and Silurian age (Brew and others, 1978; pl. IA). Mineral deposits occur in sheared tremolitic marble and in pyroxene-garnet tactite about 5-feet thick at the granitic (quartz diorite) contact. Small rich pods of bornite and other minerals in tactite were described in older reports, including Buddington, (1926, p. 56), Buddington and Chapin (1929, p. 323), Smith, (1933, p. 323), Reed (1938, p.69) and Rossman (1963-B 1121-K, p. K51). Some of the material was mined. This site has apparently been covered by landslide debris. A mineralized fault zone 10 feet thick can be followed for 50 feet before it is covered with surficial debris; MacKevett and others (1971, fig. 8) show the probable relation of the original site to the fault zone, which locally contains chalcopyrite, bornite, tetrahedrite (?), chalcocite (?), pyrite, and probably sphalerite. Malachite and pyrolusite occur as secondary minerals. MacKevett and others (1971) reported 7000 ppm copper, 1000 ppm zinc, 200 ppm antimony, 150 ppm bismuth, 20 ppm tin, and 1.46 ounce per ton silver in a selected sample of copper-stained metamorphic rock collected at or near the fault zone. Magnetite occurs locally in the tactite. A soil survey suggests the possibility of buried mineral deposits in the area (MacKevett and others, 1971, fig. 8). The U.S. Bureau of Mines (Kimball and others, 1978) reported a trace of tungsten and 7 ppm silver in float samples from the area.
Workings: Surface cuts, now concealed by landslide debris, probably date back to 2 claims staked on the island in 1923, as reported by Buddington (1926) and repeated by Smith (1933) and Reed (1938). Buddington reported gold-silver-bearing bornite in a rich sulfide pocket in the tactite. MacKevett and others (1971) obtained maximum values of 7000 ppm copper, 1000 ppm zinc, 200 ppm antimony, 150 ppm bismuth, and 1.46 ounce/ton silver in a selected sample of copper-stained rock found near the inferred buried site.
Age: Cretaceous (?).
Alteration: Marble locally contains tremolite, and at intrusive contacts is altered to pyroxene - garnet skarn. Chlorite and tremolite also occur in calc-hornfels.
Production: Possible small production, no records.
Commodities (Major) - Ag, Au, Cu, Zn; (Minor) - Bi, Sb
Development Status: No
Deposit Model: Copper skarn or tactite (Cox and Singer, 1986; model 18b).
11 entries listed. 11 valid minerals.
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Brew, D.A., Johnson, B.R., Grybeck, D., Griscom, A., Barnes, D.F., Kimball, A.L., Still, J.C., and Rataj, J.L., 1978, Mineral resources of the Glacier Bay National Monument Wilderness Study Area, Alaska: U.S. Geological Survey Open-File Report 78-494, 670 p., 7 sheets. Buddington, A.F., 1926, Mineral investigations in southeastern Alaska: U.S. Geological Survey Bulletin 783, p. 41-62. Buddington, A.F., and Chapin, Theodore, 1929, Geology and mineral deposits of southeastern Alaska: U.S. Geological Survey Bulletin 800, 398 p. 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. Rossman, Darwin, 1963, Geology of the eastern part of the Mount Fairweather quadrangle, Glacier Bay, Alaska: U.S. Geological Survey Bulletin 1121-K, p. K1-K57. Smith, P.S., 1933, Mineral industry of Alaska in 1931: U.S. Geological Survey Bulletin 844-A, p. 1-81.