Incas Mine, Juneau District, Skagway-Hoonah-Angoon Borough, Alaska, USA
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Location: The Incas vein crops out on the steep side-hill west of Reid Inlet at an elevation of about 1000 feet. Incas is about 0.75 mile south of the Sentinel vein (MF028) and 0.6 mile north-northwest of the Galena (MF031). The Incas is a vein-mineralized fault zone that can be traced for nearly 2000 feet. The coordinates given are the approximate location of workings on the vein; they are probably correct within 0.15 mile of the workings.
Geology: The Reid Inlet gold area is mainly underlain by granitic rocks of Cretaceous age (Brew and others, 1978). The area was mapped in detail by Rossman (1959, B 1058-B). The Incas is a productive north-striking, steeply-dipping vein fissure 1-3-feet thick with lenses of quartz and calcite in a shear zone. Rossman (1959) traced the vein for about 2000 feet and found free gold in all quartz lenses. Locally, gold may have been concentrated by supergene enrichment. Workings driven below the surface outcrops found only low grade quartz veins. The Incas structure is subparallel to and similar geologically to the Monarch vein fissure (MF026). Rossman (1959, plate 4) mapped another subparallel vein about 400-feet east of the Incas vein; the vein was partly covered by colluvium.
Workings: Incas was discovered in 1924 by Joe Ibach and is one of the earliest discoveries in the district. Claims were located by Ibach and famous Alaska novelist, Rex Beach, in 1936 after Glacier Bay National Monument was opened for mining (Kimball and others, 1978, p. C217). Gold, possibly enriched by supergene processes, was mined along the exposed vein fissure for about 60 feet near an elevation of 1100 feet. A crosscut tunnel was driven below the stoped area for about 170 feet. Near the face of the crosscut, the adit drifts for about 40 feet along a fissure reasonably inferred to be on the same vein mined at the surface. MacKevett found only about 0.029 oz/ton gold in narrow samples taken from the underground workings. One sample contains 20,000 ppm arsenic (MacKevett and others, table 11, location G).
Age: Tertiary.
Alteration: Rossman (1959) notes alteration (bleaching?) along the 2000-foot strike exposure of the vein fissure.
Commodities (Major) - Au; (Minor) - As
Development Status: Yes; small
Deposit Model: Low-sulfide gold-quartz vein (Cox and Singer, 1986; model 36a).
Mineral List
5 entries listed. 5 valid minerals.
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References
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. 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. Rossman, Darwin, 1959, Geology and ore deposits in the Reid Inlet area, Glacier Bay, Alaska: U.S. Geological Survey Bulletin 1058-B, p. 33-58.
