Nunatak Prospect (Muir Inlet), Juneau District, Skagway-Hoonah-Angoon Borough, Alaska, USA
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Location: The Nunatak molybdenum deposit is on the east side of upper Muir Inlet, about one mile north of Nunatak Cove. The Cove furnishes a good anchorage for access to the deposit. The deposit trends north to northwest; the coordinates are the approximate center of the outcrop of the deposit. The deposit is mainly on the west flank of the geographic feature named the Nunatak. Mineralization extends from about 1200 feet elevation on the Nunatak northwest to the shoreline. The deposit is number 43 of Cobb (1972).
Geology: The deposit is in the Muir province dominated by volcanic rocks of Silurian and Devonian age and clastic, carbonate, and volcanic rocks of Permian age. Hornfels is widespread in the terrane, especially near foliated granitic rocks of Cretaceous age (Brew and others, 1978, p. B20-21). At the deposit, calc-hornfels developed in calcic argillite and limestone of probable Paleozoic age is intruded by a quartz granodiorite (rhyodacite) porphyry, generally termed quartz monzonite porphyry (MacKevett and others, 1971, p. 74). The quartz granodiorite is a massive rock composed of phenocrystic oligoclase and lesser quartz, and hornblende in a microcrystalline background of K-feldspar, plagioclase and quartz. The largest body of the granodiorite porphyry is about 1200 feet long by as much as 400 feet across. It is aligned northerly and crops out on the northwest flank of the Nunatak. The deposit and older country rocks are cut by numerous post-mineral dikes of hornblende andesite and dacite porphyry. The quartz granodiorite porphyry is enveloped by a silicfied zone that grades outward into tactitic calc-hornfels. The hornfels is succeeded outward by fine-grained 'chert' silica rock (Moerlein, 1968; Kimball and others, 1978, pl. IV, p. C275). Moerlein proposed that these facies were alteration zones or haloes related to the intrusion of the porphyry and mineralization. Previously some of the units had been mapped as primary sedimentary facies (Twenhofel, 1946). The deposit is mainly composed of quartz-molybdenite stockworks in the silicic and tactitic zones, hornfels, and quartz porphyry. Molybdenite also locally occurs in fault zones, as rosettes on joint and fracture planes in the tactized rock, and as fine-grained disseminations in the porphyry. Chalcopyrite locally occurs with molybdenite but appears to be relatively more abundant outside the main molybdenite zone. Small amonts of enargite and bornite occur locally. Chrysocolla and malachite are developed in partly oxidized facies. Rossman (1963, B 1121-K, p. K49) reported a sample from the northeast side of the Nunatak that contained 0.04 oz/ton gold and 7.07 oz/ton silver. The silver-bearing phase is uncertain, but tetrahedrite has been reported from the property. Possibly gold forms a weak distal halo, as in many other molybdenum deposits. MacKevett and others (1971, p. 75) cite communications from Amex (American Exploration & Mining) indicating the prescence of gold and silver in Amex drill holes (1971, p. 75). Diamond drill holes of the U.S. Bureau of Mines (Sanford and others, 1949) and private exploration companies indicate mineralization extends downward to at least 300 feet below sea level. The deposit is probably of Tertiary age and is genetically related to the intrusion of a high level quartz granodiorite porphyry. The haloes and pattern of alteration suggest that the intrusive is barely unroofed.
Workings: The area was first mapped by Reed (1938) in 1936, who recognized abundant quartz veins and described hornfels, granitic dikes, and chalcopyrite and copper-iron stained veinlet mineralization. The molybdenum deposit was found in 1941 by prospectors John Johnson and Tom Smith. It was studied jointly by the U.S. Geological Survey and Bureau of Mines in 1942 (Twenhofel, 1946; Sanford and others, 1949). The first two core holes were drilled by the Bureau of Mines in 1943 (Sanford and others, 1949). The first studies recognized and concentrated on relatively rich veins or lodes within the deposit. The area was relocated by James Walper in 1964. Amex drilled three diamond core holes in 1966. In 1968, the area was remapped by consultant George M. Moerlein (1968) for Superior Oil Company. Superior drilled seven core holes with a total footage of 5,430 feet. Maximum drill hole depth of any hole was 892 feet. Limited surface mapping and sampling, including a geochemical soil survey, was carried out by MacKevett and others, 1971 (plates 11 and 12 and p. 74-78). The area was visited and studied briefly by the U.S. Bureau of Mines in 1975 and 1977 (Kimball and others, 1978, p. C274-295).
Alteration: The alteration consists of minor K-feldspar and pyrometasomatic conversion of calc-units to calc-hornfels and skarn. Actinolite is locally developed in the skarn and actinolitic skarns appear to be more favorable for ore The tactitic skarn consists mainly of orthoclase and clinozoisite, with lesser amounts of diopside, garnet, quartz, and plagioclase. Rocks adjacent to the quartz granodiorite porphyry are silicified. Phlogopite, montmorillonite, calcite, and feldspar occur in the altered zones flanking some veins (MacKevett and others, 1971, p. 75).
Production: No production.
Reserves: Approximately 145 millon tons of mineralized rock configured for surface mining and containing from 0.04 to 0.06 percent molybdenum and 0.02 percent copper exist at the Nunatak deposit. There is an inferred 9.1 million tons at or below sealevel near Muir Inlet with grades up to about 0.06 percent molybdenum and 0.02 percent Cu. The total figure (145 million tons) represents the combination of the Bureau of Mines area #1 with conspicuous molybdenite and area #2 with inconspicuous molybdenite. Rock with more molybdenum was found in the Amex No. 2 and Muir Nos. 1 & 2 drill holes. These holes indicate a discontinuous halo around the quartz granodiorite porphyry stock from 50 to 160 feet wide that grades from 0.11 to 0.143 percent molybdenum (Kimball and others, 1978, table C-53).
Commodities (Major) - Mo; (Minor) - Ag, Au, Cu
Development Status: No
Deposit Model: Porphyry molybdenum (Cox and Singer, 1986; model 21b).
18 entries listed. 16 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. 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. Moerlein, G.A., 1968, Geology and drilling results, Nunatak molybdenum prospect, Walper property, southeastern Alaska: Unpublished report available at U.S. Bureau of Land Management Library at Juneau, Alaska. 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. Sanford, R.S., Apell, G.A., and Rutledge, F.A., 1949, Investigation of Muir Inlet or Nunatak molybdenum deposits, Glacier Bay, southeastern Alaska: U.S. Bureau of Mines Report of Investigations 4421, 6 p. Twenhofel, W.S., 1946, Molybdenite deposits of the Nunatak area, Muir Inlet, Glacier Bay, in Twenhofel, W.S., Robinson, G.D., and Gault, H.R., Molybdenite investigations in southeastern Alaska: U.S. Geological Survey Bulletin 947-B, p. 9-18. Singer, D.A., Berger, V.I., and Moring, B.C. (2008): Porphyry copper deposits of the world: Database and grade and tonnage models, 2008. US Geological Survey Open-File Report 2008-1155.