Cheri & Cheri No. 1 prospects, West Arm Kendrick Bay, Kendrick Bay, Prince of Wales Island, Ketchikan District, Prince of Wales-Outer Ketchikan Borough, Alaska, USA
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Geology: This and several other nearby uranium-thorium-REE deposits (DE015 to DE028 and DE030 to DE031) are spatially and genetically related to a stock of Jurassic, peralkaline granite about 2 miles in outcrop diameter centered on Bokan Mountain. It commonly is referred to as the Bokan Mountain peralkakline granite or Bokan Mountain complex. The intrusion and its deposits have been mapped in detail several times using slightly different subdivisions of the granite (MacKevett, 1963; Thompson and others, 1980, 1982; Saint-Andre and others, 1983; Gehrels, 1992; Thompson, 1997). This description largely follows Gehrels' (1992) map units. The intrusion is a ring-dike complex with an outer border zone up to 14 meters thick of pegmatite and aplite; a nearly complete intermediate zone of aegirine granite porphyry, 15 to 180 meters thick; and a core of several varieties of riebeckite granite porphyry. It has been dated by several methods at 151 Ma to 191 Ma (Lanphere and others, 1964; Saint-Andre and others, 1983; Armstrong, 1985; Gehrels, 1992; Thompson, 1997). The peralkaline granite mainly intrudes a regionally extensive body of Silurian or Ordovician quartz monzonite, granite, and quartz diorite that makes up much of the southeast tip of Prince of Wales Island. The south and west sides of the peralkaline granite are in contact with a band up to about 3,000 feet wide of shale and argillite of the Silurian or Ordovician Descon Formation. The Bokan Mountain complex and surrounding Paleozoic rocks are cut by numerous pegmatite, andesite, dacite, and aplite dikes. The dikes are genetically related to the complex and commonly are associated with the uranium, thorium, and REE deposits. The deposits are marked by intense albitization, pervasive or fracture-controlled chloritization, calcite-fluorite replacement of aegirine, and hematitization. Three types of U-Th-REE deposits occur in the Bokan Mountain complex: 1) irregular cylindrical pipes; 2) steep, shear-zone-related pods or lenses ('veins'); and 3) quartz veins. MacKevett (1963) identified small pits and trenches at the Cheri prospects on several claims staked in 1956. The rocks in the vicinity consist of Ordovician, albitized quartz diorite and diorite, and a pendant of quartzite of the Descon Formation that cannot be delineated exactly in the heavily vegetated cover. The deposit at the Cheri prospects consists of system of radioactive andesite(?) dikes that strike about N45W and can be traced for at least 3,000 feet along strike (Warner and Barker, 1989). The zone of dikes is about 100 feet wide; it consists of several parallel dikes, 0.8 to 1.3 feet thick. The dikes are fine grained and siliceous. At one location, pyroxene-rich masses with minor pyrite, rare fluorite, and secondary uranium minerals, occur in a dike. Chlorite, magnetite, pyrite, and epidote commonly occur at the margins of the dikes. Allanite is the principal radioactive mineral and the mineralogy is probably similar to that at the nearby Carol Ann/Dotson prospects (DE027), where similar dikes occur. The radioactive minerals are mainly concentrated in quartz veinlets or along mineralized fractures. At the Cheri No. 1 prospect, allanite in the interstices of quartzite is accompanied by abundant hematite and less-abundant albite, quartz, calcite, epidote, and chlorite. Warner and Barker (1989) estimate an indicated resource of 73,000 tons of rock in two portions of the Cheri dike system that contain 91,000 pounds of columbium, 32,000 pounds of thorium, 109,000 pounds of zirconium, 13,000 pounds of beryllium, 15,000 pounds of uranium, and 349,000 pounds of REE. There is an additional indicated resource of 458,000 short tons of rock in another portion of the dike system that contains 1,122,000 pounds of columbium, 338,000 pounds of thorium, 1,602,000 pounds of yttrium, 4,397,000 pounds of zirconium, 101,000 pounds of beryllium, 153,000 pounds of uranium, and 3,593,000 pounds of REE.
Workings: Only a few prospect pits.
Age: Genetically related to the Jurassic, Bokan Mountain peralkaline granite.
Alteration: These prospects and the other uranium, thorium, and REE deposits associated with the Bokan Mountain peralkaline granite are marked by albitization, chloritization, and argillization. Minor calcite, fluorite, quartz, sulfide minerals, and tourmaline are common in the altered rocks and hematite often occurs in the periphery of high-grade ore zones.
Reserves: Warner and Barker (1989) estimate an indicated resource of 73,000 tons of rock in two portions of the Cheri dike system that contain 91,000 pounds of columbium, 32,000 pounds of thorium, 109,000 pounds of zirconium, 13,000 pounds of beryllium, 15,000 pounds of uranium, and 349,000 pounds of REE. There is an additional indicated resource of 458,000 short tons of rock in another portion of the dike system that contains 1,122,000 pounds of columbium, 338,000 pounds of thorium, 1,602,000 pounds of yttrium, 4,397,000 pounds of zirconium, 101,000 pounds of beryllium, 153,000 pounds of uranium, and 3,593,000 pounds of REE.
Commodities (Major) - Be, Cb, REE, Th, U, Y, Zr
Deposit Model: U-Th-REE deposit in dikes associated with peralkaline granite.
6 entries listed. 5 valid minerals.
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Armstrong, R. L., 1985, Rb-Sr dating of the Bokan Mountain granite complex and its country rocks: Canadian Journal of Earth Sciences, v. 22, p. 1233-1236. Cobb, E. H., 1978, Summary of references to mineral occurrences (other than mineral fuels and construction materials) in the Dixon Entrance quadrangle, Alaska: U.S. Geological Survey Open-File Report 78-863, 34 p. Collett, B., 1981, Le granite albitique hyperalcalin de Bokan Mountain, S.E. Alaska et ses mineralisations U-Th. Sa place dans la cordillere canadienne: Doct. 3 degree cycle theseis, Montpellier II University, Montpellier, France, 238 p. Denny, R. L., 1962, Operations at the Ross-Adams uranium deposit, Dixon Entrance quadrangle, in Williams, J.A., Report of the Division of Mines and Minerals for the year 1962: Alaska Division of Geological & Geophysical Surveys, Annual Report 1962, p. 89-93. Freeman, V.L., 1963, Examination of uranium prospects, 1956, in Contributions to economic geology of Alaska: U.S. Geological Survey Bulletin 1155, p. 29-33. Gehrels, G. E., 1992, Geologic map of southern Prince of Wales Island, southeastern Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-2169, 23 p., 1 sheet, scale 1:63,360. Lanphere, M. A., MacKevett, E. M., and Stern, T. W., 1964, Potassium-argon and lead-alpha ages of plutonic rocks, Bokan Mountain area, Alaska: Science, v. 145, p. 705-707. Maas, K.M., Bittenbender, P E., and Still, J.C., 1995, Mineral investigations in the Ketchikan mining district, southeastern Alaska: U.S. Bureau of Mines Open-File Report 11-95, 606 p. MacKevett, E.M., Jr., 1963, Geology and ore deposits of the Bokan Mountain uranium-thorium area, southeastern Alaska: U.S. Geological Survey Bulletin 1154, 125 p. Matzko, J.J., and Freeman, V.L., 1963 Summary of reconnaissance for Uranium in Alaska, 1955: U.S. Geological Survey Bulletin 1155, p. 33-49. Philpotts, J.A., Taylor, C.D., and Baedecker, P.A., 1996, Rare-earth enrichment at Bokan Mountain, southeast Alaska, in Moore, T.E. and Dumoulin, J.A., eds., Geologic studies in Alaska by the U.S. Geological Survey, 1994: U. S. Geological Survey Bulletin 2152, p. 89-100. Saint-Andre, Bruno de, Lancelot, J. R., and Collot, Bernard, 1983, U-Pb geochronology of the Bokan Mountain peralkaline granite, southeastern Alaska: Canadian Journal of Earth Sciences, v. 20, p. 236-245. Staatz, M. H., 1978, I and L uranium and thorium vein system, Bokan Mountain, southeastern Alaska: Economic Geology, v.73, p. 512-523. Thompson, T. B., 1988, Geology and uranium-thorium mineral deposits of the Bokan Mountain granite complex, southeastern Alaska: Fluid Inclusion Research, v. 21, p. 193-210. Thompson, T.B., 1988, Geology and uranium-thorium mineral deposits of the Bokan Mountain granite complex, southeastern Alaska, in Gabelman, J. W., ed., Unconventional uranium deposits: Ore Geology Reviews, v. 3, p 193-210. Thompson, T.B., 1997, Uranium, thorium, and rare metal deposits of Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral deposits of Alaska: Economic Geology Monograph 9, p. 466-482. Thompson, T. B., Lyttle, Thomas, and Pierson, J. R., 1980, Genesis of the Bokan Mountain, Alaska, uranium-thorium deposit: U.S.Department of Energy, Bendix Field Engineering Report GJBX-38(80), 232 p. Thompson, T. B., Pierson, J. R., and Lyttle, T., 1982, Petrology and petrogenesis of the Bokan granite complex, southeastern Alaska: Geological Society of America Bulletin, v. 93, p. 898-908. Warner, J. D., and Barker, J. C., 1989, Columbium- and rare-earth-element-bearing deposits at Bokan Mountain, southeast Alaska: U.S. Bureau of Mines Open-File Report 33-89, 196 p.