I and L; I and L Nos. 3-5 Prospects, Bokan Mountain, 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 DE022 and DE024 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. As described by MacKevett (1963) and Warner and Barker (1989), the I and L prospects are on six principal and many smaller northwest-trending, quartz-cored, pegmatite dikes that cut riebeckite granite porphyry and aegirine granite of the Bokan Mountain peralkaline stock near its southeast border. Transverse pegmatite dikes are also present. The dikes contain scattered concentrations and discrete grains of uranium, thorium, and REE minerals. The largest and northernmost dike--the No. 1 dike--is at least 900 feet long and up to 18 feet thick. The other dikes are parallel, vary from 100 to 500 feet in length, and are a few inches to 6 feet thick. The dikes are generally vertical but drilling on the No. 1 dike indicates that it is irregular in width and shape, and bends markedly at depth. Intense argillic alteration is common along the contacts of the dikes, and the better mineralization is generally associated with faults. In a study of the mineralogy of the IL & M dikes, Staatz (1978) found that the uranium is generally in thorium-bearing uraninite, whereas brannerite predominates in some transverse dikes. Thorite is the principal thorium mineral in the northwest and central parts of the prospect area; allanite predominates in the southeastern part of the area and in transverse dikes. Rare earth minerals include bastnaesite, xenotime, monazite and an unidentified fluorocarbonate. The distribution of the individual rare earth minerals varies markedly. One part of a dike may contain predominantly cerium-group minerals, for example bastnaesite; another part may contain predominantly yttrium-bearing minerals, for example xenotime. Small amounts of sulfides including galena, sphalerite, and pyrite are common in many of the dikes. Zircon and fluorite are commonly present in minor amounts and two samples contained the beryllium mineral phenacite. The gangue is mostly quartz and albite. Scanning electron microscope study by Warner and Barker (1989) shows that the REE minerals are mainly thalenite, bastnaesite, and allanite; tengerite, parisite, synchysite, an unnamed REE flurocarbonate mineral, monazite, and xenotime also occur. The columbium-bearing mineral is mainly euxenite-polycrase, although columbite-tantalite, samarskite, fergusonite, and aeschynite are present. The main radioactive mineral is thorite but uranothorite is also present. The dikes have an indicated resource of 100,000 short tons of rock that contain 181,000 pounds of columbium, 41,000 pounds of thorium, and 34,000 pounds of uranium. There is an additional inferred resource of 73,000 pounds of columbium (Warner and Barker, 1989).
Workings: The prospects have been explored by numerous pits and trenches, and in 1977 were drilled to a depth of 260 feet.
Age: Genetically related to the Jurassic, Bokan Mountain peralkaline granite.
Alteration: These prospects prospect 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: The dikes have an indicated resource of 100,000 short tons of rock that contain 181,000 pounds of columbium, 41,000 pounds of thorium, and 34,000 pounds of uranium. There is an additional inferred resource of 73,000 pounds of columbium (Warner and Barker, 1989).
Commodities (Major) - Cb, Th, U; (Minor) - Ce, Dy, Er, F, Gd, Ho, La, Nd, Pb, Y, Yb, Zn
Development Status: None
Deposit Model: U-th-REE deposit related to a peralkaline granite.
21 entries listed. 12 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.