|Quadrangle map, 1:250,000-scale||DE|
|Quadrangle map, 1:63,360-scale||D-1|
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|Location and accuracy||The Geoduck occurrences are south of the West Arm of Kendrick Bay. They are related to a dike system that extends N40-50W for about 10,000 feet, nearly diagonally through the center of section 36, T. 80 S., R. 88 E. and into the NW1/4 of section 2, T. 81 S., R. 89 E. The site is approximately at the midpoint of the most continuous and thickest dike, where resource calculations have been made.|
This and several other nearby uranium-thorium-REE deposits (DE015 to DE030) 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 peralkaline 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.
The Geoduck occurrences are related to a system of equigranular, fine- to medium-grained, andesite dikes that strike N40-50W for about 10,000 feet, west of the head of Kendrick Bay (MacKevett, 1963; Warner and Barker, 1989). The dikes have steep to vertical dips, cut Silurian or Ordovician quartz monzonite, granite, and quartz diorite, and typically have wall rocks marked by chlorite and epidote alteration. The ore mineralogy has not been worked out in detail but it is probably similar to that in the mineralized dikes at the Carol Ann/Dotson prospects (DE027), which may be extensions of the Geoduck dikes. In the northwest part of the Geoduck dike system, individual dikes average about 1.4 feet thick; in the southeast part, they average about 0.8 foot thick. Many samples contain elevated values of beryllium, thorium, yttrium, REE, columbium, uranium, and zirconium.
In 2009, Ucore Uranium (2010) drilled two holes near the Geoduck prospect for rare-earth elements. The holes are along what they term the 'Geoduck trend' that is oriented northwest along the andesite dike. The analyses for the samples were reported as the light rare-earth-element oxides or LREO (lanthanum, cerium, praesodymium, neodymium, and samarium) and the heavy rare-earth-element oxides or HREO (europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, and yttrium); together they are termed the TREO, the total rare-earth-element oxides. The most notable intercept in the two holes was 0.4 meter with 3.4 percent TREO. The ratio of the the HREO to the TREO in the three best intercepts varied between 41.9 and 46.6 percent; in other words nearly half of the rare-earth elements in the samples are the heavy rare-earth elements.Warner and Barker (1989) estimate that a 3,000-foot section of the most continuous and thickest dike, which averages 1.5 feet thick, has an indicated resource of 1,378, 000 short tons of rock that contains 278,000 pounds of beryllium, 752,000 pounds of thorium, 8,116,000 pounds of yttrium, 8,786,000 pounds of REE, 2,844,000 pounds of columbium, 358,000 pounds of U, and 12,953,000 pounds of zirconium (Warner and Barker, 1989). There is a inferred reserve in 95,28,000 short tons of rock of 1,906,000 pounds of beryllium, 3,525,000 pounds of thorium, 29,975,000 pounds of yttrium, 14,864,000 pounds of columbium, 1,944,000 pounds of uranium, and 55,262,000 pounds of zirconium.
|Geologic map unit||(-132.086946896708, 54.8883564850384)|
|Mineral deposit model||U-Th-REE deposit in dikes associated with peralkaline granite.|
|Age of mineralization||Associated with dikes that are genetically related to the Jurassic, Bokan Mountain peralkaline granite stock.|
|Alteration of deposit||The dikes are probably albitized and chloritized; the wall rocks are marked by chlorite and epidote alteration.|
|Workings or exploration||Only surface sampling by government geologists to 2008. In 2009, Ucore Uranium drilled two holes.|
|Indication of production||None|
|Reserve estimates||Warner and Barker (1989) estimate that a 3,000-foot section of the most continuous and thickest dike, which averages 1.5 feet thick, has an indicated resource of 1,378, 000 short tons of rock that contains 278,000 pounds of beryllium, 752,000 pounds of thorium, 8,116,000 pounds of yttrium, 8,786,000 pounds of REE, 2,844,000 pounds of columbium, 358,000 pounds of U, and 12,953,000 pounds of zirconium (Warner and Barker, 1989). There is a inferred reserve in 95,28,000 short tons of rock of 1,906,000 pounds of beryllium, 3,525,000 pounds of thorium, 29,975,000 pounds of yttrium, 14,864,000 pounds of columbium, 1,944,000 pounds of uranium, and 55,262,000 pounds of zirconium.|
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.
Ucore Uranium, 2010, Bokan Mountain, Alaska: http://www.ucoreuranium.com/bokan.asp (as of February 22, 2010).
|Reporters||D.J. Grybeck (Contractor, U.S. Geological Survey)|
|Last report date||4/2/2010|