I,L, and M

Prospect, Probably inactive

Alternative names

ILM
I, L, and M Nos. 1-3

Commodities and mineralogy

Main commodities Cb; REE; Th; Y; U; Zr
Ore minerals allanite; arsenopyrite; columbite-tantalite; ree minerals; uranothorite; uranothorianite; zircon
Gangue minerals albite; quartz

Geographic location

Quadrangle map, 1:250,000-scale DE
Quadrangle map, 1:63,360-scale D-1
Latitude 54.9157
Longitude -132.1488
Nearby scientific data Find additional scientific data near this location
Location and accuracy The three I, L, and M prospects are centered about 0.3 mile east of Bokan Mountain near the midpoint of the east boundary of the SE1/4 section 21, T. 80 S., R. 88 E. The prospects are aligned southeasterly over a distance of about 1,000 feet. Their location relative to the other uranium and REE prospects in the vicinity of Bokan Mountain is best shown on Plate 1 of MacKevett (1963).

Geologic setting

Geologic description

This and several other nearby uranium-thorium-REE deposits (DE015 to DE020 and DE022 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 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 I,L, and M prospects consist of a few shallow pits and trenches on 3 claims located in 1955 (MacKevett, 1963). The prospects are well within the Bokan Mountain peralkaline granite, which here is strongly jointed and cut by pegmatite dikes. The No. 1 prospect is on an altered zone about 30 feet wide and 200 feet long. The Nos. 2 and 3 prospects are associated with pegmatite dikes and irregular masses of pegmatite. All of the deposits are only weakly to moderately radioactive. Little has been done with their mineralogy. But assuming that it is similar to other deposits in the area, the major ore minerals are probably uranothorite, columbite-tantalite, zircon, and a variety of REE minerals. Arsenopyrite, allanite, and fluorite also occur locally.
Warner and Barker (1989) defined an ore zone about 1,500 feet long that contains about 586,000 short tons of rock. The ore zone contains 1,054,000 pounds of columbium, 115,000 pounds of uranium; 732,000 pounds of yttrium, 20,200,000 pounds, of zirconium, and 2,749,000 pounds of REE. The average grade of this zone is 1,230 parts per million (ppm) columbium, 3,000 ppm REE, 140 ppm uranium, 650 ppm yttrium, and 2.47 percent zirconium.
Geologic map unit (-132.150442487327, 54.9153582566931)
Mineral deposit model U-Th-REE deposit associated with peralkaline granite.
Age of mineralization Genetically related to the Jurassic, Bokan Mountain peralkaline granite.
Alteration of deposit 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.

Production and reserves

Workings or exploration Only a few small prospect pits.
Indication of production None
Reserve estimates Warner and Barker (1989) defined an ore zone about 1,500 feet long that contains about 586,000 short tons of rock. The ore zone contains 1,054,000 pounds of columbium, 115,000 pounds of uranium; 732,000 pounds of yttrium, 20,200,000 pounds, of zirconium, and 2,749,000 pounds of REE. The average grade of this zone is 1,230 parts per million (ppm) columbium, 3,000 ppm REE, 140 ppm uranium, 650 ppm yttrium, and 2.47 percent zirconium.

References

MRDS Number A010274; A010275

References

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.
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.
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.
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.
Reporters D.J. Grybeck (Applied Geology)
Last report date 9/1/2003