I and L

Prospects, Active

Alternative names

I and L Nos. 3-5

Commodities and mineralogy

Geographic location

Quadrangle map, 1:250,000-scale DE
Quadrangle map, 1:63,360-scale D-1
Latitude 54.913
Longitude -132.1345
Nearby scientific data Find additional scientific data near this location
Location and accuracy The I & L Nos. 3-5 prospects cover an area about 1,000 feet in diameter centered about 0.9 mile east-southeast of Bokan Mountain. The center of the area is just north of the midpoint of the south boundary of section 22, T. 80 S., R. 88 E, of the Copper River Meridian. As known in 2011, it is at the northwest end of the prospects along the Dotson dike and its persistent mineralization. The location of these prospects relative to the other uranium and REE prospects in the vicinity of Bokan Mountain is best shown on Plate 1 of MacKevett (1963). The location is accurate, though the degree of accuracy is not reported.

Geologic setting

Geologic description

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 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 rare-earth-element (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.
In 2007 and 2008, Ucore Uranium drilled 27 holes on the I&L vein system (Ucore Uranium, 2010). All the holes intersected uranium mineralization. Some of the best values were a 50.24-meter intercept with 0.47 percent U308, including a 3.90-meter intercept with 2.43 percent U3O8; a 46.72-meter intercept with 0.16 percent U3O8; a 5.11-meter intercept with 0.46 percent U308; a 7.56 meter intercept with 0.59 percent U3O8; and a 15.23-meter intercept with 0.42 percent U3O8. In conjunction with the drilling, an airborne geophysical survey was flown over the Bokan Mountain area in 2007 and channel samples were cut across the I & L vein system to guide the drilling. The drilling and channel sampling indicated that the uranium mineralization extends along strike for at least 100 meters, to a depth of at least 35 meters, and is open both along strike and dip (Ucore Uranium, 2008).
In 2010, Ucore drilled 20 more holes, including 13 infill holes along the Dodson Dike at roughly regular intervals from this prospect, now considered to be the northwest end of the dike, through the Atom Marietta prospect (DE024), to the Carol Ann prospect (DE027) (Ucore, 2010 [News release]; Ucore, 2011). They also dug 45 trenches across the dike. As known in early 2011, the Dodson dike has a strike length of at least 2,180 meters, averages 50 meters wide, extends from 300 meters above sea level to at least 150 meters below sea level, has remarkably persistent mineralization. and it is open along strike and down dip.
With this additional drilling on the Dodson dike, a new and considerably larger 'Conceptual Estimate' was made for the dike. The estimate was made using a model of the dike 2,425 meters long and 200 meters deep. The new figures are an 'Estimated contained mineralized tonnage' of from 3.5 to 6.5 million tonnes, with an 'Estimated grade TREO' of from 0.76 to 1.42 percent. (TREO = total rare earth elements.)
Geologic map unit (-132.136141813239, 54.9126582177153)
Mineral deposit model U-Th-REE deposit related to a peralkaline granite stock.
Age of mineralization Genetically related to the Jurassic, Bokan Mountain peralkaline granite (source not cited by previous reporter).
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. Intense argillic alteration is common along the contacts of the dikes, and the better mineralization is generally associated with faults (source not cited by previous reporter).

Production and reserves

Workings or exploration
The prospects have been explored by numerous pits and trenches, and in 1977 were drilled to a depth of 260 feet. In 2007 and 2008, Ucore Uranium drilled 23 holes, cut several channel samples across the vein system, and completed an airborne geophysical survey over the area. In 2010, Ucore drilled 13 infill holes along the Dodson Dike at roughly regular intervals from the I and L prospect which was considered to be the northwest end of the dike, through the Atom Marietta prospect (DE024) to the Carol Ann prospect (DE027) (Ucore, 2010 [News release]; Ucore, 2011). They also dug 45 trenches across the dike. As known in early 2011 through systematic drilling, the Dodson dike has a strike length of at least 2,180 meters, averages 50 meters wide, extends from 300 meters above sea level to at least 150 meters below sea level, and it is open along strike and down dip.
In 2009 and 2010, Ucore conducted airborne total magnetic field and radiometric surveys in order to map radiogenic rocks in order to directly locate uranium and rare earth mineralization with the radiometric survey, and to map structure and stratigraphy indirectly with the total magnetic field survey. In 2009, Ground total magnetic field and radiometric surveys were performed to map these zones prior to drilling. An induced polarization (IP) survey was run to attempt to discern controls on mineralization intersected in drillholes at this site. In 2010, 3,214 m of core were drilled in 13 holes (Bentzen and others, 2013). This exploration work from 2008-2010 led to the 2013 Preliminary Economic Assessment (Bentzen and others, 2013) and a 2013 updated resource estimate (Ucore Rare Metals Inc., 2013).
Although no exploration work was conducted in 2016, Ucore Rare Metals Inc. conducted mineral-separation tests on ore from their Bokan-Dotson Ridge rare-earth-element (REE) project in Southeast Alaska. The first batch of pregnant leach solution (PLS) derived from Bokan ore was treated by the SuperLig®-One Molecular Recognition Technology plant at IBC Advanced Technologies, Inc.’s Utah facility. Successful separations include: 1) separation of REE from gangue metals in the PLS; 2) greater than 99 percent purity of separation of light REE from heavy REE at greater than 99 percent recovery; 3) greater than 99 percent purity of separation and recovery of the sub-groups samarium-dysprosium (Dy sub-group) and holmium-lutetium (Ho sub-group) from the heavy REE class consisting of samarium and lutetium; and 4) greater than 99 percent separation of dysprosium from the Dy sub-group (Athey and Werdon, 2017).
Indication of production None
Reserve estimates
With the partial results of the 2010 infill drilling, a new and considerably larger 'Conceptual Estimate' was made for the Dodson dike. The estimate was made using a model of a dike 2,425 meters long and 200 meters deep. The new figures are an 'Estimated contained mineralized tonnage' of from 3.5 to 6.5 million tonnes, and an 'Estimated grade TREO' of from 0.76 to 1.42 percent. (TREO = total rare earth elements.)
In October 2013, an upgraded resource estimate was reported combining I and L, Carol Ann (DE027) and Atom Marietta (DE024). The estimate was made using a model via five alternative total rare earth oxide (TREO) cut-off grades with a baseline case employing a TREO cut-off of 0.4 percent. The deposit has indicated resources of 2,936,000 tonnes containing 39,731,596 pounds of TREO composed of 0.365 percent LREO, 0.249 percent HREO, and 0.614 percent TREO with a TREO cut-off of 0.4, and an additional inferred resources of 1,995,000 tonnes containing 26,601,729 pounds of TREO composed of 0.366 percent LREO, 0.239 percent HREO, and 0.605 percent TREO with a TREO cutoff of 0.40 (Ucore Rare Minerals Inc., 2013).
A 2013 preliminary economic assessment for Ucore Rare Metals’ Bokan property states an inferred resource of 5.228 million tonnes grading 0.653 percent total rare-earth oxide (Bentzen and others, 2013); it includes the Dotson (DE027), and I-and-L zones (DE023).

References

MRDS Number A010272

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.
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.
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 (Contractor, USGS); V.C. Zinno (Alaska Earth Sciences, Inc.); M.B. Werdon (DGGS)
Last report date 8/26/2017