Abstract

The I and L veins are noteworthy because they contain significant amounts of associated uranium, thorium, and rare earths--an uncommon combination --and because of the distribution of their rare earths. The one to at least nine steep parallel to subparallel veins that form the main vein system strike northwestward and extend for 1.6 miles (2.6 km) from the east side of Bokan Mountain to the West Arm of Kendrick Bay. Typically the veins are thin, rarely exceeding 5 feet (1.5 m) in thickness, and they are characterized by abrupt changes in thickness. They transect parts of the Upper Triassic-Lower Jurassic Bokan Mountain granite throughout their northwestern extents and Ordovician granitic rocks to the southeast. The I and L system is bounded on the north and south by several well-defined fractures that are clearly visible on aerial photographs. Several small transverse veins, which strike almost at right angles to the main group of veins, are exposed in a small area north of the main vein system.The mineralogy of the veins is complex; uranium, thorium, and the rare earths occur in several minerals from different parts of the veins. A total of 34 minerals has been recognized, although many are sporadically or locally distributed. The principal gangue minerals are quartz and albite. Uranium generally occurs in thorium-bearing uraninite, but in several of the transverse veins it is found in brannerite. Thorite is the principal thorium mineral in the northwest and central part of the vein system, whereas allanite is found in the southeastern part of the system and in the transverse veins. In addition to allanite, the rare earth minerals include bastnaesite, xenotime, monazite, and an un-named mineral. The mineralogy of the I and L veins is similar in many respects to that of thorium-bearing veins in other states.The uranium content of 43 vein samples ranges from 0.005 to 2.8 percent; thorium content of 27 samples ranges from 0.0033 to more than 10 percent; and total rare earth content of 17 samples ranges from 0.023 to more than 10 percent. The distribution of the individual rare earths is unusual: one part of a vein may contain predominantly cerium-group rare earths (for example, bastnaesite), and another part of the same vein yields predominantly yttrium-group rare earths (for example, xenotime). The I and L veins also contain abnormal amounts of beryllium, niobium, zirconium, barium, strontium, tin, lead, zinc, copper, and molybdenum. Particularly interesting are beryllium, niobium, and zirconium. The distribution of beryllium is extremely erratic, and samples with high beryllium content, as much as 0.5 percent beryllium, occur adjacent to those that contained little or no beryllium. Abundance of niobium is greatest in the northwest part of the vein system, where 6 of 10 samples yielded from 0.10 to 0.70 percent. Zirconium is widespread in the veins, with only one out of 17 analyses yielding less than 0.15 percent and two more than 2 percent.

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