West and White (1962) report that zeunerite occurs near the surface of a granite stock on the southwest flank of Brooks Mountain, Alaska. The largest deposit is at the Foggy Day prospect. Zeunerite is disseminated in hematite which partially or totally fills openings and vugs in a highly oxidized lens-shaped body of pegmatitic granite and, to a minor extent, in openings and cracks m the weathered granite enclosing the lens. Although a few specimens from the pegmatitic lens contain as high as 2.1 percent equivalent uranium, the average content of the lens rock is between 0.1 and 0. 2 per cent equivalent uranium and that of both the lens material and the surrounding zeunerite-bearing granite is about 0.07 percent equivalent uranium.
A smaller concentration of zeunerite occurs as surface coatings on a few of the quartz-tourmaline veins that occupy joint fractures in granite on Tourmaline No. 2 claim. The vein material here contains about 0.05 percent equivalent uranium. Zeunerite, in trace amounts, was identified in a sample from a site near Tourmaline No. 2 claim and in two samples from other sites near the Foggy Day prospect. The zeunerite at these three localities is probably related in source to the Tourmaline No. 2 claim and Foggy Day prospect deposits. Although no primary uranium minerals were found, it is possible that a primary mineral zone may occur below the zone of oxidation at the Foggy Day prospect.
Brooks Mountain is composed of igneous, sedimentary, and metamorphic rocks, consisting of·a granite mass and felsic and mafic dikes of Mesozoic(?) age, which intrude a formation of black slate of Cambrian or pre-Cambrian age and the Port Clarence limestone of Ordovician, Silurian, and Devonian age.Uranium in the form of zeunerite, a hydrous copper-uranium arsenate, was found concentrated at two localities and in trace amounts at three other places on Brooks Mountain. Other radioactive minerals are also present in the granite. As preliminary uranium analyses of samples from the zeunerite occurrences at Brooks Mountain closely paralleled the equivalent uranium analyses on the same samples it was believed that in the present investigation analyses for equivalent uranium would be sufficient to indicate the amount of radioactive material present.
Zeunerite is the major uranium-bearing mineral at the surface in the Brooks Mountain area. Traces of uranium occur as an impurity in some of the fluorite, black tourmaline, smoky quartz, hematite, limonite, siderite, malachite, azurite, tetrahedrite, arsenopyrite, biotite, muscovite, sericite, and a secondary bismuth mineral. Zircon, monazite, and xenotime are also radioactive, but the radioactivity of these minerals is probably due mainly to thorium. The principal points of uranium mineralization are at two localities on Brooks Mountain, although trace amounts of zeunerite have been found at three other places where the geological structure is probably related to that at the two main zeunerite deposits.
Numerous other sites, geologically similar to the two main zeunerite-bearing sites, do not contain zeunerite. Consequently, most of the quartz-tourmaline veins on the mountain contain only a very small amount of radioactive material, and no other red oxidized zones in the granite are more than slightly radioactive. The marked restriction of' principal zeunerite mineralization to the two localities indicates that uranium may have been derived from one or possibly two local primary sources within the granite.
Radiometric examination of the granite mass and laboratory studies of the granite samples have so far failed to disclose the presence of a primary uranium mineral from which the uranium in the zeunerite could have been derived.
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