Uranium in Wheeler Basin, Grand County, Colorado

Two kinds of radioactive anomalies are found in Wheeler Basin, both of which consist of biotite concentrations in Precambrian rocks, but the ones in migmatized biotite gneiss contain uraninite and the ones in Silver Plume Granite probably do not. At least 18 new uranium occurrences were found, most of which are less than a square meter. These discoveries enlarge the uraniferous area reported by Young and Hauff in 1975. Uranium in these biotite concentrations occurs in several modes: as uraninite grains; in accessory minerals, such as zircon; in fractures in plagioclase; and along grain boundaries and in cleavage openings in mica. Uranium mineralogy in the fractures, grain boundaries, and micas is not known. Yellow, secondary uranium minerals are seen locally on outcrop. Relative to crustal abundance, the radioactive biotite concentrations in migmatized biotite gneiss show depletion in Ca, Sr, Na, and, locally, Cu, but pronounced enrichment in U and Mo, and moderate enrichment in Pb, Ag, Th, and REE. The radioactive biotite concentrations in the Silver Plume Granite show pronounced enrichment in Th, and moderate enrichment in U, Sn, Zr, and Ag. Enrichment in light REE predominates over heavy REE. As U is more abundant in biotite concentrations in migmatized biotite gneiss than in biotite concentrations in Silver Plume Granite, I have concluded that U in the migmatized biotite gneiss was present before intrusion of the Silver Plume Granite, and that metamorphic effects of the Silver Plume intrusion remobilized U to form pockets of enrichment (biotite concentrations).