Geology, geochemistry, and origin of volcanic rock-hosted uranium deposits in northwestern Nevada and southeastern Oregon, USA

Northwestern Nevada and southeastern Oregon have the largest uranium (U) deposits in Tertiary volcanic rocks in the US. Most deposits are in or adjacent to calderas or rhyolite lava-dome fields and are hydrothermal. Almost all are associated with rhyolites that have high primary U concentrations (9 to 20 ppm), but the rhyolites range from peralkaline to peraluminous. Caldera-related occurrences are hosted by peralkaline rocks in the McDermitt and Virgin Valley calderas. At McDermitt, major deposits are along or just inboard of the southwestern (Kings River) and northern ring fractures (Bretz-Opalite) of the caldera complex. The Kings River deposits are strongly enriched in trace elements associated with epithermal precious-metal deposits (e.g., As, Au, Ag, Mo, and Sb). Much of the U occurs as hydrothermal uraniferous zircon, and whole rock samples contain as much as 5% Zr. Gangue minerals include quartz, adularia, fluorite, and numerous sulfides. 40Ar/39Ar dating of adjacent volcanic rocks and adularia indicates that mineralization was contemporaneous with igneous activity. The Bretz-Opalite deposits, which include the large Aurora resource (7.5 x 10(6) kg U3O8), are less enriched in epithermal trace elements and contain uraninite, coffinite, chalcedony, opal, and numerous sulfides. At Virgin Valley, deposits are mostly in tuffaceous sediments adjacent to ring-fracture rhyolites. Trace-element enrichment is minor, U is hexavalent in all identified U minerals, and associated phases include opal, organic material, and pyrite. Deposits in the Lakeview U district, Oregon are hosted by weakly peraluminous rhyolite domes and tuffs and are highly enriched in epithermal trace elements; the mineral assemblage includes coffinite, opal, and numerous sulfides. Minor U occurrences at Buff Peak, Nevada, the newly recognized, westernmost topaz (peraluminous) rhyolite in the US, are also trace element enriched; gangue minerals include quartz, adularia, and sulfides. On the basis of their geologic setting, geochemistry, age, and mineralogy, the McDermitt, Lakeview, and Buff Peak deposits are hydrothermal; sparse, published fluid inclusion data indicate temperatures between 200 degrees C and 330 degrees C. Virgin Valley deposits most likely are hydrothermal. Minor deposits in outflow ash-flow tuffs are distant from any intrusive heat source and probably formed from low-temperature groundwater. (C) 2000 Elsevier Science B.V. All rights reserved.