Relation of Mercury, Uranium, and Lithium Deposits to the McDermitt Caldera Complex, Nevada-Oregon

Abstract

The McDermitt caldera complex, located along the Nevada-Oregon border, is a Miocene collapse structure 45 kilometer in diameter. Large-volume rhyolitic and peralkaline ash-flow tuffs were erupted from 17.9 to 15.8 m.y. ago, leading to the formation of overlapping and nested calderas. Emplacement of rhyolitic ring domes, located primarily along the western margin of the calderas, represents the last phase of volcanic activity. The complex is the site of large deposits of mercury, an are deposit and several occurrences of uranium, and widespread occurrences of lithium. Mercury deposits at Cordero, McDermitt, Bretz, Ruja, and Opalite occur along ring fractures in sedimentary rocks that fill the calderas. Near the deposits the rocks are altered to zeolites and within the ore zones to potassium feldspar and silicate minerals. Although the mercury deposits contain anomalous concentrations of uranium, the most important uranium occurrences are restricted to rhyolitic ring domes emplaced along the western margin of the calderas. Lithium occurrences are located in tuffaceous rocks that are altered to zeolites and potassium feldspar. Concentrations of lithium ranging from 0.1 to 0.68 percent are associated with the clay mineral hectorite. The rhyolitic rocks erupted from the McDermitt caldera complex are enriched in mercury, uranium, and lithium and are likely source rocks for these elements in the ore deposits. Tuffaceous caldera fill-sediment averages 0.29 ppm mercury, 22 ppm uranium, and 236 ppm lithium; large-volume ash-flow tuffs contain up to 0.26 ppm mercury, 20 ppm uranium, and 300 ppm lithium.