The adjoining Aurora and Bretz uranium prospects are along the northeastern ring-fracture system of the Miocene McDermitt caldera. A series of block faults, constituting the ring-fracture system, divides the area into two contrasting terranes. The northern terrane, comprising the caldera wall and outflow facies, includes a series of mafic to silicic lavas and rhyolite ash-flow tuffs (Bretz Series). Rocks of the southern terrane (Aurora Series) represent infilling of the caldera after

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collapse. They include an enormously thickened pile of rhyolite ash-flow tuff, equivalent to the youngest Bretz Series ash-flows, intruded and overlain by rhyolite flow domes and lavas. These rhyolites are overlain by a sequence of intermediate lava flows, flow breccia, and pyroclastic breccia constuting the Aurora Lavas. Tuffaceous lacustrine sediment covers most of the southern terrane and much of the ring-fracture system.

Hydrothermal alteration associated with mercury deposits at the old Bretz mine is clearly controlled by one of the ring faults, but uranium mineralization nearby is stratigraphically controlled and not directly associated with mercury. Uranium concentrations occur along several horizons, including (1) geologic contacts, unconformities, and redox boundaries in the Bretz Series; (2) a widespread horizon in the tuffaceous lake sediments; and (3) potentially commercial deposits along the flow boundaries and interflow breccias in the Aurora Lavas.

The control of uranium by solution channelways along stratigraphic boundaries and lack of crosscutting veins, except in local areas of the Aurora Lavas, suggests that supergene mechanisms were important in ore formation. However, a combination of hydrothermal and supergene processes is favored to explain all the features observed, with the uranium in the lacustrine sediments precipitated from surficial hot springs active during mineralization.

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