Exploration for volcanogenic uranium deposits in the United States is a relatively new endeavor, but it is not new in other countries in the world. The Soviet Union, Italy, Yugoslavia, Canada, Brazil, and Mexico have substantial reserves in volcanic host environments. Few volcanogenic uranium deposits have been discovered in the United States, but they may be indicators of larger, more favorable environments.

Volcanogenic systems that evolve from mantle-derived, hydrous, alkaline, magmas that can be enriched in Li, Be, Mo, and Hg, are most likely to develop favorable host environments. Rocks that develop in these host environments exhibit regional enrichment of uranium and thorium. The tectonic settings of these systems are extensional and are related to rift systems and transverse zones in the Basin and Range province of the western United States.

Uranium mineralization processes active in the volcanogenic system produce deposits in both high-temperature and low-temperature regimes. High-temperature deposits form from pneumatolytic, magmatic-hydrothermal, and meteoric-hydrothermal processes; the deposits may be fumaroles, breccia pipes, brecciated margins of intrusives, fractured and porous country rock, and porous zones along ash-flow contacts.

Diagenetic and reduction-oxidation processes work in conjunction to form low-temperature deposits, commonly in sedimentary environments where oxidizing ground-water flow can introduce labile uranium into reducing environments.

Sierra Pena Blanca, Chihuahua, Mexico, is a preserved part of an alkaline volcanic complex that formed in a rift system active in late Eocene through early Oligocene time. Although detailed mapping is still underway, the number of uranium and thorium enriched alkaline and peralkaline ash-flows suggests several eruptive centers nearby. Enrichment of other metals in these ash-flow sheets suggests regional enrichment of lithophilic elements. Sierra Pena Blanca has pneumatolytic uranium deposits in fumarolic ash flows, magmatic and meteoric hydrothermal deposits in interbedded ash flows and invaded country rock, and low-temperature deposits forming in closed basins adjacent to the volcanic complex.

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