Abstract

Lisbon Valley is Utah’s largest uranium district. The district is located on the southwest flank of the Lisbon Valley anticline, one of several northwest trending salt anticlines in the Paradox Basin. The uranium ore deposits are in an arcuate belt, 16 miles long by one mile wide. The principal host rock is the lower member (Moss Back) of the Triassic Chinle Formation. Significant orebodies also occur in the underlying Permian Cutler Formation. Although small oxidized deposits occurred in the area, wildcat drilling by a single individual in 1952 discovered a major unoxidized deposit. The following surge of exploration drilling discovered most of the deposits by 1956.

The principal uranium ore mineral is uraninite. Significant amounts of vanadium occur in the ores, especially in the central and southeast parts of the belt. Calcite cement in the host rock prevented the oxidation of the deposits above the water table.

Annual ore production reached a record high in 1959, when over 6.6 million pounds U₃O₈ were produced, but production declined there after due to ore purchase restrictions by the U.S. Atomic Energy Commission (AEC). When the AEC program ended in 1970, Lisbon Valley had produced 53.6 million pounds U₃O_8. Most of the ore was processed at a mill in Moab, Utah.

Wildcat drilling in 1964 and 1965 on the northwestern nose of the anticline discovered ore on the down-dropped, northeast side of the Lisbon Valley fault. The Lisbon mine and mill began production from this 2,550-foot deep discovery in 1972. High uranium prices and a strong market in the mid-1970s were responsible for a second boom in the area, and several additional deposits were discovered including a major deposit in the Cutler Formation in the southwestern part of the area.

Declining prices and a weak market in the early 1980s forced many of the mines and the Moab mill to close in early 1984, and the Lisbon mine and mill to close in late 1988. During the 41 years, 1948-1988, that the uranium mines in Lisbon Valley were operating, a total of 12,778,698 tons of ore containing 77,913,499 pounds U₃O₈ were produced. Of this total, approximately eight percent came from the Cutler Formation. Should the uranium market improve, mines with reserves could reopen and the possibilities are good for additional discoveries in the Cutler.