Abstract

More than 340 million pounds (lbs) of U₃O₈ have been produced from the Grants uranium deposits in New Mexico between 1948 and 2002, and at least 403 million lbs of U₃O₈ remain as unmined resources. The Grants district is one of the largest uranium provinces in the world. The Grants district extends from east of Laguna to west of Gallup in the San Juan Basin of New Mexico.

Three types of sandstone uranium deposits are recognized: tabular, redistributed (roll-front, fault-related), and remnant-primary. The tabular deposits formed during the Jurassic Westwater Canyon time. Subsequently, oxidizing solutions moved downdip, modifying tabular deposits into redistributed roll-front and fault-related deposits. Evidence, including age dates and geochemistry of the uranium deposits, suggests that redistributed deposits could have been formed shortly after deposition in the early Cretaceous and from a second oxidation front during the mid-Tertiary.

The source of uranium is important in understanding how the Grants deposits formed. Two possible sources exist: 1) the Zuni Mountains, which lie south of the district and consist of a Proterozoic granitic highland enriched in uranium with as much as 11 parts per million, and with high heat flow; and 2) volcanic rocks erupted from a Jurassic arc volcanism, which formed southwest of the San Juan Basin, and deposited ash over much of the region. Uranium was likely leached from the Jurassic volcanic rocks, Jurassic ash, and the Precambrian granites; these leaching waters then migrated into the San Juan Basin. Leaching waters then mixed with pore water containing uranium that was leached from the detrital volcanic ash in the host sediments. The uraniferous groundwater migrated into the Westwater Canyon sandstones and precipitated in the vicinity of humate and other organic material to form the tabular uranium deposits. The recognition that there are different sources of uranium and different mechanisms of uranium deposition, aids in understanding the complexity and local variations within the tabular deposits. These deposit characteristics had a major impact on the remobilization and redistribution of uranium to form the redistributed deposits.

Although several companies continue to be active in the district, future resource development will depend upon lowering production costs, perhaps by in situ recovery techniques, and the resolution of regulatory issues.