Evolution of Genetic Concepts for Principal Types of Sandstone Uranium Deposits in the United States
Authors: Adams Samuel S. 
Published: November 2019

Genetic concepts for sandstone uranium deposits have evolved significantly over the last four decades, due in large part to higher uranium demand, government agency research programs, increased exploration and mining, and discovery of numerous deposits and districts. Genetic concepts benefited more from attention to geologic details of new deposits and their surroundings than from models imported from other mineral districts. Four sandstone uranium deposit types comprise the bulk of unexploited uranium reserves in the United States. In order of decreasing reserves, these are epigenetic humate, roll, epigenetic sulfide, and vanadium-uranium deposits. These deposit types are named for distinguishing geologic characteristics that set them apart from other deposit types and that are important in exploration.

Sandstone uranium deposits are epigenetic impregnations of uranium and other minerals and materials in the matrix of continental sandstones. Deposits are considered to have formed from formation waters, principally during sediment deposition, compaction, and diagenesis. Uranium was most likely derived from oxidized tuffaceous sediments, dissolved as bicarbonate complexes, transported by compaction and gravity flow, and precipitated through reduction by carbonaceous matter or sulfide.

Genetic concepts for vanadium-uranium deposits of the Colorado Plateau have evolved from magmatic hydrothermal to precipitation by mixing at the interface between dissimilar ground waters. One water was probably oxidizing and uranium bearing, the other reducing. In the Henry Mountains district, Utah, there is evidence that one of the waters was a brine. Roll deposits, such as occur in Wyoming Tertiary basins, are considered to have formed by the flow of oxidizing, uranium-bearing ground water into reduced sandstone and the precipitation of uranium at an oxidation-reduction boundary. Genetic concepts for epigenetic humate deposits of the Grants uranium region, northwest New Mexico, have evolved from uranium precipitation in a dead oil field to expulsion of alkaline humate-bearing fluids from tuffaceous lacustrine sediments and accumulation of humate lenses and uranium in adjacent sandstones. Genesis of epigenetic sulfide deposits, such as occur in south Texas, is now generally believed to have been due to sulfidation of host sandstones by fault-controlled brines from deep sour gas reservoirs, followed by uranium precipitation at advancing oxidation-reduction roll fronts.

Improved genetic concepts for sandstone-hosted uranium deposits will result from studies of fluvial and lacustrine tuffaceous sediments, their capacity to serve as sources of metal deposits, and conditions favorable for release of metals and organics. The roles of organisms and meta-stable species in ore-forming processes are inadequately understood. Detailed mapping within some deposits and, in particular, geologic studies outside and around deposits will identify ore-related and ore-controlling features, such as rock alteration and fluid pathways, for improved genetic concepts and exploration. This in turn will support comprehensive genetic models that consider source, transport, and accumulation, the ultimate objectives of mineral deposit studies and the ultimate tools for exploration.