Uranium Deposits in Wyoming

Published: 08th January 2010
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Precambrian rocks and in the nonconformably overlying Eocene Tepee Trail Formation. The uranium occurrence is subeconomic but of promising grade and size." He added, "The uranium is spatially related to fractures and subsidiary faults associated with the Laramide North Canning fault. Rocky Mountain Energy Company has conducted detailed drilling on the North Canning deposit."

Harris explained that mineralization occurs in the Precambrian granite and enclosed metasediments. The mineralization is said to be primarily low-temperature pitchblende and coffinite. Harris compared the North Canning deposit to nonconformity- related uranium deposits. He wrote, "It is likely that the deposit formed by processes similar to those that operated in the Athabasca and Northern Australian regions." We checked with David Miller of Strathmore Minerals (TSX: STM; Other OTC: STHJF) about their Copper Mountain holdings. He responded by email, "We own all the federal minerals in the area that covered uranium mineralization: about 75 percent of the gross uranium resources. The Canning Deposit is owned about 60 percent by us and 40 percent by Neutron. Strathmore Minerals has around 100 mining claims in the area."

The source of Wyoming's roll-front uranium deposits are open to debate and have yet to be clarified. In 1981, William Boberg wrote, "The major deposits of Wyoming occur in the Lower Cretaceous Inyan Kara Group of the Black Hills, in the Paleocene Fort Union Formation in the Powder River Basin, in correlative Eocene sandstones in all of the major uranium districts." Warren Finch later described Wyoming's roll-fronts, in his previously quoted work, "The predominant type of uranium deposit is the roll-front sandstone deposit in Tertiary continental fluvial basis developed between uplifts. These ore deposits were formed by oxidizing uranium-bearing ground waters that entered the host sandstone from the edges of the basins. Two possible sources of the uranium were (1) uraniferous Precambrian granite that provided sediment for the host sandstone and (2) overlying Oligocene volcanic ash sediments." Ray Harris appeared to lean more toward the former. William Boberg has argued more toward the latter explanation for a uranium source.

Boberg wrote, "It appears that currently available evidence is in support of a hypothesis calling for combined sources of Precambrian granites and volcanic ash falls which produce a unique, uranium-rich, ore-forming liquid that invades very porous and permeable young sediments to form large altered tongues and discrete deposits in a geologically short period of mineralization." It has been calculated that a typical altered "tongue" would take 700,000 years to form; a typical roll-front uranium deposit could be formed over 50,000 years.

Boberg speculated it was the numerous and extensive uranium-enriched ash falls from Middle Eocene volcanism, which was responsible for these deposits. He wrote, "Of greatest importance is the fact that a series of volcanic events from a variety of extrusive centers began about 50 million years ago generating tremendous volumes of ash, which was distributed across Wyoming and adjacent states for greater than a 40-million year span of time."

His explanation of the volcanic ash provides a valuable insight into how Wyoming's uranium deposits were formed:

"The volcanic ash, when flushed by the first rainfall, produced a unique fluid, which was acidic and charged with ions. The chemical reaction of the buffering on this fluid on contact with the Precambrian granites, the ash and other rocks brought the pH back to approximately neutral but leached additional uranium from the granites and probably the ash. The high rainfall and climate assured a steady supply of dissolved oxygen to the fluid resulting in the formation of a unique, oxidizing, uranium-enriched fluid, which entered the unconsolidated, reduced sediments oxidizing them and carrying the uranium to the eventual maximum extent of oxidation."

Boberg explained the development of the roll-fronts, writing, "Fluid flow through the very porous and permeable sediments would be relatively fast allowing for the development of large oxidized tongues with the young sediment as well as scattered uranium deposits at the redox (oxidized reduction) interface within approximately a million years. Deposits formed near the granitic highlands would be larger and of higher average grade because of the proximity to the dual source of granite and ash."

J.D. Love's uranium discovery in Tertiary sandstones, in 1951, was a near-surface roll-front type of redox deposit. A roll-front deposit follows a sinuous linear trend, often C-shaped. Colorado and Utah miners began calling the cross-sectional configuration a "roll" in the early 1940's. Roll-fronts occur in sandstones, bordered above and below by less permeable shales. In Wyoming, the "rolls" are bordered by altered and unaltered sandstone. It is generally concave from altered ground and convex into unaltered ground. Harris' idealized roll-front uranium deposit would have "uranium concentrations decrease abruptly away from the concave boundary, and concentrations gradually decrease away from the convex boundary in reduced rock."

Uranium is not always present everywhere along a roll front. It may be unevenly distributed and there are often other elements, such as vanadium, selenium, molybdenum, copper, silver, lead and zinc. Geologists look for where coarse-grained sandstones grade into finer grained or clay-bearing equivalents as indicators for uranium ore. As uranium geologists know with roll-front deposits, it may be mined as long as it is below the water table. Once deposits are brought above the water table, the uranium concentration can be eroded and severely modified.

It is not the roll-front uranium deposits, which interested Harris, but the tabular redox uranium occurrences found in many parts of Wyoming. He found those most prominently in the Cretaceous Inyan Kara Group in the Black Hills. Harris explained, "The uranium mines in New Mexico and many other parts of the Colorado Plateau are also tabular deposits." The tabular bodies, Harris noted, describe their irregular tabular form, and are found parallel to bedding, dissimilar to roll-front mineralization, which crosses bedding. Harris believed some of the tabular bodies in Tertiary rocks were "the limbs and detached limbs of roll fronts left in less permeable rocks at fluvial channel margins." He also said that tabular bodies could be preserved in oxidized rock due to high concentrations of other rock, such as coal or pyrite.

In any event, Harris agreed with other geologists that Wyoming is a uranium province with uranium occurring in nearly all major time divisions in the state. He concluded, "Uranium was available for mobilization during every major weathering period related to the nonconformities." In our final minutes together, he was convinced that many of the uranium development companies should sink more funds into exploration and find the elephant uranium deposits, which he pointed out in three different parts of uranium. To his way of thinking, that was more exciting that the simple ISL extraction of uranium from previously drilled areas. As with others interviewed, few of those areas will hold surprises, but instead offer the steady, cash-producing uranium extraction that help develop budding companies. That's what U.S. utilities, and utilities from other countries, are eagerly seeking right now. Wyoming uranium could fuel many of the U.S. nuclear reactors as more companies commence ISL uranium operations.

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