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Uranium of the Grants region is present predominantly in continental sandstone of the upper part of the Jurassic Morrison Formation, but significant, though lesser, deposits are in limestone of the Jurassic Todilto Formation and in black shale of the Cretaceous Dakota Formation. The deposits are disseminations that form "runs" ranging in volume from a few hundred to several million tons. The ore consists predominantly of uraninite, uraniferous carbonaceous material, coffinite, and such secondary oxidized minerals as tyuyamunite, carnotite, and uranophane. The "runs" are localized in the sandstone in conjunction with associated mudstone, interstitial carbonaceous material, and primary sandstone-channel trends. In limestone localization is related to folds and fracture zone .
Most production has come from the Ambrosia Lake mines in the Grants district and the Jackpile and Paguate mines in the Laguna district. The ores, which generally have been 0.20-0.30% U308, are treated at mills operated by Kerr-McGee Corporation, Homestake-Sapin Partners, and The Anaconda Company, all in the Grants district. In the 17 years since discovery in 1950, the deposits have yielded more than 30 million tons of concentrates valued in excess of 1 billion dollars.
Since deposition of the Jurassic host rocks, a geologic history involving tilting, faulting, erosion, changing groundwater environments, and oxidation has influenced considerably the existence and character of the deposits. It is evident that their origin is not simple. However, a pre-Dakota age is generally agreed on for the original ore runs, and at Ambrosia Lake two principal stages of ore formation, separated by a period of Laramide faulting, are recognized. All older ores have been modified by Quaternary oxidation, solution, and enrichment during which time much secondary tyuyamunite, metatyuyamunite, and carnotite formed.
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