TABLE 3.1 Global Averaged Uranium and Thorium in Different Rock Types
|Rock Type||Uranium Content (ppm)||Thorium Content (ppm)||Thorium/Uranium Ratio|
|Upper Crust Average||2.5||10||4|
SOURCE: Modified from Lassetter (2010); compiled from Rogers and Adams (1969), Woodmansee (1975), Gabelman (1977), and Rose et al. (1979).
These are the highest grade deposits in the world (generally higher than 1 percent uranium, and up to 20 percent for the McArthur River deposit). Their tonnages vary from some thousands of tonnes of uranium (tU) to more than 200,000 tU.
These deposits occur in medium- to coarse-grained sandstones deposited in continental fluvial or marginal marine sedimentary environments. The uranium is precipitated under reducing conditions associated with carbonaceous material, and/or sulfides, and/or hydrocarbons, and/or iron-magnesium minerals, disseminated within the sandstone. Four main subtypes are distinguished:
• Roll-front deposits. Uranium mineralized zones are crescent-shaped in cross section, sinuous horizontally, and localized between reduced sandstone on the hydrological gradient downside and oxidized sandstone on the hydrological gradient upside. Resources range from a few hundred tonnes to several tens of thousands of tonnes of uranium, at grades from 0.015 percent to 0.25 percent. Examples are Moynkum, Inkay, and Mynkuduk in Kazakhstan; and Crow Butte and Smith Ranch in the United States.
• Tabular deposits. Uranium minerals impregnate the sandstone matrix within tabular, irregularly shaped, lenticular masses within reduced sediments.