Lectures 34-36-Uranium Deposits
Uranium deposits can be viewed as products of the rock cycle. Uranium occurs in small amounts in the Earth’s crust. This uranium typically occurs as the mineral uraninite (UO2) or is present in other minerals as a trace component. The mineral zircon (ZrSiO4), for example, may contain uranium in concentrations in excess of 1,000 parts per million. If the crust is cycled and partially remelted, the uranium tends to be concentrated in the magma. When the magma cools the uranium will again occur as uraninite in the igneous rock. If the system contains sufficient uranium and conditions are right a uranium deposit might form in the igneous rock. The Rossing deposit of Namibia is an example of such a deposit in igneous rock. Although the Rossing deposit contains low grade uranium ore, uranium is the primary product and Rossing accounts for the bulk of Namibia’s uranium production. In modern environments uraninite will dissolve as the uranium oxidizes.
Dissolved uranium from a uranium deposit or simply from rocks that contain anomalous amounts of uranium may be carried in solution in surface waters. These waters become reduced as they percolate into the subsurface. When the uranium is reduced it will precipitate out of solution as uraninite. If sufficient fluid carrying sufficient uranium flows through a reducing “trap” a sandstone uranium deposit will form. These can be quite variable in size and grade but are never among the largest deposits. It doesn’t matter whether you look at them in terms of the amount of ore, the grade of the ore, or the amount of uranium produced, sandstone uranium deposits generally sit in the middle. Crow Butte is a sandstone uranium deposit in located near Crawford, Nebraska. It is large for a sandstone deposit and should continue producing uranium for several years. Sandstone deposits are widespread and contribute to the bulk of uranium production from Kazakhstan, Niger, and Gabon. The highest grade (richest) deposits shown on these diagrams are unconformity-related deposits. These deposits occur near unconformities at the base of some ancient sedimentary basins. The mineralization occurs in rocks above and below the unconformity that marks the basal contact of the sedimentary basin. The mineralization must be younger than the rocks immediately above the unconformity. Unconformity-related deposits are the primary reason why Canada is one of the world’s leaders in uranium production (something that is worth remembering). Their formation is still a bit of a mystery but they must involve a chemical trap where U6+ is reduced to U4+ and precipitated as uraninite. In Canada they are concentrated at the base of the Athabasca Basin in Saskatchewan. Similar deposits occur in Australia but they carry much lower uranium ore grades. Australia is another of the world’s top two uranium producers. Although Australia has some large unconformity-related deposits, more than 70% of their uranium production is derived from the giant Olympic Dam Iron oxide-Copper-Gold-Uranium (IOCG-U) deposit. Olympic Dam is one of the largest mining operations in the world; it is the largest uranium mine in the world and uranium is produced only as a byproduct of the copper and gold that they are mining. Among the lower grade uranium deposits are the quartz-pebble conglomerates or the Witwaterand-type deposits. These are quite different from the other deposits discussed above. These deposits comprise ancient (2.2 billion years old) stream gravels. If you check the images from the lecture you will see examples of rounded grains of uraninite and pyrite. These minerals are typically oxidized rapidly and dissolved in the modern world. Their persistance in these stream gravels indicates that the Earth’s atmosphere contain very little oxygen 2.2 billion years ago. Another important feature of quartz-pebble conglomerates or Witwatersrand-type deposits is their low grade. These deposits are only economic when the price of uranium is high or if the uranium can be recovered as a byproduct. Although some of these deposits contain uranium only(Blind River, Ontario) , many contain significant amounts of gold (Witwatersrand, S.A.; Jacobina Brazil). These latter deposits are mined for the gold and the uranium is recovered as a bonus (the Jacobina deposits contained no significant uranium, only gold). As a result they can afford to work with very low grades of uranium ore.
Posted: November 21st, 2009 under Study Guide, Study Guide Exam 3, Uncategorized, Uranium, Uranium Geology and Mining.
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The Orinoco delta is the area circled on the map shown here where the Orinoco River system empties into the Atlantic Ocean.
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