FIGURE 3.2 Schematic diagram illustrating the very wide range of geological processes that have resulted in uranium deposits. Average uranium concentrations of the main uranium reservoirs—the mantle (in blue), the crust (in yellow), and the upper crust are given. The circular arrows indicate the evolution of the geological cycle from surficial processes (alteration, erosion, transport by river and deposition) that produce sedimentary rocks, to deeper processes (burial of sedimentary rocks with increasing temperature and pressure) that produce metamorphic rocks; some of these rocks may be injected into the mantle during subduction. Increasing temperature leads to melting of the rocks in the continental crust and/or in the mantle and the genesis of plutonic and volcanic rocks that are injected in the Earth’s crust. Three main types of magmas can be enriched in uranium: Pal: per-aluminous magmas resulting from the partial melting of sedimentary rocks (Pal); highly potassic calc-alkaline magmas resulting from the partial melting of a mantle contaminated by subducted sediments (HKCa); and peralkaline magma resulting from very low degree of partial melting of a mantle, which can be contaminated (Pak). The main message in the schematic is the extreme variability of possible host rocks and concentration processes that can lead to potentially exploitable uranium deposits. SOURCE: Modified from Cuney (2009).

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