Uranium deposits, types granites

Intrusive Deposits. Deposits included in the intmsive deposit type are those associated with intmsive or anatectic rocks of different chemical composition, eg, alaskite, granite, monzonite, peralkaline syenite, carbonatite, and pegmatite. Examples include the uranium occurrences in the porphyry copper deposits such as Bingham Canyon and Twin Butte in the United States, the Rossing Deposit in Namibia, and Ilimaussaq deposit in Greenland, Palabora in South Africa, and the deposits in the Bancroft area, Canada (15). 

Uranium deposits in this unit are of numerous geological types, but three are typical of the Hercynian orogen (1) intra-granitic deposits related to leucogranites, (2) deposits bound to the contact-metamorphic haloes of granite intrusives in Lower Palaeozoic shales (the so-called Iberian type ) and (3) deposits bound to Permian cover rocks or Permian acid volcanics. Other types are veins in less differentiated granites, veins in metamorphic environments, sandstone-type deposits in Mesozoic or Caenozoic cover rocks in basin structures of the Hercynian space included in or adjacent to the Moldanubian zone (or its Iberian equivalent). 

Uranium vein-type deposits in metamorphic environment without apparent connexion with granites... 

Figure 19. Typical example of how uranium is released to the environment (redrawn and completed after Pfeifer et al. (1994)). By weathering and erosion U-rich ore quality material (pitchblende veins in granitic gneiss) is deposited on the slope beneath a cliff. The soil and plants devel-opping on it are enriched in uranium (up to 2500 ppm in the soils vs. 3 ppm in the reference soil outside the touched area, 50 m to the south-east, marked with r 100 mg/kg U in contaminated plants, - see Appendix A.8). The waters from this type of environment contain between 10 and 30 jxg/l U (cf. Fig. 18, Section 2.2 and Appendix A. 1).

This model for uranium mineralization associated with granites requires (1) a metalliferous Sn-U mildly alkaline type or transitional granite to rise to a level in the crust where it will create a thermal anomaly (2) adequate water of suitable chemistry for hydration and reduction of primary silicates (thus rocks of low metamorphic grade and more pelitic composition, particularly graphitic shales, will favour mineralization) and (3) a well-developed fracture system, particularly for low-temperature mineral deposition. 

---