Geochemical Peculiarities of Uranium
General abundance ratio (content in percent) of uranium in the Earth crust according to A.P. Vinogradov is 2.5*10-4%. It forms part of all rock forming the Earth crust and is also present in natural waters and living organisms. The lowest concentration of uranium is present in ultrabasic rocks, the highest — in sedimentary rocks (phosphate rock and carbon shale).
Due to radioactive decay the amount of uranium in the Earth crust in the geological history is reducing. When plutonic rock crystallizes uranium is squeezed out in the residual liquor and concentrates in accessory minerals. Proximity of ionic radius of U4+ to ionic radius of Th, Ca, TR determines a broad spectrum of isomorphous substitutions of uranium in thorium and rare-earth minerals.
During the hydrothermal process uranium actively migrates in the form of uranium-carbonate complexes. Decrease of oxidation-reduction potential of solutions, decrease of partial pressure of carbon dioxide, change of pH of solutions lead to dissipation of such complexes and precipitation of uranium in the form of mineral phases. Temperature intervals where hydrothermal uranium minerals are formed are estimated at 100 — 220оС, sometimes at 300 — 350оС. Metamorphic processes from amphibolitic to granulite facies are commonly accompanied by carry over of uranium. Ultrametamorphism sometimes is accompanied by some addition.
During exogenetic processes uranium may migrate in oxygenated waters and transform into solid phase when oxidation-reduction environment changes (geochemical barrier).