Uranium provinces and mantle anomalies

The geochemical cycle of U shows that, since 2.5 Ga ago, part of the crustal U has been returning to the mantle. This is due to the setting up of the convective cycle that altered the primordial element distribution in geospheres governed by gravity alone. Mantle material with U excess has been recognized by Pb isotopic studies in the source mantle regions for alkaline magmatism, but alkaline magmatism does not balance the U excess in the mantle. Therefore, mantle regions which are enriched in U, Th, light rare earth elements, K and other incompatible elements should be stored somewhere beneath the lithosphere. The uranium provinces representing crustal geochemical anomalies may correspond to the site of underlying mantle anomalies. This possibility has been tested in the Latium region (Central Italy), which is considered to be a very recent uranium province. The Latium region corresponds to a mantle uplift with anomalous geophysical characteristics. The magmatic suites extracted from the anomalous mantle indicate mantle metasomatic processes. The enrichment factor for incompatible elements added to the source mantle region of the alkaline magmatism has been calculated. Another supply of fluids depositing incompatible elements in shallow crustal environment has been detected and evaluated also. Regional mineralizations may correspond to this fraction of fluids supply that can percolate outside the volcanic system. Those elements that have been added to the magmatic chamber after the magma extraction from the mantle (U, Hg, Sb, As, Ba, Sr, F, S) form huge concentrations in and around the volcanic system. The isotopic composition of the different types of mineralizations and alterations shows constant values which do not relate to magmatic or to sedimentary origins. 9 refs, 4 figs, 2 tabs