Geochemical factors contributing to uranium concentration in alkalic igneous rocks

The concentration of uranium in igneous rocks is largely dependent upon its geochemical relationships to other elements. Inasmuch as uranium is predominantly a minor ionic-substitution component in accessory minerals, ionic size plays an important role in determining its distribution. Chemical association of uranium with elements such as Th, rare earths, Zr, Ti, Nb, Ta and F is an important factor in determining its distribution. Zirconium and uranium become concentrated in highly alkalic rocks, especially in syenites unsaturated with respect to silica. This is well manifested in the agpaitic rocks of the Ilimaussaq intrusive in southern Greenland. The Pocos de Caldas occurrence in Brazil and the bostonite dikes in the Front Range of Colorado, United States of America, also reflect the association. The uranium content of accessory minerals such as zircon, which is fairly ubiquitous in igneous rocks, may provide a more sensitive and contrasting measure of the uranium potential of plutons than whole-rock uranium content. Sphene may also be useful in this connection. In identifying specific plagiogranites and alkalic granites and syenites as ore-bearing intrusions, particular attention might be given to the use of element concentration ratios other than Th/U, not only in the whole rock but also in specific minerals. (author)