Notes on the geology of uranium

Abstract

Igneous processes tend to concentrate uranium in late phase differentiates of magmas, notably in pegmatites, small alkalic plutons, and veins and other epigenetic deposits. The concentrations in igneous rocks are syngenetic. They may be large but are low-grade. Vein and other epigenetic deposits associated with igneous rocks are generally relatively small, but some are high-grade. Weathering and sedimentary processes may either disperse or concentrate uranium, depending primatily on climate. Some primary , uranium-bearing minerals accumulate in placers. Others are dissolved and the uranium is transported by ground and surface waters from which it may be precipitated in favorable receptors to form epigenetic deposits. Most deposits thus formed are relatively low-grade but some are large. Some uranium in solution reaches the sea and may be extracted to form large low-grade accumulations in phosphorites or black fhales, Metamorphic processes may affect concentrations of uranium either by converting pre-existing rocks to magma, which subsequently may form uranium-rich differentiates, or by "sweating" uranium out of weakly uraniferous rocks and concentrating it in favorable geologic settings.Most of the world's important uranium deposits are clustered in a few areas or provinces that perhaps represent uranium-rich portions of an originally inhomogeneous crust. These uranium-rich provinces apparently persist through long periods of geologic time. Within them, some types of uranium-rich rocks and uranium deposits may be relatively short lived, depending on the geologic history and climate; but, as they are destroyed by erosion or metamorphism, others in equilibrium with the prevailing environment may form.