Print this pageAbstract: Uranium in Plutonic Rocks
Uranium is a strong lithopile and is concentrated in the granitic and in some alkalic rocks on the crust of the earth. In both types of rocks it is concentrated in the late-crystallizing constituents of the magma. In most granitic rocks, most of the uranium is in the essential minerals of the rock, as minute crystals of uraninite or as molecular or ionic disseminations in fractures, crystal defects, or along cleavage planes or grain boundaries. Uranium also enters tantalates, titanates, and niobates, and substitutes in the crystal structure of accessory minerals. In sodium- and potassium-rich alkalic rocks, uranium appears to be concentrated primarily in accessory minerals. Uraninite and molecular and ionic uranium are easily leachable, whereas uranium that substitutes in the crystal structure of minerals is not.
Large bodies of granitic rocks that contain 0.03 to 0.05 percent U3O8 can be ore deposits. Possible examples are Rossing, South West Africa; Charlebois Lake, Canada; and certain localities in central Idaho and northeastern Washington. Large bodies of alkalic rocks containing similar amounts of uranium also may be ore deposits but normally have more difficult metallurgic extraction problems. One possible example is the Ilimaussaq intrusion, southern Greenland, and another is the intrusive alkalic suite in the Bearpaw Mountains, Montana.
AAPG Search and Discovery Article #90971©1976 AAPG-SEPM Rocky Mountain Sections 25th Annual Meeting, Billings, Montana