Diagenetic lithification of Kombolgie sandstones followed by a metasomatic event which produced veining and replacement. Metasomatism was controlled by brecciation and bedding within the sandstones. The high-grade uranium ores are interpreted to be the product of a low-temperature (100 degrees -200 degrees C) hydrothermal system of ground water which was driven by a regional heating event also manifested by phonolite and/or diabase intrusions. The Kombolgie sandstones were an essential part of the hydrologic system which formed the high-grade orebodies, after lithification of the lower part of the thick clastic section but possibly contemporaneous with diagenetic events in the upper part. The processes operating in the Alligator Rivers uranium field were essentially similar to those in the Athabasca District of northern Saskatchewan, the primary difference probably being that no organic material was remobilized into the overlying sandstone to act as a reductant in the Australian case.--Modified journal abstract.