Over 200 diatremes occur in the Hopi Buttes volcanic province on the southwestern flank of the Black Mesa basin in northeastern Arizona. The diatremes are funnel-shaped vents with maar craters at the surface. They were formed by phreatomagmatic eruptions along existing northwest-southeast fractures during mid-Pliocene time. Anomalous concentrations of uranium occur in at least 20 of the diatremes.The diatremes and dikes which feed them are composed of lamprophyres, monchiquite, and limburgite. Tuff breccias and agglomerates rimming the vents consist of sedimentary and igneous fragments in a limburgite matrix. The maars contain waterlaid deposits of interlayered tuff, claystone, and marl.The lamprophyres are enriched in light rare earth elements and may have been derived from a slight partial melting of the upper mantle. Average uranium values are: tuff breccias, 10.54 ppm; tuffs, 23.95 ppm; limestones, 58.23 ppm. The average Th/U ratio in the lamprophyres is 2.02. The lack of correlation between U and Hf or Th indicates that uranium traveled in mobile phases and was segregated from many elements in the lamprophyres. Localization of uranium in maar-type diatremes may have occurred as fluids percolated up through fractures in the vents and mixed with ground and surface waters.