The continual expansion of the areas in which uranium finds are being reported in sedimentary rocks makes the hypothesis that the metal was transported by solutions hydrothermal in origin entirely inadequate. Solutions, which originally by their very nature were concentrated, would have had to be dispersed over vast areas with exactly the opposite effect of that necessary to make workable deposits. Weathering and erosion of Precambrian granitic rocks in volumes of many thousands of cubic miles has occurred since Pennsylvanian time in the eight western states which make up our uranium province. If we add to these granites the smaller amounts of volcanic tuffs, the uranium contained in these rocks staggers the imagination, it being at least as large as 400 million tons and probably several times this figure.Under humid conditions and without interior basins with extensive continental sediments the uranium would have been carried to the sea. But given arid or semiarid conditions, continental sediments, and a proper leaching agent, results would be quite different. Such leaching agents, which are also very common in nature, are the bicarbonates of Ca, Mg, and Na. They are able to form with uranium compounds which yield the apparently very stable U-tricarbonate ion [UO 2 (CO 3 ) 3 ] (super -4) in a solution saturated with CO 2 . Because most groundwaters contain large amounts of CO 2 and these bicarbonates, they are considered powerful reagents for the solution of uranium as shown by experiment. They could carry the metal, including also vanadium, long distances through almost neutral environments, always along paths or "channels" of least resistance until reducing conditions were met when black ores would result. If instead of precipitation the charged solutions regained the surface, the tricarbonate would decompose and the uranium would be precipitated as uranyl minerals only to be redissolved at a later date by the same process.Concentration of large deposits could proceed by several stages of oxidation-solution-migration-accretion, a kind of "recycling" action as metallurgists would call it. Orogenic movements with resulting new unconformities and new gradients would make more uranium available and, of course, add to the large number of depositional variations encountered in space and time. Whether deposition occurred in the Cutler, the Chinle or Morrison, or much later in the Wasatch or Brown's Park formations would not alter the principles involved. They would work just as well for the uranium accumulations in the lignites of the western Dakotas as for those in the Todilto limestone.

First Page Preview

First page PDF preview
You do not currently have access to this article.