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The regional distributions of elements in uranium deposits in the Salt Wash Member of the Morrison Formation of Late Jurassic age provide clues to the sources of the elements. Iron, cobalt, nickel, molybdenum, arsenic, selenium, yttrium, and to a lesser extent, zinc, tend to be more highly concentrated in deposits on the western and northwestern parts of the Colorado Plateau and in this respect conform roughly to the distribution of tuffaceous materials in unmineralized sandstones of the Salt Wash Member. Copper, silver, lead, and zinc are highly concentrated in deposits in the salt anticline area of western Colorado and eastern Utah, an area that has been extensively fractured and which includes the La Sal Mountains laccoliths. Vanadium is unique in that it is more highly concentrated in deposits on the eastern part of the plateau, with the exception of deposits near the Henry Mountains on the western part. Heavy minerals in the unmineralized sandstones of the Salt Wash Member, like vanadium in most of the uranium deposits, tend to be more abundant on the eastern part of the plateau. Because of sample bias with respect to uranium grade the regional variation of uranium in the deposits cannot be determined with available data. The compositions of the Frenchy Incline uranium deposit in the Legin area of southwest Colorado and its host sandstone and mudstone in the vicinity of the deposit have been determined from semiquantitative spectrographic analyses of 219 drill-core samples. Comparison of the compositions of the mineralized and unmineralized rocks shows that in the formation of the deposit the host rocks were enriched in vanadium, iron, uranium, strontium(?), zinc, lead, copper, chromium(?), nickel, cobalt, molybdenum, and silver, in decreasing order of abundance. In general, this is the same suite of elements which has been found to be concentrated in other uranium deposits in the Salt Wash Member of the Morrison Formation on the Colorado Plateau. Quantitative considerations show that most elements concentrated in the Frenchy Incline deposit could have been derived from altered sandstone adjacent to the deposit without changing the composition of the sandstone by any detectable amount. However, uranium and probably vanadium must have been derived from sources external to the altered part of the sandstone lens in which the deposit occurs; an unknown amount of lead in the deposit originated from the radioactive decay of uranium within the deposit. El E2 CONTRIBUTIONS TO GEOCHEMISTRY Copper, silver, some lead, and part of the zinc, because of their regional variations among all deposits in the Salt Wash Member, are interpreted to have been derived from sources external to the Salt Wash in the salt anticline area. Iron, cobalt, nickel, molybdenum, arsenic, selenium, yttrium and some zinc are interpreted to have been derived from tuffaceous materials in the Salt Wash Member, possibly in the altered sandstone adjacent to the deposits. Uranium and vanadium could have been derived from a number of sources, but there is some evidence that vanadium was derived from heavy minerals. If the elements were transported to the deposit by flowing ore solution, the required minimum concentrations of each element in the solution depends on the volume of solution which passed through the deposit. Assuming that the hydraulic gradient which caused solution flow resulted from differences in hydrostatic head caused only by the dip of the aquifer, the maximum volume of the solutions which passed through the deposit per unit time, for a given structural gradient, may be estimated from Darcy's law. The required minimum concentration of each element in the ore solution, then, becomes a function of the duration of mineralization and the structural gradient. It is possible that some of the elements concentrated in the deposit were transferred to the site of mineralization by diffusion processes rather than by flowing solutions. Thermodynamic considerations suggest that concentration gradients of iron in solution may have been established in the host sandstone during a period of relatively stagnant ground-water conditions. Iron and other elements may have been transferred by solute diffusion which occurred in response to the concentration gradients. Roll-type ore structures in the deposit can be interpreted as having resulted from diffusion of elements toward a solution interface within the host sandstone. Two hypotheses of origin of uranium deposits in the Salt Wash Member of the Morrison Formation are considered. One hypothesis involves a single stage of mineralization, whereas the other involves multiple stages. Some important similarities and some differences have been found between the distributions of elements in uranium deposits in the Salt Wash Member and in other stratigraphic units on the Colorado Plateau.

Distribution of elements in Colorado Plateau uranium deposits - a preliminary report