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The central Davis Mountains are remnants of a Tertiary eruptive center composed of layered extrusives and a few shallow intrusions. Rock types range from basalts to trachyandesites, latites, trachytes, syenites, and rhyolites.
Delayed-neutron counting was used to determine uranium abundance in 102 rock samples representative of all rock types. Thin sections of 72 samples, 30 with fission-track data, were analyzed for petrography, mineralogy, and uranium distribution and mode of occurrence. Major-element analyses for 23 samples were obtained for comparison with trends in uranium abundance.
Uranium abundance increases in general with SiO2 and K2O content, and ranges from a mean of 1 ppm in basalts and andesites to 7 ppm in rhyolites. Uranium is most abundant in welded tuffs, in contrast to lava flows and shallow intrusions. Hydrothermal alteration redistributes uranium; unaltered rocks have significantly more uranium than altered rocks of all types. Glassy rocks contain up to one-third more uranium than their crystalline counterparts. In a vertical section through three rhyolitic-welded tuff units, uranium increases progressively from the oldest unit to the youngest; within each of two of these units, uranium also increases upsection. Further, rhyolitic-welded tuffs from the southwest part of the area contain 50% more uranium than those from the eas ; the eruptive source, however, has not been located.
Within a given rock type, uranium occurs preferentially in accessory minerals, in areas surrounding hydrothermally leached zones, and in vein fillings. Coarse-grained rocks have more localized concentrations of uranium than aphanitic or glassy rocks.
Ground-water leaching of uranium from igneous rocks of the central Davis Mountains is not considered an effective mechanism for uranium redistribution and enrichment because of the low permeability of the rocks and the nature of occurrence of uranium. Therefore, the probability of occurrence of large secondary uranium deposits in the area is not high.
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