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Volcanic glass-rich mudstone and siltstone samples from the Oligocene/Miocene Catahoula formation of Jasper County, Texas, and coeval volcaniclastic rock samples from Trans-Pecos, Texas, have been compared as to U, Th, Zr, Ti, K, Rb, and Sr contents. Results are consistent with the 1977 eruption model of Sparks and Walker, in which the east Texas Catahoula samples are their "distal air-fall ash," and the Trans-Pecos samples their near-source units. Uranium is slightly greater in the distal ash (5.85 ppm U) compared to the Trans-Pecos samples (average 5.41 ppm U). Elements which are preferentially incorporated in crystallizing phases are more abundant in the crystal-rich near-source units (310 ppm Sr, 2,163 ppm Ti, 461 ppm Zr, and 22.7 ppm Th) than in the distal ash (48 pp Sr, 1,050 ppm Ti, 88 ppm Zr, and 18.1 ppm Th). Elements which tend to become enriched in the residual magma are less abundant in the near-source units (206 ppm Rb and 3.09% K) than in the distal ash (291 ppm Rb and 4.94% K). These results emphasize the close chemical affinities of Catahoula and Trans-Pecos volcanic material.
Diagenetic and pedogenetic alteration of Catahoula volcanic glass releases uranium to solution and, under favorable conditions, this uranium may accumulate to form ore bodies. Uranium has been produced from such ore bodies in south Texas, but economic deposits are not known in east Texas. Significant differences between south and east Texas include: (1) a greater amount of volcanic debris delivered to south Texas, both as air-fall ash and stream-transported material, (2) delivery of only air-fill ash to east Texas, (3) the possibility of more petroleum-related reductants such as H2S in south Texas, and (4) pervasive glass alteration with subsequent uranium release in south Texas due to late calichification. These differences argue against economic deposits of the south Texa type being found in east Texas. If economic deposits occur they are likely to be far downdip making exploration difficult and expensive.
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