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The AAPG/Datapages Combined Publications Database

South Texas Geological Society Special Publications


Contributions to the Geology of South Texas, 1986
Pages 109-125

Geology of the Nuhn Uranium Ore Body, Jackson Group, South Texas Uranium District

Richard P. McCulloh, Charles Roberts


The Nuhn ore body is a shallow, irregular, tabular uranium deposit of the Searcy trend in the lower Stones Switch sand member of the upper Jackson Whitsett Formation, South Texas uranium district. Parts of the deposit have been mined at different cutoff grades by companies since 1961. In 1979 and 1980, Conoco, Inc. undertook coring of the entire host sand interval on 100-ft. centers on much of the southeasternmost 250-acre (Korzekwa) tract to delineate ore grade (0.03 percent U3O8) mineralization by core assay. Ore lies at depths of 0 to 50 ft. in masses 2 to 20 ft. thick. Ore grades (0.03 to 0.30 percent U3O8) and disequilibrium (log conversion factors of 0.1 to greater than 50) are highly erratic, and the mineralization shows no consistent relation to either oxidation or lithology. Ore tends to be discontinuous with multiple intercepts (mineralized zones), though individual intercepts can persist for several hundred feet.

The lower Stones Switch host sand, which shows an upward-coarsening progradational sequence, probably formed in a barrier-strandplain environment. The sand crops out across the Nuhn property and contains some selectively silicified zones at depths ranging from 0 to at least 65 ft. Although silicification occurs primarily above and downdip of the ore, it may penetrate ore zones by as much as 2 ft. in places. A clay-filled channel postulated by previous workers was not confirmed by the Conoco coring program. The upper Stones Switch sand, present only in the southeastern part of the Korzekwa tract, shows characteristics of a channel sand and does not crop out at the surface. The channel sand probably represents the edge of the southwest-trending channel complex which hosts the upper Stones Switch mineralization system to the southeast.

Gamma logs are nearly uniform and show no mappable gamma front. Three gamma peaks are associated with three main zones of mineralization in the lower Stones Switch, of which the middle is the thickest and most extensive. Much of the ore is associated with carbonaceous material in the sand, and the middle and upper ore zones are associated with discontinuous lignite in places. In most places, the reduction-oxidation (redox) boundary in host sand simply represents the intersection of the base of surface oxidation with the top of the sand, but relict roll character is recognizable in an area where the boundary is convoluted in plan view. Chemical ore is only moderately displaced from radiometric ore. These features suggest partial remobilization and partial erosion of what originally was a roll-front type deposit in the recent geologic past. The middle ore zone represents the bulk of the remobilized mineralization, and the upper and lower zones probably formed by adsorption at the top and base of the host sand.

The coring program on the Korzekwa tract defined about 350,000 in-place pounds of U3O8. Conoco mined the highest grades from this portion of the Nuhn deposit in 1981 and 1982. The ore mined exceeded reserve estimates, in both tons and grade, by a combined factor of greater than two and yielded 192,292 recovered pounds of U3O8. Evaluation of old gamma-log data in light of the overall favorable disequilibrium indicates excellent mineralization potential along strike toward the Searcy-Niestroy property to the northeast, as well as in remaining unmined portions of the previously defined deposit, at the 0.03 percent U3O8 cutoff.

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