Because the chemical composition of groundwater in contact with a buried uranium deposit should be quite distinct from that of groundwater flowing through barren rock, analysis of the groundwater may provide 3-dimensional information useful to the explorationist. Because of the complex geochemistry of uranium, analysis for uranium only will not lead to an appropriate interpretation. The solution-mineral equilibria approach, using the computer program WATEQF, has shown to be useful for a regional exploration program in south Texas. The technique has outlined areas with known mineralization and with a high potential for mineralization.

Groundwater samples were collected and analyzed for a single aquifer in existing water wells on a 1–2-mi grid over an area of approximately 169 mi2 (438 km2). The WATEQF computer program uses the chemical analyses to calculate saturation indices, which describe the state of saturation of the groundwater with respect to a particular mineral. In the study area, saturation indices for the uranium minerals coffinite and uraninite were highest over the most prospective areas.

Hydrogeochemical information obtained in this study supports our geologic data that the known mineralization occurs as small reduced islands in oxidized ground. The solution-mineral equilibria approach suggests that larger deposits may exist downdip of the present-day redox front, which has moved updip since the main mineralizing event. Updip movement of the redox front may have resulted from leakage of H2S from downdip faults.

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