Sandford reported in 1994 that tabular sandstone uranium deposits constitute the largest uranium resource type in the United States. A major point of contention has been the nature and direction of the groundwater flow. This paper presents a quantitative simulation of regional groundwater flow during uranium deposition in the Westwater Canyon Member and Jackpile Sandstone Member of the Upper Jurassic Morrison Formation in the San Juan basin. Input for the four-layer finite-difference model was from thicknesses and sandstone/mudstone ratios in each layer from 1,600 drill holes and 100 measured sections and from palcotopographic reconstructions.
Topographic slope, shoreline position, and density contrasts in the lake and pore fluids controlled the directions of flow and recharge-discharge areas. The most important results for uranium ore deposit formation are that regional ground water discharged throughout the basin, regional discharge was concentrated along the shore line or playa margin, flow was dominantly gravity driven, and compaction dewatering was negligible. A strong association is found between the tabular sandstone uranium deposits and major inferred zones of mixed local and regional ground-water discharge. The results of groundwater modeling favor the brine interface model over the lacustrine-humate model for uranium deposition.
Sanford concluded that The use of paleohydrologic analysis can be a key to unlocking the mystery of the formation and localization of tabular sandstone uranium deposits. Although the deposits had been studied for more than four decades by geologists using standard stratigraphic, sedimentologic, and petrologic tools, none of these methods yielded a clear answer to the question of the patterns of ground-water flow during ore deposition. By applying the techniques of modern ground-water analysis, the pattern of regional ground-water flow can be better understood. Many details remain unsolved, some of which can be addressed by models that incorporate transient and variable-density flow. In light of this progress, modern methods of paleohyodrologic analysis should be considered as a standard tool in the analysis of a wide variety of ore deposit
types as well as in the exploration for new deposits.
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