Abstract

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 ground-water flow. This paper presents a quantitative simulation of regional ground-water 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 ground-water modeling favor the brine interface model over the lacustrine-humate model for uranium deposition.

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