Abstract

Detailed sedimentologic studies in south-central Utah demonstrate a close association between ore-bearing sandstone beds and certain thin gray carbon-bearing mudstone beds deposited in offshore lacustrine environments and named favorable gray mudstones. Tabular uranium ore-bodies in sandstone beds of the Henry Mountains mineral belt occur wherever these mudstones are present. The mudstones either lie directly above or below the ore-bearing sandstone beds, or the ore-bearing part of a sandstone bed may occur a short lateral distance of less than about 500 ft from the nearest favorable gray mudstone bed. The association is so close and so consistent that it suggests an exploration strategy based on searching for the mudstones and then for the closely associated ore deposits.

The location of the favorable gray lacustrine mudstones was determined by the interrelationship between stream energy or transport capacity, stream flow paths, and contemporaneous tectonic movements. The lakes beds are in the distal part of an alluvial plain complex where deposition was by relatively low energy braided streams, as suggested by relatively high stratification ratios and low percentages of horizontal laminations in the fluvial sandstones. Facies relations and variations in thickness of the ore-bearing depositional sequence suggest that contemporaneous tectonism produced folds that trended more or less perpendicular to the general direction of stream flow, as determined by cross-bedding studies. Evidently, when relatively low-energy or flow regime streams were present, the growing folds acted as slight local barriers to sediment distribution, allowing lakes to form in the most sheltered parts of the growing synclines. Exploration strategies based on these concepts suggest several areas in the Henry and Kaiparowits basins where the favorable mudstones and associated ore deposits may occur. The concepts may also prove useful in the seach for uranium deposits in other regions, or in other formations where the favorable gray mudstones might occur.