Abstract

Tabular-type uranium deposits occur as tabular, originally subhorizontal bodies entirely within reduced fluvial sandstones of Late Silurian age or younger. This paper proposes that belts of tabular-type uranium deposits formed in areas of mixed local and regional ground-water discharge shortly after deposition of the host sediments. The general characteristics of tabular-type uranium deposits, especially the most studied deposits, those in the Uravan mineral belt, Henry basin, and Grants uranium region, indicate that their essential feature was the formation at a density-stratified ground-water interface in areas of local and regional ground-water discharge. Reconstruction of the paleohydrogeology is the key to understanding the formation of these deposits. Gravity-driven ground water recharged in major highlands and discharged in lowlands at major concave changes in paleotopographic slope. Shallow local and deep regional ground-water systems were characterized by dilute and saline water, respectively. Typically, underlying marine rocks, especially evaporites, provided the solutes to the deep regional ground water. A density-stabilized interface existed at the ground-water divide between local and regional flow systems. Tabular-type uranium deposits formed where these divides or interfaces intersected pockets of reduction where organic matter accumulated. The precipitation of humate and uranium at an interface accounts for the tabular shape and the tendency of deposits to rise stratigraphically into the basin. Geologic ground-water controls that favor discharge, such as the pinch-out of major aquifers, are also favorable for uranium ore. The combination of topographic and geologic features that both cause discharge is most favorable for ore deposition.

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