Abstract

The Elk Ridge area of southeastern Utah contains uranium ore deposits in two lower members of the Chinle formation of Late Triassic age. Each member is mineralized in different parts of the area, and where both are present only the lower contains ore. Across the Elk Ridge area from southwest to northeast, successively younger beds lap onto the unconformity that separates the Chinle from the underlying Moenkopi formation of Triassic (?), Early and Middle (?) Triassic age. Important uranium deposits have been found only in sandstone beds of the Chinle that are in contact with the Moenkopi. Sandstone of the Chinle formation lies on the Moenkopi (or is separated from it by gray mudstone of the Chinle no thicker than the depths of most of the known paleostream channel scours) in two separate parts of the Elk Ridge area. These "favorable areas" contain all the mines and important prospects.The ore deposits are flat-lying tabular to lenticular bodies in fluvial sandstone beds of the Chinle with fine-grained black uranium minerals chiefly interstitial to sand grains and as replacement of carbonaceous material. Most of the ore-bearing sandstone beds are discontinuous lenses intertonguing with and overlain by relatively impermeable mudstone. Contact of the host sandstone with the underlying Moenkopi seems to be a prerequisite for ore as the ore bodies are generally within a few feet of such contact areas and sandstone lenses separated from the Moenkopi by gray mudstone are generally not ore bearing.The consistent association of ore deposits with the Chinle-Moenkopi contact suggests that mineralizing solutions were introduced into the host beds from their areas of contact with the Moenkopi formation. Impermeable barriers overlying ore-bearing parts of the host sandstones were probably an important control on the deposition of ore minerals from solution. Hypofiltration of metallic constituents from ascending ore solutions may have been important. It is also possible that the overlying barriers formed traps for H 2 S gas or fluid hydrocarbons and thus localized a reducing chemical environment in which ore minerals were later precipitated.

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