GEOLOGY OF EPIGENETIC URANIUM DEPOSITS IN SANDSTONE IN THE UNITED STATES

By vv ARREN I. FINCH

ABSTRACT
Epigenetic uranium deposits in sandstone and related rocks are formed by the precipitation of uranium minerals from solutions. These deposits are widespread in the United States; they have yielded most of the uranium ore produced in this country and contain nearly all the domestic ore reserves.
Most deposits are tabular layers that lie nearly parallel to the bedding and for that reason are called peneconcordant
deposits. 'ln these, the ore minerals mainly fill the pore spaces of the host rocks, but they partly replace the sand grains, plant fossils, and acccessory and cementing minerals of these rocks ; in some deposits the uranium occurs in asphaltic material that impregnates ·and partly replaces the sandstone. Other deposits are mainly localb;ed along fractures that are discordant to bedding and are referred to as vein deposits. In these, the ore minerals occupy the fracture partings and impregnate the
adjacent wallrock.
Ore bodies range from small masses only a few feet across to those that are hundreds of feet across and contain a million tons or more of ore.
Nearly all peneooncordant deposits are in beds of Devonian age or younger and correlate with the evolutionary development of woody land plants, which occur in abundance as fossils in most of the host rocks. Most of these deposits are in lenticular sandstone beds that accumulated from fresh-water streams in
one of three environments: (1) shallow depressions in foreland belts which lie ·between the stable interior of the continent and geosynclinal belts, (2) intermontane basins, and (3) coastal plains near shorelines. Drainage was poor in each of these environments ; as a result, the pore waters remained in the 'beds, at least until structural deformation caused draining or flushing
of these solutions. Through reaction with the rocks these pore waters probably became enriched in dissolved metals.
Tuffaceous rocks are commonly associated with beds containing peneconcordant deposits, and this volcanic matedal could have furnished uranium and many of the associated metals.
Vein deposits, however, occur in beds ranging in age from Precambrian to Tertiary, and are nearly as abundant in sandstone of marine origin as in that of fresh-water origin. Because of their common geologic relations, the peneconcordant uranium deposits probably had a similar origin, though one that differed in detail from place to place. The metals probably were derived from trace amounts in solution in the pore waters when the host rocks and associated strata ·began to accumulate, and from trace amounts dissolved from these rocks by pore water during diagenesis. The resulting solutions then probably moved slowly through the rocks, precipitating the metals in spots of relatively strong reducing environments.
Most deposits seem to have formed before regional deformation and have since remained stable, but others are believed to represent the redissolved and reprecipitated materials of those early deposits that became unstable owing to changes in environmental conditions. Some vein deposits, however, are probably of hydrothermal origin from a hypogene source, some probably formed by the concentration of secondary or oxidized minerals derived from nearby sources, and some are of undetermined origin.

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