--> Abstract: Environments of Formation of Uranium Deposits, by R. H. De Voto; #90972 (1976).
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Abstract: Environments of Formation of Uranium Deposits

R. H. De Voto

The geologic environments of economic and subeconomic uranium concentrations are related fundamentally to the geochemistry of uranium in the earth's crust. Uranium is a lithophile element, its average crustal abundance is 2 to 3 ppm. Because of its atomic radius (97A) and charge (+4, +6), uranium-coordination requirements do not allow it to fit into the lattice of major rock-forming minerals. Hence, on magmatic differentiation, uranium (and thorium) is concentrated in late-stage magmatic crystallization products, yielding average uranium concentrations in igneous rocks of basalt, 0.5 ppm; granodiorite, 1 to 2 ppm; granite, 3 to 4 ppm; and uranium is concentrated in some hydrothermal vein systems. Certain alkalic intrusives have uranium concentrations as high as 150 to 300 ppm. In these, most of the uranium occurs by limited substitutions for thorium and zirconium in accessory minerals. Certain granitic masses contain uranium concentrations as high as 150 to 350 ppm, mostly as finely disseminated uraninite. The Rossing deposit of southwest Africa contains 200 million pounds of U3O8 in an alaskite granite of average concentration of 350 ppm uranium.

As the earth's crust and atmosphere evolved, conditions were favorable (i.e., anoxic atmosphere) from approximately 2.2 to 3.0 billion years ago (early Proterozoic) to permit the placer accumulation of detrital uraninite in alluvial-fan and braided-river, quartz-pebble conglomerates. Two of the largest uranium districts in the world, those of Previous HitBlindTop River-Elliot Lake (Canada; greater than a billion pounds U3O8 of 0.1 percent U3O8 average grade) and the Rand (South Africa; hundreds of millions of pounds U3O8 in 250 to 300 ppm average grade) are in such alluvial deposits. With the development of an oxygen-rich atmosphere, weathering processes became capable of oxidizing uranium to its +6 valence, which caused it to form so uble carbonate complexes and not accumulate as placer concentrates.

Marine processes have been effective in extracting uranium from solution in sea water and in concentrating it in vast low-grade deposits. Bedded marine phosphorite deposits contain 50 to 150 ppm uranium, in limited substitution for calcium in the apatite lattice. Uranium has been concentrated in organic-rich, black shales deposited with slow rates of sedimentation in epicontinental seas. The Kulm Shale (Cambrian) of Sweden contains several billion pounds of U3O8 in 150 to 300 ppm concentrations; the Chattanooga Shale (Devonian) contains 40 to 75 ppm over a large area.

Mobile uranium within the constituent grains in sediments, particularly tuffaceous or acidic igneous detritus, is leached readily in oxidizing (+ Ehµ), slightly alkaline (pH 7.5 to 8.0), ground water. This early diagenetic, oxidizing, ground-water environment also commonly causes the destruction of carbonaceous debris by aerobic bacterial decay, the destruction of pyrite, and formation of red hematite staining. The dissolved uranium and associated metals migrate in the oxidizing ground water until a reducing environment is encountered. The reducing, ground-water environment commonly is characterized by ubiquitous, early-diagenetic, authigenic pyrite; carbonaceous debris; and gray colors. The uranium and associated molybdenum, vanadium, and selenium generally are precip tated and concentrated at the interface between the oxidizing and reducing environments. The humic acids of decaying carbonaceous debris collect uranium by cation exchange and assist in the reduction and precipitation of uranium. The principal uranium districts of the western United States contain high-grade (0.1 to 0.25 percent U3O8) deposits that have formed because of the early-diagenetic, oxidizing, ground-water environment leaching and moving uranium to sites of reduction within the host, nonmarine, braided-river, and meander-belt sandstones.

Other epigenetic processes cause the precipitation of uranium, such as evaporation, formation of insoluble vanadate minerals, and introduction of a reductant into an otherwise oxidizing environment.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA