The shape of roll-type uranium deposits
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Geology topics
C. G. Warren
Publications and source records attributed to C. G. Warren.
The importance of dissolved free oxygen during formation of sandstone-type uranium deposits
One factor which distinguishes t, he genesis of roll-type uranium deposits from the Uravan Mineral Belt and other sandstone-type uranium deposits may be the presence and concentration of dissolved free oxygen in the ore-forming. solutions. Although dissolved oxygen is a necessary prerequisite for the formation of roll-type deposits, it is proposed that a lack of dissolved oxygen is a prerequisite for the Uravan deposits. Solutions that formed both types of deposits probably had a supergene origin and originated as meteoric water in approximate equilibrium with atmospheric oxygen. Roll-type deposits were formed where the Eh dropped abruptly following consumption of the oxygen by iron sulfide minerals and creation of kinetically active sulfur species that could reduce uranium. The solutions that formed the Uravan deposits, on the other hand, probably first equilibrated with sulfide-free ferrous-ferric detrital minerals and fossil organic matter in the host rock. That is, the uraniferous solutions lost their oxygen without lowering their Eh enough to precipitate uranium. Without oxygen, they then. became incapable of oxidizing iron sulfide minerals. Subsequent localization and formation of ore bodies from these oxygen-depleted solutions, therefore, was not necessarily dependent on large reducing capacities.
Open-File Report↗
The concept of growth and maturity of ore-stage pyrite in roll-type uranium deposts
Roll-type uranium deposits contain both ore-stage pyrite and preore or diagenetic pyrite that was present in the host rock before the deposits began to form. Ore-stage pyrite forms as the result of redistribution and accretion from the preore pyrite. Accretion of the ore-stage pyrite seems to be governed by natural laws that limit its concentration to only a few times the concentration of the preore pyrite. Accumulations of ore-stage pyrite build up along the leading edge of a supergene oxidation zone which spreads through the host rocks, literally pushing the ore deposits ahead of it. The ore-stage pyrite probably progresses much as a wave that first grows to a nearly fixed amplitude and thereafter is steadily maintained as the mature deposit continues to advance.
Journal of Research of the U.S. Geological Survey↗
A method for discriminating between biogenic and chemical origins of the ore-stage pyrite in a roll-type uranium deposit
Some roll-type uranium deposits are marginal to an altered tongue in sandstone beds that originally contained more-or-less uniformly distributed pyrite. Mineralizing solutions percolated through the sandstone, oxidized nearly all the pre-existing pyrite, and then redeposited part of the pyrite downstream in an embryonic ore zone. The pyrite and the entire ore zone continued to migrate downstream in the sandstone, much as a sand dune migrates. The amount of pyrite in mature deposits varies systematically with the position in the ore body. It is postulated that the rate at which the pyrite was redeposited controlled the systematic variation in distribution of pyrite.Biogenic and chemical models which are described in the literature provide alternate explanations for the genesis of roll-type uranium deposits in sandstone. The different theoretical rates for the precipitation of pyrite in the two genetic models provide a distinctive distribution of pyrite that characterizes each process. The theoretical difference between the biogenic and chemical models provides a mathematical technique for identifying the origin of a deposit. Mathematical analysis of the pyrite content of a uranium deposit in the Shirley Basin, Wyoming, illustrates a practical application of the theory. Although a definite conclusion about the origin of roll-type deposits would require considerably more data than are now available, the pyrite content of this deposit does correspond to the theoretical pyrite content of the chemical model, suggesting that a disproportionation reaction was involved in its formation.
Wyoming↗
Unstable sulfur compounds and the origin of roll-type uranium deposits
Anomalous concentrations of iron sulfides found at roll fronts are believed to result from limited oxidation and mobilization of reduced sulfur species from earlier formed pyrite within the more extensively oxidized core of the roll. Laboratory experiments and chemical theory suggest that the reactions need not be biogenic, and that the sulfur of the reconstituted pyrite could be isotopically indistinguishable from biogenic sulfur. Sulfite formed by limited oxidation slowly decomposes to sulfate and sulfides, and because the sulfate-producing reaction is irreversible at low temperature, only the reduced sulfur species are available for further oxidation-reduction reactions.
Economic Geology↗
The synthesis of ferroselite from an aqueous solution at low temperature
Ferroselite , FeSe 2 , is commonly associated with sandstone-type uranium deposits. Although it has been previously synthesized, the conditions of synthesis were not applicable to the natural system, and the literature did not provide any information on the formation of natural ferroselite . The U. S. Geological Survey's Ambrosia Lake study, however, was concerned with the formation of ferroselite under more natural conditions. Thus, in this work, the synthesis of ferroselite was restricted to conditions more nearly approaching the conditions of the formation of sandstone-type uranium deposits-a low temperature and an aqueous environment. A solution of sodium seleno-sulfate, Na 2 SSeO 3 , stood in contact with freshly precipitated iron sulfide, FeS, for 10 days under an inert atmosphere at 80° C. The final product was identified by its diffraction pattern and consisted of FeSe 2 mixed with about 10 percent FeS.
Economic Geology↗