Hostetler and Garrels reported in 1962 that the uranium and vanadium in sandstone-type deposits of the western U.S. apparently have been transported to their present environment from up the groundwater gradient by low-temperature aqueous solution sand from external sources. The authors interpret to define the characteristics of aqueous solutions capable of transporting significant quantities of uranium and vanadium through continental sedimentary rocks, and the changes in these characteristics that might result in precipitation of uraninite and other ore minerals in concentrations of ore grade.
On the basis of present knowledge, the transportation environment is shown 'to be that of weakly alkaline, moderately reducing ground water, with an average or larger than average concentration of dissolved carbonate species (H2CO3 + HCO2 + CO3--). Precipitation is induced by reduction, probably by carbonaceous material or hydrogen sulfide, or both. Uranium is transported mainly in the form of the highly stable uranyl bicarbonate and tricarbonate complexes. Precipitation results from reduction of hexavalent aqueous uranium species to form uraninite, reduction of tetravalent vanadium to form montroseite, and fixation of uranyl ions by combination with potassium ions and quinquivalent vanadium to form the mineral carnotite.
The authors concluded that:
1. Eh-pH diagrams at 25°C were constructed with the aid of published thermodynamic data for the U-O2-H20 and U-O2-H2O-CO2 systems. Uraninite, UO2+x, and schoepite, UO2(OH)2.H20, are the only solid phases expected to be stable in the presence of water and variable concentrations of oxygen in the system U-O2-H20. The addition of CO2 to this system causes formation of stable uranyl carbonate complexes and a significant increase in the solubility of uraninite and schoepite.
2. Following a rough calculation of the free-energy of carnotite, Eh-pH diagrams of the U-V-K-O2-H2O-CO2, system were drawn for various values
of total carbonate (ΣCO2). These diagrams show stability fields for carnotite and other uranium and vanadium solid phases, and indicate the conditions
necessary for vanadium and uranium solubility. One set of conditions was found to indicate solubility in a mildly reducing, neutral to alkaline environment containing fairly abundant CO2, and thus is comparable with some natural groundwater.
3. Analysis of solubility controls indicates that increasing CO2 increases solubility. Oxidizing conditions cause carnotite to precipitate from solution while more reducing conditions cause montroseite and uraninite to form.
4. The possible effects of other ionic species commonly found in natural ground waters were investigated within the Eh-pH framework of aqueous transport in the U-V-K-O2-H2O-CO2 system. No significant changes in such transport were found. Sodium and, to a lesser degree, calcium and magnesium can balance electrostatically the concentrations of HCO3-, CO3-2, and other anions in solution. Activity gradients showing varying transportation conditions were presented.
5. In the presence of excess quartz the field of coffinite stability is probably very similar to that of uraninite. With favorable conditions (moderate concentration of its constituent elements) the field of formation of roscoelite should approximate that of montroseite.
6. Carnotite stability is very sensitive to changes in ionic activity rather than changes in Eh and pH (except for distinctly reducing conditions); its precipitation is critically controlled by the amounts of uranium, vanadium and CO2 in solution. The reducing environment provided by ore-solution migration into carbonaceous and/or H2S rich areas was shown to be sufficient to precipitate uraninite and montroseite from dilute solutions at 25° C.
Temperatures above 120°C might be sufficient to precipitate uraninite from the postulated ore-bearing solutions because of the breakdown of the uranyl carbonate complexes. Sulfides, carbonaceous material and trivalent vanadium minerals would not be expected to occur with uraninite formed in this way. The lack of such ore is interpreted as supporting evidence for a depositional temperature of 120° C or less.
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