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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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The almost universal association of uranium with humic-type organic matter in sedimentary deposits has long been recognized. This close relation is often overlooked in some sandstone uranium deposits owing to the difficulty in recognizing the unstructured organic material, the lack of analyses for organic carbon, or the labile nature of the immature humic substance. A colloidal suspension of humic acids derived from decaying vegetal matter can be made to flocculate by a drop in pH, by local changes in cation concentration, or by adsorption on clays. Evidence shows that the roll-type deposits are essentially "organic rolls" in which the humic acids were precipitated primarily by the drop in pH associated with the redox interface. Once precipitated the humic substances are articularly efficient in the collection of metals from very dilute solutions such as natural waters.
In actively migrating oxidation fronts, the humic matter and some of the uranium appears to be remobilized and concentrated downdip from the "radiometric front." This may explain the poor correlation between organic matter and uranium in some of the Texas deposits. The humic substances, however, are believed to be an essential prerequisite for the original accumulation of the uranium in the roll fronts.
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