Facies control of some tabular uranium deposits in sandstone implies that certain inherent features in the depositional environment set the stage for uranium mineralization. The lacustrine-humate model was developed to explain the facies control of uranium in fluvial-lacustrine units of the Salt Wash Member of the Morrison Formation in south-central Utah and in the Stockton Formation of the Newark Group in the eastern United States. In both of these areas, a close spatial relation exists between offshore-lacustrine primary gray mudstones and uranium-bearing fluvial and marginal-lacustrine sandstones. The primary gray mudstones lie directly above, below, or a short lateral distance from the tabular uranium deposits within the sandstones. This proximity suggests a model in hich alkaline pore waters containing dissolved humic substances (humic and fulvic acids) were expelled by compaction or seepage from the gray mudstones into the adjacent sandstone beds where they were fixed as tabular humate deposits. Uranium carried by groundwater that flowed toward the lakes was then concentrated by the humate to form tabular uranium deposits. Thus, the sedimentologic setting of the host rocks was an important factor in the mineralization process.

Because the dissolved humic substances are thought to be expelled from certain types of mudstones, the nature of these beds becomes important in using the model as an exploration guide. Mudstones deposited in reducing alkaline conditions are considered favorable, because reducing conditions would favor preservation of humic matter in the pore waters of the lake sediments, and alkaline conditions would favor solubilization of the humic substances so that they could be expelled with the pore fluids.

A second important aspect of the model is the means by which humic substances are fixed in the sandstone beds following their expulsion from the mudstones. Formation of organo-clay complexes, with the organic materials interacting with clay coats on sand grains, has been suggested as a possible mechanism. The nature of these complexes has been unclear because of the negative charge associated with both the clays and the humic and fulvic acid molecules. Iron and aluminum hydroxides coating clay surfaces may have formed "bridge linkages" between the clay films and the organic acids because the hydroxides carry a positive charge below pH 8. The hydroxides, abundant in near-surface sediments during early diagenesis, are most effective in fixing humic substances at pH 7, which is within th range of normal groundwaters.

The lacustrine-humate model differs from others in that the humic substances are believed to have migrated only short distances from mudstone beds that lay near the ore-bearing sandstone beds. The model is also an attempt to work within the constraints developed during facies analysis; pore-water and groundwater chemistry and flow patterns are based on reconstruction of sedimentary facies and are consistent with what would be expected in a natural system.

End_of_Article - Last_Page 843------------