Catahoula formation of the Texas Coastal Plain : origin, geochemical evolution, and characteristics of uranium deposits

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Galloway, William E.
Kaiser, W. R.

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University of Texas at Austin. Bureau of Economic Geology


The Catahoula Formation is composed of ancient fluvial sediments that controlled a wide range of water/sediment interactions responsible for uranium mobilization, transportation, and concentration. Uranium was released from volcanic glass deposited within the Catahoula through early pedogenic and diagenetic processes. Soil development produced plasmic clay cutans, oxide nodules, and vacuoles; open hydrologic system diagenesis produced shard-moldic porosity and clinoptilolite pore-filling cement. Pedogenesis was the most efficient process for mobilizing uranium. Original uranium content in fresh Catahoula glass is estimated to have averaged at least 10 ppm; about 5 ppm was mobilized after deposition and made available for migration. Uranium was transported predominantly as uranyl bicarbonate ion by oxidizing neutral to mildly basic, bicarbonate- and silica-rich ground waters. Uranium transport is continuing today in parts of the Catahoula aquifer in oxidizing (+240to +300 mV) and neutral to highly basic (pH 7 to 11) ground waters. The chemistry of modern Catahoula ground waters reflects down flow ionic evolution and localized mixing with compositionally diverse waters discharged vertically from underlying aquifers. Chlorinity mapping reveals modern ground-water flow patterns, suggests hydrodynamic interpretation of alteration-front geometry, and provides clues to flow dynamics extant during earlier aquifer evolution. Isochemical contours reproduce geometries reminiscent of alteration fronts, reveal vertical discharge of saline waters across aquitards and up fault zones, and demonstrate updip movement of sulfide-rich waters apparently intruded into shallow aquifers along faults. Six uranium deposits representative of the spectrum of Catahoula ores were studied. Uranium-bearing meteoric waters were reduced by reaction with pre-ore stage pyrite formed by extrinsically introduced fault-leaked sulfide (for example, Bruni deposit) or intrinsically by organic matter (for example, Washington-Fayette deposit). Uranium was concentrated in part by adsorption on Ca-montmorillonite cutans, amorphous TiO2, and/ or organic matter followed by uranyl reduction to U4+ in amorphous uranous silicates. Field and geochemical evidence shows that clinoptilolite, a potential adsorber of uranium, is not correlative with mineralization. Calcite is pervasive throughout host sands but shows no spatial or temporal relationship to uranium mineralization. Waters presently associated with Catahoula uranium deposits are oxidizing, alkaline waters of high ionic strength and are not appropriate models for the primary mineralizing waters, which are postulated to have been reducing, acid waters of low to moderate ionic strength. The presence of marcasite and uranium together at the alteration front strongly supports an acid pH during Catahoula mineralization. Maximum adsorption and minimum solubility of uranium occur at approximately pH 6 in carbonate-rich waters. Solution and mineral equilibria were used to test activities and mineral saturation against the occurrence of uranium in four deposits. Log activity ratios of individual waters more highly supersaturated with respect to montmorillonite, taken from montmorillonite-clinoptilolite activity diagrams, show a positive correlation with uranium mineralization. High Ca2+, Mg2+, Al(OH)-4, and H+ activities promote the formation of montmorillonite relative to clinoptilolite. High saturation ratios for montmorillonite show fair correlation with mineralization. The mineral-solution equilibria approach is a potential method of geochemical exploration.


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