Geological and Geographical Attributes of the South Texas Uranium Province

Abstract

The South Texas Uranium Province, which includes about 20 counties, mostly wraps around a formation known as the Catahoula Formation, from the Mexican border to south of San Antonio. Approximately 100 open-pit mines operated in the second half of the 20th century, but, after a hiatus of several decades, the industry is now investing in in situ recovery operations of additional uranium deposits. Important volcanic episodes, centered around Catahoula deposition times more than 20 million years ago, are credited, through ash-fall and related sediments, as being the source of uranium deposits, as well as associated elements such as arsenic and selenium. These trace elements, enriched in local sediments, were quickly mobilized in their soluble form by infiltrating waters. Then, after flowing deeper into the subsurface along stratigraphic dip within high-transmissivity, sand-rich formations deposited by rivers, uranium encountered more reducing conditions and precipitated out of solution in a pattern known as roll front. Selenium and molybdenum are also redox sensitive and accumulated in the solid phase at the redox front along with uranium. Other trace elements, such as arsenic, vanadium, boron, and fluorine, behave differently and remained mostly in solution.

Mechanisms resulting in commercial deposits are:

  • Source: Leaching of uranium and other elements (As, Mo, V, Se, B, F) from volcanic rocks (fresh ash fall and reworked ash sediments) by oxidized, slightly alkaline waters and migration downdip. Volcanic rocks are more prone to alteration because of their glassy nature and because crystals are often small and easily weathered, especially in derived sediments.

  • Migration: Controlled primarily by transmissive depositional axes (high-permeability channels) while uranium is being mobilized by oxidizing alkaline waters. In environmental conditions of Eh and pH, uranium forms mobile uranyl-carbonate complexes.

  • Trapping Mechanism: Reducing conditions due to periodic intermittent oil/gas/H2S migrating along faults (and subsequent sulfide precipitation) or, less commonly, organic-rich sediments forcing uranium to change redox state from a soluble oxidized form U(VI) to an insoluble reduced form U(IV). Uranium drops out of solution as water passes the redox flow as uraninite (UO2, uranium oxide) or coffinite (USiO4, uranium silicate). Chromatographic precipitation of other trace elements mobilized along with uranium according to redox behavior and geochemical gradients.

  • Concentrating Mechanism: Multiple occurrences of dissolution/precipitation as the redox front moves downdip and downgradient owing to a continuous stream of oxidizing recharging waters, bringing more uranium and dissolving previously precipitated uranium to be deposited together.

  • Host Rock: High-permeability sand channels allowing continuous flow unaffected by slight changes in sand porosity.

Surface alteration of some deposits when exposed to oxidizing waters can remobilize uranium and other trace elements. However, the extent of poor-quality water from ore bodies, including unmined subeconomic ore bodies, remains an open question.

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