VNIR-SWIR Spectral Analysis Sheds New Light on Uranium-Hosting Rocks in Texas

As the second-largest state by area in the United States, Texas is home to a wide variety of diverse landscapes. In the northwest part of the state, Texas is home to great plains topography, whereas the southeast contains more marshland, being in bayou country. And to the western part of the state, Texas contains desert and mountain landscapes.

South of San Antonio, Karnes and Live Oak counties contain numerous geologic formations, and these formations are home to minerals and uranium. A recent study published in the Journal of Geochemical Exploration investigated the mineralogical composition of sedimentary rocks that host uranium deposits in Karnes and Live Oak Counties, Texas (1). The study, led by Bernard E. Hubbard from the U.S. Geological Survey's (USGS) Geology, Energy & Minerals Science Center in Reston, Virginia, utilized visible-near-infrared (VNIR) and shortwave infrared (SWIR) spectral analysis to identify key minerals associated with uranium occurrences in the region (1).

A burlap bag filled with uranium ore. It has a rough surface contrasting with the dark, rough stone inside. Unrefined mineral fragments indicate radioactive potential. Generated with AI. Image Credit: © Saowanee - stock.adobe.com.

Uranium is a valuable element used in a variety of applications. Some of these applications include nuclear power, energy, agriculture, and the automobile industry (2). Because of the versatility of uranium, scientists are eager to extract it and use it as needed.

In Karnes and Live Oak counties, four geologic formations contain roll-front uranium deposits. Roll-front deposits are a type of uranium deposit found in permeable sedimentary rocks, and they are characterized by distinct oxidized and reduced zones (1). These four formations (the Tertiary Jackson Group, and the Catahoula, Oakville, and Goliad Formations) are known for their varying compositions and textures, ranging from mudstone and claystone to sandstone (1).

In their study, Hubbard and his team collected reflectance measurements in the 400 to 2500 nm wavelength range from the surfaces of 569 core and cutting samples in these four formations. These measurements were then reduced to 125 representative spectral signatures using the USGS’s Material Identification and Characterization Algorithm (MICA) (1). MICA is an advanced spectral analysis tool. Its purpose is to employ a continuum-removal procedure and least-squares linear regression to match observed spectral absorption features with those of reference mineral standards from a spectral library (1).

The reference minerals included various types of minerals. From micas, carbonates, and clays, the spectral library contained many of the common minerals the researchers were likely to find. Because the research team used MICA in their analysis, they were able to identify common minerals (1).

Looking at the data collected from all four geologic areas, the researchers made a note of where the data was similar and where it differed. For example, they noted that spectral signatures across all four formations shared common minerals, including Ca- and Na-montmorillonite, frequently matched to absorption features in the 2-μm (2000–2500 nm) wavelength range, and goethite, which often appeared in the 1-μm (400–1000 nm) range (1). Goethite's presence is indicative of limonitic iron-staining, a feature commonly associated with oxidized zones in uranium roll-front deposits (1).

The researchers also called attention to the Jackson Group formation, because of the differences in their spectral profiles compared to the others. The Jackson Group formation’s spectral profiles suggested the presence of lignite-bearing mudstone layers, a feature not observed in the Catahoula, Oakville, or Goliad units (1). In contrast, rocks from the Goliad Formation exhibited spectral features associated with dolomite, gypsum, anhydrite, and a green clay mineral potentially identified as glauconite (1).

The researchers concluded that zeolite minerals clinoptilolite and heulandite were present in Jackson Group (1). These minerals, which are often linked to the alteration of volcanic glass in tuffaceous mudstone and claystone layers, provided additional insights into the geologic history of these formations (1).

The last main finding the researchers uncovered in their study was the strong absorption features around 1135 nm. This indicates the presence of the uranium mineral coffinite. Coffinite is a key uranium-bearing mineral found in roll-front deposits, and its spectral signature overlapped somewhat with the 1157 nm zeolite absorption feature (1). This overlap highlights the challenge of distinguishing between these minerals using spectral data alone, suggesting the need for additional hyperspectral field, laboratory, or remote sensing data to improve accuracy (1).

By identifying key minerals associated with uranium, zeolites, and other alteration minerals, the VNIR-SWIR spectral data can help geologists map waste materials left over from past mining activities (1). This mapping could play a crucial role in ongoing efforts to remediate contaminated sites, particularly in areas impacted by uranium mining.

References

1. Hubbard, B. E.; Gallegos, T. J.; Stengel, V. Hyperspectral (VNIR-SWIR) Analysis of Roll Front Uranium Host Rocks and Industrial Minerals from Karnes and Live Oak Counties, Texas Coastal Plain. J. Geochem. Explo. 2024, 257, 107370. DOI: 10.1016/j.gexplo.2023.107370
2. Ulmer-Scholle, D. S. Uranium – What Is It? New Mexico Bureau of Geology and Mineral Resources. Available at: https://geoinfo.nmt.edu/resources/uranium/what.html#:~:text=Uranium%20and%20other%20radionuclides%20are,by%20the%20military%20for%20weapons. (accessed 2024-10-23).