A record of uranium-series transport at Nopal I, Sierra Peña Blanca, Mexico: Implications for natural uranium deposits and radioactive waste repositories

Abstract

Studies of uranium-series (U-series) disequilibria within and around ore deposits provide valuable information on the extent and timing of actinide mobility, via mineral-fluid interaction, over a range of spatial and temporal scales. Such information is useful in studies of analogs of high-level nuclear-waste repositories, as well as for mining and mineral extraction sites, locations of previous nuclear weapons testing, and legacy nuclear waste contamination. In this study we present isotope dilution mass spectrometry U-series measurements for fracture-fill materials (hematite, goethite, kaolinite, calcite, dolomite and quartz) from one such analog; the Nopal I uranium ore deposit situated at Peña Blanca in the Chihuahua region of northern Mexico. The ore deposit is located in fractured, unsaturated volcanic tuff and fracture-fill materials from surface fractures as well as fractures in a vertical drill core have been analyzed. High uranium concentrations in the fracture-fill materials (between 12 and 7700 ppm) indicate uranium mobility and transport from the deposit. Uranium concentrations generally decrease with horizontal distance away from the deposit but in this deposit there is no trend with depth below the surface. Isotopic activity ratios, 238 U-234 U-230 Th-232 Th linear age trends, and U-Th-Pa model ages indicate an actinide mobility evolution that can be characterized as complex and varied. There is evidence of both open and closed system behavior for uranium, depending on both sample and timescale. The 234 U/ 238 U activity ratios indicate U mobility from 1.2-0.2 Ma. In the last 0.2 Ma there is isotopic evidence for some U mobility but mostly U immobility. In almost all cases the Th-Pa radionuclides studied here have been immobile for the past 200 ka. Radium appears to have been recently (0.1-8 ka) mobile due to ongoing fluid flow in the fractures. Overall, the mainly closed system behavior for U-Th-Pa over the past ~200 ka provides an indicator of the geochemical immobility of these actinides over timescales relevant for potential nuclear waste repositories situated in fractured, unsaturated tuff.

Radioactive Wastes

“…Waste. An isotope dilution mass spectrometry U-series measurements for fracture-fill materials studied by Denton et al, (2016). Authors condudcted a study on the Nopal I uranium ore deposit situated at Peña Blanca of northern Mexico.…”

Final report of the Peña Blanca natural analogue project

“…Samples of iron oxide-oxyhydroxide fracture fillings were collected from the PB1 drill core to extend the uranium-series investigations into three dimensions. Long-lived Useries results for the surface fracture-filling materials have been summarized and compared to new U-series analyses of the drill-core fracture-filling materials by Denton et al (2016). High uranium concentrations in the fracture-fill materials of about 12 to 7700 ppm (Denton et al, 2016, Table 2) indicate uranium mobility and transport from the ore deposit in the past.…”

“…Long-lived Useries results for the surface fracture-filling materials have been summarized and compared to new U-series analyses of the drill-core fracture-filling materials by Denton et al (2016). High uranium concentrations in the fracture-fill materials of about 12 to 7700 ppm (Denton et al, 2016, Table 2) indicate uranium mobility and transport from the ore deposit in the past. Uranium concentrations generally decrease with horizontal distance away from the ore deposit.…”

“…The results of uranium analyses for both unsaturated-zone and saturated-zone groundwater, including the seepage experiment, confirm that uranium is being dissolved in relatively low amounts from the ore body and from rock near the ore body under present geochemical and geohydrologic conditions . The mainly closed-system behavior of U-Th-Pa in fracture-filling materials over the past ~200 ka Denton et al, 2016) provides an indicator of the geochemical immobility of these actinides over long time scales.…”

Evaluación de la peligrosidad de los elementos potencialmente tóxicos presentes en residuos mineros abandonados

“…Descripción de las principales características de las muestras la Tabla 9 se presenta la información geoquímica del área de estudio donde cabe destacar que no se incluyó al U debido a que este se encontró por debajo de los límites de detección (< 1 ppm). Una posible explicación a esto puede deberse a que, según Denton et al (2016)[218], en el área de estudio las concentraciones más altas de U se encuentran…”