Sources and Pathways of Radiogenic Elements in Surface Media Above the Millennium and McArthur River Uranium Deposits in the Athabasca Basin, Saskatchewan, Canada

Title: Sources and Pathways of Radiogenic Elements in Surface Media Above the Millennium and McArthur River Uranium Deposits in the Athabasca Basin, Saskatchewan, Canada
Authors: Devine, Mary Cecilia
Date: 2016
Abstract: This thesis examines the abundance of radionuclides within soil, groundwater, and fractures in shallow sandstones above two deeply buried U deposits in an effort to investigate the mechanisms that concentrate He, Rn, Ra and U within surficial material. Two high-grade, unconformity-associated, monometallic U deposits in the eastern margin of the Athabasca Basin, Saskatchewan, Canada were used as case studies; the Millennium deposit (750 m deep) and the McArthur River deposit (approx. 500 m deep). Extensive glacial sediments up to 100 m thick cover both deposits. Groundwater was sampled from diamond drill-holes and monitoring wells, and surface water was sampled from lakes, and analyzed for the abundance of 3H, minor elements (Cl, Br, F, N, S, U, Th, Pb), stable isotope composition (13C/12C, 2H/H, 18O/16O, 3He/4He), dissolved organic and inorganic carbon content, pH, conductivity and oxidation-reduction potential. B and C horizon soil samples were collected and leached by ammonium acetate at pH 5 to identify elements adsorbed or co-precipitated on secondary minerals such as carbonates, Fe-Mn-oxyhydroxides, and clays. Soils were also leached with HF-HNO3-HClO4 and measured by ICP-MS for major, trace and rare-earth elements. Rn in soil was measured from 30 cm - 80 cm deep within undisturbed soils at both sites. Coatings on drill-core fracture surfaces were leached sequentially with different strengths of nitric acid to determine the concentrations of P, Mn, Fe, Ni, Cu, As, Zr, Ba, Pb, Th, U and Ca. The Ra content of fracture coatings and soil from McArthur River was determined by γ-ray spectrometry, and the mineralogy was determined by powder X-ray diffraction (XRD) and sequential leach geochemistry. All groundwater (down to 50 m depth) at Millennium and McArthur River is <20-year old meteoric water, based on 3H results. Groundwater has undergone minor evaporation, which is evident from the greater abundance of 2H and 18O than the local meteoric water line, and is representative of Saskatchewan precipitation. The negative δ13C signature of DIC in groundwater suggests that potential sources of inorganic carbon are dissolved non-marine carbonates, the oxidation of DOC, and soil-derived CO2. The organic carbon signature is similar to that of terrestrial C3 plants (δ13C DOC = -24 to -27 ‰). Radiogenic 4He in groundwater is reported to be up to 100 times the expected value in equilibrium with the atmospheric at the Millennium deposit (Power, 2014), but is atmospheric near the McArthur River deposit. Rn radioactivity in groundwater has a similar range at both deposits; from 44 – 5342 cpm (approx. 2- 232 Bq/L) at McArthur River, and from 0.8 – 278 Bq/L at Millennium. These values are within the range of Rn radioactivity throughout the Athabasca Basin reported by Earle and Drever (1983).Drill-core fracture coatings contain Ra (≤ 32.2 pg/g) and the dominant mineral phases determined by XRD are illite, kaolin and quartz. Fe-Mn oxy-hydroxides are evident from sequential leach geochemistry. Ra is less abundant in B-horizon soils (≤ 0.220 pg/g) than fracture coatings and Ra in B-horizon soils is highest near the surface projection of the McArthur River deposit, where Rn in soil gas is also highest (6.3 kBq/m3) near drill-hole MC-251. The abundance of Ra adsorbed to secondary minerals on core fracture coatings is likely impacted by groundwater flow and the Fe and Mn concentration of fracture surfaces. Secondly, Rn in groundwater above Millennium and McArthur River U deposits is a decay product primarily from local Ra on fracture surfaces, and less so from Ra in B-horizon soils. Additionally, Rn in soil gas correlates with Ra in soils; however, Rn does not correlate with U in groundwater, soil or coatings along fractures. Furthermore, Rn in soil gas is at background levels of <10 kBq/m3 above both deposits, and therefore not an indication of U mineralization in these glaciated terrains. This study’s findings suggest that the concentration of Rn in groundwater is controlled by radon’s 3.82-day half-life, the mobility of U6+ in oxidized conditions, and radium’s affinity for Mn-oxy-hydroxides. Rn within young groundwater above the two deeply buried U deposits is a decay product from local Ra on fracture surfaces. Rn gas in oxidized, shallow groundwater does not correlate with the concentration of U in nearby soil, groundwater, or core fracture coatings.
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