Light rare earth elements and natural series radionuclides within the Morro do Ferro

Abstract As part of the Pocos de Caldas natural analogue project, data are presented on natural series radionuclides ( 232 Th, 230 Th, 228 Th, 234 U, 238 U and 231 Pa) and the light rare earth elements (LREEs) La, Ce, Pr, Nd and Sm from two boreholes within the near surface thorium-rare earth element ore body known as Morro do Ferro (MF). Under defined conditions, a study of these elements can provide useful information on the long term behavior of chemical analogues present in high level waste (HLW) and on the processes affecting radionuclide migration. Within the deeply weathered mineralized zone, Th decreases exponentially with a half thickness of 10 meters from 5.7% in the surface meter to 0.1% at 36 meters. Since no fractionation of 228 Ra is evident (based on 228 Th 232 Th equilibrium), 228 Ra is essentially decaying in-situ where it is radiogenically produced. Uranium isotopes ( mean = 35 ppm as 238 U ) are found in near equilibrium ( 234 U/ 238 U range of 0.9 to 1.1) both within the mineralization and down along the hydraulic gradient. No evidence for any significant uranium accumulation exists. Leaching of uranium by infiltrating, oxidizing rain water is postulated as the dominant removal mechanism although preferential loss of 234 U also occurs as evidenced by 234 U/ 238 U ratios of Positive and negative Ce anomalies are reported and have been interpreted as occurring from relic redox fronts which are no longer visible.