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

As part of the Pocos de Caldas natural analogue project, data are presented on natural series radionuclides (232Th, 230Th, 228Th, 234U, 238U and 231Pa) 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 228Ra is evident (based on 228Th232Th equilibrium), 228Ra is essentially decaying in-situ where it is radiogenically produced. Uranium isotopes (mean = 35 ppm as 238U) are found in near equilibrium (234U/238U 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 234U also occurs as evidenced by 234U/238U ratios of < 1.0 in rock and between 1.1 and 1.5 in ground water samples. Positive and negative Ce anomalies are reported and have been interpreted as occurring from relic redox fronts which are no longer visible.