Study of an aplite dyke from the Beira uraniferous province in Fornos de Algodres area (Central Portugal): Trace elements distribution and evaluation of natural radionuclides

• Distribution of natural radionuclides in an U-rich aplite from Central Portugal.
• INAA, XRD and gamma spectrometry were applied to various types of samples.
• Higher U values for clay/sand fractions, and core of spheroidal weathered aplite.
• Large difference between pre-Rn and post-Rn values suggests high loss of radon.
• Weathering of aplite is a potential source of radiological hazards.

A uranium-rich aplite dyke with spheroidal alteration sited in the Beira uraniferous province, in Fornos de Algodres area (Northern Central Portugal) was studied, focusing on trace element distributions, especially U, in the aplite. A vertical profile, different size fractions, and various concentrically weathered layers of rock, were examined. The main goal of the work is to better understand the geochemical behavior and distribution of natural radionuclides in fine-grained granitic rocks, and changes that occur during weathering. The rock samples obtained from this site were examined using chemical and mineralogical methods: instrumental neutron activation analysis (INAA) and the X-ray diffraction (XRD). Field gamma spectrometry (FGS) and high resolution gamma spectrometry (HRGS) in the laboratory were also performed in order to determine concentrations of the natural radionuclides (K, Th and U) responsible for terrestrial gamma radiation, and for comparison of results with INAA measurements, enabling some information concerning radon losses.The U concentration in the studied samples vary between 4.5 and 83 ppm (always higher than upper continental crust value), with the higher values detected in the clay and sand fractions of weathered aplite (residual clay) and in the rounded boulders of aplite with spheroidal weathering, especially in the core. Strong variations in natural radionuclide contents, especially observed between residual clay and soil, relate to differences in source material. The large difference between pre-Rn and post-Rn values obtained by HRGS suggests high loss of radon (40% minimum), which is in accordance with field measurements. Due to generally high concentration of U, the aplite is a potentially strong source of emission of radon to the atmosphere with consequent radiological hazards.