A-type granitoids as a major source of indoor radon and gamma radiation: Insights from the Punilla valley (Argentina)

Abstract

A field-based gamma ray spectrometric study was carried out in the Punilla valley (Argentina), where indoor radon anomalies were recently reported. Results show that the highest eU values correspond to the Achala and Capilla del Monte granitoids, in contrast to their host rocks (i.e., metasedimentary and granitic rocks) and overlying Quaternary deposits, which show generally low eU concentrations. A weak, yet statistically significant positive correlation was obtained between eU and previously published indoor 222Rn data, suggesting a strong lithological control on indoor radon. In the particular case of the Achala batholith, a well-defined positive correlation was also observed between eU and 222Rn concentration based on in situ measurements using an accumulation chamber. The studied uranium-bearing intrusions mainly correspond to A-type granitoids, in line with granitoid geochemical evidence of the GEOROC database, which shows that U-bearing granitoids (i.e., U ≥ 5 mg/kg) worldwide have a typical alkaline affinity and are emplaced in post-orogenic, extensional arc or intraplate settings. When assessing the radon risk based on the geologic hazard, regions characterized by A-type magmatism can thus be considered radon-prone areas. Areas with A-type granitoids are not only radon-prone but, in general, yield relatively high natural radiation levels of multiple sources, thus highlighting the need of systematic studies to identify potentially harmful exposure.