Study of the relationship between mineralogic types and potential Radon in rocks of the region of Serra do Gandarela - Quadrilatero Ferrico-MG

Studies on the exhalation of radon (Rn) and its contribution to the dose in indoor environments have been developed worldwide to create a database to compose a geogenic mapping of the countries and to identify areas of greatest risk related to that gas. The presence of Rn on the surface is determined by some factors such as the presence of uranium (U), thorium (Th) and radium (Ra) in rocks and soils, as well as factors related to the porosity and humidity of the geological environments in which they are found. The Serra do Gandarela, a range of hills located in the geological context of the Quadrilatero Ferrifero (Qf), Minas Gerais (MG), hosts an important occurrence of U, associated with the conglomerates of the Moeda Formation (MF), originating in Paleo-Proterozoic times, over 2.4 x 10⁹ years ago. The objective of this survey is to study the relationship between the potential of Rn gas and rocks from Serra do Gandarela, using radiation detection methodologies, radon exhalation measurement, and analytical technique to determine the precursors of the Rn. Two rock varieties were sampled in surface outcrops and geological prospecting gallery. The measurements were performed using radon monitors like AlphaGUARD, gamma spectrometers, and exhalation circuits. The presence of U and Th in the samples was determined by the neutron activation (NAA). The knowledge of the characteristic e manometric quantities of each type of rock can be an important tool for geo scientific and geo environmental studies of areas that have uranium mineralization. The contribution of this work is related to the radiological issue, which may be linked to the eventual accumulation of radon in the mining and prospecting galleries in the Serra do Gandarela region. Therefore, it was expected that the uranium present in the minerals of the uraniferous rocks (conglomerates) would contribute to a high exhaling power of the Rn to the atmospheric environment. However, the results obtained during the determination of radon exhalation rates allowed to verify that the presence of uranium minerals does not always determine, or is proportional to, the amount of radon emanated and consequently exhaled. The rock samples analyzed, although uraniferous (U contents between 2 and 43 µg.g^-1, and Th between 6 and 42 µg.g^-1), did not present high exhalation of radon, which oscillated between 50 to 300 Bq.m^-3, being the higher value attributed to the MF conglomerate taken from the prospecting gallery. The combination of the petrographic knowledge of the rocks; the results of uranium and thorium by chemical analysis; and the exhalation of Rn in a closed circuit, showed that the sample with the highest uranium content will not always present the highest exhalation. To conclude, as a final contribution to this study, it can be said that, although this is the main uraniferous region of the Qf, the radon risk of Serra do Gandarela can be considered from low to medium. (author)