Controlo geológico e mineralógico da radioatividade natural: um estudo na região de Trás-os-Montes e Alto Douro
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2017
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Na região de Trás-os-Montes e Alto Douro afloram vários grupos de granitóides, nomeadamente os granodioritos de S. Lourenço (Grupo I), o grupo II onde se integram 12 tipos de granitos duas micas e o grupo III constituído por 5 tipos de granitos biotíticos. Estes granitóides intruíram nas formações metassedimentares do Complexo Xisto Grauváquico, do Ordovícico Inferior e Superior e no Complexo de Mantos Parautóctone. A implantação destes granitos ocorreu durante as fases de deformação dúctil a frágil varisca (D3 e D4) entre os 321 e os 290 Ma. De uma forma geral, observou-se para os granitóides estudados uma correlação entre a presença e abundância de minerais acessórios como zircão, apatite, ilmenite, monazite, uraninite, torite, cofinite, alanite, xenótimo e fosfatos de tório e urânio e os teores de U e Th. Nos granitos biotíticos, a alteração hidrotermal favoreceu o aparecimento de fosfatos de urânio e tório disseminados nos óxidos de ferro que preenchem as microfracturas ou os bordos minerais. Os granitos de duas micas apresentam perfis de terras raras mais fracionados, e um enriquecimento generalizado de elementos de terras raras leves (LREE), proveniente da abundância de monazite em particular nos granitos Vale das Gatas e Felgueiras, comprovando desta forma, os elevados teores de tório obtidos (53 a 65 ppm). Quanto aos granitos biotíticos possuem perfis de terras raras menos fracionados devido a um enriquecimento em HREE, proveniente da abundância de xenótimo e elevadas quantidades de ítrio (35 a 48 ppm). Os granitóides da região em estudo apresentam um fundo radiométrico (137.6 ɳGy.h- 1), atividade em massa de 226Ra (105.6 Bq.kg-1), 222Rn (9.8 Bq.kg-1), potencial de produção de radão (172.4 Bq.m-3.h-1), radão nos solos (111 kBq.m-3) e nos edifícios (423 Bq.m-3) bastante superiores aos metassedimentos. Contudo, importa destacar que os metassedimentos apresentam mais do dobro do coeficiente de emanação (23.4 %) relativamente aos granitóides. Quanto à análise nos grupos metassedimentares verifica-se que, o Ordovícico se destaca pelo maior potencial de produção de radão obtido (117.9 Bq.m-3.h-1), devido ao elevado coeficiente de emanação manifestado (26.0 %), eventualmente através da foliação metamórfica. No entanto, o Complexo Xisto Grauváquico também se destaca no fundo radiométrico (128.9 ɳGy.h-1), radão nos solos (27 kBq.m-3) e na concentração de radão em espaços confinados (237 Bq.m-3). Quanto aos granitóides, o Grupo III destaca-se na atividade em massa do 226Ra (115.3 Bq.kg-1), 222Rn (18.8 Bq.kg-1), potencial de produção de radão (356.7 Bq.m-3.h-1), no radão nos solos (156 kBq.m-3) e edifícios (547 Bq.m-3), conferindo-lhe um maior potencial radiológico e um maior risco de exposição. Para além disso, verificou-se que as concentrações médias anuais de radão interior nos grupos II e III foram superiores, ao limite imposto de 300 Bq.m-3 pela diretiva comunitária 2013/59/EURATOM. Contudo, para todos os grupos estudados estes valores foram superiores à média mundial, ao nível recomendado por entidades internacionais e aos valores médios anuais de diversos países europeus. A presença de falhas e carreamentos próximos de algumas habitações, induziram um aumento na concentração de radão no interior das mesmas, funcionando estas vias como principais meios de condução entre as rochas ricas em urânio e a superfície, potenciando o risco radiológico nesta região. O teor de urânio e potencial de produção de radão nas rochas, a concentração de radão os solos e as principais estruturas tectónicas que permitiram a livre circulação deste gás nos solos, em concomitância com a concentração de radão nos edifícios e principais fatores que o condicionam, permitiram a classificação de um risco de exposição ao radão baixo a moderado para os grupos metassedimentares e um risco moderado a elevado nos grupos graníticos. Todas estas observações anteriores tiveram um enorme impacto na avaliação do potencial radiológico da região, pelo que se aconselha para fins estratégicos de ordenamento do território que se introduzam novas técnicas arquitetónicas em edifícios a construir, salvaguardando o licenciamento das novas construções em zonas graníticas e promovendo uma maior ventilação nos edifícios existentes.
In the study area, several groups of granitoids occur, namely granodiorites of São Lourenço (Group I), eleven two-mica granites (Group II) and five biotite granites (Group III). These granitoids outcrop in metasedimentary formations of Shale-Greywacke Complex (CXG), Lower to Upper Ordovician and Parautochthonous Nappes Complex. The emplacement of these granitoids occurred during the ductile- fragile deformation phases (D3 and D4), between 321 and 290 Ma. In general for the studied granitoids, a correlation between U and Th contents and the presence or abundance of accessory minerals such as zircon, apatite, ilmenite, monazite, uraninite, thorite, coffinite, allanite, xenotime and thorium-uranium phosphates were observed. Furthermore in biotite granites, the hydrothermal alteration induced the appearance of uranium and thorium phosphates disseminated in the iron oxides that fill the microfractures or mineral borders. Two mica granites have more fractionated rare earth element (REE) profiles and a generalized enrichment of light rare earth element (LREE), due to the abundance of monazite in Vale das Gatas and Felgueiras granites, proving by the high thorium content obtained (53 the 65 ppm). Biotite granites have less fractionated REE profiles due to heavy rare earth element (HREE) enrichment from xenotime abundance and high amount of yttrium (35 to 48 ppm). The granitoids of the study area have a radiometric backgound (137.6 ɳGy.h-1), mass activity of 226Ra (105.6 Bq.kg-1) and 222Rn (9.8 Bq.kg-1), potential of radon production (172.4 Bq.m-3.h-1), radon in soils (111 kBq.m-3) and buildings (423 Bq.m-3), much higher than the metasediments. However, it should be noted that metasediments have more than double the emanation coefficient (23.4 %) compared to granitoids. In metasedimentary groups, the Ordovician have a greatest potential of radon production (117.9 Bq.m-3.h-1) and emanation coefficient (26.0 %) expressed eventually, by metamorphic foliation. Nevertheless, CXG also highlighted in the radiometric background (128.9 ɳGy.h-1), radon in soils (27 kBq.m-3) and radon concentration in confined spaces (237 Bq.m-3). Overall, the Group III obtained mass activity of 226Ra (115.3 Bq.kg-1), 222Rn (18.8 Bq.kg-1), potential of radon production (356.7 Bq.m- 3.h- 1), radon in soils (156 kBq.m-3) and buildings (547 Bq.m-3) much elevated than the other groups, giving it a remarkable radiological potential and an increased exposure risk. In addition, the annual average indoor radon concentrations in groups II and III were higher, than the imposed limit of 300 Bq.m-3 by the EU directive 2013/59 / EURATOM. Furthermore, all groups were highest annual average indoor radon concentration than average world, at the level recommended by international entities and annual average values of several European countries. In fact, the presence of faults and overthrusts nearby of buildings, induced an increase in the indoor radon concentration. These pathways acting as the main conveying means between rocks uranium bearing and the surface, enhancing the radiological risk in this region. The uranium content and potential radon production in the rocks, the radon concentration in soils and major tectonic structures that conditioned the free circulation of this gas in soils, concomitantly with the radon concentration in dwellings and other main factors that condition it, allowed the classification of a low to moderate radon exposure risk for metasedimentary groups and a moderate to high risk in granite groups. All these previous observations have had a major impact on the assessment of the radiological potential in this region. It is therefore recommended for strategic purposes of land use planning to introduce new architectural techniques in constructed buildings, safeguarding the licensing of new constructions in granite areas and ventilation in existing buildings.
In the study area, several groups of granitoids occur, namely granodiorites of São Lourenço (Group I), eleven two-mica granites (Group II) and five biotite granites (Group III). These granitoids outcrop in metasedimentary formations of Shale-Greywacke Complex (CXG), Lower to Upper Ordovician and Parautochthonous Nappes Complex. The emplacement of these granitoids occurred during the ductile- fragile deformation phases (D3 and D4), between 321 and 290 Ma. In general for the studied granitoids, a correlation between U and Th contents and the presence or abundance of accessory minerals such as zircon, apatite, ilmenite, monazite, uraninite, thorite, coffinite, allanite, xenotime and thorium-uranium phosphates were observed. Furthermore in biotite granites, the hydrothermal alteration induced the appearance of uranium and thorium phosphates disseminated in the iron oxides that fill the microfractures or mineral borders. Two mica granites have more fractionated rare earth element (REE) profiles and a generalized enrichment of light rare earth element (LREE), due to the abundance of monazite in Vale das Gatas and Felgueiras granites, proving by the high thorium content obtained (53 the 65 ppm). Biotite granites have less fractionated REE profiles due to heavy rare earth element (HREE) enrichment from xenotime abundance and high amount of yttrium (35 to 48 ppm). The granitoids of the study area have a radiometric backgound (137.6 ɳGy.h-1), mass activity of 226Ra (105.6 Bq.kg-1) and 222Rn (9.8 Bq.kg-1), potential of radon production (172.4 Bq.m-3.h-1), radon in soils (111 kBq.m-3) and buildings (423 Bq.m-3), much higher than the metasediments. However, it should be noted that metasediments have more than double the emanation coefficient (23.4 %) compared to granitoids. In metasedimentary groups, the Ordovician have a greatest potential of radon production (117.9 Bq.m-3.h-1) and emanation coefficient (26.0 %) expressed eventually, by metamorphic foliation. Nevertheless, CXG also highlighted in the radiometric background (128.9 ɳGy.h-1), radon in soils (27 kBq.m-3) and radon concentration in confined spaces (237 Bq.m-3). Overall, the Group III obtained mass activity of 226Ra (115.3 Bq.kg-1), 222Rn (18.8 Bq.kg-1), potential of radon production (356.7 Bq.m- 3.h- 1), radon in soils (156 kBq.m-3) and buildings (547 Bq.m-3) much elevated than the other groups, giving it a remarkable radiological potential and an increased exposure risk. In addition, the annual average indoor radon concentrations in groups II and III were higher, than the imposed limit of 300 Bq.m-3 by the EU directive 2013/59 / EURATOM. Furthermore, all groups were highest annual average indoor radon concentration than average world, at the level recommended by international entities and annual average values of several European countries. In fact, the presence of faults and overthrusts nearby of buildings, induced an increase in the indoor radon concentration. These pathways acting as the main conveying means between rocks uranium bearing and the surface, enhancing the radiological risk in this region. The uranium content and potential radon production in the rocks, the radon concentration in soils and major tectonic structures that conditioned the free circulation of this gas in soils, concomitantly with the radon concentration in dwellings and other main factors that condition it, allowed the classification of a low to moderate radon exposure risk for metasedimentary groups and a moderate to high risk in granite groups. All these previous observations have had a major impact on the assessment of the radiological potential in this region. It is therefore recommended for strategic purposes of land use planning to introduce new architectural techniques in constructed buildings, safeguarding the licensing of new constructions in granite areas and ventilation in existing buildings.
Descrição
Tese de Doutoramento em Geologia
Palavras-chave
Granito , Radão , Rochas , Solos , Edifícios , Metassedimentos