3-D distribution of the radioelements in the granitic rocks of northern and central Portugal and geothermal implications

Abstract

Purpose

The main purpose of this work is to study the distribution of radioelements in both horizontal and vertical directions within the main granitic units outcropping in northern and central Portugal in order to assess the main target for geothermal energy feasibility studies.

Methods

For that, a total of 314 samples collected in the surface as well as in the subsurface were analysed by gamma-ray spectrometry techniques.

Results

The results show that the older granitic units studied (syn-orogenic pre-D3) reveal smaller concentrations of both uranium (U = 4.5 mg/kg) and thorium (Th = 9.1 mg/kg) than the youngest units (late- to post-orogenic/post-D3). This last group presents an average concentration of 7.1 mg/kg for uranium and 21.0 mg/kg for thorium. The subsurface samples belong to the youngest granitic group that was studied, and present higher uranium concentration (U = 14.3 mg/kg) but a decrease in thorium concentration (Th = 17.5 mg/kg) in comparison with the surface samples of the same lithology. Within 1000 m in depth the concentration of radioelements has a remarkably constant pattern suggesting that the distribution of radioelements is not a function of depth.

Conclusions

Based on the results, it can be concluded that the late- to post-tectonic porphyritic, biotite or biotite-muscovite granite (e.g., Beiras Granitic Batholith) is the highest radiothermal granitic group in the studied area, which makes it the main target for more detailed studies regarding its potential for geothermal energy, for power production and others exploitation options in Central Portugal.

Resumen

Objetivo

El objetivo principal de este trabajo es estudiar la distribución de radioelementos tanto en dirección horizontal como vertical dentro de las principales unidades graníticas aflorantes en el norte y centro de Portugal para evaluar cuál de ellas tiene más potencial para estudios de viabilidad de energía geotérmica.

Métodos

Para eso, un total de 314 muestras recogidas en la superficie así como en el subsuelo se analizaron mediante técnicas de espectrometría de rayos gamma.

Resultados

Los resultados muestran que las unidades graníticas más antiguas estudiadas revelan concentraciones más bajas de ambos, uranio (U = 4,5 mg/kg) y torio (Th = 9,1 mg/kg) que las unidades más recientes. Este último grupo se presenta con una concentración media de 7,1 mg/kg de uranio y 21,0 mg/kg para el torio. Las muestras de subsuelo pertenecen al grupo granítico más reciente y presentan un aumento en la concentración de uranio (U = 14,3 mg/kg), pero una disminución de torio (Th = 17,5 mg/kg) en comparación con las muestras superficiales de la misma litología. Hasta los 1000 m de profundidad la concentración de radioelementos tiene un padrón notablemente constante lo que sugiere que la distribución de los radioelementos no es una función de la profundidad.

Conclusiones

Con base en los resultados, se puede concluir que el granito profiroide, biotitico o biotitico-muscovitico tarde- a post-tectónico (e.g., Batolito Granítico das Beiras) es el grupo granítico más radiotérmico del área estudiada, lo que lo hace el principal objetivo de estudios más detallados sobre su potencial para la energía geotérmica, para la producción de energía y otras opciones de explotación en el centro de Portugal.

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Acknowledgements

This work has been framed under the Initiative Energy for Sustainability of the University of Coimbra and supported by the Energy and Mobility for Sustainable Regions Project (CENTRO-07-0224-FEDER-002004), co-funded by the European Regional Development Fund (ERDF) through the «Programa Operacional Regional do Centro 2007–2013 (PORC)», in the framework of the «Sistema de Apoio a Entidades do Sistema Científico e Tecnológico Nacional», and by the «Fundação para a Ciência e Tecnologia». The authors would like to acknowledge Prof. José António Simões Cortez and Câmara Municipal de Almeida for allowing the study of the borehole core, and Jorge Carvalho e Inês Pereira for their help in improving the manuscript.

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Correspondence to M. M. Miranda.

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Lamas, R., Miranda, M.M., Pereira, A.J.S.C. et al. 3-D distribution of the radioelements in the granitic rocks of northern and central Portugal and geothermal implications. J Iber Geol 43, 3–12 (2017). https://doi.org/10.1007/s41513-017-0001-y

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Keywords

  • Radioelements
  • Granites
  • Geothermal energy
  • Central Iberian Zone
  • Portugal

Palabras clave

  • Radioelementos
  • granitos
  • energía geotérmica
  • Zona Centroibérica
  • Portugal