Distribution of U and Th in an Iberian U-fertile granitic complex (NW, Spain): airborne-radiometry, chemical and statistical approaches

The behaviour and distribution of U and Th have been studied in an Iberian Hercynian granitic complex known as Cadalso-Casillas de Flores batholith. This batholith is composed of two granitic intrusions known as El Payo and Penaparda complexes. To explain the geochemical behaviour, the distribution of uranium and thorium and determine the fertility in U of these granites in search for mineral resources, three different approaches have been used: Interpretation of airborne radiometric data, chemical analysis of whole-rock and accessory minerals and statistical calculations. The obtained results indicate that: (a) the airborne gamma-ray spectrometry is a powerful mapping and exploration tool; (b) U/Th ratios in whole-rock samples increase with the evolution and the chemical analysis of radio-element bearing accessory minerals showed that both elements were segregated in accessory minerals during magmatic fractional crystallisation. This primary distribution of U and Th was affected by superimposed post-magmatic, hydrothermal and meteoric alterations; and (c) the statistical approach, approximately defined that the proportion of U linked to U-rich mineral phases especially uraninite differs in both granitic complexes. It also identified those independent variables that are positively correlated with uranium and hence any U not unaccounted for by these elements was interpreted as uraninite or its alteration products. The statistical analysis also highlighted the lack of positive correlations between uranium and thorium. Accordingly, the studied granites were grouped into two different genetically unrelated intrusions, where the behaviour of U and Th-bearing accessory minerals was also different and only the granitic units G2 and G3 of the El Payo complex and G6 of the Penaparda complex could be classified as fertile in U. (C) 2014 Elsevier B.V. All rights reserved.