Gamma-ray spectroscopy determination of radioactive elements in late- Hercynian plutonic rocks of Val Biandino and Val Trompia (Lombardy, Italy)

The results from natural gamma radiation measurements in main plutonic rocks outcropping in Val Biandino (VB) and Val Trompia (VT), pertaining to the central sector of the South Alpine domain, are presented and compared with data on Valle del Cervo pluton. The measurements were performed with the aid of a high-resolution gamma-ray spectroscopic system using HPGe detector. The aim is to contribute to the establishing of a baseline map of environmental radioactivity levels. In the VB samples the granite activities of 235U and 238U are only slightly higher than 232Th specific activity, while the opposite occurs in more basic rocks; moreover, U and Th contents are in agreement with the levels of rocks of calc-alkaline affinity and Hercynian age. In VT rock types the activities of 238U are in the range 48-74 Bq kg-1 and the granite shows the lowest value of activity. The 232Th activities are quite similar for granite and granodiorite. Samples coming from Torgola fluorite mine, hosted in the granitic-granodioritic rocks, show U and Th activities that do not exceed 80 and 70 Bq kg-1 respectively. The U and Th specific activities of VT rocks are not completely explained by the magmatic character of the pluton; in fact, in the granite the U activity is lower than the Th one. This opposite trend could be due to the effects of a large-scale hydrothermal system coeval with upper Palaeozoic plutono-volcanic activity and tectonism.