Hydrothermal Alteration Inferred from a Radiometric Survey on Lipari (Aeolian Islands, Italy).

Abstract

We carried out a radiometric survey on a sector of the Lipari island particularly affected by argillification due to hydrothermal processes. Preliminarily, on the basis of a wide set of field measurements over unaltered outcrops, we obtained a reference spatial distribution of U, Th and K. The concentration of these radioelements increases with the degree of magma differentiation; the U and K content shows maxima in the felsic materials, belonging to the youngest volcanic cycles (13.4 ppm and 3.7% on average, respectively), and minima in the mafic rocks of the oldest cycles (2.7 ppm and 2.0%). The Th/U ratio insignificantly varies with magma differentiation and it equals 3.2. Laboratory measurements on volcanic products from altered zones were then used to identify radiometric anomalies related to hydrothermal processes. On average, argillized rocks have radioactivity approximately five times lower than that measured in the unaltered volcanics. The Th/U ratio against U is rather uniform, whereas Th/K ratio is in the order of 3.7 in less altered rocks (K > 0.2%) and it ranges from 7 to 11 in kaolin deposits having the largest depletion in potassium.