Rare-earth element (REE) behaviour in the alteration facies of the active magmatic-hydrothermal system of Vulcano (Aeolian Islands, Italy)

This work reports a study on the rare-earth element (REE) behaviour in the high-sulfidation-type superficial alteration and in the phyllic and propyli tic alteration in the subsurface of the island of Vulcano. The results of t his study demonstrate notable differences in the REE behaviour in the diffe rent alteration facies. In silicic alteration, all REE are equally strongly depleted; in advanced argillic alteration, LREE are quite immobile whereas HREE are strongly depleted; in intermediate argillic, phyllic and propylit ic alteration, REE patterns are almost unchanged compared with their fresh rock equivalents. These features indicate that the behaviour of REE in hydr othermal alteration facies at La Fossa is mainly controlled by pH, availabi lity of complexing ions in the fluid (mainly SO42- and, in the silicic faci es, F-) and the presence of secondary minerals able to host REE in their st ructures. The origin of the acid fluids is related to the hypogenic introdu ction of prevalently magmatic acid gases produced by the degassing of the s hallow magmatic system of La Fossa volcano. In the active high-temperature fumarolic field of La Fossa, REE behaviour illustrates the transition from a dry vapor system, corresponding to the high-temperature fumaroles, to a w et condensing system in the surrounding area. The results of this research, carried out on an active high-sulfidation system, provide a useful tool fo r the study of high-sulfidation epithermal ore deposits and particularly he lp to understand the alteration processes during the main stage of hypogene wall rock alteration. (C) 1999 Elsevier Science B.V. All rights reserved.