SUBLIMATE DEPOSITION FROM HYDROTHERMAL FLUIDS AT THE FOSSA CRATER-VULCANO, ITALY

Sublimates were sampled at the Fossa crater of Vulcano Island (Sicily, Italy) in June and October 1991. Sampling was carried out by means of silica tubes inserted as deep as possible into those fumarolic vents around which lead and bismuth sulphides and sulphosalts occurred. The main mineralogical phases found within the silica tubes are native sul phur, ammonium tetrafluoroborate, lead and bismuth chlorides, oxychlor ides, sulphochlorides, sulphides and sulphosalts. The lead and bismuth mineral assemblage is interpreted as the result of the transport of l ead and bismuth as volatile chlorides. Microtextural and microanalytic al studies(SEM + EDS) on chloro-sulphides and sulphosalts suggest that the latter were deposited from metastable chlorosulphides through rea ctions involving gaseous H2S. The absence of chlorosulphides on the gr ound confirms their metastability. The lack of sulphates in the minera logical association sampled within the silica tubes is due to the redu cing conditions of volcanic fluids.