Geochemistry of metalliferous, hydrothermal deposits in the Aeolian arc (Tyrrhenian Sea)

In shallow-water areas of the submerged volcanic complex around the island of Panarea (Aeolian archipelago), hydrothermal precipitation of both low-te mperature Fe-oxyhydroxide-rich red muds and crusts, and high-temperature, s ediment-hosted massive sulfides was discovered during an integrated, high-r esolution survey. Iron-rich crusts were also found on the bathymetric high of Secca del Capo, north of Salina island. The exhalative iron-rich sedimen ts occur in small (closed) depressions or in proximity to faults and scarps at water depths ranging from 55 to 285 m. The principal chemical character istics of these deposits are high, but variable, Fe content ranging from 12 .2 to 45%, and low contents of the transition elements Mn, Cu, Zn, Ni and C o. The low contents of Cu, Ni and Co suggest a hydrothermal origin. The Fe- oxyhydroxide deposits are enriched in light rare earth element (REE) (35-11 0 x chondrite) compared to heavy REE (10-25 x chondrite). Their REE pattern s are similar to those of associated calc-alkaline volcanics: negative slop e of light REE and a horizontal distribution of the heavy ones. This contra sts with the pattern for iron and manganese oxides of hydrogenous origin, w hich have tilted trends of heavy REE paralleling that of seawater. The mine ralogy of the polymetallic sulfide deposits consists of galena, sphalerite, pyrite and barite in the form of silt-sand grains and decimeter-sized frag ments disseminated in clay, 30 cm below the seabed, at a waterdepth of 80 m . The chemistry of the Aeolian iron-rich, low-temperature deposits and of t he high-temperature, Ba-rich Pb and Zn sulfides suggests that they are gene tically analogous to, respectively, the kuroko-type iron formation ('tetsus ekiei') and the black ore exposed in the Miocene Hokuroku district of Japan . (C) 1999 Elsevier Science B.V. All rights reserved.