Hydrothermal activity in the SE Tyrrhenian Sea: an overview of 30 years of research

Abstract

Subduction-related volcanic activity in the Southeast Tyrrhenian back-arc basin has resulted in a large variety of calcalkaline rocks (ranging from arc tholeiitic to shoshonitic and rhyolitic composition), which were emplaced in both oceanic and continental crust. Hydrothermal systems in the SE Tyrrhenian Sea have leached these magmatic rocks, generating a wide range of oxyhydroxide and sulfide deposits. All the hydrothermal occurrences formed at much shallower depths than those from the mid-ocean ridges. Oxyhydroxide deposits (metalliferous sediments, crusts, and nodules) are almost pure hydrothermal formations with insignificant hydrogenetic and diagenetic contributions. They exhibit strong separation of Mn-rich and Fe-rich end members with Mn-rich occurrences dominant. Their geochemistry suggests a relative enrichment of Mn in the hydrothermal systems of the SE Tyrrhenian. The sulfides are Zn- and Pb-rich, but poor in Cu and relatively poor in Fe. This suggests either deep-seated magma chambers or boiling of the hydrothermal fluids beneath the seafloor. As a consequence: (1) Cu and a large proportion of the Fe are deposited as sulfides in the basement rocks and sediment cover; (2) the hydrothermal fluids become relatively enriched in Mn, Zn, and Pb. A number of accessory sulfosalts and selenides not typical of mid-ocean ridge massive sulfides are present.