Geochemistry of Messinian sediments of the Sardinian margin

Brightly colored mudstones within Messinian (late Miocene) and lower Pliocene successions were cored beneath the upper and lower slopes of the Sardinian continental margin of the Tyrrhenian Sea (Central Mediterranean). A total of 24 samples were analyzed for major and trace elements and 20 samples were studied by whole-rock X-ray diffraction. The colorful sediments studied include millimeter-thick limonitic laminations within coarser-grained clastic sedimentary rocks (Site 652) and local ferruginous zones near the top of the Messinian succession elsewhere (Site 653). The basal Pliocene metalliferous sediments comprise up to several tens of meters of colorful, mainly red, yellow, and brown, calcareous clays at the base of nannofossil-rich Pliocene deep-sea sediment successions (Sites 652, 653, 654).
Relative to average shale, the Messinian mudstones cored on the de Marchi Seamount, the lowest rift fault block on the Sardinian margin (Site 656), are notably enriched in lead (Pb), zinc (Zn), and to a lesser extent, in copper (Cu) and nickel (Ni), and are associated with local clasts of sulfide ore. Messinian sediments higher on the Sardinian margin (Cornaglia Terrace, Site 653) are enriched in Zn and Pb. The basal Pliocene sediments, particularly at Site 653, are strongly enriched in iron (Fe), manganese (Mn), Pb and Zn, relative to the overlying Pliocene nannofossil oozes. This Fe and Mn enrichment is believed to have resulted from oxidation under hot arid conditions in the Messinian, followed by reworking of insoluble metal oxides during the Pliocene transgression that ensued. The Pb and Zn were apparently associated with sulfide ore, of either volcanic-hosted (?Mesozoic) Kuroko-type or (?Tertiary rift-related) sediment-hosted Mississippi-type. The Fe and Mn oxides and insoluble ore material were later reworked, leading to a marked Pb-Zn metal anomaly in ferruginous basal Pliocene sediments.