Field observation of fossil manganese nodules and crusts in the the West Sicilian Jurassic limestone beds

Fossil ferromanganese crusts, nodules and pavements are contained in stratigraphically condensed pelagic limestones from the Middle Jurassic of western Sicily; and a similar association of iron-manganese deposits with condensed pelagic sequences is notable throughout the Tethyan region. The west Sicilian concretions contain goethite, haematite and todokorite as fine-grained phases; their internal structure consists of iron-manganese segregations in a calcite matrix. Electron-microprobe study indicates that the amount of iron and manganese varies considerably in nodules from different localities, notwithstanding the disparity in carbonate content; and the same is true of the minor elements. The most consistently established inter-element relationships are Ni, Ba with Mn, and Ti with Fe - but these associations are by no means invariably developed. For western Sicily, submarine volcanism seems to have been the most immediate source of supply for the formation of the concretions; and this may well have been the case elsewhere in the Alpine- Mediterranean region; however, the Jurassic was a time of widespread sedimentary iron-ore formation in epicontinental northern Europe, and river drainage must have also supplied some manganese to the Tethyan Ocean. These mineral concretions are interpreted as being formed on ancient limestone (non-volcanic) seamounts. The presence of algal stromatolites in many of the condensed sequences suggests formation within the photic zone; and this relatively shallow-water origin may have affected the minor-element composition of the concretions, particularly with reference to copper and vanadium.