Si-P impure Al-Goethite Mineralization on the island of Dugi Otok (Central Adriatic, Croatia)

Abstract

At the western seaward coast of the island of Dugi Otok, unusual goethite mineralization occurs as the infilling of a system of thin crevices in the Upper Cretaceous limestones of the Adriatic carbonate platform. Goethite is exceptionally well ordered and shows a variety of chemical compositions ranging from almost pure FeO·OH to Al-substituted composition accompanied by high Si and P impurity. The most characteristic textural type assemblages are goethite and coarse grained calcite in composite aggregates. These aggregates contain Al-substituted goethite with metamorphic silicate minerals embedded in the core, (black goethite) armoured by the XRD-pure, almost stoichiometric goethite (brown goethite). The geochemical and mineralogical features of the mineralization suggest an ascending solution from a hidden metamorphic source. We speculate that the source could have been activated by the thermal influx linked to the Pliocene intraplate magmatism in the eastern segment of Adria.