Si-Pimpure Al-goethite mineralization on the island of Dugiotok (Central Adria tic, Croatia

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 rangingfrom almost pure FeOOH to Al-substituted composition accompanied by high Si andP impurity. The most characteristic textural type assemblages are goethite and coarse grained calcite in composite aggregates. These aggregates contain Al-substituted goethite with metamor- phic silicate minerals embedded in the core (black goethite), armoured by theXRD-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.