Fe-Mn Nodular Concretions Associated with Middle Jurassic Oceanic Melange (Argolis, Greece)

Fe-Mn nodular concretions from Angelokastron and Lykotroupi areas, Northern Argolis Greece, are friable and compact types. They are associated with Middle Jurassic radiolarian red chert and red siliceous shale matrix slivers, originated and detached from a Middle Jurassic oceanic mélange. Friable Mn concretions consist of poly- or mononucleate nodules lacking primary botryoidal microstructures and possessing a unique composition. They form by the replacement of chalcedonic jasper by cryptomelane and todorokite; these concretionary crystalline manganese-structures are dissected by a birnessite phase oxidized to ntsutite and then crosscut by veinlets of hollandite and manganiferous carbonated fluroapatite during late-stage hydrothermal alteration. The resultant composition consists mostly of manganese with a very low content of iron and transition metals. The mineralogical and chemical compositions differ from those of recent or fossil manganese nodules and are related to a hydrothermal field. Compact Fe-Mn concretions consist of jasper and chert dissected by veinlets of hydrothermal todorokite. Sulphides with magnetite characterize these concretions, even when altered and silicified. Some enclose scattered fragments of magnesiochromite with Ni-rich todorokite as veinlets and as concretionary crystalline structures. Some others, such as silicified basaltic fragments, contain remnants of copper mineralization such as sulfides, oxides, and hydroxide copper minerals, generated by an older hydrothermal event with subsequent oxidation. Furthermore, a few compact concretions, which were chemically treated, revealed that they contain equal amounts of iron and manganese similar to the hydrothermal ferromanganiferous crusts on basalts. The reworked Fe-Mn nodular concretions resulted from submarine hydrothermal and fissural activity. These processes took place during the pre-emplacement period of an oceanic crust unit preserved today as the Subpelagonian ophiolite.