Pseudomorphic changes after iron sulfide concretions in Jurassic limestones, Ionian Basin, Greece: tracking changes in basinal fluids

Middle Jurassic limestones, deposited on deep-water highs in the Ionian Basin, include Fe-rich concretions. Mineralogical and textural studies by SEM show the concretions now consist of goethite with minor lepidocrocite, hematite and quartz. The cm-scale concretions include radiating spheres and pseudomorphs after ammonites, with cubic and pyritohedral pseudomorphs after pyrite. Smaller mm-scale concretions occur at limestone hardgrounds. Concretions have REE signatures and high Mo and V suggesting formation in hypoxic water overlying a local euxinic basin. Textures suggest the following sequence of events: (1) Sea floor diagenetic iron monosulphide and pyrite concretions and veins. (2) With burial, recrystallisation to cubes or pyritohedrons of pyrite. (3) Pseudomorphous alteration of pyrite to hematite, with the 5 % volume reduction resulting in sub-micron pores that filled with silica (now quartz). (4) Widespread alteration of hematite to goethite with texturally complex, coupled dissolution-reprecipitation reactions. (5) Widespread secondary porosity, with precipitation of fibrous goethite and minor lepidocrocite. (6) Late cross-cutting fractures have Mn-rich minerals, with exotic trace elements such as asbolane (Co, Ni) and romanechite (Ba). The original pyrite concretions persisted during burial diagenesis until Paleogene uplift of the basin margins allowed ingress of oxidising meteoric water, circulated by thermal anomalies related to salt domes. Pyrite was oxidized to hematite and micropores were filled by silica. Oligocene thrusting created a new fracture set, increased loading of basinal sediments and allowed ingress of abundant meteoric water creating supergene alteration. Both chert and Fe-rich concretions provide important insights into diagenetic and hydrothermal evolution of thick limestone successions. [249 words] • Fe-rich concretions and veins are common in Middle Jurassic pelagic limestones. • SEM shows Fe-rich concretions pseudomorph burial diagenesis pyrite crystal forms. • Concretions altered to hematite with silica in μm-sized pores, then to goethite. • Hematite due to ingress of meteoric water when basin margin uplifted in Paleocene. • Later goethite formed by coupled dissolution-precipitation after basin inversion.