Petrographic and geochemical characterization of the Middle?Upper Jurassic Fe–Mn crusts and mineralizations from Monte Inici (north-western Sicily): genetic implications

Abstract Fe–Mn concretions and mineralizations, associated with condensed horizons hardground, are significant archives in ancient carbonate rocks. Their petro-chemical study allows an assessment of the palaeoenvironmental context which they were formed also connected to their biotic or abiotic origin. At western side Monte Inici (Fornazzo section, north-western Sicily) a well exposed outcrop pelagic limestones (Rosso Ammonitico facies: Middle?Upper Jurassic) is well-known thoroughly studied. In this base Rosso facies consists very level rich fossils variable thickness deposited from early Bathonian early/middle Callovian. It characterized, at top, by noticeable presence concretions, typical Tethyan Jurassic related low sedimentation rates. For study, crusts mineralizations Fornazzo section investigated using X-ray diffraction, scanning electron microscopy, fluorescence, ICP stable-isotope mass spectrometry. The collected samples, submarine conditions (as testified stable oxygen carbon isotopes), have been subdivided two typologies different macroscopic mineralogical features. consist hematite, todorokite birnessite characterized relatively Mn/Fe ratio. content trace elements, rare earths yttrium (REY) compatible hydrogenetic origin involving oxy-hydroxides colloids precipitation directly seawater. Microbially mediated processes here recognition filamentous coccoid-shaped microstructures referable coexistence chemosynthetic fungi photosynthetic cyanobacteria accounting for deposition deep euphotic zone. An average growth rate ~ 8.5 mm/Myr crusts, estimated cobalt concentrations, suggests time elapsed 3.5 ± 1 Myr. This value stratigraphic gap embracing span Callovian middle Oxfordian. neighbouring limestones, deposits present form micro-dendrites mainly consisting pyrolusite, sometimes carbonato-fluorapatite. geochemical composition gives evidence prevalent diagenetic precipitation, sediment/water interface first centimeters sediments, metals diffused as consequence fluctuating redox conditions. Although well-preserved Frutexites texture commonly microbial activity, other bacterial not recognized, having probably obliterated during dendrites. Nevertheless, it possible suppose deepening bathymetry consistent involvement microorganisms formation structures.