Abstract
Fe–Mn concretions and mineralizations, associated with condensed horizons and hardground, are significant archives in ancient carbonate rocks. Their petro-chemical study allows an assessment of the palaeoenvironmental context in which they were formed also connected to their biotic or abiotic origin. At the western side of the Monte Inici (Fornazzo section, north-western Sicily) a well exposed outcrop of condensed pelagic limestones (Rosso Ammonitico facies: Middle‒Upper Jurassic) is well-known and thoroughly studied. In this section, the base of the Rosso Ammonitico facies consists of a very condensed level rich in fossils with a variable thickness deposited from the early Bathonian to the early/middle Callovian. It is characterized, at the top, by the noticeable presence of Fe–Mn concretions, typical of the Tethyan Jurassic and related to very low sedimentation rates. For this study, Fe–Mn crusts and mineralizations from the Fornazzo section were investigated using X-ray diffraction, scanning electron microscopy, X-ray fluorescence, ICP and stable-isotope mass spectrometry. The collected samples, deposited in submarine conditions (as testified by stable oxygen and carbon isotopes), have been subdivided in two typologies with different macroscopic and mineralogical features. The Fe–Mn crusts consist of hematite, todorokite and birnessite and are characterized by a relatively low Mn/Fe ratio. Their content in trace elements, rare earths and yttrium (REY) is compatible with a hydrogenetic origin involving the oxy-hydroxides colloids precipitation directly from seawater. Microbially mediated processes are here testified by the recognition of filamentous and coccoid-shaped microstructures referable to coexistence of chemosynthetic fungi and photosynthetic cyanobacteria and accounting for a deposition in the deep euphotic zone. An average growth rate of ~ 8.5 mm/Myr for the Fe–Mn crusts, estimated by cobalt concentrations, suggests a time elapsed for deposition of ~ 3.5 ± 1 Myr. This value is compatible with the stratigraphic gap embracing the time span from the early/middle Callovian to the middle Oxfordian. In the neighbouring pelagic limestones, Fe–Mn deposits are present in the form of micro-dendrites mainly consisting of pyrolusite, sometimes associated with carbonato-fluorapatite. The geochemical composition gives evidence of a prevalent early diagenetic origin with precipitation, at the sediment/water interface or in the first centimeters of sediments, of metals diffused from the crusts as consequence of fluctuating redox conditions. Although the well-preserved Frutexites texture is commonly related to a microbial activity, other bacterial microstructures have not been recognized, having probably been obliterated during the growth of the dendrites. Nevertheless, it is possible to suppose a deepening in the bathymetry consistent with the involvement of chemosynthetic microorganisms in the formation of Frutexites structures.
Highlights
The present study focuses on the petrographic and geochemical characterization of the Fe–Mn crusts and mineralizations included in the exceptionally well-exposed succession of the Fornazzo quarry (Monte Inici, western Sicily)
Limestone, represented by sample HI 1a, consists of consolidated calcilutite showing an homogeneous pink color; it is almost completely constituted by calcite (97.8%) with trace of quartz (< 1%) and hematite (1.4%), the latter identified at 2.69 Å (104 reflection)
Results from the study on the Middle‒Upper Jurassic Fe–Mn crusts and mineralizations from Fornazzo section can be summarized as follows: 1. Macroscopic observations and mineralogical analyses allow to recognize two main typologies of Fe–Mn deposits: (i) Fe–Mn crusts, characterized by parallel dense lamination and with averagely high content in hematite and todorokite, and (ii) dendrites in limestones showing columnar or dome structure and prevalence in pyrolusite sometimes associated with carbonatofluorapatite
Summary
Palmer and Wilson 1990; Fürsich et al 1992; García-Ruiz et al 1994; Clari et al 1995; Gupta 1995; Bayon et al 2004; Reolid 2011; Föllmi 2016; Jiang et al 2020) and sometimes significant potential metal source (Ehrlich 1975; Rona 2003; Polgári et al 2004; Hein et al 2013; Kuhn et al 2017; Marino et al 2019) These deposits are characterized by a complex mineralogical and chemical composition, with prevalence of iron and manganese and showing relatively high concentration of trace elements as Co, Cu, Ni, Cr, V (e.g.,Nicholson et al 1997; Rona 2008; Hein et al 2010). Similar microbial microstructures were described in marine environments such as shallow and deep water stromatolites, microbial limestones, hardgrounds, condensed pelagic limestones, and in continental setting like veins and fractures of deep subterranean environments (Rodríguez-Martínez et al 2011 and references therein)
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