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The early evolution of metazoans has been reconstructed by studies on exceptionally preserved molds in siliciclastic rocks from the Ediacaran Period. However, there remains considerable controversy regarding the formation mechanisms of this unusual ‘Ediacaran-style’ preservation. Proposed hypotheses usually include early authigenesis of minerals, but evidence for this is scarce. In a recently discovered deposit of Ediacaran biota in Brazil, we show that the classic moldic preservation is related to clay mineral authigenesis. Specifically, these clays originated from the alteration of original pyroclastic sediments, likely enhanced by microbial activity, leading to early illitization and morphological templating of the fossiliferous surfaces at a micrometric scale. Such high-fidelity preservation was made possible by rapid burial during volcanic events and the in-situ templating of tissue by clays via microbially-mediated mineralization. This newly described Lagerstätte demonstrates that a number of minerals can facilitate preservation, and that perhaps ‘Ediacaran-style’ preservation result from different processes leading to the same broad style of preservation.

The Gabes Gulf located in the South Mediterranean Sea (Southeastern part of Tunisia) is a prolific petroleum-producing area with several oil and gas fields and it's the subject of significant discoveries. The area is affected by intense tectonic events and several stages of hydrothermal activities . During drilling at the MX area in NE of the Gabes Gulf, a magmatic intrusion has been encountered within a reservoir rock (i.e., Douleb Formation). The objectives of the present study are (1) to investigate the impact of the hydrothermal activities on this magmatic intrusion and (2) to characterize the potential modifications in its mineralogy and petro-physical properties that can affect the migration/accumulation of hydrocarbons. To do so we combine optical microscopy, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectrometry, micro-Raman spectroscopy, electron microprobe analyses, and petro-physical measurements. Our results show that the initial magmatic intrusion (dolerite) is composed mainly of plagioclase , clinopyroxene , and olivine. Based on petrographic observations, this intrusion subjected to several stages of hydrothermal fluid alterations that modified the primary texture and mineralogy. Albite , calcite , ankerite , dolomite, pyrite , quartz, anatase , kaolinite, and chlorite are the main secondary mineral products. During this alteration process, carbonates, pyrite, and quartz precipitated in vesicles, forming amygdules structure with enveloped zones indicating a fluctuation of fluid chemistry during their formation. In addition, petrographic observations indicate an interaction between feldspars exsolution and metasomatic processes in the studied area. The dissolution mechanisms, as well as the textural modifications in the dolerite induced by the hydrothermal activity, are underlined by the presence of spherulites , pores, and micropores. Such modifications significantly improved the porosity of the dolerite body. Conversely, the new mineralization that filled the fractures and micro-fractures, have reduced the inter-pore connections, and thus reduced its permeability. The present work demonstrates the important role of hydrothermal activity on the petro-physical properties of magmatic intrusions and how it could facilitate the migration/accumulation of hydrocarbons. Our results open the door for further investigations to check the potential presence of hydrocarbons within this doleritic intrusion.

During the evolution of sedimentary basins, clay minerals may record changes in their mineralogy, chemistry, and texture as a response to diagenesis, fluid flows, and tectonic activities. Therefore, studying clay minerals in sedimentary rocks provides important information that may assist to decipher the basin history. The focus of the present study is the Teloua Sandstone Formation (Fm) which belongs to the Tim Mersoi Basin located in the NE of Niger republic (West Africa). The formation is cut by the major Arlit fault which is the largest tectonic structure impacting the Tim Mersoi Basin. The aim of the present work is to use the texture, mineralogy, and chemistry of clay minerals in the Teloua Fm as geochemical markers for burial diagenesis history and fluids flows (i.e., attributed mainly to the Arlit fault). Therefore, three drill holes (DH1, DH2, and DH3) were investigated as a function of their distance from the fault and samples have been collected all along the Teloua Fm (Upper Triassic-Lower Jurassic), at the unconformity surface with the underlaying Moradi Fm (Upper Permian), and at the top of the Moradi Formation. Petrographic observations demonstrate that the sediments of the Teloua Fm have undergone transformations induced by burial diagenesis, brittle deformation, and multiple stages of fluid flows. These transformations are marked by grain compaction, fragmentation, dissolution of detrital minerals, recrystallization of authigenic clay minerals, and modifications in the primary porosity. XRD and SEM results indicate no significant variation in the detrital mineralogy of the three drill holes; Teloua Fm samples are composed of quartz, feldspars (microcline and albite), micas (biotite and muscovite), minor heavy minerals such as hematite, anatase, phosphates (apatite and monazite), and zircon. The authigenic clay minerals observed in the three drill holes are rosette-like tosudite (i.e., a regular interstratified chlorite/smectite), saponite, montmorillonite, and kaolinite/smectite mixed layer. Regarding their vertical distribution, tosudite occurs throughout the Teloua Fm, whereas montmorillonite is mainly present at the top of the formation, and saponite is only observed at the unconformity between Teloua and Moradi formations. The Mg content of tosudite tends to decrease with increasing vertical distance from the basal unconformity and the lateral distance from the Arlit fault. This is interpreted as an evidence of Mg-rich fluid circulations related to the Arlit fault activity that penetrated the formation through the unconformity surface and fractures. This is also evidenced by the enrichment of dolomite at the unconformity when moving closer to the Arlit fault. In addition, the occurrence of both authigenic saponite (at the unconformity) and montmorillonite (upper part of the Teloua Fm) that post-dated tosudite is attributed to another stage of Mg-rich fluid circulation during late diagenesis. Furthermore, late downward circulation of meteoric fluids of low pH are evidenced by the partial alteration and replacement of montmorillonite by kaolinite/smectite mixed layer in the Teloua Fm (at shallow depth of the DH3). These results demonstrate the validity of clay minerals as reliable tools to provide information about fluid/rock interaction in complex geological and tectonic setting.

International audience; The Pyrenees belt features well exposed deformation structures that make it ideally suited to study thrust faults. In this paper, we present the first 40Ar/39Ar age of the Pic de Port Vieux thrust (PPVT) located in the south-western part of the Pyrenean Axial Zone. The PPVT is a secondary thrust related to the major Gavarnie thrust (GT) with a footwall comprising Upper Cretaceous limestones, and a hanging wall made of Lower-Triassic pelites and sandstones. A vertical transect in the fault hanging wall was investigated to characterize the deformation mechanism and to date the thrust activity by using 40Ar/39Ar step-heating technique on muscovite.Petrographic observations confirm that pressure solution and recrystallization/dissolution processes in presence of fluids are the main mechanisms that controlled the chemical/textural changes in the fault core zone. Three groups of muscovite are identified in the fault zone i.e. detrital, diagenetic and newly-formed muscovites. The detrital and diagenetic muscovites are mainly present in the damage zone, while the newly-formed muscovite is abundant in the core zone and is synkinematic to the fault activity. The fractions 50–16 μm, 16–2 μm, 2–0.5 μm

International audience; Synkinematic phyllosilicates in fault zones can be used to deduce the deformation mechanisms and the conditions of fault activity, as their chemical composition, crystal structure and texture can record the different stages of deformation and fluid-rock interactions. The Pic de Port Vieux, a second-order thrust related to the major Gavarnie thrust in the southern central part of the Pyrenees Axial Zone, juxtaposes Triassic pelites of the hanging wall and Cretaceous limestones of the footwall. In order to investigate the mineralogical and geochemical changes and constrain the deformation conditions of thrusting, characterization of phyllosilicates was performed along a transect in the fault hanging wall pelites. The Triassic pelites are mainly composed of quartz, calcite, phyllosilicates (chlorite and K-white mica) and ± hematite. The core fault zone thickness is estimated to be about one meter of intensively foliated green pelites, whereas the damage zone is composed of several meters of red pelites. XRD data demonstrated that the difference in color is related to hematite which is only present in the damage zone. Phyllosilicates of the damage zone are mainly inherited/diagenetic K-white mica and chlorite. In the core zone, newly formed chlorite is abundant and preferentially located in veins. It is enriched in Fe compared to the chlorite of the red pelites. The well-defined foliation in the core zone is overlaid by preferentially oriented muscovite grains which have homogeneous compositions, with less Na and relatively more Fe than mica from the red pelites. Newly formed chlorite and muscovite in the core zone are synkinematic to the fault activity. They are both related to deformation processes and fluid rock interactions. Kübler index measurements and chlorite thermometry show that synkinematic phyllosilicates have registered a temperature of 270°C ± 23°C for the damage zone and 285°C ± 28°C for the core zone which are corresponding to lower-anchizone/epizone grade conditions. Numerical modelling with the geochemical modelling program PhreeqC was performed to determine the favorable conditions for the mineralogical change between red and green pelites. It suggests that the main critical parameter favoring hematite dissolution and chlorite precipitation in the core zone, is the redox conditions. According to the model, the chemical changes in the core zone occurred due to interactions with highly reductive fluids. In addition, the hematite dissolution maybe the source of iron for newly formed phyllosilicates in the core zone that are more iron rich compared to those from the red pelites.