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Chlorite minerals, mainly in the form of clay coats, play a critical role in determining the reservoir quality of siliciclastic rocks. They can positively influence reservoir quality by preserving porosity during deep burial, but they can also play a negative role by reducing permeability through pore filling. The main aim of this research is to determine the optimal conditions for chlorite growth in sedimentary basins. This study investigates the Lower Cretaceous turbidite sandstone of the Agat Formation in the North Sea. We used a source‐to‐sink approach to investigate the impact of sediment source composition, chemical weathering and depositional environment on chlorite formation. Understanding the interplay between these processes can help refine exploration and exploitation strategies, optimise hydrocarbon recovery, and reduce exploration risks. Representative samples from two hydrocarbon fields (the Duva and Agat fields) were investigated using petrography, geochemistry, heavy mineral identification and quantification, and U–Pb geochronology of detrital zircons. Our results show a strong heterogeneity in the sediment provenance between the two turbidite systems. In the Duva field, the sandstone is derived from a mixture of mafic and felsic sources, producing Fe‐rich sediments. Intense chemical weathering generates fine fraction materials rich in kaolinite, vermiculite, and hydroxy‐interlayered clays, which are transported into shallow marine settings. Subsequent interaction with seawater results in the formation of glauconitic materials, Fe‐illite, and phosphatic concretions. These Fe‐rich materials are remobilised into deep marine settings, providing precursors for the development of authigenic Fe‐clays such as berthierine and chlorite. Conversely, in the Agat field, the sandstone is predominantly sourced from felsic rocks that underwent low chemical weathering, producing sediment rich in quartz and feldspar with a low amount of clays. With few Fe‐rich materials transported into the basin, the development of chlorite in the Agat field was less pervasive. Basin configuration and depositional environment exerted additional control on chlorite distribution. In the confined turbidite system (e.g. Duva field), chlorite is typically found as coating, whereas in less confined turbidite systems (e.g. Agat field) chlorite shows complex distribution related to depositional environment and dewatering processes. Our findings demonstrate the importance of considering the entire sediment routing system, from source to sink, when predicting chlorite occurrence and its impact on reservoir quality in deep marine settings. This integrated approach can guide exploration and development efforts in deepwater clastic reservoirs. [ABSTRACT FROM AUTHOR]

Clay mineral authigenesis at continental margins plays an important role in global marine element cycles. However, despite being increasingly used as tracers for both modern and past oceanographic conditions, the behavior of the rare earth elements (REEs) and their isotopes during marine clay authigenesis still remains poorly known. In this study, we report on a detailed geochemical investigation of glauconite from the West African continental shelf, near the mouth of the Congo River. Elemental, neodymium, and hafnium isotope analyses were conducted on both acid leachate and separated clay-size fractions of glauconite pellets, in order to investigate the behavior of REE during the formation of authigenic clays. Our data indicate that kaolinite dissolution and subsequent Fe-bearing clay authigenesis act as a net source of REEs to seawater. We show that enhanced glauconitization, as inferred from increasing Fe and K contents, is accompanied by significant decoupling of the REE toward markedly LREE-enriched shale-normalized patterns in neoformed clay separates. Using both Nd and Hf isotopes and SEM observations, we rule out any seawater influence and argue that this shift primarily reflects the progressively overwhelming presence of insoluble nanocrystals of detrital LREE-rich phosphates, which are known to occur in close association with kaolinite in tropical soils. Due to their marked insolubility in surface environments, such nanocrystals can be preserved during kaolinite dissolution and subsequently incorporated into the aggregates of authigenic green clays forming the peloids. Most strikingly, we show that the combined influence of net REE loss (due to kaolinite dissolution) and decoupling (due to subsequent entrapment of inherited LREE-bearing accessory phases into neoformed clay minerals) is accompanied by preferential release of a dissolved REE fraction characterized by seawater-like distribution patterns. These findings reinforce the emerging view that clay mineral dissolution and authigenesis at continental margins possibly play a major role in marine REE cycling. [ABSTRACT FROM AUTHOR]

• Nd-Hf isotope investigation of river clays from the Niger River basin. • Saharan dust accounts for ∼40 ± 20 % of the fine sediment exported by the Niger River. • Wet dust deposition controls regional chemical weathering patterns. • Phosphorus release in soils linked to dissolution of windblown phosphate minerals. Huge amounts of mineral dust are produced in northern Africa, representing the largest source of aerosols worldwide. Transatlantic dust transport is known to fertilize soils as far as in the Amazon Basin. Yet, the influence of Saharan dust on chemical weathering fluxes and associated nutrient release in West Africa remains largely overlooked. To address this issue, we analysed clay fractions (<2 µm) of river sediments (n = 37) from across the Niger River basin - the largest river system in West Africa - using neodymium and hafnium isotope compositions as proxies for provenance (ε Nd) and chemical weathering (Δε Hf CLAY). Compared to previously published data for corresponding sand fractions, measured ε Nd values indicate significant size-dependent decoupling for Nd isotopes in most samples, with ε Nd differences between clay and sand fractions yielding values as great as ∼26 ε-units. Using mixing models, we show that this discrepancy reflects the overwhelming presence in the studied clay fractions of Harmattan dust blown from the Bodélé Depression in Chad, which we estimate to account for about 40 % of the fine-grained sediment load exported to the Gulf of Guinea. Additionally, significant Δε Hf CLAY variability occurs across the Niger catchment, partly explained by the presence of zircon in clay-size fractions, but also by preferential alteration of dust-borne accessory phosphate minerals in the subtropical regions of the watershed. Based on these results, we propose that Saharan dust plays a major role in controlling regional patterns of chemical weathering in West Africa, suggesting that enhanced wet deposition of mineral dust in shield areas dominated by transport-limited weathering regime can result in a large increase in weatherability and associated release of phosphorus. These findings have general implications for the importance of mineral aerosols in controlling sediment yield and the supply of weathering-derived nutrients to continental areas bordering large subtropical deserts worldwide. [ABSTRACT FROM AUTHOR]

This paper describes the petrographic properties and the clay mineralogy of Eocene sandy sediments of the Chu-Saryssu basin (South-Central Kazakhstan), in which dioctahedral smectite formed during shallow burial diagenesis (eogenesis). Evidence from petrography and clay mineralogy supports the successive occurrence of kaolinite and dioctahedral smectite during the eogenetic processes, which may have resulted from a change from wet and humid to semi-arid or arid climatic conditions. An original result of this study is the predominantly beidellitic nature of the authigenic smectite, which was determined via X-ray diffraction (XRD, using the Hofmann–Klemen test) and Fourier transform infrared spectroscopy (FTIR) investigations and chemical microanalysis. The crystal-chemical investigations indicate a rather homogeneous chemical composition of the smectite at the regional scale, and the following unit formula is proposed: (Si 4 + 3.65 Al 3 + 0.35 )(Al 3 + 1.65 Fe 3 + 0.21 Mg 2 + 0.14 )O 10 (OH) 2 Na + 0.10 Mg 2 + 0.11 Ca 2 + 0.04 K + 0.07 . This type of smectite has been interpreted as representative of mixed layers of montmorillonite and beidellite. The fact that the smectite that formed primarily in the unconsolidated sandy sediments is close to beidellite, rather than montmorillonite, may have a major impact on the rate of further illitization in the Chu-Saryssu basin. Indeed, the crystal chemistry of beidellite is more favorable to illitization, and the presence of this mineral provides new insights concerning the different rates of illitization between sandstones and shale formations observed worldwide. [ABSTRACT FROM AUTHOR]

Abstract: We focus on processes possibly occurring on Mars that could affect sufficiently large volumes of rocks to be detected by remote sensing techniques. When compared with the chemical modelling of water–rock interactions, the petrographic and mineralogical characteristics of clay deposited in the prismatic joints of a lava flow from the Parana basin (Brazil) suggest that the clay fraction of lava flows may be formed at least partly during an early post-magmatic stage associated with the degassing of acid volatiles. In view of the literature concerning other contexts, such as meteorites or experimental syntheses, we conclude that the crystallization of anhydrous mafic minerals and the formation of clay mineral are not systematically mutually exclusive phenomena. While clay formation is generally related to chemical reaction pathways, it is not necessarily due to the alteration of pre-existing silicates. Such post-magmatic reactions, even if they are likely to represent only a minor contribution to Martian clay formation limited to early Noachian times, require much lower amounts of water compared to conventional hydrothermal alteration or weathering systems. The products of these reactions can be detected over large surface areas, as in the case of the Mars sites, thus allowing us to envisage a greater diversity of paleogeographic scenarios for Early Mars. [Copyright &y& Elsevier]

Abstract: Agua Rica (27°26′S–66°16′O) is a world class Cu (Mo–Au) deposit located in Catamarca, Argentina, in which the porphyry and high sulfidation epithermal stages usually distant 1km vertically, are located at the same level of erosion. Seventy one samples and 120 phyllosilicates were analyzed from the study section (E–W 6969400N) to determine distinctive mineralogical zones which differ not only by their hydrothermal history but also by their potential behavior during the ore processing operations via flotation. In the east side of the section, biotite K1.78–1.67 Na0.05–0.02(Al0.42–0.00Mg3.77–3.22Fe1.46–1.00Ti0.90–0.18Mn0.02–0.01) (Si5.42–4.82Al3.18–2.58)(OHF1.90–1.40Cl0.06–0.04)4 of the early, high temperature potassic alteration (>550°C to 370°C) is encapsulated in a phyllic halo dominated by illite K1.73–1.08 Na0.10–0.0(Al3.96–3.49Mg0.36–0.03Fe0.25–0.01Ti0.06–0.0) (Si6.53–6.07Al1.93–1.47) (OHF0.54–0.0Cl0.02–0.0)4 +quartz+pyrite+covellite+molybdenite+rutile. This mineral assemblage formed from fluids with temperatures≤350°C that attained high sulfur fugacity. The clasts of the hydrothermal and mineralized breccia bodies located at the center of the section and to the east contain widespread pyrophyllite K0.05–0.0(Al4.07–3.94Fe0.12–0.01)(Si7.92–7.50Al0.50–0.05)(OHF0.15–0.02Cl0.03–0.0)4 that had replaced illite at temperatures between 360°C and 280°C. Minor amounts of dickite and widespread kaolinite occur in the advanced argillic and phyllic halos, respectively, as late minerals in clots or in veinlets thus, fluids cooled enough (<270° and <200°C, respectively) for their formation. The presence of phyllosilicate minerals is a potential cause of loss in recovery of copper and molybdenite during the flotation process. Carefully planned and controlled comprehensive liberation and flotation testing should be undertaken in conjunction with mineralogical analysis to optimize and quantify recoveries, separations, grades and gangue deportment. [Copyright &y& Elsevier]

Clay-rich fault gouge is a highly heterogeneous material composed of clasts of host rocks surrounded by a clay matrix. Moreover, this fault rock is characterised by different layers with varying properties: clast content, quantity of shear planes and lithology of rocks. Consequently, the accurate porosity characterisation inside this heterogeneous rock is not possible using usual techniques such as microscopy or bulk physical measurements. A specimen representative of the centimetric thickness of Saint Julien fault gouge has been impregnated by a 14C marked MethylMethAcrylate resin. From the micrometric to the centimetric scale, the spatial distribution of porosity (micro to macropores) is revealed on autoradiographs of different sample sections: clasts (sandstone or pelite) and illite-rich matrix composing the gouge are distinguishable according to their porosity contrast, and the layering is highlighted through the entire gouge thickness. Quantification of porosity by a specific image analysis software shows that clast porosity (ranging from 2.7% to 8.1%) is always lower than the porosity of the clay matrix (ranging from 10.5% to 15.3%). The porosity of these two gouge components always presents the same difference in all gouge layers. A detailed observation of porosity maps reveals the spatial relations linking clast to clay matrix porosities: porosity gradients around the clasts indicate that dissolution of carbonate cements by a recent fluid circulation into the clay part of the gouge is responsible of the actual porous network. [Copyright &y& Elsevier]

Authigenic clay coats (mostly Fe-rich chlorite coats) affect sandstone reservoir qualities by inhibiting quartz overgrowth during burial diagenesis, preserving both porosity and permeability. It is still unclear what initial mineralogical assemblages and initial sedimentation conditions produce chloritized sandstone reservoirs, which is important for sandstone reservoir quality prediction. For this purpose, better link facies with chlorite coat occurences could be useful. To address these questions, sedimentological, petrographical and mineralogical analyses were carried out from sand and sandstones cores for both a deeply buried Permian estuarine sandstone reservoir (Australia) and the Gironde estuary (France). Comparisons reveal similar sedimentary facies and vertical facies associations (from a muddy bottom to cross-stratified sandier packages and to a muddy top), indicative of tidal sand bars deposited in a mud-rich estuary. These criteria can be useful for recognizing tidal deposits when describing cores. X-ray diffraction and Scanning Electron Microscope (SEM) analysis show that detrital clay minerals are composed of illite, smectite, kaolinite and chlorite while clay assemblage differs in the Permian reservoir with dickite or an illite-rich illite/smectite mixed layer (I/S). Coats are either composed of detrital clays minerals (Gironde) or Fe-rich chlorite crystals (Permian). Transformations of detrital clays into other clay minerals (such as berthierine, precursor to Fe-rich chlorite) during eogenesis can initiate well-crystallized Fe-rich chlorite formation during burial diagenesis. Detrital minerals and detrital clay grain coats observed in the Gironde estuary could be the prerequisite initial conditions for generating authigenic Fe-rich chlorite coats in estuarine sandstones during burial. This is partly due to the initial clay fraction content of 15-20%, part of which forms detrital clay grain coats. Our main conclusion shows that facies from the middle to the upper tidal sand bar at the top of the transgressive cycle were probably uncemented during burial, and might be good candidates during reservoir exploration. • Sediments of the P.U.C.H.F (Australia) were deposited in a paleo-estuary very similar to the present-day Gironde estuary. • Best reservoir facies are located from the middle to the upper sand bar areas deposited in the outer estuarine tidal bars. • Detrital clay minerals from the Gironde might promote precursor clay-mineral formation to Fe-rich chlorite coats. • Sandstones deposited at the top of the transgressive sequence.might be good candidates for reservoir exploration. • Ideal initial conditions for reservoir formation can be a maximum clay content of 20–25 wt%, with coated grain content of 15–20% and coat coverage of about 10%.. [ABSTRACT FROM AUTHOR]

Highlights • Ediacaran basalts of the East European Craton were altered by heated meteoric water. • Cyanobacteria were active on the Ediacaran land in hydrothermal systems. • Chemical changes dominated by import of Mg and H 2 O and export of Ca. • The main alteration product is a mixture of di- and trioctahedral Fe-smectites; potentially analogous to the most common Martian clays. • Most of REE in the alteration products are contained in the dioctahedral smectite. Abstract The Ediacaran continental flood basalts and associated tuffs were studied to identify and quantify alteration processes by means of XRD and chemistry, supplemented by Mössbauer and FTIR spectroscopies, petrography, oxygen and iron isotopes, K-Ar dating, and organic geochemistry. Two superimposed alteration processes were identified: the Ediacaran hydrothermal alteration, induced by meteoric waters, heated and put in motion by the cooling basalt, and the Caledonian and/or Variscan potassic alteration. The degree of basalt alteration was quantified using as an index the sum of primary minerals in the bulk rock. The sequence of minerals dissolved and crystallized during the hydrothermal alteration was established. The alteration resulted in the loss of Ca (dissolution of plagioclases), compensated by the gain of water and Mg (crystallization of clays), and proceeded from the edges of the basalt flows in an oxidizing environment, evidenced by the increasing amount of hematite and Fe3+/Fe2+ ratio of the bulk rock. Cyanobacteria were active in the hydrothermal system, most probably responsible for the measured negative δ56Fe values and more reducing conditions at the stage of intense alteration. Chlorophaeite (palagonite), following quartz as the earliest petrographically identifiable basalt alteration product was found to vary systematically from fully isotropic to birefringent. The chlorophaeite was identified as a mixture of Fe-montmorillonite and Fe-saponite, identical with griffithite and oxysmectites, probably preceded by a finer-grained ferrosaponite at the isotropic stage. REE content of chlorophaeite indicates basaltic volcanic glass (sideromelane) as the major source of material. REE in clays are contained mostly in the dioctahedral smectite, while in the bulk rock mostly in phosphates. The smectite characteristics and Mg enrichment are indicative of the hydrothermal basalt alteration process, which perhaps was dominant also on Mars. Both mineral and chemical composition of tuffs vary continuously from basaltic to felsic, the latter close to the measured rhyodacite composition, dominated by quartz and feldspars. The basaltic tuffs resemble the most altered basalts but contain also abundant albite and chlorite, indicative of higher alteration temperatures, up to 220 °C. Tuff composition indicates stronger component of felsic volcanism in the trap formation than evidenced by the preserved bodies of effusive rocks. [ABSTRACT FROM AUTHOR]

Abstract Coffinite (USiO 4) is a common uranium-bearing mineral of roll-front uranium deposits. This mineral can be identified by the visible near infrared (Vis-NIR) portable field spectrometers used in mining exploration. However, due to the low detection limits and associated errors, the quantification of coffinite abundance in the mineralized sandstones or sandy sediments of roll-front uranium deposits using Vis-NIR spectrometry requires a specific methodological development. In this study, the 1135 nm absorption band area is used to quantify the abundance of coffinite. This absorption feature does not interfere with NIR absorption bands of any other minerals present in natural sands or sandstones of uranium roll-front deposits. The correlation between the 1135 nm band area and coffinite content was determined from a series of spectra measured from prepared mineral mixtures. The samples were prepared with a range of weighted amounts of arenitic sands and synthetic coffinite simulating the range of uranium concentration encountered in roll-front uranium deposits. The methodology presented in this study provides the quantification of the coffinite content present in sands between 0.03 wt% to 1 wt% coffinite with a detection limit as low as 0.005 wt%. The integrated area of the 1135 nm band is positively correlated with the coffinite content of the sand in this range, showing that the method is efficient to quantify coffinite concentrations typical of roll-front uranium deposits. The regression equation defined in this study was then used as a reference to predict the amount of natural coffinite in a set of mineralized samples from the Tortkuduk uranium roll-front deposit (South Kazakhstan). Highlights • Field Vis-NIR spectroscopy easily detects coffinite in arenitic to arkosic sands. • Coffinite can be quantified for concentrations between 0.03 and 1.0 wt%. • Coffinite represents the main uranium-bearing mineral of studied roll-front deposit. [ABSTRACT FROM AUTHOR]

The preservation of reservoir quality in deeply buried sandstones largely depends on the presence of grain-coating clays that can inhibit quartz overgrowth and cementation. These grain-coating clays are the focus of today's hydrocarbon exploration as they enable new oil and gas discoveries in sedimentary basins. However, the origin and distribution of these clays in turbidites are poorly documented in scientific literature. Our study addresses this knowledge gap by investigating the mechanisms and parameters governing the clay coat development in turbidite sandstones, with a focus on the Agat Formation (Fm) in the Norwegian North Sea. Petrographic observations, scanning electron microscopy, and X-ray diffraction analyses were used to identify the origin of clay minerals and their distribution, as well as to understand the diagenetic evolution of the sandstone and its impact on reservoir quality. Based on the sedimentary facies description, the Agat Fm comprises four principal members deposited in various depositional environments, ranging from distal lobe fringe to amalgamated proximal lobes and weakly confined channels. Chlorite and kaolinite are the main authigenic clay minerals in these sandstones, associated with a variable amount of inherited detrital clays consisting in Fe-bearing kaolinite and mixed-layer minerals composed of illite and hydroxyl-interlayered clay minerals formed in acidic soil environments. Well-developed chlorite coats are associated with the highest reservoir quality (φ > 10%), while discontinuous chlorite coats are linked to extensive quartz cementation and poor reservoir quality (φ < 5%). Samples with abundant pore-filling chlorite also exhibit poor reservoir quality (φ < 7%). Proximal lobe deposits and weakly confined channels show well-developed chlorite coats, whereas discontinuous chlorite coats are mainly found in proximal lobe deposits associated with high velocity escaping fluids during dewatering. Pore-filling chlorites are typically present in distal lobe fringes and levee deposits. Chlorite in the Agat Fm is mainly derived from the replacement of inherited detrital clays via an intermediate berthierine precursor forming during early diagenesis. The dissolution of Fe-rich grains such as biotite and ferric detrital clays under reducing conditions governed the kinetics of berthierine/chlorite growth. The late diagenesis is characterized by extensive feldspar dissolution in a closed diagenetic system, leading to the precipitation of kaolinite and quartz cement. The presence of chlorite coatings around detrital grains preserved the porosity from extensive quartz cementation during deep burial, highlighting the role of chlorite in maintaining the quality of deeply buried turbidite reservoirs. • The distribution of clay coatings in turbidites is linked to the depositional environment and dewatering processes. • Good reservoir quality is found in weakly confined channels and in lobe deposits that have continuous grain-coating chlorites. • Poor reservoir quality is in lobes with discontinuous coatings, levees, and distal lobes dominated by pore-filling chlorite. • Chlorite forms by replacing inherited detrital clays through an intermediate berthierine precursor in early diagenesis. • Diagenetic evolution in a closed system led to kaolinite and quartz overgrowth from feldspar dissolution. [ABSTRACT FROM AUTHOR]