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Estuaries provide an excellent depositional environment to study the interaction between minerals (clays, quartz sands…) and biofilms. The estuary bottom is largely covered by biofilms that impact sediment stability, the mud and clay-coat content in sands, and sedimentary-structure stability, thus influencing sandstone properties during burial. Although numerous oil, gas, and geothermal reservoirs are exploited in estuarine heterolithic point bars, many questions remain about the origin of reservoir properties and heterogeneities in these sedimentary bodies. In order to better understand the sedimentary and microbiologic processes in estuarine systems and to better predict the reservoir quality of estuarine sandstones, this study characterizes a modern heterolithic point bar located in the Garonne estuarine channel at various scales, ranging from the microscopic (thin section) to the macroscopic (core) scale. Three piston cores 4.5 to 6.8 m long were drilled in the Bordeaux North Point Bar. Three main facies were identified in these cores: 1) sandy gravel, 2) heterolithic, medium-grained sand dunes, and 3) thin heterolithic, fine-grained sand beds with mud drapes. The sands are classified as lithic arkoses to feldspathic litharenites. Detrital clay grain coats, which at deep burial depths are transformed to permeability preserving authigenic chlorite coatings, are observed from the base to the top of the point bar. These detrital clay grain coats are mainly composed of smectite, illite, kaolinite, and chlorite, intermixed with other components, such as diatoms or pyrite. Biofilms of exopolymeric substances (EPSs), mostly produced by diatoms, are believed to control the adhesion of the clay coats to the surface of sand grains. Quantification by thin section shows that on average about 30% of the sands are coated in the point bar. The proportion of clay-coated grains appears to be independent of facies. Radiocarbon age dating measured on organic matter points to significant vertical mixing, highlighting the significance of erosion and redeposition. The activities of 137Cs and 210Pb indicate a vertical sedimentation rate of ca. 0.02 m.yr–1 in the muddy chute channel. These ages, coupled to historical maps, suggest that the present-day point bar has developed over the last 300 years with a vertical sedimentation rate ranging from 0.015 to 0.036 m.y–1 and a lateral migration rate of about 1 m.y–1. The combination of sedimentary geology, thin-section petrography, and mineralogy at high spatial and temporal resolutions highlights the potential of this study area as a modern analogue for ancient tidally influenced point-bar deposits associated with clay coatings.

In a detailed investigation of three active geothermal systems from Greece (Milos), from El Salvador (Chipilapa) and from Japan (Nigorikawa), it was found that smectites are common minerals in vapor dominated reservoirs. These smectites are formed in response to local discharge of boiling fluids in permeable horizons; they can crystallize also in pipelines of powerplants. According to the varying composition of the fluids involved in these geothermal systems (i. e. meteoric waters or sea water), beidellite, saponite and stevensite have been identified at temperature between 180 and 300°C. Montmorillonite has not been observed for these thermal conditions. The association of variable amounts of smectite rich mixed layers including illite/smectite, chlorite/smectite or stevensite/talc with pure smectites suggests that smectites are metastable phases which rapidly evolve toward illite, chlorite or talc via conversion series of mixed layers. These smectites are a potential guide for the recent evolution of vapor dominated geothermal reservoirs.

Chlorite is recognized worldwide as a key mineral that inhibits the development of quartz cement in deeply buried sandstone reservoirs. Iron-rich chlorite is mainly formed by the transformation of a precursor clay mineral; however, few studies have focused on the early stages before the crystallization of chlorite. This study analyzed shallowly buried (400–1000 m) coastal sandstones from within the Wealden Group of the Paris Basin. Berthierine, a 7 Å trioctahedral clay mineral belonging to the serpentine group, approximatively with same chemistry as chlorite but a different crystal structure, has been identified in a 900-m-deep well but not in a 400–600-m-deep well. Berthierine has mainly been observed as clay coatings around detrital grains with a typical honeycomb texture. Nanopetrographic observations suggest that the honeycomb textural organization of the clay particles found in deeper buried sandstone reservoirs (>1500 m) is acquired from a berthierine precursor at shallow depths. However, small amounts of quartz overgrowths are observed on the surface of detrital grains at shallow depths and low temperature (below 40 °C), and it is believed that precursor berthierine coatings are primarily responsible for the inhibition of quartz overgrowths before Fe-rich chlorite is formed. This suggests that the key mineral primarily controlling the reservoir quality of deeply buried sandstone reservoirs is berthierine rather than iron-rich chlorite, which challenges the commonly accepted assertion that chlorite coating is the main process that inhibits quartz overgrowths. The source-to-sink context of the Paris Basin during the Early Cretaceous was decisive with respect to the supply of sands and berthierine clay precursors (in particular kaolinite and iron-rich, hydroxy-interlayered clay minerals) to the center of the basin.

Clay minerals are the signature of hydrothermal alterations related to fluid circulation in volcanic and crystalline rocks. In the French part of the Upper Rhine Graben, in the deep-seated granites, illitic minerals (illite and I/S mixed layers (ml)) are typical products of the structurally-controlled argillic alteration in the Paleozoic granitic basement. In the new Illkirch geothermal well, GIL-1, drill-cuttings were studied with various petrographic methods to determine the characteristics of illite in paleo- and present-permeable zones, and to compare the alteration mineralogy with that of geothermal Soultz-sous-Forêts and Rittershoffen sites. Alteration petrography, crystal structure as well as the chemical composition of the illitic minerals and the altered bulk rocks were performed all along the well. This complete characterization, combined with geophysical logs and structural results, highlighted that the illitic minerals at Illkirch, Soultz-sous-Forêts, and Rittershoffen are composed of illite and illite-rich illite-smectite mixed layers (I/S ml) (&lt, 10% smectite). Two mineralogical assemblages were distinguished: chlorite + illite resulting from the propylitic alteration after the emplacement of the granitic basement under temperatures higher than 350 °C, and illite + I/S ml + carbonates + quartz resulting from the argillic alteration due to fluid circulation in the fractures at temperatures between 130 and 160 °C. Fracture zones are characterized by the occurrence of illitic minerals (illite and I/S ml), and specifically, by higher quantities of I/S ml in present-day permeable zones than in paleo-permeable zones. A conceptual model of the fracture zones at the interface between the overlying sedimentary rocks and the granitic basement is proposed. The present-day permeability distribution is controlled by the fault and fracture network, which consists of sealed zones and unsealed zones. Fluid convection in the URG implies paleo and present fluids circulating in both fractured sedimentary and crystalline reservoirs. Such circulations develop illitic minerals that could be considered as exploration guides for future geothermal sites in the URG. At Illkirch, the repartition of the present-permeable fracture zones (KFZs) in the GIL-1 well indicates that the moderately argillically altered granite distally situated from the Eschau fault is more permeable than the intensely argillically altered granite close to the Eschau fault.

This study focuses on hydrothermal alteration, and more specifically clays, in the geothermal reservoir of Cerro Pabellón in Andean Cordillera, Northern Chile. Although it is the first commercial geothermal power plant in South America with 48 MWe installed and an additional 33 MWe during 2022, the alteration mineralogy of the deep reservoir remains poorly investigated. Cuttings from two deep wells drilled between 500 and 3000 m depth were sampled. The fine-grained clay fraction was analyzed using X-ray diffraction to observe the mineral distribution and variations of crystal structure of the clays. Scanning electron microscopy coupled with energy dispersive spectroscopy allowed us to perform microanalysis of the clays. The results suggest a high heterogeneity of the reservoir in term of alteration: a pervasive alteration dominated by trioctahedral clays was observed in all the samples, whereas a fracture-controlled alteration was only observed locally, dominated by dioctahedral clays. Illite-smectite mixed layers with more than 90% illite typically occur in permeable fracture and fault as a typical clay signature. This study completed an integrated conceptual model of the high enthalpy reservoir (>250 °C). Circulations of a neutral pH Na-K-Cl fluid occur in a wide fracture system connected to highly dipping and NW-striking faults and could extend beyond the Pabelloncito graben.

The evolution of the petrophysical properties of rocks induced by hydrothermal alteration is often considered qualitatively by sorting the hydrothermal alterations encountered from low to high intensity, depending on the identified mineral paragenesis. In this paper, we studied the evolution of three different petrophysical properties (connected porosity, permeability and electrical conductivity) as a function of the argillization degree of andesites affected by argillic alteration identified in the caprock formation of the Petite Anse-Diamant hydrothermal system in Martinique. These petrophysical measurements were supplemented by quantitative mineral evaluation via scanning electron microscopy (i.e., the Quantitative Evaluation of Minerals by Scanning electron microscopy or the QEMSCAN® method), connected porosity mapping using 14C-PMMA method and measurements of methylene blue (MB) values and structural water proportions (H2O+). In the series of rocks investigated, an increasing trend of clay mineral abundances (montmorillonite and kaolinite) from 5.68 to 37.56% was revealed by the QEMSCAN® method. Based on the quantitative results, the structural water proportions were used as a proxy of argillic alteration progression. Comparison between the mineral maps provided by the QEMSCAN® system and connected porosity mapping observed in autoradiographs using 14C-PMMA method revealed a good correlation with clay mineral phases, dissolution vugs and fractures. The connected porosities evaluated with the triple weight method range from 3.74 to 35.35% and show a good correlation (R2 = 0.8835) with the H2O+ values, revealing that porosity development is mainly due to the replacement of primary phases by clay minerals. In contrast, the crystallization of silica, carbonates and, to a lesser extent, iron oxides tends to clog connected pores, inducing a local decrease in connected porosity, as revealed in autoradiographs. The Darcian permeability ranges from 7.66∙10−20 to 5.36∙10−17 m2 and shows a moderate correlation with the connected porosity. The bulk electrical conductivity measured as a function of the conductivity of the saturating solution reveals the significant contribution of surface conduction arising from montmorillonite. At a pore fluid salinity of 2 wt% (the salinity of the Eaux Ferrees thermal spring), the bulk electrical conductivity shows a relatively good correlation with the H2O+ values (R2 = 0.8591) and an even better correlation with the MB values (R2 = 0.9579). The contribution of montmorillonite to the bulk electrical conductivity was estimated with the use of the isoconductivity point, showing an increase in the BM values and a strong correlation (R2 = 0.9806).

In sedimentary environments, clay and sand are segregated by hydrodynamic processes. Yet, clay coats, thin clay envelopes lining sand grains, are abundant in modern and ancient coastal sedimentary deposits. Here, we present laboratory experiments in which we produced clay-coated quartz sands similar to those observed in modern and ancient estuarine sands. These coats were produced at ambient temperature by mixing exopolymeric substances (EPS) derived from intertidal diatom biofilms with clay minerals and quartz reference materials. The imaging of sediment-EPS mixes using cryo–scanning electron microscopy and atomic force microscopy demonstrated that EPS form organic bridges between clay and quartz. The physicochemical properties of the EPS were characterized independently through wet chemical assays and Fourier transform infrared spectroscopy. The results indicated that several EPS components (e.g., proteins, polysaccharides) had a potential to complex with quartz and clay. Our findings provide novel insights in the importance of biofilms in the aggregation of clay and sand. Detrital coats from ancient estuarine sandstones show textural similarities to the experimental clay-EPS complexes and could be considered as biosignatures of biofilm development and EPS production in past environments.

International audience; A new approach is proposed in order to spatially localize and determine the equilibrium state of natural decay chains on hand-scale geological samples, thanks to a combination of three techniques: 1) Elementary chemical mapping by microprobe; 2) Alpha autoradiograph by gaseous detectors and 3) bulk alpha particle spectrometry. The quantitative nature of alpha autoradiograph and its comparison with U chemical maps allows to locate radioactive equilibrium state in four samples. This equilibrium state was confirmed by alpha spectrometry analysis.

21 pages; International audience; In modern stromatolites, mineralization results from a complex interplay between microbial metabolisms, the organic matrix, and environmental parameters. Here, we combined biogeochemical, mineralogical, and microscopic analyses with measurements of metabolic activity to characterize the mineralization processes and products in an emergent (

International audience; The Messinian microbialites of the Terminal Carbonate Complex (TCC) from the Neogene basins of southeastern Spain show both diversified morphologies and an excellent preservation of primary microbial microstructures. Their stratigraphic architecture, fabric (micro-, meso-, and macro-fabric), and mineralogical composition were investigated in eight localities from three sedimentary basins of southeastern Spain: The Sorbas and Bajo Segura basins and the Agua Amarga depression. Two recurrent microbialite associations were distinguished. Laterally linked low relief stromatolites predominated in Microbialite Association 1 (MA1), which probably formed in low energy lagoons or lakes with fluctuating normal marine to hypersaline water. The microfabrics of MA1 reflected the predominance of microbially induced/influenced precipitation of carbonates and locally (Ca)-Mg-Al silicates. Microbialite Association 2 (MA2) developed in high energy wave and tidal influenced foreshore to shoreface, in normal marine to hypersaline water. High-relief buildups surrounded by mobile sediment (e.g., ooids or pellets) dominated in this environment. MA2 microbialites showed a significant proportion of thrombolitic mesofabric. Grain-rich microfabrics indicated that trapping and binding played a significant role in their accretion, together with microbially induced/influenced carbonate precipitation. The stratigraphic distribution of MA1 and MA2 was strongly influenced by water level changes, the morphology and nature of the substratum, and exposure to waves. MA1 favorably developed in protected areas during third to fourth order early transgression and regression phases. MA2 mostly formed during the late transgressions and early regressions in high energy coastal areas, often corresponding to fossil coral reefs. Platform scale syn-sedimentary gypsum deformation and dissolution enhanced microbial carbonate production, microbialites being thicker and more extended in zones of maximum deformation/dissolution. Microbial microstructures (e.g., microbial peloids) and microfossils were preserved in the microbialites. Dolomite microspheres and filaments showed many morphological similarities with some of the cyanobacteria observed in modern open marine and hypersaline microbialites. Dolomite potentially replaced a metastable carbonate phase during early diagenesis, possibly in close relationship with extracellular polymeric substances (EPS) degradation. Double-layered microspheres locally showed an inner coating made of (Ca)-Mg-Al silicates and carbonates. This mineral coating could have formed around coccoid cyanobacteria and indicated an elevated pH in the upper part of the microbial mats and a potential dissolution of diatoms as a source of silica. Massive primary dolomite production in TCC microbialites may have resulted from enhanced sulfate reduction possibly linked to the dissolving gypsum that would have provided large amounts of sulfate-rich brines to microbial mats. Our results open new perspectives for the interpretation of ancient microbialites associated with major evaporite deposits, from microbe to carbonate platform scales

International audience; 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.

International audience

International audience; This paper revisits the clay mineralogy of the “smectite” alteration zone in the Krafla geo-thermal field via the study of an exploratory well in which temperatures range from 40 °C to 215 °C. The clay alteration consists of several mineral assemblages superimposed in time and space, resulting from different stages of water-rock interaction. Trioctahedral clay minerals (chlorite, corrensite and smectite) are observed throughout the studied section. These minerals can form in nearly closed systems as replacements of groundmass minerals/glass after interactions with resident and nearly stagnant fluids not far from chemical equilibrium (neutral to basic pH conditions) or from direct precipitation from geothermal fluids. They are locally superimposed by Al clay phases (smectite, illite/smectite and kaolinite), which result from intense leaching of the host rocks due to their interaction with low pH fluids under strong W/R ratios. The absence of mineralogical zoning is explained by the fact that hydrothermal alteration is strongly dependent on very recent hydrodynamics. The current fluid circulation generates trioctahedral clays at depth that cannot be distinguished from pervasive earlier alteration. The only easily detectable signature of current activity and the most relevant signature for geothermal exploration is the presence of Al dioctahedral phases since it indicates leaching and intense hydrothermal activity.

The Tordillos sediment-hosted Cu deposit is located in the north-central sector of the Huincul High structure, in the Neuquén Basin, Argentina. It is hosted in sedimentary rocks of the Huincul Formation, an Upper Cretaceous redbed sequence deposited in a fluvial environment. Field mapping, petrography, and chemical analysis of the authigenic clay, sulfate, carbonate, and sulfide minerals at Tordillos have identified systematic variation in alteration and paragenesis with time and with respect to the main feeder zones. The alteration associated with Cu mineralization provides evidence of a multistage infiltration of fluids spatially associated with paleo-hydrocarbon migration driven by Tertiary tectonism. Early diagenesis formed hematite, kaolinite, and calcite1 that have been preserved in the reddish brown mudstones. Later influx of hydrocarbons and warm, reduced formation waters from deeper reservoir rocks is associated with dissolution of early cements and clasts and precipitation of new minerals. In the thickest paleo-channels of the Huincul Formation, dissolution of early cements (hematite, kaolinite, calcite1 and barite1) and clasts locally increased the porosity of sandstones up to 24% and destroyed hematite that changed sandstones and conglomerates from red to yellowish-grey. Bitumen impregnations are abundant along with calcite2, 3, and 4, pyrite, minor montmorillonite, quartz overgrowth, and coarse barite2 crystals with primary organic fluid inclusions at the contact with hydrocarbon tubes. The MnO enrichment of calcite in stages 3 and 4 reflects their precipitation under reduced conditions. Grey and greenish-grey sandstones with montmorillonite, V-hematite, smectite-chlorite group minerals and pyrite, respectively are exposed between the reddish-brown mudstones and the yellowish-grey sandstones and conglomerates. The grey and greenish-grey horizons are representative of abrupt changes in Eh-pH conditions, between regional early oxidizing solutions responsible for the formation of early diagenetic redbeds and later infiltration of reducing fluids responsible for alteration of sedimentary rocks. Copper mineralization is located inside the altered sandstones and conglomerates zones, forming halos surrounding vertical tubes containing bitumen. These tubes are interpreted as the conduits through which migrated hydrocarbons, formation waters, and subsequent chloride and metal-rich fluids. In these places, pyrite, calcite, and barite cements of the altered rocks, have been dissolved and replaced by copper-rich sulfides and then by late supergene Cu minerals. Interconnection among stacked permeable paleo-channels in the mid-section of the Huincul Formation facilitated the migration of several generations of fluids during early diagenesis, hydrocarbon up flow, and exhumation of the Neuquén Group, resulting in the development of alteration halos and Cu mineralization that can be used, both in outcrop and drill core, during exploration for sediment-hosted Cu mineralization. Fil: Pons, María Josefina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigación en Paleobiología y Geología; Argentina. Universidad Nacional del Comahue. Facultad de Ingeniería. Departamento de Geología y Petróleo; Argentina Fil: Franchini, Marta Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigación en Paleobiología y Geología; Argentina. Universidad Nacional del Comahue. Facultad de Ingeniería. Departamento de Geología y Petróleo; Argentina Fil: Giusiano, Adolfo Eugenio. Provincia de Neuquén. Dirección Provincial de Hidrocarburos y Energía; Argentina Fil: Patrier, Patricia. Université de Poitiers; Francia Fil: Beaufort, Daniel. Université de Poitiers; Francia Fil: Impiccini, Agnes. Universidad Nacional del Comahue. Facultad de Ingeniería. Departamento de Geología y Petróleo; Argentina Fil: Rainoldi, Ana Laura. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigación en Paleobiología y Geología; Argentina. Universidad Nacional del Sur. Departamento de Geología; Argentina Fil: Meinert, Lawrence. James Cook University; Australia

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; Estuarine tidal bar sandstones are complex reservoir geobodies commonly exploited by the oil and gas industry. In order to better predict the reservoir potential of these geobodies, this study provides a modern-day reservoir analogue, describing tidal bars in the inner and outer Gironde estuary from the microscopic to the macroscopic scale.The originality of this work lies in the multi-scale study of modern estuarine tidal bars based on numerous piston cores extracted in a high-energy environment. This work demonstrates that these tidal bars are composite sedimentary bodies made up of individual reservoir sand units separated by thick muddy layers. Their vertical facies associations and internal architectures are controlled by local hydrodynamic variations and seasonal river influxes. Detrital clay grain coats are notably characterized using a portable and handheld mineral spectrometer from the base to the top of the tidal bars. X-ray diffraction and electron microscopes reveal that these coats are mainly composed of di-octahedral smectite, illite, chlorite and kaolinite associated with other components such as diatoms or pyrite. The best reservoir geobodies are those with the minimum clay permeability barriers at the macro and mesoscale. An optimum coated grain content and clay fraction volume is also needed for generating authigenic clay coatings and inhibiting quartz overgrowth. These conditions are met within the tidal sand bars of the outer estuary funnel that are expected to be the best reservoir geobodies in deeply buried sandstones.

Two deep wells were drilled at Rittershoffen (Alsace, France) to produce high-temperature fluids to supply heat to a biorefinery. The GRT-2 production well was drilled to a depth of 3196 m MD and was deviated to target a permeable local fault in the granitic basement buried beneath a thick sedimentary cover. The objective of this study is to better understand the permeability of fractured reservoirs within crystalline rocks, focusing on the production well GRT-2. Based on a petrographic and mineralogical analysis of cutting samples, several granitic facies associated with hydrothermal alteration were identified on the basis of the amounts of illite, chlorite, anhydrite, secondary geodic quartz, and oxides. These observations were correlated with various geological and geophysical datasets (gamma ray, porosity, density, electrical resistivity, caliper, borehole image logs, temperature, rate of penetration, and mud losses) to localize and identify permeable fracture zones. In sections where acoustic image logs were not available, such as in the deepest part of the well, the geometries of the fracture zones were interpreted from an oriented caliper log. The caliper log interpretation detected one-third of the fractures detected by acoustic image logs. However, two major fracture sets striking N–S and dipping eastward or westward were observed. Furthermore, a synthetic resistivity log that fits the measured resistivity log relatively well was built using the Archie and Waxman and Smits models. This approach is a proxy for estimating the porosity and the mineralogical changes based on the cation exchange capacity, which is controlled by the chlorite/illite ratio, derived from electrical logs in granitic formations. The correlation of all these results allowed the identification of a resistivity signature of a permeable fracture zone that spatially fits with the temperature signature. The major contribution of this study is the identification of a hierarchy of permeable fractures based on petrophysical signatures. The geophysical signature of fracture zones with low residual permeability exhibits a broad depth extent, whereas the geophysical signature of a highly permeable fracture zone is more localized. Past hydrothermal circulation has enlarged the altered and porous zones around open fractures, and in some cases, intense illitization has plugged these fracture zones and reduced their permeabilities.

This present study provides an overview of the clay-mineral reactions involved in the chloritization process in a mixed-layer mineral series, and focuses on the properties of the resulting lowtemperature chlorites (formed at

Bleaching of red beds by the migration of reducing pore fluids is a widespread phenomenon, with most previous work on the topic focused on detailed studies in the southwestern United States, and to a lesser extent Germany. Herein, we report on widespread bleaching of Cretaceous red beds in the Neuquén Basin and the relationship of this bleaching to hydrocarbon migration. In the Cerro Granito area of the Huincul High, Neuquén Basin, the basal deposits of the Neuquén Group (Candeleros and Huincul Formations) are bleached. This alteration was apparently controlled by fluid and host-rock composition as well as by the intensity and span of the fluid-rock interaction. Red, fine-grained sandstones and mudstones of the Candeleros Formation were deposited in fluvial and swamp environments and contain authigenic hematite, Na-rich corrensite, micro- and mesoquartz, calcite, and analcite. Coarse-grained sandstones and mudstones of the Huincul Formation, which overlie the Candeleros Formation, were deposited in a braided fluvial system, under more humid and acidic conditions than the Candeleros Formation. Sandstones are similar in composition and texture to the Candeleros sandstones, except they contain less basic volcanic detritusand lack corrensite. Bleached facies in both formations contain bitumen and clusters of calcite + bitumen concretions, and partially dissolved detrital clasts and most cements, including early Fe-oxides, which resulted in an increase in secondary porosity and bleaching of the red beds. Alteration of the Candeleros Formation is confined to medium- to coarse-grained sandstone in which corrensite is partially dissolved and its interlayer charge is satisfied by Ca instead of Na, as in corrensite from the red sandstones. Extensive dissolution occurred in the Huincul Formation, which favored the precipitation of new authigenic minerals such as smectite, mixed-layer chloritesmectite, and pyrite. Bleaching and associated alteration most likely resulted from interaction of hydrocarbons with the red beds. Hydrocarbon paleomigration in the Cerro Granito area occurred in higherpermeability portions of the Candeleros and Huincul Formations. The seal provided by the Lisandro Formation was breached during uplift driven by Tertiary tectonism, resulting in the exhumation of the Huincul and Candeleros Formations. This study highlights the importance of a variety of alterations, including complex clay mineral paragenesis, contemporaneous with bleaching of the red beds. Such alterations can potentially be used to provide evidence for the passage of chemically reducing fluids associated with hydrocarbons in other regions, including cases where the original red color has been completely removed. Fil: Rainoldi, Ana Laura. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional del Sur; Argentina Fil: Franchini, Marta Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigación en Paleobiología y Geología; Argentina. Universidad Nacional del Comahue; Argentina Fil: Beaufort, Daniel. Université de Poitiers; Francia Fil: Mozley, Peter. Department of Earth and Environmental Science, New Mexico Tech; Estados Unidos Fil: Giusiano, Adolfo Eugenio. Subsecretaría de Minería e Hidrocarburos; Argentina Fil: Cesaretti, Nora Noemi. Universidad Nacional del Sur; Argentina Fil: Patrier, Patricia. Université de Poitiers; Francia Fil: Impiccini, Agnes. Universidad Nacional del Comahue; Argentina Fil: Pons, María Josefina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigación en Paleobiología y Geología; Argentina. Universidad Nacional del Comahue; Argentina

The mineralogy and chemical composition of mica-like phyllosilicates (>2 µm) and clay fraction (2 µm) and illite halo associated with phyllic and tourmaline-rich phyllic assemblages. The subhorizontal, shallow level Au-rich (>0.5 ppm Au) zone at the Quebrada de la Mina deposit coincides with the phyllic halo. The distribution of Au, Ag, Zn, Pb and As forms partially overlapping haloes toward shallower levels. Weak Cu (Mo) anomalies were found in potassic patches below the Au-rich zone. High Mn and Fe contents in clinochlore from the peripheral propylitic halo suggest a distance of ∼1 km to a porphyry Cu-(Au) mineralized core at deeper levels. White micas (>2 µm) in Quebrada de la Mina show a range of compositions between muscovite and illite. In the propylitic halo and in the patches of potassic alteration, white micas (>2 µm) show the highest values of Fe + Mg + Mn (0.40–0.55 apfu) and the lowest of Al (2.40–2.55 apfu). In the phyllic alteration zone of Quebrada de la Mina, white micas (>2 µm) with Al contents between 2.45 and 2.70 apfu and Fe + Mg + Mn between 0.35 and 0.15 apfu match the areas with high gold content (>0.5 ppm Au). The lowest values of Fe + Mg + Mn (0.11–0.05 apfu) and the highest Al contents (2.80–2.85 apfu) occur in white micas from the tourmaline-rich phyllic zone. Values of the Kübler index of illite of 0.19–0.1 2θ in the clay fraction of deep samples are indicative of higher temperature and/or higher fluid/rock ratios during illite formation, compared to values of the Kübler index > 0.2 2θ recognized in shallow samples. Thus, both the compositional variations in chlorite and white micas and the illite crystallinity could be used as a guide for exploration of copper-gold mineralization in shallow levels of porphyry systems. Fil: Maydagán, Laura. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Geológico del Sur. Universidad Nacional del Sur. Departamento de Geología. Instituto Geológico del Sur; Argentina. Universidad Nacional del Comahue. Facultad de Ingeniería. Departamento de Geología y Petróleo; Argentina Fil: Franchini, Marta Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Universidad Nacional de Río Negro. Sede Alto Valle. Instituto de Investigaciones en Paleobiología y Geología; Argentina. Universidad Nacional del Comahue. Facultad de Ingeniería. Departamento de Geología y Petróleo; Argentina Fil: Impiccini, Agnes. Universidad Nacional del Comahue. Facultad de Ingeniería. Departamento de Geología y Petróleo; Argentina Fil: Lentz, David. University of New Brunswick; Canadá Fil: Patrier, Patricia. Centre National de la Recherche Scientifique; Francia. Université de Poitiers; Francia Fil: Beaufort, Daniel. Centre National de la Recherche Scientifique; Francia. Université de Poitiers; Francia

Fil: Pons, María J. Universidad Nacional de Río Negro, Instituto de Investigación en Paleobiología y Geología. Río Negro; Argentina. Fil: Pons, María J. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Patagónico de Estudios Metalogenéticos; Argentina Fil: Pons, María J. Universidad Nacional del Comahue; Argentina Fil: Rainoldi, Ana L. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Patagónico de Estudios Metalogenéticos; Argentina Fil: Rainoldi, Ana L. Universidad Nacional del Comahue; Argentina Fil: Rainoldi, Ana L. Universidad Nacional de Río Negro; Argentina Fil: Rainoldi, Ana L. Universidad Nacional del Sur. Departamento de Geología; Argentina Fil: Franchini, Marta B. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Patagónico de Estudios Metalogenéticos; Argentina Fil: Franchini, Marta B. Universidad Nacional del Comahue; Argentina Fil: Franchini, Marta B. Universidad Nacional de Río Negro; Argentina Fil: Giusiano, Adolfo. Dirección Provincial de Hidrocarburos y Energía de la Provincia del Neuquén; Argentina Fil: Cesaretti, Nora. Universidad Nacional del Sur. Departamento de Geología; Argentina Fil: Beaufort, Daniel. Université de Poitiers, IC2MP, CNRS-UMR 7285, Hydrasa, Bâtiment B08, Rue Albert Turpin, F-86022 Poitiers Cedex, France Fil: Patrier, Patricia. Université de Poitiers, IC2MP, CNRS-UMR 7285, Hydrasa, Bâtiment B08, Rue Albert Turpin, F-86022 Poitiers Cedex, France Fil: Impiccini, Agnes. Universidad Nacional del Comahue. Centro Patagónico de Estudios Metalogenéticos; Argentina Fil: Impiccini, Agnes. Universidad Nacional de Río Negro; Argentina The Portezuelo Formation is part of a red bed sequence in the Neuquén Basin assigned to the Neuquén Group (lower Cenomanian–middle Campanian). In outcrops of the Portezuelo Formation in the Barda González region, iron oxide and hydroxide cements are only preserved in the less permeable claystone layers. Paleomigration of hydrocarbons and formation waters along the most permeable layers bleached the rocks due to partial to total dissolution of cements and grains and formation of several authigenic minerals: (1) montmorillonite and pyrite in the medium- to fine-grained sandstones and mudstones, and (2) multistage calcite and pyrite in the coarse-grained sandstones and conglomerates. The δ34SCDT (Canyon Diablo troilite) of −24.4 to −60‰ values in pyrite point to microbial reduction of a sulfate precursor. The oxidation of hydrocarbons and fluctuations in pH and carbon dioxide pressure caused by organic acids resulted in multiple stages of dissolution and precipitation of calcite. Calcite crystals host hydrocarbon-bearing fluid inclusions (± pyrite) with variable fluorescence color, indicating that hydrocarbon composition changed with time. The intermediate δ13CPDB (Peedee belemnite) (−8.11‰ to −8.52‰) and δ18OPDB(−9.76 to −9.83‰) isotope composition of calcites may have resulted from mixtures of 13C-rich CO2 after the dissolution of local calcrete with 13C-poor CO2 from oxidizing hydrocarbons. This contribution provides for the first time key information to predict the distribution of authigenic minerals in reservoir rockin the basin and improves understanding of the diagenetic history of the Portezuelo Formation. The Portezuelo Formation is part of a red bed sequence in the Neuquén Basin assigned to the Neuquén Group (lower Cenomanian–middle Campanian). In outcrops of the Portezuelo Formation in the Barda González region, iron oxide and hydroxide cements are only preserved in the less permeable claystone layers. Paleomigration of hydrocarbons and formation waters along the most permeable layers bleached the rocks due to partial to total dissolution of cements and grains and formation of several authigenic minerals: (1) montmorillonite and pyrite in the medium- to fine-grained sandstones and mudstones, and (2) multistage calcite and pyrite in the coarse-grained sandstones and conglomerates. The δ34SCDT (Canyon Diablo troilite) of −24.4 to −60‰ values in pyrite point to microbial reduction of a sulfate precursor. The oxidation of hydrocarbons and fluctuations in pH and carbon dioxide pressure caused by organic acids resulted in multiple stages of dissolution and precipitation of calcite. Calcite crystals host hydrocarbon-bearing fluid inclusions (± pyrite) with variable fluorescence color, indicating that hydrocarbon composition changed with time. The intermediate δ13CPDB (Peedee belemnite) (−8.11‰ to −8.52‰) and δ18OPDB(−9.76 to −9.83‰) isotope composition of calcites may have resulted from mixtures of 13C-rich CO2 after the dissolution of local calcrete with 13C-poor CO2 from oxidizing hydrocarbons. This contribution provides for the first time key information to predict the distribution of authigenic minerals in reservoir rockin the basin and improves understanding of the diagenetic history of the Portezuelo Formation.

A new approach is proposed in order to spatially localize and determine the equilibrium state of natural decay chains on hand-scale geological samples, thanks to a combination of three techniques: 1) Elementary chemical mapping by microprobe; 2) Alpha autoradiograph by gaseous detectors and 3) bulk alpha particle spectrometry. The quantitative nature of alpha autoradiograph and its comparison with U chemical maps allows to locate radioactive equilibrium state in four samples. This equilibrium state was confirmed by alpha spectrometry analysis.

The Imouraren uraniferous deposit is located in NW Niger and is hosted in the fluvial Tchirezrine 2 Formation (Jurassic, Tim Mersoi Basin). High-energy facies range from medium to very coarse-grained sandstones; low-energy facies are composed of fine-grained sandstones and analcimolites. Detrital material, which mainly occurs in sandstones but also occurs to a lesser extent in analcimolites, consists of quartz, K-feldspars, albite and accessory minerals. Since their deposition, these sediments have undergone transformations which were induced by two main processes. Burial diagenesis comprises the compaction and crystallization of secondary minerals, which consist of analcimes, clay minerals (i.e., chlorite, kaolins, illite), and quartz and albite overgrowths. This paragenesis suggests a burial depth of approximately 2 km, with maximum temperatures ranging from 80 to 120 °C. An alteration stage, related to the circulation of fluids, induced the alteration of diagenetic minerals, such as chlorite. This fluid/rock interaction is responsible for the deposition of uranium and the neoformations of clay minerals, such as tosudite and montmorillonite, as well as hematite, harmotome, calcite and copper sulfides. The fluids involved were U–Cu–Ba–Mg–Ca-rich, which enabled the crystallization of new minerals, and oxidizing, which destabilized chlorite and facilitated the transport of uranium.

International audience; In rocks or artificial geo-materials, radioactive isotopes emitting alpha particles are dispersed according to the mineralogy. At hand specimen scale, the achievement of quantitative chemical mapping of these isotopes takes on a specific importance. Knowledge of the distribution of the uranium and thorium series radionuclides is of prime interest to several disciplines, from the geochemistry of uranium deposits, to the dispersion of uranium mill tailings in the biosphere. The disequilibrium of these disintegration chains is also commonly used for dating. However, some prime importance isotopes, such as 226Ra, are complicated to localize in geo-materials. Because of its high specific activity, 226Ra is found in very low concentrations (~ppq), preventing its accurate localization in rock forming minerals.This paper formulates a quantitative answer to the following question: at hand specimen scale, how can alpha emitters in geo-materials be mapped quantitatively? In this study, we tested a new digital autoradiographic method (called the Beaver™) based on a Micro Patterned Gaseous Detector (MPGD) in order to quantitatively map alpha emission at the centimeter scale rock section. Firstly, for two thin sections containing U-bearing minerals at secular equilibrium, we compared the experimental and theoretical alpha count rates, measured by the Beaver™ and calculated from the uranium content, respectively. We found that they are very similar. Secondly, for a set of eight homemade standards made up of a mixture of inactive sand and low-radioactivity mud, we compared the count rates obtained by the Beaver™ and by an alpha spectrometer. The results indicate (i) a linearity between both count rates, and (ii) that the count obtained by the Beaver™ can be estimated from the count obtained by the alpha spectrometry using a factor of 0.82.

Tosudite, a regularly interstratified chlorite-smectite, crystallizes as an alteration mineral of several preexisting Al-bearing silicates (feldspars, kaolin minerals, chlorites) present in arkosic sandstones hosted in uranium deposits in Niger. X-ray diffraction patterns show a sharp superstructure at 29–29.6 Å for an air-dried state and a peak at 30.8–31.6 Å following ethylene glycol solvation. The 060 reflection at 1.507–1.509 Å indicates an overall dioctahedral character, and the very low coefficient of variation of the d00l reflections for the solvated mineral (0.03–0.13) permits validation of the regular interstratification justifying its identification as tosudite. Microprobe analysis allowed specification of the component layers of this mixed-layer mineral. The chlorite is a di-trioctahedral type analogous to sudoite (Si3Al4Mg2(OH)8), and the smectite component is a low-charge montmorillonite type Tosudite is characterized by large Al2O3 and MgO contents and small Fe content; its composition corresponds approximately to the formula where octahedral occupancy is ∼7. Scanning electron microscope (SEM) observations show that tosudite is closely associated with some uranium minerals: tosudite crystallization occurred during a late alteration event which post-dates burial diagenesis and during which uranium was remobilized by Mg-rich oxidizing fluids.

Microbialites are widespread in modern and fossil hypersaline environments, where they provide a unique sedimentary archive. Authigenic mineral precipitation in modern microbialites results from a complex interplay between microbial metabolisms, organic matrices and environmental parameters. Here, we combined mineralogical and microscopic analyses with measurements of metabolic activity in order to characterise the mineralisation of microbial mats forming microbialites in the Great Salt Lake (Utah, USA). Our results show that the mineralisation process takes place in three steps progressing along geochemical gradients produced through microbial activity. First, a poorly crystallized Mg-Si phase precipitates on alveolar extracellular organic matrix due to a rise of the pH in the zone of active oxygenic photosynthesis. Second, aragonite patches nucleate in close proximity to sulfate reduction hotspots, as a result of the degradation of cyanobacteria and extracellular organic matrix mediated by, among others, sulfate reducing bacteria. A final step consists of partial replacement of aragonite by dolomite, possibly in neutral to slightly acidic porewater. This might occur due to dissolution-precipitation reactions when the most recalcitrant part of the organic matrix is degraded. The mineralisation pathways proposed here provide pivotal insight for the interpretation of microbial processes in past hypersaline environments.

Clay minerals are widespread alteration products in most hydrothermal systems and are considered as efficient petrogenetic guides. Their distribution in active geothermal fields depends on the ability of the fluid to approach equilibrium with host rocks at any scale during the hydrothermal processes. Both illitization and chloritization reactions depend on factors such as time, water-rock ratio, deformation history... as well as the variables used to define equilibrium (i.e., temperature, composition). This paper is focussed on how the properties of clay minerals change with respect to the evolution of hydrogeological conditions during the life time of geothermal systems. The presented data are based on comparative studies performed in productive and non productive zones of several active geothermal systems related to subduction plate tectonics (Chipilapa, Milos and Bouillante). The temporal sequence of clay crystallizations can be summarized as follows. The opening stage of the geothermal system promotes a massive crystallization of metastable smectites in conditions of strong thermodynamic disequilibrium. As time and alteration progress, clay minerals of the newly sealed parts of the reservoir are transformed in more stable phases (via dissolution crystallization processes). Metastable smectites continue to precipitate in the permeable zones still subjected to strong physico-chemical disequilibrium such as boiling or mixing of fluids of contrasted composition and/or temperature, whereas clay minerals close to those predicted by thermodynamic calculations (illite, chlorite) crystallize in other zones in which the geothermal fluid can reach chemical equilibrium with the altered host rock. Downward infiltration of surficial water led to crystallization of montmorillonite in the outer part of geothermal fields. Collapsing of hydrothermal activity leads to a change in the heat flow regime with predominance of a depth dependant conductive thermal gradient. Even if such thermal conditions promote the transformation of the previously formed smectite rich metastable clay minerals and lead to a progressive emplacement of depth dependant conversion series of di- and trioctahedral smectites to respectively illite and chlorite, signatures of the ancient geothermal reservoirs are preserved in the textural or microstructural properties of the clay particles which sealed them.

The aim of this work is to provide a new efficient method of preparation of aluminium polycations/oligomers, and especially [AlO4Al12(OH)24(H2O)12]7+ (Al13). The microwave assisted forced homogeneous hydrolysis of aluminium through breakdown of urea (M–Al–Urea) is studied and compared with aluminum hydrolyzed with OH− ions (C–Al–OH). M–Al–Urea solutions are characterized by UV–Visible (‘Ferron’ kinetic assay) and 27Al NMR spectroscopies. Al13-sulphate precipitated from M–Al–Urea and C-Al-OH solutions are studied by XRD, TGA and SEM. Al-pillared montmorillonites are prepared with M–Al–Urea solutions as pillaring agent and characterized by XRD, BET, SEM-EDS, TGA and ATR techniques. The advantage of M–Al–Urea solutions as pillaring agent when compared to C–Al–OH is highlighted in the present study.

The Au-Cu (±Pb, Zn, Ag) prospects of Lavras do Sul, southernmost Brazil, are hosted in Neoproterozoic granitic and volcanogenic rocks. Mineralization occurs in structurally controlled N40°W to E-W quartz veins; sericite (±chlorite) and sulfides are the main secondary minerals in associated wall rocks. In the present contribution we use petrography (optical microscopy and scanning electron microscopy (SEM)), mineralogy (X-ray diffraction (XRD) with polytypes, FWHM, decomposition of diffraction patterns), and crystal chemistry of samples from several prospects to document the spatial and temporal evolution of sericitic alteration of veins and wall rocks associated with gold. Hexagonal, coarse-grained 2M phengite-rich alteration (± illite) is best developed with coarse-grained primary growth (comb) quartz + pyrite ± Au veins and altered wall rock from the western portion of the granitic complex (phyllic alteration). Pure phengite was recognized by narrow XRD profiles (FWHM ≤ 0.2°2θ CuKα) of the 0.9 per O(OH). Towards the eastern zones of the granitic complex and in the volcanogenic rocks, wider XRD profiles (FWHM values ≥ 0.2°2θ CuKα) were decomposed. They contain mixtures of coarse- to fine-grained, lath-like crystals of both 2M and 1M illite (non-expandable d X-ray reflections, IC between 0.85 and 0.89 per O(OH)) with expandable d reflections associated with lath-like, fine-grained crystals of ordered (R≥ 1) illite-rich 1-S (80-90% of illite; IC of ∼0.8 per O1OH)), and minor amounts of regularly ordered (R - 1), illite-rich I-S mixed layers (75% of illite; IC of ∼0.74 per O(OH)). The dioctahedral clay association of illite + illite-rich I-S mixed layers (intermediate argillic alteration) is best developed in quartz + pyrite ± Au veins, breccias, and wall-rock alteration from the eastern portion of the granitic complex and in the volcanic area. Quartz from veins and breccias has fine-grained primary growth, recrystallization, and replacement textures, similar to those in epithermal deposits. The overall distribution of the dioctahedral clays indicates that the study area represents a fracture-controlled, tilted, porphyry to epithermal deposit, with telescoping alteration features observed in the east of the mining district, Deeper levels of exposure of a large hydrothermal system are observed in the west of the mining district, as shown by higher-rank dioctahedral minerals (phengite) that crystallize at relatively high temperatures (T ≈ 300°C, phyllic alteration) associated with coarse-grained, primary-growth quartz veins, similar to those observed in porphyry deposits. On the other hand, shallower levels of exposure are observed in the east of the study area, associated with abundant, lower-rank dioctahedral clay minerals (illite + illite-rich I-S mixed layers, intermediate argillic alteration) that crystallize at relatively lower temperatures (T ≈ 120-200°C). Available data show that gold is associated with phengite, but that lower-rank, overprinting alteration characterized by illite-I-S may have locally modified the original gold grades.

The crystal structure and crystal chemistry of dioctahedral smectites in high enthalpy geothermal systems was investigated through 2 wells drilled in the Bouillante geothermal area in order to understand the factors which control their vertical variation. Smectites were examined by optical and scanning electron microscopy, electron microprobe analysis, X-ray diffraction (after Ca, Li, alkylammonium saturations), Fourier transform infrared spectrometry (after Li, NH4 + saturations) and oxygen isotope analysis. Smectites predominate within the upper part of the drill holes (up to 260m depth - temperature range : 67-160°C). The XRD, FTIR and punctual chemical analysis clearly demonstrate a transition from montmorillonite to beidellite with increasing depth and temperature which proceeded through interstratification of beidellite-like and montmorillonite-like layers. Montmorillonite predominates at temperatures lower than 100°C while beidellite predominates between 110 and 170°C. However, this transition is not explained by a thermally controlled beidellitization process but seems related to the hydrothermal solutions from which precipitated these smectites. The δ18O values of the equilibrated fluids indicate that beidellitic smectite precipitated from the hot geothermal fluid associated with minor amounts of residual solutions resulting from local boiling and that montmorillonitic smectite precipitated from reacting solutions whose origin has to be searched in the phreatic water table associated with minor amounts of liquids resulting from the condensation of vapors escaped from the boiling zones. The mixing rate of Mg-free geothermal fluid with meteoric waters exerted a major influence on the montmorillonite versus beidellite ratio of the smectite material as underlined by the irregular depth related smectite transition

Several nuclides of the 238 U ( 234 U, 230 Th, 226 Ra, 210 Pb) and 235 U ( 231 Pa, 227 Ac) decay series have been measured through gamma spectrometry in the U-bearing episyenites of the Bernardan mine (northwest Massif Central, France), in order to study recent mobilizations in the U-deposit and their time-scales. ( 230 Th/ 238 U) and ( 231 Pa/ 235 U) ratios show that U has been little affected during the last 350 ky, especially in the U-rich samples. In contrast, 226 Ra was subjected to a general redistribution, with a clear tendency for Ra to be lost from the U-rich samples and added to U-poor samples. The highest Ra excesses are present in low-U samples, and are related to adsorption onto iron hydroxides. Ra appears to have been redistributed within the mineralised episyenitic bodies, which seem to remain in 226 Ra– 238 U radioactive equilibrium, a result consistent with the earlier study of Leroy [1984. Episyenitisation dans le gisement d'uranium du Bernardan (Marche): Comparaison avec des gisements similaires du Nord-Ouest du Massif Central francais. Miner. Depos. 19, 26–35]. As shown by the 226 Ra and 210 Pb data, the Ra redistribution was very active between 8 and 0.1 ky, and this might be a consequence of the hydrological changes in groundwater circulation accompanying the Holocene climate warming.

Aluminum phosphate-sulfate (APS) minerals formed around the Athabasca unconformity-type deposits and those from their Australian counterparts are chemically very similar showing the same continuum between the diagenetic Sr-rich APS minerals of the barren sandstones and the LREE-rich composition of the APS minerals in the hydrothermally altered sandstone. The P- and LREE-rich compositions were controlled by the transport and the redistribution of P and LREE elements released from the dissolution of phosphate minerals (principally monazite) in the basement rocks and in the basin during the syn-ore alteration processes. The S/Sr ratio measured in the APS minerals from unaltered sandstone away from the unconformity and any mineralization is preserved during the syn-ore alteration processes suggesting that the fluids involved in both the deep burial diagenetic processes and the syn-ore alteration system were derived from a similar diagenetic reservoir in both the Athabasca and Kombolgie regions. The trioctahedral chlorite host-rock alteration around the Australian basement-hosted U deposits, as compared to the illite and sudoite associated with the Athabasca basement-hosted, along with the more LREE-rich APS compositions in the Australian deposits, suggests that the pH and oxygen fugacity ( f O 2 ) of the syn-ore fluids differed in the alteration systems of the two regions at the time of the U deposition.

International audience; Illite particles of

International audience; The geothermal field of Bouillante (Guadeloupe, FWI) is a high-enthalpy hydrothermal system emplaced in submarine volcanoclastic formations (hyaloclastites, scarce lava flows) and subaerial formations (andesitic lava flows, pyroclastites, lahars) which belong to the Lesser Antilles arc. Three directional wells were drilled in 2001 to optimize the productivity of the geothermal field up to 15 MWe and to investigate the vertical distribution of clay alteration from the surface area down to a depth of 1000 m where temperatures exceed 250 °C. Special attention has been paid to the "clay signature" of the fractured zones which channel the present geothermal fluids. Three successive zones, dominated, respectively by dioctahedral smectite, illite and chlorite were identified at increasing depths. Alteration petrography indicates that these mineralogical clay zones result from the spatial superimposition of at least two successive alteration stages. The first one, assimilated to a propylitic alteration stage, affected all parts of the system and consisted of crystallization of trioctahedral phyllosilicates (chlorite or corrensite), Ca-silicates (heulandite-clinoptilolite, prehnite, pumpelleyite, wairakite and epidote), quartz and minor calcite in replacement of most of the primary minerals of the intersected volcanic or volcanoclastic formations. The later stage of alteration is related to the circulation of the present geothermal fluids and is assimilated to argillic or phyllic alteration. It consists of a more or less intense argillization which results from the crystallization of aluminous dioctahedral clay phases (smectite, illite±I-S mixed layers, and accessory kaolinite) associated with quartz, calcite, hematite or pyrite. The permeable zones which channel most of the present geothermal fluids are fracture controlled and do not contain specific clay parageneses. However the illite±I-S mixed layers minerals differ from those of the surroundings by specific properties including both crystal structure and texture. These specific properties (decrease in the expandable component of the illitic material, increase of the illite crystallinity) can be controlled by the nucleation/growth rates operating in zones of active flow regime. Being mainly a product of the earlier propylitic alteration stage, chlorites are much less informative on the fracture controlled permeable levels. However, the compositional variations of chlorites recorded within the shallower fractured zone suggest a significant change in fO2 conditions related to early circulation of fluids along the major near west striking normal faults (Plateau fault).

Clay minerals associated with the unconformity-type uranium deposits of the East Alligator Rivers uranium field have been used to determine the spatial distribution of hydrothermal alteration and the conditions of uranium deposition within the altered basement underlying the Kombolgie unconformity. Alteration develops a similar pattern at regional and smaller scales, with a close relationship between the degree of alteration and the amount of structural discontinuities observed on both sides of the unconformity (e.g., faults, fractures, breccias). These discontinuities were mechanically active during the hydrothermal activity. The hydrothermal clay minerals partly to totally replace diagenetic dickite and illite in the Kombolgie sandstones and metamorphic phengite and chlorite in the basement rocks. The time-space alteration sequence established in this study can be summarized as follows: (1) early crystallization of illite ± illite-smectite (I-S) mixed layer minerals + LREE-Sr–bearing aluminum phosphate-sulfates (APS) in sandstones above the unconformity, illite ± I-S mixed layer minerals + sudoite + APS on both sides of the unconformity and in fault gouges, and illite ± I-S mixed layer minerals + chlorite ± apatite deeper in the basement; and (2) late precipitation of chlorite in the open fracture network mainly centered below the unconformity. The main stage of uranium deposition was associated with chloritization in the basement rocks. Smaller particle size, lath-shaped or flaky morphology, common interstratification with expandable layers, lower crystallinity, and Mg-rich chemical compositions can be used to distinguish the hydrothermal clays from the other diagenetic and metamorphic illite and/or chlorite. Hydrothermal illite can be distinguished from diagenetic and metamorphic illite-phengite by its poor crystallinity along the c-axis, interstratification with minor amounts of smectite layers, and a 1M polytype in which trans-vacant octahedral sites predominate (1 Mt). Chemically, the hydrothermal illite differs from diagenetic illite by a lower interlayer charge and a higher Mg content. The hydrothermal chlorite is distinguished from the metamorphic chlorite by its smaller particle size, spherulitic habit, poor crystallinity along the c-axis, and common association with corrensite and/or chlorite-corrensite mixed layers. Most of the hydrothermal chlorite is Mg rich and has a structural formula close to that of clinochlore (XFe

Aluminum phosphate–sulfate (APS) minerals occur as disseminated crystals in a wide range of geological environments near the Earth’s surface, including weathering, sedimentary, diagenetic, hydrothermal, metamorphic and also postmagmatic systems. Their general formula is AB3(XO4)2(OH)6, in which A, B and X represent three different crystallographic sites. These minerals are known to incorporate a great number of chemical elements in their lattice and to form complex solid-solution series controlled by the physicochemical conditions of their formation (Eh, pH, activities of constituent cations, P and T). These minerals are particularly widespread and spatially related to hydrothermal clay-mineral parageneses in the East Alligator River Uranium Field (EARUF) environment associated with uranium orebodies in the Proterozoic Kombolgie basin of the Northern Territories, Australia. This field contains several high-grade unconformity-related uranium deposits, including Jabiluka and Ranger. Both petrography and chemical data are used to discuss the significance of APS minerals in the alteration processes from the EARUF. The wide range of chemical compositions recorded in APS is essentially due to coupled substitutions of Sr for the LREE and of S for P at the A and X sites, respectively. The major variations of the APS solid-solution series mainly consist of the relative proportions of svanbergite, goyazite and florencite end-members. The APS minerals result from the interaction of oxidizing and relatively acidic fluids with aluminous host-rocks enriched in monazite. The spatial distribution of these minerals and their compositional variation around the uranium orebodies allow us to consider them as good tracers of redox and pH paleo-conditions responsible for the development of fronts during the alteration processes, and hence as potential tools for mineral exploration.

Surficial clay minerals as indicators of present-day geothermal activity were studied around Bouillante, Guadeloupe, French West Indies. The study is based on widespread sampling (>100 surface samples) in a zone extending 5 km north and 9 km south from Bouillante. Three types of mineral associations were distinguished: (1) dioctahedral smectites (in which the ‘beidellite–nontronite’ behavior predominates), with zeolites, calcite±quartz±kaolinite; (2) ordered illite–smectite mixed-layered clay with adularia, silica±calcite; and (3) kaolinite–smectite mixed-layered clays±halloysite±kaolinite±smectite±silica. The first association characterizes steaming grounds and areas subjected to hydrothermal fluid infiltrations along fractures or breccia zones in marine and subaerial environments. The second association was only identified in the matrix of epithermal breccias. This assemblage appears as a tracer of high-temperature fluids and is a good indicator of the proximity to the geothermal reservoir. The third assemblage was found both within and beyond the bounds of the known geothermal area, suggesting a pedogenic origin. We interpret the occurrence of the three assemblages in the Bouillante geothermal area as a background of clayey tropical weathering (kaolinite–smectite) with a hydrothermal overprint (dioctahedral smectites and illite–smectite).

Whereas the study of clay minerals (using parameters such as structure modifications, crystal chemistry) has proved to be an effective approach in order to mark the circulation zones and fluid temperatures in high-temperature systems, there is an important lack of data dealing with such problems in moderate- to low-temperature systems. This paper proposes to fill this lack by studying the site of the Lamentin Plain (Martinique), characterized by a lowtemperature geothermal activity (50‡C6 T6 90‡C). Preliminary data acquired on the three studied exploration wells (LA01, LA02 and LA03) show that the current low-temperature geothermal activity is particularly well expressed through the LA03 well. This low-temperature activity is superimposed on evidence of a high-temperature fossil activity, particularly well expressed in the non-productive levels of the LA01 and LA02 wells. After a petrographic characterization, this work focuses on the textural and microstructural properties of the low-temperature hydrothermal clay phases utilizing X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectrometry and scanning electron microscope (SEM). The petrographic observations have permitted successive hydrothermal events to be recognized, which express the thermal collapse of the system, as shown by the superposition of ancient episodes, characterized by vein and pervasive alterations of high to moderate temperatures, and recent episodes, characterized by alterations associated with present-day circulations of low-temperature fluids. The fossil activity results from several hydrothermal episodes, with fluid temperatures ranging from 100 to over 200‡C. Illitic phases (i.e. I/S (R =1 ) to illite) appear as the main markers of the fossil solution paths, due to their abundant development close to major veins and fractures, while chloritic phases (chlorite, corrensite) seem to mark a pervasive alteration, better preserved further from the fossil solution channels. On the other hand, the recent hydrothermal alteration is characterized by the massive presence of kaolinite, dioctahedral smectite and siderite. Whereas smectites are only observed abundantly farther from the current solution channels, kaolinite appears to be the dominant clay mineral in these channels, or in the surroundings of sealed fractures which do not currently drain any more hydrothermal fluids. The crystallinity estimations on XRD and FTIR patterns from the 6 2 Wm fraction, along with SEM morphological observations, clearly show that the microstructural and textural properties of the kaolinites can be related to fluid temperatures. Indeed, kaolinites with the best crystallinity, or the best-ordered, are located in the fractures carrying the hottest fluid