Mineralogy and ultrastructure of some alteration products of Yamato-86032 meteorite

White products of Yamato-86032 meteorite sample have been studied using linear localization detector method and TEM (powders examination and ultramicrotomed thin-sections). In addition to the mineral phases of the meteorite (olivine, pyroxene, labradorite), calcium sulfate and hydrous minerals have been identified. The observed glassy matrix has high silica content and the alteration products in contact with glass are mainly Fe-Al and Fe-Al-Na-K-S compounds. The presence of these mineral phases suggests that a sulfuric acid alteration has to be involved for the formation of the white products studied

Mapping U-238 decay chain equilibrium state in thin sections of geo-materials by digital autoradiography and microprobe analysis

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.Peer reviewe

A cloritização na Mina Uruguai, Minas do Camaquã, RS, Brasil

The area of Camaquã Mines is part of the camaquã basin, which has NE-NW directions and is filled out by silliciclastics sediments interbedded with volcanic rocks. Chlorite is the most abundant clay mineral in the area and appears in high amount in the halos of hydrothermal alteration present in the host's mineralization rocks. This work consists on the chlorite's petrologic, chemical and structural characterization, supplying important information on different processes and formation conditions of the hydrothermal environment. The following procedures were made: optical petrography, X-ray diffraction, difractograms modeling through the Reynolds Newmod© program, and scanning electron microscopy ( secondary electrons and microanalysis - EDS) in representative samples of the different identified hydrothermal alteration zones. Chlorites are present under three different tendencies: Chlorite I takes place with a pervasive aspect over the matrix rocks located near the mineralized veins; it is classified as Fe-chlinoclore, presents fraction ˂1 µm predominance of plytype IIb and a Mg2+  enrichment in the fraction of  ˂10 µm predominance of polytype Ib (90°) and Fetotal enrichment. Chlorite II occurs as veins filling out small fractures; it is classified as Chamosite and presents structural polytype IIb. Chlorite III occurs altering detrital minerals; it is classified as Mg-chamosite. The variation on the amount of Fe of the chlorites generated over the hydrothermal processes ( Chlorite I ˂10 µm and Chlorite II) indicates the occurrence of at least two pulses in the process of the hydrothermal alteration: one is responsible for the great matrix and clasts alternation and the other one for the generation of the late veins. The variation in the amount of Fetotal of the two different responsible fluids for the crystallization of the chlorites is was quite more enriched in iron than the early fluid that crystallized Chlorite I ˂10 µm.A região das Minas do Camaquã é a parte constituinte da Bacia do Camaquã, a qual possui direção NE-NW e é preenchida por sedimentos siliciclásticos intercalados com rochas vulcânicas. A clorita é o argilomineral mais abundante da área, ocorrendo em grande quantidade nos halos de alteração hidrotermal presentes nas rochas encaixantes nas mineralizações na Mina Uruguai. Este trabalho consiste na caracterização petrológica, química e estrutural das cloritas, que fornece importante informações sobre diferentes processos e condições de formação do ambiente hidrotermal. Para isso se realizou petrografia óptica, difratometria de raios X, modelamento dos difratogramas através do programa Reynolds Newmod © e  microscopia eletrônica de varredura (elétrons secundários e micro-análises por EDS) em amostras representativas de diferentes zonas de alteração hidrotermal identificadas. As cloritas se apresentam com três diferentes tendências: a Clorita I ocorre com aspecto pervasivo sobre matriz das rochas localizadas próximos aos filões mineralizados; é classificada como Fe-clinocloro, apresenta na fração ˂1 µm predominância do politipo IIb e um enriquecimento em Mg2+  e na fração ˂10 µm predominância do politipo Ib (90°) e enriquecimento em Fetotal .  A Clorita II ocorre com veios preenchendo pequenas fraturas; é classificada como Chamosita e apresenta politipo estrutural IIb. A Clorita III ocorre alterando minerais detríticos sendo classificada como Mg-Chamosita. A variação na quantidade de ferro das cloritas geradas por processos hidrotermais ( Clorita I ˂10 µm e Clorita II) fornece indícios da ocorrência de pelo menos dois pulsos no processo de alteração hidrotermal: um responsável pela intensa alteração da matriz e dos clastos das rochas e outro responsável pela geração de veios tardios. A variação na quantidade Fetotal  dos dois diferentes fluidos responsáveis pela cristalização das cloritas fica evidenciada pela associação de co-genecidade da Clorita II com a hematita, mostrando que o fluido final foi muito mais enriquecido em ferro que o fluido precoce cristalizou a Clorita I ˂10 µm

Microbial and diagenetic steps leading to the mineralisation of Great Salt Lake microbialites.

12 pagesInternational audienceMicrobialites 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

Influence de l'histoire thermique sur la cristallochimie et la croissance cristalline des epidotes dans les champs geothermiques: l'alteration propylitique du champ fossile de Saint-Martin

SIGLEINIST T 75314 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Signature des conditions dynamiques (thermiques, hydrodynamiques...) dans les propriétés des argiles du champ géothermique de Bouillante, Guadeloupe

L'intérêt économique des environnements hydrothermaux et leur utilisation comme analogues de stockage de déchets radioactifs justifient que ces environnements aient été largement étudiés depuis plus de 50 ans. Le champ géothermique de Bouillante, système actif situé sur la côte ouest de Basse-Terre (Guadeloupe) atteint des températures de l'ordre de 250C à 1000 m de profondeur. L'étude de la fraction argileuse montre une histoire complexe du champ géothermique de Bouillante. Les roches ont été altérées : 1) par une diffusion thermique (absence de circulation de fluides), s'accompagnant de la formation d'un cortège de minéraux argileux d'origine propylitique, de type chloritique. 2) par la circulation de fluides géothermaux conduisant à la précipitation de l'illite jusqu'à la beidellite, avec l'abaissement de la température du fluide. Enfin, vers la surface, le mélange du fluide géothermal et d'eau météorique infiltrée aboutit à la précipitation de smectites de type montmorillonite.Economic interest of the hydrothermal environments and their use as analogues of storage for radioactive waste justify that these environments have been largely studied for more than 50 years. The geothermal field of Bouillante, active system located on the West coast of Basse-Terre (Guadeloupe) reached temperatures about 250C at 1000 m of depth. The study of the clay fraction shows a complex history of the geothermal field. The rocks were altered: 1) by a thermal diffusion process (absence of circulation of fluids), being accompanied by the formation of a clay mineral procession of propylitic origin, of chloritic type. 2) by the circulation of geothermal fluids leading to the precipitation of the illite until the beidellite, with the fall in the fluid temperature. Lastly, towards surface, the mixture of the geothermal fluid with infiltrated meteoric water ends in the precipitation of smectites of the montmorillonite type.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Cristallochimie du fer dans les chlorites de basse température (implications pour la géothermométrie et la détermination des paléoconditions redox dans les gisements d'uranium)

Dans les contextes de gisements d'uranium, les conditions redox constituent le principal facteur contrôlant le dépôt de la minéralisation. Très fréquemment observées dans ces gisements, les chlorites constituent le seul minéral argileux susceptible d'enregistrer des paléo-fronts redox, du fait de la présence dans sa structure de fer à la fois sous sa forme Fe2+ et Fe3+. Toutefois, le message délivré par ces minéraux est souvent difficile à préciser du fait de la présence très fréquente dans ces environnements de plusieurs populations d'origines variées (diagénétiques, hydrothermales, métamorphiques, épisodes hydrothermaux superposés ). Grâce à la spectroscopie -XANES et à un important travail méthodologique de calibration, nous sommes maintenant capables de mesurer sur lames minces et à +- 5 % le rapport Fe3+/SFe dans les chlorites. Les mesures réalisées démontrent que le rapport Fe3+/SFe peut atteindre des valeurs voisines de 60% pour les chlorites di-trioctaédriques et 5 % à plus de 40 % pour les chlorites trioctaédriques. En contexte hydrothermal, où les chlorites se forment via un processus de dissolution/recristallisation, le rapport Fe3+/SFe diminue avec l'augmentation de la teneur en Fer globale. Les chlorites observées en contexte diagénétique qui sont issues de la transformation polymorphique de la berthiérine ont un comportement différent puisqu'il n'apparaît pas de lien entre leur teneur en Fer total et le rapport Fe3+/SFe : leur chimie est directement héritée de celle du minéral précurseur, cette transformation ne permettant pas un échange ni une réorganisation des cations au sein de la structure. Ce mode de formation explique l'inadéquation de la modélisation thermodynamique pour ces phases minérales. En vue de leur utilisation comme marqueurs de paléoconditions dans les contextes de gisements d'uranium où chlorites hydrothermales et diagénétiques peuvent être présentes, il est donc déterminant d'identifier l'origine des chlorites avant de pouvoir préciser le message délivré. Cela peut être réalisé au travers de l'analyse de leur polytype: Ib (b=90) pour les chlorites issues d'une transformation à l'état solide et IIb pour les chlorites formées via un processus de dissolution-recristallisation.In contexts of uranium deposits, redox conditions constitute the main factor controlling the uranium deposition. Often observed in these deposits, chlorites are the unique clay mineral which can be able to record in their structure the redox conditions through their Fe3+/SFe ratio. However, the common presence of several populations of chlorites makes difficult to understand the message carried out by these minerals. Thanks to -XANES spectroscopy, we are now able to measure on thin sections the Fe3+/SFe ratio in chlorites with an accuracy of 5 %. Measurements show that it can reach 60 % in di-trioctahedral chlorites and 5 % to more than 40 % for trioctahedral chlorites. In hydrothermal contexts where chlorites crystallize through a dissolution-recrystallization process, their Fe3+/SFe ratio decreases with the increase of the global Fe content. Diagenetic chlorites observed resulting from the polymorphic transformation of berthierine have a different behavior because there is no link between their total iron content and their Fe3+/SFe ratio: their chemistry is directly inherited from the one of the precursor mineral because this transformation does not allow a reorganization of cations in the structure. This transformation explains that thermodynamic models cannot work for these phases. For the use of chlorites as makers of redox paleoconditions in contexts of uranium deposits where diagenetic and hydrothermal chlorites can be present, it is decisive to determine their origin, for example analyzing their polytype: Ib (b=90) for chlorites crystallized from solid-state transformation and IIb for chlorites crystallized through dissolution-recrystallization process.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Recent mobilization of U-series radionuclides in the Bernardan U deposit (French Massif Central)

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. Episyénitisation dans le gisement d'uranium du Bernardan (Marche): Comparaison avec des gisements similaires du Nord-Ouest du Massif Central français. Miner. Depos. 19, 2635]. 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

Evolutions temporelles et spatiales des altérations argileuses des gisements d'uranium sous discordance, secteur de Shea Creek (bassin de l'Athabasca, Canada)

L'étude des paragénèses argileuses a été entreprise dans le secteur minéralisé de Shea Creek, sur plus de cent soixante échantillons provenant de contextes stériles et minéralisés. Les méthodes d'études pétrographiques, morphologiques (MEB), cristallochimiques (DRX, IRTF et microanalyses chimiques) et isotopiques (datations K-Ar) ont permis de circonscrire spatialement les phénomènes d'altération associés à l'épisode minéralisateur principal et de préciser leur mise en place. Entre environ 1600 et 1400 Ma, les formations gréseuses sont principalement affectées par une paragénèse à kaolinite-dickite diagénétique d'ampleur régionale. La première période d'illitisation (+- dravite) est circonscrite vers 1400-1350 Ma, et date l'ouverture du système diagénétique à l'incursion de fluides de socle, résultant probablement d'une réactivation des structures tectoniques. La période 1350-1150 Ma correspond à de nouvelles phases d'illitisation, certainement associées à une longue période d'activité du système de failles. Dans l'environnement étudié, les illites, via leurs propriétés microstructurales et texturales, apparaissent comme des marqueurs des conditions régnant lors de leur formation: (1) les illites de polytype 1Mc (morphologies en lattes) remplacent les minéraux du groupe kaolin, et marquent des environnements dans lesquelles les fluides, suite aux diverses interactions avec les roches, présentent des degrés de sursaturations plus faibles que pour (2) les illites de polytype 1Mt (morphologies en "cheveux"), associées à la sudoite. Ces illites ont précipitées à partir de solutions qui présentent une forte contribution des produits d'altération du socle D'un point de vue de l'exploration, le modèle d'argilisation proposé souligne la relation forte entre les dépôts économiques d'uranium et les paragenèses argileuses à illite de polytype 1Mt dominant + sudoite + dravite.A detailed study of clays minerals assemblages has been performed in the Shea Creek prospect (Saskatoon, Saskatchewan, Canada), with more than 160 samples representing the diagenetic background and the vicinity of U mineralizations. Petrographic studies, textural analysis (SEM), crystal-chemical characterisation (XRD, FTIR and microprobe analysis) and K-Ar dating allow to define spatially the alteration assemblages are associated with the main mineralization event. Their genesis has been reconstructed. Between approximately 1600 et 1400 Ma, sandstone formations were affected by a kaolinite-dickite diagenesis as observed regionally in the basin. The first illitization stage (+- dravite) is situated between 1400-1350 Ma, and dates the opening of the diagenetic system to basement fluids, resulting probably of the reactivation of tectonic structures. The period 1350-1150 Ma corresponds to new illitization phases, which are certainly associated with a long activity of the faults system. The microstructural and textural properties of illites are good indicators of the paleoconditions during their formation: (1) 1Mc-illite (lath morphologies) have grown at the expense of kaolin minerals and the fluids had lower sursaturations than (2) 1Mt-illite (hairy morphologies), associated with sudoite. This illite have grown from fluids which reflects a strong contribution of alteration products of basement rocks. From an exploration point of view, the model show a strong relation between U-mineralization and an 1Mt-illite + sudoite + dravite assemblage.POITIERS-BU Sciences (861942102) / SudocSudocFranceF