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This research involved an investigation of the application and reaction mechanisms of UV/H₂O₂ for the simultaneous removal of regulated halogenated disinfection byproducts (DBPs) and odorous aldehydic algal byproducts in the presence of geosmin and 2-methylisoborneol, which are earthy-musty odorants that commonly occur in drinking water. UV/H₂O₂ is an expensive advanced oxidation process that is used to successfully control geosmin and 2-methylisoborneol. The aqueous oxidation of odorous aldehydes and halogenated DPBs were compared to that of the earthy-musty odorants and the changes to the sensory properties of the drinking water were examined. Geosmin, 2-methylisoborneol, heptadienal, decadienal, and nonadienal, hexanal, and the two most prevalent classes of DBPs, trihalomethanes (THMs) and haloacetic acids (HAAs) were oxidized by UV photolysis alone and the UV/H₂O₂ process with 6 mg/L H₂O₂ and realistic ng/l to μg/L concentrations of the test compounds. The di-, and tri-brominated THMs and HAAs were substantially (80-99%) removed by direct UV photolysis mechanism at the same UV/H₂O₂ dose required for removing 95% of geosmin and 65% of 2-methylisoborneol with faster reaction rates for the more bromine substituted compounds. The C-Br bond cleavage is the first step of brominated HAAs degradation by UV photolysis, and followed by either of two second steps: reaction with oxygen producing peroxyl radical or interaction with water molecule causing O-H insertion/H-Br elimination. Trichloromethane and mono-, di-, and tri-chlorinated HAAs were not substantially removed under the same conditions used for the brominated compounds. The principal removal mechanism was by the reaction with hydroxyl radical for the UV/H₂O₂ process. The second order reaction rate constants were on the order of 10⁶ - 10⁸ M⁻¹ s⁻¹ with faster reaction rates for the less chlorine substituted compounds. Based on the reaction rates, hydrogen and halogen ion balance, and isotope effect, both hydrogen abstraction and electron transfer reaction were involved in the first steps of the chlorinated HAA degradation. Three odorous aldehydes - heptadienal, decadienal, and nonadienal - were removed faster than geosmin or 2-methylisoborneol, and direct UV photolysis was the principal reaction mechanism for the removal of these unsaturated aldehydes. Hexanal was poorly removed. In sensory tests, new odors such as sweet or chalky odors were produced while the concentration and initial odor intensity of these fishy/grassy-smelling aldehydes were reduced with increasing exposure time to UV/H₂O₂. Carbonyl compounds were detected as products of the UV photolysis of nonadienal. These carbonyls were not removed by further UV irradiation, which was thought to be partially related with production of new odors. The results indicate that the UV/H₂O₂ is effective to control both odorous compounds and brominated DBPs. This process can be seasonally applied to control both contaminants especially, in the warm summer when both odorants and DBPs have their higher concentrations. Removal of brominated DBPs can be a significant addition to water utilities that have difficulty in meeting regulatory levels for these highly toxic compounds. The result on the removal of odorous aldehydes indicate that new types of odors were produced from the oxidation of odorous aldehydes suggesting sensory test coupled with chemical analysis should be considered in designing oxidation process to control recalcitrant odorants. / Ph. D.

Les données électromagnétiques imagent des zones du manteau plus conductrice que l’olivine sèche. Il y a peu d’ambiguïté sur le fait qu’un liquide est thermodynamiquement stable et présent au niveau de l’asthénosphère, mais son impact sur la conductivité électrique du manteau reste débattu. Les études pétrologiques réalisées ces 30 dernières années ont montré qu’une péridotite exposée aux conditions the P-T-fO₂ de l’asthénosphère produisait des liquides riches en H₂O and CO₂, mais les conductivités électriques de ces liquides sont mal connues. Pour cette raison, des expériences de conductivité électrique ont été réalisées en piston cylindre sur des liquides riches en H₂O and CO₂. Différentes compositions de liquides ont été explorées, des liquides carbonatés aux basaltes. Les effets de la composition chimique et des volatiles sur ces liquides ont été déterminés. Les mesures de conductivités électriques ont montré que les liquides hydratés et carbonatés sont très conducteurs, et que l’incorporation de basalte décroit la conductivité. Avec ces nouvelles données, un modèle semi-empirique calculant la conductivité en fonction des teneurs en H₂O and CO₂ a été produit. Sur la base de ce modèle et de la conductivité électrique de l’olivine, des profils 1D de conductivité ont été construits. Avec ces profils, l’effet des teneurs en volatiles (partagé entre le liquide et le solide), les fractions de liquides (loi de mélange et interconnexion du liquide) et les différents régimes de température sur la conductivité ont été discutés. Ces calculs ont été considérés en milieu océanique et continental pour différents âges. La conductivité électrique du manteau est donc un outil puissant pour suivre les processus fondamentaux de la fusion du manteau, qui est à son tour étroitement liée aux cycles de H₂O and CO₂ dans le manteau supérieur. / Electromagnetic data images mantle regions more conductive than that of dry olivine. There is no doubt that melt is thermodynamically stable and present in the asthenosphere, but how they can impact on mantle electrical conductivity remains debated. Petrological studies realized some 30 years ago have shown that peridotites exposed at the P-T-fO₂ conditions of the asthenosphere produced H₂O and CO₂ rich-melts, but electrical conductivities of these melts are poorly known. Therefore, electrical conductivity experiments have been performed in piston cylinder on H₂O-CO₂ rich melts. Different melt compositions have been explored, from carbonated melts to basalts. The effects of chemical compositions and volatiles on these melts have been determined. The electrical conductivity measurements have shown that hydrous carbonated melts are very conductive, and the incorporation of basalt decreases the conductivity. With these new data, a semi-empirical law predicting the conductivity as a function of H₂O and CO₂ contents has been produced. Based on this law and the electrical conductivity of olivine, 1D conductivity profiles were constructed. With these profiles, the effect of volatile contents (partitioned between the melt and in the solids), melt fractions (mixing law and interconnection of the melt) and different temperature regimes on conductivity are discussed. These calculations are conducted on oceanic and continental settings with different ages. The electrical conductivities of the mantle is thus a powerful tool to track the fundamental process of mantle partial melting, which is in turn narrowly associated to the cycling of H₂O and CO₂ in the upper mantle.

Les conditions contraignantes provoquent la modification de l’état redox et la signalisation liée aux formes actives de l’oxygène (ROS), dont les concentrations sont régulées par des systèmes antioxydant complexes. On dénombre de plus en plus de processus qui sont affectés par la régulation redox, mais nous avons toujours des connaissances fragmentaires quant à l’importance des interactions centrales entre ROS et systèmes antioxydants pour la signalisation cellulaire chez les plantes. Cette étude a utilisé des approches de génétique classique et inverse chez l’espèce-modèle, Arabidopsis thaliana dans le but d’élucider les rôles des catalases et des systèmes NADPH-glutathion-ascorbate dans le métabolisme du H₂O₂ et la signalisation qui en dépend. Une analyse de mutants ADN-T pour les trois gènes codant la catalase a révélé que la mutation cat2, à la différence de cat1 et de cat3, a fortement affecté la croissance et le développement de la plante. Lorsqu’il était cultivé dans l’air, le mutant cat2 présentait une croissance réduite à la fois au niveau de la rosette et des racines, mais ces effets étaient absents lors de la culture des plantes sous un taux de CO₂ élevé, suggérant que la taille diminuée est causée, directement ou indirectement, par une capacité compromise de métaboliser le H₂O₂ produit par la photorespiration. Une étude de cat2 cultivé dans des photopériodes différentes a mis en évidence une forte influence de la période d’illumination sur la signalisation oxydative et ceci d’une manière qui est indépendante de l’intensité du stress. Lorsque cat2 est cultivé en jours longs, le stress oxydant induit la voie de l’acide salicylique (SA), provoquant des lésions visibles sur les feuilles. Cette réponse au stress oxydant est annulée dans un double mutant cat2 g6pd5, chez lequel l’expression d’une forme spécifique de la glucose-6-phosphate déshydrogénase (G6PDH) a également été inactivée. Une approche de génétique classique a permis d’identifier plusieurs gènes susceptibles d’être impliqués dans la régulation de la formation de lésions SA-dépendante dans ce double mutant. Afin d’explorer les rôles des monodéshydroascorbate réductases (MDHAR) spécifiques dans des conditions optimales et de stress, des mutants d’insertion pour plusieurs gènes codant la MDHAR ont été obtenus. Ces mutants présentaient un phénotype sauvage dans des conditions de culture optimales, mais à la suite de son introduction dans le fond cat2, l’un d’entre eux a fortement modifié l’induction de la voie SA par le stress oxydant. Pris dans leur ensemble, les résultats soulignent l’importance de CAT2 et permettent de dessiner un lien fonctionnel entre des G6PDH et MDHAR spécifiques dans les voies de signalisation oxydative chez Arabidopsis, lien qui pourrait s’expliquer par la production de NADPH par la G6PDH et son utilisation par la MDHAR. / Stress conditions lead to modified redox states and signaling linked to reactive oxygen species (ROS), whose cellular concentrations are regulated by complex antioxidative systems. While the list of processes subject to redox regulation continues to grow, our understanding of the importance of the core interactions between ROS and plant antioxidative systems in cell signaling remains very fragmentary. This work used forward and reverse genetics to analyze the roles of catalases and the NADPH-glutathione-ascorbate systems in H₂O₂ metabolism and related signaling in the model species, Arabidopsis thaliana. An analysis of T-DNA mutants for the three catalase-encoding genes revealed that cat2, but not cat1 or cat3, substantially impacted plant growth and development. While the cat2 mutant showed decreased shoot and root size when grown in air, both these effects were annulled by growth at high CO₂, suggesting that they were caused, directly or indirectly, by compromised capacity to metabolize photorespiratory H₂O₂. An analysis conducted in cat2 rosettes following growth in different photoperiods revealed that oxidative signaling is strongly influenced by day length in a manner that is independent of stress intensity. When cat2 is grown in long days, oxidative stress induces the salicylic acid (SA) pathway, leading to visible lesions on the leaves. This response to oxidative stress is annulled in cat2 g6pd5, which has additionally lost the function of a specific glucose-6-phosphate dehydrogenase (G6PDH). A forward genetics approach identified several genes that may be involved in regulating SA-dependent lesion formation in this double mutant. To explore the roles of specific monodehydroascorbate reductases (MDHAR) in optimal and stress conditions, insertion mutants for several MDHAR-encoding genes were obtained. While these mutants showed a wild-type phenotype in optimal growth conditions, one of them markedly altered induction of the SA pathway by oxidative stress when introduced into the cat2 background. Together, the results underline the importance of CAT2 and point to functional coupling between specific NADPH-producing G6PDH and NADPH-requiring MDHAR in oxidative stress signaling pathways in Arabidopsis.

La surface de Mars a longtemps été considérée comme inactive de nos jours. Cependant, de nombreux phénomènes actifs ont été observés ces dernières années comme : des ravines, des tornades de poussières, des écoulements ou des taches sombres. La plupart d’entre eux sont encore inexpliqués et soulèvent de nombreuses questions au regard des conditions de surface existantes actuellement. Afin d’améliorer la compréhension de ces processus et en particulier des ravines, cette thèse a pour but d’en étudier certains par une approche pluridisciplinaire combinant l’analyse de données issues des missions spatiales martiennes et par une approche expérimentale. Les données de télédétection ont permis une analyse morphologique détaillée grâce aux images provenant de la caméra HiRISE (High Resolution Imaging Science Experiment) et de leurs MNT (Modèle Numérique de Terrain). Celles-ci ont été complétées par une analyse de la composition de la surface grâce aux données spectroscopiques CRISM (Compact Reconnaissance Imaging Spectrometer for Mars).Des études expérimentales visant à mieux comprendre l’effet de la pression et des mélanges fluides/grains sur des écoulements martiens ont également été réalisés. Les études morphologiques et compositionnelles réalisées dans l’hémisphère sud de Mars, ont permis d’arriver aux conclusions suivantes : i) Les ravines ont un fonctionnement saisonnier et se différencient entre elles par leur morphologie, leur taille, leur période d’activité, leur orientation et leur pente, ii) Certaines ravines saisonnières sont très récentes : elles se créent et disparaissent actuellement, alors que d’autres fonctionnent depuis une centaine d’années martienne, iii) Le CO₂ semble être le principal agent actif pour leur création, même si l’eau liquide et les saumures ne peuvent être écartées dans certains cas, iv) Les différences d’insolation et l’accumulation de matériel en amont de l’écoulement influencent l’activité des ravines saisonnières. v) Un fluide différent de l’eau peut créer et faire évoluer des sinuosités de nos jours sur Mars. Les expériences réalisées au cours de cette thèse ont montré que : i) Les écoulements d’eau en surface et sub-surface d’un milieu granulaire, vont avoir un impact morphologique plus important à basse pression (~ 7mbar) qu’à pression terrestre (~ 1000 mbar), ii) Il existe des modes de transport des sédiments atypiques à basse pression: un processus de « lévitation » (impliquant des fluides gazeux) et de la saltation. Les morphologies qui en découlent, peu courantes sur Terre, sont susceptibles d’intervenir sur Mars, iii) Des mêmes constituants d’un écoulement peuvent créer des morphologies et des dynamiques réellement différentes. Trois paramètres semblent jouer un rôle important sur la morphologie d’un écoulement : l’épaisseur de sédiment mobilisable, les différences de températures entre la surface et l’écoulement ainsi que la quantité de matériel écoulé. L’analyse dans cette thèse d’un grand nombre d’activités saisonnières ainsi que l’approche pluridisciplinaire utilisée ont permis de mettre en évidence des écoulements actuels reliés avec des processus exotiques. La détermination plus précise des conditions de formation de ces phénomènes actuels ne pourra se faire que grâce à l’acquisition de nouvelles données sur tous ces sites et sur plusieurs saisons. De nouvelles expériences en laboratoire permettant en particulier de mieux comprendre les processus liés au CO₂ sont également nécessaires. / Mars surface has been considered like a currently inactive planet for a long time. Nevertheless, numerous active phenomena have been observed during the last few years such as: gullies, dust devils, dark and bright flows. Most of them are also unexplained and their formation processes are unclear under present Mars conditions. The aim of this PhD is to study some gullies to try to understand their formation processes on Mars at the present time. For this, martian spacecraft data were analyzed and experimental studies were performed. Thanks to HiRISE (High Resolution Imaging Science Experiment) images and HiRISE DTM (Digital Terrain Models), a morphological study was carried out. This study has been completed by a spectroscopic study of the surface thanks to CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) data. Experiments were also performed with a mix of liquid and sand to try to understand the pressure impact on martians flows. Morphological and spectroscopic studies on the southern hemisphere of Mars lead us to conclude that: i) Gullies are seasonal. They have different morphologies, sizes, timing of activities, orientations, and slopes. ii) Gullies are young. They appear and disappear on present-day. Some of them are active since 100 years. iii) CO₂ frost seems to be a good candidate for gullies creation but a participation of liquid water can’t be excluded in some cases. iv) Insolation variations and material accumulations in the upper part of the flow influence the gullies activity. v) Sinuosities can be created by another fluid than liquid water today on Mars. Experiments performed during this PhD demonstrate that: i) Water flow in granular surface and sub-surface triggers higher morphological impact under Martian pressure (~ 7mbar) and Earth pressure (~ 1000 mbar). ii) Exotic sediment transport can be observed at low pressure (~ 7mbar): “levitation” (with gas) and saltation. The morphology and dynamics are influenced by 3 main parameters: the sediment thickness, the temperature variations between the surface and the flow and the amount of flow material. Several seasonal phenomena were analyzed with different approaches. This allowed us to observe and interpret present-day flows and exotic processes. More data and new experiments at low pressure (in particular with CO₂) will help us to better constrain and better understand the formation conditions of current seasonal activities on the Mars surface.

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.

L’allumage par claquage laser non résonant des mélanges réactifs considérés à l’état gazeux et au repos est étudié dans ce travail, principalement avec des mélanges n-décane/air (C₁₀H₂₂+N₂+O₂). Ce système est considéré comme étant prometteur dans les différentes stratégies futures concernant les systèmes d’allumages équipant les moteurs à combustion interne. Le plasma d’allumage est généré en focalisant un faisceau laser de haute intensité pendant quelques nanosecondes. Le laser Nd :YAG opère à 1064 nm, il est choisi comme source laser pour l’ensemble des expériences menées en laboratoire afin de montrer l’effet des paramètres physiques, optiques, thermodynamiques (pression) et chimiques (additifs : H₂O, Ar) sur les caractéristiques de l’allumage. Cette étude met en oeuvre une approche statistique sur l’ensemble des expériences en prenant en compte l’ensemble des mesures prises lors de la combustion. Ce manuscrit offre une base de données expérimentale permettant d’appréhender la combustion et la phénoménologie de claquage laser. / Ignition by non-resonant laser breakdown of quiescent reactive mixtures was considered in this experimental study working with gaseous state. In this work, we mainly study the ignition of n-decane / air (C₁₀H₂₂+N₂+O₂) mixtures. This system is considered promising in different future strategies regarding ignitions systems for internal combustion engines. The breakdown is generated by focusing a high intensity laser beam for a few nanoseconds using Nd: YAG laser operating at 1064 nm, it is chosen as the laser source for all experiments conducted here. The experimental plan conducted allows to examine the effect of physical, optical, thermodynamic (pressure) and chemicals (additives: H₂O, Ar) on the characteristics of the laser ignition. This study implements a statistical approach on all the experimental cases taking into account all the measures during breakdown and combustion. This manuscript provides bibliographic basis for understanding combustion and laser breakdown phenomenology.

The system H₂O-CH₄ is found in a variety of geological environments in the earth’s crust, from sedimentary basins to low grade metamorphic terrains. Knowledge of the PressureVolume-Temperature-Composition (PVTX) properties of the H₂O-CH₄ system is necessary to understand the role that these fluids play in different geological environments. In this study the properties of the H₂O-CH₄ fluid system at elevated temperatures and pressures has been investigated experimentally to determine the PVTX properties of H₂O-CH₄ fluids in the P-T range equivalent to late diagenetic to low grade metamorphic environments, and XCH₄≤4mol%. A study has also been conducted to determine methane hydrate stability over the temperature range of -40~20°C. Synthetic fluid inclusions were employed in both studies as miniature autoclaves. Experimental data for the PVTX properties of H₂O-CH₄ fluids under late diagenetic to low grade metamorphic conditions was used to calculate the slopes of isoTh lines (the line connecting the P-T conditions of the inclusions at formation and at homogenization) at different PTX conditions. An empirical equation to describe the slope of iso-Th line as a function of homogenization temperature and fluid composition was developed. The equation is applicable to natural H₂O-CH₄ fluid inclusions up to 500°C and 3 kilobars, for fluid compositions ≤4 mol% CH₄. The Raman peak position of CH₄ gas is a function of the pressure and temperature. This relationship was used to determine the pressure along the methane hydrate stability curve in the H₂O-CH₄ system. The combined synthetic fluid inclusion, microthermometry and Raman spectroscopy method is a novel experimental approach to determine the P-T stability conditions of methane hydrates. The method is fast compared to conventional methods, and has the potential to be applied to study other gas hydrate systems. / Ph. D.

Le réchauffement climatique est lié aux augmentations des concentrations de gaz à effet de serre dans l'atmosphère terrestre et en particulier aux émissions anthropiques de CO₂. La séquestration géologique a la capacité et la longévité potentielles pour diminuer de façon significative les émissions anthropiques de CO₂. Cette séquestration à grande profondeur induit des risques de fuite des réservoirs géologiques. Parmi les scénarios de fuite envisagés, celui d'une fuite diffuse est le plus inquiétant puisque sans surveillance, cette fuite pourrait perdurer et entrainer des séquelles sur l'environnement ainsi que des risques pour les populations. Des outils et protocoles de surveillance doivent donc être mis au point pour la surveillance en proche surface. Ce travail de thèse s'inscrit dans le cadre de cette problématique. Il a pour objectif la caractérisation, la quantification et la modélisation du transport et des interactions du CO₂ dans une zone non saturée carbonatée. Ce travail a suivi une approche expérimentale sur un site pilote naturel à Saint-Emilion (Gironde, France), avec la réalisation de fuites diffuses en ZNS carbonatée. Cette étude aborde plusieurs thématiques: la description et l'instrumentation du site pilote naturel ; la caractérisation physico-chimique de l'hétérogénéité du réservoir carbonaté ; l'étude du fonctionnement naturel de la ZNS carbonatée et en particulier la mise en place d'une ligne de base des concentrations en CO₂ ; la caractérisation de l'extension des panaches de gaz suite à des expériences de fuite diffuse dans la ZNS carbonatée et l'étude par simulation numérique des interactions gaz-eau-roche lors d'une fuite diffuse de CO₂ dans une ZNS carbonatée. Les résultats de ces travaux montrent l'importance de la caractérisation de l'hétérogénéité du réservoir carbonaté ainsi que des techniques d'échantillonnage et d'analyse des différentes phases en présence. L'établissement de la ligne de base a une importance particulière pour permettre de distinguer les variations naturelles de celles induites par une fuite diffuse de CO₂ dans la ZNS carbonatée. Les modes de transport du CO₂ vont évoluer en fonction des paramètres physico-chimiques. Ce transport se fait par advection et/ou par diffusion. L'utilisation de gaz inertes au niveau du site de séquestration géologique est très importante puisque la détection de ces traceurs permettrait de prédire les arrivées de panaches de CO₂ en proche surface. Par ailleurs, les interactions chimiques doivent être prises en compte dans les modèles de transport afin de pouvoir définir les facteurs de retard et l'impact d'une fuite diffuse de CO₂ sur une ZNS carbonatée. / Global warming is related to atmospheric greenhouse gas concentration increase and especially anthropogenic CO₂ emissions. Geologic sequestration has the potential capacity and the longevity to significantly diminish anthropogenic CO₂ emissions. This sequestration in deep geological formation induces leakage risks from the geological reservoir. Several leakage scenarios have been imagined. Since it could continue for a long period, inducing environmental issues and risks for human, the scenario of a diffusive leakage is the most worrying. Thus, monitoring tools and protocols are needed to set up a near-surface monitoring plan. The present thesis deals with this problematic. The aims are the characterisation, the quantification and the modelling of transport and interactions of CO₂ in a carbonate unsaturated zone. This was achieved following an experimental approach on a natural pilot site in Saint-Emilion (Gironde, France), where diffusive gas leakage experiments were set up in a carbonate unsaturated zone. Different aspects were investigated during the study: natural pilot site description and instrumentation; the physical and chemical characterisation of carbonate reservoir heterogeneity; the natural functioning of the carbonate unsaturated zone and especially the set-up of a CO₂ concentrations baseline; the characterisation of gas plume extension following induced diffusive leakage in the carbonate unsaturated zone and the study of gas-water-rock interactions during a CO₂ diffusive leakage in a carbonate unsaturated zone through numerical simulations. The results show the importance of the carbonate reservoir heterogeneity characterisation as well as the sampling and analysing methods for the different phases. The baseline set-up is of main interest since it allows discrimination between the induced and the natural CO₂ concentrations variations. The transfer of CO₂ in a carbonate unsaturated zone is varying in function of physical and chemical properties. This transfer is done by diffusion and/or advection. Because the detection of the noble gases allows the prediction of CO₂ plume arrival, the use of tracers in the sequestration site is of main importance. The chemical interactions have to be taken under account in transport models in order to predict delay factors and the impact of a CO₂ leakage in a carbonate unsaturated zone.

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No. of bitstreams: 2 license_rdf: 22974 bytes, checksum: 99c771d9f0b9c46790009b9874d49253 (MD5) Dissertacao_EstudosIsotopicosZonas.pdf: 3577467 bytes, checksum: ceb7632af67c8a3b5664f57bf77a59a5 (MD5) Previous issue date: 2013 / UFPA - Universidade Federal do Pará / CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico / INCT/GEOCIAM - Instituto Nacional de Ciência e Tecnologia de Geociências da Amazônia / O depósito cupro-aurífero Visconde está localizado na Província Mineral de Carajás, a cerca de 15 km a leste do depósito congênere de classe mundial Sossego. Encontra-se em uma zona de cisalhamento de direção WNW-ESE, que marca o contato das rochas metavulcanossedimentares da Bacia Carajás com o embasamento. Nessa zona ocorrem outros depósitos hidrotermais cupro-auríferos com características similares (Alvo 118, Cristalino, Jatobá, Bacaba, Bacuri, Castanha), que têm sido enquadrados na classe IOCG (Iron Oxide Copper-Gold), embora muitas dúvidas ainda existam quanto a sua gênese, principalmente no que diz respeito à idade da mineralização e fontes dos fluidos, ligantes e metais. O depósito Visconde está hospedado em rochas arqueanas variavelmente cisalhadas e alteradas hidrotermalmente, as principais sendo metavulcânicas félsicas (2968 ± 15 Ma), o Granito Serra Dourada (2860 ± 22 Ma) e gabros/dioritos. Elas registram diversos tipos de alteração hidrotermal com forte controle estrutural, destacando-se as alterações sódica (albita + escapolita) e sódico-cálcica (albita + actinolita ± turmalina ± quartzo ± magnetita ± escapolita), mais precoces, que promoveram a substituição ubíqua de minerais primários das rochas e a disseminação de calcopirita, pirita, molibdenita e pentlandita. Dados isotópicos de oxigênio e hidrogênio de minerais representativos desses tipos de alteração mostram que os fluidos hidrotermais foram quentes (410 – 355°C) e ricos em 18O (δ18OH2O= +4,2 a 9,4‰). Sobreveio a alteração potássica, caracterizada pela intensa biotitização das rochas, a qual ocorreu concomitantemente ao desenvolvimento de foliação milonítica, notavelmente desenhada pela orientação de palhetas de biotita, que precipitaram de fluidos com assinatura isotópica de oxigênio similar à dos estágios anteriores (δ18OH2O entre +4,8 e +7,2‰, a 355°C). Microclina e alanita são outras fases características desse estágio, além da calcopirita precipitada nos planos da foliação. A temperaturas mais baixas (230 ± 11°C), fluidos empobrecidos em 18O (δ18OH2O = -1,3 a +3,7‰) geraram associações de minerais cálcico-magnesianos (albita + epidoto + clorita ± calcita ± actinolita) que são contemporâneas à mineralização. Valores de δ18DH2O e δOH2O indicam que os fluidos hidrotermais foram inicialmente formados por águas metamórficas e formacionais, a que se misturou alguma água de fonte magmática. Nos estágios tardios, houve considerável influxo de águas superficiais. Diluição e queda da temperatura provocaram a precipitação de abundantes sulfetos (calcopirita ± bornita ± calcocita ± digenita), os quais se concentraram principalmente em brechas tectônicas - os principais corpos de minério - que chegam a conter até cerca de 60% de sulfetos. Veios constituídos por minerais sódico-cálcicos também apresentam comumente sulfetos. A associação de minerais de minério e ganga indica uma assinatura de Cu-Au- Fe-Ni-ETRL-B-P para a mineralização. Os valores de δ34S (-1,2 a +3,4‰) de sulfetos sugerem enxofre de origem magmática (proveniente da exsolução de magmas ou da dissolução de sulfetos das rochas ígneas pré-existentes) e precipitação em condições levemente oxidantes. Datação do minério por lixiviação e dissolução total de Pb em calcopirita forneceu idades de 2736 ± 100 Ma e 2729 ± 150 Ma, que indicam ser a mineralização neoarqueana e, a despeito dos altos erros, permite descartar um evento mineralizador paleoproterozoico. A idade de 2746 ± 7 Ma (MSDW=4,9; evaporação de Pb em zircão), obtida em um corpo granítico não mineralizado (correlacionado à Suíte Planalto) que ocorre na área do depósito, foi interpretada como a idade mínima da mineralização. Assim, a formação do depósito Visconde teria relação com o evento transpressivo ocorrido entre 2,76 e 2,74 Ga, reponsável pela inversão da Bacia Carajás e pela geração de magmatismo granítico nos domínios Carajás e de Transição. Esse evento teria desencadeado reações de devolatilização em rochas do Supergrupo Itacaiúnas, ou mesmo, provocado a expulsão de fluidos conatos salinos aprisionados em seus intertícios. Esses fluidos teriam migrado pelas zonas de cisalhamento e reagido com as rochas (da bacia e do embasamento) pelas quais se movimentaram durante a fase dúctil. As concentrações subeconômicas do depósito Visconde devem ser resultado da ausência de grandes estruturas que teriam favorecido maior influxo de fluidos superficiais, tal como ocorreu na formação dos depósitos Sossego e Alvo 118. / The Cu-AuVisconde deposit is located in the Carajás Mineral Province, northern Brazil, about 15 km east of the world-class Sossego deposit. It lies within a regional WNW–ESE-striking shear zone that marks the contact between the ~2.76 Ga metavolcano-sedimentary rocks of the Carajás Basin and the basement units. Other Cu- Au deposits with similar characteristics (Bacaba, Castanha, Alvo 118, Cristalino, Jatobá) occur along this shear zone. They have been included in the IOCG class, although much controversy exists regarding their genesis, particularly with respect to the mineralization age and source of fluids, ligands and metals. TheVisconde deposit is hosted by Archean rocks, mainly felsic metavolcanic rocks (2968 ± 15 Ma), the Serra Dourada granite (2860 ± 22 Ma), and gabbro/diorites. These rocks are variably sheared and reveal various types of hydrothermal alteration with strong structural control. The earliest types are the sodic (albite-scapolite) and sodic-calcic alterations (albiteactinolite ± tourmaline ± quartz ± magnetite ± scapolite ± epidote), which promoted ubiquitous replacement of the rock primary minerals and precipitaton of disseminated chalcopyrite, pyrite, molybdenite and pentlandite. Oxygen isotope data of representative minerals from these stages show that the hydrothermal fluids were hot (410 – 355°C) and 18O-rich (δ18OH2O = +4.2 to +9.4‰). The following potassic stage is characterized by intense biotitization of the rocks, which developed concomitantly a mylonitic foliation highlighted by the remarkable orientation of biotite flakes. This mica precipitated from fluids with similar oxygen isotope signature to that of the previous stages (δ18OH2O = +4.8 to +7.2‰, at 355°C). Microcline and allanite are other typical minerals of this stage, in addition to chalcopyrite that deposited along the foliation planes. At lower temperatures (230 ± 11°C), 18O-depleted fluids (δ18OH2O = -1.3 to +3.7‰) generated a calcic-magnesian mineral assemblage (albite + epidote + chlorite ± actinolite ± calcite) present mostly in veins and contemporaneous with the main mineralization. The δ18OH2O and δDH2O data indicate that the hydrothermal fluids were initially formed by metamorphic and formation waters, possibly with some contribution of magmatic water. At later stages, there was a considerable influx of surface water. The resulting fluid dilution and cooling might have accounted for the abundant precipitation of sulphides (chalcopyrite ± bornite ± chalcocite ± digenite) mainly in tectonic breccias, whose matrix contains up to 60% sulphides. These breccias represent the most important ore bodies, although sulphides also occur in veins together with sodic-calcic minerals. The mineral associations assign a Cu-Au-Fe-Ni-ETRL-B-P signature to the ore. The sulphur isotope composition (δ34SCDT= -1.2 to 3.4‰) is compatible with a magmatic source for sulphur, which could have been either exsolved from a crystallizing granitic magma or dissolved from sulphides originally present in preexisting igneous rocks. Additionally, it indicates relatively reducing conditions for the fluid. Dating of chalcopyrite by Pb leaching and total dissolution techniques yielded ages of 2736 ± 100 Ma and 2729 ± 150 Ma. Despite the large errors, they point to a Neoarchean age for the mineralization and preclude a Paleoproterozoic mineralizing event. The age of 2746 ± 7 Ma (MSDW = 4.9; Pb evaporation on zircon), obtained for a non-mineralized granitic intrusion present in the deposit area and correlated to the Planalto Suite, was considered as the minimum age for the mineralization. Thus, the Visconde deposit genesis could be related to the 2.76-2.74 Ga transpressive tectonothermal event that was responsible for the inversion of the Carajás basin and generation of granitic magmatism in the Carajás and Transition domains. Such an event should have triggered devolatilazion reactions in the Itacaiunas Supergroup rocks, producing metamorphic fluids or even driving off water trapped in the pores of the basin rocks. These fluids migrated along regional shear zones and reacted with both the basin and basement rocks through which they moved during the ductile regime. The subeconomic concentrations of the Visconde deposit might be the result of the absence of prominent structures that would otherwise favor a greater influx of fluids, as it seems to have been the case in the Sossego and Alvo 118 deposits.