A dissertação tem por cerne dois textos complementares, ambos refletindo o histórico de um programa de exploração mineral no distrito ferrífero de Curral Novo do Piauí. Como trabalho executado por empresa, a discussão sobre os custos e a definição de prioridades foram aspectos continuamente considerados. Assim os textos referidos refletem essas preocupações e se referem à proposição de métodos voltados à definição dos ambientes geológicos propícios à ocorrência de formações ferríferas à magnetita e à definição de probabilidade de ocorrências destes ambientes na fase de reconaissance da pesquisa. Na contextualização geológica do tema apresenta-se aspectos da geologia e uma discussão dos tipos e origens das formações ferríferas bandadas. Nos trabalhos foram aplicadas diversas técnicas, incluindo estatística multivariada, processamento digital de imagens, mapeamento geológico sistemático de superfície e subsuperfície em testemunhos de sondagens, caracterização petrográfica de rochas, geofísica de exploração a partir da utilização de métodos potenciais (magnetometria e radiometria). / This dissertation presents two complementary manuscripts, both reflecting the history of an exploration program in the Ferriferous Mineral District of Curral Novo do Piauí. As a research performed by a company, the discussion about the costs and prioritization aspects were continuously considered. The manuscripts reflects these concerns, referring to the proposition of methods aimed at defining the geological environments conducive to the occurrence of magnetite iron formations and the definition of probability of occurrence of these environments during the research reconnaissance phase. In the introductory chapter are presented the geological context and also a discussion of types and genesis of banded iron formations. In these studies were applied several techniques, including multivariate statistics, digital image processing, systematic geological surface mapping, diamond drill surveys, petrographic characterization of rocks and exploration geophysics methods (magnetometric and radiometric).

A dissertação tem por cerne dois textos complementares, ambos refletindo o histórico de um programa de exploração mineral no distrito ferrífero de Curral Novo do Piauí. Como trabalho executado por empresa, a discussão sobre os custos e a definição de prioridades foram aspectos continuamente considerados. Assim os textos referidos refletem essas preocupações e se referem à proposição de métodos voltados à definição dos ambientes geológicos propícios à ocorrência de formações ferríferas à magnetita e à definição de probabilidade de ocorrências destes ambientes na fase de reconaissance da pesquisa. Na contextualização geológica do tema apresenta-se aspectos da geologia e uma discussão dos tipos e origens das formações ferríferas bandadas. Nos trabalhos foram aplicadas diversas técnicas, incluindo estatística multivariada, processamento digital de imagens, mapeamento geológico sistemático de superfície e subsuperfície em testemunhos de sondagens, caracterização petrográfica de rochas, geofísica de exploração a partir da utilização de métodos potenciais (magnetometria e radiometria). / This dissertation presents two complementary manuscripts, both reflecting the history of an exploration program in the Ferriferous Mineral District of Curral Novo do Piauí. As a research performed by a company, the discussion about the costs and prioritization aspects were continuously considered. The manuscripts reflects these concerns, referring to the proposition of methods aimed at defining the geological environments conducive to the occurrence of magnetite iron formations and the definition of probability of occurrence of these environments during the research reconnaissance phase. In the introductory chapter are presented the geological context and also a discussion of types and genesis of banded iron formations. In these studies were applied several techniques, including multivariate statistics, digital image processing, systematic geological surface mapping, diamond drill surveys, petrographic characterization of rocks and exploration geophysics methods (magnetometric and radiometric).

O distrito ferrífero de Curral Novo é hospedado pelo Complexo Granjeiro Sul, situado na Zona Transversal, na porção sudoeste da Província Borborema, no estado do Piauí. Nesse distrito, formações ferríferas bandadas apresentam teor médio de 27,5% de Fe em recursos de mais de um bilhão de toneladas. Dados de campo, petrográficos, isotópicos, geoquímicos e geocronológicos possibilitaram a caracterização de uma associação de gnaisses e migmatitos com afinidade TTG e uma sequência metavulcanossedimentar com evidências de atividade exalativa de fundo oceânico com idade paleoarqueana. A suíte TTG do Complexo Granjeiro Sul apresenta grande variação química. Inclui tipicamente rochas magnesianas de composição cálcica a álcali-cálcica com baixo conteúdo de minerais máficos. Essas rochas possuem baixa concentração em elementos LILE e HFSE, significativo fracionamento de ETRL (La/YbN = 9,78-139,12) e anomalias levemente negativas de Eu (Eu/Eu* = 0,62-0,97), características de magmas gerados a partir de uma fonte de baixa pressão. Idades U-Pb em zircão obtidas em gnaisses TTG indicam cristalização do magma em 3349 ±15 Ma (MSWD = 0,62). A sequência metavulcanossedimentar do Complexo Granjeiro Sul é constituída por rochas metamáficas, metaultramáficas e formações ferríferas bandadas e interpretada como de idade próxima à da geração das rochas TTG do complexo. As rochas derivadas de protólitos ultramáficos e máficos apresentam composição komatiítica a basáltica e possuem gênese associada à fase de subducção entre duas placas oceânicas, apresentando afinidade química análoga a de rochas de arcos de ilhas modernos. As formações ferríferas bandadas apresentam fácies sedimentares óxido e silicato, com forte empobrecimento em HFSE e ETR e anomalias negativas de Ce (Ce/CeN = 0,04-0,37). Todo este conjunto de rochas arqueanas foi intrudido por corpos sienograníticos de idade de 2,651.8 ±8.9 Ma (MSWD = 1,5), pós- colisionais do tipo A2. Os sienitos são quimicamente classificadas como rochas metaluminosas a peraluminosas, cálcio-alcalinas de alto-K e ferrosas. Possuem leve enriquecimento de ETRL em relação aos ETRP e apresentam altos conteúdos de LILE e HFSE. Processos de alteração hidrotermal presentes nas encaixantes do minério de ferro e nas próprias formações ferríferas bandadas incluem silicificação, albitização, potassificação, carbonatização, sulfetação e reconcentração do ferro. Assinaturas de isótopos de enxofre (\'delta\'\'POT 34\'S = -3,11 a 2,03) em sulfetos hidrotermais sugerem fontes magmáticas para o enxofre associado ao sistema hidrotermal. As rochas do Complexo Granjeiro registraram ainda forte retrabalhamento crustal no Paleoproterozoico a partir do Riaciano (ca. 2,2 Ga), que resultou em metamorfismo das rochas metavulcanossedimentares, com picos que perduram até o Estateriano (ca. 1,7 Ga). / The Southern Granjeiro Complex hosts the Curral Novo do Piauí iron district, located in the Transversal Zone of the Borborema Province. In this district, banded iron formations have an average grade of 27.5% of Fe in resources of more than one billion tons. Field, petrographic, isotopic, geochemical, and geochronological data allowed the characterization of an association of gneisses and migmatites with TTG affinity and a Paleoarchean greenstone belt sequence with evidence of ocean floor exhalative activity. The TTG suite of the Southern Granjeiro Complex includes magnesian rocks of calcic to alkali-calcic affinity and low content of mafic minerals. These rocks have a low concentration of LILE and HFSE elements, significant fractionation of ETRL (La/YbN = 9.78-139.12) and slightly negative Eu anomaly (Eu/Eu* = 0.62-0.97). These characteristics are similar to that of TTG magmas generated from low-pressure sources. LA-ICP-MS U-Pb zircon ages obtained in TTG gneisses indicate magma crystallization at 3.349 ±15 Ma (MSWD = 0.62). The Southern Granjeiro metavolcanosedimentary sequence comprises metamafic and metaultramafic volcanic rocks and banded iron formation. These rocks derived from basaltic and komatiitic protoliths with chemical affinity to those formed in modern island arcs. The banded iron formation has oxide (magnetite-quartz) and silicate (grunerite-quartz) facies, with strong impoverishment in HFSE and ETR and negative Ce anomaly (Ce/CeN = 0.04-0.37). Post-collisional A2-type syenogranites (2.651.8 ±8.9 Ma; MSWD = 1.5) crosscut the Southern Granjeiro Complex. The syenogranites are ferroan, metaluminous to peraluminous and have a high-K calc-alkaline affinity. They have a slight enrichment of ETRL concerning ETRP and high contents of LILE and HFSE. Hydrothermal alteration processes in the banded iron formations and associated volcanic rocks include silicification, albitization, potassic alteration, carbonation, sulfide formation and iron reconcentration. Sulfur isotopes signatures (\'delta\'\'POT.34\'S = -3.11 to 2.03) of hydrothermal sulfides suggest magmatic sulfur sources associated with the hydrothermal system. The rocks of the Southern Granjeiro Complex still recorded strong crustal reworking and metamorphism in the Riacian (ca. 2.2 Ga) until the Staterian (ca. 1.7 Ga).

It is the study of banded iron formations (BIFs) that provides understanding into the conditions of the Earth's oceans and atmosphere during the Archean and Early Proterozoic. The aim of this thesis is to provide a detailed mineralogical and geochemical understand of BIFs from two separate localities separated by the Archean Proterozoic boundary. Close attention is paid to their carbonate and iron oxide mineralogy.The BIFs of the 2.7 Ga Manjeri Formation, Zimbabwe and 2.4 Ga Itabira Group, Brazil were both precipitated from oxygenated mixed marine-hydrothermal fluids. This is demonstrated by the presence of nano-hematite inclusions in the chert (Itabira and Manjeri) and dolomite (Itabira only) laminae, which is interpreted as the oldest mineral phase within the samples. Additionally, focused ion beam transmission electron microscopy (FIB-TEM) reveals the presence of nano ferrihydrite platelets within the dolomitic BIFs (carbonate itabirite). The dolomite is interpreted to be a primary phase precipitated at higher temperatures (~100°C) from CO2-rich hydrothermal fluids. Positive Eu anomalies in both formations indicate a hydrothermal component, likely to be the source of the reduced iron. Facies changes in both units are the result of transgression/regression and post depositional hydrothermal events mask primary conditions. Iron-rich carbonates in both facies have different origins; diagenetic (Itabira) and post depositional hydrothermal (Manjeri). However, the iron-rich carbonates of both formations have negative ∂13C values, indicating that at least part of the carbon in the carbonates is of organic origin. Curie Balance analyses into the carbonate itabirite reveals that maghemite is the transformation product of the ferrihydrite when dolomite decomposes at ~790°C. The maghemite has a Curie temperature between 320 and 350°C and is stable up to temperatures of 925°C.FIB-TEM investigations into the martitisation process revealed two possible mechanisms from two martite samples, from Brazil and India. Depending of the cause of the martitisation, here found to be deformation and hydrothermalism, the martitisation occurs respectively via either: 1. Ordering of point defects caused by vacancies in the spinel structure of maghemite, due to the removal of excess Fe3+ ions during the oxidation of magnetite, to form twins. It is in this twinning that the martitisation mechanism occurs.2. Grain boundary migration by hematite at the expense of magnetite is due to the presence of fluid along the crystal interfaces, where maghemite forms due to excess Fe3+ produced during martitisation of the magnetite, moving towards the surface of the magnetite crystals.

L’Archéen, de 4 à 2,5 Ga, est la période qui a connu les plus grands bouleversements géologiques et biologiques de l’histoire de la Terre : formation des continents, transition d’une tectonique à composante verticale vers une tectonique des plaques horizontale, apparition de la vie, … Le but de cette thèse est d’étudier les conditions environnementales de la Terre à l’Archéen, par l’analyse des compositions isotopiques de métaux de transition (Fe, Zn) de roches provenant principalement de la province d’Isua au Groenland (3,8 Ga). Après avoir adapté le protocole de séparation du Fe, Cu, Zn à des échantillons riches en Fe, nous avons acquis les données par spectrométrie de masse à source plasma et à multicollection MC-ICPMS. Nous nous sommes d’abord intéressés au processus de serpentinisation de la croûte océanique, réaction produisant à la fois des nutriments pour la vie (CH 4 , H 2 ) et des minéraux catalyseurs (mackinawite) de la formation abiotique d’acides aminés, molécules du vivant. L’affleurement d’Isua comporte une unité ophiolitique présentant les serpentinites les plus anciennes (3.81-3.70 Ga) : leur analyse permet d’appréhender la réaction de serpentinisation à l’Archéen. Les résultats obtenus pour la composition isotopique du zinc dans ces roches et dans des serpentinites modernes ont permis d’établir une correspondance entre le processus de serpentinisation à Isua et la mise en place de volcans de boues de serpentinites à l’aplomb de la fosse des Mariannes. Nous avons ainsi pu identifier Isua comme une zone d’arrière-arc de subduction océanique, lieu d’une serpentinisation produisant des fluides de température variable (100-300°C) et de pH alcalin (9-12). Nous montrons que cette configuration atypique réunissant serpentinisation, fluides alcalins et édifices volcaniques est favorable à l’émergence du vivant. Nous avons ensuite analysé de nombreux échantillons de formations de fer rubané (BIFs), sédiments propres à l’Archéen et au début du Protérozoïque. L’évolution de la composition isotopique du zinc de ces échantillons au cours du temps a permis d’établir une chronologie de l'émersion des continents.Nos résultats sont en faveur d’une émersion débutant il y a 2,9 Ga. Enfin, nos données nous informent sur la colonisation des continents émergés par la vie à 2,6 Ga et sur la pédogenèse de sols archéens comportant un horizon organique. / During the Archean (4 to 2.5 Ga ago), the Earth experienced the biggest changes in terms of geological and biological settings – continental growth, transition from sagduction towards purely horizontal plate tectonics, emergence of life, … The purpose of the present study is to better understand the archean earth environment by measuring the isotopic composition of transition metals – Zn, Fe – of archean rocks. Most of the samples belong to the Isua supracrustal belt, in Greenland, dated 3.8 Ga. The chemical extraction protocol of Fe, Cu, Zn was adapted to our Fe-rich samples and isotopic analyses were conducted by multicollection inductively coupled plasma mass spectrometry. The serpentinization of the oceanic crust produces fuels for life (CH 4 , H 2 ) and mackinawite, which catalyses formation of complex organic compounds. Serpentinization may thus provide a suitable environment for the emergence of the first biomolecules. We analysed the oldest known serpentinites from Isua (3.81-3.70 Ga) to comprehend the archean serpentinization process. The isotopic compositions of zinc reported in this samples and in modern serpentinites attest to a strong similarity between Isua and the Mariana serpentinite mud volcanoes. We identified Isua as an oceanic forearc environment permeated by high-pH (9-12) hydrothermal solutions at medium temperature (100-300°C). We show that such an environment could have fostered the emergence of early life. We also analyzed several banded iron formations (BIF), which are sediments limited to the Archean and Proterozoic. The temporal evolution of these samples' isotopic composition shows a close relationship with the continental freeboard. Our results support the continental emersion starting 2.9 Ga ago. Besides, we identified the life colonization of continents at 2.6 Ga together with pedogenesis of archean soils with an organic horizon.

L’étude des formations de fer rubané (BIF) permet de comprendre les conditions des océans de et de l’atmosphère terrestres au cours de l’Archéen et du début du Protérozoïque. L’objectif de cette thèse est de fournir une analyse minéralogique et géochimique détaillée de BIFs de deux localités distinctes, séparées par la frontière Archéen-Protérozoïque. Une attention particulière est portée à la minéralogie de leurs carbonates et oxydes de fer. Les BIFs de 2,7 Ga de la formation Manjeri, Zimbabwe et de 2,4 Ga du Groupe Itabira, Brésil, ont dans les deux cas été précipités par mélange de fluides hydrothermaux marins oxygénés. Ceci est démontré par la présence d’inclusions de nano-hématite dans les lames de chert (Itabira et Manjeri) et de dolomite (Itabira seulement), qui sont interprétées comme la phase minérale la plus ancienne dans les échantillons. En outre, la microscopie électronique à transmission à faisceau d’ions focalisé (FIB-TEM) révèle la présence de plaquettes de nano ferrihydrite dans les BIF dolomitiques (carbonate d’itabirite). La dolomite est interprétée comme étant une phase primaire précipitée à des températures plus élevées (~100°C) de fluides hydrothermaux riches en CO2. Des anomalies positives en Eu dans les deux formations indiquent une composante hydrothermale, susceptible d’être la source du fer réduit. Les changements de faciès dans les deux unités sont le résultat de transgression/régression; et des évènements hydrothermaux post dépôt masquent les conditions primaires. Les carbonates riches en fer dans les deux faciès ont différentes origines: diagénétiques (Itabira) et hydrothermales post dépôt (Manjeri). Toutefois, les carbonates riches en fer des deux formations ont des valeurs négatives de ∂13C, ce qui indique qu’au moins une partie du carbone dans les carbonates est d’origine organique.Des analyses en balance de Curie dans le carbonate d’itabirite révèlent que la maghémite est le produit de transformation de la ferrihydrite lorsque de la dolomite se décompose à ~790°C. La maghémite a une température de Curie comprise entre 320 et 350°C et est stable jusqu’à une température de 925°C. Les analyses en FIB-TEM sur le processus de martitisation ont révélé deux mécanismes possibles à partir de deux échantillons de martite provenant respectivement du Brésil et d’Inde. En fonction de la cause de la martitisation, que nous avons déterminé être soit la déformation soit l’hydrothermalisme, la martitisation se produit respectivement par l’intermédiaire de:1. La réorganisation de défauts ponctuels, pour former des jumeaux. Ces défauts sont causés par les vacances dans la structure spinelle de la maghémite, dues à la suppression des ions Fe3+ en excès au cours de l’oxydation de la magnétite. C’est dans ce jumelage que le mécanisme de martitisation se produit.2. La migration des joints de grains par l’hématite au détriment de la magnétite, qui est due à la présence de fluide le long des interfaces du cristal. La maghémite se forme en raison de l’excès de Fe3+ produit pendant la martitisation de la magnétite, qui se déplace vers la surface des cristaux de magnétite. / It is the study of banded iron formations (BIFs) that provides understanding into the conditions of the Earth’s oceans and atmosphere during the Archean and Early Proterozoic. The aim of this thesis is to provide a detailed mineralogical and geochemical understand of BIFs from two separate localities separated by the Archean Proterozoic boundary. Close attention is paid to their carbonate and iron oxide mineralogy.The BIFs of the 2.7 Ga Manjeri Formation, Zimbabwe and 2.4 Ga Itabira Group, Brazil were both precipitated from oxygenated mixed marine-hydrothermal fluids. This is demonstrated by the presence of nano-hematite inclusions in the chert (Itabira and Manjeri) and dolomite (Itabira only) laminae, which is interpreted as the oldest mineral phase within the samples. Additionally, focused ion beam transmission electron microscopy (FIB-TEM) reveals the presence of nano ferrihydrite platelets within the dolomitic BIFs (carbonate itabirite). The dolomite is interpreted to be a primary phase precipitated at higher temperatures (~100°C) from CO2-rich hydrothermal fluids. Positive Eu anomalies in both formations indicate a hydrothermal component, likely to be the source of the reduced iron. Facies changes in both units are the result of transgression/regression and post depositional hydrothermal events mask primary conditions. Iron-rich carbonates in both facies have different origins; diagenetic (Itabira) and post depositional hydrothermal (Manjeri). However, the iron-rich carbonates of both formations have negative ∂13C values, indicating that at least part of the carbon in the carbonates is of organic origin. Curie Balance analyses into the carbonate itabirite reveals that maghemite is the transformation product of the ferrihydrite when dolomite decomposes at ~790°C. The maghemite has a Curie temperature between 320 and 350°C and is stable up to temperatures of 925°C.FIB-TEM investigations into the martitisation process revealed two possible mechanisms from two martite samples, from Brazil and India. Depending of the cause of the martitisation, here found to be deformation and hydrothermalism, the martitisation occurs respectively via either: 1. Ordering of point defects caused by vacancies in the spinel structure of maghemite, due to the removal of excess Fe3+ ions during the oxidation of magnetite, to form twins. It is in this twinning that the martitisation mechanism occurs.2. Grain boundary migration by hematite at the expense of magnetite is due to the presence of fluid along the crystal interfaces, where maghemite forms due to excess Fe3+ produced during martitisation of the magnetite, moving towards the surface of the magnetite crystals.