Academic literature on the topic 'Pilbara Craton'

New paleomagnetic and isotopic-geochronological data obtained for Neoarchean Onega granulite complex, were used to reconstruct the position of the Karelian craton in the Neoarchean supercontinent Kenorland. Geological correlations were made for the Karelian, Kaapvaal, Pilbara, Superior, and Slave cratons. Comparison of independent geological and paleomagnetic data allowed us to propose a new configuration of the Neoarchean supercontinent Kenorland. The position of the ancient core of the Karelian craton (the Vodlozero terrane), located in the North-Western margin of the supercontinent structure, reconstructed based on the previously paleomagnetic data for the Neoarchean Panozero sanukitoid massif and new one for granulite of Onega complex.

Abstract There is geological evidence for widespread deformation in the Kaapvaal craton, South Africa, between 2.2 and 2.0 Ga. In Griqualand West, post-Ongeluk Formation (ca. 2.42 Ga) and pre-Mapedi Formation (>1.91 Ga) folding, faulting, and uplift have been linked to the development of a regional-scale unconformity, weathering horizons, and extensive Fe-oxide mineralization. However, the lack of deformational fabrics and the low metamorphic temperatures (<300 °C) have hampered efforts to date this event. Here we show that metamorphic monazite in Neoarchean shales from four stratigraphic intervals from the Griqualand West region grew at ca. 2.15 Ga, >400 m.y. after deposition. Combined with previous studies, our results show that sedimentary successions across the Kaapvaal craton deposited before ca. 2.26 Ga record evidence for crustal fluid flow at ca. 2.15 Ga, which is locally associated with thrust faulting, folding, and cleavage development. The style of the deformation is similar to that of the Ophthalmian orogeny in the Pilbara craton, Australia, which is interpreted to reflect the northeast-directed movement of a fold-thrust belt between 2.22 and 2.15 Ga. Our results suggest that the Kaapvaal and Pilbara cratons, which some paleogeographic reconstructions place together as the continent Vaalbara, experienced an episode of synchronous folding and thrusting at ca. 2.15 Ga. Deformation was followed by uplift and the development of unconformities that are associated with some of Earth’s oldest oxidative weathering and with the onset of Fe-oxide mineralization.

Abstract Knowledge of the age and compositional architecture of Archean cratonic lithosphere is critical for models of geodynamics and continental growth on early Earth, but can be difficult to unravel from the exposed geology. We report the occurrence of numerous >3.7 Ga zircon crystals in 3.45 Ga rhyolites of the eastern Pilbara Craton (Western Australia), which preserve evidence for an Eoarchean meta-igneous component in the deep Pilbara crust. This inherited zircon population shares similar and distinctive age and Hf-O isotope characteristics with the oldest gneissic components of the Yilgarn Craton ∼500 km farther south, suggesting a common ca. 3.75 Ga felsic crustal nucleus to these two Archean granite-greenstone terranes. We infer a pivotal role for such ‘seeds’ in facilitating the growth and persistence of Archean continental lithosphere.

Abstract The continental crust that dominates Earth’s oldest cratons comprises Eoarchaean to Palaeoarchaean (4.0 to 3.2 Ga) felsic intrusive rocks of the tonalite-trondhjemite-granodiorite (TTG) series. These are found either within high-grade gneiss terranes, which represent Archaean mid-continental crust, or low-grade granite-greenstone belts, which represent relic Archaean upper continental crust. The Palaeoarchaean East Pilbara Terrane (EPT), Pilbara Craton, Western Australia, and the Barberton Granite-Greenstone Belt (BGGB), Kaapvaal Craton, southern Africa, are two of the best exposed granite-greenstone belts. Their striking geological similarities has led to the postulated existence of Vaalbara, a Neoarchaean-Palaeoproterozoic supercraton. Although their respective TTG domes have been compared in terms of a common petrogenetic origin reflecting a volcanic plateau setting, there are important differences in their age, geochemistry, and isotopic profiles. We present new zircon Hf isotope data from five granite domes of the EPT and compare the geochemical and isotopic record of the Palaeoarchaean TTGs from both cratons. Rare &gt;3.5 Ga EPT evolved rocks have juvenile εHf(t) requiring a chondritic source. In contrast, younger TTG domes developed via 3.5 to 3.4 and 3.3 to 3.2 Ga magmatic supersuites with a greater range of εHf(t) towards more depleted and enriched values, trace element signatures requiring an enriched source, and xenocrystic zircons that reflects a mixed source to the TTGs, which variously assimilates packages of older felsic crust and a more juvenile mafic source. EPT TTG domes are composite and record multiple pulses of magmatism. In comparison, BGGB TTGs are less geochemically enriched than those of the EPT and have different age profiles, hosting coeval magmatic units. Hafnium isotopes suggest a predominantly juvenile source to 3.2 Ga northern Barberton TTGs, limited assimilation of older evolved crust in 3.4 Ga southern Barberton TTGs, but significant assimilation of older (Hadean-Eoarchaean) crust in the ca. 3.6 Ga TTGs of the Ancient Gneiss Complex. The foundation of the EPT is younger than that for the oldest components of the Eastern Kaapvaal. Although the broader prevailing Palaeoarchaean geologic framework in which these two cratons formed may reflect similar a geodynamic regime, the superficial similarities in dome structures and stratigraphy of both cratonic terranes is not reflected in their geochemical and age profiles. Both the similarities and the differences between the crustal histories of the two cratons highlights that they are formed from distinct terranes with different ages and individual evolutionary histories. Vaalbara sensu lato represents typical Palaeoarchaean cratonic crust, not in the sense of a single homogeneous craton, but one as diverse as the continents are today.

Although Earth has a convecting mantle, ancient mantle reservoirs that formed within the first 100 Ma of Earth’s history (Hadean Eon) appear to have been preserved through geologic time. Evidence for this is based on small anomalies of isotopes such as182W,142Nd, and129Xe that are decay products of short-lived nuclide systems. Studies of such short-lived isotopes have typically focused on geological units with a limited age range and therefore only provide snapshots of regional mantle heterogeneities. Here we present a dataset for short-lived182Hf−182W (half-life 9 Ma) in a comprehensive rock suite from the Pilbara Craton, Western Australia. The samples analyzed preserve a unique geological archive covering 800 Ma of Archean history. Pristine182W signatures that directly reflect the W isotopic composition of parental sources are only preserved in unaltered mafic samples with near canonical W/Th (0.07 to 0.26). Early Paleoarchean, mafic igneous rocks from the East Pilbara Terrane display a uniform pristine µ182W excess of 12.6 ± 1.4 ppm. Fromca. 3.3Ga onward, the pristine182W signatures progressively vanish and are only preserved in younger rocks of the craton that tap stabilized ancient lithosphere. Given that the anomalous182W signature must have formed byca. 4.5 Ga, the mantle domain that was tapped by magmatism in the Pilbara Craton must have been convectively isolated for nearly 1.2 Ga. This finding puts lower bounds on timescale estimates for localized convective homogenization in early Earth’s interior and on the widespread emergence of plate tectonics that are both important input parameters in many physical models.

Vaalbara is the name given to a proposed configuration of continental blocks—the Kaapvaal craton (southern Africa) and the Pilbara craton (north-western Australia)—thought to be the Earth’s oldest supercraton assemblage. Its temporal history is poorly defined, but it has been suggested that it was stable for at least 400 million years, between 3.1 and 2.7 Ga. Here, we present an updated analysis that shows that the existence of a single supercraton between ∼2.9 and ∼2.7 Ga is inconsistent with the available palaeomagnetic data.

We generated a multi-locus phylogeny to test monophyly and distributional limits in Australian toadlets of the genus Uperoleia from the western arid zone of Australia. The molecular data were used in combination with a detailed assessment of morphological variation and some data on call structure to complete a taxonomic revision of the species that occur in this region. Our work reveals the existence of not two but five species in the region. Uperoleia russelli is restricted to the Carnarvon and Gascoyne Regions south of the Pilbara. Uperoleia micromeles is distributed from the Tanami Desert through the Great Sandy Desert and along the northern edge of the Pilbara. Uperoleia talpa was previously believed to be a Fitzroyland region endemic but it is further distributed along Dampierland and into the Roebourne Plain. Uperoleia glandulosa is a larger species than previously described as well as a greater habitat generalist, inhabiting the rocky Pilbara region and the sandy region around Port Hedland. We also describe a new species, U. saxatilis sp. nov., endemic to the Pilbara craton.

This thesis describes a systematic morphological study of minerals in feldspathic impact spherules from both Dales Gorge and Bee Gorge spherule layers in the Hamersley Basin of Pilbara Craton in Western Australia. The overarching goals are to advance interpretations on spherule formation processes. The research involves the characterisation of feldspathic spherules of the two impact horizons to propose a vapor plume model to identify where various spherule types may have formed. This includes the comparison of representative optical photomicrographs with their respective electron backscatter diffraction (EBSD) analytical results.

In the Pilgangoora Belt of the Pilbara Craton, Australia, the 3517 Ma Coonterunah Group and 3484-3468 Ma Carlindi granitoids underlie the 3458 Ma Warrawoona Group beneath an erosional unconformity, thus providing evidence for ancient emergent continental crust. The basalts either side of the unconformity are remarkably similar, with N-MORB-normalised enrichment factors for LILE, Th, U and LREE greater than those for Ta, Nb, P, Zr, Ti, Y and M-HREE, and initial e(Nd, Hf) compositions which systematically vary with Sm/Nd, Nb/U and Nb/La ratios. Geological and geochemical evidence shows that the Warrawoona Group was erupted onto continental basement, and that these basalts assimilated small amounts of Carlindi granitoid. As the Coonterunah basalts have similar compositions, they probably formed likewise, although they were deposited >60 myr before. Indeed, such a model may be applicable to most other early Pilbara greenstone successions, and so an older continental basement was probably critical for early Pilbara evolution. The geochemical, geological and geophysical characteristics of the Pilbara greenstone successions can be best explained as flood basalt successions deposited onto thin, submerged continental basement. This magmatism was induced by thermal upwelling in the mantle, although the basalts themselves do not have compositions which reflect derivation from an anomalously hot mantle. The Carlindi granitoids probably formed by fusion of young garnet-hornblende-rich sialic crust induced by basaltic volcanism. Early Archaean rocks have Nd-Hf isotope compositions which indicate that the young mantle had differentiated into distinct isotopic domains before 4.0 Ga. Such ancient depletion was associated with an increase of mantle Nb/U ratios to modern values, and hence this event probably reflects the extraction of an amount of continental crust equivalent to its modern mass from the primitive mantle before 3.5 Ga. Thus, a steady-state model of crustal growth is favoured whereby post ~4.0 Ga continental additions have been balanced by recycling back into the mantle, with no net global flux of continental crust at modern subduction zones. It is also proposed that the decoupling of initial e(Nd) and e(Hf) from its typical covariant behaviour was related to the formation of continental crust, perhaps by widespread formation of TTG magmas.

In the Pilgangoora Belt of the Pilbara Craton, Australia, the 3517 Ma Coonterunah Group and 3484-3468 Ma Carlindi granitoids underlie the 3458 Ma Warrawoona Group beneath an erosional unconformity, thus providing evidence for ancient emergent continental crust. The basalts either side of the unconformity are remarkably similar, with N-MORB-normalised enrichment factors for LILE, Th, U and LREE greater than those for Ta, Nb, P, Zr, Ti, Y and M-HREE, and initial e(Nd, Hf) compositions which systematically vary with Sm/Nd, Nb/U and Nb/La ratios. Geological and geochemical evidence shows that the Warrawoona Group was erupted onto continental basement, and that these basalts assimilated small amounts of Carlindi granitoid. As the Coonterunah basalts have similar compositions, they probably formed likewise, although they were deposited >60 myr before. Indeed, such a model may be applicable to most other early Pilbara greenstone successions, and so an older continental basement was probably critical for early Pilbara evolution. The geochemical, geological and geophysical characteristics of the Pilbara greenstone successions can be best explained as flood basalt successions deposited onto thin, submerged continental basement. This magmatism was induced by thermal upwelling in the mantle, although the basalts themselves do not have compositions which reflect derivation from an anomalously hot mantle. The Carlindi granitoids probably formed by fusion of young garnet-hornblende-rich sialic crust induced by basaltic volcanism. Early Archaean rocks have Nd-Hf isotope compositions which indicate that the young mantle had differentiated into distinct isotopic domains before 4.0 Ga. Such ancient depletion was associated with an increase of mantle Nb/U ratios to modern values, and hence this event probably reflects the extraction of an amount of continental crust equivalent to its modern mass from the primitive mantle before 3.5 Ga. Thus, a steady-state model of crustal growth is favoured whereby post ~4.0 Ga continental additions have been balanced by recycling back into the mantle, with no net global flux of continental crust at modern subduction zones. It is also proposed that the decoupling of initial e(Nd) and e(Hf) from its typical covariant behaviour was related to the formation of continental crust, perhaps by widespread formation of TTG magmas.

This study provides the first analysis of and constraints on the genesis of the Carlow Castle Cu-Co-Au deposit in the Archean Pilbara Craton of NW Western Australia. Geochronological analyses constrain the deposit’s formation to 2.95 Ga, making it the oldest of its type on Earth. Sulfur isotope analysis and thermodynamic modelling suggest an oxidised source of ore-formation. This thesis also provides the first demonstration of the utility of Cu isotopes to understand Archean ore-forming processes.

Thesis (Ph. D.)--University of Sydney, 2002.
Title from title screen (viewed Apr. 23, 2008). Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the School of Geosciences, Division of Geology and Geophysics. Degree awarded 2002; thesis submitted 2001. Includes bibliography. Also available in print form.

This research attempts to address some knowledge gaps in the formation of the Archean in age Warralong greenstone belt in the East Pilbara terrane, through geochemical and petrographic analysis. This research also addresses the possible formation mechanisms for Ocelli, a liquid immiscible texture observed in pillow lavas throughout the East Pilbara terrane and Archean greenstone belts throughout the world.

La caractérisation des environnements archéens et de la vie qui s'y développait sont encore débattus. Pour répondre à ces questions, deux forages ont été réalisés dans la formation de Dresser (3,495 Ga ; Australie) qui se compose d'un plancher océanique metakomatiitique, recouvert par des dépôt hydrothermaux primaires à barytine-sulfures, et une couverture sédimentaire à quartz-ankérite. L'ensemble a été affecté par un intense hydrothermalisme (100-200°C, pH -5,6-6, Fe-Mg) associé à un métasomatisme Si, K, Al et Ba. Toutefois, certaines ankérites des niveaux sédimentaires ont préservé en leur cœur des oxydes de fer, de la matière organique carbonée (MO) et des inclusions de quartz, calcite ou siderite, qui pourraient témoigner d'environnements marins archéens. La calcite est une phase exsolvée ; seule la siderite peut être considérée comme primaire. Sa composition témoigne d'une dissolution partielle associée à la précipitation d'oxydes de fers avant d'être piégée dans l'ankérite. Ces réactions sont intimement liées à la ƒO₂(g) dans le milieu. Les carbonates pourraient donc enregistrer l'état redox du milieu dans lequel ils précipitent. L'étude isotopique de la MO (macro- et micro-échelle) révèle deux réservoirs de δ¹³Corg, l'un à -35%o (méthanogenèse ou synthèse abiotique de type Fischer-Tropsch) l'autre à -15%o (photosynthèse anoxygénique). L'étude des carbonates de Dresser, permet de mieux contraindre la caractérisation chimique et la mise en place des carbonates extraterrestres. La spectroscopie Raman est un outil puissant permettant l'analyse chimique et structurale in situ d'assemblages carbonates-MO-oxydes de fer sur d'autres planètes
The characterization of Archean environments and ancient life is still controversial. To address these questions, two stratigraphic drill cores were performed thought the 3. 495 Gyr Dresser Formation (Australia) that consist of metakomatiitic oceanic floor, over-layered by hydrothermal barite-sulphides early deposits and quartz-ankerite sedimentary sequence. All these rocks were widely affected by hydrothermal circulations (100-200°C, pH -5,57-6, Fe-Mg) associated with Si, K, Al & Ba metasomatism. However, some organic matter (OM), iron oxides and tiny inclusions of quartz, calcite and siderite have been preserved within ankerite core from sedimentary layers and could reflect marine Archean environments. Siderite precipitates early whereas calcite is an exsolution. Siderite composition reflects a partial dissolution associated to iron-oxides precipitation before to be trapped inside ankerite. These equilibriums are closely linked to ƒO₂(g) and carbonates could register redox state during their precipitation. Isotopic study (macro- and micro-scale) of OM show two δ¹³Corg pools: one at -35%o (methanogenesis or Fischer-Tropsch-type reaction) the other at -15%o (anoxygenie photosynthesis). The study of Dresser carbonates is crucial to understand the occurrence of some extratefrestrial carbonates. Raman spectroscopy is a powerful device able to analyze in situ, chemically and structurally, carbonates-OM-iron oxides assemblages on planetary surfaces

This research led to the discovery of one of the best preserved remnants of the Earth's surficial environment 3.47 billion years ago. These ancient volcanic and sedimentary rocks contain original minerals and textures that are rare in rocks of this age. The research concentrated on chemical analysis of volcanic rocks to differentiate secondary alteration from the primary magmatic signature. This study contributes to our understanding of melting processes and geochemical reservoirs in the early Earth, which is vital for forward modelling of Earth's geodynamic evolution.

Lithium Cesium Tantalum (LCT) pegmatites are important resources for rare metals like Cesium, Lithium or Tantalum, whose demand increased markedly during the past decade. At present, Cs is known to occur in economic quantities only from the two LCT pegmatite deposits at Bikita located in Zimbabwe and Tanco in Canada. Host for this Cs mineralisation is the extreme rare zeolite group mineral pollucite. However, at Bikita and Tanco, pollucite forms huge massive, lensoid shaped and almost monomineralic pollucite mineralisations that occur within the upper portions of the pegmatite. In addition, both pegmatite deposits have a comparable regional geological background as they are hosted within greenstone belts and yield a Neoarchean age of about 2,600 Ma. Furthermore, at present the genesis of these massive pollucite mineralisations was not yet investigated in detail. Major portions of Western Australia consist of Meso- to Neoarchean crustal units (e.g., Yilgarn Craton, Pilbara Craton) that are known to host a large number of LCT pegmatite systems. Among them are the LCT pegmatite deposits Greenbushes (Li, Ta) and Wodgina (Ta, Sn). In addition, small amounts of pollucite were recovered from one single diamond drill core at the Londonderry pegmatite field. Despite that, no systematic investigations and/or exploration studies were conducted for the mode of occurrence of Cs and especially that of pollucite in Western Australia. In the course of the present study nineteen individual pegmatites and pegmatite fields located on the Yilgarn Craton, Pilbara Craton and Kimberley province have been visited and inspected for the occurrence of the Cs mineral pollucite. However, no pollucite could be detected in any of the investigated pegmatites. Four of the inspected LCT-pegmatite systems, namely the Londonderry pegmatite field, the Mount Deans pegmatite field, the Cattlin Creek LCT pegmatite deposit (Yilgarn Craton) and the Wodgina LCT pegmatite deposit (Pilbara Craton) was sampled and investigated in detail. In addition, samples from the Bikita pegmatite field (Zimbabwe Craton) were included into the present study in order to compare the Western Australian pegmatites with a massive pollucite mineralisation bearing LCT pegmatite system. This thesis presents new petrographical, mineralogical, mineralchemical, geochemical, geochronological, fluid inclusion and stable and radiogenic isotope data. The careful interpretation of this data enhances the understanding of the LCT pegmatite systems in Western Australia and Zimbabwe. All of the four investigated LCT pegmatite systems in Western Australia, crop out in similar geological settings, exhibit comparable internal structures, geochemistry and mineralogy to that of the Bikita pegmatite field in Zimbabwe. Furthermore, in all LCT pegmatite systems evidences for late stage hydrothermal processes (e.g., replacement of feldspars) and associated Cs enrichment (e.g., Cs enriched rims on mica, beryl and tourmaline) is documented. With the exception of the Wodgina LCT pegmatite deposit, that yield a Mesoarchean crystallisation age (approx. 2,850 Ma), all other LCT pegmatite systems gave comparable Neoarchean ages of 2,630 Ma to 2,600 Ma. The almost identical ages of the LCT pegmatite systems of the Yilgarn and Zimbabwe cratons suggests, that the process of LCT pegmatite formation at the end of the Neoarchean was active worldwide. Nevertheless, essential distinguishing feature of the Bikita pegmatite field is the presence of massive pollucite mineralisations that resulted from a process that is not part of the general development of LCT pegmatites and is associated with the extreme enrichment of Cs. The new findings of the present study obtained from the Bikita pegmatite field and the Western Australian LCT pegmatite systems significantly improve the knowledge of Cs behaviour in LCT pegmatite systems. Therefore, it is now possible to suggest a genetical model for the formation of massive pollucite mineralisations within LCT pegmatite systems. LCT pegmatites are generally granitic in composition and are interpreted to represent highly fractionated and geochemically specialised derivates from granitic melts. Massive pollucite mineralisation bearing LCT pegmatites evolve from large and voluminous pegmatite melts that intrude as single body along structures within an extensional tectonic setting. After emplacement, initial crystallisation will develop the border and wall zone of the pegmatites, while due to fractionated crystallisation immobile elements (i.e., Cs, Rb) become enriched within the remaining melt and associated hydrothermal fluids. Following this initial crystallisation, a relatively small portion (0.5–1 vol.%) of immiscible melt or fluid will separate during cooling. This immiscible partial melt/fluid is enriched in Al2O3 and Na2O, as well as depleted in SiO2 and will crystallise as analcime. In addition, this melt might allready contains up to 1–2 wt.% Cs2O. However, due to the effects of fluxing components (e.g., H2O, F, B) this analcime melt becomes undercooled which prevents crystallisation of the analcime as intergranular grains. Since this analcime melt exhibits a lower relative gravity when compared to the remaining pegmatite melt the less dense analcime melt will start to ascent gravitationally and accumulate within the upper portion of the pegmatite sheet. At the same time, the remaining melt will start to crystallise separately and form the inner portions of the pegmatite. This crystallisation is characterised by still ongoing fractionation and enrichment of incompatible elements (i.e., Cs, Rb) within the last crystallising minerals (e.g., lepidolite) or concentration of these incompatible elements within exsolving hydrothermal fluids. As analcime and pollucite form a continuous solid solution series, the analcime melt is able to incorporate any available Cs from the melt and/or associated hydrothermal fluids and crystallise as Cs-analcime in the upper portion of the pegmatite sheet. Continuing hydrothermal activity and ongoing substitution of Cs will then start to shift the composition from Cs-analcime composition towards Na-pollucite composition. In addition, if analcime is cooled below 400 °C it is subjected to a negative thermal expansion of about 1 vol.%. This contraction results in the formation of a prominent network of cracks that is filled by late stage minerals (e.g., lepidolite, quartz, feldspar and petalite). Certainly, prior to filling, this network of cracks enhances the available conduits for late stage hydrothermal fluids and the Cs substitution mechanism within the massive pollucite mineralisation. Furthermore, during cooling of the pegmatite, prominent late stage mineral replacement reactions (e.g., replacement of K-feldspar by lepidolite, cleavelandite, and quartz) as well as subsolidus self organisation processes in feldspars take place. These processes are suggested to release additional incompatible elements (e.g., Cs, Rb) into late stage hydrothermal fluids. As feldspar forms large portions of pegmatite a considerable amount of Cs is released and transported via the hydrothermal fluids towards the massive pollucite mineralisation in the upper portion of the pegmatite. Consequently, the initial analcime can accumulate enough Cs in order to shift its composition from the Cs-analcime member (>2 wt.% Cs2O) towards the Na-pollucite member (23–43 wt.% Cs2O) of the solid solution series. The timing of this late stage Cs enrichment is interpreted to be quasi contemporaneous or immediately after the complete crystallisation of the pegmatite melt. However, much younger hydrothermal events that overprint the pegmatite are also interpreted to cause similar results. Hence, it has been demonstrated that the combination of this magmatic and hydrothermal processes is capable to generate an extreme enrichment in Cs in order to explain the formation of massive pollucite mineralisations within LCT pegmatite systems. This genetic model can now be applied to evaluate the potential for occurrences of massive pollucite mineralisations within LCT pegmatite systems in Western Australia and worldwide
Lithium-Caesium-Tantal-(LCT) Pegmatite repräsentieren eine bedeutende Quelle für seltene Metalle, deren Bedarf im letzten Jahrzehnt beträchtlich angestiegen ist. Im Falle von Caesium sind zurzeit weltweit nur zwei LCT-Pegmatitlagerstätten bekannt, die abbauwürdige Vorräte an Cs enthalten. Dies sind die LCT-Pegmatitlagerstätten Bikita in Simbabwe und Tanco in Kanada. Das Wirtsmineral für diese Cs-Mineralisation ist das extrem selten auftretende Zeolith-Gruppen-Mineral Pollucit. In den Lagerstätten Bikita und Tanco bildet Pollucit dagegen massive, linsenförmige und fast monomineralische Pollucitmineralisationen, die in den oberen Bereichen der Pegmatitkörper anstehen. Zusätzlich befinden sich beide Lagerstätten in geologisch vergleichbaren Einheiten. Die Nebengesteine sind Grünsteingürtel die ein neoarchaisches Alter von ca. 2,600 Ma aufweisen. Die Bildung derartiger massiver Pollucitmineralisationen ist bis jetzt noch nicht detailliert untersucht worden. Große Bereiche von Westaustralien werden von meso- bis neoarchaischen Krusteneinheiten (z.B. Yilgarn Kraton, Pilbara Kraton) aufgebaut, von denen auch eine große Anzahl an LCT-Pegmatitsystemen bekannt sind. Darunter befinden sich unter anderem die LCT-Pegmatitlagerstätten Greenbushes (Li, Ta) und Wodgina (Ta, Sn). Zusätzlich wurden kleine Mengen an Pollucit in einer einzigen Kernbohrung im Londonderry Pegmatitfeld angetroffen. Ungeachtet dessen, wurden in Westaustralien bis jetzt keine systematischen Untersuchungen und/oder Explorationskampagnen auf Vorkommen von Cs und speziell der von Pollucit durchgeführt. Im Verlauf dieser Studie wurden insgesamt neunzehn verschiedene Pegmatitvorkommen und Pegmatitfelder des Yilgarn Kratons, Pilbara Kratons und der Kimberley Provinz auf das Vorkommen des Minerals Pollucit untersucht. Allerdings konnte in keinem der untersuchten LCT-Pegmatitsystemen Pollucit nachgewiesen werden. Von vier der untersuchten LCT-Pegmatitsystemen, dem Londonderry Pegmatitfeld, dem Mount Deans Pegmatitfeld, der Cattlin Creek LCT-Pegmatitlagerstätte (Yilgarn Kraton) und der Wodgina LCT-Pegmatitlagerstätte (Pilbara Kraton) wurden detailliert Proben entnommen und weitergehend untersucht. Zusätzlich wurden die massiven Pollucitmineralisationen im Bikita Pegmatitfeld beprobt und in die detailierten Untersuchungen einbezogen. Der Probensatz aus dem Bikita Pegmatitfeld dient als Referenzmaterial mit dem die Pegmatitproben aus Westaustralien verglichen werden. Die vorliegende Arbeit fasst die wesentlichen Ergebnisse der petrographischen, mineralogischen, mineralchemischen, geochemischen und geochronologischen Untersuchungen sowie der Flüssigkeitseinschlussuntersuchungen und stabilen und radiogenen Isotopenzusammensetzungen zusammen. Alle vier der in Westaustralien untersuchten LCT-Pegmatitsysteme kommen in geologisch ähnlichen Rahmengesteinen vor, weisen einen vergleichbaren internen Aufbau, geochemische Zusammensetzung und Mineralogie zu dem des Bikita Pegmatitfeldes in Simbabwe auf. Weiterhin konnten in allen LCT-Pegmatitsystemen Hinweise für späte hydrothermale Prozesse (z.B. Verdrängung von Feldspat) nachgewiesen werden, die einhergehend mit einer Anreicherung von Cs verbunden sind (z.B. Cs-angereicherte Säume um Glimmer, Beryll und Turmalin). Mit der Ausnahme der Wodgina LCT-Pegmatitlagerstätte, in der ein mesoarchaisches Kristallisationsalter (ca. 2,850 Ma) nachgewiesen wurde, lieferten die Altersdatierungen in den anderen LCT-Pegmatitsystemen übereinstimmende neoarchaische Alter von 2,630 Ma bis 2,600 Ma. Diese fast identischen Alter der LCT-Pegmatitsysteme des Yilgarn und Zimbabwe Kratons suggerieren, dass die Prozesse, die zur LCT-Pegmatitbildung am Ende des Neoarchaikums führten, weltweit aktiv waren. Ungeachtet dessen stellt das Vorhandensein von massiver Pollucitmineralisation das Alleinstellungsmerkmal des Bikita Pegmatitfeldes dar, welche sich infolge eines Prozesses gebildet haben der nicht Bestandteil der üblichen LCT-Pegmatitentwicklung ist und sich durch eine extreme Anreicherung an Cs unterscheidet. Die neuen Ergebnisse die in dieser Studie von den Bikita Pegmatitfeld und den Westaustralischen LCT-Pegmatitsystemen gewonnen wurden, verbessern das Verständnis des Verhaltens von Cs in LCT-Pegmatitsystemen deutlich. Somit ist es nun möglich, ein genetisches Modell für die Bildung von massiven Pollucitmineralisationen in LCT-Pegmatitsystemen vorzustellen. LCT-Pegmatite weisen im Allgemeinen eine granitische Zusammensetzung auf und werden als Kristallisat von hoch fraktionierten und geochemisch spezialisierten granitischen Restschmelzen interpretiert. Die Bildung von massiven Pollucitmineralisationen ist nur aus großen und voluminösen Pegmatitschmelzen, die als einzelner Körper entlang von Störungen in extensionalen Stressregimen intrudieren möglich. Nach Platznahme der Schmelze bildet die beginnende Kristallisation zunächst die Kontakt- und Randzone des Pegmatits, wobei infolge von fraktionierter Kristallisation die immobilen Elemente (v.a. Cs, Rb) in der verbleibenden Restschmelze angereichert werden. Im Anschluss an diese erste Kristallisation entmischt sich nach Abkühlung eine sehr kleine Menge (0.5–1 vol.%) Schmelze und/oder Fluid von der Restschmelze. Diese nicht mischbare Teilschmelze/-fluid ist angereichert an Al2O3 und Na2O sowie verarmt an SiO2 und kristallisiert als Analcim. Zusätzlich kann diese Schmelze bereits mit 1–2 wt.% Cs2O angereichert sein. Aufgrund der Auswirkung von Flussmitteln (z.B. H2O, F, B) wird allerdings der Schmelzpunkt dieser Analcimschmelze herabgesetzt und so die Kristallisation des Analcims als intergranulare Körner verhindert. Da diese Analcimschmelze im Vergleich zu der restlichen Schmelze eine geringere relative Dichte besitzt, beginnt sie gravitativ aufzusteigen und sich in den oberen Bereichen des Pegmatitkörpers zu akkumulieren. Währenddessen beginnt die restliche Schmelze separat zu kristallisieren und die inneren Bereiche des Pegmatits zu bilden. Diese Kristallisation ist einhergehend mit fortschreitender Fraktionierung und der Anreicherung von inkompatiblen Elementen (v.a. Cs, Rb) in den sich als letztes bildenden Mineralphasen (z.B. Lepidolit) oder der Konzentration der inkompatiblen Element in die sich entmischenden hydrothermalen Fluiden. Da Analcim und Pollucit eine lückenlose Mischungsreihe bilden, ist die Analcimschmelze in der Lage, alles verfügbare Cs von der Restschmelze und/oder assoziierten hydrothermalen Fluiden an sich zu binden und als Cs-Analcim im oberen Bereich des Pegmatitkörpers zu kristallisieren. Fortschreitende hydrothermale Aktivität und Substitution von Cs verschiebt dann die Zusammensetzung des Analcims von der Cs-Analcim- zu Na-Pollucitzusammensetzung. Zusätzlich erfährt der Analcim bei Abkühlung unter 400 °C eine negative thermische Expansion von ca. 1 vol.%. Diese Kontraktion führt zu der Bildung des markanten Rissnetzwerkes das durch späte Mineralphasen (z.B. Lepidolit, Quarz, Feldspat und Petalit) gefüllt wird. Vor der Mineralisation allerdings, erhöht dieses Netzwerk an Rissen die verfügbaren Wegsamkeiten für die späten hydrothermalen Fluide und begünstigt somit den Cs-Substitutionsmechanismus in der massiven Pollucitmineralisation. Weiterhin kommt es bei der Abkühlung des Pegmatits zu späten Mineralverdrängungsreaktionen (z.B. Verdrängung von K-Feldspat durch Lepidolit, Cleavelandit und Quarz), sowie zu Subsolidus-Selbstordnungsprozessen in Feldspäten. Diese Prozesse werden weiterhin interpretiert inkompatible Elemente (z.B. Cs, Rb) in die späten hydrothermalen Fluide freizusetzen. Da Feldspäte große Teile der Pegmatite bilden, kann somit eine beträchtliche Menge an Cs freigeben werden und durch die späten hydrothermalen Fluide in die massive Pollucitmineralisation in den oberen Bereichen des Pegmatitkörpers transportiert werden. Infolgedessen ist es möglich, dass genügend Cs frei gesetzt werden kann, um die Zusammensetzung innerhalb der Mischkristallreihe von Cs-Analcim (>2 wt.% Cs2O) zu Na-Pollucit (23–43 wt.% Cs2O) zu verschieben. Die zeitliche Einordnung dieser späten Cs-Anreicherung wird als quasi zeitgleich oder im direkten Anschluss an die vollständige Kristallisation der Pegmatitschmelze interpretiert. Es kann allerdings nicht vernachlässigt werden, dass auch jüngere hydrothermale Ereignisse, die den Pegmatitkörper nachträglich überprägen, ähnliche hydrothermale Prozesse hervorrufen können. Somit konnte gezeigt werden, dass es durch Kombination dieser magmatischen und hydrothermalen Prozessen möglich ist, genügend Cs anzureichern, um die Bildung von massiven Pollucitmineralisationen in LCT-Pegmatitsystemen zu ermöglichen. Dieses genetische Modell kann nun dazu genutzt werden, um das Potential von Vorkommen von massiven Pollucitmineralisationen in LCT-Pegmatitsystemen in Westaustralien und weltweit besser einzuschätzen

L’altération des roches supracrustales archéennes implique des conditions environnementales encore mal contraintes qu’il est nécessaire de définir afin de mieux comprendre l’évolution des enveloppes externes au Précambrien, ainsi que les conditions d’émergence et de diversification de la vie. Le premier objectif de cette thèse est l’étude des métasomatismes des formations volcano-sédimentaires du Paléoarchéen, la caractérisation des mécanismes à leur origine, ainsi que la quantification des bilans géochimiques associés. Le deuxième objectif est l’étude de la composition isotopique de l’azote dans ces mêmes formations, et sa signification pour le cycle de l’azote au Paléoarchéen. Pour cela, une étude géochimique et isotopique des systèmes volcanosédimentaires Paléoarchéens de Pilbara et de Barberton a été entreprise. Le métasomatisme potassique est issu de l’altération par l’eau de mer du matériel volcanique en un assemblage séricite - feldspath-K - quartz à des pH de 5,5-6,5 en conditions réductrices et à des températures supérieures à 70 °C. Le bilan de masse de cette altération implique un enrichissement de l’eau de mer de l’ordre de 1 mole de Fe2+, Na+, Ca2+, et de 3 moles de Mg par kg de komatiite. Trois moles de H+ et 1 mole de K+ sont incorporées dans la roche en échange. Quatre moles d’oxygène sont ainsi libérées dans l’eau de mer, impliquant au total la neutralisation de 10 moles de H+ par kg de komatiite. La silicification est un processus diagénétique précoce contrôlé par la granulométrie des particules sédimentaires, les plus fines engendrant les plus fort taux de silicification. Les particules volcano-détritiques déposées dans les bassins sédimentaires paléoarchéens adsorbent jusqu’à 5 fois leur volume de silice dissoute, produisant un flux de silice depuis l’eau de mer vers la croûte de l’ordre de plusieurs dizaines de kilomoles par kilogramme de matériel détritique. Les sédiments grossiers ne subissant qu’une silicification partielle sont carbonatisés pendant la diagenèse profonde par la précipitation de dolomite riche en Fe à partir de l’eau de mer piégée dans la porosité résiduelle durant la silicification. La carbonatisation des formations sédimentaires archéennes permet le stockage de 1,8 moles de CO2 par kg de matériel détritique. Les bilans de masse proposés montrent un profond déséquilibre entre l’eau de mer et la croûte au Paléoarchéen, résultant certainement d’une forte pression partielle de CO2 dans l’atmosphère, et de conditions globalement réductrices. Les compositions isotopiques (δ15NATM) de l’azote dans les roches sédimentaires étudiées sont comprises entre 7,1 ± 0,6 et 9,4 ± 0,4 ‰ avec des teneurs en N entre 0,8 et 5 ppm, sous forme d’ions ammonium dans les silicates potassiques. Ces compositions correspondent à des valeurs maximales de l’ammonium incorporé dans les sédiments au paléoarchéen, et témoignent d’un réservoir d’azote enrichi en 15N potentiellement représentatif de la composition de l’océan, il y a 3. 45 Ga.