Meso- to Neoarchean Lithium-Cesium-Tantalum- (LCT-) Pegmatites (Western Australia, Zimbabwe) and a Genetic Model for the Formation of Massive Pollucite Mineralisations

Autor:	Dittrich, Thomas
Jazyk:	angličtina
Rok vydání:	2017
Předmět:	info:eu-repo/classification/ddc/550 ddc:550 Pegmatit Tantal Lithium Cäsium Lagerstättenbildung Lagerstättenkunde Lithiumlagerstätte Tantallagerstätte Metallogenese Mineralisation Vorkommen Kimberleyplateau Western Australia Pilbara Pollucit Geologie Kraton Gesteinskunde Mineralogie Mineralbildung Limpopo-Gürtel Simbabwe Archaikum Cäsiumsilicate Cäsiumverbindungen Cäsium Polluzit LCT-Pegmatit Massive Polluzit Minerlisation Exploration Lithium Neoarchaisch Tantal Bikita Londonderry Cattlin Creek Wodgina Mount Deans Craton Yilgarn Pilbara Zimbabwe 2650 Ma 2600 Ma Schmelzentmischung Analzim seltene Metalle Lagerstätte Greenbushes Tanco cesium pollucite LCT-Pegmatite massive pollucite mineralisation exploration lithium neoarchean tantalum Bikita Londonderry Cattlin Creek Wodgina Mount Deans craton Yilgarn Pilbara Zimbabwe 2650 Ma 2600 Ma melt immiscibility analcim rare metal ore deposit Greenbushes Tanco
Druh dokumentu:	Text<br />Doctoral Thesis
Popis:	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.:Contents Abstract ... ... ... ... ... ... ... ... ... ... ... ... ... ... iii Zusammenfassung ... ... ... ... ... ... ... ... ... ... ... ... v Versicherung ... ... ... ... ... ... ... ... ... ... ... ... .... vii Acknowledgments ... ... ... ... ... ... ... ... ... ... ... .... ix Contents ... ... ... ... ... ... ... ... ... ... ... ... ... ... xi 1. Introduction 1 1.1. Motivation and Scope of the Thesis ... ... ... ... ... ... ... ... 1 1.2. Structure of the Thesis ... ... ... ... ... ... ... ... ... .... 5 2. Fundamentals 7 2.1. The Alkali Metal Cesium ... ... ... ... ... ... ... ... ... ... 7 2.1.1. Distribution of Cesium ... ... ... ... ... ... ... ... ... 7 2.1.2. Mineralogy of Cesium ... ... ... ... ... ... ... ... ... 8 2.1.3. Geochemical Behaviour of Cesium ... ... ... ... ... .... . 13 2.1.4. Economy of Cesium ... ... ... ... ... ... ... ... .... 15 2.2. Pollucite – (Cs,Na)2Al2Si4O12×H2O ... ... ... ... ... ... ... ... 16 2.2.1. Crystal Structure ... ... ... ... ... ... ... ... ... ... 16 2.2.2. Analcime–Pollucite–Series ... ... ... ... ... ... ... .... 17 2.2.3. Formation of Pollucite ... ... ... ... ... ... ... ... ... 17 2.2.4. Pollucite Occurences ... ... ... ... ... ... ... ... .... 21 2.3. Pegmatites ... ... ... ... ... ... ... ... ... ... ... .... . 34 2.3.1. General Characteristics of Pegmatites ... ... ... ... ... ... 34 2.3.2. Controls on Pegmatite Formation and Evolution ... ... ... .... 40 2.3.3. Pegmatite Age Distribution and Continental Crust Formation ... ... 43 3. Geological Settings of Archean Cratons 47 3.1. Zimbabwe Craton ... ... ... ... ... ... ... ... ... ... ... 47 3.1.1. Tectonostratigraphic Subdivision ... ... ... ... ... ... ... 48 3.1.2. Tectonometamorphic Evolution of the Northern Limpopo Thrust Zone. 49 3.1.3. Pegmatites within the Zimbabwe Craton ... ... ... ... .... . 52 3.1.4. Masvingo Greenstone Belt ... ... ... ... ... ... ... .... 53 3.1.5. Geological Setting of the Bikita Pegmatite District ... ... ... ... 58 3.2. Yilgarn Craton ... ... ... ... ... ... ... ... ... ... .... . 62 3.2.1. Tectonostratigraphic Framework and Geological Development .... . 62 3.2.2. Tectonic Models for the Development ... ... ... ... ... .... 70 3.2.3. Pegmatites within the Yilgarn Craton ... ... ... ... ... .... 76 3.2.4. Geological setting of the Londonderry Pegmatite Field ... ... .... 76 3.2.5. Geological Setting of the Mount Deans Pegmatite Field ... ... ... 85 3.2.6. Geological Setting of the Cattlin Creek Pegmatite Deposit ... .... . 91 3.3. Pilbara Craton ... ... ... ... ... ... ... ... ... ... .... . 99 3.3.1. Tectonostratigraphic Framework and Geological Development .... . 99 3.3.2. Tectonic Model for the Development ... ... ... ... ... .... 101 3.3.3. Pegmatites within the Pilbara Craton ... ... ... ... ... .... 105 3.3.4. Geological Setting of the Wodgina Pegmatite District ... ... .... 106 4. Fieldwork and Sampling of Selected Pegmatites and Pegmatite Fields 115 4.1. Bikita Pegmatite Field ... ... ... ... ... ... ... ... ... .... . 115 4.2. Londonderry Pegmatite Field ... ... ... ... ... ... ... ... .... 115 4.2.1. Londonderry Feldspar Quarry Pegmatite ... ... ... ... .... . 115 4.2.2. Lepidolite Hill Pegmatite ... ... ... ... ... ... ... .... . 117 4.2.3. Tantalite Hill Pegmatite ... ... ... ... ... ... ... ... ... 118 4.3. Mount Deans Pegmatite Field ... ... ... ... ... ... ... ... ... 118 4.3.1. Type I – Flat Lying Pegmatites ... ... ... ... ... ... .... . 118 4.3.2. Type II – Steeply Dipping Pegmatites ... ... ... ... ... .... 120 4.4. Cattlin Creek Pegmatite ... ... ... ... ... ... ... ... ... ... 120 4.5. Wodgina LCT-Pegmatite Deposit ... ... ... ... ... ... ... .... . 121 4.5.1. Mount Tinstone Pegmatite ... ... ... ... ... ... ... .... 123 4.5.2. Mount Cassiterite Pegmatite ... ... ... ... ... ... ... ... 123 5. Petrography and Mineralogy 139 5.1. Quantitative Mineralogy by Means of Mineral Liberation Analysis ... .... . 141 5.2. Mineralogical and Petrographical Characteristics of Individual Mineral Groups. 141 5.2.1. Feldspar ... ... ... ... ... ... ... ... ... ... .... 141 5.2.2. Quartz ... ... ... ... ... ... ... ... ... ... .... . 144 5.2.3. Mica ... ... ... ... ... ... ... ... ... ... ... ... 146 5.2.4. Pollucite ... ... ... ... ... ... ... ... ... ... .... . 147 5.2.5. Petalite ... ... ... ... ... ... ... ... ... ... .... . 149 5.2.6. Spodumene ... ... ... ... ... ... ... ... ... .... . 149 5.2.7. Beryl ... ... ... ... ... ... ... ... ... ... ... ... 154 5.2.8. Tourmaline ... ... ... ... ... ... ... ... ... ... ... 155 5.2.9. Apatite ... ... ... ... ... ... ... ... ... ... .... . 157 5.2.10. Ta-, Nb- and Sn-oxides ... ... ... ... ... ... ... .... . 157 5.3. Reconstruction of the General Crystallisation Sequence ... ... ... .... 162 6. Geochemistry 165 6.1. Major Elements ... ... ... ... ... ... ... ... ... ... .... . 165 6.2. Selected Minor and Trace Elements ... ... ... ... ... ... ... ... 174 6.3. Fractionation Indicators ... ... ... ... ... ... ... ... ... ... 175 6.4. Rare Earth Elements ... ... ... ... ... ... ... ... ... .... . 185 7. Geochronology 193 7.1. 40Ar/39Ar-Method on Mica ... ... ... ... ... ... ... ... .... . 193 7.1.1. Bikita Pegmatite Field ... ... ... ... ... ... ... ... ... 194 7.1.2. Mount Deans Pegmatite Field ... ... ... ... ... ... .... . 195 7.1.3. Londonderry Pegmatite Field ... ... ... ... ... ... .... . 195 7.1.4. Cattlin Creek Pegmatite ... ... ... ... ... ... ... .... . 195 7.1.5. Wodgina Pegmatite ... ... ... ... ... ... ... ... .... 198 7.2. Th-U-Total Pb Monazite Dating ... ... ... ... ... ... ... ... ... 201 7.2.1. Monazite Ages ... ... ... ... ... ... ... ... ... .... 201 7.3. U/Pb Dating of Selected Ta-, Nb- and Sn-Oxide Minerals ... ... ... ... 203 7.3.1. Bikita Pegmatite Field ... ... ... ... ... ... ... ... ... 203 7.3.2. Londonderry Pegmatite Field ... ... ... ... ... ... .... . 203 7.3.3. Mount Deans Pegmatite Field ... ... ... ... ... ... .... . 206 7.3.4. Cattlin Creek Pegmatite ... ... ... ... ... ... ... .... . 206 7.3.5. Wodgina Pegmatite ... ... ... ... ... ... ... ... .... 207 8. Fluid Inclusion Study 211 8.1. Bikita Pegmatite Field ... ... ... ... ... ... ... ... ... .... . 211 8.2. Wodgina Pegmatite ... ... ... ... ... ... ... ... ... ... ... 211 8.3. Carbon Isotope Analysis on Fluid Inclusion Gas of Selected Mineral Phases. . 212 9. Stable and Radiogenic Isotopes 217 9.1. Whole Rock Sm/Nd-Isotopy ... ... ... ... ... ... ... ... .... 217 9.1.1. New Whole Rock Sm/Nd Data ... ... ... ... ... ... .... 217 9.2. Lithium Isotope Analysis on Selected Mineral Phases ... ... ... ... ... 220 9.2.1. New Lithium Isotope Data ... ... ... ... ... ... ... .... 220 10.Discussion 227 10.1. Regional Geological and Tectonomagmatic Development ... ... ... ... 227 10.1.1. Constraints from Field Evidence ... ... ... ... ... ... .... 227 10.1.2. Petrographical and Mineralogical Constraints ... ... ... .... . 229 10.1.3. Geochemical Constraints ... ... ... ... ... ... ... .... 230 10.1.4. Isotopic Constraints ... ... ... ... ... ... ... ... .... 232 10.1.5. Constraints from Fluid Inclusion Data ... ... ... ... ... .... 233 10.1.6. Geochronological Constrains ... ... ... ... ... ... .... . 233 10.2. Massive Pollucite Mineralisations ... ... ... ... ... ... ... .... . 243 10.2.1. Unique Characteristics of Massive Pollucite Mineralisations ... .... 243 10.2.2. New Concepts for the Formation of Massive Pollucite Mineralisations. . 252 10.3. Genetic Model for the Formation of Massive Pollucite Mineralisations within LCT Pegmatite Systems ... ... ... ... ... ... ... ... ... ... ... 264 11.Summary and Conclusions 267 References 273 Lists of Abbreviations 309 General Abbreviations ... ... ... ... ... ... ... ... ... ... .... 309 Mineral Abbreviations ... ... ... ... ... ... ... ... ... ... .... . 310 List of Figures 311 List of Tables 315 Appendix 317 A. Legend for Topographic Maps 319 B. Sample List 323 C. Methodology 331 C.1. Quantitative Mineralogy by Means of Mineral Liberation Analysis ... .... . 331 C.2. Geochemistry ... ... ... ... ... ... ... ... ... ... ... ... 331 C.3. 40Ar/39Ar-Method on Mica ... ... ... ... ... ... ... ... .... . 335 C.4. Th-U-Total Pb Monazite Dating ... ... ... ... ... ... ... ... ... 335 C.5. U/Pb Dating of Selected Ta-, Nb- and Sn-Oxide Minerals ... ... ... ... 336 C.6. Fluid Inclusion Study ... ... ... ... ... ... ... ... ... .... . 337 C.7. Whole Rock Sm/Nd-Isotopy ... ... ... ... ... ... ... ... .... 338 C.8. Lithium Isotope Analysis on Selected Mineral Phases ... ... ... ... ... 338 D. Data – Mineral Liberation Analysis 341 E. Data – Geochemistry 345 F. Data – Geochronology 349 G. Data – Stable and Radiogenic Isotopes 353 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.:Contents Abstract ... ... ... ... ... ... ... ... ... ... ... ... ... ... iii Zusammenfassung ... ... ... ... ... ... ... ... ... ... ... ... v Versicherung ... ... ... ... ... ... ... ... ... ... ... ... .... vii Acknowledgments ... ... ... ... ... ... ... ... ... ... ... .... ix Contents ... ... ... ... ... ... ... ... ... ... ... ... ... ... xi 1. Introduction 1 1.1. Motivation and Scope of the Thesis ... ... ... ... ... ... ... ... 1 1.2. Structure of the Thesis ... ... ... ... ... ... ... ... ... .... 5 2. Fundamentals 7 2.1. The Alkali Metal Cesium ... ... ... ... ... ... ... ... ... ... 7 2.1.1. Distribution of Cesium ... ... ... ... ... ... ... ... ... 7 2.1.2. Mineralogy of Cesium ... ... ... ... ... ... ... ... ... 8 2.1.3. Geochemical Behaviour of Cesium ... ... ... ... ... .... . 13 2.1.4. Economy of Cesium ... ... ... ... ... ... ... ... .... 15 2.2. Pollucite – (Cs,Na)2Al2Si4O12×H2O ... ... ... ... ... ... ... ... 16 2.2.1. Crystal Structure ... ... ... ... ... ... ... ... ... ... 16 2.2.2. Analcime–Pollucite–Series ... ... ... ... ... ... ... .... 17 2.2.3. Formation of Pollucite ... ... ... ... ... ... ... ... ... 17 2.2.4. Pollucite Occurences ... ... ... ... ... ... ... ... .... 21 2.3. Pegmatites ... ... ... ... ... ... ... ... ... ... ... .... . 34 2.3.1. General Characteristics of Pegmatites ... ... ... ... ... ... 34 2.3.2. Controls on Pegmatite Formation and Evolution ... ... ... .... 40 2.3.3. Pegmatite Age Distribution and Continental Crust Formation ... ... 43 3. Geological Settings of Archean Cratons 47 3.1. Zimbabwe Craton ... ... ... ... ... ... ... ... ... ... ... 47 3.1.1. Tectonostratigraphic Subdivision ... ... ... ... ... ... ... 48 3.1.2. Tectonometamorphic Evolution of the Northern Limpopo Thrust Zone. 49 3.1.3. Pegmatites within the Zimbabwe Craton ... ... ... ... .... . 52 3.1.4. Masvingo Greenstone Belt ... ... ... ... ... ... ... .... 53 3.1.5. Geological Setting of the Bikita Pegmatite District ... ... ... ... 58 3.2. Yilgarn Craton ... ... ... ... ... ... ... ... ... ... .... . 62 3.2.1. Tectonostratigraphic Framework and Geological Development .... . 62 3.2.2. Tectonic Models for the Development ... ... ... ... ... .... 70 3.2.3. Pegmatites within the Yilgarn Craton ... ... ... ... ... .... 76 3.2.4. Geological setting of the Londonderry Pegmatite Field ... ... .... 76 3.2.5. Geological Setting of the Mount Deans Pegmatite Field ... ... ... 85 3.2.6. Geological Setting of the Cattlin Creek Pegmatite Deposit ... .... . 91 3.3. Pilbara Craton ... ... ... ... ... ... ... ... ... ... .... . 99 3.3.1. Tectonostratigraphic Framework and Geological Development .... . 99 3.3.2. Tectonic Model for the Development ... ... ... ... ... .... 101 3.3.3. Pegmatites within the Pilbara Craton ... ... ... ... ... .... 105 3.3.4. Geological Setting of the Wodgina Pegmatite District ... ... .... 106 4. Fieldwork and Sampling of Selected Pegmatites and Pegmatite Fields 115 4.1. Bikita Pegmatite Field ... ... ... ... ... ... ... ... ... .... . 115 4.2. Londonderry Pegmatite Field ... ... ... ... ... ... ... ... .... 115 4.2.1. Londonderry Feldspar Quarry Pegmatite ... ... ... ... .... . 115 4.2.2. Lepidolite Hill Pegmatite ... ... ... ... ... ... ... .... . 117 4.2.3. Tantalite Hill Pegmatite ... ... ... ... ... ... ... ... ... 118 4.3. Mount Deans Pegmatite Field ... ... ... ... ... ... ... ... ... 118 4.3.1. Type I – Flat Lying Pegmatites ... ... ... ... ... ... .... . 118 4.3.2. Type II – Steeply Dipping Pegmatites ... ... ... ... ... .... 120 4.4. Cattlin Creek Pegmatite ... ... ... ... ... ... ... ... ... ... 120 4.5. Wodgina LCT-Pegmatite Deposit ... ... ... ... ... ... ... .... . 121 4.5.1. Mount Tinstone Pegmatite ... ... ... ... ... ... ... .... 123 4.5.2. Mount Cassiterite Pegmatite ... ... ... ... ... ... ... ... 123 5. Petrography and Mineralogy 139 5.1. Quantitative Mineralogy by Means of Mineral Liberation Analysis ... .... . 141 5.2. Mineralogical and Petrographical Characteristics of Individual Mineral Groups. 141 5.2.1. Feldspar ... ... ... ... ... ... ... ... ... ... .... 141 5.2.2. Quartz ... ... ... ... ... ... ... ... ... ... .... . 144 5.2.3. Mica ... ... ... ... ... ... ... ... ... ... ... ... 146 5.2.4. Pollucite ... ... ... ... ... ... ... ... ... ... .... . 147 5.2.5. Petalite ... ... ... ... ... ... ... ... ... ... .... . 149 5.2.6. Spodumene ... ... ... ... ... ... ... ... ... .... . 149 5.2.7. Beryl ... ... ... ... ... ... ... ... ... ... ... ... 154 5.2.8. Tourmaline ... ... ... ... ... ... ... ... ... ... ... 155 5.2.9. Apatite ... ... ... ... ... ... ... ... ... ... .... . 157 5.2.10. Ta-, Nb- and Sn-oxides ... ... ... ... ... ... ... .... . 157 5.3. Reconstruction of the General Crystallisation Sequence ... ... ... .... 162 6. Geochemistry 165 6.1. Major Elements ... ... ... ... ... ... ... ... ... ... .... . 165 6.2. Selected Minor and Trace Elements ... ... ... ... ... ... ... ... 174 6.3. Fractionation Indicators ... ... ... ... ... ... ... ... ... ... 175 6.4. Rare Earth Elements ... ... ... ... ... ... ... ... ... .... . 185 7. Geochronology 193 7.1. 40Ar/39Ar-Method on Mica ... ... ... ... ... ... ... ... .... . 193 7.1.1. Bikita Pegmatite Field ... ... ... ... ... ... ... ... ... 194 7.1.2. Mount Deans Pegmatite Field ... ... ... ... ... ... .... . 195 7.1.3. Londonderry Pegmatite Field ... ... ... ... ... ... .... . 195 7.1.4. Cattlin Creek Pegmatite ... ... ... ... ... ... ... .... . 195 7.1.5. Wodgina Pegmatite ... ... ... ... ... ... ... ... .... 198 7.2. Th-U-Total Pb Monazite Dating ... ... ... ... ... ... ... ... ... 201 7.2.1. Monazite Ages ... ... ... ... ... ... ... ... ... .... 201 7.3. U/Pb Dating of Selected Ta-, Nb- and Sn-Oxide Minerals ... ... ... ... 203 7.3.1. Bikita Pegmatite Field ... ... ... ... ... ... ... ... ... 203 7.3.2. Londonderry Pegmatite Field ... ... ... ... ... ... .... . 203 7.3.3. Mount Deans Pegmatite Field ... ... ... ... ... ... .... . 206 7.3.4. Cattlin Creek Pegmatite ... ... ... ... ... ... ... .... . 206 7.3.5. Wodgina Pegmatite ... ... ... ... ... ... ... ... .... 207 8. Fluid Inclusion Study 211 8.1. Bikita Pegmatite Field ... ... ... ... ... ... ... ... ... .... . 211 8.2. Wodgina Pegmatite ... ... ... ... ... ... ... ... ... ... ... 211 8.3. Carbon Isotope Analysis on Fluid Inclusion Gas of Selected Mineral Phases. . 212 9. Stable and Radiogenic Isotopes 217 9.1. Whole Rock Sm/Nd-Isotopy ... ... ... ... ... ... ... ... .... 217 9.1.1. New Whole Rock Sm/Nd Data ... ... ... ... ... ... .... 217 9.2. Lithium Isotope Analysis on Selected Mineral Phases ... ... ... ... ... 220 9.2.1. New Lithium Isotope Data ... ... ... ... ... ... ... .... 220 10.Discussion 227 10.1. Regional Geological and Tectonomagmatic Development ... ... ... ... 227 10.1.1. Constraints from Field Evidence ... ... ... ... ... ... .... 227 10.1.2. Petrographical and Mineralogical Constraints ... ... ... .... . 229 10.1.3. Geochemical Constraints ... ... ... ... ... ... ... .... 230 10.1.4. Isotopic Constraints ... ... ... ... ... ... ... ... .... 232 10.1.5. Constraints from Fluid Inclusion Data ... ... ... ... ... .... 233 10.1.6. Geochronological Constrains ... ... ... ... ... ... .... . 233 10.2. Massive Pollucite Mineralisations ... ... ... ... ... ... ... .... . 243 10.2.1. Unique Characteristics of Massive Pollucite Mineralisations ... .... 243 10.2.2. New Concepts for the Formation of Massive Pollucite Mineralisations. . 252 10.3. Genetic Model for the Formation of Massive Pollucite Mineralisations within LCT Pegmatite Systems ... ... ... ... ... ... ... ... ... ... ... 264 11.Summary and Conclusions 267 References 273 Lists of Abbreviations 309 General Abbreviations ... ... ... ... ... ... ... ... ... ... .... 309 Mineral Abbreviations ... ... ... ... ... ... ... ... ... ... .... . 310 List of Figures 311 List of Tables 315 Appendix 317 A. Legend for Topographic Maps 319 B. Sample List 323 C. Methodology 331 C.1. Quantitative Mineralogy by Means of Mineral Liberation Analysis ... .... . 331 C.2. Geochemistry ... ... ... ... ... ... ... ... ... ... ... ... 331 C.3. 40Ar/39Ar-Method on Mica ... ... ... ... ... ... ... ... .... . 335 C.4. Th-U-Total Pb Monazite Dating ... ... ... ... ... ... ... ... ... 335 C.5. U/Pb Dating of Selected Ta-, Nb- and Sn-Oxide Minerals ... ... ... ... 336 C.6. Fluid Inclusion Study ... ... ... ... ... ... ... ... ... .... . 337 C.7. Whole Rock Sm/Nd-Isotopy ... ... ... ... ... ... ... ... .... 338 C.8. Lithium Isotope Analysis on Selected Mineral Phases ... ... ... ... ... 338 D. Data – Mineral Liberation Analysis 341 E. Data – Geochemistry 345 F. Data – Geochronology 349 G. Data – Stable and Radiogenic Isotopes 353
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