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The Formation of Kiruna-Type Iron Oxide- Apatite Deposits – a New Genetic Model

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The Formation of Kiruna-Type Iron Oxide- Apatite Deposits – a New Genetic Model The formation of Kiruna-type iron oxide- apatite deposits – a new genetic model Von der Naturwissenschaftlichen Fakultät der Gottfried Wilhelm Leibniz Universität Hannover zur Erlangung des Grades Doktorin der Naturwissenschaften Dr. rer. nat. genehmigte Dissertation von M.Sc. Jaayke Lynn Fiege (geb. Knipping) Erscheinungsjahr 2019 Referent: Prof. Dr. François Holtz (Leibniz Universität Hannover) Koreferent: Prof. Dr. Adam C. Simon (University of Michigan) Koreferent: Prof. Dr. Stefan Weyer (Leibniz Universität Hannover) Tag der Pomotion: 13.08.2019 Acknowledgements First, I would like to thank my advisors François Holtz, Jim Webster and especially Adam Simon, who introduced me into the exciting world of economic geology. Without the unrestricted support of each of these researches, the whole project would not have been possible, since this was not a DFG or NSF funded project and it was completely based on several stipends, scholarships and prizes, which I only received with their help and support! It was an honor to work on this very interesting project that developed into different directions over the past years and which was conducted at mainly three different great institutions. Many thanks to the whole Simon-lab-family in the Earth and Environmental Science Department at the University of Michigan (2013-2015), especially Liz, Laura, Tom, Brian, Tristan and Xiaofei! I had a great time with awesome colleagues that became all good friends! I also would like to thank Jim Webster for the full-time support at the IHPVs in the Earth and Planetary Science Department at the American Museum of Natural History in New York (2017-2018) and Shuo Ding (Echo) for fellowship in this lab! Of course I want to thank all colleagues at my home institution - Institut for Mineralogy - at Leibniz University Hannover! Special thanks to Stefan Linsler, Robert Balzer and David Neave for support at the IHPVs in Hannover, Martin Oeser for assistance at the LA-ICP-MS for in-situ Fe isotope analyses as well as discussions and Julian Feige for preparation of IR-sections! The support by Harald Behrens throughout my career - from being a Bachelor student in his lab until today - is also much appreciated! Further, I would like to thank my collaboration partners outside of these three institutions: Many thanks to Martin Reich and Fernando Barra from 1 the University of Chile for a great field trip to the Atacama Desert in 2014 and for a fantastic scientific collaboration since then, as well as with Artur Deditius from the Murdoch University in Australia, who also supplied great EPMA maps of Los Colorados samples. The assistance by Markus Wälle and Chris Heinrich at the LA-ICP-MS facility for trace element analysis at ETH Zürich in 2014 is also much appreciated. Of course I would like to acknowledge also the moral support by my parents, brother and friends (Steffi, Annika, Lars, Insa, Lennart, Sven, Franzi, Anaïs…) throughout the last years! Thanks for always being there for me! Last but not least I want to thank my wonderful husband Adrian, who always supported, motivated, and forced me to keep going! I highly appreciate his advice and our scientific discussions on many evenings! And of course I want to thank my kids Anton and Rufus, who always brought me back to what is really important in life! 2 Abstract Kiruna-type iron oxide-apatite (IOA) deposits are important sources for Fe, necessary for steel production, and other elements such as REE, crucial for new technologies. IOA deposits occur worldwide (Sweden, Chile, USA, China, Iran etc.) and range in age from Late Archean (2.5 Ga) to the present. However, their formation is still under debate. Hypotheses vary from a (magmatic-) hydrothermal origin to direct crystallization from an immiscible Fe-rich melt. In order to investigate which hypotheses works best, we measured trace element concentrations and Fe-isotope ratios in-situ in magnetites (Fe3O4) from the Cretaceous Los Colorados IOA deposit (~350 Mt Fe) in the Chilean Iron Belt. Analyses showed that magnetite cores have an igneous texture and chemistry, while the surrounding magnetite rims indicate lower temperature (magmatic-) hydrothermal formation conditions. Since a coactive cooperation between both processes could not be explained by one of the existing models, we developed a completely novel formation model for Kiruna-type IOA deposits. In our proposed scenario the decompression of an oxidized, andesitic and volatile-rich magma, typical for arc-volcanism, results in degassing of volatiles such as H2O and Cl. The exsolved fluid bubbles are expected to nucleate preferentially on surfaces of oxide crystals such as magnetite where surface tension is lower. The bulk density of these bubble-magnetite pairs is expected to be lower than the surrounding magma and will thus float upwards as a bubble-magnetite suspension that is additionally enriched in dissolved Fe due to complexation with Cl. This suspension will cause the formation of massive magnetite deposits in regional-scale transcurrent faults with magmatic-hydrothermal as well as with igneous characteristics. High temperature decompression experiments confirmed that the flotation model is physically possible and clearly showed upward accumulation of magnetite upon decompression and fluid exsolution in contrast to gravitational settling of these dense minerals expected without exsolved fluids. This flotation scenario is in agreement with the geochemical and isotopic signatures observed at Los Colorados and other Kiruna-type IOA deposits. Mineral flotation on exsolved fluid bubbles may also change classical views on crystal fractionation and thus the formation of monomineralic layers in mafic layered intrusions (e.g., Skaergaard, Bushveld complex), where dense magnetite layers overlie less dense anorthosite layers. 3 Zusammenfassung Kiruna-typ Eisenoxid-Apatit (IOA) Lagerstätten sind wichtige Quellen für Eisen und sind deshalb essentiell für die Stahlproduktion, als auch entscheidend für die Förderung von Seltenen Erden (REE), die verstärkt in neuen Technologien eingesetzt werden. IOA Lagerstätten existieren weltweit (Schweden, Chile, USA, China, Iran, etc.) und haben sich zwischen dem späten Archaikum (2.5 Ga) und der Gegenwart gebildet. Jedoch ist die Art der Entstehung dieser Lagerstätten immer noch stark umstritten. Hypothesen variieren von (magmatisch-) hydrothermalen Szenarien zu rein magmatischer Kristallisation aus Eisen-reichen Schmelzen, die sich von Silikat-Schmelzen abgetrennt haben. Um die Frage nach der tatsächlichen Entstehung letztendlich zu klären, wurden in dieser Studie Magnetite (Fe3O4) der kreidezeitlichen Los Colorados IOA Lagerstätte (~350 Mt Fe) im Chilean Iron Belt in-situ auf Spurenelemente und Fe-Isotopenverteilung ausführlich untersucht. Die analytischen Ergebnisse implizieren eine rein magmatische Bildung der Kerne, während die Kristallränder auf eine Bildung bei niedrigeren Temperaturen unter (magmatisch-) hydrothermalen Bedingungen hindeuten. Da ein direktes Zusammenwirken dieser beiden Prozesse nicht durch eines der existierenden Modelle erklärt werden konnte, haben wir ein komplett neues Modell für die Entstehung von Kiruna-typ IOA Lagerstätten entwickelt. In unserem vorgeschlagenen Scenario führt die Druckentlastung eines oxidierten, andesitischen und volatil-reichen Magmas, typisch fuer Arc- Vulkanismus, zur Entgasung von Volatilen wie H2O und Cl. Die herausgelösten Fluidblasen bilden sich bevorzugt an Oxidkristall-Oberflächen, wie z.B. Magnetit, wo die Oberflächenspannung geringer ist. Die Gesamtdichte dieser Fluidblasen- Magnetit-Paare ist geringer als das des umgebenden Magmas und würde deshalb als Fluidblasen-Magnetit-Suspension aufsteigen, welches aufgrund der Komplexierung von Fe und Cl zusätzlich an gelöstem Eisen angereichert ist. Diese Suspension wird sich als massive Magnetitlagerstätte in regionalen Blattverschiebungen niederschlagen, die sowohl (magmatisch-) hydrothermale, als auch rein magmatische Charakteristika aufweist. Hochtemperatur-Dekompressionsexperimente belegen, dass das Flotations-Modell physikalisch möglich ist und, dass nach Druckentlastung und Entgasung eine nach oben gerichtete Magnetit Ansammlung statt findet, entgegen einer gravitationsbedingten Ablagerung dieser dichten Minerale, die ohne Fluidblasen erwartet würde. Dieses Flotations-Scenario stimmt mit den geochemischen und isotopischen Signaturen überein, die in Los Colorados und in anderen IOA Lagerstatten beobachtet wurden. Flotation von dichten Mineralen an Fluidblasen verändert möglicherweise auch klassische Ansichten zur Kristallfraktionierung. Somit muss eventuell auch die Entstehung von monomineralischen Lagen in mafischen Lagenintrusionen (z.B. Skaergaard, Bushveld Komplex) überdacht werden, wo dichte Magnetitlagen weniger dichte Anorthositlagen überlagern. 4 Schlagwörter: Kiruna-typ Eisenoxid-Apatit (IOA) Lagerstätten, Magnetit, Mineral Flotation Keywords: Kiruna-type iron oxide-apatite (IOA) deposits, magnetite, mineral flotation 5 Table of Contents Acknowledgements ................................................................................. 1 Abstract ................................................................................................... 3 Zusammenfassung .................................................................................. 4 Schlagwörter/Keywords.......................................................................... 5 Table of Contents .................................................................................... 6 Chapter
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