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Mineralised Pegmatites of the Damara Belt, Namibia: Fluid Inclusion and Geochemical Characteristics with Implications for Post- Collisional Mineralisation

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Mineralised Pegmatites of the Damara Belt, Namibia: Fluid Inclusion and Geochemical Characteristics with Implications for Post- Collisional Mineralisation Mineralised Pegmatites of the Damara Belt, Namibia: Fluid inclusion and geochemical characteristics with implications for post- collisional mineralisation Luisa Ashworth A dissertation submitted to the Fa culty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 2014 DECLARATION I declare that this thesis is my own, unaided work. It is being submitted for the degree of Doctor of Philosophy at the University of the Witwatersrand, Johannesburg. It has not been submitted before for any degree or examination in any other university. _______________________ Luisa Ashworth 20 of March 2014 i ABSTRACT Namibia is renowned for its abundant mineral resources, a large proportion of which are hosted in the metasedimentary lithologies of the Damara Belt, the northeast-trending inland branch of the Neoproterozoic Pan-African Damara Orogen. Deposit types include late- to post-tectonic (~ 523 – 506 Ma) LCT (Li-Be, Sn-, and miarolitic gem-tourmaline- bearing) pegmatites, and uraniferous pegmatitic sheeted leucogranites (SLGs), which have an NYF affinity. Fluid inclusion studies reveal that although mineralization differs between the different types of pegmatites located at different geographic locations, and by extension, different stratigraphic levels, the fluid inclusion assemblages present in these pegmatites are similar; thus different types of pegmatites are indistinguishable from each other based on their fluid inclusion assemblages. Thorough fluid inclusion petrography indicated that although fluid inclusions are abundant in the pegmatites, no primary fluid inclusions could be identified, and rather those studied are pseudosecondary and secondary. Fluid inclusions are aqueo-carbonic (± NaCl), carbonic, and aqueous. It is proposed that all of the pegmatites studied share a similar late-stage evolution, with fluids becoming less carbonic and less saline with the progression of crystallisation. Oxygen isotope ratios allow the discrimination of different pegmatites into two groups, Group A (Sn-, Li-Sn-, and gem-tourmaline-bearing LCT pegmatites), and Group B (Li-Be- bearing LCT, and U-bearing NYF pegmatites). Group A pegmatites have O-isotope ratios ranging from 11 to 13 ‰ suggesting that they have an I-type affinity. These values are, however, elevated above those of typical I-type granites (7 - 9 ‰), indicating either a post- emplacement low-temperature exchange with meteoric fluid, high-temperature hydrothermal exchange with δ 18 O country rocks during emplacement, or the derivation of these pegmatites from a non-pelitic/S-type metaigneous source. Group B pegmatites have higher δ18 O ratios (δ 18 O = 15 - 16 ‰), indicative of their S-type affinity, and their derivation from metapelitic source rocks. δD values of all the pegmatites range from -40 ii ‰ to -90 ‰ indicating that the pegmatitic fluids are primary magmatic with a metamorphic fluid component. Trends in the trace element concentrations of both Group A and Group B pegmatites are very similar to each other, making the two groups indistinguishable from each other on this basis. The Damaran pegmatites also share similar geochemical trends with their country rocks. There is, however, no direct field evidence to suggest that the pegmatites were derived from the in situ anatexis of the country rocks. It is more likely that anatexis occurred some distance away from where the pegmatites were ultimately emplaced, and that the melts migrated and were finally emplaced in pre-existing structures, possibly formed during Damaran deformation. O-isotope and Ti-in-quartz geothermometry indicate that Damaran pegmatites can be subdivided into two groups based on their crystallisation temperatures. LCT pegmatites crystallised at temperatures ranging from ~ 450 - 550 ºC, while the NYF pegmatites crystallised at higher temperatures, ranging from 630 - 670 ºC. It is important to note that the subdivision of pegmatites in Groups A and B based on their O-isotope systematics does not correspond with their subdivision into the LCT and NYF pegmatite families according to their crystallisation temperatures. In addition to clarifying aspects of the emplacement and evolution of the Damaran pegmatites, this study points out that there are several discrepancies in the current classification schemes of pegmatites. It shows that in addition to the problems encountered when trying to distinguish between LCT and NYF pegmatites based on their mineralogy, they also cannot truly be distinguished from each other using their geochemical and isotopic characteristics, or their tectonic settings. It is tentatively proposed that crystallisation temperature be considered as an alternative or additional characteristic in the classification of pegmatites, and that it be considered on a regional scale rather than only in the evaluation of the highly evolved end-members of a pegmatite swarm. iii ACKNOWLEDGEMENTS I would like to thank my supervisors, Prof. J.A. Kinnaird and Dr P.A.M. Nex for giving me such an interesting and challenging project to work on, for their guidance, supervision, pearls of wisdom along the way, and for taking me to some beautiful places in the world. None of this research would have been possible without the financial assistance of the University of the Witwatersrand, the National Research Foundation, the Geological Society of South Africa, and the Society of Economic Geologists. In addition to this, I would like to thank the Geological Survey of Namibia for letting me look at, and take, their rocks, Eddie for allowing me short-notice access to the Omapyu pegmatite, and John Stewart of Licore Mining cc for allowing me access to the Rubicon pegmatite. Many thanks to Jan-Marten Huizenga and Lynette Greyling for guidance and assistance pertaining to all bubble-related matters, and to Rudolph Erasmus (School of Physics, University of the Witwatersrand), Wojciech Przybylowicz (iThemba LABS, Somerset West), and Chris Harris (Department of Geological Sciences, University of Cape Town), for allowing me the use of their laboratory facilities. Thank you also to all of the pegmatologists who I was fortunate to meet at the PEG 2011 and PEG 2013 meetings, who are abundantly generous with their knowledge, and who make pegmatites fun. Special thanks to Axel Müller at the Geological Survey of Norway for running my Ti-in- quartz samples at such short notice. I would also like to thank all technical staff at the School of Geosciences (University of the Witwatersrand) and the Department of Geological Sciences (University of Cape Town) for all the sample preparation that they did for me, especially since some of the preparation was quite challenging, and a special thanks goes to Ryan McIntosh and Michael Donze for the many, many hours they spent helping me to crush, mill, and separate minerals. Also, a special thanks to Fayrooza Rawoot at the University of Cape Town, for teaching me how not to be afraid of the silicate line. iv Many friends have helped me keep it together over the last few years, especially Natasha Barbolini and Nicole Burri, and several great listeners, including Grant Bybee, Trishya Owen-Smith, Sara Turnbull, Debra Colarossi, Traiana Naydenova, Louise Coney, my running buddy, Mary-Rose Howe-Watson, and the rest of the church gang. Thank you also to Luke Longridge and Guy Freemantle, who were so generous with their knowledge of Namibia, and with their samples. Special thanks go to Kalin Naydenov, Lorenzo Milani, and Jeremie Lehmann, William Rankin and Russell Johnson for taking such great care of me on our field trip together, and for many hours of support and good coffee. To my dance teachers and the ladies in my dance classes, thank you for helping me to keep a clear mind, and to my band mates, thanks for all the mid-thesis rock-and-roll. To the staff of GeoSpectral Imaging, thank you for your patience, and for encouraging me to see this through. I would like to thank my family for their love, support, and encouragement; mom and dad, thank you for believing in me and for thinking that what I do is interesting even though it doesn’t make a lot of sense. Lastly, and most importantly, thank you to my husband, Jason, for his unending understanding, empathy through the tough times, and several kick-starts. v TABLE OF CONTENTS DECLARATION ........................................................................................................................... I ABSTRACT ................................................................................................................................ II ACKNOWLEDGEMENTS ............................................................................................................ IV TABLE OF CONTENTS ............................................................................................................... VI LIST OF FIGURES ..................................................................................................................... XII LIST OF TABLES ..................................................................................................................... XVII CHAPTER 1 ............................................................................................................................... 1 OVERVIEW OF THE GEOLOGY OF THE DAMARA BELT, NAMIBIA .................................................. 1 1.1 INTRODUCTION ..............................................................................................................................
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