Abstract
The Archaean Zimbabwe craton of southern Africa is a classic granite-gneiss-greenstone terrain. World-class Lithium-Cesium-Tantalum (LCT) pegmatites and emerald deposits occur along the southern margin of the craton hosted within greenstone belts. The Bikita field is hosted in the Masvingo belt, whereas the Mweza field occurs in the Mweza-Buhwa belt. Research into the origin and nature of rare-element granitic pegmatites has been ongoing for more than a century. The occurrence of rare-element pegmatites in the Bikita and Mweza pegmatite fields, which are spatially associated with syn- to late-tectonic Chilimanzi and Razi granite suites, offer an excellent platform to test the hypotheses of pegmatite origin and contribute knowledge in resolving the controversy. This research project employed an integrated approach embracing desktop studies, fieldwork, mineralogical and petrographic observations, geochemical and geochronological techniques to investigate the granite-pegmatite relationships in the study areas.
The study findings are that the Chilimanzi and Razi suites are metaluminous, I-type granites with different degrees of magmatic fractionation. Broadly, the Chilimanzi Suite is more fractionated relative to the Razi Suite. The granite suites that surround the Bikita field are more evolved, altered, and exhibit elevated trace element contents including Li, Cs, Ta, Rb, Tl, Nb, W, Ga, Th, and U and simultaneous depletions in Sr, Ba and Hf and were thus more likely to spawn rare-element pegmatites relative to the plutons that envelope the Mweza field. Across both fields, the pegmatites are distributed in deformed and greenschist to upper amphibolite grade host rocks. In the Bikita field, the pegmatites were intruded into a system of regional faults and the intrusions truncate regional fabrics associated with the c. 2.63 Ga Limpopo orogeny. Pegmatites on the SW part of the Mweza field are syn-tectonic and were emplaced into shear zones, whereas pegmatites on the NE part of the field are post-tectonic. There is a narrow time window between pegmatites and granite suites magmatism with the former emplaced at c. 2630-2616 Ma, whereas the latter were intruded at c. 2635-2625 Ma. Petrographic, whole-rock, and mineral chemistry data reveal the complimentary role of magmatic and magmatic-hydrothermal transition in the petrogenesis and metallogenesis of the studied pegmatites and granites. The study established a petrographic, geochemical, and geochronological coherence which renders it conceivable for the pegmatites to be late-stage differentiates of the surrounding plutons. The main exploration implications of this study are that highly fractionated, syn-post-tectonic, I-type granites may be potential sources of LCT pegmatites, which could be explored in the rocks such as greenstones peripheral to the plutons using integrated exploration techniques.
iii
Acknowledgements
This thesis would not be possible without the permission and support of Zimbabwe Mining Development Corporation and Bikita Minerals Pvt Limited who allowed access and sample collections in their properties and presentations of results from the work. Generous bursary support from the main funder, South African Department of Science and Innovation disbursed through the Centre of Excellence for Integrated Mineral and Energy Resource Analysis (CIMERA) is highly appreciated as it covered my tuition fees, conference attendances and analytic work as well as welfare during my stay in South Africa. Financial support from the Society of Economic Geologists is greatly appreciated as it enabled attending of conferences and financing part of the analytical work.
I am greatly indebted to my project supervisor Prof. Axel Hofmann for his patience, support, encouragement, and critical review of this work, which helped me to focus and clarify views presented in the thesis. I would like to thank Prof. Paul Nex for reviewing Chapter 10; his comments and suggestions improved the thesis. Teimoor Dehkordi is thanked for reviewing most of the chapters, which certainly improved the manuscript. Ueckermann and Prof. Marlina Elburg are thanked for assistance during zircon U-Pb geochronology and whole-rock trace element geochemistry analytical sessions at the University of Johannesburg (UJ) Spectrum Analytical Facilities. I thank Dr. Christian Reinke for assistance and patience in carrying out whole-rock major and microprobe analyses at UJ Spectrum Analytical Facilities. Prof. Allan Wilson is thanked for the performing whole-rock trace element geochemistry on granite samples at the Witwatersrand’s Earth Lab. I thank Prof. Jan D. Kramers for the interpretation of vexing zircon U-Pb geochronology data and for performing muscovite 40Ar/39Ar geochronology on pegmatites as well as reviewing the subsequent write-up in Chapter 8.
I am grateful to Mr Walter Mutsauri who introduced me to the field of rare-metal pegmatites. I would like to express my thanks to the staff and fellow post graduate students in the Department of Geology at the University of Johannesburg for providing a friendly atmosphere which provided me with the energy and enthusiasm to keep going. I am very grateful to my colleague Samuel Nuno “the King” for assistance with laboratory work and final presentation of the research data. Last and certainly not least, I express my gratitude to my family- Senaniso (wife), Kimberly and Craig for their support and patience during the duration of this study as I spent a lot of time away from home and they endured all this. The support and encouragement from my father Elisha and mother Gladys are greatly appreciated.