Quantifying spatial association between mineral deposits and geology across three African crustal segments of different age, with implication for secular change in mineralization during Earth history. Tshifbiwa Mabidi Town Cape of Thesis submitted in fulfilment of the requirements for the degree of Master of Science University Department of Geological Sciences University of Cape Town February, 2006 AE::ON AFRICA EARTH OOSERVATORY NETWORK EARTH STEWARDSH P THROUGH SCIENCE The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgement of the source. The thesis is to be used for private study or non- commercial research purposes only. Published by the University of Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. University of Cape Town Declaration • I know that plagiarism is wrong. Plagiarism is to use another's work and to pretend that it is one's own. • Each significant contribution to, and quotation in, this thesis from the work, or works, of other people has been attributed, and has been cited and referenced. • This thesis is my own work. ■ I have not allowed, and will not allow, anyone to copy my work with the intention of passing it offas his/her own work. Abstract Variations in enrichment of mineralization, expressed in ore deposits, in the continental crust may be one way to test for secular changes in crustal genesis. This study collates and analyses fundamental information about mineral deposits with which to 'fingerprint' the metal endowment of African crust of different age. Three areas of juvenile African crust (e.g. mantle derived over similar lengths of time of ~500 million years, and excluding recycled older crust) of different ages with similar geology are compared. The areas range in age from 0.5 to 3.0 Ga, [e.g. the Zimbabwe Craton (2.5-3.0 Ga), the Birimian Shield (1.8-2.3 Ga), and the Arabian-Nubian Shield (0.5-1.0 Ga)]. The three areas have a total of 2671 mineral deposits, which are divided into six groups according to their geochemical affinities. Using these known deposits, mineral potential maps are created through a data- driven approach, using weights of evidence (WotE). The layers/themes used in Woffi are (1) lithology, (2) structures (faults and shear zones), and (3) lithological contacts. The analysis shows that there is strong lithology control on mineralization in all three areas. Archean crust has high predictive values compared to the younger crust. A measure of spatial association (spatial coefficient), based on the WotE approach, is also used to 'fingerprint' the met I endowment in the three selected regions of African crust. The patterns of the mineral deposits distribution within all regions shows that each region has a unique metal endowment, and that there is a greater concentration of mineral deposits in the crust of the Archean Zimbabwe Craton relative to the younger crust of the Birimian and Arabian-Nubian Shields. The analysis of this study therefore quantitatively corroborates studies that suggest older crust is more mineral diverse and more enriched in mineral deposits than younger crust. Thus, secular changes in mineralization or rates of tectonic processes, or both, are implicated, and mineral endowment in the African crust has undergone major evolutionary changes from Archean to Neoproterozoic time. Keywords: Zimbabwe Craton; Birimian Shield; Arabian-Nubian Shield; Continental crust; Secular change; Mineral deposits; Archean; Paleoproterozoic; Neoproterozoic; Weights-of­ Evidence model, Spatial analyses, GIS, Mineral potential maps 11 Acknowledgements I would like to express my special thanks to my supervisors Prof. Maarten de Wit and Prof. Christien Thiart, for the guidance, patience, mentorship and motivation that helped me combine my interest in both computing and geology. I am sincerely in debt to them for their guidance and for their continuous support at every stage throughout this study. Prof. Maarten de Wit requires special praise for his belief that it was all possible. He has inspired me to produce nothing less than my best, which was often more than I thought I could do. Maarten and Christien, you are much more than supervisors to me. This research began with data collection from various sources into a unified GIS database. Part of datasets was taken from GO-GEOID (Gondwana Geoscientific Indexing Database), now housed at AEON, that originated from a PhD thesis of Wendy Wilsher. Digital geological features of Africa used throughout this project were mostly obtained from Council for Geosciences, and De Beers Group in Centurion where I visited and I was lucky to spend some time with Lindsay Urban, Navitha Dukkan and Dr Moctar Doucoure in their GIS Department. I w uld like to thank them for the time and effort they put into my project. For this I would like to thank Dr Mike de Wit, with whom I also had a valuable discussion. I would also like to thank his group, for making my visit to Centurion not only as productive as possible, but also very pleasant. I would like to thank Dr Hielke A. Jelsma, whom I visited in De Beers-Geoscience Centre in Johannesburg and Prof. Paul. H.G.M. Dirks for sending me valuable datasets of the Zimbabwe Craton. Nick Lindenberg, the UCT-Geographical Information Systems Manager, is thanked for sharing his expertise, which has greatly improved my work and his valuable time is greatly appreciated. The support from fellow students and staff in the Department of Geological Sciences, notably my fellow AEON companions, Dr. Jacek Stankiewicz, Dr. Justine Tinker, Dr. Daud Jamal, John Decker, and Shaun Moore, are thanked for their support, encouragement and valuable ideas that I have received. I am also grateful for the support and encouragement that I have received from Robert Netshitungulwana, Simon Mabolani, James Mudau and Zibisani Bagai. Financially, this project would never happen without the generosity of Prof. Maarten de Wit. The National Research Foundation provided financial support during this project. Finally, I would like to express my special thanks to my parents, family, friends and Tshilidzi for their unbelievable support, ultra-patience and endless trust. Not forgetting the Almighty God who always being on my side throughout this study. lll TABLE OF CONTENTS Declaration ........... ... ............................................................................... .i Abstract .................................................................................................... .ii Acknowledgements ..................................... .. .... ............ ............................ .iii Table of Contents ..................................................................................... .iv List of Figures ......................................................................................... ix List of Tables ........................ ....................... ... ... ... ............... .................... xv Chapter 1: Introduction 1-1 1.1 Objectives ......................................................................................... 1-4 1.2 Definitions of a craton and a shield ....................... ... ............................... 1-5 1.3 Definition of mineral potential ................................................................. 1-5 1.4 Methodology .... .................................................................................. 1-6 Chapter 2: Geologic architecture of three different African crustal segments 2-1 2.1 The Archean Zimbabwe Craton .............................................................2-1 2.1.1 Geological Setting ................................................................. 2-4 2.1.2 Tectonic Setting .......................................... .. ........................ 2-7 2.1.3 Mineralization ...................................................................... 2-9 2.2 The Paleoproterozoic Birimian Shield ..................................................... 2-12 2. 2.1 Geological Setting ................................................................ 2-12 2. 2. 2 Tectonic Setting .. ..................................................................... 2-18 2.2.3 Mineralization .................................................................... 2-21 2.3 The Neoproteroz ic Arabian-Nubian Shield ............................................... 2-24 2. 3.1 Geological Setting ................................................................... .2-24 2.3.2 Tectonic Setting .......................................... ........................... 2-26 2.3.3 Mineralization ...................................................................... 2-29 2.4 Summary of the Mineral Deposits of three regions of African crust of different age ................................................... ... .... .... ............... .. ................2-31 IV Chapter 3: Database: acquisition and structure 3-1 3.1 Spatial Data Input ............................................................................... 3-1 3.1.1 Mineral Theme ........................................................................... 3-3 3.1.2 Geology Theme ............................................................................. 3-6 3.1.3 Faults Theme ........................................................................... 3-10 Chapter 4: Spatial odelling Methodology 4-1 4.1 Introduction .... .. ........................................................................ .. .... 4-1 4.2 Knowledge Driven Models