The Origin and Petrogenesis of the Ultramafic Enclaves at Unki Mine, Selukwe Subchamber, Great Dyke, Zimbabwe
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THE ORIGIN AND PETROGENESIS OF THE ULTRAMAFIC ENCLAVES AT UNKI MINE, SELUKWE SUBCHAMBER, GREAT DYKE, ZIMBABWE Sinikiwe Ncube A dissertation submitted to the Faculty of Science, University of Witwatersrand in the fulfillment of the requirements for the degree of Master of Science Johannesburg August, 2013 i Declaration I declare that this dissertation is my own, unaided work. It is being submitted for the Degree of Master of Science at the University of Witwatersrand, Johannesburg. It has not been submitted before for any degree or examination at any other University. ……… …………. 30th day of August 2013 in Johannesburg ii Abstract The unique Selukwe Subchamber of the Great Dyke is bounded by the Shurugwi greenstone belt (SGB) on the west side for approximately 25 km and granitoids on the east side, as compared to other subchambers of the Great Dyke that are bounded on both sides by granitoids. It is also the narrowest section of the entire Great Dyke. The extensive xenolith suite is found on the western flank and the central zone of the subchamber. This study focuses on the PAR 11 borehole and the surface xenoliths in the Selukwe Subchamber (SSC). The PAR 11 core was drilled into an anomalous sequence of ultramafic rocks situated in the Mafic Succession of the SSC. There are basically two rock types in the PAR 11 borehole: peridotites and pyroxenites. Comparison of the major and trace element geochemistry of the PAR 11 body with the MR 92 data of Coghill (1994) for the SSC reveals that they are similar but less evolved. The mineral assemblages and proportions of phases in the PAR 11 borehole samples are indicative of essentially the same composition as that which formed the layered sequence of the Great Dyke. Therefore, on the basis of the rock types and chemical compositions, the PAR 11 body and the Great Dyke cumulates appear to be petrologically and chemically similar and had the same petrogenesis. There are three rock types in the xenolith suite that have been observed in the mafic succession of the Unki area: peridotites, pyroxenites and gabbros. Major and trace elements show a wide range of compositions that have CaO/Al2O3 ~ 1, which are dissimilar to both PAR 11 and MR 92 borehole data. REE patterns show depletion of LREE, with flat HREEs indicating a different magma to that which gave rise to the Great Dyke. Such flat patterns are typical of a primitive mantle source similar to that of komatiite magma. Stowe, (1974) describes dunite and chromite in the SGB and does not describe pyroxenites and gabbros. Therefore, it is not clear in the first instance that the xenoliths were derived from the SGB. It also does not necessarily mean that these rock types did not occur in the SGB and, if they did, maybe they were derived from an intrusion within the SGB that is at depth and never been seen iii before. The xenoliths do not have mineral compositions that are similar to the Great Dyke and therefore precludes them as having been derived from the Great Dyke Marginal Facies, a possible source of such rocks. Therefore, it is concluded from this study that they were inherited from another source which also does not appear to be the SGB because there is no report of such rock types (other than peridotite) in the SGB. They are also not mantle derived. The metasedimentary rocks that occur as xenoliths are banded iron formation and quartzites and are all clearly derived from the different formations of the SGB. The quartzites are from the Mont d’Or Formation and Wanderer Formation. The BIFs are from the Upper Greenstone and Wanderer Formation. The Shurugwi Greenstones were stripped off from the western flank whereas the Archean granitoids to the eastern flank of the Great Dyke remained. The conclusion from this study is that the Shurugwi greenstones and Archean granitoids of the Selukwe area were intruded by the large volume of new magma that was the parental magma to the Great Dyke. The hot parental magma carried up with it xenoliths from outside the Great Dyke and large blocks from within the Great Dyke to the uppermost rocks of the level of the P1 pyroxenite layer and mafic unit. iv Dedication This thesis is dedicated to my mother Patricia N. Ncube and my children Shane and Sisanda Gondo. v Acknowledgements This study arose after a brief discussion with Prof. A.H. Wilson who has done great work on the Great Dyke of Zimbabwe and other layered intrusions worldwide. I would like to thank my supervisor who collected the surface samples in the early 1990s and kept them up to this day that I may do work on them. The PAR 11 borehole samples were made available by Collins Mwathahwa from Anglo Platinum Zimbabwe. I would like to thank the geological team of Unki Mine, Caston Musa, Fredrick Hlasi, Omberai Mandingaisa and Melusi Hlambelo for assisting me the times I spent at the mine during the field work and core logging. The analysis was done under the supervision of my supervisor at the Bernard Price Building, School of Geosciences, University of Witwatersrand, and Johannesburg. I would like to thank Janine Robertson who did the XRF analyses, Marlin Patchappa who carried out the ICP-MS, Musa Cebekhulu for preparing the thin and polished sections for me. All the sample preparation and analyses cost was taken care of by Prof. Allan Wilson. I would also like to thank Quartus Snyman on behalf of Anglo Platinum, South Africa for providing the financial support. I would like to thank my colleagues Irvin R. Matsheka, S’lindile S. Wela, Todani Funyufunyu, Musa Manzi, Trishya Owen-Smith, Grant Bybee. I would like to thank my officemate Masibulele Zintwana for being really helpful whenever possible. I would also like to thank my friends Nokuthula Dladla, for the love and support during the write of my thesis. At various stages of this study I have benefited for Prof. Lew Ashwell, Prof. Grant R Cawthorn and other staff and students of the School of Geosciences, University of Witwatersrand, and Johannesburg. Lastly I would like thank my mom Patricia N. Ncube, my children Shane and Sisanda to whom this project is dedicated. vi Table of Contents Declaration .................................................................................................................... ii Abstract ........................................................................................................................ iii Dedication ..................................................................................................................... v Acknowledgements ...................................................................................................... vi List of Figures ............................................................................................................. xii List of Tables.............................................................................................................. xix CHAPTER 1: INTRODUCTION ................................................................................. 1 1.1 General Geology of the Great Dyke........................................................................ 1 1.1.1 Geological Setting of the Great Dyke .............................................................. 4 1.2 General geology of the Selukwe Subchamber ........................................................ 5 1.2.1 Some Unique Characteristics of the Selukwe Subchamber ............................. 6 1.3 Previous Geological Work ...................................................................................... 8 1.4 The Aims of this Study ........................................................................................... 8 1.5 Methodology ........................................................................................................... 9 CHAPTER 2: THE GEOLOGY OF THE SELUKWE SUBCHAMBER .................. 11 2.1 Introduction ........................................................................................................... 11 2.2 Stratigraphy and Rock Types of the Selukwe Subchamber .................................. 14 2.3 Country Rocks Contacts ........................................................................................ 14 2.4 The Mafic Sequence .............................................................................................. 14 2.5 Mafic and Ultramafic contact................................................................................ 16 2.6 P1 Pyroxenite layer ............................................................................................... 16 2.7 Serpentinite Layer of Cyclic Unit 1 ...................................................................... 17 2.8 The Lower Ultramafic Succession ........................................................................ 18 2.9 Chromitites ............................................................................................................ 18 2.10 The Border Group ............................................................................................... 19 2.11 The Xenolith/Autolith Suite ................................................................................ 19 2.12 Summary and Discussion .................................................................................... 20 vii CHAPTER 3: STRATIGRAPHY AND LITHOLOGIES OF THE PAR 11 BOREHOLE ............................................................................................................... 22 3.1 Introduction ........................................................................................................... 22 3.2 Characteristic