The Petrology, Petrography and Geochemistry of Anomalous Borehole Core Sequences in the Highveld Coalfield, South Africa: a Case Study for Diatreme Activity

THE PETROLOGY, PETROGRAPHY AND GEOCHEMISTRY OF ANOMALOUS BOREHOLE CORE SEQUENCES IN THE HIGHVELD COALFIELD, SOUTH AFRICA: A CASE STUDY FOR DIATREME ACTIVITY. by BYRON VANDER WALT Submitted in fulfilment of the requirements for the degree MAGISTER SCIENTAE in GEOLOGY submitted to the FACULTV OF SCIENCE of the UNIVERSITY OF JOHANNESBURG July 2012 SUPERVISORS: PROFESSOR B. CAIRNCROSS PROFESSOR H.M. RAJESH ABSTRACT Three anomalous borehole core sequences from the north eastern Karoo Basin are examined. The boreholes are located up to 30 km from each other and are lithostratigraphically completely atypical for the Vryheid Formation, Ecca Group, Karoo Supergroup. The lithologies of the three boreholes are intensely brecciated for the most part, while all of the surrounding boreholes reveal normal stratigraphy; their sedimentary strata are normally horizontal with no faulting present. The only known disturbances to the Vryheid Formation in the study area are the occurrence of intrusive mafic dolerite sills and dykes, which are known to have been contemporaneous with and immediately following the eruption of the Drakensburg Group basaltic lavas. The borehole core lithologies are described in detail with reference to their textural, mineralogical and petrographic characteristics. Mineral and bulk rock chemical data are presented. Several modes of origin of the brecciated core sequences are considered, with the primary hypothesis that the brecciation is due to diatreme activity. A review of diatremes and their mode of emplacement is proposed with reference to their occurrence within the Karoo Igneous Province, as some diatremes in the Karoo are associated with dolerite sill emplacement. The isolated occurrences, lithologies, petrography, alteration and geochemistry of the sequences are used to argue that the Vryheid Formation, intersected in the form of the three anomalous boreholes, was disturbed by diatreme activity, which are genetically related to the late dolerite sill emplacement into the Karoo Supergroup rocks. ACKNOWLEDGEMENTS Anglo Coal Geological Services are acknowledged for providing core samples and other geological data for this thesis. Thanks to Mr. F. Botes, Mrs. U. Herrmann, Mr. M. Mattushek, Mr. J. Ndwamise and Mrs. J. Marks and all members of Anglo Coal Geological Services who assisted either directly or indirectly with the preparation of this study. Thanks go to the Coaltech Research Association and the National Research Foundation (NRF) for the financial support for this study. I am grateful to the following members of the Department of Geology at the University of Johannesburg: 1. Professor Bruce Cairncross, my supervisor, for all his guidance with this study, for his advice, assistance, editorial skills, the use of his photographic equipment, as well as approving this thesis. 2. Professor H.M. Rajesh, my co-supervisor, for all his assistance and for sharing his knowledge and expertise in the fields for igneous petrology, mineral chemistry and geochemistry. 3. Professor Fanus Viljoen, for his advice in many respects, as well as for assisting me with the use of the binocular microscope and camera used in the project. 4. Dr. Herman van Niekerk, for his advice and friendship. 5. Dr. Christian Reineke, for his assistance in using the Jeol 733 Superprobe. I would like to thank my parents David and Wilma for their support throughout all my studies and for always being there for me. I am most grateful to Nicola Skerman for always providing me with encouragement and support. ii TABLE OF CONTENTS Page No. CHAPTER 1 - INTRODUCTION 1 1.1 Coal in South Africa 1 1.2 Location 2 1.3 Objectives and Methodology 5 1.3.1 Problem Identification 5 1.3.2 Objectives 5 1.3.3 Methodology 5 1.4 Regional Geology 7 1.4.1 The Karoo Supergroup 7 1.4.2 The Karoo Igneous Province 13 1.5 Past Studies 17 CHAPTER 2- STRATIGRPHY 22 2.1 Stratigraphy of the Vryheid Formation and the Highveld Coalfield 22 2.2 Genetic stratigraphy of the Highveld Coalfield 24 2.3 Stratigraphy of the study area 27 CHAPTER 3- PETROLOGY, PETROGRAPHY AND MINERAL CHEMISTRY 42 3.1 The undeformed lithofacies in the study area 42 3.2 Petrography 51 3.2.1 Introduction 51 3.2.2 Sample Selection 51 3.2.3 Petrography of the brecciated core sequences 54 3.3 Mineral Chemistry 62 3.3.1 Introduction 62 3.3.2 Methodology 62 3.3.3 Mineral chemistry of the unaltered dolerites from the standard core sequence 63 iii 3.3.4 Mineral chemistry of the altered dolerites from the standard core sequence 65 3.3.5 Mineral chemistry of the sedimentary rocks and breccia 69 3.4 Discussion on the nature of the brecciation in Core A and Core B 71 CHAPTER 4 - GEOCHEMISTRY 75 4.1 Introduction 75 4.2 Methodology 75 4.3 Geochemistry of the unaltered and altered dolerites 77 4.3.1 Classification and tectonic setting 77 4.3.2 Comparison of geochemical characteristics of dolerite samples 83 4.3 Bulk rock geochemistry of fresh sedimentary rocks and the breccia matrix 95 4.3.1 Classification and tectonic setting 95 4.3.2 Comparison of geochemical characteristics of fresh sedimentary rocks and the breccia matrix 97 CHAPTER 5- STABLE ISOTOPES 107 5.1 Introduction to stable isotopes 107 5.2 Methodology 108 5.3 Stable isotope data 108 CHAPTER 6 - DISCUSSION 112 6.1 Possible modes of origin of the brecciated core sequences 112 6.1.1 Fault breccias 112 6.1.2 Sinkhole Collapse 112 6.1.3 Meteorite impact 113 6.1.4 Syn-sedimentary deformation 113 6.1.5 Diatreme activity 113 6.2 A review on diatremes 114 6.3 Evidence to support a diatreme model 117 6.3.1 The isolated occurrences of the brecciated borehole core sequences 117 iv 6.3.2 The lithologies of the brecciated borehole core sequences 118 6.3.3 Sediment fluidization 118 6.3.4 Variations with depth 119 6.3.5 Mineral chemical and geochemical implications 119 6.3.6 Stable isotope discrepancies 120 6.3.7 Lithological implications of a diatreme model 122 CHAPTER 7 - CONCLUSIONS 124 7.1 Motivation for a diatreme model 124 7.2 Proposed model for the formation of the brecciated core sequences 124 7.3 Time constraints for diatreme formation 125 7.4 Conclusion 125 7.5 Future studies and implications to mining activity 126 REFERENCES 127 APPENDIX A Full descriptions of the petrology and petrography of brecciated core sequence A. 140 APPENDIX B Full descriptions of the petrology and petrography of brecciated core sequence B. 190 APPENDIX C Petrology and petrography of representative samples from the standard core sequence. 232 APPENDIX D Mineral chemical analyses of the Standard Core and brecciated core sequences. 249 APPENDIX E Geochemical analyses of the Standard Core and Brecciated Core sequences. 261 v LIST OF FIGURES Page No. Figure 1.1 Locality map of the study area. 3 Figure 1.2 The distribution of the Karoo-type Basins of Southern Africa and location of the study area within the main Karoo Basin 4 Figure 1.3 The approximate relative positions of the anomalous boreholes found. 6 Figure 1.4 The areal distribution of the lithostratigraphic units in the Karoo Basin. 8 Figure 1.5 North-south cross-sections across the Karoo Basin. 11 Figure 1.6 A palaeogeographic reconstruction with major depositional environments during the No.4 seam peat accumulations 11 Figure 1.7 Map of the Karoo Igneous Province of southern Africa 14 Figure 1.8 The distribution of known diatremes and "breccia pipes" in the Karoo Basin 17 Figure 2.1 Schematic north-south section through the north-eastern part of the Ecca Group. 23 Figure 2.2 Typical and simplified stratigraphic columns in the Highveld Coalfield. 24 Figure 2.3 A typical composite stratigraphic column for the Highveld Coalfield. 26 Figure 2.4 A typical stratigraphic column for the study area. 28 Figure 2.5 The effect of pre-Karoo basement paleotopographic highs. 29 Figure 2.6 The locations of the cross-sections and fence diagram through brecciated core sequence A. 30 Figure 2.7 North-south cross section through brecciated core sequence A. 31 Figure 2.8 Southwest-northeast cross section through brecciated core sequence A. 32 Figure 2.9 Fence diagram through brecciated core sequence A. 33 vi Figure 2.10 The locations of the cross-sections and fence diagram through brecciated core sequence B. 34 Figure 2.11 Northwest-southeast cross section through brecciated core sequence B. 35 Figure 2.12 Southwest-northeast cross section through brecciated core sequence B. 36 Figure 2.13 Fence diagram through brecciated core sequence B. 37 Figure 2.14 The locations of the cross-sections through brecciated core sequence C. 38 Figure 2.15 North-south cross section through brecciated core sequence C. 39 Figure 2.16 East-west cross section through brecciated core sequence C. 40 Figure 2.17 Fence diagram through brecciated core sequence C. 41 Figure 3.1 Roundness and sphericity scale 43 Figure 3.2 Examples of the facies types found in the study area. 49 Figure 3.3 Location of samples taken within brecciated core sequence A. 52 Figure 3.4 Location of samples taken within brecciated core sequence B. 53 Figure 3.5 Representative breccia samples from Core A. 56 Figure 3.6 Representative breccia samples from Core B. 57 Figure 3.7 Unaltered dolerite core samples from the upper sections of Core A and Core B. 59 Figure 3.8 Photomicrographs of the unaltered dolerite core samples from the upper sections of Core A and Core B. 59 Figure 3.9 Comparison of unaltered and altered dolerite using cut sections of core. 60 Figure 3.10 Photomicrographs comparing unaltered and altered dolerite.

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