THE GEOLOGY OF THE ARCHAEAN GRANITOID - GREENSTONE TEERANE IN THE VICINITY OF THREE SISTERS, BARBERTON GREENSTONE BELT Ernst Alfred Kohler j A thesis submitted to the Faculty of Science, University o f the Witwatersrand, Johannesburg, for the degree o f Doctor o f Philosophy Johannesburg, 1994 ABSTRACT This thesis provides a comprehensive account of the geology of the Archaean granitoid-greenstone terrane centred around Three Sisters in the northeastern sector of the Barberton Mountain Land. The supracrustal succession in the region comprises a diverse variety of altered volcanic and sedimentary rock types that have been correlated with the principal lithostratigraphic units of the Barberton greenstone belt (BGB) as fellows: 1) schistose basic and ultrabasic lithologies correlated with the Theespruit Formation of the Gnverwacht Group are mainly developed in a narrow unit fringing the northern margin of the BGB; 2) ferruginous shale greywacke - banded iron-formation assemblages correlated with the Jheba and Belvue Road Formations constitute the dominant Fig Tree Group imks in the region. A sec «ence or silicic » .-stavolcaniclastic rocks, now altered to a variety of micaceous schists, occurs west, north ar.a northeast of Three Sisters. Viljoen and Viljoen (1970) cc rrela ted these schists ith the Theespruit Formation. In this study, the schists have been assigned'to a new lithostratigraphic unit, referred to as the Bien Venue Formation, which forms the uppermost formation of the Fig Tree Group in the northeastern part of the BGB. Isotopic studies on zircons indicate that the schists have an age of 3256 ± Ma, which is some 200 Ma younger titan the most recent age estimates for the lower portions of the Gnverwacht Group. Chemically, the silicic schists resemble calc-alkaline rocks found in modem arcs, suggesting that the Bien Venue Formation represents a period of arc-like volcanism; and 3) conglomeratic and quartzitic rocks constitute the dominant lithologies within the Moodies Group, which uncoriorroably or paraconformably overlies lithologies of the Fig Tree and Onverwacht Groups. North of the BGB is a complex suite of granitoid rocks, previously investigated by Robb et al. (1983) who defined a large (—60 kn ng and —6 km wide), elongate plutonic body of tonalitic-to-trondhjemitic composition known as the Stentor pluton. It was suggested that this pluton is correlatable with the irondhjemite gneiss plutons that intrude the southwestern parts of the BGB. Field evidence indicates, however, that the Stentor pluton forms a much smaller (* .14 km long and ~ 4 km wide) ovoid body located immediately north of the village of Louw’s Creek. Furthermore, the pluton consists of equigranular-textured graiodiorite-adamcllite, totally unlike any of the trondhjemite gneisses. In terms of texture, mineralogy and chemical composition, the Stentor pluton closely resembles the Hebron and Berlin plutohs which constitute a phase of the Nelspruit batholith. Thus, it is concluded that the Stentor pluton also forms an integral part of the batholith. Three deformation phases have been identified. The regional event affected all stratigraphic units in response to a northerly oriented compressions! stress and gave rise to east-northeasterly trending, tight-.« 5soclinal, upright and north-verging folds that are bounded by southward-dipping longitudinal reverse faults. The regional deformation occurred both prior and subsequent to the emplacement of the Stentor pluton at circa 3100 Ma. Deformation associated with the diapiric intrusion of the Stentor pluton into the greenstone assemblage during the regional deformation pnase, led to the formation of large-scale folds that have modified earlier formed structures. The final deformation episode is manifested by the presence of nortnwest- to northeast-striking, oblique-slip normal faults that exhibit both left- and right-lateral strike-slip components. v DECLARATION I, the undersigned, declare that this thesis is my own, unaided ' vork. It is being submitted for the degree of Doctor of Philosophy in the University of the 'Vv ttwatersrand, Johannesburg. It has not been submitted to any other University for the purpose of obtaining a degree. / f 99 $ E.A. KOHLER To Jennifer, for reasons beyond listing Ornne ignotum pro magnifico. - Tacitus The most incomprehensible thing about the world is that it is comprehensible. Einstein VI ACKNOWLEDGEMENTS The writer is indebted to the following persons aui organizations: Firstly, the writer would like to thank Dr, C, Frick, Director of the Council for Geoscience, Geological Survey of South Africa, and Prof. C.R. Anhaeusser, Director of the Economic Geology Research Unit, University of the Witwatersrand, for the opportunity to undertake this project. A. Schoemau, J. Elsenbroek, H, Cloete and J. Trojak of the Geological Snwey of South Africa, and V. Govender of the University of the Witwatersrand, carried out the XRF geochemical analyses. The assistance of the staff of the Schonland Research Centre for Nuclear Science, especially Dr. R J. Hart, in the analysis of samples by MAA is gratefully acknowledged. Dr. F.M. Meyer of the University of the Witwatersrand is thanked for his invaluable assistance with respect to the analytical aspects of this thesis. Dr. E.A. Relief is thanked for his advice and guidance while the writer was working in the laboratories of the Department of Earth, Marine and Atmospheric Sciences and Technology (EMAtek) of the Council for Science and Industrial Research. The writer is indebted to Prof. S.A Bowring and Dr. C, Isachsen of the Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, for carrying out the U-Pb isotopic analyses. The thesis benefited from discussions with, and technical assistance from, the following: Profs. D.R. Lowe, K. Suwa, LJ. Robb and J.R. Mclver, Drs. T. Wallmach, T.O. Reimer, B.M. Eglington, T J, Molyneux, F. Walraven, R.L. Gibson, M. Cloete, C. Heubeck, AF.M. Kisters and G. De Kock; Messers. P.D. Harris, P. Harrison, J.H.W. Ward, R. Le Roux, C. Meyer, V.J. King and N. Niemand. The writer also wishes to express his gratitude to Prof. R, Crewe the Department of Zoology at the University of the Witwatersrand for permission to use. the Pullen Farm house during the initial period of fieldwork, and the owners of the various farms mapped, in particular Messers. D. Van Graan and L. Von Johnstone, for access to their properties and hospitality. My field assistant, Mr. W, Mhlongo, is thanked for his help during the period of fieldwork. The thin sections were prepared by Messers. A. Mathebula, J. Moioisi and M.M. Lekotcko. Thanks are also due to Mr. MX). Kohler for providing photographic services. Anglo American Prospecting Services (Pty.) Ltd., General Mining, Metals and Minerals Ltd. (GENMTN) and Phelps Dodge Mining Ltd. provided berehc’e core and unpublished company information on various aspects of the geology in the region. Mr. J.H.R. Raubenheimer of GENMÏN is thanked for permission to conduct an underground visit to the Three Sisters Gold Mine. Finally, special thanks are due to my fiancée, Jennifer Rodel, for mort support during the course of this project. m CONTENTS ?age ABSTRACT I DECLARATION f f l DEDICATION IV ACKNOWLEDGEMENTS VI CONTENTS VIII LIST OF TABLES XIV l i s t o f f i g u r e s XVI 1. INTRODUCTION 1 1„1 General statement and location of the study area 1 1.2 Objectives of thé study 1 13 Typography and vegetation 3 1.4 Summary of previous work 6 1.5 Methods of investigation 7 1.5.1 Fieldwork 7 1.5.2 Petrography and X-ray diffraction analysis 8 1.5.3 X-ray fluorescence spectrometry and instrumental neutron 8 activation analysis 1.5.4 Isotopic studies 10 2. G E O LO G IC A L. 2VTEW H 2.1 Introduction 11 2.2 Nature of the earliest crust 11 2.3 Supracrustal rocks: the Barberton Supergroup 11 2.4 Granitoid rocks 15 2.5 Geochronology 16 2.5.1 Barberton Supergroup 16 2.5.2 Granitoid rocks 20 IX Page 2.6 Tectomc evolution 21 2.7 A lteration 22 3. ONVERWACHT GROUP 24 3.1 Introduction 24 3.2 Basic and ultrabasic schists 25 3.3 Amphibolites 27 3.4 Ssrpentinites 28 3.5 Silicic schists 28 3.6 Metagabbros 29 3.7 Metapyroxenites 30 3.8 Geochemistry 30 3.9 Metamorphism - metasomatism 35 4. FIG TREE GROUP 38 4.1 Introduction 38 4.2 Sheba and Belvue Road Formations 39 4.2.1 Shales and slates 39 4.2.2 Metagreywackes 43 4.2.3 Banded iron-formations and cherts 45 4.2.4 Silicic and intermediate metavolcanic-yolcaniclastic rocks 48 4.2.5 Depositional setting 51 4.3 Bien Venue Formation 52 4.3.1 Quartz-muscovite schists 54 4.3.2 Biotite-oligoslase schists 60 4.3.3 Chlorite schists, talc schists and serpentinites 62 4.3.4 Cherts and cherty dolomites 63 4.3.5 Phvllites* and slates 66 X Page 4.3.6 Stentor barite, deposit 66 4.3.7 Bien Venue massive sulphide deposit 68 43.8 Quarts ± tourmaline veins 69 4.3.9 Depositional setting 70 5. GEOCHEMISTRY AND GEOCHRONOLOGY OF THE BEEN VENUE 76 FORMATION 5.1 Introduction 76 5.2 Quartz-muscovite schists 77 5.2.1 Metamorphism - metasomatism 81 5.2.1.1 Prograde effects 83 5.2.1.2 Retrograde effects 89 5.2.2 Element distribution patterns 89 5.2.2.1 Low field-strength elements 90 5.2.22 High field-strength elements , 95 5.2.3 Comparison with rods of the Theespruit Formation 106 5,3 Biotite-oligodase schists 111 5.3.1 Comparison with rocks of the Schoongezicht Formation 113 5.4 Chlorite schists 114 5.5 Palaeotectonic setting 118 5.5.1 The role of plate tectonics in the evolution of the Earth’s 118 early crust 5.5.2 Geotectonic setting of modem silicic magmatism 121 5.5.3 Discrimination and interpretation 222 5.6 Geochronology 129 5.6.1 Analytical method 129 5,6.2 Zircon description 132 5.6.3 Results and interpretation 133 Page 6.