Remobilization and Primary Uranium Genesis in The
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REMOBILIZATION AND PRIMARY URANIUM GENESIS IN THE DAMARAN OROGENIC BELT, NAMIBIA. by ALAN GERALD MARLOW, M.Sc. Submitted in fulfilment of the requirements for the Degree of Doctor of Philosophy Department of Earth Sciences, The University of Leeds. September, 1981. ABSTRACT The Damaran belt in Namibia represents a highly eroded orogenic area, the root zone of which is exposed between Swakopmund on the coast and Karibib in the west. Primary uranium deposits located in these structurally lower regions of the orogen, have been studied in an attempt to assess the importance of geological processes associated with remobilization in the genesis of the uraniferous Damaran granitoids. The pre-Damaran basement consists of a sequence of up to 1600m of shallow water clastic and carbonate rocks with interbedded meta- basaltic and pyroclastic horizons. The 'meta-basalts' range in composition from basalt to andesite, and are characterized by chemical features diagnostic of a tholeiitic, and in some cases komatiitic affinity. This sequence, previously assigned to the Abbabis Formation, is intruded locally by 2.0 Ga gneisses which range in composition from diorites, through granodiorites to alkali-rich granites. Pb-isotope ratios of sulphides from the basement and the Damaran metasedimentary cover indicate that the basement was enriched in U/Pb at 1.7 Ga, and that the Damaran uranium province has been in existence for at least 1.7 Ga. The Damara Sequence consists of a lower Nosib Group and an upper Swakop Group. The former consists of fluviatile clastics (Etusis Formation), followed by shallow-water calcareous, feldspathic sandstones (Khan Formation). Uranium minerals, in the form of uranyl silicate inclusions, found within detrital constituents of the Etusis quartzites, provide direct evidence of radioactive material within the pre-Damaran basement prior to its erosion, transportation and deposition within the ii. Damara Sequence. The Swakop Group represents a typical geosynclinal sequence starting with carbonates, quartzites, conglomerates and pelites (Rossing Formation); followed by mixtites (Chuos Formation) and a dominantly calcareous succession (Karibib Formation); and ending with a monotonous series of biotite schists (Kuiseb Formation). Primary uranium minerals including uraninite and betafite located within schists and calc silicates of the Rossing Formation are considered to have recrystallized during the Damaran metamorphism from syngenetic uranium associated with stratabound copper deposits. The Damaran belt underwent regional deformation between about 650 and 550 Ma, and although early mafic granitoids were emplaced locally, the major granite forming event post-dates the major regional deformation. Between 540 and 46O Ma dome structures were developed by a process involving diapirism and the upward movement and subsequent ballooning of large volumes of granitic material. The Damaran granitoids may be broadly divided into syn- to post-tectonic Salem type granites and red granites, and late- to post-tectonic leucogranites and alaskites. Field relationships indicate that the Salem type granites are derived from a source deeper than the Damara Sequence, whilst the alaskites appear to be derived from migmatized basement and Damaran metasedimentary cover rocks. The earlier Damaran granitoids, which tend to be more mafic in character, show relatively low Sr-isotope ratios and contain chemical and mineralogical features in common with I-type granitoids. These various factors are considered to reflect derivation from the lower crust or upper mantle. In contrast the later granitoids are normally leucocratic in nature, commonly radioactive and occasionally mineralized. They display relatively high Sr-isotope ratios, and an affinity with S-type granitoids, and they are considered to be derived from basement and Nosib source rocks. The mineralized alaskites contain primary uranium minerals including uraninite, betafite and metamict thorite which crystallized from melts enriched in IJ and Th. Primary mineralizations are not restricted to alaskites, but also occur within red granites. Secondary uranium minerals within the mineralized Damaran granitoids include uranyl silicate, thorogummite, calciothorite, ferrothorite and uranophane. These minerals formed during a deuteric stage of alteration, and also during a recent stage of surface enrichment and secondary alteration. Dedicated to Hans Breytenbach ACKNOWLEDGEMENTS Foremost, I would like to express my sincere thanks to C. Hawkesworth and M. Coward, firstly for initiating this project and secondly for their enthusiastic supervision both in the field and the laboratory. Special thanks also go to G. Homung for his help throughout the period of research. I would like to thank the following mining companies for permission to work within their concessions. Anglo American Corp. of S.W.A., Falconbridge of S.W.A., General Mining and Finance Corp. of S.W.A., Goldfields of S.W.A., Trust and Mining Company Ltd. and Rossing Uranium Ltd. Without this permission the work would not have been done, and their help and logistical support is gratefully acknowledged- I would also like to express my gratitude to the geologists and prospectors who assisted me in the field, in particular A. Ransom, R. Miller, B. Fletcher, J. Eartleb, R. Corrans, E. Versfeld and H. Breytenbach. My thanks go to the people in Leeds for their invaluable discussions and criticisms, in particular to J. Kramers, A. Gray, E. Condliffe, P. Ruxton, K. Downing, J. Barnes, M. Powell, R. Cliff, P. Nixon and F. Johnston. Many thanks also to my mother, P. Marlow for helping with the diagrams. I am grateful to those at the Open University responsible for providing me with unpublished geochemical data. Finally, I acknowledge the financial support provided by Trust and Mining Company Ltd., and N.E.R.C. vi. DEFINITION OF NOTATIONS AND ABBREVIATIONS X, Y, Z are the three mutually perpendicular axes of the strain ellipsoid where X ^ Y ^ Z. L lineation S schistosity n no. of measurements P.P.L. plane polarised light XN crossed nicols B.E.I. back scattered electron image X.R.F. X-ray fluorescence spectrometry L.O.I. loss on ignition Fe^O^ (t) total Fe as Fe20^ Xjjk decay constant for Rb = 1.42 x 10 11 yr M.S.W.D. mean square weighted deviate (87Sr/86Sr)o initial 87Sr/86Sr R.E.E. rare earth elements vii CONTENTS Page No. ABSTRACT i DEDICATION iv A CKN OWLEDGEMENTS v DEFINITION OF NOTATIONS AND ABBREVIATIONS vi CONTENTS vii CHAPTER I. INTRODUCTION 1 1.1. Preliminary discussion 1 1.2. Location 2 1.3. Previous work 5 1.4. The aim of the study 6 1.5. Details of the field study 7 1.6. Physiography 8 CHAPTER II. THE GEOLOGY OF THE PRE-DAMARAN BASEMENT 9 11.1. Introduction 9 11.2. The metasedimentary, metavolcanic and pyroclastic sequences of the Abbabis Formation 10 (i) The quartzites 12 (ii) The schists 14 (iii) The conglomerates 16 (iv) The marbles and calc silicates 16 (v) The meta-basalts and pyroclastic rocks 17 (vi) Sulphide mineralization 21 (vii) Geochemistry of the basalts 22 (■srii) A. Major element chemistry 23 (vii) B. Trace element chemistry 28 11.3. The gneisses of the Abbabis Formation 31 (i) The granodioritic gneiss 33 (ii) The leucocratic gneiss and augen gneisses 33 (iii) Geochemistry of the Abbabis gneisses 35 11.4. The amphibolite dykes 37 11.5. Conclusions 39 CHAPTER III. THE GEOLOGY OF THE DAMARA SEQUENCE 40 111.1. Introduction 40 111.2. The Damara Sequence 42 viii Pa/^e No. (i) The Etusis Formation 42 (ii) The Khan Formation 44 (iii) The Rossing Formation 45 (iv) The Chuos Formation 49 (v) The Karibib Formation 52 (vi) The Kuiseb Formation 54 III.3. Conclusions 56 CHAPTER IV. THE GEOLOGY OF THE DAMARAN INTRUSIVES 58 IV.1. Introduction 58 IV.2. The Salem type granites 59 IV.3. The red granites 62 IV.4. The leucogranites 68 IV. 5. The alaskites 72 IV.6. The gabbros 74 IV.7. Conclusions 78 CHAPTER V. STRUCTURAL EVOLUTION 80 V.l. Introduction 80 V.2. Structural evolution of the Valencia area 81 V.3. Structural evolution of the Goanikontes area 88 V.4. Structural evolution of part of the Abbabis inlier 94 V.5. Structural evolution of the Stinkbank- Namibfontein area 99 V.6. Structural evolution of the Otjua area 101 V.7. Regional correlations 104 V.8 . Conclusions 106 CHAPTER VI. URANIUM MINERALIZATION IN THE DAMARAN BELT 108 VI.1. Introduction 108 VI.2. Uranium in the pre-Damaran basement 109 VI.3. Uranium in the Damara Sequence 113 (i) Uranium in the Etusis Formation 113 (ii) Uranium in the Khan Formation 115 (iii) Uranium in the Rossing Formation 116 VI.4. Uranium in the Damaran intrusives 121 (i) The radioactive Salem granites 122 (ii) The radioactive red granites 125 (iii) The radioactive leucogranites 131 (iv) The radioactive alaskites 132 (iv) A. The primary uranium minerals in the alaskites 133 B. The secondary uranium minerals in the alaskites 140 VI. 5- Conclusions -*-47 ix Par , e No. CHAPTER VII CHEMISTRY OF THE URANIUM MINERALS 151 VII.l. Introduction 151 VII.2. Uraninite 152 VII.3. Betafite 154 VII.4. Metamict thorite 157 VII.5. Uranyl silicate 159 VII.6. Thorogummite 162 VII.7. Uranophane 163 VII.8. Conclusions 165 CHAPTER VIII GEOCHRONOLOGY 166 VIII.1. Introduction 166 VIII.2. Rb/Sr results 168 (i) Abbabis gneiss 171 (ii) Stinkbank Salem granite 172 (iii) Otjua red granite 172 (iv) Stinkbank leucogranite 174 (v) Ida dome alaskite 174 (vi) Goanikontes alaskite 175 (vii) Valencia alaskite 180 (viii) Discussion 182 VIII.3. Pb/Pb results 184 (i) U-Pb systematics I89 (ii) Th-Pb systematics 196 VIII.4. Conclusions 198 CHAPTER IX GEOCHEMISTRY 201 IX.1. Introduction 201 IX.2. Geochemical characteristics of the Damaran granitoids 203 IX.3. Discussion 214 IX.4. Conclusions 223 CHAPTER X EVOLUTION OF THE DAMARAN URANIUM PROVINCE 225 REFERENCES 236 APPENDIX 1 . ANALYTICAL TECHNIQUES AND LABORATORY PROCEDURES 248 APPENDIX 2. CONTOUR FIGURES FOR THE STEREOGRAMS SHOWN IN FIGURES 15, 17, 18, 19 AND 20.