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The Geometry of Some Beaufort Group Sandstones and Its Relationship To THE GEOMETRY OF SOME BEAUFORT GROUP SANDSTONES AND ITS RELATIONSHIPI TO URANIUM MINERALISATION.I I I by V.v. Le Q. SStuart-Williams lliams Thesisis submitted for the degree of Master of Scienceence Universityin the Faculty of ofCape Science,ence, Town Universityiversi of Cape Town. 19811 cuzw- • ~:~_....w. .w W«=M) ihe University of CapeGape Town has been given ·. tho right to reproduce this thesis in whole t or in p<lrtpart. Copyright isIs held by the auchd.au":hi:'-r. ' '" . ,,· . •I 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 University of Cape Town FRONTISPIECEFRC>NTISPIECE - htensiveExtensive flat beddedbedded sandstone of the Teekloof rOl"Glat;onFormation wHhwith the ltJweveldNuweveld [scar\X*ntEscarpment in tnethe backgr-ound.b~ckground. AA gO%goz isis seen 5tat the right. ABSTRACT From field evidence gained whilst working on the Ptistetognathus/ Diictodon Assemblage Zone west of Beaufort West it is found that three discrete sandstone types called:- A) straight channel sandstones, B) low sinuosity channel sandstones and C)' transitional sandstones can be identified, in what has previously been considered as high sinuosity channel facies association sediments. Palaeocurrent analysis has demonstrated that the transitional sandstones were high sinuosity and were the larger fluvial systems; the straight channel and low sinuosity channel sandstones were generally much smaller and had as their names imply a much lower s i nuos i ty. The palaeo-variability of current vectors in these systems, in the study area, is such that a standard deviation of greater than ~ 40 0 is considered diagnostic of the transitional sandstones. Moreover in the study area only this type of sandstone is of importance in uranium exploration. From the palaeocurrent data and the reduction-oxidation states of the sandstone types it is proposed that the transitional sandstones represent semi-perennial fluvial systems flowing across i i an arid intracratonic basin, whilst the straight and low sinuosity channel sands are intrabasinal tributaries of the transitional sandstones. From this study of the sandstone geometry has evolved a new model of uranium mineralisation. It is proposed that the mineralisation is syngenetic and generated by reduction of uranyl carbonates on carbonaceous material. The carbonaceous material must however be lying closely below or within a weak REDOX front, since it is vital to transport complexes in an oxidising environment, and yet such an environment will not allow reduced uranium to be preserved for any length of time, as the carbonaceous material on which it reduced will eventually oxidize. Such a REDOX front, it is proposed, is created by the coalescence of two discrete sands. The upper sand isan oxidising active channel. The lower sand has been buried for some time and is weakly reducing due to anaerobic breakdown of its carbonaceous material by bacteria. Fluids in the two sands mix at the point of coalescence and uranium in transport in the upper sand is transported to and fixed at the REDOX front by carbonaceous material in the lower sand. Borehole data suggests that the correlation between coalescence of two discrete sandstones (the lower being a transitional sandstone) and mineralisation is very good. It is believed that the three sandstone types, whilst representing major fluvial systems and their tributaries, were undergoi ngwa ter Hi loss along their length such that distal portions of even the major (transitional) sandstones were likely ephemeral. Evidence for this is found in the arid climate, lack of in-channel vegetation, dominance of flat-bedded sandstones and clear evidence for bedload transport. iv CONTENTS Page 1 INTRODUCTION 1.1 GENERAL 1.2 AIMS 2 1.3 GEOGRAPHICAL SETTING AND LOCATION 2 1.4 GEOLOGICAL SETTING 4 1 .4. 1 The Karoo Supergroup - an Overview 4 1. 4.2 The Beaufort Group 9 1.5 METAMORPHIC AND TECTONIC SETTING 13 1.6 PREVIOUS WORKERS 14 1.7 SUMMARY OF CURRENT CONCEPTS 20 2 LITHOFACIES USED IN MAPPING 2. 1 INTRODUCTION 23 2.2 LITHOFACI ES A 24 2.3 LI THOFAC I ES Bl 27 2.'4 LI THOF AC I ES B2 28 2.5 LITHOFACIES B3 28 2.6 LITHOFACIES C 29 2.7 LI THOFACI ES D 32 3 SANDSTONE PETROLOGY 3. 1 GENERAL 33 3.2 MODAL ANALYSIS 34 3.3 THIN SECTION DESCRIPTIONS OF THE MAl N 37 C(Jv1PONENTS. 3.3. 1 Quartz. 37 3.3.2 Feldspar 38 3.3.3 Rock fragments 39 3.3.4 Ma tri x 40 3.3.5 Accessory Minerals 42 3.4 GRAIN SIZE ANALYSIS 42 3.5 CLASSIFICATION OF THE SANDSTONES 43 v Page 4 SECTION GEOLOGY 4.1 I NTRODUCTI ON 44 4.2 AIMS OF THE SECTION MEASURING 45 4.3 MARKOVIAN ANALYSIS 46 4.4 MUDSTONE + SILTSTONE: SANDSTONE RATIOS; 54 AN INDICATOR OF FLUVIAL AND INTERFLUVIAL ZONES? 4.5 SANDSTONE CONTINUITY 56 5 PALAEOCURRENT ANALYSIS 5. 1 INTRODUCTION 62 5.2 DATA GATHER I NG 63 . 5.3 DA TA TR EA TMENT 66 5.4 RESULTS 67 5.5 SANDSTONE SINUOSITY 69 5.6 AN ALTERNATIVE APPROACH TO SINUOSITY 75 5.7 WHAT IS THE MEAN PALAEOFLOW DIRECTION 82 OF SANDSTONE 10? 5.8 INTRABASINAL TRIBUTARIES? 83 6 PLAN GEOLOGY 6.l. I NTRODUCTI ON 85 6.2 STRAIGHT AND LOW SINUOSITY CHANNELS 85 6.3 TRANSITIONAL SANDSTONES .86 7 SANDSTONE GEOMETRY 7. 1 I NTRODUCTI ON 7.2 LOW SINUOSITY AND STRAIGHT CHANNEL SANDSTONES 7.3 TRANSITIONAL SANDSTONES 102 vi 8 URANIUM, - ITS SOURCE, MODE OF TRANSPORT AND CONCENTRATION Page 80 1 SOURCE OF THE URANIUM 114 8.2 TRANSPORT OF URANIUM IN THE SEDIMENTARY 116 ENV IRONM ENT 8 0,3 THE REDUCTION AND FIXATION OF URANIUM 119 8.4. PERMEABILITY AND WATER I~OVEMENT 121 9 URANIUM MINERALISATION-A NEW MODEL 9. 1 CURRENT MODELS OF URANIUM MINERALISATION 124 9.1 .1 Roll Front Deposits 124 9.1.2 URAVAN Type Deposits 126 9.1.3. Preservation Potential '128 9.2 A MODEL OF URANIUM MINERALISATION 129 AT MATJIESKLOOF 9.3 DISCUSSION OF THE MODEL 139 9.4 OTHER MODELS FOR URANIUM MINERALISATION 146 AT MATJIESKLOOF 10 INTERPRETATION OF THE DEPOSITIONAL ENVIRONMENT 10. 1 INTRODUCTION 149 10.2 REVIEW OF THE FLUVIAL ENVIRONMENT WITH 149 SPECIAL REFERENCE TO THE BEAUFORT GROUP 10.3 IS SANDSTONE 10 A TRANSITIONAL SAND? 151 10.4 TRANSITIONAL SYSTEMS 153 10.5 THE HYDROLOGY OF TRANSITIONAL SYSTEMS 157 10.6 CURRENT VECTOR ORIENTATION IN TRANSITIONAL 158 SYSTEMS 10.1 TRANSITIONAL SYSTEMS IN RELATION TO BEAUFORT 159 GROUP SANDSTONES 10.8 TRANSITIONAL SYSTEMS AND THE MUDSTONE + 160 SILTSTONE: SANDSTONE RATIO 10.9 TERMINAL FANS AND BEAUFORT GROUP SAND­ 161 STONES 10.10 SUMMARY 164 vii 11 APPENDIX 11 .1 PRCXlRAMMES FOR PALAEOCURRENT TREATMENT 166 USING THE HP - 33 E 11.2 THE CONTINUITY INDEX 172 12 AC KNOW LE DGEM ENTS 174 13 REFERENCES 175 LIST OF DIAGRAMS Di ag. The Location of the Study Area. 3 Diag. 2 The Measured Sections. In Backflap Diag. 3 The Location of Sections 4-26 In Backflap within the Study Area. Di ag. 4 A Tree. Diagram Showing Upward Facies 53 Transition Probabilities. Diag. 5 Illustrating how the Mudstone and 57 Siltstone: Sandstone Ratio may define Fluvial and Interf1uvia1 Zones. Di ag. 6 Illustrating the use of a Continuity 60 Index to help Define Fluvial and Interf1uvia1 Zones. Di ag. 7 Methods of Determining Sinuosity. 70 Di ag. 8 Straight Channels on the farm Pa1miet­ 74 fontein. Di ag. 9 Straight. Distribution Frequency and a 78 Rose Diagram of all Bedforms Measured on the Pa1mietfontein Sandstone. Di ag. 10 Low Sinuosity. Distribution Frequency 79 and a Rose Diagram of all Bedforms Measured on Sandstone 15. viii Di ag. 11 Transitional. Distribution Frequency 81 and a Rose Diagram of all Bedforms Measured on Sandstone 10. Diag. 12 Meanders on Sandstone 10. In Backf1ap Diag. 13 Sandstone 15 in Plan and Prof; le 96 (Dip corrected). Diag. 14 Carbonate Transport. 118 Di ag. 15 A Typical South to North Drill Section. 130 Diag. 16 A Second South to North Section. 132 Diag. 17 A Third South to North Section. 133 Di ag. 18 The Model for Syngenetic Mineralisation 134 at Matjieskl oof. Diag. 19 The proposed model in '3' Dimensions. 138 Diag. 20 The Relationship between Coalescence and 140 Mineralisation in Sandstones 10 and 1l. Di ag. 21 A Model of the Fluvial System Envisaged. 163 LIST OF TABLES Table 1 The Stratigraphy of the South-Western 5 Karoo. Tab1 e 2 Moda 1 Ana lys is of Beaufort Group Sandstones. 36 Table 3 Upward Transition Count Matrix. 48 Table 4 Upward Transition Probability Matrix (P ij). 48 Tab 1e 5 Independent Trials Probability Matrix (rij). 51 Tab le 6 Residual Transition Probabilities (d ij). 51 Table 7 Sandstone: Mud + Silt Ratio. 58 Table. 8 Sandstone Continuity. 58 Table 9 Sandstone Palaeocurrent Data. 68 ix LIST OF PLATES Page Plate I Photographs 1 - 4. 26 P1a te II Photographs 5 - 7. 3 1 P1a te III See Diagram 8. 74 Plate IV Photographs 9 - 12. 94 Plate V Photogra phs 13 - 16. 101 P1a te VI Photogra ph s 17 - 20. 105 P1 a te VII Photographs 21 - 24. 108 Plate VIII Photogra phs 25 and 26. 155 1 INTRODUCTION 1 .1 GENERAL . Over the last decade a large amount of information has been generated about Beaufort Group sediments due to the realisation that these sediments were likely targets for uranium mineralisation.
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