\ FACIES MAPPING OF THE VAAL REEF PLACER AS AN AID TO REMNANT PILLAR EXTRACTION AND STOPE WIDTH OPTIMISATION BY: A.G.O'DONOVAN This assignment is submitted as p artial ful f i l lment of the requirements for the degree of Master of Science ( Expl o ration Geology) at Rhodes University, Grahamstown. January, 1992. ABSTRACT The Vaal Reef placer is situated on the unconformable junction of the Strathmore and Stilfontein formations of the Johannesburg Subgroup. Within the South Division of the Vaal Reefs Exploration and Mining company lease, the Vaal Reef Placer is shown to be composed of several different units. Each unit exhibits its own specific characteristics and trend direction which can be used to establish distinct "Reef packages". These packages can be mapped in such a way as to provided a preliminary lithofacies map for the Vaal Reef Placer. The delineation of such geologically homogenous zones, and the development of a suitable depositional model, can be utilised in several ways. The characteristics of a particular zone are shown to influence the control of stoping width, evaluation of remnant pillars and the geostatistical methodology of evaluating current and future ore reserve blocks. TITLE PAGE ABSTRACT CONTENTS LIST OF FIGURES LIST OF TABLES LIST OF PLATES CHAPTER ONE PAGE 1. INTRODUCTION AIMS OF THE PROJECT..... ................. 1. THE WITWATERSRAND BASIN. .. ................ 2. THE KLERKSDORP GOLDFIELD... ... ............ 2. VAAL REEFS MINE. 5. THE VAAL REEF PLACER....... .. ............. 5. CHAPTER TWO 2. THE GEOLOGY OF THE VAAL REEF STRUCTURE LOCAL STRUCTURAL SETTING........ .... 9. FAULTING. 12. INTRUSIVES ..... .. ............ ,. 13. TECTONIC SETTING ................... " 14. SEDIMENTOLOGY LITHOLOGIES MB5 - FOOTWALL .................. 15. ORE ZONE. .. 1 7. MB3 - HANGINGWALL ............... 26. PEBBLE ASSEMBLAGE .................... 26. MINERALOGY. • 28. CHAPTER THREE 3. REGIONAL PATTERNS IN THE VAAL REEF VAAL REEF PROFILE DATA COLLECTION ..............••. 32. LATERAL VARIATIONS •..••.•..••••. 36. ISOPACHS AND FENCE DIAGRAMS.. • . 44. FACIES MAP. • . • . • . • . • • . •• 51. PALAEO TRENDS VALUE DISTRIBUTION ....•.....•.•• 55. PALAEOCURRENT DATA •.... • •.•••••• 62. CHAPTER FOUR 4. DEPOSITIONAL MODEL •. .. •. .. • . • .• • ... • .. • •. •. 66. CHAPTER FIVE 5. APPLICATIONS STOPE WIDTH. .. • • • . • • • • • • • • • • • • . • • • • • 78. PILLAR MINING. ... ....................... 82. NEW AREAS. • . • . • . • . • . • . .• 93. 6 . CONCLUSIONS.. ............ .... .... ..... 98 7. RECOMMENDATIONS............................. 100 7. ACKNOWLEDGEMENTS. 8. REFERENCES. 9. APPENDIX 1. Sedimentological database. LIST OF FIGURES Page. FIGURE 1.1 Stratigraphic column of the Wits Supergroup ...... 3. FIGURE 1.2 Locality plan of the Klerksdorp Goldfield .. ..... 4 . FIGURE 1.3 Stratigraphic Column of the Johannesburg Sub -group. 6. FIGURE 2.1 Simplified structure plan of the Vaal Reef ........ 10. FIGURE 2.2 Simplified N-S section through 2# and 8# ... ... .... 11. FIGURE 2.3 Possible tectonic setting for the Wits. basin . .... 16. FIGURE 2.4 Idealised profile of the Vaal Reef Ore Zone ....... lB. FIGURE 3.1 Example of a sedimentological datasheet ........... 34. FIGURE 3.2 Locality plan of sedimentological stations ... .. ... 35 . FIGURE 3 . 3 Locality plan of type sections ....... ............ 37. FIGURE 3 . 4 Typical Reef Section 2K area ...................... 3B. FIGURE 3 . 5 Typical Reef Section 8A area ...................... 38. FIGURE 3 . 6 Typical Reef Section BL area. .. .. .. .. 41. FIGURE 3.7 Typical Reef Section 9# area.. .. ..... .. ..... ..... 41. FIGURE 3 . B N-S Model section...... ................ .. ...... 43. FIGURE 3 . 9 E-W Model section................................. 43. FIGURE 3.10 Isopach plan of the VRBC. .. 45. FIGURE 3.11 Isopach plan of the UVRZ ... ... ... ................ 47. FIGURE 3.12 Locality plan of fence diagram section lines ...... 4B. FIGURE 3 . 13 NW-SE fence diagram....... .. 49. FIGURE 3.14 W-E fence diagram....................... ......... 50. FIGURE 3.15 Facies map of the VRP. .. 52. FIGURE 3.16 vertical distribution of gold within the VRP. 3.16A Bottom loaded Vaal Reef profile ...... ...... 56. 3.16B Top and Bottom loaded Vaal Reef Profile ..... 56. 3.l6C Random Vaal Reef Profile ......... .......... 57. 3.16D Top loaded Vaal Reef Profile ................ 57. 3.16E Bottom and middle loaded Vaal Reef Profile .. 57. FIGURE 3 .17 Simplified moving average Au value plan ........ ... 59 . FIGURE 3.18 Simplified moving average Uranium value plan .. .... 61 . FIGURE 3.19 Palaeocurrent directions for the VRP ...... ...... 63. FIGURE 3.20 Palaeotrends for MZ, VRBC, UVRZ, ZM ...... ........ 64. FIGURE 4.1 Deposition of the Mizpah units ........... .. ....... 67. FIGURE 4.2 Deposition of the VRBC •.• • . ••• ••.•••••••• • •••.•••• 69. FIGURE 4 . 3 Deflation of the VRBC .• • . ••• . •• •••• • • • •• • .••• . •.• • 69. FIGURE 4 . 4 Deposition of internal quartzites .... .... ..... ... 71. FIGURE 4 . 5 Deposition of the UVRZ . ............ ....... ... ... 72 . FIGURE 4 . 6' Depositi on of upper Vaal Reef quartzites ... .. ... 72. FIGURE 4 . 7 Deposition of the Zandpan units . ......... ... ... 74 . FIGURE 5.1 Example of stope width optimisation ...... .. ... .. .. 80. FIGURE 5 . 2 Flow chart of remnant pillar programme ... ..... .. 84. FIGURE 5 . 3 Histogram of potential profits - pillar A .. .... ... 89. FIGURE 5.4 Cumulative frequency distribution - pillar A . .. ... 89. FIGURE 5 . 5 Scatter plots of multiple variables - pillar A . ... 90. FIGURE 5.6 Scatter plot of tonnage Vs. % profit - pillar A ... 91. FIGURE 5 . 7 Cumulative frequency distribution - pillar B ...... 92 . FIGURE 5 . 8 Scatter plot of tonnage Vs. % profit - pillar B . .. 92. FIGURE 5 . 9 Comparative scatter plots - pillars A - D .. .. .... 95. LIST OF TABLES Page. TABLE 5.1 Input data for examples 1 and 2 ................... 87. TABLE 5.2 Input data for example 3....... ........ ..... ..... 94. LIST OF PLATES PLATE 2.1 Hand specimen of the Zandpan marker ............... 2l. PLATE 2.2 Hand specimen of the Upper Vaal Reef Zone ......... 2l. PLATE 2.3 Hand specimen of the Vaal Reef Basal Contact ...... 25 . PLATE 2.4 Hand specimen of the Mizpah Reef ...... ........... 25. PLATE 4.1 Death Valley, California, U.S.A ................... 76. CHAPTER ONE INTRODUCTION AIMS OF THE PROJECT The purpose of this project is not only to compile and assimilate the vast amounts of diverse data relating to the Vaal Reef Placer, but also to attempt to refine the current depositional model and apply this knowledge in a practical way within the production environment. To this end four goals or targets were set with the full knowledge that the achievement of such would doubtless create further questions or problems to be solved at a latter date. These targets were as follows:- 1. Sub-divide the Vaal Reef placer (VRP) into its constituent units and create a database of information using a 500 metre grid across the South Division area. 2. Produce a localised (Vaal Reefs South Division) facies/lithofacies plan defining areas of different "Reef Type", for the benefit of mine planning and the optimisation of stope width control. 3. Contour individual uni ts and investigate the possibilities of separate populations vertically and horizontally, with a view towards establishing more detailed trend directions. 4. Refine the depositional model for the VRP and comment on any changes that may be necessary to the method of statisticaly evaluating the ore zone. Investigate ways in which a more knowledgeable approach could be put to good effect in the evaluation of Remnant pillars in particular and new mining areas in general. 1 INTRODUCTION THE WITWATERSRAND BASIN The Witwatersrand basin extends from Johannesburg in the North to The Orange Free State in the South, forming an elongate basin some 320 km along its axis . The nature of the sediments within the upper part of the basin has led many workers to propose several entry pOints to the basin each forming its own discreet "Gold-Field". The Volcano-Sedimentary package wi thin the Witwatersrand basin is composed of three groups, the Dominion Group, the Witwatersrand Super-Group and the Ventersdorp Super­ Group. The Witwatersrand Super-Group is of the most interest in this particular instance (Figure 1.1) and is in turn sub-divided into the West Rand Group (lower portion) and the Central Rand Group (upper portion) . The majority of the gold producing placers occur within the Central Rand Group (CRG); the Vaal Reef Placer being no exception. The CRG attains a maximum thickness ' of 2880 metres in the Vredefort area, (Minter 1982) and consists predominantly of coarse grained Sub-greywackes with less than 10% conglomerates. THE KLERKSDORP GOLDFIELD Situated at the western edge of the Witwatersrand basin, some 160km WSW of Johannesburg, the active portion of the Klerksdorp Gold-Field is defined by the Vaal Reefs, Hartebeestfontein, Stilfontein and Buffelsfontein Gold Mines. Contiguous lease areas extend the aerial extent of the Gold-Fi eld to approximately 450Km2 (Figure 1.2). 2 THE WITWATERSRAND SUPERGROUP 1IIlIIIUS a I(OS ~. 'OIll ..... f1O/<f! ' Oil _nOlI! "["Ulil • • IrOS 1_ ...., ...;~ iI! ,- 1 ..··- 1M ( I,. SlU_ IIUH I ,." I'M IKI¥U .... t .. , IIU' ":,!;'f.' ! E\,.SII,M '" ~;;\ ILU",AO III", :'" . ~!. IIIIVU litH! . ' i~ ...:.: 01,.(.11.,00II" :.. 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