HIERDIE EKSEMPLAAT ' GEEN c m s t a n d i g h e : I BIBLIOTEEK VERWYDE?. J D U S - S R S O L - B I B L I O T E E K 0 0 9 5 4 2 8 111092712001220000018 THE RELATIONSHIP BETWEEN MINERALISATION AND STRUCTURE IN THE PILGRIM'S REST - SABIE GOLD-FIELD by ANDRIES LOUIS Z IETSMAN Dissertation submitted in fulfilment of the requirements for the degree of Doctor in the Faculty of Science, of the University of the Orange Free State, Bloemfontein. NOVEMBER, 1967. HIERDIE EKSEMPLAAR MAG ONDER GEEN OMSTANDIGHEDE UIT DIE BIBLIOTEEK VERWYDER WORD NIE Htmversiteit van die (F)ranje- ï ï rystait BLO E M FO ;TTEIN KL^ No. BIB LIO TE E K ABSTRACT The area that was i n v e s t i g a t e d , covers the m e t a l l o g e n e t i c p rovince of the S a b i e - P i l g r i m 's Rest G o l d - f i e l d in the Eastern Transvaal. The geolog i c a l formations include the Basement Granite, Godwan Formation, UJolkberg Formation, and the T r a n svaal System. D i a basic and p y r o x e n i t i c sills, p r e - m i n e r a l i s a t i o n in age, and probably genet i c a l l y related to the B u s hveld Complex, are present also. T hree g e n e r a t i o n s of dykes can be distinguished, one before, one c o n t e m p o r a n e o u s with, and the other after the mine ralisation. Three types of folding are present, i.e., tectonic folds, s u p r a t e n u o u s c o m p a c t i o n - f o l d s , and folds that are due to magmatic intrusion. At least two perio ds of tecto nic folding can be distinguished, the one super i m p o s e d upon the other. The axes of the earliest folds trend a p p r o x i m a t e l y east, while those of the later folds trend north and northeast. Both periods of folding are due to compr e s s i o n a l forces which can be a s c r i b e d to the i n t r us ion of the B u s h v e l d Complex. Both t e c tonic and n o n - t e c t o n i c faults are encountered. The te ctonic faults consist of low-an gle t h r u s t - f a u l t s , low-angle g r a v i t y - f a u l t s , and high-a n g l e normal faults. The thrusts are p robably due to the same c o m p r e á s i o n a l forces that were r e sponsible for the Viorth-iand northeast- trending f olds i . e they are prob ably related I to the i n t r u s i o n o f t h e B u s h v e l d C o m p l e x as well. ........ ...../.Evidence... (ii) Evidence of g r a v i t a t i o n a l g l i d i n g - t e c t o n i c s is provided by the presence of low- angle gravity- faults. These structures po stdate the thrust- faults. The high -a n g l e normal faults are due to te n s ional stresses and they are mostly post- m i n e r a l i s a t i o n in age. A remark a b l e d irectio nal r e l a t i o n s h i p between the linear st ruct u r e s and the folds is noticeable. The linear features, as uiell as the axes of the folds are orien tated in three directions, namely, north, east and northeast. The g o l d - b e a r i n g reefs consist of i n t e r b e d d e d as well as, tran sgressive, epigenetic ore-bodies. The i n t e r b e d d e d reefs are r e presented by b edding -thrusts. The l o c a l i s a t i o n and d e v e l o p ­ ment of these reefs mere contr o l l e d by the presence of favourable horizons and the i n t r u s i o n of sills, folding, especially sup eri m p o s e d folding, as well as the f o r m a t i o n of the b eddin g - t h r u s t s . The condit i o n s necessary for the e m p l a c e m e n t of the t r a n s g r e s s i v e reefs were the presence of early planes of w e a kness and/or dykes which are pre-—-- reef in age. The l o w - a n g l e g r a v i t y - f a u l t s resulte d in the opening of p r e - e x i s t i n g fractures such as early hig h-a n g l e faults, joints and dyke- c o n t a c t s . Two d i s t i n c t l y differe nt periods of m i n e r a l i s a t i o n are visualised, one pr eceding and the other postdating the b e d d i n g -th rusts. The first period is seen as a process of contact- m e t a s o m a t i s m , caused by the i n t r u s i o n of the basic sills. This process was probably /localised. . * (iii) locali sed and controlled by 'the folding. The second period of m i n e r a l i s a t i o n was one of f i s s u r e - f i l l i n g by so lutions which mere introduced during two phases? an early py rit e - r i c h phase, which was followed by a later copper- and bismuth- rich phase. The b e d d i n g - t h r u s t s , as well as the l o w - ang le g r a v i t y - f a u l t s , acted as c h anne l-ways for the m i n e r a l i s i n g solutions. The second period of m ineral isation, which produced ore-m i n e r a l s that are typical of m e s o t h e r m a l deposits, is v i s u a l i s e d as a m e t a l ­ l iferous front o r i ginating from the "root" of the Bushv e l d Complex. The ore-minerals, including gold, are considered to have been derived from the m a g m a - c h a m b e r itself, as well as from the adjacent s e d i m e n t a r y rocks. The deposits of c h r y s o t i l e in the area are c o n s i d e r e d to have been l o c a l i s e d due to a similar structural control as that w hich influ enced the l o c a l i s a t i o n of the i n t e r b e d d e d reefs, i.e., f o l d i n g . (iv) CONTENTS Page ABSTRACT ..... .......... ............. . ... (i) LIST OF I L L U S T R A T I O N S .................... . ( x j . i ) .1 , I N T R O D U C T I O N ........................... 1 A. GENERAL STAT EMENT ............. B. HISTORY OF G O L D - P R O D U C T I O N 2 c. PREl/I OUS G E O L O G I C A L WORK 5 GENERAL GEOLOGY ..... . ......... 7 A. B A S E MENT GRANITE ........ ...... 7 B. SEDIME N T A R Y ROCKS ........ . 8 1 . General Statement . . . . . ........ 8 2 , Godwan F o r m ati on ................ 9 3. UJolkberg F o r m ation ........ . o 4. T r a n s v a a l System ........ . 9 (a) Genera l ........ * .......... 9 (b) Bl ack Reef Series ........ 1 0 (c) D o l o mite Series ........... 10 (d) Pretoria Series ........... 13 C. INTRUSIVE ROCK ................. 13 1. Sills .......................... 1 3 2. Dykes ......................... ,15 III . S T R U C T U R A L GEOLOGY ................ 17 A. STRUCT URES IN THE FLOOR OF DEPOSI T I O N 17 1. Pr e-Go d w a n Surface ....... . 17 2. Pre-Godujan Linear S t r u c t u r e s .... 19 B. STRUCT U R E S IN THE SEDIMENT ARY F ORMAT IONS 2 0 1. G e n e r a l S t a t ement ......... 2 0 (a) Dip and Strike ...... 2 0 (b) Linear Struc t u r e s „.. 20 2. Folds . . ..... ....... . 21 (a) N o n - t e c t o n i c Folds... 21 1—I (v) (b) T e c t o n i c Folds .................. .. <. 23 (i) Gene ral ....... .........*».» 23 ( U ) D i s c u s s i o n ............ *.» . 30 (c) Folds Due to lYlagmatic Injection,, 31 3. Faults ...................... . c » 32 (a) N o n - t e c t o n i c Faults .......... .. <,. * 32 (b) T e c t o n i c Faults ........ c 34 (i) Loui-angle T h r u s t - f a u l t s c 34 (ii) Loiu-angle Normal Faults „ 35 (iii) H i g h - a n g l e Normal Faults™ 37 4. 3oints .............................. ............... 41 (a) N o r t h e r n Area ............ ................ 42 (b) C e n tra l Area ............... ... ........... 42 (c) S o u thern Area ................... ..........42 5. Dykes < 44 6 « D i s c u s s i o n ................... „ 46 (a) S t r u c t u r a l I n t e r r e l a t i o n s .............46 (b) Structural Evolu t i o n of the T r a n s v a a l System ............. , 46 II/. M I N E R A L I S A T I O N ......................................... 51 A. GENERAL STATEMENT .............................. 51 B. G O L D - B E A R I N G 0RE-B0DIE S ...........«... 52 1. C l a s s i f i c a t i o n .............................. 52 2. Interb e d d e d Reefs ................... * 53 (a) G e neral C h a r a c t e r i s t i c s ..... ......53 (b) Minera l o g y of the Ore 58 (i) O r e - m i n e r a l s ......... 59 (ii) Supe rgene Sulphide- minerál s 63 (iii) Sup ergene O x i d e - m i n e r a l s 64 (vi) Page (iv) G a n g u e - m i n e r a l s ........ 64 (v) Pr ecious Metals ....... 67 (vi) R e l a t i o n s h i p b e tween Reef- t hickness and G o l d-content. 6 8 (vii) A l t e r a t i o n of the Ulaal-rock. 69 * 3. T r a n s g r e s s i v e Ore-bo d i e s ........... ......73 (a) C l a s s i f i c a t i o n ........... ............ 73 (i) Vertical Reefs ........ 73 (ii) L e a d e r - r e e f s ............ ......73 (iii) D y k e - r e e f s ............. .......73 (iv) Blows ................ ...........74 (b) General C h a r a c t e r i s t i c s ..... 76 (i) S t r a t i g r a p h i c a l R e l a t i o n ­ ship. ....................76 (ii) A g e - r e l a t i o n s h i p ...... 77 (c) lYlineralogy ............. ............... 77 (i) O r e - m i n e r a l s 7 7 (ii) G a n g u e - m i n e r a l s .......« 79 (iii) Precious Metals ........ ......79 (d) A l t e r a t i o n of the UJall-rock .. 81 (e) C l a s s i f i c a t i o n of the Tr ixie Lode ............... ......................82 4. D i s c u s s i o n ................................... 83 (a) First Period of M i n e r a l i s a t i o n 83 (b) Second Period of M i n e r a l i s a t i o n 87 (i) G e nera l Stat ement ..... 87 (ii) Sequenc e of M i n e r a l s .. 8 8 (iii) Seque n c e of Events .... 90 (c) P r e c i p i t a t i o n of Gold and Silver 91 C. A S B E STOS ... 92 1. D i s t r i b u t i o n ...........................92 (vii) Page 2 . lïlode of O ccurrence ................. 9 3 3. R e l a t i o n s h i p to G o l d - d e p o s i t s ... 9 4 \l. ORE-C O N T R O L ................................ 96 A. GENERAL STATEMENT ................... 96 B. ORE-DEPOSITS RELATED TO THE VAALHOEK AND DIENT3IE DYKES (GROUP I)... 100 1. Deposits at Bourke's Luck .... 100 (a) Geological Environment .. ̂ 100 (b) Dientjie D y k e ............... 1 0 1 (c) Ore-bodies ................. 102 (i) Trixie Lode ....... 102 (ii) B o u r ke's Luck Reef. 105 (iii) Fern Reef ......... 105 (iv) Conta ct Reef and Stock- ujork ................. 105 (v) Dyke Reef ......... 106 (d) D i s c u s s i o n ............. . 106 (i) O r e - c o n t r o l ....... 106 (ii) Sequenc e of Events. 107 2. Vaalh o e k [Yline ..................... 109 (a) Geolog i c a l E n v i r onment ,,,, 109 (b) Vaalhoek Dyke ............... 109 (c) Struc tural Features ....... 110 (d) Ore-bodies ................... 112 (i) Vaalho ek Reef ..... 112 (ii) Thelm a Reefs ....... 114 (e) D i s c u s s i o n ................... 115 3. lUillemsoord UJest lYline ........... 115 4. Frankfort lYline .................... 116 (a) Geolo g i c a l Enviro n m e n t ... 116 (v/iii) Page (b) S t ructural Features ........... 117 (c) Q re-b odies ....................... 118 (i) Frankfort Reef ........ 118 (ii) Bevett's Reef ........ 119 (iii) Shale Reef ............. 119 (d) Discus s i o n ....................... 119 5. Other Occurrences Along the Vaalhoek Dyke ..... .................................. 120 (a) Leaders Near the Horizon of the Portuguese Reef ......... . 120 (b) M i n e r a l i s a t i o n Beloui the Frankfort Reef . ..................... 121 0RE-DEPOSITS RELATED TO THE NESTOR STRUCTURE (GROUP II) ................ ........... 122 1» Glynn's Reef South of Sabie ............ 122 (a) S t r a t i g r a p h i c a l R e l a t i o n s h i p ...... 122 (b) S t ructural Features ................... 123 (c) C h a r a c t e r and M i n e r a l o g y of the Ore-body ............ ..................... 124 2,. Rietvallei Mine .............................. 125 3. Sandstone Reef ..... .......................... 126 (a) S t r a t i g r a p h i c a l R e l a t i o n s h i p ....... 126 (b) S t ructural Features ................... 126 (c) C h a r a c t e r and M i n e r a l o g y cf the Ore-bo dies .............................. 127 4. Area Around Mac-Mac ....................... . 129 5 „ lllalidyke Mine ................................ 130 (a) Genera l Character of the Or e-b o d i e s 130 (b) S t r a t i g r a p h i c a l R e l a t i o n s h i p ....... 130 6 . Area Around the Lisbon Falls ........ 131 (a) S t r a t i g r a p h i c a l R e l a t i o n s h i p s ..... 131 (b) S t ructural Features .............. . 133 (c) Ore-b o d i e s .............................. 136 (ix) Page (i) Pigeon Reef ............ ....... 136 (ii) Betty Reef ........... 137 (iii) Gould's Reef ......... 137 (iv) Bott's Reef ........». . 137 (v) Astra Reef .............. ....... 138 (v/i) L e a d e r - r e e f s ........ „ . » 139 7. Eendrag lYline ....................... ... 140 8 . Hoyer's and UJalther's W o r k i n g s ... 141 9. Neuj Chum lYline ......................... ....... 142 (a) G e o l o g i c a l E nvir onment 142 (b) Ore-bo d i e s ....................... 143 (c) Structur al Features .................. 144 10. Other Occurr e n c e s of Group II .... 145 C. THE O R E - D E P O S I T S AROUND PILGRIM'S REST (GROUP III) ............................................. 146 1. G e nera l Statement ............. .......... ........ 146 2. G e o l o g i c a l E n vironment ................... ....... 146 3. S t r u c t u r a l Features ............... ...............147 (a) Folds ........................................... 147 (b) L a c c o l i t h i c S t r u c t u r e s ........ . <, <>. 148* (c) Linear St ructures ........................... 149 4. Ore -bo d i e s ................................. * 150 (a) lYlohlalabu Reef ........................ ....... 150 (b) Po rtug u e s e Reef ................. .... 150 (c) Slate Reef .................................... 153 (d) Beta Reef ............................... ....... 153 (e) Lo wer Theta Reef ....... .....................156 (f) Upper Theta Reef ............... ..... 161 (g) B e v e t t’s Reef .................... ............ 162 (x) Page (h) Shale Reef ................... „ 0 163 (i) Leade r - r e e f s ................ . . 165 D. ORE-DE P O S I T S OF GROUP III ................ .........165 1. General Statement ..................................165 2. Ore-bodies on Elandsdrift 220 3. T ..... ........166 (a) S t r a t i g r a p h i c a l R e l a t i o n s h i p s ... 166 (b) Ore-bodies and Struct ural Features 167 (c) Min era l o g y ............................ ........169 3. Ross Hill Mine ..................................... 171 4. Mount A n d e r s o n Area ..... ........................ 171 E. O R E - D E P O S I T S OF GROUP V .................... ....... 173 1. Occurr e n c e s in the Northern Area . 173 2. Occ urr e n c e s in the Southern Area,, 175 F. VERTICAL REEFS (GROUP VI) .... ........... ....... 176 1. General Sta tement .................... ....... 176 2. Leader Hill Mine ............................. 176 (a) Glynn's Reef .................... ....... 177 (b) L e a d e r - r e e f s .................... ....... 177 (c) Spitzk op O r e - c h a n n e l ......... ....... 178 3. R i e t f o n t e i n Reef ............................. 178 4. Sunlight Reef ................................. 180 5. Hepta Reef ..................................... 181 6 . S t r u c t u r a l C o n s i d e r a t i o n s ......... ....... 181 G. D I S C U S S I O N ........................................ ....... 182 1. Interb e d d e d Reefs ......... .......... ....... 182 (a) F avoura ble Horizons and Sills 182 (b) Folding ........................... ....... 183 (c) Refol ding ............................... 184 (d) B e d d i n g - t h r u s t s ............. . „ 184 (xi) Page (e) Loui-angle G r a v i t y - f a u l t s ....... 184 2. T r a n s g r e s s i v e Reefs .................... 185 (a) Hummocks ............................ 185 (b) Early Dykes ........................ 185 (c) Louj-angle G r a v i t y - f a u l t s ....... 186 3. O r e - c h a n n e l s .............................. 186 VI. GENESIS OF THE ORE ........................... 188 VII. CONCLUSIONS ........................................ 192 VIII. A C K N O W L E D G E M E N T ................................... 195 IX. REFERENCES .......................................... 196 (xii) LIST. OF I L L USTRATIONS FIGURES .Figure p aqe 1 « K e y -map showing area i n v e s t i g a t e d .. 3 2. R e e f - c o n t o u r s in Centra l Area ....... 26 3. Plan of r e e f - c o n t o u r s in the Southern Area ......... ........ 29 4« Schematic section through la ccol i t h i c sill on Columbia Hill ........ ......... 33 5. C o n t o u r - d i a g r a m s of normal faults .. 40 6 . C o n t o u r - d i a g r a m s of joints ...................4 3 7. C o n t o u r - d i a g r a m s of dykes ............ ........4 5 8 . C o n t o u r - d i a g r a m s of linear s tru ctures 4 7 9. Sectio n of Beta Reef in the Beta Mine 56 10. R e l a t i o n s h i p between g o l d - c o n t e n t and th i c kness of reef, section across Peach Tree Mine .................... „ . ...... . 7 Q 11. S c h e m a t i c sections i l l u s t r a t i n g w a l l -rock a l t e r a t i o n in dolomite and shale .... 72 1 2 . D i a g r a m m a t i c sections illust r a t i n g differ e n c e betwe en tabular blows and ore-ch a n n e l s ........................... 75 13. S c h e ma tic t ransverse section across T r ixi e Lode at 93 Raise, Burk e's Luck Mine ..........................80 14. The l o c a l i s a t i o n of g o l d - d e p o s i t s in the S a b i e - P i l g r i m ' s Rest Area ........ 98 15. S c h e matic section across the Dientj ie Dyke ................................ 103 1 6 . (a) Sectio n across the V/aalhoek Dyke in the Nek Section ...................... \ m (b) Section across the Vaalhoek Dyke in the River Section ........ Ill 17. G e n e r a l i s e d sch ematic secti on at the Lisbon F a l l s ............................... 132 (xiii) Figure Page 18:. S chematic section across the Astra Reef .................. . 134 19. S k e t c h - p l a n of diaba s i c dykes and leade r - r e e f s in the New Chum Mine 144 2 0. Plan and overlay showing reef- contours and r e e f - t h i c k n e s s on the P ortuguese Reef in the north- section of the J u bil ee Mine 152 2 1 .(a) Section, north-south, showing low-an gle fault in D u k e’s Hill Mine ................................ 160 (b) Section, east-west, showing low- angle fault in Duke's Hill Mine 150 2 2. T r a n s v e r s e sec tion through the E lands d r i f t Blow ..................... 170 23. S c h e matic section through Spitzkop O re-ch a n n e l ....................... 1 7 9 PLATES (In folder) Plate I . Map showing the s tructur al geology of the S a b i e - P i l g r i m 's Rest gold- field ............... I I . G e o l o g i c a l columns showing relative thickne ss of strata and lateral v a r i a t i o n from south to north ..... III. U n d e r g r o u n d plan of the V a a lhoek Mine XV. Mines and workings on the Sandsto ne Reef ........................... V. Mine and workin gs south of Sabie..... I/I , Un d e r g r o u n d plan of the Poni es k r a n t z North Mine ............................... . VII . Mines and workings on the Portugues e R e e f . VIII . Mines and worki n g s on the Beta Reef... IX. Section of the Beta Reef on the 18th level in the Beta Mine ............. . X. Mines and worki n g s on the Theta Reef.. X I . Mines and workings on the Upper Theta, B e v ett's and Shale Reefs ................ X I I . Mines and w o r k i n g s on E l a n d s d r i f t 220 0. 1. 1 • INTRODUCTION A. GENERAL STATEMENT As a result of dwindling ore-reserves in the mines of the Transvaal Gold Mining Estates, Limited, a geological research programme was considered necessary,, This work was commenced towards the end of 1961. It consisted of detailed geological mapping, an intensive study of mine records and underground plans, as well as geochemical prospecting in certain areas» These operations were undertaken by the geological department of the parent company, Rand Mines, Limited. The writer, assisted by. a field party, was called upon to do the geological work. The purpose of the project was to determine the influence of geological features on the mineral­ isation. With the kind approval of the Management of. Rand Mines, the writer was able to submit a thesis for the degree of Master of Science, at the end of 1964 (Zietsman, 1964). In this thesis, the general geology of the gold-field is dealt with. Special emphasis is placed upon the stratigraphical features in the area. Certain relationships between the geological structure and the gold-bearing deposits became apparent, even during the first stages of this investigation. Research work on these relationships was continued during 1965. It is the purpose.of this dissertation to present a detailed account of these relationships, as well as their influence on the ore- genesis and ore-control. / T h e individual ...2. - 2 - The individual occurrences of gold-bearing deposits are described in detail. Most of the mines, however, ceased production before 1961. Many of the workings had collapsed and were inaccessible to under­ ground examination. Some of these inaccessible mines have been admirably described by other authors. Many old records were available in the mine-offices of the Transvaal Gold Mining Estates. The underground plans of the old mines were found to be extremely useful. The area that is dealt with in this dissertation is shown in Figure 1. Plate I is a structural map of this area; the locations of the mines and workings are also shown on this map. B. HISTORY OF GOLD-PRODUCTION Gold was first discovered in alluvial deposits on the farms Spitzkop 195J.T. and Hendriksdal 216J.T., south of Sabie. These discoveries were made in 1872 by Messrs. MacLachlan, Valentine and Parsons (Reinecke and Stein, 1929). In 1873 another discovery was made at Mac-Mac by a certain Mr. Jan Muller and his son. In the same year, following this discovery, a certain Alec Patterson found rich alluvial gold along the Pilgrim's Creek. During the first few years, only alluvial deposits were exploited, and some large nuggets were found. The most famous of these were the "Breda" and the "Lilly" \ weighing 214 and 119 ounces 2 dwts. respectively. During the year 1881 a Mr. D.H. Benjamin secured /concessions.... - -4 - concessions from the South African Republic over the farms Poniesíerans, Ledovine, Waterhoutboom, Driekop, Grootfontein 562K«,T-0 and Belvedere. Similar concessions . were later granted over Handriksdal, Elandsdrift and Spitzkop. The following mining-companies were formed: 1 The Pilgrim»s Mining and Estate Company, The Jubilee Mines, Limited, the New Clewer Estates and Gold Mining Company and The Transvaal Gold Exploration and Land Company. These companies were purchased in 1895 by the Lydenburg Mining Estates, Limited, and in 1896 the name of this company was changed to the Transvaal Gold Mining Estates, Ltd. The latter company is still in operation. At Sabie the Glynn's Lydenburg, Limited, and Glynn's Pretoria Gold Mining Company were formed, as well as other smaller companies. These were later absorbed by the Transvaal Gold Mining Estates, Limited. In the years following the formation of the mining companies, activities gradually shifted from the stream- beds to the slopes of the hills where the "mother lodes" were discovered. The total production to date cannot be accurately determined. This is due to the fact that comprehensive records of the production of gold are not available. Visser and Verwoerd (i960, p.104) give a figure of 2,105,473 fine ounces for the period 1902 to 1958. The total gold-joutput from the mines belonging to the Transvaal Gold Mining Estates, is in the vicinity of 3,900,000 fine ounces. This was extracted from over 12,200,000 tons of ore. No records are available of the production / o f a l l u v i a l . . . 5 f ■ - ■ - 5 - of alluvial gold. Since as many as 2000 diggers were at times.Operating in the-Pilgrim's Creek, this output must have been considerable. C. PREVIOUS GEOLOGICAL WORK A large number of authors have made contributions regarding the geology of the Sabie-Pilgrim's Rest Gold- field. The present research is, however, the only comprehensive study of the structure and it's influence on the mineralisation. Many controversial theories and ideas regarding the genesis of the ores have been presented. The various authors either disregarded the influence of the structure on ore-control, or based their conclusions on a misinterpretation of it. Cohen (1873 and 1874-1875), Penning (1885), O'Donoghue (1884), Kuntz (1896), Wilson-Moore (1897), Nicol-Brown (1897) and Thord Gray (1905) were among the first to comment on the geology of the gold-bearing deposits. These publications are, however, considered to be of historical interest only. The first detailed description of the geology and the ore-bodies, was presented by Hall (1910). Hall's' •description of the geology and his map remained the standard work on the subject for many years. Much of his classification of the rocks is still recognised. His descriptions of some abandoned mines are extremely valuable. Von Dessauer (1912) concluded that the bedded reefs were metasomatic replacements of siliceous layers / in t h e ...6. in the sedimentary strata» He also suggested that the vertical reefs were feeders for the bedded reefs. Frost (1912) described the characteristics of the ore-bodies of the Mali Dyke Mine. Bell (1921) gave a general description of the ore-bodies and their stratigraphical relationships. His description of the Language Reef is very useful since these workings were inaccessible to later observers. Wvbergh (1925) gave an account of the economic geology. He added much to the general information and also produced a more detailed geological map. Reinecke (1926) made an intensive study of the farm Elandsdrift 220J.T. for the Transvaal Gold Mining Estates, Limited. Some of his results were published in collaboration with Stein (1929). These authors ascribed the mineralisation to the intrusion of a younger batholith of granite into the sedimentary rocks. This theory was conclusively disproved by later authors (Barnard, 1958 and Visser and Verwoerd, I960, p . 34). Reinecke and Stein, however, observed some very in­ teresting relationships between the mineralisation and the structure, particularly on the farm Elandsdrift 2 2 0 J.T. Swiegers (1948) made an intensive study of the ores of the gold-field. This study can be regarded as the standard work on the paragenesis of the ore-minerals. Barnard (1958) made a comprehensive study of the geology and structure of the Bourke's Luck mine. His / c o n c l u s i o n s . ..7 - 7 - conclusions regarding the paragenesis of the ores at Bourke's Luck, are in general agreement with those of Swiegers. The most recent publication on the geology of a part of the area, is that by Visser and Verwoerd .(i960). This work is a compilation of the observations made by Joubert, SShnge, van Zyl, Muller and Verwoerd between 1936 and 1959. This work deals only with the mines around Sabie and Mount Anderson. 1 1 . GENERAL GEOLOGY The general geology of the area has been described by Zietsman (1964). Plate II shows the general sequence of the beds and their stratigraphical relationships, as well as the relative positions of the interbedded reef-horizons. A . ' BASEMENT GRANITE The floor on which the sedimentary beds were laid down, is composed of granite, which is classified as Nelspruit granite. It is a massive, equigranular rock that tends to have a somewhat gneissose texture locally. A sheetlike cover of altered, sericitised granite, 5 to 50 feet in thickness, is present locally (Visser and Verwoerd, 1960, pp. 32 - 33). These authors consider the alteration to be due to weathering that took place before the deposition of the overlying sediments, followed by hydrothermal alteration after the sedimentation. / T h e ...8. \ - 8 - The sedimentary rocks were deposited on an uneven surface of granite. The presence of an un­ dulating topography is borne out by surface mapping and by certain borehole records. Reinecke and Stein (1929, p.73) interpreted this feature as being evidence of an intrusive relationship with the sedimentary rocks, and they therefore considered the granite to be a batho- lith of post-Transvaal age. Later work by Barnard (1958, p . 5) and Visser and Verwoerd (1960, p.34) conclusively prove the presence of a sedimentary contact between the granite and the overlying sedimentary rocks. B. SEDIMENTARY ROCKS 1. General Statement The sedimentation commenced with the deposition of the Godwan Formation. The depositional basin had a shoreline near Sabie (zietsman, 1964, P[i 1 1 - 1 2 ), but deepens northwards to a depth of 1200 feet at Bourke's Luck (Plate II). South of .this shoreline the Godwan Formation is present in small local basins only. The Wolkberg Formation follows un- ■.:; conformably on the Godwan Beds and is confined to the main Godwan Basin. The Black Reef Series of the Transvaal System follows unconformably on all the older rocks and.rests in turn on Basement Granite, Godwan Formation and Wolkberg Formation. The Dolomite Series of the Transvaal System follows conformably / o n the ... 9. on.the Black Reef Series, except in a few isolated areas south of Sabie, where it rests directly on granite. This is due to local elevations in the floor of. deposition. Godwan Formation The Godwan Formation usually has a basal layer of conglomerate and/or arkose. This is followed by agglomerate, lava and tuff. From Klipkraal 170JoT. northwards, the formation varies between 100 feet and 400 feet in thickness. Wolkberg Formation This formation is composed of a layer of felspathic quartzite sandwiched between two thick layers of shale. At B o u r k e’s Luck a layer of quartzite overlies the upper shale. The Wolkberg Formation is as far as gold- bearing deposits are concerned, not very important economically. Some bedded ore-bodies occur near the lower contact of the upper quartzite-member. These deposits have, however, not been exploited. A few vertical reefs are developed in the upper quartzite, as well as in the upper shale-member. Some conglomerates in the Wolkberg ' Formation are known to contain gold in economical quantities, i.e., in the old Haenertsburg Gold-field (Hall, 1907, pp. 33-60) Transvaal System (a) General The Transvaal System is economically the / m o s t . ..10. - 10 - most important stratigraphical unit. It consists of the Black Reef Series at the base, which in turn is overlain by the Dolomite Serieso The Pretoria Series is, strictly speaking, a separate system ( Zietsman, 1964, p. 13) and constitutes the top of the formation. Only the Timeball Hill Stage is present in the area under consideration, (b) Black Reef Series The Black Reef Series consists of a single layer of quartzite that ranges from a few feet to 100 feet in thickness. This quartzite usually builds the edge of the Drakensberg Escarpment, and is exposed in dip-slopes and along stream-beds. This basal member of the Transvaal System contains exploitable interbedded reefs in a few' localities. Vertical reefs are present locally. (c) Dolomite Series The Dolomite Series occupies most of the area between the edge of the escarpment and the lower slopes of the high mountains to the west.. Between P i l g r i m’s Rest and Graskop, the Dolomite Series is capped by several outliers of the Pretoria Series. The basal contact of the Pretoria Series represents a major plane of unconformity. This is due to intensive erosion of the Dolomite Series before the deposition of the overlying Pretoria Series (Zietsman, 1964, pp. 12-13). As a result of this erosion / t h e ...11. " li - the Dolpnite, Series shows a drastic thinning from north?>.tpAsouth (Plate I I ) . The Dolomite Series is divided into six zones on lithological grounds (Zietsman, 1964, pp. 27-35); Zone 1 is approximately 1000 feet in thickness, and constitutes the base of the series. This zone contains the Transition Beds which include some layers of quartzite and shale, as well as dolomite and chert. The Blyde River Quartzite and the overlying Lower Shale form the top of this zone. Zone 2, approximately 350 feet in thickness, consists of dolomite which is relatively free of chert. Zone 3 includes the Middle Shale Group. This group consists of dolomitic l i m e s t o n e - w i t h intercalated chert, several layers of black car­ bonaceous shale, and an occasional thin layer of sandstone. The thickness is approximately 300 feet. Zone4, which is approximately 350 feet in thickness, consists of dolomite and chert, capped by a layer of black spotted slate, 1 - 3 feet in thickness. This layer of slate is locally referred to as the Slate Marker. The upper 50 feet of the dolomite is known as the Bread and Butter Dolomite. Zone 5 is approximately 800 feet in thickness, and is composed of dolomitic limestone with much intercalated chert of varying thickness. The Beehive Chert Marker, six feet in thickness, forms a very consistent marker. The Middle Chert Marker, / 2 - 1 0 feet ...12. - 12 - 2-10 feet in thickness, is another prominent horizon. It occurs about 150 feet above the Beehive Chert. Zone 6 constitutes the top of the Dolomite Series. It consists of dolomite, which is often oSlitic and relatively free of chert. Several layers of calcareous shale are present also. The basal layer of shale has a maximum thickness of about 20 feet. The deposition of these zones were followed by a major period of erosion. This resulted in a gradual removal of the various zones from north to south, with the consequence that only Zones 1 and 2 are present throughout the whole area (Plate II). The Dolomite Series is, economically, the most important stratigraphical unit. Most of the larger and more consistent interbedded ore- bodies are found in this formation. This indicates that the Dolomite Series was more susceptible to mineralisation. It is further­ more noteworthy that interbedded ore-bodies prefer certain stratigraphical horizons. The ore-bodies are usually localised along the same stratigraphical horizon, or within the same zone, but are not necessarily continuous along strike. The stratigraphical zones in which the interbedded reefs are normally located, are characterised by pronounced lithological variations. Furthermore, the interbedded reefs are found mainly in the proximity of shale and/or quartzite (Plate II). : * /(d) Pretqpia S e r ie s . . . 13. - 13 - (d) Pretoria Series i The Pretoria Series is represented by the Timeball Hill Stage, known as the Nooitgedacht Stage locally. This stage, builds the slopes of the high mountains that occupy the western part of the area. Outliers of Pretoria Series cap the dolomitic hills between Pilgrim's : Rest and Graskop. The Giant Chert, which is the basal member of the Pretoria Series, is a residual sharpstone conglomerate. It is followed by the B e v e t t’s Conglomerate and the Bevett's Sandstone. The thickness of this zone, including the Giant Chert, varies from 2-60 feet. The Lower Nooitgedacht Shale, 1300-2000 feet in thickness, overlies the Bevett's Zone.._ It is followed by the Lower Nooitgedacht Quartzite (50 feet in thickness), the Upper Nooitgedacht Shale (300-800) feet, and the Upper Nooitgedacht Quartzite (100 feet). Two economical interbedded ore-bodies are present near the base of the Pretoria Series, as well as one immediately below the Lower Nooitgedacht Quartzite. In the area west of Sabie, Visser and Verwoerd (i960, p p. 73- 7 6 ) describe interbedded reefs in the Daspoort Stage. C. INTRUSIVE ROCKS 1. Sills In the Sabie-Pilgrim's Rest area, two types of / i n t r u s i v e . ..14. - 14 - intrusij^siilisr are present (zietsman, 1964, pp> 47 _ 5o). The sills that are found in the Pretoria Series and in the upper parts of the Dolomite Series are composed of altered pyroxeni'':;.c material. These sills show signs of differen­ tiation. Porphyritic dunite and peridotite are present, locally. All the other sills in the Dolomite Series, as well as those in the older formations, are diabasic in composition. The sills show certain relationships to the structure and the mineralisation. A close stratigraphical association is evident between the sills and the interbedded reefs. Plate II shows that the same stratigraphical zones were preferred by both the sills and the interbedded reefs. These intrusions are often in close proximity, either above or below these ore-bodies. In the northern area (Plate II) sills are present immediately below the Bevett's Zone and below the Upper Shale. Lower down in the succession, sills are found below the Beehive chert Marker a n d :above the-State Marker Sills were intruded above and below the Blyde River Quartzite and above and below the Black Reef Series. In each of these zones, one or more interbedded reefs occur. This relationship is even more pronounced in the central area, particularly in the upper zones. The largest number of interbedded reefs are found in this area. In the southern area, sills are also found in the proximity of interbedded reefs . (Plate II). / A second...15._________________ - 15 - A second significant characteristic of the pyroxenitic type of sills, is their irregularity in shape and size. The sill near the base of the Pretoria Series (Plate II), transgresses repeatedly through the Bevett's Zone and is not confined to a single horizon. It often splits into two sills, one above and the other below the Bevett's Zone. It shows a tendency to pinch and swell, even over short distances. This intrusive locally assumes the shape of a laccolith. Laccolithic shapes are particularly well-developed in the central area on Columbia Hill and on Theta Hill. Another occurs in the northern area, west of the Frankfort Mine, just below the Lower Nooitgedacht quartzite. These intrusives are closely related to poorly developed anticlines. Well-developed interbedded reefs accompany the structures. Mineralised trans­ gressive fractures or leader-reefs usually occur in the sedimentary beds above the intrusives. Both the pyroxenitic and diabasic sills are considered to be almost contemporaneous with the mineralisation. Both types are probably genetically associated with the early plutonic phase of the Bushveld Complex. 2. Dykes The dykes are mainly diabasic in composition. Some exceptions are, however, present. The dyke along the Grootfontein Fault, west of Brown's Hill (Plate I), is considered to be more syenitic (Zietsman, 1964, p. 51). A dyke / w h i c h . ..16. - 16 - which crosses the Blyde River some 3 miles north of Pi l g r i m ,fs Rest, has a dioritic composition. Dykes consisting of hybrid rock include the Vaalhoek Dyke (Zietsman, 1964, p. 51) and the Spitzkop Dyke (Visser and Verwoerd, 1960, p. 47). Three distinct ages of dykes are present. Some dykes are obviously older than the mineralisation, and are displaced horizontally by interbedded reefs or low-angle faults, which are closely associated with the mineralisation. Vertical or steeply inclined ore- bodies are found in some of these early dykes, either along the contacts, or within the dykes. In some places vertical ore-bodies occur in the country-rock adjacent to these dykes. The Vaalhoek Dyke, the Dientjie Dyke, and the Nestor Dyke are all known to be pre-mineralisation in age. The New Chum Dyke is an example of a dyke which is contemporaneous with the mineralisation. The younger dykes are plentiful and post­ date the mineralisation. These dykes cut through the interbedded ore-bodies, often displacing them. Wybergh (1925, p. 103) observed two sets of post-reef dykes in the New Chum Mine. Von Dessauer (1912, p. 150) came to a similar conclusion in respect of the dykes that occur in the Frankfort Mine. /visser and...17 * The spelling used is according to the list of geological terms used by the Geological Survey of. South Africa. - 17 - Visáër and Verwoerd (i960, p. 42) records dykes that are pre-Godwan in age. They trend east and are found in the Nelspruit Granite, on either side of the Sabie River. The earlier dykes of post- Transvaal age, are' considered to be older than the diabasic sills; they are probably associated with the early volcanic phase of the Bushveld Complex. III. STRUCTURAL GEOLOGY A. STRUCTURES IN THE FLOOR OF DEPOSITION 1. Pre-Godwan Surface The sedimentary formations were laid down on an undulating surface of granite. This is borne out by surface-mapping and by drilling. Barnard (1958, p p . 45-48) observed a relationship between the irregularities in the surface of the granite and the general structural trend. These irre­ gularities are usually in the form of elongated domes or hummocks that are orientated parallel to the most prominent linear structures. The most prominent of these domes are found on the farms Lisbon 531 K 0T 0 and Berlyn 506 K.T® (Plate I). The western slope of the hummock is partly exposed at the bottom of the Lisbon Falls. A borehole, which was drilled in 1895 (Barnard, 1958, p. 47), some 2000 feet east of the waterfall, confirmed the presence of the structure. The shape of this ridge of granite is revealed by the structure of the overlying strata. The sedimentary / b e d s . ..18. - 18 - beds of the Godwan Formation, Wolkberg Formation and the Black Reef Series show compaction-folding around the h u m m o c k . The sedimentary dome thus formed, has an elongated shape, and has a long axis parallel to the major linear structures in the vicinity. Drilling revealed a similar irregularity at Bourke's Luck. A borehole near the intersection of the Dientjie Dyke and the Treur River, intersected granite at a depth of 450 feet below the top of the Black Reef Series. Farther north in the Blyde River Canyon, the top of the Black Reef Series is approximately 1200 feet above the granite. The dip of the sedimentary strata is not visibly influenced by this feature. The shape and orientation of this irregularity is, therefore, uncertain. At Kowyn's Pass, south of Graskop, the presence of an elevated floor was revealed by mapping. The basal conglomerate of the Godwan Formation has a dip of 20 degrees west-northwest. Higher up in the succession, and farther west, the normal dip is six degrees west. The long axis of this hummock is apparently parallel to the linear structures in the vicinity. On the farm Klipkraal 170 J.T., an inlier of granite reveals the presence of another hummock. The relationship between the granite and the overlying sedimentary beds suggests that its shape is that of a low ridge, the long axis / w h i c h . ..19. - 19 - of which is also parallel to the main structural direction (Plate I). Vertical reefs in the sedimentary r o c k s , as well as those in the granite, are apparently associated with hummocks such as those described above, 2. Pre-Godwan Linear Structures The linear structures include wrench™ fault s. and diabasic dykes. The wrench-faults are described as shear-zones and shear-faults by Visser and Verwoerd (i960, p p. 34-35) and are characterised by an abundance of anastomizing veins and tabular masses of quartz. The faults trend either north-northwest or north-northeast; the former displaces the latter (Visser and Verwoerd, I960, p . 38). The dykes trend in two directions, namely west, and north-northwest. The dykes are displaced by the north-northwest trending faults, but apparently not by those striking north-northeast (Visser and Verwoerd, 1960, p. 38). These linear structures probably influenced the formation of the irregularities in the floor of deposition, but had no direct influence on the mineralisation. /B. STRUCTURES...20. B . STRUCT U R E S IN THE SEDIMENTARY FORMATIONS General Statement (a) Dip and Strike Very little v a r i ation in the dip and strike of the s ediment ary beds is observed. The p r e - T r a n s v a a l beds have an a v erage strike that is slightly west of north. Dips ranging from 6 - 1 4 degrees west tuere recorded. The beds of the B l a c k Reef Series and the Dolomite Series strike north. The dip of these beds st eepens from one degree west, south of Hendriksdal, to six degrees west in the B o u rke's Luck area. The strata of the Pretoria Series strike slightly west of north, and their dip varies between 4-6 degrees west. The dip of all the strata inc reases gradually wes twards towards the B u s h v e l d Complex. Linear Structures The linear s tru ctures include joints, faults and dykes. Most of these features are post-Transvaal in age, although older struc tures are known to occur. East of Sabie a horst of p r e - T r a n s v a a l age, is r eported by Visser and l/erwoerd (1960, p . 55). Along the easterly one of the two faults bounding the horst, a r e c u r r e n c e of m o v e ­ ment took place after the d e p o s i t i o n of the Tr a n s v a a l System. Most of the faults are posfe-fnineral- . isation in age, but a few predate the mineralisat ion. The latter are re pres e n t e d by the R i e t f o n t e i n R e e f ( /the f a u l t ... 21. - 21 - the fault along which the Nestor Dyke was intruded, and the fault along which the Astra Reef, east of the Lisbon Falls, was emplaced. The inter b e d d e d reefs are bedding- faults. These m o v e men ts are c o n t e m p o r a n e o u s with the later phases of the mineralisation,, Joints are common and c onspi cuous features in this area. Three joint-trends, two of which are very prominent, are present. These s tructures dip between 80-90 degrees. A large number of dykes are present. They trend mainly north to northeast, although a few strike east to southeast. The dykes normally dip at high angles, a l t hough a few with low angles are present in the northern area. 2. Folds The folds can be g e n e t i c a l l y divided into three types, viz., n o n - t e c t o n i c , tectonic, and folds that are due to m a g matic intrusion. (a ) N o n - t e c t o n i c Folds The most c onspicuous fold of non-' tectonic origin is present on the farms Lisbon 531 K.T. and Berlyn 506 K.T. It is repres e n t e d by an elongated dome of s e dimentary rocks, f.olded around a graniti c ridge in the floor of deposition. The lowe rmost sediments, i.e. those of the Godwan Formation, as well as the lower shale of the UJolkberg Formation, terminate against /the slopes ... 22. - 22 - the slopes of this ridge. The upper part of the ridge is covered by the Middle Quart zite of the UJolkberg Formation. This horizon, as well as the beds overlying it, are folded around the ridge. The indivi d u a l s edimentary beds show distinct thinning towards the crest of the dome. The geometry of the folding can, therefore, be de scribed as suprat e n u o u s (Hills, 1963, p. 251). A geometry such as this is typical of c o m p a c t i o n - f o l d s . F u r t h e r ­ more, the intensity of the folding decreas es from the crest of the ridge upwards. I m med iately west of the crest, near the bottom of the Lisbon Falls, the strata dip about 30 degrees west, while higher up, the dip is only about 10 degrees west. Normal faults of minor dis pl a c e m e n t occur east and west of the long axis of the dome, and they are almost paral lel to it. These faults are consider ed to be geneti c a l l y related to the dome. This a s s u m p t i o n is based on the results of experi m e n t s in d i f f e r e n t i a l compa c t i o n (Nevin, 1953, p . 215). At Kowyn's Pass, the abnormal dip of the basal layers of the Godwan Form ation is consid e r e d to be due to c o m p a c t i o n - f o l d i n g (Zietsman, 1964, p . 39). The beds of the Godwan Format ion in Cooper' s Creek dip 14-20 degrees west, while those of the UJolkberg Formation dip 10-12 /degrees. ... 23. - 23 - degrees west (l/isser and Verwoerd, 1960, p . 55). Farther north and higher up in the succ ession the dip of the overlying strata is normal, i.e. 4-5 degrees west. Visser and Uerwoerd (1960, p. 55) ascribe the variation in dip between the G odw an and W o l k b e r g Fo rmations to an unconformity. This varia t i o n may, however, be due to c o m p a c t i o n-fol ding, related to the granitic hummock on K l i p kraal 170 D.T. lYleasurements of b e d d i n g - p l a n e s in inliers of quar tzite on London 496 K.T. reveal slight folding (Plate I). In this localit y the dip varies between one degree east and six degrees west. Several vertical faults, showing small displacemen ts, are present. One of these fractures is c onnected with the major fault west of the dome on Lisbon 531 K.T. On L e d o u p h i n e 469 K.T. a similar fold is visible in the quartzi te of an inlier of the Black Reef Series (Plate I). In this locality the dip varies from one degree east to four degrees west. The origin of these folds is not clear, but is considered to be n o n - t e c t o n i c . (b) T e c t o n i c Folds (i) G e n e r a l . The tectonic folds are r e p r e s e n t e d by very open anticlines, synclines, domes, and depressions. The tectonic origin of the folds is indica ted by the following c h aracteri sticss /1. lYlos t . . . 24 . - 24 - 1. Most of the structur es are develo ped s t r a t i g r a p h i c a l l y 2 0 0 0 feet above the g r ani te and they can the refore not be due to compaction. 2. The axes of the different folds are not haphaz a r d l y orientated, but trend in fixed d ir ections 3. These direct i o n s are closely related to the trends of other linear structu res in the gold-field. Due to the very open nature of the folds, they are usually not visible on the surface. The only l ocalit ies where tectonic folds were observed are in the Sabie Gorge, on Jubilee Hill, in the shale above the Duke's Hill Mine, a n d in a stream-b ed on the farm L e d o u p h i n e 469 K.T. Due to s u r f a c e - c r e e p and poor e xposures it is not pra cti c a b l e to measure bedding on surface. C o m p i l a t i o n s of reef c o n t o u r - p l a n s , however, reveal the presence of several open folds. T hese plans were compile d from old m i n e - r e c o r d s . In the northe rn area folds are present in the Vaalhoek Mine, as well as on Ledoup h i n e 469 K.T. The folds in the V a a lhoek Mine are, however, poorly developed! the reef cont ours seem to / indicat e ... 25. - 25 - indicate two dire ctions of folding (Plate III). In the southern part of the mine, a structu ral terrace is present. The axis of the a n t i c inal bend of this feature trends 026 degrees. Struct u r e s such as these, are typical in areas of mild d e f o r m a t i o n (Nevin, 1953, p. 38). In the River Section of the mine, another di rection of folding, viz., 060 degrees is indicated. A small anticline, the axis of which trends 067 degrees, is exposed in a s tream-b ed in the s o u t h - w e s t e r n part of the farm Ledouphine 469 K.T. (Plate I ) . In the central area, three direct i o n s of folding can be distingu ished, namely, 076-096 degrees, 047-057 degrees and 001-014 degrees (Figure 2). In the Ponie s k r a n t z North lYline, the contours on the P o rtuguese Reef suggest the p resence of a poorly developed antic l i n e which trends 057 degrees. In the Jubilee Mine, northeast- trending folds are d e v e lope d in the Portugue se Reef, as' well as in the Lower Theta Reef. ) Althoug h these two r e e f-hori zons are over 450 feet apart vertically, the folding is remarkably similar in both horizons. Figure 2 shows that the axes of ^ h e anticl i n e s in the Lower Theta Reef are superi m p o s e d on the axes of similar anticl i n e s in the Portug u e s e Reef. /This ... . F ~ iq u r e . 2, F i e c . f r-js ir\ C A r e a . S C a /& / : j q . q o q g r~6 ̂ ce • _________f* C o f\ 6 oc/ r- tirxe. or\ F * o r - éu jc/ e .s> e . /?ee/- --------- -------/S C,or\ £ ou r /,r\e Or\ 6< ̂ - ^ e e f ------- _ ---L~T f'*6cjtjr' //ne or\. /Cotv'e-/̂ 77ve^«ï /?ee^ ■ — — i: — IV lcJ /lices. — • — • — A / £ T / ^ o / d / ^ x e 5 . ----1----1----------AJ — S P 'o/d Ajc.es>. ̂ ? & £>tirr\a6e ,d éer^$/o/\ Á,//ves o Z7 A jt ,e s ( A r̂ é tc.//̂ \e s ). - .27 - This prob'ably i n d i cate s that the a x i a l - p l a n e s are vertical.. Folds that trend east are present also but the i n c l i n a t i o n of the axial planes could not be determined. In the Desire lYline, the trend of the folding is a p p r o x i m a t e l y east. The presence of two parallel antic l i n e s is re vealed by r e e f - c o n t o u r s on the Portug u e s e Reef (Figure 2). In the Theta lYline the stru cture of the Lower Theta Reef is c o n s i d e r a b l y disturb ed by sub-s u r f a c e solution and subsequ ent slumping of the dolomite. The presence of.a dome in an undist u r b e d area could, however, be d etermin ed (Figure 2). A p o o r l y .d e v e loped an tic l i n e is also present in the Beta Reef. Anticl i n e s occur.in the Lower Theta Reef, as well as in the Beta Reef in the Brown's Hill lYline (Figure 2). The axes of these struct u r e s trend a p p r o x i m a t e l y east; the axial- plane is a p p a r e n t l y vertical. In the Peach Tree, and Beta (Ylines, an t i c l i n e s that trend east are present in the Lower Theta Reef, as well as in the Beta Reef. Althoug h these re ef-h o r i z o n s are only 300 feet apart vertically, the structure in the Theta Reef is 1500 feet north of that in the Beta Reef. It seems therefore, that they repr esent two differe nt structures. In the Duke's Hill and the Clewer Mines two directio ns of folding _______________________________/ can be . . .20 «_______________ - 26 - can be deducted (Figure 2). The most prominent d i r e c t i o n trend 001-014 degrees. The presence of several domes and basins, however, indicate another d i r e ction of folding (De Sitter, 1964, p. 260). The axis of a large irreg u l a r basin s uggests on easterly trend. It is co ncluded that the e a s t - t r e n d i n g folds were deformed by those which trend 001-014 degrees, and this resulted in the format ion of the domes and b a s i n s . In the southern area, two direction s of folding are encountered, namely 000-024 degrees and 078-114 degrees. South of the Sabie River t h e.foldin g is reveale d by contour- lines on the G l yn n's and Eland s d r i f t Reefs (Figure 3). N o r t h - t r e n d i n g anticl i n e s occur in the G l y n n’s Lydenburg Mine, as well as in the eastern part of the Comp ound Hill Mine. B e t w e e n the latter mine and t h e . lYlalieveld Section, the contours indicat e the presence of.a s tructural terrace. The axis of the a n t i c l i n a l bend of this feature strikes north. Domes ore developed, in the northern part of the G l ynn's Lydenburg Mine., as well as in the Compoun d Hill lYline. A small dome is also present in the Elan d s d r i f t Mine. In this mine, the structure is d i s t ur bed by s ub-surface solution of the dolomite. An a n t i cline is v i sible in the gorge of the Sabie River. The axis of this structure / t r e n d s ... . - 30 - trends a p p r o x i m a t e l y north. Folding is also present north of the Sabie1- Riv e r . At the Nestor lYline two parallel a n t i c l i n e s occur, one in the southern and the other in the n o r thern section of the mine (Plate II/). The axes of these struct u re s trend 080 degrees. A donie, the long axis of which trends 003 degrees (Plate I \l) , is exposed at the Rex lYline . (ii) D i s c u s s j o n . There are three direct i o n s of folding, namely 078-100 degrees, 000-024 degrees, and 045-070 degrees. The axial traces of the latter two dir ect i o n s are usually straight, while those of the former are often curved (Figure 2). This ind icates more than one period of folding, the one s u p e r i m p o s e d upon the other (Ramsay, 1958, pp. 271-307). The g e o metry of the folds which resulted from the first period of folding is altered by the later folding. The folds that trend east are consi dered to have pr eceded the other two directions. B e cau se of the a p p a r e n t l y un defo r m e d character of the latter direct i o n s they may be related to either one or two di fferent periods of deformation, or they may have resulted from diffe r e n t pulses of the same period of folding. /The folds .. . 31. -31 The folds are concentric in character. Folds such as these are of necessity the reaction of a layered mass to c o m p r e s s i o n (De Sitter, 1964, p. 168). The folding may be g enetical ly related to the i n t r usi on of the B u s h v e l d Complex. The contours in Figure 2 do not give a complet e picture of the folding in the vicinity of Pilgri m's Rest, but shouu the st ruct u r e s in isola t e d areas only. Due to practical difficu lties, the results obtained from the r e e f - c o n t o u r s could not be s u p p l e m e n t e d by direct m e a s u r e m e n t s .in the field. It is considered, however, that the e a s t - t r e n d i n g axial traces are contin u o u s as shown by the s pecul a t i v e axial traces in Figure 2 . These lines are based on t o p o g r a p h i c a l features which are c onsidered to be due to structural features. The axial •traces of the ant icl i n e s coincide with the crests of precip t o u s spurs, which are bounded by streams i.e. Pilgrim's Creek, Tiger's Creek, Clewer Creek, Darke's Gully and Kam ee l ' s Creek. (c) Folds Due to lïlaqmatic Inje ction Domical struc t u r e s that are due to magmatic inje c t i o n are found in the central area, on Colum b i a Hill and on Theta Hill. In the northern area, a similar struct ure is found in the w e stern part of the farm Frank f o r t 509 K.T. The best example of this type of fold is present on C o l umbi a Hill. It is caused by the i n t r u s i o n of a p y r o x e n i t i c sill which locally / a s s u m e s . . . 3 2 „ - 32 - assumes a l a c c o l i t h i c shape (Figure 4). A similar i n t r usion is present on T h e t a Hill. On Frankfort 509 K.T. similar doming was observed in the vicinity of the worki n g s on the Language Reef (Plate I). Northeast of this point and lower down in the succession, another fold is visible in a krantz, i m m e d i a t e l y above a p y r o x e n i t i c intrusion. The a n t i c l i n a l axes of these domes coincid e with the "axial traces" of the elon gated laccoliths. These "axial traces" again are related to the tectonic f o l d - d i r e c t i o n s ; in the n o r thern area it is a p p r o x i m a t e l y parallel to the 060 degree and in the central area to the 0 78-096 degree-trend. This probably indica tes that the formation of the l a c c o l i t h i c i n t r u s i o n s was controlle d by p r e - e x i s t i n g f o l d - s t r u c t u r e s . Faults (a) N g n - t e c t o n i c Faults The faults which are related to i r r e g u l a r i t i e s in the floor of deposition are c onsidere d to be n o n - t e c t o n i c in origin. They probably formed as the result of r e a d ­ justments due to d i f f e r e n t i a l c o m p a c t i o n in the s e d i m e n t a r y rocks. At the Lisbon Falls, several high-angle normal faults are develope d along the flanks /of a ... - 34 - of a gra nitic dome, and are roughly parallel to the long axis of the dome. They are all gravity- faults and show d i s p l a c e m e n t s in the order of 2 0 feet. One of these faults, i.e., that west of the waterfall, shows a displ a c e m e n t of over 250 feet. This fault can be traced southwa rds beyond the Sabie River and n orthwar ds to beyond the Treur River, a distance of over 30 miles. The Nestor Dyke (Plate I) is i ntru d e d along this structure. It is clear that this fault cannot be due to compac t i o n alone, but represents later t e c tonic movement along a plane originally due to compaction. The fault near Sabie, which predates the T r a n s v a a l System, and which is d e s c r i b e d by Visser and Verwoerd (1960, p. 55) may have a similar origin. (b) Tecto nic Faults There are three types of t e c t onic faults, namely, low-angle t h r u s t - f a u l t s , and high -angle normal faults. High-a ngle reverse faults are not present. This is to be expected, as no large magmatic in trusions (Hills, 1963, -p.194) occur in the area. (i) Low-a ngle T h r u s t-faults. The low-angle thrust faults are rep res e n t e d by the inter- bedded reefs and they are, therefore, / b e d d i n g - t h r u s t ... 35. - 35 - b e d d i n g - t h r u s t s „ Evidence that the in.te.r.b'edd'ed reefs occupy bedding thrust- faults, is provided by the displ a c e m e n t of an earlier dyke in the lYlalieveld Section of the Glynn's lYline (Visser and Verwoerd, 1950, p. 69). This dyke which trends 025 degrees, t er minates against the lower contact of the Glynn's Reef in one locality, and is continued above the reef, farther to the southeast. Further evidence of movement along the planes of the i n t e r b e d d e d reefs, is the presence of s l i c k e n s i d e s and breccias along these planes. Small flexures, due to faulting, have been obs erved in the Beta R e e f . As these stru c t u r e s are not as soci a t e d with major folding (Hills, 1963, pp. 196-201), they are cons i d e r e d to be g e n e t i c a l l y related to the i n t r usion of the Bushveld C o m p l e x . (ii) Low -angle Normal F a u l t s . These faults are found at B o u r k e ' s Luck in the northern area, as well as in the Duke's Hill lYline in the central area. They are nearly parallel, to the regio nal strike, and dip west at angles slightly steeper than the i n t e r b e d d e d reefs. L o w - angl e normal faults occur also on C o l u m b i a Hill, uihare they are associ a t e d with a laccol i t h i c / i n t r u s i o n . , . 3 6 . - 36 - in t r usion (Figure 4). In this locality the strike is a p p r o x i m a t e l y east, i.e. parallel to the "axial-trace" of the intrusion, and the dip is 35-45 degrees south. Accordi ng to the dynamics of faulting, only high-angle normal faults can result from a h ori zontal relief of pressure (Anderson, 1951, p . 16). The l o w - a ngle normal faults must, therefore, be ascr ibed to a condition in which only g r avity is active, i.e. g r a v i t a t i o n a l gliding. In g r a v i t a t i o n a l gliding tectonics, the i n s t a b i l i t y may be as s o c i a t e d with earlier primary t e c t o g e n e s i s , but the final mov ement is due to sliding under gravity (Hills, 1963, p. 212). Structu res such as these may be formed by gliding down the slope of an uplifted geotumor, or g r a v i t a t i o n a l gliding in a s edime n t a r y basin (Hills, 1963, p. 337). Examples of this type of d e f o r m a t i o n are d e s c r i b e d by De Sitter (1964, p p. 238-254) . The origin of the low-ang le normal faults in the area under consideration, may also be as cribed to g r a v i t a t i o n a l gliding. The structures at B o u rke's Luck and in the Duke's Hill Mine are consider ed to be related to the sagging of the floor of the Bushveld Complex. The faults on C o l u m b i a Hill (Figure 4) are believed to have resulted from g r a v i t a t i o n a l gliding, along the slopes of / a d o m i c a l .. . 3 7 . - 37 - a d o mical structure which was caused by the i n t r u s i o n of a l a c c o l i t h i c sill. ) Hiqh-anqle Normal Faults. The faults that are most commonly present, are high-angle normal faults. Although faults of this type occur throug hout the gold-field, they are most abundant in the central area. They are often a s s o c i a t e d with d i a bas ic dykes, but it is not clear whether the dykes have intruded along the faults, or whether the faulting occurred after the intrusion. The h igh-angle normal faults are shown in Plate I . Two f a u l t - d i r e c t i o n s are present, namely 352-030 degrees and 070-130 degrees. The majority of the faults have dips steeper than 80 degrees. The e a s t - t r e n d i n g faults are c h a r a c t e r i s e d by small displacements, i.e., up to 2 0 feet, and they are sometimes as s o c i a t e d with antic l i n a l folds. The i other faults show d i s p l a c e m e n t s that vary from 20-600 feet. These faults are often a c c o m p a n i e d by minor synthetic and an tith e t i c f a u l t s . The h i g h - a n g l e normal faults gave rise to a number of graben and horsts. The most prominent of these structures, is the so-cal led Fraser-IYlorgan G raben at Pilgrim's Rest. It is bounded on the west by the G r o o t f o n t e i n Fault, which shows a downthrow /of 300 f e e t . ..3 0 . - 38 * of 300 feet east (Zietsman, 1964, p. 56), and on the east by the Morgan Fault, which shows a d ownthrow of 600 feet west. Several minor parallel faults occur between the two major structures. Of these the Fraser Fault is the most important. The faults that c o nstitute the graben all show maximu m d i s p l a c e m e n t s i m m e d i a t e l y south of Pilgrim's Rest. Most of the hi gh-a n g l e faults show m a x i m u m d i s p l a c e m e n t in this area. A downfa u l t e d block is present west of the Beta Dyke, on the west bank of the Blyde River (Plate I). The faults do not continue beyond the Clewer C r e e k (Zietsman, 1964, p. 56). They are more p r o n o u n c e d southwards where they become part of a graben. On the farm In De Diep te 164 3.T. and southwards, beyond the b o u n d a r i e s of the i n vestigate d area, this graben is very prominent; Visser and V erwoerd (1960, p. 56) e r r o n e o u s l y refer to it as the F r a s e r - M o r g a n Graben. Another pr ominent graben is present on the farms Fr ankfort 509 K.T., Rotunda Creek 510 K.T., and K r u g e r s h o o p 527 K.T. (Plate I). Small, local g r aben occur in the northern area, one on H e r m a n s b u r g 495 K.T. and one on London 496 K.T. South of Sabie, a graben is d e v e lo ped near the head of the Golden Valley Creek. All the above graben trend a p p r o x i m a t e l y north. A number of small / l o c a l . . . 3 g. i .39 - local graben and horsts, which trend east, are located in the eastern part of the area, along the edge of the escarpment. The trends of both the low- and high- angle normal faults are i l l u s t r a t e d in Figure 5. The low-angle t h r u s t - f a u l t s are not included. The a z i m u t h of the most prominent trend, which shows very little variation from north to south, is 352-030 degrees. A less p rominent trend, which is conf ined to the n orthern and central areas, has an azim uth of 070-130 degrees. The trend in the northern area varies from 070-090 degrees, and that of the central area 090-130 degrees. This change in trend is cons idered to be due to l i t h o l o g i c a l d i f f e r e n c e s in the sedimentary r o c k s . There are two condi t i o n s necessary for normal faulting, viz, relief of pressure in all horiz ontal directions, and maximum pressu re in a vertical d i r e c t i o n (Nevin, 1953, p. 100). According to Anderson (1951, p. 16) the faults that result from these conditions, have angles steeper than 45 degrees. The vertical pres sure is provided by gravity, w h ereas a relief of the h oriz ontal pressu re is created by tensional stress, or by the r e l a x a t i o n of co mpr e s s i o n a l s t r e s s . The h igh-an gle normal faults, trend in /tWO 0 ..O. < W o: 2 K « 3 h ï 0 UJ K S J < K s * id Ol f-~iqi/re. 5 r~ C o r~\ tot-J r- D/a o £ A / orrr -1 <3 / f~ a CJ ! L Fi e A«a. /-~ e ; f ^ - é - S / .— C Z o C-/ . [ 3/- 60%, 2 /-30%, //-^o % , — / Corstocvr D^qr-^rr^s o f Linear £> b ure.S ■ ■ z / - s o % IgSg^ /6 - 2 0 % U-/S % Ulilllllll é - / o % EZZH ] / ~ s / , - 48 - of the geological map of South Africa, shows that the Transvaal System uias folded along ax;eis- which strike parallel to the long axis of the basin of deposition. Subsequent refolding along an axis nearly perpendicular to this direction resulted in the present structural setting. The effect of the superimposed folding, therefore, gives the impression of two separate sedimentary basins of Transvaal age. The structural evolution of the Transvaal Basin is visualised as follows; (i) The sedimentation commenced with the deposition of the Black Reef Series, followed by that of the Dolomite Series. After the deposition of the latter, the basin was elevated to above sea-level, with subsequent erosion. This is indicated by a major unconformity, i.e. the Bevett's Conglomerate and Giant Chert (Zietsman, 1964, p.41). (ii) The period of erosion was followed by the subsidence of the basin with the subsequent deposition of the Pretoria S e r i e s . (iii) The prolonged subsidence of the Transvaal Basin was sufficient to weaken the subcrust to such a degree that volcanic activity commenced. These outbursts took place in four stages during the /sedimentation...49. - 49 - sedimentation of the Pretoria Series, and finally culminated in the magmatic activity of the Bushveld Complex (Visser, 1957, i p . x x x i i i ) . (iv) The intrusion of the Bushveld Complex commenced with the emplacement of the mafic rocks and was concluded with the intrusion of the granite. The deformation of the Transvaal System was closely connected with, and to a great extent controlled by, the emplacement of the Bushveld Complex. The pyroxenitic and diabasic sills, as well as some of the dykes, i.e. the Vaalhoek, Dientjie, New Chum, Nestor and Mali D y k e , are considered to be contemporaneous with the early magmatic phase of the Bushveld Complex. The long axis of the main Bushveld Basin trends east. This direction is parallel to the east-trending folds in. the area under discussion. According to Visser (1957, p. xxxv) the emplacement of the rocks of the Critical Zone was followed;, by compression from the north, due to the collapse of the central part of the Bushveld Basin and the tilting and rising of the beds on the northern side. This compression gave rise to east-trending folds. This conclusion is contradicted by Brock (1959, p. 341), who contends that the overall picture is due to subsidence and not to magmatic activity related to the /Bushveld c,,50. - 50 - Bushveld Complex. He states that the rigidity of the floor of a basin is reduced rapidly by subsidence, followed by collapse which gives rise to horizontal compression. This mechanism is similar to the jaws of a nut-cracker (Brock, 1959, p. 344), i.e. it is the linearity of a trough that directs the pressure normal to its length. Important is the fact that the e a s t ­ ward extension of the long axis of the Bushveld Basin passes through Pilgrim's Rest. The presence of east-trending folds at Pilgrim's Rest indicates that compressive forces had acted normal to this axis. It is concluded that the forces were the result of the subsidence of the Bushveld Basin, similar to the hinge-effect (nut-cracker) as explained by Brock (1959, p. 344). As the conclusion is drawn that the mineralisation is structurally associated with the folding, this mechanism will also explain why gold-mineralisation is localised mainly near this axis, i.e., only in the S a b i e - P i l g r i m 's Rest area of the Transvaal Basin. The formation of the low-angle bedding-thrusts postdates the east-trending folds. These thrusts are due also to compressional forces, which are considered to be related to a later phase in the / magmatic ... 51. - 51 - magmatic activity. The forces acted from west to east, resulting in the formation of the bedding-thrusts, as well as the north- trending folds. The pre-reef dykes, i.e. the Nestor Dyke and several others, are displaced by the bedding-thrusts. The low-angle gravity-faults are considered to be associated with tensional stresses which superseded the compressional forces .that gave rise to the bedding-thrusts and north-trending folds. The high-angle normal faults postdate all the other structures and may be p o st-Bushveld, or even post-Karroo, in age. There is no evidence for l/isser 's conclusion (1957, p. xxxvi) that the tension-faults were formed during the emplacement of the Bushveld Granite. As the emplacement of the dykes of diabase postdates the formation of the high- angle normal faults, they are considered to be post-Bushveld in age. IV. MINERALISATION A. GENERAL STATEMENT Gold, as well as deposits of asbestos and manganese are present in the area. The latter is, however, not relevant and is not considered. The occurrence of asbestos and gold within the same area is considered to be significant. Although no direct u.o.v.J.^^ /relationship. ..52. - 52 - relationship could be established, it is believed that' both'are structurally controlled. The different mineral-localities are shown in Plate I. B. GOLD-BEARING ORE-BODIES 1. Classification The gold-bearing deposits are divided into . two main types. The first is an interbedded, roughly sheet-like body, referred to as an interbedded reef. This type of deposit is found on more than twenty different stratigraphical horizons. They occur in the Ulolkberg Formation, as well as in the three members of the Transvaal System. Most of these are found in the Dolomite Series. Swiegers (1948, p. 83) ascribes this to a higher reactivity of the d o l o m i t i c :limestone However, little or no replacement of the dolomite is observed, and the above phenomenon is there­ fore considered to be due entirely to strati­ graphical control. The second type of deposit is represented by transgressive ore-bodies. These are locally known as cross reefs, and are sub-divided into simple f i ssure-veins, sheeted zones, stockworks, dyke-veins, ladder - v e i n s , dyke-contact veins, dyke-lodes, fault-plane deposits and tabular ore-shoots (Swiegers, 1948, pp. 87-88). Visser and l/erwoerd (1960, p. 62) offer a more general /classification...53. classification, namely, vertical reefs, leaders, blows,, and irregular ore-bodies. Interbedded Reefs (a) General Characteristics The distribution and stratigraphical relationships of the interbedded reefs are shown in Plate II. It has already been shown that the mineralisation preferred planes of weakness in the sedimentary beds. Due to the topography, the outcrops of the reef-horizons are sinuous and can be traced along the hill-slopes for considerable dis t a n c e s . The interbedded reefs can be described as mineralised bedding-faults of great lateral extent. The presence of wall-rock fragments, striations and slicken- sides is evidence that these reefs were emplaced along bedding-faults (Z i e t s m a n , 1964, p. 61). In a number of places, intraformational movement has caused the displacement of pre-reef dykes and faults. Apart from the dyke in the Malieveld Mine described by Visser and Verwoerd (1960, p. 69), a dyke in the Nestor Mine, and a pre-reef compaction-fault in the Astra Mine are likewise displaced. During these displacements the movements were evidently not confined to a /single...54. - 54 - single plane, but occurred along several closely spaced b e dding-planes. The material between these planes of movement, which included early . sulphides and g a n g u e -minerals, was subjected to intense brecciation and, locally, also to flexuring. The disturbed zone was subsequently mineralised by later mineral-bearing solutions. The contacts of an inter-bedded reef are, there­ fore, almost invariably defined by slickensided planes of movement. The visible striations on the planes of movement indicate a southeasterly direction of movement, i.e. approximately perpendicular to the 060 degrees structural trend. Although the horizons of certain reefs can be traced for many miles in all directions, interbedded reefs are developed locally only, lilhere a potential inter-bedded reef is developed into a true ore-body, it is known locally as an ore-shoot. An economical ore-shoot is referred to as a pay-shoot. Ore-shoots are usually developed along the crests of very open antifi clines and domes. The shoots are generally * between 500-1000 feet in width and they may be several miles long. Interbedded reefs usually have an average thickness of 10-20 inches. In localities where they reach exceptional thicknesses of over 8 feet, the reefs are usually associated with domes. Interbedded reefs are characterised by /considerable...55. - 55 - considerable variations in thickness. The thickness within an ore-shoot is seldom less than twfl. inchë-s, and commonly swells to more than 12 inches. Away from the ore-shoot the swells become less conspicuous, with the result that the reef is eventually represented by a carbon- parting or thin layer of brecciated rock only. The characteristics of a swell are illustrated in Figure 9. It is lenticular in cross-section and is marked by an abundance of massive ore- and g a ngue-minerals. In the constrictions between adjacent swells, hardly any massive mineralisation is found. The layers of chert in the hanging- and foot-wall are curved in sympathy with the swells (Figure 9). Joints and fractures are usually present in the wall-rock above and below' the swells. The lenses are of limited lateral extent, and are elongated in an east-southeasterly direction. Structures similar to those described above are due to irregularities in the original plane of faulting (Hulin, 1929, pp. 15-49). These irregularities will cause certain parts of the fractured surface to rub together during movement, while other parts will pull apart (Newhouse, 1942, p. 6). This is admirably illustrated by Stoces and White (1935, p. 296). The lenticular character of the interbedded reefs can therefore be ascribed /to s l i g h t . . .57. F ’iCfcj r~g. v3 5 e c 6 /or\ o ^3 e 6 Siye. /* 1 I £3 ^r-/~er\ Vfe/V\ o£ «i(355//e yayr/^C 3r\c/ •é&é r-<31-\ e cĴ '6 e.. f ^yj & c ~ ir r e / ^ C L & ltz ié e . 1 C o& r& e. cJ /5s e^v>v<9 6ec/ /?y/-/V6 /rv. Cfo/onr\ i &C. . f c S S = 3 ~ 1 S / / C . X : € a s / c / e s o A yr-b\i£e I.-.1.1 c//ss> € ^ k \ / V \ < 3 c/ p t-f r~ 1 é: /̂ \ o'o/o^/^'C . jf 1~l4_ 'DoJorr\i£ ic. /irr\e *> &or\ei I•'̂ -T̂ r̂rx.J CL.Ke.r6: Q t^ cirt^ k «2/>>- ! £i//ecf wiérA c a / c / é e y / ] 5 e cor\ c/ fDe.r/ocJ o f - /r\ir\e.r-3 £/on*. - 57 - to slight undulations in the original planes of movement. These undulations may have been tectonic-,in origin, or they may.have been present in the rocks before the deformation. The apparent orientation of the lenticular bodies, however, suggest that the undulations mere due to t e ctogenesis. The abundance of massive mineralisation in the lenticular bodies is considered to be due to the following conditions? In a fissure, the presence of relatively large open spaces, which are fed through a constricting conduit, creates favourable conditions for a steep gradient of precipitation (Newhouse, 1942, p. 6). These conditions include a sudden decrease in the hydrostatic pressure and temperature, as well as a decrease in the flow-speed of the mineralising solutions. An interbedded reef may split into two or more reefs, separated by a few feet of country-rock. Disconnected reefs may develop locally a few feet above and/or below the main reef-horizon, lïlining-experience has shown that in areas where two or more reefs were present, the subsidiary reefs, as well as the main horizon, were found to be inferior, both in thickness and in gold-content. The carbon-content of the reefs is apparently influenced by the lithology. The variations are considered to be due to the presence or absence of shale. The reefs in the /shale. . ..58. - 58 - shale.of the Pretoria Series, for instance, are '' / : V-' • • . characteristically carbonaceous. The reefs in Zone One of the Dolomite Series, which contain numerous layers of shale, are generally more carbonaceous than those higher up in the succession where shale is relatively scarce. The Sandstone Reef, which occurs in quartzite, is practically free of carbonaceous matter. (b) Mineralogy of the Ore The interbedded reefs are composed of gangue- as well as ore-minerals. The following gangue-constituents are found in the sulphide- zone: quartz, calcite, dolomite, siderite, sericite and, less frequenctly, apatite. Secondary quartz and calcite are often dis­ tinguished from their hypogene counterparts. Apart from these, gypsum and associated sulphur are occasionally present in the oxidised zone (Swiegers, 1948, p. 119). The hypogene ore-minerals are pyrite, a r s e n o p y r i t e , pyrrhotite, chalco p y r i t e , tetra- hedrite, bismu t h i n i t e , native bismuth and scheelite. The supergene sulphides chalcocite and covellite are present also. The supergene ore-minerals in the oxidised zone are limonite, cuprite, native copper* malachite, azurite, chrysocolla, tenorite and b i smuth-ochre. t The precious metals gold and silver are present in various proportions as electrum. (i) Ore-minerals... 5 9 . - 59 - (i) P r e -minerals. Pyrite is the most abundant ore^mineral and is found in macroscopic quantities in all the interbedded reefs. Two types of pyrite could be distinguished; disseminated pyrite which sometimes occurs as lumps of sugary pyrite; and dense massive pyrite. The mode of occurrence of the minerals present in an interbedded reef, is illustrated in Figure 9. Disseminated pyrite is found in the wall_rocks, in fragments of wall-rock within the ore-body, as well as in the graphitic shale. It is seldom present in quartz and calcite. Lumps of sugary pyrite, associated with irregular bodies of quartz, are found in some ore-bodies. These bodies are invariably intersected by numerous graphitic slicke n s i d e s , which indicates that this type bf mineralisation predates the bedding-thrusts. The massive pyrite occurs as small blebs, massive aggregates of irregular shape, as well as veins in the gangue. It is often found in admixture with other sulphides, and less commonly, as tiny veinlets in the wall- rocks adjacent to the pre-body. A definite difference in age between the disseminated or sugary pyrite and the / massive... 60. - 60 - massive pyrite could be established. The former type is clearly earlier than the deformation. It is commonly present in mall-rock fragments and in graphitic shale, but never along slickensides or any other planes of movement. The presence of slick­ ensides in masses of sugary pyrite further­ more suggests that movement took place after this pyrite was formed. The massive type of pyrite, however, often occupies cracks and shear-planes in the ore-body. It replaces dolomite and disseminated pyrite in fragments of wall-rock. This type of pyrite, therefore, postdates the bedding- thrusts, i.e. it is later than the dis­ seminated or sugary type of pyrite. fl difference in character between the disseminated pyrite and the massive pyrite was also observed by Barnard (1958, p p, 23-24) in the ores of the Bourke's Luck Mine<.? He noticed that the crystals of the disseminated type occasionally showed pressure-shadows and were often compressed, the longer axes of the crystals being parallel to the foliation of the matrix. Pyrite has grown essentially at the expense of gangue and country-rock, and nowhere does it replace other sulphides (Swiegers, 1948, p. 98). /Arsenopyrite.,61 - 61 - Arsenopyrite occurs in much the same way as pyrite in some of the inter- bedded reefs. It is most abundant in the ores of the-Frankfort lYline, both in the Bevett's Reef and in the Frankfort Reef. Two types of arsenopyrite are present, namely a disseminated variety and a massive type- The disseminated or sugary variety, is earlier than the massive arsenopyrite, and was emplaced prior to any deformation. Massive arsenopyrite sometimes replaces massive pyrite (Swiegers, 1948, p. 98), and is therefore later than the pyrite. The arsenopyrite is in turn replaced by chalcopyrite or tetrah e d r i t e . The massive arsenopyrite is present in almost all the ore-bodies in the gold-field, while the disseminated or sugary variety is restricted to the ore-bodies on Frankfort 509 K.T. Pyrrhotite is not plentiful and is restricted to the lYlamre lYline, where it constitutes the dominating ore-mineral. Chalcopyrite is the most important copper-mineral in the ores of the gold-field (Swiegers, 1948, p. 99). This mineral is encountered in all the reefs but is most conspicuous in the Beta and Glynn's Reefs„ Swiegers (1948,fp. 100-101) observed the following characteristics and relationships in polished specimens of chalcopyrites the /mineral... 62. - 62 - mineral is invariably anhedral in form and occurs as irregular replacement-bodies, ll/here the- interstices between crystal- aggregateis of idiomorphic pyrite are completely replaced by chalcopyrite, pseudo- crystalfaces may develop . The corners of the crystals of pyrite are usually rounded, due to replacement by the chalcopyrite. The replacement of calcite by chalcopyrite is essentially controlled by the directions of cleavage. In cases of advanced replacement, evenly scattered rhomblike islands of carbonate remain in the sulphide. The chalcopyrite metasomes are irregularly developed in chert and quartz. Veins and vein-systems of chal­ copyrite are frequently encountered in gangue or in early sulphides, especially pyrite, arsenopyrite and pyrrhotite. Composite or broken veins containing chalcopyrite and tetrahedrite or bismuth- inite, or both, are commonly found in crystals of pyrite. This suggests a more or less simultaneous deposition of these three sulphides. Irregularly rounded grains of chalcopyrite are almost invariably present in massive tetrahedrite. Evenly scattered, elongated inclusions of chalcopyrite in /bismuthinite...63. -63 - bismuthinite, may resemble an intergrowth of the two. /TS, T T-e-trahedrite is an important ore- constituent in the mines around Pilgrim's Rest. It occurs as large aggregates or as small eyes in quartz in the lower levels of the Beta (Kline (Figure 9). It is found also in admixture with other massive ore- minerals like massive pyrite and chalcopyrite. Swiegers (1948, p. 103) found that tetrahedrite replaced pyrite, arsenopyrite and gangue. In general, pyrite is more readily replaced by tetrahedrite than by chalcopyrite. Bismuthinite is less commonly found in the interbedded reefs. It usually occurs in the form of isolated elongated crystals up to two inches in length. Native bismuth is usually found to be associated with bismuthinite and is often included in the latter mineral.. Other ore-minerals of minor occurrence are, galena, bornite, sphalerite and scheelite. (ii) Supergene Sulphide-minerals. The sulphides . which are secondary in' origin, are chalcocite and covellite. They are formed at or 'just below the water-table. The chalcocite is associated mainly with chalcopyrite and, / l e s s .... 64. - 64 - less frequently, with tetrahedrite (Swiegers, 1948, p. 111). This author observed that the d.e-gT'e;e-?'oT replacement of chalcopyrite by chal.cpciv£ev ranged from microscopical v/einlets of chalcocite in solid chalcopyrite, to masses of chalcocite with occasional remnants of chalcopyrite. Pyrite is occasionally also replaced by chalcocite. The covellite is extensively developed in tetrahedrite, chal­ copyrite and . c h a l c o c i t e : It also.replaces a&senopyrite and b i s m u t h i n i t e . (iii) Superqene D x ide-minerals. The oxides of iron are the most common o x ide-minerals. They are usually earthy mixtures of limonite, goethite and hematite; pseudomorphs after pyrite are sometimes present (Swiegers, 1948, p. 117). Cuprite, malachite, azurite, chryso­ colla and tenorite ore the products of oxidation of sulphides. They usually occur in close association with each other. B i s m u t h - o c h r e , which is pseudo- morphous after bismuthinite, was observed in some of the oxidised ores. It is particularly abundant in the Portuguese Reef. (iv) G a n g u e - minerals. Quartz is the most common gangue-constituent in the interbedded reefs. Other important minerals are calcite, siderite, dolomite and graphite. Minor quantities /of chlorite...65. - 65 - of chlorite, sericite and apatite are present also. Three generations of silicification can be distinguished, one before and the other two after the intraformational movement. The former is characterised by the presence of disseminated pyrite. In some localities massive pyrite is present in fractures in this quartz. Its relationship to fragments of wall-rock in the Beta Reef (Figure 9) suggests that it is pre-movement in age. Quartz which postdates the bedding- thrusts is present in the form of replacement- bodies in fragments of wall-rock, as well as in earlier quartz. Inclusions of disseminated pyrite in graphitic shale have been observed in this later quartz. The quartz is intimately associated with the massive ores and is extensively replaced by them. A third generation of silicification is indicated by veins of quartz which transect all the other minerals. The carbonates grade in composition . from pure calcite to siderite, with varying proportions of magnesia and manganese (Swiegers, 1948, p. 108). A close association exists between the carbonates and the quartz; intergrowths are often encountered, and, as in quartz, three generations of carbonates / can be ... 66. - 66 - can be distinguished. Carbonate, which is pre-movement in age, is present in the Pigeon Reef at the foot of the Lisbon Falls. The bulk of the ore-body in this locality is composed of siderite which is sheared and filled by late ore-minerals. Figure 9 shows a body of barren calcite with intergrowths of quartz, partly enclosing a layer of graphitic shale. It is not clear whether movement took place prior to the emplacement of the calcite. A thin fracture along the upper contact of the shale, also intersects the calcite, indicating slight movement after the deposition of the calcite. A third period of carbonatisation is revealed by veins of calcite which inter­ sect gangue - as well as ore-minerals of all previous generations. Graphite is present in most of the interbedded reefs. This mineral predates the formation of the b e dding-thrusts. The abundance of graphite in shaly horizons suggests that it was derived mainly from the shale. Barnard (1958, p. 33) came to a similar conclusion in connection with concentrations of graphite in the Trixie L o d e . /Chlorite,...67. - 67 - Chlorite, sericite and apatite are minor constituents of the interbedded reefs, andj^a.re, o*f-»'local occurrence only. Precious M e t a l s . The precious metals in the interbedded reefs occur as an alloy of gold and silver, i.e. electrum. The ratio of gold to silver is characteristic for the interbedded reefs, and differs from that of the transgressive reefs. The ratio in the interbedded reefs varies between 7-Í5 parts of gold to one part of silver. Swiegers (1948, p. 124) concludes that the ratio is constant for any particular ore-body but that it varies from one ore-body to another. This ratio is apparently influenced by the relative abundance of disseminated pyrite. The precious metals occur in association with sulphides. Crystalline growths, veinlets and vein-; systems - irregular replacements in pyrite, spikes and blades in bismuth-ochre, and intergrowths of gold and sulphide-minerals, were observed under the microscope (Swiegers, 1948, p p . 127-128). Visible gold is practically absent. The relationship between the gold and the disseminated pyrite is rather obscure. No gold is visible in this mineral under the microscope. Assay-r e s u l t s , however, show that the pyrite contains gold; values of up /to ... 68. - 68 - to 12 pennyweights are encountered. It mu.st therefore be concluded that the gold is pr e.se/it in sub-microscopic particles similar to some of the Barberton ores (Schweigart and Liebenberg 1965). In the later sulphides, however, the precious alloy can be distinguished under the microscope. It is generally more closely associated with chalcopyrite and tetrahedrite than with massive pyrite. In the ore of the Trixie Lode, however, the relationship between copper-minerals and gold is directly influenced by the association between chalcopyrite and pyrite. UJhere chalcopyrite is found in combination with the pyrite, moderate gold-values may be associated with high copper- v a l u e s . Where chalcopyrite is not associated with pyrite, no gold is present (Barnard, 1958, p. 27). (vi) Relationship Between Reef-thickness and G o l d - c o n t e n t . The gold-content of an interbedded reef is directly related to the relative abundance of ore-minerals which postdate the b e d d i n g-thrusts. In reefs where only disseminated pyrite is found, the average gold-content is not influenced by the thick­ ness of the ore-body. The s ulphide-minerals, other than the disseminated pyrite, are concentrated largely in the lenticular swellings, and the conditions which were responsible for the steep precipitation- gradient of these minerals, also controlled / the .0.69. - 69 - the precipitation of the gold. The gold in interbedded reefs is therefore concentrated mainly in the lenticular bodies, i.e., the go-ld-e.ontent is related to the thickness of the ore-body. This is illustrated in Figure 10. High gold-values are sometimes encountered in the thin parts of oxidised reefs, whereas the thicker parts yield inferior values. This is probably due to secondary redistribution of the gold. (vii) Alteration of the W a l l - r o c k . The alteration of the wall-rocks yielded the following minerals quartz, calcite, graphite, pyrite, and small quantities of chlorite and sericite. The dolomite may show local recrystallisation. These minerals are, however, considered to be the products of metamorphism rather than hydrothermal activity. / F igure 10 ... 70. - 71 - The alteration of the country-rocks adjacent to the interbedded reefs, which are invariably located near sills, is illustrated in Figure 11. The wall-rock adjacent to interbedded reefs located within dolomite, is usually characterised by abundant disseminated pyrite. The pyrite decreases rapidly away from the reef and usually disappears completely within 5 feet of it. The pyritic zone is also characterised by numerous "eyes" and interbedded lenses of calcite which result in a spotted appearance of the dolomite. Abundant graphite occurs near the contact of the reef, but is represented only by a few interbedded stringers away from it. An example of wall-rock alteration in shale is represented by an ore-body in the Fraser-IYlorgan lYline (Figure 11). This ore-body is located immediately above a pyroxenitic sill and also penetrates 6-15 inches into it. The ore-zone is composed of sugary pyrite with interlocking quartz. Both the quartz and the pyrite are intensely brecciated in the shale, but not in the sill. Graphite is present in great abundance, especially along slicke n s i d e s . Both graphite and disseminated pyrite decreases away from the ore-body. Numerous stringers and lenses of quartz are present within 5 feet of the ore-body. /3. Transgressive... 73. S/\ a / e. -P o /o avn / £ e 15 O fó. /r\ ^í'cknesí 4 -0 í - t - /r\ é rt\ / ck r\e.&£> Py’ /'oxsr îéic. Si/I - / JLyd/<3ry. iée~ ,5 / //. - X , X_____X V " t - ^ ''p en 6t /é & Serpe^£/>v / y/\ IrO - A Cf O / o rr\ / é e . C**\e.r~£ . c/j&se.rr\ir\,3 6 e. c/ cj r i éíS / - Ó&Conr\e. /r\cr-e&mr\^/c/ 3̂ h>ur^d^r^ £ £ovvc3 r~c/^ éA.c o/"e-6oc^. Z><=* /c=> rt-̂ iér̂ , <3r\edd>r\'V a •. • A y r/^e &/f\d t̂ Lksr-62. / A é e rv s e / y A r e c c A J í e o J w ith . r\L]rr\r I ou^, Jl̂ p/ljóc s / icke.r\£, ic/ê s 'iŜíiS i5̂'' ■ x' O e ' i s e /rv. s ill Gr\&f>b\iá.e. 5/o/\o hnzcfc/tr̂ X X X X X *£=» 'D'̂ e.rrMK̂ éecd /=-> c//-1 £ . PyroïeA/ízc s/7 /. 'vo / /. /''X í/vcXr^e55 /5 f/JIr\6 f /!lj£> 6r~c3 Ó ^Vcj// - /̂ ~g>q Â. <3l6z.r<3£ror^ />y c/oforr\f£z_ <3r\cJ sA<5>/e 5ca/e: /"== 5 ££ . f~ro^v 5er - A'/c> r~Cj eS7rv. /̂ fjlr\d_ - 73 - 3. Transgressive Pre-bodies (a) Classif icat.ion (i) Ver-t.-i-.cafet R e e f s . The vertical reefs are fissure-veins of great lateral as ujell as vertical extent. They vary between a few inches and 20 feet in thickness. These reefs are either vertical or steeply inclined, and have strikes that vary between 004-020 degrees, The vertical reefs are represented by the Rietfontein, Sunlight, Astra and Hepta Reefs. Uneconomical ore-bodies are the Bokwa and Gregory Reefs. Vertical reefs occupy old fault- planes or were emplaced along tension- fractures which show no displacement. (ii) L e ader-reefs. They are steeply inclined ore-bodies of small vertical extent. They vary between one inch and several feet in thickness, and they may extend for over 1000 feet along strike. Leader, reefs are usually characterised by smooth . well defined walls. (iii) D y k e - r e e f s . These ore-bodies are intimately associated with dykes. They ' may be emplaced along the contacts or within the dyke itself. They are either parallel or perpendicular to the contacts /The Trixie....74. - 74 - The Trixie Lode in the Bourke's Luck lYline is a dyke reef that cuts at an oblique angle through the Dientjie Dyke. The most important dyke-reefs are found in the northern area. At Bourke's Luck they are represented by the Trixie Lode, Contact Reef and Dyke Reef. At Vaalhoek the Thelma Reefs are closely associated with the Vaalhoek Dyke, while farther south, several ore-bodies of minor importance occur along this Dyke. A few ore-bodies of lesser importance are found along the New Chum Dyke and a dyke parallel to it. A few dyke-reefs are also present in the central and southern areas, i.e. along the lYlali and Nestor Dykes. B l o w s . The blows are thick ore-bodies which are greater in lateral than in vertical extent. These bodies have dimensions in the order of 5-20 feet in height, up to 200 feet in width, and to over 3000 feet in length. Two types are encountered, namely, tabular blows that are developed between two parallel interbedded reefs, and ore-channels which extend downwards from a reef-plane (Figure 12). The former type is charac­ terised by relatively smooth contacts. Blows of this type may be described as steeply inclined leaders that are greater in lateral /than in ... 76. S c á / e : / in c h . = f o fe .e r t . FriQur-e. / I2> !<=3 Q/-ei3 rrxr^^t Sec^/OA.^ / //<_j ̂ 6ns 6 /nq gy//Xêrgn.c( h >e .t t v d e r ^ . ■£&£><_j /< 3 r - b / o w & < a ^ \ a l' o * ^ e - c K s r ^ /n. ^ /3 • - 76 - than in vertical extent; in cross-section the shape resembles a parallelogram. The ore-channels are boat-shaped in cross-section (Figur.e 12). The most important tabular blow is the Elandsdrift Blow south of Sabie. The Copper Blow in the Nestor Mine is another example of this type of ore-body. Several similar, although much smaller, tabular blows ore present on London 496 K.T. and Ledouphine 469 K.T. The Spitzkop Ore-channel is the most important example of the second type. This ore-body is genetically associated with a low-angle gravity - f a u l t . (a) General Characteristics (i) Stratigraphical R e l a t i o n s h i p . While the interbedded reefs are present predominantly in relatively incompetent beds of dolomite and shale, the prominent transgressive reefs are located in competent rocks such as granite, diabase and quartzite. A close stratigraphical relationship exists between the interbedded and transgressive ore-bodies. The leader-reefs and the blows are invariably associated with interbedded reefs, and they are often developed between two interbedded reefs. Dyke-reefs occur within or along the contacts of dykes, at elevations which usually coincide with the /stratigraphical...77. - 77 - stratigraphical horizons of interbedded reefs. Vertical reefs in sedimentary rocks show enrichment in their gold-content,, at elevations which coincide with the horizons of the interbedded reefs. Ii) A g e - r e lationship. The age-relationship between the interbedded and transgressive reefs is best illustrated in the area around the Lisbon Falls. In the Little Gem Mine an early high-angle fault is displaced horizontally along the plane of an inter- bedded r e e f . The plane of the high-angle fault was opened and mineralised after the horizontal displacement took place. In the Astra Mine the same high-angle fault- plane was also displaced by a low-angle g r avity-fault, prior to the mineralisation of the fault-plane. The low-angle gravity faults in other parts of the gold-field postdate the interbedded ore-bodies, but predate the transgressive reefs. It is concluded, therefore, that the transgressive ore-bodies are younger than the interbedded reefs. (c) Mineralogy (i ) Pre-minerals. The ore-constituents in the transgressive reefs are predominantly massive sulphides and consist of pyrite, / c h a l c o p y r i t e . ..78. / - 78 - < chalcopyrite, bismuthinite and native bismuth. The transgressiv/e ore-bo.d.ie,s are characteristically rich in copper. Arsenopyrite u/as encountered in the Rietfontein Reef only. Galena is restricted to some leader-reefs on Columbia Hill, and bornite was found in the Contact Reef at Bourke's Luck. Tetrahedrite is completely absent in this type of ore-body. The disseminated and sugary varieties of pyrite are generally not represented in the transgressive ore- bodies. The Trixie Lode at Bourke's Luck, however, is an exception to this rule. Disseminated pyrite commonly occurs in the marginal zones of this ore-body. (Barnard, 1958, p. 23). Two types of massive pyrite is present in the Trixie Lode, namely a coarse subhedral type, which occurs in grains of up to half an inch in diameter, and a very fine, compact variety. The coarse variety is considered to be contemporaneous with the sugary pyrite of some interbedded reefs. This is supported by the fact that the coarse subhedral pyrite occupies the marginal zones in the Trixie Lode, and, it is replaced by the fine compact type, which forms the core of the ore-body (Barnard, 1958,pp. 23-24). /The supergene . . . 79,. - 79 - The supergene products of oxidation are the same as those of the interbedded reefs* (ii) G a ngue-minerals. Quartz is the dominant gangue—constituent in the transgressive bodies. Carbonates are seldom encountered, except in the Trixie Lode uihere it is present in the upper and lower parts of the lode. Chlorite and sericite are found in appreciable quantities, but in dyke-reefs only. Epidote, hornblende, serpentine and specularite are present in minor quantities in the Trixie Lode (Barnard, 1958, pp., 33-34). Graphite is encountered in the Trixie Lode only, and in this ore-body the graphite is not distributed throughout the lode, but is restricted to the upper and lower ends, i ._e. near the contacts of the . i Dientjie Dyke. Intergrowths of quartz and calcite are closely associated with this graphite (Figure 1-3);j- . . (iii) Precious M e t a l s . The precious metals gold and silver are present as electrum in the transgressive reefs. Coarse gold is commonly presents nuggets weighing up to 80 ounces were found in the Hepta Reef (UJybergh, 1925, p. 101). The distribution of the gold is, however, very uneven, with the result that poor /values ... . . 81. te 5 ~31 uVV. 7) aO h v i a) 5 - 81- values may be encountered in some localities, while in others, one prospector's pan of ore may yield several ounces of gold. The ratio of gold to silver is characteristically low. Ratios calculated from figures supplied by Swiegers (1948, p. 124) show a ratio of 4.3;1 for the Rietfontein lYline and l.lsl for the Bourke's Luck ores. The gold is usually associated with the ore-minerals, but in some transgressive reefs, especially in blows and l e a der-reefs, it may occur in vein quartz. (d) Alteration of the Wall-rock The country-rocks adjacent to most of the transgressive reefs are relatively unaltered. Thin sections of specimens of quartzite taken at various distances from the Astra Reef, appeared to be identical in composition and texture (Swiegers, 1948, p . 89) . The Trixie Lode at Bourke's Luck is the only transgressive ore-body that shows appreciable alteration of its wall-rocks. Barnard (1958, pp. 36-38) found that this alteration resulted in the zoning of the adjacent country-rocks (Figure 13).. The first 5-10 feet adjoining the ore-body is composed of chlorite-sericite schist in which chlorite is the dominant constituent. /Disseminated...82. - 8 2 - Disseminated pyrite, which is similar to that in the wall-rocks of the interbedded reefs, is present'^in this zone. In the next zone, which is 9-15 feet in thickness, sericite dominates over chlorite. The outer zone is characterised by an abundance of hornblende. Numerous tear-gashes are present in all three these zones of alteration. Barnard (1958, p. 38) contends that the chlorite, sericite and hornblende were not introduced by hydrothermal solutions, but resulted from the hydrothermal alteration of the diabase, which constitutes the wall-rocks of the reef. (e) Classification of the Trixie Lode Although this ore-body is structurally classified as a transgressive reef, it differs in certain mineralogical properties from all the other transgressive bodies, but shows a remarkable resemblance to the interbedded reefs. The resemblance to interbedded reefs is borne out by the following; (i) The Trixie Lode contains disseminated pyrite and coarse, subhedral crystals of pyrite which are considered to be contemporaneous with the early pyrite in the interbedded ore-bodies. This type of pyrite is absent in the other transgressive reefs. /(ii) Graphite... 8-3, - m - (ii) Graphite and intergrowths of quartz and.- calcite are prominent gangue-constituents in- the. Trixie Lode. They are found also in the interbedded reefs, but are absent in the transgressive reefs. (iii) The wall-rocks of the Trixie Lode are altered to a considerable degree without showing any evidence of the introduction of foreign material. The wall rocks of the interbedded reefs are likewise altered, while those of the transgressive ore-bodies show very little alteration. It is concluded, therefore, that the Trixie Lode was formed contemn p o r aneous: with. the interbedded reefs. 4. Discussion The structural characteristics of the interbedded and transgressive reefs indicate that they are not quite contemporaneous. This is borne out also by the distinct differences in their mineral-assemblages. The first period of the mineralisation of the interbedded ore-bodies predates the b e dding-faults. The second period is contemporaneous with that of the transgressive ore-bodies. Two distinct periods of mineralisation, different both in age and in origin, are therefore present. (a) First Period of Mineralisation The minerals that were formed durio. g/ /this . . .84 • - 84 - this period are quartz, carbonates, graphite, pyrite and arsenopyrite. During the emplacement of these minerals the following processes were involved; serpentinisation, carbonatisation, silicification, graphitisation and pyriti s a tion. These processes are considered to be genetically related to the intrusion of the pyroxenitic and diabasic sills. According to Van Biljon (1964, p. 650) the serpentinisation of dolomite along the contact of a sill is influenced by the abundance of chert and the temperature of the rock during the intrusion. In the presence of sufficient silica and water, serpentine will form directly if the temperature is below 500 degrees Centigrade. Above this temperature, however, forsterite will form instead, and this mineral will be converted to serpentine if the temperature drops below 500 degrees. A conversion such as this will result in the formation of talc and serpentine. As no appreciable talc is found in the Pilgrim's Rest Area, it is considered that serpentine was the initial product and that the temperature did not rise above 500 degrees Centigrade. This reaction is represented by the following equations- 3Ca Mg (C03)2 + 2 Si02 + 2 H20 = (Ylg3 Si2 - °5 ( ^ H ) 4 + 3CaC0 3 + 3C0 2 /The CaCOg* » . . 0 ̂ . - -85 ~ The CaCO^* as well as the CO^ thus formed may be driven out of the zone of serpentinisation, i.e. away from the source of the heat, and some of this material may be emplaced as calcite and graphite along favourable horizons. Although pyrite may be extracted from the dolomite during contact-metamorphism the presence of gold cannot be ascribed to this process, as very little gold is present in calcareous rocks (Clarke, 1924, p. 657). Lindgren (1933, p. 699) contends that magmatic solutions will enter the rocks adjacent to a magma and will produce a series of metasomatic changes, the character of which will depend upon the composition of the solutions and their temperature, as well as upon the type of country rock that is exposed to the hot solutions. Dolomite and calcareous shales are easily invaded and are very susceptible to mineralising fluids even at low temperature. If the solutions contain enough silica, sulphur, iron, and carbon dioxide, the emplacement of an ore-body consisting of quartz, pyrite and graphite will occur. In shale or other sedimentary rocks, the intrusion of a magma will cause abundant silicification, thus rendering the rock more competent and, therefore, more receptive to brecciation and introduction of fluids (Park and (ïlacDiarmid, 1964, p. 144). According to analyses shown by Clarke / ( 1 9 2 4 , p . 6 5 7 ) , . . 8 6 . - 86 - (1924, p. 657), both gold and silver are more abundant in diabase and gabbro than in sedimentary rocks. Highly acidic rocks, such as granites, are not always accompanied by ore- deposits of the contact-metamorphic type, although they may preduce widespread effects of metamorphism and a later mineralisation of veins of quartz. Gold and auriferous arsenopyrite is present in a deposit near the contact of gabbro with limestone at Hedly, Alaska (Goldschmidt, 1954, p. 198). An example of metallisation due to the intrusion of diabase is found at Cornwall, Pennsylvania (Lindgren, 1933, p. 702). In the Sab i e - P i l g r i m 's Rest Area, practically all the interbedded reefs are located in the proximity of sills. It is considered, therefore, that the early mineralisation of the interbedded reefs can be ascribed to contact- metamorphism and contact-metasomatism. The intrusion of the sills is probably contemporaneous with the first period of folding in the area. The localisation of the early mineralisation is therefore, controlled by the presence of favourable horizons, as well as folds, Mineralisation of the first period is conspicuos only where anticlinal structures exist. Away from these structures, the reef-horizon is characterised mainly by an abundance of graphitic material and occasional small bodies of quartz and ca l c i t e . /(b) Second Period...87. - '87 - (b) Second Period of Mineralisation (i) General S t a tement. The first period of mineralisation was followed by brecciation due to the formation of bedding-faults along the mineralised zones. This was followed by a second period of mineralisation, due to the introduction of hydrothernal solutions. Most previous authors recognised only one period of mineralisation. Swiegers (1948, p. 131), ascribes the mineralisation entirely to precipitation from hydro- thermal solutions. He provides the following evidence for a mesothermal origin of the ores; (1) The coarsely crystalline massive texture, rough banding and the occasional presence of vugs suggest intermediate temperature- and pressure-phenomena. (2) The wall-rocks had undergone but slight alteration, which indicates a moderate temperature for the mineralising solutions. (3) The mineralogical composition of the ore is characteristic of the intermediate type of hydrothermal deposit. Only this conclusion is considered to be reliable proof of a mesothermal origin, as the other properties are not necessarily characteristic of mesothermal deposits o n l y . /(ii) Sequence of M i n e r a l s ..88• - S8 - ) Sequence of M i n e r a l s . The sequence of .the minerals of the second period of mineral­ isation is as follows; (1) The early gangue-minerals preceded the introduction of the ore-minerals and were emplaced in channelways provided by the brecciation of the ore-locus of the first period. The introduction of these constituents was evidently accompanied by the partial replacement of the breccia thus formed. The minerals that were emplaced are mainly quartz and carbonates, the former being locally replaced by the latter (Swiegers, 1948, p. 120). (2) The early ore-minerals pyrite and a r senopyrite, accompanied by gold, commonly replace the early gangue- minerals, as well as the brecciated material of the first period of mineralisation. These constituents are invariably massive, in contrast to the disseminated or sugary character of the suphides of the first period. Arsenopyrite crystallised after the pyri t e „ (3) The late ore-minerals are mainly pyrite, chalcopyrite, tetrah e d r i t e , b i smuthinite, native bismuth, galenobismutite and the precious metals. It is difficult /to ... 0 9» - S B . - to derive a definite order of succession from the association of these ingre­ dients and they are therefore considered to be contemporaneous in age (Swiegers, 1948, p. 121). (4) The late gangue-minerals, consisting of quartz and calcite, occupy veins and fractures in all the other minerals. (5) The supergene sulphides chalcocite and covellite resulted from the alteration of the copper-minerals in the sulphide- zone. The oxide-minerals in the leached zone are contemporaneous with the supergene sulphides. (iii) Sequence of E v e n t s . The minerals of the second period of mineralisation show very little deformation. This deformation is restricted to two distinct phases. The first is represented by fractures in the pyrite and arsenopyrite, which are filled with late ore-minerals. The presence of late gangue-minerals along fractures in the late ore-minerals is indicative of the second phase. The sequence of the events during the mineralisation of both the interbedded and transgressive reefs are thus visualised as follows; (l) The infilling of openings and partial replacement by quartz and carbonates. /(2) This... r90. - 90 - (2) This phase mas followed closely by the precipitation of auriferous pyrite and arsenopyrite. (3) Slight movement resulted in a mild shattering of the early minerals. (4) Precipitation of late ore-minerals as well as gold along fractures within the earlier sulphides and g a n g u e . (5) Recurrence of mild shattering. (6) Precipitation of the late gangue- minerals along fractures in the early sulphides and gangue, as well as in the late ore-minerals. (7) Supergene alteration of the above minerals, resulting in oxides, carbonates, silicates and supergene s u l phides. Swiegers (1948, p. 119) offers a somewhat different interpretation of the paragenesis of the ore. He contends that the first phase was one of initial infilling of fractures by gangue-minerals, followed by precipitation of pyrite and arsenopyrite, both disseminated and massive. According to Swiegers (1948, p. 119), no mineralisation was present before the formation of the bedding-?f a u l t s , * *' / 0 n d * 0 e «51° - • 91 - and the alteration of the wall-rocks was caused by hydrothermal solutions which postdate these faults. This is, however, not in accordance with the facts; the presence of gangue and sulphides, as well as the products of wall-rock alteration, which were emplaced prior to the faulting, was established beyond doubt. The interpretation by Barnard (1958, pp.35-36) of the paragenesis of the Trixie Lode, is in general agreement with the interpretation by the present author. Barnard recognised the presence of two generations of pyrite, and he ascribed the origin of both to hydrothermal action. It is considered however, that the first generation of pyrite and gangue was caused by a process of contact-metasomatism, followed by two later generations of hydrothermal origin. (c) Precipitation of Gold and Silver The precious metals are considered to have been precipitated during three different phases. The first phase is contemporaneous with the contact-meta- somatic period of mineralisation. During this phase the only sulphides generated were pyrite and arsenopyrite. The second introduction of gold and silver was contemporaneous with the hydrothermal / d e p o s i t i o n . . ,92. - 92 - deposition of pyrite and arsenopyrite, i.e. after the brecciation of the early ore- minerals had taken place. The third, and most important phase is contemporaneous with the precipitation of copper- and bismuth-minerals. All three generations of gold are present in the interbedded reefs, while only the hydrothermal generations occur in the transgressive bodies. This conclusion is supported by the fact that the ratio of gold to silver in the interbedded reefs differ distinctly from that of the trans­ gressive reefs. C » ASBESTOS 1. Distribution Occurrences of chrysotile-asbestos are found in several localities in the gold-field. It is particularly interesting that the more important of these are present in areas where interbedded gold-bearing reefs are developed. Plate 1 shows the workings of asbestos in relation to gold-bearing deposits,. The bodies of asbestos are, however, small and generally not economically exploitable. The largest occurrences are those on the farms Olifantsgeraamte, Graskop and Normadale. These deposits were economically exploited between 1951 and 1954.- • During the second' world war /prospecting ...93. ^ 93 - prospecting operations mere carried out on an extensive scale. Minor quantities were found in several places on the farm Ceylon, along the banks of the Lone Creek as well as below the Bridal Veil Falls. Prospecting disclosed poorly developed fibre on the banks of the Blyde River on the farm In De Diepte. Irn the mines at Pilgrim's Rest, asbestos of inferior quality u/as frequently found in cross-cuts and in inclined shafts. These were usually restricted to patches of slip-fibre. Small lenticular patches of fibre occur west of V/aalhoek, on the farms Frankfort, Sacramento Creek and Normadale. On the farm Kaspersnek, sporadic bands and lenses of asbestos were prospected on a small scale. Mode of Occurrence The development of fibre is restricted to the zones of contact-metamorphism adjacent to some of the sills. U/ith the exception of the occurrence at Graskop, all the asbestos in the area is found in serpentinised dolomite adjacent to pyroxenite sills. They are, therefore, confined to the calcareous rocks, i.e. to the upper part of the Dolomite Series. The fibre in the Graskop Chrysotile Mine is present in a serpentinised zone located along the upper contact of a diabasic sill. The stratigraphical position of this sill is about /50 feet... 94. - 94 - 50 feet below the Blyde River Quartzite. The length of the fibre is usually very short, i.e. less than £ inch. In o feu/ places fibre of up to 1-j inches in length mas observed. The asbestos occurs in lenses which are not continuous along strike. ÍYlost of the fibre occurs within 25 inches of the contact of the intrusive sill. Farther away only scattered "eyes" of fibre are present. In some locali­ ties, serpentinisation is present within the pyroxenitic sills, usually along the contacts of t.he sills. In these localities chrysotile may be present within the intrusive rock. 3. Relationship to Gold-deposits It is considered significant that, although all the intrusive sills in the Dolomite Series are accompanied by serpentinisation of the dolomite, chrysotile has developed in certain areas only. It is furthermore an interesting fact that well-developed, although not necessarily payable, interbedded gold-bearing reefs are often found in the neighbourhood of fibre-occurrences. This probably suggests that a genetic relationship exists between these two very divergent types of mineralisation. Whereas serpentinite is practically ubiquitous along the contacts of basic sills, chrysotile asbestos is only locally developed. There are three possible modes of growth of chrysotile fibres (Van Biljon, 1954, p. 662). Firstly,, the fibres could have grown in op.e.n / cavities .... 95. - 95 - cavities, in which case growth must have taken place from both sides, and the chrysotile should therefore have a parting where the fibres meet. Secondly, the fibres could grow at the expense of the wall-rocks, i.e. replacing the serpen- tinite during its growth (Dresser, 1913). The third possibility is that the fibre grew in fractures, either by pushing the walls apart (Taber, 1916a, 1916b, 1917, 1924 and 1926), or during the gradual separation of the walls of the fractures under the influences of other forces (Keep, 1929). Evidence from the present area suggests that the asbestos is the result of tension, i.e. the fibres of chrysotile grew during the gradual opening of the fractures (Von Biljon, 1964, p. 664). Tension-fractures such as these may develop along certain portions of shear-faults, in the tensional region of folds, or as a result of the cooling of igneous rocks (Newhouse, 1942, p. 10). \Jan Biljon (1964, p. 665) observed that the horizons of fibre at the [Vlunnik Myburgh Mine near Kaapsehoop appeared to be related to folding. Around 1958 a certain Mr. Charles Bekker did some prospecting at the Graskop Chrysotile Mine. During his operations he formulated a theory regarding the localisation of exploitable fibre. He ascribed the lenticular character of the fibre-bodies to local tensional and compressional forces /after . . . 96. - 96 - after the intrusion and cooling of the sills. He observed that lenticular bodies of chrysotile mere associated with slight rolls in the contacts of sills. The deposits of chrysotile in the Sabie- Pilgrim's Rest Gold-field are not necessarily associated u/ith faults. The tension must therefore.have been caused, either by folding or by the cooling of the sills. As fibre is not everywhere developed, its formation is considered to have been controlled by the folding. It is concluded that the same structural conditions which controlled the localisation of the gold- bearing deposits in the interbedded reefs, resulted also in the formation of the fibres of chrysotile. V. ORE-CONTROL A. GENERAL STATEMENT In order to determine the relationships between the mineralisation and the structure, it is necessary to describe the geology and the structure of each deposit. For practical purposes the gold-bearing occurrences are divided into six groups. This classification is based on the association of the ore-deposits with regional structures which had apparently influenced the localisation of the ore. It must be pointed out, however, that this classification is not necessarily based ^pn /genetical ...97 - 9? - genetical considerations as some ore-bodies are related to more than one structure. The first group consists of deposits that are closely associated with the Vaalhoek and Dientjie Dykes. The occurrences of the second group are found along or near the Nestor Dyke, or the fault into which this dyke u/as intruded. The third group comprises the ore-bodies in the central Pilgrim's Rest area. The ore-deposits of the lYlount Anderson Area are classified with the fourth group. This group also includes the deposits in the Elandsdrift Area, i.e., all the occurrences that are found near the Hoppe's' and Elandsdrift Dykes. The fifth group includes those occurrences which cannot be grouped with any of the major structural features. These deposits are associated with local structures of limited extent and they are usually of minor economic significance. The last group con­ stitutes the vertical reefs along or near the edge of the escarpment. The above classification is illustrated in Figure 14. If the main structural lines in this figure are extended, a rough parallelogram is obtained. It is conspicuous that all the important gold-deposits in the Transvaal System of the Eastern Transvaal are located approximately within this parallelogram, which indicates special structural conditions. The deposits that fall outside the parallelogram are relatively / u n i m p o r t a n t . ..99. H 5. o vo. o H 0 >o A. w 3 0 0 V. £ U k0 D < 4> 5) ví N Q. 3 0 V O © H a 3 fi @ H a 3 L0 CD - 99 - unimportant, except the Rietvallei Mine. This mine is situated far to the south of the area, but is located on the approximate southward extension of the Nestor and/or Rietfontein structural features. Another interesting observation is that the centres of mineralisation are often localised at or near the intersections of the lines that describe the parallelogram. This applies also to sympathetic lines inside the parallelogram. The mineralisation at Bourke's Luck is found near the northwestern corner of the parallelogram. The deposits in the Mount Anderson Area occur near the southwestern corner. The Elandsdrift and Horseshoe Creek Mines are situated near the southeastern corner. The Hepta Reef represents the northeastern corner. At Pilgrim's Rest the largest concentration of gold-bearing mineralisation occurs near the intersection of the directions of Groups I and III. Farther east, along the direction of group III, trans­ gressive bodies are found near the intersections of Groups II, III and V I . This phenomenon is considered to illustrate, on a regional scale, the close relationship between the structure and the localisation of the mineralisation. /B. O R E-DEPOSITS..100. - 100 - B. 0RE-DEP0SITS RELATED TO THE VAALHOEK AND DIENT3IE DYKES ‘ (GROUP I ) 1. Deposits at Bourke's Luck (a) Geological Environment The ore-deposits of the Bourke's Luck Mine are closely related to the Dientjie Dyke. The dyke transects the Blyde River near the upper end of the Blyde Canyon, and the Treur River near its confluence with the former. The stratigraphical sequence, as revealed by boreholes, is as follows; The surface of the granitic floor of deposition occurs at a depth of 440 feet below the top of the Black Reef Series. The granite is overlain by 400 feet of quartzite of the UJolkberg Formation. This quartzite contains occasional shaly and gritty layers, as well as, four diabasic sills, 10 feet, 15 feet, 9 feet and 70 feet in thickness, respectively. The quartzite of the Black Reef Series, overlying the UJolkberg Formation, has a thickness of 40 feet. To the north, in the canyon of the Blyde River, the upper quartzite of the UJolkberg System is underlain by a layer of shale, which overlies a lower quartzite-member of the UJolkberg Formation, as well as the Godwan Formation. Here the granite occurs at a depth of over 1200 feet below the /Black...101. -101 - Black Reef Series. This indicates the existence of o hummock of granite in the floor of deposition. The sedimentary strata.locally dip to the west at an angle of about six degrees. The gold-bearing deposits are associated with the Dientjie Dyke and occur at an elevation which coincides with certain stratigraphical horizons in the UJolkberg Formation. These horizons are located at elevations which vary from 260-350 feet below the top of this formation. The emplacement of the ore at Bourke's Luck was controlled largely by faulting and fracturing control due to folding could not be estab l i s h e d . (b) Dientjie Dyke This dyke can be traced, on the surface, from near the southern boundary of UJillemsoord 476. K.T. northwards to beyond the Blyde Canyon and the edge of the escarpment. To the south the dyke becomes obscured by alluvium. Farther south, east of the Blyde River, several dykes occur along the line of the Dientjie Dyke. These dykes are, however, slightly different in composition and texture, and they are apparently younger in age. The Dientjie Dyke is described /by .... 102. - 102 - by Barnard (1958, p. 18) as an altered quartz- gabbro. It is approximately 120 feet thick and it dips 65-70 degrees to the east. The dyke 3 ■ is distinctly older than the earliest stages of the mineralisation. This is indicated by the fact that a fracture-plane in the dyke, i.e., the Trixie Lode, is occupied by ore-minerals which belong to the earliest stages of the mineralisation. The Dientjie Dyke is intersected by several younger dykes of diabase. (c) Ore-bodies Figure 15 is a schematic section across the Dientjie Dyke just north of its intersection with the Blyde River. The section illustrates the relationship of the ore-bodies to the dyke. (i) Trixie L o d e . This is the most important gold-bearing ore-body in the Bourke's Luck Mine. It is a lenticular body within the Dientjie Dyke and reaches a maximum thickness of 7 feet. It strikes between 020 and 025 degrees and dips 20-30 degrees east (Barnard, 1958, p. 17). The ore-body followed and exploited for nearly 19,000 feet along the dyke. At its northern end the lode fingers out into several inferior veins and stringers of quartz. To the south, mining operations were terminated due to water-difficulties and prohibitive operational costs. /The Trixie... 104o VJ&s>t £-*a -S i : f ^ i g u r e. / S ' - S c h & r r \ 6 /c. js e cL Íio r^ . ^ c r - o & a éhe. D i e -r^ t jie . D í^ k e - . (Fr~orr\ old record s) S C ts le , : - / ■ ’ / OOO e fc r e r \ c e I + ■* +\ Dier^ij,e. Z3y*e. - | S ec//menídry roc*5 o/ ÍAe l/o^foe^ © ~Trixie. /.oc/c ® Ë>o<-jrk.e.‘£> L u c k . /?eeA @ F-~e.rr>^ /^eeA © C ors-b A <9/vc/ ivor/c. © D tSC ^«re . / - O W - < s r \ j/e /ér - 104 - The Trixie Lode occupies a fracture-plane in the dyke which merges into an interbedded reef in the adjacent quartzite. The inter­ bedded reef is represented by the Bourke's Luck Reef. It seems therefore, that the Trixie Lode merely represents the continuation of an interbedded reef which had undergone deflection along a pre-existing fracture. This conclusion is supported by the fact that the Trixie Lode resembles the interbedded reefs, rather than the transgressive reefs, in m i n e r a l o g y . The Dientjie Dyke, as well as the Trixie Lode is displaced horizontally by a low-angle gravity - f a u l t . This fault divides the Trixie Lode into a so-called I-lode above, and an E-lode below the fault-plane. The fault has a dip of 13 degrees to the west, which is slightly steeper than the dip of the quartzite. As the strike of the fault is 030 degrees and that of the dyke 020 degrees, it follows automatically that the E-lode will grow at the expense of the I-lode as one goes northwards. Eventually the latter lode disappears completely. In the north, therefore, the E-lode represents the whole of the undisplaced Trixie Lode. Southwards, the opposite effect is obtained. The displacement along the fault increases from a few feet in the southern part of the mine, to more than 100 feet in the north. /(ii) Bourke's Luck Reef.,.105. - 105 - (ii) Bourke's Luck Reef (Figure 1 5 ) . This interbedded reef urns exploited on a very limited scale. Although it contains some very rich patches of ore, it is generally not payable. In the River-section of the mine the reef is exposed on the banks of the Blyde River, on both sides of the Dientjie Dyke. (iii) Fern Reef (Figure 1 5 ) . This ore-body is described by Barnard (1958, p. 13) as a flat reef with an average thickness of 5 inches. It occurs at a stratigraphical position which is some 40-50 feet below the Bourke's Luck Reef, and some 25 feet above the fault. (iv) Contact Reef and Stockwork (Figure 1 5 ) . The Contact Reef occurs below the fault-plane , along the western contact of the dyke. It spreads out immediately below the fault and follows the bedding for a short distance. The Contact Reef is usually accompanied by a stockwork of lenticular leaders located to the west of it. In some places these leaders merge with the Contact Reef to form sol-id mineralised bodies of up to 15 feet in thickness. The Contact Reef was ex­ ploited for about 5000 feet along the dyke, in the northern part of the mine. The Contact Reef and Stockwork are characteristically rick in copper-minerals such as chalcopyrite and covellite, as well / a s . ..106. as pyrite (Barnard, 1958, p. 15). Dyke R e e f . This is a vertical reef in th.e foot-wall part of the displaced dyke. It is roughly parallel to the malls of the dyke, varies in thickness between 3 inches and 2 feet, and has a vertical extent of over 90 feet (Barnard, 1958, p. 15). The mineralogy of the reef is typical of the transgressive ore-bodies in the area. Alteration of the wall-rock is practically absent. Discussion (i) Ore-control Barnard (1958, p. 10) contends that the structural conditions which were necessary for the intrusion of all the dykes are closely related to the conditions necessary for the localisation of the ore. This statement is considered to be rather vague. Although the general structure, as well as the mineralisation, is considered to be genetically related mainly to the intrusion of the Bushveld Complex, the localisation of the ore- bodies was controlled by specific conditions of structure. At Bourke's Luck this condition is the presence of an early dyke of considerable thickness. It has already been shown that early lines of weakness may have resulted from /compaction-folding,.107. - i 0 V - compaction-folding over hummocks of granite* It is possible therefore, that the fracture along which the Dientjie D yke.: i ntruded was caused by an irregularity in the floor of deposition. Although the rupture, and consequent mineralisation of the Dientjie Dyke were due to tectonic adjustments that accompanied the intrusion of the Bushveld Complex, the localisation of the ore-rbodies was controlled by the presence of the dyke. (i i ) Sequence of Events The sequence of the tectonic events in the mineralisation of the ore-bodies is visualised as follows; (l) The formation of the fracture which constitutes the Trixie Lode, was the result of shearing stresses which caused rupture on encountering the transgressive, competent Dientjie Dyke (Barnard, 1958, pp. 44-45). The present author contends that these forces were provided by the intrusion of the sills, which is approximately contemporaneous with the early folding. This structure is not considered to be a fault-deflection associated with the bedding -thrusts as the deflection is downwards and not upwards as is normally the case. /(2) T h i s...108. - 108 - (2) This event mas followed by the contact-metasomatic mineralisation of this structure as well as the Bourke's Luck and Fern Reefs. (3) The shattering of this ore-body contemporaneous with the low-angle th rust-faults. (4) The introduction of massive pyrite and gangue from hydrothermal solutions. It is considered that these solutions were fed through the Bourke's Luck R e e f . (5) The formation of a low-angl'e gravity- fault, resulting in the displacement of the Dientjie Dyke. During this event a fissure within the dyke, as well as fissures along the western side of it, were opened in the foot- wall of the fault. Mild shattering occurred within the Trixie Lode. (6) The solutions which contained the late ore-minerals found their way into these fractures to form the Dyke and Contact Reefs and the Stockwork. The Trixie Lode and interbedded reefs were enriched by these fluids. The presence of ore- minerals along the gravity-fault suggests that the solutions flowed through this structure (Barnard, 1958, p. 44). / 2 . Vaalhoek M i n e . ..109. - 109 - Vaalhoek ,/IYline (a) Geological Environment The ore-bodies of the Vaalhoek lYline are developed between 80 and 1 5 0 , feet below the Blyde River Quartzite. This reliable marker occurs about 1000 feet above the base of the Dolomite Series. The mineralisation at Vaalhoek appears to be closely related to the Vaalhoek Dyke, as well as to very open folds which are present to the west of the dyke. (b) Vaalhoek Dyke The Vaalhoek Dyke is considered to be a hybrid rock (Zietsman, 1964, p. 51). It can be traced on the surface from the IMek Section of the Vaalhoek lYline (Plate III), southwards to the central part of Rotunda Creek 510 K.T. (Plate I). The dyke varies between 100 and 180 feet in thickness. It has a strike of approximately 020 degrees and a dip of 60-70 degrees east. At the Nek Section it shows a downthrow of about 125 feet to the east. From here the displacement decreases rapidly southwards. At No. 1 Shaft it is only 25 feet (Barnard, 1958, p. 8), while south of No. 2 Shaft no displacement can be observed. The intrusion of the Vaalhoek Dyke / predates o... 110. - i i o - predates the first përiod of mineralisation. It is transected by several younger dykes of diabase, the most important of which are the so-called transverse dykes. Both these dykes show a downthrow of 10 feet to the north. The Vaalhoek Dyke has certain characteristics in common with the Dientjie Dyke. Both dykes predate the formation of the interbedded reefs, and both are similar in thickness, strike and dip. Apart from these likenesses, both have also been displaced by 'low-angle gravity- f a u l t s , Figure 16 illustrates the displacement of the Vaalhoek Dyke by a low-angle fault. In view of the similarities mentioned above, the Dientjie and Vaalhoek Dykes could possibly represent a single intrusion. Their present positions may have been the result of a horizontal dis­ placement, but due to an extensive cover of alluvium the exact relationships could not be determined. (c) Structural Features The joints in the vicinity of the Nek Section show a very distinct pattern. Three directions are distinguished, namely 325, 020 and 055 degrees. The first is the /most . „ . 112 . 3 K»9/Aoe X; /■ /" / 6 . c/t/A.e 9 l̂ á/Ẑ oeA. C 5 / y c / e / V e r ^ < / < 3 / - ^ / ^ e . - A *7"\er6 b e .J o *y ^ ^ e\y — / ^ n e ^ , £>orr\<̂ -f-irr\̂ 4> /n < *\ c /-«5»// £> e.c/. /ow~ar̂ gie. £- JD te*i £x»Z>lC. £> ÏH G T A e / < 5 l e / > 5 . O X̂ o/&/r\/̂ e <3f̂\c/ cAĉ .̂ Ôkv' -̂ Av̂ /e £<3u /£ old t-c rcêM'vt ,¥! this will yield an e 1 u v i a 1 deposit on weathering, B . ORE-DEPDSITS RELATED TO THE NESTOR STRUCTURE (GROUP II)________ This group comprises all those deposits which are apparently related to the Nestor D y k e ? the fault west of the Lisbon Falls, and the Mem Chum Dyke. South of the Nestor Mine, mainly interbedded reefs are developed, wheras trans­ gressive ore-bodies are the dominant structures north of the mine. UJith the exception of the Ledouphine Mine, all the mineral-occurrences are located within 200 feet of the base of the Dolomite Series. 1. Glynn's Reef South of Sabie (Plate V ) (a) Stratiqraphical Relationship The extent of the mineralised area south of Sabie is only slightly smaller than that of the Pilgrim's Rest Area. All the mineralisation here is confined to the Glynn's Reef. The stratigraphical position of this interbedded reef ranges from 145 to over 200 feet above the base of the Dolomite.-.: Series (Visser and l/erwoerd, 1960, p'„ 67). Two sills of diabase are present near this horizon, one above, and the other below the reef. In some localities the two sills may be 150 feet apart, with G l y n n :s Reef /occupying ... 123. - 123. - occupying any position between them (Visser and V e r w o e r d , 1960, p. 68). A thin layer of quartzite is developed 50-70 feet above the reef. This quartzite was formerly assumed to be the Blyde River Quartzite (Hall, 1910, p. 115). The quartzite-marker in the Sabie Area is, however, considered to be the equivalent of the Diggers' Sandstone of the northern area (Zietsman, 1964, p. 66), Thin layers of quartzite and black shale are present in the dolomite, above and below the reef. This suggests that the Glynn's Reef is located in the Transition Zone between the Black Reef and the Dolomite Series. (b) Structural Features Owing to the fact that the mines south of Sabie were abandoned and therefore in­ accessible, no data could be collected underground. Some information could, how­ ever, be compiled from underground-plans and records. A close relationship between the structure and the mineralisation is shown in Plate V. The two main ore-shoots are roughly parallel to local faults and dykes. This direction is parallel also to the main structural trend of the area. Minor pay-streaks within these ore-shoots, are reported by Uisser and Verwoerd (i960, p . 68). /Linear .... 124. - 124 - Linear features which predate the reefs, are considered to be present. Two dykes, which are displaced by the interbedded reef, occur (l/isser and Verwoerd, 1960, p. 69). One is found in the T.G.IÏ1.E. Malieveld Section, and the other near the South Shaft in the Glynn's Lydenburg Mine. The former dyke is considered to be the continuation of the Nestor Dyke. A study of the underground-plans provided the contour-lines shown in Plate V. The contour-lines suggest -two directions of gentle folding, i.e. an early east-west direction and a later northeasterly direction. Several small domes are present, and are considered to be the result of this super­ imposed folding. (c) Character and Mineralogy of the Ore-body The Glynn's Reef usually consists of two layers, separated by up to 4 feet of chert and dolomite. The upper layer is from 6 inches to more than 3 feet in thick - ness. The lower layer, which may be absent in places, does not exceed 6 inches in thickness (UJybergh, 1925, p. 29). The dolomite that separates the two reef-layers, may contain a network of quartz-*stringers in places. These may locally develop into a solid mineralised mass (UJybergh, 1925, p. 31). Above the upper layer of the reef, thin / v e r t i c a l ...125. - 125 - vertical veins of barren calcite extend for a few feet into the hanging. ll/ybergh's (1925, p. 31) description of the Glynn's Reef indicates that it is typical of the normal interbedded type of ore-body. Early gangue-quartz shows a laminated appearance due to thin layers of pyrite which probably belongs to the first period of mineralisation. Later quartz^ with associated massive pyrite, is more frequently found where the reef is wider. According to old mine-records the copper- and bismuth-minerals are well-represented also. Rietvallei lYline The Rietvallei lYline is situated about 20 miles south of the investigated area. It occurs along the same line as the deposits of Group II. The Rietvallei ore-body is an inter­ bedded reef which is developed some 50 feet above the Black Reef Series. In one locality in the mine, the reef lies directly on a sill of diabase. Stringers of gold-bearing quartz branch off into the sill in this locality (Visser and Verwoerd, 1960, p. 66). A prominent ore-shoot, the axis of which coincides with the axis of a shallow, but distinct, anticlinal structure in the reef, is present also (Visser and Verwoerd, 1960, p. 67). The direction of this axis is approximately north, i.e. parallel to the IMestor-structure. /3. Sandstone ... 126. - 126 - Sandstone Reef (Plate IV) (a) Stratiqraphical Relationship Although the Sandstone Reef is present in many localities in the district, it is economically exploitable in the area north of the Sabie River only. It mas mined on an extensive scale in the Nestor [Yline, as well as in the Rex Mine (Plate IV). In the Nestor Mine the reef is developed 20 feet below the top of the Black Reef Series. In the Rex Mine its position is somewhat nearer to the top of the quartzite. A layer of laminated shale, 2-6 inches in thickness, is present about 2 feet below the reef. This layer is usually over- lain by a thin layer of quartz and pyrite. (t?)... Structural features The mineralisation of the Sandstone Reef is confined to the western side of the Nestor Dyke. This mineralisation apparently terminates and spreads out against this structure, as indicated by a narrow zone of mineralisation located along the western contact of the dyke (Plate IV). The information obtained from a short tunnel driven into the dyke, suggests that the dyke is displaced along the Sandstone Reef. The North Section of the mine terminates against a normal fault which parallels the Nestor. Dyke (Visser and V-erwoerd, 1960, p. 64). This fault also intersects the South Section of the . /mine *..12 7* - 127 - mine, but in this area high gold-values are present on both sides of the fault- plane . Underground-examination of the Nestor Mine rev/eals that the Sandstone Reef is payable only along two east-trending open anticlines. The northern anticline constitutes the North Section of trie Nestor Mine, and the southern anticline, the South Section; the ore-body is apparently un­ payable in the syncline between these two s t r uctures. At the Rex Mine the mineralisation is likewise closely related to folding, in this instance a dome in the quartzite of the Black Reef Series (Plate IV). Character and Mineralogy of the Ore-bodies The Sandstone Reef normally varies between 2 and 30 inches in thickness,although figures of 40 inches and over were recorded in the Rex Mine. The Sandstone Reef consists mainly of quartz and pyrite (Visser and Verwoerd, 1960, p. 63). Copper- minerals are only present in the thicker parts of the reef. The refractory nature of the ore is probably due to the presence of cither pyrrhotite or marcasite.. Two blows, which are intimately associated with the interbedded reef, are /present... J.í',, - 128 - present. The Pyrite Blow is a tabular body, 3000 feet in length, 25 feet in width, and about 12 feet in height. It is developed above the interbedded reef only. The blow consists of large massas of sugary pyrite in places. In other localities it is composed of quartzite which is intensely inpregnated with disseminated p y r i t e . The Copper Blow occurs to the south of the Nestor Mine, and on the western side of the Nestor Dyke. It strikes roughly parallel to the Pyrite Blow. The ore is composed of massive quartz, containing late suphides, and gold (l/isser and Verwoerd, 1960, p. 86). An underground examination at the northern end of this ore-body shows that it consists of a zone of steeply inclined leaders. These leaders are, like the Pyrite Blow, only developed above the Sandstone Reef. The leaders along the edges of the ore-body measure only a few feet in height, but become progressively higher towards the centre of the ore-body. The central part of the blow consists of a massive mineralised body of quartz which is approximately 20 feet in height. This massive part of the blow is probably a coalescence of leaders. In view of the divergent nature of the mineralogy of the Pyrite and Copper / B l o w s ...129. - 129 - Blows, they are considered to be different in origin. The Pyrite Blow is probably related to the first period of mineral­ isation, while the Copper Blow is related to the second, or hydrothermal period of mineralisation. 4. Area Around lYlac-fflac The Glynn's Reef was exploited in a relatively small area, about one mile northwest of the (Yla c-IYlac Falls. The workings terminate against the eastern side of the lYlac-ÍYIac Dyke (Plate I). According to a description by Frost (1912), the reef was well-developed and well- mineralised. Immediately south of the workings, the lYlac-IYlac Dyke is transected by a transverse dyke (Plate I). This dyke contains an abnormally high percentage of pyrite in the area between the lYlac-IYlac Dyke and the Nestor Dyke. Very little pyrite is present outside this zone. Transgressive Reefs are present along the western contact of the Nestor Dyke. In the area west and southwest of the falls, evidence of extensive eluvial workings is found. These workingsare situated between the Nestor and lYlac-IYlac Dykes. They occur at a horizon which is stratigraphically just below that of the Glynn's Reef. This reef, however, does not exist here, the outcrop being farther to the west. The presence of the eluvial workings therefore / s u g g e s t s ...130. suggests that the Glynn's Reef was present and mineralised between the two dykes, but that subsequent erosion eliminated the ore- body, resulting in the formation of a residual d e p o s i t . Malidyke Mine (a ) General Character of the Ore-bodies The ore-bodies of this mine are closely related to the Mali Dyke, and are situated southwest of Graskop, between the Diggers' Creek and the Mac-Mac River. The Mali Dyke, which is pre-reef in age, is approximately parallel to the Nestor Dyke. Old mine-records reveal that the ore-bodies consisted of lenticular veins along both contacts of the dyke. Ladder- veins, and sometimes a stockwork of leaders, were developed within the dyke itself. Observations in an accessible part of the workings corroborate this description. No records of the mineralogy of the ore are available, but small remnants of the ore-bodies in certain parts of the mine consist of massive vein-quartz and l i m o n i t e . . -.fcfejL̂ -.Stratigraphida.l Relationship During the exploitation of the ore-bodies, extensive mining was carried out by means of opencasts. fin interesting / f e a t u r e ...131. - 131 - > feature, however, is that the location of these workings suggests that the ore- bodies were payable along a certain stratigraphical zone only. This zone is in the proximity. • of the horizon of the Glynn's Reef, and is probably located immediately above it. This fact is borne out also by the elevations of the under­ ground workings. The Glynn's Reef forms an out­ crop on the eastern bank of the Mac-Mac River, east of the northern end of the Malidyke Mine. This reef is, however, not payable in this locality, although it contains much quartz and limonite. 6. Area Around the Lisbon Falls The area around the Lisbon Falls is very interesting, especially from a structural point of view. From an economical point of view, however, the area is relatively unimportant. The interbedded reefs are represented by the Gould's Reef, the Betty Reef, the Bott's Reef and the Pigeon Reef; the transgressive reefs by the Astra Reef and some leader-reefs to the northwest of the falls. (a) Stratigraphical Relationships The stratigraphical positions of the ore- bodies are shown in Figure 17. The Pigeon Reef occurs near the base of the UJolkberg Formation; / t h e ... 133. - 133 - the Betty Reef is developed along the upper contact of the Black Reef Series; the Gould's Reef is located in the Transition Zone of the Dolomite Series, about 12 feet above the Betty Reef; the Bott's Reef occurs in the Transition Z o n e ; about 30 feet above the base of the Dolomite S e r i e s . The Astra Reef is a vertical reef which is developed from the top of the Block Reef Series, downwards for a vertical distance of over 200 feet. The leader- reefs to the northwest of the falls are located in the Transition Zone of the Dolomite Series. They do not exceed 30 feet in height. (b) Structural Features The most pronounced structural feature is a granitic ridge, which is Pre-Godwan in age. The axial trace of this feature trends approximately 010 degrees. Due to compaction the overlying strata were folded around this structure. During the process of compaction dis­ placement of the strata occurred on both sides of the ridge. These faults strike approximately parallel to the axial trace of the ridge, and some of them have been subjected to recurrence of movement. /A f a u l t s . ..135. F 'iqura. /e S c A e m d < iic sec7/on. d c r o s s A.e tQa^na A íe e £. «Soca'/e ! / /'acK /̂ ĝ e. rgrvc €- l~*~ -i. ~M S / / / o/~ r f A s i s s e © fí&é-r-r-r̂i£&. sSer/tS, • '• '. ‘| Ouĉ ró̂ c./ée 3/<=?c/c ̂ eeA 5er/es XX kes-h>a cJdexf f-<3U /é. - 135 - A fault east of the Lisbon Falls is occupied by the Astra Reef. An underground examination in the Astra Hfline revealed that this fault was displaced horizontally along a bedding-fault, prior to the mineralisation, as is illustrated in Figure 18. The transgressive fault which is a high-angle normal fault, shows a downthrow of over 50 feet to the east. This fault predates the mineralisation of the interbedded reefs. This is indicated by the fact that on interbedded reef in the Little Gem Mine, south of the falls, continues, without displacement, through the fault-plane. That the transgressive reef, which occupies the fault-plane, is displaced by the interbedded reef is indicated by the fact that the former apparently terminates against the lower contact of the latter (UJybergh, 1925, p. 99). The interbedded reef occupies the upper contact of the Black Reef Series, west of the fault, but it is developed some 30 feet above the contact, on the eastern side of the fault. The interbedded fault in the Astra Mine has a dip of 6 degrees east. The Astra Reef, which occupies a high-angle fault, shows a displacement of 10 feet towards the west. The interbedded fault, therefore, resembles a low-angle thrust. I /The ... 136. The low-angle thrusts in the rest of the area, however, show a relative displacement towards the east. The intur- bedded fault in the Astra Mine is therefore not a true low-angle thrust, but only resembles one due to the local easterly dip of the strata (Figure 17). Farther east and west along the same plane of m o v e ­ ment, the displacement will be that of a low-angle gravity - f a u l t . The other linear structures in the area around the Lisbon Falls, trend mainly in two directions, namely 010 and 060 degress. They are represented by faults, dykes, and leader-reefs (Plate I). Ore-bodies (i ) Pigeon R e e f . This reef was exploited on a very limited scale near the bottom of the Lisbon Falls. An examination revealed that the reef is a true inter­ bedded ore-body which occurs near the contact between the UJolkberg Formation and the underlying granite. This conclusion contradicts UJybergh's (1925, p. 99) statement that the Pigeon Reef is a transgressive reef. The ore-body dips about 30 degrees to the west, but the dip is parallel to the bedding which is disturbed locally due - 137 - to compaction (Figure 17). The Pigeon Reef has an average thickness of 10 inches. The bulk of the ore-body is composed of early siderite which is sheared and filled with later quartz and calcite, as well as chalcopyrite. (ii) Betty R e e f . This reef is developed over a large area at Lisbon Falls, but it was economically exploitable only in a narrow zone along the eastern contact of the Astra Reef. Although it is well-developed along the western contact of the vertical reef, it shows inferior values. The Betty Reef on this side of the Astra Reef occurs at a higher elevation (Figure 18). The Betty Reef is normally composed of quartz and limonite, but also contains copper-minerals to the east of the Astra R e e f . (iii) Gould's R e e f . This interbedded reef is developed only to the west of the Astra Reef. It was exploited in a few small areas to the south of the falls, but was worked on a larger scale in the Gould's Mine, some 3000 feet to the west of the falls. The average thickness of the ore- body is about 15 inches. The ore consists of quartz and pyrite. (iv) Bott's R e e f . This Reef, which was referred to also as the Little Gem Reef /Wybergh, 1 9 2 5 , p . 9 9 ) ... 138. - 138 - (UJybergh, 1925, p. 99), was mined on a relatively small scale in the Little Gem Mine where it reached a thickness of 4 feet (UJybergh, 1925, p. 96). It is developed about 30 feet above the base of the Dolomite Series, to the west of the Astra Fault and to the south of the UJaterval River (Plate I). The ore consists of quartz and limonite. This reef can be traced westwards through the Astra Reef, where it then occupies the stratigraphical position of the Betty Reef. (u ) Astra R e e f . The Astra Reef occupies a fault-plane, as shown in Figure 18. It is exposed along strike over a distance of nearly one mile. The ore-body consists of milky quartz with massive pyrite, chalcopyrite and bismut h i n i t e . Little or no alteration of the wall-rocks can be observed. Although the fault-plane is displaced along a bed d i n g - f a u l t , the ore is undisturbed by this movement. This suggests that the ore was deposited after the interbedded faulting occurred. The ore- body has an average thickness of 20 inches but reaches a thickness of over 3 feet in p l a c e s . A distinct association exists between the interbedded fault and the gold-content of the transgressive ore-body. A study /of t h e .. »139* - 139 - of the assay-plan of the mine shows that the economical gold-values were encountered only from about 10 feet below the inter­ bedded fault to about AO feet above it. Within these limits the reef maintained an average gold-content of about 13 penny­ weights per ton. Above this zone, the values diminish gradually. The reef is exposed in vertical winzes to a depth of over 100 feet below the payable zone. It maintains a reasonable thickness but shows a sudden decrease in its gold- content with depth. This relationship probably suggests that the mineralising solutions entered the transgressive fault-plane through the b e d ding-fault. The fact that the Betty Reef is payable only to the east of the Astra Reef, suggests that it was enriched by the same solutions which mineralised the vertical reef. The Betty Reef which is developed to the west of the Astra Reef, occupies a relatively higher elevation and was therefore far away from the source of the mineralisation. This explains the unpayability of this part of the reef. This phenomenon is also further proof that the mineralisation postdates the displace­ ment caused by the high-angle fault. (vi) L e a der-reefs. A series of leader-reefs are present to the northwest of the Lisbon /Falls . . .140 • - 140 - Falls (Plate I). These reefs are located near the base of the Dolomite Series. They are approximately 30 feet in height, 30 inches in thickness and can be followed along strike for distances of more than 3000 feet. They strike approximately 055 degrees, and dip 60 degrees to the southeast. One of these ore-bodies extend to the west of the UJaterval River, where it is reported to have been very rich in gold. This leader-reef,- is known as the UJet Leader. The highest gold-values were usually encountered in the lower parts of the reef, i.e. near the horizons of the Betty and Gould's Reefs. 7. Eendraq ilfline The Eendrag Mine is situated in the central part of the farm London 496 K.T. The workings are very limited in extent, and are located 1000-4000 feet to the west of a group of faults which represents the northward-continuation of the Nestor Fault (Plate I). Small tabular ore-bodies, associated with bedding-faults, were exploited in three small areas. The stratigraphical position of these ore-bodies is 100-150 feet above the base of the Dolomite Series. This horizon corresponds roughly with that of the liiillemsoord Reef (Plate I I ) . The bedding-faults, which are also slightly / m i n e r a l i s e d . ..141. - 141 - mineralised, are about 2 feet apart. Several small, parallel, tabular ore- bodies are present between the faults. The ore-bodies resemble leader-reefs that are greater in width than in height, measuring from 2-6 feet in width. The ore consists of quartz and calcite with oxide-minerals of iron and copper. 8. Hoyer's and UJalther's Workings (Plate I) Tabular ore-bodies, similar to those described above, are present approximately 10,000 feet farther north (Plate I). These workings are situated near the southeastern boundary of the farm Ledouphine 469 K.T. The ore-bodies are located between two slighly mineralised bedding-faults, about 50 feet above the Black Reef Series. The bedding-faults are developed above and below the Digger^s Sandstone, a quartzitic marker of about 2 feet in thickness. Where the tabular bodies are developed, the quartzite appears to be replaced by ore- and g a n g u e -minerals. The ore-bodies have sharply defined contacts, and in cross-section, they resemble tabular blows. The ore-bodies strike 050 degrees and dip 60 degrees to the southeast. A leader-reef, which is known as Tucker's Leader, is present some 800 feet to the south­ east of Hoyer's Workings. It strikes parallel /to t h e ...142. - 142 - to the tabular bodies, and it uias followed for more than 1000 feet along strike. It has a thickness of 15-20 inches and is about 6 feet high. The dip of this body is parallel to that of the Hoyer ore-bodies. The ore consists of calcite and quartz with copper-bearing ore-minerals. This ore- body is reported to have been extremely rich in gold (UJybergh, 1925, p. 110). 9. New Chum Mine The workings of the New Chum Mine are situated on the southwestern side of the Treur River, on the farm Ledouphine 496 K.T. (Plate I). The workings are inaccessible at present. (a) Geological Environment The ore-bodies are leader-reefs which are developed in the Transition Zone of the Dolomite Series. The leaders are all located within a zone which varies between 45 and 160 feet above the base of the Dolomite Series. This zone consists of dolomite, chert, and some layers of shale. A poorly mineralised interbedded reef, which can be correlated with the UJillemsoord Reef (Plate II), occurs near the top of the zone. The Digger's Sand­ stone, which is 18 inches in thickness, is present near the bottom of the zone. Several dykes of diabase are present / i n . ..143. in the area. Ore-bodies The ore-bodies consist, of two parallel sets of thin, but very rich leader-reefs (UJybergh, 1925, p. 101), striking 045 and 065 degrees r e spectively. They all dip 45 degrees to the southeast. These leader- reefs are 2-3 inches in thickness, up to 120 feet in height and could be followed for distances of up to 650 feet along strike. UJybergh (1925, p. 103) observed a very distinct stratigraphical control of the gold-content of the reefs. He found that the reefs were economically exploitable only where they intersected horizons of shale, and also immediately above and below the Digger's Sandstone. Structural Features The leader-reefs are apparently inti­ mately related to the New Chum Dyke, which predates one set of reefs, but postdates the other set (Figure 19). A dyke which is parallel to the New Chum Dyke, is considered by UJybergh (1925, p. 102) to be contemporaneous with it. These dykes are cut by several younger dykes which postdate all the reefs. /10. Other Occurrences ...144. F ig u r e . / 9 S k .e .tcK o f - d istxasic . ci< -/^e s s r ^ d /e^ a c /e r --re eA s ir̂ ~ t h e . fS/e.w C h c p r \ A 7 / '/\e .. (\ f í f t e . r y ber~C] K ̂ ! S>2J5t p .t0^ Z)y^:e5 o/- d/<36^se i.Xeaí/er-reeAí older V e»v CK<-trr\. Ziy/ce Xe/cJ records) z r c__/ B X L_ E I r~n zn /w e z Sca/e .-/ .'5000 F~iQur& 2.0 _ P /ar\ „*5 h.ovv' ir̂ Q r e.e. F- lo ) é &cf & r e. <3 . f-~c3l//6J c/ou/r\ét\r~c> \*s tr^c//c.3Óe.c/, Pi ee. f Cor̂ éotsr- ~ //A.e5 . Ou érc. r-o fi> af~ P?=>/-^c/^t/esc - 153 - present in the northern part of the Jubilee Mine, and that the Portuguese Reef urns best developed in the vicinity of the domes, as shown by the reef-thicknesses in the overlay. A similar conclusion was made by lliybergh (1925, p. 79), who observed that the reef reached a thickness of up to 8 feet where folding was present. The Protuguese Reef was extensively exploited in the Desire, Chi, and Chi Extension Mines (Plate \lII), where the payability of the ore-body is closely associated with anticlines. Sagging and slumping of the formation, due to weathering, resulted in very unfavourable m ining-conditions. The ore-body in the Kameel's Section, to the south of the Kameel's Creek, is characterised by narrow pay-shoots which are parallel to the axis of the anticline. (c) Slate Reef The Slate Reef is developed locally along the upper contact of the Slate Marker. It was explored along the northern side of Darke's Gully where the ore-body was proved to be thin, and, although well-mineralised, not payable. (d) Beta Reef The Beta Reef is located 150-170 feet above the Slate Marker. Diabasic sills are /present ...154 4̂ G o ) w / / / e ^ s c 0 ^ -c / 4 ~rC>KT. 4S-4-K~T LJ/ts/=K*9r^r^ irkpfiLaó ,arH Mayo /\lorfá jj Syndicate, T H E STRUCTURAL GEOLOGY S A B IE —RILGRIMS REST GOLDFIELD yriK B!d&— Nestor Mine Klei n Rex Mine Compound Hi! .M i n e j\ .Ma/ieveic M ine ( TG.M.£ 'Mali eve/d 5 EhnttsdrjPf- &ow /Vg k Sectio rs ?| *> M. 0 •I I -V -i I | ! p l a t e ttt UNDERGROUND PL AN V A A L H O E K MI NE -Sc a// é/ve V̂ «b/A.c«̂ 5 I I l v » ^ > S 6 o p e 6 o/^ 6f\e. 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