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Sampling stream sediments for gold in mineral exploration, southern British Columbia Day, Stephen John 1988

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SAMPLING STREAM SEDIMENTS FOR GOLD IN MINERAL EXPLORATION, SOUTHERN BRITISH COLUMBIA By STEPHEN JOHN DAY B . S c , The U n i v e r s i t y of B r i t i s h Columbia, 1985 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES Department of G e o l o g i c a l Sciences We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA March 1988 © C o p y r i g h t Stephen John Day, 1988 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of CjCrOt-OG-tc/tlL SC(6(0OES The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date MMCH t. l9M ABSTRACT The problems encountered by mineral e x p l o r a t i o n i s t s when sampling stream sediments f o r gold were i n v e s t i g a t e d by c o n s i d e r i n g the s p a r s i t y of f r e e gold p a r t i c l e s and t h e i r tendency to form small p l a c e r s a t c e r t a i n l o c a t i o n s i n the stream bed. Fourteen 20-kg samples of -5-mm sediment were c o l l e c t e d from c o n t r a s t i n g energy and geochemical environments i n f i v e streams d r a i n i n g gold occurrences i n southern B r i t i s h Columbia. The samples were s i e v e d to s i x s i z e f r a c t i o n s ( 420 Jim to 52 Um) and gold content was determined by neutron a c t i v a t i o n a n a l y s i s f o l l o w i n g p r e p a r a t i o n of two d e n s i t y f r a c t i o n s u s i n g methylene i o d i d e . Gold c o n c e n t r a t i o n s were converted to estimated number of f r e e gold p a r t i c l e s and the Poisson p r o b a b i l i t y d i s t r i b u t i o n was used to show that much l a r g e r f i e l d samples (>100 kg of -1 mm screened sediment) would be r e q u i r e d to reduce random v a r i a b i l i t y due to nugget e f f e c t s to a c c e p t a b l e l e v e l s . However, i n a comparison of c o n v e n t i o n a l sampling methods, the lowest p r o b a b i l i t y of f a i l i n g to d e t e c t a stream sediment gold anomaly i s obtained using the sampling method d e s c r i b e d i n t h i s study. S m a l l - s c a l e p l a c e r formation was i n v e s t i g a t e d by — i i i — c o l l e c t i n g twenty 60-kg samples of -2-mm sediment from ten l o c a t i o n s along f i v e k i l o m e t r e s of H a r r i s Creek i n the Okanagan r e g i o n , east of Vernon. Samples were prepared and analysed as d e s c r i b e d above though heavy-mineral concentrates were onl y prepared f o r two s i z e f r a c t i o n s . Gold was found to be c o n s i d e r a b l y e n r i c h e d i n sandy-gravel d e p o s i t s compared to sand d e p o s i t s , with the e f f e c t d e c r e a s i n g as sediment s i z e decreased. The l e v e l of enrichment v a r i e s on the stream i n response to changing channel slope and l o c a l h y d r a u l i c c o n d i t i o n s . Gold anomaly d i l u t i o n i s apparent i n sand d e p o s i t s but not apparent i n sandy-gravel d e p o s i t s s i n c e gold i s p r e f e r e n t i a l l y d e p o s i t e d i n g r a v e l s as channel s l o p e decreases. These r e s u l t s are presented i n the framework of H.A. E i n s t e i n ' s sediment t r a n s p o r t model. Sediment c o l l e c t e d from g r a v e l s may represent the best geochemical sample s i n c e p l a c e r - f o r m i n g processes produce high gold c o n c e n t r a t i o n s , however i n very high energy streams, the s m a l l q u a n t i t i e s of f i n e sediment i n g r a v e l s may lead to unacceptable nugget e f f e c t s . In the l a t t e r case, a sample c o l l e c t e d from a sand d e p o s i t i s a s a t i s f a c t o r y a l t e r n a t i v e . - i v -TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES v i i i LIST OF FIGURES X ACKNOWLEDGEMENTS x i v Chapter 1: INTRODUCTION 1.0 I n t r o d u c t i o n 2 1.1 P r o p e r t i e s of gold 3 1.2 Sampling problems 5 1.2.1 S t a t i s t i c a l sampling d i s t r i b u t i o n s 7 1.3 H y d r a u l i c e f f e c t s 12 1.3.1 Formation of high c o n c e n t r a t i o n s of h i g h - d e n s i t y minerals 15 1.3.1.1 S e t t l i n g or suspension s o r t i n g .''21 1.3.1.2 Entrainment s o r t i n g 22 1.3.1.3 D i s p e r s i v e s o r t i n g 26 1.3.1.4 I n t e r s t i c e t r a p p i n g 28 1.3.2 Summary of mechanisms l i k e l y to produce h i g h -d e n s i t y mineral c o n c e n t r a t i o n s 29 1.4 F i e l d sampling problems 30 1.4.1 F i e l d sample c o l l e c t i o n techniques 30 1.4.2 S e l e c t i o n of sampling l o c a t i o n 33 1.5 Conclusions 34 Chapter 2: ORIENTATION SAMPLING 2.0 I n t r o d u c t i o n 37 2.1 D e s c r i p t i o n of streams 37 2.1.1 Tsowwin Ri v e r 37 2.1.2 'Salmonberry Creek' ....39 2.1.3 F r a n k l i n R i v e r 41 2.1.4 H a r r i s Creek ...41 2.1.5 Watson Bar Creek 42 2.2 F i e l d sampling 42 2.3 La b o r a t o r y p r o c e s s i n g 44 2.3.1 S i e v i n g 44 2.3.2 Heavy mineral s e p a r a t i o n 47 2.3.3 P r e - c o n c e n t r a t i o n of -270-mesh f r a c t i o n 47 2.3.4 Chemical a n a l y s i s 48 2.3.5 V i s u a l examination of heavy-mineral co n c e n t r a t e s 50 2.3.6 Scanning e l e c t r o n microscopy 50 2.4 R e s u l t s 51 2.4.1 D i s t r i b u t i o n of sediment s i z e s i n -5 mm sediment 51 -v-2.4.2 Weights of heavy-mineral c o n c e n t r a t e s 51 2.4.3 R e l i a b i l i t y of INAA 51 2.4.4 D i s c u s s i o n of stream gold data .56 2.4.5 Reducing sampling e r r o r s 67 2.4.5.1 Combining f r a c t i o n s 67 2.4.5.2 C o l l e c t i o n of l a r g e r samples 71 2.4.4 Comparison of r e s u l t s with other sampling methods 73 2.5 Conclusions 75 Chapter 3: DETAILED SAMPLING OF HARRIS CREEK SEDIMENTS 3.0 I n t r o d u c t i o n 79 3.1 L o c a t i o n and access 79 3.2 H i s t o r y and land use 79 3.3 Climate, v e g e t a t i o n and s o i l s 82 3.4 Summary of b a s i n morphology 83 3.5 Basin geology 83 3.5.1 Quaternary geology 85 3.5.2 Source of gold 85 3.6 H a r r i s Creek seasonal d i s c h a r g e v a r i a t i o n 86 3.7 F i e l d sampling 86 3.7.1 F i e l d sampling to determine sediment t e x t u r e : problems 86 3.7.2 Sampling 87 3.7.3 Sample l o c a t i o n d e s c r i p t i o n s 93 3.8 La b o r a t o r y p r o c e s s i n g 98 3.8.1 S i e v i n g 98 3.8.2 R e l i a b i l i t y of sediment analyses 102 3.8.3 Heavy mineral s e p a r a t i o n 104 3.8.4 Magnetic m i n e r a l s e p a r a t i o n 104 3.8.5 Minus 270-mesh magnetic mineral s e p a r a t i o n 105 3.8.6 F r a c t i o n a l a n a l y s i s of -270-mesh sediment 106 3.9 Chemical analyses 106 3.9.1 R e l i a b i l i t y of INAA f o r Au 107 3.9.2 Gold analyses of s u b t r a c t i o n s of a -270-mesh sample 110 3.9.3 R e l i a b i l i t y of INAA for Hf 112 3.10 Summary 112 Chapter 4: DESCRIPTIVE GEOMORPHOLOGY AND SEDIMENTOLOGY OF THE HARRIS CREEK STUDY REACH 4.0 Geomorphology of the study reach 116 4.1 T e x t u r a l a n a l y s e s of sediment samples 118 4.1.1 T e x t u r a l f e a t u r e s of sandy-gravel samples 120 4.1.2 T e x t u r a l f e a t u r e s of sand samples 125 4.1.3 Downstream trends i n sediment t e x t u r e s 129 4.2 Magnetite (magnetic mineral) t e x t u r a l analyses 129 - v i -4.2.1 Comparison of magnetite c o n c e n t r a t i o n s i n sandy-gravel and sand samples 134 4.3 Heavy-mineral concentrate analyses 135 4 . 4 Summary '. 135 Chapter 5: GEOCHEMICAL DATA: RESULTS AND DISCUSSION 5.0 I n t r o d u c t i o n 138 5.1 Gold data 140 5.1.1 E v a l u a t i o n of h y d r a u l i c and r a r e g r a i n e f f e c t s . . 1 4 0 5.1.2 Downstream trends i n Au c o n c e n t r a t i o n 144 5.1.3 Q u a l i t a t i v e comparison of w i t h i n - s i t e and between-site v a r i a b i l i t y 147 5.1.3.1 Geometric mean c o n c e n t r a t i o n r a t i o s (GMCR)...148 5.1.3.2 One way a n a l y s i s of v a r i a n c e 151 5.1.3.3 C o e f f i c i e n t s of v a r i a t i o n 151 5.1.4 Summary of go l d r e s u l t s 155 5.2 Comparison of the behaviour of g o l d , magnetite and z i r c o n 156 5.2.1 Trends i n magnetite c o n c e n t r a t i o n s 156 5.2.2 Trends i n hafnium c o n c e n t r a t i o n s 160 5.2.3 Geometric mean c o n c e n t r a t i o n r a t i o s and c o n c e n t r a t i o n r a t i o s 163 5.3 Removal of h y d r a u l i c v a r i a b i l i t y : e m p i r i c a l data transforms 166 5.3.1 Transform * i 168 5.3.2 Transform * 2 170 5.3.3 Transform * 3 172 5.3.4 Other transforms 172 5.3.5 R a t i o s : i m p l i c a t i o n s f o r e x p l o r a t i o n 173 5.4 Conclusions 173 Chapter 6: SEDIMENT TRANSPORT AND SMALL-SCALE PLACER FORMATION, HARRIS CREEK. 6.0 I n t r o d u c t i o n 178 6.1 Database 178 6.2 A sediment t r a n s p o r t model 178 6.2.1 D e r i v a t i o n of E i n s t e i n ' s (1950) bed m a t e r i a l load f u n c t i o n 180 6.2.1.1 L i m i t a t i o n s of E i n s t e i n ' s (1950) bed load formula 183 6.2.2 Computer s o l u t i o n of the model 183 6.2.2.1 A p p l i c a t i o n of the formula 183 6.2.2.2 Numerical values f o r h y d r a u l i c parameters....184 6.3 W i t h i n - s i t e processes 186 6.3.1 Sediment motion d u r i n g a f l o o d 186 6.3.2 Densit y s o r t i n g and formation of s u r f a c e pavement 190 - v i i -6.3.3 Sand d e p o s i t s a t bar t a i l s 196 6.3.4 D i f f e r e n c e s between heavy mineral c o n c e n t r a t i o n s i n sands and sandy g r a v e l s 196 6.3.5 Sediment s o r t i n g a f t e r a f l o o d 197 6.4 Between s i t e processes 201 6.4.1 Heavy mineral trends 201 6.5 Summary and c o n c l u s i o n s 203 Chapter 7: APPLICATIONS FOR MINERAL EXPLORATIONISTS 7.0 I n t r o d u c t i o n 209 7.1 F i r s t p r i n c i p l e s 209 7.1.1 Mode of occurrence of gold 209 7.1.2 Purpose of study 209 7.2 Regional surveys 209 7.2.1 Purpose 209 7.2.2 Sampling media 210 7.2.3 Sediment f r a c t i o n 210 7.3 Follow-up surveys 211 7.3.1 Sample s i z e 211 7.3.2 Sampling media 211 7.4 Sample p r e p a r a t i o n 211 7.5 Chemical a n a l y s i s 212 7.6 Data a n a l y s i s 212 7.6.1 Nugget e f f e c t s 212 7.6.2 H y d r a u l i c e f f e c t s 212 BIBLIOGRAPHY 214 APPENDIX 223 - v i i i -LIST OF TABLES Table 1-1. Examples of l o c a t i o n s of extreme h i g h -d e n s i t y mineral c o n c e n t r a t i o n s 17 Table 2-1. Summary of c o n d i t i o n s used i n in s t r u m e n t a l neutron a c t i v a t i o n a n a l y s i s , McMaster U n i v e r s i t y r e a c t o r (X-Ray Assay L a b o r a t o r i e s ) 49 Table 2-2. D u p l i c a t e INAA analyses f o r As i n l i g h t m i n e r a l separates ( a l l values i n ppm)...55 Table 2-3A. Gold c o n c e n t r a t i o n s (ppb) of heavy mineral concentrates 59 Table 2-3B. Gold c o n c e n t r a t i o n s (ppb) of l i g h t m i neral c o n c e n t r a t e s 60 Table 2-4. C a l c u l a t e d number of g o l d p a r t i c l e s i n heavy mineral concentrates 66 Table 2-5. Weights of -16-mesh f i e l d sample and sub-sample c o n t a i n i n g 20 p a r t i c l e s of g o l d i n high and low energy environments 72 Table 2-6. P r e d i c t e d r e s u l t s of d i f f e r e n t sampling methods 76 Table 3-1. Water di s c h a r g e r a t e s (m 3/s) f o r H a r r i s Creek, i n June 1986 94 Table 3-2. Summarised June 1986 s i t e d e s c r i p t i o n s 96 Table 3-3. Summary of p h y s i c a l and sampling c h a r a c t e r i s t i c s of sample s i t e s 99 Table 3-4. Change i n the weight of -270-mesh f r a c t i o n f o l l o w i n g wet s i e v i n g of a p r e v i o u s l y d r y s i e v e d sample 101 Table 3-5. R e s u l t s of t e x t u r a l r e - a n a l y s i s of 86-SD-B2 and 86-SD-R2 103 Table 3-6. E f f e c t on Au c o n c e n t r a t i o n of adding one s p h e r i c a l g old p a r t i c l e ( d e n s i t y = 18 g/cm 3) to a 25 g sample of -270-mesh sediment 109 - i x -Table 3-7. Table 4-1. Table 4-2. Table 4-3. Table 5-1. Table 5-2. Table 5-3. Table 5-4. Table 5-5. Table 6-1. Table 6-2. Table 6-3. Gold analyses f o r s u b f r a c t i o n s of -270-mesh sediment (85-SD-10) I l l Summarised t o t a l weights, D»© and D 121 C h a r a c t e r i s t i c s of framework and matrix components of sandy g r a v e l s , the p r o p o r t i o n of framework and d i s c r i m i n a n t diameter 126 Comparison of weights of magnetite and c o n c e n t r a t i o n of magnetite i n two samples..132 Summary s t a t i s t i c s f o r heavy mineral c o n c e n t r a t i o n s i n sediment 139 GMCRs for three h i g h - d e n s i t y m i n e r a l f r a c t i o n s and s i g n i f i c a n c e t e s t e d by Duncans m u l t i p l e range t e s t 150 A n a l y s i s of v a r i a n c e r e s u l t s f o r g o l d 152 C o r r e l a t i o n matrix of c o n c e n t r a t i o n r a t i o s 164 E m p i r i c a l data transforms d i s c u s s e d i n the t e x t 167 Major s e d i m e n t o l o g i c a l and geochemical f e a t u r e s 179 Parameters to which numerical values must be assigned ( E i n s t e i n ' s (1950) bed m a t e r i a l f u n c t i o n ) 185 Sediment t e x t u r e s at s i t e C used i n computations of bed m a t e r i a l load 187 -x-LIST OF FIGURES F i g . 1-1. Example of a t y p i c a l r e g i o n a l geochemical survey f o r Au 6 F i g . 1-2. Po i s s o n frequency d i s t r i b u t i o n s f o r (A) U=0.49 and (B) u=0.12 9 F i g . 1-3. Si z e of sample r e q u i r e d t o c o n t a i n twenty s p h e r i c a l p a r t i c l e s of pure gold as a f u n c t i o n of s i z e of p a r t i c l e s and grade of p o p u l a t i o n m a t e r i a l (rock, sediment, e t c.) ( C l i f t o n et a l . , 1969) 11 F i g . 1-4. Stream sediment d i s p e r s i o n t r a i n s f o r s e l e c t e d f r a c t i o n s of some h i g h - d e n s i t y minerals 14 F i g . 1-5. (A) S h i e l d s (1936) diagram of the c r i t i c a l shear Reynolds number versus d i m e n s i o n l e s s c r i t i c a l shear s t r e s s with some experimental p o i n t s and (B) c r i t i c a l shear s t r e s s of entrainment f o r a number of s p h e r i c a l h i g h - d e n s i t y minerals i n water a t 20<>C d e r i v e d from S h i e l d s curve 23 F i g . 1-6. R e l a t i o n s h i p between c r i t i c a l d imensionless shear s t r e s s f o r motion and the r a t i o of p a r t i c l e s i z e to the median p a r t i c l e s i z e of the bed (Andrews, 1983) 27 F i g . 2-1. L o c a t i o n s of streams sampled 38 F i g . 2-2. Sampling s t a t i o n s on streams 40 F i g . 2-3. Wet-sieving apparatus 45 F i g . 2-4. Cumulative sediment s i z e d i s t r i b u t i o n s f o r sediment f i n e r than 4 mm 52 F i g . 2-5. Gold analyses (INAA) of heavy mineral concentrate s p l i t s 54 F i g . 2-6. Comparison of Au analyses determined by INAA on two d i f f e r e n t occasions ....57 F i g . 2-7. Comparison of Au c o n c e n t r a t i o n s determined by f i r e assay/atomic a b s o r p t i o n and INAA 58 - x i -F i g . 2-8. P r o b a b i l i t y p l o t of shape f a c t o r s (SF) showing i n f e r r e d component normal p o p u l a t i o n s 63 F i g . 2-9. Gold content and confidence i n t e r v a l s f o r -200+270-mesh f r a c t i o n of heavy mineral c o n c e n t r a t e s versus mean g r a i n s i z e of f i e l d sample 68 F i g . 2-10. Number of e f f e c t i v e p a r t i c l e s (N e) versus geometric midpoint of c o a r s e s t s i z e f r a c t i o n i n c l u d e d 70 F i g . 3-1. Geology of the drainage b a s i n of H a r r i s Creek and l o c a t i o n of the study reach 81 F i g . 3-2. Topography of e n t i r e drainage area of H a r r i s Creek upstream of the study reach 84 F i g . 3-3. Sample l o c a t i o n s on H a r r i s Creek 88 F i g . 3-4. T y p i c a l sandy g r a v e l sampling l o c a t i o n near the head of a po i n t bar 90 F i g . 3-5. T y p i c a l sand sampling l o c a t i o n 91 F i g . 3-6. Wooden pole with notch b u i l t up a t centre f o r c a r r y i n g samples 95 F i g . 3-7. Gold analyses f o r s p l i t s of -270-mesh sediment 108 F i g . 3-8. Hafnium analyses f o r s p l i t s of -270-mesh sediment 113 F i g . 4-1. Channel long p r o f i l e , showing average slope and channel form between open c i r c l e s . The whole s e c t i o n i s 5050 m long 117 F i g . 4-2. T y p i c a l b r a i d e d channel s e c t i o n 119 F i g . 4-3. T y p i c a l meandering channel s e c t i o n 119 F i g . 4-4. Some examples of p r o b a b i l i t y p l o t s f o r s i z e frequency d i s t r i b u t i o n s of sandy g r a v e l samples 122 F i g . 4-5. P r o b a b i l i t y p l o t f o r s i z e d i s t r i b u t i o n of 86-SD-D2 showing c h a r a c t e r i s t i c f e a t u r e s of sandy g r a v e l d e p o s i t s 124 - x i i -F i g . 4-6. Some examples of p r o b a b i l i t y p l o t s f o r s i z e frequency d i s t r i b u t i o n s of sand samples 127 F i g . 4-7. Downstream trends i n mean diameter of sediment 130 F i g . 4-8. Downstream trends of weight percent -8+4-mm sediment. F i l l e d i n c i r c l e s are sandy g r a v e l d e p o s i t s , open and h a l f f i l l e d c i r c l e s are sand d e p o s i t s 130 F i g . 4-9. Some examples of p r o b a b i l i t y p l o t s of magnetite s i z e d i s t r i b u t i o n s 133 F i g . 5-1. Poisson confidence l i m i t s f o r g o l d c o n c e n t r a t i o n s c a l c u l a t e d as d e s c r i b e d i n the t e x t f o r (A) -140+200-mesh and (B) -200 + 270-mesh 142 F i g . 5-2. Magnetite c o n c e n t r a t i o n (-70+100-mesh) versus Au c o n c e n t r a t i o n (-140+200-mesh ) 143 F i g . 5-3. Downstream trends i n Au c o n c e n t r a t i o n 145 F i g . 5-4. C o e f f i c i e n t s of v a r i a t i o n f o r Au c o n c e n t r a t i o n s by sediment d e p o s i t type and f r a c t i o n 154 F i g . 5-5. Downstream p r o f i l e s of magnetite c o n c e n t r a t i o n s 157 F i g . 5-6. Downstream p r o f i l e s of Hf concentrations....161 F i g . 5-7. Geometric mean c o n c e n t r a t i o n r a t i o s c a l c u l a t e d f o r four d e n s i t y f r a c t i o n s 162 F i g . 5-8. C o e f f i c i e n t s of v a r i a t i o n f o r t r a n s f o r m * x (Table 5-5, case x=y only) by sediment d e p o s i t type and f r a c t i o n 169 F i g . 5-9. Downstream p r o f i l e f o r * z (case x=-140+200-mesh, y=-200 + 270-mesh) . . . 170 F i g . 5-10. Downstream p r o f i l e f o r t r a n s f o r m *-» 174 F i g . 6-1. E f f e c t of i n c r e a s i n g bed roughness on sediment t r a n s p o r t r a t e s f o r sediment of d i f f e r e n t diameters 189 - x i i i -F i g . 6-2. R e l a t i o n s h i p s between (A) l o g of framework s o r t i n g and magnetite (-40+70-mesh) c o n c e n t r a t i o n and (B) l o g of mean diameter of matrix and magnetite (-40+70-mesh) c o n c e n t r a t i o n 191 F i g . 6-3. R e l a t i o n s h i p between l o g of matrix s o r t i n g and l o g of mean framework diameter 192 F i g . 6-4. Examples from a bar on Lynn Creek of (A) sub-surface g r a v e l v o i d s i n c o m p l e t e l y f i l l e d by sand and (B) g r a v e l voids completely f i l l e d by sand 194 F i g . 6-5. Comparison of t r a n s p o r t r a t i o s (low d e n s i t y / h i g h d e n s i t y ) with g r a i n s i z e , bed roughness and d e n s i t y 198 F i g . 6-6. High d e n s i t y mineral enrichment of sand sample C l due to winnowing 200 F i g . 6-7. E f f e c t of slope on magnetite t r a n s p o r t r a t i o s 204 F i g . 6-8. Diagrammatic summary of the processes i n v o l v e d i n d e p o s i t i o n of sediment i n H a r r i s Creek 206 - x i v-ACKNOWLEDGEMENTS I am g r a t e f u l to i n d u s t r y g e o l o g i s t s and geochemists D. Brabec, A. Burton, Dr. S.J. Hoffman, B. Smee, F.M. Smith, Dr. A. S o r e g o r o l i , Dr. I. Thomson, A. Troupe and R. Walker who a s s i s t e d i n s e l e c t i n g s u i t a b l e sampling l o c a t i o n s . At the U n i v e r s i t y of B r i t i s h Columbia, l a b o r a t o r y and f i e l d a s s i s t a n c e was provided a t v a r i o u s stages by B. Schroeder, H. Yuen, H. E i j g e l , G. Harrop and J . Densmore. C R . S t a n l e y and J . Knight provided l i v e l y d i s c u s s i o n s . Dr. W.K. F l e t c h e r provided guidance and encouragement as w e l l as c r i t i c a l comments on ideas and concepts. Drs. W. Barnes and A.J. S i n c l a i r c a r e f u l l y reviewed the manuscr i p t . In the f i n a l s t ages, Dr. W. McMillan gave permis s i o n t o use f a c i l i t i e s a t the B r i t i s h Columbia G e o l o g i c a l Survey Branch. Branch geochemists P.F. Matysek and J.L. Gravel helped formulate ideas through d i s c u s s i o n . Ngoc provided tea and i n s p i r a t i o n a t every stage. -1-CHAPTER 1 INTRODUCTION -2-1.0 I n t r o d u c t i o n M i n e r a l e x p l o r a t i o n i s t s use the presence of anomalous c o n c e n t r a t i o n s of an e c o n o m i c a l l y i n t e r e s t i n g mineral i n stream sediments to i n d i c a t e a bedrock occurrence of the mineral upstream of the sampling l o c a t i o n . Although there are many c o m p l i c a t i o n s t h a t can l e a d to m i s i n t e r p r e t a t i o n of geochemical data obtained from stream sediments, m o d e l l i n g of the d i s p e r s i o n of such elements as Cu, Zn, Pb and Ag has been remarkably s u c c e s s f u l (e.g. Hawkes, 1976). These elements g e n e r a l l y occur i n sediments as 1) t r a c e c o n s t i t u e n t s i n secondary c l a y s and oxides and 2) t r a c e c o n s t i t u e n t s absorbed on the s u r f a c e of Fe and Mn oxides (Rose, 1975) or (3) as t r a c e components of low d e n s i t y s i l i c a t e s (e.g. Pb i n K - f e l d s p a r ) . The behaviour of elements o c c u r r i n g predominantly as a t r a c e or major component of h i g h - d e n s i t y r e s i s t a n t minerals ( f o r example, Zn i n magnetite, Cu i n s u l p h i d e s , Au i n g o l d , Sn i n c a s s i t e r i t e , W i n s c h e e l i t e e t c.) i s not understood to the same degree. The behaviour of g o l d * i s very p o o r l y understood due to i t s high d e n s i t y and r a r i t y i n stream sediments. T h i s study addresses the e f f e c t s of these problems i n two '•"'Gold" r e f e r s to minor Ag, Cu, Hg, the mineral which i s an a l l o y of Au with Fe e t c . "Au" r e f e r s to the element o n l y . -3-s t a q e s : 1) Determination of t y p i c a l c o n c e n t r a t i o n s of gold i n stream sediments, to enable a b e t t e r understanding of sampling techniques r e q u i r e d to o b t a i n a s t a t i s t i c a l l y meaningful sample f o r d e t e r m i n a t i o n of g o l d . 2) Determination of the e f f e c t of other (non-sampling) v a r i a b l e s on the d i s t r i b u t i o n of go l d i n stream sediments. 1.1 P r o p e r t i e s of go l d The p r o p e r t i e s of gold are c o n s i d e r a b l y d i f f e r e n t from those of r e s i s t a n t s i l i c a t e and oxide m i n e r a l s . Gold i s extremely dense (pure gold (Au), d e n s i t y = 19.3 g/cm 3) though the s p e c i f i c g r a v i t y decreases with i n c r e a s e d s o l i d s o l u t i o n of s i l v e r and other elements such as copper, i r o n and mercury ( n a t u r a l g old d e n s i t y = 15 to 19 g/cm 3, Hurlbut and K l e i n , 1977). Pure g o l d i s wetted to a ve r y l i m i t e d extent; hence, attachment of a i r bubbles may lead to f l o t a t i o n of s m a l l p a r t i c l e s , overcoming h i g h - d e n s i t y e f f e c t s (Wang and P o l i n g , 1983). N a t u r a l gold w i l l show these e f f e c t s to a l e s s e r extent due to s o l i d s o l u t i o n with other metals. In common with most other n a t i v e metals, gold i s malleable and d u c t i l e which leads to a wide range of n a t u r a l shapes i n stream sediments. Although i t i s -4-commonly d e s c r i b e d as o c c u r r i n g as f l a k e s ( d i s c s ) a g r e a t many r e g u l a r and i r r e g u l a r degrees of f l a t n e s s are observed f o r g o l d i n stream sediments (Tischenko, 1981; J . Knight, pers. comm.). Other r e s i s t a n t minerals become rounded as the p o i n t s of g r a i n s are removed d u r i n g t r a n s p o r t , but g o l d may become more f l a t t e n e d as i t i s t r a n s p o r t e d and the edges can c u r l up, f o l d over and e v e n t u a l l y snap o f f ( f o r example, G i u s t i and Smith, 1984). The f l a t t e n i n g a f f e c t appears to vary s y s t e m a t i c a l l y with the s i e v e diameter of the p a r t i c l e s : coarse and f i n e g o l d p a r t i c l e s are f l a t t e n e d the l e a s t and p a r t i c l e s with diameters of approximately 1 mm are f l a t t e n e d to the g r e a t e s t e x t e n t . Although g o l d has a very l i m i t e d s o l u b i l i t y i n most n a t u r a l waters (Boyle, 1979), c o n s i d e r a b l e work has been done on the chemical m o b i l i t y of Au i n c o o l , a p p r oximately n e u t r a l , s u r f i c i a l s o l u t i o n s . With r e s p e c t to p l a c e r g o l d , the f o l l o w i n g f e a t u r e s ' have been used as evidence f o r m o b i l i t y (Boyle, 1979): 1) Gold nuggets t y p i c a l l y have a mammillary h a b i t and may show i n t e r n a l zoning suggesting chemical a c c r e t i o n . 2) S i n g l e g o l d c r y s t a l s and c r y s t a l faces on nuggets have been observed. 3) Many gold nuggets have a r i n d of high f i n e n e s s gold ( f i n e n e s s = lOOOAu/(Au+Ag+Cu+...)). -5-In t h i s study the chemical m o b i l i t y of gold was assumed n e g l i g i b l e compared to mechanical t r a n s p o r t because the sediments s t u d i e d are reworked a n n u a l l y , p r e c l u d i n g s i g n i f i c a n t chemical d e p o s i t i o n of new g o l d . The high d e n s i t y of gold and extreme v a r i a t i o n of p a r t i c l e shape and composition between p a r t i c l e s leads t o d i f f i c u l t y i n determining the behaviour of g o l d i n a c t i v e stream sediments. A f u r t h e r c o m p l i c a t i o n i s t h a t g o l d p a r t i c l e s are extremely rare i n stream sediments, hence sampling techniques must be thoroughly understood. 1•2 Sampling problems Sampling problems are evident i n most m i n e r a l e x p l o r a t i o n stream sediment surveys for g o l d . T y p i c a l l y , a l a r g e number of s m a l l samples are taken from a p r o p e r t y or r e g i o n a t a high sampling d e n s i t y (e.g. 1 to 5 samples/km 2), and on a n a l y s i s i t i s found t h a t a l l but one or two samples have background Au c o n c e n t r a t i o n s t h a t may be a t t r i b u t e d to the Au content of u b i q u i t o u s m i n e r a l s . The gold contents of the few anomalous samples are mostly extremely h i g h and non-reproducible ( g r e a t e r than 1 ppm, F i g . 1-1, f o r example, Boyle and Gleeson, 1972; Brown and H i l c h e y , 1975; Maurice, 1986a). F i g . 1-1. Example of sample d i s t r i b u t i o n f o r a t y p i c a l r e g i o n a l geochemical survey f o r Au. Samples are f i e l d panned c o n c e n t r a t e s . A l a r g e number of samples are below d e t e c t i o n l i m i t (open c i r c l e s ) whereas a few are i n excess of 2 ppm ( f i l l e d c i r c l e s ) (Boyle and Gleeson (1972), Keno H i l l Area, Yukon T e r r i t o r y ) . -7-1.2.1 S t a t i s t i c a l sampling d i s t r i b u t i o n s The sampling d i s t r i b u t i o n of v e r y r a r e p a r t i c l e s (e.g. g o l d , diamonds, c a s s i t e r i t e ) i n rocks, s o i l s and stream sediments i s t y p i c a l l y d e s c r i b e d by the P o i s s o n d i s t r i b u t i o n (Koch and L i n k , 1970). The Poisson d i s t r i b u t i o n i s d e f i n e d as P(n) = U"e-*Vn! (1-1) where U i s the expected (mean) number of p a r t i c l e s i n a sample and P(n) i s the p r o b a b i l i t y of o b t a i n i n g n p a r t i c l e s i n the sample. Confidence l i m i t s (at s i g n i f i c a n c e l e v e l <x) f o r the p o p u l a t i o n mean (H) can be estimated from the chi-squared d i s t r i b u t i o n (Zar, 1984): 2 2 where N i s an estimate of U. As p. becomes g r e a t e r than ten the simpler estimate of the 95% c o n f i d e n c e l i m i t s N - 2^N i U < N + 2vN (l-2b) i s adequate ( F l e t c h e r , 1981). The i m p l i c a t i o n s of r e l a t i o n s h i p s (1-1) and (1-2) to stream sediment sampling surveys f o r gold are i l l u s t r a t e d i n the f o l l o w i n g example. Stream sediment at a given l o c a t i o n downstream of m i n e r a l i s a t i o n i s very w e l l - s o r t e d . Gold i s d i s t r i b u t e d homogeneously as d i s c r e t e p a r t i c l e s (diameter=0.2 mm) throughout the sediment to y i e l d a bulk composition of 200 ppb Au. The gold content of the sediment not a t t r i b u t a b l e -8-to m i n e r a l i s a t i o n (that i s , sm a l l amounts of gold contained i n the l a t t i c e s of other minerals) i s n e g l i g i b l e . A sample weighing 200 g i s taken from the sediment, s u i t a b l y concentrated without s p l i t t i n g and l o s s of gold and analysed by f i r e assay. The t o t a l volume of s o l i d s (assuming d e n s i t y = 2.65 g/cm 3) i s 76 cm 3 and the expected weight of gold i s 40 p.q based on the p o p u l a t i o n c o n c e n t r a t i o n . The weight of each p a r t i c l e of gold (assuming pure Au spheres) i s 80.8 Jig, t h e r e f o r e the expected number of p a r t i c l e s (p.) i s 0.49. Using equation 1-1, the p r o b a b i l i t y t h a t the sample w i l l c o n t a i n no f r e e gold (P(0), n=0) i s 61% ( F i g . 1-2A). That i s , there i s a 61% chance t h a t the anomaly w i l l be undetected. In a more extreme case, i f the sample weighed 50 g, then u=0.12 and P(0)=87% ( F i g . 1-2B). I f one p a r t i c l e was present i n the sample ( P ( l ) = l l % ) then the a n a l y s i s would be 1700 ppb Au. Th i s l a t t e r r e s u l t has l e d to the term "nugget e f f e c t " to d e s c r i b e s u r p r i s i n g l y high gold analyses r e s u l t i n g from the chance presence of a s i n g l e or very few p a r t i c l e s of gold i n a small sample (Ingamells, 1981). The extreme r a r i t y of coarse p a r t i c u l a t e gold d e r i v e d from gold occurrences i n stream sediments leads to a st r o n g p r o b a b i l i t y t h a t an anomaly w i l l remain undetected because the true c o n c e n t r a t i o n of the sample i s g r o s s l y - 9 -n F i g . 1-2. Poisson frequency d i s t r i b u t i o n s for (A) u=0.49 and (B) u=0.12. -10-underestimated. In the example, roughly nine out of ten samples i n the extreme case would r e t u r n a background Au content whereas a r a r e but nonetheless s i g n i f i c a n t sample might c o n t a i n over 1 ppm Au. However, a repeat sample taken a t the same l o c a t i o n would probably (87% chance) not c o n t a i n d e t e c t a b l e g o l d . Equation 1-1 may be used to determine the s i z e of sample that i s r e q u i r e d to reach an ac c e p t a b l e expected number of gold p a r t i c l e s such t h a t an anomaly w i l l have an accept a b l e p r o b a b i l i t y of being d e t e c t e d q u a l i t a t i v e l y . R e l a t i o n s h i p s l - 2 a and l-2b can be used to estimate the number of gold p a r t i c l e s a sample should c o n t a i n to reduce the r e l a t i v e e r r o r of sampling to a l e v e l t h a t w i l l a l l o w gold c o n c e n t r a t i o n s of s e v e r a l samples to be s t a t i s t i c a l l y d i s t i n g u i s h a b l e with 95% c o n f i d e n c e . The necessary number of p a r t i c l e s w i l l vary c o n s i d e r a b l y with the s i t u a t i o n ( f o r example, s p a c i n g of sample l o c a t i o n s with r e s p e c t to d i s t a n c e from m i n e r a l i s a t i o n ) though a r e l a t i v e e r r o r of ±50% might be c o n s i d e r e d adequate ( C l i f t o n et. a l . , 1969). Using the r e l a t i o n s h i p s a r e l a t i v e e r r o r of 50% corresponds t o roughly 20 p a r t i c l e s of gold, though r e l a t i o n s h i p ( l - 2 a ) produces asymmetric confidence l i m i t s with a lower r e l a t i v e e r r o r of 37% and an upper l i m i t of 54% (from t a b l e s of Pearson and H a r t l e y (1966)). Some examples of sample s i z e s r e q u i r e d to o b t a i n 20 -11-100 kg 1 kg 10 g 100 mg S i z e of S a m p l e F i g . 1-3. S i z e of sample r e q u i r e d to c o n t a i n twenty s p h e r i c a l p a r t i c l e s of pure gold as a f u n c t i o n of s i z e of p a r t i c l e s and grade of p o p u l a t i o n m a t e r i a l (rock, sediment, e t c . ) ( C l i f t o n et a l . , 1969). For example, i f gold p a r t i c l e s are 100 Mm i n diameter and sediment grades 100 ppb gold then the sample r e q u i r e d weighs 2.02 kg. -12-p a r t i c l e s of gold are summarised l n F i g . 1-3 ( C l i f t o n §_t a l . , 1969). Where go l d i s present o n l y i n coarse f r a c t i o n s or f i n e sediment i s l o s t d u r i n g sampling and f i e l d and/or l a b o r a t o r y p r e - c o n c e n t r a t i o n , extremely l a r g e samples are r e q u i r e d . There are few i n d i c a t i o n s i n r e p o r t s of stream sediment surveys f o r go l d t h a t a t t e n t i o n has been paid e i t h e r to the s i z e d i s t r i b u t i o n of g o l d , or the sample s i z e r e q u i r e d , g i v e n the Au c o n c e n t r a t i o n of the sediments (e.g., K u l i k o v and Krendelov, 1976). The sampling problems d e s c r i b e d by the P o i s s o n d i s t r i b u t i o n a l s o occur i n the l a b o r a t o r y i f a sample i s s p l i t a f t e r c r u s h i n g and g r i n d i n g . T h i s leads t o s u p e r p o s i t i o n of a second Poisson d i s t r i b u t i o n onto the f i e l d sampling d i s t r i b u t i o n (Ingamells, 1981; Sutherland and Dale, 1984). 1.3 H y d r a u l i c e f f e c t s P o l i k a r p o c h i n (1971) and Hawkes (1976) proposed a simple mathematical model (Me mA m = (Me^-Meo) + AmMeB, where Me m, Me A and Me» are the metal content of m i n e r a l i s a t i o n , anomalous sediment samples and background sediment samples, r e s p e c t i v e l y and A*, and A* are the areas of exposed m i n e r a l i s a t i o n and drainage b a s i n , r e s p e c t i v e l y ) to e x p l a i n the d i l u t i o n of metal anomalies i n stream sediments downstream of an occurrence of -13-m i n e r a l i s a t i o n . The model has s e v e r a l important l i m i t a t i o n s a r i s i n g from the f o l l o w i n g assumptions: 1) constant e r o s i o n r a t e i n the b a s i n ; 2) constant l i t h o g e o c h e m i c a l and pedogeochemical background values f o r the elements of i n t e r e s t ; 3) no exchange between water and s o l i d s ( f o r example, no p r e c i p i t a t i o n b a r r i e r s ) ; 4) no sampling, a n a l y t i c a l or contamination e r r o r s ; 5) o n l y one outcrop of m i n e r a l i s a t i o n i n the b a s i n . Despite these l i m i t a t i o n s , a p p l i c a t i o n of the model t o the d i s p e r s i o n of Cu i n stream sediments compares w e l l t o observed anomaly decay p a t t e r n s (Rose et. al . . , 1979). The assumptions of the model do not preclude a p p l i c a t i o n t o such elements as W, Sn, Ba and Au hosted i n h i g h - d e n s i t y r e s i s t a t e m i n e r a l s . However, (downstream of m i n e r a l i s a t i o n ) trends of these elements are mostly extremely e r r a t i c and do not f o l l o w the model of Hawkes (1976) (e.g., F i g . 1-4B to D, W, Saxby and F l e t c h e r , 1986; Sn, F l e t c h e r et. a l . , 1987; Ba, S l e a t h and F l e t c h e r , 1982). Although these authors a t t r i b u t e the v a r i a b i l i t y of these elements to h y d r a u l i c e f f e c t s , because p a r t i c l e s of the m i n e r a l s are s u f f i c i e n t l y abundant t h a t r a r e g r a i n e f f e c t s are not important, the v a r i a b i l i t y i n most g o l d surveys can be e x p l a i n e d by the nugget e f f e c t (e.g., Taisaev, 1986, F i g . 1-4A) r e s u l t i n g from inadequate sample s i z e . -14-0.1 t i 1 r 1 r 0 1 2 3 4 5 Distance (km) (B) i i 1— 0 2 4 Distance (km) F i g . 1-4. Stream sediment d i s p e r s i o n t r a i n s f o r s e l e c t e d f r a c t i o n s of some h i g h - d e n s i t y m i n e r a l s . (A) Gold (-5 mm) (Taisaev, 1986), (B) b a r i t e (-80+270-mesh) ( S l e a t h , 1980), (C) s c h e e l i t e (-80+120-mesh) (Saxby, 1985), (D) c a s s i t e r i t e (-35 + 48-mesh) ( F l e t c h e r et a l . , 1987 ). Source of heavy mineral i n d i c a t e d by X. Hawkes' model f a i l s because i t assumes t h a t a l l components of the sediment t r a v e l a t equal r a t e s a t a l l l o c a t i o n s i n the stream. Hence/ the model r e q u i r e s t h a t there are no l o c a l stream processes t h a t tend to concentrate or e x c e s s i v e l y d i l u t e the component of i n t e r e s t . I n t u i t i v e l y , p a r t i c l e s of d i f f e r e n t d e n s i t i e s w i l l tend to t r a v e l a t d i f f e r e n t r a t e s l e a d i n g to l o c a l i s e d , h i g h l y v a r i a b l e enrichment and d i l u t i o n (over t h a t p r e d i c t e d by the d i l u t i o n model). Thus, although the supply of h i g h - d e n s i t y mineral p a r t i c l e s to a giv e n reach of a stream ( f o r example, a p o i n t bar) should be d i l u t e d a c c o r d i n g to Hawkes' (1976) model, l o c a l h y d r a u l i c processes may modify the s i z e and d e n s i t y d i s t r i b u t i o n s to d i f f e r e n t degrees at any p o t e n t i a l sampling l o c a t i o n . 1.3.1 Formation of high c o n c e n t r a t i o n s of h i g h - d e n s i t y  minerals Many d i f f e r e n t geomorphic (e.g. s l o p e , p r o x i m i t y to t r i b u t a r i e s ) and h y d r a u l i c (stream v e l o c i t y , depth to width r a t i o , c r o s s - s e c t i o n a l area) v a r i a b l e s may le a d t o orders of magnitude v a r i a b i l i t y of c o n c e n t r a t i o n s of h i g h -d e n s i t y m i n e r a l s i n a s h o r t reach of a stream ( S l i n g e r l a n d , 1984), and hence sampling a t d i f f e r e n t l o c a t i o n s w i l l produce a wide range of h i g h - d e n s i t y mineral abundances. L o c a t i o n s of extremely e l e v a t e d h i g h -d e n s i t y mineral c o n c e n t r a t i o n s ( p l a c e r s ) i n present-day -16-stream beds are we11-documented i n p r o s p e c t o r s handbooks (Anderson, 1887; Basque, 1979) and s c i e n t i f i c l i t e r a t u r e (McKay, 1921; Boyle, 1979; M i l n e r , 1983; S l i n g e r l a n d , 1984) (Table 1-1). Development of ideas on the formation of l o c a l i s e d high d e n s i t y mineral c o n c e n t r a t i o n s have centred around the concept of h y d r a u l i c e q u i v a l e n c e , o r i g i n a l l y proposed by Rubey (1933). The concept i t s e l f i s simple — any two p a r t i c l e s t h a t behave i n the same way under a g i v e n h y d r a u l i c regime are s a i d to be h y d r a u l i c a l l y e q u i v a l e n t . E a r l y use of the concept (Rubey, 1933; T o u r t e l o t , 1968) r e f e r r e d to s e t t l i n g v e l o c i t y e q u i v a l e n c e . T h e o r e t i c a l l y d e r i v e d t e r m i n a l s e t t l i n g v e l o c i t y equations (e.g. Stokes Law) p r e d i c t t h a t s m a l l h i g h - d e n s i t y p a r t i c l e s w i l l s e t t l e at the same r a t e as l a r g e l e s s dense p a r t i c l e s . T h i s law appears to s u p e r f i c i a l l y agree with the q u a l i t a t i v e o b s e r v a t i o n t h a t s m a l l gold p a r t i c l e s occur i n conglomerates i n the Witwatersrand d e p o s i t s ( P r e t o r i u s , 1981) and elsewhere. However, the s i z e d i f f e r e n c e s are g e n e r a l l y f a r too great to be accounted f o r by simple s e t t l i n g e q u i v a l e n c e . Thus, although s e t t l i n g equivalence can l e a d to the formation of l a y e r s e n r i c h e d i n h i g h -d e n s i t y m i n e r a l s , i t i s necessary to c o n s i d e r other mechanisms f o r the formation of p l a c e r s to e x p l a i n d e p o s i t s not s a t i s f y i n g s e t t l i n g e q u i v a l e n c e . - 1 7 -Tab le 1-1. Examples of l o c a t i o n s of extreme h i g h - d e n s i t y m i n e r a l c o n c e n t r a t i o n s . 1) In r i f f l e d bedrock d e p r e s s i o n s . 2) Downstream of l a r g e bou lder s and i s l a n d s . 3) Any zone of f l ow s e p a r a t i o n (Best and Brayshaw, 1985). 4) Downstream of c o n f l u e n c e s i n s u c t i o n e d d i e s . 5) Bar to bank f low convergence zones (Smith and Beukes, 1983). 6) Heads of p o i n t and channe l b a r s . 7) Rapid change of channe l c u r v a t u r e ( c f . , Pe te r son e_t a l . , 1986, c o a s t a l beach c u r v a t u r e ) . 8) Decrease of channe l g r a d i e n t . 9) Emergence of streams from canyons (abrupt v a l l e y w iden ing ) . 10) At the c o n t a c t between a l l u v i a l sediments and bedrock. 11) At " f a l s e bottoms" (above c l a y l a y e r s and pans ) . 12) In o r gan i c mats of l i c h e n s e t c . ( M i l n e r , 1983). -18-Two approaches have emerged f o r st u d y i n g h y d r a u l i c e q u i v a l e n c e : 1) beach sediment s t u d i e s and 2) stream sediment s t u d i e s . S l i n g e r l a n d (1977, 1984), Komar and Wang (1984), Trask and Hand (1984) and B a r r i e (1981) have worked with common h i g h - d e n s i t y minerals i n beach and e s t u a r y sands. In t h i s approach, i t i s assumed t h a t samples can be taken i n which the sediment p a r t i c l e s are h y d r a u l i c a l l y e q u i v a l e n t or i f s e v e r a l d e p o s i t s (e.g. lamel l a e ) must be sampled then one process has predominated i n the d e p o s i t i o n and m o d i f i c a t i o n of a l l l a y e r s . The sediment s i z e d i s t r i b u t i o n i s determined and a mean s e t t l i n g v e l o c i t y f o r the d i f f e r e n t m i n e r a l f r a c t i o n s i s obtained based on the mean sediment diameter of the v a r i o u s d e n s i t y f r a c t i o n s . S l i n g e r l a n d (1984) r a t i o s the mean t e r m i n a l s e t t l i n g v e l o c i t y of a h i g h -d e n s i t y mineral to t h a t of a low-density mineral to show the departure from s e t t l i n g e q u i v a l e n c e . T h i s method w i l l work s a t i s f a c t o r i l y f o r beach sands because the sediments are w e l l - s o r t e d and f i n e - g r a i n e d , v i r t u a l l y guaranteeing t h a t the h i g h - d e n s i t y mineral d i s t r i b u t i o n i s not trun c a t e d by a shortage of supply of the coars e r f r a c t i o n s . However, i f p a r t i c l e s of the minerals are r e s t r i c t e d to c e r t a i n s i z e s ( B a r r i e , 1980), then comparison of mean t e r m i n a l s e t t l i n g v e l o c i t i e s i s meaningless. -19-When working with stream sediments a d i f f e r e n t approach i s needed because i t cannot be assumed t h a t a complete h i g h - d e n s i t y mineral d i s t r i b u t i o n i s present i n a l l samples. A mean s e t t l i n g v e l o c i t y f o r h i g h - d e n s i t y minerals would be underestimated from that expected f o r the h y d r a u l i c regime because very l a r g e (greater than 1 mm diameter) h i g h - d e n s i t y mineral c l a s t s are r a r e l y a v a i l a b l e Thus, arguments t h a t g o l d p a r t i c l e s i n a conglomerate are too f i n e f o r s e t t l i n g equivalence are not e a s i l y v a l i d a t e d because coarse g o l d p a r t i c l e s were probably u n a v a i l a b l e . Rittenhouse (1943) and F l e t c h e r et a l . (1987) have attempted to determine equivalence i n stream sediments by l o o k i n g at i n d i v i d u a l f r a c t i o n s i n which the h i g h - d e n s i t y m i n e r a l s u i t e i n a l l samples i s assumed to be u n a f f e c t e d by supply problems. Rittenhouse (1943) used c o e f f i c i e n t s of v a r i a t i o n to determine h y d r a u l i c a l l y e q u i v a l e n t low-d e n s i t y and h i g h - d e n s i t y f r a c t i o n s . F l e t c h e r e_t a l . (1987) used a n a l y s i s of v a r i a n c e (ANOVA) to assess r e d u c t i o n of w i t h i n s i t e versus between sampling s i t e v a r i a n c e using r a t i o s of c o n c e n t r a t i o n s of low-density minerals i n one f r a c t i o n with c o n c e n t r a t i o n s of c a s s i t e r i t e i n another f r a c t i o n . R e s u l t s show t h a t s m a l l (<50 Hm) h i g h - d e n s i t y mineral p a r t i c l e s behave i n much the same way as the same s i z e low-density p a r t i c l e s . Saxby and F l e t c h e r (1986) c o l l e c t e d samples from high and low -20-energy stream micro-environments a t the sampling s t a t i o n and showed, u s i n g geometric mean c o n c e n t r a t i o n r a t i o s , t h a t h i g h - d e n s i t y mineral c o n c e n t r a t i o n s i n the f i n e s t f r a c t i o n s are v e r y s i m i l a r f o r the two environments. D i f f e r e n c e s between environments are most pronounced f o r the c o a r s e s t sediment. The two approaches (beach s t u d i e s vs. r i v e r s t u d i e s ) d i s c u s s e d above have d i f f e r e n t i m p l i c a t i o n s f o r e x p l o r a t i o n stream sediment surveys dependent on elements contained i n h i g h - d e n s i t y m i n e r a l s . The i n d i v i d u a l f r a c t i o n approach y i e l d s i n f o r m a t i o n about the a p p r o p r i a t e sampling and sample p r o c e s s i n g technique to best e l i m i n a t e h y d r a u l i c e f f e c t s i n the data. The work suggests t h a t a n a l y s i n g very f i n e sediment (<60 y.m) produces data f r e e of e f f e c t s due to l o c a l p l a c e r - f o r m i n g p r o c e s s e s . Furthermore, the work of Saxby (1985) shows t h a t d i s p e r s i o n t r a i n s f o r the f i n e s t f r a c t i o n s may be very long. However, the anomaly to background r a t i o may be low i f : (1) f i n e h i g h - d e n s i t y minerals are r a r e i n the source m i n e r a l i s a t i o n , (2) the h i g h - d e n s i t y mineral p a r t i c l e s are not comminuted or (3) d i l u t i o n by f i n e s e d i m e n t - r i c h d e p o s i t s (e.g., t i l l ) o c curs. The beach s t u d i e s are not as u s e f u l to the e x p l o r a t i o n i s t because most mountain stream sediments are completely d i f f e r e n t i n m a t u r i t y from beach sand d e p o s i t s . -21-However, these s t u d i e s do provide c o n s i d e r a b l e i n f o r m a t i o n about the processes o p e r a t i n g to produce h i g h - d e n s i t y m i n e r a l c o n c e n t r a t i o n s . Mainly as a r e s u l t of beach s t u d i e s , S l i n g e r l a n d (1984) and Reid and F r o s t i c k (1985) have proposed four major s o r t i n g mechanisms by which p l a c e r s might form, v i z . , (1) s e t t l i n g or suspension, (2) entrainment, (3) d i s p e r s i o n and (4) i n t e r s t i c e t r a p p i n g . 1.3.1.1 S e t t l i n g or suspension s o r t i n g As d i s c u s s e d e a r l i e r , s e t t l i n g e q uivalence i m p l i e s t h a t two d i f f e r e n t l y or s i m i l a r l y s i z e d and shaped p a r t i c l e s of a p p r o p r i a t e d e n s i t y c o n t r a s t s e t t l e a t the same r a t e . Suspension s o r t i n g ( S l i n g e r l a n d , 1984) i s e s s e n t i a l l y the same i n t h a t p a r t i c l e s t r a n s p o r t e d i n suspension i n t u r b u l e n t open channel flow w i l l separate i n t o d i f f e r e n t l e v e l s w i t h i n the flow a c c o r d i n g to t e r m i n a l s e t t l i n g v e l o c i t y ( E i n s t e i n , 1950). S l i n g e r l a n d (1984) a l s o suggests t h a t suspension s o r t i n g might occur downstream of dunes i n the s e p a r a t i o n zone of high turbulence and low shear v e l o c i t y . Deep pools with v e r y low flow v e l o c i t i e s downstream of p o i n t bars are s i m i l a r l a r g e s c a l e flow s e p a r a t i o n zones i n which sediment would most l i k e l y show s e t t l i n g e q u i v a l e n c e . -22-1.3.1.2 Entrainment s o r t i n g Entrainment i s the process of removing p a r t i c l e s from the bed i n t o the flow by overcoming f o r c e s tending to keep the p a r t i c l e i n place i n the bed. E a r l y s t u d i e s of the a b i l i t y of entrainment to produce h i g h - d e n s i t y m i n e r a l c o n c e n t r a t i o n s f o l l owed the flume s t u d i e s of S h i e l d s (1936) who produced a diagram r e l a t i n g the c r i t i c a l shear Reynolds number to a dimensionless c r i t i c a l shear s t r e s s for entrainment ( F i g . 1-5A). The diagram has s i n c e been used e x t e n s i v e l y to d e f i n e the s t a b i l i t y of stream beds under d i f f e r e n t h y d r a u l i c c o n d i t i o n s . I t has two shortcomings: 1) i t was e s t a b l i s h e d u s i n g p e r f e c t l y s o r t e d (uniform diameter) n a t u r a l sediments and 2) the c r i t i c a l stage of a bed (th a t i s , the p o i n t at which motion of sediment p a r t i c l e s begins) i s d i f f i c u l t to d e f i n e o b j e c t i v e l y . In n a t u r a l sediments other e f f e c t s such as shape, roundness and g r a i n o r i e n t a t i o n s cause some g r a i n s to s t a r t moving before others even though the sediment i s a l l the same s i e v e s i z e . S i m i l a r curves can be p l o t t e d f o r d i f f e r e n t bed geometries r e s u l t i n g from v a r i o u s d i f f e r e n t degrees of s o r t i n g and mean sediment diameter based on experiments, but Y a l i n (1972) notes t h a t i t i s very d i f f i c u l t to p r e d i c t the curve based on the geometry of the bed. I t i s important to understand the bedforms before any concept of g r a i n h y d r a u l i c equivalence can be - 2 3 -0 . 0 1 4 , , 0 . 0 1 0 .1 1 10 d ( m m ) F i g . 1 - 5 . ( A ) S h i e l d s ( 1 9 3 6 ) diagram of the c r i t i c a l shear Reynolds number versus dimensionless c r i t i c a l shear s t r e s s with some experimental p o i n t s and (B) c r i t i c a l shear s t r e s s of entrainment f o r a number of s p h e r i c a l h i g h -d e n s i t y m a t e r i a l s i n water at 2 0 ° C d e r i v e d from S h i e l d s curve (modified from G r i g g and Rathbun ( 1 9 6 9 ) ) . -24-formulated (Steidtmann, 1982). G r i g g and Rathbun (1969) used the S h i e l d s diagram to o b t a i n c r i t i c a l shear s t r e s s f o r p a r t i c l e s of d i f f e r e n t s i z e and d e n s i t y . The r e s u l t i n g diagram ( F i g . 1-5B) shows th a t f o r uniform mixtures of s m a l l p a r t i c l e s i z e the c r i t i c a l shear s t r e s s f o r entrainment i s independent of p a r t i c l e s i z e but f i x e d f o r a given d e n s i t y , t h i s being a r e s u l t of the 45° p o r t i o n of the S h i e l d s curve a t low Reynolds numbers ( F i g . 1-5A). Recently, n a t u r a l o b s e r v a t i o n s and t h e o r e t i c a l modelling ( S I i n g e r l a n d , 1977, 1984) have shown t h a t entrainment equivalence i s not as e a s i l y d e f i n e d as s e t t l i n g e q u i v a l e n c e . Komar and Wang (1984) and Reid and F r o s t i c k (1985) showed t h a t i n beach d e p o s i t s where entrainment s o r t i n g has occurred, the s i z e d i s t r i b u t i o n s of h i g h - d e n s i t y minerals and low-density minerals are very s i m i l a r implying t h a t s i z e , not d e n s i t y i s the most important f a c t o r t h a t determines whether or not a p a r t i c l e i s e n t r a i n e d (Steidtmann, 1982). A l a r g e p a r t i c l e p r o t r u d i n g i n t o the flow i s e n t r a i n e d i n t o the flow a t low boundary Reynolds numbers (R B = KSU*/V, K a i s a measure of bed roughness, U* i s f r i c t i o n v e l o c i t y , and v i s the kinematic v i s c o s i t y ) r e g a r d l e s s of i t s d e n s i t y ( S l i n g e r l a n d , 1977). F l e t c h e r (pers. comm.) and S l i n g e r l a n d (1984) used a -25-bedload t r a n s p o r t formula ( E i n s t e i n , 1950) to model d i f f e r e n t i a l entrainment of low-density and h i g h - d e n s i t y minerals based on d i f f e r e n t i a l t r a n s p o r t r a t e s . As the model takes i n t o account the bed roughness and r o l l i n g and s a l t a t i o n t r a n s p o r t modes, the r e s u l t s can probably be c o r r e l a t e d to s o r t i n g of bed m a t e r i a l due to entrainment. The model p r e d i c t s t h a t p a r t i c l e s c l o s e to the median diameter of the bed are t r a n s p o r t e d a t s i m i l a r r a t e s i r r e s p e c t i v e of t h e i r d e n s i t y . T r a n s p o r t r a t e c o n t r a s t s for p a r t i c l e s of d i f f e r e n t d e n s i t y are g r e a t e s t f o r the f i n e sediment s i z e s but a b s o l u t e t r a n s p o r t r a t e s are very low, thus h i g h - d e n s i t y mineral c o n c e n t r a t i o n s w i l l seldom form. Entrainment s o r t i n g w i l l , however, produce e l e v a t e d h i g h - d e n s i t y mineral c o n c e n t r a t i o n s f o r intermediate s i z e s f o r which t r a n s p o r t r a t e s are moderate and d i s s i m i l a r f o r d i f f e r e n t mineral d e n s i t i e s ( F l e t c h e r , pers. comm.). The bedload formula of E i n s t e i n (1950) was t e s t e d o r i g i n a l l y f o r r i v e r s i n which the predominant bedload i s sand. Consequently, a p p l i c a t i o n to the wide spectrum of r i v e r s t y p i c a l l y encountered when sampling fo r - h i g h -d e n s i t y minerals i s extremely l i m i t e d . The r i v e r s sampled i n t h i s study have predominantly g r a v e l beds i n which most of the movement takes place d u r i n g the s p r i n g meltwater f l o o d . Andrews (1983) showed t h a t p a r t i c l e s i n g r a v e l beds between 0.3 and 4.2 times the median diameter (Doo) -26-are e n t r a i n e d a t n e a r l y the same di s c h a r g e and that t h i s i s achieved o n l y d u r i n g extreme f l o o d s ( F i g . 1-6). T h i s r e s u l t i s c o n t r a r y to an hypothesis t h a t p r o g r e s s i v e l y l a r g e r p a r t i c l e s are e n t r a i n e d a t i n c r e a s i n g l y higher s t a g e s . Andrews' (1983) model i m p l i e s t h a t h i g h - d e n s i t y mineral c o n c e n t r a t i o n s cannot form d u r i n g the s p r i n g f l o o d because a l l t r a n s p o r t r a t e s are the same. 1.3.1.3 D i s p e r s i v e s o r t i n g D i s p e r s i v e or shear s o r t i n g was proposed by S a l l e n g e r (1979) as a mechanism for producing c o n c e n t r a t i o n s of h i g h d e n s i t y m inerals at d i f f e r e n t h o r i z o n s w i t h i n a concentrated g r a n u l a r d i s p e r s i o n , f o r example a moving bed l a y e r of a stream ( S l i n g e r l a n d , 1984). I t a r i s e s as a r e s u l t of g r a i n c o l l i s i o n s producing d i s p e r s i v e pressures ( S a l l e n g e r , 1979) or k i n e t i c s i e v i n g i n which the s m a l l e r g r a i n s f a l l through the spaces between the l a r g e r g r a i n s (Middleton, 1970). Both mechanisms produce in v e r s e grading ( l a r g e p a r t i c l e s a t the top of the d e p o s i t ) . S a l l e n g e r (1979) d e r i v e d a r e l a t i o n s h i p from the d i s p e r s i v e pressure which shows t h a t f o r the f i r s t mechanism larg e h i g h -d e n s i t y p a r t i c l e s w i l l be i n the same h o r i z o n as s m a l l low-density p a r t i c l e s . S l i n g e r l a n d (1984) shows t h a t i t i s p o s s i b l e f o r h o r i z o n s to develop i n which h i g h - d e n s i t y mineral c o n c e n t r a t i o n s are twice the mean c o n c e n t r a t i o n . -27-F i g . 1-6. R e l a t i o n s h i p between c r i t i c a l d imensionless shear s t r e s s f o r motion and the r a t i o of p a r t i c l e s i z e to the median p a r t i c l e s i z e of the bed (Andrews, 1983). -28-K i n e t i c s i e v i n g r e s u l t s i n small h i g h - d e n s i t y mineral p a r t i c l e s f a l l i n g a l l the way to the base of the d e p o s i t . Although S a l l e n g e r (1979) re c o g n i z e s t h a t d i s p e r s i v e s o r t i n g may account f o r p r o f i l e s seen on the swash faces of beaches, no workers have proposed d i s p e r s i v e s o r t i n g as a mechanism to e x p l a i n a u r i f e r o u s f l u v i a l p l a c e r s . I t i s well-known that gold p a r t i c l e s work t h e i r way to the base of a g r a v e l d e p o s i t whether the base i s bedrock or a f a l s e bottom (Basque, 1979). This has been a t t r i b u t e d to repeated s c o u r i n g of d e p o s i t s with gold f a i l i n g to be e n t r a i n e d and e v e n t u a l l y r e a c h i n g bedrock. However, d i s p e r s i v e s o r t i n g should occur d u r i n g any f l o o d provided the bed l a y e r i s completely m o b i l i s e d . 1.3.1.4 I n t e r s t i c e t r a p p i n g I n t e r s t i c e t r a p p i n g attempts to e x p l a i n h i g h - d e n s i t y mineral c o n c e n t r a t i o n s by assuming t h a t f i n e p a r t i c l e s w i l l move i n t o the i n t e r s t i c e s between coarse p a r t i c l e s a f t e r a f l o o d event, hence the f i n e matrix i s not i n h y d r a u l i c equivalence with the coarse framework. Reid and F r o s t i c k (1985) proposed the mechanism though they are not c l e a r about how h i g h - d e n s i t y minerals w i l l behave with r e s p e c t to low-density m i n e r a l s . They suggest t h a t coarsening-upwards d e p o s i t s are u n l i k e l y to produce p l a c e r d e p o s i t s because the pores at the s u r f a c e become clogged; -29-however, fining-upwards sequences would a l l o w f i n e p a r t i c l e s to move downward through i n c r e a s i n g l y l a r g e r pores. C u r r e n t l y , the o n l y b a s i s f o r assuming t h a t i n t e r s t i c e - t r a p p i n g w i l l produce p l a c e r d e p o s i t s i s t h a t gold tends to be more abundant i n the f i n e r s i z e s . 1.3.2 Summary of mechanisms l i k e l y to produce h i g h - d e n s i t y  mineral c o n c e n t r a t i o n s As with a l l n a t u r a l processes, i t i s l i k e l y t h a t s e v e r a l mechanisms combine to produce the observed n a t u r a l r e s u l t . Although entrainment s o r t i n g has r e c e i v e d the most a t t e n t i o n , because i t e x p l a i n s l a g d e p o s i t s of h i g h -d e n s i t y minerals commonly observed on beaches, Reid and F r o s t i c k (1985) p o i n t out t h a t d i s p e r s i v e s o r t i n g may be an important a n c i l l a r y process. Large low-density p a r t i c l e s are f o r c e d to the s u r f a c e a l l o w i n g them to protrude i n t o the flow and be e n t r a i n e d . T h i s leaves a l a g d e p o s i t of f i n e h i g h - d e n s i t y m i n e r a l s . S l i n g e r l a n d (1984) e x p l a i n s the flume o b s e r v a t i o n s of Brady and Jobson (1973) and McQuivey and Keefer (1969) using three processes. Entrainment s o r t i n g on the c r e s t s of r i p p l e s and dunes e x p l a i n s topset h i g h - d e n s i t y mineral l a m e l l a e ; d i s p e r s i v e s o r t i n g on the avalanche face e x p l a i n s f o r e s e t h i g h -d e n s i t y mineral lamellae and suspension s o r t i n g i n the trough e x p l a i n s bottom s e t h i g h - d e n s i t y mineral l a m e l l a e . S i m i l a r processes a c t i n g at a l l s c a l e s can be used to -30-e x p l a i n many types of p l a c e r d e p o s i t . Although experimental and t h e o r e t i c a l r e s u l t s can be brought together q u a l i t a t i v e l y , p r e d i c t i o n of the occurrence and c o n c e n t r a t i o n of p l a c e r s i s s t i l l l a r g e l y a matter of a p p l y i n g past e x p e r i e n c e . 1.4 F i e l d sampling problems The aim of e f f e c t i v e sediment sampling i n min e r a l e x p l o r a t i o n i s to provide e i t h e r : (1) an i n d i c a t i o n of gold m i n e r a l i s a t i o n i n a la r g e catchment b a s i n to warrant f u r t h e r i n v e s t i g a t i o n or (2) an i n d i c a t i o n of p r o x i m i t y to a source of anomalous c o n c e n t r a t i o n s of gold i n the sediments of a s i n g l e stream. The f i r s t problem can be so l v e d by t a k i n g an adequately l a r g e sample so that the p r o b a b i l i t y of not d e t e c t i n g the anomaly i s t o l e r a b l y low. The second problem may be approached by checking t h a t an el e v a t e d gold c o n c e n t r a t i o n i s the r e s u l t of p r o x i m i t y to m i n e r a l i s a t i o n and i s not due to l o c a l h y d r a u l i c processes. F i e l d sampling methods must d e a l with these problems or the survey w i l l be a waste of time and money. Hence, the f o l l o w i n g s e c t i o n w i l l d i s c u s s methods of f i e l d sampling. 1.4.1 F i e l d sample c o l l e c t i o n techniques Emmons (1937) recommended that prospectors should pan -31-stream sediments to determine the presence or absence of gold i n the sediment. Many bedrock gold occurrences can be l o c a t e d by determining the upstream terminus of a d i s p e r s i o n t r a i n of " c o l o u r s " (small p a r t i c l e s of gold i n panned c o n c e n t r a t e s ) . The method i s o b v i o u s l y q u i t e r a p i d i n t heory but i s extremely prone to f a i l u r e , e s p e c i a l l y i f the gold i n the stream sediments i s too f i n e to be panned s u c c e s s f u l l y . P o l i n g (1985) i n d i c a t e d t h a t the r e c o v e r y of g o ld decreases d r a m a t i c a l l y with decrease i n s i z e f o r p a r t i c l e s f i n e r than 100 Jim and i s extremely dependent on the technique and s k i l l of the panner. Despite i t s shortcomings, panning i s a powerful e x p l o r a t i o n t o o l i n the r i g h t hands. The s t a t i s t i c a l sampling problem can be overcome because a very l a r g e q u a n t i t y of sediment can be processed to y i e l d a s m a l l h i g h - d e n s i t y m i n e r a l c o n c e n t r a t e . However, i t i s important that the s i z e d i s t r i b u t i o n and the expected c o n c e n t r a t i o n of gold i n the sediment are i n v e s t i g a t e d i n a reconnaissance survey beforehand (Boyle, 1979). Boyle and Gleeson (1972) took a l a r g e number of panned concentrates from the m i n e r a l i s e d Keno H i l l a r ea, Yukon T e r r i t o r y . The r e s u l t s show t y p i c a l nugget e f f e c t s with samples from many streams r e t u r n i n g background Au content and rare samples with extremely high Au c o n c e n t r a t i o n ( F i g . 1-1). Boyle (1979) suggested that the weight of the concentrate should -32-be recorded, which may be an important step i n l a t e r assessment of r e l i a b i l i t y of Au v a l u e s . Zantop and N e s p e r e i r a (1979) compared the r e s u l t s of t r a d i t i o n a l -80-mesh (<177 p\m) geochemical techniques (e.g. Rose et. §_1., 1979) with that of panned c o n c e n t r a t e s . The geochemical method produced spot anomalies f o r Sn and W and was prone to poor r e p r o d u c i b i l i t y due to the nugget e f f e c t f o r c a s s i t e r i t e , wolframite and s c h e e l i t e . Even known m i n e r a l i s a t i o n c o u l d not be d e t e c t e d r e l i a b l y . Panned concentrates were analysed v i s u a l l y producing very r e l i a b l e r e s u l t s . They noted that panning takes more time than sampling sediments without f i e l d c o n c e n t r a t i o n but t h a t a lower sampling d e n s i t y was needed and the r e s u l t s were more meaningful. In the end the o v e r a l l c o s t i s much lower and the turnaround time i s s h o r t e r . The gold pan i s the most popular f i e l d c o n c e n t r a t i o n device because i t i s extremely p o r t a b l e and t h e o r e t i c a l l y simple to use. In c o n t r a s t Smith (1986), Maurice and Merc i e r (1986) and G i u s t i (1986) have used a s u c t i o n dredge and r i f f l e d s l u i c e p e r m i t t i n g extremely r a p i d sampling and p r o c e s s i n g of l a r g e volumes of sediment. However, Wang and P o l i n g (1983) show that the r i f f l e d s l u i c e i s extremely i n e f f i c i e n t f o r c o l l e c t i n g g old f i n e r than 0.1 mm. -33-Geochemical r e s u l t s from f i e l d panning and s l u i c i n g i n e v i t a b l y c o n t a i n c o n s i d e r a b l e v a r i a b i l i t y due to d i f f e r i n g s k i l l s of the samplers. T h i s problem can be s o l v e d by c o l l e c t i n g bulk stream sediment samples screened to a f a i r l y coarse s i z e (<2 mm) and sending the samples to a commercial l a b o r a t o r y f o r s i e v i n g and p r e p a r a t i o n of heavy mineral concentrates using h i g h - d e n s i t y l i q u i d s . T h i s method has the advantage t h a t the samples can a l l be processed i n a standard f a s h i o n . The disadvantage i s t h a t the samples must be l a r g e . There i s some evidence t h a t c o l l e c t i o n of l a r g e samples i s becoming standard procedure i n the mineral e x p l o r a t i o n i n d u s t r y , though there i s a shortage of l a b o r a t o r i e s w i l l i n g to process the samples at a reasonable c o s t . Daughtry (1986), Fipke (1986) and Smith (1986) recommend that sediment should be f i e l d -screened to between <1 mm or <2 mm to y i e l d an 8 to 10 kg sample. 1.4.2 S e l e c t i o n of sampling l o c a t i o n T h i s aspect of sampling i s u s u a l l y very p o o r l y d e s c r i b e d i n the l i t e r a t u r e d e s p i t e the i m p l i c a t i o n s to data i n t e r p r e t a t i o n with re s p e c t to h y d r a u l i c e f f e c t s . In many small streams c o l l e c t i o n or panning of a l a r g e sample can be achieved only by t a k i n g a composite sample from s e v e r a l l o c a t i o n s w i t h i n a s h o r t d i s t a n c e of each other. -34-Smith (1986) c o l l e c t s bulk samples a t l o c a t i o n s where sand f i n e r than 2 mm has accumulated, r e c o g n i z i n g the importance of t a k i n g samples from s i m i l a r environments wherever p o s s i b l e . Maurice (1986b) i n v e s t i g a t e d d i f f e r e n t types of sampling l o c a t i o n s on the Assemetquagan R i v e r , Quebec and found t h a t s i l t and sand c o l l e c t e d from between s l a t y cleavage i n the r i v e r bed y i e l d e d high numbers of gold p a r t i c l e s , however; the h i g h - d e n s i t y mineral concentrates were c o n s i d e r a b l y l a r g e r than can be analysed commercially. 1.5 Conclusions The c u r r e n t problems i n stream sediment surveys a r i s e because: 1) sampling techniques do not take i n t o account the extreme r a r i t y of gold i n stream sediments. 2) t r a d i t i o n a l sampling t o o l s (gold pan) are i n e f f i c i e n t for r e c o v e r y of f i n e sand-size gold p a r t i c l e s which may be the best f r a c t i o n to sample. 3) the s i z e d i s t r i b u t i o n and shape of gold i s not determined p r i o r to sampling. 4) very l i t t l e i s known q u a n t i t a t i v e l y about h y d r a u l i c and geomorphic processes c o n t r o l l i n g d i s p e r s i o n of gold i n gravel-bed r i v e r s , hence Au data cannot be c o r r e c t e d -35-r e l i a b l y to remove l o c a l h y d r a u l i c e f f e c t s . - 3 6 -C H A P T E R 2: O R I E N T A T I O N S A M P L I N G - 3 7 -2.0 I n t r o d u c t i o n Data on the s t a t i s t i c a l problems of sampling stream sediments for Au are not r e a d i l y a v a i l a b l e (see d i s c u s s i o n i n s e c t i o n 1.2.1). Th e r e f o r e , i n t h i s phase of the study l a r g e stream sediment samples were c o l l e c t e d from f i v e streams d r a i n i n g gold occurrences i n southern B r i t i s h Columbia. Each sample was s p l i t i n t o component f r a c t i o n s to determine the d i s t r i b u t i o n of Au i n the samples thereby p e r m i t t i n g e s t i m a t i o n of the number of gold p a r t i c l e s i n each f r a c t i o n and sampling e r r o r s due to the nugget e f f e c t . 2.1 D e s c r i p t i o n of streams In June 1 9 8 5 , f i v e streams were s e l e c t e d f o r sediment sampling based on the known occurrence of a stream sediment Au anomaly or other w e l l - d e f i n e d anomaly ( f o r example, pedo- or 1 i t h o g e o c h e m i c a l ) . L o c a t i o n s of streams are shown i n F i g . 2 - 1 . 2.1.1 Tsowwin R i v e r , near Tahsis Tsowwin R i v e r (49°48'N 1 2 6 Q 3 4 ' W , NTS 9 2 E / 1 5 ) d r a i n s i n t o T a h s i s I n l e t on the west coast of Vancouver I s l a n d . From i t s headwaters to the i n l e t the stream i s 8 km long and f a l l s through 6 0 0 m. A small electrum-quartz -38-L F i g . 2-1. L o c a t i o n s of streams sampled. l=Tsowwin R i v e r , 2='Salmonberry' Creek, 3=Franklin R i v e r , 4=Harris Creek, 5=Watson Bar Creek. -39-carbonate v e i n i n a shear zone hosted by Bonanza V o l c a n i c s (Muller e_t a l . , 1976) crops out about 5 km from the i n l e t (B. Smee, pers. comm). The stream i s y o u t h f u l , f a s t -f l o w i n g over bedrock i n i t s upper reaches but becomes braided towards T a h s i s I n l e t . The stream was sampled, 200 m below the outcrop of known m i n e r a l i s a t i o n (samples 85-SD-01, 85-SD-02), i n coarse to f i n e g r a v e l s accumulating downstream of sma l l r a p i d s . Sample 85-SD-03 was taken from a steep (slope=0.34) t r i b u t a r y about 0.5 km above the m i n e r a l i s a t i o n ( F i g . 2-2A). 2.1.2 %Salmonberry Creek', near U c l u e l e t 'Salmonberry Creek' ( u n o f f i c i a l name, 49<>01'N 125°30'W, NTS 92F/03,04) d r a i n s a complex area of Au-Sb-As-Ag quartz m i n e r a l i s a t i o n hosted i n Vancouver I s l a n d Group (Bonanza V o l c a n i c s , I s l a n d I n t r u s i v e s ) r o c k s . The creek i s very s h o r t (4 km), steep (slope=0.4) and narrow (<7 m) and flows over t i l l and f l u v i a l d e p o s i t s , e v e n t u a l l y r e a c h i n g Kennedy Lake a f t e r j o i n i n g a l a r g e r stream. Samples 85-SD-04 and 85-SD-05 were taken on a fan d e p o s i t above Kennedy Lake where the stream emerges from the h i l l s . Small g r a v e l d e p o s i t s on the bars were sampled about 4 km downstream of the m i n e r a l i s e d a r e a . A s m a l l l o g and boulder-choked t r i b u t a r y was sampled a s h o r t d i s t a n c e -40-F i g . 2-2. Sampling s t a t i o n s on streams, (A) Tsowwin R i v e r , (B) 'Salmonberry Creek', (C) F r a n k l i n R i v e r , (D) H a r r i s Creek, (E) Watson Bar Creek. D e t a i l s are from 1:50,000 NTS maps. Contours are i n metres f o r (B) and (C) and f e e t f o r (A), (D) and ( E ) . P o s s i b l e sources of gold i n the stream sediments are shown by s t a r s . -f -41-upstream of the m i n e r a l i s a t i o n on a s m a l l sand and g r a v e l bar ( F i g 2-2B). 2.1.3 F r a n k l i n R i v e r , west of P o r t A l b e r n i F r a n k l i n R i v er ( 4 9 O 0 7'N 124039'W, NTS 92F/02) i s a meandering stream with a cobble and boulder bed d r a i n i n g a r e g i o n a l Au-Cu s o i l anomaly d e r i v e d from a s u l p h i d e - i r o n formation i n the S i c k e r Group of Vancouver I s l a n d (R. Walker, pers. comm. 1985). Gold-bearing quartz v e i n s have been mined near the source of the stream ( T h i s t l e Mine). I t i s 8 km long to a major confluence and has an average slope of 0.026. As no part of the drainage could be c o n s i d e r e d background, samples 85-SD-08 and 85-SD-09 were taken about 1 km downstream of the anomaly on a small sand and g r a v e l accumulation around a log-jam ( F i g . 2-2C). 2.1.4 H a r r i s Creek, east of Vernon H a r r i s Creek (50O12'N 118055'W, NTS 82L/02) i s a mature stream a t l e a s t 40 km long to i t s confluence with Duteau Creek i n Lumby. I t d r a i n s a r o l l i n g upland blanketed by g l a c i a l d r i f t d e p o s i t s . w h i c h o v e r l i e g n e i s s , g r a n o d i o r i t e Q i n t r u s i o n s and p l a t e a u b a s a l t s . A gold anomaly i n stream sediments has been d e f i n e d though the type of m i n e r a l i s a t i o n i s unknown. Sampling was c a r r i e d out roughly 30 km from the headwaters i n the Au d i s p e r s i o n -42-t r a i n (A. Burton, pers. comm.) and background sediments. Samples 85-SD-10 and 8 5 - S D - l l were taken from a s m a l l sandy bar and beach and sample 85-SD-12 was taken from a smal l g r a v e l p o i n t bar upstream of the beginning of the d i s p e r s i o n t r a i n ( F i g . 2-2D). 2.1.5 Watson Bar Creek, north of L i l o o e t Watson Bar Creek (51<>05'N 122°15'W, NTS 920/01) i s 25 km long from the headwaters to i t s confluence with the F r a s e r R i v e r . An area of w e l l - d e f i n e d Au-Ag-Sb-As quartz carbonate m i n e r a l i s a t i o n hosted i n c l a s t i c sediments i s l o c a t e d 9 km upstream of the c o n f l u e n c e . Samples were taken 4 km downstream of the m i n e r a l i s a t i o n i n the stream bed (85-SD-13 and 85-SD-14) and from a channel bar a s h o r t d i s t a n c e upstream of the m i n e r a l i s a t i o n (85-SD-15) ( F i g . 2-2E). Downstream of the m i n e r a l i s a t i o n the stream flows through t i l l and f l u v i a l d e p o s i t s . Recently, p l a c e r mining has been c a r r i e d out upstream of the m i n e r a l i s a t i o n at S t i r r u p Creek (H. Warren, pers. comm.). 2.2 F i e l d Sampling On each stream, three samples were taken: two were taken downstream of the m i n e r a l i s a t i o n i n high energy (coarse sediment) and low energy ( r e l a t i v e l y f i n e r sediment) l o c a t i o n s w i t h i n one or two metres of each -43-other, and i f p o s s i b l e a t h i r d sample was taken upstream of the m i n e r a l i s a t i o n i n sediments expected to c a r r y background Au c o n t e n t s . Sample l o c a t i o n s were s e l e c t e d based on the a v a i l a b i l i t y of a s u f f i c i e n t volume of v i s u a l l y homogeneous sediment. U s u a l l y a p i t 1 mJ and 20 to 30 cm deep was dug to provide the sample. The sediment was wet s i e v e d through a 5 mm screen i n t o a 23 l i t r e p l a s t i c p a i l u n t i l the p a i l was t h r e e - q u a r t e r s f u l l . T h i s volume was roughl y e q u i v a l e n t to 20 kg of -5 mm sediment. I n e v i t a b l y , the p a i l became f u l l of water and overflowed but no attempt was made to ca t c h the overflow, hence v e r y f i n e sediment i n suspension was l o s t . Fine sediment was a l s o l o s t at the s i d e s of the s i e v e d u r i n g the j i g g i n g a c t i o n of s i e v i n g . A f t e r each s h o v e l f u l of sediment was t i p p e d i n t o the s i e v e the shovel blade was very c a r e f u l l y r i n s e d i n t o the s i e v e as there i s an observable tendency f o r heavy-minerals (black sand) to be r e t a i n e d . The +5 mm f r a c t i o n was d i s c a r d e d . At each sample s i t e photographs were taken of sample l o c a t i o n s and t e x t u r a l and l i t h o l o g i c a l o b s e r v a t i o n s were recorded. Sample l o c a t i o n s were s e l e c t e d c l o s e to roads to a l l o w easy removal of the samples to a v e h i c l e ; however, samples were taken upstream of nearby road b r i d g e s to a v o i d contamination by road m a t e r i a l s . -44-2.3 L a b o r a t o r y p r o c e s s i n g 2.3.1 S i e v i n g Samples were wet s i e v e d using the apparatus shown i n F i g 2-3. Seven s i e v e s were used to provide two f r a c t i o n s each spanning 2 4> (-4+16-mesh (ASTM) , -16+50-mesh, 4-mesh i s the f i e l d s i e v e ) , f i v e f r a c t i o n s each spanning 0.5 * (-50+70-mesh, -70+100-mesh, -100+140-mesh, -140+200-mesh, -200+270-mesh) and -270-mesh sediment. The s i e v e stack was mounted on a 23 l i t r e p a i l by p l a c i n g the stack on a s p e c i a l l y cut hole i n the p a i l l i d . The hole allowed the stack to f i t t i g h t l y to prevent p o s s i b l e contamination of the f i n e s t f r a c t i o n with coarse sediment, however the 270-mesh s i e v e c o u l d e a s i l y be removed. A hose provided f r e s h water at the top of the stack and a simple faucet-mounted a s p i r a t o r removed the overflow from the p a i l , p r e v e n t i n g water backup i n the s t a c k . C o l l e c t i o n of a p o r t i o n of the o v e r f l o w and f i l t r a t i o n through a 1 Um f i l t e r showed t h a t l o s s e s of f i n e sediment were about 0.7 mg/s or 30 g/sample. About 300 g of -4-mesh sediment was t i p p e d i n t o the top of the stack and washed and mixed with the f i n g e r s u n t i l the sediment was v i s i b l y f r e e of m a t e r i a l t h a t would pass through the s c r e e n . T h i s sequence was repeated f o r each s i e v e . Water tended to flow out of the j o i n t s i n the - 4 5 -cfl o > CO CO < Fresh water 16 50 70 100 140 200 270 water & fine sediment fresh water I 1 overflow water & fine sediment (plastic pail -270-mesh sediment F i g . 2 - 3 . Wet-sieving apparatus. -4 6 -s i e v e stack due to c l o g g i n g of the f i n e r s i e v e s but was channeled away from the lower j o i n t by the upward bowing of the l i d ( F i g . 2-3), thus a v o i d i n g contamination of the -270-mesh f r a c t i o n by c o a r s e r m a t e r i a l . Contamination between f r a c t i o n s was avoided by washing hands and hose before d e a l i n g with a new f r a c t i o n . The two wet 2 * f r a c t i o n s were immediately placed i n p l a s t i c bags, weighed without d r y i n g , s e a l e d , and s t o r e d . The f i v e 0.5 * f r a c t i o n s were washed i n t o g l a s s beakers and d r i e d a t 80°C and weighed. The -270-mesh f r a c t i o n was allowed to s e t t l e f o r at l e a s t 24 hours, a f t e r which the water was decanted o f f and d i s c a r d e d . The sediment was then washed i n t o s e v e r a l beakers and d r i e d a t 80<>C and weighed. At any stage when the sample had to be t r a n s f e r r e d from one c o n t a i n e r to another by repeated washing with water, extreme care was taken to ensure t h a t a s m a l l p o r t i o n of heavy minerals was not l o s t . These p a r t i c l e s (diameter g r e a t e r than 30 Vim) tend to remain i n the bottom of the o r i g i n a l c o n t a i n e r d u r i n g washing. A f t e r completion of s i e v i n g of each sample the s i e v e s were thoroughly cleaned using a p a i n t brush, a i r b r u s h and u l t r a s o n i c c l e a n e r followed by v i s u a l examination using a hand-lens to d e t e c t screen breakages. -47-2.3.2 Heavy-mineral s e p a r a t i o n Gold i n the 0.5 <t> f r a c t i o n s was concentrated i n t o heavy mineral f r a c t i o n s u sing methylene i o d i d e ( C H z I a ) a heavy l i q u i d with a s p e c i f i c g r a v i t y of 3.3. Concentrates were prepared using a s e p a r a t o r y f u n n e l with repeated manual a g i t a t i o n with a g l a s s s t i r r i n g r o d. Methylene i o d i d e was recovered from the samples by repeated washing with acetone. The r e s u l t i n g acetone-methylene i o d i d e s o l u t i o n was then washed f o r s e v e r a l hours with water to remove the acetone. Washing proceeded u n t i l the d e n s i t y of the heavy l i q u i d reached 3.3 g/cm a. Before use, r e - c y c l e d methylene i o d i d e was f i l t e r e d u s i ng Whatman 2 f i l t e r s ; t h e r e f o r e , the l i q u i d probably remained contaminated with very f i n e sediment. 2.3.3 P r e - c o n c e n t r a t i o n of -270-mesh f r a c t i o n S e v e r a l attempts were made to produce a -270-mesh heavy-mineral concentrate using methylene i o d i d e and a c e n t r i f u g e . However, c l a y i n the f r a c t i o n tends to bind the sample together and the wide range of p a r t i c l e s i z e s i n the f r a c t i o n and corresponding s e t t l i n g v e l o c i t i e s leads to p r o d u c t i o n of a concentrate of onl y the ve r y c o a r s e s t heavy minerals i n the f r a c t i o n . -48-2.3.4 Chemical a n a l y s i s Heavy-mineral c o n c e n t r a t e s , l i g h t - m i n e r a l separates and -270-mesh sediment were weighed i n t o p l a s t i c v i a l s f o r d e t e r m i n a t i o n by n o n - d e s t r u c t i v e i n s t r u m e n t a l neutron a c t i v a t i o n a n a l y s i s (INAA) of Au and 21 other elements (Na, Ca, Se, Cr, Fe, Co, N i , Zn, As, Se, Mo, Ag, Sb, Ba, La, Lu, Hf, Ta, W, Th, U). Neutron a c t i v a t i o n a n a l y t i c a l parameters (X-Ray Assay L a b o r a t o r i e s , Don M i l l s , Ontario) are summarised i n Table 2-1. Only 60 g of heavy-minerals or 25 g of l i g h t m inerals could be analysed, t h e r e f o r e l i g h t minerals and -270-mesh sediment were s p l i t u s i n g a r i f f l e s p l i t t e r . Heavy-mineral c o n c e n t r a t e s from H a r r i s Creek were a l s o l a r g e enough to be s p l i t ( c o a r s e r f r a c t i o n s ) , both s p l i t s being submitted f o r a n a l y s i s . S e v e r a l d u p l i c a t e s p l i t s of l i g h t mineral separates were a l s o analysed so t h a t 5% of a l l the samples were analysed as d u p l i c a t e s . A f t e r the samples were returned f o l l o w i n g the c o o l i n g p e r i o d to a l l o w decay of r a d i o a c t i v e isotopes to acc e p t a b l e r a d i a t i o n l e v e l s , 38 samples were resubmitted fo r a n a l y s i s by a d i f f e r e n t l a b o r a t o r y (Chemex L a b o r a t o r i e s , North Vancouver, B r i t i s h Columbia) u s i n g f i r e assay with flame atomic a b s o r p t i o n spectroscopy f i n i s h on the electrum bead. A s u i t e of samples i n t h e i r o r i g i n a l v i a l s were a l s o re-submitted to the f i r s t -49-Table 2-1. Summary of c o n d i t i o n s used i n i n s t r u m e n t a l neutron a c t i v a t i o n a n a l y s i s , McMaster U n i v e r s i t y r e a c t o r (X-Ray Assay L a b o r a t o r i e s ) Parameter Type or Value Reactor power 2 Megawatts I r r a d i a t i o n f l u x 2 x l 0 1 3 neutrons/cm 2/s I r r a d i a t i o n time 1 hour P r e - a n a l y s i s decay time 5 to 7 days A c t i v i t y i n t e g r a t i o n time 200 s Au isotope 198 Detector Ge(Li) Au Standard In-house and MA2 P o s t - a n a l y s i s decay time At l e a s t three months -50-l a b o r a t o r y to check f o r r e p r o d u c i b i l i t y . 2.3.5 V i s u a l examination of heavy-mineral concentrates Fine sand-size heavy-mineral concentrates from a l l samples were examined v i s u a l l y u s ing a b i n o c u l a r microscope a f t e r removing magnetic minerals and s p r i n k l i n g the concentrate on a black paper t r a y . A 5 cm t h i c k lead s h i e l d was c o n s t r u c t e d around the microscope to provide p r o t e c t i o n a g a i n s t l o n g e r - l i v e d ^ - p a r t i c l e - e m i t t i n g r a d i o a c t i v e i s o t o p e s . Gold p a r t i c l e s were e x t r a c t e d and r e l a t i v e dimensions and shape were recorded. Gold p a r t i c l e s were a l s o removed from the -140+200-mesh concentrates from H a r r i s Creek f o r d e t a i l e d shape analys i s . 2.3.6 Scanning e l e c t r o n microscopy (S.E.M.) Gold p a r t i c l e s from H a r r i s Creek were mounted on S.E.M. stubs by f i r s t c o a t i n g the stubs with cosmetic n a i l p o l i s h and p e r m i t t i n g the p o l i s h to d r y ( J . Knight, p e r s . comm.). P a r t i c l e s were then placed on the stubs and surrounded by acetone vapour i n a p e t r i d i s h to a l l o w the p a r t i c l e s to s e t t l e i n t o the p o l i s h . Before m i c r o s c o p i c examination, stubs were coated with carbon. A x i a l dimensions were then estimated from S.E.M. photographs. -51-2.4 R e s u l t s 2.4.1 D i s t r i b u t i o n of sediment s i z e s i n -5 mm sediment Because the weight of the +5 mm f i e l d r e j e c t was not recorded the t o t a l weight of each sample i s unknown. However, the d i s t r i b u t i o n s of the f i n e r f r a c t i o n s are known from l a b o r a t o r y p r o c e s s i n g ( F i g . 2-4A to E ) . As expected, the co a r s e r f r a c t i o n s have the g r e a t e s t weights. Comparison of medium sand and f i n e r f r a c t i o n s taken from high and low energy environments shows t h a t the weights of the l a t t e r are more than twice the weights of the former (except f o r Watson Bar Creek where environments were q u i t e s i m i l a r ) . 2.4.2 Weights of heavy-mineral c o n c e n t r a t e s Weights of heavy minerals i n each f r a c t i o n are summarised i n the appendix. The g r e a t e s t weights are found i n the c o a r s e s t f r a c t i o n (-50+70-mesh) though the hi g h e s t c o n c e n t r a t i o n s occur i n e i t h e r -70+100-mesh or -100+140-mesh. 2.4.3 R e l i a b i l i t y of INAA Non-des t r u c t i v e INAA f o r Au was s e l e c t e d as 1) i t avoids sample p r e p a r a t i o n problems encountered i n d e s t r u c t i v e techniques (e.g., gold l o s s due to smearing d u r i n g m i l l i n g ) and, 2) i t preserves the sample f o r f u t u r e - 5 2 -0-25 0.125 0.0625 0.25 0.125 0.0625 Diameter (mm) 3 A , 1 «-> 0.25 0.125 0.0625 Di ameter (mm) Fig. 2-4. Cumulative sediment size d i s t r i b u t i o n s for sediment finer than 4 mm: (A) Tsowwin River, (B) 'Salmonberry Creek', (C) Franklin River, (D) Harris Creek, (E) Watson Bar Creek. Sample numbers are shown next to each d i s t r i b u t i o n . -53-r e - a n a l y s i s or examination. However, r e p r o d u c i b i l i t y of a n alyses and s y s t e m a t i c e r r o r s (e.g., due to s e l f -s h i e l d i n g ) must be c o n s i d e r e d . Analyses of heavy-mineral s p l i t s cannot be used to provide an i n d i c a t i o n of i n s t r u m e n t a l r e p r o d u c i b i l i t y ( F i g . 2-5) because Au occurs as very r a r e d i s c r e t e p a r t i c l e s which are l i k e l y to be be unevenly p a r t i t i o n e d between s p l i t s even i f an adequate s p l i t t e r i s used. Conversely, s p l i t d e t erminations of elements not o c c u r r i n g as very r a r e d i s c r e t e mineral p a r t i c l e s (e.g. As, Table 2-2) give r e p r o d u c i b l e r e s u l t s (ANOVA, 95% c o n f i d e n c e ) . Ten samples resubmitted f o r a n a l y s i s about four months a f t e r the f i r s t batch show good r e p r o d u c i b i l i t y (±10% with 95% confidence) of Au analyses with no observable b i a s e s i n any p a r t i c u l a r f r a c t i o n , the worst case being c l o s e to the d e t e c t i o n l i m i t ( F i g . 2-6). S e l f - s h i e l d i n g occurs i n the d e t e r m i n a t i o n of heavy elements, such as g o l d , by INAA i f the c e n t r e s of l a r g e g r a i n s are s h i e l d e d from the i r r a d i a t i n g f l u x by the g r a i n margins g i v i n g an o v e r a l l u nderestimation of the true Au content of the sample. Bloom and Brooker (1986) c l a i m t h a t s e l f - s h i e l d i n g only occurs when gold p a r t i c l e s are co a r s e r than 0.1 mm. As a t e s t of p o s s i b l e under--54-Au (ppb) F i g . 2-5. Gold analyses (INAA) of heavy mineral concentrate s p l i t s . -55-Table 2-2. D u p l i c a t e INAA analyses f o r As i n l i g h t m i n e r a l separates ( a l l values i n ppm). The re p o r t e d i n s t r u m e n t a l d e t e c t i o n l i m i t i s 1 ppm. Sample # Analys i s Number F r a c t i o n F i r s t Second 01 -50+70 20 22 03 -100+140 8 6 04 -50+70 6 7 05 -70+100 4 8 06 -70+100 23 20 07 -70+100 7 11 07 -100+140 12 16 13 -200+270 81 85 14 -140+200 9 13 A n a l y s i s of v a r i a n c e on l o g a r i t h m i c data N u l l hypothesis : M i = M= • = . . . = M-s Source of Sum of Degrees of Mean F V a r i a b i l i t y Squares Freedom Squares Between P a i r s 2.1419 8 0.2677 24.56 Within P a i r s 0.0981 9 0.0109 FORXXICAL(8,9,0 .05) = 3. 23 F > F c R X T l C A L , n u l l hypothesis r e j e c t e d -56-e s t i m a t i o n of Au content by INAA, t h i r t y samples were r e -analysed by f i r e assay with atomic a b s o r p t i o n f i n i s h . R e s u l t s are comparable at high c o n c e n t r a t i o n s ( F i g . 2-7) but become s c a t t e r e d a t lower c o n c e n t r a t i o n s . However, the s c a t t e r i s p r i m a r i l y a s s o c i a t e d with the -70+100-mesh f r a c t i o n , with f i r e assay t y p i c a l l y r e t u r n i n g higher Au c o n c e n t r a t i o n s than INAA. T h i s may i n d i c a t e t h a t s e l f -s h i e l d i n g i s important f o r the c o a r s e r f r a c t i o n s as suggested Bloom and Brooker (1986). Based on the f o r e g o i n g t e s t s of r e p r o d u c i b i l i t y and freedom from b i a s , n o n - d e s t r u c t i v e INAA appears to be an adequate a n a l y t i c a l technique. 2.4.4 D i s c u s s i o n of stream Au data Gold c o n c e n t r a t i o n data f o r a l l samples are presented i n Tables 2-3A and 2-3B. The m a j o r i t y of heavy-mineral co n c e n t r a t e s (Table 2-3A) have c o n c e n t r a t i o n s i n excess of the average d e t e c t i o n l i m i t («15 ppb) whereas the m a j o r i t y of l i g h t m i n e r a l separates are at or below the d e t e c t i o n l i m i t : o n l y two analyses are s i g n i f i c a n t l y g r e a t e r than the d e t e c t i o n l i m i t . These anomalous samples probably represent inadequate heavy-mineral s e p a r a t i o n , an i n e v i t a b l e problem with the f i n e r f r a c t i o n s . The m a j o r i t y -57-F i g . 2-6. Comparison of Au analyses determined by INAA on two d i f f e r e n t o c c a s i o n s . - 5 8 -F i g . 2 - 7 . C o m p a r i s o n o f Au c o n c e n t r a t i o n s d e t e r m i n e d f i r e a s s a y / a t o m i c a b s o r p t i o n a n d I N A A . - 5 9 -Table 2-3A. Gold c o n c e n t r a t i o n s (ppb) of heavy mineral c o n c e n t r a t e s . Sample » L/H/B 1 F r a c t i o n (mesh) -50+70 -70+100 -100+140 -140+200 -200+270 Tsowwin River 01 02 03 L H B 3200 nd : nd 4000 35 11 6200 5300 13 53 23000 6000 6000 140 2100 "Salmonberry Creek" 04 L 16 05 H 3400 06 B 350 31 3700 190 3400 35 8400 27 480 19000 3300 2800 2300 F r a n k l i n R i v er 07 L 870 4100 12000 6900 10000 08 H 280 540 520 22000 500 H a r r i s Creek 10 L 11 H 12 B 455 750 730 1130 1480 590 890 720 3000 3250 1200 7400 3100 1600 3600 Watson Bar Creek 13 Ii 160 3000 13000 940 7800 14 H nd 200 nd 2900 6000 15 B 11 250 1000 23 130 (Continued) 1 L=Low energy, H=High energy, B=Background sample. * nd=not d e t e c t e d . D e t e c t i o n l i m i t s are v a r i a b l e but average 15 ppb. -60-Table 2-3B. Gold c o n c e n t r a t i o n s (ppb) of l i g h t m i neral c o n c e n t r a t e s . F r a c t i o n (mesh) Sample ft L/H/B 1 -50+70 -70+100 -100+140 -140+200 -200+270 -270 Tsowwin River 01 L n d 2 nd nd nd 20 nd 02 H nd nd 15 15 nd 34 03 B nd nd nd 20 19 nd "Salmonberrv Creek" 04 L nd nd 10 18 5 15 05 H nd nd nd nd nd 15 06 B nd nd nd nd nd 34 F r a n k l i n R i v er 07 L nd 13 14 nd 31 110 08 H 39 nd nd 26 20 59 H a r r i s Creek 10 L nd nd nd 11 210 320 11 H nd nd nd 13 nd 89 12 B nd nd nd nd 1100 nd Watson Bar Creek 13 L nd nd 30 nd nd 27 14 H 10 nd nd nd nd 50 15 B nd nd 140 nd nd 21 * L=Low energy, H =High energy , B =Background sample. a nd=not d e t e c t e d . D e t e c t i o n l i m i t s are v a r i a b l e but average 15 ppb. -61-of low a n alyses i n d i c a t e t hat Au i s not o c c u r r i n g as minute i n c l u s i o n s i n low d e n s i t y m i n e r als to any d e t e c t a b l e l e v e l . The analyses do not provide a good i n d i c a t i o n of the Au d i s t r i b u t i o n i n the s i x s i z e f r a c t i o n s : g o l d c o n c e n t r a t i o n s are e r r a t i c with no observable peaks i n any p a r t i c u l a r f r a c t i o n . Analyses of s p l i t s of heavy-mineral c o n c e n t r a t e s show l a r g e d i f f e r e n c e s implying t h a t Au i s o c c u r r i n g as d i s c r e t e p a r t i c l e s (at l e a s t f o r the samples from H a r r i s C reek). For example, i f a sample c o n t a i n s s i x p a r t i c l e s and i s s p l i t i n t o equal weight sub-samples which by chance c o n t a i n two and four p a r t i c l e s , the a n a l y s i s of the l a t t e r w i l l be twice t h a t of the former. T h i s o b s e r v a t i o n was confirmed by v i s u a l examination of samples which showed th a t Au i s o c c u r r i n g as f r e e p a r t i c l e s i n f i n e sand s i z e f r a c t i o n s from a l l f i v e streams. Based on the assumption that Au occurs as f r e e p a r t i c l e s , the Au c o n c e n t r a t i o n and sample weight can be used to estimate the average number of gold p a r t i c l e s i n each f r a c t i o n g i v e n assumptions about p a r t i c l e s i z e , shape and d e n s i t y . The s i e v e s i z e of p a r t i c l e s can be taken as the geometric midpoint of the bounding s i e v e openings because the s i e v e s used are i n a geometric sequence. However, the shape assumption r e q u i r e d a c t u a l measurement -62-of g o l d p a r t i c l e s . Gold p a r t i c l e s e x t r a c t e d from H a r r i s Creek heavy-mineral c o n c e n t r a t e s were used to determine a shape d i s t r i b u t i o n by c r e a t i n g an a r b i t r a r y shape f a c t o r (SF) so that SF=1 r e p r e s e n t s a sphere, SF>1 r e p r e s e n t s long c y l i n d r i c a l p a r t i c l e s and SF<1 r e p r e s e n t s f l a t c y l i n d r i c a l p a r t i c l e s ( " f l a k e s " ) . The r a t i o used i s SF = W D B D L . ) : (2-1) where D u , D z , and D s are the l a r g e s t , intermediate and s m a l l e s t diameters r e s p e c t i v e l y . T h i s shape f a c t o r i s s i m i l a r to the Corey shape f a c t o r (Corey, 1949, CSF = D s / v ( D x D t - ) ) though CSFSl f o r a l l shapes with no t h r e s h o l d f o r i d e n t i f y i n g long c y l i n d e r s and f l a k e s . The histogram and p r o b a b i l i t y p l o t f o r SFs of 29 p a r t i c l e s ( F i g . 2-8) i s bimodal and can be modelled with normal p o p u l a t i o n s of f l a k e s with mean SF=0.45 (standard d e v i a t i o n = 0.13) and c y l i n d e r s with mean SF=1.30 (standard d e v i a t i o n = 0.27) i n the r a t i o 65% to 35%. The mean shape f a c t o r f o r a l l p a r t i c l e s i s 0.75 (standard d e v i a t i o n = 0.46) corresponding to a c i r c u l a r d i s c with a diameter to t h i c k n e s s r a t i o of 1.8. The volume of a p a r t i c l e with these dimensions i s 0.63 times the volume of a sphere of e q u i v a l e n t s i e v e diameter ( i . e . t h a t would pass through the same mesh). For c a l c u l a t i o n s , i t was assumed t h a t the shape d i s t r i b u t i o n of gold p a r t i c l e s i n a l l streams i s the same as t h a t f o r H a r r i s Creek -63-o o CO O-H 1 1 1 1 1 1 1 r 99 95 85 70 50 30 15 5 1 Cumulative Percent F i g . 2-8. P r o b a b i l i t y p l o t of shape f a c t o r s (SF) showing i n f e r r e d component normal p o p u l a t i o n s . For p o p u l a t i o n 1 (19 p a r t i c l e s ) , mean=0.450, standard deviation=0.130. For p o p u l a t i o n 2 (10 p a r t i c l e s ) mean=1.30, standard deviation=0.270. Twenty-nine p a r t i c l e s are included i n the p l o t which was generated using S t a n l e y ' s (1987) program with the method of S i n c l a i r (1976). -64-sedintents. The d e n s i t y of gold must a l s o be determined to estimate the number of gold p a r t i c l e s i n the samples. Measurement of the f i n e n e s s (Fineness = AulOOO/(Au+Ag+Cu+...)) and, by c a l c u l a t i o n the d e n s i t y , i s p o s s i b l e u s i n g an e l e c t r o n microprobe on p o l i s h e d s e c t i o n s , however f o r the purposes of t h i s reconnaissance stage the d e n s i t y was estimated as 15 g/cm a (Fineness = 620, e l e c t r u m ) . Thus, the number of p a r t i c l e s of gold i n each heavy-mineral concentrate f r a c t i o n can be estimated by assuming that 1) a l l Au occurs as l i b e r a t e d g old p a r t i c l e s , 2) the s i e v e diameter of p a r t i c l e s i s the geometric midpoint of the bounding s i e v e openings, 3) the average volume i s 0.63 times t h a t of a sphere and 4) the d e n s i t y of the p a r t i c l e s i s 15 g/cm a. The c a l c u l a t e d number of p a r t i c l e s f o r -270-mesh r e q u i r e s a d i f f e r e n t assumption of s i z e s i n c e no s m a l l e r bounding s i e v e s i z e i s a v a i l a b l e . A r b i t r a r i l y , an average diameter of 20 Hm was s e l e c t e d . In many samples, Au content can be accounted f o r by fewer than one or very few gold p a r t i c l e s (Table 2-4). In p a r t i c u l a r , i n the two c o a r s e s t f r a c t i o n s i t appears t h a t s i n g l e p a r t i c l e s are present or, where the number i s gi v e n as l e s s than 0.1, t h a t Au content probably r e p r e s e n t s background f o r the concentrate minerals or Au present as -65-s m a l l i n c l u s i o n s . Even i n the f i n e r f r a c t i o n s , o n l y f i n e sand f r a c t i o n s from H a r r i s Creek c o n t a i n s u f f i c i e n t gold p a r t i c l e s ( i . e . , 20) to y i e l d a r e l a t i v e e r r o r of ±50%. ( S e c t i o n 1.2.1). However, -270-mesh m a t e r i a l from a l l streams p o t e n t i a l l y c o n t a i n s q u i t e high numbers of gold p a r t i c l e s with a corresponding g r e a t e r sampling r e l i a b i l i t y but lower a n a l y t i c a l p r e c i s i o n because Au c o n c e n t r a t i o n s are c l o s e to the d e t e c t i o n l i m i t (Table 2-3A). Although samples were taken upstream of known m i n e r a l i s a t i o n to provide a comparison of anomalous and background sediments, Table 2-3A shows t h a t based on c o n c e n t r a t i o n data, m i n e r a l i s a t i o n i s probably not r e s t r i c t e d to the zones d e l i n e a t e d . Comparison of low and high energy samples becomes meaningless when sampling e r r o r s are c o n s i d e r e d . For example, low energy heavy-mineral c o n c e n t r a t e s have higher gold contents than corresponding high energy heavy-mineral concentrates f o r the -200+270-mesh f r a c t i o n though almost the opposite r e l a t i o n s h i p i s seen f o r -140+200-mesh. I f 95% confidence l i m i t s ( c a l c u l a t e d using equation ( l - 2 a ) ) are c o n s i d e r e d ( F i g . 2-9) the d i f f e r e n c e s i n c o n c e n t r a t i o n become i n s i g n i f i c a n t . Table 2-4 shows that the estimated number of g o l d p a r t i c l e s f o r low energy samples i s f r e q u e n t l y g r e a t e r than the corresponding high energy -66-Table 2-4. C a l c u l a t e d number of gold p a r t i c l e s i n heavy mineral c o n c e n t r a t e s . F r a c t i o n (mesh) Sample — — # L/H/B 1 -50+70 -70+100 -100+140 -140+200 -200+270 -270 Tsowwin River 01 L 1.0 2.2 4.6 0.1 11.3 2.9 02 H 0.0 0.0 0.6 2.4 0.0 24.7 03 B 0.0 0.0 0.0 1.9 1.5 1.9 "Salmonberry Creek" 04 L 0.0 0.0 1.9 0.0 1.7 5.4 05 H 0.8 1.0 0.0 0.2 1.0 11.6 06 B 0.0 0.0 1.5 4.0 1.0 23.3 F r a n k l i n River 07 L 0.3 1.8 8.2 6.6 6.0 77.9 08 H 0.0 0.0 0.0 1.8 0.0 22.6 H a r r i s Creek 10 L 0.4 1.8 6.7 29.9 120.4 214.4 11 H 0.8 2.9 4.3 4.8 5.7 37.1 12 B 0.8 1.7 25.8 37.4 23.1 4.4 Watson Bar 13 L 0.0 1.4 9.3 0.8 14.6 16.6 14 H 0.0 0.1 0.0 1.4 4.8 43.5 15 B 0.0 0.0 0.4 0.0 0.0 10.3 1 L=Low energy, H=High energy, B=Background sample. -67-environments, hence confidence l i m i t s f o r the former are narrower. F i g . 2-4 shows t h a t high energy samples g e n e r a l l y c o n t a i n much l e s s f i n e sediment than low energy samples, hence heavy-mineral c o n c e n t r a t e s from high energy s i t e s are s m a l l and the expected number of p a r t i c l e s (ji, see s e c t i o n 1.2) i n the sample i s very low. Thus, the p r o b a b i l i t y of under-estimating the Au content of a high energy sample i s higher than f o r low energy samples even though the expected c o n c e n t r a t i o n of heavy minerals i n a p a r t i c u l a r ( f i n e ) s i z e f r a c t i o n from high energy s i t e s i s u s u a l l y higher than f o r low energy s i t e s (e.g. Saxby, 1985, F l e t c h e r et a l . , 1987). 2.4.5 Reducing sampling e r r o r s 2.4.5.1 Combining f r a c t i o n s A g r e a t e r number of gold p a r t i c l e s would be obtained by combining heavy-mineral concentrates provided t h a t the sample i s s u f f i c i e n t l y s m a l l (<60 g) t h a t i t can be analysed c o n v e n i e n t l y . Using the Au c o n c e n t r a t i o n data, N f r a c t i o n s can be combined mathematically with N d e = ( E (Mj/MJd^ 3) 1" 5 3; (2-2) 1 where de = e f f e c t i v e diameter of gold p a r t i c l e s i n a l l N f r a c t i o n s , Mj = mass of gold i n f r a c t i o n j , M = t o t a l mass of gold i n a l l N f r a c t i o n s and d j = sediment diameter of -68-100 -10 -E a a 0.1 08 02 05 2.0 07 01 04 11 10 4 1— 1.0 D (mm) I 0.5 F i g . 2-9. Gold content and confidence i n t e r v a l s f o r -200+270-mesh f r a c t i o n of heavy mineral concentrates versus mean g r a i n s i z e of f i e l d sample. E r r o r bars are 95% confidence i n t e r v a l s around the re p o r t e d gold content and t i e - l i n e s j o i n h i g h - and low-energy p a i r s from the same l o c a t i o n . Sample numbers are shown next to bars. -69-f r a c t i o n j . Hence the e f f e c t i v e number of gold p a r t i c l e s ( N e ) can be c a l c u l a t e d ( C l i f t o n §_t a i . , 1969). The e f f e c t i v e number of p a r t i c l e s g i v e s the a p p r o p r i a t e value f o r N i n c a l c u l a t i n g confidence l i m i t s about the p o p u l a t i o n mean gold c o n c e n t r a t i o n (Equation l - 2 b ) . G e n e r a l l y , equation 2-2 y i e l d s e f f e c t i v e diameters very c l o s e to the diameter of the c o a r s e s t f r a c t i o n s i n c e t h i s f r a c t i o n c o n t a i n s the g r e a t e s t weight of g o l d even though the gold may be contained i n onl y one p a r t i c l e . T y p i c a l l y , a l l m a t e r i a l p a s s i n g through one mesh i s analysed ( f o r example, the e x p l o r a t i o n -80-mesh ' s i l t * sample), hence c a l c u l a t i o n s were done to i n d i c a t e the e f f e c t i v e number of p a r t i c l e s i n a l l m a t e r i a l f i n e r than each of the f r a c t i o n s ( F i g . 2-10). R e s u l t s suggest t h a t -270-mesh (without p r e p a r a t i o n of a heavy-mineral concentrate) i s the most s u i t a b l e f r a c t i o n as i t provides the most g o l d p a r t i c l e s . As each coa r s e r heavy-mineral concentrate i s 'added* the curves f a l l s t e e p l y because a d d i t i o n a l gold i s added but i s contained i n one or two p a r t i c l e s . S e v e r a l curves f l a t t e n or have l o c a l maxima at or around -140-mesh and -100-mesh but steepen again to have very low e f f e c t i v e numbers of gold p a r t i c l e s at -50-mesh. The c a l c u l a t i o n s i n d i c a t e t h a t there i s l i t t l e p o i n t i n combining coarse and f i n e f r a c t i o n s s i n c e the coarse - 7 0 -20 62.5 88 125 177 250 Diameter (p) F i g . 2-10. Number of e f f e c t i v e p a r t i c l e s (N E) versus geometric midpoint of c o a r s e s t s i z e f r a c t i o n i n c l u d e d , e.g., f o r sample 6, N E at 20 Hm i s approximately 10 f o r the -270-mesh f r a c t i o n (assumed diameter 20 Jim) but f a l l s to about 1 i f -270-mesh and -200+270-mesh (midpoint diameter 62.5 Hm) f r a c t i o n s are combined. -71-f r a c t i o n s merely add one or two p a r t i c l e s l e a d i n g to gr e a t e r random sampling e r r o r . I f the -270-mesh f r a c t i o n c o n t a i n s d e t e c t a b l e gold c o n c e n t r a t i o n s , i t y i e l d s u s e f u l data because the number of p a r t i c l e s i n the f r a c t i o n i s high r e d u c i n g sampling e r r o r s . 2.4.5.2 C o l l e c t i o n of l a r g e r samples C o l l e c t i o n of l a r g e samples would provide more g o l d p a r t i c l e s and reduce the sampling e r r o r s . Given the c a l c u l a t e d number of gold p a r t i c l e s i n the samples, i t i s a simple procedure to determine the s i z e of sample t h a t would be r e q u i r e d to y i e l d 20 p a r t i c l e s of g o l d . However, i t i s recognized that i f there i s l e s s than a few p a r t i c l e s present the c o n c e n t r a t i o n of gold i s g r o s s l y under-estimated and, consequently, the s i z e of sample r e q u i r e d w i l l be over-estimated. T h e r e f o r e , the weights of heavy-mineral concentrate and -16-mesh sediment r e q u i r e d have been c a l c u l a t e d f o r m a t e r i a l i n f r a c t i o n s f i n e r than 140-mesh (Table 2-5). The weight of -16-mesh was s e l e c t e d because, f o r the purpose of mineral e x p l o r a t i o n , i t i s l o g i s t i c a l l y necessary to reduce sample weight by f i e l d s i e v i n g . P. Matysek (pers. comm.) and others have found that about 16-mesh i s a p r a c t i c a l f i e l d s i e v e s i z e . In many cases, a very l a r g e (»120 kg) -16-mesh sample -72-Table 2-5. Weights of -16-mesh f i e l d sample and sub-sample c o n t a i n i n g 20 p a r t i c l e s of gold i n high and low energy environments. Based on data i n Tables 2-3 and 2-4 and F i g . 2-4 f o r samples estimated to c o n t a i n more than one p a r t i c l e of g o l d . F r a c t i o n (mesh) Sample # L/H 1 -140+200 -200+270 -270 HMC -16# HMC -16» -270 -16# (g) (kg) (g) (kg) (g) (kg) 01 L * * 6.4 27 * * 02 H 4.7 45 (6.4) (18) 37.2 2 04 L * * * * ' 82.6 11 05 H * * * * 83.8 14 07 L 15.5 76 3.8 82 11.4 1 08 H (15.5) (91) (3.8) (70) 21.3 2 10 L 33.5 10 12.4 8 3.9 0.3 11 H 89 .6 140 23.6 120 14.1 2 13 L * * 4.9 35 46.3 2 14 H * * 6 . 3 89 31.2 1 x H=high energy, L=low energy. ()=Fewer than one p a r t i c l e of gold but weights estimated assuming high energy sample has same gold content as a s s o c i a t e d low energy s i t e (on b a s i s of confidence i n t e r v a l s i n F i g . 2-9) and c a l c u l a t i n g s i z e of f i e l d sample necessary to g i v e same amount of c o n c e n t r a t e . *=Fewer than 1 gold p a r t i c l e (Table 2-4) i n both high and low energy samples and t h e r e f o r e no estimated weight p o s s i b l e . -73-i s r e q u i r e d to o b t a i n s u f f i c i e n t g o l d i n the two c o a r s e s t heavy-mineral c o n c e n t r a t e s but f a l l s to more reasonable l e v e l s (1 to 2 kg) f o r -270-mesh sediment. H a r r i s Creek shows the lowest r e q u i r e d sample s i z e s i n a l l f r a c t i o n s . 2.4.4 Comparison of r e s u l t s with other sampling methods Other methods f o r e s t i m a t i n g the gold content of stream sediments d u r i n g mineral e x p l o r a t i o n were d i s c u s s e d i n chapter 1. The e f f e c t i v e n e s s of these methods can be assessed using the r e s u l t s of t h i s study by e s t i m a t i n g the p r o b a b i l i t y t h a t the sample w i l l not c o n t a i n any g o l d p a r t i c l e s (P(0), based on the Poisson d i s t r i b u t i o n ) . Two examples of e x p l o r a t i o n stream sediment sample-types were s e l e c t e d : 1) F i e l d concentrate obtained by panning 20 kg of -4-mesh m a t e r i a l . For c a l c u l a t i o n s i t was assumed t h a t g o l d f i n e r than 100-mesh would be l o s t and a l l gold i s f i n e r than 50-mesh. Ten grams of concentrate would be analysed f o r Au by f i r e assay with atomic a b s o r p t i o n f i n i s h . 2) Minus 70-mesh sediment obtained by c o l l e c t i n g sandy sediment i n the f i e l d . T h i s sample i s approximately e q u i v a l e n t to the c o n v e n t i o n a l e x p l o r a t i o n " s i l t " sample of which a 10 g p o r t i o n of -70-mesh sediment would be analysed f o r Au by f i r e assay-atomic -74-a b s o r p t i o n . Only samples taken from low energy environments were cons i d e r e d because sampling i s u s u a l l y c a r r i e d out with time c o n s t r a i n t s . The p r o b a b i l i t y t h a t a sample w i l l not c o n t a i n any gold i s c o n t r o l l e d by the f r a c t i o n c o n t a i n i n g the g r e a t e s t number of gold p a r t i c l e s (Table 2-4). The expected number of gold p a r t i c l e s i n t h i s f r a c t i o n i n the sub-sample analysed was c a l c u l a t e d and used to estimate the p r o b a b i l i t y t h a t the sample w i l l not c o n t a i n any g o l d (P(0) ) of t h i s s i z e : P(0) = e-". (2-3) R e s u l t s were compared with those obtained f o r -200+270-mesh heavy-mineral concentrates and -270-mesh sediment (Table 2-6) determined as d e s c r i b e d i n t h i s study. In every case, i f panning had been used there i s at l e a s t a 46% chance that the Au anomaly would not be d e t e c t e d . However, the r e s u l t s are probably not the best that could be expected s i n c e the volume of m a t e r i a l to be processed (20 kg) i s s m a l l though t y p i c a l i n mineral e x p l o r a t i o n . C o n s i d e r a b l e improvements i n the chance of d e t e c t i n g the anomaly would be expected i f 100 kg of -4 mm sediment i s panned. In the case of -70-mesh samples, the anomaly would c e r t a i n l y be detected i n two of the streams due to the -75-i n c l u s i o n of -270-mesh sediment. However, Au c o n c e n t r a t i o n s are l i k e l y to be f a i r l y low, f o r example a c o n c e n t r a t i o n of approximately 100 ppb Au would be obtained with a 50 Hm diameter s p h e r i c a l g o l d p a r t i c l e . A f u r t h e r improvement i n the chances of d e t e c t i n g the anomaly can be obtained with -200+270-mesh heavy mineral concentrates (Table 2-6). F i n a l l y , r e s u l t s obtained f o r -270-mesh are m a r g i n a l l y b e t t e r than -200+270-mesh c o n c e n t r a t e s . In o r i e n t a t i o n surveys, s u f f i c i e n t -270-mesh sediment f o r a n a l y s i s may be obtained from a s m a l l f i e l d sample and there i s no need to pre-concentrate the sample, thereby r e d u c i n g c o s t s a t the c o l l e c t i o n and p r e p a r a t i o n stages. However, Au c o n c e n t r a t i o n s are l i k e l y to be c l o s e to t y p i c a l c o m m e r c i a l l y - a v a i l a b l e a n a l y t i c a l d e t e c t i o n l i m i t s l e a d i n g to poor a n a l y t i c a l r e l i a b i l i t y and low geochemical c o n t r a s t . 2.5 Conclusions Large sediment samples from f i v e streams show t h a t : 1) Twenty kilograms of -4-mesh sediment i s probably adequate f o r most streams i f q u a l i t a t i v e evidence f o r an anomaly i s r e q u i r e d . Sediment f i n e r than 200-mesh should be a n a l y s e d . 2) Large -16-mesh sediment samples are r e q u i r e d (up to -76-Table 2-6. P r e d i c t e d r e s u l t s of d i f f e r e n t sampling methods. A l l r e s u l t s are based on the f i e l d sample c o l l e c t e d i n t h i s study. F u l l e x p l a n a t i o n of the method of c a l c u l a t i o n i s given i n the t e x t . Sample Number 1 Sample type Panned 2 HMC3 -70-mesh -200+270 HMC -270 P(0)*» (%) P(0) (%) P(0) (%) P(0) (%) 01 46 82 0.7 100 04 *100 41 45 15 07 55 0.0 6 . 7 0.0 10 73 0.0 0.0 0.0 13 53 11 0.2 0.1 1 A l l samples are from low energy l o c a t i o n s . 2 Panned concentrate = sediment s i z e d to -50+100-mesh. 3 HMC = heavy-mineral c o n c e n t r a t e . •* P(0) = estimated p r o b a b i l i t y of o b t a i n i n g no gold p a r t i c l e s i n the sample. -77-120 kg) i f a r e l a t i v e sampling e r r o r of ±50% i s d e s i r e d and s i n g l e coarse f r a c t i o n s are to be a n a l y s e d . 3) Lower sample s i z e s are r e q u i r e d i f -270-mesh sediment i s to be analysed though the d e t e c t a b l e presence or absence of gold i n the f r a c t i o n should be determined f i r s t . C o n c e n t r a t i o n s of Au i n the f r a c t i o n can be very low, approaching the d e t e c t i o n l i m i t of most commonly a v a i l a b l e a n a l y t i c a l techniques, hence a n a l y t i c a l r e l i a b i l i t y and geochemical c o n t r a s t i s lower than can be obtained with heavy-mineral concentrates of c o a r s e r f r a c t i o n s . 4) H a r r i s Creek samples are very anomalous i n Au, hence f i e l d sample s i z e s r e q u i r e d f o r a r e l a t i v e e r r o r of ±50% on Au c o n c e n t r a t i o n s i n the f i n e r sediment f r a c t i o n s are low. T h i s stream i s the most s u i t a b l e f o r a d e t a i l e d study of gold d i s p e r s i o n as i t i s f a i r l y l a r g e , mature and e a s i l y a c c e s s i b l e . CHAPTER 3: DETAILED SAMPLING OF HARRIS CREEK - 7 9 -3.0 I n t r o d u c t i o n H a r r i s Creek was s e l e c t e d f o r a d e t a i l e d study of the d i s t r i b u t i o n of gold i n a s i n g l e stream based on the r e s u l t s presented i n chapter 2. The primary aim of t h i s study was to c o l l e c t sediment samples that were r e p r e s e n t a t i v e of the sampling l o c a t i o n and not s e v e r e l y a f f e c t e d by sampling e r r o r s due to r a r e g r a i n e f f e c t s . Hence, the data c o l l e c t e d provide an i n d i c a t i o n of the h y d r a u l i c behavior of gold with r e s p e c t to other m i n e r a l s i z e and d e n s i t y ( f o r example, low-density minerals and magnetite) f r a c t i o n s . 3.1 L o c a t i o n and access H a r r i s Creek r i s e s i n the Okanagan Highlands (Spallumcheen P r o v i n c i a l F o r e s t ) of B r i t i s h Columbia (NTS 82L/02), flows through the town of Lumby (20 km east of Vernon) and e v e n t u a l l y i n t o the Columbia R i v e r . The reach s t u d i e d i s a c c e s s i b l e by well-maintained, paved and g r a v e l l o g g i n g roads from Lumby (about 10 km). T h i s reach i s the o n l y e a s i l y a c c e s s i b l e s e c t i o n of the stream. 3.2 H i s t o r y and land use H a r r i s Creek i s one of a number of streams i n the north -80-Okanagan r e g i o n t h a t produced p l a c e r gold i n the l a t e n i n e t e e n t h and e a r l y t w e n t i e t h c e n t u r i e s . Gold was mined from a p l a c e r d e p o s i t near the confluence with Bessette Creek ( F i g . 3-1) i n the 1930s ( B a r l e e , 1969) .though a bench about 15 m above the present stream y i e l d e d the most pr o d u c t i v e g r a v e l s . The gold occurred as coarse nuggets. Gold c o l o u r s can be panned from the stream bed a t l e a s t 3 km upstream of the main mining area and A. Burton (pers. comm.) d e l i n e a t e d a gold d i s p e r s i o n t r a i n u s i n g a s u c t i o n dredge and p o r t a b l e s l u i c e box. His work suggested t h a t the d i s p e r s i o n t r a i n ended l e s s than 4 km upstream of the Bessette Creek c o n f l u e n c e . The o r i e n t a t i o n phase of t h i s study showed t h a t the d i s p e r s i o n t r a i n extends a t l e a s t 4.5 km upstream of the co n f l u e n c e . Very l i t t l e e x p l o r a t i o n to l o c a t e a lode gold source i n the catchment b a s i n has been documented. A small g o l d -quartz v e i n was i n v e s t i g a t e d by pr o s p e c t o r s probably about the time of the p l a c e r mining a c t i v i t y ( F i g . 3-1). The exact l o c a t i o n of the showing i s unknown though M. Smith (pers. comm.) noted trenches near the upstream end of the study reach on the south bank of H a r r i s Creek. Channel g r a v e l s l o c a t e d below Miocene (Church and Seusser, 1983) pl a t e a u b a s a l t s ( F i g . 3-1) were the f o c a l p o i n t of uranium e x p l o r a t i o n i n the l a t e 1970s. However, there were no syst e m a t i c attempts to determine the gold content of the -81-F i g . 3-1. Geology of the drainage b a s i n of H a r r i s Creek and l o c a t i o n of the study reach. U n i t s : l=Monashee g n e i s s ; 1 4 , 1 5 = a n d e s i t e , a r g i l l i t e ; 18=granodiorite; 20=plateau b a s a l t (Jones 1959). *=mineral occurrence and commodity. -82-g r a v e l s d u r i n g e x p l o r a t i o n d e s p i t e the presence of a u r i f e r o u s g r a v e l s beneath p l a t e a u b a s a l t s at King Edward Creek, near Coldstream (Day, 1986). Much of the catchment b a s i n i s being logged. Most log g i n g i s on the v a l l e y slopes and not i n the v a l l e y bottoms which are used f o r c a t t l e g r a z i n g . Consequently, the stream bed i t s e l f has not been d i s t u r b e d by f a l l e n t r e e s as i s t y p i c a l of other areas of l o g g i n g i n B r i t i s h Columbia. 3.3 Climate, v e g e t a t i o n and s o i l s For three to f i v e months of the year mean temperatures are a t or below 0°C a t the weather s t a t i o n i n Lumby. January temperatures average -6°C, however extremely c o l d p e r i o d s are f a i r l y common (<-20°C). Maximum average d a i l y temperatures (180C) occur i n J u l y . Average annual p r e c i p i t a t i o n i s 547 mm with the g r e a t e s t monthly readings (approximately 60 mm/month) o c c u r r i n g i n January and February as snow (Environment Canada, 1982). Douglas f i r and l a r c h f o r e s t predominate i n the v a l l e y , however there are s m a l l areas of open range land ( B r i t i s h Columbia F o r e s t S e r v i c e , 1972). S o i l s are t y p i c a l l y r e g o s o l i c r e f l e c t i n g the steep s l o p e s though podzols are observed where the topography i s subdued ( K e l l e y and S p i l s b u r y , 1949). -83-3.4 Summary of b a s i n morphology The asymmetrically-shaped b a s i n has an area of approximately 212 km* ( F i g . 3-2). To the south, i t extends over a l a r g e area, a t e l e v a t i o n s of around 1700 m, of p o o r l y d r a i n e d t i l l on g n e i s s i c basement. To the nor t h , t r i b u t a r i e s are s h o r t e r , r i s i n g a t e l e v a t i o n s of no great e r than 1500.m. H a r r i s Creek i s f o u r t h order on the study reach when considered at a s c a l e of 1:50,000. The longest channel i n the b a s i n i s about 28 km from the south watershed to s i t e M with an average slope of 0.14. 3.5 Ba s i n geology In the upper reaches, bedrock i s Monashee Gneiss (Jones, 1959) capped on the north h a l f of the b a s i n by an Oligocene (Church and Seusser, 1983) a l l u v i a l g r a v e l and f e l s i c t u f f complex. Miocene o l i v i n e p l a t e a u b a s a l t s cap t h i s d e p o s i t . The v a l l e y of H a r r i s Creek forms a s t r o n g northwest-trending r e g i o n a l lineament that i s thought to mark the t r a c e of a s t r i k e s l i p f a u l t (Jones, 1959). Opposite banks of the reach s t u d i e d ( F i g . 3-1) have c o n t r a s t i n g geology with r e c e s s i v e a r g i l l i t e s , limestones and a n d e s i t i c v o l c a n i c s on the north bank (Jones, 1959). The south bank i s u n d e r l a i n by c l i f f - f o r m i n g g r a n i t e and -84-F i g . 3-2. Topography of the drainage area of Harris Creek upstream of the study reach (lowest sampling location shown by M in northwest corner). The contour interval is 500 feet. -85-g r a n o d i o r i t e . There are no major l i t h o l o g i c a l changes along the study reach though a sharp t u r n i n the trend of the v a l l e y occurs near the downstream end: between s i t e s A and M the north bank i s u n d e r l a i n by Monashee Gneiss. 3.5.1 Quaternary geology Outcrops of bedrock are r a r e i n the catchment b a s i n of H a r r i s Creek due to a mantle of g l a c i a l d r i f t . Mostly, the d r i f t c o n s i s t s of t i l l , however near the upper end of the study s e c t i o n a l a c u s t r i n e d e p o s i t with a s s o c i a t e d f l u v i a l d e p o s i t s occupies the north bank. No s y s t e m a t i c attempt was made to map the d r i f t geology. 3.5.2 Source of gold No s i n g l e l a r g e lode gold source has been l o c a t e d i n the b a s i n though there are numerous d e s c r i p t i o n s of s m a l l uneconomic v e i n occurrences i n the r e g i o n ( f o r example, the showing near s i t e J d e s c r i b e d i n s e c t i o n 3.2). In the b a s i n of H a r r i s Creek gold has probably been concentrated i n t o a number of f l u v i a l d e p o s i t s : 1) Oligocene g r a v e l s u n d e r l y i n g the p l a t e a u b a s a l t s , 2) Quaternary g l a c i o -f l u v i a l d e p o s i t s , 3) recent benches a s s o c i a t e d with the modern stream and 4) the bed of the modern stream. The high c o n c e n t r a t i o n of gold i n H a r r i s Creek can perhaps be a t t r i b u t e d to repeated re-working of i n c r e a s i n g l y r i c h e r -86-f l u v i a l d e p o s i t s . 3.6 H a r r i s Creek seasonal d i s c h a r g e v a r i a t i o n The study reach f r e e z e s i n November and breaks up i n March. Break-up i s f o l l o w e d i n l a t e May and e a r l y June by a s p r i n g f l o o d peak t h a t t y p i c a l l y reaches 18 rtvVs (16 r e c o r d s , 1965-1983) at the d i s u s e d gauging s t a t i o n a t Lumby. The peak, which had a maximum of 29.2 m3/s i n 1977, t y p i c a l l y decays to h a l f t h i s value i n l e s s than a week. Summer and autumn peak d i s c h a r g e s r e s u l t i n g from r a i n storms average 4.5 m3/s and l a s t f o r about one day. Sampling f o r t h i s study was c a r r i e d out i n June 1986 when unusual weather l e d to e x c e p t i o n a l d i s c h a r g e s (>30 m 3/s) and f l o o d i n g i n some s e c t i o n s of the stream. 3.7 F i e l d sampling 3.7.1 F i e l d sampling to o b t a i n sediment t e x t u r e : problems In t h i s study, the d i s t r i b u t i o n of gold with r e s p e c t to sediment t e x t u r e i s i n v e s t i g a t e d , hence f i e l d sampling n e c e s s a r i l y i n v o l v e d t e x t u r a l a n a l y s i s i n the f i e l d of a coarse d i s c a r d f r a c t i o n and c o l l e c t i o n of a screened sand and f i n e r sediment sample f o r chemical and t e x t u r a l a n a l y s i s i n the l a b o r a t o r y . S e d i m e n t o l o g i s t s have approached the problem of sampling g r a v e l s to determine sediment t e x t u r e i n a number -87-of ways ( K e l l e r h a l l s and Bray, 1971). The technique i s ' complicated because: 1) there are observable l a t e r a l v a r i a t i o n s i n the t e x t u r e of a g r a v e l d e p o s i t . 2) t y p i c a l l y a v e r y coarse s u r f a c e armour or pavement l a y e r has developed. 3) the t e x t u r a l d i s t r i b u t i o n i s bimodal and t y p i c a l l y has a s i z e range of 10 <t>. 4) the presence of very coarse sediment (>64 mm) leads to severe b i n o m i a l d i s t r i b u t i o n sampling e r r o r s f o r the coa r s e r f r a c t i o n s . When sampling stream sediments f o r gold a v e r y l a r g e sample i s r e q u i r e d , so a c e r t a i n amount of l o c a l ( v e r t i c a l and l a t e r a l ) h e t e r o g e n e i t y i s i n e v i t a b l e . For example, i t i s not p o s s i b l e to c o l l e c t a sample of the armour l a y e r without c o v e r i n g a l a r g e area ( s e v e r a l square metres) and i n c o r p o r a t i n g a v a r i e t y of h y d r a u l i c environments. 3.7.2 F i e l d sampling A d e t a i l e d sampling program was i n i t i a t e d on the study reach i n November 1985. I t was intended t h a t 20 samples i n t o t a l would be taken at ten s i t e s along the stream. Sampling was aborted a f t e r three s i t e s had been sampled because the stream began to f r e e z e due to an a i r temperature of -15°C. A second s u c c e s s f u l sampling survey - 8 8 -F i g . 3-3. Sample l o c a t i o n s on H a r r i s Creek. Flow d i r e c t i o n i s from r i g h t to l e f t . Copied from 1:20,000 map of the B r i t i s h Columbia F o r e s t S e r v i c e (1972). -89-was c a r r i e d out i n June 198S ( F i g . 3-3) though the sample l o c a t i o n s of the previous November co u l d not be re-sampled due to high water l e v e l s f o l l o w i n g the s p r i n g meltwater f l o o d . At each s i t e , high and low energy samples were taken, each c o n s i s t i n g of 60 kg of -10-mesh sediment (two f u l l f i v e g a l l o n (23 l i t r e ) p l a s t i c p a i l s ) . The high energy samples were taken from g r a v e l d e p o s i t s , t h a t i s l o c a t i o n s at which the bed i s p o o r l y s o r t e d and the s u r f a c e l a y e r (pavement) c o n s i s t s p r i m a r i l y of g r a v e l (diameter g r e a t e r than 2 mm) with very l i t t l e sand ( F i g . 3 - 4 ) . The low energy samples were taken from s h a l l o w pools with a sandy r i p p l e d bed or exposed sand d e p o s i t s ( F i g . 3-5). The f i e l d s i e v e used (10-mesh, 2 mm) i s made of nylon and f i t s snugly over the top of a 23 l i t r e p a i l (approximately 30 cm dia m e t e r ) . T h i s e l i m i n a t e s some of the l o s s e s of f i n e sediment experienced i n the f i r s t , phase. The p a i l was placed i n a g a l v a n i s e d tub d u r i n g s i e v i n g so t h a t the f i n e sediment could s e t t l e out from the overflow and be c o l l e c t e d at the end of s i e v i n g . The water i n the tub was c a r e f u l l y decanted and the two p a i l s topped up with the e x t r a sediment. I n e v i t a b l y , l o s s of very f i n e sediment occurs a t t h i s stage, but no p r a c t i c a l f i e l d method (e.g. chemical f l o c c u l e n t s ) e x i s t s to r e t r i e v e the sediment. -90-F i g . 3-4. Typical sandy gravel sampling location near the head of a point bar. On the diagram, fine s t i p p l e represents sand, coarse stipple represents sandy gravel, v e r t i c a l r u l i n g indicates a l l u v i a l banks and arrows show approximate d i r e c t i o n and magnitude of water flow. Orange marks on the rod are 20 cm long. Fig. 3-5. Typical sand sampling location. See F i g . 3-4 for explanation of symbols and scale rod. -92-A rocker s i e v e and a s e t of 1 * screens and templates were used to determine the coarse (+2 mm) f r a c t i o n d i s t r i b u t i o n up to 256 mm. In most g r a v e l l y samples a l a r g e volume of +2 mm m a t e r i a l was produced, i n which case a f i e l d s p l i t was s i e v e d . At each s i t e , t y p i c a l l y a channel or p o i n t bar, a s i t e p l a n sketch was prepared showing approximate dimensions of channel f e a t u r e s (bars and c h a n n e l s ) , l o c a l bed sediment t e x t u r e s , l o c a t i o n of samples, stream v e l o c i t i e s determined u s i n g a s m a l l p r o p e l l e r d r i v e n meter (General Oceanics, model 2030) placed a t h a l f depth, l o c a t i o n s of photographs and a stream depth p r o f i l e . These o b s e r v a t i o n s were recorded on a standard data sheet along with data on bank m a t e r i a l s and competence, water c o l o u r and t u r b i d i t y , stage of flow i n terms of b a n k f u l l c o n d i t i o n s , bedform type and dimensions a t the sample l o c a t i o n s , c o l o u r and estimated o r g a n i c (undecomposed) content of the -2 mm sample. The type of sample ( i . e . s u r f a c e , s u r f a c e l a y e r or subsurface) was a l s o recorded though most samples were a combination of a l l three types as a r e s u l t of the l a r g e sample s i z e . A comments area was used to r e c o r d p o s s i b l e sources of contamination and other p o t e n t i a l l y u s e f u l d ata. The stream depth p r o f i l e was determined u s i n g a survey c h a i n s t r e t c h e d a c r o s s the stream and a s t e e l rod with an -93-a t t a c h e d tape measure. These depth p r o f i l e s were i n e v i t a b l y not very accurate due to the r e l a t i v e l y shallow water depth and l a r g e r e l i e f of the stream bed. The p r o f i l e s t h e r e f o r e give s e m i - q u a n t i t a t i v e measurements of c r o s s - s e c t i o n a l area, wetted perimeter and bed topography a l l o w i n g d e t e r m i n a t i o n of d i s c h a r g e (from v e l o c i t y measurements) (Table 3-1). Many of the p r o f i l e s measured are incomplete due to the extremely s t r o n g c u r r e n t i n the centre of the stream d u r i n g the f l o o d . U l t i m a t e l y , removal of samples to the road proved d i f f i c u l t due to awkward shape and weight of the p a i l s . A c a r r y i n g s t i c k was c o n s t r u c t e d by b u i l d i n g up a s l o t i n the centre of a pole e n a b l i n g samples to be c o n v e n i e n t l y c a r r i e d u p - h i l l ( F i g . 3-6). 3.7.3 Sample l o c a t i o n d e s c r i p t i o n s Sample l o c a t i o n d e s c r i p t i o n s are summarised i n Table 3-2. Although two types of environment were sampled, two s u b - d i v i s i o n s of sand d e p o s i t s can be r e c o g n i z e d , to y i e l d three d i s t i n c t i v e sediment types: 1) Sandy g r a v e l d e p o s i t s . 2) Sand d e p o s i t s without v i s i b l e magnetite accumulations. 3) Sand d e p o s i t s with abundant v i s i b l e magnetite accumulations. P h y s i c a l and sampling c h a r a c t e r i s t i c s of these -94-Table 3-1. Water d i s c h a r g e r a t e s (irrVs) f o r H a r r i s Creek, i n June 1986. S i t e Date Discharge M 12/06/86 5.8 A 10/06/86 6.2 C 05/06/86 10.1 K 11/06/86 5.9 D 04/06/86 -B 09/06/86 6.3 F 06/06/86 10.2 E 08/06/86 6.3 G 03/06/86 -J 07/06/86 — - = channel depth p r o f i l e could not be measured or e s t imated - 9 5 -F i g . 3-6. Wooden pole with notch b u i l t up at centre for carry ing samples. -96-Table 3-2. Sununarised June 1986 s i t e d e s c r i p t i o n s . S i t e Sample * D e s c r i p t i o n M Ml Sand d e p o s i t below a l o g jam a t the downstream end of a p o i n t bar. R i p p l e s present with heavy minerals on c r e s t s . Heavy minerals a l s o v i s i b l e i n smal l s c o u r s . M2 Gravel d e p o s i t at the downstream end of the same bar but upstream of the l o g jam. A A l Sand d e p o s i t at the downstream end of a s m a l l p o i n t bar, i n a back eddy p o o l . A2 Gravel sample from the upstream end of a newly emerged p o i n t bar. C CI Coarse sand d e p o s i t on the downstream end of newly emerged channel bar. No v i s i b l e bedforms. C2 Gravel sample from upstream end of the same channel bar, under a few cen t i m e t r e s of water. K KI Back eddy pool surrounded by logs upstream of a g r a v e l r i f f l e (submerged p o i n t b a r ) . R i p p l e s present but very low v e l o c i t y . Minor heavy mineral c o n c e n t r a t i o n s v i s i b l e . K2 Gravel sample taken from a r i f f l e downstream of KI. D DI Sand d e p o s i t on the downstream end of a smal l p o i n t bar, c u r r e n t l y a s m a l l beach. V i s i b l e b lack sand accumulations D2 Gra v e l d e p o s i t on the upstream end of the same bar. (Continued on next page) - 9 7 -Table 3-2. (Continued) S i t e Samplel D e s c r i p t i o n B B l Sand d e p o s i t a t the downstream end of a narrow stream c u t t i n g round the back of a po i n t bar. A few r i p p l e s with b a r e l y v i s i b l e heavy mineral c o n c e n t r a t i o n s . B2 Downstream of B l i n a shallow r i f f l e , on top a submerged bar. F F l F i n e sand d e p o s i t i n a back eddy pool on the downstream s i d e of a channel bar. S l i g h t r i p p l i n g present. F2 Coarse g r a v e l d e p o s i t downstream of F l , but on the upstream end of a smal l channel bar. E E l Coarse sand d e p o s i t i n a channel f l o w i n g around the back o f , a p o i n t bar. R i p p l e s p r e s e n t . E2 Gravel d e p o s i t on the downstream end of a r e c e n t l y emerged p o i n t bar. G Gl Sand d e p o s i t i n a pool i n the middle of a bra i d e d channel bar. R i p p l e s present with b a r e l y v i s i b l e heavy mineral c o n c e n t r a t i o n s . G2 A very s h o r t d i s t a n c e upstream from Gl on a s m a l l g r a v e l r i f f l e . J J l Sand d e p o s i t at the upstream end of a p o i n t bar, formed behind a s i n g l e l o g jam. Small scours and r i p p l e s with heavy mineral c o n c e n t r a t i o n s . J2 Sand and g r a v e l d e p o s i t behind the same l o g jam, upstream of J l . 1 A number 1 i n d i c a t e s a "low energy" s i t e , a number 2 i n d i c a t e s a "high energy" s i t e . -98-environments are summarised i n Table 3-3. 3.8 L a b o r a t o r y p r o c e s s i n g 3.8.1 S i e v i n q In the f i r s t phase, wet s i e v i n g was used but was found to be extremely time-consuming s i n c e the samples were very l a r g e and were processed manually. However, compared to dry s i e v i n g , the technique has the advantage t h a t the f i n e f r a c t i o n s are v e r y w e l l s i z e d and p a r t i c l e s are disaggregated i f bound together by c l a y . Conversely, d r y s i e v i n g i s used commonly i n the a n a l y s i s of sediments because i t i s r e l a t i v e l y r a p i d . Dry s i e v i n g i s u s u a l l y c a r r i e d out f o r samples t h a t can be s i e v e d i n one batch, so s i e v i n g time i s an important parameter i n determining p u r i t y of s i z e f r a c t i o n s . McManus (1965) suggests a minimum time of 10 minutes, but Mizutani (1963) recommends th a t samples be s i e v e d f o r no l e s s than 30 minutes and the amount of sample should not exceed 40 to 50 g. In t h i s study, the l a r g e s i z e of samples would r e q u i r e them to be s p l i t and dry s i e v e d i n s e v e r a l batches. Consequently, as an a l t e r n a t i v e to e i t h e r d r y or wet s i e v i n g alone and i n an attempt to combine the advantages of both, dry s i e v i n g followed by wet s i e v i n g was e v a l u a t e d . Compared to d r y s i e v i n g i t was found that the - 9 9 -Table 3-3. Summary of p h y s i c a l and c h a r a c t e r i s t i c s of sample s i t e s . sampling Sediment type C h a r a c t e r i s t i c s Moderately to p o o r l y - s o r t e d sands ( s i x samples) (E1,F1,B1 #K1,C1,A1) R i p p l e d sand d e p o s i t s i n deep eddy pools at the t a i l end of bars. Average t o t a l sample weight: 60 kg. Time taken to sample: 30 to 45 minutes. Moderately to p o o r l y - s o r t e d magnetite-r i c h sands (four samples) (J1,G1,D1,M1) S u b - a e r i a l sand d e p o s i t s a t bar t a i l s . V i s i b l e magnetite accumulations where waves la p onto smal l beach faces and back bar s t r e a m l e t s wash acr o s s the sand. Average t o t a l sample weight: 60 kg Time taken to sample: 30 to 45 minutes. Very p o o r l y s o r t e d bimodal sandy cobble Sediment comprising most of bars. C h a r a c t e r i s e d by a coarse sand-g r a v e l s (ten samples) d e f i c i e n t s u r f a c e pavement and f i n e r t e x t u r e d s u b s u r f a c e . No v i s i b l e l a y e r i n g . Average t o t a l sample weight: 250 kg Time taken to sample: 2 to 3 hours -100-weights of the f i n e f r a c t i o n s recovered changed s i g n i f i c a n t l y (up to 670% i n c r e a s e f o r the -270-mesh f r a c t i o n , Table 3-4), p a r t i c u l a r l y i n samples c o n t a i n i n g abundant f i n e sediment. The f o l l o w i n g procedure was t h e r e f o r e adopted f o r the second phase. Each sample was f i r s t d r y s i e v e d u s i n g a Rotap automatic s i e v e r . Batches of about 800 g of -10-mesh sediment were added to the top s i e v e and s i e v e d f o r r o u g h l y seven minutes to a l l o w time f o r most of the undersized m a t e r i a l to pass the 16-mesh s i e v e . The f r a c t i o n r e t a i n e d on the top s i e v e and any f u l l lower s i e v e s were emptied i n t o p l a s t i c bags. The process was repeated f o r the whole sample with frequent c l e a n i n g of the screens with a brush. U s u a l l y the bottom two s i e v e s d i d not need to be emptied u n t i l the whole sample had been processed. Approximately s i x hours were needed to d r y s i e v e one 60 kg, -10-mesh sample. Dry s i e v e d f r a c t i o n s , s t a r t i n g with the c o a r s e s t were then cleaned up by wet s i e v i n g through a m o d i f i c a t i o n of the apparatus i n F i g . 2-3 whereby a p e r i s t a l t i c pump r e -c i r c u l a t e d the water i n the p a i l to prevent l o s s of v e r y f i n e sediment. However, samples c o n t a i n i n g a very high p r o p o r t i o n of s i l t and c l a y had to be s i e v e d with c l e a n water because the f i n e r screens became clogged. A f t e r s i e v i n g , an o r g a n i c f l o c c u l a t i n g agent ( C a t f l o c , Calgon -101-Table 3-4. Change i n the weight of -270-mesh f r a c t i o n f o l l o w i n g wet s i e v i n g of a p r e v i o u s l y d r y s i e v e d sample. Sample Weight of -270-mesh (g) Dry s i e v e Wet Sieve Change Ml - 811 -M2 - 259 -A l 304 1110 + 265 A2 298 628 + 110 C l 265 547 + 106 C2 169 339 +100 KI - 1790 -K2 - 322 -Dl 218 872 + 300 D2 524 799 + 110 B l 277 1200 + 334 B2 161 495 + 207 F l 455 1380 + 203 F2 265 830 + 213 E l 210 1630 + 676 E2 489 983 + 101 Gl 125 358 + 186 G2 124 241 + 94 J l 694 1260 + 82 J2 317 558 + 76 - = data not recorded a f t e r d r y s i e v i n g . -102-L a b o r a t o r i e s ) was added to the -270-mesh washings. A f t e r two days, the water was almost c l e a r and was drawn o f f usi n g the pump and allowed to dry a t ambient temperatures. Only f r a c t i o n s -140+200-mesh and -200+270-mesh were oven d r i e d (at 80°C), the remaining f r a c t i o n s being s t o r e d without d r y i n g i n p l a s t i c bags. 3.8.2 R e l i a b i l i t y of sediment analyses As a t e s t of r e p r o d u c i b i l i t y of sediment a n a l y s e s , the l a b o r a t o r y p o r t i o n of one sample (86-SD-B2) was recombined and re-analysed as 86-SD-R2. Comparison of i n i t i a l and f i n a l weights shows t h a t 700 g (about 1%) of the t o t a l sediment was l o s t d u r i n g recombination and r e - a n a l y s i s . The l o s s e s show as low weights of the coarser f r a c t i o n s and r e s u l t e d from a s p i l l d u r i n g t r a n s f e r of the sample from one c o n t a i n e r to another. The s p i l l e d sediment was not added to the sample to avoid contamination. Comparison of the change of weight of each f r a c t i o n (Table 3-5) shows that the r e l a t i v e e r r o r i n the coar s e r f r a c t i o n s i s q u i t e s m a l l (<1 to 3%). However, as absolute values become sm a l l e r r e l a t i v e e r r o r s become pronounced (89%, -100+140-mesh). As the s i z e of these f r a c t i o n s i n c r e a s e s i n the second a n a l y s i s , i t appears t h a t the second s i e v i n g was more e f f i c i e n t a t removing the f i n e r sediment from the coarser f r a c t i o n s , r e f l e c t i n g some of -103-Table 3-5. R e s u l t s of t e x t u r a l r e - a n a l y s i s of 86-SD-B2 and 86-SD-R2 F r a c t i o n 86-SD-B2 86-SD-R2 D i f f e r e n c e (mesh) (g) (g) (%) -10+16 10300 10000 -2.9 -16+40 39200 39100 -0.3 -40+70 12400 11600 -6.5 -70+100 1340 919 -31.4 -100+140 274 519 + 89.4 -140+200 123 159 + 29.2 -200+270 69 83 + 20.3 -104-the e r r o r s o c c u r r i n g i n r a p i d sample p r o c e s s i n g . Because re-combination of the sample to a " n a t u r a l " mix may not have occurred, i n t e r p r e t a t i o n of the r e s u l t s i s l i m i t e d . In c o n c l u s i o n , weights f o r coarse f r a c t i o n s are q u i t e r e l i a b l e , though not a l l f i n e sediment i s removed. As these f r a c t i o n s were not analysed f o r g o l d content, t h i s minor contamination " i s not important. The weight p r o p o r t i o n s of the f i n e r f r a c t i o n s (-140+200, -200+270, -270-mesh) are not as r e p r o d u c i b l e , but they do not s i g n i f i c a n t l y a f f e c t the t e x t u r a l c h a r a c t e r i s t i c s of the sample. 3.8.3 Heavy mineral s e p a r a t i o n Heavy mineral concentrates were produced fo r the two +270-mesh dry f r a c t i o n s u sing methylene i o d i d e (S.G.=3.3). The technique used was i d e n t i c a l to t h a t used i n the f i r s t phase ( s e c t i o n 2.3.2). 3.8.4 Magnetic mineral s e p a r a t i o n Magnetic minerals were separated from the two heavy mineral c o n c e n t r a t e s by repeated use of a prospectors hand p i s t o n magnet. Magnetic minerals were a l s o separated from three other d r y f r a c t i o n s (-40+70, -70+100 and -100+140 mesh) with an induced magnetic separator (Carpco high gauss model MS-1265). The separate produced c o n t a i n s -105-p a r t i c l e s t h a t are weakly magnetic and hence has to be cleaned up u s i n g a p i s t o n magnet. The magnetic f r a c t i o n produced without p r i o r heavy mineral s e p a r a t i o n i s not s t r i c t l y e q u i v a l e n t to the heavy mineral separate magnetic f r a c t i o n , because the former c o n t a i n s composite p a r t i c l e s t h a t can be e x t r a c t e d with a hand magnet but w i l l not s i n k i n the heavy l i q u i d . T h i s i s e s p e c i a l l y t rue f o r the -40+70-mesh separate which c o n t a i n s s u f f i c i e n t p o l y -m i n e r a l i c p a r t i c l e s t h a t the separate has a s p e c k l e d greenish-brown c o l o u r i n s t e a d of the usual magnetite bl a c k . 3.8.5 Minus 270-mesh magnetic mineral s e p a r a t i o n Magnetic minerals were separated from the -270-mesh f r a c t i o n by mixing a 100 g (dry) p o r t i o n with s u f f i c i e n t water to c r e a t e a suspension. A p l a s t i c coated bar magnet (of the type used i n magnetic s t i r r e r s ) was s t i r r e d manually i n the s l u r r y f o r 45 minutes and removed at f i v e minute i n t e r v a l s and washed i n t o a beaker. Manual s t i r r i n g was r e q u i r e d so that the suspension c o u l d be maintained. At the end of the s e p a r a t i o n , the magnetic concentrate was removed from the beaker using the magnet and washed i n t o a Buchner f i l t r a t i o n f u nnel (Whatman #1 f i l t e r paper). F i n a l l y , the f i l t e r paper was d r i e d under an i n f r a - r e d lamp, the separate disaggregated using a -106-s p a t u l a and then t r a n s f e r r e d to a weighing paper using the p i s t o n magnet. Each of these stages ensures that the f i n a l separate c o n s i s t s o n l y of magnetic minerals without contamination by non-magnetic m i n e r a l s . The non-magnetic separate was then d r i e d at 80°C. 3.8.6 F r a c t i o n a l a n a l y s i s of -270-mesh A p o r t i o n of a -270-mesh sample known to c o n t a i n a p p r e c i a b l e gold (85-SD-10, 320 ppb) was wet s i e v e d i n t o the f o l l o w i n g f r a c t i o n s u sing nylon s c r e e n s : -53 + 44 Jim, -44 + 30 Jim, -30 + 20 Jim, -20 + 10 Jim and -10 Jim. Each f r a c t i o n was d r i e d and magnetic minerals separated from a l l samples except -10 Jim. The d i f f e r e n t c l a y contents of the -270-mesh f r a c t i o n s are apparent from the co l o u r of the non-magnetic se p a r a t e , f o r example c l a y r i c h samples are pale yellow, pale o l i v e and o l i v e (Munsell s o i l c o l o u r s ) . Munsell c o l o u r s recorded f o r a l l -270-mesh f r a c t i o n s vary from 5Y/4/2 to 5Y/6/4. However, the Munsell s o i l c o l o u r c h a r t i s not s u f f i c i e n t l y s e n s i t i v e to give a range of i n d i c e s t h a t would give f i n e sediment content. 3.9 Chemical analyses Two non-magnetic heavy-mineral c o n c e n t r a t e s f r a c t i o n s from each sample (-140+200, -200+270-mesh), -270-mesh non--107-magnetic sediment, non-magnetic f r a c t i o n s from f r a c t i o n a t i o n of 85-SD-10 and a s e l e c t i o n of magnetic mineral separates were analysed n o n - d e s t r u c t i v e l y by in s t r u m e n t a l neutron a c t i v a t i o n a n a l y s i s (INAA) f o r the same s u i t e of elements with the same a n a l y t i c a l c o n d i t i o n s as i n the f i r s t phase (see s e c t i o n 2.3.4). 3.9.1 R e l i a b i l i t y of INAA f o r Au R e p r o d u c i b i l i t y of l a b o r a t o r y a n a l y ses was d i s c u s s e d i n chapter 2. D u p l i c a t e s p l i t s of coarse (-70+100-mesh) concentrates showed extremely poor r e p r o d u c i b i l i t y r e s u l t i n g from the "nugget e f f e c t " . D u p l i c a t e s p l i t s of -270-mesh sediment were submitted f o r a n a l y s i s but a l s o show extremely poor r e p r o d u c i b i l i t y ( F i g . 3-7). These r e s u l t s can a l s o be a t t r i b u t e d to the nugget e f f e c t a l b e i t caused by very sma l l gold p a r t i c l e s . The e f f e c t of one s p h e r i c a l gold p a r t i c l e (Density=18 g/cm 3, s e l e c t e d to account fo the observed high f i n e n e s s of H a r r i s Creek gold) of d i f f e r e n t s i z e s on a 25 g sample of -270-mesh sediment (a t y p i c a l INAA sample) can be seen i n Table 3-6. A j u s t u n d e r s i z e gold p a r t i c l e (d=53 Um) can produce a 50 ppb d i f f e r e n c e i n a n a l y s e s . -108-Au (ppb) F i g . 3-7. Gold analyses f o r s p l i t s of -270-mesh sediment. -109-Table 3-6. E f f e c t on Au c o n c e n t r a t i o n of adding one s p h e r i c a l gold p a r t i c l e ( d e n s i t y = 18 g/cm 3) to a 25 g sample of -270-mesh sediment. P a r t i c l e Diameter Au C o n c e n t r a t i o n (Um) (ppb) 50 42 40 22 30 9.2 20 2.7 10 0.34 -110-The e f f e c t decreases with p a r t i c l e s i z e but i s s i g n i f i c a n t l y pronounced f o r d=30 Um. Using t h i s t a b l e , i t i s c l e a r how s p l i t s c ould r e t u r n 24 ppb and <9 ppb i f one 40 Um gold p a r t i c l e i s present i n the f i r s t s p l i t . 3.9.2 Gold analyses of s u b f r a c t i o n s of a -270-mesh  sample R e s u l t s of a n a l y s e s of the f i v e s u b - f r a c t i o n s of -270-mesh sediment (85-SD-10) are summarised i n Table 3-7. Because the sample i s the most anomalous -270-mesh sediment encountered (bulk Au a n a l y s i s = 320 ppb) c o n c e n t r a t i o n s i n the s u b - f r a c t i o n s are above d e t e c t i o n l i m i t and i n d i c a t e d e c r e a s i n g Au c o n c e n t r a t i o n s as g r a i n s i z e decreases f o r sediment coarser than 20 Um. Sediment f i n e r than 20 Um shows increased Au content. Conversion of Au c o n c e n t r a t i o n to expected number of s p h e r i c a l g o l d p a r t i c l e s i n d i c a t e s t h a t whereas the c o a r s e s t f r a c t i o n i s c a l c u l a t e d to c o n t a i n 26 p a r t i c l e s , the medium s i l t f r a c t i o n s have low numbers of gold p a r t i c l e s . Other -270-mesh samples with much lower bulk compositions would probably c o n t a i n one or two gold p a r t i c l e s of t h i s s i z e t h e r e f o r e nugget e f f e c t s are expected. Based on t h i s a n a l y s i s of -10 Um sediment, i t appears t h a t r e l a t i v e sampling e r r o r s w i l l be the lowest o v e r a l l though the gold may not be a l l p a r t i c u l a t e as implied by the c a l c u l a t i o n . - I l l -Table 3-7. Gold analyses f o r s u b t r a c t i o n s of -270-mesh sediment (85 -SD-10). F r a c t i o n Wt of Sediment Au G o l d 1 (um) (g) (ppb) (spheres) -53+44 39.6 630 25.8 -44+30 12.6 100 2.9 -30+20 9.2 57 4.2 -20+10 10.1 42 17.7 -10 15.1 57 >110 1S.G.=18 / diameter=geometric midpoint of f r a c t i o n s (lOum for -10 um f r a c t i o n ) -112-3.9.3 R e l i a b i l i t y of INAA f o r Hf In t h i s study, the behaviour of other d e n s i t y f r a c t i o n s with r e s p e c t to gold i s c o n s i d e r e d . The c o n c e n t r a t i o n of - z i r c o n s i n the samples i s p r o p o r t i o n a l to the Hf c o n c e n t r a t i o n s i n c e other minerals i n the sediments do not c o n t a i n a p p r e c i a b l e l e v e l s of hafnium (Deer e t a l . , 1966). Z i r c o n s i n the concentrates occur as t e t r a g o n a l prisms doubly terminated by t e t r a g o n a l pyramids. The g r a i n s are p r o l a t e (height:diameter » 5) and are u s u a l l y t r a n s p a r e n t and c o l o u r l e s s though a few are reddy-brown and equant. R e p r o d u c i b i l i t y of Hf analyses can be estimated by c o n s i d e r a t i o n of -270-mesh s p l i t s . The l a r g e number of z i r c o n s i n the samples leads to very low p r e d i c t e d r e l a t i v e e r r o r s based on the Po i s s o n d i s t r i b u t i o n . However, a n a l y t i c a l e r r o r s a p p a r e n t l y lead to very poor r e p r o d u c i b i l i t y ( F i g . 3-8) even though r e p o r t e d c o n c e n t r a t i o n s are not c l o s e to the suggested a n a l y t i c a l d e t e c t i o n l i m i t (1 ppm). R e p r o d u c i b i l i t y at higher c o n c e n t r a t i o n s (500 ppm) was determined f o r s e v e r a l samples a t d i f f e r e n t times and maximum ab s o l u t e e r r o r s were l e s s than 10 ppm. 3.10 Summary Twenty-s i x -10-mesh sediment samples each weighing -113-Hf (ppm) F i g . 3-8. Hafnium analyses f o r s p l i t s of -270-mesh sediment. -114-roughly 60 kg were c o l l e c t e d from H a r r i s Creek: s i x samples were c o l l e c t e d i n November 1985 and twenty samples were c o l l e c t e d i n June 1986. In the f i e l d t e x t u r a l a n a l y s es were c a r r i e d out i n June and p e r t i n e n t h y d r o l o g i c data were c o l l e c t e d . A comparison of s i e v i n g techniques showed t h a t a combination of r a p i d dry s i e v i n g followed by wet s i e v i n g allowed each sample to be processed i n about two days, p r o v i d i n g t e x t u r a l i n f o r m a t i o n f o r the bulk sediment and magnetic minerals and three f r a c t i o n s f o r go l d and hafnium a n a l y s i s . Samples were analysed by INAA. -115-CHAPTER 4: DESCRIPTIVE GEOMORPHOLOGY AND SEDIMENTOLOGY THE HARRIS CREEK STUDY REACH. -116-4.0 Geomorphology of the study reach Examination of a 1:50,000 a e r i a l photograph taken i n August 1982 (Province of B r i t i s h Columbia, #BC 82030 204) shows that the study reach can be d i v i d e d i n t o s e v e r a l s e c t i o n s based on channel form and slope ( F i g . 4-1). Immediately upstream of s i t e J the stream i s a meandering s i n g l e channel with low s i n u o s i t y (S*1.10, S=Channel l e n g t h / V a l l e y d i s t a n c e ) . Close to s i t e J a small f i r s t order t r i b u t a r y (main channel i s f o u r t h order based on a Horton (1945) a n a l y s i s at a s c a l e of 1:50,000) j o i n s H a r r i s Creek from the south bank. At s i t e G a s u b s t a n t i a l volume of s i l t and c l a y s i z e sediment i s added from the p r e v i o u s l y d e s c r i b e d l a c u s t r i n e d e p o s i t ( S e c t i o n 3.5.1). Boulders of t h i s m a t e r i a l break down r a p i d l y to s i l t and c l a y . Downstream of s i t e G to s i t e E the stream occupies a s i n g l e channel of very low s i n u o s i t y (S«1.01) and an average channel slope ( S E ) of 0.025 ( F i g . 4-1). Downstream of s i t e F Beetle Creek j o i n s H a r r i s Creek from the south bank. Here, the main channel becomes steeper (S E=0.042), braided ( F i g . 4-2) and has a higher s i n u o s i t y (S=1.12), fo r about 1 km (to j u s t above s i t e B). Downstream of s i t e B to s i t e M the stream i s reduced to a s i n g l e meandering channel with g r a v e l p o i n t bars and a -117-F i g . 4-1. Channel long p r o f i l e , showing average slope and channel form between open c i r c l e s . The whole s e c t i o n i s 5050 m long. -118-few s t a b l e channel i s l a n d s ( F i g . 4-3). Channel g r a d i e n t s t e a d i l y decreases over t h i s s e c t i o n f o r an average of 0.022. At high flows, the channel i s a t y p i c a l a l t e r n a t i n g sequence of pools and r i f f l e s (shallows) (Andrews, 1983). On the south s i d e , the meanders cut i n t o g r a n i t e c l i f f s and smal l bedrock i s l a n d s are v i s i b l e a t low flows whereas meanders on the north s i d e i n c i s e a l l u v i u m and overbank d e p o s i t s . As a r e s u l t of the c o n t r a s t i n bank m a t e r i a l s , the channel p a t t e r n i s intermediate between r e g u l a r and c o n f i n e d meandering (Knighton, 1981). J u s t upstream of s i t e M, bends i n c i s e g n e i s s bedrock on the north bank. Downstream of s i t e M, the channel continues to meander u n t i l i t emerges i n t o Lumby v a l l e y where i t adopts an a l t e r n a t i n g - b a r channel-form (Knighton 1981) superimposed on low s i n u o s i t y meandering. 4.1 T e x t u r a l a n a l y s i s of sediment samples Data obtained from d r y and wet s i e v i n g of sediments i n the l a b o r a t o r y can be combined with data obtained by wet s i e v i n g c o a r s e r f r a c t i o n s i n the f i e l d to provide a complete t e x t u r a l a n a l y s i s from 256 mm to 0.052 mm, a range of approximately 12 *. S i z e - f r e q u e n c y data can be summarised as p h i - p r o b a b i 1 i t y p l o t s ( F i g . 4 - 4 ) , histograms, the c e n t r a l tendency measures mean (D) and -119-F i g . 4 - 3 . Typ ica l meandering channel sect ion -120-median ( D a o ) and s o r t i n g ( a t ) . However, these l a t t e r s t a t i s t i c s are the o n l y t r a d i t i o n a l s e d i m e n t o l o g i c a l s t a t i s t i c s of use here because non-Gaussian normal m u l t i -modal d i s t r i b u t i o n s are common. For t h i s reason skewness and k u r t o s i s were not c a l c u l a t e d . 4.1.2 T e x t u r a l f e a t u r e s of sandy g r a v e l samples Sandy g r a v e l samples are t e x t u r a l l y h i g h l y v a r i e d as i n d i c a t e d by t o t a l weight, D , and Do© (Table 4-1). The average sample i s a very p o o r l y s o r t e d ( ? i =2.47 *) sandy pebble g r a v e l (Folk, 1974). Histograms and p r o b a b i l i t y p l o t s show t h a t there are probably two or three sediment s i z e p o p u l a t i o n s as i s c h a r a c t e r i s t i c of t h i s type of sample from g r a v e l r i v e r s ( K e l l e r h a l l s and Bray, 1965; Graf, 1971). A c h a r a c t e r i s t i c common to many p r o b a b i l i t y p l o t s i s th a t the curve steepens r a p i d l y i n the 3 4> to 5 </> range ( F i g . 4-4). T h i s form of curve suggests e i t h e r (1) th a t there i s a p o o r l y d e f i n e d c l a y and s i l t p o p u l a t i o n c o n s t i t u t i n g l e s s than 1% of the sample or (2) that data i n t h i s p o r t i o n of the curves should not be p l o t t e d with a l o g a r i t h m i c o r d i n a t e , that i s the data r e p r e s e n t a mixture of lognormal and normal p o p u l a t i o n s ( c f . S i n c l a i r , 1976). Raw weight percentages and histograms show that there i s a small s i z e mode at the f i n e sediment extreme implying t h a t there i s a -121-Table 4-1. Summarised t o t a l weights, D s o and D. Sample Tota l Weight D s o D Number (g) (mm) (mm) Sand Samples Ml 69900 1.11 0.49 A l 60900 0.47 0.27 C l 59900 0.30 0.42 KI 67400 0.40 0.35 Dl 106000 0.76 1.44 Bl 61700 1.33 0.38 F l 61700 1.33 0.45 E l 70200 0.30 0.45 Gl 65100 1.11 0.61 J l 57800 0.41 0.33 Sandy gravel samples M2 147000 4.13 2.92 A2 239000 6.42 4.81 C2 320000 40.63 11.31 K2 236000 8.28 5.10 D2 222000 41.09 10.12 B2 160000 3.03 3.12 F2 709000 84.69 27.59 E2 223000 163.29 19.65 G2 199000 18.12 6.37 J2 332000 71.10 21.14 -122-F i g . 4-4. Some examples of p r o b a b i l i t y p l o t s f o r s i z e frequency d i s t r i b u t i o n s of sandy g r a v e l samples. Note prominent i n f l e x i o n near midpoint and i n c r e a s e of slope at the f i n e sediment extreme. -123-d i s t i n c t i v e s i l t and c l a y p o p u l a t i o n . On histograms a prominent minimum i s present i n the range -3 <t> to -1 4> which i s apparent as an i n f l e x i o n p o i n t on p r o b a b i l i t y p l o t s ( F i g . 4 - 4 ) . T h i s i n f l e x i o n p o i n t occurs c l o s e to the c u t - o f f between f i e l d and l a b o r a t o r y sample p r o c e s s i n g and might imply t h a t f i e l d s i e v i n g i s i n e f f i c i e n t f o r s e p a r a t i o n of g r a n u l e - s i z e m a t e r i a l . However, the i n f l e x i o n i s a very common fe a t u r e i n many s e d i m e n t o l o g i c a l r e p o r t s ( K e l l e r h a l l s and Bray, 1971; Beschta and Jackson, 1979) r e g a r d l e s s of sample p r o c e s s i n g method and i m p l i e s that there are two d i s t i n c t i v e s i z e p o p u l a t i o n s . Because the two p o p u l a t i o n s are w e l l - d e f i n e d the modes probably represent g r a v e l framework and sand, s i l t and c l a y matrix. Using the p r o b a b i l i t y p l o t method of S i n c l a i r (1976) and St a n l e y (1987) the c h a r a c t e r i s t i c s of the two components can be estimated. I t was assumed t h a t the components could be modelled as i d e a l phi-normal p o p u l a t i o n s , an assumption which i s j u s t i f i e d by the e x c e l l e n t f i t of model curves to the r e a l data ( F i g . 4-5). The method all o w s s e l e c t i o n of a diameter which can be used as a d i s c r i m i n a n t ( t h r e s h o l d ) f o r the two components. Overlap of the low diameter t a i l of the coarse p o p u l a t i o n and the high diameter head of the f i n e p o p u l a t i o n i s very s l i g h t ( l e s s than 2% of each d i s t r i b u t i o n ) i n a l l but two -124-Fr ame work component Q \ \ 72% « E CO 5 4 -Matrix component 28% 5 i 1 1 1 r 30 50 70 95 99 Cumulative % coarser F i g . 4-5. P r o b a b i l i t y p l o t f o r s i z e d i s t r i b u t i o n of 86-SD-D2 showing c h a r a c t e r i s t i c f e a t u r e s of sandy g r a v e l d e p o s i t s . Mean diameter and s o r t i n g (standard d e v i a t i o n ) f o r framework and matrix components can be read from the graph. -125-cases. Thus, the estimated d i s c r i m i n a n t diameter and the mean s i z e and s o r t i n g of the matrix and framework are e a s i l y - e s t i m a t e d , p o t e n t i a l l y important s e d i m e n t o l o g i c a l c h a r a c t e r i s t i c s (see s e c t i o n 6.3.2) (Table 4-2). On average matrix m a t e r i a l i s p o o r l y - s o r t e d coarse sand, whereas framework m a t e r i a l i s p o o r l y - s o r t e d pebble g r a v e l . F i n a l l y , the average weight p r o p o r t i o n of matrix m a t e r i a l i s 30% (Table 4-2) which i s ve r y c l o s e to t h a t expected f o r rhombohedrally-packed framework-supported g r a v e l (Leeder, 1982). 4.1.3 T e x t u r a l f e a t u r e s of sand samples Samples show an extreme v a r i e t y of s i z e d i s t r i b u t i o n s on p r o b a b i l i t y p l o t s and histograms. Sand samples are mostly p o o r l y - s o r t e d (cr4=1.24 #) with an average diameter of 0.46 mm (coarse sand). Four p l o t s suggest the presence of a t l e a s t three sediment sub-groups: a moderately t o p o o r l y s o r t e d sand component with a low median diameter ( t y p i c a l l y 0.5 to 0.25 mm), a very p o o r l y s o r t e d p o o r l y d e f i n e d pebble component ( F i g . 4-6) and a p o o r l y - d e f i n e d s i l t and c l a y component. The g r a v e l component t y p i c a l l y i s represented by extreme steepening of p r o b a b i l i t y curves towards the coarse l i m i t . In common with sandy g r a v e l samples, the curves commonly steepen toward the f i n e sediment l i m i t , implying that there i s a d i s t i n c t i v e -126-Table 4-2. C h a r a c t e r i s t i c s of framework and matrix components of sandy g r a v e l s , the p r o p o r t i o n of framework and d i s c r i m i n a n t diameter Sample Matr i x D (mm) Framework D (mm) D i s c r i m i n a n t Diameter (mm) Framework P r o p o r t i o n (%) M2 1.1 10.2 2.6 55 A2 0.6 12.6 1.9 75 C2 1.0 12.5 5.0 78 K2 1.0 16.5 4.0* 60 D2 0.6 40.0 4.5 73 B2 0 . 8 10.0 2.2 1 60 F2 0.5 43.1 2.8 96 E2 1. 0 166.0 25.1 70 G2 1.0 32.1 7.9 58 J2 1. 3 63 .1 8.9 76 1 Samples K2 and B2 have poor d i s c r i m i n a n t diameters, t h a t i s , there i s s i g n i f i c a n t o v e r l a p of framework and matrix s i z e frequency d i s t r i b u t i o n s . -127-1 1 1 1 1 1 - I 0.01 1 10 30 50 70 90 99 99.99 Cumulative % coarser F i g . 4 - 6 . Some examples of p r o b a b i l i t y p l o t s for s i z e frequency d i s t r i b u t i o n s of sand samples. Note extreme steepening of C l towards the coarse l i m i t and presence of three i n f l e x i o n p o i n t s i n the curve f o r D l . -128-medium to f i n e s i l t and c l a y component ( F i g . 4-6). The g r a v e l component (granule and c o a r s e r ) r e s u l t s from a f i e l d sampling problem. Sand d e p o s i t s l o c a l l y occur as a very t h i n (0 to 15 cm) veneer over p r e v i o u s l y d e p o s i t e d g r a v e l s . However, the co n t a c t between the d e p o s i t s i s g e n e r a l l y d i f f u s e , i n which case i t i s easy to contaminate the sample with very coarse sediment. Contamination of t h i s s o r t was recorded i n f i e l d notes (samples C l , F l , E l , J l ) and c o i n c i d e s with the t r i m o d a l s i z e d i s t r i b u t i o n s observed. As the g r a v e l appears t o be a d i f f e r e n t sedimentary horizon,. the m a t e r i a l was e l i m i n a t e d from the t e x t u r a l a n a l y s i s by assuming that the coarse end of the s i z e d i s t r i b u t i o n would be the same as the non-contaminated samples. As the l a t t e r samples can be modelled as phi-normal d i s t r i b u t i o n s , the coarse contaminant was removed u n t i l p r o b a b i l i t y p l o t s had a constant slope a t the coarse l i m i t . T o t a l sand sample weights do not vary much from 60 kg, except Dl which i s anomalous i n t h a t i t was taken from a sand d e p o s i t but has a wide range of sediment s i z e s ( F i g . 4-6). For t h i s reason Dl has the hig h e s t mean diameter (5=1.443 mm) whereas the remainder vary around D=0.5 mm (Table 4-1). -129-4.1.4 Downstream trends i n sediment t e x t u r e s Downstream trends i n the two c e n t r a l tendency measures fo r sandy g r a v e l samples are p o s i t i v e l y c o r r e l a t e d . There i s a s l i g h t tendency f o r i n c r e a s i n g f i n e n e s s downstream ( F i g . 4-7). Downstream trends f o r sand samples are not as obvious though D appears to decrease downstream with departures f o r s i t e s recognized as l o c a t i o n s of magnetite-enrichment ( s i t e s J , D and M). Trends of weight percentages of i n d i v i d u a l f r a c t i o n s of sandy g r a v e l samples vary from p o s i t i v e to negative c o r r e l a t i o n with D and D a o. In p a r t i c u l a r , the weight percents of -128+64 mm sediment (cobbles) and the mean s i z e of framework c l a s t s decrease downstream, whereas weight percents of the -16+8 mm and -8+4 mm (pebbles) f r a c t i o n s i n crease downstream ( F i g . 4-8). 4.2 Magnetite (magnetic m i n e r a l 1 ) t e x t u r a l analyses Magnetic mineral analyses f o r s i x f r a c t i o n s formed by the combination of data f o r h i g h - d e n s i t y magnetic minerals (density>3.3 g/cm 3, -140+200, -200+270 mesh) and t o t a l *Although magnetite was not separated from the sediment, the weight of magnetic p a r t i c l e s i s approximately p r o p o r t i o n a l to the weight of magnetite. - 1 3 0 -F i g . 4 - 7 . Downstream trends i n mean diameter of sediment. F i l l e d i n c i r c l e s are sandy g r a v e l d e p o s i t s , open and h a l f f i l l e d c i r c l e s are sand d e p o s i t s . 20n 1 1—i 1—i 1 1 1 1 F i g . 4 - 8 . Downstream trends of weight percent -8+4 mm sediment. F i l l e d i n c i r c l e s are sandy g r a v e l d e p o s i t s , open and h a l f f i l l e d c i r c l e s are sand d e p o s i t s . -131-magnetic minerals ( i . e . , i n c l u d i n g composite, low d e n s i t y (density<3.3 g/cm 3) magnetic p a r t i c l e s , -40+70, -70+100, -100+140, -270 mesh) can be summarised as e i t h e r (1) c o n c e n t r a t i o n of magnetite i n each f r a c t i o n or (2) weight of magnetite i n each f r a c t i o n . The l a t t e r a llows comparison of h i g h - and low-density mineral d i s t r i b u t i o n s to determine e q u i v a l e n t s e t t l i n g v e l o c i t i e s (e.g. Reid and F r o s t i c k , 1985). Comparison of magnetite c o n c e n t r a t i o n s f o r t y p i c a l sandy g r a v e l and sand samples are summarised i n Table 4-3. Both sample types show a peak magnetite c o n c e n t r a t i o n . However, the weight of magnetite decreases as p a r t i c l e s i z e decreases; thus the f r a c t i o n with the lowest c o n c e n t r a t i o n (-40+70 mesh) has the g r e a t e s t a b s o l u t e weight of magnetite due to the high t o t a l f r a c t i o n weight. P r o b a b i l i t y p l o t s with sediment diameter expressed i n phi f o r cumulative percent magnetite weights were used i n an attempt to determine the amount of t r u n c a t i o n (the percentage of the d i s t r i b u t i o n not determined by s e p a r a t i o n of magnetic p a r t i c l e s ) a t the coarse l i m i t of magnetic s e p a r a t i o n s (0.5 mm). T r u n c a t i o n i s i n d i c a t e d where the p l o t becomes asymptotic to h o r i z o n t a l at the extremes and a normal d i s t r i b u t i o n model i s a p p r o p r i a t e . The curvature of the curve can be used to estimate the -132-Table 4-3. Comparison of weights of magnetite and c o n c e n t r a t i o n of magnetite i n two samples F r a c t i o n (mesh) Sand (Ml) Sandy g r a v e l (C2) Cone. (%) Wt. (g) Cone. (%) Wt. (g) -40+70 0.6 94 2.5 218 -70+100 2.4 48 6.8 103 -100+140 4 . 5 68 6.2 22 -140+200 5.2 20 4.4 9.4 -200+270 2 . 7 12 3.3 2.2 -270 0.8 6 . 5 1.6 5.4 -133-F i g . s i z e 4-9. Some examples of p r o b a b i l i t y p l o t s of magnetite d i s t r i b u t i o n s . -134-percent t r u n c a t i o n and subsequently a normal model of the frequency d i s t r i b u t i o n of the weights of magnetite ( S i n c l a i r , 1976). The p r o b a b i l i t y p l o t s show very l i t t l e f l a t t e n i n g a t the coarse end of the d i s t r i b u t i o n , s u g g e s t i n g t h a t t r u n c a t i o n at the coarse end i s minimal ( F i g . 4-9). However, the problem i s complicated f o r sandy g r a v e l samples because the bulk sediment p r o b a b i l i t y p l o t s are t r i m o d a l . In these samples there i s no i n d i c a t i o n t h a t the magnetite p r o b a b i l i t y p l o t i s t r i m o d a l . Due to these fundamental u n c e r t a i n t i e s about magnetite s i z e frequency d i s t r i b u t i o n s , the approach of Reid and F r o s t i c k (1985), S l i n g e r l a n d (1977) and others a p p l i e d to beach sediments cannot be a p p l i e d to very coarse f l u v i a l sediments. Most curves are steep at the f i n e sediment end of the d i s t r i b u t i o n implying that the s i l t and c l a y p o p u l a t i o n present i n the t o t a l sediment d i s t r i b u t i o n i s a l s o present i n the magnetite d i s t r i b u t i o n . 4.2.1 Comparison of magnetite c o n c e n t r a t i o n s i n sandy  g r a v e l and sand samples Magnetite c o n c e n t r a t i o n s i n both sand samples and sandy g r a v e l samples r i s e smoothly to maxima i n one f r a c t i o n (Table 4-3) f o r most samples. The f r a c t i o n i n which the peak c o n c e n t r a t i o n occurs i s f i n e r f o r sand samples -135-(-140+200-mesh or -200+270-mesh) compared to sandy g r a v e l samples (-70+100-mesh). O v e r a l l magnetite c o n c e n t r a t i o n s i n g r a v e l samples are gr e a t e r than i n sand samples. The lowest c o n c e n t r a t i o n s occur i n -40+70-mesh samples f o r sand samples while i n sandy g r a v e l samples the minimum occurs i n -270-mesh. 4.3 Heavy mineral concentrate analyses Weights and c o n c e n t r a t i o n s of non-magnetic heavy mineral c o n c e n t r a t e s show very s i m i l a r trends t o magnetite. O v e r a l l weights of non-magnetic heavy-mineral concentrates are s l i g h t l y (1.05 times) g r e a t e r than the corresponding magnetite weights. 4.4 Summary The reach sampled can be d i v i d e d i n t o three s e c t i o n s based on channel p a t t e r n (braided and meandering) and channel slope (0.02 to 0.04). A steep braided s e c t i o n i n the middle of the reach produces an upper i n c r e a s i n g channel slope s e c t i o n and a lower d e c r e a s i n g s l o p e s e c t i o n . Sediment t e x t u r a l analyses show t h a t most sediment i n bars i s very p o o r l y s o r t e d sandy cobble g r a v e l whereas at bar t a i l pools there i s a smal l amount of moderately to p o o r l y s o r t e d sand. In both types of sample there i s a -136-minor (<1% by weight) but d i s t i n c t i v e s i l t and c l a y component. Gravel samples show t y p i c a l sand and g r a v e l s i z e modes. Since there i s a t h r e s h o l d diameter that d i s c r i m i n a t e s between the two components t h e i r t e x t u r a l p r o p e r t i e s can be determined s e p a r a t e l y . Magnetite c o n c e n t r a t i o n s show coherent trends r i s i n g to s i n g l e peaks i n one f r a c t i o n . However, the s i z e d i s t r i b u t i o n s of magnetite cannot be used to i n v e s t i g a t e h y d r a u l i c equivalence using the methods of S l i n g e r l a n d (1977) and others due to absence of pure magnetite p a r t i c l e s i n the c o a r s e r f r a c t i o n s . -137-CHAPTER 5 GEOCHEMICAL DATA: RESULTS AND DISCUSSION -138-5.0 I n t r o d u c t i o n Complete t a b l e s of high d e n s i t y mineral data are g i v e n i n the Appendix, however summary s t a t i s t i c s f o r the data are provided i n Table 5-1. Gold c o n c e n t r a t i o n s were determined as p a r t s per b i l l i o n i n the non-magnetic heavy mi n e r a l concentrates (HMCs) but to be c o n s i s t e n t with magnetite, the data were converted to gold c o n c e n t r a t i o n i n the whole f r a c t i o n : A U T O T A L = AUMMC . Wturtc (5-1) Wt T O T A L . based on the assumption t h a t there i s very l i t t l e g o ld i n the low d e n s i t y mineral f r a c t i o n (see Table 2-3B). The same tr a n s f o r m was a p p l i e d to Hf data. A l l Hf and Au data were logtransformed to c o r r e c t f o r p o s i t i v e l y skewed d i s t r i b u t i o n s which b i a s s t a t i s t i c s i n favour of high c o n c e n t r a t i o n s . Samples r e p o r t e d as below a n a l y t i c a l d e t e c t i o n l i m i t s were coded at h a l f the l i m i t . Hafnium and Au c o n c e n t r a t i o n s i n non-magnetic -270-mesh sediment were transformed using equation (5-1). As a l l minus 270-mesh samples were analysed i n d u p l i c a t e , the analyses of the s p l i t s were combined i n a weighted average. -139-Table 5-1. Summary s t a t i s t i c s f o r Au, magnetite and Hf c o n c e n t r a t i o n s i n sediment. F r a c t i o n (mesh) Au Magnetite Hf (ppb) (%) X (%) CV (%) X (ppm) CV (%) Sandy q r a v e l samples (n = 10) . -40+70 - - 3. 7 70 — _ -70+100 - - 10. 2 57 - --100+140 - - 9 . 8 50 - --140+200 243 20 7. 2 59 46 20 -200+270 150 23 3. 9 42 28 13 -270 20 37 1. 5 67 21 8 Black sand--enriched sand samples (n = 4) -40+70 1. 2 50 _ _ -70+100 - - 4. 9 40 - --100+140 - - 6. 7 29 - --140+200 80 34 6. 0 24 46 10 -200+270 80 26 4. 3 27 33 7 -270 13 42 1. 9 67 19 2 Black sand poor sand samples (n= -40+70 - - 0. 5 34 — ' --70+100 - - 1. 6 21 - --100+140 - - 2. 3 26 - --140+200 4.0 122 2. 7 22 16 10 -200+270 7.8 87 2. 5 35 16 14 -270 8 . 3 28 0. 9 48 17 13 'Geometric mean fo r Au and Hf, a r i t h m e t i c mean f o r magnetite. ^ C o e f f i c i e n t of v a r i a t i o n ( i . e . standard deviation/mean). - = f r a c t i o n not determined. -140-5.1 Gold data 5.1.1 E v a l u a t i o n of h y d r a u l i c and r a r e g r a i n e f f e c t s In t h i s study, c o l l e c t i o n of l a r g e samples has been emphasised to minimize s t a t i s t i c a l sampling e r r o r s due to the occurrence of g o l d as d i s c r e t e p a r t i c l e s . Before p r e s e n t i n g r e s u l t s , i t i s necessary to e s t a b l i s h that t h i s c o n d i t i o n has been met and t h a t a high p r o p o r t i o n of w i t h i n and between s i t e v a r i a b i l i t y can be a t t r i b u t e d to h y d r a u l i c e f f e c t s . As a f i r s t s t e p i n the e v a l u a t i o n of t h i s problem, 95% confidence l i m i t s f o r g o l d a n a l y s e s based on the Poisson d i s t r i b u t i o n can be c a l c u l a t e d to determine the extent of sampling problems. Overlap of confidence i n t e r v a l s i m p l i e s t h a t there i s a reasonable chance t h a t two samples have the same gold c o n c e n t r a t i o n and apparent d i f f e r e n c e s between them are simply due to the nugget e f f e c t . N i n e t y - f i v e percent confidence l i m i t s can be estimated f o r the Poisson d i s t r i b u t i o n u s i n g the chi-squared d i s t r i b u t i o n (Zar, 1984): Z S C l - a / 2 > , 2 I M £ ]d £ X ' Q I / Z , 2 C f>»<- 1 > (5~2) 2 2 a f t e r e s t i m a t i n g the number of p a r t i c l e s of g o l d (N) i n the sample and a=.95. C a l c u l a t i o n of the number of gold p a r t i c l e s f o r samples i n the f i r s t phase ( S e c t i o n 2.4.3) was based on gold with a s p e c i f i c g r a v i t y of 15. However, the s t r o n g y e l l o w c o l o u r of gold p a r t i c l e s from H a r r i s Creek i m p l i e s a -141-higher f i n e n e s s and d e n s i t y . A d e n s i t y of 18 g/cm 3, corresponding to a f i n e n e s s of 900 (assuming only Au and Ag present) was s e l e c t e d for c a l c u l a t i o n s . Shape d i s t r i b u t i o n of gold p a r t i c l e s i n H a r r i s Creek sediments was determined i n the f i r s t phase ( s e c t i o n 2.4.2). The "average" gold p a r t i c l e was found to be 0.63 times the volume of a sphere of the same s i e v e diameter ( i . e . i d e n t i c a l intermediate a x e s ) . As p r e v i o u s l y , the intermediate diameter of p a r t i c l e s of a f r a c t i o n was taken as the geometric midpoint of the bounding s i e v e openings. Gold i n -270-mesh sediment was i n i t i a l l y assumed to have an average diameter of 20 Um, however analyses of sub-f r a c t i o n s of 85-SD-10 (Table 3-7) permits a b e t t e r estimate of the e f f e c t i v e diameter to be made using dE = (E (Mj/M)dj 3) 1 / " 3 (5-3) where n o t a t i o n i s given i n s e c t i o n 2.4.5.1. Due to the high c o n c e n t r a t i o n of Au i n the c o a r s e s t s u b - f r a c t i o n (-53+44 Um) the e f f e c t i v e diameter, f o r the purpose of confidence l i m i t e s t i m a t i o n , i s 47 um. Using the shape and s i z e assumptions d e s c r i b e d above, gold c o n c e n t r a t i o n confidence l i m i t s have been c a l c u l a t e d f o r the two very f i n e sand f r a c t i o n s (-140+200-mesh, -200+270-mesh) from sandy g r a v e l and sand d e p o s i t s ( F i g . 5-1). In both f r a c t i o n s , confidence l i m i t s f o r sandy g r a v e l s are very narrow ( r e f l e c t i n g t h e i r high number of -142-( A ) Au (ppb) 1 10 100 1000 i i i i M 0 A e C e • K e D B — — e • F 9 #  E 9 G © J C ( B ) Au (ppb) 1 10 100 1000 i | i _ • M C  A e C 0 •== K © D B 9 # F E e G V — J F i g . 5-1. Poisson confidence l i m i t s f o r gold c o n c e n t r a t i o n s c a l c u l a t e d as d e s c r i b e d i n the t e x t f o r (A) -140+200-mesh and (B) -200+270-mesh. S o l i d c i r c l e s are sandy g r a v e l samples, open c i r c l e s are magnetite-poor sand samples, h a l f - f i l l e d c i r c l e s are m a g n e t i t e - r i c h sand samples. - 1 4 3 -161 # 12 -o o o I <D +-"5 c O) co 2 8 -0.1 o cP ft, 1 10 100 A u -140+200 ( P P b ) 1000 F i g . 5-2. Magnetite concentration (-70+100-mesh) versus Au concentration (-140+200-mesh). Solid c i r c l e s are sandy gravel samples, open c i r c l e s are magnetite-poor sand samples, h a l f - f i l l e d c i r c l e s are magnetite-rich sand samples. -144-qold p a r t i c l e s ) and o v e r l a p i n o n l y a few cases with the much wider confidence l i m i t s f o r sandy sediments. Trends i n these f r a c t i o n s are t h e r e f o r e b e l i e v e d to r e f l e c t h y d r a u l i c e f f e c t s . T h i s i s c o r r o b o r a t e d by the very s i m i l a r trends i n magnetite c o n c e n t r a t i o n s ( F i g . 5-2) which are too high to be s e r i o u s l y a f f e c t e d by the occurrence of magnetite as d i s c r e t e p a r t i c l e s . In the case of the -270-mesh f r a c t i o n s , due to the lar g e estimated e f f e c t i v e diameter of -270-mesh g o l d p a r t i c l e s , confidence l i m i t s o v e r l a p i n a l l cases. Conclusions about h y d r a u l i c e f f e c t s on t h i s f r a c t i o n are more l i m i t e d . 5.1.2 Downstream trends i n Au c o n c e n t r a t i o n s The two very f i n e sand-size f r a c t i o n s show s i m i l a r trends i n Au c o n c e n t r a t i o n ( F i g . 5-3A, 5-3B). Minus 270-mesh Au trends ( F i g . 5-3C) are d i s s i m i l a r to the two c o a r s e r f r a c t i o n s , perhaps due to sampling and a n a l y t i c a l u n c e r t a i n t i e s d i s c u s s e d e a r l i e r ( s e c t i o n s 3.9.1 and 5.1.1). Samples from sandy g r a v e l d e p o s i t s show Au c o n c e n t r a t i o n s from approximately 10 ppb to 1000 ppb and there are no marked i n c r e a s i n g or d e c r e a s i n g trends though a s l i g h t low p o i n t i s d e t e c t a b l e i n the s e c t i o n of steep -145-0.1-1— I 1 — I 1 — I 1 1 1 J G E F B D K C A M F i g . 5-3. Downstream trends i n Au c o n c e n t r a t i o n i n (A) -140+200-mesh sediment, (B) -200+270-mesh sediment... (Continued next page). -146-1000 J G E F B D K C A M F i g . 5-3. (continued from previous page) (C) -270-mesh. F i l l e d c i r c l e s = sandy g r a v e l d e p o s i t s , Open c i r c l e s = magnetite-poor sand d e p o s i t s , h a l f - f i l l e d c i r c l e s m agnetite-enriched sand d e p o s i t s . S i t e J i s the upstream end of the reach which i s 5050 m i n l e n g t h . -147-channel and b r a i d i n g . Gold c o n c e n t r a t i o n s i n sand samples vary between approximately 0.1 ppb and 1000 ppb and show a d e c r e a s i n g downstream t r e n d , though major departures occur f o r magnetite-enriched samples, p a r t i c u l a r l y DI. T h i s r e s u l t s i n a gap between Au c o n c e n t r a t i o n s of sandy g r a v e l and sand d e p o s i t s t h a t i n c r e a s e s downstream and appears to d i v e r g e more r a p i d l y a f t e r s i t e B corresponding to the change from braided to meandering channel. T h i s d i f f e r e n c e between Au c o n c e n t r a t i o n s i n d i f f e r e n t sediment d e p o s i t s decreases as s i e v e diameter of sediment decreases, becoming very small f o r -270-mesh sediment. However, even i n t h i s f r a c t i o n Au c o n c e n t r a t i o n s i n g r a v e l d e p o s i t s are g r e a t e r than sand d e p o s i t s toward the downstream end of the reach ( F i g . 5-3C). 5.1.3 Q u a l i t a t i v e comparison of w i t h i n - s i t e and between  s i t e v a r i a b i l i t y Comparison of F i g s . 5-3A, 5-3B and 5-3C i n d i c a t e s t h a t the f o l l o w i n g sources of environmental v a r i a b i l i t y should be e v a l u a t e d : 1) Gold c o n c e n t r a t i o n s i n sandy g r a v e l d e p o s i t s are mostly g r e a t e r than gold c o n c e n t r a t i o n s i n adjacent sand d e p o s i t s , but the d i f f e r e n c e decreases as sediment diameter decreases. 2) Between s i t e v a r i a b i l i t y appears to be smaller than -148-w i t h i n s i t e v a r i a b i l i t y f o r any of the three f r a c t i o n s cons i d e r e d . 3) Within a sediment d e p o s i t - t y p e between s i t e v a r i a b i l i t y f o r d i f f e r e n t sediment f r a c t i o n s i s not uniform, f o r example gold c o n c e n t r a t i o n i n -140+200-mesh sediment from sand d e p o s i t s appear to show more e r r a t i c between-s i t e v a r i a t i o n s than does -200+270-mesh sediment. Three s t a t i s t i c a l methods were used to assess v a r i a b i l i t y : 1) Geometric mean c o n c e n t r a t i o n r a t i o s (Saxby, 1985), 2) a n a l y s i s of v a r i a n c e and 3) c o e f f i c i e n t s of v a r i a t i o n . 5.1.3.1 Geometric mean c o n c e n t r a t i o n r a t i o s (GMCR) Geometric mean c o n c e n t r a t i o n r a t i o s (Saxby and F l e t c h e r , 1986) provide a means of e v a l u a t i n g between f r a c t i o n w i t h i n s i t e v a r i a b i l i t y . The mean high to low energy gold c o n c e n t r a t i o n r a t i o i s determined using the usual formula f o r the geometric mean: n GMCR = a n t i l o g [ ( E l o g C R i ) / n ] , 1 where C R i = A U s o , I . X A U o , * , x n = number of s i t e s and A u s a . i . x , A u a . i . x are gold c o n c e n t r a t i o n s i n f r a c t i o n X from s i t e i sandy g r a v e l and sand d e p o s i t s , - I r -r e s p e c t i v e l y . The geometric mean i s used so th a t o v e r a l l d i l u t i o n of sandy g r a v e l d e p o s i t s with r e s p e c t to sand d e p o s i t s (GMCR<1) i s weighted the same as o v e r a l l enrichment (GMCR>1). No net o v e r a l l d i l u t i o n or enrichment i s g i v e n by GMCR=1. Geometric mean c o n c e n t r a t i o n r a t i o s can then be compared with each other using Duncan's m u l t i p l e range t e s t (Ott, 1984), thus p r o v i d i n g an i n d i c a t i o n of the merits of using d i f f e r e n t s i z e f r a c t i o n s to reduce l o c a l h y d r a u l i c e f f e c t s . C a l c u l a t i o n of GMCRs f o r Au shows t h a t the g r e a t e s t d i f f e r e n c e between environments i s for the c o a r s e s t f r a c t i o n (Table 5-2) and the l e a s t d i f f e r e n c e i s f o r the f i n e s t f r a c t i o n (-270-mesh). However, the m u l t i p l e range t e s t shows t h a t GMCRs f o r the two f i n e sand s i z e f r a c t i o n s are not s i g n i f i c a n t l y d i f f e r e n t at the 95% confidence l e v e l . T h i s i s due to the extreme w i t h i n f r a c t i o n v a r i a b i l i t y of CRs r e s u l t i n g from the mixture of magnetite-poor and m a g n e t i t e - r i c h sand d e p o s i t s and the downstream in c r e a s e of CRs. S i t e s at which m a g n e t i t e - r i c h sands were sampled y i e l d the lowest CRs, d i p p i n g to l e s s than 1 f o r s i t e D (Table 5-2). Removal of these four samples y i e l d s higher GMCRs but adjacent f r a c t i o n s s t i l l do not have s i g n i f i c a n t l y d i f f e r e n t GMCRs (Table 5-2) at the 95% confidence l e v e l . -150-Table 5-2. GMCRs f o r three h i g h - d e n s i t y mineral f r a c t i o n s and s i g n i f i c a n c e t e s t e d by Duncans m u l t i p l e range t e s t . F r a c t i o n (mesh) D e n s i t y F r a c t i o n -40+70 -70+100 -100+140 -140+200 -200+270 -270 Au 18.1 7.4 1.8 Au 1 35.6 10.8 2.3 Magnetite 4. 3 3.7 2.4 1.7 1.3 1.2 Zi r c o n (Hf) 1.9 1.3 1.0 Bars are under f r a c t i o n s whose GMCRs are i n s i g n i f i c a n t l y d i f f e r e n t at the 95% confidence l e v e l . *GMCRs c a l c u l a t e d f o r s i t e s where sands are magnetite-poor . -151-Thus, GMCRs are s i g n i f i c a n t l y lower fo r the f i n e s t sediment implying t h a t h y d r a u l i c processes tending to produce d i f f e r e n t i a l gold contents at d i f f e r e n t l o c a t i o n s i n the stream bed are subdued when a c t i n g on f i n e sed iment. 5.1.3.2 One way a n a l y s i s of v a r i a n c e One way a n a l y s i s of v a r i a n c e can be used to assess the importance of w i t h i n s i t e versus between s i t e v a r i a b i l i t y of gold c o n c e n t r a t i o n s ( S i n c l a i r 1984). If the n u l l hypothesis of the t e s t i s r e j e c t e d then i t can be concluded t h a t w i t h i n s i t e v a r i a b i l i t y i s s u f f i c i e n t l y low that trends between s i t e s are meaningful. However, due to the extreme v a r i a b i l i t y of Au c o n c e n t r a t i o n s w i t h i n s i t e s the n u l l hypothesis i s accepted (at 95% confidence l e v e l ) for a l l three f r a c t i o n s (Table 5-3). For p r a c t i c a l purposes, the a n a l y s i s of v a r i a n c e shows that i f gold c o n c e n t r a t i o n between s i t e s i s of importance, f o r example when attempting to d e l i n e a t e a d i s p e r s i o n t r a i n , then the type of sediment d e p o s i t sampled must be c o n s i d e r e d a s i g n i f i c a n t source of environmental v a r i a b i l i t y . 5.1.3.3 C o e f f i c i e n t s of v a r i a t i o n (CVs) C o e f f i c i e n t s of v a r i a t i o n ( r e l a t i v e standard d e v i a t i o n ) : -152-Table 5-3. A n a l y s i s of va r i a n c e r e s u l t s f o r g o l d . N u l l h y p o t h e s i s : U M = U<°» = = U c = . . . = U F r a c t i o n / Source of Sum of v a r i a t i o n Squares Degrees of Mean Sum Freedom of Squares -140+200-mesh Between s i t e s 4.36 Within s i t e s 15.53 9 10 0.484 1.353 F C A L C = 2.80 F(10,9,0.05) = 5. 26 . N u l l hypothesis accepted -200+270-mesh Between s i t e s 4.46 Within s i t e s 8.10 9 10 0.495 0.810 F C A L C = 1.64 F(10,9,0.05) = 5. 26 . N u l l hypothesis accepted -270-mesh Between s i t e s 1.40 Within s i t e s 2.05 9 10 0.156 0.205 F C A L C = 1.32 F(10,9,0.05) = 5. 26 . N u l l hypothesis accepted -153-CV (%) = standard d e v i a t i o n .10 0 Mean provide a means of comparing between s i t e v a r i a b i l i t y f o r Au c o n c e n t r a t i o n s i n d i f f e r e n t f r a c t i o n s from any given environment. However, comparisons between sediment d e p o s i t s are not u s e f u l because downstream trends i n Au c o n c e n t r a t i o n s f o r sands versus g r a v e l s are completely d i f f e r e n t ( F i g . 5-3). In magnetite-poor sands, h i g h e s t CVs are found i n -140+200-mesh sediment ( F i g . 5-4, curve 1) p r i m a r i l y due to the d i s t i n c t i v e downstream decay of Au c o n c e n t r a t i o n s . In minus 200+270-mesh sediment, t h i s decay i s l e s s pronounced y i e l d i n g a lower c o e f f i c i e n t of v a r i a t i o n . F i n a l l y , i n minus 270-mesh sediment the CV i s lowest f o r t h i s type of d e p o s i t . The overwhelming components of v a r i a b i l i t y i n t h i s f r a c t i o n are presumably random e r r o r s due to the nugget e f f e c t ( s e c t i o n 5.1.1) and poor inst r u m e n t a l p r e c i s i o n c l o s e to the INAA d e t e c t i o n l i m i t ( s e c t i o n 3.9.1). Only four samples of m a g n e t i t e - r i c h sands were analysed for each f r a c t i o n , however CVs ( F i g . 5-4, curve 2) are low as downstream decay of Au c o n c e n t r a t i o n s i s not apparent. Gold c o n c e n t r a t i o n s i n the two f i n e sand f r a c t i o n s i n sandy g r a v e l s show low CVs ( F i g . 5-4, curve 3) r e f l e c t i n g the absence of decay of Au c o n c e n t r a t i o n s as w e l l as good -154-160H 120-§ 80-40-140+200 -200+270 Fraction (mesh) -270 F i g . 5-4. Coef f i c i ents of v a r i a t i o n for Au concentrations by sediment deposit type and f r a c t i o n . Curve 1 = magnetite-poor sand deposi ts , curve 2 = magneti te-r ich sand deposi ts , curve 3 = sandy gravel depos i ts . -155-a n a l y t i c a l p r e c i s i o n and low random sampling e r r o r s due to very high Au c o n c e n t r a t i o n s . C o e f f i c i e n t s of v a r i a t i o n f o r Au c o n c e n t r a t i o n s i n -270-mesh sediment are a l l i n a narrow range (30 to 40%) i n d i c a t i n g t h a t a high p r o p o r t i o n of v a r i a b i l i t y can perhaps be a t t r i b u t e d to random sampling e r r o r s and INAA problems near the d e t e c t i o n l i m i t . 5.1.4 Summary of gold r e s u l t s C o n s i s t e n t l y high Au c o n c e n t r a t i o n s with low e r r o r s due to the nugget e f f e c t and low between s i t e v a r i a b i l i t y are observed i n f i n e sand f r a c t i o n s (-140+200-mesh, -200+270-mesh) from sandy g r a v e l d e p o s i t s . Gold c o n c e n t r a t i o n s i n magnetite-poor sand d e p o s i t s show r a p i d decay to sub-d e t e c t i o n l e v e l s over the 5 km long study reach. A n a l y s i s of v a r i a n c e and geometric mean c o n c e n t r a t i o n s r a t i o s i n d i c a t e t h at Au c o n c e n t r a t i o n data obtained from f i n e sand f r a c t i o n s i n sandy g r a v e l d e p o s i t s and sand d e p o s i t s should not be mixed. In t h i s case w i t h i n s i t e (environmental) v a r i a b i l i t y w i l l obscure between s i t e ( d i s p e r s i o n t r a i n ) t r e n d s . Minus 270-mesh sediment appears to o f f e r an a l t e r n a t i v e to the environment s e n s i t i v e c o a r s e r f r a c t i o n s because the GMCR approaches u n i t y and CVs are s i m i l a r r e g a r d l e s s of sampling environment. However, Au c o n c e n t r a t i o n are c l o s e -156-to the d e t e c t i o n l i m i t , i n c r e a s i n g v a r i a b i l i t y due to poor a n a l y t i c a l p r e c i s i o n and random e r r o r s are very high due to s m a l l - s c a l e nugget e f f e c t s . Low gold c o n c e n t r a t i o n s may perhaps be a t t r i b u t a b l e to d i l u t i o n by f i n e sediment from the l a c u s t r i n e d e p o s i t near s i t e G (see s e c t i o n 4.0). Thus, although the f r a c t i o n i s not as s e n s i t i v e to sampling environment as the c o a r s e r f r a c t i o n s , anomaly c o n t r a s t w i l l be low and the chances of being below the d e t e c t i o n l i m i t are high. 5.2 Comparison of the behaviour of g o l d , magnetite and  z i r c o n 5.2.1 Trends i n magnetite c o n c e n t r a t i o n s Magnetite c o n c e n t r a t i o n s i n the coarser f r a c t i o n s of sandy g r a v e l s i n i t i a l l y decrease downstream ( F i g . 5-5) as channel slope i n c r e a s e s ( F i g . 4-1) then increase as channel slope decreases. Conversely, magnetite c o n c e n t r a t i o n s i n sand d e p o s i t s f l u c t u a t e about the mean va l u e . Consequently, as f o r Au, magnetite c o n c e n t r a t i o n s from d i f f e r e n t sediment d e p o s i t s diverge downstream of s i t e F ( F i g . 5-3). In the m a g n e t i t e - r i c h sand d e p o s i t s magnetite c o n c e n t r a t i o n s g r a d u a l l y increase r e l a t i v e to sandy g r a v e l samples as sediment diameter decreases. S i t e D r e p r e s e n t s the most extreme example with the magnetite content of the sand being g r e a t e r than the corresponding g r a v e l f o r -157-F i g . 5-5. Downstream p r o f i l e s of magnetite c o n c e n t r a t i o n s i n (A) -40+70-mesh sediment, (B) -70+100-mesh sediment... (continued on next page). -158-P i g . 5-5. (C) -100+140-mesh sediment, (D) -140+200-mesh sediment... (continued on next page). -159-P i g . 5-5. (E) -200+270-mesh sediment, (F) -270-mesh sediment. F i l l e d c i r c l e s = sandy g r a v e l d e p o s i t s , Open c i r c l e s = magnetite-poor sand d e p o s i t s , h a l f - f i l l e d c i r c l e s = magnetite-enriched sand d e p o s i t s . S i t e J i s the upstream end of the reach which i s 5050 m i n l e n g t h . -160-f r a c t i o n s f i n e r than 70-mesh. Magnetite c o n c e n t r a t i o n s i n -270-mesh s h a r p l y decrease at s i t e G, then g r a d u a l l y i n c r e a s e downstream ( F i g . 5-5F). T h i s e f f e c t may be s i m i l a r to t h a t observed f o r the coars e r f r a c t i o n s or, a l t e r n a t i v e l y , may be due to d i l u t i o n of the f r a c t i o n by magnetite-barren f i n e sediment from the l a c u s t r i n e d e p o s i t near the sampling l o c a t i o n at s i t e G. In the l a t t e r case, magnetite c o n c e n t r a t i o n s i n c r e a s e s l i g h t l y downstream as m a g n e t i t e - r i c h f i n e sediment i s added from the banks. 5.2.2 Trends i n hafnium c o n c e n t r a t i o n s High hafnium c o n c e n t r a t i o n s r e f l e c t the presence of abundant z i r c o n s i n the non-magnetic heavy-mineral c o n c e n t r a t e s . Thus, Hf c o n c e n t r a t i o n s are p r o p o r t i o n a l to the c o n c e n t r a t i o n of z i r c o n s , assuming that a l l z i r c o n s have the same Hf content. Hafnium contents f o r a l l three f r a c t i o n s analysed are mostly between 1 ppm and 100 ppm and show s t r o n g s i m i l a r i t i e s to trends i n magnetite c o n c e n t r a t i o n s i n the two f i n e sand f r a c t i o n s ( F i g . 5-6). Conc e n t r a t i o n s i n -270-mesh sediment are not meaningful as i n d i c a t e d i n s e c t i o n 3.9.3. -161-1000 E a. a 100-J G E F 100 E a a. E F F i g . 5-6. Downstream p r o f i l e s of Hf c o n c e n t r a t i o n s i n (A) -140+200-mesh and (B) sandy g r a v e l d e p o s i t s , d e p o s i t s , h a l f - f i l l e d d e p o s i t s . S i t e J i s i s 5050 m i n l e n g t h . -200+270-mesh. F i l l e d c i r c l e s Open c i r c l e s = magnetite-poor sand c i r c l e s = magnetite-enriched sand the upstream end of the reach which -162-100H -70+100 -140+200 Fraction (mesh) F i g . 5-7. Geometric mean c o n c e n t r a t i o n r a t i o s c a l c u l a t e d f o r four d e n s i t y f r a c t i o n s as d e s c r i b e d i n the t e x t . Curve 1 = Au, curve 2 = magnetite, curve 3 = Hf and curve 4 = bulk sediment. -163-5.2.3 Geometric mean c o n c e n t r a t i o n r a t i o s and  c o n c e n t r a t i o n r a t i o s Geometric mean c o n c e n t r a t i o n r a t i o s were determined f o r magnetite, z i r c o n and a l l sediment i n a f r a c t i o n u sing equation 5-2 (Table 5-2, F i g . 5-7). As d e n s i t y of the mineral f r a c t i o n decreases GMCRs f o r a given f r a c t i o n decrease. Geometric mean c o n c e n t r a t i o n r a t i o s approach u n i t y f o r a l l d e n s i t y f r a c t i o n s as sediment gets f i n e r . However, although Duncans m u l t i p l e range t e s t shows t h a t the o v e r a l l t rend i s s i g n i f i c a n t f o r magnetite t h i s i s not so f o r z i r c o n (Table 5-2). A matrix of c o r r e l a t i o n c o e f f i c i e n t s (r) determined f o r l o g a r i t h m i c CRs (Table 5-4) shows a number of i n t e r e s t i n g f e a t u r e s s i m i l a r to those noted by Saxby and F l e t c h e r (1986), though they determined CRs r e l a t i v e to the weight of -10-mesh i n a sample r a t h e r than the weight of the same f r a c t i o n (equation 5-2). A high, s t a t i s t i c a l l y s i g n i f i c a n t c o r r e l a t i o n c o e f f i c i e n t i m p l i e s t h a t the combination of processes l e a d i n g to v a r i a b l e h i g h - d e n s i t y mineral c o n c e n t r a t i o n s between the two types of sediment sampled are s i m i l a r f o r two s i z e and/or d e n s i t y f r a c t i o n s . A l l c o r r e l a t i o n c o e f f i c i e n t s were evaluated by examining b i v a r i a t e s c a t t e r p l o t s to check for o u t l i e r s . The h i g h e s t c o r r e l a t i o n s occur between adjacent s i z e f r a c t i o n s f o r the same d e n s i t y (r=0.902 for Au CRs f o r -140+200-mesh T a b l e 5-3. C o r r e l a t i o n m a t r i x of c o n c e n t r a t i o n r a t i o s . C a l c u l a t i o n d e s c r i b e d In t h e t e x t . V a l u e s g r e a t e r t h a n 0.632 a r e s i g n i f i c a n t w i t h 95% c o n f i d e n c e . Ten samples were used t o g e n e r a t e m a t r i x . G o l d Magnet i t e Z i r c o n - 140+200 -200+270 - 270 -40+70 -70+100 -100+140 -140+200 200+270 -270 -140+200 -200+270 -270 G o l d -140+200 1. .0000 -200+270 0 . 9020 1. 0000 -270 0. 4517 0. 3540 1. 0000 Magnet i t e -40+70 0. .6977 0. 7358 0 . 6509 1. 0000 -70+100 0 . 8613 0 . 8271 0 . 5038 0. 8864 1. 0000 -100+140 0 . 8983 0 , .8089 0 . 4429 0 . 7899 0 . ,9788 1. 0000 -140+200 0. .7349 0. .6049 0 .  3988 0 . 6385 0 . 8717 0. 8965 1.0000 -200+270 0 . 3543 0 .4151 0 .3457 0. .7413 0. .7187 0. .6146 0.5656 1 .0000 -270 -0 .2386 -0 .1298 0 . 2423 0. . 2297 -0 . 0309 -0 . , 1881 -0.0752 0 . ,2414 1. . 0000 Z i r c o n -140+200 0 . 7953 n . 5968 0 . 3442 0 . 5622 0 .8327 0 . 8873 0.9645 0 .4651 -0. .1352 1.0000 -200+270 0 . 4328 0 . 5959 0 .1274 0. .5647 0 .6715 0. .6033 0.5085 0 .8429 0. .0184 0.4046 1.0000 -270 0 . 1997 0 . 2788 0 .1059 0 . 6896 0 .4867 0 . 3645 0.2783 0 .6314 0. .4194 0.1923 0.4032 1, . 0000 -165-and -200+270-mesh). Magnetite shows th a t as the d i f f e r e n c e between diameter of sediment i n c r e a s e s the r -values decrease, but remain s i g n i f i c a n t with 95% con f i d e n c e . T h i s i n d i c a t e s t h a t the behaviour of s i m i l a r s i z e f r a c t i o n s i s comparable as would be expected. C o r r e l a t i o n c o e f f i c i e n t s f o r gold i n -140+200-mesh and -200+270-mesh with magnetite are high f o r magnetite f r a c t i o n s -70+100-mesh and -100+140-mesh. These r e s u l t s suggest t h a t f i n e sand s i z e gold behaves s i m i l a r l y to medium sand s i z e magnetite. There i s i n s u f f i c i e n t data t o draw the same c o n c l u s i o n f o r z i r c o n , however r - v a l u e s with magnetite i n c r e a s e as sediment s i z e i n c r e a s e s . Thus GMCRs i n d i c a t e t h a t coarse s i l t and f i n e r h i g h -d e n s i t y minerals show s i m i l a r behaviour i n sandy g r a v e l d e p o s i t s and sand d e p o s i t s . However, c o r r e l a t i o n c o e f f i c i e n t s f o r l o g a r i t h m i c CRs i n d i c a t e that coarse intermediate d e n s i t y minerals (magnetite) behave s i m i l a r l y to f i n e high d e n s i t y minerals ( g o l d ) . The r e s u l t s are i n t e r n a l l y c o n s i s t e n t as can be seen i n F i g . 5-7: a l i n e drawn for GMCR=4 i n t e r s e c t s the magnetite and go l d curves at d i f f e r e n t s i z e f r a c t i o n s which w i l l be h y d r a u l i c a l l y s i m i l a r . The f i n e sand s i z e magnetic minerals do not share GMCRs with any of the gold f r a c t i o n s c o n s i d e r e d . S i m i l a r l y , a high c o r r e l a t i o n c o e f f i c i e n t between z i r c o n i n -140+200-mesh and magnetite i n the same f r a c t i o n -166-corresponds to almost i d e n t i c a l GMCRs for the two f r a c t i o n s . 5.3 Removal of hydraul ic v a r i a b i l i t y : empir ica l data  transforms Comparison of downstream p r o f i l e s of raw data, shows that at a given s i t e high concentrations of one s ize or densi ty f r a c t i o n are matched by high concentrations of another densi ty and s ize f r a c t i o n . This observation shows that r a t i o i n g of fract ions might lead to e l iminat ion of hydraul ic e f fects permitt ing downstream anomaly decay patterns to be become v i s i b l e . S i m i l a r l y , negative corre la t ions between concentration of gold and weight of heavy-mineral concentrate observed elsewhere (D. Brabec, pers. comm.) suggests that a product transform might be useful though there is no evidence that a d i l u t i o n ef fect of th i s type is seen in Harris Creek sediments. Determination of su i table ra t io s has impl icat ions to inves t igat ion of hydraul ic equivalence since a useful r a t i o implies that the fract ions used are behaving in a s imi lar fashion. As well as e l iminat ing hydraul ic e f fec t s , a potent ia l transform must be determinable from e a s i l y co l l ec ted data . Thus, only transforms involv ing gold , magnetite, non-magnetic heavy mineral concentrate and fine sediment weights have been invest igated. The data are supplied -167-Table 5-5. Empir i ca l data transforms discussed in the text . = log W t A u , ^ Wt 101 *z . x . v- = log(C A«_« , x ) / C t l A Q , y 3. »< , y - log W t e E D , y 10' where Wt*, j = weight of mineral i in f r a c t i o n j . C i . . j = concentration of mineral i in f rac t ion j . -168-o r i q i n a l l y as c o n c e n t r a t i o n of Au i n non-magnetic heavy-min e r a l c o n c e n t r a t e s and re p r e s e n t s a simple, but l o g i c a l r a t i o (<frx, Table 5-5). A tran s f o r m suggested by downstream p r o f i l e s i s * 3 s i n c e l o g a r i t h m i c g o l d c o n c e n t r a t i o n s and no n - l o g a r i t h m i c magnetite c o n c e n t r a t i o n s show s i m i l a r t r e n d s . Behaviour of go l d r e l a t i v e to low-density minerals was i n v e s t i g a t e d using t r a n s f o r m * 3 . The a b i l i t y of a tr a n s f o r m to e l i m i n a t e h y d r a u l i c e f f e c t s can be evaluated u s i n g : 1) v i s u a l examination of downstream p r o f i l e s and 2) geometric mean c o n c e n t r a t i o n r a t i o s . The best transform y i e l d s a low GMCR (imp l y i n g r e d u c t i o n of w i t h i n s i t e v a r i a b i l i t y ) and v i s u a l l y shows an anomaly decay curve. 5.3.1 Transform <t>x T h i s r a t i o r e p r e s e n t s r e - c o n v e r s i o n of the data back to the o r i g i n a l form rep o r t e d by the l a b o r a t o r y (case x=y, Table 5-5). I n t u i t i v e l y , GMCRs should be lower than f o r gold c o n c e n t r a t i o n expressed r e l a t i v e to the weight of sediment s i n c e the d e n s i t y of heavy-mineral concentrates i s c l o s e r to th a t of gold and a d d i t i o n a l random v a r i a b i l i t y due to l a b o r a t o r y p r o c e s s i n g e r r o r s d u r i n g s i e v i n g and weighing are not i n c l u d e d . However, GMCRs are reduced but remain high (GMCR*14 f o r -140+200-mesh). -169-' 1 1 - 1 4 0 - 2 0 0 - 2 0 0 - 2 7 0 - 2 7 0 Fraction (mesh) F i g . 5-8. C o e f f i c i e n t s of v a r i a t i o n f o r t r a n s f o r m (Table 5-5, case x=y only) by sediment d e p o s i t type and f r a c t i o n . Curve 1 = magnetite-poor sand d e p o s i t s , curve 2 = m a g n e t i t e - r i c h sand d e p o s i t s , curve 3 = sandy g r a v e l d e p o s i t s . -170-C o e f f i c i e n t s of v a r i a t i o n were c a l c u l a t e d to produce a s i m i l a r diagram ( F i g . 5-8) to t h a t presented i n F i g . 5-4. CVs are reduced by up to 60% i n some cases. Very low CVs (R;10%) were obtained f o r -140 + 200-mesh heavy-mineral concentrates from sandy g r a v e l d e p o s i t s (Curve 3, F i g . 5-8) implying a c o n s i d e r a b l e r e d u c t i o n i n between s i t e v a r i a b i l i t y . Thus, t h i s f r a c t i o n appears to be the best f o r s t u d i e s where between s i t e v a r i a b i l i t y must be low. For cases where the f u l l range of x and y are used (Table 5-5), i t was found t h a t there i s no a d d i t i o n a l advantage i n r a t i o i n g to the weight of concentrate i n some f r a c t i o n other than t h a t analysed. 5.3.2 Transform 4>2 T h i s t r a n s f o r m i s suggested by downstream p r o f i l e s i n which a r i t h m e t i c magnetite trends are very s i m i l a r to l o g a r i t h m i c gold content t r e n d s , thus the r a t i o should e l i m i n a t e h y d r a u l i c e f f e c t s i f the e f f e c t s a ct s i m i l a r l y on a l l p a r t s of the reach. C a l c u l a t i o n of GMCRs i s not p o s s i b l e i n t h i s case s i n c e a l o g a r i t h m of a l o g a r i t h m must be taken. However, as an i n d i c a t i o n of the d i f f e r e n c e between sediment types mean t r a n s f o r m a t i o n r a t i o s were c a l c u l a t e d : MTR = E(T R t / n ) ; where TR± = $ 2 , 8 A N D Y S R A W E L . $ 2 , B A N D -171-F i g . 5-9. Downstream p r o f i l e for *a (case x=-140+200-mesh, y=-200+270-mesh). F i l l e d c i r c l e s = sandy g r a v e l d e p o s i t s , Open c i r c l e s = magnetite-poor sand d e p o s i t s , h a l f - f i l l e d c i r c l e s = magnetite-enriched sand d e p o s i t s . S i t e J i s the upstream end of the reach which i s 5050 m i n l e n g t h . -172-The MTRs f o r a l l p o s s i b l e combinations of x and y are very c l o s e to u n i t y with low standard d e v i a t i o n implying t h a t w i t h i n s i t e v a r i a b i l i t y i s reduced. Downstream p r o f i l e s of * 2 show that g r a v e l and sand r e s u l t s are ve r y s i m i l a r , e x p l a i n i n g the low MTRs ( F i g . 5-9). In a d d i t i o n , there i s a peak i n values of the tr a n s f o r m at S i t e F followed by steady downstream decay. T h i s t r a n s f o r m appears to i n d i c a t e t h a t the c o n d i t i o n s l e a d i n g to the formation of h i g h - d e n s i t y mineral c o n c e n t r a t i o n s i n sandy g r a v e l s and sands are l i n k e d . 5.3.3 Transform 4>3 I t i s a n t i c i p a t e d that c e r t a i n gold f r a c t i o n s are h y d r a u l i c a l l y e q u i v a l e n t to low d e n s i t y mineral (quartz and f e l d s p a r ) f r a c t i o n s . However, no a d d i t i o n a l i n f o r m a t i o n can be gained from cases where x/y (Table 5-5) over the case where x=y. 5.3.4 Other transforms I n t e r - r e l a t i o n s h i p s between sandy g r a v e l and sand d e p o s i t s sediments are shown by r a t i o s such as *t>*t- — I J t A u , - 1 4 0 + 2 0 0 , S A N D Y G R A V E L W t s E D , - 4 0 + 7 0 , S A N D The r e s u l t i n g downstream p r o f i l e i s very s i m i l a r to the downstream p r o f i l e f o r Au i n -140+200-mesh sediment being -173-very smooth, with major departures f o r m a g n e t i t e - r i c h sands ( F i g . 5-10). 5.3.5 R a t i o s : i m p l i c a t i o n s f o r e x p l o r a t i o n E m p i r i c a l data transforms suggested by raw data trends i n d i c a t e t h a t : 1) Gold c o n c e n t r a t i o n s i n -140+200-mesh sediment from sandy g r a v e l d e p o s i t s expressed r e l a t i v e to the weight of heavy mineral concentrate i n the same f r a c t i o n have the lowest between s i t e v a r i a b i l i t y 2) Within s i t e v a r i a b i l i t y can be reduced c o n s i d e r a b l y by d i v i d i n g l o g transformed gold c o n c e n t r a t i o n s by magnetite c o n c e n t r a t i o n . 3) No a d d i t i o n a l d i s p e r s i o n t r a i n i n f o r m a t i o n can be obtained by e x p r e s s i n g gold c o n c e n t r a t i o n i n one f r a c t i o n r e l a t i v e to the weight of sediment i n another f r a c t i o n . 5.4 Conclus ions 1) At H a r r i s Creek s i x t y kilograms of -10-mesh sediment a l l o w a s u f f i c i e n t r e d u c t i o n i n rare g r a i n e f f e c t s t h a t gold c o n c e n t r a t i o n trends i n two f i n e sand f r a c t i o n s can be d i s c u s s e d with r e f e r e n c e to h y d r a u l i c e f f e c t s . 2) Gold c o n c e n t r a t i o n s i n -270-mesh sediments are p o o r l y r e p r o d u c i b l e due to the occurrence of Au i n the -174-0.20 F i g . 5-10. Downstream p r o f i l e f o r t r a n s f o r m c a l c u l a t i o n d e s c r i b e d i n t e x t . -175-f r a c t i o n as one or two s i l t s i z e d gold p a r t i c l e s . The s c a r c i t y of gold i n t h i s f r a c t i o n i s probably due to d i l u t i o n by s i l t - and c l a y - s i z e d sediment from the l a c u s t r i n e d e p o s i t at s i t e G. 3) Gold c o n c e n t r a t i o n s are p o s i t i v e l y c o r r e l a t e d with magnetite c o n c e n t r a t i o n s . However, Au c o n c e n t r a t i o n s i n sandy g r a v e l s do not in c r e a s e or decrease with d i s t a n c e downstream whereas magnetite c o n c e n t r a t i o n s i n c r e a s e downstream. Conversely, Au c o n c e n t r a t i o n s i n sand d e p o s i t s decrease r a p i d l y downstream, whereas magnetite c o n c e n t r a t i o n s remain f a i r l y c o n s t a n t . 4) Geometric mean c o n c e n t r a t i o n r a t i o s (Saxby and F l e t c h e r , 1986) show t h a t the r a t i o between heavy mineral c o n c e n t r a t i o n s f o r sandy g r a v e l and sand d e p o s i t s are grea t e r than u n i t y f o r the coarser f r a c t i o n s s t u d i e d . However, GMCRs approach u n i t y f o r a l l heavy mineral c o n c e n t r a t i o n s i n the -270-mesh sediment f r a c t i o n . For a given f r a c t i o n , GMCRs decrease as d e n s i t y decreases. 5) A n a l y s i s of v a r i a n c e and GMCRs i n d i c a t e t h a t w i t h i n s i t e v a r i a b i l i t y i s s u f f i c i e n t to mask between s i t e t r e n d s . Thus, i n e x p l o r a t i o n surveys, Au data obtained from both sandy g r a v e l and sand d e p o s i t s must not be combined f o r i n t e r p r e t a t i o n purposes. However, w i t h i n s i t e v a r i a b i l i t y can be reduced c o n s i d e r a b l y by -176-r a t i o i n g l o g transformed gold c o n c e n t r a t i o n s to the c o n c e n t r a t i o n of magnetite. The longest d e t e c t a b l e d i s p e r s i o n t r a i n , lowest between s i t e v a r i a b i l i t y and g r e a t e s t Au c o n c e n t r a t i o n s are found f o r gold c o n c e n t r a t i o n s i n -140+200-mesh non-magnetic heavy mineral c o n c e n t r a t e s . -177-CHAPTER 6: S E D I M E N T TRANSPORT AND S M A L L - S C A L E P L A C E R F O R M A T I O N , H A R R I S CREEK -178-6.0 I n t r o d u c t i o n R e s u l t s have been presented i n the previous chapter to show that heavy mineral c o n c e n t r a t i o n s a t H a r r i s Creek vary s y s t e m a t i c a l l y . In t h i s chapter, geochemical and sediment t e x t u r e data are used to generate a semi-q u a n t i t a t i v e h y d r a u l i c model for t r a n s p o r t of sediment and formation of s m a l l - s c a l e p l a c e r s i n g r a v e l bed r i v e r s . Major f e a t u r e s d i s c u s s e d i n the model can be d i v i d e d i n t o w i t h i n s i t e and between s i t e ( d i s p e r s i o n t r a i n ) f e a t u r e s (Table 6-1). 6.1 Database In a d d i t i o n to s e d i m e n t o l o g i c a l , geochemical and stream morphological data c o l l e c t e d at H a r r i s Creek i n 1986, Lynn Creek i n North Vancouver, B r i t i s h Columbia was v i s i t e d b r i e f l y i n March 1987 to o b t a i n f u r t h e r data on stream bed geometry at d i f f e r e n t l o c a t i o n s a c r o s s g r a v e l bars. 6.2 A sediment t r a n s p o r t model ( E i n s t e i n , 1950). Graf (1971) and Knighton (1974) reco g n i z e three types of sediment t r a n s p o r t model. However, the model of E i n s t e i n (1950) u n i q u e l y recognizes t h a t sediment t r a n s p o r t i s a s t o c h a s t i c process due to the nature of sediment p a r t i c l e movement. For t h i s reason, the model -179-Table 6-1. Major s e d i m e n t o l o g i c a l and geochemical f e a t u r e s . A. Within s i t e f e a t u r e s : Sandy g r a v e l : - t r i m o d a l s i z e frequency d i s t r i b u t i o n s w e l l - d e f i n e d t h r e s h o l d diameter between sand and g r a v e l , s u r f a c e pavement. bimodal s i z e frequency d i s t r i b u t i o n s t h r e s h o l d diameter between sand and s i l t . c o n c e n t r a t i o n i n g r a v e l g r e a t e r than c o n c e n t r a t i o n i n sand, sands are o c c a s i o n a l l y heavy mi n e r a l r i c h . heavy mineral c o n c e n t r a t i o n s are p o s i t i v e l y c o r r e l a t e d , f o r a l l sediment types. GMCRs decrease as sediment diameter decreases. GMCRs f o r a given diameter decrease as d e n s i t y of minerals decrease, f i n e high d e n s i t y m i n e r a l p a r t i c l e s (gold) appear to behave the same as coars e r intermediate d e n s i t y p a r t i c l e s . B. D i s p e r s i o n t r a i n f e a t u r e s - c o n c e n t r a t i o n r a t i o s i n c r e a s e downstream. - gold CRs increase at the expense of de c r e a s i n g c o n c e n t r a t i o n s i n sands whereas magnetite CRs increase due to i n c r e a s i n g c o n c e n t r a t i o n s i n g r a v e l s . Sand: Heavy mi n e r a l s : -180-shows the best agreement with n a t u r a l measurements i n both sand and g r a v e l r i v e r s (Graf, 1971; Parker et a l . , 1982) though Y a l i n (1972) o f f e r s some fundamental c r i t i c i s m s and m o d i f i c a t i o n s . In the model, p r o b a b i l i t y and h y d r a u l i c theory and e m p i r i c a l o b s e r v a t i o n s from flumes are combined to produce a bed m a t e r i a l load f u n c t i o n . That i s , c a l c u l a t i o n s y i e l d the t r a n s p o r t r a t e s of sediment that a t some time makes co n t a c t with the bed. These r a t e s are determined by the geometry of the bed at the l o c a t i o n of i n t e r e s t , whereas the r a t e of t r a n s p o r t of sediment i n suspension but not i n cont a c t with the bed (washload) i s determined by the c h a r a c t e r i s t i c s of the drainage b a s i n upstream of the l o c a t i o n . In g r a v e l bed r i v e r s almost a l l coarse sediment moves by con t a c t with the bed. 6.2.1 D e r i v a t i o n of E i n s t e i n ' s (1950) bed m a t e r i a l  f u n c t i o n The f u l l d e r i v a t i o n of the f u n c t i o n i s very long, t h e r e f o r e o n l y the important p h y s i c a l aspects w i l l be presented here. The formula i s based on experimental evidence t h a t i n d i c a t e s that there i s an intimate r e l a t i o n s h i p between the bed and the bedload (Graf, 1971): 1) A steady and i n t e n s i v e exchange of p a r t i c l e s i s observed between the moving bed load and the bed. 2) I n d i v i d u a l p a r t i c l e s move i n quick steps with -181-long r e s t p e r i o d s . 3) The average s t e p made by any p a r t i c l e appears to be independent of the flow c o n d i t i o n , the t r a n s p o r t r a t e , and the bed composition. 4) D i f f e r e n t t r a n s p o r t r a t e s can be achieved by a change of the average time between two steps and of the t h i c k n e s s of the moving l a y e r . Thus, E i n s t e i n (1950) assumes t h a t the number of p a r t i c l e s d e p o s i t e d per u n i t area i s c o n t r o l l e d by the u n i t jump l e n g t h of a p a r t i c l e . The p r o b a b i l i t y of e r o s i o n depends on the l e v e l of turbulence s i n c e i f the instantaneous shear s t r e s s exceeds the c r i t i c a l shear s t r e s s the p a r t i c l e w i l l be e n t r a i n e d . The d i s t r i b u t i o n of shear s t r e s s due to t u r b u l e n t f l u c t u a t i o n s i s modelled using a Gaussian normal d i s t r i b u t i o n . If the p r o b a b i l i t y of e r o s i o n of a p a r t i c l e i s s m a l l (e.g. d u r i n g low sediment t r a n s p o r t ) , then d e p o s i t i o n i s p o s s i b l e anywhere, however at g r e a t e r t r a n s p o r t r a t e s d e p o s i t i o n a f t e r u n i t jump length i s l e s s probable due to i n t e r a c t i o n with other p a r t i c l e s . Under these c o n d i t i o n s , p o t e n t i a l d e p o s i t i o n a l s i t e s are r a r e ( S l i n g e r l a n d and Smith, 1986). E i n s t e i n uses t h i s hypothesis to i n c l u d e the e f f e c t of r e l a t i v e sediment t r a n s p o r t i n t e n s i t y on the average jump length e x p r e s s i o n . Hence, a dimensionless bedload equation i s d e r i v e d i n terms of the p r o b a b i l i t y of e r o s i o n (p). -182-A n a l y t i c a l l y , a second e x p r e s s i o n f o r p i s found by n o t i n g t h a t the l i f t must overcome the e f f e c t i v e weight. Using three e m p i r i c a l c o r r e c t i o n s : (1) f o r the flow v e l o c i t y d i s t r i b u t i o n adjacent to h y d r a u l i c a l l y smooth and rough boundaries, (2) f o r the l i f t a c t i n g on s m a l l p a r t i c l e s s h i e l d e d by l a r g e r p a r t i c l e s and (3) f o r the l i f t due to d i f f e r e n t roughnesses expressed as a c h a r a c t e r i s t i c roughness diameter ( D a s ) , E i n s t e i n a r r i v e s at a bed load f u n c t i o n . The f u n c t i o n i s expressed i n terms of dimensionless sediment t r a n s p o r t i n t e n s i t y (*») and dimensionless stream flow i n t e n s i t y (ip*) ( F i g . 6-1). f B*W,-i/ n o 1 - 1 e~*" dt = p = A*** VK J - B * ^ _ i / n o 1+A*** where A*, B* and n C T are s o - c a l l e d " u n i v e r s a l c o n s t a n t s " to be determined by flume experiments. The d e t e r m i n a t i o n of t r a n s p o r t r a t e s i s a long, but s t r a i g h t f o r w a r d c a l c u l a t i o n . E i n s t e i n i n t r o d u c e s a suspended bed m a t e r i a l load f u n c t i o n by assuming t h a t the l i m i t of bed load t r a n s p o r t i s a t a height above the bed equal to two times the diameter of the sediment. The r e f o r e , the E i n s t e i n bed m a t e r i a l load f u n c t i o n i s not a true bed m a t e r i a l load formula s i n c e sediment t r a n s p o r t i s not determined as a continuum. -183-6.2.1.1 L i m i t a t i o n s of E i n s t e i n ' s (1950) bed load formula The f o l l o w i n g major l i m i t a t i o n s are of importance to t h i s study: (1) the formula was developed f o r non-cohesive sand - s i z e sediment though Parker e_t al^. (1982) i n d i c a t e that i t i s probably adequate f o r g r a v e l bed stream. (2) The geometry of a g r a v e l bed may be c o n s i d e r a b l y d i f f e r e n t from that of a sand bed thus E i n s t e i n ' s l i f t c o r r e c t i o n s may be erroneous. (3) The 4>*-^ U r e l a t i o n s h i p has been t e s t e d e x p e r i m e n t a l l y o n l y f o r p a r t i c l e d e n s i t i e s i n the range 1.052 to 4.22 (Chein, 1954) and, d e v i a t i o n s a t ve r y high d e n s i t i e s are unknown. (4) Choice of D a a as the c h a r a c t e r i s t i c diameter of the bed i s based on sand s i z e d i s t r i b u t i o n s and may be too low f o r bimodal sand and g r a v e l sediments. The same o b j e c t i o n a p p l i e s to the s e l e c t i o n of Des as the bed roughness d e s c r i p t o r . 6.2.2 Computer s o l u t i o n of the model 6.2.2.1 A p p l i c a t i o n of the formula Manual s o l u t i o n of the formula i s extremely t e d i o u s r e q u i r i n g i t e r a t i v e s o l u t i o n s and use of s e v e r a l e m p i r i c a l graphs. S h u l i t s and H i l l (1968) and Burkham §_t a l . (1977) wrote F o r t r a n programs using l i n e segment approximations f o r the graphs. F l e t c h e r (1986) modified the program f o r a microcomputer. For the purpose of t h i s study the program was r e w r i t t e n -184-i n TurboPascal to permit r a p i d m o d i f i c a t i o n of code and reduce the time taken f o r a bed load d e t e r m i n a t i o n . The program was checked f o r e r r o r s u s i n g E i n s t e i n ' s o r i g i n a l example (Big Sand Creek, M i s s o u r i ) . S e v e r a l m o d i f i c a t i o n s to the program were attempted to provide a b e t t e r model of behaviour i n g r a v e l bed streams, p a r t i c u l a r l y with r e s p e c t to bed geometry. However, understanding of sediment t r a n s p o r t mechanisms under these c o n d i t i o n s i s l i m i t e d and cannot be used to improve the model. 6.2.2.2 Numerical values f o r h y d r a u l i c parameters Parameters to which values must be assigned are summarised i n Table 6-2. Normally, sediment t r a n s p o r t would be c a l c u l a t e d for the whole width of the stream at a gi v e n s t a t i o n . However, i n g r a v e l r i v e r s , e s p e c i a l l y a t sharp meander bends the range of sediment t e x t u r e s i s so wide t h a t no s i n g l e sample could c h a r a c t e r i s e the range of beds. T h e r e f o r e , a s i n g l e channel bar ( s i t e C) was s e l e c t e d to provide geometric parameters and sediment diameter frequency d i s t r i b u t i o n s . The width («6.1 m) of the bar was used to d e f i n e the wetted perimeter, and v e l o c i t y and depth were estimated f o r v a r i o u s stages and used to d e f i n e the d i s c h a r g e . Channel slope was v a r i e d over the range encountered i n the study reach (0.02 to -185-Table 6-2. Parameters to which numerical values must be assigned ( E i n s t e i n ' s (1950) bed m a t e r i a l f u n c t i o n ) . E q u i l i b r ium  F l u i d p r o p e r t i e s Kinematic v i s c o s i t y (1x10 s m 2/s) Density (1000 kg/m 3) Flow p r o p e r t i e s Discharge (Q) V e l o c i t y (v) Wetted perimeter ( P B ) Slope ( S E) Sediment p r o p e r t i e s S p e c i f i c g r a v i t y ( S s ) Size d i s t r i b u t i o n ( f ( D ) ) P e r c e n t i l e s ( D a s , D & S 5 / o p t i o n a l l y p re-defined) F a l l v e l o c i t y ( v Q / from Y a l i n (1972), p.70) Other p r o p e r t i e s G r a v i t y (9.8 m/s 2) "Non-equilibrium" ( i t e r a t i v e s o l u t i o n ) Above parameters Number of i t e r a t i o n s Percent sediment removed each i t e r a t i o n P e r c e n t i l e s ( D 3 9 , D B S constant or v a r y i n g as bed modified) -186-0.04). C l e a r l y , v a r i a t i o n of h y d r a u l i c r a d i u s ( s l o p e , d i s c h a r g e , v e l o c i t y ) i m p l i e s t h a t the s i z e d i s t r i b u t i o n of the bed changes, however c e r t a i n c h a r a c t e r i s t i c s must be kept constant to monitor v a r i a b l e parameters. Th e r e f o r e , sediment t e x t u r e s at s i t e C were used as the b a s i s f o r a l l m o d e l l i n g (Table 6-3). 6.3 With-in s i t e processes 6.3.1 Sediment motion d u r i n g a f l o o d In November 1985, d u r i n g average stream d i s c h a r g e sediment was not v i s i b l y moving over the bed of H a r r i s Creek. S i m i l a r l y , i n March 1987, Lynn Creek was not t r a n s p o r t i n g sand-size or coars e r m a t e r i a l . Andrews (1982) presents a histogram of sediment s i z e s caught i n a sediment t r a p i n d i c a t i n g t h at at low di s c h a r g e s s m a l l amounts of the f i n e s t sediment move over the g r a v e l bed. At very high d i s c h a r g e s a l l sediment s t a r t s t o move d u r i n g a very s h o r t i n t e r v a l of time. These o b s e r v a t i o n s i n d i c a t e t h a t v e r y coarse g r a v e l pavement prevents e r o s i o n of subsurface f i n e sediment at low d i s c h a r g e s . However, as d i s c h a r g e i n c r e a s e s d u r i n g a f l o o d , the bed shear s t r e s s may e v e n t u a l l y exceed the c r i t i c a l shear s t r e s s of the pavement members. E r o s i o n of the pavement exposes sediment i n the subsurface, an e f f e c t reproduced i n Lynn Creek by a r t i f i c i a l l y d i s l o d g i n g c o b b l e - s i z e p a r t i c l e s . -187-Table 6-3. Sediment t e x t u r e s a t s i t e C used i n computations of bed m a t e r i a l load Sediment Sand Sandy g r a v e l Diameter (mm) Low High Low High D e n s i t y D e n s i t y Dens i t y D e n s i t y (% of (% of (% of (% of t o t a l ) t o t a l ) t o t a l ) t o t a l ) 128 0.0 0.0 8.6 0.0 64 0.0 0.0 23 .0 0.0 32 0.0 0.0 21.8 0.0 16 0.0 0.0 14 . 5 0.0 8 0.0 0.0 8 . 5 0.0 4 1.1 0.0 4 . 4 0.3 2 1.6 3.4 7 .1 3 . 4 1 46.8 16 . 3 8.7 16.3 0.5 40.1 36.1 2.2 36.0 0.25 6.7 29 .1 0.8 29 .0 0.125 2.4 11. 5 0.2 11.5 0.0625 0.8 1.7 0.1 1.7 0.0312 0.5 1.8 0.1 1.8 T o t a l (%) 100 .0 100.0 100 .0 100.0 Wt. (g) 60070 1121 999300 1125 -188-In the experiment san d - s i z e p a r t i c l e s were e n t r a i n e d from the r e s u l t i n g d e p r e s s i o n , t r a n s p o r t e d downstream a sh o r t d i s t a n c e and trapped between pavement stones. Thus, d u r i n g a f l o o d both sand and g r a v e l p a r t i c l e s may be m o b i l i s e d . E i n s t e i n ' s model does not agree with these o b s e r v a t i o n s i f the t o t a l sediment mixture i s permitted to d e f i n e the bed roughness. In t h i s case, sediment t r a n s p o r t r a t e s f o r the f i n e r sediment are very low ( F i g . 6-1) because the model a n t i c i p a t e s that the p a r t i c l e s cannot be eroded e a s i l y . T h i s i s due to the s h i e l d i n g e f f e c t of the l a r g e r p a r t i c l e s . By lowering the roughness diameter ( D S B ) by an order of magnitude, t r a n s p o r t r a t e s f o r sa n d - s i z e sediment are r a i s e d ( F i g . 6-1) i n d i c a t i n g that the bed roughness i s decreased by e r o s i o n of pavement c l a s t s . The model a l s o shows that sediment f i n e r than 0.5 mm i s t r a n s p o r t e d i n suspension although bed m a t e r i a l load t r a n s p o r t r a t e s f o r sediment f i n e r than 0.1 mm are very low. In r e a l i t y v ery f i n e sediment would probably pass through the s e c t i o n without c o n t a c t i n g the bed (washload). As the i n t e n s i t y of a f l o o d i n c r e a s e s , g r a v e l p a r t i c l e s are c o n t i n u a l l y eroded and w i l l r o l l i n t e r m i t t e n t l y down the stream whereas s a n d - p a r t i c l e s w i l l move by s a l t a t i o n and suspension. Discharges capable of d i s l o d g i n g l a r g e pavement c l a s t s probably l a s t f o r a few days at H a r r i s Creek d u r i n g the -189-3H CO -34 1 1 3 1 • 0.01 0.1 1 10 100 1000 Sediment Diameter (mm) F i g . 6-1. E f f e c t of i n c r e a s i n g bed roughness on sediment t r a n s p o r t r a t e s f o r sediment of d i f f e r e n t diameters. A l l curves were c a l c u l a t e d with v=2 m/s, Q=9.32 m=Vs, S E=0.03 / P B=6.1 m, S B=2.65. Sample C2 (sandy g r a v e l ) used to d e f i n e bed composition. Curve (1) Des=2 mm, D 3 a = l mm, (2) Dees=4 mm, D 3 =»=l mm, (3) D & a=8 mm, D3==l mm (4) Bed d e f i n e d geometry, i . e . D 6 a=40.6 mm, D 3 = s=13.0 mm. -190-s p r i n g f l o o d . As water flows decrease the sequence of d e p o s i t i o n of sediment determines the t e x t u r a l and geochemical c h a r a c t e r i s t i c s . T r a d i t i o n a l l y , i t i s impli e d t h a t a l l sediment ceases to move at the same time producing c h a o t i c , p o o r l y - s o r t e d d e p o s i t s . T h i s concept i s incompatible with observed heavy-mineral s o r t i n g i n g r a v e l d e p o s i t s and cannot e x p l a i n the genesis of pavement. 6.3.2 De n s i t y s o r t i n g and formation of s u r f a c e pavement Sev e r a l f e a t u r e s of g r a v e l d e p o s i t s i n d i c a t e s i z e and d e n s i t y s o r t i n g of sediment (Table 6-1). For example, c o n c e n t r a t i o n s of gold and magnetite are p o s i t i v e l y c o r r e l a t e d ( F i g . 5-2) and there i s a very c l e a r d i s t i n c t i o n between the framework and matrix components of the g r a v e l d e p o s i t s ( F i g . 4-5). Furthermore, there are l i n e a r r e l a t i o n s h i p s between magnetite c o n c e n t r a t i o n s and s o r t i n g of framework and mean s i z e of matrix ( F i g . 6-2A,B), (see s e c t i o n 4.1.2 f o r d e s c r i p t i o n of e s t i m a t i o n of these parameters) and there i s a p o s i t i v e c o r r e l a t i o n between matrix s o r t i n g and mean s i z e of framework ( F i g . 6-3). E i n s t e i n (1968) and Bechta and Jackson (1979) have used flume s t u d i e s to i n v e s t i g a t e the movement of f i n e sediment i n t o the spaces between g r a v e l c l a s t s . As the coar s e r 12 -191-# 9-o + O I CD c CO CO • M2 (A) • F2 0 02 OA 0i6 Log Framework Sorting (mm) 12i 9-+• o 5 6.-1 C CO CO 2 3-0.8 (B) M2 -0.3 -0.2 -0.1 0.0 Log Matrix Mean Diameter (mm) ~oi F i g . 6-2. R e l a t i o n s h i p s between (A) l o g of framework s o r t i n g and magnetite (-40+70-mesh) c o n c e n t r a t i o n (samples M2 and F2 appear to be o u t l i e r s from a c u r v i l i n e a r trend) and (B) l o g of mean diameter of matrix and magnetite (-40+70-mesh) c o n c e n t r a t i o n (omission of o u t l i e r M2 y i e l d s a Pearson l i n e a r c o r r e l a t i o n c o e f f i c i e n t (0.511) s i g n i f i c a n t l y d i f f e r e n t from zero with 86% c o n f i d e n c e ) . Data f o r a l l ten sandy g r a v e l sample are shown. -192-E S 2.5i CD •» CD E CO 5 c co CD 2 O CD E co O) o 2.0-1.5-1.0. B2 0.3 0.4 0.5 Log Matrix Sorting (mm) 0.6 F i g . 6-3. R e l a t i o n s h i p between log of matrix s o r t i n g and l o g of mean framework diameter. The Pearson l i n e a r c o r r e l a t i o n c o e f f i c i e n t (0.821) i s s i g n i f i c a n t l y d i f f e r e n t from zero with b e t t e r than 99% c o n f i d e n c e . Data f o r a l l ten sandy g r a v e l samples are shown. -193-sediment ceases to move i t w i l l form a porous framework through which water and f i n e sediment can flow. At high flow d i s c h a r g e s , the spaces between g r a v e l c l a s t s r e p r e s e n t l o c a t i o n s a t which d e n s i t y and s i z e s o r t i n g can occur. I t i s apparent that under these c o n d i t i o n s o n l y sediment s m a l l e r than the average s i z e of the spaces between the g r a v e l c l a s t s w i l l move i n t o the voids i n the sub s u r f a c e . Thus, the composite sediment d e p o s i t formed by f i l l i n g the voids has a d i s t i n c t i v e break i n the frequency d i s t r i b u t i o n of c l a s t s i z e s ( F i g . 4-5). The v o i d s w i l l f i l l from the base upwards (Bechta and Jackson, 1979) as i t i s here that flow v e l o c i t i e s w i l l be lowest. The spaces w i l l continue to f i l l u n t i l e i t h e r (a) there i s no sand to f i l l the v o i d s when the flow i s s u f f i c i e n t l y reduced or (b) f u r t h e r sediment d e p o s i t e d i n the s u r f a c e h a l f - v o i d s w i l l be eroded p r e v e n t i n g a d d i t i o n a l accumulation of sand. Evidence f o r the former was found a t Lynn Creek near the head of a channel bar. The g r a v e l d e p o s i t v o i d s contained a few p a r t i c l e s of sand to a depth of s e v e r a l centimetres ( F i g . 6-4A) whereas at a d i f f e r e n t l o c a t i o n the voids were completely f i l l e d ( F i g . 6-4B). At H a r r i s Creek i n some cases even the pavement i n t e r s t i c e s f i l l e d with sediment. E i n s t e i n s model shows th a t as flow v e l o c i t y i s reduced the same sediment load -194-F i g . 6-4. Examples from a bar on Lynn Creek of (A) sub-surface gravel voids incompletely f i l l e d by sand and (B) gravel voids completely f i l l e d by sand. Pen is approximately 15 cm long. -195-can be t r a n s p o r t e d i f the bed roughness i s decreased. T h i s can be achieved by f i l l i n g the l a r g e g r a v e l voids with sand. Thus, pavement formation i s e x p l a i n e d i n terms of lack of f i l l i n g of s u r f a c e v o i d s r a t h e r than the t r a d i t i o n a l concept of winnowing of s u r f a c e f i n e sediment (Graf, 1971), a process e a s i l y demonstrated i n a flume. The t r a d i t i o n a l , favoured process i s i n r e a l i t y very u n l i k e l y because i t r e q u i r e s that' sand i s d e p o s i t e d under high d i s c h a r g e c o n d i t i o n s and then subsequently removed under low flow c o n d i t i o n s (Hjulstrom, 1935). Parker and Klingemann (1982) proposed that pavement i s a f e a t u r e observable at a l l d i s c h a r g e stages and that the extreme p r o t r u s i o n of l a r g e c l a s t s coupled with h i d i n g of s m a l l c l a s t s i s r e q u i r e d to balance the m o b i l i t y of g r a v e l and sand s i z e m a t e r i a l . F o l l o w i n g Milhous (1973) they c l a i m that " v e r t i c a l winnowing" occurs. Removal of l a r g e c l a s t s a llows f i n e sediment to f a l l i n t o the hole and reduce the p r o b a b i l i t y of r e - e r o s i o n , thus f i n e sediment burrows to the s u b s u r f a c e . T h i s mechanism i s c o n t r a r y to the o b s e r v a t i o n a t Lynn Creek, mentioned e a r l i e r t h a t removal of l a r g e c l a s t s leads to e r o s i o n of f i n e sediment. Furthermore, by E i n s t e i n ' s (1950) arguments, there i s a high p r o b a b i l i t y t h a t the hole w i l l be f i l l e d by another c l a s t as i t r o l l s forward, l e a d i n g to a subsurface -196-d e p l e t e d i n f i n e sediment d u r i n g the the f l o o d peak. 6.3.3 Sand d e p o s i t s a t bar t a i l s Deep pools at bar t a i l s form due to p r e f e r e n t i a l e r o s i o n of g r a v e l s d u r i n g the f l o o d peak (Richards 1982). As water l e v e l s drop but flow v e l o c i t i e s g e n e r a l l y remain high enough to t r a n s p o r t sand, these pools become l o c a l s i t e s of d e p o s i t i o n due to very low flow v e l o c i t i e s . The sediment r e a c h i n g these pools may i n c l u d e some sediment t h a t f a i l e d to be d e p o s i t e d i n the g r a v e l s a s h o r t d i s t a n c e upstream but mostly r e p r e s e n t s m a t e r i a l swept around the deep pool on the o u t s i d e of the meander bed i n suspension. T h i s sediment presumably was not a f f e c t e d by the geometry of the stream bed i n the pool and i s probably s i m i l a r to the sand i n overbank d e p o s i t s observed a t H a r r i s Creek. 6.3.4 D i f f e r e n c e s between heavy mi n e r a l c o n c e n t r a t i o n s i n  sands and sandy g r a v e l s The m a j o r i t y of sand moving downstream w i l l not come i n t o c o n t a c t with the bed and sand caught i n bar t a i l p ools s e t t l e s out under g r a v i t y . A l l sediment s e t t l e s i n the near zero v e l o c i t y c o n d i t i o n s , thus enrichment of high d e n s i t y minerals cannot occur. However, sand t h a t passes c l o s e to or through the g r a v e l subsurface comes i n t o c o n t a c t with bed roughness elements p e r m i t t i n g s o r t i n g by -197-d e n s i t y . Because heavy minerals are p r e f e r e n t i a l l y d e p o s i t e d from sand p a s s i n g through the voids of the g r a v e l , t h i s n o n - e q u i l i b r i u m process cannot s t r i c t l y be modelled by an e q u i l i b r i u m model such as t h a t of E i n s t e i n . However, the r a t i o of l i g h t and heavy mineral t r a n s p o r t r a t e s p r i o r to r e a c h i n g the bed can be estimated ( F i g . 6-5) f o r a given g r a i n s i z e and compared to the same r a t i o f o r the rough regime encountered as the sediment and water pass through the g r a v e l bed. I t i s apparent t h a t heavy minerals are t r a n s p o r t e d at a r e l a t i v e l y lower r a t e and are trapped i n the g r a v e l bed ( F i g . 6-5A). The remaining sediment c o n t a i n s few heavy minerals and may be d e p o s i t e d i n a bar t a i l pool thereby enhancing low heavy m i n e r a l c o n c e n t r a t i o n s i n sand d e p o s i t s . Comparison of d e n s i t i e s shows that gold (S.G.=18) i s trapped to a much gr e a t e r extent than magnetite, which e x p l a i n s the d i f f e r e n c e i n geometric mean c o n c e n t r a t i o n r a t i o s ( F i g . 6-5B). Furthermore, as sediment diameter decreases, t r a n s p o r t r a t i o s do not change a p p r e c i a b l y as roughness i n c r e a s e s ( c f . curves 3 and 4 f o r magnetite, F i g . 6-5B) i n d i c a t i n g t h at GMCRs w i l l approach u n i t y a t small diameters. 6.3.2 Sediment s o r t i n g a f t e r a f l o o d A f t e r pavement has developed f u l l y , g r a v e l beds cannot -198-6 -I 0.01 0.1 1 10 100 Diameter (mm) Diameter (mm) F i g . 6-5. Comparison of t r a n s p o r t r a t i o s (low d e n s i t y / h i g h d e n s i t y ) with g r a i n s i z e , bed roughness and d e n s i t y . A l l curves computed with v=l m/s, Q=1.83 m 3/s, P©=6.1 m and D 3 S=1 mm. Since only sand i s i n motion, sample CI d e f i n e d the t r a n s p o r t a b l e sediment. (A) Transport r a t i o s f o r magnetite (S s=5.2). Curve (1) D&==8.0 mm, (2) 4.0 mm (3) 2.0 mm. (B) Transport r a t i o s for gold (So=18, Curves 1 (De»a=8 mm) and 2 (De=(=4 mm)) and magnetite (Curves 3 (Ds==8 mm) and 4 (Ds==4 mm)). - 1 9 9 -be m o d i f i e d f u r t h e r u n t i l t h e n e x t m a j o r f l o o d . H o w e v e r , s a n d d e p o s i t s a t b a r t a i l s become s i t e s o f e r o s i o n when w a t e r l e v e l s f a l l t o t h e e x t e n t t h a t s m a l l b a c k b a r c h a n n e l s wash a c r o s s t h e s a n d a n d waves f r o m t h e m a i n c h a n n e l l a p o n t o t h e d e p o s i t s . A t t h i s t i m e , v i s i b l e m a g n e t i t e l a g d e p o s i t s f o r m e x c e p t i o n a l h e a v y m i n e r a l c o n c e n t r a t i o n s . W . K . F l e t c h e r ( p e r s . comm.) m o d e l l e d s i m i l a r w i n n o w i n g p r o c e s s e s i n a M a l a y s i a n s t r e a m b y a d a p t i n g E i n s t e i n s m o d e l . A l t h o u g h t h e m o d e l w i l l n o t p e r m i t w i n n o w i n g t o be s t u d i e d b e c a u s e i t a s s u m e s t h a t a l l e r o s i o n i s b a l a n c e d b y d e p o s i t i o n , F l e t c h e r s i m u l a t e s d e p a r t u r e f r o m e q u i l i b r i u m b y a s s u m i n g a s m a l l a m o u n t o f n e t e r o s i o n . T h e d i f f e r e n c e b e t w e e n s e d i m e n t t r a n s p o r t r a t e s means t h a t l i g h t m i n e r a l s a r e r e m o v e d s l i g h t l y f a s t e r t h a n h e a v y m i n e r a l s , a n d o v e r t h o u s a n d s o f i t e r a t i o n s o f t h e m o d e l c a n l e a d t o s u b s t a n t i a l h e a v y m i n e r a l e n r i c h m e n t . As s i z e s o r t i n g a l s o o c c u r s , t h e m o d e l r e - c a l c u l a t e s t h e s e d i m e n t t e x t u r e p a r a m e t e r s ( D e = , D 3 = ) . S e v e r a l t e s t s o f t h e m o d e l were c o n d u c t e d t o i n v e s t i g a t e t h e c o n d i t i o n s a t w h i c h e x t r e m e h e a v y m i n e r a l c o n c e n t r a t i o n s ( s u c h a s t h a t a t s i t e D) w o u l d d e v e l o p . S a m p l e C I was u s e d a s t h e i n i t i a l s e d i m e n t m i x t u r e . The m o s t i m p o r t a n t r e s u l t was t h a t h i g h " c h a n n e l " s l o p e s (Se) w e r e n e e d e d t o d e v e l o p t h e h i g h h e a v y m i n e r a l c o n c e n t r a t i o n s o b s e r v e d a t some s i t e s ( F i g . 6 - 6 ) . T h e s e -200-10-Diameter (mm) F i g . 6-6. High density mineral enrichment of sand sample CI due to winnowing. Enrichment factor is expressed as concentration of high density mineral of a given diameter af ter 3000 i t era t ions (5% erosion of sediment at each i t era t ion) div ided by concentration before erosion begins. A l l curves determined with u=0.5 m/s and Q=0.305 m 3 . Curve (1), S E=0.2, Sa=18; (2), S e=0.2, S s=5.2; (3) S E=0.1, S e=18; (4) Se=0.1, S a=5.2. -201-s l o p e s are s e v e r a l times g r e a t e r than the average slope of H a r r i s Creek showing that l o c a l v a r i a b l e s c o n t r o l the g e n e r a t i o n of heavy mineral e n r i c h e d sands. Thus, extreme v a r i a b i l i t y i s introduced i n t o gold analyses of sand d e p o s i t s by p o s t - f l o o d processes, f o r example c o n c e n t r a t i o n r a t i o s f o r s i t e s where heavy mineral enrichment has occurred are l e s s than u n i t y , whereas CRs at non-enriched s i t e s are as high as 1291. The sampler g e n e r a l l y has no c o n t r o l over e s t i m a t i o n of these var i a b l e s . 6.4 Between s i t e processes 6.4.1 Heavy mineral trends The most apparent between s i t e v a r i a b l e i s channel slope ( F i g . 4-1). In the upper reaches, the slope averages 0.025, which s t e a d i l y i n c r e a s e s to 0.042 between s i t e s E and B. Slope then decreases to 0.022. Although three s e c t i o n s of d i f f e r i n g slope e x i s t , f o r the purpose of using a model such as E i n s t e i n ' s where departures from e q u i l i b r i u m may be i n v e s t i g a t e d by c o n s i d e r i n g changes i n important v a r i a b l e s , the reach may be c o n s i d e r e d as one s e c t i o n of i n c r e a s i n g slope followed by a s e c t i o n of d e c r e a s i n g s l o p e . Slope a f f e c t s the geometry of the bed. Therefore, any sediment not coming i n t o c o n t a c t with the bed w i l l not be -202-affected by small changes in channel s lope. Sand t r a v e l l i n g downstream in suspension w i l l only be contro l l ed by slope to the extent that flow v e l o c i t y w i l l s lowly decrease as slope decreases. It is expected that sediment caught in bar t a i l pools w i l l not r e f l e c t changes in channel slope and w i l l only r e f l e c t changes in supply of sediment to the reach. Thus, magnetite, which is supplied constantly to the study reach by the granodiori te on the south bank w i l l not be d i l u t e d or enriched in sand deposits with distance downstream ( F i g . 5-5). Conversely, gold is probably not supplied to the study reach in appreciable quant i t ies and w i l l be continuously d i l u t e d with distance downstream as indicated by the steep decay of the gold anomaly in sand deposits from over 100 ppb to sub-detection levels at s i t e s A and C ( F i g . 5-3 ) . The ef fect of slope on deposit ion of sediment in gravel deposits is complicated by how the geometry of the sediment subsurface is affected by slope changes. In th i s study, i t is assumed that sediment motion through a gravel bed is very s imi lar to sediment motion over a bed. E i n s t e i n ' s model indicates that as discharge of the stream decreases density transport ra t io s (low densi ty to high density) increase implying net deposi t ion of heavy minerals in the subsurface voids thereby decreasing bed -203-roughness. The model shows t h a t t r a n s p o r t r a t i o s would then decrease b a l a n c i n g the increase caused by d e c r e a s i n g d i s c h a r g e ( r e - e s t a b l i s h m e n t of e q u i l i b r i u m ) . Now, the e f f e c t of d e c r e a s i n g slope i s to increase t r a n s p o r t r a t i o s ( F i g . 6-7). At low slopes the stream cannot t r a n s p o r t heavy minerals to the same extent as at high slopes ( c f . curve A and B, F i g . 6-7). O v e r a l l , i g n o r i n g d i f f e r i n g supply of heavy minerals to d i f f e r e n t l o c a t i o n s , a s e c t i o n of i n c r e a s i n g slope r e s u l t s i n d e c r e a s i n g heavy mineral c o n c e n t r a t i o n s whereas a s e c t i o n of d e c r e a s i n g s l o p e causes enrichment of heavy minerals i n g r a v e l d e p o s i t s . T h i s l a t t e r r e s u l t agrees with the pr o s p e c t o r ' s r u l e t h a t a break i n slope i s a good l o c a t i o n f o r formation of heavy mineral p l a c e r s (Table 1-1). Magnetite c o n c e n t r a t i o n s decrease i n t o the br a i d e d s e c t i o n due to i n c r e a s i n g slope (to s i t e F) but in c r e a s e r a p i d l y i n the c o a r s e s t f r a c t i o n s (-35+70, -70+100-mesh, F i g . 5-5) downstream of s i t e F as slope decreases. On the other hand, gold which i s c o n t i n u a l l y d i l u t e d maintains constant c o n c e n t r a t i o n s downstream of s i t e F because extreme enrichment due to d e c r e a s i n g slope i s balanced by anomaly d i l u t i o n . 6.5 Summary and c o n c l u s i o n s Although g r a v e l d e p o s i t s commonly are d e s c r i b e d as very -204--1-1 i , r-0.01 0.1 1 10 Diameter (mm) F i g . 6-7. Effe c t of slope on magnetite transport r a t i o s . Conditions used for a l l curves are u=l m/s, Q=1.83m3/s/ DS!3=4 mm, D 3 a=l mm. Since only sand is transportable, CI was used to define the bed. (1) SE=0.02, (2) SE=0.03, (3) SE=0.04. - 2 0 5 -p o o r l y s o r t e d and have a c h a o t i c appearance, w e l l -c o r r e l a t e d heavy mineral c o n c e n t r a t i o n s f o r g o l d , magnetite and z i r c o n i n d i c a t e t h a t d e n s i t y s o r t i n g i n the f i n e f r a c t i o n s has occurred. T h i s o b s e r v a t i o n can be e x p l a i n e d by assuming that the d i s t i n c t i v e g r a v e l framework component i s mobile f o r a s h o r t p e r i o d of time a l l o w i n g f i n e sand and s i l t to become suspended i n the f l o o d waters. As the g r a v e l - s i z e c l a s t s cease motion they form a porous framework through which water and sediment can flow. The space between the g r a v e l c l a s t s are s i t e s s i m i l a r to s u r f a c e roughness that w i l l p r e f e r e n t i a l l y entrap heavy minerals ( F i g . 6-8). Thus, a l l g r a v e l d e p o s i t s are e n r i c h e d i n heavy m i n e r a l s . Sand d e p o s i t s mostly are depleted i n heavy minerals because they are l o c a t i o n s where p a r t i c l e s i n suspension are trapped and s e t t l e out under g r a v i t y without p r e f e r e n t i a l enrichment of heavy m i n e r a l s . An unknown p r o p o r t i o n of the sediment i n these d e p o s i t s r e p r e s e n t s the r e s i d u e of low d e n s i t y minerals f o l l o w i n g enrichment of heavy minerals i n g r a v e l s d e p o s i t s . However, sand d e p o s i t s may become s i t e s of heavy mineral enrichment i f they are exposed to wave a c t i o n from the main stream. This r e s u l t s i n winnowing of l i g h t m i n e r a l s . G e n e r a l l y , very high channel sl o p e s are r e q u i r e d to produce two or three orders of magnitude of enrichment, thus extremely -206-Coarse magnetite & fine Au trapped in gravel bed Fine magnetite, very fine Au & sand overpass bed STEEP GRADIENT No pronounced affect on washload Heavies transported GENTLE GRADIENT c 2 2 O * # « Heavies trapped F i g . 6-8. Diagrammatic summary of the processes i n v o l v e d i n d e p o s i t i o n of sediment i n H a r r i s Creek. (A) Within s i t e (at a p o i n t bar) and (B) between s i t e s (as channel g r a d i e n t d e c r e a s e s ) . -207-r i c h d e p o s i t s may be produced under c o n d i t i o n s completely d i f f e r e n t from those present i n the main channel. Downstream d i s p e r s i o n of heavy minerals appears to be a f u n c t i o n of both heavy mineral supply and channel slope ( F i g . 6-8). Magnetite i s s u p p l i e d c o n t i n u o u s l y from the south bank of the stream whereas gold presumably i s s u p p l i e d from a p o i n t source upstream of the study reach. Sediment t r a n s p o r t modelling shows t h a t as channel slope i n c r e a s e s the d i f f e r e n c e between t r a n s p o r t r a t e s of l i g h t and heavy minerals d i m i n i s h e s ( F i g . 6-8). Thus g r a v e l s tend to be magnetite-enriched toward the upstream end of the reach, but become deplet e d as slope i n c r e a s e s i n t o a braided s e c t i o n at s i t e F. As slope decreases f u r t h e r g r a v e l s become enr i c h e d i n magnetite. Because gold i s d i l u t e d with d i s t a n c e downstream, enrichment due to d e c r e a s i n g slope i s not v i s i b l e , r e s u l t i n g i n n e a r l y constant Au c o n c e n t r a t i o n s i n sandy g r a v e l d e p o s i t s . Since heavy minerals are not e n r i c h e d i n sand d e p o s i t s , these sediments r e f l e c t the supply of heavy minerals to any sampling s t a t i o n , t h e r e f o r e gold c o n c e n t r a t i o n s decay r a p i d l y but magnetite c o n c e n t r a t i o n s remain co n s t a n t . - 2 0 8 -CHAPTER 7: A P P L I C A T I O N S FOR M I N E R A L E X P L O R A T I O N I S T S -209-7.0 I n t r o d u c t i o n T h i s study p r o v i d e s the f o l l o w i n g general g u i d e l i n e s for o p t i m i s i n g the chances of success of stream sediment surveys i n e x p l o r a t i o n f o r coarse, n a t i v e gold d e p o s i t s . 7.1 F i r s t p r i n c i p l e s 7.1.1 Mode of occurrence of gold Before a sampling s t r a t e g y can be formulated the mode of occurrence of Au i n the t a r g e t m i n e r a l i s a t i o n must be understood. In common with a l l other geochemical surveys, an o r i e n t a t i o n study to determine the s i z e d i s t r i b u t i o n and mode of occurrence of gold should be c a r r i e d out. S p e c i f i c a l l y , the sampling problems encountered w i l l be s e v e r e l y complicated i f Au occurs as coarse, n a t i v e gold p a r t i c l e s . 7.1.2 Purpose of survey The purpose of the survey must be c l e a r l y d e f i n e d . That i s , w i l l the survey d e l i n e a t e broad r e g i o n a l trends or l o c a t e a mineral occurrence? 7.2 Regional surveys 7.2.1 Purpose Regional surveys g e n e r a l l y attempt to determine -210-presence or absence of gold i n a l a r g e drainage b a s i n . In which case, the chance that a sample f a i l s to c o n t a i n gold i n a m i n e r a l i s e d area must be e v a l u a t e d . 7.2.2 Sampling media Ten to 20 kg bulk sediment samples should be c o l l e c t e d by f i e l d s c r e e n i n g g r a v e l l y sediments to approximately 10-mesh. T h i s medium optimises the n a t u r a l enrichment of gold that occurs i n g r a v e l s . In regions where f i n e sediment i s scarce (e.g. Vancouver I s l a n d ) , the i n f o r m a t i o n d e r i v e d from low energy sandy d e p o s i t s may be b e t t e r than t h a t obtained from g r a v e l s . However, i t i s very important that the same type of sediment i s sampled c o n s i s t e n t l y otherwise w i t h i n s i t e v a r i a b i l i t y due to gold enrichment i n g r a v e l s w i l l mask between s i t e t r e n d s . 7.2.3 Sediment f r a c t i o n The sediment f r a c t i o n analysed depends on s e v e r a l f a c t o r s i n c l u d i n g the s i z e d i s t r i b u t i o n of gold i n the m i n e r a l i s a t i o n sought and the sediment s i z e d i s t r i b u t i o n of l o c a l s u r f i c i a l m a t e r i a l s such as t i l l and l a c u s t r i n e d e p o s i t s . However, i n t h i s study the -200+270-mesh and -270-mesh f r a c t i o n s allowed the number of gold p a r t i c l e s i n the sample to be optimised f o r s e v e r a l streams. There i s no p r a c t i c a l d i f f i c u l t y i n l a b o r a t o r y s i e v i n g of -200--211-mesh sediment from bulk f i e l d - s c r e e n e d sediment samples. 7.3 Follow-up surveys 7.3.1 Sample s i z e Very l a r g e samples may be r e q u i r e d i f a d i s p e r s i o n t r a i n i s to be d e l i n e a t e d . The s i z e of sample needed must be s e l e c t e d such t h a t random e r r o r s due to the nugget e f f e c t do not mask between s i t e anomaly d i l u t i o n t r e n d s . Sample s i z e can o n l y be adequately determined by an o r i e n t a t i o n survey. 7.3.2 Sampling media Screened samples c o l l e c t e d from sand d e p o s i t s appear to show Au anomaly d i l u t i o n , though i n t h i s study the anomaly decayed to s u b - d e t e c t i o n l e v e l s v ery r a p i d l y . Furthermore Au c o n c e n t r a t i o n s were low, reducing anomaly c o n t r a s t and a n a l y t i c a l r e l i a b i l i t y . Samples c o l l e c t e d from g r a v e l d e p o s i t s maintained high Au c o n c e n t r a t i o n s but d i d not show anomaly decay, however a sharp break i n the d i s p e r s i o n t r a i n curve would probably be more obvious than i n sands. Consequently, i t i s recommended t h a t g r a v e l s be sampled. 7.4 Sample p r e p a r a t i o n Samples must be p r e - c o n c e t r a t e d , f o r example using -212-methylene i o d i d e so t h a t a very l a r g e sample i s analysed, reducing the nugget e f f e c t . Removal of magnetite and other minerals f u r t h e r enhances Au c o n c e n t r a t i o n s . G r i n d i n g of samples i s not recommended as smearing of gold and between-sample contamination may r e s u l t . F i e l d sample and sediment f r a c t i o n weights should be recorded to a i d i n data i n t e r p r e t a t i o n . 7.5 Chemical a n a l y s i s Instrumental neutron a c t i v a t i o n a n a l y s i s c u r r e n t l y allows Au to be determined i n a 60 g heavy m i n e r a l concentrate p e r m i t t i n g f u r t h e r r e d u c t i o n of the nugget e f f e c t . 7.6 Data a n a l y s i s 7.6.1 Nugget e f f e c t s Using the weight of a sample and i t s gold c o n c e n t r a t i o n , the number of gold p a r t i c l e s i n the sample can be estimated. T h i s c a l c u l a t i o n provides an i n d i c a t i o n of random e r r o r s due to the nugget e f f e c t and shows whether or not ranking of gold anomalies f o r the purpose of follow-up i s s i g n i f i c a n t . 7.6.2 H y d r a u l i c e f f e c t s H y d r a u l i c e f f e c t s w i l l i ncrease v a r i a b i l i t y (noise) i n - 2 1 3 -the data, e s p e c i a l l y i f d i f f e r e n t sediment d e p o s i t s are sampled. Thus, these e f f e c t s can be minimised i n i t i a l l y d u r i n g f i e l d sampling. E x p r e s s i n g gold c o n c e n t r a t i o n s r e l a t i v e to the c o n c e n t r a t i o n of heavy minerals or magnetite w i l l f u r t h e r reduce data v a r i a b i l i t y due to l o c a l heavy mineral s o r t i n g . -214-B I B L I O G R A P H Y -215-Anderson, J.W. 1887. The p r o s p e c t o r s handbook. Crosby, Lockwood and Company. 132 pages. Andrews, E.D. 1983. 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P r e n t i c e - H a l l Inc. 718 pages. - 2 2 3 -APPENDIX: CHEMICAL AND SEDIMENTOLOGICAL DATA -224-ORIENTATION SAMPLING WEIGHT OF SEDIMENT FRACTIONS (GRAMS) (CHAPTER 2) FRACTION (ASTM MESH) SAMPLE — _ NUMBER -4 -16 -50 -70 -100 -140 -200 -270 + 16 +50 +70 +100 +140 +200 +270 85 -SD- 01 13080 6660 374 .0 183 .0 105 .0 55 .3 49 .0 154 .0 85 -SD- 02 16200 2380 106 . 0 48 .3 25 .6 10 .9 42 .4 42 . 4 85 -SD- 03 15670 2140 106 .0 56 .8 35 . 3 19 .9 19 .7 73 .0 85 -SD- 04 14400 5460 316 .0 110 .0 61 .0 24 .7 19 .4 44 .9 85 -SD- 05 15000 3 46 0 154 .0 59 . 7 30 . 3 12 .7 9 . 5 22 .0 85 -SD- 06 12800 4760 336 . 0 145 .0 86 .4 34 . 8 29 .7 82 .5 85 -SD- 07 9850 10200 644 . 0 283 . 0 152 . 0 65 .3 39 . 2 172 .0 85 -SD- 08 11910 4100 137 .0 51 . 6 27 .3 13 .8 10 . 6 49 .7 85 -SD- 10 3510 15700 2060 .0 1080 . 0 641 . 0 209 .0 144 .0 292 .0 85 -SD- 11 12400 13900 1000 .0 441 . 0 225 .0 75 .1 46 . 8 96 .9 85 -SD- 12 14800 10100 995 .0 550 .0 330 . 0 122 .0 124 .0 585 .0 85 -SD- 13 14400 9090 948 . 0 479 .0. 289 .0 111 . 0 112 . 0 308 .0 85 -SD- 14 11500 8310 658 . 0 327 . 0 189 .0 86 .9 81 . 4 238 .0 85 -SD- 15 13200 14000 2820 . 0 639 .0 332 .0 193 .0 67 .8 53 .2 WEIGHT OF HEAVY MINERAL CONCENTRATES (CHAPTER 2) FRACTION (ASTM MESH) SAMPLE — — — NUMBER -50 -70 -100 -140 -200 +70 +100 +140 +200 +270 85-SD-01 17 . 50 10 . 90 6 .03 2. 330 1. 810 85-SD-02 3. 02 1. 54 0 . 77 0 . 269 0 . 146 85-SD-03 6 . 78 3 . 70 2 .04 0 . 797 0 . 635 85-SD-04 21. 90 8. 94 4 .08 1. 100 0 . 449 85-SD-05 13 . 20 5. 78 2 .68 0 . 817 0. 323 85-SD-06 5. 78 2. 83 1 . 31 0 . 528 0 . 393 85-SD-07 21. 40 8 . 99 4 .99 2 . 350 0 . 532 85-SD-08 4 . 38 1. 44 0 .65 0. 202 0 . 086 85-SD-10 204 . 00 252 . 00 219 .00 67 . 300 33 . 200 85 -SD- l l 117. 00 77 . 20 41 .40 10. 100 3. 090 85-SD-12 122 . 00 115. 00 60 . 30 12 . 100 5. 410 85-SD-13 12. 70 9 . 22 5 .25 2. 220 1. 600 85-SD-14 7 . 74 5. 53 3 .06 1. 150 0 . 696 85-SD-15 2 . 15 2 . 44 2 . 73 1. 190 0. 209 -225-GOLD IN HEAVY MINERAL CONCENTRATES (PPB). REPORTED DETECTION LIMIT IS 5 PPB OR GREATER DEPENDING ON THE SAMPLE (CHAPTER 2) FRACTION (ASTM MESH) SAMPLE — NUMBER -50 -70 -100 -140 -200 +70 +100 +140 +200 +270 85-SD-01 3200 4000 6200 53 6000 85-SD-02 5 35 5300 23000 150 85-SD-03 6 11 13 6000 21 85-SD-04 16 31 3400 27 3300 85-SD-05 3400 3700 35 480 2800 85-SD-06 350 190 8400 19000 2300 85-SD-07 870 4100 12000 6900 10000 85-SD-08 280 540 520 22000 500 85-SD-10 18/910 960/2300 380/1400 3700/2800 3100 85-SD- l l 1500/12 2100/850 720 1200 1600 85-SD-12 940/520 190/990 3000 7400 3600 85-SD-13 160 3000 13000 940 7800 85-SD-14 5 200 5 2900 6000 85-SD-15 5 250 1000 23 130 -226-GOLD IN LIGHT MINERAL SEPARATES (PPB) DETECTION LIMIT IS 5 PPB (CHAPTER 2). REPORTED SAMPLE NUMBER FRACTION (ASTM MESH) -50 + 70 -70 + 100 -100 + 140 -140 + 200 -200 + 270 •270 85-SD-Ol 5/5 5/5 5 5 5 5 85-SD-02 5 5 15 5 7 34 85-SD-03 5/5 5 5/5 20 19 5 85-SD-04 5/5 8 10 18 5 15 85-SD-05 5/5 5/5 5 5 5 5 85-SD-06 5 11/16 8 ,„ 5 5 34 85-SD-07 5 5/18 14/21 5 31 110 85-SD-08 39 5 5 26 20 59 85-SD-10 5/9 5 5 11 210 320 85-SD- l l 5 5 5 13 5 89 85-SD-12 5/5 5 5/5 5/5 1100 5 85-SD-13 5 5 30 5 5/5 27 85-SD-14 10 5 8 5/5 5 50 85-SD-15 5 5 140 5 5 21 -227-AU (PPB) AND HF(PPM) RE-ANALYSES OF SELECTED SAMPLES BY INAA. ALL +270 FRACTIONS ARE HEAVY MINERAL CONCENTRATES. SAMPLE FRACTION AU HF (ASTM MESH) 1ST 2ND 1ST 2ND 85 -SD- 02 -200+270 150 180 45 42 85 -SD- 04 -200+270 3300 3000 210 200 85 -SD- 05 -50+70 3400 3900 3 2 85 -SD- 07 -70+100 4100 3200 1 1 85 -SD- 08 -100+140 520 520 4 4 85 -SD- 08 -140+200 22000 20000 6 5 85 -SD- 11 -270 89 80 17 19 85 -SD- 10 -270 320 380 43 55 85 -SD- 14 -270 50 57 4 4 85 -SD- 15 -50+70 12 25 10 8 -228-AU (PPB) RE-ANALYSIS OF SELECTED FRACTIONS BY FIRE ASSAY/ATOMIC ABSORPTION (FA/AA) SAMPLE FRACTION AU NUMBER (H=HMC) (L=NMS) FA/AA INAA 85 -SD- 08 -50+70H 260 280 85 -SD- 13 -50+70H 175 160 85 -SD- 14 -50+70H 140 140 85 -SD- 01 -50+70H 4720 3200 85 -SD- 10 -70+100H 50 6 85 -SD- 02 -70+100H 380 35 85 -SD- 14 -70+100L 40 6 85 -SD- 14 -70+100H 205 200 85 -SD- 03 -70+100H 350 11 85 -SD- 04 -70+100H 65 31 85 -SD- 08 -70+100H 20 250 85 -SD- 05 -70+100H 4225 3700 85 -SD- 06 -70+100H 300 190 85 -SD- 07 -100+140H 13320 12000 85 -SD- 01 -100+140H 6725 6200 85 -SD- 13 -100+140H 14395 13000 85 -SD- 02 -140+200H 20000 23000 85 -SD- 14 -140+200H 4650 2900 85 -SD- 05 -140+200H 740 380 85 -SD- 11 -140+200H 1885 1200 85 -SD- 14 -140+200H 10 6 85 -SD- 03 -140+200H 6500 6000 85 -SD- 06 -140+200H 22550 19000 85 -SD- 07 -200+270H 5580 10000 85 -SD- 05 -200+270L 5 9 85 -SD- 07 -200+270L 35 31 85 -SD- 10 -200+270H 3335 3100 85 -SD- 12 -200+270L 1000 1100 85 -SD- 12 -200+270H 3935 3600 85 -SD- 01 -200+270H 5950 6000 85 -SD- 06 -270 30 34 85 -SD- 02 -270 40 34 -229-HARRIS CREEK HARRIS CREEK FIELD SIEVED FRACTION WEIGHTS (GRAMS) FRACTION (MM) SAMPLE NUMBER -128 -64 -32 -16 -8 -4 +128 +64 +32 +16 +8 +4 +2 86 -SD -Ml 0 0 0 0 0 0 0 86 -SD--M2 0 0 4200 20400 22200 18400 14800 86 -SD - A l 0 0 0 0 0 0 0 86 -SD--A2 0 0 22200 39000 51000 39200 20000 86 -SD - C l 0 2000 6800 4200 2100 900 650 86 -SD--C2 0 27500 73800 69800 50400 27200 14200 86 -SD -KI 0 0 0 0 800 1000 950 86 -SD--K2 0 7400 29600 32000 31000 32800 35800 86 -SD -D l 0 1000 725 4625 6000 6900 6600 86 -SD -D2 0 35000 41500 40000 30500 15500 7000 86 -SD - B l 0 0 0 0 0 0 500 86 -SD -B2 0 4600 3600 13800 29000 27200 19200 86 -SD - F l 0 4400 1100 1200 1100 2700 6400 86 -SD -F2 81600 88500 105600 94200 67200 42600 20400 ,8 6 -SD - E l 7300 5100 400 350 600 750 1600 86 -SD -E2 46200 77000 6000 12800 10800 9200 7400 86 -SD - G l 0 0 0 500 500 1650 3175 86 -SD -G2 0 5800 36000 38000 29000 9000 18200 86 -SD - J l 0 0 1000 300 575 700 850 86 -SD -J2 112400 76000 58600 33800 25400 15000 10800 -230-HARRIS CREEK LAB SIEVED FRACTION WEIGHTS (GRAMS) FRACTION (ASTM MESH) SAMPLE NUMBER -10 -16 -40 -70 -100 -140 -200 -270 +16 +40 +70 +100 +140 +200 +270 86 -SD--Ml 2350 46900 15500 1990 1510 393 439 811 86 -SD -M2 13900 47200 3830 835 256 127 135 259 86 -SD-- A l 228 5790 41500 8540 3160 493 120 1110 86 -SD -A2 12800 29400 19600 2980 1410 395 140 628 86 -SD - C l 960 28100 24100 4010 1165 262 86 547 86 -SD -C2 22600 27800 8700 1520 352 213 66 339 86 -SD -KI 1350 16200 35300 3540 3620 2600 275 1790 86 -SD -K2 21300 27400 14600 2260 1050 267 87 322 86 -SD -D l 7540 29700 23100 3800 1310 288 127 872 86 -SD -D2 10600 21400 14000 4610 642 194 100 799 86 -SD - B l 1800 24800 25000 5100 2400 772 157 1200 86 -SD -B2 10000 39100 11600 919 519 159 83 495 86 -SD - F l 11800 20600 19400 5310 2520 493 200 1380 86 -SD -F2 13000 14300 9850 1940 789 172 80 830 86 -SD - E l 4590 27000 25400 4980 2760 406 158 1630 86 -SD -E2 8160 30800 9220 3230 1390 216 99 983 86 -SD - G l 6440 29400 20200 2360 250 174 75 358 86 -SD -G2 25100 26500 9680 1060 234 141 63 241 86 -SD - J l 1630 15900 26200 6370 3620 1570 362 1260 86 -SD -J2 18600 25000 13600 2419 406 292 130 558 -231-HARRIS CREEK NONMAGENTIC HEAVY MINERAL CONCENTRATE (NMHMC) WEIGHTS (GRAMS), AU CONCENTRATIONS (PPB) AND HF - CONCENTRATIONS (PPM). AU CONCENTRATIONS IN -270 MESH ARE GIVEN IN NON-MAGNETIC SEDIMENT (NMS). REPORTED AU DETECTION LIMIT IS 5 PPB. REPORTED HF DETECTION LIMIT IS 1 PPM. FRACTION (ASTM MESH) SAMPLE — NUMBER NMHMC WT NMHMC AU NMS AU NMHMC HF NMS HF -140 -200 -140 -200 270 -140 -200 270 + 200 + 270 + 200 + 270 A B + 200 + 270 A B 86 -SD- -Ml 23 .20 16 . 30 300 560 5 5 950 800 18 17 86 -SD -M2 11 .73 7 .93 3300 8900 140 62 1300 1100 26 25 86 -SD - A l 22 .10 5 .49 5 5 25 5 390 300 40 20 86 -SD -A2 20 .40 3 .99 19000 26000 20 11 1000 880 25 22 86 -SD -CI 11 .50 2 . 29 5 28 5 5 390 910 24 22 86 -SD ~C2 12 .72 2 .27 1500 590 5 13 540 780 14 22 86 -SD--KI 37 .50 17 . 20 170 350 5 11 500 470 21 18 86 -SD -K2 28 .90 3 .22 7500 350 30 51 1200 1000 23 16 86 -SD -DI 22 . 20 5 . 53 7200 13000 32 42 940 830 23 14 86 -SD -D2 16 . 00 7 . 17 5600 7000 31 60 620 570 24 19 86 -SD -BI 52 .63 3 . 43 90 3500 13 9 400 1400 22 19 86 -SD -B2 7 .28 1 .63 2600 2100 5 94 760 1200 18 32 86 -SD -FI 19 .60 7 . 49 1000 4700 5 5 420 320 19 31 86 -SD -F2 4 . 43 1 .14 1100 980 5 5 370 940 14 13 86 -SD - E l 19 . 80 5 .19 400 3400 16 15 440 400 16 16 86 -SD -E2 14 . 30 2 .34 5700 850 24 5 880 1000 11 17 86 -SD - G l 7 .60 2 .89 2400 9200 55 5 770 740 21 20 86 -SD -G2 5 .24 1 .06 5000 1300 5 0 1200 2300 25 -86 -SD - J l 60 .80 15 . 60 1025 3400 14 11 850 1100 36 31 86 -SD -J2 17 .10 3 .99 9900 6600 41 47 1200 890 27 28 -232-MAGNETIC MINERAL CONCENTRATIONS (%) IN HARRIS CREEK SEDIMENT FRACTION (ASTM MESH) SAMPLE — NUMBER -40 -70 -100 -140 -200 -270 +70 +100 +140 +200 +270 86-SD-Ml 0.6 2.4 4.5 5.8 2.7 0.8 86-SD-M2 9 . 4 23.2 17.4 14.1 7.7 1.9 86-SD-A1 0.4 1.2 1.7 2.4 2.6 1.4 86-SD-A2 3.7 13.0 11.3 5.6 3.4 1.2 86-SD-Cl 0.5 2.0 2.8 3.2 3.1 1.3 86-SD-C2 2 . 5 6 . 8 6.2 4 . 4 3.3 1.6 86-SD-K1 0.8 1.3 2.2 1.9 3.9 0.7 86-SD-K2 4 . 6 12 . 5 14 . 5 15 . 4 5.2 0.8 86-SD-D1 1.4 6.9 9.2 8.2 4 . 8 1.0 86-SD-D2 3 . 3 6 . 2 5.6 4 . 3 3 . 3 2.2 86-SD-B1 0.5 1.6 2.4 2.6 1.4 0.7 86-SD-B2 1.8 9.9 9.9 5.3 2.8 0.6 86-SD-F1 0.6 1.6 1.5 2.7 2.0 0.8 86-SD-F2 0.8 1.8 1.4 2.2 1.6 0.6 86-SD-E1 0.7 2.0 3.0 3.6 2.2 0.3 86-SD-E2 2.1 7.6 7.4 7 . 0 3 . 4 0.6 86-SD-G1 0.9 4 . 3 6.6 5.3 4.2 1.8 86-SD-G2 2 . 3 8 . 0 8 . 8 6 .1 3 . 7 2.0 86-SD-Jl 2.0 5.8 8.7 5.4 5.3 4.0 86-SD-J2 6 . 7 13 . 2 15.0 7 . 6 4 . 8 3.9 GOLD (PPB) IN HARRIS CREEK MAGNETITE FRACTIONS SAMPLE FRACTION AU NUMBER (ASTM MESH) 86-SD-Jl -140+200 5/23 86-SD-Jl -200+270 5 86-SD-Jl -270 5 -233-GOLD (PPB) IN WET-SIEVED SUB-FRACTIONS OF 85-SD-10 -270 MESH. FRACTION (U) WEIGHT (G) AU -53+44 39 .6 630 -44 + 30 12 .6 100 -30+20 9.2 57 -20+10 10.1 42 -10 15.1 57 

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