UNIVERSITY OF NEVADA Reno PLACER EXAMINATION PRINCIPLES AND PRACTICE A thesis submitted in partial fulfillment of the requirements for the degree of Engineer of Mines by John H. Wells June 1970 The thesis of John H. Wells is approved: Thesis advisor University of Nevada Reno June 1970 placer examination PRINCIPLES AND PRACTICE TECHNICAL BULLETIN 4 UNITED STATES DEPARTMENT OF THE INTERIOR BUREAU OF LAND MANAGEMENT Washington, D.C. July 1969 placer examination PRINCIPLES AND PRACTICE by John H. Wells Mining Engineer Bureau o f Land Management CONTENTS Page PREFACE ................................................................. ................. .......................... vii PART I - REVIEW OF PLACER THEORY AND GEOLOGY ACKNOWLEDGEMENTS 1. Placers Defined ........................................................................................ 3 The author is indebted to the Bureau o f Land 2. Study of Placers — General ................................................................. 3 Management for making the publication o f this book 3. Sources o f Valuable Mineral ................................................................. 3 possible. a) Lodes or mineralized z o n e s ............................................................. 3 A book of this type would not be complete without the b) Erosion o f pre-existing placer deposits ......................................... 3 illustrated description o f placer drilling and related data c) Low-grade auriferous conglomerates or glacial debris .............. 4 contained in Appendix D. These were provided by Mr. Cy d) Magmatic segregations and associated basic rocks ..................... 4 Ostrom of C. Kirk Hillman Company, whose generosity is e) Regional rocks containing particles of valuable mineral .... 4 appreciated. 4. Weathering and Release Processes ...................................................... 4 a) Ground water ..................................................................................... 4 b) Temperature change ........................................................................ 4 c) Plant growth ..................................................................................... 5 d) Surface erosion .................................................................................. 5 5. Stream Processes Related to Placers ................................................... 5 6. Concentration of Valuable Minerals ................................................... 6 a) Bedrock concentrations.................................................................... 6 b) Types of bedrock .............................................................................. 6 c) Pay streaks ......................................................................................... 6 7. Preservation o f the Deposit ................................................................. 6 a) A b a n d on m en t..................................................................................... 7 b) Regional uplift .................................................................................. 7 c) Burial ................................................................................................... 7 PART II - TYPES OF PLACERS 1. Residual Placers ..................................................................................... 11 2. Eluvial Placers ......................................................................................... 11 3. Stream Placers ......................................................................................... 11 a) Gulch placers ..................................................................................... 11 b) Creek placers ..................................................................................... 12 c) River d e p o s it s ..................................................................................... 12 d) Gravel plain deposits ........................................................................ 12 4. Bench P la c e r s ............................................................................................ 12 5. Flood Gold Deposits .............................................................................. 12 6. Desert Placers . 7. Tertiary Gravels 8. Miscellaneous Types a) Beach placers b) Glacial deposits For salo by the Superintendent ot Documents, c) Eolian placers . U.S. Qovornmont Printing Offlce Washington, D.C. 20402 - Price $1.50 PART III - SAMPLING AND EVALUATION PART VII - CHECK LIST FOR PLACER INVESTIGATIONS 1. General C on sid eration s............................................................................. 21 1. General Considerations ............................................................................ 79 a) Problems .............................................................................................. 21 2. Field Guide and Check List (example) ............................................. 80 b) Industry practice ................................................................................. 22 c) Minerals other than gold ................................................................... 22 PART VIH - GLOSSARY OF PLACERS TERMS ................................... 93 d) Other factors to be considered ........................................................ 22 e) In brief .................................................................................................. 23 APPENDIX 2. Sampling Guides ........................................................................................ 23 A. Placer Sampling Forms ............................................................................ 115 a) Reconnaissance .................................................................................... 23 B. Gold Price and Value Data ..................................................................121 b) Choosing a sampling method ............................................................ 23 C. Water Data .................................................................................................... 125 c) Number and size o f samples ............................................................ 24 D. Placer Drilling Data ...................................................................................129 E. General Information and Cost Data ....................................................143 3. Sampling Methods .................................................................................... 25 a) Existing exposures ............................................................................. 25 F. Conversion Tables ...................................................................................147 b) Hand-dug e x ca v a tio n s.......................................................................... 26 c) Machine-dug shafts ............................................................................. 28 d) Backhoe excavators ............................................................................. 31 e) Bulldozer trenches ............................................................................. 32 f) Sampling with churn drills ............................................................... 32 g) Bulk samples ........................................................................................ 43 h) Grab samples ........................................................................................ 43 i) Drift mine sampling ............................................................................. 44 j) Hydraulic mine sampling ................................................................... 44 4. Special Problems ........................................................................................ 45 a) Large rock or boulders ...................................................................... 45 b) Erratic high values ............................................................................. 46 5. Uncased or Small-diameter Drill H o l e s ................................................. 47 6. Salting ......................................................................................................... 48 7. When Sampling Fails ................................................................................. 49 PART IV - SAMPLE WASHING EQUIPMENT 1. General C on sid eration s............................................................................. 53 2. Miner’s Pan .................................................................................................. 53 3. Sluice Box .................................................................................................. 53 4. R o c k e r s ......................................................................................................... 55 5. Special Machines ........................................................................................ 58 6. Dry Washers .............................................................................................. 58 PART V - PANNING AND ASSAY PROCEDURES 1. P a n n in g ......................................................................................................... 63 2. General Notes on Panning with Suggestions For Improving Procedure ...................................................................... 64 3. Fire Assay o f Placer Samples — Misleading Results ......................... 65 4. Procedure For Determining Recoverable Gold in Placer Samples . 66 PART VI - NOTES ON GENERAL PRACTICE 1. Description o f S a m p le s ............................................................................