yi_ xu_n--n. REESE LIBRARY _- n n n_ % UNIVERSITY -OF. CALIFORNIA. ] - '-hrassion No. ' THE METALLURGY OF GOLD, NEW METALLURGICAL SERIES. EDITED BY W. C. ROBERTS- AUSTEN, C.B., F.R.S., Chemist and Assayer of the Royal Mint ; Prof, of Metallurgy, Royal Cottege of Science. 1. INTRODUCTION TO THE STUDY OF METALLURGY. By Prof. ROBERTS- AUSTEN, C.B., F.B.S. FOURTH EDITION 15s. 2. GOLD. By. T. K. ROSE, Assoc.R.S.M., D.Sc., Assistant- Assayer of the Royal Mint. THIRD EDITION. 3. IRON. By THOS. TURNER, Assoc.R.S.M., F.I.C. 16s. 4. STEEL. By F. W. HARBORD, Assoc.R.S.M., F.I.C. 5. SILVER AND LEAD. By HENRY F. COLLINS, Assoc.R.S.M., Assoc. Mem.Inst.C.E. 6. METALLURGICAL MACHINERY. BY H. JENKINS, Wh.Sc., Assoc.R.S.M., Assoc. Mem. Inst. C.E., of the Royal Mint. 7. ALLOYS. By the EDITOR. *** Other Volumes in Preparation. ELECTRIC SMELTING AND REFINING. A Practical Manual of the Extras tion and Treatment of Metals by Electrical Methods. Being the "ELEKTRO- METALLURGIE" of Dr. W. BOUCHERS. Translated from the Second German Edition by WALTER G. M'MILLAN, F.I.C., F.C.S., Secretary to the Institution of Electrical Engineers. With numerous Illustrations and Three Folding Plates. 21s > ASSAYING. By J. J. BE RINGER, F.C.S., F.I.C., and C. BERINGER, F.C.S. FIFTH EDITION. 10s. 6d. ELEMENTS OF METALLURGY; The Art of Extracting Metals from their Ores. By J. ARTHUR PHILLIPS, C.E., F.C.S., F.G.S., and H. BAUERMAN, F.G.S. With numerous Illustrations. THIRD EDITION. 36s. run miiikimbw . L IMWAAVUMJ *^^****Jwfc *w J- nvo* w* VJUA?] , SETTLERS, and all interested in t'ie Opening up and Development of 3. By S. HERBERT Cox, Assoc.R.S.M., M. Inst, M.M., F.G.S. With is. 5s. Leather, 6s. 6d. (Griffin's " New Land Series.") PROSPECTING FOR MINERALS : A PRACTICAL HANDBOOK for PROSPECTORS, EXPLORERS, SETTLERS L and all interested in t'ie Opening_up and Development of New lands. Illustrations. ORE AND STONE MINING. By C. LE NEVE FOSTER, D.Sc., F.R.S., Prof, of Mining, Royal College of Science. With very numerous Illustrations. In Large 8vo. SECOND EDI'J ION. 34s. ELEMENTARY MINING AND QUARRYING. By Prof. LE NEVE FOSTER, F.R S. In Small 8vo. With Illustrations. COAL MINING. By H. W. HUGHES, F.G.S., Assoc.R.S.M. With 490 Illustrations. THIRD EDITION. 18s. PRACTICAL GEOLOGY (Aids in). By G. A. J. COLE, F.G.S., Professor of Geology, Royal College of Science, Dublin. With numerous Illustrations. THIRD EDITION, Kevised and partly Re-written. ICs. 6d. MINE SURVEYING. By BENNETT H. BROUGH, F.G.8., formerly Instructor of Mine Surveying, Royal School of Mines. SIXTH EDITION. Illustrated. 7s. 6d. TRAVERSE TABLES ; computed to Four Places Decimals for every Minute of Angle up to 100 of Distance. By RICHARD LLOYD GURDEN, Authorised Surveyor for the Governments of !New South Wales and Victoria. FOURTH EDITION. 21s. MINE ACCOUNTS AND MINING BOOK-KEEPING. With very Numerous EXAMPLES taken from the ACTUAL PRACTICE of leading Mining Companies throughout the world. By JAMES G. LAWN, Assoc.R.S.M., Professor of Mining at the South African School of Mines, Capetown, Kimberley, and Johannesburg. Edited by Prof. Le Neve Foster. Large 8vo. 10s. 6d. BLASTING AND THE USE OF EXPLOSIVES. By 0. GUTTMAN, A.M.Inst. C.E. With Folding Plates and Illustrations. 10s. 6d. LONUOJf : CHARLES GRIFFIN & CO., LIMITED; EXETER STREET, S1HAJND. rS ^S I THE METALLURGY OF GOLD. T. KI11KE ROSE, D.Sc., ASSOCIATE OP THE ROYAL SCHOOL OP MINES ; FELLOW OP THE CHEMICAL SOCIETY ASSISTANT ASSAYER OP THE ROYAL MINT. BEING ONE OF A SERIES OF TREATISES ON METALLURGY, WRITTEN BY ASSOCIATES OF THE BOYAL SCHOOL OF MINES. EDITED BY prot TO. WITH NUMEROUS ILLUSTRATIONS. THIRD EDITION, REVISED AND ENLARGED. LONDON: CHARLES GRIFFIN AND COMPANY, LIMITED; EXETER STREET, STRAND. 1898. [All Rights Reserved.'} PBEPACE. IN the preparation of the THIRD EDITION, the text has again been revised and considerable additions made to it, in order to keep pace with the progress of the metallurgy of gold. In particular, the chapters on the Cyanide Process will be found to have undergone great changes. The frontispiece is from a photograph kindly lent by Mr. Alfred James, late of Johannesburg, and several other new illustrations have been added. ROYAL MINT, April, 1898. PREFACE TO THE SECOND EDITION, IN preparing this edition, the whole book has been care- fully revised, due attention being paid to some useful suggestions contained in the various reviews of the first edition. Eight new illustrations have been added, in- cluding the frontispiece, which is from a photograph kindly lent by Messrs. Eraser & Chalmers. The rapid progress made in improving the cyanide process has rendered it necessary to make considerable alterations in, and additions to, the chapters devoted to that subject, and a new chapter on Economic Considerations has also been added. The author desires to express his thanks to the old students of the Royal School of Mines and others, who have kindly sent information on work connected with the metallurgy of gold, now in progress in various parts of the world. ROYAL MINT, August, 1896. INTRODUCTION TO THE FIKST EDITION. As this is the first of a series of treatises devoted to indi- vidual metals, which is being prepared under my guidance it may be well to offer a few introductory remarks. Associates of the Royal School of Mines have taken their full share in conducting mining and metallurgical operations in all parts of the world, but, notwithstanding the wide experience they have gained, no treatise claiming to give a general account of the Metallurgy of Gold and adequately dealing with modern processes could hitherto have been attributed to a Student of the School. It may be claimed that in this country Dr. Percy founded the literature of Metallurgy, but his volume on " Silver and Gold," although unrivalled in accuracy of detail, is only a splendid fragment, and gold is alone dealt with in the sections devoted to the refining of bullion and to assaying. A large amount of valuable information concerning new methods and machinery used in the treatment of gold ores has appeared in various official publications during the last twenty years, and much is either scattered over the pages of the scientific press or incorporated in the proceedings of various learned societies. Any attempt, however, to study the subject as a whole from Vlll INTRODUCTION. these sources of information would be hopeless for those whose time is limited, or who have not good libraries at their disposal. Mr. Rose gained his practical experience of gold and silver extraction in the Western States of America, and has written a volume which should prove to be very useful, as its careful and conscientious preparation entitles it to confidence. W. C. ROBERTS-AUSTEN. ROYAL MINT, March, 1894. PREFACE TO THE FIRST EDITION. IN the present volume an effort has been made to supply a succinct summary of the existing condition of the metallurgy of gold, for the use of students and others who are interested in the industries connected with the precious metals. It has been said that mill-managers as a class have little to learn from books ; but, even if this be so, they will still need to keep themselves acquainted with the progress of their art in far distant countries. To them the bibliography appended will be useful. Although, in this book, brief accounts are given of some typical machines, attention has been directed rather to methods of procedure than to details of machinery, which will be dealt with in a separate volume in the series. The practice in particular extraction works has been described at some length, as such information, wherever it is available, is among the most valuable that can be given. Some recently devised methods of great importance in the metallurgy of gold, such as the MacArthur-Forrest cyanide process, the new barrel chlorination process, and the improved Gutzkow parting process, are given for the first time in a manual. Particular attention has been paid to the assay of gold bullion, the system in use at the Royal Mint and the precautions necessary to ensure X PREFACE. the highest attainable accuracy being described. Little space is devoted to the geographical and geological dis- tribution of gold ores, as this ground has been amply covered by other works. Moreover the processes of smelting, leaching, and pan-amalgamation used in the treatment of silver ores, whether they contain gold or not, have either been omitted or merely sketched in outline, as belonging to the metallurgy of silver. I arn indebted to the President and Council of the Institute of Civil Engineers for leave to reproduce Figs. Nos. 14, 16, 17, 32, 33, 35, and 36, and to Mr. John Murray, for Figs. 58, 60, 61, 62, 63, 64, 66, and 67, which are copied from Dr. Percy's Metallurgy. I also take this opportunity of expressing my thanks to my colleague, Mr. F. W. Bayly, and to other friends for their kind assistance while certain sections were going through the press. In conclusion, I desire gratefully to acknowledge the kindness of Prof. Roberts - Austen in giving valuable advice throughout the progress of the work. It was at his suggestion that the task was undertaken, and it is hoped that it will contribute to the realisation of his wish that the experience of School of Mines men should be collected in a number of works which will together form a comprehensive metallurgical series. T. K. ROSE. ROYAL MINT, March, 1894. CONTENTS. CHAPTER I. THE PROPERTIES OF GOLD AND ITS ALLOYS. PAGE INTRODUCTION, . . . 1 Physical and chemical proper- ties of gold, ... 2 Introduction, ... 2 Colour, .... 2 Malleability and ductility, . Hardness, 3 Tenacity, .... 3 Specific gravity, ... 4 Cohesion, .... 4 Specific heat, ... 4 Fusibility, .... 4 Spectrum, .... 4 Latent heat, . . 4 Magnetism, . Conductivity and expansion Atomic weight and volume, Volatility, . Crystallisation, Solubility, . Preparation of pure gold, Allotropic forms of gold, . Alloys of gold, . Amalgams, . Gold and silver, . copper, . Liquation, . PAGE 4 5 5 5 9 10 11 12 13 15 16 17 IS CHAPTER II. CHEMISTRY OF THE COMPOUNDS OF GOLD. Compounds of gold, Haloid compounds, Chlorides of gold, Proto- chloride, Auro-aurichloride Trichloride, . Chlor-auric acid, Bromides of gold, Proto-bromide, Auro-auric bromide, Tribromide, . Iodides of gold, Cyanides of gold, 20 21 21 21 21 21 27 21 27 28 28 28 28 Aurous cyanide, . Auro- cyanide of potassium, Auri-cyanide of potassium, Oxides of gold, Aurous oxide, Auric oxide, Aurates, . Fulminating gold, . Sulphites of gold, Hyposulphites of gold, Silicates of gold. Sulphides of gold, . Purple of Cassius, . 28 28 29 29 29 29 30 30 30 31 33 33 34 Xll CONTENTS. CHAPTER III. MODE OF OCCURRENCE AND DISTRIBUTION OF GOLD. Forms in which gold occurs in nature, .... 35 Vein gold, .... 35 Placer gold and nuggets, . 37 Calaverite, . . . .38 Sylvanite, .... 38 Petzite Nagyagite, . Composition of native gold, Geographical distribution of gold, . . . . Origin of gold ores, . PAGE 3* 38 39 39 42. CHAPTER IV. PLACER MINING SHALLOW DEPOSITS. Placer deposits, ... 43 Methods of obtaining gravel from shallow placers, . 44 Appliances used in washing the gravel, . .45 Pan, ... .45 Batea, ... .45 Prospecting trough, . 46 Horn- spoons, . . 46 Cradle, . .' .46 Long-torn, . .47 Puddling-tub, ." .47 Siberian trough, . . 48 Sluice, ... . 50 Sluicing, . . . .51 Use of drops, . . 52 ,, undercurrents, . 52 ,, mercury, . . 53 Cleaning-up, . .53 Tail-race, .... 54 Ground-sluice, ... 54 Booming, .... 54 Dry diggings, . . , .55 Cement gravels, ... 55 Tail sluices, .... 55 Fly catchers, .... 56 River mining, . . . .56 1. River mining proper, . 56 2. Dredging, ... 58 3. Deep bar mining, . . 58 Methods of working Siberian placers, .... 59 1. Siberian sluice, . -.. .. 60 2. Trommel, . . .61 3. Pan, . . . . 61 Beach mining, .... 62 New method of working shallow placer deposits, . . .62 CHAPTER V. DEEP PLACER DEPOSITS. Nature and mode of origin of deposits, .... Distribution of gold in the gravels, .... Origin of gold in the gravels, Minerals occurring in the gravels, Methods of working, " Hydraulicking," Commencement of opera- tions, .... Supply of water, Breaking down the bank, Sluicing gravel, *. . Cleaning-up, Disposal of tailings, . Drift mining, Shaft ,, Hydraulic elevator, Economic conditions, Shallow placer deposits, Deep 72 75 77 80 81 82 82 84 86 86 CONTENTS. X11I CHAPTER VI. QUARTZ CRUSHING IN THE STAMP BATTERY. Primitive methods of crushing and amalgamating, . Mortar, .... Maray, . . .91 Chilian mill, . . . Arrastra, Iron prospecting arrastra, Stamp battery, Rock-breakers, . Blake, Dodge, . Gates, Schranz, Material employed for crushing surfaces, . Position and use of rock- breakers, . . ",.' Battery proper, . ... Foundations, . . . Framework, PAGE 89 91 91 92 92 95 95 101 101 102 103 104 105 106 107 108 108 Mortar, , . ' Splash-box, Dies, PAGE 108 110 110 111 111 112 113 114 114 115 115- 120 12a 121 122" 123 124 126 > . 127 Cam-shaft Cams, Tappets, Stamp, Guides, Height of drop, Screens, . Order of fall of stamps, Automatic feeding, Stanford's feeder, Challenge Water supply, Amalgamated plates, . Mill site, General arrangement of th< stamp mill, . CHAPTER VII. AMALGAMATION IN THE STAMP BATTERY. Treatment of the amalgamated plates, 129 Discoloration of the copper plates, 130 Chemicals used to promote amalgamation, . . 131 Use of mercury, . . 132 Grade of plates, . .'132 Muntz metal plates, . 133 Shaking copper plates, . 134 Corrugated plates, . . 135 Mercury wells, . . 135 Galvanic action in amalgama- tion, . . . Designolle process, . Cleaning- up, . The clean-up barrel, pan, Retorting, Loss of mercury, ,, gold, . Non-amalgamable gold, . Gold in pyrites, 135 13& 137 13S 138 140 142 145 151 152 CHAPTER VIII. OTHER FORMS OF CRUSHING AND AMALGAMATING MACHINERY. Special forms of stamps, . Husband, Steam, . _ . . Elephant, Morison's high-speed stamp Huntington mill, Crawford ball mill, . Other ball mills, Tyrolean mill, . . . 155 ( Schemnitz mill, . 155 j Lazzlo amalgamator, 156 5 Pan-amalgamation, . 158 158 159 163 165 165 The old system, The Boss continuous system, Treatment of concentrates, . Molloy's hydrogen amalgamator, Jordan's amalgamator, . 165 166 167 168 171 172 173 174 XIV CONTENTS. CHAPTER IX. CONCENTRATION IN STAMP MILLS. PAGE Concentration, . . .175 Settling boxes, . . .177 Classification according to size, 178 Screens, . . . .178 Pointed boxes, . . .179 Early concentrating machinery, 180 Blanket strakes, . . .181 Riffled sluices, . . .182 Raising-gate concentrator, . ] 82 Round buddle, . . . Ib2 Centrifugal concentrators, . 183 Hendy s, . . .183 PAGE Duncan's, . . .183 Percussion tables, . . ] 84 Gilpin County concentrator, 184 Frue vanner, . . .185 Method of working, . 189 Riffle belted, . . 192 Embrey concentrator, . 192 Luhrig vanner, . .193 Hartzjigs, . . , 196 Pneumatic jig, . . 197 Clarkson and Stanfield's con- centrator, . . . .197 CHAPTER X. STAMP BATTERY PRACTICE IN PARTICULAR LOCALITIES. In California, . . . .199 In Colorado, .... 204 On free-milling ores in Australia and New Zealand, . . 208 In the Thames Valley, New Zealand, . . . .212 In Dakota, . . , . 215 In the Transvaal, . . . 216 CHAPTER XI. CHLORINATION : THE PREPARATION OF ORE FOR TREATMENT. The Plattner process, . . 224 Origin, . . . .224 Method of working at Reich- enstein, .... 225 Modern practice in chlorination, 226 Crushing, . . . .227 Krom's rolls, . . . 228 Rolls at Rapid City, Dakota, 231 Comparison between rolls and stamps, . . . 232 Drying the ore, . . . 233 Roasting, .... 234 Reverberatory furnace, . 235 Chemistry of oxidisingroasting, 238 Decomposition of various minerals, .... 240 Elimination of arsenic and antimony, . . .241 Use of salt, .... 242 Losses of gold, . . . 244 Mechanical furnaces, . . 246 1. With mechanical stirrers, 247 (a) O'Hara, . . 247 (b) Spence, . . 248 (c) Pearce Turret, 249 (d) Brown horse shoe, 250 (e i Ropp straight line, 252 2. With rotating bed, 252 3. Revolving cylinders, 255 (a) Bruckner, . 255 (b) Hofmann, . * 256 (c) White, . . 258 (d) White-Howell, 259 Use of producer gas in roast ing, . . . . . 260 CHAPTER XII. CHLORINATION : THE VAT PROCESS. Construction of vats, Charging-in, . Generation of chlorine, Impregnation, Reactions in vat, 261 263 264 266 267 Amount of chlorine required, . 269 Leaching, .... 270 Precipitation of gold, . . 271 Cost of working, . . .273 CONTENTS. XV CHAPTER XIII. CHLORINATION : THE BARREL PROCESS. PAGE History, 274 Mears process, . . . 275 Thies process, .... 277 Construction of barrel, . 280 Charging-in, . . .280 Amounts of chemicals re- quired, . . . .282 Method of leaching, . . 283 Mechanical difficulties in leach- ing, 284 Leaching by pressure, . . 284 Riotte's method, . . . 285 Precipitation of gold, . . 285 1. Soluble precipitants, . 286 Ferrous sulphate, . 286 PAGE Organic substances, . 287 Sulphuretted hydrogen, 287 Sulphurous acid, . 288 2. Solid precipitants, . . 289' Charcoal, . . .289 Insoluble sulphides, . 290 Metals, . . .291 Modern patent processes of chlorination, . . . 292 Newbery-Vautin, . . 292 Pollok, .... 292 Swedish, .... 29& Cassel, 295 Julian, . . . .295 Greenwood, . . . 295 CHAPTER XIV. CHLORINATION : Vat process 296 1. Butters' Mill, Kennel, Cali- fornia, .... 296 2. Plymouth Mill, Amador Co., California, . . 297 3. Treadwell Mine, Alaska, 298 Barrel process 1. Mears process at Deloro, Canada, . . .298 2. Thies process in Carolina, 302 PRACTICE IN PARTICULAR MILLS. Cost of working, . . 3. Modern barrel process at the Golden Reward Works, Deadwood, Dakota, . Cost of working, . . 4. Bromination Mill, Rapid City, Dakota, . . The Cassel -Hinman bromine process, .. Future of chlorination, 303 310- 311 314 CHAPTER XV. THE CYANIDE PROCESS. History, 319 Mac Arthur- Forrest process, . 321 1. Preparation of ore, . .321 2. Dissolving the gold, . 322 Vats, . . . .322 Charging-in, . . . 323 Separation of slimes, . 324 Leaching, . . .324 Agitation, . . .325 Strength of cyanide solu- tion, . . . .328 Disposal of tailings, . 330 3. Precipitation of gold, . 331 Cleaning-up, . . . 332 4. Production of bullion from precipitate, . . . 333 Plant required, . . . 337 Treatment of ore slimes, . 337 ,, accumulated slimes, . 340 Testing of ores, . . .341 Direct treatment of Rand ore, 342 Use of cyanide in stamp bat- tery, . . . .343 Method of treatment At the Robinson mine, . 344 Sylvia mine, . . 344 ,, New Primrose mine, 34ti Siemens-Halske process, . 347 Results of process In South Africa, . . 351 ,, other parts of the world, 353- -xvi CONTENTS. CHAPTER XVI. CHEMISTRY or THE CYANIDE PROCESS. Action of potassium cyanide on gold and other metals, . 354 Decomposition of potassium cyanide, . . . .360 Reactions in the zinc boxes, . 361 Action of potassium cyanide On metallic salts and minerals, 364 On oxidised pyrites, . . 366 The soda solution, . . . 368 Re-precipitation of gold and silver in leaching vats, . 369 Testing strength of solution, . 369 Estimation of sulphides in potassium cyanide, . .371 PAOE Strength of solution required, 372 Consumption of cyanide, . 372 Methods of increasing the speed of action of potas- sium cyanide, . . . 373 The Hood process, . . . 374 ,, Sulman-Teed process, . 376 ,, Molloy process, . . 377 ,, Pelatan-Clerici process, . 377 ,, Johnston process, . . 378 Wilde process, . . 378 Ores suitable to process, . . 378 CHAPTER XVII. PYRITIC SMELTING, . 380 CHAPTER XVIII. THE REFINING AND PARTING OF GOLD BULLION. General considerations, . . 383 Refining, . . . .384 Composition of bullion, . 384 Melting furnace, . . . 386 Crucibles, . . . .387 Melting, . . - . .387 Refining, .... 388 Toughening, . . . 390 Casting, . . . .391 Losses of bullion, . . . 393 Uetining by sulphur, . . 395 Osmiridium in gold bars, . 395 ^Parting processes, . . . 396 Cementation, . . . 397 Parting by sulphide of anti- mony, .... 397 Parting by sulphur, . . 397 Nitric acid process, . . 398 1. Granulation of alloys, . 398 2. Dissolving granulations, 399 3. Treatment of gold resi- dues, . . . .400 4. Treatment of silver solu - tion, . . . .401 5. Reduction of silver chloride, . . . 401 Parting by sulphuric acid, . 402 1. Mixing and granulating alloys, . . . 402 2. Dissolving silver, . . 403 3. Melting gold residue, . 406 4. Precipitation of silver, . 407 5. Crystallisation of sul- phate of copper, . 407 Combined process, . . 408 Gutzkow process, ; . 409 New Gutzkow process, . 411 Miller's chlorine process, . 414 Original method, . -415 Later improvements, . 417 Modern practice at Mel- bourne, . . -420 Modern practice at Sydney, 429 Refining brittle gold by chlorine, . . .431 Parting by electrolysis, . 432 Parting of gold from plati- num, iridium. &c., . 435 CHAPTER XIX. THE ASSAY OF GOLD ORES. General considerations, . Blowpipe assay, Sampling and crushing the ore Metallics, . Crucible method of assay, Fusion, Assay-ton weights, . General charges, 437 437 438 439 440i 440 441 441 Fluxes, . Methods of operation, Roasting before fusion, Cleaning slag, . Treatment of base ores, Cupellation, Influence of base metals, 442 444 447 448 448 448 451 CONTENTS. XV11 CHAPTER XIX. Continued. Inquartation and parting, . 452 Examination of assay materials, . . . 454 Examination of cupel, . . 454 Assay by scorification, . . 455 Detection of gold in minerals, . 457 Estimation of gold in dilute solution, .... 458 Special methods of assay, . 458 1. Mixed wet and dry method, 458 PAGE 459 459 461 461 461 461 6. Assay of purple of Cassius, 462 7. ,, a Mint sweep, . 462 2. Amalgamation, 3. Chlorination, . 4. Whitehead's method, 5. Assay of pyrites, (a) Swartz's method, (b) Staptf's CHAPTER XX. THE ASSAY OF GOLD BULLION. (Parting assay, .... 462 1. Selection of sample, . 463 2. Preparation of assay-piece for cupellation, . . 464 3. Cupellation, . . .467 Assay furnace, . . . 467 Cupels, . . . .469 Method of operation, . 469 Flashing, . . . .470 Temperature of muffle, . 471 4. Preparation of cupelled buttons for parting, . 474 5. Parting, . . . .475 (a) In parting flasks, . 475 (b) In platinum trays, . 476 Relative advantages of these, . . .477 6. Weighing the cornets, . 478 Losses of gold in bullion assay- ing, 478 Silver retained in cornets, . 480 Occluded gases, . . .481 Checks or proofs, . . .481 Limits of accuracy in assay, . 482 Parting by sulphuric acid, . 482 Preliminary assay, . . . 433 Assay by cadmium,. . . 433 Assay of alloys of gold, silver, and copper, . . . 483 Effects of other metals, . . 484 Assay of various gold alloys, . 484 A. Alloys requiring scorifi- cation, . . 484 Arsenic and antimony, . 484 Iron and manganese, . 485 Cobalt and nickel, . . 485 Zinc, . . . .485 Tin, . . . . .485 Aluminium, . . . 485 B. Amalgams, . . . 486 C. Platinum group, . . 486 (1) Platinum, . . .486 (2) Palladium, . . .487 (3) Rhodium and iridium, 487 D. Tellurium compounds, . 488 Wet methods of assay, . . 488 Other methods, . . .489 1. Touchstone, . . . 489 2. Colour and hardness, . 490 3. Density, . . . .490 4. Spectroscope, . . . 490 5. Electrolysis, . . . 491 6. Induction balance, . .491 Auxiliary balance for weighing cornets, .... 492 CHAPTER XXI. ECONOMIC CONSIDERATIONS. Management of gold mills, . 493 ost of production of gold, . 493 Annual production of gold, past and present, . . . 495 Table of production in different countries, . . . 498 Amount produced by different processes, . . . 500 Consumption of gold, 501 XVlll CONTENTS. BIBLIOGRAPHY. PAGE Periodicals, .... 503 General, 504 Properties of gold and its alloys, 505 Distribution of gold, . . 507 Placer mining, . . . 508 Crushing and amalgamation, . 509 Concentration, . . . 510 PAGE Roasting 511 Chlorination, . . . .511 Cyanide process, . . .512 Pyritic smelting, . . . 512 Refining and parting of bullion, 512 Assaying, . . . .513 INDEX, 515 THE METALLURGY OF GOLD. CHAPTER I. THE PROPERTIES OF GOLD AND ITS ALLOYS. PHYSICAL AND CHEMICAL PROPERTIES OF GOLD. Introduction. From very early times the ancients were attracted by the beautiful colour, the brilliant lustre, and the indestructibility of gold, and spared no pains in the endeavour to acquire it. In the code of Menes, who reigned in Egypt in 3600 B.C. or about 2000 years before Moses, the ratio of value between gold and silver is mentioned, one part of gold being declared equal in value to two and a half parts of silver, and it is, therefore, clear that the extraction of both metals from the deposits containing them must have been carried on before that time. It is, indeed, probable that gold was the first metal observed and collected, since it occurs in fragments of all sizes in loose sand, and the operations of collecting the larger pieces and melting them together are so simple. Among the rock carvings of Upper Egypt there are several illustrative of the art of washing auriferous sands by stirring and working them up by the hand in hollowed-out stone basins, and subsequently melting the gold in simple furnaces with the aid of mouth blow-pipes. The earliest of these carvings is supposed to date back to about 2500 B.C. However, in ancient times gold appears to have been mainly derived from India, and that country continued to supply most of the gold used in Europe until the discovery of America by Columbus. In order to collect alluvial gold, the sands were washed down over smooth sloping rocks by means of running water, and the particles of gold, sinking to the bottom of the stream by reason of their high density, were entangled and caught in the hair of raw hides spread on the rocks. Among the hides used were 1 THE METALLURGY OP GOLD. sheepskins, and hence originated the form of the legend of the Golden Fleece. Stripped of its heroic dress, this legend merely describes a successful piratical expedition about 1200 B.C. to win gold, which was being laboriously obtained from streams with the help of sheepskins by the inhabitants of what is now Armenia. Similar expeditions have not been unknown in much later times, and the method of obtaining gold by washing river sands is still practised, with improvements in matters of detail, in many parts of the world. Metallurgists are almost pro- verbially conservative in their methods. Hides are even now occasionally employed to catch the gold, but sheepswool, when used, is generally in the form of blankets. At the present day, however, when auriferous sands are washed, the aid also is invoked of what Baron Born called in 1786 the "elective affinity" of mercury for gold when mixed with impurities. The ease with which gold-amalgam can be collected, in spite of its being less dense than gold itself, is due to the fact that it is miscible in all proportions with mercury, so that, under proper conditions, large globules of liquid alloy are formed by the running-together of smaller particles, and the former are readily caught in suitable crevices. In the history of gold, it is also of interest to the metallurgist to remember that the earliest dawn of the science of chemistry was heralded by the study of the properties of gold, and by the efforts which were made to invest other matter with these properties. From the fourth to the fifteenth century, chemistry, which was first called "chemia" (x^sia), and then "alchemy," was defined as the art of transmuting base metals into gold and silver, almost all the labours of philosophers being intended to aid directly or indirectly in solving this problem. At the end of this period, while Paracelsus was giving to chemistry a new aim that of investigating the composition of drugs, and their effect on the human body Agricola was reducing to order the numerous empirical facts which together made up the art of metallurgy, and although alchemy died hard, its era of usefulness may be said to have ended here. Gold has doubtless been the cause of many of the wars and marauding expeditions from which the world has suffered, but on the other hand it has been instrumental, in a far greater degree than most other commodities, in promoting the growth of civilisation, the efforts of the alchemists having laid the foundations of the science of chemistry, and those of the gold-seekers having resulted in the discovery of new countries, and in the spread of knowledge of all kinds. Colour. The lustre and fine colour of gold have given rise to most of the words which are used to denote it in different languages. The word "gold" is probably connected with the Sanscrit word "jvalita," which is derived from the verb "jval" to shine. It is the only metal which has a yellow colour when TiiE PJROPEKFIES OF GOLD. 3 in mass and in a state of purity. Impurities greatly modify this colour, small quantities of silver lowering the tint, while copper raises it. In a finely divided state, when prepared by volatilisa- tion or precipitation, gold assumes various colours, such as deep violet, ruby and reddish-purple, the tint varying to brownish- purple and thence to dark brown and black. This purple colour has been supposed by some experimenters (viz., Guy ton de Morveau, Biichner, Desmarest, Creuzbourg and Berzelius) to be due to the formation of a coloured oxide of gold of unknown com- position, but Buisson, Proust, Figuier and, more recently, Kriiss have shown that no oxygen can be obtained from this coloured material, and that it probably consists of metallic gold. Similar colours are seen in purple of Cassius, and in Roberts- Austen's purple alloy of aluminium and gold, the colour in each case being probably due to a particular form of finely divided gold. The ruby coloured gold, suspended in a liquid in which it has been precipitated by ether and phosphorus, is present in the most finely divided state obtainable, and perhaps it then approaches the condition of separate atoms. Such gold shows no tendency to settle by gravity even after being kept undisturbed for several years. Somewhat less finely divided gold gives a faint blue tinge to light transmitted through the liquid in which it is suspended. Gold precipitated from its solution as bromide is in a different molecular condition from that formed from chloride, giving out 3-2 calories in passing into the latter state.* The surface colour of small particles of native gold is often apparently reddened by being coated with translucent films of oxides of iron. Very thin plates of gold are translucent, and appear green by transmitted light, while remaining yellow by reflected light. On heating, the green colour changes to ruby red, but is restored by the pressure of a hard substance by which the state of aggregation is again altered (Faraday). Molten gold is green, and its vapour is also probably greenish. Malleability and Ductility. Malleability and ductility are possessed by gold at all temperatures to a far higher degree than by any other metal. A single grain of gold can be drawn out into a wire over 500 feet long, and leaves of not more than -g-Q ^ Q Q of an inch in thickness can be obtained by beating. Faraday has shown that the thickness of these leaves may be still further reduced by floating them in a dilute solution of potassic cyanide by which they are partly dissolved. Hardness. The hardness of gold lies between that of alu- minium and that of silver, corresponding to the number 979 in Bottone's scale, in which the diamond is 3,010. Tenacity. The purest gold obtainable has a tenacity of 7 tons * Thomsen, Thermochemische Untersuchungen, vol. iii., p. 412. 4 THE METALLURGY OF GOLD. per square inch and an elongation of 30-8 per cent., but the presence of as little as ^rnnj" ^ other elements, especially bismuth, tellurium, lead, and other metals with high atomic volumes, greatly lowers these constants, as well as the malle- ability and ductility of the metal, while its hardness is increased.* Gold containing ^^ of bismuth can almost be crumbled in the fingers. Specific Gravity. The specific gravity of gold when pre cipitated from solution by oxalic acid is 19-49 (G. Rose) ; f when cast it varies from 19*29 to 19-37, but this can be raised by compression to over 19-48.| Henry Louis has shown that the specific gravity of unannealed "parted" gold (i.e., the residue left after boiling silver-gold alloys in nitric acid) is 20-3, its density being lowered by the process of annealing. When precipitated by ferrous sulphate, its density may be as high as 20-72 (G. Rose). Cohesion. On heating, gold can be welded like iron below the point of fusion, and finely divided gold agglomerates on heating without being subjected to pressure. Pressure alone is also sufficient to make gold dust cohere, while a true flow of the particles of gold can be induced in the case of the pure metal and some of its alloys. Specific Heat. The specific heat of gold is -0324 (Regnault) or -0316 (Yiolle). Fusibility. Gold fuses, after passing through a pasty stage, at a clear cherry-red heat, just below the fusing point of copper and much above that of silver. The metal expands considerably on fusing and contracts again on solidifying. Carnelley gives the temperature of fusion as 1,037, Yiolle as 1,045, whilst recent determinations have given it as 1,061-7 (Hey cock and Neville), 1,050 to 1,060 (Le Chatelier), and 1,072 (Holborn and Wien). Spectrum. In the gold spectrum Huggins saw 23 lines, the wave lengths of the most important ones being 523-1, 583*5, and 627-6 respectively.)) Latent Heat. The latent heat of fusion of gold is 16-3, and the normal lowering of the freezing point for 1 atom of impurity in 100 atoms of gold is 10 -6, but the presence of from O'l to 4*0 per cent, silver does not cause any alteration in the freezing, point. Magnetism. Gold is diamagnetic, its specific magnetism being 3-47 (Becquerel), if that of iron is taken as 100. * Roberts- Austen, Phil. Trans. Royal Soc., vol. clxxix. (1888), p. 339. t Pogg. Ann., vol. Ixxiii. (1848), p. 1, and vol. Ixxv. (1848), p. 403. t Eighth Report of the Royal Mint, 1877, p. 43. Trans. Am. Inst. of a H ^ iH OO O O O* CO o 1 I So^ .0 ipo>p O * i + S 68 A L L I ^ iO *O pH GO ^5 ^^ * : oJ -2 S 43 ft o + 4- + + + + 08 | JZ si fell I ** 5 * HH~ * * ^H oi C^ c^ '^ 1| do?o t o ^+4.+ 4- 4S (jj CO Tj< "^ Od r i ^ ^ .. 43 A s s + + + + + ^^j. a 6 cS 3^^ t 1 * .a .5 .s C a Ofl ' a .a a s's ' . 1 |'i s a a sg 2 jq 5 EHI ^ *"* ^ "^ ^ &: i i s ^ "* -^ 3 P* 2 3 it - ^ PN j^ SB 5 G* 2 * ^ ^ -2 - ^ s ^ S) 18 J ^c3"|c3c3c3{8 ^ c3 |1| 5^ sO'SO^O 03 O 00-Q-O^ ^0 i 54 ^H (M CO <* O CO 1^ 00 1 8 THE METALLURGY OF GOLD. The volatilisation of gold-copper alloys merits further in- vestigation, as it is of great industrial importance. The high percentage of the losses shown in the table is, no doubt, due to the smallness of the masses treated, and to the comparatively large surfaces exposed in consequence of this. A number of experiments were also made at the same time on various gold alloys to determine the relative effect of the presence of other metals on the volatilisation of gold. The temperatures and conditions used were similar to those described above. It was found that the volatility of gold was increased by the pre- sence of any metallic impurity, even by the non-volatile metals, such as platinum. The tellurium alloys suffered the heaviest losses, gold containing five per cent, of tellurium losing from 16*5 to 39*5 parts per 1,000 by volatilisation per hour at a temperature of 1,245 (i.e., 200 above the melting point of pure gold), a point of much importance in connection with the metal- lurgy of telluric gold. Lead and platinum had a very slight effect in increasing the volatility of gold ; copper and zinc a more marked effect, while five per cent, of antimony or mercury caused losses amounting to about two parts per 1,000 of gold per hour at 1,245. Further conclusions which were drawn were as follows : 1. Although the temperatures employed were higher than those at which pure zinc, cadmium and tellurium, and probably antimony and bismuth, are distilled, yet these elements were never completely volatilised. The " temperature of dissociation of the alloys," as in the case of the bismuth-arsenic alloy investigated by Edward Matthey in a paper read before the Royal Society, January 26, 1893, is much higher than the ordinary distillation point of the more volatile constituent. Thus zinc boils at 950, but its gold alloy loses little or no zinc at 1,120, and still retains part at 1,250, whilst antimony was not driven off to any extent by the highest temperature attained. Copper appears to pass off more easily than some of the so-called volatile metals, perhaps because the dissociation of its alloy with gold may not form an initial stage of the operation. 2. The amount of gold lost depends partly on the volatility of the alloying metal, but perhaps too much stress has been laid on this factor in the past, the results of heating alloys of mercury, zinc, antimony and copper pointing to that conclusion. A metal with a strong attraction for gold, such as copper, may carry it off, perhaps as a vaporised alloy, more easily than one which mixes with it less intimately. 3. It is observable that those impurities which reduce the surface tension of a button of liquid gold appear to increase the vapour pressure of the metal as indicated by the loss on heating. This was to be expected 4. A current of air or coal gas insufficient to disturb the THE PROPERTIES OF GOLD. 9 surface of the liquid metal does not appear to increase the volatilisation. It may be added that Hellot stated that if an alloy of one part of gold and seven parts of zinc is heated in air, the whole of the gold comes off in the fumes. Crystallisation of Gold. Gold crystallises in the cubic system, occurring frequently in nature in the form of cubes, octahedra and rhombic dodecahedra. Cleavage is never exhibited. Single detached crystals are comparatively rare, and the crystals are usually attached end to end, forming strings, and branching, arborescent, or moss-like masses, which are composed of micro- Fig. 3. Fig. 2. scopic crystals, usually octahedra. These forms occur frequently in quartz veins, but the single crystals, which are usually of larger size, viz., from inch to 1J inch in diameter, are mainly found in drift deposits. They are rarely perfect or of brilliant lustre, although such crystals were found at the Princeton Gold Mine, Mariposa County, California, but occur more frequently with rounded angles, raised edges, and cavernous faces, which are often marked with parallel striations, and possess little or no lustre (Fig. 1). The octahedra found in California are usually flattened parallel to two opposite faces, or elongated, or otherwise dis- torted. Still more frequently they are only partially developed, as in Figs. 2 and 3. In all these cases " the incomplete crystals 10 THE METALLURGY OF GOLD. have the appearance of a failure for lack of material " (W. P. Blake). Crystals of greater complexity, containing many modi- fying faces, occur chiefly in Siberia, Transylvania and Brazil. The most common forms occurring naturally in Australia are the octahedron and the rhombic dodecahedron.* Artificial crystals can be obtained in several ways, but with great difficulty. The slow cooling of an ingot of gold from fusion usually gives faces and sometimes angles of octahedra on the surface of the metal, f The presence of small quantities of copper prevents this crystallisation. Feathery crystalline plates are precipitated in the electrolysis of a solution of chloride of gold and ammonium. By keeping an amalgam containing 5 per cent. of gold at a temperature of 80 for eight days, and then di- gesting it at 80 with nitric acid of specific gravity 1'35, and subsequently subjecting the residue to a red heat, bright crystals of gold which are, however, usually microscopic can be obtained. | In the Percy collection are some gold crystals found in the mercury troughs at the foot of the " blanket strakes," in an amalgamation mill. The troughs are placed so as to catch any stray particles of gold that may pass the blankets. As the amount of gold recovered in this way is very small, it is not worth while to clean out the troughs frequently, and in this case they had remained undisturbed for nine months, at the end of which time all the amalgam was found to be crystallised. The mercury has been dissolved off by nitric acid, and the gold crystals remain. The smaller crystals are rather indefinite in shape, but amongst the larger ones (which are about half the size of a pea) are well-defined combinations of the octahedron, rhombic dodecahedron, and cube. Solubility of Gold. Gold is readily soluble in aqua regia, or in any other mixture producing nascent chlorine, among such mixtures being solutions of (1) nitrates, chlorides, and sulphates e.g., bisulphate of soda, nitrate of soda, and common salt; (2) chlorides and some sulphates e.g., ferric sulphate ; (3) hy- drochloric acid and potassium chlorate ; (4) bleaching powder and acids, or salts such as bicarbonate of soda. The action is much more rapid if heat is applied or if the gold is alloyed with one of the base inetals than if it is pure. The presence of silver in the gold retards the process, a scale of insoluble chloride of silver being formed over the metal, and the action may even- tually be completely stopped if the percentage of silver present is large. Gold is also dissolved by liquids containing chlorine and bromine, but the action is much slower than that of aqua regia; and subject to the same difficulties if silver is present; * For a full account of the crystalline forms of native gold, see a Paper by W. P. Blake, in Precious Metals of the U.S.A., 1884, p. 573. t Chester in Am. Journ. of Science and Arts, vol. xvi., July, 1878, p. 29. Krafft, Encyclopaedia. Brit., article "Crystallisation." THE PROPERTIES OP GOLD. 11 heat assists the dissolution. Iodine only dissolves gold if it is nascent, or if heated with gold and water in a sealed tube to 50. Metallic gold dissolves in hot strong sulphuric acid, especially if a little nitric acid is added (the precipitated metal dissolving most readily), forming a yellow liquid, which, when diluted with water, deposits the metal as a violet or brown powder. The solution also becomes covered with a shining film of reduced metal on. exposure to moist air. On addition of hydrochloric acid or a metallic chloride, auric chloride is formed, no longer precipitable by water.* Gold is also attacked when used as the positive pole of the battery in the electrolysis of strong sulphuric acid, but is immediately reduced again by the evolved hydrogen, f According to Nickles, J the easily decomposable metallic per- chlorides, perbromides and periodidep are capable of dissolving gold, lower chlorides, by the excavator, and thence is charged into a revolving trommel * Prod. Free. Met., U.S.A., 1884, p. 557. DEEP PLACER DEPOSITS. .. 63 placed inside the trough. Here the disintegration of the gravel is effected, and the fine material falls through into the trough, while the stones are discharged outside at the end of the trommel. At the bottom of the trough there is a water pipe, carrying water at high pressure, and in that pipe a series ot jets pointing alternately forwards and backwards. The result is to give a series of eddies or whirlpools in the water in the trough, and the sand and fine gold is continually carried up to the surface near the middle line of the trough, and in descending again near the sides it comes in successive contact with several of the plates, which form a series of steps. The fine sand is eventually discharged at the end of the trough. It is stated by the Bucyrus Steam Shovel and Dredge Company, by which the machinery is manufactured, that such a machine erected in Montana has a capacity of from 600 to 800 cubic yards of gravel per day, all the water required being supplied by a pump raising 500 gallons per minute, the proportion being only three of water to one of gravel, which seems much too small. It is stated that gravel containing only 12 cents of gold (i.e., about 3 grains) to the ton has been treated successfully by this machine, but exact records of continuous work done by it are wanting. It will be noted that special means must be adopted in each case for the handling of the tailings. CHAPTER V. DEEP PLACER DEPOSITS. Nature and Mode of Origin of Deposits. This discussion is necessary in order that the description of the methods of treating the deep placer gravels may be intelligible. Both, in Australia and California, besides the superficial placer deposits situated in or near the existing rivers, which in the deep canons of the Klamath and other rivers in the extreme north of California attain a thickness of 250 feet, there exist auriferous gravels which bear no apparent relation to the present drainage of .the country. These gravels often attain enormous thicknesses,,. and are in many places covered by volcanic rocks, consisting of basaltic lavas and tuffs, which are sometimes; interbedded with gravel and loam This latter circumstance shows that inter,- mittent action of the volcanic vents, with long intervals of repose, has taken place. There has been some difficulty in accounting for the origin of these deep placers, and it has been ascribed in succession to the agency of the sea, of ice, and (for California) of a huge river flowing from north to south at right angles to the direction of flow of the existing rivers. Kone of these views are now entertained, and the " fluviatile " theory is 64 THE METALLURGY OF GOLD. generally accepted, the origin of the gravel being ascribed to the depositions of ancient rivers flowing in courses roughly parallel to those of existing rivers. The geological age of these ancient rivers has not yet been determined with certainty, but though they may be Pleistocene, the balance of palaeontological evidence is perhaps in favour of Whitney's view that the deposits were formed in the Pliocene period. The ancient Californian rivers probably had their sources at somewhat higher altitudes than those now existing, and had more uniform general grades, the slope of their beds corre- sponding more nearly to the general slope of the country. The existing rivers, on the other hand, have steep grades in the upper parts of their courses, followed by comparatively level stretches below. The old rivers, however, like their successors, had rapids, falls and level stretches, the grade varying from 5 feet to- 250 feet or more per mile. The Pliocene rivers ran in valleys which were broad and shallow in comparison with the present deep precipitous canons, and the volume of water was in general much greater than that delivered by their representatives of to-day. The width of the valleys varied from 100 feet to fully 1J miles (which is the width at Columbia Hill), and the depth must have been often over 1,000 feet. These valleys were already partly filled up by accumulations of gravel, when the outbreak of volcanic activity in many cases filled up the remain- der, and the streams were deflected into other channels, which often lie close alongside the old canons. These new channels have been excavated by the running water until they now lie much below the level of the beds of the Pliocene rivers, and consequently the gravels which were deposited in the old valleys- now sometimes crown the highest ground in the district, the general level of the country having been greatly reduced in height. The new channels have been cut partly in the old country rock and partly in the Pliocene auriferous gravels and their covering of volcanic rocks. Sometimes the course of the present canons cuts that of the old at several points owing to the sinuosity of both, see Fig. 8, in which A represents the modern river, and B the ancient one. The result is that sections of the old valley from bed-rock to surface are exposed in the sides of the caftons, usually at some height above the present level of the water, and it was at such points as these that the discovery of the existence of the deep placers was first made. The hard covering of basalt has served to protect the more friable gravels, which have been for the most part removed in those places where the lava has been worn away or has never existed, so that DEEP PLACER DEPOSITS. 65 the largest tracts of gravels still existent lie beneath the volcanic rocks. Fig. 9 represents a section across two ancient channels (B, B) and a modern canon, that of the American river. Here, A is the volcanic capping, which is 800 feet thick above the Red Point channel; B, B are the auriferous gravel channels; C, C are deposits of gravel on the " rims," containing gold in places ; D is the bed-rock, consisting of dark-blue slates ; E is a barren deposit of angular debris and boulders ; F F are prospecting tunnels, which were put in at too high an altitude ; F' is the tunnel bored with the object of reach mg the bottom of the gravel de- posit ; H are prospecting winzes sunk in order to discover the position of the gravel. The space included within the dotted lines N M Y M' N' has been obviously denuded since the deposi- tion of the volcanic cappings, the soft slate rims N M R and N' M' Fig. 9. having been worn away, while the hard lava has resisted erosion. The vertical depth from M to the American river is about 1,800 or 2,000 feet. This condition of things is that prevailing in California, but in Victoria the structure closely resembles that just described, with the exceptions that the old valleys were smaller and that the erosive action of the rivers since the deposition of the basalt has been comparatively slight, owing to the slight grades of the streams caused by the low elevation of the country and to the small amount of the rainfall. In consequence of this the basalt has usually not been worn through, and the " deep leads " or old river bottoms are often below the level of the present streams, so that although a larger proportion of the Pliocene gravel remains, it is more difficult and expensive to mine. The shallow placers, at any rate in California, have resulted in 66 THE METALLURGY OF GOLD. the main from the erosion of these deep placers, the materials of which, having undergone a natural concentration in the ground sluices afforded by the river beds, furnished the wonderfully rich river-bed and bar deposits, which yielded so much gold between 1848 and 1860. The deep level gravels vary greatly in thickness, as has already been stated, being only 2 feet thick at Table Mountain, Tuolumne County, and over 600 feet thick at Columbia Hill, Nevada County, California, and averaging from 100 to 300 feet, the thickness varying with the nature of the river bed, and the subsequent erosion. The gravel consists in slaty districts chiefly of quartzose sand, the fine materials furnished by the disintegration of the slate having been for the most part swept away, and the products of the quartz veins contained in the slate being left. Near bed-rock, but at no higher level, there is often a collection of large boulders, varying in size up to 10 feet in diameter, consisting mainly of quartz, but sub- ordinate to these are others usually similar in character to the bed-rock. These boulders, though rounded, are too large to have been transported far by running water, and have probably been polished by the attrition of the sand carried over them. At the Forest Hill Divide, Placer County, California, the gravels consist almost entirely of pure white quartz, but at other places such quartz is rare. It is to be noted that it is only in slaty districts, where the gravels are mainly quartzose, that rich auriferous de- posits occur. In granite districts, where the gravels are composed of more heterogeneous materials, and in cases where they consist of volcanic boulders and detritus, little or no gold is found. The lower parts of the gravels are often cemented into a conglomerate, called " cement," by infiltration of silica, oxides or sulphides of iron, or, rarely, carbonate of lime ; when the gravels are covered with lava, the whole thickness is in some cases converted into cement. The upper parts of the gravels often contain pipe-clay in greater or less quantity, either in pure beds or mixed with sand. Fossil leaves occur in the clays, and drift wood occurs throughout the whole of the deposits in extraordinary abund- ance, particularly in Australia ; this wood is for the most part silicified or replaced by sulphide of iron. The higher portions of the gravels are often altered by the action of air and water on the iron sulphide, thus forming ferrous salts, haematite and hydrous sesquioxides, which colour the gravels red and brown respectively. The upper gravels are hence called " red gravels." The lowest layers, being protected from alteration from above, are coloured dark blue -grey by the ferrous sulphide contained in them, and are hence called "blue gravels," their occurrence giving origin to the old "blue lead" theory, owing to their uniformity of colour over wide areas in California. The sul- phide of iron which incrusts fossil bones and teeth found in the gravel, and replaces the substance of drift wood, was formerly DEEP PLACER DEPOSITS. 67 believed to be derived from below, as the result of metamorphic action going on in the country rock. Distribution of Gold in the Gravels. The gold is found chiefly either in contact with or just above bed-rock. If this con- sists of soft slate, and especially if the planes of cleavage are at a high angle to the horizon, particles of gold are often found in the natural riffles thus formed, and are disseminated through the rock to the depth of a foot or two. If depressions, pot-holes, or fissures exist in the old river bottom, they are usually very rich in gold. Where, as often happens, there is a channel, or " gutter," to adopt the Australian expression, cut by the stream in the lowest part of the valley, the gravel filling it is usually much richer than that found elsewhere. Such rich portions, often only a few feet wide, and of insignificant depth, but extend- ing to considerable distances in the direction of the stream, are called "leads." As a result of these circumstances, the "blue gravels " happen to be richer in general than the " red gravels," from which arose the old theory that only blue gravel pays to work. Coarse gold and nuggets chiefly occur near bed-rock in the deeper parts of the channel, but the " rim-rock " gravels are also often rich. The " rim-rock " is that portion of the bed- rock which forms the sides of the old valley, thus lying consider- ably higher than the central channel. The richness of gravels here doubtless arises from the existence of old bench or terrace gravels, which are consequently the oldest of the whole series, being formed before even the gutter gravels. Rich streaks also occur at various levels in the gravels, often resting on "false bottoms," which consist of impermeable beds of clay or some similar material. Sometimes these streaks are richer than those encoun- tered at bed-rock, as, for example, at the Paragon Mine, Placer County, California. Although it is concentrated in this manner at various points, gold nevertheless occurs disseminated through the greater portion of the red gravel, where, however, it is in a finer state of division and less abundant. Besides existing as free particles, gold may occur in quartz boulders, although this is rare. For instance at the Polar Star Mine, Dutch Flat, Oalifornia, a white quartz boulder was found, which contained 288 ozs. of gold. Gold may also occur, together with pyrites, replacing the substance of drift wood. The amount and position of the gold varies, as in the case of the present rivers, with the grade, the shape of the valley, the volume of water, the amount of gravel being carried down, &c. "An underloaded current i.e., a current charged with less detritus than it is well able to carry is apt to cut its bed, and prevent the accumulation of gravel. A greatly overloaded current will deposit too rapidly to admit of the concentration of the gold dust " * Under conditions intermediate between these * Ross E. Browne, Tenth Report, of the Gal. State Mineralogist, p. 448. 68 THE METALLURGY OF GOLD. extreme states, the current may be just strong enough to keep its bed clear from all accumulations except a small quantity of coarse gravel and the coarse gold, which is caught in the natural riffles, and thus all the conditions necessary to form a rich bed of pay-dirt may be present. If, however, the bed consist of granite or other rock which wears in smooth and rounded shapes, little gold will be caught. Slates, consisting of layers of uneven hardness, wear irregularly, and afford a good gold catching surface. The condi- tions noted above as necessary to form rich gravels cannot be expected to have been prevalent over great distances. " An increase of grade or narrowing of the channel will cause an increase of velocity, and the same stream may be underloaded in a narrow steep section, and overloaded in a broad flat section." * The difference of velocity between the middle and sides of a stream, and between the inside and outside of a bend, may give the right conditions in one part of a river bed and not in another. Thus with high grades, rich gravels should occur in the less rapid, and with low grades, in the more rapid parts of a stream. Having regard to such considerations, the richer parts of existing rivers can be pointed out with little trouble. When, as in the Pliocene rivers, the beds are buried to a depth of hundreds of feet, the richer parts are more difficult to find. The history of the Pliocene rivers began with a period when ex- cavation exceeded deposition, when the rivers were underloaded for at least a portion of each year, and the channel was constantly being deepened. Some bench or terrace gravels were formed at this time, and being at the sides of an underloaded river tended to be rich. The river bed, although rocky and comparatively free from sand, would perhaps accumulate some coarse gold, which as the channel deepened was no doubt in part ground up into fine particles and carried off, but at the time when the excavation had reached its lowest point, some of this coarse gold would certainly be present. When the underloading of the stream ceased, what- ever caused the cessation, a pause must in many cases have occurred before the gravel proved too much for the stream to carry. During this pause the conditions for gold catching were favourable, and hence rich gravels were formed on bed-rock in the gutters or channels. Then, as the streams became overloaded, sand and gravel accumulated rapidly, so that little concentration of the gold in them could take place. The rivers flowed over thick sand banks and, in consequence, frequently changed their courses. The sands, being deposited by overloaded rivers, of course con- tained fewer and smaller boulders, and the thick masses of poor sand thus went on accumulating until the volcanic outbursts put an end to the process. Origin of the Gold in the Placers. The origin of the gold in deep placers has long been a vexed question. It was * Ross E. Browne, Tenth Report of the Col. State Mineralogist, p. 448. DEEP PLACER DEPOSITS. 69 formerly accepted without question that the erosion of auriferous quartz lodes existing at higher altitudes furnished both gravel .and gold. In support of this it w<*s urged that the same districts which furnished auriferous gravels abounded in quartz veins at higher levels, while "Whitney pointed out that numerous lodes were intersected by the valleys and were still to be seen in the bed-rock. On the other hand, the fact that the nuggets found in the drift are much larger than any masses of gold encoun- tered in veins, and that the placer gold is of superior fineness, .are difficulties in the way of accepting this theory. Moreover, Egleston states that nuggets as large as a man's fist have been found embedded in the midst of fine sand, whither they could not have been carried by the action of running water, but authentic instances of such finds seem to be lacking, nuggets usually occur- ring in coarse gravel, among boulders. It is further declared by the opponents of the erosion theory that if a small quantity of soft material like gold, mixed with lumps of hard quartz, were washed down by water, then, long before the quartz could be reduced by grinding to the condition of grains of sand, the gold would be worn down to such a fine state of division that none of it could lodge in the river bed at all. In opposition to this con- tention, it may be urged that the extreme malleability of fine gold would make this comminution very slow, and scales of the metal are said to have their edges blunted and thickened by the pounding action of dry sand moved by the wind, instead of having them worn away. However, it is certain that the form of placer gold is different from what might have been expected if it con- sisted of water-worn vein gold. Nuggets are mammillary in form, and generally appear more like concretions than water- worn fragments, in spite of the fact that they often include more or less quartz. In 1864, in order to account for these and other facts, Mr. A. C. Selwyn, of Victoria, suggested a theory of solution in which it is supposed that the gold disseminated through the rocks and drifts is dissolved by percolating waters which contain acids and salts in solution, and is reprecipitated around certain centres. Selwyn considered that the waters capable of dissolving gold, must have acquired this property by passing through the beds of basalt, &c., overlying the drifts, inasmuch as large nuggets occur in districts where basaltic eruptions have taken place, while, where these are absent, the gold is very fine, and nuggets can scarcely be said to exist. The fact has long been known that gold is soluble in certain dilute solutions of salts, likely to be met with in nature, such as a mixture of nitrates with chlorides, bromides or iodides, or as the haloid ferric salts. This has been firmly established by the researches of Skey, Daintree, Egleston and others. Also, the precipitation of gold from these solutions around nuclei consisting of particles of gold, pyrites,