THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA GIFT OF George C* Linton THE METALLURGY OF GOLD. NEW METALLURGICAL SERIES EDITED BY W. C. ROBERTS-AUSTEN, C.B., F.R.S., st and Assayer of the Royal Mint; Professor of Metallurgy in the College of Science. In Large Svo, Handsome Cloth. With Illustrations. 1. INTRODUCTION to the STUDY of METALLURGY. By the EDITOR. THIRD EDITION. "No English text-book at all approaches this in the COMPLETENESS with which the most modern views on the subject are dealt with. Professor Austen's volume will be INVALUABLE." Chemical News. 2. GOLD (The Metallurgy of). By T. KIRKE ROSE, D.Sc., Assoc. R.S.M., F.C.S., Assist. -Assayer of the Royal Mint. SECOND EDITION. " The Four chapters on Chlorination, written from the point of view alike of the practical man and the chemist, TEEM WITH CONSIDERATIONS HITHERTO UNRECOGNISED, and constitute an addition to the literature of Metallurgy, which will prove to be of classical value." Nature. 3. COPPER (The Metallurgy of). By THOMAS GIBB, Assoc. R.S.M. 4. IRON and STEEL (The Metallurgy of). By THOMAS TURNER, Assoc. R.S.M., F.I.C. Vol. L, IRON. "A MOST VALUABLE SUMMARY of useful knowledge relating to every method and stage in the manufacture of cast and wrought iron down to the present moment . . . particularly rich in chemical details. ... An EXHAUSTIVE and REALLY NEEDED compilation by a MOST CAPABLE and THOROUGHLY UP-TO-DATE metallurgical authority." Bulletin of the American Iron and Steel A ssociation. 5. METALLURGICAL MACHINERY : the application of Engineering to Metallurgical Problems. By HENRY CHARLES JENKINS, Wh.Sc., Assoc. R.S.M., Assoc. M.Inst.C.E., of the Royal Mint. 6. ALLOYS. By the Editor. %* Other Volumes in Preparation. 04 3 30 THE METALLURGY OF GOLD. BY T. KIRKE BOSE, D.Sc., ASSOCIATK OP THE ROYAL SCHOOL OP MINES; FELLOW OF THE CHEMICAL SOCIETY; ASSISTANT ASSAYER OF THE ROYAL MINT. BE I NO ONE OF A SERIES OF TREATISES ON METALLURGY, WRITTEN BY ASSOCIATES OF TIIE ROYAL SCHOOL OF MINES. EDITED BY fcrot TO. (L 1Roberts*Busten, rove that the composition is not uniform, but this lack of uni- formity is doubtless due to the presence of some other element in addition to gold, silver and copper. In support of this view it may be mentioned that Louis Janin Jr. instances the case of three ingots from an Idaho mine, which were melted with borax in plumbago pots, and on cooling showed evidences of liquation.* Dip samples were taken after vigorous stirring, and granulated, and assay cuts were also taken from the diagonally opposite corners of the top and bottom faces of the bar after pouring. On assaying these samples the fol- lowing results were obtained : Gold. Silver. Total. Bar No. 1 Cuts, . . . 4 808 181 989 Dips, . r . . - 784 191 975 Bar No. 2 Cuts, . . . 7 809 180 989 Dips, . 786 184 970 Bar No. 3 Cuts, . . ,. -i 805 190 995 Dips, . . ...', 778 .201 979 No further experiments were made on the composition of these bars, so that the nature of the base metals present was not determined, although it is probable that the cause of liquation was to be looked for here. It will be observed that silver moved towards the centre in these cases, and gold in a far greater degree towards the outside of the ingots. James 0. Booth, of the U.S. Mint, Philadelphia, has stated f that liquation of gold-copper alloys is induced by the presence of small quantities of base metals, the greatest effect being produced by antimony, bismuth and arsenic, while lead, tin and zinc act in the same way to a less degree, but no direct experi- ments have been adduced in support of this view. Alloys containing much lead, bismuth and zinc cannot be obtained perfectly homogeneous. It has been pointed out by the author J that segregation takes place in standard gold rendered crystalline by small quantities of lead or bismuth, the presence of 0'2 per cent, of either of these metals causing the centre of a sphere 3 inches in diameter to be enriched in gold to the extent of about one part per thou- sand. This is due to the fact, proved by Roberts-Austen, that an eutectic alloy of gold and lead remains molten after the remainder of the mass has solidified, and is consequently driven towards * Eng. and Mng. Journ., vol. liv., 1892, p. 317. *t* Condemnation of Ingot At fits. Washington, 1875. J Chem. Soc. Journ., vol. Ixvii., 1895, p. 552. CHEMISTRY OF THE COMPOUNDS OF GOLD. 19 the centre of the sphere, and that this fusible alloy contains much gold but very little copper. Arnold's micrographic re- sults* tend to confirm the view which follows from this that the brittleness of crystalline gold is due to the presence of films composed of such eutectic alloys separating the crystals of gold from each other. It has been shown by Edward Matthey f that when gold ingots containing members of the platinum group are cooled from a state of fusion an alloy rich in the more fusible element (gold) falls out first, driving the less fusible constituent to the centre. Thus the assay of an outside cut of such an ingot gives a result too high in gold, sometimes by several per cent. It has long been known, moreover, that iridium and osmium become concen- trated towards the bottom of the mass. The reason for this is that, at the temperature of fusion of gold, these refractory elements, either free or alloyed with gold, sink in the molten metal and are left in the state of small crystalline particles. CHAPTER II. CHEMISTRY OF THE COMPOUNDS OF GOLD. Compounds of Gold. Gold is characterised chemically by an extreme indifference to the action of all bodies usually met with in nature. Its compounds are formed with difficulty, and decom- pose readily, their heat of combination being in general small, while some are formed by endothermic actions. The result of this condition of things is that gold is found in nature chiefly in the metallic form, and the mineralogist has, therefore, few compounds to consider. Nevertheless, the laws governing the formation of artificially-prepared compounds of gold are of great importance to the metallurgist, as the processes of extraction are all based on these laws, and in particular, some knowledge of the reactions and general behaviour of gold in its compounds is essential to those who are engaged in extracting gold by wet processes. A short account of some of the compounds of gold is, therefore, appended, special attention being paid to those bodies, which are most likely to be of interest to the metallurgist. Gold forms two series of compounds, having the general formulae AuR and AuR 3 , while doubtful compounds corre- sponding to AuR.,, AuR 4 , and AuR 5 , have been declared to exist by Thomsen, Prat, Figuier, and others. These two series are denominated aurous and auric respectively. * Engineering, vol. Ixi. (1896), p. 176. t Proc. Roy. Soc., vol. li. (1892), p. 447, and Phil. Trans. Roy. Soc. t vol. clxxxiii. (1892), A. pp. 629-652. See also Proc. Roy. Soc,, 1896. THE METALLURGY OF GOLD. Compounds of Gold with the Halogens. Gold forms two series of compounds with the halogens, the general formulae being AuR and AuR 3 respectively. Supposed compounds having the formula AuR 2 have been described, but are probably mix- tures of the series denoted by AuR and AuR 3 . All these bodies are very unstable, existing throughout only low ranges of tem- perature, whether in the dry state or in aqueous solution. The chlorides are the most easily formed and the least unstable, the bromides coming next, as might be predicted from the heats of combination which are given in the subjoined table in calories: Chloride. Bromide. Iodide. AuR, solid, . AuR 3 , solid, AuR 3 , in solution, + 5-8 + 22-8 + 27-3 - O'l ? + 5-1 - 5-5 '> CHLORIDES OF GOLD. Gold Monochloride or Aurous Chloride, AuCl. This salt is prepared by heating the trichloride to 185 in air for twelve hours. It is non-volatile and unaltered at ordinary tem- peratures and pressures by dry air, even when exposed to light, but begins to decompose at temperatures above 160, and the decomposition is complete if it is heated at 175 to 180 for six days, or at 250 for one hour. Its density is 74. Water con- verts aurous chloride into a mixture of gold and gold trichloride. It is a citron-yellow amorphous powder. Auro-Aurichloride, Au 2 Cl 4 . A dark red compound having this composition is said, by Thomsen, to be obtained by heating finely-divided gold in a current of chlorine to 140 to 170. According to Kriiss and Lindet it is merely a mixture of AuCl and AuCl 3 . It yields gold and AuCl 3 if brought in contact with water, ether, or alcohol. Auric Chloride or Gold Trichloride, AuCl 3 . Preparation. Trichloride of gold can be prepared, according to Debray,* by heating finely-divided gold in a current of pure dry chlorine in a glass tube at a temperature of 300. The chloride formed sublimes at this temperature and is deposited in the cooler part of the tube in fine red prisms and needles crystallising in the triclinic system. Kriiss tried to repeat Debray's experiments without confirming his results, f He found that at first brown- ish vapours of AuCl 3 were given off freely at 140 to 150, and were condensed in the cold part of the tube, but that after some time the sublimation ceased, and the whole mass was trans- formed to brownish -red trichloride. On continuing to raise the * Compt. Rend., vol. xix., p. 985. t Berichte, vol. xx., p. 212. CHEMISTRY OF THE COMPOUNDS OF GOLD. 21 temperature AuCl 3 began to decompose at about 180 in spite of the presence of an excess of chlorine, and protochloride of gold, AuCl, of a greenish-yellow colour was formed. At 220 the AuOl was completely decomposed, leaving metallic gold, while up to this temperature a little AuCl 3 continued to be sublimed, but towards 300 all action ceased, and the gold remained un- attacked by the chlorine at all higher temperatures. On lowering the temperature, Kriiss observed in succession the formation of AuCl at 220, and of AuCl 3 at 180. The exact point at which gold ceases to be attacked by chlorine and the rate of volatilisation of the chloride are of great import- ance in connection with the loss of gold on roasting auriferous materials with salt.* The matter has, therefore, been lately investigated by the author f with the following results. Gold unites with chlorine if placed in the gas at atmospheric pressure at all temperatures up to a white heat, but the subsequent decomposition of the chloride is rapid above 300. The absorption of chlorine by gold with the formation of chlorides at first in- creases in rapidity as the temperature rises, and reaches its maximum at about 225. The fact that gold is attacked by chlorine, and that the chloride is subsequently volatilised at all temperatures between 180 and 1,100, was proved by means of Deville's hot and cold tubes, which enable part of the sublimed chloride to be collected. The rate of volatilisation at various temperatures is as follows : Gold Volatilised. Number. Temperature. Lost from Porcelain Vessel containing the Gold. Recovered from Water Tube. Percentage Volatilised m 30 minutes. 1 180 Grammes. Grammes. 0-00105 0-007 -2 230 ... 0-0522 0-35 3 300 0-3473 2-32 4 390 0-2731 0-1748 1-82 5 480 0-1325 0-0884 0-88 6 580 0-0907 0-0624 0-60 ft i 590 0-0864 0-0590 0-58 8 805 0-0753 0-0518 0-50 9 965 0-2441 0-1672 1-63 10 1100 0-2895 0-2037 1-93 For purposes of comparison, it may be added that when gold ie heated in air or coal gas, no gold is volatilised below 1,050, * See Prof. Christy's experiments, Section on " Roasting," Chap. XI. t Chem. Soc. Journ., vol. Ixvii. (1895), p. 881. 22 THE METALLURGY OF GOLD. and only about O02 per cent, in 30 minutes at 1,100,* or about one-hundredth part of that volatilised in chlorine. In the table the amounts " lost " by volatilisation include the amounts recovered from the water tube and the gold condensed on the inside of the outer tube ; the latter sublimate was not recovered separately after each experiment. In every case the gold recovered from the water tube was associated with a little less chlorine than was required to form gold trichloride, and therefore presumably contained either some metallic gold or AuCl, or both ; these doubtless resulted from the dissociation of some of the trichloride when in the form of vapour. The amounts volatilised vary according to two different factors (1) The vapour pressure of gold trichloride, AuCl 3 , which of Fig. 3a. course increases continuously as the temperature rises ; and (2) the pressure of dissociation of the trichloride, which also rises continuously with the temperature, but not at the same rate as the vapour pressure. The rise of vapour pressure tends to raise, and that of the pressure of dissociation to reduce, the amount of gold volatilised as chloride. The vapour pressure increases more rapidly than the pressure of dissociation at temperatures below 300, and also above 900, but less rapidly at intermediate tem- peratures. Hence the curve (Fig. 3a) showing the variation of volatilisation with temperature is irregular, passing through a * Chem. Soc. Journ., vol. Ixiii. (1893), p. 717. CHLORIDES OF GOLD. 23 maximum near 300, and a minimum at a point somewhere below the melting point of gold. The first-named change in the direction of the curve possibly occurs at the melting point of the chloride, namely 288. The second change is perhaps caused by the change of sign of the heat of formation of the trichloride AuCl 3 ; when this becomes negative, the pressure of dissociation of the compound would decrease, in accordance with the law of van't Hofl ? and Le Chatelier. However this may be, it is certain that when gold is heated in chlorine at atmospheric pressure, trichloride of gold is formed and volatilised at all temperatures above 180, up to, and probably far beyond, 1,100. The usual method adopted for the preparation of auric chloride is to dissolve gold in aqua regia, and then to evaporate the liquid to dry ness, keeping the temperature above 100 to pre- vent the formation of the hydrochloride AuCl 3 HCl. A brownish- red mass is thus formed, consisting of AuCl 3 mixed with more or less of the protochloride and of hydrochloric acid. On taking up with water the protochloride is decomposed into gold and trichloride, but the hydrochloric acid can only be eliminated by shaking with ether, which withdraws the trichloride from its solu- tion in water. If an attempt is made to drive off the hydrochloric acid by heat, a partial decomposition of the trichloride results. Auric chloride exists both in the anhydrous state and in combination with two equivalents of water, AuCl 3 . 2 H 2 O, when it occurs in orange-red crystals. The anhydrous salt is of a brilliant-red colour, crystallising in needles belonging to the triclinic system and melting at 288 under a pressure of two atmospheres of chlorine. It can be prepared by drying the hydrated salt at 150. The anhydrous and hydrated salts are both hygroscopic, and dissolve readily in water with elevation of temperature ; they are also soluble in alcohol and ether, and in some acid chlorides, such as AsCl 3 , SbCl 5 , SnCl 2 , SiCl 4 , &c. Auric chloride is readily decomposed by heat. Lowe states* that 4 grammes of the trichloride, when heated in a porcelain basin on a boiling water bath, can be completely transformed into the monochloride, although not until after the lapse of several days. On the other hand, as has already been mentioned, Kriiss states that the decomposition of auric chloride, in an atmosphere of chlorine, begins at 180. According to the experiments of the author,! auric chloride is observed to suffer slow decomposition at as low a temperature as 165 in an atmosphere consisting of chlorine, about 1-6 per cent, being converted into monochloride in four hours at this temperature ; the decomposition is about five times more rapid at 190. The decomposition in air can be readily observed at 100, although it does not seem to be so rapid as was indicated by Lowe. In * Dingler's polyt. Jown., 1891, vol. cclxxix., p. 167. t Ghem. Soc. Journ., vol. Ixvii. (1895), p. 902. 24 THE METALLURGY OF GOLD. seven days only 6 '6 per cent, of the trichloride was decomposed, the initial rate of decomposition being 0-041 per cent, per hour. At 165, however, the initial rate of decomposition appeared to be 3-2 per cent, per hour, and the conversion into monochloride was complete in four or five days at 160 and in ten hours at 190. The rate of decomposition of the trichloride in air at various temperatures can be calculated from the above data by the help of Harcourt and Esson's formula a y a ^ = ( Tl / r ) , where 15 2 are the rates of decomposition at the absolute temperatures r v rg respectively. The value of the constant ra for this chemical action is found to be about 27, and by substituting this number for m in the equation, the rate of decomposition of gold trichloride is calculated to be 0-365 per cent, in a year at 15. The decomposition begins to be observable at 70 in air, when the monochloride is formed, about twenty- five years, however, being required for the complete change at this tem- perature. The observed rate of decomposition shows that a similar change would require about 1,000 days at 100, while it results from calculation, using Harcourt and Esson's formula, that at 200, thirty-six hours, and at the melting point namely, 288 less than one minute suffices for the complete decomposi- tion of AuCl 3 in air. Whether dry or in aqueous solution, the trichloride is also decomposed by light, gold being deposited in scales in the latter case, but the presence of free hydrochloric acid prevents this decomposition. Weak voltaic currents precipitate metallic gold from the solution of the trichloride upon the negative pole. The solution of trichloride of gold is also decomposed by many re- ducing agents, such as most organic substances, metals and protosalts; heating the solution in every case hastens the decomposition. The reduction by organic matter is assisted by the action of light, which is especially efficacious in the presence of starchy and saccharine compounds, or of charcoal or ether. In direct sunlight the last reagent deposits a bright mirror of metallic gold, but under ordinary conditions excessively finely divided gold (Faraday's gold) is precipitated. Alkalies also quicken the action of organic matter, and it may be said that all organic compounds reduce gold chloride on boiling in the presence of potash or soda,* while Miiller states that a mixture of glycerine and soda lye is one of the best precipitants for gold chloride, separating the metal completely in highly dilute solutions. According to Kriiss, if potash and soda are quite free from organic matter, they have no action on solutions of auric chloride, whether cold or hot. If a small quantity of organic matter is present, sub-oxide of gold is precipitated ; if larger quantities are present, both metallic gold and sub-oxide * Fr&ny, Ency. Chim., vol. iii., 16e Cahier, p. 74. CHLORIDES OP GOLD. 25 are precipitated in the cold, but gold alone at boiling point. Alkaline carbonates are without action on cold solutions, but if they are hot, then half the gold is precipitated as hydrate, while the other half remains in solution in the form of a double chloride of gold and the alkali. The precipitation by means of charcoal is of especial importance in view of its adoption in practice. It has been stated that a current of hydrogen gas will preci- pitate gold completely, especially on boiling the solution, but Kriiss has proved that, if the hydrogen is quite pure, it has no- effect either on cold or hot solutions. Sulphur, selenium, phos- phorus and arsenic all precipitate gold on boiling the solution of the trichloride. Many metals reduce chloride of gold, the action being, of course, most rapid in the case of the most highly electro-positive metals, such as zinc and iron. It seems probable from the author's experiments * that turnings of these metals would make good precipitants for use on a commercial scale in the chlorination process. Lead sometimes gives fine dendritic plates of gold. Sulphuretted hydrogen precipitates sulphide of gold from both neutral and acid solutions, all traces of gold being readily removed from a solution by this reagent, whilst phos- phoretted, arseniuretted, and antimoniuretted hydrogen all precipitate metallic gold. The lower oxides of nitrogen, nitrous acid and many other " ous " acids and oxides effect the same decomposition. Sulphur dioxide is a convenient reagent, and is often used in the laboratory, being almost equally effica- cious in cold and hot solutions. The reaction is 2AuCl 3 + 3S0 2 + 6H 2 = 2Au + 6HC1 + 3H 2 S0 4 . Various protosalts also reduce trichloride of gold. Ferrous sulphate is often used to detect the presence of gold in solution as chloride ; this reagent gives the solution a pale blue colour by transmitted light, and brown by reflected light, owing to the formation of finely divided precipitated gold. The reaction is. represented by the following equation 2AuCl 3 + 6FeS0 4 = 2Au + 2Fe 2 (S0 4 ) 3 + Fe 2 Cl 6 . To test a dilute solution for gold, a test tube filled with the liquid is held in the hand side by side with a test tube filled with distilled water and a few drops of a clear solution of ferrous sulphate are added to each. On looking down through the length of the test tubes from above, with a white surface as background, any slight changes of colour may be detected by comparison and the liquids may also be compared with the original solution in a test tube. In this way, by a little prac- tice, the presence of gold in the proportion of only T^Vinr (1 dwt. per ton of water), or even less can be detected. The * See Section on " Precipitation of Gold," Chap. xiii. 26 THE METALLURGY OF GOLD. method is often used in the chloriiiation process, but it is better to use protochloride of tin, SnCl 2 . This substance gives a brown precipitate of variable composition in concentrated solutions, but if mixed with the tetrachloride, SnCl 4 , it gives a precipi- tate of purple of Cassius. The reaction is very sensitive, and by its means a violet coloration by transmitted light can be obtained in a solution containing 1 part of gold in 500,000 parts of water, while by special means the presence of 1 part of gold in 100,000,000 parts of water can be detected, as de- scribed below : * The liquid supposed to contain gold is raised to boiling, and poured suddenly into a large beaker contain- ing 5 to 10 c.c. of saturated solution of stannous chloride, and the liquids agitated so as to effect complete mixture. A yellowish-white precipitate of tin hydrate forms, which settles rapidly, and can be readily separated from the bulk of the liquid by decantation. If the solution originally contained at least 1 part of gold in 5,000,000 of water (3 grs. per ton), the precipitate is coloured purplish-red or blackish-purple, accord- ing to the nature of the solution, and the condition of the precipitant. The colour can be seen without comparing it with other precipitates. If less gold than this is present it is better to compare the precipitate with one obtained by the use of boiling distilled water, and to increase the quantity of liquid used while adhering to the same amount of stannous chloride. In this way the presence of 1 part of gold in 100,000,000 parts of water (1 grain of gold in 6 tons of water) can be detected, the amount of liquid required in this case being about 3 litres. The gold is concentrated in the precipitate in which a distinct colour is caused by less than OO5 per cent, of the precious metal. Chlor-auric Acid, H AuCl 4 . Gold trichloride in the presence of free hydrochloric acid is supposed to form this compound, which crystallises out on evaporation in vacuo in long, yellow needles, having the composition HAuCl 4 + 4H 2 O, and since gold chloride unites with many other soluble chlorides to form double chlorides, this hydrochloric acid compound is regarded as an acid. It is more stable than gold trichloride. The chlor- aurates, having the general formula M'AuCl 4 , or AuCl 3 . M'Cl, are readily soluble bodies which can be crystallised, and which decompose with about the same readiness as chlor-auric acid. BROMIDES OF GOLD. Gold Protobromide, AuBr, is a yellowish-green powder ob- tained by heating the tribromide to about 140. It is insoluble in water, but is decomposed by it, metallic gold and the tribro- * T. K. Rose, in Chem. News, 1892, vol. Ixvi., p. 271. CYANIDES OF GOLD. .. 27 mide being formed ; the change is especially rapid on boiling, and is hastened by the presence of hydrobrornic acid. Auro-auric Bromide, Au 2 Br 4 , is produced by the action of bromine on finely-divided gold in the cold, some tribromide being simultaneously formed. Water breaks up this bromide into AuBr and AuBr 3 , and, according to some authorities, it is only a mixture of these bodies.' G-old Tribromide, AuBr 3 , is produced by the action of a mixture of bromine and water 011 gold, particularly on the application of heat. Auric tribromide resembles the trichloride in most of its properties. It crystallises in blackish needles or scarlet plates. It is deliquescent, and very soluble in water, and suffers decompositions similar to those noted in describing AuCl 3 , its solutions being still less stable than those of the chloride. A solution of gold tribromide is gradually decolorised by sulphur dioxide, being completely reduced to the state of monobromide before any precipitate of metallic gold is formed. It is prepared in a pure state by heating finely-divided gold in sealed tubes with bromine and arsenic bromide, AsBr 3 , to 126. Gold tri- bromide forms intensely coloured brownish-red aqueous solutions, the presence of a mere trace of the salt in a solution being observable in this way. Double bromides exist analogous to the chlor-aurates. The bromides are not volatile. The Iodides of Gold are of little interest to the metallurgist. They are prepared with difficulty, and decompose more readily than the chlorides and bromides and are not likely to become of practical importance. The tri-iodide is formed if gold is heated with water and iodine to 50, particularly in direct sunshine. CYANIDES OP GOLD.* Cyanogen and gold unite in two proportions forming aurous and auric cyanides, but the latter is only known with certainty in combination. Aurous Cyanide, AuCy, is obtained by heating aurocyanide of potassium, KAuCy 2 , with hydrochloric or nitric acid. It is a lemon-yellow crystalline powder, insoluble in water, and unaltered by exposure to air. It is decomposed by heat, yield- ing metallic gold and cyanogen, and is soluble in ammonia, in alkaline cyanides, and in hyposulphite of soda. It is unattacked by the mineral acids, except by aqua regia, but is decomposed when boiled with potash, metallic gold being thrown down. Aurocyanide of Potassium, KAuCy 2 , is obtained by crystal- lisation from its solution, which is prepared by dissolving metallic gold, auric oxide or aurous cyanide in a solution of potassium cyanide. It is slightly soluble in water and the * See also chapter xvi. 2C THE METALLURGY OP GOLD. aqueous solution, especially if hot, gilds copper or silver without the agency of a battery, the gold being replaced in solution by the other metal. Precipitates are also formed on the addition of salts of zinc, tin, iron, or silver, no precipitates being formed if potassium cyanide is present in excess. Auricyanide of Potassium, AuCy 3 . KCy, is formed by adding potassium cyanide to a solution of trichloride of gold, the pre- cipitate first formed being redissolved. The solution is com- pletely decolorised, and on cooling deposits colourless crystals of AuCy 3 . KCy + 3H 2 O. These effloresce in air, giving up two molecules of water ; and, on heating, the third molecule of water and some cyanogen are given off, aurocyanide of potassium being formed, and this in its turn is decomposed at a slightly higher temperature. OXIDES OF GOLD. Aurous Oxide, Au 2 O. This oxide is prepared by decomposing aurous chloride, AuCl, or the corresponding bromide by potash in the cold (Berzelius), when a violet precipitate forms which is blackish when moist, but greyish when dry. When freshly precipitated it is soluble both in alkalies and in cold water, forming an indigo blue solution, with brownish fluorescence, and on warming the solution slightly the corresponding hydrate is pre- cipitated. It is also prepared by the action of nitrate of mercury on the trichloride, and by boiling aurate of potash with organic compounds, such as citrates or tartrates, or by boiling a solution of the trichloride with the potassium salts of these acids. When prepared according to these methods, aurous oxide always contains a certain proportion of metallic gold. Kriiss obtained the oxide pure by reducing brom-aurate of potassium at by SO 2 , passing in the gas only until the solution became colourless, after which an excess of gas would have precipitated metallic gold. Aurous hydrate is then precipitated by potash, and, after being agglomerated by boiling, it is filtered, washed with cold water, dried, and heated to 200 to expel the water of hydration. At 250 it is resolved into gold and oxygen. Hydrochloric acid decomposes aurous oxide into metallic gold and auric salts, slowly in the cold, quickly at a boiling temperature; aqua regia dis- solves the oxide, but sulphuric and nitric acids are without action on it, while weak bases at once decompose it. An intermediate oxide, AuO, is prepared as a black powder by dissolving metallic gold in aqua regia containing an excess of hydrochloric acid, then adding an excess of carbonate of potash, and afterwards filtering and drying the precipitate. It has been little studied, but the temperature at which it decomposes has been fixed at 205 and its hydrate has been prepared. Auric Oxide, Au 2 O 3 . This, the best known oxide, is a black OXIDES OF GOLD. 29 powder when anhydrous, and is precipitated from solutions of auric chloride in the form of a hydrate by the caustic alkalies, the carbonates of the alkalies, and hydrates of the alkaline earths or zinc. The readiest method of preparation of this compound is to add caustic potash, little by little, to a hot solution of gold chloride, until the yellow precipitate first formed is dissolved to a brown liquid. Then a slight excess of sulphuric acid or some Glauber's salt is added, the precipitate filtered off, washed and purified from potash by being redissolved in concentrated nitric acid, and reprecipitated by dilution with water. On drying this precipitate in vacuo, the hydrate Au 2 O 3 . H 2 O, an ochreous powder, results. If it is heated to 110, oxygen begins to be given off. At 160, AuO remains, and on heating for some time ;it 250, metallic gold remains. Trioxide of gold dissolves in concentrated sulphuric and nitric acids, from which it is partly reprecipitated on boiling or on dilution, and these solutions are supposed to contain sulphates and nitrates of gold respectively. Double nitrates of gold and the alkalies have been obtained as crystals. Hydrochloric and hydrobromic acids dissolve the trioxide forming the haloid salts, but hydriodic acid decomposes it on boiling, giving iodine and metallic gold. Gold trioxide dissolves in boiling solutions of alkaline chlorides, giving aurates and chlor-aurates, while it also combines with metallic oxides to form aurates. It is easily reduced by hydrogen, carbon and carbonic oxide, with the aid of very gentle heat. Boiling alcohol reduces it, yielding minute spangles of gold which were formerly used in miniature painting. Aurates. The aurates of potash and soda have the general formula Au 2 O 3 . R' 2 O or K' 2 Au 2 O 4 assigned to them. They are readily soluble, crystallisable compounds, and are formed when alkalies are added in excess to solutions of gold chloride. The aurates of calcium, magnesium and zinc are insoluble in water, but soluble in hydrochloric acid. Fulminating Gold is a compound of auric oxide with am- monia, Au 2 O 3 (NH 3 ) 4 , which is formed by precipitating gold chloride with ammonia or its carbonate, or by the action of ammonia on gold trioxide. When prepared by the former method its composition is variable, but the fulminate is always a fearful explosive, decomposing with violence at 145, or 011 being struck, and sometimes even spontaneously. It is de- composed without explosion by sulphuretted hydrogen, and by protochloride of tin. It is a grey pulverulent powder, insoluble in water, but soluble in potassium cyanide, auricyanide of potassium being formed. Sulphites of Gold. Alkaline sulphites, or sulphur dioxide, which reduce gold trichloride easily, do not produce the same effect on a solution of an alkaline aurate. If sodic bisulphite is 30 THE METALLURGY OF GOLD. added to a boiling solution of sodic aurate (NaAuO 2 ) a yellowish precipitate is formed, soluble in excess of sodic bisulphite, and consisting of a double sulphite of gold and sodium, or sodic auro-sulphite, having the composition 3Na 2 SO 3 . Au 2 SO 3 + 3H 2 O. It is obtained pure by precipitating the corresponding baric salt with Bad,,, and decomposing the precipitate with the minimum quantity of sodic carbonate. Double sulphites of potassium and ammonium with gold also exist. These salts are decomposed by acids, sulphite of gold being deposited, and also on boiling their aqueous solutions, but the addition of sulphuretted hydrogen or alkaline sulphides has no effect on them. Hyposulphites of Gold. These compounds are now called thiosulphates by chemists, but the old name is retained as being universally employed by metallurgists. They are especially interesting to the metallurgist, as on their formation depends the extraction of gold from auriferous silver ores, when these are treated by the ordinary hyposulphite, or by the Russell process. The soluble double hyposulphites of gold with the alkalies and alkaline earths have the general formula 3R"S 2 O 3 . Au 2 S 2 O 3 + 4H O. The double compounds of gold with sodium, potassium, calcium, magnesium and barium, are all known. The sodic salt is prepared by adding a dilute solution of gold trichloride little by little to a concentrated solution of sodic hyposulphite, when the following reaction occurs : 8Na 2 S 2 3 + 2AuCl 3 = Au 2 S 2 3 . 3Na 2 S 2 3 + 2Na 2 S 4 O 6 + GNaCl. The double hyposulphite may be separated by precipitation with strong alcohol, with which it is also washed, or it may be purified by repeated solution in water and precipitation with alcohol. Thus prepared, it consists of colourless crystalline needles, highly soluble in water, but almost insoluble in alcohol. The solution, which possesses a sweetish taste, decomposes under the influence of heat, the action being much more rapid when nitric acid is present; metallic gold and sulphate of soda are formed. Gold, however, is not reduced from its solution as double hyposulphite by either stannous chloride, ferrous sulphate or oxalic acid, although sulphuretted hydrogen and alkaline sulphides give a black precipitate of Au 2 S 3 . The addition of hydrochloric acid or of dilute sulphuric acid does not immediately cause an evolution of sulphur dioxide and a deposit of sulphur, as in the case of ordinary hyposulphites. Since, therefore, the double sulphite of soda and gold does not present the characteristics of either aurous salts or of hyposulphurous acid, it has been sug- gested that it contains a compound radical, and has a composition expressed by either Na 3 S 4 O G Au or Na,S 4 O 6 Au + 2H 2 O. The addition of any dilute acid soon effects the decomposition of this body in solution, gold sulphide being precipitated j the reaction is accelerated by heat. OXIDES OF GOLD. 31 The double hyposulphites of potassium, calcium, barium and magnesium present similar characteristics. If the barium salt is treated with the amount of sulphuric acid required by theory, a solution of the acid auro-hyposulphite, 3H 2 S 2 O 3 . Au 2 S 2 O 3 , is obtained, but it cannot be crystallised. It has been supposed that the calcium salt is more easily formed than the sodic salt,, and, therefore, that calcium hyposulphite is more suitable than, sodium hyposulphite for use in the leaching process, whenever gold is present in perceptible quantities. According to a series of experiments conducted by Russell,* this is not the case, very little difference existing in the case of formation of the calcic, sodic, magnesic and potassic salts. Russell has demonstrated f that finely divided gold is soluble- to a limited extent (i.e., OO02 gramme in 1,000 c.c. in 48 hours),, in solutions of sodic hyposulphite of all degrees of concentra- tion. The action depends on the oxidation of the gold by the air present in the solution, the soluble double hyposulphite, Au 2 S 2 O 3 . 3Na 2 S 2 O 3 + 4H 2 O, and caustic soda being formed. The formation of this hyposulphite by the action of the sodic- salt on gold sulphide is far more complete and rapid. In 24 hours in the cold, 0*066 gramme of gold, and in 2 hours at 65, 0*117 gramme of gold were dissolved in dilute solutions. Since an alkaline sulphide will again precipitate Au 2 S 3 from the solution, these facts seem to be at variance, and Stetefeldt suggests that the gold sulphide originally contained a small quantity of metallic gold in an excessively fine state of division, and that on heating more gold was set free. He instances Level's- statement that sulphuretted hydrogen precipitates metallic gold,, and not gold sulphide, from a boiling solution of the trichloride. He believes J that it is only the free gold which is attacked, and that the results obtained on attacking the sulphide were better than those in which metallic gold was used, owing to the much finer state of division in which the gold existed in the former case. Another reason for the inverse reactions may of course exist in the influence of mass, the small quantity of sodic sulphide which is formed by the reaction being insufficient to precipitate any gold, which comes down on further additions of the same reagent. In some further experiments which Russell conducted, it was proved that gold sulphide is far more soluble in a solution containing the molecular quantities, 4Na.,S 2 O 3 . 3Cu 2 S 2 O 3 (hypo- sulphite of copper and sodium), than in sodic hyposulphite alone, especially if the solutions are cold. If the solution is heated the reduction of the gold sulphide to metallic gold renders it less soluble, and less rapid dissolution occurs. The composition of the double salts obtained by the use of this " extra " solution has not been worked out. * Stetefeldt's Lixiviation of Silver Ores, New York, 1888, p. 90. t/6/rf., p. 19. ilbid., p. 22. 02 THE METALLURGY OP GOLD. SILICATES OF GOLD. The existence of auro-silicates is now admitted without dis- pute, and gold has for centuries been used to impart colour to glasses, the method used being as follows : A solution of chloride of gold is added to a mixture of sand with alkalies and alkaline earths or lead, and the whole is then fused, and colourless or yellow transparent silicates of gold thus formed. These are decomposed by being reheated gently to low redness, oxides of gold, or more probably metallic gold, being set free, and red or purple colorations thus obtained. The occurrence of silicates of gold in nature seems to be doubtful. Experiments conducted by E. Cumenge* tend to show that the alkaline auro-silicates, obtained in the wet way, may have played an important part in the formation of auriferous quartz. The following conclusions have been established by these investigations : 1. If an alkaline aurate, obtained by dissolving auric ses- quioxide in caustic soda, is mixed with an alkaline solution of silicate of soda (soluble glass), the mixture may be concentrated by evaporation until it has attained a syrupy consistency with- out being decomposed. Auro-silicate of soda is, therefore, fairly stable, so long as there is an excess of alkali present. 2. The decomposition of this auro-silicate is effected by the addition of hydrochloric acid to it, by which gelatinous silica is precipitated. This carries down a certain proportion of gold which gives a rose colour to the white magma. 3. This decomposition may also be completely effected by the action of an aqueous solution of carbonic acid under pressure. Thus, if the syrupy, alkaline auro-silicate is introduced into a bottle of seltzer water, which is then hermetically closed, the decomposition can be seen to be gradually going on without the semi-fluid mass being dissolved, and the latter is replaced at the end of some days by coherent silica, which, on exposure to the air, assumes a white opaline appearance tinged with rose colour. 4. When gelatinous silica, obtained by the decomposition of an alkaline auro-silicate, is heated to redness in a current of steam, it assumes either a beautiful, unalterable rose colour, or a reddish tint with visible grains of gold, according to the pro- portion of precious metal present, and the conditions under which the precipitation has been effected. SULPHIDES OF GOLD. These compounds are prepared as brown or black precipitates by passing sulphuretted hydrogen through a solution of gold * Frgmy, Ency. Chim., vol. iii., L'or, p. 62. SULPHIDES OF GOLD. 33 chloride. The exact composition of the precipitate varies with the temperature and degree of concentration of the solution, and the amount of free acid present. Levol and Kriiss state that Au. 7 S is precipitated in the cold, but that only metallic gold and free sulphur are thrown down from boiling solutions. According to others, Au 2 S 3 is precipitated from cold and Au 2 S from boiling solutions. It seems probable, however, that free sulphur is always formed in considerable quantities, and whether the solutions are hot or cold, dilute or concentrated, definite compounds are not precipitated, variable mixtures of the two sulphides with free sulphur and metallic gold being formed. Similar precipitates are formed by alkaline sulphides and by sulphides of most of the heavy metals. The sulphides are soluble to some extent in a saturated solution of sulphuretted hydrogen, and are easily soluble in hot solutions of alkaline sulphides or alkalies, forming double salts so that precipitation from alkaline solutions is never complete. The sulphides are readily decomposed into gold and sulphur by the action of heat, the decomposition being complete below 270. Sulphide of gold is also dissolved at ordinary temperatures by potassic cyanide, and is slowly attacked by mercury with formation of mercury sulphide. Purple of Cassius. This body was discovered by Cassius of Ley den in 1683. It contains gold and oxide of tin, and is used to colour glass and glazes, various shades of violet, red and purple being thus obtainable. Several methods of prepara- tion are used, of which the following is that employed at the factory at Sevres""" : Half a gramme of gold is dissolved in aqua regia composed of 16*8 grammes of hydrochloric and 10*2 grammes of nitric acid, and the solution is then diluted with 14 litres of water. To this solution is added, drop by drop, a solution of a mixture of protochloride and tetrachloride of tin, prepared as follows : 3 grammes of finely divided tin is dropped, little by little, into 18 grammes of aqua regia (constituted as above, with the addition of 5 c.c. water), the reaction is checked by cooling if it is too violent, and the solution of chloride of tin formed is allowed to cool. The precipitate of purple oxide thus obtained is finely coloured when it has been washed with boiling water. The purple precipitate obtained by Miiller, by reducing chloride of gold with glucose in an alkaline solution containing tin oxide in suspension, differs from that prepared by the foregoing method in losing its colour at a red heat, while the true purple of Cassius becomes brick-red under such conditions. The true colour is seen when metallic tin acts on trichloride of gold, or when alloys of gold and tin are attacked with nitric acid. The composition of purple of Cassius has been the subject of much discussion. Some chemists have considered it to be a compound of aurous oxide with the oxides of tin. Debray * Fr^my's Ency. Chim., vol. v., L'or, p. 63. 3 34 THE METALLURGY OF GOLD. regards it as a lake of stannic acid coloured by finely divided gold. If this latter view is correct then the gold may be present in an allotropic modification, since (1) the purple of Cassius is completely dissolved by ammonia, a purple solution being formed, and the solution may be kept unaltered for weeks although it is decomposed by light, or on heating, becoming bluish, and finally depositing metallic gold; the ammonia may be removed by dialysis, leaving a purple aqueous solution which contains both gold and stannic oxide;* (2) the purple of Cassius does not yield any gold on treatment with mercury. Miiller confirms Debray's views, showing that fine purple compounds can be made with gold and magnesia, lime, baryta, sulphate of barium, &c., the colour depending on the presence of finely divided gold and not on the other constituent. The purple colour possessed by (possibly allotropic) gold, when in a finely divided condition, is further attested by the purple stain given to the fingers by a solution of gold trichloride, and by the colour of Roberts- Austen's aluminium alloy, AuAl 2 . CHAPTER III. MODE OF OCCURRENCE AND DISTRIBUTION OF GOLD. Forms in which Gold occurs in Nature. Gold is obtained from two very different sources, viz., (1) from veins in rock for- mations, and (2) from placers, or the alluvial deposits of ancient and modern streams. Vein Gold. In this case the metal, whenever it is present in visible grains or masses, has sharp angular edges, and although usually not distinctly crystalline, it frequently penetrates the rock irregularly in various directions, and is completely inter- woven with, and attached to the matrix, usually quartz, so that the metal cannot be separated from the rock without crushing the latter. The gold in lodes is sometimes in the form of crystallisations, which are, however, exceedingly rare, and crystals of gold are still probably unknown to most miners, although they occur more fre- quently in placer deposits. Arborescent branching and dendritic masses of crystalline gold are more common than single crystals in both quartz lodes and placer deposits. The crystalline forms met with have already been described, p. 8. In the Transyl- vanian lodes, gold occurs chiefly in thin sheets or plates, often as * Chem. Soc. Journ., vol. Ixiv. (1893) p. 575. OCCURRENCE AND DISTRIBUTION OF GOLD. 35 much as from half an inch to two or more inches in breadth. Such plates are rarely thicker than- a visiting card, and are generally covered with crystalline lines and markings, revealing a distinct geometrical structure. Gold also occurs in wire-like forms, sometimes penetrating crystals of other minerals, such as calcite and dolomite. The matrix in which the gold is contained is usually quartz, intersecting as veins or interlaminated with sub-crystalline, slaty, or schistose rocks, especially hydromica and chloritic slates. Gold also occurs sparingly in similar veins in granite and gneiss, and has been detected in the trachytes of Colorado, and in silurian and carboniferous trachytes, as well as in some lime- stones. With regard to the distribution of gold, W, P. Blake ob- serves* " There is a much greater dissemination of gold in a ragged granular condition, in situ, in fine particles in the midst of rock formations, and without any obvious connection with veins, than is generally supposed. Prominent examples are found in the belts or zones of layers of soft slate in Georgia, and in North Carolina. ... The Boly- Fields gold vein, Lumpkin County, Georgia, is an example of the occurrence of coarse ragged gold in the midst of a mass of slate, without any defined quartz vein. The gold is closely associated with bornite, pyrites, and dolomite." The dissemination of gold in the schistose rocks of North Carolina has also been noted by Professor Kerr,f and by Dr. Emmons, and similar occurrences have been observed in Texas, Nova Scotia, and in other parts of the world. At the Contention Mine, Tombstone, Arizona, free metallic gold is found in the thin cracks and cleavage surfaces of partially decayed porphyry, and appears to have been deposited there from solution, and not mechanically. It occurs in thin subcrystalline flakes and scales, and may have been derived from the decomposition of the iron pyrites with which the adjoining sedimentary formations are charged. Gold also occurs in small quantities (1 part in 1,124,000) in the bed of clay on which the city of Philadelphia is built. Its occurrence in solution in sea water has been proved by Sonstadt.J The wide distribution of gold in minute quantities throughout the world was pointed out by W. E. Dubois, an Assayer in the United States Mint, in 1861, and is further attested by a large number of specimens now in the Percy collection. These con- sist of small specks of gold of different sizes which have been obtained from the most varied sources. Thus, samples of Pattin- son's crystallised and uncrystallised lead, pig lead from all * Prod. Gold and Silver in United States, 1884, p. 581. t Trans. Am. Inst. Mng. Eng., vol. x., p. 475. J Chem. News, vol. xxvi., p. 159. Journ. Am. Phil. Soc., June 1861. 36 THE METALLURGY OF GOLD. countries, lead fume, red lead, litharge, white lead, precipitated carbonate of lead and acetate of lead were all found to contain gold, which seems to be invariably present in galena. Moveover, it appears to be impossible to procure samples of copper in which gold cannot be detected, although the Lake Superior copper contains less than 1 part in 1,000,000; the bronze and copper coins of all nations are usually found to contain much greater quantities of gold than this. Similar evidence has been adduced which tends to show that all ores of silver, antimony, and bismuth contain gold. * Placer Gold and Nuggets. Placer gold is usually in the form of small scales, but pellets or rounded grains also occur, while larger masses or nuggets are usually of a rounded mamrnil- lated form. The chief difference between the appearance of placer and vein gold lies in the fact that the former is always rounded, showing no sharp edges, even the crystals having their angles smoothed and rounded off. This has been pointed to by the advocates of the erosion theory of the origin of placer gold, as evidence in favour of their views, the roundness of the fragments being taken to prove that abrasion of the gold has been effected by attrition with water and grains of sand. The largest masses of gold yet discovered have been found in auriferous gravel. The "Blanch Barkley" nugget, found in South Australia, weighed 146 pounds, and only six ounces of it were gangue; and one still larger, the " Welcome " nugget, from Victoria, weighed 2,195 ounces, or 183 pounds, and yielded gold to the value of 8,376 10s. 6d. In Eussia a mass was found in 1842 near Miask, weighing 96 pounds troy. The largest mass from California is given in the State Mineralogist's report as weigh- ing 2,340 ounces, or 195 pounds, but no authentic cases seem to be on record of nuggets from this State weighing more than 20 pounds. The minerals most common in auriferous quartz lodes or in the placer deposits are platinum, iridosmine, magnetite, iron pyrites, galena, ilmenite, copper pyrites, blende, tetradymite, zircon, garnets, rutile and barytes ; wolfram, scheelite, brookite and diamonds are less common. Diamonds are associated with gold in Brazil, and also occasionally in the Urals and in the United States. The sulphides present in auriferous quartz frequently contain gold ; the gold in such an ore is usually in part quite free, disseminated through the quartz, in which visible grains of the metal often occur, and in part locked up in the pyrites, whence but little can in general be extracted by mercury. It is, however, in all probability in the metallic state in pyrites, although this is not completely established. The subject is discussed in Section on " Gold in Pyrites," Chap. vii. * Loc. cit., and E. A. Smith on Bismuth, &c., Journ. Soc. Chem. Ind. t vol. xii. (1893), No. 1. OCCURRENCE AND DISTRIBUTION OF GOLD. 37 Among minerals other than sulphides which contain gold, the following may be mentioned, although none are of great importance in metallurgy : Calaverite is a bronze - yellow gold telluride, usually containing a little silver, occurring in certain mines in California and Colorado. One analysis gave tellurium 55 -5 per cent., gold 44'5 per cent., corresponding to the formula AuTe. 2 . Sylvanite, called also graphic tellurium, is a telluride of gold and silver, supposed to correspond to (AuAg)Te 3 . It sometimes contains antimony and lead in ad- dition. It is from white to brass-yellow in colour, and the arrangement of the crystals sometimes bears a resemblance to writing characters, whence the name graphic. It occurs in Transylvania, in Calaveras County, California, and in Colorado. Petzite is a telluride of silver, Ag 2 Te, in which the silver is partly replaced by gold. A specimen from the Golden Rule Mine, according to Genth, contained tellurium 32*68 per cent., silver 41-86 per cent., and gold 25-60 per cent. It occurs in Transylvania, Chili, California, Colorado, and Utah. Nagyagite or Foliated Tellurium is remarkable for being foliated like graphite, which it also resembles in its colour, a blackish lead-grey, and in having a hardness of from 1 to 1-5 only Its density, however, is above 7. It occurs in Transylvania, and contains tellurium 32*2, lead 54'0, gold 9-0 to 13-0 per cent., some- times with silver, copper and sulphur in addition. Other gold tellurides and some native gold amalgams are occasionally met with, but these minerals in which gold is an essential part are rarely of much importance as ores, as they seldom occur in sufficient abundance to be regarded as anything but specimens for collectors. In some few mines, however, notably at the Cripple Creek district, Colorado, in Transylvania, in Boulder County, Colorado, and at the Bassick mine in the same State, the value of the ore depends on the tellurides of gold con- tained in it. One of the most striking differences between the ores of gold and those of all other metals lies in the extremely small propor- tion which the desired material bears to the worthless gangue with which it is accompanied. Occasionally hand specimens of vein stuff are found containing several per cent, of the precious metal, but these are of quite exceptional occurrence and have not the slightest economic importance. The greater part of the vein gold now being produced is derived from ores containing only about one part of gold in seventy or eighty thousand, whilst, under exceptional circumstances, a yield of one part in half a million parts of gangue may give handsome profits. Placer deposits are usually much less rich than this; the average amount of gold contained in, those now worked does not exceed one part in one million, and in California deposits of gravel with only one part of gold in fifteen millions have proved susceptible of successful treatment by the " hydraulicking " method on a large scale. 38 THE METALLURGY OF GOLD. Composition of Native Gold. Native gold usually contains silver, which occurs in varying proportions, the colour becoming paler with the increase of silver. The finest native gold yet found is that from the Mount Morgan Mine, Queensland, which is 997 fine. The finest Russian gold was that formerly obtained at Katerineburg in the Urals, and yielded gold 989'6, silver 1*6, copper 3-5, and iron 0*5 (G. Rose). Gold dust from West Africa has been found to contain 978 '1 of fine gold, the remainder being silver. The gold found in the British Isles varies from 800 to 900 fine, the remainder being silver ; the specimens from the district around Dolgelly are sometimes a little over 900 fine. Particulars of the fineness of gold from Australia, California, &c., are given in the chapter on Refining, Chap, xviii. Gold is occasionally found alloyed with copper, and sometimes also with iron, bismuth, palladium, or rhodium. Rhodic-gold from Mexico was found to be of the specific gravity 15*5 to 16*8 and contained 34 to 43 per cent, of rhodium. Bismuthic-gold has been called Maldonite.* Geographical Distribution of Gold. Gold occurs in lodes in many districts composed of partially metamorphosed rocks such as slates or schists, while its occurrence in holocrystalline- metamorphic, or igneous rocks is comparativaly rare. Among sedimentary rocks, its occurrence is almost confined to the sands of rivers which run for a part of their course through crystalline formations, or more particularly through districts in which gold occurs in quartz veins. Such river sands are rarely quite free from gold. The beds of ancient rivers no longer existent are also frequently auriferous. In spite of the fact that the sea contains gold in solution, the aggregate amount perhaps exceeding that contained in the accessible portion of the earth's crust, nevertheless, unaltered marine deposits seldom or never contain a perceptible quantity of the metal, except in one or two cases of beach deposits formed by the erosion of auriferous land- formed gravels. In the British Isles, gold is found in some of the streams of Cornwall and in lodes and river gravels near Dolgelly and in other parts of Wales, in Sutherlandshire and near Leadhills in Scotland, and in the County of Wicklow. The total amount which has been obtained from these localities is small, probably not exceeding 40,000 ounces, and little is now being produced. On the Continent of Europe, gold is most abundant in Hungary and Transylvania, where the gold occurs in quartz lodes contained in eruptive rocks of tertiary age, chiefly propylite, porphyry, diorite and granite. The minerals occurring with the gold are galena, blende and pyrites. In the German Empire, the gold obtained is chiefly derived from the smelting of argentiferous galena in which small quantities of the more precious metal are contained. * Dana's Mineralogy, p. 108. OCCURRENCE AND DISTRIBUTION OF GOLD. 39 In Italy the only important mines are those of Pestarena and Val Toppa in North Piedmont, near Monte Rosa. The pyritic ores from these mines are treated by amalgamation. Gold is also found in the sands of the Rhine, the Reuss, the Aar, and other rivers, and in small quantities in Sweden. The gold-bearing districts of R-ussia are (1 ) the Urals, (2) Siberia, Eastern and Western, whilst an insignificant amount is' also derived from Finland and from the Caucasus. The gold was for- merly derived chiefly from lodes both in the Urals and in Western Siberia. In the Urals, quartz-mining began in 1745, and the output from this source continually increased up to the year 1810, when it began to fall off, and has been trifling since 1838. The placers, which are mainly situated on the eastern slope of the range, were discovered in 1774, and the yield has continually increased since then up to the present time. The Siberian placers were discovered in 1829, although quartz-mining had been prose- cuted since 1704.* The output from these continually increases, in spite of the exhaustion of the old placers and the reduction in the percentage of gold contained in those still worked ; this increase of output is in consequence of the discovery of new placers further east. The auriferous gravels are all thin, shallow deposits, ranging from 3 to 20 feet in thickness, and as they are worked out, other gravels are opened-up further east, so that operations are being gradually transferred from west to east. When the exhaustion of these placers has proceeded further it may be expected that more attention will again be paid to the quartz lodes. The relative amounts produced by the different districts are given as follows : f Ural, per Cent. Western Siberia, per Cent. Eastern Siberia, per Cent. Finland, per Cent. 1876-80, . 20-0 6-0 74-0 0-02 1890, . 28-0 6-4 65-4 0-04 Almost the whole of the production is now derived from shallow alluvial deposits by means of sluicing operations, as described on pp. 58-61. In India, almost all the gold now being produced is derived from the quartz lodes of the Colar gold-field, Mysore, in Southern India, in which work was begun in the year 1880. The ore is free milling, and is treated by amalgamation in the stamp battery. A little gold also comes from the Presidency of Madras. In China and Japan considerable quantities of gold are produced, * Mineral Industry for 1892, p. 203. t Prod, of Knld and Silver in U.S.A., 1886, p. 81 : Mineral Ind., 1892, p. 203. 40 THE METALLURGY OF GOLD. chiefly by various primitive methods ; little is known of the methods used and of the amount produced in the former country. From the United States a large percentage of the total gold production of the world is obtained. The chief producing States are California, Colorado, Dakota, Montana, Nevada, Idaho and Oregon, but smaller amounts come from many other States. The produce is now far more from lodes than from placer deposits, and in the treatment of auriferous quartz and pyritic ores almost all the known methods of treatment are applied in different localities.* Mexico produces considerable quantities of gold, and gold ores are also found in various parts of Canada, Colombia, Bolivia, Chili, Venezuela, Brazil, Peru, and the small States of Central America. The production of several of these countries was formerly much larger than it is at the present day, the reduction being especially marked in the cases of Brazil and Venezuela. On the other hand, the gold districts of British and French Guiana, of the Argentine Republic and of Uruquay are now being developed, and may eventually prove to be of considerable importance. Gold is somewhat widely distributed in Africa, the chief sources of production in former times being the placer deposits of the Gold Coast and Abyssinia. The discoveries of auriferous quartz deposits in the Transvaal since 1884 have converted that region into one of the most important among gold producing countries. The richest deposits are in the Banket formation on the Witwatersrand, whence about nine-tenths of the gold obtained in the Transvaal is derived, the De Kaap field being next in importance. No placer deposits of value have been discovered, and the gold is mainly obtained by stamp-battery amalgamation, followed by treatment of the tailings by means of the cyanide process. The produce from both sources is being continually increased. The production by means of treatment with cyanide is now about 30 per cent, of the whole. Gold is found in all the Colonies of Australia, together with Tasmania and New Zealand; the largest amount is now pro- duced by Victoria, and Queensland, New South Wales, and New Zealand follow in the order named. The chief gold pro- ducing districts of Queensland are Charters-Towers, Rockhampton (where the Mount Morgan mine is situated), Croydon, and Gyinpie. Almost all the gold is produced from the quartz mines, the placers having been practically exhausted ; thus in 1891, out of a total production of 576,439 ozs., 560,418 ozs. were produced from quartz, and only 16,021 ozs. from alluvial deposits, while in 1877 the amounts were Placer gold, 164,778 ozs.; quartz gold, 188,488 ozs. total, 353,266 ozs.f * For full details as to the geographical and geological distribution and the nature of the gold ores found in the United States, the student is referred to the Annual Reports of the Director of the United States Mint, and of the Californian State Mineralogist. t Mineral Jndu*ti-y, 1892, p. 192. OCCURRENCE AND DISTRIBUTION OF GOLD. 4} The production of gold in Victoria is now increasing, but is far less than formerly, owing to the exhaustion of the alluvial deposits, although the yield of the quartz mines has continuously increased. In 1894 the production was divided as follows : Alluvial gold, 254,308 ozs.; reef gold, 419,371 ozs. total, 673,680 ozs. The chief producing districts are Ballarat, Sandhurst (Bendigo), Beech worth, Maryborough, Castlemaine, Gippsland and Ararat. In 1894 in Victoria the average yield per ton of quartz crushed was 8 dwts. 8 grains of gold, this being slightly less than the mean yield during the last decade. In New South Wales about half the gold is obtained from quartz lodes ; the pyritic ores are not yet effectively treated, and the river bank deposits have not up to the present been exploited on a large scale.* In New Zealand, only quartz lodes are worked in the North Island, alluvial workings being confined to the Middle Island. The most important districts are those of Kuaotuna, Thames, Coromandel, Waihi and Reefton in the North Island, and Ross and Kuraara in the Middle Island. f Origin of G-old Ores. The origin of mineral veins, includ- ing those in which gold is contained, has long been discussed by geologists. The old theory that the quartz of veins was originally in a molten condition and was ejected from below into fissures is no longer maintained, although in 1860 H. Rosales brought forward evidence in its favour as far as the Victorian lodes are concerned. It is now admitted by all that the materials forming the veins have been transported in aqueous solution and precipitated where they occur. In a few excep- tional cases, sublimation may have played a part. The view that the solutions found their way downwards from above has been abandoned, but the ascensional theory and the lateral secretion theory both have many adherents. The last-named theory has found its principal supporter during the last twenty years in Prof. F. von Sandberger, who pointed out that the: gangue of many lodes varies in composition if the nature of the rocks through which they pass is changed, and claimed to have proved by analysis that the materials forming vein-stone are derived from the adjacent country rocks. He stated, more- over, that such minerals as augite, hornblende, mica, and olivine, which are essential constituents of crystalline rocks, contain small quantities of the heavy metals occurring in veins. | Although Sandberger did not try to detect gold in the silicates, this metal is not likely to be an exception. Prof. A. Stelzner objected to these conclusions, urging that small quantities of the sulphides of the heavy metals were probably mechanically mixed * Report of the Dept. of Mines and Agriculture, N.S. W., 1894. t Annual 'Report of the Mining Commissioner of New Zealand^ 1891. J Untersvchungen uber Erzgange. Wiesbaden, 1882 and 1885. Useful abstracts are given in Phillips' Ore Deposits and in Le Neve Foster's Ore and stone Mining. 42 THE METALLURGY OF GOLD. with the crystals of minerals which Sandberger analysed in the belief that they were pure. Stelzner advocated the retention of the ascensional theory, which alone affords a satisfactory expla- nation of the difference in composition observable in neighbour- ing lodes passing through the same rocks, and apparently formed at different periods. The two theories are, however, not con- tradictory, and perhaps neither need be entirely rejected, the solutions being supposed to pass more or less freely in the plane of the lode, after they have been impregnated. For the origin of placer gold, see p. 67. CHAPTER IV. TREATMENT OF SHALLOW PLACER DEPOSITS. THE deposits grouped together under the name of " placers " comprise sands, gravels, or any loosely coherent or non-co- herent alluvial beds containing gold. They have accumulated, owing to the action of running water, in the beds of rivers, or on the adjojiiing inundation plains, or on sea beaches. They fall naturally into two groups, between which no strict line of demarcation exists. These are (1) Shallow or modern placers, which are in or near existing rivers, and have not yet been covered by other deposits. (2) Deep level or ancient placers, which now lie buried beneath an accumulation of debris or coherent rock, the rivers by which they were formed having often been deflected into other channels by more or less extensive changes in the physical geography of the district in which they existed. Beach deposits occur in each subdivision. ] n this chapter, the first of these groups will be considered. From the earliest times to the present day, shallow placer deposits have probably yielded more gold in the aggregate than has been derived from all other sources put together. Shallow placers consist of loose aggregations of sand, gravel, loam and clay, accumulated by the action of existing rivers and streams, and not extending to a greater depth than 10 or 15 feet from the surface. They contain metallic gold in fragments of all sizes, ranging from the finest dust to nuggets weighing thousands of ounces. Auriferous sands are found in the beds of most rivers which flow during any part of their course through a region composed of crystalline rocks. If the rivers have rocky beds, gold may be found in the crevices, caught in natural riffles, and the whole may subsequently be covered by beds of sand. SHALLOW PLACER DEPOSITS. 43 Gold also occurs in river bars and banks, in river " flats," or inundation plains, in the dry beds of streams which only flow after heavy rains (" gulch diggings "), in terrace gravels on the sides of valleys high above the present level of the water ("bench diggings "), and 011 the sides and tops of hills (" hill diggings "). The last two subdivisions are evidently ancient rather than modern deposits. The gravels may contain boulders of any size, up to several feet in diameter, or may shade off into fine sand, while sandy clays, especially if on the bed-rock, are frequently very rich. In the Urals, the placer deposits often consist of heavy clays, while others are formed of waterworn fragments of auriferous quartz, talcose and chloritic schists, serpentine, greenstone, &c. Gold occurs under very various conditions in these deposits. It may occur in the grass roots on inundation plains, or near the surface of the gravels in river beds, or dispersed through the whole thickness of a stratum. More commonly, however, the lowest part of the superficial beds, just above " bed-rock " (the country rock of the district), is richest. In hollows, cracks, and crevices of the bed-rock, or, if it is soft and decomposed, in the substance of the upper part of the rock itself, to the depth of 1 or 2 feet, gold occurs in the greatest quantities, vln pipeclays just above bed-rock, in Victoria, it was not uncommon to find 12 ozs. of gold or more in a single tubful of " dirt," and similar rich bed-rock deposits occur in California. The depth at which bed-rock is found varies greatly ; it may crop out at the surface, or it may be buried beneath hundreds of feet of gravel, and 'great variations occur even in a single district. In the Urals, however, the thickness of the gravel is usually less than 3 feet thick, and is rarely more than 12 feet. Methods of obtaining Gravel from Shallow Placers. The gravel obtained from any placer deposit is, with exceptions, ultimately all treated alike, but the mode of winning the dirt varies with the necessities of the case. On flats and bars, the surface gravel, if rich enough, is loosened with pick and shovel, and then washed. If only the part just above the bed-rock will pay for treatment, it is reached by "stripping," or, if covered Iby too great a thickness of barren material, shafts are sunk, and short levels run from the bottoms of them in all directions. If the nature of the ground permits of it, tunnels are run without shafts, and the rich gravel is then followed from the surface, wherever it is found. This system was much practised in the early days in California, although now seldom to be seen in operation ; it was called " coyoting," from the coyote, which lives in holes in the ground. When water was encountered in the shaft, it was drawn out by a bucket, until it came in too fast, when the claim was abandoned. In California, in somewhat later times, efforts were made to reach the gold in the river beds 44 THE METALLURGY OF GOLD. by deflecting the streams of water from their courses, and in other ways. These methods will be briefly described under the head " River Mining." Methods of Washing the Gravel. The Pan. When the existence of gold in the placers of California and Australia first became known, the diggers were not acquainted with any appar- atus which was well adapted to extract the metal. The house- hold pan was used everywhere to wash the gravel, and though, in its original form, it was a difficult implement to use efficiently, it has retained its place in both countries for prospecting and also for washing small quantities of rich material, however they may have been obtained. The pan is usually made of stiff sheet-iron, is flat-bottomed and circular, and at bottom about 14 inches in diameter. The sides slope outwards at an angle of about 30 to the bottom, and are about 5 inches wide. A riffle is a useful addition, formed by the thickening or bulging inwards of the side, situated about half-way up the latter and running about half-way round the pan. The method of using this pan embraces several operations. First, it is filled to about two-thirds of its capacity with pay-dirt, of which it then contains from one- fifth to one-quarter of a cubic foot. It is then placed at the bottom of a water-hole or convenient stream, and the dirt is thoroughly broken up with both hands, care being taken not to leave any lumps of clay. As soon as the contents of the pan are reduced to the consistency of soft mud, the pan is grasped with both hands a little behind its greater diameter, inclined away from the operator, raised until the dirt is only just covered with water, and shaken from side to side, while a slight oscillatory circular motion is also imparted to it. The mud and fine sand is soon obtained in suspension in the water, and gradually passes over the far edge, which is lowered more and more, until little but the stones, coarse particles of sand, black sand and gold is left. The larger stones lie on the top and are removed by hand. The final stage consists in lifting the pan with a little water in it, and by a movement of the wrist, something like that used in vanning, causing the material to be spread out by the water, in a comet shape, in the angle of the pan. The "colours" i.e., yellow specks of gold are seen at the extreme head of the comet, and also occur in the succeeding inch or two, mixed with the black sand, while the quartz-sand forms the remainder of the tail and is scraped or washed off. The gold is separated from the black sand by (a] amalgamation with mercury, or (6) drying and blowing away the black sand, a wasteful process. Liquid amalgam is readily separated from sand, and the mercury is then driven off by heat. The Batea differs from the miner's pan in not having a flat bottom. It is of wood turned in a lathe, about 20 inches in diameter, conical, or more rarely basin shaped, and about 3 SHALLOW PLACER DEPOSITS. 45 inches deep in the centre, so that the angle at the apex is about 160. The gold collects at the lowest point and clings to the wooden surface under conditions when it would slide over iron. The batea consequently is more rapid and effective in obtaining a " prospect" than the pan, especial 'y when the gold is fine, but is less frequently used in the United States and Australia. It had its origin in South A merica. It is usually now made of enam- elled iron with a hole in the centre fitted with a cork, when it is for use in countries other than South America. It is con- sidered by MM. Oumenge and Fuchs to be especially favoured by the negro race. Prospecting Trough. This instrument is used in the far east, especially by the Chinese, Malays, Annamites, and tunnels have been made on the American and Feather Rivers to permanently drain large reaches and deliver the water at a lower point. None of these undertakings have as yet been strikingly successful, from various causes. River mining is probably subject to more uncertainty than any other branch of gold mining. The whole capital invested may be lost, and all works and machinery swept away by a flood before the pay-dirt is sighted, while numerous instances are on record in which the alluvium on the river-bed, after having been laid bare at great expense, was not rich enough to pay for sluicing. Between the years 1857 and 1880 the Californian river-beds were covered so deep by the tailings from hydraulicking that they could not be worked with advantage. Since the suspension of hydraulicking, however, and the gradual working down of the debris, some places have again become worthy of attention.* * For a full account of river mining, with details of dam-construction, &c., the student is referred to the article 011 the subject by R. L. Dunn in the Ninth Annual Report of the Cali/ornifin State Mineralogist, 1889, pp. 262-281. See also the Eleventh Report, 1892, pp. 150-153. SHALLOW PLACER DEPOSITS. 57 2. Dredging. This method of winning the gravel has seldom met with any success, for the reasons that the gold, even when it exists in the river bed, cannot be got at in such a simple way. The bed-rock cannot by this means be cleaned and creviced, and if the poor gravels on the surface are thick, they cave in and slide down into the pit made by the dredge, so that the pay-dirt is never reached. Moreover, boulders impede or prevent the work. The favourite implements are various forms of vacuum, lifts and pumps, the power being sometimes supplied by a hy- draulic elevator. Water, gravel and stones are brought into the barge together, and the stuff is usually then and there washed and dumped into the river again. The system has made somewhat more progress in New Zealand than elsewhere, and the following details of recent work there are taken from a Government report on mines.* On the Molyneux River in New Zealand the centre bucket dredger has done good service, the difficulty of raising big stones being that most severely felt. Here it is found that little of the fine scaly gold of the rivers is caught in ordinary sluices, and the washing is done by separating all the coarser material by means of trommels, and passing the fine sand by itself over wide tables in a very shallow stream. The volume of water must be just enough to keep the tables free from sand, as if the latter begins to collect, it is believed that the fine gold is not being caught. The tables are covered with cocoa-nut matting, baize, blanketing, or even plush, each material having its advo- cates. On the Shotover River, the Sand Hills Dredging Company separates the stones by passing the gravel through a revolving cylinder 3 feet in diameter and 10 feet long, set with fine holes ; 30 tons per hour are treated by this trommel. The washing is done on inclined tables which are 64 inches wide, and about 27 feet long, having a grade of 18 inches in 12 feet. These tables have a superficial area of 144 square feet only, which is insufficient for the treatment of 30 tons per hour. At the Waipapa Creek Dredging Company's works, which are situated on the sea coast, a Welman dredge is used, which acts on the centrifugal suction system. The pump is 3 feet 6 inches in diameter, and the delivery tube 13 inches in diameter, and the pump raises gravel from an area having a radius of 40 feet. All large stones are caught and separated from the fine stuff by a riddled hopper-plate. The gold is very finely divided, and is caught on plush mats which are washed every eight hours. The water for washing is supplied from a reservoir by means of an 18-inch pipe. Stones of 56 Ibs. weight are lifted by this pump, which may be used with advantage on all low wet ground. In Northern Italy a dredging plant has also been successfully at work for some years. 3. Deep Bar Mining. Deep bar mining, as conducted by * New Zealand Mng. Comm. Report, 1891. 58 THE METALLURGY OF GOLD. the old methods, consisted in sinking shafts in the bank and running drifts below water level, either under the river bed itself or under deep bars, for the purpose of digging out the pay- dirt from the surface of the bed-rock and taking it to the surface to wash. It was mostly unsuccessful owing to the amount of water, quicksands, &c., encountered, although to-day these diffi- culties could be often overcome by the present improved practice in quicksand digging. As, however, this kind of digging is better suited to the efforts of a few individuals than to under- takings on a large scale, it is not likely to be largely reverted to. In particular, working in loose detrital matter under the river bed itself must always be hazardous to the workers, and the chances of success very dubious, although profits have been obtained where the pay-dirt was only worth 70 cents per cubic yard. Deep bars, however, are now being worked profitably by the hydraulic elevator, which has become of great import- ance in placer mining. This machine may be more conveniently described after hydraulicking has been considered. Method of Working Siberian Placers. The methods and apparatus employed in Siberia differ so markedly from those which have been adopted elsewhere, that they are well worth a special description, although they cannot usually be applied to placers found in other parts of the world, owing to the difference in economic conditions. In California the valleys are narrow and the grade steep, so that watercourses are usually close to the auriferous deposits, and the sluices can be made of almost any length, while still conforming to the general slope of the soil. In Siberia the slope of the valleys is so gradual that the flow of the water is almost imperceptible, and takes place through wide marshy tracts. The result of this is that the sluices must be short, being usually less than 100 feet long, and their upper ends must be raised on trestles. As a further consequence, also, the gravel must be excavated by hand and carried in waggons to the sluice, and the tailings removed in a similar way, the flow of water acting by gravity not being available for these purposes. The excavation is made in benches or terraces, working up the valley. The height of each bench above the lower one is about 5 feet, and the gravel is picked down from the face of each bank and shovelled into carts by which it is conveyed to the washing establishments. The additional expense entailed by these causes is balanced by the low cost of labour in the country, and an inci- dental advantage lies in the fact that the washing apparatus can be placed outside the limits of the river during flood time, and so may remain for a number of years undisturbed, while all the gravel in the district is being washed. Only surface deposits are exploited, no deep workings existing in the country. There are three types of apparatus employed, each designed for washing a particular kind of deposit. They are : SHALLOW PLACER DEPOSITS. 59 1. The Siberian sluice, which is used to wash light sand. 2. The Trommel, used for loamy sands. 3. The Pan, used for gravel which is cemented together by means of compact clay. 1. The Siberian Sluice. The apparatus at Voltchanka, which may be taken as a type, consists of a head sluice and three secondary sluices, which are placed at right angles to the head sluice, and which leave it at different points and converge to a common centre, where the tailings are discharged. The head sluice begins at a height of 1 3 feet from the ground ; it is about 90 feet long by 2 feet wide, and has a fall of about 1 in 14. The sands are dumped from the waggons on to a wooden platform situated above the sluice-head, and shovelled into the latter, a stream of water being turned in at the same time. After passing through a grizzly, the gravel runs over a series of cast-iron cross-bar riffles, which form a number of rectangular depressions (or pigeon- holes) in the bed of the sluice, by which the disintegration is favoured. The stream then flows over an iron screen, through which a part of it falls into the first secondary sluice, while the remainder continues its course over more pigeon-hole riffles. This arrangement resembles the Californian undercurrent. A second and a third screen open on to the other secondary sluices, and the part of the gravel (consisting chiefly of small stones) which has resisted disintegration, and has not passed through the screens, is then let fall into a hopper, whence it is removed to the tailings waggons. The secondary sluices are wider than the head sluice, and have a steeper grade, and the amount of water and auriferous material passed is of course much less in each one than on the principal sluice. The sands first pass over a number of trans- verse riffles, and then over about 30 feet of blanketing, the sluice being widened at the same time, and subdivided by longitudinal wooden partitions, and the grade being as much as 1 in 6, while small drops are introduced at intervals of a few feet. The third sluice has a more gentle inclination than the others. The tail- ings fall into a shallow sump, and are instantly lifted out of it by a bucket elevator or " tailings-wheel " operated by water power, and stored in a hopper, whence they fall into waggons, by which they are removed to the dumping ground. No mercury is used in these sluices, and only the production of " grey concentrates " is attempted, this work being continued from 6 a.m. to 7.30 p.m. every day, after which the concentrates are collected from the riffles. They consist of gold in scales and plates, magnetic iron oxide, pyrites, rutile, together with some quartz, tfec. They are treated with mercury in the Siberian trough or on inclined tables, the method being that described on p. 47. The apparatus described above treats 500 tons of gravel per 60 THE METALLURGY OF GOLD. day, the labour required being furnished by twenty men and ten horses. The gravel treated contains an average of from 12 to 15 grains of gold per ton, rarely falling below 6 grains per ton ; the exceptional richness of 3| dwts. per ton has been observed. The gold is chiefly found in the head sluice, where 70 per cent, is caught, 30 per cent, being caught on the secondary sluices. At a similar establishment at Tchernaia-Retchka, however, where the gold is less finely divided, 97 per cent, was caught on the head sluice, and only 3 per cent, on the secondary sluices. The amount of water used at Yoltchanka is about six times the weight of the gravel. The cost of construction of the works was 70,000 roubles, or about 7,000. 2. The Trommel. Gravels which are too compact for satisfac- tory disintegration in the short sluices described above are sub- jected to a preliminary treatment by a trommel. At Berezovsk the trommel is of sheet iron of 9 mm. thick, having holes in it of about 1 mm. in diameter. The trommel is about 12 feet long, 3^ feet in diameter at one end, and 4i feet at the other, and is set inside with denticulated plates of iron to assist in the disin- tegration effected by the water. The machine is driven by a water wheel, and is sufficient for the disintegration of from 400 to 500 tons of gravel per day, requiring the expenditure of about 3 horse-power to drive it. The amount of water used in the trommel and on the tables is 67 '5 litres per second, or about seven and a-half times the weight of the ore. The washing is effected on inclined tables only 30 feet long and 12 feet wide, and with a grade of about 1 in 4, placed with the incline at right angles to the length of the trommel. Near the head of the table, and stretching across it, is a deep trough-like depression, and below this there are a number of transverse riffles, in which grey concentrates are caught and treated as usual. The Bere- zovsk establishment employs twenty-five men and fourteen horses, constantly ; the trommel usually lasts for two seasons. 3. Pan Washings. Sandy clays cannot be economically disintegrated in a trommel, and are, therefore, treated in a washing pan, which bears a strong resemblance to the cement pans employed in California. The pan usually consists of cast iron, and is from 8 to 16 feet in diameter, with vertical sides from 1 to 5 feet high. The bottom is of cast iron or sheet iron, and has numerous holes in it of about T V inch in diameter, widening downwards. The bottom is divided into 25 sectors, between which are deep grooves for the collection of the pebbles. Through a circular opening in the centre of the pan there passes a revolving axis to which are suspended eight horizontal arms studded with vertical iron teeth, some of these being shaped like plough-shares. The revolution of these arms effects the disintegration of the sandy clays, which are fed into the pan together with water and puddled until fine enough to pass SHALLOW PLACER DEPOSITS. 61 through the holes in the bottom, and the stones are removed at intervals by opening little gates placed opposite the radial grooves. The disintegrated gravel falls from the pan on to concentration tables, similar to those used after disintegration in the trommel. At Berezovsk the pan is 11 J feet in diameter and 5 feet deep, and the arms revolve at the rate of 25 turns per minute. From 50 to 55 tons of material are treated in twelve hours, the water consumed, including that required for power, being ten times the volume of the sand. Beach Mining. Beach mining is a comparatively unimport- ant form of shallow placer mining. The sea beaches on parts of the coasts of California, Australia and New Zealand contain small quantities of gold, which has been proved, in all cases in which the matter has been investigated, to be derived from the cliffs, which mostly contain a still smaller quantity. Some streaks of black sand, however, in the " Gold Bluff," California, have yielded $135 or 6| ozs. per ton by actual working.* The waves of the sea wash down and partially concentrate the poor sands, and, under certain rather exceptional circumstances, as the tide goes out the surface of the beach is left covered with black sand, in which numerous specks of gold occur. This is carefully scraped up and transported inland to be washed, as sea water is not well adapted for the purpose, although it is used by one Californian company. The next tide usually washes away all the valuable material which has not been collected, or else covers it with barren sand. There is great difficulty in washing the black sand in California, as it consists largely of rounded grains of magnetite, the density of which is about 5-0, while the gold is in minute flakes and scales, which can be seen under the microscope to be oblong in shape, and thicker at the sides than in the middle (a shape due to continued pounding of a malleable material). This form is so easily moved and buoyed up by water ' that it is difficult to get a "colour" with the pan, and the amount caught by the mercury in sluices or long-toms is usually an insignificant proportion of the total assay value of the sand. The industry is generally a languishing one. Treatment of Shallow Placer Gravels by Steam Shovels and Amalgamated Plates. A plant for the treatment of placer deposits, which are similar in nature to those worked in Siberia, has been devised in America, where labour-saving contrivances are indispensable in all such cases, owing to the high rate of wages. The apparatus consists essentially of a combination of a steam shovel or navvy, and an amalgamator consisting of a wide waggon- shaped wrought-iron trough loosely lined with silver-plated amal- gamated copper plates, which form a series of steps or riffles at the sides of the trough. The material is elevated into a hopper by the excavator, and thence is charged into a revolving trommel * Prod. Free. Met., U.S.A., 1884, p. 557. 62 THE METALLURGY OF GOLD. 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 of 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 thickness, 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. None of these views are now entertained, and the " fluviatile " theory is DEEP PLACER DEPOSITS. 63 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 palseontological 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- ier, and the streams were deflected into other channels, which Dften lie close alongside the old cailons. 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 cailons 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 Fig. g. that sections of the old valley from bed-rock to surface are exposed in the sides of the canons, 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 64 THE METALLURGY OF GOLD. 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 reaching 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 II 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 DEEP PLACER DEPOSITS. 65 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 5 66 THE METALLURGY OF GOLD. 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, California, 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,