OCCURENCE OF GOLD IN NATURE.
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THE MINER'S GUIDE.
OCCURENCE OF GOLD IN NATURE.
Gold is always found in the metallic state, but never quite pure; it is always alloyed with silver, and sometimes with copper, or iron, or some of the rarer metals, the proportion of the baser metals varying from a few tenths to 45 per cent. The proportion of gold contained in an alloy is expressed in "carats." A "mark" is divided into 21 carats, and if 24 grains of the alloy contain 22 grains of pure gold and 2 grains of other metals, the alloy is called 22 carat fine. The rough estimate of the carating, or proportion of pure gold in an alloy, is performed by means of the touch-stone. This is a very fine-grained, black, silicious slate, ground quite smooth. A streak is made across the stone with the gold to be tested; then other streaks are drawn beside it with needles composed of alloys with silver of known composition called "trial needles," and the gold is considered to be of the same fineness as that of the needle whose streak approaches nearest in colour to that of the gold to be tested. More accurate estimates are made by assaying, which process is described at page 23.
The geological conditions under which gold occurs in the earth are not so various as might have been expected, considering the almost universal distribution of the metal. Both practically and geologically two important divisions are recognised, viz., gold in situ, in the rock, and gold in alluvium. We will take each of these cases separately.
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Formerly, gold in situ was thought to be found only in rocks of Palaeozoic age, but of late years this opinion has had to be modified, for in 1860 Mr. David Forbes reported to the Geological Society of London, that he had found, in South America, gold in rocks belonging to the Cretaceo-oolitic, or upper secondary formation, and Professor Whitney, in his "Report on the Geology of California," states that, in 1863, fossils of Jurassic age were found in the auriferous slates of that country; and that gold also occurs there in many localities in rocks as new as the Cretaceous. But in all these cases two conditions invariably occur, first that the newer auriferous rocks rest upon, or are connected physically with, Palaeozoic rocks, and secondly that the newer, as well as the older rocks, are pierced by igneous dykes, generally composed of diorite, so that the following conclusions, taken from the last edition of Sir Roderick Murchison's "Siluria," will probably express the present opinion of most geologists on this point.
1. That looking to the world at large, the auriferous vein-stones of the lower silurian rocks contain the greatest quantity of gold.
2. That where certain igneous eruptions penetrated the secondary deposits, the latter have been rendered auriferous for a limited distance only beyond the junction of the two rocks.
3. That the general axiom remains that all secondary and tertiary deposits (except the auriferous detritus in the latter) not so specially affected, never contain gold.
4. That the granites and diorites have been the chief gold-producers.
Gold is nearly always associated with quartz in veins, but occasionally (as in South America) in mica and quartz schists; and, in minute quantities, it has been detected in slate and granite in the
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immediate neighbourhood of quartz-veins. Iron pyrites, or "mundic," also often contains more or less traces of gold, but so finely distributed that it cannot be got out by the ordinary process of crushing and amalgamating. Auriferous quartz-veins have been found in granite, gneiss, mica-slate, and other schistose rocks, serpentine, quartzite, sandstone, limestone, diorite, and volcanic tufa, but they occur most plentifully in clay-slate, and talcose and chlorite schist; and are nearly always associated with iron pyrites.
The opinion of geologists as to the origin of these auriferous quartz-veins is still much divided; for while, on the one hand, Sir R. Murchison maintains that, "as no unaltered purely aqueous sediment ever contains gold, the argument for the igneous origin of that metal is prodigiously strengthened," on the other Professor Jukes says "that the great auriferous veins of white quartz, which traverse the rocks of many parts of Australia and other countries, must almost certainly have been formed through the agency of water." Here, as in most cases, the truth probably lies between the two extremes, and the greater number of geologists and chemists are now beginning to think, that hot water in the form of thermal springs, has been the chief agent in producing them. All gold-bearing regions can be shewn to have been at one time subjected to long continued heat, although never perhaps very intense. The igneous and metamorphic rocks, found in all auriferous districts, are a sufficient proof of this, and the only semi-metamorphism of some of the rocks, in many cases, also shews that the heat could never have been sufficiently great to melt quartz, one of the most refractory substances in nature. But, by long-continued heat, water would dissolve silica freely, and would be forced up through fissures to the surface depositing silica as quartz, as
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well as any metals it might have in solution (probably as chlorides), as it cooled, If the region were also a volcanic one, sulphur would also be brought up as sulphurous acid, or sulphuretted hydrogen, which, acting on the iron present in nearly all rocks, would form iron pyrites, or "mundic." The general occurence of the gold disseminated through the body of the quartz is conclusive evidence that both were deposited contemporaneously; but frequent examples also occur of fine threads of gold lying, as it were, entangled between the points of well formed crystals of quartz, and in these cases we must suppose that the deposition of the quartz had ceased before the gold was introduced.
There is also another method by which auriferous quartz-veins might have been formed. In a volcanic country sulphur and gold might have been sublimed, and, permeating the rocks while they were hot, might have formed with the iron they came in contact with, auriferous iron pyrites. At a subsequent period, water, percolating through, may have decomposed both the pyrites and the bed rock, forming a solution of silica and the sulphide of the alkali contained in the rock, which would dissolve the gold, and both it and the silica might then find their way into the fissures, and form auriferous quartz-veins; indeed it seems almost impossible that those veins, which die out as they descend, could have been formed from below.
This distinction between the two methods of forming the veins is of considerable importance to the miner. In the first place, it is evident that those veins which have been filled from below must necessarily have been fissures of great depth, and therefore probably long and wide in proportion, forming the true "reefs;" while water, charged with silica, percolating laterally through the rock, would
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fill up all the small cracks and fissures formed by previous drying and contracting of the rock, and those cracks will be usually small and of uncertain length and direction, forming the so called "leaders" of the miner. In the second place, there generally occurs, between either side of the vein and the bed rock, a soft band called the "casing." Now this casing is nothing but the decomposed bed rock, and in veins formed by the first method, it is difficult to see how any gold could get into the casing without quartz finding its way there also; but by the second method we can readily suppose that some of the gold might be left behind in the casing or bed rock, while the quartz passed through into the fissure, and therefore veins formed by the second method will be more likely to contain gold in the casing than those formed by the first method.
The uncertain direction of the leaders, as well as the common occurence of gold in the casing, and the quantity of mundic found through the bed rock, as well as the volcanic origin of that rock itself, all point to the conclusion that the auriferous veins at the Thames have been formed by the second method.
From the foregoing observations it will be seen that, in exploring for a gold-bearing region, the following particulars should be kept in view.
Of the first importance.
1. The country should be composed of clay-slate, or metamorphic schists: or, in the case of the North Island of New Zealand, of volcanic tufa lying on primary rocks.
2. The rocks should be traversed by quartz-veins.
Of secondary importance.
8. The rocks should be impregnated with iron pyrites.
4. They should be associated with other rocks of igneous origin, especially granite or diorite.
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5. Among the rocks impregnated with iron pyrites, those portions should be more particularly examined where the pyrites has been decomposed into the red oxide of iron. Quartz veins here will most likely contain gold if any exists in the district.
The second manner in which gold is found is as water-worn grains or lumps in alluvial deposits. No gold has, we believe, as yet been found in gravel or sand of Miocene age, or older; certainly this is the case in Australia and New Zealand, so that time is thrown away in examining such deposits and it is only the more recent washings, brought down by rain or rivers, that are likely to repay a search.
The origin of these deposits is evident. A range of hills or large mountain tract, when exposed to the action of the weather, is decomposed at the surface, and fragments of the rock so decomposed are then washed down by the rain into the rivers,and by them are carried into the valleys and low lands. If the mountain range contains auriferous quartz veins, portions of these will be carried down among the rest, and the constant rolling and friction that they undergo on their passage down, gradually separates the gold from the quartz, and forms it into more or less rounded grains; its weight then causes it to sink to the bottom of the deposit, and to remain in any hole or cavity it may come across in the river bed. The distance gold has come from its original place may be guessed at by the amount of rubbing it has undergone, and its more or less complete disentanglement from its quartz matrix.
This process of nature, although very favourable to the digger, by giving him his gold already crushed, is of course a very wasteful one, as much of the gold must be lost by the process of rubbing, and
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all that is too fine to see with the unassisted eye is never recovered; moreover if the district from which the gold is derived is not situated at some distance inland, the greater part of the gold will reach the sea while still imbedded in quartz, and so will be washed away before its weight has had an opportunity of acting.
To secure then an extensive alluvial gold-field two things are necessary.
1. A large district of gold bearing rocks fully exposed to the weather, or that has been so exposed since the Miocene epoch, and
2. That this district should be situated so far from the sea that there is ample time for the gold to get rid of the quartz while travelling down the streams.
We are now in a position to make a definition that is sometimes of great importance to the digger, viz: What is alluvial gold?
Alluvial gold is gold that has been moved, piece by piece, from its former position in a rock, by water, or atmospheric causes acting on the surface, and re-deposited in some other place without any regard to the former relative position of its several pieces. It is not therefore necessary that alluvial gold should be waterworn or free from quartz, as the gold found in the detritus at the foot of a cliff would come under the term; but, on the other hand, if the whole face of the cliff had slipped down in mass, as a land slip, the gold in it would not be alluvial, as it would not have been moved by water or atmospheric causes acting on the surface, neither would its various pieces have been re-deposited without any regard to their former relative position. Whether the rock is hard and undecomposed, or soft and decomposed into clay, has nothing at all to do with it.
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CHEMISTRY OF GOLD.
Gold is a soft metal, having a beautiful yellow colour. It surpasses all other metals in malleability, the thinnest gold leaf not exceeding 1/282000 inch in thickness. It may also be drawn into very fine wire. Gold has a density of 19.5. It melts at a temperature a little above the fusing point of silver. Neither air nor water effect it in the least at any temperature; the ordinary acids fail to attack it singly. A mixture of two of hydrochloric acid to one of nitric acid however dissolves gold with ease, the active agent being the liberated chlorine. Gold is also dissolved by chlorine, sulphide of potassium or sodium, and cyanide of potassium. The light transmitted through gold-leaf is green.
Protochloride of Gold is produced by evaporating the terchloride to dryness, and exposing it to a heat of 440 deg. F. It is a yellowish white mass, insoluble in water, but decomposes slowly into metallic gold and terchloride.
Terchloride of Gold. --This is the most important compound of the metal. It is always produced when gold is dissolved in aqua-regia. It is, when crystallised, of a red colour, very deliquescent, and soluble in water, alcohol, and ether.
Protoxide of Gold is produced when caustic potash in solution is poured upon the protochloride. It is a green powder partly soluble in the alkaline liquid. It decomposes rapidly into metallic gold and teroxide, which remains dissolved.
Teroxide of Gold, or Auric Acid. --When magnesia is added to the terchloride of gold, and the sparingly-soluble aurate of magnesia well washed and digested with nitric acid, the teroxide is left as an insoluble reddish yellow powder, which, when dry, becomes chestnut-brown. It is easily reduced by heat, or by mere exposure to the light.
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When digested with ammonia it yields fulminating gold.
Gold is precipitated from solution by the following:-- Sulphuretted Hydrogen, as black persulphuret of gold,
Potassa, as a reddish yellow peroxide,
Protosalts of Iron, as metallic gold, but in a form of a very fine brown powder, which acquires metallic lustre when rubbed.
Protochloride of Tin produces, even in extremely dilute solutions of gold, a purple red color known as the purple of Cassius. It is used for coloring glass and porcelain.
Organic substances also reduce solutions of gold, light assisting the action. If the skin be touched with a solution of gold it soon becomes stained of a dull purple color.
Chloride of Gold is reducible by heat alone.
Cyanide of Potassium acts upon gold forming a cyanide of gold which, although insoluble in water, is freely soluble in a solution of cyandine of potassium, and it is this solution that is chiefly used in gilding by electrolysis.
Gilding on copper is generally performed by dipping the articles into a solution of nitrate of mercury and then shaking them with a small lump of soft gold amalgam; the articles are then heated to expel the mercury, and afterwards burnished. Gilding on steel is done by applying a solution of chloride of gold in ether, or by roughening the surface of the metal, heating it, and applying gold-leaf with a burnisher.
The consumption of gold-leaf in the arts in Birmingham amounts to 1000 ozs. a week, and in London to 400 ozs., of which not one-tenth is ever recovered. For gilding by electrolysis more than 10,000 ozs. are used annually. The consumption
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of gold in the Staffordshire potteries for gilding, and making crimson and rose colours, varies from 7,000 ozs. to 10,000 ozs. per annum.
DISTINGUISHING GOLD FROM OTHER SUBSTANCES.
Gold is easily distinguished from all other natural substances, there being only three that are at all likely to be mistaken for it. These three are iron pyrites, or mundic, copper pyrites, and yellow mica.
Whether the substance under examination is gold or iron or copper pyrites is most readily ascertained by heat. Both iron and copper pyrites are compounds of sulphur, and if the stone that is thought to contain gold is heated in a fire to a red heat for a few minutes, all the sulphur will be driven off, and the metal previously combined with it will be changed into an oxide, in which state it cannot be confounded with gold. If however the substance was really gold, the fire would only make it brighter than ever. Care must be taken not to heat the stone to a white heat, for, as gold melts at a temperature of 2016 deg. F., it might be in danger of being lost. Nitric acid will also dissolve both iron and copper pyrites, while it does not affect gold, but this test is much more likely to load to mistakes than the burning.
To the inexperienced eye, mica is not so readily distinguished from gold as either of the foregoing, for it is affected neither by heat nor acids. It must however be remembered that mica is not a metal, but a silicate of alumina and potash, and can be recognised by its scaly appearance with semi-transparent edges, by its lustre being pearly and not metallic, by its frequent occurrence in six-sided plates (a form gold never assumes), and, when in quantity, by its comparative lightness. Mica
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moreover, although common in granite, mica-slate, and some sandstones, is never found embedded in vein quartz. No mica has as yet been found at the Thames.
If, however, any uncertainty should still exist the following process must be resorted to.
The substance to be examined must be reduced to as fine a powder as possible, then mixed with just sufficient water to make it into a paste, and ground to an impalpable powder in a mortar. It must then be placed in a porcelain crucible, and roasted over a spirit lamp until all traces of sulphur are driven off. When this is accomplished, the dry powder must be transferred to a glass flask, or test tube, and diluted hydrochloric, or muriatic acid poured over it, and allowed to remain for 20 minutes or half an hour. It is necessary that the acid used should contain no trace of free chlorine, and to prove this some of the acid should previously have been boiled with a few drops of a solution of sulphate of indigo: if the solution, when boiling, retains its blue color the acid contains no free chlorine. The solution must now be carefully decanted off and clean water poured into the flask: when the powder has again settled this water must be decanted off. This operation can be repeated two or three times, until the powder is well washed. A mixture of two parts of hydrochloric acid to one of nitric acid is now added, and it is boiled carefully over a spirit lamp for five minutes. Water is then added, and as soon as the residue has settled down, the solution is decanted off into another clean flask or test tube: this solution will contain the gold if any existed in the ore examined. To prove its presence add to the solution a small quantity of solution of protochloride of tin, which will give the mixture a purple red color if any gold was in the substance.
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GOLD MINING.
The processes adopted for extracting gold bearing quartz from the rocks in which it is found are similar to those applied to other mineral veins, but as the auriferous veins are generally more variable in direction than the ordinary metallic lodes, the arrangement of the shafts, levels, and winzes is seldom so regular in the former as it is in the latter. When the conformation of the country admits of working by means of a day-level, it is driven in from some convenient valley in the neighbourhood of the vein, and the rock is obtained by stoping in the usual way. If, on the contrary, there be no facilities for this method of commencing the work, shafts are at once sunk from the surface, either perpendicularly, so as to intersect the reef at some convenient depth, or an inclined shaft is put down on the course of the vein itself. Inclined shafts are less expensive, and take less time, than vertical ones, but ultimately they are the least advantageous, as they are not well adapted for the application of pumps and machinery, an evil that increases much with the depth of the mine. When the mine is worked by means of an adit level, the rock, broken in the interior, is trammed through it into the open air for subsequent treatment; but if the extraction be conducted by the aid of shafts, the auriferous quartz is drawn to the surface by skips, or waggons running on inclined tramways in connection with a horse-whim or steam engine. In the case of very narrow veins, some of the enclosing rock has frequently to be broken with the lode in order to afford room for the miner, whilst in wide ones, it often happens that a portion only is sufficiently rich to pay the expenses of extraction and treatment, and the remainder is consequently allowed to remain in the mine. Sometimes also the walls of a vein, and
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particularly the foot wall, contain a sufficient amount of gold to make its extraction a matter of importance, and when this occurs a portion of the enclosing rock is necessarily excavated. In some veins the rock is, generally speaking, barren, containing traces merely of the precious metal, the gold only occurring in paying quantities in pockets at considerable distances from each other. In others the "pay-rock" forms bands or streaks, running more or less parallel within the walls of the vein, and frequently separated from the non-productive portion by a distinct heading, or bands, of country rock. Other veins are productive throughout their entire width, but seldom contain visible gold. The most profitable leads are usually those which afford a large supply of rock, obtainable at a cheap rate, and uniformly yielding an amount of gold in excess of the cost of extraction and treatment.
The chief gold quartz mining countries are California, Australia, Hungary, and Brazil, although mines are also worked in Wales, Ural Mountains, Tyrol, Virginia, North Carolina, South Carolina, New Granada, Nova Scotia, and New Zealand. Some of the mines in California are very deep, that of "Hayward's" being 1,250 feet, and the "North Star" 750 feet in 1866, and the yield of gold does not fall off within the depth as, at one time, it was expected to do. The average cost of raising and treating the quartz per ton in California is 7.70 dollars, while the average yield per ton is 22.87 dollars, giving an average profit of 16.17 dollars on each ton raised. In Australia the cost of crushing and amalgamating a ton of quartz at first amounted to £2, and as the cost of extraction of the ore had to be added to this, it followed that a very small proportion of the auriferous veins in the country could be worked with advantage; but now that the total expense of raising and treating a ton of rook
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by steam power has, under favourable circumstances, been reduced to about 16s. 4d., there are numerous reefs throughout the gold mining region affording satisfactory results, and many others that would do the same if extensively and judiciously worked. The miners generally, in common with those of other countries, have come to the conclusion that better results, with regard to profit, are to be obtained by crushing large quantities of only moderately rich stuff, with powerful and well arranged machinery, than from treating a limited amount of very rich rock on a small scale.
The following is the average produce of some of the Victorian quartz gold-mining districts for 1860.
| DISTRICT.
|
Ozs..
|
Dwts.
|
Grs.
|
| Ballarat
|
0
|
12
|
13 per ton
|
| Beechworth Mines
|
3
|
14
|
10 " "
|
| Sandhurst
|
2
|
7
|
12 " "
|
| Maryborough
|
1
|
7
|
21 " "
|
| Castlemaine
|
1
|
2
|
11 " "
|
| Ararat
|
1
|
11
|
15 " "
|
| Average
|
1
|
16
|
2 " "
|
As in California, the yield of the Victorian quartz veins, speaking generally, has not been found to decrease in depth, and those which have been wrought below 500 feet from the surface have experienced no diminution in their produce. The same may also be said in Hungary, where some of the lodes are worked at depths exceeding 1,500 feet.
In Nova Scotia considerable excitement prevailed in 1861, in consequence of the gold discoveries; and portions of the country were pegged off into claims, and more or less worked, at a very shallow
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depth. Several companies were also established, with a view to quartz mining on a more extensive scale; but, although many of the veins were found sufficiently auriferous to have warranted more extensive and systematic exploration, the larger companies were not usually successful. This result appears to have been in a great measure the effect of the injudicious nature of the mining laws framed by the local government, which limited the size of the claims to an exceedingly small area; and a somewhat heavy tax charged on each, rendered the prosecution of deep mining all but impossible. Nearly every man in the colony thus became the possessor of one or more claims, which could only be taken up in accordance with certain maps prepared by the county surveyor, by whom each district was laid out in parellelograms, almost irrespective of the nature and position of the leads, and consequently the larger companies, if desirous of obtaining a mining field of suitable extent, had to buy out, at a ruinous expense, numerous small claimholders, who, although they could not work their several minute holdings, did not scruple to demand for them an exorbitant sum of money. It also frequently happened that some claimowner, whose fragmentary holding was so situated as to render it a necessity for the efficient working of a large concern, took advantage of his position to refuse to sell except for an extortionate amount. It is evident that such a system was totally incompatible with the rapid development of the mineral resources of the colony, which, as a natural consequence, have been most materially affected thereby. Quartz mining requires a considerable amount of capital, and is subject to all the usual fluctuations of mining enterprises, excepting such as are influenced by the varying prices of metals; but, in spite of all its disadvantages, it has been found a highly remunerative occupation.
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METALLURGY OF GOLD.
Pulverisation. --After the quartz has been brought to the surface, it must be finely pulverised before the extraction of the gold that it contains can be effected. Various contrivances for this purpose have been proposed.
Arrastre.--The arrastre, which is much used in Mexico, consists of a circular pavement of stone surrounded by a wooden framework. It is generally about 12 feet in diameter, and the ore is ground on the stone pavement by means of large stones, or "mullers " dragged continually round its surface by means of mules harnessed to a beam of wood connected with a shaft in the centre of the arrastre. This beam of wood, revolving, drags the mullers after it. The arrastre does its work slowly, and consumes a large amount of power in proportion to the quantity of rock crushed, but it is an excellent amalgamator, and often valuable for the purpose of testing newly-discovered veins, and ascertaining their approximate yield.
Chilian Mill consists of one or two vertical runners of iron or stone, revolving on horizontal arms, which project from a perpendicular shaft, to which motion is given either by water or steam power. The basin in which the runners revolve is usually slightly conical, and made of cast iron. In this arrangement the grinding area is regulated by the difference of the circumference of the circles described on the bed by the inner and outer edges of the runners.
Berdan. ---This machine consists of a cast iron basin placed obliquely, and caused to revolve on an oblique axis. Inside the basin two cast iron cannon balls of different sizes are placed, which, rolling to maintain their place while the basin revolves, crush the ore.
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Stamping. --None of the foregoing, nor any of the many other ingenious inventions, can in any way compare in efficiency to the ordinary stamping mill. This consists of a series of heavy iron stamps, each of which is alternately lifted by means of a cam, and then let fall with its full weight upon the ore to be crushed, which is placed in an iron rectangular battery box. The lift of the stamps is from nine to twelve inches; their weight varies from 5501bs. to 9001bs., the number of blows struck is from 6O to 80 a minute, and the quantity of ore crushed by each is from 1 to 2.8 tons in the 24 hours. The rock to be crushed is put into the battery-box with a shovel, and a stream of water being also introduced, it washes the ore, when sufficiently pounded, through a fine iron grating. It takes about 8 gallons of water per stamp per minute to work the machine effectively. The iron grating through which the pounded ore is washed generally contains about 250 holes to the square inch.
Separation. --The pulverised ore, escaping from the battery-box, is generally conducted over blankets spread on the bottoms and sides of shallow troughs, or sluices, inclined at an angle of between three and four degrees with the horizon. It is necessary that the bottoms of these troughs should be quite level transversely, in order that an equal depth of water may flow over every portion of the surface of the blanket, and so prevent a rapid current on one side while an accumulation of sand is taking place on the other. A large portion of the gold and iron pyrites thus becomes entangled in the fibres of the wool, and remains behind, whilst the lighter particles of quartz are carried off by the force of the current. About every quarter of an hour the blankets are taken up and washed, and others put down. It is estimated that the gold
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retained in the battery, together with that collected on the blankets, represents about nine-tenths of the total amount obtained from the rock, nevertheless, a certain quantity both of gold and pyrites passes over, and other means are necessary for collecting it.
Amalgamation. -- The sand and water passing over the blankets is next conducted through troughs with mercury in them, which arrests the gold, while the "tailings" pass on into some other contrivance for separating pyrites from the sand. Formerly, the mercury in these troughs was placed along the upper side of battens of wood, called "riffles" or "ripples," nailed across it. This system is however very wasteful, as so small an amount of surface of mercury is presented in proportion to the quantity employed, and accordingly it has quite given way to amalgamated copper plates. These plates are of various sizes, and are considered as effective for saving fine gold as an equal surface of pure mercury, and are not only cheaper, but more easily managed. The amalgamation of the copper plate is effected by first washing over its upper surface with dilute nitric acid, and then, with a rag, rubbing on quicksilver, on which a little dilute nitric acid has been first poured, so as to form a certain amount of nitrate of mercury. When a plate has been thus once well covered, the operation need never be repeated, it only being necessary to sprinkle its surface occasionally with a little fresh quicksilver, in proportion as the gold caught converts it into a solid amalgam. In order that these plates should act satisfactorily, it is essential that the current should be slow, and the water shallow, since otherwise a principal portion of the gold might escape without coming into contact with the face of the plate. When a newly amalgamated plate is first used, its surface is apt to
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become tarnished by the formation of subsalts of copper, which, forming a green slime, interferes with the amalgamation of the gold. This should be carefully scraped off, and the place from whence it has been removed rubbed with a little fresh mercury. To remove the amalgam, the plate is heated over a fire, when the amalgam softens, and can be easily removed by scraping. The plate, after cooling, may then be again rubbed with mercury, without the aid of nitric acid, and is again ready for use.
The separation of the gold from the matters caught on the blankets, and collected in the washing tanks, is generally effected in California by Atwood's amalgamator, which consists of two wooden rollers, eight inches in diameter, and two feet long, furnished on their circumference with numerous small flat pieces of iron, working in cisterns containing mercury. These rollers are worked by a belt stretched so as to make them both revolve in the same direction, but contrary to the stream of water running through the apparatus. The sand is introduced by means of a hopper, and, after it has passed through the amalgamator, is conducted over a ripple board covered with amalgamated copper plates, at the end of which is a cistern for retaining the pyrites.
Occasionally, mercury is introduced into the battery where the ore is being crushed. When the proportion has been properly adjusted, the amalgamation, in this case, is very complete, except when the ore contains large quantities of lead or antimony, and has been previously burned for the purpose of expelling its more volatile constituents, by which treatment the particles of gold often become coated in such a way as to interfere with their combination with mercury. The amalgam, however, thus obtained, is generally of a light spongy character,
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difficult to collect by any of the appliances commonly employed for the purpose. The loss of mercury by battery-amalgamation is also a serious item; and, as there is no evidence that it possesses, under any circumstances, a decided advantage, it is now seldom resorted to.
Concentration of Tailings. --After leaving the riffle boards, the tailings are collected in tyes or settling pits, and concentrated by various contrivances.
In the earlier days of quartz mining, the pyrites and sulphides were generally allowed to escape with the sand, although sometimes an attempt was made to extract the gold from them by roasting before passing the ore into the stamping mill. This, however, was found to be of little practical advantage, and quartz now is always stamped without burning, but much care is devoted to recovering from the tailings the largest practicable proportion of auriferous material. The most usual arrangement for concentrating tailings is the "rocker," which consists of a trough about 12 feet long, 14 inches wide, and 10 or 12 deep, placed on two short legs, of different lengths, rounded at the bottom like rockers. The trough is rocked at the rate of 45 strokes a minute, with a one inch throw, by means of a crank or eccentric. The bottom is lined with sheet iron to prevent wearing. The concave or Borlase's buddle, is also found very effective in Victoria, and the ore after passing through it is sometimes further concentrated by a rocker, hand buddle, or shaking table.
Extraction of Gold from Tailings. --The pyrites in many establishments in California is collected and sold as an auriferous sulphide of iron, but in others, as well as in Australia, it is treated on the spot. It has been found that simply grinding pyrites to an impalpable powder with mercury only extracts
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about one-fourth of the gold contained in it; it is now therefore almost always first roasted in a reverberatory furnace, until all traces of sulphur disappear, and is then ground with mercury either in an arrastre, Chilian mill, or Berdan, or else in the Hepburn and Peterson, as well as the Wheeler, Varney, and other pans. Of these the Chilian mill is usually considered the worst, and the Wheeler and Varney pans the best. Wheeler's pan takes two-and-a-half or three horse power to work it efficiently, Varney's about the same, while the Hepburn and Peterson costs twice as much money as the Wheeler, and requires four or five horse power to work it. It is however an excellent grinder, and is much used in the silver mines on the coast of the Pacific. The common pan will work with one horse power, and will amalgamate one-and-a-half to two tons of ore in the 21 hours.
In the Grass Valley District, in California, an arrastre is used, consisting of a basin of cast iron four feet in diameter, with two mullers, generally of iron, but sometimes of stone, set in motion by a central shaft connected by a belt with the other machinery of the mill.
Several establishments in the neighbourhood of Grass Valley also employ on a small scale the chlorination process, or the extraction of gold by means of chlorine. The concentrated tailings are roasted until all the sulphur is driven off. They are then moistened, and put into large wooden tubs with false bottoms, into which chlorine gas, made from a mixture of salt, binoxide of manganese, and sulphuric acid, is introduced and allowed to permeate through the mass. After the expiration of 12 or 15 hours, clean water is introduced, and the liquid, containing chloride of gold in solution, is drawn off into glass carboys. Solution of green sulphate of iron is then added, which precipitates
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the gold in the form of a dark brown powder, easily separated by decantation and nitration. This process, when the gold is in a finely divided state, yields good results, but the larger particles of metal, not being dissolved in the time allowed for effecting the solution of the smaller ones, are often only partially attacked, and, unless great caution be exercised, a loss is the result.
Retorting. --The amalgam of gold and mercury collected during these processes is first filtered through buckskin, chamois leather, wet canvas, or flannel, and formed into balls about the size of large apples. These are placed into an iron retort, and the mercury is driven over by heat into a receiver, leaving the gold behind of a light color and spongy appearance. In order to obtain satisfactory results, the retort should be slightly covered on the inside, by means of a rag attached to a stick, with a paste made of water and clay, or sifted wood ashes, to prevent the adhesion of gold, in case of too much heat being accidentally applied. The retort should also be heated gradually, kept a long time at a black-red heat, and allowed to cool before being opened.
Smelting. --The gold is now melted in block lead crucibles by means of a coke or charcoal furnace, and run into bars, called ingots.
Refining. --The gold alloy is first mixed with silver, so as to make the proportion of gold as 1 to 2.5. It is then melted and granulated by being poured into water, and then attacked by boiling in a platinum vessel with twice its weight of sulphuric acid. When the operation is finished, the vessel is withdrawn from the fire, and its contents allowed to settle. In this way the gold falls to the bottom, and the liquid is decanted off into another receiver, and diluted with water. The gold is again boiled with acid, and then melted into bars. The solution
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containing the silver is introduced into a large leaden cistern, heated by steam, in which are hung copper plates, by which the silver is precipitated in the form of minute granular crystal.
This process has been rendered so inexpensive, that bars containing but a two-thousandth part of gold can be refined at a profit.
The foregoing is a description of the usual mode of treating gold ores in California and Australia, the countries in which this industry is carried to the greatest perfection; but in other places alterations may possibly have to be made to meet peculiar circumstances under which the gold may occur. Thus at the Thames, and more especially at Tapu, where gold is often found in a clay casing so soft that it can be washed like alluvial gold, some treatment to get rid of the clay before sending the ore to the mill appears to be necessary. Where a plentiful supply of water can be obtained, some modification of the sluice would be most economical; but where water is scarce, the puddling machine, so much used in Australia, would perhaps be preferable. This machine consists of a large shallow tub with an upright shaft standing in its centre, provided with strong rake-like arms, which are set in motion by a mitre wheel attached to the perpendicular shaft.
ASSAY OF MINERALS CONTAINING GOLD.
In the assay of minerals containing gold, the object sought is to obtain that metal in the form of an alloy of lead, which is afterwards placed in a muffle, and cupelled, with various precautions.
Fusion. --About 400 or 600 grains of finely powdered ore are mixed on a sheet of glazed writing paper, with its own weight of litharge, or red lead, 200 grains of dry carbonate of soda, and 8 to 10 grains of dry and finely powdered charcoal. This
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mixture is introduced into an earthen crucible, and covered with a thin layer of borax, and fused in an assay furnace, care being taken to withdraw the crucible from the fire as soon as a liquid, and perfectly homogeneous slag has been obtained, since the unreduced litharge might cut through the pot and spoil the experiment.
The slags thus formed contain the whole of the excess of litharge added, whilst the button of alloy produced is extracted by breaking the crucible, after it has been allowed to cool, and is then subjected to the process of cupellation.
The oxide of lead should always be in excess, since, if the slags contain any trace of an alkaline sulphide, the whole of the gold will not be present in the button of alloy obtained. The objection to this method of assaying minerals containing much sulphides is the large amount of lead produced for cupellation, which should not exceed 200 grains. This inconvenience may however be obviated by effecting the partial oxidation of the mineral by the aid of saltpetre, but in many cases the process of scorification is to be preferred.
Scorification. --In this process, the oxidation of the substances to be removed is produced by the action of air, instead of by the action of the oxygen contained in the litharge. A certain weight of the finely powdered ore to be operated on is intimately mixed with a certain quantity of finely granulated lead, and placed in a small saucer formed of close grained fire-clay. These saucers, or scorifiers, are then placed in a muffle, which is a hollow semi-cylinder of fire-clay, closed at one end, and furnished with slits on the sides; the muffle containing the scorifiers is now introduced into the furnace, and the door closed. When the lead is melted the mouth of the muffle is opened, and the current of heated air which passes through the muffle immediately
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begins the process of roasting. The effect of this is to produce a slag on the surface of the metallic bath; those slags gradually become soft, and finally remain in a perfectly liquid state. The operation may be considered complete when a small iron rod, previously heated to redness, is placed in the mixture, and found, on withdrawing it, covered with a slight film of scoria, which runs off without forming a solid drop at the end. When this point has been attained, the scorifier is withdrawn from the fire by means of tongs, and the alloy is poured into a mould. When cold, the metallic button is easily separated from the slags, and may be passed on to a cupel. The process of scorification is one of the most exact methods that can be employed; for however small may be the proportion of lead, the slags never contain any bi-sulphides at the close of the operation, and therefore rarely retain the most minute trace of either gold or silver.
Cupellation. --In order to ascertain the amount of gold and silver contained in the buttons of lead obtained by the foregoing operations, they are subjected to a process called cupellation. This process is founded on the fact that neither gold nor silver are sensibly oxidised when exposed in a state of fusion to the action of the air, while lead is. The button of alloy is therefore placed in a small cup, called a cupel, made of bone ash, tightly consolidated by pressure; and the cupels are placed in a muffle, and introduced into the furnace. The door is closed until the alloy is fused, it is then opened, and the air rushing through rapidly converts the lead into litharge, which is absorbed by the bone-ash of the cupel as fast as it is produced. When nearly the whole of the lead has been converted into litharge, and absorbed, the remaining bed of rich alloy appears to be agitated by a rapid circular movement, by which it seems to be made to revolve
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with great rapidity. At this stage of the operation the agitation will be observed to cease suddenly, and the button, after having for a moment emitted a bright flash, becomes brilliant and immovable. This is a sign that the process is complete, and that the remaining button in the cupel is composed only of gold and silver.
Parting. --The button obtained from the cupel is now treated with nitric acid, to dissolve out the silver. In order, however, to obtain quite exact results, it has been found that the alloy must contain a little less than three parts of silver to one of gold, since, if the silver be not present in sufficient quantity, the mixture is not completely attached by nitric acid; whilst, on the other hand, when too large a proportion of this metal is added, the gold remains in a pulverulent form, very difficult to be collected for the purpose of weighing. The operation of adding the proper amount of silver to the alloy of gold is called inquartation. The quantity to be added is judged by the touch-stone, and the inquarted button is flattened with a polished hammer on a steel anvil, and then put in a test-tube, and nitric acid, with a specific gravity of 1.28, is poured over it, and boiled from ten minutes to a quarter of an hour. The acid is then poured off, and the gold, after being carefully washed, is transferred to a thin porcelain capsule, from which the water is partly removed by pouring, and partly evaporated by heat.
After being heated to redness, the gold may be weighed directly in an accurate balance, or be folded in a small piece of poor lead-foil, and again passed to the cupel, so as to obtain it in the form of a pure metallic globule.
It is of the greatest importance that the acid employed for the above operation should be perfectly free from chlorine.
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In quartz-crushing establishments, the assays of ores and tailings may be conveniently conducted in the furnace used for retorting and smelting, which is generally of sufficient size to admit of three or four fusions being carried on at the same time.
The chief use of assaying to the miner is to enable him, from time to time, to test the tailings, and find out what amount of gold he is losing, and so to see if the machinery is working satisfactorily.
Geology of the Coromandel Peninsula.
The Peninsula of Coromandel has been so little explored by the geologist, that its general structure even is not well determined, and it will, no doubt, be some time before it is satisfactorily made out in detail. The following sketch, however, will shew what is at present known on the subject.
The main range, which divides Hauraki Gulf and the valley of the Lower Thames from the Bay of Plenty, is composed of a series of green and blue slates and sandstones, belonging to the upper Palaeozoic period. This formation runs from Cape Colville in a southerly direction to beyond Ohinimouri, where it is broken through by the volcano of Aroha, and deeply buried beneath volcanic ejections. The range has a bold serrated outline, varying in height from 1,000 to 2,600 feet, and is, for the most part covered with dense bush. The slates of which it is composed are much disturbed, and tilted up, being in places quite vertical, and their upper surface is worn into hills and valleys, which sometimes have different forms and directions to those at present existing.
Upon the up tilted edges of these primary rocks, and filling up many of the valleys, rests a vast mass of trachytic tufa of various colors, and in the most different states of decomposition, from the hardest
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rock to a soft clayey mass, and varying from a fine grained tufa to a course angular breccia. It is this rock that, previous to the visit of Dr. Hochstetter, was mistaken for granite and porphyry, and it is still often erroneously called granite by miners. In general it is a hard blue granular rock, becoming first soft and grey, and ultimately white, through the influence of the weather, and when in this state it is often stained red with peroxide of iron, by the decomposition of the iron pyrites with which it is strongly impregnated. The decomposed tufa, when free from iron, would probably make an excellent clay for fine earthenware or even porcelain, and it can be obtained in any quantity. The point north of Tararu is composed of felstone, and a little beyond it, and to the east, up the Waiohanga Creek, blue silicious primary slates are seen, which however do not extend into the valley of the Tararu Creek. With this exception, the whole of the country from Shortland to Hastings, or Tapu, is composed of the trachytic tufa, and from Tapu it is found in patches as far as north of Coromandel. In it all the claims at Shortland, and most of those at Tapu, are situated. Above it lies another tufa containing large boulders of a dark colored trachyte, which in its turn, is capped by the trachytic breccia that forms the summit of Castle Hill.
It is still an unsettled point whether the two trachytic tufas are one and the same formation, or whether they are widely separated with respect to age. The upper one, containing the rounded boulders, no doubt belongs to a late tertiary period, while the lower one, in its hard decomposed state, has much the appearance of a primary rock; but appearances in rocks are very deceitful, and chemical analysis shews that there is no essential difference between the two as regards their mineralogical composition, while hand specimens from both, in
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their decomposed state, cannot be distinguished, and we have no other evidence of any volcanic outbursts on this Island previous to the Miocene period. Both rocks contain veins of crystalline and drusy quartz, while, at present, carnelian, agate, and jasper appear to be confined to certain localities in the upper one. The published analysis, by Mr. W. Skey, of the upper tufa from Keven's point, at Coromandel, also shews that it sometimes contains traces of gold.
On the tops of many of the hills between Shortland and Tararu, another tufa, younger than any of the foregoing, is found. It is a soft red and white sandy rock, and does not contain any gold.
The whole of the rocks in the district, with the exception of this last tufa, are occasionally pierced by dykes of igneous rocks, such as diorite, dolerite, and sometimes of a more silicious rock, like compact trachyte or pitch-stone. Two good examples may be seen up the Moanataiari Creek.
The greater number of the auriferous quartz-veins or leaders have hitherto been found in the finer grained portion of the tufa, none having as yet been discovered in it where it assumes a brecciated structure. A few at Tapu--e. g. McIsaac's and the Duke of Wellington--are in the primary slates, and, as gold of very various descriptions has been found in different parts of the peninsula, it seems reasonable to expect that auriferous veins will be found in various rocks, and it is likely that the purer kind of gold, found at Kennedy's Bay and other places, may be confined to the primary slates, and be of a much older date than the alloy of gold and silver found at Shortland, Tapu, and Coromandel, which was probably formed during the close of the Miocene period.
The quartz-veins containing gold at Shortland are, for the most part, only a few inches in thickness,
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and their direction and duration is extremely variable. There is no point of the compass to which leaders may not be found running, and their bearing does not seem to have any effect on the quantity of gold they contain. Their want of regularity is however made up for by their great number, for amongst the 2,000 claims at the Thames, there are few that have not opened out one or more, the greater part of which have been proved to be more or less auriferous. The origin of these veins is this:-- The tufa in which they are found was evidently deposited under water; the long-continued heat which it afterwards underwent, as proved by its altered character, and the dykes of igneous rocks cutting through it, would partly dry it, and, in drying, it would contract, and split in all directions; the remaining portion of the water, heated by volcanic fires, would dissolve the silica, and percolating through the rocks, would deposit it as quartz in the cracks, as explained in the first chapter, and as those cracks would have no particular direction, the quartz veins filling them will have none, and they will not lead to any definite reefs. In one or two instances however, such as in the Dawn of Hope claim, and in Smith's claim, on the Hape, the leaders are slightly different. In both of these cases, the crack, or fissure, seems to have extended to the surface, and to have been partly filled up by broken fragments of rock, which have fallen into them from above, and the whole has been subsequently bound together by a silicious cement, the gold of course, being found in the quartz cement, and not in the enclosed fragments of rock. At Tapu Creek, some of the leads are larger and hotter defined, their general direction being from N. to N. N. W., and almost always dipping to the East. Such are McIsaac's, Golden Point, Golden Valley, Lady Bowen, the Prospector's, and Quin
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and Cashell's. These seem to be fissures, caused by a fracturing of the rock on a larger scale, owing to mechanical pressure. These leads also often shew on their casings what is known by the name of "sliken-sides." This is a marking, or grooving, in parallel lines, in the direction of the dip of the veins, and has been caused by a sliding or rubbing movement of the different sides of the vein against one another. The fact of an unequal movement of the two sides having taken place, may be taken as good evidence that the fissure is either of considerable extent, or that two or more join together to insulate the piece that has moved; for unless it were so, such a movement appears to be impossible.
The alluvial deposits on the Karaka Flat have been exposed by various shafts and borings to a depth of about 90 feet below the sea level, and have been found to be composed as follows:--
The youngest, or uppermost, is a sand full of broken sea shells, about 10 feet thick. Next in age to this (although sometimes rising above it to a height of 170 feet), is a blue or yellow clay, formed by the decomposition and washing down of the surrounding hills, and contains blocks of tufa, dark colored trachyte, and occasionally fragments of auriferous quartz. It is this formation that forms the terraces round the mouth of the Karaka, and at the back of Shortland. Below this comes a sandy gravel, consisting of rounded stones of tufa, dolerite, trachyte, obsidian, and rhyolite, a peculiar kind of lava not found nearer than the hot lakes or Taupo districts, in the centre of the island. Below this, again, comes pumice sand, which has never been bottomed.
From this description we can glean the following facts in the geological history of the Coromandel Peninsula. Sometime subsequent to the deposition of the slates, the whole of the country was raised,
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with much squeezing together and fracturing, above the sea to a considerably higher level than it now stands. At this time it probably formed part of a large mountain district extending over the greater part of the Island. This mountain range must have been for a long time exposed to the rain and weather, which, in the course of time, hollowed out valleys, some of which we can now recognise. The largest of these was the valley of the Thames, emptying itself into the sea between Cape Rodney and the Great Barrier, and with smaller gorges running into it from either side. This state of things lasted until the commencement of the Miocene period, when the whole land sank about 2,000 feet below its present level. Submarine volcanic eruptions on a stupendous scale then broke out on the north from the Great and Little Barrier Islands, on the east from Cuvier's Island, Mercury Islands, Mayor's Island, and other places, and on the south from Aroah. These eruptions filled up the valleys, and covered the whole district with volcanic ejections. As a consequence, probably, of this energetic volcanic action, the land was again elevated to a height of perhaps 200 feet more than at present, and many of the volcanos, still active, reared their heads above the water. This was the time, the close of the Miocene period, when the rocks were impregnated with iron pyrites, and probably when the gold first began to accumulate in the quartz veins. A second time the rain and frost acted upon the surface of the land, re-scooping out the Thames valley, and denuding the whole of the volcanic rocks off from some parts of the slate rocks, and leaving only patches here and there remaining. At this time the Thames ran as far north as Coromandel, bringing down with it the pumice and gravel, containing obsidion and rhyolite, that underlies the town of Shortland. Again the land sank until the
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sea overflowed the flat by Tookey's Town, and covered it with broken shells. Once more it rose until it attained its present level. Whether it is now rising, sinking, or stationary, there are no data for determining; but, whichever it may be, its progress is so slow that, in all probability, generations will pass away before any change will be perceptible.
It will be thus seen that the present configuration of the country is owing to the valleys having been slowly scooped out by running water, and not to each of the hills having been violently thrust up, notwithstanding that the uptilted position of the older strata might naturally give the idea that such had been the case.
It has been sometimes said, that the discovery of gold at the Thames was quite contrary to previous geological experience, and that it would revolutionise the science. Such however is not at all the case, for it so happens that some of the oldest mines in the world, viz., those of northern Hungary, present a most remarkable similarity.
The mining districts of Schemnitz and Kremnitz lie about 80 miles north of Burda, and are in the midst of a group of mountains covered with forest. The whole of the auriferous veins occur in rocks of various kinds of trachyte, and trachytic tufa, partially decomposed, of Miocene age, and resting unconformably on slates and limestone. The rocks are largely charged with iron pyrites, and the veins are rarely interrupted by faults. With the exception of the underlying limestone, this description might answer, word for word, for the Thames. There are however some points of difference; the veins in Hungary do not always exhibit distinct walls, but the ores seem sometimes distributed through the rock in bands with very little quartz, and they contain more galena and sulphide of silver than
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gold; at Kremnitz, however, the veins are more decided, more quartzy, and contain more gold.
Schemnitz is situated 1680 feet above the sea, and some of the mines are sunk to 250 fathoms. The yield of gold is now falling off, but we must remember that they have been worked regularly for the last 800 years, a fact which speaks volumes for the permanency of the Thames gold-field.
The "Great Comstock" silver vein, in Nevada, California, also appears to be in very similar rocks to those at the Thames.
The following metals have been found in the Thames district:--
1. Gold.
2. Silver.
3. Mercury (Cinnabar, in small grains.)
4. Lead (galena, small quantities.)
5. Antimony (stibnite.)
6. Copper (copper pyrites.)
7. Arsenic (native and mispickel.)
8. Iron (iron pyrites, siderite, hematite, and magnetite.)
9. Tin? (cassiterite?)
10. Titanium (Titanic iron sand and rutile?)
