GOLD MINING, AND THE GOLD DISCOVERIES MADE SINCE 1851.

Otago Daily Times, Issue 219, 2 September 1862, Page 6

GOLD MINING, AND THE GOLD DISCOVERIES MADE SINCE 1851.

BY J. ARTHUR PHILLIPS, E?Q. at the Society of Arts, on Wednesday, May 14.) . (Continued from our laat.) Methods for Extracting Gold from its Matrix.—The most simple au<J, at the same time, mo<tancient method for obtaining gold is undoubtedly by washing the sands nud dirt with which it is found associated. On a small scale this may be performed cither in a bowl or tin pan ; but when greater expedition is sought, recourse must be had to appliances a somewhat more complicated nature. Among the easier miners in California and Australia the "c< adle' J was much employed. This instrument appears to ha^e been introduced from Virginia and Carolina, and consists of an oblong inclined box, having a sieve at its upper extremity mounted on rockers, so that, by means of a hnudle, it may be swayed from side to side. .The interior of this, case is provided with'a sloping diaphram of tightly-stretched canvas, and the bottom is divided into partitions by means of wooden cleets. Washing by the cradle, is, howe-er, a very slow operation, and requires a great deal of manual labour since, besides rocking, it is necessary to supply it with water by means of a clipper, and to continually stir the fresh-brought sfcutf deposited" on the sieve. The gold and other heavy bodies retained between, these wooden divisions are finally te-wobhed in a tin pan, and the metal thus obtained in a pare state. Tlie loss of fine gold attending this operation i is very great. j

The arrangement which next came into general use among Oalifornian miners was the "ions; torn." This consists of a long, roughly-made wooden case, having a considerable inclination, and provided it-its lower extremity with a sieve, made ot perforated sheet iron, t;eneath which in placed a " riflle-box/' divided into compartments, as in the case or the cradle, by means < .slips of wood. In the upper trough a stream of water is so directed as to fall with considerable force upon the auriferous drift with which it is charged, and this being continually stirred" with a shovel, the finer particles are washed through the sieve over the riffle-box, whilst the coarser fragments are, from tiiiie to time, removed, after being duly examined for any nuggets they may contaiu. The stuff retaim dby the rifiies is afterwards washed in a pan. and the clean gold is thus separated. The torn has the advantage over the cradle of getting through a much larger amount of work withiu a given time ; but it requires a much more plentiful supply of water, and the lo.ssot fine gold is great. f When convenience exists for its introduction, the .'■' sluice" has now generally superseded the torn. This arrangement is nothing more than a ioug run of wooden troughs, provided with false bottoms,. in which augerhol-s have been bored to a certain depth, and in which mercury is generally placed. .Through these inclined troughs the " pay dirt " is washed, and the metal, from its greater density, settling1 in the depressions at the bottom, and combining with the mercury placed there for that purpose, is thus retained. These false bottoms are oc-casioaaliy removed and the merenry separated from the gold by filtration and subsequent distillation. This process" a!t*\oiigli a certain portion of the gold is still lost, is generally much preferred to either of those above described. It is also now customary, whenever a sufficient fall of water can be obtained, to direct a stream, by me.'tns either of metallic tubes or canvass hose, again-1 the bench of pay dirt it is intended to remove. A powerful stream playing against the side, of a hill will in a short time disintegrate a large quantity of dirt. The rubbish thus detached i-- conducted through a .sluice in the usual way, und the cold is in-thi* manner separated and collected. This method of proceeding is known by the name of " Hydraulic mining," and is, generally spealcing, considered ihe most economical that can be adopted. When, instead of being found in deposits of pay dirt, the gold occurs in veins, associated with other metals, it. becomes necessary to reduce thegangue to a state of fine division, before it can be extracted. Two distinct methods are ememployed for the separation of this metal from the matrix -with which it is associated—viz., waßhiugand amalgamation.

lv some countries, and particularly in Mexico, the " arrastra "is much employed for the treatment of auriferous minerals. This consists of a vertical axis, provided with cross arms, to which are attached, by means either of ropes or thongs of untanned leather, two or more heavy masses of porphyry. Mules are harnessed to one of the projecting arms, and a rotatory motion given to the shaft. The stones thus set in motiou are dragged over a well'paved bed, and thus, by an action somewhat resembling that of the common muller and slab, the ore is gradually reduced. Mercury and water are added to the ores operated on and the resulting amalgam is from time to time passed to the retort. In some instances the ores are introduced into the arrastra in fragments of about the size of peas; but in large establishments it is nrst coarsely ground in a stamping mill. It is needless to say that grinding by means of the arrastra is a slow and expensive operatiou. In Chili the "trapiche" is much used. This is nothing more than a grinding-mill, like the ordinary edge-runner. The roller runs on a proved bedstone, .in .which a certain quaittitv of mercury is placed, and by the continual trituration of the*revolving runner tho ove is gradually reduced and amalgamation effected This is. however, like the foregoing, a tedious and costly operatiou. In some cases a mill like that commonly employed for grinding corn bus been made use of, anil found* to answer remarkably well. fn one establishment where apparatus of both constructions is in operation the ratio "of the cost of grinding by the horizontal mill', as compared to the edge runners," is as 2s, 3,!. to ds. lOd. The ordinary roller, crushing-mill hasber-n .also employed for the reduction of gol! quar'z previoui to amalgamation, but it cannot be considered to be well adapted for this purpose. In the first place, the whole of the stuff coming from the mill lias to be passed through sieves of fine wire-gauze, and the*c become so rapidly worn by the rougher fragments' which are being returned to the raff-wheel as to render repairs constantly .necessary, and the operation very expensive. Then, again, unless the ore ha remarkably dry these seives choke, and the stuff is carried round aad round without passing through • and, finally, if the ore be dry, such a dust is created as to nearly choke those attending to the crusher. Among the quartz miners of California and Australia the stamping-mill is now the machine, almost universally employed. The oe is o~ien calcined ' D heaps or kilns, and, after stamping, the rediK-td-mineral is passed through apparatus of various forms for the separation of the gold. The calcination of the quartz, although not always adopted, is frequently productive of advantageoue results. Hard quartz is rendered much more friable by this treatmeut and when a large proportion of sulphides is present the expulsion of sulphur by the operation of roastin" is likewise beneficial. It is also probable that when gold occurs in thin finely-divided lamince the ignition^ of the quartz causes such agglutination of its particles as to cause them to offer less surface to the action of the water, and that the loss of " float cold " is thereby diminished. The metal is separated from the stamped ore either by washing alone, or by washing and amalgamation. Wnen the former process is reported to, the stuff flowing Irom the stamping-mill is either allowed to pass over rirae-boxes, or is conducted over blankets or skins on which the hair is retained. These are occasionally washed in proper vessels, and the metal retained by them thus collected. The gold so obtained is, m most instances, concentrated by washing in a batea,' or otherwise, and finally amalgamated, or less frequently fused with litharge, or an ore of lead and finally cupelled. When amalgamation is employed the riffle boxes may be changed with mercury, or the auriferous sands produced can bepawed through triturating apparatus containing mercury, with which the gold is caused to combine. In some cases barrel amalgamation is resorted to. The diagram on the wall exhibits a combination of three of the most efficient amalgamating appliances used by the miners of California and Australia, which, is well calculated to separate the precious metal from ordinary gold quartz The ore flowing from the mill first passes over a lip through a triturator, not unlike that employed at Zell in the Tyrol, and then falls into an apparatus, the action, of which is similar to that of the amalgamating barrel. Finally, the whole of the stuff, before passing over the riffle-boxes or blankets, is agitated in a column of mercury, through which it is made to decend. In some instances/where wateris not plentiful that from which the tailings have settled is a«dn pumped round. In this case a little woo ! ashes should rrom time to time be thrown into the mill. This is mployed for the purpose of saponifying any oil or other fatty matter which, if present in even the most tnmuteproportions when quicksilver is used, would, by preveatmg the prrtides of gold from uniting with the mercury, mate ully interfere with the results obtained. It is, therefor^, of greatimportance in all quartz crushing and amalgamating establishments that proper care be taken to prevent any dropping of oil from the bearings into the apparatus, since the^result of such an : accident wunhlinevitably be a notable falling off m the produce.or gold obtained. In order to prevent 1< ss occurring ih: o.,gh this cause.it would as before stated, be found a^vant-eeors to throw from time to time into tho n:i I a little wood ashes or Eome other alkaline body, for the purpose of removing any greasy matter which may have become accidentally introduced. When the quartz contains an ' appreciable-quantity of auriferous sulphides it would In many cases be found advantageous to separate these from the tailings by means of Hundt's buddle applied to the end of the riffles. The sulphides thus collected might be treated either by fusion with oxide of lead, and the produce cuplled for gold, or after preliminary roasting be again subjected to amalamation. The former process -will, however, in many instances prove the jaost advantageous.

As an instance of the small yield of gold w lric~-, even in Australia, is at the present time found reininei'iitiye, 1 wou'd quote the following re-ults of tho Colonial raid I'uitPiiiliip Company/ It must however, be observe.l that, to olnaiti a satisfactory profit from ores of tuis class, it is necessary «r,fc only tint large quantities should be treated, but also that the greatest economy should be observed in every depavtment of the manipulation. Tlie quantity oi quartz embed by this oinpany* between Oct. 1, ISGO, and Sept. 30, ISGi, was 32,258 tons, from whicli the produce was-24,330 ozs. G dwts, being an average of 15-2 dwts. per ton. The quantity crashed duting the preceding year was 21,093 tons, and the produce 17,400 w^., being an average of 16 dwts. per ton, shewing an increase in crushing of 10,563 tons, and on yield of gold of 0,870 ozs, overthe same period, of.the previous year. It will be perceived that the yield of gold per ton had experienced a variation of 22 grains, equal to 5| per cent. The total expenditure per ton has been 12s : in the preceding year it was lUs.v The profit on the quartz crushing for the year ending September 30th, was 22,958 Us. sd.

Assay of Ores Containing Gold.—Minerals containing gold arc in most instances assayed in precisely th° same way as those affording silver. To make an assay of auriferous quartz, the sample to be operated on is first finely pulverised, and a given weight subsequently welt mixed with litharge, carbonate of soda, borax and an-amount, of chareoul dual sufficient for the production of a button of load of a convenient size for cupella ■"• >n. The metallic globule remaining on the cupel after this operation will contain all the gold present in the ore, together with any silver that may be associated with it. as well as a certain minute portion of that metal derived from the had of the reduced litharge. In ea&e of Ibe poorer ores containing less than ten dwfs. of line gold to the ton, the silver derived. from the litharge will frequently be found amply sufficient for the purposes of inquartation, ■whilst for the richer varieties the addition of a little pure silver at the time of" placing . the button on the | cupel is often necessary.. When in addition to gold, | the ore contains smail quantities of iron pyrites," or ; other sulphurised mineral, it not uiifiequently hap- [ pens that the admixture of charcoal or .any other rej riucing agent becomes unnecessary, and the fusion may be made with litharge alone. '■When, .however, pyrites, blende, or other metallic sulphides are present in large quantities the sample must either he I treated by sconfication, or it must be first roasted i until all the smell cf su'phur- has disappeared, and then treated as in the case ofsubstancfsiiot containing that body, but with this addition of a large quantity of- borax. It is, huwevar, to be remembered Urn"' i when any of tlirse" compounds contain sulphur, it is |of importance that the whole of it should be removed either before or during the process of obtaining the leaden button, eiuca otherwise, and particularly in •he presence of alkaline fluxes, a portion of the gold may enter in combination with the slags in such a way as not to le entirely separated from them by the action of metallic lead.

It may l> e proper to remark here, t'uat although nothing is more easy than to estimate with great accuracy, the fimouat of j^oltl contained in any Riven speoinien of pold quartz,it wcon.-i(lerabsyworoiliilieii!t to.obtain a fair average sample of the usual produce of a vein. When the metal i.i in a tine state of division, and equally disseminated throughout the gangue, this presents less difficulty ; but wnen, on the contrary, it occurs in pockets and irregular deposits it fre quently requires the exercise of great care in order to avoid falling into very serious errors. It is consequently of the highest 'importance that whenever ore? are to be assayed for gold, the greatest care should be obvei ved in preparing the samples on which the cpemtion is to be eaidueerd. With this view, the heaps or piles should be fairly cut through, twj or three tons being taken from eacY parcel of importance, and reduced to fragments not exceeding the size of beans; this may be (-fleeted, when crushing machinery is not available, by breaking the ore by the aid of properly-shaped hammers on iron plates. This operation is technically called "bucking." The ore thus prepared is now to be thoroughly mixed, again made into a pile, and again cut through, taking out of it this time from 3 ewts. to A ewts., which are reduced to the state of liiio. powder, either in a large mortar or by grinding on an iron plate. After well mixing, this powder is again cut through, and about 20 lbs. weight taken for the purpose of beinir still further reduced in size, and passed through i» sieve of fine wire gauze. Should any flattened particles of gold remain in the sieve they an; to be carefully collected, cupelled, and parted", mi! due niloiv.we made for them in the result obtained by direct a.ssay. Of the finely-divided ore which has passed through the sieve at least six different assays of 100U grains each should be made, and their mean Jesuittaken a.-s representing the produce of the parcel of ore of which it is the object to determine the value. By operating in this way almost absolute accuracy may be 'insured ; but when a less decree of exactitude h necessary the quantity of crushed ore may be reduced, and the number of assays few, r.

If after thus accurately testing the produce of a parcel of ore, it be pessed through' (he most efficient crushing and amalgamating machinery with which we .ire acquaint^--!, it will be inn rid that the total amount of gold originally joes; nt in the stuff is never obtained and if thisd. licit besought for in the tailings resulting from the operation it will bo dii covemi that a certain f-'mall quantity of the precious metal still remains unaccounted for. This 'deficiency would appear to be due to the circumstance of minute particles of flattened gold having floated off on the .surliuM? of iho water, and frequently amounts to neaily 2 ilwts per ton of ore treat.").!. When the ore to be examined contains silver in addition to gold, and it is desirable to ascertain its amount, it bm)in..s necessary first to cupel the butt-in of lead without the addition ot silver : the metallic globule thus obtained is weighed, and-its weight noted, deductions being made for the weight of silver derived from the reduced litharge, whi-h mart be ascertained by a distinct ampliation. If mere silver is required for the operation of parting, it is added, ami the button, together with the fragment otsnver, enveloped in apiece of pure lead foil, and again cm pel led. Lastly,- the resulting globule is di&soived in nitric acid, and the gold weighed. The.weight ol silver present in the ore will consequently be represented by that of the button of alloy obtained from* the first <:upe)iation,-Jess tli'e united weights of the gold and the silver resulting from t'>e reduced litharge In concluding this fiuhjeV. I cannot butter express the LTfat importance' of obtaining fair sampler, than by quoting the words made u'.se of by Dr. Percy, in a lecture delivered in 1852, at the School of Mines, who, when .speakin;; ( .f gold assays, s'iid— ' Above everything b«. p.riiciiiar in obtaining "an honest and f.ur sample. This Ls a matter of paramount importance, and of no .sunll difficulty in many cases but let there be hon.utv of intention, and this difficulty will be g« nerally surmounted."

Gor.D Mijsino SpEcur ations of l&Vi.—Shortly after the discovery of the #>ld deposits ot California and Australia numerous associations were organised in the United Kingdom for the pm-paso of'working gold mine.-; in tlio.se countries. Janil I regret to say that, in almost every instance, these have resulted' in the total loss of the capital so embarked. Various circumstances hiwc contributed to produce these disastrous results, but none more so than the fact that, in too many instances, sufficient attention had not been paid to obtaining samples fairly representing the average produce of tho various veins which it wa? intended to work. The specimens which reached this country were often picked samples, and on beinjr placed in Ihe hands of the asaayev yielded a produce which v.a« f.u* from realised when fair average samples of the leads came to bo tested on a large scale. Then, too, it was not unfrequently fonnd that quartz veins] producing what should have been a remunerative amount of the precious metal, were situated in localities in which, eidier from the want of water or some other cause, thair exploitation was attended with extraordinary difficulties. And, above all, the excessive price of labor, and all other minintr requisites which then prevailed- was, in most e^es, a sufficient barrier to anything like remuuciative returns to the proprietary. It is sufficiently evident■ that in a country where an e<r«- sells for a shilling and a fowl tor a guinea, a much larger amount of gold will be required in order to afford remunerative results, than in one in which the necessaries of life can be procured at a more moderate rate; and it h also much less difficult to control the .labor-of a hux<> staff of operatives where employment is comparatively scarce, than where any man of moderate indnstry'in'av, W workin<r on his own account, earn nearly 20s", pnday. The natural efit-cf; of such a state cf soci-'ty being that, in the first place, work of all kinds is necessarily expensive, arid, secondly, that- the supply of labor is very precarious.

In all rich nnd newly-discovered gold districts, which have for the most part a very limited resident population, the alluvial and easily-worked deposits afford for a considerable time 'a superabundance of remunerative occupation for the newly-arrived immigrant, but as these gradually but slowly become exhausted, * something more than meremuscular slreniith becomes necessary in o-der to keep up the returns, a more systematic method of mining is adopted, a thorough combination of labor and the investment of larger capitals are required. It must, however be remembered that these changes, although gradual even in a new colony, are infinitely more rapid than thos* who have always resided in Kuropeah. countries generally imagine. Ten years in the life of a colony and particularly a pod-bearing one, effect greater ° ,an* « mite coiamercia nnd .social relations than a ct'nturv

(To be (Qntiwitd.)