Mining.

Otago Witness, Issue 987, 29 October 1870, Page 10

Mining.

ORIGIN OF GOLD NUGGETS AND GOLD DUST.

Mr Andrew Murray, F.L.S., writes on this subject as follows :—: —

The origin of gold nuggeta and gold dust is not so simple or clear as at first sight it appears to be. The natural explanation of the production of gold duat is, that it is the golden portion of the debris of rocks which have originally had gold disseminated through them. As the wear 'and tear of ages has crumbled into dust mountains so composed, part of the dust becomes sand or quartz, or whatever else the basis of the rock may be, and the other part is the liberated gold from which the quartz has been rubbed away ; and if we accept this as the explanation of the production of gold dust, the same hypothesis should explain that of gold nuggets which are found associated with it. But there are various circumstances which it is difficult to reconcile with this theory. One of these is the occurrence in the drift of nuggets of a larger size and less intermixed with foreign substances than have yet been discovered in any quartz reef. As most people are aware, the gold in reefs is usually disseminated in particles and strings through the quartz veins or rock, instead of lying in pockets or masses. Another still more remarkaole fact, applicaole both to gold dust and gold nuggets, is that alluvial gold is generally of a higher standard than that obtained from the reefs. It is needless to say that if it is merely the gold washed or crumbled out of these reefs, it ought to be of identically the same standard and quality. Another objection to the dust being merely the degraded particles released from the rocks is the size of the particles — nr>t nuggets, but particles of dust. Gold being so much softer than quartz, its particles, afier being subjected to the same degree of attrition, ought to be vastly smaller ; although of greater toughness than quartz, and possessed of ductility and tenacity which quartz wholly wants, it is very soft, and under the influence of the attrition from running water and its accompaniments ought to be pounded and torn into the minutest fragments ; but this is not bo. There is, moreover, a marked difference in the appearance of the gold dust from different drifts in different countries. In some it is like dust or sand, in others it is like scales. If subjected to the same influences in all, there seems no reason why the same shape should not obtain in all.

These peculiarities would suggest that some other influence than mere degradation of gold-charged rocks has been the agent in producing gold-dust ; but in any and every view we think it cannot be disputed that degradation most have had some share in the work. It is plain that if a gold-charged rock is reduced to gravel, sand, or powder, particles of gold of some size or other, or gold in some shape or other, must form part of the debris. These gold remnants should be found in greater quantity and in greater size the nearer tney lie tp the source from which they

were drawn; and this we believe also to be the case. The general similarity between gold - producing districts, by which a Oalifornian miner could detect a likely spot for gold in Australia or Kildonan, probably depends rather on the character of the mountains out of which the gold has come than on the mode of production of the manufactured dust, if we may call it so. We imagine that the truth will be found to be that the result is referable to two causes, only one of which may in some cases have been present, at others both ; the first, the ordinary processof degradation, andgriudingtherock to fragments ; the other, as suggested by Mr Selwyn, the Government geologist of Victoria, tha' gold has also been taken up in solution by the water permeating the wold-boarmg rocks, and that in passing through tin drift, in which minute particles of gold lay, it has from some c*use become decomposed, and the gold held in solution been precipitated and deposited around the most congenial nuclei presented to it, which would undoubtedly generally be the particles or pieces of reef gold, or any other metallic Bubstances for which it had an affinity.

We find an interesting paper on this subject in the Transactions of the Royal Society of Victoria (1«67), by Mr C. Wilkinson, in which he mentions some facts bearing on the subject. It appears that Mr Daintree, formerly of the Geological Survey of Victoria, had on one occasion prepared for photographic uses a solution of chloride of gold, leaving in it asmall piece of metallic gold undissolved. Accidentally some extraneous substance, supposed to be a piece of cork, had fallen into the solution, decomposing it, and causing the j a;old to precipitate, which made a deposit in the metallic state, as in the electro-plating process, around the | small piece of undissolved gold, increasing it in size to two or three times its original dimensions. Considering this accidental experiment of Mr Daintree's as in some measure supporting Mr Selwyn's theory, Mr Wilkinson followed it up by a few simple experiments in the same direction, which he details in his paper. In his exI periments a small chip of wood was genei rally used as the decomposing agent. In j one instance he used a bit of leather. All through the wood and leather gold was disseminated in fine particles, and when cut through, the characteristic metallic lustre was highly reflected. From various experiments it would appear that organic matter iis the necessary agent for decomposing i a solution of the chloride of gold in order to precipitate the gold as a coherent coating around a nucleus ; and that, so far as Mr Wilkinson had yet tried, iron, copper, and arsenical pyrites, galena, antimony, molybdenite, blend, wolfram, and metallic gold constitute essentially favourable nuclei to determine this chemical reaction. It is to be observed, too, that organic substances, such as fragments of wood, roots of trees, &c, occur abundantly in the gold drifts of Australia. If water holding gold in solution circulates through the rocks and drifts, all the conditions necessary for the production of gold-dust and nuggets by deposit are present. Does the water so ci culating now hold gold in solution ? One would think it would not be difficult for a chemist in Australia or California to determine the fact by direct experiment, but it does not appear that it has ever been tried. Mr C. Wilkinson, however, quotes facts which lend probability to the view that when the trial is made, the question will tie solved in the affirmative, in testing a solid mass of iron pyrites, Mr Daintree found gold throughout. This mass retained the structure of a treestem, in which the organic structure was replaced by pyrites. It had been taken from the Ballarat drift, and the same experiment was repeated by Mr Newbury, the Geological Survey analyst, on another s'em taken from the same drift, with a like result. Gold in such deposits assumes a mammillary form, which appears analogous to that presented by the surface of nuggets — a point of some importance, for, in the first place, it is a question whether a mpmmillary surface is the kind of surface that would be produced by abrasion and attrition ; and, in the next place, abrasion or attrition can certainly have nothing to do with its appearance in these golden petrifactions. We cannot avoid attaching the greatest importance in relation to the question, to the presence of gold in pyrites that has been formed in wood embedded in auriferous drifts. The gold must have been in solution when so deposited, and everything will then depend on the age of the so petrified wood. If contemporaneous with the drift, the question is answered. Another fact to the same effect is, that sometimes gold encloses a nucleus of brown iron, &c. This is obviously quite inconsistent with such pieces of gold having been abraded, as they are out of crumbling rocks, such nucleated pieces of gold are never found in reefs. It is the old puzzle of a reel in a bottle.

In relation to this we may remark that we believe that nuggets have never been

ound in the goldfields of Brazil. We lia<e the authority of Mr Harding (a gentleman well known for his great practical knowledge of gold mines and mining in that country) that he never met with nor heard of a nugget, properly so palled, in all his many years' experience in the gold district of Brazil ; but on the other hand, it is there almost invariably found in veins in connection with, or in the vicinity of, some other metal — generally iron. In -what is probably the most prolific mine of gold that has ever been known in the whole world, that of San Juan del Rev (he value of which was not very lon« since so seriously depreciated by the accidental destruction by fire of the wooden ladders, supports, and machinery), the gold is found in a ma rix of porous iron or agglutinated iron sand called Jacotinga, which consists of a bed or vein, not a foot in widrh, but so incredibly rich that on one occasion when our informant was on a visit to the manager, there was brought in on an assiette, as a sort of desert for the eyes after dinner, a lumn of gold ore that had been extracted that day from the mine. It was about the Bize of a lare fowl not so big as a turkey, but bigger than a duck. It was a mass of Jacotinsa-iron with gold all mingled and streaked through it. The gold when afterwards extracted was found to amcuint to 301b weight. On the previous day the amount of poH obtained from the Jacotinga had reen 671b, and on the day Mlcwinsr 1301b, equal in value to about LBOOO. We only mention it as a corroborative instance of the concurrent presence of gold and iron. Lastly, aa pointed out by Mr Wilkinson, it must be admitted tha<- th^ fact that gold may be greatly purified by dissolving and reprecipitating it, is very suggestive of the generally higher standard of alluvial over reef gold being due to a similar cause.

Marking out claims and selling them to Chinese, is now a branch of commercial enterprise in the Lake district. The Cromwell Company, Bendigo Gully, retorted a few da\ s aao. when they obtained 365 ounces of gold for fourteen days' crushing The Company* battery of ten sampers is now steadily at work. Satisfactory accounts are received from the qnartz reefn at Corcroy's and Butcher's Gullies near Alexandra, rushing machinery wlli. it is said, be erected shor ly, and it is believed thit the reefs will yield remuneratively for ten years to come. Mining operations have, with the return of mid weather, bpen resumed at Clark's. At Surface Hill the ground recently opened has not come up to the expectations that were entertained regard ng it. and a number of the claims h^ve been abandoned in conquence. In spite of their numerous disappointments, the people of Wellington have not yet given up bopas of discovering a payable quartz reef in the neiehbourhood of that city. Prospecting is still going on, and a small crushing machine has been constructed for one of the local companies. We learn that the Duke of Edinburgh Company's quartz-crushing machine at Macraes has been steadily at work since Friday last, and it is hoped that it will now continue to do so without interruption, as the dry weather has allowed of a large quantity of coal being brought to the spot where it is at work. The dredges at Alexandra have all stopped working ; some on account of the rise of the river, and others because they are at law. Iv spite of the ill luck they have had this season, some of the dredges seem to have done pretty well. The owners of the Galatea dredge, in particular, are reported to have made L 35 per man per week, clear of workingexpenses, duri-gtheseason. Itissaid, however, that thiß claim is now worked out. A correspondent of the Oamaru Times writes, regarding mining matters at Bendigo Grully, as follows :— "The reefs are not turning out so well as was anticipated, with the exception of Logan's. The Aurora had a trial crushing recen'ly, and the result being unsatisfactory, the price of shares has considerably receded. The Alta reef will, it is expected, turn out very we'l. The Company are waiting the arrival of the necessary machinery, which is oa the road from Dunedin. before commencing work. ColclougVs retf is nenrly *t a standstill in conseqnei cc of the dilatoriness of the Dunedin shareholders in forwarding the machinery It is understood tint the stono looks very promising, and is likely to yield good returns. General business at Bendigo Q-ully is very quiet, indeed there is nothing of importance going on. Since the Chinese left there has been less deep sinking, the Europe in mi"ers not being ab e to make it pay so well as their Asiatic brethren " During the month of September there were only 8 days on which rain fell in Dnnedin, the total rainfall being 1 096 irch, and the greatest rainfall in 24 hours 0 234 irch on the 12 h. The mean daily veloeitv of the wind during the same period was 157 mile*, the yrf'ites-t velocity observtd for 24 bourjI'eini/ 500 miles on the Ist. Hail fell on thr^e days, viz , the lsfc, 11th, and 12th. The highest maximum temperature registered during the month was 67 degrees on the 29th, and the lowest

minimum temperature 33 degrees on the Ist;, giving an extreme range for the month of 34 degrees. Ihe mean daily range, however, was only 13.6 degrees, and the ap proximate mean temperature of the month was 47.8 degrees. The barometrical observations for the some period show the highest atmospheric pressure to have been 30 548 inches on the 6h, and the lowest atmospheric pressure 29.438 inches on the 13th, giving a range for the month, of 1.110 inch. The mean atmospheric pressure during the period was 30.074 inches.