Report on the Goldfields of Otago.

Bruce Herald, Volume VIII, Issue 701, 18 May 1875, Page 7

Report on the Goldfields of Otago.

(Continued, from our last.) Br G..H. P. TJjdbicii, Esq., F.G.S

Report on the Auriferous Quartz Keefs and CrushingMachines of the Province of Otago, New Zealand, -with Remarks on Auriferous Drifts and Occurrences of Copper Ore, Ciunabar, Grey Antimony, and Brown Coal, in Different l'arts of the Province. Chances op Prospecting foe. New AußiFEßiotrs Eeefs.

As regards the chances of the occurrence of other auriferous reefs in the districts under notice, I consider them to be very good. In the neighborhood of Saddle Hill Reef, several (4) good-look-ing, strong reef's crop out, which as superficial prospecting has already proved them to be slighlty auriferous, might likely yield payable stoar on being tried at other points in their line of strike. But besides this, there is, in my opinion a very good chance of the existence of an auriferous reef about the head of a highly auriferous gully, worked abreast of the Saddle Hill Reef, on the right hand side of the main road leading from Dunedin to the Taieri. In the neighborhood of the Canada Reef, Tokomairiro, the finding of rich quartz specimens outside the lines of the reefs opened, coupled with that of rich, nuggetty gold in the north branch of the Tokomairiro River, is pretty good evidence of the occurrence of other auriferous reefs in that district. That the Gabriel's Gully Reef should be the only auriferous one in the Tuapeka district is also very unlikely. And, touching the country round Waipori, where no systematic prospecting seems to have been carried on at all, I feel quite convinced that good reefs exsist besides the one in work— not only in the neighborhood of the latter, but also in the ranges on the opposite side 6f and aboifc the head of the Waipori River, — in order to account for the gold in the drift of the "VTaippri Valley, the angular specimens of auriferous quartz, and the patches of highly auriferous angular drift found here and there in the ranges. At Bendigo, the Carrick Range, and Skipper's Creek, where the tracing of auriferous gullies and quartz specimens led, I was told, to the discovery of most of -the principal reefs .worked, though some showed by no means plain outcrops at the surface, partly by reason of their mullocky character, partly on account of being covered by detritus or being disturbed. And these features are the most serious obstacles the intelligent prospector has to contend with in "these districts, for that the latter are promising -fields".- 1 for further discoveries of quartz reefs is plainly indicated by numerous auriferous alluvial gullies and creeks, and the occurrence of quartz specimens outside the drainage range of the known Hnebofreef.'Therecent discovery by tracing specimens of a new reef in the Bendigo district presents a case in point. The same reasoning, based upon similar data, applies more or less to all the other reefing districts (Arrow, Bough Ridge, etc). And not to them alone, but also to the north and south elopes of the Old Man Eange, and to the rich alluvial diggings dotted along; the edges of the Manukerikia, Idaburn, Upper Taieri, and other main valleys, though the reefs that in the latter cases supplied the gold to thß drift would most likely lie in the ranges at the heads of and bounding the permanent creeks and rivers, entering the main valleys at, or in the neighborhood of, the diggings. With regard to" accepting the occurrence of jnassiv© auriferous drift or of mere surface

at any place in rangy country as a promising indication of the existence of auriferous quartz reefs in the neighbor-., hood, the prospector ought to pay special attention, however, to one point, namely, to the nature of both the drift and the gold, i.e., whether the pebbles and goldbpecks are waterworn or not. For, if the first is the case, the deposit may be a remnant of a former terrace formation far transported from its original source, whilst the more angular the stones, the more hackly and crystalline the goldspecks are, the nearer lies their place of derivation. There is one district (I had no time to visit it) hitherto only known as an alluvial diggings, which, according to information kindly afforded me by Messrs L. H. Ereston, Jenkins, McDougall, and Smith, of Arrow, presents first-class: indications of being a good reefing; district. This refers to the 12-Mile Creek diggings, and mainly to the Great Barrier Range in which the Arrow River and 12-Mile Creek take their rise, and that forms the watershed between them and the Shotover River, the position of the tract being pretty nearly in the line. of strike of the Skipper's Creek reefs,' A big reef runs right along the top of the rarge, and in the drift of the top part of the 12 -Mile Creek, Scanlan's Gully, Specimen Point, and in that of Rodger's Gully and Tobin's Point, at the head of the Arrow River, rich quartz specimens were frequently found, and, shown at the stores and banks of Arrowtown, whilst at certain points in the 12-Mile Creek, large quartz specimens can be seen— one of nearly 1£ ton in weight — showing fine specks of gold all over. The gold obtained from Rodger's Gully was generally so quartzose that it required careful crushing and separation of the quartz before it could be sold. Lower down country, about •§• mile from the junction of the 12-Mile Creek and Arrow River, a mullock reef, full of quartz veins, appearing as a wall- like formation on the mountain side, was cut through by a race, in which it proved to be 8 feet thick, showing well defined walls, and dipping nearly vertical. Its promising appearance i led a party of miners some time ago to prospect it. but, though gold was found in the stuff, they deserted the reef again and no work has been done on it since. Strong evidences of auriferous reefs are said to abound in many other places on the Arrow River and over the 12-Mile Hill, and only await the pick of the enterprising prospector for their development. Concluding with a general comparison between this Province and Victoria as to the facilities and chances of prospecting, I lnusfc say that this work — looking at it in a strictly mining point of view only — is here much more difficult than in Victoria ; for, whilst there, reefs consist mostly of massive white quartz, and are plainly exposed on the surface, they are here more frequently of a mullocky character, and more or less covered over by detrital matter. And there is besides one striking difference between the two countries, namely, that whilst in Victoria reef's abound all through the goldfields, but, as the saying g<-es, ninety-nine in a hundred prove barren, here they are comparatively very scarce, but with this redeeming feature, that nearly every one hitherto | found has proved to be auriferous, and therefore we may fairly assign the same chance to any new ones discovered in future. CRUSHING MACHINERY AND GOLD'SAYING APPLIANCES. The number of crushing machines erected and in course of erection at the different goldfields I visited, amounts to twenty-one, eighteen of which (including two near completion) are for quartzcrushing, whilst the remaining three (including two in course of erection) are for crushing the cement of the Blue Spur. Crushing Machinery. — Speaking of the crushing machinery proper, all these machines are, with but slight variations, constructed after the same model, viz., they consist of batteries of revolving stamps, four or five in a battery, working in cast-iron coffers on false bottoms, and hieing partly fed by hand, and partly supplied with self- feeding hoppers. As far as I could see, they are all well and substantially erected, the comparative 1 freedom from jar of those I found at I work proving their stability, and that due care had been bestowed upon the preparation of their foundations. At one or two machines the discs on the shanks were not gui c in order, and the wiperß itoo loDg, but the managers knew of these defects, and were going to repair them. On the, at one time iv Victoria, much discussed question, ! whether round stamps are equal in crushitfg power to square ones, I need only remark that careful experiments under equal conditions have proved that they are nearly, if not quite, as effective as the latter, whilst their wear and tear, in consequence of the turning, is in the average less. In the weight and lift of the stamps and the speed at which they were driven, there were considerable differences between the machines, the weight ranging from four to seven cwt., the lift from five to eight inches, and the speed from fiftysix, to eighty- five blows per minute.- As regards the most advantageous weight for stamps, opinions are still- divided in Victoria j still, heavy stamps are, on account of greater efficiency on the generally hard quartz, the most in use. In my- opinion, a medium of about 5|- to 6 CWt. would beat s^fc the character of the stuff of moat of the Otago reefs, though for such, consisting principally of .hard, more or less solid quartz— as, for instance, Logan's Reef, the Canada Reef, &c.~- stamps up to 8 cwt. would no doubt be preferable. ...■■• Touching the height. of drop and speed of the stamps, the "former Bhould not'be less than seven incheßy and might ady an-; tageoualy b> " increased fa n i ne inches,

especially if the stampß are light ; whilst, touching the speed, it is generally considered best at the rate of 75 to 80 blowß per minute. On the amount of stuff crushed during a certain time I could not obtain any definite information, but thefigures given seemed to me to be rather^ low, and in order, therefore to show what ought to be done under certain conditions, I may mention that at the Port Phillip Company, Olunes, stamps of 6 c wt, driven at a Bpeed of 75 blows per minute, and with a drop of eight to nine inches, crush in the average 2 tons 4 cwts. per 24 hours ; whilst others of 8 cwt., with the same fall; and speed as just given, are expected to reduce each up to 4 tons per diem. Regarding the iron coffers, they seemed to me rather, if not too shallow for botheconomic and effective working. They allowed hardly one inch of loose quartz to be put beneath the false bed plates, and it would, therefore, require the greatest care in the placing of these plates, the feeding, &c, to prevent the plates from working unevenly into the shallow gravel bed. In fact, I think it can hardly be avoided that ttfey (the plates) come frequently in contact with the iron bottom, the result of which, of course, is, as the sound of the blows already indicates, ineffective working and great wear and tear-^-sometimes even an unexpected breaking of the coffers. On this account I think it would certainly be advisable to have the latter, say, about two inches deeper, so as to allow a gravel bed of three inches beneath the false bottoms. Considering that the office of these latter is not only the Baying of the coffers from wear and tear, but mainly to prevent the gold from being smashed too fine or " beaten dead," as it is called, they should be only of the same size as the stamp heads, in order to leave sufficient space around them for the liberated gold particles to get into the gravel out of reach of the stampers. The provision of self-feeding hoppers — a great desideratum for Baying labor — has been neglected at a great number of the machines, and would deserve early I attention. Many practical quartz crushi era consider, and no doubt rightly, that hand feeding, if properly executed, is more effective j still it is extremely doubtful whether, especially in the case of small machines, the value of the increase in the quantity of stuff crushed is equal to that of a man's labor ; in my opinion, this labor is far more profitably applied to attending upon the gold-saving appliances. With only one or two exceptions, I found the great defect of the coffers having only front discharge ; for it must stand to reason that, as it is, or ought to be, the aim to get the finely crushed material quickly out of the boxes, large escapes both in front and at the back are the most effective. Of course, such a double discbarge arrangement necessitates a corresponding increase in the quantity of water to be supplied per stamp-head — a supply of from five to eight gallons per head per minute, regulated according to the weight of the heads and the nature of the crushing stuff, would, however, satisfy all requirements. The gauge of the gratings, varying at the different machines from 122 to 144 holes per square inch, is, I think, scarcely well adapted to the nature of the stuff treated ; for, as the gold is mostly very fine, the gratings should be very fiae also, in order to ensure the necessary degree of reduction for a satisfactory liberation of the gold particles. Gratings with 169 to 196 holes per square inch would certainly be safer. Gold Saving Appliances. — As regards the gold saving appliances in use, they consist, with the exception of two machines, which have deep drop-ripples attached, of amalgamated copperplate tables, in some instances with improvementa in the arrangements of the ripples, and all, except the cement crushing machines, have various lengths of blanketstrakes succeeding. During work, it is the regular cubtom of putting quicksilver into the stamper boxes. For the treatment of blanket sand, serve the common revolving barrel with shaking tables or ties attached, dolly tubn, small Berdan machines and simple ties, though the latter inferior appliances at a few machines only. Although generally well constructed and carefully attended to, as I found these appliances at the machines in work, and as they respectively are, and were said to be at those at a standstill, most of the managers I came in contact with were well aware that they lost a considerable percentage of the gold, and in some instances notable quantities of quicksilver besides. Considering this general losa, which, in some establishments was occasionally much increased through an insufficient Bupply of, and the use of muddy water, I attribute it mainly to three causes, viz., the U3e of amalgamated copperplates, too strong an inclination and insufficient length of the blanket strakes, and last, though not least, to the introduction of mercury into the stamper boxes. It would lead too far here to enter into a discussion upon the merits or otherwise of copperplate tables generally, suffice it to say that tables of the same pattern as those under notice were, at one time, in high favor in Victoria, but careful trials soon proved their inefficiency in many respects, and they have at all the principal crushing establishments been long ago discarded in favor of more perfect appliances which I shall mention further on. According to Kustel and other authorities on the subject, they have also suffered a similar fate long since in California — the country where they were first introduced. The putting of quicksilver into the boxes is no doubt a great improvement in case of copperplates being \we<\, but it is fundamentally detrimental in the crushing of stuff so highly charged with pyrites, as -most of the Otago reefs produce. For it is a well-known fact that pyrites generally, though certain fcinda

more than others (and these abound in the latter reefs), cause a flouring of the mercury and amalgam in the boxes, and |for the saving of such floured stuff no method has as yet been discovered. On i this account I would therefore strongly | advise to abandon the practice, even in case of copperplates being retained : for the loss both in quicksilver and. gold | caused by it alone is, perhaps, much [ larger than what would be sustained by less efficient working of .the plates through its disuse. I may at this place take the opportunity to remark, that I hold the practice of special harm with regard to the cement crushing machines at Blue Spur, on account of the abundant occurrence in the bottom portions of that cement of secondary pyrites, ie., such formed in the drift, a kind that though easy decomposition is very prone strongly to flour the mercury. Considering also that the generality of the gold crushed from the cement is dirty, i.e., more or less coated with oxide of iron or pyrites, and that, therefore, the copperplate tables (which I found only in use) have but a poor- chance of retaining it, I think the attachment of blankeistrakes, or perhaps better of a well-con-Btructed tail-race, would be found very profitable. Touching the blanket-strakes, they are, at most machines, from ten to fourteen feet in length, and their inclination is seldom less than one and a half inches, and reaches two inches per foot. Both figures, which, considering that the supply of water is mostly rather copious, are certainly not calculated to ensure a satisfactory saving of the fine gold and amalgam escaping from the copperplates ; irrespective of that, at some machines, the blankets are not, or have not been, washed frequently enough. Having herewith given my opinion on the point from whence at least the greater percentage of the loss in gold and mercury proceeds, I would recommend the exchange of the present appliances and system for, or respectively their modification according to, those for years successfully in use at the Port Phillip Company's works. Clunes — an establishment which in Victoria occupies the foremost place in satisfactory gold extraction, mainly because the practice there introduced of daily taking and assaying samples of the tailings serves not only for controlling and guiding the working of the appliances adopted, butinthecase of any new invention in gold saving being tried, it affords also the best proof of the merits or otherwise of the latter. The system of appliances used at dunes simply consists — starting from the battery — of three connected quicksilver troughs — the first with a 10 inch drop, the second with a, 9 inch, and the third, or lowest, with an 8 inch drop— through which the material passes in succession, to run next over 24 to 27 feet of blanket-strakes, laid at a pitch of only one foot in 16, and ultimately to pass from the blankets through another quicksilver trough before it reaches the waste channel. This last trough is only cleared, however, at intervals of several months, whilst the upper troughs are cleared every week. In order to keep any coarse stuff from entering these latter, and also for even distribution of the material, a perforated plate is fixed right in front of the battery, through which both back and front discharge pass on to an apron which leads it (the material) into the firat trough. An important part of each trough is the splash-board, which, reaching down to within about one and a half inches of the bottom (of the trough) near to the surface of the quicksilver compels the material, in its drop, to pass more or lesa through the latter before rising over the lip of the trough. All the troughs are supplied with tap-holes on one side, by meanß of which the quicksilver and amalgam can be drawn off when required. The whole system will be easily understood by reference to appended Plan 1., Fig. 1, which represents a longitudinal section, whilst Fig. 2 is a section of the troughs on a larger scale with the principal measurements marked. As to the blanket-strakes, their small inclination^ requires the supply of water to be up to eight gallons per stamp-head per minute, according to the nature of the stuff, in order to keep them free from sandy settlement. The rate at which the blankets are washed at Clunes is generally the upper row every hour, the second row every two hours, and of the remaining length of the strakeß the blankets of the upper half every six, those of the lower half every twelve hours Considering the nature of the stuff of the Otago reefs, I think, however, that partly because the more or less slimy stuff from the mullock reefs renders the surface of the blankets quickly inactive, partly on account of the great amount of pyrites contained in the quartz, a more frequent changing of the blankets than the above is advisable. Touching the treatment of the blanketsand, the method in use at most of the machines, viz., by revolving barrel and shaking table, gives, if properly carried I out, the most satisfactory results, and deserves, therefore, general adoption. | In the proper working of the barrel, upon which depends most, certain rules require to be followed, however, and as I bad no opportunity of judging of the mode of operation at any 'of the machines, I give the following particulars on this head for comparison and guidance. Assuming the barrel to be about 4 feet long by 2 feet in diameter, it ehould be charged with 8 to 10 cwt. of damp sand, and 2 to 3001 be. mercury, and set to revolve for about 8 hours at a speed of from 14 to 16. revolutions por minute. After this, it ehould be filled with hot water and set to revolve (again for another 4 hours at the rate of 5 to 6 revolutions per minute, when the operation is finished, and the charge —^ quicksilver first— may be d'awu off." Having been informed that at peveral of the machines th 3 practice is followed of putting round stones or pieces of iron into the barrel, in order to grind the sand finer and aid the amalgamation of the

gold, I have to remark that I consider this proceeding likely to prove more . harmful than advantageous, on account of the large quantity of pyrites generally present in the sand, which through the grinding is very liable to sicken or flour the mercury and amalgam, and this invariably is followed by a loss of mercury and gold afterwards. A determination of the exact loss of mercury in this and the main gold- saving process by carefully weighing the metal at short intervals, is not practised, as far as I could glean, .at any of the machines, but as it iB of the greatest importance in testing the comparative efficiency of the amalgamating appliances, it ought certainly not to be neglected in future. Having herewith noticed all the principal points touching the saving of gold from the crushed material, it remains to draw attention to the saving and treatment of the pyrites of but a few of the reefs, and trials on the large scale of that of only one reef (Southberg's Reef, Skipper's Creek) proving the payably auriferous character of the ore, still I think there can hardly be a doubt, judging from Victorian experience, that the pyrites of all the other auriferous reefs of the Province is more or less payably auriferous also, and it would be highly advisable, therefore, after the truth of this supposition has been established by fire assays*, to take early steps towards the concentration of and the extraction of the gold from the ore. Both these operations are not, however, very easy ones, but require, for profitable and satisfactory execution, much care, favorable natural conditions, and a considerable amount of capital in the construction of the necessary works. The saving operation has to be effected at each single crushing mill, and for it the so-cailed Borlase's huddle, with Munday's patent scrapers, is the most approved in Victoria, but where capital and favorable conditions are wanting lor the erection of these rather cumbrous machines, the use of a good length of blanket strakes -say 20 to 30 ft — and careful attention to, and more frequent washing |of the blankets than hitherto practised, would at least save the greater quantity of the pyrites. With regard to the after treatment of the latter for the extraction of the gold, roasting furnaces and certain kinds of amalgamators are required, the working of which has to be specially learned. In fact, it may be said to constitute a special industry which not every company or reefowner might care or be able to enter into. In Victoria nearly all the larger companies have their own pyrites- works, but there are also special private establishments of the kind, unconnected with crushing mills, at which miners or companies can either have their pyrites treated at a fixed rate per ton, or can sell it at a certain reduction on its gold-value, which latter is ascertained by careful metallurgical sample assay. A similar course must be left for private enterprise to follow in Otago. In view oi the fact that in the reefs of Victoria — and this as already stated, will most likely also happen in the reefs of Ofcago — the pyrites generally increases in quantity in depth, whilst the free gold correspondingly decreases, and that, moreover, the latter is the more diffcult to save, the more pyrites the stuff contains ; the pyrites question, aa it is called, referring to the best modes of concentration and after treatment of the ore, sanitary precautions connected with the roasting, etc., has there for years been one of increasing importance, and the Government appointed some time ago a commission to fully investigate the subject in all its bearings. The recently published report of this Commission contains full information, with drawings of machinery, etc., on the best methods of pyrites concentration, and gold extraction, and as, if I entered upon a description of the respective processes, I should merely have to recount what is stated in it, I append a copy of this report instead. Considering in conclusion the general working effect of the crushing machines throughout the goldfields of the Province, there are two serious natural disadvantages under which all suffer, though some in a stronger degree than others. I refer to the hard frost in winter and the general low temperature of the water throughout the greater part of the year. The former compels the actual stoppage of the works for several months (five to six months at Skipper's Creek), the latter affects the liveliness of the quicksilver, and thereby impairs, as it were, its amalgamating power. Against the first nothing can be done, and to ameliorate the second difficulty the introduction of hot water into the coffers can only conveniently be practised at those machines driven by eceatn engines. However, I think that,

whilst the introduction of long blanketstrakes would prove a great safeguard agaiLst any loss of gold caused by # imperfect amalgamation, the most rational way to meet the frost difficulty would be to have all parts of the machinery sound and in best working<order, so as to guard against interruptions in crushing day and night through the summer months, in order to work up as well the crushing material accumulated during winter stoppage, as alao'that concurrently produced. (To he Continued.)