DREDGING UPON THE KAWARAU AND SHOTOVER RIVERS.

Otago Witness, Issue 1935, 26 March 1891, Page 12

DREDGING UPON THE KAWARAU AND SHOTOVER RIVERS.

(By Our Special Reporter.) The Shotover has the advantage of running through country which is famed for its gold deposits It is the central depression in the most auriferous

portion of tbe Wakatipu goldflald, and has been proved to have contained by far the richest run of gold the field has ever known. Within its watershed is found gold in all the degrees of fineness, from nuggets downwards. The region of the coarse and nuggety gold extends from the left-hand branch (geographically the right-hand branch) dowu stream as far as Londonderry Terrace, where a nugget lloz in weight was found. Very fine float-gold occurs at the amphitheatre near the branches, at Stony Greek, and at Tucker Beach ia paying quantities; but the general run of the gold is what is familiarly described as

SHOTTY GOLD. like grains of wheat slightly flattened. This is the gold that weighs, and which is found In leads or runs, that count as the most permanent and dependable, that are not patchy, and do not give out suddenly without some apparent reason. The great portion of the gold found in tbe Shotover valley has been obtained from the terraces, aad for many years past river workings have been almost completely discarded. Taken on an average, the gold obtained from the river bed maybe said to have cost £6 an oz to produce, and with tbe exception of a very\few oases, in whioh the physical surroundings favoured its promotion, did not pay for the labour expended to get it, owing chiefly to floods and other difficulties in the way of working the gold, and not that the gold was not there to get. Dredging has been employed in times gone by in several parts of the river with varying suooeai, bub never became a favourito or a profitable means of winning the gold. These dredges, of course, were spoon dredges, raising, probably, not more than from six to ten tons of stuff per day. Though ia many respeots they may have been inferior to the machines of modern days, they worked regularly without much lost of time from breakages, baing constructed by practical men who, guided by their experience, knew how to meet tbe requirements expected of the machines they undertook to build, and who were consequently fairly successful in their endeavours. The efficiency of ttiese older machines contrasts very strikingly with some of the mechanical burlesques of modern times, for which it is said the soi disant scientific expert is accountable. In the history of

THE PRESENT DREDGING BOOM nothing has beeu more conspicuous than breakages and disasters of all kinds, keeping the patient shareholders constantly in hot water and in dreadful expec tatio aof " What next ? " They have had all the excitement that mining ventures give rise to, with the exception of that which flows from dividends. That is still to come.

However, amongst a number of abortions there have been some machines^ which show that dredges capable of doing the work required of them can be produced in Otago, and such structures as the Frankton Beach, Horseihoe Bend, and Sew Hoy Company's dredges prove that the difficulties, great as they may be, are to be overoome by properly proportioned and constructed dredges. With such appliances the Shotover river should offar an exceptionally profitable field, for as the gold is easily saved, because of its heavy nature, the machines may be worked up to their full capaoity, the rule that the more stuff put through the greater the result in gold holding good uuder suoh conditions, whioh is nod the case iv places where the gold is five. Indeed, it is |the very reverse, and frequently the more stuff treated the less the quantity of gold saved. So far as tbiDgs have gone, however, no very tangible

results have become apparent, which is no doubt owing to the difficulties {mentioned in previous articles- -namely, the lilting upol the river bed from various causes, such as land slips damming back the river, and tailings paid into it from the terrace workings, both of whioh add to the cost of winning the gold. At present the attention of those interested in dredging is chiefly directed towards the improvement and extension of the

APPARATUS FOB SAVING THB GOLD,

Unfortunately these efforts, almost without exception, run upon old lines worn threadbare yean ago, and have degeuerated into hair-aplittiag trivialities. So long as gold is ' ' washed " it is clear that It must be separated from the gravel with whioh it is mixed by virtue of its specific gravity. It is only of late years that the proceis of sifting has been adopted in the treatment of washdirfc in large quantities, |by " undercurrents," as is done in sluicing operations. By " sifting "—that is, separating the coarse material from the finer and the gold and then treating the latter two for the gold— two purposes are served. One is to reduce tbe bulk of the whole aud the other to simplify tbe manipulation of the remainder. The cradle and the " long Tom " are familiar illustration* of the sifting of stuff, and both are excellent in their way. The principle has been adopted in slu'oing works, in the shape of what is known as the " undercurrent." and it is not too much to say that some claims are now worked with very satisfactory results, owing chiefly to the application of the " undercurrent," and which without It wouldnotpay to work.

From this it is clear that sifting the stuff aids in the saving of fine gold, and I think it is safe to say that the sifting should be carried correspondingly farther at the fineness of the gold increases. Next to a clear lively catch for the gold, uniformity in the flow of the water which washes the gold is of the greatest importance. The value of this uniformity of flow is well known to managers of quartz crushing batteries where even fi aer gold than that saved by the dredges is arrested upon the blanket strakes. independent of the quicksilver. If pieces of stone the size of a pea were allowed to ruu down the blanket strakes the uniformity of tbe flow would be disturbed, with the consequence that tbe lighter particles of the gold would ba carried off and lost unless other means were adopted for its arrest after they leave the blanket strakes.

Arguing from this, it may be accepted that tbe process of sifting the stuff is not carried far enough upon the dredges, whioh have to deal with very fine gold, and nothing would be easier or less expensive than to provide a finer screen underneath the present one now in use. A scteen made of steel bars running longitudinally, say Jin in thickneis, and Jin, or even less, apart, and one-third tbe circumference of the revolving screen, would do the business. It could be made to move sideways, like a shaking table, and to discharge the small atones Into the same stone shoot which carries off the discharge from the revolving screen. The same water would answer for both, and the total coit of the dredge tins would be trifling, while it is clear that a greater quantity of stuff could be treated in a given time, and that more gold would be saved from it than is the oase at present.' These are the tangible results which would ba received by shareholders with the greatest appreciation, while to the dredgemaster the proposed innovation would simplify matters greatly, and bring the treatment of the fine stuff much more under his control than can be expected under the existing state of affairs.

The arrangement of the tables and the boxes as adopted by the Frankton Beach dredge 'is generally to be commended as most efficient, but it might in one or two particulars be improved upon with very good results. As for instance, the stuff a« it leaves the roof-shaped tables may be made to pass over perforated plates, with a view of further sifting it, aud also to more evenly pay it into the longitudinal boxes, in which the principle of the undercurrent could be extended with much advantage. As>lready stated,

THE CAPABILITIES OF DBEDGISQ are of a promising and hopeful nature, aud no stone should be left unturned to make the enterprise a success. Wo other class of gold, mining processes are likely to be so permanent, and therefore so profitable, as dredging, ia the success of which the goldsaving apparatus plays the first and principal part, and this unfortunately 1b the most difficult to deal with. After a few more abortive dredges not half strong and powerful enough to do the work required of them have been built, the discovery will probably become general that it does not pay to go to work with a cheap thing simply because it costs Icbs than a really efficient machine. And so much -will ba clear gain. But with the gold-saving apparatus it is different. No one plant can be expected to answer every case. In every instance there must be modifications to adapt it to the peouliar nature of the gold and the wash it Is mixed with. This requires experience to determine, and experiments to demomtrate. But tbe case is not hopeless, nor beset with any very great difficulties, nor does it require a heavy outlay. Experience gained in the battery treatment it unfortunately of little use here, for the reason that tbe difficulties presenting themselves are not analogous. The chief difficulty of the battery— viz., the admixture of foreign minerals, does not come at all into play in dredging, so that the dredgemaster must fight out his own battle upon his own ground. There are

THBEE OTHER DRAWBACKS to the success of dredging. There are the cost of steam, the manner of and the method of emptying the buckets lifting the stuff. The cost of steam Is estimated at from £20 to £30 per week This is an item of very grave consideration, and efforts to improve upon it have been made by the Upper Shotover dredges, whioh have preferred eleotrioity to steam, which latter in their oase could hardly be thought of. Bffeotivo electrical power, it is calculated, could be supplied to the Kawarau dredges at a reduotion of from 200 to 300 per cent., which would be saving what would amount to a handsome dividend at the end of a year, so that it should commend itself to those dredging companies affected by it. The drawback on account of tho cost of steam is one of calculation only, and will no doubt be removed when tbe question comes to be praotioally considered ; but that on account of

IitPIING THE STUFF is like the gold-saving apparatus, one of great nicety and beset with many technicalities. For these reasons I would adviae those dredgemaaters who jumped up In a night, and the scientific experts who are never done jumping up, not to read what follows. Only practical men will ba able to understand what there is iv tbe lifting of the stuff. Much has been said, and very muoh to the point too, about the dredges lifting the gold at one end to pay it back iuto the river at the other. True as this fs it is not nearly the whole truth, for even a graver defect militates against the success of the dredge*

upon the Kawarau and Shotover riven, and that Is, that a great portion of the heavier and coarser gold Is never lilted at all t Anyone who has tried to lift small— not to say flue— gold under water with a sbovel out of a heap or out of the solid without' having a proper bottom to work upon will know that it is next to impossible to take up all the gold that properly belongs to every shovelful of stuff removed ; and in a dredge the buckets are doing the work of a shovel on a larger scale, ihe loss is owing to the fact that so soon as toe sbovel eaters the stuff it moves and disturbs more (travel than it can lift, and that through all the stuff so moved the gold Bettles to below the level at which the shovel lifts At, so that there is a loss In gold for every shovelful Taken up. With the buckets of a dredging maohine this loss, or rather waste, is even greater than in the case of the shovel, and it is safe to say that this circumstance alone aocounts more than any other for the indifferent suocess met with. Wherever dredging operations have been fairly successful there always existed what is known as a " false bottom," composed generally of a dense homogenous clay that allowed the gold washed down upon it to be scraped up clean, leaving little behind, though even under these conditions there is always a considerable waste. Now, unfortunately there is very little of this bottom in the Shotover river, or in] that part of the Kawarau which' is at present the favourite hunting ground for our dredges. It seems to be all wash, and the waste from the cause described is considerable ; certainly more in weight than what is lost over the tail of the boxes. I believe some of the dredgemasters try to lessen this waste by "cleaning up the bottoms" after a piece of ground has been stripped, which ii about the only means that can be adopted, but Is at best only a modification of the wasteful process. EMPLOYING THB BUCKETS. Another waste is going on in the emptying of the buckets. Any observant miner will have noticed in shovelling gravel or other material, whether dry or not, containing fine or small gold, that the droppings, especially that portion falling off the shovel after it has been emptied, or in etnpting it, are much richer in gold than the average of the material. The explanation of this is obvious, especially in wet stuff ; as in moving the stuff the gold, as the heaviest particles in it, sinks to the lowest possible level, which in our present case is the shovel blade or the outer wall of the dredge bucket. Again, aB tbe heaviest particles of the stuff move, all the specs of gold leave the shovel or the dredge bucket last ; and once more, as the heaviest particles, they drop at a shorter periphery or arc from the shovel and dredge bucket mouths than tbe much lighter gravel and sand. Then, considering that tbe specs of gold bo dropping and wasted are amongst the heaviest in the material treated, the loss from this source is alto well worthy of study and consideration, and more especially so in dredging operations, where all tbe material lifted is wet. It is not too muoh to say that in nearly all dredges the gold lost by the manner of emptying the buckets is sufficient in quantity to make the difference-be-tween paying and non-paying operations. If any dredgemaster is not satisfied with this statement it is an easy matter for him to submit what has been said to practical test, and save the droppings from the buckets, trying them separately. - What has be m said above la neither scientific nor learned, and does therefore concern neither the SJientifio expert nor the learned gold-saver; It is simple practical experience, which every miner of ordinary observation can corroborate, but it is submitted to all who are concerned in making dredging

pay. Speaking generally, the two sources of loss of gold touched upon - namely, the waste of heavy gold in raising the stuff, and the loss of heavy gold in the emptying of the buckets— do not as a rule receive the attention of dredgemasters to which their importance entitles them. There is also some reason to fear that experts when prospecting the Kawarau lost sight of these items, and through not allowing for them are at a loss to aocount for the discrepancy between the prospects they obtained and the actual result of dredging operation!. It is clear their arithmetic was not at fault, and it is therefore also clear that something else mint have gone wrong. When a mere working miner prospects ground and obtains gold enough to make him think seriomly of going to the necessary expense of profiting by It, he makes allowances for the lose of gold, especially when it is fine, and be does not count on saving the same quantity of the same kind of gold in his sluice as he did in tbe trial prospects ; but tho scientific export is above adopting such precautions. Besides, caloulat ing from full prospeots leads to better results— in the calculation ; and that is all that Is required by tbe promoter, while the shareholder pays for the mild imposition without knowing anything about the bland and childlike tricks with whioh the prospectuses are built up. The frequant references to the "scientific" expert, and occasional allusion to " sciance " asapp'ied to tbe manipulation of mining companies in the ahare market made iv this and preceding articles, which herewith come to a close, are not aimed at legitimate science, but at that bogus article that begin* with mistakes and ends with explanations— whioh promises everything and fulfils nothing.