ON THE LOSING AND SATING OF FINE GOLD.

Otago Witness, Issue 1892, 8 May 1890, Page 13

ON THE LOSING AND SATING OF FINE GOLD.

By J. A. Miller,

THE SAVING OF' GOLD FROM ALLUVIAL DEPOSITS.

11. Whatever the origin of alluvial gold may have been, there can be no doubt but that it was distributed and deposited in the rivers of the present day, as well as in those of past geological ages, by the action of running water. If the gold-bearing, moraine-like deposits at the confluent of the head waters, and the gravel beds at the junction of branch streams, as also those near the embouchure of auriferous rivers into ancient lakes are exempted, the above theory must be accepted without further cavil. It therefore follows that all the fine gold contained ia these deposits was lodged where it is now found by it? own gravity, assisted by mechanical means, and that there is no evidence that either chemistry or electricity played a conspicuous or indeed any part whatever in its deposition. Here then we come upon

THE FIBST GREAT FACT to guide us in our investigation, meeting at tne same time the greatest difficulty very fine gold — that ip, " float gold " — gives to the gold-saver. Every method Nature adopted for the lodgment of this kind of gold can unfortunately not be imitated in an artificial manner where wholesale treatment of large quantities is required. It will be found that this "float gold" occurs wherever there was still water, such as tb!e moraine deposits at the heads of rivers, the gravel beds at the junction of branch stream's with main channels, near the mouths of rivers emptying into lakes of still water, or where there were eddies under the shelter of projecting rocks or other protecting causes, none of which it would be practicable to reproduce in a tail race, ground sluica, or any other artificial channel designed for the arrest of fine gold. However, the same purpose may be attained by other means, of which more in its proper place.

THE DEPOSITION OF GOLD IN BIVEBS indicates that the metal must have beeD native to its banks, and washed down to lower levels as the rivers deepened their beds. In proof of this it may be mentioned that had the gold been fed into the watercourses at their heads the heavier gold would be founJ in its upper portions, the lighter and finer gold lower down the stream, anil that the very fine gold had travelled the greatest distance ; and such is found to be the case in all artificial sluices and tail races. In river beds the case is very different.

For the sake of illustration as well as for argument the Shotover river may be quoted. Perhaps there is no better example extant anywhere, certainly not in Otago, than this river— in fact, as an example it Is perfect. The river has bad its glacial lake at the coi.flux of its head waters ; its whole course is one vast ground sluice, flanked by confining walls ; there is a& perfect a series of terrace formations up its banks as can be found anywhere ; numerous branch streams, which corroborate in every detail the geological history of the parent stream, Bbow at their junction with the main river the gravel beds already mentioned, notably at the left hand branch (geographically the " right " hand branch), Skippers, Pleasant and Stony creeks ; the river has broken its way through stupendous gorges, to end and empty itself near Arthur's Point into the ancestral Wakatinu Lake of a departed geological age, forming from its beginning to its end a unique and complete illustration of a mountain torrent.

A SUGGESTION may be submitted to the effect, in case Parliament decide that New Zealand be represented at the forthcoming Mining Exhibition at Londou, that the Shotover river with its tributaries be modelled aud exhibited. Such a model would be of immense and permanent value as au educational appliance in the Dunedin School of Mines for the aid of mining students, as showing views of comprehensive terrace formations, and illustrating the action of water, which latter item is of the utmost imnnrtance to engineers. But this is digressing from our argument. In the Shotover river, then, FINE AND FLOAT GOLD

occurs at all the points where the physical conditions of the valley point to its presence. The geueral character of the Shotovr-r gold is small granular pellets, not by any means difficult to save ; but while coarse nuggetty gold is found at the left hand branch, Skippers creek, and Londonderry terrace, there are also large deposits of "float gold" at the Amphitheatre, near the head of the river ; at Stony terrace, at about midway of its course; and at Tucker beach, near thra mouth ot' the river Now the most remarkable thing in this distribution of fine gold is that the finest of the fine is lodgpd near the head of the river ab tho so-called Amphitheatre So light is this gold that up to the present no means have been d< visrd to work the ground profitably, and to fully understand the import of t i< Hta^empn!; it mus*; be mlded that neither fall nor water are wanting to employ any of the mcaus at present in use for tho arrest of the fine aid " float gcW." In the treatment of this Idr.d of gold

ANOrHUK CONSIDERATION

obtrudes itself, it wai stated abovo that the still water of which Nature avails herself for the lodgment of " float gold " could not be prac-

ticably employed in a sluice ; but a secondary agent, which plays no mean part in the business, offers itself, and this agent is the introduction of a stream of clear water into the main sluice. Such a stream falling from a height of from one to two feet, according to its volume, and in proportion to that running in the sluice, will not only have the effect of aiding the settlement of fine gold by reducing the carrying power of muddy or otherwise dirty water, but will also be a means of breaking, or at least interrupting, the onward sweep of the current, thus in a mild degree creating still water.

The next observation will be the manner in which

NATUBE EMPLOYS THE EIPPLE for the saving of fine gold. The most obvious form of its application in Nature is the bar extending either quite or partially across the width of the current. It must be borne in mind that the ripple operates in a twofold way up a current, first by hindering and then by accelerating it— that is, breaking the current by reducing its speed, and next by increasing it. And this the bar in a river most effectually

does. Above the bar the fall in the river is reduced, consequently the ourrent slackens ; on the brink of the bar the current precipitates itself with increased speed, scooping out a pocket below the bar. Now every artificial ripple in a sluice, be it formed by a paving stone or by the unevenness of coconnut matting, acts in exactly the same manner upon the current down the sluice, and the same is also the case with the stones and gravel forming the bar in a river, the anevenness of which may be likened to that of the cocoanut matting.

This comparison of a natural river with an artificial sluice is, so far as it goes, analytically correct, but in copying it miners have overlooked one important fact, and that is that the bar breaks the whole current of the river, while they bave given to their sluices one uniform rate of fall from the top end to the bottom, and this presents to us

THE SECOND OHBAT FACT, which has been universally overlooked, in copying Nature's plan of saving fine gold. We have now done with running water and its action as a current. The next step will be to examine THE PHYSICAL CONDITION OP THE NATURAL CHANNEL

in which the current does its work. Miners, without exception, consider it of the utmost importance that their tail ra-es are perfectly level ie their cross sections, so that one side be not higher than the other, and that the bottom of the tail race or sluice box form sharp angles with the sides. When tail races are cut through the rock pick after pick is blunted, broken, or knocked up, gads are hammered by the hour, and dynamite is wasted to secure sharp angles and a perfectly level bottom, that is measuring from aide to side. Now what do we see in the natural channel ? I venture to say that none, be he miner or anything else, has ever seen a natural channel cut by a stream through material subject to its action in which the bottom formed a sharp angle with either of its sides. The fact is that sharp angles in a watercourse, if they do not form direct obstructions to the flow of the water, do so indirectly, and in 'no way increase the capacity of the race.

In tail races sharp angles are objectionable on account of their harbouring stones, &c, reducing the gold-catching surface of the race, and confining too much of the sweep of the current to narrowed channels down the sluice, thereby reducing the gold-saving efficiency very considerably, and thus we have

THE THIBD GREAT TACT — namely, that a tail race or ground sluice must resemble in its physical construction the natural channel of a watercourse, in order to catch and retain the fine gold Nature has shown us can be saved.

N.B.— What has been said above about sharp angles in watercourses applies as a matter of course to head races and all ditches constructed for [the purpose of conveying water. A head race with a rounded bottom and no angles at the sides at all will carry just as much water aa a race in which the angles are sharply out, provided that the main width and depth of the races be the same in both oases. Another advantage is that a race with a rounded bottom is not nearly so liable to be cut or scooped out by the current as one with sharp angles and a flat bottom. These facts are capable of scientific demonstration, which, however, will be superfluous, as every miner who has ever worked a race must know that, however sharp and fine the angles of his race may have been cub out, in a short time they were filled up with fine sediment, showing plainly that there could have been no current in the angles without any scientific demonstration.