GOLD MINING

Manawatu Standard, Volume LII, Issue 3, 3 December 1931, Page 2

GOLD MINING

WHERE METAL IS OBTAINED, j In view of the continued rise in the j price of gold (it is now quoted at £6 j <i> 7d per oz) a brief description of i the sources of suppiy of this noble metal maybe of interest. Gold is obtained almost entirely from two sources, i alluvial gravels or placers, and reefs j or lodes. , ■ i ‘ The alluvial gravels have been laid down in benches or terraces by the action of glaciers or rivers in the prehistoric past, and in them the goal is found in layers, usually at the bottom on a bedding of rock or marine formation. These layers are known as wash or paystreaks. It is generally believed that this gold, which is metallic, and generally comparatively pure, has been transported from lodes in the mountains of which in many cases there is now no trace left. Wo all know that gold has a high specific gravity, approximately 19, whilst most kinds of rock from which gravels are formed run about two, and this difference in the specific gravities of the two is the chief contributing factor in the segregation of the gold in the paystreaks. This fact is also made use of in the washing of the gold from the gravels of the paystreaks. The methods used in freeing the gold from the associated paystreak gravels vary according to circumstances. Where the paystreak is capped by. a deep deposit of unprofitable gravels or overburden, as it is known, and the topography permits the method known as driving out is largely resorted to. If the terrain is fiat, as at Ross, recourse must be had to the sinking of a shaft. If, however, the paystreak can be reached by a tunnel, this is always preferred. The tunnel is usually continued to the limit of the paystreak most distant from the portal. Here active mining operations are commenced on a plan very similar to that used in the coal mines (the room and pillar system). These mining operations consist of extracting the gravels of the paystreak from the rooms and later from the pillars, and removing them to the tunnel portal. If no fills is available to support the roof it is allowed to cave in. In the case where a shaft is used the same method is adopted, the ground nearest to the shaft being the last to be removed. When the gravel reaches the portal it is washed over ripples where the gold is saved. The gold on account of its high specific gravity settles in the ripples, and the gravel passes on to the tailings dump. In the Yukon and Alaska much of this driving out. is done in the winter partly because the ground, being frozen, is very easy and safe to work, and partly because of the scarcity of water for washing the gravels and the difficulty of actual washing in the severe cold when subzero temperatures are the rule rather than the exception. With the advent of summer the dump is washed, and its gold content saved. When the gravels containing the gold are shallow, and on the surface, the method of ground sluicing was largely practised in New Zealand. In this method a small ditch or series of ditches contured the adjacent slopes to conserve the stormwater which was then conveyed by iron pipes or even canvas hose to the shallow workings. Here again the wash was passed over ripples to trap the gold and discard the gravel. With the advent of hydraulic sluicing a tremendous advance was made. In this method a considerable volume of water was brought to the face of gravel to be opened up under considerable pressure. In-cases the hydrostatic head was sometimes as much as 600 feet. The water emerges from the penstock or pipe line through a giant nozzle fitted with two universal joints, and the powerful jet of water can then he directed against any part of “the working face, and by its erosive action soon tears away the gold bearing gravels and their overburden. The whole stream of sludge, water, gravel and gold is then carried away in a tailrace. This is a box-like structure, in some cases eventually reaching a length of Half a mile, and is usually paved with blocks which are set about half an inch apart. This tailrace has a grizzly of iron bars at its intake end to carry over the coarse stones. Immediately under the grizzly through which pass the finer wash and gold is a set of ripples and often sheets of cocoanut matting or similar material to trap the gold. As a matter of fact, in all cases the bulk of the gold is saved iu the first 12 feet of boxes of the tail race. Under certain circnmstances a modification of this method is used, and is known as a blow-up or elevating plant. In all sluicing operations a fairly considerable fall is required for the tailrace, and where this fall is not available the blow-up is used to elevate the tailings to a height where sufficient fall for the tailrace is available. The chief point of interest in this method is the ingenuity of the elevating device. In the claim itself all operations are carried on as in an ordinary hydraulic sluicing claim, except that only a very short tailrace ending in a sump is used. Into this sump is set the bottom of the elevating pipe which is open. This pipe, the size of which depends on the quantity of wash to be raised, lias led into it a smaller pipe ending in a nozzle directed upward, and through this smaller pipe is discharged a jet of water under considerable pressure. This discharge tends to create a vacuum in the larger elevating pipe through which the sluge from the sump is now rapidly elevated to an upper tailrace. Hi this method, which was used extensively, at Barrytown, near Grevmouth, the gold saving is done, of course, in the lower tail race.

All along the West Cqast of New Zealand from the Mohikinui River to Jackson’s Bay the beaches at one time supported large numbers of miners engaged in winning gold from the blacksands of the beach-leads. These sands are cast up by the ocean, and consist largely of magnetite, ail ore of iron. The gold associated with them is usually very fine grained, and most of these sands were washed by some primitive hand device, such as a rocker or cradle. But even by these laborious methods immense quantities of gold were won. Okarito and Gillespie’s Beaches in South Westland were world famous. At Addisons, near Westport, these old beach sands are covered by a later overburden of unprofitable gravels, and shaft sinking and driving out have been resorted to.