METHODS OF MINING.
The more common form of alluvial gold is as grains, or scales, or dust, varying in size from that of ordinary gunpowder to a minuteness that is invisible to the naked eye. Sometimes indeed the particles are so small that they are known as 'paint' gold, forming a scarcely perceptible coating on fragments of rock. When the gold is very fine or in very thin scales, much of it is lost in the ordinary processes for treating gravels, by reason of the fact that it will actually float on water for a considerable distance.
From what has been already said it will be evident that gold-mining must be an industry presenting several distinct phases. These may be classed as alluvial mining, vein-mining, and the treatment of auriferous ores.
In alluvial mining natural agencies, such as frost, rain, &c., have, in the course of centuries, performed the arduous tasks of breaking up the matrix which held the gold, and washing away much of the valueless material, leaving the gold concentrated into a limited area by virtue of its great specific gravity. Hence it is never safe to assume that the portion of the veins remaining as such will yield anything like so great an equivalent of gold as the alluvials formed from the portion which has been disintegrated. As water has been the chief (but not the only) agent in distributing the gold and gravel constituting alluvial diggings or placers, the banks and beds of running streams in the neighbourhood of auriferous veins are likely spots for the prospector, who finds in the flowing water of the stream the means of separating the heavy grains of gold from the much lighter particles of rock, sand, and mud. Often the brook is made to yield the gold it transports by the simple expedient of placing in it obstacles which will arrest the gold without obstructing the lighter matters. Jason's golden fleece was probably a sheepskin which had been pegged down in the current of the Phasis till a quantity of gold grains had become entangled among the wool. To this day the same practice is followed with ox-hides in Brazil, and with sheepskins in Ladakh, Savoy, and Hungary. This may be deemed the simplest form of 'alluvial mining.' If the gold deposited in holes and behind bars in the bed of the stream is to be recovered, greater preparations are needed. Either the river-bed must be dredged by floating dredgers, worked by the stream or otherwise; or the gravel must be dug out for washing while the bed is left dry in hot weather; or the river must be diverted into another channel (natural or artificial) whilst its bed is being stripped. The first-named method is best adapted to large volumes of water, but probably is least productive of gold, passing over much that is buried in crevices in the solid bed-rock. The second plan is applicable only to small streams, and entails much labour. The third is most efficient, but very liable to serious interference by floods, which entail a heavy loss of plant.
In searching for placers it is necessary to bear in mind that the watercourses of the country have not always flowed in the channels they now occupy. During the long periods of geological time many and vast changes have taken place in the contour of the earth's surface. Hence it is not an uncommon circumstance to find beds of auriferous gravel occupying the summits of hills, which must, at the time the deposit was made, have represented the course of a stream. In the same way the remains of riverine accumulations are found forming 'terraces' or 'benches' on the flanks of hills. Lacustrine beds may similarly occur at altitudes far above the reach of any existing stream, having been the work of rivers long since passed away.
Another form of alluvial digging occurs in Western America and New Zealand, where the sea washes up auriferous sands. These are known as 'ocean placers' or 'beach diggings,' and are of minor importance.
Whilst most placers have been formed by flowing water, some owe their origin to the action of ice, and are really glacial moraines. Others are attributed to the effects of repeated frost and thaw in decomposing the rocks and causing rearrangement of the component parts. Yet another class of deposits is supposed to have been accumulated by an outpouring of volcanic mud. And, finally, experts declare that some of the rich banket beds of the Transvaal became auriferous by the infiltration of water containing a minute proportion of gold in solution.
In all cases the recovery of alluvial gold is in principle remarkably simple. It depends on the fact that the gold is about seven times as heavy, bulk for bulk, as the material forming the mass of the deposit. The medium for effecting the separation is water in motion. The apparatus in which it is applied may be a 'pan,' a 'cradle,' or a 'tom,' for operations on a very small scale, or a 'sluice,' which may be a paved ditch or a wooden 'flume' of great length, for large operations. The method is the same in all: flowing water removes the earthy matters, while obstructions of various kinds arrest the metal. As a rule, it is more advantageous to conduct the water to the material than to carry the material to water. In many cases a stream of water, conveyed by means of pipes, and acting under the influence of considerable pressure, is utilised for removing as well as washing the deposit. This method is known as 'piping' or 'hydraulicing' in America, where it has been chiefly developed, but is now forbidden in many localities, because the enormous masses of earth washed through the sluices have silted up rivers and harbours, and caused immense loss to the agricultural interest by burying the rich riverside lands under a deposit that will be sterile for many years to come. The plan permits of very economical working in large quantities, but is extremely wasteful of gold. The water-supply is of paramount importance, and has led to the construction of reservoirs and conduits, at very heavy cost, which in many places will have a permanent value long after gold-sluicing has ceased. These large water-supply works are often in the hands of distinct parties from the miners, the latter purchasing the water they use. To give an example of the results attained in alluvial mining, it may be mentioned that in a three-months' working in one Victorian district in 1888, over 33,500 tons of wash-dirt were treated for an average yield of 18½ grains of gold per ton, or, say, one part in 700,000. Where water cannot be obtained recourse is had to a fanning or winnowing process for separating the gold from the sand, which, however, is less efficacious.
Hydraulic Gold-mining.
Vein-mining for gold differs but little from working any other kind of metalliferous lode. When the vein-stuff has been raised it is reduced to a pulverulent condition, to liberate the gold from the gangue. In some cases roasting is first resorted to. This causes friability, and facilitates the subsequent comminution. When the gold is in a very fine state, too, it helps it to agglomerate. But if any pyrites are present the effect is most detrimental, the gold becoming coated with a film of sulphur or a glazing of iron oxide. The powdering of the vein-stuff is usually performed in stamp batteries, which consist of a number of falling hammers. While simple in principle, the apparatus is complicated in its working parts, and is probably destined to give way to the improved forms of crushing-rolls and centrifugal roller mills, which are less costly, simpler, more efficient, and do not flatten the gold particles so much. One of the most effective is that by Jordan. When the vein-stuff has been reduced to powder, it is akin to alluvial wash-dirt, and demands the same or similar contrivances for arresting the liberated gold and releasing the tailings—that is, mercury troughs, amalgamated plates, blanket strakes, &c.; but, in addition, provision is made for catching the other metalliferous constituents, such as pyrites, which almost always carry a valuable percentage of gold. These pyrites or 'sulphurets' are cleansed by concentration in various kinds of apparatus, all depending on the greater specific gravity of the portion sought to be saved.
Of the metals and minerals with which gold is found intimately associated in nature are the following: Antimony, arsenic, bismuth, cobalt, copper, iridium, iron, lead, manganese, nickel, osmium, palladium, platinum, selenium, silver, tellurium, tungsten, vanadium, and zinc, often as an alloy in the case of palladium, platinum, selenium, silver (always), and tellurium. The methods of separation vary with the nature of the ore and the conditions of the locality.