280 



GOLD 



has been barren. Felspar in Colorado and felsite 

 magnesian slate in Newfoundland carry gold. 



The physical conditions under which gold occurs 

 are extremely variable. Popularly speaking, the 

 most familiar form is the 'nugget,' or shapeless 

 mass of appreciable size. These, however, con- 

 stitute in the aggregate but a small proportion of 

 the gold yielded by any field, and were much more 

 common in the early days of placer-mining in 

 California and Australia than they are now. The 

 largest ever found, the Welcome Nugget, discovered 

 in 1858 at Bakery Hill, Ballarat, weighed 2217 oz. 

 16 dwt., and sold for 10,500, whilst not a few 

 have exceeded 1000 ounces. The origin of these 

 large nuggets has been a subject for discussion. 

 Like all placer or alluvial gold, they have been 

 in part at least derived from the auriferous veins 

 traversing the rocks whose disintegration fur- 

 nished the material forming the gravel beds in 

 which the nuggets are found. But no mass of gold 

 has ever been discovered in a vein equal in size to 

 many of the nuggets unearthed from the gravels. 

 Hence has arisen a theory that in the course of 

 ages nuggets have ' grown ' in the gravels that 

 is to say, nodular fragments of gold have gradually 

 accumulated and attached to themselves smaller 

 fragments with which they came in contact, and 

 perhaps helped to cause the re-deposition of gold 

 held in suspension or solution by mineral waters 

 which have percolated through the superincumbent 

 mass of gravel. Gold nuggets have been artificially 

 formed in the laboratory by decomposing solutions 

 of the chforide or sulphide. In the earliest experi- 

 ments organic matter was added to effect the 

 decomposition e.g. a piece of wood ; but it has 

 been found that the presence of organic matter is 

 by no means necessary, and that fragments of 

 pyrites and other mineral bodies common in 

 auriferous formations are very suitable nuclei on 

 which the gold accumulates in a concretionary 

 state, resembling natural nuggets. 



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 coat- 

 ing 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 oh water for a 

 considerable ' d istance. 



Vein-gold is often crystalline in structure, the 

 elementary form being cubical. In some localities 

 too, notably in Hungary, it assumes most beautiful 

 leaf-like forms, such fetching a high price among 

 collectors for mineral cabinets. In the ores of 

 other metals, such as pyrites, galena, &c., gold 

 very commonly occurs as an accessory, but cannot 

 be detected except by assay. Whether, as in all 

 other cases, the gold exists in the native state in 

 such ores is open to some doubt. It is never found 

 absolutely pure ; some silver is always present as 

 an alloy, and occasionally also bismuth, lead, and 

 tellurium. 



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 eres. 



In alluvial mining natural agencies, such as frost, 

 rain, &c., have, in the course of centuries, per- 

 formed 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 prospec- 

 tor, 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 ma.de to yield the gold it 

 transports by the simple expedient of placing in it 

 obstacles which will arrest the gold without ob- 

 structing 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 other- 

 wise ; 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 uncom- 

 mon 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 ' ter- 

 races ' or ' benches ' on the flanks of hills. Lacus- 

 trine beds may similarly occur at altitudes far 

 above the reach of any existing stream, having been 

 the work of rivers long since passed away. 



So far, account has been taken only of gravels 

 lying practically within view. But in many in- 

 stances an enormously thick covering of more 

 recently distributed material, resulting from the 

 denudation of non-auriferous rocks, hides the earlier 

 gravel, which is auriferous. Such a phenomenon 

 was not suspected until the first instance of the 

 kind was discovered by some miners who, in follow- 

 ing a gravel patch formed by an existing water- 

 course, were led to burrow into the side of the 

 adjacent hill, under which the golden ground con- 

 tinued to be found, and then men realised that 

 the modern stream was only redistributing the 

 rich accumulation made by a river belonging to a 

 system that had ceased to exist. As prospecting 

 extended and became a subject for scientific study, 

 such instances rapidly multiplied, and to these ' deep 

 leads ' or ' dead rivers ' is due the bulk of the 

 placer gold found in Australasia and California. 

 Generally the watersheds in the extinct system run 

 at right angles to the present, so that operations 

 often extend under modern hill-ranges. A more 

 surprising discovery was that many of the ancient 

 river-beds had been filled up by flows of volcanic 

 rock, and in not a few cases several streams of 

 molten matter had at varying intervals displaced 



