GOLD 



3583 



GOLD 



particularly the first, where 

 have been changed by con- 



more 

 they 



tact with irruptive igneous rocks. 

 The fissures formed in these sedi- 

 mentary rocks by the eruptive 

 forces to which they were exposed 

 were subsequently filled by de- 

 posits of quartz which probably 

 carried the gold with it where 

 it is now found concentrated. 



Sometimes, however, gold is also 

 found diffused through the masses 

 of these rocks, both the sedimen- 

 tary and the igneous. Occasion- 

 ally it appears in granite, but the 

 great supplies of the world have 

 been derived immediately or re- 

 motely from the quartz veins of 

 the altered rocks, chiefly of the 

 Lower Silurian period, though it 

 has been pointed out that the 

 great Californian gold-bearing 

 deposits are as recent as the 

 Jurassic age. The gold-bearing 

 formations of the Witwatersrand 

 present peculiar ^features ; they 

 consist of beds of sandstone, quart- 

 zite, conglomerates, and frequently 

 shales. The gold occurs in the 

 beds of conglomerate, locally 

 described as " banket," which 

 consist of masses of waterworn 

 quartz pebbles cemented together 

 by quartz sand, clayey and talcose 

 matter, and oxide of iron ; but it 

 is found, not in the quartz pebbles, 

 but in the cementing material. 

 Sources of Supply 



The veins of gold-bearing quartz 

 in normal formations vary in 

 thickness from that of a sheet of 

 writing-paper to several feet, and 

 may extend for a few yards or for 

 many miles. Generally the thinner 

 veins are richer than the thicker 

 ones. The veins may be super- 

 ficial or descend to great depths. 

 Mines in S. Africa are now being 

 worked at nearly 2,000 ft. below 

 the surface, while the famous 

 Morro Velho mine of Brazil has 

 reached a depth of over 5,000 ft. 



The gold of commerce is to-day 

 obtained from three sources : (1) 

 alluvial deposits, (2) quartz rock, 

 and (3) telluride ores, the great 

 bulk from the first two, and mostly 

 from the second. The alluvial de- 

 posits are in the beds and banks 

 of existing or of ancient streams 

 or rivers or their estuaries, the 

 gold being found in sands and 

 gravels. These deposits are un- 

 doubtedly derived from quartz 

 rock formations which in the 

 remote past have been exposed 

 and broken up by atmospheric 

 agencies, by earth movements or 

 volcanic action, and their contents 

 carried by water or glaciers into 

 their present situations, which 

 may, however, be high above 

 existing rivers, and running in 

 quite different directions. These 



deposits may be superficial, but in 

 some regions they are of great 

 depths, e.g. in the " deep placer " 

 mines of California, where the de- 

 posits attain a depth or thickness 

 of 500 ft. 



Australian Gold Fields 



Found in nearly all parts of the 

 world, these deposits were the 

 sources whence primitive man 

 obtained his gold, and whence 

 came most of the precious metal 

 accumulated by the ancient civili- 

 zations. Many streams of the 

 north of Scotland and of Ireland 

 have furnished such deposits from 

 which appreciable amounts of gold 

 have been obtained in. the past. 

 Indeed, in streams and rivers all 

 over the world traces of gold may 

 at any time be found. The open- 

 ing up of the great gold-producing 

 regions of the world has nearly 

 always been begun by discoveries, 

 usually accidental, of rich deposits 

 of this class. Thus the gold in- 

 dustry of A-ustralia really dates 

 from 1851, when E. H. Hargreaves 

 announced his discovery of gold 

 at Summer Hill Creek and other 

 places near Bathurst, about 150 m. 

 from Sydney, New South Wales, 

 although the existence of gold in 

 Australia had been known for 

 some years. The gold presents 

 itself in these placer or alluvial 

 workings mostly in very fine 

 grains, " dust " practically, but 

 from time to time nuggets weigh- 

 ing from 8 oz. or 10 oz. upwards 

 arc found. The largest known 

 nugget ever found was the " Wel- 

 come Stranger," 21 ins. long and 

 10 ins. thick, and weighing 2,520 

 oz., the melted gold amounting to 

 2,268 oz. 10 dwt. 14 grs. 



The precious metal is usually 

 evenly distributed throughout con- 

 siderable masses of these alluvial 

 deposits, but occasionally is found 

 in remarkable concentrations. 

 From a few sq. ft. of such a deposit 

 a value exceeding 10,000 has been 

 recovered in many instances. 

 Gold is still extracted from such 

 placers by individuals working on 

 their own account by simple wash- 

 ing by means of the primitive 

 appliances of the old-time miner 

 the pan, the cradle, the batea, and 

 the torn (q.v.) but this is mostly 

 in remote regions or where Chinese 

 or other cheap labour can find a 

 sufficient return for its industry in 

 very poor deposits. 



The great bulk of the gold from 

 alluvial deposits is now obtained 

 by much more elaborate ma- 

 chinery, particularly dredging, 

 excavating, and hydraulicking. 

 Dredging is now practised on a 

 large scale on some of the rivers of 

 N. and S. America, Africa, Aus- 

 tralia, and the Far East. The ma- 



chines used are identical in all es- 

 sentials with the dredgers used in 

 harbours for removing or prevent- 

 ing accumulations of sand or mud 

 likely to incommode navigation. 

 (See Dredger.) The mouths of the 

 rivers and other parts where the 

 velocity of the water is reduced 

 are selected as the grounds most 

 likely to prove profitable, as the 

 gold brought down by the river 

 will settle at such parts. 



Where the deposits are not in the 

 beds or on the banks of existing 

 rivers, but in those of ancient 

 rivers, arrangements are some- 

 times made to bring water to the 

 site in sufficient quantity to float a 

 dredger and carry off its spoil. The 

 " pay-dirt " recovered by dredgers 

 must be subjected to treatment for 

 the extraction of the gold. . This 

 treatment begins with a rough and 

 ready concentration, which con- 

 sists in simply washing away some 

 of the worthless mud by streams of 

 water, followed by amalgamation, 

 chlorination, or cyaniding, or a 

 combination of the first and third 

 of these processes. 



Hydraulic Mining 



The most remarkable method of 

 recovering gold from the elevated 

 placer deposits is that of hydraulic 

 mining or hydraulicking. The 

 illustration shows the operation of 

 this system ; a is the hydraulic 

 main by which the water is brought 

 down from some elevated source, 

 which may be 500 ft. above the 

 site shown ; b is a distributing box 

 provided with valves by which the 

 water is served to c, c', c", which 

 are nozzles through which the 

 water is directed on to the rock. 



These nozzles are called " moni- 

 tors," and are constructed to 

 swivel through a certain arc so as 

 to command a considerable section 

 of the deposit face ; e, e', e," are 

 channels which carry off the water 

 with the material washed out 

 (more generally a tunnel takes the 

 place of these open channels) ; 

 /, /' is the sluice stream, having at 

 intervals drops, as at g, the object 

 of which is to break up boulders, 

 and at some point a grizzly, h, a 

 grill of iron bars so placed that 

 stones above a certain size cannot 

 pass it, but are rolled over the top 

 and discharged into a dump down 

 the side of the hill. 



At j is an undercurrent, the idea 

 of which is to take a certain 

 amount of the water in the sluice 

 above from a point below its sur- 

 face where it may be supposed 

 some proportion of gold is being 

 carried along, and to spread this 

 water out over a large area, thus 

 reducing its velocity and permit- 

 ting the rich dirt to settle before 

 the water again rejoins the main 



