GOLD 



the river, whicli aftei \\anU rcsii I its course and 



it, li.il'its, HO tlmt tli- extraordinary feature i 

 encountered <>i -e\.-ral superposed l>eds of aurii'-i 

 ous gravel alternating with layers of lava. 



Another form of alluvial digging occurs in 

 W'->tern America and New /calami, where the sea 

 \\a-lies up auriferoiiH sands. These are known as 

 ocean placers' or 'beach diggings,' and are of 

 minor importance. 



Whilst iini-t placers have been formed by Mowing 

 water, some owe their origin to the action of ice, 

 aii' I are really glacial moraines. Others are attrib- 

 uted to the effects of repeated frost and thaw in 

 decomposing the rocks and causing rearrangement 

 uf the rompiment parts. Yet another class of 

 deposits is supposed to have been accumulated 

 by an outpouring of volcanic mud. And, finally, 

 expert* declare that some of the rich banket beds 

 of the Transviial became auriferous by the infiltra- 

 tion 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 'torn,' for opera- 

 tions 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 itn/tfu.f u/icniiiil/ is the same 

 in all : flowing water removes 

 the earthy matters, while obstruc- 

 tions 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 con- 

 siderable pressure, is utilised for 

 removing as well as washing the 

 deposit. This method is known 

 a< ' piping' or ' hydraulicing ' in 

 America, where it has been ehietty 

 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 ]>ermits of 

 very economical working in large quantities, but 

 i- extremely wasteful of gold. The water-supply 

 is of paramount importance, and has led to the 

 construction of reservoirs arid conduits, at very 

 heavy cost, which in many places will have a per- 

 manent .value long after gold-sluicing has ceased. 

 These large water-supply works are often in the 

 hands of distinct parties from the miners, tin- 

 latter purchasing the water they use. To give 

 an example of the results attained in alluvial 

 mining, it may be mentioned that in a tliree- 

 months' working in one Victorian district in 1888 

 over 33,500 tons of wash-dirt were treated for an 

 average yield of 18J grains of gold per ton, or -;i\ , 

 one part in 700,000. Where water cannot }>e ob- 

 tained recourse is had to a fanning or winnowing 

 process for separating the gold from the sand, which, 

 however, is less efficacious. 



Vein-mining for gold differs but little from work- 



ing any other kind of im-talliferouM lode. When the 

 vein stulf ha> been mixed it i* reduced to a pulveru- 

 lent condition, toliU-rate the gold from the gangue. 

 In some CHS.., masting U first resorted tot. Thin 

 'au-'-s frialiility, and facilitate** the Hiihttequent 

 comminution. When the gold in in a very fine 

 state, txx, it helpM it to agglomerate. Hut if any 

 pviiies is present the effect in most detrimental, 

 I lie gold iM'coming coated with a film of sulphur or 

 a gla/ing of iron oxide. The powdering of the vein- 

 stutl is usually )>erformcd in stamp balterie-. which 

 consist of a number of falling hammer*. While 

 simple in principle, the apparatus is complicated in 

 its \\orking parts, and is probably destined to give 

 \\.-iy to ilie improved forms of crushing-rolls and 

 centrifugal roller mills, which are less costlv, 

 simpler, more efficient, and do not llatten the gold 

 particles so much. One of the most effective is 

 that by Jordan. Whrn 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 tail- 

 ings i.e. mercury troughs, amalgamated plates, 

 blanket strakes, &c. ; but, in addition, provision is 



Hydraulic Mining, Devil's Creek, Reefton, New Zealand. 



made for catching the other metalliferous constitu- 

 ents, such as pyrites, which almost always carry 

 a valuable percentage of gold. These pyrites or 

 'snlphurets arc cleansed b\ concentration in vari- 

 ous kinds of apparatus, all depending on the greater 

 specific gravity of the portion sought to le saved. 



Of the metals and minerals with which gold is 

 found intimately associated in nature are tne fol- 

 lowing . antimony, arsenic, bismuth, cobalt, copper, 

 iridium, iron, lead, manganese, nickel, osmium, 

 palladium, platinum, selenium, silver, tellurium, 

 tungsten, vanadium, and /inc. often as an allov in 

 the case of palladium, platinum, selenium, silver 

 (always ), aim tellurium. The met boils of separation 

 vary with the nature of the ore and the conditions 

 of the locality. In the case of sulphides of some 

 of the base metals the sulphur can be oxidised by 

 burning in suit aide kilns, so as to afford sulphurous 

 or sulphuric acid, leaving the gold and other metals 

 in the 'cinders,' whence they can be recovered by 

 solution. Where the base metal is volatile it may 

 be obtained by condensing the fume-. To get rid of 

 the sulphur and arsenic in the ore (with or without 



