140 Meclianical Assay of Quartz. 



■occasions on v»'hicli the indications of the assa}^ are most 

 required, namely : — 



1st. When we have found a quartz reef, and before we 

 have commenced to work it. 



And 2nd. After the extraction in the mill, when we 

 especially require to assay the quartz tailings, so as to find 

 out how much gold we have left in them. 



Let me suggest that there is one particular in which gold 

 extraction differs from the treatment of the ores of most 

 other metals ; namely, in the small proportion which the 

 gold bears to the matrix. An iron ore may contain thirty, 

 or even seventy two per cent, of metallic iron, while pay 

 able quartz may hold no more than a few pennyweights 

 of gold to the ton. If 5 dwts. per ton is the contents, 

 the proportion in this instance is 5 times 24, equal to 

 120 grains, in the 15 millions GSO thousand grains con- 

 stituting the ton avoirdupois ; about one part of gold in 

 130,000 of the ore. 



The gold, it is true, is in the metallic state, in isolated 

 particles distributed through the stone ; and the distinctive 

 physical properties of quartz and gold are such as to be, on 

 the whole, favourable even for the separation of so small a 

 proportion of metal from so large a relative proportion of 

 matrix. But oftentimes other minerals, arsenical iron, iron 

 and copper pyrites, and many other species are present in 

 the quartz, and it is a fact that from these natural admixtures 

 and from other causes, the whole of the gold is not extracted 

 on the large scale. A proportion only is obtained from the ore, 

 ^nd a proportion is left in the tailings ; indeed, the technical 

 problem is rather how to obtain the largest profit in working a 

 given sample of quartz, than how to extract, at all cost, the 

 whole of its gold. If the last pennyweight which passes out 

 of the mill could be extracted at an additional cost of four 

 shillings, we do better to abandon it, instead of bearing a 

 loss which would be incidental to its recovery. The 

 original gold contents of the ore- expresses rather the 

 extreme possibilities of its yield, than the return which we 

 ought to expect from its treatment, but, nevertheless, there 

 is no more valuable guide to the quartz- mill owner, or miner, 

 than the assay, especially the assay of the tailings, collected 

 as they pass out of the mill. 



In the assay of a sample of quartz, all the gold which it 

 contains can be separated, because there is not the same 

 limitation of expense as that obtaining on the large scale ; 



