1098 A TREATISE ON METAMORPHISM. 



Bearing in the same direction are facts given by Kemp: "From Mr. 

 Bancroft the writer learns that the actual ore at Kalg'oorlie consists of 

 calaverite and native gold. In a specimen kindly given the writer there is 

 a yellow telluride and a silvery one. In the midst of the latter a bit of wire 

 gold is embedded. There is no sign of alteration, and the native metal and 

 the telluride must have crystallized together."" 



The very general occurrence of tellurides in gold deposits leads me to 

 the belief that the precipitation of gold by tellurides is probably an impor- 

 tant reaction in the first concentration by ascending waters. The work of 

 Hall and Lenher shows' conclusively that where gold-bearing- solutions 

 enter deposits containing tellurides, whatever their origin, the gold will be 

 rapidly precipitated by the tellurides. In this connection the manner in 

 which the tellurium travels and is precipitated is important, and is discussed 

 with the tellurides. (See pp. 1119-1125.) 



Hall and Lenher 6 have shown that silver selenide also reduces gold to 

 the metallic form almost as rapidly as selenium does. The reaction takes 

 place only slightly in the cold, but readily on warming. Whether or not 

 this has any importance with reference to ore deposits is uncertain. 



In many cases the precipitation of gold is not produced by a single 

 one of the causes given, but by some combination of them. For instance, it 

 has already been pointed out that the abundance of ons solutions and dimin- 

 ishing temperature work together. Ag-ain, the reducing action of organic 

 material and that of base sulphides or tellurides, or both, may work together. 

 In still other cases all of the above favorable conditions may occur simulta- 

 neously. The combination of organic matter with sulphides or tellurides is 

 believed to be very common. 



Such a combination is probably illustrated by the deposits of New 

 South Wales, in which the gold is associated both with organic matter and 

 with pyrite. In this case the organic matter was probably the chief agent 

 in precipitating the base sulphides from the sulphates. In consequence of 

 this reaction there are two compounds present capable of precipitating the 

 gold, both the pyrite and the carbonaceous material. Since the work of 

 Liversidge shows that the sulphides precipitate gold more readily than 



a Kemp, J. F., Geological occurrence and associates of the telluride gold ores: Mineral Industry, 

 1898, p. 319. 



6 Hall and Lenher, Action of tellurium and selenium on gold and silver salts: Jour. Am. Chem. 

 Soc, vol. 24, 1902, p. 927. 



