1122 A TREATISE ON METAMORPHISM. 



These facts strongly favor the precipitation of the gold as a first concentrate 

 simultaneously in the sulphides and the tellurides. It is believed (see pp. 

 1172-1174) that the abundant native gold at the surface and in the tellu- 

 rides at intermediate zones to the exclusion of the sulphides is due to 

 secondary reaction by descending water. 



It may be that the key to the problem of the deposition of the 

 tellurides lies in their association with sulphides. We have already seen 

 that telluric salts of the type of TeCl 4 may travel with gold in solutions. 

 It has already been pointed out that gold, in most cases, probably also 

 travels as a chloride, and thus solutions of auric chloride and telluric 

 chloride may together enter trunk channels which contain sulphides. In 

 such trunk channels the reaction of the sulphides might reduce both the 

 gold and the tellurium simultaneously and thus produce tellurides of gold; 

 or, by the reaction of the sulphides upon the telluric salts, these may be 

 reduced to tellurous salts, which, as already explained, would decompose 

 into metallic tellurium and telluric salts, and the tellurium would precipitate 

 the gold. Under these or some other conditions, the gold and tellurium go 

 down together, with a definite composition, and thus form the tellurides. 



The invariable association of silver with the tellurides of gold 

 suggests the possibility that the presence of silver salts is essential for the 

 formation of the tellurides of gold. But in some cases the silver is very 

 low in the tellurides, although rarely as low as 0.8, as in the case of one 

 specimen of calaverite from Kalgoorlie," which makes it improbable that the 

 presence of silver salts is essential for the precipitation of the tellurides. 



The association of the tellurides with fluorite has already been men- 

 tioned, and this association suggests a genetic relation between the two; 

 but bearing in the opposite direction are the observations made by Penrose 

 at Cripple Creek, where the abundance of fluorite has no definite relation 

 to the abundance of tellurides. 6 Possibly pointing in the same direc- 

 tion is the experimental work of Lenher, who has shown that gold and 

 gold oxide are absolutely insoluble in hydrofluoric acid. Not only is this 

 the case, but when any gold salt is brought into contact with a soluble 

 fluoride the gold is immediately precipitated as a hydrate. This is true 



"Bickard, T. A., Telluride ores of Cripple Creek and Kalgoorlie: Trans. Am. Inst. Min. Eng., 

 vol. 30, 1901, p. 711. 



6 Penrose, cit., pp. 158-159. 



" Lenher, Victor, Fluoride of gold: Jour. Am. Chem. Soc, vol. 25, 1903, pp. 1136-1138. 



