94 T ran motions. 



(c.) By flepositioii of valuable metals on primary sulphides- 

 in those portions of the vein subject to the influence 

 of circulating surface waters. In this case the pri- 

 mary sulphides may form the nuclei for the deposit 

 of the secondary sulphides. 



Manifestly the first operation in the process of secondary 

 enrichment is the chemical weathering and oxidation of the 

 metallic contents of the vein. 



The oxidation of base sulphides can be seen in operation 

 every day. In a mass of mixed sulphides of iron, copper, zinc, 

 and galena, the iron will be the first to be affected, from its affi- 

 nity for oxygen. Iron-pyrites is decomposed and forms ferrous 

 sulphate, which is changed intoFe(OH)3, Fe2(S04)3, and H2SO4. 

 The H2SO4 attacks fresh iron-sulphide and forms ferrous sul- 

 phate, liberating HjS, which at once combines with free oxygen 

 to form H2SO4. The ferrous sulphate changes to the ferric 

 sulphate, which attacks gold and sulphides of copper, lead, zinc, 

 and silver. The process of dissolution is necessarily slow, on 

 account of the extreme dilution of the solutions. 



For many years it was believed that the only secondary en- 

 richment that could take place was the formation of rich bon- 

 anzas of carbonate ores and chloride of silver, in the zone above 

 water-level. But careful investigation has shown that primary 

 sulphides have been enriched by the deposition of secondary 

 sulphides even in places below the present water-level. 



It was proved experimentally by Skey* in 1870 and Liver- 

 sidgef in 1893 that gold is more readily precipitated from its 

 solutions by metallic sulphides than by organic matter. Fur- 

 thermore, Skey showed that sulphides of the base metals were 

 readily precipitated from their alkaline sulphide solutions in a 

 solid coherent form in the presence of iron-p\Tites, galena, 

 blende, stibnite, &c. 



The descending acid solutions formed in the zone of oxida- 

 tion will attack the constituents of the rocks through which they 

 percolate, producing alkaline silicates and sulphides. 



Gold dissolved by ferric sulphate would be also carried down 

 and deposited as leaf, scale, or wire gold in cracks in sulphide 

 ore, thereby causing local enrichment. 



It is maintained by some writers that secondary sulphides 

 have been found below water-level. The evidence on this ques- 

 tion is not quite conclusive. Changes of water-level may have 

 taken place since the secondary sulphides were deposited. 



* Skey. Trans. X.Z. Inst., Vol. iii, 1879, p. 226. 



t Liversidge. Proc. Roy. Soc. N.S.W., Vol. xxvii, 1893, p. 2S7. 



