PRECIPITATION OF GOLD. 1093 



various gold deposits it seems to me very probable that in many instances 

 the ferrous salts have been imjaortant reducing agents, and the same may 

 be true of cuprous compounds. 



It has been seen on a previous page that the ic compounds capable of 

 dissolving gold are produced extensively in the early part of the journey 

 of underground water. It has further been fully explained (p. 1085), that 

 deep in the belt of cementation, where organic matter and sulphides are 

 abundant, ic salts are changed to ous salts, which are the prevalent com- 

 pounds deep within the belt. Therefore the trunk channels which carry 

 the gold in solution are likely to receive contributions from other solutions 

 which carry ous salts in large amount. Here the conditions are just the 

 reverse of those existing in the early part of the water's journey. The 

 law of mass action now demands the reduction of the gold to its metallic 

 condition. For instance, if the gold were a chloride, and ferrous sulphate 

 were present, the following reaction would take place : 



2AuCl 3 +6FeSO i =2Au+2Fe 2 (S0 4 ) 3 +2FeCl 3 



Similar reactions are written for ferrous chloride and cuprous chloride on 

 the preceding page. It is also to be remembered that in trunk channels the 

 solutions are likely to be ascending. Therefore the temperature is falling, 

 and as pointed out by Stokes all of these reactions are promoted by this 

 condition. Thus an abundance of reducing solutions and the temperature 

 both work together to produce precipitation in the trunk channels. 



In connection with these reactions the question naturally arises as to 

 which of the mis salts is probably the most important. Since iron is the 

 most abundant of all the metals carried in underground solutions, such sul- 

 phates would be more likely to be sulphates of iron than any other. If the 

 salts formed in the belt of weathering were ferric sulphates, they would be 

 likely to be reduced to the ferrous condition at depth by organic matter or 

 by sulphides. Indeed, analyses of mineral waters which bear sulphates also 

 ordinarily show ferrous iron. a Therefore ascending waters bearing ferrous 

 sulphate or other ous salt might be brought into a lode by side streams and 

 there precipitate the gold. Such side channels entering through lateral 

 cracks may, in many cases, cause the extreme irregularity in the distribution 

 of the gold. 



aPeale, A. C, Mineral waters of the United States: Bull. U. S. Geol. Survey No. 32, 1886. 



