1118 A TREATISE ON METAMORPHISM. 



with other sulphides previously precipitated or with other reducing- agents. 

 Any of these factors, or any combination of them, may cause the precipita- 

 tion of the sulphides. 



A case due to the combination of these factors is the precipitation of 

 sulphides which later may react to precipitate the metal of the oxidized salts. 

 For instance, in the early stage of the development of a deposit, the sul- 

 phides of iron and other base metals may be precipitated by hydrogen 

 sulphide or some other compound, and later these previously precipitated 

 sulphides take part in the precipitation of the sulphides of copper, silver, a,nd 

 mercury from the oxidized solutions of these compounds, such as sulphates. 



While it is believed that sulphides are generally segregated in the first 

 concentration by upward-moving waters, this is not supposed to be univer- 

 sal. Nature's processes are always too complex to be coverd by a single 

 general statement. As a result of mingling* solutions at various places, and 

 of reactions between solutions and walls, many lateral-moving and downward- 

 moving streams doubtless deposit rather than dissolve sulphides. Indeed, 

 in the frequent case where in downward-moving waters sulphites or sul- 

 phates are reduced by organic matter to sulphides, precipitation of a portion 

 of the sulphide is usual. Still the statement would hold true that upon the 

 average more sulphides are dissolved than deposited by descending waters, 

 and more sulphides are deposited than dissolved by ascending waters. 



I conclude, therefore, that sulphide ores, 'whatever their source, as first 

 concentrates are generally deposited by ascending waters in the trunk channels. 



It appears that whether sulphur-bearing compounds reach trunk chan- 

 nels as sulphides or as oxidized salts, they are likely to be precipitated in 

 such channels because solutions from various sources mingle there and 

 come into contact with previously precipitated sulphides. The sulphides 

 thus produced in the trunk channels may by denudation again reach the 

 belt of weathering- when the cycle is complete. These sulphides may be 

 again oxidized to sulphate, and so on. It is therefore clear that sulphur, as 

 sulphide and sulphate, may again and again take part in the deposition of 

 ores," but the first chief source of the sulphur is probably the sulphides of 

 the original crystallized rocks and of magmas. 



«Le Conte, Joseph, Genesis of metalliferous veins: Am. Jour. Sci., 3d ser., vol. 26, 1883, p. 13. 



