PRINCIPLES CONTROLLING ORE DEPOSITS 76 1 



Parallel sets of reactions could be, and indeed are written in 

 my full paper upon the subject of ore deposits, which explain 

 the formation of the rich sulphides of lead, zinc, and silver 

 through the reactions of the oxidized products £>f these metals 

 upon sulphide of iron, producing rich sulphides of lead, zinc, 

 and silver. However time does not suffice to present these this 

 evening. The particular reaction in an individual case will 

 depend upon the relative solubilities of the various compounds 

 present, upon the law of mass action, upon the pressure and 

 temperature, and upon various other factors. 



Now I do not assert of the equations which have been written 

 for copper and the other metals that the reactions represented 

 occur exactly as written, but I do assert that reactions of the gen- 

 eral character represented occur by which the oxidized products 

 of the metals in solution are thrown down by the lean sulphurets, 

 producing rich sulphurets. I have no doubt that many other reac- 

 tions besides those written take place. It is exceedingly difficult 

 to ascertain the particular reactions which occur at a given time 

 and place ; but I think it is perfectly clear that reactions occur 

 of the type of those written. I cannot attempt to give you all the 

 evidence on the point, but to me the case is demonstrative. If 

 this be correct we now have an explanation of the fact that a 

 great many ore deposits are rich at high levels and become 

 poorer with depth. These ore deposits have undergone two con- 

 centrations, a first concentration deposited by ascending waters 

 and a second concentration deposited by descending waters. 

 The supplies for the first concentration were obtained from the 

 widely dispersed and small amounts of material disseminated 

 through the rocks. The supplies for the second concentration 

 were derived from an earlier concentration. 



In the foregoing statements the second concentration of 

 metals by solution, downward transportation, and precipitation 

 by reactions upon the sulphides of an earlier concentration has 

 been emphasized. However, it is not supposed that this is the 

 only process which may result in enrichment of the upper parts 

 of ore-deposits by descending waters. The enrichment of this 



