PRINCIPLES CONTROLLING ORE DEPOSITS 7 $9 



Now this has been held to be a mere unverified assumption 

 by some geologists, but it seems to me that they have not fully 

 considered the certain effects of the chemical laws concerned. 

 We know if in a laboratory a solution of copper sulphate or 

 other copper salt be placed in contact with iron sulphide, that 

 copper will be thrown down as copper sulphide. If the cop- 

 per solution be placed in contact with a lean copper-iron sul- 

 phide, a sulphide richer in copper will be produced. And if 

 these reactions occur in the chemical laboratory, will they not 

 as certainly occur in the laboratory of nature, although perhaps 

 more slowly ? 



At this point it is to be recalled that in many copper deposits 

 above the level of groundwater oxides and carbonates occur, 

 while below the level of groundwater are sulphides. Moreover, 

 at high levels these sulphides are rich in copper, and they usually 

 become poorer in copper sulphide and richer in iron sulphide at 

 the lower levels. You will remember at Butte, Mont., at and for 

 a distance below the level of the groundwater, are rich copper 

 sulphurets which grade at depth into leaner copper sulphides 

 containing correspondingly large amounts of iron sulphide. 

 You will remember the same is true for the entire Appalachian 

 region. You will remember that frequently above the level of 

 groundwater gold lodes are exceedingly rich. What is the 

 explanation of these and similar facts ? What is the explana- 

 tion of the exceptional or even extraordinary richness of the 

 deposits at and near the level of groundwater, and of the low 

 grade of cupriferous pyrites deep below the level of ground- 

 water. In my opinion the only plausible explanation is that the 

 rich parts of the deposits have received two concentrations, the 

 first by ascending waters and the second by descending waters. 

 The metals of the rich portions of the deposits were largely con- 

 tributed by the parts of the deposit above, or once above, the 

 rich parts. In some cases portions of the depleted veins remain, 

 as at Butte ; but frequently the depleted parts of the veins have 

 been removed by erosion. The remote source of the material 

 was, therefore, the metals deposited by the first concentration. 



