COPPER ORES. n 1165 



While in. some cases the sulphides were precipitated by reactions in 

 which oxygen is present and in other cases by those in which it was absent, 

 in still other cases I have no doubt that the reactions utilized some oxygen, 

 but not as much as indicated by the equations as written. In such instances 

 the real changes when precipitation took place are represented by equations 

 which combine in various proportions those in which oxygen is represented 

 as present and those in which it is absent. Moreover, for a certain part of 

 a deposit the conditions may for a time have been those in which the reac- 

 tions were without oxygen, later those in which it was insufficient, and 

 finally those in which it was sufficient. Thus, all combinations of the reac- 

 tions with and without oxygen may have taken place in the production of a 

 single deposit. 



The vertical relations of the richer sulphides which appear in passing 

 from depth to the surface are very different in various regions. In some 

 districts there niay be somewhat regular gradations from the poor 

 sulphurets at depth to the very rich sulphurets at or near the level 

 of ground water. In other cases this change may be very abrupt. 

 For instance, at Ducktown, Tenn., the lean cupriferous pyrrhotite changes 

 rapidly into a zone of very rich sulphuret at and near the level 

 of ground water. At Butte, Mont., the rich sulphurets are found at a 

 much greater depth, and in the deeper workings of some of the mines 

 secondary fracture openings in the lean sulphurets now contain small veins 

 of rich sulphides of various kinds, even bornite or chalcocite, which have 

 evidently been reduced by the lean sulphides adjacent. These illustrations 

 show that the various sulphides overlap one another. In passing upward 

 from the poor material, bornite may appear before the iron sulphide has 

 been replaced largely by chalcopyrite, and at the place where bornite has 

 become reasonably abundant chalcocite may be found. If the dominating 

 material be iron sulphide, the copper mineral which is present is likely to be 

 chalcopyrite rather than the richer sulphurets. Chalcopyrite is likely to 

 be associated on the one hand with pyrite or pyrrhotite and on the other 

 hand with bornite, or even chalcocite. Bornite and chalcocite are likely to 

 be associated with each other and with chalcopyrite. 



At still higher levels in a mine, a moderate distance below the 

 level of ground water, oxidized and carbonated products may appear 

 with the sulphurets. These mixed products, sometimes called oxysul- 



