1166 A TREATISE ON METAMORPHISM. 



phurets, are well illustrated in the Appalachian, Arizona, and Montana 

 deposits." Still higher, and especially above the level of ground water, 

 the oxidized and hydrated products may become dominant, for there 

 the rich sulphurets which have emerged from the ground water have been 

 directly acted upon by the oxygen, carbon dioxide, and water. A series 

 of transformations now take place which may result in metallic copper, 

 cuprite, tenorite, azurite, and malachite, as already described. (See pp. 

 1158-1159.) The oxidized products may largely remain in place, fur- 

 nishing - rich ores, or they may be almost wholly dissolved and carried to 

 lower levels, to react on the sulphides, as already explained. 



Therefore rich sulphurets are first formed largely by the oxidation 

 and the reaction of the oxidized products on lean sulphurets. Later, when 

 in consequence of denudation, these rich sulphurets pass into the belt of 

 weathering, rich oxidized compounds are produced. Thus in the upper 

 few hundred meters of a vein there may be concentrated a large part of 

 the copper produced by a first concentration in a much greater distance. 



I do not mean to imply that each rich copper sulphide deposit has 

 gone through the entire history above detailed, or that other factors have 

 not been concerned in the development of the rich ore. Indeed, it will be 

 seen that the general process outlined is much modified in many instances. 

 (See pp. 1182-1189). However, it is held that processes of secondary 

 concentration, due to descending water, have been very important factors 

 in the production of rich copper deposits at many localities. 



ASSOCIATION OF SILVER AND GOLD WITH BASE METALS. 



The association of lead, zinc, and iron, and of copper and iron — two 

 common cases — have now been considered. A similar set of transforma- 

 tions can be traced out in either of these classes of deposits when they 

 contain silver and gold. 



SILVER. 



In the case of silver it is well known that the original forms are gener- 

 ally sulphides, sulphantimonites, sulphantimonates, sulpharsenites, sulph- 

 arsenates, and silver replacing a part of the iron, lead, zinc, or copper in the 

 sulphides of these metals. Silver is one of the compounds which holds 



"Douglas, James, The copper resources of the United States: Trans. Am. Inst. Min. Eng., 

 vol. 19, 1891, p. 688-695. 



