1142 A TREATISE ON METAMORPHISM. 



dantly present in the driest mines of the Southwest, but at Bisbee, a more 

 humid area, it is absent." Probably the reaction producing the sulphate 

 has taken place extensively in both areas, but in the driest area there was 

 not sufficient amount of water to dissolve the calcium sulphate formed and 

 hence it was precipitated as gypsum. The sulphates may even react upon 

 the aluminum salts and form hydrous sulphate of aluminum. This occurs 

 in the gold veins of California. 6 



The oxidized belt in some cases may be altogether above the level of 

 ground water, but in other cases where the descending oxidizing solutions 

 are strong may extend somewhat into the belt of cementation. An excel- 

 lent case illustrating the almost perfect coincidence of the oxidized belt 

 with the belt of weathering- is furnished by the Monte Cristo district, 

 described by Spurr. He says, "the zone between the surface of the ground 

 and the lowest level of the ground-water surface is practically coincident 

 with the zone of oxidation." e 



It has been shown how the upper oxidized belt is produced, and next 

 the manner of formation of the rich sulphide and telluride belt will be consid- 

 ered. The oxidized products, oxides, carbonates, and sulphates, may either 

 in situ react upon the unaltered sulphides, or the carbonates, sulphates, and 

 other oxidized compounds may be transported downward in solution and 

 there react upon the sulphurets. In either case the result is to produce a 

 richer sulphuret. The reactions may be between an oxide or a salt of a 

 metal and its sulphide, for instance between the oxide or sulphate of cop- 

 per and the sulphide of copper, as shown by the following equations : 



6CuS+2Cu 2 0=5Cu 2 S+S0 2 



and 



6CuS+2Cu 2 S0 4 +3H 2 0=5Cu 2 S+2H 2 S0 4 +H 2 S0 3 



The reaction, however, may be between the oxide or salt of one metal and 

 the sulphide of a second, as for instance, between zinc oxide, sulphate, or 

 carbonate and iron sulphide. In this case a sulphide of the first metal is 



"Douglas, James, The Copper Queen mine, Arizona: Trans. Am. Inst. Min. Eng., vol. 29, 1900, 

 p. 535. 



& Lindgren, Waldemar, The gold-quartz veins of Nevada City and Grass Valley, California: Seven- 

 tenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1896, p. 120. 



« Spurr, J. E., The ore deposits of Monte Cristo, Washington: Twenty-second Ann. Rept. U. S. 

 Geol. Survey, pt. 2, 1901, p. 859. 



