COROCORO COPPER DISTRICT OF BOLIVIA 71 



and gypsum can be explained by assuming a basin in which 

 copper sulphate watery came in contact with calcium 

 carbonate. The sulphuric acid attacked the carbonate and 

 formed gypsum and the copper was precipitated as the 

 metal. After the deposition of the first veta, silt carried in 

 by a flood of new waters covered it and protected the 

 copper from oxidation. A repetition of this process gave 

 rise to the various ore beds, and the pressure of the 

 accumulating sediments consolidated more and more the 

 underlying and earliest formed beds of the series. 



Sotomayor attributed the copper of the Corocoro deposits 

 to the reduction of sulphate of copper, which probably 

 represented solutions originating from the decomposition 

 of the cupriferous iron sulphides so abundant in the metalli- 

 ferous deposits of both chains of the Andes, by ferrous 

 sulphate which in turn would decompose calcium carbonate 

 and produce the gypsum and iron hydroxide which the 

 metalliferous sandstones enclose. Aside from the improba- 

 bility of the source of the copper sulphate, this explanation 

 has the weakness that the iron hydroxide is least prominent 

 where the mineralization has occurred ; and the fact that it 

 explains the deposition of gypsum is no asset for its validity 

 because gypsum, as noted in a preceding paragraph, is in no 

 wise restricted in its occurrence to the mineralized rocks. 



Domeyko makes the somewhat fantastic suggestion that 

 the discordant juxtaposition of the two systems of beds, 

 composed of strata permeable to liquids and united to the 

 two chains of the Andes, would seem to have formed an 

 enormous battery. The sources of emission of electricity 

 would be perhaps the two ranges which enclose metallic 

 substances undergoing decomposition, and the electrodes 

 would be the strata themselves, at the extremities of which 

 the immense deposit of copper has been reduced. 



Lorenzo Sundt discussed the genesis of these deposits at 

 some length. He presents four arguments in support of 

 their epigenetic origin. First, the sheets of copper filling- 

 fractures in the beds are naturally younger than the beds 



