BOLIVIAN COPPER DEPOSITS 
371 
which so commonly accompanies these deposits. But it might be 
added in comment on this suggestion that gypsum is not confined 
to the mineralized parts of the Corocoro rocks but is quite wide¬ 
spread and abundant in its occurrence beyond the limits of copper 
mineralization. 
Mossbach * thinks that the association of native copper and 
gypsum can be explained by assuming a basin in which copper 
sulphate waters 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 accum¬ 
ulating 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 metalliferous 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 improbability of the source of the copper sulphate, this ex¬ 
planation has the weakness that the iron hydroxide is least prom¬ 
inent 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. 
4 Die Gruben von Corocoro und Chasavilla un Bolivia (Sud-Amerika), Berggeist, 
1873. 
5 Annales de la Junta de Mineria de Copiapo, 1877. 
eAnnales de Mines, (7), t. XVIII, pp. 531-537, 1880. 
