378 
BOLIVIAN COPPER DEPOSITS 
copper precipitation would be relatively easy to write. It is true 
that many of the beds in the vetas contain carbonized plant re¬ 
mains, but they are not coextensive with the cupriferous beds of 
the Vetas and this material is lacking in the Ramos. Hence it can 
not be called upon as the precipitating agent. On the other 
hand, the solutions reduced ferric oxide or dissolved it wherever 
they deposited copper. Consequently ore deposition took place 
in the presence of ferric oxide and probably the mineralizing 
solutions were being oxidized by it. The balance between the 
deposition of native copper and copper sulphide seems to have 
been delicate as both were deposited in large quantity. Just what 
was the chemical character of the mineralizing solutions and just 
what were the reactions that caused the precipitation of the native 
copper throughout most of the cupriferous beds are questions that, 
in the light of present knowledge, can only be speculated on but 
not convincingly or unequivocally answered. Steinmann’s theory 
of the oxidation of sulphides in those solutions by the ferric oxide 
and the reaction of the resulting sulphuric acid with the alkaline 
earths of the impregnated beds, leaves the copper in a state and 
under conditions favorable to the deposition of native copper. 
It is the most plausible of the theories that have been reviewed. 
The source of the mineralizing solutions may be ascribed with 
reasonable certainty to an underlying dioritic magma of which the 
Comanche rock is an offshoot. Evidence of igneous activity dur¬ 
ing the period of the geologic history of the Corocoro district with 
which we have had to deal was presented in the account of the 
geology of the district. The period of mineralization coincided 
with the period of consolidation of that magma and the mineraliz¬ 
ing solutions doubtless bore the usual relations to it which are so 
generally recognized in the case of epigenetic deposits associated 
with igneous rocks. 
14 Economic Geology, Vol. I, pp. 646-647, 1906. 
15 Economic Geology, Vol. II, pp. 580-581, 1907. 
