370 
BOLIVIAN COPPER DEPOSITS 
with a tenor of 45 per cent in copper, contain only 3 ounces of 
silver. 
An epigenetic origin of the Corocoro deposits and a deep-seated 
source of the mineralizers was suggested by Baron C. A. de la 
Ribette.^ He considered the ore deposition to have taken place 
at the time of formation of the Andes, the elevation of which 
was accompanied by the emission of metallic vapors from the 
bosom of the earth, which elsewhere produced regular veins, but 
in this case penetrated the softer beds which they encountered. 
The genesis of the Corocoro ores was considered more fully by 
Forbes.^ He regarded the copper content syngenetic, but its re¬ 
duction to the metallic state he ascribed to sulphurous fumes 
emitted at the time of intrusion of the dioritic rocks. Forbes says 
the problem would have been easier if the deposits could have been 
shown to have had their cupriferous contents injected into them 
at the time of the dioritic intrusions as in the case of the copper 
veins of Bolivia, Peru, and Chile; but he believed the facts in 
hand point to the copper as originally present in the sedimentary 
beds, probably not as metallic copper, but in a state of combin¬ 
ation, and subsequently reduced to the metallic state. What facts 
he alludes to he does not specify, and his own description of the 
occurrence is at variance with such an interpretation. He recog¬ 
nizes clearly that the copper content is confined to the bleached 
parts of the strata, and he ascribes the bleaching to the magmatic 
exhalations. Hence the more natural assumption would seem to 
be that they also introduced the metal. The discoloration of the 
mineralized rock he concluded was “caused by the evolution of 
sulphurous fumes, disengaged, and penetrating into the pores of 
the strata, at the time of the eruption of the dioritic rocks of Co¬ 
manche and the Cerro de las Esmeraldas, situated respectively to 
the north and south of the metalliferous district of Corocoroand 
the protusion of these rocks through the Corocoro strata he 
thought caused the fault and the accompanying dislocations of the 
strata. More specifically he considered the ore bodies to have 
been calcareous sandstones impregnated with copper oxide or car¬ 
bonate. The sulphurous fumes reduced the copper to the metallic 
form and were themselves thereby oxidized to sulphuric acid. The 
latter reacting with the calcium carbonate produced the gypsum 
2 I^a Gaceta de Gobierno, Vol. LVII, Aug. 2, 1846. 
3 Quart. Jour. Geol. Soc., London, Vol. XVII, pp. 7-84, 1861. 
