BOLIVIAN COPPER DEPOSITS 
375 
copper deposits of the western Andes are associated with dioritic 
rocks, usually with granular texture, but in part porphyritic, and 
are genetically connected with the magmas from which they were 
derived. Forbes recognized two zones of dioritic rocks, a wester¬ 
ly zone running along the Pacific slope of the Andes and an 
easterly which extends from the Atacama region through Es- 
meraldas and Comanchi to Lake Titicaca. Corocoro and the 
other similar copper districts of the Bolivian high plateau lie in 
this second zone. Hence Steinmann concludes that the rocks of 
the dioritic zone exist in depth beneath the Corocoro district and 
that the mineralizing solutions originated from the same magma. 
Further it might be mentioned that Steinmann correlates these 
intrusions in age with the porphyries of the eastern Andes of 
Bolivia and concludes that the period of intrusion was late Miocene 
or early Pliocene. If Steinmann was correct in his determina¬ 
tions of the age of the Comanche rocks, then he himself presents 
evidence against his opinion of the Cretaceous age of the Vetas 
for they contain fragments of that igneous rock. 
Straus ^ says only that: “the mineralization appears to be due 
to the reduction effected by organic matter, as well as the replace¬ 
ment of the cementing lime that filled the interstitial spaces in the 
sandstone.’^ 
Douglas does not enter into the question of the origin of the 
Corocoro deposits but remarks: “it can hardly be doubted that the 
presence of copper in the metallic state is due to the intrusion of 
the dioritic rocks.^^ 
Singewald and Miller consider the close association of the 
mineralization with the Corocoro fault as indicative of some rela¬ 
tion between the two, and think that the parent magma of the 
diorities was the source of the mineralizers which deposited the 
ores. 
The Corocoro copper deposits are often spoken of as analogous 
to the Lake Superior copper deposits. These have been more 
closely studied than the Bolivian occurrence and it would seem that 
an explanation of their genesis might be applicable to or at least 
suggest the explanation of the origin of the Corocoro deposits. 
A comparison of the geologic features and the ore deposits of 
9 Mineralogical Magazine, Vol. VII, p. 208, London, 1912. 
10 Quart. Jour. Geol. Soc., London, Vol. LXX, p. 28, 1914. 
11 Eng. and Min. Jour., Vol. CIII, pp. 171-176, 1917. > 
