COROCORO COPPER DISTRICT OF BOLIVIA 75 



would be sufficiently soluble to be carried away, the less 

 soluble calcium sulphate would remain. In this way would 

 be explained the bleaching of the sandstone, the formation 

 of gypsum, and the deposition of metallic copper ; and the 

 result would be accomplished by solutions of such a 

 chemical character as Steinmann believed formed the other 

 Andean copper deposits. He adds that with an excess of 

 sulphur in the solutions over the available ferric oxide, the 

 copper might be precipitated in part or entirely as the 

 sulphide. 



The final genetic query raised by Steinmann is concern- 

 ing the source of the metalliferous solutions. He points 

 out that the 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 recog- 

 nized two zones of dioritic rocks, a westerly zone running 

 along the Pacific slope of the And.es and an easterly which 

 extends from the Atacama region through Esmeraldas 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 por- 

 phyries 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 determinations 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 (p. 208) says only that: "the mineralization 

 appears to be due to the reduction effected by organic 

 matter, as well as the replacement of the cementing lime 

 that filled the interstitial spaces in the sandstone." 



Douglas (p. 28) does not enter into the question of the 



