74 STUDIES IN GEOLOGY, No. i 



the first conclusion he cites certain characteristics of the 

 copper veins that are widely distributed along the west 

 slope of the western Andes from central Chile to Peru. 

 Most important is the small amount of the common gangue 

 minerals which they contain, from which fact he concludes 

 that the metalliferous solutions to which they owed their 

 origin were relatively deficient in silica, alkali earths, etc. 

 Except in the abundance of gypsum, the Corocoro deposits 

 are analagous to the other Andean copper deposits in this 

 respect. The occurrence of chalcocite, bornite, and domey- 

 kite in the Corocoro ores is another point of similarity 

 between them and the West Coast copper deposits. On 

 these grounds Steinmann refers the metalliferous solutions 

 of this district back to the same source and considers them 

 of the same general character as the other cupriferous 

 mineralizing solutions of the Andes. 



Why then, he asks, is the copper not united with sulphur 

 and with arsenic as in the West Coast copper veins and why 

 is it in the native state? That the copper was introduced 

 as some salt or sulposalt will be conceded. Previous 

 explanations have assumed that it entered as a carbonate 

 or chloride, and hence a reducing agent had to be postulated 

 to explain the deposition of native copper. Herein Stein- 

 mann believes a fundamental error was made, and thinks 

 the mineralizing solutions were characterized by a scarcity 

 of oxygen, that is, of sulphates, as compared with sulphur 

 and arsenic. In that event, in order to explain the precipi- 

 tation of native copper, one must seek oxidizing agencies 

 rather than reducing. These, he said, were at hand in the 

 form of ferric oxide of the Corocoro strata. On entering 

 these beds the sulphides of the metalliferous solutions would 

 be oxidized at the expense of the iron oxide and the beds 

 thereby bleached. The resulting sulphuric acid having 

 greater affinity for lime, magnesia, and iron, which it 

 encountered in the beds, than for copper would form 

 sulphates of those elements and the copper would be pre- 

 cipitated in the native state. Iron and magnesium sulphates 



