374 
BOLIVIAN COPPER DEPOSITS 
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 domeykite in the Coro¬ 
coro 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 cuprifer¬ 
ous 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 sulphosalt will be conceded. Previous explanations have as¬ 
sumed that it entered as a carbonate or chloride, and hence a re¬ 
ducing agent had to be postulated to explain the deposition of na¬ 
tive copper. Herein Steinmann 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 sul¬ 
phur 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 sul¬ 
phides 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 precip¬ 
itated in the native state. Iron and magnesium sulphates 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 accom¬ 
plished 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 precipated in part or entirely as the 
sulphide. 
The final genetic query raised by Steinmann is concerning the 
source of the metalliferous solutions. He points out that the 
