66 STUDIES IN GEOLOGY, No. i 



come to the surface and the ramos are found in depth; but 

 a mile and a half north of the city the ramos also outcrop. 

 The copper-bearing sandstones are closely spaced and 

 separated by beds of shale. They vary in thickness from 

 one inch to 25 feet, averaging 4 feet. The ore body in the 

 fault plane, which is now worked out, Lincoln thinks may 

 possibly have been drag from the vetas and ramos. This 

 would make the Corocoro fault later than the mineralization, 

 a conclusion with which we do not agree. The intensity 

 of the mineralization varies in different ore-bearing beds 

 and in different parts of the same bed and nowhere does 

 the entire bed consist of ore. The copper-bearing beds are 

 generally white in color and are always free of copper 

 where they contain red patches. The white sandstone is 

 more friable than the red, as a result of having been leached 

 of its original cementing material and of having had this 

 replaced in part only by ore. The principal ore mineral is 

 copper. In the ramos this has been oxidized to an average 

 depth of 100 feet to malachite, azurite, and chrysocolla. 

 The vetas also contain native copper in depth but a zone 

 averaging 300 feet deep from the surface contains chalcocite, 

 which to an average depth of 50 feet has been oxidized 

 chiefly to bronchantite. Other ore minerals of minor 

 importance are cuprite, domeykite, native silver, and galena. 

 The principal gangue mineral is gypsum. Small amounts 

 of hematite, barite, and celestite are also found. Lincoln 

 says a polished section of ore containing both native copper 

 and chalcocite showed these minerals as a cementing 

 material between the grains of sand, and intergrown in 

 such a manner as to indicate contemporaneous deposition. 



SUMMARY ACCOUNT OF THE ORE DEPOSITS 



The principal mines of the Corocoro district have a 

 longitudinal extent along the Corocoro fault of a little more 

 than 4 km. from the Challcoma mine I km. south of the 

 city to the Libertad mine 3 km. north of the city. With 



