COROCORO COPPER DISTRICT OF BOLIVIA 61 



description of the Corocoro ore deposits taken from Soto- 

 mayor. The vetas are mineralized sandstones which contain 

 in addition to disseminated native copper, sub-sulphate, 

 oxide, sulphide, and arsenide of copper. Locally native 

 silver occurs instead of copper. The rainos are not so rich 

 in copper and are more argillaceous. The richest vetas are 

 those nearest the plane of contact of the Vetas and Ramos 

 and the mines extend for 3 km. along that contact. A sketch 

 shows the plane of contact parallel to the vetas and truncat- 

 ing the ramos beds, but he does not refer to it as a fault. 



Denegri states that in 1888 silver ores were no longer 

 being produced and that the principal vetas were the 

 Umacoya, the San Jose, and San Marcos. 



Lorenzo Sundt says the Corocoro ores are encountered 

 on both sides of the fault plane, in the vetas and in the 

 ramos. They are merely certain strata of these rocks 

 impregnated with native copper, and very rarely native 

 silver. The metals are accompanied by calcium sulphate, 

 barium sulphate, and a little calcium carbonate, and in the 

 vetas, but not in the rainos, by arsenides and sulpharsenides 

 of copper. These substances constitute the cement of the 

 sandstones and vary in size with that of the grains of sand 

 and the interstices that separate them. The grains of copper 

 are usually from pin-point to pin-head in size, but in coarse 

 sandstones they are much larger. The barite sometimes 

 forms nodules impregnated with native copper. Calcium 

 sulphate often traverses the barite in thin fibrous veins and 

 glistens in the sandstones. Native copper is encountered 

 principally in the sandstone beds intercalated in the shales, 

 but in the ramos series it is also found in the shales as 

 slender sheets, in grains up to the size of hazel nuts, and in 

 nodules impregnated with sulphides and carbonates. In the 

 vetas native copper associated with barite fills fissures cutting 

 the stratification perdendicularly and obliquely as thick 

 sheets, called "charque/' which attain to weights of 2-^/2 to 

 5 tons. The fractures, or at least the copper fillings, never 

 extend beyond the cupriferous beds whose width varies 



