COROCORO COPPER DISTRICT OF BOLIVIA 69 



tenor is 7 to 8 per cent copper. In the zone of oxidation 

 it alters chiefly to the basic sulphate bronchantite, but also 

 to malachite and azurite ; and especially in fractures and 

 along bedding planes, cuprite forms. Hand-sorted mixed 

 oxidized and sulphide ores carry 18 to 25 per cent copper. 



The charque, or sheets and arborescent forms of native 

 copper, occurs in fractures, along bedding planes, and in 

 the principal fault planes. The thickness of the sheets 

 varies with the width of the fracture and according to 

 whether gangue minerals are associated with the copper 

 or not. In some cases the native metal occurs as a single 

 sheet completely filling the opening; in other cases it is 

 enclosed in gangue, or is intergrown with gangue. The 

 commonest gangue mineral is gypsum, but celestite occurs 

 in considerable quantity. Celestite also occurs in tabular 

 crystals lining druses in which there has been no copper 

 deposition. 



No additional data can be given concerning the occurrence 

 of silver ores, as such ores are no longer produced and 

 silver is a very subordinate constituent of the copper ores 

 now worked. The native copper concentrates, with a tenor 

 of 85 per cent in copper, contain only 6 oz. ag ; and the 

 sulphide concentrates, with a tenor of 45 per cent in copper, 

 contain only 3 oz. ag. 



GENESIS OP THE COROCORO COPPER DEPOSITS 



An epigenetic origin of the Corocoro deposits and a 

 deep-seated source of the mineralizers was suggested by 

 Baron C. A. de la Ribette. He considered the ore deposi- 

 tion to have taken place at the time of formation of the 

 Andes, the elevation of which was accompanied by the 

 emission of metallic vapors from the bosom of the earth, 

 which elsewhere produced regular veins, but in this case 

 penetrated the softer beds which they encountered. 



The genesis of the Corocoro ores was considered more 

 fully by Forbes. He regarded the copper content syn- 



