COROCORO COPPER DISTRICT OF BOLIVIA 59 



which is explained by its occurrence in fine-grained sand- 

 stones instead of coarse and porous beds of grit and con- 

 glomerate. In both Vetas and Ramos the ore is seldom 

 continuous for any great distance, but is scattered through 

 the metalliferous sandstones in irregular patches of white or 

 greenish color full of small grains of metallic copper which 

 contrast strikingly with the red color of the rest of the bed. 

 Forbes also mentions the occurrence of crystalline and 

 beautiful dendritic forms of copper in sheets' and plates in 

 the planes of stratifiation of the sandstone, and of native 

 copper pseudomorphs after aragonite in the Umacoya vein. 



Reck says the Corocoro ore beds usually range from 

 }/2 to 2 m. in thickness and that the copper occurrence is 

 local and sporadic. The Umacoya was the principal vein 

 and had a maximum thickness of 4 m. and average of 2 m. 

 The ramos differed from the vetas in the finer grain of the 

 impregnated sandstone and in the absence of native silver. 

 Next in importance to the Umacoya was the Buen Pastor 

 which near the surface carried oxidic, sulphidic and arsenical 

 ores of copper and in depth native copper and native silver. 

 The occurrence of the native silver was in small, rich, but 

 irregularly distributed pockets. The silver production for 

 some years prior to 1860 is given at 25 tons of native silver 

 concentrate. 



Forbes in 1866 gave a few additional notes in describing 

 the occurrence of domeykite in the Buen Pastor mine, it 

 occurred as irregular nodules imbedded in the sandstone 

 along with metallic copper and minute grains of metallic 

 silver. The nodules vary in size from that of a pea to 3 or 

 4 inches in diameter and are irregularly imbedded in a com- 

 paratively soft red sandstone which is bleached in their 

 vicinity. The nodules are an intimate mixture of quartz 

 grains and domeykite. 



Mossbach describes 14 ramos and 4 vetas. The richest 

 ores are found above 100 m. Above that level shoots or 

 pockets of rich ore are more frequent ; below that level they 

 are less numerous and not as high grade. The ramos range 



