62 STUDIES IN GEOLOGY, No. i 



from a few centimeters to 3 to 4 meters. To a depth of a 

 few meters the copper has been altered to the oxide, car- 

 bonate, and silicate. Otherwise there is no change in the 

 ore in depth down to 380 meters, the lowest level reached 

 in the mine. 



Dereims adds little to the description of the deposits but 

 says in a vertical interval of 350 m. in the Guallatiri mine 

 he saw 13 workable beds of copper ore ranging from .8 to 

 15 m. wide. 



Steinmann describes the cupriferous beds as occupying 

 an area on both sides of the Corocoro fault with a width of 

 2 km. and a. length of 3-4 km. Those in the vetas are 

 chiefly thick beds of coarse sandstone and those in the 

 ramos thinner beds of fine-grained sandstone and shales. 

 Wherever the strata are metalliferous they are bleached, 

 so that the irregular distribution and pockety character 

 of the mineralization is strikingly apparent. For the most 

 part ore deposition occurred as an impregnation of the 

 mineralized beds with native copper but veinlets of native 

 copper intersect the strata in all directions. In the sand- 

 stone the metal fills the interstices between the sand grains 

 as a binder; and when they are widely spaced, the copper 

 appears as a sponge-like mass enclosing countless sand 

 grains. In the shales the copper is usually a fracture filling 

 and occurs as sheets and plates in cross fractures or along 

 bedding planes. The ores usually consist solely of native 

 copper. In the upper levels of the vetas silver ores were 

 also formerly worked associated with which were chalcocite, 

 domeykite, and other sulphur and arsenic compounds. Barite 

 is occasionally encountered and more rarely calcite intimate- 

 ly intergrown with native copper. Large crystals of bluish 

 celestite Steinmann says occur as a primary mineral in 

 druses in the red shales of the ramos, but are not related to 

 the ore deposits. On the other hand, gypsum is a never 

 failing companion of the copper. It especially occurs 

 between the plates of charque and the inclosing sandstone. 



The Corocoro deposits are briefly described in a paper 



