COROCORO COPPER DISTRICT OF BOLIVIA 27 



horizons and usually also contain plant remains. Copper 

 impregnations and plant-bearing beds range through the 

 entire section of 1058111. 



The sandstones and conglomerates of the Vetas forma- 

 tion vary much in mineralogical and petrographic com- 

 position. In the vicinity of Corocoro, quartz grains are 

 more abundant than other constituents and are usually 

 fairly well rounded and water worn. Associated with the 

 quartz grains are subangular grains of feldspar, usually 

 plagioclase, which in some instances are undergoing kaoli- 

 nization, but frequently are perfectly fresh and glassy. The 

 amount of feldspar varies greatly and in many places is in 

 excess of the quartz. In the coarser rocks many of the 

 grains and pebbles are fragments of igneous rocks, and most 

 commonly of hornblende-andesite porphyry, or hornblende 

 diorite porphyry, very much like the Mirikiri rock described 

 in a subsequent section. The matrix of the sandstones is 

 more feldspathic than the grains, and also includes consid- 

 erable chlorite and frequently calcite. These beds are, con- 

 sequently, always arkosic and often are really arkose rather 

 than sandstones and conglomerates. Further, the more 

 abundant the fragments of igneous rock, the less water-worn 

 and more angular are the constituent grains, so that locally 

 the beds represent but slightly water-sorted tuffs. 



South of the Pontezuelo River no very coarse beds made 

 up of fragments of igneous rocks were seen. North of that 

 river, however, about 10 km. from Corocoro, and about I km. 

 northwest of Ballivian station on the Arica-La Paz railroad, 

 at the Carmen mine, a much coarser facies of the Vetas 

 formation is encountered. Whereas the pebbles in the 

 conglomerates about Corocoro include fragments of sand- 

 stone, quartzite and shales, at this locality they are made up 

 of igneous rock varying from quartz porphyry, to andesite 

 in composition. Large fragments of hornblende diorite 

 porphyry, like that of Mirikiri, are particularly abundant. 

 The fragments range in size up to pieces several inches in 

 diameter. They are sufficiently subangular to show the 



