COROCORO COPPER DISTRICT OF BOLIVIA 53 



Jancocata are best reserved for separate accounts of those 

 localities, since the above mentioned correlation of them 

 with the Desagnadero series is only conjectural. 



IGNEOUS ROCKS 



No igneous rocks outcrop within the Corocoro district. 

 The participation of fragments of igneous rocks in the 

 composition of the Vetas is described in a preceding section 

 and also the occurrence of coarse clastic beds at the base of 

 the Vetas exposed at the Carmen mine north of Corocoro 

 made up of igneous material. Hence at the time the lowest 

 outcropping horizons of the Vetas were being laid down the 

 region had already been the seat of igneous intrusions and 

 volcanic outbursts. 



The nearest outcrops of igneous rocks are on Cerro 

 Chukapaca, 15 km. west of Corocoro, and- on the Cerro de 

 Comanche at Mirikiri, 20 km. to the north. 



A specimen of the Chukapaca rock obtained at Corocoro 

 is a rhyolite porphyry. It has a light gray, dense porcelain- 

 like groundmass with phenocrysts of sanidine, orthoclase, and 

 quartz averaging from 2 to 3 mm. in diameter and with a 

 maximum of about 6 mm. The rock also contains minute 

 phenocrysts of biotite. At one end of the specimen are 

 veinlets of micaceous specular hematite. 



The Mirikiri rock is much more coarsely crystalline than 

 that of Chukapaca. It consists of over 50 per cent of felds- 

 par phenocrysts, 10 per cent of hornblende phenocrysts, and 

 less than 40 per cent groundmass. Its color is medium gray. 

 The groundmass is made up chiefly of feldspar grains rang- 

 ing from .03 to .1 mm. in diameter. The feldspar pheno- 

 crysts are from y 2 to 3 mm. in length and characterized by 

 very pronounced zonal growths. Their composition is 

 between that of oligoclase and andesine. Some of the horn- 

 blende phenocrysts are larger than 2 x y 2 mm. in cross- 

 section, but most of them lie between .45 x .15 mm. and 

 3 x .1 mm. Some facies of the rock are more porphyritic 



