GOLD AND SILVER DEPOSITS LINDGREN. 169 



The sierras of the pampas, like that extending from San Luis to 

 Cordova, contain a feeble pre-Cambrian or early Cambrian miner- 

 alization, referred to above, but these quartz veins appear to be poor 

 in gold and silver. In the same vicinity there is also evidence of a 

 much later development of gold deposits, perhaps connected with 

 the effusion of Tertiary andesitic lavas, but these veins which have 

 the character of crushed or sheeted zones are also poor in gold. 1 



The whole eastern slope of the Andes from the Bolivian plateau 

 to the latitude of Santiago de Chile shows a relatively feeble mineral- 

 ization. The slopes of the central cordillera and the pre-Cordilleras 

 are largely composed of sedimentary rocks folded, overturned, and 

 overthrusted toward the east, with relatively small and inconspicu- 

 ous areas of igneous rocks, 3 which are designated as andesites and 

 dacites, but which in reality seem to be holocrystalline intrusives. 

 There are also smaller areas of granular rocks of Tertiary age, which 

 were designated as "Anden diorite " by Stelzner. 



Gold, silver, and copper prospects are rather abundant, but at very 

 few places has serious work been undertaken. The most important 

 deposit, located at Famatina, is a copper-bearing vein with sulphar- 

 senides and antimonides of copper and very little gold and silver. 



The eastern slope of the Andes in the northern half of the Argen- 

 tine Republic is comparable in a way to the eastern Rocky Mountain 

 chain of Canada. Both show overturned folds and overthrusts 

 toward the east, with comparatively little of intrusive rocks and 

 attendant mineralization. The gold-silver-tin belt of the 

 Bolivian eastern cordillera apparently does not enter the Argentine 

 territory. 



No lode deposits are reported south of Mendoza, except on the 

 headwaters of Neuquen River, at about the latitude of Concepcion 

 in Chile, where there is a mining district of gold-bearing quartz veins 

 in granite of uncertain age. Considerable work has been done on 

 these, but the expected production does not seem to have been real- 

 ized. The ore is apparently of low grade. The only other precious 

 metal deposits reported from the eastern slope of the Andes in 

 Patagonia are placers of doubtful value on the headwaters of Chu- 

 but, Rio Gallegos, and other streams. Placers and some lode mines 

 have been taken up at various places on the Argentine Tierra del 

 Fuego, but little information is available as to their values. 



As observed above, the Mesozoic beds of the Patagonian cordillera 

 and eastern cordillera are intruded by laccolithic and' batholithic 

 masses of granitic rocks, and careful prospecting might well yield 



X E. Gerth: Constitucion geologica de la Provincia de San Luis (Anales Ministerio de 

 Agrieultura, Secgion Geologico, Tomo x, No. 2, 1914). 



2 R. Stappenbeck : La Pre-Cordillera de San Juan y Mendoza (Ibid., Tomo iv. No. 3). 



