GOLD AND SILVER DEPOSITS — LINDGREN. 153 



It is almost forgotten at the present time that the placers of south- 

 ern Brazil yielded heavily in the eighteenth century, particularly 

 from 1700 to 1775, and this production was particularly welcome at 

 a time when the gold from the Americas seemed exhausted and the 

 treasures of the northern Cordilleras were as yet undreamed of. 

 During the period named, these placers yielded from $1,000,000 to 

 $2,000,000* annually. The total yield during the eighteenth century 

 is variously given from $200,000,000 to much higher figures. After 

 this period the production languished, but a few quartz mines con- 

 tinued to be operated and a little placer gold was washed. At the 

 present time Brazil maintains its output of gold at from $2,000,000 

 to $3,800,000, but this is practically derived from three deep mines 

 in Minas Geraes, of which the Morro Velho is the most important, 

 besides having the distinction of being the deepest mine in the world 

 (vertical depth 6,200 feet). 



The deposits are quartz veins of a deep-seated type, allied in 

 places to pegmatite dikes. They occur in part in Archean schists, 

 gneisses, and granites, but most of them are found in a thick sedi- 

 mentary series of schists and quartzite, which is older than the Cam- 

 brian but overlies the Archean. This series contains no intrusives, 

 except some pegmatite dikes, and the Brazilian veins are in this re- 

 spect markedly different from most other pre-Cambrian occurrences. 

 It is believed that igneous intrusions took place in the rocks under- 

 lying the pre-Cambrian sediments and that only pegmatitic dikes 

 and quartz veins reached up into the covering series. 1 



Similar geological conditions prevail in Eio Grande do Sul, be- 

 yond which the gold-bearing region continues into Uruguay, where 

 the most southerly mines are found near Cunapiru. Uruguay yields 

 annually up to $100,000 in gold. 



The most southerly representatives of this older class of gold de- 

 posits appear in the Sierras of the pampas, for instance, in that ex- 

 tending from San Luis to Cordova in Argentina. The old crystal- 

 line schists, granites, and pegmatites here emerge from under the 

 pampas formation and the Permo-Triassic beds, and contain de- 

 posits of tungsten, gold, and silver, but the latter two metals are not 

 present in quantities sufficient for economic mining. 



While it is possible that some deposits of this kind occur in the 

 pre-Cambrian of the Andean region, which is exposed in Colombia 

 and in the northernmost provinces of Argentina, it is improbable 

 that they contribute perceptibly to the total production. 



To sum up: The old gold deposits yield the total production of 

 Venezuela, the Guianas, Brazil, and Uruguay and at the present 



1 E. C. Harder and C. K. Leith : The Geology of Central Minas Geraes, Brazil (Journal 

 of Geology, vol. xxiii, pp. 341 to 378, 385 to 424 (1915)). 



