GEOLOGY OF CENTRAL MINAS GERAES, BRAZIL 413 



This alloy has been encountered in a number of places, but 

 is nowhere very abundant. 



Quartz in veins or lenses is very common in the iron formation, 

 and in places contains gold. It is very often found where dis- 

 seminated gold occurs, and it is possible that some genetic relation 

 exists between the gold in the quartz and that disseminated in the 

 iron formation. As the masses of vein quartz in this association 

 are generally small, irregular, and discontinuous, it seems possible 

 that, because of the porous nature of the formation, the gold- 

 bearing waters did not follow regular channels but impregnated the 

 iron formation for considerable distances on both sides of the main 

 lines of flow and thus deposited their gold in a more or less dissemi- 

 nated condition. At any rate, from its form and occurrence it is 

 certain that the gold is a secondary concentration within the iron 

 formation. 



Gold occurs in the gravels of most streams rising in, or flowing 

 through, the gold-bearing district (Fig. 19). These occurrences are 

 simply placer deposits whose gold has been derived in part from 

 gold-bearing quartz veins, but mainly from the great masses of iron 

 formation, the disseminated gold of which, because of its wide- 

 spread occurrence, offers abundant opportunity for concentration 

 along streams. 



At present small gold-washing operations are conducted by 

 natives along many of the streams at the end of each rainy season, 

 since the flood waters caused by the heavy tropical rains bring 

 new material down the valleys each year. 



DIAMONDS 



The principal diamond fields of Brazil are those of central 

 Minas Geraes near Diamantina, of Western Minas Geraes near 

 Bagagem, and of Bahia near the towns of Sincora, Lencoes, and 

 Jacobina. Of these the Diamantina field is the oldest and best 

 known and has yielded most of the best Brazilian diamonds. 

 The Bagagem field has yielded most of the large diamonds found 

 in Brazil, while the Bahia field supplies nearly all the world's 

 demand for carbonados. 



The Diamantina field is located in the Serra do Espinhago 

 quartzite belt, the principal workings lying north, east, and south 



