GEOLOGY OF CENTRAL MINAS GERAES, BRAZIL 411 



origin of these and their relation to the minerals of the gold-bearing 

 lode has not yet been determined. 



Summarizing the foregoing statements we may separate the 

 various minerals into the following groups: 



Original Vein 



Contact 



Later Vein 



Geode 



Minerals 



Minerals 



Minerals 



Minerals 



Quartz 



Garnet 



Tourmaline Arsenopyrite 



Calcite 



Oligoclase 



Biotite 



Pyrrhotite 



(surface etched) 



(largely altered 



(locally- 



Pyrite 



Siderite 



to white mica 



altered) 



Secondary 



(surface dissolved) 



and calcite) 



Staurolite 



Minerals 



Albite 





Cyanite 



Sericite 



Quartz 





Apatite 



Calcite 



Muscovite 





Graphite 





(with sericite) 





(associated 









with contact 









minerals) 







The famous Morro Velho mine at Villa Nova de Lima, which has 

 been the greatest gold-producer in Minas Geraes, is operating on a 

 vein in the Piracicaba schist. This vein is parallel to the bedding 

 which here dips toward the southeast at an angle of about 45°. 

 It has an average width of 5 to 10 meters, and this notable width 

 continues with some variation to the greatest depth yet reached, 

 which in 191 2 was more than 1,600 meters below the surface. The 

 lode reaches a length of more than 150 meters along the strike. 



While the country rock is the Piracicaba schist, the gangue 

 material is a fine-textured mixture of carbonates (siderite, dolo- 

 mite, and calcite) with quartz, and, according to Dr. Derby, a 

 small amount of albite.^ The ore consists principally of pyrrhotite 

 with smaller amounts of arsenopyrite, pyrite, and chalcopyrite. 

 A notable feature of the vein is its constancy in width, mineraliza- 

 tion, and values through the numerous levels down to the lowest one 

 yet opened.^ 



^ O. A. Derby, "Notes on Brazilian Gold Ores," Trans. Am. Inst. Mining Eng., 

 XXXIII (1902), p. 282-87. 



^ For a brief history and general statistics of these mines see: H. K. Scott, "The 

 Gold-field of the State of Minas Geraes, Brazil," Trans. Am. Inst. Min. Eng., XXXIII 

 (1902), 406-44. 



