GEOLOGY OF CENTRAL MINAS GERAES, BRAZIL 395 



cracks as veins and stringers. In some places manganese oxide 

 (psilomelane) is associated with this redeposited material. 



The lenses and beds of carbonate rocks in the lower part of the 

 Piracicaba formation in places contain a noticeable percentage of 

 iron and manganese carbonates. On decomposition these form 

 impure residual deposits of iron and manganese oxides. 



Origin of the bedded ores. — From the structure and shape of the 

 bedded ore deposits and their relation to the inclosing rocks it is 

 necessary to conclude that they are original sedimentary deposits, 

 laid down in a measure similar to limestone beds. They are simply 

 a part of the iron-formation beds, similar in origin to the itabirite 

 with which they are associated, except that they contain a smaller 

 proportion of mechanical sediments, such as quartz sand. It is, 

 therefore, best to explain the origin of the iron formation as a whole 

 and note the differences in origin between the itabirite portions and 

 the iron ore portions. 



As has been stated, the iron formation is, in the main, an iron- 

 oxide-bearing sandstone or quartzite. It varies in different portions 

 from a rock consisting largely of quartz sand to one consisting of 

 pure iron oxide. The siliceous portions of the iron formation are 

 called itabirite, just as the siliceous portions of the Lake Superior 

 iron formation are called ferruginous chert, jaspilite, or taconite. 

 The portions rich in iron oxide are termed massive ore, powdery ore, 

 or laminated ore, according to their texture. The line between the 

 itabirite and the iron ore, especially the laminated ore, is arbitrary, 

 depending on the percentage of metallic iron present. At the 

 present state of the iron industry, iron formation with 50 per cent 

 or more of metallic iron may be termed iron ore and iron formation 

 containing below 50 per cent metallic iron itabirite. In the 

 Brazilian iron formation there are no equivalents of the cherty 

 iron carbonate and the greenalite rock of the Lake Superior district 

 which in that region represent the original rock from which the 

 iron ore, ferruginous chert, and associated rocks have been formed 

 by weathering. In the Brazilian iron fields there has been no such 

 alteration, the itabirite and iron ore being the original rocks. ^ 



^ C. K. Leith, and E. C. Harder, "The Hematite Ores of Brazil and a Comparison 

 with the Hematite Ores of Lake Superior," Econ. Geol., VI (1911) 670-86; E. C. 

 Harder, "The 'Itabirite' Iron Ores of Brazil," Econ. Geol., IX (1914), loi-ii. 



