GEOLOGY OF CENTRAL MINAS GERAES, BRAZIL 393 



been cemented together by iron oxide deposited from solution, 

 both fragments and cementing material being derived from the 

 same source — the iron formation. The cementing material is 

 largely hydrated hematite and hmonite, red or yellow in color, 

 giving different portions of canga deposits colors varying from light 

 yellow to dark red. The fragments decrease in size and abundance 

 as the distance from the source increases, so that in one place a 

 canga deposit may consist largely of cemented fragments and 

 elsewhere of finely textured chemically deposited iron oxide. 

 The latter is most common over areas outside the iron formation 

 or over soft portions of the iron-formation belts where erosion 

 yields no coarse fragments. In the latter localities, however, 

 much fine fragmental material is intermixed. In places at a 

 distance from the iron-formation areas the canga is very low grade, 

 containing much clay and quartz sand, the iron oxide present being 

 principally limonitic cement. 



The ordinary canga of which the great portion of the canga 

 deposits are composed averages between 60 and 65 per cent metalHc 

 iron and up to 0.3 per cent phosphorus, rarely containing less than 

 0.1 per cent phosphorus. This concentration of phosphorus is 

 due to the same cause as the concentration of the phosphorus in 

 the surface laminated ore. The outlying portions of canga deposits 

 at a distance from the iron-formation belt are so low in metallic iron 

 that they do not constitute an iron ore. 



A canga blanket may vary in thickness in different portions 

 from a few centimeters to 20 meters or more. It is generally 

 thickest on the lower slopes and at the base of iron-formation ridges, 

 where the conditions for accumulation are best, and decreases in 

 thickness both up the slope and toward the valley. A single con- 

 tinuous blanket of canga may cover several square kilometers. 



Stream deposits of iron oxide in the form of sand, pebbles, and 

 larger fragments occur in greater or less purity along most of the 

 streams rising in or flowing through areas of iron formation. The 

 material is distributed over the width of the valley floors and in 

 places occurs in terraces several meters above the present valley 

 bottom. The stream deposits do not constitute a very high-grade 

 ore, always being mixed with more or less quartz sand and other 



