GEOLOGY OF CENTRAL MINAS GERAES, BRAZIL 405 



scattered through the basement complex without any apparent 

 regularity, but most of them appear to have either gneiss or crystal- 

 line schist on one or both bounding walls. 



The manganese oxide composing these lenses is usually in the 

 amorphous form, occurring mainly as psilomelane and wad, though 

 pyrolusite also is found with these. According to detailed studies 

 made by Dr. Derby,^ it appears that these oxides are surface 

 decomposition products of other manganese minerals which have 

 in one or two cases been encountered below the zone of oxidation. 

 Of these minerals the principal ones are the manganese silicates, 

 tephroite and spessartite, and with these occur rhodochrosite, the 

 manganese carbonate, and sparingly rhodonite, another manganese 

 silicate. These minerals occur intimately intermixed in varying 

 proportions, one being more abundant in one place and another 

 elsewhere, and together they form a reddish manganese silicate 

 and carbonate rock. The relation of the manganese rock to the 

 inclosing crystalline rocks has not been definitely determined; it 

 may be interlayered with the gneiss or crystalline schist or perhaps 

 it is intrusive into them. 



From the one or two instances noted it is judged that all the 

 manganese oxide deposits in the areas of basement complex are 

 surface oxidation products of such masses of manganese silicate 

 and carbonate rock. In many of the deposits where the original 

 rock has not been encountered, the oxide ores have textures which 

 are duplicated in the manganese silicate and carbonate rock else- 

 where, and therefore suggest a similar origin. During the process 

 of decomposition more or less solution and redeposition takes 

 place, with the result that certain portions of a deposit are composed 

 of high-grade manganese oxide, while other portions contain admix- 

 tures of other products of decomposition, such as clay and quartz 

 sand. Most of the ore is hard, but soft material, mainly wad and 

 pyrolusite, also occurs abundantly, being irregularly intermixed 

 with it. 



Manganese ores associated with igneous rocks, such as those 

 described above, occur abundantly in India and are also found 

 locally in the eastern United States. 



' O. A. Derby, Am. Jour. Sci., XXV (1908), 215-16. 



