22 0. A. Derby — Manganese Ore Deposits 



oxide, in the form of magnetite, ilmenite or hematite, could be 

 detected as an original element in the sections or residues. 



At Sao Goncalo the main ore body, composed exclusively of 

 hard secondary material similar in appearance and composition 

 to that of Piquiry, has the appearance of a heavy intercalated 

 bed with an inclination of 30-40° in a decomposed schistose 

 rock. Sound granite occurs a few hundred meters away, but 

 none was seen in immediate contact with the ore. The prin- 

 cipal working near the top of a high hill exposes about three 

 meters of the foot wall, consisting of banded clay of predomi- 

 nant red color with white streaks, which on a carefully scraped 

 surface shows a characteristic gneissic structure with indications 

 of small included fragments, or segregations, differing some- 

 what in texture and color from the body of the rock. On 

 washing, this clay gives a very abundant argillaceous slime, 

 leaving a residue of tolerably abundant white mica (bleached 

 biotite?) with a very moderate amount of quartz and in the 

 heavy portion rare grains of transparent red (secondary?) hema- 

 tite and tourmaline. Next to the ore body the clay for the 

 space of about 20 centimeters is lighter colored with white and 

 yellow streaks, and this shows very distinctly an augen-gneiss 

 structure. Its residue does not differ materially from that of 

 the underlying reddish clay except that the micaceous portion 

 has more the appearance of secondary sericite. 



From the above observations it is tolerably evident that the 

 foot wall at Sao Goncalo was a somewhat micaceous gneiss poor 

 in quartz and without characteristic clastic or granitic accesso- 

 ries, that is to say, it was most probably a sheared basic erup- 

 tive presumably of dioritic or gabbroitic type. 



At the base of the ore body comes a layer about 30 centime- 

 ters thick of quartz rock charged with manganese oxide and 

 with the appearance of shattered vein quartz. This give a 

 moderate residue of garnet without other recognizable acces- 

 sories. 



Above the quartzose layer and separating it from the heavy 

 mass of secondary oxide that constitutes the ore body proper, 

 comes a layer about 40 centimeters thick of a banded yellowish 

 clay with tolerably distinct traces of original feldspathic and 

 micaceous elements giving a well-defined gneissic structure. 

 This gives on sliming a residue of secondary (?)mica and earthy 

 grains of manganese oxide. 



Higher up in the ore body is another clay layer from 2 to 3 

 meters thick which on a scraped surface shows a granitoid 

 aspect, with small scattered patches of earthy manganese and 

 iron oxides that appear to occupy the place of some original 

 manganese-bearing bisilicate element. This also gives a very 

 abundant argillaceous slime with a residue of granular kaolin, 



