316 O. A. Derby— Magnetite Ore Districts in Brazil. 



latter is. for the most part, a granular mixture of small irreg- 

 ular grains (often perfect crystals in the leaner ores) of mag- 

 netite and decomposed mica, that passes abruptly to a mica- 

 ceous clay almost free from iron. In this, as in general appear- 

 ance, abundant residue of apatite and other particulars, it 

 agrees with the decomposed rock of Jacupiranga, the only notice- 

 able differences beiug in the less apparent schistosity and 

 absence of perofskite.* Unfortunately no sound rock of this 

 type could be found at Ipanema, but in view of the similarity 

 in the decomposition products, the close correspondence in the 

 eruptive rocks of other types in the two districts and the lack 

 of any primary mica-magnetite type to which the Ipanema 

 rock can be referred, it seems quite safe to consider the latter 

 as decomposed jacupirangite and to conclude for the eruptive 

 origin at both localities. 



The above described mode of occurrence is the only one 

 that has been observed in the Jacupiranga district. At 

 Ipanema on the contrary only a small portion of the ore in 

 sight can be referred to the Jacupirangite type of rock. The 

 greater part occurs as a superficial drift-like deposit resting 

 ( where the underlying rock has been seen, in the extension of 

 about 100 meters i on micaceous clay which according to all the 

 indications results from the decomposition in situ jacupirangite 

 free from iron. The ore masses are rounded as if water-worn 

 and vary in size from shot-like grains to blocks of half a cubic 

 meter or more. These for the most part differ in aspect from 

 the jacupirangite ores and. when impure, contain, instead of 

 decomposed mica, nests of granular apatite and masses of sec- 

 ondary silica (chalcedony, quartz and rarely tridymite). Large 

 rounded bowlders of secondary silica with or without inclu- 

 sions of ore are abundant. In these the only recognizable 

 minerals, aside from the magnetite, are decomposed mica, rare 

 prisms of green pyroxene (acraite) and exceedingly fresh 

 enstatite in large grains much invaded by chalcedony. It is 

 evident from the inspection of the blocks of this superficial 

 deposit that, with the exception of a small part that can be 

 referred satisfactorily to the jacupirangite type, the magnetite 

 occurs as segregations in a rock extremely subject to alteration 

 with separation of secondary silica. This rock contained mag- 

 netite, apatite and acmite as primary constituents without 

 original free silica. Whether the mica and enstatite are pri- 

 mary constituents or not could not be determined, but it is 

 strongly suspected that they are not. In this case it may be 

 surmised that they point to original pyroxene and a magnesium 

 silicate probably olivine. 



* This is in accord with the general character of the ores, that of Ipanema 

 being a pure magnetite, while that of jacupiranga is highly titaniferous. 



