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The aeromagnetic survey of the State of Minas Gerais, being executed in cooperation between the Governments of the Federal Republics of Brazil and Germany, is besides its economical value, of large interest as to its consequences to the geological structure of the region. Based on the isoanomaly-maps, till now completed, which cover most of Minas Gerais, a subdivision of the area according to the character of the magnetic field can be undertaken, which partly corresponds to proved geological units. The central part shows a magnetic field, which corresponds to that of known old shields. It therefore can be interpreted as hint at the Sao Francisco Craton, so proved by Magnetics. The "Craton-area" is surrounded by marginal zones in the north and south, in which possibly the material in the underground corresponds to that of the craton, but which are modified by younger tectonics. Southeast of the craton the magnetic field is determined by the itabirites and magmatites of the Minas Series. A possible swell region separates it from areas of different magnetic character in the east. In the south strong anomalies occur over areas, which can be correlated to migmatite zones. The geological interpretation is completed by a quantitative interpretation of single anomalies as to the shape, depth and situation of the source bodies.

The area studied is situated in the southwest part of the state of Bahia, Brazil. From Urandi to Brejinho das Ametistas, about 70 kilometers to the north, occurs an elongated sequence of metamorphic rocks. It forms part of the major geotectonic unit known as the Espinhaco Geosyncline. This part of the Brazilian Platform was probably formed during the Brazilian Orogenic Cycle. The ages of the rocks in this region, based on K/Ar determinations, lie in the range 520 to 790 m.y.. These ages correspond to the youngest rocks situated in the east of the Sao Francisco craton. The Precambrian rocks of the area comprise a lower sequence of gnaisses, schists and amphibolites and an upper sequence of phyllites, green schists, metaconglomerates, quartzites and limestones. The lense like shaped deposits of manganese ore are always intercaleted between the schists and phyllites. The lenses are conformable with the regional schistosity which strikes northeast and dips SE.