Jacobsita occurs in metamorphic oxide-silicatic manganese protore in Itacima, in the manganese district of Pacatuba, Ceara, Brazil. Precambriam gneisses and schists are the host rocks of the lenses of the manganese protore which was weathered at the surface, forming small ore deposits. All the Precambrian rocks in this area belong to the Maranguabe Metamorphic Complex and are located in the Jaguaribeana folding belt. The assemblage of primary manganese minerals in the protore consists of rhodonite, spessartite, tephroite, jacobsite and hausmannite. Manganese silicates and hausmannite were altered to aMnO2, but the jacobsite shows only incipient alteration. In spite of the presence of two micas in the gneissic country rock, the metamorphism is believed to be of high grade, as evidenced by the assemblage of manganese metamorphic minerals. Probably the regional metamorphic rocks underwent two different phases of metamorphism. The manganese content of the manganiferous rocks varies from 10.0 to 38.0%. The magnetometric anomalies were very useful in establishing the form and dimensions of both the manganese ore and protore. The Mn-deposits of Itacima have very small reserves, consequently their resources have no economic interest.
