Petrology of the Barro Vermelho Fe-Ti ore body and its metamafic hostrocks, Custodia - PE, northeast Brazil

Abstract

The Barro Vermelho Fe-Ti ore body is located 12 km east of the town of Custodia and 320 km west of the city of Recife in the State of Pernambuco (PE), Northeast Brazil. It is part of many enclave entrained within granitic to tonalitic orthogneisses that constitute the basement of the Proterozoic Pajeu-Paraiba foldbelt in the Borborema Province. The field relationships, petrographic features, geochemistry and U/Pb data on zircons suggest that the gneisses of granitic composition formed at 2.01 Ga by migmatization of the 2.44 Ga old tonalites. Many decimetric to metric and a few hectometric metamorphosed enclaves of anorthosite, gabbro to gabbronorite, banded diorite to gabbroic amphibolites, calc-silicate rocks and trondhjemites are randomly distributed within the orthogneisses. The Fe-Ti ore body occurs as an almost concordant tabular massive unit, 0.8 m thick and 60 m to 80 m long, enclosed in gabbro-anorthosite, banded amphibolite and trondhjemite that together form a hectometric mega-enclave within the orthogneisses. Some apophysis of the ore body, formed by martitized magnetite and illmenite, crosscut the mafic wallrock. The banded amphibolite enclosing the ore body are identical to those found as synplutonic dykes and enclaves in the tonalitic orthogneisses, while the gabbroanorthosite are similar to those found as xenoliths in the orthogneisses. Petrographic features, variation diagrams, and trace element and REE distribution patterns strongly suggest that the gabbro-anorthosites, gabbronorites, trondhjemites, and banded amphibolites as enclaves are differentiation products of a tholeiitic magma of oceanic affinity or originated by the melting of a crustal slab composed of volcanic arc tholeiites. The metagabbronorites appear to represent the most primitive magma from which the anorthosites are interpreted to have formed by fractional crystallization of plagioclase while the protholiths of amphibolites may have crystallized from the residual melt. The ore body may represent the most evolved melt of this suite.