IMPLICATION OF THE CARAÍBA DEPOSIT'S STRUCTURAL CONTROLS ON THE EMPLACEMENT OF THE CU-BEARING HYPERSTHENITES OF THE CURAÇÁ VALLEY, BAHIA-BRAZIL

Abstract

The Caraiba deposit of Bahia, Brazil, is a Paleoproterozoic sill-like intrusion consisting of magnetite-bearing hypersthenites, melanorites and norites mainly mineralized with disseminated-type Cu-ore (chalcopyrite and bornite) exploited since 1978. The intrusion is the largest of a set of 100's mafic-ultramafic bodies found within the Curaca Valley terrane, a N-S trending strip of high grade rocks in the northern part of the Sao Francisco Craton. This terrane comprises supracrustals and gneisses of their probable basement, the mafic-ultramafic rocks emplaced as sills, all intruded by G (sub 1) -G (sub 3) early-to late-tectonic tonalitic-granodioritic orthogneisses and granites related to highly ductile deformation events between 2.25 to 2.05 Ga (U-Pb ages of zircons). Final and localized events under greenschist metamorphism lasted up to 1.9 Ga (whole rock Sm-Nd isochron on hypersthenites). A penetrative, layer-parallel amphibolite-facies metamorphic banding (S (sub 1) ) occurs in all mentioned rocks--apart from G (sub 2) and G (sub 3) --and is affected by two younger folding events respectively under granulite and amphibolite facies. As a result of this evolution, the Caraiba Cu-deposit is a N-S trending, tight-isoclinal and asymmetric F (sub 3) synform, with 1000 m of amplitude and axial plane dipping 70 degrees -75 degrees to the W, and lies in the western limb of a normal, regional-scale F (sub 3) antiform plunging about 18 degrees S. However, because of the interference pattern of F (sub 3) with E-W trending F (sub 2) folds, the deposit has a nearly 5 km-long and 800 m-wide mushroom shape. Brittle fracturing and boudinage disrupted the orebody throughout the structural evolution, so mining planning and underground exploitation are difficult in some parts of the mine. Because the Caraiba orebody was a relatively rigid barrier to the intense, ductile and melt assisted D (sub 3) event, the structural evolution of the Curaca Valley high grade terrane, where most of the earlier structures were tightened and/or brought into parallelism with the S (sub 3) foliation. The structural controls of Caraiba also bear implications for the genesis of the orebody.