Geologia e controle estrutural do depósito cuprífero Caraíba, Vale do Curaçá, Bahia, Brasil

Abstract

The Caraíba deposit, located in the northern part of Bahia State, in the Curaçá river valley, is a chalcopirite/bornite-bearing mafic/ultramafic sill, derived from a tholeiitic magma, which was intruded into a volcanic-sedimentary sequence composed of quartz- feldspar gneisses, leptinites, banded iron formation, calcsilicate rocks and amphibolites. Probably between 2.6 and 2.0 Ga, that sequence was deposited and submited to at least three main tectonic-magmatic events. The first two deformational events were thrust-undesthrusting types, producing a crustal thickening by interleaving of the layers and injection of several G1 and G2 orthogneissic sheet-like intrusions, tonalitic/trondhjemitic and granodioritic in composition. Amphibolite and granulite facies metamorphism acomppanied the first and second phases, resulting in a mixed pile with a strong metamorphic S1 foliation with transposed N-S trending D1 intrafolial folds, followed by N60°W trending tight folds.

After the horizontal tectonic regime a strong E-W compressive stress field resulted in a regional sequence of tight to open D3 folds with N-S 80 S axial planes and 16° to 20°S plunging regional axes. M3 metamorphism reached high-amphibolite to locally granulite facies and, together with a strong deformation, created a very strong and penetrative foliation, S3 , marked by oriented quartz-plagioclase- biotite-hornblende crystals. Many of syntectonic pottassic lens shaped granitic bodies, were intruded during F3 , including the huge Itiúba syenite, all of them strongly foliated and with a characteristic pink-red colour. As a result, the Caraiba copper deposit is now alobate inter- ference pattern (type 2 of Ramsay, 1967) between a D3 tight synform positioned on the 70°W dipping limb of the major N-S trending D3 Caraiba antiform, refolding the recumbent tight D2 folds. The sul- phide mineralization is now concentrated along vertical and disrupted rods which marked originally a horizontal N60°W lineation (or B2 ) . Because of this poliphase tectonic-metamorphic history with associated strong migmatization, the copper content is very heter- ogeneously distributed inside the pyroxenitic/noritic host -rocks, adding difficulties to the mining works, mainly the underground ope- rations. Two later events of shearing are also described and probably one fourth folding fase, but not important for the ore control. A very hypothetical regional tectonic rift-valley system is proposed for the crustal evolution of the Curaçá region, Itiúba syenite and the Cr-belt on its eastern side, and the Jacobina Group, all of them enclosed between two Archean blocks.