Implication of the Caraíba Deposit's Structural Controls on the Emplacement of the Cu-Bearing Hypersthenites of the Curaçá Valley, Bahia-Brazil

Abstract

RESUMO IMPLICATES DOS CONTROLES ESTRUTURAIS DO DEPÓSITO CARAÍBANA COLOCAÇÃO DOS CORPOS CUPRÍFEROS DO VALE DO CURAÇÁ, BAHIA -BRASIL O depósito cupríferoCaraíba é o maior dentre diversos corpos de rochas máficas-ultramáficas encontradas no Vale do Curaçá (norte do Estado da Bahia) na parte norte do Cráton do São Francisco. O depósito, que vem sendo explorado desde 1978, consiste de hiperstenitos e noritos mineralisados a calcopirita e bornita disseminadas e/ou em agregados disformes. O Vale do Curaçá é um terreno de alto grau metamórfico compreendendo supracrustais, gnaisses do seu provável embasamento e as rochas máficas/ultramáficas colocadas como sills, sendo todas intrudidas por ortognaisses tonalíticos/granodioríticos e granites (G 1 -G 3 )Cedo-a tardi-tectônicos a eventos deformacionais altamente dúcteis ocorridos entre 2.25 e 2.05 Ga (idade U-Pb de zircões). Eventos finals e localisados ocorreram sob fácies xisto verde até cerca de 1,9 Ga atrás (isócrona Sm-Nd de rocha total em hiperstenitos). A exceção de G 2 e G 3 todas as rochas já mencionadas apresentam um bandamento metamórfico penetrative (Si) no fácies anfibolito afetado por duas fases de dobramentos respectivamente associados a metamorfismo de fácies granulito e anfibolito. Devido à interferência desses dobramentos mais jovens o depósito Caraíba adquiriu a forma de um cogumelo, com cerca de 5 km de comprimento N-S e 800 m de largura E-W, e encontra-se alojado em sinforma com piano axial fortemente inclinado para oeste e com caimento duplo de eixo para N ou S, fazendo parte do flanco oeste da estrutura maior -a antiforma Caraíba. A complexa geometria do depósito, aliada ao intense desmembramento rúptil-dúctil que o afetou durante a evolução estrutural da área, tem dificultado o planejamento e as operações de lavra. Uma vez que o corpo mineral atuou como barreira relativamente resistente à deformação intensa e rica em mobilisados da terceira fase, as estruturas da segunda fase puderam ser melhor preservadas nas cercanias de Caraíba, e este fator contribuiu sobremaneira para o entendimento da evolução tectônica do Vale do Curaçá. Os controles estruturais da mineralização cuprífera tem importantes implicações na gênese dos depósitos cupríferos da região, como discutido em detalhe. Palavras-chaves:Evolução estrutural e tectônica, Mina Caraíba, terrenes granulíticos, Vale do Curaçá, Cráton do São Francisco Paleoproterozóico, idade U-Pb, minério de cobre ABSTRACT The Caraíba 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 Curaçá Valley terrane, a N-S trending strip of high grade rocks in the northern part of the São Francisco Craton. This terrane comprises supracrustais and gneisses of their probable basement, the mafic-ultramafic rocks emplaced as sills, all intruded by G 1 -G 3 ear...

U–Pb and Sm–Nd geochronology of amphibolites from the Curaçá Belt, São Francisco Craton, Brazil: Tectonic implications

“…Its westernmost part is covered by (meta) sediments deformed in the Neoproterozoic Brasiliano orogeny and by Quaternary limestones, whereas the eastern part includes (to the west of the town of Bonfim, Fig. 1b) the Campo Formoso Cr-bearing maficultramafic layered body, a major intrusion that is unconformably overlain by the Jacobina Group, both of which are intruded by the Late Paleoproterozoic Campo Formoso granite (Silva, 1996;Leite, 2002). According to a summary in Silva (1996) the Jacobina Group is part of a much longer Paleoproterozoic riftlike volcanic-sedimentary basin extending to the boundary between the Gavião and Jequié blocks.…”

“…1b) the Campo Formoso Cr-bearing maficultramafic layered body, a major intrusion that is unconformably overlain by the Jacobina Group, both of which are intruded by the Late Paleoproterozoic Campo Formoso granite (Silva, 1996;Leite, 2002). According to a summary in Silva (1996) the Jacobina Group is part of a much longer Paleoproterozoic riftlike volcanic-sedimentary basin extending to the boundary between the Gavião and Jequié blocks. The Serrinha block consists of amphibolite-facies 2900-3500 Ma ortho and paragneisses of granodiorite composition, as well as migmatites and amphibolites (Barbosa and Sabaté, 2004).…”

“…The Serrinha block consists of amphibolite-facies 2900-3500 Ma ortho and paragneisses of granodiorite composition, as well as migmatites and amphibolites (Barbosa and Sabaté, 2004). The western margin is in contact with the Itiúba syenite, whereas the eastern margin encloses Paleoproterozoic greenstone sequences, and arc-like orthogneiss and granites that are indicators of a back-arc basin system within the Serrinha block (Silva, 1996). Farther to the east, this block underlies Neoproterozoic (meta)sediments and, outside of Fig.…”

“…The second unit consists of Cu-poor or simply barren gabbros, gabbronorites, leucogabbros, and hyperites, together with Cu-rich hypersthenites, melanorites, and norites. A vast amount of geology and structural data collected regionally, and also on surface and in underground sites of the Caraíba Mine and other small bodies, drove Lindenmayer (1981), Lindenmayer et al (1984), D'el-Rey Silva (1984Silva ( , 1985, and lately Mayer and Barnes (1996) and D'el-Rey Silva et al (1988Silva et al ( , 1994Silva et al ( , 1996 to state that the Cubearing rocks are sill-like structures. It is noteworthy that the Caraíba orebody is a Cu-only, chalcopyrite and bornite-rich deposit and associated with deep ocean rocks such as banded iron formation and evaporites (anhydrate) forsterite-marble.…”

“…Resolution Ion Microprobe -SHRIMP -data in Oliveira et al, 2004); 2248±36Ma for syn-D2 tonalite in the Caraíba Airport outcrop; and 2051±16 Ma for syn-D3 granite intrusive in the Caraíba orebody (both U-Pb data, but respectively from zircons and monazite; D'el-Rey Silva et al, 1996), whereas the age of the supracrustals was not directly determined. This paper presents the results of U-Pb and Sm-Nd integrated studies carried out on samples of amphibolite, tonalite and granite of the Caraíba Airport outcrop, situated ∼5 km to the north of the Caraíba orebody (Fig.…”