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Marques, J.C.1; Frantz, J. C.1; Chemale Jr.; F.1, Brito, R.S.C.2; Appollo, J.F.H.1
1. Instituto de Geociências, UFRGS , Av. Bento Gonçalves, 9500, Prédio 43126, Porto Alegre - RS, 91501-970 - juliana.marques@ufrgs.br 2. CPRM Serviço Geológico do Brasil, SGAN Quadra 603, Conj. J, Parte A, 1o andar, Brasília - DF, 70830-030
AbstractThe Jacurici Complex, located to the NE sector of the São Francisco Craton, hosts the largest chromite deposit of Brazil. The Complex is constituted by several NS-oriented bodies, which are possible fragments of a single larger sill disrupted during deformation. Ni-Cu sulfide mineralization was recently described in the north part of the Complex. The sulfide mineralization shows stratigraphic association with the chromitites. The parental magma was very primitive, originated from high mantle extraction, and the petrologic evolution of the mafic-ultramafic rocks suggests that a high volume of magma flowed through the sill, which acted as a dynamic conduit. Geochemical and isotope data strongly suggest that crustal contamination triggered the chromitite crystallization and, possibly, the Ni-Cu sulfide mineralization. Among other regional aspects, we emphasize that the Complex may be larger than originally assumed and have lateral variations as exemplified by some characteristics, such as larger amount of mafic rocks, sulfidation and higher deformation to the north. The understanding on the metamorphic and deformation processes, as well as on the age of the Jacurici Complex still require further studies. Key-words: mafic-ultramafic rocks, chromite deposit, Ni-Cu sulfidesIntroduction The Jacurici Complex has been considered as a swarm of chromite-rich mafic-ultramafic sills that outcrop along a NS trend in the Eastern part of the Itabuna-Salvador-Curaçá belt, to the NE sector of the São Francisco Craton, Brazil (Figure 1). Alternatively, the Jacurici Complex may be a former larger single sill that has been disrupted during deformation. The host rocks of the Complex and the regional geologic evolution are still under debate, and two main options are reported: (1) the Complex intrudes the granulite-gneiss terrane of the Caraíba Granulite Complex (Barbosa et al., 1996); or (2) the Complex is hosted by the Archean, medium-grade, gneiss-migmatitic and granite-greenstone sequences of the Serrinha Block (Teixeira et al., 2000; Oliveira et al. 2004). The Jacurici Complex parallels the Cu-mineralized mafic-ultramafic deformed intrusions of the Caraíba Complex (Oliveira et al. 2004), and both areas bear a similar geologic evolution. Nevertheless, the ages reported for the two complexes are different. SHRIMP U-Pb dating of zircons yielded a 2580 ± 10 Ma age for the Caraíba intrusions and a 2085 ± 5 Ma age for the Jacurici intrusions (Oliveira et al., 2004). The intrusions are geographically separated by the 2084 ± 16 Ma Itiúba Syenite (Oliveira et al. 2004).
The estimated chromite reserves in the Jacurici Complex amount to more than 30 Mt (Marinho et al., 1986), making it the largest chromite deposit in Brazil. The ore occurs in most of the bodies and is mined from layers up to 8 meters thick. Recently, Ni-Cu sulfide-rich bodies were investigated in the north part of the swarm (Marques et al., subm.). The relationships of these mineralized bodies and their metallogenetic features are considered here. The Jacurici ComplexThe Jacurici Complex has been considered as outcropping in a 70 km long to 20 km wide trend. Nevertheless, detailed mapping and extensive drilling carried out by the Mineração Vale do Jacurici (FERBASA Group) shows that the Complex is much larger and possibly extends to the Uauá region to the east, and to Santa Luz to the south. The bodies that outcrop to the west of the Itiúba Syenite may also be correlated with this magmatism, but this still requires further investigations. The drilling also has provided fresh rocks from the complete stratigraphy of the mafic-ultramafic bodies. Primary igneous textures and minerals are preserved in many places, except for the olivine-rich rocks (dunites) that are highly serpentinized. The noritic-gabbroic rocks show commonly major transformations because of the regional amphibolite to granulite facies metamorphism.
Generally, all bodies are composed of similar rock types along the stratigraphy. The bodies can be divided into two main zones, from bottom to top: an Ultramafic Zone up to 300 meters thick and a Mafic Zone up to 60 meters thick. The zonal subdivision was mainly based on rock composition. The Ultramafic Zone is subdivided into three members: (1) the Lower Ultramafic Unit (LUU); (2) the Main Chromitite Layer (MCL); and (3) the Upper Ultramafic Unit (UUU). The LUU is 100 to 250 m thick, and consists mainly of dunite interlayered with minor harzburgite and pyroxenite and rare thin chromitite seams. The MCL is 5 to 8 m thick, and is mainly constituted by massive chromitite with minor chain-textured chromitite layers. The UUU is up to 50 m thick and is comprised of harzburgite and pyroxenite interlayered with minor dunite and thin chromitite layers. Magmatic intercumulus amphibole (hornblende-tschermakite to Mg-hornblende) occurs in both LUU and UUU, but is more abundant in the UUU. The Mafic Zone, at the upper part of the bodies, consists of up to 60 m thick norite and local gabbro-norite, with variable proportions of orthopyroxene, clinopyroxene and plagioclase with meso- to adcumulate primary igneous texture. When metamorphic transformation occurs the assemblage normally consists of plagioclase, hornblende, biotite and garnet, indicating the regional amphibolite to granulite metamorphic facies of the area. The Jacurici Complex shows some important distinct features from south to north. The Ultramafic Zone rocks that outcrop in the south part of the trend are predominantly composed by dunite with minor harzburgite and rare orthopyroxenite. Clinopyroxene is rare in these rocks, and magmatic amphibole occurs in some layers, mainly in the UUU. Most of bodies are chromite-rich and sulfide-poor. On the other hand, the Ultramafic Zone rocks that outcrop to the north are enriched in pyroxene in both LUU and UUU. Pyroxenite is very common and clinopyroxene is relatively frequent. Amphibole is also common, mainly in close association with the ore minerals. Most of the northern bodies have experienced sulfidation, and concentrations of Ni-Cu sulfides occur in the northernmost bodies. Chromitites occur in the majority of these northern bodies, at the same stratigraphic level that they are found in the southern bodies. The Mafic Zone is thicker toward the north. Furthermore, from the middle to the north of the Complex, the deformation is more intense and the cumulate rocks become locally highly foliated and enriched in phlogopite. The Jacurici Complex is crosscut by normal faults and shows many brittle structures related to them, which are generally filled by alkaline pegmatites that seem to be related to the Itiúba Syenite. Regarding the metallogenesis, two major bodies
deserve attention: (1) the Cr-rich Ipueira-Medrado Sill, one of the
largest bodies of the region, which was studied in detail by Marques
& Ferreira-Filho (2003) and Marques et al. (2003); and (2) the
Cr and Ni-rich Várzea do Macaco Sill, which was recently investigated
by Marques et al. (subm.). The Ipueira-Medrado sill crops out as a 7 km long, 0.5 km wide and 300 m thick synformal structure (Figure 1b). The sill is oriented parallel with the regional basement foliation, and is disrupted into two segments. These segments are conformable with the underlying quartzofeldspathic gneiss and with the overlying rocks, including olivine-bearing marble, calc-silicate rocks and metachert (Deus & Viana, 1982). The compositional variation of olivine and orthopyroxene from harzburgite samples allows identification of two different magmatic regimes. Regime 1 occurs through the LUU, and is characterized by a slow upward increase in MgO/FeO (Fo 89-93.5; En 88-94.5), which is intensified immediately below the MCL and is interpreted as an open magmatic system with concomitant injections of fresh primitive magma and extrusion of fractionated melt. Regime 2 is expressed in the UUU, and is typified by a fast upward decrease in MgO/FeO (Fo 90-84; En 90- 82), and is considered as a result of a closed magmatic system with minor injections of fresh magma (Marques & Ferreira Filho, 2003). The parental magma was enriched in LREE, LILE and Zr, and depleted in Ta. Sm-Nd isotopes are consistent with a 2 Ga age, but suggest variable initial Nd isotopic composition that correlates well with the abundance of amphibole. The more negative eNd (mean of -6.5) of the amphibole-rich intervals indicates crustal contamination, although the eNd of the amphibole-free samples (mean of –4.4) suggests an old and enriched subcontinental lithospheric source. Chromite separates have initial gOs that range from -4.6 to +3. The negative gOs are typical of old, Re-depleted lithospheric peridotitic mantle, and yield Re-depletion model ages of up to 2.75 Ga (Marques et al., 2003). The Várzea do Macaco SillThe Várzea do Macaco Sill is a 7 to 8 km long body that outcrops to the north part of the Jacurici Complex. It is disrupted into five blocks, which are laterally dislocated by late faults. The stratigraphy is inverted and layers are conformable with the steep dipping basement rocks. Comparing to Ipueira-Medrado sill, three main differences can be immediately pointed out: (1) the presence of disseminated and interstitial sulfides in a large portion of the sill, mainly close to the MCL; (2) the relative enrichment in pyroxene throughout the sill; (3) the strongly deformation that caused intense foliation, and (4) thicker mafic layers. The Ni-Cu-sulfide mineralization, shows an apparent primary stratigraphic control. It occurs as disseminated and interstitial ore associated with a ca. 5 meters thick pyroxenite layer, below the MCL and a pyroxene-rich harzburgite layer above the MCL. The disseminated and interstitial sulfides are predominantly pyrrhotite with pentlandite and minor chalcopyrite. Massive sulfide lenses crosscut the pyroxenite and harzburgite layers at this stratigraphic level. These massive lenses are 5 centimeters thick and are also constituted by pyrrhotite with pentlandite, with minor chalcopyrite. Additionally, the intensive deformation that affected some layers have remobilized the sulfides along foliation and concentrated some of these minerals within small folds. Veinlets also crosscut the basement rocks. The Ni and PGE grades are still off the record, under a confidential business agreement. A thoughtful study of this sill is currently being performed, including MEV, geochemical and isotope studies. The aim is to constrain the petrology of the Várzea do Macaco sill, make a better comparison with the Ipueira-Medrado Sill and understand the Cr and Ni-Cu-sulfide mineralizations.
The Origin of the Mineralizations The origin of the 5-8m thick MCL is difficult to explain considering current models and one single mechanism. Petrographic and chemical data from the Ipueira-Medrado Sill pointed to a very primitive parental magma (very hot, Cr-rich, which resulted from high mantle extraction) undergoing changes in physical-chemical conditions at the interval where the MCL formed. The presence of amphibole closely associated with chromite and becoming more abundant above the MCL indicated that crustal contamination played an important role in the formation of the chromitites. Additional evidence for crustal contamination beginning at this level was provided by both the Nd and Os isotopic data (Marques et al., 2003). An integrated assessment suggested that the contamination triggered the chromitite crystallization. Nevertheless, it is also important to emphasize that the crystallization of such thick layer was only possible if the sill acted as a conduit to allow a large volume of magma flow. Considering all these features (primitive magma, open system with large volume of magma passing through, and crustal contamination), it would be reasonable expect a sulfide segregation with Ni and possibly PGE mineralization. Nevertheless, sulfidation is absent at the Ipueira-Medrado Sill. On the other hand, at the Várzea do Macaco Sill sulfidation took place exactly at the stratigraphic level of the chromitite formation and Ni-Cu- mineralization occurs. The reasons of this metallogenetic differences between the south and north parts of the Jacurici Complex are under studies, but one possible reason would be differences in the composition of the contaminants, which could be sulfur-rich in the north segments of the Complex.
Final Remarks The Jacurici Complex hosts the largest chromite deposit of Brazil and is constituted by several bodies, possible fragments of a single larger sill, which has been disrupted during deformation. Detailed studies performed by Mineração Vale do Jacurici show that the Complex is larger, but further investigations must be carried out. Considering the main trend (70 km long and 20 km wide) of outcropping bodies that parallels the Itiúba Syenite to the east, important differences can be pointed out from south to north, despite the fact that the bodies show similar stratigraphic units (LUU, MCL and UUU, followed by a Mafic Zone). To the north, occurs: (1) an increase in the amount of pyroxenite; (2) a decrease in orthopyroxene concomitant with an increase of clinopyroxene; (3) sulfidation; (4) an increase in the amount of mafic rocks; and (5) an intensification of deformation. Detailed studies characterized the parental magma as very primitive, originated from high mantle extraction and originally undersaturated in sulfur. The petrologic evolution of the mafic-ultramafic rocks suggests that a high volume of magma flowed through the sill, which acted as a dynamic conduit, and crustal contamination triggered both chromite and Ni-Cu-sulfide mineralizations. The sulfidation occurred only in the northern part of the Complex, probably because of the composition of the host rocks that possibly provided sulfur-rich contaminants during the crystallization. Considering the changes that occur to the north, particularly the increase in the amount of mafic rocks, increase in deformation and occurrence of sulfidation, the long-time dispute about the relationship between the Cr-rich Jacurici Complex and the Cu-rich Caraíba Complex emerges again, despite the different ages recently reported by Oliveira et al. (2004). The Jacurici Complex shows metamorphic and deformation features similar to the basement rocks indicating that it has intruded before the peak of the tectonic events that affected this region. Nevertheless, the age reported for the Complex is virtually the same of the Itiúba Syenite, which shows lower deformation and apparently crosscut the Complex after the peak of the metamorphism and deformation. In our understanding, further geochronologic and metamorphic studies of the Jacurici Complex are strongly recommended for better regional geological understanding.
Acknowledgments Mineração Vale do Jacurici (FERBASA Group) and its staff are gratefully acknowledged for field support, geological information and allowance for sampling the drill cores. This project was partially supported by grants and scholarships from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES-PRODOC).
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