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Jacurici Complex, Ipueira-Medrado Sill, Campo Formoso Complex, Coitezero, Cascabulhos |
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Bahia, Brazil |
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Cr
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Super Porphyry Cu and Au
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The chromitite bearing Ipueira-Medrado Sill, some 220 km NNW of the city of Salvador and the Coitezero Mine in the Campo Formoso Complex, ~75 km to the WSW, are located in the São Francisco Craton of northern Bahia State, north-eastern Brazil.
The Ipueira-Medrado Sill was emplaced in the northeastern sector of the São Francisco Craton and belongs to the regionally extensive Jacurici Complex of mafic-ultramafic intrusions emplaced into an Archaean granulite gneiss terrane of the Caraíba granulite complex. The Jacurici Complex intrusions are distributed over a north-south elongated area of 70 x 20 km. Limited dating of the mafic-ultramafic intrusions of the complex suggests they are around 2038±19 Ma in age. The Jacurici Complex parallels the copper bearing Caraíba mafic complex (see the Caraiba-Jaguari copper mine record), separated by the 2.1 Ma Itiúba Syenite, while the larger Campo Formoso mafic-ultramafic intrusion is located some 50 km to the west.
The Jacurici mafic-ultramafic bodies have a similar stratigraphic succession in all studied segments and can be subdivided using the same scheme divisions as seen in the Ipueira sill as described below (cf. Marques and Ferreira Filho, 2003), with the exception of the absence of the Marginal Zone at the Monte Alegre Sul and Várzea do Macaco segments which are located 20 to 25 km and 35 to 40 km farther north, respectively from the Ipueira underground mine (Marques et al., 2017).
The Ipueira-Medrado Sill is a single intrusion that has been tectonically disrupted by faulting and folding into two segments that outcrop on the limbs of a synform. It is conformable with the enclosing quartzo-feldsapthic gneisses, which in the hangingwall include olivine bearing marble, calcsilicates and metachert.
The sill is mainly composed of ultramafic rocks with dunite, harzburgite and pyroxenites making up 80% of the body. It has been divided into three zones as follows from the base:
i). Marginal Zone, 5 to 20 m thick - sheared gabbro and orthopyroxene rich harzburgite.
ii). Ultramafic Zone, 250 m thick - subdivided into
- Lower Ultramafic Unit, composed of dunite and minor harzburgite with chain textured chromitite,
- Main Chromitite Layer, comprising massive and chain textured chromitites
- Upper Ultramafic Unit, mainly of harzburgite but also with minor chain chromitite and dunite.
Pyroxene increases upwards in the Ultramafic Zone to produce a pyroxenite at the top, while magmatic intercumulus amphibole makes up around 20% of the zone.
iii). Mafic Zone, up to 40 m thick - consists of leuconorites to melanorites that are partially metamorphosed to amphibolite facies.
The ore in the Ipueira-Medrado Sill is mined from a single sinuous layer that is continuously developed (but structurally disrupted) throughout the 7 km long sill and varies from 5 to 8 m in thickness within the 300 m thick sequence of cumulate mafic-ultramafic rocks that make up the sill as described above.
The measured remaining reserves of chromite ore in the Ipueira-Medrado Sill total 4.5 Mt @ 30 to 40% Cr2O3 and have been mined since 1974.
The geologic resource in the Jacurici Complex is estimated to be around 30 Mt @ a similar grade.
The Campo Formoso Complex is located ~75 km WSW of the Ipueira-Medrado Sill. The complex is or has been exploited by nine mines, the most significant of which is Coitezero. The mineralised complex has an arcuate outcrop, sandwiched between the subdued topography of the 2.0 Ga Palaeoproterozoic Campo Formoso Granite and the ridge formed by the overlying Palaeoarchaean Jacobina Group quartzite. It immediately overlies and partly intrudes gneisses that are similar to those of the Caraíba Group (de Deus et al., 1982). The complex is exposed over a strike length of ~40 km, and tapers to the NE from a width of ~1.1 km to ~100 m. Detrital chromite layers in overlying quartzite indicate a deeply eroded upper surface to the intrusion (Hedlund et al., 1974; de Deus et al., 1982).
The chromite seams are concentrated in the southwestern tip of the ultramafic complex, with the principal open-pit mines, from west to east, being the Cascabulhos, Camarinha, Campinhos, Pedrinhas, Coitezeiro and Limoeiro. Coitezeiro is the main producer. It comprises a stratiform complex that contains more than seven different economic chromite seams, composed of varying proportions of disseminated, net-textured, layered, and massive ores. Each seam is commonly 5 to 15 m thick and dips at ~50°E. Whilst the preserved total thickness of chromite-bearing ultramafics decreases to the NE, the individual chromite layers and the intervening olivine-serpentinite sequences appear to thicken. The seams of principal economic importance 4, 5 and 6. The highest exploited seam 7 at Cascabulhos is 580 m above the floor of the intrusion, has a low Cr/Fe ratio, and is hosted by serpentinised pyroxenite. Greenschist facies metamorphism of the ultramafic host rocks is complete, with the exception of orthopyroxene relicts in an upper unit at the Cascabulhos mine (Lord et al., 2004).
At the Cascabulhos mine the chromitite-bearing serpentinite after an olivine protolith in the lower sections of the intrusion are dark greenish-black in color. Talc-carbonate alteration increases in intensity upward culminating in a sheared talcose zone above the highest serpentinite-hosted chromitites. Higher, the sequence becomes a greyish mottled serpentinite that lacks pyroxene pseudomorphs, although it is geochemically similar to the altered pyroxenite found higher in the sequence. The latter is mottled and grey, or greenish-yellow where weathered, with a granular texture due to 2 to 4 mm pyroxene cleavage surfaces, pseudomorphs, or local platy bastites (Lord et al., 2004).
Microprobe studies of chromite-rich specimens from representative stratigraphic sections of each intrusion suggest that unusually low TiO2 levels (<0.9 wt,%) combined with high Cr2O3 grades of 45 to 55 wt.% reflect the primitive parental magma composition in the intrusions. The resource of the Campo Formoso Complex has been quoted at 3 Mt @ 38% Cr2O3 (Lord et al., 2004; Student Chapter of the Federal University of Rio Grande do Sul [SEG-UFRGS] field trip report, 2019).
The most recent source geological information used to prepare this decription was dated: 2019.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Friedrich, B.M., Marques, J.C., Ribeiro Olivo, G., Frantz, J.C., Joy, B. and Queiroz, W.J.A., 2020 - Petrogenesis of the massive chromitite layer from the Jacurici Complex, Brazil: evidence from inclusions in chromite: in Mineralium Deposita v.55, pp. 1105-1126.
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Lord, R.A., Prichard, H.M., Sa, J.H.S. and Neary, C.R., 2004 - Chromite Geochemistry and PGE Fractionation in the Campo Formoso and Ipueira-Medrado Sill, Bahia State, Brazil: in Econ. Geol. v.99, pp. 339-363.
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Marques, J.C. and Feirreira Filho, C.F., 2003 - The Chromite deposit of the Ipueira-Medrado Sill, Sao Francisco Craton, Bahia State, Brazil: in Econ. Geol. v.98, pp. 87-108.
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Marques, J.C., Dias, J.R.V.P., Friedrich, B.M., Frantz, J.C., Queiroz, W.J.A., Botelho, N.F., 2017 - Thick chromitite of the Jacurici Complex (NE Craton Sao Francisco, Brazil): Cumulate chromite slurry in a conduit: in Ore Geology Reviews v.90, pp. 131-147.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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