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Santa Luz, Pedras Pretas |
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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 Santa Luz or Pedras Pretas chromite deposit is hosted within the Santa Luz Peridotite Complex, also known as the Pedra Preta Mafic-ultramafic Complex, and is located 2.7 km southeast of the town of Santa Luz, and ~175 km NNW of Salvador, in Bahia, Brazil, within the northeastern São Francisco Craton.
Chromite mineralisation is associated with several small bodies of Palaeoproterozoic Rhyacian ultramafic rocks which include pyroxenite, serpentinite, peridotite and dunite, as well a associated gabbro and anorthosite. These bodies intrude poly-deformed grey gneisses and migmatites of the Mesoarchaean Serrinha Block basement and supracrustal rocks of the Paleoproterozoic Rio Itapicuru greenstone belt. They are distributed along the border of the Rio Itapicuru greenstone belt and are older than 2085 ±12 Ma but younger than 2983 ±8 Ma, based on U/Pb ages obtained respectively on aplite dyke and host banded gneiss (Oliveira et al., 2007).
The main mineralised body of the complex is ~400 m long by ~100 m wide, conformably interleaved with NW-trending, tightly folded, nearly upright banded gneisses
composed of bands of alternating amphibolite and felsic gneisses that range from a few tens of centimetres to a few metres in thickness. The contact between the country rock gneisses and the ultramafic body is tectonic and marked by narrow shear zones that are in places enriched in biotite-phlogopite. The ultramafic body is predominantly composed of serpentinite and serpentinised harzburgite containing both disseminated and massive chromite layers, gabbroic pockets and amphibolite dykes/veins with epidote-rich margins. The massive layers are compact or friable. Compact chromitites show major element chemistry and platinum group element signatures similar to ophiolite chromitites (Oliveira et al., 2007).
[NOTE: no reference to the thickness of chromitite layers has been encountered in the available sources].
Quartz veins, aplites, pegmatites and mafic dykes crosscut these rocks. Fractures and shear zones are common within the peridotites, usually associated with alteration of the ultramafic rocks to dark mica and tremolite-rich bands.
During the First World War, the deposit produced between 25 and 30 thousand tonnes of chromite concentrate (Carvalho Filho et al., 1986).
The 'current' chromite mine as described by Teixeira et al. (2010) was exploiting a main orebody that was ~250 m long, associated with much smaller bodies, which are all contained in a NNW band about 1 km long. Chromite reserves were estimated at 1.49 Mt @ 40% Cr2O3, containing 0.59 Mt of Cr2O3 (Carvalho Filho et al., 1986).
The most recent source geological information used to prepare this decription was dated: 2010.
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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Oliveira, E.P., Escayola, M., Souza, Z.S., Bueno, J.F., Araujo, M.G.S. and McNaughton, N., 2007 - The Santa Luz chromite-peridotite and associated mafic dykes, Bahia-Brazil: remnants of a transitional-type ophiolite related to the Paleoproterozoic (>2.1 Ga) Rio Itapicuru greenstone belt?: in Revista Brasileira de Geociencias v.37, (Sup. 4), pp. 28-39.
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Teixeir, J.B.G., Silva, M.G., Misi, A., Cruz, S.C.P. and Sa, J.H.S., 2010 - Geotectonic setting and metallogeny of the northern Sao Francisco craton, Bahia, Brazil: in J. of South American Earth Sciences v.30, pp. 71-83.
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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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