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Bacuri Complex |
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Amapa, Brazil |
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Cr
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Super Porphyry Cu and Au
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The Bacuri layered mafic-ultramafic complex is located within the eastern part of the Archaean to Palaeoproterozoic Guyana Shield, in Amapa State, north-eastern Brazil, 80 km northwest of Macapa which is situated on the northern bank of the Amazon River.
The Guyana Shield in Amapá comprises Archaean to Palaeoproterozoic granite-gneiss-granulite terranes (gneiss, migmatite, granite, tonalite and both mafic and felsic granulites), layered mafic-ultramafic complexes, Palaeoproterozoic greenstone belt sequences and Phanerozoic cover.
The Bacuri mafic-ultramafic complex was intruded into a granite-gneiss-granulite terrane and was metamorphosed to amphibolite facies by the 2.0 Ga Transamazonian cycle. The complex is overlain in turn by the greenstones and supracrustals of the Palaeoproterozoic Vila Nova Group (which hosts the Serra do Navio manganese deposits).
The Bacuri Complex has been weathered and oxidised to a depth of 130 m. It is mapped over an east-west trending strike length of at least 30 km and comprises interlayered mafic and ultramafic rocks, mainly represented by amphibolite, serpentinite, tremolite-serpentinite, tremolite and chromitite, subdivided from the base into:
Lower Mafic Zone: >500 m of leucoamphibolite (plagioclase and hornblende with minor biotite and accessory ilmenite and titanite).
Ultramafic Zone: occuring as several discontinuous and elongated bodies over strike intervals of 100 to 1200 m and thicknesses of 30 to 150 m, composed of interlayered chromitite and serpentinite (occuring as bands of dark serpentinite with mottled tremolite-serpentinite) suggestive of interbanded dunite-peridotite protoliths. The serpentinite is composed of serpentine, chlorite, magnetite, chromite, talc, tremolite and anthophyllite.
Upper Mafic Zone: >300 m of (plagioclase, hornblende and minor biotite) amphibolite.
There are sharp contacts between the three bands, which have also been tectonically disrupted.
At least 11 discontinuous chromitite bodies have been delineated, concentrated in the south-eastern part of the complex. These have been subjected to severe ductile deformation to produce a variety of shapes from planar to lensoid and variably folded forms.
The main chromitite layer is always a thick (3 to 30 m thick, averaging 12 m), massive chromitite at the base of the Ultramafic Zone, directly above the top of the sharp contact with the Lower Mafic Zone. Continuity is good, persisting to depths of at least 300 m in sub-vertical bodies, with lenses traced laterally for up to 250 m. Several laminae and layers up to 4 to 5 m thick are found higher in the Ultramafic Zone.
The chromitites are generally massive, with more than 60% of 0.1 to 3 mm (averaging 0.2 mm) diameter cumulus chromite in a matrix of serpentine, chlorite and tremolite.
Mining has been confined to the weathered zone, with three types of ore, namely, lateritic, friable and lumpy.
The complex contains an estimated 8.8 Mt @ 34% Cr2O3 (Spier and Ferreira, 2001).
For more detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2001.
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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Spier C A, Filho C F F 2001 - The Chromite deposits of the Bacuri mafic-ultramafic layered complex, Guyana Shield, Amapa State, Brazil: in Econ. Geol. v96 pp 817-835
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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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