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Tapira |
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Minas Gerais, Brazil |
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P Ti
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The Tapira alkaline-carbonatite complex is located ~35 km south of Araxá and ~300 km west of Belo Horizonte, in the state of Minas Gerais, Brazil.
The Tapira complex is located ~30 km SSE of the similar Barreiro complex (Araxá) on the northeastern margin of the Palaeozoic-Mesozoic Paraná Basin, and has been dated at 69.5±3.5 Ma, i.e., uppermost Cretaceous (K-Ar on biotite). It comprises an elliptical, NNE elongated, plutonic body with a diameter of ~6 km and a surface area of ~33 sq. km, intruded into metasediments of the Brasilia fold belt to the west of the São Francisco craton. The intruded basement comprises a sequence of quartzites and schists of the Mesoproterozoic Canastra Group, a member of the Espinhaço Supergroup that is deposited within an aulacogen (rift basin) to the west of, and lapped onto the São Francisco craton to the east, and was most likely metamorphosed during both the ~1100 Ma Uruaçu and 600 Ma Brasiliano orogenies.
The Tapira complex is a layered intrusion, characterised by a cumulate series with layers ranging from a few mm to several cm in thickness, formed by the amalgamation of several intrusions of dominant plutonic silicate rocks, with less voluminous carbonatites and ultramafic dykes, and composed mainly of alkaline-silicate and silicate-carbonatite rocks and very subordinate carbonatite, overlain by a thick (30 to 200 m) lateritic cover.
The silicate plutonic-rock series consists mainly of bebedourite (an alkaline clinopyroxenite), with subordinate wehrlite and syenite, and rare dunite and melilitolite. The silicate plutonic-rock series occurs as at least two separate bodies of coarse-grained ultramafic rocks, one of which (B1) occupies most of the central portion of the complex and is partially surrounded, on the northern margin, by the second (B2). Coarse- to medium-grained syenites occur as an intrusive body in the northern portion of the complex, and as smaller plugs elsewhere.
The bebedourite phase generally has a medium to coarse-grained to pegmatitic granular texture and consists of subhedral to euhedral green clinopyroxene (diopside) and perovskite, and euhedral apatite, anhedral phlogopite, calcite, ilmenite and rare titaniferous magnetite. Cumulus phases in the dunite are represented by olivine and, occasionally, clinopyroxene (<5%) whereas the intercumulus phases consist mainly of apatite, phlogopite, calcite, ilmenite, variable amounts of perovskite, and occasionally, titaniferous magnetite, with subordinate schorlomitic garnet and sulphides. The wehrlite paragenesis is very similar to that of dunite, except for the amount of subhedral clinopyroxene.
Five episodes of carbonatite intrusion (C1 to C5) were identified. The earliest and largest carbonatite (C1), which includes significant occurrences of phoscoritic rocks, occurs at the centre of the complex, intruding the main body of coarse-grained ultramafic rocks. C1 carbonatites are granular, medium-grained, and vary from homogeneous to locally banded, and are essentially composed of calcite, phlogopite, magnetite and apatite, with subordinate dolomite, sulphides, barite and pyrochlore. The C2 carbonatites occupy an intrusive body spatially associated with syenites, in the northern part of the complex, but occur also as scattered dykes and as minor diatreme-like breccias elsewhere. C2 rocks are usually finer-grained than C1 and consist essentially of carbonate, with accessory magnetite and very small (in most cases negligible) amounts of phlogopite, amphibole, zirconolite, barite and pyrochlore. C3 and C4 are small intrusions, respectively located near the northern and southern margins of the complex, while C5 comprises widespread late-stage dykes and veinlets.
There is a general progression in composition observed from C1 to C4, commencing with dolomite-bearing sövites (C1 and C2) and evolving towards pure sövite (C3 and C4), while dolomite-rich carbonatites (beforsite) are recurrent in the late-stage C5 dykes.
All of the plutonic rock-types are crosscut by fine-grained ultramafic dykes and veins of variable thickness and composition which represent about 15% of the complex. These dykes and veins are carbonate-rich and may contain carbonate ocelli, implying immiscibility between carbonatite and silicate liquids occurred early in the evolution of the complex. Two varieties are recognised, the most primitive of which are phlogopite picrites, and comprise olivine phenocrysts in a groundmass of phlogopite, carbonate, perovskite, oxides and apatite. The second are bebedouritic, typically lacking olivine and Cr-rich spinel, and are compositionally similar to the coarse-grained bebedourites, representing more evolved (although still ultramafic) liquids.
Bebedourite and glimmerite are the dominant rocks in the Tapira complex, mainly composed of Fe3+-rich phlogopite (30 to 50%), with variable amounts of clinopyroxene (>20% in bebedourite and 0 to 20% in glimmerite), and subordinate perovskite, apatite, calcite, ilmenite and titaniferous magnetite.
The unweathered bebedourite (clinopyroxenite) contains >5% P2O5, with substantial resources developed in the lateritic mantle, which averages 30 m in thickness. Five main weathering ore zones of anatase and phosphate are known, based on cut-offs of 15% TiO2 and 5% P2O5.
Reserves in 2006 (DNPM 2006 Mineral Annuary) were 1.309 Gt @ 7.69% P2O5, concentrated to grades of 35.5% P2O5.
This summary was drawn from Brod et al., 2005, Brigatti et al. 1996 and Northolt et al., 1989.
The most recent source geological information used to prepare this decription was dated: 2005.
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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Brigatti M F, Medici L, Saccani E and Vaccaro C, 1996 - Crystal chemistry and petrologic significance of Fe3+-rich phlogopite from the Tapira carbonatite complex, Brazil: in American Mineralogist v.81 pp. 913-927
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Brod J A, Gaspar J C, Diniz-Pinto H S and Junqueira-Brod T C, 2005 - Spinel chemistry and petrogenetic processes in the Tapira alkaline-carbonatite complex, Minas Gerais, Brazil: in Revista Brasileira de Geologia v.35 pp. 23-32
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Carvalho, L.M.M., Pires, A.C.B. and Vidotti, R.M., 2015 - Phosphate prospection in the alkaline province of Alto Do Paranaiba, Brazil, based on airborne geophysical data: in Revista Brasileira de Geofisica (Brazilian Journal of Geophysics), v.33, pp. 237-249.
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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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