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Limoeiro |
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Pernambuco, Brazil |
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Ni Cu Pt Pd
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Super Porphyry Cu and Au
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The Limoeiro low grade nickel-copper-PGE deposit is located near the town of Limoeiro in the state of Pernambuco in north-eastern Brazil, ~75 km NW of Recife (#Location; 7° 44' 28"S, 35° 24' 24"W).
The deposit lies within a high grade mobile belt, the Borborema Province, a branch of the Neoproterozoic Brasiliano orogenic system. Within a pre-Gondwana rift reconstruction, the province is interpreted to continue into central-west Africa through the Pan-African belts of Nigeria and Cameroon. It lies between the Amazon-West Africa craton to the north, and the São Francisco-Congo craton to the south. The Borborema province is characterised by an abundance of significant shear zones that juxtapose different Achaean, Palaeo- and Mesoproterozoic terranes during the amalgamation of the Gondwana Supercontinent in the late Neoproterozoic (650 to 500 Ma).
The Limoeiro deposit lies within the Rio Capibaribe Terrane (Santos and Medeiros, 1999; Brito Neves et al., 2000) of the Borborema Province. This east-west oriented, wedge shaped, fault bounded terrane, essentially comprises a basement of orthogneisses of the Salgadinho complex, and paragneisses with amphibolites (metavolcanic rocks) of the Vertentes complex. The latter, which is intruded by the 1718±20 Ma Passira Anorthositic Complex, partly underlies an allochthonous thrust-klippe composed of metapelite and meta-calcareous rocks of the younger Neoproterozoic Surubim Complex which contains detrital zircons as young as 640 to 620 Ma (Neves et al., 2009). The best age constraint for the Rio Capibaribe Terrane is an U-Pb age of 1.97 Ga from prismatic zircons of the Salgadinho complex (Sá et al., 2002). Neves et al. (2006) summarised the Rio Capibaribe Terrane geologic evolution as follows: i). juvenile magmatism from 2.15 to 2.10 Ga; ii). Transamazonian peak metamorphism from 2.05 to 2.03 Ga; iii). late orogenic magmatism from 1.99 to 1.97 Ga; iv). deposition of supracrustal sequences; v). Brasiliano-Pan African cycle syntectonic granite intrusion from 650 to 500 Ma (Santos and Medeiros, 1999); and vi). high-gradeBrasiliano metamorphism from 630 to 610 Ma.
The ultramafic rocks that host the Limoeiro deposit were intruded into a sequence of sulphide-bearing paragneiss of the Vertentes Complex (e.g., Santos et al., 2010), which is mainly composed of quartz-feldspathic schist and gneiss, with minor amphibolite (plagioclase and hornblende, in contrast to the monomineralic amphibolite rimming the mineralised ultramafic intrusion) and sulphide-bearing calc-silicate rocks. The most common country rocks are medium-grained, interlayered K feldspar-sillimanite-quartz bearing biotite schist and biotite-garnet-quartz-K feldspar-bearing gneiss containing from <1 to 2 vol.% sulphides (pyrite and pyrrhotite). These rocks are with textures varying progressively from lepidoblastic in biotite schist to granoblastic in garnet gneiss, with frequent irregular coarse-grained migmatitic granite gneiss bands.
The Limoeiro deposit is hosted by an intrusion with a tub-like (chonolithic) form that has been interpreted to represent a magma conduit cutting a high grade paragneiss and schist sequence. The absolute age of the intrusion has not been determined, although regional geological constraints suggest it belongs to a terrane older than 650 to 500 Ma. During the Brasiliano orogeny, the intrusion and country rock were metamorphosed to upper amphibolite to granulite facies at a temperature of between 700 and 850°C (Mota-e-Silva et al., 2015). The ultramafic rocks have also been affected by subsequent low temperature hydrothermal alteration (Mota-e-Silva et al., 2013).
The ultramafic intrusion has a combined length in excess of >4 km, thickness of 150 to 200 m and width of 350 to 800 m. It trends roughly east-west, plunges very shallowly to the west, and is exposed over an area of ~800 m diameter in the east with two areas of gossan after sulphides. Several steep north-south and NW-SE ductile-brittle normal faults crosscut the intrusion and divide it into four main domains, from west to east of the Parnazo, Retiro, Piçarra and Bofe (Mota-e-Silva et al., 2013).
It is composed of two main distinct sequences, an upper and lower suite. Each comprises a core of harzburgite (olivine-orthopyroxene ±chromititic cumulates), enveloped by orthopyroxenite (orthopyroxene-olivine±chromitic cumulates) with an irregular and discontinuous thin (from 0 to ~10 metres thick) outer shell of amphibolite. The amphibolite rim has a metamorphic origin, as evidenced by its granoblastic texture consisting of equigranular medium-grained amphibole crystals with polygonal contacts (Mota-e-Silva et al., 2015).
Where preserved, the ultramafic rocks are coarse-grained, with up to 4 cm orthopyroxene and 2 cm olivine crystals, commonly comprising dark-coloured aggregates of olivine crystals and irregular whitish patches of orthopyroxene and amphibole. These coarse-grained olivine and orthopyroxene crystals are usually partially replaced by medium- to fine-grained aggregates of metamorphic minerals. Olivine commonly occurs as anhedral crystals with corroded borders included in larger orthopyroxene crystals, suggesting that orthopyroxene grew through olivine reaction with the magma. Chromite is a cumulus accessory mineral, usually ranging from 1 to 4 but up to 6 vol.%, in orthopyroxenite and harzburgite of both Lower and Upper suites. Chromite occurs mainly as fine-grained (5 to 200 µm) euhedral crystals included in orthopyroxene or olivine (Mota-e-Silva et al., 2013).
The intrusion has been largely metamorphosed to an assemblage of anthophyllite, hornblende, phlogopite, chlorite, AI-Fe-Mg-Cr spinel, coexisting with deformed and partially recystalised relicts of larger orthopyroxene and olivine crystals. The ultramafic rocks are also affected by a later low temperature hydrothermal alteration, which is characterised by antigorite with magnetite, talc, chlorite and calcite (Mota-e-Silva et al., 2013).
Although the two ultramafic suites have a similar composition and structure, they have distinctly different sulphur contents as a result of the presence of disseminated Ni-Cu sulphide mineralisation which is restricted to the upper suite. These suites are believed to have been the result of injection of two separate pulses of magma with differing sulphide saturation histories. Geological and geochemical data suggest the direction of magma flux was from west to east (Mota-e-Silva et al., 2013).
On a broad scale, the mineralisation preserves its primary magmatic geometry, comprising thick (up to 150 m) and elongated (up 1 km) bodies of disseminated sulphides (2 to
10 vol.%), broadly concordant with the chonolithic structure (Mota-e-Silva et al., 2013).
The majority of the mineralisation in all four orebodies comprises thick (up to 150 m) and elongated (up to 1 km) bodies containing disseminated sulphides that are broadly concordant with the chonolithic structure. Three different types of ore occur in the deposit (after Mota-e-Silva et al., 2013):
• Disseminated, which predominates in all orebodies and represents about 97 vol % of the Limoeiro deposit. It consists of between 2 and 20 vol.% interstitial sulphides and/or sulphide blebs, hosted by orthopyroxenite and harzburgite<;br>
• Massive, which only occurs in the Retiro and Piçarra domains and represents about 3 vol.% of the total deposit. It contains >60 vol.% sulphide in a single thin (usually <1 m thick) but continuous layer located in the basal zone of the orebody in the Retiro, and scattered, multiple, and less continuous layers in the Piçarra orebody;
• Tectonically remobilised stringers and massive sulphide bodies, which occur in tectonically disturbed portions of the deposit, especially in the Piçarra domain. This type of ore is characterised by crosscutting sharp contacts with host harzburgite or orthopyroxenite, or by pervasive tectonic foliation concordant with highly transformed foliated ultramafic rocks.
The deposit has estimated resources of 35 Mt @ 0.25% Ni, 0.27% Cu, 0.40 g/t Pd, 0.16 g/t Pt (Votorantim Metals unpublished - quoted by Mota-e-Silva et al., 2015).
The most recent source geological information used to prepare this decription was dated: 2015.
Record last updated: 25/11/2015
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.
Limoeiro
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Selected References:
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Mota-e-Silva, J, Ferreira Filho, C.F. and Della Giustina, M.E.S., 2013 - The Limoeiro Deposit: Ni-Cu-PGE Sulfide Mineralization Hosted Within an Ultramafic Tubular Magma Conduit in the Borborema Province, Northeastern Brazil : in Econ. Geol. v. 108, pp. 1753-1771
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Mota-e-Silva, J,. Prichard, H.M, Ferreira Filho, C.F., Fisher, P. C. and McDonald, I., 2015 - Platinum-group minerals in the Limoeiro Ni-Cu-(PGE) sulfide deposit, Brazil: the effect of magmatic and upper amphibolite to granulite metamorphic processes on PGM formation: in Mineralium Deposita v.50, pp. 1007-1029
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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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