Grota do Cirilo - Xuxa, Barreiro, Murial, Lavra do Meio

Minas Gerais, Brazil

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The Grota do Cirilo lithium-cesium-tantalum pegmatite deposit cluster is located in the municipalities of Araçuaí and Itinga, in Minas Gerais State, Brazil, ~25 km NE of the town of Araçuaí and 450 km NE of Belo Horizonte (#Location: Xuxa - 16° 43' 55"S, 41° 53' 17"W).

These deposits lie within the Eastern Brazilian Pegmatite Province that covers an area of ~150 000 km2, stretching from the state of Bahia, through Minas Gerais to Rio de Janeiro state. They are hosted within the crystalline core of the Araçuaí Fold Belt, associated with one of the five granitic supersuites (G1 to G5) of the north-south oriented Rio Doce Magmatic Arc that is associated with that core. The core and arc are at the centre of the Araçuaí rift basin which opened between the São Francisco and Congo cratons in the mid Neoproterozoic and was then inverted. The Rio Doce volcano-sedimentary arc sequence was developed during the inversion and is accompanied by the coeval 630 to 580 Ma plutonic G1 supersuite. The metamorphosed volcano-sedimentary sequence comprises pelitic schists to paragneisses, interbedded with proximal dacitic to rhyolitic meta-volcanic and explosive meta-volcaniclastic rocks dated at 595 ±13 and 584 ±5 Ma (zircon U-Pb); followed by psammitic to pelitic meta-turbidites with lenses of calc-silicate rocks, with a maximum depositional age of ~594 Ma and an arc-related lithochemical signature. These are overlain by predominantly quartz sandstones and micaceous to feldspathic sandstones, with a maximum sedimentation age of ~620 Ma, representing a fluvial to continental shelf setting (Novo et al., 2018). The G1 Supersuite is a calc-alkaline, magnesian, I-type pre-collisional rock-assemblage, mostly composed of tonalite to granodiorite. The Rio Doce Magmatic Arc is succeeded by and/or overlaps another 4 granitoid supersuites as follows: the most voluminous G2 Supersuite, a series of batholiths and stocks of syn- to late-collisional S-type peraluminous granites intruded between 590 and 540 Ma to the east and north of the Rio Doce Magmatic Arc. They are mostly biotite-garnet syenogranite to alkali-feldspar granite with minor monzogranite to tonalite, rich in garnet, and garnet-two-mica granite, locally with sillimanite; G3 Supersuite, late- to post-collisional, 545-500 Ma alkali feldspar granite to syenogranite with cordierite and/or garnet, poor to free of biotite, occurring as undeformed dykes and small stocks, unaffected by the regional foliation, mostly cutting migmatites and G2; G4 Supersuite, are more restricted in Minas Gerais, being confined to a narrow zone to the north and NW of the northern extremity of the Rio Doce Magmatic Arc. They are characterised by post-collisional granites emplaced between 530 and 500 Ma, mainly leucogranites to two-mica granites, generally with garnet, pegmatoid granite and minor biotite granite occurring as balloon shaped plutons, with minor sill like bodies, generally free of any regional deformation fabric; G5 Supersuite, 520 to 480 Ma high-K metaluminous A- and I-type granitoids, mainly alkali feldspar granite to granodiorite and corresponding orthopyroxene-bearing (charnockitic) rocks, with minor enderbite and gabbronorite. Rich in magma mingling textures with mafic to intermediate enclaves. They area generally co-extensive with G2, extending further to the east, although individual batholiths are smaller (Soares et al., 2020; Pedrosa-Soares et al., 2011).

The Grota do Cirilo pegmatite swarm is interpreted to be related to the G4 supersuite, in particular, the Piaui batholith (Soares et al., 2009). The pegmatite bodies are typically hosted in a grey biotite-quartz schist and form bodies that are generally concordant with the schist foliation but can also cross-cut foliation. The country rock is a medium-grey biotite-quartz schist, interpreted to be a metamorphosed flysch of the Eocambrian Salinas Formation (Quéméneur and Lagache, 1999). It typically has millimetre to centimetre-sized cordierite porphyroblasts and finely disseminated, stretched, iron-sulphide crystals with a preferred orientation sub-parallel to foliation. Minor intercalations of calcsilicate rocks are locally evident. Where weathered, the schist contains sericite-rich zones and micro-crystalline quartz-calcite intercalations that include dark green, disseminated, sub- to millimetre-sized amphibole and pink garnet crystals.

Two types of pegmatite swarm are found within the Neoproterozoic Araçuaí Fold Belt: i). anatectic, which are directly formed by the partial melting of the country rock, and ii). residual, which are fluid rich final silicate melts resulting from the fractional crystallisation of a parent magma. The pegmatites in the Grota do Cirilo district are interpreted to be residual pegmatites and are further classified as lithium-cesium-tantalum types.

The pegmatite dykes of the Araçuaí and Itinga district are sub-horizontal to shallow-dipping sheeted tabular bodies, with widths, thicknesses and lengths that vary widely, generally ranging from a few metres up to 40 m or more in thickness. They typically have sharp contacts with the enclosing country rock and have discontinuous, thin, fine-grained chilled margins, without concentric zoning around a quartz core (e.g. Simmons et al., 2003). Instead, they display a characteristic layered anisotropic internal fabric (London, 1992). In detail, the schist-pegmatite contact zone exhibits recrystallisation features such as biotite eyes within cordierite masses, and the development of millimetre-sized, black tourmaline needles that are almost always perpendicular to the schist foliation. Spodumene commonly comprises about 28 to 30% of the dyke, microcline and albite ~30 to 35%, with microcline content dominant over albite, and white micas ~5 to 7%, with the remainder of the rock mass being quartz. The spodumene is pale green, with crystals that are elongate or tabular, ranging from millimetre to centimetres in scale, up to 10 to 20 cm in length, but are locally occassionally metre-scale in outcrop. Spodumene cuts the microcline matrix, with intergrowths of spodumene and quartz, sometimes in association with muscovite, being common. Tantalite, columbite and cassiterite can occur in association with albite and quartz. The primary lithium-bearing minerals are spodumene and petalite. Spodumene can contain as much as 3.73% Li, equivalent to 8.03% Li
2O, whereas petalite, can contain as much as 2.09% lithium, equivalent to 4.50% Li2O. Late-stage mineralisation includes sphalerite and pyrite.

The pegmatite deposits of the cluster for which mineral resources have been estimated, include the following:
Xuxa - hosted within biotite-quartz schist with a well-developed crenulation cleavage. Schist xenoliths ranging from a few centimetres to a metre across occur within the pegmatite. The pegmatite/schist contact is frequently hornfelsed. The pegmatite is concordant to the regional foliation, striking NE-SW with a dip of 45 to 55°SE, and a strike length of 1700 m, averaging 12 to 13 m, but locally as much as 20 m in thickness. It persists to a vertical depth of at least 260 m, but remains open in all directions. The pegmatite comprises ~20% spodumene, 40 to 50% microcline and albite, ~30% quartz and 5% muscovite. Spodumene occurs as pale green to colourless, elongated, tabular, crystals that can range in size from a millimetre to as much as 80 cm in length and be as wide as 10 cm. The spodumene laths are set in a medium- to very coarse-grained groundmass of colourless albite, translucent quartz and pale grey perthitic microcline. Pale yellow-green medium- to coarse-grained muscovite may also be present. Poikilitic spodumene and quartz textures are common. Tantalite-columbite and cassiterite can occur in association with albite.

Barreiro - 5.5 km to the south of Xuxa, also intruding biotite-quartz schist. The schist contains pale greenish-grey coloured, microcrystalline quartz-feldspar intercalations up to several centimetres across, with disseminated green, sub- to one-millimetre-sized amphibole and pink garnet crystals. Schist xenoliths occur within 3 m of the dyke edge and can range from a centimetre to as much as a metre across. The pegmatite dyke strikes NE-SW and dips 30 to 35°SE. It is at least ~600 m long and 800 m down dip, with an average thickness of 30 to 35 m. It is open to the NE and at depth, having been tested to a depth of 374 m. It is apparently discordant to the host crenulated biotite schist as seen at surface, but at depth, can be concordant. The dyke is weakly zoned into distinct spodumene-rich and albite-rich domains and is divided into a marginal and a central zone. Spodumene, overall, comprises ~20 to 24%, while albite-microcline is ~32 to 40%, with ~10 to 18% muscovite. The marginal zone is ~45 cm thick and comprises fine-grained albite, quartz and muscovite, with heavy minerals, such as cassiterite and tantalite occurring in association with albite rich intervals. The central zone is spodumene-rich and comprises albite and spodumene crystals that are typically 10 to 25 cm in length, but can rarely be as much as a metre in length. Spodumene crystals are also present as short, prismatic, elongated laths. The spodumene laths are colourless or pale green, sometimes displaying a poikilitic texture of fine- to medium-grained quartz and/or pale green sericite. Petalite occurs sporadically, as both colourless, translucent to transparent, coarse to very coarse-grained crystalline aggregates. It can also be present as cryptocrystalline, translucent masses.

Murial - ~2.5 km ENE of Barreiro, hosted by a similar schist. The pegmatite dyke strikes north-south and has varying westerly dips, ranging from 70 to 85° in the south, to 25 to 35° in the north. It is ~750 m long, 200 m down dip, and has an average thickness of 15 to 20 m, but remains open to the north, south and at depth. The southern end of the dyke generally has lower lithium contents, while to the north, the lithium grades increase, and the dyke flattens and becomes more planar. The dyke is zoned into a marginal albite-rich, intermediate spodumene-rich and central spodumene and petalite domain. The fine-grained matrix of the marginal zone can include tantalite and cassiterite mineralisation.

Lavra do Meio - ~1 km SW of Murial, hosted by a schist that is similar, with garnet and tourmaline developed near the pegmatite-schist contact. The pegmatite dyke is concordant with the schist foliation, and strikes north-south, with a dip of 75 to 80°E. It is ~300 m long, 250 m down dip, and has an average thickness of 12 to 15 m, but remains open at depth, with the deepest drill hole intersecting mineralisation to 270 m. The mineralisation within the pegmatite is moderately to highly homogeneous, but is mostly in the centre and deeper sections. The upper and lower contacts are characterised by albite, quartz and mica. In the albite-rich marginal zone, tantalite and cassiterite can occur interstitial to fan-shaped albite lamellae. In the pegmatite core, medium to very coarse-grained laths of typically pale green spodumene and coarse to very coarse-grained, colourless, translucent to transparent, petalite crystal aggregates and cryptocrystalline masses occur and comprise ~20% of the lithium-bearing minerals. Both spodumene and petalite are set within a micro-fractured, medium to coarse-grained matrix composed of quartz, mica, albite and microcline. The micro-fractures are infilled with pyrolusite.

Published Mineral Resource estimates at a 0.5% Li
2O cut-off, are as follows (Ferreira et al., 2021, NI 43-101 Techical Report):
  Measured + Indicated Resource - 17.414 Mt @ 1.55% Li
    Inferred Resource - 3.802 Mt @ 1.58% Li
  Measured + Indicated Resource - 20.485 Mt @ 1.43% Li
    Inferred Resource - 1.909 Mt @ 1.44% Li
  Measured + Indicated Resource - 5.564 Mt @ 1.14% Li
    Inferred Resource - 0.669 Mt @ 1.06% Li
Lavra do Meio
  Measured + Indicated Resource - 2.275 Mt @ 1.09% Li
    Inferred Resource - 0.261 Mt @ 0.87% Li

Sigma Lithium Corporation commenced production of battery-grade lithium concentrate on a pilot scale in 2018, with a planned larger-scale commercial operation with a annual capacity for 0.22 Mt containing 33 000 tonnes of lithium carbonate equivalent (LCE) annually in Phase I, rising to 0.44 Mt containing 65 000 tonnes of LCE in Phase II (Sigma Lithium website, viewed 2022). The pegmatites of the district have previously, between 1957 and the 1980s, been exploited for the production of a, cassiterite/tantalite concentrate with by-products of feldspar and lithium minerals. Mining was focused on near surface, weathered zones, with excavations ranging from 100 to 700 m in length. During the 1980s to 2000, small scale mining of surface exposures produced 6 to 6.5% Li
2O spodumene concentrate and a 3.5 to 4% Li2O petalite concentrate. Artisanal mining continued until Sigma Lithium began an organised exploration program in 2012, leading to the current operation.

This district and deposit scale section of this summary is drawn from: "Ferreira, G.G., Laporte, M.-A., Parent, J., Quinn, J., Rodriguez, P.C., Veliz, P.P. and Wang, J., 2021 - Grota do Cirilo lithium project Araçuaí and Itinga regions, Minas Gerais, Brazil, Phase 2 (Barreiro) update; An NI 43-101 Technical Report prepared by Primero Group Americas Inc., SGS Geological Services, GE21 Consultoria Mineral and Promon Engenharia Ltda. for Sigma Lithium Corporation, 358p."

The most recent source geological information used to prepare this summary was dated: 2021.    
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.

  References & Additional Information

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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