Summary:
The Bandeira Project lies in the Eastern Brazilian Pegmatite Province (EBPP), in terranes of the Araçuaí Orogen. The EBPP, one of the largest spegmatitic populations in the world, with about 150,000 km2, containing 12 pegmatite districts in eastern Minas Gerais (ca. 90% of the whole province), southeastern Bahia, and Espírito Santo States of Brazil.
Deposit Types
Two Lithium deposits comprise the Araçuaí Pegmatite District:
• The spodumene-rich deposits of the Araçuaí Pegmatite District include the non-zoned to poorly zoned SRPs and spodumene-petalite pegmatite, and the simple-zoned perthitespodumene pegmatite (Pedrosa-Soares et al., 2025). Among them, the SRP deposits are by far the most important in terms of resource volume, lithium grade, high lithium recovery, and low mining and processing complexities, due to their relatively simple structure and mineralogy, simple and little variable chemistry, and large quantities of spodumene widely disseminated throughout the ore bodies. The proof-of-concept for this are CBL’s spodumene mine, producing spodumene through an underground mine since 1992; Sigma’s mine, producing spodumene from an open-pit mine since 2023, and all the recently discovered significant spodumene deposits in Brazil, such as MGLIT’s Bandeira, Baixa Grande, and Outro Lado SRP deposits. Therefore, the SRP deposit model has proven to be highly efficient in successful knowledge-based exploration of spodumene on formerly unknown to poorly known targets, leading to the discovery of large spodumene deposits by the use of geological mapping, soil geochemistry, trenching, and continuous core drilling;
• The complex-zoned LCT pegmatites with diversified mineralogy, typically displaying a roughly concentric zoning with marginal (granitic), wall (graphic), and intermediate (blocky K-feldspar) zones, and one or more quartz cores, with very variable amounts of secondary units (e.g., fracture fillings, albite-rich replacement bodies, miarolitic cavities, gem-bearing pockets). Despite their relatively large dimensions and recurrent gemstone (mostly colored tourmaline), cassiterite, tantalite, and industrial minerals production, such complex LCT pegmatites may contain little spodumene in internal zones, quartz cores and/or albite-rich units, although none of them has ever been found to be relevant targets for exploration of spodumene in economic terms (Paes et al., 2016; Pedrosa-Soares et al., 2011, 2025). Therefore, the classical model for complex zoned LCT pegmatites (e.g., Cerný, 1991; Cerný et al., 2012) must be discarded if spodumene is indeed the lithium ore mineral sought in exploration work in the Araçuaí Pegmatite District, although this model can be applied to exploration for lithium-mica (e.g., lepidolite), lithium-phosphate (e.g., montebrasite), and/or petalite.
Bandeira’s Mineralization Model
A soil geochemistry campaign revealed lithium anomalies roughly parallel to CBL’s Cachoeira pegmatite swarm. This, together with lithological and structural data from outcrops, old diggings, and new exploration trenches, provided the basis for a very successful drilling campaign that discovered a new swarm of spodumene-rich pegmatites (SRP) extending from near surface up to 500 m deep.
Following the regional NE–SW structural trend, the Bandeira deposit comprises SRP swarms of NEstriking ore bodies mostly hosted by and concordant with the NW-dipping schistosity (S1), but also some discordant SRP bodies emplaced along the SE-dipping S2 spaced cleavage, as well as a few SRP bodies hosted by late, flat-lying joints. The Bandeira pegmatites are tabular bodies with convex lens-shaped terminations, arranged in tight and staggered (en-echellon) swarms, locally with branched connections linking ore bodies. Single SRP bodies normally reach hundreds of metres long along the strike, ranging from a few decametres to decimetres thick, with the discordant SRP bodies tending to be thicker than the concordant ones. With known down-dip width up to 800 m. The exploration drilling work revealed two main sectors in the SRP swarm of the Bandeira deposit: i) the northern sector, with thicker, longer, and wider SRP bodies concordant with the S1 foliation of host rocks; and ii) the southern sector, with somewhat smaller SRP bodies.
The host rocks of SRP ore bodies in the Bandeira deposit are banded to laminated cordierite-quartzmica schists, locally containing disseminated sulfide and/or graphite-rich bands, with intercalations of massive calcsilicate rock. Most cordierite forms ellipsoidal (eggshaped) stretched poikiloblasts syn-kinematic to the regional S1 schistosity. The banded to laminated quartz-mica schists represent metamorphosed sand-mud sediments, and the calcsilicate rocks are metamorphosed calcium-rich carbonate-mud sediments (marls). They show sharp contacts with the SRP ore bodies that generally are concordant to the regional S1 foliation (often parallel to the compositional layering S0), but are also hosted by the S2 cleavage or foliation. The host schists may be enriched in decussate muscovite and/or biotite, black to green tourmaline, and recrystallized cordierite along narrow (cm to dm) fringes of contact metamorphism imposed by pegmatites. Although the host schists may be anomalous in lithium content close to pegmatites, they show no lithium-ore mineral.
The scarce accessory minerals (mainly montebrasite, and Nb-Sn-Ta oxides) and secondary minerals (cookeite, sericite, zabuyelite, Fe-Mn oxides, clay minerals) generally comprise less than 5 vol%. In drill cores, the spodumene crystals are mostly free of hydrothermal and weathering alterations and very poor in mineral inclusions. Conversely, surface outcrops, shallow diggings and exploration trenches cutting SRP bodies generally show weathered spodumene, forming pseudomorphs composed of white clay (kaolinite and montmorillonite). Rare spodumene-quartz intergrowth (SQUI) may be found associated with spodumene crystals. Petalite has been found in SRP's drill cores and thin sections, mostly occurring in the matrix as very fine- to fine-grained (sub-millimetre to 1 cm) crystals and, more rarely, as coarser crystals locally found in rather restricted intervals.
The thicker SRP bodies may show a lithium-barren and thin marginal zone rich in albite, generally rather discontinuous, followed inwards by a thick internal zone rich in disseminated spodumene (although spodumene may also be more concentrated in some domains than others along the internal zone). Owing to the upward migration of water-rich fluids, flat-lying SRP sections close to the hangingwall contact, as well as the top termination (head) of high-angle dip bodies, may show metasomatic units with miarolitic cavities that partially replaced the primary mineral assemblage. Many SRP bodies lack the external lithium-barren zone, showing disseminated spodumene along virtually the whole ore body. Unidirectional solidification textures outlined by tabular to telescope-shaped spodumene crystals are common in Bandeira’s SRP ore bodies. Thin albite-rich pegmatites, barren to poor in lithium, are also found in the Bandeira pegmatite swarms.
In summary, the mineralization model for the Bandeira deposit consists of a dense swarm of non-zoned spodumene-rich pegmatite dikes and sparse albite-rich/spodumene-barren pegmatite dikes. The Bandeira SRP dykes show a simple mineralogy, with spodumene, albite, perthitic microcline (perthitic K-feldspar), and quartz totalling up to 95 vol%, and rather scarce accessory minerals (Al-Li mica, lithium-phosphates, petalite, zabuyelite, and others). These spodumene-rich pegmatites can generally be correlated with undeformed and non-zoned albite-spodumene pegmatites Cerný 1991, Cerný et al., 2012).