Summary:
According to the most accepted petrologic-metallogenetic classification of pegmatites (Cerný, 1991; Cerný & Ercit, 2005; Cerný et al., 2012), all the spodumene-rich pegmatites found within the Bandeira deposit, as well as in the whole Cachoeira Pegmatite Group, belong to the rare element class, lithium subclass, and albite-spodumene type.
Although generally included in the LCT (Lithium-Cesium-Tantalum) family, the non- to poorly zoned spodumene-rich pegmatites (SRP) found in the Bandeira deposit, as well as all the orebodies mined in CBL’s Cachoeira Mine since the 1990’s, the Xuxa and other spodumene-rich deposits of Sigma Lithium (Sá, 1977; Delboni et al., 2023), and the Outro Lado deposit of Lithium Ionic, are rather poor both in Ta and Cs when compared with the complex zoned LCT pegmatites.
Geological Setting and Mineralization
The Bandeira Project lies in the Middle Jequitinhonha River valley, northeastern Minas Gerais—the Lithium Valley. The region is part of the Eastern Brazilian Pegmatite Province (EBPP), one of the largest pegmatite provinces in the world, with about 150,000 km2 (Pedrosa-Soares et al., 2011, 2023). The EBPP resulted from the magmatic and tectono-metamorphic events that formed the Araçuaí Orogen from the Early Ediacaran (ca. 630 Ma) to the Late Cambrian (ca. 490 Ma). The major EBPP pegmatite populations found within the Araçuaí Orogen have been grouped into twelve pegmatite districts that include residual pegmatites (representing late silicate melts released by fractional crystallization of parent granites) or anatectic pegmatites (formed directly from partial melting of country rocks). Among them, the Araçuaí Pegmatite District includes hundreds of residual pegmatites of distinct subclasses, types, and sub-types of the rare-element class (B, Be, Cs, Li, Sn, Ta). They comprise two main groups of rare-element pegmatites:
• The generally thick (up to 100 m), zoned, complex lithium–cesium–tantalum (LCT) pegmatites with several lithium minerals (e.g., elbaite, lepidolite, Li-phosphates, petalite, or spodumene) and other rare-element minerals (e.g., beryl, Bi-minerals, cassiterite, pollucite, schorlite, Ta-minerals), displaying roughly concentric to irregularly shaped primary zones (marginal, graphic or wall, and intermediate zones, and quartz cores) cut by albite-bearing replacement bodies and fracture fillings with gem-bearing pockets.
• The relatively thinner, non-zoned to poorly zoned, spodumene-rich pegmatites (SRP) with rather simple mineralogical assemblages that include spodumene (up to 35 vol%), albite, perthite, quartz, and muscovite (together forming up to 90 vol%–95 vol%), and accessory minerals, such as cookeite, Li- phosphates, petalite, cassiterite, Nb-Ta oxides, graphite, Fe-Mn oxides, and zabuyelita.
Both LCT pegmatites and SRP bodies commonly show unidirectional solidification textures outlined by minerals (e.g., mica, spodumene, tourmaline) oriented roughly orthogonal to the contacts with the host rocks (or to any other lower-temperature surface inside the pegmatite, such as host rock xenoliths). The rare-element pegmatites of the Araçuaí District are related to granitic intrusions, mostly composed of peraluminous (S-type), sub-alkaline to alkaline, muscovite-bearing leucogranites with pegmatoid cupolas, of the Cambrian (535–500 Ma) post-collisional (post-tectonic) G4 supersuite of the Araçuaí Orogen.
The Itinga Pegmatite Field, in the central part of the Araçuaí Pegmatite District, contains the most important discoveries of lithium deposits in Brazil since the 1950s, both in terms of economic resources and geological potential. As with other lithium-rich pegmatite populations worldwide, the favourable geological conditions for the outstanding abundance of both SRP and LCT pegmatites in the Itinga Field are due to:
• The relatively low-pressure and high-temperature regimes of the regional and contact metamorphisms, recorded by the dominant country rocks (quartz-mica schists with andalusite or cordierite or sillimanite).
• The profusion of two-mica granite intrusions with pegmatoid cupolas emplaced in relatively shallow crustal levels.
The Itinga Pegmatite Field includes the spodumene mines and deposits of CBL and Sigma Lithium, as well as MGLIT´s properties of its Bandeira Project. The lithium ore bodies exploited since the early 1990s in CBL’s underground mine display a closely spaced swarm of relatively narrow (6 m thick on average) but long (up to 700 m along strike) non-zoned SRPs. In the Sigma Lithium properties, where several large SRPs are found (e.g., Barreiro, Murial, and Xuxa), an open pit mine is being developed on the Xuxa SRP deposit (15 m thick x 1,800 m long x 500 m). Regardless of their sizes, most pegmatites of the Itinga Field are (sub-)parallel to the prominent northeast–southwest structural trend outlined by the regional ductile foliation (the schistosity S1: NE strike/NW dip) and late-spaced cleavage (S2: NE strike/SE dip). However, flat-lying, or high-angle dip joint systems can also host some lithium-mineralized pegmatites.
Also following the regional northeast–southwest structural trend, the Bandeira deposit comprises northeast-striking swarms of SRPs, including concordant SRP bodies, hosted by the north-westdipping schistosity (S1), and discordant SRP bodies, emplaced along a southeast-dipping fracture system (the S2 spaced cleavage), as well as a few mineralized pegmatites hosted by late, flat-lying joints. They show sharp contacts with a cordierite–quartz–mica schist that may be enriched in decussate micas, tourmaline, and cordierite porphyroblasts, recrystallized along narrow (cm to dm) fringes of contact metamorphism which may also be anomalous in lithium content. 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 along strike, ranging in thickness from a few decameters to decimeters, with the discordant SRP bodies tending to be thicker than the concordant ones. With known downdip-width up to 800 m, several Bandeira SRP bodies remain open in depth. The Bandeira ore bodies show a rather simple mineralogical assemblage, consisting of medium (3 cm–10 cm) to very coarse-grained (>30 cm) spodumene crystals (up to 30 vol%) within a fine- to medium-grained matrix composed of albite, perthitic microcline, quartz, and muscovite, with generally scarce (<5 vol% in total) accessory (montebrasite, Nb-Sn-Ta oxides) and secondary minerals (cookeite, sericite, zabuyelita, Fe-Mn oxides, clay minerals). Petalite has been found in some drill cores and thin sections, mostly occurring in the SRP matrix as very fine- to fine-grained (sub-millimetre to 1 cm) crystals and, more rarely, in coarse-grained crystals locally found in a few core intervals. The thicker SRP bodies generally show a barren external zone rich in albite (which can be rather discontinuous), followed inwards by an internal zone rich in disseminated spodumene (although spodumene may also be more concentrated in some domains than others along the internal zone). The thinner SRP bodies generally lack the external lithium-barren zone, showing disseminated spodumene along the whole ore body. Unidirectional solidification textures outlined by tabular to telescope-shaped spodumene crystals are common in the Bandeira SRP ore bodies. Thin albite-rich pegmatites, barren to poor in lithium, are also found in the Bandeira SRP swarms. The exploration drilling work revealed two main SRP swarms in the Bandeira deposit: the northern swarm, with thicker and longer SRP bodies; and the southern swarm, with smaller SRP bodies.