INSIGHT INTO THE FORMATION OF SPODUMENE-RICH PEGMATITES: THE ARAÇUAÍ PEGMATITE DISTRICT, BRAZIL

Spodumene-rich pegmatites are among the most important sources of lithium, but the processes controlling their formation remain debated. In particular, the origin of Li-enriched pegmatitic melts may involve extreme fractional crystallization of granitic magmas, direct crustal anatexis, source pre-enrichment, or multi-stage enrichment processes. Once emplaced, this primary record may be further modified by internal pegmatite differentiation, country-rock interaction, and late- to post-magmatic pro-cesses. This thesis investigates these issues through the study of the Outro Lado spod-umene-rich pegmatite and associated Li-barren pegmatites and two-mica granite from the Itinga Pegmatite Field, Araçuaí Pegmatite District, Minas Gerais, Brazil. The study combines field and drill-core observations, petrography, major- and trace-element mineral chemistry, and U-Pb geochronology of apatite, cassiterite, monazite and zircon. The sampling strategy was designed to compare mineralized and non-mineralized pegmatitic bodies within the same geological setting, with particular fo-cus on the mineralogical expression of Li enrichment, internal variability of the spod-umene-rich pegmatite, and its chronological relationship with local granitic magma-tism. The Outro Lado SRP is simply zoned and consists of a volumetrically limited, spodu-mene-free border zone and a volumetrically dominant internal zone characterized by a relatively stable spodumene-bearing assemblage. Spodumene is the main Li-bearing mineral, whereas petalite and lepidolite are rare or accessory. Together with the lim-ited development of F-rich Li minerals in the associated Li-barren pegmatites and two-mica granite, this assemblage indicates a generally F-poor pegmatite-granite associa-tion, in which the SRP represents the spodumene-dominated Li-mineralized end mem-ber. Although the studied lithologies share an overall LCT affinity, muscovite chemis-try distinguishes the spodumene-rich pegmatite from Li-barren pegmatites and the two-mica granite, with SRP muscovite enriched in Cs, B, Sn, Rb and locally Li, and depleted in Ti, Zr and Mg. Internal muscovite variability does not define a continuous margin-to-centre differentiation trend. Instead, the strongest deviations are concentrat-ed in samples and local domains in contact, or proximal, with country-rock schist. The SRP-schist contact records early bidirectional metasomatism: tourmalinization of the schist indicates outward transfer of pegmatitic components, whereas Ba-Mg-Sr-Eu-Na2O-Pb enrichment in border-zone SRP muscovite indicates inward transfer of com-ponents from the country rock. This exchange occurred during emplacement or before the onset of border-zone crystallization, suggesting very early fluid saturation and/or fluid-assisted diffusion at the pegmatite-country rock interface. U-Pb geochronology defines two main chronological stages. Monazite and zircon from the Teixeirinha two-mica granite yield overlapping crystallization ages of 527 ± 9 Ma and 526 ± 11 Ma, respectively, whereas monazite from one Li-barren pegmatite gives an indistinguishable age of 524 ± 10 Ma. Cassiterite from the spodumene-rich pegma-tite yields a younger age of 496 ± 13 Ma, interpreted as recording crystallization of the Li-mineralized body. Apatite ages across the studied lithologies define a coherent age population with a weighted mean of 487 ± 11 Ma, interpreted as cooling in the granite and at least part of the Li-barren pegmatite system, and as crystallization in the spod-umene-rich pegmatite. These results indicate that Li mineralization in the studied system is associated with a younger cassiterite-apatite age interval of 476-509 Ma, interpreted as the uncertainty envelope of SRP crystallization and closely related thermal evolution, rather than with the older monazite-zircon crystallization interval of 514-537 Ma recorded by the dated two-mica granite and one Li-barren pegmatite. This age offset makes the dated Teixeirinha Granite unlikely to represent the immediate parental source of the SRP-forming melt. At the same time, the shared LCT affinity of the studied pegmatite-granite association indicates that the older Teixeirinha Granite and Li-barren pegma-tite represent an LCT-affiliated but non-mineralized stage, rather than a chemically unrelated system. Petrographic and mineral-chemical constraints further indicate that the studied Li de-posit likely formed from a melt already sufficiently enriched in Li to saturate spodu-mene, rather than through strong internal differentiation within the exposed pegmatite body. The source of this Li-rich melt remains unresolved, but the timing of SRP crys-tallization places Li mineralization within a late post-collisional magmatic-thermal in-terval during which the Araçuaí-West Congo Orogen was still cooling through inter-mediate-temperature conditions of ~425-550 °C and was affected by widespread re-gional hydrothermal circulation. In this framework, the data are compatible with a younger Li-enriched melt derived either from an unexposed evolved granite or from renewed melting/reworking of previously Li-enriched crustal material, potentially linked to the older G4 granitic-pegmatitic stage.