Mechanisms of formation of spodumene-rich pegmatites in the Araçuaí Pegmatite District, Minas Gerais, Brazil

Spodumene is the main player in the extraction of lithium from hard rock deposits. In Brazil, the primary lithium deposits are the spodumene-rich pegmatites (SRPs) of the Araçuaí Pegmatite District. These pegmatites contain up to 30% of coarse-grained spodumene crystals dispersed in a quartz-feldspars-muscovite matrix and form rather homogeneous ore bodies. In the study area SRPs are closely spatially associated to large volumes of barren pegmatites and two-mica granites. The relation between barren and mineralized pegmatites as well as the formation mechanism of SRPs in this region and elsewhere remains puzzling. We present U-Pb geochronological data on columbite-tantalite and compositional data for muscovite, apatite and columbite-tantalite. Columbite-tantalite U-Pb analyses from SRP yielded a lower intercept age of 525 ± 16 Ma, overlapping the crystallization age of the two-mica granites (528 ± 13 Ma). The Fe/Mn in columbite-tantalite and muscovite, Th/U of apatite, and the Sn and Zn elemental concentration in muscovite and apatite are all chemical proxies that can be used to distinguish between SRP, barren pegmatites, and two-mica granite as well as between different samples along the profile of the SRP. Instead, Rb/Cs and Nb/Ta largest variations are expressed internal to single samples, or even single crystals, as observed via trace element mapping of muscovite. The co-variation in the concentration of fluid mobile and immobile elements (Zn, Sn vs U, Th, Rb) in muscovite and apatite support the limited importance of a free fluid phase during the crystallization of the SRP. Geochronological data support a coeval origin for SRP and two-mica granite; on the contrary, mineral chemistry data are not compatible with a direct genetic link between these lithologies. The chemical composition of muscovite, apatite and columbite-tantalite is a valid indicator of the degree of differentiation and rare-elements enrichment among various products of granitic magmatism, and permitted to address important questions regarding the genesis of SRPs. We stress as the absence (or limited importance) of a free fluid, and a depletion in some fluxing elements (i.e. B and F), are fundamental characteristics in the genesis of SRPs, favouring the crystallization of spodumene as the main Li-bearing phase.