The fluid inclusions of pegmatites with lithium mineralisation of the Aracuai pegmatite district (Eastern Pegmatite Province, Minas Gerais, Brazil)

The pegmatites of the pegmatite district of Aracuai (Minas Gerais, Brazil) have a significant economic potential for their Li, Be, Nb, and Ta mineralisation and for gemstones, e.g. multi-coloured tourmaline, aquamarine, morganite, and kunzite. Three types of fluid inclusions are contained in quartz, tourmaline and beryl from the highly differentiated and zoned LCT-type pegmatites of Urubu, Barreiro, Maxixe and Morro Redondo. Type A fluid inclusions contain an aluminosilicate-rich melt ±CO₂, type B fluid inclusions contain mostly CO₂ with variable, but always low H₂O contents, and type C fluid inclusions are of H₂O-type and contain occasionally daughter minerals. Th (fluid), Te(Tc), and Tm(ice) were determined for the H₂O-type fluid inclusions as well as Th(CO(2)) and Tm(CO(2)) for the CO₂-bearing fluid inclusions. Raman spectrometry was used to check the composition of the CO₂-rich phase. The fluid evolution in the pegmatites started with a very H₂O poor, but CO₂-bearing aluminosilicate-rich melt and changed with time into a H₂O-dominated hydrothermal fluid. The fluid evolution and the P,T path are similar to what is given in the literature for other highly differentiated pegmatites. The change in fluid composition from aluminosilicate-rich to hydrothermal during the early stage of the pegmatite crystallisation can be attributed to the early crystallisation of tourmaline which reduced the solubility of H₂O in the silicate melt. The increase in salinity, observed in fluid inclusions with decreasing crystallisation temperature of the pegmatite, could be explained by the incorporation of water in the late (secondary) micas. The salinity increased also due to the late 'argillic' alteration of petalite.