Spodumene from rare-metal pegmatites of the Kolmozero lithium world-class deposit on the Fennoscandian shield: trace elements and crystal-rich fluid inclusions
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Abstract The paper investigates trace elements and crystal-rich fluid inclusions in spodumene from rare-metal pegmatites of the Kolmozero lithium deposit in the Kola region, Russia. The main lithium mineral in the pegmatites is spodumene, which occurs in three generations, designated as Spd-I, Spd-II and Spd-III. Iron, Na and Mn are the most typical element impurities in spodumene. The Fe/Mn ratio is 7.1 in Spd-I, 12.3 in Spd-II and 13.2 in Spd-III. Spd-II contains fluid and crystal-rich fluid inclusions. The crystal-rich fluid inclusions in Spd-II originally trapped CO 2 -bearing aqueous fluids with dissolved alkali carbonates. The crystal-rich fluid inclusions contain zabuyelite (Li 2 CO 3 ) and cristobalite (SiO 2 ) as solid phases, which have not been reported previously from the Kolmozero rare-metal pegmatites. These minerals are assumed to have resulted from a reaction between a CO 2 -bearing aqueous fluid and host Spd-II and are not related to the mineral-forming system of pegmatites.Keywords:
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Rare-metal pegmatites, in which spodumene is a major mineral of lithium, form vein series, or occur as individual veins in pegmatite fields. One such example is the Soldat-Myl'k pegmatite field located in the south-eastern Kola rare-metal pegmatite belt of the Baltic Shield (Kola Peninsula, Russia). Geologically the pegmatite field is situated on the Kolmozero-Voronya greenstone belt, and includes a vein of albite-spodumene pegmatites, veins of Ta-Nb feldspar pegmatites, and veins of ore-free pegmatites. The new data on a rare element content in rare-metal pegmatites determined using the ICP-MS method have been obtained. The study results have indicated that albite-spodumene pegmatites correspond to LCT rare-metal pegmatites by their mineralogical and geochemical features. Compared to a granite clarke, albite-spodumene pegmatites are enriched in Li, Nb, Ta, Be, Cs, Rb, Hf, U, depleted in Ba, Sr, Y, Th, REE, and have low values of indicators of ratios, i.e. Mg/Li (0.43), Nb/Ta (1.29), Zr/Hf (8.3), Ba/Rb (0.01), Sr/Rb (0.02). Ore minerals in pegmatites occur as spodumene, beryl, and minerals of the columbite group. Spodumene content in pegmatites is ∼20% of the total volume of veins. Albite-spodumene pegmatites of the Soldat-Myl'k pegmatite field differ from rare-metal pegmatites of the Kolmozero lithium deposit by higher contents of boron (29-fold enrichment), phosphorus, medium (Gd, Tb, Dy) and heavy lanthanoids, lower contents of lithium (17-fold depletion), and a weakly differentiated spectrum of lanthanoid dissemination. Feldspar pegmatites with the Ta-Nb mineralisation have high contents of Ta, Nb, Rb, Hf, U, B, and are depleted in Ba, Sr, Y, Th, REE. There is a lowering (ppm) of LREE (1.91 and 1.07, respectively), MREE (2.11 and 0.61, respectively), LREE (0.29 and 0.19, respectively) from Ta-Nb feldspar pegmatites to albite-spodumene pegmatites. Mean values of indicators of ratios, i.e. Mg/Li, Nb/Ta, Zr/Hf, Ba/Rb, Sr/Rb, also indicate a characteristic lowering from feldspar pegmatites with the Ta-Nb mineralisation (9.74, 1.70, 10.9, 0.05, 0.06, respectively) to albite-spodumene pegmatites with the Be-Nb-Ta mineralisation (0.43, 1.29, 8.3, 0.01, 0.02, respectively). Results of geochemical studies taking into account the mineralogical data indicate that rare-metal pegmatites of the Soldat-Myl'k pegmatite field can be of practical interest in relation to the rare-metal mineralisation.
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Abstract The Nanyangshan Li-Cs-Ta (LCT) pegmatite is the largest of hundreds of pegmatite dikes in the eastern Qinling orogenic district, North China. The Nanyangshan pegmatite is strongly zoned into a contact zone, border zone, wall zone, intermediate zone, and core, with Li mineralization occurring predominantly in the intermediate zone. Inward through the intermediate zone, Li mineralization is divided into subzones of Spd (spodumene), Mbs (montebrasite), Elb (elbaite), and Lpd (lepidolite). Lithium minerals include spodumene, montebrasite, lithiophilite, elbaite, lepidolite, and possible former petalite. Paragenetic assemblages of Li minerals are variable, with spodumene ± Li-phosphates (montebrasite and lithiophilite), Fe-rich elbaite, lepidolite, and possible former petalite in the Spd subzone; Li-phosphates (main montebrasite and rare lithiophilite) + spodumene + Fe-bearing elbaite + lepidolite in the Mbs subzone; Fe-poor elbaite + lepidolite ± montebrasite in the Elb subzone; and lepidolite ± Fe-poor elbaite in the Lpd subzone. Whole-rock contents of Li2O, P2O5, B2O3, and F are consistent with the high contents of various Li minerals. Spodumene was formed first and dominantly from a Li-saturated melt in the Spd subzone (1.66 wt% Li2O). This subzone graduates into the P-rich Mbs subzone (3.75 wt% P2O5) with montebrasite gradually succeeding Li-aluminosilicates, followed by the appearance of abundant Fe-poor elbaite in the Elb subzone (1.04 wt% B2O3), reflecting the consumption of P in the melt. Lepidolite formed after early-formed Li phases in the F-rich Lpd subzone (2.03 wt% F), as indicated by replacement textures. Among the numerous LCT pegmatites worldwide, the Li mineralization sequence can be suggested as Li-aluminosilicates (commonly spodumene and less commonly petalite) → Li-phosphates (montebrasite-amblygonite and triphylite-lithiophilite) → elbaite → lepidolite, and can be regarded as a general sequence for Li mineralization.
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