Rare-metal pegmatites of the Soldat-Myl’k pegmatite field: geology, geochemistry of rare elements

Abstract

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.