preprints.org > environmental and earth sciences > geochemistry and petrology > doi: 10.20944/preprints202311.0404.v1

Preprint Article Version 1 Preserved in Portico This version is not peer-reviewed

Version 1 : Received: 3 November 2023 / Approved: 7 November 2023 / Online: 7 November 2023 (06:34:15 CET)

In this paper we present textural and compositional data for columbite group minerals (CGM) and associated Nb-Ta-Sn oxides from lithium-beryllium-tantalum pegmatite deposits of the Kolmozero-Voronja belt, NW Russia, with the aim of deciphering the above mentioned features for minerals from deposits with different mineral signatures and lithium ore grade. Minerals from four deposits, including two of world-class (Kolmozero and Polmostundra), are examined. It is shown that the main controlling factors are the diversity and rate of magmatic fractionation, hydrothermal overprint and mineral paragenesis followed from the specific geochemical signature of the different pegmatite deposits. CGM from Kolmozero include several mineral species (columbite-(Fe), columbite-(Mn), tantalite-(Fe), tantalite-(Mn)) showing large compositional variations, mainly controlled by Nb-Ta fractionation (Ta/(Ta+Nb) = 0.16-0.70; Mn/(Mn + Fe) = 0.45-0.63). Textural patterns are extremely various (oscillatory, homogeneous, patchy, spongy, somewhere with over-growing Ta-rich rims) and indicate the involvement of numerous magmatic and hydrothermal processes. The Polmostundra CGM are represented by columbite-(Fe) with Ta/(Ta+Nb) ranging from 0.05 to 0.39 with homogeneous, mottled, oscillatory, patchy and irregular reverse textures. The Okhmylk CGM are irregular normal, patchy and homogeneous columbite-(Fe) and columbite-(Mn) with Ta/(Ta+Nb) = 0.09-0.24 and Mn/(Mn + Fe) = 0.29-0.92, indicating the suppressed magmatic fractionation and iron drop due to precipitation of Fe minerals. Columbite-(Fe) and columbite-(Mn) from the Be-Ta Shongui deposit are less evolved with Ta/(Ta+Nb) = 0.07-0.23 and Mn/(Mn + Fe) = 0.31-0.55. The minerals are characterized by progressive normal, oscillatory, homogeneous and ir-regular reverse patterns. Associated pyrochlore minerals occur both as early magmatic (Kolmozero) and late hydrothermal (Polmostundra, Okhmylk). Cassiterite is found only in the Okhmylk dykes and is apparently of hydrothermal origin. CGM from Li pegmatites have impurities of Ti (0.01-0.05 apfu) and W (up to 0.02 apfu), whereas CGM from Be pegmatites contains elevated Ti (up to 0.09 apfu). The mineral systems analysis presented here is relevant for exploration.

columbite group minerals; pyrochlore; cassiterite; pegmatite; lithium; beryllium; NW Russia

Environmental and Earth Sciences, Geochemistry and Petrology

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