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Source of boron in the Palokas gold deposit, northern Finland: evidence from boron isotopes and major element composition of tourmaline

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The recently discovered Palokas gold deposit is part of the larger Rompas-Rajapalot gold-mineralized system located in the Paleoproterozoic Peräpohja Belt, northern Finland. Tourmaline is an important gangue mineral in the Palokas gold mineralization. It occurs as tourmalinite veins and as tourmaline crystals in sulfide-rich metasomatized gold-bearing rocks. In order to understand the origin of tourmaline in the gold-mineralized rocks, we have investigated the major element chemistry and boron isotope composition of tourmaline from three areas: (1) the Palokas gold mineralization, (2) a pegmatitic tourmaline granite, and (3) the evaporitic Petäjäskoski Formation. Based on textural evidence, tourmaline in gold mineralization is divided into two different types. Type 1 is located within the host rock and is cut by rock-forming anthophyllite crystals. Type 2 occurs in late veins and/or breccia zones consisting of approximately 80% tourmaline and 20% sulfides, commonly adjacent to quartz veins. All the studied tourmaline samples belong to the alkali-group tourmaline and can be classified as dravite and schorl. The δ11B values of the three localities lie in the same range, from 0 to −4‰. Tourmaline from the Au mineralization and from the Petäjäskoski Formation has similar compositional trends. Mg is the major substituent for Al; inferred low Fe3+/Fe2+ ratios and Na values (<0.8 atoms per formula unit (apfu)) of all tourmaline samples suggest that they precipitated from reduced, low-salinity fluids. Based on the similar chemical and boron isotope composition and the Re–Os age of molybdenite related to the tourmaline-sulfide-quartz veins, we propose that the tourmaline-forming process is a result of a single magmatic-hydrothermal event related to the extensive granite magmatism at around 1.79–1.77 Ga. Tourmaline was crystallized throughout the hydrothermal process, which resulted in the paragenetic variation between type 1 and type 2. The close association of tourmaline and gold suggests that the gold precipitated from the same boron-rich source as tourmaline.

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This work was supported by the K.H. Renlund Foundation, Mawson Resources Limited, and Advanced Material Doctoral Program in the University of Oulu. We thank Bernd Lehmann and one anonymous reviewer who gave useful comments and suggestions that significantly improved this paper.

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Correspondence to Jukka-Pekka Ranta.

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Editorial handling: P. Eilu

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Ranta, J., Hanski, E., Cook, N. et al. Source of boron in the Palokas gold deposit, northern Finland: evidence from boron isotopes and major element composition of tourmaline. Miner Deposita 52, 733–746 (2017). https://doi.org/10.1007/s00126-016-0700-x

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  • Inductively Couple Plasma Mass Spectrometry
  • Tourmaline
  • Gold Mineralization
  • Molybdenite
  • Boron Isotope