University of Oulu

Tapio, J., Ranta, J.-P., Cook, N., Lahaye, Y., & O’Brien, H. (2021). Paleoproterozoic Rajapalot Au-Co system associated with evaporites: Chemical composition and boron isotope geochemistry of tourmaline, and sulfur isotopes of sulfates, Peräpohja belt, northern Finland. Precambrian Research, 365, 106410.

Paleoproterozoic Rajapalot Au-Co system associated with evaporites : chemical composition and boron isotope geochemistry of tourmaline, and sulfur isotopes of sulfates, Peräpohja belt, northern Finland

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Author: Tapio, Juho1,2; Ranta, Jukka-Pekka2; Cook, Nick1;
Organizations: 1Mawson Gold Ltd., Vancouver, BC, Canada
2Oulu Mining School, University of Oulu, P.O. Box 3000, FI-90014, Finland
3Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 35.4 MB)
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Language: English
Published: Elsevier, 2021
Publish Date: 2021-09-21


The Raja Au-Co mineralization in the Paleoproterozoic Peräpohja belt in northern Finland is part of the wider Rompas-Rajapalot mineralized area with several known Au-Co occurrences. The area is characterized by distinct tourmaline occurrences, spatially associated with Au-Co mineralization. Four texturally distinct tourmaline types were identified although all analysed tourmalines belong to the alkali-group and are classified as dravite. δ ¹¹B values and fractionation modelling indicates that at least two distinct fluids were responsible for the tourmaline formation with initial end member δ¹¹B values of the fluids at −8‰ and −1‰. Possible sources for boron rich fluids are Svecofennian orogeny related (ca. < 1.9 Ga) metamorphic fluids and magmatic-hydrothermal fluids related to the late-orogenic ca. 1.78 Ga granitoids. Structural data from the quartz-tourmaline veins outlines a strong linear trend towards the Raja high-grade mineralization trend. This is interpreted as evidence for structural control of the Raja mineralization, which possibly is contained in shear-zones or within the hinge regions of local high degree folding.

A distinctive rock unit with bright purple anhydrite layers and white gypsum veins has been intersected by drilling. While the sulfate-rich unit is unmineralized, the mineralogy and regional geological setting suggests an evaporitic origin. Anhydrite has δ³⁴S values in a narrow range from 8.1 to 9.8 ‰. Gypsum has slightly heavier δ³⁴S from 10.6 to 12.2 ‰. Together with isotope fractionation constraints, textural evidence suggests the authigenic formation of gypsum with SO₄²⁻ sourced from anhydrite. Significant similarities in textures and sulfur isotope values to well-known evaporite successions of the Onega basin, western Russia, further supports the presence of evaporitic strata within the Peräpohja belt.

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Series: Precambrian research
ISSN: 0301-9268
ISSN-E: 1872-7433
ISSN-L: 0301-9268
Volume: 365
Article number: 106410
DOI: 10.1016/j.precamres.2021.106410
Type of Publication: A1 Journal article – refereed
Field of Science: 1171 Geosciences
Funding: This work was supported by the Mawson Gold Ltd. We thank Marko Moilanen for the assisting during electron microprobe analyses. Two anonymous reviewers are thanked for their constructive comments which significantly improved the quality of the manuscript. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Copyright information: © 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (