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
Tapio, Juho; Ranta, Jukka-Pekka; Cook, Nick; Lahaye, Yann; O’Brien, Hugh (2021-09-21)
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. https://doi.org/10.1016/j.precamres.2021.106410
© 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
https://creativecommons.org/licenses/by/4.0/
https://urn.fi/URN:NBN:fi-fe2021092146694
Tiivistelmä
Abstract
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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