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

Juho Tapio,Jukka-Pekka Ranta,Nick Cook,Yann Lahaye,Hugh O'Brien

doi:10.1016/j.precamres.2021.106410

Juho Tapio, Jukka-Pekka Ranta + Show 3 more

Open Access

https://doi.org/10.1016/j.precamres.2021.106410

Copy DOI

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.

Highlights

Tourmaline is a mineral commonly associated with gold deposits
Tourmaline grains display optical and color zonation from transparent pale green or blue in cores to brown towards the rims
Contrary to the observations by Ranta et al (2017) for the nearby Palokas mineralization, the majority of the coarse tourmaline grains occur in the Raja mineralization hanging wall rocks and do not make up a significant proportion of the mineral ized intervals

Summary

Introduction

Tourmaline is a mineral commonly associated with gold deposits G. Slack et al, 1996; Goldfarb et al, 2005; Baksheev et al, 2015; Sciuba et al, 2021). 1996; Goldfarb et al, 2005; Baksheev et al, 2015; Sciuba et al, 2021) It is the major boron sink in hydrothermal systems and can retain the isotopic signature of the source of boron throughout hydro thermal processes Tourmaline is an abundant mineral associated with or in close proximity to the mineralized rocks of the Rajapalot Au-Co prospect area within the northern part of the Perapohja belt (Fig. 1a), and can provide valuable information about the nature and the source of hydrothermal fluids responsible for the alter ation and mineralization. Tourmaline major- and trace element compositions and B-isotope results are presented and the po tential source of boron, conditions during tourmaline crystallization, and tourmaline relationships to the Au-Co mineralization are discussed

Results

Discussion

Conclusion