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.