Geochronological study of the Savukoski group in Central Lapland Belt northern Finland and its implication for regional mineral exploration

Abstract

The Savukoski Group in the Central Lapland Belt (CLB) represents a key sedimentary sequence for understanding Paleoproterozoic tectonic, sedimentary, and ore formation processes. This study focuses on the Matarakoski Formation, a sulfur-rich black shale unit, with the aim of precisely constraining its depositional timing and investigating its paleoenvironmental conditions. Determining the age of these black shales is critical for refining the stratigraphy of the Savukoski Group and for assessing their role in sulfur assimilation processes during the emplacement of mafic-ultramafic magmas, such as the economically significant Kevitsa deposit.

The study employs a combination of petrographic analysis, Re-Os isotopic dating, and geochemical characterization. Eight thin sections spanning the depth interval of 49–103 meters were analyzed to assess the mineralogical and textural characteristics of the black shales. Re-Os isotopic analysis was conducted on fine-grained black shale samples, while U-Pb dating was attempted on detrital zircons from coarser sandy intervals.

The results yield a model age of 2129 ± 17 Ma for the Matarakoski Formation, providing the most precise depositional age to date and aligning with the Lomagundi-Jatuli Event. The absence of U-Pb zircon data suggests limited detrital input or low zircon concentrations in the studied interval. Mineralogical analysis indicates a high degree of homogeneity, dominated by clay minerals (clinochlore, muscovite), quartz, scapolite, and graphite, with sulfides primarily comprising pyrrhotite and chalcopyrite. Geochemical evidence from redox-sensitive elements suggests anoxic to euxinic depositional conditions with localized oxygenation events.

These findings have significant implications for understanding the metallogenic evolution of the CLB. The sulfur-rich nature of the Matarakoski Formation likely influenced the sulfur budget, and likely also reduction material, during mafic-ultramafic magmatism, with potential implications for the genesis of magmatic sulfide deposits. The study provides a robust temporal and paleoenvironmental framework for the Savukoski Group, contributing to broader efforts to clarify the geological history of Paleoproterozoic sedimentary basins and their economic potential. The methods and findings are broadly applicable to similar settings worldwide, highlighting the value of integrated geochronological and geochemical approaches in sedimentary basin analysis.