Petrophysical cobalt exploration indicators in Kuusamo, northern Finland

• Cobalt’s connection with monoclinic pyrrhotite demonstrates in the petrophysical data. • Co ores are typically ferromagnetic with high remanent magnetisation intensity. • Co ores are related to higher electrical conductivities in comparison to Au ores. • Co–Au ore mixtures are common with combined, bimodal petrophysical properties. The increasing global demand for cobalt, a critical green transition metal, encourages the development of improved exploration workflows, as Co typically occurs as a by-product of other metals, and the exploration and processing methods are rarely optimised for it. This paper investigates the possibility of extracting geophysical cobalt exploration indicators from petrophysical properties in Kuusamo, one of the most prominent cobalt-enriched areas in the largest European Co producer, Finland. The study region hosts 20 Au–Co deposits and occurrences classified as orogenic gold deposits with atypical metal association. Two separate mineralization stages have been distinguished: a pyrrhotite-dominated Co-stage and a pyrite-dominated Au–Co stage. The assumption in this study was that Co-enriched samples could be distinguished from Au-enriched samples and host rocks in petrophysical data due to the mineral associations. The investigation used an extensive previously acquired petrophysical and geochemical drill hole dataset from six targets complemented with new thermomagnetic data for magnetic mineral phase detection. The samples were divided into ore classes for the petrophysical data analysis. The results indicate that cobalt’s association with monoclinic pyrrhotite is a key factor for geophysical applications: the high remanent magnetisation characteristic of monoclinic pyrrhotite suggests that Co-enriched ores should typically associate with magnetic anomaly sources. Co enrichment is also related to higher electrical conductivities in comparison to Au. The workflow applied in this study illustrates how key petrophysical parameters can be connected with ore-bearing minerals and rocks, and the results indicate how cobalt could be explored more directly and effectively in the study area.