Revisiting the Pyhäsalmi volcanic complex, western Finland: new insights from volcanic-facies mapping

The Pyhäsalmi Volcanogenic Massive Sulfide (VMS) deposit, hosted by volcanic rocks of the 1.93–1.91 Ga Vihanti–Pyhäsalmi–Rautalampi belt, is the largest base metal sulfide deposit in Finland (75.7 Mt at 0.9 wt.% Cu, 1.9 wt.% Zn, 0.4 g/t Au, and 14.1 g/t Ag, production 1962–2022). Although significant geological research and mineral exploration projects have been conducted since the 1950s, a comprehensive reconstruction of the volcanic setting and the linkages between volcanism and ore-forming processes has been missing. Based on outcrop mapping and relogging of old drill core in the Pyhäsalmi area, we identify 18 volcanic, sedimentary, and intrusive lithofacies with distinctive lithological and structural characteristics. The study area is dominated by rhyolitic lavas and/or domes, mafic lavas and intrusions, and a volcanogenic sedimentary facies composed of interbedded mudstone and sandstone turbidites. Volcanic facies associations suggest that the eruptive style of felsic volcanism was submarine and non-explosive, forming coherent rhyolitic flows and dome complexes with quench-fragmented hyaloclastic dome margins. Minor resedimented autoclastic volcanic breccias are also found. Volcaniclastic rocks were emplaced as turbiditic mass transport deposits in a submarine slope environment, and are intercalated with hemipelagic sediments, indicating a deep water below-wave-base depositional environment. The Pyhäsalmi ore formation is linked to the permeable autobrecciated facies of rhyolite domes/lavas which are found in contact with sedimentary facies. In contrast, the adjacent Mullikkoräme deposit (1.15 Mt) may represent a less-permeable dome/lava setting with no associated sedimentary facies. Volcanic facies associations (e.g., peperites and hyaloclastites) suggest shallow intrusive (cryptodome) to extrusive felsic volcanism in both the Pyhäsalmi and Mullikkoräme successions, followed by (extension-related) extrusive and locally weakly pyroclastic mafic magmatism on the seafloor. The study highlights new belt-scale exploration potential in areas of similar volcanic and sedimentary lithofacies associations and allow more detailed comparison with other base metal sulfide deposits and occurrences within Vihanti–Pyhäsalmi–Rautalampi belt.