Research topics:

1. Formation of Paleoproterozoic orthomagmatic sulfide deposits in the Vammala Belt, southwest Finland.

This project investigates the petrogenesis and ore formation of the ultramafic Kylmäkoski and Stormi deposits in southwestern Finland, which intruded approximately 1.89 billion years ago. The Vammala Belt hosts several Paleoproterozoic orthomagmatic Ni–Cu–(Co) sulfide deposits whose formation processes are still only partially understood.

These sulfide deposits formed during the Svecofennian orogeny (1.87–1.89 Ga) along a subduction zone. The mineralized ultramafic cumulates represent deep crustal levels. The main ore minerals are pyrrhotite, pentlandite, chalcopyrite, and cubanite, occurring in peridotites, amphibolites, and pyroxenites.

In this study, geochemical analyses (major and trace elements, whole-rock composition, δ³⁴S isotopes) are combined with petrological and textural observations to determine how melt evolution, temperature, pressure, water content, and oxygen fugacity influenced sulfide saturation.

The results suggest that assimilation of external sulfur from sedimentary rocks was likely not the primary mechanism for sulfide saturation, as is often assumed for orthomagmatic sulfide deposits in subduction settings. Instead, fractional crystallization of the silicate melt led to sulfur enrichment. The crystallization of magnetite and the associated decrease in oxygen fugacity resulted in late-stage sulfide precipitation.

Because the redox conditions of the Paleoproterozoic Kylmäkoski and Stormi magmas resemble those of modern subduction-related magmas, Phanerozoic orthomagmatic Ni–Cu deposits may have formed through similar mechanisms. This suggests that assimilation of crustal rocks is not always required to generate economically significant magmatic sulfide deposits.

Methods used:

• Electron Probe Microanalysis (EPMA)
• Scanning Electron Microscopy (SEM)
• X-ray Fluorescence (XRF)
• Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)
• Secondary Ion Mass Spectrometry (SIMS)
• Element Analyzer – Isotope Ratio Mass Spectrometry (EA-IRMS)

2. Mechanisms of Ni and Co enrichment in hydrothermal sulfides of the ultramafic Kylylahti VMS deposit, Finland

The Kylylahti deposit is an ultramafic volcanogenic massive sulfide (VMS) deposit and part of the Outokumpu mining district in eastern Finland. A striking feature is the strong enrichment of nickel and cobalt compared to modern hydrothermal systems on the ocean floor (“black smokers”). The mechanisms responsible for this enrichment remain unclear and are the focus of this research project.

The main ore minerals include pyrite, chalcopyrite, sphalerite, pyrrhotite, magnetite, pentlandite, and cobaltian pentlandite. Particularly notable is the concentration of cobaltian pentlandite, which occurs abundantly in a “high-Co zone” at the center of the ore body and can be traced into the “low-Co zone” at its margins.

Temperature is considered a key control on Co enrichment: toward the center of the ore body, increasing temperatures correspond to higher Co/Ni ratios in pentlandite and cobaltian pentlandite. Lead and sulfur isotope analyses indicate that the metamorphic fluids responsible for metal enrichment likely originated from the surrounding black shales.

The project aims to investigate the origin of the Ni and Co enrichment and the metamorphic processes responsible for metal remobilization in detail. Using EBSD (Electron Backscatter Diffraction), deformation structures in rocks and mineral grains will be analyzed to identify zones of elevated stress and pressure shadows. This approach may help link the formation of secondary sulfide phases more precisely to metamorphic and tectonic processes, thereby improving our understanding of their origin.

Methods used:

• Electron Probe Microanalysis (EPMA)
• Scanning Electron Microscopy (SEM)
• X-ray Fluorescence (XRF)
• Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)
• Element Analyzer – Isotope Ratio Mass Spectrometry (EA-IRMS)
• Electron Backscatter Diffraction (EBSD)