To better interpret both recent and historical crystalline basement drill cores from Estonia, we traveled north for specialized training. The Geological Survey of Finland welcomed us to their research center in Loppi, where we focused on studying and describing metamorphic rock cores and learning from the expertise of our northern colleagues.

A particular emphasis was placed on rocks associated with volcanic massive sulfide (VMS) deposits. These deposits are among the most valuable mining targets, as they concentrate metals like copper, zinc, gold, and silver. Given our shared geological history with Finland, similar mineralizations may also be present in Estonia’s basement rocks. These formations date back approximately 1.9 billion years, to a time when the Baltic continent was still forming and tectonically active. Back then, black smokers on the seafloor released metal-rich hydrothermal fluids, which, upon mixing with cold seawater, precipitated sulfur compounds containing various metals.

There was much to learn, as neither students nor professional geologists encounter such rocks very often, making this training especially valuable. Some of the key takeaways included:

• Identifying the protolith (original rock type) is essential for understanding the alteration and mineralization environment and locating ore bodies.
• Core samples alone are often insufficient for accurately interpreting rock structures.Examining the same rocks in outcrops often provides critical context (a luxury unfortunately not available in Estonian geology).
• Systematic chemical and mineralogical analysis is necessary for core logging.
• Portable instruments like handheld XRF, XRD, and LIBS are efficient tools, but one must consider their limitations.
• Alongside portable instruments, classical sampling for chemical and mineralogical identification, as well as thin section microscopy, remain crucial components of core logging.
• Preliminary studies are necessary before launching large-scale surveys to ensure an efficient workflow.
• Quality assurance and control (QA/QC) should be integrated into every stage of the core logging project.

Despite Estonian geologists’ extensive experience in describing sedimentary rock cores, metamorphic rocks often left us scratching our heads. On the second-to-last day, Lauri Joosu, the senior geologist from EGT, pointed to one core section and confidently told our instructor, the all mighty basement expert at GTK, Janne Hokka, “These rocks haven’t been metamorphosed at all.” Janne threw up his hands and jokingly said, “That's it! I’m going home.”

Describing metamorphic rocks in drill cores is no easy task. Still, by the end of the week, we had grasped the basic principles. Furthermore, the Finnish experts were asked to come over in winter and review our cores at the Arbavere drill core storage in Estonia as well.

After long days of training, we relaxed in Hämeenlinna, where the highlights included a medieval castle and a Nepalese restaurant with portion sizes that defied all human limits.

One morning, on our way to Loppi, we passed a truck whose cargo had spilled onto the road. The cargo turned out to be trout heads and backbones left over from filleting. Even two days later, while driving past the same spot, our colleague Johannes Vind said he could still smell rotting fish. The general consensus in the car, however, was that there was no smell at all and Johannes had suffered a psychological trauma.

In the end, we returned home with two things: a basic grasp of metamorphic core logging, and the unshakable knowledge that fish bones decompose far slower than expected. Both lessons, in their own way, will probably stay with us for life.