Geochemical Insights into Pyrite from the Karoli Orebody, Idrija Mercury Deposit (Slovenia): An LA-ICP-MS Trace Element Study

The Idrija mercury deposit represents one of the largest mercury formations globally, ranking second only to the Almadén deposit in Spain. The deposit has been exploited for more than five centuries and represents one of the most historically significant and extensively studied mercury mines worldwide. The Karoli orebody is characterized by a high abundance of pyrite (50 to 90 vol.% of the rock) and exceptionally rich cinnabar mineralization, with contents reaching up to 78 wt.% Hg locally. This study investigates the trace-element composition of Py3 pyrite from the Karoli orebody using LA-ICP-MS analysis to examine variations within Py3 pyrite, revealing insights into ore-forming processes and mineralization characteristics. Trace-element analysis of pyrite was performed and complemented by microscopic examination of thin sections. Three different pyrite types were identified: fine-grained framboidal Py1, subhedral to euhedral Py2, and larger, well-developed euhedral Py3. LA-ICP-MS analysis of Py3 pyrite grain revealed low trace-element contents, with maximum values remaining below 100 ppm. These observations, combined with published sulfur and mercury isotope data, suggest that Py3 pyrite crystallized under stable growth conditions from mercury-rich, low-salinity hydrothermal fluids. Our research provides insights into Py3 pyrite formation and the characteristics of the hydrothermal fluids in the Karoli orebody, serving as a solid foundation for further studies. Future research is envisioned to include the analysis of Py2 grains to complement the current dataset, with further investigations of fluid composition, salinity, and fluid inclusions.