Preliminary Mineralogical Characterization of Mercury Ore from the Karoli Orebody, Idrija Deposit (Slovenia): Implications for Ore Genesis

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 (from 50 to 90%) and exceptionally rich cinnabar mineralization, with mercury contents locally reaching up to 78%. This study aims to provide a preliminary yet detailed characterization of the ores from the Idrija mercury deposit and to establish their textural and genetic characteristics. We implemented microscopic examination of thin sections, complemented by LA-ICP-MS trace element analyses of pyrite. 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 grains revealed low trace element concentrations, with maximum values remaining below 100 ppm. These observations suggest that the hydrothermal fluids were mercury-rich but of low salinity, which may have limited the incorporation of trace elements into pyrite. Our research provides new insights into the formation of the Karoli orebody. We envision that further investigations of fluid composition, salinity, and fluid inclusions could significantly enhance understanding of the Idrija mercury formation and evolution.