Selective trace element uptake in iron sulfides during burial metamorphism of coal: Insights from the coal-bearing Socka Beds, Northeastern Slovenia

The coal-bearing basal section of the Late Eocene Socka Beds in northeastern Slovenia exhibits iron sulfide mineralization associated with coal. Detailed petrographic analyses have identified five distinct textural and geochemical varieties of pyrite and marcasite. Geochemical analyses reveal that pyrite is notably enriched in Cu, Zn, and Mo, while marcasite shows enrichment in Co, Ni, As, Se, and Sb. Geochemical proxies suggest that marcasite formed under relatively higher temperatures, significantly more reducing conditions, and lower pH compared to pyrite, a distinction that is reflected in their contrasting geochemical signatures. Pyrite incorporates trace elements linked to seawater incursions, while marcasite contains trace elements associated with the progressive degradation of organic matter and the release of organically-bound trace elements. This study demonstrates that iron sulfide speciation is not confined to a single precipitation phase but involves multiple generations of minerals that reflect shifts in physicochemical conditions over time. Additionally, it demonstrates that the incorporation of trace elements is primarily governed by the availability of metals and fluid chemistry, rather than by mineral-specific selective sequestration. These findings offer new insights into the physicochemical pathways that govern sulfide formation in coal-forming environments.