Decoding hydrothermal alteration link to epithermal vein systems: Insights from the Pb-Zn-Ag Freiberg vein district, Germany

The Freiberg district represents a key example of a Late Paleozoic epithermal precious and base metal deposit in Europe. Despite extensive research focusing on the ore-bearing vein infill, the nature and extent of hydrothermal alteration in the surrounding wall rocks remain poorly understood. This study presents new petrographic and mineral chemical data from altered host rocks adjacent to mineralized veins across different parts of the Freiberg district. The data are supplemented by Ar-Ar geochronology of muscovite (sericite), which has been identified in both the alteration halos and as part of vein assemblages. Our results reveal a complex alteration pattern involving sericitic, chloritic, and silicic overprints associated with the epithermal mineralization. The Ar-Ar dating of the coarse-grained muscovite within veins and fine-grained sericite in altered host rocks yielded ages between 316.7±3.2 Ma and 306.8±3.1 Ma. These values are older than the previously reported Rb-Sr age of 276±16 Ma on sphalerite but align well with field-based cross-cutting relationships and recent evidence for multiple rhyolitic magmatic pulses in the area (326–297 Ma). Some samples returned anomalously old ages, likely due to excess argon. The data support a multi-stage mineralization history and demonstrate the complex interplay between regional geodynamics, felsic magmatism, and hydrothermal processes in the Erzgebirge Metallogenic province.