Geological Characterisation and 3D Modelling of the Schwarzleo Mining District/Leogang

Leogang has a mining history spanning over 3,500 years. As early as the Celtic period, copper was prospected in the Schwarzleo Valley. The mining industry reached its golden era in the 17th and 18th centuries, when most adits were driven to extract raw materials. The most significant mining areas were Nöckelberg, Vogelhalte, Inschlagalm, and the Schwarzleo district. Mining operations in the Schwarzleo area ceased in 1833, while mining at Nöckelberg continued until the World War I, with exploration work even taking place during the World War II. Magnesite production at Inschlagalm continued until the 1970s. The historical deposit is located in the northern Greywacke Zone, which mainly consists of Paleozoic carbonates, siliciclastic sediments, and volcanic metasediments. Tectonically, it belongs to the Hochhörndler Schuppenzone, positioned between the Wildseeloder and Glemmtal units. The primary sulphide mineralisation is exclusively hosted in Devonian carbonates, known as the "Südfaziesdolomit". The Schwarzleo district is divided into three sub-areas: Mine West, Mine Central, and Mine East. The Mine West and Mine Central were fully surveyed, resulting in detailed mine maps and a 3D model. The West Mine was geologically mapped, and samples of both the mineralisation and host rock were collected. In addition, various surface investigations¿such as magnetic surveys and LiDAR flights¿were conducted. Based on both, newly acquired and historical data, a geological 3D model of the three areas was created, covering the entire southern region around Schwarzleo. This model integrates directly accessible areas with interpolated regions derived from historical maps and geological data. The Mine West is characterised by a fahlore paragenesis. The fahlore minerals, classified as tennantite, are associated with various minerals containing Fe, Ag, Hg, Ni, Bi, Pb, and Cu, along with numerous secondary phases. The mineralisation is closely linked to a shear zone that dips approximately with 45° to the south. The host rock consists of various carbonates. The most significant is breunnerite¿an iron-rich magnesite (>10 wt% Fe)¿which occurs in both the footwall and hanging wall of the shear zone. These brown-weathering, porous rocks form the bulk of the West Mine. Additionally, a massive dolomite is associated with the shear zone, with a thickness up to 10 m. Overall, the West Mine appears to be a breunnerite body with associated sulphide mineralisa- tion. Trace element analyses and metasomatic structures indicate a three-stage formation process: pre-existing Südfaziesdolomit was transformed into breunnerite through hydrothermal processes; subsequently, in areas of increased fluid flow along the shear zone, the breunnerite was redolomit- ised into the competent dolomite. The sulphide mineralisation occurred along the shear zone.