Exploration of karst bauxite deposits in the Dinarides using ground geophysical methods - possibilities and limitations

The research was conducted in the Dinarides in two test areas, Posušje and Jajce in Bosnia and Herzegovina, where bauxite has been intensively exploited since the second half of the 20^th century. The explored bauxite deposits were formed in two emersions, terrestrial geological phases in the geological history. In the Jajce area, the deposits originate from the horizon within the Middle Cretaceous, while in the Posušje area they were formed in the emersion between the Upper Cretaceous and Paleogene rocks. Two basic goals of the research in the test areas were set. The first goal is to determine the possibility of directly discovering bauxite deposits, and the second is to determine lithological and structural relationships in very complex geological models. The expected result of the research is to increase the efficiency of geophysical methods and thus reduce the overall costs of exploring bauxite deposits. Ground geophysical methods (GGM) were applied, that could contribute to solving the problem, on the basis of previous experiences in exploring karst areas. These are the methods based on the determination of the inverse resistivity model, electrical resistivity tomography (ERT) and the magneto-telluric method (Controlled Source Audio-frequency Magnetotellurics - CSAMT), and seismic refraction which gives an inverse velocity model.

Research at already discovered bauxite deposits at several micro-sites in the Posušje area showed the deposits are outlined as geophysical anomalies on inverse resistivity and velocity models, that is, bauxite deposits can be directly detected by ERT and seismic refraction if Paleogene limestones are in the hanging wall. However, if there are clastic Paleogene-Neogene deposits, it is very difficult to discover bauxite deposits. In the Jajce area, bauxite deposits could not be recognized on geophysical models, since the hanging wall of the deposits mainly consists of clastic rocks whose resistivities and velocities overlap with the bauxite deposits.

GGM can significantly contribute to the determination of very complex geological models in bauxite exploration. In both research areas, Jajce and Posušje, it was shown that ERT should be considered as a basic research method in determining generally very complex geological models. In combination with other geological data, from the surface and from boreholes, the effectiveness of overall investigations can be significantly increased. One of the main tasks is the mapping of the weathered carbonate bedrock with possible bauxite deposits, especially when clastic rocks, Cretaceous or Paleogene-Neogene, are found in the hanging wall. In the case of a deeper carbonate bedrock, greater than 90 m, the CSAMT method should be applied due to the limited depth penetration of ERT.

Acknoledgments

This exploration was carried out in the AGEMERA project (Agile Exploration and Geo-modelling for European Critical Raw Materials) - the European Union's Horizon Europe research and innovation programme - grant agreement No 101058178.