Assessment of geophysical methods in the discovery of karst bauxite deposits in the Dinarides

 

Geophysical exploration was carried out on karst bauxite deposits in the Posušje area in Bosnia and Herzegovina, which generally represent the problem of researching bauxite deposits in the Dinarides and similar geological models in the Mediterranean. Karst-type bauxite deposits are found in very complex geological models and were deposited in depressions in carbonate bedrock during numerous emersions on the Adriatic carbonate plate. Carbonate or clastic rocks can be found as the hanging wall of the deposits. Due to the complex lithological and structural relations, very irregular shapes of the deposits and their relatively small dimensions, the discovery of karst-type bauxite deposits is a very demanding geophysical task. That is why the published literature falls short in offering solutions for this very complex problem. So, the fundamental question is whether very irregular bauxite deposits whose dimensions are generally small can be detected by geophysical exploration?

Basic near surface geophysical methods, electrical resistivity tomography and seismic refraction, as well as magnetometry were applied in the exploration, which was carried out in two phases. In the first phase, geophysical methods were applied to already discovered bauxite deposits in order to determine whether geophysical responses correlate with bauxite deposits and to evaluate the efficiency of each method. Geophysical measurements were performed at several microlocations, and in the area of Mratnjača were carried out immediately after the deposit discovery, which was subsequently mapped with a very dense network of exploratory boreholes and was very well defined. In the second phase, measurements were performed in an microlocation selected by geological prospecting in order to discover potentially new bauxite deposit.

The characteristic responses of bauxite deposits are expressed on the resistivity models of tomographic profiles as zones of lower resistivities within carbonate rocks, and on the velocity models of refraction profiles as velocity inversions. The responses are much clearer on resistivity models, so the electrical tomography should be considered as a fundamental method in the exploration of karst bauxite deposits. Seismic refraction can contribute to a better characterization of deposits and reduce the interpretation ambiguity, thereby increasing the efficiency of geophysical exploration. In the last case, the electrical tomography can be applied independently to give satisfactory exploration results. On the other hand, the seismic refraction should be combined with the electrical tomography, because in some cases the depth coverage is greatly reduced due to distinct velocity inversions. Unfortunately, magnetometric measurements showed there are no magnetic anomalies that could be associated with bauxite deposits, that is, there are no magnetic minerals in the deposit.

Acknoledgments

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