Mineralogical study of dedolomitization in cave environments –the Mravljetovo brezno v Gošarjevih rupah cave, central Slovenia

Dedolomitization is a geological process where dolomite undergoes transformation into calcite via the influx of aggressive Ca-rich solutions derived from various sources (e.g., evaporite dissolution). This conversion can take place in diverse environments. Our study focuses on a unique occurrence of dedolomite within a cave environment in the Mravljetovo brezno v Gošarjevih rupah cave located in central Slovenia. The cave walls exhibit distinctive yellowish and reddish crusts and covers that originated from dedolomitization process developed within this cave system. Macro- and microscopic observations reveal that yellowish lithologies are of sedimentary origin, where dominating dedolomite (i.e., replacive calcite) is associated by mainly detrital particles with a high content of quartz, muscovite, clay minerals, and lithoclasts. In contrast, red lithologies are associated with the transitional zone between sedimentary layers and the host rock dolomite, featuring peculiar calcite crystals with abundant pore space and intercrystalline iron oxides and clay. The transitional zone itself consists of a calcite mosaic with smaller dolomite remnant embedded within the calcite structure. The primary objective of this study is to conduct a comprehensive mineralogical analysis (XRD, SEM-EDS, EMPA, μXRF, CL, and Raman spectroscopy), aiming to recognize their interrelationships and utilize them as proxies for understanding the processes leading to dedolomitization. Dedolomitization is evidenced by the abundance of corroded dolomite rhombs infilled with calcite, coupled with the development of moldic (dissolution-related) porosity, and subsequent crystallization of vein calcite. Detrital quartz reveals embayed crystal boundaries, as well as rounded lithoclasts enriched in clay-group species (e.g., illite). Subordinate detrital apatite, rutile, and zircon were also recognized. Besides, some mineral phases of authigenic origin were detected based on their habits and textural relationships. These include clay-group species (i.e., kaolinite, illite, and/or possible mixed-layer clays), as well as S- and Na-rich apatite, and fluorite. The formation of dedolomite was followed by the decrease of Mg and increase of Fe as evidenced by elemental mapping. These changes can reflect oxidizing conditions and subsequent precipitation of Fe3+ in hematite or goethite/lepidocrocite mineral phases that stained dedolomitized regions red to yellow. Furthermore, we seek to investigate the potential influence of these mineralogical dynamics on cave formation and speleogenesis. All these findings can contribute to a better understanding of geological and geochemical mechanisms governing dedolomitization processes within cave environments. This work is funded by the Slovenian and Polish research agencies (ARRS and NCN) through the bilateral Polish-Slovenian research project CEUS (project code in Slovenia: N1-0226; project code in Poland: 2020/39/I/ST10/02357).