| Sažetak (engleski) | The exploitation of bauxite in the wider area of Posušje in Bosnia and Herzegovina has been known since the first half of the 20th century, and to date more than 1100 deposits or occurrences of various sizes and structures have been discovered. Bauxite deposits of Posušje represent typical karst bauxites, formed during the emersion phase between Upper Cretaceous rudist limestones in the footwall and various carbonate and clastic rocks of Paleocene/ Eocene and Oligocene ages in the hanging wall. The most intensive mineralogical and geochemical bauxite research in these areas was carried out during the 70s and 80s. Afterwards, despite numerous newly discovered deposits, no petrographic, mineralogical and geochemical analysis on the bauxites has been published. The present study is focused on recently discovered bauxite deposits in Posušje and their petrographic and mineralogical description. The bauxites show ooidic to pelitomorphic, but in places also clastic textures. Micro-ooids and spheroids are mostly smaller than 0.1 mm, and in places fragments of the former ooid can be found. In some bauxites, pebbles of resedimented bauxite prevail, mostly larger than 2 mm but sometimes larger than 1 cm, which define their conglomerate texture. Gibbsite and boehmite are the main Al-bearing minerals of the bauxites, and diaspore has not been observed. The ratio of gibbsite to boehmite varies between deposits, and in some deposits boehmite is the only aluminium phase present. Where present, the gibbsite is developed into relatively large hypidiomorphic crystals larger than 0.25 mm, with clearly defined polysynthetic lamellae. Hematite is the dominant Fe-phase in all deposits. Although rarely, goethite-rich bauxites can be found in the form of isolated layers, with just traces or no hematite. Zircon, apatite and calcite are minor and accessory minerals, and X-ray analysis indicated significant amounts of anatase and rutile. Kaolinite was detected in only one sample. Chemical analyses indicate a negative correlation of Al and Fe content. The Al2O3 content ranges from 53.4 to 63.9 wt.%, and Fe2O3 from 22.5 to 35.2 wt.%. The SiO2 and TiO2 content varies from 1.6 to 5.6 wt.% and 3.5 to 4.6 wt.%, respectively. The chemical composition of the analysed samples defines them as ferrous bauxites. A change in the chemical composition was observed in relation to the footwall distance. The relatively high content of Zn (up to 627 ppm) gradually decreases towards host carbonates of the footwall, and a similar trend is observed in the Fe, Cr and Zr content, while Al, P, Ca and Ti, on the other hand, increase closer to the footwall. Mineralogical, petrological and chemical studies of the recently opened bauxite mines in Posušje revels heterogenic properties and indicate a complex genesis of the deposits, with possibly diverse source material. |
