Mineralogy, petrology, geochemistry and U-Pb zircon age of Upper Cretaceous bauxites from Jajce, Bosnia and Herzegovina

The area of Jajce in Bosnia and Herzegovina is historically one of the most important localities for bauxite production in the wider region. It has huge production and exploration potential; however, it lacks scientific data, especially in terms of mineralogy and geochemistry. Our study provides the first detailed mineralogical and geochemical, as well as U-Pb dating data from this area. Petrographic analyses revealed mostly ooidic to conglomeratic textures. Hematised black pebbles attract attention as a textural element but are also shown to be economically interesting as they have the largest concentrations of trace elements. The major Al-phase is boehmite with minor occurrences of diaspore. A variety of accessory minerals were detected, indicating multiple and diverse protolith sources, ranging from ultrabasic to acidic magmatic and metamorphic rocks. Chemical composition corresponds to ferritic bauxite with up to 61 wt% of Al₂O₃. Hematite is the primary scavenger of trace elements (REE, Sc, V, Cr, U, Th and Pb). REE content reaches up to 1353 ppm in the whole rock, primarily stored in xenotime, monazite, synchysite and parisite. Most of the REE carbonates indicate an authigenic origin, reflecting slightly alkaline and oxidative conditions. Synchysite hosts Ga and Ge, while kaolinite concentrates Li. U-Pb zircon dating yielded concordant ages ranging mainly from 84 ± 1.4 to 92.6 ± 2.1 Ma, corresponding to the Santonian to Turonian stages of the Late Cretaceous period. These ages constrain the timing of bauxitisation in the Jajce area, indicating that it must have occurred after the deposition of the youngest zircon populations (∼84 Ma). Based on the zircon crystallisation age and geochemical similarities, the source material for the Jajce bauxites likely originates from the Apuseni-Banat-Timok-Srednogorie Belt of the Carpathian-Balkan orogen. Contributions from the contemporary Sava Zone cannot be excluded, although dated volcanic activity in that region is reported to be slightly younger.