Geological and geochemical evolution of the Bolkardagi bauxite deposits, Karaman, Turkey: Transformation from shale to bauxite

摘要

The Bolkardagi bauxite province is located in the Tauride Belt of southern Turkey. The region hosts many lateritic- and karstic-type bauxite deposits within the Bolkardagi Unit (BU), the Namrun Tectonic Unit (NTU) and the Aladag Unit (AU). The lateritic-type deposits occur on the Upper Triassic sericite-chlorite schist of the BU, on the Lower Triassic slates, and on the Upper Triassic phyllites of the NTU. The gradual transition from the protolith to saprolite and, eventually, to bauxite is observed in the lateritic-type deposits in the field. The karstic-type deposits occur in the carbonates of the BU, the NTU and the AU and were accumulated by clastic transportation from the lateritic material. The deposits within the BU and the NTU are metamorphosed and consist mainly of diasporite, haematite, pyrophyllite, chlorite, chamosite, and anatase; the deposits within the AU are not metamorphosed and consist mainly of boehmite and kaolinite, haematite, goethite, chlorite and anatase. The mean compositions of the deposits are 53% Al2O3,19% Fe2O3, 12% SiO2, and 3% TiO2. The lateritic-type deposits have higher silica content (e.g., Camizalani Bauxite Zone; mean 21% SiO2) than the karstic-type, indicating higher mobilisation of silica in the karstic-type deposits because of alkaline pH conditions. The SREE content varies between 273 ppm and 1693 ppm, and the SREE content increases with an increase in the content of La, Ce and Nd. The mass change calculations according to the Ti index element show that Rb, K, Ba, Ce, Si, Y, Sr, P, Ca, Mg and U were mobile during the bauxitisation processes. The Zr, Ta, Nb and Hf elements are relatively immobile in the first stage of bauxitisation in the saprolite zone but were slightly mobile at a later stage of bauxitisation. Fe, V, Pb, Ni, and Cr are the elements that were gained in the bauxite zone. From field evidence and geochemical data, it is concluded that the first stages of bauxitisation in the Bolkardagi Bauxite Province started during the post-Upper Triassic by the formation of a kaolinite-rich bauxitic soil via chemical weathering of the Lower Triassic pelitic rocks and Upper Triassic shales under acidic conditions. The kaolinite-rich bauxitic soil was transformed to boehmite during low water activity in hot and humid climatic conditions. The Aalenian-Bajocian transgression submerged the bauxite, causing burial diagenesis. At the time of the closure of the Tethyan Ocean during the Upper Cretaceous, ophiolite obductions over the continent gave rise to imbrication and further burial of the bauxite-bearing units. During this time, the bauxite deposits and their host units underwent a very low grade metamorphism in relation to their burial depths and thrusting. Thus, kaolinite was transformed to dickite, nacrite and pyrophyllite at 240-260 °C in the saprolite zone, and boehmitic bauxite was transformed to diasporite in the deeply buried BU and the NTU. The boehmite was preserved as a primary bauxite mineral in the deposit which avoided burial metamorphism in the uppermost tectonic unit, the AU.