Low-temperature geochemistry Research Papers - Academia.edu (original) (raw)

The Bolkardağı bauxite province is located in the Tauride Belt of southern Turkey. The region hosts many
lateritic- and karstic-type bauxite deposits within the Bolkardağı Unit (BU), the Namrun Tectonic Unit
(NTU) and the Aladağ 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., Camızalanı 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
ΣREE content varies between 273 ppm and 1693 ppm, and the ΣREE 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
Bolkardağı 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.