Geochemical characteristics of bauxites: the Permian Shahindezh horizon, NW Iran

The bauxite horizon of Permian age to the east of Shahindezh, NW Iran, is a part of Irano-Himalayan karst bauxite belt. The horizon was developed as 14 discontinuous stratified layers and lenses with varying thicknesses (4-16 m) extending over 6.2 km within carbonate rocks. Diaspore, boehmite, kaolinite, and hematite are the major minerals components in the ores, with small amounts of goethite, rutile, illite, montmorillonite, chlorite, and quartz. The textural features of ores suggest that this residual horizon has an authigenic origin. Geochemical data indicates that diabasic rocks (whose remnants are still present along the contact of this horizon with carbonate bedrock) are most likely the parent rocks for this horizon. Calculated values of absolute weathering index (AWI) across a selected profile illustrates that Si, Ca, Mg, Na, K, Mn, P, Rb, Cs, Ba, Sr, Cu, and HREE were leached from the weathered system during bauxitization, whereas Al, Nb, Hf, Zr, Ta, and Ga were enriched. Nevertheless, elements such as Fe, U, Th, Y, V, Cr, Co, Ni, and LREE underwent both leaching and fixation. The overall data obtained from calculated AWI values along with elemental ratios indicate that distribution and behavior of elements during bauxitization to the east of Shahindezh were controlled by factors such as pH and Eh variations in weathering solutions, groundwater chemical characteristics, Fe concentration variation in the weathering profile, and adsorption processes.