Critical metals distribution in the late Triassic–early Jurassic Nasr-Abad bauxite deposit, Irano–Himalayan karst bauxite belt, NW Iran

Abstract

In the late Triassic–early Jurassic Nasr-Abad deposit, bauxite ores are bauxitic iron ore and Fe-rich bauxite, grading into bauxite and clayey bauxite upward the succession. The ores mainly consist of a mineral assemblage of diaspore, hematite, kaolinite, and rutile. Conservative elemental ratios of provenance, such as Eu/Eu*, Sm/Nd, and Nb/Ta, suggest a derivation from a mafic protolith, although, in some cases, the Eu/Eu* ratio indicates a felsic protolith, probably derived from Precambrian rock units. R-mode factor analysis indicates that most of the variance of the dataset is explained by a factor related to climatic and redox paleo-environmental conditions. This factor, which controls some critical metals, such as Co, V, and LREE, accounts for the competition between SiO2 and the Fe2O3 + P2O5 pool. The second factor is related to the accumulation of Al and Ti (oxyhydr)oxides, and low-solubility critical metals, such as Ga, Nb, Hf, and Ta, during late stages of bauxitization under dry conditions. The Nasr-Abad bauxite ores are remarkably enriched in critical metals Ti, Nb, Ta, V, and Co with respect to the average Upper Continental Crust. Among these, Ti, Nb, and Ta are particularly enriched in the uppermost Al-rich bauxite and clayey bauxite ores, whereas V and Co are particularly enriched in the bauxitic iron ores.