Clayey bauxite from the Irano-Himalayan belt
The Semirom karst type clayey bauxite is located in the Zagros zone, a part of the Irano-Himalayan belt, and occurs between the Sarvak and Ilam Formations. The Albian to Cenomanian limestone bedrock of the Sarvak Fm is a part of a platform ramp environment including clay bearing components. Diaspore, boehmite, kaolinite, and hematite are the major ore-forming minerals. Elemental mobility determinations reveal that Th and the critical metals Co, Ni, and Cr are enriched in the bauxitic horizons relatively to the Upper Continental Crust. Further, only one sample is outlier for more than one critical metal (Cr, Co, Ni, and Ga) likely recording a specific event during bauxitization. The outlier is enriched in Ni, Co, Ga, and Th, is depleted in Cr, and shows a large positive Ce anomaly (Ce/Ce = 3.03). The outlier may record an event characterized by more acidic and oxidizing conditions and may have a regional relevance. R-mode factor analysis suggests that Cr and Co are likely adsorbed onto oxy-hydroxides probably in horizons also concentrating secondary phosphates hosting HREE and Th. Nickel is instead likely adsorbed onto titanium oxides which concentrate in the pool of highly insoluble detrital phases also including zircon whereas diaspore and boehmite are important in controlling both Ga and LREE. The low contents of Ni and Cr associated to the Semirom clayey bauxite exclude a provenance from a mafic protolith. Further, most of the samples have Eu/Eu values close to those observed in the Sarvak Fm marly limestone, suggesting that the clayey component within the underlying limestone acted as protolith. The oxygen and hydrogen isotopic composition of the kaolinite-rich separates from the Semirom deposit indicates temperature never lower than 30 °C, supporting the idea of the Zagros basin close to the equator during the Cenomanian-Turonian under a warm and humid tropical climate with heavy rainfall.