Geochemistry of Upper cretaceous bauxite deposits, Zagros Fold Thrust Belt, SW Iran: Paleoenvironment and provenance constraints

Iranian's karst bauxites, as part of the Irano-Himalayan bauxite belt, are related to four structural zones, including the Zagros Fold-Thrust Belt (ZFTB). The ZFTB consists of 7–12 km of the Paleozoic to Pliocene carbonate sediments deposited on the Precambrian basement and Arabian continental margin. In the ZFTB occur several karst bauxite deposits between the Cenomanian-Turonian shallow-marine argillaceous of the Sarvak Formation and the Santonian-Campanian carbonates of the Ilam Formation. Compositional data of karst bauxite deposits may address paleogeographic restoration and record paleoclimate, and here we discuss a dataset including the Mombi, Bidgol, Tang-e Pirzal, and Bagoushi bauxite deposits in the ZFTB. Although the ZFTB karst bauxites show similar texture, they are different in composition, reflecting different paleoenvironmental formation conditions. In most of the studied bauxites, the composition of ooidal concretions indicates the occurrence of long periods of humid tropical climate followed by short and intermittent dry-to-wet conditions. In the Bidgol deposit, a different pattern is recognized, reflecting the predominance of dry climates and characterized by low aqueous silica activity and low water activity. The formation of iron oxyhydroxide minerals is likely due to a multistage process involving the circulation of Fe-rich solutions in micro-cracks or veins due to pyrite and chamosite dissolution and Fe2+ oxidization. In the ZFTB bauxites, two main trends of Ce/Ce* distribution with depth, a proxy of paleo-redox evolution during bauxitization, are recognized. In the Bidgol and Mombi deposits, Ce/Ce* is negative in the basal portion and increases upward. The positive Ce-anomalies in the upper parts of these deposits are due to ceranite formation under oxidizing environment near the surface. In the Bagoushi and Tang-e Pirzal deposits, Ce/Ce* is close to the unit with the notable exceptions of a few samples characterized by significant and positive Ce/Ce* anomalies. This feature suggests that cerianite, originally ubiquitous in these deposits, was affected by congruent dissolution through cerium reduction with Ce3+ transportation by acidic downward percolating solutions as fluoride complexes (REEF2+ and REEF2+), carbonate complexes (REECO3+) and carbonate–fluoride complexes (REECO3F) followed by fluorocarbonate minerals precipitation. This process has been repeated several times with maximum distribution throughout the profile due to environmental changes, including periodic fluctuations in the groundwater level. Finally, the Sm/Nd, Nb/Ta, and Eu/Eu* provenance proxies indicate that the protolith(s) from which the ZFTB bauxite deposits are derived is UCC-like, thus excluding supply of mafic material and pointing toward a source material primarily derived from the bedrock argillaceous sediments. In addition, several samples record provenance from siliciclastic craton-derived sediment, suggesting that the ZFTB was connected with a continental margin during the Jurassic-early Cretaceous.