Geochemistry and genesis of rare earth minerals in the Tange Pirzal karstic bauxite deposit, Zagros Folded Belt, SW (Iran)

The present study focuses on the geochemistry and genesis of REE minerals in the Tang-e Pirzal karst bauxite deposit. The deposit is located about 45 km east of Dehdasht city in the central part of the Zagros Fold Belt. The ore bodies comprise a series of contiguous lenses that occur as infills of deep karst cavities, depressions, and fractures within the upper neritic carbonates of the Cenomanian–Turonian Sarvak Formation and are structurally controlled by strike-slip fault systems. The most frequent authigenic REE minerals are fluorocarbonates of bastnäsite group that form a solid solution series between end members parisite (CaCe2(CO3)3F2) and bastnasite (CeCO3F). The formation of bastnasite-Ce and parisite-Ce suggests that cerium as fluoride and/or carbonate–fluoride complexes were simply leached by acidic downward solutions and finally precipitated on the geochemical barrier of the carbonate bedrock under alkaline and reducing conditions. Cerianite (CeO2) occurs mainly as fine grains in the porous aggregates of the cryptocrystalline matrix. Cerianite precipitation may occur due to the predominance of acidic/oxide conditions in the upper parts of the bauxite profile. Examination of the vertical distribution of the ∑REE, (La/Yb)N, Ce/Ce*, and Eu/Eu* indexes indicates progressive enrichment of the REE and slight LREE/HREE fractionation toward the lower parts of the bauxite profile. Cerium behaves differently from the other REEs and shows fluctuating behavior throughout the Tang-e Pirzal profile, which can be related to periodical fluctuations of the groundwater table in response to environmental changes.