The tetrad-effect in rare earth elements distribution patterns of titanium-rich bauxites
The titanium-rich bauxite horizon of the Kanigorgeh bauxite (northeast of Bukan, NW Iran) is a part of the globally signiﬁcant known Irano-Himalaya karst bauxite belt. This bauxite horizon was developed as discontinuous stratiﬁed layers and/or lenses within carbonate rocks of the Ruteh Formation of Upper Permian age. Studies of whole rock geochemistry along a selected proﬁle across the horizon revealed that Ce anomaly values of the bauxite ore samples vary from 0.22 (negative) to 7.34 (strongly positive). The remarkable variations in ratios of some isovalent elements such as Y/Ho, Nb/Ta, and Zr/Hf along the proﬁle illustrate non-CHARAC behavior which can be attributed to so-called tetrad-eﬀect phenomenon in this residual horizon. The chondritenormalizedREEdistributionpatternsrepresenttheirregularshapesknownasM-andW-shapetetrad-eﬀects.The illustration of diagrams of fourth tetrad-eﬀects versus Ce anomaly values and ratios of Y/Ho, Nb/Ta, and Zr/Hf indicated that the studied residual horizon comprises two distinct zones with diﬀerent geochemical behaviors. The upper zone, from top surface down to ~7.3 m, was deposited under oxidizing conditions represented by the bauxitic composition.Thelowerzone,startingfrom~7.3 mdownto thebottomofthehorizon, wasprecipitated under reducing conditions manifested by the kaolinitic composition. The obtained results led us to deduce that the tetrad-eﬀects can be used as a geochemical indicator to evaluate the depositional conditions of bauxite deposits. It has been further concluded that certain mechanisms such as mineral crystallization, REE-complexation, penetration of meteoric acidic surface waters, and lateritization intensity had striking inﬂuence upon occurrence of the tetrad-eﬀects in the studied titanium-rich bauxite ores in the Kanigorgeh district.