Geochemical constraints on the middle Triassic Kani Zarrineh karst bauxite deposit, Irano–Himalayan belt, NW Iran: Implications for elemental fractionation and parental affinity

Abstract

The middle Triassic Kani Zarrineh bauxite deposit, in the Irano–Himalayan belt, located 20 km northwest of Saqqez city in northwestern Iran, occurs in depressions of the early Triassic bedrock carbonates of the Elika Formation. The ores formed during the Carnian Pluvial Event under high atmospheric pCO2 conditions and humid episodes. Powder-XRD analyses revealed that the Kani Zarrineh bauxite is mainly composed of diaspore, hematite, and kaolinite. There is a broad inverse relationship between hematite abundance and diaspore abundance in the profile with a downward increase of hematite and an upward increase of diaspore. Among minor constituents, single crystals rich in high field strength elements, such as florencite, monazite-(Ce), xenotime-(Y), and zircon were detected by SEM-EDS analyses. R-mode factor analysis indicates that most of the variance of the dataset is explained by a factor accounting for the competition between the Al2O3–TiO2–Ga–HFSE pool and the SiO2–Fe2O3–U–first row transition metals pool, and related to the coupled effect of a change of redox conditions and climatic regime. The other two factors are related to the importance of phosphate minerals (e.g., florencite and monazite) in hosting LREE, and to the influence that the chemistry of the soil solutions, especially pH, exerted in promoting the preferential mobility of HREE. Both these factors contribute to explaining LREE/HREE fractionation in the Kani Zarrineh bauxite ores. A negative correlation between the Ce/Ce* and the TOC (r = –0.91) corroborates the fact that bauxitization occurred in an environment characterized by a progressive increase in Eh upward. The coupled Ce/Ce* increase and TOC decrease upward are likely related to soil aeration, a progressive increase in soil temperature, and a reduction in soil moisture. An upward decrease of soil moisture is also consistent with an increase in the abundance of Al hydroxides that require drier conditions than those necessary for the formation of Fe oxyhydroxides. Finally, the evaluation of elemental mobility and results of the R-mode factor analysis indicate that, in addition to Al, high field strength elements Nb and Ta can be used confidently for assessing parental affinity of the Kani Zarrineh bauxite ores. The Al/Ta and Nb/Ta ratios and the Sm/Nd and Eu/Eu* provenance proxies reveal that mafic rocks interbedded in the early Triassic bedrock carbonates of the Elika Formation are likely protoliths of the Kani Zarrineh bauxite ores. This finding is consistent with what is observed for some bauxite deposits from the Mediterranean-type karst bauxite belt, especially from northwestern Iran of different ages that basalt rocks were the parent rock of these deposits.