Geochemistry and secular trends in the middle–late Permian karst bauxite deposits, northwestern Iran

This study focuses on the middle–late Permian bauxite deposits found within karstic depressions and sinkholes of carbonates of the Ruteh and Nessen Formations, northwestern Iran. Data concerning five karst bauxite deposits, each related to a different stratigraphic position within the bedrock, were selected to address the chemical distribution and inter-elemental relationships, especially for critical elements, factors that control bauxite formation, and the parental affinity. The analyzed samples, mainly clayey bauxite and bauxitic clay, are characterized by large compositional fluctuations involving both major and trace elements. The major element composition is dominated by SiO2, TiO2, Al2O3, and Fe2O3. Among the critical elements, compositional fluctuations induce the occurrence of outliers for Co, Nb, Ta, LREE, and HREE, similar to other trace elements, such as Ba, Sr, Cr, U, and Zr. Elemental mobility indicates that most of elements are generally depleted. Among the critical elements, Co is enriched only in several samples of the older deposits, and ΣREE are enriched in some samples of the younger deposits. Factor analysis suggests that, due to the observed Al–Ga covariance, fairly acidic conditions were never attained during bauxitization, and that detrital resistant minerals concentrated during a bauxitization stage, promoting Al–Ti accumulation. Furthermore, the U co-variance with low soluble elements (Nb, Ta, and Th) suggests that U was mostly conservative during the formation of deposits. Cobalt covaries with Cr, and these elements accumulated during later bauxitization stages, when the massive leaching of silica occurred. The CIA, CIW, and PIA paleo-weathering proxies indicate an upward weathering increase, which suggests a trend toward more extreme weathering conditions in a wetter and warmer climate, according to the Permian global warming. This is consistent with an upward increasing Ba/Sr ratio and the inspection of the A–CN–K plot, where the younger deposits fall close to the A–K edge, involving a clayey precursor requires a rainfall increase. Interestingly, the upward trend toward a warmer and wetter climate was also coupled with paleo-redox conditions promoting cerium oxidation, as depicted by the Ce/Ce* values. Finally, the Eu anomaly and the Sm/Nd ratio indicate that the studied bauxite deposits were mainly derived from a mafic protolith that formed in the middle–late Permian, as a consequence of the volcanism that affected the entire Iranian platform. A cluster of karst bauxites, however, has Eu/Eu* values lower than the average-standard deviation range of the mafic rocks, suggesting a possible contribution of a more differentiated and less mafic protolith. Similarly, the U/ Th ratio, at large, supports the hypothesis of a mafic protolith for the middle–late Permian bauxite deposits, although some samples show the U/Th values well above the average-standard deviation range of the mafic rocks, likely indicating that processes promoting the accumulation of resistant minerals with high U/Th ratios cannot be excluded. This means that, in this case, the U/Th ratio does not necessarily reflect the protolith of karst bauxites and that, more in general, this proxy needs to be carefully interpreted when used to assess the parental affinity of residual rocks.