Ali Asghar Calagari

The limestones of the Tizkuh Formation, deposited along the southern margin of the Neo-Tethys Ocean during the Cretaceous, provide an opportunity to investigate REE behavior, depositional processes, and the extent to which primary marine geochemical features may have been retained despite post-depositional modification. In this study, integrated mineralogical (XRD and SEM–EDS) and geochemical (ICP–AES and ICP–MS) data from 15 limestone samples were used to evaluate the controls on major- and trace-element and REE distributions and to assess the preservation of selected marine and redox-related features. The limestones are dominated by calcite, with subordinate kaolinite, quartz, dolomite, illite, hematite, goethite, plagioclase, and locally identified apatite. Negative correlations of CaO with SiO₂, Al₂O₃, TiO₂, Ga, Nb, Zr, Ta, and Th indicate that variations in the relative proportions of carbonate and non-carbonate materials were an important control on whole-rock geochemical variability. Positive associations among Ti, Zr, Nb, Ta, and Ga further support a contribution from siliciclastic materials to the non-carbonate fraction. Total REE concentrations (ΣREE = 31.54–62.18 ppm) are within the range of those commonly reported for marine carbonate rocks and do not indicate anomalous bulk-rock REE enrichment. The positive correlations of ΣREE with Al₂O₃, SiO₂, Th, Zr, Y, and P₂O₅ denote that both siliciclastic and phosphate-bearing components contributed to the bulk-rock REE inventory. The positive relationship between ΣREE and Al₂O₃ (r = 0.55), together with the broadly similar Al-normalized REE patterns among the studied samples, signifies that variations in absolute REE abundances were partly associated with differences in relative proportions of the carbonate and non-carbonate components. However, the consistency of the Al-normalized patterns suggests that the overall REE fractionation pattern cannot be attributed solely to variations in terrigenous dilution. The SEM–EDS data support the occurrence of apatite in the selected samples. The PAAS-normalized REE patterns show limited LREE enrichment relative to HREE, indicating that the REE system does not represent an unmodified seawater signature and was influenced by the non-carbonate components and post-depositional processes. Weak negative Ce anomalies (Ce/Ce* = 0.76–0.94), together with Pr/Pr* values close to unity and relatively low U/Th (0.04–0.11) and V/Cr (0.77–1.90) ratios, are compatible with relatively oxidizing marine depositional conditions. However, the preservation of a primary seawater Ce signal remains tentative because Ce/Ce* is associated with the non-carbonate and REE-bearing components, while U and V may be affected by diagenetic redistribution and other post-depositional processes. Positive Eu anomalies (Eu/Eu* = 1.44–1.98) are interpreted cautiously and may reflect a combination of mineralogical controls, limited detrital feldspar contribution, and possible early diagenetic and/or fluid–rock interaction. Overall, the Tizkuh limestones represent a mixed and partially modified carbonate geochemical archive in which selected depositional and redox-related features may have been retained. Consequently, reconstruction of the Cretaceous paleoenvironment requires a multi-proxy approach integrating REE patterns, Ce and Eu anomalies, Y/Ho ratios, U/Th, V/Cr, detrital and phosphate indicators, Al-normalized REE patterns, geochemical relationships, and mineralogical evidence.

The Vali–Janlou kaolin deposit is located in the northern part of the Urmia–Dokhtar magmatic belt, central-northern Iran, and is hosted by middle Eocene rhyodacitic volcanic rocks. The Vali–Janlou kaolin deposit is one of the most important sources of raw material for ceramics industries in Iran. No trace element geochemical characterizations of this deposit have been conducted in detail before, and this is the main objective of the current research work. Kaolinite and quartz are the major mineral phases present in this deposit, accompanied by some minor phases like illite, rutile, pyrophyllite, dickite, alunite, diaspore, and chlorite. The calculation of mass balance changes revealed that the kaolinization of the rhyodacitic rocks was accompanied by the enrichment of Sr, Zr, Hf, Ta, Nb, U, Th, Y, La, and Pr, leaching–fixation of Sm, Nd, and HREEs, and depletion of Rb, Cs, Ba, Pb, V, Cr, Zn, Eu, and Ce. The behavior of trace elements during kaolinization was controlled by factors such as variation in the pH and temperature of the hydrothermal fluids, the residual concentration, and the presence of mineral phases resistant to alteration. The occurrence of negative Eu anomalies during kaolinization indicates plagioclase destruction by high-temperature hydrothermal solutions and also the liberation of Eu²⁺ during a decreasing intensity of hydrothermal alteration. The presence of diaspore, dickite, and pyrophyllite together with the differentiation of HREEs from one to another, the occurrence of robust negative Ce anomalies, the strong positive correlation between P₂O₅ and LOI, and geochemical parameters like Ce + La + Y, Nb + Cr, Rb + Sr, and Y/Ho are all indicative of the effective role of hypogene processes in the evolution of this deposit.