The Taragheh titanium-rich karst bauxite deposit, northwestern Iran: Constraints on REE fractionation, Ce anomaly, and provenance

Abstract

The Taragheh Ti-rich bauxite ores occur as distinct stratiform layers at the contact between the middle–late Permian limestones of the Ruteh Formation and the Triassic dolomites of the Elika Formation. These ores are primarily composed of diaspore, hematite, pyrophyllite, and chlorite, with smaller amounts of boehmite, rutile, anatase, goethite, ilmenite, siderite, amesite, and illite. Cerianite and parisite-(Ce) were identified in the ores by SEM–ESD analysis. The studied bauxite samples exhibit ooidic, colloform, and nodular textures. The Al₂O₃–SiO₂–Fe₂O₃ ternary diagram indicates that the destruction of kaolinite and deferruginization were crucial in the formation and evolution of these ores. Variations in the La/Y ratio (0.35–4.51) along the profile suggest a change in pH of depositional environment from acidic to alkaline conditions downward. The increasing downward (LREE/HREE)_N and (La/Yb)_N ratios are attributed to bicarbonate complexation and the preferential adsorption of LREE onto hematite and goethite. Positive Ce anomalies in the upper part of the profile reflect the precipitation of cerianite, due to the oxidation of Ce³⁺ to Ce⁴⁺. Following the fall of the underground water table and the occurrence of Ce in the form of CeCO₃F⁰_(aq) close to carbonate bedrocks, parisite-(Ce) forms in the bottom of the deposit under alkaline conditions. Additionally, positive Ce anomalies in the basal part of the profile may result from the preferential adsorption of Ce onto hematite. Geochemical evidence suggests that the Eu/Eu* and Sm/Nd ratios were not affected by diagenetic processes. The shales of the Ruteh Formation and basaltic rocks are proposed as the provenance for the Taragheh karst bauxite deposit.