Rare earth element geochemical characteristics of the late Permian Badamlu karst bauxite deposit, NW Iran

Abstract

The Badamlu karst bauxite deposit, located 55 km southeast of Miandoab city in northwestern Iran occurs in karstic depressions of carbonates of the late Permian Nessen Formation. The studied bauxitic ores have a dominant pelitomorphic texture, and locally developed ooidic, pisoidic, and clastic textures in a pelitomorphic matrix. Mineralogically, the bauxite ores are composed mostly of diaspore, hematite, and kaolinite, as well as lesser amounts of rutile, chlorite, goethite, and illite. In addition, REE minerals, such as churchite and cerianite were detected. Cerianite occurs in the core of ooids, and churchite is surrounded by a kaolinite-rich matrix. Churchite and cerianite formed in acidic and oxidizing conditions in the uppermost part of the weathered profile, respectively. Total rare earth elements La-Lu contents in the bauxite ores range widely from 29.1 to 2138.2 ppm (average = 746.5 ppm, std = 722.3 ppm), with Ce being the dominant REE (1.3–1825.0 ppm, average = 606.7 ppm, std = 645.1 ppm). The ΣREE contents, (LREE/HREE)ch ratio, and Ce anomaly in the bauxite ores increase upward through the profile, in which LREE and HREE are La–Eu and Gd–Lu, respectively. The Ce anomaly in the bauxite ores has a bimodal distribution with higher Ce anomalies in the upper 7.5–m-thick succession (2.7–20.8, average 10.5), compared to the lower and middle 4.5–m-thick succession (0.1–0.3, average 0.1). The downward decrease in the Ce anomaly may be due to the lack of cerianite and a decrease in the oxidation state. The REE distribution patterns normalized to chondrite exhibit a variable depletion/enrichment of LREE and HREE, with a negative Eu anomaly (0.3–0.7).