Geochemical and mineralogical features of karst bauxite deposits from the Alborz zone (Northern Iran): Implications for conditions of formation, behavior of trace and rare earth elements and parental affinity

Abstract

The present study focuses on three outcrops of Upper Permian to Lower Jurassic karst bauxite deposits in the Alborz zone, north of Iran. The deposits occur as sharp semi-continuous lenses within the Upper Permian Ruteh Formation (Gheshlagh) and the Upper Triassic Elika Formation (Siahrudbar and Separdeh) carbonate rocks. Textural characteristics such as pelitomorphic matrix, local ooidic-pisoidic, fluidal, and clastic textures indicate authigenic-allothigenic origins. Mineralogically, kaolinite, boehmite, goethite, and hematite in the Gheshlagh, diaspore, hematite, and kaolinite in the Siahrudbar, and kaolinite, hematite, goethite, diaspore, siderite, and chamosite in the Separdeh deposits are the main phases. The distribution patterns of major elements normalized to the upper continental crust (UCC) depict enrichments of Al, Fe, and Ti in all three deposits. The three deposits are variably enriched in transition trace elements (Cr, Ni, Co, V), U, Th, and high field strength elements (Ga, Hf, Zr, Nb, Y) relative to UCC. The studied bauxites have average concentrations of rare earth elements (ΣREE, La-Lu) between 305.97 and 373.69 ppm. Geochemical data reveal progressive enrichment of REE and an intense (LREE/HREE)N fractionation toward the lower parts of the profile in the Separdeh deposit due to the alkaline conditions of the carbonate bedrock, and irregular distribution patterns at the Gheshlagh and Siahrudbar deposits due to water level and pH fluctuations. The positive Ce anomalies at Separdeh and Siahrudbar suggest oxidizing conditions in the upper parts of the profiles. At Gheshlagh, the negative to nil Ce anomalies reflect low intensity of oxidation. Based on previous geochemical analysis data, the Upper Permian basalts in the Gheshlagh, and the Upper Triassic basalts in the Separdeh and Siahrudbar areas were the most likely source materials. The local uplift of the Permian-Upper Triassic series led to the formation of karstic topography filled with materials derived from weathering of the basaltic rocks under humid tropical paleoclimate. The initial weathering converted the source rocks to kaolinitic soils characterized by the enrichment of Al, Fe, and Ti. Then, they were transferred to the karstic depressions and sinkholes and underwent in situ leaching and desilicification bringing about a relatively higher ore grade. The overall field and petrographic evidence demonstrate that these deposits were formed in shallow and swampy environments. In the eastern part of the Alborz zone (at Siahrudbar and Gheshlagh), better drainage conditions have enhanced the quality of ores.