Gheshlagh Permo-Triassic bauxite deposit in the Eastern-Alborz Zone: Geology and petrography

The Alborz structural zone in northern Iran is the host of a number of bauxite deposits. The Gheshlagh bauxite deposit is one of them with more than 2 km long and a thickness of about 20 m in the eastern part of the Alborz structural zone. This deposit has been developed as a stratiform horizon along the contact zone of Permian limestones and Triassic dolomitic limestones. The basal contact zone of the bauxite horizon is mainly undulatory, whereas the upper contact zone is concordant with the hanging-wall dolomitic limestones. Based on mineralogical and textural evidences, the horizon can be divided from top to bottom into five distinct units: (1) upper bauxite unit with a thickness of about 5 m, is composed of boehmite, hematite, kaolinite, rutile and svanbergite, 2) upper kaolinite unit approximately 2-3 m thick and is composed mainly of clay minerals, 3) hard bauxite unit with a thickness of approximately 1.5 m, mainly consists of hematite, kaolinite, boehmite and diaspore, 4) lower kaolinite unit approximately 1-4 m thick and is composed of kaolinite, boehmite and diaspore, and 5) lower bauxite unit about 4-6 m thick. Textural analyses indicate both allochthonous and autochtonous origins for these deposits. Combination of geological, textural and mineralogical data of the Gheshlagh bauxite deposit correspond to a boehmitic-diasporic karst bauxite deposit type.