Stratigraphic, Petrographic and Facies Characteristics of Tash and Astaneh Bauxite Deposits in Easthern Alborz: Paleoenvironmental Implications

Tash and Astaneh bauxite deposits are located about 40 km north of Shahroud and 30 km northwest of Damghan, respectively. The Tash deposit has been developed as a stratiform horizon along the contact zone of Ruteh limestones and Elika dolomitic limestones, whereas the Astaneh deposit located along the contact zone of Triassic dolomitic limestones and Jurassic shales and sandstones. Pelitomorphic and microgranular textures are the most abundant matrix textural elements and ooids, pisoids and pellets are the main separated textural elements in these deposits. Textural analyses indicate both allochthonous and autochtonous origins for both deposits. Based on petrographic and facies evidences the Tash and Astaneh deposits can be divided into five and three distinct units, respectively. The Tash deposit is mainly composed of diaspore, anatase, kaolinite, hematite, goethite and berthierine, whereas the quartz, hematite, goethite kaolinite and anatas are the most abundant minerals in Astaneh. Combination of stratigraphic, petrographic and facies data show that Tash and Astaneh deposits formed in three and two main stages, respectively. First, bauxite materials and clay minerals were developed as authigenic bauxitization processes of the parent rock. During the second stage, these materials were transported to karst depressions, where they accumulated as a relatively thick bauxite horizon. Finally, during the third stage which has occurred only in Tash deposit, the ore upgraded by in situ leaching and desilicification. Based on geological, mineralogical and textural evidences both deposits can be grouped as karst bauxite type.