The Shirinabad clay-bauxite deposit with more than 1 km long and about 8 m in
thickness is located in 60 km south-east of Gorgan. The Shirinabad deposit has been
developed as a stratiform horizon along the contact zone of Triassic dolomitic limestones
and Jurassic shales and sandstones. The basal contact zone of the horizon is mainly
undulatory, whereas the upper contact zone is concordant with the hanging-wall shales
and sandstones. The rocks within the horizon show pelitomorphic, microgranular, oolitic
and pisolitic textures. Textural analysis indicates both allochthonous and autochtonous
origins for the Shirinabad deposit. Based on textural and mineralogical evidences the
deposit can be divided into four distinct units. Kaolinite, anatase, routile, bohemite,
hematite, goethite and berthierine are the principal constituents. From geochemical data,
it is concluded that the Shirinabad deposit probably originated from basaltic volcanic
rocks. Combination of mineralogical and geochemical data shows that the Shirinabad
deposit formed in two stages. First, bauxite materials and clay minerals were developed as
authigenic bauxitization processes of alkaline basaltic parent rock. Then, these materials
were transported to karst depressions and formed the Shirinabad clay-bauxite deposit.
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