Abstarct: Tash bauxite deposit is located in 40 km NW Shahrood in NE of the Alborz structural zone. the study area are affected by faults and folds with general trend of NE – SW. The Tash bauxite in the Aliabad 1/100000 geological sheet is situated between 54° 41' to 54° 48 ' eastern longitudes, and 36° 32' to 36° 37' northern latitudes. The main stratigraphic Formations in this area are including Mubarak, Dorud , Ruteh Elika and Shemshak F.. Bauxite is created by erosion and weathering of basalts. Basalt rocks resulted in Eo-Cimmerian orogenic phase in Upper Triassic. The recent rocks limited stratigraphicaly to baxse of Shemshak F. The bauxite horizon over lying karstic carbonates of Elika F. and its upper border as angular unconformity is covered by Shemshak F.. Thin sections mineralogical studies display that oolitic, pisolitic, plitomorphic, collomorphic-fluidal and dissolution textures indicating to authigenic origin. The results of X-ray diffraction studies show that the the main minerals are diaspore, boehmite, kaolinite, hematite, shamosite, anatase and quartz. Also polished sections identify pyrite, chalcopyrite, goethite and hematite as ore minerals. Textural and mineralogical observations indicate that bauxite laxyer are individed into four separate units: Lower kaolinite, shale bauxite, hard bauxite and upper kaolinite units. The geological and geochemical documents and calculations of accumulation coefficient of trace elements demonstrate that source rocks of bauxite have the characteristics of intermediate mafic rocks. Silica and iron releaching as secondary and epigenetic processes is also observed in area. According to the proposed genetic model, under suitable climatic conditions in the Late Triassic, basalt rocks weathering created the soil bauxite. The subsequent stage the soil bauxite carried in short – distance and finally deposited into karstic cavities of Elika Formation.